Long T methamphetamine schedules produce circadian ensuing drug activity in rats

Eight female Sprague-Dawley rats were housed in isolated continuous 24-h/da y environments under conditions of constant dim light and a rate-limited fe eding schedule. Following 2 months of free-running activity, all animals we re administered methamphetamine (MA) i.p. (2 mg/kg) once every 31 h for 24 injection cycles. Average wave forms of wheel-running activity showed that animals did not anticipate the 31-h schedule of MA injections, but rather d isplayed circadian ensuing drug activity (CEDA) between 24 and 28 h followi ng the injections. Post-injection meals failed to meet reliably the thresho ld necessary to achieve food-engendered anticipatory or ensuing activity. C osinor analysis showed that the intensity of CEDA was strongly influenced b y the relative phase of the free-running rhythm. CEDA was moderately influe nced by the size of the post-injection bout of activity. Because injection times rotated daily throughout local time without repeating a time of day, CEDA resulting from a long T schedule of MA administration appeared to be b ased on one-trial resetting of a circadian-related mechanism by a major dru g of abuse. (C) 2000 Elsevier Science Inc. All rights reserved.