Capsaicin-sensitive Primary Afferents Research Articles

Contribution of capsaicin-sensitive innervation to the continuous eruption of the rat mandibular incisors.

A major component of tooth innervation is made of capsaicin-sensitive primary afferents (CSPA). These fibers play a key role in tooth pain and inflammation; little is known, however, about the role of CSPA in tooth eruption. The aim of this study was to examine the role of the capsaicin-sensitive afferents in the process of eruption of intact rat incisors. CSPA fibers in several rat groups, were subjected to one of the following experimental procedures: systemic chemical ablation, systemic ablation followed by chemical sympathectomy and localized activation. The observed effects on incisor eruption were compared to those made on controls. The total amount of eruption in control/naïve rats, measured over a total period of 144h, was 3.18 ± 0.07mm and decreased to 2.43 ± 0.08mm (n = 7; p < 0.001) following systemic ablation of CSPA. Further decrease to 2.24 ± 0.08mm (n = 7; p < 0.001) was noticed when chemical sympathectomy was added to CSPA ablation. The average rate of eruption was 1.7 ± 0.25mm following CSPA activation, compared to an average of 0.8 ± 0.07mm for controls (n = 7; p < 0.001). Capsaicin sensitive fibers play an important role in tooth homeostasis, and intact neural supply is required for tooth growth under normal conditions.

УЧАСТИЕ TRPV1 В РЕГУЛЯЦИИ ПРОДУКЦИИ БИКАРБОНАТОВ И КРОВОТОКА В ЖЕЛУДКЕ ПРИ УВЕЛИЧЕНИИ ПОЛОСТНОЙ КИСЛОТНОСТИ, "Российский физиологический журнал им. И.М. Сеченова"

Целью работы было выяснить, как влияет взаимодействие ванилоидных рецепторов переменного потенциала 1-го типа TRPV1 с эндотелиальной (eNOS) и нейрональной (nNOS) изоформами синтазы оксида азота на секрецию бикарбонатов (HCO) и кровоток в теле желудка на фоне аппликации кислоты на слизистую оболочку желудка (СОЖ). У наркотизированных крыс концентрация HCO3 в полостном перфузате желудка рассчитывалась на основе измеренных значений pH/PCO2, а объемный кровоток в стенке органа регистрировался с помощью лазерной допплеровской флоуметрии. Раздражение СОЖ подкисленным гипертоническим раствором (1 M NaCl, pH 2.0) значительно усиливало секрецию HCO3. Деструкция капсаицин-чувствительных первичных афферентов большими дозами капсаицина устраняла эту реакцию. Блокада TRPV1 не влияла на продукцию HCO и увеличение кровотока, вызванные внутрижелудочной аппликацией кислоты. Тем не менее было показано, что TRPV1 активируют eNOS, что приводит к усилению желудочного кровотока и ослаблению транспорта HCO3 на поверхность СОЖ. На активность nNOS, усиливающую продукцию HCO3, TRPV1 влияния не оказывали.

Experimental colitis in rats induces de novo synthesis of cytokines at distant intestinal sites: role of capsaicin-sensitive primary afferent fibers.

Increased levels of pro- and anti-inflammatory cytokines were observed in various segments of histologically-intact small intestine in animal models of acute and chronic colitis. Whether these cytokines are produced locally or spread from the inflamed colon is not known. In addition, the role of gut innervation in this upregulation is not fully understood. To examine whether cytokines are produced de novo in the small intestine in two rat models of colitis; and to investigate the role of capsaicin-sensitive primary afferents in the synthesis of these inflammatory cytokines. Colitis was induced by rectal instillation of iodoacetamide (IA) or trinitrobenzene sulphonic acid (TNBS) in adult Sprague-Dawley rats. Using reverse transcriptase (RT) and real-time PCR, TNF-α, and IL-10 mRNA expression was measured in mucosal scrapings of the duodenum, jejunum, ileum and colon at different time intervals after induction of colitis. Capsaicin-sensitive primary afferents (CSPA) were ablated using subcutaneous injections of capsaicin at time 0, 8 and 32 h, and the experiment was repeated at specific time intervals to detect any effect on cytokines expression. TNF-α mRNA expression increased by 3-40 times in the different intestinal segments (p<0.05 to p<0.001), 48h after IA-induced colitis. CSPA ablation completely inhibited this upregulation in the small intestine, but not in the colon. Similar results were obtained in TNBS-induced colitis at 24 h. Intestinal IL-10 mRNA expression significantly decreased at 12 h and then increased by 6-43 times (p<0.05 to p<0.001) 48h after IA administration. This increase was abolished in rats subjected to CSPA ablation except in the colon, where IL-10 further increased by twice (p<0.05). In the TNBS group, there was 4-12- and 4-7-fold increases in small intestinal IL-10 mRNA expression at 1 and 21 days after colitis induction, respectively (both p<0.01). This increase was not observed in rats pretreated with capsaicin. Capsaicin-treated and untreated rats had comparable visual ulcer scores after colitis induction. Inflammatory cytokines are produced de novo in distant intestinal segments in colitis. CSPA fibers play a key role in the upregulation of this synthesis.

Involvement of mast cells and proteinase-activated receptor 2 in oxaliplatin-induced mechanical allodynia in mice

The chemotherapeutic agent oxaliplatin induces neuropathic pain, a dose-limiting side effect, but the underlying mechanisms are not fully understood. Here, we show the potential involvement of cutaneous mast cells in oxaliplatin-induced mechanical allodynia in mice. A single intraperitoneal injection of oxaliplatin induced mechanical allodynia, which peaked on day 10 after injection. Oxaliplatin-induced mechanical allodynia was almost completely prevented by congenital mast cell deficiency. The numbers of total and degranulated mast cells was significantly increased in the skin after oxaliplatin administration. Repetitive topical application of the mast cell stabilizer azelastine hydrochloride inhibited mechanical allodynia and the degranulation of mast cells without affecting the number of mast cells in oxaliplatin-treated mice. The serine protease inhibitor camostat mesilate and the proteinase-activated receptor 2 (PAR2) antagonist FSLLRY-NH2 significantly inhibited oxaliplatin-induced mechanical allodynia. However, it was not inhibited by the H1 histamine receptor antagonist terfenadine. Single oxaliplatin administration increased the activity of cutaneous serine proteases, which was attenuated by camostat and mast cell deficiency. Depletion of the capsaicin-sensitive primary afferents by neonatal capsaicin treatment almost completely prevented oxaliplatin-induced mechanical allodynia, the increase in the number of mast cells, and the activity of cutaneous serine proteases. These results suggest that serine protease(s) released from mast cells and PAR2 are involved in oxaliplatin-induced mechanical allodynia. Therefore, oxaliplatin may indirectly affect the functions of mast cells through its action on capsaicin-sensitive primary afferents.

Contribution of capsaicin-sensitive primary afferents to mechanical hyperalgesia induced by ventral root transection in rats: the possible role of BDNF

Objective: A recent study showed that brain-derived neurotrophic factor (BDNF) may play a role in the development of the neuropathic pain resulting from injury to motor efferent fibres, such as that in the ventral root transection (VRT) model. Capsaicin stimulation of afferent fibres was also shown to result in the release of BDNF into the spinal cord. Here, the effects of ablation of capsaicin-sensitive primary afferents (CSPAs) by local application of capsaicin on the sciatic nerve on VRT-induced mechanical hyperalgesia were observed.Methods: The paw withdrawal mechanical threshold (PWMT) was measured before and then 1 and 3 days and 1, 2, 3, 4 and 6 weeks after VRT.Results: The results showed that local application of capsaicin significantly inhibited the decrease in the PWMT induced by VRT, suggesting the inhibitory effect of locally delivered capsaicin. Furthermore, intrathecal administration of exogenous BDNF not only produced mechanical hyperalgesia but also significantly blocked the inhibitory effect of capsaicin.Conclusion: Taken together, the results of this study suggest that CSPA fibres may contribute to mechanical hyperalgesia in the VRT model.

The Preventive Effect on Ethanol-Induced Gastric Lesions of the Medicinal Plant Plumeria rubra: Involvement of the Latex Proteins in the NO/cGMP/K ATP Signaling Pathway.

Plumeria rubra (Apocynaceae) is frequently used in folk medicine for the treatment of gastrointestinal disorders, hepatitis, and tracheitis, among other infirmities. The aim of this study was to investigate the gastroprotective potential of a protein fraction isolated from the latex of Plumeria rubra (PrLP) against ethanol-induced gastric lesions and describe the underlying mechanisms. In a dose-dependent manner, the pretreatment with PrLP prevented ethanol-induced gastric lesions in mice after single intravenous administration. The gastroprotective mechanism of PrLP was associated with the involvement of prostaglandins and balance of oxidant/antioxidant factors. Secondarily, the NO/cGMP/KATP pathway and activation of capsaicin-sensitive primary afferents were also demonstrated as part of the mechanism. This study shows that proteins extracted from the latex of P. rubra prevent gastric lesions induced in experimental animals. Also, the results support the use of the plant in folk medicine.

Hydrogen Sulfide Plays a Key Role in the Inhibitory Neurotransmission to the Pig Intravesical Ureter

According to previous observations nitric oxide (NO), as well as an unknown nature mediator are involved in the inhibitory neurotransmission to the intravesical ureter. This study investigates the hydrogen sulfide (H2S) role in the neurogenic relaxation of the pig intravesical ureter. We have performed western blot and immunohistochemistry to study the expression of the H2S synthesis enzymes cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS), measurement of enzymatic production of H2S and myographic studies for isometric force recording. Immunohistochemical assays showed a high CSE expression in the intravesical ureter muscular layer, as well as a strong CSE-immunoreactivity within nerve fibres distributed along smooth muscle bundles. CBS expression, however, was not consistently observed. On ureteral strips precontracted with thromboxane A2 analogue U46619, electrical field stimulation (EFS) and the H2S donor P-(4-methoxyphenyl)-P-4-morpholinylphosphinodithioic acid (GYY4137) evoked frequency- and concentration-dependent relaxations. CSE inhibition with DL-propargylglycine (PPG) reduced EFS-elicited responses and a combined blockade of both CSE and NO synthase (NOS) with, respectively, PPG and NG-nitro-L-arginine (L-NOARG), greatly reduced such relaxations. Endogenous H2S production rate was reduced by PPG, rescued by addition of GYY4137 and was not changed by L-NOARG. EFS and GYY4137 relaxations were also reduced by capsaicin-sensitive primary afferents (CSPA) desensitization with capsaicin and blockade of ATP-dependent K+ (KATP) channels, transient receptor potential A1 (TRPA1), transient receptor potential vanilloid 1 (TRPV1), vasoactive intestinal peptide/pituitary adenylyl cyclase-activating polypeptide (VIP/PACAP) and calcitonin gene-related peptide (CGRP) receptors with glibenclamide, HC030031, AMG9810, PACAP6–38 and CGRP8–37, respectively. These results suggest that H2S, synthesized by CSE, is involved in the inhibitory neurotransmission to the pig intravesical ureter, through an NO-independent pathway, producing smooth muscle relaxation via KATP channel activation. H2S also promotes the release of inhibitory neuropeptides, as PACAP 38 and/or CGRP from CSPA through TRPA1, TRPV1 and related ion channel activation.

Hydrogen Sulfide Mediated Inhibitory Neurotransmission to the Pig Bladder Neck: Role of KATP Channels, Sensory Nerves and Calcium Signaling

Because neuronal released endogenous H2S has a key role in relaxation of the bladder outflow region, we investigated the mechanisms involved in H2S dependent inhibitory neurotransmission to the pig bladder neck. Bladder neck strips were mounted in myographs for isometric force recording and simultaneous measurement of intracellular Ca(2+) and tension. On phenylephrine contracted preparations electrical field stimulation and the H2S donor GYY4137 evoked frequency and concentration dependent relaxation, which was reduced by desensitizing capsaicin sensitive primary afferents with capsaicin, and the blockade of adenosine 5'-triphosphate dependent K(+) channels, cyclooxygenase and cyclooxygenase-1 with glibenclamide, indomethacin and SC560, respectively. Inhibition of vanilloid, transient receptor potential A1, transient receptor potential vanilloid 1, vasoactive intestinal peptide/pituitary adenylyl cyclase-activating polypeptide and calcitonin gene-related peptide receptors with capsazepine, HC030031, AMG9810, PACAP6-38 and CGRP8-37, respectively, also decreased electrical field stimulation and GYY4137 responses. H2S relaxation was not changed by guanylyl cyclase, protein kinase A, or Ca(2+) activated or voltage gated K(+) channel inhibitors. GYY4137 inhibited the contractions induced by phenylephrine and by K(+) enriched (80 mM) physiological saline solution. To a lesser extent it decreased the phenylephrine and K(+) induced increases in intracellular Ca(2+). H2S produces pig bladder neck relaxation via activation of adenosine 5'-triphosphate dependent K(+) channel and by smooth muscle intracellular Ca(2+) desensitization dependent mechanisms. H2S also promotes the release of sensory neuropeptides and cyclooxygenase-1 pathway derived prostanoids from capsaicin sensitive primary afferents via transient receptor potential A1, transient receptor potential vanilloid 1 and/or related ion channel activation.

Capsaicin‐Sensitive Primary Afferent (CSPA) Fibers are Involved in the Regulation of Glucose Transport and the Expression of its Transporters in the Small Intestine in Rats

Previous studies have shown that CSPA fibers are involved in the regulation of alanine absorption in the small intestine. However, their effect on glucose absorption has not been yet fully characterized. This study examines the effect of CSPA fibers on glucose absorption and determines their effect on mRNA expression of glucose transporters (GLUT). Glucose absorption was measured in a jejunal segment perfused with 20mM glucose using the single-pass perfusion technique. CSPA ablation was achieved by subcutaneous injections of 1% capsaicin under light anesthesia at different time intervals. Sham rats received vehicle injections. Two weeks post-treatment, verification of desensitization was done by standard method. Glucose absorption in the jejunum showed a 68% increase in capsaicin-treated rats compared to sham. mRNA expressions of sodium-glucose cotransporter (SGLT1), GLUT2 and Na+-K+-ATPase were determined in jejunal samples by real-time PCR with actin as the housekeeping gene. A 14-fold increase in SGLT1 and a 7-fold increase in Na+-K+-ATPase mRNA expression were detected in capsaicin-treated rats as compared to sham. No significant increase in GLUT2 mRNA expression was noted in capsaicin-treated rats. Our results show that ablation of CSPA fibers increased glucose absorption by increasing the expression of its transporter. This indicates that CSPA fibers exercise an inhibitory tone in the small intestine that regulates epithelial absorption of glucose. This work is partially supported by the Lebanese National Council for Scientific Research.

Glucosylsphingosine Induces Itch-Scratch Responses in Mice

Pruritus is one of major symptoms in atopic dermatis. The pathophysiological mechanism of pruritus is unclear. The search for pruritogen is important in elucidating the pathophysiological mechanism of pruritus in atopic dermatitis. Glucosylsphingosine (Gsp) is upregulated in the strateum corneum of atopic dermatitis patients. We investigated to determine whether Gsp induces itch-scratch responses (ISRs) in mice. Intradermal administration of Gsp induces ISRs. Gsp dose-dependently induced scratching response at 50-500 nmol/site range. Pretreatment with naltrexone, an opioid receptor antagonist, and capsaicin, a TrpV1 receptor agonist, inhibited Gsp-induced ISRs. Additionally, Gsp-induced ISRs were also suppressed by cyproheptadine, an antagonist of serotonin receptor. These findings suggest that Gsp-induced scratching might be at least partly mediated by capsaicin-sensitive primary afferents, and the opioids receptor systems might be involved in transmission of itch signaling in the central nervous system. Furthermore, our findings suggest that Gsp-induced ISRs may be attributable to the serotonin-mediated pathways and Gsp is not any more one of byproducts of abnormal skin barrier but can lead to induce pruritus, one of typical symptoms of atopic dermatitis.

The role of capsaicin-sensitive primary afferents in experimental sciatic pain

To determine the effect of destroying capsaicin-sensitive primary afferents (CSPA) fibers on paw withdrawal mechanical threshold (PWMT) induced by the direct compression of L5 nerve root with autologous disc. The procedure used autologous disc of the rats from the coccygeal intervertebral discs to apply direct pressure to the L5 dorsal root. PWMT was measured at the different time points post-surgery and pre-surgery. The changes in spatial expression pattern of c-fos protein in the spinal cord were also determined at 3 weeks when PWMT decreased to the peak. The pretreatment with capsaicin produced a complete prevention of mechanical hyperalgesia induced by disc compression. The direct compression of L5 nerve root produced an obvious expression of fos-like immunoreactivity neurons in the dorsal horn of the spinal cord, which was significantly decreased by pretreatment with capsaicin. The study shows that CSPA fibers, which mainly terminated in superficial layers of dorsal horn, may play a key role in mechanical hyperalgesia in the new sciatica model.

Amino acids in the rat intestinal lumen regulate their own absorption from a distant intestinal site

Intestinal nutrient transport is altered in response to changes in dietary conditions and luminal substrate level. It is not clear, however, whether an amino acid in the intestinal lumen can acutely affect its own absorption from a distant site. Our aim is to study the effect of an amino acid present in rat small intestinal segment on its own absorption from a proximal or distal site and elucidate the underlying mechanisms. The effect of instillation of alanine (Ala) in either jejunum or ileum on its own absorption at ileal or jejunal level was examined in vivo. The modulation of this intestinal regulatory loop by the following interventions was studied: tetrodotoxin (TTX) added to Ala, subdiaphragmatic vagotomy, chemical ablation of capsaicin-sensitive primary afferent (CSPA) fibers, and IV administration of calcitonin gene-related peptide (CGRP) antagonist. In addition, the kinetics of jejunal Ala absorption and the importance of Na+-dependent transport were studied in vitro after instilling Ala in the ileum. Basal jejunal Ala absorption [0.198 +/- 0.018 micromol x cm(-1) x 20 min(-1) (means +/- SD)] was significantly decreased with the instillation of 20 mM Ala in the ileum or in an adjacent distal jejunal segment (0.12 +/- 0.015; P < 0.0001 and 0.138 +/- 0.014; P < 0.002, respectively). Comparable inhibition was observed in the presence of proline in the ileum. Moreover, basal Ala absorption from the ileum (0.169 +/- 0.025) was significantly decreased by the presence of 20 mM Ala in the jejunum (0.103 +/- 0.027; P < 0.01). The inhibitory effect on jejunal Ala absorption was abolished by TTX, subdiaphragmatic vagotomy, neonatal capsaicin treatment, and CGRP antagonism. In vitro studies showed that Ala in the ileum affects Na+-mediated transport and increases K(m) without affecting Vmax. Intraluminal amino acids control their own absorption from a distant part of the intestine, by affecting the affinity of the Na+-mediated Ala transporter, through a neuronal mechanism that involves CSPA and CGRP.

S1736 EP4 Receptor Blockade Inhibits Epithelial Ion Secretion Induced By PGE2, TNFα and Il1β in Human Colon

Introduction: Increased levels of proand anti-inflammatory cytokines were observed in various segments of histologically intact small intestines in animal models of acute and chronic colitis. We recently demonstrated that these cytokines are produced de novo shortly after the induction of colitis. The trigger for this upregulation is not known. Aim: To study the possible neural involvement in the synthesis of inflammatory cytokines in remote areas from the ulcer site in rat models of colitis. Methods: Colitis was induced by rectal instillation of trinitrobenzenesulphonic acid (TNBS) or iodoacetamide in adult Sprague-Dawley rats. The capsaicin sensitive primary afferents (CSPA) were ablated using subcutaneous injections of capsaicin at time 0 (25mg/kg), 8 and 32 hours (50mg/kg). Using real time PCR, TNF-α and IL-10 mRNA expression was measured in mucosal scrapings of the duodenum, jejunum, ileum and colon at different time intervals after induction of colitis. Results: TNF-α mRNA expression increased by 3-40 times in the different intestinal segments (p<0.05) 48h after iodoacetamide induced colitis. CSPA ablation completely inhibited this upregulation in the small intestine but not in the colon. Similar results were obtained in TNBS induced colitis. Intestinal IL-10 mRNA expression increased by 6-43 times (p<0.01) 48h after iodoacetamide administration. This increase was abolished in rats subjected to CSPA ablation except in the colon where IL-10 further increased by 2 times (p<0.05). In TNBS group, there was respectively a 4-12 and 4-7 folds increase of small intestinal IL-10 mRNA expression at 1 and 21 days after colitis induction (both p<0.01). This increase was not observed in rats pretreated with capsaicin. Both capsaicin treated and untreated rats elicited a similar visual ulcer score after colitis induction. Conclusion: Intestinal CSPA fibers play a key role in the induction of a de novo synthesis of inflammatory cytokines in intestinal segments distant from the site of colitis. The study was supported by the medical practice plan (MPP) and university research board (URB), American University of Beirut-Lebanon.

S1733 The Role of the IL-6/STAT3/SOCS3 Pathway in Ulcerative Colitis Related Carcinogenesis

Introduction: Increased levels of proand anti-inflammatory cytokines were observed in various segments of histologically intact small intestines in animal models of acute and chronic colitis. We recently demonstrated that these cytokines are produced de novo shortly after the induction of colitis. The trigger for this upregulation is not known. Aim: To study the possible neural involvement in the synthesis of inflammatory cytokines in remote areas from the ulcer site in rat models of colitis. Methods: Colitis was induced by rectal instillation of trinitrobenzenesulphonic acid (TNBS) or iodoacetamide in adult Sprague-Dawley rats. The capsaicin sensitive primary afferents (CSPA) were ablated using subcutaneous injections of capsaicin at time 0 (25mg/kg), 8 and 32 hours (50mg/kg). Using real time PCR, TNF-α and IL-10 mRNA expression was measured in mucosal scrapings of the duodenum, jejunum, ileum and colon at different time intervals after induction of colitis. Results: TNF-α mRNA expression increased by 3-40 times in the different intestinal segments (p<0.05) 48h after iodoacetamide induced colitis. CSPA ablation completely inhibited this upregulation in the small intestine but not in the colon. Similar results were obtained in TNBS induced colitis. Intestinal IL-10 mRNA expression increased by 6-43 times (p<0.01) 48h after iodoacetamide administration. This increase was abolished in rats subjected to CSPA ablation except in the colon where IL-10 further increased by 2 times (p<0.05). In TNBS group, there was respectively a 4-12 and 4-7 folds increase of small intestinal IL-10 mRNA expression at 1 and 21 days after colitis induction (both p<0.01). This increase was not observed in rats pretreated with capsaicin. Both capsaicin treated and untreated rats elicited a similar visual ulcer score after colitis induction. Conclusion: Intestinal CSPA fibers play a key role in the induction of a de novo synthesis of inflammatory cytokines in intestinal segments distant from the site of colitis. The study was supported by the medical practice plan (MPP) and university research board (URB), American University of Beirut-Lebanon.

Modulation of Noradrenergic Neurotransmission in Isolated Rat Radial Artery

The present study was designed to characterize the neurogenic contraction of rat radial artery. Electrical field stimulation (EFS) evoked frequency-dependent contraction that was abolished by tetrodotoxin (neuronal Na+ channel blocker), guanethidine (sympathetic neuron blocker), or phentolamine (α-adrenoceptor blocker). The α1-adrenoceptor antagonist prazosin inhibited endothelium-independent contractions to EFS, noradrenaline (NA), and the α1-adrenoceptor agonist phenylephrine. Rauwolscine, an α2-adrenoceptor antagonist, augmented nerve-mediated contractions and reduced sensitivity to NA and the α2-adrenoceptor agonist BHT-920. The β-adrenoceptor antagonist propranolol diminished EFS-elicited contractions, while sensitivity to NA was enhanced by propranolol. Relaxations evoked by isoproterenol, a β-adrenoceptor agonist, were abolished by propranolol. NG-Nitro-L-arginine (L-NOARG), a nitric oxide (NO) synthase inhibitor, increased both nerve-mediated and NA-induced responses in endothelium-intact, but not in endothelium-denuded arteries. Moreover, endothelium-dependent responses to BHT-920 and isoproterenol were modified by L-NOARG. Tetraethylammonium (TEA) or 4-amynopyridine, the Ca2+-activated (KCa) or voltage-dependent K+ (KV) channel blockers, respectively, enhanced the neurogenic contractions observed. TEA but not 4-amynopyridine increased NA-induced contractions. The ATP-sensitive K+ (KATP)–channel blocker glibenclamide failed to modify adrenergic contractions. Blockade of capsaicin-sensitive primary afferents increased EFS-induced contractions. In conclusion, adrenergic contractions are predominantly mediated by muscular α1-adrenoceptors, while endothelial α2- and β-adrenoceptors play a minor role. Presynaptic α2- and β-adrenoceptors cannot be precluded. Noradrenergic neurotransmission in rat radial artery seems to be modulated by both stimulation of endothelial NO, KCa, and KV channels and sensory C-fiber activation.

The Role of Capsaicin-Sensitive Primary Afferents in Experimental Sciatica Induced by Disc Herniation in Rats

The topical capsaicin treatment of the sciatic nerve, which was proved to destroy capsaicin-sensitive primary afferent (CSPA) fibers, was performed to determine the effect on decreases in paw withdrawal mechanical threshold (PWMT) and changes in spatial expression pattern of spinal c-Fos protein induced by the direct compression of L5 nerve root with autologous disc. To investigate the role of CSPA fibers in the development of mechanical hyperalgesia in the new sciatica model. To date, CSPA fibers have been shown to be involved in development of thermal hyperalgesia in various pain models. But the controversy still exists as to whether CSPA fibers are involved in the development of mechanical hyperalgesia in different pain models. To our best knowledge, the role of CSPA in sciatica was not investigated. Therefore, the present study was designed to determine the role of CSPA fibers in the newly developed sciatica model. All surgeries were performed in Sprague-Dawley rats. PWMT was measured at the different time points postsurgery and presurgery. The changes in spatial expression pattern of c-Fos protein in the spinal cord were also determined at 3 weeks when PWMT decreased to the peak. The pretreatment with capsaicin produced a complete prevention of mechanical hyperalgesia induced by disc compression. The direct compression of L5 nerve root produced an obvious expression of Fos-like immunoreactivity neurons in the dorsal horn of the spinal cord, which was significantly decreased by pretreatment with capsaicin. We conclude that CSPA fibers, which mainly terminated in superficial layers of dorsal horn, may play a key role in mechanical hyperalgesia in the new sciatica model.

Depletion of capsaicin sensitive afferents prevents lamina-dependent increases in spinal N-methyl- d-aspartate receptor subunit 1 expression and phosphorylation associated with thermal hyperalgesia in neuropathic rats

Phosphorylation of the N-methyl- d-aspartate (NMDA) receptor NR1 subunit (pNR1) in the spinal cord is associated with increased neuronal responsiveness, which underlies the process of central sensitization. Because of the importance of NR1 in central sensitization, the first goal of this study was to examine both time- and lamina-dependent changes in spinal NR1 and pNR1 expression in a chronic constriction injury (CCI) model of neuropathic pain. Increased excitability of capsaicin sensitive primary afferents (CSPAs), which express TRPV1 receptors, also contributes to central sensitization. Thus, we next examined whether the depletion of CSPAs with resiniferatoxin (RTX) modified the change of spinal NR1 and pNR1 expression induced by CCI. Experimental rats were euthanized at 1, 3, 7, 14, and 28 days post-CCI surgery and spinal cords processed for NR1 or pNR1 immunostaining. The number of NR1 or pNR1-immunoreactive neurons was significantly increased in all lamina (I–VI) of the ipsilateral L4/L5 dorsal horn from 1 or 7 days post-CCI, respectively. Pretreatment with RTX (0.3 mg/kg, s.c. in the scruff of the neck or intraplantar) 2 days prior to CCI completely prevented induction of thermal hyperalgesia, but not mechanical allodynia in neuropathic rats. Interestingly, RTX treatment significantly attenuated the CCI-induced upregulation of NR1 and pNR1 in spinal laminae I–II and V–VI, but not laminae III–IV as compared with that of vehicle-treated CCI rats. These findings demonstrate that the increased expression of NR1 and pNR1 in spinal laminae I–II and V–VI is dependent on activation of CSPAs, which ultimately contribute to the development of thermal hyperalgesia in neuropathic rats.

Determination of current threshold for paw withdrawal with sine-wave electrical stimulation in rats: Effect of drugs and alteration in acute inflammation

Electrical stimulation is widely used to assess the function of sensory nerves in humans. In the present study, the threshold current (CT) required to evoke a paw withdrawal response in rats was assessed with stepwise increases in current delivered as sinusoidal stimulation at frequencies of 2000 Hz (CT2000), 250 Hz (CT250) and 5 Hz (CT5). Baseline CT was 840 ± 3 μA for CT2000, 267 ± 2 μA for CT250 and 165 ± 1 μA for CT5 ( n = 59). Intrathecal administration (1–10 μg/rat) of morphine selectively increased CT5 and CT250 (efficacy order was CT5 > CT250 ≫ CT2000 = 0), although systemic morphine (1–5 mg/kg, S.C.) affected all three CTs (CT5 > CT250 > CT2000 > 0). Intrathecal pretreatment at day −3 of capsaicin (75 μg/rat) increased the thermal nociceptive threshold and selectively increased CT5 (CT5 ≫ CT250, CT2000 = 0). Intraplantar carrageenan injection progressively decreased CT250 and CT5, but increased CT2000 for a 3 h period. Intraperitoneal pretreatment with indomethacin (20 mg/kg) attenuated carrageenan evoked CT alterations as well as progression of paw swelling and thermal hyperalgesia. In conclusion, low, but not high, frequency stimulation activated a withdrawal response which appears mediated by morphine and capsaicin sensitive primary afferents and this threshold was reduced in the presence of inflammation. These data suggest the validity of such stimulation in defining drug action in a nontissue injurious fashion.

Capsaicin‐sensitive sensory fibers in the islets of Langerhans contribute to defective insulin secretion in Zucker diabetic rat, an animal model for some aspects of human type 2 diabetes

The system that regulates insulin secretion from beta-cells in the islet of Langerhans has a capsaicin-sensitive inhibitory component. As calcitonin gene-related peptide (CGRP)-expressing primary sensory fibers innervate the islets, and a major proportion of the CGRP-containing primary sensory neurons is sensitive to capsaicin, the islet-innervating sensory fibers may represent the capsaicin-sensitive inhibitory component. Here, we examined the expression of the capsaicin receptor, vanilloid type 1 transient receptor potential receptor (TRPV1) in CGRP-expressing fibers in the pancreatic islets, and the effect of selective elimination of capsaicin-sensitive primary afferents on the decline of glucose homeostasis and insulin secretion in Zucker diabetic fatty (ZDF) rats, which are used to study various aspects of human type 2 diabetes mellitus. We found that CGRP-expressing fibers in the pancreatic islets also express TRPV1. Furthermore, we also found that systemic capsaicin application before the development of hyperglycemia prevents the increase of fasting, non-fasting, and mean 24-h plasma glucose levels, and the deterioration of glucose tolerance assessed on the fifth week following the injection. These effects were accompanied by enhanced insulin secretion and a virtually complete loss of CGRP- and TRPV1-coexpressing islet-innervating fibers. These data indicate that CGRP-containing fibers in the islets are capsaicin sensitive, and that elimination of these fibers contributes to the prevention of the deterioration of glucose homeostasis through increased insulin secretion in ZDF rats. Based on these data we propose that the activity of islet-innervating capsaicin-sensitive fibers may have a role in the development of reduced insulin secretion in human type 2 diabetes mellitus.

Roles of capsaicin-sensitive primary afferents in differential rat models of inflammatory pain: A systematic comparative study in conscious rats

To characterize the role of capsaicin-sensitive primary afferents in inflammatory pain, the effects of subcutaneous (s.c.) injection of 0.15% capsaicin on different chemical irritants-induced pathological nociception including persistent spontaneous nociception, primary thermal and mechanical hyperalgesia, and inflammatory response were systematically investigated in unanesthetized conscious rats. Four different animal models of inflammatory pain: the bee venom (BV) test, the formalin test, the carrageenan model, and the complete Freund’s adjuvant (CFA) model, were employed and compared. Local pre-treatment with capsaicin produced a significant inhibition on the s.c. BV and formalin induced long-lasting persistent spontaneous nociception. However, this capsaicin-induced inhibitory effect on spontaneous nociception in the BV test was only found within the late phase (tonic nociception; 11–60 min), but not the early phase (acute nociception; 0–10 min). A complete preventing effect of capsaicin on the decreased thermal paw withdrawal latency was found in the BV, carrageenan, and CFA models. Nevertheless, pre-treatment with capsaicin only produced complete blocking effects on the decreased mechanical paw withdrawal threshold in the BV and carrageenan models, but not in the CFA model. For inflammatory response, a significant inhibition of the BV-elicited paw swelling was found following capsaicin treatment. In marked contrast, capsaicin did not produce any effects on the paw inflammation during exposure to carrageenan, CFA, and formalin. These data suggest that capsaicin-sensitive primary afferents may play differential roles in the induction and development of pathological nociception in differential inflammatory pain models. In contrast to other chemical irritants, BV-induced long-term spontaneous nociception, facilitated nociceptive behavior, and inflammation are modulated by peripheral capsaicin-sensitive afferents.