Antagonistic action of pitrazepin on human and rat GABA(A) receptors

1 Pitrazepin, 3-(piperazinyl-1)-9H-dibenz(c,f) triazolo(4,5-a)azepin is a p iperazine antagonist of GABA in a variety of electrophysiological and in, v itro binding studies involving GABA and glycine receptors. In the present s tudy we have investigated the effects of pitrazepin, and the GABA(A) antago nist bicuculline, on membrane currents elicited by GABA in Xenopus oocytes injected with rat cerebral cortex mRNA or cDNAs encoding alpha(1)beta(2) or alpha(1)beta(2)gamma(2S) human GABA(A) receptor subunits. 2 The three types of GABA receptors expressed were reversibly antagonized b y bicuculline and pitrazepin in a concentration-dependent manner. GABA dose -current response curves for the three types of receptors were shifted to t he right, in a parallel manner, by increasing concentrations of pitrazepin. 3 Schild analyses gave pA(2) values of 6.42+/-0.62, n = 4, 6.41 +/- 1.2, n = 5 and 6.21 +/- 1.24, n = 6, in oocytes expressing rat cerebral cortex, al pha(1)beta(2) or alpha(1)beta(2)gamma(2S) human GABA(A) receptors respectiv ely (values are given as means +/- s.e.mean), and the Hill coefficients wer e all close to unity. All this is consistent with the notion that pitrazepi n acts as a competitive antagonist of these GABA(A) receptors; and that the ir antagonism by pitrazepin is not strongly dependent on the subunit compos ition of the receptors here studied. 4 Since pitrazepin has been reported to act also at the benzodiazepine bind ing site, we studied the effect of the benzodiazepine antagonist Ro 15-1788 (flumazenil) on the: inhibition of alpha(1)beta(2)gamma(2S) receptors by p itrazepin. Go-application of Ro 15-1788 did not alter the inhibiting effect of pitrazepin. Moreover, pitrazepin did not antagonize the potentiation of GABA-currents by flunitrazepam. All this suggests that pitrazepin does not affect the GABA receptor-chloride channel by interacting with the benzodia zepine receptor site.