Early emergence of three 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 chicken, respectively, challenged the established duality (D-1A and D-1B) of the dopamine D-1 receptor class in vertebrates. To ascertain the molecular diversity of this gene family in early diverging vertebrates, we isolated four receptor-encoding sequences from the European eel Anguilla 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 subtype. Additional sequence was orthologous to the Xenopus D-1C receptor and to several other previously unclassified fish D-1-like receptors. When expressed in COS-7 cells, eel D-1A and D-1B receptors display affinity profiles for dopaminergic ligands similar to those of other known vertebrate homologues. The D-1C receptor exhibits pharmacological characteristics virtually identical to its Xenopus homologue. Functionally, while all eel D-1 receptors stimulate adenylate cyclase, the eel D-1B receptor exhibits greater constitutive activity than either D-1A 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 of the eel brain. Taken together, these data suggest that the D-1A, D-1B, and D-1C receptors arose prior to the evolutionary divergence of fish and tetrapods and exhibit molecular, pharmacological, and functional attributes that unambiguously allow for their classification as distinct D-1 receptor subtypes in the vertebrate phylum.