Abstract

Behavioral sensitization refers to the increase of the behavioral activating effects of a drug following its repeated administration. Here I examined the effects of varying the length of the daily cycle on the expression of sensitization to cocaine. Rats were entrained to light cycles consisting of a variable dark phase and a fixed (0.5h), light phase (i.e., T cycles). One group was placed on a 24-h T cycle (T24) and another was placed on a 26-h T cycle (T26). Each group received cocaine (10 mg/kg) or saline for five days, at the same local time daily. Thus, injections were given at the same circadian time (CT) in the T24 but not the T26 group. Only the T24 group expressed sensitization. To assess whether injections given at varying CTs blocked sensitization in the T26 group, in a subsequent experiment the timing of the injections during pretreatment was delayed each day by two hours. Thus, the T26 but not the T24 group received injections at the same CT. Once again, the T26 group failed to express sensitization whereas the T24 group showed robust sensitization. In a third experiment, all rats were pretreated under a T26; following pretreatment half were switched to T24. Only animals switched to T24 expressed sensitization. Thus, entrainment to a long cycle prevents the expression rather than the induction of sensitization. These results open up a previously under-appreciated perspective on the influence that temporal features of the environment have on the behavioral effects of a drug.