Dietary self-selection in insulin-injected hamsters

Physiology&Behavior,Vol. 35, pp. 1-8. Copyright©Pergamon Press Ltd., 1985. Printed in the U.S.A.

0031-9384/85 $3.00 + .00

Dietary Self-Selection in Insulin-Injected Hamsters D A V I D D I B A T T I S T A 1 A N D K. J E N N I F E R

HELM

Department o f Psychology, Concordia University, Montreal, Quebec, Canada R e c e i v e d 23 J u l y 1984 DIBATTISTA, D. AND K. J. HELM. Dietary self-selection in insulin-injected hamsters. PHYSIOL BEHAV 35(1) I-8, 1985.--Adult male golden hamsters were given access to a variety of nutrient sources and were observed following the administration of regular insulin. It was hypothesized that if insulin produced hyperphagia in hamsters by the activation of a glucoprivic feeding mechanism, a selective increase in carbohydrate consumption would be observed. All animals received subcutaneous injections of 10, 30, 50 and 100 units/kg of insulin as well as a control injection of saline. Food consumption was recorded at +3, +6 and +24 hours after injections. In Experiment 1 hamsters having continuous access to Purina lab chow, fat (Crisco) and sucrose (sugar cubes) increased their total caloric consumption in response to insulin, but did not do so by selectively increasing their carbohydrate intake. In Experiment 2 hamsters maintained on Purina chow and sugar cubes consistently increased their carbohydrate intake as well as their total caloric consumption in response to insulin, but again the increase in carbohydrate intake was not selective; increased consumption of both sugar cubes and Purina chow occurred, and neither the proportion of total calories derived from carbohydrate nor the proportion of total calories derived from sugar cubes was affected by insulin administration. The results support the conclusion that insulin-induced hyperphagia in hamsters results from the activation of a non-glucoprivic feeding mechanism. Golden hamster

Insulin

Dietary self-selection

A number of studies have indicated that experimentally induced glucoprivation produces increases in food consumption in a variety of species. F o r example, insulin administration causes marked decreases in plasma glucose (PG) concentration and increases in food consumption [11, 22, 27, 32]. Furthermore the glucose analogs 2-deoxyglucose (2DG) adn 5-thioglucose (5TG), which produce glucoprivation by interfering with the normal metabolism of glucose [2, 3, 35, 36], also cause hyperphagia [11, 12, 26]. Taken together, such evidence supports the glucostatic theory of food intake, which proposes that any significant reduction in the availability of glucose stimulates specialized receptor cells in the brain, which in turn directly trigger the feeding response [20]. Interestingly, most o f the available evidence suggests that glucoprivation is not an effective hyperphagia-inducing stimulus in the golden hamster. Indeed the feeding behaviour of hamsters differs in a number of ways from that of more commonly studied laboratory animals. F o r example, hamsters do not increase their food consumption after periods of food deprivation under a wide variety of experimental circumstances [28,31], although rats, in contrast, compensate quite well even for frequent and prolonged periods of deprivation [17,31]. Hamsters also appear to lack an opiatesensitive feeding mechanism, as neither opiate agonists nor antagonists influence hamster food consumption [18]. With respect to glucoprivation, several studies have shown that hamsters do not increase their food consumption in response to either 2DG [25, 27, 29, 30] or 5TG [5]. However, like other

Hyperphagia

Glucoprivation

species examined, hamsters do become hyperglycemic when these glucose analogs are administered [5, 11, 12, 25, 27, 29], so the hamster's failure to increase food intake cannot be ascribed simply to a failure to detect the metabolic consequences of 2DG and 5TG. There are indications that hamster feeding behaviour may, under appropriate circumstances, be influenced by reductions in glucose availability. Hamsters do become hyperphagic in response to insulin administration [6, 7, 25, 27], and while the pattern of the hyperphagic response differs from that of rats [7], it does appear that insulin-induced hyperphagia occurs in hamsters only in conjunction with marked hypoglycemia [6,7]. In fact, there is a significant direct relationship between the hyperphagic and the glucose-lowering effects of various doses of insulin [7]. While the data concerning insulin-induced hyperphagia in the hamster are clearly compatible with the glucostatic theory, it is noteworth that insulin produces other metabolic and physiological effects, in addition to hypoglycemia, which may contribute to the occurrence of the hyperphagic response. First, insulin administration in hamsters causes decreases in plasma concentrations of free fatty acids and probably also of ketones [6,29], both of which may serve as alternative sources of metabolic fuel [9,33]. In addition, a hyperphagiainducing dose of insulin markedly accelerates emptying of both the pregastric and the gastric pouches in the hamster [8]. Because either of these effects may influence food consumption, it is not possible to ascribe insulin-induced hyper-

1Requests for reprints should be addressed to David DiBattista, Department of Psychology, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec, H4B 1R6.

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DIBATTISTA A N D H E L M

phagia in the hamster exclusively and directly to the effects of hypoglycemia. In summary, the existence of a glucoprivic control of feeding in the hamster remains open to question. While evidence from studies involving insulin administration is compatible with the glucostatic theory, data from 2DG and 5TG experiments suggest that a glucoprivic control does not exist in the hamster. Because a new approach to the investigation of this issue may prove worthwhile, it is suggested here that if a glucoprivic control of feeding does exist in the hamster, insulin-injected hamsters given access to a fractionated diet should selectively increase their consumption of carbohydrate. Such a preference for carbohydrate would be adaptive because, of the three macronutrients (protein, fat and carbohydrate), carbohydrate would be expected to most rapidly and efficiently offset the insulin-induced glucoprivation. Indeed, evidence from other species supports the conclusion that insulin administration generally causes selective increases in carbohydrate consumption. Working with humans, Mayer-Gross and Walker [21] found that insulininduced hypoglycemia caused an increased preference for more concentrated sucrose solutions, which these investigators suggested was due to an adaptive decrease in sensitivity for sweet tastes.. Several experiments with rodents have provided clear evidence of selective increases in carbohydrate intake in response to both regular and long-acting insulin. When allowed to choose among macronutrient sources, mice increase their consumption of a highcarbohydrate diet in response to regular, short-acting insulin [1]. Rats on a dietary self-selection regimen have also been found to selectively increase their intake of a pure carbohydrate source (sucrose) when injected with regular insulin [ 19]. Richter [24] had previously observed that rats receiving protamine zinc insulin and offered both a standard diet and a 40% dextrose solution selectively increased their intake of the dextrose solution. More recently, Kanarek, MarksKaufman, and Lipeles [15] allowed rats access to a fractionated diet and administered daily injections of NPH insulin over 19 days. They observed that insulin-injected animals more than doubled their carbohydrate consumption, but did not increase their intake of either protein or fat over baseline values. Given this background of evidence it is entirely reasonable to propose that, if hamsters possess a glucoprivic control of feeding, they too will demonstrate selective increases in carbohydrate consumption in response to insulin administration. On the other hand, if insulin-induced hyperphagia depends upon the activation of a non-glucoprivic mechanism in hamsters, as has previously been suggested [8,25], one would expect that increases in macronutrient consumption would be relatively nonselective. In the present studies, the nutrient choices of hamsters injected with regular insulin were examined.

Animals were assigned to either of two groups (N=9/group) on the basis of body weight. Throughout the experiment, the Chow group had access only to Purina Rodent Chow No. 5012-7 (22.5% protein, 5% fat, 51.1% carbohydrate; 3.39 kcal/g). The Choice group received continuous free access to a diet consisting of Purina chow, as well as sugar cubes (Atlantic Sugar, Montreal, Quebec; 99.96% sucrose by manufacturer's analysis; 4 kcal/g) and solid vegetable shortening (Crisco, Proctor and Gamble; 100% fat; 9 kcal/g). Purina chow and sugar cubes were placed on the floor of the cage, which was covered with a f'me wire mesh screening to prevent loss of small pieces of food. Fat was presented in glass food cups. Spillage was accounted for throughout the experiments. Procedure Four weeks after arriving in the laboratory, during which time all hamsters received Purina chow, the Choice group was begun on the self-selection diet. Because these animals rapidly gained weight over the next three weeks, the actual experiment was not begun until the rate of body weight gain for the Choice group had stabilized relative to the Chow groups. Mean body weights at this time were 143 g and 122 g for the Choice and Chow groups, respectively. On subsequent feeding test days, hamsters in both groups were injected subcutaneously with either regular insulin (Insulin Toronto; Connaught Laboratories, Toronto) or sterile physiological saline (0.9%). Insulin was administered in dosages of 10, 30, 50 and 100 units/kg, diluted in all cases with saline to a final concentration of 10 units/ml. The volume of the saline control injection was 5 ml/kg, which was equivalent t o the volume of the 50 unit/kg dosage of insulin. The various injections were administered to all animals in a counterbalanced order at intervals of 3 to 7 days. All feeding tests began at approximately 1000 hr. Before the start of the test, the hamster was weighed and injected with the solution under investigation. The animal was then immediately returned to its home cage with access to fresh portions of its usual diet, either Chow or Choice. Food consumption was determined to the nearest 0.01 g for all dietary components at +3, +6 and +24 hr after the injection. Several weeks after the last feeding test, all hamsters were humanely sacrificed with an overdose of Nembutal, and determinations were made of carcass weight, naso-anal length, and weights of testes, kidneys, and epididymal and retroperitoneal fat pads. Data were analyzed by t-tests, the analysis of variance, and by appropriate tests for individual comparisons [13,23]. Data collected at the three post-injection intervals were analyzed separately. RESULTS

EXPERIMENT 1 METHOD Subjects and Diets Eighteen adult male golden hamsters (Mesocricetus auratus) were obtained from Charles River Canada (St. Constant, Quebec) and housed individually in hanging wire cages in a room with lights on from 0800 to 2200 hr. Room temperature was 21-24 degrees Celsius, and water was available ad lib at all times.

Total Caloric Consumption of Chow and Choice Groups Total caloric consumption increased as a function of insulin dosage for both the Chow and Choice groups (Fig. 1). Two-way analyses of variance (Diet × Dosage) revealed significant Dosage main effects at +3, +6 and +24 hours post-injection, F(4,64)=5.19, 9.69 and 4.59, respectively, p<0.01 in each case. Dunnett's test for comparing a control group to experimental groups indicated that most of the significant increases in caloric consumption in both the Chow and Choice groups occurred in response to the larger dosages of insulin, especially at +6 hr (see Fig. 1).

N U T R I E N T C H O I C E IN H A M S T E R S

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FIG. 2. Cumulative calories derived from (a) fat, (b) carbohydrate, and (c) protein at +3, +6 and +24 hours following injection of regular insulin. Hamsters had access to a variety of macronutrient sources. Values are means_+SEM. *Significantly different (p<0.05) from saline injection (=0 units/kg).

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FIG. 1. Cumulative caloric consumption at +3, +6 and +24 hours following injection of regular insulin. Hamsters had access to either (a) Purina Chow only or (b) a variety of macronutrient sources. Values are means_+SEM. *Significantly different (p<0.05) from saline injection (=0 units/kg).

A significant effect of diet was detected at +24 hours, F(1,16)=11.90, p<0.01. Further analysis by T u k e y ' s multiple comparison method indicated that hamsters in the Choice group consumed significantly more calories than animals in the Chow group in response to all injections except the 100 unit/kg dosage o f insulin (o<0.05). This difference in the total daily caloric consumption may have been due, at least in part, to the greater body weights of hamsters in the Choice group. Indeed, when the +24 hour data were expressed in terms o f caloric consumption p e r unit of body weight, no significant difference was found between the Choice and Chow groups (data not presented).

Macronutrient Consumption in the Choice Group As indicated above, the total caloric consumption of the Choice group was increased above control values for various insulin dosages at both +3 and +6 hours post-injection. However, an analysis of the number of calories derived from

each of the three macronutrients (Fig. 2) indicated that the insulin-induced increase in total caloric consumption was not always achieved by means of increased carbohydrate ,consumption, as would be expected according to the glucostatic hypothesis. One-way analyses of variance indicated significant increases (O<0.05) at both +3 and +6 hours postinjection both for carbohydrate consumption, F(4,32)=2.83 and 3.16 respectively, and for fat consumption, F(4,32) =4.09 and 2.91 respectively. More specifically, carbohydrate calories were found to be increased (O<0.05) at +3 hours following injection of 10 units/kg of insulin and at +6 hours following 30 and 100 units/kg of insulin, while the number of calories derived from fat was increased at both +3 and + 6 hours in response to 50 units/kg of insulin. The number of calories derived from protein did not change as a function of insulin dosage at any of the post-injection intervals examined. The fact that the insulin-induced increase in total caloric consumption was not due to a selective increase in carbohydrate consumption is highlighted by the finding that sugar consumption was not significantly affected by insulin administration at any post-injection time, F(4,32)=1.53, 0.85, and 0.72 for the three successive post-injection intervals, p >0.05 in each case. On the other hand, analysis of variance of the Crisco consumption data revealed significant effects at both +3 and +6 hours post-injection, F(4,32)=4.10 and 2.70, respectively, p <0.05 in both cases. Dunnett's test further indicated that Crisco consumption was significantly elevated in response to 50 units/kg of insulin at both of these times (O<0.05).

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DIBATTISTA A N D H E L M

Body Measurements

Hamsters in the Choice group were significantly heavier than animals in the Chow group at the end of the experiment (151.4 g vs. 129.0 g, t(16)=2.96, p<0.01), the magnitude of the difference between groups being almost identical to that which was observed just before the experiment had begun (see the Method section). Despite the sizable difference in body weight, no differences were observed either in body length or in weights of kidneys and testes. However both the epididymal and retroperitoneal fat deposits of the Choice animals were more than twice as heavy as those of the Chow animals (4.7 g vs. 2.2 g, t(16)=5.99, p<0.01, and 2.9 g vs. 1.3 g, t(16)=4.62, p<0.01, respectively).

quite obese as a function of their dietary regimen before the insulin experiments had even begun. While these animals did demonstrate insulin-induced hyperphagia, it is possible that their nutrient consumption patterns in response to insulin may not be the same as would those of normal weight hamsters. Because of the presence of these complicating factors in Experiment I, a second experiment was performed in which hamsters were maintained on a regimen of Purina chow and sugar cubes. The use of such a diet eliminates the complications mentioned above, yet it still provides hamsters with the opportunity to selectively increase their carbohydrate consumption in response to insulin without increasing their intake of the other dietary macronutrients.

DISCUSSION The major finding of Experiment 1 is that the insulininduced increase in caloric consumption which occurs in hamsters having access to a choice of nutrient sources is not achieved by a selective increase in carbohydrate consumption. This pattern of results is quite different from that which has been observed previously in other species. When allowed to choose among macronutrient sources, rats selectively increase their carbohydrate consumption in response to either acute [19] or chronic [15,24] insulin administration. Furthermore, acute glucoprivation induced by 2DG also causes rats to increase their carbohydrate intake, while reducing fat consumption [16]. These results suggest that the hyperphagia which occurs in rats as a consequence of either insulin or 2DG administration is the result of a specific hunger for carbohydrate. Moreover, the changes which occur in the pattern of macronutrient selection in response to insulin and 2DG are not specific to either the acute or the chronic situation, nor do they appear to vary as a function of the nature of the giucoprivic stimulus. Hamsters, on the other hand, did not always increase their carbohydrate intake even when their total caloric intake did increase as a function of insulin administration. Indeed, fat consumption sometimes increased significantly while carbohydrate consumption did not, a pattern which has not been observed in other species. These behavioural differences between species cannot be attributed merely to differences in the nature of the dietary carbohydrate sources used in the various studies. In both the present study and in that of Matsuo et al. [19], a pure carbohydrate source was provided in the form of sucrose, yet hamsters and rats demonstrated very different patterns of macronutrient choice in response to the administration of short-acting regular insulin. The failure of the hamsters in the Choice group to selectively increase their carbohydrate intake in response to acute administration of insulin supports the conclusion that a glucoprivic control for feeding does not exist in the hamster, but certain other factors must also be considered before accepting this interpretation. F o r example, hamsters in the Choice group ordinarily derived over 50% of their total calories from Crisco, but only about 10% from sugar. This initial strong preference for pure fat as compared to pure sugar may have been sufficient to influence the outcome of Experiment 1. That is, even assuming the existence of a glucoprivic feeding mechanism in hamsters, the insulin-injected hamsters may not have always made the presumably more adaptive response of increasing their sugar consumption because of an overriding preference for pure fat. Another possible complication in this experiment involves the fact that the hamsters in the Choice group were

EXPERIMENT 2 METHOD Twelve adult male golden hamsters ranging in weight from 103 to 118 g were obtained from Charles River Canada in a separate batch from those used in Experiment 1. Several weeks after their arrival, these animals were all given continuous access to a diet consisting of Purina chow and sugar cubes, except as noted below. After a three week period of adaptation to this regime, during which time these animals did not gain significantly more weight than an untreated group of animals from the same shipment, ten injections were administered to each animal in a counterbalanced order at intervals of 3-10 days. Each animal was injected subcutaneously on two separate occasions with each of the following: saline, and 10, 30, 50 and 100 units/kg of regular insulin. Following one of the injections at a given dosage, the hamster was allowed access only to Purina chow for the duration of the 24 hr feeding test, while on the other occasion both Purina chow and sugar cubes were available as usual. Other details regarding the feeding tests were similar to those of Experiment 1. RESULTS Total Caloric Intake

Total caloric intake increased as function of insulin dosage under both the chow-only and the chow-plus-sugar conditions (Fig. 3). Two-way analyses of variance (Diet x Dosage) indicated significant Dosage main effects at +3, +6 and +24 hours post-injection, F(4,44)=5.74, 17.99 and 2.85 respectively, p < 0 . 0 5 in each case. Dunnett's test further revealed that caloric consumption was elevated at +3 and +6 hours at almost all insulin dosages regardless of dietary regimen (p<0.05); however at +24 hours, none of the insulin dosages caused increased caloric intake for either dietary condition. Diet main effects were also significant at +3, +6 and +24 hours, F(1,11) =25.34, 37.50 and 64.09 respectively, p<0.01 in each case. Thus hamsters consumed significantly more total calories when having access to both Purina chow and sugar cubes than when allowed to consume only Purina chow. This result must be interpreted cautiously. It must not be taken to mean that the animals of Experiment 2, which were maintained on Purina chow and sugar cubes, were chronically consuming more total calories than hamsters normally maintained on Purina chow alone. Wade [34] has found that hamsters given continuous access to Purina chow and a 32% sucrose solution derived more than 60% of their total calories from sucrose, but neither gained more weight

NUTRIENT CHOICE IN HAMSTERS

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FIG. 4. Cumulative calories derived from carbohydrate at +3, +6 and +24 hours following injection of regular insulin. Hamsters had access to Purina chow and sugar cubes. Values are means-SEM. *Significantly different (p <0.05) from saline injection (=0 urLits/kg).

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FIG. 3. Cumulative caloric consumption at +3, +6 and +24 hours following injection of regular insulin. Hamsters had access to either (a) Purina chow only or (b) Purina chow and sugar cubes. Values are means_+SEM. *Significantly different (0<0.05) from saline injection (=0 units/kg).

nor consumed more total calories than a chow-maintained control group; thus the availability of sucrose in addition to the regular diet does not lead to hyperphagia and obesity in hamsters as it typically does in rats [4,14]. In support of this interpretation, it will be recalled that the hamsters of Experiment 2 did not gain more weight than a control group maintalned on Purina chow even after having had access to sugar cubes for three weeks. Furthermore there was no difference in the 24 hour total caloric consumption following saline administration of the hamsters of Experiment 1 which were maintained on Purina chow only (Fig. la) and the hamsters o f Experiment 2 which had access to both Purina chow and sugar cubes (Fig. 3b). It appears therefore that the difference in total caloric consumption between the chow-only and the chow-plus-sugar conditions of Experiment 2 was not due to chronic caloric overconsumption due to the availability of sucrose, but rather resulted from a temporary depression in caloric consumption during the several 24 hour periods in which the hamsters had access only to Purina chow instead of to their normal regimen o f Purina chow and sugar cubes.

Carbohydrate Consumption Under the Chow-Plus-Sugar Condition Data regarding carbohydrate consumption for animals with access to both Purina chow and sugar cubes are summarized in Fig. 4. One-way analyses o f variance revealed significant Dosage effects at +3 and +6 hours, F(4,44)=2.72 and 7.97 respectively, p < 0 . 0 5 in each case, but not at +24 hours, F(4,44)= 1.33. Because total carbohydrate intake may have increased as the result of a rise in consumption of either Purina chow, which is a mixed nutrient source, or sugar, which is pure carbohydrate, it was also necessary to examine the patterns of chow and sugar consumption (Fig. 5). Sugar consumption was significantly influenced by insulin dosage at both +3 and +6 hours post-injection F(4,44)=3.13 and 3.32 respectively, p < 0 . 0 5 in both cases. Dunnett's test further indicated that sugar intake was elevated above baseline at both +3 and + 6 hr for the 10 units/kg dosage and at +6 hr for the 100 units/kg dosage. However, a significant effect of insulin dosage upon chow consumption was also observed at +6 hours, F(4,44)=6.58, p<0.001, with Dunnett's test indicating significant increases in chow consumption at the 30, 50 and 100 units/kg dosages. This pattern of results suggests that hamsters did not selectively increase their intake of the pure carbohydrate source in order to increase their total caloric consumption. Although increases in sugar consumption did sometimes occur, on other occasions it was the intake of Purina chow which increased, and in these cases increased consumption of both protein and fat also necessarily occurred due to the fact that Purina chow consists of fLxed proportions of each of the three macronutrients. The conclusion that insulin did not influence the overall pattern of macronutrient selection, although it did produce increases in total caloric consumption, is greatly strengthened by examination o f Figs. 6 and 7. Analyses of variance indicated that there were no significant effects of insulin dosage at any post-injection interval either upon the proportion of total calories derived from carbohydrate (Fig. 6) or upon the proportion of total calories derived from sugar cubes (Fig. 7). It may be concluded therefore that the insulin-induced increases in consumption of Purina chow and of sugar cubes were of rather similar proportions for the various dosages and time periods under investigation.

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DIBATTISTA A N D H E L M

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FIG. 5. Cumulative consumption of (a) Purina chow and (b) sugar cubes at +3, +6 and +24 hours following injection of regular insulin. Hamsters had free access to both nutrients. Values are means-+SEM. *Significantly different (o<0.05) from saline injection (=0 units/kg).

DISCUSSION

The results of Experiment 2 confLrm the finding that hamsters do not selectively increase their carbohydrate consumption in response to acute insulin administration. This failure to demonstrate a specific hunger for carbohydrate is not a function of body weight, as both the obese hamsters of Experiment 1 and the normal weight hamsters of Experiment 2 demonstrated the same general pattern of results. The lack of selective effect of insulin upon carbohydrate intake cannot be attributed merely to a relative unpalatibility of the sugar cubes, as the hamsters of Experiment 2 ordinarily consumed a substantial proportion (about 30%) of their total daily caloric intake in the form of sugar cubes. Nor can the negative results be ascribed to the inability of hamsters to increase their sugar consumption a b o v e baseline levels, as such increases were in fact observed on several occasions (Fig. 5). In addition, the Choice group of Experiment 1 typically consumed only about 1 g/day o f sugar cubes, as compared to the 2.5 g/day of the animals of Experiment 2, and were thus clearly capable o f consuming far more sugar than they actually did when injected with insulin. Moreover, hamsters having continuous access to both Purina chow and sugar cubes (the dietary regimen of Experiment 2) consistently increase their intake of sugar by over 35% when subjected to partial chow deprivation (DiBattista, unpublished observations). Thus the failure of hamsters to selectively increase their carbohydrate consumption in response to insulin cannot be attributed either to unpalatibility of the pure carbohydrate source or to a global inability of

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FIG. 7. Proportion of total calories derived from sugar cubes at +3, +6 and +24 hours following injection of regular insulin. Values are means_+SEM.

hamsters to increase their consumption of pure carbohydrate above the baseline levels in either Experiment 1 or 2. The data from the present experiments therefore support the conclusion that hamsters do not possess a glucoprivic feeding system. However it must be noted that selective increases in carbohydrate consumption in response to glucoprivation have been adequately demonstrated only in two closely related species, mice [1] and rats [15, 16, 19]. F o r this reason, conclusions based upon the failure of insulin-injected hamsters to demonstrate a specific hunger for carbohydrate must be somewhat tentative. It would certainly be helpful to have evidence regarding the effects of insulin and other glucoprivic stimuli upon the macronutrient consumption patterns of a wider variety of animal species, for such information would help to clarify the interpretation of the relevant hamster data and would also be of theoretical importance with respect to the generality of application o f the glucostatic hypothesis. Despite this caveat and the inevitable difficulty of confirming the null hypothesis, the data presented here add further weight to the already existing body of evidence which suggests that a glucoprivic feeding mechanism does not exist in hamsters. Not only do hamsters fail to increase their food intake when injected with either 21)(3 [25, 27, 29, 30] and 5TG [5], but there is also now considerable evidence to support the suggestion of Ritter and Balch [25] that insulininduced hyperphagia in hamsters actually results from the activation o f a non-glucoprivic mechanism. F o r example, several researchers have proposed, in keeping with the glucostatic hypothesis, that insulin-induced hyperphagia

N U T R I E N T CHOICE IN HAMSTERS

7

serves as a behavioral mechanism for glucose homeostasis by increasing the supply of nutrients from the gastrointestinal tract (e.g., [15]). It has been found in hamsters however that hyperphagia does not serve to counteract insulininduced hypoglycemia in either the short or the long term [8]. In addition, insulin-induced hyperphagla is achieved in hamsters by an increase in meal frequency [7], and the pregastric pouch, which normally fills and empties with a cycle similar to that of spontaneous meal consumption, empties far more rapidly than normal under the influence of a hyperphagia-inducing dosage of insulin [8]. These findings suggest that the hyperphagic effect of insulin in hamsters may be mediated largely by an acceleration of stomachemptying rather than by the direct stimulation of a glucoprivic feeding mechanism. Finally, the fact that appreciable quantities of volatile fatty acids are produced in the fermentative pregastric pouch and absorbed into the circulation [10] suggests that glucose may play a less important role as an energy source in hamsters than it does in other rodents. For example, the data of Experiment 1 demonstrate that hamsters allowed access to a variety of nutrient sources derive more than two-thirds of their total daily caloric intake from fat and only one-quarter from carbohydrate. This contrasts sharply with the pattern observed in rats, which derive approximately equal proportions of their total daily caloric intake from each of these macronutrients when maintained on a fractionated diet [16,19]. Furthermore, insulin-injected hamsters with very low circulating glucose levels frequently

show no signs of neurological impairment [8,27], perhaps because of the continuing availability of some alternate metabolic fuel(s) capable of preventing the occurrence of the behavioural deficits which typically accompany hypoglycemia. The prediction derived from the glucostatic hypothesis that selective increases in carbohydrate consumption will occur in response to glucoprivation has previously been confirmed in rats [15, 16, 19] and mice [1]. In contrast, the relatively non-selective increase in macronutrient consumption observed in the present experiments in hamsters does not support this hypothesis, but is entirely compatible with the interpretation that insulin-induced hyperphagla in the hamster is largely dependent upon an acceleration of stomach-emptying [8]. The results presented here thus further support the conclusion that insulin-induced hyperphagia in hamsters does not result from the activation of a glucoprivic feeding mechanism, and that such a mechanism may be lacking in this species.

ACKNOWLEDGEMENTS This research was supported by Grant A7873 from the Natural Sciences and Engineering Research Council of Canada and by a grant from the Committee on Aid to Scholarly Activity of Concordia University. The authors wish to thank Nancy Brennan for typing the manuscript and Carolyn DiBattista for preparing the figures.

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