Differential effects of d-amphetamine on vigilance in younger and older male rats.

After training to comparable levels of performance on a two-choice, discrete-trial vigilance task, younger (9 mo) and older (26 mo) male F344xBN rats were tested after SC injections of d-amphetamine (0.125, 0.25, 0.50, and 1.0 mg/kg). Relative to their saline treatment performance levels, both groups exhibited decreases in choice latencies under the lower doses of amphetamine and an increase in food retrieval latencies after 1.0 mg/kg amphetamine. The percentage of correct responses in the older animals was lower than in the younger animals at all doses of amphetamine, and the groups differed significantly at the 0.25 and 0.50 mg/kg doses. There were no significant differences between the groups in either of the latency measures at any of the doses of amphetamine. These results suggest, as has been demonstrated with cocaine, that the alertness-altering properties of amphetamine are qualitatively different in older and younger adult organisms.