Differences in the negative allosteric modulation of gamma-aminobutyric acid receptors elicited by 4'-chlorodiazepam and by a beta-carboline-3-carboxylate ester: a study with natural and reconstituted receptors.
Cl- currents elicited by gamma-aminobutyric acid (GABA) application were recorded with the whole-cell tight-seal technique from voltage-clamped cortical neurons of neonatal rats in primary culture. The peripheral benzodiazepine recognition site ligand 4'-chlorodiazepam [Ro 5-4864; 7-chloro-1,3-dihydro-1-methyl-5-(4-chlorophenyl)-2H-[1,4]-benzodiazep in-2- one] inhibited the GABA-generated currents in a dose-dependent manner. Also, a beta-carboline (DMCM; 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate methyl ester), acting as a negative allosteric modulator of GABAA receptors, reduced the intensity of GABA-generated currents with similar efficacy but greater potency. Flumazenil (Ro 15-1788; 8-fluro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo-[1,5-a] [1,4]-benzodiazepine-3-carboxylate ethyl ester) antagonized DMCM inhibition but not that elicited by 4'-chlorodiazepam. The isoquinoline carboxamide PK 11195, an antagonist of 4'-chlorodiazepam effects in other systems, failed to antagonize the action of 4'-chlorodiazepam. The transient expression of various molecular forms of GABAA receptors in the human embryonic kidney cell line 293 allowed a study of the minimal structural requirements for the inhibition of GABA-induced Cl- currents by bicuculline, picrotoxin, 4'-chlorodiazepam, and DMCM. GABA-elicited Cl- currents in cells coexpressing alpha 1 and beta 1 subunits of GABAA receptors were inhibited by bicuculline and picrotoxin, but not by DMCM or 4'-chlorodiazepam. Conversely, the GABA currents in cells coexpressing alpha 1 beta 1 and gamma 2 subunits were inhibited by bicuculline, picrotoxin, 4'-chlorodiazepam, and DMCM. Since the Cl- currents generated by GABA in some molecular forms of GABAA receptors are inhibited by bicuculline and picrotoxin only, 4'-chlorodiazepam cannot be acting isosterically with picrotoxin.
Puia
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FIDIA-Georgetown Institute for the Neurosciences, Georgetown University, Washington, DC 20007.
Santi
M R
MR
Vicini
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S
Pritchett
D B
DB
Seeburg
P H
PH
Costa
E
E
D000818:Animals; D001570:Benzodiazepinones / Q000494:pharmacology*; D002243:Carbolines / Q000494:pharmacology*; D002460:Cell Line; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000502:physiology; D018118:Chloride Channels; D002712:Chlorides / Q000502:physiology*; D003292:Convulsants / Q000494:pharmacology; D006801:Humans; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology; D007668:Kidney; D008565:Membrane Proteins / Q000502:physiology*; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D011963:Receptors, GABA-A / Q000187:drug effects* / Q000235:genetics / Q000502:physiology; D014162:Transfection; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D001570:Benzodiazepinones; D002243:Carbolines; D018118:Chloride Channels; D002712:Chlorides; D003292:Convulsants; D007473:Ion Channels; D008565:Membrane Proteins; D011963:Receptors, GABA-A; C034818:methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate; C028513:4'-chlorodiazepam; D005680:gamma-Aminobutyric Acid