Phy~tology and Behavror Vol. 3, pp. 667-671 Pergamon Press, 1968. Printed in Great Britain
Food-Deprivation Polydipsia in Gerbils
(Meriones unguiculatus) C H A R L E S L. K U T S C H E R , R O B E R T D. S T 1 L L M A N A N D I R A P. W E I S S
Psychology Department, Syracuse University, Syracuse, N.Y. (Received 18 March 1968) KUTSCHER, C. L., R. D. STILLMAN AND I. P. WEISS. Food deprivation polydipsia in gerbils. (Mertones unguiculatus). PHYSIOL. BEHAV. 3 (5) 667--671, 1968.--After 2-5 days of food deprivation, gerbils develop polydipsia which ends when they are returned to normal adlibitum food intake. This polydipsia is not due to N a + deficiency since maintaining normal daily N a + intake while restricting food did not prevent polydipsia. No renal dysfunction was noted. Polydipsic gerbils excreted extremely dilute urine as they showed almost complete conservation of N a + and K + . When water deprived, gerbils were able to curb urine outputs as well as non-polydipsic control animals. When salt injected, polydipsic gerbils excreted hypertonic urine as well as controls.
A PREVIOUS STUDY  showed that pronounced species differences exist in regard to the effect of food deprivation on daily water intake. Under total food deprivation, guinea pigs became almost totally adipsic, rats reduced water intake to about 40 per cent ad libitum baseline, hamsters showed first reduced water intake and then polydipsia, and gerbils showed vigorous and sustained polydipsia. When food was restored ad libitum, rat and guinea pig water intakes rose sharply and hamster and gerbil intakes declined sharply toward predeprivation baselines. The reduction in water intake with food deprivation in rats has been reported before [1, 13, 14] and has been interpreted as decreased need for water since the food-deprived rat is losing less by formation of digestive juices, osmotic shifts of water into the gut, and formation of feces. Food-deprivation polydipsia in hamsters and gerbils apparently has not been reported prior to the work of Kutscher , but Cizek  and Huang  studied it in male rabbits, and Cizek  noted it also in guinea pigs. Cizek  initially attributed the rabbit polydipsia to salt deficiency resulting from increased urinary N a + loss during food deprivation because polydipsia was abolished by the substitution of 0.85 per cent or 0.45 per cent NaCI for drinking water. Later Cizek, Nocenti, and Oparil  found that sex hormones are implicated also since diethylstilbestrol or estradiol injections into the food-deprived and polydipsic male rabbits caused a significant reduction in water intake and urine output without changing the elevated N a + excretion. Food-deprivation polydipsia apparently still needs an adequate explanation and this explanation may be different from species to species. The following experiments were done to determine the effect of food deprivation on Na + balance in the gerbil (thus making possible a comparison of gerbil and rabbit polydipsia) and to determine whether food-deprived, polydipsic gerbils suffer from a dysfunction of the kidney which might allow
for an explanation of polydipsia as a secondary reaction to a primary polyuria. EXPERIMENT 1 This experiment was designed to impose food restriction without any decrease in mean daily Na + intake. Polydipsia developing under these conditions must be attributed to something other than Na + deficiency. METHOD
Subjects were 10 naive, adult male gerbils (mean wt = 78.5 g) obtained from Tumblebrook Farms, Brant Lake, New York. Gerbils were removed from colony cages and placed in individual cages of steel and hardware cloth (10 × 12 × 9 in.) which were so constructed that the gerbils could not see each other. Room temperature was maintained at 72 ± 2°F. Overhead lights were timed for 9 hr darkness and 15 hr light. Food was Purina chow mash given in spill-proof glass cups which were weighed to determine food intake. Tap water was provided in inverted 100 ml eudiometer tubes, graduated in 0.2 ml units and fitted with stainless steel drinking nozzles. Daily body weights were measured throughout the experiment. Gerbils were allowed 3 days to adapt to the individual cages, food cups and drinking tubes. Food and water were given ad libitum for 7 days. Mean food intake for this period was 7.2 g. On day 8 all animals were given salted Purina chow ad libitum to see if animals would readily eat it and to determine the effect of salted chow on water intake when the animals were not food deprived. Sodium chloride was added to the Purina chow (0.07 g of NaCI to 1.0 g chow) so that mean daily intake of Na + could be maintained when food intake was later reduced to 1 g/day. Ordinary chow was given for 8 more days to allow any residual effects of the 667
KUTSCHER, STILLMAN AND WEISS
increased salt ingestion of day 8 to dissipate. Then the 10 gerbils were given 1 g of food per day for 5 days to elicit polydipsia. Water was always available adlibitum. Five gerbils were given ordinary and 5 were given the salted chow. Animals were returned to ordinary chow ad libitum for a 10 day recovery period. Five weeks later, the animals were again subjected to 5 days of 1 g/day food restriction. Those who had previously been given salted chow during food restriction were now given ordinary chow and vice versa. RESULTS
Figure I shows that polydipsia developed during food restriction when ordinary chow was given, as expected, as well as when salted chow was given (thus maintaining normal Na + intake). When ordinary chow was given, mean dally water intake on day 5 of the food-restriction period was 24.4 ml compared to 10.5 ml on the last day of ad libitum food intake. A t-test for related measures showed that this
~ad lib. I1 gm/daYl ad lib. ol
I I I I III I I I III I I I I DAYS
FIG. 1. Mean dady water intakes for gerbils given food ad hbitum (5 days), restricted to 1 g/day (5 days), and returned to food ad libiturn (5 days). During the food-restriction period gerbils were given normal chow (N) or salted chow (S).
difference was significant (p < 0.01). Under the salted-chow condition, mean daily water intake on day 5 of the foodrestriction period was 37.2 ml compared to 11.4 for the last day of ad libitum food intake (p < 0.001). On day 5 of the food-restriction period, water intake was significantly higher (p < 0.01) for the salted chow condition (37.2 ml) than for the ordinary chow condition (24.4 ml). Increasing the salt content of the chow, thus preventing reduction in mean daily N a + intake, not only failed to prevent polydipsia, but actually enhanced it. Presentation of salted chow ad libitum on day 8 of the experiment resulted in a water intake of 14.8 ml compared to 10.1 ml on day 7 when ordinary chow was given. This difference was significant (p < 0.05). There was no statistical difference between the intake of salted chow on
day 8 (6.7 g) and the intake of ordinary chow on day 7 (7.1 g). Figure 1 also shows the time course of polydlpsia. Water intake increased with days of food restriction and dropped sharply toward normal when food was restored. EXPERIMENT 2 A study of urine output characteristics was undertaken in order to determine if the abnormally high water intakes under food restriction were artifacts due to the gerbil spilling water from the water tubes and to determine if the gerbil, like the rabbit , shows an abnormally high urinary Na + output during food restriction. The latter condition would invalidate the conclusions of the first experiment. METHOD
Subjects were l0 adult, naive male gerbils (mean w t - 64.1 g). They were removed from the colony cage and isolated in 9½ × 5 × 7 in. aluminium cages with hardware cloth floors and PIexiglass tops. After 2 days adaptation to the cages, daily water intakes were measured for 7 days of a d hbitum food intake. On the fourth day of this period, water intake and urine output were measured at 30 min intervals for 24 hr. Cages were thoroughly cleaned and were suspended above a sheet of clean polyethylene plastic. When urine samples were voided, they beaded-up on the polyethylene and were removed immediately with a syringe and needle. Samples were deposited into graduated centrifuge tubes for determination of urine output. Any water spilled from the drinking tubes was distinguished from urine since even the dilute urine was pale yellow noticeably distinct from the drops of water which sometimes dripped from the water spout. At the end of the 7 day food ad libitum period, the gerbils were given 1 g of ordinary chow each day to elicit polydipsia. On the fifth day of food restriction, urine outputs and water intakes for the 10 gerbils were measured at 30-rain intervals. All urine samples were analyzed for Na + and K + content on a Process and Instruments internal standard flame photometer. RESULTS
All data comparisons are between the fifth day of food restriction (1 g/day) and a day o f a d libitum food intake 3 days previous to the introduction of food restriction. All differences were evaluated with t-tests for related data. Table 1 shows that food restriction increased water intake from 7.4 ml to 26.6 ml and urine volume from 5.4 to 24.7 ml. The mean difference between water intake and urine output was approximately 2 ml under both food-restriction and ad libitum conditions. Under the food ad libitum conditions, N a + concentration in the urine was 203.1 mEq/L, but under the polydipsia and polyuria resulting from food restriction, mean urine N a + concentration fell to 7.1 mEq/L. Even during polyuria gerbils were able to reduce urinary N a + concentration so greatly that mean N a + loss (0.17 mEq) on the fifth day of food restriction was still less than mean N a + intake in the 1 g of food (0.20 mEq). Under food restriction, gerbils are clearly able to conserve N a + and to produce very dilute urine. As Table 1 shows, urine K + concentration and total K + excretion were also greatly reduced during foodrestriction polydipsla.
POLYDIPSIA IN GERBILS
TABLE 1 URINE CHARACTERISTICS (MEAN 4- SD) AND WATER INTAKES FOR GERBILS WITH FOOD ad libitum AND ON THE FIFTH DAY OF FOODRESTRICTION (1 G/DAY)
Characteristics Water intake (ml) Urine volume (ml) N a + concentration (mEq/L) N a + amount (mEq) K + concentration (mEq/L) K + amount (mEq)
Food ad lib
gerbils showed a 100 per cent increase in water intake from the last pre-deprivation day (mean intake = 9.6 ml) to the third day of food-deprivation (mean intake = 24.5 ml).
7.4 4- 2.5 5.4 4- 1.7
26.6 4- 10.8 24.7 ± 10.0
URINE CHARACTERISTICS (MEAN+ SD) OF POLYDIPSlC AND CONTROL GERBILS BEFORE AND AFTER AN INTRAPERITONEAL INJECTION OF 5 ~/o NACL
203.1 4- 52.3 1.03 4- 0.21
7.1 4- 3.4 0.17 4- 0.08
Pre-injection period (18-hr)
234.2 4- 50.7 t.20 4- 0.20
20.0 4- 17.6 0.37 4- 0.08
EXPERIMENT 3 Experiments 1 and 2 have shown that the food-deprivation polydipsia is not due to N a + deficiency and that the gerbil, unlike the rabbit  shows, almost complete N a + conservation in the presence of food-deprivation. The possibility remained that the polydipsia was secondary to a polyuria, possibly caused by deficiency of antidiuretic hormone (ADH). In such a case one would expect food-deprived, polydipsic gerbils to show higher urine outputs when water deprived than control animals previously maintained on food and water a d libitum. A n A D H deficiency should also result in the inability of the food-restricted gerbil to form hypertonic urine. The latter was studied with injections of hypertonic NaCI. METHOD
Subjects were 14 naive, adult male gerbils (mean wt = 83.0 g). Animals were isolated in individual cages of steel and hardware cloth (12 × 6½ × 10 in.). After 3 days of adaptation to cages and water tubes, daily water intake was measured for 7 days. Then, 7 of the animals were deprived of food for 3 days (water ad libitum) and consequently became polydipsic. The other 7 gerbils (controls) were allowed food and water ad libitum. Then cages were thoroughly cleaned and both groups were subjected to the same deprivation conditions: 18 hr of food and water deprivation. Cages were suspended above sheets of polyvinyl plastic (found to be superior to polyethylene plastic for this purpose) and urine samples were collected in syringes as voided. At the end of the 18-hr deprivation period, all animals were injected with 0.3 ml of 5 per cent NaCI intraperitoneally. Urine samples were collected for an additional 4 hr. Following the above test, gerbils were returned to food and water a d libitum for 20 days before the tests were run again. Then those animals previously used as the control group were deprived of food for 3 days to elicit polydipsia and those previously polydipsic were used as the control group. RESULTS
Since each animal was tested under both the control condition and after the development of polydipsia, all statistical comparisons were made with t-tests for related data. Total food deprivation effectively elicited polydipsia. All 14 animals showed a higher water intake on the third day of food deprivation than on the last pre-deprivation day. Thirteen
Characteristic Urine volume (ml) N a + concentration (mEq/L) N a + amount (mEq) K + concentration (mEq/L) K4- amount (mEq)
1.73 -t- 0.22
1.68 ± 0.70
95.2 ~ 47.2 0.18 4- 0.09
44.1 ± 19.3 0.08 4- 0.08
172.3 -4- 71.3 0,28 4- 0.07
87.7 4- 46.7 0.13 4- 0.07
Post-injection period (4-hr) Urine volume (ml) N a + concentration (mEq/L) N a + amount (mEq) K + concentration (mEq]L) K + amount (mEq)
0.28 i 0.09
0.38 4- 0.10
277.3 4- 120.4 265.6 ± 85.7 0.08 4- 0.03 0.10 4- 0.04 194.9 4- 43.0 0.06 4- 0.02
135.64- 25.9 0.05 4- 0.02
<0.01 n.s. n.s. <0.01 n.s.
Table 2 shows the characteristics of urine outputs during 18 hr of food and water deprivation (pre-injection) and during a subsequent 4..hr period following the intraperitoneal NaC1 injection (post-injection) for animals which became polydipsic during the 3 days of food deprivation (polydipsic) and for the same animals maintained till the deprivation period on food and water ad libitum (control). When stressed by water-deprivation, polydipsic gerbils did not show continued polyuria, but reduced urine volume just as much as they did under control conditions (18-hr control urine volume = 1.73 ml; polydipsic urine volume = 1.68 ml). Under both polydipsic and control conditions urine output was significantly lower during the third 6-hr segment of the test period than during the first 6-hr segment, indicating increased renal conservation of water. Under polydipsic conditions, urine output was reduced from 1.04 ml (first 6 hr) to 0.29 ml (second 6 hr; p < 0.01). Under control conditions, urine output was reduced from 0.90 ml to 0.38 ml (p < 0.001) for these time periods. Urine N a + and K + concentration and total N a + and K + excreted during this 18-hr period were greater under the control condition than under the polydipsic condition. This is to be expected since the animals under the polydipsic condition were tested in the fourth day of food deprivation and hence bad less electrolyte to excrete than when under the control condition. Following the salt injection, mean urine N a + concentration was not significantly different for the polydipsic condition (265.6 rnEq/L) and the control condition (277.3 mFxl/L). Both groups were able to excrete urine with N a + content decidedly hypertonic to plasma and both groups increased urine N a + concentrations significantly following salt injection (t7 < 0.01).
KUTSCHER, STILLMAN AND WEISS EXPERIMENT 4
It Is clear that food-deprived gerbils show polydlpsm in 2-4 days if one measures 24 hr water intakes. In the exper|ment below, water was offered only for short (1-hr) drinking periods at the end of a water or water plus food deprivation period. In rats, Verplank and Hayes  showed that l-hr water intakes were much lower for animals 22-hr food and water deprived than for rats 22-hr water deprived only. If polydlpsia appears in the short drinking periods, one might expect that differences between polydipsic and normal animals might be studied profitably in short term measurements of bar-pressing, activity, or runway performance.
Subjects were 40 adult, male, naive, gerbils (mean wt = 57.8 g). Ten animals were randomly assigned to the following deprivation groups: 24, 48, and 96 hr. Each gerbil was tested twice: once after water deprivation with food allowed ad libitum and once after both food and water deprivation. Ten animals were never deprived before the test and served as a non-deprived control group. Animals were removed from colony cages and isolated in individual cages of steel and hardware cloth (12 × 9 "< 10 in.). After 3 days of adaptation to the cages, half of the animals within each group were water deprived and half were water and food deprived. At the end of the deprivation interval, water was given (without food) for the 1-hr test period. Gerbils were then returned to food and water ad libitum. Approximately 25 days later, deprivation conditions were
reversed, so that those animals previously water deprived were now water and food deprived and vice versa, and tests were repeated. This time water intakes (in the absence of food) were measured after 1 and after 24 hr. RE SU L T S
The l-hr water retakes following the various deprivation conditions are shown m the lower portion of Fig. 2. Analysis of the data with t-tests showed that 1-hr water intakes were greater for water-deprived gerbils than for water and fooddeprived gerbils in the 24-hr (p < 0.01), 48-hr (p < 0.05), and 96-hr (p < 0.05) groups. One-hr drinking for the animals 24-hr water deprived was greater (p < 0.001) than that of the non-deprived group as was that of animals 24-hr water and food deprived (p < 0.01). A one-way analysis of variance performed on 1-hr water intakes under water-deprivation only showed significant differences due to hr of deprivation (p < 0.01). One-hr drinking under water and food deprivation was not significantly affected by hr of deprivation. Clearly, food deprivation inhibits drinking of water deprived animals when the test period is as short as 1 hr. When the gerbils were allowed to drink for 24 hr, food-deprivation clearly increased water intake in water-deprived animals (upper portion of Fig. 2). A two-way analysis of variance on this data revealed that 24-hr water intakes differed asa function of type of deprivation (p < 0.001), water vs. water plus food, but not as a function of hr of deprivation. The interaction was not significant. The 24-hr water and food deprivation animals drank significantly more (p < 0.01) in the 24-hr test period than did the control group, but the 24-hr water deprived gerbils did not. Under these experimental conditions then, food deprivation inhibits drinking in water-deprived gerbils in a 1-hr test period, but results in polydipsia if a 24-hr drinking period is allowed.
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FIG. 2. Lower portion of figure: mean water intakes during a 1-hr drinking period (with no food available) following various durations of water (W) or water and food (W + F) deprivation. Upper portion of figure: mean water intakes during a 24-hr drinking period (no food available) following various durations of deprivation.
The evidence presently indicates that gerbil starvation polydipsia is not due to N a + deficiency, as is the case with the rabbit, or to any lack of ability of the kidney to produce concentrated urine, or to conserve body water or electrolytes. The gerbils showed strong conservation of K + during polydipsia indicating that N a + was not saved at the expense of K + . This is important since polydipsia has been reported in rats  and dogs  maintained on a K + deficient diet. The fact that urinary K + concentration slightly exceeded N a + concentration under polydipsic conditions may result from the release of K + due to cell breakdown during food deprivation. Even after polydipsia was well developed, gerbils conserved water well when exposed to 18 hr of food and water deprivation. During the last 6 hr of the 18-hr urine collection period, the mean urine output was only 0.29 and 0.38 ml for the polydipsic gerbils and control gerbils, respectively. The evidence thus far indicates that the polydipsic animals excrete as urine more than 90 per cent of the high water intakes and probably do not store excess water in the body. If polydipsic gerbils were prone to diabetes insipidus, possibly due to an A D H deficiency, one might expect much higher urine outputs during water restriction than were actually observed. An A D H deficiency explanation is also difficult to reconcile with the fact that the return to ad libitum feeding causes a sharp decline in drinking and the termination of
POLYDIPSIA IN GERBILS
polydipsia unless one assumes a rapid synthesis of A D H upon refeeding. Furthermore, the polydipsic gerbils were capable of producing hypertonic urine, a process believed to require A D H , when stressed with moderately concentrated salt injections. Conclusions regarding A D H are tentative at this time since no A D H assays were made. In short, the foodrestricted and polydipsic gerbils were able to excrete hypertonic and very hypotonic urine and were able to greatly restrict the volume of urine as conditions required. There seems to be no evidence of renal dysfunction. Increased urine volume with food deprivation has been observed in another species. Morrison, et al.  reported urine volume greatly increased for the first 48 hr of food deprivation in three strains of rats even though water intakes were below normal for two of these strains. Since this polyuria could be curtailed by water restriction, Morrison concluded that the rats were drinking more than was needed to meet their needs (since the cessation of eating freed a considerable amount of water to move from the gut, back into other body compartments) and thus were exhibiting a relative polydipsia. This food-deprivation polyuria and apparent relative polydipsia were later observed in hooded rats  and on the first day of food deprivation in gerbils [unpublished data]. The latter finding does not provide an adequate explanation for the high absolute polydipsia observed in gerbils, however, since hooded rats do not show absolute polydipsia as do gerbils , even though hooded rats exhibit high polyuria during food deprivation and sustain
considerable losses in carcass water and circulating plasma volume . It thus seems a paradox that an animal capable of existing in the arid regions of Asia and Africa  and capable of existing for long periods on dried grain without drinking water  should, when faced with food deprivation, gorge itself on water which it apparently does not need. In the present experiment, some gerbils have been observed to drink their weight in water in one day. The possibility remains that polydipsia in the gerbil may result from food restriction causing severe changes m internal environmental stimuli related to water intake, such as increased plasma osmolality or decreased plasma volume, which result in polydipsia (followed by secondary polyuria). Changes in serum osmolality and plasma volume with food restriction have yet to be determined in the gerbil, but it is known that the gerbil conserves plasma volume much better than the rat when water deprived . One might expect that any such alterations in the internal environment resulting from food deprivation might affect drinking in a short 1-hr time period, as would be true for salt injections or water deprivation (Fig. 2). In short, starvation polydipsias in different species seem to be qualitatively different. Starvation polydipsia in gerbils is absolute, increases with time and is enhanced by N a + supplement; in rabbits it is absolute, persistent and is prevented by N a + supplement ; in rats it is usually only relative, transient and is enhanced by N a ÷ supplement .
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