Capsaicin-sensitive C-fibers Research Articles

Effect of Tamsulosin on Bladder Blood Flow and Bladder Function in a Rat Model of Bladder Over Distention/Emptying Induced Bladder Overactivity

Decreased bladder blood flow was the subject of a recent study as a pathophysiological cause of bladder overactivity. We developed a rat model of bladder over distention/emptying induced bladder overactivity and investigated the effect of the α(1)-adrenoceptor antagonist tamsulosin on bladder blood flow and bladder function in this model. The bladder was distended with 2 ml saline using anesthesia for 2 hours (over distention) and then emptied. Bladder blood flow was measured using a perfusion imager. Micturition behavior and parameters were observed using a metabolic cage and a cystometry method, respectively, from 2 hours after bladder emptying. After model establishment was confirmed we examined the participation of afferent C-fibers and the effects of tamsulosin in rats pretreated with capsaicin (Sigma-Aldrich®) (125 mg/kg) and tamsulosin (1 μg/kg per hour), respectively, using a metabolic cage. Decreased bladder blood flow was observed upon over distention with partial recovery at emptying. Bladder over distention/emptying increased micturition frequency and decreased mean voided volume in the micturition recording study, and decreased the intercontraction interval and voided volume without affecting micturition pressure, threshold pressure or post-void residual volume in the cystometry study. Capsaicin pretreatment did not affect bladder overactivity. However, 1-week continuous treatment with tamsulosin increased bladder blood flow after bladder emptying, resulting in decreased micturition frequency and increased voided volume. Bladder over distention/emptying induced bladder blood flow decrease/partial recovery and caused bladder overactivity via a mechanism other than capsaicin sensitive C-fiber activation. Findings in tamsulosin treated rats confirmed the potency of tamsulosin to increase bladder blood flow and ameliorate bladder overactivity.

Functional implications of the multiple afferent pathways regulating cough

At least 2 airway vagal afferent nerve subtypes can directly initiate coughing upon activation. The capsaicin-insensitive, acid-sensitive mechanoreceptors innervating the larynx, trachea and large bronchi regulate coughing in both conscious and anesthetized animals. Activation of capsaicin-sensitive C-fibers innervating these airways will also produce coughing, but C-fiber dependent cough is prevented entirely by anesthesia. The different stimuli activating these afferent nerve subtypes and their differential sensitivity to anesthesia implies the existence of 2 parallel pathways for cough, and by extension, 2 types of cough, one essential and homeostatic, the second nonessential and pathophysiologic. The basic properties of the afferent nerves regulating cough, their interactions both centrally and peripherally and their responsiveness to tussive stimuli are briefly reviewed. Also reviewed is evidence against the notion of 2 completely separate types of cough regulated by parallel afferent pathways, asserting instead that multiple afferent nerve subtypes contribute to all types of cough.

Vagal control of mucociliary clearance in murine lungs: A study in a chronic preparation

We conducted several experiments that focused on the effect of vagal control on mucociliary clearance (MCC) in murine lungs. We hypothesized that loss of vagal control by chronic denervation (i.e. vagotomy) would reduce both basal MCC and the increase in MCC typically observed upon stimulation of capsaicin sensitive C-fibers. Vagotomy was performed on the right side of C57BL/6 mice and MCC was measured 5 days later. Mucociliary clearance was measured by gamma scintigraphy after oropharyngeal aspiration of the radioisotope 99mtechnetium and was expressed as the amount of radioactivity removed from the right lung 6 h later. Baseline MCC was unaffected by vagotomy, averaging 6.5 ± 4.9% and 6.8 ± 5.8%, in 6 vagotomized and 6 non-vagotomized mice (controls), respectively. Mucociliary clearance increased significantly to 12.7 ± 5.9% in 9 non-vagotomized mice treated with 1.6 × 10 − 9 M capsaicin, a vagally-mediated, nociceptor stimulus ( p = 0.041). Capsaicin was admixed with 99mtechnetium and administered by oropharyngeal aspiration. In contrast, MCC was unchanged from control values in 9 vagotomized, capsaicin-treated animals, averaging 6.0 ± 5.5% ( p = 0.024). These findings suggest that loss of vagal control through denervation does not affect basal MCC in C57BL/6 mice, but does appear to reduce the capacity of mice to respond to nociceptor agents that stimulate MCC. These data could have implications for patients whose lungs are denervated due to lung transplantation, since they may be at risk for an inadequate MCC response to inhaled irritants and inflammatory mediators, which are also nociceptor stimuli.

The Effects of Sacral Acupuncture on Acetic Acid-induced Bladder Irritation in Conscious Rats

To investigate the effects of sacral acupuncture on acetic acid-induced bladder irritation using cystometry in conscious rats, with particular focus on effects on afferent fibers of the bladder. A total of 40 female Sprague-Dawley rats weighing 200-270 g were used. The animals were divided into 5 groups: (1) rats with bladder overactivity induced by acetic acid, without sacral acupuncture stimulation (irritated bladder group); (2) rats treated with sacral acupuncture stimulation after induction of bladder overactivity by acetic acid (acupuncture stimulation group); (3) rats treated with sacral acupuncture stimulation after capsaicin desensitization (capsaicin-desensitized group); (4) rats treated with atropine (atropine injection group); and (5) rats treated with sacral acupuncture stimulation after nonacetic acid infusion (nonirritated bladder group). Cystometry was carried out without anesthesia, and the following variables were measured in all animals: intercontraction interval (ICI), basal pressure, threshold pressure, and micturition pressure. In the acupuncture stimulation group, sacral acupuncture stimulation resulted in a 140.1 +/- 5.9% increase in ICI after acetic acid-induced irritation to an interval similar to baseline (P = .072). A significant difference in the percent of ICI increase was observed between the acupuncture stimulation and irritated bladder groups (P < .01). In the capsaicin-desensitized groups and acupuncture stimulation groups, no significant differences in ICI occurred, before and after acupuncture. Sacral acupuncture could contribute to improve acetic acid-induced bladder irritation through inhibition of capsaicin-sensitive C-fiber activation.

Involvement of prostaglandin F2α receptor in ATP-induced mechanical allodynia

Nociceptive primary afferents have the capacity to induce a state of increased excitability in the dorsal horn neurons of the spinal cord. It is well accepted that capsaicin-sensitive C-fibers transduce noxious stimulation and acute pain and that capsaicin-insensitive Aβ-fibers are responsible for touch and innocuous sensation. It has been reported that the intrathecal (i.t.) administration of prostaglandin F2α (PGF2α) and ATP induces mechanical allodynia via the capsaicin-insensitive primary afferent pathway. In the present study, we investigated the interaction of purinoceptor P2X and the PGF2α receptor (FP) in the induction of allodynia by use of mice lacking FP (FP−/−). Both PGF2α and the P2X receptor agonist αβ-methylene ATP administered i.t. strongly induced allodynia for 50 min by tactile stimuli to the flank of mice. The allodynia induced by αβ-methylene ATP, but not that by PGF2α, was suppressed by simultaneous i.t. administration of P2X receptor antagonists pyridoxalphosphate-6-azophenyl-2,4-disulphonic acid and A-317491. In contrast, the allodynia induced by αβ-methylene ATP as well as that by PGF2α was not observed in FP−/− mice. Immunostaining of β-galactosidase, a reporter knocked into the endogenous FP locus in FP−/− mice, showed that the FP receptor was co-localized with P2X2 and P2X3 receptors in neurons of the spinal cord. αβ-Methylene ATP evoked a transient or sustained [Ca2+]i increase in most of the PGF2α-responsive cells in the deeper layer of the spinal cord, and the αβ-methylene ATP-evoked increase was blocked by the FP receptor antagonist AL-8810 in two-thirds of the cells. Neither PGF2α nor αβ-methylene ATP induced the activation of spinal microglia. The present study demonstrates that the αβ-methylene ATP-evoked allodynia is mediated by the FP receptor, possibly via the functional coupling between the activation of P2X2/3 receptors on the central terminal of capsaicin-insensitive fibers and FP receptors on spinal neurons.

Involvement of capsaicin-sensitive afferent nerves in the proteinase-activated receptor 2–mediated vasodilatation in the rat dura mater

Neurogenic inflammation of the dura mater encephali has been suggested to contribute to the mechanisms of meningeal nociception and blood flow regulation. Recent findings demonstrated that the rat dura mater is innervated by trigeminal capsaicin-sensitive peptidergic nociceptive afferent nerves which mediate meningeal vascular responses through activation of the transient receptor potential vanilloid type 1 (TRPV1) receptor. The present work explored the functional significance of the capsaicin-sensitive subpopulation of dural afferent nerves via their contribution to the meningeal vascular responses evoked through activation of the proteinase-activated receptor 2 (PAR-2). The vascular responses of the dura mater were studied by laser Doppler flowmetry in a rat open cranial window preparation. Topical applications of trypsin, a PAR-2-activator, or Ser–Leu–Ile–Gly–Arg–Leu–amide (SLIGRL-NH 2), a selective PAR-2 agonist peptide, resulted in dose-dependent increases in meningeal blood flow. The SLIGRL-NH 2-induced vasodilatation was significantly reduced following capsaicin-sensitive afferent nerve defunctionalization by prior systemic capsaicin treatment and by pretreatment of the dura mater with the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP 8-37. Nω-nitro- l-arginine methyl ester hydrochloride ( l-NAME) an unspecific inhibitor of nitric oxide (NO) production, but not 1-(2-trifluoromethylphenyl) imidazole (TRIM), a neuronal NO synthase inhibitor, also inhibited the vasodilator response to SLIGRL-NH 2. The vasodilator responses elicited by very low concentrations of capsaicin (10 nM) were significantly enhanced by prior application of SLIGRL-NH 2. The present findings demonstrate that activation of the PAR-2 localized on capsaicin-sensitive trigeminal nociceptive afferent nerves induces vasodilatation in the dural vascular bed by mechanisms involving NO and CGRP release. The results indicate that the PAR-2-mediated activation and sensitization of meningeal capsaicin-sensitive C-fiber nociceptors may be significantly implicated in the pathophysiology of headaches.

Deletion of the mouse α-calcitonin gene-related peptide gene increases the vulnerability of the heart to ischemia-reperfusion injury

Calcitonin gene-related peptide (CGRP), a potent vasodilator released from capsaicin-sensitive C-fiber and Adelta-fiber sensory nerves, has been suggested to play a beneficial role in myocardial ischemia-reperfusion (I/R) injury. Because most previous studies showing a cardioprotective role of CGRP employed pharmacological experiments, the purpose of this study was to utilize a genetic approach by using mice with a targeted deletion of the alpha-CGRP gene to determine whether this neuropeptide had a modulatory function on the severity of I/R injury. To accomplish this goal, isolated, perfused hearts from alpha-CGRP knockout (KO) and wild-type (WT) mice were subjected to 30 min of ischemia followed by 5, 15, and 30 min of reperfusion. Cardiac functional parameters, including coronary flow rates, left ventricular developed pressure, maximum rates of pressure development, and left ventricular end-diastolic pressure, were measured before and after I/R injury, as were levels of creatine kinase, to assess myocardial damage, and malonaldehyde, to assess oxidative stress. Following I/R injury, cardiac performance was significantly reduced in the hearts from the alpha-CGRP KO mice compared with their WT counterparts. The marked reduction in myocardial function in the alpha-CGRP KO hearts compared with WT hearts after I/R injury was associated with a significant elevation in creatine kinase release into the perfusates and malonaldehyde production in the cardiac tissue. Therefore, these data indicate that, in this in vitro setting, deletion of alpha-CGRP makes the heart more vulnerable to I/R injury, possibly due, at least in part, to increased oxidative stress.

Expression of neurotransmitters and neurotrophins in neurogenic inflammation of the rat retina.

Antidromic stimulation of the rat trigeminal ganglion triggers the release of substance P (SP) and calcitonin gene-related peptide (CGRP) from sensory nerve terminals of the capsaicin sensitive C-fibers. These pro-inflammatory neuropeptides produce a marked hyperemia in the anterior segment of the eye, accompanied by increased intraocular pressure, breakdown of the blood-aqueous barrier and myosis. To assess the effects of neurogenic inflammation on the retina, specifically on the immunostaining of neurotransmitters and neurotrophins, as well as on the expression of neurotrophin receptors in the retina. RT-PCR was also accomplished in control and stimulated animals to confirm the immunohistochemical results. In the electrically stimulated eyes, immunostaining for SP, CGRP, VIP and nNOS demonstrated a marked increase in the RPE/POS (Retinal Pigment Epithelium/Photoreceptor Outer Segments), in the inner and outer granular layers and in the ganglion cells in comparison to the control eyes. CGRP and SP were found increased in stimulated animals and this result has been confirmed by RT- PCR. Changes in neurotrophin immunostaining and in receptor expression were also observed after electric stimulation of trigeminal ganglia. Decrease of BDNF and NT4 in the outer and inner layers and in ganglion cells was particularly marked. In stimulated rat retinas immunostaining and RT-PCR showed a NGF expression increase. Neurotrophin receptors remained substantially unchanged. These studies demonstrated, for the first time, that antidromic stimulation of the trigeminal ganglion and subsequent neurogenic inflammation affect immunostaining of retinal cell neurotransmitter/neuropeptides and neurotrophins as well as the expression of neurotrophin receptors.

Roles of Peripheral and Central Nicotinic Receptors in the Micturition Reflex in Rats

We investigated the effects of nicotinic acetylcholine receptor activation in the bladder and central nervous system on the micturition reflex in urethane anesthetized rats. The effects of nicotinic acetylcholine receptor activation on bladder activity were examined during continuous infusion cystometrogram. Nicotine with or without the nicotinic acetylcholine receptor antagonist mecamylamine (Sigma Chemical Co., St. Louis, Missouri) was administered intravesically, intrathecally or intracerebroventricularly in normal or capsaicin pretreated rats. We also examined nicotine induced responses in dissociated bladder afferent neurons from L6 to S1 dorsal root ganglia that were sensitive to capsaicin using whole cell patch clamp recordings. Intravesical nicotine (1 to 10 mM) significantly decreased intercontraction intervals in dose dependent fashion. This excitatory effect was abolished by co-application of mecamylamine (3 mM) as well as by capsaicin pretreatment. On patch clamp recordings 300 muM nicotine evoked rapid inward currents that were antagonized by mecamylamine in capsaicin sensitive bladder afferent neurons. Intrathecal and intracerebroventricular administration of nicotine (10 mug) decreased and increase intercontraction intervals, respectively. Each effect was antagonized by mecamylamine (50 mug) administered intrathecally and intracerebroventricularly. The spinal excitatory effect was significantly inhibited by the N-methyl-D-aspartate receptor antagonist (+)-MK-801 hydrogen maleate (20 mug) given intrathecally or by capsaicin pretreatment, although the effects of capsaicin pretreatment were significantly smaller than those of (+)-MK-801 hydrogen maleate. These results indicate that nicotinic acetylcholine receptor activation in capsaicin sensitive C-fiber afferents in the bladder can induce detrusor overactivity. In the central nervous system nicotinic acetylcholine receptor activation in the spinal cord and brain has an excitatory and an inhibitory effect on the micturition reflex, respectively. In addition, the nicotine induced spinal excitatory effect may be mediated by the activation of glutamatergic mechanisms.

DIFFERENTIAL EFFECTS OF ACTIVATION OF PERIPHERAL AND SPINAL TACHYKININ NEUROKININ 3 RECEPTORS ON THE MICTURITION REFLEX IN RATS

We clarified the roles of tachykinin neurokinin (NK)3 receptors in the bladder or spinal cord for control of the micturition reflex in rats. In female rats under urethane anesthesia repetitive bladder contractions were elicited by saline infusion into the bladder through intravesical bladder catheters. The effects of peripheral receptor activation were first examined by topical application of the tachykinin NK3 receptor agonist [MePhe]-NKB (Calbiochem, Darmstadt, Germany) in normal rats and rats pretreated with capsaicin (Sigma Chemical Co., St. Louis, Missouri) 4 days before the experiments. Subsequently the effects of spinal NK3 receptor activation were examined by intrathecal administration of [MePhe]-NKB via implanted intrathecal catheters. The effects of the tachykinin NK3 receptor antagonist SB235375 and the opioid receptor antagonist naloxone on changes in bladder activity induced by [MePhe]-NKB were also investigated. Topical application of [MePhe]-NKB onto the bladder surface decreased intercontraction intervals and bladder capacity, and increased baseline bladder pressure in dose dependent fashion. [MePhe]-NKB induced bladder overactivity was inhibited by simultaneous topical administration of SB235375 or by capsaicin pretreatment. In contrast, intrathecal injection of [MePhe]-NKB increased intercontraction intervals in dose dependent fashion and at a high dose it induced overflow incontinence or inefficient voiding. These inhibitory effects of [MePhe]-NKB in the spinal cord were antagonized by the intrathecal injection of SB235375 or naloxone. These results indicate that the tachykinin NK3 receptor mediated neural control of the micturition reflex has dual actions depending on the location of receptor activation. Activation of tachykinin NK3 receptors located in the bladder can induce bladder overactivity at least in part via the activation of capsaicin sensitive C-fiber afferents, while tachykinin NK3 receptor activation in the spinal cord can inhibit the micturition reflex through an opioid mechanism.

Deafferentation affects short-term but not long-term control of food intake

Deafferentation affects short-term but not long-term control of food intake (PHYSIOL BEHAV XX(X) 000-000, 2005). Rats were treated neonatally with capsaicin (CAP) to investigate the involvement of vagal afferents in food intake control and body weight regulation. In the first set of experiments, rats were offered increasing concentrations of sucrose (10–15–20–40%) in short-term feeding tests of 1 h. At the end, 10% was offered again to see whether CAP rats modified their intake after repeated exposure to different concentrations of sucrose solution. Results demonstrated that CAP animals overconsume persistently compared to vehicle (VEH) controls. This overconsumption is most pronounced and variable at 10% trials. Hypertonic 40% sucrose solution resulted in a small but significant drop in intake in CAP rats. Overall, if the concentration of sucrose solution is more than 10%, sucrose ingestion of CAP and VEH rats does not depend on the concentration of sucrose solution and remains relatively constant during all trials. In another experiment, rats were exposed to a high-fat condensed milk suspension (CMS) for 5 days. CAP rats initially overconsumed from this CMS compared to VEH. This was accompanied by a decreased intake in chow. However, over the 5 day period CAP animals adjusted their CMS and chow intake to control levels. During both experiments there were no differences in body weight gain between CAP and VEH. Together, these results suggest that capsaicin-sensitive vagal C-fibers are involved in the control of volume ingestion and short-term food intake control but are not required for long-term control of energy intake.

Cold response of the bladder in guinea pig: Involvement of transient receptor potential channel, TRPM8

To address the physiologic role of TRPM8, one of the transient receptor potential channels, we investigated the bladder cooling reflex and the effect of menthol on it in the guinea pig. Single cystometry in female Hartley guinea pigs was performed with high-speed infusion (60 mL/hr) under urethane anesthesia (1 g/kg intraperitoneally). The volume threshold for micturition (VT) and micturition pressure were determined. The distribution of TRPM8 in the S1 dorsal root ganglion (DRG) was also examined by immunostaining. Intravesical infusion of saline containing menthol (0.6 mM) at 38 degrees C markedly decreased the VT and increased micturition pressure. Although cold saline itself (4 degrees C) had little effect on VT or micturition pressure, the VT was significantly decreased in a temperature-dependent manner when the bladder was pretreated with menthol. This decrease in the VT was not observed in animals that received hexamethonium pretreatment (10 mg/kg intravenously), which blocks the spinal reflex, or capsaicin (1 mM intravesically), which causes deafferentation of capsaicin-sensitive C-fiber afferent. Immunohistochemical analysis revealed that TRPM8 is expressed in small-diameter neurons in guinea pig S1 dorsal root ganglions. The results of our study showed that the bladder cooling reflex is observed in guinea pigs if the animals were pretreated with menthol. This reflex was sensitive to ganglion blockade or capsaicin-sensitive C-fiber deafferentation and might be mediated by C-fiber activation through TRPM8.

DIFFERENTIAL ROLES OF PERIPHERAL AND SPINAL ENDOTHELIN RECEPTORS IN THE MICTURITION REFLEX IN RATS

We investigated the effects of endothelin (ET) receptor activation in the bladder and the spinal cord on the micturition reflex in urethane anesthetized rats. The effects of ET receptor activation on bladder activity were examined during continuous infusion cystometrograms. ET-1 was administered intrathecally or intravesically in normal rats or rats pretreated with capsaicin. The effects of intravenous injection of the selective ETA receptor antagonist ABT-627, or selective ETB receptor antagonist A-192621 (Abbott Laboratories, Abbott Park, Illinois) intrathecal injection of the opioid receptor antagonist naloxone hydrochloride on changes in bladder activity induced by intravesical or intrathecal ET-1 administration were investigated. Intravesical injection of ET-1 (0.1 to 10 microM) induced detrusor overactivity, as evidenced by a decrease in intercontraction intervals, in a dose dependent manner. ET-1 induced detrusor overactivity was suppressed by intravenous application of ABT-627 (0.1 mg/kg) as well as capsaicin pretreatment but not by A-192621. In contrast, intrathecal injection of ET-1 (0.5 to 50 fmol) increased intercontraction intervals dose dependently and ET-1 at a higher dose (50 fmol) induced urinary retention. These inhibitory effects were antagonized by ABT-627 (10 mg/kg) and also by intrathecal application of naloxone but not by A-192621. These results indicate that the activation of ETA receptors in capsaicin sensitive C-fiber afferents in the bladder can induce detrusor overactivity, while ETA receptor activation in the spinal cord can inhibit the micturition reflex via activation of a spinal opioid mechanism. Thus, targeting peripheral ETA receptors could be an effective treatment for bladder overactivity and/or painful conditions.

Antitussive effect of NS-398, a selective cyclooxygenase-2 inhibitor, in guinea pigs

Several reports have demonstrated that the number of capsaicin-induced coughs is increased in the presence of prostaglandins in the airway. Moreover, it has been reported that the expression of cyclooxygenase-2, which converts arachidonic acid to prostaglandins, was found in cultured human airway epithelial cells in the absence of inflammatory cytokine stimulation. Thus, it is possible that cyclooxygenase-2 inhibitor may produce an antitussive effect. To test this hypothesis, we investigated the effects of N-[2-(cyclohexyloxy)-4-nitrofenyl]-methane sulfonamide (NS-398), a selective cyclooxygenase-2 inhibitor, and 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethyl-pyrazole (SC-560), a selective cyclooxygenase-1 inhibitor, on capsaicin-induced coughs in guinea pigs. NS-398 (1–10 mg/kg, p.o.) dose-dependently and significantly reduced the number of capsaicin-induced coughs. In contrast, SC-560 (10 mg/kg, p.o.) did not reduce the number of capsaicin-induced coughs. The antitussive effect of NS-398 (10 mg/kg, p.o.) was not antagonized by pretreatment with methysergide (3 mg/kg, i.p.), a non-selective serotonin (5-HT) receptor antagonist, or glibenclamide (10 mg/kg, i.p.), an ATP-sensitive K + channel blocker. Furthermore, although NS-398 did not significantly affect the cough reflex induced by substance P (10 -16 M), it significantly reduced the capsaicin-induced release of substance P in bronchoalveolar lavage fluid (BALF). The present findings clearly show that cyclooxygenase-2 inhibitor, but not cyclooxygenasez-1 inhibitor, has a potent antitussive effect. Furthermore, it is possible that the antitussive action of NS-398 does not depend on centrally acting mechanisms, since 5-HT receptors play an important role in the cough-depressant activities of centrally acting antitussive drugs. NS-398 may exert peripheral antitussive effects by inhibiting the release of substance P from capsaicin-sensitive afferent C-fibers in the airways. These results suggest that cyclooxygenase-2 inhibitors may have a therapeutic benefit in reducing coughs.

Effect of Preemptive Treatment of Capsaicin or Resiniferatoxin on the Development of Pre-micturition Contractions After Partial Urethral Obstruction in the Rat

Neuroplasticity in afferent pathways, including C-fiber bladder afferents, is believed to be one of the major causes of changes in bladder function after partial urethral obstruction. We determined if capsaicin sensitive C-fiber bladder afferents are involved in the development of pre-micturition contractions after partial urethral obstruction in the rat. Female Wistar rats were preemptively treated with capsaicin (100 mg/kg subcutaneously) or resiniferatoxin (0.3 mg/kg subcutaneously) prior to the creation of obstruction. Before and 6 weeks after obstruction micturition profiles were compared among the 3 groups, including obstructed rats preemptively treated with capsaicin (BO/CAP) or resiniferatoxin (BO/RTX) and untreated obstructed rats (BO/-). In addition, conscious filling cystometry was performed after obstruction. Instillation of capsaicin solutions into the eye or bladder induced significant irritative reactions in BO/- but not in BO/CAP or BO/RTX, indicating that the effect of pretreatment with capsaicin or resiniferatoxin lasted 6 weeks. Voided volume per micturition on micturition profiles was decreased after partial urethral obstruction in all 3 groups. On conscious filling cystometry a significant increase in bladder capacity, voided volume and micturition threshold pressure was noted in BO/CAP but not in BO/RTX compared with BO/-. In contrast, there was no significant change in residual volume, voiding efficiency or micturition pressure among the 3 groups. The prevalence of pre-micturition contractions was 100% in BO/CAP and BO/RTX, and 83% in BO/-. Capsaicin sensitive C-fiber afferents are not essential to induce pre-micturition contractions but they are involved in functional alterations in bladder afferent pathways after partial urethral obstruction in rats.

Effect of olvanil on the afferent and efferent function of capsaicin-sensitive C-fibers in guinea pig airways

The aim of the present study was to examine the ability of the nonpungent vanilloid VR1 receptor agonist, olvanil, to activate the afferent and efferent function of capsaicin-sensitive C-fibers in guinea pig airways. We found that while capsaicin (10 nM–10 μM) and resiniferatoxin (0.1 nM–1.0 μM) evoked a robust contraction of the guinea pig trachea in vitro, olvanil (10 nM–10 μM) was a weak spasmogen. In addition, pretreatment with olvanil caused only a minor reduction of subsequent responses to capsaicin or resiniferatoxin. Using single fiber recording from guinea pig airway C-fibers, we found that olvanil (10 μM) did not evoke action potential discharge although these fibers responded vigorously to capsaicin after prolonged treatment with olvanil (10 μM). These findings are indicative of significant differences in the relative sensitivity of vanilloid VR1 receptor-transfected cells and the peripheral terminals of airway C-fibers to pungent and nonpungent vanilloid VR1 receptor agonists.

Role of primary afferent nerves in allodynia caused by diabetic neuropathy in rats

Both myelinated and unmyelinated afferents are implicated in transmitting diabetic neuropathic pain. Although unmyelinated afferents are generally considered to play a significant role in diabetic neuropathic pain, pathological changes in diabetic neuropathy occur mostly in myelinated A-fibers. In the present study, we first examined the role of capsaicin-sensitive C-fibers in the development of allodynia induced by diabetic neuropathy. We then studied the functional changes of afferent nerves pertinent to diabetic neuropathic pain. Diabetes was induced in rats by i.p. streptozotocin. To deplete capsaicin-sensitive C-fibers, rats were treated with i.p. resiniferatoxin (300 μg/kg). Mechanical and thermal sensitivities were measured using von Frey filaments and a radiant heat stimulus. Single-unit activity of afferents was recorded from the tibial nerve. Tactile allodynia, but not thermal hyperalgesia, developed in diabetic rats. Resiniferatoxin treatment did not alter significantly the degree and time course of allodynia. Post-treatment with resiniferatoxin also failed to attenuate allodynia in diabetic rats. The electrophysiological recordings revealed ectopic discharges and a higher spontaneous activity mainly in Aδ- and Aβ-fiber afferents in diabetic rats regardless of resiniferatoxin treatment. Furthermore, these afferent fibers had a lower threshold for activation and augmented responses to mechanical stimuli. Thus, our study suggests that capsaicin-sensitive C-fiber afferents are not required in the development of allodynia in this rat model of diabetes. Our electrophysiological data provide substantial new evidence that the abnormal sensory input from Aδ- and Aβ-fiber afferents may play an important role in diabetic neuropathic pain.

Bladder afferents and their role in the overactive bladder

The role of afferent innervation of the bladder in the pathophysiology of urinary incontinence has become the focus of intense interest. In normal health, the afferent pathway is mediated largely by Aδ-fibers, which ultimately send information about the state of bladder fullness to the pontine micturition center via the periaqueductal gray matter. However, after spinal disruption, a different type of afferent pathway emerges, mediated by capsaicin-sensitive C-fibers that drive a spinal segmental reflex pathway, causing neurogenic detrusor overactivity. The common sources of afferent information for either pathway are likely to be afferents from the urothelium, lamina propria, and afferents that originate in the bladder wall. Ultrastructural investigations of the constituent neural elements of these structures contribute to our knowledge of their role in both health and disease and help provide a rational approach to treatment strategies. Evidence of the involvement of capsaicin-sensitive C-fibers in the spinal reflex pathway has been supported by the successful treatment of patients with neurogenic incontinence with intravesical capsaicin or its ultrapotent nonpungent analog, resiniferatoxin. On the other hand, capsaicin has not been shown to be clearly effective in treatment of overactive bladder caused by detrusor overactivity or suprapontine pathology without the emergent C-fiber-mediated reflex. It is hoped that continued investigation of neurotoxins that have the potential to act on afferent innervation will lead to other treatment strategies for bladder disorders and other disorders involving afferent dysfunction.

Sensory nerves and neuropeptides in uterine cervical ripening

At the time of parturition (fetal delivery) the uterine cervix must “ripen,” becoming soft, pliable, and dilated to accommodate the fetus’ delivery. The fundamental processes underlying cervical ripening remain poorly understood. Knowledge that abundant autonomic and sensory nerves supply the uterine cervix, that transection of afferent nerves supplying the cervix blocks parturition, and that some of the changes in the cervix resemble those seen in inflammatory reactions suggests nerves may have a role in the cervical ripening changes. The present study utilized immunohistochemistry, plasma extravasation, and solution hybridization-nuclease protection assay to elucidate the complement of primary afferent nerves and some receptors in the rat cervix during pregnancy, and to determine if they may have roles in the ripening process at term. This study revealed an abundance of nerves associated with the cervical vasculature and myometrial smooth muscle containing immunoreactivity for substance P, calcitonin gene-related peptide, secretoneurin, and nitric oxide synthase throughout pregnancy. Many of these are small unmyelinated capsaicin-sensitive C-fibers. Substance P- (NK1-) and calcitonin gene-related peptide receptors were apparent on uterine cervix vasculature from pregnant, parturient, and postpartum rats. NK1 receptor mRNA was maximal at 20 days of pregnancy. Plasma extravasation of i.v. administered Evans Blue or Monastral Blue was most pronounced at parturition (shortly after NK1 mRNA is maximal); this was similar to plasma extravasation evoked by i.v. administration of substance P or capsaicin-treatment. This study revealed new data about the nervous system of the rat uterine cervix and that these nerves and their transmitters could very well be part of a neurogenic inflammatory process involved in cervical ripening.

Involvement of vanilloid receptor VR1 and prostanoids in the acid-induced writhing responses of mice

We found that intraperitoneal injection of organic acids, such as propionic and lactic acid, are able to develop writhing responses in mice similarly as that of acetic acid. These acid-induced writhing reactions were significantly attenuated by capsazepine, a VR1 receptor-specific antagonist, but the phenylbenzoquinone-induced one was not, suggesting that the acids but not phenylbenzoquinone activate the VR1 receptor, which is involved in polymodal pain perception. Hoe 140, a bradykinin B 2 receptor antagonist, also suppressed the acid-induced writhing response. Furthermore, these writhing responses were significantly suppressed after neonatal treatment with capsaicin, which treatment is known to destroy peripheral sensory afferent C-fibers. Capsazepine and Hoe 140 did not further attenuate the already reduced writhing responses of capsaicin-treated mice, suggesting that the acids stimulate the VR1 and the bradykinin B 2 receptor in the pathway comprising sensory afferent C-fibers. On the other hand, indomethacin further significantly suppressed the writhing number of the capsaicin-treated animals, suggesting that the acid-induced pain perception requires prostanoid receptors not only in the pathway via capsaicin-sensitive C-fibers but also in other sensory pathways. These results provide the first evidence for the involvement of the vanilloid receptor in the acid-induced inflammatory pain perception via sensory C-fibers in addition to the known mediators bradykinin, neurokinins, and prostanoids.