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

Abstract

1. γ-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 ρ₁ 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 μM). In contrast, trans-4-amino-2-methylbut-2-enoic acid was found to be a moderately potent antagonist (IC₅₀=31.0 μM and K_B=45.5 μM). 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 μM), 4-amino-2-methylenebutanoic acid (K_D=182 μM) and 4-amino-2-chlorobutanoic acid (K_D=285 μM) 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 μM and 101 μM, respectively.

5. The sulphinic acid analogue of GABA, homohypotaurine, was found to be a potent partial agonist (K_D=4.59 μM, 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.

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