Methylphenidate potentiates morphine-induced antinociception, hyperthermia, and locomotor activity in young adult rats.

The goal of this study was to determine if the exaggerated morphine-induced conditioned place preference (CPP) response seen in adult rats after preweanling methylphenidate exposure is unique to reward-mediated behaviors or is indicative of generalized changes in opioid-mediated behaviors. Rats were exposed to saline or methylphenidate (2.0 or 5.0 mg/kg) for 10 consecutive days starting on postnatal (PD) 11 with testing beginning on PD 60. In Experiment 1, morphine-induced (0, 2.5, 5.0 or 10.0 mg/kg) antinociception was assessed using the tail immersion and hot plate tasks. In Experiment 2, morphine-induced (0, 2.5, 5.0, or 10.0 mg/kg) hyperthermia and locomotor activity were measured. Morphine caused an increase in antinociception, with early methylphenidate (5.0 mg/kg) exposure potentiating the effects of 5.0 mg/kg morphine. Rectal temperatures were elevated after morphine, with the greatest increase occurring in male rats. Methylphenidate potentiated the hyperthermic effects of morphine (10.0 mg/kg) but only in males. Moderate doses (2.5 and 5.0 mg/kg) of morphine increased the locomotor activity of adult rats, while a higher dose (10.0 mg/kg) decreased locomotion. Interestingly, methylphenidate-pretreated females showed increased locomotor activity relative to controls. These results suggest that early methylphenidate exposure induces general changes in opioid system functioning that are not specific to reward-mediated behaviors.