Pharmacological evidence for N-methyl-D-aspartate receptors on nigrostriatal dopaminergic nerve terminals

The efflux of tritium from rat striatal synaptosomes labelled with [³H]dopamine was utilized as an index of dopamine (DA) release for the purpose of characterizing the receptors underlying the effects of L-glutamate. N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA), and kainate each induced DA release in the absence of Mg²⁺, though NMDA was much more efficacious and only the NMDA response was inhibited by Mg²⁺. The response to L-glutamate was potentiated in a concentration-dependent manner by glycine. Further, it was completely inhibited by the competitive NMDA antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid and by the NMDA channel blocker phencyclidine. Finally, the response to L-glutamate was unaffected by either tetrodotoxin or the kainate-AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. These data demonstrate the presence of NMDA receptors on dopaminergic nerve terminals that mediate the ability of L-glutamate to release DA and suggest an additional mechanism by which information from the nigrostriatal and corticostriatal pathways may be integrated.