Enantiomers of cis-constrained and flexible 2-substituted GABA analogues exert opposite effects at recombinant GABA_C receptors

The effects of the enantiomers of a number of flexible and cis-constrained GABA analogues were tested on GABA_C receptors expressed in Xenopus laevis oocytes using two-electrode voltage-clamp electrophysiology. (1S,2R)-cis-2-Aminomethylcyclopropane-1-carboxylic acid ((+)-CAMP), a potent and full agonist at the ρ1 (EC₅₀ ≈ 40 μM, I_max ≈ 100%) and ρ 2 (EC₅₀ ≈ 17 μM, I_max ≈ 100%) receptor subtypes, was found to be a potent partial agonist at ρ3 (EC ₅₀ ≈ 28 μM, I_max ≈ 70%). (1R,2S)-cis-2- Aminomethylcyclopropane-1-carboxylic acid ((-)-CAMP), a weak antagonist at human ρ1 (IC₅₀ ≈ 890 μM) and ρ2 (IC₅₀ ≈ 400 μM) receptor subtypes, was also found to be a moderately potent antagonist at rat ρ3 (IC₅₀ ≈ 180 μM). Similarly, (1R,4S)-4- aminocyclopent-2-ene-1-carboxylic acid ((+)-ACPECA) was a full agonist at ρ1 (EC₅₀ ≈ 135 μM, I_max ≈ 100%) and ρ2 (EC ₅₀ ≈ 60 μM, I_max ≈ 100%), but only a partial agonist at ρ3 (EC₅₀ ≈ 112 μM, I_max ≈ 37 %), while (1S,4R)-4-aminocyclopent-2-ene-1-carboxylic acid ((-)-ACPECA) was a weak antagonist at all three receptor subtypes (IC₅₀ ≫ 300 μM). 4-Amino-(S)-2-methylbutanoic acid ((S)-2MeGABA) and 4-amino-(R)-2-methylbutanoic acid ((R)-2MeGABA) followed the same trend, with (S)-2MeGABA acting as a full agonist at the ρ1 (EC₅₀ ≈ 65 μM, I_max ≈ 100%), and ρ2 (EC₅₀ ≈ 20 μM, I_max ≈ 100%) receptor subtypes, and a partial agonist at ρ3 (EC₅₀ ≈ 25 μM, I_max ≈ 90%). (R)-2MeGABA, however, was a moderately potent antagonist at all three receptor subtypes (IC₅₀ ≈ 16 μM at ρ1, 125 μM at ρ2 and 35 μM at ρ3). On the basis of these expanded biological activity data and the solution-phase molecular structures obtained at the MP2/6-31+G* level of ab initio theory, a rationale is proposed for the genesis of this stereoselectivity effect.