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 (14)C at a specific activity of 53 mCi/mmol by synthesis from [(14)C]thiourea. [(14)C]ZAPA was taken up into rat cortical slices having an affinity for the carrier about one third that of GABA (ZAPA K(m) 89 ?M; GABA K(m) 26 ?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, ?-alanine. Specific binding of [(14)C]ZAPA (0.3 ?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.