Recurrent aggressive episodes entrain ultradian heart rate and core temperature rhythms

The present study was designed to investigate the effects of recurrent aggressive episodes on the synchrony of autonomic circadian and ultradian rhythms. Eight aggressive male rats were entrained to a reverse 12 h:12 h light-dark cycle and then implanted with telemetry senders to continuously monitor heart rate (HR) and core temperature (Tc). The amplitude and the time of the peak (acrophase) for each of the circadian and ultradian oscillations were quantified by nonlinear, least-squares, multioscillator cosinor analysis that included the first four harmonics of the circadian rhythm. After recovery from surgery, the 3- and 5-cycle/day ultradian rhythms of HR and Tc were the prominent ultradian components that were synchronized to the light-dark cycle. First, the resident males confronted a male intruder daily at lights-off (0800 hours) for a period of 3 weeks. Second, after a 3-week recovery period, 15 daily aggressive confrontations were scheduled, with the intruders being introduced at 1200 hours. During the course of the confrontations the amplitude of 3- and 5-cycle/day oscillations in HR and Tc decreased, whereas the hemicircadian (2 cycles/day) rhythm amplitude doubled with minor changes of the circadian amplitude. The hemicircadian acrophase coincided with the time of the confrontation most clearly, and this alignment lasted for more than 1 week after the last social confrontation, even in the absence of a reminder. We interpret the synchronization of the hemicircadian acrophases to the time point of social confrontations as anticipating the physiological demands of the aggressive encounters.