Early Emergence of Three Dopamine D1 Receptor Subtypes in Vertebrates

Abstract

The existence of dopamine D1C and D1D receptors in Xenopus and chicken, respectively, challenged the es-tablished duality (D1A and D1B) of the dopamine D1 receptor class in vertebrates. To ascertain the molecular diversity of this gene family in early diverging verte- brates, we isolated four receptor-encoding sequences from the European eel Anguilla anguilla . Molecular phylogeny assigned two receptor sequences (D1A1 and D1A2) to the D1A subtype, and a third receptor to the D1B subtype. Additional sequence was orthologous to the Xenopus D1C receptor and to several other previously unclassified fish D1-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 iden- tical to its Xenopus homologue. Functionally, while all eel D 1 receptors stimulate adenylate cyclase, the eel D 1B receptor exhibits greater constitutive activity than ei- ther D 1A or D 1C receptors. Semiquantitative reverse transcription-polymerase chain reaction reveals the dif- ferential distribution of D 1A1 ,D 1A2 ,D 1B , and D 1C recep- tor 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 evolution- ary divergence of fish and tetrapods and exhibit molec- ular, pharmacological, and functional attributes that unambiguously allow for their classification as distinct D 1 receptor subtypes in the vertebrate phylum.