Although skeletal pain is a leading cause of chronic pain and

Although skeletal pain is a leading cause of chronic pain and disability relatively little is known about the specific populations of nerve fibers that innervate the skeleton. whole mount preparations of the normal adult mouse femur were analyzed using immunohistochemistry and confocal microscopy. Analysis of these preparations revealed that 80% of the unmyelinated/thinly myelinated sensory nerve fibers that express calcitonin gene-related peptide (CGRP) and innervate the periosteum mineralized bone and bone marrow also express TrkA. Similarly the majority of myelinated sensory nerve fibers that express neurofilament 200 kDa (NF200) which innervate the periosteum mineralized bone and bone marrow CHC also co-express TrkA. In the normal femur the relative density of CGRP+ NF200+ and TrkA+ sensory nerve fibers per unit volume is usually: periosteum > bone marrow > mineralized bone > cartilage with the respective relative densities being 100: 2: 0.1: 0. The observation that the majority of sensory nerve fibers innervating the skeleton express TrkA+ may in part explain why therapies that block NGF/TrkA pathway are highly efficacious in attenuating skeletal pain. animal and human experiments aimed at attenuating skeletal pain by blocking NGF or TrkA. Sequestration of NGF has been shown to reduce mouse (Inglis et al. 2008 rat (Shelton et al. CHC 2005 and human (Lane et al. 2010 arthritis pain fracture pain in mice (Jimenez-Andrade et al. 2007 complex regional pain syndrome type I in rats Rabbit Polyclonal to PKCB1. (Sabsovich et al. 2008 and bone cancer pain due to sarcoma (Mantyh et al. 2010 or canine prostate tumor cells growing in mouse bones (Jimenez-Andrade et al. 2010 Similarly recent studies have reported the ability of TrkA inhibitors to attenuate mouse fracture pain (Ghilardi et al. 2010 and mouse bone cancer pain (Ghilardi et al. 2010 with comparable efficacy (approximately a 50% reduction) as was seen with anti-NGF therapies in mouse and human studies thereby suggesting that it is the conversation of NGF with TrkA that drives skeletal pain. Potential limitations and conclusions This study experienced several potential limitations. First this study examined the sensory innervation of mineralized bone marrow periosteum and cartilage of the mouse femur. In order to determine whether this pattern of innervation of the femur applies to all long and flat bones additional studies will be needed. Second decalcification of the bone using EDTA generally causes a decrease in the antigenicity of molecules in the bone tissue (Mullink et al. 1985 Schwei et al. 1999 resulting in an underestimation of nerve fibers that express a labile antigen such as TrkA or Space43. However studies performed in our lab CHC show that this density of TrkA+ nerve fibers in whole mount preparations that did not undergo decalcification is similar to CHC those found in the periosteum of bone tissues that did undergo the standard decalcification treatment for 2 weeks. Third the present study was conducted on femurs obtained from young adult mice and these findings need to be confirmed in both middle-aged and aged animals. Due to increased prevalence of skeletal diseases with aging future studies are clearly needed to investigate how the density phenotype and response characteristics of sensory nerve fibers that innervate bone change with age. In conclusion the majority of sensory nerve fibers in bone express TrkA which may in part explain why blockade of NGF or TrkA CHC is effective in attenuating skeletal pain. Understanding how the density morphology phenotype and response characteristics of skeletal sensory nerve fibers change following injury disease and aging may help in the development of more targeted therapies to treat acute and/or chronic skeletal pain. Acknowledgments We thank Marvin Landis and the University or college of Arizona Information Technology Service for generating the 3D renderings of the innervated bone. This work was supported by the National Institutes of Health grant (NS23970) by the Department of Veteran Affairs Veteran Health Administration Rehabilitation Research and Development Support Grants (04380-I and A6707-R) and by the Calhoun Fund for Bone Pain. None of the authors of this study claim a discord of interest. Gabriela Casta?eda-Corral is a Conacyt fellow from Mexico. LIST OF ABBREVIATIONS ╬╝CTMicro-computed tomography5-HTSerotoninANOVAAnalysis of varianceASICAcid-sensing ion channelsBDNFBrain-derived neurotrophic factorCGRPCalcitonin.