Neuropeptides in Asthma: How Substance P and CGRP Drive Inflammation
Airway Neuropeptides
2 key peptides
Substance P and CGRP are the primary neuropeptides released from sensory C-fibers that drive neurogenic inflammation in asthmatic airways.
Atanasova & Bhatt, Respiratory Research, 2018
Atanasova & Bhatt, Respiratory Research, 2018
View as imageAsthma is typically discussed in terms of immune cells, cytokines, and allergens. But there is a parallel system driving airway inflammation that most patients never hear about: sensory nerves. The airways are densely innervated by C-fiber sensory neurons that release neuropeptides, primarily substance P (SP) and calcitonin gene-related peptide (CGRP), when activated by irritants, allergens, or inflammatory mediators.[1] This process, called neurogenic inflammation, causes bronchoconstriction, plasma extravasation, mucus secretion, and immune cell recruitment independently of the classical adaptive immune response. The interplay between neural and immune pathways in asthma represents a dimension of the disease that current treatments largely ignore. For the broader context of how neuropeptides create airway inflammation, see the pillar article on neurogenic inflammation in airways.
Key Takeaways
- Substance P released from airway C-fibers causes bronchoconstriction, plasma extravasation, and mucus hypersecretion through NK1 receptor activation on airway smooth muscle, endothelium, and submucosal glands (Atanasova and Bhatt, 2018)
- CGRP upregulates substance P receptor (NK1R) expression in airway tissues, creating a positive feedback loop that amplifies neurogenic inflammation (Wu et al., 2007)
- Capsaicin-sensitive sensory afferents are required for endotoxin-induced airway inflammation and bronchial hyperreactivity in mouse models (Elekes et al., 2007)
- In asthmatic airways, allergen challenge increases CGRP expression with a similar pattern to substance P, suggesting coordinated neuropeptide release from the same sensory neurons (Wu et al., 2007)
- Despite decades of research on airway neuropeptides, no approved asthma drug specifically targets the neurogenic inflammation pathway
- CGRP has paradoxical roles in the lung, acting as both a pro-inflammatory mediator and a bronchodilator depending on the receptor population activated
The Sensory Nerve System in Your Airways
Your airways contain a dense network of sensory nerve fibers, the majority of which are unmyelinated C-fibers that express the capsaicin receptor TRPV1.[3] These fibers serve as the lung's alarm system, detecting irritants, temperature changes, and inflammatory mediators. When activated, they do two things simultaneously: they send signals to the brainstem (triggering cough and bronchoconstriction reflexes) and they release neuropeptides locally from their peripheral terminals through a mechanism called the axon reflex.
The axon reflex works like this: an irritant activates one branch of a sensory nerve ending, and the electrical signal travels antidromically (backward) down other branches of the same neuron, triggering neuropeptide release at sites distant from the original stimulus.[6] This means that a localized trigger can cause neuropeptide release across a wide area of airway tissue. The primary neuropeptides released are substance P, neurokinin A (NKA), and CGRP. Together, these peptides produce the hallmarks of neurogenic inflammation: vasodilation, plasma leakage, immune cell recruitment, and smooth muscle contraction.
Substance P: The Pro-Inflammatory Driver
Substance P is an 11-amino-acid tachykinin peptide that acts primarily through the neurokinin-1 receptor (NK1R). In the airways, NK1R is expressed on smooth muscle cells, endothelial cells, submucosal glands, immune cells (mast cells, macrophages, T cells, and eosinophils), and epithelial cells.[1]
What Substance P Does in Asthmatic Airways
Bronchoconstriction. SP contracts airway smooth muscle directly through NK1R activation and indirectly by stimulating mast cell degranulation and histamine release.[2]
Plasma extravasation. SP causes postcapillary venule endothelial cells to contract, opening gaps between cells and allowing plasma proteins to leak into airway tissue. This produces the edema and swelling characteristic of acute asthma attacks.[6]
Mucus hypersecretion. SP stimulates goblet cells and submucosal glands to increase mucus output, contributing to airway obstruction.[1]
Immune cell recruitment and activation. SP promotes chemokine synthesis in eosinophils, mast cells, and neutrophils. The SP-NK1R axis enhances Th2 immune responses, the same T-helper cell pathway that drives allergic asthma.[2] SP also stimulates macrophage activation and cytokine production, amplifying the inflammatory cascade.
Bronchial hyperreactivity. Beyond acute effects, SP contributes to the exaggerated airway narrowing in response to triggers (cold air, exercise, allergens) that defines asthma as a chronic disease.
For more on substance P's role beyond the airways, see substance P and pain.
CGRP: A More Complex Story
CGRP is a 37-amino-acid peptide co-released with substance P from the same C-fiber terminals. Its role in asthma is more nuanced than substance P's because CGRP has both pro-inflammatory and potentially protective effects depending on the tissue compartment and receptor population involved.
Pro-Inflammatory Effects
CGRP is a potent vasodilator in the bronchial vasculature, increasing blood flow to inflamed airway tissue and enhancing plasma extravasation when combined with SP.[7] CGRP also enhances Th2 and Th9 immune responses in the lungs, amplifying the allergic inflammation that characterizes eosinophilic asthma.[2]
The Feedback Loop with Substance P
A 2007 study by Wu and colleagues demonstrated that CGRP upregulates NK1R (the substance P receptor) expression in airway tissues.[4] CGRP and SP expression exhibited similar temporal and spatial patterns in allergen-challenged airways, with a strong correlation between the two. The upregulation of NK1R by CGRP creates a positive feedback loop: CGRP makes airway cells more responsive to substance P, which in turn amplifies neurogenic inflammation. This cooperative action between two neuropeptides released from the same nerve terminal may explain why neurogenic inflammation can escalate so rapidly during asthma exacerbations.
Paradoxical Bronchodilation
Counterintuitively, CGRP can also relax airway smooth muscle. In isolated airway preparations, CGRP has been shown to inhibit substance P-induced bronchoconstriction in a dose-dependent manner.[9] This creates a paradox: the same peptide that promotes vascular inflammation and immune activation in the airways can also oppose the bronchoconstriction caused by its co-released partner substance P. The net effect likely depends on the relative receptor density, the local concentration of each peptide, and the disease state of the tissue. In severely inflamed asthmatic airways, the pro-inflammatory effects of CGRP appear to dominate.
CGRP's role in migraine (where it is the primary therapeutic target for anti-CGRP antibodies) is covered in CGRP and migraines. The mechanisms overlap with airway neurogenic inflammation, but the clinical outcomes have diverged dramatically: CGRP-targeted drugs are now first-line for migraine prevention, while no equivalent breakthrough has occurred for asthma.
Capsaicin-Sensitive Nerves: The Evidence From Animal Models
The importance of sensory neuropeptides in airway inflammation has been demonstrated through capsaicin desensitization experiments. Capsaicin, the compound that makes chili peppers hot, activates and eventually depletes TRPV1-expressing C-fibers. By removing these nerves, researchers can assess their contribution to airway disease.
Elekes and colleagues (2007) showed that capsaicin-sensitive afferents are essential for endotoxin-induced airway inflammation and bronchial hyperreactivity in mice.[5] Mice with desensitized C-fibers (unable to release neuropeptides) showed significantly reduced airway inflammation, leukocyte infiltration, and bronchial hyperreactivity after endotoxin challenge. Mice lacking the TRPV1 receptor genetically showed intermediate protection, suggesting that both TRPV1-dependent and TRPV1-independent sensory pathways contribute.
Specifically, desensitized mice showed reduced peribronchial inflammatory cell infiltration, lower myeloperoxidase activity (a marker of neutrophil presence), and attenuated airway resistance changes in response to methacholine challenge.[5]
This finding is particularly relevant because endotoxin (bacterial lipopolysaccharide) is a common environmental trigger for asthma exacerbations. The fact that neurogenic mechanisms are required for the full inflammatory response to endotoxin suggests that neuropeptides are not just secondary amplifiers of immune-driven inflammation; they are necessary components of the inflammatory cascade itself.
The Neuroimmune Interface in Asthma
The distinction between "neural" and "immune" inflammation in asthma is increasingly artificial. Pavon-Romero and colleagues (2021) mapped the extensive crosstalk between sensory neurons and immune cells in asthmatic airways.[2] Sensory neuropeptides activate immune cells, but immune mediators also sensitize sensory nerves, creating reciprocal amplification loops:
- Mast cell-derived histamine and prostaglandins lower the activation threshold of C-fibers, causing neuropeptide release at lower stimulus intensities
- Eosinophil-derived major basic protein damages nerve sheaths, exposing sensory terminals to direct allergen contact
- SP and CGRP released from nerves promote eosinophil chemotaxis and survival, maintaining the eosinophilic infiltrate characteristic of allergic asthma
- Nerve growth factor (NGF), produced by inflamed epithelial cells, increases neuropeptide content in sensory neurons, priming them for greater release during the next trigger
Atanasova and Bhatt (2018) described how this neuroimmune crosstalk extends to adaptive immunity: substance P enhances antigen presentation by dendritic cells, promotes B-cell immunoglobulin class switching toward IgE (the antibody class responsible for allergic reactions), and stimulates T-cell proliferation.[1] CGRP, meanwhile, modulates macrophage polarization and may contribute to the airway remodeling (subepithelial fibrosis, smooth muscle thickening) that occurs in chronic severe asthma.[7]
This bidirectional relationship means that chronic asthma is partly a disease of sensory nerve plasticity. Inflamed airways contain remodeled nerves with increased neuropeptide stores, lower activation thresholds, and greater branching density.[8] The nervous system "learns" to overreact, and each episode of neurogenic inflammation makes the next one more likely.
Why There Are No Approved Neuropeptide Drugs for Asthma
Barnes identified neurogenic inflammation as a therapeutic target in asthma in 1991.[6] More than three decades later, no approved asthma medication specifically targets the neuropeptide pathway. Several factors explain this gap:
NK1R antagonists failed in asthma trials. Despite clear preclinical evidence that substance P drives airway inflammation, NK1R antagonists showed minimal clinical benefit in human asthma studies. The redundancy of tachykinin signaling (substance P also activates NK2R and NK3R, and neurokinin A provides parallel signaling) may explain why blocking a single receptor is insufficient.[1]
Species differences complicate translation. Guinea pigs have prominent airway neurogenic inflammation resembling the human pattern, but rats and mice have less robust neuropeptide expression in airway nerves. The relative contribution of neurogenic versus immune-mediated inflammation differs across species, making it difficult to predict human outcomes from standard rodent asthma models.[7]
CGRP's dual nature is a problem. Anti-CGRP drugs work brilliantly for migraine, but in the lungs, blocking CGRP could remove its bronchodilatory effects while also removing its inflammatory effects. The net clinical impact is unpredictable. Some researchers have raised concerns that anti-CGRP migraine drugs (erenumab, fremanezumab, galcanezumab) could worsen asthma in susceptible patients, though this has not been confirmed in clinical data.[10]
Current asthma therapies work well enough. Inhaled corticosteroids and long-acting beta-agonists effectively control most asthma, reducing the commercial incentive to develop novel neuropeptide-targeted approaches. The patients who might benefit most from anti-neurogenic therapy are those with severe, corticosteroid-resistant asthma driven by neural rather than immune mechanisms, but this subpopulation has not been well-characterized.
For related peptide approaches to airway disease, see VIP and bronchodilation and peptide approaches to COPD, which explore protective rather than pro-inflammatory neuropeptide pathways.
The Bottom Line
Substance P and CGRP released from airway sensory C-fibers drive neurogenic inflammation in asthma through bronchoconstriction, plasma extravasation, mucus secretion, and immune cell activation. CGRP amplifies substance P's effects by upregulating NK1R expression while paradoxically also possessing bronchodilatory properties. Animal studies confirm that capsaicin-sensitive sensory nerves are required for full airway inflammatory responses. Despite over three decades of research, no approved asthma drug targets the neuropeptide pathway, largely due to receptor redundancy, species translation challenges, and the dual nature of CGRP in the lungs.