Age-dependent induction and maintenance of sensitization to methamphetamine-induced hyperactivity in mice

Repeated administrations of methamphetamine (2 mg/kg, s.c.), 10 times at 3-day intervals, induced ambulatory sensitization in all groups of mice that were 13-, 15-, 19-, 23- and 36-weeks-old at the start of methamphetamine administration. The most prominent sensitization was observed in the 19-week-old mice. Among five groups of mice, even though the mice of 36 weeks old showed the highest sensitivity to methamphetamine at the first administration, they exhibited the lowest sensitization during the latter stage of repeated methamphetamine administration. Methamphetamine sensitization once established was well reproduced by the post-sensitization period of 8 weeks. Furthermore, the group of mice given methamphetamine with post-sensitization interval of 8 weeks (19-week-old mice) exhibited further enhancement of the sensitization. In contrast, the groups of mice given methamphetamine with post-sensitization intervals of 12 and 25 weeks (the 23- and 36-week-old, respectively) showed a significant reduced sensitization, and the latter group failed to reach the level of sensitization previously established. These results suggest that the induction of and maintenance of methamphetamine sensitization are dependent on the age of the mice, and that methamphetamine sensitization once established completely persists for up to 8 weeks.