Bradykinin and capsaicin stimulate cyclic GMP production in cultured rat dorsal root ganglion neurons via a nitrosyl intermediate

Peripheral KV7 channels regulate visceral sensory function in mouse and human colon

Background

Chronic visceral pain is a defining symptom of many gastrointestinal disorders. The K_V7 family (K_V7.1–K_V7.5) of voltage-gated potassium channels mediates the M current that regulates excitability in peripheral sensory nociceptors and central pain pathways. Here, we use a combination of immunohistochemistry, gut-nerve electrophysiological recordings in both mouse and human tissues, and single-cell qualitative real-time polymerase chain reaction of gut-projecting sensory neurons, to investigate the contribution of peripheral K_V7 channels to visceral nociception.

Results

Immunohistochemical staining of mouse colon revealed labelling of K_V7 subtypes (K_V7.3 and K_V7.5) with CGRP around intrinsic enteric neurons of the myenteric plexuses and within extrinsic sensory fibres along mesenteric blood vessels. Treatment with the K_V7 opener retigabine almost completely abolished visceral afferent firing evoked by the algogen bradykinin, in agreement with significant co-expression of mRNA transcripts by single-cell qualitative real-time polymerase chain reaction for KCNQ subtypes and the B₂ bradykinin receptor in retrogradely labelled extrinsic sensory neurons from the colon. Retigabine also attenuated responses to mechanical stimulation of the bowel following noxious distension (0–80 mmHg) in a concentration-dependent manner, whereas the K_V7 blocker XE991 potentiated such responses. In human bowel tissues, K_V7.3 and K_V7.5 were expressed in neuronal varicosities co-labelled with synaptophysin and CGRP, and retigabine inhibited bradykinin-induced afferent activation in afferent recordings from human colon.

Conclusions

We show that K_V7 channels contribute to the sensitivity of visceral sensory neurons to noxious chemical and mechanical stimuli in both mouse and human gut tissues. As such, peripherally restricted K_V7 openers may represent a viable therapeutic modality for the treatment of gastrointestinal pathologies.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Final report on the safety assessment of capsicum annuum extract, capsicum annuum fruit extract, capsicum annuum resin, capsicum annuum fruit powder, capsicum frutescens fruit, capsicum frutescens fruit extract, capsicum frutescens resin, and capsaicin

Capsicum-derived ingredients function as skin-conditioning agents--miscellaneous, external analgesics, flavoring agents, or fragrance components in cosmetics. These ingredients are used in 19 cosmetic products at concentrations as high as 5%. Cosmetic-grade material may be extracted using hexane, ethanol, or vegetable oil and contain the full range of phytocompounds that are found in the Capsicum annuum or Capsicum frutescens plant (aka red chiles), including Capsaicin. Aflatoxin and N-nitroso compounds (N-nitrosodimethylamine and N-nitrosopyrrolidine) have been detected as contaminants. The ultraviolet (UV) absorption spectrum for Capsicum Annuum Fruit Extract indicates a small peak at approximately 275 nm, and a gradual increase in absorbance, beginning at approximately 400 nm. Capsicum and paprika are generally recognized as safe by the U.S. Food and Drug Administration for use in food. Hexane, chloroform, and ethyl acetate extracts of Capsicum Frutescens Fruit at 200 mg/kg resulted in death of all mice. In a short-term inhalation toxicity study using rats, no difference was found between vehicle control and a 7% Capsicum Oleoresin solution. In a 4-week feeding study, red chilli (Capsicum annuum) in the diet at concentrations up to 10% was relatively nontoxic in groups of male mice. In an 8-week feeding study using rats, intestinal exfoliation, cytoplasmic fatty vacuolation and centrilobular necrosis of hepatocytes, and aggregation of lymphocytes in the portal areas were seen at 10% Capsicum Frutescens Fruit, but not 2%. Rats fed 0.5 g/kg day-1 crude Capsicum Fruit Extract for 60 days exhibited no significant gross pathology at necropsy, but slight hyperemia of the liver and reddening of the gastric mucosa were observed. Weanling rats fed basal diets supplemented with whole red pepper at concentrations up to 5.0% for up to 8 weeks had no pathology of the large intestines, livers, and kidneys, but destruction of the taste buds and keratinization and erosion of the gastrointestinal (GI) tract were noted in groups fed 0.5% to 5.0% red pepper. The results of 9-and 12-month extension of this study showed normal large intestines and kidneys. In rabbits fed Capsicum Annuum Powder at 5 mg/kg day-1 in the diet daily for 12 months damage to the liver and spleen was noted. A rabbit skin irritation test of Capsicum Annuum Fruit Extract at concentrations ranging from 0.1% to 1.0% produced no irritation, but Capsicum Frutescens Fruit Extract induced concentration-dependent (at 25 to 500 microg/ml) cytotoxicity in a human buccal mucosa fibroblast cell line. An ethanol extract of red chili was mutagenic in Salmonella typhimurium TA98, but not in TA100, or in Escherichia coli. Other genotoxicity assays gave a similar pattern of mixed results. Adenocarcinoma of the abdomen was observed in 7/20 mice fed 100 mg red chilies per day for 12 months; no tumors were seen in control animals. Neoplastic changes in the liver and intestinal tumors were observed in rats fed red chili powder at 80 mg/kg day-1 for 30 days, intestinal and colon tumors were seen in rats fed red chili powder and 1,2-dimethyl hydrazine, but no tumors were observed in controls. In another study in rats, however, red chile pepper in the diet at the same dose decreased the number of tumors seen with 1,2-dimethylhydrazine. Other feeding studies evaluated the effect of red chili peppers on the incidence of stomach tumors produced by N-methyl-N'-nitro-N-nitrosoguanidine, finding that red pepper had a promoting effect. Capsicum Frutescens Fruit Extract promoted the carcinogenic effect of methyl(acetoxymethyl)nitrosamine (carcinogen) or benzene hexachloride (hepatocarcinogen) in inbred male and female Balb/c mice dosed orally (tongue application). Clinical findings include symptoms of cough, sneezing, and runny nose in chili factory workers. Human respiratory responses to Capsicum Oleoresin spray include burning of the throat, wheezing, dry cough, shortness of breath, gagging, gasping, inability to breathe or speak, and, rarely, cyanosis, apnea, and respiratory arrest. A trade name mixture containing 1% to 5% Capsicum Frutescens Fruit Extract induced very slight erythema in 1 of 10 volunteers patch tested for 48 h. Capsicum Frutescens Fruit Extract at 0.025% in a repeated-insult patch test using 103 subjects resulted in no clinically meaningful irritation or allergic contact dermatitis. One epidemiological study indicated that chili pepper consumption may be a strong risk factor for gastric cancer in populations with high intakes of chili pepper; however, other studies did not find this association. Capsaicin functions as an external analgesic, a fragrance ingredient, and as a skin-conditioning agent--miscellaneous in cosmetic products, but is not in current use. Capsaicin is not generally recognized as safe and effective by the U.S. Food and Drug Administration for fever blister and cold sore treatment, but is considered to be safe and effective as an external analgesic counterirritant. Ingested Capsaicin is rapidly absorbed from the stomach and small intestine in animal studies. Subcutaneous injection of Capsaicin in rats resulted in a rise in the blood concentration, reaching a maximum at 5 h; the highest tissue concentrations were in the kidney and lowest in the liver. In vitro percutaneous absorption of Capsaicin has been demonstrated in human, rat, mouse, rabbit, and pig skin. Enhancement of the skin permeation of naproxen (nonsteroidal anti-inflammatory agent) in the presence of Capsaicin has also been demonstrated. Pharmacological and physiological studies demonstrated that Capsaicin, which contains a vanillyl moiety, produces its sensory effects by activating a Ca2 +-permeable ion channel on sensory neurons. Capsaicin is a known activator of vanilloid receptor 1. Capsaicin-induced stimulation of prostaglandin biosynthesis has been shown using bull seminal vesicles and rheumatoid arthritis synoviocytes. Capsaicin inhibits protein synthesis in Vero kidney cells and human neuroblastoma SHSY-5Y cells in vitro, and inhibits growth of E. coli, Pseudomonas solanacearum, and Bacillus subtilis bacterial cultures, but not Saccharomyces cerevisiae. Oral LD50 values as low as 161.2 mg/kg (rats) and 118.8 mg/kg (mice) have been reported for Capsaicin in acute oral toxicity studies, with hemorrhage of the gastric fundus observed in some of the animals that died. Intravenous, intraperitoneal, and subcutaneous LD50 values were lower. In subchronic oral toxicity studies using mice, Capsaicin produced statistically significant differences in the growth rate and liver/body weight increases. Capsaicin is an ocular irritant in mice, rats, and rabbits. Dose-related edema was observed in animals receiving Capsaicin injections into the hindpaw (rats) or application to the ear (mice). In guinea pigs, dinitrochlorobenzene contact dermatitis was enhanced in the presence of Capsaicin, injected subcutaneously, whereas dermal application inhibited sensitization in mice. Immune system effects have been observed in neonatal rats injected subcutaneously with Capsaicin. Capsaicin produced mixed results in S. typhimurium micronucleus and sister-chromatid exchange genotoxicity assays. Positive results for Capsaicin were reported in DNA damage assays. Carcinogenic, cocarcinogenic, anticarcinogenic, antitumorigenic, tumor promotion, and anti-tumor promotion effects of Capsaicin have been reported in animal studies. Except for a significant reduction in crown-rump length in day 18 rats injected subcutaneously with Capsaicin (50 mg/kg) on gestation days 14, 16, 18, or 20, no reproductive or developmental toxicity was noted. In pregnant mice dosed subcutaneously with Capsaicin, depletion of substance P in the spinal cord and peripheral nerves of pregnant females and fetuses was noted. In clinical tests, nerve degeneration of intracutaneous nerve fibers and a decrease in pain sensation induced by heat and mechanical stimuli were evident in subjects injected intradermally with Capsaicin. An increase in mean inspiratory flow was reported for eight normal subjects who inhaled nebulized 10(-7) M Capsaicin. The results of provocative and predictive tests involving human subjects indicated that Capsaicin is a skin irritant. Overall, studies suggested that these ingredients can be irritating at low concentrations. Although the genotoxicity, carcinogenicity, and tumor promotion potential of Capsaicin have been demonstrated, so have opposite effects. Skin irritation and other tumor-promoting effects of Capsaicin appear to be mediated through interaction with the same vanilloid receptor. Given this mechanism of action and the observation that many tumor promoters are irritating to the skin, the Panel considered it likely that a potent tumor promoter may also be a moderate to severe skin irritant. Thus, a limitation on Capsaicin content that would significantly reduce its skin irritation potential is expected to, in effect, lessen any concerns relating to tumor promotion potential. Because Capsaicin enhanced the penetration of an anti-inflammatory agent through human skin, the Panel recommends that care should be exercised in using ingredients that contain Capsaicin in cosmetic products. The Panel advised industry that the total polychlorinated biphenyl (PCB)/pesticide contamination should be limited to not more than 40 ppm, with not more than 10 ppm for any specific residue, and agreed on the following limitations for other impurities: arsenic (3 mg/kg max), heavy metals (0.002% max), and lead (5 mg/kg max). Industry was also advised that aflatoxin should not be present in these ingredients (the Panel adopted < or =15 ppb as corresponding to "negative" aflatoxin content), and that ingredients derived from Capsicum annuum and Capsicum Frutescens Plant species should not be used in products where N-nitroso compounds may be formed. (ABSTRACT TRUNCATED)

Dissociation of dorsal root ganglion neurons induces hyperexcitability that is maintained by increased responsiveness to cAMP and cGMP

Injury or inflammation affecting sensory neurons in dorsal root ganglia (DRG) causes hyperexcitability of DRG neurons that can lead to spontaneous firing and neuropathic pain. Recent results indicate that after chronic compression of DRG (CCD treatment), both hyperexcitability of neurons in intact DRG and behaviorally expressed hyperalgesia are maintained by concurrent activity in cAMP-protein kinase A (PKA) and cGMP-protein kinase G (PKG) signaling pathways. We report here that when tested under identical conditions, dissociation produces a pattern of hyperexcitability in small DRG neurons similar to that produced by CCD treatment, manifest as decreased action potential (AP) current threshold, increased AP duration, increased repetitive firing to depolarizing pulses, increased spontaneous firing and resting depolarization. A novel feature of this hyperexcitability is its early expression-as soon as testing can be conducted after dissociation (approximately 2 h). Both forms of injury increase the electrophysiological responsiveness of the neurons to activation of cAMP-PKA and cGMP-PKG pathways as indicated by enhancement of hyperexcitability by agonists of these pathways in dissociated or CCD-treated neurons but not in control neurons. Although inflammatory signals are known to activate cAMP-PKA pathways, dissociation-induced hyperexcitability is unlikely to be triggered by signals released from inflammatory cells recruited to the DRG because of insufficient time for recruitment during the dissociation procedure. Inhibition by specific antagonists indicates that continuing activation of cAMP-PKA and cGMP-PKG pathways is required to maintain hyperexcitability after dissociation. The reduction of hyperexcitability by blockers of adenylyl cyclase and soluble guanylyl cyclase after dissociation suggests a continuing release of autocrine and/or paracrine factors from dissociated neurons and/or satellite cells, which activate both cyclases and help to maintain acute, injury-induced hyperexcitability of DRG neurons.

Nicotinic receptor alpha 7-subunits are coupled to the stimulation of nitric oxide synthase in rat dorsal root ganglion neurons

In dorsal root ganglia (DRG) intraganglionic communication takes place both among neurons and between neurons and satellite cells. One diffusible substance involved in this signalling is nitric oxide (NO), and acetylcholine (ACh) is a candidate for the stimulation of intraganglionic NO synthesis. DRG neurons react to ACh-receptor stimulation with NO-dependent cGMP production. Here, we investigated the role of the alpha 7-subunit containing Ca(2+)-permeable nicotinic ACh receptors (nAChR) in this process. The alpha 7-nAChR mRNA and the protein were expressed in virtually all lumbar DRG neurons as evidenced by laser-assisted cell picking and oligo cell RT-PCR, in situ hybridisation and immunohistochemistry. Strong alpha 7-nAChR immunoreactivity was present in vanilloid receptor 1-immunoreactive, i.e. nociceptive, neurons. A neuronal production of NO in response to nicotine could be demonstrated in DRG slice preparations utilising the NO-sensitive fluorescent indicator diaminofluorescein diacetate (DAF-2DA). This stimulation of NO production was sensitive to inhibition of alpha 7-nAChR by mecamylamine and alpha-bungarotoxin, to inhibition of nitric oxide synthase (NOS) with L-NAME and L-NMMA, and to the blockade of voltage-operated Ca(2+) channels by verapamil. The results show the presence of the alpha 7-nAChR subunit in nociceptive rat DRG neurons and provide evidence for its coupling to NOS activation, indicating a role of this pathway in the intraganglionic communication in sensory ganglia.

Neutral endopeptidase knockout induces hyperalgesia in a model of visceral pain, an effect related to bradykinin and nitric oxide

Neutral endopeptidase (EC3.4.24.11, NEP, enkephalinase) is a zinc-metalloendopeptidase, cleaving a variety of substrates like enkephalins, substance P, and bradykinin. In the brain, NEP is a key enzyme in the degradation of enkephalins. Pharmacological inhibition of NEP-activity causes analgesia resulting from enhanced extracellular enkephalin concentrations. Recently, transgenic mice lacking the enzyme NEP have been developed (Lu, 1995). The present study was designed to investigate the nociceptive behavior of these NEP-knockout mice. Interestingly, NEP-deficient mice did not respond with decreased pain perception, but exhibited hyperalgesia in the hot-plate jump, warm-water tail-withdrawal, and mostnotablyin theacetic-acid writhing test. Inhibition of aminopeptidase N by bestatin reduced writhing in both strains, whereas NEP-inhibition by thiorphan reduced writhing selectively in wild-type mice. Naloxone increased writhing in wild-type but not in knockouts, whereas the bradykinin B2-receptor antagonist HOE140 reduced writhing selectively in NEP-knockouts. Similarly, the nitric oxide synthase inhibitor L-NAME reduced writhing in NEP-knockouts. These results indicate that genetic elimination of NEP, in contrast to pharmacological inhibition, leads to bradykinin-induced hyperalgesia instead of enkephalin-mediated analgesia. Nitric oxide (NO) is suggested to be involved in this process.

Evidence that nitric oxide-induced synthesis of cGMP occurs in a paracrine but not an autocrine fashion and that the site of its release can be regulated: studies in dorsal root ganglia in vivo and in vitro

As nitric oxide is a gas, it cannot be stored and has to be synthesized as required. This suggests that it could be released wherever nitric oxide synthase (NOS) is activated and due to its unstable state will react with appropriate targets at this site of production. In both dissociated dorsal root ganglion (DRG) cultures and in acutely isolated, but intact, DRG, treatment with capsaicin or bradykinin caused cGMP synthesis, which could be blocked by NOS inhibitors. The cGMP was synthesized in cells different from those expressing the neuronal isoform of NOS (nNOS). In dissociated cultures many of the cells stimulated to produce cGMP were neurons, whereas in isolated ganglia they were always satellite glia cells. Surprisingly, the satellite glia cells surrounding the nNOS-containing neurons did not contain cGMP. Following nerve section in adult rats, many axotomized ganglion neurons expressed nNOS. Again in these axotomized ganglia, most cGMP was expressed in the satellite glia surrounding nNOS-negative neurons. However, an nNOS-selective inhibitor reduced the cGMP present in these axotomized ganglia, suggesting that the cGMP synthesized is stimulated by NO (nitrogen monoxide) produced by nNOS. In both dissociated cultures and axotomized ganglia, nNOS-containing processes were observed close to cGMP-positive cells. These observations lead to the suggestion that NO acts in a paracrine fashion when stimulating the synthesis of cGMP and may not be synthesized at all sites containing nNOS.

Inhibitory effect of nitric oxide on voltage-dependent calcium currents in rat dorsal root ganglion cells

The effect of nitric oxide (NO) on calcium current (I(Ca)) and intracellular calcium concentration ([Ca(2+)](i)) in primarily cultured dorsal root ganglion (DRG) neurons was investigated from neonatal rats. I(Ca) and [Ca(2+)](i) were simultaneously recorded using perforated-patch technique in combination with fluorescence measurement from single DRG neurons. NO donors, sodium nitroprusside (SNP) and S-nitro-N-acetylpenicillamine (SNAP), inhibited I(Ca) in small-diameter neurons without significant change in voltage-dependence of activation and activation time constants. SNP and SNAP also reduced the transient [Ca(2+)](i) peak accompanied by I(Ca). Inhibition by NO was reproducible, but gradually desensitized. In some DRG neurons, SNP and SNAP increased basal [Ca(2+)](i) in concentration of 10 microM with little effect on NO-induced inhibition of I(Ca). 8-Br-cGMP, a permeable cGMP analog, mimicked the effects of SNP and SNAP. These results suggest that, in DRG neurons, NO has inhibitory effect on I(Ca), which is independent of NO-induced increase of basal [Ca(2+)](i), through cGMP-dependent pathway.

Capsaicin-evoked release of immunoreactive calcitonin gene-related peptide from the spinal cord is mediated by nitric oxide but not by cyclic GMP

Recent data support a role for nitric oxide (NO) in pain processing at the level of the spinal cord, possibly via regulation of neuropeptide release. The goal of this study was to determine whether capsaicin, which selectively activates primary afferent neurons and evokes neuropeptide release, acts in an NO-dependent manner. Our results indicate that capsaicin (1 microM)-evoked release of immunoreactive calcitonin gene-related peptide (iCGRP) is significantly reduced in the presence of the NO synthase inhibitor, L-NAME (10-400 nM; F(3,45)=68.38; P<0.001) and, the selective nNOS inhibitor, 3-bromo-7-nitroindazole (170-680 nM; F(5,48)=56.2; P<0. 01). D-NAME (200 nM) had no effect on capsaicin-evoked iCGRP release. Hemoglobin (an extracellular scavenger of NO; 3 mg/ml) significantly reduced the effect of capsaicin on the release of iCGRP (F(1,8)=9.12; P<0.05). The NOS substrate, L-arginine, effectively reversed the inhibitory effect of 3-bromo-7-nitroindazole on capsaicin-evoked iCGRP release. To determine whether the NO-mediated release was NMDA-driven, we superfused spinal cord slices with competitive and non-competitive NMDA antagonists in the presence and absence of capsaicin. MK-801 (0. 1-10 microM; F(4,33)=8.49; P<0.0001) and AP-5 (0.01-10 microM; F(4, 38)=3.34; P<0.05) reduced capsaicin-evoked iCGRP release. CNQX, an AMPA/kainate antagonist (10 nM-10 microM), significantly decreased capsaicin-evoked release of iCGRP (F(6,42)=8.76; P<0.01) in a dose-dependent fashion. Additionally, our results demonstrate that while capsaicin-evoked release is significantly reduced in the presence of LY-83583 (10 microM; F(2,18)=3.46; P<0.01; a cyclic GMP lowering agent), there is no effect of ODQ (a potent and selective inhibitor of guanylate cyclase). Moreover, the application of a cell permeable analog of cyclic GMP (8-bromo-cGMP; 0.01-1000 microM) is without effect on both basal and evoked iCGRP release. Finally, we observed no colocalization of immunoreactive neuronal NOS (nNOS) with CGRP in the dorsal horn. In summary, these data indicate that capsaicin evokes the release of iCGRP, in part, via the production of NO which enters the extracellular space prior to having an effect. Moreover, iCGRP and nNOS are produced in distinct populations of neurons within the dorsal horn. We conclude that capsaicin-evoked release involves the activation of the NMDA receptor but is also modified by the activation of AMPA or kainate receptors. Finally, these data suggest that while capsaicin-evoked iCGRP release is modified by NO, this release does not require the activation of guanylate cyclase and subsequent production of cyclic GMP.

The role of prostaglandins in the bradykinin-induced activation of serosal afferents of the rat jejunum in vitro

1. This study was performed to elucidate the role of prostaglandins in the action of bradykinin on serosal afferent neurones supplying the rat jejunum. Extracellular recordings of multi-unit activity were made from serosal afferents in isolation, using a novel in vitro preparation. The discharge of single afferents within the multi-unit recording was monitored using waveform discrimination software. 2. All afferents tested were both mechano- and capsaicin sensitive. Application of bradykinin elicited increases in whole nerve discharge in a concentration-dependent manner. The agonist potency estimate (EC50) was 0.62 +/- 0.12 microM and is consistent with an interaction at the B2 receptor subtype. 3. The stimulatory effect of bradykinin on serosal afferents was antagonized by a specific antagonist of the B2 receptor, HOE140. In contrast, a selective B1 receptor antagonist, [des-Arg10]HOE140, had no effect. The IC50 estimate obtained for HOE140 was 1.6 nM and again consistent with an interaction at B2 receptors. 4. The response to a submaximal concentration of bradykinin (1 microM) was significantly reduced to 24.4 +/- 54.9 % of control following blockade of cyclo-oxygenase activity with naproxen (10 microM). The addition of 1 microM prostaglandin E2 (PGE2), in the presence of naproxen, had no direct effect on afferent activity, but fully restored the response to bradykinin in 15 single afferents. 5. In summary, bradykinin stimulates serosal afferents by a direct action on kinin B2 receptors that are present on serosal afferent terminals. The response to bradykinin is dependent on the presence of prostaglandins, particularly PGE2. We suggest that bradykinin has a self-sensitizing action, whereby it stimulates the release of PGE2, which in turn sensitizes the endings of serosal afferent neurones responsive to bradykinin.

Cyclic guanosine 3',5'-monophosphate-mediated neuroprotection by nitric oxide in dissociated cultures of rat dorsal root ganglion neurones

In dissociated cultures of dorsal root ganglia (DRG) derived from 15-day-old rats, many neurones expressed nitric oxide synthase (NOS) and this expression was found to be reduced by nerve growth factor. The application of blockers of NOS caused selective death of those neurones expressing NOS. The soluble guanylate cyclase (sGC) blocker ODQ also caused neuronal death. The appearance of the neurones undergoing cell death was typical of apoptosis. This suggests that NO has a neuroprotective action in DRG neurones which is probably mediated by its activation of cyclic guanosine 3',5'-monophosphate. These observations are discussed in relation to the developmental and neuropathic changes in NOS expression by DRG neurones.

Fractional Ca2+ currents through capsaicin- and proton-activated ion channels in rat dorsal root ganglion neurones

1. Capsaicin and protons cause excitation and sensitization of primary nociceptive afferents. In a subset of dorsal root ganglion (DRG) neurones, which probably represent nociceptive neurones, both capsaicin and protons induce slowly inactivating non-selective cation currents. Whole-cell as well as single channel currents activated by these two stimuli share many biophysical and physiological properties in these neurones. This has lead to the suggestion that protons and capsaicin might activate the same ion channels. 2. In this study we simultaneously measured fluorescence signals and whole-cell currents activated by capsaicin or protons in acutely isolated DRG neurones filled with a high concentration (1 mM) of the Ca2+ indicator dye fura-2. From these measurements the fractional contribution of Ca2+ (Pf; the portion of the whole-cell current carried by Ca2+) to capsaicin- and two types of proton-induced (fast and slowly inactivating) membrane currents was determined. 3. Capsaicin- and slowly inactivating proton-induced currents were accompanied by a change in fluorescence that was dependent on the presence of extracellular Ca2+. With 1.6 mM extracellular Ca2+ and at a holding potential of -80 mV Pf of capsaicin-induced currents (at pH 7.3) was 4.30 +/- 0.17% (mean +/- S.E.M.; no. of experiments, n = 16) and of slowly inactivating proton-induced currents (at pH 5.1) was 1.65 +/- 0.11% (n = 17). Pf of fast inactivating proton-induced currents was negligible. 4. Pf of capsaicin- and slowly inactivating proton-induced currents increased with increasing extracellular Ca2+ concentration (0.5-4.8 mM). 5. Pf of both current types decreased linearly with decreasing extracellular pH by about 0.7% per pH unit over the pH range investigated. When determined at the same extracellular pH Pf values were significantly different for the two current types at all pH values tested. 6. In summary, our results provide evidence that capsaicin and protons activate ion channels which are markedly permeable to Ca2+. The fractional contribution of Ca2+, however, was significantly different for capsaicin- and slowly inactivating proton-induced currents. This strongly suggests that the two stimuli activate different populations of ion channels and supports the possibility that Ca2+ influx through these channels may be important for Ca(2+)-dependent sensitization of primary nociceptive neurones.

Multiple capsaicin-evoked currents in isolated rat sensory neurons

The response to capsaicin in functional assays suggests multiple sites of capsaicin action. This hypothesis is supported by the results of the present patch-clamp study of isolated dorsal root ganglion cells of the rat. The response to a prolonged application of capsaicin of different concentrations in an external solution with different ion compositions was investigated. Capsaicin evoked up to three distinct current components. The first and second current components could be activated independently. The third component occurred only in the presence of sodium and only in cells in which the second component was also elicited. In an extracellular solution with a physiological composition of ions and 300 nM capsaicin, the peaks of the three components, when evoked, occurred at 10.1 +/- 1.35 s (mean +/- S.E.M., n = 9), 44.0 +/- 2.64 s (n = 16) and 79.0 +/- 8.10 s (n = 5). The activation of the first and/or second current component depended on the concentration of capsaicin. A low concentration predominantly elicited the second component, while a high concentration activated the first and suppressed the second one. The third component seems to be a secondary response of the cell and was not investigated in detail. The activation and decay phases of the first two current components could be fitted by single exponential functions, whereas those of the third component could not. The first and second current components were carried by sodium and calcium. After tachyphylaxis, if the extracellular medium was then acidified to a pH of 6.3, the second component alone could then be elicited by capsaicin. The results demonstrate that capsaicin can elicit different current components that are distinguishable by their time-course, by the effects of acidification of the extracellular solution and by the concentration of capsaicin required to activate these currents. We postulate two distinct binding sites of capsaicin causing two distinct current components. This may account for the variety of physiological responses evoked by capsaicin and the variations in these responses between species.

Epicardial bradykinin B2 receptors elicit a sympathoexcitatory reflex in rats

Bradykinin may be generated in the heart during ischemia and is involved in nociception. We tested the hypothesis that bradykinin elicits a sympathoexcitatory reflex in rats by stimulating cardiac afferent nerve fibers. Rats were implanted with femoral catheters for measurement of blood pressure and heart rate, a bipolar electrode for measurement of renal sympathetic nerve activity, and a pericardial catheter for intrapericardial injection of substances. Rats were slightly anesthetized with hexobarbital so pain reactions were prevented. Graded doses of bradykinin (2.5, 12, 25 micrograms) were injected intravenously or intrapericardially into control rats, intrapericardially after vagotomy, intrapericardially after intrapericardial pretreatment with the bradykinin B2 receptor antagonist Hoe 140, and intrapericardially after cardiac autonomic blockade (intrapericardial pretreatment with 10% procaine). For comparison, the serotonin 5-HT3 agonist phenylbiguanide, a substance known to elicit sympathoinhibitory reflexes by cardiac vagal afferents, and adenosine, putatively inducing sympathoexcitatory responses via the heart, were applied intrapericardially. Bradykinin increased blood pressure when administered intrapericardially but decreased blood pressure when injected intravenously; both intrapericardial and intravenous bradykinin increased renal sympathetic nerve activity. Intrapericardial adenosine had no effect on circulatory control. Intrapericardial pretreatment with the B2 receptor antagonist Hoe 140 completely inhibited the increases of blood pressure and renal sympathetic nerve activity in response to intrapericardial bradykinin but did not affect the responses to intrapericardial phenylbiguanide. Bilateral cervical vagotomy abolished the decreases of blood pressure, heart rate, and renal sympathetic nerve activity after intrapericardial phenylbiguanide but did not influence the responses to intrapericardial bradykinin. Cardiac autonomic blockade with intrapericardial procaine abolished all responses to bradykinin and phenylbiguanide. We conclude that cardiac bradykinin elicits a sympathoexcitatory reflex by epicardial B2 receptors in rats. The afferent portion of the reflex is most likely contained within sympathetic cardiac afferent fibers. Bradykinin may contribute to increased sympathetic nerve activity in pathophysiological situations of coronary artery disease and cardiac ischemia.

Possible role of nitric oxide in the antinociceptive action of intraventricular bradykinin in mice

The i.c.v. administration of bradykinin (4, 8 and 16 micrograms) induced antinociception in mice which was resistant to naloxone; furthermore, the induction of tolerance to morphine by a single s.c. injection (100 mg/kg, 24 h before test doses of the peptide) did not affect antinociception. Since bradykinin is known to increase nitric oxide (NO) in peripheral tissues, we studied the possibility that its antinociceptive action may be related to NO effects in the central nervous system. Bradykinin effects were antagonized by previous treatment with NG-nitro-L-arginine or concomitant i.c.v. administration of bradykinin and methylene blue. The immediate precursor of NO, L-arginine, which by itself produces analgesia, also reduced bradykinin effects; moreover, tolerance to L-arginine significantly decreased the response to the peptide. These results suggest that NO is involved in antinociception induced by i.c.v. administration of bradykinin.

Effect of spinal nitric oxide inhibition on capsaicin-induced nociceptive response

Pretreatment with the nitric oxide synthase (NOS) inhibitor L-NG-nitro arginine methyl ester (L-NAME), injected intraperitoneally (i.p.) or intrathecally (i.t.), produced a significant antinociception in the mouse assessed by the capsaicin-induced paw licking procedure. Varying the administration time of an effective dose of L-NAME (160nmol, i.t.) resulted in a significant decrease of the brief nociceptive behavioral response induced by capsaicin, even when L-NAME was given 2 hr before capsaicin. L-NAME, injected i.p. or i.t., produced a dose-related reduction in paw licking in the second phase of the formalin (2.0%) response without affecting the first phase. L-Arginine (600 mg/kg, i.p.) but not D-arginine (600 mg/kg, i.p.) reversed the antinociceptive effect of L-NAME in the capsaicin test. Antinociceptive effect of L-NAME, injected i.p. or i.t., was more potent in the second phase response of formalin-induced paw licking than in capsaicin-induced nociceptive response. The inhibitory action of L-NAME was reversed by L-arginine but not D-arginine in the second phase response. L-Arginine alone was without affecting capsaicin- and formalin-induced nociceptive responses. These results suggest that spinal nitric oxide (NO) may be involved in the mechanisms of capsaicin-induced brief nociceptive stimuli, but not in the first, acute phase of the formalin-induced response in mice.

Cyclic GMP regulates activation of phosphoinositidase C by bradykinin in sensory neurons

Prior exposure of cultured neonatal rat dorsal root ganglion (DRG) neurons to bradykinin resulted in marked attenuation of bradykinin-induced activation of phosphoinositidase C (PIC). The (logconcentration)-response curve for bradykinin-induced [3H]inositol trisphosphate ([3H]IP3) formation was shifted to the right and the maximum response was reduced. Bradykinin increases cyclic GMP (cGMP) in DRG neurons [Burgess, Mullaney, McNeill, Coote, Minhas and Wood (1989) J. Neurochem. 53, 1212-1218] and treatment of the neurons with dibutyryl cGMP (dbcGMP) had a similar, inhibitory, effect on bradykinin-induced [3H]IP3 formation. NG-Nitro-L-arginine (LNNA) blocked bradykinin-induced formation of cGMP. It prevented the functional uncoupling induced by pretreatment with bradykinin, but not the inhibitory effect of dbcGMP on [3H]IP3 formation. The ability of LNNA to prevent desensitization was reversed by excess L-arginine, indicating that its actions were mediated through inhibition of nitric oxide synthase. In addition to functional desensitization, exposure to bradykinin reduced the number of cell-surface receptors detected with [3H]bradykinin, without affecting its KD value for the remaining sites. In contrast to bradykinin, pretreatment with dbcGMP had no effect on either the KD or B(max) for [3H]bradykinin binding. This implies that the inhibitory effect of dbcGMP was down-stream from the binding of bradykinin to its receptor and upstream of IP3 formation. The lack of effect of dbcGMP on [3H]bradykinin binding suggests that the decrease in receptor number induced by bradykinin was mediated by a different mechanism and was not a key factor in the rapid phase of desensitization in these cells.

Involvement of endogenous nitric oxide in the mechanism of bradykinin-induced peripheral hyperalgesia

1. When NG-nitro-L-arginine methyl ester (L-NAME, 0.1-10 nmol) or NG-monomethyl-L-arginine (L-NMMA, 10 nmol-1 mumol) was intradermally administered with bradykinin (BK, 3 nmol) into the instep of rat hind-paws, a dose-related suppression of BK-induced hyperalgesia, assessed by the paw-pressure test, was produced. 2. L-Arginine (1 mumol) but not D-arginine (1 mumol) reversed the suppressive effects of L-NAME (10 nmol) and L-NMMA (1 mumol) on BK-induced hyperalgesia. 3. Concomitant intradermal administration of BK (3 nmol) with haemoglobin (1 nmol) significantly suppressed BK-induced hyperalgesia in the paw-pressure test. The BK-induced hyperalgesia was abolished by concomitant intradermal administration of either a guanylate cyclase inhibitor, methylene blue (10 nmol), or LY83583 (1 nmol). In addition, KT5823 (1 nmol) or Rp-8-bromoguanosine-3':5'-cyclic monophosphothioate (Rp-8-Br-cGMPS; 1 nmol), an inhibitor of cyclic GMP-dependent protein kinase, also significantly suppressed BK-induced hyperalgesia. 4. The carrageenin-induced hyperalgesia was significantly attenuated by L-NAME in a dose-dependent manner. 5. L-Arginine (1 mumol), sodium nitroprusside (1 mumol), dibutyryl cyclic GMP (1 mumol) or 8-bromo cyclic GMP (1 mumol) all failed to produce any significant relieving effect on the nociceptive threshold of rodent hind-paws. Concomitant administrations of each agent with a sub-threshold dose (0.1 nmol) of BK induced significant hyperalgesia. 6. Rp-adenosine 3':5'-cyclic monophosphothioate (Rp-cAMPS; 1 nmol), an inhibitor of cyclic AMP-dependent protein kinase, significantly suppressed BK-induced mechanical hyperalgesia. Concomitant administration of forskolin (1 nmol) with 8-bromo cyclic GMP (100 nmol) induced significant hyperalgesia. 7. In the superfusion experiment of a blister base on the instep of rodent hind-paws, intradermally administered BK (3 nmol) significantly increased the outflow of both cyclic GMP and cyclic AMP from the blister base. Concomitant administrations of L-NAME (10 nmol) with BK significantly reduced the BK-induced outflow of cyclic GMP without affecting the cyclic AMP content. 8. These results suggest that the NO-cyclic GMP pathway is involved in the mechanism of BK-induced hyperalgesia, and an activation of both cyclic GMP-and cyclic AMP-second messenger system plays an important role in the production of peripherally induced mechanical hyperalgesia.

Inflammation modulates the contribution of receptor-subtypes to bradykinin-induced hyperalgesia in the rat

While B2 receptors mediate pain and hyperalgesia induced by bradykinin, in normal rats, recent reports indicate that, in the setting of inflammation, B1 receptors also mediate pain and hyperalgesia. Since bradykinin-induced hyperalgesia in normal rats is mediated by prostaglandins released from the postganglionic sympathetic neurons, we have evaluated the contribution of the sympathetic nervous system to the hyperalgesia induced by bradykinin, a preferential B2-receptor agonist, and des-Arg9-bradykinin, a major metabolite of bradykinin and a selective B1-receptor agonist. Mechanical hyperalgesia was quantified by the Randall-Selitto paw-withdrawal method. Inflammation was induced by injecting Complete Freund's Adjuvant into the left hindpaw of the rat and testing mechanical nociceptive threshold in the right hindpaw after injecting B1 or B2 agonists and/or antagonists. Sympathectomy was achieved by surgically removing sympathetic ganglia L1-L4. Rats were used 48 h post-adjuvant injection. In the normal rat, intradermal injection of bradykinin but not des-Arg9-bradykinin, into the dorsal surface of the hindpaw, produced a dose-dependent decrease in mechanical nociceptive threshold. NPC 17731, a B2-receptor antagonist, but not des-Arg9-[Leu8]-bradykinin, a B1-receptor antagonist, almost completely inhibited the decrease in mechanical threshold, suggesting that bradykinin hyperalgesia in the normal rat hindpaw was mediated by B2 receptors. In rats whose left paws were treated, 48 h earlier, with adjuvant, intradermal injection of bradykinin or des-Arg9-bradykinin, into the right paw produced dose-dependent hyperalgesia. Bradykinin hyperalgesia was partially inhibited by NPC 17731, and the residual part by des-Arg9,[Leu8]-bradykinin. des-Arg9-bradykinin hyperalgesia was inhibited by des-Arg9,[Leu8]-bradykinin but not by NPC17731. These results suggest that in the setting of inflammation, bradykinin hyperalgesia was mediated by both B1 and B2 receptors, and that des-Arg9-bradykinin hyperalgesia was mediated by the B1 receptor. Forty-eight hours after injection of complete Freund's adjuvant, in sympathectomized rats, bradykinin or des-Arg9-bradykinin failed to produce hyperalgesia, suggesting that intact sympathetic postganglionic neurons are required for the hyperalgesia produced by these agents in this model. These results are consistent with the suggestions that B2 receptors mediate bradykinin-induced cutaneous hyperalgesia in the normal rat hindpaw. The hyperalgesia induced by bradykinin, 48 h post injection of complete Freund's adjuvant is mediated by both B1 and B2 receptors, that by des-Arg9-bradykinin is mediated by B1 receptors. The hyperalgesia induced by both agents is dependent on the presence of intact sympathetic postganglionic neurons.

Regulation of bradykinin sensitivity in peripheral sensory fibres of the neonatal rat by nitric oxide and cyclic GMP

Bradykinin-induced activation of peripheral sensory fibres was studied using an in vitro preparation of the neonatal rat spinal cord with attached tail. Noxious heat stimulation, as well as the applications of bradykinin and capsaicin, to the tail evoked reproducible responses recorded as a depolarization of a lumbar ventral root. Prolonged administration of a supramaximal concentration of bradykinin invariably induced a complete but selective desensitization to a subsequent bradykinin challenge. Bradykinin-induced desensitization was significantly attenuated by concanavalin-A and the effect of concanavalin-A was prevented by alpha-methyl mannoside. Both cyclic GMP and sodium nitroprusside induced a long lasting reduction of bradykinin responsiveness in peripheral fibres. The effect of nitroprusside was prevented by concanavalin-A, and by methylene blue, an inhibitor of guanylyl cyclase. Methylene blue also reduced bradykinin-induced desensitization. L-arginine, but not D-arginine, induced a desensitization to bradykinin. On the other hand, 7-nitroindazole (7-NI, 200-500 nM), an inhibitor of NOS, reduced the desensitization of bradykinin responses but higher concentrations of 7-NI (IC50 = 6.7 +/- 0.9 microM) selectively attenuated responses to bradykinin. The effects of 7-NI were attenuated by L-arginine pretreatment. These data suggest that bradykinin-induced desensitization of peripheral sensory fibres is mediated in part via NO and cyclic GMP dependent mechanisms: possibly NO production is required for guanylate cyclase activation.

Nitric oxide and cyclic guanosine 3',5'-monophosphate do not alter neuropeptide release from rat sensory neurons grown in culture

Recent studies demonstrate that nitric oxide and cyclic guanosine 3',5'-monophosphate may mediate hyperalgesia induced by N-methyl-D-aspartate at the level of the spinal cord. One possible mechanism for this action is that nitric oxide increases transmitter release from the primary afferent nociceptors that synapse in the dorsal horn of the spinal cord. To address this possibility, we investigated whether various nitric oxide donors and cyclic guanosine 3',5'-monophosphate could alter the release of substance P and calcitonin gene-related peptide from rat sensory neurons in culture. Sodium nitroprusside (100 nM to 100 microM) had little effect on basal release of either peptide, but it significantly increased the release of substance P and calcitonin gene-related peptide induced by 50 nM capsaicin. In contrast, sodium nitroprusside did not alter release evoked by 100 nM bradykinin or 30 mM KCl. Two other nitric oxide-donating compounds, S-nitroso-N-acetylpenicillamine and 3-morpholinosydnonimine did not enhance resting or capsaicin-evoked peptide release, although they induced a marked elevation in the intracellular cyclic guanosine 3',5'-monophosphate levels. Pretreating the cultures with 8-bromo-cyclic guanosine 3',5'-monophosphate, (0.5 or 0.1 mM for 30 or 60 min) did not result in the enhancement of capsaicin-induced release from sensory neurons. Moreover, pretreating the cells with the nitric oxide synthase inhibitor, NG-nitro-L-arginine (100 microM), abolished the rise in cyclic guanosine 3',5'-monophosphate induced by capsaicin without altering capsaicin-stimulated release of either peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic cholinergic stimulation of the nitric oxide-cyclic GMP signaling system in cultured rat sensory neurons

Acetylcholine or carbachol stimulated cyclic GMP production in neuronal cultures from embryonic rat dorsal root ganglia but not in non-neuronal dorsal root ganglia cultures. Acetylcholine stimulation of cyclic GMP production was mediated by muscarinic receptors and required extracellular Ca2+. Basal cyclic GMP production and acetylcholine-evoked cyclic GMP production were attenuated by Methylene Blue, suggesting the involvement of soluble guanylate cyclase and nitric oxide synthase. L-NG-Monomethyl arginine attenuated basal, acetylcholine or carbachol-stimulated cyclic GMP production; this inhibition of acetylcholine and carbachol stimulation of cyclic GMP was reversed by L-arginine. These results suggest that a nitrosyl factor mediates basal, as well as acetylcholine- and carbachol-stimulated, cyclic GMP production. Selective destruction of small diameter neurons by capsaicin pretreatment of dorsal root ganglion neuronal cultures abolished acetylcholine and capsaicin stimulation of cyclic GMP, but did not affect sodium nitroprusside stimulation of cyclic GMP. These results suggest that acetylcholine evoked production of a nitrosyl factor in capsaicin-sensitive (small diameter) sensory neurons, which subsequently stimulated a soluble guanylate cyclase and cyclic GMP production in adjacent neuronal and/or non-neuronal cells. These results demonstrate that muscarinic agonists stimulate the nitric oxide-cyclic GMP signaling system in capsaicin-sensitive sensory neurons. Thus, the noxious character of acetylcholine when administered peripherally may be mediated by nitric oxide-cyclic GMP.

Cutaneous vasodilatation induced by nitric oxide-evoked stimulation of afferent nerves in the rat

1. The site of action at which nitric oxide (NO) may contribute to neurogenic vasodilatation in the hindpaw skin of urethane-anaesthetized rats was examined by the use of NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthase. 2. Skin blood flow was measured by laser Doppler flowmetry, and neurogenic vasodilatation was evoked either by topical application of mustard oil (5%) or antidromic electrical stimulation of the saphenous nerve (antidromic vasodilatation). 3. L-NAME (60 mumol kg-1, i.v.) attenuated the hyperaemia evoked by mustard oil in an enantiomer-specific manner but failed to reduce antidromic vasodilatation and the vasodilatation due to i.v. injected calcitonin gene-related peptide (CGRP) and substance P (0.1-1 nmol kg-1 each), two proposed mediators of neurogenic vasodilatation. 4. Pretreatment of rats with capsaicin (125 mg kg-1, s.c. 2 weeks beforehand), to defunctionalize afferent neurones, reduced the hyperaemic response to mustard oil and prevented L-NAME from further decreasing the vasodilatation evoked by mustard oil. 5. Intraplantar infusion of sodium nitroprusside (SNP, 0.15 nmol in 1 min), a donor of NO, induced hyperaemia which was significantly diminished by the CGRP antagonist CGRP8-37 (50 nmol kg-1, i.v.) and by capsaicin pretreatment. The ability of CGRP8-37 to inhibit the vasodilator response to SNP was lost in capsaicin-pretreated rats. 6. Taken together, these data indicate that NO does not play a vasorelaxant messenger role in neurogenic vasodilatation but can contribute to activation of, and/or transmitter release from, afferent nerve fibres in response to irritant chemicals.