44 KiB
Neurotransmitter receptors
- Embedded in the plasma membrane of post-synaptic cell
- Two classes of neurotransmitter receptors–
- receptors that are ion channels themselves (ionotropic or 'ligand-gated' ion channel)
- receptors that interface with separate ion channels (metabotropic, or G-protein coupled receptors)
- Ultimately, the binding of neurotransmitter results in the opening of ion channels and ion flux. This leads to either depolarization or hyperpolarization of the membrane potential depending on the types of ions flowing through the channel pores and the ions' respective electrochemical driving forces
Note:
Today we will dive a bit deeper into the structure and function of neurotransmitter receptors... last time was a warm up
For synaptic transmission, neurotrans receps are generally located in the post-synaptic membrane (though there are exceptions, e.g. some transmitter receptors may be located on pre-synaptic membrane or at non synaptic site in the cell).
Two classes...
In either case, neurotransmitter binding will result in ion channels opening and ion flux across the post-synaptic membrane. Whether this results in hyperpolarization or depolarization of the membrane will be due to the types of ions flowwing through the channels and their respective electrical/chemical driving forces (Nernst)
Ionotropic neurotransmitter receptors
- Neurotransmitter binds receptor
- Channel opens, 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.
- neurotransmitter binds
- channel opens
- ions flow across membrane
Metabotropic neurotransmitter receptors
Note:
Metabotropic transmitter receptors are G-protein coupled receptors, also known as seven-transmembrane domain receptors in you cell biology courses.
- neurotransmitter binds
- g protein is activated
- g protein subunits or intracellular messengers modulate ion channels
- ion channel opens
- ions flow across membrane
Neurotransmitter receptors video summary
Note:
Nicotinic acetylcholine receptors (nAChR)
- Ionotropic receptor
- ACh binds the nAChR– opens the channel
- ACh causes nAChR 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 transmembrane potential of the muscle, the end plate potential (EPP), which triggers an action potential
Note:
So to understand the properties of ionotropic neurotransmitter receptors lets start with the nicotinic ACh receptor (abbreviated nAChR).
nACh Receptors are ionotropic or ligand-gated receptors where the ligand is ACh 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 in muscle cells.
ACh causes...
Patch clamping shows ACh gated currents through nicotinic ACh receptors
Note:
The binding of a neurotransmitter to its receptor usually opens (sometimes closes) ion channels.
The figure shows a simple case. In the absence of ACh, the nAChR is closed. In the presence of high [ACh] (the channel always has ACh bound), the channel opens and closes. These repeated brief openings are seen as downward deflections corresponding to inward current. Notice the current amplitudes in this patch clamp trace below are unitary or quantal indicating that a single channel is being recorded in this case...
These look like microscopic currents you get in single channel patch clamp recordings like we discussed previously.
If this piece of membrane and channel is from a muscle cell than a bunch of these currents put together are the ones that give rise to the end plate potentials we for muscle cells before.
Activation of nAChR at neuromuscular synapses
Note:
Indeed imagine we are doing an experiment where we stimulate a motor neuron and we record end plate currents in a muscle cell...
...then these traces on the left show inward currents through these ionotropic ACh channels in the muscle cell, showing the currents stemming from a single channel, 10 channels, and hundreds of thousands of channels. Notice the amplitudes of the currents scale.
...and this panel on the right shows postsynaptic potential change or end plate potential produced by the EPC as we discussed previously
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/:
- acetylcholine causes opening of a cation channel in the receptor capable of transmitting 15,000 – 30,000 Na⁺ or K⁺ ions a millisecond
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-
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.
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How do we figure out what ions flow through the nicotinic ACh receptor?
- Recall from Nernst equation– the equilibrium potential of a cell for ion x is the potential at which the electrochemical driving forces is balanced for ion x (i.e 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 membrane potential (Vm) where current is 0. This is called the reversal potential or Erev
- The end plate current (EPC) at the muscle cell must therefore be IACh and is equal to the driving force on an ion multiplied by its permeability (remember Ohm's 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:
Now using 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 Vm on end plate currents
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
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) then the current-voltage relationship would look like...
11Na, 12Mg, 17Cl, 19K, 20Ca
Ca2+ ions flow through CaV channels at a rate of ~106 ions s−1, but Na+ conductance is 500fold less through CaV channels
extracellular [Na+] is nearly 70fold higher than Na+, thus Ca2+ selectivity is crucial
Ca2+ and Na+ have nearly identical diameters (~2 Å)
Ca2+ selectivity from high affinity binding, preventing Na+ permeability. Multi site pore, with knock on mechanism to push Ca2+ ions through [#Tang:2014]
[#Tang:2014]: Tang, L., Gamal El-Din, T. M., Payandeh, J., Martinez, G. Q., Heard, T. M., Scheuer, T., Zheng, N., and Catterall, W. A. (2014). Structural basis for Ca2+ selectivity of a voltage-gated calcium channel, Nature, 505(7481), 56-61. PMID 24270805
Influence of the postsynaptic Vm on end plate currents
Note:
These little transients are just stimulus artifacts, but look at the postsynaptic end plate currents in these at these different Vms. Look what happens when Vm is at 0mV, there is no current and then above 0 mV it flips from being inward to net outward current...
We already know that ACh is essential for the end plate currents-- therefore we can say that this EPC is IAch. Therefore what is the Erev for IAch?
Influence of the postsynaptic Vm on end plate currents
Note:
[#Takeuchi:1960]: Takeuchi, A. and Takeuchi, N. (1960). On the permeability of end-plate membrane during the action of transmitter, J Physiol, 154(), 52-67. PMID 13774972
Influence of the postsynaptic Vm on end plate currents
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.
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.
Na⁺ and K⁺ movements during EPCs and EPPs
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.
In fact the Na⁺ and K⁺ permeabilities of the nAChR channel are similar, therefore the magnitudes of the Na⁺ and K⁺ currents depends on the driving forces present for each ion
Na⁺ and K⁺ movements during EPCs and EPPs
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.
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
nAChR summary
- When the nAChR opens at normal resting potentials many Na⁺ ions rush in and a few K⁺ rush out. This causes a depolarizing EPP in the muscle cell. As the Vm during the EPP approaches Erev, outward K⁺ flux is equal to inward Na⁺ flux. Therefore if the nACh receptor is open long enough, it will drive Vm to Erev.
- If Erev is above action potential threshold, the probability of an action potential occurring is increased
- If Erev is below action potential threshold, the probability of an action potential occurring decreased
Note:
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.
Similar mechanisms exist at all chemical synapses
For synapses between neurons:
- Postsynaptic current (PSC) is similar to an end plate current
- Post synaptic potential (PSP) is similar to an end plate potential
- Excitatory PSP (EPSP)– increases likelihood of an action potential occurring
- Inhibitory PSP (IPSP)– decreases likelihood of an action potential occurring
Note:
So now let's generalize the properties that we’ve learned about EPCs through ionotropic AChR and their effects on EPPs at the neuromuscular junction to the case of chemical synapses between any pair of neurons...
But instead of the so called EPPs, we'll call the postsynaptic potentials between neurons we call excitatory PSP if it increases the likelihood of an AP firing in a postsynaptic cell and inhibitory PSP if it decr the probability of an AP occurring in a postsynaptic cell.
EPSP summation
- Unlike the neuromuscular junction– at synapses between neurons an individual EPSP is usually not very strong, typically well below threshold.
- Multiple EPSPs need to be summed together for the neuron's Vm to reach threshold. Individual neurons can receive thousands synapses. It's the summation of EPSPs and IPSPs that determine whether or not an action potential occurs.
Note:
EPSP
Note:
So imagine an experiment like we were doing before...
IPSP type 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
Note:
IPSP type 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
Note:
Imagine if a separate EPSP input brought Vm of this neuron to -41 mV, just below -40mV threshold. Since this is now postive to the ECl of -50mV, further activity at the IPSP synapses will now hyperpolarize the neuron back towards -50mV.
This can also be called shunting inhibition. In this case Na⁺ channels could persistently be in a state of inactivation due to small ongoing depolarizing and hyperpolarzing pulses keeping the neurons Vm below threshold.
So just remember, the key is that 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 demonstrated 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 cotransporter (NKCC) and K-Cl cotransporter (KCC). NKCC1 imports 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).
Summation of postsynaptic potentials
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
- 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:
Summation of postsynaptic potentials video
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Events from neurotransmitter release to postsynaptic excitation or inhibition
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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
- 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 permeable to Na+, K+, and Ca2+
from [#Picciotto:2000]:
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]
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 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
- 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, β1, γ, δ in fetal mammals vs. 2α1, β1, 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 0.3 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:
Ligand gated channels in general are made up of 4 or 5 subunit monomers.
Muscarinic ACh receptors
- Muscarine, a poisonous mushroom alkaloid, is an agonist
- Metabotropic (G-protein coupled receptors), mediates most ACh effects in the brain
- typically linked to K⁺ channel opening that results in IPSPs
- 5 or so isoforms
- mAChR blockers are used for pupil dilation (atropine), motion sickness (scopolamine) and asthma treatment (ipratropium)
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.
- 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
- atropine
- from deadly nightshade family
- dilate pupils, treat slow heart rate
- anticholinergic, muscarinic antagonist
- inhibits parasympathetic nervous system
- WHO essential medicine
- scopolamine
- colorless, odorless alkaloid drug
- competitive antagonist, antimuscarinic
- 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 : 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 Erev 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
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??3 classes, 8 subunits??
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Kainate receptors, or KARs, are ionotropic receptors that respond to the neurotransmitter glutamate.
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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,
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Domoic acid is a structural analog of kainic acid and proline.
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Domoic acid (DA) is a kainic acid analog neurotoxin that causes amnesic shellfish poisoning
Glutamate receptor subunit types
| AMPA | Kainate | NMDA | Metabotropic |
|---|---|---|---|
| GluR1 | GluR5 | NR1 | mGluR1 |
| GluR2 | GluR6 | NR2A | mGluR5 |
| GluR3 | GluR7 | NR2B | mGluR2 |
| GluR4 | KA1 | NR2C | mGluR3 |
| KA2 | NR2D | mGluR4 | |
| NR2D | mGluR6 | ||
| NR3A | mGluR7 | ||
| NR3B | mGluR8 |
Note:
AMPA/Kainate receptors
- ionotropic glutamate receptors that allow Na⁺ or K⁺ ion flow
- multi-subunit channels (typically as heterotetramers from a pair of GluR2 plus a pair of GluR1, GluR3, or GluR4)
- evoke EPSPs that are large and fast
- AMPA receptors are more common than Kainate receptors
Note:
- Each AMPAR is composed of 4 subunits and has four sites to an agonist like glutamate can bind (one per subunit)
- alternative splicing of each of the 4 subunit genes can result in a number of more isoforms
- GluR1 and GluR2 especially important in synaptic plasticity by being upregulated
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
- 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
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
- Mg²⁺ blocks pore– removed by depolarization
- This is possible because AMPA and NMDA receptors are often at the same synapse
Note:
NMDA receptors can open only during depolarization
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. Erev 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
Note:
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
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
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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
Note:
Start at around 1:23
from: 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
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
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 much 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.
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: