The effects of a 17-day spaceflight duration on serotonergic measures in various parts of rat brain have been studied (flight-SHAM group). The contribution of the activation of the hypothalamo-pituitary-adrenal axis (HPA) related to the response of the central serotonin system was evaluated in adrenalectomized with chronic corticosterone replacement rats (flight-ADX+CORT group). These two groups of rats were compared to their respective ground-based controls. Physiological parameters (body, adrenal and thymus weights) and corticosterone levels were measured. In flight-SHAM group as compared to controls, adrenal hypertrophy and elevation in plasma corticosterone levels (174%) were observed, without change in thymus mass. In most brain areas studied, significant decreases in TRP, 5-HTP and 5-HIAA were found associated with lower levels of 5-HT in cortex, thalamus and striatum. Conversely, there were elevations in TRP, 5-HTP levels in striatum and increases in 5-HIAA/5-HT ratios, an index of 5-HT turnover, in cortex, striatum and olfactory bulb while the hypothalamus was the sole region where a fall was observed. In ADX rats with chronic corticosterone replacement these effects were not observed in the majority of brain areas. It is concluded that a 17-day spaceflight exerted an inhibitory effect on serotonin metabolism, probably by activation of the HPA axis. The results could not distinguish between the effects of microgravity and the stress associated with landing.
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