Continuous circadian measurements of ventilation in behaving adult rats

Body temperature (Tb) and oxygen consumption (V-O2) are important determina nts of ventilation (VE). While the circadian rhythms in Tb and V-O2 have be en well described, the daily pattern of VE has not due to limitations in th e available methods for measuring be. 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 p ump 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 how for long-term recording in freely moving, undisturbed small anim als. This system was then used to test the hypothesis that VE oscillates wi th a circadian pattern similar to that of Tb Measurements of tidal volume ( VT) and breathing rate (f), in combination with Tb and activity by telemetr y, were made in eight adult rats over 4-6 days under 12:12 light:dark condi tions. Both VT, f, and thus ire, showed a circadian pattern similar to that of Tb and activity; that is, values were higher during the dark compared t o the light phase. The differences in ire levels according to the time of t he day suggest that mechanisms involved in the control of breathing may als o have circadian patterns. (C) 2000 Elsevier Science B.V. All rights reserv ed.