Respiratory effects of intrathecal capsaicin in arthritic and non-arthritic rats

The study determined the effects of intrathecal injection of 50 micrograms of capsaicin on respiration in rats with adjuvant arthritis as well as in control animals. Whole body plethysmographic measurements of steady-state frequency, tidal volume, and minute volume of respiration were made 3 hours and for up to 11 days after intrathecal injection. Capsaicin increased minute volume within 3 hours of its intrathecal injection in control animals. Intrathecal capsaicin also reduced the respiratory response to adjuvant arthritis in the experimental animals; the latter effect was apparent 11 days after injection. This biphasic pattern of capsaicin effects is consistent with a possible role of substance P in the chronic pain which is presumably associated with adjuvant arthritis in the rat.