At the GABA-A receptor chloride channel, how does the action of barbiturates differ from that of benzodiazepines?

• A Barbiturates increase the frequency of channel opening, benzodiazepines increase the duration
• B Barbiturates increase the duration of channel opening, benzodiazepines increase the frequency ✓
• C Both act only in the absence of GABA, directly opening the channel
• D Barbiturates block the chloride channel, benzodiazepines open it directly

Explanation

Benzodiazepines bind at the alpha-gamma subunit interface and increase the frequency of GABA-gated chloride channel opening, whereas barbiturates bind at different sites and prolong the duration of each opening, and at high concentrations open the channel directly without GABA. This direct gating underlies the lower safety margin of barbiturates in overdose compared with benzodiazepines, which are ineffective without ambient GABA.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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