Age-related changes in the ventilatory response to inspired CO2 in neonatal rats

The objective of this study was to determine whether there is an age-related ventilatory response to transient increases in inspired CO2 in unanesthetized rat pups. Using plethysmography, ventilatory responses to 30 sec of 0, 2, 4, 6, and 8% inspired CO2 were measured in 21 rat pups from two litters. Recordings were made 1, 2, 3, 5, 7, 9 and 12 days after the day of birth (day 0). On day 1 there was a significant dose-related decrease in mean ventilatory frequency in response to each of the inspired CO2 concentrations. On day 2 there was no significant change in breathing frequency in response to 2 or 4% CO2 and a significant increase in frequency in response to 6 and 8% CO2. On days 3, 5, 7, 9 and 12 there was generally a significant increase in frequency in response to each of the inspired CO2 concentrations. Tidal volume was not significantly affected by the CO2 stimuli on any of the test days. Minute ventilation exhibited a significant decrease, on day 1, in response to 6 and 8% CO2. Litter, sex or weight of the rat pups was not correlated with the ventilatory depressions observed on day 1. These results show that in neonatal rats the ventilatory response to inspired CO2 is age-related and indicates a possible link between upper airway CO2 chemoreceptors, an inhibition of breathing, and SIDS.