Cingulate lesions and behavioral adaptation to amino acid imbalanced diets

The effect of anterior cingulate cortex lesions on dietary intake and adaptation of disproportionate amounts of amino acids was examined. Rats with bilateral electrolytic lesions in the anterior cingulate cortex and sham-operated rats were fed, in turn, amino acid basal, imbalanced or devoid diets involving threonine and isoleucine as the growth limiting amino acids, and then a low protein (6% casein) followed by a high protein (75% casein) diet. Lesions of the anterior cingulate cortex did not prevent the initial depression in food intake of the amino acid imbalanced diets, but shortened the duration of anorexia associated with dietary amino acid imbalances. Cingulate lesions did not influence the food intake of rats fed amino acid devoid diets. When switched from a low protein to a high protein diet, animals bearing lesions and sham-operated controls reduced markedly their initial food intake and adapted to the high protein diet in similar manner. It was concluded that the initial food intake depression associated with a dietary amino acid imbalance is a direct response to postingestive cues which influence food intake. Moreover, that the difference in adaptive intakes of the cingulate cortex lesioned animals who ingested a diet of imbalanced amino acids or of high protein, indicates that separate mechanisms act to control food intake of animals fed diets containing imbalanced amino acid mixtures or diets with excessive amounts of protein.     

Effect of type of protein on food intake of rats fed high protein diets

The effects of type of protein on intake of a high protein diet after adapting rats to a low protein diet were examined in rats trained to eat a 5.2% (N X 6.25) protein diet containing a mixture of casein, lactalbumin, egg white and soy protein, in a single 3-hour period per day. Food intake was measured from 0-15, 15-30, 30-90, and 90-180 minutes. After a 2-week adjustment period, rats were presented with a 40% (N X 6.25) protein purified diet containing only one of the 4 proteins mentioned above or a mixture of these 4 proteins. During the first 15-minute interval, rats eating diets containing protein mixture, lactalbumin, egg white or soy protein depressed their intake significantly compared with the average intake of the 3-day pre-test period, whereas rats eating casein diet increased their intake. During the last 90-minute interval of the first day, all rats depressed their intake, those rats eating casein the least and those rats eating egg white the most. On the second day, rats offered lactalbumin depressed their intake 52.5% for the 3-hour period and rats offered casein depressed their intake 34.3%. Rats eating soy protein, egg white and protein mixture increased their intake from day 1 to day 2. These experiments show that type of protein affects rats' initial intake when they are offered a high protein diet.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of hippocampal lesions on adaptive intake of diets with disproportionate amounts of amino acids

Bilateral double electrolytic overlapping lesions were placed in dorsal-lateral hippocampus of male 230 g rats, and their food intake responses to the ingestion of diets containing disproportionate amounts of amino acids were examined. Rats with such lesions and intact control rats maintained their normal intakes of the 6% casein basal diet or a threonine basal amino acid diet postoperatively. However, they exhibited marked initial food intake depression, similar to that of intact rats, when fed the threonine imbalanced amino acid diet. Also, animals with lesions in certain areas of the dorsal-lateral hippocampus showed facilitated adaptation to the amino acid imbalanced diet. Similar severe reduction in food intake with relative lack of adaptation were observed in both the intact controls and rats with hippocampal lesions when fed amino acid diets completely devoid of threonine. Initial food intake of rats with hippocampal lesions was inhibited drastically as was the case with the intact controls when fed a 75% casein high protein diet. All rats, either intact or lesioned, showed similar slow adaptation patterns with the prolonged ingestion of the high protein diet. The initial food intake responses and facilitated adaptation of the animals bearing lesions in certain areas of the hippocampus suggest that such areas are not crucially involved in the inhibition of food intake of rats fed disproportionate amounts of dietary amino acids. Rather, such areas of lesions in the hippocampus may play a role in a system governing the behavioral adaptation of the intake of amino acid imbalanced diets but not of diets containing amino acids in general excess. This would also indicate that different mechanisms control the intake of amino acid imbalanced diets and diets containing amino acids in excess.     

Protein selection, food intake, and body composition in response to the amount of dietary protein

Though not universally observed, moderately low-protein diets have been found to increase caloric intake and body fat. It appears that animals overeat in calories in order to obtain more dietary protein. For animals to control protein intake, they must be able to distinguish between two isocaloric diets containing different percentages of protein and make the appropriate dietary selection on the basis of their previous history of protein intake. Experiment 1 examined the 24-h diet selection (5 vs. 35% casein) of Sprague-Dawley rats that had been previously fed diets containing various percentages of dietary protein (5, 10, 20, 35, or 60% casein). Animals fed 5, 10, or 20% dietary protein showed a preference for the higher protein selection diet. In contrast, no significant diet preference was found in animals pre-fed the two higher levels of dietary protein (35 or 60% casein). In this study, daily food intake and body fat of rats fed the low-protein diets (5 and 10% casein) were similar to rats fed the 20% casein diet. Experiment 2 examined the effects of the level of methionine supplementation on rats fed 10% casein. In this study, food intake and body fat were increased by approximately 20% in rats fed 10% casein diets, regardless of the level of methionine supplementation (0.3 vs. 0.15%). Together, the results suggest that the presence of low-protein-induced hyperphagia helps maintain body protein levels in the face of moderately low dietary protein and promotes an increase in the amount of body fat and energy.     

Effects of monosodium glutamate on eating and drinking behavior in rats

The effect of monosodium glutamate (MSG) on eating and drinking behavior of rats was studied by means of preference tests in free choice situations. Low or high protein casein diets containing up to 3% MSG were selected indifferently compared to plain casein diet in 7 day trials. The acceptability of diets containing 7% MSG, particularly those of low protein level, was significantly reduced. Except for Day 1, the proportional intake (MSG flavored diet intake/total diet intake) of 9% protein--7% MSG diet was consistently lower than 18% protein--7% MSG diet. In brief exposure tests (10 min) solutions of MSG (0.02-8%) were preferred over deionized water. The acceptance of solutions containing higher concentrations of MSG was significantly reduced. The total volume intake of both choices (MSG flavored water and water) was significantly increased in tests using solutions containing 3-8% MSG. In long-term tests (1-14 days), solutions containing 0.05-1% MSG were preferred over water. The acceptance of a solution containing 5% MSG was significantly reduced. For maximum intake of moles of MSG, rats selected solutions which offered 3-5 x 10(-1) M MSG whether the experimental period was 10 min or up to 2 weeks. In a series of two choice preference tests using solutions where Na+ content and pH were kept equal, MSG was preferred over sodium acetate and over sodium glutamate but was less preferred than monosodium aspartate. The results of this study can be explained in terms of sensory quality.     

The effects of early dietary experience on subsequent protein selection in the rat

Two series of experiments were designed to investigate the effects of protein composition of the first solid food consumed by young rat pups on their subsequent protein intake after weaning. In the first series of experiments, pups were allowed access to the maternal diet between 17 and 21 days of age, then weaned to a choice of 10% and 60% casein diets at 21 days. Pups with access to a maternal diet containing 39% casein selected high levels of protein (calculated as % of total calories selected as protein: 50 +/- 1% (access) vs. 29 +/- 3.5% (no access), p less than 0.05), whereas lower levels of protein were selected following access to a maternal diet containing 26% casein (16 +/- 1.1% (access) vs. 23 +/- 1% (no access), p less than 0.05) or 10% casein (27 +/- 3.9% (access) vs. 39 +/- 3.7% (no access), p less than 0.05). In a second series of experiments, the influence of early diet composition on subsequent protein selection was confirmed by prematurely weaning rats at 17 days of age to either 39% or a 10% casein diet. When given a choice of 10% and 60% casein diets at 21 days, pups selected higher levels of protein if weaned to a 39% casein diet as compared to a 10% casein diet (37 +/- 5.0% vs. 19 +/- 3.5% (p less than 0.05) for the first experiment and 43 +/- 3.0% vs. 26 +/- 4.0% (p less than 0.05) in a second experiment). It was concluded that the level of protein selected by young rats is positively correlated with the protein concentration of their first solid food.     

Effects of dietary nutrients and foraging costs on meal patterns of rats

Rats were fed either a cereal-based or a purified casein-based diet in a foraging paradigm in which the costs of procurement and consumption were varied. The group offered the cereal-based diet consumed about 10% more calories than the group offered the casein-based diet, but both groups grew at the same rate. The intake of a control group offered a choice between the two diets was approximately 80% from the casein diet, and the growth of this group did not differ from that of the experimental groups. Variations in the cost of procurement and the cost of consumption affected the patterning of meals differentially for the two diets: changes in meal patterns tended to control the time and/or energy spent feeding. These results show that (1) meal patterns in the foraging paradigm are sensitive to subtle differences in diets, and (2) the amount of diet consumed (acceptance) and the choice between diets (preference) are determined by the economics of feeding and the nutritive quality of the foods, as well as by their palatability.     

Effect of pre-feeding ammonium acetate on food intake of rats fed high protein diets

The effect on intake of a 75% casein diet after prefeeding for one week a 6% casein basal diet with additional 0%, 2%, 5%, 8% or 15% ammonium acetate was examined in rats trained to eat in three hours per day. Food intake was measured from 0-15, 15-30, 30-90, and 90-180 minutes for the first two days that the ammonium acetate diets were presented. Rats eating 5% and 8% or 15% ammonium acetate diet depressed their intake significantly for one day and for four days respectively. Rats eating 2%, 5%, 8%, or 15% ammonium acetate diets depressed their intake significantly from 0-30 minutes. When presented with the 75% casein diet, rats prefed 0% to 5% and 8% and 15% ammonium acetate diets ate 55% to 58% and 72% and 94% of their respective baseline intakes. It is suggested that prefeeding 15% ammonium acetate apparently induces sufficient metabolic adaptation to ammonia intake so that the rat is able to offset the metabolic consequences of intake of the 75% casein diet, thus preventing the usual food intake depressing effect of the high protein diet.     

The ability of pigs to control their protein intake when fed in three different ways

The ability of pigs to control their protein intake was tested in three different ways. When pigs were offered access to single feeds of different protein contents, at a constant liveweight of 20 kg, they increased their daily feed intake as the protein content of their feed was decreased (long-term protein intake regulation). When the protein content of their feed was altered daily, pigs consumed consistently higher amounts of feed when they had access to a low protein feed rather than to a high protein one (short-term protein intake regulation). When, finally, they were given a choice between two feeds of different protein contents, pigs selected a diet that met their protein requirements (as judged by their growth rates and feed efficiencies) and avoided excess of protein intake, but only if they had had previous experience of both feeds. It is suggested that pigs first need to learn about the feeds that are subsequently offered as a choice, before they can make correct dietary choices.     

Effects of dietary protein on food and water intake in spontaneously hypertensive rats

We have been studying the development of hypertension in spontaneously hypertensive rats (SHR) fed a low protein diet. The effects of a low protein diet upon food and water intake were examined. Body weight gain, food and water intake were measured in three to twenty-three week-old SHR and Wistar Kyoto rats (WKY) fed diets containing 8%, 15% or 25% casein. Body weights of SHR and WKY fed an 8% casein diet were significantly lower at 23 weeks than rats on the higher protein diets, although both groups on the 8% diet consumed more food and water per g of body weight. In addition, SHR fed an 8% casein diet drank less water per gram of food than WKY or SHR fed 15% and 25% casein diets. These results indicate that changes in food and water intake, as a consequence of low protein diets, should be an additional consideration when examining the effects of dietary protein on the development of hypertension.     

Protein selection by rats adapted to high or moderately low levels of dietary protein.

After preliminary studies on flavor acceptability, patterns and indices of subsequent feeding behavior were monitored by computer in young rats which were adapted to 15% or 70% casein diets before being offered, sequentially, choices between flavored diet pairs in which the proportions of percentage casein were 5/65, 5/55, 5/45, 5/35 and 5/25. Similarly adapted rats received these choices in the reverse sequence. Rats adapted to 15% casein usually ate randomly from the diet pairs and selected approximately 15-30% casein; individual behaviors were prominent. The 70% casein groups avoided the higher casein diet, often within minutes (except for the first-offered 5/25 choice), and seldom selected more than 10% casein; individual differences were infrequent. Such rats also distinguished between flavored 70% and 65% casein diets. Sizes and numbers of meals and rates of eating differed for the paired diets, especially for rats adapted to 70% casein. A flavor added to the 70% casein adaptation diet was not avoided when present only in the 5% casein diet of a 5/65 choice. Rats adapted to 70% soy protein before receiving flavored 5/65 to 5/25 choices selected 20-28% soy protein, a level far above those of casein selections by rats adapted to 70% casein. Dietary adaptation and type of protein thus affect subsequent diet selection and feeding patterns and indices.     

Acquisition of dietary self-selection in rats with normal and impaired oral sensation

To study the mechanisms of dietary self-selection, the dietary choice behavior of rats was observed for up to 5 weeks, beginning on the first day of exposure to two nutritionally different diets, a high-protein diet (44% protein) and a protein-free carbohydrate diet. In Experiment 1 normal rats selected equal amounts of the two selection diets at first and over a 7-day period gradually modified the choice ratio until a stable protein intake of 14.6% (SE,0.4) of total intake was reached. In Experiment 2 rats were subjected to partial trigeminal deafferentation, which impairs oral somatosensory input (touch, temperature, pain), before the two selection diets were introduced. The deafferented animals did not develop a stable selection pattern; their protein ratio varied over the entire possible range (0-44%) throughout the experiment. It is hypothesized that quantitative protein/carbohydrate selection involves an associative learning process in which somatosensory inputs from the feeding activity and/or from the properties of the food link dietary choice behavior to later metabolic consequences.     

Maternal malnutrition in the rat: Effects on food intake and body weight

The effects of dietary protein level on food intake and body weight were examined in adult female rats during a 35-day pre-mating period and during gestation and lactation. During the pre-mating period, no differences in daily food intake were observed among rats fed a 6% casein, 8% casein or 25% casein diet. However, during this period, rats fed the 6% casein diet gained significantly less weight than those with ad lib access to the 8% or 25% casein diets or than rats pair-fed the 25% casein diet in amounts equivalent to that consumed by rats in the 6% or 8% casein groups. Additionally, rats fed the 6% casein diet displayed decreased efficiency of energy utilization, calculated as weight gain per 100 kilocalories consumed, relative to rats fed the 8% or 25% casein diets. No differences in food intake were observed among the groups during gestation. However, rats fed the 6% casein diet gained less weight than rats fed the 8% or 25% casein diets. During lactation rats fed either the 6% or 8% casein diet consumed significantly less food than animals given the 25% casein diet ad lib. During the second week of lactation, rats receiving ad lib access to the 25% casein diet gained weight while those receiving the 6% or 8% casein diets continued to lose weight. At parturition, body weights of pups did not differ as a function of dietary condition. However, by day 12 of life, pups whose dams had ad lib access to the 25% casein diet weighed significantly more than pups whose dams consumed the 6% or 8% casein diet or whose dams were pair-fed the 25% casein diet in amounts equivalent to those consumed by rats fed the 6% or 8% casein diet.     

Dietary self-selection and the regulation of protein and energy intake in chicks

Studies were conducted to determine whether chicks could regulate their protein intake independent of total energy intake in self-selection feeding trials. Day-old White Mountain cockerels were reared in electrically-heated battery brooders and given access to either a 23% protein control ration (no choice) or two diets containing 10% or 60% protein with or without supplemental amino acids. The latter were added to either improve the dietary amino acid balance or to alter plasma and brain levels of free large neutral amino acids (tryptophan, isoleucine, leucine, valine, phenylalanine, and tyrosine) which have been implicated in the neuroregulation of feed intake. Both feed and water were provided ad lib, and the location of the feed troughs within each pen were changed daily. Body weights and feed intakes were measured daily, and total calorie and protein intakes were calculated. Chicks offered 10% and 60% protein diets with no supplemental amino acids exhibited reduced weight gains and markedly higher protein intakes as compared to birds fed either the control ration or those given a choice between 10% and 60% protein diets supplemented with methionine. The higher protein consumption by chicks fed the unsupplemented diets most likely was a result of an attempt to compensate for a dietary methionine deficiency. Chicks fed the 10% and 60% protein diets supplemented with amino acids grew at a slower rate than those fed the 23% protein control diet. In general, plasma and brain data did not support a proposed relationship between certain large neutral amino acid ratios and protein or energy intake.     

Dietary composition alters gentamicin-induced nephrotoxicity in rats

Previous studies have shown temporal variations in gentamicin-induced renal toxicity characterized by a peak when administered during the resting period and a trough during the active period. This time-dependent toxicity was also altered according to the macronutrient composition of dietary regimens offered to female rats. In the present study, adult female Sprague-Dawley rats were adapted to semipurified isocaloric diets containing 20% casein or soy-protein (10% fat each) or to a standard chow diet (18.1% mixed proteins; 4.5% fat). The animals were then chronically treated for 10 days with a nephrotoxic dose of gentamicin sulfate (40 mg/kg/day ip) or a saline solution administered in the middle of their resting period (1200 h) or in the middle of their activity period (0000 h). Body weights of rats injected in the middle of their resting period decreased over the last 6 days of gentamicin treatment. Total 12-h light and 12-h dark food intakes were decreased in gentamicin-treated rats. Rats fed the standard chow diet had significantly lower corticocellular regeneration, serum creatinine and blood urea nitrogen compared to those fed the casein- and soy-containing diets. The present study demonstrates that chronic gentamicin-induced renal toxicity varies temporally according to the time of administration and that a mixed protein diet containing a lower fat level can protect against gentamicin-induced nephrotoxicity.     

Meal composition influences subsequent food selection in the young rat

Food selection one-half hour after the ingestion (2 g) of either a protein containing or a protein-free (carbohydrate) diet was studied in young rats. Following a 12 hr fast the rats were allowed 15 minutes to consume the meal (premeal). Thirty minutes later, they had access to two isocaloric diets that differed only in protein and carbohydrate content. During the first hour of ad lib feeding, protein intake and protein concentration selected were lower in the 45% casein prefed group compared to the carbohydrate group. A further reduction in protein intake and protein concentration occurred when the protein content of the premeal was increased to 70% from 45% casein. Rats prefed with the 70% casein diet significantly reduced their daily total food intake (12 hr) compared to either the 45% casein or carbohydrate prefed group. It is concluded that both quantity and composition of food selected is influenced by the composition of the preceding meal.     

Influence of dietary protein level on protein self-selection and plasma and brain amino acid concentrations

Control of protein intake was studied in young rats that were allowed to choose between either protein-free and 55% casein diets or 15% and 55% casein diets. Animals on the protein-free vs. 55% casein regimen exhibited a lower weight gain, a lower cumulative energy intake and a greater cumulative total protein intake during the 13-day study compared to rats selecting between 15% and 55% casein. The daily average proportion of total food selected as casein by animals choosing between protein-free and 55% casein diets increased from 15% to 38% during the course of the study. In contrast, rats choosing between 15% and 55% casein chose 18-22% of total food as protein throughout the entire study. Long-term protein intake or protein selection did not correlate significantly with whole-brain contents of 5-HT or 5-HIAA. Our results suggest that protein intake is not regulated at a constant proportion of total calories, but is controlled between a minimum level that will support rapid growth and a maximum that, if exceeded, would require the animal to undergo substantial metabolic adaptation. The mechanism controlling protein selection may involve diet-induced changes in the brain content of total free indispensable amino acids.     

The evaluation of energy intake adjustments and preferences in juvenile rainbow trout fed increasing amounts of lipid

This study aimed at a better understanding of the feed intake (FI) regulation in rainbow trout by dietary digestible energy (DE). The DE contents of the three diets (20.5, 23.0 and 24.7 kJ per g dry diet) were modified by supplementing different amounts of fish oil. The crude lipid and protein levels were 13%, 26% and 34% and 64%, 54% and 48%, respectively (% dry diet). The daily FI was measured by means of self-feeders in groups of rainbow trout (32-55 g, initial BW) during two 5 to 6-week trials. Their eventual preference for one of the lipid levels was evaluated by offering the choice between a low and higher lipid diet. The results indicated that fish of a similar body mass had a similar FI without apparent energy intake compensations. It is believed that the excessive energy intakes with the higher lipid diets were not caused by a higher palatability of these diets since the trout did not express any particular preference. The trout fed the lipid rich diets had a higher level of body adiposity, but a similar protein growth. The observation that the trout did not reduce FI when fed the high lipid diet implies a low negative feedback by the ingested fat or by the increase in body adiposity. The similarities in lean carcass growth favour the idea that growing animals regulate their FI in order to meet the demand for maximal protein growth rather than to satisfy a predetermined energy requirement.     

Effect of amygdaloid lesions on dietary intake of disproportionate amounts of amino acids

Male rats with bilateral electrolytic lesions in the anterior, medial or posterior aspects of the ventral amygdala and groups of intact rats were fed, in turn, basal, imbalanced or deficient amino acid diets involving threonine or isoleucine as the limiting amino acid, and then a low protein (6% casein) followed by a high protein (75% casein) diet. No change in food intake was observed in animals fed the threonine basal diet postoperatively. When the threonine or isoleucine imbalanced diet was substituted for the respective basal diet, animals with lesions in certain areas of the medial amygdala showed little or no depression in food intake of the imbalanced diets, while all other rats with amygdala lesions reduced their food intake markedly, as did intact controls, when fed such diets. All animals, however, curtailed their food intake of the deficient or high protein diets. The lack of responsiveness of the animals with medial amygdaloid lesions to the imbalanced diets suggests that these areas may be involved in a system regulating food intake of animals fed diets containing imbalanced amino acid mixtures.     

Hepatic vagotomy disrupts somatotropin-induced protein selection

Rats treated with somatotropin (STH) and allowed to self-select between diets varying in protein content will consume more of the high-protein diet. The objective of this study was to determine the role of the hepatic vagus nerve in this ability to select protein. Female Sprague-Dawley rats (n=40) received a hepatic vagotomy (HVAGX) or a sham surgery. Postsurgery, the rats were maintained on pelleted diets for 2 weeks, after which the rats were adapted to selecting between powdered diets with 5% casein and 30% casein. After a 7-day adaptation to diet selection, rats in each surgical treatment group were treated with STH (4 mg/day) or physiological saline for 14 days. Body weight and intake were recorded daily. STH treatment increased growth rate to a similar degree in both sham and HVAGX groups. Despite causing an increase in total food intake, there was no effect of HVAGX alone on body weight. Relative to the sham-saline group, sham-STH in treated rats had greater total food intake that was accounted for entirely by increased consumption of the 30% protein diet and no change in intake of the 5% diet. In contrast, HVAGX+STH rats exhibited 20-30% increases in consumption of both the 5% and 30% protein diets. Thus, the HVAGX+STH rats recognized an increased need for protein, but were unable to distinguish between the high- and low-protein diets and selected more of both. The data suggest that the ability to alter diet selection in response to a stimulation of protein accretion is at least partially mediated through the liver and hepatic branch of the vagus nerve.