High-affinity GABA uptake by neuronal GAT1 transporters provokes release of [3H]GABA by homoexchange and through GAT1-independent Ca(2+)-mediated mechanisms

High-affinity uptake of GABA into nerve terminals may have functions other than recapture of the neurotransmitter. Synaptosomes purified from mouse cerebellum were prelabelled with [3H]GABA and then superfused with GABA and drugs selective for some presynaptic targets. Influx of GABA through GAT1 transporters stimulated efflux of [3H]GABA in a concentration-dependent manner (EC50 ~ 3 mM). The efflux of the transmitter occurred in part by GAT1 reversal through the so called homoexchange. The ion fluxes (particularly Naþ influx) accompanying GABA uptake triggered intraterminal Ca2þ signals through both plasmalemmal Naþ/Ca2þ exchangers, sensitive to KB-R7943 or to ifenprodil and mitochondrial Naþ/Ca2þ exchangers, sensitive to CGP37157. These Ca2þ signals likely facilitated GABA release from nerve terminals via niflumic acid- and NPPB-sensitive anion channels. The results show that GABA, at concentrations corresponding to the high-affinity uptake, can evoke GABA release which occurs in part by the expected GAT1-mediated homoexchange, while the transporter-independent component of the GABA uptake-evoked GABA release takes place by hitherto unsuspected mechanisms which include Naþ/Ca2þ exchangers and anion channels. The significance of the novel function of the GABA high-affinituptake here identified deserves further multidisciplinary investigation.