Glycine stimulates [3H]noradrenaline release by activating a strychnine-sensitive receptor present in rat hippocampus

Rat hippocampus slices were prelabeled with [3H]noradrenaline ([3H]NA) and depolarized by superfusion with KCl. The release evoked by 12 mM K+ was totally calcium-dependent and more than 90% tetrodotoxin (TTX)-sensitive. Glycine (0.1-1 mM) increased the K(+)-evoked [3H]NA overflow in a concentration-dependent manner. The effect of 1 mM glycine reached 300%. Strychnine (0.3 microM) shifted to the right the concentration-response curve for glycine. The effect of glycine (0.1 or 1 mM) was totally abolished by 3 microM strychnine but was unaffected by the GABAA receptor antagonist, bicuculline (10 microM), or by 100 microM of 1-hydroxy-3-aminopyrrolidone-2 (HA-966), a proposed antagonist of glycine at the strychnine-insensitive site located on the N-methyl-D-aspartate (NMDA) receptor. The effect of glycine was mimicked by L-serine, although less potently; the release of [3H]NA was enhanced by 200% in presence of 3 mM L-serine. At this concentration D-serine was ineffective. Strychnine shifted to the right the concentration-response curve for L-serine. Glycine (1 mM) had only a minor effect (less than 20% potentiation) on the release of [3H]NA evoked by 12 mM KCl in hippocampal synaptosomes. While the effect of glycine in slices was increased by decreasing the depolarizing concentration of K+ (about 500% potentiation at 9 mM K+), the response of synaptosomes remained minimal, even in presence of 9 mM KCl. Hippocampal synaptosomes prelabeled with [3H]glycine released the radiolabeled amino acid when exposed to superfusion with 12 mM KCl. The release of [3H]glycine was more than 75% calcium-dependent. The results suggest that the release of NA in rat hippocampus may be enhanced by glycine through the activation of a strychnine-sensitive receptor. This receptor does not seem to be located on noradrenergic terminals.