Space flight affects magnocellular supraoptic neurons of young prepuberal rats: transient and permanent effects

Effects of microgravity on postural control and volume of extracellular flu ids as well as stress associated with space flight may affect the function of hypothalamic neurosecretory neurons. Since environmental modifications i n young animals may result in permanent alterations in neuroendocrine funct ion, the present study was designed to determine the effect of a space flig ht on oxytocinergic and vasopressinergic magnocellular hypothalamic neurons of prepuberal rats. Fifteen-day-old Sprague-Dawley female rats were flown aboard the Space Shuttle Columbia (STS-90, Neurolab mission, experiment 150 ) for 16 days. Age-matched litters remained on the ground in cages similiar to those of the flight animals. Six animals from each group were killed on the day of landing and eight animals from each croup were maintained under standard vivarium conditions and killed 18 weeks after landing. Several si gns of enhanced transcriptional and biosynthetic activity were observed in magnocellular supraoptic neurons of flight animals on the day of landing co mpared to control animals. These include increased c-Fos expression, larger nucleoli and cytoplasm, and higher volume occupied in the neuronal perikar yon by mitochondriae, endoplasmic reticulum, Golgi apparatus, lysosomes and cytoplasmic inclusions known as nematosomes. In contrast, the volume occup ied by neurosecretory vesicles in the supraoptic neuronal perikarya was sig nificantly decreased in flight rats. This decrease was associated with a si gnificant decrease in oxytocin and vasopressin immunoreactive levels, sugge stive of an increased hormonal release. Vasopressin levels, cytoplasmic vol ume and c-Fos expression returned to control levels by 18 weeks after landi ng. These reversible effects were probably associated to osmotic stimuli re sulting from modifications in the volume and distribution of extracellular fluids and plasma during flight and landing. However, oxytocin levels were still reduced at 18 weeks after landing in flight animals compared to contr ols. This indicates that space flight during prepuberal age may induce irre versible modifications in the regulation of oxytocinergic neurons, which in turn may result in permanent endocrine and behavioral impairments. (C) 200 1 Elsevier Science B.V. All rights reserved.