INDIRECT EVIDENCE OF CNS ADRENERGIC PATHWAYS ACTIVATION DURING SPACEFLIGHT

Background: Microgravity causes cephalad fluid shift and compensatory mechanisms. Hormonal changes suggestive of peripheral sympathetic (cat echolaminergic) nervous system activation have been recently found in astronauts during flight. Simulation Studies showed increased perivasc ular sympathetic fiber density in the rat brain. Hypothesis: Intracran ial microcirculatory adaptations might also occur in astronauts, invol ving an increase in the turnover rate of catecholamines, i.e., norepin ephrine (NE) and its precursor, Dopamine (DA). DA is known to inhibit prolactin (PRL) release and to enhance growth hormone (GH) secretion b y the pituitary. Therefore, increased brain dopaminergic activity woul d result into lower circulating PRL concentrations. At the same time, plasma levels of GH and of its effector insulin-like growth factor-1 ( IGF1)would increase during flight. Methods: Circulating cortisol (CS), PRL, GH and ICF-I levels were measured 2 d preflight, inflight (4-5 d after launch) and on different days postflight in four astronauts inv olved in the Spacelab D-2 mission. Results: No significant changes wer e found in CS concentrations. PRL decreased while GH and IGF-l increas ed inflight (p < 0.05). After flight no statistically relevant hormona l changes were found with respect to preflight. Conclusion: The observ ed hormonal changes were consistent with the original hypothesis that spaceflight might activate CNS adrenergic pathways. They occurred in t he absence of two typical markers of stress, namely CS and PRL increas e, thus ruling out any non-specific effect of acute stress on the resu lts. in agreement with the most recent results of real and simulated m icrogravity studies performed in both the experimental animal and in m an, these data lend support to the hypothesis that the CNS adrenergic pathways are also activated in the human during spaceflight.