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Abstract
Persistent autonomic disturbances following stressful events suggest that the rhythmical
nature of homeostatic functioning may be disrupted by these experiences. We assessed
the effects of two different stressors on circadian and ultradian rhythms of Long-Evans
rats by using nonlinear multi-oscillator cosinor analysis. Heart rate and intraperitoneal
temperature were monitored continuously in 5-min intervals in two groups of animals
via radio-telemetry for 15 days after surgery (n = 9) and 15 days following social
defeat (n = 6). Circadian amplitude of heart rate and temperature increased significantly
for the first nine days of the recovery from surgery but only circadian temperature
amplitude increased following social defeat. Circadian acrophase of temperature but
not heart rate changed significantly for a similar period following the surgery but
not after the social defeat. A mathematical model incorporating the first five harmonics
of the circadian rhythm was found to fit the data significantly better than a circadian
model alone with rhythms of 3 and 5 cycles/day in temperature and heart rate entraining
significantly to the light-dark schedule. Full recovery of the circadian and ultradian
rhythms did not occur until a minimum of nine to twelve days after surgery or social
defeat. The results suggest that rhythms with multiple periodicities are involved
in homeostatic functioning and that models incorporating these rhythms may aid in
understanding an organisms adaptive response to surgical intervention and social defeat,
long after the challenges have terminated.