Feeding patterns and meal microstructure during development of a taste aversion to a threonine devoid diet

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.

Plasma Trp/LNAA ratio increases during chronic ingestion of an alpha-lactalbumin diet in rats

Brain serotonin synthesis depends on the uptake of its precursor, tryptophan (Trp), and is correlated to the plasma ratio of Trp to large neutral amino acids (LNAA) which compete for the same transporter system in the brain. As the plasma Trp/LNAA ratio decreases when the dietary protein content exceeds 5%, we tested whether a diet containing 17% of a Trp-rich protein, namely alpha-lactalbumin (LAC), might increase the plasma Trp/LNAA ratio over a long period. Blood samples were obtained at different days (-1, 3, 6 and 9) from rats receiving either a LAC or casein (CAS) diet, and plasma amino acids and insulin concentrations were determined. The increase in plasma Trp concentration was much higher during the LAC diet (49 vs 26%; P<0.001), while the plasma LNAA concentration remained fairly constant. Consequently, the plasma Trp/LNAA ratio increased by 40% during the LAC diet while it decreased by 15% during the CAS diet (P<0.001). The above results were not related to plasma insulin concentration differences during these diets. These data suggest that a balanced diet containing a natural Trp rich-protein increases the plasma Trp/LNAA ratio over a long period, leading to a probable increase in brain serotonin activity.

Is the early increase in leptinemia one of the anorectic signals induced by an essential amino acid-deficient diet in the rat?

Rats start decreasing their food intake as early as 70 min after the first ingestion of a food deficient in threonine. A decrease of the limiting essential amino acid (EAA) in the plasma was proposed to be the first anorectic signal. Because many hormones regulate feeding behavior, we studied the effect of a meal (46 kJ) that was either devoid of threonine or was corrected for the deficiency, on plasma leptin, insulin and glucagon levels using a radio-immunoassay, at 0 to 180 min after the meal. One hour after ingestion of the threonine-devoid meal, a larger increase in insulinemia (22+/-1 vs. 15+/-1 microU/ml) and leptinemia (7.8+/-0.5 vs. 4.4+/-0.6 ng/ml; p<0.001) was observed than after ingestion of the corrected meal. The area under the curve of the threonine-devoid meal group was 3 and 1.34 fold larger than for the corrected meal group for insulin and leptin respectively. Glucagonemia was not different between the two groups. We propose that the rise in leptinemia, perhaps in synergy with rise in plasma insulin, might serve as one early signal to brain structures, participating in the anorectic mechanism following ingestion of an EAA-deficient diet.

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.

Postprandial metabolism and aversive response in rats fed a threonine-devoid diet

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.

Conditioned taste aversion in rats for a threonine-deficient diet: demonstration by the taste reactivity test

Rats avoid a diet that is deficient in one or more essential amino acids (EAAs). This phenomenon is thought to involve the development of a "learned aversion" for the sensory properties or spatial placement associated with the deficient diet. The dietary self-selection technique has been widely used to show this avoidance of the deficient diet. Because avoidance does not necessarily imply taste aversion, we used the Taste Reactivity Test initially created by Grill and Norgren (1978) to analyze the affective reactivity pattern of rats that ingested a threonine-deficient diet. The results showed that there was an increase in the aversive responses when ingesting the threonine-deficient (Thr-Dev) diet, compared to a control diet, without changes in the hedonic responses. The aversive reactions were mainly gaping, and to a lesser extent chin rubbing and head shaking. This asymmetrical shift in the Thr-Dev diet palatability is consistent with a two-dimensional hypothesis of palatability, indicating that the aversive palatability of the deficient diet was increased while the positive palatability did not change. Further evidence indicates that rats do not develop a normal behavioral satiety sequence after ingesting the threonine-deficient diet. These results indicate that a true aversion is formed to the taste of a diet that is deficient in an essential amino acid.