Neuroscience Letters, 126 (1991) 131-133 0 1991 Elsevier Scientific Publishers Ireland Ltd. 0304-3940/91/$03.50 ADONIS030439409100233G NSL 01152 131 Ruthenium red decreases capsaicin and citric acid-induced cough in guinea pigs D.C. Bolser, S.M. Aziz, and R.W. Chapman Schering-Plough Research, Bloomjeld, NJ (U.S.A.) (Received 17 December 1990; Revised version received 15 February 1991; Accepted 18 February 1991) Key words: Ruthenium red; Cough; Guinea pig The influence of aerosols of Ruthenium red (RR) on capsaicin- and citric acid-induced cough was investigated in guinea pigs. Aerosols of RR (0.3, 1, 3%) reduced capsaicin-induced cough in dose-dependent manner. Inhalation of RR also reduced cough produced by low, (200 mM) but not high (550 mM), concentrations of citric acid. These data suggest that RR is not a specific capsaicin antagonist and that citric acid and capsaicin share a common mechanism for activation of airway C-fibers that is RR sensitive. Furthermore, high concentrations of citric acid can elicit cough through a RR-insensitive mechanism. Aerosols of capsaicin, a stimulant of thin fiber prim- ary afferents [15, 161, induce cough in humans [6] and guinea pigs [9, lo]. The tussigenic effect of capsaicin is most likely due to stimulation of airway C-fibers [lo]. Citric acid aerosols also elicit cough in human [4] and guinea pigs [19]. This effect of citric acid is blocked by capsaicin desensitization, suggesting that citric acid- induced cough is mediated by capsaicin-sensitive C- fibers in the airways [lo]. Ruthenium red (RR), an inorganic dye, has recently been shown to be a specific capsaicin antagonist in a var- iety of preparations. It has no effect on responses elicited by substance P, acetylcholine, bradykinin or nicotine [2, 3, 11, 181. Therefore, the availability of RR afforded us the opportunity to investigate in greater detail the mechanism of capsaicin and citric acid-induced cough. A total of 216 male Dunkin Hartley guinea pigs were used in this study. Unanesthetized animals (400-600 g) were placed into a transparent Plexiglas chamber and exposed to aerosols of RR (0.3, 1, or 3%) or physiologi- cal saline vehicles at an airflow of 4 L/min. The aerosol was generated by a jet nebulizer and the volume of solution aerosolized was approximately 0.4 ml/min. An unpaired paradigm was used such that each group of animals was exposed to vehicle or a different dose of RR. More specifically, the animals were exposed to RR or Correspondence: D.C. Bolser, Department of Allergy, Schering-Plough Research, 60 Orange St., Bloomfield, NJ 07003, U.S.A. vehicle for 4 min. and then exposed to capsaicin (0.07 or 0.3 mM) or citric acid (200 or 550 mM) aerosols for 4 min to induce coughing. These doses of citric acid and capsaicin are consistent with concentrations used in pre- vious reports to elicit cough in guinea pigs [9, lo]. Coughs were detected by a microphone placed in the chamber and connected to an audio monitor and chart recorder. The number of coughs elicited during the 4 min of exposure to capsaicin or citric acid were counted by visual inspection of the chart record. Inhaled irritants can produce sneeze as well as cough [14]. Codeine inhibits cough but not sneeze [13]. Codeine is a very effective antitussive agent in this model, both in our laboratory (unpublished observations) and in the laboratories of others [l, 121. The antitussive effect of codeine provides evidence that the events we detect were in fact cough and not sneeze. Capsaicin (Sigma) was dissolved in 10% ethanol and 90% physiological saline. Citric acid and RR (Sigma), were dissolved in physiological saline. Ruthenium red doses were corrected for the purity (36%) of the com- pound. Data are expressed as mean f S.E.M. Statistical ana- lysis was performed using Duncan’s multiple range sta- tistic. Differences were considered significant if P< 0.05. Prior treatment with RR aerosol significantly de- creased cough induced by low (0.07 mM) and high (0.3 mM) concentrations of capsaicin aerosol in a dose-de- pendent manner (Fig. 1A). RR appeared to cause a par- allel shift in the dose-response to capsaicin, suggestive of