GABAA receptor-mediated positive inotropism in guinea-pig isolated left atria: evidence for the involvement of capsaicin-sensitive nerves

Capsaicin and Gut Microbiota in Health and Disease

Capsaicin is a widespread spice known for its analgesic qualities. Although a comprehensive body of evidence suggests pleiotropic benefits of capsaicin, including anti-inflammatory, antioxidant, anti-proliferative, metabolic, or cardioprotective effects, it is frequently avoided due to reported digestive side-effects. As the gut bacterial profile is strongly linked to diet and capsaicin displays modulatory effects on gut microbiota, a new hypothesis has recently emerged about its possible applicability against widespread pathologies, such as metabolic and inflammatory diseases. The present review explores the capsaicin–microbiota crosstalk and capsaicin effect on dysbiosis, and illustrates the intimate mechanisms that underlie its action in preventing the onset or development of pathologies like obesity, diabetes, or inflammatory bowel diseases. A possible antimicrobial property of capsaicin, mediated by the beneficial alteration of microbiota, is also discussed. However, as data are coming mostly from experimental models, caution is needed in translating these findings to humans.

Capsaicin‐sensitive sensory neurons regulate myocardial function and gene expression pattern of rat hearts: a DNA microarray study

We have previously shown that capsaicin-sensitive sensory nerves contribute to the regulation of normal cardiac function and to the development of cardiac adaptation to ischemic stress; however, the underlying molecular mechanisms remain unknown. Therefore, here we assessed cardiac functional alterations and relative gene expression changes by DNA microarray analysis of 6400 genes in rat hearts 7 days after the end of systemic capsaicin treatment protocol leading to selective sensory chemodenervation. Capsaicin pretreatment resulted in a cardiac dysfunction characterized by elevation of left ventricular end-diastolic pressure and led to altered expression of 80 genes of known function or homology to known sequences. Forty-seven genes exhibited significant up-regulation and 33 genes were down-regulated (changes ranged from -3.9 to +4.8-fold). The expression changes of 10 selected genes were verified, and an additional 11 genes were examined by real-time quantitative PCR. This is the first demonstration that gene expression changes in the heart due to capsaicin pretreatment included vanilloid receptor-1 (capsaicin receptor), transient receptor potential protein, GABA receptor rho-3 subunit, 5-hydroxytryptamine 3 receptor B, neurokinin receptor 2, endothelial nitric oxide synthase, matrix metalloproteinase-13, cytochrome P450, farnesyl-transferase, ApoB, and leptin. None of the genes have been previously shown to be involved in the mechanism of the cardiac functional effects of sensory chemodenervation by capsaicin. We conclude that capsaicin-sensitive sensory nerves play a significant role in the regulation of a variety of neuronal and non-neuronal genes in the heart and possibly in other tissues as well.

GABAergic mechanisms of gastroprotection in the rat: role of sensory neurons, prostaglandins, and nitric oxide

gamma-Aminobutyric acid (GABA) is a neurotransmitter found in both the central and the peripheral nervous systems including the gastrointestinal tract. The aims of the present studies were to examine mechanisms by which GABA exerts gastroprotective effects against ethanol- and water-restraint stress (WRS)-induced gastric mucosal injury in the rat. GABA, administered intragastrically (400 mg/kg), induced gastroprotection against ethanol and WRS by activating gastric sensory neurons to release calcitonin gene-related peptide (CGRP) and promote nitric oxide (NO) synthesis and release. Furthermore, these protective effects of GABA were associated with an increase in gastric mucosal blood flow (GMBF) that was dependent on sensory neuron and NO systems. GABA-mediated protection involved GABAA receptor activation and prostaglandin generation. In conclusion, intraluminal GABA protects the stomach against ethanol- and WRS-induced injury by mechanisms which involve sensory neuron/CGRP/NO pathways and increases in GMBF and prostaglandin generation.

Capsazocaine: a capsaicin-sensitive functional antagonist displays an argument on sensory capsaicin receptor

1. Intravenous infusion of capsazocaine (CAPBZ), a molecular fusion product of irritant synthetic capsaicin and local analgesic benzocaine, at 100 micrograms/kg/min for 15 min inhibited capsaicin (10 micrograms/kg, IV)-induced spinal release of substance P-like immunoreactivity and vagus reflex responses in blood pressure and heart rate changes in rats. 2. Intrathecal perfusion of CAPBZ (1.0 nM) also reversed retrograde epigastric intraarterial capsaicin (10 micrograms/kg)-induced hypotensive spinal reflex. 3. In isolated guinea pig tissues, CAPBZ (1.0-100.0 microM) inhibited capsaicin (1.0 microM)-sensitive sensory and functional activities, including cardiatonic, bronchial, tracheal and ileal contractilities. CAPBZ is suggested to be a capsaicin antagonist.

Prejunctional modulation by nociceptin of nerve-mediated inotropic responses in guinea-pig left atrium

The superimposition of a train of electrical stimuli (15 Hz. 1 ms, 60 V for 2.5 s) to the electrically driven (3 Hz) isolated left atria from reserpine-pretreated guinea pigs in the presence of atropine (1 microM) produces a delayed positive inotropic response due to the antidromic activation of capsaicin-sensitive primary afferents and the release of the sensory neuropeptide, calcitonin gene-related peptide (CGRP). The novel opioid peptide nociceptin, inhibited (Emax 88% inhibition at 1 microM) in a concentration-dependent manner (10 nM-1 microM) (EC50 33 nM) the delayed positive inotropic response induced by train electrical field stimulation, without affecting the positive inotropic response produced by exogenous CGRP (10 nM) or capsaicin (30 nM). The inhibitory effect of nociceptin on the delayed positive inotropic response induced by train electrical field stimulation was not antagonized by the opioid receptor antagonists naloxone, naltrindole and nor-binaltorphimine (1 microM each) nor was it modified by a cocktail of peptidase inhibitors (bestatin, captopril and thiorphan, 1 microM each). A significant inhibition by nociceptin (1 microM) was also observed toward the sympathetic positive inotropic response produced by EFS at 5 Hz in the presence of atropine (1 microM) and after in vitro capsaicin desensitization and toward the parasympathetic negative inotropic response produced by EFS at 10 Hz in atria from reserpine-pretreated guinea pigs and after in vitro capsaicin desensitization. We conclude that nociceptin exerts a prejunctional inhibitory effect on evoked release of CGRP from capsaicin-sensitive sensory nerve terminals in guinea-pig left atria. The effect of nociceptin occurs independently from the activation of mu-, delta- or kappa-opioid receptors. Nociceptin, at appropriate frequency of stimulation, appears to exert a general inhibitory neuromodulation on transmitters release in guinea-pig left atria.

Multiple sensory and functional effects of non-phenolic aminodimethylene nonivamide: an approach to capsaicin antagonist

1. Hexylaminodimethylene nonivamide (CAPCNC6, 0.1-10 microM) inhibited the contractility of isolated guinea pig right atria, toxically revealed positive inotropic, chronotropic and then a cardiac arrest effect at 100 microM and inhibited capsaicin (1.0 microM)-induced cardiotonic effects. 2. CAPCNC6 (0.1-10 microM)-induced aorta contractions were inhibited in the presence of flunarizine, atropine, phentolamine, Ca(2+)-free solution and pre-treatment of the animal with capsaicin. 3. CAPCNC6 (1.0-300 microM)-induced trachea contractions were inhibited in the presence of capsazepine, ruthenium red, hCGRP8-37 and pre-treatment of the animal with capsaicin.

Adenosine inhibits efferent function of extrinsic capsaicin-sensitive sensory nerves in the enteric nervous system

Capsaicin (1-3 microM) and electrical stimulation of mesenteric nerves in the presence of hexamethonium and guanethidine antidromically stimulate extrinsic sensory nerve fibers to produce a specific slow depolarizing response of myenteric neurons and a contractile response of muscles in the isolated guinea-pig ileum, mediated by release of substance P and acetylcholine. Adenosine (1-100 microM) inhibited the response to mesenteric nerve stimulation. Adenosine (10-100 microM) suppressed the contractile response to a threshold concentration of capsaicin (1 microM) while leaving the contractile response to a submaximal concentration of substance P (1 nM) and acetylcholine (0.1 microM) intact. Adenosine (1-10 microM) inhibited dose dependently the capsaicin 10 microM)-induced depolarization of myenteric neurons, but did not inhibit the depolarizing response to exogenous substance P. The adenosine P1 receptor antagonist, 8-phenyltheophylline (1-10 microM), antagonized the inhibitory effect of adenosine (1-10 microM) on the mechanical responses. We conclude that adenosine-induced prejunctional inhibition of the mechanical responses is mediated by adenosine P1 receptors.

Non-adrenergic non-cholinergic (NANC) neural control of the atrial myocardium

1. Current concepts in the regulation of atrial contractility by non-adrenergic non-cholinergic (NANC) sensory nerves are reviewed. 2. There is now evidence that in addition to sympathetic and parasympathetic innervation capsaicin-sensitive sensory nerves contribute to the local control of atrial contractility by releasing NANC transmitters, such as calcitonin gene-related peptide (CGRP). 3. Certain chemical and physical stimuli affect atrial contractility by inducing the release of CGRP from sensory nerves. In addition, as widely recognized for the sympathetic and vagal atrial innervation, NANC neurotransmission is under the inhibitory control of several endogenous modulators. 4. Cardioexcitatory actions of NANC neurotransmission on the atrial myocardium are considered. 5. Pharmacological modulation of NANC neurotransmission and functional evidence for cross-talk between NANC and sympathetic neurones are also discussed.

Alpha-adrenoceptor modulation of the efferent function of capsaicin-sensitive sensory neurones in guinea-pig isolated atria

1. Transmural nerve stimulation of guinea-pig atria, obtained from animals pretreated with reserpine (5 mg kg-1, i.p.), in the presence of atropine 1 microM and of the beta-adrenoceptor blocker CGP 20712A 1 microM, induced a positive inotropic effect which was reduced by the calcitonin gene-related peptide (CGRP) antagonist hCGRP-(8-37) and abolished by pretreatment with capsaicin 1 microM. 2. Noradrenaline concentration-dependently (0.01-10 microM) reduced the increase in cardiac contractility induced by transmural nerve stimulation. The inhibitory effect of noradrenaline was antagonized by yohimbine (0.5-1 microM), in a dose-dependent manner. Prazosin (0.5-1 microM) antagonized the effect of noradrenaline and this effect was independent of concentration. 3. In the presence of yohimbine, the lower part of the inhibitory-response curve for noradrenaline was slightly but significantly shifted by prazosin. A similar degree of antagonism was observed in the presence of 1 microM phenoxybenzamine. 4. The selective alpha 2 agonists BHT 920 and clonidine reduced, in the same concentration-range (0.01-1 microM), the cardiac response to transmural nerve stimulation in a yohimbine-sensitive fashion. 5. Phenylephrine (0.1-100 microM) and methoxamine (1-300 microM) also induced an inhibitory effect on transmural nerve stimulation. The effect of phenylephrine was antagonized by yohimbine (1 microM) more efficiently than by prazosin (0.5 microM). 6. These results are in keeping with the presence of inhibitory prejunctional alpha 2-adrenoceptors on cardiac sensory nerve endings which modulate the efferent function of capsaicin-sensitive neurones.

Inhibitory actions of prostaglandin E1 on neurogenic plasma extravasation in rat airways

To determine whether neurogenic inflammation can be inhibited by prostaglandin E1 (PGE1), that is suggested to have an inhibitory effect on neuropeptide release from airway sensory nerves, we examined plasma extravasation in the airways of anesthetized rats in vivo with Evans blue due as a marker. Neurogenic inflammation was produced by an i.v. injection of capsaicin (100 micrograms/kg) or by antidromic electrical stimulation of the right vagus nerve (4 Hz, 1 ms, 4 V for 1 min). Capsaicin injection significantly increased leakage of dye in the trachea and main bronchi. Similar increases in leakage were seen in the trachea and right bronchus on electrical stimulation of the right vagus nerve. PGE1 (1-1000 micrograms/kg) inhibited the leakage induced by capsaicin in the trachea and bronchi concentration dependently with complete inhibition at a concentration of 1000 micrograms/kg. Likewise, PGE1 (1000 micrograms/kg) significantly inhibited electrical stimulation-induced leakage in the trachea and right bronchus (P less than 0.01). I.v. substance P (SP; 1 microgram/kg) increased Evans blue dye extravasation in the same way as the leakage induced by capsaicin and electrical stimulation but PGE1 (1000 micrograms/kg) failed to inhibit SP-induced leakage in the trachea and main bronchi (P greater than 0.20). These results suggest that PGE1 inhibits neurogenic plasma leakage by presynaptic inhibition of the release of neuropeptides from sensory nerves.

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.

The neurotransmitter role of calcitonin gene-related peptide in the rat and guinea-pig ureter: effect of a calcitonin gene-related peptide antagonist and species-related differences in the action of omega conotoxin on calcitonin gene-related peptide release from primary afferents

In the rat and guinea-pig isolated ureter electrical field stimulation of intrinsic nerves (10 Hz for 10 s) produces transient inhibition of evoked (20 mM KCl or 0.1-1 microM neurokinin A) rhythmic contractions by releasing transmitter(s) from peripheral endings of capsaicin-sensitive primary afferents. The C-terminal fragment of human calcitonin gene-related peptide (8-37) blocked the inhibitory effect of electrical field stimulation as well as that produced by exogenous calcitonin gene-related peptide, while leaving unaffected the inhibitory response to isoprenaline. Human calcitonin gene-related peptide (8-37) was devoid of any inhibitory activity of its own but enhanced the amplitude and frequency of KCl-evoked rhythmic contractions in the rat ureter, probably by antagonizing the inhibitory effect of endogenous calcitonin gene-related peptide released by KCl. Omega conotoxin fraction GVIA, a peptide which possesses a potent blocking activity of N-type voltage-sensitive calcium channels, prevented the inhibitory response to electrical stimulation in the guinea-pig ureter, while leaving the response unaffected in the rat ureter. Conotoxin had no effect toward the inhibition produced by exogenous calcitonin gene-related peptide indicating its prejunctional site of action, demonstrated previously in the guinea-pig ureter [Maggi et al. (1990) Neurosci, Lett. 114, 203-206]. Dermorphin, an amphibian peptide with potent agonist activity on mu-type opioid receptors, inhibited the response to electrical stimulation in the guinea-pig ureter but had no effect in the rat ureter.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAA and GABAB receptors modulate the K(+)-evoked release of sensory CGRP from the guinea pig urinary bladder

Superfusion of mucosa-free muscle slices of guinea-pig urinary bladder with 40 mM K+ produced a remarkable increase in calcitonin gene-related peptide-like immunoreactivity (CGRP-LI), that in this organ is entirely contained in capsaicin-sensitive nerves. GABA (1 mM) did neither affect the basal nor the 40 mM K+ evoked CGRP-LI release. Baclofen (0.1 mM) or muscimol (1 mM) did not affect the basal CGRP-LI outflow. However, baclofen (0.1 mM) significantly reduced by 32% and muscimol (0.1-1 mM) significantly increased by 60% and 70%, respectively the K(+)-evoked CGRP-LI release. These findings add neurochemical evidence to the functional data suggesting the existence of GABAA and GABAB receptors which modulate the efferent function of capsaicin-sensitive afferents.

GABA agonists and omega conotoxin GVIA modulate responses to nerve activation of the perfused rat mesentery

The modulatory actions of gamma-aminobutyric acid (GABA) receptor agonists and omega-conotoxin GVIA (CTX) on sympathetic and sensory nerves were examined on contractile responses of the perfused rat mesentery to transmural nerve stimulation (TNS). GABA and baclofen, a selective GABAB receptor agonist, significantly inhibited vasoconstrictor responses to TNS, while muscimol, a selective GABAA receptor agonist, had no effect. In the guanethidine treated and methoxamine-contracted mesentery, TNS caused a vasodilator response which was unaffected by GABA. CTX (10(-8) M) markedly suppressed the vasoconstrictor response to TNS, but did not affect vasodilator responses. These findings suggest that in the rat mesentery: (1) GABA receptors modulate the activity of sympathetic nerves via prejunctional GABAB receptors, but do not influence sensory nerves, and (2) calcium channels which participate in sympathetic nerve activation have different properties than calcium channels in capsaicin-sensitive sensory nerves.

Neurochemical evidence for the involvement of N-type calcium channels in transmitter secretion from peripheral endings of sensory nerves in guinea pigs

In the guinea pig ureter, substance P-(SP) and calcitonin gene-related peptide-(CGRP) like immunoreactivity (LI) were depleted by systemic capsaicin pretreatment, indicating that they are entirely stored in peripheral endings of primary afferent neurons. Electrical field stimulation (20 Hz, 60 V, 0.5 ms) evoked the simultaneous release of SP- and CGRP-LI from superfused guinea pig ureters which was abolished by tetrodotoxin (0.3 microM). omega-Conotoxin (0.1 microM), a potent blocker of N-type voltage-sensitive calcium channels, reduced by 50-70% the evoked release of both peptides. These findings provide direct neurochemical evidence indicating that conotoxin-sensitive calcium channels play a role in transmitter secretion evoked by antidromic invasion of peripheral terminals of capsaicin-sensitive primary afferents.

Human isolated ileum: motor responses of the circular muscle to electrical field stimulation and exogenous neuropeptides

(1) Circularly-oriented muscle strips from the human ileum responded to electrical field stimulation (1-50 Hz) with frequency-related primary relaxation at low frequency and primary contractions at high frequencies of stimulation. Both responses were abolished or markedly reduced by tetrodotoxin (1 microM). (2) Atropine (3 microM) or omega conotoxin (0.1 microM) reduced but dit not abolish contraction to electrical field stimulation and enhanced the relaxation. Omega conotoxin (0.1 microM) did not affect carbachol-induced contraction nor isoprenaline-induced relaxation. (3) Neurokinin A and substance P (1 nM-1 microM) produced a concentration-dependent contraction. The NK-1 receptor selective agonist, [Pro9]SP sulfone and the NK-2 receptor selective agonist [beta Ala8]NKA(4-10) produced a contraction superimposable to that of substance P and neurokinin A, respectively. On the other hand, [MePhe7]-neurokinin B, an NK-3 receptor selective agonist was ineffective up to 1 microM. The response to substance P or neurokinin A was unaffected by atropine (3 microM). (4) Galanin, up to 0.1 microM, produced a weak and inconsistent contraction. (5) Vasoactive intestinal polypeptide (10 nM-1 microM) produced a concentration-dependent relaxation while human alpha calcitonin gene-related peptide exerted a weak and inconsistent relaxant effect. (6) These findings indicate that both cholinergic excitatory and non-cholinergic inhibitory nerves affect the motility of the circular muscle of the human small intestine. Transmitter release from excitatory nerves seems largely mediated by activation of omega conotoxin-sensitive (N-type) calcium channels. Tachykinins exert a potent contractile effect, independently of cholinergic nerves, via NK-1 and NK-2 receptors.

Opioid receptors and prejunctional modulation of capsaicin-sensitive sensory nerves in guinea-pig left atrium

1. In the isolated electrically driven left atria from reserpine-pretreated guinea-pigs and in presence of 1 microM atropine, electrical field stimulation (EFS) at 10 Hz produces a delayed positive inotropic response (DPIR) involving activation of capsaicin-sensitive afferents. 2. Opioids inhibited the DPIR with the following order of potency: dermorphin greater than [D-Ala2,N-MePhe4, Gly5-ol]-enkephalin (DAGO) greater than or equal to [D-Ala2,D-Leu5]-enkephalin (DADLE) greater than morphine greater than dynorphin A (1-13) greater than [D-Pen2,D-Pen5]-enkephalin (DPDPE). U-50488 was ineffective up to 10 microM. 3. Opioids also inhibited resting inotropism (3 Hz) with the following rank order of potency: DADLE greater than DAGO greater than U-50488 = dynorphin A (1-13) = morphine = DPDPE. 4. Both inhibition of the DPIR and inhibition of resting inotropism were prevented by 10 microM naloxone. 5. Neither dermorphin (0.1 microM) nor DAGO (0.3 microM) or DADLE (1 microM) inhibit responses produced by capsaicin (30 nM) or calcitonin gene-related peptide (3 nM). 6. These findings indicate that capsaicin-sensitive nerves in the guinea-pig atrium are endowed with mu opioid receptors which inhibit transmitter release when sensory nerve terminals are activated by EFS but not by capsaicin.

The 'efferent' function of capsaicin-sensitive nerves: ruthenium red discriminates between different mechanisms of activation

We have investigated the ability of Ruthenium Red, an inorganic dye with calcium entry blocking properties, to interfere with the 'efferent' function of capsaicin-sensitive sensory nerves. These nerves were activated in the guinea-pig isolated bronchus (atropine in the bath) or left atria (reserpine-pretreated animals, atropine in the bath) by electrical field stimulation or with capsaicin. Both stimuli produced a contraction of the bronchus and a positive inotropic response in the atria, responses which are mediated by endogenous neuropeptides (tachykinins in the bronchus, calcitonin gene-related peptide in the atria) released from sensory nerves. Ruthenium Red (10 microM for 20 min in both cases) selectively inhibited the responses produced by the administration of capsaicin, while leaving the responses to electrical field stimulation unaffected. Likewise, the bronchoconstrictor response to exogenous neurokinin A and the atrial positive inotropic response to calcitonin gene-related peptide were unaffected by Ruthenium Red. A prejunctional site of action of Ruthenium Red was confirmed in release experiments where the dye strongly inhibited the capsaicin-evoked outflow of calcitonin gene-related peptide, which is taken as a marker of activation in sensory nerves. Together with other observations, these findings support the concept that there are two independent mechanisms for activating the 'efferent' function of sensory nerves, one of which is activated by capsaicin and is Ruthenium Red-sensitive but omega-conotoxin-resistant, while the other is activated by propagated action potentials (field stimulation) and is omega-conotoxin-sensitive and Ruthenium Red-resistant.

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.

Multiple mechanisms in the motor responses of the guinea-pig isolated urinary bladder to bradykinin

1. Bradykinin (1 nm-1 microM) produced a contraction of bladder strips excised from the dome of the guinea-pig urinary bladder, an effect which was greatly enhanced by removal of the mucosal layer or by thiorphan (10 microM). All subsequent experiments were performed in mucosa-free strips and in the presence of thiorphan. 2. In carbachol (5 microM)-contracted strips, bradykinin produced a concentration (1 nm-1 microM)-dependent transient relaxation. 3. Kallidin was slightly more potent than bradykinin in producing a contraction and a relaxation of the carbachol-induced tone. By contrast, [des-Arg9]-bradykinin, a selective B1 receptor agonist was barely effective up to 1 microM. 4. The contractile response to bradykinin was: (a) unaffected by either tetrodotoxin (1 microM), in vitro capsaicin desensitization (10 microM for 30 min) or apamin (0.1 microM); (b) antagonized by indomethacin (5 microM), the prostaglandin receptor antagonist SC-19220 (100 microM) or the B2 receptor antagonist [D-Arg0, Hyp3, Thi5,8, Phe7]-bradykinin (10 micron) and (c) almost abolished by nifedipine (1 microM). 5. The antagonism of the contractile response to bradykinin produced by indomethacin and SC-19220 was non-additive while that produced by indomethacin and the B2 receptor antagonist was additive. 6. The relaxant response to bradykinin was unaffected by tetrodotoxin, in vitro capsaicin desensitization or indomethacin but antagonized in a competitive manner by the B2 receptor antagonist. Further, this response was abolished by apamin (0.1 microM) but unaffected by glibenclamide (1 microM). 7. Bradykinin (10 microM) produced a consistent release of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) but not substance P-LI from the guinea-pig bladder muscle. CGRP-LI release by bradykinin was greatly reduced in bladders exposed to indomethacin. [des-Arg9]-bradykinin (10 microM) was ineffective. 8. We conclude that: (a) bradykinin-induced contraction involves activation of both B2 receptors and prostanoid synthesis, via distinct mechanisms which act by inducing calcium influx via nifedipine-sensitive channels; (b) bradykinin-induced relaxation involves activation of B2 receptors and opening of apamin-sensitive potassium channels; (c) bradykinin stimulates sensory nerves in this tissue largely via prostanoid production.