A PHOTOAFFINITY LIGAND FOR DOPAMINE D2 RECEPTORS - AZIDOCLEBOPRIDE

  • 1 January 1985
    • journal article
    • research article
    • Vol. 27  (2) , 193-199
Abstract
In order to label D2 dopamine receptors selectively and covalently by means of a photosensitive compound, azidoclebopride was synthesized directly from clebopride. The Kd of clebopride for the D2 dopamine receptor (canine brain striatum) was 1.5 nM, while that for azidoclebopride was 21 nM. The affinities of both clebopride and azidoclebopride were markedly reduced in the absence of NaCl. In the presence of UV light, azidoclebopride inactivated D2 dopamine receptors irreversibly, as indicated by the inability of the receptors to bind [3H]spiperone. Maximal photoinactivation of .apprx. 60% of the D2 dopamine receptors occurred at 1 .mu.M azidoclebopride; 30% of the receptors were inactivated at 80 nM azidoclebopride (pseudo-IC50 [concentration needed to give 50% inhibition]). Dopamine agonists selectively protected the D2 receptors from being inactivated by azidoclebopride, the order of potency being (-)-N-n-propylnorapomorphine > apomorphine > (.+-.)-6, 7-dihydroxy-2-aminotetralin > (+)-N-n-propylnorapomorphine > dopamine > noradrenaline [norepinephrine] > serotonin. Similarly, dopaminergic antagonists prevented the photoinactivation of D2 receptors by azidoclebopride with the following order of potency: spiperone > (+)-butaclamol > haloperidol > clebopride > (-)-sulpiride > (-)-butaclamol. The degree of D2 dopamine receptor photoinduced inactivation by azidoclebopride was not significantly affected by scavengers such as p-aminobenzoic acid and dithiothreitol. Irradiation of striatal membranes with a concentration of azidoclebopride sufficient to inactivate dopamine D2 receptors by 60% did not significantly reduce dopamine D1, serotonin (S2), benzodiazepine, .alpha.1- or .beta.-noradrenergic receptors. This study describes the use of a novel and selective photoaffinity ligand for brain dopamine D2 receptors. The molecule, in radiolabeled form, may aid in the molecular characterization of these receptors.

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