Lecture 17: Ionotropic Receptors

Ionotropic and Metabotropic Receptors

ionotropic:

• rapid onset of effects

• rapid termination of effects

• 1:1 relationship between action and response

• effects limited by the type of ion channel that is part of the receptor protein

• often bind neurotransmitter in the uM range(low affinity)

• often located near site of NT release

metabotropic:

• slow onset of effects

• slow termination of effects

• >1:1 amplification of response by G-proteins and second messangers

• Diverse possible effects from a single NT due to a multitude of second messenger-mediated signaling pathways

• usually bind neurotransmitter in the nM range(high affinity)

  • far from AZ so you want them to be able to respond to small amounts of NT

• often located at some distance from site of NT release(perisynaptic)

Families of ionotropic receptors

• a: pentamers(5 subunits), 4 full TM segments

  • nAChRs, GABA, 5-HT, Glycine, ZAC

• c: trimers(3) subunits, only 2 TM segments, 1 large protein chain

  • ATPr, ASIC

• b: glutamate receptors(tetramers: 4 subunits)

  • 3 full TM segments, 1 half

• d: TRP receptors(tetramers: 4 subunits)

  • 6 TM segments, p-loop between segment 5 and 6

Nicotinic Acetylcholine Receptors(nAChRs)

• helped characterize ACh+ receptors

• stingray has many AChRs

nAChR subunits

• pentamer=5 subunits

• each color is a different TM segment

• M2 faces the pore(no p-loop)

• needs 2 molecules ACh to bind for AChR to open

  • ACh binding sites are on a-subunits

  • therefore, must have 2 a-subunits

ex. of combinations of neuronal nAChR subunits

• not all subunits are the same

• must have 2 a-subunits

Ligand binding

• binding of a ligand to a receptor is probabilistic; so binding frequency depends on [ligand]

• amount of time a ligand spends bound to a receptor binding site varies by lingand KD

Dissociation constants(KD): a measure of the strength of binding affinity

• calculated by the ratio of the unbinding rate(Koff) divided by the binding rate (Kon)

• KD value indicates concentration at which 50% of binding sites are occupied

• KD and affinity are inversely related

  • therefore, a ligand with high affinity for a receptor has a low KD

  • a ligand with low binding affinity for a receptor has a high KD

Dissociation constants for muscle vs neuronal AChRs

• muscle needs more ACh to bind to 50% of ligand receptors so lower affinity

• neurons have higher affinity

Ligand terminology: agonists and antagonists(blockers

• agonist: a compound that elicits the same biological effects as the endogenous(naturally-occuring) ligand when it binds to a receptor

  • ACh/nicotine to nAChR

• antagonist: a compound that reduces or eliminates the effect of an agonist when bound to a receptor

  • tubocurarine/a-bungarotoxin to nAChR

Ligand terminology: Competitive and non-competitive antagonists

• can overcome competitive antagonist by pouring in agonist

• competitive antagonist: binds to the same site as agonist(orthosteric) binding) but doesn’t activate the receptor; reduces or prevents activation of the channel by the agonist

• non-competitive antagonists: binds to the receptor at a different site from an agonist(allosteric binding) but prevents or reduces activation of the receptor. referred to as a NAM(Negative allosteric modulator

Ligand terminology: reversible and irreversible antagonists

• reversible antagonist: binds non-covalently to the receptor, so can come off the receptor by “washing off”

• irreversible antagonists: binds covalently so cannot be displaced by either competing ligands or “washing off”

GABA receptors

• 2 types of GABA receptors: A and B

• B: metabotropic

• A: in pentameric family with nAChRs; ligand-gated chloride channels

  • direction of chlorine depends on DF which depends on eq. pot

Effects of GABA-A receptors: hyperpolarizing inhibition

• DF on GABA receptor current determined by Ecl because receptors are chloride channels

  • DFgaba = Vm-Ecl

• chloride current through GABAa receptor will ALWAYS move membrane potential towards Ecl

• Even though when it’s depolarization, it is still inhibitory since the most depolarization it can make is -70mV which is hyperpolarized compared to AP threshold

Effects of GABAa receptors: shunting inhibition

• open GABA receptors provide path

• membrane potential is determined by relative permeability of the membrane to different ions

• with GABAa receptors activated: significant permeability to Cl- so wants to move membrane potential to -70mV=influences membrane potential

shunting inhibition

• inactive inhibitory synapse: smaller EPSP in soma

• active inhibitory synapse: no EPSP in soma bc shunting

  • when inhibitory synapse active near soma

Glycine Receptors(GlyRs)

• also chloride channels like GABA just activated by Glycine

• faster(in being activated+being desensitized so shuts off faster)

• GABA is slower to desensitize so response lasts longer

  • stays open and passes more current

Ionotropic glutamate receptor structure

• only 4 subunits

• clam shell

• extracellular structure has some neurotransmitter binding sites and modifying sites

• 3 full TM segments, M2(half, functions as a p-loop, not fully TM, is a-helix so more fancy than p-loop)

• ligand binding domains: closer to plasma membrane, where ligand binds

• amino-terminal domain: further from plasma membrane, where modulating agents bind

Types of ionotropic glutamate receptors

• NMDA vs nonNMDA(kainate and AMPA)

• kainate less involved in directly mediating EPSPs

• AMPA is fast because right at AZ

• NMDA blocked by Mg2+ at rest

• NMDA needs 2 agonists(glutamate and glycine(co-agonist))

  • why focus on glutamate: most synapses have glycine so no need to worry about their presence

• actual glycine receptors only respond to vesicularly released glycine

Mg2+ block of NMDA glutamate receptors

• pore of NMDA channel has Mg2+ in it at negative potentials

• Mg2+ expelled upon depolarization

• glutamate synapse: pre releasing glutamate, post expressing NMDA receptors, nothing happens since Mg2+ still there

• NMDA receptors need simultaneous pre(glutamate release) and post(need AMPA to cause depol to remove Mg2+ to allow NMDA to pass current) synaptic activity to be activated,, often called coincidence detectors

Mg2+ block of NMDA glutamate receptors: IV plot

• no Mg2+: works like AMPA receptor

  • permeable to Na, K, Ca2+

  • rev potential of NMDA=0 since extra permeability to Ca2+ which has a + eq potential so inward DF

    • AChR = -10

• Mg2+ block at - potentials even if glutamate is present

  • as depolarized current slowly turns on

  • -10 mV: all Mg2+ block is gone, can pass current normally