Cough reflex responses during pulmonary C-fibre receptor activation in anesthetized rabbits

The effect of stimulation and unloading of baroreceptors on cough in experimental conditions

Cough, the main airway defensive process, is modulated by multiple sensory inputs from the respiratory system and outside of it. This modulation is one of the mechanisms that contributes to the sensitization of cough pathways at the peripheral and/or central level via neuroplasticity and it manifests most often as augmented coughing. Cardiorespiratory coupling is an important mechanism responsible for a match between oxygenation and cardiac output and bidirectional relationships exist between respiration and cardiovascular function. While the impact of cough with the robust swings of the intrathoracic pressure on haemodynamic parameters and heart electrophysiology are well characterized, little is known about the modulation of cough by haemodynamic parameters - mainly the blood pressure. Some circumstantial findings from older animal studies and more recent sophisticated analysis confirm that baroreceptor stimulation and unloading alters coughing evoked in experiments. Clinical relevance of such findings is not presently known.

GABAA- and glycine-mediated inhibitory modulation of the cough reflex in the caudal nucleus tractus solitarii of the rabbit

Cough-related sensory inputs from rapidly adapting receptors (RARs) and C fibers are processed by second-order neurons mainly located in the caudal nucleus tractus solitarii (NTS). Both GABAA and glycine receptors have been proven to be involved in the inhibitory control of second-order cells receiving RAR projections. We investigated the role of these receptors within the caudal NTS in the modulation of the cough reflex induced by either mechanical or chemical stimulation of the tracheobronchial tree in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections (30-50 nl) of the receptor antagonists bicuculline and strychnine as well as of the receptor agonists muscimol and glycine were performed. Bicuculline (0.1 mM) and strychnine (1 mM) caused decreases in peak abdominal activity and marked increases in respiratory frequency due to decreases in both inspiratory time (Ti) and expiratory time (Te), without concomitant changes in arterial blood pressure. Noticeably, these microinjections induced potentiation of the cough reflex consisting of increases in the cough number associated with decreases either in cough-related Ti after bicuculline or in both cough-related Ti and Te after strychnine. The effects caused by muscimol (0.1 mM) and glycine (10 mM) were in the opposite direction to those produced by the corresponding antagonists. The results show that both GABAA and glycine receptors within the caudal NTS mediate a potent inhibitory modulation of the pattern of breathing and cough reflex responses. They strongly suggest that disinhibition is one important mechanism underlying cough regulation and possibly provide new hints for novel effective antitussive strategies.

Suppression of the cough reflex by α 2-adrenergic receptor agonists in the rabbit

The α₂-adrenergic receptor agonist clonidine has been shown to inhibit citric acid-induced cough responses in guinea pigs when administered by aerosol, but not orally. In contrast, oral or inhaled clonidine had no effect on capsaicin-induced cough and reflex bronchoconstriction in humans. In addition, intravenous administration of clonidine has been shown to depress fentanyl-induced cough in humans. We investigated the effects of the α₂-adrenergic receptor agonists, clonidine and tizanidine, on cough responses induced by mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nL) into the caudal nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) as well as administered intravenously in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections of clonidine into the cNTS or the cVRG reduced cough responses at 0.5 mmol/L and abolished the cough reflex at 5 mmol/L. Bilateral microinjections of 0.5 mmol/L tizanidine into the cNTS completely suppressed cough responses, whereas bilateral microinjections of 5 mmol/L into the cVRG only caused mild reductions in them. Depressant effects on the cough reflex of clonidine and tizanidine were completely reverted by microinjections of 10 mmol/L yohimbine. Intravenous administration of clonidine (80–120 μg/kg) or tizanidine (150–300 μg/kg) strongly reduced or completely suppressed cough responses. These effects were reverted by intravenous administration of yohimbine (300 μg/kg). The results demonstrate that activation of α₂-adrenergic receptors in the rabbit exerts potent inhibitory effects on the central mechanism generating the cough motor pattern with a clear action at the level of the cNTS and the cVRG.

Modulation of the cough reflex by GABA(A) receptors in the caudal ventral respiratory group of the rabbit

We have previously shown that the caudal ventral respiratory group (cVRG) is a possible site of action of some antitussive drugs and plays a crucial role in determining both the expiratory and inspiratory components of the cough motor pattern. In addition, it has been reported that medullary expiratory neurons of the cVRG are subject to potent GABAergic gain modulation. This study was devoted to investigate the role of cVRG GABA_A receptors in the control of baseline respiratory activity and cough responses to mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. To this purpose, bilateral microinjections (30–50 nl) of bicuculline or muscimol were performed into the cVRG of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bicuculline (1 mM) increased peak abdominal activity and respiratory frequency due to decreases in T_E. Cough responses were potentiated mainly owing to increases in the cough number. The recovery was observed within ~2 h. On the contrary, muscimol (0.3 mM) abolished abdominal activity and decreased respiratory frequency due to increases in T_E. In addition, cough responses were progressively reduced and completely suppressed within ~20 min. Partial recovery of cough responses was achieved after ~3 h or within ~5 min following bicuculline microinjections at the same locations. The sneeze reflex induced by mechanical stimulation of the nasal mucosa persisted following bicuculline and muscimol microinjections. However, the number and intensity of expiratory thrusts were enhanced by bicuculline and suppressed by muscimol. The results provide evidence that a potent GABA_A-mediated inhibitory modulation is exerted at the level of the cVRG not only on respiratory activity, but also on cough and sneeze reflex responses.

Suppression of the cough reflex by inhibition of ERK1/2 activation in the caudal nucleus tractus solitarii of the rabbit

The caudal nucleus tractus solitarii (cNTS), the predominant site of termination of cough-related afferents, has been shown to be a site of action of some centrally acting antitussive agents. A role of ERK1/2 has been suggested in acute central processing of nociceptive inputs. Because pain and cough share similar features, we investigated whether ERK1/2 activation could also be involved in the central transduction of tussive inputs. For this purpose, we undertook the present research on pentobarbital sodium-anesthetized, spontaneously breathing rabbits by using microinjections (30–50 nl) of an inhibitor of ERK1/2 activation (U0126) into the cNTS. Bilateral microinjections of 25 mM U0126 caused rapid and reversible reductions in the cough responses induced by both mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. In particular, the cough number and peak abdominal activity decreased. Bilateral microinjections of 50 mM U0126 completely suppressed the cough reflex without affecting the Breuer-Hering inflation reflex, the pulmonary chemoreflex, and the sneeze reflex. These U0126-induced effects were, to a large extent, reversible. Bilateral microinjections of 50 mM U0124, the inactive analog of U0126, at the same cNTS sites had no effect. This is the first study that provides evidence that ERK1/2 activation within the cNTS is required for the mediation of cough reflex responses in the anesthetized rabbit. These results suggest a role for ERK1/2 in the observed effects via nontranscriptional mechanisms, given the short time involved. They also may provide hints for the development of novel antitussive strategies.

Blood pressure changes alter tracheobronchial cough: computational model of the respiratory-cough network and in vivo experiments in anesthetized cats

We tested the hypothesis, motivated in part by a coordinated computational cough network model, that alterations of mean systemic arterial blood pressure (BP) influence the excitability and motor pattern of cough. Model simulations predicted suppression of coughing by stimulation of arterial baroreceptors. In vivo experiments were conducted on anesthetized spontaneously breathing cats. Cough was elicited by mechanical stimulation of the intrathoracic airways. Electromyograms (EMG) of inspiratory parasternal, expiratory abdominal, laryngeal posterior cricoarytenoid (PCA), and thyroarytenoid muscles along with esophageal pressure (EP) and BP were recorded. Transiently elevated BP significantly reduced cough number, cough-related inspiratory, and expiratory amplitudes of EP, peak parasternal and abdominal EMG, and maximum of PCA EMG during the expulsive phase of cough, and prolonged the cough inspiratory and expiratory phases as well as cough cycle duration compared with control coughs. Latencies from the beginning of stimulation to the onset of cough-related diaphragm and abdominal activities were increased. Increases in BP also elicited bradycardia and isocapnic bradypnea. Reductions in BP increased cough number; elevated inspiratory EP amplitude and parasternal, abdominal, and inspiratory PCA EMG amplitudes; decreased total cough cycle duration; shortened the durations of the cough expiratory phase and cough-related abdominal discharge; and shortened cough latency compared with control coughs. Reduced BP also produced tachycardia, tachypnea, and hypocapnic hyperventilation. These effects of BP on coughing likely originate from interactions between barosensitive and respiratory brainstem neuronal networks, particularly by modulation of respiratory neurons within multiple respiration/cough-related brainstem areas by baroreceptor input.

Role of excitatory amino acids in the mediation of tracheobronchial cough induced by citric acid inhalation in the rabbit

We investigated the role of ionotropic glutamate receptors located within the caudal portions of the nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) in the mediation of coughing evoked by citric acid inhalation in spontaneously breathing rabbits under pentobarbitone anaesthesia. Bilateral microinjections (30-50nl) of 10mM CNQX and 10mM D-AP5 were performed to block non-NMDA and NMDA receptors, respectively. An attempt was also made to investigate the effects of ionotropic glutamate receptor blockade within the cVRG on sneezing induced by mechanical stimulation of the nasal mucosa. Blockade of non-NMDA receptors within the cNTS abolished coughing and associated tachypneic responses, while blockade of NMDA receptors only reduced cough responses. Blockade of non-NMDA receptors within the cVRG always abolished spontaneous rhythmic abdominal activity as well as coughing and associated tachypneic responses; blockade of NMDA receptors only reduced spontaneous rhythmic abdominal activity and coughing. As to sneezing, blockade of non-NMDA receptors within the cVRG suppressed the expiratory thrusts without affecting the inspiratory preparatory bursts, while blockade of NMDA receptors only strongly attenuated the expiratory thrusts. This study is the first to provide evidence that ionotropic glutamate receptors, and especially non-NMDA receptors, are involved in the mediation of coughing induced by citric acid inhalation and to suggest that citric acid-activated cough-related afferents terminate within the cNTS. Present data also corroborate the notion that the cVRG is involved in the generation of the whole cough motor pattern, but seems to represent merely an expiratory output system for sneezing.

Central regulation of the cough reflex: therapeutic implications

In many species including humans, antagonists of NMDA-type glutamate receptors such as dextromethorphan, when used at sufficient doses, have been found to be relatively safe and effective antitussives. Similarly, now in five different species (guinea pigs, rabbits, cats, dogs and pigs), neurokinin receptor antagonists have also proven to be safe and effective antitussive agents. Both of these classes of drugs act centrally to prevent cough. A brief review of what is known about the central encoding of cough is presented, as are the advantages of centrally acting antitussives. Also discussed are new insights into cough and NMDA receptor signaling that may lead to the development of more effective antitussive agents with limited side effects and broad application in treating cough associated with a variety of aetiologies.