Mild stress influences sex differences in exploratory and amphetamine-enhanced activity in rats

The locomotor activity of experimentally naive male and female rats was monitored in a novel environment during two sessions one week apart. Half of the animals were handled for 5 days before testing, and all animals were injected before each activity session either with saline or with D-amphetamine (0.25 mg/kg). Overall, there were no sex differences in activity. However, activity was increased both by prior handling and by amphetamine treatment, and these effects were larger in females than in males. The finding that the activity-increasing effects of handling were greater in females than in males suggested that sex differences in behavior may be influenced by previous experience and that this should be considered in the design of behavioral experiments involving naive animals.