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.
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CNS Diseases Research, Upjohn Company, Kalamazoo, MI 49001, USA.
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Carter
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McKinley
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D001463:Barbiturates / Q000494:pharmacology; D002243:Carbolines / Q000494:pharmacology*; D002460:Cell Line; D018118:Chloride Channels / Q000187:drug effects; D003292:Convulsants / Q000494:pharmacology*; D005442:Flumazenil / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D058785:GABA-A Receptor Agonists; D006801:Humans; D007668:Kidney / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010957:Plasmids; D011963:Receptors, GABA-A / Q000235:genetics; D013256:Steroids / Q000494:pharmacology
D001463:Barbiturates; D002243:Carbolines; D018118:Chloride Channels; D003292:Convulsants; D018756:GABA Antagonists; D058785:GABA-A Receptor Agonists; D011963:Receptors, GABA-A; D013256:Steroids; C034818:methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate; D005442:Flumazenil