Role of learning in the selection of dietary protein in the golden hamster (Mesocricetus auratus)

FGF21 as a mediator of adaptive changes in food intake and macronutrient preference in response to protein restriction

Free-feeding animals navigate complex nutritional landscapes in which food availability, cost, and nutritional value can vary markedly. Animals have thus developed neural mechanisms that enable the detection of nutrient restriction, and these mechanisms engage adaptive physiological and behavioral responses that limit or reverse this nutrient restriction. This review focuses specifically on dietary protein as an essential and independently defended nutrient. Adequate protein intake is required for life, and ample evidence exists to support an active defense of protein that involves behavioral changes in food intake, food preference, and food motivation, likely mediated by neural changes that increase the reward value of protein foods. Available evidence also suggests that the circulating hormone fibroblast growth factor 21 (FGF21) acts in the brain to coordinate these adaptive changes in food intake, making it a unique endocrine signal that drives changes in macronutrient preference in the context of protein restriction.

Protein Appetite at the Interface between Nutrient Sensing and Physiological Homeostasis

Feeding behavior is guided by multiple competing physiological needs, as animals must sense their internal nutritional state and then identify and consume foods that meet nutritional needs. Dietary protein intake is necessary to provide essential amino acids and represents a specific, distinct nutritional need. Consistent with this importance, there is a relatively strong body of literature indicating that protein intake is defended, such that animals sense the restriction of protein and adaptively alter feeding behavior to increase protein intake. Here, we argue that this matching of food consumption with physiological need requires at least two concurrent mechanisms: the first being the detection of internal nutritional need (a protein need state) and the second being the discrimination between foods with differing nutritional compositions. In this review, we outline various mechanisms that could mediate the sensing of need state and the discrimination between protein-rich and protein-poor foods. Finally, we briefly describe how the interaction of these mechanisms might allow an animal to self-select between a complex array of foods to meet nutritional needs and adaptively respond to changes in either the external environment or internal physiological state.

The neurochemistry of social reward during development: What have we learned from rodent models?

Social rewards are fundamental to survival and overall health. Several studies suggest that adequate social stimuli during early life are critical for developing appropriate socioemotional and cognitive skills, whereas adverse social experiences negatively affect the proper development of brain and behavior, by increasing the susceptibility to develop neuropsychiatric conditions. Therefore, a better understanding of the neural mechanisms underlying social interactions, and their rewarding components in particular, is an important challenge of current neuroscience research. In this context, preclinical research has a crucial role: Animal models allow to investigate the neurobiological aspects of social reward in order to shed light on possible neurochemical alterations causing aberrant social reward processing in neuropsychiatric diseases, and they allow to test the validity and safety of innovative therapeutic strategies. Here, we discuss preclinical research that has investigated the rewarding properties of two forms of social interaction that occur in different phases of the lifespan of mammals, that is, mother-infant interaction and social interactions with peers, by focusing on the main neurotransmitter systems mediating their rewarding components. Together, the research performed so far helped to elucidate the mechanisms of social reward and its psychobiological components throughout development, thus increasing our understanding of the neurobiological substrates sustaining social functioning in health conditions and social dysfunction in major psychiatric disorders.

FGF21 and the Physiological Regulation of Macronutrient Preference

The ability to respond to variations in nutritional status depends on regulatory systems that monitor nutrient intake and adaptively alter metabolism and feeding behavior during nutrient restriction. There is ample evidence that the restriction of water, sodium, or energy intake triggers adaptive responses that conserve existing nutrient stores and promote the ingestion of the missing nutrient, and that these homeostatic responses are mediated, at least in part, by nutritionally regulated hormones acting within the brain. This review highlights recent research that suggests that the metabolic hormone fibroblast growth factor 21 (FGF21) acts on the brain to homeostatically alter macronutrient preference. Circulating FGF21 levels are robustly increased by diets that are high in carbohydrate but low in protein, and exogenous FGF21 treatment reduces the consumption of sweet foods and alcohol while alternatively increasing the consumption of protein. In addition, while control mice adaptively shift macronutrient preference and increase protein intake in response to dietary protein restriction, mice that lack either FGF21 or FGF21 signaling in the brain fail to exhibit this homeostatic response. FGF21 therefore mediates a unique physiological niche, coordinating adaptive shifts in macronutrient preference that serve to maintain protein intake in the face of dietary protein restriction.

Fooled by savouriness? Investigating the relationship between savoury taste and protein content in familiar foods

Selecting savoury foods after consuming a protein depleted diet has been suggested to reflect protein seeking behaviour. The modern diet contains a large number of processed foods, many of which are highly savoury to taste, but not necessarily high in protein. The present two studies aimed to investigate the relationship between savoury taste and protein content (actual and participant estimated). Participants (S1 n = 20, S2 n = 37) completed 100 mm VAS ratings of sensory and nutritional qualities of 18 familiar foods, categorised as sweet low protein, savoury low protein and savoury high protein. In study 2, the individual foods were blended to a fine consistency to disguise their identity and ensure ratings were based primarily on taste. Multilevel linear regression was used to test associations between savoury taste and actual protein content. Protein content did not predict savoury taste rating, irrespective of category. The results also indicated that participants were generally accurate at estimating the protein content of foods, although there was a tendency towards overestimation. The magnitude of this error was increased in low protein savoury foods. Specifically, there was a shift in the spread of estimation scores which showed a greater level of overestimation in some blended compared to unblended foods, and predominantly in savoury foods which participants could not identify. These results provide evidence that savoury taste and protein content are not well linked in the current food environment, but taste may guide nutrient estimations about certain unidentified foods.

Conditioned feeding responses of sheep towards flavoured foods associated with the administration of ruminally degradable and/or undegradable protein sources

The main objective of the experiment was to investigate the conditioned responses of sheep towards food flavours associated with the administration of ruminally degradable protein (RDP) and ruminally undegradable, but readily digestible protein (DUP) sources given either alone or in combination. The experiment consisted of three consecutive periods during which sheep were conditioned to associate a flavoured food with a nutritive stimulus (or water, W). Two foods (basal and novel test) with different crude protein (CP; 92 and 64 g/kg dry matter (DM) respectively) and similar metabolizable energy (≊ 9 MJ/kg DM) contents were used on a total of 48 Texel ✕ Greyface female sheep. The basal food was offered during non-experimental (rest) days whereas the test food was used in combination with two flavours, orange and aniseed, during experimental days. Food was presented for 8 h (09:00 to 17:00 h) daily throughout the experiment. Two nutritive stimuli (casein, C, and formaldehyde treated casein, FC) were chosen such as to provide major contrasts in their RDP and DUP contents, on an isonitrogenous basis. Each dose (50 g) of a particular nutritive stimulus was administered by gavage through a stomach tube twice daily (at 10:00 and 14:00 h). Sheep were randomly assigned to one of four (C v . W, FC v . W , C v . FC, C v . FC + C) treatments (no. = 12 per treatment). For the first 2 days (days 1 + 2) of each conditioning period half of the sheep within each treatment were offered one flavoured food paired with the administration of C (treatments C v . W , C v . FC and C v. FC + C) or FC (treatment FC v. W). The other half were offered the opposite flavoured food paired with the administration of water (treatments C v . W and FC v . W), FC (treatment C v . FC) or C + FC (treatment C v . FC + C). There followed 2 days (days 3 + 4) of rest and for the 2 days subsequently (days 5 and 6) received the opposite flavoured food and the opposite stimuli to that received earlier. In the morning of day 7 sheep were offered a choice between the two flavoured foods for 20 min. After the completion of the preference test sheep were offered the basal food. The same procedure was followed for each of three conditioning periods (i.e. each animal followed the same flavour/stimulus association throughout the experiment). The design was balanced for order of flavour and stimulus presentation. Sheep preferred the flavoured food associated with C (P < 0·05) or FC (P < 0·01) over the opposite flavoured food associated with water in C v . W and FC v . W treatments respectively. In the C v . FC treatment sheep showed a strong preference for food flavours associated with the administration of FC to those associated with C (P < 0·05). In the C v. FC + C treatment sheep showed equal preference towards the food flavours associated with either stimuli. These results: (i) support the view that sheep are able to form learned preferences for food flavours associated with the administration of protein, and (ii) suggest that sheep are able to distinguish between food flavours associated with the administration of both RDP and DUP sources. Sheep preferred flavours associated with DUP administration only over flavours associated with RDP administration only; however, such preferences did not develop when DUP was administered concurrently with RDP. Given the learned responses of sheep towards flavours associated with RDP and DUP the expectation is that they may be able to select their diet on the basis of these qualities when they are offered a choice.

The brain's response to an essential amino acid-deficient diet and the circuitous route to a better meal

The essential (indispensable) amino acids (IAA) are neither synthesized nor stored in metazoans, yet they are the building blocks of protein. Survival depends on availability of these protein precursors, which must be obtained in the diet; it follows that food selection is critical for IAA homeostasis. If even one of the IAA is depleted, its tRNA becomes quickly deacylated and the levels of charged tRNA fall, leading to disruption of global protein synthesis. As they have priority in the diet, second only to energy, the missing IAA must be restored promptly or protein catabolism ensues. Animals detect and reject an IAA-deficient meal in 20 min, but how? Here, we review the molecular basis for sensing IAA depletion and repletion in the brain's IAA chemosensor, the anterior piriform cortex (APC). As animals stop eating an IAA-deficient meal, they display foraging and altered choice behaviors, to improve their chances of encountering a better food. Within 2 h, sensory cues are associated with IAA depletion or repletion, leading to learned aversions and preferences that support better food selection. We show neural projections from the APC to appetitive and consummatory motor control centers, and to hedonic, motivational brain areas that reinforce these adaptive behaviors.

Neural and metabolic regulation of macronutrient intake and selection

There is considerable disagreement regarding what constitutes a healthy diet. Ever since the influential work of Cannon and Richter, it was debated whether the 'wisdom of the body' will automatically direct us to the foods we need for healthy lives or whether we must carefully learn to eat the right foods, particularly in an environment of plenty. Although it is clear that strong mechanisms have evolved to prevent consumption of foods that have previously made us sick, it is less clear whether reciprocal mechanisms exist that reinforce the consumption of healthy diets. Here, we review recent progress in providing behavioural evidence for the regulation of intake and selection of proteins, carbohydrates and fats. We examine new developments in sensory physiology enabling recognition of macronutrients both pre- and post-ingestively. Finally, we propose a general model for central neural processing of nutrient-specific appetites. We suggest that the same basic neural circuitry responsible for the homoeostatic regulation of total energy intake is also used to control consumption of specific macro- and micronutrients. Similar to salt appetite, specific appetites for other micro- and macronutrients may be encoded by unique molecular changes in the hypothalamus. Gratification of such specific appetites is then accomplished by engaging the brain motivational system to assign the highest reward prediction to exteroceptive cues previously associated with consuming the missing ingredient. A better understanding of these nutrient-specific neural processes could help design drugs and behavioural strategies that promote healthier eating.

Role of gut nutrient sensing in stimulating appetite and conditioning food preferences

The discovery of taste and nutrient receptors (chemosensors) in the gut has led to intensive research on their functions. Whereas oral sugar, fat, and umami taste receptors stimulate nutrient appetite, these and other chemosensors in the gut have been linked to digestive, metabolic, and satiating effects that influence nutrient utilization and inhibit appetite. Gut chemosensors may have an additional function as well: to provide positive feedback signals that condition food preferences and stimulate appetite. The postoral stimulatory actions of nutrients are documented by flavor preference conditioning and appetite stimulation produced by gastric and intestinal infusions of carbohydrate, fat, and protein. Recent findings suggest an upper intestinal site of action, although postabsorptive nutrient actions may contribute to flavor preference learning. The gut chemosensors that generate nutrient conditioning signals remain to be identified; some have been excluded, including sweet (T1R3) and fatty acid (CD36) sensors. The gut-brain signaling pathways (neural, hormonal) are incompletely understood, although vagal afferents are implicated in glutamate conditioning but not carbohydrate or fat conditioning. Brain dopamine reward systems are involved in postoral carbohydrate and fat conditioning but less is known about the reward systems mediating protein/glutamate conditioning. Continued research on the postoral stimulatory actions of nutrients may enhance our understanding of human food preference learning.

Homeostatic regulation of protein intake: in search of a mechanism

Free-living organisms must procure adequate nutrition by negotiating an environment in which both the quality and quantity of food vary markedly. Recent decades have seen marked progress in our understanding of neural regulation of feeding behavior. However, this progress has occurred largely in the context of energy intake, despite the fact that food intake is influenced by more than just the energy content of the diet. A large number of behavioral studies indicate that both the quantity and quality of dietary protein can markedly influence food intake. High-protein diets tend to reduce intake, low-protein diets tend to increase intake, and rodent models seem to self-select between diets in order to meet protein requirements and avoid diets that are imbalanced in amino acids. Recent work suggests that the amino acid leucine regulates food intake by altering mTOR and AMPK signaling in the hypothalamus, while activation of GCN2 within the anterior piriform cortex contributes to the detection and avoidance of amino acid-imbalanced diets. This review focuses on the role that these and other signaling systems may play in mediating the homeostatic regulation of protein balance, and in doing so, highlights our lack of knowledge regarding the physiological and neurobiological mechanisms that might underpin such a regulatory phenomenon.

Preference for novel flavors in adult golden hamsters (Mesocricetus auratus)

Although animals generally prefer to eat foods with familiar rather than unfamiliar flavors, adult golden hamsters (Mesocricetus auratus) were found to do the opposite. After having prolonged exposure to a food with a particular flavor, hamsters were allowed to select between the food with the familiar flavor and the same food with a novel flavor. Hamsters consistently ate more of the food with the novel flavor, and this preference was long-lasting and resistant to extinction. Furthermore, the novelty effect was robust, being manifested in both sexes and under a variety of experimental circumstances. In contrast, rats tested under identical conditions consistently preferred the food with the familiar flavor. The origins of the novelty effect in hamsters remain to be determined.

Operant responding for dietary protein in the golden hamster (Mesocricetus auratus)

Golden hamsters housed in operant chambers over a period of weeks had ad lib access to a maintenance diet that was either nutritionally complete (NCMD) or protein-free (PFMD), and they were required to press a lever on a fixed-ratio (FR) schedule to obtain 20-mg high-protein pellets. As the FR requirement increased, hamsters maintained on the NCMD made fewer lever presses and ate fewer pellets, and at the highest FR levels, they earned very few pellets. In contrast, hamsters maintained on PFMD increased the number of lever presses as the FR requirement increased, and they only slightly reduced the number of pellets eaten. Even at the highest FR requirement levels, PFMD hamsters still derived an average of 11-12% of total calories from protein, a level of intake that is either adequate for adult hamsters, or very nearly so. Previous research has shown that hamsters make adaptive behavioural adjustments in response to time-restricted access to dietary protein, and the present findings demonstrate that protein-restricted hamsters that must press a lever to obtain protein-rich pellets also make adaptive behavioural adjustments when challenged with increases in the FR requirement.

The effect of fat and carbohydrate content of the diet on voluntary ethanol intake in golden hamsters

Golden hamsters, which avidly consume ethanol solutions, had continuous access to food and water and to either 15% or 30% ethanol solution (v/v) over a period of weeks. Hamsters consumed approximately equal amounts of absolute ethanol when maintained on either a Purina Chow diet or a high-carbohydrate, low-fat diet (7.6 and 7.1 g/kg/day, respectively), but they consumed substantially less ethanol when maintained on a high-fat, low-carbohydrate diet (4. 4 g/kg/day). However, the pattern of differences among groups of hamsters maintained on different diets was not the same at the two ethanol concentrations. Thus, at the 15% concentration, HC hamsters consumed more than twice as much ethanol as HF hamsters, but at the 30% concentration, the difference in ethanol consumption was greatly reduced and did not prove to be statistically significant. These results indicate that it is important to consider the concentration of the ethanol solution when studying the effects of dietary macronutrient content on ethanol consumption.