From 20646ee58ac26453df0c85166a4e8787d33db5bd Mon Sep 17 00:00:00 2001 From: ackman678 Date: Mon, 17 Oct 2016 11:19:27 -0700 Subject: [PATCH] lecture07,08 init --- 2016-10-03-lecture04.md | 5 + 2016-10-03-lecture05.md | 42 +- 2016-10-10-lecture06.md | 706 +++++++++++++++++ 2016-10-16-lecture07.md | 761 ++++++++++++++++++ 2016-10-16-lecture08.md | 1670 +++++++++++++++++++++++++++++++++++++++ 5 files changed, 3167 insertions(+), 17 deletions(-) create mode 100644 2016-10-10-lecture06.md create mode 100644 2016-10-16-lecture07.md create mode 100644 2016-10-16-lecture08.md diff --git a/2016-10-03-lecture04.md b/2016-10-03-lecture04.md index c205af5..3a834bb 100644 --- a/2016-10-03-lecture04.md +++ b/2016-10-03-lecture04.md @@ -464,6 +464,11 @@ Active and Passive current flow. Note: +Might here in other classes, especially human physiology about absolute and relative refractory periods. + +Refractory period we've been discussing here is mostly the absolute refractory period-- due to Na+ channel inactivation. + +Relative refractory period is the transient hyperpolarization during the undershoot when potassium conductance is still greater than normal-- when K+ conductance returns to resting value the relative refractory period is over. Until this time, a greater secondary depolarizing stimulus will be required to reach AP threshold. -- diff --git a/2016-10-03-lecture05.md b/2016-10-03-lecture05.md index 252a57e..c62a932 100644 --- a/2016-10-03-lecture05.md +++ b/2016-10-03-lecture05.md @@ -416,23 +416,6 @@ from wikipedia: ## Structure of the bacterial K⁺ channel -
Each subunit has 2 transmembrane domains, 4 subunits make a channel
Neuroscience 5e Fig. 4.7
- -Note: - -(Doyle et al, Science 280:69, 1998) - - - - ---- - -## Structure of the bacterial K⁺ channel - * Bacteria have K⁺ channels that are very similar in structure to mammalian K⁺ channels. Main difference is that they are not gated by voltage * Could be crystallized in the bacterial membrane * 3D structure tells us a lot about function @@ -459,6 +442,24 @@ Note: (Doyle et al, Science 280:69, 1998) +--- + +## Structure of the bacterial K⁺ channel + +
Each subunit has 2 transmembrane domains, 4 subunits make a channel
Neuroscience 5e Fig. 4.7
+ +Note: + +(Doyle et al, Science 280:69, 1998) + + + + + --- ## Structure of a bacterial K⁺ channel determined by crystallography @@ -598,6 +599,8 @@ Note: Yellow are voltage sensing tm domains +4–8 positively-charged amino acids in the S4 domain. Experiences force in a transmembrane electric field. Is the electric-field sensor for voltage-dependent gating. + K channels are more diverse - Kv2.1 show little inactivation and are closely related to the delayed rectifier K channels involved in AP repolarization @@ -722,10 +725,15 @@ charybdotoxin from scorpions K channels ## Diseases caused by altered ion channels +
+
+ EA1: episodic ataxia type 1 (abnormal limb movements and severe ataxia) BFNC: benign familial neonatal convulsion. Frequent brief seizures starting in first postnatal week then disappearing in a few months. Mutation mapped to two K⁺ channel genes +
+
Neuroscience 5e Box 4D
diff --git a/2016-10-10-lecture06.md b/2016-10-10-lecture06.md new file mode 100644 index 0000000..d9dcb1f --- /dev/null +++ b/2016-10-10-lecture06.md @@ -0,0 +1,706 @@ +## Synaptic transmission + +* Synapses– functional contacts between neurons +* Two general classes– chemical and electrical synapses +* Chemical– neurons talk to each other by release of neurotransmitters +* Electrical– direct, passive flow of current between neurons + + +Note: + +Thus far we’ve discussed how neurons generate action potentials that propagate down axons with high fidelity over cm’s to to meters of space and the ion channels in the membrane that underly voltage dependent excitability. + +But is through synapses that neurons actually talk with one another and it is also through synapses that the nervous system effects behavior function enabling us to interact with the world around us– in other words there are synapses between pairs of neurons that form the basis of inter-neuronal communication as well as synapses on muscle fibers that neurons use to get our muscles to contract. + +Now there are two general classes of synapses, chemical... +and electrical... + +*todo: motor neuron - muscle fiber model* + + + +--- + +## Electrical and chemical synapses differ in their transmission mechanisms + +
chemical synapse
Neuroscience Box 5A
+
electrical synapse
Neuroscience 5e Fig. 5.1
+
electrical synapse
Neuroscience 5e Fig. 5.1
+ + +Note: + + +--- + +## Electrical synapses + +* Less common than chemical synapses +* The cell membranes of two cells are linked together via gap junctions +* Current flows **directly** from one neuron to another via gap junctions– form large pores between cells made up of connexin proteins +* The signal is very fast– the only limit is diffusion +* Signals can go in both directions +* Are used to synchronize electrical activity among populations of neurons + +Note: + +These electrical synapse or gap junction synapses are thought to be more common among inhibitory interneurons in the brain— + +quadrillion synapses, 10^15 in our nervous system + +--- + +## Gap junctions allow current to flow from one cell to the next + +
Neuroscience 5e Fig. 5.1
+ + + +Note: + +* connexins— extracellular loops and disulfide bridges +* 3.5nm separating the apposed lipid bilayers connected through connexon hemichannels +* 20-40nm separation at a chemical synaptic cleft + +* passive ionic current flow, small substance like ATP and second messengers + + + + +Current in the presynaptic cell is not felt directly by post-synaptic cell for a chemical synapse. + +-- + +## Electrical synapses + +
Neuroscience 5e Fig. 5.2
+ + +Note: + +In Crayfish an action potential in one neuron spreads quickly to the next + +-- + +## Electrical synapses + +
Neuroscience 5e Fig. 5.2
+ +Note: + +In hippocampal neurons gap junctions can make neurons fire in synchrony + +--- + +## Electrical Synapses: putative functions + +* Synchronization of the electrical activity of large populations of neurons + * the large populations of neurosecretory neurons that synthesize and release biologically active peptide neurotransmitters and hormones are extensively connected by electrical synapses +* Synchronization may be required for neuronal development, including the development of chemical synapses +* Synchronization may be important in functions that require instantaneous responses, such as reflexes and pacemakers + +Note: + +quadrillion synapses, 10^15 in our nervous system + +important in diseases of pathological oscillations/synchrony like childhood epilepsy, etc + +--- + +## Chemical synapses + +* The majority of connections use chemical synapses +* They form at the synaptic cleft +* Presynaptic cells have synaptic vesicles that have neurotransmitters in them +* Post-synaptic cells have neurotransmitter receptors on the plasma membrane + +Note: + + +--- + +## Synapse structure as seen by electron microscopy + +
chemical synapse, type 1
[SynapseWeb, Kristen M. Harris, PI](https://synapseweb.clm.utexas.edu)
+ +
chemical synapse, type 2
[SynapseWeb, Kristen M. Harris, PI](https://synapseweb.clm.utexas.edu)
+ +
synaptic vesicles
[SynapseWeb, Kristen M. Harris, PI](https://synapseweb.clm.utexas.edu)
+ +
synaptic cleft
[SynapseWeb, Kristen M. Harris, PI](https://synapseweb.clm.utexas.edu)
+ +Note: + +* synapse, Gray type 1 is asymmetrical synapse. Usually excitatory synapse. Spherical vesicles. +* synapse, Gray type 2 is symmetrical synapse. Usually inhibitory synapse. Elongated vesicles. + +--- + +## 11 steps of synaptic transmission + +
+
+ +1. Neurotransmitter is synthesized and packaged into vesicles +1. An action potential invades the presynaptic terminal +1. Depolarization causes opening of voltage-gated calcium channels +1. There is a rapid influx of Ca²⁺. 1000x concentration difference across the membrane(1x10⁻⁴ mM inside, 1 mM outside) +1. Calcium causes vesicles to fuse with membrane +1. Neurotransmitter is released into cleft +1. Transmitter binds to receptors on postsynaptic cell +1. This opens or closes postsynaptic channels +1. Postsynaptic current flows inside post-synaptic cell +1. Removal of neurotransmitter by glia uptake or enzymatic degradation +1. Retrieval of membrane via endocytosis + +
+ +Note: + + +--- + +## Synaptic transmission + +
+ + +Note: + +* Action potential in the presynaptic neuron opens voltage-gated Ca²⁺ channels +* Ca²⁺ influx raises [Ca²⁺]i in the nerve terminal +* Elevated [Ca²⁺]i triggers the fusion of synaptic vesicles to the plasma membrane of the presynaptic neuron and exocytosis +* Neurotransmitter is released into the synaptic cleft where it diffuses about +* Neurotransmitter binds to specific receptors in the postsynaptic neuron causing channels in that cell to open or close +* Direct action on ligand gated channels +* Indirect action on G-protein coupled channels +* The neurotransmitter is inactivated and/or removed from the synaptic cleft (active transport into presynaptic neuron or glial cells or both) +* The vesicular membrane is recovered by endocytosis and recycled + +--- + + +## The discovery of the neurotransmitter acetylcholine + +* Otto Loewi– wanted to figure out how stimulation of vagus nerve caused the heart to slow down + * Vagus nerve (cranial nerve X) has both sensory and motor axons. Regulates heartbeat +* Loewi transfered a solution generated from one heart to slow down another heart even without stimulation +* Demonstrated a diffusible substance was released upon stimulation + +Note: + +The vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve). It also has some afferent fibers that innervate the inner (canal) portion of the outer ear (via the auricular branch, also known as Alderman's nerve) and part of the meninges. + +The vagus nerve (/ˈveɪɡəs/ vay-gəs), historically cited as the pneumogastric nerve, is the tenth cranial nerve or CN X, and interfaces with parasympathetic control of the heart and digestive tract. The vagus nerves are paired; however, they are normally referred to in the singular. + +The vagus nerve supplies motor parasympathetic fibers to all the organs except the suprarenal (adrenal) glands, from the neck down to the second segment of the transverse colon. The vagus also controls a few skeletal muscles, notable ones being: + +* Cricothyroid muscle +* Levator veli palatini muscle +* Salpingopharyngeus muscle +* Palatoglossus muscle +* Palatopharyngeus muscle +* Superior, middle and inferior pharyngeal constrictors +* Muscles of the larynx (speech). + +*This means that the vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve).* + +*It also has some afferent fibers that innervate the inner (canal) portion of the outer ear (via the auricular branch, also known as Alderman's nerve) and part of the meninges. This explains why a person may cough when tickled on the ear, such as when trying to remove ear wax with a cotton swab.[citation needed]* + +*Afferent vagus nerve fibers innervating the pharynx and back of the throat are responsible for the gag reflex.* + + +--- + +## The discovery of acetylcholine + +
Neuroscience 5e Fig. 5.4
+
Neuroscience 5e Fig. 5.4
+ + +Note: + +Free acetylcholine acts on **muscarinic receptors** which **hyperpolarize** the cells of the SA node and slow the conduction of the action potential through the AV node. This slows heart rate. Acetylcholine also decreases Ca2+ influx which lowers the heart's force of contraction. + +-- + +## The discovery of acetylcholine + +
+
+ +Otto Loewi (Austrian)– on the discovery of vagus nerve substance: + +>"In the night of Easter Saturday, 1921, I awoke, turned on the light, and jotted down a few notes on a tiny slip of paper. Then I fell asleep again. It occurred to me at six o'clock in the morning that during the night I had written down something most important, but I was unable to decipher the scrawl. That Sunday was the most desperate day in my whole scientific life. During the next night, however, I awoke again, at three o'clock, and I remembered what it was. This time I did not take any risk; I got up immediately, went to the laboratory, made the experiment on the frog's heart, described above, and at five o' clock the chemical transmission of nervous impulse was conclusively proved." + +
+ +Note: + + +--- + +## Acetylcholine (ACh) shown to be the vagus factor + +* Sir Henry Dale purified ACh (1914) and showed that it is vagus nerve substance +* Can apply ACh to muscle and evoke an end plate potential (EPP) +* ACh action has same pharmacology as vagus nerve substance in that it is sensitive to curare (a plant poison that kills by preventing muscle contraction). Competes with curare for receptor binding +* Henry Dale and Otto Loewi shared Nobel prize (1936): + +
+
+ +>"for their discoveries relating to chemical transmission of nerve impulses" + +
+ +Note: + +*Curare was used as a paralyzing poison by South American indigenous people. The prey was shot by arrows or blowgun darts dipped in curare, leading to asphyxiation owing to the inability of the victim's respiratory muscles to contract.* + +*Curare /kʊˈrɑːri/[1] or /kjʊˈrɑːri/[2] is a common name for various plant extract alkaloid arrow poisons originating from Central and South America. These poisons function by competitively and reversibly inhibiting the nicotinic acetylcholine receptor (nAChR), which is a subtype of acetylcholine receptor found at the neuromuscular junction. This causes weakness of the skeletal muscles and, when administered in a sufficient dose, eventual death by asphyxiation due to paralysis of the diaphragm.* + +--- + +## Formal criteria that define a neurotransmitter + +1. Must be present in the presynaptic neuron +2. Must be released in response to a depolarization and be Ca²⁺ dependent +3. Must have specific receptors localized on the post-synaptic cell + * Note– It does not have to function uniquely as a neurotransmitter (it may have other functions). e.g. glutamate, glycine, ATP + + +Note: + +There are a few criteria that define a neurotransmitter... + +--- + +## Criteria that define a neurotransmitter + +
present in presynaptic cell
Neuroscience 5e Box 5A
+
calcium dependent release
Neuroscience 5e Box 5A
+
specific receptors on post-synaptic cell
Neuroscience 5e Box 5A
+ +Note: + +Criteria depicted here + +[https://www.quora.com/How-many-types-of-neurotransmitters-are-there-in-a-human-brain](https://www.quora.com/How-many-types-of-neurotransmitters-are-there-in-a-human-brain) + +It depends on how you count, but maybe 30 - 100 different molecule types, with 10 of them doing 99% of the work. More than 100 different neurotransmitters have been identified. + +There are two main broad categories of neurotransmitters: "Small molecule" neurotransmitters (glutamate, GABA, acetylcholine, biogenic amines (dopamine, serotonin, noradrenaline, and histamine)) and neuropeptides (opioid peptides, substance P). ATP/purines and unsaturated fatty acids like endocannabinoids (anandamide, 2-AG) also can act as neurotransmitters. + + + +--- + +## Synaptic transmission is quantal + +* Synaptic transmission is quantal (composed of discrete units) +* The initial evidence was obtained from studying the release of acetylcholine at neuromuscular junctions +* The synapses between spinal motor neurons and skeletal muscle are simple, large, and peripherally located. Easy to study + * These motor synapses form structures at the neuromuscular junction called **end plates**. This is where the action happens + +Note: + +How have we come to learn about the properties of chemical synaptic transmission? + +--- + +## Neuromuscular junction + +
+ +
+ +Note: + +motor unit is a motor neuron’s axon terminals and all the skeletal muscle fibers it innervates (10 for extraocular muscles, 1000 for thigh muscles). Motor pool is a bunch of motor units of same fiber type. + +--- + +## Muscle action potentials + +* Muscles have action potentials too– triggered by stimulus from motor neurons at the neuromuscular junction +* The regenerative action potential travels away from the neuromuscular junction along the muscle fiber + +
+ + +Note: + + +-- + +## Muscle action potentials + +* Recordings in the junction reveal local potential changes at the end plate before a regenerative action potential is produced + +
+ + +Note: + +-- + +## Muscle action potentials + +* These local potentials are called end plate potentials (EPPs) +* End plate potentials are generated **at the end plate** + +
+ + +Note: + + + +--- + +## End plate potential + +A presynaptic action potential releases a lot of ACh, opening channels in the muscle cell. The resulting depolarization is called an end plate potential (EPP). + +
Neuroscience 5e Fig. 5.6
+ +
Neuroscience 5e Fig. 5.6
+ + +Note: + +End plate potentials evoked by motor neuron stimulation almost are almost always above threshold and result in an action potential along the muscle fiber + +--- + +## Miniature end plate potentials (MEPPs) + +* Spontaneous changes in potential even in the absence of an action potential +* Same shape as EPPs but smaller (1 mV vs 50+ mV) +* Sensitive to agents that block ACh receptors +* Removing Ca²⁺ from media reduces EPPs to MEPPs +* Thus EPPs are a bunch of MEPPs added up + +Note: + +--- + +## Comparison of MEPPs and subthreshold EPPs + +
Neuroscience 5e Fig. 5.6
+ + +Note: + +* in the absence of stimulation there is spontaneous postsynaptic membrane transients called minature EPPs. Small amplitude. +* Bath in low calcium and stimulate you get small subthreshold EPPs that are about the same size as the MEPPs. +* Examination of the muscle membrane potential at high gain reveals small, spontaneous depolarizations. These are miniature end plate potentials (MEPPs) + +--- + +## Quantal neurotransmission + +* By lowering Ca²⁺ one can reduce the amount of transmitter released by an AP +* Here [Ca²⁺] is so low that *many* presynaptic APs fail to release any ACh +* Other APs release 1 to 6 quanta + +
Neuroscience 5e Fig. 5.7
+ + +Note: + + +If you measure the amplitudes of these small low calcium EPPs and plot their distribution, e.g. this histogram here you can see a certain statistical distribution that indicates these amplitudes fall into discrete steps or quanta showing that the smallest amplitude ones that are about the same size as the spontaneous MEPPs must be result of neurotransmitter release from single synaptic vesicles. + + +--- + +## Quantal neurotransmission + +* The **MEPP is the quantal event of neurotransmission**. It represents the postsynaptic response to the release of a single vesicle of neurotransmitter +* The EPP is the result of the synchronized release of many vesicles. It is the sum of many MEPPs +* Bernard Katz Nobel prize (1970) + +
Bernard Katz
+ +Note: + + + +--- + +## One MEPP = one synaptic vesicle + +* Synaptic vesicles are full of neurotransmitter +* In motor neuron one vesicle contains approximately 10,000 molecules of neurotransmitter +* About the same amount needed to invoke an MEPP + +Note: + + + +--- + +## Synaptic vesicles recycle + +* All that vesicle fusion– why doesn’t the membrane keep growing and growing? +* Synaptic vesicle membranes get recycled quickly +* Are endocytosed in clathrin coated vesicles which fuse to endosome and bud off again +* Can use a pulse chase experiment to show this + +Note: + + + +-- + +## Local recycling of synaptic vesicles in presynaptic terminals + +
Neuroscience 5e Fig. 5.9
+ + +Note: + +(Heuser and Reese, 1973) + +--- + +## Local recycling of synaptic vesicles in presynaptic terminals + +
Neuroscience 5e Fig. 5.9
+ + + +Note: + + + +--- + +## Calcium is required for synaptic vesicle fusion + +* Voltage clamping shows that there is an inward Ca²⁺ flux in presynaptic cells that is voltage dependent +* Ca²⁺ can be visualized entering cell after depolarization +* Injection of Ca²⁺ into the presynaptic neuron can drive a post-synaptic potential +* Chelating Ca²⁺ in the presynaptic cell can inhibit post-synaptic potential + +Note: + + + +-- + +## The role of Ca²⁺ + +
+
+ +* If extracellular Ca²⁺ is removed or Ca²⁺ entry is blocked, there will be no release +* Voltage-gated Ca²⁺ channels in the presynaptic membrane provide Ca²⁺ to trigger the release of neurotransmitter + +
+ +
Voltage-clamp presynaptic neuron and block Na⁺/K⁺ currents with TTX/TEA
Neuroscience 5e Fig. 5.10
+ + +Note: + +(Augustine and Eckert 1984) + + +-- + +## The role of Ca²⁺ + +
+
+ +* Intracellular injection of Ca²⁺ into the presynaptic terminal will stimulate release +* Intracellular injection of Ca²⁺ chelator will inhibit release + +
+ +
microinjection of Ca²⁺ into presynaptic terminal
Neuroscience 5e Fig. 5.11
+
microinjection of Ca²⁺ chelator BAPTA into presynaptic terminal
Neuroscience 5e Fig. 5.11
+ + +Note: + +* microinjection of Ca²⁺ into squid giant axon presynaptic terminal (Miledi, 1973) +* microinjection of Ca²⁺ chelator BAPTA into squid giant axon presynaptic terminal (Adler et al, 1991) + +*Fluorescent dye that binds calcium (Smith et al 1993)* + +*squid giant axon from contacts the contractile muscular mantle responsible for water expulsion and squid jet propulsion* + +--- + +## There are lots of proteins involved in synaptic vesicle cycling + +* Many specific proteins have been isolated from presynaptic terminals +* Some of these proteins are required for different steps of vesicle cycling: budding, docking, priming, fusion + +Note: + + + +-- + +## Presynaptic proteins implicated in synaptic vesicle cycling + +
Molecular model of a synaptic vesicle
Neuroscience 5e Fig. 5.13
+ + +Note: + +Model after Takamori et al 2006 + +-- + +## Presynaptic proteins implicated in synaptic vesicle cycling + +
The vesicle trafficking cycle
Neuroscience 5e Fig. 5.13
+ + +Note: + +NSF: ATPase NSF important for fusion of vesicle with membranes of the golgi apparatus. NEM senstivie fusion protein. + +snaps: soluble NSF-attachment proteins + +snares: SNAP receptors + +Model after Takamori et al 2006 + +--- + +## Molecular mechanisms of synaptic vesicle exocytosis + +* SNAP-25 is a plasma membrane SNARE that regulates the assembly of two other SNAREs +* Syntaxin is a plasma membrane SNARE +* Synaptobrevin is a vesicle SNARE +* Together they tether the vesicle to the plasma membrane +* Synaptotagmin is a vesicle Ca²⁺ sensor and helps trigger vesicle fusion + +
Vesicle bound to plasma membrane
Neuroscience 5e Fig. 5.14
+ + +Note: + +NSF +: NEM-sensitive fusion protein (orig found to be important for fusion of vesicles with membranes of Golgi apparatus) +: ATPase + +SNAPs +: soluble NSF attachment proteins + +SNARES +: 'SNAP receptors' + + + +--- + +## Molecular mechanisms of synaptic vesicle exocytosis + +
Neuroscience 5e Fig. 5.14
+ + +Note: + + + +--- + +## Vesicle proteins are the targets of many toxins + +* Tetanus toxin– cleaves synaptobrevin +* Botulinum toxins– cleave syntaxin and snap25 (causes botulism) +* alpha-latrotoxin– black widow causes a massive exocytosis of vesicles. Somehow bypasses Ca²⁺ requirement, likely affecting synaptotagmin + +Note: + +[from https://en.wikipedia.org/wiki/Botulinum_toxin:](https://en.wikipedia.org/wiki/Botulinum_toxin:) + +>Cleavage of the SNARE proteins inhibits release of acetylcholine.[45] Hence, botulinum toxins A, B, and E specifically cleave SNAREs, preventing "neurosecretory vesicles" from docking/fusing with the interior surface of the plasma membrane of the nerve synapse, and so block release of neurotransmitter. In inhibiting acetylcholine release, nerve impulses are blocked, causing the flaccid (sagging) paralysis of muscles characteristic of botulism[45] + + + +--- + +## Synaptic vesicle toxins + +Tetanus toxin and various types of botulinum toxin act by preventing exocytosis. + +
SNARE protein sites cleaved by tetanus and botulinum toxins
Neuroscience 5e Box 5B
+ + +Note: + +NSF +: NEM-sensitive fusion protein (orig found to be important for fusion of vesicles with membranes of Golgi apparatus) +: ATPase + +SNAPs +: soluble NSF attachment proteins + +SNARES +: 'SNAP receptors' + + + +--- + +## Botox + +* Dermatologists have been using botulinum toxin (or Botox) for cosmetic purposes +* When injected locally into a particular muscle or surrounding area, Botox causes a paralysis of that muscle due to a blockade of ACh release from the incoming motor nerve fibers. This leads to a reduction of wrinkle lines, although effective for only a few months + +
+ + +Note: + +when botox is injected in small amounts, it can effectively weaken a muscle for a period of three to four months + + +--- + +## Synaptic transmission summary video + +
Neuroscience 5e Animation 5.1
+ +Note: + +--- + +## Midterm thursday + +* Similar format as the practice midterm +* 100 points total, 25% of your grade. +* Covers material in lectures 1–6 + * today's material covers Chapter 5, pages 77-95 +* Hannah's office hrs this week: Wednesday 3:30 – 5:30pm Biomed 101 + +Note: + +--- diff --git a/2016-10-16-lecture07.md b/2016-10-16-lecture07.md new file mode 100644 index 0000000..bd9686c --- /dev/null +++ b/2016-10-16-lecture07.md @@ -0,0 +1,761 @@ +## Neurotransmitters + +* Many different kinds, over 100 +* There are two main types– small molecule neurotransmitters and neuropeptides +* Abnormalities of neurotransmitter function contributes to wide range of neurological diseases and psychiatric disorders + +Note: + +So we already defined what a neurotransmitter is. It is a substance that must be present inside a presynaptic neuron, it’s release must be dependent on calcium flux from an AP, and it must have specific receptors on the postsynaptic neuron. + +--- + +## Major categories of neurotransmitters + +* Small molecule neurotransmitters– amino acids, purines, biogenic amines +* Peptide neurotransmitters– 3-36 amino acid polypeptides, often derived from longer polypeptides + + +Note: + + +--- + +## Examples of small-molecule neurotransmitters + +
+
+
+ + +Note: + + +--- + +## Examples of small-molecule neurotransmitters + +share hydroxylated benzene ring + +
+ +Note: + +most of which share a hydroxylated benzene ring + + +* -Catechol, also known as pyrocatechol or 1,2-dihydroxybenzene, is an organic compound with the molecular formula C6H4(OH)2 + +--- + +## Examples of peptide neurotransmitters + +Endogenous opioid peptide. + +
+ +Note: + + + +--- + +## Neurotransmitter release can be regulated at many steps + +* Synthesis– small molecules are generated from biosynthetic enzymes +* Neuropeptides are generated by translation followed by protein processing +* Packaging into vesicles– requires specific transporters on vesicle membrane, there are different types of vesicles, small clear-core (e.g. ACh and amino acids) and large dense core (neuropeptides), biogenic amines do both. Location in synapses is different +* Release– small vesicles release fast, large-dense take more effort + +Note: + + +--- + +## The synthesis, packaging, secretion, and removal of neurotransmitters + +
+ +Note: + + +--- + +## Small molecule neurotransmitters are synthesized at the presynaptic terminal + +Raw materials are collected by active transport. Neurotransmitter is synthesized and packaged at terminus. + +
+ +Note: + + + +--- + +## Neuropeptides are synthesized in the cell body + +Neuropeptides are synthesized in the nerve cell body, loaded into vesicles and transported down the axon via microtublules. + +
+ +Note: + + +--- + +## Small molecule neurotransmitters + +* Acetylcholine +* Amino acids +* Glutamate +* Aspartate +* GABA +* Glycine +* Purines (ATP) +* Biogenic amines + * Dopamine + * Norepinephrine + * Epinephrine + * Serotonin + * histamine + +Note: + + +--- + +## Acetylcholine + +* The neurotransmitter used at the neuromuscular junction. Also used at synapses in visceral motor system and at some CNS synapses– called cholinergic neurons +* Synthesized from acetyl CoA and choline by choline acetyl transferase (ChAT)– its presence is a good indication that the neuron is cholinergic +* Removed from synapse by acetylcholine esterase (AChE) has high activity can cleave 5000 molecules per second +* Sarin “nerve gas” is a AChE inhibitor + +Note: + +ACh: skeletal muscle excitation vs release from vagus nerve that slows down heart beat (cardiac muscle)— + +* -Ligand gated channel that depolarizes skeletal muscle fibers vs g-protein coupled receptor that results in hyperpolarization of cardiomyocytes. + +--- + +## Acetylcholine + +acetylcholineesterase (degradation) + +choline acetyltransferase (synthesis) + +
+ +Note: + +from krebs cycle you get Acetyl CoA. Na-Choline cotransporter exchanges Na ions for choline. + +choline acetyltransferase… + +VAChT packs ACh into vesicles. + +--- + +## AChE Inhibition + +* Sarin and Soman: toxic irreversible AChE inhibitors. Also known as “nerve gases” for use in chemical warfare. +* Designed to dispersed as a vapor cloud or spray, which allows their entry into the body through skin contact or inhalation. Drug quickly penetrates into bloodstream and is distributed to all organs, including the brain. +* Symptoms: profuse sweating and salivating, uncontrollable vomiting, gasping for breath, convulsing, and gruesome death . These are due to rapid accumulation of ACh and overstimulation of cholinergic synapses throughout the CNS and PNS. Death occurs through asphyxiation due to paralysis of the muscles of the diaphragm. + +
+ +Note: + +--- + +## Acetylcholine synthesis video summary + +
Neuroscience 5e Animation 6.1
+ +Note: + +--- + +## Glutamate + +* Most important transmitter for normal brain function. +* Nearly all excitatory neurons in the CNS are glutamatergic. +* Does not cross the blood brain barrier. +* Glutamine is most common precursor glutaminase converts it to glutamate. +* Retrieved from synapse by glutamate transporters in glia and neurons. Glia (astrocytes) turn glutamate to glutamine and spit it back out +* Too much glutamate can kill the post-synaptic neuron (excitotoxicity). A major problem after damage due to stroke. + +Note: + +Most important neurotransmitter for normal brain function. Almost all excitatory neurons in CNS are glutamatergic. Half of all synapses estimated to use this transmitter. Glutamate is non-essential a.a. (by that I mean non-essential per dietary requirements) that does not cross the blood brain barrier. Synthesized inside neurons by local precursors. + +histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. + +Monosodium glutamate (MSG, also known as sodium glutamate) is the sodium salt of glutamic acid + +--- + +## Glutamate + +
+ +Note: + +system A transporter 2 (SAT2) transports glutamine into presynaptic terminal. Metabolized into glutamate by mitochondrial enzyme glutaminase. Also glucose metabolism from Krebs cycle can also produce glutamate. Packaged into vesicles by vesicular glutamate transporters (VGLUT). 3 different VGLUTs identified. + +Removed from cleft by excitatory a.a. transporters (EAATs). These are family of 5 Na⁺ dependent glutamate cotransporters. Some in glial cells, some in presynaptic terminals. Glutamate in glial cells by EAAT converted into glutamine by enzyme glutamine synthetase. Then transporter out by different transporter system N transporter 1 (SN1) then back into nerve cells by SAT2. + +essential AA: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. + +--- + +## Glutamate + +
+ +Note: + +histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. + +--- + +## Glutamate synthesis video summary + +
Neuroscience 5e Animation 6.2
+ +Note: + +ACh role in Alzheimers: basal forebrain innervation to neocortex vs hippocampus. Cholinergic neuron degradation vs local postsynaptic neuron effects… + +--- + +## GABA and glycine + +* Most inhibitory neurons use one or the other. +* Inhibits the ability to fire action potentials. +* GABA (gamma-aminobutyric acid) made from glutamate by glutamic acid decarboxylase (GAD), requires Vitamin B6 as cofactor. B6 deficiency can lead to loss of synaptic transmission. +* Glycine– about 1/2 of neurons in spinal cord use glycine. +* Both GABA and glycine are rapidly taken up by glia and neurons. +* Hyperglycinemia– defect in glycine uptake and removal leading to severe mental retardation. + +Note: + +As many as a third of synapses in the brain use GABA as an inhibitory transmitter. Most commonly found in local circuit neurons. + +glycine encephalopathy: + +[http://ghr.nlm.nih.gov/condition/glycine-encephalopathy](http://ghr.nlm.nih.gov/condition/glycine-encephalopathy) + +>Glycine encephalopathy, which is also known as nonketotic hyperglycinemia or NKH, is a genetic disorder characterized by abnormally high levels of a molecule called glycine. This molecule is an amino acid, which is a building block of proteins. Glycine also acts as a neurotransmitter, which is a chemical messenger that transmits signals in the brain. Glycine encephalopathy is caused by the shortage of an enzyme that normally breaks down glycine in the body. A lack of this enzyme allows excess glycine to build up in tissues and organs, particularly the brain, leading to serious medical problems. + +--- + +## Glycine + +* Inhibitory neurotransmitter +* Makes the post-synaptic membrane more permeable to Cl-. Can result in hyperpolarization of the post-synaptic cell +* Glycine receptor is primarily found in the ventral spinal cord +* Strychnine +* Glycine antagonist which can bind to the receptor without opening the Cl- channel +* (i.e. it inhibits inhibition) +* spinal hyperexcitability + +
+ +Note: + + +--- + +## Synthesis, release, and reuptake of the inhibitory neurotransmitters GABA and glycine + +
+ +Note: + +transported into vesicles by vesicular inhibitory amino acid transporter (VIAAT) + +Removal by neurons and glia by Na⁺ dependent cotransporters for GABA called GATs + +--- + +## Synthesis, release, and reuptake of the inhibitory neurotransmitters GABA and glycine + +
+ +Note: + +--- + +## Biogenic amines + +* Catecholamines– dopamine, norepinephrine, and epinephrine +* Histamine +* Serotonin +* All derived from tyrosine. Tyrosine hydroxylase is the rate limiting step and is a good histological marker for catecholaminergic neurons +* Are implicated in many complex behaviors + +Note: + +Biogenic amines regulate many functions in the CNS and PNS. Ranging from homeostatic functions to cognition and attention. + +* All come from same synthesis pathway +* defects in function implicated in many psychiatric disorders. +* targets of many drugs of abuse + +*Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group.* + +*reserpine used as antipsychotic, depletes Norep at synaptic terminals by blocking vesicle loading* + +* organic structure template: R—NH2* + +--- + +## Catecholamine synthesis + +Neuroscience 5e 6.10 + +
+ +Note: + +--- + +## Dopamine + +* Produced by the enzyme DOPA decarboxylase +* Made by substantia nigra pars compacta (which connects to corpus striatum for coordination of body movements). +* Does not cross the blood brain barrier, but levadopa (L-DOPA) does. +* Parkinson’s treatments include L-DOPA plus degradation enzyme inhibitors +* Cocaine inhibits uptake of dopamine (inhibits DAT) + +
+ +Note: + +Synthesized in cytoplasm of presynaptic terminals. + +Loaded into synaptic vesicles by vesicular monoamine transporter (VMAT). Dopamine in synaptic cleft is terminated by reuptake of dopamine into nerve terminals or glia cells by a Na-dependent dopamine cotransporter called DAT. Cocaine works by inhibiting DAT, increasing dopamine concentrations in synaptic cleft. + +Amphetamine also inhibits DAT as well as a transporter for norepinephrine + +* Catabolized by monoamine oxidase and catechol O-methyltransferase (COMT). Both neurons and glia contain mitochondrial MAO and cytoplasmic COMT. Inhibitors of these enzymes are targets of some kinds of antidepressants (phenelzine and tranylcypromine) +* Acts throught GPCRs. D3 parallels that of other metabotropic receptors like mAChR. Subtypes act by activating or inhibiting adenylyl cyclase. +* Activation leads to complex behaviors. Antagonists can cause catalepsy (state where difficult to initiate voluntary movement). +* L-DOPA is the precursor to the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline) collectively known as catecholamines. +* it is converted into dopamine by the enzyme aromatic L-amino acid decarboxylase, also known as DOPA decarboxylase. + +*Encephalitis lethargica, sleeping sickness, 40 yrs later Oliver Sacks in NYC treats them with L-DOPA* + +* neostriatum +* Part of + * Basal ganglia[1] + * Reward system[2][3] +* Components + * Ventral striatum[2][3][4 + * Dorsal striatum[2][3][4] + +The corpus striatum, a macrostructure which contains the striatum, is composed of the entire striatum and the globus pallidus. The lenticular nucleus refers to the putamen together with the globus pallidus. + +--- + +## PET scans before and after cocaine + +Red means lots of unoccupied dopamine receptors + +before + +after + +
+ +
+ +Note: + +striatum. + +>Imaging studies in humans show that low striatal D2 receptor binding in cocaine abusers in the striatum correlates with decreases in glucose metabolism in the orbito-frontal cortex and cingulate gyrus, which process drive and affect, and may lead to continued drug-taking behavior (Volkow et al., 1993, 1999) + +anterior cingulate cortex + +--- + +## Projections from dopaminergic neurons in the human brainstem + +
+ +Note: + + +--- + +## Dopamine synthesis video summary + +
Neuroscience 5e Animation 6.3
+ +Note: + + +--- + +## Norepinephrine + +* also called noradrenaline +* Comes from dopamine by way of dopamine-β-hydroxylase +* Sympathetic ganglion cells use it– project to visceral motor system (fight or flight response) +* Used as a transmitter from locus coeruleus in brainstem– projects to areas that are involved in sleep, attention, and feeding +* Its reuptake mechanism, the norepinephrine transporter (NET), is a target of amphetamines + +Note: + +VMAT for loading into vesicles + +Norep transporter (NET) is a Na⁺ depedent cotranporter. NET is a target of amphetamines. + +alpha and beta adrengergic receptors. GPCRs. Some alphas lead to slow depolarization. Some lead to slow hyperpolarization (acting on different K⁺ channels). + +--- + +## Projections from noradrenergic neurons in the human brainstem + +
+ +Note: + + +--- + +## Norepinephrine synthesis video summary + +
Neuroscience 5e Animation 6.4
+ +Note: + + +--- + +## Epinephrine + +* Adrenaline– present at lower levels than the others +* Made by neurons in rostral medulla. Project to thalamus and hypothalamus + +Note: + +--- + +## Projections from adrenergic neurons in the human brainstem + +
+ +Note: + +--- + +## Serotonin + +* 5-hydroxytryptamine (5-HT) +* Made from tryptophan +* Reuptake by specific serotonin transporters +* Many antidepressants act by inhibiting serotonin reuptake (selective serotonin reuptake inhibitors-SSRIs). Prozac, Zoloft +* Found primarily in groups of neurons in the raphe region of the pons and upper brainstem +* The raphe nucleus projects widespread in forebrain areas that are implicated in sleep and wakefulness and mood + +Note: + +VMAT loads this (as well as other monoamines) into synaptic vesicles. + +turkey/tryptophan—> sleep? Yes— but not really, you’d have to eat a lot more (3x more according to tryptophan supplements) than typically at thanksgiving meal. + +[http://www.snopes.com/food/ingredient/turkey.asp](http://www.snopes.com/food/ingredient/turkey.asp) + +Chicken and ground beef contain almost the same amount of tryptophan as turkey — about 350 milligrams per 4-ounce serving. + +Swiss cheese and pork actually contain more tryptophan per gram than turkey, + +The amount of tryptophan in a single 4-ounce serving of turkey (350 milligrams) is also lower than the amount typically used to induce sleep. The recommendations for tryptophan supplements to help you sleep are 500 to 1,000 milligrams. + +[http://www.webmd.com/food-recipes/the-truth-about-tryptophan?page=2](http://www.webmd.com/food-recipes/the-truth-about-tryptophan?page=2) + +>The small, all-carbohydrate snack is tryptophan's ticket across the blood-brain barrier, where it can boost serotonin levels. So have your turkey, Somer says, because it will increase your store of tryptophan in the body, but count on the carbohydrates to help give you the mood boost or the restful sleep. + +>"Research shows that a light, 30 gram carbohydrate snack just before bed will actually help you sleep better," Somer says. + +--- + +## Histamine + +* Made from histidine, metabolized by monoamine oxidase +* Made by neurons in hypothalamus that send projections to all regions of the brain and spinal cord. +* Mediates arousal and attention. +* Histamine receptors are in the immune system and in the CNS. The sedative side effects of Benadryl act through the CNS. + +Note: + +* H1 receptors (antagonists used for treating motion sickness because role in vestibular function) +* H2 receptors control secretion of gastrci acid in digestive system + +*transported into vesicle by VMAT as catecholamines* + +--- + +## Synthesis of histamine and serotonin + +
+ +Note: + +--- + +## Widespread projections from histaminergic and serotonergic neurons in the human brain + +
+ +Note: + + +--- + +## Serotonin synthesis video summary + +
Neuroscience 5e Animation 6.5
+ +Note: + + +--- + +## Peptide neurotransmitters + +* 3-36 or so amino acids, cleaved from larger precursor proteins +* Catabolized by peptidases +* 5 general classes, brain/gut peptides, opioid peptides, pituitary peptides, hypothalamic releasing hormones, all others. +* Packaged into large dense core vesicles (amino acids are packaged into small clear core vesicles). +* Generally used as co-transmitters + +Note: + +* Many peptide known to be hormones also act as neurotransmitters +* melanocyte-stimulating hormone, adrenocorticotropin, Beta-endorphin regulate complex responses to stress +* substance P and opioid peptides involved in the perception of pain + +--- + +## Amino acid sequences of peptide neurotransmitters + +
+ +Note: + + +--- + +## Examples of peptide neurotransmitters + +Endogenous opioid peptide. + +Note: + + +--- + +## Synthesis of neuropeptides + +Neuropeptides are synthesized as pre-propeptides in the nerve cell bodies. + +This includes a signal sequence that targets the peptides to the inside of the endoplasmic reticulum. + +The signal sequence is cleaved to form the propeptide. + +Note: + + +--- + +## Synthesis of neuropeptides + +ACTH– adrenocorticotripic hormone + +modulation of pain + +Note: + +Proteolytic processing of the pre-propeptides, pre-proopiomelanocortin and pre-proenkaphalin + +Processing the polypeptides that make the final neuropeptdies happens in an neurons cell body. Propeptide packaged into vesicles in golgi network. Final peptide processing occurs after packaging into vesicles. Multiple neuroactive peptides can be released from a single vesicle. + +melanocyte-stimulating hormone, adrenocorticotropin, Beta-endorphin regulate complex responses to stress + +--- + +## Synthesis of neuropeptides + +
+ +Note: + +Proteolytic processing of the pre-propeptides, pre-proopiomelanocortin and pre-proenkaphalin + +--- + +## Peptide dense core vesicles + +
+ +Note: + +Neurons very often make both a conventional neurotransmitter (such as glutamate, GABA or dopamine) and one or more neuropeptides. Peptides are generally packaged in large dense-core vesicles, and the co-existing neurotransmitters in small synaptic vesicles. + +The large dense-core vesicles are often found in all parts of a neuron, including the soma, dendrites, axonal swellings (varicosities) and nerve endings, whereas the small synaptic vesicles are mainly found in clusters at presynaptic locations. + +This refers to the larger amount of material inside the dense-core vesicles, which contain not only neurotransmitters, but also proteases and other peptide chains that have been cleaved from the active neurotransmitter. + +Greater electron scattering in EM: + +Chemical fixation – for biological specimens aims to stabilize the specimen's mobile macromolecular structure by chemical crosslinking of proteins with aldehydes such as formaldehyde and glutaraldehyde, and lipids with osmium tetroxide. + +--- + +## Clear core vesicles release upon a single action potential + +Neuroscience 5e 5.12 + +
+ +Note: + +release of small molecule transmitters inside clear core vesicles + +--- + +## Large core release after multiple action potentials + +Neuroscience 5e 5.12 + +Note: + +release of both types of neurotransmitter + +--- + +## Examples of peptide neurotransmitters + +* Substance P– 16 amino acid peptide +* Present in human hippocampus, neocortex, and GI tract (hence a brain-gut peptide) +* Involved in the perception of pain +* Released from C-fibers which carry information about pain and temperature + +Note: + +accidental discovery of substance P. ominous sounding compound from Area 51? No. It was an unidentified component of power extracts from brain and intestine. High conc. in hippocampus, neocortex, and GI tract. A brain/gut peptide. Release of Subst P in cfibers can be inhibited by spinal interneurons releasing opioid peptides. + +--- + +## Opioids + +* Bind to same post-synaptic receptors as opium +* Family with more than 20 members, three basic groups: endorphins, enkephalins, and dynorphins +* Often co-localized with GABA and serotonin +* Tend to act as depressants, used for analgesics +* Repeated use often leads to tolerance and addiction + +Note: + +Opioids are named because they bind to same postsynaptic receptors as opium. + +* -opium poppy cultivated for 5000 yrs +* -opium contains a variety of plant alkaloids, predominantly morphine. Morpheus, greek god of dreams. Very effective analgesic. Fentanyl, synthetic opiate with 80 times analgesic potency of morphine + +Opioid peptides distributed throughout the brain. Colocalize with GABA and 5-HT. Tend to be depressants. They act like analgesics when injected intracerebrally. Initiate effects through GPCRs. Activate at low concentrations (nM to uM). mu, delta, kappa opioid receptor subtypes play role in reward and addiction. mu-receptor is primary site for opiate drugs. + +--- + +## Cannabinoids + +* Cannabinoids +* Δ9-tetrahydrocannabinol (THC) +* Endocannabinoids +* anandamide +* 2-arachidonylglycerol (2-AG) +* Bind to G-protein coupled receptors (GPCRs): CB1 & CB2 +* CB1 enriched in substantia nigra, caudate putamen, neocortex, hippocampus, cerebellum + +
+ +
+ +
+ +Note: + +used for hemp (fiber, oil, seed) + +cannabis sativa + +cannabis indica + +* -A hybrid Cannabis strain (White Widow) (which contains one of the highest amounts of Cannabidiol), flower coated with trichomes, which contain more THC than any other part of the plant + +phytocannabinoids (85 active identified in cannabis) + +THC: + +-agonist of both CB1 and CB2 +-mild to moderate analgesic effects (dorsal root ganglion and PAG), antiemetic (anti-nausea) +-tolerance appears to be irregular throughout mouse brain areas +-possesses mild antioxidant activity + +* Bioavailability10–35% (inhalation), 6–20% (oral)[3] +* Protein binding97–99%[3][4][5] +* MetabolismMostly hepatic by CYP2C[3] +* Biological half-life1.6–59 h,[3] 25–36 h (orally administered dronabinol) +* Excretion65–80% (feces), 20–35% (urine) as acid metabolites[3] + +cannabidiol: a major phytocannabinoid, accounting for up to 40% of the plant's extract. More complex effects than THC, may potentiate effects through CB1 density increases, inhibition of FAAH. Allosteric modulator of mu-opioid receptors. Less understood. + +cannabinol: higher affinity for CB2 (but weaker than THC) + +Unconventional neurotransmitters. released from neurons, regulated by Ca²⁺, and have specific receptors, but not released from synapses by exocytotic vesicle mechanisms. Often unconventional NTs are associated with retrograde signaling from post to pre. + +[from https://en.wikipedia.org/wiki/Anandamide](https://en.wikipedia.org/wiki/Anandamide) + +>Anandamide, also known as N-arachidonoylethanolamine or AEA, is an essential fatty acid neurotransmitter derived from the non-oxidative metabolism of eicosatetraenoic acid (arachidonic acid) an essential ω-6 polyunsaturated fatty acid + +>Anandamide's effects can occur in either the central or peripheral nervous system. These distinct effects are mediated primarily by CB1 cannabinoid receptors in the central nervous system, and CB2 cannabinoid receptors in the periphery.[6] The latter are mainly involved in functions of the immune system. + +These endocannabinoids are actually unsaturated fatty acids from enzymatic digestion of membrane lipids. Production stimulated by second messengers within postsynaptic neuron, typically a rise in postsynaptic Ca²⁺ concentration. + +-anandamide + +-2-arachidonylglycerol (2-AG) + +Mechanism of release not clear, but likely that these hydrophobic signals diffuse through the postsynaptic membrane to reach cannabinoid receptors on nearby cells. Action terminated by carrier mediated transport into postsynaptic neuron and hydrolyzed by enzyme fatty acid hydrolase (FAAH). + +-rimonabant, synthetic drug + +GPCRs: + +CB1 enriched in substantia nigra, caudate putamen, neocortex, hippocampus, cerebellum + +CB2 expressed in cells throughout the immune system. T cells, macrophages, B cells, peripheral nerve terminals (relief of pain), microglial cells + +major CB2 targets are: >immune and immune-derived cells (e.g. leukocytes, various populations of T and B lymphocytes, monocytes/macrophages, dendritic cells, mast cells, microglia in the brain, Kupffer cells in the liver, etc. + +>multiple intracellular signal transduction pathways are activated. At first, it was thought that cannabinoid receptors mainly inhibited the enzyme adenylate cyclase (and thereby the production of the second messenger molecule cyclic AMP), and positively influenced inwardly rectifying potassium channels (=Kir or IRK).[25] However, a much more complex picture has appeared in different cell types, implicating other potassium ion channels, calcium channels, protein kinase A and C, Raf-1, ERK, JNK, p38, c-fos, c-jun and many more.[#Demuth:2006] + +inhibits inhibition on presynaptic GABAergic neurons. Inhibits IPSCs. disinhibitory effect. + +[#Demuth:2006]: Demuth DG, Molleman A (2006). "Cannabinoid signalling". Life Sci. 78 (6): 549–63. doi:10.1016/j.lfs.2005.05.055. PMID 16109430. + + +--- + +## Summary + +
+ +Note: + +--- diff --git a/2016-10-16-lecture08.md b/2016-10-16-lecture08.md new file mode 100644 index 0000000..f1f5253 --- /dev/null +++ b/2016-10-16-lecture08.md @@ -0,0 +1,1670 @@ +## Neurotransmitter receptors + +* Embedded in the plasma membrane of post-synaptic cell. + +* Either are ion channels themselves (ionotropic, or ligand-gated ion channel) or interface with ion channels (metabotropic, or G-protein coupled receptors). + +* Ultimately, the binding of neurotransmitter causes opening of channels and ion flux. This can lead to depolarization or hyperpolarization of the membrane potential depending on the ion concentrations and the particular ion species flowing in or out. + +2016-02-01 16:12:13 + +Note: + + + + + + + + + + + + + + + +Today we will dive a bit deeper into the structure and function of neurotransmitter receptors. + + + + + +--- + +## Ionotropic neurotransmitter receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +Neurotransmitter binds receptor + +Channel open allowing ions to flow through + +
+ +Note: + + + + + +The ionotropic receptors are the ones you’ve probably seen in our synaptic diagrams so far, where NT binds directly to an ion channel pore, causing it to open and allow ions to move through the pore. + +--- + +## Ionotropic neurotransmitter receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + +neurotransmitter binds + +channel opens + +ions flow across membrane + + + +--- + +## Metabotropic neurotransmitter receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + +neurotransmitter binds + +g protein is activated + +g protein subunits or intracellular messengers modulate ion channels + +ion channel opens + +ions flow across membrane + + + +--- + +## Title Text + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +[http://courses.pbsci.ucsc.edu/mcdb/bio125/Animation05-03IonotropicandMetabotropicReceptors.mov](http://courses.pbsci.ucsc.edu/mcdb/bio125/Animation05-03IonotropicandMetabotropicReceptors.mov) + +
+ +Note: + + + +--- + +## Nicotinic acetylcholine (nACh) receptors + +* Ion-channel receptor (ionotropic) + +* ACh binds nACh receptor– opens up + +* ACh causes nACh receptor to open transiently and stochastically (patch clamp studies). + +* An action potential causes lots of ACh molecules to be released simultaneously, transiently opening many nACh receptors. + +* The summed current flow into the muscle cell is called the end plate current (EPC). Current flow changes the potential of the muscle, the EPP, which can trigger an action potential. + +Note: + +nACh Receptors are ionotropic receptors and are the receptor you’ve heard the most thus far, being the one that underlies end plate currents at the neuromuscular junction that cause end plate potentials. + +--- + +## Outside-out patch clamping showing ACh gated currents + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +Neuroscience 5e 5.17 + +Channels open for various amounts of time at a given potential. + +
+ +Note: + +The binding of a neuro-transmitter to its receptor usually opens (sometimes closes) ion channels. + +The figure shows a simple case. In the absence of ACh, the channel is closed. In the presence of high ACh (the channel always has ACh bound), the channel opens and closes. These repeated breif openings are seen as downward deflections corresponding to inward current. + + + + + + + +--- + +## Activation of nACh receptors at neuromuscular synapses + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +post-synaptic muscle cell voltage clamped to look at currents + +not voltage clamped, inward EPC causes depolarizing EPP in the muscle cell + +Neuroscience 5e 5.17 + +
+ +
+ +Note: + +The traces show inward currents through these ionotropic ACh channels, showing the currents stemming from a single channel, 10 channels, and a million channels. + + + +As we will learn in a few minutes, the channel opened by ACh lets mostly Na+ through resulting in these inward currents that depolarize the muscle cell, resulting in EPPs and typically resulting in APs as we’ve discussed before. + + + + + +[from http://www.ncbi.nlm.nih.gov/books/NBK21586/: ](http://www.ncbi.nlm.nih.gov/books/NBK21586/) + +* acetylcholine causes opening of a cation channel in the receptor capable of transmitting 15,000 – 30,000 Na+ or K+ ions a millisecond + +* - >Two factors greatly assisted in the characterization of the nicotinic acetylcholine receptor. First, this receptor can be rather easily purified from the electric organs of electric eels and electric rays; these organs are derived from stacks of muscle cells (minus the contractile proteins) and thus are richly endowed with this receptor. (In contrast, this receptor constitutes a minute fraction of the total membrane protein in most nerve and muscle tissues.) Second, α-bungarotoxin, a neurotoxin present in snake venom, binds specifically and irreversibly to nicotinic acetylcholine receptors. + +--- + +## How do we figure out what ions flow through the nACh receptor? + +* From Nernst equation– the equilibrium potential of a cell is the potential at which there is a balance between the concentration gradient and the electrochemical gradient. + +* In other words– there is no net flow of ion x at the equilibrium potential, Ex. + +* Thus if one measured the ACh dependent current flow at different potentials, one could determine the potential that current flow was 0. This is called the reversal potential or Erev. + +* The end plate current (EPC) is therefore IACh and is equal to the driving force on an ion multiplied by its permeability (remember Ohms law: I = gV). + +* IACh = gACh(Vm-Erev) + +* Predicts that current will be inward at potentials more negative than Erev, becomes small at potentials approaching Erev, and then becomes outward at potentials more positive then Erev. + +Note: + +Use our good friend the Nernst eqn, which you can recall is… + +Since we know there isn’t any net flow of an ion x, at the Ex, we can measure the ACh dependent currents at different potentials and figure out the potentials at which current flow is 0. + + + +When we are talking about the potential at which postsynaptic currents like the endplate current reverses from inward net ion flux to outward net ion flux, we call this potential the reversal potential denoted Erev. + + + +We can call the endplate current then the IAch or the current flowing through the ACh receptor at skeletal muscle endplate membrane and IAch is therefore equal to the driving force (which is the difference between Vm and Erev) multiplied by the permeability for ACh gAch. + + + +This would then predict that current will be inward at potentials more negative than Erev… + +* - Predicts that current will be negative (inward) at potentials more negative than Erev, becomes small at potentials approaching Erev, and becomes positive (outward) at potentials more positive then Erev. + + + + + + + + + +--- + +## Influence of the postsynaptic membrane potential on end plate currents + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + +A postsynaptic muscle fiber is voltage clamped to control the muscle fiber’s membrane potential, while the presynaptic neuron is stimulated to cause ACh release at the end plate synapse. + + + +--- + +## Hypothetical ion channel selectivities and the reversal potential + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + +So let’s imaging what the current-voltage relationships would look like for different channel selectivities. Remember the reversal potential is when there there is no net ion flux, so it 0 nA on all these graphs and if a channel is selective to only K, it would be equal to the Ek. + + + +If the channel was selective only to Na, than the Erev would be equal to ENa. Same for chloride. + + + +If the channel was a non-selective cation channel (passing both K and Na) than + + + + + + + + + +11Na, 12Mg, 17Cl, 19K, 20Ca + + + + + +--- + +## What ions flow through the nACh receptor? + +* Voltage clamping experiments show that there are large inward currents at -110 mV, smaller currents at -60 mV and no current at 0 mV. Outward currents at +70 mV. Therefore Erev = 0. + +* Erev is not at any of the equilibrium potentials for a single ion, lies in between K+ (-100 mV) and Na+ (+70 mV). + +* Altering the K+ concentration or the Na+ concentration will change the membrane potential. Therefore both Na+ and K+ are permeable through the nACh receptor. + +* nACh receptor can conduct both Na+ and K+ ions. The direction of flow is dependent on the membrane potential. The normal resting state of muscle is -100 mV, well below 0 mV (Erev) therefore normally at rest Na+ rushes in with very little K+ rushing out. + +Note: + +As we will see in a minute voltage clamp experiments show that there is a… + +Erev… + +Furthermore, altering… + +Therefore we can conclude that the nAChR can conduct both Na and K ions. + + + + + +--- + +## Influence of the postsynaptic membrane potential on end plate currents + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + + + +--- + +## The effect of ion channel selectivity on the reversal potential + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +K+ only permeable channel + +Cl– only permeable channel + +Na+ only permeable channel + +
+ +
+ +Note: + + + +--- + +## Influence of the postsynaptic membrane potential on end plate currents + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + +So it seems that the ACh activated ion channels are equally permeable to Na and K and this was tested in 1960 by Akira and Noriko Takeuchi by changing the extracellular concentration of these ions. As predicted, lowering [Na] shifts Erev to the left and and raising the external [K] shifts Erev to the right. + +--- + +## Na+ and K+ movements during EPCs and EPPs + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +-90 typical resting potential of a muscle + +depolarization + +hyperpolarization + +nothing + +Neuroscience 5e 5.20 + +
+ +Note: + +Even though these ionotropic channels opened by ACh are permeable to both Na and K, at the resting membrane potential the EPC is generated primarily by Na influx because of the reduced driving force on K since at Vrest the membrane potential is closer to Ek. + +--- + +## Na+ and K+ movements during EPCs and EPPs + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +EPC: in or out + +EPP:depolarzing or + +hyperpolarizing + +Neuroscience 5e 5.20 + +
+ +Note: + +Here is the key: you get inward currents at potentials more negative the Erev and you get outward currents at potentials more positive than Erev. + + + +The resulting EPPs depolarize postsynaptic cell at potentials more negative than Erev and potentials more positive than Erev hyperpolarize the cell. + + + + + +--- + +## Na+ and K+ movements during EPCs and EPPs + +* Since the Na+ and K+ permeabilities of this channel are similar, the magnitudes of the Na+ and K+ currents depends on the driving forces present for each ion + +
+ +Note: + + + +--- + +## Think about it + +* At normal resting potentials as the nACh receptor opens, many Na+ ions rush in and a few K+ rush out. This causes the cell to depolarize. As the potential goes toward Erev, as many K+ go out as Na+ goes in. Therefore the nACh receptor if open long enough would drive the potential to Erev. If Erev is above threshold, the probability of an action potential happening is increased and is called an excitatory postsynaptic potential (EPSP) + +* If Erev is below threshold the probability of an action potential is decreased. Called an inhibitory postsynaptic potential (IPSP). + +Note: + + + + + + + +[http://www.nature.com/nrd/journal/v1/n6/full/nrd821.html: ](http://www.nature.com/nrd/journal/v1/n6/full/nrd821.html) + +>In the case of this modified muscle nAChR, the conductance of the pore is sensitive to the presence of negative charge at three locations that form three negatively charged rings in and near the M2 domain56. So, intensive studies of the M2 segment have been carried out to determine the amino acids that are responsible for the cationic or anionic selectivity of receptors. + + + +--- + +## Postsynaptic potentials between neurons + +* Excitatory postsynaptic potentials (EPSP) increases the likelihood that an action potential will be initiated in the post synaptic cell. + +* Inhibitory postsynaptic potentials (IPSP) decreases the likelihood that an action potential will be initiated in the post synaptic cell. + +
+ +Note: + +In fact we can generalize the properties that we’ve learned about EPCs through ionotropic AChR and their effects on EPPs at the neuromuscular junction to the general case of chemical synapses between pairs of neurons. + + + +Instead of so called EPPs, the postsynaptic potentials between neurons we call excitatory if it increases the likelihood of an AP firing in a postsynaptic cell and inhibitory if it decr the probability of an AP occurring in a postsynaptic cell. + + + +This plot shows two pretend neurotransmitters D and H that can depolarize or hyperpolarize the cell and their corresponding Erevs. This one causes an EPSP and inward current from Vrest, whereas this one causes an IPSP and an outward current from Vrest. + + + +--- + +## Similar mechanisms exist at all chemical synapses + +* Instead of end plate current called postsynaptic current (PSC). + +* Instead of end plate potential called post synaptic potential (PSP). + +* excitatory PSP– EPSP increases likelihood of an action potential + +* inhibitory PSP– IPSP decreases likelihood of an action potential + +Note: + + + +--- + +## EPSP summation + +* Unlike the neuromuscular junction– at synapses between neurons an EPSP is usually not very strong, usually well below threshold. What is needed is a bunch of EPSPs to sum together. Typically neurons receive more than a thousand synapses. It is basically the summation of EPSPs and IPSPs that determine whether or not an action potential occurs. If sum is above threshold, an AP happens. + +Note: + + + +--- + +## EPSP + +* Here is an EPSP mediated by glutamate activating nonselective cation channels. + +Neuroscience 5e 5.21 + +
+ +Note: + + + +--- + +## IPSP #1 + +* Here is an IPSP mediated by GABA activating Cl- selective channels. + +* The reversal potential for the Cl current is negative to the resting potential and threshold. + +* Activation of Cl channels hyperpolarizes the neuron. + +ECl + + + +Neuroscience 5e 5.21 + +Time (ms) + +Note: + + + +--- + +## IPSP #2 + +* The reversal potential for the Cl– current is positive to the resting potential but negative to threshold. + +* Activation of Cl– channels depolarizes the neuron. Stabilizes membrane potential below threshold. + +ECl + + + +Neuroscience 5e 5.21 + +Time (ms) + +Note: + + + + + +Also called shunting inhibition. Na+ channels persistently in state of inactivation due to small depolarizing pulses. + + + +So just remember if the Erev for the neurotransmitter receptor is more positive than threshold than it is excitatory. If it is more negative than threshold than it is inhibitory. + + + + + + + +>Blocking NKCC1 with bumetanide disrupts excitatory synapse development in the cortex + +Bumetanide, a selective NKCC1 inhibitor, has been demon- strated to suppress certain forms of epileptiform activity in vitro and in vivo, presumably by attenuating the depolarizing effect of GABA (Dzhala et al., 2005; Kilb et al., 2007) + + + +>effect of GABA on membrane polarity depends on the Cl gradient created by the expression of Na -K -2Cl cotrans- porter (NKCC) and K -Cl cotransporter (KCC). NKCC1 im- ports Cl and is expressed from the embryonic stage until the first postnatal week, whereas KCC2 exports Cl and is weakly expressed at birth and upregulated as the brain matures (Plotkin et al., 1997; Rivera et al., 1999; Li et al., 2002). The temporal expression patterns of these two transporters correspond to the switch of GABA from being excitatory to inhibitory during the first few weeks of rodent postnatal life (Delpire, 2000). + +--- + +## Remember in neurons an EPSP is not driven from one pulse + +* Activation of ionotropic receptors opens nonselective cation channels. + +* The first stimulation does not reach threshold. + +* More intense stimulation yields a larger EPSP and an AP. + +
+ +Note: + + + +--- + +## IPSP + +* Activation of ionotropic receptors opens Cl- channels. + +* The first stimulation does not reach threshold. + +* More intense stimulation yields a longer IPSP but not a larger one. + +Note: + + + +--- + +## Summation of postsynaptic potentials + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +Neuroscience 5e 5.22 + +
+ +
+ +Note: + + + +--- + +## Summation + +* In general EPSPs in neurons are small 0.2–0.4 mV. + +* Most neurons are somewhere between 10–20 mV below threshold. If everything was linear that it would take the sum of 50 or so inputs to trigger AP + +* Not so simple. Some inputs are bigger than others, the inputs can be summed differently– spatially or temporally. + +* A single neuron can have as many as 10,000 different synapses. Some excitatory some inhibitory, some strong some weak. Some at the tips of dendrites, some near the cell body. + +* A neuron integrates all this information and either fires a spike or not. + +Note: + +Of course we are greatly simplifying everything here, a single neuron may have as many as 10K synaptic inputs. + +--- + +## Neural integration + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + + + +--- + +## Neural integration + +* How does a neuron integrate all the information it is getting? + +* In most motor neurons and interneurons the decision to initiate an action potential is at the axon hillock. Contains a high density of voltage dependent Na+ channels. Contains membrane with lowest threshold. + +* Axon hillock is senses the local state of the cell, which is the combination of all the EPSPs and IPSPs going on at one time. + +* This is mostly due potentials that spread passively. + +* Temporal summation, process by which consecutive synaptic potentials at the same site are added together. Different synapses will have different time constants. + +* Length constant of the cell determines the degree to which a depolarization current decreases as it spreads passively. Easier to sum inputs on the same dendritic branch than on different branches. + +* Some dendrites even have voltage gated Na+ channels, these can amplify inputs. + +Note: + + + + + + + +some neurons in the globus pallidus have voltage gated Na channels. + + + + + +--- + +## Title Text + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +[http://courses.pbsci.ucsc.edu/mcdb/bio125/Animation05-02SummationofPostsynapticPotentials.mov](http://courses.pbsci.ucsc.edu/mcdb/bio125/Animation05-02SummationofPostsynapticPotentials.mov) + +
+ +Note: + + + +--- + +## Events from neurotransmitter release to postsynaptic excitation or inhibition + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + + + +--- + +## Ionotropic neurotransmitter receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +Note: + +neurotransmitter binds + +channel opens + +ions flow across membrane + + + +--- + +## Metabotropic neurotransmitter receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +Note: + +neurotransmitter binds + +g protein is activated + +g protein subunits or intracellular messengers modulate ion channels + +ion channel opens + +ions flow across membrane + + + +--- + +## Metabotropic neurotransmitter receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +Ligand binding site + +G-protein binding site + +
+ +Note: + +neurotransmitter binds + +g protein is activated + +g protein subunits or intracellular messengers modulate ion channels + +ion channel opens + +ions flow across membrane + + + +--- + +## Cholinergic receptors + +* Best studied– the nicotinic ACh receptor (nAChR) + +* Pentamer-5 subunits to make a pore. Selective for cations. + +* Nicotine can mimic ACh to stimulate receptor, this is called an agonist. Most effects of nicotine go through this receptor. Nicotine is not cleared very well so receptor stays open longer which leads to larger EPSPs + +* nACh receptors produce EPSPs. + +* Many toxins specifically bind to and block nicotinic receptors called antagonists. + +* alpha-bungarotoxin (snake venom)– binds to alpha subunit of nAChR very tightly and prevents ACh from activating it. + +Note: + +As we’ve shown in our examples earlier the nAChR receptor is a non-selective cation channel. Or another way to think of it is that it is selective for cations. + + + +5 subunits + +--- + +## nAChR + +* Green is motor axons, red is where Bungarotoxin binds, defines the endplates + +
+ +Note: + + + +--- + +## Structure of the nACh receptor + +* 5 subunits come together to make a pore. + +* Each subunit has 3-4 membrane spanning domains. + +* In muscles the receptor has 2 α, β,γ,ε subunits. The α subunits bind ACh, both need to be bound for channel to open. α subunits also binds bungarotoxin and nicotine. + +* Multiple isoforms for each subunit, depending on which isoform is in channel get different properties + +* In neurons its slightly different. 5 subunits 3α:2β. Bungarotoxin only inhibits muscle nACh receptors. + +
+ +Note: + +The alpha subunits bind ACh. + +--- + +## Muscle nAChR + +* Pentamers of 2α1, ß1, γ, δ in fetal mammals vs. 2α1, ß1, δ, ε in adult mammal + +* ACh, nicotine, curare, and bungarotoxin binding sites are on the α1 subunits + +Pore diameter 10x bigger than Na+ channels + +(3 nm vs .3 nm) + + + +
+ +Note: + +Changes in subunit composition during development. + + + +curare is a competitive antagonist. + + + +--- + +## Ligand Gated Ion Channels + +* Built up of 4 or 5 monomers + +* Each monomer spans the membrane 3 or 4 times + +* Each monomer contributes properties + +* Mixing and matching from a large pool of monomer isoforms creates receptors with different properties + +
+ +Note: + + + +--- + +## Muscarinic ACh receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + + + +--- + +## Muscarinic ACh receptors + +* Muscarine, a poisonous mushroom alkaloid, is an agonist. + +* Metabotropic, mediates most of ACh effects in the brain. + +* 5 or so isoforms + +* mACh blockers are used for pupil dilation (atropine), motion sickness (scopolamine) and asthma treatment (ipratropium). + +Note: + +- seven transmembrane spanning domains. + +- coupled to G proteins + +cause variety of slow postsynatpic responses. + +highly expr in sttiatum and varous forebrain regions. + +activate inward rectifier K+ channels (allow more K current at hyperpolarized potentials) + +or Ca2+ activated K+ channels + +exert inhibitory influence on dopamine mediated motor effects + +in hippocampus mAChRs are excitatory, acting by closing KCNQ type K+ channels + +* mutations in four out of five KCNQ genes underlie diseases including cardiac arrhythmias, deafness and epilepsy. + +* [http://www.ncbi.nlm.nih.gov/pubmed/11252765](http://www.ncbi.nlm.nih.gov/pubmed/11252765) + +* KCNQ/M (Kv7) very slow voltage-gated K channels, suppress repetitive firing + +* Inhibited by ACh and many neurotransmitters, but enhanced by others + +* [http://physiolgenomics.physiology.org/content/22/3/269](http://physiolgenomics.physiology.org/content/22/3/269) + + + +Also found in ganglia of PNS. Mediate peripheral cholinergic responses of autonomic effector organs like heart, smooth muscle, exocrine glands. Inhibition of heart rate by vagus nerve. + +--- + +## Muscarinic ACh receptors + +* Muscarine, a poisonous mushroom alkaloid, is an agonist. + +* Metabotropic, mediates most of ACh effects in the brain. + +* 5 or so isoforms + +* mACh blockers are used for pupil dilation (atropine), motion sickness (scopolamine) and asthma treatment (ipratropium). + +* [Also used for bad things http://www.rense.com/general38/frug.htm](http://www.rense.com/general38/frug.htm) + +Note: + + + +--- + +## Glutamate receptors + +* Both ionotropic and metabotropic + +* Ionotropic– NMDA receptors, AMPA receptors, and Kainate receptors (named after the agonists that stimulate them). + +* All are non-selective ion channels with Erev close to 0 (above threshold therefore excitatory). + +* Formed from an association of many subunits, that can combine to create many isoforms. + +Note: + +tetramers + + + +3 classes, 8 subunits + + + + + +Kainate receptors, or KARs, are ionotropic receptors that respond to the neurotransmitter glutamate. + + + +Kainic acid (kainate) is a natural marine acid present in some seaweed. Kainic acid is a potent neuroexcitatory amino acid that acts by activating receptors for glutamate, + +* Domoic acid is a structural analog of kainic acid and proline. + +* Domoic acid (DA) is a kainic acid analog neurotoxin that causes amnesic shellfish poisoning + + + + + +--- + +## Glutamate receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + + + +--- + +## AMPA/Kainate receptors + +* glutamate receptors that allow Na+ or K+ ions across. + +* multi-subunit channels + +* evoke EPSPs that are large and fast + +* AMPA receptors are more common than Kainate receptors + +
+ +Note: + + + +--- + +## NMDA receptor + +* Glutamate receptors that allow flow of Ca2+ as well as Na+ and K+. As a result EPSPs produced by NMDA receptors can increase the Ca2+ concentration in the neuron. Acts as a second messenger to activate cellular processes. + +* Needs a co-agonist, glycine to open channel + +* Blocked by Mg2+ in the pore during hyperpolarizing conditions. Depolarization can remove block. Needs either a bunch of presynaptic cells to fire at the same time or repeated firing of presynaptic cell to open channel. + +* Key component of a model for learning. + +* Evoke EPSPs that are slow and long lasting. + +* PCP “angel dust” binds and clogs channel. Get symptoms similar to schizophrenia. + +Note: + + + + + + + + + +PCP “angel dust” binds and clogs channel. Get symptoms similar to schizophrenia. Some hypothesize NMDA receptor is involved in this disease. + + + +--- + +## NMDA receptor currents requires glutamate, glycine, and removal of voltage-gated Mg2+ block + +* Glycine is a co-agonist-no glycine no current. + +* Mg2+ blocks pore-is removed by depolarization. + +* This can happen because AMPA and NMDA receptors are often in the same synapse. + +Neuroscience 5e 6.6 + +
+ +Note: + + + +--- + +## NMDA receptor currents require glycine and removal of Mg2+ block + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + + + + + + + +fig from: + +[http://www.bris.ac.uk/synaptic/info/glutamate.html](http://www.bris.ac.uk/synaptic/info/glutamate.html) + + + +--- + +## The NMDA receptor channel can open only during depolarization + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + +chp 8 more on NMDA-R mediated mechanisms involved in learning and memory, adv neuroscience. + + + +--- + +## Building a brainier mouse + +* NMDA receptor consists of four subunits, each constructed separately. + +* Receptors with NR2B subunits stay open longer than those with NR2A. + +* Genetically engineered mice to produce NR2B receptors, the DOOGIE mouse. + +* The genetically engineered mice showed stronger synaptic connections, faster fear learning, and better water maze learning. + +Tsien, J.Z (2000) Building a brainier mouse. + +Scientific American, April , Vol 282, pp62-68. + +
+ +
+ +Note: + +Conductances during early infancy and as an adult + +--- + +## Neurobiology: Young receptors make smart mice + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +Figure 1 Object-recognition task. a, In an initial training session the mouse explores two + +objects in a box, devoting roughly equal time to each. + + b, When the mouse is then re-exposed to one of these objects, together with a new object, + + it spends more time exploring the new object. Tang et al.3 find that this bias is enhanced in + +transgenic mice (inset) over expressing the NR2B + +subunit of the NMDA receptor, indicating improved recognition memory. + +subunit NR2A in adults subunit NR2B during development + +
+ +Note: + + + +--- + +## Metabotropic glutamate receptors mGluRs + +* Large class of receptor subtypes + +* G-protein coupled + +* Often leads to inhibition of postsynaptic Ca2+ and Na+ channels + +* But sometimes inhibitory sometimes excitatory + +Note: + + + +--- + +## GABA receptors + +* Three types of GABA receptors: A, B and C. + +* A and C are ionotropic, B is metabotropic. + +* A and C are inhibitory because their channels are permeable to Cl-. The flow of Cl- into the cell lowers the potential. Erev is less than the threshold potential. + +* Pentamers, subunit diversity as well as variable stoichiometry, allows for variable functions of GABA receptors. + +* Glycine receptors have generally the same properties as GABA receptors + +Note: + +\pentameric + +--- + +## Ionotrophic GABA Receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +Note: + +In this example Erev>Vm so chloride goes from + +inside to outside + + + +[from: https://en.wikipedia.org/wiki/Picrotoxin](https://en.wikipedia.org/wiki/Picrotoxin) + +picrotoxin + +>Found primarily in the fruit of the climbing plant Anamirta cocculus, it has a strong physiological action. It acts as a non-competitive channel blocker for the GABAA receptor chloride channels.[3] It is therefore a channel blocker rather than a receptor antagonist. + + + +--- + +## Ionotrophic GABA Receptors + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +current due to many channels + +opening + +step nature shows individual + +channels closing. + +In this example Erev>Vm so Cl– goes from + +inside to outside + +
+ +Note: + +In this example Erev>Vm so chloride goes from + +inside to outside + + + +--- + +## Examples of GABA receptor-mediated IPSPs recorded at different membrane potentials + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + + + + + +Reversal potential is at the Nernst potential for Cl- ions. + +(In this case about –78 mV) + + + +Figure from Coombs et al. 1955) + +
+ +Note: + + + +--- + +## GABA induced IPSPs + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +Stimulate GABA producing interneuron- record from post-synaptic neuron + +
+ +Note: + + + +--- + +## The GABA receptor binds many interesting things + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +[http://www.youtube.com/watch?v=L6dzUOYTQtQ-](http://www.youtube.com/watch?v=L6dzUOYTQtQ-) + +
+ +Note: + + + +[from: https://en.wikipedia.org/wiki/Barbiturate#Mechanism_of_action](https://en.wikipedia.org/wiki/Barbiturate#Mechanism_of_action) + +>Barbiturates act as positive allosteric modulators, and at higher doses, as agonists of GABAA receptors. + + + +[from: https://en.wikipedia.org/wiki/Benzodiazepine#Pharmacology](https://en.wikipedia.org/wiki/Benzodiazepine#Pharmacology) + +>Benzodiazepines work by increasing the efficiency of a natural brain chemical, GABA, to decrease the excitability of neurons. + + + + + +[from: http://thebrain.mcgill.ca/flash/i/i_03/i_03_m/i_03_m_par/i_03_m_par_alcool.html](http://thebrain.mcgill.ca/flash/i/i_03/i_03_m/i_03_m_par/i_03_m_par_alcool.html) + + + +>GABA’s effect is to reduce neural activity by allowing chloride ions to enter the post-synaptic neuron. These ions have a negative electrical charge, which helps to make the neuron less excitable. This physiological effect is amplified when alcohol binds to the GABA receptor, probably because it enables the ion channel to stay open longer and thus let more Cl- ions into the cell. + + + +>Still other substances block a natural neuromediator. Alcohol, for example, blocks the NMDA receptors. + + + +>It has now been established that all substances that trigger dependencies in human beings increase the release of a neuromediator, dopamine, in a specific area of the brain: the nucleus accumbens. + +--- + +## Serotonin receptors + +* Large family of receptors called 5-HT 1-7. + +* 5-HT3 is a ligand gated non-selective cation channel, thus it is excitatory. + +* Same basic structure as nACh receptor. + +* All others are metabotropic– likely that perturbations in these receptors are involved in many neural disorders. + +Note: + +most receptors are metabotropic + + + + + + + + + +--- + +## Catecholamine receptors + +* Act exclusively by activating G-protein coupled receptors. Contribute to complex behaviors. + +* Norepinephrine and epinephrine each act on α and β adrenergic receptors. + +* Mostly used to control smooth muscles, especially cardiovascular. + +* B-blockers are used to treat hypertension, anxiety, and panic. + +Note: + + + +--- + +## Peptide receptors + +* Virtually all mediate their effects by activating G-protein coupled receptors. + +* Neuropeptide-Y receptor important in food intake/ obesity. + +* Opiate receptors have been identified and shown to be important in addiction (e.g. µ-opioid receptor). + +Note: + + + + + + + +Opioid peptides distributed throughout the brain. Colocalize with GABA and 5-HT. Tend to be depressants. They act like analgesics when injected intracerebrally. Initiate effects through GPCRs. Activate at low concentrations (nM to uM). mu, delta, kappa opioid receptor subtypes play role in reward and addiction. mu-receptor is primary site for opiate drugs. + + + +--- + +## ATP and other purines (adenosine) + +* ATP is contained in all synaptic vesicles + +* Has specific receptors on post-synaptic cells + +* P2X + +* A2A adenosine receptor (blocked by caffeine) + +* Generally excitatory in nature + +* Used in spinal cord, motor neurons, and other ganglia. + +Note: + +Another neurotransmitter that we didn’t talk about last time is + + + + + +Receptors for ATP and adenosine are widely distributed through the nervous system as well as other tissues. + + + +One class of purinergic receptors for ATP and adenoscie are P2X-receptors which are ionotropic non-selective cation receptors. Others are GPCRs like A2A adenosine receptor throughout brain and heart, adipose tissue, and kidney. Xanthines like caffeine and theophylline block adenosine receptors and this is thought to be the cause of its stimulant effects. + + + +--- + +## Title Text + +* Body Level One + +* Body Level Two + +* Body Level Three + +* Body Level Four + +* Body Level Five + +
+ +
+ +Note: + + + +--- + +## Summary + +* Neurotransmitter receptors bind neurotransmitters. Tremendous diversity but with commonalities. + +* Two types– ionotropic (ligand-gated ion channel) and metabotropic (G-protein coupled receptor). + +* Both types lead to opening or closing of ion channels. These conductance changes can either increase or decrease the probability of firing an action potential. + +* Because postsynaptic neurons are usually innervated by many different inputs, it is the combination of EPSP and IPSPs that determines whether a cell fires and if an action potential occurs. + +Note: + + + +--- + +--- +