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

Non-selective benzodiazepines, such as diazepam, interact with equivalent affinity and agonist efficacy at GABAA receptors containing either an alpha 1, alpha 2, alpha 3 or alpha 5 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 etectroshock (MES)-induced seizures in mice containing point mutations in single (alpha 1H101R, alpha 2H101R or alpha 6H105R) or multiple (alpha 25H ->)R) alpha subunits that render the resulting GABA(A) receptors diazepam-insensitive. Furthermore, the anticonvulsant properties of diazepam, the alpha 1- and alpha 3-selective compounds zotpidem and TPOO3, respectively, and the alpha 2/alpha 3 preferring compound TP13 were studied against PTZ-induced seizures. In the transgenic mice, no single subtype was responsible for the anticonvutsant effects of diazepam in either the PTZ or MES assay and neither the alpha nor alpha 5 subtypes appeared to confer anticonvulsant activity. Moreover, whereas the alpha 1 and alpha 2 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 alpha 2-containing GABA(A) 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.