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.     

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.     

Norepinephrine and amino acids in prepyriform cortex of rats fed imbalanced amino acid diets

Monoamines and amino acids were measured in anterior prepyriform cortex (PPC) and anterior cingulate cortex (CC) of male Sprague-Dawley rats after they were offered basal, imbalanced (IMB) or corrected amino acid diets, limited in threonine (THR) or isoleucine (ILE). In the THR study, brains were taken after 2.5 hr of feeding, when intake of THR-IMB was just depressed. In the ILE study the brains were taken after 3.5 hr on ILE-IMB, a less severely imbalanced ration, before the onset of food intake depression. The PPC has been shown to be involved in the acute response of animals to imbalanced amino acid diets. In the PPC from the IMB diet groups, NE was reduced by 30%, but the other monoamines were unchanged. In CC, an area involved in the adaptive, but not the acute feeding response to imbalanced diets, the monoamines were unchanged in the IMB diet groups. In both studies, in both tissues, the limiting amino acids were decreased in the IMB groups, although the decrease of ILE in the CC failed to reach significance. The remaining indispensable amino acids, added to create the imbalance, were slightly reduced in the THR-IMB group, but not in the ILE-IMB group in both tissues. Thus, the amino acid patterns were altered in the PPC and CC, as they are in whole brains from animals fed imbalanced amino acid diets. These results also suggest that the concentration of NE in the PPC may be associated with the initial food intake response of animals to imbalanced amino acid diets.     

Increase in plasma ammonia and amino acids when rats are fed a 44% casein diet

Rats were trained to eat a 6% casein basal diet during a 3-hour period per day. They were then fed either the same 6% casein diet or a 44% casein diet for 3 hours. No food intake depression was observed in the rats eating 44% casein diet during the 3-hour period. Plasma ammonia and amino acids and brain amino acids were measured at 0, 4, 12 and 24 hours after presentation of the 6% or 44% casein diets. Plasma ammonia rose to 134 (p less than 0.01) and 110 micromolar (p less than 0.05) in the 44% casein fed rats at 4 and 12 hours, respectively, as compared to 67 and 53 micromolar, respectively, for the 6% casein fed rats. All plasma amino acid concentrations except methionine and glutamate were elevated (p less than 0.05) at 4 hours. In the brain, threonine, glutamine and tyrosine concentrations were elevated (p less than 0.05) at 4 hours after diet presentation. At 24 hours, valine, isoleucine, leucine, phenylalanine, and methionine concentrations were also elevated (p less than 0.05). Because intake of the 44% casein diet decreases the second day of its presentation, as noted in an earlier experiment, the increases in plasma ammonia and its possible entry into the brain as reflected by increased brain glutamine together with changes in amino acid concentrations should be considered collectively among possible metabolic signals affecting intake of high protein diets.     

Avoidance of GABA-containing diets by olfactory bulbectomized rats

In order to test the importance of olfaction in the avoidance of GABA-containing diets by intact rats, olfactory bulbectomized and sham-operated control rats were allowed to choose between a low protein control diet and this diet supplemented with 2.5% GABA; the specificity of the response was examined by also testing for responses to other amino acids. Both groups of rats markedly avoided the GABA diet initially; the bulbectomized rats later tended to increase their intake of this diet. The bulbectomized rats chose similar amounts from the control diet and one supplemented with alanine whereas the control rats ultimately strongly preferred the latter diet. Both groups only moderately avoided a threonine-supplemented diet. The results suggest that (1) the odor of GABA is not critical in the avoidance of diets containing this amino acid; and (2) patterns of food selection from diets containing GABA differ from choices from diets containing other small neutral amino acids such as alanine or threonine.     

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.     

Threonine-imbalanced diet alters first-meal microstructure in rats

Diets limiting in an essential amino acid have long been known to suppress food intake. The purpose of this study was to examine the microstructure of feeding behavior of rats within the very first meal of an imbalanced diet. Rats were preconditioned for 12 days on a Baseline diet and were then given a test diet with either a corrected amino acid profile or a diet imbalanced with respect to the essential amino acid threonine. Overall, first-meal intake and first-meal duration were robustly and significantly reduced by the Imbalanced diet but not altered by the Corrected diet. The Corrected diet caused an increase in the number of feeding bouts during the first meal. The Imbalanced diet increased the duration of pauses during the first meal. Most rats in the Imbalanced group stopped eating after just 15 min of exposure to the diet, but those still eating after this time tended to have a lower rate of eating compared to those eating the Corrected diet. On the basis of these results, we conclude that changes in microstructure and meal duration contribute to the reduction in food intake upon exposure to amino-acid-deficient diets.     

Effect of amino acid imbalance and deficiency on dietary choice patterns of rats

Detailed dietary choice patterns were determined with a computerized feeding monitoring system in groups of Sprague Dawley rats kept on a 12:12 hr light-dark cycle and offered in sequence a series of dietary choice regimens involving amino acid-imbalanced or deficient diets with threonine as the most limiting amino acid. Animals established their preference for a threonine-basal diet over a threonine-imbalanced or a threonine-devoid (devoid of threonine) diet shortly (within 2-3 hr) after the consumption of small quantities of either diet in the beginning of the first dark-cycle. An intensive sampling process characterized by frequent small bouts was evident throughout the light period. Both the meal size and the meal frequency of the imbalanced or devoid diet were curtailed after prolonged choices. Animals preferred the threonine-corrected (imbalance corrected by threonine supplementation) over the threonine-basal diet initially with an increase in meal frequency. But no clear choice for either diet was observed thereafter. Animals did not establish their preference for the threonine-corrected diet when paired with the threonine-devoid diet until after 5 days with a steady decrease in the meal size of the devoid diet but not the meal frequency. When the protein-free diet was introduced as an alternative for the threonine-imbalanced diet, animals selected the protein-free diet during the first dark-cycle after consuming a small amount of the imbalanced diet. Initially there was a drastic reduction in meal size of the imbalanced diet and subsequently a decrease in meal frequency as well. Nevertheless, animals immediately rejected the protein-free diet and chose the threonine-basal diet when it replaced the imbalanced diet as an alternative. The almost exclusive preference for the basal diet occurred in the beginning of the first dark-cycle with an increase in meal size but no change in meal frequency. The sampling bouts of small quantities, which followed the first introduction of the diets in the choice regimens, may be an inherent investigative behavior whereby the physical or oropharyngeal properties of the diets are recognized. The establishment of the choices for the alternative diets in the present experiments provides additional information about the rapid time course of the food intake control mechanisms in rats fed amino acid-imbalanced or deficient diets.     

Dietary amino acid imbalance and neurochemical changes in three hypothalamic areas

The impact of feeding imbalanced amino acid diets on monoamine, metabolite and amino acid concentrations was measured in the ventromedial hypothalamus (VMH), lateral hypothalamus (LH) and paraventricular nucleus (PVN). After rats were fed either an isoleucine imbalanced diet, a threonine imbalanced diet, or the appropriate basal or corrected control diets, regional differences were found in neurochemical concentrations. Contrary to our expectations, the limiting amino acid was unchanged in the imbalanced groups, tending to be decreased only in the isoleucine imbalanced-diet group in the PVN. This is the first report that the limiting amino acid was not reduced uniformly in the brain after imbalanced amino acid feeding. In the VMH, norepinephrine (NE) was increased by 22% and 63% in the threonine and isoleucine imbalanced-diet groups, respectively. Since the concentration of NE was affected even before the decrease in feeding, both in the VMH, and, as previously reported, in the prepyriform cortex, the NE system may be involved in very early responses to imbalanced amino acid diets.     

Induction of conditioned taste aversion in rats by GABA or other amino acids

GABA included in the diet is known to reduce food intake and growth of rats fed a low protein diet. Experiments were designed to determine if GABA or other small neutral amino acids would affect food intake if they were administered separately from the diet, and if such amino acids could induce a conditioned taste aversion (CTA) to saccharin. Intubated or injected GABA or alpha-aminoisobutyric acid (AIB), a non-metabolizable isomer of GABA, reduced food intake. When rats were fed a low protein diet, IP injection of threonine (2 mmoles/200 g rat) induced CTA but did not depress food intake; serine (3 mmoles/200 g rat) induced CTA and caused only a small reduction in food intake. Another isomer, alpha-amino-n-butyric acid did not affect food intake or induce CTA at the tested concentrations. Adaptation to a high protein diet, which increases enzymatic degradation of many amino acids including GABA and serine, lessened severity of GABA-induced CTA and eliminated that caused by serine. CTA to saccharin can be induced by certain amino acids; the mechanism is unknown but may involve malaise or other adverse sensations.     

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 sardine protein lowers insulin resistance, leptin and TNF-α and beneficially affects adipose tissue oxidative stress in rats with fructose-induced metabolic syndrome

The present study aims at exploring the effects of sardine protein on insulin resistance, plasma lipid profile, as well as oxidative and inflammatory status in rats with fructose-induced metabolic syndrome. Rats were fed sardine protein (S) or casein (C) diets supplemented or not with high-fructose (HF) for 2 months. Rats fed the HF diets had greater body weight and adiposity and lower food intake as compared to control rats. Increased plasma glucose, insulin, HbA1C, triacylglycerols, free fatty acids and impaired glucose tolerance and insulin resistance was observed in HF-fed rats. Moreover, a decline in adipose tissues antioxidant status and a rise in lipid peroxidation and plasma TNF-α and fibrinogen were noted. Rats fed sardine protein diets exhibited lower food intake and fat mass than those fed casein diets. Sardine protein diets diminished plasma insulin and insulin resistance. Plasma triacylglycerol and free fatty acids were also lower, while those of α-tocopherol, taurine and calcium were enhanced as compared to casein diets. Moreover, S-HF diet significantly decreased plasma glucose and HbA1C. Sardine protein consumption lowered hydroperoxide levels in perirenal and brown adipose tissues. The S-HF diet, as compared to C-HF diet decreased epididymal hydroperoxides. Feeding sardine protein diets decreased brown adipose tissue carbonyls and increased glutathione peroxidase activity. Perirenal and epididymal superoxide dismutase and catalase activities and brown catalase activity were significantly greater in S-HF group than in C-HF group. Sardine protein diets also prevented hyperleptinemia and reduced inflammatory status in comparison with rats fed casein diets. Taken together, these results support the beneficial effect of sardine protein in fructose-induced metabolic syndrome on such variables as hyperglycemia, insulin resistance, hyperlipidemia and oxidative and inflammatory status, suggesting the possible use of sardine protein as a protective strategy against insulin resistance and related situations.     

Parabrachial lesions disrupt responses of rats to amino acid devoid diets, to protein-free diets, but not to high-protein diets

Normal rats "reduce" intake of diets that lack an essential amino acid (THR-DEV), are protein free (PO%), or contain a high proportion of protein (P75%). We tested the importance of the parabrachial nuclei (PBN) in signaling such adjustments of food intake by placing electrophysiologically guided lesions in these nuclei at points that responded to gustatory stimuli. When fed the THR-DEV diet, rats with PBN lesions (PBNx) decreased their food intake significantly less than the controls (78.5 vs. 44.4%). When put on a P0% diet, PBNx animals decreased their intake only 8% compared with 23% for our CONT group. When put on a P75% diet, however, both groups decreased their intake in an equivalent amount. These experiments show that the PBN is involved in the learned aversion to an amino acid devoid diet.     

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.     

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.     

Dietary protein level alters gentamicin-induced nephrotoxicity in rats

Aminoglycosides (AG) such as gentamicin are antimicrobial drugs widely used in the hospital setting due to their efficacy in the treatment of severe gram-negative bacterial infections. However, all AG have the potential to cause nephrotoxicity. Two studies have been conducted (1) to assess the protein level of a diet that would give the best renal outcome with gentamicin administration, and (2) to get a better idea about the rhythms of food ingestion associated with the different protein levels. Adult female Sprague–Dawley rats fully adapted to a standard chow diet, the standard chow with 20% or 55% added casein were chronically treated for 10 days with a nephrotoxic dose of gentamicin sulfate (40 mg/kg/day, i.p.) or a saline solution. Food ingestion patterns of rats were recorded every hour using a Diet Scan system and gentamicin nephrotoxicity indices were measured. The second study used rats that were fed the same diets and given a sham injection. Corticosterone was assayed to quantify the stress of the animals. Results showed that chronic gentamicin treatment leads to a decrease in food intake and flattening of the rhythms of food ingestion. Also, chow feeding and the 20% casein diet were found to be more protective against gentamicin-induced nephrotoxicity than the 55% casein diet. Therefore, while a protein-rich diet can be protective against gentamicin-induced nephrotoxicity, the present study demonstrates that a diet too high in protein might rather be harmful to the kidneys.     

Lack of diet-induced thermogenesis following lesions of paraventricular nucleus in rats

The effects of electrolytic lesions in the hypothalamus paraventricular nucleus were studied in adult male and female Sprague-Dawley rats, fed different diets, consisting of either palatable human food plus chow (cafeteria diet) or chow alone. The results showed that both cafeteria diet and lesions induced an increase in energy intake and weight gain in rats of both sexes. Oxygen consumption rate and colonic temperature were significantly decreased by lesions, while cafeteria diet increased the same parameters only in intact animals. The lesion decreased weight, protein and DNA, and temperature of brown adipose tissue, while cafeteria diet increased the values considered in brown adipose tissue of sham-injured rats, but not in lesioned animals. The response to norepinephrine administration was significantly greater in intact rats and those fed cafeteria diet. The results suggest that the larger body weight gain observed in lesioned rats, particularly evident in rats fed cafeteria diet, is partly due to the disappearance of diet-induced thermogenesis that depends on the reduced mass and functional activity of brown adipose tissue.     

Amino acid and NaCl appetite, and LHA neuron responses of lysine-deficient rat

Rats' preferences for amino acids and NaCl in their drinking behavior were noted when they were fed either a control diet of gluten plus 20% purified egg protein, or (same rats, different time) a lysine-deficient diet. In control, the order of the rats' preferences was arginine greater than saline greater than monosodium L-glutamate (MSG, umami) greater than glycine greater than water greater than threonine greater than histidine greater than lysine. When fed a lysine-deficient diet the order of preference was lysine greater than saline greater than MSG greater than glycine greater than threonine greater than water greater than arginine greater than histidine. To relate neural activity with preference for amino acids and NaCl, activity of lateral hypothalamic (LHA) neurons was recorded during ingestion of MSG, lysine, arginine, glycine, saline, glucose, or water by the control and lysine-deprived condition, following a different cue tone. When the diet was lysine deficient, some neurons responded specifically to lysine ingestion. More neurons responded nondifferentially during licking in control, and responded to lysine, but fewer to other amino acid ingestion during lysine deficiency. Responses to cue tone were associated with those during licking. The present results suggest that preference for deficient amino acids might be mediated in the LHA.     

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.     

Effect of amino acid supplementation to a low-protein diet on brain neurotransmitters and memory-learning ability of rats

The supplementation of methionine and threonine to a 10% soy protein isolate diet caused sharp decreases in the concentrations of brain tryptophan, serotonin and 5HIAA (5-hydroxyindole acetic acid) (e.g., in the hypothalamus, hippocampus, amygdala, locus coeruleus and brain stem). The serum tryptophan ratio (i.e., the ratio of the serum concentration of tryptophan to the sum of the concentrations of other large neutral amino acids, such as tyrosine, phenylalanine, leucine, isoleucine, valine and methionine) significantly decreased. The changes in catecholamines were small on the supplementation of amino acids to the soy protein diet. In the brightness-discrimination learning test, the number of total responses (R⁺, correct; R⁻, incorrect) of rats fed the amino acid-supplemented diet increased as compared with that of rats fed the nonsupplemented diet. The correct response ratio in the primary learning test did not change, but in the reverse learning test, the response ratio of rats fed the amino acid-supplemented diet increased. Therefore, it may be considered that the learning ability is correlated with the nutritional state.