Capsaicin-sensitive Primary Afferent Neurons Research Articles

Functional roles of capsaicin-sensitive intrinsic neural circuit in the regulation of esophageal peristalsis in rats: in vivo studies using a novel method

A well-developed myenteric plexus exists in the esophagus composed of striated muscle layers, but its functional role in controlling peristaltic movements remains to be clarified. The purpose of this study was to clarify the role of a local neural reflex consisting of capsaicin-sensitive primary afferent neurons and intrinsic neurons in esophageal peristalsis. We firstly devised a method to measure peristaltic movement of esophagus in vivo in rats. Rats were anesthetized with urethane, and esophageal intraluminal pressure and propelled intraluminal liquid volume were recorded. In the experimental system, an intraluminal pressure stimulus evoked periodic changes in intraluminal pressure of the esophagus, which were consistently accompanied by intraluminal liquid propulsion. Bilateral vagotomy abolished changes in intraluminal pressure as well as liquid propulsion. These results indicate that the novel method is appropriate for inducing peristalsis in the esophagus composed of striated muscles. Then, by using the method, we examined functional roles of the local reflex in esophageal peristalsis. For that purpose, we used rats in which capsaicin-sensitive neurons had been destroyed. The esophagus of capsaicin-treated rats showed a multiphasic rise in intraluminal pressure, which may due to noncoordinated contractions of esophageal muscles, whereas a monophasic response was observed in the intact rat esophagus. In addition, destruction of capsaicin-sensitive neurons increased the propelled liquid volume and lowered the pressure threshold for initiating peristalsis. These results suggest that the local neural reflex consisting of capsaicin-sensitive neurons and intrinsic neurons contributes to coordination of peristalsis and suppresses mechanosensory function of vagal afferents in the esophagus.

Propagation of the Electrical Impulse in Reversible Unilateral Ureteral Obstruction as Determined at High Electrophysiological Resolution

We investigated the propagation of electrical impulses in a reversible, complete or partial unilateral ureteral obstruction model in vivo. In Wistar rats the left mid ureter was completely (8) or partially (7) occluded and released after 24 hours. We recorded electrical activity of the left and right ureter before, during and after obstruction at different stages up to 2 weeks after obstruction using a high resolution, 64 extracellular electrode probe. Complete obstruction in the left proximal ureter caused an immediate increase in frequency from a mean ± SEM of 14.8 ± 1.3 to 18.6 ± 1.7 per minute (p <0.05), followed by a 1.4 ± 0.9 per minute decrease (p <0.001). Within the first 2 days after reversal velocity gradually decreased from 1.82 ± 0.12 to 0.79 ± 0.17 cm per second (p <0.001). Release of obstruction gradually restored frequency and velocity, which returned to baseline at 2 weeks. Generally the alterations in rats with complete and partial obstruction were similar but they were less marked in those with partial obstruction. Distal to the obstruction site the impulses disappeared (38%) or propagated retrograde (43%) at some stage in the post-obstruction period. These abnormal impulse propagations also gradually disappeared in the post-obstruction stage. After complete or partial ureteral obstruction there were immediate, significant changes in the propagation of electrical impulses in the proximal and distal left ureter, which were generally less marked after partial than after complete obstruction. Reversal of obstruction resulted in the gradual disappearance of this abnormality in 2 weeks.

The role of the transient receptor potential (TRP) superfamily of cation‐selective channels in the management of the overactive bladder

• The pathophysiology of lower urinary tract symptoms (LUTS), detrusor overactivity (DO), and the overactive bladder (OAB) syndrome is multifactorial and remains poorly understood. • The transient receptor potential (TRP) channel superfamily has been shown to be involved in nociception and mechanosensory transduction in various organ systems, and studies of the LUT have indicated that several TRP channels, including TRPV1, TRPV2, TRPV4, TRPM8, and TRPA1, are expressed in the bladder, and may act as sensors of stretch and/or chemical irritation. • However, the roles of these individual channels for normal LUT function and in LUTS/DO/OAB, have not been established. • TRPV1 is the channel best investigated. It is widely distributed in LUT structures, but despite extensive information on morphology and function in animal models, the role of this channel in normal human bladder function is still controversial. Conversely, its role in the pathophysiology and treatment of particularly neurogenic DO is well established. • TRPV1 is co-expressed with TRPA1, and TRPA1 is known to be present on capsaicin-sensitive primary sensory neurones. Activation of this channel can induce DO in animal models. • TRPV4 is a Ca(2+)-permeable stretch-activated cation channel, involved in stretch-induced ATP release, and TRPV4-deficient mice exhibit abnormal frequencies of voiding and non-voiding contractions in cystometric experiments. • TRPM8 is a cool receptor expressed in the urothelium and suburothelial sensory fibres. It has been implicated in the bladder-cooling reflex and in idiopathic DO. • The occurrence of other members of the TRP superfamily in the LUT has been reported, but information on their effects on LUT functions is scarce. There seem to be several links between activation of different members of the TRP superfamily and LUTS/DO/OAB, and further exploration of the involvement of these channels in LUT function, normally and in dysfunction, may be rewarding.

Phenotypes and peripheral mechanisms underlying inflammatory pain-related behaviors induced by BmK I, a modulator of sodium channels

The integrated mechanisms of dynamic signaling of sodium channels involved in clinical pain are still not yet clear. In this study, a new rat inflammatory pain model was developed by using the unilateral intraplantar injection of BmK I, a receptor site 3-specific modulator of sodium channels from the venom of scorpion Buthus martensi Karsch (BmK). It was found that BmK I could induce several kinds of inflammatory pain-related behaviors including spontaneous pain companied with unique episodic paroxysms, primary thermal hypersensitivity, and mirror-image mechanical hypersensitivity with different time course of development, which could be suppressed by morphine, indomethacin, or bupivacaine to a different extent. The dramatic attenuation by pretreatment with resiniferatoxin (RTX), an ultrapotent analog of capsaicin, on BmK I-induced pain-related behaviors, paw edema, and spinal L4–L5 c-Fos expression demonstrated that capsaicin-sensitive primary afferent neurons played important roles in pain induced by BmK I. Furthermore, the electrophysiological recordings showed that BmK I persistently increased whole-cell and tetrodotoxin-resistant (TTX-R) peak sodium currents and significantly delayed the inactivation phase of whole-cell sodium currents but could not enhance capsaicin-evoked inward currents, in acute isolated small dorsal root ganglion neurons of rat. The results strongly suggested that the dynamic modulation of BmK I on sodium channels located in peripheral primary afferent neurons, especially in capsaicin-sensitive neurons, mediated pain sensation. Thus, BmK I may be a valuable pharmacological tool to understand the sodium channel-involved pain mechanisms.

The Role of Capsaicin in Spontaneous Pacemaking Activity in Gastrointestinal Tract

Interstitial cells of Cajal (ICCs) are the pacemaking cells in the gastrointestinal (GI) tracts that generate the rhythmic oscillations in the membrane potential which were known as slow waves recorded in native smooth muscle tissue preparations in the past. Auto-rhythmicity in GI tracts is an exclusive property of ICCs and the spread of slow waves through GI tracts occurs electrically, and not by the active regeneration properties of ICCs and smooth muscles. Pacemaking ICCs are organized into electrically coupled networks within discrete areas of the stomach, small intestine, and colon.1,2 ICCs were originally identified by Santiago Ramon y Cajal and were characterized by morphological criteria until the discovery that these cells express the receptor tyrosine kinase Kit.3 Subsequent studies determined that Kit immunoreactivity in the muscularis propria of the GI tract is restricted to 2 cell types: mast cells4 and ICCs. Kit-positive ICCs are distributed throughout the GI tract as well as in other smooth muscle tissues.5 All regions of the GI tract contain ICCs but the location within the muscularis propria varies according to region or species.6 In the myenteric plexus region, ICCs form a network between the muscle layers forming a mesh around the ganglia and are considered as major pacemaker cells. A network of deep muscular plexus ICCs is present in the small intestine between the inner and outer circular muscle layers.6 In the colon and parts of the gastric antrum, submuscular ICCs are located outside the circular muscle layer.7 Intramuscular ICCs are distributed through the longitudinal and circular muscle layers and are considered to be involved in neurotransmission from nerve terminals. Septal ICCs are found between the fascicles of muscle in humans and other large species. These can be considered as one type of intramuscular ICCs.8 Stellate, subserosal ICCs are observed on the boundary between the longitudinal muscle and the serosa in the colon of mice.9 Capsaicin (8-methyl-N-vanillyl-6-ninenamide) activates afferent fiber endings and capsaicin sensitive primary afferent neurons participate in the regulation of GI motility.10 Furthermore, capsaicin induces contraction in the guinea-pig ileum11 and relaxation in human small and large intestines.10 Namely, it is known that capsaicin may have excitatory action on animal GI motility but inhibitory on human. However, information about the effect of capsaicin on human GI motility is somewhat controversial. Some reports showed the oral administration of capsaicin to fail in showing a significant effect on GI motility in human12-14 but there are reports that capsaicin could contribute to protection of the esophagus and could promote retention of the irritant in the stomach and faster transit through the small bowel.15 However, there are no reports on the direct effect of the capsaicin on ICCs that have important roles for regulation of GI motility as pacemaker cells. In this issue of the Journal of Neurogastroenterology and Motility, Choi et al examined the role of capsaicin on ICC in mouse small intestine. They showed that capsaicin inhibited the pacemaker activity of ICC dose-dependently. The effect of capsaicin, however, is not through the transient receptor potential of the vanilloid type 1 (TRPV1) channel because capsazepine did not block the effect of capsaicin. Consistently with this result, they also showed in their previous study that TRPV1-immunopositive cells were not co-localized with c-Kit positive tissue layer.16 The effect of capsaicin was not through nitric oxide release from ICCs, because there was no influence on capsaicin-induced action by L-NAME, an inhibitor of nitric oxide synthase in their study. Choi et al. proposed that further studies are needed to investigate the direct effect of capsaicin on pacemaker ion channels like TRPC4 or TRPM7 responsible for pacemaking activity in ICCs.17 Interestingly, when the action of capsaicin was examined in the intracellular calcium oscillation, capsaicin completely abolished the calcium oscillation in ICCs. Recently, 3 research groups showed simultaneously the calcium activated Cl channels to be required for pacemaking activity using TMEM16A knockout mice.18-20 Capsaicin might inhibit pacemaking activity of ICCs through the decreased activity of calcium activated Cl channels due to completely abolished calcium oscillations by application of capsaicin to ICCs. The effect of capsaicin seems more complicated because capsaicin acts on ICCs as well as afferent nerve endings of GI tract. In the future, more sophisticated studies are needed to clarify the role of capsaicin in GI tract considering ICCs and species differences.

Role of Capsaicin-Sensitive Primary Afferent Neurons and Non-protein Sulphydryl Groups on Gastroprotective Effect of Amifostine Against Ethanol-Induced Gastric Damage in Rats

Amifostine has been widely tested as a cytoprotective agent against a number of aggressors in different organs. Recently, a gastroprotective effect was observed for this drug in a model of indomethacin-induced gastric injury. Our objective was to investigate the effect of amifostine on ethanol-induced gastric injury and the role played in this mechanism by afferent sensory neurons, non-protein sulfhydryl groups, nitric oxide, ATP-sensitive potassium channels, and cyclooxygenase-2. Rats were treated with amifostine (22.5, 45, 90, or 180 mg/kg, PO or SC). After 30 min, the rats received absolute ethanol (5 ml kg(-1), PO). One hour later, gastric damage was quantified with a planimeter. Samples from the stomach were also taken for histopathological assessment and for assays of non-protein sulfhydryl groups. The other groups were pretreated with L-NAME (10 mg kg(-1), IP), glibenclamide (10 mg kg(-1), PO), or celecoxib (10 mg kg(-1), PO). After 30 min, the animals were given amifostine (90 mg kg(-1), PO or SC), followed 30 min later by gavage with absolute ethanol (5 ml kg(-1)). Other rats were desensitized with capsaicin (125 mg kg(-1), SC) 8 days prior to amifostine treatment. Amifostine administration PO and SC significantly and dose-dependently reduced ethanol-induced macroscopic and microscopic gastric damage by restoring glutathione levels in the stomach mucosa. Amifostine-promoted gastroprotection against ethanol-induced stomach injury was reversed by pretreatment with neurotoxic doses of capsaicin, but not by L-NAME, glibenclamide, or celecoxib. Amifostine protects against ethanol-induced gastric injury by increasing glutathione levels and stimulating the afferent sensory neurons in the stomach.

Hydrogen Sulfide Prevents Ethanol-Induced Gastric Damage in Mice: Role of ATP-Sensitive Potassium Channels and Capsaicin-Sensitive Primary Afferent Neurons

The aim of this study was to evaluate the protective effect of hydrogen sulfide (H(2)S) on ethanol-induced gastric lesions in mice and the influence of ATP-sensitive potassium (K(ATP)) channels, capsaicin-sensitive sensory afferent neurons, and transient receptor potential vanilloid (TRPV) 1 receptors on such an effect. Saline and L-cysteine alone or with propargylglycine, sodium hydrogen sulfide (NaHS), or Lawesson's reagent were administrated for testing purposes. For other experiments, mice were pretreated with glibenclamide, neurotoxic doses of capsaicin, or capsazepine. Afterward, mice received L-cysteine, NaHS, or Lawesson's reagent. After 30 min, 50% ethanol was administrated by gavage. After 1 h, mice were sacrificed, and gastric damage was evaluated by macroscopic and microscopic analyses. L-cysteine, NaHS, and Lawesson's reagent treatment prevented ethanol-induced macroscopic and microscopic gastric damage in a dose-dependent manner. Administration of propargylglycine, an inhibitor of endogenous H(2)S synthesis, reversed gastric protection induced by L-cysteine. Glibenclamide reversed L-cysteine, NaHS, or Lawesson's reagent gastroprotective effects against ethanol-induced macroscopic damage in a dose-dependent manner. Chemical ablation of sensory afferent neurons by capsaicin reversed gastroprotective effects of L-cysteine or H(2)S donors (NaHS or Lawesson's reagent) in ethanol-induced macroscopic gastric damage. Likewise, in the presence of the TRPV1 antagonist capsazepine, the gastroprotective effects of L-cysteine, NaHS, or Lawesson's reagent were also abolished. Our results suggest that H(2)S prevents ethanol-induced gastric damage. Although there are many mechanisms through which this effect can occur, our data support the hypothesis that the activation of K(ATP) channels and afferent neurons/TRPV1 receptors is of primary importance.

TRP family proteins in the lower urinary tract: translating basic science into new clinical prospective.

The lower urinary tract (LUT) is densely innervated by capsaicin-sensitive primary afferent neurons, a sub set of sensory nerves, in a number of species including humans. These fibers exhibit both a sensory (afferent) function, including the regulation of the micturition reflex and the perception of pain, and an 'efferent' function, involved in the detrusor smooth muscle contractility and plasma protein extravasation. The discovery of specific binding sites for capsaicin, the pungent ingredient of red chilli, initiated a rush that ended up with the cloning of the 'vanilloid receptor', which belongs to the TRP (transient receptor potential) family. Here we reviewed the knowledge about the presumable functions of TRP family proteins in the LUT as regulators of bladder reflex activity, pain perception and cell differentiation. This review will focus on experimental evidence and promising clinical applications of targeting these proteins for the treatment of detrusor overactivity and bladder pain syndrome. As TRP receptor ligands may promote cellular death, and inhibit the growth of normal and neoplastic cells, the translation of basic science evidence into new clinical prospective for bladder and prostate cancer will be shown.

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

Our previous finding, that the capsaicin- and KCl-induced Ca2+-dependent production of the intra- and intercellular signaling molecule N-arachidonoyl ethanolamine (anandamide) in cultured primary sensory neurons could be abolished and reduced by ∼2/3 by capsaicin-induced degeneration of capsaicin-sensitive neurons, respectively suggests that a major sub-population of capsaicin-sensitive cells together with a group of non-capsaicin-sensitive cells should express enzymes involved in Ca2+-dependent anandamide synthesis. N-acyl phosphotidylethanolamine phospholipase D (NAPE-PLD) is known to be involved in Ca2+-dependent anandamide production. Hence, here, we used reverse transcriptase and quantitative real time polymerase chain reaction to study NAPE-PLD expression in dorsal root ganglia and to clarify the sub-population of cells expressing this enzyme. Cultures prepared from mouse dorsal root ganglia were grown either in the absence or presence of the neurotoxin, capsaicin (10 μM) overnight. We report, that NAPE-PLD is expressed both in dorsal root ganglia and cultures prepared from dorsal root ganglia and grown in the absence of capsaicin. Furthermore, we also report that capsaicin application downregulates the expression of NAPE-PLD as well as the capsaicin receptor, transient receptor potential vanilloid type 1 ion channel, by about 70% in the cultures prepared from dorsal root ganglia. These findings indicate that a major sub-population of capsaicin-sensitive primary sensory neurons expresses NAPE-PLD, and suggest that NAPE-PLD is expressed predominantly by capsaicin-sensitive neurons in dorsal root ganglia. These data also suggest that NAPE-PLD might be a target to control the activity and excitability of a major sub-population of nociceptive primary sensory neurons.

Impaired Basal Thermal Homeostasis in Rats Lacking Capsaicin-sensitive Peripheral Small Sensory Neurons

We studied the effects of selective loss of capsaicin-sensitive primary sensory neurons on thermosensation and thermoregulation in rats. Neonatal capsaicin treatment in rats caused a remarkable decrease in the number of small-diameter neurons in the dorsal root ganglion (DRG) compared with their number in the control rats. Gene expression analysis for various thermo-sensitive transient receptor potential (TRP) channels indicated marked reductions in the mRNA levels of TRPV1 (70%), TRPM8 (46%) and TRPA1 (64%), but not of TRPV2, in the DRG of capsaicin-treated rats compared with those in the control rats. In addition to the heat and cold insensitivity, capsaicin-treated rats showed lower rectal core temperature, higher skin temperature and decreased sensitivity to ambient temperature alteration under normal housing at room temperature, suggesting impaired thermosensation and change in thermoregulation in the rats. Uncoupling protein 1 (UCP1) expression and the thermogenic ability in brown adipose tissues were attenuated in the capsaicin-treated rats. These results indicate a critical role of capsaicin-sensitive sensory neurons in both heat and cool sensation and hence in basal thermal homeostasis, which is balanced by heat release and production including UCP1 thermogenesis, following sensation of the ambient temperature.

Hypoglycemic detection at the portal vein is mediated by capsaicin-sensitive primary sensory neurons

To elucidate the type of spinal afferent involved in hypoglycemic detection at the portal vein, we considered the potential role of capsaicin-sensitive primary sensory neurons. Specifically, we examined the effect of capsaicin-induced ablation of portal vein afferents on the sympathoadrenal response to hypoglycemia. Under anesthesia, the portal vein was isolated in rats and either capsaicin (CAP) or the vehicle (CON) solution applied topically. During the same surgery, the carotid artery (sampling) and jugular vein (infusion) were cannulated. One week later, all animals underwent a hyperinsulinemic hypoglycemic clamp, with glucose (variable) and insulin (25 mU x kg(-1) x min(-1)) infused via the jugular vein. Systemic hypoglycemia (2.76 +/- 0.05 mM) was induced by minute 75 and sustained until minute 105. By design, no significant differences were observed in arterial glucose or insulin concentrations between groups. When hypoglycemia was induced in CON, the plasma epinephrine concentration increased from 0.67 +/- 0.05 nM at basal to 36.15 +/- 2.32 nM by minute 105. Compared with CON, CAP animals demonstrated an 80% suppression in epinephrine levels by minute 105, 7.11 +/- 0.55 nM (P < 0.001). A similar response to hypoglycemia was observed for norepinephrine, with CAP values suppressed by 48% compared with CON. Immunohistochemical analysis of the portal vein revealed an 85% decrease in the number of calcitonin gene-related peptide-reactive nerve fibers following capsaicin-induced ablation. That the suppression in the sympathoadrenal response was comparable to our previous findings for total denervation of the portal vein indicates that hypoglycemic detection at the portal vein is mediated by capsaicin-sensitive primary sensory neurons.

Intrinsic sensory deprivation induced by neonatal capsaicin treatment induces changes in rat brain and behaviour of possible relevance to schizophrenia

Schizophrenia is considered to be a neurodevelopmental disorder with origins in the prenatal or neonatal period. Brains from subjects with schizophrenia have enlarged ventricles, reduced cortical thickness (CT) and increased neuronal density in the prefrontal cortex compared with those from normal subjects. Subjects with schizophrenia have reduced pain sensitivity and niacin skin flare responses, suggesting that capsaicin-sensitive primary afferent neurons might be abnormal in schizophrenia. This study tested the hypothesis that intrinsic somatosensory deprivation, induced by neonatal capsaicin treatment, causes changes in the brains of rats similar to those found in schizophrenia. Wistar rats were treated with capsaicin, 50 mg kg(-1) subcutaneously, or vehicle (control) at 24-36 h of life. At 5-7 weeks behavioural observations were made, and brains removed, fixed and sectioned. The mean body weight of capsaicin-treated rats was not significantly different from control, but the mean brain weight of male, but not female, rats, was significantly lower than control. Capsaicin-treated rats were hyperactive compared with controls. The hyperactivity was abolished by haloperidol. Coronal brain sections of capsaicin-treated rats had smaller cross-sectional areas, reduced CT, larger ventricles and aqueduct, smaller hippocampal area and reduced corpus callosum thickness, than brain sections from control rats. Neuronal density was increased in several cortical areas and the caudate putamen, but not in the visual cortex. It is concluded that neonatal capsaicin treatment of rats produces brain changes that are similar to those found in brains of subjects with schizophrenia.

Dopamine type 2 receptor expression and function in rodent sensory neurons projecting to the airways

Agonists of the dopamine receptors have been demonstrated to have bronchodilatory properties in pathologically constricted airways. The mechanism by which these agonists induce bronchodilatation is thought to involve airway sensory nerves. In this study, the expression and function of dopamine D(2) receptor were examined in sensory ganglia supplying the airways. Neuronal dopamine D(2) receptor mRNA expression was demonstrated by single-cell RT-PCR following laser-assisted microdissection. The projection of the neurons to the airways was confirmed by retrograde neuronal labeling. In functional studies, dopamine D(2) receptor agonists (AR-C65116AB and ropinirole) inhibited intraneuronal calcium mobilization in rat capsaicin-sensitive primary sensory neurons and capsaicin-induced plasma extravasation in the rat trachea. Our results provide support to the hypothesis that dopamine D(2) receptor activation inhibits neurogenic inflammation and proinflammatory reflex responses.

The vanilloid receptor and hypertension1

Mammalian transient receptor potential (TRP) channels consist of six related protein sub-families that are involved in a variety of pathophysiological function, and disease development. The TRPV1 channel, a member of the TRPV sub-family, is identified by expression cloning using the "hot" pepper-derived vanilloid compound capsaicin as a ligand. Therefore, TRPV1 is also referred as the vanilloid receptor (VR1) or the capsaicin receptor. VR1 is mainly expressed in a subpopulation of primary afferent neurons that project to cardiovascular and renal tissues. These capsaicin-sensitive primary afferent neurons are not only involved in the perception of somatic and visceral pain, but also have a "sensory-effector" function. Regarding the latter, these neurons release stored neuropeptides through a calcium-dependent mechanism via the binding of capsaicin to VR1. The most studied sensory neuropeptides are calcitonin gene-related peptide (CGRP) and substance P (SP), which are potent vasodilators and natriuretic/diuretic factors. Recent evidence using the model of neonatal degeneration of capsaicin-sensitive sensory nerves revealed novel mechanisms that underlie increased salt sensitivity and several experimental models of hypertension. These mechanisms include insufficient suppression of plasma renin activity and plasma aldosterone levels subsequent to salt loading, enhancement of sympathoexcitatory response in the face of a salt challenge, activation of the endothelin-1 receptor, and impaired natriuretic response to salt loading in capsaicin-pretreated rats. These data indicate that sensory nerves counterbalance the prohypertensive effects of several neurohormonal systems to maintain normal blood pressure when challenged with salt loading. The therapeutic utilities of vanilloid compounds, endogenous agonists, and sensory neuropeptides are also discussed.

Pharmacological investigation of hydrogen sulfide (H 2S) contractile activity in rat detrusor muscle

We have investigated the mechanism through which hydrogen sulfide (H 2S) stimulates capsaicin-sensitive primary afferent neurons in the rat isolated urinary bladder. Sodium hydrogen sulfide (NaHS), a donor of H 2S, produced concentration-dependent contractile responses (pEC 50=3.5±0.1) that were unaffected by the transient receptor potential vanilloid receptor 1 (TRPV1) antagonist capsazepine (30 μM) and SB 366791 (10 μM) and by the N-type Ca 2+ channel blocker omega-conotoxin GVIA (ω-CTX; 100 nM). In contrast, the unselective transient receptor potential (TRP) cation channels blocker ruthenium red (30 μM) almost abolished NaHS-induced contractions. Ruthenium red (30 μM) greatly reduced capsaicin-induced contractions, whereas it did not attenuate the contractile response to neurokinin A. The putative TRPV1 receptor antagonist iodo-resiniferatoxin, from 100 nM upward, produced agonist responses per se, and could not be tested against NaHS. We conclude that H 2S either acts at TRPV1 receptorial sites unblocked by capsazepine or SB 366791, or stimulates a still unidentified transient receptor potential-like channel co-expressed with TRPV1 on sensory neurons.

Differential Regulation of 3-Beta-Hydroxysteroid Dehydrogenase and Vanilloid Receptor TRPV1 mRNA in Sensory Neurons by Capsaicin and NGF

It was the aim of the present study to investigate by RT-PCR the regulation of the mRNA of the neurosteroid-synthesizing enzyme 3β-hydroxysteroid dehydrogenase (3β-HSD) and of the vanilloid receptor TRPV1 in dorsal root ganglia (DRGs) of rats during the process of capsaicin denervation of primary sensory neurons and the following regeneration. The expression of 3β-HSD in DRG was increased 3 days after the capsaicin treatment, and it remained at that level during a 22 day observation period. The expression of TRPV1, a specific marker of capsaicin-sensitive small sensory neurons connected to C- and Aδ-fibers, was markedly reduced 3 days after the capsaicin treatment. It slowly recovered during the 22 days observation period reaching almost control levels on day 22. When the capsaicin-treated rats received 5 intraplantar injections of nerve growth factor (NGF), the prototypical neurotrophin for capsaicin-sensitive neurons, on day 1, 2, 3, 5 and 6, both the 3β-HSD and the TRPV1 mRNA had returned to control levels at the time point 8 days after capsaicin. The present results demonstrate that both 3β-HSD and TRPV1 are markers for neurodegeneration and neuroregeneration in capsaicin-sensitive primary afferent neurons, and that NGF is an effective tool to induce recovery after peripheral nerve injury.

Antiinflammatory and analgesic effects of somatostatin released from capsaicin‐sensitive sensory nerve terminals in a Freund's adjuvant–induced chronic arthritis model in the rat

We previously demonstrated that somatostatin (SOM) released from the activated peripheral terminals of capsaicin-sensitive primary sensory neurons inhibits acute inflammation and nociception. This study was undertaken to examine this systemic "sensocrine" function of neuronally derived somatostatin in chronic inflammation in the Freund's complete adjuvant (CFA)-induced arthritis model. Arthritis of the tibiotarsal joint of Lewis rats was evoked by subcutaneous injection of CFA into the left hind paw and the tail root. For 3 weeks, the volume of the paws was measured by plethysmometry, and the mechanonociceptive thresholds were measured by esthesiometry. Plasma concentrations of SOM were determined by radioimmunoassay, and histologic studies of the joints were performed. To impair the function of capsaicin-sensitive afferents, the capsaicin receptor (VR1/TRPV1) agonist resiniferatoxin (RTX) was injected subcutaneously (30, 70, and 100 microg/kg on 3 subsequent days) 7 days before CFA administration. The SOM receptor antagonist cyclosomatostatin (c-SOM; 20 microg/kg) or, in another group, the synthetic heptapeptide agonist TT-232 (2 x 50-400 microg/kg) was administered intraperitoneally every day. RTX pretreatment or c-SOM injection significantly increased edema and mechanical hyperalgesia of both CFA-treated and contralateral paws. The histologic score based on synovial thickening, cell infiltration, cartilage destruction, and bone erosion was also significantly higher both in the RTX- and the c-SOM-injected groups. These parameters were dose-dependently decreased by TT-232. Plasma SOM-like immunoreactivity increased 4-fold on the twenty-first day, and was inhibited by RTX pretreatment, as well as by daily administration of TT-232. Our data suggest that SOM released into the circulation from capsaicin-sensitive afferents in response to prolonged activation exerts systemic antiinflammatory and analgesic effects. TT-232 can open new perspectives in the treatment of chronic arthritis.

Capsaicin sensitive-sensory nerves and blood pressure regulation.

Capsaicin (8-methyl-N-vannillyl-6-nonenamide), via binding to the vanilloid receptor subtype 1 (VR1), stimulates a subpopulation of primary afferent neurons that project to cardiovascular and renal tissues. These capsaicin-sensitive primary afferent neurons are not only involved in the perception of somatic and visceral pain, but also have a "sensory-effector" function. Regarding the latter, these neurons release stored neuropeptides through a calcium-dependent mechanism via the binding of capsaicin to the VR1. A subset of capsaicin-sensitive sensory nerves contains calcitonin gene-related peptide (CGRP) and substance P (SP). These sensory neuropeptides are potent vasodilators and natriuretic/diuretic factors. Neonatal degeneration of capsaicin-sensitive sensory nerves has revealed novel mechanisms that underlie increased salt sensitivity and several experimental models of hypertension. These mechanisms are reviewed, which include insufficient suppression of plasma renin activity and plasma aldosterone levels subsequent to salt loading, enhancement of sympathoexcitatory response in the face of a salt challenge, activation of the endothelin-1 receptor, and impaired natriuretic response to salt loading in capsaicin-pretreated rats. These data indicate that sensory nerves counterbalance the prohypertensive effects of several neuro-hormonal systems to maintain normal blood pressure when challenged with salt loading. Mechanisms underlying pneumotoxicity and pulmonary hypertension as revealed by degeneration of capsaicin-sensitive nerves are also discussed. Finally, the therapeutic utilities of capsaicin, endogenous anandamide, and CGRP agonists are assessed.

Role of Peripheral Tachykinin Receptors in Neurogenic Inflammation of the Respiratory, Genitourinary and Gastrointestinal Systems

A wide distribution of tachykinin-containing projections of capsaicin-sensitive primary afferent neurons is present in the peripheral organs of respiratory, gastrointestinal and genitourinary systems. Tachykinins, along with calcitonin gene-related peptide (CGRP) and other neuropeptides, can be released by adequate stimuli having pathological rather than physiological relevance. In most cases these stimuli are represented by chemical irritants penetrating through the respiratory tree or alimentary channel to the body, or being removed through the urinary system. Tachykinins produce in the visceral organs part of the responses obtained at somatic level by antidromic activation of capsaicin-sensitive nerve terminals and known as “neurogenic inflammation” that is, increase in vascular permeability and plasma protein leakage followed by oedema. In addition tachykinins produce at visceral level smooth muscle contraction, neuronal stimulation, mucus secretion and recruitment / activation of immune cells. The physiological significance of these effects is to afford protection toward chemical irritants or noxious stimuli by facilitating their removal from the body. Nonetheless, a large body of preclinical evidence exists indicating that tachykinins participate in the genesis and/or maintenance of symptoms accompanying various human diseases, such as asthma / bronchial hyperreactivity, cystitis of various aetiologies, inflammatory bowel diseases and irritable bowel syndrome. Tachykinin receptor antagonists, proved capable of preventing/reducing tachykinin-mediated effects in experimental animal models of these pathologies, are expected to afford therapeutically relevant effects in humans.

Transganglionic transport of choleragenoid by capsaicin-sensitive c-fibre afferents to the substantia gelatinosa of the spinal dorsal horn after peripheral nerve section

Choleratoxin B subunit-binding thick myelinated, A-fibre and unmyelinated, capsaicin-sensitive nociceptive C-fibre primary afferent fibres terminate in a strict topographic and somatotopic manner in the spinal cord dorsal horn. Injection of choleratoxin B subunit–horseradish peroxidase conjugate into injured but not intact peripheral nerves produced transganglionic labelling of primary afferents not only in the deeper layers (Rexed’s laminae III–IV), but also in the substantia gelatinosa (Rexed’s laminae II) of the spinal dorsal horn. This was interpreted in terms of a sprouting response of the Aβ-myelinated afferents and suggested a contribution to the pathogenesis of neuropathic pain [Nature 355 (1992) 75; J Comp Neurol 360 (1995) 121]. By utilising the selective neurotoxic effect of capsaicin, we examined the role of C-fibre sensory ganglion neurons in the mechanism of this phenomenon. Elimination of these particular, capsaicin-sensitive C-fibre afferents by prior intrathecal or systemic capsaicin treatment inhibited transganglionic labelling by the choleratoxin B subunit–horseradish peroxidase conjugate of the substantia gelatinosa evoked by chronic sciatic nerve section. More importantly, prior perineural capsaicin treatment of the transected nerve proximal to the anticipated site of injection of choleragenoid 12 hours later prevented the labelling of the substantia gelatinosa, but not that of the deeper layers. Electron microscopic examination of the dorsal roots revealed no significant difference in the proportion of labelled myelinated fibres relating to the intact (54.4±5.5%) and the transected (62.4±5.4%) sciatic nerves. In contrast, the proportion of labelled unmyelinated dorsal root axons relating to the transected, but not the intact nerves showed a significant, six-fold increase after sciatic nerve transection (intact: 4.9±1.3%; transected: 35±6.7%). These observations indicate that peripheral nerve lesion-induced transganglionic labelling of the substantia gelatinosa by choleratoxin B subunit–horseradish peroxidase may be primarily accounted for by the uptake and transganglionic transport of choleragenoid by injured capsaicin-sensitive C-fibre afferents rather than a sprouting response of A-fibre afferents. The present findings suggest an essential role of capsaicin-sensitive primary sensory neurons in lesion-induced spinal neuroplastic changes and provide further support for C-fibre nociceptor neurons being promising targets for the development of new strategies in pain management.