PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Lung. Author manuscript; available in PMC 2010 June 9.
Published in final edited form as:
Published online 2007 December 15. doi:  10.1007/s00408-007-9054-6
PMCID: PMC2882536
NIHMSID: NIHMS124308

The cough reflex in animals: Relevance to human cough research

Abstract

All mammalian species studied cough or display some similar respiratory reflex upon aerosol challenge with tussigenic stimuli such as citric acid or capsaicin. Animals cough to the same stimuli that evoke coughing in humans, and therapeutic agents that display antitussive effects in human studies also prevent coughing in animals. The many invasive procedures and complementary in vitro studies possible in animals but readily reproduced in human subjects, along with the proven predictive value of cough studies in animals provides the rationale for animal modeling of human cough. The advantages and disadvantages of studying cough in animals are discussed.

Keywords: capsaicin, bradykinin, nucleus of the solitary tract, vagus

The cough reflex protects the airways and lungs from airborne and inhaled pathogens, allergens, aspirate and other irritants (figure 1). Humans that have a compromised cough reflex due to neuromuscular disorders or stroke are highly susceptible to pulmonary infections and aspiration (1-6). It would seem reasonable to expect that most or perhaps all mammalian species would have a similar respiratory reflex subserving the same role in lung defense. Indeed, although direct evidence for their protective role in animals has not been published, every mammalian species studied to date displays a cough reflex or some similar forceful expiratory reflex evoked by airway irritation (7-13) (figure 2). Given the similar physiologic patterning of these respiratory efforts and that the same stimuli that evoke coughing in humans also evoke coughing in animals, studying cough in animals is likely to provide insight into the physiology and pathophysiology of cough in humans. Rather than compare, contrast and critique the various animal models used to study cough, this review will discuss the rationale behind animal modeling of human cough, the advantages of studying cough in animals and the several disadvantages of studying this and other respiratory reflexes in animals.

Figure 1
The neural pathways that regulate the cough reflex are depicted. Each component of this reflex arc functions similarly in all species including humans. Studies carried out in animals allow for more mechanistic experimentation at each site of regulation, ...
Figure 2
Coughing and expiration reflexes (labeled with asterisks) in awake guinea pigs evoked by aerosol challenges with 10 mg/ mL bradykinin. Tracing depicts changes in pressure within a chamber containing the guinea pig and filled with a bradykinin aerosol. ...

Why study cough in animals?

Human physiology and consciousness is probably sufficiently unique amongst vertebrates that human pathophysiology is also likely to be distinct from that in other species. It would follow logically from the above assertion that animal models of human disease and/ or pathophysiology are imperfect, and thus, whenever possible, research related to these diseases should be carried out using humans. But the symptoms and causes of human diseases including those diseases associated with cough are regulated by cells, organ systems and reflex pathways that have remained remarkably unchanged amongst animal species. Coughing, for example can be evoked in all species studied by mechanically stimulating the airways mucosa or by inhalation of acidic saline or capsaicin (7-9, 14-16). The latter 2 stimuli acts on the ion channel and receptor TRPV1, which is preferentially localized to distinct subsets of nociceptive sensory nerves innervating somatic and visceral tissues and encoded by a gene that shows upwards of 80% homology across species (17-21). The biophysical and pharmacological properties of TRPV1 are similar if not identical in different mammalian species. Thus, using animals to identify stimuli that do and do not evoke coughing and to evaluate the efficacy of putative antitussives has had good predictive value for the results of human studies (Tables (Tables11 and and22).

Table 1
Stimuli Evoking Cough in Humans and Animals.
Table 2
Stimuli that do not reliably evoke cough in humans or animals.

The predictive value of animal studies of cough is nevertheless insufficient justification for animal experimentation relating to cough. Rather, the primary advantages of studying cough in animals are the many interventions, treatments and conditions under which and/ or following which cough can be studied in animals that are simply not feasible in human subjects (7, 9, 10, 11, 16, 22-28). This allows experimenters precise control over the induction of cough and the conditions under which cough is evoked, and thus better insight into the physiology and pharmacology of cough. These physiological studies can be extended to in vitro settings, allowing for even more refined (reduced) approaches to studying the synaptic, cellular and molecular interactions and processes relevant to this respiratory reflex (23-25, 27, 28). With this experimental flexibility, new insights into the physiology and pharmacology of cough are routinely recorded and incorporated into our working models of the physiology and pathophysiology of cough. Such studies may reveal previously unrecognized causes of cough and perhaps new treatments or at least a rational basis for evaluating new treatments. Given the consensus that new, specific and effective treatments for cough remains a critically important, valued and yet unmet need in clinical practice and the advantages to studying cough in animals detailed above provides the rationale for continued study of the cough reflex in animals.

The advantages of studying cough in animals

The cough reflex follows a fairly simple reflex arc. Chemical or mechanical irritants primarily in the larynx, trachea and large bronchi activate vagal sensory nerves that form action potentials that conduct up the vagus nerves to the brain. When the action potential invades the central terminals of these sensory nerves in the brainstem (primarily in the nucleus of the solitary tract; nTS), synaptic vesicles release their neurotransmitters onto nTS relay neurons in varying quantities depending on the frequency of action potential formation (impulses/ sec), the duration of activation and the number of sensory neurons activated. These transmitters (e.g. glutamate, substance P) act postsynaptically on nTS relay neurons, that in turn form action potentials and encode information projected elsewhere in the brainstem, resulting in respiratory reflexes. If the specificity and intensity of the afferent nerve activation is sufficient, the peculiarities of the cough reflex (enhanced inspiratory effort, closed glottis and respiratory muscle contraction during the compressive phase of the cough, opened glottis and sustained respiratory muscle contraction during the expulsive phase) are transmitted through the motor nerves innervating the diaphragm and related respiratory muscles (15, 29, 30). Each aspect of this reflex arc can be studied at the systems, synaptic, cellular and molecular level in animals using physiologic, pharmacologic, immunohistochemical and molecular techniques that are not readily duplicated in human studies.

Insights derived from multiple experimental approaches in animals reveal potential causes of cough and multiple physiologic and pharmacologic concepts that may lead to novel and rational therapeutic approaches to treating cough. Gastroesophageal reflux disease (GERD), for example, is a common cause of chronic cough in human subjects (31, 32). It has not been firmly established why GERD precipitates coughing, but it is thought to occur secondary to aspiration/ microaspiration and direct activation of the airway nerves regulating cough, or alternatively, following effects on esophageal afferent nerves that in turn modulate cough centrally (15, 33, 34). Studies in awake and anesthetized animals confirm that acid challenge to the tracheal or laryngeal mucosa or inhalation of acidic aerosols readily evokes coughing in animals (12-16, 25, 26). Electrophysiological studies have identified the afferent nerve subtypes innervating the larynx, trachea and bronchi that are responsive to acid, and have identified the ion channels (TRPV1, acid sensing ion channels) that may regulate the excitability of these sensory nerves (35, 36). Insights from these electrophysiological analyses have lead to the development of potent and selective inhibitors of these ion channels, and studies in whole animals confirm their ability to prevent coughing initiated by acidification of the airways (16, 37, 38). With respect to the notion that esophageal afferent nerve activation might modulate coughing, we’ve shown that acid or capsaicin challenge to the esophagus does not induce coughing, but increases the sensitivity of the airways to tussive stimuli (unpublished observations). This sensitization of the cough reflex occurs centrally and is analogous to the process of central sensitization that is known to be fundamental to the onset of chronic pain (39). We’ve reported a similar interaction between airway afferent nerve subtypes, suggesting such synergistic effects amongst afferent nerves may be one rational therapeutic target for treating cough (15, 26, 39).

Therapeutic targets at the sensory nerve terminals have also been described in studies carried out in animals. Allergen and a number of autacoids associated with airways inflammation have been shown to sensitize sensory nerve terminals to subsequent activation and to exacerbate experimentally induced coughing in animals (12-15, 40-47). These autacoids may be targeted in subsets of patients, as in cough variant asthma (48-50). Studies of cough in anesthetized animals and in vitro electrophysiological and immunohistochemical analyses have also identified a number of regulatory ion channels and pumps that may be targeted therapeutically to reduce coughing, including an isozyme of the sodium pump (15), Ca++- activated Cl- channels (16, 51-53), and perhaps TTX-insensitive Na+ channels (54, 55).

Recent studies in animals might also encourage further study of the CNS as it relates to human cough. As mentioned above, our studies of airway and esophageal afferent nerve interactions highlights the importance of the central nervous system in regulating sensitivity to tussive stimuli. Other studies of the altered cough reflex following cigarette smoke exposure and studies in guinea pigs, cats and dogs reveal other central mechanisms by which cough can be enhanced or inhibited (26, 56-58). More recent studies in guinea pigs describe a conceptual basis for new antitussive therapies for treating cough. In these studies, cough was evoked experimentally by electrically stimulating the tracheal mucosa. When the stimulation intensity and stimulation frequency were suprathreshold, cough could be reliably evoked. Interestingly, however, the stimulation frequency required for induction of cough (≥8 Hz) and the stimulus duration required (3-10 seconds) were quite high (55). The implications of these results are that centrally acting therapeutics that simply reduce but not necessary abolish synaptic efficacy in the CNS may greatly reduce coughing. Consistent with this notion, we have shown that NMDA- type glutamate receptor selective antagonists can nearly abolish coughing in anesthetized guinea pigs while having little or no effect on basal respiratory rate and other respiratory reflexes (15).

The disadvantages of modeling altered cough reflexes in animals

There are several disadvantages to studying cough in animals. As mentioned previously, there are no perfect animal models of the human diseases most often associated with acute or chronic cough. Although existing models approximate these human conditions, the peculiarities of GERD and asthma and COPD and various respiratory tract infections are not reliably reproduced. Since coughing in humans during illness is spontaneous, it would be ideal to study animals that had developed spontaneous cough. But this is essentially never done, with coughing in animals typically studied in response to artificial delivery of a tussive stimulus. The physiology and pharmacology of spontaneous coughing and induced coughing is likely to be different (59). Another problem with modeling cough in animals is that just as there are peculiarities of human physiology, there are also peculiarities of animal physiology that can negatively impact the predictive value of cough research. The profound role of tachykinins and the axon reflex in guinea pigs and rats and their limited role in humans is one example (15). The complete inability to study subjective endpoints such as urge to cough and dyspnea are also limitations to using animals. Finally, one of the primary reasons that animal experimentation allows for more invasive approaches is that many of these interventions can be carried out following anesthesia. The many benefits afforded by anesthesia and the interventions then possible are detailed above, but the detrimental effects of anesthesia on cough are not so obvious. Naturally, the reason animals are given anesthetic is to blunt or eliminate their response or sensitivity to noxious stimuli. Many of the stimuli used to evoke cough in conscious animals would evoke pain when administered to somatic tissues. These same stimuli fail to evoke pain when administered to somatic tissues following anesthesia and similarly do not evoke coughing in anesthetized animals (7, 9, 15, 25). Thus, although it is possible to study the cough reflex in anesthetized animals, a complete and fully functional cough reflex and coughing pattern is never achieved except in awake animals. In the absence of anesthesia, many of the benefits of studying cough in animals are lost.

Conclusions

We still have a limited understanding of the causes of chronic cough. A consequence of this gap in our existing knowledge is that therapeutic approaches used for treating cough are minimally effective and nonspecific. Better therapies based on rational approaches are needed. There is no precedent for drug discovery based entirely or even largely on human experimentation, and animal models have been and will remain essential to our progress in understanding and treating this chronic, and often troublesome respiratory reflex.

References

1. Horner J, Massey EW, Riski JE, Lathrop DL, Chase KN. Aspiration following stroke: clinical correlates and outcome. Neurology. 1988;38(9):1359–1362. [PubMed]
2. Nakagawa T, Sekizawa K, Arai H, et al. High incidence of pneumonia in elderly patients with basal ganglia infarction. Arch Intern Med. 1997;157(3):321–324. [PubMed]
3. Addington WR, Stephens RE, Gilliland KA. Assessing the laryngeal cough reflex and the risk of developing pneumonia after stroke: an interhospital comparison. Stroke. 1999;30(6):1203–1207. [PubMed]
4. Hammond CA Smith, Goldstein LB, Zajac DJ, Gray L, Davenport PW, Bolser DC. Assessment of aspiration risk in stroke patients with quantification of voluntary cough. Neurology. 2001;56(4):502–506. [PubMed]
5. Niimi A, Matsumoto H, Ueda T, Takemura M, Suzuki K, Tanaka E, Chin K, Mishima M, Amitani R. Impaired cough reflex in patients with recurrent pneumonia. Thorax. 2003;58:152–153. [PMC free article] [PubMed]
6. Perrin C, Unterborn JN, Ambrosio CD, Hill NS. Pulmonary complications of chronic neuromuscular diseases and their management. Muscle Nerve. 2004;29(1):5–27. [PubMed]
7. Tatar M, Webber SE, Widdicombe JG. Lung C-fibre receptor activation and defensive reflexes in anaesthetized cats. J Physiol. 1988;402:411–420. [PubMed]
8. Kamei J, Iwamoto Y, Suzuki T, Misawa M, Nagase H, Kasuya Y. Antitussive effects of naltrindole, a selective delta-opioid receptor antagonist, in mice and rats. Eur J Pharmacol. 1993;249(2):161–165. [PubMed]
9. Tatar M, Sant’Ambrogio G, Sant’Ambrogio FB. Laryngeal and tracheobronchial cough in anesthetized dogs. J Appl Physiol. 1994;76(6):2672–2679. [PubMed]
10. Deep V, Singh M, Ravi K. Role of vagal afferents in the reflex effects of capsaicin and lobeline in monkeys. Respir Physiol. 2001;125(3):155–168. [PubMed]
11. Adcock JJ, Douglas GJ, Garabette M, et al. RSD931, a novel anti-tussive agent acting on airway sensory nerves. Br J Pharmacol. 2003;138(3):407–416. [PMC free article] [PubMed]
12. Bolser DC. Experimental models and mechanisms of enhanced coughing. Pulm Pharmacol Ther. 2004;17(6):383–388. [PMC free article] [PubMed]
13. Lewis CA, Ambrose C, Banner K, et al. Animal models of cough: literature review and presentation of a novel cigarette smoke-enhanced cough model in the guinea-pig. Pulm Pharmacol Ther. 2007;20(4):325–333. [PubMed]
14. Karlsson JA, Fuller RW. Pharmacological regulation of the cough reflex--from experimental models to antitussive effects in Man. Pulm Pharmacol Ther. 1999;12(4):215–228. [PubMed]
15. Canning BJ, Mori N, Mazzone SB. Vagal afferent nerves regulating the cough reflex. Respir Physiol Neurobiol. 2006;152(3):223–242. [PubMed]
16. Canning BJ, Farmer DG, Mori N. Mechanistic studies of acid-evoked coughing in anesthetized guinea pigs. Am J Physiol Regul Integr Comp Physiol. 2006;291(2):R454–R463. [PubMed]
17. Tominaga M, Caterina MJ, Malmberg AB, et al. The cloned capsaicin receptor integrates multiple pain-producing stimuli. Neuron. 1998;21(3):531–543. [PubMed]
18. Hayes P, Meadows HJ, Gunthorpe MJ, et al. Cloning and functional expression of a human orthologue of rat vanilloid receptor-1. Pain. 2000;88(2):205–215. [PubMed]
19. Savidge J, Davis C, Shah K, et al. Cloning and functional characterization of the guinea pig vanilloid receptor 1. Neuropharmacology. 2002;43(3):450–456. [PubMed]
20. Correll CC, Phelps PT, Anthes JC, Umland S, Greenfeder S. Cloning and pharmacological characterization of mouse TRPV1. Neurosci Lett. 2004;370(1):55–60. [PubMed]
21. Gavva NR, Klionsky L, Qu Y, et al. Molecular determinants of vanilloid sensitivity in TRPV1. J Biol Chem. 2004;279(19):20283–20295. [PubMed]
22. Bolser DC. Fictive cough in the cat. J Appl Physiol. 1991;71(6):2325–2331. [PubMed]
23. Ricco MM, Kummer W, Biglari B, et al. Interganglionic segregation of distinct vagal afferent fibre phenotypes in guinea-pig airways. J Physiol. 1996;496(Pt 2):521–530. [PubMed]
24. Undem BJ, Chuaychoo B, Lee MG, et al. Subtypes of vagal afferent C-fibres in guinea-pig lungs. J Physiol. 2004;556(Pt 3):905–917. [PubMed]
25. Canning BJ, Mazzone SB, Meeker SN, et al. Identification of the tracheal and laryngeal afferent neurones mediating cough in anaesthetized guinea-pigs. J Physiol. 2004;557(Pt 2):543–558. [PubMed]
26. Mazzone SB, Mori N, Canning BJ. Synergistic interactions between airway afferent nerve subtypes regulating the cough reflex in guinea-pigs. J Physiol. 2005;569(Pt 2):559–573. [PubMed]
27. Bonham AC, Chen CY, Sekizawa S, Joad JP. Plasticity in the nucleus tractus solitarius and its influence on lung and airway reflexes. J Appl Physiol. 2006;101(1):322–327. [PubMed]
28. Chuaychoo B, Lee MG, Kollarik M, Pullmann R, Jr, Undem BJ. Evidence for both adenosine A1 and A2A receptors activating single vagal sensory C-fibres in guinea pig lungs. J Physiol. 2006;575(Pt 2):481–490. [PubMed]
29. Shannon R, Baekey DM, Morris KF, Li Z, Lindsey BG. Functional connectivity among ventrolateral medullary respiratory neurones and responses during fictive cough in the cat. J Physiol. 2000;525(Pt 1):207–24. [PubMed]
30. Bolser DC, Poliacek I, Jakus J, Fuller DD, Davenport PW. Neurogenesis of cough, other airway defensive behaviors and breathing: A holarchical system? Respir Physiol Neurobiol. 2006;152(3):255–265. [PMC free article] [PubMed]
31. Irwin RS, Baumann MH, Bolser DC, et al. Diagnosis and management of cough executive summary: ACCP evidence-based clinical practice guidelines. Chest. 2006;129(1 Suppl):1S–23S. [PMC free article] [PubMed]
32. Morice AH, Fontana GA, Belvisi MG, et al. ERS guidelines on the assessment of cough. Eur Respir J. 2007;29(6):1256–1276. [PubMed]
33. Ing AJ, Ngu MC, Breslin AB. Pathogenesis of chronic persistent cough associated with gastroesophageal reflux. Am J Respir Crit Care Med. 1994;149(1):160–167. [PubMed]
34. Wu DN, Yamauchi K, Kobayashi H, et al. Effects of esophageal acid perfusion on cough responsiveness in patients with bronchial asthma. Chest. 2002;122(2):505–509. [PubMed]
35. Kollarik M, Undem BJ. Mechanisms of acid-induced activation of airway afferent nerve fibres in guinea-pig. J Physiol. 2002;543(Pt 2):591–600. [PubMed]
36. Gu Q, Lee LY. Characterization of acid signaling in rat vagal pulmonary sensory neurons. Am J Physiol Lung Cell Mol Physiol. 2006;291(1):L58–L65. [PMC free article] [PubMed]
37. Lalloo UG, Fox AJ, Belvisi MG, et al. Capsazepine inhibits cough induced by capsaicin and citric acid but not by hypertonic saline in guinea pigs. J Appl Physiol. 1995;79(4):1082–1087. [PubMed]
38. Trevisani M, Milan A, Gatti R, et al. Antitussive activity of iodo-resiniferatoxin in guinea pigs. Thorax. 2004;59(9):769–772. [PMC free article] [PubMed]
39. Mazzone SB, Canning BJ. Central nervous system control of the airways: pharmacological implications. Curr Opin Pharmacol. 2002;2(3):220–228. [PubMed]
40. Fox AJ, Lalloo UG, Belvisi MG, et al. Bradykinin-evoked sensitization of airway sensory nerves: a mechanism for ACE-inhibitor cough. Nat Med. 1996;2(7):814–817. [PubMed]
41. Xiang A, Uchida Y, Nomura A, et al. Effects of airway inflammation on cough response in the guinea pig. J Appl Physiol. 1998;85(5):1847–1854. [PubMed]
42. Liu Q, Fujimura M, Tachibana H, et al. Characterization of increased cough sensitivity after antigen challenge in guinea pigs. Clin Exp Allergy. 2001;31(3):474–484. [PubMed]
43. Xiang A, Uchida Y, Nomura A, et al. Involvement of thromboxane A(2) in airway mucous cells in asthma-related cough. J Appl Physiol. 2002;92(2):763–770. [PubMed]
44. House A, Celly C, Skeans S, et al. Cough reflex in allergic dogs. Eur J Pharmacol. 2004;492(2-3):251–258. [PubMed]
45. El-Hashim AZ, Amine SA. The role of substance P and bradykinin in the cough reflex and bronchoconstriction in guinea-pigs. Eur J Pharmacol. 2005;513(1-2):125–133. [PubMed]
46. Gatti R, Andre E, Amadesi S, et al. Protease-activated receptor-2 activation exaggerates TRPV1-mediated cough in guinea pigs. J Appl Physiol. 2006;101(2):506–511. [PubMed]
47. Nishitsuji M, Fujimura M, Oribe Y, Nakao S. Effect of montelukast in a guinea pig model of cough variant asthma. Pulm Pharmacol Ther. In press. [PubMed]
48. Fujimura M, Kamio Y, Kasahara K, et al. Prostanoids and cough response to capsaicin in asthma and chronic bronchitis. Eur Respir J. 1995;8(9):1499–1505. [PubMed]
49. Dicpinigaitis PV, Dobkin JB, Reichel J. Antitussive effect of the leukotriene receptor antagonist zafirlukast in subjects with cough-variant asthma. J Asthma. 2002;39(4):291–297. [PubMed]
50. Spector SL, Tan RA. Effectiveness of montelukast in the treatment of cough variant asthma. Ann Allergy Asthma Immunol. 2004;93(3):232–236. [PubMed]
51. Oh EJ, Weinreich D. Bradykinin decreases K(+) and increases Cl(−) conductances in vagal afferent neurones of the guinea pig. J Physiol. 2004;558(Pt 2):513–526. [PubMed]
52. Lee MG, Macglashan DW, Jr, Undem BJ. Role of chloride channels in bradykinin-induced guinea pig airway vagal C-fibre activation. J Physiol. 2005;566(Pt 1):205–212. [PubMed]
53. Mazzone SB, McGovern AE. Na+-K+-2Cl- cotransporters and Cl- channels regulate citric acid cough in guinea pigs. J Appl Physiol. 2006;101(2):635–643. [PubMed]
54. Carr MJ. Influence of mexiletine on action potential discharge and conduction in nodose Adelta afferent neurons innervating guinea pig isolated trachea. Pulm Pharmacol Ther. 2006;19(4):258–263. [PubMed]
55. Canning BJ. Encoding of the cough reflex. Pulm Pharmacol Ther. 2007;20(4):396–401. [PMC free article] [PubMed]
56. Bolser DC, Hey JA, Chapman RW. Influence of central antitussive drugs on the cough motor pattern. J Appl Physiol. 1999;86(3):1017–1024. [PubMed]
57. Joad JP, Munch PA, Bric JM, et al. Passive smoke effects on cough and airways in young guinea pigs: role of brainstem substance P. Am J Respir Crit Care Med. 2004;169(4):499–504. [PubMed]
58. Poliacek I, Corrie LW, Wang C, Rose MJ, Bolser DC. Microinjection of DLH into the region of the caudal ventral respiratory column in the cat: evidence for an endogenous cough-suppressant mechanism. J Appl Physiol. 2007;102(3):1014–1021. [PMC free article] [PubMed]
59. Dicpinigaitis PV. Experimentally induced cough. Pulm Pharmacol Ther. 2007;20(4):319–324. [PubMed]