Some determinants of morphine effects on intracranial self-stimulation in rats: dose, pretreatment time, repeated treatment, and rate dependence.

Intracranial self-stimulation (ICSS) is a procedure used to evaluate the abuse liability of drugs. The μ opioid receptor agonist morphine is an acknowledged drug of abuse, and this study examined factors that may influence expression of abuse-related morphine effects on ICSS in rats. Adult male rats were equipped with intracranial electrodes targeting the medial forebrain bundle, and 10 stimulus frequencies (56-158 Hz in 0.05 log increments) were available during each daily session under a continuous reinforcement schedule. The primary dependent variable was the ICSS rate at each frequency. Under baseline conditions, the ICSS rate increased with frequency. After acute morphine (1-10 mg/kg), rate-decreasing effects predominated at early pretreatment times (10-30 min) and rate-increasing effects predominated at later pretreatment times (100-180 min). Acute morphine effects dissipated after 300 min. Repeated morphine (3.2-18 mg/kg/day×7 days at each dose) produced tolerance to rate-decreasing effects, enhanced expression of rate-increasing effects, and enhanced rate dependency of morphine effects. Withdrawal from repeated morphine produced small but significant dose-dependent decreases in ICSS. These results show that the magnitude and valence of morphine effects on rates of ICSS in rats are strongly influenced by morphine dose and pretreatment time, history of morphine exposure, and baseline ICSS rate.