Inhibition of cholinergic and non-adrenergic, non-cholinergic bronchoconstriction in the guinea pig mediated by neuropeptide Y and alpha 2-adrenoceptors and opiate receptors

Reciprocal sympatho-sensory control: functional role of nucleotides and calcitonin gene-related peptide in a peripheral neuroeffector junction

The rat vas deferens has scattered sensory afferens plus a dense network of sympathetic motor efferens; these fibers are not known to interact functionally. We ascertained whether sensory fibers modulate the release of sympathetic transmitters through the release of calcitonin gene-related peptide (CGRP) and reciprocally assessed whether sympathetic transmitters modulate the overflow of ir-CGRP from sensory fibers. The tissue overflow of electrically evoked sympathetic co-transmitters (ATP/metabolites, noradrenaline (NA), and immunoreactive neuropeptide tyrosine (ir-NPY)) and the motor responses elicited were quantified following either exogenous CGRP or capsaicin application to elicit peptide release. Conversely, the outflow of ir-CGRP was examined in the presence of sympathetic transmitters. Exogenous CGRP reduced in a concentration-dependent manner the electrically evoked outflow of ATP/metabolites, NA, and ir-NPY with EC(50) values of 1.3, 0.18, and 1.9 nM, respectively. CGRP also reduced the basal NA overflow. The CGRP-evoked modulation was blocked by CGRP8-37 or H-89. Release of endogenous CGRP by capsaicin significantly reduced the basal overflow of NA, ir-NPY, and the electrically evoked sympathetic transmitter release. ADP, 2-methylthioadenosine-5'-O-diphosphate (2-MeSADP), or UTP decreased the electrically evoked ir-CGRP overflow, whereas clonidine, α,β-methyleneadenosine 5'-triphosphate (α,β-mATP), or adenosine (ADO) were inactive. CGRP acting postjunctionally also reduced the motor responses elicited by exogenous NA, ATP, or electrically evoked contractions. We conclude that CGRP exerts a presynaptic modulator role on sympathetic nerve endings and reciprocally ATP or related nucleotides influence the release of ir-CGRP from sensory fibers, highlighting a dynamic sympatho-sensory control between sensory fibers and sympathetic nerve ending. Postjunctional CGRP receptors further contribute to reduce the tissue sympathetic motor tone implying a pre and postjunctional role of CGRP as a sympathetic tone modulator.

Sensory nerves and airway irritability

The lung, like many other organs, is innervated by a variety of sensory nerves and by nerves of the parasympathetic and sympathetic nervous systems that regulate the function of cells within the respiratory tract. Activation of sensory nerves by both mechanical and chemical stimuli elicits a number of defensive reflexes, including cough, altered breathing pattern, and altered autonomic drive, which are important for normal lung homeostasis. However, diseases that afflict the lung are associated with altered reflexes, resulting in a variety of symptoms, including increased cough, dyspnea, airways obstruction, and bronchial hyperresponsiveness. This review summarizes the current knowledge concerning the physiological role of different sensory nerve subtypes that innervate the lung, the factors which lead to their activation, and pharmacological approaches that have been used to interrogate the function of these nerves. This information may potentially facilitate the identification of novel drug targets for the treatment of respiratory disorders such as cough, asthma, and chronic obstructive pulmonary disease.

The modulation of voltage-gated potassium channels by anisotonicity in trigeminal ganglion neurons

Voltage-gated potassium channels (VGPCs) play an important role in many physiological functions by controlling the electrical properties and excitability of cells. Changes in tonicity in the peripheral nervous system can activate nociceptors and produce pain. Here, using whole cell patch clamp techniques, we explore how hypo- and hypertonicity modulate VGPCs in cultured rat and mouse trigeminal ganglion (TG) neurons. We found that hypo- and hypertonicity had different effects on slow-inactivating K+ current (IK) and fast-inactivating K+ current (IA): hypotonicity increased IK but had no effect on IA while hypertonicity depressed both IK and IA. The increase of IK by hypotonicity was mimicked by transient receptor potential vanilloid 4 (TRPV4) receptor activator 4alpha-phorbol-12,13-didecanoate (4alpha-PDD) but hypotonicity did not exhibit increase in TRPV4-/- mice TG neurons, suggesting that TRPV4 receptor was involved in hypotonicity-induced response. We also found that inactivation of PKC selectively reversed the increase of IK by hypotonicity, whereas antagonism of G-protein selectively rescued the inhibitions of IK and IA by hypertonicity, indicating that different intracellular signaling pathways were required for the modulation by hypo- and hypertonicity. In summary, changes in osmolality have various effects on IK and IA and different receptors and second messenger systems are selective for the modulation of VGPCs induced by hypo- versus hypertonicity.

Expression of substance P and nitric oxide synthase in vagal sensory neurons innervating the mouse airways

INTRODUCTION

Airway sensory nerves have the capacity to release neuromediators such as substance P and nitric oxide to control airway functions. The aim of the present study was to investigate substance P and neuronal nitric oxide synthase (NOS-1) expression in airway-specific sensory neurons.

METHODS

Airway-projecting neurons in the jugular-nodose ganglia were investigated for NOS-1 and substance P expression by neuronal tracing and double-labelling immunoreactivity.

RESULTS

Of the Fast blue labelled neurons, 14.6+/-1.8% (mean+/-S.E.M.) were immunoreactive only for NOS-1, 3.0+/-0.3% for NOS-1 and substance P, 2.7+/-0.3% only for substance P, and 79.7+/-1.7% of the labelled neurons were nonimmunoreactive for substance P or NOS-1 but were partly positive for I-B4-lectin-binding. Fast blue labelled NOS and/or substance P-positive neurons were small to medium sized (<20 microm).

CONCLUSION

Based on the expression of substance P and nitric oxide synthase in airway neurons, the present study suggests that there may be substance P and NO biosynthesis and release following a peripheral activation of the afferents, there could be a triggering of substance P and NO-mediated phenomena, including those related to airway inflammation, such as plasma extravasation and vasodilatation.

Central pathways for cough in man--unanswered questions

Central processing of afferent cough impulses occurs in a putative 'cough centre' in the dorsal medulla where the reflex is subject to considerable cortical control. Little is known about the central neurotransmitters and mediators which mediate cough in humans. Previous animal and human studies suggest that the antitussive effect of opiates may be mediated at central 5-HT receptors. In three studies in healthy human volunteers, we have investigated the potential role of central cholinergic and dopaminergic receptors in the mediation of cough, and the potential role of 5-HT receptors in the antitussive action of opiates. Intravenous administration of atropine or physostigmine had no effect on capsaicin-induced cough. Similarly, oral administration of L-dopa, bromocriptine or haloperidol had no effect on capsaicin-induced cough. Compared with saline, intravenous morphine significantly suppressed capsaicin-induced cough and increased drowsiness. Compared with placebo, pretreatment with oral pizotifen significantly attenuated the antitussive effect of morphine, but not the sedative effect. This suggests that in humans, an agonist action at 5-HT2 and/or 5-HT1 receptors may be involved in the antitussive effect of morphine, but not its sedative effect. Further knowledge of central cough pathways in humans must await the availability of more selective receptor agonists and antagonists for human studies. This offers the promise of effective antitussive therapy. The challenge is to find an antitussive agent which can return the abnormal sensitivity of the cough reflex to normal, without adverse effects.

The effect of sympathetic activity on thermal hyperalgesia in capsaicin-treated skin during body cooling and warming

An adrenergic mechanism is thought to contribute to pain in conditions that sometimes develop during chronic inflammation and after nerve or tissue injury. There is some doubt, however, about whether adrenergic activity influences nociception in acute inflammation. To investigate this issue, the noncompetitive alpha-(alpha) adrenergic antagonist phenoxybenzamine was introduced by iontophoresis into the skin of 16 healthy volunteers either before or after the topical application of capsaicin. When applied before capsaicin, phenoxybenzamine increased thermal hyperalgesia at normal ambient temperatures and during body warming. These findings suggest that phenoxybenzamine blocked an analgesic mechanism when applied before the onset of inflammation. However, this effect disappeared during body cooling. When applied after capsaicin, phenoxybenzamine inhibited thermal hyperalgesia at normal ambient temperatures, and during body warming and cooling. Thus, phenoxybenzamine blocked a hyperalgesic mechanism when applied after the onset of inflammation. It was concluded that the presence of inflammation influences the nociceptive effect of alpha-adrenergic blockage, possibly by increasing access to excitatory adrenergic receptors on nociceptive afferents. An excitatory adrenergic influence on nociception may overcome an inhibitory adrenergic influence during inflammation.

Neuropeptide Y1- and Y2-receptor-mediated cardiovascular effects in the anesthetized guinea pig, rat, and rabbit

Neuropeptide Y (NPY) causes vasoconstriction through Y1-receptors and inhibits vagal bradycardia through presynaptic Y2-receptors. These effects of NPY were investigated in anesthetized guinea pigs, rats, and rabbits to find the most suitable species for evaluation of Y1- and Y2-active agents in vivo. The increase in blood pressure (through Y1) of lower doses of NPY was similar in the three species (ED50, 0.9 +/- 0.13, 0.8 +/- 0.39, and 0.6 +/- 0.09 nmol/kg, respectively), but higher doses had depressor effects in four of six rats. Vagal bradycardia, induced by electrical stimulation of the right cervical vagus nerve, was inhibited by NPY in the guinea pig and in the rat (ED35, 3.5 +/- 0.46 and 11.2 +/- 1.79 nmol/kg, respectively; p < 0.05) but not in the rabbit. In the guinea pig, the Y2-receptor-preferring fragment NPY(3-36) and the selective Y1-receptor antagonist H 409/22 were used to confirm that the increase in blood pressure was mediated solely through the Y1-receptor and the vagal inhibition solely through the Y2-receptor. Aside from the cardiovascular effects, NPY caused a decrease in the body temperature and inhibited vagal bronchoconstriction in this species. Considering that NPY may cause depressor effects in the rat and has no effect on the vagal bradycardia in the rabbit, the guinea pig is preferable to both these species for assessment of Y1- and Y2-receptor-active agents in vivo.

Vagal neurotransmission to the ferret lower oesophageal sphincter: inhibition via GABA(B) receptors

GABA(B) receptors modulate the function of the lower oesophageal sphincter (LOS) in vivo by inhibiting neurotransmitter release in the vagal pathway controlling LOS relaxation. We aimed to determine whether this effect was mediated peripherally on vagal motor outflow to the ferret LOS in vitro. The LOS, with intact vagal innervation, was prepared from adult ferrets and LOS tension measured. Vagal stimulation (0.5 - 10 Hz, 30 V) evoked a tetrodotoxin-sensitive, frequency-dependent relaxation. Both GABA (3x10(-4) M) and (+/-)baclofen (2x10(-4) M) inhibited vagally-stimulated LOS relaxation. The potent GABA(B) receptor-selective agonist 3-APPA dose-dependently inhibited vagally-stimulated LOS relaxation, with an EC(50) value of 0.7 microM Decreased responses following vagal stimulation in the presence of (+/-)baclofen or 3-APPA were reversed with the potent GABA(B) receptor antagonist CGP 62349. Neither CGP 62349 nor muscimol (GABA(A) receptor agonist) alone affected LOS responses following vagal stimulation. Agonists of other G protein-coupled receptors (clonidine (alpha(2)-adrenoceptor) (5x10(-6) M), U50488 (kappa opioid) (10(-5) M), neuropeptide Y (10(-6) M)) did not affect vagally-mediated LOS relaxation. The present study supports a discrete presynaptic inhibitory role for GABA(B) receptors on vagal preganglionic fibres serving inhibitory motorneurones in the ferret LOS.

Nociceptin inhibits capsaicin-induced bronchoconstriction in isolated guinea pig lung

The isolated perfused guinea pig lung was used to investigate the effect of nociceptin against bronchoconstriction elicited by endogenous and exogenous tachykinins. The opioid receptor-like 1 (ORL1) receptor agonist, nociceptin/orphanin FQ (0.001-1 microM) produced a dose-related inhibition of the capsaicin-induced bronchoconstriction (10(-5)-10(3) microg) in isolated guinea pig lung (P<0.05), a response mediated by the release of endogenous tachykinins from lung sensory nerves. The new ORL1 receptor antagonist 1-[(3R, 4R)-1-Cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1, 3-dihydro-2H-benzimidazol-2-one (J-113397) (0.3 microM) significantly blocked the inhibitory effect of nociceptin/orphanin FQ (0.01 microM) on capsaicin-induced bronchoconstriction, whereas the non-selective opioid receptor antagonist naloxone (1 microM) had no effect. Nociceptin/orphanin FQ (1 microM) did not affect the bronchoconstriction induced exogenously by the tachykinin NK2 receptor agonist neurokinin A. In conclusion, the present data provide evidence that nociceptin inhibits capsaicin-evoked tachykinin release from sensory nerve terminals in guinea pig lung by a prejunctional mechanism. This inhibitory action occurs independently from activation of opioid receptors. The present study also indicates that J-113397 is a potent ORL1 receptor antagonist.

Effects of aging on neurogenic vasodilator responses evoked by transcutaneous electrical nerve stimulation: relevance to wound healing

We have previously shown an age-related decline in the modulation of skin vascular reactivity by sensory nerves that correlates with a decline in wound repair efficacy. This study was designed to examine the possibility that improving the functional ability of aged sensory nerves using noninvasive transcutaneous electrical nerve stimulation (TENS) could also accelerate tissue repair. TENS of the sciatic nerve, combined with measuring blood flow responses in the rat hind-footpad using laser Doppler flowmetry, was used to establish the vascular effects. Following TENS (using parameters 20V, 5 Hz for 1 min), similar increases in vascular responses were obtained in both young (13.2+/-0.9 cm2) and old rats (11.6+/-2.3 cm2). In contrast, capsaicin-pretreated rats showed markedly diminished responses. Sympathetic fibers did not appear to modulate these sensory nerve responses. In the second part, a thermal wound was induced (using a CO2 laser) in the interscapular region of old rats (under anesthesia). In the active treatment group, TENS was applied twice daily for the initial 5 days, and the sham group received inactive TENS. Using the healing endpoint as the time when full wound contraction occurred, the active group required 14.7+/-0.2 days for complete healing, a significant improvement over the sham group (21.8+/-0.3 days). We contend that low-frequency TENS can improve the vascular response of old rats. In addition, wound healing in aged rats can be accelerated by peripheral activation of sensory nerves at low-frequency electrical stimulation parameters.

Influence of TASP-V, a novel neuropeptide Y (NPY) Y2 agonist, on nasal and bronchial responses evoked by histamine in anaesthetized pigs and in humans

1. In nine anaesthetized pigs we have studied the influence of intranasal or intrabronchial pretreatment with TASP-V, a neuropeptide Y (NPY) Y2 agonist formed by the attachment of NPY 21-36 to a template-assembled synthetic peptide (TASP), on the functional responses to subsequent intranasal or intrabronchial histamine challenge. 2. In a parallel study, subjective and objective nasal airway resistance (NAR) increase following intranasal histamine challenge was evaluated in 11 healthy volunteers after TASP-V or placebo pretreatment. 3. In pigs, increase in sphenopalatine blood flow induced by histamine dihydrochloride nasal spray (0.25 mg kg(-1) in 3 ml of saline) was significantly reduced by 65% (P<0.05) following intranasal pretreatment with 10 microg kg(-1) of TASP-V. Bronchoconstriction induced by histamine dihydrochloride nebulization (0.5 mg kg(-1) in 3 ml of saline) was significantly attenuated by 25 and 55% following aerosolized pretreatment with TASP-V analogue at 10 and 20 microg kg(-1), respectively. 4. In healthy volunteers, objective increase in NAR and reduction in nasal minimal cross section area (MCSA) induced by intranasal spray of histamine dihydrochloride (15 microg kg(-1) in 200 microl of saline) were significantly attenuated by 50% following local pretreatment with 1.275 microg kg(-1) of TASP-V when compared with saline. 5. It is concluded that intranasal or intrabronchial pretreatment with TASP-V reduced nasal obstruction and bronchoconstriction evoked by histamine challenge in the pig. In healthy human volunteers, this agent attenuated NAR increase and MCSA reduction induced by intranasal application of histamine.

Role of adrenergic nervous system in cigarette smoke-induced bronchoconstriction in guinea pigs

The goal of this study was to clarify the role of the adrenergic nervous system in bronchoconstriction induced by exposure to cigarette smoke in guinea pigs. Artificially ventilated animals were exposed to 160 puffs of smoke for 8 min. Bronchoconstriction was assessed as a percentage of the baseline total pulmonary resistance (RL). The effects of pretreatment with phentolamine (0.1 mg/kg, i.v.), propranolol (1 mg/kg, i.v.), and/or atropine (1 mg/kg, i.v.) were evaluated. Exposure to cigarette smoke caused significant bronchoconstriction. Phentolamine, an alpha-adrenoceptor antagonist, significantly inhibited cigarette smoke-induced bronchoconstriction, while propranolol, a beta-adrenoceptor antagonist, significantly enhanced it. Combined use of these compounds further enhanced the bronchoconstriction. All of modulations of the bronchoconstriction by adrenoceptor antagonists were completely abolished by pretreatment with atropine. Phentolamine and/or propranolol had no effect on the bronchoconstriction induced by inhaled acetylcholine. Pretreatment with yohimbine (0.5 mg/kg, i.v.), a selective alpha2-adrenoceptor antagonist, showed modulatory effects similar to those of phentolamine on cigarette smoke-induced bronchoconstriction. These results suggest that cigarette smoke-induced bronchoconstriction is regulated by the prejunctional modulation of the cholinergic system via alpha- and beta-adrenoceptors. This mechanism may be modulated by the autoregulation of adrenergic nerves via the alpha2-autoreceptor.

Prejunctional control of pH 6-induced bronchoconstriction by NK1, NK2, mu-opioid, alpha2-adrenoceptor and glucocorticoid receptors in guinea-pig isolated perfused lung

This study investigated the release of calcitonin-gene related peptide-like (CGRP) immunoreactivity and bronchoconstriction induced by pH 6 buffer in guinea-pig isolated perfused lung. Both pH 6-induced CGRP-like immunoreactivity and bronchoconstriction were completely abolished after systemic pretreatment with capsaicin. Pretreatment with the NK2 receptor antagonist SR 48968 (5 x 10(-7)M) completely inhibited bronchoconstriction and significantly reduced the immunoreactivity induced by the pH 6 buffer. The NK1 antagonist SR 140333 (5 x 10(-7)M) and, to a lesser extent the NK1 antagonist CP 96345, morphine (5 x 10(-6)M), the alpha2-adrenoceptor agonist UK 14304 (10(-7)M) and betamethasone (10(-6)M) significantly reduced both pH 6-induced bronchial response and CGRP-like immunoreactivity overflow. The effects of morphine and UK14304 were partially reversed by naloxone (5 x 10(-5)M) and idazoxan (5 x 10(-50M). Therefore, NK1, NK2, mu-opioid, alpha2-adrenoceptor and glucocorticoid receptors seemed to have a prejunctional action on pH 6 buffer-induced CGRP-like immunoreactivity and bronchoconstriction.

Comparison of ozone-induced effects on lung mechanics and hemodynamics in the rabbit

The effects of rabbit exposure to ozone (O3)(0.4 ppm for 4 h) on pulmonary mechanical properties and hemodynamics have been investigated on the isolated perfused lung model. Tracheal pressure, airflow, and tidal volume were measured in order to calculate lung resistance (RL) and dynamic compliance (Cdyn). Using the arterial/venous/double occlusion method, the total pressure gradient (deltaPT) was partitioned into four components (arterial, pre-, postcapillary and venous). Dose-response curves to acetylcholine (ACh), substance P (SP), and histamine were constructed in lungs isolated from rabbits immediately or 48 h after air or O3 exposure O3 induced a significant increase in the baseline value of deltaPt, more markedly 48 h after the exposure. Immediately after the exposure, O3 partly inhibited the ACh-, SP-, and histamine-induced decreases in Cdyn and increases in RL. This inhibitory effect was still in part present 48 h after O3 treatment. In the groups studied immediately after exposure, O3 did not significantly modify the ACh-, SP-, and histamine-induced vasoconstriction. Forty-eight hours after exposure, O3 induced a contractile response to ACh and SP in the arterial segment but decreased the response to histamine. We conclude that O3 can induce direct vascular constriction. Directly, but also 48 h after exposure, O3 can inhibit the ACh-, SP-, and histamine-induced changes in lung mechanical properties. Ozone can also induce some changes in the intensity and in the location of the vascular responses to ACh, SP, and histamine.

Adenosine reduces airway excitatory non-cholinergic (e-NC) contraction through both A1 and A2 adenosine receptor activation in the guinea pig

The influence of adenosine and selective A1 and A2 agonists and antagonists was investigated on the cholinergic and the excitatory non-cholinergic (e-NC) contractions induced by electrical field stimulation in the guinea-pig bronchi. Adenosine (10 nM-1 mM) induced a concentration-dependent inhibition of the e-NC contraction (EC50 = 90 +/- 14 microM), whereas the cholinergic peak was only slightly affected. Preincubation of the tissue with the adenosine uptake blocker dipyridamole (10 microM) significantly shifted the concentration-inhibition curve to adenosine to the left (EC50 = 10 +/- 1 microM), suggesting an interaction with extracellular adenosine receptors of A1 and/or A2 subtype. To characterize the receptor type involved in this effect, selective adenosine derivatives were studied. The agonist to both A1 and A2 adenosine receptors, 5'-N-ethylcarboxamidoadenosine (NECA) was more potent than the selective A1 agonist, (-)-R-6-phenylisopropyladenosine (R-PIA), in inhibiting the e-NC contraction (EC50 = 0.10 +/- 0.04 and 0.60 +/- 0.12 microM, respectively, with a maximal inhibition of 70 and 45%, respectively). The concentration-response curve to NECA was shifted to the right by the A2 receptor selective antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) (10 microM) (EC50 = 1.4 +/- 0.5 microM) as well as by the specific A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (10 microM) (EC50 = 0.7 +/- 0.3 microM). The inhibitory effect induced by the association of both antagonists, DPCPX and DMPX, was considerably potentiated (EC50 > 22 +/- 2.5 microM). The effect of R-PIA was also shifted to the right by DPCPX (EC50 = 8.2 +/- 1.6 microM) but was not modified by DMPX. The contractile response to exogenous substance P was unaffected by NECA pretreatment (0.3 microM). Altogether, these results suggest that adenosine-induced inhibition of e-NC contraction of guinea-pig bronchi is mediated through activation of both A1 and A2 adenosine receptors linked to inhibition of the release of neuropeptides from C-fibre nerve endings.

Alpha 2-adrenergic receptor activation inhibits calcitonin gene-related peptide expression in cultured dorsal root ganglia neurons

Calcitonin gene-related peptide (CGRP), a potent vasodilator, is produced in dorsal root ganglia (DRG) neurons which extend nerves peripherally to blood vessels and centrally to the spinal cord. We previously reported that neuronal CGRP expression is significantly reduced in the spontaneously hypertensive rat (SHR) which could contribute to the elevated BP. Other studies suggest that the enhanced activity of the sympathetic nervous system in the SHR may mediate, at least in part, this reduction in neuronal CGRP expression via activation of alpha 2-adrenoreceptors (alpha 2-AR) on DRG neurons. To test this hypothesis in vitro we employed primary cultures of adult rat DRG neurons. Neuronal cultures were initially exposed (24 h) to either the alpha 2-AR agonist UK 14,304 (10(-6) M) or vehicle; however, no changes in CGRP mRNA content or immunoreactive CGRP (iCGRP) release were observed. Using the rationale that in vivo DRG neurons receive a continuous supply of target tissue derived nerve growth factor (NGF), which stimulates CGRP synthesis, the cultured neurons were treated (24 h) with either vehicle, NGF (25 ng/ml) alone, or NGF plus UK. NGF treatment increased CGRP mRNA accumulation 5.5 +/- 0.9-fold (p < 0.001) and iCGRP release 2.9 +/- 0.4-fold (p < 0.001) over control levels. The stimulatory effects of NGF were markedly attenuated, but not abolished, by UK (NGF + UK vs. control, CGRP mRNA, 2.9 +/- 0.4-fold, p < 0.05; iCGRP, 1.7 +/- 0.2-fold, p < 0.05). These values were also significant (p < 0.05) when compared to NGF treatment alone. Experiments performed using the alpha 2-antagonist yohimbine confirmed that the effects of UK were mediated by the alpha 2-AR. These results, therefore, demonstrate that alpha 2-AR activation attenuates the stimulatory effects of NGF on CGRP expression in DRG neurons.

Ozone-induced stimulation of pulmonary sympathetic fibers: a protective mechanism against edema

Tropospheric ozone exerts well-described toxic effects on the respiratory tract. Less documented, by contrast, is the ability of ozone to induce protective mechanisms against agents that are toxic to the lungs. In particular, interactions between ozone and the sympathetic nervous system have never been considered. Using a model of permeability edema in isolated perfused rabbit lungs, we report here that, immediately after exposure of rabbits to 0.4 ppm ozone for 4 hr, the pulmonary microvascular responses to acetylcholine and substance P are completely blocked. Several lines of evidence, including partial inhibition of the ozone-induced protective effect by several drugs (alpha2- and beta-adrenergic antagonists, neuropeptide Y antagonist, guanethidine), measured levels of released catecholamines in blood and urine and the in vitro response of isolated lungs exposed to 0.4 ppm ozone all seem to suggest that ozone can stimulate pulmonary adrenergic fibers and induce the local release of catecholamines and neuropeptide Y, this resulting in transient protection against pulmonary edema. We also showed that, 48 hr after the exposure, ozone increased the baseline microvascular permeability and the response to low concentrations of acetylcholine.

Post-exercise nasal vasoconstriction and hyporeactivity: possible involvement of neuropeptide Y

Neuropeptide Y (NPY) is co-localized with noradrenaline (NA) in perivascular sympathetic nerve and is a vasoconstrictor. Pre-treatment with exogenous NPY markedly reduced nasal airway obstruction and rhinorrhea induced by the irritant capsaicin in control subjects. The aim of the present experiments was to study the time course variations of plasma concentrations of NA and NPY during and after intense exercise in 17 healthy volunteers. In parallel, changes in nasal airway resistance (NAR) were recorded. Nasal obstruction and rhinorrhea induced by capsaicin were compared after 30 min of rest and after 30 min of exercise. Both subjective and objective NAR were significantly reduced (p < 0.05) for over 15 min after the end of exercise. Plasma levels of NPY remained increased for more than 15 min after exercise whereas NA returned to basal values within less than 10 min. The increases of NAR and mucus production evoked by capsaicin were markedly attenuated for 30 min after exercise (p < 0.05). Variations of plasma NPY concentrations over time correlated better with post-exercise nasal vasoconstriction and hyporeactivity to capsaicin than NA. These observations suggest that endogenous NPY could be involved in the prolonged post-exercise nasal vasoconstriction and acts as a modulator of nasal airways reactivity.

Intranasal administration of neuropeptide Y in man: systemic absorption and functional effects

1. Exogenous neuropeptide Y (NPY, 10 nmol, 50 nmol and 100 nmol) and its vehicle (NaCl 0.9%) were administered in a double blind, randomized and controlled manner by intranasal spray in 7 healthy volunteers. Variations of plasma NPY concentration over time were measured during 120 min. Forty min after the administration of 50 nmol and 100 nmol of exogenous NPY, plasma NPY increased from 5.5 +/- 1.1 pM to 9.8 +/- 2.3 pM (P < 0.05) and from 9.06 +/- 5.1 pM to 20.8 +/- 6.16 pM (P < 0.001), respectively. There was no significant modification of the mean arterial blood pressure and no subjective discomfort was reported. 2. Nasal airway resistance (NAR) was measured by anterior rhinomanometry and was reduced by 25 +/- 3% and 32 +/- 5% after the spray of 50 nmol and 100 nmol, respectively, for about 90 min. 3. Double-blind, randomized, placebo-controlled and 3-way crossover design experiments were performed in 8 healthy volunteers to evaluate the influence of intranasal pretreatment with NPY (20 nmol) and the mixed alpha 1/alpha 2-adrenoceptor agonist oxymetazoline (20 nmol) on the functional effects of subsequent local irritation evoked by capsaicin (3.3 x 10(-4) mol). Subjective evaluation of NAR and local intensity of discomfort were evaluated by means of a visual analogue scale. Nasal secretions were collected and objective NAR was recorded by rhinomanometry. 4. Subjective NAR, nasal secretions and rhinomanometry recordings were not modified by intranasal application of saline, NPY or oxymetazoline. Subjective nasal obstruction, local discomfort, nasal secretions and NAR increase evoked by capsaicin were markedly reduced by NPY pretreatment (P < 0.05) when compared to saline or oxymetazoline. 5. It is concluded that intranasal application of exogenous NPY has very low systemic absorption but induced long lasting nasal vasoconstriction without cardiovascular effects. Pretreatment of the nasal mucosa with exogenous NPY reduces both secretagogue and vasodilator responses to subsequent application of capsaicin.

Neural mechanisms and axon reflexes in asthma. Where are we?

For many years, asthma has been classified as a "neural" disease, with an imbalance between constrictor and dilator nerves being responsible for the symptomatology. Although, nowadays, asthma is recognized as an inflammatory disorder of the airways, neural mechanisms remain very important; axon reflexes, in particular, have received a lot of attention in recent years. In this commentary, an overview is given on the innervation of the airways and its relevance in asthma, and potential new insights in airways innervation are discussed. In a second part, the role of axon reflexes is highlighted. Although neuropeptides such as substance P and neurokinin A are present in human airways, where they produce many of the features characteristic of asthma, and although there is an elevation of their content in induced sputum from asthmatics, there is still no clear direct evidence for the existence of operational axon reflexes in human airways. Most of the research focused on this subject is performed in guinea pigs, where such an axon reflex clearly operates in the airways. In these animals, different receptors have been identified on C-fiber endings, which, upon stimulation, cause inhibition of neuropeptide release. Some of these receptors have also been identified on human airway nerves. Therefore, it has been suggested that modulation of axon reflexes could be of potential benefit in asthma treatment. Indeed, some drugs (e.g. sodium cromoglycate, nedocromil sodium, and ketotifen), which have been demonstrated to partially inhibit neuropeptide release in guinea pig airways, have anti-inflammatory effects on neuropeptide release in guinea pig airways, do not seem to have any anti-inflammatory effects in human asthma. Other drugs, however, such as beta2-mimetics, which have a much more pronounced inhibitory effect in asthma. In conclusion, although there is a lot of indirect evidence for the existence of axon reflex mechanisms in human airways, most of the data now available are derived from animal studies. The key question of whether axon reflexes are operational in human airways remains unanswered. Hopefully, the near future will bring a solution to this enigma with the introduction of very potent tachykinin antagonists for the treatment of human asthma.

Effects of adenosine on NANC bronchoconstriction in anaesthetized guinea-pigs

The present work assesses the effects of the acute administration of adenosine on tachykinergic bronchoconstriction induced in different ways (exogenously administered capsaicin or substance P and vagal electrical stimulation) in anaesthetized and curarized guinea-pigs. Adenosine (30-3000 micrograms kg-1, i.v.) enhanced significantly and dose-relatedly the airway narrowing induced by a single dose of capsaicin (0.5-2 micrograms kg-1, i.v.), both in normal and in vagotomized animals. A smaller and less dose-dependent enhancement by the nucleoside of the pulmonary resistance increase induced by substance P (5-15 micrograms kg-1, i.v.) was observed. This effect was almost completely prevented by the H1 antagonist diphenhydramine (1 mg kg-1, i.v.), which also unmasked an inhibitory action of adenosine at the highest doses. Diphenhydramine, on the contrary, did not significantly modify the potentiation by adenosine of capsaicin-mediated bronchoconstriction. Finally, the nucleoside dose-dependently inhibited the atropine-resistant bronchospasm following vagal electrical stimulation. The use of the selective adenosinic agonists R-N6-[2-phenylisopropyl]adenosine (1-100 micrograms kg-1, i.v.) and 5'-N-methylcarboxamidoadenosine (1-100 micrograms kg-1, i.v.) before the administration of capsaicin, revealed the ability of the first to reproduce the enhancement induced by adenosine, while the second had an inhibitory effect. It is concluded that adenosine has both excitatory and inhibitory modulatory effects on airway responsiveness to excitatory non-adrenergic non-cholinergic (e-NANC) stimuli. The excitatory effects, revealed with substance P and capsaicin, support the hypothesis that adenosine may play a role as an asthma mediator.

KW-4679-induced inhibition of tachykininergic contraction in the guinea-pig bronchi by prejunctional inhibition of peripheral sensory nerves

1. Sensory mechanisms play an important role in the vagal regulation of tracheobronchial smooth muscle tone. We examined the effect of KW-4679, an anti-allergic drug, on guinea-pig tachykinin-mediated contractile responses induced by electrical field stimulation (EFS) in guinea-pig bronchial muscles. 2. EFS (8 Hz, 0.5 ms, 15 V, for 15 s) evoked biphasic contractile responses in the guinea-pig isolated main bronchus in the presence of 5 microM indomethacin. The contractions consisted of a fast phase of an atropine-sensitive transient contraction and a slow phase of a sustained contraction which was inhibited by a combination of the tachykinin NK1 receptor antagonist, (+/-)-CP-96,345 (1 microM) and the NK2 receptor antagonist, SR 48969 (0.1 microM). 3. KW-4679 preferentially inhibited the slow phase in a concentration-dependent manner by 43.2 +/- 7.7% at 10 microM, whereas the drug had no effect on the fast phase at concentrations up to 10 microM. KW-4679, at a concentration of 100 microM, inhibited not only the slow phase by 49.2 +/- 11.4%, but also the fast phase by 36.8 +/- 9.3% [corrected]. 4. KW-4679 (10 microM and 100 microM) did not affect the substance P-induced or neurokinin A-induced contraction. Against the acetylcholine-induced contractile responses, 100 microM KW-4679 had a marked effect producing a 10.2 fold shift to the right in the curve. 5. The inhibitory effect of KW-4679 (10 microM) on the slow phase contraction was not influenced by treatment with naloxone (100 nM), propranolol (1 microM), thioperamide (1 microM), saclofen (50 microM), yohimbine (1 microM), methiothepin (1 microM) or methysergide (1 microM). 6. The inhibitory effect of KW-4679 (10 microM) on the slow phase contraction was not influenced by treatment with intermediate or large conductance Ca(2+)-activated K+ channel blockers (charybdotoxin (10 nM) or iberiotoxin (10 nM)), but suppressed by treatment with small conductance Ca(2+)-activated K+ channel blockers, apamin (500 nM) or scyllatoxin (300 nM). Apamin or scyllatoxin per se did not influence the slow phase contractions. 7. The results suggest that KW-4679 preferentially inhibits the release of tachykinins from the bronchial sensory nerves through activation of small conductance Ca(2+)-activated K+ channels.

Production, characterization, and expression of neuropeptide Y by human pheochromocytoma

Neuropeptide Y (NPY) levels are increased in plasma and tumors of patients with pheochromocytoma. The present study was designed to evaluate plasma and tissue NPY levels simultaneously as well as to study its release and expression in patients with either adrenal or extraadrenal pheochromocytomas. Plasma NPY levels were higher (P < 0.01) in patients with adrenal tumors than in matched normal subjects and patients with extraadrenal tumors. NPY levels were also higher (P < 0.05) in adrenal than in extraadrenal tumors. Bioactive NPY (1-36) was the predominant form in plasma and tumors of patients with adrenal pheochromocytomas. In contrast, patients with extraadrenal pheochromocytomas had an abundance of NPY fragments. NPY mRNA was abundant in 11 of 13 adrenal tumors but in only 1 of 6 extraadrenal tumors. Moreover, NPY was coreleased with NE with manipulation of adrenal but not extraadrenal tumors. These findings indicate that increased NPY gene expression in adrenal pheochromocytomas accounts for the greater biosynthesis and storage of NPY in these tumors and that increased release of NPY results in elevated plasma NPY. Factors regulating NPY gene expression in pheochromocytoma and the role of NPY in the clinical manifestations of the disease remain to be elucidated.

Modulation of acetylcholine, capsaicin and substance P effects by histamine H3 receptors in isolated perfused rabbit lungs

The modulatory role of histamine H3 receptors in pulmonary oedema induced by acetylcholine, capsaicin and by exogenous substance P was investigated in isolated, ventilated rabbit lungs. Endothelial permeability was evaluated by measuring the capillary filtration coefficient (Kf,c). Acetylcholine (10(-8) to 10(-4) M), substance P (10(-10) to 10(-6) M), capsaicin (10(-4) M) and 5-hydroxytryptamine (5-HT) (10(-4) M) induced an increase in the Kf,c. Carboperamide, a novel histamine H3 receptor antagonist, induced a significant leftward shift of the concentration-response curve to acetylcholine and also enhanced the effect of capsaicin on the Kf,c, while it had no significant effect on the response to substance P and 5-HT. Imetit, a new histamine H3 receptor agonist, strongly inhibited the effects of acetylcholine and capsaicin. Imetit also strongly protected the lung against substance P effects but did not prevent the 5-HT-induced increase in the Kf,c. Carboperamide completely blocked the inhibitory effect of Imetit on the acetylcholine response. (R)-alpha-Methylhistamine, an other histamine H3 receptor agonist, had the same protective effect against acetylcholine response as Imetit. We conclude that histamine H3 receptors could protect the lung against acetylcholine- and capsaicin-induced oedema via a prejunctional modulatory effect on the C-fibres. However, since the response to exogenous substance P was also inhibited by histamine H3 receptor stimulation, the presence of such receptors at a postsynaptic level, probably on mast cells, was also suggested.

Effects of three alpha 2-adrenoceptor agonists, rilmenidine, UK 14304 and clonidine on bradykinin- and substance P-induced airway microvascular leakage in guinea-pigs

The effects of three alpha 2-adrenoceptor agonists, clonidine, rilmenidine and UK 14304 on the increase of microvascular permeability induced by bradykinin or substance P in guinea-pigs airways have been studied in vivo. Extravasation of intravenously (i.v.) injected Evans blue dye was used as index of permeability. The effects of the three alpha 2-adrenoceptor agonists on the contraction induced by bradykinin (0.3 micrograms.kg-1, i.v.) and substance P (0.3 micrograms.kg-1, i.v.) were also studied on the isolated guinea-pig trachea. The increase of plasma exudation induced by bradykinin (0.3 micrograms.kg-1, i.v.) was inhibited partially by rilmenidine and UK 14304 (20 micrograms and 100 micrograms, intratracheally) respectively. These two substances had no action on the effects of substance P. The effects of rilmenidine and UK 14304 were abolished by alpha 2-blockers (idazoxan 1mg.kg-1 i.v. and RX 821001 100 micrograms.kg-1, i.v.), but they were not altered by the alpha 1-blocker prazosin (30 micrograms.kg-1, i.v.). Under similar conditions, clonidine or the alpha 1-adrenoceptor agonist methoxamine were without significant effects. In vitro, rilmenidine and UK 14304 inhibited partially the contractile effects of bradykinin but not those of substance P. To conclude, both rilmenidine and UK 14304 inhibit the bradykinin-induced increase of vascular permeability in the airways, and they probably do so on peptidergic nerve endings at the prejunctional level since these substances are without effect on substance P. The absence of activity of clonidine in our study might be due to a difference in spectrum of action on the several types of alpha 2-adrenergic or imidazole receptors.

Modulatory effect of neuropeptide Y on acetylcholine-induced oedema and vasoconstriction in isolated perfused lungs of rabbit

1. The modulatory role of neuropeptide Y (NPY) on pulmonary oedema induced by acetylcholine and capsaicin was investigated. The effects of NPY on the haemodynamic response to acetylcholine, phenylephrine and substance P were also investigated. 2. Isolated, ventilated, exsanguinated lungs of the rabbit were perfused with a constant flow of recirculating blood-free perfusate. The double/arterial/venous occlusion method was used to partition the total pressure gradient (delta Pt) into four components: the arterial gradient (delta Pa), the pre- and post-capillary gradients (respectively delta Pa' and delta Pv') and the venous pressure gradient (delta Pv). Endothelial permeability was evaluated by measuring the capillary filtration coefficient (Kf,c). 3. Acetylcholine (10(-8) M to 10(-4) M) and substance P (SP, 10(-10) M to 10(-6) M) induced a concentration-dependent increase in the Kf,c. Capsaicin (10(-4) M) and 5-hydroxytryptamine (5-HT) (10(-4) M) also increased this parameter. NPY (10(-8) M) completely inhibited the effects of acetylcholine and capsaicin on the Kf,c, without preventing the effects of substance P and 5-HT. 4. Acetylcholine induced concentration-dependent vasoconstriction in the precapillary segment. The effect was inhibited by NPY and aspirin, an inhibitor of cyclo-oxygenase, while ketanserin, a 5-HT2 receptor antagonist, and SR140333, a new NK1 antagonist, had no protective effect. Phenylephrine increased delta Pa at high concentration, an effect also inhibited by NPY and aspirin. Substance P had no significant haemodynamic effect. When injected together with NPY, substance P (10(-6) M) induced a significant increase in the total pressure gradient. 5. It was concluded that NPY can protect the lung against acetylcholine- and capsaicin-induced oedemavia a prejunctional modulatory effect on the C-fibres. NPY also inhibits acetylcholine-evoked precapillary and phenylephrine-induced arterial vasoconstriction, probably by interfering with cyclo-oxygenase products synthesis.

Sympathetic and parasympathetic interaction in vascular control of the nasal mucosa in anaesthetized cats

1. In cats anaesthetized with pentobarbitone sodium (45 mg kg-1), electrical stimulation of the parasympathetic nerve fibres to the nasal mucosa evoked frequency-dependent increases in nasal arterial blood flow whereas stimulation of the superior cervical sympathetic nerve induced marked vasoconstriction. 2. Sympathetic nerve stimulation for 3 min at 10 Hz evoked significant (P < 0.05) and prolonged (> 30 min) attenuation of the vasodilatory response to subsequent parasympathetic stimulation. 3. Combined pretreatment with adrenergic and cholinergic blockers reduced the vasoconstrictory effect of sympathetic stimulation by 28 +/- 4% (mean +/- S.E.M.) and the parasympathetically evoked vasodilatation by 20 +/- 5%. 4. The vasodilatory effects of exogenous vasoactive intestinal polypeptide, peptide histidine isoleucine and galanin, and the vasoconstrictory effects of exogenous neuropeptide Y (NPY) and alpha,beta-methylene adenosine 5'-triphosphate were not altered by adrenoceptor antagonists and atropine whereas the effects of exogenous noradrenaline and acetylcholine were virtually abolished. 5. The atropine-resistant parasympathetic vasodilatation remained significantly attenuated for more than 30 min after the non-adrenergic sympathetically evoked vasoconstriction. 6. Exogenous NPY (25 x 10(-9) mol) mimicked the effect of sympathetic stimulation in attenuating subsequent parasympathetically evoked vasodilatation.

Characterization of sensory neurotransmission and its inhibition via alpha 2B-adrenoceptors and via non-alpha 2-receptors in rabbit iris

To find out whether, and which type of, adrenoceptors mediate prejunctional inhibition of sensory neurotransmitter release from trigeminal fibres, the modulation of twitch response to electrical field stimulation on rabbit isolated iris was investigated. Evoked iris sphincter contractions consisted of a minor fast cholinergic and a large slow component. The latter was unaffected by atropine and guanethidine, hence nonadrenergic noncholinergic in nature (NANC), but nearly completely abolished by capsaicin pretreatment and by the neurokinin receptor antagonist spantide. The response was probably not mediated by NK2 receptors as SR 48,968, an NK2 selective nonpeptide antagonist, failed to reduce the response to the release of the endogenous neurokinin(s) (and exogenous substance P), but in part due to NK1 receptor activation as shown by a reduction of response by CP 96,345, an NK1 selective non-peptide antagonist, and in part perhaps mediated by NK3 receptors. A small neurokinin receptor antagonist- and capsaicin-insensitive NANC contraction is probably not mediated by CGRP receptors. The alpha 2-adrenoceptor agonist oxymetazoline inhibited the evoked NANC response (22 nmol/l, IC20; about 40%, maximum inhibition) without affecting the cholinergic response (up to 1 mumol/l) or the postjunctional iris sensitivity to exogenous substance P. The inhibition was antagonized by rauwolscine (apparent -log KB 8.04) and by the relatively alpha 2B-adrenoceptor selective antagonist ARC-239 (-log KB 8.51). The alpha 2- and imidazoline receptor agonist aganodine inhibited the evoked NANC response (0.25 mumol/l, IC20; about 30%, maximum inhibition) without affecting the postjunctional substance P responses. Rauwolscine 0.3 mumol/l failed to antagonize this effect. It is concluded that the release of sensory neurotransmitter(s) from trigeminal fibres in the rabbit eye may be inhibited by alpha 2B-adrenoceptors and by a non-alpha 2-receptor, perhaps an imidazoline receptor.

Inhibition by alpha 2-adrenoceptor agonists of contractions of rabbit isolated colon elicited by pelvic nerve stimulation

1. Segments of rabbit distal colon were set up for isotonic recording and the extrinsic pelvic parasympathetic nerves were stimulated for 30 s periods at 2 Hz. 2. Atropine (100 nM) abolished acetylcholine-induced contractions, but only partly reduced responses to nerve stimulation. 3. Clonidine and related compounds, UK14819, UK14304, UK15121, UK11957 and UK42620, inhibited nerve stimulation-evoked contractions at concentrations which had no effect on the response to exogenous acetylcholine, suggesting that the compounds inhibited transmitter release. 4. The imidazolidine compounds UK14819, UK14304 and UK15121 had three to five times the potency of clonidine. Another imidazolidine, UK11957, was about three times less potent than clonidine and appeared to have lower intrinsic activity. The morpholinocatechol UK42620 was about 100 times less potent than clonidine. Phenylephrine was about 10,000 times less potent than clonidine. 5. Idazoxan (1-10 microM) and yohimbine (300 nM-3 microM) reversed the depressant effects of clonidine and the UK compounds, indicating that the effects were exerted on alpha 2-adrenoceptors. This was confirmed by the finding that the slope of Schild plot of idazoxan against UK14304 was close to unity with the pA2 value = 7.14. 6. Atropine-resistant neurogenic contractions were completely abolished by UK14819 (100 nM) and this effect was completely reversed by idazoxan (10 microM). 7. Depending on other (especially centrally mediated) effects, the more potent inhibitors of neurogenic colonic motor activity, with their higher alpha 2-adrenergic agonist activity, may have advantages over clonidine as antidiarrhoeal drugs.

Neuronal control of airways smooth muscle

Sensory afferent nerves relay impulses from the airways to the central nervous system so that appropriate changes in bronchomotor tone and breathing patterns may occur. The dominant efferent control of airways smooth muscle is exerted via bronchoconstrictor parasympathetic cholinergic nerves. In some species this is opposed by bronchodilator sympathetic noradrenergic nerves. In addition, there exist both excitatory bronchoconstrictor and inhibitory bronchodilator non-adrenergic, non-cholinergic pathways. This review examines the role of the different branches of the autonomic nervous system in the control of airways smooth muscle tone with particular reference to modulation of these branches and the interactions which may exist between them.

Neuropeptide Y suppresses the neurogenic inflammatory response in the rabbit eye; mode of action

Ocular injury in the rabbit causes miosis and breakdown of the blood aqueous barrier (aqueous flare response, AFR), reflecting a sensory nerve-mediated inflammatory response, elicited by the release of tachykinins and calcitonin gene-related peptide (CGRP) from C-fibers. Neuropeptide Y (NPY) occurs in sympathetic fibers in the eye. The study was designed to examine whether NPY and related peptides interfere with the inflammatory response to ocular injury in the rabbit in vivo. The isolated rabbit iris was studied with respect to NPY binding sites and second messenger coupling. The AFR and the miotic response to a standardized injury (infrared irradiation (IR) of the iris) were suppressed dose-dependently by NPY (0.01-1.0 nmol) injected intravitreally 30 min prior the trauma. The treated eye was compared with the contralateral eye, which received 0.9% saline and IR. The Y1 receptor agonist [Pro34]NPY, the Y2 receptor agonist NPY 13-36 and the structurally related peptide YY (1 nmol each) suppressed the AFR in response to IR. Injection of either NPY or the Y1 and Y2 receptor agonists (0.3 nmol each) suppressed the AFR evoked by exogenously applied CGRP (0.15 nmol). Saturation studies with 125I-NPY revealed both high and 'moderate' affinity binding sites in the iris. The Bmax values were 26 and 321 fmol/mg protein, respectively. NPY suppressed the forskolin-stimulated adenylate cyclase activity (IC50 value 19 nM). NPY did not affect basal or noradrenaline-induced accumulation of inositol phosphates in the iris. In conclusion, the rabbit iris seems to be rich in NPY receptors linked to inhibition of adenylate cyclase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of sympathetic nerves exerts an inhibitory influence on afferent nerve-induced vasodilation unrelated to vasoconstriction in rat dental pulp

In order to elucidate a possible influence of the sympathetic nervous system on afferent nerve function, rat mandibular incisors were electrically stimulated and blood flow changes monitored in the incisor pulp of untreated and sympathectomized animals by a laser Doppler flowmeter. Monopolar electrical stimulation of the tooth (200 microA, 5 ms, 40 Hz, 1 s) in normal animals resulted in a transient reduction in pulpal blood flow (PBF) (16% reduction, n = 10) followed by a small but long-lasting increase (11% increase). After administration of phenoxybenzamine or phentolamine (3 mg kg-1, i.v.) the initial dip in PBF was reduced by 59% (P < 0.001) while the subsequent increase was enhanced by 185% (P < 0.001). Similarly, infusion of prazosin (50 micrograms kg-1, i.v.) and idazoxan (0.5 mg kg-1, i.v.) significantly enhanced the increase in PBF by 118 and by 79%, respectively. In chronically sympathectomized animals the increase in PBF was 250% larger than that seen in untreated animals (P < 0.001). This increase in PBF was not further enhanced after alpha-adrenergic blockade. Acute resection of the superior cervical sympathetic ganglion, also resulted in some enhancement (by 56%) of the stimulation-induced increase in PBF (P < 0.01, n = 6). The increase in PBF was unaffected by infusion of timolol (150 micrograms kg-1) and atropine (1 mg kg-1) but was totally abolished by intravenous pre-treatment with capsaicin (1-3 mg kg-1). The present results suggest that activation of sympathetic nerves exerts inhibitory effects on the afferent nerve-induced vasodilation in the rat incisor pulp unrelated to sympathetic vasoconstriction.

Modulation of non-adrenergic non-cholinergic inhibitory neurotransmission in rat gastric fundus by the alpha 2-adrenoceptor agonist, UK-14,304

1. The influence of the alpha 2-adrenoceptor agonist, UK-14,304, on non-adrenergic non-cholinergic (NANC) relaxation induced by electrical field stimulation was investigated in longitudinal muscle strips of the gastric fundus of reserpinized rats. 2. In tissues where tone was raised by 3 x 10(-7) M prostaglandin F2 alpha (PGF2 alpha), the inhibitory effect of 10(-6) M UK-14,304, on the NANC relaxations induced by short train stimulation (40 V, 1 ms, 20 s) was inversely related to the stimulus frequency (1-4-16 Hz). UK-14,304 (10(-6) M) did not influence relaxations induced by administration of exogenous nitric oxide (NO, 2 x 10(-6) M-10(-4) M). The inhibitory effect of UK-14,304 on the electrically induced relaxations was antagonized by 10(-6) M rauwolscine but not by 10(-6) M prazosin. 3. UK-14,304 (10(-6) M) also reduced the amplitude of the sustained NANC relaxation, induced by electrical field stimulation (40 V, 1 ms, 4 Hz) for 5 min. The effect of UK-14,304 was also antagonized by 10(-6) M rauwolscine but not by 10(-6) M prazosin. UK-14,304 (10(-6) M) did not reduce the relaxation induced by 3 x 10(-9) M vasoactive intestinal polypeptide (VIP). 4. These results suggest that the release of the inhibitory NANC neurotransmitter during short train stimulation, thought to be NO, and during sustained stimulation, thought to be VIP, is inhibited by stimulation of presynaptic alpha 2-adrenoceptors in the rat gastric fundus.

An opioid peptide inhibits capsaicin-sensitive vasodilatation in the pig's skin

Microcirculatory effects of electrical stimulation of nerves through a pair of needle electrodes in the skin of anaesthetized pigs were studied by using the laser Doppler flowmetric method. Electrical stimulation (0.3-30 Hz) evoked a short-lasting decrease in capillary blood flux (vasoconstriction) followed by an increase (vasodilatation), of longer duration. Vasoconstriction was inhibited by local guanethidine, but not by capsaicin pretreatment, whereas vasodilatation was blocked by local capsaicin, but not by guanethidine. Both phases of the response were suppressed by local application of tetrodotoxin. Thus, vasoconstriction due to electrical stimulation seems to be of sympathetic origin, while vasodilatation is a result of a release of vasoactive substances from capsaicin-sensitive nerve endings. Vasodilatation due to electrical stimulation was strongly and dose-dependently inhibited by the opioid peptide [D-Met2,Pro5] enkephalinamide, while vasoconstriction remained apparently unchanged. At both doses of the opioid peptide tested (0.03 and 0.15 mumol/kg i.m.) inhibition of vasodilatation was larger at lower than at higher frequencies of stimulation. Guanethidine pretreatment did not influence the inhibitory action of [D-Met2,Pro5] enkephalinamide. Naloxone (1.5 mumol/kg i.m.) reversed or prevented the inhibitory action of the opioid peptide; naloxone on its own did not influence responses due to 0.3-30 Hz stimulation. [D-Met2,Pro5] enkephalinamide (0.15 mumol/kg i.m.) did not influence basal blood flux in the skin, mean arterial blood pressure, respiratory minute volume or respiratory frequency. It was concluded that stimulation of opioid receptors by [D-Met2,Pro5] enkephalinamide is likely to inhibit stimulation-evoked vasodilatation by reducing the release of vasoactive substances from capsaicin-sensitive afferent neurons, an effect that does not depend on functional integrity of sympathetic nerves. Endogenous opioids probably do not modulate the capsaicin-sensitive vasodilatation.

Effects of inhaled alpha 2-adrenoceptor and GABAB receptor agonists on citric acid-induced cough and tidal volume changes in guinea pigs

The effects of alpha 2-adrenoceptor and GABA receptor agonists on citric acid-induced cough and increased tidal volume were investigated in conscious guinea pigs. Inhalation of low doses of B-HT 920 (5-allyl-2-amino 5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine dihydrochloride), and xylazine significantly inhibited citric acid-induced cough and tidal volume increases. Intraperitoneal administration of higher doses of B-HT 920 than those given by aerosol were ineffective. The inhibitory effects of B-HT 920 were antagonised by prior intraperitoneal administration of yohimbine, but not atropine. Inhalation of GABA or baclofen inhibited tidal volume increases, but had no effect on cough. Inhaled alpha 2-adrenoceptor or GABA agonists had no effect on the reduced respiratory rate after citric acid inhalation. It is concluded that alpha 2-adrenoceptor agonists inhibit cough via a mechanism which may not be related to their ability to reduce citric acid-induced tidal volume increases, since GABA and baclofen inhibited tidal volume increases but not cough. We suggest that alpha 2-adrenoceptor agonists may have therapeutic potential in the treatment of cough.

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.

Role of K+ channels in the modulation of cholinergic neural responses in guinea-pig and human airways

1. Several agonists modulate cholinergic neurotransmission in airways raising the possibility that there may be a common inhibitory mechanism, such as the activation of a common K+ channel in the nerve ending. To test this hypothesis, we examined whether blockers of K+ channels are able to depress the prejunctional inhibitory modulation of cholinergic contractile responses by various agonists in guinea-pig and human airways in vitro. 2. Electrical field stimulation (40 V, 0.5 ms) was applied to guinea-pig (0.5 Hz) or human (1 Hz) tracheal strips every 4 min to elicit cholinergic neural responses. The effects of the K+ channel blockers, charybdotoxin (ChTX, 10 nM), apamin (100 nM) and glibenclamide (1 microM), on the prejunctional inhibition of cholinergic contraction evoked by neuropeptide Y (NPY, 100 nM), an alpha 2-agonist, clonidine (10 nM), a mu-opioid agonist, [D-Ala2, NMePhe4, Gly-ol5]-enkephalin (DAMGO, 100 nM), and a KATP channel opener, lemakalim (300 nM) were tested in guinea-pigs. In human tissues, the effect of ChTX (10 nM) on the mu-opioid (DAMGO, 300 nM)-induced inhibition of cholinergic nerves was examined. 3. In guinea-pigs, ChTX (10 nM) significantly reversed the prejunctional inhibition of cholinergic contraction by NPY (84.2 +/- 16.2%), clonidine (71.9 +/- 22.4%), DAMGO (67.3 +/- 13.1%) and lemakalim (20.9 +/- 9.4%) (n = 5, P < 0.05, respectively), while apamin (100 nM) had no effect. Glibenclamide (10 microM) reduced only the lemakalim-induced inhibitory modulation. ChTX (10 nM) itself potentiated cholinergic contraction (24.6 +/- 9.4%, n = 5, P < 0.05) without affecting exogenously applied acetylcholine dose-response curves. Pretreatment with ChTX (10 nM) significantly reduced the inhibitory modulation of cholinergic nerves by NPY, clonidine and DAMGO, but not by lemakalim. 4. In human tissues, ChTX significantly reduced DAMGO-induced prejunctional inhibition of cholinergic contraction (13.6 +/- 8.5% with and 46.5 +/- 5.5% without ChTX, respectively; n = 5, P < 0.05). 5. These results may support a hypothesis that the activation of ChTX-sensitive K+ channels is involved in the inhibitory modulation of cholinergic neuro-transmission by agonists acting on presynaptic receptors in guinea-pig and human airways.

Neuropeptide Y in the rabbit maxillary sinus modulates cholinergic acceleration of mucociliary activity

The distribution of neuropeptide Y (NPY)-immunoreactivity was investigated in the rabbit maxillary sinus and adjacent ganglia. A moderate supply of NPY-containing nerve fibers occurred around seromucous glands and a denser supply around small blood vessels. Only a few immunoreactive nerve fibers were seen beneath the epithelium. Double immunostaining showed that vasoactive intestinal peptide (VIP) coexisted with NPY in the nerve fibers surrounding blood vessels and seromucous glands. NPY-containing nerve cell bodies were numerous in the superior cervical ganglion, and moderately numerous in the sphenopalatine ganglion. The finding of NPY-containing neurons in the latter parasympathetic ganglion suggests that NPY may influence the cholinergic regulation of mucociliary activity. The effect of NPY on the mucociliary activity of the maxillary sinus in connection with cholinergic stimulation has therefore been investigated in vivo using a photoelectric technique. At dosages of 2.5 and 5.0 micrograms/kg, the ganglionic stimulant nicotine bitartrate, which increases mucociliary activity by a cholinergic pathway, accelerated mucociliary activity by 28.0 +/- 7.5% and 36.8 +/- 6.2%, respectively. In the same experiment repeated during infusion of NPY (0.1 microgram/kg/min), the increase in mucociliary activity was reduced to 10.8 +/- 2.3% and 28.9 +/- 7.1%, respectively. Infusion of NPY did not affect the stimulating effect on mucociliary activity by bolus injections (0.1 and 0.5 microgram/kg) of the cholinergic agonist, methacholine. It is concluded that NPY-like immunoreactivity is present in nerve fibers in the rabbit maxillary sinus and in neurons in the sympathetic and parasympathetic ganglia that supply the nose and paranasal sinuses. NPY attenuates the effect of nicotine on mucociliary activity, probably via a prejunctional mechanism, and may act as a modulator of cholinergic regulation of the mucociliary system.

Neurogenic inflammation and asthma

The release of neurotransmitters may exacerbate the inflammatory response. Such neurogenic inflammation has been documented in a number of inflammatory diseases. Neurogenic inflammation due to release of neuropeptides from sensory nerves has been demonstrated in airways of several species, particularly rodents, and may contribute to the inflammatory response in asthmatic airways. Tachykinins (substance P and neurokinin A) released from airway sensory nerves may cause bronchoconstriction, vasodilatation, plasma exudation, and mucus secretion, whereas another sensory neuropeptide, calcitonin generelated peptide, may contribute to hyperemia of inflammation. Airway epithelial damage in asthma exposes sensory nerves which may become sensitized by inflammatory products (including prostaglandins and cytokines) so that neuropeptides are released via a local reflex trigger such as bradykinin, resulting in exaggerated inflammation. The effects of tachykinins may be amplified further by loss of the major degrading enzyme, neutral endopeptidase, from epithelial cells. Direct evidence for neurogenic inflammation in asthma is still awaited, however. Several strategies for reducing neurogenic inflammation are possible, particularly inhibition of neuropeptide release from sensory nerves by stimulating prejunctional receptors such as mu-opioid receptors.

Release of calcitonin gene-related peptide from sensory neurons

CGRP is released from capsaicin-sensitive sensory neurons in a Ca(2+)-dependent manner in a variety of peripheral organs as well as from central terminals. The mechanisms for CGRP release by low concentrations of capsaicin, electrical antidromic nerve stimulation, and bradykinin have several similar characteristics regarding sensitivity to TTX, CTX, and alpha 2-adrenoceptor activation. High capsaicin concentration and nicotine evoke CGRP release via other mechanisms. The effects of capsaicin, resiniferatoxin, and SO2 are blocked by RR, which probably inhibits ion fluxes associated with capsaicin receptor activation. CGRP released upon irritation of peripheral branches of primary afferents may evoke a variety of cardiovascular actions and influence motility in the gastrointestinal and urogenital tracts.

Inhibitory action of neuropeptide Y on agonist-induced responses in isolated guinea pig trachea

The effect of neuropeptide Y (NPY) was tested on isolated guinea pig trachea. At 30 nM, NPY induced a weak but significant contractile response which was on average less than 6% of responses elicited by standard spasmogens. This myotropic action of NPY was blocked by indomethacin. In addition to its contractile activity, NPY greatly reduced the maximal response to vasoactive intestinal peptide (VIP), noradrenaline (NA), substance P (SP) and 5-hydroxytryptamine (5-HT), without affecting their pD2 values. However, NPY did not influence the response induced by histamine and carbamylcholine. Pretreatment of tracheal spirals with indomethacin (10(-6) M) abolished the NPY-evoked inhibition of VIP, SP and 5-HT responses but failed to reduce the action of NPY on NA-elicited relaxation. This latter effect was however blocked in the presence of tetrodotoxin. In conclusion, NPY inhibits responses elicited by various agonists in the guinea pig trachea. This effect seems to be mediated at both pre- and postjunctional levels. The postjunctionally mediated inhibitory action of NPY appears to be expressed especially with agents that generate prostaglandins concomitantly with inducing their response. In contrast, the NPY-evoked inhibition of NA-evoked relaxation seems to be mediated via a prejunctional mechanism.

Calcium-activated potassium channels mediate prejunctional inhibition of peripheral sensory nerves

Activation of several receptors, including mu-opioid, alpha 2-adrenergic, and neuropeptide Y receptors, inhibits excitatory nonadrenergic noncholinergic (NANC) neural responses in airways, which were mediated by the release of peptides from capsaicin-sensitive sensory nerves. This raises the possibility of a common inhibitory mechanism, which may be related to an increase in K+ conductance in sensory nerves. To examine this hypothesis, we have studied whether K(+)-channel blockers inhibit the effects of neuromodulators of sensory nerves in guinea pig bronchi by using selective K(+)-channel blockers. Charybdotoxin (ChTX; 10 nM), which blocks large conductance Ca(2+)-activated K(+)-channel function, completely blocked and reversed the inhibitory effects of a mu-opioid agonist, neuropeptide Y, and an alpha 2-adrenoceptor agonist on excitatory NANC responses. Neither inhibitors of ATP-sensitive K+ channels (BRL 31660 or glibenclamide, both at 10 microM) nor an inhibitor of small conductance Ca(2+)-activated K+ channels (apamin; 0.1 microM) were effective. This suggests that ChTX-sensitive K(+)-channel activation may be a common mechanism for the prejunctional modulation of sensory nerves in airways. This may have important implications for the control of neurogenic inflammation.

Nicotine- and capsaicin-, but not potassium-evoked CGP-release from cultured guinea-pig spinal ganglia is inhibited by Ruthenium red

In the present study we have investigated the effects of nicotine, capsaicin, potassium, glutamate and aspartate on release of calcitonin gene-related peptide (CGRP)-like immunoreactivity (-LI) from guinea-pig dorsal root ganglion (DRG) cultures. In addition the possible influence of Ruthenium red (RR), neuropeptide Y (NPY) and noradrenaline (NA) on the CGRP-LI outflow has been evaluated. Nicotine, capsaicin and potassium, but not glutamate or aspartate, evoked a Ca(2+)-dependent increase in the culture medium, suggesting release of CGRP-LI. RR inhibited the effect of both capsaicin and nicotine but did not influence potassium-induced CGRP-LI release. Furthermore, the nicotine- but not capsaicin-evoked CGRP-LI release was inhibited by NPY. It is concluded that DRG cultures represent an experimental model where regulation of CGRP-LI release can be studied. The ability of RR to inhibit not only capsaicin but also nicotine effects indicate that the proposed selectivity of RR may depend on the agent used to evoke peptide release and/or concentrations used.

Selective inhibition by dactinomycin of NANC sensory bronchoconstriction and [125I]NKA binding due to NK-2 receptor antagonism

In the present study, dactinomycin (10(-5) M) inhibited the non-adrenergic, non-cholinergic bronchoconstriction upon antidromic vagal nerve stimulation (1 Hz for 1 min) in the isolated perfused guinea-pig lung by 84%. The release of calcitonin gene-related peptide was unchanged, however, suggesting a postjunctional action. Dactinomycin (10(-5), 5 x 10(-5) M) also reduced non-adrenergic non-cholinergic bronchial contractions (maximally by 75%) induced by electrical field stimulation or capsaicin, while the cholinergic component and non-adrenergic non-cholinergic relaxation remained intact. The neurokinin-2 receptor antagonist L-659,877 (10(-6) M) had a similar effect as dactinomycin, inhibiting the non-adrenergic non-cholinergic bronchial contractions by 69%, while the neurokinin-1 receptor antagonist CP-96,345 (10(-6) M) had no effect. The bronchoconstriction evoked by neurokinin A, the selective neurokinin-2 receptor agonist Nle10neurokinin A (4-10) and capsaicin was markedly inhibited by dactinomycin while the contraction induced by substance P (SP), the selective neurokinin-1 receptor agonist Sar9Met(O2)11SP, endothelin-1 and acetylcholine was not affected. In autoradiographic experiments on guinea-pig lung, [125I]neurokinin A-labelled sections showed dense binding in the bronchial smooth muscle layer. Dactinomycin inhibited the specific binding of [125I]neurokinin A in a concentration-dependent manner (IC50 = 6.3 x 10(-6) M) and 66% of [125I]neurokinin A total binding was inhibited by 10(-4) M dactinomycin. In the rat colon, [125I]neurokinin A binding to neurokinin-2 sites on circular smooth muscle was inhibited by dactinomycin with an IC50 value of 7.9 x 10(-6) M. Dactinomycin failed to reduce increased nerve-evoked contractions or those caused by Nle10neurokinin A (4-10) per se in the rat vas deferens, which are considered to be mediated by neurokinin-2 receptor activation. In the rat portal vein, dactinomycin did not influence the contractions caused by the neurokinin-3 selective agonist Pro7neurokinin B. In conclusion, dactinomycin selectively inhibited neurokinin-2 receptor activation in guinea-pig lung and rat colon, but not in rat vas deferens, which may depend on the existence of different neurokinin-2 receptor subtypes. Neurokinin A is most likely the main endogenous excitatory non-adrenergic non-cholinergic transmitter in guinea-pig bronchi.

Variable α2-adrenoceptor-mediated inhibition of bronchoconstriction and peptide release upon activation of pulmonary afferents

In the present study, we evaluated the possible regulation by alpha 2-receptor agonists (SKF 35886 and UK 14304) of peptide release and functional responses upon sensory nerve activation in the guinea-pig lung. The peptide release and bronchoconstriction caused by antidromic vagal nerve stimulation (low frequency, 1 Hz), and a low concentration of capsaicin (10(-8) M) and resiniferatoxin (3 x 10(-10) M) were attenuated by alpha 2-adrenoceptor agonists. The effects of capsaicin and nicotine in high concentrations and high frequency nerve stimulation (10 Hz) were influenced to a much smaller extent by alpha 2-adrenoceptor stimulation. The calcitonin gene-related peptide release evoked by bradykinin but not the functional effects was inhibited by alpha 2-adrenoceptor activation. It is concluded that alpha 2-adrenoceptor stimulation mainly inhibits the release of mediator and/or the bronchoconstriction caused by moderate activation of sensory nerves. It is necessary to measure mediator release directly to reveal prejunctional effects and not to rely only on indirect functional evidence.

Modulation by adrenergic transmitters of the efferent function of capsaicin-sensitive nerves in cardiac tissue

In atrial preparations obtained from reserpine-pre-treated guinea-pigs, incubated in the presence of 1 microM atropine plus 1 microM CGP 20712A (a beta 1 blocking drug), a positive inotropic effect due to CGRP release from capsaicin-sensitive sensory neurons was induced by electrical field stimulation (EFS). This response was concentration-dependently reduced by noradrenaline (0.01-3 microM), neuropeptide Y (NPY, 3-300 nM) and adenosine triphosphate (ATP, 1-30 microM). On the other hand, the overflow of [3H]-noradrenaline from sympathetic nerve terminals induced by EFS in isolated atria obtained from normal untreated animals was not modified in 10 nM calcitonin gene-related peptide (CGRP). Substance P (SP) and neurokinin A (NKA), at concentrations ranging from 0.01 to 1 microM did not affect the cardiac response to field stimulation of adrenergic terminals of atrial tissue. These findings demonstrate that all the co-transmitters stored in adrenergic nerve terminals have a modulatory role on the efferent function of cardiac capsaicin-sensitive sensory neurons, while cardiac adrenergic neurotransmission is not influenced by the peptidergic transmitters released from sensory neurons.