The effects of methamphetamine on fine motor control in rhesus monkeys

Six rhesus monkeys were trained to extend their arms through a tube to press a lever with between 25 and 40 g of force for 3 or 5 sec. Responding was maintained by the delivery of 1.5 cc of water. Stimulus lights indicated whether the exerted force was below 25 g. between 25 and 40 g (i.e., correct) or above 40 g. Sessions were terminated after 50 water deliveries or 30 min had elapsed. Performance was well-maintained in all monkeys. Allowing the animals access to water prior to the session had no effect on performance. Discontinuing sessions for two weeks disrupted some aspects of performance but responding improved within 5 sessions. Single injections of methamphetamine (0.06-0.5 mg/kg) were given IM 20 min prior to the session. The highest dose of 0.5 mg/kg totally eliminated responding. Lower doses decreased rate of responding somewhat and increased phasic activity (i.e., tremors) in a dose-dependent manner. The procedure seems ideally suited for investigating the effects of psychotropic drugs on fine motor control in rhesus monkeys.