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A Stevenson, PB Wingrove, PJ Whiting and KA Wafford
Merck Sharp & Dohme Research Laboratories, Harlow, Essex, UK.
The benzodiazepine site on the gamma-aminobutyric acid(A) (GABAA) receptor is the principle site of action for a number of structurally diverse compounds, including the beta-carbolines, many of which bind with high affinity. The apparent reversal of inhibition and potentiation by high concentrations of methyl-6,7-dimethoxy-4-ethyl- beta-carboline (DMCM) and other beta-carbolines has been reported by several groups and is insensitive to the benzodiazepine antagonist Ro 15-1788. By using alpha 6-containing receptors, which have low affinity for benzodiazepines, we observed robust potentiation of GABAA responses by micromolar concentrations of DMCM and other beta-carbolines that is dependent on the beta subunit variant. The beta subunit-dependent potentiation by the anticonvulsant loreclezole is dependent on a single amino acid in the putative transmembrane 2 region. By using single point mutations that discriminate the loreclezole site, we show that potentiation by DMCM is also dependent on the presence of the same amino acid, Asn290, in beta 2 or beta 3 (serine in beta 1), providing evidence that the low affinity site for beta-carboline potentiation is the loreclezole site. The potentiation is independent of the alpha subunit and is more pronounced on alpha 6-containing receptors due to the lack of DMCM inhibition via the benzodiazepine site. In addition, the potentiation observed is competitive with that of loreclezole, and other beta-carbolines, such as ethyl-beta-carboline-3-carboxylate and propyl-beta-carboline-3-carboxylate, act in a similar manner. The finding that beta-carbolines can act via the loreclezole site as well as the benzodiazepine site suggests that a wider variety of compounds may act via this site and shows that compounds can interact with more than one modulatory site on the GABAA receptor.
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