Adolescent morphine exposure increases nociceptive behaviors in rat model of formalin test

The number of adolescents who use illicit drugs has increased dramatically. Adolescence is a critical period for brain development and maturation. The importance of the study of pain perception and the possible mechanisms involved is crystal clear. Up until now, there has been no evidence regarding the long-term effect of adolescence morphine administration on pain perception. The objective of the present study was to investigate long-lasting effect of adolescent morphine exposure on pain perception as well as analgesic response to a single dose of morphine injection. Adolescent and adult rats received morphine or saline, and then after 30 days of washout period, formalin test was performed. To evaluate morphine analgesia, in a separate group of animals, formalin test was performed after injection of a single dose of morphine during adulthood. The results demonstrated that the adolescent rats treated by morphine exhibited higher pain-related behaviors compared to the control group, while the same results were not observed in adult rats that had been treated by morphine. Moreover, there was no significant difference in analgesic response to a single dose of morphine between two experimental groups. This study demonstrates enduring effect of morphine exposure during adolescence on pain perception.