Physiology & Behavior, Vol. 28, pp. 885-891. Pergamon Press and Brain Research Publ., 1982. Printed in the U.S.A. Nocturnal Food-Related Hyperdipsia in the Adult Spontaneously Hypertensive Rat F. SCOTT KRALY, ALAN F. MOORE, LISA A. MILLER AND ANNE DREXLER Department of Psychology, Colgate University, Hamilton, N Y 13346 and Pharmacology Section, Norwich-Eaton Pharmaceuticals, Norwich, NY 13815 Received 19 November 1981 KRALY, F. S., A. F. MOORE, L. A. MILLER AND A. DREXLER. Nocturnal food-related hyperdipsia in the adult spontaneously hypertensive rat. PHYSIOL. BEHAV. 28(5) 885-891, 1982.--Male adult spontaneously hypertensive rats (SHR) ate the same but drank more and had a higher water to food ratio (W:F) than did Wistar-Kyoto (WKY) rats in 24-hr when they had continuous access to standard laboratory pellets and tap water. When rats ate in the day phase of a 12:12 light/dark cycle after 24-hr food deprivation, SHR rats ate and drank the same as did WKY rats in a 60-min test. When. the same rats ate at night after 24-hr food deprivation, however, SHR rats were hyperdipsic: They ate the same as did WKY rats, but SHR rats drank more and had a higher W:F. This relative hyperdipsia reflected the increased ability of ingestion of food to stimulate drinking in SHR, because when food was absent for a 60-min test at night SHR drank the same as did WKY rats. Three dipsogens which are candidate components for eating-elicited drinking in the rat, cellular dehydration, histamine and angiotensin II, elicited drinking differentially in SHR and WKY rats: SHR drank more than did WKY rats in response to (1) cellular dehydration produced by IP hypertonic saline, (2) large doses of SC histamine, and (3) SC angiotensin II. These results demonstrate that SHR exhibit a nocturnal food-related hyperdipsia which may reflect differ- ential sensitivity to stimuli important for eating-elicited drinking such as increased osmolality and endogenous histamine or angiotensin. Food-related drinking SHR Cellular dehydration Drinking Hypertension Angiotensin II Hyperdipsia Histamine THE precise role of ingestion in the development and main- tenance of high blood pressure in humans or in spontane- ously hypertensive rats (SHR) is unclear. While there is evi- dence that increased intake of salt may contribute to the development and/or maintenance of hypertension in humans [15] and SHR [6, 14, 29, 31] little else is known about appe- tite and dietary habits of the SHR--a useful animal model of human essential or idiopathic hypertension. We have begun a study of ingestion in adult SHR and report here an eating- elicited hyperdipsia and disordered drinking in SHR. EXPERIMENT 1 This experiment was designed to provide a description of eating and drinking of hypertensive SHR and genetically- similar normotensive adult Wistar-Kyoto (WKY) rats main- tained under standard labotatory conditions. METHOD Animals Age-matched male SHR (n=10) and WKY (n=6) rats (Taconic Farms, Germantown, NY) were housed individually in conventional wire-mesh cages on a 12:12 hr light/dark cycle. Rats had continuous access to Charles River pellets on the cage floor and tap water available from a stainless-steel spout at- tached to a graduated cylinder. Rats were 5 months of age at the time of testing; SHR weighed significantly (p<0.05) less than did WKY rats and were fiankly hypertensive [31]. Blood pres- sure was not measured until the end of the experiments re- ported here because we did not wish to introduce the con- found of a reliable invasive procedure (e.g., catheterization) or a less reliable procedure that might introduce trauma (e.g., tail cuff)---procedures which could have jeopardized the sensitive and reliable measurement of behavior in these experiments. (Data were routinely gathered in age-matched Sprague-Dawley (SD) males in these experiments. Select comparisons of SD rats to SHR and WKY are provided when appropriate.) Procedure Rats were made accustomed to being handled, weighed and offered fresh food and water dally at 1:00 p.m.-- approximately 6 hr into the light phase. Measured amounts of food and water were offered at this time. Twenty-four hr later uneaten food was removed from the cage and spillage was recovered from a tray beneath the cage; food intake was determined by subtracting the amount of uneaten food plus spillage from the amount of food originally offered (to the nearest 0. I g). Water intake was recorded by reading the Copyright © 1982 Brain Research Publications Inc.--0031-9384/82/080885-07503.00/1