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Postprandial metabolism and aversive response in rats fed a threonine-devoid diet. Am J Physiol Regulatory Integrative Comp Physiol 279: R248-R254, 2000.--Lack of an indispensable amino acid in the diet induces a rapid reduction in food intake. In this study, we assessed whether the anorectic signal after ingestion of a meal lacking threonine originated from either direct perception of the decrease in plasma threonine or from an indirect effect related to increased postprandial amino acid catabolism and energy expenditure. We observed that 3 g of such a meal was sufficient to induce an aversive response to the diet within 2 h. Postprandial changes to plasma ammonia and urea, urinary urea, and energy metabolism did not differ from those measured after a control meal. In contrast, plasma threonine levels fell within 1 h after the meal. It is concluded that an increase in postprandial energy expenditure is not involved in the anorectic response to eating a threonine-devoid diet. The drop in plasma threonine levels may be a potential signal, but the fact that the decrease in food intake occurred 1 h after the decrease in plasma threonine questions a direct causal relationship. energy expenditure; urea; ammonia; dietary proteins

Food intake decreases and a conditioned taste aversion is induced when rats are fed a diet that is devoid of an indispensable amino acid. The purpose of this study was to characterize the meal patterns associated with (1) the onset of anorexia after the initial recognition of a threonine deficiency and (2) after the development of the conditioned taste aversion to this deficient diet. When rats ate the threonine-devoid diet for the first time, meal patterns were characterized by an increase in intermeal interval (IMI) between 3 and 6 h after food presentation, which was followed by a decrease in meal size and ingestion rate, between 6 and 12 h. Meal patterns on days 2 and 10 were associated with expression of the taste aversion, characterized by meals of smaller size, longer duration and by a reduction in ingestion rate, without variations in either IMI or meal frequency. Meals of the threonine-deficient group were composed of more frequent bouts, smaller size and shorter duration, with large within-meal pauses, which accounted for the reduced ingestion rate. This study presents the first analysis in terms of feeding patterns and meal microstructure of a conditioned taste aversion induced by a food rather than a toxin. [ABSTRACT FROM AUTHOR]

Among food proteins, alpha-lactalbumin (LAC) has the highest ratio of tryptophan (Trp) over its competitor amino acids. Consequently, contrary to casein (CAS), LAC ingestion increases Trp access to the brain leading to enhanced serotonin (5-HT) synthesis. As an index of serotonergic activity, we assessed extracellular 5-HT in response to LAC ingestion, using microdialysis, and performed behavioural tests in rats in order to characterise the suggested improvements of mood observed in humans after ingestion of this protein. Rats were fed with diets enriched either in LAC or CAS as control, acutely (30 min meals) or chronically (3 and 6 days). A 30 min LAC meal significantly increased 5-HT release in the medial hypothalamus. This effect disappeared after 3 and 6 days of diet. The basal premeal 5-HT levels were increasingly enhanced by the LAC diet. Compared to a CAS meal, LAC increased the percentage of time spent on the open arms of the elevated plus maze and the number of visits to the centre of the open field, suggesting an anxiolytic-like effect. A single LAC meal decreased sucrose consumption, while 3 or 6 days diets enhanced it, reflecting an appetitive and/or rewarding action. In conclusion, LAC ingestion induces anxiolytic-like and rewarding effects possibly related to serotonergic activation. Shifting transiently, the commonly consumed CAS-enriched to LAC-enriched diets may induce beneficial effects on mood.

Rats are able to choose appropriately between two versions of a novel diet, when one is amino-acid devoid and the other corrected. Recognition of the deficiency has been reported to occur within hours and to initiate a strong conditioned aversion. For that purpose the rat can use either oro-sensory cues or another alternative as the conditioned stimulus (CS) with which to associate the unconditioned stimuli (US) of either the adequate diet or the devoid diet. The present investigation was designed to determine whether rats have the ability to use place as a cue in amino-acid preference/aversion. In order to avoid interfering with any other than spatial sensory discrimination between the devoid diet and its corrected version, rats were offered two food cups containing an identical threonine-devoid diet. This was supplemented with an intra-gastric delivery of either threonine or its vehicle, that varied according to the position of the feeder from which the rats had chosen to eat. After three choice sessions, rats chose the food container on the side corresponding to the threonine load. Our results showed that rats use place as a cue for an amino-acid replete nutritional state by learning a conditioned place preference/aversion and that they achieve this type of learning in spite of the long delay elapsing between CS and US.