Antinociceptive effects of neurotropin in a rat model of painful peripheral mononeuropathy

A non-protein extract isolated from the inflamed skin of rabbits inoculated with vaccinia virus (Neurotropin, NTP), has been clinically used in Japan for the treatment of chronic painful conditions such as low back pain, osteoarthritis, postherpetic neuralgia, subacute myelo-opticoneuropathy, and so on, Recent studies have shown its efficacy on patients with complex regional pain syndrome (CRPS). The chronic constriction injury (CCl) model described by Bennett and Xie has been thought to show similar painful conditions to those seen in CRPS patients. Thus, the antinociceptive effects of NTP were tested in CCl model. In rats with mechanical hyperalgesia 2 weeks after nerve injury, i.p, injection of NTP (100 Neurotropin Unit, NU/kg) produced an analgesic effect that lasted for at least 50 min. An analgesic effect lasting up to 30 min. was observed in rats with heat hyperalgesia 2 weeks after nerve injury. Seven daily i.p, injections (50 NU/kg) of NTP commencing 1 week after surgery produced an early recovery from heat hyperalgesia. Prior studies suggest that NTP produces analgesia by activation of a descending pain inhibitory system. Thus, our findings suggest the possibility that the dysfunction of the descending pain inhibitory system could be related to the hyperalgesia in the nerve injury model, and perhaps also in people who suffer from painful peripheral neuropathies.