Components of the brain's dopaminergic system, such as dopamine receptors, undergo final maturation in adolescence. Exposure to social stress during human adolescence contributes to substance abuse behaviors. We utilized a rat model of adolescent social stress to investigate the neural mechanisms underlying this correlation. Rats exposed to repeated social defeat in adolescence (P35-P39) exhibited increased conditioned place preference (CPP) for amphetamine (1 mg/kg) in adulthood (P70). In contrast, rats experiencing foot-shock during the same developmental period exhibited amphetamine CPP levels similar to non-stressed controls. Our previous experiments suggested adolescent defeat alters dopamine activity in the mesocorticolimbic system. Furthermore, dopamine receptors have been implicated in the expression of amphetamine CPP. Therefore, we hypothesized that alteration to dopamine receptor expression in the mesocorticolimbic system may be associated with to heightened amphetamine CPP of adult rats exposed to adolescence defeat. We measured D1 and D2 dopamine receptor protein content in the medial prefrontal cortex, nucleus accumbens (NAc), and dorsal striatum following either adolescent social defeat or foot-shock stress and then adult amphetamine CPP. In controls, amphetamine CPP training reduced D2 receptor protein content in the NAc core. However, this down-regulation of NAc core D2 receptors was blocked by exposure to social defeat but not foot-shock stress in adolescence. These results suggest social defeat stress in adolescence alters the manner in which later amphetamine exposure down-regulates D2 receptors. Furthermore, persistent alterations to adult D2 receptor expression and amphetamine responses may depend on the type of stress experienced in adolescence.
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