Cough is a complex neurophysiological reflex. Cough receptors, in the form of receptors and ion channels, are located at the sensory nerve endings of the airway. Upon stimulation, they generate signals that are transmitted via the vagus nerve to the cough center in the brainstem, and then activate corresponding muscle groups through efferent nerves to produce cough. Therefore, targeting cough receptors has become a hotspot in the development of new antitussive drugs.

1、Peripheral Neuronal Targets

Exogenous stimuli induce functional or phenotypic changes in peripheral sensory neurons, which upregulate the host's cough response. Research on drugs targeting peripheral nerves can effectively avoid the side effects brought by the central nervous system. Up to now, purinergic P2X3 receptors and the transient receptor potential family are promising peripheral therapeutic targets.

1.1 Transient Receptor Potential Channels (TRP)

The TRP family comprises a wide range of non-selective cation channels that function as cellular sensors, responding to various stimuli in the cellular environment, such as temperature, chemicals, stretch, osmolarity, pH, and oxidation. The TRP channels associated with cough include TRPV1, TRPA1, TRPV4, and TRPM8.

（1）TRPV1/TRPA1. Both TRPV1 and TRPA1 are calcium-permeable channels. TRPV1 is mainly distributed on unmyelinated C fibers and myelinated Aδ fibers and can be directly activated by capsaicin, low pH, resiniferatoxin, and endogenous mediators. TRPA1 is mainly distributed on unmyelinated C fibers and can be directly activated by natural products such as cinnamaldehyde, oxidative stress products, and environmental irritants like ozone. TRPV1 and TRPA1 channels are closely linked in structure and function, with Ca2+ influx through TRPV1 channels activating TRPA1 channels. The TRPA1 antagonist AP-18 can partially inhibit cough induced by inhaled cinnamaldehyde in guinea pigs, but when combined with a TRPV1 antagonist, it can completely eliminate cinnamaldehyde-induced cough. Blocking both TRPV1 and TRPA1 channels or the cyclooxygenase (COX) and 12-lipoxygenase (12-LOX) receptors can significantly inhibit bradykinin-induced cough and airway obstruction, indicating that simultaneous blocking of TRPV1 and TRPA1 channels has a synergistic inhibitory effect on cough and airway obstruction.

（2）TRPV4. TRPV4 is an osmolarity sensor, primarily expressed in trigeminal ganglia and dorsal root ganglia in sensory nerves, with less expression in nociceptors of the vagus nerve. It can be directly activated by endogenous substances or external chemicals, or indirectly activated through intracellular signaling pathways. Research has shown that TRPV4 activation is more closely related to the release of ATP from accessory cells near the vagal nerve terminals than to nerve cells. The selective P2X3 antagonist AF-353 has been shown to inhibit neural depolarization and cough induced by TRPV4 in animals and humans, indicating that TRPV4 can mediate ATP release. Pannexin 1 is a large conductance ion pore that allows ATP efflux from the cell. In vagal nerve tissues of Pannexin 1 knockout mice, the response to TRPV4 is abolished, suggesting that TRPV4-mediated ATP release may require the participation of Pannexin 1. Targeting the Pannexin 1-TRPV4-ATP axis may provide a novel therapeutic approach for chronic cough.

（3）TRPM8. The TRPM8 channel belongs to the family of ligand-gated cold-sensing ion channels, possessing voltage-sensing and ion-selective functions. It is widely distributed in dorsal root ganglia, trigeminal ganglia, and airway vagal ganglia, regulating the cough reflex by controlling intracellular Ca2+ concentration. Under temperatures ranging from 8°C to 28°C, TRPM8 channels can be activated by natural or synthetic cold mimetic agents such as menthol, eucalyptol, and spearmint oil. The TRPM8 channel agonist menthol has been widely used in antitussive treatments, but menthol can also affect other ion channels besides TRPM8. Some research suggests that the antitussive effect of nasal inhalation of menthol is due to the activation of neuronal subpopulations expressing TRPA1-/TRPV1-/TRPM8+ on nasal trigeminal afferents, rather than the activation of vagal bronchopulmonary sensory nerves. However, this study cannot exclude the possibility of other mechanisms contributing to the observed effects.

1.2 P2X3 Receptor

The P2X3 receptor is an ATP-activated cation channel located on vagal sensory fibers, primarily distributed in sensory nerve cells, including the trigeminal nerve, dorsal root ganglia, and nodose ganglia. In inflamed respiratory tracts, due to cell damage, hypoxia, or stress, ATP is released into the extracellular space in large quantities and acts as a signaling molecule to promote disease and inflammation. ATP can also regulate inflammation through alternative purinergic receptors expressed on immune cells. Since ATP binds to the P2X3 receptor, generating action potentials that induce coughing, P2X3 receptor inhibitors such as Gefapixant, Eliapixant, and Sivopixant have become research hotspots. Results from the phase III clinical trial of Gefapixant showed significant improvements in cough frequency, severity, and quality of life for patients, with mild and reversible taste-related adverse events. Importantly, protective cough was not affected by the treatment. Therefore, Gefapixant is an acceptable and safe treatment option for refractory or idiopathic chronic cough. Recently, Gefapixant has been approved for marketing in Japan and is also seeking approval in the European Union and the United States. Eliapixant, as a P2X3 receptor antagonist, is considered an effective treatment for chronic cough, significantly reducing cough frequency and severity, with high selectivity and low incidence of taste-related adverse reactions, demonstrating good tolerability. Researchers speculate that Eliapixant has universal applicability, but further research is needed. Sivopixant is also a highly selective P2X3 receptor antagonist that significantly reduces cough frequency per hour within 24 hours, improves patients' quality of life, rarely causes taste-related adverse reactions, and has no patients stopping treatment due to taste disorders.

2、Central Neuron Targets

Repeated peripheral stimulation can lead to changes in the central nervous system, but regardless of the way in which the vagal afferent nerves are activated, they provide input to the brainstem nuclei, primarily the nucleus tractus solitarii (NTS). The central neural plasticity changes involved in respiratory tract sensory processing are important drivers of coughing. Therefore, the central nervous system has potential advantages compared to peripheral nervous system targets, potentially having the ability to modulate brainstem and higher-level central pathways.

2.1 Voltage-Gated Sodium Channel (NaV)

NaV is essential for the initiation and conduction of action potentials. As a subtype of NaV channels, NaV 1.8 plays a significant role in the excitability and action potential firing capability of vagal sensory neurons. NAV 1.8 can be activated in the presence of inflammation and specific inflammatory mediators. Research has found that using a conscious guinea pig cough model, acute exposure to prostaglandin E2 (PGE2) via intravenous injection led to sensitization of the cough reflex. Concurrent administration of the EP3 antagonist L-798,106 significantly reduced the cough response, and the NaV 1.8 antagonist A-803467 also dose-dependently inhibited the cough response. This study suggests that PGE2 activates the cough reflex through EP3 receptor-dependent NaV 1.8 channels. Therefore, targeting the central EP3 receptor and the NaV 1.8 subtype channel may be a new approach for treating cough hypersensitivity syndrome.

2.2 Neurokinin-1 Receptor (NK-1)

Tachykinins are a class of excitatory neuropeptides, including human substance P (SP), neurokinin A (NKA), neurokinin B (NKB), and others. These tachykinins are mainly produced and released by vagal nerve fibers into the periphery of the airways, with widespread distribution in both the central and peripheral nervous systems. Prolonged exposure of guinea pigs to secondhand smoke enhances synaptic transmission, which can be reversed by NK-1 antagonists. Cough responses induced by citric acid can also be reversed by NK-1 antagonists. Animal studies have shown that SP and NK-1 receptors play an important role in neurotransmission in the central nervous system. Additionally, SP can activate vagal nerve fibers, and this effect can be blocked by NK-1 antagonists, indicating that this mechanism also functions in the peripheral nervous system. Patients with idiopathic pulmonary fibrosis (IPF) and acute cough exhibit increased cough responses after inhalation of SP, and patients with cough have elevated levels of SP in their bodies, suggesting that SP may be an important mediator of the cough response, and NK-1 receptors are crucial therapeutic targets.

2.3 Neuronal Nicotinic Acetylcholine Receptor (nAChR) α7

Among the various subtypes of nAChRs distributed in the central nervous system, α4β2 and α7 are the most prevalent. α7 nAChR is a unique subtype within the nAChRs family. When an agonist binds to the ligand-binding domain of α7 nAChR, it can rapidly induce the opening of a central ion channel. Additionally, α7 nAChR has a very high permeability to Ca2+, which can trigger intracellular signal transduction leading to cough through Ca2+ influx or non-Ca2+-dependent pathways. Research has demonstrated the antitussive effect of nicotine receptor subtypes in guinea pigs. Specifically, the α4β2-selective agonist Tc-6683 had no effect on the induced cough response in guinea pigs, while the α7-selective agonist PHA543613 inhibited induced cough in a dose-dependent manner.

2.4 Gamma-Aminobutyric Acid Receptor (GABA)

GABA is widely distributed and utilized in the central nervous system. Gabapentin is a structural analog of GABA. Studies have reported that gabapentin can significantly improve cough symptoms, reduce cough frequency and severity, and enhance patients' quality of life. However, it has been found that approximately 40% of patients do not respond to gabapentin treatment. Therefore, there is a need to strengthen the selection of patient characteristics that respond well to gabapentin in order to improve the success rate of using this drug.

References

[1] Liu Jia, Luo Yun, Pan Yunfeng, et al. Research Progress on Potential Therapeutic Targets for Chronic Cough [J]. Chinese Pharmacological Bulletin, 2023, 39(08): 1426-1429.

[2] Yao Yue, Wu Hu, Zhang Jiashuo, et al. Research Progress on Therapeutic Targets for Cough and New Drugs [J]. Chinese Journal of New Drugs, 2023, 32(15): 1538-1545.

About the Author

Xiaomichong, a pharmaceutical quality researcher, has been committed to pharmaceutical quality research and drug analysis method validation for a long time. Currently employed by a large domestic pharmaceutical research and development company, she is engaged in drug inspection and analysis as well as method validation.