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The effect of acute morphine on delay discounting in dependent and non-dependent rats
Colin Harvey-Lewis1, Keith B J Franklin
1Department of Psychology, McGill University, Montreal, QC, H3A 1B1, Canada.
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.
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