Are there long-term changes in the basal or evoked Fos expression in the dorsal horn of the spinal cord of the mononeuropathic rat?

The long-term changes in Fos like-immunoreactivity (Fos-LI) in the dorsal h orn of the spinal cord following various peripheral nerve lesions remain co ntroversial. This study considers such an approach with chronic constrictio n injury rats (CCI: loose ligations of the sciatic nerve), at 2 weeks after the surgery, when changes in spontaneous and evoked behaviour were clearly described. All rats used for Fos studies displayed allodynia to mechanical stimulation (decrease of 32% of the vocalization threshold to paw pressure ). In CCI rats, which displayed 'spontaneous pain-related behaviour', the n umber of Fos-LI neurones, in the absence of any intentional stimulation, wa s very low and comparable with that observed in normal and sham-operated ra ts (<10 neurones/40 mu m section). Thus, in this model, the expression of F os protein is not a reliable index of spontaneous pain. Surprisingly, despi te the fact that in this model numerous anatomical studies described a dram atic loss of large and unmyelinated primary afferent fibers, we were unable to detect changes in the number and distribution of Fos-LI evoked by vario us modalities of peripheral noxious stimulation (noxious thermal stimuli, n oxious mechanical stimuli and carrageenin induced inflammation). For exampl e, the stimulus-response curves for the number of Fos-LI neurones evoked by a series of heat stimuli (40, 45, 48, 52, 55 degrees C) were almost superi mposable for CCI, sham-operated and normal rats. In contrast, seeking of th e nerve-injured paw induced a significant expression of Fos-LI in the super ficial laminae (I-II) of the dorsal horn of CCI rats (19.5 +/- 3/sections, P = 0.027) which was greater than that observed in sham-operated (6.5 +/- 3 /sections) or in normal rats (3.5 +/- 2/section). These modifications may r eflect mechanical allodynia observed in behavioural studies and could be re lated to AP fibers, which are known to be severely affected after the const riction of the nerve. These results suggest that this approach could be use ful to study, at the cellular level, in freely moving rats, some pharmacolo gical aspects of neuropathic pain. (C) 1999 International Association for t he Study of Pain. Published by Elsevier Science B.V.