Postnatal development of right atrial injection of capsaicin-induced apneic response in rats

Abstract

Apnea and respiratory failure often occur in infants with pulmonary disease. Bronchopulmonary C-fiber (PCF)-mediated apnea is an important component of respiratory dysfunction. This study was undertaken to define the postnatal development of PCF-mediated apnea. The experiments were conducted in five groups of anesthetized, tracheotomized, and spontaneously breathing rats with ages at postnatal days P1–3, P7–9, P14–16, P21–23, and P56–58. Right atrial bolus injection of three doses of capsaicin (Cap), equivalent to 2, 4, and 8 μg/kg used previously in 450-g rats, was applied to stimulate PCFs. We found that 1) Cap-induced apneic response [percent change from the baseline expiratory duration (Te) values (ΔTe%)] and the sensitivity of this response (ΔTe%·μg−1) were significantly greater in the rats <P10 than those >P10; 2) the Cap-induced apneas were vagally dependent in all rats tested; and 3) bivagotomy-induced prolongation of Te was much greater in the rats <P10 than those >P10. From these findings we concluded that, compared with the older rats (>P10), the newborn rats have a stronger PCF-mediated respiratory inhibition that may contribute to infants' vulnerability to respiratory failure.