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

Estrous cycle state-dependent renewal of appetitive behavior recruits unique patterns of Arc mRNA in female rats

Introduction

Renewal is a behavioral phenomenon wherein extinction learning fails to generalize between different contextual environments, thereby representing a significant challenge to extinction-based rehabilitative therapies. Previously, we have shown that renewal of extinguished appetitive behavior differs across the estrous cycle of the female rat. In this experiment that effect is replicated and extended upon to understand how the estrous cycle may modulate contextual representation at the neuronal population level to drive renewal.

Methods

Estrous cycle stage [i.e., proestrus (P, high hormone) or metestrus/diestrus (M/D, low hormone)] was considered during two important learning and behavioral expression windows: at extinction training and during long-term memory (LTM)/renewal testing. Cellular compartment analysis of temporal activity using fluorescence in situ hybridization (catFISH) for Arc mRNA was conducted after the distinct context-stimulus exposures.

Results

Rats in P during context-dependent extinction training but in a different stage of the estrous cycle during LTM and renewal testing (P-different) were shown to exhibit more renewal of conditioned foodcup (but not conditioned orienting) behavior compared to rats in other estrous cycle groups. Importantly, we discovered this depends on the order of tests. P-different rats showed differential Arc mRNA expression in regions of the prefrontal cortex (PFC), amygdala, and hippocampus (HPC). For each case P-different rats had more co-expression (i.e., expression of both nuclear and cytoplasmic) of Arc mRNA compared to other groups; specific to the dorsal HPC, P-different rats also had a more robust Arc mRNA response to the extinction context exposure.

Conclusion

These data suggest female rats show estrous cycle state-dependent renewal of appetitive behavior, and differences in context and conditioned stimulus representation at the neuronal level may drive this effect.

ABA, AAB and ABC renewal with Pavlovian Conditioning of Tentacle Lowering procedure in the snail Cornu aspersum

This study assesses the recovery of the conditioned response (CR) due to a contextual change (renewal effect) in the Cornu aspersum, using the appetitive Pavlovian Conditioning of Tentacle Lowering procedure. Snails experienced an odorous conditioned stimulus (CS) paired with food (conditioning), followed by the exposition to the CS without any consequence (extinction). Then, they were exposed to the CS in a different context from the extinction one (renewal test). The contexts were three types of illumination. In Experiment 1a, the conditioning was performed in context A, the extinction was conducted in context B and the renewal test was performed in context A. For Experiment 1b, the conditioning and extinction were conducted in context A and renewal was performed in context B. In Experiment 1c, three dissimilar contexts were used for each experimental phase: context A for the conditioning, context B for the extinction and context C for the renewal. In Experiment 2, the renewal magnitude was compared among the three paradigms (ABA, AAB and ABC). Experiments 1a, 1b and 1c showed a recovery of the CR when subjects experienced a contextual change and Experiment 2 showed equivalent levels of renewal in the three paradigms. Learning processes and theories involved are discussed.

Learning processes in relapse to alcohol use: lessons from animal models

RATIONALE

Alcohol use is reliably preceded by discrete and contextual stimuli which, through diverse learning processes, acquire the capacity to promote alcohol use and relapse to alcohol use.

OBJECTIVE

We review contemporary extinction, renewal, reinstatement, occasion setting, and sex differences research within a conditioning framework of relapse to alcohol use to inform the development of behavioural and pharmacological therapies.

KEY FINDINGS

Diverse learning processes and corresponding neurobiological substrates contribute to relapse to alcohol use. Results from animal models indicate that cortical, thalamic, accumbal, hypothalamic, mesolimbic, glutamatergic, opioidergic, and dopaminergic circuitries contribute to alcohol relapse through separable learning processes. Behavioural therapies could be improved by increasing the endurance and generalizability of extinction learning and should incorporate whether discrete cues and contexts influence behaviour through direct excitatory conditioning or occasion setting mechanisms. The types of learning processes that most effectively influence responding for alcohol differ in female and male rats.

CONCLUSION

Sophisticated conditioning experiments suggest that diverse learning processes are mediated by distinct neural circuits and contribute to relapse to alcohol use. These experiments also suggest that gender-specific behavioural and pharmacological interventions are a way towards efficacious therapies to prevent relapse to alcohol use.

Optogenetic stimulation of infralimbic cortex projections to the paraventricular thalamus attenuates context-induced renewal

Contexts associated with prior reinforcement can renew extinguished conditioned responding. The prelimbic (PL) and infralimbic (IL) cortices are thought to mediate the expression and suppression of conditioned responding, respectively. Evidence suggests that PL inputs to the paraventricular nucleus of the thalamus (PVT) drive the expression of cue-induced reinstatement of drug seeking and that IL inputs to the PVT mediate fear extinction retrieval. However, the role of these projections in renewal of appetitive Pavlovian conditioned responding is unknown. We trained male and female Long-Evans rats to associate a conditioned stimulus (CS; 10 s white noise) with delivery of a 10% sucrose unconditioned stimulus (US; .2 ml/CS) to a fluid port in a distinct context (Context A). We then extinguished responding by presenting the CS without the US in a different context (Context B). At test, rats were returned to Context A, and optogenetic stimulation was delivered to either the IL-to-PVT or PL-to-PVT pathway during CS presentations. Optically stimulating the IL-to-PVT, but not the PL-to-PVT pathway, attenuated ABA renewal of CS port entries, and this effect was similar in males and females. Further, rats self-administered optical stimulation of the IL-to-PVT but not the PL-to-PVT pathway suggesting that activation of the IL-to-PVT pathway is reinforcing. The effectiveness of optical stimulation parameters to activate neurons in the IL, PL and PVT was confirmed using Fos immunohistochemistry. These findings provide evidence for novel neural mechanisms in renewal of responding to a sucrose-predictive CS, as well as more generally in contextual processing and appetitive associative learning.

Estradiol and progesterone in female reward-learning, addiction, and therapeutic interventions

Sex steroid hormones like estradiol (E2) and progesterone (P4) guide the sexual organization and activation of the developing brain and control female reproductive behavior throughout the lifecycle; importantly, these hormones modulate functional activity of not just the endocrine system, but most of the nervous system including the brain reward system. The effects of E2 and P4 can be seen in the processing of and memory for rewarding stimuli and in the development of compulsive reward-seeking behaviors like those seen in substance use disorders. Women are at increased risk of developing substance use disorders; however, the origins of this sex difference are not well understood and therapeutic interventions targeting ovarian hormones have produced conflicting results. This article reviews the contribution of the E2 and P4 in females to functional modulation of the brain reward system, their possible roles in origins of addiction vulnerability, and the development and treatment of compulsive reward-seeking behaviors.

Stress and sex-dependent effects on conditioned inhibition of fear

Anxiety and stress-related disorders are highly prevalent and are characterized by excessive fear to threatening and nonthreatening stimuli. Moreover, there is a large sex bias in vulnerability to anxiety and stress-related disorders-women make up a disproportionately larger number of affected individuals compared with men. Growing evidence suggests that an impaired ability to suppress fear in the presence of safety signals may in part contribute to the development and maintenance of many anxiety and stress-related disorders. However, the sex-dependent impact of stress on conditioned inhibition of fear remains unclear. The present study investigated sex differences in the acquisition and recall of conditioned inhibition in male and female mice with a focus on understanding how stress impacts fear suppression. In these experiments, the training context served as the "fear" cue and an explicit tone served as the "safety" cue. Here, we found a possible sex difference in the training requirements for safety learning, although this effect was not consistent across experiments. Reductions in freezing to the safety cue in female mice were also not due to alternative fear behavior expression such as darting. Next, using footshock as a stressor, we found that males were impaired in conditioned inhibition of freezing when the stress was experienced before, but not after, conditioned inhibition training. Females were unaffected by footshock stress when it was administered at either time. Extended conditioned inhibition training in males eliminated the deficit produced by footshock stress. Finally, exposing male and female mice to swim stress impaired safety learning in male mice only. Thus, we found sex × stress interactions in the learning of conditioned inhibition and sex-dependent effects of stress modality. The present study adds to the growing literature on sex differences in safety learning, which will be critical for developing sex-specific therapies for a variety of fear-related disorders that involve excessive fear and/or impaired fear inhibition.

Sex differences in the immediate extinction deficit and renewal of extinguished fear in rats

Extinction learning is central to exposure-based behavioral therapies for reducing fear and anxiety in humans. However, patients with fear and anxiety disorders are often resistant to extinction. Moreover, trauma and stress-related disorders are highly prone to relapse and are twice as likely to occur in females compared to males, suggesting that females may be more susceptible to extinction deficits and fear relapse phenomena. In this report, we tested this hypothesis by examining sex differences in a stress-induced extinction learning impairment, the immediate extinction deficit (IED), and renewal, a common form of fear relapse. In contrast to our hypothesis, there were no sex differences in the magnitude of the immediate extinction deficit in two different rat strains (Long-Evans and Wistar). However, we did observe a sex difference in the renewal of fear when the extinguished conditioned stimulus was presented outside the extinction context. Male Wistar rats exhibited significantly greater renewal than female rats, a sex difference that has previously been reported after appetitive extinction. Collectively, these data reveal that stress-induced extinction impairments are similar in male and female rats, though the context-dependence of extinction is more pronounced in males.

The role of context on responding to an alcohol-predictive cue in female and male rats

In male rats, physical contexts that are associated with alcohol can amplify the response to a discrete, alcohol-predictive conditioned stimulus (CS), and amplify prime-induced reinstatement. Here, we examined these effects as a function of biological sex. Male and female Long-Evans rats were acclimated to drinking ethanol (15% v/v) in their home cages. Next, they were trained to associate an auditory conditioned stimulus (CS) (10 s; white noise or clicker; 15 trials per session) with ethanol delivery (0.2 mL per CS; 3.0 mL per session) into a fluid port for oral intake. Training occurred in a distinctive context containing specific visual, olfactory, and tactile stimuli. During alternating sessions, rats were exposed to a second context wherein they did not receive ethanol. At test, CS trials occurred in both contexts without ethanol delivery. Rats then underwent extinction using repeated unreinforced presentations of the CS in both contexts. An alcohol-primed reinstatement test was then conducted, in which 0.2 mL of ethanol was presented at the start of the session and during the first CS trial, after which no ethanol was delivered for the remainder of the session. At both test and reinstatement, male rats made significantly more CS port-entries in the context associated with alcohol delivery than in the context in which alcohol was never experienced. Unlike males, female rats made a similar number of CS port-entries at the test in both the alcohol context and the neutral context. The reinstatement observed in female rats was also not affected by context. These findings suggest that the capacity of an alcohol-associated context to modulate responding to a discrete, alcohol-predictive cue is less pronounced in female than male rats.

A review of sex differences in the mechanisms and drivers of overeating

Disordered eating is often associated with marked psychological and emotional distress, and severe adverse impact on quality of life. Several factors can influence eating behavior and drive food consumption in excess of energy requirements for homeostasis. It is well established that stress and negative affect contribute to the aetiology of eating disorders and weight gain, and there is substantial evidence suggesting sex differences in sub-clinical and clinical types of overeating. This review will examine how negative affect and stress shape eating behaviors, and how the relationship between the physiological, endocrine, and neural responses to stress and eating behaviors differs between men and women. We will examine several drivers of overeating and explore possible mechanisms underlying sex differences in eating behavior.

Contextual Fear Memory Maintenance Changes Expression of pMAPK, BDNF and IBA-1 in the Pre-limbic Cortex in a Layer-Specific Manner

Post-traumatic stress disorder (PTSD) is a debilitating and chronic fear-based disorder. Pavlovian fear conditioning protocols have long been utilised to manipulate and study these fear-based disorders. Contextual fear conditioning (CFC) is a particular Pavlovian conditioning procedure that pairs fear with a particular context. Studies on the neural mechanisms underlying the development of contextual fear memories have identified the medial prefrontal cortex (mPFC), or more specifically, the pre-limbic cortex (PL) of the mPFC as essential for the expression of contextual fear. Despite this, little research has explored the role of the PL in contextual fear memory maintenance or examined the role of neuronal mitogen-activated protein kinase (pMAPK; ERK 1/2), brain-derived neurotrophic factor (BDNF), and IBA-1 in microglia in the PL as a function of Pavlovian fear conditioning. The current study was designed to evaluate how the maintenance of two different long-term contextual fear memories leads to changes in the number of immune-positive cells for two well-known markers of neural activity (phosphorylation of MAPK and BDNF) and microglia (IBA-1). Therefore, the current experiment is designed to assess the number of immune-positive pMAPK and BDNF cells, microglial number, and morphology in the PL following CFC. Specifically, 2 weeks following conditioning, pMAPK, BDNF, and microglia number and morphology were evaluated using well-validated antibodies and immunohistochemistry (n = 12 rats per group). A standard CFC protocol applied to rats led to increases in pMAPK, BDNF expression and microglia number as compared to control conditions. Rats in the unpaired fear conditioning (UFC) procedure, despite having equivalent levels of fear to context, did not have any change in pMAPK, BDNF expression and microglia number in the PL compared to the control conditions. These data suggest that alterations in the expression of pMAPK, BDNF, and microglia in the PL can occur for up to 2 weeks following CFC. Together the data suggest that MAPK, BDNF, and microglia within the PL of the mPFC may play a role in contextual fear memory maintenance.

Influence of ovarian hormones on value-based decision-making systems: Contribution to sexual dimorphisms in mental disorders

Women and men exhibit differences in behavior when making value-based decisions. Various hypotheses have been proposed to explain these findings, stressing differences in functional lateralization of the brain, functional activation, neurotransmitter involvement and more recently, sex hormones. While a significant interaction of neurotransmitter systems and sex hormones has been shown for both sexes, decision-making in women might be particularly affected by variations of ovarian hormones. In this review we have gathered information from animal and human studies on how ovarian hormones affect decision-making processes in females by interacting with neurotransmitter systems at functionally relevant brain locations and thus modify the computation of decision aspects. We also review previous findings on impaired decision-making in animals and clinical populations with substance use disorder and depression, emphasizing how little we know about the role of ovarian hormones in aberrant decision-making.

Dorsal hippocampus plays a causal role in context-induced reinstatement of alcohol-seeking in rats

Drug addiction is a chronic mental disorder characterized by frequent relapses. Contextual cues associated with drug use to play a critical causal role in drug-seeking behavior. The hippocampus has been implicated in encoding drug associative memories. Here we examine whether the dorsal hippocampus mediates context-induced reinstatement of alcohol-seeking. Male Long-Evans rats were trained to self-administer alcohol in Context A. Alcohol self-administration was extinguished in a distinct context (Context B). On the test day, animals were re-exposed to the alcohol Context A or the extinction Context B. Next, to assess a causal role for the dorsal hippocampus in context-induced alcohol-seeking, on the test day, we injected cobalt chloride (CoCl2; a nonselective synapse inhibitor) or vehicle into the dorsal hippocampus, and 15 min later, rats were tested by re-exposing them to the drug-associated context. The re-exposure to the alcohol-associated Context A reinstated alcohol seeking and increased Fos-positive cells in the dorsal hippocampus neurons (CA1, CA3, and Dentate Gyrus). Pharmacological inactivation with cobalt chloride of the dorsal hippocampus attenuated the reinstatement of alcohol-seeking. Our data suggest that the dorsal hippocampus may be involved in context-induced alcohol-seeking behavior.

The effects of novelty on food consumption in male and female rats

Novelty powerfully impacts feeding behavior and can override homeostatic and hedonic drives, because consumption of a new food could lead to illness or even death. New foods and new feeding environments can decrease or inhibit feeding, but how the two interact and whether there are sex differences has not been determined. The current study examined consumption of a palatable (high sucrose) novel food compared to a familiar food in adult male and female rats that were fed in a familiar or a novel environment. Rats were deprived of food for 20 h prior to each of eight tests. During the first test, male and female rats that were tested in a familiar environment showed robust taste neophobia, as they mainly consumed familiar food. Across repeated tests, these rats increased consumption of the novel food, which indicated that they habituated to the novel taste and developed a preference for the novel food. In contrast, all rats tested in a novel feeding environment ate very little of both foods during the initial test. Across repeated tests, male rats habituated to the novel food faster than females and by the fourth test ate more of the novel than familiar food. In contrast, females showed sustained, suppressed consumption across habituation tests. These results demonstrated robust differences in feeding behavior depending whether rats were fed at home or in a novel feeding environment, and robust sex differences in habituation to eating in a new environment. These findings suggest that novel context has a greater impact on female consumption than male consumption. This difference may be relevant to sex differences in avoidant behaviors in maladaptive circumstances and the development of psychopathology. Therefore, the behavioral profile outlined in this study for consumption under novelty provides an important starting point for investigation of the underlying neural substrates of novelty processing.

Renewal of conditioned tentacle lowering by circadian contextual cues in snails Cornu aspersum

Previous experiments using tentacle lowering conditioning in terrestrial snails Cornu aspersum have shown extinction and recovery of the conditioned response (CR) as a consequence of both inserting a delay between the extinction and test (spontaneous recovery) and of re-exposing the animal to the unconditioned stimulus after extinction (reinstatement). Two experiments that examined recovery of the CR due to a change in context (renewal effect) were carried out to continue this line of research. In Experiment 1, subjects received conditioning with an odour (CS) followed by extinction in the presence of another odour (CS + C), before being exposed to the original one (CS). In Experiment 2, conditioning and extinction of an odour CS took place in the presence of different circadian contextual cues (hour of the day and presence of light). The results showed that a return to the original context of conditioned training, after the extinction in a different context, either defined by an odour (Experiment 1) or by circadian cues (Experiment 2), produce a recovery of the CR compared to suitable control groups. These results can be interpreted as an instance of ABA renewal effect and they provide information about psychological mechanisms involved in extinction processes.

Comparing ABA, AAB, and ABC Renewal of Appetitive Pavlovian Conditioned Responding in Alcohol- and Sucrose-Trained Male Rats

Conditioned responding can be renewed by re-exposure to the conditioning context following extinction in a different context (ABA renewal) or by removal from the extinction context (AAB or ABC renewal). ABA renewal is robust in Pavlovian and operant conditioning paradigms. However, fewer studies have investigated AAB and ABC renewal of appetitive conditioning, and those that did predominantly used operant conditioning tasks. Renewal has theoretical relevance for extinction and for exposure-based treatments for substance use disorders that aim to extinguish reactivity to drug-predictive cues. We therefore investigated ABA, AAB, and ABC renewal of Pavlovian conditioned responding to cues that predicted either alcohol or sucrose. Male, Long-Evans rats (Charles River) were exposed to either 15% ethanol (Study 1: “alcohol”) or 10% sucrose (Study 2: “sucrose”) in their home cages. Next, they were trained to discriminate between two auditory stimuli (white noise and clicker; 10 s) in conditioning chambers equipped with distinct olfactory, visual, and tactile contextual stimuli (context A). One conditioned stimulus (CS+) was paired with fluid delivery (0.2 ml/CS+; 3.2 ml/session; alcohol or sucrose in separate experiments), and the second CS (CS−) was not. In all sessions (conditioning, extinction, and test), each CS was presented 16 times/session on a variable-time 67-s schedule, and entries into the fluid port were recorded. CS+ port entries were then extinguished by withholding fluid delivery either in context A or in a second, different context (context B). Next, we assessed ABA, AAB, and ABC renewal in the absence of fluid delivery. During extinction, CS+ port entries were initially elevated in context A relative to context B. ABA renewal of CS+ port entries occurred in both alcohol- and sucrose-trained rats. ABC renewal approached statistical significance when data from both experiments were combined. No AAB renewal was observed, and, in fact, alcohol-trained rats showed AAB suppression. These results corroborate the reliability of ABA renewal and suggest that ABC renewal is a modest effect that may require greater statistical power to detect. From a treatment perspective, the lack of AAB renewal suggests that exposure-based treatments for substance use disorders might benefit from implementation in real-world, drug-use contexts.

Considering Drug-Associated Contexts in Substance Use Disorders and Treatment Development

Environmental contexts that are reliably associated with the use of pharmacologically active substances are hypothesized to contribute to substance use disorders. In this review, we provide an updated summary of parallel preclinical and human studies that support this hypothesis. Research conducted in rats shows that environmental contexts that are reliably paired with drug use can renew extinguished drug-seeking behavior and amplify responding elicited by discrete, drug-predictive cues. Akin to drug-associated contexts, interoceptive drug stimuli produced by the psychopharmacological effects of drugs can also influence learning and memory processes that play a role in substance use disorders. Findings from human laboratory studies show that drug-associated contexts, including social stimuli, can have profound effects on cue reactivity, drug use, and drug-related cognitive expectancies. This translationally relevant research supports the idea that treatments for substance use disorders could be improved by considering drug-associated contexts as a factor in treatment interventions. We conclude this review with ideas for how to integrate drug-associated contexts into treatment-oriented research based on 4 approaches: pharmacology, brain stimulation, mindfulness-based relapse prevention, and cognitive behavioral group therapy. Throughout, we focus on alcohol- and tobacco-related research, which are two of the most prevalent and commonly misused drugs worldwide for which there are known treatments.

Cue-alcohol associative learning in female rats

The ability of environmental cues to trigger alcohol-seeking behaviors is believed to facilitate problematic alcohol use. We previously showed that the development of this cue-evoked alcohol approach reflects cue-alcohol learning and memory in the adult male rat; however, we do not know whether the same is true for similarly aged female rats. Consequently, adult Long-Evans female rats were allowed to drink unsweetened alcohol in the home cage (Monday, Wednesday, Friday; 24-h two-bottle choice; 5 weeks) and were subsequently split into two experimental groups: Paired and Unpaired. Groups were matched for ingested doses and alcohol bottle preference across the pre-conditioning home cage period. Both groups were trained in conditioning chambers using a Pavlovian procedure. For the Paired group, the chamber houselight was illuminated to signal access to an alcohol sipper. Houselight onset was yoked for the Unpaired group, but access to the alcohol sipper was scheduled to occur only during the intervening periods (in the absence of light). We found that in the Paired, but not Unpaired group, an alcohol approach reaction was conditioned to houselight illumination, and the level of cue-conditioned reactivity predicted drinking behavior within trials. Groups experienced equivalently low but non-negligible blood alcohol concentrations over the course of conditioning sessions. We conclude that cue-triggered alcohol-seeking behavior in adult female rats reflects associative learning about the relationship between alcohol availability and houselight illumination.

Updating gender differences in the control of homeostatic and hedonic food intake: Implications for binge eating disorder

In an obesity pandemic context, eating disorders (ED) have arisen as serious illnesses associated with severe disturbances and has a clear gender dependent bias. In this manuscript, we provide an overview of the oestrogen role in the homeostatic and hedonic control of food intake. We draw attention to the role of oestrogens in the various reward processes and their possible implication in the development of ED, a condition much more common in women. In here, we have summarized the most relevant studies conducted in animal models over the last few years. In particular, we want to emphasize on the importance of continuing thorough investigations in female animal models. We believe that understanding the molecular mechanisms that regulate gender differences in food intake may provide new potential targets for ED treatment.

Examining the Impact of Estrogen on Binge Feeding, Food-Motivated Behavior, and Body Weight in Female Rats

OBJECTIVE

Binge-eating disorder is associated with diminished self-control, emotional distress, and obesity. In this context, women are nearly twice as likely to develop binge-eating disorder and depression relative to men. Here, the physiological, psychological, and endocrine parameters were characterized in female rats subjected to a binge-eating protocol.

METHODS

Nonrestricted female Long Evans rats (n = 8/group) received 2-hour restricted access to a high-fat diet (HFD) (4.54 kcal/g) every day or every third day. The progression of estrous cycling, the functional relevance of estrogen signaling for binge feeding, and binge-induced changes in food motivation were measured.

RESULTS

Female rats developed a binge pattern of feeding that included alternation between caloric overconsumption and compensatory voluntary restriction without impacting estrous cycling. Notably, rats that received daily HFD exposure progressively decreased binge meals. Estrogen replacement in normal cycling or ovariectomized rats mimicked the reduction in body weight in female rats that received daily HFD access. Operant responding was unaffected by binge feeding; however, estrogen augmented operant performance in HFD-exposed rats.

CONCLUSIONS

Collectively, these data suggest that estrogen protects against binge-induced increases in body weight gain without affecting food motivation in female rats.

The relationship between neuroticism and appetitive conditioning

Appetitive conditioning is considered a central mechanism for the vulnerability to psychiatric disorders. However, the investigation of individual differences that are related to altered appetitive learning has been almost neglected so far. The aim of this study was to investigate the link between neuroticism and appetitive conditioning processes. 79 subjects participated in a differential conditioning procedure in which a conditioned stimulus (CS+) was paired with a reward (money) after a fast behavioral response, while a second conditioned stimulus (CS-) was never followed by a reward, irrespective of the behavioral response. As a main result, neuroticism correlated negatively with the underlying neural processes of appetitive conditioning in females, but not in males. In detail, higher levels of neuroticism were associated with decreased neural responses in the left (p = .001) and right amygdala (p = .011), left (p = .063) and right (p = .019) nucleus accumbens, and left (p = .002) and right (p = .021) orbitofrontal cortex (all results are family-wise-error-corrected). The present results support previous findings, which also showed an inverse sex-specific effect in the context of neuroticism and emotional processing in females. In addition, the findings suggest that neuroticism is not solely linked to increased amygdala sensitivity during the processing of negative stimuli but also to decreased neural responses when processing rewarding stimuli. Possible explanations for the sex differences and implications are discussed.

Distinct recruitment of the hippocampal, thalamic, and amygdalar neurons projecting to the prelimbic cortex in male and female rats during context-mediated renewal of responding to food cues

Persistent responding to food cues may underlie the difficulty to resist palatable foods and to maintain healthy eating habits. Renewal of responding after extinction is a model of persistent food seeking that can be used to study the underlying neural mechanisms. In context-mediated renewal, a return to the context in which the initial cue-food learning occurred induces robust responding to the cues that were extinguished elsewhere. Previous work found sex differences in context-mediated renewal and in the recruitment of the ventromedial prefrontal cortex (vmPFC) during that behavior. Males exhibited renewal of responding to food cues and had higher Fos induction in the prelimbic area (PL) of the vmPFC, while females failed to exhibit renewal of responding and had lower Fos induction in the PL. The main aim of the current study was to determine key components of the PL circuitry mediating renewal. The focus was on inputs from three areas important in appetitive associative learning and contextual processing: the amygdala, ventral hippocampal formation, and the paraventricular nucleus of the thalamus. The goal was to determine whether neurons from these areas that send direct projections to the PL (identified with a retrograde tracer) are selectively activated (Fos induction) during renewal and whether they are differently recruited in males and females. The Fos induction patterns demonstrated that the PL-projecting neurons in each of these areas were recruited in a sex-specific way that corresponded to the behavioral differences between males and females. These pathways were selectively activated in the male experimental group-the only group that showed renewal behavior. The findings suggest the pathways from the ventral hippocampal formation, paraventricular nucleus of the thalamus, and basolateral amygdala to the PL mediate renewal in males. The lack of recruitment in females suggests that under activation of these pathways may underlie their lack of renewal.

Ventromedial prefrontal cortex mediates sex differences in persistent cognitive drive for food

Contemporary environments are saturated with food cues that stimulate appetites in the absence of hunger, which leads to maladaptive eating. These settings can induce persistent drive to eat, as learned behaviors can reappear after extinction. Behavioral paradigms of responding renewal provide a valuable framework to study how food cues contribute to the inability to resist palatable foods and change maladaptive eating habits. Using a rat model for this persistent food motivation, we determined sex differences in the causal function for the ventromedial prefrontal cortex (vmPFC) during context-mediated renewal of responding to food cues. Previously, we found behavioral sex differences (only males exhibited renewal) and differential recruitment within the vmPFC (increased Fos induction in males but decreased in females). Here, we used DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) to silence vmPFC neurons in males and to stimulate vmPFC neurons in females specifically during renewal. Silencing vmPFC neurons in males disrupted renewal of responding to a food cue, while stimulating vmPFC neurons in females induced this behavior. These findings demonstrate sex differences in the vmPFC function in a model of food seeking relevant to environmentally driven appetites contributing to obesity and eating disorders.

Considering sex differences in the cognitive controls of feeding

Women are disproportionately affected by obesity, and obesity increases women's risk of developing dementia more so than men. Remarkably little is known about how females make decisions about when and how much to eat. Research in animal models with males supports a framework in which previous experiences with external food cues and internal physiological energy states, and the ability to retrieve memories of the consequences of eating, determines subsequent food intake. Additional evidence indicates that consumption of a high-fat, high-sugar diet interferes with hippocampal-dependent mnemonic processes that operate to suppress eating, such as in situations of satiety. Recent findings also indicate that weakening this form of hippocampal-dependent inhibitory control may also extend to other forms of learning and memory, perpetuating a vicious cycle of increased Western diet intake, hippocampal dysfunction, and further impairments in the suppression of appetitive behavior that may ultimately disrupt other types of memorial interference resolution. How these basic learning and memory processes operate in females to guide food intake has received little attention. Ovarian hormones appear to protect females from obesity and metabolic impairments, as well as modulate learning and memory processes, but little is known about how these hormones modulate learned appetitive behavior. Even less is known about how a sex-specific environmental factor - widespread hormonal contraceptive use - affects associative learning and the regulation of food intake. Extending learned models of food intake to females will require considerably investigation at many levels (e.g., reproductive status, hormonal compound, parity). This work could yield critical insights into the etiology of obesity, and its concomitant cognitive impairment, for both sexes.

Estradiol is a critical regulator of food-reward behavior

Food intake is reduced by estrogenic hormones, levels of which vary throughout life and fluctuate throughout the ovarian cycle in females. However, estrogens have also been shown to increase reward derived from drugs of abuse, where motivational properties of drugs and progression to addiction are potentiated by estrogens. Whether reward derived from food, and specifically motivational properties of food, are altered by estrogens remains unknown. Here we investigated the effect of the estrous cycle on food reward behavior and show estrous cycle dictated variability in food motivation, measured by progressive ratio operant conditioning, in female rats. Reward behavior was lowest on days associated with high estrogen signaling. We therefore also examined the actions of subcutaneously administered β-estradiol on food reward and found that β-estradiol reduced food reward behavior. The ventral tegmental area (VTA) is a crucial node of the neurocircuitry underlying motivated behavior and estrogen receptors are expressed in this nucleus. Thus, we examined whether the effects of estrogens on reward were exerted directly at the level of the VTA. Intra-VTA microinjection of β-estradiol led to a significant reduction in food-motivated behavior. Interestingly, this effect was not accompanied by a reduction in chow intake or body weight, nor did it alter locomotor activity. Importantly, removal of the ovaries produced a potent and lasting elevation in food reward and food-seeking behavior, suggesting that ovarian sex steroids are critical for maintenance of normal food reward behavior. These data reveal a novel role for estrogens in the control of food reward behavior.​.

Sex specific recruitment of a medial prefrontal cortex-hippocampal-thalamic system during context-dependent renewal of responding to food cues in rats

Renewal, or reinstatement, of responding to food cues after extinction may explain the inability to resist palatable foods and change maladaptive eating habits. Previously, we found sex differences in context-dependent renewal of extinguished Pavlovian conditioned responding to food cues. Context-induced renewal involves cue-food conditioning and extinction in different contexts and the renewal of conditioned behavior is induced by return to the conditioning context (ABA renewal). Male rats showed renewal of responding while females did not. In the current study we sought to identify recruitment of key neural systems underlying context-mediated renewal and sex differences. We examined Fos induction within the ventromedial prefrontal cortex (vmPFC), hippocampal formation, thalamus and amygdala in male and female rats during the test for renewal. We found sex differences in vmPFC recruitment during renewal. Male rats in the experimental condition showed renewal of responding and had more Fos induction within the infralimbic and prelimbic vmPFC areas compared to controls that remained in the same context throughout training and testing. Females in the experimental condition did not show renewal or an increase in Fos induction. Additionally, Fos expression differed between experimental and control groups and between the sexes in the hippocampal formation, thalamus and amygdala. Within the ventral subiculum, the experimental groups of both sexes had more Fos compared to control groups. Within the dorsal CA1 and the anterior region of the paraventricular nucleus of the thalamus, in males, the experimental group had higher Fos induction, while both females groups had similar number of Fos-positive neurons. Within the capsular part of the central amygdalar nucleus, females in the experimental group had higher Fos induction, while males groups had similar amounts. The differential recruitment corresponded to the behavioral differences between males and females and suggests the medial prefrontal cortex-hippocampal-thalamic system is a critical site of sex differences during renewal of appetitive Pavlovian responding to food cues. These findings provide evidence for novel neural mechanisms underlying sex differences in food motivation and contextual processing in associative learning and memory. The results should also inform future molecular and translational work investigating sex differences and maladaptive eating habits.

Gender-related Differences in Food Craving and Obesity

Food craving is often defined as a strong desire to eat. Much work has shown that it consistently and prospectively predicts eating and weight-related outcomes, contributing to the growing obesity epidemic. Although there are clear gender differences in the prevalence and health consequences of obesity, relatively little recent work has investigated gender differences in craving, or any sex-hormone-based differences as they relate to phases of the menstrual cycle. Here, we propose that gender-related differences in food craving contribute to gender-related differences in obesity. Drawing on findings in the addiction literature, we highlight ways to incorporate gender-based differences in food craving into treatment approaches, potentially improving the efficacy of obesity and weight loss treatment. Overall, this review aims to emphasize the importance of investigating gender differences in food craving, with a view towards informing the development of more effective treatments for obesity and weight loss.