Localization and function of the D3 dopamine receptor.

A novel dopamine receptor has been recently cloned, which differs from D1 and D2 receptors by its sequence, localization, pharmacology and possibly signalling system, hence its designation as D3 receptor. The D3 receptor cDNA was obtained by a combination of screenings of DNA libraries and PCR polymerase chain reaction experiments. It encodes a protein with a predicted structure consisting in 7 transmembrane domains indicating that it belongs to the G-protein coupled receptor family. Its global homology is 52% with the D2 receptor but 78% if only transmembrane domains are considered. Two shorter transcript variants, in addition to the full-length cDNA were detected by PCR in various rat brain regions. The shorter transcripts are generated by alternative splicing and encode two putative proteins respectively interrupted after the second transmembrane domain and lacking the second extracytoplasmic loop. After transfection of this latter isoform into cultured cells, no dopaminergic activity could be detected. These shorter splice variants may regulate the number of active D3 receptors. The human D3 receptor was also cloned using similar approaches with rat D3 receptor cDNA probes and was found highly homologous to the rat receptor, except in the third intracytoplasmic loop. The human D3 receptor gene was assigned to the chromosome 3 at q13.3 band. Visualisation of D3 receptor mRNA in rat brain by in situ hybridization indicated a predominant expression of the message in the ventral striatum and other "limbic" areas. There is no overlap in the distributions of D2 and D3 receptor mRNAs in discrete structures, suggesting that the two transcripts are expressed by different cells.(ABSTRACT TRUNCATED AT 250 WORDS)