EARLY EMERGENCE OF 3 DOPAMINE D-1 RECEPTOR SUBTYPES IN VERTEBRATES - MOLECULAR PHYLOGENETIC, PHARMACOLOGICAL, AND FUNCTIONAL CRITERIA DEFINING D-1A, D-1B, AND D-1C RECEPTORS IN EUROPEAN EEL ANGUILLA-ANGUILLA

The existence of dopamine D-1C and D-1D receptors in Xenopus and chick en, respectively, challenged the established duality (D-1A and D-1B) o f the dopamine D-1 receptor class in vertebrates. To ascertain the mol ecular diversity of this gene family in early diverging vertebrates, w e isolated four receptor-encoding sequences from the European eel Angu illa anguilla. Molecular phylogeny assigned two receptor sequences (D- 1A1 and D-1A2) to the D-1A subtype, and a third receptor to the D-1B s ubtype. Additional sequence was orthologous to the Xenopus D-1C recept or and to several other previously unclassified fish D-1-like receptor s. When expressed in COS-7 cells, eel D-1A and D-1B receptors display affinity profiles for dopaminergic ligands similar to those of other k nown vertebrate homologues. The D-1C receptor exhibits pharmacological characteristics virtually identical to its Xenopus homologue. Functio nally, while all eel D-1 receptors stimulate adenylate cyclase, the ee l D-1B receptor exhibits greater constitutive activity than either D-1 A or D-1C receptors. Semiquantitative reverse transcription-polymerase chain reaction reveals the differential distribution of D-1A1, D-1A2, D-1B, and D-1C receptor mRNA within the hypothalamic-pituitary axis o f the eel brain. Taken together, these data suggest that the D-1A, D-1 B, and D-1C receptors arose prior to the evolutionary divergence of fi sh and tetrapods and exhibit molecular, pharmacological, and functiona l attributes that unambiguously allow for their classification as dist inct D-1 receptor subtypes in the vertebrate phylum.