Aging changes the effects of cocaine on vigilance task performance of rats

To investigate age-related behavioral changes with the administration of cocaine, young (6-10 mo), middle-aged (12-18 mo), young-old (21-24 mo), and old (25-36 mo) rats that had been trained on a 2-choice, discrete trial vigilance task were tested under different doses of cocaine. The young and middle-aged rats exhibited significantly increased accuracy and decreased choice latencies following 2.5 mg/kg cocaine. A 15.0 mg/kg dose increased variability in these measures and increased food retrieval latencies. The young-old and old rats exhibited no significant changes in accuracy or choice latency to 1.25, 2.5, and 5.0 mg/kg cocaine; however, a dose of 15.0 mg/kg significantly reduced accuracy in both these groups and increased choice latencies in the old animals. Also, doses of 5.0 and 15.0 mg/kg tended to increase food retrieval latencies more in the two older groups than in the two younger groups. These results indicate that, rather than a simple increase or decrease in sensitivity, there is a qualitative change in cocaine's behavioral effects as rats age. These findings may reconcile long-standing discrepancies in the literature regarding age-related changes in the behavioral effects of amphetamine, with actions and effects very similar to those of cocaine.