Mechanisms of Analgesic Action of Neurotropin on Chronic Pain in Adjuvant-Induced Arthritic Rat: Roles of Descending Noradrenergic and Serotonergic Systems

Neurotropin®, a non-protein extract from the inflamed skin of rabbits inoculated with vaccinia virus, has been clinically used as an analgesic drug for treatment of chronic pain. In this study, we investigated the analgesic mechanisms of Neurotropin in the adjuvant-induced arthritic rat, a chronic pain model with inflammation. Neurotropin caused dose-dependent inhibition of hyperalgesia in the adjuvant-induced arthritic rat after single intravenous (10 – 100 NU/kg) and oral (30 – 200 NU/kg) administration. The analgesic effect of Neurotropin (intravenous 100 NU/kg and oral 200 NU/kg) was significantly inhibited by intrathecal injections of the α2-adrenoceptor antagonist yohimbine (30 nmol/animal) and the selective 5-HT3 serotonin receptor antagonist MDL72222 (30 nmol/animal), and slightly inhibited by the non-selective serotonin receptor antagonist methysergide (100 nmol/animal). The results suggest that the analgesic action of Neurotropin is at least in part due to the enhancement of noradrenergic and serotonergic descending pain inhibitory pathways. Neurotropin may be useful for the clinical management of chronic pain diseases such as a rheumatoid arthritis and osteoarthritis. Keywords:: Neurotropin, adjuvant-induced arthritis, descending pain inhibitory pathway, serotonin receptor, α2-adrenoceptor