Forebrain networks and the control of feeding by environmental learned cues

The Outward Spiral: A vicious cycle model of obesity and cognitive dysfunction

Chronic failure to suppress intake during states of positive energy balance leads to weight gain and obesity. The ability to use context - including interoceptive satiety states - to inhibit responding to previously rewarded cues appears to depend on the functional integrity of the hippocampus. Recent evidence implicates energy dense Western diets in several types of hippocampal dysfunction, including reduced expression of neurotrophins and nutrient transporters, increased inflammation, microglial activation, and blood brain barrier permeability. The functional consequences of such insults include impairments in an animal's ability to modulate responding to a previously reinforced cues. We propose that such deficits promote overeating, which can further exacerbate hippocampal dysfunction and thus initiate a vicious cycle of both obesity and progressive cognitive decline.

Early life exposure to obesogenic diets and learning and memory dysfunction

Obesogenic dietary factors, such as simple sugars and saturated fatty acids, have been linked to memory impairments and hippocampal dysfunction. Recent evidence suggests that the brain may be particularly vulnerable to the effects of obesogenic diets during early life periods of rapid growth, maturation, and brain development. Investigations utilizing rodent models indicate that early life exposure to "high fat diets" (40-65% kcal derived from fat) or simple sugars (sucrose or high fructose corn syrup) can impair hippocampus-dependent learning and memory processes. In some cases, these deficits occur independent of obesity and metabolic derangement and can persist into adulthood despite dietary intervention. Various neurobiological mechanisms have been identified that may link early life consumption of obesogenic dietary factors with hippocampal dysfunction, including increased neuroinflammation and reduced neurotrophin mediated regulation of neurogenesis and synaptic plasticity. Age, duration of exposure, and dietary composition are key variables contributing to the interaction between early life diet and cognitive dysfunction, however, more research is needed to unravel the precise critical windows of development and causal dietary factors.

Enhanced Glutamatergic Synaptic Plasticity in the Hippocampal CA1 Field of Food-Restricted Rats: Involvement of CB1 Receptors

The endogenous endocannabinoid system has a crucial role in regulating appetite and feeding behavior in mammals, as well as working memory and reward mechanisms. In order to elucidate the possible role of cannabinoid type-1 receptors (CB1Rs) in the regulation of hippocampal plasticity in animals exposed to food restriction (FR), we limited the availability of food to a 2-h daily period for 3 weeks in Sprague-Dawley rats. FR rats showed a higher long-term potentiation at hippocampal CA1 excitatory synapses with a parallel increase in glutamate release when compared with animals fed ad libitum. FR rats showed a significant increase in the long-term spatial memory determined by Barnes maze. FR was also associated with a decreased inhibitory effect of the CB1R agonist win55,212-2 on glutamatergic field excitatory postsynaptic potentials, together with a decrease in hippocampal CB1R protein expression. In addition, hippocampal brain-derived neurotrophic factor protein levels and mushroom dendritic spine density were significantly enhanced in FR rats. Altogether, our data suggest that alterations of hippocampal CB1R expression and function in FR rats are associated with dendritic spine remodeling and functional potentiation of CA1 excitatory synapses, and these findings are consistent with increasing evidence supporting the idea that FR may improve cognitive functions.

Orexin/hypocretin receptor 1 signaling mediates Pavlovian cue-food conditioning and extinction

Learned food cues can drive feeding in the absence of hunger, and orexin/hypocretin signaling is necessary for this type of overeating. The current study examined whether orexin also mediates cue-food learning during the acquisition and extinction of these associations. In Experiment 1, rats underwent two sessions of Pavlovian appetitive conditioning, consisting of tone-food presentations. Prior to each session, rats received either the orexin 1 receptor antagonist SB-334867 (SB) or vehicle systemically. SB treatment did not affect conditioned responses during the first conditioning session, measured as food cup behavior during the tone and latency to approach the food cup after the tone onset, compared to the vehicle group. During the second conditioning session, SB treatment attenuated learning. All groups that received SB, prior to either the first or second conditioning session, displayed significantly less food cup behavior and had longer latencies to approach the food cup after tone onset compared to the vehicle group. These findings suggest orexin signaling at the 1 receptor mediates the consolidation and recall of cue-food acquisition. In Experiment 2, another group of rats underwent tone-food conditioning sessions (drug free), followed by two extinction sessions under either SB or vehicle treatment. Similar to Experiment 1, SB did not affect conditioned responses during the first session. During the second extinction session, the group that received SB prior to the first extinction session, but vehicle prior to the second, expressed conditioned food cup responses longer after tone offset, when the pellets were previously delivered during conditioning, and maintained shorter latencies to approach the food cup compared to the other groups. The persistence of these conditioned behaviors indicates impairment in extinction consolidation due to SB treatment during the first extinction session. Together, these results demonstrate an important role for orexin signaling during Pavlovian appetitive conditioning and extinction.

Interacting Neural Processes of Feeding, Hyperactivity, Stress, Reward, and the Utility of the Activity-Based Anorexia Model of Anorexia Nervosa

Anorexia nervosa (AN) is a psychiatric illness with minimal effective treatments and a very high rate of mortality. Understanding the neurobiological underpinnings of the disease is imperative for improving outcomes and can be aided by the study of animal models. The activity-based anorexia rodent model (ABA) is the current best parallel for the study of AN. This review describes the basic neurobiology of feeding and hyperactivity seen in both ABA and AN, and compiles the research on the role that stress-response and reward pathways play in modulating the homeostatic drive to eat and to expend energy, which become dysfunctional in ABA and AN.

Orexin/Hypocretin-1 Receptor Antagonism Selectively Reduces Cue-Induced Feeding in Sated Rats and Recruits Medial Prefrontal Cortex and Thalamus

The orexin/hypocretin system is important for reward-seeking behaviors, however less is known about its function in non-homeostatic feeding. Environmental influences, particularly cues for food can stimulate feeding in the absence of hunger and lead to maladaptive overeating behavior. The key components of the neural network that mediates this cue-induced overeating in sated rats include lateral hypothalamus, amygdala, and medial prefrontal cortex (mPFC), yet the neuropharmacological mechanisms within this network remain unknown. The current study investigated a causal role for orexin in cue-driven feeding, and examined the neural substrates through which orexin mediates this effect. Systemic administration of the orexin-1 receptor (OX1R) antagonist SB-334867 had no effect on baseline eating, but significantly reduced cue-driven consumption in sated rats. Complementary neural analysis revealed that decreased cue-induced feeding under SB-334867 increased Fos expression in mPFC and paraventricular thalamus. These results demonstrate that OX1R signaling critically regulates cue-induced feeding, and suggest orexin is acting through prefrontal cortical and thalamic sites to drive eating in the absence of hunger. These findings inform our understanding of how food-associated cues override signals from the body to promote overeating, and indicate OX1R antagonism as a potential pharmacologic target for treatment of disordered eating in humans.

Renewal of conditioned responding to food cues in rats: Sex differences and relevance of estradiol

Cues associated with food can stimulate food anticipation, procurement, and consumption, independently of hunger. These and other behaviors driven by learned cues are persistent and can reappear after extinction, because the original learned associations continue to exist. Renewal, or reinstatement, of extinguished conditioned behavior may explain the inability to change maladaptive eating habits driven by food cues, similar to the mechanisms of drug use relapse. Here, we investigated sex differences in context-induced renewal of responding to food cues, and the role of estradiol in females in a Pavlovian conditioning preparation. We compared adult male and female rats because there is evidence for sex differences in learning and memory and in the control of feeding. Context-induced renewal involves conditioning and extinction in different contexts and the renewal of conditioned behavior is induced by return to the conditioning context ("ABA renewal"; experimental groups). Control groups remain in the same context during conditioning, extinction, and test. In Experiment 1, male and female rats were trained to associate a tone with food pellets during acquisition, and after extinction with tone only presentations, were tested for renewal of responding to the tone. Learning was assessed through the expression of the conditioned response, which included approach and activity directed at food receptacle (food cup behavior). Males and females learned the acquisition and extinction of tone-food associations similarly, but there were sex differences during renewal of the conditioned responses to the food cue. Males showed robust renewal of responding, while renewal in intact females was inconsistent. Males in the experimental group had significantly higher food cup behavior compared to males in the control group, while females in both groups showed similar levels of food cup behavior during the tone. In Experiment 2, we examined a potential role of estradiol in renewal, by comparing intact females with ovariectomized females with, and without, estradiol replacement. Rats in all groups acquired and extinguished tone-food associations similarly. During the test for renewal, the ovariectomized rats with estradiol replacement in the experimental group showed renewal of responding, evidenced by significantly higher food cup behavior compared to the control group. Intact and ovariectomized rats in the experimental groups had similar rates of food cup behavior as their corresponding control groups. These results provide novel evidence for sex differences and relevance of estradiol in renewal of responding to food cues and more broadly in contextual processing and appetitive associative learning, potentially relevant to maladaptive eating habits and eating disorders.

Overlap in Eating Disorders and Obesity in Adolescence

While eating disorders and obesity have traditionally been conceptualized as separate conditions, recent research suggests important overlap in several areas including etiology, comorbidity, risk factors, and prevention approaches. Examining the commonality among these conditions is particularly important as adolescents who present with both eating disorder symptomology and obesity demonstrate poorer outcomes within weight control treatments and are at greater risk for future development of full threshold eating disorders and additional weight gain. The purpose of this paper is to review the research examining the overlap in prevalence rates for eating disorders and obesity in adolescents, as well as shared etiology, risk factors, and psychiatric and medical comorbidities. Current preventive and treatment approaches also will be discussed, while highlighting the need for more integrated assessment, prevention, and treatment efforts that focus on maladaptive eating and activity patterns shared by both eating disorders and obesity.

High vitamin A intake during pregnancy modifies dopaminergic reward system and decreases preference for sucrose in Wistar rat offspring

High multivitamin (HV) content in gestational diets has long-term metabolic effects in rat offspring. These changes are associated with in utero modifications of gene expression in hypothalamic food intake regulation. However, the role of fat-soluble vitamins in mediating these effects has not been explored. Vitamin A is a plausible candidate due to its role in gene methylation. Vitamin A intake above requirements during pregnancy affects the development of neurocircuitries involved in food intake and reward regulation. Pregnant Wistar rats were fed AIN-93G diets with the following content: recommended multivitamins (1-fold multivitamins: RV), high vitamin A (10-fold vitamin A: HA) or HV with only recommended vitamin A (10-fold multivitamins, 1-fold vitamin A: HVRA). Body weight, food intake and preference, mRNA expression and DNA methylation of hippocampal dopamine-related genes were assessed in male offspring brains at different developmental windows: birth, weaning and 14weeks postweaning. HA offspring had changes in dopamine-related gene expression at all developmental windows and DNA hypermethylation in the dopamine receptor 2 promoter region compared to RV offspring. Furthermore, HA diet lowered sucrose preference but had no effect on body weight and expression of hypothalamic genes. In contrast, HVRA offspring showed only at adulthood changes in expression of hippocampal genes and a modest effect on hypothalamic genes. High vitamin A intake alone in gestational diets has long-lasting programming effects on the dopaminergic system that are further translated into decreased sucrose preference but not food intake.

Organization of connections between the amygdala, medial prefrontal cortex, and lateral hypothalamus: a single and double retrograde tracing study in rats

The amygdala and medial prefrontal cortex (mPFC) are highly interconnected telencephalic areas critical for cognitive processes, including associative learning and decision making. Both structures strongly innervate the lateral hypothalamus (LHA), an important component of the networks underlying the control of feeding and other motivated behaviors. The amygdala-prefrontal-lateral hypothalamic system is therefore well positioned to exert cognitive control over behavior. However, the organization of this system is not well defined, particularly the topography of specific circuitries between distinct cell groups within these complex, heterogeneous regions. This study used two retrograde tracers to map the connections from the amygdala (central and basolateral area nuclei) and mPFC to the LHA in detail, and to determine whether amygdalar pathways to the mPFC and to LHA originate from the same or different neurons. One tracer was placed into a distinct mPFC area (dorsal anterior cingulate, prelimbic, infralimbic, or rostromedial orbital), and the other into dorsal or ventral LHA. We report that the central nucleus and basolateral area of the amygdala send projections to distinct LHA regions, dorsal and ventral, respectively. The basolateral area, but not central nucleus, also sends substantial projections to the mPFC, topographically organized rostrocaudal to dorsoventral. The entire mPFC, in turn, projects to the LHA, providing a separate route for potential amygdalar influence following mPFC processing. Nearly all amygdalar projections to the mPFC and to the LHA originated from different neurons suggesting amygdala and amygdala-mPFC processing influence the LHA independently, and the balance of these parallel pathways ultimately controls motivated behaviors.

Ghrelin regulates phasic dopamine and nucleus accumbens signaling evoked by food-predictive stimuli

Environmental stimuli that signal food availability hold powerful sway over motivated behavior and promote feeding, in part, by activating the mesolimbic system. These food-predictive cues evoke brief (phasic) changes in nucleus accumbens (NAc) dopamine concentration and in the activity of individual NAc neurons. Phasic fluctuations in mesolimbic signaling have been directly linked to goal-directed behaviors, including behaviors elicited by food-predictive cues. Food-seeking behavior is also strongly influenced by physiological state (i.e., hunger vs. satiety). Ghrelin, a stomach hormone that crosses the blood-brain barrier, is linked to the perception of hunger and drives food intake, including intake potentiated by environmental cues. Notwithstanding, whether ghrelin regulates phasic mesolimbic signaling evoked by food-predictive stimuli is unknown. Here, rats underwent Pavlovian conditioning in which one cue predicted the delivery of rewarding food (CS+) and a second cue predicted nothing (CS-). After training, we measured the effect of ghrelin infused into the lateral ventricle (LV) on sub-second fluctuations in NAc dopamine using fast-scan cyclic voltammetry and individual NAc neuron activity using in vivo electrophysiology in separate groups of rats. LV ghrelin augmented both phasic dopamine and phasic increases in the activity of NAc neurons evoked by the CS+. Importantly, ghrelin did not affect the dopamine nor NAc neuron response to the CS-, suggesting that ghrelin selectively modulated mesolimbic signaling evoked by motivationally significant stimuli. These data demonstrate that ghrelin, a hunger signal linked to physiological state, can regulate cue-evoked mesolimbic signals that underlie food-directed behaviors. Cues that predict food availability powerfully regulate food-seeking behavior. Here we show that cue-evoked changes in both nucleus accumbens (NAc) dopamine (DA) and NAc cell activity are modulated by intra-cranial infusions of the stomach hormone ghrelin--a hormone known to act centrally to promote food intake. These data demonstrate that hormones associated with physiological state (i.e., hunger) can affect encoding of food-predictive cues in brain regions that drive food-motivated behavior.

Anorexia nervosa as a motivated behavior: Relevance of anxiety, stress, fear and learning

The high comorbidity between anorexia nervosa (AN) and anxiety disorders is well recognized. AN is a motivated behavioral disorder in which habit formation is likely to contribute to the persistence of abnormal eating and exercise behaviors. Secondary alterations in brain circuitry underlying the reward value of food and exercise, along with disturbances in neuroendocrine hunger and satiety signaling arising from starvation and excessive exercise, are likely contributors to the maintenance of anorectic behaviors in genetically vulnerable individuals. The potential role of fear conditioning in facilitating onset of AN, or of impaired fear extinction in contributing to the high relapse rates observed following weight restoration, is of interest. Evidence from animal models of anxiety and human laboratory studies indicate that low estrogen impairs fear extinction. Low estradiol levels in AN may therefore play a role in perpetuating fear of food and fat in recently weight restored patients. Translational models including the activity based anorexia (ABA) rodent model of AN, and neuroimaging studies of fear extinction and conditioning, could help clarify the underlying molecular mechanisms and neurocircuitry involved in food avoidance behaviors in AN. Moreover, the adaptation of novel treatment interventions with efficacy in anxiety disorders may contribute to the development of new treatments for this impairing disorder.

Stress exposure, food intake and emotional state

This manuscript summarizes the proceedings of the symposium entitled, "Stress, Palatable Food and Reward", that was chaired by Drs. Linda Rinaman and Yvonne Ulrich-Lai at the 2014 Neurobiology of Stress Workshop held in Cincinnati, OH. This symposium comprised research presentations by four neuroscientists whose work focuses on the biological bases for complex interactions among stress, food intake and emotion. First, Dr Ulrich-Lai describes her rodent research exploring mechanisms by which the rewarding properties of sweet palatable foods confer stress relief. Second, Dr Stephanie Fulton discusses her work in which excessive, long-term intake of dietary lipids, as well as their subsequent withdrawal, promotes stress-related outcomes in mice. Third, Dr Mark Wilson describes his group's research examining the effects of social hierarchy-related stress on food intake and diet choice in group-housed female rhesus macaques, and compared the data from monkeys to results obtained in analogous work using rodents. Finally, Dr Gorica Petrovich discusses her research program that is aimed at defining cortical-amygdalar-hypothalamic circuitry responsible for curbing food intake during emotional threat (i.e. fear anticipation) in rats. Their collective results reveal the complexity of physiological and behavioral interactions that link stress, food intake and emotional state, and suggest new avenues of research to probe the impact of genetic, metabolic, social, experiential and environmental factors on these interactions.

Regulation of the Motivation to Eat

Although food intake is necessary to provide energy for all bodily activities, considering food intake as a motivated behavior is complex. Rather than being a simple unconditioned reflex to energy need, eating is mediated by diverse factors. These include homeostatic signals such as those related to body fat stores, to food available and being eaten, and to circulating energy-rich compounds like glucose and fatty acids. Eating is also greatly influenced by non-homeostatic signals that convey information related to learning and experience, hedonics, stress, the social situation, opportunity, and many other factors. Recent developments identifying the intricate nature of the relationships between homeostatic and non-homeostatic influences significantly add to the complexity underlying the neural basis of the motivation to eat. The future of research in the field of food intake would seem to lie in the identification of the neural circuitry and interactions between homeostatic and non-homeostatic influences.

Predictive models of glucose control: roles for glucose-sensing neurones

The brain can be viewed as a sophisticated control module for stabilizing blood glucose. A review of classical behavioural evidence indicates that central circuits add predictive (feedforward/anticipatory) control to the reactive (feedback/compensatory) control by peripheral organs. The brain/cephalic control is constructed and engaged, via associative learning, by sensory cues predicting energy intake or expenditure (e.g. sight, smell, taste, sound). This allows rapidly measurable sensory information (rather than slowly generated internal feedback signals, e.g. digested nutrients) to control food selection, glucose supply for fight-or-flight responses or preparedness for digestion/absorption. Predictive control is therefore useful for preventing large glucose fluctuations. We review emerging roles in predictive control of two classes of widely projecting hypothalamic neurones, orexin/hypocretin (ORX) and melanin-concentrating hormone (MCH) cells. Evidence is cited that ORX neurones (i) are activated by sensory cues (e.g. taste, sound), (ii) drive hepatic production, and muscle uptake, of glucose, via sympathetic nerves, (iii) stimulate wakefulness and exploration via global brain projections and (iv) are glucose-inhibited. MCH neurones are (i) glucose-excited, (ii) innervate learning and reward centres to promote synaptic plasticity, learning and memory and (iii) are critical for learning associations useful for predictive control (e.g. using taste to predict nutrient value of food). This evidence is unified into a model for predictive glucose control. During associative learning, inputs from some glucose-excited neurones may promote connections between the 'fast' senses and reward circuits, constructing neural shortcuts for efficient action selection. In turn, glucose-inhibited neurones may engage locomotion/exploration and coordinate the required fuel supply. Feedback inhibition of the latter neurones by glucose would ensure that glucose fluxes they stimulate (from liver, into muscle) are balanced. Estimating nutrient challenges from indirect sensory cues may become more difficult when the cues become complex and variable (e.g. like human foods today). Consequent errors of predictive glucose control may contribute to obesity and diabetes.

Role of the lateral hypothalamus in submandibular salivary secretion during feeding in rats

To evaluate the role of the lateral hypothalamic area (LH) in the masticatory-salivary reflex, we investigated submandibular salivary secretion and the electromyographic (EMG) activity of the jaw-closer masseter muscle in sham-operated rats and rats with unilateral LH lesions. One week prior to surgery and recording, the rats were given daily experience of eating pellets; powder; or hard, medium or soft mash, all of which were composed of laboratory chow. Salivary secretion was induced during eating and grooming behavior. During eating, the powdered food induced the highest salivary flow rate, and the soft (wet) mash induced the lowest salivary flow rate. Conversely, the amount of food consumed (dry weight) was greatest when soft mash was provided and lowest when the powder or pellets (a dry diet) were provided. The EMG activity of the masseter muscle during eating was greatest during consumption of the pellets and weakest during consumption of the powder. LH lesions that were ipsilateral to the examined submandibular gland reduced salivary secretion to about 20-30% of the control value, whereas contralateral LH lesions reduced it to about 40-50% of the control value. Neither masseter muscle EMG activity nor food consumption was markedly affected by the presence of an LH lesion. These results suggest that the texture of food, especially its water content, affects the flow rate of saliva and that the LH is heavily involved in the masticatory-salivary reflex.

The G protein-coupled receptor GPR162 is widely distributed in the CNS and highly expressed in the hypothalamus and in hedonic feeding areas

The Rhodopsin family is a class of integral membrane proteins belonging to G protein-coupled receptors (GPCRs). To date, several orphan GPCRs are still uncharacterized and in this study we present an anatomical characterization of the GPR162 protein and an attempt to describe its functional role. Our results show that GPR162 is widely expressed in GABAergic as well as other neurons within the mouse hippocampus, whereas extensive expression is observed in areas related to energy homeostasis and hedonic feeding such as hypothalamus, amygdala and ventral tegmental area, regions known to be involved in the regulation of palatable food consumption.

Reward components of feeding behavior are preserved during mouse aging

Eating behavior depends on associations between the sensory and energetic properties of foods. Healthful balance of these factors is a challenge for industrialized societies that have an abundance of food, food choices and food-related cues. Here, we were interested in whether appetitive conditioning changes as a function of age. Operant and pavlovian conditioning experiments (rewarding stimulus was a palatable food) in male mice (aged 3, 6, and 15 months) showed that implicit (non-declarative) memory remains intact during aging. Two other essential components of eating behavior, motivation and hedonic preference for rewarding foods, were also found not to be altered in aging mice. Specifically, hedonic responding by satiated mice to isocaloric foods of differing sensory properties (sucrose, milk) was similar in all age groups; importantly, however, this paradigm disclosed that older animals adjust their energy intake according to energetic need. Based on the assumption that the mechanisms that control feeding are conserved across species, it would appear that overeating and obesity in humans reflects a mismatch between ancient physiological mechanisms and today's cue-laden environment. The implication of the present results showing that aging does not impair the ability to learn stimulus-food associations is that the risk of overeating in response to food cues is maintained through to old age.

Food consumption and weight gain after cessation of chronic amphetamine administration

Cessation of drug use often coincides with increased food consumption and weight gain in recovering addicts. However, it is not known whether this phenomenon (particularly the weight gain) is uniquely human, or whether it represents a consequence of drug cessation common across species. To address this issue, rats (n = 10/group) were given systemic injections of D-amphetamine (3 mg/kg) or an equal volume of saline vehicle for 9 consecutive days. Beginning 2 days after the final injection, rats were given free access to a highly palatable food mixture (consisting of sugar and butter) along with their standard chow diet, and food consumption and body weight were measured every 48 h for 30 days. Consistent with clinical observations, amphetamine-treated rats showed a greater increase in body weight over the course of the 30 days relative to vehicle-treated rats. Surprisingly, there was no difference in highly palatable food consumption between amphetamine- and vehicle-treated groups, but the amphetamine-treated group consumed significantly more standard chow than the control group. The finding that a history of chronic amphetamine exposure increases food consumption is consistent with previous work in humans showing that withdrawal from drugs of abuse is associated with overeating and weight gain. The current findings may reflect amphetamine-induced sensitization of mechanisms involved in reward motivation, suggesting that weight gain following drug cessation in humans could be due to similar mechanisms.

Human cognitive function and the obesogenic environment

Evidence is accumulating which suggests that, in addition to leading to unprecedented rates of obesity, the current food environment is contributing to the development of cognitive impairment and dementia. Recent experimental research indicates that many of the cognitive deficits associated with obesity involve fundamental inhibitory processes that have important roles in the control of food intake, implicating these cognitive impairments as a risk factor for weight gain. Here, we review experiments that link obesity with deficits in memory, attentional, and behavioral control and contemplate how these deficits may predispose individuals to overeat. Specifically, we discuss how deficits in inhibitory control may reduce one's ability to resist eating when confronted with the variety of foods and food cues that are ubiquitous in today's environment. Special attention is given to the importance of memory inhibition to the control of eating and appetitive behavior, and the role of the hippocampus in this process. We also discuss the potential etiology of both obesity and obesity-related cognitive impairment, highlighting non-human animal research which links both of these effects to the consumption of the modern "Western" diet that is high in saturated fats and simple carbohydrates. We conclude that part of what makes the current food environment "obesogenic" is the increased presence of food cues and the increased consumption of a diet which compromises our ability to resist those cues. Improving control over food-related cognitive processing may be useful not only for combating the obesity epidemic but also for minimizing the risk of serious cognitive disorder later in life.

Remembering to eat: hippocampal regulation of meal onset

A wide variety of species, including vertebrate and invertebrates, consume food in bouts (i.e., meals). Decades of research suggest that different mechanisms regulate meal initiation (when to start eating) versus meal termination (how much to eat in a meal, also known as satiety). There is a very limited understanding of the mechanisms that regulate meal onset and the duration of the postprandial intermeal interval (ppIMI). In the present review, we examine issues involved in measuring meal onset and some of the limited available evidence regarding how it is regulated. Then, we describe our recent work indicating that dorsal hippocampal neurons inhibit meal onset during the ppIMI and describe the processes that may be involved in this. We also synthesize recent evidence, including evidence from our laboratory, suggesting that overeating impairs hippocampal functioning and that impaired hippocampal functioning, in turn, contributes to the development and/or maintenance of diet-induced obesity. Finally, we identify critical questions and challenges for future research investigating neural controls of meal onset.

Feelings about food: the ventral tegmental area in food reward and emotional eating

Overconsumption of high caloric food plays an important role in the etiology of obesity. Several factors drive such hedonic feeding. High caloric food is often palatable. In addition, when an individual is sated, stress and food-related cues can serve as potent feeding triggers. A better understanding of the neurobiological underpinnings of food palatability and environmentally triggered overconsumption would aid the development of new treatment strategies. In the current review we address the pivotal role of the mesolimbic dopamine reward system in the drive towards high caloric palatable food and its relation to stress- and cue-induced feeding. We also discuss how this system may be affected by both established and potential anti-obesity drug targets.