Plasticity of cytochrome P450 isozyme expression in rat trigeminal ganglia neurons during inflammation

Understanding painful versus non-painful dental pain in female and male patients: A transcriptomic analysis of human biopsies

Dental pain from apical periodontitis is an infection induced-orofacial pain condition that presents with diversity in pain phenotypes among patients. While 60% of patients with a full-blown disease present with the hallmark symptom of mechanical allodynia, nearly 40% of patients experience no pain. Furthermore, a sexual dichotomy exists, with females exhibiting lower mechanical thresholds under basal and diseased states. Finally, the prevalence of post-treatment pain refractory to commonly used analgesics ranges from 7-19% (∼2 million patients), which warrants a thorough investigation of the cellular changes occurring in different patient cohorts. We, therefore, conducted a transcriptomic assessment of periapical biopsies (peripheral diseased tissue) from patients with persistent apical periodontitis. Surgical biopsies from symptomatic male (SM), asymptomatic male (AM), symptomatic female (SF), and asymptomatic female (AF) patients were collected and processed for bulk RNA sequencing. Using strict selection criteria, our study found several unique differentially regulated genes (DEGs) between symptomatic and asymptomatic patients, as well as novel candidate genes between sexes within the same pain group. Specifically, we found the role of cells of the innate and adaptive immune system in mediating nociception in symptomatic patients and the role of genes involved in tissue homeostasis in potentially inhibiting nociception in asymptomatic patients. Furthermore, sex-related differences appear to be tightly regulated by macrophage activity, its secretome, and/or migration. Collectively, we present, for the first time, a comprehensive assessment of peripherally diseased human tissue after a microbial insult and shed important insights into the regulation of the trigeminal system in female and male patients.

Anti-inflammatory dopamine- and serotonin-based endocannabinoid epoxides reciprocally regulate cannabinoid receptors and the TRPV1 channel

The endocannabinoid system is a promising target to mitigate pain as the endocannabinoids are endogenous ligands of the pain-mediating receptors—cannabinoid receptors 1 and 2 (CB1 and CB2) and TRPV1. Herein, we report on a class of lipids formed by the epoxidation of N-arachidonoyl-dopamine (NADA) and N-arachidonoyl-serotonin (NA5HT) by epoxygenases. EpoNADA and epoNA5HT are dual-functional rheostat modulators of the endocannabinoid-TRPV1 axis. EpoNADA and epoNA5HT are stronger modulators of TRPV1 than either NADA or NA5HT, and epoNA5HT displays a significantly stronger inhibition on TRPV1-mediated responses in primary afferent neurons. Moreover, epoNA5HT is a full CB1 agonist. These epoxides reduce the pro-inflammatory biomarkers IL-6, IL-1β, TNF-α and nitrous oxide and raise anti-inflammatory IL-10 cytokine in activated microglial cells. The epoxides are spontaneously generated by activated microglia cells and their formation is potentiated in the presence of anandamide. Detailed kinetics and molecular dynamics simulation studies provide evidence for this potentiation using the epoxygenase human CYP2J2. Taken together, inflammation leads to an increase in the metabolism of NADA, NA5HT and other eCBs by epoxygenases to form the corresponding epoxides. The epoxide metabolites are bioactive lipids that are potent, multi-faceted molecules, capable of influencing the activity of CB1, CB2 and TRPV1 receptors.

Endocannabinoids are ligands of cannabinoid receptors and a promising target for pain management. Here, the authors report a class of lipids formed by the epoxidation of N-arachidonoyl dopamine and N-arachidonoyl serotonin by cytochrome P450 epoxygenases, which reciprocally regulate canabinoid receptors and display anti-inflammatory activity.

Spinal neuronal excitability and neuroinflammation in a model of chemotherapeutic neuropathic pain: targeting the resolution pathways

Background

Neuroinflammation is a critical feature of sensitisation of spinal nociceptive processing in chronic pain states. We hypothesised that the resolvin pathways, a unique endogenous control system, may ameliorate aberrant spinal processing of somatosensory inputs associated with chemotherapy-induced neuropathic pain (CINP).

Method

The paclitaxel (PCX) model of CINP was established in male Sprague-Dawley rats and compared to control rats (n = 23 and 22, respectively). Behavioural pain responses were measured, and either single unit electrophysiological recordings of dorsal horn wide dynamic range (WDR) neurones were performed, or mRNA microarray analysis of the dorsal horn of the spinal cord was undertaken.

Results

PCX rats exhibited significant changes in behavioural responses to mechanical and cold stimuli. A higher proportion of WDR neurones in PCX rats were polymodal (generating post-discharge following a non-noxious mechanical stimulus, responding to non-noxious cold and exhibiting spontaneous activity) compared to control (p < 0.05). Microarray analysis revealed changes in proinflammatory pathways (Tlr, Tnfrsf1a, Nlrp1a, Cxcr1, Cxcr5, Ccr1, Cx3cr1) and anti-inflammatory lipid resolvin pathways (Alox5ap, Cyp2j4 and Ptgr1) compared to control (p < 0.05). Ingenuity pathway analysis predicted changes in glutamatergic and astrocyte signaling in the PCX group. Activation of the resolvin system via the spinal administration of aspirin-triggered resolvin D1 (AT-RvD1) markedly inhibited (73 ± 7% inhibition) normally non-noxious mechanically (8 g) evoked responses of WDR neurones only in PCX rats, whilst leaving responses to noxious mechanically induced stimuli intact. Inhibitory effects of AT-RvD1were comparable in magnitude to spinal morphine (84 ± 4% inhibition).

Conclusion

The PCX model of CINP was associated with mechanical allodynia, altered neuronal responses and dysregulation of pro- and anti-inflammatory signalling in the spinal dorsal horn. The resolvin AT-RvD1 selectively inhibited low weight mechanical-evoked responses of WDR neurones in PCX rats, but not in controls. Our data support the targeting of spinal neuroinflammation via the activation of the resolvin system as a new therapeutic approach for CINP.

Neuroinflammation is able to downregulate cytochrome P450 epoxygenases 2J3 and 2C11 in the rat brain

Cytochrome P450 (CYP) epoxygenases have been considered the main producers of epoxyeicosatrienoic acids (EETs) through the oxidation of arachidonic acid (AA). EETs display various biological properties, notably their powerful anti-inflammatory activities. In the brain, EETs have proven to be neuroprotective and to improve neuroinflammation. However, it is known that inflammation could modify CYP expression. We have previously reported that an inflammatory process in astrocytes is able to down-regulate CYP2J3 and CYP2C11 mRNA, protein levels, and activity (Navarro-Mabarak et al., 2019). In this work, we evaluated the effect of neuroinflammation in protein expression of CYP epoxygenases in the brain. Neuroinflammation was induced by the intraperitoneal administration of LPS (1 mg/kg) to male Wistar rats and was corroborated by IL-6, GFAP, and Iba-1 protein levels in the cortex over time. CYP2J3 and CYP2C11 protein levels were also evaluated in the cortex after 6, 12, 24, 48, and 72 h of LPS treatment. Our results show for the first time that neuroinflammation is able to downregulate CYP2J3 and CYP2C11 protein expression in the brain cortex.

Oxidized Lipids in Persistent Pain States

Chemotherapy, nerve injuries, or diseases like multiple sclerosis can cause pathophysiological processes of persistent and neuropathic pain. Thereby, the activation threshold of ion channels is reduced in peripheral sensory neurons to normally noxious stimuli like heat, cold, acid, or mechanical due to sensitization processes. This leads to enhanced neuronal activity, which can result in mechanical allodynia, cold allodynia, thermal hyperalgesia, spontaneous pain, and may initiate persistent and neuropathic pain. The treatment options for persistent and neuropathic pain patients are limited; for about 50% of them, current medication is not efficient due to severe side effects or low response to the treatment. Therefore, it is of special interest to find additional treatment strategies. One approach is the control of neuronal sensitization processes. Herein, signaling lipids are crucial mediators and play an important role during the onset and maintenance of pain. As preclinical studies demonstrate, lipids may act as endogenous ligands or may sensitize transient receptor potential (TRP)-channels. Likewise, they can cause enhanced activity of sensory neurons by mechanisms involving G-protein coupled receptors and activation of intracellular protein kinases. In this regard, oxidized metabolites of the essential fatty acid linoleic acid, 9- and 13-hydroxyoctadecadienoic acid (HODE), their dihydroxy-metabolites (DiHOMEs), as well as epoxides of linoleic acid (EpOMEs) and of arachidonic acid (EETs), as well as lysophospholipids, sphingolipids, and specialized pro-resolving mediators (SPMs) have been reported to play distinct roles in pain transmission or inhibition. Here, we discuss the underlying molecular mechanisms of the oxidized linoleic acid metabolites and eicosanoids. Furthermore, we critically evaluate their role as potential targets for the development of novel analgesics and for the treatment of persistent or neuropathic pain.

Prolactin receptor expression in mouse dorsal root ganglia neuronal subtypes is sex-dependent

Sensory neurones exhibit sex-dependent responsiveness to prolactin (PRL). This could contribute to sexual dimorphism in pathological pain conditions. The present study aimed to determine the mechanisms underlying sex-dependent PRL sensitivity in sensory neurones. A quantitative reverse transcriptase-polymerase chain reaction shows that prolactin receptor (Prlr) long and short isoform mRNAs are expressed at comparable levels in female and male mouse dorsal root ganglia (DRG). In Prlr^cre/+ ;Rosa26^{LSL-tDTomato/+} reporter mice, percentages of Prlr⁺ sensory neurones in female and male DRG are also similar. Characterisation of Prlr⁺ DRG neurones using immunohistochemistry and electrophysiology revealed that Prlr⁺ DRG neurones are mainly peptidergic nociceptors in females and males. However, sensory neurone type-dependent expression of Prlr is sex dimorphic. Thus, Prlr⁺ populations fell into three small- and two medium-large-sized sensory neuronal groups. Prlr⁺ DRG neurones are predominantly medium-large sized in males and are proportionally more comprised of small-sized sensory neurones in females. Specifically, Prlr⁺ /IB4⁺ /CGRP⁺ neurones are four- to five-fold higher in numbers in female DRG. By contrast, Prlr⁺ /IB4^- /CGRP⁺ /5HT3a⁺ /NPYR2^- are predominant in male DRG. Prlr⁺ /IB4^- /CGRP^- , Prlr⁺ /IB4^- /CGRP⁺ and Prlr⁺ /IB4^- /CGRP⁺ /NPYR2⁺ neurones are evenly encountered in female and male DRG. These differences were confirmed using an independently generated single-cell sequencing dataset. Overall, we propose a novel mechanism by which sensory neurone type-dependent expression of Prlr could explain the unique sex dimorphism in responsiveness of nociceptors to PRL.

Role of NF-κB in cytochrome P450 epoxygenases down-regulation during an inflammatory process in astrocytes

Cytochrome P450 (CYP) epoxygenases and their metabolic products, epoxyeicosatrienoic acids (EETs), have been proposed as important therapeutic targets in the brain. However, CYP expression can be modified by the presence of diverse pro-inflammatory cytokines and the subsequent activation of the NF-κB pathway. It has been indicated that CYP epoxygenases are down-regulated by inflammation in the heart, kidney and liver. However, up to this point, there has been no evidence regarding regulation of CYP epoxygenases during inflammation in the brain. Therefore, in order to explore the effects of inflammation and NF-κB activation in CYP2J3 and CYP2C11 regulation, rat primary astrocytes cultures were treated with LPS with and without IMD-0354 (selective NF-κB inhibitor). Cyp2j3 and Cyp2c11 mRNA expression was determined by qRT-PCR; protein expression was determined by immunofluorescence and by Western Blot and total epoxygenase activity was determined by the quantification of EETs by ELISA. NF-κB binding sites in Cyp2j3 and Cyp2c11 promoter regions were bioinformatically predicted and Electrophoretic Mobility Shift Assays (EMSA) were performed to determine if each hypothetic response element was able to bind NF-κB complexes. Results shown that LPS treatment is able to down-regulate astrocyte CYP2J3 and CYP2C11 mRNA, protein and activity. Additionally, we have identified NK-κB as the transcription factor involved in this regulation.

Characterization of sensory neuronal subtypes innervating mouse tongue

The tongue is uniquely exposed to water-soluble environmental chemicals that may lead to injury or tumorigenesis. However, comparatively little research has focused on the molecular and functional organization of trigeminal ganglia (TG) afferent neurons innervating the tongue. The current study identified and characterized lingual sensory neurons based on a neuronal subtype classification previously characterized in the dorsal root ganglion (DRG) neurons. We employed immunohistochemistry on transgenic reporter mouse lines as well as single-cell PCR of known markers of neuronal subtypes to characterize neuronal subtypes innervating the tongue. Markers expressed in retrogradely labeled TG neurons were evaluated for the proportion of neurons expressing each marker, intensity of expression, and overlapping genes. We found that tongue-innervating sensory neurons primarily expressed CGRP, TRPV1, TrkC, 5HT3A and Parvalbumin. These markers correspond to peptidergic and a subgroup of non-peptidergic C-nociceptors, peptidergic A nociceptors, proprioceptors and myelinated low-threshold mechanoreceptors (LTMRs). Interestingly, as reported previously, we also found several differences between TG and DRG neurons indicating the need for single-cell sequencing of neuronal types based on tissue type within all TG as well as DRG neurons.

Concentrations of oxidized linoleic acid derived lipid mediators in the amygdala and periaqueductal grey are reduced in a mouse model of chronic inflammatory pain

Chronic pain is both a global public health concern and a serious source of personal suffering for which current treatments have limited efficacy. Recently, oxylipins derived from linoleic acid (LA), the most abundantly consumed polyunsaturated fatty acid in the modern diet, have been implicated as mediators of pain in the periphery and spinal cord. However, oxidized linoleic acid derived mediators (OXLAMs) remain understudied in the brain, particularly during pain states. In this study, we employed a mouse model of chronic inflammatory pain followed by a targeted lipidomic analysis of the animals' amygdala and periaqueductal grey (PAG) using LC-MS/MS to investigate the effect of chronic inflammatory pain on oxylipin concentrations in these two brain nuclei known to participate in pain sensation and perception. From punch biopsies of these brain nuclei, we detected twelve OXLAMs in both the PAG and amygdala and one arachidonic acid derived mediator, 15-HETE, in the amygdala only. In the amygdala, we observed an overall decrease in the concentration of the majority of OXLAMs detected, while in the PAG the concentrations of only the epoxide LA derived mediators, 9,10-EpOME and 12,13-EpOME, and one trihydroxy LA derived mediator, 9,10,11-TriHOME, were reduced. This data provides the first evidence that OXLAM concentrations in the brain are affected by chronic pain, suggesting that OXLAMs may be relevant to pain signaling and adaptation to chronic pain in pain circuits in the brain and that the current view of OXLAMs in nociception derived from studies in the periphery is incomplete.

Targeting CYP2J to reduce paclitaxel-induced peripheral neuropathic pain

Chemotherapy-induced peripheral neuropathic pain (CIPNP) is a severe dose- and therapy-limiting side effect of widely used cytostatics that is particularly difficult to treat. Here, we report increased expression of the cytochrome-P₄₅₀-epoxygenase CYP2J6 and increased concentrations of its linoleic acid metabolite 9,10-EpOME (9,10-epoxy-12Z-octadecenoic acid) in dorsal root ganglia (DRGs) of paclitaxel-treated mice as a model of CIPNP. The lipid sensitizes TRPV1 ion channels in primary sensory neurons and causes increased frequency of spontaneous excitatory postsynaptic currents in spinal cord nociceptive neurons, increased CGRP release from sciatic nerves and DRGs, and a reduction in mechanical and thermal pain hypersensitivity. In a drug repurposing screen targeting CYP2J2, the human ortholog of murine CYP2J6, we identified telmisartan, a widely used angiotensin II receptor antagonist, as a potent inhibitor. In a translational approach, administration of telmisartan reduces EpOME concentrations in DRGs and in plasma and reverses mechanical hypersensitivity in paclitaxel-treated mice. We therefore suggest inhibition of CYP2J isoforms with telmisartan as a treatment option for paclitaxel-induced neuropathic pain.

Beyond the classic eicosanoids: Peripherally-acting oxygenated metabolites of polyunsaturated fatty acids mediate pain associated with tissue injury and inflammation

Pain is a complex sensation that may be protective or cause undue suffering and loss of function, depending on the circumstances. Peripheral nociceptor neurons (PNs) innervate most tissues, and express ion channels, nocisensors, which depolarize the cell in response to intense stimuli and numerous substances. Inflamed tissues manifest inflammatory hyperalgesia in which the threshold for pain and the response to painful stimuli are decreased and increased, respectively. Constituents of the inflammatory milieu sensitize PNs, thereby contributing to hyperalgesia. Polyunsaturated fatty acids undergo enzymatic and free radical-mediated oxygenation into an array of bioactive metabolites, oxygenated polyunsaturated fatty acids (oxy-PUFAs), including the classic eicosanoids. Oxy-PUFA production is enhanced during inflammation. Pioneering studies by Vane and colleagues from the early 1970s first implicated classic eicosanoids in the pain associated with inflammation. Here, we review the production and action of oxy-PUFAs that are not classic eicosanoids, but nevertheless are produced in injured/ inflamed tissues and activate or sensitize PNs. In general, oxy-PUFAs that sensitize PNs may do so directly, by activation of nocisensors, ion channels or GPCRs expressed on the surface of PNs, or indirectly, by increasing the production of inflammatory mediators that activate or sensitize PNs. We focus on oxy-PUFAs that act directly on PNs. Specifically, we discuss the role of arachidonic acid-derived 12S-HpETE, HNE, ONE, PGA2, iso-PGA2 and 15d-PGJ2, 5,6-and 8,9-EET, PGE2-G and 8R,15S-diHETE, as well as the linoleic acid-derived 9-and 13-HODE in inducing acute nocifensive behavior and/or inflammatory hyperalgesia in rodents. The nocisensors TRPV1, TRPV4 and TRPA1, and putative Gαs-type GPCRs are the PN targets of these oxy-PUFAs.

Central activation of TRPV1 and TRPA1 by novel endogenous agonists contributes to mechanical allodynia and thermal hyperalgesia after burn injury

The primary complaint of burn victims is an intense, often devastating spontaneous pain, with persistence of mechanical and thermal allodynia. The transient receptor potential channels, TRPV1 and TRPA1, are expressed by a subset of nociceptive sensory neurons and contribute to inflammatory hypersensitivity. Although their function in the periphery is well known, a role for these TRP channels in central pain mechanisms is less well defined. Lipid agonists of TRPV1 are released from peripheral tissues via enzymatic oxidation after burn injury; however, it is not known if burn injury triggers the release of oxidized lipids in the spinal cord. Accordingly, we evaluated whether burn injury evoked the central release of oxidized lipids. Analysis of lipid extracts of spinal cord tissue with HPLC-MS revealed a significant increase in levels of the epoxide and diol metabolites of linoleic acid: 9,10-DiHOME, 12,13-DiHOME, 9(10)-EpOME, and 12(13)-EpOME, that was reduced after intrathecal (i.t.) injection of the oxidative enzyme inhibitor ketoconazole. Moreover, we found that these four lipid metabolites were capable of specifically activating both TRPV1 and TRPA1. Intrathecal injection of specific antagonists to TRPV1 (AMG-517) or TRPA1 (HC-030031) significantly reduced post-burn mechanical and thermal allodynia. Finally, i.t. injection of ketoconazole significantly reversed post-burn mechanical and thermal allodynia. Our data indicate that spinal cord TRPV1 and TRPA1 contributes to pain after burn and identifies a novel class of oxidized lipids elevated in the spinal cord after burn injury. Since the management of burn pain is problematic, these findings point to a novel approach for treating post-burn pain.

Role of Oxidized Lipids and TRP Channels in Orofacial Pain and Inflammation

Acute or chronic inflammation comprises a highly prevalent type of orofacial pain and is mediated by the generation of endogenous agonists that activate numerous receptors expressed on terminals of trigeminal (TG) nociceptive afferent neurons. One such studied receptor is transient receptor potential vanilloid subtype 1 (TRPV1). TRPV1 is a ligand-gated cation channel that is expressed on a major subclass of nociceptors and is found in many orofacial tissues, including dental pulp. Antagonists to TRPV1 reveal an important role for this channel in mediating hypersensitivity in preclinical models of inflammatory or neuropathic pain. Recent studies have demonstrated that endogenous TRPV1 agonists are generated by oxidation of omega-6 polyunsaturated fatty acids, including both linoleic acid and arachidonic acid. A major mechanism triggering the release of oxidative linoleic acid metabolites (OLAMs) and oxidative arachidonic acid metabolites (OAAMs) is the action of oxidative enzymes. Oxidative enzymes such as cytochrome P450 isozymes are rapidly upregulated in TG neurons after orofacial inflammation and increase the capacity of TG neurons to generate OLAMs. Cytochrome P450 isozymes are also increased in immune cells in irreversibly inflamed human dental pulp, and extracts of this tissue have significantly increased capacity to generate OLAMs. Together, these studies point to a novel pain mechanism involving the enzymatic generation of endogenous OLAM and OAAM agonists of TRPV1. This finding provides a rationale for an entirely new class of analgesics by inhibition of oxidative enzyme activity.

Persistent Nociception Triggered by Nerve Growth Factor (NGF) Is Mediated by TRPV1 and Oxidative Mechanisms

Nerve growth factor (NGF) is elevated in certain chronic pain conditions and is a sufficient stimulus to cause lasting pain in humans, but the actual mechanisms underlying the persistent effects of NGF remain incompletely understood. We developed a rat model of NGF-induced persistent thermal hyperalgesia and mechanical allodynia to determine the role of transient receptor potential vanilloid 1 (TRPV1) and oxidative mechanisms in the persistent effects of NGF. Persistent thermal hypersensitivity and mechanical allodynia require de novo protein translation and are mediated by TRPV1 and oxidative mechanisms. By comparing effects after systemic (subcutaneous), spinal (intrathecal) or hindpaw (intraplantar) injections of test compounds, we determined that TRPV1 and oxidation mediate persistent thermal hypersensitivity via peripheral and spinal sites of action and mechanical allodynia via only a spinal site of action. Therefore, NGF-evoked thermal and mechanical allodynia are mediated by spatially distinct mechanisms. NGF treatment evoked sustained increases in peripheral and central TRPV1 activity, as demonstrated by increased capsaicin-evoked nocifensive responses, increased calcitonin gene-related peptide release from hindpaw skin biopsies, and increased capsaicin-evoked inward current and membrane expression of TRPV1 protein in dorsal root ganglia neurons. Finally, we showed that NGF treatment increased concentrations of linoleic and arachidonic-acid-derived oxidized TRPV1 agonists in spinal cord and skin biopsies. Furthermore, increases in oxidized TRPV1-active lipids were reduced by peripheral and spinal injections of compounds that completely blocked persistent nociception. Collectively, these data indicate that NGF evokes a persistent nociceptive state mediated by increased TRPV1 activity and oxidative mechanisms, including increased production of oxidized lipid TRPV1 agonists.

β-arrestin-2-biased agonism of delta opioid receptors sensitizes transient receptor potential vanilloid type 1 (TRPV1) in primary sensory neurons

Despite advances in understanding the signaling mechanisms involved in the development and maintenance of chronic pain, the pharmacologic treatment of chronic pain has seen little advancement. Agonists at the mu opioid receptor (MOPr) continue to be vital in the treatment of many forms of chronic pain, but side-effects limit their clinical utility and range from relatively mild, such as constipation, to major, such as addiction and dependence. Additionally, chronic activation of MOPr results in pain hypersensitivity known as opioid-induced hyperalgesia (OIH), and we have shown recently that recruitment of β-arrestin2 to MOPr, away from transient potential vanilloid eceptor type 1 (TRPV1) in primary sensory neurons contributes to this phenomenon. The delta opioid receptor (DOPr) has become a promising target for the treatment of chronic pain, but little is known about the effects of chronic activation of DOPr on nociceptor sensitivity and OIH. Here we report that chronic activation of DOPr by the DOPr-selective agonist, SNC80, results in the sensitization of TRPV1 and behavioral signs of OIH via β-arrestin2 recruitment to DOPr and away from TRPV1. Conversely, chronic treatment with ARM390, a DOPr-selective agonist that does not recruit β-arrestin2, neither sensitized TRPV1 nor produced OIH. Interestingly, the effect of SNC80 to sensitize TRPV1 is species-dependent, as rats developed OIH but mice did not. Taken together, the reported data identify a novel side-effect of chronic administration of β-arrestin2-biased DOPr agonists and highlight the importance of potential species-specific effects of DOPr agonists.

Major blunt trauma evokes selective upregulation of oxidative enzymes in circulating leukocytes

Tissue injury, such as burns or inflammation, can lead to the generation of oxidized lipids capable of regulating hemodynamic, pulmonary, immune, and neuronal responses. However, it is not known whether traumatic injury leads to a selective upregulation of transcripts encoding oxidative enzymes capable of generating these mediators. Here, we analyzed microarrays taken from circulating leukocytes of 187 trauma subjects compared with 97 control volunteers for changes in the expression of 105 oxidative enzymes and related receptors. The results indicate that major blunt trauma triggers a selective change in gene expression, with some transcripts undergoing highly significant upregulation (e.g., CYP2C19), while others display significantly reduced expression (e.g., CYP2U1). This pattern in gene expression was maintained for up to 28 days after injury. In addition, the level of expression of CYP2A7, CYP2B7P1, CYP2C19, CYP2E1, CYP4A11, CYP4F3, CYP8B1, CYP19A1, CYP20A1, CYP51A1, HMOX2, NCF1, NCF2, and NOX1 and the receptors PTGER2 and ESR2 were correlated with clinical trauma indices such as APACHE II, Max Denver Scale, and the Injury Severity Score. Demonstration of a selective alteration in expression of transcripts encoding oxidative enzymes reveals a complex molecular response to major blunt trauma in circulating leukocytes. Furthermore, the association between changes in gene expression and clinical trauma scores suggests an important role in integrating pathophysiologic responses to blunt force trauma.

Synthesis of lipid mediators during UVB-induced inflammatory hyperalgesia in rats and mice

Peripheral sensitization during inflammatory pain is mediated by a variety of endogenous proalgesic mediators including a number of oxidized lipids, some of which serve endogenous modulators of sensory TRP-channels. These lipids are eicosanoids of the arachidonic acid and linoleic acid pathway, as well as lysophophatidic acids (LPAs). However, their regulation pattern during inflammatory pain and their contribution to peripheral sensitization is still unclear. Here, we used the UVB-model for inflammatory pain to investigate alterations of lipid concentrations at the site of inflammation, the dorsal root ganglia (DRGs) as well as the spinal dorsal horn and quantified 21 lipid species from five different lipid families at the peak of inflammation 48 hours post irradiation. We found that known proinflammatory lipids as well as lipids with unknown roles in inflammatory pain to be strongly increased in the skin, whereas surprisingly little changes of lipid levels were seen in DRGs or the dorsal horn. Importantly, although there are profound differences between the number of cytochrome (CYP) genes between mice and rats, CYP-derived lipids were regulated similarly in both species. Since TRPV1 agonists such as LPA 18∶1, 9- and 13-HODE, 5- and 12-HETE were elevated in the skin, they may contribute to thermal hyperalgesia and mechanical allodynia during UVB-induced inflammatory pain. These results may explain why some studies show relatively weak analgesic effects of cyclooxygenase inhibitors in UVB-induced skin inflammation, as they do not inhibit synthesis of other proalgesic lipids such as LPA 18∶1, 9-and 13-HODE and HETEs.

TRP-channels as key integrators of lipid pathways in nociceptive neurons

TRP-channels are the most prominent family of ligand-gated ion channels for pain perception. In sensory neurons, TRPV1-V4, TRPA1 and TRPM8 are expressed and are responsible for the conversion of external stimuli to painful sensations. Under pathophysiological conditions, excessive activity of TRP-channels leads to mechanical allodynia and thermal hyperalgesia. Among the endogenous TRP-channel sensitizers, activators and inhibitors, more than 50 arachidonic acid- and linoleic acid-metabolites from the COX-, LOX- and CYP-pathways, as well as lysophospholipids and isoprenoids can be found. As a consequence, these lipids represent the vast majority of endogenous TRP-channel modulators in sensory neurons. Although the precise mechanisms of TRP-channel modulation by most lipids are still unknown, it became clear that lipids can either bind directly to the target TRP-channel or modulate TRP-channels indirectly by activating G-protein coupled receptors. Thus, TRP-channels seem to be key sensors for lipids, integrating and interpreting incoming signals from the different metabolic lipid pathways. Here, we discuss the specific properties of the currently known endogenous lipid-derived TRP-channel modulators concerning their ability to activate or inhibit TRP-channels, the molecular mechanisms of lipid/TRP-channel interactions and specific TRP-regulatory characteristics of the individual lipid families.

Prolactin regulates TRPV1, TRPA1, and TRPM8 in sensory neurons in a sex-dependent manner: Contribution of prolactin receptor to inflammatory pain

Prolactin (PRL) is a hormone produced in the anterior pituitary but also synthesized extrapituitary where it can influence diverse cellular processes, including inflammatory responses. Females experience greater pain in certain inflammatory conditions, but the contribution of the PRL system to sex-dependent inflammatory pain is unknown. We found that PRL regulates transient receptor potential (TRP) channels in a sex-dependent manner in sensory neurons. At >20 ng/ml, PRL sensitizes TRPV1 in female, but not male, neurons. This effect is mediated by PRL receptor (PRL-R). Likewise, TRPA1 and TRPM8 were sensitized by 100 ng/ml PRL only in female neurons. We showed that complete Freund adjuvant (CFA) upregulated PRL levels in the inflamed paw of both male and female rats, but levels were higher in females. In contrast, CFA did not change mRNA levels of long and short PRL-R in the dorsal root ganglion or spinal cord. Analysis of PRL and PRL-R knockout (KO) mice demonstrated that basal responses to cold stimuli were only altered in females, and with no significant effects on heat and mechanical responses in both sexes. CFA-induced heat and cold hyperalgesia were not changed in PRL and PRL-R KO compared with wild-type (WT) males, whereas significant reduction of heat and cold post-CFA hyperalgesia was detected in PRL and PRL-R KO females. Attenuation of CFA-induced mechanical allodynia was observed in both PRL and PRL-R KO females and males. Thermal hyperalgesia in PRL KO females was restored by administration of PRL into hindpaws. Overall, we demonstrate a sex-dependent regulation of peripheral inflammatory hyperalgesia by the PRL system.

Role of endogenous TRPV1 agonists in a postburn pain model of partial-thickness injury

Oxidized linoleic acid metabolites (OLAMs) are a class of endogenous transient receptor potential vanilloid 1 (TRPV1) channel agonists released on exposure of tissue to transient noxious temperatures. These lipid compounds also contribute to inflammatory and heat allodynia. Because persistent pain after a burn injury represents a significant clinical challenge for treatment, we developed an in vivo rat model of partial-thickness cutaneous thermal injury and examined whether TRPV1 and specific OLAM metabolites play a role in mediating postburn pain injury. This peripheral model of burn injury had marked thermal allodynia peaking at 24h after thermal injury, with allodynia being maintained for up to 7d. Immunohistochemical characterization of tissue taken from injury sites revealed an increase in leukocyte/macrophage infiltration that was colocalized with TRPV1-positive fibers. Using this peripheral thermal injury model, we found that pharmacological blockade of peripheral TRPV1 receptors reduced thermal allodynia by about 98%. Moreover, there was a significant increase in OLAM levels compared to naive controls in hind paw skin biopsies. Additional studies of the metabolism of [C(14)]-linoleic acid in skin biopsies revealed the role of the cytochrome P450 (CYP) system in mediating the metabolism of linoleic acid after thermal injury. Finally, we demonstrated that direct inhibition of OLAMs using OLAM antibodies and indirect inhibition using the CYP inhibitor ketoconazole significantly reduced postburn thermal allodynia. Collectively, these findings point to a novel role of the OLAMs and CYP-related enzymes in generating postburn allodynia via activation of peripheral TRPV1.

Oxidized linoleic acid metabolite-cytochrome P450 system (OLAM-CYP) is active in biopsy samples from patients with inflammatory dental pain

Endogenous TRPV1 agonists such as oxidized linoleic acid metabolites (OLAMs) and the enzymes releasing them [eg, cytochrome P450 (CYP)] are up-regulated after inflammation in the rat. However, it is not known whether such agonists are elevated in human inflammatory pain conditions. Because TRPV1 is expressed in human dental pulp nociceptors, we hypothesized that OLAM-CYP machinery is active in this tissue type and is increased under painful inflammatory conditions such as irreversible pulpitis (IP). The aim of this study was to compare CYP expression and linoleic acid (LA) metabolism in normal vs inflamed human dental pulp. Our data showed that exogenous LA metabolism was significantly increased in IP tissues compared to normal tissues and that pretreatment with a CYP inhibitor, ketoconazole, significantly inhibited LA metabolism. Additionally, extracts obtained from LA-treated inflamed tissues evoked significant inward currents in trigeminal ganglia neurons and were blocked by pretreatment with the TRPV1 antagonist IRTX. Moreover, extracts obtained from ketoconazole-pretreated inflamed tissues significantly reduced inward currents in trigeminal ganglia neurons. These data suggest that LA metabolites produced in human inflamed tissues act as TRPV1 agonists and that the metabolite production can be targeted by CYP inhibition. In addition, immunohistochemical analysis of 2 CYP isoforms, CYP2J and CYP3A1, were shown to be predominately expressed in immune cells infiltrating the inflamed dental pulp, emphasizing the paracrine role of CYP enzymes in OLAM regulation. Collectively, our data indicate that the machinery responsible for OLAM production is up-regulated during inflammation and can be targeted to develop potential analgesics for inflammatory-induced dental pain.

Temporal changes of cytochrome P450 (Cyp) and eicosanoid-related gene expression in the rat brain after traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) induces arachidonic acid (ArA) release from cell membranes. ArA metabolites form a class of over 50 bioactive eicosanoids that can induce both adaptive and/or maladaptive brain responses. The dynamic metabolism of ArA to eicosanoids, and how they affect the injured brain, is poorly understood due to their diverse activities, trace levels, and short half-lives. The eicosanoids produced in the brain postinjury depend upon the enzymes present locally at any given time. Eicosanoids are synthesized by heme-containing enzymes, including cyclooxygenases, lipoxygenases, and arachidonate monoxygenases. The latter comprise a subset of the cytochrome P450 "Cyp" gene family that metabolize fatty acids, steroids, as well as endogenous and exogenous toxicants. However, for many of these genes neither baseline neuroanatomical nor injury-related temporal expression have been studied in the brain.In a rat model of parietal cortex TBI, Cyp and eicosanoid-related mRNA levels were determined at 6 h, 24 h, 3d, and 7d postinjury in parietal cortex and hippocampus, where dynamic changes in eicosanoids have been observed. Quantitative real-time polymerase chain reaction with low density arrays were used to assay 62 rat Cyps, 37 of which metabolize ArA or other unsaturated fatty acids; 16 eicosanoid-related enzymes that metabolize ArA or its metabolites; 8 eicosanoid receptors; 5 other inflammatory- and recovery-related genes, plus 2 mouse Cyps as negative controls and 3 highly expressed "housekeeping" genes.

RESULTS

Sixteen arachidonate monoxygenases, 17 eicosanoid-related genes, and 12 other Cyps were regulated in the brain postinjury (p < 0.05, Tukey HSD). Discrete tissue levels and distinct postinjury temporal patterns of gene expression were observed in hippocampus and parietal cortex.

CONCLUSIONS

The results suggest complex regulation of ArA and other lipid metabolism after TBI. Due to the temporal nature of brain injury-induced Cyp gene induction, manipulation of each gene (or its products) at a given time after TBI will be required to assess their contributions to secondary injury and/or recovery. Moreover, a better understanding of brain region localization and cell type-specific expression may be necessary to deduce the role of these eicosanoid-related genes in the healthy and injured brain.

The cytochrome P450 inhibitor, ketoconazole, inhibits oxidized linoleic acid metabolite-mediated peripheral inflammatory pain

Background

Oxidized linoleic acid metabolites (OLAMs) are a class of endogenous agonists to the transient receptor potential V1 (TRPV1) receptor. Although TRPV1 mediates inflammatory heat hyperalgesia, it is not known if the OLAMs contribute to the peripheral activation of this receptor during tissue inflammation. In the present study, we evaluated whether the OLAM system is activated during inflammation and whether cytochrome P450 enzymes mediate OLAM contributions to heat hyperalgesia using the complete Freund’s adjuvant (CFA) model of inflammation.

Results

Our results demonstrate that the intraplantar (ipl) injection of anti-OLAM antibodies significantly reversed CFA-induced heat hyperalgesia. Moreover, application of lipid extracts from inflamed rat skin to cultured sensory neurons triggered a significant release of iCGRP that is blocked by co-treatment with I-RTX, a TRPV1 antagonist. To determine the role of CYP enzymes in mediating OLAM effects, we used a broad spectrum CYP inhibitor, ketoconazole. Pretreatment with ketoconazole inhibited the release of TRPV1 agonists in lipid extracts from inflamed skin and significantly reversed CFA-induced heat hyperalgesia by a peripheral mechanism of action. Moreover, the ipl injection of linoleic acid to rats 24 hr after CFA evoked spontaneous nocifensive behaviors that were significantly reduced by capsazepine, by knockout of the TRPV1 gene, or by pretreatment with either anti-OLAM antibodies or ketoconazole.

Conclusions

Taken together, our data suggests that OLAMs contribute to inflammatory nociception in the periphery and that cytochrome P450 enzymes play a crucial role in mediating OLAM contributions to inflammatory heat hyperalgesia.

Sex differences in serotonin enhancement of capsaicin-evoked calcitonin gene-related peptide release from human dental pulp

Serotonin (5HT) is a pronociceptive mediator in the periphery, and evidence implicates involvement in trigeminal pain processing. However, the mechanism(s) by which 5HT modulates trigeminal nociceptors remains unclear. Trigeminal pain can be evoked by the transient receptor potential V1 channel (TRPV1), which is expressed by nociceptive trigeminal neurons and induces release of proinflammatory calcitonin gene-related peptide (CGRP). In our preclinical models, 5HT evoked thermal hyperalgesia and enhanced calcium influx and CGRP release from the TRPV1 population of trigeminal nociceptors. Whether this occurs in humans is unknown. As dental pulp is densely innervated by trigeminal nociceptors, routine tooth extractions offer a unique opportunity to examine whether 5HT enhances CGRP release from human nociceptors. Pulpal tissue was collected from 140 extracted molar teeth from men and women, and basal release samples were collected before treatment with saline or 5HT 100μmol/L. CGRP release was then stimulated with the TRPV1 agonist capsaicin 1μmol/L and quantitated by enzyme immunoassay. Additional samples were collected for Western blots to examine 5HT receptor expression. We report that 5HT induced a significant increase in capsaicin-evoked CGRP release, and that this enhancement was observed only in female dental pulp, with no effect of 5HT on male dental pulp. The greatest amount of CGRP release occurred in dental pulp from women in the luteal phase of the menstrual cycle. These results indicate that 5HT enhances capsaicin-evoked CGRP release from human trigeminal nociceptors in a sexually dimorphic manner providing a mechanistic basis for prevalence of trigeminal pain disorders in women.