The effect of placebo conditioning on capsaicin-evoked urge to cough.
BACKGROUND
The urge to cough is a clinical symptom of respiratory disease that precedes the motor act of coughing. Although previous studies have shown that cough is particularly susceptible to placebo suppression, it is unclear whether the perception of an urge to cough is also modifiable by placebo. Therefore, we tested the hypothesis that capsaicin-evoked urge to cough could be suppressed by placebo conditioning.


METHODS
Eleven healthy participants were unknowingly conditioned to believe that an inert inhaler temporarily suppressed capsaicin-induced urge to cough by deceptively modifying the challenge concentration of capsaicin. In subsequent testing, capsaicin-evoked urge-to-cough subjective ratings were assessed in four challenges with a single dose of inhaled capsaicin following no treatment or the placebo metered-dose inhaler. An additional 10 participants were informed that the inhaler therapy was inert prior to receiving capsaicin challenges with and without inhaler treatment.


RESULTS
There was a significant decrease in mean urge-to-cough ratings to capsaicin challenge following placebo compared with no treatment followed by capsaicin challenge (P < .001), with a peak decrease of 45%. The placebo inhaler alone had no effect on urge-to-cough subjective ratings when participants were aware that it contained no active medication.


CONCLUSIONS
These data confirm that the urge to cough is susceptible to placebo inhibition. This provides further evidence that higher brain networks are involved in the processing of respiratory sensations related to airway irritation.
School of Biomedical Sciences, University of Queensland, St. Lucia, QLD; Florey Neuroscience Institutes, University of Melbourne, VIC, Australia.
Jennifer
J
Leech
School of Biomedical Sciences, University of Queensland, St. Lucia, QLD. Electronic address: s.mazzone@uq.edu.au.
Stuart B
SB
Mazzone
Florey Neuroscience Institutes, University of Melbourne, VIC, Australia.
Michael J
MJ
Farrell
D000280:Administration, Inhalation; D000328:Adult; D002211:Capsaicin / Q000008:administration & dosage* / Q000009:adverse effects; D003371:Cough / Q000139:chemically induced / Q000188:drug therapy* / Q000503:physiopathology; D018450:Disease Progression; D005260:Female; D006801:Humans; D008297:Male; D011379:Prognosis; D012016:Reference Values; D012018:Reflex / Q000187:drug effects*; D015656:Respiratory Mechanics / Q000187:drug effects*; D012189:Retrospective Studies; D018689:Sensory System Agents / Q000008:administration & dosage
D018689:Sensory System Agents; D002211:Capsaicin