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Pharmacology of GAB A ρ1 and GAB A α/β receptors expressed in Xenopus oocytes and COS cells
Author(s) -
Kusama T.,
Spivak C.E.,
Whiting P.,
Dawson V.L.,
Schaeffer J.C.,
Uhl G.R.
Publication year - 1993
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1993.tb13554.x
Subject(s) - muscimol , receptor , gabaa receptor , chemistry , bicuculline , agonist , xenopus , gaba receptor antagonist , gaba receptor , metabotropic receptor , gamma aminobutyric acid , pharmacology , stereochemistry , biochemistry , biology , gene
1 The ρ1 protein, which we previously cloned from retina, assembles as a homooligomer that transduces the binding of γ‐aminobutyric acid (GABA) into robust chloride currents. However, its insensitivity to bicuculline, pentobarbitone and benzodiazepines, all potent agents at typical GABA A receptors, suggested that it may react atypically to other GABA agonists and antagonists. 2 cDNAs for the ρ1 and the α 5 β 1 receptors for GABA were expressed as homo‐ and heterooligomers, respectively, in Xenopus oocytes. The selectivities of the respective receptors for various agonists were investigated using concentration‐response experiments in voltage clamped cells. 3 The most potent agonists at the ρ1 receptor were trans ‐4‐aminocrotonic acid (TACA) > GABA > muscimol; at the α 5 β 1 receptor the rank order was muscimol > GABA > 4,5,6,7‐tetrahydroisoxazole[4,5‐c]pyridine‐3‐ol (THIP). The most specific agonists were cis ‐(2‐(aminomethyl)‐cyclopropyl‐carboxylic acid (CAMP) and THIP for the ρ1 and the α 5 β 1 receptors, respectively. 4 Comparing GABA, TACA and cis ‐aminocrotonic acid (CACA) at ρ1 receptors expressed in COS cells gave results almost indistinguishable from those found at oocytes; the pharmacology of ρ1 seems independent of the expression system. 5 Agonists THIP, piperidine‐4‐sulphonic acid (P4S), and isoguvacine, whose C‐C‐C‐N chains are constrained by rings into a folded conformation and were potent at the α 5 β 1 receptor, were among the weakest at the ρ1 receptor. However CACA and CAMP, which align better with the extended than the folded conformation, were weakest at the α 5 β 1 receptor but moderately potent at the ρ1 receptor. These findings suggest that the ρ1 receptor recognizes agonists in the extended conformation, in contrast to GABA A receptors, which are believed to recognize agonists in the partially folded conformation. 6 In contrast to the α 5 β 1 receptor, gradations in maximum responses were apparent in the ρ1 receptor, suggesting various degrees of partial agonism. In particular, imidazole‐4‐acetic acid (I4AA), whose maximum response was only 3% of GABA's maximum, had an apparent K d for activating the ρ1 receptor of 16 μ m ; but it had an apparent K d for competitively blocking the receptor of 0.64 μ m . This difference suggests that steric constraints in the activated (open channel) receptor are tighter than in the resting receptor. 7 Hill coefficients approached 2 at the ρ1 receptor, but were closer to unity at the α 5 β 1 receptor. Thus, the ρ1 receptor displayed higher cooperativity. 8 Unlike typical GABA A receptors, the ρ1 receptor was insensitive to the competitive antagonists bicuculline, SR95531, securinine, and (+)‐tubocurarine.