Modulation of GABAA receptor-mediated currents by phenazepam and its metabolites

The effects of 7-bromo-5-(2-chlorophenyl)-1,3-dihydro-2H-1,4-benzodiazepin-2-one (phenazepam, PNZ), a 1,4-benzodiazepine derivative, and its physiological metabolites on GABA-activated whole-cell currents were studied in enzymatically isolated rat Purkinje neurones. PNZ, its hydroxylated metabolite (HPNZ) and a reference benzodiazepine, diazepam, potently enhanced (up to 200% of control) peak amplitude of currents activated by 10 microM GABA with EC50s of 6.1 +/- 0.8, 10.3 +/- 1.4 and 13.5 +/- 1.9 nM respectively. Both PNZ and HPNZ caused a parallel leftwards shift of the concentration/effect relationship for GABA. Another metabolite, 6-bromo-(2-chlorophenyl) quinazoline-2-one (QNZ), augmented responses to 10 microM GABA with a maximal efficacy similar to that of the 1,4benzodiazepines tested, although its EC50 was 2.4 +/- 0.2 microM. A further metabolite, 5-bromo-(2-chlorophenyl)-2-aminobenzophenone (ABPH), had only minimal effects on the responses elicited by 10 microM GABA. Incubation with QNZ and ABPH had biphasic effects on the concentration/effect relationship for GABA. These compounds enhanced peak amplitudes of currents activated by low concentrations of GABA, but inhibited responses to saturating concentrations of the agonist. This effect could, in part, be explained by the acceleration of the desensitisation process by those substances. It is concluded that both PNZ and HPNZ can be referred to as full positive modulators of GABAA receptors and that they are primarily responsible for GABAergic effects of therapeutic doses of PNZ.