2.**metabotropic**, a G-protein coupled receptors that modulate separate ion channels
* Neurotransmitter receptor activation following ligand (neurotransmitter) binding results in the opening of ion channels and ionic flux. This ion flux is the postsynaptic current (or 'end plate' current for a muscle cell)
* These postsynaptic currents result in depolarization or hyperpolarization of the membrane potential (postsynaptic potential or 'end plate' potential) depending on the **types of ions** flowing through the channel pores and the ions' respective **electro-chemical driving forces**
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.
In physiological solution, calcium flux estimated to be 2% of total current through nAchR. For comparison calcium flux is estimated to be 7% of the current in the voltage gated L-type calcium ion channel. But with high density clustering of many nAchRs at muscle end plate synapses, total calcium flux through these channels could influence the local environment significantly https://doi.org/10.1523/JNEUROSCI.10-10-03413.1990
This Ca^2+^ permeability depends on subunit composition of the nAchR pentamer. mammalian α9α10 receptors receptors show higher calcium ion selectivity (important function in cochlear hair cells) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4245820/
>some subtypes of nAChR in the brain (those containing the b2 subunit) are located diffusely throughout the membrane of the neuron, with no obvious concentration at the synaptic junction (Hill et al. 1993).
a number of alpha and beta subunits have expression throughout brain (medulla, superior colliculus, cortex, beta2 subunit expression 'very high' in thalamus). Only alpha3 KO mice have high mortality [#Picciotto:2000].
[^Picciotto:2000]: Picciotto, M. R., Caldarone, B. J., King, S. L., and Zachariou, V. (2000). Nicotinic receptors in the brain. Links between molecular biology and behavior, Neuropsychopharmacology, 22(5), 451-65. PMID 10731620
Low (nM) concentrations of nicotine are found in the blood of moderate smokers (Henningfield et al. 1983). These are sufficient to enhance excitatory transmission in cultures of neurons from the medial habenula or the hippocampus (Gray et al. 1996; McGehee et al. 1995) [#Picciotto:2000]
Many effects of nicotine probably through presynaptic or preterminal nAChRs instead of through postsynaptic AChRs (Léna et al. 1993; Marshall et al. 1997; McGe- hee et al. 1995; Summers and Giacobini 1995; Vidal and Changeux 1993; Wonnacott et al. 1990; Yang et al. 1996) [#Picciotto:2000]
* In muscles the receptor has 2α, β, δ, γ, ε subunits. The α subunits bind ACh, both need to be bound for channel to open. α subunits also binds bungarotoxin and nicotine
<div><img src="figs/Neuroscience5e-Fig-06.04-1R_copy_e029b6f.jpg" height="200px"><figcaption>Neuroscience 5e Fig. 6.4</figcaption></div>
Note:
- seven transmembrane spanning domains
- coupled to G proteins
- causes variety of slow postsynatpic responses
- highly expr in striatum 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
- though in hippocampus mAChRs are excitatory, acting by closing KCNQ type K⁺ channels
*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.*
* KCNQ...
* mutations in four out of five KCNQ genes underlie diseases including cardiac arrhythmias, deafness and epilepsy.
: motion sickness, postoperative nausea and vomiting
: WHO essential medicine
: from flowering plant genus *Scopolia*
ipratropium
: opens up medium and large airways of lungs by causing smooth muscles to relax
: anticholinergic and muscarinic antagonist
: treats obstructive pulmonary disease and asthma
: WHO essential medicine
*Clitocybe dealbata*
: muscarine can occur in this species sufficient concentrations to be deadly
: commonly found growing in lawns in North America an Europe
: white flat topped
[*Amanita muscaria*, Onderwijsgek, CC BY-SA 3.0 nl](https://commons.wikimedia.org/w/index.php?curid=21983879)
: red mushroom with white speckles
: muscarine first isolated from this species in 1869
: muscarine actually only in trace amounts in this species
: muscimol is a predominent compound from this mushroom though
---
## Glutamate receptors
* Both ionotropic and metabotropic
* Ionotropic– AMPA/Kainate receptors and NMDA receptors (named after the agonists that stimulate them)
* All are non-selective ion channels with E<sub>rev</sub> close to 0 (above threshold therefore excitatory)
* Formed from an association of 4 subunits. There are a variety of possible subunits which can combine to create many receptor isoforms
Note:
* form tetramers
* 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
* Non-selective cation channel like nAChr, but tetramer and less calcium permeability. mRNA editing of a intramembrane domain of the GluR2 subunit switches a glutamine to an arginine. This post translational modification results in AMPA receptors that have resistance to calcium permeability. Though AMPA-R usually contain one or more GluR2 subunits, ones that are missing GluR2 subunits do have more calcium permeability and may be important in developing neurons and early forms of synaptic plasticity in some neurons https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3092818/
* 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
* Formed as a heterotetramer of 4 subunits (typically 2 NR1 and 2 NR2 subunits)
* 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
</div>
Note:
* NR1 has the glycine agonist binding site
* NR2 has the glutamate binding site
* NR2B predominant in developing brain before switching to NR2B being predominant in adults
* PCP “angel dust” binds and clogs channel. Get symptoms similar to schizophrenia. Some hypothesize NMDA receptor is involved in this disease.
---
## NMDA receptors require removal of a voltage-dependent Mg²⁺ block
<div style="font-size:0.8em;width:500px;">
<div></div>
* Mg²⁺ blocks pore– removed by depolarization
* This is possible because AMPA and NMDA receptors are often at the same synapse
</div>
<div style="margin:0 15px;"><img src="figs/Neuroscience5e-Fig-06.06-2R_95ba51c.jpg" height="450px"><figcaption>Neuroscience 5e Fig. 6.6</figcaption></div>
## NMDA receptors can open only during depolarization
<figure><img src="figs/Neuroscience5e-Fig-08.10-0_59fb457.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 8.10</figcaption></figure>
Note:
chp 8 more on NMDA-R mediated mechanisms involved in learning and memory, adv neuroscience.
---
## 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:
* group I (mGluR1, mGluR5) associated with IP3 signaling and ER Ca2+ channel opening. Also associated with Na+ and K+ channels. Can result in EPSPs but can also result in IPSPs.
* activated selectively by 3,5-dihydroxyphenylglycine (DHPG) (but not other groups)
* group II mGluRs 2 and 3 prevent formation of cAMP (by activating Gi that inhibits adenylyl cyclase) and result in presynaptic inhibition (not apparently affecting PSPs directly)
* group III, including mGluRs 4, 6, 7, and 8 prevent formation of cAMP and have similar functional pathway and consequences as group II
---
## 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. E<sub>rev</sub> is less than the threshold potential
* Pentamers, subunit diversity as well as variable stoichiometry, allows for variable functions of GABA receptors
* Glycine receptors generally have the same properties as GABA receptors
Note:
* pentameric
* GABAB metabotropic receptors always inhibitory. Coupled indirectly to K+ channels and can decreased Ca2+ conductance resulting in less cAMP production. Baclofen is a potent and selective GABAB agonist. GABA responses that are insensitive to bicuculline and baclofen are termed GABAC responses.
* GABAA: muscimol potent agonist from mushrooms. Bicuculline classical antagonist and convulsant.
---
## Ionotrophic GABA Receptors
<figure><img src="figs/Neuroscience5e-Fig-06.09-1R_69a0993.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 6.9</figcaption></figure>
>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.
---
## Examples of IPSPs recorded at different membrane potentials
<figure><figcaption class="big">Erev is at the Nernst potential for Cl⁻ (e.g. –80 mV)</figcaption><img src="figs/Coombs-JPhysiol1955-Fig1_copy_1932d79.jpg" height="400px"><figcaption>Coombs et al., J Physiol 1955 Fig. 1</figcaption></figure>
Note:
Coombs, Eccles, Fatt 1955: double barreled pipete, inject small currents through one barrel (for voltage clamp) in biceps motorneuron (crustacean) to hold Vm while stimulating afferent nerve inputs to get IPSPs. Erev was found to be close to ECl. Notice hyperpolarization when Vm was above -78 mV, small depolarizations when Vm below -80mV. They found that messing with Cl- concentrations would correspondingly alter the IPSPs but not when messing with Na or K concentrations. Thus Cl- ion flux is necessary for the IPSPs.
[#Coombs:1955]: Coombs, J. S., Eccles, J. C., and Fatt, P. (1955). The specific ionic conductances and the ionic movements across the motoneuronal membrane that produce the inhibitory post-synaptic potential, J Physiol, 130(2), 326-74. PMID 13278905
---
## Ionotropic GABA receptor mediated IPSPs
<figure><figcaption>Stimulate GABA producing interneuron, record from post-synaptic neuron</figcaption><img src="figs/Neuroscience5e-Fig-06.09-2R_9a77707.jpg" height="300px"><figcaption>Neuroscience 5e Fig. 6.9</figcaption></figure>
Note:
Chavas and Marty performed Gramacidin perforated patch recordings from young rat cerebellum interneurons and purkinje cells. *Interneurons had more depolarized GABAA reversal potentials than purkinje cells at matched ages (e.g. P12, likely from higher [Cl-]intra for interneurons compared to purkinje cells).*
[#Chavas:2003]: Chavas, J. and Marty, A. (2003). Coexistence of excitatory and inhibitory GABA synapses in the cerebellar interneuron network, J Neurosci, 23(6), 2019-31. PMID 12657660
---
## GABA receptors bind many interesting things
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Start at around 1:23
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<div><img src="figs/ch16f2_ed1a4dc.jpg" height="300px"><figcaption>Basic Neurochemistry 6e Fig. 16.2</figcaption></div>
>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
* 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– ionotropic non-selective cation channel
* A2A– adenosine receptor (blocked by caffeine)
* Used in spinal cord, motor neurons, and other ganglia
Note:
Another neurotransmitter that we didn’t talk much about last time is ATP.
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 adenosine are P2X-receptors which are ionotropic non-selective cation receptors.
Other purinergic receptors are metabotrobic GPCRs like A2A adenosine receptor throughout brain and heart, adipose tissue, and kidney. Xanthines (e.g. caffeine and theophylline) block adenosine receptors. This is thought to be the cause of its stimulant effects.
---
## Summary
* Two types o neurotransmitter receptors– ionotropic (ligand-gated ion channel) and metabotropic (G-protein coupled receptor)
* Both lead to opening or closing of ion channels. Ionic currents then either increase or decrease the probability of firing an action potential
* Postsynaptic neurons are usually innervated by many different inputs– it is the combination of EPSP and IPSPs that determines if an action potential occurs
