GABA neurotransmission Flashcards
Name 2 fast inhibitory neurotransmitters in the brain and list their receptors
• GABA GABAA, GABAB • Glycine GlyR o No metabotropic Gly receptor known
GABA(A)R pharmacology - agonists:
- GABA
- Muscimol from Amanita muscaria
- Gabadoxadol
GABA(A)R pharmacology - antagonists:
• Bicuculline (competitive)
• GABAzine (competitive)
• Picrotoxin (non-competitive; combination of picrotoxinin and picrotin)
U93631
GABA(A)R pharmacology - modulators:
- Benzodiazepines (BZDs)
- Barbiturates
- Neurosteroids
- Anaesthetics
- Alcohol
Where are GABA(A)Rs found?
Not on dendritic spines - usually found in the membrane of the postsynaptic neuron. They signal in the normal mechanism for an ionotropic receptor
Explain feed-forward inhibition, using a diagram
• Excitatory neuron stimulates GABAergic and Glutamatergic neurons –> EPSP, then IPSP (caused by GABA release)
diagram: https://neurology.mhmedical.com/data/books/1049/kan_ch2_f008.png
Using a diagram, explain feedback (recurrent) inhibition
• 2nd excitatory cells feeds back onto itself
• Is ‘doubly inhibited’
• There are 3 synapses; 2 of them inhibitory
Diagram: https://neurology.mhmedical.com/data/books/1049/kan_ch2_f008.png
Using a diagram, explain inhibition of inhibition and what it leads to
–> excitation (overall)
GABA neuron inhibits GABA neuron – cancels out release of GABA from 2nd neuron; prevents inhibition ∴ allows excitation
Diagram: https://neurology.mhmedical.com/data/books/1049/kan_ch2_f008.png
Explain why inhibitions are needed
GABA antagonists induce seizures Epilepsy in rats Give PTZ --> seizures ∴ GABA agonists can be used as anticonvulsants Zebrafish assay • Tetanus toxin --> lockjaw/tetanus o Without inactivation with formaldehyde --> muscle spasm o Stops inhibitory synapses from working
Explain how strychnine achieves its inhibitory effects and their consequences:
Antagonises GlyRs Alkaloid Comes from Quaker’s buttons Inhibits synapses of spinal inhibition of motor neurons --> spasms Renshaw cell: Glycinergic Sits in spinal cord α muscle neurons excite these; Renshaw cells feed back and inhibit motor neurons (FF inhibition via Gly)
With the aid of a diagram, explain Glycine receptor structure and subunit composition
‘Cys-loop’ family members 4 TMDs long intracellular loop connects TM3 and 4 either: 5 α subunits Or 2α + 3β
diagram: https://www.researchgate.net/figure/Structure-of-the-glycine-receptor-A-Membrane-topology-of-the-a-subunit-with-the-four_fig1_11904296
List the features of a glycine receptor
Ligand-gated ion channel 5 subunit genes: α1-4 No α4 in humans β1 Large single channel conductance – 50-110pS Anion pore; selects for Cl- ions Activated by Glycine (β-alanine + taurine) Potently inhibited by: Strychnine RU5135 Pore can be blocked by picrotoxinin, as with GABAAR
Explain GABA structure (chirality), synthesis, transport, and receptors
• GABA has no chiral centre – no L/D form
• Not usually on dendritic spines
• Synthesized by GAD67 (in cell) and GAD65 (nerve terminal)
o GAD = Glu decarboxylase
• GAT1 and 3 transport GABA into cells
• VGAT gets GABA (and Gly) into vesicles
• GABAA,B,C receptors bind GABA
• GABA function terminated by diffusion and uptake by GATs
With the aid of a diagram, explain GABA(A)R structure and potential subunit composition, as well as the 3 most common conformations
Ligand-gated ion channel assembly Channel is inherent to receptor Ligand-binding enables channel opening Pentameric - α,β,γ 19 subunits: α1-6 β1-3 γ1-3 δ ϵ π θ ρ1-3 Most receptors have 2α, 2β, 1γ/δ subunit Other conformations include: α1β2γ2, α1βγ2 different subunits = different localisations – α6 is only in the cerebellum
• In the transmembrane domain the terms “plus side”, which is formed by parts of TM2 and TM3 of the principal subunit; and “minus side”, formed by parts of TM2 and TM1 of the complementary subunit, are also commonly used
Diagram: https://upload.wikimedia.org/wikipedia/commons/3/3a/GABAa_receptor.gif
Use a diagram to show where on the GABA(A)R benzodiazepines bind, and explain their potential effects
Bind α-γ subunit interface
Don’t work on α4,6-containing receptors
Classically, are PAMs of GABA-mediate channel activation:
They increase frequency of bursts of channel opening
Are anticonvulsants; promote sleep; reduce anxiety
Can be very addictive
Temazepam sleeping pills
Drug users tend to prefer short-acting and intermediate-acting barbiturates. The most commonly used are amobarbital (Amytal), pentobarbital (Nembutal), and secobarbital (Seconal). A combination of amobarbital and secobarbital (called Tuinal) is also highly used. Short-acting and intermediate-acting barbiturates are usually prescribed as sedatives and sleeping pills. These pills begin acting fifteen to forty minutes after they are swallowed, and their effects last from five to six hours.
Are also BDZ-site inverse agonists
Anxiogenic
Seizure-promoting
E.g. DCM
And competitive antagonists:
Flumazenil
Treats overdose
Some BDZ-site ligands have subtype selectivity:
Can lead to more focused therapeutic use with fewer side-effects
α1,5 –> sedation (ataxia, amnesia)
α2,3 –> anxiolytic (stop anxiety)
Barbiturates and (neuro)steroids
Different binding site to BDZs:
PAMs at [low]
Activate receptors directly at [higher]
Tend to increase duration of bursts of channel opening
Sodium thiopental/pentobarbital – used in lethal injections (as with midazolam)
