EXPRESSION OF THE XENOPUS D2 DOPAMINE RECEPTOR - TISSUE-SPECIFIC REGULATION AND 2 TRANSCRIPTIONALLY ACTIVE GENES BUT NO EVIDENCE FOR ALTERNATIVE SPLICING

In the amphibian Xenopus laevis the D2 dopamine receptor is involved i n the regulation of the melanotrope cells of the intermediate pituitar y 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 gener ates two molecular forms of the D2 receptor. These isoforms differ by the presence or absence of 29 amino acids in the third cytoplasmic loo p which is thought to be involved in guanine-nucleotide-binding-regula tory-protein (G-protein) binding of the receptor. We previously descri bed a cDNA encoding the large isoform of the Xenopus D2 receptor. Here we report on the isolation of a brain cDNA encoding a second, structu rally different Xenopus D2 dopamine receptor. Both Xenopus receptors c orrespond to the large isoform of the D2 receptor and they display a h igh degree of sequence identity with their mammalian counterparts. The ir occurrence reflects the expression of two Xenopus D2 receptor genes and they are expressed to approximately the same level. In contrast t o mammals, PCR analysis gave no evidence for alternative splicing duri ng D2 receptor expression in Xenopus brain and pituitary. Tissue-speci fic expression of the Xenopus D2 receptor was observed in the pituitar y during background adaptation. The low level of receptor mRNA in mela notrope cells of white animals compared to that of black animals may b e caused by chronic dopamine stimulation of melanotrope cells in white animals with consequent cellular desensitization and down regulation of the D2 receptor gene.