Locomotor activity and opiate effects in male and female hamsters

Locomotor activity of golden Syrian hamsters was investigated in three experiments. In Experiment 1, running wheel activity of male and female hamsters was compared under the following conditions: no injection, saline injection, morphine injection (15 mg/kg) and naltrexone injection (1 mg/kg). During two hour test sessions, females maintained high levels of activity, whereas males slowed down considerably during the second hour. The difference between males and females was evident under all conditions except following morphine which produced a biphasic time-effect pattern in both sexes. Naltrexone, however, had no detectable effects on either males or females. Experiment 2 investigated the effects of four doses of naltrexone (0.3, 1, 3, 10 mg/kg) on hamster locomotion. Results indicated that none of the doses tested had an effect. Experiment 3 demonstrated that a 1 mg/kg dose of naltrexone antagonizes two of morphine's (15 mg/kg) effects on activity. First, naltrexone partially blocked morphine elicited sedation. Second, naltrexone blocked the increase in activity that characterizes recovery from morphine.