Analogues of γ-aminobutyric acid (GABA) and trans-4-aminocrotonic acid (TACA) substituted in the 2 position as GABAC receptor antagonists

1 gamma-Aminobutyric acid (GABA) and trans-4-aminocrotonic acid (TACA) have been shown to activate GABA(C) receptors. In this study, a range of C2, C3, C4 and N-substituted GABA and TACA analogues were examined for activity at GABA(C) receptors. 2 The effects of these compounds were examined by use of electrophysiological recording from Xenopus oocytes expressing the human rho(1) subunit of GABA(C) receptors with the two-electrode voltage-clamp method. 3 trans-4-Amino-2-fluorobut-2-enoic acid was found to be a potent agonist (K-D = 2.43 mu M). In contrast, trans-4-amino-2-methylbut-2-enoic acid was found to be a moderately potent antagonist (IC50=31.0 mu M and K-B=45.5 mu M) These observations highlight the possibility that subtle structural substitutions may change an agonist into an antagonist. 4 4-Amino-2-methylbutanoic acid (K-D = 189 mu M), 4-amino-2-methylenebutanoic acid (K-D = 182 mu M) and 4-amino-2-chlorobutanoic acid (K-D = 285 mu M) were weak partial agonists. The intrinsic activities of these compounds were 12.1%, 4.4% and 5.2% of the maximal response of GABA, respectively. These compounds more effectively blocked the effects of the agonist, GABA, giving rise to K-B values of 53 mu M and 101 mu M, respectively. 5 The sulphinic acid analogue of GABA, homohypotaurine, was found to be a potent partial agonist (K-D=4.59 mu M, intrinsic activity 69%). 6 It was concluded that substitution of a methyl or a halo group in the C2 position of GABA or TACA is tolerated at GABA(C) receptors. However, there was dramatic loss of activity when these groups were substituted at the C3, C4 and nitrogen positions of GABA and TACA. 7 Molecular modelling studies on a range of active and inactive compounds indicated that the agonist/competitive antagonist binding site of the GABA(C) receptor may be smaller than that of the GABA(A) and GABA(B) receptors. It is suggested that only compounds that can attain relatively flat conformations may bind to the GABA(C) receptor agonist/competitive antagonist binding site.