Julie Keeble

The functions of TRPA1 and TRPV1: moving away from sensory nerves

The transient receptor potential vanilloid 1 and ankyrin 1 (TRPV1 and TRPA1, respectively) channels are members of the TRP superfamily of structurally related, non‐selective cation channels. It is rapidly becoming clear that the functions of TRPV1 and TRPA1 interlink with each other to a considerable extent. This is especially clear in relation to pain and neurogenic inflammation where TRPV1 is coexpressed on the vast majority of TRPA1‐expressing sensory nerves and both integrate a variety of noxious stimuli. The more recent discovery that both TRPV1 and TRPA1 are expressed on a multitude of non‐neuronal sites has led to a plethora of research into possible functions of these receptors. Non‐neuronal cells on which TRPV1 and TRPA1 are expressed vary from vascular smooth muscle to keratinocytes and endothelium. This review will discuss the expression, functionality and roles of these non‐neuronal TRP channels away from sensory nerves to demonstrate the diverse nature of TRPV1 and TRPA1 in addition to a direct role in pain and neurogenic inflammation.

Pharmacology and potential therapeutic applications of nitric oxide‐releasing non‐steroidal anti‐inflammatory and related nitric oxide‐donating drugs

This review examines the biological significance, therapeutic potential and mechanism(s) of action of a range of nitric oxide‐releasing non‐steroidal anti‐inflammatory drugs (NO‐NSAID) and related nitric oxide‐releasing donating drugs (NODD). The slow release of nitric oxide (NO) from these compounds leads to subtle changes in the profile of pharmacological activity of the parent, non‐steroidal anti‐inflammatory drugs (NSAID). For example, compared with NSAID, NO‐NSAID cause markedly diminished gastrointestinal toxicity and improved anti‐inflammatory and anti‐nociceptive efficacy. In addition, nitroparacetamol exhibits hepatoprotection as opposed to the hepatotoxic activity of paracetamol. The possibility that NO‐NSAID or NODD may be of therapeutic benefit in a wide variety of disease states including pain and inflammation, thrombosis and restenosis, neurodegenerative diseases of the central nervous system, colitis, cancer, urinary incontinence, liver disease, impotence, bronchial asthma and osteoporosis is discussed.

A distinct role for transient receptor potential ankyrin 1, in addition to transient receptor potential vanilloid 1, in tumor necrosis factor α–induced inflammatory hyperalgesia and Freund's complete adjuvant–induced monarthritis

OBJECTIVE To investigate the involvement of transient receptor potential ankyrin 1 (TRPA1) in inflammatory hyperalgesia mediated by tumor necrosis factor α(TNFα) and joint inflammation. METHODS Mechanical hyperalgesia was assessed in CD1 mice, mice lacking functional TRP vanilloid 1 (TRPV1-/-) or TRPA1 (TRPA1-/-), or respective wildtype (WT) mice. An automated von Frey system was used, following unilateral intraplantar injection of TNFα or intraarticular injection of Freunds complete adjuvant (CFA). Knee swelling and histologic changes were determined in mice treated with intraarticular injections of CFA. RESULTS TNFα induced cyclooxygenase-independent bilateral mechanical hyperalgesia in CD1 mice. The selective TRPV1 receptor antagonist SB-366791 had no effect on mechanical hyperalgesia when it was coinjected with TNFα, but intrathecally administered SB- 366791 attenuated bilateral hyperalgesia, indicating the central but not peripheral involvement of TRPV1 receptors. A decrease in pain sensitivity was also observed in TRPV1-/- mice. Intraplantar coadministration of the TRPA1 receptor antagonist AP-18 with TNFα inhibited bilateral hyperalgesia. Intrathecal treatment with AP-18 also reduced TNFα-induced hyperalgesia. CFA-induced mechanical hyperalgesia in CD1 mice was attenuated by AP-18 (administered by intraarticular injection 22 hours after the administration of CFA). Furthermore, intraarticular CFA–induced ipsilateral mechanical hyperalgesia was maintained for 3 weeks in TRPA1 WT mice. In contrast, TRPA1-/- mice exhibited mechanical hyperalgesia for only 24 hours after receiving CFA. CONCLUSION Evidence suggests that endogenous activation of peripheral TRPA1 receptors plays a critical role in the development of TNFα-induced mechanical hyperalgesia and in sustaining the mechanical hyperalgesia observed after intraaarticular injection of CFA. These results suggest that blockade of TRPA1 receptors may be beneficial in reducing the chronic pain associated with arthritis.

The paradoxical role of the transient receptor potential vanilloid 1 receptor in inflammation.

The transient potential receptor vanilloid 1 (TRPV1) receptor is a non-selective cation channel that is chemically activated by capsaicin, the pungent component of hot peppers. In addition, endogenous compounds, in particular the endogenous cannabinoid receptor activator, anandamide, have been demonstrated to activate TRPV1 in vivo. TRPV1 receptors are also activated by temperatures within the noxious range (>43 degrees C) and low pH (<pH 6.0). TRPV1 receptors are predominantly expressed in primary afferent fibres which are peptidergic sensory neurones, such as the thinly myelinated A-delta and unmyelinated C-fibres. TRPV1 receptors have also been demonstrated to be present in non-neuronal cells. Historically, TRPV1 has been considered as a pro-inflammatory receptor due to its key role in several conditions, including neuropathic pain, joint inflammation and inflammatory bowel disease, amongst others. However, the purpose of this review is to underline the emerging new evidence which demonstrate paradoxical, protective functions for this unique receptor in vivo. For example, in experimentally induced sepsis, TRPV1 null mice demonstrated elevated levels of pathological markers in comparison to wild-type mice. In addition to the pro-inflammatory and protective roles of TRPV1 in pathophysiological states, TRPV1 has also been shown to have important functions under normal physiological conditions, for example in urinary bladder function, thermoregulation and neurogenesis. The emerging functions of TRPV1 highlight the necessity for further research in light of increasing reports of potential TRPV1 antagonists undergoing pre-clinical experimentations.

Hydrogen peroxide is a novel mediator of inflammatory hyperalgesia, acting via transient receptor potential vanilloid 1-dependent and independent mechanisms

Abstract Inflammatory diseases associated with pain are often difficult to treat in the clinic due to insufficient understanding of the nociceptive pathways involved. Recently, there has been considerable interest in the role of reactive oxygen species (ROS) in inflammatory disease, but little is known of the role of hydrogen peroxide (H2O2) in hyperalgesia. In the present study, intraplantar injection of H2O2‐induced a significant dose‐ and time‐dependent mechanical and thermal hyperalgesia in the mouse hind paw, with increased c‐fos activity observed in the dorsal horn of the spinal cord. H2O2 also induced significant nociceptive behavior such as increased paw licking and decreased body liftings. H2O2 levels were significantly raised in the carrageenan‐induced hind paw inflammation model, showing that this ROS is produced endogenously in a model of inflammation. Moreover, superoxide dismutase and catalase significantly reduced carrageenan‐induced mechanical and thermal hyperalgesia, providing evidence of a functionally significant endogenous role. Thermal, but not mechanical, hyperalgesia in response to H2O2 (i.pl.) was longer lasting in TRPV1 wild type mice compared to TRPV1 knockouts. It is unlikely that downstream lipid peroxidation was increased by H2O2. In conclusion, we demonstrate a notable effect of H2O2 in mediating inflammatory hyperalgesia, thus highlighting H2O2 removal as a novel therapeutic target for anti‐hyperalgesic drugs in the clinic.

The transient receptor potential vanilloid 1 (TRPV1) receptor protects against the onset of sepsis after endotoxin

Transient potential vanilloid 1 (TRPV1) receptor is an ion channel receptor primarily localized on sensory nerves and activated by specific stimuli to initiate and amplify pain and inflammation, as typified by murine models of scald and arthritis. Little is known of the role of TRPV1 in sepsis, an infective disease associated with inflammation. Through use of a suble‐thal murine model of lipopolysaccharide‐induced peritoneal sepsis, we provide novel evidence that genetic deletion of TRPV1 leads to an enhanced onset of various pathological components of systemic endotox‐emia. Paired studies of TRPV1 knockout (KO) and wild‐type mice demonstrate significantly enhanced hypotension (56±2% vs. 38±6% decrease in blood pressure, n=12), hypothermia (13±3% vs. 7±1% decrease in core temperature, n=6), and peritoneal exudate mediator levels (TNF‐α, 0.78±0.2 vs. 0.38±0.1 ng/ml; nitrite, for NO, 35±10 vs. 15±3 μM; n=8) in TRPV1 KO mice, indicating loss of protective effect. Findings correlated with liver edema and raised plasma levels of aspartate aminotransferase in TRPV1 KO mice. These data suggest that TRPV1 may play an important regulatory role in sepsis independent of the major sensory neuropeptide substance P. The findings are relevant to developing strategies that increase the beneficial, and reduce the harmful, components of sepsis to prevent and treat this often fatal condition.—Clark, N., Keeble, J., Fernandes, E. S., Starr, A., Liang, L., Sugden, D., de Winter, P., Brain, S. D. The transient receptor potential vanilloid 1 (TRPV1) receptor protects against the onset of sepsis after endotoxin. FASEB J. 21, 3747–3755 (2007)

The complex effects of the slow-releasing hydrogen sulfide donor GYY4137 in a model of acute joint inflammation and in human cartilage cells

The role of hydrogen sulfide (H2S) in inflammation remains unclear with both pro‐ and anti‐inflammatory actions of this gas described. We have now assessed the effect of GYY4137 (a slow‐releasing H2S donor) on lipopolysaccharide (LPS)‐evoked release of inflammatory mediators from human synoviocytes (HFLS) and articular chondrocytes (HAC) in vitro. We have also examined the effect of GYY4137 in a complete Freunds adjuvant (CFA) model of acute joint inflammation in the mouse. GYY4137 (0.1–0.5 mM) decreased LPS‐induced production of nitrite (NO2−), PGE2, TNF‐α and IL‐6 from HFLS and HAC, reduced the levels and catalytic activity of inducible nitric oxide synthase (iNOS) and cyclooxygenase‐2 (COX‐2) and reduced LPS‐induced NF‐κB activation in vitro. Using recombinant human enzymes, GYY4137 inhibited the activity of COX‐2, iNOS and TNF‐α converting enzyme (TACE). In the CFA‐treated mouse, GYY4137 (50 mg/kg, i.p.) injected 1 hr prior to CFA increased knee joint swelling while an anti‐inflammatory effect, as demonstrated by reduced synovial fluid myeloperoxidase (MPO) and N‐acetyl‐β‐D‐glucosaminidase (NAG) activity and decreased TNF‐α, IL‐1β, IL‐6 and IL‐8 concentration, was apparent when GYY4137 was injected 6 hrs after CFA. GYY4137 was also anti‐inflammatory when given 18 hrs after CFA. Thus, although GYY4137 consistently reduced the generation of pro‐inflammatory mediators from human joint cells in vitro, its effect on acute joint inflammation in vivo depended on the timing of administration.

A role for substance P in arthritis

Substance P is a neuropeptide that is released from sensory nerves and which has a number of pro-inflammatory effects. In this article, we review the evidence for a role of substance P in arthritis, both in experimental animal models and rheumatoid arthritis patients. Substance P expression is altered in the joint and dorsal horn of arthritic animals, exogenous substance P and neurokinin 1 (NK(1)) receptor antagonists modulate responses in the joint, and there is some evidence for a role of substance P in human joint disease. However, the therapeutic potential of NK(1) receptor antagonists in the treatment of rheumatoid arthritis remains controversial.

TRPV1 Deletion Enhances Local Inflammation and Accelerates the Onset of Systemic Inflammatory Response Syndrome

The transient receptor potential vanilloid 1 (TRPV1) is primarily localized to sensory nerve fibers and is associated with the stimulation of pain and inflammation. TRPV1 knockout (TRPV1KO) mice show enhanced LPS-induced sepsis compared with wild type (WT). This implies that TRPV1 may have a key modulatory role in increasing the beneficial and reducing the harmful components in sepsis. We investigated immune and inflammatory mechanisms in a cecal ligation and puncture (CLP) model of sepsis over 24 h. CLP TRPV1KO mice exhibited significant hypothermia, hypotension, and organ dysfunction compared with CLP WT mice. Analysis of the inflammatory responses at the site of initial infection (peritoneal cavity) revealed that CLP TRPV1KO mice exhibited: 1) decreased mononuclear cell integrity associated with apoptosis, 2) decreased macrophage tachykinin NK1-dependent phagocytosis, 3) substantially decreased levels of nitrite (indicative of NO) and reactive oxygen species, 4) increased cytokine levels, and 5) decreased bacteria clearance when compared with CLP WT mice. Therefore, TRPV1 deletion is associated with impaired macrophage-associated defense mechanisms. Thus, TRPV1 acts to protect against the damaging impact of sepsis and may influence the transition from local to a systemic inflammatory state.

Tumour necrosis factor α mediates transient receptor potential vanilloid 1-dependent bilateral thermal hyperalgesia with distinct peripheral roles of interleukin-1β, protein kinase C and cyclooxygenase-2 signalling

ABSTRACT TNFα plays a pivotal role in rheumatoid arthritis (RA) but little is known of the mechanisms that link the inflammatory and nociceptive effects of TNFα. We have established a murine model of TNFα‐induced TRPV1‐dependent bilateral thermal hyperalgesia that then allowed us to identify distinct peripheral mechanisms involved in mediating TNFα‐induced ipsilateral and contralateral hyperalgesia. Thermal hyperalgesia and inflammation were assessed in both hindpaws following unilateral intraplantar (i.pl.) TNFα. The hyperalgesic mechanisms were analysed through pharmacogenetic approaches involving TRPV1−/− mice and TRPV1 antagonists. To study the mediators downstream of TNFα, cyclooxygenase (COX) and PKC inhibitors were utilised and cytokine and prostaglandin levels assessed. The role of neutrophils was determined through use of the selectin inhibitor, fucoidan. We show that TNFα (10 pmol) causes thermal hyperalgesia (1–4 h) in the ipsilateral inflamed and contralateral uninjured hindpaws, which is TRPV1‐dependent. GF109203X, a PKC inhibitor, suppressed the hyperalgesia indicating that PKC is involved in TRPV1 sensitisation. Ipsilateral COX‐2‐derived prostaglandins were also crucial to the development of the bilateral hyperalgesia. The prevention of neutrophil accumulation with fucoidan attenuated hyperalgesia at 4 but not at 1 h, indicating a role in the maintenance but not in the induction of bilateral hyperalgesia. However, TNFα‐induced IL‐1β generation in both paws and the presence of local IL‐1β in the contralateral paw were essential for the development of bilateral hyperalgesia. These results identify a series of peripheral events through which TNFα triggers and maintains bilateral inflammatory pain. This potentially allows a better understanding of mechanisms involved in TNFα‐dependent pain pathways in symmetrical diseases such as arthritis.