Sex difference in reward asymmetry and effects of cocaine

Male and female rats with bilaterally implanted electrodes in the lateral hypothalamus were tested daily for self-stimulation of each side of the brain, and rotation (circling behavior) was recorded concomitantly. All rats rotated in a preferred direction, regardless of the side of the brain stimulated, and all rats had asymmetries in the sensitivity for self-stimulation (thresholds and rate-intensity functions) related to the direction of rotation. However, the direction of the asymmetry of self-stimulation was sex-dependent: the side of the brain contralateral to the preferred direction of rotation was the more sensitive side in female rats, as reported previously, whereas the ipsilateral side was the more sensitive side in male rats. In both sexes, cocaine acted predominantly on the contralateral side: that is, cocaine lowered the thresholds and shifted the rate-intensity functions to the left on both sides of the brain, but the effects were much greater on the side of the brain contralateral to the preferred direction of rotation. When compared to baseline parameters, cocaine enhanced the asymmetry of self-stimulation in female rats, but reduced or reversed the asymmetry of self-stimulation in male rats; similar effects were produced by d-amphetamine. It is suggested that at least two separate systems (both lateralized) mediate self-stimulation of the lateral hypothalamus, that the relative importance of these systems differs between the sexes and that cocaine selectively affects one of these systems.