Effects of apomorphine on self-stimulation responding: does the drug mimic the current?

The effects of apomorphine (APO) on self-stimulation responding were examined as a function of drug dose and stimulation current intensity. The lowest dose tested (0.02 mg/kg) significantly elevated the stimulation threshold, presumably reflecting the drug's preferential affinity for presynaptic dopamine (DA) receptors at this dose. At a dose which stimulates postsynaptic DA receptors (0.2 mg/kg), the rats responded with a pattern of behavior that resembled that obtained when non-drugged animals are given non-contingent current. However, during extinction, drugged animals were identical to non-drugged rats in their pattern of responding. These data indicate an interaction of APO with the stimulating current and suggest that APO may be mimicking only one component of the stimulation--perhaps the rewarding but not the motivating one.