Lysine deficient diet and lysine replacement affect food directed operant behavior

Preference for and learning of amino acids in larval Drosophila

Relative to other nutrients, less is known about how animals sense amino acids and how behaviour is organized accordingly. This is a significant gap in our knowledge because amino acids are required for protein synthesis - and hence for life as we know it. Choosing Drosophila larvae as a case study, we provide the first systematic analysis of both the preference behaviour for, and the learning of, all 20 canonical amino acids in Drosophila We report that preference for individual amino acids differs according to the kind of amino acid, both in first-instar and in third-instar larvae. Our data suggest that this preference profile changes across larval instars, and that starvation during the third instar also alters this profile. Only aspartic acid turns out to be robustly attractive across all our experiments. The essentiality of amino acids does not appear to be a determinant of preference. Interestingly, although amino acids thus differ in their innate attractiveness, we find that all amino acids are equally rewarding. Similar discrepancies between innate attractiveness and reinforcing effect have previously been reported for other tastants, including sugars, bitter substances and salt. The present analyses will facilitate the ongoing search for the receptors, sensory neurons, and internal, homeostatic amino acid sensors in Drosophila.

Mated Drosophila melanogaster females consume more amino acids during the dark phase

To maintain homeostasis, animals must ingest appropriate quantities, determined by their internal nutritional state, of suitable nutrients. In the fruit fly Drosophila melanogaster, an amino acid deficit induces a specific appetite for amino acids and thus results in their increased consumption. Although multiple processes of physiology, metabolism, and behavior are under circadian control in many organisms, it is unclear whether the circadian clock also modulates such motivated behavior driven by an internal need. Differences in levels of amino acid consumption by flies between the light and dark phases of the day:night cycle were examined using a capillary feeder assay following amino acid deprivation. Female flies exhibited increased consumption of amino acids during the dark phase compared with the light phase. Investigation of mutants lacking a functional period gene (per⁰), a well-characterized clock gene in Drosophila, found no difference between the light and dark phases in amino acid consumption by per⁰ flies. Furthermore, increased consumption of amino acids during the dark phase was observed in mated but not in virgin females, which strongly suggested that mating is involved in the rhythmic modulation of amino acid intake. Egg production, which is induced by mating, did not affect the rhythmic change in amino acid consumption, although egg-laying behavior showed a per⁰-dependent change in rhythm. Elevated consumption of amino acids during the dark phase was partly induced by the action of a seminal protein, sex peptide (SP), on the sex peptide receptor (SPR) in females. Moreover, we showed that the increased consumption of amino acids during the dark phase is induced in mated females independently of their internal level of amino acids. These results suggest that a post-mating SP/SPR signal elevates amino acid consumption during the dark phase via the circadian clock.

Bilateral dopaminergic lesions in the ventral tegmental area of rats influence sucrose intake, but not umami and amino acid intake

The role of the dopaminergic cells in the ventral tegmental area (VTA) in response to natural rewards is important in understanding palatability-induced feeding behavior. In this study, we first investigated whether dopaminergic lesions in the VTA would influence the taste preferences of rats for sodium chloride (NaCl), monosodium glutamate (MSG), disodium inosine-5'-monophosphate (IMP), disodium guanine-5'-monophosphate (GMP) and sucrose. Among these taste stimuli, only the preference of sucrose solutions decreased significantly in the VTA lesioned rats, preferences for the other taste stimuli were unaffected. Secondly, we tested whether VTA lesioned rats made slightly deficient in the amino acid lysine (by feeding rats a lysine deficient diet for five days) would detect the deficient amino acid in a choice test. Both the VTA lesioned rats and the control rats chose to consume the lysine solution and there was no difference between these two groups. These results suggest that the dopaminergic neurons in the VTA influence sucrose consumption, but do not alter the consumption of palatable umami compounds and salt. They also do not inhibit the animal's ability to recognize the essential amino acid lysine when the animal is deficient in it. Different central pathways must underlie sucrose preference and preferences for these amino acids and ribonucleotides.

Relationships between insulin release and taste

Tasting sweet food elicits insulin release prior to increasing plasma glucose levels, known as cephalic phase insulin release (CPIR). The characteristic of CPIR is that plasma insulin secretion occurs before the rise of the plasma glucose level. In this experiment, we examined whether taste stimuli placed on the tongue could induce CPIR. We used female Wistar rats and five basic taste stimuli: sucrose (sweet), sodium chloride (salty), HCl (sour), quinine (bitter) or monosodium glutamate (umami). Rats reliably exhibited CPIR to sucrose. Sodium chloride, HCl, quinine, or monosodium glutamate did not elicit CPIR. The non-nutritive sweetener saccharine elicited CPIR. However, starch, which is nutritive but non-sweet, did not elicit CPIR although rats showed a strong preference for starch which is a source of glucose. In addition, we studied whether CPIR was related to taste receptor cell activity. We carried out the experiment in rats with bilaterally cut chorda tympani nerves, one of the gustatory nerves. After sectioning, CPIR was not observed for sweet stimulation. From these results, we conclude that sweetness information conducted by thistaste nerve provides essential information for eliciting CPIR.

The influence of nutritional status on the feeding behaviour of the field slug, Deroceras reticulatum (Müller)

The field slug, Deroceras reticulatum, a common pest of agricultural and horticultural crops, is a generalist herbivore with distinct preferences for particular food items. However, these preferences are not fixed, but are influenced by the recent dietary history of the slugs. In particular, slugs tend to select novel food items ('neophilia'). We investigated the basis of such influences, using artificial diets in which protein and carbohydrate composition could be controlled and non-nutritional ('secondary') chemicals added as flavours. The slugs showed no general inclination for neophilia based on taste alone. There was a possible learned association between protein content and taste, but this was weakly expressed. However, the slugs selected food items, when given a choice, containing nutrients that were deficient in earlier diets, even though all the food items contained the same familiar secondary taste chemicals. Injections of missing nutrients into the haemocoel of slugs inhibited such changes in food preference, indicating that slugs' feeding preferences are influenced directly by their internal nutritional status. We suggest that neophilia in D. reticulatum is a physiological response to a nutritional imbalance arising from a suboptimal diet. Copyright 2000 The Association for the Study of Animal Behaviour.

Effect of inhibin, follistatin, or activin infusion into the lateral hypothalamus on operant behavior of rats fed lysine deficient diet

To identify brain mechanisms which mediate hunger for amino acid (e.g. L-lysine; Lys) deficiency, rats were trained to bar press (FR30 schedule) to receive 50 mg pellets of a complete diet. Rats given a lysine deficient (Lys-def) diet ad libitum maintained a high rate of bar pressing but when allowed ad libitum access to 0.4 M Lys to drink had a significant decrease in pressing. Also, Lys continuously infused by minipump into the lateral hypothalamic area (LHA) inhibits pressing by rats given a Lys-def diet. The threshold maximal dose is between 0.1-0.5 nmol Lys/h. Therefore, animals lacking dietary Lys will work to receive complete diet, but replacement of Lys by voluntary consumption or by direct infusion into the LHA inhibits bar pressing for complete diet. The ratio of brain activin and inhibin may modulate motivation to work for a complete diet, since continuous inhibin or follistatin, but not activin, infusion into the LHA was found to inhibit bar pressing, which is normally quite strong in rats maintained on Lys-def diet. The inhibitory effect of LHA inhibin infusion was replicated, and concurrent availability of Lys solution ad libitum was additive with LHA inhibin infusion to depress responding further. This inhibitory effect of inhibin or follistatin did not result from altered ad lib. consumption of Lys-def diet. Although LHA Lys infusion did decrease consumption of a concurrently available Lys solution, inhibin did not change ad libitum Lys consumption. This indicates that inhibin may work in the LHA to inhibit bar pressing for complete diet via other mechanisms than sensation of Lys deficiency.

Recognition and neural plasticity responding to deficient nutrient intake scanned by a functional MRI in the brain of rats with L-lysine deficiency

Each L-amino acid (AA) in plasma and brain remains unchanged while normal diet is available. Once L-lysine (Lys) deficient diet was offered to rats, Lys in plasma and brain declined, and anorexia occurred. When solutions of AAs were offered, they selected the Lys solution, and their food intake and growth normalized. The single neuron activity in the lateral hypothalamic area of these rats suggested that neural plasticity occurred, specifically responding to Lys, both by iontophoretic application and during ingestion of AA. The recognition site for deficient nutrient intake in the brain of rats with Lys deficiency was identified by non-invasive magnetic resonance imaging (MRI 4.7 tesla, 40 cm bore in diameter) developed to monitor changes in cerebral blood flow and oxygenation in rat brain. Wistar strain young male rats fed with Lys deficient diet for 4 days, were adapted to be settled in the center of the bore. When they received a Lys injection intraperitoneally (0.2 M, 10 mL/kg), a signal intensity decrease in the medial and lateral hypothalamus appeared 30 minutes later in T2 weighted images, reflecting increased oxygenation which lasted for 30 minutes, and then gradually recovered. These changes never occurred in any other areas of the brain of rats with Lys deficiency, i.e., the thalamus, the cortex, the hippocampus, etc. There were no changes in the signal intensity with control injection of saline. In addition, oxygen consumption in the brain of rats without Lys deficiency was not altered by intraperitoneal Lys injection. The present results suggest that in essential AA deficiency, the medial and lateral hypothalamus may play important roles in recognition responses to particular deficient nutrients in order to maintain homeostasis.

Effect of continuous infusion of lysine via different routes and hepatic vagotomy on dietary choice in rats

The effect of continuous L-lysine (Lys) infusion on dietary choice between Lys deficient and protein-free diets in Sprague-Dawley rats was studied to determine the sensing site of Lys deficiency. After daily intake of each diet became constant, Lys was continuously infused for 11 days via intraperitoneal (IP), intragastric (IG) or intracerebroventricular (ICV) route, with an osmotic pump. Daily intake of each diet was measured. Intake of the Lys deficient diet compared with protein-free diet in either IP or IG Lys-infused group increased significantly (p < 0.001) vs. the intake in the baseline period. The selection of the Lys deficient diet was quite comparable between IP and IG groups. But the intake of the ICV group was unchanged. Hepatic vagotomy during IP infusion transiently delayed selection of the Lys deficient diet. These results imply the roles of postabsorptive mechanisms in sensing an amino acid deficiency, and possible involvement of the hepatic branch of the vagus in the sensing. However, sensing in the brain or indeed in the intestine was not excluded.