WALKING EVOKES A DISTINCTIVE PATTERN OF FOS-LIKE IMMUNOREACTIVITY IN THE CAUDAL BRAIN-STEM AND SPINAL-CORD OF THE RAT

We have evaluated the pattern of c-fos expression induced in the rat s pinal cord, caudal brainstem and cerebellum by a behavior that is asso ciated with non-noxious inputs transmitted over large-diameter primary afferent fibers, namely walking for Ih on a rotating rod. Walking on the rotating rod induced a large increase in the number of Fos-like im munoreactive neurons in regions of the cervical and lumbar spinal cord gray matter that contain neurons that respond to non-noxious stimuli: the inner part of the substantia gelatinosa (lamina IIi), the nucleus proprius and the medial parts of laminae V and VI. We also observed c onsiderable labeling in lamina VII and in ventral horn motoneurons. We did not record an increased number of Fos-like immunoreactive neurons in lamina I, in the outer substantia gelatinosa (lamina IIo), or in t he lateral, reticulated portion of lamina V, regions that contain neur ons predominantly responsive to noxious stimulation. Unilateral sensor y deafferentation of the forelimb, by multiple dorsal rhizotomies, sig nificantly decreased the number of Fos-like immunoreactive neurons in the ipsilateral spinal cord, suggesting that afferent input contribute d to the walking-induced pattern of labeling. In rats that walked on t he Rota-Rod, we also recorded increased labeling in the dorsal column nuclei. Unilateral cervical deafferentation reduced the labeling in th e cuneate nucleus; this reduction was paralleled by decreased cytochro me oxidase activity. Finally, we found that there was a significant in crease in the number of Fos-like immunoreactive neurons in the cerebel lum of rats that walked on the Rota-Rod. Northern blot analysis reveal ed that the increase in Fos-like immunoreactivity was associated with an increase in c-fos messenger RNA. The pattern of labeling observed i n the rats that walked on the Rota-Rod was distinct from that observed when rats are exposed to a noxious stimulus [Presley et al. (1990) J. Neurosci. 10, 323-335]. This result reinforces the conclusion that by monitoring the evoked expression of the c-fos proto-oncogene, it is p ossible to identify unique populations of neurons that are specificall y related to the modality of the stimulus or to behaviour occurring du ring the stimulus presentation.