This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Psychiatry
Which drug acts through alpha subunit of GABA? 
Select and explain. 
A. Benzodiazepine, B. Barbiturate, C. Haloperidol, D. TCA
A: Benzodiazepine
Answer: A: Benzodiazepine 
Explanation: The exact subunit structures of native GABA receptors are still unknown, but it is thought that most GABA receptor are composed of α, β and γ subunits that coassemble with some uncertain stoichiometry.
Binding site of GABA is on β -subunit.
Benzodiazepine site is located on the a subunit but the stabilization or completion of that site in the assembled structure requires also the γ subunit.

"The photo - affinity labeling of the benzodiazepine site has confirmed, using isoform - antibodies, to be on an α and not a γ subunit".

So, BZD binding site is present on α subunit at α - γ interface, and both α & γ units are required for BZDs action.
Barbiturates act on Picrotoxin site that is located usually on β- subunit at α -β interface. Both α & β subunit are required for barbiturates action.



GABA → β - subunit.
BZD → α subunit at α - γ interface, i.e. α / γ site.
Barbiturate → β subunit at α - β interface, i.e. α / β site.