Interaction of beta-carboline inverse agonists for the benzodiazepine site with another site on GABAA receptors.

1. We examined the effects of methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), a beta-carboline inverse agonist for the benzodiazepine site, on gamma-aminobutyric acid (GABA)-induced Cl-currents in several cloned rat GABAA 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 microM) occupying only the benzodiazepine site decreased GABA-induced Cl currents in the alpha 1 beta 2 gamma 2 and alpha 3 beta 2 gamma 2 subtypes as expected from an inverse agonist, but produced no change in the alpha 6 beta 2 gamma 2 subtype (perhaps a neutral antagonist). The drug at higher concentrations (> 0.5 microM) enhanced Cl- currents in all the subtypes with a half maximal concentration of 6 to 20 microM, depending on the alpha isoform. In the alpha 1 beta 2 subtype, which is without the benzodiazepine site, DMCM monophasically increased Cl- currents with a half maximal concentration of 1.9 microM. 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 3 alpha, 21-dihydroxy-5 alpha-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 beta-carboline analogues, methyl-beta-carboline-3-carboxylate and propyl-beta-carboline-3-carboxylate markedly enhanced GABA-induced Cl currents in the alpha 1 beta 2 gamma 2 subtype, while N-methyl-beta-carboline-3-carboxamide and 1-methyl-7-methoxy-3,4-dihydro-beta-carboline did not. It appears that the 3-carboxyl ester moiety is necessary for beta-carbolines to interact with a novel site on GABAA receptors as agonists.