Visualization of antipsychotic drug binding to living mesolimbic neurons reveals D2 receptor, acidotropic, and lipophilic components.

To examine the binding of antipsychotic drugs to living neurons, we applied fluoroprobe derivatives of the D2 antagonist spiperone to mesolimbic system neurons in postnatal culture. We found that rhodamine-N-(p-aminophenethyl)spiperone (rhodamine-NAPS) stereospecifically labeled the plasma membranes of 38 +/- 6% of ventral tegmental area neurons, 22 +/- 7% of which were dopaminergic, and 50 +/- 6% of medium-sized putatively GABAergic nucleus accumbens neurons, with a time constant of approximately 8 min. In contrast, the BODIPY derivative of NAPS rapidly labeled intracellular sites in all neurons in a punctate pattern, consistent with acidotropic uptake. Native antipsychotics also show acidotropic uptake, which we visualized by their displacement of the fluorescent weak base vital dye acridine orange from acidic intracellular compartments. We found that acidotropic uptake correlated best with the partition coefficients of the drugs. With a time constant of 23 min, rhodamine-NAPS labeled all neurons in a pattern suggestive of lipophilic solvation. Thus, initially rhodamine-NAPS makes possible visualization of D2 receptors on living neurons; however, acidotropic uptake and lipophilic solvation obscure receptor labeling and may account for time-dependent factors in the action of antipsychotic drugs, as well as affect their use as radioreceptor ligands.