Pharmacology of GAB A ρ1 and GAB A α/β receptors expressed in Xenopus oocytes and COS cells

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. cDNAs for the ρ1 and the α₅β₁ 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. The most potent agonists at the ρ1 receptor were trans‐4‐aminocrotonic acid (TACA) > GABA > muscimol; at the α₅β₁ 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 α₅β₁ receptors, respectively. 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. 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 α₅β₁ 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 α₅β₁ 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. In contrast to the α₅β₁ 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 μm; but it had an apparent K_d for competitively blocking the receptor of 0.64 μm. This difference suggests that steric constraints in the activated (open channel) receptor are tighter than in the resting receptor. Hill coefficients approached 2 at the ρ1 receptor, but were closer to unity at the α₅β₁ receptor. Thus, the ρ1 receptor displayed higher cooperativity. Unlike typical GABA_A receptors, the ρ1 receptor was insensitive to the competitive antagonists bicuculline, SR95531, securinine, and (+)‐tubocurarine. 1993 British Pharmacological Society