Modulation of GABA(A) 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 physiolog ical metabolites on GABA-activated whole-cell currents were studied in enzy matically isolated rat Purkinje neurones. PNZ, its hydroxylated metabolite (HPNZ) and a reference benzodiazepine, diazepam, potently enhanced (up to 2 00% of control) peak amplitude of currents activated by 10 muM GABA with EC (50)s 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 rela tionship for GABA. Another metabolite, 6-bromo-(2-chlorophenyl) quinazoline -2-one (QNZ), augmented responses to 10 muM GABA with a maximal efficacy si milar to that of the 1,4-benzodiazepines tested, although its EC50 was 2.4 +/-0.2 muM. A further metabolite, 5 -bromo-(2-chlorophenyl)-2-aminobenzophe none (ABPH), had only minimal effects on the responses elicited by 10 muM G ABA. Incubation with QNZ and ABPH had biphasic effects on the concentration/effe ct relationship for GABA. These compounds enhanced peak amplitudes of curre nts activated by low concentrations of GABA, but inhibited responses to sat urating concentrations of the agonist. This effect could, in part, be expla ined 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 GABA(A) receptors and that they are primarily responsible for GABAergic effects of therapeutic doses of PNZ.