Rat striatal and mesencephalic neurons contain the long isoform of the D2 dopamine receptor mRNA.

Cloning of the D2 dopamine receptor gene has demonstrated that two D2 mRNA isoforms are generated through alternative splicing: a short mRNA (D2(415)) and a long one (D2(444)) that differs by an additional exon transcript. D2 receptor gene expression was demonstrated in striatal dopamine terminal fields as well as in the mesencephalon by in situ hybridization using a probe that does not differentiate the two isoforms or a probe specific for the D2(444)) mRNA. We report here that the long D2(444)) mRNA isoform is present in all the neurons expressing the D2 receptor gene in the striatum and the mesencephalon. In the striatum, medium-sized neurons (previously identified as enkephalinergic) and large-sized neurons (previously identified as cholinergic) contain the D2(444)) mRNA. Our results with the D2(444)) probe confirm that 20% of the cholinergic neurons in the striatum do not express the D2 gene. The present results also demonstrate that the neurons expressing the D2 receptor gene in the substantia nigra are dopamine neurons and that D2 mRNA can be present in proximal dendrites.