Evidence for a role of endogenous opioids in the nigrostriatal system: influence of naloxone and morphine on nigrostriatal dopaminergic supersensitivity.

The effects of morphine and naloxone on nigrostriatal function were evaluated by their influence on rotational behavior in rats with unilateral lesions of the substantial nigra. Two different rotational syndromes which result from different lesion placements, were examined. Rats with the contraversive syndrome, when given apomorphine, rotate away from the lesioned side, while rats with the ipsiversive syndrome rotate toward the lesioned side. In both syndromes, rats rotate toward the lesioned side when given amphetamine. Morphine or naloxone, alone, was without effect in either syndrome. Morphine antagonized rotation by either apomorphine or amphetamine in both syndromes. Naloxone stimulated apomorphine-induced rotation in contraversive rats and antagonized amphetamine-induced rotation in ipsiversive rats. These findings support a functional role of endogenous opioids in this dopaminergic system. The effects of morphine and naloxone on apomorphine-induced rotation indicate that opiates act at a postsynaptic site in this system. Finally, the different responses to naloxone and morphine in the two rotational syndromes suggest that an enkephalinergic asymmetry may underlie the differences in behavioral responses between these two syndromes.