Osteoclasts play a part in pain due to the inflammation adjacent to bone
Introduction
Bone disorders with increased osteoclastic bone resorption such as metastatic bone diseases [1], Paget's disease of bone [2], osteoporosis [3], fibrous dysplasia [4] and osteogenesis imperfecta [5] are frequently associated with bone pain. Of note, bisphosphonates (BPs), potent and specific inhibitors of osteoclasts, have been shown to reduce bone pain complicated with these bone disorders [1], [2], [3], [4], [5]. BPs also diminish pain in rheumatoid arthritis [6] in which osteoclastic bone resorption is secondary to the neighboring synovial inflammation [7]. In addition, other types of inhibitors of osteoclasts including calcitonin [8], mithramycin [9] and gallium nitrate [10] that have different mechanism of action from BP are also shown to decrease bone pain. These results collectively led us to hypothesize that osteoclasts play a role in causing bone pain in the pathological conditions associated with increased bone resorption/destruction.
Here, we attempted to prove this notion and study the underlying mechanism using an animal model that has been widely used to study inflammatory pain and advanced our understanding of the mechanism of pain over the last 2 decades. A single bolus injection of the complete Freund's adjuvant (CFA) into the plantar surface of the unilateral hind-paw induced an intense and persistent inflammation characterized by the edema, redness and hyperalgesia (hyper-responsiveness to noxious stimuli) that appear within 2 h, peak at 6–24 h and last for approximately 2 weeks only on the CFA-injected hind-paw [11], [12]. These symptoms are in part attributable to increased production of pro-inflammatory cytokines such as inteleukin-1, interleukin-6 and tumor necrosis factor-α following CFA-injection [13], [14]. Of note, these pro-inflammatory cytokines are potent stimulators of osteoclastic bone resorption [15]. It therefore seems likely that increased osteoclastic bone resorption as a consequence of CFA-induced inflammatory reactions contributes to the CFA-induced hyperalgesia. However, the role of osteoclasts in the CFA-induced hyperalgesia is unknown.
Osteoclasts degrade bone minerals by secreting protons through the vacuolar H+-ATPase, making neighboring microenvironments acidic [15], [16]. Acid is a well-known cause of pain [17], [18], [19]. Recently, the two main classes of acid-sensing nociceptors, the acid-sensing ion channels (ASICs) and the transient receptor potential channel vanilloid subfamily member 1 (TRPV1), are found to be expressed in the sensory neurons innervating bone [20]. ASICs and TRPV1 elicit pain signals upon activation by acid [17], [18], [19], [21]. We reasoned that the acidic microenvironment created by bone-resorbing osteoclasts activates these acid-sensing nociceptors, causing bone pain. To examine this, we investigated the expression and activation of ASICs and TRPV1 in the primary sensory neurons in the dorsal root ganglions (DRGs). Activation of these acid-sensing receptors was evaluated by examining nuclear c-Fos expression in the sensory neurons in the spinal dorsal horn. Earlier studies showed that c-Fos expression in the spinal dorsal horn was up-regulated following CFA injection into the hind-paw [22], [23]. Elevated nuclear c-Fos expression in the spinal dorsal horn in response to noxious stimuli has been used as a molecular indicator of neural activation [24], [25], [26]. Subsequently, we attempted to study the activation of these acid-sensing nociceptors by acid using the F-11 rat DRG cells in culture [27], [28]. Finally, we examined the effects of bafilomycin A1, an inhibitor of the vacuolar H+-ATPase [16], [29], on the CFA-induced hyperalgesia in this model.
Our results are consistent with the notion that the acidosis created by osteoclasts plays a part in causing inflammatory pain adjacent to bone through an activation of acid-sensing receptors including ASICs and TRPV1.
Section snippets
Reagents
ZOL (2-(imidazol-1-yl)-1-hydroxyethylidene-1,1-bisphosphonate) and ALN (4-amino-1-hydroxybutylidene bisphosphonate) were provided as dehydrated disodium or monosodium salt by Novartis Pharma AG (Basel, Switzerland) and Merck and Co., Inc. (Rahway, NJ), respectively. Recombinant human OPG (rhOPG) was a generous gift from Amgen Inc. (Thousand Oaks, CA). Synthetic human parathyroid hormone-related protein (PTH-rP), bafilomycin A1 and amiloride hydrochloride hydrate were purchased from Peptide
CFA-induced osteoclastic bone resorption and hyperalgesia
As previously described [12], inflammation and swelling became evident in the CFA-injected ipsi-lateral hind-paws and the volume of the inflamed paw was apparently larger than that of the non-CFA-injected contra-lateral paw 24 h after the injection (day 1) (data not shown). Of note, histological and histomorphometrical examinations showed that an injection of CFA stimulated bone resorption with increased TRAP-positive osteoclasts in the metatarsal bones (Fig. 1). Hyperalgesia determined by the
Discussion
Here, we conducted in vivo and in vitro studies to obtain experimental evidence for the involvement of osteoclasts in the pathophysiology of pain due to the inflammation adjacent to bone and to determine the role of acidosis created by osteoclasts in causing pain.
Using a well-characterized animal model [11], [12], [31], we verified that CFA caused inflammatory pain. In addition, our data show that CFA increased osteoclastic bone resorption at the injected site. Of note, ZOL, ALN and OPG, of
Acknowledgments
This study was supported in part by Grants in aid 14771020, 16791123 (to T.H.), 12137205 and 21st century Center of Excellence (COE) program (to T.Y.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, Senri Life Science Foundation Research Grant (to T.H.) and Osaka Cancer Society Research Grant (to T.H.).
References (45)
- et al.
The role of bisphosphonates in the treatment of painful metastatic bone disease: a review of phase III trials
Pain
(1998) - et al.
Unilateral inflammation of the hindpaw in rats as a model of prolonged noxious stimulation: alterations in behavior and nociceptive thresholds
Pharmacol. Biochem. Behav.
(1988) - et al.
Osteoclastic acidification pathways during bone resorption
Bone
(2002) - et al.
Molecular physiology of proton transduction in nociceptors
Curr. Opin. Pharmacol.
(2001) - et al.
Origins of skeletal pain: sensory and sympathetic innervation of the mouse femur
Neuroscience
(2002) - et al.
A parametric study of electroacupuncture on persistent hyperalgesia and Fos protein expression in rats
Brain Res.
(2004) - et al.
Fos protein induction in the medullary dorsal horn and first segment of the spinal cord by tooth-pulp stimulation in cats
Pain
(1998) - et al.
Inhibition of osteoclast proton transport by bafilomycin A1 abolishes bone resorption
Biochem. Biophys. Res. Commun.
(1990) - et al.
A new and sensitive method for measuring thermal nociception in cutaneous hyperalgesia
Pain
(1988) - et al.
Intrathecal morphine in mice: a new technique
Eur. J. Pharmacol.
(1980)
The ASICs: signaling molecules? Modulators?
Trends Neurosci.
Influence of bisphosphonates on the production of pro-inflammatory cytokines by activated human articular chondrocytes
Cytokine
Short-term intravenous therapy with neridronate in Paget's disease
Clin. Exp. Rheumatol.
Analgesic effect of intravenous pamidronate on chronic back pain due to osteoporotic vertebral fractures
Clin. Rheumatol.
Bisphosphonate therapy in fibrous dysplasia
Clin. Orthop.
Beneficial effect of long term intravenous bisphosphonate treatment of osteogenesis imperfecta
Arch. Dis. Child
Efficacy of disodium-clodronate in the management of joint pain in rheumatoid arthritis. Six month open study
Minerva Med.
Bone loss in inflammatory arthritis: mechanism and treatment strategies
Curr. Opin. Rheumatol.
Use of calcitonin in the treatment of bone pain associated with osteoporosis
Calcif. Tissue Int.
Effect of mithramycin on widespread painful bone metastases in cancer of the breast
Cancer Treat Rep.
Gallium nitrate for the treatment of bone metastases
Cancer
Inflammatory models of pain and hyperalgesia
ILAR J.
Cited by (138)
Bone formation recovery with gold nanoparticle-induced M2 macrophage polarization in mice
2021, Nanomedicine: Nanotechnology, Biology, and MedicineCitation Excerpt :These results obtained in vivo are consistent with the in vitro results (Figure 2), which indicated that AuNPs inhibited the persistent inflammation by inducing macrophage differentiation to the M2 subtype. Fractures are the most common and most serious consequence of localized bone destruction caused by a variety of factors, including inflammation,30 aging,31 post-menopause,32 and others. The ensuing bone destruction can be greatly alleviated with existing treatment options by inhibiting the inflammatory process and preventing further bone erosion.33
Comparative analysis of anti-osteoporosis medications in preventing vertebral body fractures after balloon kyphoplasty
2024, Archives of OsteoporosisNociceptor mechanisms underlying pain and bone remodeling via orthodontic forces: toward no pain, big gain
2024, Frontiers in Pain ResearchThe role of acid-sensing ion channels in monosodium urate-induced gouty pain in mice
2024, Pflugers Archiv European Journal of Physiology