The aim of the present study was to investigate the voluntary suppression of cough in response to capsaicin inhalation in healthy volunteers, and to determine if the dose-response curve to capsaicin was significantly altered when volunteers were asked to suppress their cough response. The quantification of the degree of voluntary suppression of induced cough could provide a new methodology for screening antitussive agents as antitussives may act by influencing voluntary control of cough. Cough was induced by inhalation of capsaicin. Two challenges were given 5 min apart, each comprising five ascending concentrations of capsaicin (1 x 10(-5) M-3.33 x 10(-4) M). During one of these challenges the volunteer was allowed to cough when required, and during the other they were asked to suppress cough. These two conditions were given in random order. The cough response was recorded by means of a microphone with the integrated sound trace displayed on a chart recorder. A dose-response relationship was obtained on administration of ascending concentrations of capsaicin. In the non-suppressed challenge 23/24 subjects coughed on inhalation of capsaicin (3.33 x 10(-4) M) with a mean number of coughs of 2.92 +/- 0.34, whereas in the suppressed challenge only 3/24 subjects coughed with a mean number of coughs of 0.29 +/- 0.18 (P < 0.001). These results demonstrate that cough induced by inhalation of capsaicin can be voluntarily suppressed. The mechanism of voluntary suppression of cough is discussed in relation to capsaicin challenge and the screening of antitussive medications.

Opioids exert an analgesic action by mimicking the effects of endogenous neurotransmitter substances in the central nervous system. Opioids are widely used as antitussives, and it is reasonable to assume that endogenous opioids are involved in the control of cough. In order to investigate this hypothesis, a parallel design study was carried out to examine the effects of 50 mg codeine (opioid agonist), 50 mg naltrexone (opioid antagonist) and placebo on capsaicin-induced cough in 80 healthy volunteers (mean age 25 yrs).Volunteers received two capsaicin challenge units (each consisting of five inhalations of different concentrations of capsaicin, 0.00-3.33×10 -4 M). On one challenge unit subjects were instructed to suppress cough, and on the other challenge unit subjects coughed freely. Coughs were recorded on a tape cassette player and later played back into a pen recorder to produce integrated sound traces.The number of coughs in the suppression challenge unit was significantly reduced in all three treatment groups compared to that recorded in the non-suppression challenge unit. Comparisons between the three treatment groups showed that there was no statistical difference between the three groups both before and 90 min after treatment for the total coughs in the suppression challenge unit and for the total coughs in the non-suppression challenge unit.These results demonstrate that capsaicin-induced cough can be voluntarily suppressed, but that both suppressed and non-suppressed cough were unaffected by treatment with codeine, naltrexone or placebo. These results do not provide any support for the hypothesis that capsaicin-induced cough is influenced by endogenous opioid substances.

The aim of the present study was to determine whether the ability to suppress cough voluntarily is an index of cough severity in upper respiratory tract infection. Cough was measured by means of a microphone linked to a pen recorder and subjects were instructed to voluntarily suppress cough in order to determine cough suppression time. Subjective scores of symptom severity, mood and psychological parameters were made prior to cough measurements. The baseline frequency of cough showed a distribution towards the higher frequencies, with a median of 2.1 (lower quartile 1.2, upper quartile 3.2) coughs.min-1. The results for cough suppression fell into two distinct groups, one group reaching a breaking point within 12.6 min; and another group which did not cough during the 20 min cough suppression period. In the group of subjects which broke from the cough suppression, there was an inverse relationship between the cough suppression time and the baseline frequency of cough. The median frequency of cough following cough suppression was significantly greater than the baseline median frequency of cough. The subjects who reached a breaking point had a greater baseline frequency of cough and a greater symptom severity score, and they also felt more feeble, clumsy, sad and antagonistic than the group which did not reach a breaking point. The subjects who reached a breaking point had significantly greater scores for the psychology parameter of obsessional symptoms than the group which did not reach a breaking point.(ABSTRACT TRUNCATED AT 250 WORDS)