Synthesis and characterization of biotinylated and photoactivatable neuroleptics. Novel bifunctional probes for dopamine receptors.

We have synthesized and characterized a series of novel derivatives of established antagonists of the neurotransmitter dopamine, i.e. butyrophenones, hexahydrocarbolines and phenothiazines. All derivatives were biotinylated, some of them carried an additional (photoactivatable) azido group. In the case of butyrophenones, the structural modifications were introduced at the aliphatic keto group and/or the heterocyclic ring system, both modifications resulting in significant decreases in binding affinity to dopamine D2 and dopamine D1 receptor subtypes. Biotinylation of hexahydrocarbolines significantly increased their binding affinity to D1 receptors, with the affinity for D2 receptors increasing only slightly, or remaining approximately the same, as compared to the parent compound. As a consequence, the derivatized hexahydrocarbolines behaved as nonselective antagonists of dopamine. Biotinylation of phenothiazines increased their binding affinity to both main subtypes of dopamine receptors by at least one order of magnitude, resulting in binding affinities in the nM range. These derivatives bound to both D1 and D2 receptor subtypes. In three of the biotinylated derivatives the photoactivatable azido group was introduced. These compounds bound to synaptosomal membranes from bovine caudate nuclei with similar affinity and subtype specificity as the biotinylated derivatives, and photoaffinity labelling was shown to proceed under mild conditions and selectively. These novel bifunctional ligands may become useful tools in the purification and characterization of dopamine receptors including their visualization and localization in the central nervous system and in tissue culture.