Capsaicin-induced reflex bronchoconstriction in the newborn

Area of Science:

• Neonatal Physiology
• Respiratory Medicine
• Neuroscience

Background:

• Previous studies suggested weak or absent bronchoconstriction to C-fibre stimulation in newborns.
• Pulmonary resistance and compliance in response to C-fibre stimulation have not been previously measured in neonates.
• C-fibre stimulation involves sensory nerve activation, which can trigger physiological responses.

Purpose of the Study:

• To test the hypothesis that capsaicin reflexly activates pulmonary efferent outflow to airway smooth muscle in newborns.
• To investigate the role of C-fibre stimulation in neonatal airway function.
• To measure changes in inspiratory lung resistance and dynamic compliance in response to capsaicin.

Main Methods:

• Breath-by-breath measurements of inspiratory lung resistance (RLi) and dynamic compliance (CLdyn) in 13 newborn dogs and 4 newborn pigs.
• Administration of saline, capsaicin, and acetylcholine (ACh) via right heart injection.
• Assessment of desensitization by repeating capsaicin injections at 5-minute intervals in newborn dogs.
• Pharmacological blockade using atropine to investigate muscarinic receptor involvement.

Main Results:

• A brisk bronchoconstriction was observed in all newborn dogs and 2 of 4 newborn pigs, with significant increases in RLi and decreases in CLdyn.
• Capsaicin injections elicited a long-latency increase in heart rate in both species, with varying effects on heart rate (tachycardia/bradycardia) after acetylcholine administration.
• Responses, including bronchoconstriction and bradycardia, were reduced with repeated capsaicin injections, indicating desensitization.
• Atropine abolished capsaicin-induced bronchoconstriction and bradycardia, but not tachycardia, suggesting a muscarinic pathway for these responses.

Conclusions:

• Newborns possess the neural circuitry to exhibit a bronchoconstrictor response to C-fibre stimulation.
• The observed bronchoconstriction may serve to reduce dead space and stabilize highly compliant airways during rapid, shallow breathing typical of C-fibre activation.
• These findings challenge previous assumptions about neonatal airway responses to C-fibre stimulation.