P75, a low affinity neurotrophic receptor, is expressed during development, in only specific motoneuron groups, and following injury in adult animals. In previous studies we have demonstrated that p75 immunoreactivity (IR) is associated with specific projections in the most lateral portion of the intermediolateral (IML) nucleus, as well as in the dorsal horn and commissure in the normal adult rat lumbosacral spinal cord (Westerlund et al., SfN abstract 497.16, 2003). We also showed that p75 expression in the lumbosacral cord was not altered after a mid-thoracic transection of the spinal cord, suggesting that the input source is not from the descending tracts. Here, we address whether the p75 IR is from the dorsal afferent roots and whether those dorsal fibers are associated with calcitonin gene-related peptide (CGRP) and substance P fibers. We performed a unilateral dorsal rhizotomy of the L4-S2 roots on adult male Sprague-Dawley rats and allowed a survival time of 5 days postoperatively. We performed immu...