GABA release provoked by disturbed Na+, K+ and Ca2+ homeostasis in cerebellar nerve endings: Roles of Ca2+ channels, Na+/Ca2+ exchangers and GAT1 transporter reversal

Abstract

GABA release provoked by ion dysregulations typical of some neuropathological conditions was analyzed in purified cerebellar synaptosomes pre-labeled with [3H]GABA and exposed in superfusion to KCl, 4-aminopyridine (4-AP) or veratridine. The overflows caused by relatively low concentrations of the releasers were almost totally external Ca2+-dependent. Higher concentrations of KCl or veratridine, but not 4-AP, involved also external Ca2+-independent mechanisms. The GABA overflows evoked by veratridine and, less so, the overflow evoked by high K+, occurred in part by reversal of the GAT1 transporter. None of the depolarizing agents activated store-operated or transient receptor potential or L-type Ca2+ channels. Only the overflow caused by 4-AP occurred in part by N- and P/Q-type voltage-sensitive calcium channel-dependent exocytosis. Significant portions of the external Ca2+-dependent overflows evoked by the three releasers involved reversal of plasmalemmal Na+/Ca2+ exchangers. The overflows evoked by high K+ or veratridine, but not by 4-AP were evoked by Ca2+ originated through mitochondrial Na+/Ca2+ exchangers. Ca2+-induced Ca2+ release mediated by inositoltrisphosphate receptors participated exclusively in the GABA release stimulated by high KCl which also occurred in a modest portion through anion channels. Important differences could be observed between the release mechanisms of GABA here described and those previously reported for glycine, in spite of the abundant vesicular co-localization of the two transmitters in cerebellar interneurons.