C-fos Induction in the Spinal Cord after Peripheral Nerve Lesion.

Immunocytochemical localization of a product of the proto-oncogene c-fos, Fos protein, was used to map the activity of a subset of rat spinal neurons at 3 days, 3 weeks and 3 months following section of the sciatic nerve. In a well-established experimental paradigm, the gene was induced by activation of primary afferent fibres with brief noxious sensory stimulation under anaesthetic. Central sciatic projections were demonstrated with isolectin B4 counterstain and GAP-43 immunocytochemistry. In Rexed's lamina II of the spinal cord, in which there is somatotopic organization of afferent terminals, Fos-positive neurons were largely restricted to the projection area of intact peripheral nerves. Three days after a sciatic nerve lesion, the number of Fos-positive neurons in a cord region innervated by the saphenous nerve was similar to control levels, but was markedly increased by 3 weeks, remaining elevated at 3 months. Three weeks after sciatic nerve section the lectin stain in the area of sciatic representation had almost completely disappeared, and conversely GAP-43 staining had greatly intensified. There was no evidence of invasion by Fos-immunoreactive cells of the area of sciatic representation. After 3 months both the size and the intensity of the lectin gap, and of the corresponding area of increased GAP-43 immunoreactivity, appeared reduced. Thus a peripheral nerve lesion was followed by a delayed increase in excitability of the spinal cord as assessed by c-fos expression, so that greater numbers of second-order neurons were activated by sensory stimulation of an adjacent intact nerve. These changes may be related to the sensory abnormalities which follow nerve damage.