The capsaicin-sensitive vanilloid receptor (VR1) was recently shown to play an important role in inflammatory pain (hyperalgesia), but the underlying mechanism is unknown. We hypothesized that pain-producing inflammatory mediators activate capsaicin receptors by inducing the production of fatty acid agonists of VR1. This study demonstrates that bradykinin, acting at B2 bradykinin receptors, excites sensory nerve endings by activating capsaicin receptors via production of 12-lipoxygenase metabolites of arachidonic acid. This finding identifies a mechanism that might be targeted in the development of new therapeutic strategies for the treatment of inflammatory pain. V R1, a cloned capsaicin receptor, is a nonspecific cation channel expressed preferentially in small sensory neurons and activated by the vanilloids, capsaicin and resiniferatoxin (1). Because VR1 is also activated by heat and acid (1, 2), it is now considered to be a molecular sensor that detects a variety of painful stimuli. Indeed, recent experiments performed in mice that lack VR1 demonstrated that the receptor is essential for inflammation-induced heat hyperalgesia (3, 4). Therefore, understanding the cellular mechanisms by which capsaicin receptors are activated by inflammatory mediators may be a key to identifying novel therapeutic targets for pain treatment. Because of the presence of VR1 in sensory neurons and an apparent role in inflammatory hyperalgesia, endogenous activators of VR1 have been suspected. We recently demonstrated that products of the lipoxygenase pathway of arachidonic acid (AA) metabolism can activate capsaicin receptors (5). Among the eicosanoids tested, the 12-lipoxygenase product, 12-hydroperoxyeicosatetraenoic acid (12-HPETE), structurally similar to capsaicin, was the most potent VR1 agonist. Thus, metabolic products of lipoxygenases become candidates for the endogenous capsaicinlike substances. However, the upstream signals that stimulate lipoxygenase and activate VR1 are elusive.Bradykinin (BK) is a potent inflammatory mediator that causes pain and hyperalgesia. BK is known to activate as well as sensitize sensory neurons to other stimuli. Various signaling pathways have been suggested to mediate the sensitizing effect of BK on sensory neurons (6, 7). However, activation mechanism by BK is not known. BK is now known to stimulate the production of AA in sensory neurons (8), a key substrate of lipoxygenases. Therefore, on the basis of previous observations that products of lipoxygenase activate VR1 (5), we hypothesized that BK excites sensory neurons by opening the capsaicin receptor via production of 12-lipoxygenase products of AA metabolism. Materials and MethodsCell Culture. Experiments were carried out according to the Ethical Guidelines of the International Association for the Study of Pain and approved by the research ethics committee for the use of animals of the Seoul National University and the University of California, San Francisco. Thoracic and lumbar dorsal root ganglia (DRGs) were dissected from 1-to 2 day-...

DEL-1 is an endothelial cell-secreted protein that regulates LFA-1-integrin–dependent leukocyte recruitment and inflammation in various tissues. Here we identified a novel regulatory mechanism of DEL-1 in osteoclast biology. Specifically, we showed that DEL-1 is expressed by human and mouse osteoclasts and regulates their differentiation and resorptive function. Mechanistically, DEL-1 inhibited the expression of NFATc1, a master regulator of osteoclastogenesis, in a Mac-1-integrin–dependent manner. In vivo mechanistic analysis has dissociated the anti-inflammatory from the anti-bone resorptive action of DEL-1 and identified structural components thereof mediating these distinct functions. Importantly, locally administered human DEL-1 blocked inflammatory periodontal bone loss in nonhuman primates—a relevant model of human periodontitis. The ability of DEL-1 to regulate both upstream (inflammatory cell recruitment) and downstream (osteoclastogenesis) events that lead to inflammatory bone loss paves the way to a new class of endogenous therapeutics for treating periodontitis and perhaps other inflammatory disorders.

Mechanosensitive (MS) ion channels are present in a variety of cells. However, very little is known about the ion channels that account for mechanical sensitivity in sensory neurons. We identified the two most frequently encountered but distinct types of MS channels in 1390 of 2962 membrane patches tested in cultured dorsal root ganglion neurons. The two MS channels exhibited different thresholds, thus named as low-threshold (LT) and high-threshold (HT) MS channels, and sensitivity to pressure. The two channels retained different single-channel conductances and current-voltage relationships: LT and HT channels elicited large- and small-channel conductance with outwardly rectifying and linear I-V relationships, respectively. Both LT and HT MS channels were permeable to monovalent cations and Ca2+ and were blocked by gadolinium, a blocker of MS channels. Colchicine and cytochalasin D markedly reduced the activities of the two MS channels, indicating that cytoskeletal elements support the mechanosensitivity. Both types of MS channels were found primarily in small sensory neurons with diameters of <30 microm. Furthermore, HT MS channels were sensitized by a well known inducer of mechanical hyperalgesia, prostaglandin E2, via the protein kinase A pathway. We identified two distinct types of MS channels in sensory neurons that probably give rise to the observed MS whole-cell currents and transduce mechanical stimuli to neural signals involved in somatosensation, including pain.

Vanilloid receptor 1 (VR1), a ligand-gated ion channel activated by vanilloids, acid, and heat, is a molecular detector that integrates multiple modes of pain. Although the function and the biophysical properties of the channel are now known, the regions of VR1 that recognize ligands are largely unknown. By the stepwise deletion of VR1 and by chimera construction using its capsaicin-insensitive homologue, VRL1, we localized key amino acids, Arg-114 and Glu-761, in the N-and C-cytosolic tails, respectively, that determine ligand binding. Point mutations of the two key residues resulted in a loss of sensitivity to capsaicin and a concomitant loss of specific binding to [ 3 H]resiniferatoxin, a potent vanilloid. Furthermore, changes in the charges of the two amino acids blocked capsaicin-sensitive currents and ligand binding without affecting current responses to heat. Thus, these two regions in the cytoplasmic tails of VR1 provide structural elements for its hydrophilic interaction with vanilloids and might constitute a long-suspected binding pocket.Capsaicin, the principal pungent ingredient of hot peppers, excites sensory neurons by opening an ion channel, the vanilloid receptor 1 (VR1), 1 thereby causing pain. VR1 is a ligand-gated, cationic channel that is present mainly in small nociceptive sensory neurons (1-3). The presence of VR1 in sensory neurons leads to questions concerning the existence of endogenous capsaicin-like substances, and various lipid metabolic products of lipoxygenases or anandamide have been suggested as candidates, because they activate VR1 and are structurally similar to capsaicin (4, 5). Accordingly, a role for lipoxygenase products in the activation of VR1 during inflammation was suggested (5), and in fact, bradykinin, a potent pain-causing inflammatory mediator, is now known to activate VR1 via the lipoxygenase/VR1 pathway (6). In addition, bradykinin also has a potential to sensitize VR1 via a phospholipase C or protein kinase C pathway (7-9).VR1 is also activated by acid or heat at over 43°C, a threshold temperature for pain (3, 10 -12). Moreover, because ischemic or inflamed tissues become acidic, the acid activation of VR1 is a pathologically relevant event (13). More direct evidence for the pathophysiological role of VR1 in the production of inflammatory pain came from knock-out experiments. In mice lacking VR1, thermal hyperalgesia evoked by inflammation is reduced (14, 15). Furthermore, hyperalgesia induced by the key inflammatory mediators, bradykinin and nerve growth factor, is reduced in mice lacking VR1 (8). Thus, VR1 is now considered a primary molecular transducer that mediates inflammatory hyperalgesia (13).The putative topology of VR1 indicates that it belongs to a class of transient receptor potential channels possessing six transmembrane domains and two cytosolic domains at each Nand C terminus (3, 16). VR1 appears to form a homotetramer when expressed heterologously (17). However, VR1 may form a heteromultimer with another temperature-sensitive channel, transient rec...