The microstructure of ingestive behavior

Negative feedback control of hypothalamic feeding circuits by the taste of food

The rewarding taste of food is critical for motivating animals to eat, but whether taste has a parallel function in promoting meal termination is not well understood. Here, we show that hunger-promoting agouti-related peptide (AgRP) neurons are rapidly inhibited during each bout of ingestion by a signal linked to the taste of food. Blocking these transient dips in activity via closed-loop optogenetic stimulation increases food intake by selectively delaying the onset of satiety. We show that upstream leptin-receptor-expressing neurons in the dorsomedial hypothalamus (DMHLepR) are tuned to respond to sweet or fatty tastes and exhibit time-locked activation during feeding that is the mirror image of downstream AgRP cells. These findings reveal an unexpected role for taste in the negative feedback control of ingestion. They also reveal a mechanism by which AgRP neurons, which are the primary cells that drive hunger, are able to influence the moment-by-moment dynamics of food consumption.

Licking microstructure in response to novel rewards, reward devaluation and dopamine antagonists: Possible role of D1 and D2 medium spiny neurons in the nucleus accumbens

Evidence on the effect of dopamine D1 and D2-like antagonists and of manipulations of reward value on licking microstructure is reanalysed considering recent findings on the role of nucleus accumbens (NAc) medium spiny neurons (MSNs) in the control of sugar intake. The results of this analysis suggest that D1 MSN activation, which is involved in the emission of licking bursts, might play a crucial role in response to novel rewards. D2 MSN activation, which results in reduction of burst size and suppression of licking, might mediate the response to reward devaluation. Elucidating the neural mechanisms underlying the licking response might lead to a better definition of its microstructural measures in behaviourally and psychologically meaningful functional terms. This could further support its use as a behavioural substrate in the study of the neural mechanisms of ingestive behaviour and motivation, as well as in animal models of pathological conditions such as eating disorders and obesity.

Fluid transitions

Water is critical for survival and thirst is a powerful way of ensuring that fluid levels remain in balance. Overconsumption, however, can have deleterious effects, therefore optimization requires a need to balance the drive for water with the satiation of that water drive. This review will highlight our current understanding of how thirst is both generated and quenched, with particular focus on the roles of angiotensin II, glucagon like-peptide 1, and estradiol in turning on and off the thirst drive. Our understanding of the roles these bioregulators play has benefited from modern behavioral analyses, which have improved the time resolution of intake measures, allowing for attention to the details of the patterns within a bout of intake. This has led to behavioral interpretation in ways that are helpful in understanding the many controls of water intake and has expanded our understanding beyond the dichotomy that something which increases water intake is simply a "stimulator" while something that decreases water intake is simply a "satiety" factor. Synthesizing the available information, we describe a framework in which thirst is driven directly by perturbations in fluid intake and indirectly modified by several bioregulators. This allows us to better highlight areas that are in need of additional attention to form a more comprehensive understanding of how the system transitions between states of thirst and satiety.

Neural circuits regulation of satiation

Terminating a meal after achieving satiation is a critical step in maintaining a healthy energy balance. Despite the extensive collection of information over the last few decades regarding the neural mechanisms controlling overall eating, the mechanism underlying different temporal phases of eating behaviors, especially satiation, remains incompletely understood and is typically embedded in studies that measure the total amount of food intake. In this review, we summarize the neural circuits that detect and integrate satiation signals to suppress appetite, from interoceptive sensory inputs to the final motor outputs. Due to the well-established role of cholecystokinin (CCK) in regulating the satiation, we focus on the neural circuits that are involved in regulating the satiation effect caused by CCK. We also discuss several general principles of how these neural circuits control satiation, as well as the limitations of our current understanding of the circuits function. With the application of new techniques involving sophisticated cell-type-specific manipulation and mapping, as well as real-time recordings, it is now possible to gain a better understanding of the mechanisms specifically underlying satiation.

Dopamine, activation of ingestion and evaluation of response efficacy: a focus on the within-session time-course of licking burst number

RATIONALE

Evidence on the effect of dopamine D1-like and D2-like receptor antagonists on licking microstructure and the forced swimming response led us to suggest that (i) dopamine on D1-like receptors plays a role in activating reward-directed responses and (ii) the level of response activation is reboosted based on a process of evaluation of response efficacy requiring dopamine on D2-like receptors. A main piece of evidence in support of this hypothesis is the observation that the dopamine D2-like receptor antagonist raclopride induces a within-session decrement of burst number occurring after the contact with the reward. The few published studies with a detailed analysis of the time-course of this measure were conducted in our laboratory.

OBJECTIVES

The aim of this review is to recapitulate and discuss the evidence in support of the analysis of the within-session burst number as a behavioural substrate for the study of the mechanisms governing ingestion, behavioural activation and the related evaluation processes, and its relevance in the analysis of drug effects on ingestion.

CONCLUSIONS

The evidence gathered so far suggests that the analysis of the within-session time-course of burst number provides an important behavioural substrate for the study of the mechanisms governing ingestion, behavioural activation and the related evaluation processes, and might provide decisive evidence in the analysis of the effects of drugs on ingestion. However, further evidence from independent sources is necessary to validate the use and the proposed interpretation of this measure.

The estrogenic reduction in water intake stimulated by dehydration involves estrogen receptor alpha and a potential role for GLP-1

It is well documented that estrogens inhibit fluid intake. Most of this research, however, has focused on fluid intake in response to dipsogenic hormone and/or drug treatments in euhydrated rats. Additional research is needed to fully characterize the fluid intake effects of estradiol in response to true hypovolemia. As such, the goals of this series of experiments were to provide a detailed analysis of water intake in response to water deprivation in ovariectomized female rats treated with estradiol. In addition, these experiments also tested if activation of estrogen receptor alpha is sufficient to reduce water intake stimulated by water deprivation and tested for a role of glucagon like peptide-1 in the estrogenic control of water intake. As expected, estradiol reduced water intake in response to 24 and 48 h of water deprivation. The reduction in water intake was associated with a reduction in drinking burst number, with no change in drinking burst size. Pharmacological activation of estrogen receptor alpha reduced intake. Finally, estradiol-treatment caused a leftward shift in the behavioral dose response curve of exendin-4, the glucagon like peptide-1 agonist. While the highest dose of exendin-4 reduced 10 min intake in both oil and estradiol-treated rats, the intermediate dose only reduced intake in rats treated with estradiol. Together, this series of experiments extends previous research by providing a more thorough behavioral analysis of the anti-dipsogenic effect of estradiol in dehydrated rats, in addition to identifying the glucagon like peptide-1 system as a potential bioregulator involved in the underlying mechanisms by which estradiol reduces water intake in the female rat.

Negative feedback control of hunger circuits by the taste of food

The rewarding taste of food is critical for motivating animals to eat, but whether taste has a parallel function in promoting meal termination is not well understood. Here we show that hunger-promoting AgRP neurons are rapidly inhibited during each bout of ingestion by a signal linked to the taste of food. Blocking these transient dips in activity via closed-loop optogenetic stimulation increases food intake by selectively delaying the onset of satiety. We show that upstream leptin receptor-expressing neurons in the dorsomedial hypothalamus (DMHLepR) are tuned to respond to sweet or fatty tastes and exhibit time-locked activation during feeding that is the mirror image of downstream AgRP cells. These findings reveal an unexpected role for taste in the negative feedback control of ingestion. They also reveal a mechanism by which AgRP neurons, which are the primary cells that drive hunger, are able to influence the moment-by-moment dynamics of food consumption.

An evaluation of hedonic responses in taste-potentiated odor aversion using the analysis of licking microstructure and orofacial reactivity

Two experiments examined the hedonic responses conditioned to odor cues in the phenomenon of taste-potentiated odor aversion. Experiment 1 analyzed the microstructure of licking behavior during voluntary consumption. A tasteless odor (amyl acetate) was delivered to rats either diluted in water or mixed with saccharin before being injected with LiCl. At test, subjects which had received the odor-taste compound during conditioning showed both lower odor consumption and lick cluster size, a result indicating an increased negative evaluation of the odor. Experiment 2 examined the orofacial reactions elicited by the odor as index of its hedonic impact. During conditioning, the rats were intraorally infused with either the odor alone or the odor-saccharin compound before being injected with LiCl. At test, they were infused with the odor and their orofacial responses video recorded. More aversive orofacial responses were elicited by the odor cue in rats that had compound conditioning, again a result indicating a strengthened negative hedonic reactivity compared to animals experiencing odor aversion conditioning alone. Taken together, these results indicate that taste-mediated potentiation of odor aversion conditioning impacts on the acquisition of conditioned hedonic reactions as well as consumption.

Slc12a2 loss in insulin-secreting β-cells links development of overweight and metabolic dysregulation to impaired satiation control of feeding

Male mice lacking the Na+-K+-2Cl- cotransporter Slc12a2 (Nkcc1) specifically in insulin-secreting β-cells (Slc12a2βKO) have reduced β-cell mass and mild β-cell secretory dysfunction associated with overweight, glucose intolerance, insulin resistance, and metabolic abnormalities. Here, we confirmed and extended previous results to female Slc12a2βKO mice, which developed a similar metabolic syndrome-like phenotype as males, albeit milder. Notably, male and female Slc12a2βKO mice developed overweight without consuming excess calories. Analysis of the feeding microstructure revealed that young lean Slc12a2βKO male mice ate meals of higher caloric content and at a relatively lower frequency than normal mice, particularly during the night. In addition, overweight Slc12a2βKO mice consumed significantly larger meals than lean mice. Therefore, the reduced satiation control of feeding precedes the onset of overweight and is worsened in older Slc12a2βKO mice. However, the time spent between meals remained intact in lean and overweight Slc12a2βKO mice, indicating conserved satiety responses to ad libitum feeding. Nevertheless, satiety was intensified during and after refeeding only in overweight males. In lean females, satiety responses to refeeding were delayed relative to age- and body weight-matched control mice but normalized in overweight mice. Since meal size did not change during refeeding, these data suggested that the satiety control of eating after fasting is impaired in lean Slc12a2βKO mice before the onset of overweight and independently of their reduced satiation responses. Therefore, our results support the novel hypothesis that reduced satiation precedes the onset of overweight and the development of metabolic dysregulation.NEW & NOTEWORTHY Obesity, defined as excess fat accumulation, increases the absolute risk for metabolic diseases. Although obesity is usually attributed to increased food intake, we demonstrate that body weight gain can be hastened without consuming excess calories. In fact, impaired meal termination control, i.e., satiation, is detectable before the development of overweight in an animal model that develops a metabolic syndrome-like phenotype.

Disentangling the role of NAc D1 and D2 cells in hedonic eating

Overeating is driven by both the hedonic component ('liking') of food, and the motivation ('wanting') to eat it. The nucleus accumbens (NAc) is a key brain center implicated in these processes, but how distinct NAc cell populations encode 'liking' and 'wanting' to shape overconsumption remains unclear. Here, we probed the roles of NAc D1 and D2 cells in these processes using cell-specific recording and optogenetic manipulation in diverse behavioral paradigms that disentangle reward traits of 'liking' and 'wanting' related to food choice and overeating in healthy mice. Medial NAc shell D2 cells encoded experience-dependent development of 'liking', while D1 cells encoded innate 'liking' during the first food taste. Optogenetic control confirmed causal links of D1 and D2 cells to these aspects of 'liking'. In relation to 'wanting', D1 and D2 cells encoded and promoted distinct aspects of food approach: D1 cells interpreted food cues while D2 cells also sustained food-visit-length that facilitates consumption. Finally, at the level of food choice, D1, but not D2, cell activity was sufficient to switch food preference, programming subsequent long-lasting overconsumption. By revealing complementary roles of D1 and D2 cells in consumption, these findings assign neural bases to 'liking' and 'wanting' in a unifying framework of D1 and D2 cell activity.

Validation of computational models to characterize cumulative intake curves from video-coded meals

Introduction

Observational coding of eating behaviors (e.g., bites, eating rate) captures behavioral characteristics but is limited in its ability to capture dynamic patterns (e.g., temporal changes) across a meal. While the Universal Eating Monitor captures dynamic patterns of eating through cumulative intake curves, it is not commonly used in children due to strict behavioral protocols. Therefore, the objective of this study was to test the ability of computational models to characterize cumulative intake curves from video-coded meals without the use of continuous meal weight measurement.

Methods

Cumulative intake curves were estimated using Kisslieff's Quadratic model and Thomas's logistic ordinary differential equation (LODE) model. To test if cumulative intake curves could be characterized from video-coded meals, three different types of data were simulated: (1) Constant Bite: simplified cumulative intake data; (2) Variable Bite: continuously measured meal weight data; and (3) Bite Measurement Error: video-coded meals that require the use of average bite size rather than measured bite size.

Results

Performance did not differ by condition, which was assessed by examining model parameter recovery, goodness of fit, and prediction error. Therefore, the additional error incurred by using average bite size as one would with video-coded meals did not impact the ability to accurately estimate cumulative intake curves. While the Quadratic and LODE models were comparable in their ability to characterize cumulative intake curves, the LODE model parameters were more distinct than the Quadradic model. Greater distinctness suggests the LODE model may be more sensitive to individual differences in cumulative intake curves.

Discussion

Characterizing cumulative intake curves from video-coded meals expands our ability to capture dynamic patterns of eating behaviors in populations that are less amenable to strict protocols such as children and individuals with disordered eating. This will improve our ability to identify patterns of eating behavior associated with overconsumption and provide new opportunities for treatment.

Odor-taste pairings lead to the acquisition of negative hedonic qualities by the odor in aversion learning

Three experiments examined the affective responses conditioned to an odorous stimulus in the taste-mediated odor aversion learning paradigm. Experiment 1 analyzed the microstructure of licking behavior during voluntary consumption. Before conditioning, water-deprived rats had access to a bottle containing either a tasteless odor (0.01% amyl acetate) diluted in water or mixed with 0.05% saccharin. Next, the rats were injected with either LiCl or saline immediately after drinking saccharin. At test, they received the odor and taste solutions on separate days. Lick cluster size was used as a direct measure of the hedonic response to the odor cue. Rats receiving odor-taste pairings prior to the saccharin devaluation showed both lower consumption and lick cluster size, reflecting a reduced hedonic evaluation of the odor. Experiments 2a and 2b used the orofacial reactivity method. After pretraining in the drinking boxes with the odor alone or mixed with saccharin, the rats were intraorally infused with saccharin before injection with LiCl or saline. At test, they were infused in separate sessions with the odor and taste and their orofacial reactions video recorded. There were increased aversive orofacial responses to the odor in rats that had prior odor-taste experience, a result indicating a negative hedonic evaluation of the odor. These results provide evidence of conditioned changes in affective value of odor cues through taste-mediated learning and are consistent with the idea that odor-taste pairings lead to the acquisition of taste qualities by the odor.

The role of glucagon-like peptide-1 (GLP-1) in fluid and food intakes in vasopressin-deficient Brattleboro rats

Eating and drinking co-occur and many of the same mechanisms that control one are involved in the control of the other, making it difficult to isolate specific mechanisms for the control of fluid intake. Glucagon-like peptide-1 (GLP-1) is a peptide that seems to be involved in the endogenous control of both ingestive behaviors, but we lack a thorough understanding of how and where GLP-1 is acting to control fluid intake. Vasopressin-deficient Brattleboro rats are a model of hereditary hypothalamic diabetes insipidus that have been used extensively for the study of vasopressin actions in behavior and physiology. Here, we propose that these rats, that eat normally but drink excessively, provide a useful model to dissociate central controls of food and fluid intakes. As an initial step toward establishing this model for these purposes, we focused on GLP-1. Similar to the effect observed after treatment with a GLP-1 receptor (GLP-1R) agonist, the intake difference between wildtype and Brattleboro rats was largely a function in the number of licking bursts, indicating differences in post-ingestive feedback (e.g., satiation). When given central injections of a GLP-1R agonist, the effect on feeding was comparable between wildtype and Brattleboro rats, but the effect of drug on fluid intake was markedly exaggerated in Brattleboro rats. Additionally, Brattleboro rats did not respond to GLP-1R antagonism, whereas wildtype rats did. Taken together, these results suggest that Brattleboro rats exhibit a selective disruption to GLP-1's control of water intake. Overall, these experiments provide foundational studies of the ingestive behavior of Brattleboro rats and demonstrate the potential to use these rats to disentangle the effects of GLP-1 on food and fluid intakes.

Serotonergic control of feeding microstructure in Drosophila

To survive, animals maintain energy homeostasis by seeking out food. Compared to freely feeding animals, food-deprived animals may choose different strategies to balance both energy and nutrition demands, per the metabolic state of the animal. Serotonin mediates internal states, modifies existing neural circuits, and regulates animal feeding behavior, including in humans and fruit flies. However, an in-depth study on the neuromodulatory effects of serotonin on feeding microstructure has been held back for several technical reasons. Firstly, most feeding assays lack the precision of manipulating neuronal activity only when animals start feeding, which does not separate neuronal effects on feeding from foraging and locomotion. Secondly, despite the availability of optogenetic tools, feeding in adult fruit flies has primarily been studied using thermogenetic systems, which are confounded with heat. Thirdly, most feeding assays have used food intake as a measurement, which has a low temporal resolution to dissect feeding at the microstructure level. To circumvent these problems, we utilized OptoPAD assay, which provides the precision of optogenetics to control neural activity contingent on the ongoing feeding behavior. We show that manipulating the serotonin circuit optogenetically affects multiple feeding parameters state-dependently. Food-deprived flies with optogenetically activated and suppressed serotonin systems feed with shorter and longer sip durations and longer and shorter inter-sip intervals, respectively. We further show that serotonin suppresses and enhances feeding via 5-HT1B and 5-HT7 receptors, respectively.

Expression of c-Fos following voluntary ingestion of a novel or familiar taste in rats

Taste neophobia, the rejection of novel tastes or foods, involves an interplay of various brain regions encompassing areas within the central gustatory system, as well as nuclei serving other functions. Previous findings, utilising c-Fos imaging, identified several brain regions which displayed higher activity after ingestion of a novel taste as compared to a familiar taste. The present study extends this analysis to include additional regions suspected of contributing to the neurocircuitry involved in evoking taste neophobia. Our data show increased c-Fos expression in the basolateral amygdala, central nucleus of the amygdala, gustatory portion of the thalamus, gustatory portion of the insular cortex and the medial and lateral regions of the parabrachial nucleus. These results confirm the contribution of areas previously identified as active during ingestion of novel tastes and expose additional areas that express elevated levels of c-Fos under these conditions, thus adding to the neural network involved in the detection and initial processing of taste novelty.

Gut microbiota suppress feeding induced by palatable foods

Feeding behaviors depend on intrinsic and extrinsic factors including genetics, food palatability, and the environment.^1,2,3,4,5 The gut microbiota is a major environmental contributor to host physiology and impacts feeding behavior.^{6,7,8,9,10,11,12} Here, we explored the hypothesis that gut bacteria influence behavioral responses to palatable foods and reveal that antibiotic depletion (ABX) of the gut microbiota in mice results in overconsumption of several palatable foods with conserved effects on feeding dynamics. Gut microbiota restoration via fecal transplant into ABX mice is sufficient to rescue overconsumption of high-sucrose pellets. Operant conditioning tests found that ABX mice exhibit intensified motivation to pursue high-sucrose rewards. Accordingly, neuronal activity in mesolimbic brain regions, which have been linked with motivation and reward-seeking behavior,³ was elevated in ABX mice after consumption of high-sucrose pellets. Differential antibiotic treatment and functional microbiota transplants identified specific gut bacterial taxa from the family S24-7 and the genus Lactobacillus whose abundances associate with suppression of high-sucrose pellet consumption. Indeed, colonization of mice with S24-7 and Lactobacillus johnsonii was sufficient to reduce overconsumption of high-sucrose pellets in an antibiotic-induced model of binge eating. These results demonstrate that extrinsic influences from the gut microbiota can suppress the behavioral response toward palatable foods in mice.

Eating rate and bite size were related to food intake across meals varying in portion size: A randomized crossover trial in adults

Serving larger portions leads to increased food intake, but behavioral factors that influence the magnitude of this portion size effect have not been well characterized. We investigated whether measures of eating microstructure such as eating rate and bite size moderated the portion size effect. We also explored how sensory-specific satiety (SSS; the relative hedonic decline of a food as it is eaten) was affected by eating microstructure and larger portions. In a randomized crossover design, 44 adults aged 18-68 y (66% women; 46% with overweight and obesity) ate lunch in the laboratory once a week for 4 weeks. The meal consisted of pasta that was varied in portion size (400, 500, 600, or 700 g) and 700 g of water. Meals were video-recorded to assess bite count and meal duration, which were used to calculate mean eating rate (g/min) and mean bite size (g/bite). At each meal participants also completed an assessment of SSS. The results showed that as larger portions were served, meal intake increased in a curvilinear manner (p < 0.0001). Measures of eating microstructure did not moderate the portion size effect but were related to intake across all portions; faster eating rate, larger bite size, higher bite count, and longer meal duration were associated with greater consumption at all meals (all p < 0.0001). SSS was not influenced by any measure of eating microstructure or by portion size (all p > 0.10). In summary, the portion size effect was not moderated by eating microstructure, but relatively faster eating rates and larger bite sizes at meals, along with large portions, combined to increase food intake.

A new apparatus to analyze meal-related ingestive behaviors in rats fed a complex multi-food diet

The measurement of the size and timing of meals provides critical insight into the processes underlying food intake. While most work has been conducted with a single food or fluid, the availability of food choices can also influence eating and interact with these processes. The 5-Item Food Choice Monitor (FCM), a device that continuously measures eating and drinking behaviors of rats provided up to 5 foods and 2 fluids simultaneously, was designed to allow study of food choices simultaneously with meal patterns. To validate this device, adult male and female (n = 8 each) Sprague-Dawley rats were housed in the FCM. Food and fluid intake were measured continuously (22-h/day) while rats were presented water and powdered chow. Then a cafeteria diet of 5 foods varying in macronutrient content, texture, and flavors were offered along with water. Lastly, the 5 foods were offered along with 0.3 M sucrose and water. Analyses were conducted to find optimal criteria for parceling ingestive behavior into meals, and then meal patterns were quantified. Total intake, as assessed by FCM software, was in good concordance with that measured by an independent scale. A minimum meal size of 1 kcal and a meal termination criterion of 15-min accounted for >90% of total intake and produced meal dynamics that were in register with the literature. Use of the cafeteria diet allowed comparisons between meal patterns with a single food versus a multi-food diet, as well as analyses of macronutrient-related food choices across subsets of meals. The FCM proved to accurately measure food intake over a 22-h period and was able to detect differences and similarities in the meal patterns of rats as a function of sex and food choice availability. Combined with any number of experimental manipulations, the FCM holds great promise in the investigation of the physiological and neural controls of ingestive behavior in a dietary environment that allows food choices, more closely emulating human eating conditions.

Burst-pause criterion derivation for drinkometer measurements of ingestive behavior

The drinkometer is a promising device for the study of ingestive behavior of liquid meals in humans. It can be used to investigate behavior in different target populations. However, ingestive behavior has a great variability across study participants. Therefore, a new analytical approach is required for the extraction and analysis of drinkometer-derived data that could account for this variability. We developed an optimized protocol to predict an optimal burst-pause criterion (PC) for the extraction of PC-dependent microstructural parameters of ingestive behavior. These describe the microstructure of bursts, while PC-independent parameters describe the microstructure of sucks. Therefore, a PC is required to analyze separately two physiologically different parts of behavior. To accomplish this burst-pause criterion derivation (BPCD), a Gaussian Mixture Model (GMM) was built for estimation of two probability density functions (PDFs). These model the distribution of inter-suck intervals (ISIs) and inter-burst intervals (IBIs), respectively. The PC is defined at the intersection point of the two density functions. A Kaplan-Meier (KM) survival analysis was performed for post-hoc verification of the fit of the predicted optimal PC to the ISI distribution. In this protocol paper, we present a walkthrough of the data analysis of drinkometer-derived data for the measurement of microstructure of ingestive behavior based on previous results published by our group [1].•Standardization of the burst-pause criterion derivation for drinkometer measurements of ingestive behavior.•All codes are publicly available in a repository.•The method can be easily adapted to studies with larger sample size or more than one study stimulus.

Child meal microstructure and eating behaviors: A systematic review

Observational coding of children's eating behaviors and meal microstructure (e.g., bites, chews) provides an opportunity to assess complex eating styles that may relate to individual differences in energy intake and weight status. Across studies, however, similar terms are often defined differently, which complicates the interpretation and replication of coding protocols. Therefore, this study aimed to compile methods of coding meal microstructure in children. To limit bias and ensure a comprehensive review, a systematic search was conducted in January 2021 across three databases (PubMed, PsychInfo, Web of Science) resulting in 46 studies that coded at least one meal-related behavior in healthy children (i.e., no medical/psychological disorders) who were able to self-feed (i.e., no spoon-, breast-, or bottle-feeding). While the majority of studies had good interrater reliability, the details reported about study foods and the clarity of the definitions used for behavioral coding varied considerably. In addition to reported microstructure behaviors, a non-exhaustive review of individual differences was included. While few studies reported individual differences related to age and sex, there was a larger literature related to weight status that provided evidence for an 'obesogenic' style of eating characterized by larger Bites, faster Eating and Bite Rates, and shorter Meal Durations. However, some studies may not have been optimally designed or powered to detect individual differences because they did not set out a priori to examine them. Based on this systematic review, best practices for the field are recommended and include reporting more details about foods served and coded eating behaviors to improve reproducibility. These suggestions will improve the ability to examine patterns of individual differences across studies, which may help identify novel targets for intervention.

Meal Patterns and Food Choices of Female Rats Fed a Cafeteria-Style Diet Are Altered by Gastric Bypass Surgery

After Roux-en-Y gastric bypass surgery (RYGB), rats tend to reduce consumption of high-sugar and/or high-fat foods over time. Here, we sought to investigate the behavioral mechanisms underlying these intake outcomes. Adult female rats were provided a cafeteria diet comprised of five palatable foodstuffs varying in sugar and fat content and intake was monitored continuously. Rats were then assigned to either RYGB, or one of two control (CTL) groups: sham surgery or a nonsurgical control group receiving the same prophylactic iron treatments as RYGB rats. Post-sur-gically, all rats consumed a large first meal of the cafeteria diet. After the first meal, RYGB rats reduced intake primarily by decreasing the meal sizes relative to CTL rats, ate meals more slowly, and displayed altered nycthemeral timing of intake yielding more daytime meals and fewer nighttime meals. Collectively, these meal patterns indicate that despite being motivated to consume a cafeteria diet after RYGB, rats rapidly learn to modify eating behaviors to consume foods more slowly across the entire day. RYGB rats also altered food preferences, but more slowly than the changes in meal patterns, and ate proportionally more energy from complex carbohydrates and protein and proportionally less fat. Overall, the pattern of results suggests that after RYGB rats quickly learn to adjust their size, eating rate, and distribution of meals without altering meal number and to shift their macronutrient intake away from fat; these changes appear to be more related to postingestive events than to a fundamental decline in the palatability of food choices.

Microstructural changes in human ingestive behavior after Roux-en-Y gastric bypass during liquid meals

BACKGROUND

Roux-en-Y gastric bypass (RYGB) decreases energy intake and is, therefore, an effective treatment of obesity. The behavioral bases of the decreased calorie intake remain to be elucidated. We applied the methodology of microstructural analysis of meal intake to establish the behavioral features of ingestion in an effort to discern the various controls of feeding as a function of RYGB.

METHODS

The ingestive microstructure of a standardized liquid meal in a cohort of 11 RYGB patients, in 10 patients with obesity, and in 10 healthy-weight adults was prospectively assessed from baseline to 1 year with a custom-designed drinkometer. Statistics were performed on log-transformed ratios of change from baseline so that each participant served as their own control, and proportional increases and decreases were numerically symmetrical. Data-driven (3 seconds) and additional burst pause criteria (1 and 5 seconds) were used.

RESULTS

At baseline, the mean meal size (909.2 versus 557.6 kCal), burst size (28.8 versus 17.6 mL), and meal duration (433 versus 381 seconds) differed between RYGB patients and healthy-weight controls, whereas suck volume (5.2 versus 4.6 mL) and number of bursts (19.7 versus 20.1) were comparable. At 1 year, the ingestive differences between the RYGB and healthy-weight groups disappeared due to significantly decreased burst size (P = 0.008) and meal duration (P = 0.034) after RYGB. The first-minute intake also decreased after RYGB (P = 0.022).

CONCLUSION

RYGB induced dynamic changes in ingestive behavior over the first postoperative year. While the eating pattern of controls remained stable, RYGB patients reduced their meal size by decreasing burst size and meal duration, suggesting that increased postingestive sensibility may mediate postbariatric ingestive behavior.

TRIAL REGISTRATION

NCT03747445; https://clinicaltrials.gov/ct2/show/NCT03747445.

FUNDING

This work was supported by the University of Zurich, the Swiss National Fund (32003B_182309), and the Olga Mayenfisch Foundation. Bálint File was supported by the Hungarian Brain Research Program Grant (grant no. 2017-1.2.1-NKP-2017-00002).

The anti-dipsogenic and anti-natriorexigenic effects of estradiol, but not the anti-pressor effect, are lost in aged female rats

Estradiol (E2) inhibits fluid intake in several species, which may help to defend fluid homeostasis by preventing excessive extracellular fluid volume. Although this phenomenon is well established using the rat model, it has only been studied directly in young adults. Because aging influences the neuronal sensitivity to E2 and the fluid intake effects of E2 are mediated in the brain, we tested the hypothesis that aging influences the fluid intake effects of E2 in female rats. To do so, we examined water and NaCl intake in addition to the pressor effect after central angiotensin II treatment in young (3-4 months), middle-aged (10-12 months), and old (16-18 months) ovariectomized rats treated with estradiol benzoate (EB). As expected, EB treatment reduced water and NaCl intake in young rats. EB treatment, however, did not reduce water intake in old rats, nor did it reduce NaCl intake in middle-aged or old rats. The ability of EB to reduce blood pressure was, in contrast, observed in all three age groups. Next, we also measured the gene expression of estrogen receptors (ERs) and the angiotensin type 1 receptor (AT1R) in the areas of the brain that control fluid balance. ERβ, G protein estrogen receptor (GPER), and AT1R were reduced in the paraventricular nucleus of the hypothalamus in middle-aged and old rats, compared to young rats. These results suggest the estrogenic control of fluid intake is modified by age. Older animals lost the fluid intake effects of E2, which correlated with decreased ER and AT1R expression in the hypothalamus.

Fluid intake, what's dopamine got to do with it?

Maintaining fluid balance is critical for life. The central components that control fluid intake are only partly understood. This contribution to the collection of papers highlighting work by members of the Society for the Study of Ingestive Behavior focuses on the role that dopamine has on fluid intake and describes the roles that various bioregulators can have on thirst and sodium appetite by influencing dopamine systems in the brain. The goal of the review is to highlight areas in need of more research and to propose a framework to guide that research. We hope that this framework will inspire researchers in the field to investigate these interesting questions in order to form a more complete understanding of how fluid intake is controlled.

Bidirectional effects of estradiol on the control of water intake in female rats

The inhibitory effect of estradiol (E2) on water intake has been recognized for 50 years. Despite a rich literature describing this phenomenon, we report here a previously unidentified dipsogenic effect of E2 during states of low fluid intake. Our initial goal was to test the hypothesis that the anti-dipsogenic effect of E2 on unstimulated water intake is independent of its anorexigenic effect in female rats. In support of this hypothesis, water intake was reduced during estrus, compared to diestrus, when food was present or absent. Water intake was reduced by E2 in ovariectomized rats when food was available, demonstrating a causative role of E2. Surprisingly, however, when food was removed, resulting in a significant reduction in baseline water intake, E2 enhanced drinking. Accordingly, we next tested the effect of E2 on water intake after an acute suppression of intake induced by exendin-4. The initial rebound drinking was greater in E2-treated, compared to Oil-treated, rats. Finally, to reconcile conflicting reports regarding the effect of ovariectomy on water intake, we measured daily water and food intake, and body weight in ovariectomized and sham-operated rats. Predictably, ovariectomy significantly increased food intake and body weight, but only transiently increased water intake. Together these results provide further support for independent effects of E2 on the controls of water and food intake. More importantly, this report of bidirectional effects of E2 on water intake may lead to a paradigm shift, as it challenges the prevailing view that E2 effects on fluid intake are exclusively inhibitory.

Microstructure analysis of sucrose ingestion in the course of chronic treatment with imipramine

The analysis of licking microstructure provides measures which might be interpreted in terms of psychological constructs, such as pleasure and motivation, relevant for the interpretation of the effects of antidepressant drugs. The aim of this study was to characterise the effect of the prototypical antidepressant imipramine on the microstructure of licking for a 10% sucrose solution. In particular, ten 30-min sessions were performed in the course of a daily 21 day treatment with imipramine - 5, 10 and 20 mg/kg/die administered intraperitoneally. To interpret drug effects in relation to the presumed concentration of imipramine and its active metabolite desipramine, the experimental sessions were performed in an alternate order either 1-h or 24-h after imipramine administration. In the sessions performed 1-h after drug administration, the results showed a dose-dependent reduction of sucrose ingestion, accounted for by a reduction of the licking burst number. Moreover, reduced intra-burst lick rate and increased latency to lick were observed with the highest doses. Imipramine effect in the sessions performed 24-h after drug administration was similar but less pronounced. These results are consistent with the hypothesis that the reduction of sucrose ingestion might be due to reduced motivation and/or to a potentiation of satiety signals. These effects appear to be related, at least in part, to brain drug levels at testing time, and do not seem related to the mechanisms underlying the antidepressant therapeutic effect. However, these results might be relevant in explaining the effects of imipramine in models of drug-seeking and on body weight.

Memantine effects on ingestion microstructure and the effect of administration time: A within-subject study

In a between-subject comparison of two memantine administration schedules we observed that treatment with the NMDA receptor antagonist memantine before testing sessions reduced ingestion of a 10% sucrose solution in rats, due to reduced licking burst size, thus suggesting a blunted hedonic response. Conversely, daily post-session administration reduced burst number, indicating a reduced level of behavioural activation, likely due to the development of conditioned taste aversion (CTA). In this study, the effect of pre-session and post-session memantine administration was investigated within-subjects. Memantine was administered in daily intraperitoneal injections for 13 days, on alternate days, either 1-h before–“before testing” sessions—or immediately after a 30-min session–“after testing” sessions. The effects on the microstructure of licking for a 10% sucrose solution were examined in the course of treatment and for 21 days after treatment discontinuation. The results show reduced burst size in the “before testing” sessions, without effects on the intra-burst lick rate, an index of motoric effects. Moreover, burst number was reduced since the third session of both administration conditions until the end of treatment. Interestingly, the effect of memantine of reducing the activation of ingestive behaviour was less pronounced in this study with respect to that observed with the previous study post-session administration schedule, in spite of the longer treatment. This apparent paradox might be explained if one considers these effects as instances of a memory-related effect, such as the development of CTA. In the framework of this hypothesis, the “before testing” sessions, not being followed by memantine administration, can be considered as extinction sessions performed every other day. Moreover, the animals treated with memantine at the highest dose failed to recover to pre-treatment ingestion levels 21 days after treatment discontinuation, while the animals treated after testing sessions in the previously published study showed a complete recovery well before the 15^th day test. Within the same interpretative framework, this might depend by the reduced number and frequency of the extinction trials—i.e. the number of the sessions run after treatment discontinuation—in the present study. These results provide further support to the conclusion that memantine administration before sessions reduce burst size, an effect which is likely due to blockade of NMDA receptors occurring during behavioural testing. The observation that this effect can be obtained even in absence of a reduced intra-burst lick rate, which rules out the involvement of motor impairment, provides an important piece of evidence in support to the interpretation of this effect as a blunted hedonic response. Moreover, these results provide further evidence that burst number reduction is due to a memory-related effect induced by memantine administration after sessions.

Wistar Kyoto Rats Display Anhedonia In Consumption but Retain Some Sensitivity to the Anticipation of Palatable Solutions

The Wistar Kyoto (WKY) rat has been proposed as a model of depression-like symptoms. However, anhedonia-a reduction in the response to normatively rewarding events-as a central depression symptom has yet to be fully assessed in this model. We compared WKY rats and Wistar controls, with stress-susceptibility examined by applying mild unpredictable stress to a subset of each group. Anhedonia-like behavior was assessed using microstructural analysis of licking behavior, where mean lick cluster size reflects hedonic responses. This was combined with tests of anticipatory contrast, where the consumption of a moderately palatable solution (4% sucrose) is suppressed in anticipation of a more palatable solution (32% sucrose). WKY rats displayed greatly attenuated hedonic reactions to sucrose overall, although their reactions retained some sensitivity to differences in sucrose concentration. They displayed normal reductions in consumption in anticipatory contrast, although the effect of contrast on hedonic reactions was greatly blunted. Mild stress produced overall reductions in sucrose consumption, but this was not exacerbated in WKY rats. Moreover, mild stress did not affect hedonic reactions or the effects of contrast. These results confirm that the WKY substrain expresses a direct behavioral analog of anhedonia, which may have utility for increasing mechanistic understanding of depression symptoms.

Challenges in the interpretation and therapeutic manipulation of human ingestive microstructure

This minireview focuses on the interpretative value of ingestive microstructure by summarizing observations from both rodent and human studies. Preliminary data on the therapeutic manipulation of distinct microstructural components of eating are also outlined. In rodents, the interpretative framework of ingestive microstructure mainly concentrates on deprivation state, palatability, satiation, and the role of learning from previous experiences. In humans, however, the control of eating is further influenced by genetic, psychosocial, cultural, and environmental factors, which add complexity and challenges to the interpretation of the microstructure of meal intake. Nevertheless, the presented findings stress the importance of microstructural analyses of ingestion, as a method to investigate specific behavioral variables that underlie the regulation of appetite control.

Microinjection of urotensin II into the pedunculopontine tegmentum leads to an increase in the consumption of sweet tastants

The pedunculopontine tegmentum (PPTg) plays a role in processing multiple sensory inputs and innervates brain regions associated with reward-related behaviors. The urotensin II receptor, activated by the urotensin II peptide (UII), is selectively expressed by the cholinergic neurons of the PPTg. Although the exact function of cholinergic neurons of the PPTg is unknown, they are thought to contribute to the perception of reward magnitude or salience detection. We hypothesized that the activation of PPTg cholinergic neurons would alter sensory processing across multiple modalities (ex. taste and hearing). Here we had three aims: first, determine if cholinergic activation is involved in consumption behavior of palatable solutions (sucrose). Second, if so, distinguish the impact of the caloric value by using saccharin, a zero calorie sweetener. Lastly, we tested the UII-mediated effects on perception of acoustic stimuli by measuring acoustic startle reflex (ASR). Male Sprague-Dawley rats were bilaterally cannulated into the PPTg, then placed under food restriction lasting the entire consumption experiment (water ad lib.). Treatment consisted of a microinjection of either 1 μL of aCSF or 1 μL of 10 μM UII into the PPTg, and the rats were immediately given access to either sucrose or saccharin. For the remaining five days, rats were allowed one hour access per day to the same sweet solution without any further treatments. During the saccharin experiment rats were tested in a contact lickometer which recorded each individual lick to give insight into the microstructure of the consumption behavior. ASR testing consisted of a baseline (no treatment), treatment day, and two additional days (no treatment). Immediately following the microinjection of UII, consumption of both saccharin and sucrose increased compared to controls. This significant increase persisted for days after the single administration of UII, but there was no generalized arousal or increase in water consumption between testing sessions. The effects on ASR were not significant. Activating cholinergic PPTg neurons may lead to a miscalculation of the salience of external stimuli, implicating the importance of cholinergic input in modulating a variety of behaviors. The long-lasting effects seen after UII treatment support further research into the role of sensory processing on reward related-behaviors at the level of the PPTg cholinergic neurons.

Habitual Ethanol Seeking and Licking Microstructure of Enhanced Ethanol Self-Administration in Ethanol-Dependent Mice

BACKGROUND

A significant component of ethanol (EtOH) dependence is the disruption to decision-making processes. Prior work has shown EtOH dependence biases habitual seeking of EtOH and disrupts neural mechanisms supporting decision-making. This has contributed to the hypothesis that habitual EtOH seeking in EtOH dependence may promote excessive habitual or compulsive EtOH consumption. However, decision-making and behavioral processes underlying seeking and consummatory behaviors differ. Here, we examine the microstructure of EtOH consummatory behavior in the context of habitual EtOH seeking.

METHODS

Following home cage pre-exposure to EtOH, C57Bl/6J mice underwent 4 rounds of chronic intermittent EtOH (CIE) or air exposure. Following acute withdrawal, mice began training for operant self-administration of 15% EtOH. Training consisted of 16-hour sessions in which mice were trained in a random ratio (RR) schedule of reinforcement for 30-second access to the EtOH sipper. To test for CIE-induced changes in action control, we used sensory-specific satiation and assessed the effect of outcome devaluation on EtOH seeking. Importantly, the use of a lickometer during operant training allowed us to measure the microstructure of lick behavior.

RESULTS

Prior induction of EtOH dependence led to increased EtOH seeking, consumption, and an insensitivity to outcome devaluation, the latter indicative of habitual EtOH seeking. We also found altered consummatory lick patterns in CIE-exposed mice compared to Air controls. While CIE mice had significantly more licks in a burst and a longer burst duration, there were no differences in the total number of bursts compared to Air controls. Furthermore, these EtOH consummatory behaviors correlated with blood EtOH concentrations (BECs), while EtOH-seeking responses did not.

CONCLUSIONS

Our results confirm that EtOH dependence can produce habitual EtOH seeking and suggests the increased EtOH consummatory behaviors following EtOH dependence are separable from decision-making processes controlling EtOH seeking.

Daily memantine treatment blunts hedonic response to sucrose in rats

RATIONALE

Preclinical and clinical studies suggest the potential use of memantine in the treatment of binge eating disorder. The aim of this study was to further investigate the mechanisms by which memantine influences the motivational aspects of ingestion through the analysis of licking microstructure. To interpret treatment effects in relation to drug action at specific functionally relevant times, we compared the effect of two different administration schedules.

METHODS

Memantine was administered daily for a week, either 1 h before or immediately after a 30-min daily session. The effects on the microstructure of licking for a 10% sucrose solution in rats were examined in the course of treatment and for 15 days after treatment discontinuation.

RESULTS

Treatment before testing reduced ingestion due to reduced burst size and increased latency in the first session. However, a progressive increase in burst number across sessions led to a full recovery of ingestion levels by the end of treatment. Daily post-session administration induced a dramatic decrease of activation of licking behaviour, indicated by reduced burst number, accompanied to reduced burst size. A slow recovery of ingestion took place after treatment discontinuation.

CONCLUSION

These results suggest a reduced hedonic/reward evaluation response, an effect likely due to NMDA receptor blockade occurring during the testing time and support the hypothesis that memantine interferes with the hedonic/non-homeostatic mechanisms regulating food intake and food-seeking. The effect of post-session administration might be explained by the development of conditioned taste aversion.

Role of dopamine D1-like and D2-like receptors in the activation of ingestive behaviour in thirsty rats licking for water

RATIONALE

Analysis of lick pattern for sucrose and NaCl and of the forced swimming response after dopamine antagonist administration led us to suggest that dopamine on D₁-like receptors is involved in behavioural activation, and the level of activation is "reboosted" on the basis of an evaluation process involving D₂-like receptors. Although some studies investigated licking microstructure for water after dopamine antagonists, the within-session time course of their effect was never investigated.

OBJECTIVES

The aims of this study were to further investigate the role of dopamine receptors in the mechanisms governing water ingestion, focussing on the within-session time course of the microstructure parameters, and to test the proposed hypothesis.

MATERIALS AND METHODS

The effects of the dopamine D₁-like receptor antagonist SCH 23390 (0.01-0.04 mg/kg) and of the dopamine D₂-like receptor antagonist raclopride (0.025-0.25 mg/kg) on licking microstructure for water were examined in 20-h water-deprived rats in 30-min sessions.

RESULTS

As previously observed with sucrose and NaCl, SCH 23390 reduced licking by reducing burst number, suggesting reduced behavioural activation. Moreover, it resulted in an increased burst size. Raclopride reduced the size of licking bursts, while their number was either increased or decreased depending on the dose.

CONCLUSION

The results support the suggestion that D₁ receptors are involved in behavioural activation and D₂ receptors are involved in a related evaluation process. Within the framework of the proposed hypothesis, the increased burst size after D₁-like receptor blockade might be interpreted as a pro-hedonic effect consequent to the increased cost of the activation of the licking response.

The homeostatic dynamics of feeding behaviour identify novel mechanisms of anorectic agents

Better understanding of feeding behaviour will be vital in reducing obesity and metabolic syndrome, but we lack a standard model that captures the complexity of feeding behaviour. We construct an accurate stochastic model of rodent feeding at the bout level in order to perform quantitative behavioural analysis. Analysing the different effects on feeding behaviour of peptide YY3-36 (PYY3-36), lithium chloride, glucagon-like peptide 1 (GLP-1), and leptin shows the precise behavioural changes caused by each anorectic agent. Our analysis demonstrates that the changes in feeding behaviour evoked by the anorectic agents investigated do not mimic the behaviour of well-fed animals and that the intermeal interval is influenced by fullness. We show how robust homeostatic control of feeding thwarts attempts to reduce food intake and how this might be overcome. In silico experiments suggest that introducing a minimum intermeal interval or modulating upper gut emptying can be as effective as anorectic drug administration.

Affective biases and their interaction with other reward-related deficits in rodent models of psychiatric disorders

Major depressive disorder (MDD) is one of the leading global causes of disability. Symptoms of MDD can vary person to person, and current treatments often fail to alleviate the poor quality of life that patients experience. One of the two core diagnostic criteria for MDD is the loss of interest in previously pleasurable activities, which suggests a link between the disease aetiology and reward processing. Cognitive impairments are also common in patients with MDD, and more recently, emotional processing deficits known as affective biases have been recognised as a key feature of the disorder. Studies in animals have found similar affective biases related to reward. In this review we consider these affective biases in the context of other reward-related deficits and examine how affective biases associated with learning and memory may interact with the wider behavioural symptoms seen in MDD. We discuss recent developments in how analogues of affective biases and other aspects of reward processing can be assessed in rodents, as well as how these behaviours are influenced in models of MDD. We subsequently discuss evidence for the neurobiological mechanisms contributing to one or more reward-related deficits in preclinical models of MDD, identified using these behavioural assays. We consider how the relationships between these selective behavioural assays and the neurobiological mechanisms for affective bias and reward processing could be used to identify potential treatment strategies.

Differential effects of dopamine D1-like and D2-like receptor agonists on water drinking behaviour under thirsty conditions in mice with reduced dopamine secretion

The mesolimbic dopamine system is important for reward-oriented behaviours, such as drinking and eating. However, the precise involvement of dopaminergic neurons and dopamine receptors in water drinking behaviour remains unclear. Here, we generated triple transgenic mice harbouring Slc6a3(DAT)-icre/ERT2, Camk2a-loxP-STOP-loxP-tetracycline transactivator and tetO-tetanus toxin constructs, in which the release of dopamine is blocked by tetanus toxin. These mice, referred to as dopamine secretion interference mice, had reduced dopamine secretion in the striatum (61.4%) and the nucleus accumbens (54.5%). They showed adequate limb strength and food consumption, similarly to control mice, but exhibited motor control impairment in a challenging rotarod test. Dopamine secretion interference mice made fewer licks and had fewer bursts than control mice during a licking test under thirsty conditions. To elucidate the influence of dopamine receptors in the altered drinking behaviour, a dopamine D1 or D2/D3 receptor agonist (A68930 or ropinirole, respectively) was administered prior to the licking microstructure analysis. Treatment with the D1 agonist restored the total number of licks but not the burst number in dopamine secretion interference mice. By contrast, the D2/3 agonist impeded water drinking behaviour in both transgenic and control mice. The present findings indicate that D1 receptor activation partially ameliorates the altered drinking behaviour of the dopamine secretion interference mice and suggest that D1 receptor activity impacts drinking under thirsty conditions.

Drinking despite adversity: behavioral evidence for a head down and push strategy of conflict-resistant alcohol drinking in rats

Compulsive alcohol drinking, where intake persists regardless of adverse consequences, plays a major role in the substantial costs of alcohol use disorder. However, the processes that promote aversion-resistant drinking remain poorly understood. Compulsion-like responding has been considered automatic and reflexive and also to involve higher motivation, since drinking persists despite adversity. Thus, we used lickometry, where microstructural behavioral changes can reflect altered motivation, to test whether conflict-resistant intake [quinine-alcohol (QuiA)] reflected greater automaticity or motivation relative to alcohol-only drinking (Alc). Front-loading during QuiA and Alc suggested incentive to drink in both. However, the relationship between total licking and intake was less variable during QuiA, as was lick volume, without changes in average responding. QuiA bout organization was also less variable, with fewer licks outside of bouts (stray licks) and fewer gaps within bouts. Interestingly, QuiA avoidance of stray licking continued into short bouts, with fewer short and more medium-length bouts, which was striking given their minor impact on intake. Instead, more effort at bout onset could allow short bouts to persist longer. Indeed, while QuiA licking was overall faster, QuiA bouts were especially fast at bout initiation. However, few QuiA changes individually predicted greater intake, perhaps suggesting an overarching strategy during aversion-resistant responding. Thus, our results indicate that aversion-resistant intake exhibited less variability, where increased automaticity could decrease need for awareness, and stronger bout initiation, which might prolong responding despite adversity. This may reflect a collective strategy, which we call Head Down and Push responding that facilitates conflict-resistant, compulsion-like intake.

Partial reinforcement and conditioned taste aversion: No evidence for resistance to extinction

The partial reinforcement extinction effect (PREE) is the observation that, following training in which a response is followed by reward on only a subset of trials, the response is more resistant to extinction following the total removal of reward than it is after training in which reward is presented on all trials. The PREE is almost ubiquitous in instrumental conditioning procedures but only inconsistently observed in Pavlovian conditioning. In his classic review of animal learning, Mackintosh attributes the bulk of the PREE to generalisation decrement relating to the fact that partial reinforcement typically ensures that acquisition of responding has taken place in conditions similar to that of extinction (e.g., in the absence of the reinforcer). We report here that extinction of a conditioned taste aversion is not retarded by partial reinforcement in terms of either consumption of the taste or hedonic reactions to it (assessed through the analysis of licking microstructure). These results are consistent with Mackintosh’s analysis of the PREE and the way in which it might differ between instrumental and Pavlovian conditioning.

Limbic control over the homeostatic need for sodium

The homeostatic need for sodium is one of the strongest motivational drives known in animals. Although the brain regions involved in the sensory detection of sodium levels have been mapped relatively well, data about the neural basis of the motivational properties of salt appetite, including a role for midbrain dopamine cells, have been inconclusive. Here, we employed a combination of fiber photometry, behavioral pharmacology and c-Fos immunohistochemistry to study the involvement of the mesocorticolimbic dopamine system in salt appetite in rats. We observed that sodium deficiency affected the responses of dopaminergic midbrain neurons to salt tasting, suggesting that these neurons encode appetitive properties of sodium. We further observed a significant reduction in the consumption of salt after pharmacological inactivation of the nucleus accumbens (but not the medial prefrontal cortex), and microstructure analysis of licking behavior suggested that this was due to decreased motivation for, but not appreciation of salt. However, this was not dependent on dopaminergic neurotransmission in that area, as infusion of a dopamine receptor antagonist into the nucleus accumbens did not alter salt appetite. We conclude that the nucleus accumbens, but not medial prefrontal cortex, is important for the behavioral expression of salt appetite by mediating its motivational component, but that the switch in salt appreciation after sodium depletion, although detected by midbrain dopamine neurons, must arise from other areas.

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.

Inhibiting gustatory thalamus or medial amygdala has opposing effects on taste neophobia

Taste neophobia is a feeding system defense mechanism that limits consumption of an unknown, and therefore potentially dangerous, edible until the post-ingestive consequences are experienced. We found that transient pharmacological inhibition (induced with the GABA agonists baclofen and muscimol) of the gustatory thalamus (GT; Experiment 1), but not medial amygdala (MeA; Experiment 2), during exposure to a novel saccharin solution attenuated taste neophobia. In Experiment 3 we found that inhibition of MeA neurons (induced with the chemogenetic receptor hM4DGi) enhanced the expression of taste neophobia whereas excitation of MeA neurons (with hM3DGq) had no influence of taste neophobia. Overall, these results refine the temporal involvement of the GT in the occurrence of taste neophobia and support the hypothesis that neuronal excitation in the GT is necessary for taste neophobia. Conversely, we show that chemogenetically, but not pharmacologically, inhibiting MeA neurons is sufficient to exaggerate the expression of taste neophobia.

The role of the gustatory cortex in incidental experience-evoked enhancement of later taste learning

The strength of learned associations between pairs of stimuli is affected by multiple factors, the most extensively studied of which is prior experience with the stimuli themselves. In contrast, little data is available regarding how experience with "incidental" stimuli (independent of any conditioning situation) impacts later learning. This lack of research is striking given the importance of incidental experience to survival. We have recently begun to fill this void using conditioned taste aversion (CTA), wherein an animal learns to avoid a taste that has been associated with malaise. We previously demonstrated that incidental exposure to salty and sour tastes (taste preexposure-TPE) enhances aversions learned later to sucrose. Here, we investigate the neurobiology underlying this phenomenon. First, we use immediate early gene (c-Fos) expression to identify gustatory cortex (GC) as a site at which TPE specifically increases the neural activation caused by taste-malaise pairing (i.e., TPE did not change c-Fos induced by either stimulus in isolation). Next, we use site-specific infection with the optical silencer Archaerhodopsin-T to show that GC inactivation during TPE inhibits the expected enhancements of both learning and CTA-related c-Fos expression, a full day later. Thus, we conclude that GC is almost certainly a vital part of the circuit that integrates incidental experience into later associative learning.

Pattern of access determines influence of junk food diet on cue sensitivity and palatability

AIMS

Like drug addiction, cues associated with palatable foods can trigger food-seeking, even when sated. However, whether susceptibility to the motivating influence of food-related cues is a predisposing factor in overeating or a consequence of poor diet is difficult to determine in humans. Using a rodent model, we explored whether a highly palatable 'junk food' diet impacts responses to reward-paired cues in a Pavlovian-to-instrumental transfer test, using sweetened condensed milk (SCM) as the reward. The hedonic impact of SCM consumption was also assessed by analyzing licking microstructure.

METHODS

To probe the effects of pattern and duration of junk food exposure, we provided rats with either regular chow ad libitum (controls) or chow plus access to junk food for either 2 or 24 h per day for 1, 3, or 6 weeks. We also examined how individual susceptibility to weight gain related to these measures.

RESULTS

Rats provided 24 h access to the junk food diet were insensitive to the motivational effects of a SCM-paired cue when tested sated even though their hedonic experience upon reward consumption was similar to controls. In contrast, rats provided restricted, 2 h access to junk food exhibited a cue generalization phenotype under sated conditions, lever-pressing with increased vigor in response to both a SCM-paired cue, and a cue not previously paired with reward. Hedonic response was also significantly higher in these animals relative to controls.

CONCLUSIONS

These data demonstrate that the pattern of junk food exposure differentially alters the hedonic impact of palatable foods and susceptibility to the motivating influence of cues in the environment to promote food-seeking actions when sated, which may be consequential for understanding overeating and obesity.

Contributions of Pavlovian incentive motivation to cue-potentiated feeding

Cues signaling the availability of palatable food acquire the ability to potentiate food seeking and consumption. The current study employed a combination of behavioral, pharmacological, and analytical techniques to probe the role of Pavlovian incentive motivation in cue-potentiated feeding. We show that a cue paired with sucrose solution (CS+) can transfer its control over feeding to stimulate sucrose consumption at a new receptacle, and that this effect depends on activation of D1 dopamine receptors, which is known to modulate other forms of cue-motivated behavior but not taste palatability. Microstructural analyses of sucrose-licking behavior revealed that the CS+ tended to increase the frequency with which rats engaged in active bouts of licking behavior without having a reliable effect on the duration of those licking bouts, a measure that was instead associated with sucrose palatability. Furthermore, we found that individual differences in CS+ elicited increases in bout frequency were associated with total sucrose intake at test, supporting the view that this process was related to meaningful dysregulation of eating behavior. The current study, therefore, (1) demonstrates that a dopamine-dependent Pavlovian incentive motivational process can mediate cue-potentiated feeding, and (2) lays out an experimental and analytical approach for parsing this aspect of behavior.

Handling method alters the hedonic value of reward in laboratory mice

Mice are the most widely used model species for drug discovery and scientific research. Consequently, it is important to refine laboratory procedures and practices to ensure high standards of welfare and scientific data quality. Recent studies have identified that the standard practice of handling laboratory mice by their tails increases behaviours indicative of anxiety, which can be overcome by handling mice using a tunnel. However, despite clear negative effects on mice’s behaviour, tunnel handling has yet to be widely implemented. In this study, we provide the first evidence that tail handling also reduces mice’s responses to reward. Anhedonia is a core symptom of clinical depression, and is measured in rodents by assessing how they consume a sucrose solution: depressed mice consume less sucrose and the size of their licking bouts when drinking (their ‘lick cluster sizes’) also tend to be smaller. We found that tail handled mice showed more anhedonic responses in both measures compared to tunnel handled mice, indicative of a decreased responsiveness to reward and potentially a more depressive-like state. Our findings have significant implications for the welfare of laboratory mice as well as the design and interpretation of scientific studies, particularly those investigating or involving reward.

Sex Differences in the Behavioral Desensitization of Water Intake Observed After Repeated Central Injections of Angiotensin II

Previous in vivo and in vitro studies demonstrate that the angiotensin type 1 receptor rapidly desensitizes after exposure to angiotensin II (AngII). Behaviorally, this likely underlies the reduced drinking observed after acute repeated central injections of AngII. To date, this phenomenon has been studied exclusively in male subjects. Because there are sex differences in the dipsogenic potency of AngII, we hypothesized that sex differences also exist in desensitization caused by AngII. As expected, when male rats were pretreated with AngII, they drank less water after a test injection of AngII than did rats pretreated with vehicle. Intact cycling female rats, however, drank similar amounts of water after AngII regardless of the pretreatment. To probe the mechanism underlying this sex difference, we tested the role of gonadal hormones in adult and developing rats. Gonadectomy in adults did not produce a male-like propensity for desensitization of water intake in female rats, nor did it produce a female-like response in male rats. To test if neonatal brain masculinization generated a male-like responsiveness, female pups were treated at birth with vehicle, testosterone propionate (TP), or dihydrotestosterone (DHT). When tested as adults, TP-treated female rats showed a male-like desensitization after repeated AngII that was not found in vehicle- or DHT-treated rats. Together, these data reveal a striking sex difference in the behavioral response to elevated AngII that is mediated by organizational effects of gonadal hormones and provide an example of one of the many ways that sex influences the renin-angiotensin-aldosterone system.