Ballicules.com

GABA-A PAM

• ↑ GABA-A chloride current for the same GABA
• Sedation, anticonvulsant, ataxia, amnesia
• Respiratory depression and dependence depend on the binding site

What a GABA-A PAM is

The receptor, the transmitter, and the modulator

• GABA-A is a ligand-gated chloride channel: five subunits (usually two α, two β and a γ or δ) around a pore. GABA binds at the two β/α interfaces and opens it.
• Chloride flows in, the neuron hyperpolarizes and fires less — the brain’s main fast inhibitory signal.
• A positive allosteric modulator (PAM) binds elsewhere on the receptor and strengthens GABA’s signal. At ordinary doses it does not open the channel alone.
• Binding site — benzodiazepine, barbiturate, neurosteroid or anesthetic — determines ceiling or no ceiling, phasic or tonic action, and reversibility.
• One mechanism id, thirteen site-specific GABA-A PAM peg designs.

Frequency vs duration — why the ceiling matters

The single most useful distinction in the family

• Benzodiazepine-site drugs raise how OFTEN the channel opens once GABA is bound. They need GABA and cannot do more than GABA can — a ceiling.
• Barbiturates, neurosteroids, propofol and etomidate prolong how LONG each opening lasts, and at higher concentration open the channel with no GABA at all — no ceiling.
• Ceiling: a benzodiazepine overdose alone is rarely fatal. No ceiling: a barbiturate overdose stops breathing, and there is no antidote.
• Phasic vs tonic: synaptic αβγ2 receptors give brief phasic inhibition; extrasynaptic αβδ receptors carry a standing tonic current. Neurosteroids, ethanol and anesthetics reach the δ receptors; benzodiazepines cannot (no γ2 subunit, no benzodiazepine site).

Same receptor, thirteen pegs

Benzodiazepine site — benzodiazepines

Diazepam, lorazepam, alprazolam, clonazepam, midazolam, clobazam, remimazolam …

• Bind at the α/γ2 interface on the outer face of the receptor — the benzodiazepine site. GABA must be present; opening frequency rises.
• Subunit by effect: α1 → sedation, amnesia, anticonvulsant; α2/α3 → anxiolysis, muscle relaxation; α5 (hippocampus) → memory.
• Ceiling effect: rarely fatal alone. Risk increases with opioids or alcohol.
• Flumazenil is a competitive antagonist at this site — it reverses the drug, and can precipitate seizures in a dependent patient.
• Sedation tolerance within days to weeks; dependence; withdrawal can cause seizures — taper, never stop.

Benzodiazepine site — Z-drugs

Zolpidem, zaleplon, eszopiclone

• Not chemically benzodiazepines; same binding site, ceiling and flumazenil reversal.
• α1-preferring: a hypnotic with less anxiolysis, muscle relaxation or anticonvulsant effect. Eszopiclone is the least selective of the three.
• Short half-lives built for sleep onset; the complex sleep behaviors (sleep-driving, sleep-eating) carry a boxed warning.
• Tolerance and rebound insomnia with sustained use; Schedule IV like the benzodiazepines.

Barbiturate site — barbiturates

Phenobarbital, primidone, pentobarbital, secobarbital, butalbital, thiopental, methohexital

• Their own site, in the transmembrane part of the β subunit. They prolong the DURATION of each opening and at high dose open the channel with no GABA — no ceiling.
• A second brake: barbiturates also block AMPA/kainate glutamate receptors.
• Overdose is respiratory arrest with no antidote; the margin between sedation and coma is narrow.
• Phenobarbital and primidone are strong CYP inducers. Tolerance, dependence, and a withdrawal with seizures and delirium.
• One site, every speed: thiopental and methohexital induce anesthesia in seconds; pentobarbital and secobarbital are short; phenobarbital lasts days and is the anticonvulsant.

Barbiturate-like sites — carbamates and stiripentol

Meprobamate, carisoprodol, stiripentol

• Meprobamate — and carisoprodol, which is metabolized to it — act like barbiturates: direct opening at high concentration, sedation with real abuse liability, and a barbiturate-type withdrawal.
• Carisoprodol’s “muscle relaxation” reflects central sedation, not a muscle effect.
• Stiripentol is a direct PAM with a preference for α3-containing receptors and prolongs channel opening — a Dravet syndrome adjunct given with clobazam.
• Stiripentol is also a CYP2C19/3A4 inhibitor that raises clobazam and norclobazam, so its GABA effect and its interaction both add to clobazam’s.
• Ethchlorvynol, glutethimide and methaqualone are older sedatives at barbiturate-like sites; they carry the family peg.

Neurosteroid site

Allopregnanolone (brexanolone), zuranolone, ganaxolone — and the body’s own progesterone metabolites

• Endogenous progesterone metabolites and their synthetic copies bind transmembrane α/β sites distinct from the benzodiazepine and barbiturate sites.
• They reach the extrasynaptic δ receptors → TONIC inhibition as well as phasic; at high concentration they open the channel directly.
• Allopregnanolone falls abruptly after childbirth and swings with the cycle — the rationale for brexanolone (a 60-hour infusion) and zuranolone (14 days by mouth) in postpartum depression. Ganaxolone treats CDKL5 deficiency seizures.
• Sedation, dizziness and loss of consciousness during infusion (brexanolone has a REMS); flumazenil does not reverse them.

Anesthetic sites — propofol, etomidate, the volatiles

Propofol, etomidate, isoflurane, sevoflurane, desflurane

• Propofol binds a β-subunit transmembrane site (β M286): it potentiates GABA and, at anesthetic concentration, opens the channel directly, tonic receptors included — unconscious within a minute, no analgesia. Redistribution ends it in minutes; propofol infusion syndrome with prolonged high-dose infusion.
• Etomidate is selective for β2/β3-containing receptors at the same site family — the most GABA-A-selective induction agent. Hemodynamically stable induction; the price is adrenal suppression (11β-hydroxylase) after even one dose, and myoclonus.
• Volatiles potentiate GABA-A at transmembrane sites as one target among several (glycine, two-pore K+ and NMDA receptors). Potency tracks lipid solubility (Meyer–Overton); amnesia and hypnosis are the GABA-A part, immobility is largely spinal.

Ethanol

The most used GABA-A PAM of all

• Low-potency potentiation, proposed to be strongest at the extrasynaptic α4/α6βδ receptors that carry tonic inhibition.
• GABA-A is one target among several — NMDA antagonism, glycine, nicotinic and K+ channels.
• Chronic use down-regulates GABA-A and up-regulates NMDA. Remove the alcohol and the brain is hyperexcitable: tremor, seizures, delirium tremens.
• Cross-tolerance with benzodiazepines and barbiturates is why benzodiazepines (and phenobarbital) treat the withdrawal.

Antiseizure drugs with a GABA-A hand

Topiramate, cenobamate, felbamate — not the benzodiazepine site

• Topiramate potentiates GABA-A at a site that is NOT the benzodiazepine site — flumazenil-insensitive, no benzodiazepine-type dependence. It is one of four mechanisms (Na+ channel block, AMPA/kainate antagonism, carbonic anhydrase inhibition). The tolls are cognitive slowing and paresthesias, not respiratory depression.
• Cenobamate potentiates GABA-A at a non-benzodiazepine site alongside its block of the persistent Na+ current; it needs slow titration for DRESS.
• Felbamate potentiates GABA-A and blocks NMDA (NR2B); aplastic anemia and hepatic failure limit it to refractory cases.
• None of the three sedates like a benzodiazepine, and none is a controlled substance — the GABA-A effect is a contribution, not the whole drug.

What it costs — the family, then the site

Shared by all; decided by the site

• All of them: sedation, ataxia, anterograde amnesia, an anticonvulsant effect, and falls in the elderly.
• Respiratory depression: benzodiazepine-site drugs mainly in combination (opioids, alcohol); barbiturate-site and anesthetic-site drugs on their own.
• Tolerance and withdrawal: benzodiazepines, Z-drugs, barbiturates, carbamates and ethanol — cross-tolerant with each other. Not topiramate, not the anesthetics, and not (so far) the neurosteroids.
• Reversal: flumazenil for the benzodiazepine site only. For every other site there is no antidote — support the airway and ventilate.
• Interactions: phenobarbital and primidone induce CYPs; stiripentol inhibits CYP2C19/3A4; and every member adds to every other.

Drugs with this mechanism (61)

Self-check

• How do benzodiazepines alter GABA-A channel opening when GABA is present?
  They increase opening frequency.
• How do barbiturates alter GABA-A channel opening duration?
  They prolong each opening.

All mechanisms · Open GABA-A PAM in the interactive console ↗