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

Abstract

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.

High-affinity GABA uptake by neuronal GAT1 transporters provokes release of [3H]GABA by homoexchange and through GAT1-independent Ca2+-mediated mechanisms

“…Evidence was provided for vesicular exocytotic release independent of external Ca 2þ but triggered by Ca 2þ ions mobilized from intraterminal stores (for a review, see Berridge, 1998). Internal Ca 2þ can be mobilized from different pools, depending on the stimulus applied to nerve endings and it can mediate transmitter release by different modes (Luccini et al, 2008;Romei et al, 2011). Cytosolic Na þ increases can lead to transmitter release by mobilizing internal Ca 2þ through mitochondrial Na þ /Ca 2þ exchangers (Rizzuto and Pozzan, 2006;Palty et al, 2012).…”

“…The releasing effect of 10 mM GABA was significantly prevented by 5 mM KB-R7943, an inhibitor of the plasmalemmal Na þ /Ca 2þ exchangers (NCXs) when they work in reverse to import Ca 2þ ions. Because KB-R7943 was reported to be only relatively selective for reverse plasmalemmal NCXs (see, for references, Romei et al, 2011), we tested ifenprodil which has recently been found to inhibit reverse Na þ /Ca 2þ exchangers in neurons (Brittain et al, 2012). When added at 5 mM, the drug significantly inhibited the GABA-evoked [ 3 H]GABA release by 20% (629 ± 43 vs 786 ± 59; n ¼ 5).…”

Neocortical GABA release at high intracellular sodium and low extracellular calcium: an anti‐seizure mechanism

“…Furthermore, KB‐R7943 reduced the veratridine‐evoked release of inhibitory glycine (Romei et al . ). Conversely, NCX inhibition increased aspartate release (Amoroso et al .…”

“…; Romei et al . ; Chen and Li ). This reversal, based on increased [Na + ] i , should become impossible when there is not enough extracellular Ca 2+ to be exchanged for intracellular Na + .…”

Beta Amyloid Differently Modulate Nicotinic and Muscarinic Receptor Subtypes which Stimulate in vitro and in vivo the Release of Glycine in the Rat Hippocampus

“…Increasing evidence demonstrate that glycine (GLY) is an important aminoacid at hippocampal level which may have a dual role acting as an inhibitory neurotransmitter, when interacting with the strychnine-sensitive receptors, and playing a stimulatory role, when co-activating excitatory N -Methyl- d -aspartic acid receptors together with glutamate (Johnson and Ascher, 1987; Luccini et al, 2008; Romei et al, 2009, 2011; Zappettini et al, 2011). No data are available so far on the possible effects of Aβ on the cholinergic receptors which modulate GLY release at the hippocampal level.…”