Six flight rats and ground control rats (six flight simulated and six vivarium controls) were utilized in one set of experiments. Twelve flight rats and two sets of ground control rats (12 simulated and 12 vivarium) were used in another set of experiments. The rodents were implanted with biotelemetry units to monitor circadian rhythms of body temperature, heart rate, and activity. In addition, eating and drinking was monitored for all rats. To examine the space flight effect on circadian and homeostatic regulation of physiological and behavioral functions, rats were exposed to 24-hour light/dark cycles (LD) consisting of 12 hours of light (approximately 30 lux) followed by 12 hours of darkness. The circadian rhythms and daily mean of rat body temperature, heart rate, feeding, drinking and activity (as a measure of homeostasis) were compared. To examine the space flight effect on free-running circadian rhythm characteristics, a subset of rats was maintained in a constant light condition (LL). Data were analyzed to determine circadian rhythms and results were compared to those of rats exposed to 24 -hour light/dark cycles. To examine the space flight effect on the pattern of c-Fos synthesis in hypothalamic nuclei, and whether it alters the effect of light pulses to induce c-Fos expression in the central nervous system, one group of rats received a one-hour pulse of light during the dark cycle. Another group was not exposed to a pulse of light. The brains and eyes from both groups were then removed and sections of the hypothalamus were histochemically stained for c-Fos reactive neurons.

Results:

The LL flight rats exhibited an increase (p<0.001) in free-running period of body temperature and heart rate relative to controls. The periods returned to preflight values after landing. The LL flight animals maintained internal phase angle relationships between rhythms compared with controls. The LD flight rats remained entrained to the LD cycle; however, they evidenced a pronounced phase delay in body temperature, suggesting an increase in period, compared to controls. The LD flight rats also demonstrated a decrease in body temperature and a change in the daily waveform compared to controls. Both the LD and LL flight rats and controls exhibited an increase in heart rate, suggesting a possible caging effect. Finally, the Flight Day 2 flight animals demonstrated a reduced sensitivity to light as evidenced by highly attenuated c-Fos immunoreactivity in the SCN compared to controls. The sensitivity to light of the flight animals returned to preflight and control levels by Flight Day 14. These findings demonstrate that microgravity affects the circadian clock, including the clock�s ability to maintain temporal organization and to properly entrain to an external LD cycle.