Introduction: Natural rewards are essential for survival. However, drug-seeking behaviors 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. Methods: 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 both 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, SC). To induce natural reward, we used two different protocols. In the first one, the rats were deprived of food for 24 h. In the other method, the rats were restricted to food for 14 days. During the conditioning period, the animals received daily chow, biscuits, or popcorn as a reward inducer. Results: Results revealed that CPP was not induced 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 biscuits during a 7-day conditioning period was more than that of the morphine group. Conclusion: In conclusion, food restriction could be a better protocol than food deprivation to facilitate food reward.

Drug abuse and dependence have progressively become a dramatic problem in human societies. Since the exact mechanism (s) of the effect of related drugs in the brain has not been explored, therefore, this problem has not been resolved yet. Several lines of evidence indicated that nitric oxide (NO) plays an important role in the process of drug dependence. The role of NO within the different regions of the brain has previously been reported in a condition of morphine dependence. The previous studies were carried out by means of conditioned place preference paradigm, a simple and useful method for evaluating the effect of drug abuse in an animal model. Therefore, the role of NO within hippocampus, central nucleus of amygdala, nucleus accumbens and ventral tegmental area was investigated in the process of drug dependence. For this purpose, L-arginine (0.3, 1, and 3 mg/rat), the NO precursor and L-NAME (0.3, 1, and 3 mg/rat), the NOS inhibitor were used as NOergic drugs, which were administered to male Wistar rats (250-300 g). Male rats (n=8/group) were anesthetized and bilaterally cannulated above the mentioned regions. Five days after the cannulation, animals were trained in an un-biased conditioned place preference apparatus for five consecutive days. The NOergic drugs were injected into the animals in two ways: first; the animals were trained with morphine and received L-arginine or L-NAME at 5th day of experiments just before the test. Second group received L-arginine or L-NAME before morphine injection. At the 5th day of the experiments, each animal was placed in the apparatus and its behavior was recorded for 10 min. The results showed that NO plays a crucial role in the above-mentioned regions and the role of NO in morphine dependence should be considered in treatment of morphine addicts.