Repeated cocaine administration does not alter morphine-induced rotational behavior in nigrally denervated rats

Repeated administration of morphine to rats increases their sensitivity to behavioral effects of morphine as well as to those of psychomotor stimulants, such as cocaine and amphetamine. Conversely, stimulant-induced sensitization to behavioral effects of stimulants often results also in sensitization to behavioral effects of morphine. However, in nigrally lesioned rats, repeated injections of morphine produce sensitization to morphine-induced turning but not to turning induced by cocaine or amphetamine. The present study was performed to determine whether giving repeated cocaine injections to nigrally lesioned rats would produce cross-sensitization to morphine-induced turning. Daily injections of 10 mg/kg cocaine (i.p.) enhanced the turning response to cocaine by day 8, but not the turning response to 3.0 mg/kg morphine (s.c.). The response to morphine increased equally in both cocaine- and saline-treated animals after they had received morphine once. Dose–response curves for morphine (1.0–10 mg/kg) and for cocaine (3.0–30 mg/kg), determined during weeks 3 and 4, were the same in rats receiving daily injections of cocaine or daily injections of saline. Thus, although repeated exposure to cocaine or morphine resulted in sensitization to turning induced by each drug, respectively, there was no cross-sensitization between the two drugs. In contrast to other behaviors, rotational behavior does not seem to exhibit cross-sensitization between morphine and psychomotor stimulants.