Nerve growth factor selectively decreases activity-dependent conduction slowing in mechano-insensitive C-nociceptors

Nerve growth factor (NGF) induces acute sensitization of nociceptive sensory endings and long-lasting hyperalgesia. NGF modulation of sodium channel expression might contribute to neurotrophin-induced hyperalgesia. Here, we investigated NGF-evoked changes of the activity-dependent slowing of conduction in porcine C-fibers. Animals received intradermal injections of NGF (2 mu g or 8 mu g) or saline in both hind limbs. Extracellular recordings from the saphenous nerves were performed 1 week later. Based on sensory thresholds and electrically induced activity-dependent slowing (ADS) of axonal conduction, C-fibers were classified as mechano-sensitive afferents, mechano-insensitive afferents, cold nociceptors, and sympathetic efferents. NGF (2 mu g) increased conduction velocity in C-fibers from 1.0 +/- 0.05 m/s to 1.2 +/- 0.07 m/s. In mechano-insensitive afferents, NGF (8 mu g) reduced activity-dependent slowing of conduction, from 5.3 +/- 0.2% to 3.2 +/- 0.5% (0.125-0.5 Hz stimulation) and from 28.5 +/- 1.3% to 20.9 +/- 1.9% (2 Hz stimulation), such that ADS no longer differentiated between mechano-sensitive and mechano-insensitive fibers. Accordingly, the number of fibers with pronounced ADS decreased but more units with pronounced ADS were mechano-sensitive. Spontaneously active C-fibers were increased above the control level (1%) by NGF 8 mu g (8%). The results demonstrate that NGF changes the functional axonal characteristics of mechano-insensitive C-fibers and enhances spontaneous activity thereby possibly contributing to hyperalgesia. (C) 2011 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.