Continuous circadian measurements of ventilation in behaving adult rats

Body temperature (Tb) and oxygen consumption (V(O(2))) are important determinants of ventilation (VE). While the circadian rhythms in Tb and V(O(2)) have been well described, the daily pattern of VE has not due to limitations in the available methods for measuring VE. Here we describe an adaptation of the barometric method using a chamber in which a large flow through very small restrictions was generated by the combined action of a positive pressure pump on the entrance and a negative pressure pump at the outlet. In this way the chamber effectively behaved as a closed system, despite having a high enough flow for long-term recording in freely moving, undisturbed small animals. This system was then used to test the hypothesis that VE oscillates with a circadian pattern similar to that of Tb(.) Measurements of tidal volume (VT) and breathing rate (f), in combination with Tb and activity by telemetry, were made in eight adult rats over 4-6 days under 12:12 light:dark conditions. Both VT, f, and thus VE, showed a circadian pattern similar to that of Tb and activity; that is, values were higher during the dark compared to the light phase. The differences in VE levels according to the time of the day suggest that mechanisms involved in the control of breathing may also have circadian patterns.