1 The pl protein, which we previously cloned from retina, assembles as a homooligomer that transduces the binding of y-aminobutyric acid (GABA) into robust chloride currents. However, its insensitivity to bicuculline, pentobarbitone and benzodiazepines, all potent agents at typical GABAA receptors, suggested that it may react atypically to other GABA agonists and antagonists.2 cDNAs for the pl and the a5p1 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 p1 receptor were trans-4-aminocrotonic acid (TACA) > GABA > muscimol; at the a5p, 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)-cyclopropylcarboxylic acid (CAMP) and THIP for the pl and the a53, receptors, respectively. 4 Comparing GABA, TACA and cis-aminocrotonic acid (CACA) at pl receptors expressed in COS cells gave results almost indistinguishable from those found at oocytes; the pharmacology of p1 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 a501 receptor, were among the weakest at the pl receptor. However CACA and CAMP, which align better with the extended than the folded conformation, were weakest at the a5pI receptor but moderately potent at the pl receptor. These findings suggest that the pl receptor recognizes agonists in the extended conformation, in contrast to GABAA receptors, which are believed to recognize agonists in the partially folded conformation.6 In contrast to the o5p1 receptor, gradations in maximum responses were apparent in the pl 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 Kd for activating the pl receptor of 16f1M; but it had an apparent Kd for competitively blocking the receptor of 0.64 1AM. 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 pl receptor, but were closer to unity at the (0I1 receptor. Thus, the pl receptor displayed higher cooperativity. 8 Unlike typical GABAA receptors, the pl receptor was insensitive to the competitive antagonists bicuculline, SR95531, securinine, and (+)-tubocurarine.
