Compensatory Role of Insulin in the Extinction but Not Reinstatement of Morphine-Induced Conditioned Place Preference in the Streptozotocin-Induced Diabetic Rats.

Insulin receptors are distributed in the whole brain, including different parts of the reward circuit that modulate dopamine as the primary neurotransmitter implicated in addiction. The goal of the current study was to illuminate the role of insulin in the extinction period and reinstatement of morphine-induced conditioned place preference (CPP) in the naïve and diabetic rats. One hundred and twelve male rats were randomly divided into two naïve and diabetic groups. Diabetes was induced by one dose administration of streptozotocin (STZ; 60 mg/kg; IP) ten days before the conditioning procedure. To evaluate the insulin's role in the duration of extinction period of morphine-CPP, naïve and diabetic rats received insulin (10 U/kg; IP) before each morphine injection (5 mg/kg; sc) during the 3-day conditioning phase. All rats that passed the conditioning phase and then underwent the extinction period. Morphine priming-induced reinstatement was determined in both naïve and diabetic rats by injection of different ineffective doses of morphine (0.5 and 1 mg/kg; sc) in extinguished rats. In the following experiments, three groups of diabetic rats received insulin during the conditioning, expression, or reinstatement phase to illustrate insulin's effect on the morphine-induced reinstatement and the duration of the extinction period (insulin was only treated during the acquisition phase). The results showed that the extinction period and reinstatement of morphine were potentiated in the STZ-induced diabetic rats. The obtained findings also revealed that insulin replacement shortened the extinction period of morphine-induced CPP in STZ-diabetic rats. However, insulin replacements in conditioning, expression, and reinstatement phases did not affect morphine priming-induced reinstatement in diabetic animals.