In the present experiment, under drinking of 0.01% methamphetamine (MA), young rats exhibited free-running locomotor rhythm ranging from 24.6 to 30.9 h period under light-dark cycle. In contrast, 24-month-old rats showed a low activity and did not exhibit MA-induced free-running rhythm. When MA was injected at a fixed time of day (14:00), young rats exhibited intense locomotor activity from 1-2 h before drug-injection time. This preinjection activity was still detected on the withdrawal day. In addition, intense locomotor activity was observed for 12:00-17:00, even on the withdrawal day (MA-associated anticipatory activity). Thus, MA-associated activity of the withdrawal day became an another good index for MA anticipatory activity rhythms whereas old rats did not exhibit this anticipatory activity. The present study suggests that aging strongly impairs the manifestation of MA-induced free-running and anticipatory activity rhythms in rats.
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