Potentiation of morphine-elicited circling by dopaminergic uptake blockade.

Contraversive circling induced by unilateral infusions of morphine (7.5 nanomoles or 5 micrograms) into the ventral tegmental area was studied under conditions of dopamine uptake inhibition. Dopamine uptake blockade with either nomifensine (2.5 mg/kg) or GBR 13069 (2.0 mg/kg) resulted in a 3-fold increase in the rate of circling. Naloxone (1.5 mg/kg) attenuated but did not completely block the potentiated circling. These data are consistent with the hypothesis that morphine causes contraversive circling through local activation or disinhibition of dopaminergic cell firing.