Cloning of the D2dopamine receptor gene has demonstrated that two D2mRNA isoforms are generated through alternative splicing: a short mRNA (D2415) and a long one (D2444) that differs by an additional exon transcript. D2receptor 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 D2444mRNA. We report here that the long D2444mRNA isoform is present in all the neurons expressing the D2receptor 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 D2444mRNA. Our results with the D2444probe confirm that 20% of the cholinergic neurons in the striatum do not express the D2gene. The present results also demonstrate that the neurons expressing the D2receptor gene in the substantia nigra are dopamine neurons and that D2mRNA can be present in proximal dendrites. © 1991.
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