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It has been known for many years that weightlessness induces changes in numerous physiological systems: the cardiovascular system declines in both aerobic capacity and orthostatic tolerance; there is a reduction in fluid and electrolyte balance, hematocrit, and certain immune parameters; bone and muscle mass and strength are reduced; various neurological responses include space motion sickness and posture and gate alterations. These responses are caused by the hypokinesia of weightlessness, the cephalic fluid shift, the unloading of the vestibular system, stress, and the altered temporal environment.

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NASA: Over the last five years, with the advent of flights of U.S. Shuttle/Spacelab missions dedicated entirely to life sciences research, the opportunities for conducting serious studies that use a fully outfitted space laboratory to better understand basic biological processes have increased. The last of this series of Shuttle/Spacelab missions, currently scheduled for 1998, is dedicated entirely to neuroscience and behavioral research. The mission, named Neurolab, includes a broad range of experiments that build on previous research efforts, as well as studies related to less mature areas of space neuroscience. The Neurolab mission provides the global scientific community with the opportunity to use the space environment for investigations that exploit microgravity to increase our understanding of basic processes in neuroscience. The results from this premier mission should lead to a significant advancement in the field as a whole and to the opening of new lines of investigation for future research. Experiments under development for this mission will utilize human subjects as well as a variety of other species. The capacity to carry out detailed experiments on both human and animal subjects in space allows a diverse complement of studies that investigate functional changes and their underlying molecular, cellular, and physiological mechanisms. In order to conduct these experiments, a wide array of biomedical instrumentation will be used, including some instruments and devices being developed especially for the mission.^ieng

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BACKGROUND: Biological clocks time many physiological parameters with periodicities close to 24 h; those which persist in the absence of environmental cues are circadian. An earlier shuttle experiment (STS-9) examined circadian pacemaker function and growth rate of Neurospora crassa and demonstrated damped rhythm amplitudes, increased variability in period lengths and altered growth rates. HYPOTHESIS: Postflight studies suggested that accelerative forces of launch could have induced rhythm alterations. Differences in growth rate may have been due to an alteration of metabolic rate. METHODS: Race tubes inoculated with bd or csp strains were flown aboard STS-32, exposed to ambient mid-deck temperatures throughout flight, and exposed to light only during marking procedures. Period, rhythm amplitude, and growth rate were determined and compared to orbital environmental controls (OES) and 25 degrees C ground controls (GC). RESULTS: Unlike the previous flight exposurement, the rhythm persisted quite normally. bd flight and OES cultures each displayed lengthened periods of a similar magnitude when compared to GC. The lengthened periods of csp flight cultures while longer than GC, were shorter than OES. Shuttle temperatures were relatively warm, however the increased period length in space was greater than predicted by the known Q10. Growth rates also increased substantially during flight, which could not be accounted for by thermal mechanisms alone. CONCLUSION: It is likely that some of the cultures may have entrained to the unexpected diurnal temperature variations; however, other cultures did not entrain, yet retained rhythmicity with increased periods. The results also suggest an increased metabolic rate during spaceflight.

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Prolonged bed rest, undertaken by volunteers or resulting from injury and disease, can impair bone and muscle function and structure; extended travel in space also induces these effects. Fluid shifts and disrupted fluid balance may also contribute to observed musculoskeletal aberrations in the weightless environment. Some molecular and cellular events involved in the loading and unloading of the musculoskeletal system are under neural and endocrine influence or control, whereas other events are influenced by local growth factors. Studies are in progress to develop interventions that preserve or improve musculoskeletal integrity in 1g. The NIAMS and NASA are interested in basic and clinical studies of the influence of microgravity on the musculoskeletal system. The interagency workshop results form the basis for new collaborative and cooperative research emphases for the biomedical community under a broad agreement between the National Institutes of Health and NASA.

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This study examines the activity, axillary temperature (T(ax)), and ankle skin temperature (Tsk) of two male Rhesus monkeys exposed to microgravity in space. The animals were flown on a Soviet biosatellite mission (COSMOS 1514). Measurements on the flight animals, as well as synchronous flight controls, were performed in the Soviet Union. Additional control studies were performed in the United States to examine the possible role of metabolic heat production in the T(ax) response observed during the spaceflight. All monkeys were exposed to a 24-h light-dark cycle (LD 16:8) throughout these studies. During weightlessness, T(ax) in both flight animals was lower than on earth. The largest difference (0.75 degree C) occurred during the night. There was a reduction in mean heart rate and Tsk during flight. This suggests a reduction in both heat loss and metabolic rate during spaceflight. Although the circadian rhythms in all variables were present during flight, some differences were noted. For example, the amplitude of the rhythms in Tsk and activity were attenuated. Furthermore, the T(ax) and activity rhythms did not have precise 24.0 hour periods and may have been externally desynchronized from the 24-h LD cycle. These data suggest a weakening of the coupling between the internal circadian pacemaker and the external LD synchronizer.

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Future research in the neurosciences can best be understood in the context of NASA's life sciences goals in the near term (1990-95), mid term (1995-2000), and long term (2000 and beyond). Since NASA is planning short-duration Spacelab and International Microgravity Laboratory (IML) flights for many years to come, the acute effects of exposure to microgravity will continue to be of experimental and operational interest in the near term. To this end, major new areas of research will be devoted to ground-based studies of preflight adaptation trainers and their efficacy in preventing or reducing the incidence of space motion sickness. In addition, an extensive series of studies of the vestibular system will be conducted inflight on the IML-1 mission The IML-2 mission will emphasize behavior and performance, biological rhythms, and further vestibular studies. In the mid-term period, Spacelab missions will employ new technology such as magnetic recording techniques in order to evaluate changes in the processing of sensory and motor inputs at the brainstem and cortical level during exposure to microgravity. Two Space Life Sciences (SLS) missions planned for the mid to late 1990's, SLS-4 and SLS-5, will utilize an onboard centrifuge facility that will enable investigators to study the effects of partial gravity on sensory and motor function. In the long term (2000 and beyond), Space Station Freedom and long-duration missions will provide opportunities to explore new options in the neurosciences, such as sensory substitution and augmentation, through the use of physical sensors to provide three-dimensional tactile-visual, tactile-auditory and tactile-somatosensory inputs. The use of this technology will be extremely important in the area of robotic telepresence. Finally, Space Station Freedom and proposed LifeSat missions will provide neuroscientists the opportunity to study the effects of partial gravity and microgravity on neuronal plasticity.

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In an effort to determine the inductive component(s) of photic input in long day seasonal breeders, adult male Syrian hamsters (Mesocricetus auratus) were exposed to one of nine lighting conditions for a duration of 10 weeks: a light-dark cycle of 14 hours of light followed by 10 hours of dark (LD 14:10, a long photoperiod); LD 10:14 (a short photoperiod); a high frequency light-dark cycle of 1 hour of light and 1 hour of dark (LD 1:1); a higher frequency light-dark cycle of 1 minute of light and 1 minute of dark (LD 1m:1m); constant light (LL); constant dark (DD); feedback lighting (LDFB; a condition that illuminates the cage in response to locomotor activity); a feedback lighting neighbor control (LDFB NC; the animal receives the same light pattern as a paired animal in LDFB, but has no control over it); or reverse feedback lighting (rLDFB; a condition that darkens an illuminated cage in response to locomotor activity). Exposure to LL, LD 1:1, LD 1m:1m, LDFB and rLDFB significantly and similarly lengthened the free-running period of the locomotor rhythm when compared to the period of animals in DD. The paired tests and accessory reproductive glands weights, spermiogenesis, seminiferous tubule diameter and serum concentrations of testosterone, prolactin, LH and FSH, suggest that LD 14:10, LL, LD 1:1, rLDFB and LDFB NC maintain reproductive function in the Syrian hamster, while LD 10:14, DD, LD 1m:1m and LDFB do not. It is known that as little as two 1-second pulses of light are stimulatory if coincident with the subjective night (17.22). Thus, it is not surprising that LD 1:1 is stimulatory. LD 1m:1m is not stimulatory, however, despite an identical quanta of light per 24 hours and similar phase relationship. It appears that mere light exposure during the subjective night is not necessarily reproductively inductive. It would also appear that behaviorally generated light-dark cycles can be (i.e., LDFB), but are not necessarily (i.e., rLDFB) inhibitory to the maintenance of the reproductive system in long day breeders. Furthermore, the lighting pattern derived from LDFB is stimulatory if given exogenously (i.e., LDFB NC). Although it is not understood why light exposure that is coincident with the subjective night (i.e., LD 1m:1m and LDFB) is not stimulatory in long day breeders, a possible hypothesis is that an internal coincidence model is involved in the photoperiodic response and that multiple transitions during the subjective night may cause a dissociation of internal oscillations which must be in phase for light to be stimulatory.

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The circadian rhythm of conidiation in Neurospora crassa is thought to be an endogenously derived circadian oscillation; however, several investigators have suggested that circadian rhythms may, instead, be driven by some geophysical time cue(s). An experiment was conducted on space shuttle flight STS-9 in order to test this hypothesis; during the first 7-8 cycles in space, there were several minor alterations observed in the conidiation rhythm, including an increase in the period of the oscillation, an increase in the variability of the growth rate and a diminished rhythm amplitude, which eventually damped out in 25% of the flight tubes. On day seven of flight, the tubes were exposed to light while their growth fronts were marked. Some aspect of the marking process reinstated a robust rhythm in all the tubes which continued throughout the remainder of the flight. These results from the last 86 hours of flight demonstrated that the rhythm can persist in space. Since the aberrant rhythmicity occurred prior to the marking procedure, but not after, it was hypothesized that the damping on STS-9 may have resulted from the hypergravity pulse of launch. To test this hypothesis, we conducted investigations into the effects of altered gravitational forces on conidiation. Exposure to hypergravity (via centrifugation), simulated microgravity (via the use of a clinostat) and altered orientations (via alterations in the vector of a 1 g force) were used to examine the effects of gravity upon the circadian rhythm of conidiation.

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Mammals have evolved under the influence of many selective pressures. Two of these pressures have been the static force of gravity and the daily variations in the environment due to the rotation of the earth. It is now clear that each of these pressures has led to specific adaptations which influence how organisms respond to changes in either gravity or daily time cues. However, several unpredicted responses to altered gravitational environments occur within the homeostatic and circadian control systems. These results may be particularly relevant to biological and medical issues related to spaceflight. This paper demonstrates that the homeostatic regulation of rat body temperature, heart rate, and activity become depressed following exposure to a 2 G hyperdynamic field, and recovers within 5-6 days. In addition, the circadian rhythms of these same variables exhibit a depression of rhythm amplitude; however, recovery required a minimum of 7 days.

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Feedback lighting provides illumination primarily during the subjective night (i.e., the photosensitive portion of the circadian cycle) in response to a given behavior. This technique has previously been used to test the nonparametric model of entrainment in nocturnal rodents. In three species (Rattus norvegicus, Mesocricetus auratus, and Mus musculus), the free-running period of the locomotor activity rhythm was similar whether the animals were exposed to continuous light or discrete light pulses occurring essentially only during the subjective night (i.e., feedback lighting). In the current experiments, feedback lighting was presented to squirrel monkeys so that light fell predominantly during the subjective night. Feedback lighting was linked to the drinking behavior in this diurnal primate so that when the animal drank, the lights went out. Despite the seemingly adverse predicament, the monkeys maintained regular circadian drinking rhythms. Furthermore, just as the period of the free-running activity rhythms of nocturnal rodents exposed to continuous light or feedback lighting were similar, the period of the drinking rhythms of the squirrel monkeys in continuous light and feedback lighting were comparable (25.6 +/- 0.1 and 25.9 +/- 0.1 hours, respectively), despite a substantial decrease in the total amount of light exposure associated with feedback lighting. The free-running period of monkeys exposed to continuous dark (24.5 +/- 0.1 hours) was significantly shorter than either of the two lighting conditions (P < 0.001). The results presented for the drinking rhythm were confirmed by examination of the temperature and activity rhythms. Therefore, discrete light pulses given predominately during the subjective night are capable of simulating the effects of continuous light on the free-running period of the circadian rhythms of a diurnal primate. The response of squirrel monkeys to feedback lighting thus lends further support for the model and suggests that the major entrainment mechanisms are similar in nocturnal rodents and diurnal primates.

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We examined light effects on the circadian timing system of the squirrel monkey. A phase-response curve to 1-h pulses of light was constructed for the drinking rhythm of six animals. The phase-response curve was the same type as that exhibited by nocturnal rodents, with phase delays occurring early in the subjective night and phase advances late in the subjective night. The range of entrainment of 10 monkeys to days with 1 h light and x h dark was determined. Five monkeys used to generate the phase-response curve were also used in the range of entrainment determination. For short light-dark cycles the range of entrainment was smaller than that expected, with no monkey entraining to a day length of less than 23.5 h.

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The rhythms of drinking and body temperature of 4 male owl monkeys (Aotus trivirgatus) were examined under conditions of LD 12:12 (L = 100 lx, D = 0.1 lx), DD (0.1 lx) and LL (100 lx). For all 4 monkeys, the circadian pattern expressed in LD 12:12 continued in DD, with a free-running period averaging 23.6 hr. In LL the circadian component of both rhythms decayed and, in one monkey, a low frequency pattern arose. In at least two aspects, masking and persistence, the owl monkey circadian timing system appears to be unlike that of its diurnal relative, the squirrel monkey. Circadian rhythms of owl monkeys also differ in some respects from those of other nocturnal mammals.

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To study heat production and heat loss in determination of the daily body temperature rhythm, we examined colonic temperature, skin (tail, foot and abdomen) temperatures and oxygen consumption in chair-restrained squirrel monkeys maintained in isolation in an environmental chamber with a 24-hr light-dark cycle (LD 12:12), maintained at a constant thermoneutral temperature (26 degrees C). In all experiments repeated high amplitude (2 degrees C) diurnal rhythms in colonic temperature were observed. Heat loss, estimated from changes in skin temperature, also displayed a circadian rhythm, although there was considerable variation in waveform. On average, a rhythm in heat production, indicated by changes in the rate of oxygen consumption, was also present. However, a large degree of variability was seen in oxygen consumption, and in several cycles from various animals there were no observable 24-hr rhythms. The circadian body temperature rhythm is thus not simply a consequence of daily changes in metabolism, but rather a regulated response that involves both heat production and heat loss.

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To test the functioning of circadian rhythms removed from periodicities of the earth's 24-hour rotation, the conidiation rhythm of the fungus Neurospora crassa was monitored in constant darkness during spaceflight. The free-running period of the rhythm was the same in space as on the earth, but there was a marked reduction in the clarity of the rhythm, and apparent arrhythmicity in some tubes. At the current stage of analysis of our results there is insufficient evidence to determine whether the effect seen in space was related to removal from 24-hour periodicities and whether the circadian timekeeping mechanism, or merely its expression, was affected.

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In order to evaluate the function of the circadian timing system in space, the circadian rhythm of conidiation of the fungus Neurospora crassa was monitored in constant darkness on the STS 9 flight of the Space Shuttle Columbia. During the first 7 days of spaceflight many tubes showed a marked reduction in the apparent amplitude of the conidiation rhythm, and some cultures appeared arrhythmic. There was more variability in the growth rate and circadian rhythms of individual cultures in space than is usually seen on earth. The results of this experiment indicate that while the circadian rhythm of Neurospora conidiation can persist outside of the Earth's environment, either the timekeeping process or its expression is altered in space.

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In the absence of other environmental cycles, daily variations in auditory stimuli are normally not capable of entraining the circadian rhythms of drinking behavior in the squirrel monkey (Saimiri sciureus). However, the drinking rhythm appears to become entrainable by previously ineffective auditory cues after lesions are placed which destroy only the caudal portion of the hypothalamic suprachiasmatic nuclei. The results suggest specificity of function within the SCN and an increased influence of auditory stimuli in animals with impaired SCN function.

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