Age and sex differences in impulsive action in rats: the role of dopamine and glutamate.

Although impulsive behavior is considered to peak during adolescence, relatively little empirical work has examined this issue. Sex differences in impulsivity across development are also poorly understood. We examined age and sex differences in impulsive action with a simplified version of the 5-choice serial reaction time test, using only 2 choices. Adolescent and adult male and female Sprague-Dawley rats were trained to respond to one of two possible brief light stimuli to receive a food reinforcer. Responding before the onset of the light stimulus (a premature response) was considered a measure of impulsive action. We also investigated age and sex differences in the impulsivity-inducing effects of drugs that (1) target neurotransmitter systems still developing during adolescence and (2) increase premature responding in adult rats. To this end, we examined the effects of increasing the inter-trial interval (ITI; 9s) and amphetamine (dopamine releaser; 0, 0.25, 0.5mg/kg; Experiment 1), Ro 63-1908 (glutamate NMDA receptor NR2B subunit antagonist; 0, 0.3, 1.0mg/kg; Experiment 2 & 3), and nicotine (nicotinic receptor agonist and indirect dopamine releaser; 0, 0.15, 0.3mg/kg; Experiment 3) on premature responding. Adolescent rats were more impulsive than adults. In response to a long ITI and amphetamine, adolescent males and adult females also made more premature responses compared to adult males. No consistent age or sex differences were observed for Ro 63-1908 or nicotine. These findings suggest that impulsive action is heightened in adolescents compared to adults. Further, age and sex differences in impulsive action may be mediated by dopamine.