Appearance of central dipsogenic mechanisms induced by dehydration in near-term rat fetus

Cellular dehydration of central osmoreceptors evokes an integration of beha vioral (i.e. drinking) and endocrinologic (i.e. arginine vasopressin secret ion) responses to maintain body fluid balance. These osmoregulatory mechani sms have been intensely investigated in adult models. However, there has be en limited research of the fetal development of neural mechanisms regulatin g responses to dehydration. Although behavioral and neuroendocrine response s to dehydration have been demonstrated in utero in precocial species (e.g. ovine), there has been no study to date demonstrating that these responses develop before the neonatal period of altricial species (e.g. rat). This s tudy is the first to use the near-term rat fetus to investigate the effects of maternal subcutaneous hypertonic (2 M NaCl) or isotonic (0.15 M NaCl) s aline injection on fetal plasma osmolality and brain FOS-immunoreactivity ( FOS-ir). Maternal subcutaneous hypertonic saline significantly increased ma ternal and fetal plasma osmolality to similar levels (328+/-6 and 326+/-6 m osM/kg, respectively). In response to plasma hypertonicity, maternal and fe tal brain FOS-ir increased significantly in the regions including the lamin a terminalis, and the supraoptic and paraventricular nuclei (SON and PVN) o f the hypothalamus. Together, these data indicate that central mechanisms f or dipsogenic and arginine vasopressin secretory responses to hypertonicity are present and responsive in the fetal rat brain at near-term gestation. However, differences between fetal and maternal FOS-ir mapping suggest that fetal osmoreceptor development is not yet completed near term. (C) 2000 El sevier Science B.V. All rights reserved.