Six differences in the locomotor-activating effects of amphetamine: role of circulating testosterone in adulthood.

The effects of circulating testosterone (T) on sex differences in locomotor activity elicited by both acute and repeated amphetamine (AMPH) administration were evaluated in adult rats. Male and female rats were gonadectomized in adulthood and implanted with Silastic capsules containing either T or cholesterol (CHOL). In the preexposure period, locomotor activity in response to IP injections of either AMPH (1.5 or 1.3 mg/kg) or saline (1.0 ml/kg) was measured for 2 h, every third day on five occasions. In a subsequent test for sensitization, all animals received AMPH (0.75 or 0.65 mg/kg). Results indicate that regardless of the presence of circulating T, females showed higher levels of activity in response to AMPH than males. In male animals, T suppressed AMPH-induced activity on the first day of the preexposure period, but this effect was lost with repeated testing. In female animals, T enhanced AMPH-induced activity during the first hour of testing. The presence of circulating T did not influence the degree of sensitization in either sex as determined by the difference between AMPH preexposed and SAL preexposed animals on the test day for sensitization.