Glycine release provoked by disturbed Na+, K+ and Ca2+ homeostasis in cerebellar nerve endings: roles of Ca2+ channels, Na+/Ca2+ exchangers and GlyT2 transporter reversal

J. Neurochem. (2011) 119, 50–63. Abstract Glycine release provoked by ion dysregulations typical of some neuropathological conditions was analyzed in cerebellar synaptosomes selectively pre-labelled with [3H]glycine through GlyT2 transporters and exposed in superfusion to KCl, 4-aminopyridine (4-AP) or veratridine. The overflows caused by relatively low concentrations of the releasers were largely external Ca2+-dependent. Higher concentrations of KCl (50 mM) or veratridine (10 μM), but not of 4-AP (1 mM), involved also external Ca2+-independent mechanisms. GlyT1-mediated release could not be observed; only the external Ca2+-independent veratridine-evoked overflow occurred significantly by GlyT2 reversal. None of the three depolarizing agents activated store-operated or transient receptor potential or L-type Ca2+ channels. The overflows caused by KCl or 4-AP occurred in part by N- and P/Q-type voltage-sensitive calcium channel-dependent exocytosis. Significant portions of the external Ca2+-dependent overflow evoked by KCl or 4-AP (and all that caused by veratridine) were mediated by reverse plasmalemmal Na+/Ca2+ exchangers. Significant contribution to the overflows evoked by KCl or veratridine came from Ca2+ originated through mitochondrial Na+/Ca2+ exchangers. Ca2+-induced Ca2+ release (CICR) mediated by inositoltrisphosphate receptors (InsP3Rs) represents the final trigger of the glycine release evoked by high KCl. The overflows evoked by 4-AP or, less so, by veratridine also involved InsP3R-mediated CICR and, in part, CICR mediated by ryanodine receptors. To conclude, ionic dysregulations typical of ischemia and epilepsy caused glycine release in cerebellum by multiple differential mechanisms that may represent potential therapeutic targets.