Localization of the mRNAs for the short and long isoforms of the dopamine D2 receptor in embryonic striatal grafts: predominance of the short isoform.

The expression of mRNA coding for the long (D2L) and short (D2S) isoforms of the dopamine D2 receptor was measured by in situ hybridization histochemistry in embryonic striatal tissue grafts implanted into the ibotenic acid-lesioned neostriatum of adult rats. The intact, lesioned and grafted neostriata were evaluated 3 months after transplantation using 35S-labelled 'antisense' oligonucleotide probes (45-mer) specific for each mRNA isoform. In the adult neostriatum each mRNA isoform exhibited a strong lateral-to-medial gradient in expression in terms of both the number of expressing cells and the hybridization signal (grain density) per cell. Although there were no significant differences in the numbers of cells expressing the two isoforms in each striatal compartment, the hybridization signal per cell for D2S was significantly greater than that for D2L in the lateral and central striatum. The mRNA expression of each isoform was markedly reduced by the lesions. In the striatal grafts, the expression of both D2L and D2S occurred in 'patches' that corresponded to patches of intense acetylcholinesterase activity, and which are believed to involve reaggregation of striatal-like tissues within the graft. In contrast to the situation in the intact neostriatum the striatal grafts contained significantly greater numbers of cells expressing D2S mRNA; however, similar to the findings in the intact striatum the mRNA signal per cell was significantly greater than that for D2L mRNA. These results indicate that the D2 receptor isoforms are differentially expressed in mature striatal tissue grafts, and suggest that the dopamine regulation of striatal grafts via D2 signal transduction mechanisms may be similar to that observed in the normal neostriatum.