INTERACTION OF BETA-CARBOLINE INVERSE AGONISTS FOR THE BENZODIAZEPINESITE WITH ANOTHER SITE ON GABA(A) RECEPTORS

1 We examined the effects of methyl 6,7-dimethoxy-4-ethyl-beta-carboli ne-3-carboxylate (DMCM), a beta-carboline inverse agonist for the benz odiazepine site, on gamma-aminobutyric acid (GABA)-induced Cl- current s in several cloned rat GABAA receptor subtypes expressed in human emb ryonic kidney cells. The Cl- currents were measured in the whole cell configuration of patch clamp techniques.2 DMCM at low concentrations ( < 0.5 mu M) occupying only the benzodiazepine site decreased GABA-indu ced 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 chang e in the alpha 6 beta 2 gamma 2 subtype (perhaps a neutral antagonist) . The drug at higher concentrations (> 0.5 mu M) enhanced Cl- currents in all the subtypes, with a half maximal concentration of 6 to 20 mu M, depending on the a isoform. In the alpha 1 beta 2 subtype, which is without the benzodiazepine site, DMCM monophasically increased Cl- cu rrents with a half maximal concentration of 1.9 mu M. 3 Ro 15-1788 (a classical benzodiazepine antagonist) had no effect on Cl- current enha ncement by DMCM and, in fact, increased the current level through bloc king current inhibition by DMCM via the benzodiazepine site. Also, Cl- current enhancement by pentobarbitone or by 3 alpha, 21-dihydroxy-5 a lpha-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 b enzodiazepines, barbiturates and neurosteroids. 4 Among beta-carboline analogues, methyl-beta-carboline-3-carboxylate and propyl-beta-carbol ine-3-carboxylate markedly enhanced GABA-induced Cl currents in the al pha 1 beta 2 gamma 2 subtype, while N-methyl-beta-carboline-3-carboxam ide and 1-methyl-7-methoxy-3,4-dihydro-beta-carboline did not. It appe ars that the 3-carboxyl ester moiety is necessary for beta-carbolines to interact with a novel site on GABAA receptors as agonists.