Heat hyperalgesia after incision requires TRPV1 and is distinct from pure inflammatory pain

Is chronic pain caused by central sensitization? A review and critical point of view

Chronic pain causes disability and loss of health worldwide. Yet, a mechanistic explanation for it is still missing. Frequently, neural phenomena, and among them, Central Sensitization (CS), is presented as causing chronic pain. This narrative review explores the evidence substantiating the relationship between CS and chronic pain: four expert researchers were divided in two independent teams that reviewed the available evidence. Three criteria were established for a study to demonstrate a causal relationship: (1) confirm presence of CS, (2) study chronic pain, and (3) test sufficiency or necessity of CS over chronic pain symptoms. No study met those criteria, failing to demonstrate that CS can cause chronic pain. Also, no evidence reporting the occurrence of CS in humans was found. Worryingly, pain assessments are often confounded with CS measures in the literature, omitting that the latter is a neurophysiological and not a perceptual phenomenon. Future research should avoid this misconception to directly interrogate what is the causal contribution of CS to chronic pain to better comprehend this problematic condition.

Epidural administration of large dose of opioid μ receptor agonist may impair cardiac functions and myocardial viability via desensitizing transient receptor potential vanilloid 1

The incidence of postoperative myocardial injury remains high as the underlying pathogenesis is still unknown. The dorsal root ganglion (DRG) neurons express transient receptor potential vanilloid 1 (TRPV1) and its downstream effector, calcitonin gene-related peptide (CGRP) participating in transmitting pain signals and cardiac protection. Opioids remain a mainstay therapeutic option for moderate-to-severe pain relief clinically, as a critical component of multimodal postoperative analgesia via intravenous and epidural delivery. Evidence indicates the interaction of opioids and TRPV1 activities in DRG neurons. Here, we verify the potential impairment of myocardial viability by epidural usage of opioids in postoperative analgesia. We found that large dose of epidural morphine (50 μg) significantly worsened the cardiac performance (+dP/dtmax reduction by 11% and -dP/dtmax elevation by 24%, all P < 0.001), the myocardial infarct size (morphine vs Control, 0.54 ± 0.09 IS/AAR vs. 0.23 ± 0.06 IS/AAR, P < 0.001) and reduced CGRP in the myocardium (morphine vs. Control, 9.34 ± 2.24 pg/mg vs. 21.23 ± 4.32 pg/mg, P < 0.001), while induced definite suppression of nociception in the postoperative animals. It was demonstrated that activation of μ-opioid receptor (μ-OPR) induced desensitization of TRPV1 by attenuating phosphorylation of the channel in the dorsal root ganglion neurons, via inhibiting the accumulation of cAMP. CGRP may attenuated the buildup of ROS and the reduction of mitochondrial membrane potential in cardiomyocytes induced by hypoxia/reoxygenation. The findings of this study indicate that epidurally giving large dose of μ-OPR agonist may aggravate myocardial injury by inhibiting the activity of TRPV1/CGRP pathway.

Increases in local skin temperature correlate with spontaneous foot lifting and heat hyperalgesia in both incisional inflammatory models of pain

Background

This study investigated if a localized increase in skin temperature in rat models of incisional and inflammatory pain correlates with the intensity of spontaneous and evoked pain behaviors.

Methods

Anesthetized rats received either a 20-mm longitudinal incision made through the skin, fascia, and muscle of the plantar hind paw or an injection of complete Freund adjuvant into the plantar hind paw of anesthetized rats to induce local inflammation. Spontaneous and evoked pain behaviors were assessed, and changes in skin temperature were measured using a noncontact infrared thermometer.

Results

There were no differences in skin temperature between the ipsilateral and contralateral hind paw before the incision or inflammation. Skin temperature increased at 2 hours after hind paw plantar incision or 1 day after inflammation of the affected paw, which gradually returned to baseline by the first day and fourth days after treatment, respectively. The increase in skin temperature correlated with the intensity of spontaneous pain behaviors and heat but not with mechanical allodynia.

Conclusions

Our results suggest that a simple measurement of localized skin temperature using a noncontact infrared thermometer could measure the extent of spontaneous pain behaviors and heat hyperalgesia following plantar incision or inflammation in animals. In the absence of a reliable objective marker of pain, these results are encouraging. However, studies are warranted to validate our results using analgesics and pain-relieving interventions, such as nerve block on skin temperature changes.

[Postoperative pain experience after proximal femur fracture in dementia]

BACKGROUND

The present study aimed to assess the postoperative pain experience in cognitive deficit patients with special reference to sensory or affective pain quality.

METHODS

Nineteen patients with normal cognition up to cognitive impairments according to the DemTect screening-tool were studied regarding their postoperative pain experience after proximal femur fracture. The numerical rating scale (NRS), the cognitive DemTect questionnaire, the pain sensation questionnaire (SES), and a quantitative sensory test (QST) were used as examination instruments.

RESULTS

The mean ± SD age of the patients was 83.8 ± 10.0 years. Of the 19 patients, 6 (31.6%) had normal cognitive abilities. In 4 patients (21.1%) there were indications of mild cognitive impairments, and in 9 patients (47.4%) the suspicions of the presence of dementia arose. The mean postoperative pain intensity (NRS) was 4.0 (1.6). With comparable analgesic therapy, the reported pain intensities did not differ between the three patient groups with different cognitive impairments and the first three postoperative treatment days. There were no statistically significant differences between the groups for the sensory or affective total scores of the pain sensation scale. The QST parameters deep pain (PPT), superficial mechanical pain after needle stimulation (MPT), and the superficial sensitivity to light touch stimuli (MDT) showed a significantly increased sensitivity of the operated side. For the sensation of vibration (VDT) no differences between operated and healthy extremities could be proven.

DISCUSSION

The postoperative pain experience does not differ between patients with normal and limited cognition. The quantitative sensory testing showed mechanical hyperalgesia in the operated area. The study points to the importance of adequate postoperative pain management even in those with dementia.

High-fat diet causes mechanical allodynia in the absence of injury or diabetic pathology

Understanding the interactions between diet, obesity, and diabetes is important to tease out mechanisms in painful pathology. Western diet is rich in fats, producing high amounts of circulating bioactive metabolites. However, no research has assessed how a high-fat diet (HFD) alone may sensitize an individual to non-painful stimuli in the absence of obesity or diabetic pathology. To investigate this, we tested the ability of a HFD to stimulate diet-induced hyperalgesic priming, or diet sensitization in male and female mice. Our results revealed that 8 weeks of HFD did not alter baseline pain sensitivity, but both male and female HFD-fed animals exhibited robust mechanical allodynia when exposed to a subthreshold dose of intraplantar Prostaglandin E₂ (PGE₂) compared to mice on chow diet. Furthermore, calcium imaging in isolated primary sensory neurons of both sexes revealed HFD induced an increased percentage of capsaicin-responsive neurons compared to their chow counterparts. Immunohistochemistry (IHC) showed a HFD-induced upregulation of ATF3, a neuronal marker of injury, in lumbar dorsal root ganglia (DRG). This suggests that a HFD induces allodynia in the absence of a pre-existing condition or injury via dietary components. With this new understanding of how a HFD can contribute to the onset of pain, we can understand the dissociation behind the comorbidities associated with obesity and diabetes to develop pharmacological interventions to treat them more efficiently.

TRPV1 in Pain and Itch

Transient receptor potential vanilloid type 1 (TRPV1) is a nonselective cation channel that is intensively expressed in the peripheral nerve system and involved in a variety of physiological and pathophysiological processes in mammals. Its activity is of great significance in transmitting pain or itch signals from peripheral sensory neurons to the central nervous system. The alteration or hypersensitivity of TRPV1 channel is well evidenced under various pathological conditions. Moreover, accumulative studies have revealed that TRPV1-expressing (TRPV1⁺) sensory neurons mediate the neuroimmune crosstalk by releasing neuropeptides to innervated tissues as well as immune cells. In the central projection, TRPV1⁺ terminals synapse with the secondary neurons for the transmission of pain and itch signalling. The intense involvement of TRPV1 and TRPV1⁺ neurons in pain and itch makes it a potential pharmaceutical target. Over decades, the basis of TRPV1 channel structure, the nature of its activity, and its modulation in pathological processes have been broadly studied and well documented. Herein, we highlight the role of TRPV1 and its associated neurons in sensing pain and itch. The fundamental understandings of TRPV1-involved nociception, pruriception, neurogenic inflammation, and cell-specific modulation will help bring out more effective strategies of TRPV1 modulation in treating pain- and itch-related diseases.

TRPV1: Role in Skin and Skin Diseases and Potential Target for Improving Wound Healing

Skin is innervated by a multitude of sensory nerves that are important to the function of this barrier tissue in homeostasis and injury. The role of innervation and neuromediators has been previously reviewed so here we focus on the role of the transient receptor potential cation channel, subfamily V member 1 (TRPV1) in wound healing, with the intent of targeting it in treatment of non-healing wounds. TRPV1 structure and function as well as the outcomes of TRPV1-targeted therapies utilized in several diseases and tissues are summarized. In skin, keratinocytes, sebocytes, nociceptors, and several immune cells express TRPV1, making it an attractive focus area for treating wounds. Many intrinsic and extrinsic factors confound the function and targeting of TRPV1 and may lead to adverse or off-target effects. Therefore, a better understanding of what is known about the role of TRPV1 in skin and wound healing will inform future therapies to treat impaired and chronic wounds to improve healing.

Modality-specific facilitation of noninjurious sharp mechanical pain by topical capsaicin

We had previously shown that a "blunt blade" stimulator can mimic the noninjurious strain phase of incisional pain, but not its sustained duration. Here, we tested whether acute sensitization of the skin with topical capsaicin can add the sustained phase to this noninvasive surrogate model of intraoperative pain. Altogether, 110 healthy volunteers (55 male and 55 female; 26 ± 5 years) participated in several experiments using the "blunt blade" (0.25 × 4 mm) on normal skin (n = 36) and on skin pretreated by a high-concentration capsaicin patch (8%, Qutenza; n = 36). These data were compared with an experimental incision (n = 40) using quantitative and qualitative pain ratings by numerical rating scale and SES Pain Perception Scale descriptors. Capsaicin sensitization increased blade-induced pain magnitude and duration significantly (both P < 0.05), but it failed to fully match the sustained duration of incisional pain. In normal skin, the SES pattern of pain qualities elicited by the blade matched incision in pain magnitude and pattern of pain descriptors. In capsaicin-treated skin, the blade acquired a significant facilitation only of the perceived heat pain component (P < 0.001), but not of mechanical pain components. Thus, capsaicin morphed the descriptor pattern of the blade to become more capsaicin-like, which is probably explained best by peripheral sensitization of the TRPV1 receptor. Quantitative sensory testing in capsaicin-sensitized skin revealed hyperalgesia to heat and pressure stimuli, and loss of cold and cold pain sensitivity. These findings support our hypothesis that the blade models the early tissue-strain-related mechanical pain phase of surgical incisions.

Antinociceptive Effects of Sinomenine Combined With Ligustrazine or Paracetamol in Animal Models of Incisional and Inflammatory Pain

The management of postoperative and inflammatory pain has been a pressing challenge in clinical settings. Sinomenine (SN) is a morphinan derived alkaloid with remarkable analgesic properties in various kinds of pain models. The aim of the current study is to investigate if SN can enhance the effect of ligustrazine hydrochloride (LGZ) or paracetamol (PCM) in animal models of postoperative and inflammatory pain. And to determine if the combined therapeutic efficacies can be explained by pharmacokinetics changes. Pharmacological studies were performed using a rat model of incisional pain, and a mouse model of carrageenan induced inflammatory pain. Pharmacokinetic studies were performed using a microdialysis sampling and HPLC-MS/MS assay method to quantify SN, LGZ, and PCM levels in blood and extracellular fluid in brain. We found that SN plus LGZ or SN plus PCM produced marked synergistic analgesic effects. However, such synergy was subjected to pain modalities, and differed among pain models. Pharmacological discoveries could be partially linked to pharmacokinetic alterations in SN combinations. Though further evaluation is needed, our findings advocate the potential benefits of SN plus LGZ for postoperative pain management, and SN plus PCM for controlling inflammatory pain.

Phospholipase Cβ3 Expressed in Mouse DRGs is Involved in Inflammatory and Postoperative Pain

Background

Previous studies suggested that phospholipase Cβ3 (PLCβ3), which is a common downstream component in the signaling cascade, plays an important role in peripheral mechanisms of perception including nociception. However, detailed profiles of PLCβ3-expressing dorsal root ganglion (DRG) neurons and involvement of PLCβ3 in inflammatory and postoperative pain have not been fully investigated.

Purpose

We evaluated neurochemical char0acteristics of PLCβ3-expressing DRG neurons in mice and then we examined the effects of selective knockdown of PLCβ3 expression in DRGs on inflammatory and postoperative pain.

Methods

Male C57BL/6-strain mice were used. For the inflammatory model, each mouse received subcutaneous injection of complete Freund’s adjuvant (CFA) in the left hindpaw. For the postoperative pain model, a plantar incision was made in the left hindpaw. PLCβ3 antisense oligodeoxynucleotide or PLCβ3 mismatch oligodeoxynucleotide was intrathecally administered once a day for three consecutive days in each model. The time courses of thermal hyperalgesia and mechanical hyperalgesia were investigated. Changes in PLCβ3 protein levels in DRGs were evaluated by Western blotting.

Results

Immunohistochemical analysis showed that high proportion of the PLCβ3-positive profiles were biotinylated isolectin B4-positive or transient receptor potential vanilloid subfamily 1-positive. PLCβ3 protein level in DRGs during CFA-induced inflammation was comparable to that at baseline. Intrathecal administration of PLCβ3 antisense oligodeoxynucleotide, which significantly suppressed PLCβ3 expression in DRGs, did not affect pain thresholds in normal conditions but inhibited CFA-induced thermal and mechanical hyperalgesia both at the early and late phases compared to that in mismatch oligodeoxynucleotide-treated mice. Intrathecal administration of PLCβ3 antisense oligodeoxynucleotide also inhibited surgical incision-induced thermal and mechanical hyperalgesia.

Conclusion

Our results uncover a unique role of PLCβ3 in the development and maintenance of inflammatory pain induced by CFA application and in those of surgical incision-induced pain, although PLCβ3 does not play a major role in thermal nociception or mechanical nociception in normal conditions.

Optogenetic Inhibition of CGRPα Sensory Neurons Reveals Their Distinct Roles in Neuropathic and Incisional Pain

Cutaneous somatosensory neurons convey innocuous and noxious mechanical, thermal, and chemical stimuli from peripheral tissues to the CNS. Among these are nociceptive neurons that express calcitonin gene-related peptide-α (CGRPα). The role of peripheral CGRPα neurons (CANs) in acute and injury-induced pain has been studied using diphtheria toxin ablation, but their functional roles remain controversial. Because ablation permanently deletes a neuronal population, compensatory changes may ensue that mask the physiological or pathophysiological roles of CANs, particularly for injuries that occur after ablation. Therefore, we sought to define the role of intact CANs in vivo under baseline and injury conditions by using noninvasive transient optogenetic inhibition. We assessed pain behavior longitudinally from acute to chronic time points. We generated adult male and female mice that selectively express the outward rectifying proton pump archaerhodopsin-3 (Arch) in CANs, and inhibited their peripheral cutaneous terminals in models of neuropathic (spared nerve injury) and inflammatory (skin-muscle incision) pain using transdermal light activation of Arch. After nerve injury, brief activation of Arch reversed the chronic mechanical, cold, and heat hypersensitivity, alleviated the spontaneous pain, and reversed the sensitized mechanical currents in primary afferent somata. In contrast, Arch inhibition of CANs did not alter incision-induced hypersensitivity. Instead, incision-induced mechanical and heat hypersensitivity was alleviated by peripheral blockade of CGRPα peptide-receptor signaling. These results reveal that CANs have distinct roles in the time course of pain during neuropathic and incisional injuries and suggest that targeting peripheral CANs or CGRPα peptide-receptor signaling could selectively treat neuropathic or postoperative pain, respectively.SIGNIFICANCE STATEMENT The contribution of sensory afferent CGRPα neurons (CANs) to neuropathic and inflammatory pain is controversial. Here, we left CANs intact during neuropathic and perioperative incision injury by using transient transdermal optogenetic inhibition of CANs. We found that peripheral CANs are required for neuropathic mechanical, cold, and heat hypersensitivity, spontaneous pain, and sensitization of mechanical currents in afferent somata. However, they are dispensable for incisional pain transmission. In contrast, peripheral pharmacological inhibition of CGRPα peptide-receptor signaling alleviated the incisional mechanical and heat hypersensitivity, but had no effect on neuropathic pain. These results show that CANs have distinct roles in neuropathic and incisional pain and suggest that their targeting via novel peripheral treatments may selectively alleviate neuropathic versus incisional pain.

NOD-like receptor protein 3 inflammasome drives postoperative mechanical pain in a sex-dependent manner

Postoperative pain management continues to be suboptimal because of the lack of effective nonopioid therapies and absence of understanding of sex-driven differences. Here, we asked how the NLRP3 inflammasome contributes to postoperative pain. Inflammasomes are mediators of the innate immune system that are responsible for activation and secretion of IL-1β upon stimulation by specific molecular signals. Peripheral IL-1β is known to contribute to the mechanical sensitization induced by surgical incision. However, it is not known which inflammasome mediates the IL-1β release after surgical incision. Among the 9 known inflammasomes, the NLRP3 inflammasome is ideally positioned to drive postoperative pain through IL-1β production because NLRP3 can be activated by factors that are released by incision. Here, we show that male mice that lack NLRP3 (NLRP3) recover from surgery-induced behavioral and neuronal mechanical sensitization faster and display less surgical site inflammation than mice expressing NLRP3 (wild-type). By contrast, female NLRP3 mice exhibit minimal attenuation of the postoperative mechanical hypersensitivity and no change in postoperative inflammation compared with wild-type controls. Sensory neuron-specific deletion of NLRP3 revealed that in males, NLRP3 expressed in non-neuronal cells and potentially sensory neurons drives postoperative pain. However, in females, only the NLRP3 that may be expressed in sensory neurons contributes to postoperative pain where the non-neuronal cell contribution is NLRP3 independent. This is the first evidence of a key role for NLRP3 in postoperative pain and reveals immune-mediated sex differences in postoperative pain.

TRPV1 Antagonists as Novel Anti-Diabetic Agents: Regulation of Oral Glucose Tolerance and Insulin Secretion Through Reduction of Low-Grade Inflammation?

With a global prevalence among adults over 18 years of age approaching 9%, Type 2 diabetes mellitus (T2DM) has reached pandemic proportions and represents a major unmet medical need. To date, no disease modifying treatment is available for T2DM patients. Accumulating evidence suggest that the sensory nervous system is involved in the progression of T2DM by maintaining low-grade inflammation via the vanilloid (capsaicin) receptor, Transient Receptor Potential Vanilloid-1 (TRPV1). In this study, we tested the hypothesis that TRPV1 is directly involved in glucose homeostasis in rodents. TRPV1 receptor knockout mice (Trpv1^−/−) and their wild-type littermates were kept on high-fat diet for 15 weeks. Moreover, Zucker obese rats were given the small molecule TRPV1 antagonist, N-(4-Tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl)tetrahydropyrazine-1(2H)-carbox-amide (BCTC), per os twice-a-day or vehicle for eight days. Oral glucose tolerance and glucose-stimulated insulin secretion was improved by both genetic inactivation (Trpv1^−/− mice) and pharmacological blockade (BCTC) of TRPV1. In the obese rat, the improved glucose tolerance was accompanied by a reduction in inflammatory markers in the mesenteric fat, suggesting that blockade of low-grade inflammation contributes to the positive effect of TRPV1 antagonism on glucose metabolism. We propose that TRPV1 could be a promising therapeutic target in T2DM by improving glucose intolerance and correcting dysfunctional insulin secretion.

In Vivo Methods to Study ThermoTRP Channels in Rodents

Ion channels participate in several biological processes. Among these channels, the ionotropic TRP family is the most prominent group being TRPV1 the most studied. The activation of these channels can elicit pain sensation; thus, the development of blockers for these channels is receiving increasing attention. TRP channels are the responsible for thermonociception but also, they are involved in osmolarity, taste, and chemical substances perception such as capsaicin or menthol which can evoke pain. The needed of testing new compounds implies the use of animal models of pain and nociceptive tests in order to evaluate their potential efficacy for the treatment of painful symptoms. Several methods have been developed. Here, I describe the standard, current, and available tests to explore nociception in rodents, especially when thermal or mechanical stimuli are applied.

Sex differences and mechanisms of muscle pain

Clinical conditions resulting in musculoskeletal pain show important sex differences in both prevalence and degree of functional disability. The underlying mechanisms for these distinctions in pain manifestation are not fully known. However, recent preclinical studies have shown at the primary afferent level that males and females present fundamental differences in their peripheral response properties and injury-related gene expression patterns that may underlie observed afferent sensitization. At the spinal cord level, studies in various models of pain suggest important roles for the immune system, glutamate signaling and hormones in modulating sex differences. While preclinical studies have been able to characterize some of the basic underlying molecular mechanisms of sex differences in muscle pain, human studies have relied mainly on functional brain imaging studies to explain differences. Further complicating our understanding of how sex influences muscle pain is the notion that the type of injury sustained, or clinical condition may differentially activate distinct mechanisms of muscle pain development in males versus females. More research is necessary to better understand how the sexes differ in their perception of muscle pain. This review highlights recent advances in both human and animal studies of sex differences in muscle pain.

Selective inhibition of CaV3.2 channels reverses hyperexcitability of peripheral nociceptors and alleviates postsurgical pain

Pain-sensing sensory neurons of the dorsal root ganglion (DRG) can become sensitized or hyperexcitable in response to surgically induced peripheral tissue injury. We investigated the potential role and molecular mechanisms of nociceptive ion channel dysregulation in acute pain conditions such as those resulting from skin and soft tissue incision. We used selective pharmacology, electrophysiology, and mouse genetics to link increased current densities arising from the CaV3.2 isoform of T-type calcium channels (T-channels) to nociceptive sensitization using a clinically relevant rodent model of skin and deep tissue incision. Furthermore, knockdown of the CaV3.2-targeting deubiquitinating enzyme USP5 or disruption of USP5 binding to CaV3.2 channels in peripheral nociceptors resulted in a robust antihyperalgesic effect in vivo and substantial T-current reduction in vitro. Our study provides mechanistic insight into the role of plasticity in CaV3.2 channel activity after surgical incision and identifies potential targets for perioperative pain that may greatly decrease the need for narcotics and potential for drug abuse.

Role of Trpv1 and Trpv4 in surgical incision-induced tissue swelling and Fos-like immunoreactivity in the central nervous system of mice

Pain management remains a major concern regarding the treatment of postoperative patients. Transient receptor potential (TRP) channels are considered to be new therapeutic targets for pain control. We investigated whether the genes Trpv1 and Trpv4 are involved in hind paw swelling caused after surgical incision in mice or in incision-induced Fos-like immunoreactivity (Fos-LI) levels in the central nervous system. Mice were divided into four groups: wild-type (WT) control, WT incision, Trpv1 knockout (Trpv1^-/-) incision, and Trpv4 knockout (Trpv4^-/-) incision. Mice were anesthetized, and only those in the incision, and not control, groups received a surgical incision to their right plantar hind paw. Changes in paw diameter and in Fos-LI levels in the dorsal horn of the spinal cord, paraventricular nucleus of the hypothalamus (PVN), paraventricular nucleus of the thalamus, and central amygdala were evaluated 2 h after the incision. There was no significant difference in the paw diameter among groups. In contrast, in laminae I-II of the dorsal horn of the spinal cord and PVN, Fos-LI was significantly higher in all incision groups than in the WT control group. A significant increase in Fos-positive cells was also observed in the dorsal horn laminae III-IV in Trpv1^-/- and Trpv4^-/- incision groups compared with the WT incision group. Our results indicate that surgical incision activates the PVN and that Trpv1 and Trpv4 might be involved in neuronal activity in the dorsal horn laminae III-IV after surgical incision.

Analyses of Synthetic N-Acyl Dopamine Derivatives Revealing Different Structural Requirements for Their Anti-inflammatory and Transient-Receptor-Potential-Channel-of-the-Vanilloid-Receptor-Subfamily-Subtype-1 (TRPV1)-Activating Properties

We studied the chemical entities within N-octanoyl dopamine (NOD) responsible for the activation of transient-receptor-potential channels of the vanilloid-receptor subtype 1 (TRPV1) and inhibition of inflammation. The potency of NOD in activating TRPV1 was significantly higher compared with those of variants in which the ortho-dihydroxy groups were acetylated, one of the hydroxy groups was omitted ( N-octanoyl tyramine), or the ester functionality consisted of a bulky fatty acid ( N-pivaloyl dopamine). Shortening of the amide linker (ΔNOD) slightly increased its potency, which was further increased when the carbonyl and amide groups (ΔNODR) were interchanged. With the exception of ΔNOD, the presence of an intact catechol structure was obligatory for the inhibition of VCAM-1 and the induction of HO-1 expression. Because TRPV1 activation and the inhibition of inflammation by N-acyl dopamines require different structural entities, our findings provide a framework for the rational design of TRPV1 agonists with improved anti-inflammatory properties.

Transcriptional Changes in Dorsal Spinal Cord Persist after Surgical Incision Despite Preemptive Analgesia with Peripheral Resiniferatoxin

BACKGROUND

Peripheral nociceptors expressing the ion channel transient receptor potential cation channel, subfamily V, member 1, play an important role in mediating postoperative pain. Signaling from these nociceptors in the peri- and postoperative period can lead to plastic changes in the spinal cord and, when controlled, can yield analgesia. The transcriptomic changes in the dorsal spinal cord after surgery, and potential coupling to transient receptor potential cation channel, subfamily V, member 1-positive nociceptor signaling, remain poorly studied.

METHODS

Resiniferatoxin was injected subcutaneously into rat hind paw several minutes before surgical incision to inactivate transient receptor potential cation channel, subfamily V, member 1-positive nerve terminals. The effects of resiniferatoxin on postincisional measures of pain were assessed through postoperative day 10 (n = 51). Transcriptomic changes in the dorsal spinal cord, with and without peripheral transient receptor potential cation channel, subfamily V, member 1-positive nerve terminal inactivation, were assessed by RNA sequencing (n = 22).

RESULTS

Peripherally administered resiniferatoxin increased thermal withdrawal latency by at least twofold through postoperative day 4, increased mechanical withdrawal threshold by at least sevenfold through postoperative day 2, and decreased guarding score by 90% relative to vehicle control (P < 0.05). Surgical incision induced 70 genes in the dorsal horn, and these changes were specific to the ipsilateral dorsal horn. Gene induction with surgical incision persisted despite robust analgesia from resiniferatoxin pretreatment. Many of the genes induced were related to microglial activation, such as Cd11b and Iba1.

CONCLUSIONS

A single subcutaneous injection of resiniferatoxin before incision attenuated both evoked and nonevoked measures of postoperative pain. Surgical incision induced transcriptomic changes in the dorsal horn that persisted despite analgesia with resiniferatoxin, suggesting that postsurgical pain signals can be blocked without preventing transcription changes in the dorsal horn.

Involvement of glycine receptor α1 subunits in cannabinoid-induced analgesia

Some cannabinoids have been shown to suppress chronic pain by targeting glycine receptors (GlyRs). Although cannabinoid potentiation of α3 GlyRs is thought to contribute to cannabinoid-induced analgesia, the role of cannabinoid potentiation of α1 GlyRs in cannabinoid suppression of chronic pain remains unclear. Here we report that dehydroxylcannabidiol (DH-CBD), a nonpsychoactive cannabinoid, significantly suppresses chronic inflammatory pain caused by noxious heat stimulation. This effect may involve spinal α1 GlyRs since the expression level of α1 subunits in the spinal cord is positively correlated with CFA-induced inflammatory pain and the GlyRs antagonist strychnine blocks the DH-CBD-induced analgesia. A point-mutation of S296A in TM3 of α1 GlyRs significantly inhibits DH-CBD potentiation of glycine currents (I_Gly) in HEK-293 cells and neurons in lamina I-II of spinal cord slices. To explore the in vivo consequence of DH-CBD potentiation of α1 GlyRs, we generated a GlyRα1^S296A knock-in mouse line. We observed that DH-CBD-induced potentiation of I_Gly and analgesia for inflammatory pain was absent in GlyRα1^S296A knock-in mice. These findings suggest that spinal α1 GlyR is a potential target for cannabinoid analgesia in chronic inflammatory pain.

Effect of Testosterone on TRPV1 Expression in a Model of Orofacial Myositis Pain in the Rat

Recent clinical studies have revealed sex differences in response to transient receptor potential vanilloid 1 (TRPV1) agonist-induced pain. However, the mechanism of these differences in TRPV1-related chronic pain remains unclear. In the present study, we investigate the effects of inflammation and gonadal hormones on TRPV1 expression in trigeminal ganglia. Inflammatory pain was modeled by injecting complete Freund's adjuvant (CFA) into the left masseter muscle in rats. TRPV1 mRNA and protein levels in the trigeminal ganglia of male and female rats following CFA injection were assessed. CFA-induced changes in TRPV1 mRNA and protein expression in the trigeminal ganglia from orchidectomized (ODX) male rats and testosterone-replaced ODX rats were examined. Additionally, TRPV1 mRNA levels in the trigeminal ganglia from ovariectomized (OVX) female and ODX male rats treated with tamoxifen were assessed. We found that the levels of TRPV1 mRNA and protein in the trigeminal ganglia from female rats following CFA injection were significantly higher than in the ganglia from naïve female rats. CFA-induced inflammatory hyperalgesia did not alter TRPV1 expression in the trigeminal ganglia from male rats. The TRPV1 mRNA and protein expression levels in the ODX male trigeminal ganglia were significantly upregulated on day 3 following the initiation of inflammation. However, CFA-induced inflammatory pain had no significant effect on TRPV1 mRNA or protein expression in testosterone-replaced ODX rats. Furthermore, tamoxifen was unable to inhibit the upregulation of TRPV1 expression in OVX female and ODX male rats after CFA injection. In summary, these data indicate that gender differences in TRPV1 function may be, in part, mediated by sex-dependent TRPV1 expression in sensory ganglia. Testosterone plays a key role in the inhibition of TRPV1 expression in this rat chronic inflammatory pain model.

Transient Receptor Potential Vanilloid 1 Antagonists Prevent Anesthesia-induced Hypothermia and Decrease Postincisional Opioid Dose Requirements in Rodents

Background

Intraoperative hypothermia and postoperative pain control are two important clinical challenges in anesthesiology. Transient receptor potential vanilloid 1 has been implicated both in thermoregulation and pain. Transient receptor potential vanilloid 1 antagonists were not advanced as analgesics in humans in part due to a side effect of hyperthermia. This study tested the hypothesis that a single, preincision injection of a transient receptor potential vanilloid 1 antagonist could prevent anesthesia-induced hypothermia and decrease the opioid requirement for postsurgical hypersensitivity.

Methods

General anesthesia was induced in rats and mice with either isoflurane or ketamine, and animals were treated with transient receptor potential vanilloid 1 antagonists (AMG 517 or ABT-102). The core body temperature and oxygen consumption were monitored during anesthesia and the postanesthesia period. The effect of preincision AMG 517 on morphine-induced reversal of postincision hyperalgesia was evaluated in rats.

Results

AMG 517 and ABT-102 dose-dependently prevented general anesthesia-induced hypothermia (mean ± SD; from 1.5° ± 0.1°C to 0.1° ± 0.1°C decrease; P &lt; 0.001) without causing hyperthermia in the postanesthesia phase. Isoflurane-induced hypothermia was prevented by AMG 517 in wild-type but not in transient receptor potential vanilloid 1 knockout mice (n = 7 to 11 per group). The prevention of anesthesia-induced hypothermia by AMG 517 involved activation of brown fat thermogenesis with a possible contribution from changes in vasomotor tone. A single preincision dose of AMG 517 decreased the morphine dose requirement for the reduction of postincision thermal (12.6 ± 3.0 vs. 15.6 ± 1.0 s) and mechanical (6.8 ± 3.0 vs. 9.5 ± 3.0 g) withdrawal latencies.

Conclusions

These studies demonstrate that transient receptor potential vanilloid 1 antagonists prevent anesthesia-induced hypothermia and decrease opioid dose requirements for the reduction of postincisional hypersensitivity in rodents.

Estradiol Sensitizes the Transient Receptor Potential Vanilloid 1 Receptor in Pain Responses

Sex differences exist in chronic pain pathologies, and gonadal estradiol (E2) alters the pain sensation. The nocisensor transient receptor potential vanilloid 1 (TRPV1) receptor plays a critical role in triggering pain. Here we examined the impact of E2 on the function of TRPV1 receptor in mice sensory neurons in vitro and in vivo. Both mechano- and thermonociceptive thresholds of the plantar surface of the paw of female mice were significantly lower in proestrus compared with the estrus phase. These thresholds were higher in ovariectomized (OVX) mice and significantly lower in sham-operated mice in proestrus compared with the sham-operated mice in estrus phase. This difference was absent in TRPV1 receptor-deficient mice. Furthermore, E2 potentiated the TRPV1 receptor activation-induced mechanical hyperalgesia in OVX mice. Long pretreatment (14 hours) with E2 induced a significant increase in TRPV1 receptor messenger RNA expression and abolished the capsaicin-induced TRPV1 receptor desensitization in primary sensory neurons. The short E2 incubation (10 minutes) also prevented the desensitization, which reverted after coadministration of E2 and the tropomyosin-related kinase A (TrkA) receptor inhibitor. Our study provides in vivo and in vitro evidence for E2-induced TRPV1 receptor upregulation and sensitization mediated by TrkAR via E2-induced genomic and nongenomic mechanisms. The sensitization and upregulation of TRPV1 receptor by E2 in sensory neurons may explain the greater pain sensitivity in female mice.

A randomized study to evaluate the analgesic efficacy of a single dose of the TRPV1 antagonist mavatrep in patients with osteoarthritis

BACKGROUND/AIMS

Transient receptor potential vanilloid type 1 (TRPV1) receptor antagonists have been evaluated in clinical studies for their analgesic effects. Mavatrep, a potent, selective, competitive TRPV1 receptor antagonist has demonstrated pharmacodynamic effects consistent with target engagement at the TRPV1 receptor in a previous single-dose clinical study. The current study was conducted to evaluate the analgesic effects of a single dose of mavatrep.

METHODS

In this randomized, placebo- and active-controlled, 3-way crossover, phase 1b study, patients with painful knee osteoarthritis were treated with a single-dose of 50mg mavatrep, 500mg naproxen twice-daily, and placebo. Patients were randomized to 1 of 6 treatment sequences. Each treatment sequence included three treatment periods of 7 days duration with a 7 day washout between each treatment period. The primary efficacy evaluation was pain reduction measured by the 4-h postdose sum of pain intensity difference (SPID) based on the 11-point (0-10) Numerical Rating Scale (NRS) for pain after stair-climbing (PASC). The secondary efficacy evaluations included 11-point (0-10) NRS pain scores entered into the Actiwatch between clinic visits, the Western Ontario and McMaster Universities Arthritis Index subscales (WOMAC) questionnaire, and use of rescue medication. Safety and tolerability of single oral dose mavatrep were also assessed.

RESULTS

Of 33 patients randomized, 32 completed the study. A statistically significantly (p<0.1) greater reduction in PASC was observed for mavatrep versus placebo (4-h SPID least square mean [LSM] [SE] difference: 1.5 [0.53]; p=0.005 and 2-h LSM [SE] difference of PID: 0.7 [0.30]; p=0.029). The mean average daily current pain NRS scores were lower in the mavatrep and naproxen treatment arm than in the placebo arm (mavatrep: 7 day mean [SD], 3.72 [1.851]; naproxen: 7 day mean [SD], 3.49 [1.544]; placebo: 7 day mean [SD], 4.9 [1.413]). Mavatrep showed statistically significant improvements as compared with placebo on the WOMAC subscales (pain on days 2 [p=0.049] and 7 [p=0.041], stiffness on day 7 [p=0.075]), and function on day 7 [p=0.077]). The same pattern of improvement was evident for naproxen versus placebo. The mean (SD) number of rescue medication tablets taken during the 7-day treatment period was 4.2 (6.49) for mavatrep treatment, 2.8 (5.42) for naproxen, and 6.3 (8.25) for placebo treatment. All patients that received mavatrep reported at least 1 treatment emergent adverse event (TEAE). Feeling cold (79%), thermohypoesthesia (61%), dysgeusia (58%), paraesthesia (36%), and feeling hot (15%) were the most common TEAEs in the mavatrep group. Total 9% patients receiving mavatrep experienced minor thermal burns. No deaths or serious AEs or discontinuations due to AEs occurred.

CONCLUSION

Overall, mavatrep was associated with a significant reduction in pain, stiffness, and physical function when compared with placebo in patients with knee osteoarthritis. Mavatrep's safety profile was consistent with its mechanism of action as a TRPV1 antagonist.

IMPLICATIONS

Further studies are required to evaluate whether lower multiple doses of mavatrep can produce analgesic efficacy while minimizing adverse events, as well as the potential for improved patient counselling techniques to reduce the minor thermal burns related to decreased heat perception.

TRIAL REGISTRATION

2009-010961-21 (EudraCT Number).

Functional interactions between transient receptor potential M8 and transient receptor potential V1 in the trigeminal system: Relevance to migraine pathophysiology

Background

Recent genome-wide association studies have identified transient receptor potential M8 (TRPM8) as a migraine susceptibility gene. TRPM8 is a nonselective cation channel that mediates cool perception. However, its precise role in migraine pathophysiology is elusive. Transient receptor potential V1 (TRPV1) is a nonselective cation channel activated by noxious heat. Both TRPM8 and TRPV1 are expressed in trigeminal ganglion (TG) neurons.

Methods

We investigated the functional roles of TRPM8 and TRPV1 in a meningeal inflammation-based migraine model by measuring the effects of facial TRPM8 activation on thermal allodynia and assessing receptor coexpression changes in TG neurons. We performed retrograde tracer labeling to identify TG neurons innervating the face and dura.

Results

We found that pharmacological TRPM8 activation reversed the meningeal inflammation-induced lowering of the facial heat pain threshold, an effect abolished by genetic ablation of TRPM8. No significant changes in the heat pain threshold were seen in sham-operated animals. Meningeal inflammation caused dynamic alterations in TRPM8/TRPV1 coexpression patterns in TG neurons, and colocalization was most pronounced when the ameliorating effect of TRPM8 activation on thermal allodynia was maximal. Our tracer assay disclosed the presence of dura-innervating TG neurons sending collaterals to the face. Approximately half of them were TRPV1-positive. We also demonstrated functional inhibition of TRPV1 by TRPM8 in a cell-based assay using c-Jun N-terminal kinase phosphorylation as a surrogate marker.

Conclusions

Our findings provide a plausible mechanism to explain how facial TRPM8 activation can relieve migraine by suppressing TRPV1 activity. Facial TRPM8 appears to be a promising therapeutic target for migraine.

Postoperative pain-from mechanisms to treatment

INTRODUCTION

Pain management after surgery continues to be suboptimal; there are several reasons including lack of translation of results from basic science studies and scientific clinical evidence into clinical praxis.

OBJECTIVES

This review presents and discusses basic science findings and scientific evidence generated within the last 2 decades in the field of acute postoperative pain.

METHODS

In the first part of the review, we give an overview about studies that have investigated the pathophysiology of postoperative pain by using rodent models of incisional pain up to July 2016. The second focus of the review lies on treatment recommendations based on guidelines and clinical evidence, eg, by using the fourth edition of the "Acute Pain Management: Scientific Evidence" of the Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine.

RESULTS

Preclinical studies in rodent models characterized responses of primary afferent nociceptors and dorsal horn neurons as one neural basis for pain behavior including resting pain, hyperalgesia, movement-evoked pain or anxiety- and depression-like behaviors after surgery. Furthermore, the role of certain receptors, mediators, and neurotransmitters involved in peripheral and central sensitization after incision were identified; many of these are very specific, relate to some modalities only, and are unique for incisional pain. Future treatment should focus on these targets to develop therapeutic agents that are effective for the treatment of postoperative pain as well as have few side effects. Furthermore, basic science findings translate well into results from clinical studies. Scientific evidence is able to point towards useful (and less useful) elements of multimodal analgesia able to reduce opioid consumption, improve pain management, and enhance recovery.

CONCLUSION

Understanding basic mechanisms of postoperative pain to identify effective treatment strategies may improve patients' outcome after surgery.

TRPV1 antagonist JNJ-39439335 (mavatrep) demonstrates proof of pharmacology in healthy men: a first-in-human, double-blind, placebo-controlled, randomized, sequential group study

This double-blind, randomized, placebo-controlled, sequential group, phase 1 study was designed to assess in healthy men, the safety, tolerability, pharmacokinetics, and translational pharmacodynamics of JNJ-39439335 (mavatrep), a transient receptor potential vanilloid subtype 1 antagonist; it was preceded by a translational preclinical study which assessed the ability of JNJ-39439335 to block capsaicin-induced flare in rats, providing predictive pharmacokinetic and pharmacodynamic data that informed the subsequent phase 1 clinical study. The clinical study consisted of 2 parts: part 1 assessed pharmacokinetics and pharmacodynamics, including heat pain detection threshold and heat pain tolerance, of JNJ-39439335, and part 2 assessed pharmacodynamic effect of JNJ-39439335 on capsaicin-induced flare and sensory testing on naïve and UVB-sensitized skin in humans. Plasma concentrations of JNJ-39439335 peaked at approximately 2 to 4 hours postdose, then declined multiexponentially, with a prolonged terminal phase (half-life: 30-86 hours). Renal clearance of JNJ-39439335 was negligible. JNJ-39439335 treatment resulted in clear, consistent dose-related increases in heat pain detection threshold, heat pain tolerance, and heat pain latency. JNJ-39439335 reduced the capsaicin-induced flare area and flare intensity, with complete blocking observed in the 50-mg dose group at 144 hours postdose. This was consistent with the capsaicin flare results observed with JNJ-39439335 in rats. The most common adverse events observed in the clinical study were related to increases in body temperature after JNJ-39439335 treatment; these were predominately mild to moderate in severity with no evidence of exposure dependence up to 225 mg. JNJ-39439335 was well tolerated at single doses up to 225 mg, recommending its suitability for further clinical development.

Safety, Tolerability and Pharmacokinetic and Pharmacodynamic Learnings from a Double-Blind, Randomized, Placebo-Controlled, Sequential Group First-in-Human Study of the TRPV1 Antagonist, JNJ-38893777, in Healthy Men

BACKGROUND AND OBJECTIVE

Nociceptive and neuropathic pain, one of common reasons of disability and loss of quality life, are often undertreated due to safety concerns with current therapies. This study assessed the safety, tolerability, pharmacokinetics and pharmacodynamics of JNJ-38893777, a potent and selective transient receptor potential vanilloid 1 (TRPV1) channel antagonist in healthy men.

METHODS

In a single-center, double-blind, placebo-controlled, sequential group, single-ascending-dose phase 1 study, 80 healthy men (18-45 years old; body mass index 18.5 to <30 kg/m(2)), randomized to two groups, received either JNJ-38893777 (n = 6) or placebo (n = 2) in a dose-escalation manner. The study was designed in two parts: Part 1, an early tablet formulation was administered under fasting conditions at 5, 15, 45, 125, 250, or 500 mg; Part 2, a new tablet formulation was administered in a fasting state (250 mg) and a high-fat fed state (250 mg, 375 mg, or 500 mg). Serial plasma and urine samples (collected over 120 h post-dose) were analyzed using LC-MS/MS for pharmacokinetic evaluations.

RESULTS

JNJ-38893777 concentrations peaked from 3.0 to 5.5 h (median) post-administration, and then declined multi-exponentially with a prolonged terminal phase. Renal clearance was negligible. Maximum concentration (C max) and area under the concentration-time curve from time zero to infinity (AUC∞) of the early formulation increased with increasing doses but less than dose-proportionally over 5-500 mg (fasted) doses. The new tablet formulation showed no improvements in the fasting state but showed an 11- to 22-fold increase in JNJ-38893777 exposure; interindividual variability reduced from 73-85% to 23-24%, and a significant increase (P < 0.05) in heat pain detection threshold (~3 °C) was observed in the fed state. Mild to moderate adverse events were observed, with no evidence of exposure dependence up to 500 mg (fed). Concentration-related increases in body temperature or changes in Fridericia-corrected QT interval (QTcF) were not observed.

CONCLUSION

JNJ-38893777 was tolerated at single doses up to 500 mg (fed) and is suitable for further clinical development.

100-Hz Electroacupuncture but not 2-Hz Electroacupuncture is Preemptive Against Postincision Pain in Rats

Preemptive analgesia involves introducing an analgesic before noxious stimulation. Electroacupuncture (EA) activates descending mechanisms that modulate nociceptive inputs into the spinal dorsal horn. This study evaluated whether preoperative EA is more effective than postoperative EA in reducing incision pain in rats. The nociceptive threshold to mechanical stimulation was utilized to examine the effects of an intraperitoneal injection of saline (0.1 mL/kg) or naloxone (1 mg/kg) on antinociception induced by a 20-minute period of 2-Hz or 100-Hz EA applied to the Zusanli (ST36) and Sanyinjiao (SP6) acupoints before surgical incision, or 10 minutes after or 100 minutes after surgical incision of the hind paw. The extent of mechanical hyperalgesia after the incision was significantly attenuated by the application of 100-Hz EA preoperatively, but not by its application at 10 minutes or 100 minutes postoperatively. By contrast, 2-Hz EA was effective against postoperative hyperalgesia when applied 10 minutes or 100 minutes after surgery but not when it was applied preoperatively. Only the effect of 2-Hz EA applied 10 minutes after surgery was sensitive to naloxone. The present study showed for the first time that 100-Hz EA, but not 2-Hz EA, exerts a nonopioidergic preemptive effect against postincision pain in rats.

Multiple impairments of cutaneous nociceptor function induced by cardiotoxic doses of Adriamycin in the rat

Besides their deleterious action on cardiac muscle, anthracycline-type cytostatic agents exert significant neurotoxic effects on primary sensory neurons. Since cardiac sensory nerves confer protective effects on heart muscle and share common traits with cutaneous chemosensitive nerves, this study examined the effects of cardiotoxic doses of adriamycin on the function and morphology of epidermal nerves. Sensory neurogenic vasodilatation, plasma extravasation, and the neural CGRP release evoked by TRPV1 and TRPA1 agonists in vitro were examined by using laser Doppler flowmetry, the Evans blue technique, and ELISA, respectively. Carrageenan-induced hyperalgesia was assessed with the Hargreaves method. Immunohistochemistry was utilized to study cutaneous innervation. Adriamycin treatment resulted in profound reductions in the cutaneous neurogenic sensory vasodilatation and plasma extravasation evoked by the TRPV1 and TRPA1 agonists capsaicin and mustard oil, respectively. The in vitro capsaicin-, but not high potassium-evoked neural release of the major sensory neuropeptide, CGRP, was markedly attenuated after adriamycin treatment. Carrageenan-induced inflammatory hyperalgesia was largely abolished following the administration of adriamycin. Immunohistochemistry revealed a substantial loss of epidermal TRPV1-expressing nociceptive nerves and a marked thinning of the epidermis. These findings indicate impairments in the functions of TRPV1 and TRPA1 receptors expressed on cutaneous chemosensitive nociceptive nerves and the loss of epidermal axons following the administration of cardiotoxic doses of adriamycin. Monitoring of the cutaneous nociceptor function in the course of adriamycin therapy may well be of predictive value for early detection of the deterioration of cardiac nerves which confer protection against the deleterious effects of the drug.

The preventive effect of resiniferatoxin on the development of cold hypersensitivity induced by spinal nerve ligation: involvement of TRPM8

BACKGROUND

Resiniferatoxin (RTX) is a potent analog of capsaicin and activates transient receptor potential (TRP) vanilloid type (TRPV) 1. In the current study, we investigated the preventive effect of perineural RTX on the development of cold hypersensitivity induced by spinal nerve ligation (SNL) in rats. Furthermore, we examined the association between the expression level of TRPV1, TRP ankyrin type (TRPA) 1 and TRP melastatin type (TRPM) 8 in the dorsal root ganglion (DRG) and cold hypersensitivity after SNL.

RESULTS

RTX pretreatment prevented the development of SNL-induced hypersensitivity to mechanical, thermal, and cold stimuli. Western blot analysis 4 weeks after RTX pretreatment showed that RTX pretreatment decreased the protein expression level of SNL-induced TRPM8, but not TRPV1 or TRPA1, in the DRG of SNL rats. Immunofluorescent analysis revealed that up-regulated TRPM8-stained neurons after SNL co-localized with neurofilament 200-positive neurons located in the DRG.

CONCLUSIONS

Pretreatment with perineural RTX significantly inhibits SNL-induced mechanical, thermal, and cold hypersensitivity. The antinociceptive effect of perineural RTX, especially on cold hypersensitivity, may be related to the suppression of TRPM8 expression in DRG.

Antinociceptive effect of hydroalcoholic extract and isoflavone isolated from Polygala molluginifolia in mice: evidence for the involvement of opioid receptors and TRPV1 and TRPA1 channels

PURPOSE

The plants of the genus Polygala (Polygalaceae) have been used for a long time in folk medicine to treat pain and inflammation. The species Polygala molluginifolia is native to southern Brazil and is popularly known as "cânfora". The presented study analyzes the antinociceptive effect of hydroalcoholic extract from Polygala molluginifolia (HEPm) and an isoflavone (ISO) isolated from the extract, in behavioral models of pain in mice, as well as the mechanism underlying this effect.

MATERIALS AND METHODS

The phytochemical analysis of HEPm was performed through a capillary electrophoresis analysis and colorimetric test. The antinociceptive effects of HEPm and ISO (10-1000 mg/kg, i.g.) were evaluated by applying the formalin test; mechanical and thermal hyperalgesia to postoperative pain in mice. The possible involvement of opioid receptors, TRPV1 and TRPA1 channels in the antinociceptive effect of HEPm and ISO were also evaluated. Finally, the nonspecific effects of HEPm and ISO were evaluated by measuring locomotor activity (Open-field Test) and corporal temperature.

RESULTS

The 5,3',4'-trihydroxy-6″,6″-dimethylpyrano[2″,3″:7,6] isoflavone (ISO) was identified in HEPm by capillary electrophoresis analysis and selected for the experimental tests. The oral administration of HEPm or of ISO significantly inhibited the neurogenic and inflammatory phases of formalin-induced pain, edema formation and local hyperemia, without causing any change to locomotor activity. Acute and repeated treatment of animals with HEPm reduced mechanical and thermal (heat and cold) hyperalgesia in the postoperative pain. In addition, administering HEPm or ISO markedly reduced nociceptive behavior induced by the peripheral and central injection of TRPV1 and TRPA1 channels activators. Finally, the antinociception provided by the administration of HEPm or ISO was reversed by the preadministration of naloxone.

CONCLUSIONS

Taken together, these results provide the first experimental evidence of the significant antinociceptive effect of HEPm and ISO in animal models of acute pain without causing sedation or locomotor dysfunction. This effect appears to be mediated, at least in part, by the activation of opioid receptors and/or by the inhibition of TRPV1 and TRPA1 channels. Moreover, this study adds new scientific evidence and highlights the therapeutic potential of the medicinal plant Polygala molluginifolia in the development of phytomedicines with analgesic properties.

Transient receptor potential ion channels in primary sensory neurons as targets for novel analgesics

The last decade has witnessed an explosion in novel findings relating to the molecules involved in mediating the sensation of pain in humans. Transient receptor potential (TRP) ion channels emerged as the greatest group of molecules involved in the transduction of various physical stimuli into neuronal signals in primary sensory neurons, as well as, in the development of pain. Here, we review the role of TRP ion channels in primary sensory neurons in the development of pain associated with peripheral pathologies and possible strategies to translate preclinical data into the development of effective new analgesics. Based on available evidence, we argue that nociception-related TRP channels on primary sensory neurons provide highly valuable targets for the development of novel analgesics and that, in order to reduce possible undesirable side effects, novel analgesics should prevent the translocation from the cytoplasm to the cell membrane and the sensitization of the channels rather than blocking the channel pore or binding sites for exogenous or endogenous activators.

TRPV1 antagonist attenuates postoperative hypersensitivity by central and peripheral mechanisms

BACKGROUND

Acute postoperative pain is one of the frequent reasons for pain treatment. However, the exact mechanisms of its development are still not completely clear. Transient receptor potential vanilloid 1 (TRPV1) receptors are involved in nociceptive signaling in various hypersensitive states. Here we have investigated the contribution of TRPV1 receptors expressed on cutaneous peripheral nociceptive fibers and in the spinal cord on the development and maintenance of hypersensitivity to thermal and mechanical stimuli following surgical incision. A rat plantar incision model was used to test paw withdrawal responses to thermal and mechanical stimuli. The effect of the TRPV1 receptor antagonist SB366791 was investigated 1) by intrathecal injection 15 min before incision and 2) intradermal injection before (30 min) and immediately after the surgery. Vehicle-injected rats and naïve animals treated identically were used as controls.

RESULTS

Plantar incision induced mechanical allodynia and hyperalgesia and thermal hyperalgesia. A single intrathecal administration of SB366791 significantly reduced postincisional thermal hyperalgesia and also attenuated mechanical allodynia, while mechanical hyperalgesia remained unaffected. Local intradermal SB366791 treatment reduced thermal hyperalgesia and mechanical allodynia without affecting mechanical hyperalgesia.

CONCLUSIONS

Our experiments suggest that both peripheral and spinal cord TRPV1 receptors are involved in increased cutaneous sensitivity following surgical incision. The analgesic effect of the TRPV1 receptor antagonist was especially evident in the reduction of thermal hyperalgesia. The activation of TRPV1 receptors represents an important mechanism in the development of postoperative hypersensitivity.

Pain related channels are differentially expressed in neuronal and non-neuronal cells of glabrous skin of fabry knockout male mice

Fabry disease (FD) is one of the X-linked lysosomal storage disorders caused by deficient functioning of the alpha-galactosidase A (α-GalA) enzyme. The α-GalA deficiency leads to multi-systemic clinical manifestations caused by the preferential accumulation of globotriaosylceramide in the endothelium and vascular smooth muscles. A hallmark symptom of FD patients is peripheral pain that appears in the early stage of the disease. Pain in FD patients is a peripheral small-fiber idiopathic neuropathy, with intra-epidermal fiber density and integrity being used for diagnosing FD in humans. However, the molecular correlates underlying pain sensation in FD remain elusive. Here, we have employed the α-GalA gene KO mouse as a model of FD in rodents to investigate molecular changes in their peripheral nervous system that may account for their algesic symptoms. The α-GalA null mice display neuropathic pain as evidenced by thermal hyperalgesia and mechanical allodynia, with histological analyses showing alterations in cutaneous innervation. Additionally, KO mice showed a decreased and scattered pattern of neuronal terminations consistent with the reduction in neuronal terminations in skin biopsies of patients with small fiber neuropathies. At the molecular level KO animals showed an increase in the expression of TRPV1 and Nav1.8, and a decrease in the expression of TRPM8. Notably, these alterations are observed in young animals. Taken together, our findings imply that the α-GalA KO mouse is a good model in which to study the peripheral small fiber neuropathy exhibited by FD patients, and provides molecular evidence for a hyperexcitability of small nociceptors in FD.

TRPV1 Antagonism: From Research to Clinic

The capsaicin receptor, TRPV1, has been one of the most extensively studied molecules in sensory research. Its contribution to the sensation of pain in numerous pre-clinical inflammatory and neuropathic paradigms has been well-established and expression analysis suggests a potential role clinically in pain and bladder conditions. The field has now reached an exciting point in time with the development of a number of high quality TRPV1 antagonist drug candidates and the release of clinical data. What has become apparent from this work is that inhibition of TRPV1 function brings with it the potential liabilities of increased body temperature and altered thermal perception. However, there is cause for optimism because it appears that not all antagonists have the same properties and compounds can be identified that lack significant on-target side-effects whilst retaining efficacy, at least pre-clinically. What is perhaps now more critical to address is the question of how effective the analgesia provided by a TRPV1 antagonist will be. Although tantalizing clinical data showing effects on experimentally-induced pain or pain following molar extraction have been reported, no clear efficacy in a chronic pain condition has yet been demonstrated making it difficult to perform an accurate risk-benefit analysis for TRPV1 antagonists. Here we provide an overview of some of the most advanced clinical candidates and discuss the approaches being taken to avoid the now well established on-target effects of TRPV1 antagonists.

Intrathecal AAV serotype 9-mediated delivery of shRNA against TRPV1 attenuates thermal hyperalgesia in a mouse model of peripheral nerve injury

Gene therapy for neuropathic pain requires efficient gene delivery to both central and peripheral nervous systems. We previously showed that an adenoassociated virus serotype 9 (AAV9) vector expressing short-hairpin RNA (shRNA) could suppress target molecule expression in the dorsal root ganglia (DRG) and spinal cord upon intrathecal injection. To evaluate the therapeutic potential of this approach, we constructed an AAV9 vector encoding shRNA against vanilloid receptor 1 (TRPV1), which is an important target gene for acute pain, but its role in chronic neuropathic pain remains unclear. We intrathecally injected it into the subarachnoid space at the upper lumbar spine of mice 3 weeks after spared nerve injury (SNI). Delivered shTRPV1 effectively suppressed mRNA and protein expression of TRPV1 in the DRG and spinal cord, and it attenuated nerve injury-induced thermal allodynia 10-28 days after treatment. Our study provides important evidence for the contribution of TRPV1 to thermal hypersensitivity in neuropathic pain and thus establishes intrathecal AAV9-mediated gene delivery as an investigative and potentially therapeutic platform for the nervous system.

TRPV1, but not TRPA1, in primary sensory neurons contributes to cutaneous incision-mediated hypersensitivity

Background

Mechanisms underlying postoperative pain remain poorly understood. In rodents, skin-only incisions induce mechanical and heat hypersensitivity similar to levels observed with skin plus deep incisions. Therefore, cutaneous injury might drive the majority of postoperative pain. TRPA1 and TRPV1 channels are known to mediate inflammatory and nerve injury pain, making them key targets for pain therapeutics. These channels are also expressed extensively in cutaneous nerve fibers. Therefore, we investigated whether TRPA1 and TRPV1 contribute to mechanical and heat hypersensitivity following skin-only surgical incision.

Results

Behavioral responses to mechanical and heat stimulation were compared between skin-incised and uninjured, sham control groups. Elevated mechanical responsiveness occurred 1 day post skin-incision regardless of genetic ablation or pharmacological inhibition of TRPA1. To determine whether functional changes in TRPA1 occur at the level of sensory neuron somata, we evaluated cytoplasmic calcium changes in sensory neurons isolated from ipsilateral lumbar 3–5 DRGs of skin-only incised and sham wild type (WT) mice during stimulation with the TRPA1 agonist cinnamaldehyde. There were no changes in the percentage of neurons responding to cinnamaldehyde or in their response amplitudes. Likewise, the subpopulation of DRG somata retrogradely labeled specifically from the incised region of the plantar hind paw showed no functional up-regulation of TRPA1 after skin-only incision. Next, we conducted behavior tests for heat sensitivity and found that heat hypersensitivity peaked at day 1 post skin-only incision. Skin incision-induced heat hypersensitivity was significantly decreased in TRPV1-deficient mice. In addition, we conducted calcium imaging with the TRPV1 agonist capsaicin. DRG neurons from WT mice exhibited sensitization to TRPV1 activation, as more neurons (66%) from skin-incised mice responded to capsaicin compared to controls (46%), and the sensitization occurred specifically in isolectin B4 (IB4)-positive neurons where 80% of incised neurons responded to capsaicin compared to just 44% of controls.

Conclusions

Our data suggest that enhanced TRPA1 function does not mediate the mechanical hypersensitivity that follows skin-only surgical incision. However, the heat hypersensitivity is dependent on TRPV1, and functional up-regulation of TRPV1 in IB4-binding DRG neurons may mediate the heat hypersensitivity after skin incision injury.

Randomized clinical trial: inhibition of the TRPV1 system in patients with nonerosive gastroesophageal reflux disease and a partial response to PPI treatment is not associated with analgesia to esophageal experimental pain

OBJECTIVE

Many patients with nonerosive reflux disease (NERD) have insufficient relief on proton pump inhibitors (PPIs). Some patients have a hypersensitive esophagus and may respond to transient receptor potential vanilloid 1 (TRPV1) antagonists. Aim. To investigate the effect of the TRPV1 antagonist AZD1386 on experimental esophageal pain in NERD patients.

MATERIAL AND METHODS

Enrolled patients had NERD and a partial PPI response (moderate-to-severe heartburn or regurgitation ≥3 days/week before enrolment despite ≥6 weeks' PPI therapy). Fourteen patients (21-69 years, 9 women) were block-randomized into this placebo-controlled, double-blinded, crossover study examining efficacy of a single dose (95 mg) of AZD1386. On treatment days, each participant's esophagus was stimulated with heat, distension, and electrical current at teaching hospitals in Denmark and Sweden. Heat and pressure pain served as somatic control stimuli. Per protocol results were analyzed.

RESULTS

Of 14 randomized patients, 12 were treated with AZD1386. In the esophagus AZD1386 did not significantly change the moderate pain threshold for heat [-3%, 95% confidence interval (CI), -22;20%], distension (-11%, 95% CI, -28;10%), or electrical current (6%, 95% CI, -10;25%). Mean cutaneous heat tolerance increased by 4.9°C (95% CI, 3.7;6.2°C). AZD1386 increased the maximum body temperature by a mean of 0.59°C (95% CI, 0.40-0.79°C), normalizing within 4 h.

CONCLUSIONS

AZD1386 had no analgesic effect on experimental esophageal pain in patients with NERD and a partial PPI response, whereas it increased cutaneous heat tolerance. TRPV1 does not play a major role in heat-, mechanically and electrically evoked esophageal pain in these patients. ClinicalTrials.gov identifier: D9127C00002.

Activation of p38 mitogen-activated protein kinase in the dorsal root ganglion contributes to pain hypersensitivity after plantar incision

BACKGROUND

The phosphorylation of p38 mitogen-activated protein kinase (MAPK) in the dorsal root ganglion (DRG) promotes primary afferent sensitization. The role of p38MAPK signaling in the DRG in the pathogenesis of plantar incision hyperalgesia has not been investigated.

RESULTS

Levels of phosphorylated p38MAPK (p-p38MAPK) obviously increased in the DRG after plantar incision. Unmyelinated and myelinated DRG neurons that express p-p38MAPK contained small to medium cell bodies, suggesting that p-p38MAPK expression is induced in neurons with C- and Aδ-fibers. The p-p38MAPK inhibitors FR167653 or SB203580 inhibited incision-induced mechanical hypersensitivity and spontaneous pain behavior. The systemic administration of tumor necrosis factor-α (TNF-α) inhibitor prevented subsequent incision-induced activation of p38MAPK in the DRG and alleviated mechanical hypersensitivity after the incision.

CONCLUSIONS

p38MAPK signaling in the DRG plays a crucial role in the development of primary afferent sensitization and pain behavior caused by plantar incision.

Oleaginous extract from the fruits Pterodon pubescens Benth induces antinociception in animal models of acute and chronic pain

ETHNOPHARMACOLOGICAL RELEVANCE

Pterodon pubescens Benth is a medicinal plant commonly used for therapeutic purposes in folk medicine for rheumatic diseases' treatment. In the present work we analyzed the chemical composition of the oleaginous extract of P. pubescens Benth (OEPp) and extended the antinociceptive effect of OEPp evaluating its role on animal models of acute and chronic pain.

MATERIALS AND METHODS

The antinociceptive and antiedematogenic effects of OEPp (3-100mg/kg, i.g.) were evaluated in the formalin test; mechanical allodynia in the postoperative pain and complex regional pain syndrome type-I (CRPS-I) animal models; and thermal hyperalgesia was induced by plantar incision. Finally, we performed a phytochemical analysis of OEPp.

RESULTS

The chemical composition of OEPp was analyzed by mass spectrometry (GC/MS) and eight sesquiterpene compounds were identified, i.e. three major sesquiterpene (E-cariofilene, γ-muurolene, biciclogermacrene), and nine vouacapane diterpenes, four of which showed in major concentration (6α-acetoxyvouacapane, 6α,7β-dimetoxivouacapan-17-ene, 6α-acetoxy,7β-hidroxyvouacapane, 6α,7β-diacetoxycouacapane). Furthermore, the results of the present study demonstrate, for the first time, that the OEPp reduced mechanical allodynia in the postoperative pain and CRPS-I animal models. OEPp also increased the paw withdrawal latency in hot- and cold-plate tests in the postoperative pain model. In addition, the present work confirms and extends previous data from literature showing that systemic administration of OEPp caused significant inhibition against both phases of pain response to formalin intraplantar injection and edema formation.

CONCLUSIONS

Together, present and previous findings show that OEPp given intra-gastrically caused significant inhibition against both phases of formalin intraplantar injection and effectively inhibited mechanical and thermal hyperalgesia in the postoperative pain and CRPS-I animal models.

Skin incision-induced receptive field responses of mechanosensitive peripheral neurons are developmentally regulated in the rat

Maturation of the nervous system results in changes in both central and peripheral processing. To better understand responses to injury in the young, developmental differences in the acute response to incision were investigated in both tactile and nociceptive myelinated peripheral mechanosensitive afferent neurons in vivo. Neuronal intrasomal recordings were performed in juvenile and infant rats in 34 L5 dorsal root ganglia, and each neuron was phenotypically defined. Neurons had a mechanosensitive receptive field in the glabrous skin on the plantar surface of the hind paw, which was characterized at baseline and for up to 45 min after incision. Fundamental maturational differences in the effect of incision were clear: in high-threshold nociceptive mechanoreceptors, the mechanical threshold decreased immediately and the receptive field size increased rapidly in juvenile rats but not in infant rats. Additionally, a divergence in changes in the instantaneous response frequency of tactile afferents occurred between the two ages. These differences may help explain maturational differences in responses to peripheral injury and suggest that differences in central nervous system responses may be partially mitigated by spatially confined and frequency-dependent differences resulting from tactile and nociceptive mechanosensitive input.