Activation of γ‐aminobutyric acid GAT‐1 transporters on glutamatergic terminals of mouse spinal cord mediates glutamate release through anion channels and by transporter reversal

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Abstract
The effects of γ‐aminobutyric acid (GABA) on the release of glutamate from mouse spinal cord nerve endings have been studied using superfused synaptosomes. GABA elicited a concentration‐dependent release of [3H]D‐aspartate ([3H]D‐ASP; EC50 = 3.76 μM). Neither muscimol nor (–)baclofen mimicked GABA, excluding receptor involvement. The GABA‐evoked release was strictly Na+ dependent and was prevented by the GABA transporter inhibitor SKF89976A, suggesting involvement of GAT‐1 transporters located on glutamatergic nerve terminals. GABA also potentiated the spontaneous release of endogenous glutamate; an effect sensitive to SKF89976A and low‐Na+‐containing medium. Confocal microscopy shows that the GABA transporter GAT‐1 is coexpressed with the vesicular glutamate transporter vGLUT‐1 and with the plasma membrane glutamate transporter EAAT2 in a substantial portion of synaptosomal particles. The GABA effect was external Ca2+ independent and was not decreased when cytosolic Ca2+ ions were chelated by BAPTA. The glutamate transporter blocker DL‐TBOA or dihydrokainate inhibited in part (∼35%) the GABA (10 μM)‐evoked [3H]D‐ASP release; this release was strongly reduced by the anion channel blockers niflumic acid and NPPB. GABA, up to 30 μM, was unable to augment significantly the basal release of [3H]glycine from spinal cord synaptosomes, indicating selectivity for glutamatergic transmission. It is concluded that GABA GAT‐1 transporters and glutamate transporters coexist on the same spinal cord glutamatergic terminals. Activation of these GABA transporters elicits release of glutamate partially by reversal of glutamate transporters present on glutamatergic terminals and largely through anion channels.