Regulation of the nigrostriatal pathway by metabotropic glutamate receptors during development

Dopamine neurons in the substantia nigra heavily innervate the striatum, making it the nucleus with the highest levels of dopamine in the adult brain. The present study analyzes the time course and the density of striatal innervation by nigral dopamine neurons and characterizes the role of the neurotransmitter glutamate during the development of the nigrostriatal pathway. For this purpose, organotypic cultures containing the cortex, the striatum, and the substantia nigra (triple cultures) were prepared from rat brains at postnatal day (PND) 0-2 and were cultured for up to 60 d in vitro (DIV). Dopamine fibers and neurons were labeled using tyrosine hydroxylase (TH) immunohistochemistry. Striatal TH-ir fiber density was quantitatively analyzed using confocal laser scanning microscopy (CLSM). In long-term triple cultures (44 +/- 3 DIV), the striatal dopamine fiber density was high and was weakly correlated with the number of nigral dopamine neurons. The high striatal dopamine fiber density mainly resulted from an enhanced ingrowth and ramification of dopamine fibers from nigral neurons during 8-17 DIV. The metabotropic glutamate receptor (mGluR) antagonist L(+)-2-amino-3-phosphonopropionic acid (L-AP-3) selectively inhibited this dopaminergic innervation of the striatum, whereas ionotropic GluR antagonists had no effect. The L-AP-3-mediated inhibition was prevented by the mGluR agonist 1S, 3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). The inhibition of the striatal dopaminergic innervation by L-AP-3 was further confirmed by anterograde tracing of the nigrostriatal projection with Phaseolus vulgaris leucoagglutinin. These results indicate that glutamate, by acting on group I mGluRs, plays an important "trophic" role for the development of the nigrostriatal dopamine pathway.