Mitochondrial changes in preoptic neurons after capsaicin desensitization of the hypothalamic thermodetectors in rats

Resiniferatoxin: Nature's Precision Medicine to Silence TRPV1-Positive Afferents

Resiniferatoxin (RTX) is an ultrapotent capsaicin analog with a unique spectrum of pharmacological actions. The therapeutic window of RTX is broad, allowing for the full desensitization of pain perception and neurogenic inflammation without causing unacceptable side effects. Intravesical RTX was shown to restore continence in a subset of patients with idiopathic and neurogenic detrusor overactivity. RTX can also ablate sensory neurons as a "molecular scalpel" to achieve permanent analgesia. This targeted (intrathecal or epidural) RTX therapy holds great promise in cancer pain management. Intra-articular RTX is undergoing clinical trials to treat moderate-to-severe knee pain in patients with osteoarthritis. Similar targeted approaches may be useful in the management of post-operative pain or pain associated with severe burn injuries. The current state of this field is reviewed, from preclinical studies through veterinary medicine to clinical trials.

Bioactive Properties, Bioavailability Profiles, and Clinical Evidence of the Potential Benefits of Black Pepper (Piper nigrum) and Red Pepper (Capsicum annum) against Diverse Metabolic Complications

The consumption of food-derived products, including the regular intake of pepper, is increasingly evaluated for its potential benefits in protecting against diverse metabolic complications. The current study made use of prominent electronic databases including PubMed, Google Scholar, and Scopus to retrieve clinical evidence linking the intake of black and red pepper with the amelioration of metabolic complications. The findings summarize evidence supporting the beneficial effects of black pepper (Piper nigrum L.), including its active ingredient, piperine, in improving blood lipid profiles, including reducing circulating levels of total cholesterol, low-density lipoprotein cholesterol, and triglycerides in overweight and obese individuals. The intake of piperine was also linked with enhanced antioxidant and anti-inflammatory properties by increasing serum levels of superoxide dismutase while reducing those of malonaldehyde and C-reactive protein in individuals with metabolic syndrome. Evidence summarized in the current review also indicates that red pepper (Capsicum annum), together with its active ingredient, capsaicin, could promote energy expenditure, including limiting energy intake, which is likely to contribute to reduced fat mass in overweight and obese individuals. Emerging clinical evidence also indicates that pepper may be beneficial in alleviating complications linked with other chronic conditions, including osteoarthritis, oropharyngeal dysphagia, digestion, hemodialysis, and neuromuscular fatigue. Notably, the beneficial effects of pepper or its active ingredients appear to be more pronounced when used in combination with other bioactive compounds. The current review also covers essential information on the metabolism and bioavailability profiles of both pepper species and their main active ingredients, which are all necessary to understand their potential beneficial effects against metabolic diseases.

N-Arachidonoyl Dopamine Modulates Acute Systemic Inflammation via Nonhematopoietic TRPV1

N-Arachidonoyl dopamine (NADA) is an endogenous lipid that potently activates the transient receptor potential vanilloid 1 (TRPV1), which mediates pain and thermosensation. NADA is also an agonist of cannabinoid receptors 1 and 2. We have reported that NADA reduces the activation of cultured human endothelial cells by LPS and TNF-α. Thus far, in vivo studies using NADA have focused on its neurologic and behavioral roles. In this article, we show that NADA potently decreases in vivo systemic inflammatory responses and levels of the coagulation intermediary plasminogen activator inhibitor 1 in three mouse models of inflammation: LPS, bacterial lipopeptide, and polymicrobial intra-abdominal sepsis. We also found that the administration of NADA increases survival in endotoxemic mice. Additionally, NADA reduces blood levels of the neuropeptide calcitonin gene-related peptide but increases the neuropeptide substance P in LPS-treated mice. We demonstrate that the anti-inflammatory effects of NADA are mediated by TRPV1 expressed by nonhematopoietic cells and provide data suggesting that neuronal TRPV1 may mediate NADA's anti-inflammatory effects. These results indicate that NADA has novel TRPV1-dependent anti-inflammatory properties and suggest that the endovanilloid system might be targeted therapeutically in acute inflammation.

Vagal neurocircuitry and its influence on gastric motility

A large body of research has been dedicated to the effects of gastrointestinal peptides on vagal afferent fibres, yet multiple lines of evidence indicate that gastrointestinal peptides also modulate brainstem vagal neurocircuitry, and that this modulation has a fundamental role in the physiology and pathophysiology of the upper gastrointestinal tract. In fact, brainstem vagovagal neurocircuits comprise highly plastic neurons and synapses connecting afferent vagal fibres, second order neurons of the nucleus tractus solitarius (NTS), and efferent fibres originating in the dorsal motor nucleus of the vagus (DMV). Neuronal communication between the NTS and DMV is regulated by the presence of a variety of inputs, both from within the brainstem itself as well as from higher centres, which utilize an array of neurotransmitters and neuromodulators. Because of the circumventricular nature of these brainstem areas, circulating hormones can also modulate the vagal output to the upper gastrointestinal tract. This Review summarizes the organization and function of vagovagal reflex control of the upper gastrointestinal tract, presents data on the plasticity within these neurocircuits after stress, and discusses the gastrointestinal dysfunctions observed in Parkinson disease as examples of physiological adjustment and maladaptation of these reflexes.

Effect of capsaicin on thermoregulation: an update with new aspects

Capsaicin, a selective activator of the chemo- and heat-sensitive transient receptor potential (TRP) V1 cation channel, has characteristic feature of causing long-term functional and structural impairment of neural elements supplied by TRPV1/capsaicin receptor. In mammals, systemic application of capsaicin induces complex heat-loss response characteristic for each species and avoidance of warm environment. Capsaicin activates cutaneous warm receptors and polymodal nociceptors but has no effect on cold receptors or mechanoreceptors. In this review, thermoregulatory features of capsaicin-pretreated rodents and TRPV1-mediated neural elements with innocuous heat sensitivity are summarized. Recent data support a novel hypothesis for the role of visceral warmth sensors in monitoring core body temperature. Furthermore, strong evidence suggests that central presynaptic nerve terminals of TRPV1-expressing cutaneous, thoracic and abdominal visceral receptors are activated by innocuous warmth stimuli and capsaicin. These responses are absent in TRPV1 knockout mice. Thermoregulatory disturbance induced by systemic capsaicin pretreatment lasts for months and is characterized by a normal body temperature at cool environment up to a total dose of 150 mg/kg s.c. Upward differential shift of set points for activation vasodilation, other heat-loss effectors and thermopreference develops. Avoidance of warm ambient temperature (35°C, 40°C) is severely impaired but thermopreference at cool ambient temperatures (Tas) are not altered. TRPV1 knockout or knockdown and genetically altered TRPV1, TRPV2 and TRPM8 knockout mice have normal core temperature in thermoneutral or cool environments, but the combined mutant mice have impaired regulation in warm or cold (4°C) environments. Several lines of evidence support that in the preoptic area warmth sensitive neurons are activated and desensitized by capsaicin, but morphological evidence for it is controversial. It is suggested that these neurons have also integrator function. Fever is enhanced in capsaicin-desensitized rats and the inhibition observed after pretreatment with low i.p. doses does not support in the light of their warmth sensitivity the concept that abdominal TRPV1-expressing nerve terminals serve as nonthermal chemosensors for reference signals in thermoregulation.

TRPV1

TRPV1 is a well-characterised channel expressed by a subset of peripheral sensory neurons involved in pain sensation and also at a number of other neuronal and non-neuronal sites in the mammalian body. Functionally, TRPV1 acts as a sensor for noxious heat (greater than ~42 °C). It can also be activated by some endogenous lipid-derived molecules, acidic solutions (pH < 6.5) and some pungent chemicals and food ingredients such as capsaicin, as well as by toxins such as resiniferatoxin and vanillotoxins. Structurally, TRPV1 subunits have six transmembrane (TM) domains with intracellular N- (containing 6 ankyrin-like repeats) and C-termini and a pore region between TM5 and TM6 containing sites that are important for channel activation and ion selectivity. The N- and C- termini have residues and regions that are sites for phosphorylation/dephosphorylation and PI(4,5)P2 binding, which regulate TRPV1 sensitivity and membrane insertion. The channel has several interacting proteins, some of which (e.g. AKAP79/150) are important for TRPV1 phosphorylation. Four TRPV1 subunits form a non-selective, outwardly rectifying ion channel permeable to monovalent and divalent cations with a single-channel conductance of 50-100 pS. TRPV1 channel kinetics reveal multiple open and closed states, and several models for channel activation by voltage, ligand binding and temperature have been proposed. Studies with TRPV1 agonists and antagonists and Trpv1 (-/-) mice have suggested a role for TRPV1 in pain, thermoregulation and osmoregulation, as well as in cough and overactive bladder. TRPV1 antagonists have advanced to clinical trials where findings of drug-induced hyperthermia and loss of heat sensitivity have raised questions about the viability of this therapeutic approach.

Capsaicin and sensory neurones: a historical perspective

Capsaicin, the pungent ingredient of red pepper has become not only a "hot" topic in neuroscience but its new target-related unique actions have opened the door for the drug industry to introduce a new chapter of analgesics. After several lines of translational efforts with over 1,000 patents and clinical trials, the 8% capsaicin dermal patch reached the market and its long-lasting local analgesic effect in some severe neuropathic pain states is now well established. This introductory chapter outlines on one hand the historical background based on the author's 50 years of experience in this field and on the other hand emphasizes new scopes, fascinating perspectives in pharmaco-physiology, and molecular pharmacology of nociceptive sensory neurons. Evidence for the effect of capsaicin on C-polymodal nociceptors (CMH), C-mechanoinsensitive (CHMi), and silent C-nociceptors are listed and the features of the capsaicin-induced blocking effects of nociceptors are demonstrated. Common and different characteristics of nociceptor-blocking actions after systemic, perineural, local, intrathecal, and in vitro treatments are summarized. Evidence for the misleading conclusions drawn from neonatal capsaicin pretreatment is presented. Perspectives opened from cloning the capsaicin receptor "Transient Receptor Potential Vanilloid 1" (TRPV1) are outlined and potential molecular mechanisms behind the long-lasting functional, ultrastructural, and nerve terminal-damaging effects of capsaicin and other TRPV1 agonists are summarized. Neurogenic inflammation and the long-list of "capsaicin-sensitive" tissue responses are mediated by an unorthodox dual sensory-efferent function of peptidergic TRPV1-expressing nerve terminals which differ from the classical efferent and sensory nerve endings that have a unidirectional role in neuroregulation. Thermoregulatory effects of capsaicin are discussed in detail. It is suggested that since hyperthermia and burn risk due to enhanced noxious heat threshold are the major obstacles of some TRPV1 antagonists, they could be overcome. The special "multisteric" gating function of the TRPV1 cation channel provides the structural ground for blocking chemical activation of TRPV1 without affecting its responsiveness to physical stimuli. A new chapter of potential analgesics targeting nociceptors is now already supported for pain relief in persistent pathological pain states.

A critical re-evaluation of the specificity of action of perivagal capsaicin

Perivagal application of capsaicin (1% solution) is considered to cause a selective degeneration of vagal afferent C fibres and has been used extensively to examine the site of action of many gastrointestinal (GI) neuropeptides. The actions of both capsaicin and GI neuropeptides may not be restricted to vagal afferent fibres, however, as other non-sensory neurones have displayed sensitivity to capsaicin and brainstem microinjections of these neuropeptides induce GI effects similar to those obtained upon systemic application. The aim of the present study was to test the hypothesis that perivagal capsaicin induces degeneration of vagal efferents controlling GI functions. Experiments were conducted 7-14 days after 30 min unilateral perivagal application of 0.1-1% capsaicin. Immunohistochemical analyses demonstrated that, as following vagotomy, capsaicin induced dendritic degeneration, decreased choline acetyltransferase but increased nitric oxide synthase immunoreactivity in rat dorsal motor nucleus of the vagus (DMV) neurones. Electrophysiological recordings showed a decreased DMV input resistance and excitability due, in part, to the expression of a large conductance calcium-dependent potassium current and the opening of a transient outward potassium window current at resting potential. Furthermore, the number of DMV neurones excited by thyrotrophin-releasing hormone and the gastric motility response to DMV microinjections of TRH were decreased significantly. Our data indicate that perivagal application of capsaicin induced DMV neuronal degeneration and decreased vagal motor responses. Treatment with perivagal capsaicin cannot therefore be considered selective for vagal afferent C fibres and, consequently, care is needed when using perivagal capsaicin to assess the mechanism of action of GI neuropeptides.

Extract of grains of paradise and its active principle 6-paradol trigger thermogenesis of brown adipose tissue in rats

Grains of paradise (GP) is a species of the ginger family, Zingiberaceae, extracts of which have a pungent, peppery taste due to an aromatic ketone, 6-paradol. The aim of this study was to explore the thermogenic effects of GP extracts and of 6-paradol. Efferent discharges from sympathetic nerves entering the interscapular brown adipose tissue were recorded. Intragastric injection of a GP extract or 6-paradol enhanced the efferent discharges of the sympathetic nerves in a dose-dependent manner. The enhanced nerve discharges were sustained for as long as 3h. The rats did not become desensitized to the stimulatory effects these compounds on sympathetic nerve activity. The tissue temperature of brown adipose tissue showed significant increase in rats injected with 6-paradol. These results demonstrate that GP extracts and 6-paradol activate thermogenesis in brown adipose tissue, and may open up new avenues for the regulation of weight loss and weight maintenance.

The transient receptor potential vanilloid-1 channel in thermoregulation: a thermosensor it is not

The development of antagonists of the transient receptor potential vanilloid-1 (TRPV1) channel as pain therapeutics has revealed that these compounds cause hyperthermia in humans. This undesirable on-target side effect has triggered a surge of interest in the role of TRPV1 in thermoregulation and revived the hypothesis that TRPV1 channels serve as thermosensors. We review literature data on the distribution of TRPV1 channels in the body and on thermoregulatory responses to TRPV1 agonists and antagonists. We propose that two principal populations of TRPV1-expressing cells have connections with efferent thermoeffector pathways: 1) first-order sensory (polymodal), glutamatergic dorsal-root (and possibly nodose) ganglia neurons that innervate the abdominal viscera and 2) higher-order sensory, glutamatergic neurons presumably located in the median preoptic hypothalamic nucleus. We further hypothesize that all thermoregulatory responses to TRPV1 agonists and antagonists and thermoregulatory manifestations of TRPV1 desensitization stem from primary actions on these two neuronal populations. Agonists act primarily centrally on population 2; antagonists act primarily peripherally on population 1. We analyze what roles TRPV1 might play in thermoregulation and conclude that this channel does not serve as a thermosensor, at least not under physiological conditions. In the hypothalamus, TRPV1 channels are inactive at common brain temperatures. In the abdomen, TRPV1 channels are tonically activated, but not by temperature. However, tonic activation of visceral TRPV1 by nonthermal factors suppresses autonomic cold-defense effectors and, consequently, body temperature. Blockade of this activation by TRPV1 antagonists disinhibits thermoeffectors and causes hyperthermia. Strategies for creating hyperthermia-free TRPV1 antagonists are outlined. The potential physiological and pathological significance of TRPV1-mediated thermoregulatory effects is discussed.

Brain TRPV1: a depressing TR(i)P down memory lane?

On sensory neurons, the capsaicin receptor TRPV1 (transient receptor potential, vanilloid subfamily, member 1) functions as a molecular integrator of noxious stimuli and represents a novel target for analgesic drugs. The presence of TRPV1 in the brain is now well established but, despite intensive research, its function is only beginning to be understood. New evidence implies an unexpected role for hippocampal TRPV1 in neuropsychiatric disorders. For instance, it was hypothesized that the effects of the cannabinoid-receptor antagonist rimonabant on mood might be due to its capability to antagonize TRPV1 receptors at high doses. Most studies, however, imply a positive (e.g. anxiolytic) outcome for TRPV1 antagonism. With potent small-molecule TRPV1 antagonists undergoing clinical trials, the effect of brain TRPV1 blockade might determine the future of this class of novel analgesic drugs. Clearly, more research is needed to delineate the biological role of brain TRPV1 receptors.

The pharmacological challenge to tame the transient receptor potential vanilloid-1 (TRPV1) nocisensor

The transient receptor potential vanilloid-1 (TRPV1) cation channel is a receptor that is activated by heat (>42 degrees C), acidosis (pH<6) and a variety of chemicals among which capsaicin is the best known. With these properties, TRPV1 has emerged as a polymodal nocisensor of nociceptive afferent neurones, although some non-neuronal cells and neurones in the brain also express TRPV1. The activity of TRPV1 is controlled by a multitude of regulatory mechanisms that either cause sensitization or desensitization of the channel. As many proalgesic pathways converge on TRPV1 and this nocisensor is upregulated and sensitized by inflammation and injury, TRPV1 is thought to be a central transducer of hyperalgesia and a prime target for the pharmacological control of pain. As a consequence, TRPV1 agonists causing defunctionalization of sensory neurones and a large number of TRPV1 blockers have been developed, some of which are in clinical trials. A major drawback of many TRPV1 antagonists is their potential to cause hyperthermia, and their long-term use may carry further risks because TRPV1 has important physiological functions in the peripheral and central nervous system. The challenge, therefore, is to pharmacologically differentiate between the physiological and pathological implications of TRPV1. There are several possibilities to focus therapy specifically on those TRPV1 channels that contribute to disease processes. These approaches include (i) site-specific TRPV1 antagonists, (ii) modality-specific TRPV1 antagonists, (iii) uncompetitive TRPV1 (open channel) blockers, (iv) drugs interfering with TRPV1 sensitization, (v) drugs interfering with intracellular trafficking of TRPV1 and (vi) TRPV1 agonists for local administration.

Body-temperature maintenance as the predominant function of the vanilloid receptor TRPV1

Agonists of the transient receptor potential vanilloid type 1 (TRPV1), such as capsaicin, cause pain and a drop in body temperature (hypothermia). Conversely, antagonists of TRPV1 block pain behaviors in rodent models of inflammation, osteoarthritis and cancer. Efforts that evaluate TRPV1 antagonists in on-target challenge models have uncovered that TRPV1 blockade elicits an increase in body temperature (hyperthermia) from rodents to primates, revealing the intimate relationship between the role of TRPV1 in pain and body-temperature maintenance. This evolutionarily conserved function of TRPV1 in body-temperature maintenance became a hurdle for clinical development of one antagonist, AMG 517. However, several other TRPV1 antagonists are currently being evaluated in the clinic and soon-to-be-published results should shed light on the potential of managing antagonist-induced hyperthermia while developing them as therapeutics.

Intracerebroventricular capsaicin influences the body weight increasing of rats

Adult rats were treated for ten days with capsaicin or with NaCl 0.9% directly injected into the lateral cerebral ventricles through a surgically implanted cannula. A third group of rats was implanted with the same cannula but did not receive any treatment. The food intake and the body weight were recorded for at least six weeks after stopping the treatment. The animals were always kept at constant ambient temperature of 22 °C. The body weight of the capsaicin-treated group was reduced by the treatment, and showed a regular but lower degree of recovery trend than the control groups after the treatment period. In fact the capsaicin treated animals never reached the body weight of the controls. Nevertheless, food intake did not significantly vary after the capsaicin treatment. On the basis of these and previous findings, we can assume that capsaicin injected into the cerebral ventricles to rats kept at constant ambient temperature can acts on hypothalamic neurons, but a permanent action on metabolic pathways can not be excluded.

Capsaicin (TRPV1 Agonist) therapy for pain relief: farewell or revival?

OBJECTIVE

In this review, we explain our current understanding of the molecular basis for pain relief by capsaicin and other transient receptor potential vanilloid subfamily, member 1 (TRPV1) agonists. We summarize disease-related changes in TRPV1 expression and its implications for therapy and potential adverse effects. Last, we provide an overview of the current clinical uses of topical and injectable TRPV1 agonist preparations in both oncologic and nononcologic populations.

METHOD

Search of MEDLINE and other databases.

RESULTS

The capsaicin receptor TRPV1 is a polymodal nociceptor exhibiting a dynamic threshold of activation that could be lowered under inflammatory conditions. Consistent with this model, TRPV1 knock-out mice are devoid of post-inflammatory thermal hyperalgesia. TRPV1 desensitization of primary sensory neurons is a powerful approach to relieve symptoms of nociceptive behavior in animal models of chronic pain. However, over-the-counter capsaicin creams have shown moderate to poor analgesic efficacy. This is in part related to low dose, poor skin absorption, and compliance factors. Recently developed site-specific capsaicin therapy with high-dose patches and injectable preparations seem to be safe and reportedly provide long-lasting analgesia with rapid onset.

CONCLUSIONS

We argue that TRPV1 agonists and antagonists are not mutually exclusive but rather complimentary pharmacologic approaches for pain relief and we predict a "revival" for capsaicin and other TRPV1 agonists in the clinical management of pain associated with inflammation, metabolic imbalances (eg, diabetes), infections (HIV), and cancer, despite the current focus of the pharmaceutical industry on TRPV1 antagonists.

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)

The vanilloid receptor TRPV1: 10 years from channel cloning to antagonist proof-of-concept

The clinical use of TRPV1 (transient receptor potential vanilloid subfamily, member 1; also known as VR1) antagonists is based on the concept that endogenous agonists acting on TRPV1 might provide a major contribution to certain pain conditions. Indeed, a number of small-molecule TRPV1 antagonists are already undergoing Phase I/II clinical trials for the indications of chronic inflammatory pain and migraine. Moreover, animal models suggest a therapeutic value for TRPV1 antagonists in the treatment of other types of pain, including pain from cancer. We argue that TRPV1 antagonists alone or in conjunction with other analgesics will improve the quality of life of people with migraine, chronic intractable pain secondary to cancer, AIDS or diabetes. Moreover, emerging data indicate that TRPV1 antagonists could also be useful in treating disorders other than pain, such as urinary urge incontinence, chronic cough and irritable bowel syndrome. The lack of effective drugs for treating many of these conditions highlights the need for further investigation into the therapeutic potential of TRPV1 antagonists.

The vanilloid receptor TRPV1 is tonically activated in vivo and involved in body temperature regulation

The vanilloid receptor TRPV1 (transient receptor potential vanilloid 1) is a cation channel that serves as a polymodal detector of pain-producing stimuli such as capsaicin, protons (pH <5.7), and heat. TRPV1 antagonists block pain behaviors in rodent models of inflammatory, neuropathic, and cancer pain, suggesting their utility as analgesics. Here, we report that TRPV1 antagonists representing various chemotypes cause an increase in body temperature (hyperthermia), identifying a potential issue for their clinical development. Peripheral restriction of antagonists did not eliminate hyperthermia, suggesting that the site of action is predominantly outside of the blood-brain barrier. Antagonists that are ineffective against proton activation also caused hyperthermia, indicating that blocking capsaicin and heat activation of TRPV1 is sufficient to produce hyperthermia. All TRPV1 antagonists evaluated here caused hyperthermia, suggesting that TRPV1 is tonically activated in vivo and that TRPV1 antagonism and hyperthermia are not separable. TRPV1 antagonists caused hyperthermia in multiple species (rats, dogs, and monkeys), demonstrating that TRPV1 function in thermoregulation is conserved from rodents to primates. Together, these results indicate that tonic TRPV1 activation regulates body temperature.

Transient receptor potential ion channels as participants in thermosensation and thermoregulation

Living organisms must evaluate changes in environmental and internal temperatures to mount appropriate physiological and behavioral responses conducive to survival. Classical physiology has provided a wealth of information regarding the specialization of thermosensory functions among subclasses of peripheral sensory neurons and intrinsically thermosensitive neurons within the hypothalamus. However, until recently, the molecular mechanisms by which these cells carry out thermometry have remained poorly understood. The demonstration that certain ion channels of the transient receptor potential (TRP) family can be activated by increases or decreases in ambient temperature, along with the recognition of their heterogeneous expression patterns and heterogeneous temperature sensitivities, has led investigators to evaluate these proteins as candidate endogenous thermosensors. Much of this work has involved one specific channel, TRP vanilloid 1 (TRPV1), which is both a receptor for capsaicin and related pungent vanilloid compounds and a "heat receptor," capable of directly depolarizing neurons in response to temperatures >42 degrees C. Evidence for a contribution of TRPV1 to peripheral thermosensation has come from pharmacological, physiological, and genetic approaches. In contrast, although capsaicin-sensitive mechanisms clearly influence core body temperature regulation, the specific contribution of TRPV1 to this process remains a matter of debate. Besides TRPV1, at least six additional thermally sensitive TRP channels have been identified in mammals, and many of these also appear to participate in thermosensation. Moreover, the identification of invertebrate TRP channels, whose genetic ablation alters thermally driven behaviors, makes it clear that thermosensation represents an evolutionarily conserved role of this ion channel family.

Forty years in capsaicin research for sensory pharmacology and physiology

Capsaicin, the pungent ingredient of chilli peppers has become a "hot" topic in neuroscience with yearly publications over half thousand papers. It is outlined in this survey how this exciting Hungarian research field emerged from almost complete ignorance. From the initial observation of the phenomenon of "capsaicin desensitization", a long-lasting chemoanalgesia and impairment in thermoregulation against heat, the chain of new discoveries which led to the formulation of the existence of a "capsaicin receptor" on C-polymodal nociceptors is briefly summarized. Neurogenic inflammation is mediated by these C-afferents which are supplied by the putative capsaicin receptor and were termed as "capsaicin sensitive" chemoceptive afferents. They opened new avenues in local peptidergic regulation in peripheral tissues. It has been suggested that in contrast to the classical axon reflex theory, the capsaicin-sensitive sensory system subserves a "dual sensory-efferent" function whereby initiation of afferent signals and neuropeptide release are coupled at the same nerve endings. Furthermore, in the skin at threshold stimuli which do not evoke sensation elicit already maximum efferent response as enhanced microcirculation. In isolated organ preparations large scale of new type of peptidergic capsaicin-sensitive neurogenic smooth muscle responses were revealed after the first one was described by ourselves on the guinea-pig ileum in 1978. Recently the "capsaicin receptor" has been cloned and it is now named as the "transient receptor potential vanilloid 1" (TRPV1). Hence, capsaicin research led to the discovery of the first temperature-gated ion channel gated by noxious heat, protons, vanilloids and endogenous ligands as anandamide, N-oleoyldopamine and lipoxygenase products. Another recent achievement is the discovery of a novel "unorthodox" neurohumoral regulatory mechanism mediated by somatostatin. Somatostatin released from the TRPV1-expressing nerve endings reaches the circulation and elicits systemic antiinflammatory and analgesic "sensocrine" functions with counter-regulatory influence e.g. in Freund's adjuvant-induced chronic arthritis. Nociceptors supplied by TRPV1 and sst4 somatostatin receptors has become nowadays promising targets for drug development.

Daily body temperature rhythm and heat tolerance in TRPV1 knockout and capsaicin pretreated mice

Daily body temperature (DBT) rhythm of mice lacking one of the transient receptor potential (TRP) family of proteins, the capsaicin receptor or TRPV1, was recorded by biotelemetry and found to have significantly higher amplitude than that of wild-type mice. Capsaicin-desensitized wild-mice exhibited an even higher DBT amplitude than did TRPV1 deficient mice. A standard heat load (radiant temperature of 36-37 degrees C) resulted in similar rises in body core temperature in wild-type mice and in TRPV1 deficient mice, while capsaicin-desensitized wild-type mice exhibited a robust heat-intolerance. The lack of TRPV1 slightly modifies amplitude of daily body temperature rhythm but does not seem to influence physiological heat defence in mice. In vivo evidence for a TRP protein functioning in the physiological heat-defence range is still lacking.

Neonatal anandamide treatment results in prolonged mitochondrial damage in the vanilloid receptor type 1-immunoreactive B-type neurons of the rat trigeminal ganglion

Capsaicin acting on the vanilloid type 1 receptor (VR1) excites a subset of primary sensory neurons. Systemic capsaicin treatment of adult or neonatal rats results in selective damage of the B-type neurons in the rat sensory ganglia by causing a long-lasting mitochondrial lesion that has been described in detail in previous studies. The endocannabinoid, anandamide, exhibits an agonist effect on VR1 receptors. The physiological role of anandamide as a VR1 agonist is still uncertain. This study addresses whether high doses of anandamide induce similar ultrastructural changes to those described for capsaicin. The effect of neonatally administered anandamide (1 mg/kg) on neurons of the trigeminal ganglia and the hippocampal formation was examined in the light and electron microscope from the first day after injections to the 20th week after treatment. Anandamide was found to cause mitochondrial damage of the B-type neurons of trigeminal ganglia similar to what has been described for capsaicin. The time course of damage was also comparable. In addition to the cells of the trigeminal ganglia, B-type cells of dorsal root ganglia were also damaged. A-type neurons and satellite glial cells were not affected either in the trigeminal or in the dorsal root ganglia. In the hippocampal formation, where a subpopulation of local circuit neurons is known to contain cannabinoid type 1 (CB1) but not VR1 receptors, anandamide did not cause morphological changes of mitochondria either in the dentate gyrus or in Ammon's horn. At 3 weeks of age, all VR1-immunoreactive neurons in the trigeminal ganglia of animals treated neonatally with anandamide displayed swollen mitochondria. The results suggest that anandamide, at pharmacologically relevant doses, acts on the VR1 receptor and causes prolonged and selective mitochondrial damage of B-type sensory neurons, as has previously been described for capsaicin.

Neonatal capsaicin treatment results in prolonged mitochondrial damage and delayed cell death of B cells in the rat trigeminal ganglia

Capsaicin acts on the vanilloid receptor subtype 1, a noxious heat-gated cation channel located on a major subgroup of nociceptive primary afferent neurons. Following the systemic capsaicin treatment of neonatal rats, the loss of B-type sensory neurons in trigeminal ganglion of adult rats with chemoanalgesia and abolition of neurogenic inflammation was investigated. Our quantitative morphometric analysis revealed that in the trigeminal ganglion of neonatal rats treated with 50 mg/kg s.c. capsaicin, the total number of neurons, morphology of B-type cells and cell-size histograms did not differ from that of the controls 1 or 5 days after treatment. These observations indicate that early cell death does not play a significant part in the loss of B-type cells, which in our sample was 39.4% on the 19th day. However under the electron microscope pronounced selective mitochondrial swelling with disorganized cristae was observed in B-type neurons at 1-20 weeks after capsaicin treatment. Daily treatment with nerve growth factor (NGF, 10 x 100 microg/kg s.c.), started 1 day after capsaicin injection, prevented the loss of B-type cells but did not counteract the development of long-lasting mitochondrial damage. After NGF treatment, partial restitution of chemonociception to capsaicin instillation into the eye occurred but capsaicin-induced inhibition of neurogenic plasma extravasation in the hindpaw evoked by topical application of mustard oil remained unaltered. We conclude, that capsaicin treatment in neonatal rats, as in the adults, destroys terminal parts of the sensory neurons supplied by vanilloid receptors and induces long-lasting mitochondrial swelling in the soma. We hypothesize that loss of NGF uptake results in delayed cell death of B-type neurons in neonates.

Distribution of mRNA for vanilloid receptor subtype 1 (VR1), and VR1-like immunoreactivity, in the central nervous system of the rat and human

The cloned vanilloid receptor VR1 has attracted recent attention as a molecular integrator of painful stimuli on primary sensory neurons. The existence of vanilloid-sensitive neurons in the brain is, however, controversial. In this study, we have used an antibody and a complementary RNA probe to explore the distribution of neurons that express VR1 in rat and in certain areas of human brain. In the rat, we observed VR1-expressing neurons throughout the whole neuroaxis, including all cortical areas (in layers 3 and 5), several members of the limbic system (e.g., hippocampus, central amygdala, and both medial and lateral habenula), striatum, hypothalamus, centromedian and paraventricular thalamic nuclei, substantia nigra, reticular formation, locus coeruleus, cerebellum, and inferior olive. VR1-immunopositive cells also were found in the third and fifth layers of human parietal cortex. Reverse transcription-PCR performed with rat VR1-specific primers verified the expression of VR1 mRNA in cortex, hippocampus, and hypothalamus. In the central nervous system, neonatal capsaicin treatment depleted VR1 mRNA from the spinal nucleus of the trigeminal nerve, but not from other areas such as the inferior olive. The finding that VR1 is expressed not only in primary sensory neurons but also in several brain nuclei is of great importance in that it places VRs in a much broader perspective than pain perception. VRs in the brain (and putative endogenous vanilloids) may be involved in the control of emotions, learning, and satiety, just to name a few exciting possibilities.

Distribution of mRNA for vanilloid receptor subtype 1 (VR1), and VR1-like immunoreactivity, in the central nervous system of the rat and human

The cloned vanilloid receptor VR1 has attracted recent attention as a molecular integrator of painful stimuli on primary sensory neurons. The existence of vanilloid-sensitive neurons in the brain is, however, controversial. In this study, we have used an antibody and a complementary RNA probe to explore the distribution of neurons that express VR1 in rat and in certain areas of human brain. In the rat, we observed VR1-expressing neurons throughout the whole neuroaxis, including all cortical areas (in layers 3 and 5), several members of the limbic system (e.g., hippocampus, central amygdala, and both medial and lateral habenula), striatum, hypothalamus, centromedian and paraventricular thalamic nuclei, substantia nigra, reticular formation, locus coeruleus, cerebellum, and inferior olive. VR1-immunopositive cells also were found in the third and fifth layers of human parietal cortex. Reverse transcription-PCR performed with rat VR1-specific primers verified the expression of VR1 mRNA in cortex, hippocampus, and hypothalamus. In the central nervous system, neonatal capsaicin treatment depleted VR1 mRNA from the spinal nucleus of the trigeminal nerve, but not from other areas such as the inferior olive. The finding that VR1 is expressed not only in primary sensory neurons but also in several brain nuclei is of great importance in that it places VRs in a much broader perspective than pain perception. VRs in the brain (and putative endogenous vanilloids) may be involved in the control of emotions, learning, and satiety, just to name a few exciting possibilities.

Selective neurotoxins, chemical tools to probe the mind: the first thirty years and beyond

For centuries, starting with the advent of the microscope, cytotoxins have been known to non-selectively destroy nerves and other tissue cells. However, neurotoxins restricted in effect to one kind of neuron are an invention of the 20th century. One might reasonably trace the origins of this field to 1960 when the Nobel Laureates, R. Levi- Montalcini and S Cohen, showed that an antibody to nerve growth factor effectively prevented development of sympathetic nerves in the absence of overt changes in dorsal root ganglia and other neural and non-neural tissues. The year 1967 marks discovery of 6-hydroxydopamine, the first of dozens of chemically-selective neurotoxins. As stated by the physiologist W.B. Cannon, neural function can be deduced by denoting absence-deficits. A wealth of knowledge in neuroscience has been realized through use of neurotoxins. In the 21st century we foresee neurotoxins for virtually all neurochemically-identifiable or receptor-specific neurons, acting at/via functional proteins or characteristic DNA sites. These tools will provide us with a better means to probe the mind and thereby lead to a fuller understanding of the intricate roles of identifiable neuronal systems in integrative neuroscience.

Existence of capsaicin-sensitive glutamatergic terminals in rat hypothalamus

Capsaicin has been suggested to act not only on thin primary afferents but also on the hypothalamus, but the neurotransmitter(s) of central capsaicin-sensitive neurons are unknown. The present study was conducted to determine whether any central, especially hypothalamic, glutamatergic terminals were sensitive to capsaicin. Capsaicin evoked glutamate release from slices of hypothalamus and lumbar dorsal horn, but not cerebellum. Such capsaicin action was Ca2+ dependent and inhibited by the capsaicin antagonist capsazepine. Vanilloid receptor subtype 1 mRNA was widely distributed in the brain, with a marked level in the hypothalamus and cerebellum, but not in the spinal cord. The results suggest that there are glutamatergic terminals sensitive to capsaicin in the hypothalamus.

Inhibitors of NADH-ubiquinone reductase: an overview

This article provides an updated overview of the plethora of complex I inhibitors. The inhibitors are presented within the broad categories of natural and commercial compounds and their potency is related to that of rotenone, the classical inhibitor of complex I. Among commercial products, particular attention is dedicated to inhibitors of pharmacological or toxicological relevance. The compounds that inhibit the NADH-ubiquinone reductase activity of complex I are classified according to three fundamental types of action on the basis of available evidence and recent insights: type A are antagonists of the ubiquinone substrate, type B displace the ubisemiquinone intermediate, and type C are antagonists of the ubiquinol product.

Specific vanilloid responses in C6 rat glioma cells

Capsaicin and its ultrapotent analog resiniferatoxin (RTX) act through specific vanilloid receptors on sensory neurons. Here, we describe specific vanilloid responses in rat C6 glioma cells. Capsaicin and RTX stimulated 45Ca uptake in a similar fashion to that found for cultured rat dorsal root ganglion neurons (DRGs); this response was antagonized by the antagonists capsazepine and ruthenium red. As in DRGs, pretreatment of C6 cells with capsaicin or RTX produced desensitization to subsequent stimulation of 45Ca uptake. The potency for desensitization by RTX in the C6 cells corresponded to that for 45Ca uptake, whereas in DRGs it occurred at significantly lower concentrations corresponding to that for the high affinity [3H]RTX binding site. Consistent with this difference, in C6 cells we were unable to detect [3H]RTX binding. These characteristics suggest the presence of C-type but not R-type vanilloid receptors on C6 cells. After 2 day treatment, capsaicin but not RTX inhibited the proliferation and altered the differentiation of the cells and produced apoptosis. In the long term experiments, capsazepine, instead of antagonizing the effect of capsaicin, acted as an agonist. Moreover, capsazepine displayed these effects with higher potency than that of capsaicin. The different potencies and structure activity relations suggest a distinct mechanism for these long-term vanilloid effects. Our finding that C6 cells can respond directly to capsaicin necessitates a reevaluation of the in vivo pathway of response to vanilloids, and highlights the importance of the neuron-glial network.

Specific binding of [3H]resiniferatoxin by human and rat preoptic area, locus ceruleus, medial hypothalamus, reticular formation and ventral thalamus membrane preparations

Specific [3H]resiniferatoxin (RTX) binding detects the vanilloid (capsaicin) receptors and provides a biochemical means for exploring their pharmacology. In the present study we demonstrate specific vanilloid (RTX) binding sites in various brain areas not known to be innervated by primary afferent neurons. Specific high-affinity binding of [3H]RTX could be detected in membrane preparations of the posterior ("hypothalamic") and anterior ("septal") parts of the preoptic area, locus ceruleus, medial hypothalamus, brainstem reticular formation and ventral thalamic nuclei from naive rats. The determined levels of binding at 4 nM [3H]RTX were 23.0 +/- 4.5, 7.1 +/- 1.6, 29.9 +/- 2.3, 23.5 +/- 2.4, 9.9 +/- 2.2 and 8.1 +/- 1.9 fmol/mg, respectively; unfortunately, the high levels of non-specific binding (higher than 80%) in the present experiments made it impossible for us to characterize fully the binding properties of the receptors. However, no detectable specific [3H]RTX binding was present in membranes of brain nuclei from rats pretreated with 300 mg/kg capsaicin, a treatment which causes loss of response to capsaicin. Significant specific [3H]RTX binding was also absent in membrane preparations of the midbrain central gray matter, somatosensory cortex and cerebellum either from naive or capsaicin treated rats. In human brain specific [3H]RTX binding measured at 4 nM [3H]RTX showed a pattern of distribution similar to that in the rat brain. The corresponding levels of specific [3H]RTX binding in the preoptic area, locus ceruleus, medial hypothalamus, reticular formation and ventral thalamus were 44.9 +/- 2.4, 50.6 +/- 3.0, 36.1 +/- 2.9, 9.4 +/- 2.8 and 8.4 +/- 2.4 fmol/mg, respectively. Our findings corroborate previous biological evidence that vanilloid receptors are present in brain as well as in sensory afferent neurons.

Vanilloid (capsaicin) receptors in the rat: distribution in the brain, regional differences in the spinal cord, axonal transport to the periphery, and depletion by systemic vanilloid treatment

Vanilloid (capsaicin) receptors were visualized by [3H]resiniferatoxin (RTX) autoradiography in the brain of newborn as well as adult (both control and colchicine-treated) rats. Specific labelling was seen in the brain stem only, in the nucleus of the solitary tract extending into the area postrema and the spinal sensory nucleus of the trigeminal nerve. Also, a strong signal was seen in the dorsal horn, dorsal root, trigeminal and nodose ganglia. Membranes obtained from the cervical, thoracic, and lumbar segments of the spinal cord showed similar affinities for RTX and likewise for capsaicin and capsazepine; maximal receptor density was similar in the cervical and thoracic segments (approximately 70 fmol/mg protein) but was twice as high in the lumbar segment. 24 h after ligation of the vagal or the sciatic nerves, a strong accumulation of specific RTX binding sites was observed mainly proximal to the ligature, implying intraaxonal receptor transport from the nodose and dorsal root ganglia, respectively, to the periphery. Systemic (s.c.) vanilloid treatment depleted specific [3H]RTX binding sites from the brain stem, the sensory (dorsal root as well as trigeminal) ganglia, and the spinal cord. RTX was approximately 200-fold more potent than capsaicin for eliminating vanilloid receptors from the spinal cord. The present results suggest a discrete expression of vanilloid receptors in the brain stem (sensory nuclei); although intrinsic vanilloid receptor-expressing neurons are though to exist in the rat brain, they remain undetected by the present [3H]RTX autoradiography methodology.

The vanilloid (capsaicin) receptor: receptor types and species differences

1. Capsaicin was postulated to exert its pharmacological actions by interacting at a specific recognition site (receptor) expressed predominantly by primary afferent neurons. 2. The actual existence of this long-sought "capsaicin-receptor" has recently been demonstrated by the specific binding of [3H]resiniferatoxin (RTX), an ultrapotent capsaicin analog with a unique spectra of actions. 3. Since homovanillic acid is the key structural motif shared by capsaicin and RTX, their recognition site appears to be best termed the vanilloid receptor. 4. Central (sensory ganglia and spinal cord) vanilloid receptors of the rat bind RTX with high affinity in a cooperative fashion; moreover, they recognize capsaicin with higher affinity than the competp6ive antagonist, capsazepine. Peripheral (urinary bladder, urethra, airways, colon) vanilloid receptors, by contrast, bind RTX with lower affinity in a noncooperative manner. An opposite affinity for capsazepine relative to capsaicin appears to distinguish vanilloid receptors in the urinary bladder from those present in the airways or colon. These findings imply heterogeneity in the properties of vanilloid receptors. 5. The affinity of [3H]RTX binding in vitro is influenced by reducing agents, suggesting an in vivo modulatory role for endogenous reducing agents in vanilloid receptor functions. 6. The size of central vanilloid receptors (270 kDa) as measured by radiation inactivation and the cooperative binding both suggest a receptor cluster with cooperating subunits. 7. RTX binds to vanilloid receptors with orders of magnitude higher affinity than capsaicin; its ability to induce cooperative binding is also more pronounced. These differences in receptor binding along with the pharmacokinetical differences in tissue equilibration and in plasma binding may form a rational basis to explain the peculiar spectrum of actions of RTX. 8. Guinea pig spinal cord and airway membranes bind RTX with lower affinity than rat tissues. The receptor density is, however, higher in the guinea pig in keeping with the marked sensitivity of this species to vanilloid actions. 9. The apparently low level of specific [3H]RTX binding sites in the hamster and rabbit is in accord with the resistance of these species to vanilloid actions. 10. In post-mortem human spinal cord specific [3H]RTX binding sites can be detected; their binding parameters are similar to those determined in guinea pig spinal cord. 11. The vanilloid receptor appears to display both intraspecies heterogeneity and marked interspecies differences. 12. As yet, it is known whether the vanilloid receptor is operated by endogenous ligands. It is not known either which receptor superfamily (if any) it belongs to. The [3H]RTX binding assay has, however, the potential of answering these questions.(ABSTRACT TRUNCATED AT 400 WORDS)

[3H]resiniferatoxin binding by the vanilloid receptor: species-related differences, effects of temperature and sulfhydryl reagents

Specific binding of [3H]resiniferatoxin (RTX) is thought to represent the vanilloid (capsaicin) receptor. In the present study, we have used this binding assay to elucidate the contribution of differential receptor expression to the capsaicin-resistance of hamsters and rabbits; binding parameters were compared to those of species (rats, mice) regarded as capsaicin-sensitive. Whereas the 5-fold lower affinity for [3H]RTX binding in the hamster (100 pM) as compared to the rat (20 pM) is unlikely to account for the 100-fold difference in the in vivo responses of RTX-induced inflammation and hypothermia, the lack of detectable specific [3H]RTX binding sites in the rabbit might represent the predominant mechanism of capsaicin-resistance in this species. Regulation of the vanilloid receptor was further characterized in the rat. In accord with the temperature dependence of both in vivo and in vitro capsaicin actions, we found a marked temperature dependence for association rates. Dissociation turned out to have complex kinetics dependent on time and receptor occupancy. Low pH (5.5-7.0) did not affect receptor binding. Preincubation with heavy metal cations and other sulfhydryl-reactive agents inhibited specific [3H]RTX binding indicating that the vanilloid receptor is a thiol-protein, and that free sulfhydryl groups play an essential role in agonist binding activity. Preliminary characterization suggested noncompetitive inhibition.

Age-related changes in capsaicin-induced degeneration in rat brain

Previous results indicate that the pattern of capsaicin-induced degeneration in the rat central nervous system is age-related. Experiments utilizing capsaicin's selective neurodegenerative effects to study the function of central neural circuits will therefore require a detailed understanding of capsaicin's central neurotoxicity in rats of different ages. The goal of this experiment was to characterize the degeneration induced in the rat brain by systemic treatment with capsaicin at different ages (10, 15, 20, 25, 30 or 75 days, or 11 months), using a cupric silver stain to label degenerating neurons. Results revealed degenerating cell bodies in the ventromedial brainstem in capsaicin-treated rats of all age groups, though they were more numerous in adult rats than in pups. In addition, many areas contained capsaicin-induced nerve terminal degeneration both in rat pups and in adult rats. These areas were the substantia gelatinosa of the spinal cord dorsal horn; the solitary tract; the nucleus of the solitary tract, visceral portion; the area postrema; the trigeminal nerve and spinal trigeminal nucleus; the medial nucleus of the inferior olive; the rostral, dorsomedial and dorsolateral interpeduncular subnuclei and overlying interfascicular nucleus; the supramammillary area; the lateral septal nucleus; the bed nucleus of the stria terminalis, anterior medial portion; the optic nerve and tract; the suprachiasmatic nucleus, ventroposterolateral portion; the magnocellular subnucleus of the ventrolateral geniculate nucleus; the intergeniculate leaf; the medial pretectal nucleus and the olivary pretectal nucleus. In several, but not all of these areas, the apparent density of degenerating terminals was significantly less in adult rats than in pups. In other brain sites, capsaicin-induced degeneration was observed only in rats younger than 30 days of age. These areas were the lateral habenula, medial part; the sphenoid nucleus; and the stria medullaris. Still other brain sites lost their sensitivity to capsaicin sometime between 30 and 75 days of age. These areas were the bed nucleus of the stria terminalis, medial posteromedial part; the medial preoptic nucleus, central part; the septohypothalamic nucleus; the ventral reuniens area; and the ventromedial hypothalamic nucleus. Adult rats 75 days and 11 months of age did not differ detectably in their response to capsaicin. Thus, loss or attenuation of capsaicin sensitivity is not progressive throughout life. It does not occur in all capsaicin-sensitive sites. Where it does occur, loss of sensitivity occurs prior to adulthood and follows a distinct and reproducible time course that may differ for different sites.

Capsaicin affects aggressive behavior, but not hot plate responding, of adult male mice

Adult male mice of albino Swiss-derived CD-1 strain were used to assess the effects of capsaicin (a powerful agent that produces a marked depletion of the undecapeptide substance P) on both intraspecific aggressive behavior (induced by 8 weeks of individual housing) and pain sensitivity. Capsaicin was given SC, 48 h before behavioral testing. Aggressive behavior, scored during a 5-min session under red light, was significantly enhanced by capsaicin treatment (50 or 100 microliters of a 7.5 mg/ml solution). In fact, Total Aggressive Episodes, Attacks, and Upright Offensive Posture were significantly higher in the two capsaicin-treated groups, while Latency to the first Attack was decreased, when compared to both vehicle or unhandled controls. A concomitant decrease in Submissive Postures and Flee was also evident in capsaicin mice. Hot plate testing (55 +/- 0.1 degrees C, cutoff time 30 s), carried out on nonisolated mice, did not reveal any difference among the two capsaicin groups (same doses) and vehicle or unhandled controls.

Capsaicin as a tool for studying sensory neuron functions

The exceptional selectivity with which capsaicin acts on a population of peptide-containing thin primary afferent neurons has made this drug an important tool with which to investigate the neuroanatomical, neurochemical and functional implications of these neurons. As a consequence, the use of capsaicin has enabled a substantial furthering of our understanding of the physiological and pathophysiological roles of thin primary sensory neurons. With appropriate controls, both the acute excitatory and long-term neurotoxic actions of capsaicin can be utilized in these studies but it is important to know the advantages and disadvantages and the limitations of each of the different experimental approaches. Table 1 is a brief checklist of the caveats that should be considered and that have been dealt with in this article.

Capsaicin-induced neuronal degeneration in the brain and retina of preweanling rats

Capsaicin is a neurotoxin known for its ability to cause degeneration of small unmyelinated primary sensory neurons in both spinal and cranial nerves. Although lower motor neurons do not degenerate following capsaicin treatment, the extent to which capsaicin may damage neurons in the brain has not been thoroughly evaluated. This study examines the effects of systemic capsaicin (50-150 mg/kg) on the central nervous system of 10-day-old rats. Rat pups were injected with capsaicin or the injection vehicle and sacrificed 6 hours-10 days later. Brains, spinal cords, and retinas were stained with cupric silver to label degenerating neurons. As previously reported for capsaicin-treated rats, degenerating nerve terminals were present in areas receiving primary afferent input: the spinal cord dorsal horn, spinal trigeminal nucleus, nucleus of the solitary tract, and area postrema. However, degenerating terminals were also present in areas not known to receive primary sensory innervation: the inferior olivary nucleus, sphenoid nucleus, medial and olivary pretectal nuclei, interpeduncular nucleus, interfascicular nucleus, caudal linear, dorsal, median, and paramedian raphe nuclei, supramammillary area, lateral habenula, ventrolateral geniculate nucleus, ventral reuniens nucleus, ventromedial hypothalamic nucleus, lateral hypothalamic and preoptic areas, suprachiasmatic nucleus, septohypothalamic nucleus, bed nucleus of the stria terminalis, lateral septal nucleus, accumbens shell, olfactory bulb, and retina. Some areas where capsaicin caused degeneration in rat pups do not appear to be capsaicin-sensitive in adult rats. Results indicate that (1) capsaicin's neurotoxicity is not limited to primary sensory neurons and (2) developmental factors may alter the capsaicin sensitivity of some neuronal projections within the brain.

The effects of capsaicin on emotional responses to odors in the rat

Capsaicin is described as disturbing the autonomic responses to stress-inducing environments. The effects of capsaicin (130 mg/kg in 2 series of subcutaneous injections) on emotionality responses were studied in 19 Sprague-Dawley male rats using the open-field test. Eleven rats treated with isotonic saline served as controls. Emotionality (E) measured before capsaicin treatment in the open-field ventilated with deodorized air was similar in the 2 groups of rats. Nine out of the 19 treated rats survived. Their E was significantly higher than that of there 10 rats that died from capsaicin. When a frightening odor (fox feces) was added to the open-field E increased in the controls but remained unchanged in the capsaicin-surviving rats. The ability to discriminate palatable food or female odor was similar in the two groups. The results suggest that; 1) Highly emotional rats survived subcutaneous capsaicin injections; 2) Reaction to an emotionality-inducing environment was decreased in the capsaicin-surviving rats; 3) Olfactory discrimination was not impaired by capsaicin treatment.

Capsaicin and its analogs inhibit the activity of NADH-coenzyme Q oxidoreductase of the mitochondrial respiratory chain

Capsaicin and its analogs with different acyl moieties were found to inhibit the electron-transfer activity of NADH-coenzyme Q oxidoreductase isolated from beef heart mitochondria. The inhibitory potency of capsaicin was lower than those of dihydrocapsaicin and analogs with heptanoyl, capryl, undecanoyl, and lauroyl moieties, but was higher than those of analogs with palmitoyl and stearoyl moieties. The analog with the lauroyl moiety showed the strongest inhibition. These results suggest that hydrophobicity and the appropriate carbon chain length of the acyl moiety are important for the binding of compounds to the enzyme. On the other hand, capsaicin and its analogs did not interrupt rotenone-insensitive electron transfer from NADH to menadione. Furthermore, these compounds had almost no effect on the spectral properties and EPR signals arising from iron-sulfur clusters of the NADH-treated enzyme. Kinetic analyses with double-reciprocal plots showed that these compounds were competitive inhibitors with respect to coenzyme Q1, an electron acceptor. These results strongly suggest that capsaicin and its analogs bind to the coenzyme Q1 binding site of the enzyme.

Effects of subacute capsaicin treatment on local cerebral glucose utilization in the rat

The autoradiographic 2-deoxy-D-[1-14C]glucose method was used to map the effect of subacute capsaicin administration on local cerebral glucose utilization, an index of brain function. After treatment with an 80 mg/kg, subcutaneous, cumulative dose of capsaicin over 3 days, a challenge dose of 20 mg/kg capsaicin stimulated glucose utilization in dorsal column and brainstem nuclei which receive primary sensory afferent input or are important in autonomic functions. Glucose utilization in the medial septum was simultaneously reduced. Following a 280 mg/kg cumulative dose of capsaicin over 5 days, a capsaicin challenge dose of 20 mg/kg did not stimulate glucose utilization in the hindbrain, but the decrement in the medial septum was maintained and extended into the lateral septum. The findings provide evidence for a central component of the stimulation and subsequent insensitivity observed with continued capsaicin treatment, and suggest that the deoxyglucose procedure is useful in elucidating the neuroanatomical areas involved in several of the sensory and autonomic effects of capsaicin.

Responses of anterior hypothalamic-preoptic thermosensitive neurons to locally applied capsaicin

The effects of local application of capsaicin on the activity of single thermosensitive neurons in the anterior hypothalamic-preoptic area were studied in the urethane-anesthetized rat. Local injection of capsaicin through a cannula to the vicinity of the neurons increased the activity in 15 of 28 warm-units, decreased the activity in 2 of 4 cold-units and had no effect on 5 of 10 thermally-insensitive units. Electrophoretic application of capsaicin with the use of multibarrelled microelectrodes excited 16 of 27 warm-units, inhibited 12 of 17 cold-units and had no effect on 35 of 60 thermally-insensitive units. Progressive decreases in the responsiveness of the neurons to both capsaicin and the hypothalamic temperature were observed with repeated applications of capsaicin. Many neurons ceased firing after showing excitatory or inhibitory responses to single or repeated applications of capsaicin either by local injection or electrophoretic application. The results may explain the acute thermolytic response, as well as the subsequent decrease in responsiveness to the injection of capsaicin into the anterior hypothalamic-preoptic area, on the basis of changes in the activity of thermosensitive neurons in the anterior hypothalamic-preoptic area.

Capsaicin-induced excitation of locus coeruleus neurons

The noradrenergic pontine nucleus locus coeruleus (LC) seems to be involved in sensory processing. In the present study, the effect of capsaicin, a drug which specifically interferes with chemosensitive primary afferents, on LC firing rate was analysed, utilizing electrophysiological techniques. In control rats, low doses of capsaicin (1-8 micrograms kg-1 i.v.) caused a marked excitation of LC units. The effect was instantaneous in onset but short-lasting and no signs of tachyphylaxis were observed. The excitation was maintained in adult rats treated as neonates with high doses of capsaicin (50 mg kg-1 s.c.) but almost totally prevented by pretreatment of adult rats with high doses of capsaicin (300 mg kg-1 s.c.). According to our histological data, using selective silver impregnation techniques, the LC seems not to receive innervation by sensory primary afferents. It is proposed that the capsaicin-induced excitation of LC neurons is a centrally mediated effect and might, in part, be involved in the analgetic effect induced by the drug.

Cholecystokinin-octapeptide-induced hypothermia in rats: dose-effect and structure-effect relationships, effect of ambient temperature, pharmacological interactions and tolerance

Subcutaneous injection of cholecystokinin octapeptide (CCK-8) (0.005-1.25 mg/kg) elicited dose-dependent hypothermia in rats. The threshold of the response was between 0.01 and 0.05 mg/kg and the dose-response curve levelled off at doses larger than 0.2-0.5 mg/kg. Warm and cold ambient temperatures decreased and increased the response, respectively. Pretreatment with capsaicin, morphine, naloxone, atropine, haloperidol or propranolol did not affect the response to CCK-8, whereas pretreatment with phenoxybenzamine and a large dose of proglumide, an antagonist for CCK-receptors, attenuated the hypothermia. It seems that neither capsaicin-sensitive thermal and non-thermal afferents, nor opiate mechanisms are involved in the response, but alpha-adrenoceptors might be of some importance in the hypothermia. Non-sulphated-CCK-8, the C-terminal tetrapeptide and hexapeptide, [D-Ala4]-CCK-8 and [D-Met6]-CCK-8 were ineffective. Chronic treatment with CCK-8 resulted in the development of tolerance to the thermoregulatory effect, while the hypothermic responses to apomorphine and capsaicin were not affected. It seems that the tolerance cannot be attributed to conditioned homeostatic reactions.