Neural substrates of fear-induced hypophagia in male and female rats

Sex differences in the rodent medial prefrontal cortex - What Do and Don't we know?

The prefrontal cortex, particularly its medial subregions (mPFC), mediates critical functions such as executive control, behavioral inhibition, and memory formation, with relevance for everyday functioning and psychopathology. Despite broad characterization of the mPFC in multiple model organisms, the extent to which mPFC structure and function vary according to an individual's sex is unclear - a knowledge gap that can be attributed to a historical bias for male subjects in neuroscience research. Recent efforts to consider sex as a biological variable in basic science highlight the great need to close this gap. Here we review the knowns and unknowns about how rodents categorized as male or female compare in mPFC neuroanatomy, pharmacology, as well as in aversive, appetitive, and goal- or habit-directed behaviors that recruit the mPFC. We propose that long-standing dogmatic concepts of mPFC structure and function may not remain supported when we move beyond male-only studies, and that empirical challenges to these dogmas are warranted. Additionally, we note some common pitfalls in this work. Most preclinical studies operationalize sex as a binary categorization, and while this approach has furthered the inclusion of non-male rodents it is not as such generalizable to what we know of sex as a multidimensional, dynamic variable. Exploration of sex variability may uncover both sex differences and sex similarities, but care must be taken in their interpretation. Including females in preclinical research needs to go beyond the investigation of sex differences, improving our knowledge of how this brain region and its subregions mediate behavior and health. This article is part of the Special Issue on "PFC circuit function in psychiatric disease and relevant models".

Activation patterns in male and female forebrain circuitries during food consumption under novelty

The influence of novelty on feeding behavior is significant and can override both homeostatic and hedonic drives due to the uncertainty of potential danger. Previous work found that novel food hypophagia is enhanced in a novel environment and that males habituate faster than females. The current study's aim was to identify the neural substrates of separate effects of food and context novelty. Adult male and female rats were tested for consumption of a novel or familiar food in either a familiar or in a novel context. Test-induced Fos expression was measured in the amygdalar, thalamic, striatal, and prefrontal cortex regions that are important for appetitive responding, contextual processing, and reward motivation. Food and context novelty induced strikingly different activation patterns. Novel context induced Fos robustly in almost every region analyzed, including the central (CEA) and basolateral complex nuclei of the amygdala, the thalamic paraventricular (PVT) and reuniens nuclei, the nucleus accumbens (ACB), the medial prefrontal cortex prelimbic and infralimbic areas, and the dorsal agranular insular cortex (AI). Novel food induced Fos in a few select regions: the CEA, anterior basomedial nucleus of the amygdala, anterior PVT, and posterior AI. There were also sex differences in activation patterns. The capsular and lateral CEA had greater activation for male groups and the anterior PVT, ACB ventral core and shell had greater activation for female groups. These activation patterns and correlations between regions, suggest that distinct functional circuitries control feeding behavior when food is novel and when eating occurs in a novel environment.

Lateral hypothalamic glutamatergic inputs to VTA glutamatergic neurons mediate prioritization of innate defensive behavior over feeding

The lateral hypothalamus (LH) is involved in feeding behavior and defense responses by interacting with different brain structures, including the Ventral Tegmental Area (VTA). Emerging evidence indicates that LH-glutamatergic neurons infrequently synapse on VTA-dopamine neurons but preferentially establish multiple synapses on VTA-glutamatergic neurons. Here, we demonstrated that LH-glutamatergic inputs to VTA promoted active avoidance, long-term aversion, and escape attempts. By testing feeding in the presence of a predator, we observed that ongoing feeding was decreased, and that this predator-induced decrease in feeding was abolished by photoinhibition of the LH-glutamatergic inputs to VTA. By VTA specific neuronal ablation, we established that predator-induced decreases in feeding were mediated by VTA-glutamatergic neurons but not by dopamine or GABA neurons. Thus, we provided evidence for an unanticipated neuronal circuitry between LH-glutamatergic inputs to VTA-glutamatergic neurons that plays a role in prioritizing escape, and in the switch from feeding to escape in mice.

The influence of the subcortex and brain stem on overeating: How advances in functional neuroimaging can be applied to expand neurobiological models to beyond the cortex

Functional neuroimaging has become a widely used tool in obesity and eating disorder research to explore the alterations in neurobiology that underlie overeating and binge eating behaviors. Current and traditional neurobiological models underscore the importance of impairments in brain systems supporting reward, cognitive control, attention, and emotion regulation as primary drivers for overeating. Due to the technical limitations of standard field strength functional magnetic resonance imaging (fMRI) scanners, human neuroimaging research to date has focused largely on cortical and basal ganglia effects on appetitive behaviors. The present review draws on animal and human research to highlight how neural signaling encoding energy regulation, reward-learning, and habit formation converge on hypothalamic, brainstem, thalamic, and striatal regions to contribute to overeating in humans. We also consider the role of regions such as the mediodorsal thalamus, ventral striatum, lateral hypothalamus and locus coeruleus in supporting habit formation, inhibitory control of food craving, and attentional biases. Through these discussions, we present proposals on how the neurobiology underlying these processes could be examined using functional neuroimaging and highlight how ultra-high field 7-Tesla (7 T) fMRI may be leveraged to elucidate the potential functional alterations in subcortical networks. Focus is given to how interactions of these regions with peripheral endocannabinoids and neuropeptides, such as orexin, could be explored. Technical and methodological aspects regarding the use of ultra-high field 7 T fMRI to study eating behaviors are also reviewed.

Connections of the mouse subfornical region of the lateral hypothalamus (LHsf)

The lateral hypothalamus is a major integrative hub with a complex architecture characterized by intricate and overlapping cellular populations expressing a large variety of neuro-mediators. In rats, the subfornical lateral hypothalamus (LHsf) was identified as a discrete area with very specific outputs, receiving a strong input from the nucleus incertus, and involved in defensive and foraging behaviors. We identified in the mouse lateral hypothalamus a discrete subfornical region where a conspicuous cluster of neurons express the mu opioid receptor. We thus examined the inputs and outputs of this LHsf region in mice using retrograde tracing with the cholera toxin B subunit and anterograde tracing with biotin dextran amine, respectively. We identified a connectivity profile largely similar, although not identical, to what has been described in rats. Indeed, the mouse LHsf has strong reciprocal connections with the lateral septum, the ventromedial hypothalamic nucleus and the dorsal pre-mammillary nucleus, in addition to a dense output to the lateral habenula. However, the light input from the nucleus incertus and the moderate bidirectional connectivity with nucleus accumbens are specific to the mouse LHsf. A preliminary neurochemical study showed that LHsf neurons expressing mu opioid receptors also co-express calcitonin gene-related peptide or somatostatin and that the reciprocal connection between the LHsf and the lateral septum may be functionally modulated by enkephalins acting on mu opioid receptors. These results suggest that the mouse LHsf may be hodologically and functionally comparable to its rat counterpart, but more atypical connections also suggest a role in consummatory behaviors.

The Function of Paraventricular Thalamic Circuitry in Adaptive Control of Feeding Behavior

The paraventricular nucleus of the thalamus (PVT) is a complex area that is uniquely embedded across the core feeding, reward, arousal, and stress circuits. The PVT role in the control of feeding behavior is discussed here within a framework of adaptive behavioral guidance based on the body’s energy state and competing drives. The survival of an organism depends on bodily energy resources and promotion of feeding over other behaviors is adaptive except when in danger or sated. The PVT is structurally set up to respond to homeostatic and hedonic needs to feed, and to integrate those signals with physiological and environmental stress, as well as anticipatory needs and other cognitive inputs. It can regulate both food foraging (seeking) and consumption and may balance their expression. The PVT is proposed to accomplish these functions through a network of connections with the brainstem, hypothalamic, striatal, and cortical areas. The connectivity of the PVT further indicates that it could broadcast the information about energy use/gain and behavioral choice to impact cognitive processes—learning, memory, and decision-making—through connections with the medial and lateral prefrontal cortical areas, the hippocampal formation, and the amygdala. The PVT is structurally complex and recent evidence for specific PVT pathways in different aspects of feeding behavior will be discussed.

Cardiac vagal imbalance to the isometric sustained weight test in adolescents with emotional eating behavior

PURPOSE

To assess the relationship between emotional eating behavior and heart rate variability in Spanish adolescents during an isometric exercise test.

METHODS

Participants included 52 adolescents aged between 13 and 18 years old. Heart rate was continuously recorded at rest (2 minutes) and during the sustained weight test (2 minutes). Linear and nonlinear methods of heart rate variability were assessed and related to the emotional eating behavior divided in two clusters.

RESULTS

Statistically significant differences were observed in linear and non-linear parameters of heart rate variability comparing rest and sustained weight test. An increase in the value of emotional eating in overweight adolescents was founded. During the sustained weight test, there were differences between the two emotional eating clusters regarding the variables peak high frequency power, normalized low frequency power, normalized high frequency power, low frequency/high frequency ratio, and sample entropy. A positive correlation between the emotional eating behavior and the peak high frequency power was observed, though the prediction capacity of the high frequency waves is low it is observed that there is a good fit to the regression line.

CONCLUSION

Results of this study shows that there was a relationship between vagal tone and emotional eating behavior in adolescents during an isometric exercise, with excessive parasympathetic predominance and sympathetic withdrawal during a physical effort.

The neurobiological basis of sex differences in learned fear and its inhibition

Learning that certain cues or environments predict threat enhances survival by promoting appropriate fear and the resulting defensive responses. Adapting to changing stimulus contingencies by learning that such cues no longer predict threat, or distinguishing between these threat-related and other innocuous stimuli, also enhances survival by limiting fear responding in an appropriate manner to conserve resources. Importantly, a failure to inhibit fear in response to harmless stimuli is a feature of certain anxiety and trauma-related disorders, which are also associated with dysfunction of the neural circuitry underlying learned fear and its inhibition. Interestingly, these disorders are up to twice as common in women, compared to men. Despite this striking sex difference in disease prevalence, the neurobiological factors involved remain poorly understood. This is due in part to the majority of relevant preclinical studies having neglected to include female subjects alongside males, which has greatly hindered progress in this field. However, more recent studies have begun to redress this imbalance and emerging evidence indicates that there are significant sex differences in the inhibition of learned fear and associated neural circuit function. This paper provides a narrative review on sex differences in learned fear and its inhibition through extinction and discrimination, along with the key gonadal hormone and brain mechanisms involved. Understanding the endocrine and neural basis of sex differences in learned fear inhibition may lead to novel insights on the neurobiological mechanisms underlying the enhanced vulnerability to develop anxiety-related disorders that are observed in women.

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.

Feeding Behavior Survival Circuit: Anticipation & Competition

A decision to eat or not to eat can be beneficial or detrimental to an organism, depending on internal and external conditions. Because feeding is essential for survival, as it replenishes energy and nutrients, in safe environments, its expression is prioritized over other behaviors. Under threat, responding to danger is a higher priority for survival and feeding is paused even in hungry states. Thus, successful expression of feeding behavior requires adaptive control that utilizes cognitive processes to dynamically assess and update internal drives and environmental changes. Recently identified key circuit components, which are important in anticipatory responding based on food memories and predictions and in resolving feeding versus threat avoidance competition, will be discussed within a connectional schema.