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 24 h 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 (1.5 mg/kg/infusion) under a DT3 procedure (three trials/h) 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 (tau) of 24.14 +/- 0.07 h in standard 12 h light:12 h 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 h. Exposure to constant-light conditions (1000 lux, LL) lengthened tau to 26.46 +/- 0.23 h; 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.
Department of Physiology and Pharmacology, Wake Forest University Health Sciences, Winston-Salem, North Carolina 27101, USA. cbass@wfubmc.edu
Caroline E
CE
Bass
Heiko T
HT
Jansen
David C S
DC
Roberts
D000818:Animals; D001522:Behavior, Animal; D001683:Biological Clocks; D002940:Circadian Rhythm; D003042:Cocaine / Q000008:administration & dosage*; D008027:Light; D008029:Lighting / Q000379:methods*; D008297:Male; D017440:Photoperiod; D051381:Rats; D017207:Rats, Sprague-Dawley; D012420:Running; D012646:Self Administration / Q000379:methods
D003042:Cocaine