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 anorex ia develops in response to drinking hypertonic (2.5%) saline for 4 days; an d second, to determine the organization of ingestive behaviors after access to water is resumed. Body weights, food, and fluid intake were measured mo rning and evening before, during, and after a 4-day period of dehydration c aused by drinking hypertonic saline. A profile of the behaviors expressed i mmediately after rehydration was determined. The data make three points. Fi rst, dehydration-associated anorexia does not emerge until the second night of dehydration when the composition of the fluid compartments can no longe r 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 signific antly 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 S imultaneously activates two sets of circuits within the brain that will ind ependently stimulate or inhibit feeding. Eating is inhibited during dehydra tion through the action of a set of inhibitory circuits, which masks the ou tput of circuits that stimulate eating. However, when drinking water resume s, sensory inputs to these circuits rapidly release the inhibition and allo w eating to proceed freely. (C) 1999 Elsevier Science Inc.