THE OPIOID AGONIST CODEINE AND ANTAGONIST NALTREXONE DO NOT AFFECT VOLUNTARY SUPPRESSION OF CAPSAICIN-INDUCED COUGH IN HEALTHY-SUBJECTS

Opioids exert an analgesic action by mimicking the effects of endogeno us neurotransmitter substances in the central nervous system. Opioids are widely used as antitussives, and it is reasonable to assume that e ndogenous opioids are involved in the control of cough. In order to in vestigate this hypothesis, a parallel design study was carried out to examine the effects of 50 mg codeine (opioid agonist), 50 mg naltrexon e (opioid antagonist) and placebo on capsaicin-induced cough in 80 hea lthy volunteers (mean age 25 yrs). Volunteers received two capsaicin c hallenge units (each consisting of five inhalations of different conce ntrations of capsaicin, 0.00-3.33x10(-4) M). On one challenge unit sub jects were instructed to suppress cough, and on the other challenge un it subjects coughed freely. Coughs were recorded on a tape cassette pl ayer and later played back into a pen recorder to produce integrated s ound traces. The number of coughs in the suppression challenge unit wa s significantly reduced in all three treatment groups compared to that recorded in the non-suppression challenge unit. Comparisons between t he three treatment groups showed that there was no statistical differe nce between the three groups both before and 90 min after treatment fo r the total coughs in the suppression challenge unit and for the total coughs in the non-suppression challenge unit. These results demonstra te that capsaicin-induced cough can be voluntarily suppressed, bat tha t both suppressed and non-suppressed cough were unaffected by treatmen t with codeine, naltrexone or placebo. These results do not provide an y support for the hypothesis that capsaicin-induced cough is influence d by endogenous opioid substances.