Capsaicin-pretreated Rats Research Articles

Effects of intravesical administration of sensory neuron-specific receptor agonist on voiding function in rats with cyclophosphamide-induced cystitis

IntroductionThe aim of this study was to investigate the urodynamic effects of intravesical administration of bovine adrenal medulla 8-22 (BAM8-22), a selective rat sensory neuron-specific receptor 1 agonist, on the micturition reflex in normal rats and rats with cyclophosphamide-induced bladder overactivity.Material and methodsContinuous cystometrograms (0.04 ml/min) were performed in urethane-anesthetized rats. After stable micturition cycles were established, vehicle (saline) or BAM8-22 was instilled intravesically and changes in bladder activity were monitored. The experiments using BAM8-22 were also performed in capsaicin-pretreated rats. In another experiment, vehicle (saline) or BAM8-22 was instilled intravesically and changes in bladder activity were monitored in cyclophosphamide-treated rats. Continuous cystometrograms were performed 48 hours after cyclophosphamide injection. Cystometric parameters were recorded and compared before and after intravesical drug administration.ResultsIntravesical administration of BAM8-22 significantly increased the intercontraction interval and threshold pressure in urethane-anesthetized rats, but did not affect the basal pressure or maximum pressure at any doses tested. The inhibitory effects of intravesical administration of BAM8-22 were not inhibited by capsaicin pretreatment. Intravesical administration of BAM8-22 also significantly increased intercontraction interval in the cyclophosphamide-treated rats.ConclusionsThe current results indicate that intravesical administration of a selective rat sensory neuron-specific receptor 1 agonist can inhibit the micturition reflex and can ameliorate cyclophosphamide-induced bladder overactivity in rats.

An investigation of the neural mechanisms underlying the efficacy of the adjustable gastric band

BackgroundThe mechanisms via which adjustable gastric band (AGB) surgery provides effective and durable weight loss remain unclear. ObjectivesThis study defines the role of sensory vagal fibers in the efficacy of the adjustable gastric banding using capsaicin to eliminate unmyelinated afferent fibers in the vagus nerve in a rodent model. SettingUniversity. MethodsA miniaturized AGB was fitted at the gastroesophageal junction of obese rats with either intact or sensory fiber depleted vagus nerves where deafferentation involved intraperitoneal (125 mg/kg) or topical (1% to the stomach) application of capsaicin. The extent of sensory fiber lesion was assessed using c-fiber-mediated reduction in cholecystokinin-induced feeding. Food intake, weight, and composition, as well as shifts in central neural activity (measured by elevation of Fos protein), were assessed after either control or AGB inflation with or without vagal deafferentation. ResultsAGB inflation caused a significant reduction in food intake, weight, and fat mass (P<.05) in obese rats. The effect of AGB on these parameters was prevented in capsaicin pretreated (vagal sensory lesioned) rats. Elevations in neural activity in the nucleus of the solitary tract and parabrachial nucleus after AGB inflation were ameliorated in capsaicin-treated rats. ConclusionVagal sensory fibers are integral to the efficacy of the AGB.

Cardiovascular Effects of a Novel Synthetic Analogue of Naturally Occurring Piperamides

Cardiovascular responses to intravenous administration of a piperamide analogue, LASSBio 365, were investigated in anesthetized rats. LASSBio 365 [62.5-1000 microg/kg, intravenously (IV)] has potent cardiovascular effects that include hypotension and bradycardia, accompanied by a brief pressor effect and apnea. Bilateral vagotomy or atropine injection (2 mg/kg, IV) completely abolished the bradycardia. A drop in blood pressure was abolished in bivagotomized rats. However, it was only inhibited in atropine-treated rats. The apnea was inhibited by both treatments. The Bezold-Jarisch reflex (ie, hypotension, bradycardia, and apnea) induced by LASSBio 365 is altered neither by 5-HT3 antagonist (tropisetron, 0.1 mg/kg, intraarterially) nor by the P2x antagonist (PPADS, 8.6 mg/kg, IV). The pressor component was affected neither by any of these interventions nor by the 5-HT2 antagonist (ritanserin, 0.5 mg/kg, i.a.). In capsaicin-pretreated rats (50 mg/kg, subcutaneously), all responses evoked by LASSBio 365 were abolished, including the pressor effect, which was inhibited. The data show that LASSBio 365 evokes the Bezold-Jarish reflex, neither via serotonergic receptors nor purinergic receptors but perhaps via the vanilloid pathway.

Hypotension induced by activation of the transient receptor potential vanilloid 4 channels: role of Ca2+-activated K+ channels and sensory nerves

To examine the mechanisms involved in hypotension induced by transient receptor potential vanilloid 4 (TRPV4) activation. Wistar rats were given 50 mg/kg capsaicin subcutaneously 1-2 days postnatally to cause degeneration of capsaicin-sensitive sensory nerves. Vehicle was given to the corresponding newborn rats that formed the control group. After being weaned, male rats were picked for further investigation. At the age of 8 weeks, mean arterial pressure and its response to 4alpha-phorbol 12,13-didecanoate [4alpha-PDD, a selective TRPV4 activator, 2.5 mg/kg, intravenous(ly) or i.v.] with or without CGRP8-37 (1 mg/kg per min, i.v.), an antagonist of calcitonin gene-related peptide (CGRP, a potent vasodilator released from sensory nerves), in vehicle or capsaicin-pretreated rats anesthetized with sodium pentobarbital [50 mg/kg, intraperitoneal(ly)] were monitored to observe the contributions of neuropeptides released from sensory nerves to the 4alpha-PDD-induced hypotension. To detect the roles of various vasodilating factors released by vascular endothelium in the hypotensive effect induced by TRPV4 activation, the corresponding inhibitors/blockers, including indomethacin (a cyclooxygenase inhibitor, 10 mg/kg, i.v.), Nomega-nitro-L-arginine (L-NA, a nitric oxide synthase inhibitor, 20 mg/kg, i.v.), apamin [a blocker of small conductance Ca2+-activated K+ (MaxiK) channels, 50 microg/kg, i.v.] combined with charybdotoxin (a blocker of intermediate and large conductance MaxiK channels, 50 microg/kg, i.v.), were used at various time before 4alpha-PDD injection. Plasma CGRP and substance P levels of rats before or after administration were measured using the corresponding radioimmunoassays. At last, immunohistochemistry stainings were performed to observe expression of TRPV4/CGRP/MaxiK in mesenteric resistance arteries and sensory neurons/nerve fibers. Intravenous administration of 4alpha-PDD produced remarkable hypotension in vehicle-pretreated rats. The depressor effect was attenuated by degeneration of capsaicin-sensitive sensory nerves (P < 0.05) or administration of CGRP8-37 (P < 0.05). In both vehicle and capsaicin-pretreated rats, the combined administration of apamin and charybdotoxin markedly reduced the 4alpha-PDD-induced hypotensive effect (P < 0.05), but i.v. administration of indomethacin and Nomega-nitro-L-arginine did not produce the similar effect. Intravenous administration of 4alpha PDD increased plasma CGRP but not substance P levels in vehicle-pretreated rats only (P < 0.05), which was not affected by indomethacin, Nomega-nitro-L-arginine, or apamin and charybdotoxin. Immunohistochemistry staining showed that TRPV4 colocalized with MaxiK channels in endothelium of mesenteric resistance arteries and with CGRP in sensory neurons/nerve fibers. Our data show that the hypotensive effect induced by TRPV4 activation attributes to, at least in part, activation of MaxiK channels and CGRP receptors upon CGRP release from sensory nerves.

Effects of cholinesterase inhibition in supraspinal and spinal neural pathways on the micturition reflex in rats

To investigate whether activation of brain and spinal cholinergic pathways affects the micturition reflex in rats. The effects of intracerebroventricular (i.c.v.) or intrathecal (i.t.) administration of neostigmine as a cholinesterase inhibitor and oxotremorine-M (OXO-M) as a muscarinic acetylcholine receptor (mAChRs) agonist, on the micturition reflex were evaluated by infusion cystometrography (CMG) in urethane-anaesthetized untreated rats or rats pretreated with capsaicin. Neostigmine injected i.c.v. increased bladder capacity (BC) and pressure threshold (PT) dose-dependently, with an increase in maximum voiding pressure (MVP) and a decrease in voiding efficiency (VE) at higher doses. Also, neostigmine injected i.t. increased the BC and PT dose-dependently without changing MVP or VE, and these effects were not apparent in capsaicin-pretreated rats. In both routes, atropine as an antagonist of mAChRs, but not mecamylamine as a nicotinic-AChR antagonist, almost completely antagonized the effects of neostigmine. The rank order of potencies of the antagonists for increasing effects of BC induced by 1 nmol of neostigmine was: pirenzepine (an M(1) mAChR antagonist) = atropine > 4-DAMP (an M(3) mAChR antagonist) " methoctramine (an M(2) mAChR antagonist) and tropicamide (an M(4) mAChR antagonist) via the i.c.v. route; and atropine > methoctramine > pirenzepine > tropicamide and 4-DAMP via the i.t. route, respectively. OXO-M injected via i.c.v. and i.t. had the same effects on BC, PT, MVP and VE as neostigmine by i.c.v. and i.t., respectively. These results indicate that activation of muscarinic cholinergic mechanisms by the cholinesterase inhibitor in the brain and spinal cord can inhibit the micturition reflex, mainly by affecting afferent pathways. These mAChR-induced inhibitory effects seem to be mediated through M(1)/M(3) receptor subtypes in the brain, while in the spinal cord, the M(1)/M(2) receptor subtypes might be involved in inhibitory effects, which are mediated via inhibition of mechanoceptive C-fibre afferent pathways.

Comparison of voiding function and nociceptive behavior in two rat models of cystitis induced by cyclophosphamide or acetone

Nociceptive behavior and its relationship with bladder dysfunction were investigated in two cystitis models, which were induced by intraperitoneal (i.p.) injection of cyclophosphamide (CYP) or intravesical instillation of acetone, using freely moving, non-catheterized conscious rats. Female Sprague-Dawley rats were used. Cystitis was induced by i.p. injection of CYP (100 and 200 mg/kg) or intravesical instillation of acetone (10%, 30%, and 50%) via a polyethylene catheter temporarily inserted into the bladder through the urethra. Then the incidence of nociceptive behavior (immobility with decreased breathing rates) was scored. Voided urine was collected simultaneously and continuously to measure bladder capacity. The plasma extravasation in the bladder was quantified by an Evans blue (EB) dye leakage technique. CYP (100 mg/kg, i.p.) induced nociceptive behavior without affecting bladder capacity or EB concentration in the bladder. A higher dose of CYP (200 mg/kg, i.p.) decreased bladder capacity and increased EB levels as well as nociceptive behavior. In contrast, intravesical instillation of acetone (30%) decreased bladder capacity and increased EB levels, but evoked nociceptive behavior less frequently compared with CYP-treated animals. In capsaicin-pretreated rats, nociceptive behavior induced by CYP or acetone was reduced; however, the overall effects of CYP or acetone on bladder capacity and bladder EB levels were unaffected. These results suggest that there is a difference in the induction process of nociceptive behavior and small bladder capacity after two different types of bladder irritation, and that C-fiber sensitization is more directly involved in pain sensation than reduced bladder capacity.

The anatomy of the cremaster muscle during inguinoscrotal testicular descent in the rat

Extrapolation of rat testicular descent studies to humans has been criticized because of anatomical differences of the cremaster muscle. Human cremaster is described as a thin strip rather than a large, complete sac as in rats, which is proposed to be more important in propelling the testis during descent. This study investigated cremaster muscle anatomy and ontogeny in both normal and cryptorchid rat models. Gubernacula from 4 groups of neonatal rats were sectioned longitudinally and transversely: normal Sprague-Dawley, capsaicin pretreated, flutamide pretreated, and congenital cryptorchid rats. Gubernacula were stained with hematoxylin-eosin, Masson trichrome, and desmin immunohistochemistry to study muscle development. Myoblasts are more numerous at the gubernacular tip, whereas the most differentiated muscle is proximal. Rat cremaster develops as an elongated strip rather than a complete sac derived from abdominal wall muscles. Flutamide and capsaicin pretreatment disrupts development. Rat cremaster muscle develops as a strip, bearing close resemblance to human cremaster muscle, permitting extrapolation of cremaster function to human testicular descent. The cremaster muscle appears to differentiate from the gubernacular tip during elongation to the scrotum, and requires intact sensory innervation and androgen.

Early capsaicin intervention for neurogenic bladder in a rat model of spinal cord injury

We explored capsaicin pretreatment, prior to spinal trauma, as a method to prevent the development of neurogenic detrusor overactivity (NDO) and urethral-bladder dyssynergia reflex after spinal cord injury (SCI). In addition, the duration of effect of capsaicin therapy on NDO in a rat model of SCI was investigated. Two sets of experiments were performed on female Sprague Dawley rats transected at the T9-T10 spinal level. First, SCI rats received capsaicin (125 mg/kg s.q.) 3-4 days before and 4-5 days after SCI. Cystometrograms (CMG) was performed 4 weeks after injury. In the second set of experiments, serial CMG in the same SCI animal was performed after one time injection of capsaicin (125 mg/kg s.q.) 4 weeks after spinalization. There were no differences in intercontraction intervals, voiding efficiency, or voiding pressure between the capsaicin pretreated and control SCI rats. However, the number of uninhibited detrusor contractions decreased 4 weeks after injury. We found that a single dose of capsaicin suppressed uninhibited detrusor contractions for 34 days in the chronic SCI animals. Early therapy with capsaicin was able to prevent/reduce detrusor hyperreflexia in spinal cord injured animals 4 weeks after injury. Early vanilloid therapy may prevent development of urologic sequelae after SCI.

Effects of neonatal capsaicin treatment in the neutrophil production, and expression of preprotachykinin-I and tachykinin receptors in the rat bone marrow

Bone marrow is richly innervated with both myelinated and non-myelinated nerve fibers, but the role of this innervation on hemopoiesis is poorly understood. Therefore, the aim of this study was to investigate the role of C-fibers on hematopoiesis. Wistar rats were neonatally injected with either capsaicin or its vehicle, and used at adult ages (8–10 weeks). In capsaicin-pretreated rats, the levels of substance P (SP) in bone marrow fluid were markedly reduced in comparison with the vehicle group (13.1±4.5pg/ml versus 47.3±5.5pg/ml, p<0.05). In bone marrow, the number of total leukocytes was 28% higher (p<0.05) in capsaicin-pretreated group, and this accompanied by a higher number of neutrophils, particularly of the immature forms. The mononuclear cell and eosinophils counts did not differ significantly among vehicle and capsaicin groups. In peripheral blood, the number of circulating neutrophils in the capsaicin group increased by 53.8% (p<0.05), whereas the number of mononuclear cells did not change significantly among groups. Eosinophils were virtually absent in the circulating blood in both groups. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed that both preprotachykinin (PPT)-I mRNA and the tachykinin neurokinin (NK)-1 mRNA expression in bone marrow cells significantly increased in capsaicin group, whereas the NK-2 mRNA expression was unchanged after capsaicin pretreatment. In conclusion, our data show that chronic neuropeptide depletion enhance the neutrophil proliferation and differentiation in the rat bone marrow by mechanisms involving upregulation of PPT-I gene and NK-1 receptors.

Partial C-fiber ablation modulates diphenylmethane-4,4′-diisocyanate (MDI)-induced respiratory allergy in Brown Norway rats

Brown Norway (BN) rats were topically sensitized to polymeric diphenylmethane-diisocyanate (MDI) and challenged with MDI-aerosol approximately every 2 weeks over a time period of 2 months. Half of the sensitized animals were pretreated with capsaicin for partial C-fiber defunctionalization. After the fourth challenge inflammatory and pro-inflammatory factors in bronchoalveolar lavage (BAL) fluid and cells and physiological delayed-onset breathing patterns were analyzed. The latter endpoint was examined in the capsaicin pretreated group before and after each challenge. Findings were compared against naïve but repeatedly MDI-challenged BN rats. BAL-neutrophils, -protein, and -LDH as well as lung weights were significantly increased in the MDI-sensitized and challenged rats relative to the naïve, challenged control rats. With regard to these endpoints, capsaicin pretreatment did not affect the responsiveness to MDI-aerosol. In contrast, pro-inflammatory cytokines, the Th2 cell cytokine IL-4, and the CC-chemokine MCP-1 were significantly increased in BAL-cells of capsaicin pretreated and MDI-sensitized rats, whilst in the normal MDI-sensitized rats markedly less pronounced changes (if any) occurred. In the former group, IL-4 and MCP-1 were also significantly increased in the lung draining lymph nodes. Time-related increased frequencies of delayed-onset responses were observed in MDI-sensitized rats after subsequent MDI-challenges, however, differences between capsaicin pretreated and normal rats were not found. Despite the remarkable differences between normal and capsaicin pre-treated rats in the concentrations of pro-inflammatory and Th1-/Th2-cell specific cytokines, the inflammatory endpoints in BAL as well as the physiological measurements did not identify appreciable differences amongst these groups. This study included an ancillary study addressing the analysis of the modulating effect of capsaicin pre-treatment of naïve Wistar rats exposed for single 6 h to MDI-aerosol. The results indicated more pronounced changes on endpoints in the BAL-fluid of capsaicin-pretreated rats as compared to rats with intact C-fibers. This complex picture appears to suggest that C-fibers may modulate the allergic inflammatory response elicited by MDI-challenge. It appears that tachykinergic sensory C-fibers modulate the protective pathways against irritant-related lung inflammation and, similarly, also pro-inflammatory immunological factors modulating allergic inflammation. Although difficult to disentangle unequivocally the mechanisms involved, neuro-immunological factors may be important in triggering and maintaining this complex disease and cytokine/chemokine patterns may not necessarily predict the functional outcome of test.

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.

Pivotal role of endogenous tachykinins and the NK 1 receptor in mediating leukocyte accumulation, in the absence of oedema formation, in response to TNFα in the cutaneous microvasculature

Tachykinins including substance P (SP) are well known to play a role in influencing oedema formation and leukocyte accumulation during tissue insult and inflammation. Cutaneous inflammatory models to characterize a TNFα-dependent mechanism where endogenous SP act via the NK 1 receptor to promote leukocyte accumulation in the absence of oedema formation were used. We found that TNFα induced dose-dependent leukocyte accumulation at 4 h, which returned towards basal levels at 8 h in NK 1 +/+ mice. This response was absent in both the NK 1 +/+ mice treated with an NK 1 receptor antagonist and NK 1 −/− mice. At the highest dose IL-6 induced a significant accumulation in NK 1 +/+ and NK 1 −/− mice but IL-12 was ineffective. SP induced skin oedema but none of the cytokines did. Either co-injection of SP with low dose of TNFα (0.3 pmol/site) or SP previously injected (30 min) to TNFα evoked a significant increase in MPO activity when compared with that induced by the cytokine alone. In contrast, SP injected i.d. 3.5 h after TNFα failed to produce additive response. Control, but not capsaicin-pretreated rats (to deplete sensory nerves), exhibited a marked increase in MPO activity in response to TNFα. Histological analysis showed that TNFα caused tissue infiltrate of leukocytes in NK 1 +/+ mice, whilst leukocytes accumulated at intravascular sites in NK 1 −/− mice, but did not appear to emigrate, suggesting a defect in trans-endothelial migration. Interestingly, monocytes in addition to neutrophils accumulated 4 h post TNFα injection. In conclusion, the NK 1 receptor plays a functional role in mediating leukocyte accumulation independently of the historically important NK 1 mediated oedema formation. It seems that TNFα directly activates sensory nerve in addition to its chemoattractant activity. The NK 1 receptor agonist influences the accumulation of monocytes in addition to that of PMN by 4 h, thus revealing an important influence of the NK 1 receptor on TNFα mediated events in mouse skin.

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.

Neurokinin A Is a Main Constituent of Sensory Neurons Innervating the Anterior Segment of the Eye

To study the innervation pattern of the anterior segment of the eye by neurokinin (NK)-A-immunoreactive nerves and to determine their sensory origin. The presence and distribution of NKA was examined in human eyes by radioimmunoassay and immunofluorescence. The source of nerves was determined by measuring the concentration of NKA in the trigeminal ganglion (TG) in comparison with that of the classic sensory peptides substance P (SP) and calcitonin gene-related peptide (CGRP) and in eye tissues in capsaicin-pretreated rats versus control subjects. The NKA-like immunoreactivities were further characterized by reversed phase HPLC in the rat TG and the human iris-ciliary body complex. The presence of gamma-PPT-A mRNA was studied in the rat TG by in situ hybridization. The levels of NKA in human eye tissues were approximately 10 times higher than those of SP but lower than those of CGRP. Nerve fibers were visualized in the cornea, the trabecular meshwork, the iridial stroma, and, prominently, in the sphincter muscle, the ciliary body stroma and muscle and processes, and the choroidal stroma and surrounding blood vessels. In the rat TG, the concentration of NKA was approximately five times higher than that of SP. Capsaicin led to a >60% decrease of the concentration of the peptide in the rat TG and rat eye tissues except for the retina. NKA-like immunoreactivities were present in a single peak corresponding to synthetic NKA, both in the rat TG and in the human iris-ciliary body complex, and numerous ganglion cells of small size were labeled by a gamma-PPT-A probe in the rat TG. The present results clearly demonstrate that NKA is a main constituent of sensory neurons innervating the anterior segment of the eye. The presence of the peptide in C fibers in ocular tissues indicates a participation in sensory transmission and an involvement in the irritative response in the eye, a model for neurogenic inflammation in lower mammals.

Effects of capsaicin and nitric oxide synthase inhibitor on increase in cerebral blood flow induced by sensory and parasympathetic nerve stimulation in the rat

Effects of electrical stimulation of the nerve bundles including sensory and parasympathetic nerves innervating cerebral arteries on cerebral blood flow (CBF) and mean arterial blood pressure (MABP) were investigated with a laser-Doppler flowmeter and a blood pressure monitoring system in anesthetized rats pretreated with and without capsaicin. The electrode was hooked on the nerve bundles including the distal nasociliary nerve from trigeminal nerve and parasympathetic nerve fibers from sphenopalatine ganglion. In control rats, the nerve stimulation for 30 s increased CBF in the ipsilateral side and MABP. Hexamethonium attenuated the increase in CBF and abolished that in MABP. Under treatment with hexamethonium, N(G)-nitro-L-arginine (L-NNA, 1 mg/kg) significantly attenuated the stimulation-induced increase in CBF, which was restored by the addition of L-arginine. Although the dose of L-NNA was raised up to 10 mg/kg, the stimulation-induced increase in CBF was not further inhibited and was never abolished. In capsaicin-pretreated rats, magnitudes of the stimulation-induced increases in CBF and MABP were lower than those in control rats. Hexamethonium attenuated the increase in CBF and abolished that in MABP. Under treatment with hexamethonium, L-NNA abolished the stimulation-induced increase in CBF in capsaicin-pretreated rats. In conclusion, nitric oxide released from parasympathetic nerves and neuropeptide(s) released antidromically from sensory nerves may be responsible for the increase in CBF in the rat. The afferent impulses by nerve stimulation may stimulate the trigeminal nerve and lead to the rapid increase in MABP, which partly contributes to the increase in CBF.

Mechanisms Involved in the Enhancement of Allergic Airways Neutrophil Influx by Permanent C-Fiber Degeneration in Rats

This study was undertaken to clarify the mechanisms by which C-fiber degeneration at neonatal stages exacerbates the inflammatory responses of rat airways. Rats were treated with capsaicin at neonatal stages and immunized with ovalbumin (OVA) at adult ages. Challenge of capsaicin-pretreated rats with OVA promoted a higher influx of neutrophils in bronchoalveolar lavage (BAL) fluid compared with the vehicle group. No significant differences were found for the other cell types. The increased adhesion of N-formyl-methionyl-leucyl-phenylalanine (fMLP; 0.1 microM)- and phorbol myristate acetate (PMA; 1 microM)-treated neutrophils to fibronectin-coated wells did not differ among vehicle- and capsaicin-pretreated rats. Additionally, fMLP (10 microM), platelet-activating factor (0.1 microM), and substance P (50 microM) induced a significant neutrophil chemotaxis, but no differences were found among vehicle and capsaicin groups. Increased levels of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, IL-10, and leukotriene B4 in BAL fluid as well as higher expression of cytokine-induced neutrophil chemoattractant (CINC)-3 in lung homogenates were detected in the capsaicin group compared with vehicle group. In the capsaicin group, chronic treatment with compound 48/80 restored the TNF-alpha levels to control values and prevented the neutrophil influx in BAL fluid. The enhanced production of TNF-alpha, superoxide anion, and nitrite by isolated alveolar macrophages in response to lipopolysaccharide (3 microg/ml), PMA (10 nM), and/or zymosan (100 particles/cell) did not differ between vehicle- and capsaicin-pretreated rats. In conclusion, chronic neuropeptide depletion promoted by neonatal capsaicin treatment up-regulates airways mast cells, which upon activation by antigen at adult ages, release large amounts of cytokines such as TNF-alpha and CINC-3 that accounts for the massive airways neutrophil infiltration.

Selective sensory denervation by capsaicin aggravates adriamycin-induced cardiomyopathy in rats

Capsaicin-sensitive sensory nerves that contain calcitonin gene-related peptide (CGRP) contribute significantly to cardioprotective mechanisms. In this study, the possible role of capsaicin-sensitive afferent nerves in the development of congestive heart failure was examined in an established model of adriamycin-induced experimental cardiomyopathy in rats. Systemic treatment with capsaicin was utilized to deplete sensory neuropeptides from cardiac afferent nerves. Echocardiography was applied to assess the cardiac function in adriamycin-treated rats pretreated with capsaicin or its vehicle. In control rats, adriamycin treatment produced a reduction in the fractional shortening of the left ventricle and an increase in the ratio of the left atrial diameter and the aortic diameter, indicative of a decreased myocardial contractility and heart failure only at 3-4 weeks post-treatment. In contrast, in capsaicin-pretreated rats, a deterioration of the cardiac function was already evident 1 week after the cessation of adriamycin administration, while the clinical signs associated with cardiomyopathy were more severe and displayed a significantly more rapid progression. Immunohistochemistry revealed a complete depletion of calcitonin gene-related peptide from cardiac sensory nerves after systemic capsaicin treatment. This study has demonstrated that elimination of capsaicin-sensitive afferent nerves promotes the development and progression of adriamycin-induced myocardial dysfunction. The results suggest that interfering with capsaicin/vanilloid receptor function and/or perturbation of the myocardial CGRP metabolism may open up new perspectives concerning prevention and/or alleviation of the pathological changes that follow adriamycin treatment.

Selective C-fiber deafferentation of the spinal dorsal horn prevents lesion-induced transganglionic transport of choleragenoid to the substantia gelatinosa in the rat

The effect of neonatal capsaicin treatment, producing selective elimination of almost all unmyelinated C-fiber sensory axons, was studied on lesion-induced transganglionic labelling of the substantia gelatinosa of the spinal cord by choleragenoid. In both control and capsaicin-pretreated rats, the injection of choleragenoid-horseradish peroxidase conjugate into the intact sciatic nerves resulted in intense labelling only of the deeper layers of the spinal dorsal horn. In the control but not the capsaicin-pretreated rats, the injection of the tracer into sciatic nerves transected 2 weeks previously produced an intense homogeneous labelling of the substantia gelatinosa. It is concluded that the uptake and axonal transport of choleragenoid by capsaicin-sensitive C-fiber afferents may be accounted for by the lesion-induced transganglionic labelling of the substantia gelatinosa, rather than by A-fiber sprouting.

Capsaicin-sensitive neurogenic sensory vasodilatation in the dura mater of the rat.

The neurogenic sensory vascular responses of the dura mater encephali are considered to contribute significantly to the mechanisms of meningeal nociception and headache. Although the fundamental role of capsaicin-sensitive afferent nerves in the development of the neurogenic inflammatory responses of a variety of tissues is well established, their participation in meningeal vascular reactions is unclear. In the present study, the effects of the topical application of capsaicin on the dural blood flow and on the morphology of the dural nerve fibres were examined in control and capsaicin-pretreated rats by means of laser Doppler flowmetry and electron microscopy, respectively. In the control rats, the dural application of capsaicin at concentrations of 50 and 100 nM induced significant increases in blood flow in the branches of the medial meningeal artery. This capsaicin-induced vasodilatation was abolished by capsazepine, a transient receptor potential vanilloid 1 (TRPV1) receptor antagonist, and by hCGRP8-37, a calcitonin gene-related peptide (CGRP) receptor antagonist. Administration of capsaicin at higher concentrations (1 and 10 microM) resulted in marked, dose-dependent decreases in dural blood flow. The capsaicin-induced vasodilatation was abolished, whereas vasoconstriction was augmented, by systemic pretreatment of the animals with capsaicin. Electron microscopy revealed degenerating unmyelinated axons in the dura mater after an acute exposure to capsaicin (10 microM), providing support for the existence and possible functional role of capsaicin-sensitive dural afferent nerves. The results indicate that capsaicin-induced vasodilatation in the rat dura mater is mediated by the release of CGRP from the sensory nerves, whereas the vasoconstrictor response may be attributed to a direct action of capsaicin on the vascular smooth muscle. The present study demonstrates for the first time that capsaicin-sensitive nociceptive afferent nerves contribute significantly to the dural vasodilatory responses and suggests an important role in meningeal nociception.

Tachykinin substance P signaling involved in diesel exhaust-induced bronchopulmonary neurogenic inflammation in rats.

This study characterizes the molecular neurotoxicity of diesel exhaust (DE) on the tachykinin substance P (SP) signaling system in the lungs. A total of 96 female Fischer 344/NH rats (approximately 175 g, approximately 4 weeks old) were randomly assigned to eight groups in a 2 x4 factorial design: capsaicin versus non-capsaicin (vehicle) pretreatment, and filtered room air versus two exposure levels of DE with diesel engine room control. The rats were exposed nose-only to room air or low (35.3 micro g/m(3)) and high concentrations (669.3 micro g/m(3)) particulates directly from a Cummins N14 research engine at 75% throttle for 4 h/day, 5 days/week, for 3 weeks. The findings showed that exposure to DE dose-dependently induced bronchopulmonary neurogenic inflammation, both in capsaicin- and vehicle-pretreated rats, as measured by plasma extravasation, edema, and inflammatory cells. DE inhalation affected the SP signaling processes, including stored SP depletion and the gene/protein overexpression for neurokinin-1 receptor. DE also significantly reduced the activity of neutral endopeptidase, a main degradation enzyme for SP. Consequently, these changes may be regarded as critical factors that switched neurogenic pulmonary responses from their protective functions to a detrimental role that perpetuates lung inflammation. These changes may possibly be associated with the mass concentration of DE particles due to their physico-chemical characteristics. Moreover, capsaicin-pretreated rats had more sensitivity to these levels of DE exposure due to stimulation of bronchopulmonary C-fibers. However, the effects of capsaicin treatment were not consistent and apparent in this study. Taken together, our findings suggest that neurokininergic mechanisms may possibly be involved in DE-induced lung inflammation, but that bronchopulmonary C-fibers did not dominate DE-induced inflammatory abnormalities.