Potent cocaine analogs inhibit [3H]dopamine uptake in rat mesencephalic cells in primary cultures: pharmacological selectivity of embryonic cocaine sites.

The cellular localization of the cocaine binding sites in primary cultures of embryonic rat mesencephalic cells was previously reported to differ from that observed in adult rat brain. In order to know whether this different localization was associated with a different pharmacological selectivity, we tested the effect of new cocaine analogs on tritiated dopamine ([3H]DA) uptake in primary cultures of rat embryonic mesencephalic cells. In these cultures, [3H]DA was taken up by a nomifensine-sensitive, but desipramine and fluoxetine-insensitive process, reflecting selective uptake by the dopaminergic transporter. 3 beta-(4-Chlorophenyl)tropan-2 beta-carboxylic acid methyl ester (RTI-COC-31) was by far the most potent inhibitor of the [3H]DA uptake, presenting an IC50 of 3.8 nM, while the corresponding analog with an unsubstituted phenyl ring (WIN 35,065-2) was 38 times less potent. The enantiomer of WIN 35,065-2, namely WIN 35,065-3, was 30 times less potent than the former. A similar pattern was found for the relative ability of these compounds to inhibit binding of the radiolabeled cocaine derivative [125I]RTI-55 to membranes prepared from mesencephalic cultures. The order of potencies found for the three cocaine analogs on mesencephalic cultures was similar to that previously obtained in [3H] WIN 35,428 binding experiments and [3H]DA uptake inhibition in adult rat striatum, suggesting that the pharmacological selectivity of cocaine sites functionally related to the DA transporter in cultured embryonic neurons does not differ from that obtained in adult rat brain.