Effects of acute and chronic morphine on delay discounting in pigeons

When reinforcers of different magnitudes are concurrently available, choice is greater for a large reinforcer; that choice can be reduced by delaying its delivery, a phenomenon called delay discounting and represented graphically by a delay curve in which choice is plotted as a function of delay to the large reinforcer. Morphine, administered acutely, can alter responding for large, delayed reinforcers. In this study, the impact of morphine tolerance, dependence and withdrawal on choice of delayed reinforcers was examined in six pigeons responding to receive a small amount of food delivered immediately or a larger amount delivered immediately or after delays that increased within sessions. Acutely, morphine decreased responding for the large reinforcer, and the effect was greater when morphine was administered immediately, rather than 6 hr, before sessions. During 8 weeks of daily administration, morphine produced differential effects across pigeons, shifting the delay curve downward in some and upward in others. In all pigeons, tolerance developed to the response-rate-decreasing effects of morphine but not to its effects on delay discounting. When chronic morphine treatment was discontinued, rate of responding decreased in four pigeons, indicating the emergence of withdrawal; choice of the large reinforcer increased, regardless of delay, in all pigeons, an effect that persisted for weeks. These data suggest that chronic morphine administration has long-lasting effects on choice behavior, which might impact vulnerability to relapse in opioid abusers.