Waking and sleeping following water deprivation in the rat

Wake-sleep (W-S) states are affected by thermoregulation. In particular, REM sleep (REMS) is reduced in homeotherms under a thermal load, due to an impairment of hypothalamic regulation of body temperature. The aim of this work was to assess whether osmoregulation, which is regulated at a hypothalamic level, but, unlike thermoregulation, is maintained across the different W-S states, could influence W-S occurrence. Sprague-Dawley rats, kept at an ambient temperature of 24 °C and under a 12 h:12 h light-dark cycle, were exposed to a prolonged osmotic challenge of three days of water deprivation (WD) and two days of recovery in which free access to water was restored. Two sets of parameters were determined in order to assess: i) the maintenance of osmotic homeostasis (water and food consumption; changes in body weight and fluid composition); ii) the effects of the osmotic challenge on behavioral states (hypothalamic temperature (Thy), motor activity, and W-S states). The first set of parameters changed in WD as expected and control levels were restored on the second day of recovery, with the exception of urinary Ca++ that almost disappeared in WD, and increased to a high level in recovery. As far as the second set is concerned, WD was characterized by the maintenance of the daily oscillation of Thy and by a decrease in activity during the dark periods. Changes in W-S states were small and mainly confined to the dark period: i) REMS slightly decreased at the end of WD and increased in recovery; ii) non-REM sleep (NREMS) increased in both WD and recovery, but EEG delta power, a sign of NREMS intensity, decreased in WD and increased in recovery. Our data suggest that osmoregulation interferes with the regulation of W-S states to a much lesser extent than thermoregulation.