A system for studying the microstructure of ingestive behavior in mice

Prolonged Consumption of glucose syrup enhances glucose tolerance in mice

There is debate about the metabolic impact of sugar-sweetened beverages. Here, we tested the hypothesis that ad lib consumption of glucose (Gluc) or high-fructose (HiFruc) syrups improves glucose tolerance in mice. We provided C57BL/6 mice with a control (chow and water) or experimental (chow, water and sugar solution) diet across two consecutive 28-day exposure periods, and monitored changes in body composition, glucose tolerance, cephalic-phase insulin release (CPIR) and insulin sensitivity. The sugar solutions contained 11% concentrations of Gluc or HiFruc syrup; these syrups were derived from either corn starch or cellulose. In Experiment 1, consumption of the Gluc diets reliably enhanced glucose tolerance, while consumption of the HiFruc diets did not. Mice on the Gluc diets exhibited higher CPIR (relative to baseline) by the end of exposure period 1, whereas mice on the control and HiFruc diets did not do so until the end of exposure period 2. Mice on the Gluc diets also exhibited higher insulin sensitivity than control mice at the end of exposure period 2, while mice on the HiFruc diets did not. In Experiment 2, we repeated the previous experiment, but limited testing to the corn-based Gluc and HiFruc syrups. We found, once again, that consumption of the Gluc (but not the HiFruc) diet enhanced glucose tolerance, in part by increasing CPIR and insulin sensitivity. These results show that mice can adapt metabolically to high glucose diets, and that this adaptation process involves upregulating at least two components of the insulin response system.

Short-term Housing in Metabolic Caging on Measures of Energy and Fluid Balance in Male C57BL/6J Mice ( Mus musculus)

Metabolic caging is an important tool for quantitative urine and feces collection in rodents, although significant limitations and problems accompany its use. Despite strong opinions among investigators regarding the effects of metabolic caging on energy and fluid homeostasis, careful quantitative analysis of the impact of this caging type-particularly when used for mice-is lacking. The current study assessed the effects of metabolic caging, with or without modifications such as plastic platform inserts, on ingestive behaviors, energy expenditure, accuracy of urine and fecal collection, and ambulatory activities in male C57BL/6J mice. Housing mice in metabolic cages, regardless of platform inclusion, increased energy expenditure without modifying food intake, presumably due to the inability of mice to perform normal thermoregulatory behaviors (burrowing and huddling). Surprisingly, mice in metabolic cages actively avoided platforms, and the inclusion of platforms modified the behavior of the mice and had position-dependent effects that reduced the accuracy of urine collection. Moving mice from cohousing to individual housing in home cages also increased ingestive behaviors and energy expenditure. We conclude that single housing of male C57BL/6J mice increases energy expenditure, that this increase is potentiated in metabolic caging conditions, and that platforms in metabolic cages alter mouse behavior and urine collection. Additional future work is needed to determine the potential benefits of using higher ambient temperature for studies of mice in metabolic caging and whether the above effects occur in females and other strains of mice and other rodent species.

Antioxidants Promote Intestinal Tumor Progression in Mice

Dietary antioxidants and supplements are widely used to protect against cancer, even though it is now clear that antioxidants can promote tumor progression by helping cancer cells to overcome barriers of oxidative stress. Although recent studies have, in great detail, explored the role of antioxidants in lung and skin tumors driven by RAS and RAF mutations, little is known about the impact of antioxidant supplementation on other cancers, including Wnt-driven tumors originating from the gut. Here, we show that supplementation with the antioxidants N-acetylcysteine (NAC) and vitamin E promotes intestinal tumor progression in the ApcMin mouse model for familial adenomatous polyposis, a hereditary form of colorectal cancer, driven by Wnt signaling. Both antioxidants increased tumor size in early neoplasias and tumor grades in more advanced lesions without any impact on tumor initiation. Importantly, NAC treatment accelerated tumor progression at plasma concentrations comparable to those obtained in human subjects after prescription doses of the drug. These results demonstrate that antioxidants play an important role in the progression of intestinal tumors, which may have implications for patients with or predisposed to colorectal cancer.

Systematic analysis of differential rhythmic liver gene expression mediated by the circadian clock and feeding rhythms

The circadian clock and feeding rhythms are both important regulators of rhythmic gene expression in the liver. To further dissect the respective contributions of feeding and the clock, we analyzed differential rhythmicity of liver tissue samples across several conditions. We developed a statistical method tailored to compare rhythmic liver messenger RNA (mRNA) expression in mouse knockout models of multiple clock genes, as well as PARbZip output transcription factors (Hlf/Dbp/Tef). Mice were exposed to ad libitum or night-restricted feeding under regular light-dark cycles. During ad libitum feeding, genetic ablation of the core clock attenuated rhythmic-feeding patterns, which could be restored by the night-restricted feeding regimen. High-amplitude mRNA expression rhythms in wild-type livers were driven by the circadian clock, but rhythmic feeding also contributed to rhythmic gene expression, albeit with significantly lower amplitudes. We observed that Bmal1 and Cry1/2 knockouts differed in their residual rhythmic gene expression. Differences in mean expression levels between wild types and knockouts correlated with rhythmic gene expression in wild type. Surprisingly, in PARbZip knockout mice, the mean expression levels of PARbZip targets were more strongly impacted than their rhythms, potentially due to the rhythmic activity of the D-box-repressor NFIL3. Genes that lost rhythmicity in PARbZip knockouts were identified to be indirect targets. Our findings provide insights into the diurnal transcriptome in mouse liver as we identified the differential contributions of several core clock regulators. In addition, we gained more insights on the specific effects of the feeding-fasting cycle.

Barriers in translating preclinical rodent exercise metabolism findings to human health

Physical inactivity and low aerobic capacity are primary drivers of chronic disease pathophysiology and are independently associated with all-cause mortality. Conversely, increased physical activity and exercise are central to metabolic disease prevention and longevity. Although these relationships are well characterized in the literature, what remains incompletely understood are the mechanisms by which physical activity/exercise prevents disease. Given methodological constraints of clinical research, investigators must often rely on preclinical rodent models to investigate these potential underlying mechanisms. However, there are several key barriers to applying exercise metabolism findings from rodent models to human health. These barriers include housing temperature, nutrient metabolism, exercise modality, exercise testing, and sex differences. Increased awareness and understanding of these barriers will enhance the ability to impact human health through more appropriate experimental design and interpretation of data within the context of these factors.

High Fat Rodent Models of Type 2 Diabetes: From Rodent to Human

Poor dietary habits contribute to increased incidences of obesity and related co-morbidities, such as type 2 diabetes (T2D). The biological, genetic, and pathological implications of T2D, are commonly investigated using animal models induced by a dietary intervention. In spite of significant research contributions, animal models have limitations regarding the translation to human pathology, which leads to questioning their clinical relevance. Important considerations include diet-specific effects on whole organism energy balance and glucose and insulin homeostasis, as well as tissue-specific changes in insulin and glucose tolerance. This review will examine the T2D-like phenotype in rodents resulting from common diet-induced models and their relevance to the human disease state. Emphasis will be placed on the disparity in percentages and type of dietary fat, the duration of intervention, and whole organism and tissue-specific changes in rodents. An evaluation of these models will help to identify a diet-induced rodent model with the greatest clinical relevance to the human T2D pathology. We propose that a 45% high-fat diet composed of approximately one-third saturated fats and two-thirds unsaturated fats may provide a diet composition that aligns closely to average Western diet macronutrient composition, and induces metabolic alterations mirrored by clinical populations.

Reappraisal of the optimal fasting time for insulin tolerance tests in mice

Objective

Most studies routinely use overnight or 6 h of fasting before testing metabolic glucose homeostasis in mice. Other studies used empirically shorter fasting times (<6 h). We attempted to determine the shortest fasting time required for optimal insulin responsiveness while minimizing metabolic stress.

Methods

A course of fasting for up to 24 h (0, 2, 4, 6, 12, and 24 h) was conducted in C57Bl/6J male mice. Body weight, metabolic parameters, and insulin tolerance were measured in each experimental group. The organs were collected at the same time on separate occasions and glycogen and metabolic gene expression were measured in the liver and skeletal muscle.

Results

Our data show that blood glucose levels do not significantly change during a 6 h fast, while plasma insulin levels decrease to similar levels between 2 h and 6 h of fasting. During overnight (12 h) and 24 h fasts, a robust decrease in blood glucose and plasma insulin was observed along with a profound depletion in liver glycogen content. Insulin tolerance was comparable between baseline and 6 h fasts while 4 h and 6 h fasts were associated with a greater depletion of liver glycogen than 2 h fasts, impacting the glucose counter-regulatory response. Fasting induced progressive weight loss that was attenuated at thermoneutrality. Fasting longer than 4 h induced major body weight loss (>5%) and significant changes in catabolic gene expression in the liver and skeletal muscle.

Conclusion

Collectively, these data suggest that 2 h of fasting appears optimal for the assessment of insulin tolerance in mice as this duration minimizes major metabolic stress and weight loss.

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Fasting in mice induces time-dependent metabolic stress and weight loss.

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Fasting promotes profound changes in catabolic gene expression in liver and muscles.

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Fasting-induced weight loss is attenuated at thermoneutrality.

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Two hours fasting appears to be optimal prior to assessing insulin tolerance in mice.

Nutrient homeostasis - translating internal states to behavior

Behavioral neuroscience aims to describe a causal relationship between neuronal processes and behavior. Animals' ever-changing physiological needs alter their internal states. Internal states then alter neuronal processes to adapt the behavior of the animal enabling it to meet its needs. Here, we describe nutrient-specific appetites as an attractive framework to study how internal states shape complex neuronal processes and resulting behavioral outcomes. Understanding how neurons detect nutrient states and how these are integrated at the level of neuronal circuits will provide a multilevel description of the mechanisms underlying complex feeding and foraging decisions.

Fasting of male mice - Effects of time point of initiation and duration on clinical chemistry parameters and animal welfare

Fasting of mice is a common procedure, which can affect the outcome of the study as well as animal welfare. In this study, we assess the effects of fasting, fasting duration and fasting initiation time in relation to light schedule and present suggestions for optimization of fasting. Male C57BL/6NCrl mice were fasted for 0, 3, 6, 12, 18 and 24 hours initiated either in the light period (photophase) or the dark period (scotophase). Body weight, gastric content, body temperature, corticosterone and 19 routine clinical chemistry parameters were evaluated. Fasting caused significant changes in most of the measured parameters. Increasing duration of fasting resulted in increasing physiological changes. Fasting initiated in the scotophase caused more significant changes than fasting initiated in the photophase. To cause the least physiological changes in mice and increase animal welfare, mice should preferably be fasted in the photophase and for the shortest possible period allowed by the experimental purpose of fasting.

Altering salivary protein profile can increase acceptance of a novel bitter diet

Bitter taste is often associated with toxins, but accepting some bitter foods, such as green vegetables, can be an important part of maintaining a healthy diet. In rats and humans, repeated exposure to a bitter stimulus increases acceptance. Repeated exposure allows an individual the opportunity to learn about the food's orosensory and postingestive effects. It also alters the salivary protein (SP) profile, which in turn alters taste signaling. We have hypothesized that altering the salivary proteome plays a role in the increased acceptance after repeated exposure. Here we test this and attempt to disentangle the contribution of learning during dietary exposure from the contribution of SPs in increased acceptance of bitter diet. Dietary exposure to quinine or tannic acid and injection of isoproterenol (IPR) result in similar salivary protein profiles. Here we used either the bitter stimulus tannic acid or IPR injection to upregulate a subset of SPs before exposing animals to a novel diet containing quinine (0.375%). Control animals received either a control diet before being exposed to quinine, or a diet containing sucrose octaacetate, a compound that the animals avoid but does not alter SP profiles. The treatments that alter SP expression increased rate of feeding on the quinine diet compared to the control treatments. Additionally, tannic acid exposure altered intake and meal size of the quinine diet. These data suggest that SPs, not just learning about bitter food, increase acceptance of the bitter diet.

Measurement of Behavioral Taste Responses in Mice: Two-Bottle Preference, Lickometer, and Conditioned Taste-Aversion Tests

The natural like and dislike of foods based on taste is one of the most easily observed behaviors in animals. Animals eat palatable foods and reject aversive foods, which makes measurement of taste perception possible using various behavioral techniques. Three different methods to accurately measure taste behavior are described here. First, two-bottle preference tests evaluate whether a taste compound (tastant) is preferred over water. Second, lickometer tests quantify the like and dislike for multiple concentrations of the same tastant or multiple tastants at the same time. Finally, conditioned taste aversion tests accurately determine the perceived taste threshold for palatable tastants. Together, these diverse methods enable researchers to observe and measure behavioral taste responses in mice to any tastant. © 2016 by John Wiley & Sons, Inc.

MSG-Evoked c-Fos Activity in the Nucleus of the Solitary Tract Is Dependent upon Fluid Delivery and Stimulation Parameters

The marker of neuronal activation, c-Fos, can be used to visualize spatial patterns of neural activity in response to taste stimulation. Because animals will not voluntarily consume aversive tastes, these stimuli are infused directly into the oral cavity via intraoral cannulae, whereas appetitive stimuli are given in drinking bottles. Differences in these 2 methods make comparison of taste-evoked brain activity between results that utilize these methods problematic. Surprisingly, the intraoral cannulae experimental conditions that produce a similar pattern of c-Fos activity in response to taste stimulation remain unexplored. Stimulation pattern (e.g., constant/intermittent) and hydration state (e.g., water-restricted/hydrated) are the 2 primary differences between delivering tastes via bottles versus intraoral cannulae. Thus, we quantified monosodium glutamate (MSG)-evoked brain activity, as measured by c-Fos, in the nucleus of the solitary tract (nTS; primary taste nucleus) across several conditions. The number and pattern of c-Fos neurons in the nTS of animals that were water-restricted and received a constant infusion of MSG via intraoral cannula most closely mimicked animals that consumed MSG from a bottle. Therefore, in order to compare c-Fos activity between cannulae-stimulated and bottle-stimulated animals, cannulated animals should be water restricted prior to stimulation, and receive taste stimuli at a constant flow.

Postoral glucose sensing, not caloric content, determines sugar reward in C57BL/6J mice

Recent studies suggest that because of their energy value, sugars are more rewarding than non-caloric sweeteners. However, intragastric infusion data indicate that sugars differ in their postoral appetite-stimulating effects. We therefore compared the preference for isocaloric 8% sucrose, glucose, and fructose solutions with that of a non-caloric sweetener solution (0.8% sucralose) in C57BL/6J mice. Brief 2-bottle tests indicated that sucralose was isopreferred to sucrose but more preferred than glucose or fructose. Yet, in long-term tests, the mice preferred sucrose and glucose, but not fructose to sucralose. Additional experiments were conducted with a non-caloric 0.1% sucralose + 0.1% saccharin mixture (S + S), which does not have the postoral inhibitory effects of 0.8% sucralose. The S + S was preferred to fructose in brief and long-term choice tests. S + S was also preferred to glucose and sucrose in brief tests, but the sugars were preferred in long-term tests. In progressive ratio tests, non-deprived and food-deprived mice licked more for glucose but not fructose than for S + S. These findings demonstrate that the nutrient-specific postoral actions, not calories per se, determine the avidity for sugar versus non-caloric sweeteners. Furthermore, sweet taste intensity and potential postoral inhibitory actions must be considered in comparing non-caloric and caloric sweeteners.

The regulation of glucose metabolism: implications and considerations for the assessment of glucose homeostasis in rodents

The incidence of insulin resistance and type 2 diabetes (T2D) is increasing at alarming rates. In the quest to understand the underlying causes of and to identify novel therapeutic targets to treat T2D, scientists have become increasingly reliant on the use of rodent models. Here, we provide a discussion on the regulation of rodent glucose metabolism, highlighting key differences and similarities that exist between rodents and humans. In addition, some of the issues and considerations associated with assessing glucose homeostasis and insulin action are outlined. We also discuss the role of the liver vs. skeletal muscle in regulating whole body glucose metabolism in rodents, emphasizing the importance of defective hepatic glucose metabolism in the development of impaired glucose tolerance, insulin resistance, and T2D.

Fructose- and glucose-conditioned preferences in FVB mice: strain differences in post-oral sugar appetition

Recent studies indicate that, unlike glucose, fructose has little or no post-oral preference conditioning actions in C57BL/6J (B6) mice. The present study determined whether this is also the case for FVB mice, which overconsume fructose relative to B6 mice. In experiment 1, FVB mice strongly preferred a noncaloric 0.1% sucralose + 0.1% saccharin (S+S) solution to 8% fructose in a 2-day choice test but switched their preference to fructose after separate experience with the two sweeteners. Other FVB mice displayed a stronger preference for 8% glucose over S+S. In a second experiment, ad libitum-fed FVB mice trained 24 h/day acquired a significant preference for a flavor (CS+) paired with intragastric (IG) self-infusions of 16% fructose over a different flavor (CS-) paired with IG water infusions. IG fructose infusions also conditioned flavor preferences in food-restricted FVB mice trained 1 h/day. IG infusions of 16% glucose conditioned stronger preferences in FVB mice trained 24- or 1 h/day. Thus, fructose has post-oral flavor conditioning effects in FVB mice, but these effects are less pronounced than those produced by glucose. Further studies of the differential post-oral conditioning effects of fructose and glucose in B6 and FVB mice should enhance our understanding of the physiological processes involved in sugar reward.

Fasting of mice: a review

Fasting of mice is a common procedure performed in association with many different types of experiments mainly in order to reduce variability in investigatory parameters or to facilitate surgical procedures. However, the effects of fasting not directly related to the investigatory parameters are often ignored. The aim of this review is to present and summarize knowledge about the effects of fasting of mice to facilitate optimization of the fasting procedure for any given study and thereby maximize the scientific outcome and minimize the discomfort for the mice and hence ensure high animal welfare. The results are presented from a number of experimental studies, providing evidence for fasting-induced changes in hormone balance, body weight, metabolism, hepatic enzymes, cardiovascular parameters, body temperature and toxicological responses. A description of relevant normal behaviour and standard physiological parameters is given, concluding that mice are primarily nocturnal and consume two-thirds of their total food intake during the night. It is argued that overnight fasting of mice is not comparable with overnight fasting of humans because the mouse has a nocturnal circadian rhythm and a higher metabolic rate. It is suggested that because many physiological parameters are regulated by circadian rhythms, fasting initiated at different points in the circadian rhythm has different impacts and produces different results.

GPR40 and GPR120 fatty acid sensors are critical for postoral but not oral mediation of fat preferences in the mouse

In addition to orosensory signals, postoral actions of fat stimulate appetite and condition flavor preferences, but the gut sensors mediating these responses are unknown. Here, we investigated the role of the fatty acid sensors GPR40 and GPR120 in postoral and oral preferences for a soybean oil emulsion (Intralipid). Mice were trained to drink a flavored solution (CS+) paired with intragastric (IG) oil infusions and another flavored solution (CS-) paired with water infusions. Knockout (KO) mice missing GPR40 or GPR120 sensors increased their CS+ intake in one-bottle tests (1 h/day) but less so than wild-type (WT) mice. The KO mice also preferred the CS+ to CS- in a two-bottle test, but the preference was attenuated in GPR40 KO mice. Double-knockout (DoKO) mice missing both GPR40 and GPR120 displayed attenuated stimulation of CS+ intake and only a marginal CS+ preference. The DoKO mice developed a more substantial CS+ preference when tested 24 h/day, although weaker than that of WT mice. The DoKO mice also consumed less of the CS+ paired with IG Intralipid, as well as less Intralipid in oral tests. However, DoKO mice, like GPR40 KO and GPR120 KO mice did not differ from WT mice in their preference for Intralipid over water at 0.001%-20% concentrations. In contrast to prior results obtained with mice missing the CD36 fatty acid sensor, these findings indicate that, together, GPR40 and GPR120 play a critical role in the postoral stimulation of appetite by fat but are not essential for oral fat preferences.

Flavor preferences conditioned by intragastric monosodium glutamate in mice

The consumption of monosodium glutamate (MSG) solutions has been shown to reinforce preferences for MSG and for MSG-paired flavors in mice. These effects appear to have a strong postoral component, such that MSG detected in the gut is associated with concurrently consumed flavors. Two experiments investigated postoral MSG reward by infusing 400mM MSG intragastrically (IG) to C57BL/6 mice as they consumed a conditioned stimulus (CS+) flavor. An alternate CS- flavor was paired with IG water. In Experiment 1, the grape and cherry CS flavors were unsweetened, and intakes and preferences for the CS+ flavor were modest. Experiment 2 attempted to generate stronger preferences by adding 0.05% saccharin to the CS flavors. Sweet taste did enhance intakes during training and testing but did not significantly increase percent CS+ intake or persistence of the preference. However, only conditioning with the sweet CS+ resulted in the mice expressing a preference for oral MSG in an initial choice test with water. These findings extend recent studies demonstrating postoral MSG conditioning in rats.

Ghrelin increases the motivation to eat, but does not alter food palatability

Homeostatic eating cannot explain overconsumption of food and pathological weight gain. A more likely factor promoting excessive eating is food reward and its representation in the central nervous system (CNS). The anorectic hormones leptin and insulin reduce food reward and inhibit related CNS reward pathways. Conversely, the orexigenic gastrointestinal hormone ghrelin activates both homeostatic and reward-related neurocircuits. The current studies were conducted to identify in rats the effects of intracerebroventricular ghrelin infusions on two distinct aspects of food reward: hedonic valuation (i.e., "liking") and the motivation to self-administer (i.e., "wanting") food. To assess hedonic valuation of liquid food, lick motor patterns were recorded using lickometry. Although ghrelin administration increased energy intake, it did not alter the avidity of licking (initial lick rates or lick-cluster size). Several positive-control conditions ruled out lick-rate ceiling effects. Similarly, when the liquid diet was hedonically devalued with quinine supplementation, ghrelin failed to reverse the quinine-associated reduction of energy intake and avidity of licking. The effects of ghrelin on rats' motivation to eat were assessed using lever pressing to self-administer food in a progressive-ratio paradigm. Ghrelin markedly increased motivation to eat, to levels comparable to or greater than those seen following 24 h of food deprivation. Pretreatment with the dopamine D1 receptor antagonist SCH-23390 eliminated ghrelin-induced increases in lever pressing, without compromising generalized licking motor control, indicating a role for D1 signaling in ghrelin's motivational feeding effects. These results indicate that ghrelin increases the motivation to eat via D1 receptor-dependent mechanisms, without affecting perceived food palatability.

Antidepressant response to chronic citalopram treatment in eight inbred mouse strains

RATIONALE

The antidepressant response exhibits a characteristic delay. BALB/cJ mice respond to chronic, but not subchronic, treatment with selective serotonin reuptake inhibitors (SSRIs), providing a model of antidepressant onset. Identification of other mouse strains exhibiting this phenotype will provide additional tools for studying mechanisms of the antidepressant response.

OBJECTIVES

We aimed to identify inbred mouse strains that respond to chronic, but not subchronic, SSRI treatment in the forced swim test (FST). We also assessed whether response correlated with genotype at the functional C1473G polymorphism in tryptophan hydroxylase-2 (Tph2).

METHODS

BALB/cJ, three closely related strains (BALB/cByJ, SEA/GnJ, A/J), and four distantly related strains (C57BL/6J, C57BL/10J, CAST/EiJ, SM/J) received the SSRI citalopram (0-30 mg/kg/day in drinking water) for ~4 weeks and were assessed for locomotion and FST behavior. Citalopram-responsive strains were assessed identically following ~1 week of treatment. C1473G genotypes were determined.

RESULTS

BALB/cJ and related strains carried the 1473G allele and responded to chronic citalopram treatment in the FST. BALB/cJ, BALB/cByJ, and SEA/GnJ mice showed either no response or an attenuated response to subchronic treatment. Distantly related strains carried the 1473C allele and showed no response to citalopram. No relationship was found between the antidepressant response and baseline immobility or locomotion.

CONCLUSIONS

BALB/cJ and related strains exhibit an antidepressant response to chronic SSRI treatment that emerges over time and is likely a heritable trait. This antidepressant response is associated with carrying the 1473G allele in Tph2. In conclusion, BALB/cJ and related strains provide valuable models for studying the therapeutic mechanisms of SSRIs.

Activation of brain somatostatin 2 receptors stimulates feeding in mice: analysis of food intake microstructure

We recently reported that the oligosomatostatin receptor agonist, ODT8-SST increases food intake in rats via the somatostatin 2 receptor (sst(2)). We characterized ingestive behavior following intracerebroventricular (icv) injection of a selective sst(2) agonist in freely fed mice during the light phase. The sst(2) agonist (0.01, 0.03, 0.1, 0.3 or 1μg/mouse) injected icv under short inhalation anesthesia dose-dependently increased cumulative light phase food intake over 4h compared to vehicle with a 3.1-times increase at 1μg/mouse (p<0.05). Likewise, the sst(2,3,5) agonist octreotide (0.3 or 1μg/mouse) dose-dependently increased 4-h food intake, whereas selective sst(1) or sst(4) agonists at 1μg/mouse did not. In vehicle-treated mice, high fat diet increased caloric intake/4h by 2.8-times compared to regular diet (p<0.05) and values were further increased 1.4-times/4h by the sst(2) agonist. Automated continuous assessment of food intake established a 6.6-times higher food intake during the dark phase due to increased number of meals, meal size, meal duration and rate of ingestion compared to non-treated mice during the light phase. During the first 4h post icv sst(2) agonist injection, mice had a 57% increase in number of meals with a 60% higher rate of ingestion, and a 61% reduction in inter-meal intervals, whereas meal sizes were not altered compared to vehicle. These data indicate that the activation of brain sst(2) receptors potently stimulates the light phase ingestive behavior under basal or high fat diet-stimulated conditions in mice. The shortened inter-meal interval suggests an inhibitory effect of the sst(2) agonist on "satiety", whereas "satiation" is not altered as indicated by normal meal size.

Central urocortin 3 administration decreases limited-access ethanol intake in nondependent mice

Stress and alcohol abuse are co-related. Acute alcohol is anxiolytic and stress is cited as a factor in relapse to alcohol use. A primary mediator of the stress response is the neuropeptide corticotropin-releasing factor (CRF). The CRF family of endogenous ligands includes urocortin 3 (Ucn 3), which binds selectively to the CRF type 2 receptor and has been implicated in ethanol consumption in dependent and withdrawing rats. The objective of this study was to examine the effect of Ucn 3, delivered centrally to nondependent mice, on limited-access ethanol consumption. Adult C57BL/6J mice were trained to self-administer 10% ethanol during daily, 2-h limited-access sessions, using lickometers to assess drinking patterns for both ethanol and water. Sterile saline or 0.3, 1, or 3 nmol of Ucn 3 was microinjected into the lateral ventricle immediately before the limited-access session in a within-subjects design. There was a significant decrease in ethanol (both ml and g/kg), but not water, intake following Ucn 3 treatment, explained by a change in size of the largest lick run. Food intake at both 2 h and 24 h after injection was statistically unaffected by Ucn 3 administration. These results establish a role for CRF type 2 receptors in a nondependent mouse model of ethanol self-administration.

Meal patterns of mice under systematically varying approach and unit costs for food in a closed economy

Several field and experimental studies have investigated the behavioral economics of food intake. In the laboratory, operant behavior has been used to emulate cost and to generate demand functions that express the relationship between the price of food and amount consumed. There have been few such studies of motivated food seeking and intake in mice, and none has reported demand functions. Using albino (CD1) male mice, the present study compares food intake and meal patterns across a series of ratio cost schedules. The first experiment examined unit price. A closed economy was used in which the mice were in the test chambers for 23 h/day and earned all of their food via either a nose poke or lever press response under fixed (FUP5, FUP10, FUP25, FUP50), variable (VUP10, VUP20, VUP50), and progressive (PUP1.25, PUP1.5, PUP1.75) unit prices. Mice were run for 4 days at each cost. There were no consistent differences between the first and last day indicating that behavioral adjustments to schedule changes occurred rapidly. When averaged across all price schedules, mice in the nose poke group consumed more food than their lever press counterparts but the overall shapes of the demand curves did not differ between the two operant responses, with intake decreasing as price increased. The number of meals taken per day differed between two meal-defining criteria that we applied, and there were some differences between the types of unit price schedule. In the second experiment, approach cost in the form of nose poke responses was required to activate a response device (lever) on which a fixed unit price for food was in force. These approach and unit costs were varied systematically. Meal number decreased, and meal size increased, with increasing approach cost even though nose pokes accounted for only a small fraction of the total response activity. Thus, meal patterns in mice are sensitive to approach cost while total amount consumed is more sensitive to unit price. These data are discussed in terms of the concept of foraging cost as either a unitary or a multidimensional variable.

Normal feeding and body weight in Fischer 344 rats lacking the cholecystokinin-1 receptor gene

A large body of evidence has demonstrated that one mechanism by which cholecystokinin (CCK) inhibits food intake through activation of CCK1 receptors (CCK1R) on vagal afferent neurons that innervate the gastrointestinal tract and project to the hindbrain. OLETF rats, which carry a spontaneous null mutation of the CCK1R, are hyperphagic, obese, and predisposed to type 2 diabetes. Recently, by introgressing the OLETF-derived, CCK1R-null gene onto a Fischer 344 genetic background, we have been able to generate a CCK1R-deficient, congenic rat strain, F344.Cck1r(-/-), that in contrast to OLETF rats, possesses a lean and normoglycemic phenotype. In the present study, the behavioral and neurobiological phenotype of this rat strain was characterized more fully. As expected, intraperitoneal injections of CCK-8 inhibited intake of chow and Ensure Plus and induced Fos responses in the area postrema and the gelatinosus, commissural and medial subdivisions of the nucleus tractus solitarius of wild-type F344.Cck1r(+/+) rats, whereas CCK-8 was without effect on food intake or Fos induction in the F344.Cck1r(-/-) rats. F344.Cck1r(-/-) and F344.Cck1r(+/+) rats did not differ in body weight and showed comparable weight gain when maintained on Ensure Plus for 2 weeks. Also, no difference was found in 24-h food intake, and dark-phase meal frequency or meal size between F344.Cck1r(+/+) and F344.Cck1r(-/-) rats. As expected, blockade of endogenous CCK action at CCK1R increased food intake and blocked the effects of peripheral CCK-8 in wild-type F344.Cck1r(+/+) rats. These results confirm that in rats with a F344 background, CCK-1R mediates CCK-8-induced inhibition of food intake and Fos activation in the hindbrain and demonstrate that selective genetic ablation of CCK1R is not associated with altered meal patterns, hyperphagia, or excessive weight gain on a palatable diet.

A robust automated system elucidates mouse home cage behavioral structure

Patterns of behavior exhibited by mice in their home cages reflect the function and interaction of numerous behavioral and physiological systems. Detailed assessment of these patterns thus has the potential to provide a powerful tool for understanding basic aspects of behavioral regulation and their perturbation by disease processes. However, the capacity to identify and examine these patterns in terms of their discrete levels of organization across diverse behaviors has been difficult to achieve and automate. Here, we describe an automated approach for the quantitative characterization of fundamental behavioral elements and their patterns in the freely behaving mouse. We demonstrate the utility of this approach by identifying unique features of home cage behavioral structure and changes in distinct levels of behavioral organization in mice with single gene mutations altering energy balance. The robust, automated, reproducible quantification of mouse home cage behavioral structure detailed here should have wide applicability for the study of mammalian physiology, behavior, and disease.

Oxytocin knockout mice demonstrate enhanced intake of sweet and nonsweet carbohydrate solutions

Oxytocin knockout (OT KO) mice display enhanced intake of nutritive and nonnutritive sweet solutions (i.e., sucrose and saccharin) compared with wild-type (WT) mice of the same C57BL/6 background strain. The present study further investigated the differential behavioral response of OT KO and WT mice to sucrose solutions and also examined intake preferences of OT KO and WT mice for palatable but nonsweet isocaloric solutions of carbohydrate and fat. A progressive ratio operant licking procedure demonstrated that OT KO and WT mice display a similar motivational drive to consume 10% sucrose. A series of two-bottle intake tests revealed that OT KO mice consume significantly larger amounts of both sweet and nonsweet carbohydrate solutions (i.e., sucrose, Polycose, and cornstarch) compared with WT cohorts. Intake pattern analyses revealed that OT KO mice overconsume carbohydrate solutions by initiating more drinking bouts compared with WT mice; bout sizes did not differ between the genotypes. In contrast, OT KO and WT mice did not differ in their intake of Intralipid, a palatable soybean oil emulsion. These findings indicate that the absence of OT in mice does not affect their appetitive drive to consume palatable sucrose solutions. Instead, the absence of OT may increase daily intake of palatable sweet and nonsweet solutions of carbohydrate (but not fat) by selectively blunting or masking processes that contribute to postingestive satiety.

A low-cost solution to measure mouse licking in an electrophysiological setup with a standard analog-to-digital converter

Licking behavior in rodents is widely used to determine fluid consumption in various behavioral contexts and is a typical example of rhythmic movement controlled by internal pattern-generating mechanisms. The measurement of licking behavior by commercially available instruments is based on either tongue protrusion interrupting a light beam or on an electrical signal generated by the tongue touching a metal spout. We report here that licking behavior can be measured with high temporal precision by simply connecting a metal sipper tube to the input of a standard analog/digital (A/D) converter and connecting the animal to ground (via a metal cage floor). The signal produced by a single lick consists of a 100-800 mV dc voltage step, which reflects the metal-to-water junction potential and persists for the duration of the tongue-spout contact. This method does not produce any significant electrical artifacts and can be combined with electrophysiological measurements of single unit activity from neurons involved in the control of the licking behavior.

Sucrose motivation in sweet "sensitive" (C57BL/6J) and "subsensitive" (129P3/J) mice measured by progressive ratio licking

As compared to C57BL/6J mice, 129P3/J mice show weaker preferences for and lower intakes of dilute sugar solutions. These differences have been attributed to genetic differences in their sweet taste receptor. The two mouse strains do not differ, however, in their intake of concentrated sugar solutions. The post-oral satiating effect of concentrated sugar solutions may mask strain differences in the avidity for these solutions. This hypothesis was investigated using fixed ratio (FR, low demand) and progressive ratio (PR, high demand) operant licking tests (22h/day) to measure sugar appetite. In Experiment 1, sucrose-experienced 129 mice licked less than did B6 mice for 4% but not for 16% sucrose in free access bottle tests and FR operant tests. Yet, in PR tests the 129 mice licked as much for 4% sucrose and more for 16% sucrose than did B6 mice. In Experiment 2, sucrose-naive 129 mice licked less than did B6 mice in FR and PR tests with 0.4% saccharin but the strains did not differ in PR licking in their first test with 16% sucrose. After they were given unconstrained bottle access to 16% sucrose for 3 days, the 129 mice now licked more than B6 mice in a second sucrose PR test. Thus, despite having a less sensitive sweet taste receptor, 129 mice are as much or more motivated to obtain sucrose than are B6 mice and appear to be more influenced by prior experience with sugar. This suggests that the strains differ in their central reward processing of sweet taste.

Behavioral seizure correlates in animal models of epilepsy: a road map for assay selection, data interpretation, and the search for causal mechanisms

A broad spectrum of learning/memory, social interaction, and affective behavioral measures serve as functional correlates for neurobiological changes in seizure-prone animals as well as in epileptic clinical populations. The utility of such measures is demonstrated by their ability to distinguish anomalous characteristics in developing organisms predisposed to seizure onset, as well as to discriminate prior seizure history in organisms with established pathology. For instance, typical findings that generalize across species suggest that seizure-experienced organisms exhibit a variety of deficits in cognitive function as well as inappropriate social neglect and aggression. Behavioral testing batteries have also proven useful in assessing neural mechanisms for seizure induction, subcortical neural circuits, and neuropeptide modulators, for example, as well as in identifying neural pathology resulting from prior seizure activity. However, the wanton application of behavioral tests can also produce false positives in the identification of seizure-related disorders unless alternative performance and motivational hypotheses are discounted effectively. Accordingly, the present review attempts to provide the reader interested in behavioral phenotyping and characterization of seizure-prone rats and mice with a roadmap for rational selection, implementation, and interpretation of data from behavior assays while highlighting potential successes and pitfalls inherent in employing functional correlates of brain activity using animal models of epilepsy.

c-Kit mutant mouse behavioral phenotype: altered meal patterns and CCK sensitivity but normal daily food intake and body weight

The mouse W/Wv mutation of the c-Kit receptor causes extensive loss of gastrointestinal interstitial cells of Cajal and vagal intramuscular arrays (IMAs; one of the two putative mechanoreceptors in gastrointestinal smooth muscle). To characterize the behavioral phenotype of the c-Kit mouse and to evaluate the roles of these mechanoreceptors in controlling food intake, meal patterns and daily intakes of W/Wv mice and controls were examined using solid (20-mg pellets) and liquid (Isocal) maintenance diets. After the meal pattern experiments, CCK (0.5, 1, 2, 4, 8, and 16 microg/kg ip) was administered to examine the role of the interstitial cells and vagal IMA mechanoreceptors in relaying peripheral signals of satiety activated by CCK-A receptors, whereas the specificity of the response was assessed with the antagonist devazepide (300 microg/kg ip). On both diets, the W/Wv mice ate smaller meals for shorter durations, with a compensatory increase in meal number, resulting in daily intakes and body weights similar to the controls. After CCK injections, the mutant mice consistently suppressed intake more ( approximately 2x) in 30-min tests, regardless of the test diet (12.5% glucose, chow, pellets, and Isocal). The increased sensitivity of W/Wv mice to CCK reflected an increased potency of the hormone (c-Kit mouse ED50 = 2.4 microg/kg; control ED50 = 6.4 microg/kg) and a shift of the dose-response curve to the left. Devazepide blocked the CCK suppression of ingestion. These results indicate that the selective loss of the interstitial cells and IMAs disrupts short-term feeding of the W/Wv mice by inducing an earlier satiety, possibly by altering gastric accommodation and/or emptying, without affecting the long-term mechanisms controlling overall intake or body weight. The results also suggest that the reduction of interstitial cells and IMAs augments the sensitivity to or increases the efficiency of exogenous CCK.

Meal patterns of lean and leptin-deficient obese mice in a simulated foraging environment

C57BL/6J lean and obese (lep -/-) mice were studied in a closed economy operant protocol that simulates foraging. A predetermined number of presses on a procurement lever (PFR) activated a consumatory lever on which presses would produce 20-mg food pellets. Mice could eat as much as they wished but, once no responding occurred for an elapsed 10-min period, the consumatory lever was inactivated and the procurement or foraging cycle began again. Under these conditions, as has been shown for rats and other species, mice initiated relatively discrete meals (about nine per day) at the lowest PFR, and the number of meals initiated per day decreased with increasing PFR. Meal size increased reciprocally, so that total intake was conserved across the range of PFR examined. Obese mice ate larger meals than lean mice at low PFR, and showed further increases but only at the highest PFRs. The small and inconsistent literature on meal patterns in mice is reviewed, and we discuss the utility of the present protocol to study the interactions between genetic and environmental economic factors, and their implications for the etiology of human obesity.

Mouse feeding behavior: ethology, regulatory mechanisms and utility for mutant phenotyping

Ingestive behaviors, feeding and drinking, constitute unconditioned, obligatory functions that are tightly regulated in the rodent according to demands of the external and internal milieu. Dependent measures of food intake have been used extensively in rats to infer the identity and function of neurochemical pathways, which mediate energy balance. A recent interest in application of appetitive measures in mice can be attributed jointly to the discovery of novel markers of energy balance in genetically obese mice as well as systematic targeting of known feeding regulatory pathways in bioengineered mutant mice. Accordingly, this review will attempt to provide the reader interested in behavioral phenotyping of knockout or transgenic mice with information regarding the ethology of mouse eating behavior, known mechanisms of appetitive regulation and examples of successes and pitfalls encountered when studying food intake in mutant mice.

A new apparatus for studying feeding and drinking in the mouse

The quantity of powder food consumed by individual mice was gauged with a newly developed apparatus that includes a specialized feeding station, an electric scale, and an interface to a computer that records the weight of the powder food jar. Using the measurements that exceeded the cutoff value, that is, the threshold between a mouse feeding or drinking event and scale noise, the reconstructed data were presented as the daily pattern of feeding and drinking in time resolution of 9 to 30 min. In this system, the ratio of noise to total consumption value was less than 4%. The fractal structure and fitting curve of this time series data were also analyzed by the nonlinear least-squares method, combined with the maximum entropy method. These analyses demonstrated that the mouse feeding event has circadian and ultradian periodicity. This apparatus and system are useful tools in studying the daily feeding pattern of mice.

Feed pecking in young chickens: new techniques of evaluation

Three techniques were compared: automated recording (A) of 2 h of feeding activities conveyed to a computer by constantly connected electronic balances, videotaping (V) of a closeup of the head of a chick during a feed-pecking session analyzed by focal sampling at reduced speed (16 times slower), strength of pecking (S) at feed particles recorded from a feeder-weight signal conveyed to a computer by a customized electronic balance at rapid speed (24 times/s). These techniques were applied to 16-18-day-old chicks fed either a complete feed or a split diet (whole grain wheat + a complementary feed). The two feeds had similar pellet forms. The complementary feed particles were eaten at a slower rate than the complete feed particles (A and V techniques). Wheat grains were pecked with a weaker measured strength than the pellets (technique S). Two pecks of three did not result in prehension of a feed particle and were categorized as "exploratory" pecks. For 75% of the time during a continuous pecking session the head of the chick was in a static position, suggesting a long period of observation of the feed between 2 consecutive pecks. Videotaping with slow-motion focal sampling (V) offers potential development for the study of food intake behavior of chickens.

A computerized system for the continuous recording and analysis of feeding, drinking, diuresis, and locomotor activity

A system has been designed to study the intake of liquids and solid foods in rats that allows for the analysis of feeding and drinking episodes over time. In addition, it monitors locomotor activity and diuresis. This system is particularly useful for the study of the differential effects of drugs on various parameters of feeding and drinking. Because it allows for the monitoring of all behaviors for periods of 24 consecutive hours without disturbing the animals, it can be applied to the study of light/dark cycles of ingestive behavior. The apparatus consists of rat cages (equipped with photocells, bottles, electronic balances, and a funnel to collect urine), computer interfaces, an AT-compatible microcomputer for data collection, and a VAX system for analysis.