GABA A Receptors, Anxiolytics, Sedatives, Hypnotics Outline • Physiology of GABA A responses • How is GABA synthesized, released and metabolized? – Vitamin B6 – Stiff-man syndrome • Pharmacology: – Agonists, competitive & non-competitive antagonists: How to study? • Sedatives, anxiolytics & hypnotics • Benzodiazepine agonists, competitive antagonists, inverse agonists (allosteric modulators) • Next lecture: How to identify the GABA receptor subunits that are responsible for the different effects of benzodiazepines -Aminobutyric Acid (GABA) • GABA occurs in brain tissue, trace amount in other tissue • GABA functions as an inhibitory transmitter in many different CNS pathways • 20% of CNS neurons are GABAnergic – Most are short interneurons – But long GABAnergic tracts run to cerebellum and striatum • GABA serves as a transmitter at about 30-40% of all the synapses in the CNS AP may be initiated AP not initiated. AP less likely to be initiated
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GABAA Receptors,Anxiolytics, Sedatives,
Hypnotics
Outline• Physiology of GABAA responses• How is GABA synthesized, released and metabolized?
– Vitamin B6– Stiff-man syndrome
• Pharmacology:– Agonists, competitive & non-competitive antagonists: How to
• GABAA receptors – located postsynaptically– Serves as main fast inhibitory neurotransmitter in the brain– Selectively permeable to Cl- ,hyperpolarizes membrane & inhibits
action potential firing
• GABAB receptors – located pre- and postsynaptically– GPCR; functional receptor is dimer– Inhibitory action– Stimulates opening of K+ channels & inhibits voltage-gated Ca2+
channels
GABAA Receptor• 19 subunits encoded by 19 distinct
genes – 1-6, 1-3, 1-3, 1-2– Selectively permeable to Cl-
• Each subunit– 4 transmembrane segments – Both amino and carboxyl termini
extracellular – These extracellular segments form the
binding sites for GABA and for allosteric modulators
• Member of the Cys-loop superfamily(related to nAChR and Glycine R)
GABAA Receptor: Phasic vs. Tonic
Neurotherapeutics. 2007, 4:18
Phasic inhibition• at synapse• Fast transient inhibitory postsynaptic
currents due to synaptically released GABA• α2β2γ subunits• Benzodiazepine sensitive
Tonic inhibition• at extrasynaptic sites• Responds to background GABA
• Antagonists– Bicuculline: naturally occurring convulsant compound, is a
specific antagonist that blocks fast inhibitory synaptic potential in most CNS synapses
– Gabazine: synthetic GABA analogue
• Channel blocker– picrotoxin: convulsant, acts by blocking Cl- channel pore of
the GABAA receptor, thus blocking the postsynaptic inhibitory effect of GABA
Drugs acting on GABAA Receptors
• Benzodiazepines– Four among the 200 most prescribed drugs in US (alprazolam (Xantax),
clonazepam (Klonopin), diazepam (Valium), lorazepam (Ativan))– Selectively potentiate the effects of GABA on GABAA receptors– Bind with high affinity to a separate site on GABAA receptor that is
different from the agonist binding site for GABA – Binding of GABA is facilitated and its agonist effect is enhanced.– Not all GABAA receptors are affected by benzodiazepines– Sedative benzodiazepines such as diazepam (Valium) are benz agonists– Convulsant analogues: flumazenil is a benzodiazepine antagonist; β-
carbolines are benzodiazepine inverse agonists• Barbiturates
– CNS depressant• Anesthetic agents and neurosteroids
– IV anesthetic for fast-acting induction of anesthesia—propofol, etomidate, potentiate the effect of GABA
– Neurosteroids: allosterically enhance activation of GABAA R, as well as activating conventional intracellular steroid receptors
Allosteric Modulators of GABAA Receptors Anxiolytics, Sedatives, Hypnotics• Anxiolytics
– Reduce anxiety– Short-term relief of agitation due to fear or stress, preferably
Can a non-sedating benzodiazepine be developed that retains anxiolytic and/or anticonvulsant effects?
Goal: Identify receptor subtypes for anxiolytic, anti-convulsant and hypnotic effects.
Strategy to Identify Benzodiazepine Receptor with anxiolytic function:
Make diazepam-insensitive subunits.Is anxiety still reduced by diazepam?
Low et al. (2000) Molecular and Neuronal Substrate for the Selective Attention of Anxiety, Science 290, 131-4
Creating a “Knock-in” Mouse
Low et al. (2000)
Are protein levels normal for the mutant animals?•Yes, determined by immunoblot.
Are the mutant GABA receptor subunits expressed in the correct tissues?•Compare immunocytochemical localization of Ro15-4513 (benzodiazepam inverse agonist that binds normally) + diazepam
What is the physiological response of the mutant GABA receptors?
– Like benzodiazepines, Zolpidem & Eszopiclone often associated with tolerance & dependence—note that receptors with α1 subunit are associated with addictive behavior
– Potential for abuse
– Some Z drugs have less or no disruption of sleep architecture, less risk of rebound insomnia upon discontinuation (unlike benzodiazepines that actually worsen sleep architecture)
Summary-Anxiolytics, Sedatives, Hypnotics• Benzodiazepines, barbiturates & 5HT1A agonists are anxiolytics
• Benzodiazepines:– increase the affinity of GABA for binding to the GABAA receptor;
– increase frequency of channel opening;
– only effective in the presence of GABA
• GABAA receptor: pentameric
• Agonists bind at interface
• Benzodiazepines bind at interface
• “Knock-in” receptor mutation in transgenic mice allows study of receptor subunits involved in responses to benzodiazepines
• 1 subunit in GABAA receptors mediates benzodiazepine effects of sedation & memory impairment (Z-drugs selective for this type)
• 2 subunit in GABAA receptors mediates benzodiazepine effects of anti-anxiety
• 5 subunit in GABAA receptors – inverse benzodiazepine agonists may enhance cognition
• Inhibitory—hyperpolarizes cell membrane• Anion channel• pentameric structure related to GABAA R
Tetanus toxin:inhibits release of glycine in spinal cord.
contract
relax
relax
contract
Crossed Extensor reflex & Flexor/Withdrawal reflex:withdraw from painful stimulus while maintaining balance
Glycine
Painful stimulus:
Stretch Reflex:Balance, walking
Stretch Reflex: maintain balance, walking
Glycine
Stretch Reflex:Balance, homeostasis
Tendon (Inverse Stretch) Reflex:Prevent overstretching of muscle
+ -
Glycine
Spinal Motor Neurons
Renshaw Inhibition:Fine motor control
+
- - -
Glycine & GABA
Syntaxin
SNAP-25
VAMP/Synaptobrevin
Jahn and Scheller (2006)
Purves et al (2007)
SNARE Complex-Vesicle Fusion
Tetanus & Botulinum Toxins: inhibit release of neurotransmitters vesicles
• Produced by anaerobic Clostridium bacteria
• Each toxin is cleaved to produce:– a heavy chain that binds to surface of target nerve cells. Toxin then is
endocytosed, & low endosomal pH aids in translocating light chain across the membrane into the cytosol
– a light chain that is a proteases that cleaves proteins of the SNARE complex involved in vesicle fusion, thereby blocking neurotransmitter release
• Tetanus toxin – taken up by inhibitory interneurons in spinal cord– Cleaves vesicle SNARE synaptobrevin
• Botulinum toxin – taken up by excitatory motor neurons in spinal cord– Types B,D,F&G: cleave vesicle SNARE synaptobrevin– Type C: cleaves plasma membrane SNARE syntaxin– Types A & E: cleaves plasma membrane SNARE SNAP-25
Glycine Receptor• Glycine receptors mediate the major inhibitory
actions in the spinal cord
• Released by inhibitory interneurons involved in spinal reflexes
• Anion channels that allow chloride influx upon glycine binding—hyperpolarizes the membrane potential
• Strychnine is competitive antagonist—causes spinal convulsions