The transient receptor potential vanilloid 1 (TRPV1) channel is the principal detector of noxious heat in the peripheral nervous system. TRPV1 is expressed in many nociceptors and is involved in heat-induced hyperalgesia and thermoregulation. The precise mechanism or mechanisms mediating the thermal sensitivity of TRPV1 are unknown. Here, we have shown that the oxidized linoleic acid metabolites 9-and 13-hydroxyoctadecadienoic acid (9-and 13-HODE) are formed in mouse and rat skin biopsies by exposure to noxious heat. 9-and 13-HODE and their metabolites, 9-and 13-oxoODE, activated TRPV1 and therefore constitute a family of endogenous TRPV1 agonists. Moreover, blocking these substances substantially decreased the heat sensitivity of TRPV1 in rats and mice and reduced nociception. Collectively, our results indicate that HODEs contribute to the heat sensitivity of TRPV1 in rodents. Because oxidized linoleic acid metabolites are released during cell injury, these findings suggest a mechanism for integrating the hyperalgesic and proinflammatory roles of TRPV1 and linoleic acid metabolites and may provide the foundation for investigating new classes of analgesic drugs. IntroductionThe TRP family of ligand-gated ion channels consists of several subgroups, including the vanilloid subfamily (transient receptor potential vanilloid [TRPV]). The first member of the subfamily to be discovered, TRPV1, is an extensively studied channel (1-5) that is expressed in a substantial proportion of pain-sensing sensory neurons, termed nociceptors. TRPV1 can be activated by a variety of endogenous lipids (including lipoxygenase and phospholipase D metabolites of arachidonic acid) and by exogenous substances such as capsaicin (the pungent compound in hot chili peppers) (6). We recently discovered that linoleic acid metabolites are synthesized in the spinal dorsal horn following the afferent barrage due to stimuli such as peripheral inflammation and constitute what we believe to be a novel, physiologically active family of endogenous TRPV1 ligands that mediates central sensitization to mechanical stimuli (7).In the periphery, TRPV1 also serves as a detector for noxious heat (> 43°C) (6), and pharmacological and gene deletion studies have shown that TRPV1 is important in inflammatory heat hyperalgesia and thermoregulation (8-9). However, the precise mechanism of heat activation of TRPV1 remains unknown. We found that endogenous TRPV1 agonists are formed on exposure of cell membranes to noxious heat. The released compounds activate TRPV1 and contribute to the thermal responsiveness of this channel.

Transient receptor potential vanilloid 1 (TRPV1) plays a major role in hyperalgesia and allodynia and is expressed both in the peripheral and central nervous systems (CNS). However, few studies have evaluated mechanisms by which CNS TRPV1 mediates hyperalgesia and allodynia after injury. We hypothesized that activation of spinal cord systems releases endogenous TRPV1 agonists that evoke the development of mechanical allodynia by this receptor. Using in vitro superfusion, the depolarization of spinal cord triggered the release of oxidized linoleic acid metabolites, such as 9-hydroxyoctadecadienoic acid (9-HODE) that potently activated spinal TRPV1, leading to the development of mechanical allodynia. Subsequent calcium imaging and electrophysiology studies demonstrated that synthetic oxidized linoleic acid metabolites, including 9-HODE, 13-HODE, and 9 and 13-oxoODE, comprise a family of endogenous TRPV1 agonists. In vivo studies demonstrated that intrathecal application of these oxidized linoleic acid metabolites rapidly evokes mechanical allodynia. Finally, intrathecal neutralization of 9-and 13-HODE by antibodies blocks CFA-evoked mechanical allodynia. These data collectively reveal a mechanism by which an endogenous family of lipids activates TRPV1 in the spinal cord, leading to the development of inflammatory hyperalgesia. These findings may integrate many pain disorders and provide an approach for developing analgesic drugs.inflammation ͉ pain T ransient receptor potential vanilloid 1 (TRPV1) plays a pivotal role in many pain models, leading to the development of hyperalgesia and/or allodynia (1-2). TRPV1 is expressed in the peripheral as well as central nervous systems (CNS) including several areas involved in nociceptive transmission (3). Numerous studies have demonstrated that peripheral TRPV1 is activated by noxious heat and protons and is regulated by endogenous ligands (2), contributing to peripheral mechanisms of heat hyperalgesia (4). However, TRPV1 is also expressed in the CNS, where comparatively little is known about the role of this receptor in mediating central pain mechanisms.Recent studies have used TRPV1 antagonists to evaluate the role of CNS TRPV1 in inflammatory hyperalgesia. Interestingly, the systemic/spinal administration of TRPV1 antagonists blocks inflammation-induced heat hyperalgesia, as well as mechanical allodynia (5-6). These findings were unexpected since TRPV1 is not activated by mechanical stimuli, suggesting that spinal TRPV1 activation leads to central sensitization to both thermal and mechanical stimuli (7-8). Based upon these observations, we hypothesized that the afferent barrage resulting from peripheral tissue injury leads to generation of endogenous TRPV1 ligands in the spinal cord that activate TRPV1 in the CNS resulting in central sensitization. Results Depolarization of Spinal Cord Results in the Release of EndogenousTRPV1 Ligands. To evaluate the hypothesis that endogenous TRPV1 agonists are released from spinal cord neurons, freshly isolated male rat spinal cords w...

Peripheral opioid analgesia is increased substantially after inflammation. We evaluated the hypothesis that an inflammatory mediator, bradykinin (BK), evokes functional competence of the ␦-opioid receptor (DOR) for inhibiting trigeminal ganglia (TG) sensory neurons. We also evaluated whether BK evokes trafficking of the DOR to the plasma membrane. Rat TG cultures were pretreated with BK (

Sex dependency in pain perception is well documented and is thought to be attributable to the effect of reproductive hormones on nociceptive processing. In the present study, we evaluated whether estradiol alters gene transcription in the trigeminal ganglia (TG) of ovariectomized rats (OVX). These experiments demonstrated a dramatic (40-fold) upregulation of prolactin (PRL) expression in TG by 17-␤-estradiol (E2). PRL expression was restricted to TG neurons and was highly overlapped with transient potential receptor vanilloid type 1 (TRPV1) (ϳ90%) in TG. Additionally, PRL is released from neurons during stimulation. Both forms of PRL receptors (PRLRs), short and long, were also present in TG neurons. Moreover, expression of the long PRLRs was under control of estradiol. We next evaluated the novel hypothesis that PRL acts as a neuromodulator of sensory neurons. PRL pretreatment significantly enhanced capsaicin-evoked inward currents, calcium influx, and immunoreactive calcitonin gene-related peptide release from cultured TG neurons. This PRL modulation of capsaicin responses was abolished by withdrawal of E2 from TG cultures. Biochemical analysis demonstrated that PRL increased (Ͼ50%) phosphorylation levels of TRPV1 in TG. In a behavioral test, PRL pretreatment significantly potentiated capsaicin-evoked nocifensive behavior in female rats at proestrous and in OVX rats after E2 treatment. The in vivo potentiating effect of PRL on capsaicin responses was also dependent on E2. Collectively, these data demonstrate that PRL is a novel modulator of sensory neurons tightly regulated by E2. These findings are consistent with the hypothesis that PRL could contribute to the development of certain pain disorders, possibly including those modulated by estrogen.

Supplemental Digital Content is Available in the Text. Severe acute respiratory syndrome coronavirus 2's spike protein promotes analgesia by interfering with vascular endothelial growth factor-A/NRP1 pathway, which may affect disease transmission dynamics.

The Transient Receptor Potential channel subtypes V1 (TRPV1) and A1 (TRPA1) play a critical role in the development of hyperalgesia in inflammatory pain models. Although several studies in animals and humans have demonstrated that capsaicin (CAP), a TRPV1-specific agonist, and mustard oil (MO), a TRPA1 agonist, evoke responses that undergo functional cross-desensitization in various models, the mechanisms mediating this phenomenon are largely unknown. In the present study, we evaluated the mechanisms underlying homologous and heterologous desensitization between CAP and MO responses in peripheral nociceptors using an in vitro neuropeptide release assay from acutely isolated rat hindpaw skin preparation and in vivo behavioral assessments. The pretreatment with CAP or MO significantly inhibited (50-60%) both CAP-and MO-evoked CGRP release indicating homologous and heterologous desensitization using this assay. Further studies evaluating the requirement of calcium in these phenomena revealed that homologous desensitization of CAP responses was calcium-dependent while homologous desensitization of MO responses was calciumindependent. Moreover, heterologous desensitization of both CAP and MO responses was calciumdependent. Further studies evaluating the role of calcineurin demonstrated that heterologous desensitization of CAP responses was calcineurin-dependent while heterologous desensitization of MO responses was calcineurin-independent. Homologous and heterologous desensitization of CAP and MO was also demonstrated using in vivo behavioral nocifensive assays. Taken together, these results indicate that TRPV1 and TRPA1 could be involved in a functional interaction that is regulated via different cellular pathways. The heterologous desensitization of these receptors and corresponding inhibition of nociceptor activity might have potential application as a therapeutic target for developing novel analgesics.

Cannabinoids are known to have multiple sites of action in the nociceptive system, leading to reduced pain sensation. However, the peripheral mechanism(s) by which this phenomenon occurs remains an issue that has yet to be resolved. Because phosphorylation of TRPV1 (transient receptor potential subtype V1) plays a key role in the induction of thermal hyperalgesia in inflammatory pain models, we evaluated whether the cannabinoid agonist WIN 55,212-2 (WIN) regulates the phosphorylation state of TRPV1. Here, we show that treatment of primary rat trigeminal ganglion cultures with WIN led to dephosphorylation of TRPV1, specifically at threonine residues. Utilizing Chinese hamster ovary cell lines, we demonstrate that Thr 144 and Thr 370 were dephosphorylated, leading to desensitization of the TRPV1 receptor. This post-translational modification occurred through activation of the phosphatase calcineurin (protein phosphatase 2B) following WIN treatment. Furthermore, knockdown of TRPA1 (transient receptor potential subtype A1) expression in sensory neurons by specific small interfering RNA abolished the WIN effect on TRPV1 dephosphorylation, suggesting that WIN acts through TRPA1. We also confirm the importance of TRPA1 in WIN-induced dephosphorylation of TRPV1 in Chinese hamster ovary cells through targeted expression of one or both receptor channels. These results imply that the cannabinoid WIN modulates the sensitivity of sensory neurons to TRPV1 activation by altering receptor phosphorylation. In addition, our data could serve as a useful strategy in determining the potential use of certain cannabinoids as peripheral analgesics.Cannabinoids have been shown to exert anti-inflammatory and anti-hyperalgesic effects via peripheral site(s) of action in several pain models (1-5). These effects are thought to be mediated by cannabinoid type 1 (CB1) 4 and/or 2 (CB2) receptor activation, both peripherally and centrally (4 -7). Cannabinoids could exert their effects by acting on CB1/CB2 receptors located on sensory neurons and/or other peripheral cells influencing sensory neuronal function (8). However, there is a Ͻ5-10% co-localization of metabotropic CB1/CB2 receptors with nociceptive neuronal markers such as TRPV1 (transient receptor potential subtype V1) and calcitonin gene-related peptide in trigeminal and dorsal root ganglion neurons (9 -11), suggesting that cannabinoids could act on nociceptors through non-CB1/CB2 receptor mechanism(s). Certain cannabinoids have been shown to activate channels such as TRPV1, including arachidonyl-2-chloroethylamide (ACEA) (12), N-arachidonoyldopamine (13), and anandamide (14), as well as TRPA1 (transient receptor potential subtype A1), including ⌬ 9 -tetrahydrocannabinol (15). In addition, the synthetic cannabinoid R(ϩ)-WIN 55,212-2 (WIN) has demonstrated non-CB1/CB2 receptor activities in trigeminal ganglia (11). The results from these studies suggest that cannabinoids may activate calcium channel function similar to non-cannabinoid transient receptor potential agonists, including the...

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