After mechanical injury of a peripheral nerve some axotomized afferent neurons develop spontaneous activity, which is thought to trigger abnormal pain behavior in rats and neuropathic pain in humans. Here, we analysed the ectopic activity in axotomized afferent fibers recorded from the L5 dorsal root in different time periods after L5 spinal nerve lesion and the effects of sympathectomy on it. The following results were obtained: (1) Up to 6 hours after spinal nerve transection there was almost no spontaneous activity in axotomized afferents, except short-lasting injury discharges at the time of transection; (2) Three to 8 days following spinal nerve lesion, the rate of spontaneous activity was 7.3+/-7.7 imp/s (mean+/-SD, median 5.0 imp/s, n=204); 41.6% of the spontaneously active afferent neurons exhibited a bursting pattern with interspike intervals of 32.4+/-18.3 ms; (3) Twenty to 53 days after nerve lesion the rate of spontaneous activity had decreased significantly to 3.4+/-4.3 imp/s (median 2.6 imp/s, n=120). The frequency of bursting and non-bursting neurons remained roughly the same; (4) In sympathectomized rats, 15-45 days following spinal nerve lesion, the mean discharge rate was 3.8+/-4.3 imp/s (median 2. 3 imp/s, n=255). However, the percentage of bursting neurons and the intraburst frequency decreased significantly; (5) Spontaneous activity occurred in afferent A-fibers but not in afferent C-fibers. These results suggest that ectopic activity in axotomized afferent neurons develops within the first days after L5 spinal nerve lesion, decreases with time and is only marginally dependent on the sympathetic innervation. There was a positive correlation between this ectopic activity and the allodynia-like behavior in spinal nerve-lesioned rats.

1. Spontaneous activity and ectopic mechanical excitability of axotomized unmyelinated and myelinated fibers in the sural nerve were examined in anesthetized rats. The analysis was performed within 30 h after the nerve lesion using single-fiber recordings that were performed proximal to the severed nerve end. 2. Among all unmyelinated fibers tested (n = 865), 4-8% exhibited persistent spontaneous activity of low and irregular frequency. The percentage of spontaneously active C fibers did not change significantly during the first 30 h. Only 6 of 796 A fibers had spontaneous activity. 3. Mechanical stimulation of the cut nerve end excited 5-8% of all C fibers under investigation. No development with time could be detected in the frequency of mechanically excitable C fibers. In contrast, beginning 6 h after nerve transection, the number of mechanically excitable A fibers rose with time, reaching 27% after 22-30 h. 4. Among the A fibers (C fibers) that exhibited mechanical excitability or spontaneous activity, only 4% (25%) had both properties, whereas 96% (75%) were either mechanosensitive or spontaneously active. 5. With time after the nerve lesion, the mean discharge rate of all spontaneously discharging C fibers decreased significantly from 49 imp/min (0.5-9 h after nerve lesion) to 11 imp/min after 22-30 h. The mean discharge rate of C fibers exhibiting solely spontaneous activity and those C fibers that were additionally mechanosensitive did not differ significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Many axotomized myelinated as well as unmyelinated cutaneous nerve fibers are sensitive to mechanical stimuli applied to the cut nerve end within a few hours after nerve lesion. Here we investigated the influence of inflammatory mediators on this ectopic mechanosensitivity after cutting and ligating the sural nerve in anesthetized rats. Neural activity was recorded from single axons in filaments teased from the sural or sciatic nerve proximally to the lesion site 2-33 hr after axotomy. Using calibrated von Frey hairs (1.0-128.5 mN), 30 sec trains of phasic stimuli were applied to the cut nerve end immediately before and after local application of a mixture of inflammatory mediators [inflammatory soup (IS), consisting of bradykinin, 5-HT, prostaglandin E2, histamine (all 10 microM), and K+ 7 mM, pH 7.0] for 2 min. Before as well as after IS application, von Frey thresholds were significantly lower in myelinated (A) fibers than in unmyelinated (C) fibers. IS application enhanced the ectopic mechanical excitability, as expressed in reduced von Frey thresholds and increased response magnitudes, of most severed mechanosensitive C fibers (77%) and some mechanosensitive A fibers (46%). The sensitization lasted for 10-40 min after a 2 min IS application. Additionally, among axotomized nerve fibers unresponsive to probing of the nerve lesion site before IS application, 1 of 63 (1.6%) A and 3 of 106 (2.8%) C fibers became mechanosensitive immediately after IS application. The results indicate that after axotomy, inflammatory processes augment touch-evoked ectopic activity in lesioned sensory nerve fibers. Because many affected afferents are presumably of nociceptive function, their enhanced neural barrage may contribute to neuropathic pain states.

The direct consequences of a peripheral nerve injury at the lesion site itself are often twofold: axons of afferent (and efferent) nerve fibres are transected and the tissue surrounding the nerve injury site is inflamed. Recent studies have shown that a few hours after nerve transection, axotomised myelinated (A) and unmyelinated (C) afferents may respond to mechanical and thermal stimuli applied to the cut nerve end. Here, 5-24 h after sural nerve ligation and transection we studied the ectopic excitability of axotomised cutaneous A and C fibres by chemical agents, most of which excite afferent terminals in skin. Topical application of bradykinin (BK; 10(-4) M) to the nerve stump excited 7.3% of all C fibres tested. Application of prostaglandin E2 (PGE2), a solution with increased proton concentration (pH 6.0) or a combination of inflammatory mediators ('inflammatory soup', containing histamine, 5-HT, BK and PGE2 (all 10(-5) M) at a pH of 7.0) activated 2.7-4.3% of all C fibres tested. Hypertonic saline solution (HS; 4.5%) and capsaicin, painful irritants, excited 8.3% and 5.0% of the C fibres, respectively. Among the axotomised A fibres tested, between 0.8% and 1.7% were excited by BK, PGE2, inflammatory soup (IS) or HS. Capsaicin and acid pH did not excite cut A fibres. In total, the number of chemically excited C fibres (50/547) significantly exceeded the number of activated A fibres (10/469). Local norepinephrine application (0.5-2.4.10(-3) M) did not activate A or C fibres (234 and 224 fibres tested, respectively). The results indicate that already during the first hours after transection of a peripheral skin nerve a significant proportion of axotomised afferents can be excited by topical chemical stimulation. This evoked activity is preferentially found in unmyelinated fibres, many of which have nociceptive functions. Chemically evoked discharges as described in the present study may therefore contribute to the induction of pain and paraesthesias in patients with peripheral nerve lesion when the injury site is inflamed.