Lecture 17- Ionotropic Receptors

Ionotropic receptors vs metabotropic- MEMORIZE PIC

ionotropic: NT binds, ion channel opens, done

happens on a ms timescale

effects are limited by the type of ion channel it is

they have low affinity for nt binding molecules because they only want to produce EPSPs in response to big bolus’s of molecules

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Metabotropic- g protein couples- involves a number of different reactions

is not itself an ion channel

communicator that links NT eC to g protein IC

• look at pic

activation of pathways take a lot longer so effecrs take longer to happen

termination of effects is slower too bc g protein can still be active after NT unbinds

located some distance from active zone

• they already dont see large amounts of NT, so they will only be activated when acetylcholine leaves the cleft

Families of ionotropic receptors

• are ion channels

  1. Pentameric channels - nicotinic ACH receptors, 5-HT receptors, GABAa receptors, Glycine, and zinc activated receptors

     • all comprised of 5 subunits

  2. Trimers- ATP RECEPTORS acid sensing ion channels

  3. (Tetramers)Glutamate Receptor Family- NMDA, AMPA, Kainate

  4. TRP Receptor Family (Tetramers)- TRP receptors - transient receptor potential- not involved in synaptic transmission

TRP Tetramer family vs Glutamate Receptor family

TRP tetramers have 6 subunits- p loop between 5 and 6

Glutmate- 3 complete transmembrane segments

second segment hooks into the membrane

Trimers description

elaborate EC configuration of the protein, but only have 2 full transmembrane segments

Nicotinic ACH receptors- stingray

stingrays uses ach to generate electricity to zap prey

electric organs are filled with ACH receptors

finding this was essential to classify ACH receptor - need a lot of protein in pure form

nACHR structure

top down view of closed channels- colors are diff transmembrane segments

4- transmembrane segments make up one subunit of it

M2 segments face thre pore of the channel- instead of pore lining segment

acHR subunits

alpha, beta, delta, epsilon

in order for receptor to open, two molecules of ACH have to bind on ALPHA SUBUNITYS ONLY- every receptor has to have atleast two alpha subunits

Examples of combinations of nACHR units

Homomeric nACHR’s - all made of the same subunits

heteromeric- not all of the subunits are the same

*still have to have 2 alphas

Ligand Binding - what does binding frequency depend on

ligand binding to receptor is purely probabalistic - depends on the concentration of the ligand

nicotine only binds to ionotropic ACH recpetors - specific agonist for it

ligand not always bound to receptor— it is on and off binding

Kd information

measure of the strength binding

D= dissasociation

Greater kd, the more dissociating ligand from receptor

Kd tells you concentration that will induce binding at 50% of the receptors

when Conc=kd

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Unbinding rate/binding rate

PIC: higher Kd- requires more concentration for 50% of receptors to be occupied

Agonist - ligand

compound that elicits the same biological effects as the naturally occuring (endogenous) ligand when it binds to receptor

nACHR agonists- nicotine

produces same EPSP

Antagonist - ligand

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

ex. curare or alpha bungarotoxin- reducing the effect of the agonist or eliminating

compeitive antagonist

binds to the same site as an agonist(orthosteric binding) but does not activate the receptor. This reduces or prevents activation of the channel by an agonist

• effects can be overcome by putting alot of agonist- outcompeting

Non-competitive antagonist

Allosteric binding

binds to the receptor at a different binding site from an agonist, but prevents or reduces activation of the receptor

*negative allosteric modulator (NAM)

reversible vs irreversible antagonist

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

non-reversible: binds covalently - so cannoy be washed off

GABA recpetors

ionotropic GABA receptors are GABA A receptors

5 subunits, 4 transmem

in pentameric family

GABA activated chloride channels

Effects of GABA receptors - what is driving force on these receptors determined by - why is it always inhibitory

equilibrium potential is affected by driving force on the cl- ions

equilibrium potential of cl- is the one that changes most

DF= VM-Ecl

eq CL=-70

-40mV- Df increasing, increase current inward

at -80 mV you would see a membrane depolarization- cl- efflux out of cell

STILL IS INHIBITORY — the most depolarized the cell can get from it is -70mV

Shunting inhibition

open GABA receptors are providing another place for current to go and for ions to cross membrane

LOWERING resistance of the cell by opening ion channels

if put ach on cell when GABA receptors are activated it still shunts- WHY?

• the membrane potential at any given moment is determined by the permeability of ions the cell is permeable to

• as long as GABA receptors open, they have the -70 MV influence on cell

Shunting inhibition shown on neuron

inhibitory synapses right near the soma- strong on memebrane potential bc they do not have to travel very far

EPSP in dendrite and smaller in SOMA if nothing stops it

if you activate inhibitory synapse- excitatory synapse produces same epsp in dendrite, but no change in membrane potential in soma because opposite inhibitory response

Glycine receptors

Glycine receptors are also chloride channels

same family, same structure

OPEN FASTER and shuts off faster

activated by glycine

Glutamate receptor family - explain structure of channel and types of receptors that exist within this family

Tetramers - 4 subunits to them

extracelluarly- 2 layers of globs

clamshell type structures with binding sites within them

EC portion- has NT binding sites and places where modifying agents can bind to them

NMDA, AMPA and kainate- receptors are ALSO activated by these compounds

Structure of glutamate receptors

3 transmembrane segments- M2 segments creeps into membrane

represents a P loop- pore lining region

clamshell 1- ligand binding

clamshell 2- amino terminal domain is where modulating agents bind

Types of ionotropic glutamate receptors - why is glycine not acknowledged in glutamate receptors and separate ones are called glycine receptors

Non NMDA vs NMDA

COMPARE THEM LOOK AT PICTURE

AMPA- fast, excitatory neurotransmission

low conductance

na, k, ca- sometimes ca2+ permeability

No Mg2+ block

at most active zones, reacting immediately to release of glutamate

NMDA

• high conductance

• slow gating speed

• Na, k, ca- significant ca2+ permeability

  yes mg2+ block

• ca2+ flouresces when its bind to

AT REST, NMDA receptors are blocked by magnesium

pore of channel has mg2+ ion in it

• Need two agonists to respective binding sites to activate- glutamate and glycine

• Why do you focus on glutamate and ignore glycine: in most synapses, glycine is abundant. There is enough glycine to be bound to active sites- just waiting on glutamate

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Why do glycine recpetors separately exist?

• at post synaptic glycine synapses where glycine exists, glycine receptors only react to released glycine from vesicle and not free calcium all around the cell

Mg2+ block of glutamate receptors

• At negative membrane potentials, there is Mg2+ in the pore

• expelled upon depolarization

• when inside of the cell is negative, mg2+ is drawn into pore, when depolarizes, repulsion happens

• REFER TO PIC

What presynaptic and post synaptic activity has to happen for NMDA glutamate receptors to work

glutamate is released from the synapse (however there is still MG2+ block)

Need multiple glutamate releases, and AMPA receptors at the same synapse

1. Glutamate binds to AMPA receptors

2. EPSP produced, post synaptic neuron is depolarized

3. Mg2+ block relieved

4. second glutamate release- NMDA receptors can work

IV plot of NMDA receptors and Mg2+ block- one with EC magnesium and one withh

without- similar to AMPA receptor

• current determined by DF

reversal potential 0MV- calcium eq potential positive

WITH Mg2+

• negative potentials, has mag2+ block

• even if it has glutamate, receptor will not pass any current

by -10MV, all Mg2+ block gone, cell would pass the same current as if there were no mg2= at all

How does mg2+ presence affect current

little mg2+, a lot of current through NMDA receptor, and vice versa