The pharmacology of adenylyl cyclase modulation by GABAB receptors in rat brain slices.
GABAB receptor activation inhibits forskolin-stimulated adenylyl cyclase activity but augments noradrenaline-stimulated adenylyl cyclase activity. The present study investigated the pharmacology of these two GABAB receptor mediated responses. In a cross-chopped rat cortical slice preparation, it was confirmed that (-)baclofen inhibited forskolin-stimulated adenylyl cyclase activity and augmented noradrenaline-stimulated adenylyl cyclase activity. The potency of five further agonists was investigated (SKF97541, CGP47656, CGP44533, 3-APA and CGP44532). Of these agonists two compounds were significantly more potent as inhibitors of forskolin-stimulated adenylyl cyclase than as augmenters of noradrenaline-stimulated adenylyl cyclase activity, these were (-)baclofen (pEC50 = 6.07 +/- 0.29 and 5.04 +/- 0.17, respectively (p < 0.05)), and CGP47656 (pEC50 = 6.44 +/- 0.05 and 4.48 +/- 0.26, respectively (p < 0.05)). It is possible to explain this difference in potency by proposing that these compounds have low intrinsic efficacy, and the augmentation of noradrenaline-stimulated adenylyl cyclase has a low receptor reserve. In addition six antagonists (CGP49311A, CGP46381, CGP45024, CGP45397, CGP36742) were also tested for their ability to antagonize 10 microM (-)baclofen in these two assays. These antagonists ranged in potency as inhibitors of forskolin-stimulated adenylyl cyclase activity from CGP49311A (pEC50 = 5.45 +/- 0.30) to CGP36742 (pEC50 = 3.87 +/- 0.16). Each antagonist had similar potency in the two assays, suggesting that these two responses are mediated by pharmacologically similar receptors.
Knight
A R
AR
Department of Pharmacology, School of Pharmacy, London, U.K.
Bowery
N G
NG
D000262:Adenylyl Cyclases / Q000187:drug effects*; D000818:Animals; D001418:Baclofen / Q000494:pharmacology*; D001921:Brain / Q000187:drug effects*; D005576:Colforsin / Q000494:pharmacology*; D000242:Cyclic AMP / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D008297:Male; D009638:Norepinephrine / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D018080:Receptors, GABA-B / Q000187:drug effects*
D018080:Receptors, GABA-B; D005576:Colforsin; D000242:Cyclic AMP; D000262:Adenylyl Cyclases; D001418:Baclofen; D009638:Norepinephrine