PHAR 426 - Lecture 4: Fast Synaptic Inhibition: GABA and Glycine

GABA

inhibitory neurotransmitter throughout CNS and PNS; can’t cross BBB

Vigabatrin

non competitive inhibitor of GABA transaminase (break down GABA); antiepileptic

GABA synthesis starts from

glutamate

GABA transporters

GAT1-3 and BGT1

GAT1 location

found on neurons

GAT2 location

found in CNS during development and non neural tissues

GAT3 location

astrocytes

BGT1 location

astrocytes in BBB

nipecotic acid, homo-b-proline

competitive GAT1 and 3 inhibitor GABA_a receptor agonist

guvacine

non selecetive GAT inhibitor, GABA_a receptor agonist

Tiagabine

selective GAT1 inhibitor

GAT inhibitors

increase conc of GABA in synapse

GABA receptor groups

inotropic + metabotropic

inotropic GABA_A receptors 

anion channel for Cl and HCO3; influx of Cl into neurons = hyperpolarization = harder to fire APs

metabotropic GABA_B receptors

G-protein coupled; results in decreased Ca influx + increased K efflux = neuron less excitable

synaptic GABA_A receptors

pentameric chloride ion channel; synaptic receptor w/ 2 alpha, 2 beta and gamma subunits

activation of synaptic receptors GABA_a receptors produces

large transient Cl current

extra-synaptic receptors

gamma subunit is delta subunit = increases receptor affinity for GABA; creases small sustained Cl current when activated

muscimol

GABA_a selective agonist; binds to same site as GABA

gaboxadol

GABA selective agonist preference to extra-synaptic GABA_a receptors

extrasynaptic GABA_A receptors have

role in effects of general anesthetics and depressants

benzodiazepines and barbiturates

allosteric GABA_A modulators; modify current flow through channel

benzodiazapines MOA

gamma subunit on GABA_A; increases number of channel openings

barbiturates MOA

bind to beta subunit on GABA_A, increases amplitude of current produced during channel openings; at high conc = activate GABA_Ar

picrotoxin

non competitive GABA_A receptor antagonist + glycine receptor

picrotoxin MOA

blocks chloride channel when chloride channel is OPEN

bicuculline

competitive GABA_A antagonist; competes with GABA for binding site

gabazine

competitive gaba antagonist

flumazenil

competitive inhibitor at benzodiazepine binding site (gamma subunit) (like naloxone)

effect of GABA_A activation depends on

intra and extracellular chloride concentration

NKCC1

uptake Cl with Na, K, 2Cl

KCC2

extrudes chloride with K, Cl 

Cl concentration in soma

kept low with KCC2 so that GABA_A activation = hyperpolarization (inhibition)

Cl in afferent fibers

kept high by NKCC1 so that GABA_A activation = depolarization

GABA_A activation in presynaptic terminal

chloride flows out = depolarization; primary afferent depolarization = decreases amplitude of AP invading terminal → decreased neurotransmitter release

post synaptic GABA_A activation

chloride flows in = hyperpolarization = cell less excitable

GABA_b receptor

g protein coupled receptor; heterodimer with 2 subunits;

GABA binding site on GABA_B

B1 unit

G protein binding site for GABA_B

B2 unit

GABA_B receptor activation

inhibits adenylate cyclase (= lower cAMP levels) and inositol triphosphate synthesis; decreases VGCC opening

presynaptic GABA_B activation

lowers calcium influx = decreases neurotransmitter release

beta gamma subunit of g protein GABA_B activation

inward rectifying potassium channel (GIRK) opens = K efflux = hyperpolarization

less calcium influx + increased GIRK =

slow and long lasting synaptic inhibition

baclofen

GABA_b selective agonist; skeletal muscle relaxant

GHB (gamma hydroxybutyrate)

low affinity GABA_b partial agonist

CGP7930

positive allosteric modulator; binds to B2 subunit to increase effect of GABA

phacolfen

competitive GABA antagonist

saclofen

competitive GABA binding site

nerve injury increases expression of which produces chronic pain how

P2RX4 ATP receptor → atp activates microglia to make BDNF = decreased expression of KCC2 = less chloride out (more depolarization = more pain) = less inhibitory effect

decreased mechanical pain sensitivity is due to

GABA_A

GABA analgesic effect moa

decrease in neurotransmitter release + decreased excitability of dorsal horn neurons (afferent)

glycine

fast inhibitory neurotransmitter in spinal cord, brainstem, cerebellum and parts of cortex, modulates muscle tone and coordination by inhibition of motoneurons; modulates respiratory rhythm, modulates pain

glycine release, uptake, and removal

released through GlyT2 transporter, acts on GlyR and removed from cleft by GlyT, uptake into presynaptic vesicles by VGAT

glycine receptors

pentameric chloride permeable channel

functional glycine receptors at synapses

composed of a1/b combos or (less likely) a3/B

extra synaptic receptors 

contain only a subunits (homomeric)

glycine receptors are found

spinal cord, brainstem, cerebellum (not on primary afferents)

ethanol

allosteric modulators for glycine

tropisetron

increases glycine receptor mediated currents; antinausea drug

Zn

increases Cl currents at low conc; inhibits at high

volatile anesthetics

potentiate glycine receptor currents

glycine receptor in inflammatory pain

prostaglandin E2 reduces glycine receptor response; PGE2 acts on EP2 receptor → activates phosphokinase A to phosphorylate a3 subunit on receptor = decreased Cl flux

strychnine

competitive and relatively selective glycine receptor antagonist

glycine reuptake transporters

GlyT1 and T2

GlyT1

expressed by astrocytes; removes released glycine; can be reversed

GlyT2

expressed by glycinergic neurons; unidirectional

inhibition of GlyT1 reuptake

increases extracellular glycine conc, inhibits nociceptive transmission, increases activation of glycine site on NMDA receptor, repeat administration = decreased NMDA expression

compounds that inhibit GlyT1

sarcosine, org25935, alx5407, bitopertin, N-ethyl glycine

opiranserin

inhibit GlyT2, PX3, and 5-HT2A

pros of glycine transport inhibitor analgesics

sparsity of receptors in forebrain = reduced central side effects

cons of glycine transport inhibitors

associated with significant motor and respiratory deficits, seizures