Methamphetamine (MAP) was administered to rats through drinking water repeatedly (three sessions, one session:administration for 60 days followed by withdrawal of 30 days) in order to examine whether or not MAP-induced disorganization of daily activity rhythm is sensitized. Each session (60 days) was divided into six blocks of 10 days. In the 1st session, daily locomotor activity rhythm of rats became disorganized around at 40 days (4th block) after the start of MAP drinking. However, MAP-induced disorganization of daily activity rhythm appeared at 20 days (2nd block) in the 2nd session and at 10 days (1st block) in the 3rd session following re-start of MAP drinking. On the other hand, the amount of MAP intake was decreased on the 2nd and 3rd sessions as compared with the 1st session. These results indicate that the mechanism of MAP-induced disorganization of daily activity rhythm may involve sensitization. Copyright (C) 1999 Elsevier Science B.V.
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