The cDNA encoding D2 dopamine receptor was cloned from the distal lobe of the bullfrog pituitary. The deduced amino acid sequence of the bullfrog D2 dopamine receptor (bfD2A) spanned 444 amino acids and exhibited typical features of those of D2 dopamine receptors cloned in other animals to date. It showed a high similarity of 75-87% with rat, turkey, Xenopus and tilapia counterparts. Further analysis of nucleotide sequence of the cDNA revealed the presence of putative truncated D2 dopamine receptor isoforms, bfD2B and bfD2C, of which nucleotide sequences lacked 12 and 99 nucleotides of the coding region for bfD2A, respectively. The alignment analysis indicated that putative bfD2C isoform was close to D2S subtype cloned in mammals and birds, whereas bfD2A and putative bfD2B isoforms were close to mammalian and avian D2L subtype and homologous to two isoforms of Xenopus. This is the first report of the presence of mRNAs for two D2L-like isoforms and one D2S-like isoform in a single species. The amino acid sequence responsible for producing isoforms is present in the third intracellular loop, which has been shown to play an important role in the coupling with G protein. Accordingly, differences in the mode of coupling with G protein among three isoforms were suggested. The expression of three isoforms mRNA in organs and tissues was analyzed by RT-PCR. In the brain, pars distalis and pars neurointermedia, mRNAs for three isoforms were invariably expressed, whereas only putative bfD2C mRNA was expressed in peripheral organs and tissues.
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