Effects of resiniferatoxin desensitization of capsaicin-sensitive afferents on detrusor over-activity induced by intravesical capsaicin,acetic acid or ATP in conscious rats

Recent studies have provided evidence for a major role of urothelially released ATP acting on a subpopulation of pelvic afferent nerves in mechano-afferent transduction in the bladder. We investigated whether desensitization of capsaicin-sensitive nerve fibres by systemic resiniferatoxin (RTX)-pretreatment can counteract the detrusor over-activity induced by intravesical capsaicin, acetic acid or ATP. Cystometric investigations were performed on awake female Sprague-Dawley rats before and 24 h after injection of RTX (0.3 mg/kg s.c.) or vehicle. The effects of intravesically instilled ATP (0.1 or 1.0 mM), capsaicin (30 microM) or acetic acid (pH 4.0) were compared with those of intravesical saline. RTX, but not its vehicle, significantly increased threshold pressure, voiding interval, micturition volume and bladder capacity. In the vehicle-pretreated rats, intravesical instillation of capsaicin or acetic acid significantly decreased voiding interval, micturition volume, and bladder capacity. However, in the RTX-pretreated rats, neither capsaicin nor acetic acid affected any parameter. On the other hand, intravesical ATP (0.1 mM) significantly decreased voiding interval and micturition volume in both groups of animals. At 1.0 mM, ATP also increased basal pressure and decreased the pressure threshold for micturition in both groups. The present results support the view that increased extracellular ATP has a role in mechano-afferent transduction in the rat bladder and that ATP-induced facilitation of the micturition reflex is mediated, at least partly, by nerves other than capsaicin-sensitive afferent nerves.     

Pharmacological analysis of the local and reflex responses to bradykinin on rat urinary bladder motility in vivo.

1. The topical application of bradykinin (BK) (0.05-5000 pmol/rat) onto the serosal surface of the urinary bladder in urethane-anaesthetized rats, evoked low amplitude tonic contractions (not exceeding 25 mmHg) or high amplitude (about 50 mmHg), phasic reflex contractions (chemoceptive micturition reflex) which were abolished by bilateral ablation of the pelvic ganglia. In ganglionectomized rats, BK induced only a local, tonic-type contraction. 2. Systemic capsaicin pretreatment (164 mumol kg-1, 4 days before) reduced the incidence of chemoceptive reflex induced by BK (500 pmol/rat) but had no effect on the magnitude of the tonic-type contraction elicited by BK in ganglionectomized rats. Indomethacin (11 mumol kg-1, 20 min before) reduced the incidence but not the amplitude of the reflex contractions induced by topical application of BK (500 pmol/rat). In ganglionectomized rats, indomethacin (11 mumol kg-1, 20 min before) decreased the amplitude of the tonic contraction evoked by BK. Indomethacin did not affect the chemoceptive reflex induced by topical application of capsaicin (15 nmol/rat) onto the bladder. 3. Intrathecal administration of the tachykinin NK1 receptor antagonists, RP 67,580 (10 nmol/rat) or SR 140,333 (10 nmol/rat), abolished the chemoceptive reflex induced by BK without modifying the magnitude of the tonic contraction. SR 140,333 (10 nmol/rat) also abolished the occurrence of the chemoceptive reflex induced by capsaicin. 4. Intravenous administration of the B2 receptor antagonist, Hoe 140 (35 nmol kg-1, 10 min before) abolished the reflex and local effects induced by BK on bladder motility but failed to modify the chemoceptive reflex induced by topical application of capsaicin (15 nmol/rat).(ABSTRACT TRUNCATED AT 250 WORDS)     

The contribution of capsaicin-sensitive sensory nerves to xylene-induced visceral pain in conscious,freely moving rats

1. Intravesical instillation of xylene (10-100%, dissolved in silicone oil) through a catheter implanted into the bladder of conscious, freely-moving rats produced behavioural effects (licking of lower abdomen or perineal region) suggestive of intense visceral pain, not mimicked by topical application of the irritant on the urethral outlet. 2. The xylene-induced visceral pain was prevented, to the same extent, by systemic desensitization to capsaicin (50 mg/kg s.c.) performed in either adult or newborn rats, as well as by extrinsic bladder denervation (pelvic ganglionectomy), thus indicating the involvement of primary afferents in the bladder wall. 3. Other behavioural responses induced by xylene instillation into the bladder (hind limb hyperextension, grooming) were not affected by systemic capsaicin desensitization in either adult or newborn rats, but were abolished by bladder denervation. 4. Systemic capsaicin desensitization produced an almost complete depletion of substance P-, neurokinin A-like and calcitonin gene-related peptide-like immunoreactivity in the rat urinary bladder. 5. These findings indicate that, in addition to their role in activating reflex micturition, the neuropeptides-containing capsaicin-sensitive sensory nerves of the rat bladder are involved in chemogenic visceral pain.     

Vesico-inhibitory responses and capsaicin-sensitive afferents in rats

Summary (1) The effect of perineal pinching and distension of a balloon inserted into the colon on motility of the urinary bladder has been investigated in adult urethane-anesthetized rats pretreated with capsaicin (50 mg/kg s.c.) or its vehicle 4 days before the experiments. (2) At bladder volumes which were sufficient to elicit reflex micturition, perineal pinching or colonic distension transiently inhibited the ongoing bladder voiding contraction. The somato-vesical inhibitory response was markedly reduced or even abolished by division of pudendal nerves. Neither the somato-vesical nor the colovesical inhibitory response were modified by desensitization with systemically administered capsaicin. (3) Intraurethral administration of capsaicin produced a transient inhibition of the reflexly-activated bladder contractions. A second administration of the drug was less effective, indicating desensitization. Intravenously administered capsaicin had a similar inhibitory effect on bladder motility. (4) The vesico-inhibitory response produced by intraurethral administration of capsaicin was not affected by phentolamine, propranolol, guanethidine, picrotoxin or naloxone, while it was greatly reduced or even abolished by bilateral section of the pudendal nerves. (5) These findings provide evidence that capsaicin-sensitive chemoreceptors in the rat urethra are involved in generating a vesico-inhibitory response via pudendal nerves. On the other hand, no evidence was found for the participation of capsaicin-sensitive nerves in the generation of the somato- or colo-vesical inhibitory response. Send offprint requests to C. A. Maggi at the above address     

Evidence that toluene diisocyanate activates the efferent function of capsaicin-sensitive primary afferents

Isocyanates are an important cause of occupational asthma. The mechanism of isocyanate-induced asthma is still unknown. To determine whether toluene diisocyanate stimulates the 'efferent' function of peripheral endings of capsaicin-sensitive sensory nerves, we investigated the effect of toluene diisocyanate in the rat isolated urinary bladder, a preparation in which the action of capsaicin has been well characterized. Toluene diisocyanate (0.03-3 mM) produced a concentration-dependent contraction of the bladder strips. Its maximal effect was about 50% of the response to capsaicin (1 microM). Previous exposure of the strips to capsaicin followed by washing out produced complete unresponsiveness, both to the first exposure to toluene diisocyanate and to a second exposure of capsaicin. Further, the response to both toluene diisocyanate and capsaicin was completely prevented by extrinsic bladder denervation, achieved by bilateral removal of pelvic ganglia (72 h before). Repeated exposure of the rat bladder to toluene diisocyanate reduced the capsaicin-evoked release of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI), taken as biochemical marker of activation of these sensory nerves. These experiments provide the first evidence that toluene diisocyanate activates directly or indirectly the efferent function of capsaicin-sensitive primary sensory nerves.     

The role of the capsaicin-sensitive innervation of the rat urinary bladder in the activation of micturition reflex

Capsaicin applied on the serosal surface of the urinary bladder in urethane-anaesthetized rats produces two distinct types of motor effects: a tetrodotoxin-, hexamethonium- and lidocaine-insensitive 'tonic' contraction and a series of tetrodotoxin-, hexamethonium- and lidocaine-sensitive rhythmic contractions. Both 'tonic' and rhythmic contractions are abolished by bladder denervation indicating their neurogenic origin. The rhythmic but not the 'tonic' component of the contractile effect of capsaicin is abolished by spinal cord transection indicating activation of a supraspinal micturition reflex. The motor effects of topical capsaicin are unaffected by pretreatment with indomethacin or diphenhydramine plus cimetidine. Pretreatment with a large dose of subcutaneous (SC) capsaicin increases both volume and pressure threshold for micturition while amplitude of micturition contraction is unaffected. Moreover the spinal somatovesical reflex elicited by pinching of the perineal skin is unaffected by capsaicin-desensitization. The intracerebroventricular (ICV) administration of capsaicin reproduces the effects of SC capsaicin on the bladder response to saline filling. Rats pretreated with ICV capsaicin are as sensitive as controls in reacting to noxious heat (hot plate test) while the wiping response to instillation of capsaicin into one eye was abolished. These findings provide functional evidence for the presence in the rat urinary bladder of a capsaicin-sensitive innervation which subserves a sensory function in relaying volume/pressure information from detrusor muscle to central nervous system. Information carried through these capsaicin-sensitive fibers appears to be relevant for initiation of a supraspinal vesico-vesical micturition reflex. Functional evidence indicates that these fibers may terminate at supraspinal level.     

Cadmium chloride induces contractions of the rat isolated urinary bladder by activation of capsaicin-sensitive sensory nerves

Cadmium chloride (CdCl2)(30 microM-1 mM) produced a concentration-related contraction of the rat isolated urinary bladder which was abolished by tetrodotoxin (1 microM) or extrinsic bladder denervation (72 h before). Complete cross-desensitization was observed between the contractile response to Cd and capsaicin, indicating that, at the peripheral level, this inorganic calcium channel blocker can activate the 'efferent' function of capsaicin-sensitive sensory nerves.     

Visceromotor responses to calcitonin gene-related peptide (CGRP) in the rat lower urinary tract: evidence for a transmitter role in the capsaicin-sensitive nerves of the ureter

Either intra-arterial or topical administration of calcitonin gene-related peptide (CGRP) had little effect on motility of the urinary bladder in urethane-anaesthetized rats. Only a high concentration (50 microM) of topical CGRP activated the micturition reflex and potentiated the response to exogenous substance P (SP). In the isolated rat bladder CGRP had inconsistent effects on spontaneous or field-stimulated contractions. CGRP neither produced any significant plasma extravasation (Evans blue leakage) in the rat lower urinary tract, nor potentiated the response to exogenous SP. CGRP inhibited motility in the rat isolated proximal urethra and ureters and counteracted the contractile response to neurokinins. An inhibitory effect of capsaicin on stimulated motility of the urethra was observed in all preparations and a small contractile response was evident in about 40% of cases. Lack of desensitization to the action of CGRP prevented the study of its interaction with capsaicin. The inhibitory effect of CGRP in the ureter exhibited a specific desensitization: if the preparations were pre-exposed to exogenous CGRP, the inhibition of motility produced by antidromic activation of the capsaicin-sensitive nerve terminals (field stimulation) as well as the response to capsaicin (1 microM) was prevented but the inhibitory response to isoprenaline was unaffected. These findings indicate that CGRP is able to influence markedly the motility of the rat lower urinary tract, but exhibits marked regional differences in its action. Endogenous CGRP could be the inhibitory transmitter which, when released from capsaicin-sensitive fibers, participate in the control of ureteral motility.     

The inhibitory effect of nociceptin on the micturition reflex in anaesthetized rats

1. We have investigated the effect of nociceptin on the micturition reflex evoked by distension or topical application of capsaicin on the urinary bladder of urethane-anaesthetized rats.
2. Nociceptin produced a dose-dependent (3–100 nmol kg⁻¹ i.v.) transient suppression of the distension-evoked micturition reflex: its effect was not modified by guanethidine (68 μmol kg⁻¹ s.c.) nor by bilateral cervical vagotomy, alone or in combination, and by naloxone (1.2 μmol kg⁻¹ i.v.).
3. Nociceptin (100 nmol/kg i.v.) slightly (about 30%) inhibited the contractions of the rat bladder produced by pre- or postganglionic electrical stimulation of the pelvic nerve.
4. Nociceptin almost totally abolished the reflex component of the response to topical capsaicin (1 μg in 50 μl).
5. In the rat isolated bladder, submaximal contractions produced by electrical field stimulation were slightly reduced (25±4% inhibition) by 1 μM nociceptin. Nociceptin did not affect the contraction of the rat bladder induced by acetylcholine (10 μM) or ATP (1 mM).
6. These findings indicate that nociceptin exerts a naloxone-resistant suppression of the volume-evoked micturition reflex which involves inhibition of transmitter release from postganglionic bladder nerves. An inhibitory effect on bladder afferent nerves is also suggested.

     

Excitatory and inhibitory urinary bladder reflexes induced by stimulation of cervicovaginal capsaicin-sensitive sensory fibers in rats

We have investigated the effect of intravaginal application of capsaicin on micturition reflex in female rats. Urinary bladder contractility was measured by transurethral pressure recording at isovolumetric and subthreshold conditions in anaesthetized rats. The intravaginal application of capsaicin (15 mug/50 mul rat) induced reproducible bladder phasic contractions, without desensitization upon repeated applications, that were blocked by intravenous atropine (1 mg/kg) or hexamethonium (5 mg/kg) and prevented by removal of paracervical ganglia or systemic capsaicin pretreatment (125 mg/kg, s.c.). The inhibition of sympathetic transmission by guanethidine (30 mg/kg, s.c.) produced significant increase of the bladder reflex contractions activated by intravaginal capsaicin. Intravenous administration of the TRPV1 antagonist, capsazepine (3 mg/kg), significantly reduced the excitatory reflex response to capsaicin. Intravaginal administration of capsaicin (15 mug/50 mul), during distension-induced reflex bladder contractions, produced a transient block of reflexes, unaffected by guanethidine pretreatment. In conclusion, the stimulation of capsaicin-sensitive sensory nerve endings in the rat cervix-vagina induced a dual excitatory or inhibitory bladder response in anaesthetized female rats depending on the degree of bladder distension.     

Prostanoids modulate reflex micturition by acting through capsaicin-sensitive afferents

Topical application of exogenous prostanoids (PGE2, TBX B2) on the serosal surface of the urinary bladder of urethane-anaesthetized rats activated reflex micturition. Likewise, intravesical instillation of PGE2 during the cystometrogram lowered the threshold for reflex micturition. Both effects were prevented by systemic capsaicin desensitization (50 mg/kg s.c., 4 days before). Indomethacin pretreatment and systemic capsaicin desensitization each increased the micturition threshold without affecting the amplitude of micturition contraction. However, the effect of the two treatments combined was not greater than the effect of either alone. These findings support the idea that endogenous prostanoids facilitate reflex micturition by stimulating or sensitizing, directly or indirectly, the subset of bladder mechanoreceptors which is capsaicin-sensitive in adult rats.     

Capsaicin-sensitive afferents in the rat urinary bladder activate a spinal sympathetic cardiovascular reflex

Summary In urethane-anesthetized rats with an intact spinal cord, application of capsaicin on the outer surface of the urinary bladder produced a transient bradycardia, hypotension and negative cardiac inotropism which were neither prevented by i. v. atropine (0.5 mg/kg) nor by cervical vagotomy. In acute spinal rats (C2-C3) application of capsaicin (0.2 and 2 pg in 25 pl) on the urinary bladder induced a transient hypertension, tachycardia and positive cardiac inotropism. A second application (30 min later) induced minor cardiovascular effects, expecially with the higher dose, indicating desensitization. All cardiovascular responses to topical capsaicin were abolished by systemic capsaicin desensitization (50 mg/kg s. c., 4 days before). The excitatory cardiovascular response to capsaicin in acute spinal rats was markedly reduced by bilateral section of pelvic but not hypogastric nerves. Further, it was abolished by pretreatment with hexamethonium (20 mg/kg i.v.) or reserpine (5 mg/kg i. p., 2 days before) and reduced, at various extent for the different components, by phentolamine (0.5 mg/kg i. v.) or propranolol (1 mg/kg). In rats with pelvic and hypogastric nerves intact, section of the cord at a level (T12-L1), just above the medullary segments which receive primary afferent input from the bladder (L6-S1), abolished the excitatory cardiovascular response to application of capsaicin on the bladder. In spinal rats (C2-C3) rapid distension of the urinary bladder with saline produced transient tachycardia, hypertension and positive cardiac inotropism similar to that evoked by capsaicin. These responses were not observed in rats systemically pretreated with capsaicin. These findings indicate that certain bladder afferents which are susceptible to capsaicin desensitization in adult rats activate a spinal reflex having excitatory influence on cardiovascular function. This response is apparently mediated by spinal centers located above the site of entry of bladder pelvic afferents into the cord and most likely involves excitation of preganglionic sympathetic neurons in the spinal cord.Send offprint requests to S. Giuliani at the above address     

Further studies on the mechanisms of the tachykinin-induced activation of micturition reflex in rats: evidence for the involvement of the capsaicin-sensitive bladder mechanoreceptors

The relative ability of substance P, neurokinin A, neurokinin B and kassinin to activate the micturition reflex was investigated in urethane-anaesthetized rats. When administered topically neurokinin A, neurokinin B and kassinin were 14, 36 and 280 times, respectively, more potent than substance P to activate micturition. On the other hand substance P, neurokinin A and kassinin were practically equipotent (and neurokinin B was about 3-4 times less potent than substance P) to stimulate the contraction of the rat isolated bladder and to potentiate the contractions induced by electrical field stimulation. This indicates that neither a direct action on muscle cells nor a potentiating effect on efferent neurotransmission can account for the rank order of potency of tachykinins for activation of the micturition reflex. The ability of topical tachykinins to activate the micturition reflex was largely impaired in 2 months old rats pretreated with capsaicin (50 mg/kg s.c.) on their second day of life, indicating that integrity of the capsaicin-sensitive bladder mechanoreceptors is essential for the production of this effect. These findings indicate that an NK-B receptor, possibly located on sensory nerves in the bladder wall, participates in the tachykinin-induced activation of reflex micturition.     

Prejunctional modulatory action of neuropeptide Y on peripheral terminals of capsaicin-sensitive sensory nerves.

1. We have determined the effect of neuropeptide Y (NPY) on motor responses produced by activation of capsaicin-sensitive primary afferents in the guinea-pig isolated left atria (reserpine-pretreatment, atropine in the bath) and bronchi (atropine and indomethacin in the bath) using capsaicin itself and electrical field stimulation as stimuli. 2. In both preparations, NPY inhibited or suppressed the response produced by electrical field stimulation while leaving the response to a submaximal concentration of capsaicin unaffected. 3. NPY had no effect on motor responses produced by a submaximal concentration of calcitonin gene-related peptide (atria) or neurokinin A (bronchi), the putative endogenous mediators of the responses produced by activation of the 'efferent' function of sensory fibres in these preparations. 4. We conclude that NPY exerts a prejunctional inhibitory action on transmitter release from peripheral endings of capsaicin-sensitive nerves. Failure of NPY to modulate responses activated by capsaicin provides further evidence for the existence of two independent modes of activation of the 'efferent' function of capsaicin-sensitive sensory nerves.     

Effect of ruthenium red on responses mediated by activation of capsaicin-sensitive nerves of the rat urinary bladder

Summary (1) Topical administration of Ruthenium Red (10–100 M in saline) to the serosal surface of the urinary bladder in urethane-anesthetized rats prevented the motor response of the urinary bladder to topical administration of capsaicin and protected the sensory fibers from capsaicin desensitization, but had no effect on the volume-evoked contractions (micturition reflex). At 1 mM increased bladder capacity and decreased amplitude of micturition contraction were observed. (2) At 100 M, topical Ruthenium Red prevented the blood pressure rise produced by topical administration of capsaicin onto the bladder but did not affect the blood pressure rise produced by sudden bladder distension in spinal rats. (3) After intrathecal administration, Ruthenium Red (80–800 ng/rat) produced a long lasting inhibition of the micturition reflex in urethane-anesthetized rats, this effect being evident in both vehicleor capsaicin- (50 mg/kg s. c. 4 days before) pretreated rats. At 800 ng/rat, intrathecal Ruthenium Red did not affect the blood pressure rise produced by topical administration of capsaicin onto the rat bladder nor that produced by bladder distension. (4) These findings provide further evidence that Ruthenium Red acts quite selectively as a capsaicin antagonist preventing both reflex and efferent responses activated by peripherally administered capsaicin. By contrast, sensory impulse generation by a natural stimulus such as bladder distension is apparently unaffected by Ruthenium Red. The marked inhibition of the micturition reflex observed after intrathecal administration of Ruthenium Red does probably not involve an interaction with primary afferents in the spinal cord.     

Prejunctional modulatory action of neuropeptide Y on responses due to antidromic activation of peripheral terminals of capsaicin-sensitive sensory nerves in the isolated guinea-pig ileum.

1. The effect of neuropeptide Y (NPY) on motor responses produced by activation of capsaicin-sensitive primary afferents in the guinea-pig isolated ileum was determined by use of capsaicin itself and electrical mesenteric nerve stimulation as stimuli. 2. NPY inhibited or suppressed the cholinergic contractile response produced by electrical mesenteric nerve stimulation while leaving the contractile response to a threshold concentration of capsaicin. 3. NPY had no effect on motor responses produced by a submaximal concentration of substance P, the putative endogenous mediator of the 'efferent' function of sensory fibres in this preparation. 4. It is concluded that NPY exerted a prejunctional inhibitory action on transmitter release from peripheral endings of capsaicin-sensitive nerves at interneuronal synapses.     

Connections between P2 purinoceptors and capsaicin-sensitive afferents in the intestine and other tissues.

Relations between P2 purinoceptors and capsaicin-sensitive sensory neurons include an excitatory action of P2 purinoceptor agonists on spinal afferent neurons, as well as release of ATP from afferents at their central and peripheral endings, and a possible participation of ATP in nociception and/or in 'local efferent' responses mediated by sensory nerves at the periphery. The present paper briefly summarizes available evidence on these interrelations. Ample evidence shows that ATP and other P2 purinoceptor agonists can activate primary afferent neurons, through P2X3 receptors and probably other purinoceptors as well, but evidence for an involvement of P2 purinoceptors in nociception or in 'local efferent' responses due to activation of primary afferents is, at best, circumstantial. The possibility is also dealt with that P2 purinoceptor activation may cause small intestinal contraction with the mediation of capsaicin-sensitive sensory neurons and that the motor response to capsaicin in this tissue may involve the release of a P2 purinoceptor stimulant from sensory nerves. Our data show that cholinergic contractions of the guinea-pig ileum in response to the P2 purinoceptor agonist alpha,beta-methylene ATP (alpha,beta-meATP) are blocked by atropine, but not by in vitro capsaicin pretreatment (which completely blocks the contractile action of capsaicin). Cholinergic ileum contractions due to capsaicin (2 microM) are insensitive to suramin (a P2 purinoceptor antagonist; 100 microM). In the presence of antagonists acting at tachykinin NK1 and NK2 receptors, however, suramin (100 microM) causes a significant inhibition of the capsaicin-evoked contraction. These data indicate that capsaicin-sensitive nerves are not involved in the excitatory effect of alpha,beta-methylene ATP on myenteric neurons. On the other hand, ATP is probably involved in the 'non-tachykininergic' component of the capsaicin-induced excitatory response of the small intestine. ATP may originate from sensory neurons and probably acts as activator of myenteric nerves.     

Motor and inflammatory effect of hyperosmolar solutions on the rat urinary bladder in relation to capsaicin-sensitive sensory nerves

1. Intravesical administration of hyperosmolar NaCl or urea solutions produced a concentration-dependent stimulatory action on the micturition reflex in urethane-anesthetized rats. This effect was not modified in rats pretreated with capsaicin as adults (50 mg/kg s.c. 4 days before). 2. Hyperosmolar NaCl also produced Evans blue leakage (plasma extravasation) in the rat bladder. This effect was greatly reduced by extrinsic bladder denervation and in rats desensitized to capsaicin as newborns but not as adults. 3. Cumulative addition of NaCl produced a concentration-dependent increase in tone and biphasic effects on neurogenic contractions of the rat isolated bladder. These effects were not modified by in vitro capsaicin desensitization. 4. These findings do not support the idea that true osmoreceptors are present in the rat urinary bladder. The neurogenic component of the inflammatory response to hyperosmolar NaCl could involve activation of a subpopulation of bladder sensory fibers susceptible to the neurotoxic action of capsaicin in the early postnatal period only.     

The effects of topical capsaicin on rat urinary bladder motility in vivo

The effects of topical capsaicin on urinary bladder motility were investigated following saline-induced distension of the bladder wall in urethane-anaesthetized rats and compared to the effects of topical substance P and acetylcholine. Capsaicin and substance P produced similar excitatory effects in both quiescent and rhythmically contracting bladders, i.e., a TTX resistant tonic contraction followed by a series of rhythmic, TTX sensitive, phasic contractions. Acetylcholine, in doses equieffective in producing TTX resistant contractions was less effective than capsaicin or substance P in triggering neurogenic rhythmic contractions of bladder muscle. Atropine pretreatment prevented the neurogenic component of the excitatory effect of both capsaicin and substance P. Repeated applications of capsaicin but not of substance P led to desensitization. Bladders of animals pretreated (4 days before) with a large dose of s.c. capsaicin developed insensitivity to topical capsaicin and a larger volume of saline was required to trigger neurogenic rhythmic contractions of the detrusor muscle. These results suggest that capsaicin acts by interfering with the mechanism(s) regulating the threshold for the micturition reflex to occur.     

Peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) activates capsaicin-sensitive primary afferent nerves in guinea-pig atria and urinary bladder.

1. We have investigated the ability of the N-formyl-methionyl-leucyl-phenylalanine (FMLP) a synthetic analogue of a chemotactic peptide derived from a variety of bacteria, to activate capsaicin-sensitive primary afferents in the guinea-pig atria and urinary bladder. 2. In the isolated, electrically-driven left atria from reserpine-pretreated guinea-pigs (atropine in the bath), FMLP (3 nM-1 microM) produced a biphasic positive inotropic response. The late component of this response was selectively abolished by in vitro capsaicin pretreatment while both the early and late responses were abolished by indomethacin. 3. The inotropic response to FMLP in the guinea-pig atria was unaffected by ruthenium red. The late but not the early response was strongly inhibited or abolished by tetrodotoxin (TTX), omega-conotoxin (CTX) or by the C-terminal fragment (8-37) of human alpha-calcitonin gene-related peptide (hCGRP). hCGRP-(8-37) acts as competitive antagonist at CGRP receptors. 4. In the guinea-pig isolated bladder, FMLP (10 nM-10 microM) produced a concentration-dependent contraction which was unchanged by previous in vitro capsaicin, TTX or CTX pretreatment. The response to low concentrations of FMLP was suppressed by indomethacin, irrespective of the capsaicin pretreatment. 5. FMLP (10 microM) produced a significant increase in the outflow of CGRP-like immunoreactivity (CGRP-LI) from superfused guinea-pig atria or urinary bladder. CGRP-LI outflow induced by FMLP was blocked by indomethacin or in vitro capsaicin pretreatment. 6. These findings indicate that FMLP activates the 'efferent' function of capsaicin-sensitive primary afferents via prostanoid generation.(ABSTRACT TRUNCATED AT 250 WORDS)