Tana M. Hoban-Higgins

Alterations in endogenous circadian rhythm of core temperature in senescent Fischer 344 rats

We assessed whether alterations in endogenous circadian rhythm of core temperature (CRT) in aging rats are associated with chronological time or with a biological marker of senescence, i.e., spontaneous rapid body weight loss. CRT was measured in male Fischer 344 (F344) rats beginning at age 689 days and then continuously until death. Young rats were also monitored. The rats were housed under constant dim red light at 24-26°C, and core temperature was recorded every 10 min via biotelemetry. The CRT amplitude of the body weight-stable (presenescent) old rats was significantly less than that of young rats at all analysis periods. At the onset of spontaneous rapid weight loss (senescence), all measures of endogenous CRT differed significantly from those in the presenescent period. The suprachiasmatic nucleus (a circadian pacemaker) of the senescent rats maintained its light responsiveness as determined by an increase in c- fos expression after a brief light exposure. These data demonstrate that some characteristics of the CRT are altered slowly with chronological aging, whereas others occur rapidly with the onset of senescence.

Influence of gravity on the circadian timing system

The circadian timing system (CTS) is responsible for daily temporal coordination of physiological and behavioral functions both internally and with the external environment. Experiments in altered gravitational environments have revealed changes in circadian rhythms of species ranging from fungi to primates. The altered gravitational environments examined included both the microgravity environment of spaceflight and hyperdynamic environments produced by centrifugation. Acute exposure to altered gravitational environments changed homeostatic parameters such as body temperature. These changes were time of day dependent. Exposure to gravitational alterations of relatively short duration produced changes in both the homeostatic level and the amplitude of circadian rhythms. Chronic exposure to a non-earth level of gravity resulted in changes in the period of the expressed rhythms as well as in the phase relationships between the rhythms and between the rhythms and the external environment. In addition, alterations in gravity appeared to act as a time cue for the CTS. Altered gravity also affected the sensitivity of the pacemaker to other aspects of the environment (i.e., light) and to shifts of time cues. Taken together, these studies lead to the conclusion that the CTS is indeed sensitive to gravity and its alterations. This finding has implications for both basic biology and space medicine.

Gender differences in the circadian rhythms of rhesus monkeys.

Studies investigating gender differences in human circadian rhythms report equivocal results. In addition, many of these studies have been limited to examination of one circadian variable. This study examined gender differences in circadian rhythms of multiple physiological variables of rhesus monkeys under highly controlled conditions. Under general anesthesia, eight female and seven male rhesus were implanted with a biotelemetry transmitter to measure body temperature (T(b)) and heart rate. An external accelerometer was used to measure physical activity. The Psychomotor Task System (PTS) provided environmental enrichment and delivered a pelletized diet and water was available ad libitum. Data were collected continuously under LD 16:8 for a minimum of 31days. Mean, phase and amplitude of each rhythm were calculated and compared between genders. Although there were no significant differences between genders in mean or amplitude, circadian rhythms in females were significantly delayed compared to males in all variables (p range 0.001 to 0.030). The consistent pattern of delay suggests that a fundamental gender difference may be present in the circadian timing system. Mechanisms underlying this difference require further exploration.