Pharmacological modulation of the diazepam-insensitive recombinant gamma-aminobutyric acidA receptors alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2.
We characterized modulation of the gamma-aminobutyric acid (GABA)-evoked responses of the diazepam-insensitive alpha 4 beta 2 gamma2 and alpha 6 beta 2 gamma 2 recombinant GABAA receptors. The partial agonist bretazenil potentiated the responses of both receptors with similar dose dependence but with a higher maximal enhancement at the alpha 4 beta 2 gamma 2 receptor. The bretazenil-induced potentiation was reduced by the benzodiazepine antagonist flumazenil. At a high concentration (10 microM), flumazenil was a weak potentiator of the GABA response. The partial agonist imidazenil was inactive. The imidazobenzodiazepine inverse agonist Ro 15-4513, which is known to bind with high affinity to the alpha 6 beta 2 gamma 2 receptor, potentiated the GABA responses of the alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptor subtypes with similar dose dependence over the concentration range of 0.1-10 microM. Methyl-6, 7-dimethoxy-4-ethyl-beta-carboline, a beta-carboline inverse agonist, had a similar potentiating effect when tested at a concentration of 10 microM. The alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptor-mediated currents had equal sensitivities to furosemide and Zn2+ ions, both of which reduced the GABA-evoked responses. The alpha 6 beta 2 gamma 2 receptor but not the alpha 4 beta 2 gamma 2 receptor exhibited a low level of spontaneous activity in the absence of GABA; this resting current could be directly potentiated by Ro 15-4513, methyl-6,7-dimethoxy-4-ethyl-beta-carboline, bretazenil and flumazenil and was blocked by picrotoxin. Thus, although the alpha 4 beta 2 gamma 2 receptors are insensitive to benzodiazepine binding site full agonists, such as diazepam, they can be modulated by certain ligands acting as partial and inverse agonists at diazepam-sensitive receptors and thereby contribute to the respective pharmacological profiles.
Knoflach
F
F
Institute of Pharmacology, University of Zürich, Switzerland.
Benke
D
D
Wang
Y
Y
Scheurer
L
L
Lüddens
H
H
Hamilton
B J
BJ
Carter
D B
DB
Mohler
H
H
Benson
J A
JA
D000818:Animals; D001386:Azides / Q000378:metabolism / Q000494:pharmacology; D001569:Benzodiazepines / Q000378:metabolism / Q000494:pharmacology; D001570:Benzodiazepinones / Q000378:metabolism / Q000494:pharmacology; D001665:Binding Sites; D002243:Carbolines / Q000378:metabolism / Q000494:pharmacology; D003975:Diazepam / Q000494:pharmacology*; D005665:Furosemide / Q000378:metabolism / Q000494:pharmacology; D018755:GABA Agonists / Q000378:metabolism / Q000494:pharmacology; D018756:GABA Antagonists / Q000378:metabolism / Q000494:pharmacology; D018757:GABA Modulators / Q000494:pharmacology*; D007093:Imidazoles / Q000378:metabolism / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D011869:Radioligand Assay; D051381:Rats; D011963:Receptors, GABA-A / Q000145:classification* / Q000187:drug effects* / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D012680:Sensitivity and Specificity; D015032:Zinc / Q000378:metabolism / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D001386:Azides; D001570:Benzodiazepinones; D002243:Carbolines; D018755:GABA Agonists; D018756:GABA Antagonists; D018757:GABA Modulators; D007093:Imidazoles; D011963:Receptors, GABA-A; D011994:Recombinant Proteins; D001569:Benzodiazepines; C034818:methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate; D005680:gamma-Aminobutyric Acid; D005665:Furosemide; C082661:imidazenil; C042957:Ro 15-4513; D015032:Zinc; C054626:bretazenil; D003975:Diazepam