Topic 12: GABA and Glycine

Question 1

What is gamma-amino butyric acid (GABA)?

Answer 1

GABA is the main inhibitory neurotransmitter in the CNS (10-40% of neurons in cortex, hippocampus, and substantia nigra)

GABA increases the conductance of chloride ions across cell membranes

Glycine has comparable but limited function as an inhibitory neurotransmitter

Question 2

How is GABA synthesized?

Answer 2

glutamate is synthesized into GABA by glutamic acid decarboxylase (GAD)

Question 3

What is the vesicular transport of GABA?

Answer 3

GABA and glycine share a vesicular transporter

vesicular GABA transporter (VGAT) or vesicular inhibitory amino acid transporter (IAAT)

VGAT identifies both GABAergic and glycinergic neurons in the CNS

Question 4

In what way are inhibitors of GABA convulsants?

Answer 4

GAD has several antagonists used experimentally: allylglycine, thiosemicarbazone, and 3-mercaptopropionic acid

inhibition of GAD decreases GABA levels and leads to convulsive activity

many drugs that decrease GABAergic activity are limited in use to in vitro studies

Question 5

What are GABA transporters?

Answer 5

GABA transporters (GAT) are found on astrocyte and neuronal membranes at the synapse

GAT-1 is located on neurons and astrocytes

Gat-2 and -3 are principally astrocytic

Question 6

What are GABAergic AEDs?

Answer 6

drugs that increase GABA activity are anticonvulsant

tiagabine is a selective antagonist of GAT-1 and elevates GABA levels in the synapse

Tiagabine (Gabitril) is approved as a adjunctive AED for epilepsy

vigabatrin is a irreversible inhibitor of GABA-T and elevates GABA levels in the brain by blocking breakdown

Vigabratrin (Sabril) is approved as an adjunctive or primary AED for epilepsy

Question 7

In what way is GABA widespread through the brain?

Answer 7

GABA is widely used in inhibitory interneurons throughout the brain

chandelier cells of the cortex synapse onto the axonal initial segment of pyramidal cells

basket cells of the cerebellum, hippocampus, and cortex form axo-somatic synapses onto target cells

Question 8

What are the types of synapses formed at GABAergic neurons?

Answer 8

in addition to axo-dendritic synapses GABAergic synapses are often axo-somatic or axo-axonal

axo-somatic synapses control excitability of cell body

axo-axonal synapses at the axon initial segment influence signal integration

Question 9

What is GABAergic output from the cerebellum?

Answer 9

Purkinje cells are large GABAergic projection neurons of the cerebellum: provide the sole output of motor coordination from the cerebellar cortex

Purkinje cells are under inhibitory control from GABAergic interneurons: basket cells from axo-axonal synapses at the axon initial segment, stellate cells form axo-dendritic synapses

degeneration of Purkinje neurons is termed Holmes cerebellar degeneration: impaired fine hand movement, speech deficits, tremors, and ataxia while walking

Question 10

What is the GABAergic control of motor initiation?

Answer 10

medium spiny neurons comprise 90-95% of the neurons in the striatum: inputs from neocortex (all except visual and auditory), outputs to globus pallidus and substantia nigra

involved in two pathways that control inhibition of motor activity in the basal ganglia

direct pathway: excitatory input from cortex causes excitation of upper motor neurons in motor cortex

indirect pathway: excitatory input from cortex causes inhibition of upper motor neurons in motor cortex

Question 11

What is the direct pathway of motor initiation?

Answer 11

excitatory input from cortex (glutamatergic) to medium spiny neurons (MS) in striatum

inhibitory output from medium spiny neurons to the internal globus pallidus & substantia nigra pars reticula (SNpr)

GABAergic MSN inhibits tonic inhibitory output from globus pallidus –> ventral thalamus (VTh) and form SNpr –> superior colliculus

disinhibits outputs

VTh: excitatory projections to upper motor neurons of cortex

superior colliculus: controlling eye saccades

Question 12

What is the indirect pathway of motor initiation?

Answer 12

medium spiney neurons project to the external globus pallidus which forms a loop with the subthalamic nuclei

subthalamic nuclei (STN) has excitatory glutamatergic projections to the internal globus pallidus

indirect pathway activation leads to disinhibition of STN projections and this inhibition of motor output (dis-disinhibitory pathway)

Question 13

What is the dopaminergic balance between the direct and indirect pathways of motor initiation?

Answer 13

dopamine plays a gating role and balances activity between the direct and indirect pathways

activation of nigrostriatal dopamine pathways promotes the direct pathway (D1 - excitatory) over the indirect pathway (D2 - inhibitory)

in Parkinson’s the loss of dopaminergic projections shifts activity to the indirect pathway

Question 14

What is the cholinergic balance between the direct and indirect pathways of motor initiation?

Answer 14

cholinergic interneurons in the striatum receive excitatory inputs from the cortex

cholinergic interneurons act directly on the direct pathway

M4AChR antagonists and AChE inhibitors are useful therapeutics in early Parkinson’s as they compensate for decreased dopaminergic input

Question 15

What are ionotropic (GABAA) receptors?

Answer 15

classic ligand gated ion channel permeable to Cl-

5 subunits form the channel pore

originally characterized by sensitivity to bicucculine (comp. antagonist)

Question 16

What are metabotropic (GABAB) receptors?

Answer 16

G-protein coupled receptors

Gi - inhibits adenylate cyclase (decreased cAMP)

G-beta-gamma: opens G-protein coupled K+ channel (GIRK)

originally characterized by sensitivity to baclofen (specific agonist)

Question 17

What are the characteristics of GABAA?

Answer 17

pentameric channel (5 subunits) through combination of 19 different genes

normal channel form is at least 1 each of alpha and beta: usually 2 alpha, 2 beta, and 1 of something else

special case is GABAAP which only forms homopentameric channels with itself

Question 18

What are the GABAA receptor binding sites?

Answer 18

GABAA has 4 binding sites for endogenous and exogenous ligands

GABA site: binds two molecules of GABA at the interface between alpha and beta subunits

benzodiazepine site: binds benzodiazepines (tranquilizers) as positive allosteric modulators

barbiturate site: binds barbiturates (sedative & anxiolytics) as positive or negative allosteric modulators

picrotoxin is a non-competitive channel blocker

Question 19

What binds to the non-competitive sites of the GABAA receptor?

Answer 19

pentylenetetrazol binds in the pore at the same site as picrotoxin and was used as a convulsant for depression therapy (discontinued due to high risks of spontaneous seizures, widely replaced with electroconvulsive therapy in 1939)

GABA site: competitive antagonist is bicucculine - potent convulsant, classic agonist if muscimol

Question 20

What is Amanita muscaria (fly agaric)?

Answer 20

source of the muscarinic AChR agonist muscarine

source of the GABAA agonist muscimol

potent hallucination: induces macroscopia, perception of objects being larger than they are

consumption of fly agaric has serious peripheral side-effects due to muscarinic cholinergic effects at NMJ and parasympathetic effects

Gaboxidol is a synthetic version of muscimol with reduced psychotropic effects: anxiolytic and analgesic, investigated for insomnia treatment

Question 21

What are benzodiazepines?

Answer 21

sedative-hypnotic, anxiolytic

Diazepam (Valium) one of the best known

better safety margin than barbiturates

binding causes increased probability of pore opening

high risks of drug interactions at the GABAA receptor

orphan receptor site: endogenous ligand not known

Question 22

What are barbiturates?

Answer 22

sedative-hypnotic, anaesthetic

Phenobarbitol best known

narrow safety margin: high potential for abuse, high risk of overdose

binding prolongs open time of Cl- pore

used in physician-assisted suicide and euthanasia

sodium amytal (amobarbital) is a barbiturate known as a “truth serum”: helps to circumvent inhibitions

Question 23

How does ethanol interact with the GABAA receptor?

Answer 23

ethanol is a potent positive allosteric modulator of GABAA binding to a site on the transmembrane surface of the delta-subunit

ethanol exerts many of it’s sedative, euphoric, and addictive effects through modulation of GABAA

ethanol binds GABAA with very high affinity - binding even at doses that would be considered moderate, social levels

Question 24

How does propofol interact with the GABAA receptor?

Answer 24

propofol is a potent anaesthetic that interacts with the transmembrane surface of the beta-subunit of GABAA

positive allosteric modulator that increases channel open time

Question 25

What are GABAB receptors?

Answer 25

primarily affect excitability by coupling to GIRK

GIRK activation is inhibitory by allowing potassium efflux which hyperpolarizes the cell

GABAA is responsible for fast, weak inhibitory postsynaptic potential (IPSP) signaling: reversal potential Cl- (~70 mV)

GIRK is responsible for slow, strong component of IPSP: reversal potential K+ (~90 mV)

Question 26

How does baclofen interact with the GABAB receptor?

Answer 26

baclofen is a specific agonist of GABAB and is a muscle relaxant and antispastic

Question 27

How does gamma-hydroxybutyric acid (GHB) interact with the GABAB receptor?

Answer 27

GHB is a weak GABAB agonist

excitatory at the GHB receptor at lower doses –> recreational drug use (euphoria, disinhibition, empathogenic)

inhibitory at GABAB at higher doses –> “date rape drug” (sedation, nausea, dizziness, and unrouseable sleep)

Question 28

What is the G-protein coupled inward rectifying K+ channel (GIRK)?

Answer 28

K+ channel activated during GPCR signaling

GIRK opens on binding of G-beta-gamma (the otherwise regulatory component of the G-protein complex)

K+ exits the cell causing hyperpolarization of the cell membrane

GIRK signaling inhibits subsequent depolarizing stimuli

Question 29

What is the GABAA-rho receptor?

Answer 29

unique - formed exclusively as a homopentamer of the rho-subunit (Cl- channel); formerly considered GABAC receptor

insensitive to baclofen and bicucculine - lacks binding sites for benzodiazepines, barbiturates, and neurosteroids

more sensitive to GABA (having 5 GABA binding sites)

found in bipolar cells of the retina: receive inhibitory signals from amacrine and horizontal cells

mutations in GABAA-rho are associated with heritable cases of retinitis pigmentosa

Question 30

How is GABA developed in the prefrontal cortex?

Answer 30

GABA development in the prefrontal cortex is a late developmental step and is associated with maturation of impulse control, working memory, and executive function

Question 31

How is GABA involved in anxiety?

Answer 31

generalized anxiety disorder, social anxiety disorder, panic disorder, post-traumatic stress disorder

GABA agonists and positive allosteric modulators are anxiolytic

Question 32

What are the possible models of GABA function in anxiety?

Answer 32

anxiety is caused by secretion of endogenous inverse agonists of GABAR –> inhibition of GABAR increases anxiety?

ligand activity at GABAR is shifted in anxiety (subunit alterations?)

secretion of endogenous agonists of benzodiazepine site during stressful conditions –> deficit in anxiety disorders?

Question 33

What are the developmental effects of GABA?

Answer 33

high levels of GABA and developmental changes in GABA activity (excitatory/inhibitory switch)

GABA may contribute to cell proliferation, survival, and motility

excitatory/inhibitory balance important in normal brain development: E/I balance is affected in conditions such as Down’s syndrome and Autism

Question 34

How is GABA involved in epilepsy?

Answer 34

excitatory/inhibitory balance is implicated in seizure disorders

drugs that decrease GABA levels or inhibit GABAR function are convulsant

drugs that increase GABA levels or increase GABAR function are anticonvulsant

E/I imbalance in Down’s syndrome and Autism correlate with increased risk of seizure disorders

Question 35

How is GABA involved in psychiatric disorders?

Answer 35

GABA has been implicated or suggested to play a role in numerous neuropsychiatric & neurodegenerative disorders

developmental disorders (ASD), addiction, learning disorders, schizophrenia, tardive dyskinesia, Huntington’s disease, Parkinson’s disease

generally proposed to contribute to hyperactivity through decreased inhibitory signaling