BEHAVIORAL BIOLOGY 24, 474-480 (1978)

Saline Intake in Hamsters

RODERICK WONG AND WAYNE JONES 1

Department of Psychology, University of British Columbia, Vancouver, Canada V6T 1 W5

Experiment 1 assessed the preference-aversion function of hamsters for water and 0.08, 0.15, 0.35, and 0.71 M NaC1 solutions in the two-bottle test. Unlike rats, the hamsters did not show a strong preference for isotonic (0.15 M) saline, nor did they exhibit any preference for the other solutions. Experiment 2 exam- ined the effects of hydrochlorothiazide on the hamsters" intake of 0.71 M NaC1 solution. The results indicated that this diuretic agent did not cause the hamsters to increase their saline intake relative to the baseline level. Experiment 3 also indicated that a high (5 rag) dose of desoxycorticosterone acetate failed to produce sodium appetite in the hamsters. These results contrast with those found with rats.

The golden hamster (Mesocricetus auratus ) is a rodent native to Syria that has adapted to the natural selection pressures of a dry, xerophytic, desert environment (Schmidt-Nielsen, 1964). Although psychologists have conducted extensive studies on the agonistic, social, sexual, and open-field field behavior of members of this species, there are much less data on the hamster's mode of adaptation to fluid requirements. Experi- mental data on the determinants of fluid ingestion in hamsters would enable us to evaluate the generality of models about the regulation of consumption found in rodents from mesophytic niches. Unfortunately, most of our knowledge about fluid ingestion in rodents has been derived from rat experiments.

Pioneer studies on species differences in water regulation had origi- nated in Kutscher's (1969) laboratory. His research group had demon- strated that following food deprivation, guinea pigs become adipsic, rats show restricted drinking, and gerbils and hamsters develop polydipsic behavior. Such results certainly indicate that the rat's physiological and behavioral systems operate on principles different than those upon which other rodents operate. In another comparative study, Carpenter (1956) indicated that hamsters differ from rats, cats, and rabbits in their be-

1 This research was aided by Grant 67-0247 from the National Research Council of Canada and by the University of British Columbia Natural, Applied and Health Sciences Grants Committee. We thank Amyra Carsh for her help in running and analyzing the data of Experiment 3.

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0091-6773/78/0244-0474502,00/0 Copyright @ 1978 by Academic Press, lnc, All rights of reproduction in any form reserved.

SALINE INTAKE IN HAMSTERS 475

havioral responses to salt solutions. Unlike the other species in Carpen- ter's experiment, hamsters showed no preference for NaC1 solutions over water in the two-bottle test. Since the time of CarpenteVs study, numer- ous experiments have shown that rats exhibit greater preference for isotonic saline than for any other concentration of the solution or of water (see Young, 1966). In Carpenter's experiment, the hamsters' intake of hypotonic saline was equivalent to that of water. When the saline was increased to 0.2 M, the hamsters drank more water than saline solution. The animals showed progressively greater intake of water and decreased intake of saline when the tonicity of the NaC1 solution was increased from 0.2 to 1.0 M.

The Carpenter (1956) results with hamsters are particularly interesting because their NaC1 preference-aversion function was clearly different from that typically observed in rats. However, there are a number of methodological features of Carpenter's experiment that may be problem- atic. Carpenter took only a single reading of the hamsters' intake of a number of NaC1 solutions varying in concentration. The results of many saline preference studies in our laboratory with rats (Wong, 1977a; Wong and Kraintz, 1977) and gerbils (Wong, 1977b) have indicated that a daily reading of the animals' intake over a substantial number of sessions provides a more stable estimate of an animal's preference than that obtained with a "one-shot" test. We also believe that Carpenter should have presented and analyzed his data in terms of a saline preference score (percentage of saline intake relative to total fluid intake) in addition to that based upon absolute intake.

The present experiment was designed to ascertain the preference- aversion function of the hamster taking into account the methodological features previously discussed. Of particular importance is the fact that we tested the animals with isotonic (0.15 M) NaC1 in addition to testing them with hypotonic and hypertonic solutions. Under normal conditions, rats invariably prefer isotonic saline over any other solution (Young, 1966). Unfortunately, Carpenter did not study the hamster's intake of this solu- tion relative to water.

EXPERIMENT 1

The subjects were eight male and six female golden hamsters (Meso- cdcetus am'atus) ranging in weight from 96 to 130 g at the beginning of the experiment. They were housed in single stainless-steel cages measuring 17.8 x 24.2 17.8 cm. Purina Lab Chow was placed on the wire mesh floor instead of in the food hopper on the side of the cage because the animals had difficulties in biting between the wire mesh wall adjacent to the hopper.

We placed a bottle of water in each cage and maintained their daily intake for 10 days. Then we tested the two-bottle preferences of the

476 WONG AND JONES

hamsters by placing one bottle of water and one of saline through the front of the cage. The left-right position of the bottles was alternated from day to day, and the daily intake from each bottle was recorded around the same time (0930 hr). The animals were first tested for 10 days with a bottle each of water and 0.15 M NaC1. Then they were given water only for the subsequent 3 days before being tested with a bottle of water and one of 0.08 M NaCi for 10 days. Following another 3 days during which water was the only source of fluid, the hamsters were given a choice between water and 0.35 M NaC! for another 10 days. After the 3-day water-only period, the animals were given 10 days to choose between water and 0.71 M NaC1 solution.

During the initial 10-day period, the hamsters' mean water intake was 9.72 ml. Data from the two-bottle test were first analyzed in terms of a preference score consisting of (saline intake/saline + water intake) x 100. This score minimizes the error due to individual variation in total fluid consumption within and between subjects. The saline preference of the hamsters over the 10 test days for each NaCI concentration is shown in Fig. 1. Sex differences were not evident (P > 0.05), so the data were pooled.

An examination of the data indicates that, unlike rats, hamsters do not show a strong preference for 0.15 M NaC1 solution. In contrast to the rats' tendency to show a saline preference of about 70% (Devenport, 1973;

35

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20

' ~ .3'5 ' .08 .15 .71

NaCI Mo lar i ty

FIG. I. Percentage intake of 0.08, 0.15, 0.35, and 0.71M NaCI solutions by hamsters in the two-bottle test.

SALINE INTAKE IN HAMSTERS 477

Wong, 1977a), the hamsters showed 37.2% preference for isotonic saline. This figure is even lower than the 53% saline preference shown by gerbils in the Wong (1977b) experiment. When the hamsters were tested with 0.08 M (0.45%, w/v) NaC1 solution, they showed 36.5% saline preference. When the hamsters were tested with 0.35 M (2.0%, w/v) NaC1, their saline preference was 31%, and when tested with 0.71 M (4.0%, w/v) NaC1, their salt preference dropped to 24.2%.

A repeated-measures analysis of variance where the mean saline pref- erence for each solution was dealt with as a within-subject variable indicated significant differences arising from the tonicity of the test so- lutions [F (3, 39) = 4.36, P < 0.01]. When multiple comparisons were performed between the pairs of test conditions, the results indicated that the saline preference of the hamsters differed only when they were tested under the 0.08 and 0.71 M NaCI solutions (Tukey T = 5.14, P < 0.05). None of the other pairwise differences was statistically significant.

The data on the hamsters" absolute intake of water and saline were also recorded and analyzed. Their mean intake of 0.15 M NaC1 during the 10 test days was 4.67 ml in contrast to their intake of 9.75 ml of water. When offered a choice between 0.08 M NaCI and water, the animals' mean intake of saline was 4.51 ml and that of water was 7.44 ml. When tested with 0.35 M NaC1 and water, the hamsters drank 4.45 ml of saline and 9.14 ml of water. However, when exposed to 0.71 M NaCI and water, the hamsters" mean intake of saline was 3.02 ml and that of water was 8.95 ml. A repeated-measures analysis of variance of the saline intake indicated a significant effect due to conditions IF (3, 39) = 4.25, P < 0.05]. Multiple comparisons of the pairs of test conditions indicated that the hamsters' intake of 0.71 M NaC1 was significantly less than their intake of 0.08, 0.15 and 0.35 M NaCI solutions (P < 0.05). Analyses of the hamsters' water intake under the various test conditions did not reveal any significant differences (P > 0.05).

The present results indicate that hamsters show little preference for NaCI solutions of varying tonicity. Their intake of hypotonic, isotonic, and 0.35 M hypertonic NaCI saline is at a similar low level. When offered a highly hypertonic solution (0.71 M NaC1), hamsters show a significant decrease in saline preference relative to the other concentrations. In some respects our results with hypertonic solutions are similar to those found by Carpenter (1956). However, we did not replicate Carpenter's finding that hamsters show an equivalent intake of hypotonic saline and water. These interexperimental differences may be due to the different proce- dures (i.e., estimation of saline preference from a "one-shot" vs a repeated-measures assessment). At any rate, the preference-aversion function for NaC1 among hamsters is clearly different from that among rats and gerbils.

478 WONG AND JONES

EXPERIMENT 2

The results of Experiment 1 indicate that hamsters show a relatively low level of acceptance of 0.71 M NaCI under normal, nondeprived conditions. The present experiment was designed to determine whether sodium depletion would increase the hamsters' preference for this solu- tion. We believe that this issue is important because of a recent finding by Salber and Zucker (1974). They reported an absence of salt appetite among adrenalectomized hamsters. Adrenalectomy produces sodium def- icits through a continuous loss of body sodium in urine and other excre- tions. Thus, we would expect the animal to show a compensatory increase in available salt.

In the present experiment we injected the hamsters with a diuretic agent, hydrochtorothiazide, which acts upon the kidneys to produce the loss of sodium excreted in the urine. This drug has been shown to increase the spontaneous appetite for NaC1 solution of rats (Fregly, 1967; Fregly and Kim, 1970). We assumed that with the loss of body sodium, the hamsters should show a compensatory increase in NaC1 consumption similar to that shown by rats in the Fregly studies.

With the exception of one female that died between experiments, the subjects were the same hamsters used in Experiment 1. Eight male and five female hamsters served in the present experiment. Following 6 days when their intake of 0.71 M NaC1 and water was assessed, we gave each hamster daily injections of 3.0 mg of hydrochlorothiazide dissolved in 0.25 ml of water and Tween 80 (Baker) for 3 consecutive days. We recorded the hamsters' intake of water and 0.71 M NaC1 during this period as well as during the six postinjection sessions.

During the 6 preinjection days, the mean saline preference score of the hamsters was 19.0%. During the subsequent 3 days (Days 7-9), there appeared to be a substantial increase in the hamsters' saline preference ()7 = 31.8%). During the 6 postinjection days (Days 10-15), the hamsters' saline preference (J = 19.7%) returned to their preinjection baseline level. A repeated-measures analysis of variance, where the mean saline preference score during Days 1 to 6, 7 to 9, and 10 to 15 was dealt with as a within-subject variable, indicated a significant effect IF (2, 24) = 10.96), P < 0.01].

An examination of the absolute intake of water and saline of the hamsters during the various phases of the experiment produced the fol- lowing results. During the 6 preinjection days, the mean saline intake of the hamsters was 1.97 ml and that of water was 8.55 ml. During Days 7 to 9 the hamsters' mean saline intake was 3.07 ml and that of water was 6.16 ml. During the 6 postinjection days the hamsters showed a mean saline intake of 2.10 ml in contrast to a mean water intake of 8.49 ml. A repeated-measures analysis of variance, where the mean saline intake of the animals during Days 1 to 6, 7 to 9, and 10 to 15 was dealt with as a

SALINE INTAKE IN HAMSTERS 479

within-subject variable, did not yield any significant effects (P > 0.05). A similar analysis of the animals' water intake during the three phases of the experiment yielded a significant effect IF (2, 24) = 10.52, P < 0.01]. Multiple comparisons of the pairs of test conditions indicated that the hamsters drank less water during Days 7 to 9 than they did during the periods before and after the injections.

The results of Experiment 2 are interesting in that sodium-depleted hamsters appeared to show an increased preference for hypertonic saline even though they really did not show a compensatory increase in saline consumption. The apparent change in saline preference arising from the treatment was due to an artifact. The hamsters showed a decrease in water intake during the injection days but maintained their saline intake at the level they had exhibited during the pre- and postinjection periods. One might wonder if we had administered the appropriate dose of hy- drochlorothiazide to the hamsters. The amount of the diuretic (3 mg/100 g body weight) used in the present experiment was comparable to the amount (2 rag/100 g body weight) used in the Wong and Wilson (1973) and Wot~g and Hardisty (1974) studies that had demonstrated sodium appetite in rats.

On the basis of these results one may also question whether hamsters respond to diuretics like rats by showing a significant sodium loss in their urine. A study in progress (Kraintz et al., 1978) indicates that hamsters show a significant increase in urine volume but no significant change in sodium output following diuretic injections. In view of such results, the absence of sodium appetite in hamsters following diuretic injections is not surprising.

EXPERIMENT 3

In addition to studying the effects of adrenalectomy on the hamster's saline intake, Salber and Zucker (1974) also assessed the effects of mineralcorticoid injections on these animals. They found an absence of sodium appetite when the animals were injected with 2.5 mg of desoxy- corticosterone acetate (DOCA). In the present experiment we as- sessed the effects of a DOCA injection that was twice the dose used by Salber and Zucker.

This experiment was conducted 3 months after Experiment 2 had been completed and the subjects were the 11 surviving members (seven males and four females). Following 3 days during which their intake of 0.71 M NaCI and water was assessed, we gave each hamster daily injections of 5.0 mg of DOCA dissolved in 0.25 ml of peanut oil over 3 consecutive days. We recorded the hamsters' intake of water and 0.71 M NaCI during this period as well as during the three postinjection sessions.

In general, the results indicated no significant effects of DOCA injec- tions on either the saline preference or the absolute saline intake of the

480 WONG AND JONES

hamsters (P > 0.05). These results support Sa lber and Zucker ' s (1974) fai lure to produce sodium appet i te in the animals with a lower dose of DOCA. These exper imenters had suggested that the ancestors of the present laboratory hamsters had inhabited a desert env i ronment where salt was readi ly avai lable. For this reason hamsters did not exper ience salt def ic iencies that would select for those who evo lve adrenal salt -retaining capabi l i t ies. Even when depleted of body sodium (Exper iment 2), hamsters do not show the adapt ive react ion of seeking and consuming hyperton ic saline that is character is t ic of most mammals .

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