Feeding refers to the process that begins from when a mouse searches for food to when it ingests it, in order to stay healthy and maintain its energy levels. Feeding behaviors are the behaviors exhibited to make the process of feeding successful.
Therefore, feeding behaviors are one of the maintenance behaviors which a mouse exhibits in order to stay healthy and maintain its energy levels.
Also, since feeding is an active behavior defined by the presence of motion, it is typically observed during a mouse’s awake-phase.
A mouse’s feeding behaviors revolve around the consumption of food. For a mouse to be successful, it must be capable of performing various behaviors in order to find the food and consume it.
When eating, the mouse will typically use its forepaws to hold the food up steadily while biting it with its incisors. Then, the food moves deeper into the mouth and is chewed with the molars prior to swallowing.
Feeding behaviors also involve the storage of food.
In scientific literature, ‘feeding behaviors’ and ‘eating behaviors’ are usually used interchangeably.
Brain Circuitry Associated with Feeding Behaviors
The drive to eat is thought to be regulated by two different brain pathways which are called homeostatic and hedonic. Therefore, through scientific research and interventions, these two pathways can be targeted and studied closely. Orexinergic neurons, found in these pathways, are located in the hypothalamus and are known for regulating metabolism and the organism’s sense of feeding and reward. The brain’s suprachiasmatic clock drives the orexinergic cells’ rhythmic activity, thus controlling the circadian regulation of feeding. Therefore, these neurons have the ability to control both physiological (homeostatic) and hedonic appetite. Other brain circuits which are also crucial for successful feeding behaviors include motor, sensorimotor, and memory circuits.
Table of Content
- Behaviors Associated with Feeding
- Function of Feeding Behaviors
- Application of Feeding Behaviors
- Research Techniques
- Behavioral tests
- Pharmacological Studies
- Mouse Strains and Feeding Behaviors
- Abnormality in Feeding Behaviors
- Disease Models of Feeding Behaviors
Behaviors Associated with Feeding Behaviors
There are many behaviors which (in one way or another) can be classified as feeding behaviors. When a mouse is trying to find food, the general sequence of the feeding behaviors is as follows:
- Search: A mouse is moving through its environment attempting to encounter a food source. Searching is an exploratory behavior that enables the mouse to spot and find food.
- Approach: Then, once the mouse has identified a food source through its searching efforts, it will approach and orient its locomotion towards it. Therefore, approach is another type of exploratory behavior which serves the purpose of acquiring information about the stimulus at hand.
- Chase: In a natural setting, a mouse may have to chase its food source which would require the mouse to move quickly, following the path trajectory of the insect, for example, that it is trying to catch. In this situation, chase is classified as an agonistic behavior since the mouse is trying to gain control of a resource which will aid in its survival. Learn more in our mouse behavior: chase
- Sniff: A mouse will sniff its food in order to gather information about it. Olfactory cues are important to mice because they enable them to make a decision as to whether they should eat or not. Therefore, sniffing is a specific type of investigative behavior. When mice are still pups, they sniff and use their sense of smell by following their mother’s olfactory cues, in order to suckle.
- Bite: Biting is the feeding behavior which accounts for the mechanical consumption of food. In order to acquire energy and ingest its food, a mouse needs to be physically capable of chewing the food.
Also, feeding behaviors which are related to the storage of food may also be observed, such as:
- Bury: A mouse will remove or displace bedding material by using its forepaws and snout with the ultimate aim of covering and concealing its food.
- Dig: In order for a mice to bury, cache, and hoard food, it must be physically capable of digging which is a behavior that requires the energetic alteration of the forepaws and occasional use of the hind limbs to remove the accumulated or piled up materials. Learn more in our mouse behavior: dig article
- Hoarding: Hoarding is a commonly studied feeding behavior wherein mice secure their food supply for future use, typically located either in or close to their homes, or nests. Sometimes, hoarding may be referred to as ‘caching’ but, generally speaking, hoarding is more commonly used to describe this gathering behavior in mice while caching is usually used for birds.
Other behaviors which are also related to feeding behaviors include:
- Nose poking: Nose poking occurs when the mouse is using its nose to press something. Typically, such a behavior can be observed in laboratory settings where the mouse is trying to feed itself by pressing the pellet dispenser’s lever.
- Jumping: Mice, if motivated enough, may jump in order to reach a food pellet that is elevated above the ground.
- Sucking: When it comes to mouse pups, suckling and nursing are bound to be observed since those are the sole way for pups to feed and acquire nutrients.
- Drinking: Although drinking is a behavior that is studied as its own maintenance behavior, drinking-related behavioral tests may be sometimes administered to mice in order to assess their overall profile of feeding behaviors.
Function of Feeding Behaviors
Feeding behaviors are important to a mouse for many reasons, most of which are associated with survival. Feeding behaviors are necessary for:
- Providing energy: Feeding is the intake of food and food is a major energy source. Therefore, the major function of feeding behaviors is to enable the mouse to feed itself and provide its body with sufficient energy levels in order to maintain homeostasis
- Acquiring nutrients: Some nutrients are not readily made or provided by the body’s own metabolic processes. The same physiological limitation exists in humans. Therefore, feeding behaviors are necessary because they enable the organism to meet its nutrient requirements.
- Promoting normal development: By obtaining the necessary nutrition that it needs, a mouse can develop normally and function properly in its immediate environment in order to survive for as long as possible.
- Guarding or storing food: In the case of burying, hoarding, or caching, a mouse will store its food in response to aversive stimuli (such as air puffs and shock prods) or simply in order to access it at a later point in time. Being able to store food for a later time or in response to noxious stimuli is an evolutionary advantage because it delays food consumption until a safer or more optimal point in time.
Application of Feeding Behaviors
Feeding behaviors occur across a variety of contexts. A combination of feeding behaviors in mice is likely to be observed:
- During laboratory feeding: Laboratory feeding protocols are bound to elicit a mouse’s feeding behaviors. Feeding behaviors are also crucial for model induction and intervention studies. It is common to give mice pharmaceutical substances or drugs through food pellets, thus feeding behaviors are observed in the laboratory as well.
- After wheel running: Physical activity and wheel running are factors which lead to increased feeding behavior. The mouse is more physically active, using its available resources rapidly, so it will need to elicit more feeding behaviors in order to supply energy for cellular metabolism.
- Under chronic stress conditions: Chronic stress has been demonstrated to induce higher instances of feeding behavior in mice. Stress makes changes to the hypothalamic-pituitary-adrenal (HPA) axis which results in altered feeding behaviors and disorders (which will be discussed under the Disease Models section). Thus, researchers have used this association for the sake of studying feeding behaviors.
- Due to periods of starvation: When mice are starving, they are likely to display high-risk behaviors which would in any other context be considered as maladaptive. Starving mice perform high-risk feeding behaviors by moving toward open areas which are easily accessible to predators. Therefore, starvation evokes feeding behaviors which differ from the feeding behaviors which a mouse exhibits when it is simply hungry.
- After fasting periods: Fasting for 4-6 hours or even overnight will enhance a mouse’s food consumption once given food again. Sometimes, scientists will subject mice to long fasting periods, in order to elicit any of the feeding behaviors.
- During the winter: Mice will hoard their food when there are tough seasonal changes. By hoarding their food and creating a large storage supply close to their homes, mice are able to feed during cold winter nights without the risk of freezing or getting trapped in a snowstorm.
- During the dark-phase: Since mice are nocturnal creatures, the majority (about 70%) of feeding behaviors will be observed during their dark-phase with only a small fraction occurring during the light-phase.
- In direct response to aversive stimuli: Aversive stimuli are usually associated with causing the mouse to become aggressive, freeze, or flee. However, studies have found that aversive stimuli are also likely to cause a mouse to bury its food.
Since a mouse has many different types of feeding behaviors in its repertoire, various types of research techniques can be used for studying them, including:
- Behavioral Tests: Behavioral tests are the go-to method for assessing feeding behaviors, since feeding behaviors are, after all, behaviors. By subjecting mice to various behavioral tests, feeding behaviors can be measured, assessing facets such as hoarding, motivation, and consumption patterns. More about behavioral tests will be discussed in the subsequent section.
- Brain Lesions: Brain lesions can be performed surgically in order to remove certain areas of the brain. Usually, neural circuits and brain regions are targeted which are thought to be related to the feeding behavior. Then, by removing or manipulating this brain region, alterations in feeding behaviors are compared between experimental and sham-operated mice.
- Electrical Stimulation Studies: Electrical stimulation studies apply controlled electricity to certain regions of the brain which leads to either neuronal excitation or inhibition depending on the protocols used. Thus, researchers are able to study the various feeding behaviors by stimulating relevant neural circuits and subsequently observing the consequences.
- Genetic Studies: Genetic studies, such as genome-wide association and deletion studies, enable researchers to get a closer look at how genetics influence feeding behaviors. This knowledge, in turn, is applied to intervention efforts aiming to restore normal feeding behavioral patterns in disease models where feeding behaviors are impaired.
- Pharmaceutical Studies: Pharmaceutical studies take advantage of mouse feeding behaviors by targeting specific receptors, for example, which are known or believed to be involved in regulating a feeding behavior of interest. In experimental setting, mice can be given drugs which are incorporated into the food pellets. (More about pharmaceutical studies will be elaborated on later.)
Some commonly used behavioral tests to study feeding behaviors are:
- Two-Feeding Choice Test for Food Consumption: Sweet delectable snacks like chocolate are offered to mice along with the usual laboratory chow food for a whole 3 weeks. In order to avoid location preference, the food position is switched left and right daily. The intake and preference for both kinds of food (regular laboratory food and sweet snacks) are measured every 6 hours in Kcal and percent respectively. Sometimes, high-fat diet pellets will be offered to mice (instead of sweet snacks) in order to assess preference. Healthy mice, in some situations (as when presented with a high-fat diet and regular chow), will not show a food preference. But, under disease models or abnormalities, they may begin to show a steady preference for a particular type of diet. This preference choice is then used to assess behavior, metabolic profiles, and short-term consumption patterns. In other contexts, a lack of preference may be equated with anhedonia, an abnormality associated with a lack of motivation for desirable foods.
- Food-Motivation Test: In a setup familiar to mice, 2 grams of chocolate is placed hanging high above the ground, thus making it difficult for the mice to reach. Then on day 2, regular food is suspended. On days 3-5 chocolate is suspended once more. The behavior of mice towards the food is video-recorded for 5 minutes interval, in order to collect data and determine whether the motivation to reach food will be altered based on the type of food that is suspended above the ground. By challenging mice to acquire food, their level of motivation, desire, and drive of feeding behaviors can be quantitatively assessed.
- Sucrose Deprivation Test: This test makes use of two bottles containing water which mice are given unlimited access to for the span of 2 days. Thereafter, the test changes slightly and mice will have access to 2 new water bottles, each of which contains 10% sucrose. Then, sucrose access will be discontinued for 3 days (leaving only water available) and will be reinstated once more at a 10% concentration for 5 days. Before and after the 3 days of sucrose deprivation, sucrose intake is measured in order to deduce what the mice prefer. The sucrose deprivation test is used to measure mice’s preference for tasty diets and their motivation for acquiring such diets. Using this test, it is possible to measure what the effect of being deprived of sucrose will be. While normal, wild-type mice will not have a significantly different sucrose intake level before and after deprivation, abnormal mice are likely to consume more sucrose after being deprived of it.
- Two-Bottle Choice Test for Sucrose Consumption: Mice will have access to two bottles with either water or water containing 10% sucrose for the span of about 1 week. Then, after the mice familiarize with the bottles, the test begins and the bottles will change position on a daily basis. Therein, the mice’s preference is measured based on the quantity of sucrose intake (in ml) in order to gauge their preference. This preference test is also used, sometimes, in order to study depression. Mice which are depressed do not show the same preference to sucrose as wild-type do. In other diseases, such as obesity, experimental mice may show more preference for sucrose than wild-type mice do.
- Digging and Marble Burying Test: In the Digging Test, wood chip bedding is used to cover the floor of the cage in the depth of 5 cm firm and flat. The wood chip can be used more than once in testing different mice as the repeated use does not seem to have an influence on their burying and digging activities. Testing can be done with two side by side cages with a mouse in each cage. The test lasts for 3 minutes during which the mice’s latency to start the digging activity and number and duration of digging spells are recorded. The setup is the same for the Marble Burying test, but glass marbles are placed on the surface in a regular pattern about 4 cm apart. The mice are left each in one cage for 30 minutes after which the number of marbles buried to ⅔ of their depth is counted. This test can be used to assess digging and burying behaviors and, in combination with other behavioral tests, the results can be used in such a way so as to comment on a mouse’s feeding behaviors.
- Hoarding Apparatus: The hoarding apparatus is a rectangular acrylic box divided into 8 chambers, each connected to a hoarding tube made of acrylic plastic and mesh. For the hoarding test, 100 grams of food pellets are placed at the far end of the tube but mice are not immediately given access to the food. At the start of the 12-hour light-phase, a mouse is to be placed in each chamber. Then, just before the start of the 12-hour dark-phase, mice are given access to the hoarding tube and food pellets by removing the wooden plug that was closing the tube. With the start of the light phase the next day, the hoarded food pellets in the chamber are collected and weighed. Thus, the hoarding apparatus is a useful behavior test for measuring a mouse’s hoarding tendency and determine whether this relates or correlates with other feeding behaviors based on results acquired from other behavioral tests.
- Operant-Responding Task: Using the Operant Chamber, mice are taught to press a lever under certain conditions in order to receive a food reward. Sometimes, eating behaviors are studied by pairing them with a stimulus. Other times, mice may be given access to the lever simply as a means to access food. The latter approach makes it easy for researchers to quantify feeding behaviors based on how many times the mouse presses the lever during the experimental trial. Thus, the operant-responding task can be a simple way to measure the food-seeking behavior in mice.
- Vogel Conflict with Food Reinforcement: Although the Vogel Conflict Task was initially developed with regards to drinking behaviors, recent modifications have expanded it to also include feeding behaviors. Mice are deprived of food for several hours, thus become very hungry and motivated to find food. Then, to create conflict, whenever a mouse is presented with food and begins to approach it, it will get electrified from the grid floor of the chamber they are in. Therefore, the Vogel conflict measures how long it takes for a mouse to learn to override its instincts (hunger) due to punishment. The Vogel conflict by creating a conflicting situation assesses the extent to which a mouse will go in order to feed.
Ghrelin is an appetite stimulant, thus is classified as an orexigenic hormone. In order to induce feeding behaviors, ghrelin is injected into mice. Sometimes, in addition to the injection, mice may be subjected to fasting. Wild-type mice that are given ghrelin injections have an increased preference for saccharin-flavored foods. This is somewhat parallel to how ghrelin acts in humans. In fact, one experiment using functional magnetic resonance imaging (fMRI) demonstrated that when human participants who received ghrelin were shown food pictures, they had increased neural responses in brain regions which are known to be associated with hedonic feeding.
Intra-VTA Insulin Suppresses Food Approach Behaviors
The Ventral Tegmental Area (VTA) is a brain region that is important for motivation and learning environmental cues. Furthermore, this region of the brain is filled with dopaminergic neurons which, if stimulated and primed by stimuli such as rewarding food, will have an increased excitatory synaptic density which will last for a few days and will influence behavior by increasing food-seeking behavior. However, if insulin is injected into the brain’s VTA, the local dopamine neurons’ excitatory synaptic transmission will be suppressed. In fact, C57BL/6J mice which are injected with intra-VTA insulin will have reduced food approach behaviors given the reduced distance traveled in the food zone of the Light/Dark Box and the decreased number of food zone entries they exhibit if compared with controls injected only with saline.
ST-1283 Reduces Feeding Latency
ST-1283 antagonizes or blocks histamine H3 receptors (H3R). H3Rs have been found to be implicated in many brain functions, including cognition, stress, emotion, and feeding. Thus, current research efforts are trying to establish how H3Rs can be used for therapeutic applications. One recent study, for example, demonstrated that synthetic ST-1283 is able to decrease anxiety-related behaviors while increasing feeding behaviors in a novelty suppressed feeding test wherein mice typically do not eat since they are experiencing high anxiety levels from being introduced in a novel setting. Therefore, ST-1283 is implicated in affecting feeding behaviors which may otherwise have been modulated by anxiety.
Simvastatin Reduces High-Fat Diet-Related Anhedonia
One of the hallmark effects of a high-fat diet in mice is anhedonia. This demonstrates how feeding behaviors which favor the high-fat diet can, in the long-term, affect the emotional tendencies of a mouse. While normal, wild-type mice prefer sucrose in the sucrose preference test, mice given a high-fat diet do not exhibit such a preference, demonstrating anhedonia which affects their food choice preference and feeding behaviors.
Simvastatin, which in clinical studies has been demonstrated as able to reduce the risk of depression following cardiac intervention, is also being studied as a potential drug-intervention for ameliorating problems associated with feeding behaviors. Current research trends using mice show that simvastatin may be able to reverse the anhedonia towards sucrose in mice given a high-fat diet.
Fluoxetine Reduces Feeding Latency
Fluoxetine is an antidepressant drug classified as a selective serotonin reuptake inhibitor and is able to influence emotions, behavior, and cognition. Thus, it is used clinically to treat patients that have been diagnosed with major depressive disorder, panic disorder, bulimia nervosa, obsessive-compulsive disorder, and premenstrual disorder. In animal studies using mice, fluoxetine is able to reduce the feeding latency time that it takes for mice to start eating in a novel environment. When mice are placed in a novel setting, their feeding behaviors are bound to decrease because they are experiencing stress and anxiety. Fluoxetine is able to reverse this effect so that mice take as much time to initiate their first feeding episode in a novel environment as they would have in a familiar setting.
Mouse Strains and Feeding Behaviors
C57BL/6J mice are commonly used for conducting energy homeostasis experiments. Since this mouse strain was the first to have its genome fully sequenced, it is favored for experimental studies which study the genetics of feeding behaviors. Also, diet-induced obesity is easily modeled using C57BL/6J mice which is useful since a lot of biomedical research is trying to address the contemporary problem of high levels of obesity. The downside of using C57BL/6J mice when studying feeding behaviors is that they produce small litters, which is a disadvantage if the research is designed to make use of knock-out or transgenic animals.
Abnormalities in Feeding Behaviors
HDAC4A778T Mutation Affects Female Mice’s Feeding Behaviors
The rare missense mutation affecting the gene for the repressor histone deacetylase 4 (HDAC4) is thought to be a risk factor in humans for developing eating disorders. HDAC4A778T mice have this genetic mutation at position 778 of the HDAC4. While male heterozygous HDAC4A778T mice do not exhibit changes to their behavioral and metabolic profiles, female equivalents have multiple eating disorder-related feeding deficits. Female mice showed low willingness and motivation to obtain high-fat food, a behavioral profile which resembles anorexia in humans, characterized by reduced preference and desire for foods which have high-fat content. To test this behavior, an operant-responding task chamber is used where mice must poke a button or a lever in order to receive a high-fat pellet. On the other hand, wild-type control mice will perform more nose pokes and earn a greater number of high-fat rewards when compared to female HDAC4A778T mice, suggesting that the genetic mutation modeled in the HDAC4A778T mice affects feeding behaviors.
Increased Food-Seeking in MCH1-/- Mice
Melanin-concentrating hormone 1 (MCH1) receptors are functionally involved in feeding behaviors. Therefore, mice which have MCH1 deleted, as MCH1-/- mice do, are bound to have a compromised ability when it comes to controlling their feeding behaviors. In fact, MCH1-/- mice have an increased motivational drive when it comes to food-seeking when compared to mice that have their MCH1 receptors intact. Furthermore, MCH1-/- mice consume more calories daily when adjusted for bodyweight than their counterparts do. Even when the MCH1-/- mice were subjected to operant conditioning wherein an aversive stimulus was paired each with feeding behaviors, these mice still continued to eat more than controls.
Histamine H4 KO Mice Have Increased Food Consumption
H4Rs are known to be involved in important immunological processes, interacting with hematopoietic cells throughout the body. However, it has recently been discovered that H4Rs can also be found in the brain. Up to this point, brain science has focused on the following neuronal receptors: H1R, H2R, and H3R. Thus, researchers are trying to get a better understanding of H4Rs’ role in the central nervous system. Histamine H4 knockout (KO) mice have their histamine H4 receptors (H4R) deleted and have been utilized for the purpose of determining whether H4Rs are in any way involved in feeding behaviors as well. After a 4-hour food deprivation test, H4R KO mice will consume more food than wild-type controls when re-introduced to food. However, after a 12-hour food deprivation test, H4R KO will eat the same amount of food as wild-type mice.
BDNFff, Cre Mice
Brain-derived neurotrophic factor (BDNF) is a protein that is crucial for neuronal growth, higher cognitive functions, and long-term memory. The homozygous BDNF‐floxed Cre‐positive (BDNFff, Cre) mice have a CA-3 restricted knocking out of BDNF. This knockout (KO) strain shows high feeding behaviors in the presence of an unfamiliar conspecific, a behavior that is not observed typically in their wild-type counterparts. Typically, wild-type mice do not eat in the presence of an unfamiliar mouse since an unfamiliar mouse can make them feel stressed due to its novelty. BDNFff, Cre mice do not seem to be bothered by unfamiliar mice when it comes to feeding. This mutant strain of mice demonstrates how the removal of BDNF restricted to a certain region of the brain can change feeding behaviors in mice within a social context.
REV-ERBα Mutant Mice Show Highly Motivated Feeding Behaviors
The REV-ERBα protein is a nuclear receptor protein which is expressed throughout the body’s organs and in the brain’s suprachiasmatic nucleus, the region of the hypothalamus which controls circadian rhythms and homeostasis. Therefore, the REV-ERBα protein is a crucial molecular link for understanding and manipulating the relationship between metabolism, circadian rhythms, and homeostatic behaviors like feeding. In fact, REV-ERBα mutant mice, which have had the REV-ERBα protein genetically knocked out, demonstrate highly motivated feeding behaviors. Compared to wild-type mice, REV-ERBα mice have increased sucrose preference and will consume more than control mice (while both mice have an equal intake of non-caloric sweet solution). REV-ERBα mice also have a high preference for palatable foods like chocolate when compared to controls, and consumption of such foods is associated with significant and progressive weight gain that is much higher than what wild-type controls will exhibit. Also, the mutant mice will have higher plasma leptin concentration and adipose tissue levels when fed on a diet with chocolate. Furthermore, REV-ERBα mice will place a greater effort and exert more behaviors, like jumping, in the food-motivation test in order to obtain a palatable reward.
Disease Models of Feeding Behaviors
Anorexia nervosa is considered to be the most common eating disorder and is most prevalent amongst teenage girls going through puberty. Anorexia nervosa feeding behaviors are related to having a distorted self-image, a strong fear of weight gain, excessive weight loss, and chronic food refusal. Although these individuals feel hungry, they will deny themselves food intake. Furthermore, anorexia nervosa is associated with high activity levels and mental alertness which, in turn, creates a harmful positive feedback/reward cycle causing them to engage in excessively high levels of exercise and caloric restriction. In many cases, anorexia nervosa is comorbid with depressive, anxiety, and obsessive-compulsive disorders.
In mice, anorexia models are induced through the following four methods:
- Self-starvation/activity-based anorexia model
- Stress models
- The diet restriction model
- Genetic models
Self-Starvation/Activity-Based Anorexia Nervosa
In self-starvation/activity-based anorexia (ABA), mice are given access to two rewards, one is food and the other can be something else, like exercise or brain stimulation. Mice have a ‘free choice’ of picking between the two. ABA is thought to be induced when the mouse consistently chooses the other reward (i.e., not food) and experiences weight loss as a result of not eating. The ABA model is advantageous because it reflects some characteristic aspects of anorexia, including reduced food intake even in the presence of hunger, physiological evidence of malnutrition, hyperactivity, and weight loss.
Stress Models of Anorexia Nervosa
Stress models affect the hypothalamic-pituitary-adrenal axis (HPA) which may affect food intake. Such models represent the portion of eating disorders which developed as a result of a life stressor which led to hormonal imbalances. Commonly used methods and techniques to create stress-induced weight loss include tail pinching, cold swimming, and direct brain stimulation. Recent developments in this area include the novelty environment where mice are constantly introduced to surroundings which are novel or unfamiliar to them, leading to an increase in corticotropin-releasing factor and a decrease in peripheral levels of orexigenic peptides.
Diet Restriction Model of Anorexia Nervosa
The diet restriction model limits the amount of food that the laboratory mice have available to them. This model affects the mouse physiologically, causing changes which resemble those found in the endocrine and central nervous systems found in anorexic patients. However, the major drawback of this model is that it does not reflect the voluntary aspect of anorexia nervosa.
Genetic Model of Anorexia Nervosa
Genetic models in mice are also used to study the disordered feeding behaviors associated with anorexia nervosa. Animal models are used to mutate target genes, in order to identify which genes could be involved in regulating energy balance and food intake. However, this has proven to be challenging because there is limited evidence of possible genetic correlates which could influence the manifestation of anorexia nervosa in humans. Currently, a genetic model does not exist for inducing anorexia nervosa in mice. In fact, anx/anx mice which have an autosomal recessive anx mutation have been reported to have such decreased food intake behaviors that death may result by the time mice are just 20-30 days old. Furthermore, anx/anx mice demonstrate hyperactivity, reduced body weight, head weaving, body tremors, and an uncoordinated gait.
Binge Eating/Bulimia Nervosa Models
Binge disorder is a feeding disorder wherein an individual: eats more rapidly than normal, consumes large quantities of food even when not physically hungry, eats until feeling uncomfortably full, and feels disgusted or depressed with oneself due to overeating. Bulimia nervosa, just like binge eating disorder, also involves the lack of control over eating episodes but is distinguished by subsequent compensatory behaviors such as fasting, misuse of laxatives, excessive exercise, or self-induced vomiting.
Even though binge eating disorder and bulimia nervosa are feeding disorders with distinct characteristics in humans, they are oftentimes grouped together in animal models. Commonalities in clinical symptoms are the main basis for this. Also, the traits which distinguish the two disorders such as the compensatory behaviors observed in binge eating disorder, are basically impossible to induce in animal models.
Food Restriction Model of Binge Eating/Bulimia Nervosa
Food restriction models are commonly used to represent these feeding disorders in mice. It is known that food restriction which lasts for more than two hours will increase food consumption, thus creating a “binge-like” episode where high food intake persists for a few hours. Also, when rodents lose about 20-35% of their baseline weight, binge-like feeding behaviors are more likely to occur. The major disadvantage of this approach is that it does not reflect the context in which binge eating occurs in humans since humans are not typically motivated by physical hunger during binge-eating episodes. Yet, even in individuals without a history of binge eating disorders or bulimia nervosa, food restriction has been linked to an increase in the chance of a binge eating episode.
Obesity can be classified as an eating disorder if it is not caused by a metabolic imbalance. Therefore, in some circumstances, when obesity is related to binge-eating disorder, it can be classified as an eating disorder. Since obesity is defined as an energy imbalance between the calories consumed and the total calories expended, the hypothalamus is implicated in this disorder since it’s one of the brain’s key regions for regulating energy balance.
Genetic Model of Obesity
Genetic deletion studies are commonly used when studying obesity as an eating disorder since they can target numerous peptides and affect multiple signaling cascades within the hypothalamus which, in turn, affects the observable feeding behaviors. The two genetic models of obesity are represented by db/db and ob/ob mice. The db/db mouse has a leptin receptor mutation while the ob/ob mouse has a spontaneous mutation in the obese gene which is responsible for encoding leptin. The two mouse models are similar in that their phenotypes capture profound early onset of obesity, hyperphagia, hyperglycemia, type-2 diabetes, and insulin resistance.
- Feeding behaviors are active, maintenance behaviors.
- A mouse’s eating behaviors revolve around the consumption of food. In order to be able to eat, a mouse must be able to perform a series of behaviors in order to find the food and consume it.
- Feeding behaviors are related to many brain pathways.
- Many behaviors fall under the scope of feeding behaviors, including search, approach, chase, sniff, and bite.
- Also, some behaviors which are likely to be observed while studying feeding behaviors include bury, caching, dig, hoarding, nose poking, jump, suck, and drink.
- Feeding behaviors have many functions, such as to: provide energy, acquire nutrients, promote normal development, and store food.
- There are many opportunities or points in time in which feeding behaviors may be observed. Common applications of feeding behaviors include during laboratory feeding, after wheel running, under chronic stress conditions, due to periods of starvation, after fasting periods, in the winter, in response to aversive stimuli, and during the dark-phase.
- Some research techniques which are used to study feeding behaviors include brain lesions, behavioral tests, electrical stimulation studies, genetic studies, and pharmaceutical studies.
- Handy behavioral tests for studying feeding behaviors include the two-feeding choice test for food consumption, the food-motivation test, the sucrose deprivation test, the two-bottle choice test for sucrose consumption, the digging and marble burying test, the hoarding apparatus, the operant-responding fast, and the Vogel conflict scenario.
- Pharmaceutical findings have revealed that ghrelin injection increase appetite, intra-VTA injections suppress food approach behaviors, ST-1283 will reduce feeding latency, simvastatin reduces anhedonia related to a high-fat diet, and fluoxetine reduces feeding latency.
- C57BL/6J mice are commonly used for conducting homeostasis experiments designed to look into feeding behaviors.
- Many genetic mutations can be linked to altered feeding behaviors and abnormalities.
- Disease models of feeding behaviors are modeled in mice to reflect anorexia nervosa, obesity, binge eating and bulimia nervosa which are found in humans.
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