Activation of α6 GABAA receptors on depolarized cerebellar parallel fibers elicits glutamate release through anion channels

Rat cerebellar synaptosomes labeled with [H-3]D-aspartate ([H-3]D-ASP) were exposed in superfusion to muscimol. The GABA(A) receptor agonist did not affect [H-3]D-ASP basal release or the overflow provoked by 15 mM K+; muscimol potentiated the 35 mM K+-evoked overflow of [H-3]D-ASP or endogenous glutamate. Membrane potential measured by Rhodamine 6G fluorescence was -65 mV under resting conditions and -32 mV in the presence of 35 mM K+. The membrane potential was not significantly affected by muscimol. The muscimol effect on the K+(35 mM)-evoked [H-3]D-ASP overflow was not inhibited by omitting external Ca2+ or by entrapping BAPTA to chelate cytosolic Ca2+. Muscimol lost its ability to release glutamate following superfusion with D-aspartate to deplete cytosolic glutamate by heteroexchange suggesting that GABA(A) receptor activation elicits release of cytosolic glutamate. The non-transportable glutamate carrier blockers dihydrokainate or DL-TBOA did not reduce the muscimol potentiation. This was abolished by the anion channel blockers niflumic acid and NPPB. To conclude, when cerebellar parallel fiber terminals are sufficiently depolarized, activation of alpha6 GABA(A) receptors on these terminals mediates glutamate release in addition to that evoked by depolarization. This extra-release does not occur by exocytosis or transporter reversal but involves the opening of anion channels present on parallel fiber terminals. (C) 2001 Elsevier Science Ltd. All rights reserved.