Size, neurochemistry, and segmental distribution of sensory neurons innervating the rat tibia

Retrograde labeling has been used to identify sensory neurons in the lumbar dorsal root ganglia (DRG) that innervate the rat tibial periosteum, medullary cavity, and trabecular bone. The size, neurochemical profile [isolectin B4 (IB4) binding, substance P (SP), calcitonin gene-related peptide (CGRP), and NF200 immunoreactivity (-IR)], and segmental distribution of sensory neurons innervating each of these bony compartments are reported. After injections of fast blue into the periosteum, medullary cavity, and trabecular bone (epiphysis), retrogradely labeled neurons were observed throughout the ipsilateral (but not contralateral) lumbar DRG. They were predominantly small (<800 microm(2)) or medium-sized (800-1,800 microm(2)) neurons. CGRP-IR and SP-IR were found in 23% and 16% of the retrogradely labeled neurons, respectively. IB4 binding was observed in 20% and NF200-IR in 40% of the retrogradely labeled neurons. There were no significant differences in the percentage of neurons labeled with any one of the antisera following injections into each of the three bony compartments. To allow a direct comparison with sensory neurons innervating cutaneous tissues, injections of fast blue were also made into the skin overlying the tibia. The percentage of CGRP-IR neurons innervating bone was significantly lower than the percentage of CGRP-IR neurons innervating skin (ANOVA; P < 0.05). No other significant differences in the neurochemical profiles of neurons labeled from bone vs. skin were observed. The findings of the present study show that the periosteum, medullary cavity, and trabecular bone are all innervated by sensory neurons that have size and neurochemical profiles consistent with a role in nociception.