Circadian and sex differences in hyperactivity produced by amphetamine in rats

Hyperactivity produced in rats with d-amphetamine HCl was measured using a residential maze. Groups of 4 male or 4 female rats were kept on a 12-hr light, 12-hr dark schedule. Amphetamine was given 4 hr after onset of either the light or dark cycle and activity measured for the following 8 hr. For all doses of amphetamine (in light or dark), females displayed significantly greater increases over control activity than males. The results suggested that the greater activity of female rats was not due solely to the recognized sex difference in amphetamine metabolism, but rather there was a sex-related difference in the CNS. Previous experiments have shown that neonatal asphyxia and x-irradiation in utero produce greater hyperactivity in female than in male rats. It is proposed that female rats are more likely to develop hyperactive behavior.