The effect of acute morphine on delay discounting in dependent and non-dependent rats

Rationale Chronic opiate use is associated with increased impulsivity in both humans and animals, and previous studies suggest that acute morphine can increase impulsivity in non-dependent rats. However, the extent to which chronic opiate usage modulates the effect of acute morphine is unknown. Methods Rats were trained to delay discount 20 % sucrose solution and then randomly assigned to either a dependent group that received a nightly 30 mg/kg subcutaneous dose of morphine or a non-dependent group that received a nightly saline injection. Once dependence was established, rats were then assigned to one of four acute morphine doses (0, 1.25, 2.5, 5 mg/kg). For 5 days, delay discounting curves were determined 22.5 h after maintenance doses and 1 h after their prescribed acute injections. Results In non-dependent rats, 2.5 and 5 mg/kg doses of morphine caused decreased preference for the large reward at all delays. Acute morphine had no effect on discounting curves in dependent rats. Conclusions Morphine dependence can cause tolerance to the effects of acute morphine on delay discounting.