5.3 GABAergic Transmission & Anxiolytics, Sedative Hypnotics

Question 1

Where is GABA distributed in?

Answer 1

cerebral cortex, cerebellum, hippocampus, corpus striatum, hypothalamus, dorsal horn of spinal cord (influences information from primary afferent fibres entering the cord) and the peripheral nervous system (minor):

Question 2

• Most neurones (at 30% of synapses) in the brain respond to GABA → those utilising GABA as a neurotransmitter are short inhibitory interneurones (short axons)
o Function: _____________
• GABA is also involved in the longer GABAergic tracts (e.g. striato-nigral tract – descending tract from ______________- , cerebellar tract – incoming afferent cerebellar tracts)
• Possesses widespread inhibitory actions in the CNS at pre- and post-synaptic sites

Answer 2

regulate brain activity;

corpus striatum to substantia nigra

Question 3

The original precursor (______________-) for GABA biosynthesis comes from the Krebs cycle in the GABAergic neurones:
• Converted to glutamate via ________________, then to GABA via __________________ → specific for inhibitory GABAergic neurones
• After GABA performs its function, it is metabolised by GABA-T into ________________, which is converted into succinate by succinic semialdehyde dehydrogenase (SSDH) → returns to the Krebs cycle
• GABA shunt accounts for 10% of the activity of the Krebs cycle in GABAergic neurones

Answer 3

α-oxoglutarate;

GABA transaminase (GABA-T);

glutamate decarboxylase (GAD);

succinic semialdehyde

Question 4

GABA is stored in presynaptic vesicles after synthesis, which have _____________ to take up more GABA into the vesicles (high concentration of GABA accumulates):
• Arrival of action potentials down the short axons of the inhibitory interneurone causes membrane depolarisation and opening of voltage-sensitive Ca2+ channels
• Influx of Ca2+ causes exocytosis of GABA into the synaptic cleft and binding to postsynaptic GABAergic receptors

Answer 4

surface transporters

Question 5

GABA is inactivated by reuptake into ________________ via protein carrier molecules (saturable):
• Na+-dependent process requiring energy as it generates a concentration gradient within the cells
• Metabolised after uptake into the neurones or glial cells → acted on by __________________ (mitochondrial enzymes)
• Inhibition of GABA-T or SSDH causes a large increase in the concentration of GABA in the brain → enhances GABA-mediated inhibition
• ___________________ act partially via inhibition of GABA metabolism to slow down its breakdown → used as anticonvulsants/antiepileptics

Answer 5

GABAergic neurones and surrounding glial cells;

neurones or glial cells;

GABA-T and SSDH;

Sodium valproate (Epilim) or vigabatrin (Sabril)

Question 6

What is the type, structure and mechanism of a GABA A receptor?

Answer 6

• Type 1 (ionotropic)
• Pentameric (commonly 2α1β2γ2)
• Linked to Cl- channel → conformational change leads to opening of the channels → Cl- influx (hyperpolarisation; IPSP) → inhibition of firing

Question 7

What is the type, structure and mechanism of a GABA B receptor?

Answer 7

• Type 2 (G-protein coupled)
• 7 transmembrane domains (coupled to Gi and adenylyl cyclase)
• Gi negatively linked to adenylate cyclase → reduced cAMP levels → reduced Ca2+ conductance → reduced neurotransmitter release

Question 8

What is the definition of autoreceptors?

Answer 8

inhibit GABAergic neurone release of GABA (negative feedback)

Question 9

What is the definition of heteroreceptors?

Answer 9

localised on nerve terminals which use other neurotransmitters (e.g. increase dopaminergic neurone release of dopamine) → regulates concentration of other neurotransmitters

Question 10

What are some specific GABA A agonists?

Answer 10

• GABA (non-selective)
• Muscimol (selective)
• Benzodiazepines, barbiturates (clinically useful)

Question 11

What are some specific GABA B agonists?

Answer 11

• GABA (non-selective)
• Baclofen (selective; therapeutically used as a muscle relaxant and anti-spastic drug):
  –> Muscle relaxant: acts in the spinal cord to reduce the outflow of action potential to skeletal muscle fibres
  –> Anti-spastic drug: treat spasticity
  following e.g. multiple sclerosis/spinal
  cord damage

Question 12

The GABAA receptor has a pentameric organisation (consists of 5 main proteins) which allows Cl- ions to pass through the centre (contains Cl- channel) into the postsynaptic cell. What are the 5 proteins?

Answer 12

GABA receptor protein, benzodiazepine receptor protein, barbiturate receptor protein, Cl- channel protein and GABA modulin protein (links GABA receptor protein to benzodiazepine receptor protein)

Question 13

What are the effects of activating the GABA receptor protein?

Answer 13

Triggers linkage between GABA receptor protein and benzodiazepine receptor protein by GABA modulin:
• Allows opening of the Cl- channel protein → Cl- influx → hyperpolarisation → inhibition

Question 14

What are the effects of activating the Benzodiapene receptor protein?

Answer 14

1. Facilitates underlying action of GABA (increases Cl- conductance)
2. Enhances binding of GABA to GABA receptor site (reciprocal arrangement – binding of benzodiazepine to benzodiazepine receptor protein is also enhanced in presence of GABA)

Question 15

What are the effects of activating the Barbituate receptor protein?

Answer 15

1. Facilitates action of GABA (enhances Cl- influx caused by GABA binding to GABA receptor)
2. Enhances binding of GABA to GABA receptor (not reciprocal)
3. Increases Cl- influx into postsynaptic cell (direct action on Cl- channel)

Question 16

Bisculline is a GABA A antagonist. How does it inhibit?

Answer 16

Competitive GABAA receptor antagonist (binds to GABAA receptor → competes with GABA for binding)

Question 17

Picrotoxin is a GABA A antagonist. How does it inhibit?

Answer 17

non-competitive; binds to Cl- channel and inhibits flow into postsynaptic neurone

Question 18

Flumazenil is a GABA A antagonist. How does it inhibit?

Answer 18

Competitive benzodiazepine antagonist (competes with benzodiazepines for the binding site)

Question 19

How is BDZ distributed?

Answer 19

binds plasma proteins strongly, highly lipid-soluble (wide distribution)

Question 20

How is BDZ administered?

Answer 20

oral (well absorbed; peak plasma conc. achieved in about 1h), IV (e.g. for status epilepticus)

Question 21

How is BDZ metabolised?

Answer 21

usually extensive (in liver)

Question 22

How is BDZ excreted?

Answer 22

in urine via the kidneys (as glucuronide conjugates)

Question 23

Benzodiazepines have many advantages (makes them more clinically useful):
1. Wider margin of safety (overdose results in prolonged sleep → patient still rousable) compared to death in overdose of BARBs
• ____________: IV antidote to overdose (competitive BDZ antagonist)
2. Only mild effect on ____________
3. Does not induce ________________

Answer 23

Flumazenil;

REM sleep;

liver enzymes (less drug interactions)

Question 24

What are the ideal characteristics of the drugs?

Answer 24

1. Wide margin of safety
2. Not depress respiration
3. Produce natural sleep (hypnotics)
4. No drug interactions
5. Not produce hangovers
6. Not produce dependence

Question 25

What BDZ so you use in an axiolytic?

Answer 25

Long acting BDZs

• Diazepam (Valium)
• Chlordiazepoxide (Librium)
• Nitrazepam

Question 26

What BDZ should you use in patients with hepatic impairment + as an antiolytics?

Answer 26

short-acting BDZs (e.g. oxazepam – t1/2 8 hours) may be more useful → metabolised more slowly)

Question 27

What BDZs do you use as a sedative/ hypnotic

Answer 27

• Temazepam
• Oxazepam
• Nitrazepam (t1/2 28 hours) used for hypnotic activity (for patients who also require a daytime anxiolytic effect)
• Zopiclone (t1/2 5 hours) → BDZ-like drug (not BDZ) → actions on BDZ receptors and useful hypnotic activity

Question 28

Which barbituate is used as an anaesthetic?

Answer 28

thiopentone – very lipid soluble; used in induction of GA

Question 29

What barbituate can you use as a hypnotic in patients not responding to BDZ and has severe, intractable insomnia?

Answer 29

Amobarbital (t1/2 20 – 25 hours)

Question 30

what would you use as a sedative for children and the elderly?

Answer 30

Chloral hydrate (pro-drug) → converted to the active trichloroethanol in the liver
(mechanism unknown; but wide margin of safety for children and the elderly)

Question 31

Other anxiolytic:
1) Buspirone (__________________ agonist; slow onset of action → maximal effects may take days/weeks, few side effects)

2) ___________ (non-selective β receptor antagonist → improves physical symptoms of anxiety like β1-mediated tachycardia and β2-mediated tremor)
3) ____________ (mechanism unknown; therapeutically useful anxiolytic activity and relatively mild side effects (e.g. sedation, ataxia) → usually used as a backup for BDZs

Answer 31

5HT1A/serotonin;

Propranolol;

Pregabalin

Question 32

What are the side effects of BDZ?

1. Sedation (when used as an ___________) and confusion
2. Ataxia (impaired manual skills)
3. Potentiates other CNS depressants (alcohol, opioids, BARBs)
4. __________ (less marked than BARBs)
5. Dependence (withdrawal syndrome similar to BARBs but less intense) → withdraw slowly (over days to weeks)
6. Free [plasma] of BDZ increases when co-administered with other drugs that bind to plasma proteins (e.g.______________) → BDZs displaced from plasma proteins

Answer 32

anxiolytic;

Tolerance ;

aspirin, heparin

Question 33

What are the side effects of barbituates?

1. Low safety margin (depress _____________) → overdose is lethal
2. Alters natural sleep (reduced ___________) → hangovers/irritability
3. Enzyme inducers (long-term use → increased activity of liver microsomal enzymes → drug interactions with those metabolised by same system)
4. Potentiates other CNS depressants (alcohol, opioids)
5. Tolerance (may develop with repeated administration)
6. Dependence: withdrawal syndrome (insomnia, anxiety, tremor, convulsions, death)

Answer 33

respiration;

REM