Nerve growth factor (NGF) sensitizes nociceptors and induces hyperalgesia by peripheral and central mechanisms. A major role for the induction of spinal sensitization and secondary hyperalgesia play the mechano-insensitive nociceptors which characteristically exhibit a large activity-dependent slowing of conduction velocity. NGF modulates nociceptors expression of sodium channels, including the axonal Nav1.7, and is therefore expected to influence their axonal excitability. We demonstrated that pig and human C-fibres have similar axonal properties. Here, we investigated subacute effects of NGF on the axonal excitability of porcine C-fibres.

Animals were assigned into 4 groups. Two groups received NGF (2mg, respectively 8mg). One group received saline and another group was untreated. Injections were administered intradermally and bilaterally at five standardized sites along the antero-medial aspect of the hindlimbs (100ml/limb). In vivo extracellular recordings from the saphenous nerves were performed 4 to 7 days later.

Single units were characterized based on mechanical response and latency changes upon repetitive electrical stimulation. Two protocols were employed: 1) low-frequency stimulation, with gradual increases in frequency: 0.128–0.25–0.5 Hz; 2) high-frequency stimulation, at 2Hz for 3min. According to criteria previously established, units were classified as: mechano-sensitive afferents (CM, cutoff 150mN); mechano-insensitive afferents (CMi; unresponsive to 150mN); sympathetic efferents (SYMP, mechano-insensitive and characteristic reversal of the latency changes). Saline did not significantly alter the amount of slowing in any fibre type. Therefore, similar fibres from saline-treated and untreated pigs were pooled as "control". NGF reduced the conduction slowing specifically in CMi fibres. Effects were dose- and time-dependent. Thus, total slowing during the low-frequency stimulation decreased in CMi from 5.6 1% (control; n= 22) to 4.7 0.8% (NGF 2mg; n= 10) and 3.3 1% (NGF 8mg; n= 10). Similar results were obtained after 2Hz stimulation: 30.7 4.5% (control CMi; n= 24) vs. 23.7 3.5% (NGF 2mg; n= 11), respectively 20.2 6.3% (NGF 8mg; n= 9). NGF-induced decrease in slowing progressed over time, such as at 7 days after NGF administration, CMi slowed similarly to the CM fibres.

No significant dose- or time-dependent axonal excitability changes in the CM and SYMP fibres were detected upon NGF treatment. However, mechanical thresholds significantly decreased in the polymodal nociceptors from a median of 100mN (control; n= 17) to 60mN (NGF 8mg; n= 10).

In conclusion, our data provide direct evidence that NGF not only sensitizes receptive nerve endings of nociceptors, but also induces axonal hyperexcitability of the mechano-insensitive C-nociceptors, thereby perhaps contributing to the initiation and maintenance of hyperalgesic states.