Circadian activity precedes daily methamphetamine injections in the rat

Scheduled daily injections of methamphetamine (MA) produced locomotor activity that preceded and followed the usual time of injection in rats housed under conditions of constant, moderately dim light and temporally distributed feeding. A circadian basis for pre-injection time activity was supported by its anticipatory timing in the apparent absence of reliable preceding external cues and by its persistence on a test day on which the rats remained undisturbed. Post-injection time locomotor activity also persisted on the test day, occurring from 24 to 29 h after the final MA injection. These results indicate that MA injections engage circadian processes underlying locomotor activity, and they raise the possibility that intake of drugs of abuse by humans may facilitate drug taking or relapse at times of day related to previous drug use.