Artemin, a member of the glial-derived neurotrophic factor family, promotes robust regeneration of sensory axons after dorsal root crush. We report here that several classes of sensory axons regenerate to topographically appropriate regions of the dorsal horn with artemin treatment. Projections of regenerated muscle and cutaneous myelinated sensory afferents are restricted to the correct spinal segments and to appropriate regions within spinal gray matter. Regenerated unmyelinated axons expressing calcitonin gene-related peptide project only to superficial laminae of the dorsal horn, where uninjured nociceptive afferents project normally. In contrast, intraventricular infusion of a soluble form of the Nogo receptor that blocks the action of several myelin-associated inhibitory proteins promotes relatively unrestricted regeneration of sensory axons throughout the dorsal white and gray matter of the spinal cord. These results demonstrate that cues capable of guiding regenerating axons to appropriate spinal targets persist in the adult mammalian cord, but only some methods of stimulating regeneration allow the use of these cues by growing axons.artemin | central nervous system regeneration | dorsal root | soluble Nogo receptor peptide | specificity G rowth of damaged axons in the adult spinal cord is inhibited by the presence of myelin-associated proteins, up-regulation of proteoglycan expression, and the absence of appropriate growth factors. Several agents that can partially overcome this inhibition permit some regeneration of spinal axons in contusion or transaction models of spinal cord injury (SCI) (1-4); however, the limited regeneration and difficulty in labeling specific subclasses of axons with known projection patterns within the spinal cord have impeded progress in determining whether these regenerated projections are specific.The regeneration of sensory axons into the spinal cord after dorsal root (DR) crush provides a useful preparation for assessing the precision with which regenerating axons project back to appropriate target areas within the central nervous system (CNS). Regrowth of damaged sensory axons within the spinal cord can be visualized by injecting neurotracers into peripheral nerves. Identification of individual classes of axons can be achieved by making injections close to target tissues where nerve branches contain single classes of sensory afferents. Because only the DRs are damaged, the architecture of the spinal cord is left intact, allowing clear identification of the central projections of regenerated axons.Without treatment, sensory axons regenerate only to the DR entry zone (DREZ), where they encounter inhibitory barriers within the CNS (5, 6). Application of two agents-a soluble Nogo receptor peptide (sNgR), which binds to Nogo receptor ligands and abrogates their inhibitory effect (7), and artemin (ART), a member of the glial-derived neurotrophic factor familypromote robust regeneration of sensory axons after DR crush (8, 9). The specificity of projections of specific classes of...