Rat Meynert neurons were acutely isolated using a dissociation technique that maintains functional GABAergic presynaptic boutons. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded under voltage-clamp conditions using whole cell patch-clamp recordings. Using the frequency of mIPSCs as a measure of presynaptic terminal excitability, the existence of a Na+/Ca2+ exchanger (NCX) in these GABAergic nerve terminals was clearly demonstrated. Both the frequency and the amplitude of mIPSCs were unaffected by replacement of extracellular Na2+. However, in this Na+-free external solution, ouabain could now induce a transient increase of mIPSCs frequency, which was not inhibited by adding Cd2+ or cyclopiazonic acid but was inhibited by removing external Ca2+. This indicates that this transient potentiation was dependent on external Ca2+, but that this Ca2+ influx was not via voltage-dependent Ca2+ channels. KB-R7943, an inhibitor of NCX, at a concentration of 3 × 10-6 M, reduced this transient increase of mIPSCs frequency without affecting mIPSCs amplitude and the response to exogenoug GABA. These results demonstrate the existence of NCX in these GABAergic nerve terminals. In zero external Na+, ouabain causes an accumulation of intraterminal Na+ and a resultant influx of Ca2+ through the reversed mode operation of NCX. However, under more physiological conditions, NCX may also operate in a forward mode and serve to maintain low intracellular [Ca2+] in nerve terminals.
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