PHOTOAFFINITY-LABELING OF D-1 AND D-5 DOPAMINE-RECEPTORS

Although human D-1 and D-5 dopamine receptors are encoded by distinct genes and share only 50% sequence homology at the amino acid level; th eir pharmacological properties are identical. Using a selective D-1 re ceptor photoaffinity radioligand, 4-azidophenyl)-2,3,4,5-tetrahydro-1H -3-benzazepine ([I-125]MAB), we have further probed the molecular prop erties of these receptors in transfected GH(4)C(1) rat pituitary cells . Under reversible, non-covalent binding conditions, [I-125]MAB bound to both the D, and the D, receptors with identical affinities, dopamin ergic selectivity and stereospecificity. Upon photoactivation of the b ound [I-125]MAB, the label was incorporated into a similar to 64,000 m el. wt protein corresponding to the D-1 dopamine receptor. However, th ere was no specific photoincorporation of the ligand observed in D-5 r eceptors. The lack of [I-125]MAB photolabeling of D-5 receptors was in dependent of the cell line chosen, since similar results were obtained using other transfected cells. The data suggest that although both D- 1 and D-5 receptors share structurally similar binding sites, the prot ein domains around the sites are different. Thus, although there are c urrently no specific compounds which bind preferentially to D-1 or D-5 receptors, these receptors can be distinguished from one another by t he inability of [I-125]MAB to photolabel D-5, but not D-1, receptors. Such selective targeting of a specific receptor may be useful in under standing the functional importance and/or interaction between closely related members of the same receptor family when co-expressed in the s ame cell. (C) 1997 IBRO. Published by Elsevier Science Ltd.