DOPAMINE-RECEPTOR SUBTYPES EXPRESSED IN VERTEBRATE (CARP AND EEL) RETINAE - CLONING, SEQUENCING AND COMPARISON OF 5 D-1-LIKE AND 3 D-2-LIKERECEPTORS

Eight dopamine receptor-like cDNA crones were isolated from the carp ( Cyprinus carpio) retina and four dopamine receptor-like cDNA clones we re isolated from the European eel (Anguilla anguilla) retina. These cD NA clones show high sequence and structural homology to the known dopa mine receptor subtypes. The sequence similarity and phylogenetic analy sis revealed that five subtypes (D-1A3, D-1A4, D-1B, D-1C and D-1X) in the carp retina and four subtypes (D-1A1, D-1A2, D-1B and D-1C) in th e eel retina are D-1-like receptor subtypes, and three (D-2, D-4A and D-4B) in the carp retina are D-2-like receptor subtypes; no D-2-like r eceptor was found in the eel. Carp D-1A3 and D-1A4, carp D-4A and D-4B , and eel D-1A1 and D-1A2 are highly homologous pairs of receptors whi ch show significant, domain-specific differences to each other and to their species homologues. The structure of the third cytoplasmic loop in the carp D-1X receptor was particularly different from the other D- 1-like receptors. The implications of these structural differences in terms of dopamine receptor activation and signalling are discussed. It is suggested that the known diverse physiological and pharmacological effects of dopamine on the retinal neurones are likely to be mediated through these multiple receptor subtypes which may be coupled to diff erent signal transduction pathways.