Respiratory effects of capsaicin occur beyond the lung vagi in anaesthetized rats

The effects of an intravenous capsaicin challenge on the respiratory pattern and ventilation were studied in 15 urethane/chloralose-anaesthetized, spontaneously breathing rats. Bolus injection of capsaicin at a dose of 5 micrograms/kg into the right femoral vein evoked respiratory arrest in all animals (both prior to and after bilateral midcervical vagotomy), which effect was abolished by ruthenium red pretreatment. Breathing that followed the apnoea was of enlarged tidal volume and initially increased respiratory rate, which resulted in an augmented ventilation. The capsaicin-induced respiratory changes were independent of vagal integrity and may depend on stimulation of vanilloid receptors within the nodose ganglia.