Sodium-dependent release of exogenous glycine from preloaded rat hippocampal synaptosomes

The effect of high potassium, veratridine, and ouabain stimulation upon the release of exogenously-loaded [3H]glycine was evaluated in crude synaptosomal preparations from rat hippocampi by means of a superfusion technique in the presence of media with different ionic compositions and of tetrodotoxin (TTX). Four minute superfusion of synaptosomes with 30 mM KCl, 10 microM veratridine or 0.4 mM ouabain caused a significant increase in the [3H]glycine efflux which averaged 6.6 +/- 0.2, 25.5 +/- 1.0, and 8.9 +/- 1.0% of the total radioactivity present in the synaptosomes, respectively. The omission of Ca2+ ions in the superfusion medium markedly decreased K(+)-evoked [3H]glycine efflux (2.5 +/- 0.5%), did not appreciably modify that evoked by veratridine (24.2 +/- 2.0%) and significantly increased that evoked by ouabain (18.5 +/- 0.5%). The superfusion of synaptosomes with Na(+)-free media always resulted in a drastic decrease of the depolarization-stimulated [3H]glycine efflux, whereas the omission of Cl- generally resulted in a moderate increase of [3H]glycine efflux. TTX (0.8 microM) markedly affected the stimulatory effect of veratridine (2.5 +/- 0.9%) and ouabain (2.2 +/- 0.5%), but failed to modify significantly that evoked by high potassium (6.5 +/- 0.7%). Finally, [3H]glycine was seen readily to exchange in a partially sodium-dependent way with unlabelled glycine present in the medium. On the whole these findings appear to be consistent with the neurotransmitter character of the glycine release from hippocampal synaptosomes.