Hypersensitivity Induced by Activation of Spinal Cord PAR2 Receptors Is Partially Mediated by TRPV1 Receptors

Medial and dorsal lateral septum involving social disruption stress-primed escalation in acid-induced writhes

Introduction

Stress may cause prospective escalations in abdominal pain magnitude and accumbal TRPV1 expression, while central neural circuits mediating these stress effects remain unclear.

Methods

Using retrograde tracing methods, we first demonstrated the existence of a medial septal-dorsal lateral septal -accumbal circuit very likely involving social disruption stress-primed escalations in acid-induced writhes and accumbal TRPV1 level. An intersectional viral strategy and virus-carrying hM3Dq and hM4Di DREADDs were, then, employed to selectively modulate GABAergic and cholinergic neuronal activity in medial and dorsal lateral septum.

Results

Exciting medial septal GABAergic neuron was found to prevent social disruption stress-primed escalations in acid-induced writhes and accumbal TRPV1 and PKCε expressions. Likewise, inactivating dorsal lateral septal cholinergic neurons was also effective in abolishing these stress-primed escalations. Inactivating GABAergic neuron in non-stressed animals' medial septum was found to reproduce the stress-primed effects in causing heightened acid-induced writhes and accumbal TRPV1 and PKCε levels.

Discussion

These results, taken together, prompt us to conclude that social disruption stress may produce plastic changes in a newly-identified medial septal-dorsal lateral septal-accumbal circuit. Moreover, medial septal GABAergic hypoactivity and dorsal lateral septal cholinergic hyperactivity are, at least, two likely causes reflecting such stress-produced escalations in abdominal pain magnitude and pain transduction-related protein over-expression in nucleus accumbens.

TRP Channels as Molecular Targets to Relieve Cancer Pain

Transient receptor potential (TRP) channels are critical receptors in the transduction of nociceptive stimuli. The microenvironment of diverse types of cancer releases substances, including growth factors, neurotransmitters, and inflammatory mediators, which modulate the activity of TRPs through the regulation of intracellular signaling pathways. The modulation of TRP channels is associated with the peripheral sensitization observed in patients with cancer, which results in mild noxious sensory stimuli being perceived as hyperalgesia and allodynia. Secondary metabolites derived from plant extracts can induce the activation, blocking, and desensitization of TRP channels. Thus, these compounds could act as potential therapeutic agents, as their antinociceptive properties could be beneficial in relieving cancer-derived pain. In this review, we will summarize the role of TRPV1 and TRPA1 in pain associated with cancer and discuss molecules that have been reported to modulate these channels, focusing particularly on the mechanisms of channel activation associated with molecules released in the tumor microenvironment.

Social disruption-induced stress pre-exposure aggravates, while the presence of conspecifics diminishes, acetic acid-induced writhing

RATIONALE AND OBJECTIVE

This study was undertaken to assess the modulating effects of (1) pre-exposure to repeated social disruption and (2) group testing on writhing associated with visceral pain induced by intraperitoneal administration of acetic acid.

MATERIALS AND METHODS

Six consecutive days of social disruption were used to prime for stress, while group testing referred to 3 mouse cage-mates receiving the acetic acid-induced writhing test as a group.

RESULTS

Social disruption-induced stress-pre-exposed mice displayed a greater number acid-induced writhes compared to mice not receiving the pre-exposure. However, mice displayed fewer acid-induced writhes in a triad group vs. individually, suggesting group-mediated writhing-reducing effects. Likewise, group testing prevented the stress pre-exposure escalation in acid-induced writhes. Additional studies revealed that the stress-pre-exposed mice had increased expression in accumbal TRPV1 receptors. Systemic (0.25 mg/kg) and bilateral intra-accumbal (0.2 ng/0.2 µl/side) administration of SB366791, a TRPV1 receptor antagonist, reliably prevented the stress pre-exposure escalation in acid-induced writhing; SB366791 treatment alone did not affect acid-induced writhing, stress pre-exposure anxiety-like behavior, or the group testing effects. Furthermore, lower neuronal activation was found in the medial septal nucleus in group vs. individual tested mice. Intra-medial septum (0.2 µg/0.5 µl) infusion with bicuculline, a GABAA receptor antagonist, effectively prevented group-mediated writhing-reducing effects, but not individual acid-induced writhing effects.

CONCLUSIONS

These findings suggest that social disruption-induced stress pre-exposure may upregulate accumbal TRPV1 receptor expression and consequently aggravate acid-induced writhing. Group testing prevents such stress pre-exposure escalation of acid-induced writhing most likely by strengthening the GABAergic inhibition on local neural activity in the medial septum.

TRPV1 Channel Activated by the PGE2/EP4 Pathway Mediates Spinal Hypersensitivity in a Mouse Model of Vertebral Endplate Degeneration

Low back pain (LBP) is the primary cause of disability globally. There is a close relationship between Modic changes or endplate defects and LBP. Endplates undergo ossification and become highly porous during intervertebral disc (IVD) degeneration. In our study, we used a mouse model of vertebral endplate degeneration by lumbar spine instability (LSI) surgery. Safranin O and fast green staining and μCT scan showed that LSI surgery led to endplate ossification and porosity, but the endplates in the sham group were cartilaginous and homogenous. Immunofluorescent staining demonstrated the innervation of calcitonin gene-related peptide- (CGRP-) positive nerve fibers in the porous endplate of LSI mice. Behavior test experiments showed an increased spinal hypersensitivity in LSI mice. Moreover, we found an increased cyclooxygenase 2 (COX2) expression and an elevated prostaglandin E2 (PGE2) concentration in the porous endplate of LSI mice. Immunofluorescent staining showed the colocalization of E-prostanoid 4 (EP4)/transient receptor potential vanilloid 1 (TRPV1) and CGRP in the nerve endings in the endplate and in the dorsal root ganglion (DRG) neurons, and western blotting analysis demonstrated that EP4 and TRPV1 expression significantly increased in the LSI group. Our patch clamp study further showed that LSI surgery significantly enhanced the current density of the TRPV1 channel in small-size DRG neurons. A selective EP4 receptor antagonist, L161982, reduced the spinal hypersensitivity of LSI mice by blocking the PGE2/EP4 pathway. In addition, TRPV1 current and neuronal excitability in DRG neurons were also significantly decreased by L161982 treatment. In summary, the PGE2/EP4 pathway in the porous endplate could activate the TRPV1 channel in DRG neurons to cause spinal hypersensitivity in LSI mice. L161982, a selective EP4 receptor antagonist, could turn down the TRPV1 current and decrease the neuronal excitability of DRG neurons to reduce spinal pain.

Transient receptor potential vanilloid subtype 1 depletion mediates mechanical allodynia through cellular signal alterations in small-fiber neuropathy

Transient receptor potential vanilloid subtype 1 (TRPV1) is a polymodal nociceptor that monitors noxious thermal sensations. Few studies have addressed the role of TRPV1 in mechanical allodynia in small-fiber neuropathy (SFN) caused by sensory nerve damage. Accordingly, this article reviews the putative mechanisms of TRPV1 depletion that mediates mechanical allodynia in SFN. The intraepidermal nerve fibers (IENFs) degeneration and sensory neuronal injury are the primary characteristics of SFN. Intraepidermal nerve fibers are mainly C-polymodal nociceptors and Aδ-fibers, which mediated allodynic pain after neuronal sensitization. TRPV1 depletion by highly potent neurotoxins induces the upregulation of activating transcription factor 3 and IENFs degeneration which mimics SFN. TRPV1 is predominately expressed by the peptidergic than nonpeptidergic nociceptors, and these neurochemical discrepancies provided the basis of the distinct pathways of thermal analgesia and mechanical allodynia. The depletion of peptidergic nociceptors and their IENFs cause thermal analgesia and sensitized nonpeptidergic nociceptors respond to mechanical allodynia. These distinct pathways of noxious stimuli suggested determined by the neurochemical-dependent neurotrophin cognate receptors such as TrkA and Ret receptors. The neurogenic inflammation after TRPV1 depletion also sensitized Ret receptors which results in mechanical allodynia. The activation of spinal TRPV1(+) neurons may contribute to mechanical allodynia. Also, an imbalance in adenosinergic analgesic signaling in sensory neurons such as the downregulation of prostatic acid phosphatase and adenosine A1 receptors, which colocalized with TRPV1 as a membrane microdomain also correlated with the development of mechanical allodynia. Collectively, TRPV1 depletion-induced mechanical allodynia involves a complicated cascade of cellular signaling alterations.

Hypersensitivity Induced by Intrathecal Bradykinin Administration Is Enhanced by N-oleoyldopamine (OLDA) and Prevented by TRPV1 Antagonist

Transient receptor potential vanilloid 1 (TRPV1) channels contribute to the development of several chronic pain states and represent a possible therapeutic target in many painful disease treatment. Proinflammatory mediator bradykinin (BK) sensitizes TRPV1, whereas noxious peripheral stimulation increases BK level in the spinal cord. Here, we investigated the involvement of spinal TRPV1 in thermal and mechanical hypersensitivity, evoked by intrathecal (i.t.) administration of BK and an endogenous agonist of TRPV1, N-oleoyldopamine (OLDA), using behavioral tests and i.t. catheter implantation, and administration of BK-induced transient thermal and mechanical hyperalgesia and mechanical allodynia. All these hypersensitive states were enhanced by co-administration of a low dose of OLDA (0.42 µg i.t.), which was ineffective only under the control conditions. Intrathecal pretreatment with TRPV1 selective antagonist SB366791 prevented hypersensitivity induced by i.t. co-administration of BK and OLDA. Our results demonstrate that both thermal and mechanical hypersensitivity evoked by co-administration of BK and OLDA is mediated by the activation of spinal TRPV1 channels.

N-Acetylcysteine causes analgesia in a mouse model of painful diabetic neuropathy

N-Acetylcysteine, one of the most prescribed antioxidant drugs, enhances pain threshold in rodents and humans by activating mGlu2 metabotropic glutamate receptors. Here, we assessed the analgesic activity of N-acetylcysteine in the streptozotocin model of painful diabetic neuropathy and examined the effect of N-acetylcysteine on proteins that are involved in mechanisms of nociceptive sensitization. Mice with blood glucose levels ≥250 mg/dl in response to a single intraperitoneal (i.p.) injection of streptozotocin (200 mg/kg) were used for the assessment of mechanical pain thresholds. Systemic treatment with N-acetylcysteine (100 mg/kg, i.p., either single injection or daily injections for seven days) caused analgesia in diabetic mice. N-acetylcysteine-induced analgesia was abrogated by the Sxc− inhibitors, sulfasalazine (8 mg/kg, i.p.), erastin (30 mg/kg, i.p.), and sorafenib (10 mg/kg, i.p.), or by the mGlu2/3 receptor antagonist, LY341495 (1 mg/kg, i.p.). Repeated administrations of N-acetylcysteine in diabetic mice reduced ERK1/2 phosphorylation in the dorsal region of the lumbar spinal cord. The analgesic activity of N-acetylcysteine was occluded by the MEK inhibitor, PD0325901 (25 mg/kg, i.p.), the TRPV1 channel blocker, capsazepine (40 mg/kg, i.p.), or by a cocktail of NMDA and mGlu5 metabotropic glutamate receptor antagonists (memantine, 25 mg/kg, plus MTEP, 5 mg/kg, both i.p.). These findings offer the first demonstration that N-acetylcysteine relieves pain associated with diabetic neuropathy and holds promise for the use of N-acetylcysteine as an add-on drug in diabetic patients.

Spinal PAR2 Activation Contributes to Hypersensitivity Induced by Peripheral Inflammation in Rats

The mechanisms of inflammatory pain need to be identified in order to find new superior treatments. Protease-activated receptors 2 (PAR2) and transient receptor potential vanilloid 1 (TRPV1) are highly co-expressed in dorsal root ganglion neurons and implicated in pain development. Here, we examined the role of spinal PAR2 in hyperalgesia and the modulation of synaptic transmission in carrageenan-induced peripheral inflammation, using intrathecal (i.t.) treatment in the behavioral experiments and recordings of spontaneous, miniature and dorsal root stimulation-evoked excitatory postsynaptic currents (sEPSCs, mEPSCs and eEPSCs) in spinal cord slices. Intrathecal PAR2-activating peptide (AP) administration aggravated the carrageenan-induced thermal hyperalgesia, and this was prevented by a TRPV1 antagonist (SB 366791) and staurosporine i.t. pretreatment. Additionally, the frequency of the mEPSC and sEPSC and the amplitude of the eEPSC recorded from the superficial dorsal horn neurons were enhanced after acute PAR2 AP application, while prevented with SB 366791 or staurosporine pretreatment. PAR2 antagonist application reduced the thermal hyperalgesia and decreased the frequency of mEPSC and sEPSC and the amplitude of eEPSC. Our findings highlight the contribution of spinal PAR2 activation to carrageenan-induced hyperalgesia and the importance of dorsal horn PAR2 and TRPV1 receptor interactions in the modulation of nociceptive synaptic transmission.

Calcium Increase and Substance P Release Induced by the Neurotoxin Brevetoxin-1 in Sensory Neurons: Involvement of PAR2 Activation through Both Cathepsin S and Canonical Signaling

Red tides involving Karenia brevis expose humans to brevetoxins (PbTxs). Oral exposition triggers neurotoxic shellfish poisoning, whereas inhalation induces a respiratory syndrome and sensory disturbances. No curative treatment is available and the pathophysiology is not fully elucidated. Protease-activated receptor 2 (PAR2), cathepsin S (Cat-S) and substance P (SP) release are crucial mediators of the sensory effects of ciguatoxins (CTXs) which are PbTx analogs. This work explored the role of PAR2 and Cat-S in PbTx-1-induced sensory effects and deciphered the signaling pathway involved. We performed calcium imaging, PAR2 immunolocalization and SP release experiments in monocultured sensory neurons or co-cultured with keratinocytes treated with PbTx-1 or P-CTX-2. We demonstrated that PbTx-1-induced calcium increase and SP release involved Cat-S, PAR2 and transient receptor potential vanilloid 4 (TRPV4). The PbTx-1-induced signaling pathway included protein kinase A (PKA) and TRPV4, which are compatible with the PAR2 biased signaling induced by Cat-S. Internalization of PAR2 and protein kinase C (PKC), inositol triphosphate receptor and TRPV4 activation evoked by PbTx-1 are compatible with the PAR2 canonical signaling. Our results suggest that PbTx-1-induced sensory disturbances involve the PAR2-TRPV4 pathway. We identified PAR2, Cat-S, PKA, and PKC that are involved in TRPV4 sensitization induced by PbTx-1 in sensory neurons.

The Transient Receptor Potential Channel Vanilloid 1 Is Critical in Innate Airway Epithelial Responses to Protease Allergens

The airway epithelium plays a critical role in innate responses to airborne allergens by secreting IL-1 family cytokines such as IL-1α and IL-33 as alarmins that subsequently orchestrate appropriate immune responses. Previous studies revealed that epithelial IL-33 secretion by allergens such as Alternaria alternata or house dust mite involves Ca²⁺-dependent signaling, via initial activation of ATP-stimulated P2YR2 (type 2 purinoceptor) and subsequent activation of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase DUOX1. We sought to identify proximal mechanisms by which epithelial cells sense these allergens and here highlight the importance of PAR2 (protease-activated receptor 2) and TRP (transient receptor potential) Ca²⁺ channels such as TRPV1 (TRP vanilloid 1) in these responses. Combined studies of primary human nasal and mouse tracheal epithelial cells, as well as immortalized human bronchial epithelial cells, indicated the importance of both PAR2 and TRPV1 in IL-33 secretion by both Alternaria alternata and house dust mite, based on both pharmacological and genetic approaches. TRPV1 was also critically involved in allergen-induced ATP release, activation of DUOX1, and redox-dependent activation of EGFR (epidermal growth factor receptor). Moreover, genetic deletion of TRPV1 dramatically attenuated allergen-induced IL-33 secretion and subsequent type 2 responses in mice in vivo. TRPV1 not only contributed to ATP release and P2YR2 signaling but also was critical in downstream innate responses to ATP, indicating potentiating effects of P2YR2 on TRPV1 activation. In aggregate, our studies illustrate a complex relationship between various receptor types, including PAR2 and P2YR2, in epithelial responses to asthma-relevant airborne allergens and highlight the central importance of TRPV1 in such responses.

Pain responses to protease-activated receptor-2 stimulation in the spinal cord of naïve and arthritic rats

There is strong evidence showing that the activation of peripheral proteinase-activated receptors type 2 (PAR-2) can initiate hyperalgesic and inflammatory responses in the joint. However, to date, there is no report of functional spinal PAR-2 receptors in arthritis models. The primary aim of this study was to evaluate the activity of PAR-2 receptors at the spinal cord by using a potent agonist (FLIGRL) in naïve animals, and an antagonist (GB83) in different models of joint pain. Saline or FLIGRL (10 nmol) were injected intrathecally in naïve animals and nociceptive behaviour was evaluated over a 24 h time period by von Frey hair algesiometry. Paw withdrawal threshold decreased from 3 to 24 h and this allodynic effect was blocked by GB83 (90 nmol; i.p.). Acute inflammatory joint pain was induced by injecting 0.5 % kaolin/carrageenan (50 μL each) into the right knee joint of male Wistar rats (24 h recovery). Chronic inflammatory joint pain was modelled by intraarticular injection of Freund's complete adjuvant (FCA; 50 μL; 7 days recovery) or chronic osteoarthritis pain by sodium monoiodoacetate (MIA; 3 mg; 14 days recovery). Animals were then treated with either intrathecal vehicle or 10 nmol of GB83 (10 μL); joint pain was evaluated throughout the subsequent 3 h period. The acute inflammatory pain induced by kaolin/carrageenan was not affected by treatment with GB83. Conversely, both chronic arthritis models demonstrated increased hind paw withdrawal threshold after spinal injection of the PAR-2 antagonist. Based on these results, spinal PAR-2 receptors are involved in joint nociceptive processing in chronic but not acute arthritic conditions.

Cathepsin S acts via protease-activated receptor 2 to activate sensory neurons and induce itch-like behaviour

Chronic itch is a debilitating condition characterised by excessive scratching and is a symptom frequently reported in skin diseases such as atopic dermatitis. It has been proposed that release of the cysteine protease Cathepsin S (CatS) from skin keratinocytes or immune cells resident in or infiltrating the skin could act as a pruritogen in chronic itch conditions. CatS is known to activate protease-activated receptor 2 (PAR2). We therefore hypothesised that enzymatic activation of neuronally expressed PAR2 by CatS was responsible for activation of sensory neurons and transmission of itch signals. Intradermally-injected human recombinant (hr)-CatS or the PAR2 agonist, SLIGRL-NH2 behaved as pruritogens by causing scratching behaviour in mice. Hr-CatS-induced scratching behaviour was prevented by CatS inhibitors and PAR2 antagonists and reduced by 50% in TRPV1-/- mice compared with wild-type mice, whilst no significant reduction in scratching behaviour was observed in TRPA1-/- mice. Cultured dorsal root ganglion (DRG) cells showed an increase in [Ca2+]i following incubation with hr-CatS, and the percentage of neurons that responded to hr-CatS decreased in the presence of a PAR2 antagonist or in cultures of neurons from TRPV1-/- mice. Taken together, our results indicate CatS acts as a pruritogen via PAR2 activation in TRPV1-expressing sensory neurons.