Neurogenic Inflammation in Airways: The Peptide Cascade
Airway Neuropeptide Biology
3 key neuropeptides
Substance P, neurokinin A, and CGRP released from airway sensory C-fibers drive bronchoconstriction, plasma exudation, and mucus hypersecretion in neurogenic airway inflammation.
Barnes, Respiration Physiology, 2001
Barnes, Respiration Physiology, 2001
View as imageNeurogenic inflammation in airways occurs when sensory nerve fibers release pro-inflammatory neuropeptides directly into airway tissue. Substance P, neurokinin A, and calcitonin gene-related peptide (CGRP) are the primary mediators, released from unmyelinated C-fiber endings in the bronchial mucosa when these nerves are activated by irritants, allergens, or inflammatory mediators. Barnes described this peptide cascade in a series of publications spanning 1991 to 2001, establishing that neuropeptide release from sensory nerves produces bronchoconstriction, vasodilation, plasma exudation, and mucus secretion through activation of specific tachykinin and CGRP receptors on airway smooth muscle, blood vessels, and glands (Barnes, Respiration Physiology, 2001; Barnes, International Archives of Allergy, 1991). This mechanism adds a neural dimension to airway inflammation that operates alongside and amplifies conventional immune responses.
Key Takeaways
- Substance P activates NK1 receptors on airway cells and promotes bronchial asthma progression through the PI3K/AKT/NF-kB signaling pathway (Li et al., 2022)
- CGRP released from sensory C-fibers causes potent arteriolar vasodilation in airway mucosa, contributing to the edema and hyperemia characteristic of asthma attacks
- VIP functions as a counter-regulatory bronchodilator and anti-inflammatory peptide, with genetic deletion causing spontaneous pulmonary arterial hypertension in mice (Said, 2008)
- Mast cells express a specific receptor (MRGPRB2/MRGPRX2) that directly senses neuropeptides, creating a neuroimmune bridge that amplifies allergic inflammation (Green et al., 2019)
- Neuropeptide Y modulates airway immune responses by regulating dendritic cell, macrophage, and T cell function in respiratory disease (Itano et al., 2024)
- The gut-lung axis involves microbiota-coordinated TRPV1 neuroimmune crosstalk that modulates neuropeptide release in asthma pathogenesis (Zeng et al., 2026)
The Sensory Nerve Network in Airways
Human airways contain a dense network of sensory nerve fibers that extend from the trachea to the terminal bronchioles. The majority of airway sensory neurons are unmyelinated C-fibers with cell bodies in the vagal nodose and jugular ganglia. These neurons synthesize and store neuropeptides including substance P, neurokinin A (both tachykinins), and CGRP within synaptic vesicles at their peripheral terminals.
When activated, C-fibers release neuropeptides through a mechanism called the axon reflex. Rather than transmitting a signal centrally and receiving an efferent command, the activated nerve terminal releases peptides locally from collateral branches, creating inflammation at the site of stimulation and in adjacent tissue. Sousa-Valente and Brain provided a comprehensive historical account of this process in 2018, tracing the concept from Bayliss's 1901 observation of antidromic vasodilation through to modern understanding of TRPV1-mediated neuropeptide release.[1]
TRPV1 (transient receptor potential vanilloid 1) is the primary ion channel that gates neuropeptide release from airway C-fibers. When TRPV1 opens, calcium flows into the nerve terminal, triggering vesicle fusion and peptide release. TRPV1 responds to multiple stimuli: capsaicin (the active component of chili peppers), protons (low pH, as occurs during tissue inflammation), heat above 43 degrees Celsius, and endogenous lipid mediators including anandamide and certain prostaglandins. TRPA1, a related channel often co-expressed with TRPV1, responds to environmental irritants including acrolein, chlorine, and oxidative stress products. Together, these channels act as molecular sensors that convert chemical and physical threats into neuropeptide release.
The triggers for neuropeptide release in asthmatic airways include capsaicin, bradykinin, prostaglandins, histamine, and environmental irritants including cigarette smoke and air pollutants. Histamine plays a particularly important role in amplifying neurogenic inflammation. Rosa and Fantozzi documented in 2013 how histamine released from mast cells activates H1 receptors on sensory nerve terminals, triggering further neuropeptide release and creating a positive feedback loop between mast cell degranulation and neurogenic inflammation.[2]
Substance P and Neurokinin A: The Pro-Inflammatory Tachykinins
Substance P and neurokinin A are the principal pro-inflammatory neuropeptides in airway neurogenic inflammation. They act through two G-protein-coupled receptors: neurokinin 1 (NK1), which preferentially binds substance P, and neurokinin 2 (NK2), which preferentially binds neurokinin A. Both receptors are expressed on airway smooth muscle, submucosal glands, blood vessels, and immune cells.
Li et al. demonstrated in 2022 that substance P promotes bronchial asthma progression through a specific intracellular signaling cascade. Binding of substance P to NK1 receptors activates the PI3K/AKT/NF-kB pathway, which upregulates inflammatory gene expression and promotes immune cell recruitment to airway tissue.[3] This pathway represents a direct molecular link between neuropeptide signaling and the transcriptional programs that sustain chronic airway inflammation.
The effects of tachykinins on airway function are dose-dependent and receptor-specific. Substance P acting through NK1 receptors primarily causes vasodilation and plasma protein extravasation in the bronchial microvasculature, leading to mucosal edema. Neurokinin A acting through NK2 receptors is the more potent bronchoconstrictor, directly contracting airway smooth muscle. Lundberg and colleagues established this receptor-specific pharmacology in the 1990s, showing that tachykinins from sensory nerves coordinate multiple inflammatory responses simultaneously (Lundberg, Canadian Journal of Physiology and Pharmacology, 1995). For a detailed examination of how substance P and CGRP work together in asthma, see our article on neuropeptides in asthma.
Petersen et al. further characterized tachykinin receptor pharmacology in 2025, demonstrating how truncated neuropeptide variants exhibit different activity and signaling bias at NK1 and NK2 receptors.[4] These findings have implications for drug development: selective NK1 or NK2 antagonists could block specific components of the neurogenic inflammatory cascade without suppressing the entire tachykinin system.
Chen et al. examined the substance P/NK1 receptor system in the context of central sensitization, documenting how sustained substance P signaling can establish persistent pain and inflammatory states that outlast the original stimulus.[5] In airways, an analogous sensitization process may underlie the chronic hyperresponsiveness that characterizes persistent asthma, where neurogenic inflammation becomes self-sustaining even after allergen exposure ends.
Substance P also plays a role in host defense. Hsieh et al. demonstrated that substance P contributes to pulmonary clearance of bacteria following lung injury, showing that the same peptide driving pathological inflammation also participates in antimicrobial defense.[6] This dual role complicates therapeutic targeting: complete suppression of substance P signaling could compromise host defense while reducing inflammation. This tension between host defense and pathological inflammation is also relevant to how defensins protect the lungs.
CGRP: Vasodilation and Immune Modulation
CGRP is co-released with substance P from sensory C-fiber terminals but acts through a distinct receptor complex (the CLR/RAMP1 heterodimer). In airways, CGRP is one of the most potent vasodilators known, causing sustained arteriolar dilation that contributes to the mucosal hyperemia and edema observed during asthma exacerbations. While substance P increases vascular permeability (allowing plasma proteins to leak into tissue), CGRP increases blood flow to the affected area, together producing the vascular changes characteristic of neurogenic inflammation.
Ge et al. demonstrated that CGRP/TSP1 signaling can also dampen inflammation and fibrosis in certain tissue contexts, showing that CGRP's role is not uniformly pro-inflammatory.[7] In corneal wound healing, CGRP signaling reduced inflammatory markers and fibrotic remodeling. Whether analogous anti-inflammatory effects occur in airways during the resolution phase of asthma attacks remains an open question, but it suggests CGRP plays a more nuanced role than simple inflammation amplification.
Ramachandran et al. reviewed the broader biology of neurogenic inflammation and CGRP in 2018, documenting the signaling pathways through which CGRP modulates both vascular tone and immune cell function.[8] CGRP can suppress macrophage inflammatory responses while simultaneously promoting eosinophil recruitment and T cell adhesion, creating a complex immunomodulatory profile that defies simple pro- or anti-inflammatory classification.
The success of anti-CGRP monoclonal antibodies (erenumab, fremanezumab, galcanezumab) in treating migraine has generated interest in whether similar approaches could address CGRP-driven airway inflammation. However, the anti-inflammatory properties of CGRP in certain contexts raise concerns about systemic CGRP blockade. Complete suppression of CGRP signaling could theoretically impair the resolution of airway inflammation or compromise wound healing in the bronchial mucosa. Any therapeutic strategy targeting CGRP in airways would need to account for its dual pro- and anti-inflammatory roles, potentially requiring tissue-targeted delivery rather than systemic antibody administration.
The Mast Cell-Nerve Axis
A critical amplification mechanism in airway neurogenic inflammation is the bidirectional communication between sensory nerves and mast cells. Mast cells reside in close anatomical proximity to sensory nerve terminals throughout the airway mucosa. Green et al. identified a specific mast cell receptor, MRGPRX2 (MRGPRB2 in mice), that directly senses neuropeptides including substance P.[9] Activation of this receptor triggers mast cell degranulation, releasing histamine, tryptase, and prostaglandins that further stimulate sensory nerves to release more neuropeptides.
This creates a self-amplifying cycle: neuropeptide release activates mast cells, mast cell mediators activate sensory nerves, and the resulting neuropeptide release activates more mast cells. The cycle can escalate rapidly, converting a localized irritant exposure into widespread airway inflammation within minutes. Coll et al. investigated pharmacological approaches to interrupt this cycle, demonstrating that natural alpha,beta-unsaturated lactones could inhibit neuropeptide-induced mast cell activation in an in vitro model of neurogenic inflammation.[10]
The mast cell-nerve axis is particularly relevant to allergic asthma, where IgE-mediated mast cell activation is a primary trigger. Once mast cells degranulate in response to allergen cross-linking of IgE, the released mediators activate nearby sensory nerves, adding a neurogenic inflammatory component on top of the allergic immune response. This explains why asthma attacks often feature both immune-mediated and neurogenic inflammatory characteristics simultaneously.
The MRGPRX2 receptor identified by Green et al. is distinct from the classical NK1 receptor through which substance P was traditionally thought to activate mast cells. MRGPRX2 has a broader ligand profile, responding to multiple cationic peptides and even some small-molecule drugs (including fluoroquinolone antibiotics and neuromuscular blocking agents). This broader sensitivity means that the mast cell-nerve communication channel is not limited to classical neuropeptides but can be activated by a range of endogenous and exogenous cationic molecules, potentially explaining why some drugs trigger pseudo-allergic reactions that mimic neurogenic airway inflammation.
VIP and Counter-Regulatory Neuropeptides
Not all airway neuropeptides promote inflammation. Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are released from parasympathetic and sensory nerve fibers and exert predominantly anti-inflammatory and bronchodilatory effects. VIP relaxes airway smooth muscle, inhibits mucus secretion from submucosal glands, and suppresses T cell proliferation and cytokine production.
Said demonstrated in 2008 that the VIP gene is a key modulator of pulmonary vascular remodeling and inflammation.[11] Mice lacking the VIP gene developed spontaneous pulmonary arterial hypertension with features of airway inflammation, demonstrating that VIP is essential for maintaining normal pulmonary homeostasis. The finding established VIP not merely as a therapeutic candidate but as an endogenous protective factor whose deficiency creates pathology.
Leceta et al. investigated VIP's immunoregulatory mechanisms in 2021, showing that VIP modulates autoimmune responses through effects on dendritic cells, macrophages, and regulatory T cells.[12] In the airway context, VIP shifts the immune balance away from the Th2-dominant responses that drive allergic asthma. For a comprehensive look at VIP's bronchodilatory properties, see our dedicated article on VIP and bronchodilation.
VIP's therapeutic potential for asthma has been explored through inhaled delivery. Early clinical observations noted that inhaled VIP produced bronchodilation in asthmatic patients, though the effect was shorter-lived than conventional beta-agonists due to rapid peptide degradation. Modified VIP analogs with enhanced protease resistance and longer half-lives have been developed in preclinical programs, and PACAP-based analogs are also under investigation for their combined bronchodilatory and anti-inflammatory properties.
The balance between pro-inflammatory neuropeptides (substance P, neurokinin A, CGRP) and anti-inflammatory neuropeptides (VIP, PACAP) determines the net effect of neurogenic activity on airway function. In healthy airways, this balance favors homeostasis. In asthma, several factors shift the balance toward inflammation: increased sensory nerve density and peptide content, reduced expression of peptide-degrading enzymes like neutral endopeptidase (NEP), epithelial damage that exposes nerve terminals to luminal irritants, and reduced VIP expression in inflammatory conditions. The result is a neuropeptide environment skewed toward pro-inflammatory signaling, which compounds the effects of conventional immune-mediated inflammation.
Neuropeptide Y and the Broader Peptide Network
The airway neuropeptide network extends beyond the classical tachykinin-CGRP-VIP triad. Neuropeptide Y (NPY), released from sympathetic nerve fibers, modulates airway immune responses through effects on multiple cell types. Itano et al. reviewed NPY's role in respiratory disease in 2024, documenting how NPY regulates dendritic cell maturation, macrophage polarization, and T cell function in the airways.[13] NPY receptor subtypes Y1, Y2, and Y5 are expressed on different immune cell populations, creating opportunities for selective immunomodulation through receptor-specific targeting.
Chen et al. measured expression levels of VIP, substance P, neurokinin A, and neurokinin B simultaneously in airway tissue, demonstrating that these peptides change in coordinated patterns during disease states rather than independently.[14] This coordinated regulation suggests that the airway peptide milieu functions as an integrated signaling network rather than a collection of independent mediators. Measuring a single neuropeptide in isolation provides limited diagnostic information; profiling the entire peptide network captures the balance between pro-inflammatory and protective signaling that determines disease trajectory.
The neuropeptide network also includes endogenous opioid peptides (endorphins and enkephalins), which modulate sensory nerve excitability and can suppress neuropeptide release from C-fibers. Opioid receptors on airway sensory neurons provide a natural braking mechanism that limits neurogenic inflammation. This endogenous suppression system may explain why some individuals tolerate airway irritants without developing symptoms: their opioid tone is sufficient to prevent runaway neuropeptide release. The research also has implications for conditions beyond asthma, including COPD, where different neuropeptide imbalances contribute to distinct pathological features including mucus hypersecretion, small airway remodeling, and impaired mucociliary clearance.
The Gut-Lung Axis: Microbiome Connections
Recent research has revealed unexpected connections between gut microbiome composition and airway neurogenic inflammation. Zeng et al. described a gut-lung axis mechanism in 2026 involving microbiota dysbiosis that coordinates PLA2-TRPV1 neuroimmune crosstalk in asthma pathogenesis.[15] In this model, disrupted gut microbial communities alter lipid mediator production, which sensitizes TRPV1-expressing sensory neurons in both the gut and the airways. The sensitized neurons have a lower threshold for neuropeptide release, predisposing to exaggerated neurogenic inflammation in response to airway irritants.
Al-Keilani et al. investigated the relationship between pulmonary exacerbations in cystic fibrosis patients and circulating neuropeptide levels, finding that serum neuropeptide concentrations changed during acute respiratory events.[16] This observation supports the concept that airway neuropeptide signaling has systemic readouts that could serve as biomarkers for respiratory disease activity.
The gut-lung axis concept has particular relevance for pediatric asthma. Epidemiological studies have consistently shown that early-life antibiotic exposure and disrupted gut microbiome development increase asthma risk in childhood. If microbial metabolites modulate TRPV1 sensitivity in airway sensory neurons, then microbiome disruption could lower the threshold for neurogenic inflammation in developing airways. This framework connects seemingly disparate observations about antibiotic exposure, microbiome composition, and asthma risk through a neuropeptide-mediated mechanism. It also suggests that microbiome-targeted interventions (prebiotics, probiotics, or postbiotics) could potentially modulate neurogenic airway inflammation by altering the metabolite environment that regulates sensory nerve excitability.
Enzyme Degradation and the NEP Hypothesis
A critical regulatory mechanism in airway neurogenic inflammation is the enzymatic degradation of released neuropeptides. Neutral endopeptidase (NEP, also called neprilysin or CD10) is expressed on airway epithelial cells and rapidly cleaves substance P, neurokinin A, and other peptide mediators into inactive fragments. When epithelial damage strips away NEP-expressing cells, neuropeptides persist longer in the tissue and reach higher concentrations, amplifying the inflammatory response.
This "NEP hypothesis" explains why asthma attacks worsen with epithelial shedding: the same damage that exposes sensory nerve terminals also removes the enzyme that limits neuropeptide activity. Viral respiratory infections, which damage airway epithelium, may trigger asthma exacerbations partly through this mechanism. The loss of NEP creates a permissive environment for neurogenic inflammation that compounds the direct effects of viral immune activation. For context on how airway peptide defense works against respiratory viruses, see our article on defensins against influenza and COVID.
The concept also has implications for inhaled peptide drug delivery. Any peptide-based therapeutic delivered to the airways will encounter NEP and other peptidases that degrade it. Understanding the airway peptidase landscape is essential for designing stable inhaled peptide drugs that can resist degradation and reach their intended targets. Strategies including D-amino acid substitution, peptide cyclization, and PEGylation have been applied to extend the half-life of therapeutic peptides in the airway environment, though each modification must be balanced against potential changes in receptor binding affinity and selectivity.
Unresolved Questions
Despite decades of research in animal models, the contribution of neurogenic inflammation to human asthma remains incompletely defined. Most evidence comes from guinea pig models, where the airway sensory innervation pattern differs from humans. Human studies have been limited by the difficulty of measuring neuropeptide levels in airway tissue during acute asthma attacks. Bronchoalveolar lavage can capture peptides in airway fluid, but the procedure itself may trigger neuropeptide release, confounding results.
The failure of NK1 receptor antagonists in clinical asthma trials in the 1990s and 2000s raised questions about whether tachykinin signaling is a viable therapeutic target in humans, or whether redundant inflammatory pathways compensate when one is blocked. These trials used systemically administered antagonists that may not have achieved sufficient concentrations in airway tissue, and they did not select patients based on neurogenic inflammation biomarkers. More selective approaches targeting specific components of the cascade, or combination strategies addressing multiple neuropeptide receptors simultaneously, have not been adequately tested. Inhaled NK1 antagonists that achieve high local concentrations in airway tissue without systemic exposure represent one untested approach. Another is targeting the TRPV1 channel directly with airway-restricted antagonists, which would prevent neuropeptide release rather than blocking individual receptors downstream. The biologic revolution in asthma treatment has demonstrated that pathway-specific therapies can succeed where broad anti-inflammatory approaches fall short, but applying this principle to neurogenic inflammation requires identifying the right patients, the right target, and the right delivery route.
The role of TRPV1 channel polymorphisms in determining susceptibility to neurogenic airway inflammation is another open area. Genetic variation in TRPV1 and related channels could explain why some individuals develop asthma in response to irritant exposures while others do not, but population-level genetic studies have not confirmed this hypothesis.
Biomarker development for neurogenic airway inflammation is also in its early stages. Measuring substance P, CGRP, or VIP levels in sputum, exhaled breath condensate, or blood could theoretically identify patients with neurogenic-predominant asthma phenotypes who might respond to neuropeptide-targeted therapies. However, these peptides are rapidly degraded by tissue peptidases, have short half-lives in biological fluids, and are present at low concentrations that challenge current clinical assay sensitivity. Developing stable, reliable biomarkers for neurogenic airway inflammation remains a prerequisite for precision medicine approaches that match patients to neuropeptide-targeted treatments.
The emergence of biologics targeting specific inflammatory pathways in severe asthma (anti-IL-5, anti-IL-4/13, anti-TSLP) has shifted the treatment landscape toward phenotype-specific therapy. Neurogenic inflammation may define a distinct asthma endotype that responds poorly to current biologics but could benefit from neuropeptide-targeted approaches. Identifying this endotype clinically would require the biomarker tools that are still under development.
The Bottom Line
Neurogenic inflammation in airways involves a peptide cascade triggered by sensory C-fiber activation. Substance P, neurokinin A, and CGRP drive bronchoconstriction, vasodilation, plasma exudation, and mucus secretion through NK1, NK2, and CGRP receptors. Counter-regulatory peptides including VIP provide bronchodilatory and anti-inflammatory balance. The mast cell-nerve axis amplifies these responses through bidirectional signaling. While animal data strongly supports neurogenic inflammation as a contributor to asthma pathophysiology, translation to human disease remains an active area of investigation.