Phrenic and iliohypogastric nerve discharges during tussigenic stimulation in paralyzed and decerebrate guinea pigs and rats

Although effects of antitussive drugs have been examined in inbred small animals using a whole body plethysmography, neuronal mechanisms underlying the cough reflex are not fully understood. The present study analyzed the reflex discharge patterns of the phrenic (PN) and iliohypogastric nerves (IHN) evoked in decerebrate and paralyzed guinea pigs and rats. In guinea pigs, electrical stimulation of the superior laryngeal nerve, chemical stimulation with capsaicin and mechanical stimulation to the intratracheal mucosa equally produced a serial PN-IHN response. This response was characterized by an increased PN discharge and following spindle-shaped burst of the IHN. The evoked discharges overlapped for 20 ms. In rats, by contrast, mechanical stimulation was without effect while capsaicin and electrical stimulation produced two types of responses, both of which differed from that observed in guinea pigs. The first type consisted of an augmented burst of the IHN that was immediately followed by an increased PN discharge. The second type was a large spindle-shaped burst of the IHN that occurred 80 ms after the end of the preceding PN discharge. Codeine (3 mg/kg i.v.) depressed all types of responses evoked in guinea pigs and rats. The present study demonstrated that the fictive cough comparable with those induced in other experimental animals was produced consistently in guinea pigs, but not in rats. Therefore, guinea pigs are suitable for investigation of the neuronal mechanisms underlying the cough reflex and assessment of antitussive drugs.

Introduction

Cough is a protective reflex that eliminates inhaled irritants from the airway. This reflex is characterized by a rapid, large expiration combined tightly with a preceding deep inspiration. Hence, sequential changes in the inspiratory (diaphragm) and expiratory (abdominal muscles) motor acts are directly correlated to the cough reflex [7], [8], [12], [42], [43]. Moreover, various antitussive drugs have been introduced to manage the cough reflex, since excessive and continual occurrence of cough reflexes sometimes causes serious burdens to the cardiovascular and respiratory organs. For assessment of the antitussive action of drugs, a whole body plethysmographic recording has been usually performed on unanesthetized and unrestrained guinea pigs [5], [6], [13], [20], [28], [45], rats [16], [17] and mice [18]. Those studies utilized inhalation of capsaicin or citric acid aerosol to induce the cough reflex. However, inhalation of these irritants occasionally produced various respiratory-related reflexes other than cough, such as apnea, sneeze, sigh, expiratory reflex and augmented breath [19], [23], [39], [44]. This often makes it difficult to differentiate cough from other defensive reflexes.

To investigate neuronal mechanisms underlying the cough reflex, Tomori and Widdicombe [40] developed a fictive cough model in paralyzed and artificially ventilated cats. Bolser [2] recently modified this model and recorded discharge activities of the inspiratory and expiratory efferent nerves during electrical stimulation of the superior laryngeal nerve (SLN) or mechanical stimulation of the intrathoracic trachea. Moreover, behaviors of bulbar respiratory neurons during fictive cough have been extensively investigated using the cat's model [15], [30], [35], [36], [37]. These studies demonstrated that the central respiratory network was involved in multifunctional motor acts including coughing, sneezing, vomiting and swallowing [14], [34]. However, such neuronal analyses have never been performed on guinea pigs and rats, although the use of inbred small experimental animals is in recent times indispensable in neurophygiological and pharmacological studies on cough reflexes. Furthermore, it is still controversial whether a cough reflex is inducible in rats as in cats or guinea pigs [1], [16], [17], [23], [39]. This suggests that the central neural circuitry responsible for cough motor acts may be different among animal species.

Therefore, the present study was designed to resolve important issues regarding the identity and motor patterns of cough reflex in guinea pigs and rats. According to the cat's model [2], a fictive cough model was developed in paralyzed and decerebrate guinea pigs and rats, and the reflex discharges were recorded from the inspiratory (phrenic nerve; PN) and expiratory nerves (iliohypogastric nerve; IHN). We employed three tussigenic stimuli; electrical stimulation of the SLN, chemical stimulation of the tracheal mucosa with capsaicin and mechanical stimulation with the intratracheal tube. These stimuli have been shown to activate the specific cough-related receptors and nerve fibers in the airway [2], [9], [10], [23], [27], [33], [44]. In addition, codeine, a selective and centrally acting antitussive [11], was used to verify pharmacologically whether the induced response was related to the cough reflex.