Hypoxic depression of circadian oscillations in sino-aortic denervated rats

We hypothesized that hypoxia depresses the circadian oscillations of body temperature (T(b)) and oxygen consumption (V(O(2))) even in the absence of inputs from the peripheral chemoreceptors. Adult rats were sino-aortic denervated bilaterally (SAX, N=17) or sham-operated (Sham, N=17). Ten 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 ('free-running') conditions. Hypoxia almost eliminated the T(b) circadian oscillations, 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 expected phase of the cycle. The hypoxic effects did not differ between Sham and SAX. During hypoxia the amplitude of the circadian oscillation of activity was reduced by approximately 25%, and that of V(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 two groups. We conclude that activation of the peripheral chemoreceptors is not required for the manifestation of the hypoxic depression of the metabolic and temperature circadian oscillations. The results are compatible with the view that hypoxia depresses thermogenesis acting on the thermoregulatory centers of the hypothalamus.