Twenty-four hours, or five days, of continuous sleep deprivation or experimental sleep fragmentation do not alter thirst or motivation for water reward in rats

Abstract

Sleep disruption results in an increased demand for energy, which typically causes hyperphagia in an attempt to redress the energy metabolism imbalance. Therefore, experiments combining food reward and sleep disruption may underestimate the effect of sleep disruption due to their contradictory influences on behavior (for example on operant measures of attention). In contrast, water is not a central component of energy metabolism and thus thirst may not be affected by sleep disruption. However, little work has been done examining the effect of sleep disruption on thirst and motivation for water. The effect of total sleep deprivation (SD) and experimental sleep fragmentation (SF) on thirst and motivation for water was assessed. In experiment 1 (using 22 month old male Fisher-Norway rats) the amount of water consumed during a 15 min period immediately following a period of 24 h SD or SF (in which water was not available) was measured, and, in a separate session, the amount of water consumed during the 24 h of SD or SF was measured. Thereafter, the effect of 5 days SD or SF on motivation for water was assessed with the progressive ratio task (using water reward), which is widely used to assess motivation. Experiment 2 (using 6 month, and 22 month, old male Sprague- Dawley rats) followed an identical design except that the SF condition was dropped (due to a lack of any difference between the SD and SF conditions in experiment 1), and only the 6 month old rats experienced the full 5 day SD condition. Daily measurements of body weight and food consumption were recorded in experiment 2 in order to confirm previously published findings that food consumption goes up and body weight declines in sleep deprived rats. In both experiments the quantity of water rats consumed during a 15 min period immediately following the 24 h period of sleep disruption, or consumed during the 24 h period of SD or SF, did not change compared to control rats. Furthermore, 5 days of SD or SF had no effect on breakpoint in the progressive ratio task indicating that 5 days of SD or SF did not alter motivation for water reward. As previously reported, food consumption increased and body weight decreased during the 5 days of SD. in experiment 2. The findings indicate that although sleep disruption increases food consumption and decreases body weight, it does not alter thirst or motivation for water reward. Thus, water restriction is well suited for experiments examining the effect of sleep disruption on reward motivated behavioral tests in rats.