Spontaneous activity of axotomized afferent neurons after L5 spinal nerve injury in rats

After mechanical injury of a peripheral nerve some axotomized afferent neur ons develop spontaneous activity, which is thought to trigger abnormal pain behavior in rats and neuropathic pain in humans. Here, we analysed the ect opic activity in axotomized afferent fibers recorded from the L5 dorsal roo t 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 hour s after spinal nerve transection there was almost no spontaneous activity i n axotomized afferents, except short-lasting injury discharges st 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) Twent y to 53 days after nerve lesion the rate of spontaneous activity had decrea sed significantly to 3.4 +/- 4.3 imp/s (median 2.6 imp/s, n = 120). The fre quency of bursting and non-bursting neurons remained roughly the same; (4) In sympathectomized rats, 15-45 days following spinal nerve lesion, the mea n 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 efferent A-fibers but not in afferent C-fibers. These results suggest that ectopic activity in ax otomized afferent neurons develops within the first days after L5 spinal ne rve lesion, decreases with time and is only marginally dependent on the sym pathetic innervation. There was a positive correlation between this ectopic activity and the allodynia-like behavior in spinal nerve-lesioned rats. (C ) 2000 International Association for the Study of Pain. Published by Elsevi er Science B.V.