The effect of morphine dependence on impulsive choice in rats

In the human opiate-dependent population, the most consistently reported deficit in executive functioning is impulsivity. Previous research has shown that acute and chronic exposure to drugs of abuse can increase impulsive choice; however, the extent to which opiate dependence contributes to increased impulsivity has not been examined. We report here the effects of morphine dependence on rats' delay discounting (DD) of a sucrose reward. We assigned rats randomly to either a dependent group that received a nightly 30 mg/kg subcutaneous dose of morphine or a morphine-naive group that received a nightly saline injection. DD of a sucrose reward was examined in rats prior to initiation of the dosing regimen, 22.5 h after the daily maintenance dose, and after a 14-day abstinence period. The groups did not differ at baseline, but rats showed accelerated DD while dependent on morphine. After withdrawal from morphine, DD in previously dependent rats was not significantly different from that of naive rats. Since dependent rats also showed reduced motivation to acquire the sucrose reinforcer, we performed a separate experiment to test whether such a decrease in motivation could cause an increase in impulsivity. We found that food-deprived rats switched to a free-feeding diet did not differ in DD from rats maintained at 85 % of free-feeding weight. An increase in impulsivity can result from physical dependence on morphine and cannot be attributed to changes in motivation to acquire sucrose-reinforced responses.