Morphine may mimic the apomorphine cue by inhibiting dopaminergic autoinhibition.

Rats were trained to discriminate the cue produced by 0.25 mg/kg apomorphine from that produced by saline, using a two-lever, drug discrimination task. This cue was blocked by haloperidol but not by DPI, ergometrine, bromocriptine, sulpiride, naloxone or naltrexone; none of these agents substituted for (mimicked) apomorphine. Morphine and levorphanol did, however, mimic apomorphine, whereas dextrorphan did not; pentazocine induced responding unlike either apomorphine or saline. The substitution of morphine for apomorphine was blocked by naltrexone or bromocriptone, and was partially antagonized by a low of ergometrine, but was unaffected by haloperidol or sulpiride. To explain these results, it was proposed that morphine mimics apomorphine by inhibiting dopaminergic autoregulation involving D2-receptors.