Capsaicin Desensitization Research Articles

Reassessment of capsaicin desensitization in the rodent spinal dorsal horn

Capsaicin activates primary afferent transient receptor potential vanilloid 1 (TRPV1) in the spinal dorsal horn and induces exaggerated glutamate release. This capsaicin action is followed by a lasting refractory state referred to as “capsaicin desensitization”, which is considered a presynaptic event. In this study, using whole-cell recordings and holographic photostimulation, we reassessed this notion by investigating presynaptic glutamate release and the postsynaptic glutamate response during capsaicin administration. We found that both presynaptic synchronous glutamate release and the postsynaptic glutamate response were largely attenuated in this refractory state; in contrast, asynchronous release was exaggerated. Further behavioral studies revealed a quick increase in the mechanical pain threshold with intrathecal capsaicin administration. Taken together, both presynaptic synchronous glutamate release and the postsynaptic response are downregulated during capsaicin desensitization, and this desensitization may transiently increase the pain threshold. Since both presynaptic synchronous release and postsynaptic glutamate responses are attenuated, the traditional electrophysiological evidence supporting capsaicin desensitization as a presynaptic event should be reassessed.

TRPV1 corneal neuralgia mutation: Enhanced pH response, bradykinin sensitization, and capsaicin desensitization

The factors that contribute to pain after nerve injury remain incompletely understood. Laser-assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) are common surgical techniques to correct refractive errors. After LASIK or PRK, a subset of patients suffers intense and persistent pain, of unknown origin, described by patients as feeling like shards of glass in their eye. Here, we evaluated a TRPV1 variant, p.V527M, found in a 49-y-old woman who developed corneal pain after LASIK and subsequent PRK enhancement, reporting an Ocular Surface Disease Index score of 100. Using patch-clamp and Ca2+ imaging, we found that the V527M mutation enhances the response to acidic pH. Increasing proton concentration induced a stronger leftward shift in the activation curve of V527M compared to WT, resulting in channel activity of the mutant in acidic pH at more physiological membrane potentials. Finally, comparing the responses to consecutive applications of different agonists, we found in V527M channels a reduced capsaicin-induced desensitization and increased sensitization by the arachidonic acid metabolite 12-hydroxyeicosatetraenoic acid (12-HETE). We hypothesize that the increased response in V527M channels to protons and enhanced sensitization by 12-HETE, two inflammatory mediators released in the cornea after tissue damage, may contribute to the pathogenesis of corneal neuralgia after refractive surgery.

Targeting TRPV1 for Cancer Pain Relief: Can It Work?

Chronic intractable pain affects a large proportion of cancer patients, especially those with metastatic bone disease. Blocking sensory afferents for cancer pain relief represents an attractive alternative to opioids and other drugs acting in the CNS in that sensory nerve blockers are not addictive and do not affect the mental state of the patient. A distinct subpopulation of sensory afferents expresses the capsaicin receptor TRPV1. Intrathecal resiniferatoxin, an ultrapotent capsaicin analog, ablates TRPV1-expressing nerve endings exposed to the cerebrospinal fluid, resulting in permanent analgesia in women with cervical cancer metastasis to the pelvic bone. High-dose capsaicin patches are effective pain killers in patients with chemotherapy-induced peripheral neuropathic pain. However, large gaps remain in our knowledge since the mechanisms by which cancer activates TRPV1 are essentially unknown. Most important, it is not clear whether or not sensory denervation mediated by TRPV1 agonists affects cancer progression. In a murine model of breast cancer, capsaicin desensitization was reported to accelerate progression. By contrast, desensitization mediated by resiniferatoxin was found to block melanoma growth. These observations imply that TRPV1 blockade for pain relief may be indicated for some cancers and contraindicated for others. In this review, we explore the current state of this field and compare the analgesic potential of TRPV1 antagonism and sensory afferent desensitization in cancer patients.

Inducible desensitization to capsaicin with repeated low-dose exposure in human volunteers

Responses to capsaicin are reduced following repeated exposure, a phenomenon known as capsaicin desensitization. Heavy consumers of chilies consistently report reduced oral burn relative to infrequent consumers, presumably due to chronic desensitization. However, the mechanism(s) underlying capsaicin desensitization remain poorly understood. We hypothesized that reduced response to capsaicin due to repeated oral exposure may result from a change in the expression of the capsaicin receptor (TRPV1) gene. To test this, we conducted two longitudinal desensitization studies in healthy human volunteers. In Study 1, 51 adults completed a 17-day capsaicin desensitization protocol. The study consisted of three in-person visits where they were asked to sample stimuli, including 3, 6, and 9 ppm capsaicin, and rate intensity on a general labeled magnitude scale (gLMS). Between days 3 & 17, participants rinsed at home with 6 ppm capsaicin (n = 31) or a control (n = 20) solution (20 uM sucrose octaccetate; SOA) twice a day. Before and after the oral exposure protocol, a clinician collected fungiform papillae. Participants randomized to the capsaicin rinse showed a statistically significant reduction in oral burn ratings that was not observed in controls, indicating repeated low-dose exposure can systematically induce desensitization. TRPV1 expression was not associated with reported capsaicin burn, and there was no evidence of a decrease in TRPV1 expression following capsaicin exposure. In Study 2, participants (n = 45) rinsed with 6 ppm capsaicin in a similar protocol, rating capsaicin, vanillyl butyl ether (VBE), cinnamaldehyde, ethanol, menthol, and sucrose on days 1, 3, & 17. Burn from capsaicin, VBE, cinnamaldehyde, and ethanol all showed a statistically significant change – capsaicin, VBE and cinnamaldehyde burn all dropped ∼20 %, and a larger reduction was seen for ethanol – while menthol cooling and sucrose sweetness did not change. Collectively, this suggests reductions in oral burn following chronic capsaicin exposure generalizes to other stimuli (i.e., cross desensitization) and this cannot be explained by a change in TRPV1 mRNA expression. More work is needed to elucidate the underlying mechanism for capsaicin desensitization in the oral cavity.

Optimization of capsaicin-induced dermal blood flow measurement by laser Doppler imaging in cynomolgus macaque.

Capsaicin is used in several areas of non-human primate research including allodynia and dermal blood flow (DBF). The capsaicin-induced DBF increase was measured using laser Doppler imaging (LDI), but this response is known to diminish upon repeated topical applications. Refinement of the experimental procedures could improve the rigor and reproducibility of the DBF migraine model. Optimal anatomical site in cynomolgus was determined, and conditions and experimental settings for DBF measurement using LDI were established. Then, two study design trial structures were compared. Medial thigh was the preferrable site, and an ethanol-Tween 20 formulation of capsaicin was desirable. A 1-week washout for contralateral side or 2-week washout for ipsilateral side was necessary to eradicate capsaicin desensitization. With the established technicality in DBF measurements in cynomolgus macaques, the capsaicin-induced DBF model may be utilized in translational medical research in developing migraine therapeutics.

Voiding behavior in awake unrestrained untethered spontaneously hypertensive and Wistar control rats.

The spontaneously hypertensive rat (SHR), a genetic model of high blood pressure, has also been studied as a potential model of overactive bladder. In vivo studies have confirmed the presence of surrogate markers of overactive bladder, including detrusor overactivity, increased urinary frequency, decreased bladder capacity and voided volume (VV), and afferent hypersensitivity to bladder irritation. However, these observations were during awake cystometry using implanted bladder catheters tethered to an infusion pump and artificially filled. We conducted experiments in awake unrestrained untethered age-matched female SHRs and Wistar rats to quantify naïve consumption and voiding behavior and the effect of capsaicin desensitization on consumption and voiding behavior. Food and water consumption, body weight, voiding frequency, and VV were recorded. Rats were placed in metabolism cages for 24 h, up to twice a week, from 17 to 37 wk of age. Compared with Wistar rats, SHRs exhibited decrease in VV and did not exhibit diurnal variation in VV between light and dark periods, suggesting that SHRs may have bladder hypersensitivity. Furthermore, SHRs may also have smaller bladder capacities, as they consumed less water, voided less volume (regardless of light cycle), and had equal urinary frequencies compared with age-matched Wistar rats. We detected no change in SHR voiding behavior following capsaicin desensitization, which was in contrast to a prior awake in vivo cystometry study describing increased VV and micturition interval in SHRs and suggests that C-fiber activity may not contribute to bladder hypersensitivity in SHRs.NEW & NOTEWORTHY We characterized the long-term (20 wk) voiding, defecation, and consumption behavior of age-matched spontaneously hypertensive and Wistar rats without the influence of anesthesia or catheters. Spontaneously hypertensive rats exhibited bladder hypersensitiviy that persisted for the 20-wk duration and was unaffected by capsacin desensitization.

The Mysteries of Capsaicin-Sensitive Afferents

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

Effects of intravesical prostaglandin E2 on bladder function are preserved in capsaicin-desensitized rats.

Prostaglandin E2 (PGE2) instilled into the bladder generates symptoms of urinary urgency in healthy women and reduces bladder capacity and urethral pressure in both humans and female rats. Systemic capsaicin desensitization, which causes degeneration of C-fibers, prevented PGE2-mediated reductions in bladder capacity, suggesting that PGE2 acts as an irritant (Maggi CA, Giuliani S, Conte B, Furio M, Santicioli P, Meli P, Gragnani L, Meli A. Eur J Pharmacol 145: 105-112, 1988). In the present study, we instilled PGE2 in female rats after capsaicin desensitization but without the hypogastric nerve transection that was conducted in the Maggi et al. study. One week after capsaicin injection (125 mg/kg sc), rats underwent cystometric and urethral perfusion testing under urethane anesthesia with saline and 100 µM PGE2. Similar to naïve rats, capsaicin-desensitized rats exhibited a reduction in bladder capacity from 1.23 ± 0.08 mL to 0.70 ± 0.10 mL (P = 0.002, n = 9), a reduction in urethral perfusion pressure from 19.3 ± 2.1 cmH2O to 10.9 ± 1.2 cmH2O (P = 0.004, n = 9), and a reduction in bladder compliance from 0.13 ± 0.020 mL/cmH2O to 0.090 ± 0.014 mL/cmH2O (P = 0.011, n = 9). Thus, changes in bladder function following the instillation of PGE2 were not dependent on capsaicin-sensitive pathways. Further, these results suggest that urethral relaxation/weakness and/or increased detrusor pressure as a result of decreased compliance may contribute to urinary urgency and highlight potential targets for new therapies for overactive bladder.

Capsaicin-Induced Skin Desensitization Differentially Affects A-Delta and C-Fiber-Mediated Heat Sensitivity.

Localized neuropathic pain can be relieved following the topical application of high-concentration capsaicin. This clinical effect is thought to be related to the temporary desensitization of capsaicin- and heat-sensitive epidermal nociceptors. The objective of the present study was to examine whether the changes in thermal sensitivity induced by high-concentration topical capsaicin can be explained entirely by desensitization of capsaicin-sensitive afferents. For this purpose, we characterized, in 20 healthy human volunteers, the time course and spatial extent of the changes in sensitivity to thermal stimuli preferentially activating heat-sensitive A-fiber nociceptors, heat-sensitive C-fiber afferents, and cool-sensitive A-fiber afferents. The volar forearm was treated with a high-concentration capsaicin patch for 1 h. Transient heat, warm and cold stimuli designed to activate Aδ- and C-fiber thermonociceptors, C-fiber warm receptors, and Aδ-fiber cold receptors were applied to the skin before and after treatment at days 1, 3, and 7. Reaction times, intensity ratings, and quality descriptors were collected. The stimuli were applied both within the capsaicin-treated skin and around the capsaicin-treated skin to map the changes in thermal sensitivity. We found that topical capsaicin selectively impairs heat sensitivity without any concomitant changes in cold sensitivity. Most interestingly, we observed a differential effect on the sensitivity to thermal inputs conveyed by Aδ- and C-fibers. Reduced sensitivity to Aδ-fiber-mediated heat was restricted to the capsaicin-treated skin, whereas reduced sensitivity to C-fiber-mediated heat extended well beyond the treated skin. Moreover, the time course of the reduced sensitivity to C-fiber-mediated input was more prolonged than the reduced sensitivity to Aδ-fiber-mediated input.

Effects of Cinnamaldehyde on Smooth Muscle Preparations

The effects of cinnamaldehyde (CNA), known as a transient receptor potential ankyrin 1 (TRPA1) agonist, on guinea-pig ileum and urinary bladder were studied in isolated organ experiments. Contractile effects were found to be present on both preparations. In the ileum, both cholinergic and purinergic (pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid tetrasodium salt-sensitive) mechanisms are involved; the TRPA1 antagonist A967079 (1 µmol/L) significantly reduced the response. The contractile response to CNA in the bladder, but not in the ileum, was significantly reduced by in vitro capsaicin desensitization. In the bladder A967079 or the TRPV1 antagonist, BCTC failed to reduce the response. A direct relaxation on the smooth muscle was detected in the precontracted ileum. In the precontracted urinary bladder, CNA also caused relaxation that was insensitive to capsaicin pretreatment. It is suggested that CNA excites the muscles of the bladder via activation of capsaicin-sensitive nerves; in the ileum, it may interact with TRPA1 located on tissue elements that initiate both purinergic and cholinergic mechanisms. The relaxant effects of CNA may be due to the direct inhibition of the smooth muscles.

Effect of lipopolysaccharide on the responsiveness of equine bronchial tissue

Recurrent airway obstruction (RAO) is a main characteristic of horses with severe equine asthma syndrome. The presence of bacterial lipopolysaccharide (LPS) in the airways of horses is thought to play a crucial role in the clinical expression of this disorder. This study pharmacologically characterized the effect of LPS on the responsiveness of equine bronchial tissue. Equine isolated bronchi were incubated overnight with LPS (0.1–100 ng/ml) and then stimulated by electrical field stimulation (EFS). The role of capsaicin sensitive-sensory nerves (capsaicin desensitization treatment), neurokinin-2 (NK2) receptors (blocked by GR159897), transient receptor potential vanilloid type 1 receptors (TRPV1; blocked by SB366791), and neurokinin A (NKA) were investigated. Untreated bronchi were used as control tissues. LPS (1 ng/ml) significantly increased the EFS-evoked contractility of equine bronchi compared with control tissues (+742 ± 123 mg; P < 0.001). At higher concentrations LPS induced desensitization to airways hyperresponsiveness (AHR; EC50: 5.9 ± 2.6 ng/ml). Capsaicin desensitization and GR159897 significantly prevented AHR induced by LPS at EFS1–50Hz (−197 ± 25%; P < 0.01). SB366791 inhibited AHR at very low EFS frequency (EFS1Hz −193 ± 29%; P < 0.01 vs. LPS-treated bronchi). LPS (1 ng/ml) significantly (P < 0.01) increased 3.7 ± 0.7 fold the release of NKA compared with control bronchi. LPS induces biphasic dysfunctional bronchial contractility due to the stimulation of capsaicin sensitive-sensory nerves, increased release of NKA, and activation of NK2 receptors, whereas TRPV1 receptors appear to play a marginal role in this response. The overnight challenge with low concentrations of LPS represents a suitable model to investigate pharmacological options that may be of value in the treatment of equine RAO.

Evidence for a novel, neurohumoral antinociceptive mechanism mediated by peripheral capsaicin-sensitive nociceptors in conscious rats

Stimulation of capsaicin-sensitive peripheral sensory nerve terminals induces remote anti-inflammatory effects throughout the body of anesthetized rats and guinea-pigs mediated by somatostatin. As somatostatin has also antinociceptive effects, the study aimed at investigating whether similar remote antinociceptive effects can be demonstrated in awake animals.In conscious rats, nociceptive nerve endings of the right hind paw decentralized by cutting the sciatic and saphenous nerves 18h before were chemically stimulated, and drop of the noxious heat threshold (heat hyperalgesia) induced by prior (18h before) plantar incision was measured on the contralateral, left hind paw using an increasing-temperature water bath. 18h after nerve transection, mustard oil-evoked plasma extravasation was not significantly reduced in the right hind paw as tested by in vivo fluorescence imaging. Applying agonist of either transient receptor potential vanilloid 1 (TRPV1) or transient receptor potential ankyrin 1 (TRPA1) receptor (capsaicin or mustard oil, respectively) to the nerve-transected paw inhibited the plantar incision-induced drop of the noxious heat threshold on the contralateral paw. The onset of these remote antihyperalgesic effects was 10–20min. A similar contralateral inhibitory effect of capsaicin or mustard oil treatment was observed on neuropathic mechanical hyperalgesia evoked by partial sciatic nerve injury 2days before nerve transection and measured by a Randall–Selitto apparatus. The remote thermal antihyperalgesic effect was prevented by chronic (5days) denervation or local capsaicin desensitization of the stimulated paw; reduced by intraperitoneally applied antagonist of somatostatin (cyclosomatostatin) or opioid receptors (naloxone). The response was mimicked by intraperitoneally applied somatostatin and associated with a 72±27% increase in plasma somatostatin-like immunoreactivity that was absent after chronic (5days) denervation.In conclusion, chemical activation of decentralized peripheral capsaicin-sensitive nociceptors evokes remote antihyperalgesic responses initiated outside the central nervous system and mediated by somatostatin and endogenous opioids.

Endovanilloids are potential activators of the trigeminovascular nocisensor complex.

BackgroundIn the dura mater encephali a significant population of trigeminal afferents coexpress the nociceptive ion channel transient receptor potential vanilloid type 1 (TRPV1) receptor and calcitonin gene-related peptide (CGRP). Release of CGRP serves the central transmission of sensory information, initiates local tissue reactions and may also sensitize the nociceptive pathway. To reveal the possible activation of meningeal TRPV1 receptors by endogenously synthetized agonists, the effects of arachidonylethanolamide (anandamide) and N-arachidonoyl-dopamine (NADA) were studied on dural vascular reactions and meningeal CGRP release.MethodsChanges in meningeal blood flow were measured with laser Doppler flowmetry in a rat open cranial window preparation following local dural applications of anandamide and NADA. The release of CGRP evoked by endovanilloids was measured with ELISA in an in vitro dura mater preparation.ResultsTopical application of NADA induced a significant dose-dependent increase in meningeal blood flow that was markedly inhibited by pretreatments with the TRPV1 antagonist capsazepine, the CGRP antagonist CGRP8–37, or by prior systemic capsaicin desensitization. Administration of anandamide resulted in minor increases in meningeal blood flow that was turned into vasoconstriction at the higher concentration. In the in vitro dura mater preparation NADA evoked a significant increase in CGRP release. Cannabinoid CB1 receptors of CGRP releasing nerve fibers seem to counteract the TRPV1 agonistic effect of anandamide in a dose-dependent fashion, a result which is confirmed by the facilitating effect of CB1 receptor inhibition on CGRP release and its reversing effect on the blood flow.ConclusionsThe present findings demonstrate that endovanilloids are potential activators of meningeal TRPV1 receptors and, consequently the trigeminovascular nocisensor complex that may play a significant role in the pathophysiology of headaches. The results also suggest that prejunctional CB1 receptors may modulate meningeal vascular responses.

The TRPA1 Activator Allyl Isothiocyanate (AITC) Contracts Human Jejunal Muscle: Pharmacological Analysis.

The contractile effect of AITC (300 μM) on human jejunal longitudinal strips was inhibited by the TRPA1 antagonist HC 030031 and atropine or scopolamine, but was insensitive to tetrodotoxin, purinoceptor antagonists or capsaicin desensitization. It is concluded that TRPA1 activation stimulates a cholinergic mechanism in a tetrodotoxin-resistant manner.

Ureteral relaxation through calcitonin gene-related peptide release from sensory nerve terminals by hypotonic solution.

To evaluate the influence of hypotonic solutions on ureteral relaxation mediated by the release of calcitonin gene-related peptide from intramural sensory nerve endings. Urine osmolarity of Sprague-Dawley rats drinking water low in salt content (Fiuggi water) or a reference water for 7 days was measured. Release of calcitonin gene-related peptide-like immunoreactivity from slices of rat ureter and urinary bladder by hypotonic solutions was assessed by an immunometric assay. The mechanism through which hypotonic solutions inhibit neurokinin A-induced phasic contractions of isolated rat ureters was evaluated by organ bath studies. A 7-day consumption of Fiuggi water in rats reduced urine osmolarity by ~40%. Exposure to hypotonic solutions released calcitonin gene-related peptide-like immunoreactivity from slices of rat ureter. This response was abated in a calcium-free medium, after capsaicin desensitization, and in the presence of the unselective transient receptor potential channel antagonist, ruthenium red. Exposure of isolated rat ureteral preparations to a hypotonic solution inhibited neurokinin A-evoked phasic contraction. This response was attenuated by capsaicin desensitization and in the presence of the calcitonin gene-related peptide receptor antagonist, calcitonin gene-related peptide8-37 . Transient receptor potential vanilloid 1 or transient receptor potential vanilloid 4 antagonists did not affect the neurogenic and calcitonin gene-related peptide-dependent relaxation. Present data show that hypotonic solution evokes calcitonin gene-related peptide release from capsaicin-sensitive intramural sensory nerves, thus inhibiting ureteral contractility, through a transient receptor potential-dependent mechanism. However, this mechanism does not involve transient receptor potential vanilloid 1 or transient receptor potential vanilloid 4. Future studies with appropriate in vivo models should investigate the hypothesis that hypostenuric urine diffusing into the ureteral tissue might favor ureteral relaxation through this novel mechanism.

Contribution of sensory nerves to LPS-induced hyperresponsiveness of human isolated bronchi

Bacterial lipopolysaccharide (LPS) can induce bronchial hyperresponsiveness (BHR), but the underlying mechanisms remain to be determined. Here, the possible contribution of sensory nerves to LPS-induced BHR was examined in human isolated bronchi to pharmacologically identify the mechanisms underlying this phenomenon. Human isolated bronchial tone was induced by electrical field stimulation (EFS). The responses of airways to LPS, with or without capsaicin desensitization or thiorphan treatment were studied and the transient receptor potential vanilloid type 1 (TRPV1) expression was assessed. We performed similar experiments in the presence of a TRPV1 or a neurokinin (NK) 2 receptor antagonist using SB366791 and GR159897, respectively. LPS increased (≃2.3-fold, P<0.001) the contraction induced by EFS, compared to control tissues. Acute administration of capsaicin enhanced (≃2.3-fold, P<0.001) the EFS-mediated contraction, but did not potentiate the effect of LPS. Thiorphan increased (≃1.3-fold, P<0.05) the contractile response of LPS treated tissues and, at lower frequencies, it enhanced (≃1.7-fold, P<0.001) the capsaicin-induced contraction. In capsaicin-desensitized bronchi, LPS did not modify (P>0.05) the EFS contractile response, nor after treatment with thiorphan. Capsaicin desensitization reduced (≃0.4-fold, P<0.001) the LPS-induced BHR. SB366791 and GR159897 prevented the LPS-induced BHR and the release of NKA. LPS increased (+85.3±9.5%, P<0.01) the surface membrane expression of TRPV1 in parasympathetic ganglia. Our results demonstrate the involvement of capsaicin-sensitive sensory nerves and neutral endopeptidases in LPS-induced BHR of the human bronchi, associated with an upregulation of TRPV1 and release of NKA.

The effects of juvenile capsaicin desensitization in rats: Behavioral impairments

Capsaicin desensitization leads to behavioral changes, some of which are related to schizophrenia, but investigations into these effects have been scarce. The goal of this study was to characterize the consequences of juvenile capsaicin desensitization on different functions: acute and inflammation-induced thermal and mechanical sensitivity, urinary bladder capacity and thermoregulation, and also on the potentially schizophrenia-related impairments in sensory-motor gating, motor activity and cognitive functioning.Male Wistar rats desensitized with increasing doses of subcutaneous capsaicin after weaning were investigated.Heat and mechanical pain sensitivity did not change significantly; however, morphine produced a prolonged decrease in the nociceptive response to inflammation in desensitized animals. Ultrasound examination of the bladder revealed enhanced bladder volume in treated animals.Capsaicin-treated animals had higher body temperature at 22°C in both dark and light periods, and they also showed prolonged hyperthermia in new environmental circumstances. Warm environment induced a profound impairment of thermoregulation in desensitized animals. The treated animals also showed higher levels of activity during the active phase and at both cool and warm temperatures.The amplitude of the responses to auditory stimuli and prepulse inhibition did not differ between the two groups, but the desensitized animals showed learning impairments in the novel object recognition test.These results suggest that juvenile capsaicin desensitization leads to sustained changes in several functions that may be related to schizophrenia. We propose that capsaicin desensitization, together with other interventions, may lead to an improved chronic animal model of schizophrenia.

Nitro-oleic acid desensitizes TRPA1 and TRPV1 agonist responses in adult rat DRG neurons

Nitro-oleic acid (OA-NO2), an electrophilic fatty acid nitroalkene byproduct of redox reactions, activates transient receptor potential ion channels (TRPA1 and TRPV1) in primary sensory neurons. To test the possibility that signaling actions of OA-NO2 might modulate TRP channels, we examined: (1) interactions between OA-NO2 and other agonists for TRPA1 (allyl-isothiocyanate, AITC) and TRPV1 (capsaicin) in rat dissociated dorsal root ganglion cells using Ca2+ imaging and patch clamp techniques and (2) interactions between these agents on sensory nerves in the rat hindpaw. Ca2+ imaging revealed that brief application (15–30s) of each of the three agonists induced homologous desensitization. Heterologous desensitization also occurred when one agonist was applied prior to another agonist. OA-NO2 was more effective in desensitizing the response to AITC than the response to capsaicin. Prolonged exposure to OA-NO2 (20min) had a similar desensitizing effect on AITC or capsaicin. Homologous and heterologous desensitizations were also demonstrated with patch clamp recording. Deltamethrin, a phosphatase inhibitor, reduced the capsaicin or AITC induced desensitization of OA-NO2 but did not suppress the OA-NO2 induced desensitization of AITC or capsaicin, indicating that heterologous desensitization induced by either capsaicin or AITC occurs by a different mechanism than the desensitization produced by OA-NO2. Subcutaneous injection of OA-NO2 (2.5mM, 35μl) into a rat hindpaw induced delayed and prolonged nociceptive behavior. Homologous desensitization occurred with AITC and capsaicin when applied at 15minute intervals, but did not occur with OA-NO2 when applied at a 30min interval. Pretreatment with OA-NO2 reduced AITC-evoked nociceptive behaviors but did not alter capsaicin responses. These results raise the possibility that OA-NO2 might be useful clinically to reduce neurogenic inflammation and certain types of painful sensations by desensitizing TRPA1 expressing nociceptive afferents.

The Contractile Effect of Anandamide in the Guinea‐Pig Small Intestine is Mediated by Prostanoids but not TRPV1 Receptors or Capsaicin‐Sensitive Nerves

Although exogenous and endogenous cannabinoid receptor agonists have well-documented inhibitory effects on gastrointestinal motility, a TRPV1 receptor-mediated excitatory action of anandamide (arachidonoyl ethanolamide, AEA) in the guinea-pig ileum strip has also been described. We used in vitro capsaicin desensitization for assessing the possible participation of sensory neurons in the contractile effect of anandamide on the guinea-pig whole ileum, as well as autonomic drugs and a cyclooxygenase inhibitor for characterizing this response. Isolated organ experiments were used with isotonic recording. Contractions induced by anandamide (1 or 10 μM) were strongly inhibited by tetrodotoxin, indomethacin or atropine plus a tachykinin NK(1) receptor antagonist, but weakly to moderately reduced by atropine alone and partly diminished by the fatty acid amide hydrolase inhibitor URB 597. Neither capsaicin pre-treatment nor the TRPV1 receptor antagonist BCTC, the ganglionic blocking drug hexamethonium or cannabinoid (CB1 or CB2 ) receptor antagonists, influenced the effect of anandamide. It is concluded that the capsaicin-insensitive, neuronal excitatory effect of anandamide in the intestine is most probably mediated by cyclooxygenase products. Such a mechanism may also play a role at other sites in the mammalian body.

N-Glycosylation Determines Ionic Permeability and Desensitization of the TRPV1 Capsaicin Receptor

The balance of glycosylation and deglycosylation of ion channels can markedly influence their function and regulation. However, the functional importance of glycosylation of the TRPV1 receptor, a key sensor of pain-sensing nerves, is not well understood, and whether TRPV1 is glycosylated in neurons is unclear. We report that TRPV1 is N-glycosylated and that N-glycosylation is a major determinant of capsaicin-evoked desensitization and ionic permeability. Both N-glycosylated and unglycosylated TRPV1 was detected in extracts of peripheral sensory nerves by Western blotting. TRPV1 expressed in HEK-293 cells exhibited various degrees of glycosylation. A mutant of asparagine 604 (N604T) was not glycosylated but did not alter plasma membrane expression of TRPV1. Capsaicin-evoked increases in intracellular calcium ([Ca(2+)](i)) were sustained in wild-type TRPV1 HEK-293 cells but were rapidly desensitized in N604T TRPV1 cells. There was marked cell-to-cell variability in capsaicin responses and desensitization between individual cells expressing wild-type TRPV1 but highly uniform responses in cells expressing N604T TRPV1, consistent with variable levels of glycosylation of the wild-type channel. These differences were also apparent when wild-type or N604T TRPV1-GFP fusion proteins were expressed in neurons from trpv1(-/-) mice. Capsaicin evoked a marked, concentration-dependent increase in uptake of the large cationic dye YO-PRO-1 in cells expressing wild-type TRPV1, indicative of loss of ion selectivity, that was completely absent in cells expressing N604T TRPV1. Thus, TRPV1 is variably N-glycosylated and glycosylation is a key determinant of capsaicin regulation of TRPV1 desensitization and permeability. Our findings suggest that physiological or pathological alterations in TRPV1 glycosylation would affect TRPV1 function and pain transmission.