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

Many recent findings suggest that the nervous system has efferent effects o n bone. A putative role of the sensory innervation has been assessed by usi ng 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 w ere processed for histometric evaluation of resorption along the cortex and of calcitonin gene-related peptide (CGRP)- and substance P (SP)-immunoreac tive (IR) fibres in the dental pulp. CGRP-LR and SP-IR fibres were signific antly reduced in numbers by the capsaicin treatment (by 58 and 49%, respect ively), confirming the success of sensory denervation. The resorption surfa ce was significantly reduced (P < 0.005) versus the sham-treated animals. A lthough the size of the osteoclast population recruited in the site was not modified, the number of actively resorbing osteoclasts was significantly r educed (P < 0.03). However, the activity of the resorbing cells was not mod ified. Non-specific esterase-positive osteoclast precursors were also signi ficantly few after capsaicin treatment. These data show that the sensory ne rvous system is involved in the control of bone resorption at two different levels: (1) in the recruitment of osteoclast precursors, and (2) in regula ting the access of recruited cells to the bone surface.