Respiration Physiology 122 (2000) 61 – 69
Hypoxic depression of circadian oscillations in sino-aortic
denervated rats
Karine Fenelon, Erin L. Seifert, Jacopo P. Mortola *
Department of Physiology, McGill Uniersity, McIntyre Basic Sciences Building, Room 1121, 3655 Drummond Street, Montreal,
Que ´bec, Canada H3G 1Y6
Accepted 13 March 2000
Abstract
We hypothesized that hypoxia depresses the circadian oscillations of body temperature (T
b
) and oxygen consump-
tion (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 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. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Chemoreceptors, peripheral, circadian rhythm; Hypoxia, circadian oscillations; Mammals, rat; Metabolism, circadian
rhythm; hypoxia; Oxygen, consumption; Temperature, body, circadian rhythm, hypoxia
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1. Introduction
Physiological parameters often oscillate around
the mean value with cycles of variable periods.
Among the most studied are the cycles of body
temperature (T
b
) and oxygen consumption (V
O
2
),
which show high and low values during the daily
dark and light phases. The endogenous origin of
these rhythms can be revealed by their persistence
in absence of the daily dark–light cycle or of any
other external synchronizers, i.e. in free-running
conditions. In these cases the periods are close,
* Corresponding author. Tel.: +1-514-3984335; fax: +1-
514-3987452.
E-mail address: jacopo@med.mcgill.ca (J.P. Mortola).
0034-5687/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII:S0034-5687(00)00118-3