Neuropeptides and airway smooth muscle function

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Calcitonin gene-related peptide as inflammatory mediator

Sensory neuropeptides have been proposed to play a key role in the pathogenesis of a number of respiratory diseases such as asthma, chronic obstructive pulmonary disease or chronic cough. Next to prominent neuropeptides such as tachykinins or vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP) has long been suggested to participate in airway physiology and pathophysiology. CGRP is a 37 amino-acid peptide which is expressed by nerve fibers projecting to the airways and by pulmonary neuroendocrine cells. The most prominent effects of CGRP in the airways are vasodilatation and in a few instances bronchoconstriction. A further pulmonary effect of CGRP is the induction of eosinophil migration and the stimulation of beta-integrin-mediated T cell adhesion to fibronectin at the site of inflammation. By contrast, CGRP inhibits macrophage secretion and the capacity of macrophages to activate T-cells, indicating a potential anti-inflammatory effect. Due to the complex pulmonary effects of CGRP with bronchoconstriction and vasodilatation and diverse immunomodulatory actions, potential anti-asthma drugs based on this peptide have not been established so far. However, targeting the effects of CGRP may be of value for future strategies in nerve modulation.

The G glycoprotein of respiratory syncytial virus depresses respiratory rates through the CX3C motif and substance P

Respiratory syncytial virus (RSV) infection in the neonate can alter respiratory rates, i.e., lead to episodes of apnea. We show that RSV G glycoprotein reduces respiratory rates associated with the induction of substance P (SP) and G glycoprotein-CX3CR1 interaction, an effect that is inhibited by treatment with anti-G glycoprotein, anti-SP, or anti-CX3CR1 monoclonal antibodies. These data suggest new approaches for treating some aspects of RSV disease.

The link between the nose and lung, perennial rhinitis and asthma--is it the same disease?

Perennial rhinitis and asthma are clinical syndromes representing a range of overlapping pathologies; accurate classification should therefore precede any comparison. Although the sinonasal cavities, trachea and bronchi have a common respiratory mucosa, there are also anatomical differences. For example, the nose has a capacitance vessel network and the lower airways possess smooth muscle, both of which are responsive to neurohumoral influences. The prevalence of rhinitis and asthma has increased over the last three decades. Rhinitis occurs in around 75% of allergic asthmatics while 20% of perennial allergic rhinitics develop asthma. Eosinophils, and their associated proteins and cytokines, may play a central role in both perennial rhinitis and asthma with and without atopy. The characteristic pathology of asthma can be summarized as a chronic, desquamating, eosinophilic bronchitis. Non-allergic rhinitis with eosinophilia is recognized, but without consistent evidence of epithelial damage. Eosinophils are also present in rhinosinusitis with polyposis, particularly in patients with aspirin sensitivity, in whom asthma also often occurs. Increased mast cell activation and mediator release is evident in both perennial rhinitis and asthma following allergen challenge. The importance of mast cells in non-atopic asthma and polyposis is also recognized. Adhesion molecules may also be upregulated, with an increased number and activation of TH2 lymphocytes. However, allergen-resultant T-cell activation may be less marked in the nose than in the lung. Autonomic imbalance also plays a role in both conditions via changes in neural tone to effector tissues, release of neuropeptides, and interplay with cellular recruitment. Pharmacological manipulation of rhinitis and asthma also illustrates the pathological similarities and differences.

Coexpression of vasoactive intestinal peptide, calcitonin gene-related peptide and substance P immunoreactivity in parasympathetic neurons of the rhesus monkey lung

By the use of light microscopic immunohistochemistry, the present study investigates whether substance P (SP) and calcitonin gene-related peptide (CGRP), which are well documented neurotransmitter candidates in primary sensory fibers, are also expressed in parasympathetic neurons of the rhesus monkey lung. A combination of double fluorescence immunohistochemistry and staining of adjacent sections revealed triple coexistence of SP, CGRP and the cholinergic co-transmitter vasoactive intestinal peptide (VIP) in a large number of neuronal cell bodies in intrinsic peribronchial ganglia. In addition, there was co-localization of SP and CGRP in choline acetyl-transferase (ChAT)-positive neurons. These data suggest that SP and CGRP, in addition to their sensory role, are cholinergic cotransmitter candidates in the postganglionic parasympathetic innervation of primate lung. Co-release and co-function of SP and CGRP with VIP and acetylcholine may be important in the regulation of pulmonary physiology and in pulmonary pathophysiology.

Autonomic innervation of the airways

A review is given of the literature concerning the autonomic innervation of airway smooth muscle. The cholinergic, adrenergic and non-cholinergic non-adrenergic (NANC) systems in humans and several animal species are discussed. The diagnostic and therapeutic possibilities and limitations of new receptor specific agonists and antagonists are also discussed.

Effect of passive sensitization on the mechanical activity of human isolated bronchial smooth muscle induced by substance P, neurokinin A and VIP

1. The effect of passive sensitization on the mechanical activity of human isolated bronchial smooth muscle induced by the following neuropeptides substance P (SP), neurokinin A (NKA) and vasoactive intestinal peptide (VIP) was studied both in the absence and in the presence of the neutral endopeptidase (NEP) inhibitor, phosphoramidon. 2. Cumulative concentration-response curves (CCRC) to these neuropeptides were constructed in human passively sensitized isolated bronchial rings and compared to those in paired controls. Passively sensitized human isolated bronchial rings were tissues incubated overnight in serum from asthmatic patients atopic to Dermatophagoides pteronyssinus and paired controls were tissues originating from the same lung specimens but incubated overnight in serum from healthy donors. 3. In the absence of phosphoramidon, passive sensitization significantly increased the amplitude of the contractile responses to SP and NKA including that to the maximal concentration given from 50 +/- 5% to 76 +/- 6% (n = 5, P < 0.05) and from 70 +/- 7% to 101 +/- 6% (n = 5, P < 0.05) of the maximal response to acetylcholine, respectively. Passive sensitization significantly shifted to the left the CCRC for both tachykinins as measured by the geometric means dose-ratios which were 8.5 (95% confidence limits (CL): 3.1-13.9) and 7.3 (95% CL: 4.2-10.3) for SP and NKA, respectively. 4. In the presence of phosphoramidon (10 microM), passive sensitization still increased significantly the amplitude of the contractile responses to SP and NKA including that to the maximal concentration given from 74 +/- 4% to 115 +/- 7% (n = 5, P<0.05) and from 104 +/- 9% to 146 +/- 16% (n = 5, P<0.05)of the maximal response to acetylcholine, respectively. Passive sensitization still significantly shifted to the left the CCRC for both tachykinins as measured by the dose-ratios which were 9.0 (95% CL:4.3-13.6) and 5.4 (95% CL: 2.9-7.9) for SP and NKA, respectively.5. The relaxant response to the maximal concentration of VIP given in tissues precontracted with histamine (0.5 mM) was significantly reduced by passive sensitization from 41 +/- 4% to 25 +/- 3% (n = 5,P <0.05) of the amplitude of the precontraction in the absence of phosphoramidon and from 72 +/- 1%to 49 +/- 4% (n = 5, P<0.05) in the presence of phosphoramidon (10 microM). Passive sensitization significantly shifted to the right the CCRC for VIP as measured by the dose-ratios which were 10.4(95% CL: 6.6-14.1) and 6.4 (95% CL: 3.0-9.8) in the absence and in the presence of phosphoramidon,respectively.6. We conclude that passive sensitization enhances the mechanical response to neuropeptides which contract human isolated bronchial smooth muscle and reduces that to a neuropeptide which relaxes it.The mechanism of passive sensitization-induced changes in the mechanical activity appears to be independent of a decrease in NEP activity since these changes persist in the presence of the NEP inhibitor, phosphoramidon.

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.

Bronchoconstrictor action of neuropeptide Y (NPY) in isolated guinea pig airways

Responses of various preparations of guinea pig and rat airways to synthetic porcine neuropeptide Y were analysed in vitro. NPY in doses up to 10(-6) M, induced a dose dependent contraction in trachea, bronchi and lung parenchymal strips of the guinea pig but did not have any effect in similar preparations obtained from the rat. In guinea pig airways the contractile responses to NPY were small in size and characterized by a slow onset and a long duration. This effect of the peptide was not dependent on pre-junctional nerve stimulation but rather mediated through the secondary generation of cyclooxygenase products. It is concluded that NPY may contribute per se to regulating the resting tone of guinea pig airways.