Free-running rhythms of cocaine self-administration in rats held under constant lighting conditions

Using a discrete trials (DT) procedure, we have previously shown that rats exhibit variations in their pattern of cocaine self-administration relative to the time-of-day, often producing a daily rhythm of intake in which the majority of infusions occur during the dark phase of the 24h light-dark cycle. We have sought to determine if cocaine self-administration demonstrates free-running circadian characteristics under constant-lighting conditions in the absence of external environmental cues. Rats self-administering cocaine (1.5mgkginfusion) under a DT3 procedure (three trialsh) were kept in constant-dim (<2 lux, DIM) conditions, and the pattern of intake was analyzed for free-running behavior. We show that cocaine self-administration has a period length (τ) of 24.14±0.07h in standard 12h light:12h dark conditions, which is maintained for at least five days in constant-dim conditions. With longer duration DIM exposure, cocaine self-administration free-runs with a τ of approximately 24.92±0.16h. Exposure to constant-light conditions (1000 lux, LL) lengthened τ to 26.46±0.23h; this was accompanied by a significant decrease in total cocaine self-administered during each period. The pattern of cocaine self-administration, at the dose and availability used in this experiment, is circadian and is likely generated by an endogenous central oscillator. The DT procedure is therefore a useful model to examine the substrates underlying the relationship between circadian rhythms and cocaine intake.