Differential contribution of GABAA receptor subtypes to the anticonvulsant efficacy of benzodiazepine site ligands

Abstract

Non-selective benzodiazepines, such as diazepam, interact with equivalent affinity and agonist efficacy at GABAA receptors containing either an α1, α2, α3 or α5 subunit. However, which of these particular subtypes are responsible for the anticonvulsant effects of diazepam remains uncertain. In the present study, we examined the ability of diazepam to reduce pentylenetetrazoLe (PTZ)-induced and maximal electroshock (MES)-induced seizures in mice containing point mutations in single (α1H101R, α2H101R or α5H105R) or multiple (α125H→R) α subunits that render the resulting GABAA receptors diazepam-insensitive. Furthermore, the anticonvulsant properties of diazepam, the α1- and α3-selective compounds zoLpidem and TP003, respectively, and the α2/α3 preferring compound TP13 were studied against PTZ-induced seizures. In the transgenic mice, no single subtype was responsible for the anticonvulsant effects of diazepam in either the PTZ or MES assay and neither the α3 nor α5 subtypes appeared to confer anticonvulsant activity. Moreover, whereas the α1 and α2 subtypes played a modest role with respect to the PTZ assay, they had a negligible role in the MES assay. With respect to subtype-selective compounds, zolpidem and TP003 had much reduced anticonvulsant efficacy relative to diazepam in both the PTZ and MES assays whereas TP13 had high anticonvulsant efficacy in the PTZ but not the MES assay. Taken together, these data not only indicate a role for α2-containing GABAA receptors in mediating PTZ and MES anticonvulsant activity but also suggest that efficacy at more than one subtype is required and that these subtypes act synergistically.