Age-dependent effects of repeated methamphetamine exposure on locomotor activity and attentional function in rats

Many adolescents use amphetamines which are the second most common abused illegal drugs. Methamphetamine (Meth), as a potent amphetamine affects attentional functions. However, the most significant factor for susceptibility to Meth is the age of exposure, most studies have examined the effects of Meth after early adolescence stage. The present experiment was aimed to investigate some possible short- and long-term effects of Meth at two distinct points of adolescence stage (early versus late) on 1) locomotor activity in adolescent rats and 2) attentional functions in their adulthood. Rats received Meth (5 mg/kg, i.p., for consecutive 10 days) during early adolescence (postnatal days (PND) 30-39) or late adolescence (PND 50-59). Locomotor activity was assessed after the first and tenth injections. Then, in adulthood, rats were trained and tested on the Five-choice serial reaction time task (5-CSRTT) to display possible attentional impairments. The first Meth administration in early exposed adolescent (EEA) group produced the highest level of activity, compared with the first exposure in late exposed adolescent (LEA) group and tenth administrations in both groups. In adulthood, LEA group significantly delayed learning the 5-CSRTT and exhibited attentional impairments, as demonstrated by significant reduced response accuracy and increased omission errors under pharmacological challenge, compared with control group. The susceptibility to Meth depends on the age of exposure and Meth administration during late adolescence stage may cause prolonged attentional deficits in adulthood.