Dehydration-associated anorexia: development and rapid reversal.

Dehydration in rats results in anorexia that is proportional to the degree of dehydration. The aims of this study were first, to determine when anorexia develops in response to drinking hypertonic (2.5%) saline for 4 days; and second, to determine the organization of ingestive behaviors after access to water is resumed. Body weights, food, and fluid intake were measured morning and evening before, during, and after a 4-day period of dehydration caused by drinking hypertonic saline. A profile of the behaviors expressed immediately after rehydration was determined. The data make three points. First, dehydration-associated anorexia does not emerge until the second night of dehydration when the composition of the fluid compartments can no longer be homeostatically buffered. Second, dehydration reduces the amount food eaten nocturnally, but leaves diurnal food consumption largely unaffected. Animals very rapidly return to predehydration nocturnal ingestion patterns, whereas the amounts of food and water ingested during the day are significantly increased. Increased diurnal food intake may play a significant role in normalizing metabolism after dehydration. Finally, anorexia is reversed within minutes of rehydration. The data suggest a model where dehydration simultaneously activates two sets of circuits within the brain that will independently stimulate or inhibit feeding. Eating is inhibited during dehydration through the action of a set of inhibitory circuits, which masks the output of circuits that stimulate eating. However, when drinking water resumes, sensory inputs to these circuits rapidly release the inhibition and allow eating to proceed freely.