Modulation of the cough reflex by antitussive agents within the caudal aspect of the nucleus tractus solitarii in the rabbit

Abstract

We have previously shown that ionotropic glutamate receptors in the caudal portion of the nucleus tractus solitarii (NTS), especially in the commissural NTS, play a prominent role in the mediation of tracheobronchial cough and that substance P potentiates this reflex. This NTS region could be a site of action of some centrally acting antitussive agents and a component of a drug-sensitive gating mechanism of cough. To address these issues, we investigated changes in baseline respiratory activity and cough responses to tracheobronchial mechanical stimulation following microinjections (30 -50 nl) of centrally acting antitussive drugs into the caudal NTS of pentobarbitone-anesthetized, spontaneously breathing rabbits. [D-Ala 2 ,N-Me-Phe 4 ,Gly 5 -ol]-enkephalin (DAMGO) and baclofen decreased baseline respiratory frequency because of increases in the inspiratory time only at the higher concentration employed (5 mM and 1 mM, respectively). DAMGO (0.5 mM) and baclofen (0.1 mM) significantly decreased cough number, peak abdominal activity, peak tracheal pressure, and increased cough-related total cycle duration. At the higher concentrations, these agents suppressed the cough reflex. The effects of these two drugs were counteracted by specific antagonists (10 mM naloxone and 25 mM CGP-35348, respectively). The neurokinin-1 (NK1) receptor antagonist CP-99,994 (10 mM) abolished cough responses, whereas the NK2 receptor antagonist MEN 10376 (5 mM) had no effect. The results indicate that the caudal NTS is a site of action of some centrally acting drugs and a likely component of a neural system involved in cough regulation. A crucial role of substance P release in the mediation of reflex cough is also suggested.-opioid receptors; GABAB receptors; neurokinin receptors; control of breathing; airway defensive reflexes PREVIOUS STUDIES LED TO THE conclusion that some antitussive drugs act centrally since they inhibit cough when administered by intravertebral arterial or intracerebroventricular route in guinea pigs and cats (7,8). These drugs comprise the opioid receptor agonist codeine, the -opioid receptor agonist morphine, the neurokinin-1 (NK 1 ) receptor antagonist (ϩ)(2R,3R)-3-(2-methoxybenzylamino)-2-phenylpiperidine (CP-99,994), the NK 2 receptor antagonist {(ϩ)-N-methyl-[4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-di-dichlorophenyl) butyl]benzamide} (SR48968), and the GABA B receptor agonist baclofen. A prominent central antitussive effect has been proven for the selective -opioid receptor agonist [D-Ala 2 ,N-Me-Phe 4 ,Gly 5 -ol]-enkephalin (DAMGO) by using intracerebroventricular administration in the rat (30). However, the central responsive structures and the mechanism of action of these drugs remain to be investigated.It is widely agreed that tracheobronchial rapidly adapting receptors (RARs) are involved in cough mediation, while the role of bronchopulmonary C-fibers and A␦-nociceptive pulmonary afferent fibers in this reflex is controversial (see e.g., 36, 49, 62; for further references, also see Ref. 43). We have ...

The cough reflex is upregulated by lisinopril microinjected into the caudal nucleus tractus solitarii of the rabbit

“…It has already been shown that substance P microinjected into the caudal NTS markedly potentiates cough reflex responses (Mazzone et al, 2005;Mutolo et al, 2007) while 10 mM CP-99,994 microinjected into the same site abolishes them (Mutolo et al, 2008). Thus, we searched for a concentration of the NK 1 receptor antagonist that injected into the caudal NTS did not cause any antitussive effect under control conditions, but could abolish lisinopril-induced potentiation.…”

“…All efforts were made to minimize both the number of animals used and their suffering. Experimental procedures and details about the methods employed have previously been described Mutolo et al, 2007Mutolo et al, , 2008Mutolo et al, , 2009Mutolo et al, , 2010Mutolo et al, , 2012Mutolo et al, , 2013Mutolo et al, , 2014.…”

“…Only one of these drugs was tested in each preparation unless otherwise stated. Drug concentrations were in the same range as those previously used in in vivo preparations (Arnold et al, 2010;Caligiorne et al, 1996;Kasparov et al, 1998;Mutolo et al, 2008). In particular, the concentrations of lisinopril and losartan were based on the knowledge that the antihypertensive potency of lisinopril is ∼5 times higher than that of losartan (see e.g., Mutolo et al, 2013).…”

“…The average values of cough-related variables were taken as single measurements for subsequent statistical analysis (GraphPad Prism 5, La Jolla, CA, USA). On some occasions, an expiration reflex could occur as the first motor event in a cough epoch (Korpáš and Tomori, 1979;Widdicombe and Fontana, 2006 for further details see Tatar et al, 2008;Mutolo et al, 2007Mutolo et al, , 2008Mutolo et al, , 2009Mutolo et al, , 2012Cinelli et al, 2013). Expiration reflexes were not considered for data analysis.…”

“…Similarly, no consistent changes in arterial blood pressure were observed (mean arterial pressure was always between 96 and 105 mmHg). A general evaluation of cardiorespiratory variables under control conditions has been provided in previous reports (Mutolo et al, 2007(Mutolo et al, , 2008(Mutolo et al, , 2009). The correspondence of lisinopril injection sites to the NTS regions involved in cardiovascular regulation, where glutamate microinjections cause decreases in arterial blood pressure (e.g., Cheng et al, 2012;Mosqueda-Garcia et al, 1990), was also investigated.…”