Title

 

Authors

Romei C¹and Raiteri L<sup>1,2

1</sup> Department of Experimental Medicine, Section of Pharmacology and Toxicology
² Center of Excellence for Biomedical Research, University of Genoa – Italy

 

Abstract

Glycine is an inhibitory neurotransmitter when it activates strychnine-sensitive receptors, especially in spinal cord, brainstem and cerebellum (Legendre 2001; Xu and Gong 2010). In addition, in these regions, glycine is an obligatory coagonist of glutamate in activating excitatory NMDA receptors (Traynelis et al. 2010). Glycinergic neurons, glycine transporters and glycine receptors are widely expressed in the cerebellum (Zafra et al. 1997; Duguè et al. 2005; Bagnall et al. 2009). To our knowledge, however, no detailed studies on the mechanisms of glycine release from cerebellar glycinergic neurons have been reported.
Glycine release from purified cerebellar nerve endings selectively prelabelled through GlyT2 transporters was analyzed. Particular attention was focussed on the glycine overflows provoked by stimuli known to cause ionic dysregulations typical of some neurological diseases.
The overflows of [³H]glycine from cerebellar synaptosomes exposed in superfusion to relatively low concentrations of KCl, 4-aminopyridine (4-AP) or veratridine were largely external Ca²⁺-dependent. Higher concentrations of KCl (50 mM) or veratridine (10 μM), but not of 4-AP (1 mM) involved external Ca²⁺-independent mechanisms. The veratridine-evoked overflow occurred by transporter reversal, but the overflow provoked by 50 mM KCl was not carrier-mediated. Minor portions of the overflows caused by KCl or 4-AP ( but not by veratridine ) occurred by VSCC-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. Calcium-induced Calcium release (CICR) mediated by InsP₃ receptors represents the final trigger of the glycine release evoked by high KCl. The overflows evoked by 4-AP or veratridine also involved InsP₃ receptor-mediated CICR and, less so, CICR mediated by ryanodine receptors. To conclude, ionic dysregulations typical of ischemia and epilepsy are able to evoke glycine release in cerebellum by multiple differential mechanisms.
 
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