Hypoxic depression of circadian oscillations in sino-aortic denervated rats

We hypothesized that hypoxia depresses the circadian oscillations of body t emperature (T-b) and oxygen consumption ((V) over dot O-2) even in the abse nce of inputs from the peripheral chemoreceptors. Adult rats were sine-aort ic denervated bilaterally (SAX, N = 17) or sham-operated (Sham, N = 17). Te n rats of each group were instrumented for measurements of T-b and activity by telemetry. Animals were exposed to normoxia (21% O-2), hypoxia (10.5% O -2), and again normoxia, each for a 5-day duration, in constant light ('fre e-running') conditions. Hypoxia almost eliminated the T-b circadian oscilla tions, mostly by abolishing the daily rise in T-b. Upon return to normoxia T-b rapidly increased and the normal oscillation was reestablished at the e xpected phase of the cycle. The hypoxic effects did not differ between Sham and SAX. During hypoxia the amplitude of the circadian oscillation of acti vity was reduced by similar to 25%, and that of (V) over dot O-2, measured by an open flow method in the remaining Sham and SAX rats (N = 7 each), was reduced by almost 50%. In all cases there was no difference between the tw o groups. We conclude that activation of the peripheral chemoreceptors is n ot required for the manifestation of the hypoxic depression of the metaboli c and temperature circadian oscillations. The results are compatible with t he view that hypoxia depresses thermogenesis acting on the thermoregulatory centers of the hypothalamus. (C) 2000 Elsevier Science B.V. All rights res erved.