Abstract

On initial inspection, bone remodeling, the process whereby the skeleton adapts through time, appears to be relatively simple. Two cell types, the bone-forming osteoblasts and the bone-resorbing osteoclasts, interact to keep bone mass relatively stable throughout adult life. However, the complexity of the regulatory influences on these cells is continuing to expand our understanding of the intricacy of skeletal physiology and also the interactions between other organ systems and bone. One such example of the broadening of understanding in this field has occurred in the last decade with study of the central, neural regulation of bone mass. Initial studies of an adipose-derived hormone, leptin, helped define a direct, sympathetic pathway involving efferent neural signals from the hypothalamus to receptors on the osteoblast. Since the leptin-mediated pathway has been continuously modified to reveal a complex system involving neuromedin U, cocaine- and amphetamine-related transcript and serotonin interacting within the hypothalamus and brainstem to regulate both bone formation and resorption in cancellous bone, a number of other systems have also been identified. Neuropeptide Y, acting through hypothalamic Y2 receptors, is capable of skeleton-wide modulation of osteoblast activity, with important coordination between body weight and bone mass. Cannabinoids, acting through central cannabinoid receptor 1 and bone cell cannabinoid receptor 2 receptors, modulate osteoclast activity, thereby identifying pathways active on both aspects of the bone remodeling process. This review explores the key central pathways to bone and explores the complexity of the interactions being revealed by this emergent field of research.