Dopaminergic and cholinergic influences on morphine self-administration in rats

Various doses of apomorphine, haloperidol, pilocarpine and scopolamine were administered i.p. to rats self-administering i.v. morphine (0.4 mg/kg/infusion). Apomorphine, at low doses, increased rates of morphine self-administration and, at high doses, decreased rates. Haloperidol decreased self administration rates with increasing dose. Both pilocarpine and scopolamine produced only small decreases in rates at high doses. When administered in combination, the effects of apomorphine and haloperidol were antagonistic as were the effects of pilocarpine and scopolamine; the effects of apomorphine or haloperidol predominated, however, when either drug was administered in combination with pilocarpine or scopolamine. The results are consistent with previous work implicating an important role for dopaminergic mechanisms in morphine's reinforcing property.