Synthetic double-stranded RNA polyinosinic-polycytidylic acid augments morphine-induced conditioned place preference in rats.

Three experiments were performed to study the effects of immune challenge on the rewarding properties of opiates. Intraperitoneal injection of polyinosinic-polycytidylic acid (Poly I:C, 1 mg/kg) was used to trigger an immune challenge. Conditioned place preference (CPP) in rats trained with alternating subcutaneous injections of morphine (5 mg/kg) and saline was used to assess the rewarding effect of morphine. Poly I:C administered before CPP training had no effects on CPP acquisition. Poly I:C administered during CPP training enhanced CPP acquisition. Poly I:C administered after morphine-induced CPP acquisition retarded CPP extinction. These results show that the immune challenge by Poly I:C augmented morphine CPP in rats depending on the onset time of the challenge. The findings suggest that immune challenge may enhance the rewarding properties of opiates.