Clozapine and some other antipsychotic drugs may preferentially block the same subset of GABA(A) receptors.

Selective blockade of a subset of GABA(A) receptors may be involved in the antipsychotic effects of Clozapine and several other antipsychotic drugs. Seven antipsychotic drugs, and 11 drugs classified as antidepressants that only partially reverse the inhibitory effect of 1 microM GABA on [35S]TBPS binding, do not yield additive reversal when tested pairwise with Clozapine, which also only partially reverses the inhibitory effect of GABA. This suggests that all of these antipsychotic/antidepressant drugs may block a common subset of GABA(A) receptors. DMCM and Ro 5-4864 are also partial reversers of GABA's inhibitory effect, but they yield additive reversals when tested pairwise with the antipsychotic/antidepressant drugs, and also with each other, suggesting that DMCM, Ro 5-4864, and the antipsychotic drugs define three heterogeneous subsets of GABA(A) receptors, with variable overlap, depending on the drug. Several potent ligands for benzodiazepine binding sites can block the GABA inhibitory effects of DMCM and Ro 5-4864, but with different patterns: the ligands generally blocked DMCM less potently, but more completely than Ro 5-4864. Ro 5-4864 was not blocked by Flumazenil or CGS-8216, ligands that potently blocked DMCM. Nine additional antipsychotic/antidepressant drugs, as well as Clozapine, and 7 "classical" GABA(A) receptor blockers, all of which reversed GABA nearly completely, when tested at lower concentrations that only reverse approximately 20-35%, yielded almost complete additivity when tested pairwise with DMCM or Ro 54864. Another convulsant benzodiazepine, KW-1937, a positional isomer of Brotizolam, fully reverses the inhibitory effect of 1 microM GABA. At a lower concentration yielding about 50% reversal, KW-1937 is completely additive with DMCM, but entirely nonadditive with Ro 5-4864. The 50% reversal obtained with KW-1937 was potently blocked by Triazolam, but with a plateau similar to that obtained with Ro 5-4864. The results with KW- 1937 suggest that its 50% reversal largely corresponds to the reversal obtained with Ro 5-4864, and that virtually all of the [35S]TBPS binding sites inhibited by 1 microM GABA are coupled to benzodiazepine binding sites. The fraction of GABA(A) receptors preferentially blocked by all the antipsychotic/antidepressant drugs, roughly 25% of the [35S]TBPS binding sites inhibited with 1 microM GABA, are sensitive to KW-1937, but not to DMCM or to Ro 5-4864.