Stimulation of [3H]GABA and β-[3H]Alanine Release from Rat Brain Slices by cis-4-Aminocrotonic Acid

Abstract: cis -4-Aminocrotonic acid (CACA; 100 µ M ), an analogue of GABA in a folded conformation, stimulated the passive release of ³H]GABA from slices of rat cerebellum, cerebral cortex, retina, and spinal cord and of β-³H]alanine from slices of cerebellum and spinal cord without influencing potassium-evoked release. In contrast, CACA (100 µ M ) did not stimulate the passive release of ³H]taurine from slices of cerebellum and spinal cord or of d -³H]aspartate from slices of cerebellum and did not influence potassium-evoked release of ³H]taurine from the cerebellum and spinal cord and d -³H]aspartate from the cerebellum. These results suggest that the effects of CACA on GABA and β-alanine release are due to CACA acting as a substrate for a β-alanine-sensitive GABA transport system, consistent with CACA inhibiting the uptake of β-³H]alanine into slices of rat cerebellum and cerebral cortex. The observed K _i for CACA against β-³H]alanine uptake in the cerebellum was 750 ± 60 µ M . CACA appears to be 10-fold weaker as a substrate for the transporter system than as an agonist for the GABA_c receptor. The effects of CACA on GABA and β-alanine release provide indirect evidence for a GABA transporter in cerebellum, cerebral cortex, retina, and spinal cord that transports GABA, β-alanine, CACA, and nipecotic acid that has a similar pharmacological profile to that of the GABA transporter, GAT-3, cloned from rat CNS. The structural similarities of GABA, β-alanine, CACA, and nipecotic acid are demonstrated by computer-aided molecular modeling, providing information on the possible conformations of these substances being transported by a common carrier protein.