Eur J Biochem 1993 May 01;2133:1349-54.

Expression of the Xenopus D2 dopamine receptor. Tissue-specific regulation and two transcriptionally active genes but no evidence for alternative splicing.

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In the amphibian Xenopus laevis the D2 dopamine receptor is involved in the regulation of the melanotrope cells of the intermediate pituitary during background adaptation of the animal. The Xenopus D2 receptor has been found to be pharmacologically different from the mammalian D2 receptor. In a number of mammalian species alternative splicing generates two molecular forms of the D2 receptor. These isoforms differ by the presence or absence of 29 amino acids in the third cytoplasmic loop which is thought to be involved in guanine-nucleotide-binding-regulatory-protein (G-protein) binding of the receptor. We previously described a cDNA encoding the large isoform of the Xenopus D2 receptor. Here we report on the isolation of a brain cDNA encoding a second, structurally different Xenopus D2 dopamine receptor. Both Xenopus receptors correspond to the large isoform of the D2 receptor and they display a high degree of sequence identity with their mammalian counterparts. Their occurrence reflects the expression of two Xenopus D2 receptor genes and they are expressed to approximately the same level. In contrast to mammals, PCR analysis gave no evidence for alternative splicing during D2 receptor expression in Xenopus brain and pituitary. Tissue-specific expression of the Xenopus D2 receptor was observed in the pituitary during background adaptation. The low level of receptor mRNA in melanotrope cells of white animals compared to that of black animals may be caused by chronic dopamine stimulation of melanotrope cells in white animals with consequent cellular desensitization and down regulation of the D2 receptor gene.

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