Contribution of the alpha1-GABA(A) receptor subtype to the pharmacological actions of benzodiazepine site inverse agonists.

A histidine-to-arginine point-mutation at position 101 in the alpha1-subunit of gamma-aminobutyric acid (GABA)(A) receptors has been shown to switch the in vitro efficacy of Ro 15-4513 from inverse agonism to agonism. In order to assess the consequences of this pharmacological switch in vivo, the motor and proconvulsant effects of Ro 15-4513 were analyzed in knock-in mice containing point-mutated alpha1(H101R)-GABA(A) receptors. Furthermore the influence of the alpha1(H101R) substitution on the efficacy of the beta-carboline inverse agonist DMCM was examined both in vitro and in vivo. Ro 15-4513 (10 mg/kg) increased baseline locomotion and potentiated the convulsant effect of pentylenetetrazole in wild type mice. In alpha1(H101R) mice, Ro 15-4513 decreased locomotion and, at a higher dose (30 mg/kg) it displayed an anticonvulsant action. In vitro, DMCM acted as an inverse agonist at recombinant alpha1beta2gamma2 receptors whereas it potentiated GABA-evoked chloride currents at alpha1(H101R)beta2gamma2 receptors. DMCM was inactive as a convulsant in alpha1(H101R) mice. In keeping with the major contribution of these receptors to the sedative and anticonvulsant properties of benzodiazepine site agonists, the present findings identify the alpha1-GABA(A) receptors as the molecular targets for the allosteric modulation by benzodiazepine site ligands in either direction with regard to the behavioral outputs, sedation/motor stimulation and anticonvulsion/proconvulsion.