CHARACTERIZATION OF BICUCULLINE BACLOFEN-INSENSITIVE (RHO-LIKE) GAMMA-AMINOBUTYRIC-ACID RECEPTORS EXPRESSED IN XENOPUS OOCYTES .2. PHARMACOLOGY OF GAMMA-AMINOBUTYRIC ACIDA AND GAMMA-AMINOBUTYRIC ACIDB RECEPTORAGONISTS AND ANTAGONISTS

Poly(A)+ RNA from mammalian retina expresses bicuculline/baclofen-inse nsitive gamma-aminobutyric acid (GABA) receptors in Xenopus oocytes wi th properties similar to those of homooligomeric GABA(rho1) receptors. The pharmacological profile of these rho-like receptors was extended by measuring sensitivities to various GABA(A) and GABA(B) receptor lig ands. For direct comparison the same compounds were also assayed with GABA(A) receptors expressed by rat brain RNA. The potency sequence for heterocyclic GABA analogues at the GABA(rho)-like receptors was GABA (1.3) > muscimol (2.3) > isoguvacine (1 00) (approximate EC50 in paren theses; all EC50 and K(b) values given in mum). Both muscimol and isog uvacine were partial agonists at the rho-like receptors. 4,5,6,7-Tetra hydroisoxazolo[5,4-c]pyridin-3-ol (K(b) congruent-to 32), piperidine-4 -sulfonic acid (K(b) congruent-to 85), and isonipecotic acid (K(b) con gruent-to 1000) acted primarily as competitive antagonists, showing li ttle or no activity as agonists. The sulfonic acid GABA analogue 3-ami nopropanesulfonic acid was also a competitive antagonist (K(b) congrue nt-to 20). Conformationally restricted GABA analogues trans- and cis-4 -aminocrotonic acid (TACA and CACA) were agonists at the rho-like rece ptors. TACA (EC50 = 0.6) had twice the potency of GABA and was 125 tim es more Potent than CACA (EC50 congruent-to 75). Z-3-(Amidinothio)prop enoic acid, an isothiouronium analogue of GABA, had little activity as an agonist but instead acted as a competitive antagonist (K(b) congru ent-to 20). At concentrations of >1 00 mum, bicuculline did have some weak competitive inhibitory effects on the GABA(rho)-like receptors (K (b) congruent-to 6000), but it was at least 5000 times more potent at GABA(A) receptors. Strychnine (K(b) congruent-to 70) and SR-95531 (K(b ) congruent-to 35) also were competitive inhibitors of the rho-like re ceptors but were, respectively, 20 and 240 times more potent at GABA(A ) receptors. The GABA(B) receptor ligands baclofen, phaclofen, and sac lofen (1-100 mum) had no appreciable effects on the rho-like receptors . In contrast, 3-aminopropylphosphonic acid, the phosphonic acid analo gue of GABA(B) acted as a competitive antagonist (K(b) congruent-to 1 0), and 3-aminopropylphosphinic acid and 3-aminopropyl(methyl)-phosphi nic acid were moderately potent antagonists (K(b) congruent-to 1.7 and 0.8, respectively). Delta-Aminovaleric acid was also an antagonist (K (b) congruent-to 20), whereas 4-aminobutylphosphonic acid, the phospho nic acid analogue of delta-aminovaleric acid, was only a weak inhibito r (K(b) congruent-to 600). In terms of structure-activity relationship s, our experiments suggest that incorporation of the carboxyl or amino groups of GABA into 3-isoxazolol or piperidine rings either reduces a gonist potency at the GABA(rho)-like receptors or results in analogues that act as competitive antagonists. Similarly, substitution of the c arboxyl group for sulfonic acid or of the amino group for isothiouroni um generates antagonists. The relative activities of TACA and CACA cle arly suggest that GABA interacts with the rho-like receptors in extend ed conformations and appears to distinguish these receptors from previ ously postulated GABA(C) receptor sites. The 4-chlorophenyl substituen t of baclofen and related GABA(B) receptor antagonists almost wholly p rohibits functionally relevant interactions with the rho-like receptor s. However, some phosphonic and phosphinic analogues of GABA, normally catagorized as selective GABA(B) receptor agonists, show a clear pote ntial for acting as antagonists. These results should be useful for de signing drugs that interact selectively with mammalian bicuculline-bac lofen-insensitive GABA receptors and for investigating the mechanisms by which ligands interact with GABA-gated Cl-channels.