COMPLETE RECOVERY BY NERVE GROWTH-FACTOR OF NEUROPEPTIDE CONTENT AND FUNCTION IN CAPSAICIN-IMPAIRED SENSORY NEURONS

In the present study the ability of nerve growth factor (NGF) to facil itate the recovery of peptidergic primary sensory C-fibers after an ac ute capsaicin treatment (50 mg/kg s.c.) was investigated in adult rats . NGF (4 mu g 1/day for 3 days) was injected into the plantar of one h ind paw starting 24 h after the capsaicin treatment. Without NGF, ther e was a significant reduction of calcitonin gene-related peptide (CGRP ) and substance P content of the paw skin and the sciatic nerve. CGRP and substance P levels were completely replenished in the NGF-treated paw skin and in the innervating sciatic nerve they even increased over control levels as determined 40 h after the last injection of NGF. CG RP levels also recovered in the contralateral paw and sciatic nerve, b ut no recovery was observed in other tissues such as the front paw, th e auricle, or the urinary bladder. Mustard oil-induced neurogenic plas ma extravasation, taken as a functional parameter for peptidergic prim ary sensory C-fibers, was significantly decreased after the capsaicin treatment and showed a complete recovery by NGF in the injected paw as well as in the contralateral paw skin. These results show that NGF no t only was able to reverse the decrease of transmitter content caused by capsaicin but also restored the peripheral function of primary affe rent neurons.