Dehydration followed by sham rehydration contributes to reduced neuronal activation in vasopressinergic supraoptic neurons after water deprivation. Abbreviated title: Sham rehydration and the SON Authors: W

This experiment tested the role of oropharyngeal and gastric afferents on hypothalamic activation in dehydrated rats instrumented with gastric fistulae and allowed to drink water or isotonic saline compared with euhydrated controls (CON). Rats were water deprived for 48h (48 WD) or 46h WD with 2h rehydration with water (46+W) or isotonic saline (46+S). 46+W and 46+S rats were given water with fistulae open (46+WO/46+SO, sham) or closed (46+WC/46+SC). Compared to CON, water deprivation increased and water rehydration decreased plasma osmolality while sham rehydration had no effect. Water deprivation increased Fos staining in the lamina terminalis. However, none of the sham or rehydration treatments normalized Fos staining in the lamina terminalis. Analysis of AVP and Fos positive neurons in the SON revealed reduced co-localization in 46+WO and 46+SC rats compared to 48 WD and 46+SO rats. However, 46+WO and 46+SC rats had higher Fos staining in the SON than 46+WC or CON rats. Examination of Fos in the perinuclear zone (PNZ) revealed that sham and rehydrated rats had increased Fos staining to CON while 48 WD and 46+SO rats had little or no Fos staining in this region. Thus, pre-absorptive reflexes contribute to the regulation of AVP neurons in a manner independent of Fos expression in the lamina terminalis. Further, this reflex pathway may include inhibitory interneurons in the PNZ region surrounding the SON.