Evidence for inhibition of sympathetic neurotransmission by endogenously released acetylcholine in the guinea-pig trachea

High-frequency electrical stimulation of cervical vagi reduces airway response to methacholine

AIM: To test whether high-frequency electrical stimulation (HES) of the bilateral cervical vagus nerves reduces the airway responses to methacholine (MCh).

METHODS: Guinea pigs were pretreated with saline (Sal, n = 9) or ovalbumin (Ova, n = 10) aerosol for two weeks (5 min/d, 5 d/wk) and subsequently anesthetized, paralyzed, tracheotomized and artificially ventilated. Both total lung resistance (R_L) and dynamic pulmonary compliance (C_dyn) were recorded. In addition, the effects of vagal low-frequency electrical stimulation (LES, monophasic, 50 Hz) and HES (monophasic and biphasic, 1 and 2.5 kHz) for about 10 s or 2 min on the responses of R_L and C_dyn to MCh aerosol-induced bronchoconstriction were compared in both groups of guinea pigs. In a few guinea pigs, the impact of bivagotomy on the R_L responses to MCh was assessed.

RESULTS: Before MCh challenge, LES, but not HES, significantly increased R_L by about 30% (P < 0.01) and decreased C_dyn by about 20% (P < 0.01) similarly in both groups. MCh aerosol for 2 min elevated R_L and diminished C_dyn more in Ova- than Sal-treated animals (R_L: 313% ± 52% vs 113% ± 17%, P < 0.01; C_dyn: -56% ± 7% vs -21% ± 3%, P < 0.01). During MCh-induced airway constriction, LES further enhanced, but HES decreased R_L and this decrease was greater in Ova- (about 45%) than Sal-treated animals (about 34%, P < 0.01) with little change in cardiovascular activity. On the other hand, LES further reduced whereas HES increased C_dyn more in Ova- (about 20%) than Sal-treated animals (about 13%, P < 0.01). In addition, bivagotomy almost eliminated the R_L and C_dyn responses to MCh.

CONCLUSION: We conclude that vagal HES is able to alleviate the bronchoconstriction induced by MCh in anesthetized guinea pigs, likely via reversible inhibition/blockade of vagal conduction.

IFNγ Increases M2 Muscarinic Receptor Expression in Cultured Sympathetic Neurons

M2 muscarinic receptors are expressed on both parasympathetic and sympathetic nerve endings where they function as autoinhibitory receptors to limit release of acetylcholine and norepinephrine, respectively. M2 muscarinic receptor expression on parasympathetic nerves is decreased by viral infection and by gamma-interferon (IFNγ) and increased by dexamethasone; and these effects are of clinical relevance in the etiology and treatment of asthma. Whether IFNγ and dexamethasone similarly modulate M2 receptor expression on sympathetic nerves is not known. To address this question, we examined the effects of IFNγ and dexamethasone on M2 receptor expression at the mRNA and protein level in primary cultures of sympathetic neurons dissociated from the rat superior cervical ganglia (SCG). Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) indicated that neither IFNγ nor dexamethasone altered M2 receptor transcript levels. However, western blot analyses demonstrated that IFNγ, but not dexamethasone, increases M2 receptor protein expression in sympathetic neurons. Increased expression did not significantly alter subcellular localization of M2 receptors in sympathetic neurons as determined using immunocytochemistry. These findings indicate that M2 receptors are differentially regulated in different types of autonomic neurons, and they suggest a novel mechanism by which IFNγ may contribute to airway hyperreactivity in viral-induced asthma.

Nerve growth factor expression in parasympathetic neurons: regulation by sympathetic innervation

Interactions between sympathetic and parasympathetic nerves are important in regulating visceral target function. Sympathetic nerves are closely apposed to, and form functional synapses with, parasympathetic axons in many effector organs. The molecular mechanisms responsible for these structural and functional interactions are unknown. We explored the possibility that Nerve Growth Factor (NGF) synthesis by parasympathetic neurons provides a mechanism by which sympathetic-parasympathetic interactions are established. Parasympathetic pterygopalatine ganglia NGF-gene expression was examined by in situ hybridization and protein content assessed by immunohistochemistry. Under control conditions, NGF mRNA was present in approximately 60% and NGF protein was in 40% of pterygopalatine parasympathetic neurons. Peripheral parasympathetic axons identified by vesicular acetylcholine transporter-immunoreactivity also displayed NGF immunoreactivity. To determine if sympathetic innervation regulates parasympathetic NGF expression, the ipsilateral superior cervical ganglion was excised. Thirty days postsympathectomy, the numbers of NGF mRNA-positive neurons were decreased to 38% and NGF immunoreactive neurons to 15%. This reduction was due to a loss of sympathetic nerve impulse activity, as similar reductions were achieved when superior cervical ganglia were deprived of preganglionic afferent input for 40 days. These findings provide evidence that normally NGF is synthesized by parasympathetic neurons and transported anterogradely to fibre terminals, where it may be available to sympathetic axons. Parasympathetic NGF expression, in turn, is augmented by impulse activity within (and presumably transmitter release from) sympathetic axons. It is suggested that parasympathetic NGF synthesis and its modulation by sympathetic innervation provides a molecular basis for establishment and maintenance of autonomic axo-axonal synaptic interactions.

Effects of five different airway smooth muscle relaxants on inhibitory neurotransmission in isolated guinea-pig trachea in vitro

Pharmacodynamic effects produced by terbutaline (10 nM), theophylline (10 microM), sodium nitroprusside (30 nM), levcromakalim (0.3 microM) or isradipine (1 nM) on frequency-dependent relaxations induced by electric field stimulation of either proximal or distal parts of isolated guinea-pig trachea were studied in vitro. Preparations were depleted for tachykinins by capsaicin, pretreated with atropine (0.1 microM) and contracted by histamine (2 microM). Drug effects were studied in preparations with combined adrenergic and inhibitory non-adrenergic non-cholinergic (NANC) innervation and in preparations with inhibitory NANC innervation either with or without additional treatment with N(G)-nitro-L-arginine methyl ester (L-NAME) (100 microM). In preparations with combined adrenergic and inhibitory NANC innervation terbutaline, sodium nitroprusside, levcromakalim and isradipine significantly reduced relaxant responses to electric field stimulation in proximal preparations, whereas distal preparations were only affected by terbutaline. In preparations with inhibitory NANC innervation without L-NAME pretreatment, terbutaline significantly enhanced relaxant responses to electric field stimulation only in distal preparations, whereas theophylline, sodium nitroprusside and levcromakalim significantly augmented responses to electric field stimulation in both proximal and distal preparations. In preparations with inhibitory NANC innervation pretreated with L-NAME, theophylline significantly inhibited relaxant responses in distal preparations, whereas sodium nitroprusside, levcromakalim and isradipine significantly augmented relaxant responses to electric field stimulation in proximal preparations. It was concluded that drugs used in the present study can modulate the effects of inhibitory autonomic and NANC neurotransmission in isolated guinea-pig trachea. Furthermore, it was shown that some variation in drug effects exists in relation to proximal and distal parts of guinea-pig trachea.

Parasympathetic varicosity proliferation and synaptogenesis in rat eyelid smooth muscle after sympathectomy

Parasympathetic innervation to eyelid smooth muscle inhibits sympathetic neurotransmission pre-junctionally without appreciable direct post-junctional effects. However, 5 weeks after sympathectomy, parasympathetic stimulation elicits substantial cholinergically mediated contractions. This study examined ultrastructural changes accompanying the conversion to parasympathetic excitation. In intact muscles, 64+/-9 nerve varicosities were encountered per 104 micron2. Most were close to muscle cells and not fully enclosed by supporting cells. Axo-axonal synapses were observed occasionally. Two days following sympathectomy, varicosity numbers were reduced by 97% and, relative to controls, remaining varicosities were farther from muscle cells and more frequently fully enclosed by supporting cells, but contained greater numbers of small spherical and large dense vesicles. By 6 weeks post-sympathectomy, numbers of varicosities per unit muscle volume increased to 14% of controls. These varicosities differed from those at 2 days in being closer to smooth muscle cells, less frequently enclosed, and having fewer small vesicles. These findings indicate that intact eyelid smooth muscle varicosities are predominantly sympathetic, but a small number of parasympathetic varicosities are present, some of which may form pre-junctional synapses with sympathetic nerves. Between 2 days and 6 weeks post-sympathectomy, varicosities increased in number and established appositions with smooth muscle cells. This suggests that parasympathetic nerves are capable of re-innervating an atypical smooth muscle target after sympathectomy, and that parasympathetic synaptogenesis is likely to contribute to conversion from pre-junctional inhibition to post-junctional excitation after sympathectomy.

Pancreatic sympathetic nerves contribute to increased glucagon secretion during severe hypoglycemia in dogs

To determine if pancreatic sympathetic nerves can contribute to increased glucagon secretion during hypoglycemia, plasma glucagon and pancreatic glucagon secretion in situ were measured before and during insulin-induced hypoglycemia in three groups of halothane-anesthetized dogs. All dogs were bilaterally vagotomized to eliminate the input from pancreatic parasympathetic nerves. One group of dogs received only vagotomy (VAGX). A second group was vagotomized and adrenalectomized (VAGX + ADX). A third group received vagotomy, adrenalectomy, plus surgical denervation of the pancreas (VAGX + ADX + NERVX) to prevent activation of pancreatic sympathetic nerves. In dogs with VAGX only, hypoglycemia increased plasma epinephrine (Epi), pancreatic norepinephrine (NE) output (+320 +/- 140 pg/min, P < 0.05), arterial plasma glucagon (+28 +/- 12 pg/ml, P < 0.01), and pancreatic glucagon output (+1,470 +/- 370 pg/min, P < 0.01). The addition of ADX eliminated the increase of Epi but did not increase pancreatic NE output (+370 +/- 190 pg/min, P < 0.025), arterial plasma glucagon (+20 +/- 5 pg/ml, P < 0.01), or pancreatic glucagon output (+810 +/- 200 pg/min, P < 0.01). In contrast, the addition of pancreatic denervation eliminated the increase of pancreatic NE output (-20 +/- 40 pg/min, P < 0.05 vs. VAGX), the arterial glucagon (+1 +/- 2 pg/ml, P < 0.01 vs. VAGX), and pancreatic glucagon output responses (+210 +/- 280 pg/min, P < 0.025 vs. VAGX) to hypoglycemia. Thus activation of pancreatic sympathetic nerves can contribute to the increased glucagon secretion during severe insulin-induced hypoglycemia in dogs.

Beta-adrenoceptors mediate inhibition of [3H]-acetylcholine release from the isolated rat and guinea-pig trachea: role of the airway mucosa and prostaglandins

1. Rat or guinea pig isolated tracheae were labelled with [3H]-choline to measure evoked tritium outflow, which reflects neuronal release of [3H]-acetylcholine. Tritium outflow was evoked either by electrical stimulation of the extrinsic vagal nerve (rat tracheae) or by 27 mM potassium (guinea pig tracheae). 2. In rat tracheae isoprenaline (0.01, 0.1 microM) inhibited evoked [3H]-acetylcholine release, whereas beta 2-adrenoceptor-selective agonists (fenoterol, formoterol, salbutamol) were ineffective. 3. The inhibitory effect of isoprenaline was abolished under the following conditions: (i) presence of propranolol (1 microM) or of the beta 1-selective antagonist CGP 20712 A (0.1 microM); (ii) removal of the mucosa at the start of the experiments; (iii) blockade of cyclooxygenase activity by 3 microM indomethacin. 4. In rat isolated tracheae prelabelled with [3H]-arachidonic acid, isoprenaline (0.1 microM) but not formoterol (0.01 microM) enhanced the outflow of [3H]-prostaglandins (PGD2, PGE2). This effect was blocked by 0.1 microM CGP 20712 A. 5. In guinea pig tracheae electrical stimulation of the extrinsic vagal nerve did not cause a constant release of [3H]-acetylcholine, but 27 mM potassium elicited a reproducible release of [3H]-acetylcholine. In this species both isoprenaline (0.1 microM) and formoterol (0.01 microM) inhibited evoked [3H]-acetylcholine release. Inhibition was abolished under the following conditions: (i) presence of propranolol (1 microM) or of the beta 2-selective antagonist ICI 118551 (0.3 microM); (ii) removal of the mucosa at the start of the experiments; (iii) blockade of cyclooxygenase activity by 3 microM indomethacin. 6. In conclusion, the present experiments have demonstrated that activation of beta-adrenoceptors localized in the mucosa mediates inhibition of [3H]-acetylcholine release from the neuroeffector junctions of the pulmonary, parasympathetic nerves most probably by the liberation of inhibitory prostaglandins from the airway mucosa. The adrenoceptor subtype involved differs in rat (beta 1 subtype) and guinea pig (beta 2 subtype) airways.

Beta-adrenoceptor-mediated facilitation of endogenous noradrenaline release from rat isolated trachea

Overflow of endogenous noradrenaline from rat isolated trachea was evoked by electrical field stimulation (3 Hz, 540 pulses) in the presence of yohimbine, desipramine and tyrosine. Isoprenaline 100 nmol/l increased the evoked overflow of noradrenaline by about 65%. This effect was antagonized by propranolol (100 nmol/l) and the beta 2-selective adrenoceptor antagonist ICI 118,551 (100 nmol/l), but not by the beta 1-selective adrenoceptor antagonist CGP 20712 A (100 nmol/l). The beta 2-selective adrenoceptor agonist formoterol (1-100 nmol/l) also facilitated the evoked overflow of noradrenaline, but maximally by only about 25% at 10 nmol/l, i.e. formoterol behaved as a partial agonist at these facilitatory beta-adrenoceptor. This assumption is also supported by the observation that formoterol (10 nmol/l) acted as antagonist against isoprenaline (100 nmol/l). Mechanical removal of the mucosa resulted in a 30% decrease in tissue noradrenaline and a 55% reduction of the evoked overflow of noradrenaline. In mucosa-denuded preparations isoprenaline failed to facilitate noradrenaline overflow. In the presence of indomethacin (3 mumol/l) the evoked overflow of noradrenaline from mucosa containing preparations was increased by about 50%, but isoprenaline still further facilitated the evoked noradrenaline overflow by about 40%. In conclusion, the overflow of noradrenaline in the rat trachea is facilitated via beta 2-adrenoceptors, an effect which requires an intact airway mucosa.

Muscarinic inhibition of endogenous noradrenaline release from rabbit isolated trachea: receptor subtype and receptor reserve

The aim of the present study was to characterize putative muscarine receptors on sympathetic nerve terminals in the rabbit trachea. Release of endogenous noradrenaline from in vitro incubated rabbit trachea was evoked by electrical field stimulation (3 Hz, 540 pulses) and quantified by high performance liquid chromatography with electrochemical detection. The muscarine receptor agonist oxotremorine inhibited the evoked release of noradrenaline completely at 1 mumol/l (EC50: 64 nmol/l). The concentration response curve was very steep (Hill coefficient of 2.3). Scopolamine shifted the concentration response curve of oxotremorine to the right (-log KB 8.48) demonstrating specific, inhibitory muscarine receptors. Several subtype-preferring muscarine receptor antagonists also shifted the concentration response curve of oxotremorine to the right. The rank order of potency was (-log KB or pA2*): scopolamine (8.48) > AF-DX 384 (7.88*; slope of Schild plot 1.1) > (R)-trihexyphenidyl (7.87) > 4-DAMP (7.85) > AQ-RA 741 (7.77) >> methoctramine 6.18) > pirenzepine (6.0) > p-fluoro-hexahydrosiladifenidol (p-FHHSiD, 5.68). When these affinity constants were plotted against reported -log Ki values determined in binding studies on human cloned muscarine receptor subtypes (m1-m5), the best correlation was obtained for m2. Indomethacin (3 mumol/l), which on its own increased the evoked noradrenaline release by about 45%, affected neither the inhibitory effect of oxotremorine nor the antagonistic potency of methoctramine or p-FHHSiD.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of indomethacin on muscarinic inhibition of endogenous noradrenaline release from rat isolated trachea

The release of endogenous noradrenaline from rat isolated tracheae was evoked by electrical field stimulation (3 Hz, 540 pulses) in the presence of yohimbine, desipramine and tyrosine. The muscarine receptor agonist oxotremorine concentration-dependently inhibited the evoked release of noradrenaline by 95% at 1 mumol/l, EC50 values in two series of experiments 41 and 57 nmol/l, respectively. The effect of oxotremorine was antagonized by the non-selective muscarine receptor antagonist scopolamine (10-1000 nmol/l) in a manner suggesting a simple competitive interaction (slope of Schild plot -0.94; pA2 value 8.88). However, the M2 selective muscarine receptor antagonist methoctramine (0.1-10 mumol/l) affected the action of oxotremorine in a manner suggesting a complex interaction (slope of Schild plot -0.47). Addition of indomethacin (3 mumol/l) caused an increase of the evoked release of noradrenaline by 45% and low concentrations of oxotremorine (0.01 and 0.1 mumol/l, but not 1 mumol/l) became less effective resulting in a slight shift to the right of the concentration response curve (EC50 169 nmol/l). Moreover, in the presence of indomethacin methoctramine (0.1-10 mumol/l) antagonized the effects of oxotremorine in a manner suggesting a simple competitive interaction (slope of Schild plot -0.93, pA2 value 7.61). In the presence of indomethacin, the concentration response curve of oxotremorine was only slightly shifted to the right in the presence of the M1 receptor selective antagonist pirenzepine (1 mumol/l, -log KB 6.1) and not significantly affected by the M3 receptor selective antagonist p-fluoro-hexahydrosiladifenidol (1 mumol/l). In conclusion, the release of noradrenaline in the rat trachea is inhibited via presynaptic muscarine heteroreceptors of the M2 subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

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.

Relaxant innervation of the guinea-pig trachealis: demonstration of capsaicin-sensitive and -insensitive vagal pathways

1. The guinea-pig trachea was isolated with its extrinsic innervation intact and placed in a water-jacketed dissecting dish containing warmed, oxygenated Krebs solution. The trachea was not separated from the oesophagus. Two adjacent cartilage rings of the rostral portion of the trachea were cut open opposite the trachealis and prepared for isometric tension measurements. 2. Following the addition of atropine and contraction of the trachealis with prostaglandin F2 alpha (PGF2 alpha), stimulation of the cervical sympathetic trunks elicited relaxations that were abolished by propranolol or hexamethonium. Stimulation of the vagus nerves caudal to the nodose ganglia also elicited relaxations. These vagally mediated relaxations were unaffected by propranolol but were abolished by hexamethonium or by cutting the recurrent laryngeal nerves. 3. After cutting the vagi caudal to the nodose ganglia, stimulation of the vagi rostral to the nodose ganglia elicited relaxations of the trachealis that were not significantly affected by either propranolol or hexamethonium but were abolished by cutting the superior laryngeal nerves. Stimulation of right vagi which had undergone supranodose vagotomy 14 days prior to experimentation was without effect on the smooth muscle of the guinea-pig trachea while the response to stimulation of the left vagus was unchanged. 4. Acute capsaicin desensitization abolished relaxations of the guinea-pig trachealis elicited by stimulation of the vagal fibres carried by the superior laryngeal nerves. In contrast, capsaicin desensitization only modestly inhibited relaxations elicited by stimulation of the preganglionic parasympathetic fibres carried by the recurrent laryngeal nerves and had no effect on sympathetic nerve-induced relaxations. 5. Removing the oesophagus selectively abolished relaxations elicited by stimulation of both vagal pathways of non-adrenergic relaxant innervation. Non-adrenergic relaxations of the trachealis elicited by electrical field stimulation were unaffected by removing the oesophagus. Oesophagus removal also had no effect on the parasympathetic-cholinergic contractile innervation or the sympathetic relaxant innervation of the trachealis. 6. The results indicate that the guinea-pig trachealis receives non-adrenergic relaxant innervation from both parasympathetic and capsaicin-sensitive vagal pathways. The results also suggest that the neurones mediating non-adrenergic relaxations of the trachea are sensitive to oesophagus removal. The observation that oesophagus removal abolishes parasympathetic relaxations of the trachealis while having no effect on parasympathetic contractions supports the hypothesis that the guinea-pig trachealis receives excitatory and inhibitory innervation from distinct vagal parasympathetic pathways.

Endogenous noradrenaline release from guinea-pig isolated trachea is inhibited by activation of M2 receptors

Overflow of endogenous noradrenaline (NA) from guinea-pig isolated tracheae was evoked by electrical field stimulation (3 Hz, 540 pulses). The muscarinic receptor agonist oxotremorine inhibited the evoked overflow of NA in a concentration-dependent manner (EC50 84 nM). Methoctramine, pirenzepine and p-fluoro-hexahydrosiladiphenidol (each 1 microM) shifted the concentration-response curves of oxotremorine to the right with apparent pA2 values of 7.60, 6.74 and 6.18, respectively. It is concluded that sympathetic nerve terminals in the guinea-pig trachea are endowed with inhibitory muscarinic M2 receptors.