N-METHYL-D-ASPARTIC ACID/GLYCINE INTERACTIONS ON THE CONTROL OF 5-HYDROXYTRYPTAMINE RELEASE IN RAPHE PRIMARY CULTURES

Glutamic acid and glycine were quantified in cells and medium of cultured rostral rhombencephalic neurons derived from fetal rats. In the presence of 1 mM Mg2+, NMDA (50 muM) significantly stimulated (by 69%) release of newly synthesized 5-[H-3]hydroxytryptamine ([H-3]5-HT). D-2-Amino-5-phosphonopentanoate (AP-5; 50 muM) blocked the stimulatory effect of NMDA. AP-5 by itself inhibited [H-3]5-HT release (by 25%), suggesting a tonic control of 5-HT by glutamate. In the absence of Mg2+, basal [H-3]5-HT release was 60% higher as compared with release with Mg2+. AP-5 blocked the increased [H-3]5-HT release observed without Mg2+, suggesting that this effect was due to the stimulation of NMDA receptors by endogenous glutamate. Glycine (100 muM) inhibited [H-3]5-HT release in the absence of Mg2+. Strychnine (50 muM) blocked the inhibitory effect of glycine, indicating an action through strychnine-sensitive inhibitory glycine receptors. The [H-3]5-HT release stimulated by NMDA was unaffected by glycine. In contrast, when tested in the presence of strychnine, glycine increased NMDA-evoked [H-3]5-HT release (by 22%), and this eff ect was prevented by a selective antagonist of the NMDA-associated glycine receptor, 7-chlorokynurenate (100 muM). 7-Chlorokynurenate by itself induced a drastic decrease in [H-3]5-HT release, indicating that under basal conditions these sites were stimulated by endogenous glycine. These results indicate that NMDA stimulated [H-3]5-HT release in both the presence or absence of Mg2+. Use of selective antagonists allowed differentiation of a strychnine-sensitive glycine response (inhibition of [H-3]5-HT release) from a 7-chlorokynurenate-sensitive response (potentiation of NMDA-evoked [H-3]5-HT release).