Search

Capsaicin is the source of spicy taste in pepper fruits. Capsaicin stimulates nerve endings and reacts with some molecules to produce bioelectric pulses, which trigger the transient receptor potential vanilloid 1 (TRPV1) channal in human nerves, and then transmit the signal to the brain to produce a spicy feeling. It is found that capsaicin synergizes with or inhibits taste, and the interaction between them varies depending on the concentration of capsaicin. Therefore, this paper summarizes the types, chemical properties and applications of capsaicinoids, describes the biological mechanism by which capsaicin causes a spicy sensation, namely activating the capsaicin receptor TRPV1 channel, and discusses the interaction of spiciness with sour, sweet and bitter taste. Next, this review points out the shortcomings of the current research on the interaction between spiciness and taste and the potential underlying mechanism, and provides an outlook on future research directions. It is expected that this review will provide theoretical support for the further development of the spicy foods industry.

Introduction: To date, a multitude of neural circuits within the central nervous system have been recognized for their roles in pain modulation. Notably, the ventrolateral periaqueductal gray (vlPAG) region of the midbrain emerges as a pivotal element within the supraspinal pain modulation network. This region's significance has been thoroughly documented across diverse animal pain models. In our study, we concentrated on exploring the functions of capsaicin receptors in this specific area, particularly their involvement in mediating antinociceptive effects. Methods: In this study, male Wistar rats were utilized to examine the antinociceptive effects of capsaicin when directly administered into the ventrolateral periaqueductal gray (vlPAG) region of the midbrain. The efficacy of this intervention was evaluated using the tail-flick test, conducted five minutes after injection. The research compared the outcomes of intravlPAG capsaicin administration between healthy control rats and those with diabetes. Results: In the control groups, capsaicin induced a swift and temporary analgesic effect, but it did not produce antinociceptive effects in diabetic rats. Conclusion: Acute microinjections of capsaicin failed to elicit significant antinociceptive effects in the diabetic animal group. To derive more precise conclusions, it is advisable to also examine the long-term impacts of these compounds. [ABSTRACT FROM AUTHOR]

The design of TRPV1 antagonists and agonists has reached a new era since TRPV1 structures at near-atomic resolution are available. Today, the ligand-binding forms of several classical antagonists and agonists are known; therefore, the specific role of key TRPV1’s residues in binding of ligands can be elucidated. It is possible to place the well-defined pharmacophore of TRPV1 ligands, conformed by head, neck, and tail groups, in the right pocket regions of TRPV1. It will allow a more thorough use of molecular modelling methods to conduct more effective rational drug design protocols. In this work, important points about the interactions between TRPV1 and capsaicin-like compounds are spelled out, based on the known pharmacophore of the ligands and the already available TRPV1 structures. These points must be addressed to generate reliable poses of novel candidates and should be considered during the design of novel TRPV1 antagonists and agonists.

Copyright of Shipin Kexue/ Food Science is the property of Food Science Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Introduction: Up to now, numerous neural circuits within the central nervous system have been identified as participants in the modulation of pain. Among these, the ventrolateral periaqueductal gray (vlPAG) region of the midbrain stands out as a crucial component of the pain modulation network at supraspinal levels. Its involvement has been extensively characterized across various animal pain models. In this research, we focused on investigating the roles of orexin A and capsaicin receptors within this region in mediating antinociceptive responses. Methods: In this study, male Wistar rats were utilized to explore the antinociceptive effects of administering orexin-A and capsaicin directly into the ventrolateral periaqueductal gray (vlPAG) region of the midbrain. The substances were administered both separately and in combination. Then, their antinociceptive effects were assessed using the von Frey and hot plate tests post-injection. The experiments compared the outcomes of intra-vlPAG drug administration in both healthy control and diabetic animal models. Results: Microinjections of orexin-A and capsaicin, whether administered individually or in combination directly into the PAG, failed to produce antinociceptive effects on mechanical allodynia or thermal hyperalgesia in both healthy and diabetic rats. Conclusion: Acute microinjections of orexin-A and capsaicin did not produce significant anti-nociceptive effects in either healthy or kindled animal groups. To draw more accurate conclusions, it is recommended to investigate the long-term effects of these compounds as well. [ABSTRACT FROM AUTHOR]

The design of TRPV1 antagonists and agonists has reached a new era since TRPV1 structures at near-atomic resolution are available. Today, the ligand-binding forms of several classical antagonists and agonists are known; therefore, the specific role of key TRPV1's residues in binding of ligands can be elucidated. It is possible to place the well-defined pharmacophore of TRPV1 ligands, conformed by head, neck, and tail groups, in the right pocket regions of TRPV1. It will allow a more thorough use of molecular modelling methods to conduct more effective rational drug design protocols. In this work, important points about the interactions between TRPV1 and capsaicin-like compounds are spelled out, based on the known pharmacophore of the ligands and the already available TRPV1 structures. These points must be addressed to generate reliable poses of novel candidates and should be considered during the design of novel TRPV1 antagonists and agonists. [ABSTRACT FROM AUTHOR]

Although TRPV1 ion channel has been attracting researchers' attention for many years, its functions in animal organisms, the principles of regulation, and the involvement in pathological processes have not yet been fully clarified. Mutagenesis experiments and structural studies have identified the structural features of the channel and binding sites for its numerous ligands; however, these studies are far from conclusion. This review summarizes recent achievements in the TRPV1 research with special focus on structural and functional studies of the channel and on its ligands, which are extremely diverse in their nature and interaction specificity to TRPV1. Particular attention was given to the effects of numerous endogenous agonists and antagonists that can fine-tune the channel sensitivity to its usual activators, such as capsaicin, heat, acids, or their combination. In addition to the pain sensing not covered in this review, the TRPV1 channel was found to be involved in the regulation of many important physiological and pathological processes and, therefore, can be considered as a promising therapeutic target in the treatment of various diseases, such as pneumonia, ischemia, diabetes, epilepsy, schizophrenia, psoriasis, etc. [ABSTRACT FROM AUTHOR]

Prokineticins ( PKs), consisting of PK1 and PK2, are a pair of newly identified regulatory peptides. Two closely related G-protein coupled receptors, PKR1 and PKR2, mediate the signaling of PKs. PKs/PKRs participate in the regulation of diverse biological processes, ranging from development to adult physiology. A number of studies have indicated the involvement of PKs/PKRs in nociception. Here we show that PK2 is a sensitizer for nociception. Intraplantar injection of recombinant PK2 resulted in a strong and localized hyperalgesia with reduced thresholds to nociceptive stimuli. PK2 mobilizes calcium in dissociated dorsal root ganglion (DRG) neurons. Mice lacking the PK2 gene displayed strong reduction in nociception induced by thermal and chemical stimuli, including capsaicin. However, PK2 mutant mice showed no difference in inflammatory response to capsaicin. As the majority of PK2-responsive DRG neurons also expressed transient receptor potential vanilloid (TRPV1) and exhibited sensitivity to capsaicin, TRPV1 is likely a significant downstream molecule of PK2 signaling. Taken together, these results reveal that PK2 sensitize nociception without affecting inflammation.

The naked mole-rat is a subterranean rodent lacking several pain behaviors found in humans, rats, and mice. For example, nerve growth factor (NGF), an important mediator of pain sensitization, fails to produce thermal hyperalgesia in naked mole-rats. The sensitization of capsaicin-sensitive TRPV1 ion channels is necessary for NGF-induced hyperalgesia, but naked mole-rats have fully functional TRPV1 channels. We show that exposing isolated naked mole-rat nociceptors to NGF does not sensitize TRPV1. However, the naked mole-rat NGF receptor TrkA displays a reduced ability to engage signal transduction pathways that sensitize TRPV1. Between one- and three-amino-acid substitutions in the kinase domain of the naked mole-rat TrkA are sufficient to render the receptor hypofunctional, and this is associated with the absence of heat hyperalgesia. Our data suggest that evolution has selected for a TrkA variant that abolishes a robust nociceptive behavior in this species but is still compatible with species fitness.

The transient receptor potential cation channel subfamily V member 1 (TRPV1) provides the sensation of pain (nociception). However, it remains unknown whether TRPV1 is activated after peripheral nerve injury, or whether activation of TRPV1 affects neural regeneration. In the present study, we established rat models of unilateral sciatic nerve crush injury, with or without pretreatment with AMG517 (300 mg/kg), a TRPV1 antagonist, injected subcutaneously into the ipsilateral paw 60 minutes before injury. At 1 and 2 weeks after injury, we performed immunofluorescence staining of the sciatic nerve at the center of injury, at 0.3 cm proximal and distal to the injury site, and in the dorsal root ganglia. Our results showed that Wallerian degeneration occurred distal to the injury site, and neurite outgrowth and Schwann cell regeneration occurred proximal to the injury. The number of regenerating myelinated and unmyelinated nerve clusters was greater in the AMG517-pretreated rats than in the vehicle-treated group, most notably 2 weeks after injury. TRPV1 expression in the injured sciatic nerve and ipsilateral dorsal root ganglia was markedly greater than on the contralateral side. Pretreatment with AMG517 blocked this effect. These data indicate that TRPV1 is activated or overexpressed after sciatic nerve crush injury, and that blockade of TRPV1 may accelerate regeneration of the injured sciatic nerve.

Background/Aims: Ocular surface health depends on conjunctival epithelial (HCjE) layer integrity since it protects against pathogenic infiltration and contributes to tissue hydration maintenance. As the same increases in tear film hyperosmolarity described in dry eye disease can increase corneal epithelial transient receptor potential vanilloid type-1 (TRPV1) channel activity, we evaluated its involvement in mediating an osmoprotective effect by L-carnitine against such stress. Methods: Using siRNA gene silencing, Ca2+ imaging, planar patch-clamping and relative cell volume measurements, we determined if the protective effects of this osmolyte stem from its interaction with TRPV1. Results: TRPV1 activation by capsaicin (CAP) and an increase in osmolarity to ≈ 450 mOsM both induced increases in Ca2+ levels. In contrast, blocking TRPV1 activation with capsazepine (CPZ) fully reversed this response. Similarly, L-carnitine (1 mM) also reduced underlying whole-cell currents. In calcein-AM loaded cells, hypertonic-induced relative cell volume shrinkage was fully blocked during exposure to L-carnitine. On the other hand, in TRPV1 gene-silenced cells, this protective effect by L-carnitine was obviated. Conclusion: The described L-carnitine osmoprotective effect is elicited through suppression of hypertonic-induced TRPV1 activation leading to increases in L-carnitine uptake through a described Na+-dependent L-carnitine transporter.

Summary The naked mole-rat is a subterranean rodent lacking several pain behaviors found in humans, rats, and mice. For example, nerve growth factor (NGF), an important mediator of pain sensitization, fails to produce thermal hyperalgesia in naked mole-rats. The sensitization of capsaicin-sensitive TRPV1 ion channels is necessary for NGF-induced hyperalgesia, but naked mole-rats have fully functional TRPV1 channels. We show that exposing isolated naked mole-rat nociceptors to NGF does not sensitize TRPV1. However, the naked mole-rat NGF receptor TrkA displays a reduced ability to engage signal transduction pathways that sensitize TRPV1. Between one- and three-amino-acid substitutions in the kinase domain of the naked mole-rat TrkA are sufficient to render the receptor hypofunctional, and this is associated with the absence of heat hyperalgesia. Our data suggest that evolution has selected for a TrkA variant that abolishes a robust nociceptive behavior in this species but is still compatible with species fitness. [ABSTRACT FROM AUTHOR]

A novel taste biosensor based on ligand–receptor interaction was developed through fixing taste-bud tissues of SD rats to a glassy carbon electrode. Using the sodium alginate-starch gel as a fixing agent, taste-bud tissues of SD rats were fixed between two nuclear microporous membranes to make a sandwich-type sensing membrane. With the taste biosensor, the response current induced by capsaicin and gingerol stimulating the corresponding receptors was measured. The results showed that the lowest limit of detection of this biosensor to capsaicin was 1×10 −13 mol/L and the change rate of response current was the highest at the concentration of 9×10 −13 mol/L, indicating that the capsaicin receptor was saturated at this point. The lowest limit of detection of this biosensor to gingerol was 1×10 −12 mol/L, and the gingerol receptor was saturated when the concentration of gingerol was 3×10 −11 mol/L. It was demonstrated that the interaction curves of capsaicin and gingerol with their respective receptors exhibited high correlation ( R 2 : 0.9841 and 0.9904). The binding constant and dissociation constant of gingerol with its receptor were 1.564×10 −11 and 1.815×10 −11 respectively, which were all higher than those of capsaicin with its receptor (1.249×10 −12 and 2.078×10 −12 ). This study, for the first time, made it possible to quantitatively determine the interaction of the taste receptor and pungent substances with a new biosensor, thus providing a simple approach for monitoring pungent substances and investigating the mechanism of ligand–receptor interaction. [ABSTRACT FROM AUTHOR]

Background/Aims: Ocular surface health depends on conjunctival epithelial (HCjE) layer integrity since it protects against pathogenic infiltration and contributes to tissue hydration maintenance. As the same increases in tear film hyperosmolarity described in dry eye disease can increase corneal epithelial transient receptor potential vanilloid type-1 (TRPV1) channel activity, we evaluated its involvement in mediating an osmoprotective effect by L-carnitine against such stress. Methods: Using siRNA gene silencing, Ca2+ imaging, planar patch-clamping and relative cell volume measurements, we determined if the protective effects of this osmolyte stem from its interaction with TRPV1. Results: TRPV1 activation by capsaicin (CAP) and an increase in osmolarity to ≈ 450 mOsM both induced increases in Ca2+ levels. In contrast, blocking TRPV1 activation with capsazepine (CPZ) fully reversed this response. Similarly, L-carnitine (1 mM) also reduced underlying whole-cell currents. In calcein-AM loaded cells, hypertonic-induced relative cell volume shrinkage was fully blocked during exposure to L-carnitine. On the other hand, in TRPV1 gene-silenced cells, this protective effect by L-carnitine was obviated. Conclusion: The described L-carnitine osmoprotective effect is elicited through suppression of hypertonic-induced TRPV1 activation leading to increases in L-carnitine uptake through a described Na+-dependent L-carnitine transporter. © 2014 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The powerful plant-derived irritant allyl isothiocyanate (AITC, aka mustard oil) induces hyperalgesia to heat in rodents and humans through mechanisms that are not yet fully understood. It is generally believed that AITC activates the broadly tuned chemosensory cation channel transient receptor potential cation channel subfamily A member 1 (TRPA1), triggering an inflammatory response that sensitizes the heat sensor transient receptor potential cation channel subfamily V member 1 (TRPV1). In the view of recent data demonstrating that AITC can directly activate TRPV1, we here explored the possibility that this compound sensitizes TRPV1 to heat stimulation in a TRPA1-independent manner. Patch-clamp recordings and intracellular Ca imaging experiments in HEK293T cells over-expressing mouse TRPV1 revealed that the increase in channel activation induced by heating is larger in the presence of AITC than in control conditions. The analysis of the effects of AITC and heat on the current-voltage relationship of TRPV1 indicates that the mechanism of sensitization is based on additive shifts of the voltage dependence of activation towards negative voltages. Finally, intracellular Ca imaging experiments in mouse sensory neurons isolated from Trpa1 KO mice yielded that AITC enhances the response to heat, specifically in the subpopulation expressing TRPV1. Furthermore, this effect was strongly reduced by the TRPV1 inhibitor capsazepine and virtually absent in neurons isolated from double Trpa1/ Trpv1 KO mice. Taken together, these findings demonstrate that TRPV1 is a locus for cross sensitization between AITC and heat in sensory neurons and may help explaining, at least in part, the role of this channel in AITC-induced hyperalgesia to heat. [ABSTRACT FROM AUTHOR]

Calcitonin gene-related peptide (CGRP) is a predominant neurotransmitter from capsaicin-sensitive sensory nerves, which are widely distributed in the gastrointestinal system. These sensory nerves are reported to be involved in the protection of gastric mucosa against damage by various stimuli, and CGRP is a potential mediator in this process. In addition to increase in gastric mucosal blood flow, the beneficial effects of CGRP on gastric mucosa include inhibition of gastric acid secretion, prevention of cellular apoptosis and oxidative injury. The synthesis and release of CGRP is regulated by the capsaicin receptor which is known as transient receptor potential vanilloid subfamily member 1 (TRPV1) and the agonists of TRPV1 have the potential for gastric mucosal protection. So far, multiple TRPV1 agonists, including capsaicin, capsiate, anandamide and rutaecarpine are reported to exert beneficial effects on gastric mucosal injury induced by various stimuli. Therefore, the TRPV1/CGRP pathway represents a novel target for therapeutic intervention in human gastric mucosal injury. [ABSTRACT FROM AUTHOR]

Gigantoxin I, isolated from sea anemone Stichodactyla gigantea, was previously described as the first epidermal growth factor (EGF)-like toxin from natural origin. In this study, we discovered the interaction between the transient receptor potential vanilloid subtype I (TRPV1) channels and gigantoxin I. The TRPV1 channel is a non-selective cation channel involved in pain sensation and is described as pharmacological target of cnidaria venom. Our results highlight the involvement of the epidermal growth factor receptor/phospholipaseA2 /arachidonic acid/lipoxygenase (EGFR/PLA2/AA/ LOX) pathway in the indirect activation of TRPV1 channels by gigantoxin I. This is the first time that this pathway is described in the indirect activation of TRPV1 channels by toxins. This knowledge not only gives insights into the possible induced effects by this new group of toxins, but also leads to a better understanding of the regulatory mechanism of TRPV1 channels themselves. [ABSTRACT FROM AUTHOR]

Electrophysiological studies demonstrate that transient receptor potential vanilloid subtype 1 (TRPV1) is involved in neuronal transmission. Although it is expressed in the peripheral as well as the central nervous system, the questions remain whether TRPV1 is present in synaptic structures and whether it is involved in synaptic processes. In the present study we gathered evidence that TRPV1 can be detected in spines of cortical neurons, that it colocalizes with both pre- and postsynaptic proteins, and that it regulates spine morphology. Moreover, TRPV1 is also present in biochemically prepared synaptosomes endogenously. In F11 cells, a cell line derived from dorsal-root-ganglion neurons, TRPV1 is enriched in the tips of elongated filopodia and also at sites of cell-cell contact. In addition, we also detected TRPV1 in synaptic transport vesicles, and in transport packets within filopodia and neurites. Using FM4-64 dye, we demonstrate that recycling and/or fusion of these vesicles can be rapidly modulated by TRPV1 activation, leading to rapid reorganization of filopodial structure. These data suggest that TRPV1 is involved in processes such as neuronal network formation, synapse modulation and release of synaptic transmitters. [ABSTRACT FROM AUTHOR]

In ancient times, physicians had a limited number of therapies to provide pain relief. Not surprisingly, plant extracts applied topically often served as the primary analgesic plan. With the discovery of the capsaicin receptor (transient receptor potential cation channel, subfamily V, member 1 [TRPV1]), the search for “new” analgesics has returned to compounds used by physicians thousands of years ago. One such compound, capsaicin, couples the paradoxical action of nociceptor activation (burning pain) with subsequent analgesia following repeat or high-dose application. Investigating this “paradoxical” action of capsaicin has revealed several overlapping and complementary mechanisms to achieve analgesia including receptor desensitization, nociceptor dysfunction, neuropeptide depletion, and nerve terminal destruction. Moreover, the realization that TRPV1 is both sensitized and activated by endogenous products of inflammation, including bradykinin, H+, adenosine triphosphate, fatty acid derivatives, nerve growth factor, and trypsins, has renewed interest in TRPV1 as an important site of analgesia. Building on this foundation, a new series of preclinical and clinical studies targeting TRPV1 has been reported. These include trials using brief exposure to high-dose topical capsaicin in conjunction with prior application of a local anesthetic. Clinical use of resiniferatoxin, another ancient but potent TRPV1 agonist, is also being explored as a therapy for refractory pain. The development of orally administered high-affinity TRPV1 antagonists holds promise for pioneering a new generation of analgesics capable of blocking painful sensations at the site of inflammation and tissue injury. With the isolation of other members of the TRP channel family such as TRP cation channel, subfamily A, member 1, additional opportunities are emerging in the development of safe and effective analgesics. [ABSTRACT FROM AUTHOR]

The capsaicin receptor TRPV1 is a polymodal sensory transducer molecule in the pain pathway. TRPV1 integrates noxious heat, tissue acidosis and chemical stimuli which are all known to cause pain. Studies on TRPV1-deficient mice suggest that TRPV1 is essential for acid sensing by nociceptors and for thermal hyperalgesia in inflammation of the skin, but not for transducing noxious heat. After TRPV1, other TRPV channels were cloned with polymodal properties and sensitivity to noxious heat, named TRPV2, TRPV3 and TRPV4. While TRPV3 and TRPV4 are predominantly warm sensors, TRPV2’s threshold is in the noxious range (>52°C). However, mice deficient of TRPV2 and TRPV1 or TRPV3 or TRPV4 show no major impairment of noxious heat sensing. Ruthenium red, a water soluble polycationic dye, was found to block the pore of the capsaicin-operated cation channel TRPV1 thus interfering with all polymodal ways of TRPV1 activation. Antagonistic effects of the dye were subsequently described on many other TRP-channels, especially on the heat-sensitive ones of the vanilloid family, TRPV2, TRPV3 and TRPV4. In this study, we used the rat skin-nerve preparation to define the possible actions of ruthenium red on the proton, capsaicin and noxious heat activation of native polymodal nociceptors. Ruthenium red was found to suppress only the capsaicin-induced excitation and desensitization of these nerve endings. On the contrary, the proton and heat-induced discharge responses of the single fibres were not influenced. Additionally, we found that the dye concentration dependently increases the excitability of the neurons resulting in ongoing activity and burstlike discharge. These differential results are discussed in the light of recent findings from transgenic mouse models, and they point once more to major (pharmacological) differences between cellular models of nociception, including spinal ganglion neuron and transfected cell lines, and the real native nerve endings. [ABSTRACT FROM AUTHOR]

Feng N-H, Lee H-H, Shiang J-C, Ma M-C. Transient receptor potential vanilloid type 1 channels act as mechanoreceptors and cause substance P release and sensory activation in rat kidneys. Am J Physiol Renal Physiol 294: F316-F325, 2008. First published November 21, 2007; doi:10.1152/ajprenal.00308.2007.—Stimulation of capsaicin receptors results in an increase in afferent renal nerve activity (ARNA), but it is unclear how capsaicin contributes to sensory activation intrarenally. Here, we studied the relationships between capsaicin receptor activation, substance P (SP) release, and the sensory response in the rat renal pelvis. Immunoblots showed that one of the capsaicin receptors, transient receptor potential vanilloid type 1 channel (TRPV1), was found in various renal tissues and was especially abundant in the renal pelvis, where most sensory nerve fibers originate. Interestingly, immunolabeling showed colocalization of TRPV1, SP, and the panneuronal marker PGP9.5 in the renal pelvis. Electrophysiological recordings showed that SP and capsaicin activated the same mechanosensitive ARNA in a single-unit preparation. Intrapelvic administration of capsaicin or a specific TRPV1 agonist, resiniferatoxin, resulted in a dose-dependent increase in multi-unit ARNA and SP release, and these effects were blocked by the TRVPI blocker capsazepine. Inhibition of the SP receptor by L-703,606 largely prevented capsaicin- or resiniferatoxin-induced ARNA. Capsazepine also prevented intrapelvic pressure (IPP)-dependent ARNA activation and contralateral diuresis/natriuresis in the renorenal reflex at an IPP of 20 mmHg, but had no effect at an IPP of 50 mmHg. These data indicate that TRPV1, a low-pressure baroreceptor, is present in the renal pelvis and exclusively regulates neuropeptide release from primary renal afferent C-fibers in response to mechanostimulation. [ABSTRACT FROM AUTHOR]

Recent evidence implicates the neuronal transient receptor potential vanilloid receptor 1 (TRPV1), expressed on sensory C-fibers, as playing an important endogenous protective role in limiting the damaging effects of myocardial I/R injury. In neurons the 12-lipoxygenase (12-LOX) arachidonic acid (AA) metabolite, 12(S)-HpETE, has been proposed as the endogenous ligand for TRPV1. However, whether 12(S)-HpETE underlies TRPV1 channel activation during I/R is unknown. Treatment of isolated Langendorff rat hearts with a 12-LOX/AA cocktail significantly attenuated I/R injury (∼40% inhibition of infarct size), an effect reversed by the 12-LOX inhibitor baicalein or after chemical desensitization of local sensory C-fiber afferents using capsaicin. Both 12(S)-HpETE and AA caused dose-dependent coronary vasodilatation (∼EC50s of 6 x 10-19 and 1 x 10-7, respectively) that was profoundly suppressed by the TRPV1 antagonist capsazepine, in hearts of TRPV1 knockout mice compared with wild-type mice, or by treatment with a CGRP antagonist. In addition, I/R itself stimulates up-regulation of TRPV1 expression in both the cell bodies located within the dorsal root ganglia and locally within the myocardium. Together, our data identify a novel 12-LOX/AA/TRPV1 pathway activated and up-regulated during I/R injury, providing an endogenous damage-limiting mechanism whose targeting may prove useful in treating myocardial infarction. [ABSTRACT FROM AUTHOR]

Abstract: Premature activation of digestive enzymes within the pancreas which leads to autodigestion of the gland is an early step in the pathogenesis of pancreatitis. Pancreatic injury is followed by other manifestations of inflammation including plasma extravasation, edema, and neutrophil infiltration which constitute the features of pancreatitis. Recent studies indicate that neural innervation of the pancreas may play an important role in the initiation and maintenance of the inflammatory response to injury. The pancreas is innervated by vagal, sympathetic and parasympathetic neurons, as well as sensory neurons. Activation of pancreatic primary sensory neurons causes the release of inflammatory neuropeptides both in the spinal cord to signal pain and in the pancreas itself where they produce plasma extravasation and neutrophil infiltration. Recent studies indicate that primary sensory neurons of the pancreas express transient receptor potential V1 (TRPV1) channels whose activation induces pancreatic inflammation. Moreover, blockade of these TRP channels significantly ameliorates experimental pancreatitis. This review describes our current understanding of the role of TRPV1 channels in pancreatitis and illustrates how this mechanism might be used to direct future treatments of pancreatic diseases. [Copyright &y& Elsevier]

Extracellular acidification contributes to pain by activating or modulating nociceptor activity. To evaluate acidic signaling from the colon, we characterized acid-elicited currents in thoracolumbar (TL) and lumbosacral (LS) dorsal root ganglion (DRG) neurons identified by content of a fluorescent dye (DiI) previously injected into the colon wall. in 13% of unidentified LS DRG neurons (not labeled with DiI) and 69% of LS colon neurons labeled with DiI, protons activated a sustained current that was significantly and reversibly attenuated by the transient receptor potential vanilloid receptor 1 (TRPV1) antagonist capsazepine. In contrast, 63% of unidentified LS DRG neurons and 4% of LS colon neurons exhibited transient amiloride-sensitive acid-sensing ion channel (ASIC) currents. The peak current density of acid-elicited currents was significantly reduced in colon sensory neurons from TRPV1-null mice, supporting predominant expression of TRPV1 in LS colon sensory neurons, which was also confirmed immunohistochemically. Similar to LS colon DRG neurons, acid-elicited currents in TL colon DRG neurons were mediated predominantly by TRPV1. However, the pH producing half-activation of responses significantly differed between TL and LS colon DRG neurons. The properties of acid-elicited currents in colon DRG neurons suggest differential contributions of ASICs and TRPV1 to colon sensation and likely nociception. [ABSTRACT FROM AUTHOR]

The capsaicin receptor, also known as ‘transient receptor potential vanilloid receptor subtype 1’ (TRPV1, VR1), is an ion channel subunit expressed in primary afferent nociceptors, which plays a critical role in pain transduction and thermal hyperalgesia. Increases in nociceptor TRPV1 mRNA and protein are associated with tissue injury–inflammation. As little is understood about what controls TRPV1 RNA transcription in nociceptors, we functionally characterized the upstream portion of the rat TRPV1 gene. Two functional rTRPV1 promoter regions and their transcription initiation sites were identified. Although both promoter regions directed transcriptional activity in nerve growth factor (NGF) treated rat sensory neurons, the upstream Core promoter was the most active in cultures enriched in sensory neurons. Because NGF is a key modulator of inflammatory pain, we examined the effect of NGF on rTRPV1 transcription in PC12 cells. NGF positively regulated transcriptional activity of both rTRPV1 promoter regions in PC12 cells. We propose that the upstream regulatory region of the rTRPV1 gene is composed of a dual promoter system that is regulated by NGF. These findings support the hypothesis that NGF produced under conditions of tissue injury and/or inflammation directs an increase of TRPV1 expression in nociceptors in part through a transcription-dependent mechanism. [ABSTRACT FROM AUTHOR]

Abstract: To examine the role of the vanilloid receptor TRPV1 in neuropathic pain, we assessed the effects of the receptor antagonist thioxo-BCTC and antisense oligonucleotides against the TRPV1 mRNA in a rat model of spinal nerve ligation. In order to identify accessible sites on the mRNA of TRPV1, the RNase H assay was used, leading to the successful identification of binding sites for antisense oligonucleotides. Cotransfection studies using Cos-7 cells were employed to identify the most effective antisense oligonucleotide efficiently inhibiting the expression of a fusion protein consisting of TRPV1 and the green fluorescent protein in a specific and concentration-dependent manner. In an in vivo rat model of spinal nerve ligation, intravenous application of the TRPV1 antagonist thioxo-BCTC reduced mechanical hypersensitivity yielding an ED50 value of 10.6mg/kg. Intrathecal administration of the antisense oligonucleotide against TRPV1, but not the mismatch oligonucleotide or a vehicle control, reduced mechanical hypersensitivity in rats with spinal nerve ligation in a similar manner. Immunohistochemical analysis revealed neuropathy- and antisense-associated regulation of TRPV1 protein expression in spinal cord and dorsal root ganglia. Our data demonstrate comparative analgesic effects of a TRPV1 anatagonist and a rationally designed TRPV1 antisense oligonucleotide in a spinal nerve ligation model of neuropathic pain and thus, lend support to the validation of TRPV1 as a promising target for the treatment of neuropathic pain. [Copyright &y& Elsevier]

Abstract: Our previous recordings from dorsal root ganglion and spinal lamina V neurons from TRPV1-mutant mice showed dramatic decreases in responses to temperatures near the activation threshold of this channel (43–49°C). Somewhat unexpectedly, we only observed behavioral deficits in these mice at higher temperatures (50–58°C). In the present study, we tested the hypothesis that the noxious heat-evoked pain behavior that persists in TRPV1-mutant mice reflects residual responsiveness of neurons in the superficial, but not deep, dorsal horn. To this end, we performed in vivo extracellular recordings of spinal nociresponsive neurons in laminae I and V in wild type (WT) and TRPV1 mutant mice. Neurons in WT and mutant mice from both laminae did not differ in their spontaneous activity or evoked responses to mechanical or cold stimuli. By contrast, most lamina I neurons from mutant mice responded to noxious heat with significantly higher thresholds than in WT mice. In contrast, lamina V neurons from mutant mice were virtually unresponsive to noxious heat before and after topical mustard oil-induced tissue injury. Interestingly, lamina I neurons in mutant mice displayed thermal sensitization following tissue injury, comparable in magnitude, but of shorter duration, than in WT mice. We conclude that TRPV1 is necessary for noxious heat-evoked responses of lamina V neurons, both before and after tissue injury. It is also an essential contributor to the normal activation threshold of lamina I neurons to noxious heat and for the full duration of thermal sensitization of lamina I neurons following injury. Finally, our results suggest that the processing of noxious thermal messages by neurons in lamina I involves convergent inputs from a heterogeneous population of primary afferent thermal nociceptors. [Copyright &y& Elsevier]

Abstract: We have previously shown that the neurosteroid pregnenolone sulfate (PS) inhibits the capsaicin receptor-mediated current in rat dorsal root ganglion neurons. Here, we examined the effect of intradermal injection of PS into the rat hindpaw on capsaicin-induced nociception. Results revealed that PS co-injected with capsaicin dose-dependently inhibited the capsaicin-induced nocifensive response. In contrast, injections of PS into one hindpaw and capsaicin into the contralateral hindpaw had no effect on the capsaicin-induced nocifensive response, suggesting that PS produced its effect locally but not systemically. Moreover, PS inhibition of the capsaicin-induced nocifensive response was not significantly reduced by a nonselective opioid receptor antagonist or by cannabinoid receptor antagonists, indicating that neither an opioid- nor a cannabinoid-dependent mechanism mediated the effect of PS. These data demonstrate that PS acts peripherally to attenuate capsaicin-induced nociception through an opioid- and cannabinoid-independent mechanism and suggest a new therapeutic potential for PS in pain management. [Copyright &y& Elsevier]

A number of subfamilies of the capsaicin receptor, collectively called TRP, have been reported since the discovery of vanilloid receptor 1 (VR1). The term ‘TRP’ is derived from ‘transient receptor potential’, which means the transient and rapid defect of reaction following long stimulation with light in the photoreceptor cells of mutant Drosophila. The common features of TRP family members are the centrally situated six transmembrane domain, in which an ion channel is located, three to four ankirin repeats at the N-terminus and a TRP domain comprising 25 amino acids at the C-terminus. The TRP family members are present in animals, including invertebrates and vertebrates, and in the cells in various tissues in individual animals. During evolution, the original TRP seems to have acquired a wide variety of functions related to sensing the inner or outer environment (e.g. to sensing light ( Drosophila), osmolarity, protons, temperature, ligands and mechanical force). In mammals, the TRPV subfamily is exclusively expressed in small- to medium-sized primary sensory neurons that also co-express some chemical markers (i.e. isolectin B4 (IB4), fluoride-resistant acid phosphatase (FRAP), the P2X3 purinoceptor (a receptor provoked by ATP-induced nociception) and Ret, a glial cell line-derived neurotrophic factor receptor). There is a paradox in that regardless of the marked or complete loss of noxious, small sensory neurons (polymodal nociceptors) in mice treated with capsaicin during the neonatal period, as well as in VR1 (TRPV1)-deficient knock-out mice, the responses to noxious heat are normal. Regarding the paradox in mice treated with capsaicin as neonates, our explanation is that although capsaicin probably reduces the number of a subgroup of small neurons (IB4–, VR1+), the remaining IB4+ (VR1–) neurons can sense noxious heat normally. One working hypothesis is that mice lacking TRPV1/2 can sense noxious heat under normal conditions, presumably via another still unknown pathway, and TRPV1 has been suggested to be involved in noxious heat transduction under pathological conditions, such as inflammation and tissue injury. Further studies will be required to clarify these complexities. Mice treated with capsaicin as neonates would provide a model to investigate the above paradoxes, as would TRPV1-knock-out mice, although different mechanisms may be operating in the two models. [ABSTRACT FROM AUTHOR]

We describe the expression of the capsaicin receptor (TRPV1) in human peripheral nervous system (PNS) and its changes in sural nerve and skin nerve fibers of patients with painful neuropathy. Dorsal root ganglion (DRG), root, and spinal cord autopsy specimens from subjects without PNS diseases were immunoassayed with anti-TRPV1 antibodies. Bright-field and confocal microscope studies using anti-TRPV1, protein gene product 9.5 (PGP 9.5), and unique-β-tubulin (TuJ1) antibodies were performed in skin biopsies from 15 healthy subjects and 10 painful neuropathies. The density of intraepidermal nerve fiber (IENF) labeled by each antibody was quantified. Sural nerve biopsies from three patients with painful, one patient with nonpainful diabetic neuropathy, and two patients with multifocal motor neuropathy used as controls were immunoassayed with anti-TRPV1 antibodies and investigated by immunoelectron microscopy. TRPV1 strongly labeled laminae I and II of dorsal horns, most small-size and some medium-size DRG neurons, and small-diameter axons of dorsal roots. In sural nerve, TRPV1 was expressed within the cytoplasm of most unmyelinated and some small myelinated axons, in the muscular lamina of epineural vessels, and in the endothelium of endoneurial vessels. The density of IENF labeled by TRPV1, PGP 9.5, and TuJ1 did not differ. TRPV1 colocalized with TuJ1 in all IENF and dermal nerve bundles. Painful neuropathies showed a diffuse loss of TRPV1-positive axons both in the sural nerve and in the skin. Our findings demonstrated that TRPV1 is normally expressed throughout the nociceptive pathway of PNS and that TRPV1-positive peripheral nerve fibers degenerate in painful neuropathies. [ABSTRACT FROM AUTHOR]

Copyright of Der Schmerz is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: TRPV1 gene disruption results in a loss of capsaicin and proton responsiveness, but has minimal effects on heat-induced nocifensive behavior, suggesting that sensory transduction of heat is independent of TRPV1. TRPV3, another heat-activated ion channel but insensitive to capsaicin, was shown to be expressed in keratinocytes as well as in sensory neurons projecting to the skin. Recently, 2-aminoethoxydiphenyl borate was introduced as a TRPV3 agonist, but its selectivity was questioned by showing that it activated recombinant TRPV1 and TRPV2 as well. We used the isolated mouse skin-saphenous nerve preparation and whole-cell patch-clamping of cultured dorsal root ganglia neurons from TRPV1−/− and wildtype mice. We found no phenotypic differences between the heat responses of polymodal C-fibers, whereas cultured dorsal root ganglia neurons of TRPV1−/− hardly showed any heat-activated currents. Only C-fibers of wildtype but not TRPV1−/− mice were clearly sensitized to heat by 2-aminoethoxydiphenyl borate 10 and 100 μM; heat-activated current in wildtype neurons was only facilitated at 100 μM. Noxious heat-induced calcitonin gene-related peptide release showed clear deficits (<50%) in TRPV1 deficient skin, but the stimulated calcitonin gene-related peptide release from the isolated skull dura was unaffected. In both models, 2-aminoethoxydiphenyl borate was able to potentiate the heat response (46°C, 5 min) in a concentration-dependent manner, again, only in wildtype but not TRPV1−/− mice, suggesting that TRPV2/3 are not involved in this sensitization to heat. The results further suggest that TRPV1 is not responsible for the normal heat response of native nociceptors but plays the essential role in thermal sensitization and a prominent one in controlling dermal calcitonin gene-related peptide release, i.e. neurogenic inflammation. [Copyright &y& Elsevier]

Abstract: Immunohistochemistry for substance P, somatostatin and vanilloid receptor subtype 1 as well as receptors for somatostatin and opioids was performed on the trigeminal ganglion in wild-type and Brn-3a knockout mice at postnatal day 0. In wild-type mice, the trigeminal ganglion contained abundant substance P-, vanilloid receptor subtype 1-, sst2A receptor- and delta-opioid receptor-immunoreactive neurons, while the ganglion had only a few mu-opioid receptor-immunoreactive neurons. The Brn-3a deficiency had an effect on the cell size but not the number of substance P-immunoreactive neurons. In knockout mice, the proportion of small immunoreactive neurons markedly increased and that of medium- to large-sized immunoreactive ones correspondingly decreased (mean±S.D.=54.7±29.1μm2, range=10.9–220.8μm2) compared to wild-type mice (mean±S.D.=116.6±58.6μm2, range=27.3–400.7μm2). As for vanilloid receptor subtype 1-immunoreactive neurons, the number and cell size was barely affected by the deficiency. On the other hand, the loss of Brn-3a caused a decrease in the number of sst2A receptor- or delta-opioid receptor-immunoreactive neurons (more than 95% reduction) and an increase in the number of mu-opioid receptor-immunoreactive neurons (9.3-fold increase). Somatostatin-immunoreactive neurons were not detected in the trigeminal ganglion of wild-type or mutant mice at postnatal day 0. The present study suggests that Brn-3a deficiency may have effects on the survival of trigeminal nociceptors and their expression of some neurochemical substances. [Copyright &y& Elsevier]

Using whole-cell patch-clamp methods, we examined the hypothesis that serotonin [5-hydroxytryptamine (5-HT)] receptor activation enhances TRPV1 function in mouse colon sensory neurons in lumbosacral dorsal root ganglia, which were identified by retrograde labeling with Dil (1,1'-dioctadecyl-3,3,3',3-tetramethlindocarbocyanine methanesulfonate) injected into multiple sites in the wall of the descending colon. 5-HT increased membrane excitability at a temperature below body temperature in response to thermal ramp stimuli in colon sensory neurons from wild-type mice, but not from TRPV1 knock-out mice. 5-HT significantly enhanced capsaicin-, heat-, and proton-evoked currents with an EC50 value of 2.2 µM. 5-HT (1 µM) significantly increased capsaicin-evoked (100 nM) and proton-evoked (pH 5.5) currents 1.6- and 4.7-fold, respectively, and significantly decreased the threshold temperature for heat current activation from 42 to 38°C. The enhancement of TRPV1 by 5-HT was significantly attenuated by selective 5-HT2 and 5- HT4 receptor antagonists, but not by a 5-HT3 receptor antagonist. In support, 5-HT2 and 5-HT4 receptor agonists mimicked the facilitating effects of 5-HT on TRPV1 function. Downstream signaling required G-protein activation and phosphorylation as intracellularly administered GDP-β-S [guanosine 5'-0-(2thiodiphosphate], protein kinase A inhibitors, and an A-kinase anchoring protein inhibitor significantly blocked serotonergic facilitation of TRPV1 function; 5-HT2 receptor-mediated facilitation was also inhibited by a PKC inhibitor. We conclude that the facilitation of TRPV1 by metabotropic 5-HT receptor activation may contribute to hypersensitivity of primary afferent neurons in irritable bowel syndrome patients. [ABSTRACT FROM AUTHOR]

The vanilloid receptor VR1 is a nonselective cation channel activated by capsaicin as well as increases in temperature and acidity, and can be viewed as molecular integrator of chemical and physical stimuli that elicit pain. The distribution of VR1 receptors in peripheral and central processes of rat primary vagal afferent neurons innervating the gastrointestinal tract was investigated by immunohistochemistry. Forty-two percent of neurons in the nodose ganglia retrogradely labeled from the stomach wall expressed low to moderate VR1 immunoreactivity (VR1-IR). VR1-IR was considerably lower in the nodose ganglia as compared to the jugular and dorsal root ganglia. In the vagus nerve, strongly VR1-IR fibers ran in separate fascicles that supplied mainly cervical and thoracic targets, leaving only weakly VR1-IR fibers in the subdiaphragmatic portion. Vagal afferent intraganglionic laminar endings (IGLEs) in the gastric and duodenal myenteric plexus did not express VR1-IR. Similarly, VR1-IR was contained in fibers running in perfect register with vagal afferents, but was not colocalized with horseradish peroxidase in the same varicosities of intramuscular arrays (IMAs) and vagal afferent fibers in the duodenal submucosa anterogradely labeled from the nodose ganglia. Only in the gastric mucosa did we find evidence for colocalization of VR1-IR in vagal afferent terminals. In contrast, many nerve fibers coursing through the myenteric and submucosal plexuses contained detectable VR1-IR, the majority of which colocalized calcitonin gene-related peptide immunoreactivity. In the dorsal medulla there was a dense plexus of VR1-IR varicose fibers in the commissural, dorsomedial and gelatinosus subnuclei of the medial NTS and the lateral aspects of the area postrema, which was substantially reduced, but not eliminated on the ipsilateral side after supranodose vagotomy. It is concluded that about half of the vagal afferents innervating the gastrointestinal tract express low levels of VR1-IR, but that presence in most of the peripheral terminal structures is below the immunohistochemical detection threshold. [ABSTRACT FROM AUTHOR]

Recent investigations suggest that vanilloid receptor-1 (VR1) immunoreactivity occurs in the intestine. We have determined and quantified this immunoreactivity in the myenteric plexus with respect to cholinergic and neurofilament protein-positive neurones. Guinea-pig and rat preparations were dual-labelled with specific antibodies raised in rabbit or goat against vanilloid receptor-1 and against other neurochemical markers. In the rat ileum, both vanilloid receptor antibodies were co-distributed, whereas in the guinea-pig ileum and colon, tertiary fibres were also detected with the goat antibody. In the guinea-pig, all vanilloid receptor-1-immunoreactive cell bodies were choline acetyltransferase-immunopositive (100%) and showed some immunoreactivity to neurofilament proteins (NFP-200 kDa (79%) or triplet (10.8%)) or calretinin. Immunoreactive fibres in the secondary plexus co-localised with calcitonin gene-related peptide (CGRP) and with substance P, calretinin and synapsin I in the tertiary plexus. Subpopulations of cholinergic neurones including sensory, interneuronal and secretory neurones express vanilloid receptor-1. Co-localisation with substance P and calretinin in fibres suggests that vanilloid receptor-1 may be expressed by excitatory motor neurones. The association of vanilloid receptors with calcitonin gene-related peptide and synaptic protein in fibres implies a role for vanilloid receptors in neurotransmitter/neuropeptide release. Although it is likely that at least some of the vanilloid receptor-bearing fibres originate in immunopositive myenteric soma, the origin of all these fibres cannot be identified in the present study. [Copyright &y& Elsevier]