Effects of capsaicin-induced sensory denervation on osteoclastic resorption in adult rats

Many recent findings suggest that the nervous system has efferent effects on bone. A putative role of the sensory innervation has been assessed by using a synchronised rat model of bone resorption after treating adult animals with the neurotoxin capsaicin. Fourteen days after capsaicin treatment (50 mg kg-1) the right maxillary molars were extracted to activate a wave of resorption along the mandibular cortex. The rats were killed 4 days later (i.e. at the peak of resorption in this model), and their right mandibles were processed for histometric evaluation of resorption along the cortex and of calcitonin gene-related peptide (CGRP)- and substance P (SP)-immunoreactive (IR) fibres in the dental pulp. CGRP-IR and SP-IR fibres were significantly reduced in numbers by the capsaicin treatment (by 58 and 49%, respectively), confirming the success of sensory denervation. The resorption surface was significantly reduced (P < 0.005) versus the sham-treated animals. Although the size of the osteoclast population recruited in the site was not modified, the number of actively resorbing osteoclasts was significantly reduced (P < 0.03). However, the activity of the resorbing cells was not modified. Non-specific esterase-positive osteoclast precursors were also significantly few after capsaicin treatment. These data show that the sensory nervous system is involved in the control of bone resorption at two different levels: (1) in the recruitment of osteoclast precursors, and (2) in regulating the access of recruited cells to the bone surface.