Capsaicin-induced neurotoxicity in cultured dorsal root ganglion neurons: involvement of calcium-activated proteases

Galacturonic acid-capsaicin prodrug for prolonged nociceptive-selective nerve blockade

There is an urgent clinical need to develop nerve-blocking agents capable of inducing long duration sensory block without muscle weakness or paralysis to treat post-operative and chronic pain conditions. Here, we report a galacturonic acid-capsaicin (GalA-CAP) prodrug as an effective nociceptive-selective axon blocking agent. Capsaicin selectively acts on nociceptive signaling without motor nerve blockade or disruption of proprioception and touch sensation, and the galacturonic acid moiety enhance prodrug permeability across the restrictive peripheral nerve barriers (PNBs) via carrier-mediated transport by the facilitative glucose transporters (GLUTs). In addition, following prodrug transport across PNBs, the inactive prodrug is converted to active capsaicin through linker hydrolysis, leading to sustained drug release. A single injection of GalA-CAP prodrug at the sciatic nerves of rats led to nociceptive-selective nerve blockade lasting for 234 ± 37 h, which is a sufficient duration to address the most intense period of postsurgical pain. Furthermore, the prodrug markedly mitigated capsaicin-associated side effects, leading to a notable decrease in systemic toxicity, benign local tissue reactions, and diminished burning and irritant effects.

Identification of compounds that cause axonal dieback without cytotoxicity in dorsal root ganglia explants and intervertebral disc cells with potential to treat pain via denervation

Low back pain, knee osteoarthritis, and cancer patients suffer from chronic pain. Aberrant nerve growth into intervertebral disc, knee, and tumors, are common pathologies that lead to these chronic pain conditions. Axonal dieback induced by capsaicin (Caps) denervation has been FDA-approved to treat painful neuropathies and knee osteoarthritis but with short-term efficacy and discomfort. Herein, we propose to evaluate pyridoxine (Pyr), vincristine sulfate (Vcr) and ionomycin (Imy) as axonal dieback compounds for denervation with potential to alleviate pain. Previous literature suggests Pyr, Vcr, and Imy can cause undesired axonal degeneration, but no previous work has evaluated axonal dieback and cytotoxicity on adult rat dorsal root ganglia (DRG) explants. Thus, we performed axonal dieback screening using adult rat DRG explants in vitro with Caps as a positive control and assessed cytotoxicity. Imy inhibited axonal outgrowth and slowed axonal dieback, while Pyr and Vcr at high concentrations produced significant reduction in axon length and robust axonal dieback within three days. DRGs treated with Caps, Vcr, or Imy had increased DRG cytotoxicity compared to matched controls, but overall cytotoxicity was minimal and at least 88% lower compared to lysed DRGs. Pyr did not lead to any DRG cytotoxicity. Further, neither Pyr nor Vcr triggered intervertebral disc cell death or affected cellular metabolic activity after three days of incubation in vitro. Overall, our findings suggest Pyr and Vcr are not toxic to DRGs and intervertebral disc cells, and there is potential for repurposing these compounds for axonal dieback compounds to cause local denervation and alleviate pain.

"ThermoTRP" Channel Expression in Cancers: Implications for Diagnosis and Prognosis (Practical Approach by a Pathologist)

Temperature-sensitive transient receptor potential (TRP) channels (so-called "thermoTRPs") are multifunctional signaling molecules with important roles in cell growth and differentiation. Several "thermoTRP" channels show altered expression in cancers, though it is unclear if this is a cause or consequence of the disease. Regardless of the underlying pathology, this altered expression may potentially be used for cancer diagnosis and prognostication. "ThermoTRP" expression may distinguish between benign and malignant lesions. For example, TRPV1 is expressed in benign gastric mucosa, but is absent in gastric adenocarcinoma. TRPV1 is also expressed both in normal urothelia and non-invasive papillary urothelial carcinoma, but no TRPV1 expression has been seen in invasive urothelial carcinoma. "ThermoTRP" expression can also be used to predict clinical outcomes. For instance, in prostate cancer, TRPM8 expression predicts aggressive behavior with early metastatic disease. Furthermore, TRPV1 expression can dissect a subset of pulmonary adenocarcinoma patients with bad prognosis and resistance to a number of commonly used chemotherapeutic agents. This review will explore the current state of this rapidly evolving field with special emphasis on immunostains that can already be added to the armoire of diagnostic pathologists.

Spicy food intake predicts Alzheimer-related cognitive decline in older adults with low physical activity

A plausible association exists among spicy food consumption, physical activity, and Alzheimer's disease (AD) or cognitive decline, but it remains poorly investigated. We aimed to examined the association between spicy food and AD-related memory decline or global cognitive decline in older adults under the moderating effect of physical activity. Total 196 non-demented older adults were included. Participants underwent comprehensive dietary and clinical assessments including spicy food intake, AD-related memory, global cognition, and physical activity. The strength of spicy food was stratified into three categories: 'not spicy' (reference), 'low spiciness', and 'high spiciness'. Multiple linear regression analyses were performed to examine the relationships between spicy level and cognition. The spicy level was the independent variable in each analysis; it was entered as a stratified categorical variable using the three categories. We found a significant association between a high level of spiciness in food and decreased memory ([Formula: see text] - 0.167, p < 0.001) or global cognition ([Formula: see text] - 0.122, p = 0.027), but not non-memory cognition. To explore the moderating effects of age, sex, apolipoprotein E ε4 allele-positivity, vascular risk score, body mass index, and physical activity on the associations between spicy level and memory or global cognition, the same regression analyses were repeated including two-way interaction terms between the spicy level and each of the six variables as an additional independent variable. An interactive effect was detected between a high level of spiciness in food and physical activity on the memory ([Formula: see text] 0.209, p = 0.029) or global cognition ([Formula: see text] 0.336, p = 0.001). Subgroup analyses showed that the association between a high level of spiciness in food and a lower memory ([Formula: see text] - 0.254, p < 0.001) and global score ([Formula: see text] - 0.222, p = 0.002) was present only in older adults with low physical activity, but not in older adults with high physical activity. Our findings suggest that spicy food intake is predictive of AD-related cognitive decline, i.e., episodic memory; this relationship is worsened by physically inactive lifestyle.

Capsicum annuum L. and its bioactive constituents: A critical review of a traditional culinary spice in terms of its modern pharmacological potentials with toxicological issues

Capsicum annuum L., commonly known as chili pepper, is used as an important spice globally and as a crude drug in many traditional medicine systems. The fruits of C. annuum have been used as a tonic, antiseptic, and stimulating agent, to treat dyspepsia, appetites, and flatulence, and to improve digestion and circulation. The article aims to critically review the phytochemical and pharmacological properties of C. annuum and its major compounds. Capsaicin, dihydrocapsaicin, and some carotenoids are reported as the major active compounds with several pharmacological potentials especially as anticancer and cardioprotectant. The anticancer effect of capsaicinoids is mainly mediated through mechanisms involving the interaction of Ca²⁺ -dependent activation of the MAPK pathway, suppression of NOX-dependent reactive oxygen species generation, and p53-mediated activation of mitochondrial apoptosis in cancer cells. Similarly, the cardioprotective effects of capsaicinoids are mediated through their interaction with cellular transient receptor potential vanilloid 1 channel, and restoration of calcitonin gene-related peptide via Ca²⁺ -dependent release of neuropeptides and suppression of bradykinin. In conclusion, this comprehensive review presents detailed information about the traditional uses, phytochemistry, and pharmacology of major bioactive principles of C. annuum with special emphasis on anticancer, cardioprotective effects, and plausible toxic adversities along with food safety.

The triple function of the capsaicin-sensitive sensory neurons: In memoriam János Szolcsányi

This paper is dedicated to the memory of János Szolcsányi (1938-2018), an outstanding Hungarian scientist. Among analgesics that act on pain receptors, he identified capsaicin as a selective lead molecule. He studied the application of capsaicin and revealed several physiological (pain, thermoregulation) and pathophysiological (inflammation, gastric ulcer) mechanisms. He discovered a new neuroregulatory system without sensory efferent reflex and investigated its pharmacology. The authors of this review are his former Ph.D. students who carried out their doctoral work in Szolcsányi's laboratory between 1985 and 2010 and report on the scientific results obtained under his guidance. His research group provided evidence for the triple function of the peptidergic capsaicin-sensitive sensory neurons including classical afferent function, local efferent responses, and remote, hormone-like anti-inflammatory, and antinociceptive actions. They also proposed somatostatin receptor type 4 as a promising drug target for the treatment of pain and inflammation. They revealed that neonatal capsaicin treatment caused no acute neuronal death but instead long-lasting selective ultrastructural and functional changes in B-type sensory neurons, similar to adult treatment. They described that lipid raft disruption diminished the agonist-induced channel opening of the TRPV1, TRPA1, and TRPM8 receptors in native sensory neurons. Szolcsányi's group has developed new devices for noxious heat threshold measurement: an increasing temperature hot plate and water bath. This novel approach proved suitable for assessing the thermal antinociceptive effects of analgesics as well as for analyzing peripheral mechanisms of thermonociception.

The Association between Spicy Food Consumption and Psychological Health in Chinese College Students: A Cross-Sectional Study

BACKGROUND

Capsaicin is the main active ingredient in chili peppers and spicy food. Animal studies provide contradictory results on the role of capsaicin in psychiatric disorders. There are no epidemiological studies to investigate the relationship between spicy food consumption and psychological health.

METHODS

A cross-sectional online survey was conducted. Psychological health was assessed with the Depression Anxiety Stress Scale, and spicy food consumption was assessed as frequency, strength, and duration of consumption. Multivariable logistic regression was conducted to determine the associations between spicy food consumption and psychological symptoms.

RESULTS

Our sample comprised 1771 participants (male = 674, mean age = 21.97 years). The odds of having depressive, anxiety, and stress symptoms were 34.0%, 46.5%, and 19.1% in Chinese college students, respectively. After adjusting for a series of covariates, compared with non-consumers, the odds ratios (ORs) of depressive symptoms across spicy food consumption were 1.13 (95% CI: 0.87-1.46) for 1-2 days/week and 1.38 (95% CI: 1.02-1.86) for ≥3 days/week. With regard to anxiety symptoms, the ORs were 0.99 (95% CI: 0.78, 1.27) for 1-2 days/week and 1.50 (95% CI: 1.13-1.99) for ≥3 days/week. For stress symptoms, the ORs were 0.90 (95% CI: 0.66-1.23) for 1-2 days/week and 1.27 (95% CI: 0.89-1.80) for ≥3 days/week. The ORs for the depressive symptoms associated with different intensities of spicy food consumption were 1.00 (reference) for the reference group (non-consumers), 1.17 (95% CI: 0.90-1.52) for eating weakly spicy food, and 1.34 (95% CI: 1.01-1.78) for moderately to strongly spicy food.

CONCLUSION

The findings suggested a positive association between frequently spicy food consumption and depressive/anxiety symptoms in adolescents, and no such association was found for stress symptoms.

Extraction, purification, bioactivity and pharmacological effects of capsaicin: a review

Capsaicin (trans-8-methyl-N-vanillyl-6-nonenamide), a well-known vanilloid, which is the main spicy component in chili peppers, showing several biological activities and the potential applications range from food flavorings to therapeutics. Traditional extraction of capsaicin by organic solvents was time-consuming, some new methods such as aqueous two-phase method and ionic liquid extraction method have been developed. During past few decades, an ample variety of biological effects of capsaicin have been evaluated. Capsaicin can be used in biofilms and antifouling coatings due to its antimicrobial activity, allowing it has a promising application in food packaging, food preservation, marine environment and dental therapy. Capsaicin also play a crucial role in metabolic disorders, including weight loss, pressure lowing and insulin reduction effects. In addition, capsaicin was identified effective on preventing human cancers, such as lung cancer, stomach cancer, colon cancer and breast cancer by inducing apoptosis and inhibiting cell proliferation of tumor cells. Previous research also suggest the positive effects of capsaicin on pain relief and cognitive impairment. Capsaicin, the agonist of transient receptor potential vanilloid type 1 (TRPV1), could selectively activate TRPV1, inducing Ca²⁺ influx and related signaling pathways. Recently, gut microbiota was also involved in some diseases therapeutics, but its influence on the effects of capsaicin still need to be deeply studied. In this review, different extraction and purification methods of capsaicin, its biological activities and pharmacological effects were systematically summarized, as well as the possible mechanisms were also deeply discussed. This article will give an updated and better understanding of capsaicin-related biological effects and provide theoretical basis for its further research and applications in human health and manufacture development.

Quaternary Lidocaine Derivatives: Past, Present, and Future

Local anesthetics have the advantage of complete analgesia with fewer side effects compared to systemic analgesics. However, their clinical use is limited due to their short duration of action. Thus, local anesthetics with fast onset, long duration of action, selective nociceptive block, and low local and systemic toxicity are highly desirable. In the past electrophysiological studies, quaternary lidocaine derivatives (QLDs) showed these characteristics. Here, we review electrophysiological properties of QLDs and their pharmacodynamic characteristics to shed light on potential problems.

Calpain-Inhibitors Protect Frataxin-Deficient Dorsal Root Ganglia Neurons from Loss of Mitochondrial Na+/Ca2+ Exchanger, NCLX, and Apoptosis

Calpains are calcium-dependent proteases activated in apoptotic cell death and neurodegeneration. Friedreich Ataxia is a neurodegenerative rare disease caused by frataxin deficiency, a mitochondrial protein. Dorsal root ganglion (DRG) sensory neurons are among the cellular types most affected in this disease. We have previously demonstrated that frataxin-deficient DRGs show calpain activation, alteration in calcium levels and decreased content of the Na+/Ca2+ exchanger (NCLX). This transporter is involved in mitochondrial calcium efflux. In this study, we have performed a time-course analysis of several parameters altered in a frataxin-deficient DRGs. These include decline of NCLX levels, calcium accumulation, mitochondrial depolarization, α-fodrin fragmentation and apoptotic cell death. Furthermore, we have analysed the effect of the calpain inhibitors MDL28170 and Calpeptin on these parameters. We have observed that these inhibitors increase NCLX levels, protect sensory neurons from neurite degeneration and calcium accumulation, and restore mitochondrial membrane potential. In addition, calpain 1 reduction alleviated neurodegeneration in frataxin-deficient DRG neurons. These results strengthen the hypothesis of a central role for calcium homeostasis and calpains in frataxin-deficient dorsal root ganglia neurons.

Fight fire with fire: Neurobiology of capsaicin-induced analgesia for chronic pain

Capsaicin, the pungent ingredient in chili peppers, produces intense burning pain in humans. Capsaicin selectively activates the transient receptor potential vanilloid 1 (TRPV1), which is enriched in nociceptive primary afferents, and underpins the mechanism for capsaicin-induced burning pain. Paradoxically, capsaicin has long been used as an analgesic. The development of topical patches and injectable formulations containing capsaicin has led to application in clinical settings to treat chronic pain conditions, such as neuropathic pain and the potential to treat osteoarthritis. More detailed determination of the neurobiological mechanisms of capsaicin-induced analgesia should provide the logical rationale for capsaicin therapy and help to overcome the treatment's limitations, which include individual differences in treatment outcome and procedural discomfort. Low concentrations of capsaicin induce short-term defunctionalization of nociceptor terminals. This phenomenon is reversible within hours and, hence, likely does not account for the clinical benefit. By contrast, high concentrations of capsaicin lead to long-term defunctionalization mediated by the ablation of TRPV1-expressing afferent terminals, resulting in long-lasting analgesia persisting for several months. Recent studies have shown that capsaicin-induced Ca²⁺/calpain-mediated ablation of axonal terminals is necessary to produce long-lasting analgesia in a mouse model of neuropathic pain. In combination with calpain, axonal mitochondrial dysfunction and microtubule disorganization may also contribute to the longer-term effects of capsaicin. The analgesic effects subside over time in association with the regeneration of the ablated afferent terminals. Further determination of the neurobiological mechanisms of capsaicin-induced analgesia should lead to more efficacious non-opioidergic analgesic options with fewer adverse side effects.

Modulation on tetrodotoxin-resistant sodium current of loureirin B in rat dorsal root ganglion neurons via cyclic AMP-dependent protein kinase A

To search the modulation mechanism of loureirin B, a flavonoid is extracted from Dracaena cochinchinensis, on tetrodotoxin-resistant (TTX-R) sodium channel in dorsal root ganglion (DRG) neurons of rats. Experiments were carried out based on patch-clamp technique and molecular biological methods. We observed the time-dependent inhibition of loureirin B on TTX-R sodium currents in DRG neurons and found that neither occupancy theory nor rate theory could well explain the time-dependent inhibitory effect of loureirin B on TTX-R sodium currents. It suggested that a second messenger-mediated signaling pathway may be involved in the modulation mechanism. So the cyclin AMP (cAMP) level of the DRG neurons before and after incubation with loureirin B was tested by ELISA Kit. Results showed that loureirin B could increase the cAMP level and the increased cAMP was caused by the enhancement of adenylate cyclase (AC) induced by loureirin B. Immunolabelling experiments further confirmed that loureirin B can promote the production of PKA in DRG neurons. In the presence of the PKA inhibitor H-89, the inhibitory effect of loureirin B on TTX-R sodium currents was reversed. Forskolin, a tool in biochemistry to raise the levels of cAMP, also could reduce TTX-R sodium currents similar to that of loureirin B. These studies demonstrated that loureirin B can modulate the TTX-R sodium channel in DRG neurons via an AC/cAMP/PKA pathway involving the activation of AC and PKA, which also can be used to explain the other pharmacological effects of loureirin B.

High Chili Intake and Cognitive Function among 4582 Adults: An Open Cohort Study over 15 Years

We aimed to examine the association between chili intake and cognitive function in Chinese adults. This is a longitudinal study of 4852 adults (age 63.4 ± 7.7) attending the China Health and Nutrition Survey during 1991 and 2006. Cognitive function was assessed in 1997, 2000, 2004 and 2006. In total, 3302 completed cognitive screening tests in at least two surveys. Chili intake was assessed by a 3-day food record during home visits in each survey between 1991 and 2006. Multivariable mixed linear regression and logistic regression were used. Chili intake was inversely related to cognitive function. In fully adjusted models, including sociodemographic and lifestyle factors, compared with non-consumers, those whose cumulative average chili intake above 50 g/day had the regression coefficients (and 95% CI) for global cognitive function of -1.13 (-1.71-0.54). Compared with non-consumers, those with chili consumption above 50 g/day had the odds ratio (and 95% CI) of 2.12(1.63-2.77), 1.56(1.23-1.97) for self-reported poor memory and self-reported memory decline, respectively. The positive association between chili intake and cognitive decline was stronger among those with low BMI than those with high BMI. The longitudinal data indicate that higher chili intake is positively associated with cognitive decline in Chinese adults in both genders.

Ion Channel Pharmacology for Pain Modulation

A large series of different ion channels have been identified and investigated as potential targets for new medicines for the treatment of a variety of human diseases, including pain. Among these channels, the voltage gated calcium channels (VGCC) are inhibited by drugs for the treatment of migraine, neuropathic pain or intractable pain. Transient receptor potential (TRP) channels are emerging as important pain transducers as they sense low pH media or oxidative stress and other mediators and are abundantly found at sites of inflammation or tissue injury. Low pH may also activate acid sensing ion channels (ASIC) and mechanical forces stimulate the PIEZO channels. While potent agonists of TRP channels due to their desensitizing action on pain transmission are used as topical applications, the potential of TRP antagonists as pain therapeutics remains an exciting field of investigation. The study of ASIC or PIEZO channels in pain signaling is in an early stage, whereas antagonism of the purinergic P2X₃ channels has been reported to provide beneficial effects in chronic intractable cough. The present chapter covers these intriguing channels in great detail, highlighting their diverse mechanisms and broad potential for therapeutic utility.

Transient Receptor Potential Vanilloid 1 Expression Mediates Capsaicin-Induced Cell Death

The transient receptor potential (TRP) ion channel family consists of a broad variety of non-selective cation channels that integrate environmental physicochemical signals for dynamic homeostatic control. Involved in a variety of cellular physiological processes, TRP channels are fundamental to the control of the cell life cycle. TRP channels from the vanilloid (TRPV) family have been directly implicated in cell death. TRPV1 is activated by pain-inducing stimuli, including inflammatory endovanilloids and pungent exovanilloids, such as capsaicin (CAP). TRPV1 activation by high doses of CAP (>10 μM) leads to necrosis, but also exhibits apoptotic characteristics. However, CAP dose-response studies are lacking in order to determine whether CAP-induced cell death occurs preferentially via necrosis or apoptosis. In addition, it is not known whether cytosolic Ca²⁺ and mitochondrial dysfunction participates in CAP-induced TRPV1-mediated cell death. By using TRPV1-transfected HeLa cells, we investigated the underlying mechanisms involved in CAP-induced TRPV1-mediated cell death, the dependence of CAP dose, and the participation of mitochondrial dysfunction and cytosolic Ca²⁺ increase. Together, our results contribute to elucidate the pathophysiological steps that follow after TRPV1 stimulation with CAP. Low concentrations of CAP (1 μM) induce cell death by a mechanism involving a TRPV1-mediated rapid and transient intracellular Ca²⁺ increase that stimulates plasma membrane depolarization, thereby compromising plasma membrane integrity and ultimately leading to cell death. Meanwhile, higher doses of CAP induce cell death via a TRPV1-independent mechanism, involving a slow and persistent intracellular Ca²⁺ increase that induces mitochondrial dysfunction, plasma membrane depolarization, plasma membrane loss of integrity, and ultimately, cell death.

Targeting nociceptive transient receptor potential channels to treat chronic pain: current state of the field

Control of chronic pain is frequently inadequate and/or associated with intolerable adverse effects, prompting a frantic search for new therapeutics and new therapeutic targets. Nearly two decades of preclinical and clinical research supports the involvement of transient receptor potential (TRP) channels in temperature perception, nociception and sensitization. Although there has been considerable excitement around the therapeutic potential of this channel family since the cloning and identification of TRPV1 cation channels as the capsaicin receptor more than 20 years ago, only modulators of a few channels have been tested clinically. TRPV1 channel antagonists have suffered from side effects related to the channel's role in temperature sensation; however, high dose formulations of capsaicin have reached the market and shown therapeutic utility. A number of potent, small molecule antagonists of TRPA1 channels have recently advanced into clinical trials for the treatment of inflammatory and neuropathic pain, and TRPM8 antagonists are following closely behind for cold allodynia. TRPV3, TRPV4, TRPM2 and TRPM3 channels have also been of significant interest. This review discusses the preclinical promise and status of novel analgesic agents that target TRP channels and the challenges that these compounds may face in development and clinical practice.

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.

Activation and expression of μ-calpain in dorsal root contributes to RTX-induced mechanical allodynia

Background

Calpain is a calcium-dependent cysteine protease, and inhibition of calpain by pre-treatment with MDL28170 attenuated the rat mechanical allodynia in a variety of pain models. Postherpetic neuralgia (Shingles) is a neuropathic pain conditioned with the presence of profound mechanical allodynia. Systemic injection of resiniferatoxin can reproduce the clinical symptoms of postherpetic neuralgia. In this study, we determined to study whether activation of calpain contributes to cleave the myelin basic protein of dorsal root and is involved in resiniferatoxin-induced mechanical allodynia of postherpetic neuralgia animal model.

Results

Resiniferatoxin up-regulated the expression and activation of µ-calpain in dorsal root. The expression of µ-calpain was located in Schwann cell of dorsal root, and resiniferatoxin increased the expression of µ-calpain in Schwann cell in L4–L6 dorsal root at six weeks after injection. Resiniferatoxin also induced myelin basic protein degradation in L4–L6 dorsal root at six weeks after injection. Moreover, intraperitoneal injection of calpain inhibitor MDL28170 prevented the degradation of myelin basic protein and then reduced the sprouting of myelinated afferent fibers into spinal lamina II, thus relieving resiniferatoxin-induced mechanical allodynia.

Conclusions

Up-regulation and activation of µ-calpain located in Schwann cell may be the mechanism underlying resiniferatoxin-mediated proteolysis of myelin basic protein in dorsal root. Calpain inhibitor MDL28170 prevents resiniferatoxin-induced sprouting of myelinated afferent fibers and mechanical allodynia through inhibition of degradation of the myelin basic protein in dorsal root. Our results indicate that inhibition of pathological µ-calpain activation may present an interesting novel drug target in the treatment of postherpetic neuralgia.

Static magnetic field enhances the anticancer efficacy of capsaicin on HepG2 cells via capsaicin receptor TRPV1

Static magnetic field (SMF) has shown some possibilities for cancer therapies. In particular, the combinational effect between SMF and anti-cancer drugs has drawn scientists’ attentions in recent years. However, the underlying mechanism for the drug-specific synergistic effect is far from being understood. Besides, the drugs used are all conventional chemotherapy drugs, which may cause unpleasant side effects. In this study, our results demonstrate for the first time that SMF could enhance the anti-cancer effect of natural compound, capsaicin, on HepG2 cancer cells through the mitochondria-dependent apoptosis pathway. We found that the synergistic effect could be due to that SMF increased the binding efficiency of capsaicin for the TRPV1 channel. These findings may provide a support to develop an application of SMF for cancer therapy. The present study offers the first trial in combining SMF with natural compound on anti-cancer treatment, which provides additional insight into the interaction between SMF and anti-cancer drugs and opens the door for the development of new strategies in fighting cancer with minimum cytotoxicity and side effects.

N-Arachidonoyl Dopamine: A Novel Endocannabinoid and Endovanilloid with Widespread Physiological and Pharmacological Activities

N-arachidonoyl dopamine (NADA) is a member of the family of endocannabinoids to which several other N-acyldopamines belong as well. Their activity is mediated through various targets that include cannabinoid receptors or transient receptor potential vanilloid (TRPV)1. Synthesis and degradation of NADA are not yet fully understood. Nonetheless, there is evidence that NADA plays an important role in nociception and inflammation in the central and peripheral nervous system. The TRPV1 receptor, for which NADA is a potent agonist, was shown to be an endogenous transducer of noxious heat. Moreover, it has been demonstrated that NADA exerts protective and antioxidative properties in microglial cell cultures, cortical neurons, and organotypical hippocampal slice cultures. NADA is present in very low concentrations in the brain and is seemingly not involved in activation of the classical pathways. We believe that treatment with exogenous NADA during and after injury might be beneficial. This review summarizes the recent findings on biochemical properties of NADA and other N-acyldopamines and their role in physiological and pathological processes. These findings provide strong evidence that NADA is an effective agent to manage neuroinflammatory diseases or pain and can be useful in designing novel therapeutic strategies.

Optogenetic control of mitochondrial metabolism and Ca2+ signaling by mitochondria-targeted opsins

Key mitochondrial functions such as ATP production, Ca²⁺ uptake and release, and substrate accumulation depend on the proton electrochemical gradient (ΔμH⁺) across the inner membrane. Although several drugs can modulate ΔμH⁺, their effects are hardly reversible, and lack cellular specificity and spatial resolution. Although channelrhodopsins are widely used to modulate the plasma membrane potential of excitable cells, mitochondria have thus far eluded optogenetic control. Here we describe a toolkit of optometabolic constructs based on selective targeting of channelrhodopsins with distinct functional properties to the inner mitochondrial membrane of intact cells. We show that our strategy enables a light-dependent control of the mitochondrial membrane potential (Δψ_m) and coupled mitochondrial functions such as ATP synthesis by oxidative phosphorylation, Ca²⁺ dynamics, and respiratory metabolism. By directly modulating Δψ_m, the mitochondria-targeted opsins were used to control complex physiological processes such as spontaneous beats in cardiac myocytes and glucose-dependent ATP increase in pancreatic β-cells. Furthermore, our optometabolic tools allow modulation of mitochondrial functions in single cells and defined cell regions.

Capsaicin protects cortical neurons against ischemia/reperfusion injury via down-regulating NMDA receptors

Capsaicin, the ingredient responsible for the pungent taste of hot chili peppers, is widely used in the study and management of pain. Recently, its neuroprotective effect has been described in multiple studies. Herein, we investigated the underlying mechanisms for the neuroprotective effect of capsaicin. Direct injection of capsaicin (1 or 3nmol) into the peri-infarct area reduced the infarct volume and improved neurological behavioral scoring and motor coordination function in the middle cerebral artery occlusion (MCAO)/reperfusion model in rats. The time window of the protective effect of capsaicin was within 1h after reperfusion, when excitotoxicity is the main reason of cell death. In cultured cortical neurons, administration of capsaicin attenuated glutamate-induced excitotoxic injury. With respect to the mechanisms of the neuroprotective effect of capsaicin, reduced calcium influx after glutamate stimulation was observed following capsaicin pretreatment in cortical neurons. Trpv1 knock-out abolished the inhibitory effect of capsaicin on glutamate-induced calcium influx and subsequent neuronal death. Reduced expression of GluN1 and GluN2B, subunits of NMDA receptor, was examined after capsaicin treatment in cortical neurons. In summary, our studies reveal that the neuroprotective effect of capsaicin in cortical neurons is TRPV1-dependent and down-regulation of the expression and function of NMDA receptors contributes to the protection afforded by capsaicin.

Ca2+ and calpain mediate capsaicin-induced ablation of axonal terminals expressing transient receptor potential vanilloid 1

Capsaicin is an ingredient in spicy peppers that produces burning pain by activating transient receptor potential vanilloid 1 (TRPV1), a Ca²⁺-permeable ion channel in nociceptors. Capsaicin has also been used as an analgesic, and its topical administration is approved for the treatment of certain pain conditions. The mechanisms underlying capsaicin-induced analgesia likely involve reversible ablation of nociceptor terminals. However, the mechanisms underlying these effects are not well understood. To visualize TRPV1-lineage axons, a genetically engineered mouse model was used in which a fluorophore is expressed under the TRPV1 promoter. Using a combination of these TRPV1-lineage reporter mice and primary afferent cultures, we monitored capsaicin-induced effects on afferent terminals in real time. We found that Ca²⁺ influx through TRPV1 is necessary for capsaicin-induced ablation of nociceptive terminals. Although capsaicin-induced mitochondrial Ca²⁺ uptake was TRPV1-dependent, dissipation of the mitochondrial membrane potential, inhibition of the mitochondrial transition permeability pore, and scavengers of reactive oxygen species did not attenuate capsaicin-induced ablation. In contrast, MDL28170, an inhibitor of the Ca²⁺-dependent protease calpain, diminished ablation. Furthermore, overexpression of calpastatin, an endogenous inhibitor of calpain, or knockdown of calpain 2 also decreased ablation. Quantitative assessment of TRPV1-lineage afferents in the epidermis of the hind paws of the reporter mice showed that EGTA and MDL28170 diminished capsaicin-induced ablation. Moreover, MDL28170 prevented capsaicin-induced thermal hypoalgesia. These results suggest that TRPV1/Ca²⁺/calpain-dependent signaling plays a dominant role in capsaicin-induced ablation of nociceptive terminals and further our understanding of the molecular mechanisms underlying the effects of capsaicin on nociceptors.

The effect of capsaicin on expression patterns of CGRP in trigeminal ganglion and trigeminal nucleus caudalis following experimental tooth movement in rats

Objectives

The aim of this study was to explore the effect of capsaicin on expression patterns of calcitonin gene-related peptide (CGRP) in the trigeminal ganglion (TG) and trigeminal subnucleus caudalis (Vc) following experimental tooth movement.

Material and Methods

Male Sprague-Dawley rats were used in this study and divided into small-dose capsaicin+force group, large-dose capsaicin+force group, saline+force group, and no force group. Closed coil springs were used to mimic orthodontic forces in all groups except for the no force group, in which springs were inactivated. Capsaicin and saline were injected into periodontal tissues. Rats were euthanized at 0 h, 12 h, 1 d, 3 d, 5 d, and 7 d following experimental tooth movement. Then, TG and Vc were obtained for immunohistochemical staining and western blotting against CGRP.

Results

Immunohistochemical results indicated that CGRP positive neurons were located in the TG, and CGRP immunoreactive fibers were distributed in the Vc. Immunohistochemical semiquantitative analysis and western blotting analysis demonstrated that CGRP expression levels both in TG and Vc were elevated at 12 h, 1 d, 3 d, 5 d, and 7 d in the saline + force group. However, both small-dose and large-dose capsaicin could decrease CGRP expression in TG and Vc at 1 d and 3 d following experimental tooth movement, as compared with the saline + force group.

Conclusions

These results suggest that capsaicin could regulate CGRP expression in TG and Vc following experimental tooth movement in rats.

Use of Capsaicin to Treat Pain: Mechanistic and Therapeutic Considerations

Capsaicin is the pungent ingredient of chili peppers and is approved as a topical treatment of neuropathic pain. The analgesia lasts for several months after a single treatment. Capsaicin selectively activates TRPV1, a Ca²⁺-permeable cationic ion channel that is enriched in the terminals of certain nociceptors. Activation is followed by a prolonged decreased response to noxious stimuli. Interest also exists in the use of injectable capsaicin as a treatment for focal pain conditions, such as arthritis and other musculoskeletal conditions. Recently injection of capsaicin showed therapeutic efficacy in patients with Morton’s neuroma, a painful foot condition associated with compression of one of the digital nerves. The relief of pain was associated with no change in tactile sensibility. Though injection evokes short term pain, the brief systemic exposure and potential to establish long term analgesia without other sensory changes creates an attractive clinical profile. Short-term and long-term effects arise from both functional and structural changes in nociceptive terminals. In this review, we discuss how local administration of capsaicin may induce ablation of nociceptive terminals and the clinical implications.

The effect of intra-articular vanilloid receptor agonists on pain behavior measures in a murine model of acute monoarthritis

Arthritis is the most common cause of disability in the US, and the primary manifestation of arthritis is joint pain that leads to progressive physical limitation, disability, morbidity, and increased health care utilization. Capsaicin (CAP) is a vanilloid agonist that causes substance P depletion by interacting with vanilloid receptor transient receptor potential V1 on small unmyelinated C fibers. It has been used topically for analgesia in osteoarthritis with variable success. Resiniferatoxin (RTX) is an ultra potent CAP analog. The aim of this study was to measure the analgesic effects of intra-articular (IA) administration of CAP and RTX in experimental acute inflammatory arthritis in mice. Evoked pain score (EPS) and a dynamic weight bearing (DWB) device were used to measure nociceptive behaviors in a murine model of acute inflammatory monoarthritis. A total of 56 C57B16 male mice underwent EPS and DWB testing – 24 nonarthritic controls and 32 mice with carrageenan-induced arthritis. The effects of pretreatment with 0.1% CAP, 0.0003% RTX, or 0.001% RTX were measured. Nociception was reproducibly demonstrated by increased EPS and reduced DWB measures in the affected limb of arthritic mice. Pretreatment with 0.001% RTX resulted in statistically significant improvement in EPS and DWB measures when compared with those observed in carrageenan-induced arthritis animals. Pretreatment with IA 0.0003% RTX and IA 0.01% CAP resulted in improvement in some but not all of these measures. The remaining 24 mice underwent evaluation following treatment with 0.1% CAP, 0.0003% RTX, or 0.001% RTX, and the results obtained were similar to that of naïve, nonarthritic mice.

Experimental evidence for alleviating nociceptive hypersensitivity by single application of capsaicin

The single application of high-concentration of capsaicin has been used as an analgesic therapy of persistent pain. However, its effectiveness and underlying mechanisms remain to be further evaluated with experimental approaches. The present study provided evidence showing that the single application of capsaicin dose-dependently alleviated nociceptive hypersensitivity, and reduced the action potential firing in small-diameter neurons of the dorsal root ganglia (DRG) in rats and mice. Pre-treatment with capsaicin reduced formalin-induced acute nocifensive behavior after a brief hyperalgesia in rats and mice. The inhibitory effects of capsaicin were calcium-dependent, and mediated by the capsaicin receptor (transient receptor potential vanilloid type-1). We further found that capsaicin exerted inhibitory effects on the persistent nociceptive hypersensitivity induced by peripheral inflammation and nerve injury. Thus, these results support the long-lasting and inhibitory effects of topical capsaicin on persistent pain, and the clinic use of capsaicin as a pain therapy.

A review of the high-concentration capsaicin patch and experience in its use in the management of neuropathic pain

In the European Union, the high-concentration capsaicin patch is licensed for the management of neuropathic pain conditions in nondiabetic patients, including postherpetic neuralgia (PHN) and HIV-associated distal sensory polyneuropathy (HIV-DSP). However, in the USA, the Food and Drug Administration approved its use only in PHN patients. Capsaicin is a transient receptor potential vanilloid-1 agonist, which increases the intracellular calcium ion concentration. This triggers calcium-dependent protease enzymes causing cytoskeletal breakdown and leads to the loss of cellular integrity and 'defunctionalization' of nociceptor fibres. Efficacy and therapeutic effect has been shown in several clinical studies of PHN and HIV-DSP. The high-concentration capsaicin patch and its practical application are different from low-concentration creams; one application can help for up to 3 months. The process of setting up of a service to use the capsaicin 8% patch is also discussed.

Effects of inhibitors on the synergistic interaction between calpain and caspase-3 during post-mortem aging of chicken meat

Calpain has been considered to be the most important protease involved in tenderization during the conversion of muscle into meat. However, recent evidence suggests the possible involvement of the key apoptosis protease, caspase, on post-mortem tenderization. This study used inhibitors of calpain and caspase-3 to treat chicken muscle immediately after slaughter and followed the changes in caspase-3 and calpain activities together with their expression during 5 days of aging. Addition of calpain inhibitors to the system resulted in significantly higher caspase-3 activities (p < 0.01) during storage. Western blot analysis of pro-caspase-3 and α-spectrin cleavage of the 120 kDa peptide (SBDP 120) showed that the addition of calpain inhibitors resulted in the formation of higher amounts of the active form of caspase-3 compared with the control (p < 0.01). Inclusion of inhibitors of caspase-3 led to lower calpain activities (p < 0.01) and dramatically reduced the expression of calpain-1 and calpain-2 (p < 0.01). Concomitantly, this inhibition resulted in greater calpastatin expression compared with the control (p < 0.01). The findings of this investigation show that calpain prevented the activation of caspase-3, whereas caspase-3 appeared to enhance the calpain activity during post-mortem aging through inhibition of calpastatin. It is therefore suggested that there is a relationship between caspase-3 and calpain which contributes to the tenderizing process during the conversion of muscle tissue into meat.

Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patch

Topical capsaicin formulations are used for pain management. Safety and modest efficacy of low-concentration capsaicin formulations, which require repeated daily self-administration, are supported by meta-analyses of numerous studies. A high-concentration capsaicin 8% patch (Qutenza™) was recently approved in the EU and USA. A single 60-min application in patients with neuropathic pain produced effective pain relief for up to 12 weeks. Advantages of the high-concentration capsaicin patch include longer duration of effect, patient compliance, and low risk for systemic effects or drug-drug interactions. The mechanism of action of topical capsaicin has been ascribed to depletion of substance P. However, experimental and clinical studies show that depletion of substance P from nociceptors is only a correlate of capsaicin treatment and has little, if any, causative role in pain relief. Rather, topical capsaicin acts in the skin to attenuate cutaneous hypersensitivity and reduce pain by a process best described as 'defunctionalization' of nociceptor fibres. Defunctionalization is due to a number of effects that include temporary loss of membrane potential, inability to transport neurotrophic factors leading to altered phenotype, and reversible retraction of epidermal and dermal nerve fibre terminals. Peripheral neuropathic hypersensitivity is mediated by diverse mechanisms, including altered expression of the capsaicin receptor TRPV1 or other key ion channels in affected or intact adjacent peripheral nociceptive nerve fibres, aberrant re-innervation, and collateral sprouting, all of which are defunctionalized by topical capsaicin. Evidence suggests that the utility of topical capsaicin may extend beyond painful peripheral neuropathies.

Characterization of capsaicin induced responses in mice vas deferens: evidence of CGRP uptake

Calcitonin gene-related peptide (CGRP) is extensively distributed in primary afferent sensory nerves, including those innervating the genitourinary tract. Capsaicin can stimulate the release of CGRP from intracellular stores of these nerves, but this phenomenon has not been investigated in-depth in isolated preparations. The present study sets out to study and characterize the capsaicin as well as CGRP-induced responses in isolated mouse vas deferens. The effects of capsaicin and CGRP family of peptides were studied on electrically-induced twitch responses in the absence or presence of transient receptor potential cation channel vanilloid subfamily member 1 (TRPV1) antagonist and CGRP receptor antagonists. Twitch responses were attenuated by capsaicin (1nM-30nM) and CGRP family of peptides. The potency order was CGRP>intermedin-long (IMDL)~[Cys(Et)(2,7)]αCGRP~adrenomedullin (AM)>[Cys(ACM)(2,7)]αCGRP>amylin (AMY). These responses were disinhibited by the CGRP receptor antagonists and TRPV1 antagonists. The addition of CGRP receptor antagonists caused a transient potentiation of the twitch response and this potentiation was blocked by pretreatment with capsaicin and enhanced by incubation with exogenous CGRP. During the second consecutive cumulative concentration-response curve with capsaicin, the first phase of concentration-response curve disappeared and this was partially restored when the mouse vas deferens was preincubated with CGRP, suggesting the uptake of exogenous CGRP by nerves. Besides showing capsaicin-induced CGRP releases this study shows that exogenous CGRP can be taken up in vas deferens and can be re-released. CGRP uptake will add another dimension in understanding the homeostasis of this neuropeptide.

TRPV1 receptors modulate retinal development

We investigated the possible participation of TRPV1 channels in retinal apoptosis and overall development. Retinas from newborn, male albino rats were treated in vitro with capsazepine, a TRPV1 antagonist. The expression of cell cycle markers was not changed after TRPV1 blockade, whereas capsazepine reduced the number of apoptotic cells throughout the retina,increased ERK1/2 and p38 phosphorylation and slightly reduced JNK phosphorylation. The expression of BAD, Bcl-2, as well as integral and cleaved capsase-3 were similar in all experimental conditions. Newborn rats were kept for 2 months after receiving high doses of capsazepine. In their retinas, calbindin and parvalbumin protein levels were upregulated, but only the number of amacrine-like, parvalbumin-positive cells was increased. The numbers of calretinin, calbindin, ChAT, vimentin, PKC-alpha and GABA-positive cells were similar in both conditions. Protein expression of synapsin Ib was also increased in the retinas of capsazepine-treated rats. Calretinin, vimentin, GFAP, synapsin Ia, synaptophysin and light neurofilament protein levels were not changed when compared to control values. Our results indicate that TRPV1 channels play a role in the control of the early apoptosis that occur during retinal development, which might be dependent on MAPK signaling. Moreover, it seems that TRPV1 function might be important for neuronal and synaptic maturation in the retina.

TRPV1 receptors are involved in protein nitration and Müller cell reaction in the acutely axotomized rat retina

We report here the protein expression of TRPV1 receptor in axotomized rat retinas and its possible participation in mechanisms involved in retinal ganglion cell (RGC) death. Adult rats were subjected to unilateral, intraorbital axotomy of the optic nerve, and the retinal tissue was removed for further processing. TRPV1 total protein expression decreased progressively after optic nerve transection, reaching 66.2% of control values 21 days after axotomy. The number of cells labeled for TRPV1 in the remnant GCL decreased after 21 days post-lesion (to 63%). Fluoro-Jade B staining demonstrated that the activation of TRPV1 in acutely-lesioned eyes elicited more intense neuronal degeneration in the GCL and in the inner nuclear layer than in sham-operated retinas. A single intraocular injection of capsazepine (100 μM), a TRPV1 antagonist, 5 days after optic nerve lesion, decreased the number of GFAP-expressing Müller cells (72.5% of control values) and also decreased protein nitration in the retinal vitreal margin (75.7% of control values), but did not affect lipid peroxidation. Furthermore, retinal explants were treated with capsaicin (100 μM), and remarkable protein nitration was then present, which was reduced by blockers of the constitutive and inducible nitric oxide synthases (7-NI and aminoguanidine, respectively). TRPV1 activation also increased GFAP expression, which was reverted by both TRPV1 antagonism with capsazepine and by 7-NI and aminoguanidine. Given that Müller cells do not express TRPV1, we suppose that the increased GFAP expression in these cells might be elicited by TRPV1 activation and by its indirect effect upon nitric oxide overproduction and peroxynitrite formation. We incubated Fluorogold pre-labeled retinal explants in the presence of capsazepine (1 μM) during 48 h. The numbers of surviving RGCs stained with fluorogold and the numbers of apoptotic cells in the GCL detected with TUNEL were similar in lesioned and control retinas. We conclude that TRPV1 receptor expression decreased after optic nerve injury due to death of TRPV1-containing cells. Furthermore, these data indicate that TRPV1 might be involved in intrinsic protein nitration and Müller cell reaction observed after optic nerve injury.

Pharmacology of capsaicin-, anandamide-, and N-arachidonoyl-dopamine-evoked cell death in a homogeneous transient receptor potential vanilloid subtype 1 receptor population

BACKGROUND

Transient receptor potential vanilloid subtype 1 (TRPV1) receptor is a primary pain-sensing relay at peripheral sensory nerve endings and is also widespread in the brain, where it is implicated in neurodegeneration. Previous studies of TRPV1 neurotoxicity have utilized heterogeneous receptor populations, non-selective ligands, or non-neuronal cell types. Here, we explored the pharmacology of TRPV1-induced cytotoxicity in a homogeneous, neurone-like cellular environment.

METHODS

Cell death was examined in a human neurone-like cell line, stably expressing recombinant human TRPV1. Cytotoxicity was quantified in terms of nuclear morphology and mitochondrial complex II activity. Immunocytochemical markers of apoptotic cell death were also examined.

RESULTS

The TRPV1-selective agonist capsaicin, and the endovanilloids anandamide and N-arachidonoyl-dopamine (NADA), induced TRPV1-dependent delayed cell death in a concentration- and time-dependent manner. Capsaicin exposure time was significantly correlated with potency (r(2)=0.91, P=0.01). Release of cytochrome c from mitochondria, activation of caspase-3, and condensed nuclear chromatin were evident 6 h after capsaicin exposure, but cytotoxicity was unaffected by a pan-caspase inhibitor (zVAD-fmk, 50 microM).

CONCLUSIONS

We conclude that capsaicin, anandamide, and NADA can initiate TRPV1-dependent delayed cell death in neurone-like cells. This is an apoptosis-like process, but independent of caspase activity.

Psychophysical evaluation of a sanshool derivative (alkylamide) and the elucidation of mechanisms subserving tingle

Previous studies investigated the neural and molecular underpinnings of the tingle sensation evoked by sanshool and other natural or synthetic alkylamides. Currently, we sought to characterize the psychophysical properties associated with administration of these compounds. Like other chemesthetic stimuli, the synthetic tingle analog isobutylalkylamide (IBA) evoked a sensation that was temporally dynamic. Repeated IBA application at short (30 sec) interstimulus intervals (ISI) resulted in a tingle sensation that increased across trials. Application at longer ISIs (approximately 30 min) resulted in a sensation of decreased intensity consistent with self-desensitization. Prior treatment with the TRPV1 or TRPA1 agonists, capsaicin and mustard oil did not cross-desensitize the tingle sensation evoked by IBA suggesting that neither TRPV1 nor TRPA1 participate in the transduction mechanism sub-serving tingle. When evaluated over 30-min time period, lingual IBA evoked a sensation that was described initially as tingling and pungent but after approximately 15 min, as a cooling sensation. Further, we found that the sensation evoked by lingual IBA was potentiated by simultaneous application of cold (0 degrees C) and cool (21 degrees C) thermal stimuli but was unaffected by warm (33 degrees C) and hot (41 degrees C) temperatures. Finally, to test the hypothesis that the tingling sensation is subserved by the activation of mechanosensitve fibers, we evaluated lingual tactile thresholds in the presence and absence of lingual IBA. The presence of IBA significantly raised lingual tactile thresholds, whereas capsaicin did not, identifying a role for mechanosensitive fibers in conveying the tingle sensation evoked by sanshool-like compounds. Collectively, these results show that lingual alkylamide evokes a complex sensation that is temporally dynamic and consistent with in vitro and in vivo experiments suggesting these compounds activate mechanosensitve neurons via blockade of KCNK two-pore potassium channels to induce the novel tingling sensation.

Capsaicin-sensitive primary sensory neurons in the mouse express N-Acyl phosphatidylethanolamine phospholipase D

Our previous finding, that the capsaicin- and KCl-induced Ca(2+)-dependent production of the intra- and intercellular signaling molecule N-arachidonoyl ethanolamine (anandamide) in cultured primary sensory neurons could be abolished and reduced by approximately 2/3 by capsaicin-induced degeneration of capsaicin-sensitive neurons, respectively suggests that a major sub-population of capsaicin-sensitive cells together with a group of non-capsaicin-sensitive cells should express enzymes involved in Ca(2+)-dependent anandamide synthesis. N-acyl phosphotidylethanolamine phospholipase D (NAPE-PLD) is known to be involved in Ca(2+)-dependent anandamide production. Hence, here, we used reverse transcriptase and quantitative real time polymerase chain reaction to study NAPE-PLD expression in dorsal root ganglia and to clarify the sub-population of cells expressing this enzyme. Cultures prepared from mouse dorsal root ganglia were grown either in the absence or presence of the neurotoxin, capsaicin (10 muM) overnight. We report, that NAPE-PLD is expressed both in dorsal root ganglia and cultures prepared from dorsal root ganglia and grown in the absence of capsaicin. Furthermore, we also report that capsaicin application downregulates the expression of NAPE-PLD as well as the capsaicin receptor, transient receptor potential vanilloid type 1 ion channel, by about 70% in the cultures prepared from dorsal root ganglia. These findings indicate that a major sub-population of capsaicin-sensitive primary sensory neurons expresses NAPE-PLD, and suggest that NAPE-PLD is expressed predominantly by capsaicin-sensitive neurons in dorsal root ganglia. These data also suggest that NAPE-PLD might be a target to control the activity and excitability of a major sub-population of nociceptive primary sensory neurons.

Interdisciplinary review for correlation between the plant origin capsaicinoids, non-steroidal antiinflammatory drugs, gastrointestinal mucosal damage and prevention in animals and human beings

BACKGROUND

The plant origin capsaicinoids (capsaicin, dihydrocapsaicin, norcapsaicin, dihydrocapsaicin, homocapsaicin, homodihydrocapsaicin) are well known and used as nutritional additive agents in the every day nutritional practice from the last 9,500 years; however, we had have a very little scientifically based knowledge on their chemistry, physiology and pharmacology in animal observations, and in humans up to the mid-twentieth century. Our knowledge about their chemistry, physiology, pharmacology entered to be scientifically based evidence from the year 1980, dominantly in animal observations. The human observations with capsaicin (capsaicinoids), in terms of good clinical practice, have been started only in the last 10-year period (from 1997) in randomized, prospective, multiclinical studies. The name of "capsaicin" used only in the physiological and pharmacological research both in animal experiments and in human observation. The "capsaicin" (as a "chemically" used natural compound) modifies the so-called capsaicin-sensitive afferent nerves depending on their applied doses.

AIMS

The specific action of capsaicin (capsaicinoids) on sensory afferent nerves modifying gastrointestinal (GI) function (under very specific conditions) offers a possibility for the production of an orally applicable drug or for other drug combinations, which can be used in the human medical therapy. The production of new drug is based on the critical interdisciplinary review of the results obtained with capsaicinoids.

MATERIALS AND METHODS

This paper gives an interdisciplinary and critical overview on the chemical, physiological, pharmacological and toxicological actions of the natural origin capsaicinoids (from the point of drug production) under conditions of acute, subacute and chronic administration in animal experiments and human observations, toxicology, pharmacokinetics). This interdisciplinary review covers the following main chapters: (1) physiological and pharmacological research tool by capsaicin in the animals and human beings, (2) capsaicin research in animals (including the acute, subacute toxicology and chronic toxicology metabolism, genotoxicology), (3) capsaicin observation with capsaicin in human beings.

CONCLUSION

(1) The capsaicin used in the physiological and pharmacological observations (in animals and human beings) chemically represents different chemical compounds, which can be obtained from the plants (paprika, chilli, etc.), (2) capsaicinoids are able to modify the capsaicin-sensitive afferent nerves, which have principle roles in the defence of different organs (including the gastrointestinal tract [against the different chemicals, heat, strech, chemical millieu-induced damage], (3) the application of capsaicin (capsaicinoids) can be repeated for the beneficial effects on the gastrointestinal tract as those in animal experiments. After this interdisciplinary and critical review, this paper demonstrates the well-planned research pathways of the discoveries of capsaicinoids from plant chemistry, via physiology, pharmacology and toxicology in animal experiments and human observations.

Submembraneous microtubule cytoskeleton: biochemical and functional interplay of TRP channels with the cytoskeleton

Much work has focused on the electrophysiological properties of transient receptor potential channels. Recently, a novel aspect of importance emerged: the interplay of transient receptor potential channels with the cytoskeleton. Recent data suggest a direct interaction and functional repercussion for both binding partners. The bi-directionality of physical and functional interaction renders therefore, the cytoskeleton a potent integration point of complex biological signalling events, from both the cytoplasm and the extracellular space. In this minireview, we focus mostly on the interaction of the cytoskeleton with transient receptor potential vanilloid channels. Thereby, we point out the functional importance of cytoskeleton components both as modulator and as modulated downstream effector. The resulting implications for patho-biological situations are discussed.

Vanilloid-induced conduction analgesia: selective, dose-dependent, long-lasting, with a low level of potential neurotoxicity

Vanilloid agonists (capsaicin, resiniferatoxin, [RTX]) applied to the peripheral nerves provide conduction blockade. In contrast to the analgesic component of conduction anesthesia produced by local anesthetics, vanilloid agonists provide conduction analgesia not associated with suppression of motor or sensory functions not related to pain. Vanilloid agonists provide conduction analgesia selectively because their effect on the nerve trunks is limited to C- and ADelta-fibers. RTX is much more potent than capsaicin and has a wider therapeutic window. In rat experiments, perineural RTX produced a long-lasting thermal and mechanical hypoalgesia with a very wide separation between effective concentrations (from 0.00003% to 0.001%) providing an effect lasting from several hours to several weeks. A nerve block with RTX prevented the development of thermal and mechanical hyperalgesia as well as pain behavior in a model of incisional pain. RTX-induced conduction blockade has an inherent drawback of TRPV1 agonists, the initial excitation (pain); therefore, a local anesthetic should be injected to prevent it. When RTX was applied to the rat's sciatic nerve in doses necessary to provide conduction analgesia, the frequency of unmyelinated fiber degeneration was more than an order of magnitude lower than that with the therapeutic concentration of lidocaine. These promising results should be confirmed by experiments in species other than rodents (pigs, sheep). Taken together, the data indicate possible clinical applicability of vanilloid-induced conduction analgesia.

Inhibition of calpains improves memory and synaptic transmission in a mouse model of Alzheimer disease

Calpains are calcium-dependent enzymes that determine the fate of proteins through regulated proteolytic activity. Calpains have been linked to the modulation of memory and are key to the pathogenesis of Alzheimer disease (AD). When abnormally activated, calpains can also initiate degradation of proteins essential for neuronal survival. Here we show that calpain inhibition through E64, a cysteine protease inhibitor, and the highly specific calpain inhibitor BDA-410 restored normal synaptic function both in hippocampal cultures and in hippocampal slices from the APP/PS1 mouse, an animal model of AD. Calpain inhibition also improved spatial-working memory and associative fear memory in APP/PS1 mice. These beneficial effects of the calpain inhibitors were associated with restoration of normal phosphorylation levels of the transcription factor CREB and involved redistribution of the synaptic protein synapsin I. Thus, calpain inhibition may prove useful in the alleviation of memory loss in AD.

Comparative effects of sodium pyrithione evoked intracellular calcium elevation in rodent and primate ventral horn motor neurons

Oral administration of sodium pyrithione (NaP) causes hindlimb weakness in rodents, but not in primates. Previous work using Aplysia neurons has demonstrated that NaP produces a persistent influx of Ca(2+) ions across the plasma membrane. To determine whether this also occurs in mammalian neurons and whether this could underlie the inter-species difference between rodents and primates, we have tested the effects of NaP on intracellular Ca(2+) levels ([Ca(2+)](i)) in rat and monkey motor neurons in vitro. Motor neurons present in spinal cord slices from rhesus monkey embryos (E37 and 56) and from rat E16 were dissected and cultured on glass coverslips. Following 2 weeks (rhesus) or 2-3 days (rat) in culture, neurons were loaded with fura-PE3/AM, and examined for [Ca(2+)](i) changes in response to NaP. Rhesus motor neurons were identified by immunostaining for Islet-1 (MN specific antigen) and neuron specific enolase (NSE). Motor neurons from both species exhibited dose-dependent NaP-evoked increases in [Ca(2+)](i) However, the dose-response curve for the Rhesus motor neurons was significantly shifted to the right of the rat dose-response curve, whereas the overall amplitude of the Ca(2+) rise was similar in both species. As shown previously for the Aplysia neurons, the action of NaP is attenuated by SKF 96365, an inhibitor of store-operated calcium entry. In contrast the action of NaP is unaffected by nifedipine and tetrodotoxin, blockers of voltage-dependent Ca(2+) and Na(+) channels, respectively, or by ouabain, an inhibitor of the plasma membrane Na(+)/K(+) ATPase. Our results indicate that the NaP-induced increase in [Ca(2+)](i) is conserved across species and suggest that the toxicological sensitivity of rodent over primate to pyrithione could be due to the enhanced sensitivity of rodent motor neurons to NaP-evoked intracellular Ca(2+) elevation.

Sciatic nerve block with resiniferatoxin: an electron microscopic study of unmyelinated fibers in the rat

BACKGROUND

Perineural administration of the naturally occurring vanilloids (capsaicin, resiniferatoxin [RTX]) produces selective nociceptive blockade. Studies using perineural vanilloids in high concentrations suggest that they can cause a degeneration of unmyelinated fibers. However, electron microscopic studies of local vanilloid toxicity produced conflicting outcomes. In the present study, we sought to determine whether RTX-induced reversible sciatic nerve block results in the degenerative changes of unmyelinated fibers.

METHODS

In rat experiments, RTX was administered percutaneously at the sciatic nerve. The effect of RTX was monitored by measuring the rat's response to noxious heat. The sciatic nerves were removed 48 h after the blockade initiation. Quantitative electron microscopic evaluation of the unmyelinated fibers was performed in three groups of animals: RTX 0.0001% (0.1 microg), RTX 0.001% (1 microg), and control (RTX vehicle, 0.1 mL).

RESULTS

Cross-sections of the sciatic nerve 48 h after the initiation of RTX-induced reversible nerve blockade appeared essentially normal. One rarely observed finding was the irregularly compacted membranous deposits in the unmyelinated axons. The frequency of this finding was approximately one per thousand fibers with both concentrations of RTX.

CONCLUSIONS

The results of the study suggest that a selective and long-lasting sciatic nerve block (up to 2 wk) can be provided by RTX without any significant damage to the unmyelinated nerve fibers.

Final report on the safety assessment of capsicum annuum extract, capsicum annuum fruit extract, capsicum annuum resin, capsicum annuum fruit powder, capsicum frutescens fruit, capsicum frutescens fruit extract, capsicum frutescens resin, and capsaicin

Capsicum-derived ingredients function as skin-conditioning agents--miscellaneous, external analgesics, flavoring agents, or fragrance components in cosmetics. These ingredients are used in 19 cosmetic products at concentrations as high as 5%. Cosmetic-grade material may be extracted using hexane, ethanol, or vegetable oil and contain the full range of phytocompounds that are found in the Capsicum annuum or Capsicum frutescens plant (aka red chiles), including Capsaicin. Aflatoxin and N-nitroso compounds (N-nitrosodimethylamine and N-nitrosopyrrolidine) have been detected as contaminants. The ultraviolet (UV) absorption spectrum for Capsicum Annuum Fruit Extract indicates a small peak at approximately 275 nm, and a gradual increase in absorbance, beginning at approximately 400 nm. Capsicum and paprika are generally recognized as safe by the U.S. Food and Drug Administration for use in food. Hexane, chloroform, and ethyl acetate extracts of Capsicum Frutescens Fruit at 200 mg/kg resulted in death of all mice. In a short-term inhalation toxicity study using rats, no difference was found between vehicle control and a 7% Capsicum Oleoresin solution. In a 4-week feeding study, red chilli (Capsicum annuum) in the diet at concentrations up to 10% was relatively nontoxic in groups of male mice. In an 8-week feeding study using rats, intestinal exfoliation, cytoplasmic fatty vacuolation and centrilobular necrosis of hepatocytes, and aggregation of lymphocytes in the portal areas were seen at 10% Capsicum Frutescens Fruit, but not 2%. Rats fed 0.5 g/kg day-1 crude Capsicum Fruit Extract for 60 days exhibited no significant gross pathology at necropsy, but slight hyperemia of the liver and reddening of the gastric mucosa were observed. Weanling rats fed basal diets supplemented with whole red pepper at concentrations up to 5.0% for up to 8 weeks had no pathology of the large intestines, livers, and kidneys, but destruction of the taste buds and keratinization and erosion of the gastrointestinal (GI) tract were noted in groups fed 0.5% to 5.0% red pepper. The results of 9-and 12-month extension of this study showed normal large intestines and kidneys. In rabbits fed Capsicum Annuum Powder at 5 mg/kg day-1 in the diet daily for 12 months damage to the liver and spleen was noted. A rabbit skin irritation test of Capsicum Annuum Fruit Extract at concentrations ranging from 0.1% to 1.0% produced no irritation, but Capsicum Frutescens Fruit Extract induced concentration-dependent (at 25 to 500 microg/ml) cytotoxicity in a human buccal mucosa fibroblast cell line. An ethanol extract of red chili was mutagenic in Salmonella typhimurium TA98, but not in TA100, or in Escherichia coli. Other genotoxicity assays gave a similar pattern of mixed results. Adenocarcinoma of the abdomen was observed in 7/20 mice fed 100 mg red chilies per day for 12 months; no tumors were seen in control animals. Neoplastic changes in the liver and intestinal tumors were observed in rats fed red chili powder at 80 mg/kg day-1 for 30 days, intestinal and colon tumors were seen in rats fed red chili powder and 1,2-dimethyl hydrazine, but no tumors were observed in controls. In another study in rats, however, red chile pepper in the diet at the same dose decreased the number of tumors seen with 1,2-dimethylhydrazine. Other feeding studies evaluated the effect of red chili peppers on the incidence of stomach tumors produced by N-methyl-N'-nitro-N-nitrosoguanidine, finding that red pepper had a promoting effect. Capsicum Frutescens Fruit Extract promoted the carcinogenic effect of methyl(acetoxymethyl)nitrosamine (carcinogen) or benzene hexachloride (hepatocarcinogen) in inbred male and female Balb/c mice dosed orally (tongue application). Clinical findings include symptoms of cough, sneezing, and runny nose in chili factory workers. Human respiratory responses to Capsicum Oleoresin spray include burning of the throat, wheezing, dry cough, shortness of breath, gagging, gasping, inability to breathe or speak, and, rarely, cyanosis, apnea, and respiratory arrest. A trade name mixture containing 1% to 5% Capsicum Frutescens Fruit Extract induced very slight erythema in 1 of 10 volunteers patch tested for 48 h. Capsicum Frutescens Fruit Extract at 0.025% in a repeated-insult patch test using 103 subjects resulted in no clinically meaningful irritation or allergic contact dermatitis. One epidemiological study indicated that chili pepper consumption may be a strong risk factor for gastric cancer in populations with high intakes of chili pepper; however, other studies did not find this association. Capsaicin functions as an external analgesic, a fragrance ingredient, and as a skin-conditioning agent--miscellaneous in cosmetic products, but is not in current use. Capsaicin is not generally recognized as safe and effective by the U.S. Food and Drug Administration for fever blister and cold sore treatment, but is considered to be safe and effective as an external analgesic counterirritant. Ingested Capsaicin is rapidly absorbed from the stomach and small intestine in animal studies. Subcutaneous injection of Capsaicin in rats resulted in a rise in the blood concentration, reaching a maximum at 5 h; the highest tissue concentrations were in the kidney and lowest in the liver. In vitro percutaneous absorption of Capsaicin has been demonstrated in human, rat, mouse, rabbit, and pig skin. Enhancement of the skin permeation of naproxen (nonsteroidal anti-inflammatory agent) in the presence of Capsaicin has also been demonstrated. Pharmacological and physiological studies demonstrated that Capsaicin, which contains a vanillyl moiety, produces its sensory effects by activating a Ca2 +-permeable ion channel on sensory neurons. Capsaicin is a known activator of vanilloid receptor 1. Capsaicin-induced stimulation of prostaglandin biosynthesis has been shown using bull seminal vesicles and rheumatoid arthritis synoviocytes. Capsaicin inhibits protein synthesis in Vero kidney cells and human neuroblastoma SHSY-5Y cells in vitro, and inhibits growth of E. coli, Pseudomonas solanacearum, and Bacillus subtilis bacterial cultures, but not Saccharomyces cerevisiae. Oral LD50 values as low as 161.2 mg/kg (rats) and 118.8 mg/kg (mice) have been reported for Capsaicin in acute oral toxicity studies, with hemorrhage of the gastric fundus observed in some of the animals that died. Intravenous, intraperitoneal, and subcutaneous LD50 values were lower. In subchronic oral toxicity studies using mice, Capsaicin produced statistically significant differences in the growth rate and liver/body weight increases. Capsaicin is an ocular irritant in mice, rats, and rabbits. Dose-related edema was observed in animals receiving Capsaicin injections into the hindpaw (rats) or application to the ear (mice). In guinea pigs, dinitrochlorobenzene contact dermatitis was enhanced in the presence of Capsaicin, injected subcutaneously, whereas dermal application inhibited sensitization in mice. Immune system effects have been observed in neonatal rats injected subcutaneously with Capsaicin. Capsaicin produced mixed results in S. typhimurium micronucleus and sister-chromatid exchange genotoxicity assays. Positive results for Capsaicin were reported in DNA damage assays. Carcinogenic, cocarcinogenic, anticarcinogenic, antitumorigenic, tumor promotion, and anti-tumor promotion effects of Capsaicin have been reported in animal studies. Except for a significant reduction in crown-rump length in day 18 rats injected subcutaneously with Capsaicin (50 mg/kg) on gestation days 14, 16, 18, or 20, no reproductive or developmental toxicity was noted. In pregnant mice dosed subcutaneously with Capsaicin, depletion of substance P in the spinal cord and peripheral nerves of pregnant females and fetuses was noted. In clinical tests, nerve degeneration of intracutaneous nerve fibers and a decrease in pain sensation induced by heat and mechanical stimuli were evident in subjects injected intradermally with Capsaicin. An increase in mean inspiratory flow was reported for eight normal subjects who inhaled nebulized 10(-7) M Capsaicin. The results of provocative and predictive tests involving human subjects indicated that Capsaicin is a skin irritant. Overall, studies suggested that these ingredients can be irritating at low concentrations. Although the genotoxicity, carcinogenicity, and tumor promotion potential of Capsaicin have been demonstrated, so have opposite effects. Skin irritation and other tumor-promoting effects of Capsaicin appear to be mediated through interaction with the same vanilloid receptor. Given this mechanism of action and the observation that many tumor promoters are irritating to the skin, the Panel considered it likely that a potent tumor promoter may also be a moderate to severe skin irritant. Thus, a limitation on Capsaicin content that would significantly reduce its skin irritation potential is expected to, in effect, lessen any concerns relating to tumor promotion potential. Because Capsaicin enhanced the penetration of an anti-inflammatory agent through human skin, the Panel recommends that care should be exercised in using ingredients that contain Capsaicin in cosmetic products. The Panel advised industry that the total polychlorinated biphenyl (PCB)/pesticide contamination should be limited to not more than 40 ppm, with not more than 10 ppm for any specific residue, and agreed on the following limitations for other impurities: arsenic (3 mg/kg max), heavy metals (0.002% max), and lead (5 mg/kg max). Industry was also advised that aflatoxin should not be present in these ingredients (the Panel adopted < or =15 ppb as corresponding to "negative" aflatoxin content), and that ingredients derived from Capsicum annuum and Capsicum Frutescens Plant species should not be used in products where N-nitroso compounds may be formed. (ABSTRACT TRUNCATED)