A neural circuit for excessive feeding driven by environmental context in mice

Conditioned overconsumption is dependent on reinforcer type in lean, but not obese, mice

Associative learning can drive many different types of behaviors, including food consumption. Previous studies have shown that cues paired with food delivery while mice are hungry will lead to increased consumption in the presence of those cues at later times. We previously showed that overconsumption can be driven in male mice by contextual cues, using chow pellets. Here we extended our findings by examining other parameters that may influence the outcome of context-conditioned overconsumption training. We found that the task worked equally well in males and females, and that palatable substances such as high-fat diet and Ensure chocolate milkshake supported learning and induced overconsumption. Surprisingly, mice did not overconsume when sucrose was used as the reinforcer during training, suggesting that nutritional content is a critical factor. Interestingly, we also observed that diet-induced obese mice did not learn the task. Overall, we find that context-conditioned overconsumption can be studied in lean male and female mice, and with multiple reinforcer types.

Current perspectives on brain circuits involved in food addiction-like behaviors

There is an emerging view that the increased availability of energy-dense foods in our society is contributing to excessive food consumption which could lead to food addiction-like behavior. Particularly, compulsive eating patterns are predominant in people suffering from eating disorders (binge-eating disorder, bulimia and anorexia nervosa) and obesity. Phenotypically, the behavioral pattern exhibits a close resemblance to individuals suffering from other forms of addiction (drug, sex, gambling). Growing body of evidence in neuroscience research is showing that excessive consumption of energy-dense foods alters the brain circuits implicated in reward, decision-making, control, habit formation, and emotions that are central to drug addiction. Here, we review the current understanding of the circuits of food addiction-like behaviors and highlight the future possibility of exploring those circuits to combat obesity and eating disorders.

Single-neuron projectomes of mouse paraventricular hypothalamic nucleus oxytocin neurons reveal mutually exclusive projection patterns

Oxytocin (OXT) plays important roles in autonomic control and behavioral modulation. However, it is unknown how the projection patterns of OXT neurons align with underlying physiological functions. Here, we present the reconstructed single-neuron, whole-brain projectomes of 264 OXT neurons of the mouse paraventricular hypothalamic nucleus (PVH) at submicron resolution. These neurons hierarchically clustered into two groups, with distinct morphological and transcriptional characteristics and mutually exclusive projection patterns. Cluster 1 (177 neurons) axons terminated exclusively in the median eminence (ME) and have few collaterals terminating within hypothalamic regions. By contrast, cluster 2 (87 neurons) sent wide-spread axons to multiple brain regions, but excluding ME. Dendritic arbors of OXT neurons also extended outside of the PVH, suggesting capability to sense signals and modulate target regions. These single-neuron resolution observations reveal distinct OXT subpopulations, provide comprehensive analysis of their morphology, and lay the structural foundation for better understanding the functional heterogeneity of OXT neurons.

Internal-state-dependent control of feeding behavior via hippocampal ghrelin signaling

Hunger is an internal state that not only invigorates feeding but also acts as a contextual cue for higher-order control of anticipatory feeding-related behavior. The ventral hippocampus is crucial for differentiating optimal behavior across contexts, but how internal contexts such as hunger influence hippocampal circuitry is unknown. In this study, we investigated the role of the ventral hippocampus during feeding behavior across different states of hunger in mice. We found that activity of a unique subpopulation of neurons that project to the nucleus accumbens (vS-NAc neurons) increased when animals investigated food, and this activity inhibited the transition to begin eating. Increases in the level of the peripheral hunger hormone ghrelin reduced vS-NAc activity during this anticipatory phase of feeding via ghrelin-receptor-dependent increases in postsynaptic inhibition and promoted the initiation of eating. Together, these experiments define a ghrelin-sensitive hippocampal circuit that informs the decision to eat based on internal state.

Conditioned overconsumption is dependent on reinforcer type in lean, but not obese, mice

Associative learning can drive many different types of behaviors, including food consumption. Previous studies have shown that cues paired with food delivery while mice are hungry will lead increased consumption in the presence of those cues at later times. We previously showed that overconsumption can be driven in male mice by contextual cues, using chow pellets. Here we extended our findings by examining other parameters that may influence the outcome of context-conditioned overconsumption training. We found that the task worked equally well in males and females, and that palatable substances such as high-fat diet and Ensure chocolate milkshake supported learning and induced overconsumption. Surprisingly, mice did not overconsume when sucrose was used as the reinforcer during training, suggesting that nutritional content is a critical factor. Interestingly, we also observed that diet-induced obese mice did not learn the task. Overall, we find that context-conditioned overconsumption can be studied in lean males and female mice, and with multiple reinforcer types.

Acute inhibition of hunger-sensing AgRP neurons promotes context-specific learning in mice

OBJECTIVE

An environmental context, which reliably predicts food availability, can increase the appetitive food drive within the same environment context. However, hunger is required for the development of such a context-induced feeding (CIF) response, suggesting the neural circuits sensitive to hunger link an internal energy state with a particular environment context. Since Agouti related peptide (AgRP) neurons are activated by energy deficit, we hypothesised that AgRP neurons are both necessary and sufficient to drive CIF.

METHODS

To examine the role of AgRP neurons in the CIF process, we used fibre photometry with GCaMP7f, chemogenetic activation of AgRP neurons, as well as optogenetic control of AgRP neurons to facilitate acute temporal control not permitted with chemogenetics.

RESULTS

A CIF response at test was only observed when mice were fasted during context training and AgRP population activity at test showed an attenuated inhibitory response to food, suggesting increased food-seeking and/or decreased satiety signalling drives the increased feeding response at test. Intriguingly, chemogenetic activation of AgRP neurons during context training did not increase CIF, suggesting precise temporal firing properties may be required. Indeed, termination of AgRP neuronal photostimulation during context training (ON-OFF in context), in the presence or absence of food, increased CIF. Moreover, photoinhibition of AgRP neurons during context training in fasted mice was sufficient to drive a subsequent CIF in the absence of food.

CONCLUSIONS

Our results suggest that AgRP neurons regulate the acquisition of CIF when the acute inhibition of AgRP activity is temporally matched to context exposure. These results establish acute AgRP inhibition as a salient neural event underscoring the effect of hunger on associative learning.

Feeding neurons integrate metabolic and reproductive states in mice

Balance between metabolic and reproductive processes is important for survival, particularly in mammals that gestate their young. How the nervous system coordinates this balance is an active area of study. Herein, we demonstrate that somatostatin (SST) neurons of the tuberal hypothalamus alter feeding in a manner sensitive to metabolic and reproductive states in mice. Whereas chemogenetic activation of SST neurons increased food intake across sexes, ablation decreased food intake only in female mice during proestrus. This ablation effect was only apparent in animals with low body mass. Fat transplantation and bioinformatics analysis of SST neuronal transcriptomes revealed white adipose as a key modulator of these effects. These studies indicate that SST hypothalamic neurons integrate metabolic and reproductive cues by responding to varying levels of circulating estrogens to modulate feeding differentially based on energy stores. Thus, gonadal steroid modulation of neuronal circuits can be context dependent and gated by metabolic status.

Is Type 2 Diabetes Mellitus a Behavioural Disorder? An Evidence Review for Type 2 Diabetes Mellitus Prevention and Remission through Lifestyle Modification

The prevalence of type 2 diabetes mellitus (T2DM) is steadily rising worldwide due to an increasingly sedentary lifestyle combined with unhealthy food habits. Currently, the burden of diabetes on healthcare systems is unprecedented and rising daily. Several observational studies and randomized controlled trials provide clinical evidence that T2DM remission is possible by adopting dietary interventions and a strict exercise training protocol. Notably, these studies provide ample evidence for remission in patients with T2DM or for prevention in those with risk factors for the disease through various non-pharmacological behavioural interventions. In this article, we present two clinical cases of individuals who showed remission from T2DM/prediabetes via behavioural changes, especially through the adoption of a low-energy diet and exercise. We also discuss the recent advances in T2DM and obesity research, focusing on nutritional interventions and exercise and their benefits for weight loss, improved metabolic profile, enhanced glycaemic control and remission of diabetes.

An iterative neural processing sequence orchestrates feeding

Feeding requires sophisticated orchestration of neural processes to satiate appetite in natural, capricious settings. However, the complementary roles of discrete neural populations in orchestrating distinct behaviors and motivations throughout the feeding process are largely unknown. Here, we delineate the behavioral repertoire of mice by developing a machine-learning-assisted behavior tracking system and show that feeding is fragmented and divergent motivations for food consumption or environment exploration compete throughout the feeding process. An iterative activation sequence of agouti-related peptide (AgRP)-expressing neurons in arcuate (ARC) nucleus, GABAergic neurons in the lateral hypothalamus (LH), and in dorsal raphe (DR) orchestrate the preparation, initiation, and maintenance of feeding segments, respectively, via the resolution of motivational conflicts. The iterative neural processing sequence underlying the competition of divergent motivations further suggests a general rule for optimizing goal-directed behaviors.

Feeding Neurons Integrate Metabolic and Reproductive States in Mice

Trade-offs between metabolic and reproductive processes are important for survival, particularly in mammals that gestate their young. Puberty and reproduction, as energetically taxing life stages, are often gated by metabolic availability in animals with ovaries. How the nervous system coordinates these trade-offs is an active area of study. We identify somatostatin neurons of the tuberal nucleus (TNSST) as a node of the feeding circuit that alters feeding in a manner sensitive to metabolic and reproductive states in mice. Whereas chemogenetic activation of TNSST neurons increased food intake across sexes, selective ablation decreased food intake only in female mice during proestrus. Interestingly, this ablation effect was only apparent in animals with a low body mass. Fat transplantation and bioinformatics analysis of TNSST neuronal transcriptomes revealed white adipose as a key modulator of the effects of TNSST neurons on food intake. Together, these studies point to a mechanism whereby TNSST hypothalamic neurons modulate feeding by responding to varying levels of circulating estrogens differentially based on energy stores. This research provides insight into how neural circuits integrate reproductive and metabolic signals, and illustrates how gonadal steroid modulation of neuronal circuits can be context-dependent and gated by metabolic status.

Acts of appetite: neural circuits governing the appetitive, consummatory, and terminating phases of feeding

The overconsumption of highly caloric and palatable foods has caused a surge in obesity rates in the past half century, thereby posing a healthcare challenge due to the array of comorbidities linked to heightened body fat accrual. Developing treatments to manage body weight requires a grasp of the neurobiological basis of appetite. In this Review, we discuss advances in neuroscience that have identified brain regions and neural circuits that coordinate distinct phases of eating: food procurement, food consumption, and meal termination. While pioneering work identified several hypothalamic nuclei to be involved in feeding, more recent studies have explored how neuronal populations beyond the hypothalamus, such as the mesolimbic pathway and nodes in the hindbrain, interconnect to modulate appetite. We also examine how long-term exposure to a calorically dense diet rewires feeding circuits and alters the response of motivational systems to food. Understanding how the nervous system regulates eating behaviour will bolster the development of medical strategies that will help individuals to maintain a healthy body weight.

A discrete parasubthalamic nucleus subpopulation plays a critical role in appetite suppression

Food intake behavior is regulated by a network of appetite-inducing and appetite-suppressing neuronal populations throughout the brain. The parasubthalamic nucleus (PSTN), a relatively unexplored population of neurons in the posterior hypothalamus, has been hypothesized to regulate appetite due to its connectivity with other anorexigenic neuronal populations and because these neurons express Fos, a marker of neuronal activation, following a meal. However, the individual cell types that make up the PSTN are not well characterized, nor are their functional roles in food intake behavior. Here, we identify and distinguish between two discrete PSTN subpopulations, those that express tachykinin-1 (PSTNTac1 neurons) and those that express corticotropin-releasing hormone (PSTNCRH neurons), and use a panel of genetically encoded tools in mice to show that PSTNTac1 neurons play an important role in appetite suppression. Both subpopulations increase activity following a meal and in response to administration of the anorexigenic hormones amylin, cholecystokinin (CCK), and peptide YY (PYY). Interestingly, chemogenetic inhibition of PSTNTac1, but not PSTNCRH neurons, reduces the appetite-suppressing effects of these hormones. Consistently, optogenetic and chemogenetic stimulation of PSTNTac1 neurons, but not PSTNCRH neurons, reduces food intake in hungry mice. PSTNTac1 and PSTNCRH neurons project to distinct downstream brain regions, and stimulation of PSTNTac1 projections to individual anorexigenic populations reduces food consumption. Taken together, these results reveal the functional properties and projection patterns of distinct PSTN cell types and demonstrate an anorexigenic role for PSTNTac1 neurons in the hormonal and central regulation of appetite.

Brain circuits for promoting homeostatic and non-homeostatic appetites

As the principal means of acquiring nutrients, feeding behavior is indispensable to the survival and well-being of animals. In response to energy or nutrient deficits, animals seek and consume food to maintain energy homeostasis. On the other hand, even when animals are calorically replete, non-homeostatic factors, such as the sight, smell, and taste of palatable food, or environmental cues that predict food, can stimulate feeding behavior. These homeostatic and non-homeostatic factors have traditionally been investigated separately, but a growing body of literature highlights that these factors work synergistically to promote feeding behavior. Furthermore, recent breakthroughs in cell type-specific and circuit-specific labeling, recording, and manipulation techniques have markedly accelerated the discovery of well-defined neural populations underlying homeostatic and non-homeostatic appetite control, as well as overlapping circuits that contribute to both types of appetite. This review aims to provide an update on our understanding of the neural circuit mechanisms for promoting homeostatic and non-homeostatic appetites, focusing on the function of recently identified, genetically defined cell types.

Research on the neural circuit mechanisms underlying feeding behaviors is critical to identifying therapeutic targets for food-related disorders like obesity and anorexia. Sung-Yon Kim and colleagues at Seoul National University, South Korea, reviewed the current understanding of neural circuits promoting feeding behavior, which is regulated by homeostatic and non-homeostatic appetites. In response to deficits in energy (caloric) or nutrients, specific populations of neurons sensitive to hormones leptin and ghrelin generate homeostatic appetite and promote feeding. In addition, diverse neural populations stimulate non-homeostatic appetite in the absence of immediate internal needs and are thought to drive overconsumption in the modern obesogenic environment. These appetites extensively interact through overlapping neural circuits to jointly promote feeding behaviors.

Neural circuit control of innate behaviors

All animals possess a plethora of innate behaviors that do not require extensive learning and are fundamental for their survival and propagation. With the advent of newly-developed techniques such as viral tracing and optogenetic and chemogenetic tools, recent studies are gradually unraveling neural circuits underlying different innate behaviors. Here, we summarize current development in our understanding of the neural circuits controlling predation, feeding, male-typical mating, and urination, highlighting the role of genetically defined neurons and their connections in sensory triggering, sensory to motor/motivation transformation, motor/motivation encoding during these different behaviors. Along the way, we discuss possible mechanisms underlying binge-eating disorder and the pro-social effects of the neuropeptide oxytocin, elucidating the clinical relevance of studying neural circuits underlying essential innate functions. Finally, we discuss some exciting brain structures recurrently appearing in the regulation of different behaviors, which suggests both divergence and convergence in the neural encoding of specific innate behaviors. Going forward, we emphasize the importance of multi-angle and cross-species dissections in delineating neural circuits that control innate behaviors.

Two opposing hippocampus to prefrontal cortex pathways for the control of approach and avoidance behaviour

The decision to either approach or avoid a potentially threatening environment is thought to rely upon the coordinated activity of heterogeneous neural populations in the hippocampus and prefrontal cortex (PFC). However, how this circuitry is organized to flexibly promote both approach or avoidance at different times has remained elusive. Here, we show that the hippocampal projection to PFC is composed of two parallel circuits located in the superficial or deep pyramidal layers of the CA1/subiculum border. These circuits have unique upstream and downstream connectivity, and are differentially active during approach and avoidance behaviour. The superficial population is preferentially connected to widespread PFC inhibitory interneurons, and its activation promotes exploration; while the deep circuit is connected to PFC pyramidal neurons and fast spiking interneurons, and its activation promotes avoidance. Together this provides a mechanism for regulation of behaviour during approach avoidance conflict: through two specialized, parallel circuits that allow bidirectional hippocampal control of PFC.

A circuit from lateral septum neurotensin neurons to tuberal nucleus controls hedonic feeding

Feeding behavior is regulated by both the homeostatic needs of the body and hedonic values of the food. Easy access to palatable energy-dense foods and the consequent obesity epidemic stress the urgent need for a better understanding of neural circuits that regulate hedonic feeding. Here, we report that neurotensin-positive neurons in the lateral septum (LS^Nts) play a crucial role in regulating hedonic feeding. Silencing LS^Nts specifically promotes feeding of palatable food, whereas activation of LS^Nts suppresses overall feeding. LS^Nts neurons project to the tuberal nucleus (TU) via GABA signaling to regulate hedonic feeding, while the neurotensin signal from LS^Nts→the supramammillary nucleus (SUM) is sufficient to suppress overall feeding. In vivo calcium imaging and optogenetic manipulation reveal two populations of LS^Nts neurons that are activated and inhibited during feeding, which contribute to food seeking and consumption, respectively. Chronic activation of LS^Nts or LS^Nts→TU is sufficient to reduce high-fat diet-induced obesity. Our findings suggest that LS^Nts→TU is a key pathway in regulating hedonic feeding.

Targeting the T-type calcium channel Cav3.2 in GABAergic arcuate nucleus neurons to treat obesity

OBJECTIVE

Cav3.2, a T-type low voltage-activated calcium channel widely expressed throughout the central nervous system, plays a vital role in neuronal excitability and various physiological functions. However, the effects of Cav3.2 on energy homeostasis remain unclear. Here, we examined the role of Cav3.2 expressed by hypothalamic GABAergic neurons in the regulation of food intake and body weight in mice and explored the underlying mechanisms.

METHODS

Male congenital Cana1h (the gene coding for Cav3.2) global knockout (Cav3.2KO) mice and their wild type (WT) littermates were first used for metabolic phenotyping studies. By using the CRISPR-Cas9 technique, Cav3.2 was selectively deleted from GABAergic neurons in the arcuate nucleus of the hypothalamus (ARH) by specifically overexpressing Cas9 protein and Cav3.2-targeting sgRNAs in ARH Vgat (Vgat^ARH) neurons. These male mutants (Cav3.2KO-Vgat^ARH) were used to determine whether Cav3.2 expressed by Vgat^ARH neurons is required for the proper regulation of energy balance. Subsequently, we used an electrophysiological patch-clamp recording in ex vivo brain slices to explore the impact of Cav3.2KO on the cellular excitability of Vgat^ARH neurons.

RESULTS

Male Cav3.2KO mice had significantly lower food intake than their WT littermate controls when fed with either a normal chow diet (NCD) or a high-fat diet (HFD). This hypophagia phenotype was associated with increased energy expenditure and decreased fat mass, lean mass, and total body weight. Selective deletion of Cav3.2 in Vgat^ARH neurons resulted in similar feeding inhibition and lean phenotype without changing energy expenditure. These data provides an intrinsic mechanism to support the previous finding on ARH non-AgRP GABA neurons in regulating diet-induced obesity. Lastly, we found that naringenin extract, a predominant flavanone found in various fruits and herbs and known to act on Cav3.2, decreased the firing activity of Vgat^ARH neurons and reduced food intake and body weight. These naringenin-induced inhibitions were fully blocked in Cav3.2KO-Vgat^ARH mice.

CONCLUSION

Our results identified Cav3.2 expressed by Vgat^ARH neurons as an essential intrinsic modulator for food intake and energy homeostasis, which is a potential therapeutic target in the treatment of obesity.

Somatostatin receptor 4 agonism normalizes stress-related excessive amygdala glutamate release and Pavlovian aversion learning and memory in rodents

Background

Excessive processing of aversive life events is a major pathology in stress-related anxiety and depressive disorders. Current pharmacological treatments have rather nonspecific mechanisms of action. Somatostatin is synthesized and released as an inhibitory co-neurotransmitter by specific GABA (gamma-aminobutyric acid) interneurons, and one of its receptors, SSTR4 (somatostatin receptor 4), is localized in brain regions involved in adaptive aversion processing and implicated in negative valence neuropathology, including the amygdala.

Methods

Rat and mouse experiments were conducted to investigate effects of specific SSTR4 agonism on neurobehavioral aversion processing, including any normalization of stress-related hyperresponsiveness. A mouse experiment to investigate stress and SSTR4 agonism effects on reward processing was also conducted.

Results

In male rats (n = 5–10/group) fitted with glutamate biosensors in basolateral amygdala, SSTR4 agonism attenuated glutamate release to restraint stress in control rats and particularly in rats previously exposed to chronic corticosterone. In male mice (n = 10–18/group), SSTR4 agonism dose-dependently attenuated Pavlovian tone/footshock learning and memory measured as freezing behavior, in both control mice and mice exposed to chronic social stress, which induces excessive Pavlovian aversion learning and memory. Specificity of SSTR4 agonism effects to aversion learning/memory was demonstrated by absence of effects on discriminative reward (sucrose) learning/memory in both control mice and mice exposed to chronic social stress; SSTR4 agonism did increase reward-to-effort valuation in a dose-dependent manner and in both control mice and mice exposed to chronic social stress, which attenuates reward motivation.

Conclusions

These neuropsychopharmacological findings add substantially to the preclinical proof-of-concept evidence for SSTR4 agonism as a treatment in anxiety and depressive disorders.