beta-Carboline gamma-aminobutyric acidA receptor inverse agonists modulate gamma-aminobutyric acid via the loreclezole binding site as well as the benzodiazepine site.
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.
Stevenson
A
A
Merck Sharp & Dohme Research Laboratories, Harlow, Essex, UK.
Wingrove
P B
PB
Whiting
P J
PJ
Wafford
K A
KA
D000818:Animals; D000927:Anticonvulsants / Q000378:metabolism* / Q000494:pharmacology; D001569:Benzodiazepines / Q000378:metabolism; D001665:Binding Sites; D002243:Carbolines / Q000378:metabolism / Q000494:pharmacology*; D003292:Convulsants / Q000378:metabolism* / Q000494:pharmacology; D004357:Drug Synergism; D005260:Female; D018755:GABA Agonists / Q000378:metabolism / Q000494:pharmacology*; D007536:Isomerism; D046911:Macromolecular Substances; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D017354:Point Mutation; D018079:Receptors, GABA / Q000235:genetics / Q000502:physiology*; D014230:Triazoles / Q000378:metabolism* / Q000494:pharmacology; D014981:Xenopus; D005680:gamma-Aminobutyric Acid / Q000378:metabolism / Q000494:pharmacology*
D000927:Anticonvulsants; D002243:Carbolines; D003292:Convulsants; D018755:GABA Agonists; D046911:Macromolecular Substances; D018079:Receptors, GABA; D014230:Triazoles; D001569:Benzodiazepines; C034818:methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate; D005680:gamma-Aminobutyric Acid; C066440:loreclezole