Pharmacology of the capsaicin receptor, transient receptor potential vanilloid type-1 ion channel

Simultaneous hyperbaric oxygen therapy during systemic chemotherapy reverses chemotherapy-induced peripheral neuropathy by inhibiting TLR4 and TRPV1 activation in the central and peripheral nervous system

BACKGROUND AND OBJECTIVES

Chemotherapy-induced peripheral neuropathy (CIPN) is considered one of the most common sequelae in patients with cancer who experience consistent abnormal sensations or pain symptoms during or after paclitaxel (PAC) chemotherapy. Transient receptor potential vanilloid 1 (TRPV1) and toll-like receptor 4 (TLR4) have been reported to interact in the nervous system in patients with CIPN. The antinociceptive effects of hyperbaric oxygen therapy (HBOT) on CIPN was demonstrated in this study through behavior tests. Using a CIPN rat model, we examined the effects of simultaneous HBOT (SHBOT) administration during chemotherapy and discovered that SHBOT achieved better reversal effects than chemotherapy alone.

MATERIALS AND METHODS

Twenty-four rats were randomly allocated to four groups: control, PAC, SHBOT, and HBOT after PAC groups. Behavior tests were performed to evaluate mechanical allodynia and thermal hyperalgesia status. Tissues from the spinal cord and dorsal root ganglions were collected, and TLR4 and TRPV1 expression and microglial activation were investigated through immunofluorescence (IF) staining.

RESULTS

The mechanical and thermal behavior tests revealed that HBOT intervention during PAC treatment led to the early alleviation of CIPN symptoms and inhibited CIPN deterioration. IF staining revealed that TLR4, TRPV1, and microglial activation were all upregulated in PAC-injected rats and exhibited early and significant downregulation in SHBOT-treated rats.

CONCLUSION

This study is the first to demonstrate that the use of SHBOT during PAC treatment has potential for the early suppression of CIPN initiation and deterioration, indicating that it can alleviate CIPN symptoms and may reverse CIPN in patients undergoing systemic chemotherapy.

TRPV1 feed-forward sensitisation depends on COX2 upregulation in primary sensory neurons

Increased activity and excitability (sensitisation) of a series of molecules including the transient receptor potential ion channel, vanilloid subfamily, member 1 (TRPV1) in pain-sensing (nociceptive) primary sensory neurons are pivotal for developing pathological pain experiences in tissue injuries. TRPV1 sensitisation is induced and maintained by two major mechanisms; post-translational and transcriptional changes in TRPV1 induced by inflammatory mediators produced and accumulated in injured tissues, and TRPV1 activation-induced feed-forward signalling. The latter mechanism includes synthesis of TRPV1 agonists within minutes, and upregulation of various receptors functionally linked to TRPV1 within a few hours, in nociceptive primary sensory neurons. Here, we report that a novel mechanism, which contributes to TRPV1 activation-induced TRPV1-sensitisation within ~ 30 min in at least ~ 30% of TRPV1-expressing cultured murine primary sensory neurons, is mediated through upregulation in cyclooxygenase 2 (COX2) expression and increased synthesis of a series of COX2 products. These findings highlight the importance of feed-forward signalling in sensitisation, and the value of inhibiting COX2 activity to control pain, in nociceptive primary sensory neurons in tissue injuries.

The Mysteries of Capsaicin-Sensitive Afferents

A fundamental subdivision of nociceptive sensory neurons is named after their unique sensitivity to capsaicin, the pungent ingredient in hot chili peppers: these are the capsaicin-sensitive afferents. The initial excitation by capsaicin of these neurons manifested as burning pain sensation is followed by a lasting refractory state, traditionally referred to as "capsaicin desensitization," during which the previously excited neurons are unresponsive not only to capsaicin but a variety of unrelated stimuli including noxious heat. The long sought-after capsaicin receptor, now known as TRPV1 (transient receptor potential cation channel, subfamily V member 1), was cloned more than two decades ago. The substantial reduction of the inflammatory phenotype of Trpv1 knockout mice has spurred extensive efforts in the pharmaceutical industry to develop small molecule TRPV1 antagonists. However, adverse effects, most importantly hyperthermia and burn injuries, have so far prevented any compounds from progressing beyond Phase 2. There is increasing evidence that these limitations can be at least partially overcome by approaches outside of the mainstream pharmaceutical development, providing novel therapeutic options through TRPV1. Although ablation of the whole TRPV1-expressing nerve population by high dose capsaicin, or more selectively by intersectional genetics, has allowed researchers to investigate the functions of capsaicin-sensitive afferents in health and disease, several "mysteries" remain unsolved to date, including the molecular underpinnings of "capsaicin desensitization," and the exact role these nerves play in thermoregulation and heat sensation. This review tries to shed some light on these capsaicin mechanisms.

The impact of capsaicinoids on APP processing in Alzheimer's disease in SH-SY5Y cells

The vanilloid capsaicin is a widely consumed spice, known for its burning and "hot" sensation through activation of TRPV1 ion-channels, but also known to decrease oxidative stress, inflammation and influence tau-pathology. Beside these positive effects, little is known about its effects on amyloid-precursor-protein (APP) processing leading to amyloid-β (Aβ), the major component of senile plaques. Treatment of neuroblastoma cells with capsaicinoids (24 hours, 10 µM) resulted in enhanced Aβ-production and reduced Aβ-degradation, leading to increased Aβ-levels. In detailed analysis of the amyloidogenic-pathway, both BACE1 gene-expression as well as protein-levels were found to be elevated, leading to increased β-secretase-activity. Additionally, γ-secretase gene-expression as well as activity was enhanced, accompanied by a shift of presenilin from non-raft to raft membrane-domains where amyloidogenic processing takes place. Furthermore, impaired Aβ-degradation in presence of capsaicinoids is dependent on the insulin-degrading-enzyme, one of the major Aβ-degrading-enzymes. Regarding Aβ-homeostasis, no differences were found between the major capsaicinoids, capsaicin and dihydrocapsaicin, and a mixture of naturally derived capsaicinoids; effects on Ca2+-homeostasis were ruled out. Our results show that in respect to Alzheimer's disease, besides the known positive effects of capsaicinoids, pro-amyloidogenic properties also exist, enhancing Aβ-levels, likely restricting the potential use of capsaicinoids as therapeutic substances in Alzheimer's disease.

Capsaicin Exerts Anti-convulsant and Neuroprotective Effects in Pentylenetetrazole-Induced Seizures

The transient receptor potential vanilloid-1 (TRPV1) receptor has been implicated in the development of epileptic seizures. We examined the effect of the TRPV1 agonist capsaicin on epileptic seizures, neuronal injury and oxidative stress in a model of status epilepticus induced in the rat by intraperitoneal (i.p.) injections of pentylenetetrazole (PTZ). Capsaicin was i.p. given at 1 or 2 mg/kg, 30 min before the first PTZ injection. Other groups were i.p. treated with the vehicle or the anti-epileptic drug phenytoin (30 mg/kg) alone or co-administered with capsaicin at 2 mg/kg. Brain levels of malondialdehyde (MDA), reduced glutathione (GSH), nitric oxide, and paraoxonase-1 (PON-1) activity, seizure scores, latency time and PTZ dose required to reach status epilepticus were determined. Histopathological assessment of neuronal damage was done. Results showed that brain MDA decreased by treatment with capsaicin, phenytoin or capsaicin/phenytoin. Nitric oxide decreased by capsaicin or capsaicin/phenytoin. GSH and PON-1 activity increased after capsaicin, phenytoin or capsaicin/phenytoin. Mean total seizure score decreased by 48.8% and 66.3% by capsaicin compared with 78.7% for phenytoin and 69.8% for capsaicin/phenytoin treatment. Only phenytoin increased the latency (115.7%) and threshold dose of PTZ (78.3%). Capsaicin did not decrease the anti-convulsive effect of phenytoin but prevented the phenytoin-induced increase in latency time and threshold dose. Neuronal damage decreased by phenytoin or capsaicin at 2 mg/kg but almost completely prevented by capsaicin/phenytoin. Thus in this model of status epilepticus, capsaicin decreased brain oxidative stress, the severity of seizures and neuronal injury and its co-administration with phenytoin afforded neuronal protection.

The Pharmacology of Pain Associated With the Monoiodoacetate Model of Osteoarthritis

The high incidence of osteoarthritis (OA) in an increasingly elderly population anticipates a dramatic rise in the number of people suffering from this disease in the near future. Because pain is the main reason patients seek medical help, effective pain management-which is currently lacking-is paramount to improve the quality of life that OA sufferers desperately seek. Good animal models are, in this day and age, fundamental tools for basic research of new therapeutic pathways. Several animal models of OA have been characterized, but none of them reproduces entirely all symptoms of the disease. Choosing between different animal models depends largely on which aspect of OA one aims to study. Here, we review the current understanding of the monoiodoacetate (MIA) model of OA. MIA injection in the knee joint leads to the progressive disruption of cartilage, which, in turn, is associated with the development of pain-like behavior. There are several reasons why the MIA model of OA seems to be the most adequate to study the pharmacological effect of new drugs in pain associated with OA. First, the pathological changes induced by MIA share many common traits with those observed in human OA (Van Der Kraan et al., 1989; Guingamp et al., 1997; Guzman et al., 2003), including loss of cartilage and alterations in the subchondral bone. The model has been extensively utilized in basic research, which means that the time course of pain-related behaviors and histopathological changes, as well as pharmacological profile, namely of commonly used pain-reducing drugs, is now moderately understood. Also, the severity of the progression of pathological changes can be controlled by grading the concentration of MIA administered. Further, in contrast with other OA models, MIA offers a rapid induction of pain-related phenotypes, with the cost-saving consequence in new drug screening. This model, therefore, may be more predictive of clinical efficacy of novel pharmacological tools than other chronic or acute OA models.

Combination of a Rapidly Penetrating Agonist and a Slowly Penetrating Antagonist Affords Agonist Action of Limited Duration at the Cellular Level

The capsaicin receptor TRPV1 (transient receptor potential vanilloid 1) has been an object of intense interest for pharmacological development on account of its critical role in nociception. In the course of structure activity analysis, it has become apparent that TRPV1 ligands may vary dramatically in the rates at which they interact with TRPV1, presumably reflecting differences in their abilities to penetrate into the cell. Using a fast penetrating agonist together with an excess of a slower penetrating antagonist, we find that we can induce an agonist response of limited duration and, moreover, the duration of the agonist response remains largely independent of the absolute dose of agonist, as long as the ratio of antagonist to agonist is held constant. This general approach for limiting agonist duration under conditions in which absolute agonist dose is variable should have more general applicability.

Capsaicin: Physicochemical properties, cutaneous reactions and potential applications in painful and inflammatory conditions

Capsaicin is a natural protoalkaloid recognized as the main pungent component in hot peppers (Capsicum annuum L.). The capsaicin receptor is highly expressed in the unmyelinated type C nerve fibers originating from small diameter sensory neurons in dorsal root ganglia and cranial nerve ganglia correspondents. Capsaicin and related vanilloids have a variety of effects on primary sensory neurons function, from sensory neuron excitation characterized by local burning sensation and neurogenic inflammation, followed by conduction blockage accompanied by reversible ultrastructural changes of peripheral nociceptive endings (desensitization), going as far as irreversible degenerative changes (neurotoxicity). The main role in capsaicin-induced neurogenic inflammation relies on the capsaicin sensitive, small diameter primary sensory neurons, therefore its evaluation could be used as a diagnostic instrument in functional alterations of cutaneous sensory nerve fibers. Moreover, capsaicin-induced desensitization and neurotoxicity explain the analgesic/anti-nociceptive and anti-inflammatory effects of topical capsaicin and its potential use in the management of painful and inflammatory conditions. In this study, we describe the effects of capsaicin on neurogenic inflammation and nociception, as well as its potential diagnostic value and therapeutic impact in various conditions involving impairment of sensory nerve fibers.

Pharmacological enhancement of KCC2 gene expression exerts therapeutic effects on human Rett syndrome neurons and Mecp2 mutant mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the methyl CpG binding protein 2 (MECP2) gene. There are currently no approved treatments for RTT. The expression of K+/Cl- cotransporter 2 (KCC2), a neuron-specific protein, has been found to be reduced in human RTT neurons and in RTT mouse models, suggesting that KCC2 might play a role in the pathophysiology of RTT. To develop neuron-based high-throughput screening (HTS) assays to identify chemical compounds that enhance the expression of the KCC2 gene, we report the generation of a robust high-throughput drug screening platform that allows for the rapid assessment of KCC2 gene expression in genome-edited human reporter neurons. From an unbiased screen of more than 900 small-molecule chemicals, we have identified a group of compounds that enhance KCC2 expression termed KCC2 expression-enhancing compounds (KEECs). The identified KEECs include U.S. Food and Drug Administration-approved drugs that are inhibitors of the fms-like tyrosine kinase 3 (FLT3) or glycogen synthase kinase 3β (GSK3β) pathways and activators of the sirtuin 1 (SIRT1) and transient receptor potential cation channel subfamily V member 1 (TRPV1) pathways. Treatment with hit compounds increased KCC2 expression in human wild-type (WT) and isogenic MECP2 mutant RTT neurons, and rescued electrophysiological and morphological abnormalities of RTT neurons. Injection of KEEC KW-2449 or piperine in Mecp2 mutant mice ameliorated disease-associated respiratory and locomotion phenotypes. The small-molecule compounds described in our study may have therapeutic effects not only in RTT but also in other neurological disorders involving dysregulation of KCC2.

Neuroprotective Effects of Low Dose Anandamide in Pentylenetetrazole-Induced Kindling in Rats

Anandamide (N-arachidonoylethanolamine) is an endogenous cannabinoid receptor CB1 ligand that exhibits neuroprotective effects in the brain. In this study, the effect of exogenously given anandamide on pentylenetetrazole (PTZ)-induced chemical kindling oxidative stress and brain damage in rats was studied. Rats were intraperitoneally (i.p.) injected with 35 mg/kg PTZ once every 48 hours for 12 times to induce seizures. Anandamide was i.p. given. 30 min prior to PTZ injection at 100 or 200 mg/kg. Injections of PTZ induced significant increase in brain lipid peroxidation (malondialdehyde: MDA), and nitric oxide associated with marked decrease in brain reduced glutathione (GSH). There were also significant decrements in acetylcholinesterase (AChE) concentration, butyrylcholinesterase (BChE) and paraoxonase-1 (PON-1) activities in brain tissue of PTZ injected rats. Meanwhile, there was no significant effect for PTZ on the concentration of brain neutrophil elastase. Anandamide administered at 100 and 200 mg/kg significantly decreased MDA and increased GSH contents and at 200 mg/kg significantly decreased nitric oxide in brain of PTZ-treated rats. The drug also caused significant increments in AChE concentration and PON-1 activity but had no significant effect on BChE or neutrophil elastase in rats treated with PTZ. Anandamide given at the dose of 200mg/kg significantly decreased the mean seizure scores over the study period by 22.3% and the frequency of myoclonic jerks and rearing (stage 3) by 56.7% compared with the vehicle-treated group. Anandamide given at 100 and 200 mg/kg completely inhibited the development of generalized tonic-clonic seizures (stage 5). It is concluded that in the PTZ-induced seizures, the cannabinoid receptor CB1 agonist anandamide decreases brain oxidative stress, neuronal injury, and exerts an antiepileptic activity.

Mechanical allodynia and enhanced responses to capsaicin are mediated by PI3K in a paclitaxel model of peripheral neuropathy

Paclitaxel chemotherapy treatment often leads to neuropathic pain resistant to available analgesic treatments. Recently spinal Toll-like receptor 4 (TLR4) and the transient receptor potential cation channel subfamily V member 1 (TRPV1) were identified to be involved in the pro-nociceptive effect of paclitaxel. The aim of this study was to investigate the role of phosphatidylinositol 3-kinase (PI3K) and serine/threonine kinases in this process, with the use of their antagonists (wortmannin, LY-294002, and staurosporine). The single paclitaxel administration (8 mg/kg i.p.) in mice induced robust mechanical allodynia measured as a reduced threshold to von Frey filament stimulation and generated reduced tachyphylaxis of capsaicin-evoked responses, recorded as changes in mEPSC frequency in patch-clamp recordings of dorsal horn neurons activity in vitro, for up to eight days. Paclitaxel application also induced increased Akt kinase phosphorylation in rat DRG neurons. All these paclitaxel-induced changes were prevented by the wortmannin in vivo pretreatment. Acute co-application of wortmannin or LY-294002 with paclitaxel in spinal cord slices also attenuated the paclitaxel effect on capsaicin-evoked responses. Staurosporine was effective in the acute in vitro experiments and on the first day after the paclitaxel treatment in vivo, but in contrast to wortmannin, it did not have a significant impact later. Our data suggest that the inhibition of PI3K signaling may help alleviate pathological pain syndromes in the paclitaxel-induced neuropathy.

Beneficial effects of curcumin and capsaicin on cyclophosphamide-induced premature ovarian failure in a rat model

Background

In recent years, cancer rates have been rising among reproductive-age women. Thus, chemotherapy exposure has become an important cause of premature ovarian failure (POF). There has been growing interest regarding the preservation and restoration of ovarian function before and after oncological treatment because of the reproductive risk of chemotherapeutics and improved long-term survival of cancer patients. In this study, we sought to analyze the effects of curcumin (CRC) and capsaicin (CPS) on cyclophosphamide-induced POF in a rat model.

Methods

POF in rats was induced by intraperitoneal injection of 200 mg/kg cyclophosphamide on day 1 and then 8 mg/kg/day for the following 14 days. After 14 days of cyclophosphamide administration, rats were randomly divided into three groups as follows (n = 10/group): POF, POF + CRC (100 mg/kg/day), and POF + CPS (0.5 mg/kg/day) to determine the effects of CRC and CPS on the cyclophosphamide-induced POF rat model. Biochemical, hormonal, and histopathological evaluations were performed on blood and tissue samples 14 days after the CRC and CPS treatments.

Results

Malonaldehyde levels were significantly reduced, and glutathione levels and superoxide dismutase activity were significantly increased, in ovarian tissues in the POF + CRC and POF + CPS groups compared with the POF group. In the POF group, we observed hemorrhage and prominent mononuclear cell infiltration beneath the germinative epithelium, vascular congestion in ovarian stroma, hemorrhage around the corpus luteum, and atresia in ovarian follicles. This histopathological damage was significantly improved by treatment with CRC and CPS. There was a significant reduction in serum follicle-stimulating hormone and luteinizing hormone levels in rats treated with CRC and CPS compared with the POF group. Moreover, the levels of estradiol and anti-mullerian hormone in rats treated with CRC and CPS were significantly increased compared with the control group.

Conclusions

In conclusion, CRC and CPS treatment of rats with cyclophosphamide-induced POF had a beneficial effect on reducing ovarian damage by improving tissue oxidative stress marker levels, ovarian reserve marker levels, and histopathological parameters. The significant improvements in ovarian tissue histopathological damage and hormonal levels detected in this study indicate that treatment with CRC or CPS might be a conservative treatment approach for cyclophosphamide-induced POF.

TRPV1 receptors contribute to paclitaxel-induced c-Fos expression in spinal cord dorsal horn neurons

Transient receptor potential vanilloid type 1 (TRPV1) receptors are important in the development of different pathological chronic pain states. Here we examined the role of spinal cord TRPV1 receptors in the mechanisms leading to activation of dorsal horn neurons after paclitaxel (PAC) treatment. PAC is a widely used chemotherapeutic drug that often leads to development of painful neuropathy. Immunohistochemical analysis of c-Fos protein expression in dorsal horn neurons was used as a marker of neuronal activation. Rat spinal cord slices were processed for in vitro incubation with PAC (100 nM) and TRPV1 receptor antagonists (SB366791 and AMG9810; 10 microM). PAC treatment induced significant upregulation of c-Fos nuclear expression in superficial dorsal horn neurons that was diminished by TRPV1 receptor antagonists pre-incubation. These results further substantiated the role of spinal TRPV1 receptors in the development of paclitaxel-induced neuropathic pain and contribute to better understanding of the pathological mechanisms involved.

Peripheral inflammation affects modulation of nociceptive synaptic transmission in the spinal cord induced by N-arachidonoylphosphatidylethanolamine

BACKGROUND AND PURPOSE

Endocannabinoids play an important role in modulating spinal nociceptive signalling, crucial for the development of pain. The cannabinoid CB₁ receptor and the TRPV1 cation channel are both activated by the endocannabinoid anandamide, a product of biosynthesis from the endogenous lipid precursor N-arachidonoylphosphatidylethanolamine (20:4-NAPE). Here, we report CB₁ receptor- and TRPV1-mediated effects of 20:4-NAPE on spinal synaptic transmission in control and inflammatory conditions.

EXPERIMENTAL APPROACH

Spontaneous (sEPSCs) and dorsal root stimulation-evoked (eEPSCs) excitatory postsynaptic currents from superficial dorsal horn neurons in rat spinal cord slices were assessed. Peripheral inflammation was induced by carrageenan. Anandamide concentration was assessed by mass spectrometry.

KEY RESULTS

Application of 20:4-NAPE increased anandamide concentration in vitro. 20:4-NAPE (20 μM) decreased sEPSCs frequency and eEPSCs amplitude in control and inflammatory conditions. The inhibitory effect of 20:4-NAPE was sensitive to CB₁ receptor antagonist PF514273 (0.2 μM) in both conditions, but to the TRPV1 antagonist SB366791 (10 μM) only after inflammation. After inflammation, 20:4-NAPE increased sEPSCs frequency in the presence of PF514273 and this increase was blocked by SB366791.

CONCLUSIONS AND IMPLICATIONS

While 20:4-NAPE treatment inhibited the excitatory synaptic transmission in both naive and inflammatory conditions, peripheral inflammation altered the underlying mechanisms. Our data indicate that 20:4-NAPE application induced mainly CB₁ receptor-mediated inhibitory effects in naive animals while TRPV1-mediated mechanisms were also involved after inflammation. Increasing anandamide levels for analgesic purposes by applying substrate for its local synthesis may be more effective than systemic anandamide application or inhibition of its degradation.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Health-promoting properties of compounds derived from Capsicum sp. A review

This article presents multidirectional effects of capsaicin and its natural derivatives as well as natural and synthetic analogs in term of their therapeutic properties. Active agents present in various Capsicum genus plants exert analgesic, anti-inflammatory, antibacterial, antioxidant and gastroprotective effects. Furthermore, capsaicin positively influences the metabolism of lipids. Numerous research show that capsaicinoids inhibit proliferation and migration process of cancer cells, what makes them molecules of high interest in oncology. Among broad range of positive activities, we have focused only on those properties that have already found application in medicine or seemed to be the most probably used in the near future. Even if in low or single doses this compound has been reported successful in numerous therapies, the negative consequences of high doses or prolonged administration is also discussed in the review.

Topical Treatments for Localized Neuropathic Pain

Purpose of Review

Topical therapeutic approaches in localized neuropathic pain (LNP) syndromes are increasingly used by both specialists and general practitioners, with a potentially promising effect on pain reduction. In this narrative review, we describe the available compounds for topical use in LNP syndromes and address their potential efficacy according to the literature.

Recent Findings

Local anaesthetics (e.g., lidocaine, bupivacaine and mepivacaine), as well as general anaesthetic agents (e.g., ketamine), muscle relaxants (e.g., baclofen), capsaicin, anti-inflammatory drugs (e.g., diclofenac), salicylates, antidepressants (e.g., amitriptyline and doxepin), α2 adrenergic agents (e.g., clonidine), or even a combination of them have been tested in various applications for the treatment of LNP. Few of them have reached a sufficient level of evidence to support systematic use as treatment options.

Summary

Relatively few systemic side effects or drug–drug interactions and satisfactory efficacy seem to be the benefits of topical treatments. More well-organized and tailored studies are necessary for the further conceptualization of topical treatments for LNP.

Phosphorylated Histone 3 at Serine 10 Identifies Activated Spinal Neurons and Contributes to the Development of Tissue Injury-Associated Pain

Transcriptional changes in superficial spinal dorsal horn neurons (SSDHN) are essential in the development and maintenance of prolonged pain. Epigenetic mechanisms including post-translational modifications in histones are pivotal in regulating transcription. Here, we report that phosphorylation of serine 10 (S10) in histone 3 (H3) specifically occurs in a group of rat SSDHN following the activation of nociceptive primary sensory neurons by burn injury, capsaicin application or sustained electrical activation of nociceptive primary sensory nerve fibres. In contrast, brief thermal or mechanical nociceptive stimuli, which fail to induce tissue injury or inflammation, do not produce the same effect. Blocking N-methyl-D-aspartate receptors or activation of extracellular signal-regulated kinases 1 and 2, or blocking or deleting the mitogen- and stress-activated kinases 1 and 2 (MSK1/2), which phosphorylate S10 in H3, inhibit up-regulation in phosphorylated S10 in H3 (p-S10H3) as well as fos transcription, a down-stream effect of p-S10H3. Deleting MSK1/2 also inhibits the development of carrageenan-induced inflammatory heat hyperalgesia in mice. We propose that p-S10H3 is a novel marker for nociceptive processing in SSDHN with high relevance to transcriptional changes and the development of prolonged pain.

The pharmacology of palmitoylethanolamide and first data on the therapeutic efficacy of some of its new formulations

Palmitoylethanolamide (PEA) has emerged as a potential nutraceutical, because this compound is naturally produced in many plant and animal food sources, as well as in cells and tissues of mammals, and endowed with important neuroprotective, anti-inflammatory and analgesic actions. Several efforts have been made to identify the molecular mechanism of action of PEA and explain its multiple effects both in the central and the peripheral nervous system. Here, we provide an overview of the pharmacology, efficacy and safety of PEA in neurodegenerative disorders, pain perception and inflammatory diseases. The current knowledge of new formulations of PEA with smaller particle size (i.e. micronized and ultra-micronized) when given alone or in combination with antioxidant flavonoids (i.e. luteolin) and stilbenes (i.e. polydatin) is also reviewed.

LINKED ARTICLES

This article is part of a themed section on Principles of Pharmacological Research of Nutraceuticals. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.11/issuetoc.

Spatial Distribution of the Cannabinoid Type 1 and Capsaicin Receptors May Contribute to the Complexity of Their Crosstalk

The cannabinoid type 1 (CB1) receptor and the capsaicin receptor (TRPV1) exhibit co-expression and complex, but largely unknown, functional interactions in a sub-population of primary sensory neurons (PSN). We report that PSN co-expressing CB1 receptor and TRPV1 form two distinct sub-populations based on their pharmacological properties, which could be due to the distribution pattern of the two receptors. Pharmacologically, neurons respond either only to capsaicin (COR neurons) or to both capsaicin and the endogenous TRPV1 and CB1 receptor ligand anandamide (ACR neurons). Blocking or deleting the CB1 receptor only reduces both anandamide- and capsaicin-evoked responses in ACR neurons. Deleting the CB1 receptor also reduces the proportion of ACR neurons without any effect on the overall number of capsaicin-responding cells. Regarding the distribution pattern of the two receptors, neurons express CB1 and TRPV1 receptors either isolated in low densities or in close proximity with medium/high densities. We suggest that spatial distribution of the CB1 receptor and TRPV1 contributes to the complexity of their functional interaction.

Harnessing the Therapeutic Potential of Capsaicin and Its Analogues in Pain and Other Diseases

Capsaicin is the most predominant and naturally occurring alkamide found in Capsicum fruits. Since its discovery in the 19th century, the therapeutic roles of capsaicin have been well characterized. The potential applications of capsaicin range from food flavorings to therapeutics. Indeed, capsaicin and few of its analogues have featured in clinical research covered by more than a thousand patents. Previous records suggest pleiotropic pharmacological activities of capsaicin such as an analgesic, anti-obesity, anti-pruritic, anti-inflammatory, anti-apoptotic, anti-cancer, anti-oxidant, and neuro-protective functions. Moreover, emerging data indicate its clinical significance in treating vascular-related diseases, metabolic syndrome, and gastro-protective effects. The dearth of potent drugs for management of such disorders necessitates the urge for further research into the pharmacological aspects of capsaicin. This review summarizes the historical background, source, structure and analogues of capsaicin, and capsaicin-triggered TRPV1 signaling and desensitization processes. In particular, we will focus on the therapeutic roles of capsaicin and its analogues in both normal and pathophysiological conditions.

TRPV1 Channel: A Potential Drug Target for Treating Epilepsy

Epilepsy has 2-3% incidence worldwide. However, present antiepileptic drugs provide only partial control of seizures. Calcium ion accumulation in hippocampal neurons has long been known as a major contributor to the etiology of epilepsy. TRPV1 is a calcium-permeable channel and mediator of epilepsy in the hippocampus. TRPV1 is expressed in epileptic brain areas such as CA1 area and dentate gyrus of the hippocampus. Here the author reviews the patent literature on novel molecules targeting TRPV1 that are currently being investigated in the laboratory and are candidates for future clinical evaluation in the management of epilepsy. A limited number of recent reports have implicated TRPV1 in the induction or treatment of epilepsy suggesting that this may be new area for potential drugs targeting this debilitating disease. Thus activation of TRPV1 by oxidative stress, resiniferatoxin, cannabinoid receptor (CB1) activators (i.e. anandamide) or capsaicin induced epileptic effects, and these effects could be reduced by appropriate inhibitors, including capsazepine (CPZ), 5'-iodoresiniferatoxin (IRTX), resolvins, and CB1 antagonists. It has been also reported that CPZ and IRTX reduced spontaneous excitatory synaptic transmission through modulation of glutaminergic systems and desensitization of TRPV1 channels in the hippocampus of rats. Immunocytochemical studies indicated that TRPV1 channel expression increased in the hippocampus of mice and patients with temporal lobe epilepsy. Taken together, findings in the current literature support a role for calcium ion accumulation through TRPV1 channels in the etiology of epileptic seizures, indicating that inhibition of TRPV1 in the hippocampus may possibly be a novel target for prevention of epileptic seizures.

The Cancer Chemotherapeutic Paclitaxel Increases Human and Rodent Sensory Neuron Responses to TRPV1 by Activation of TLR4

Peripheral neuropathy is dose limiting in paclitaxel cancer chemotherapy and can result in both acute pain during treatment and chronic persistent pain in cancer survivors. The hypothesis tested was that paclitaxel produces these adverse effects at least in part by sensitizing transient receptor potential vanilloid subtype 1 (TRPV1) through Toll-like receptor 4 (TLR4) signaling. The data show that paclitaxel-induced behavioral hypersensitivity is prevented and reversed by spinal administration of a TRPV1 antagonist. The number of TRPV1(+) neurons is increased in the dorsal root ganglia (DRG) in paclitaxel-treated rats and is colocalized with TLR4 in rat and human DRG neurons. Cotreatment of rats with lipopolysaccharide from the photosynthetic bacterium Rhodobacter sphaeroides (LPS-RS), a TLR4 inhibitor, prevents the increase in numbers of TRPV1(+) neurons by paclitaxel treatment. Perfusion of paclitaxel or the archetypal TLR4 agonist LPS activated both rat DRG and spinal neurons directly and produced acute sensitization of TRPV1 in both groups of cells via a TLR4-mediated mechanism. Paclitaxel and LPS sensitize TRPV1 in HEK293 cells stably expressing human TLR4 and transiently expressing human TRPV1. These physiological effects also are prevented by LPS-RS. Finally, paclitaxel activates and sensitizes TRPV1 responses directly in dissociated human DRG neurons. In summary, TLR4 was activated by paclitaxel and led to sensitization of TRPV1. This mechanism could contribute to paclitaxel-induced acute pain and chronic painful neuropathy. Significance statement: In this original work, it is shown for the first time that paclitaxel activates peripheral sensory and spinal neurons directly and sensitizes these cells to transient receptor potential vanilloid subtype 1 (TRPV1)-mediated capsaicin responses via Toll-like receptor 4 (TLR4) in multiple species. A direct functional interaction between TLR4 and TRPV1 is shown in rat and human dorsal root ganglion neurons, TLR4/TRPV1-coexpressing HEK293 cells, and in both rat and mouse spinal cord slices. Moreover, this is the first study to show that this interaction plays an important role in the generation of behavioral hypersensitivity in paclitaxel-related neuropathy. The key translational implications are that TLR4 and TRPV1 antagonists may be useful in the prevention and treatment of chemotherapy-induced peripheral neuropathy in humans.

Mechanisms involved in the development of chemotherapy-induced neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a debilitating and painful condition seen in patients undergoing treatment with common agents such as vincristine, paclitaxel, oxaliplatin and bortezomib. The mechanisms of this condition are diverse, and include an array of molecular and cellular contributions. Current research implicates genetic predispositions to this condition, which then may influence cellular responses to chemotherapy. Processes found to be influenced during CIPN include increased expression of inflammatory mediators, primarily cytokines, which can create cascading effects in neurons and glia. Changes in ion channels and neurotransmission, as well as changes in intracellular signaling and structures have been implicated in CIPN. This review explores these issues and suggests considerations for future research.