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 light cycle. We have sought to determine if cocaine self-administration demonstrates free-running circadian characteristics when held under constant lighting conditions in the absence of external environmental cues. Rats self-administering cocaine (1.5 mg/kg/infusion) under a DT3 procedure (three trials per hour) were kept in constant dim (<5lux, DIM) conditions and the pattern of intake analyzed for free-running behavior. We show that cocaine self-administration has a period length (TAU) of 24.13 ± 0.07 hours in standard 12-hr light-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 TAU of approximately 24.92 ± 0.16 hours. Exposure to constant light conditions (1000lux, LL) lengthened TAU to 26.46 ± 0.23 hours; this was accompanied by a significant decrease in total cocaine obtained 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