Operant responding for dietary protein in the golden hamster (Mesocricetus auratus).

Golden hamsters housed in operant chambers over a period of weeks had ad lib access to a maintenance diet that was either nutritionally complete (NCMD) or protein-free (PFMD), and they were required to press a lever on a fixed-ratio (FR) schedule to obtain 20-mg high-protein pellets. As the FR requirement increased, hamsters maintained on the NCMD made fewer lever presses and ate fewer pellets, and at the highest FR levels, they earned very few pellets. In contrast, hamsters maintained on PFMD increased the number of lever presses as the FR requirement increased, and they only slightly reduced the number of pellets eaten. Even at the highest FR requirement levels, PFMD hamsters still derived an average of 11-12% of total calories from protein, a level of intake that is either adequate for adult hamsters, or very nearly so. Previous research has shown that hamsters make adaptive behavioural adjustments in response to time-restricted access to dietary protein, and the present findings demonstrate that protein-restricted hamsters that must press a lever to obtain protein-rich pellets also make adaptive behavioural adjustments when challenged with increases in the FR requirement.

Role of learning in the selection of dietary protein in the golden hamster (Mesocricetus auratus).

Hamsters (Mesocricetus auratus) were kept for several weeks on maintenance diets (MDs) that were either nutritionally complete or protein deficient, and had periodic access to protein-free and high-protein conditioning diets (CDs) with marker flavors (anise and clove). In Experiment 1, protein-restricted hamsters came to prefer the flavor of high-protein CDs but did not prefer unflavored high-protein CD. Thus, hamsters learned to select dietary protein by attending to the flavor of the CD. In Experiment 2, a within-subjects design was used, and MDs also had marker flavors (garlic and sage). Hamsters came to prefer the flavor of high-protein CD when protein restricted, and they showed this preference even in the absence of protein restriction if reexposed for only 90 min to merely the flavor of a protein-deficient MD. Thus, learned associations between the flavor and the usual postingestional consequences of a recently ingested MD can affect short-term dietary protein selection.

The effect of fat and carbohydrate content of the diet on voluntary ethanol intake in golden hamsters.

Golden hamsters, which avidly consume ethanol solutions, had continuous access to food and water and to either 15% or 30% ethanol solution (v/v) over a period of weeks. Hamsters consumed approximately equal amounts of absolute ethanol when maintained on either a Purina Chow diet or a high-carbohydrate, low-fat diet (7.6 and 7.1 g/kg/day, respectively), but they consumed substantially less ethanol when maintained on a high-fat, low-carbohydrate diet (4. 4 g/kg/day). However, the pattern of differences among groups of hamsters maintained on different diets was not the same at the two ethanol concentrations. Thus, at the 15% concentration, HC hamsters consumed more than twice as much ethanol as HF hamsters, but at the 30% concentration, the difference in ethanol consumption was greatly reduced and did not prove to be statistically significant. These results indicate that it is important to consider the concentration of the ethanol solution when studying the effects of dietary macronutrient content on ethanol consumption.

Dietary protein restriction and selective preference for a protein-containing diet in the golden hamster (Mesocricetus auratus).

Two experiments were performed to examine protein appetite in golden hamsters (Mesocricetus auratus). In Experiment 1, hamsters were maintained for 10 days on either a protein-free or a nutritionally complete maintenance diet, and they were also given access to protein-rich and carbohydrate-rich test diets for 6 h/day. Hamsters maintained on the protein-free diet strongly preferred the protein test diet, but hamsters on the complete diet showed no such preference even when their caloric intake was matched to that of hamsters on the protein-free diet. In Experiment 2, hamsters that had developed a preference for the protein test diet while maintained on the protein-free diet were given Purina Chow for 25 days to permit them to recover from their protein deficiency. When later maintained on the complete diet, these hamsters did not demonstrate a preference for the protein test diet when maintained on the complete diet, but did so when returned to the protein-free maintenance diet. These findings indicate that dietary protein restriction causes hamsters to develop a strong preference for a protein-rich diet and that this preference may be manifested only in response to a physiological need for protein.

Enhanced preference for a protein-containing diet in response to dietary protein restriction.

Rats were maintained for 10 days on either a protein-free or a nutritionally complete maintenance diet, and they were also given access to protein-rich and carbohydrate-rich test diets during separate daily 2-h test sessions. In Experiment 1, rats maintained on the protein-free diet gradually and selectively increased their intake of the protein test diet, and eventually derived 16% of their daily energy intake from the protein test diet. Rats maintained on a nutritionally complete diet ate similar amounts of the two test diets even when their total caloric intake was matched to that of rats maintained on the protein-free diet. In Experiment 2, rats that developed a preference for the protein test diet while maintained on a protein-free diet were given Purina Chow for 25 days to allow them to recover from their protein deficiency. When these rats were later returned to the protein-free diet for 10 days, their preference for the protein test diet was immediate and sustained. However, if they were maintained on the nutritionally complete diet after the 25-day recovery period, they initially preferred the protein test diet, but this preference diminished over days. Results of these studies are consistent with other findings showing that rats can learn to compensate for macronutrient deficiencies by using oral-sensory cues. In particular, the rats' diet selection was consistent with their having learned a preference for the cues paired with dietary protein.

Dietary energy shortage and ethanol intake in golden hamsters.

Adult male golden hamsters, which are avid consumers of ethanol solutions, were maintained on powdered Purina chow and tap water, and they were permitted continuous access to either a 15% or a 30% ethanol solution (v/v). In two experiments, the effect of chronic dietary energy shortage on ethanol consumption was examined. Energy shortage was produced either by food restriction (Experiment 1) or by dilution of the diet with nonnutritive cellulose (Experiment 2). Dietary energy shortage caused increases of up to 50% in ethanol consumption, and the energy derived from ethanol offset the short-fall in food-derived energy by up to 50%. When normal feeding conditions were reinstated, hamsters maintained their enhanced intake of ethanol solutions, consuming up to 15.4 g/kg/day of ethanol. Possible factors underlying the enhanced intake of ethanol solution both during and after dietary energy shortage are discussed.

Surgical removal of the pregastric pouch does not affect voluntary ethanol consumption in golden hamsters.

Golden hamsters underwent either surgical removal of the pregastric pouch (PGX) or sham surgery. Following recovery from surgery, hamsters were given free access to 15% (v/v) ethanol solution for 40 days, and then to 30% ethanol solution for a further 32 days. PGX and sham-operated hamsters ingested similar amounts of absolute ethanol throughout the study, with intake exceeding 15 g/kg/day when 30% ethanol solution was available. The failure to find a significant effect of surgical treatment on ethanol intake was not attributable to a lack of statistical power, but rather suggests that other factors, such as the hamster's high levels of hepatic alcohol dehydrogenase, are responsible for the hamster's unusual tolerance for ethanol.

Dietary salt and voluntary ethanol consumption in golden hamsters.

Adult male golden hamsters were maintained on powdered Purina chow and tap water, and were permitted continuous access to either a 15% or a 30% ethanol solution (v/v); after an initial 4-5 weeks of ethanol availability, hamsters had stabilized their intakes and were deriving an average of 1.25 and 1.96 g/day of absolute ethanol from the 15% and 30% solutions, respectively. When salt was added to the diet in increasing concentrations ranging from 4% to 10% over a period of 40 days, hamsters reduced chow-derived calories by up to 35%, increased tap water consumption by up to 50%, and increased consumption of ethanol solutions by up to 100%; when unadulterated Purina chow was reinstated, intakes of chow-derived calories, tap water, and ethanol solutions returned to baseline levels. Hamsters that were continuously maintained on unadulterated Purina chow, but with chow-derived calories matched to that of animals on the salt-adulterated diet, significantly increased their ethanol intake, but not their tap water intake; the increase in their ethanol intake was only about half as large as that of hamsters that had salt added to the diet, but the increase persisted even after ad lib feeding was reinstated. The results indicate that the addition of salt to the diet of hamsters produces large increases in ethanol consumption; furthermore, the increased ethanol intake is not simply the result either of a nonselective increase in fluid consumption or of the reduction in food intake that accompanies the addition of salt to the diet. Results are related to the possible role of the renin-angiotensin system in the control of ethanol consumption in the golden hamster.

Effects of time-restricted access to protein and of oral-sensory cues on protein selection.

The effects on protein consumption of restricting access to protein and of varying the oral-sensory properties of protein diets were measured. During the initial phase of the study, rats were maintained on a self-selection diet in which three different macronutrient sources (carbohydrate, fat, and either soy-based or casein-based protein diets) were continuously available. For the remaining 9 days of the study, half of the rats were protein deprived for 23 h each day and the other half continued to receive the same protein diet during this 23-h period. The remaining 1 h of each day was a test period in which all rats had access to a protein diet that was either the same as or different from the one they had received in the initial phase. Compared to the nonrestricted rats, the protein-restricted rats consumed diet during 1-h test periods. For the nonrestricted rats, those that received diet during 1-h test periods. For the nonrestricted rats, those that received a different protein diet during the 1-h test periods consumed 60% more of the protein diet than did those that received the same protein diet. These results indicate that increases in protein consumption following protein deprivation can be attributed, at least in part, to the oral-sensory properties of diets and not necessarily to a specific protein appetite.

Dietary variety enhances meal size in golden hamsters.

The effect of dietary variety on meal size was investigated in golden hamsters. Hamsters ate meals (four courses x 12 min/course) in which either the same food was offered in all four courses (one of powdered Purina chow, shortcake cookie, process cheddar cheese, or milk chocolate) or four different foods were offered in successive courses. Total energy intake was significantly greater in the variety condition than in any of the single-food conditions. Thus, although previous research indicates that the meal size of hamsters is relatively resistant to change, hamsters do increase meal size in response to dietary variety. This finding indicates that the satiety mechanisms of golden hamsters share at least some important features with those of other species.

Surgical removal of the pregastric pouch reduces voluntary lactose consumption in golden hamsters.

Unlike other adult mammals, golden hamsters voluntarily consume substantial amounts of the disaccharide sugar lactose. It was hypothesized that the hamster's readiness to consume lactose depends upon its possession of a pregastric pouch in which ingested lactose is broken down into its constituent monosaccharides, thus preventing the aversive gastrointestinal symptoms that monogastric animals experience following lactose ingestion. Adult golden hamsters underwent either surgical removal of the pregastric pouch or sham surgery. Following recovery from surgery, hamsters were given continuous free access for 14 days to tap water, Purina Rodent Chow, a 24% sugar solution (either lactose or sucrose), and a Purina chow-based diet containing 30% added sugar (for each hamster, the same sugar that was provided in solution). Surgical removal of the pregastric pouch caused a 40% reduction in total lactose consumption: this reduction resulted from equivalent decreases in consumption of the lactose solution and the lactose-containing diet. Pouch removal had no effect on total sucrose consumption, and sham surgery had no effect on either lactose or sucrose consumption. These results support the conclusion that the adult hamster's unusually high level of voluntary lactose consumption depends upon its possession of a pregastric pouch, which plays an important role in lactose digestion.

Primary school teachers' beliefs and advice to parents concerning sugar consumption and activity in children.

The available body of scientific evidence does not support the notion that sugar consumption has significant adverse effects on children's behaviours; however, responses to a mailed questionnaire indicated that more than 80% of 389 Canadian primary school teachers believed that sugar consumption contributes to increasing activity of normal children and to the behavioural problems of hyperactive children. Moreover, in the previous three years, 55% of all respondents had counselled the parents of children whom they believed to be hyperactive to consider reducing their child's sugar consumption to control the child's activity, and parents frequently did so. These results indicate that teachers need to be provided with accurate and up-to-date information about the effects of sugar on children's behaviour and about effective treatments for hyperactivity. Possible origins of beliefs about the reputed adverse behavioural effects of sugar and about factors that may serve to perpetuate these beliefs are discussed.

Voluntary lactose ingestion in gerbils, rats, mice, and golden hamsters.

Over a period of 20 days, adult male gerbils, rats, mice, and hamsters were allowed to choose between tap water and a sugar solution (either sucrose, glucose, or lactose) presented in increasing concentrations (maximum concentration = 24% weight/volume). Rats, mice, and hamsters preferred both glucose and sucrose solutions to water across a wide range of concentrations; gerbils preferred sucrose solutions at concentrations of 8% and above, but preferences for glucose solutions were not significant. Gerbils, mice, and rats did not prefer lactose solutions to water at any concentration, and actually preferred water at higher lactose concentrations; in contrast, hamsters preferred lactose solutions to water at concentrations of 4% and above, and never preferred water to lactose solutions. As solution concentrations increased, all species consumed increasing amounts of glucose and sucrose (i.e., solute). The lactose intake of gerbils, rats, and mice tended to remain quite low even as solution concentration increased; in contrast, the lactose intake of hamsters was substantially greater than that of other species and increased to a maximum of 1.95 g/100 g body weight/day at the 24% concentration. These results indicate that gerbils and mice, like rats, have a low preference for lactose and consume very little of this disaccharide, and confirm that golden hamsters are exceptional in demonstrating both a preference for lactose solutions and an apparent tolerance to the effects of ingestion of substantial amounts of lactose.

Examination of the negative alcohol-deprivation effect in the golden hamster (Mesocricetus auratus).

When ethanol-consuming animals are denied access to their ethanol solution for a period of days, there is typically a temporary but substantial increase in their ethanol consumption when the solution is returned. Golden hamsters are unusual in that they actually decrease their consumption of a 7% ethanol solution (v/v) under these circumstances. There experiments were therefore undertaken to further investigate this unusual negative alcohol-deprivation effect (ADE) in hamsters. In Experiment 1, the negative ADE was observed across a wide range of ethanol concentrations; adult male hamsters were given access to food, water, and either a 7.5, 15, or 30% (v/v) ethanol solution, and when the ethanol solution was withdrawn for seven days and then returned, ethanol consumption decreased significantly for several days and then recovered. Experiment 2 demonstrated that similar negative deprivation effects occur with glucose (15% w/v) and saccharin (0.1%) solutions, suggesting that the nutritional and pharmacological properties of ethanol do not play an important role in the negative ADE of hamsters. In Experiment 3, when hamsters with continuous access to either an ethanol, glucose, or saccharin solution were switched to an alternate-days access schedule, their intake of solutions decreased substantially, supporting the conclusion that a common mechanism accounts for the golden hamster's negative deprivation responses to ethanol solutions and to other solutions, both nutritive and nonnutritive. Hypotheses relating to the mechanism underlying negative deprivation effects are presented and discussed.

Lactose ingestion in the adult golden hamster (Mesocricetus auratus).

Adult mammals generally demonstrate a lower preference for the disaccharide sugar lactose than for any other common sugar, and because adults typically have low levels of the intestinal enzyme lactase, lactose ingestion may cause gastrointestinal distress. The lactose intake of adult golden hamsters was examined in three experiments; the main findings were: (a) hamsters allowed to choose between tap water and lactose solutions (maximum concentration = 32% weight/volume) over a 20-day period showed a clear preference for lactose solutions and ingested substantial quantities of lactose (up to 3 g/100 g body weight/day) without noticeable adverse effects; (b) hamsters consuming a single diet with lactose added (maximum concentration = 50%) over a period of days ingested up to 3.42 g/100 g body weight/day of lactose without noticeable adverse effects; (c) both hamsters with prior exposure to lactose solutions and those without such exposure consumed similar amounts of 32% lactose solution over an 8-day period, suggesting that hamsters' lactose intake does not depend on the occurrence of adaptation. It is suggested that the fermentative capacity of the hamster's pregastric pouch may underlie this animal's unusual tolerance for lactose.

Effects of time-restricted access to protein and to carbohydrate in adult mice and rats.

Specific appetites for protein and carbohydrate were investigated by allowing mice and rats time-restricted access to either protein or carbohydrate. Animals were maintained over a period of weeks on a self-selection regimen consisting of three separate macronutrient sources (carbohydrate, fat, and protein). After a baseline phase during which all three diets were freely available, protein-restricted animals were allowed access to the protein diet only during a one-hour period each day, but other diets were always available; access of carbohydrate-restricted animals to the carbohydrate diet was restricted in a similar fashion. Control animals had continuous free access to all three diets throughout the experiment. During the restriction phase, protein-restricted animals showed substantial selective increases in protein intake during the one-hour periods of protein availability, consuming 40-45% of their normal daily protein intake during this phase of the experiment. In contrast, carbohydrate-restricted rats demonstrated no evidence for the development of a carbohydrate appetite, and the carbohydrate appetite of mice was weak and transient. These results are related to current notions concerning specific appetites.

Effects of thirst-inducing stimuli on consumption of ethanol solutions by golden hamsters.

Two experiments were performed to examine the acute effects of thirst-inducing stimuli upon the intake of tap water and ethanol solutions by golden hamsters, a species which avidly consumes ethanol solutions. In Experiment 1, three groups of adult male hamsters (n = 6/group) were maintained on Purina chow and tap water; hamsters in two of the groups also had access to one of two ethanol solutions (15% or 30%, v/v). Animals were deprived at various times of either one or both fluids for 24 hr, and then either one or both fluids were presented during a 2-hr drinking test. Total water intake increased substantially following both selective water deprivation and total fluid deprivation whenever tap water was available during the drinking test, but no significant changes occurred when only the ethanol solution was available. Both total fluid deprivation and selective ethanol deprivation produced similar increases in ethanol consumption, but selective water deprivation did not, suggesting that the temporary removal of tap water has little direct effect upon ethanol intake in hamsters, at least at the ethanol concentration levels studied here. In Experiment 2, thirst was induced by a subcutaneous injection of 10% saline (1 ml/100 g). This procedure produced large increases in total water intake whenever tap water was available during the drinking test, but ethanol intake did not change under any circumstances. These results suggest that factors that acutely enhance water intake have little or no effect upon the ethanol consumption of golden hamsters.

Conditioned taste avoidance induced by lactose ingestion in adult rats.

Because adult rats have very low levels of the intestinal enzyme lactase, the ingestion of appreciable quantities of the disaccharide lactose may cause gastrointestinal distress. The present experiment was designed to demonstrate that adult rats will learn to avoid previously neutral stimuli which have been paired with lactose ingestion. Adult rats ingested both a novel solution [either tap water (WA) or 0.1% saccharin (SA)] and a novel food substance (49% powdered lab chow + 50% added disaccharide + 1% saccharin) during a single conditioning session. The added disaccharide was either sucrose (group SU-SA), lactose (groups HL-SA and HL-WA), or equal amounts of these two disaccharides (group LL-SA); a fifth group (LC-SA) consumed a sucrose-containing diet to which lithium chloride was added (5 mg per 1 g of diet). Separate feeding tests and drinking tests, carried out over several weeks, were used to assess the extent of conditioned taste avoidance. In the four feeding tests, rats were allowed to ingest powdered lab chow with added saccharin (but without added disaccharide), while in the four drinking tests, rats chose between tap water and a 0.1% saccharin solution. Group HL-SA demonstrated a substantial conditioned avoidance in both feeding and drinking tests, but group HL-WA showed avoidance only in feeding tests. Conditioned avoidance was weak in group LL-SA; the strongest avoidance was observed in lithium chloride-treated rats (group LC-SA). Results are related to previous research and to the hypothesis that a learned avoidance of milk may facilitate the weaning process in mammals.

Voluntary ethanol consumption during pregnancy and lactation in the golden hamster.

Golden hamsters voluntarily consume substantial amounts of ethanol without developing dependence, apparently because ethanol metabolism is rapid and efficient. Six adult female hamsters were given continuous free access to Purina chow, water and a 20% (v/v) ethanol solution before and during pregnancy and during lactation; six control females did not receive the ethanol solution. Intakes of food and water were not elevated during pregnancy, but increased markedly for both groups during lactation. Ethanol consumption remained substantial but unchanged throughout the experiment, with ethanol consumers taking an average of 3.8 g (4.4 kcal) of ethanol solution daily. No significant differences were observed in the size and weight of litters either at delivery or at 16 days postpartum.

Conditioned taste aversion produced by 2-deoxy-D-glucose in rats and hamsters.

The glucose antimetabolite 2-deoxy-D-glucose (2DG) reliably causes hyperphagia in rats, but has consistently proven ineffective in producing overeating in golden hamsters. It was hypothesized that hamsters do not overeat following 2DG administration because of unusually strong aversive effects of the drug in this species. To test this hypothesis, rats and hamsters were tested in a conditioned taste aversion (CTA) paradigm, in which a novel 0.1% saccharin solution was paired on three occasions with intraperitoneal injections of either saline, lithium chloride (LiCl; 50 mg/kg), or 2DG (either 350 or 750 mg/kg). CTA was measured in 16 twenty-minute, two-bottle preference tests which were conducted at 2-3 day intervals following conditioning. LiCl and 2DG both produced strong and long-lasting aversions to saccharin solution in rats. However, 2DG was significantly less effective than LiCl in producing CTA in hamsters. It is unlikely, therefore, that the failure of 2DG to produce hyperphagia in hamsters is due primarily to an unusual sensitivity to the aversive effects of the drug.