Capsaicin-sensitive nerves modulate resting blood flow and vascular tone in rat gut

Regulation of early and delayed radiation responses in rat small intestine by capsaicin-sensitive nerves

PURPOSE

Mast cells protect against the early manifestations of intestinal radiation toxicity, but promote chronic intestinal wall fibrosis. Intestinal sensory nerves are closely associated with mast cells, both anatomically and functionally, and serve an important role in the regulation of mucosal homeostasis. This study examined the effect of sensory nerve ablation on the intestinal radiation response in an established rat model.

METHODS AND MATERIALS

Rats underwent sensory nerve ablation with capsaicin or sham ablation. Two weeks later, a localized segment of ileum was X-irradiated or sham irradiated. Structural, cellular, and molecular changes were examined 2 weeks (early injury) and 26 weeks (chronic injury) after irradiation. The mast cell dependence of the effect of sensory nerve ablation on intestinal radiation injury was assessed using c-kit mutant (Ws/Ws) mast cell-deficient rats.

RESULTS

Capsaicin treatment caused a baseline reduction in mucosal mast cell density, crypt cell proliferation, and expression of substance P and calcitonin gene-related peptide, two neuropeptides released by sensory neurons. Sensory nerve ablation strikingly exacerbated early intestinal radiation toxicity (loss of mucosal surface area, inflammation, intestinal wall thickening), but attenuated the development of chronic intestinal radiation fibrosis (collagen I accumulation and transforming growth factor beta immunoreactivity). In mast cell-deficient rats, capsaicin treatment exacerbated postradiation epithelial injury (loss of mucosal surface area), but none of the other aspects of radiation injury were affected by capsaicin treatment.

CONCLUSIONS

Ablation of capsaicin-sensitive enteric neurons exacerbates early intestinal radiation toxicity, but attenuates development of chronic fibroproliferative changes. The effect of capsaicin treatment on the intestinal radiation response is partly mast cell dependent.

Mesenteric afferent nerves are sensitive to vascular perfusion in a novel preparation of rat ileum in vitro

Using novel in vitro preparations of vascularly perfused rat ileum, we investigated mesenteric afferent sensitivity to vascular perfusion. Gut (GPP) and vascular (VPP) perfusion pressures were recorded simultaneously with afferent discharge (AD). After preconstriction (L-phenylephrine), capsaicin (100 microM, gut lumen) caused a transient increase in AD and a sustained fall in VPP, supporting afferent modulation of vascular tone. In turn, AD was affected by vascular perfusion rate (VPR). Increasing VPR step-wise (0.6 to 1.0, 1.4 and 1.8 ml/min) caused concomitant falls in AD, returning at 0.6 ml/min. Terminating flow (5 min) increased AD. Afferent responses were independent of changes in GPP, vascular O2, or the gut "tube" ("gut-off"). In gut-off studies, where capsaicin (100 nM ia) still reduced VPP, flow-associated falls in AD were abolished by the enzyme neuraminidase (0.2 U/ml ia or extravascularly over 20 min). In contrast, increased AD after stopped flow was unaffected. We propose that mesenteric afferents "sense" changes in vascular perfusion. The precise stimuli (pressure and/or flow) and the physiological relevance to control of local circulation remain to be determined.

Capsaicin inhibits the pentagastrin-stimulated gastric acid secretion in anaesthetized rats with acute gastric fistula

The effect of capsaicin on basal and pentagastrin-stimulated gastric acid secretion was investigated in the urethane anaesthetized acute gastric fistula rat. Gastric acid secretion was measured by flushing of the gastric lumen with saline every 15 min or by continuous gastric perfusion. Capsaicin given into the rat stomach at 120 ng x mL(-1) prior to pentagastrin (25 microg x kg(-1), iv) reduced gastric acid secretory response to pentagastrin by 24%. Intravenous (iv) capsaicin (0.5 microg x kg(-1)) did not reduce the pentagastrin-stimulated gastric acid secretion. After topical capsaicin desensitization (3 mg x mL(-1)), basal gastric acid secretion and that in response to pentagastrin (25 microg x kg(-1), intraperitonaeally) was unaltered compared with the control group. Data indicate that topical capsaicin inhibits gastric acid secretion stimulated with pentagastrin in anaesthetized rats.

Capsaicin-sensitive afferent sensory nerves in modulating gastric mucosal defense against noxious agents

In the rat stomach, evidence has been provided that capsaicin-sensitive sensory nerves (CSSN) are involved in a local defense mechanism against gastric ulcer. In the present study capsaicin or resiniferatoxin (RTX), a more potent capsaicin analogue, was used to elucidate the role of these sensory nerves in gastric mucosal protection, mucosal permeability, gastric acid secretion and gastrointestinal blood flow in the rat. In the rat stomach and jejunum, intravenous RTX or topical capsaicin or RTX effected a pronounced and long-lasting enhancement of the microcirculation at these sites, measured by laser Doppler flowmetry technique. Introduction of capsaicin into the rat stomach in very low concentrations of ng-microg x mL(-1) range protected the gastric mucosa against damage produced by topical acidified aspirin, indomethacin, ethanol or 0.6 N HCl. Resiniferatoxin exhibited acute gastroprotective effect similar to that of capsaicin and exerted marked protective action on the exogenous HCl, or the secretagogue-induced enhancement of the indomethacin injury. The ulcer preventive effect of both agents was not prevented by atropine or cimetidine treatment. Capsaicin given into the stomach in higher desensitizing concentrations of 6.5 mM markedly enhanced the susceptibility of the gastric mucosa and invariably aggravated gastric mucosal damage evoked by later noxious challenge. Such high desensitizing concentrations of capsaicin, however, did not reduce the cytoprotective effect of prostacyclin (PGI2) or beta-carotene. Capsaicin or RTX had an additive protective effect to that of atropine or cimetidine. In rats pretreated with cysteamine to deplete tissue somatostatin, capsaicin protected against the indomethacin-induced mucosal injury. Gastric acid secretion of the pylorus-ligated rats was inhibited with capsaicin or RTX given in low non-desensitizing concentrations, with the inhibition being most marked in the first hour following pylorus-ligation. Low intragastric concentrations of RTX reduced gastric hydrogen ion back-diffusion evoked by topical acidified salicylates. It is concluded that the gastropotective effect of capsaicin-type agents involves primarily an enhancement of the microcirculation effected through local release of mediator peptides from the sensory nerve terminals. A reduction in gastric acidity may contribute to some degree in the gastric protective action of capsaicin-type agents. The vasodilator and gastroprotective effects of capsaicin-type agents do not depend on vagal efferents or sympathetic neurons, involve prostanoids, histaminergic or cholinergic pathways.

Mechanism of acid-induced mesenteric hyperemia in rats

The mesenteric hyperemia induced by intraduodenal application of hydrochloric acid (HCl) is mediated in part by capsaicin-sensitive afferent nerves. Antagonist of capsaicin-sensitive receptors (capsazepine) and blocker of capsaicin-sensitive cation channels (ruthenium red) have been described. We employed these tools to dissect the mechanism of regulation of mesenteric hyperemia induced by intraduodenal administration of HCl. Subcutaneous 100 micromol/kg capsazepine or intraduodenal 0.1% ruthenium red was administered to pentobarbital anesthetized rats. Then, 2.5 ml/kg of 640 microM capsaicin or 0.1 N HCl was administered intraduodenally. The mesenteric hyperemic responses were recorded. The results demonstrated that in a dose that decreased the mesenteric hyperemia induced by intraduodenal capsaicin, capsazepine failed to attenuate the mesenteric vasodilatory effect of intraduodenal HCl. Ruthenium red significantly attenuated the mesenteric hyperemia after intraduodenal capsaicin and HCl. These in vivo data provide the first functional evidence for the existence of capsazepine-sensitive capsaicin receptors and cation channel complexes in the rat duodenal and intestinal mucosa. The capsaicin- and HCl-sensitive receptors are unlikely to be functionally identical in these locations. The ruthenium red-sensitive cation channels appear to mediate the capsaicin- and HCl-induced mesenteric hyperemia.

Intraintestinal capsaicin transiently reduces CGRP-like immunoreactivity in rat submucosal plexus

Intraintestinal infusion of the sensory neurotoxin, capsaicin, transiently abolishes behavioral responses to chemical stimulation of the intestine. This desensitizing action of capsaicin may be due to an action on CGRP-containing nerve terminals, which are postulated to serve a sensory function in the enteric plexuses. To determine whether intraintestinal capsaicin treatment alters CGRP-like immunoreactivity (CGRP-li) in the enteric plexuses, we performed immunohistochemical analyses of the small intestinal submucosal and myenteric plexuses of rats at various times after intestinal infusion of capsaicin (5 mg) or its vehicle. Intestinal capsaicin treatment, but not vehicle treatment, reduced CGRP-li, but not substance-P-like immunoreactivity (SP-li), in nerve fibers of the submucosal plexus. CGRP-li was reduced in submucosal interganglionic connectives and in nerve fibers associated with submucosal blood vessels. CGRP-li of submucosal connectives was reduced by 1 h post-infusion. Reduction of CGRP-li in the submucosal fibers also was pronounced 24 h after intraintestinal capsaicin treatment. By 48 h after intestinal capsaicin infusion, CGRP-li was not distinguishable from vehicle-treated animals. There were no consistent immunohistochemical changes in CGRP-li or SP-li in the myenteric plexus at any time. These results indicate that intestinal capsaicin selectively induces transient reduction of CGRP-li in nerve fibers of the submucosal plexus. The chronology of depletion and reappearance of CGRP-li is congruent with previously reported, transient impairment of sensory function observed following intestinal capsaicin infusion.

Capsaicin and the stomach. A review of experimental and clinical data

Capsaicin, the pungent principle of hot pepper, because of its ability to excite and later defunctionalize a subset of primary afferent neurons, has been extensively used as a probe to elucidate the function of these sensory neurons in a number of physiological processes. In the rat stomach, experimental data provided clear evidence that capsaicin-sensitive (CS) sensory nerves are involved in a local defense mechanism against gastric ulcer. Stimulation of CS sensory nerves with low intragastric concentrations of capsaicin protected the rat gastric mucosa against injury produced by different ulcerogenic agents. High local desensitizing concentrations of capsaicin or systemic neurotoxic doses of the agent markedly enhanced the susceptibility of the rat gastric mucosa to later noxious challenge. Resiniferatoxin, a potent analogue of capsaicin possesses an acute gastroprotective effect similar to that of capsaicin in the stomach. The gastroprotective effect of capsaicin-type agents involves an enhancement of the microcirculation effected through the release of mediator peptides from the sensory nerve terminals with calcitonin gene-related peptide being the most likely candidate implicated. They do not depend on vagal efferent or sympathetic neurons or involve prostanoids. The gastric mucosal protective effect of prostacyclin is retained after systemic or topical capsaicin desensitization. Capsaicin-sensitive fibers are involved in the repair mechanisms of the gastric mucosa. A protective role for CS sensory nerves has also been demonstrated in the colon. In most studies, capsaicin given into the stomach of rats or cats inhibited gastric acid secretion. In humans, although recent studies provide evidence in favor of a beneficial effect of capsaicin on the gastric mucosa, an exact concentration-related assessment of the effect of the agent is still lacking.

Primary afferent nerves mediate in part beta-adrenoceptor stimulation-induced mesenteric hyperemia in rats

We tested the hypothesis that in the rat duodenum and intestine the mesenteric hyperemia due to beta-adrenoceptor stimulation is mediated by capsaicin-sensitive afferent nerves. Superior mesenteric artery blood flow was measured by pulsed Doppler flowmetry in the anesthetized rat. Functional ablation of afferent nerves was accomplished by subcutaneous 125 mg/kg capsaicin pretreatment 10 to 14 days before blood flow studies. Blockade of capsaicin-sensitive cation-selective ion channels of the duodenal and intestinal mucosal afferent nerves was achieved by intraduodenal 0.1% ruthenium red given 15 min prior to the intraduodenal administration of 5 mg/kg isoproterenol. Functional ablation of the afferent nerves and blockade of the capsaicin-sensitive cation-selective ion channels alone or in combination resulted in a significant reduction of mesenteric hyperemia induced by intraduodenal isoproterenol. These data support the hypothesis that beta-adrenoceptor stimulation by intraduodenal isoproterenol induces mesenteric hyperemia in part through afferent nerves in the rat duodenal and intestinal mucosa. The results suggest for the first time a link between beta-adrenoceptor function and peripheral capsaicin-sensitive afferent nerve-mediated mechanism in the rat gut.

Beneficial effect of a novel pentadecapeptide BPC 157 on gastric lesions induced by restraint stress, ethanol, indomethacin, and capsaicin neurotoxicity

Very recently, the integrity of capsaicin somatosensory neurons and their protection were suggested to be related to the activity in nociception of a newly discovered 15-amino acid peptide, BPC 157, shown to have strong beneficial effect on intestinal and liver lesions. Therefore, from this viewpoint, we have studied the gastroprotective effect of the pentadecapeptide BPC 157, on gastric lesions produced in rats by 96% ethanol, restraint stress, and indomethacin. The possible involvement of sensory neurons in the salutary actions of BPC 157 (10 micrograms/kg, 10 ng/kg intraperitoneally) was studied with capsaicin, which has differential effects on sensory neurons: a high dose in adult (125 mg/kg subcutaneously, 3 months old) or administration (50 mg/kg subcutaneously) to neonatal animals (age of the 7 days) destroys sensory fibers, whereas a low dose (500 micrograms/kg intraperitoneally) activates neurotransmitter release and protective effects on the mucosa. In the absence of capsaicin, BPC 157 protected gastric mucosa against ethanol, restraint, and indomethacin application. In the presence of neurotoxic doses of capsaicin, the negative influence of capsaicin on restraint, ethanol, or indomethacin lesions consistently affected salutary activity of BPC 157. However, BPC 157 protection was still evident in the capsaicin-treated rats (either treated as adults or as newborns) in all of these assays. Interestingly, after neonatal capsaicin treatment, a complete abolition of BPC gastroprotection was noted if BPC 157 was applied as a single nanogram-regimen, but the mucosal protection was fully reversed when the same dose was used daily. In line with the excitatory dose of capsaicin the beneficial effectiveness of BPC 157 appears to be increased as well. Taken together, these data provide evidence for complex synergistic interaction between the beneficial effectiveness of BPC 157 and peptidergic sensory afferent neuron activity.

Aging impairs afferent nerve function in rat intestine. Reduction of mesenteric hyperemia induced by intraduodenal capsaicin and acid

The high incidence of peptic ulcer disease despite decreased acid secretion in the elderly suggests an impairment of mucosal defense mechanism with aging. Stimulation of the intestinal mucosal afferent nerves by intraduodenal application of capsaicin or hydrochloric acid (HCl) increases superior mesenteric artery (SMA) blood flow and protects the duodenal mucosa against deep damage. We tested the hypothesis that the intestinal hyperemia induced by intraduodenal capsaicin or HCL is significantly reduced in older (12 months) rats compared with younger (2 months) rats. Mesenteric blood flow was measured by pulsed Doppler flowmetry in anesthetized rats with the flow probe around the SMA. Two milliliters per kilogram of 160 microM capsaicin or 0.1 N HCl administered intraduodenally increased SMA blood flow significantly in both age groups. The peak response in SMA blood flow, however, was significantly smaller in the older rats than in the younger rats. These observations support the hypothesis that impairment of afferent nerve function occurs with aging in the rat intestine.

Inhibition of endogenous nitric oxide reduces basal mesenteric vascular tone but does not alter intraduodenal hydrochloric acid-induced intestinal hyperemia in rats

There are conflicting reports on the role of endogenous nitric oxide (NO) in the regulation of basal intestinal blood flow. The effect of inhibition of NO in intraduodenal hydrochloric acid (HCl) induced intestinal hyperemia remains to be confirmed. We investigated the effect of inhibition of endogenous NO on basal intestinal blood flow, HCl-induced intestinal hyperemia, and duodenal villous injury. Superior mesenteric artery blood flow in rats was measured by pulsed Doppler flowmetry and duodenal villous injury evaluated by histology. Intravenous NG-nitro-L-arginine methyl ester (L-NAME), or L-arginine or D-arginine followed by L-NAME, was given to show inhibition, reversal of inhibition of endogenous NO synthase, and stereospecificity, respectively. An intraduodenal 2 ml/kg bolus or perfusion for 30 min of 0.1 N HCl was given 15 min after L-NAME or vehicle. Mean arterial blood pressure was increased by L-NAME, which also significantly reduced intestinal blood flow under basal condition and after intraduodenal HCl. Basal mesenteric blood flow was not altered by L- or D- arginine. The L-NAME-induced increase in blood pressure and decrease in basal blood flow was attenuated by L- but not D-arginine. The villous damage and the magnitude of the peak hyperemia was unchanged by L-NAME, L- or D-arginine. Inhibition of endogenous NO by L-NAME is suggested by the significant rise in blood pressure. The rise in blood pressure and reduction in blood flow are attenuated by L- but not D-arginine, indicating stereospecificity. Inhibition of endogenous NO reduces basal mesenteric vascular tone but does not alter intraduodenal HCl-induced intestinal hyperemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide mediates intestinal hyperaemic responses to intraluminal bile-oleate

It has long been recognized that intestinal blood flow increases at mealtimes. Mesenteric hyperaemia is also evoked by activation of sensory peptidergic nerves. Our studies explored the possible role of endogenous nitric oxide (NO) in the rat intestinal vasodilator response to luminal instillation of an oleic acid plus bile mixture before and after acute intrajejunal instillation of capsaicin and after chronic pretreatment with capsaicin. In anaesthetized rats we measured jejunal blood flow (BF) with an ultrasonic Doppler flowmeter and systemic arterial pressure (AP) with a pressure transducer. Intestinal perfusion with 80 mM oleic acid in bile increased BF by 98 +/- 12%. Instillation of 4 mg of capsaicin into the jejunal lumen initially increased BF by 42 +/- 9% but was followed by vasoconstriction. Inhibition of NO synthase with 25 mg/kg i.v. N-nitro-L-arginine (L-NNA) decreased BF by 27 +/- 5% and increased AP by 37 +/- 11%. After treatment with L-NNA and after acute and chronic administration of capsaicin, the bile-oleate-induced maximal increases in BF above control levels were 42 +/- 7%, 65 +/- 12%, and 58 +/- 8%, respectively. The observed inhibitory effect of L-NNA on the intestinal hyperaemic response to the bile-oleate mixture was reversed by pretreatment with L-arginine (100 mg/kg i.v.). In capsaicin pretreated rats the subsequent bile-oleate-induced hyperaemia was reduced in magnitude but the inhibitory effects of L-NNA were proportionately the same as in animals not receiving capsaicin. These findings support the hypothesis that NO is involved with bile-oleate-induced mesenteric hyperaemia.

Adrenergic, purinergic, and endothelial mediators and modulators of norepinephrine-induced mesenteric autoregulatory escape

We evaluated the effects of potential factors in autoregulatory escape from norepinephrine-induced vasoconstriction in rat anterior mesenteric artery. We determined mesenteric artery blood flow velocity with a pulsed Doppler, sonic flowmeter, and systemic arterial blood pressure with a transducer. A 4-min norepinephrine infusion (0.125-1.0 x 10(-8) M/min) intravenously evoked a dose-dependent, initial vasoconstriction that was followed by rapid escape of blood flow toward or above the control value during sustained norepinephrine administration. Neonatal capsaicin treatment enhanced vasoconstrictor responses to norepinephrine but failed to affect escape parameters. Propranolol decreased norepinephrine-induced escape dose dependently. Adenosine deaminase attenuated escape, and the combination of this enzyme plus propranolol nearly abolished escape from norepinephrine-induced vasoconstriction. Methylene blue also diminished autoregulatory escape. These findings suggest that norepinephrine-induced autoregulatory escape involves simultaneous beta-adrenoceptor, purinergic, and endothelial mediation. Norepinephrine-evoked mesenteric vasoconstriction appears to involve predominantly alpha 2-adrenoceptors and is modulated by peptidergic sensory nerves and adenosine.

Protective function of extrinsic sensory neurons in acute rabbit experimental colitis

BACKGROUND/AIMS

Sensory nerves appear to have a protective effect against acute injury in the gastric mucosa. Their function in the intestine is unclear.

METHODS

In this study an immune-complex model of colitis was used to induce inflammation in the distal colon with and without functional ablation of sensory neurons by capsaicin pretreatment.

RESULTS

Colitis was more severe in the capsaicin-pretreated group than in the vehicle group 48 and 96 hours after induction of colitis. Neutrophil infiltration, expressed as inflammatory index, was significantly increased to 4.25 +/- 0.4 vs. 1.83 +/- 0.5 at 48 hours and to 2.66 +/- 0.6 vs. 1.65 +/- 0.3 at 96 hours in the capsaicin group and the vehicle group, respectively. The microscopic ulcer index also was significantly increased in the capsaicin-pretreated group compared with the vehicle group (63.3 +/- 10.6 vs. 3.3 +/- 2.4 at 48 hours, 20.0 +/- 8.4 vs. 1.5 +/- 1.1 at 96 hours). Immunoreactive substance P (SP) and calcitonin gene-related peptide (CGRP) contents were decreased in extracts of inflamed compared with uninflamed colon.

CONCLUSIONS

These data suggest that sensory neurons have a protective role in an acute rabbit model of experimental colitis by release of sensory neuropeptides (SP, CGRP), which may modulate vascular tone and mucosal blood flow.

Capsaicin sensitive nerves in the jejunum of Nippostrongylus brasiliensis-sensitized rats participate in a cardiovascular depressor reflex

Superfusion of capsaicin onto the serosal surface of jejunum of Nippostrongylus brasiliensis-sensitized rats induces a short-lasting (1-3 min), dose-dependent (2 to 20 micrograms) decrease in blood pressure which ranges from -5.3 +/- 1.4% to -22.6 +/- 2.2%. The hypotension evoked by capsaicin was more marked in sensitized rats than in unsensitized animals, which responded only to the highest dose (20 mg) of capsaicin tested. The hypotensive effects of capsaicin were not affected by intravenous injections of mepyramine (10 mg/kg), a histamine receptor antagonist, or by the cyclooxygenase inhibitor indomethacin (10 mg/kg). However, an intravenous injection of a platelet-activating factor (PAF) antagonist, BN 52021 (20 mg/kg), or an intraperitoneal injection of guanethidine (8 mg/kg) 18 h prior to experimentation, to functionally impair the sympathetic nerves, abolished the capsaicin-induced drop in blood pressure. Treatment of neonatal rats with capsaicin reduced by 75% the hypotensive effects of capsaicin, whereas the capsaicin antagonist, ruthenium red, reduced non-significantly the hypotensive action of capsaicin. It is concluded that the activation of jejunal sensory nerves in N. brasiliensis-sensitized rats by capsaicin induced a reflex hypotension that is dependent upon PAF release from mast cells and functional sympathetic nerves. In addition, the afferent function of the sensory nerves are not totally blocked by ruthenium red as capsaicin elicits the reflex hypotension in the presence of this blocker of sensory nerve efferent function.

Primary sensory nerves mediate in part the protective mesenteric hyperemia after intraduodenal acidification in rats

BACKGROUND

The mechanism of intraduodenal HCl-induced mesenteric hyperemia is unknown. In anesthetized rats, the hypothesis that the primary sensory nerves mediate the intraduodenal HCl-induced protective mesenteric hyperemia was tested.

METHODS

The hyperemic response in the superior mesenteric artery (SMA) and superficial villus damage after intraduodenal bolus administration of saline, 0.03N, or 0.1N HCl were measured. These changes induced by 0.1N HCl after mucosal anesthesia (1% lidocaine) or afferent nerve ablation (125 mg/kg subcutaneous capsaicin) were evaluated. The duodenal villus damage induced by intraduodenal perfusion of 0.1N HCl after mucosal afferent nerve stimulation by intraduodenal capsaicin or afferent nerve ablation by subcutaneous capsaicin was examined.

RESULTS

Intraduodenal bolus administration of HCl produced a dose-related increase in SMA blood flow and villus tip damage. The mesenteric hyperemia induced by 0.1N HCl was significantly reduced, but the villus tip damage was not altered by prior intraduodenal lidocaine or subcutaneous capsaicin. The deep duodenal villus damage produced by intraduodenal perfusion of 0.1N HCl was decreased by intraduodenal but increased by subcutaneous capsaicin.

CONCLUSIONS

The capsaicin-sensitive afferent nerves mediate in part the HCl-induced mesenteric hyperemia. They protect against the deep but not the superficial duodenal villus damage induced by HCl.

Involvement of capsaicin-sensitive neurones in the haemodynamic effects of exogenous vasoactive peptides: studies in conscious, adult Long Evans rats treated neonatally with capsaicin

1. The regional haemodynamic effects of i.v. bolus injections of bradykinin (0.05 or 0.5 nmol), cholecystokinin (0.175 or 1.75 nmol), substance P (0.01 or 0.1 nmol) and calcitonin gene-related peptide (0.05 or 0.5 nmol) were assessed in conscious, adult Long Evans rats that had been treated neonatally with either capsaicin (50 mg kg-1, s.c.) or vehicle. 2. In vehicle-treated rats, both doses of bradykinin were without effect on blood pressure, but caused tachycardia and hindquarters vasodilatation. Moreover, after the higher dose there were dilatations in the renal and superior mesenteric vascular beds. In capsaicin-treated rats the hindquarters vasodilator effects elicited by both doses of bradykinin were significantly reduced, while the tachycardia and responses in the renal and superior mesenteric vascular beds were unchanged. 3. In vehicle-treated rats, cholecystokinin caused dose-dependent increases in blood pressure accompanied by renal, superior mesenteric and hindquarters vasoconstriction followed, after the higher dose, by a hindquarters vasodilatation. The lower dose produced a tachycardia, while there was a bradycardia followed by a tachycardia after the higher dose. In capsaicin-treated rats, the pressor response, as well as the renal vasoconstrictor effects of cholecystokinin, were greater than in vehicle-treated rats, while the heart rate, superior mesenteric or hindquarters responses were not different. 4. In vehicle-treated rats, substance P produced a dose-dependent depressor response and tachycardia accompanied by dilatations in the renal and hindquarters vascular beds and constriction in the superior mesenteric vascular bed. In capsaicin-treated rats, the responses to the lower dose of substance P were not different from those in vehicle-treated rats, while the depressor response to the higher dose of substance P was slightly less than in vehicle-treated rats and the renal vasodilatation was absent.5. In vehicle-treated rats, calcitonin gene-related peptide caused dose-dependent hypotensive and tachycardic effects associated with dilatations in renal and hindquarters vascular beds and a constriction in the superior mesenteric vascular bed. After the higher dose, the renal vasodilatation was followed by a modest vasoconstriction. In capsaicin-treated rats, the depressor responses to both doses of calcitonin generelated peptide were slightly more prolonged than in vehicle-treated animals, whereas the heart rate and renal and mesenteric vascular conductance changes were not significantly different. However, there was a more sustained hindquarters vasodilator response to the higher dose of calcitonin gene-related peptide in the capsaicin-treated rats.6. The results suggest that peripheral, capsaicin-sensitive neurones are involved in the cardiovascular responses to exogenous bradykinin and cholecystokinin in conscious rats. It does not appear that the extent of involvement of these neurones is underestimated on account of development of marked supersensitivity to the peptides they normally release, since responses to such peptides (e.g. substance P and calcitonin gene-related peptide) are relatively normal in capsaicin-treated rats.

Capsaicin-sensitive nerves mediate inhibitory junction potentials and dilatation in guinea-pig mesenteric artery

1. The present study examined the effects of repetitive nerve stimulation on membrane potential and on contractile responses to noradrenaline in the guinea-pig inferior mesenteric artery and its distal branches. 2. Repetitive stimulation of perivascular nerves evoked slow inhibitory junction potentials (IJPs) and dilator responses. Individual nerve shocks elicited excitatory junction potentials (EJP)s. 3. Stimulation-evoked IJPs were abolished in the presence of tetrodotoxin (0.3 microM) or a low-Ca2+ (0.5 mM) superfusion solution. 4. The amplitudes and durations of IJPs were dependent on the frequency and duration of repetitive nerve stimulation. Nerve stimulation delivered at 5 Hz for 5 s induced IJPs which had an average amplitude of 2 mV and an average duration of 130 s. When the time interval between successive stimulation periods was less than 4 min, the amplitudes of IJPs were reduced in a time-dependent manner. 5. Stimulation-evoked IJPs were unaffected following endothelium removal. Furthermore, stimulation-evoked IJPs were not affected by atropine (1 microM), indomethacin (20 microM), prazosin (0.5 microM), phentolamine (10 microM), propranolol (0.5 microM) or alpha,beta-methylene ATP (0.2 microM). 6. Pre-treatment of arteries with guanethidine (30 microM) or 6-hydroxydopamine (0.4 mM) abolished stimulation-evoked EJPs but had no effect on stimulation-evoked IJPs. 7. In a similar manner to repetitive nerve stimulation, capsaicin (10 microM) itself induced membrane hyperpolarization and dilatation in mesenteric arteries. Moreover, following application of capsaicin (10 microM), stimulation-evoked IJPs and dilator responses were abolished. 8. EJPs evoked during stimulation-induced IJPs were reduced in amplitude, compared to EJPs evoked under resting conditions. 9. These findings suggest that, in addition to an excitatory sympathetic innervation, mesenteric arteries receive an inhibitory, capsaicin-sensitive innervation which is activated by low-frequency repetitive stimulation.