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How Time Flies - The Effects of Spaceflight on the Circadian Timing System.  C.A. Fuller.  University of California, Davis, CA.

   Altered circadian function has been documented in species ranging from fungi to humans during exposure to the microgravity of spaceflight.  Such observations include changes in all of the fundamental properties of the circadian clock, including:  endogenous period, internal and external phases, and photic sensitivity.  One of the most extensive demonstrations of such changes occurred during NASA STS-90 Neurolab Spaceflight , which provided a unique opportunity to evaluate the direct impact of microgravity on the circadian timing system and the circadian pacemaker.  In this study, rats were exposed to 16 days of microgravity.  These rats were chronically implanted with biotelemetry transmitters to record body temperature (T_b) and heart rate (HR).  Six of the rats were exposed to constant light (LL) and 18 of the rats were exposed to a 24-hr light-dark cycle (LD 12:12).  The ability of light to induce c-Fos in the suprachiasmatic nucleus (SCN), the mammalian circadian pacemaker, was examined in animals sacrificed on flight day 2 (FD2), FD14, recovery day 2 (R1) and R13.  The flight rats in LL exhibited persisting free-running rhythms, with a change in circadian period relative to ground controls.  The period changes returned to control values one week after landing.  The flight animals in LL maintained internal phase angle relationships between rhythms compared with ground controls.  The flight rats in LD remained entrained to the LD cycle; however, they evidenced a pronounced phase delay in T_b, but not HR compared to controls.  The flight rats also demonstrated a decrease in T_b and a change in the daily waveform compared to controls.  Finally, early in-flight, the flight animals demonstrated a reduced sensitivity to light as evidenced by a highly attenuated c-Fos immunoreactivity compared to controls.  These findings suggest and further, corroborate with previous space-flight data, that microgravity affects the circadian clock, including the clock’s ability to maintain temporal organization and to properly entrain to an external LD cycle.  More recent studies, conducted in ground-based experiments, suggest that most, if not all, of these circadian responses are a result of altered vestibular signaling.