Inflammation with no axonal damage of the rat saphenous nerve trunk induces ectopic discharge and mechanosensitivity in myelinated axons.

Inflammation along a nerve trunk with no frank axonal nerve damage produced by complete Freund's adjuvant (CFA) or Carrageenan is known to induce a painful peripheral neuropathy. In the present study, we examined the electrophysiological properties of myelinated axons (spontaneous discharge and mechanical sensitivity) at the inflamed nerve site. The rat saphenous nerves were exposed at mid-thigh level and wrapped in 2 mm wide bands of haemostatic oxidized cellulose (Oxycel) that were saturated with undiluted CFA. In the control rats the Oxycel) was saturated with saline. At postoperative days (PODs) 2-5 and 6-10, fine axon bundles were teased from the nerve, and electrophysiological recordings performed. At both time points spontaneous activity at the site of the application in CFA rats (PODs 2-5=9.9+/-2.5%: PODs 6-10=6.1+/-1.4%) was significantly higher than in the control animals (PODs 2-5=2.9+/-1.1%: PODs 6-10=1.6+/-1.4%: P=0.03, P=0.02, respectively). Mechanical sensitivity at both time points was significantly higher in CFA rats (PODs 2-5=12.6+/-3.1%: PODs 6-10=10.3+/-3.1%) than in saline rats (PODs 2-5=3.4+/-2.91%: PODs 6-10=0.8+/-1.0%: P=0.03, P=0.04, respectively). This study clearly shows that perineural inflammation with no axonal nerve damage induced by CFA application around the nerve trunk elevates spontaneous activity and induces mechanosensitivity in myelinated axons.