A microstructural analysis of sodium and water intake in male and female rats

Analysis of fluid intake microstructure is a powerful tool to provide insight into the mechanisms underlying ingestion. Foundational work demonstrated that changes in the number of licking bursts reflect changes in negative feedback related to satiety, whereas the average size of those bursts reflects changes in the hedonic value of the substance being consumed. These conclusions, however, were drawn primarily from studies of sucrose intake, and it is unclear if the interpretation can be extended to intake of water or sodium. To address this open question, we first tested and found support for the hypothesis that dehydrated rats would show a concentration-dependent decrease in sodium intake that is the result of reduced burst size, without differences in burst number. Next, we tested if a preference for cold water over tepid water was a function of burst size. We found that rats preferred 10°C water over 40°C water, and that the difference in intake was a function of both burst size and burst number. To test this in a condition with a reduced influence of satiation, we repeated this experiment using polydipsic Brattleboro rats. Like the rats in the second experiment, we found that Brattleboro rats preferred 10°C water over 40°C water, but the difference was mediated only by burst size and not burst number. Together, these findings support the conclusion that burst size can be used to measure changes in the hedonic value of sodium and water.