NaCl preference and water intake effects of food availability in median eminence polydipsia

Lesions in the median eminence (ME) produce a well-known neurological model of polydipsia and polyuria in rats. The effect of food availability (ad lib/deprivation) on the polydipsic/polyuric behaviour of animals was tested. As expected, all lesioned rats developed strong polyuria and polydipsia during the first postoperative 24h. This effect was maintained during day 2 when food was available ad lib (experiment A), but both polyuria and polydipsia were abolished when animals were deprived of food during the next 24h (day 3). Animals deprived of food from the first post-operative day (experiment B) showed a significant reduction in the initial polyuria and polydipsia (day 1) on day 2, but these effects were again observed on day 3 when food was available ad lib. Finally, when food-deprived animals were able to choose between a 1.5% sodium chloride solution and water (experiment C), they preferentially chose (82% of total liquid consumed) the hypertonic saline solution (day 1); during the next 24h (day 2), when only water was available, the polyuric/polydipsic effect was abolished but returned when food became available ad lib on day 3. Hence, the polyuria/polydipsia effect produced by ME lesions appears to be consistent during the first 24h but might later be related to the availability of standard food and is completely abolished under food deprivation conditions. Preference for the hypertonic solution supports the volemic component of this syndrome and demonstrates the need for appropriate amounts of hypertonic nutrients to be consumed during the first 24h.