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Apnea frequency and the weak ventilatory response to hypoxia are a major clinical correlates of the immaturity of respiratory control system in preterm neonates. Rats are frequently used as model to study the respiratory control during development. However, little is known about the postnatal ages that best represent these respiratory irregularities and the hypoxic ventilatory response. Using plethysmography, we assessed baseline minute ventilation, ventilatory response to moderate hypoxia (FiO2 = 12%, 20 min) and apnea frequency in awake and non-anesthetized rats at the postnatal ages of 1, 4, 7, 12, 21 and 90 days old (P1, P4, P7, P12, P21, and P90, respectively). Baseline minute ventilation slightly increased in P4 (~25% vs P1) then gradually decreased with age (age effect: p < 0.05). The lowest level of ventilation was observed in P90 (p < 0.01 vs all ages). Minute ventilation (% from baseline) in response to hypoxia showed the well-known biphasic pattern in all rats at 12 days old or less. Minute ventilation at the initial phase of the hypoxic response was not significantly different between P1, P4, between P7, P12 and between P21, P90. The late phase of the hypoxic response was similar between P1, P4, and between P21, P90, but was significantly different between P7 and P12 (p < 0.05). Under baseline or hypoxic condition, the higher number of apnea frequency (spontaneous and post- sigh) was observed in P1, it then decreased progressively with age (age effect: p < 0.01 for baseline; p < 0.001 for hypoxia). These results suggest that when P4, P7 and P12 are selected to represent the age-dependent changes of the hypoxic ventilatory response in rats, the P1 rats should be included to better describe the age-dependence of apnea frequency

Selecting representative ages for developmental changes of respiratory irregularities and hypoxic ventilatory response in rats