Irreversible binding of [3H](-)N-chloroethylnorapomorphine to mammalian brain tissue

A proposed dopamine (DA) receptor labeling agent, [3H](-)N-chloroethylnorapomorphine (3H-NCA) underwent relatively little chemical change at 25 degrees C and pH 6.4 up to an hour of incubation. At low (nM) concentrations it bound rapidly and avidly to a membrane preparation of calf caudate nucleus, but the binding did not saturate over two hours of incubation or at ligand concentrations between 0.2 nM and 10 microM. While similarly bound [3H]-(-)apomorphine was rapidly displaced by DA and other agents that interact with DA receptors, 3H-NCA could not be displaced by unlabeled DA, apomorphine and (+)butaclamol, nor by denaturation of the tissue with trichloracetic acid (TCA). There was no evidence of selectivity of binding of 3H-NCA in regions of rat brain, and binding occurred even to TCA-denatured caudate tissue. Catechol-aporphines prevented binding of 3H-NCA to calf caudate membranes by up to 30%, but this effect was not stereoselective and was lost at concentrations of 3H-NCA above 100 nM. In contrast, DA and ADTN as well as neuroleptics and adrenergic agonists had no such effect. The results suggest that while 3H-NCA may bind irreversibly, and possibly covalently, it does not have high selectivity for labeling dopamine D-3 or D-2 receptor sites, but may be partially selective for an aporphine binding site.