Amino acid balance and food intake: effect of amino acid infusions on plasma amino acids

Basolateral and central amygdaloid lesions leave aversion to dietary amino acid imbalance intact

Expression of c-fos is increased in the central amygdaloid nucleus (CE) of rats ingesting a diet with a severely imbalanced essential amino acid profile (IMB), at a time associated with development of a conditioned taste aversion (CTA). The CE and the basolateral amygdaloid nucleus (BL) both are reported to be involved in the development of CTA. Large amygdaloid lesions involving CE and BL mitigate the normal decrease in intake of IMB; this treatment also impairs CTA to a flavor cue associated with gastrointestinal discomfort. To differentiate their potential roles in aversive responses to IMB, we electrolytically lesioned CE and BL separately. Neither lesion attenuated IMB-induced anorexia, or prevented the avoidance of flavored solutions previously paired with IMB. In contrast, after saccharin-LiCl pairing, CE-lesioned animals showed attenuated CTA to saccharin solution in a two-bottle test. We conclude that neither the CE nor the BL is essential for the reduction of IMB intake, or for CTA associated with IMB. Furthermore, these results suggest that the aversive consequences of IMB intake do not involve gastrointestinal malaise-evoked neurotransmission involving the CE.

Effects of threonine injections in the lateral hypothalamus on intake of amino acid imbalanced diets in rats

Previous work from this laboratory suggests that animals decrease their intake of an amino acid imbalanced diet (IMB), due in part to a drop in the concentration of the dietary limiting amino (DLAA) in the anterior piriform cortex (APC). Administration of the DLAA, but not of a non-limiting amino acid into the APC, blocks the anorectic response to IMB. To our knowledge, the effects of DLAA injections on intake of a diet devoid of the DLAA (DEV), have not been examined in areas outside the APC. We hypothesized that the LH is a potential chemosensory area for DLAA. Our objectives were: (1) to determine whether injections of the DLAA threonine into the lateral hypothalamus (LH) alter intake of a threonine-devoid diet (DEV); and (2) to examine the dose-response effects of threonine injections into the LH on intake of threonine-corrected diet (COR). Administration of threonine into the LH stimulated DEV intake during the first 6 h at the 0.25 and 1-nmol doses by approximately 26 and 24%, respectively. Threonine (0.25, 2.5 nmol) did not alter COR intake at any time during the first 12 h. Our results suggest that: (1) the LH, along with the APC, likely acts as a chemosensory brain area for indispensable amino acids; and (2) both the APC and LH are part of a circuit that is involved in the short term anorectic response to amino acid imbalanced diets.

Effects of infusions of lysine, leucine and ammonium chloride into the hepatic portal vein of chickens on voluntary food intake

1. Adolescent cockerels of a laying strain were prepared with catheters whose tip lay in the hepatic portal vein, to study the effect of 3-h infusions of nutrients on food intake. 2. Lysine, infused into the hepatic portal vein at rates of 150-450 mg/3 h reduced 3-h food intake by up to 58%, for a period of 6 h in previously starved birds, but had no effect on birds allowed free access to food. Infusions made into the jugular vein had no effect, suggesting a role for the liver in monitoring lysine levels. 3. Portal infusion of leucine had a delayed effect while ammonium chloride, infused at isosmolar rates to those of the lysine infusions, had very little effect on intake. 4. The results support the concept of liver sensitivity to amino acids, but the mode of action is not clear; it appears not to be via the effects of ammonia.

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.

Vagal unitary responses to intestinal amino acid infusions in the anesthetized cat: a putative signal for protein induced satiety

Single unitary discharges in the nodose ganglia were recorded extracellularly in chloralose anesthetized cats while amino acid solutions were being perfused through the small intestine via implanted cannulae. Test infusions consisted of either amino acid mixtures (12 amino acids; 120 mM in all) or individual amino acids (50 mM each) dissolved in Krebs Henseleit buffer. Units which were activated by amino acid infusions were also tested with 10% glucose infusions performed in the same way. Control infusions consisted of either buffer alone or a physiological saline solution isotonic to the test solution. All perfusions were performed at 38 degrees C, pH 7.4 by means of a syringe over a 10 second period. Out of 1250 vagal units activated by electrical vagal stimulation, 92 units showed an increased firing rate in response to amino acid intestinal perfusions. Of these, only 1/7 were also responsive to glucose perfusions. Osmotic, thermal or mechanical stimuli associated with infusions did not modify vagal responses to the amino acids. Among vagal units responding only to amino acid but not to glucose infusion, some were activated in a specific manner, depending on the specific amino acid infused intraduodenally. These neurons illustrated a very strict specificity regarding the nature of chemical stimuli. The very short latency, mean of 9 sec +/- 0.7 (SE) of these vagal neurons to amino acid infusions unequivocally indicates that chemoreceptors are located at the preabsorptive level. The corresponding fibers were non-myelinated (conduction velocities: 0.8/1.4 m/sec.) and were of the C type. The functional characteristics of these vagal amino acid receptors are discussed in terms of the role of intestinal signals in short term protein satiety.

Methionine hydroxy analog supplementation of low protein calf rations

Thirty-six Holstein calves were randomly assigned at 3 days of age to one of three complete calf rations: ration A, B, C containing 0, .25, or .50% methionine hydroxy analog. All three rations were isonitrogenous at 13.5% crude protein (dry basis) and were offered adlibitum until calves were 12 wk of age. Calves were fed 3.64 kg whole milk oncedaily for 4 wk. Average daily gains (kg) and dry matter intakes (kg) from 4 to 12 wk were .88, 2.49; .77, 2.50; and .56, 1.98 for calves fed A, B, and C. Weight gains differed between all rations while intake was higher for A and B than for C. Dry matter intakes (kg per kg gain) from 4 to 12 wk were 2.85, 3.32, and 3.54. Serum amino acids for five calves on each treatment were similar for A and B. Calves fed C had lower serum values for all individual essential amino acids compared to A and B and 28% lower circulating total essential amino acids. Serum cysteine increased as dietary methionine hydroxy analog increased. Anamino acid imbalance or toxicity may have been produced which severely reduced gain, intake, and serum amino acids on ration C but only reduced gain on ration B.

The rejection of a diet which has been associated with a single administration of an histidine-free amino acid mixture

1. A protein-free diet, to which an odour had been added, was offered to rats immediately after giving a gastric load of an histidine-free, but otherwise balanced, amino acid mixture. The same diet with a different odour was offered to the rats on another day, after administration of a control load of saline or a balanced amino acid mixture. After access to stock diet for 6 h of 1 d, the rats were offered two samples of protein-free diet, each with one of the two odours.

2. The rate of consumption of odorized protein-free diet was depressed 2–4 h after administration of the histidine-free load. In the later preference test, the dietary sample with odour which had been offered after the deficient load was rejected in favour of the sample with the odour which had been offered after the control load.

3. Rejection of the deficiency-paired odour in the final preference test did not occur when the histidine-free load had been given 2 h before the rats were first offered odorized diet. Also, there was in these instances no depression of rate of food consumption 2–4 h after loading. This indicated that aversive reactions to the odour were established by association of the odour with some effect of the histidine-free load which had occurred within 2 h of its administration, and that the early depression of intake and the much later rejection during choice were both expressions of these acquired reactions.

4. This rapid conditioning of selective rejection did not depend on previous prolonged protein deprivation or on the use of immature rats but did depend upon an intermittent supply of amino acids during 3 d before conditioning. Subcutaneous injection of deficient amino acid mixture did not establish rejection, suggesting that conditioning depended on rapid delivery of the deficient mixture into the circulation.

5. It is concluded that the critical biochemical events which lead to the rejection of diets that are imbalanced or deficient in essential amino acids occur soon after ingestion of the diet, and may have been effective in producing a conditioned aversion before any suppression of food intake appears. It is suggested that the depression of food consumption, which is the normal response to an imbalanced diet, is in this instance the result of conditioned response to sensory qualities of the diet rather than to the direct effect of biochemical stimuli.

Olfactory conditioning by association with histidine-free or balanced amino acid loads in rats

An histidine-devoid but otherwise balanced amino acid mixture depressed food intake from 2 hr after its gastric intubation. It induced conditioned aversion to an odour incorporated in a protein-free diet presented for 6 h following intubation. In other rats, a balanced amino acid mixture established conditioned preference for odour presented in the same diet for 6 h following intubation. The degree of preference was considerably less than the degree of aversion.

Food preferences acquired by association with variations in amino acid nutrition

Protein-deprived rats were given, on one day, a balanced mixture of amino acids followed by access to protein-free food having a distinctive odour. On another day, an imbalanced (histidine-free) amino acid mixture was given just before food having another odour. The rats afterwards preferred the balance-paired odour to the imbalance-paired odour. The preference was acquired whether the duration of odour presentation, or the amount of odourised food taken, was kept constant on the two conditioning days. Retention of the preference seemed unattenuated after 4 weeks. An attraction to the balance-paired odour (relative to odours paired with a water load) contributed to the acquired preference. There was also a relative aversion to unfamiliar odours when they had been paired with imbalance. Such acquired chemosensory control of preferences, together with an anorexigenic effect of imbalanced amino acid mixtures, can account for characteristics of feeding behaviour under conditions in which the diet is deficient in an essential amino acid.