Interaction of β‐carboline inverse agonists for the benzodiazepine site with another site on GABA A receptors

1 We examined the effects of methyl 6,7‐dimethoxy‐4‐ethyl‐β‐carboline‐3‐carboxylate (DMCM), a β‐carboline inverse agonist for the benzodiazepine site, on γ‐aminobutyric acid (GABA)‐induced Cl − currents in several cloned rat GABA A receptor subtypes expressed in human embryonic kidney cells. The Cl − currents were measured in the whole cell configuration of patch clamp techniques. 2 DMCM at low concentrations (<0.5 μ m ) occupying only the benzodiazepine site decreased GABA‐induced Cl currents in the α1β2γ2 and α3β2γ2 subtypes as expected from an inverse agonist, but produced no change in the α6β2γ2 subtype (perhaps a neutral antagonist). The drug at higher concentrations (> 0.5 μ m ) enhanced Cl − currents in all the subtypes, with a half maximal concentration of 6 to 20 μ m , depending on the α isoform. In the α1β2 subtype, which is without the benzodiazepine site, DMCM monophasically increased Cl − currents with a half maximal concentration of 1.9 μ m . 3 Ro 15–1788 (a classical benzodiazepine antagonist) had no effect on Cl − current enhancement by DMCM and, in fact, increased the current level through blocking current inhibition by DMCM via the benzodiazepine site. Also, Cl − current enhancement by pentobarbitone or by 3α, 21‐dihydroxy‐5α‐pregnan‐20‐one was additive to that by DMCM at saturating doses. It appears that the agonist site for DMCM is distinct from those for benzodiazepines, barbiturates and neurosteroids. 4 Among β‐carboline analogues, methyl‐β‐carboline‐3‐carboxylate and propyl‐β‐carboline‐3‐carboxylate markedly enhanced GABA‐induced Cl currents in the α1β2γ2 subtype, while N‐methyl‐β‐carboline‐3‐carboxamide and 1‐methyl‐7‐methoxy‐3,4‐dihydro‐β‐carboline did not. It appears that the 3‐carboxyl ester moiety is necessary for β‐carbolines to interact with a novel site on GABA A receptors as agonists.