ZAPA, A SUBSTRATE FOR THE NEURONAL HIGH-AFFINITY GABA UPTAKE SYSTEM IN RAT-BRAIN SLICES

The GABA analogue ZAPA, a potent GABA(A) receptor agonist, is a substrate for the GABA high affinity neuronal uptake system. ZAPA was labelled with C-14 at a specific activity of 53 mCi/mmol by synthesis from [C-14]thiourea. [C-14]ZAPA was taken up into rat cortical slices having an affinity for the carrier about one third that of GABA (ZAPA K(m) 89-mu-M; GABA K(m) 26 mu-M). ZAPA uptake could be inhibited by the relatively selective GABA neuronal uptake inhibitor, nipecotic acid, but not by the relatively selective glial uptake inhibitor, beta-alanine. Specific binding of [C-14]ZAPA (0.3-mu-M) to GABA receptor sites was observed only under GABA(A) conditions with 23% of the total binding being displaced by 1 mM GABA. ZAPA would need to be labelled to higher specific activity to enable a more extensive study of its binding interactions with GABA(A) receptors.