Barbiturates differ from benzodiazepines in their mechanism of action at GABA-A receptors. The key difference is:

• A Barbiturates increase the frequency of Cl- channel opening; benzodiazepines increase duration
• B Both increase frequency and duration equally but barbiturates also block NMDA receptors
• C Barbiturates directly open Cl- channels at high doses; benzodiazepines never do
• D Barbiturates increase the duration of Cl- channel opening; benzodiazepines increase frequency ✓

Explanation

Benzodiazepines act at their own modulatory site on GABA-A receptors to increase the frequency of chloride channel opening in the presence of GABA. Barbiturates, in contrast, bind to a separate site and increase the duration of chloride channel opening. At high (anaesthetic/toxic) doses, barbiturates can also open chloride channels in the absence of GABA, which underlies their greater lethality in overdose compared to benzodiazepines.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.