Differential Aspects of Natural and Morphine Reward-Related Behaviors in Conditioned Place Preference Paradigm

Natural rewards are essential for animal survival. On the other hand, drug-seeking behaviours can be maladaptive and endanger survival. The present study was conducted to enhance our understanding of how animals respond to food and morphine as natural and drug rewards, respectively, in a conditioned place preference (CPP) paradigm. We designed a protocol to induce food-CPP and compare it as a natural reward with morphine-CPP in rats. The protocol for reward induction in all groups (foods and morphine) consisted of three phases: pre-test, conditioning, and post-test. In morphine groups, we injected morphine as a reward (5 mg/kg, s.c.), and for inducing natural-reward, we used two different protocols, by one of the rats were deprived of food for 24h. In the other method, rats were restricted for food for 14 days. During the conditioning period, animals received daily chow, biscuit, or popcorn as a reward inducer. Results revealed that CPP did not induce in food-deprived rats. A combination of food restriction (as a facilitator) and a biscuit or popcorn induced reward using CPP. In contrast, food deprivation did not facilitate food-CPP in response to regular food. Interestingly the CPP score of the group which received biscuit during a 7-day conditioning period was more than that of the morphine group. In conclusion, food restriction could be a better protocol than food deprivation to facilitate food reward.