Background: Nerve growth factor (NGF) is known to induce inflammation and pain; however its role in opioid-induced tolerance has not been studied. This study investigated the effects of an anti-NGF neutralizing antibody on the development of tolerance following chronic morphine treatment in naïve rats.
Methods: Four groups of rats were used in this study; one treated with saline alone, one with 10 mg/kg of morphine, one with 10 μg of anti-NGF and the other with 10 mg/kg of morphine + 10 μg of anti-NGF, twice per day for 5 days. The route of treatment was subcutaneous (S.C.) for morphine and saline, and intraperitoneal (i.p.) for anti-NGF. Response to a noxious thermal stimulus during the course of drug treatment was assessed (Hargreaves' test). Further, the change in the NGF levels in the lumbar spinal cord was measured by ELISA.
Results: Our results showed that repeated administration of morphine produced an apparent tolerance which was significantly attenuated by co-administration of anti-NGF (P < 0.001). Additionally, the area under the curve (AUC) of the analgesic effect produced by the combination of morphine and anti-NGF was significantly (P < 0.001) greater than for saline controls and chronic morphine treated rats. Moreover, the level of NGF in the spinal cord of chronic morphine treated rats was significantly higher (P < 0.05) than in both the saline control group and the group receiving simultaneous administration of anti-NGF with morphine. These results indicate that anti-NGF has the potential to attenuate morphine-induced tolerance behavior by attenuating the effects of NGF at the spinal level.
Conclusion: Taken together, our study strongly suggests that the NGF signaling system is a potential novel target for treating opioid-induced tolerance.
Keywords: Anti-NGF; Hargreaves’ test; Morphine; Nerve growth factor; Tolerance.