The effect of anti-NGF and NGF on the responses to noxious heat in mice treated with capsaicin in the neonatal period

Unexpectedly, despite the profound loss of C-fibers in the dorsal roots of mice injected capsaicin on day 2 of life, they showed normal responses to nociceptive heat. To examine the cause of this conflicting phenomenon, we studied the effect of anti-NGF and NGF on the response to noxious heat in mice pretreated with capsaicin. Capsaicin (50 mg/kg) was injected on the 2nd day of life, and 5 days later, anti-NGF (3 μl/g) was daily injected for 30 days. Capsaicin was also injected on day 10 of life, and NGF (3 μl/g) was daily injected for 30 days from 5 days after treatment. From 10 days after capsaicin treatment, the mice were stimulated by radiant heat (Hargreaves' method) every 10 days up to 60 days and every 20 days from 60 to 80 days. The same test was undertaken in age-matched mice. Withdrawal latencies of the mice treated with both capsaicin and anti-NGF were significantly retarded compared with those of the animals treated with only capsaicin. Thus, it was suggested that the treatment of capsaicin on day 2 induces sprouting from the remaining primary sensory neurons. On the other hand, the latencies of the mice treated with both capsaicin and NGF were short compared with those of the animals treated with only capsaicin. The results may be attributed to the reinforcement of the sprouting from remaining neurons. However, we must not neglect the effect of individual treatment with NGF or anti-NGF.