Elucidation of the anatomy of a satiety network: Focus on connectivity of the parabrachial nucleus in the adult rat

Concerning neuromodulation as treatment of neurological and neuropsychiatric disorder: Insights gained from selective targeting of the subthalamic nucleus, para-subthalamic nucleus and zona incerta in rodents

Neuromodulation such as deep brain stimulation (DBS) is advancing as a clinical intervention in several neurological and neuropsychiatric disorders, including Parkinson's disease, dystonia, tremor, and obsessive-compulsive disorder (OCD) for which DBS is already applied to alleviate severely afflicted individuals of symptoms. Tourette syndrome and drug addiction are two additional disorders for which DBS is in trial or proposed as treatment. However, some major remaining obstacles prevent this intervention from reaching its full therapeutic potential. Side-effects have been reported, and not all DBS-treated individuals are relieved of their symptoms. One major target area for DBS electrodes is the subthalamic nucleus (STN) which plays important roles in motor, affective and associative functions, with impact on for example movement, motivation, impulsivity, compulsivity, as well as both reward and aversion. The multifunctionality of the STN is complex. Decoding the anatomical-functional organization of the STN could enhance strategic targeting in human patients. The STN is located in close proximity to zona incerta (ZI) and the para-subthalamic nucleus (pSTN). Together, the STN, pSTN and ZI form a highly heterogeneous and clinically important brain area. Rodent-based experimental studies, including opto- and chemogenetics as well as viral-genetic tract tracings, provide unique insight into complex neuronal circuitries and their impact on behavior with high spatial and temporal precision. This research field has advanced tremendously over the past few years. Here, we provide an inclusive review of current literature in the pre-clinical research fields centered around STN, pSTN and ZI in laboratory mice and rats; the three highly heterogeneous and enigmatic structures brought together in the context of relevance for treatment strategies. Specific emphasis is placed on methods of manipulation and behavioral impact.

Neural circuits regulation of satiation

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

Multiple metabolic signals in the CeA regulate feeding: The role of AMPK

BACKGROUND

The central nucleus of the amygdala (CeA) is part of the dopaminergic reward system and controls energy balance. Recently, a cluster of neurons was identified as responsive to the orexigenic effect of ghrelin and fasting. However, the signaling pathway by which ghrelin and fasting induce feeding is unknown. AMP-activated protein kinase (AMPK) is a cellular energy sensor, and its Thr172 phosphorylation (AMPKThr172) in the mediobasal hypothalamus regulates food intake. However, whether the expression and activation of AMPK in CeA could be one of the intracellular signaling activated in response to ghrelin and fasting eliciting food intake is unknown.

AIM

To evaluate the activation of AMPK into CeA in response to ghrelin, fasting, and 2-deoxy-D-glucose (2DG) and whether feeding accompanied these changes. In addition, to investigate whether the inhibition of AMPK into CeA could decrease food intake.

METHODS

On a chow diet, eight-week-old Wistar male rats were stereotaxically implanted with a cannula in the CeA to inject several modulators of AMPKα1/2Thr172 phosphorylation, and we performed physiological and molecular assays.

KEY FINDINGS

Fasting increased, and refeeding reduced AMPKThr172 in the CeA. Intra-CeA glucose injection decreased feeding, whereas injection of 2DG, a glucoprivation inductor, in the CeA, increased food intake and blood glucose, despite faint increases in AMPKThr172. Intra-CeA ghrelin injection increased food intake and AMPKThr172. To further confirm the role of AMPK in the CeA, chronic injection of Melanotan II (MTII) in CeA reduced body mass and food intake over seven days together with a slight decrease in AMPKThr172.

SIGNIFICANCE

Our findings identified that AMPK might be part of the signaling machinery in the CeA, which responds to nutrients and hormones contributing to feeding control. The results can contribute to understanding the pathophysiological mechanisms of altered feeding behavior/consumption, such as binge eating of caloric-dense, palatable food.

The parasubthalamic nucleus refeeding ensemble delays feeding initiation and hastens water drinking

The parasubthalamic nucleus (PSTN) is activated by refeeding after food deprivation and several PSTN subpopulations have been shown to suppress feeding. However, no study to date directly addressed the role of PSTN neurons activated upon food access in the control of ensuing food consumption. Here we identify consumption latency as a sensitive behavioral indicator of PSTN activity, and show that, in hungry mice, the ensemble of refeeding-activated PSTN neurons drastically increases the latency to initiate refeeding with both familiar and a novel, familiar food, but does not control the amount of food consumed. In thirsty mice, this ensemble also delays sucrose consumption but accelerates water consumption, possibly reflecting anticipatory prandial thirst, with again no influence on the amount of fluid consumed. We next sought to identify which subpopulations of PSTN neurons might be driving these latency effects, using cell-type and pathway-specific chemogenetic manipulations. Our results suggest a prominent role of PSTN Tac1 neurons projecting to the central amygdala in the hindrance of feeding initiation. While PSTN Crh neurons also delay the latency of hungry mice to ingest familiar foods, they surprisingly promote the consumption of novel, palatable substances. Furthermore, PSTN Crh neurons projecting to the bed nucleus of the stria terminalis accelerate rehydration in thirsty mice. Our results demonstrate the key role of endogenous PSTN activity in the control of feeding and drinking initiation and delineate specific circuits mediating these effects, which may have relevance for eating disorders.

Withdrawal from repeated nicotine vapor exposure increases somatic signs of physical dependence, anxiety-like behavior, and brain reward thresholds in adult male rats

Nicotine vapor consumption via electronic nicotine delivery systems has increased over the last decade. While prior work has shed light on the health effects of nicotine vapor inhalation, its unique effects on the brain and behavior have not been thoroughly explored. In this study we assessed markers of withdrawal following 14 days of nicotine vapor exposure. For Experiment 1, 21 adult male rats were exposed to ambient air or 6, 12, or 24 mg/mL nicotine vapor for 14 consecutive days. Following exposure on day 14, rats were injected with the nicotinic receptor antagonist mecamylamine (3.0 mg/mL) and assessed for somatic withdrawal signs and anxiety-like behavior in the elevated plus maze. For Experiment 2, 12 adult male rats were tested for intracranial self-stimulation (ICSS) immediately following exposure to vehicle vapor (50%/50%, vegetable glycerin/propylene glycol) or 24 mg/mL nicotine vapor, for 14 consecutive days. ICSS behavior was assessed for an additional 14 days, following cessation of repeated vapor exposure. Results reveal that rats with repeated nicotine vapor exposure display an increase in behavioral indicators of withdrawal following injection of mecamylamine (precipitated withdrawal). Additionally, increases in ICSS stimulation thresholds, indicative of reduced brain reward sensitivity, persist following cessation of repeated nicotine vapor exposure (spontaneous withdrawal). These data suggest that repeated e-cigarette use leads to nicotine dependence and withdrawal that affects behavior and brain reward function. Further characterization of the health effects of nicotine vapor is necessary to improve treatment strategies for nicotine use disorder and public health policies related to novel nicotine delivery systems.

Parasubthalamic calretinin neurons modulate wakefulness associated with exploration in male mice

The parasubthalamic nucleus (PSTN) is considered to be involved in motivation, feeding and hunting, all of which are highly depending on wakefulness. However, the roles and underlying neural circuits of the PSTN in wakefulness remain unclear. Neurons expressing calretinin (CR) account for the majority of PSTN neurons. In this study in male mice, fiber photometry recordings showed that the activity of PSTN^CR neurons increased at the transitions from non-rapid eye movement (non-REM, NREM) sleep to either wakefulness or REM sleep, as well as exploratory behavior. Chemogenetic and optogenetic experiments demonstrated that PSTN^CR neurons were necessary for initiating and/or maintaining arousal associated with exploration. Photoactivation of projections of PSTN^CR neurons revealed that they regulated exploration-related wakefulness by innervating the ventral tegmental area. Collectively, our findings indicate that PSTN^CR circuitry is essential for the induction and maintenance of the awake state associated with exploration.

Glycemic Challenge Is Associated with the Rapid Cellular Activation of the Locus Ceruleus and Nucleus of Solitary Tract: Circumscribed Spatial Analysis of Phosphorylated MAP Kinase Immunoreactivity

Rodent studies indicate that impaired glucose utilization or hypoglycemia is associated with the cellular activation of neurons in the medulla (Winslow, 1733) (MY), believed to control feeding behavior and glucose counterregulation. However, such activation has been tracked primarily within hours of the challenge, rather than sooner, and has been poorly mapped within standardized brain atlases. Here, we report that, within 15 min of receiving 2-deoxy-d-glucose (2-DG; 250 mg/kg, i.v.), which can trigger glucoprivic feeding behavior, marked elevations were observed in the numbers of rhombic brain (His, 1893) (RB) neuronal cell profiles immunoreactive for the cellular activation marker(s), phosphorylated p44/42 MAP kinases (phospho-ERK1/2), and that some of these profiles were also catecholaminergic. We mapped their distributions within an open-access rat brain atlas and found that 2-DG-treated rats (compared to their saline-treated controls) displayed greater numbers of phospho-ERK1/2+ neurons in the locus ceruleus (Wenzel and Wenzel, 1812) (LC) and the nucleus of solitary tract (>1840) (NTS). Thus, the 2-DG-activation of certain RB neurons is more rapid than perhaps previously realized, engaging neurons that serve multiple functional systems and which are of varying cellular phenotypes. Mapping these populations within standardized brain atlas maps streamlines their targeting and/or comparable mapping in preclinical rodent models of disease.

Parabrachial-to-parasubthalamic nucleus pathway mediates fear-induced suppression of feeding in male mice

Feeding behavior is adaptively regulated by external and internal environment, such that feeding is suppressed when animals experience pain, sickness, or fear. While the lateral parabrachial nucleus (lPB) plays key roles in nociception and stress, neuronal pathways involved in feeding suppression induced by fear are not fully explored. Here, we investigate the parasubthalamic nucleus (PSTN), located in the lateral hypothalamus and critically involved in feeding behaviors, as a target of lPB projection neurons. Optogenetic activation of lPB-PSTN terminals in male mice promote avoidance behaviors, aversive learning, and suppressed feeding. Inactivation of the PSTN and lPB-PSTN pathway reduces fear-induced feeding suppression. Activation of PSTN neurons expressing pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide enriched in the PSTN, is sufficient for inducing avoidance behaviors and feeding suppression. Blockade of PACAP receptors impaires aversive learning induced by lPB-PSTN photomanipulation. These findings indicate that lPB-PSTN pathway plays a pivotal role in fear-induced feeding suppression.

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

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

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

At the heart of the interoception network: Influence of the parasubthalamic nucleus on autonomic functions and motivated behaviors

The parasubthalamic nucleus (PSTN), a small nucleus located on the lateral edge of the posterior hypothalamus, has emerged in recent years as a highly interconnected node within the network of brain regions sensing and regulating autonomic function and homeostatic needs. Furthermore, the strong integration of the PSTN with extended amygdala circuits makes it ideally positioned to serve as an interface between interoception and emotions. While PSTN neurons are mostly glutamatergic, some of them also express neuropeptides that have been associated with stress-related affective and motivational dysfunction, including substance P, corticotropin-releasing factor, and pituitary adenylate-cyclase activating polypeptide. PSTN neurons respond to food ingestion and anorectic signals, as well as to arousing and distressing stimuli. Functional manipulation of defined pathways demonstrated that the PSTN serves as a central hub in multiple physiologically relevant networks and is notably implicated in appetite suppression, conditioned taste aversion, place avoidance, impulsive action, and fear-induced thermoregulation. We also discuss the putative role of the PSTN in interoceptive dysfunction and negative urgency. This review aims to synthesize the burgeoning preclinical literature dedicated to the PSTN and to stimulate interest in further investigating its influence on physiology and behavior.

Dissecting a disynaptic central amygdala - parasubthalamic nucleus neural circuit that mediates cholecystokinin-induced eating suppression

Objective

Cholecystokinin (CCK) plays a critical role in regulating eating and metabolism. Previous studies have mapped a multi-synapse neural pathway from the vagus nerve to the central nucleus of the amygdala (CEA) that mediates the anorexigenic effect of CCK. However, the neural circuit downstream of the CEA is still unknown due to the complexity of the neurons in the CEA. Here we sought to determine this circuit using a novel approach.

Methods

It has been established that a specific population of CEA neurons, marked by protein kinase C-delta (PKC-δ), mediates the anorexigenic effect of CCK by inhibiting other CEA inhibitory neurons. Taking advantage of this circuit, we dissected the neural circuit using a unique approach based on the idea that neurons downstream of the CEA should be disinhibited by CEA^PKC-δ+ neurons while being activated by CCK. We also used optogenetic assisted electrophysiology circuit mapping and in vivo chemogenetic manipulation methods to determine the circuit structure and function.

Results

We found that neurons in the parasubthalamic nucleus (PSTh) are activated by the activation of CEA^PKC-δ+ neurons and by the peripheral administration of CCK. We demonstrated that CEA^PKC-δ+ neurons inhibit the PSTh-projecting CEA neurons; accordingly, the PSTh neurons can be disynaptically disinhibited or “activated” by CEA^PKC-δ+ neurons. Finally, we showed that chemogenetic silencing of the PSTh neurons effectively attenuates the eating suppression induced by CCK.

Conclusions

Our results identified a disynaptic CEA-PSTh neural circuit that mediates the anorexigenic effect of CCK and thus provide an important neural mechanism of how CCK suppresses eating.

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A unique approach combining a genetically identified neuron population with an anorexigenic agent for circuits mapping.

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Dissected a disynaptic central amygdala-parasubthalamic nucleus neural circuit for the function of cholecystokinin.

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Identified a previously understudied brain region that regulates the anorexigenic effect of cholecystokinin.

Understanding the Significance of the Hypothalamic Nature of the Subthalamic Nucleus

The subthalamic nucleus (STN) is an essential component of the basal ganglia and has long been considered to be a part of the ventral thalamus. However, recent neurodevelopmental data indicated that this nucleus is of hypothalamic origin which is now commonly acknowledged. In this work, we aimed to verify whether the inclusion of the STN in the hypothalamus could influence the way we understand and conduct research on the organization of the whole ventral and posterior diencephalon. Developmental and neurochemical data indicate that the STN is part of a larger glutamatergic posterior hypothalamic region that includes the premammillary and mammillary nuclei. The main anatomic characteristic common to this region involves the convergent cortical and pallidal projections that it receives, which is based on the model of the hyperdirect and indirect pathways to the STN. This whole posterior hypothalamic region is then integrated into distinct functional networks that interact with the ventral mesencephalon to adjust behavior depending on external and internal contexts.

Projections from the lateral parabrachial nucleus to the lateral and ventral lateral periaqueductal gray subregions mediate the itching sensation

ABSTRACT

Lateral and ventral lateral subregions of the periaqueductal gray (l/vlPAG) have been proved to be pivotal components in descending circuitry of itch processing, and their effects are related to the subclassification of neurons that were meditated. In this study, lateral parabrachial nucleus (LPB), one of the most crucial relay stations in the ascending pathway, was taken as the input nucleus to examine the modulatory effect of l/vlPAG neurons that received LPB projections. Anatomical tracing, chemogenetic, optogenetic, and local pharmacological approaches were used to investigate the participation of the LPB-l/vlPAG pathway in itch and pain sensation in mice. First, morphological evidence for projections from vesicular glutamate transporter-2-containing neurons in the LPB to l/vlPAG involved in itch transmission has been provided. Furthermore, chemogenetic and optogenetic activation of the LPB-l/vlPAG pathway resulted in both antipruritic effect and analgesic effect, whereas pharmacogenetic inhibition strengthened nociceptive perception without affecting spontaneous scratching behavior. Finally, in vivo pharmacology was combined with optogenetics which revealed that AMPA receptor-expressing neurons in l/vlPAG might play a more essential role in pathway modulation. These findings provide a novel insight about the connections between 2 prominent transmit nuclei, LPB and l/vlPAG, in both pruriceptive and nociceptive sensations and deepen the understanding of l/vlPAG modulatory roles in itch sensation by chosen LPB as source of ascending efferent projections.

Female rats display greater nicotine withdrawal-induced cellular activation of a central portion of the interpeduncular nucleus versus males: A study of Fos immunoreactivity within provisionally assigned interpeduncular subnuclei

BACKGROUND

The interpeduncular nucleus (>1840) (IPN) has been shown to modulate the behavioral effects of nicotine withdrawal in male rodents. To date, the contribution of this brain structure to sex differences in withdrawal is largely unexplored.

METHODS

This study compared neuronal activation, as reported by observable Fos expression in the IPN of nicotine-dependent female and male rats experiencing withdrawal. We provisionally localized the Fos-expressing cells to certain IPN subnuclei within Swanson's standardized brain atlas (2018). Adult female and male rats were prepared with a pump that delivered nicotine (3.2 mg/kg/day; base) continuously. Controls received a sham surgery. Fourteen days later, the rats received administration of saline or the nicotinic receptor antagonist, mecamylamine (3.0 mg/kg; salt), and physical signs and anxiety-like behavior were assessed. The rats were then euthanized and brain sections containing the IPN were processed for Fos immunofluorescence to infer the possible IPN subnuclei displaying differential activation between sexes.

RESULTS

Both female and male rats displayed withdrawal-induced Fos expression within the IPN. Compared to males, female rats displayed greater numbers of withdrawal-induced Fos-positive cells within a circumscribed portion of the IPN that may fall within the cytoarchitectural boundaries of the central subnucleus (>1840) (IPNc). The withdrawal-induced activation of the IPN was correlated with negative affective states in females, but not males.

CONCLUSION

These data suggest that a centrally located group of IPN cells, presumably situated partly or completely within the IPNc, play a role in modulating sex differences in negative affective states produced by withdrawal.

Extended amygdala-parabrachial circuits alter threat assessment and regulate feeding

An animal's evolutionary success depends on the ability to seek and consume foods while avoiding environmental threats. However, how evolutionarily conserved threat detection circuits modulate feeding is unknown. In mammals, feeding and threat assessment are strongly influenced by the parabrachial nucleus (PBN), a structure that responds to threats and inhibits feeding. Here, we report that the PBN receives dense inputs from two discrete neuronal populations in the bed nucleus of the stria terminalis (BNST), an extended amygdala structure that encodes affective information. Using a series of complementary approaches, we identify opposing BNST-PBN circuits that modulate neuropeptide-expressing PBN neurons to control feeding and affective states. These previously unrecognized neural circuits thus serve as potential nodes of neural circuitry critical for the integration of threat information with the intrinsic drive to feed.

Whole-brain activation signatures of weight-lowering drugs

OBJECTIVE

The development of effective anti-obesity therapeutics relies heavily on the ability to target specific brain homeostatic and hedonic mechanisms controlling body weight. To obtain further insight into neurocircuits recruited by anti-obesity drug treatment, the present study aimed to determine whole-brain activation signatures of six different weight-lowering drug classes.

METHODS

Chow-fed C57BL/6J mice (n = 8 per group) received acute treatment with lorcaserin (7 mg/kg; i.p.), rimonabant (10 mg/kg; i.p.), bromocriptine (10 mg/kg; i.p.), sibutramine (10 mg/kg; p.o.), semaglutide (0.04 mg/kg; s.c.) or setmelanotide (4 mg/kg; s.c.). Brains were sampled two hours post-dosing and whole-brain neuronal activation patterns were analysed at single-cell resolution using c-Fos immunohistochemistry and automated quantitative three-dimensional (3D) imaging.

RESULTS

The whole-brain analysis comprised 308 atlas-defined mouse brain areas. To enable fast and efficient data mining, a web-based 3D imaging data viewer was developed. All weight-lowering drugs demonstrated brain-wide responses with notable similarities in c-Fos expression signatures. Overlapping c-Fos responses were detected in discrete homeostatic and non-homeostatic feeding centres located in the dorsal vagal complex and hypothalamus with concurrent activation of several limbic structures as well as the dopaminergic system.

CONCLUSIONS

Whole-brain c-Fos expression signatures of various weight-lowering drug classes point to a discrete set of brain regions and neurocircuits which could represent key neuroanatomical targets for future anti-obesity therapeutics.

A basal ganglia-like cortical-amygdalar-hypothalamic network mediates feeding behavior

The insular cortex (INS) is extensively connected to the central nucleus of the amygdala (CEA), and both regions send convergent projections into the caudal lateral hypothalamus (LHA) encompassing the parasubthalamic nucleus (PSTN). However, the organization of the network between these structures has not been clearly delineated in the literature, although there has been an upsurge in functional studies related to these structures, especially with regard to the cognitive and psychopathological control of feeding. We conducted tract-tracing experiments from the INS and observed a pathway to the PSTN region that runs parallel to the canonical hyperdirect pathway from the isocortex to the subthalamic nucleus (STN) adjacent to the PSTN. In addition, an indirect pathway with a relay in the central amygdala was also observed that is similar in its structure to the classic indirect pathway of the basal ganglia that also targets the STN. C-Fos experiments showed that the PSTN complex reacts to neophobia and sickness induced by lipopolysaccharide or cisplatin. Chemogenetic (designer receptors exclusively activated by designer drugs [DREADD]) inhibition of tachykininergic neurons (Tac1) in the PSTN revealed that this nucleus gates a stop "no-eat" signal to refrain from feeding when the animal is subjected to sickness or exposed to a previously unknown source of food. Therefore, our anatomical findings in rats and mice indicate that the INS-PSTN network is organized in a similar manner as the hyperdirect and indirect basal ganglia circuitry. Functionally, the PSTN is involved in gating feeding behavior, which is conceptually homologous to the motor no-go response of the adjacent STN.

Hedonic Eating: Sex Differences and Characterization of Orexin Activation and Signaling

Palatable taste can stimulate appetite in the absence of hunger, and individual differences in hedonic eating may be critical to overeating. Women are more prone to obesity and binge eating than men, which warrants comparisons of hedonic versus physiological consumption and the underlying neural substrates in both sexes. The current study examined palatable (high-sugar) food consumption in male and female rats under physiological hunger and satiety, and the role of the neuropeptide orexin/hypocretin (ORX). Across multiple tests, females consistently consumed similar amounts of palatable food regardless of whether they were hungry or sated prior to testing. In contrast, males typically adjusted their consumption according to their hunger/satiety state. This difference was specific to palatable food consumption, as both sexes ate standard chow according to their hunger state. ORX is important in food motivation and reward behaviors. Thus, to begin to determine the neuronal mechanisms of hedonic eating, we examined activation and signaling of ORX neurons. We systematically characterized Fos induction patterns of ORX neurons across the entire rostrocaudal extent of the lateral hypothalamus and found that they were activated by food and by fasting in both sexes. Then, we showed that systemic blockade of ORX receptor 1 signaling with SB-334867 decreased palatable food consumption in hungry and sated rats of both sexes. These results demonstrate sex differences in hedonic eating; increased susceptibility in females to overeat palatable food regardless of hunger state, and that ORX is a critical neuropeptide mechanism of hedonic eating in both sexes.

Distributions of hypothalamic neuron populations coexpressing tyrosine hydroxylase and the vesicular GABA transporter in the mouse

The hypothalamus contains catecholaminergic neurons marked by the expression of tyrosine hydroxylase (TH). As multiple chemical messengers coexist in each neuron, we determined if hypothalamic TH-immunoreactive (ir) neurons express vesicular glutamate or GABA transporters. We used Cre/loxP recombination to express enhanced GFP (EGFP) in neurons expressing the vesicular glutamate (vGLUT2) or GABA transporter (vGAT), then determined whether TH-ir neurons colocalized with native EGFP^Vglut2 - or EGFP^Vgat -fluorescence, respectively. EGFP^Vglut2 neurons were not TH-ir. However, discrete TH-ir signals colocalized with EGFP^Vgat neurons, which we validated by in situ hybridization for Vgat mRNA. To contextualize the observed pattern of colocalization between TH-ir and EGFP^Vgat , we first performed Nissl-based parcellation and plane-of-section analysis, and then mapped the distribution of TH-ir EGFP^Vgat neurons onto atlas templates from the Allen Reference Atlas (ARA) for the mouse brain. TH-ir EGFP^Vgat neurons were distributed throughout the rostrocaudal extent of the hypothalamus. Within the ARA ontology of gray matter regions, TH-ir neurons localized primarily to the periventricular hypothalamic zone, periventricular hypothalamic region, and lateral hypothalamic zone. There was a strong presence of EGFP^Vgat fluorescence in TH-ir neurons across all brain regions, but the most striking colocalization was found in a circumscribed portion of the zona incerta (ZI)-a region assigned to the hypothalamus in the ARA-where every TH-ir neuron expressed EGFP^Vgat . Neurochemical characterization of these ZI neurons revealed that they display immunoreactivity for dopamine but not dopamine β-hydroxylase. Collectively, these findings indicate the existence of a novel mouse hypothalamic population that may signal through the release of GABA and/or dopamine.

Cognitive and Non-Cognitive Outcomes Associated with Student Engagement in a Novel Brain Chemoarchitecture Mapping Course-Based Undergraduate Research Experience

Course-based undergraduate research experiences (CUREs) engage emerging scholars in the authentic process of scientific discovery, and foster their development of content knowledge, motivation, and persistence in the science, technology, engineering, and mathematics (STEM) disciplines. Importantly, authentic research courses simultaneously offer investigators unique access to an extended population of students who receive education and mentoring in conducting scientifically relevant investigations and who are thus able to contribute effort toward big-data projects. While this paradigm benefits fields in neuroscience, such as atlas-based brain mapping of nerve cells at the tissue level, there are few documented cases of such laboratory courses offered in the domain. Here, we describe a curriculum designed to address this deficit, evaluate the scientific merit of novel student-produced brain atlas maps of immunohistochemically-identified nerve cell populations for the rat brain, and assess shifts in science identity, attitudes, and science communication skills of students engaged in the introductory-level Brain Mapping and Connectomics (BM&C) CURE. BM&C students reported gains in research and science process skills following participation in the course. Furthermore, BM&C students experienced a greater sense of science identity, including a greater likelihood to discuss course activities with non-class members compared to their non-CURE counterparts. Importantly, evaluation of student-generated brain atlas maps indicated that the course enabled students to produce scientifically valid products and make new discoveries to advance the field of neuroanatomy. Together, these findings support the efficacy of the BM&C course in addressing the relatively esoteric demands of chemoarchitectural brain mapping.

The analgesic effect of refeeding on acute and chronic inflammatory pain

Pain is susceptible to various cognitive factors. Suppression of pain by hunger is well known, but the effect of food intake after fasting (i.e. refeeding) on pain remains unknown. In the present study, we examined whether inflammatory pain behavior is affected by 24 h fasting and 2 h refeeding. In formalin-induced acute inflammatory pain model, fasting suppressed pain behavior only in the second phase and the analgesic effect was also observed after refeeding. Furthermore, in Complete Freund’s adjuvant-induced chronic inflammatory pain model, both fasting and refeeding reduced spontaneous pain response. Refeeding with non-calorie agar produced an analgesic effect. Besides, intraperitoneal (i.p.) administration of glucose after fasting, which mimics calorie recovery following refeeding, induced analgesic effect. Administration of opioid receptor antagonist (naloxone, i.p.) and cannabinoid receptor antagonist (SR 141716, i.p.) reversed fasting-induced analgesia, but did not affect refeeding-induced analgesia in acute inflammatory pain model. Taken together, our results show that refeeding produce analgesia in inflammatory pain condition, which is associated with eating behavior and calorie recovery effect.

Interleukin-1 reduces food intake and body weight in rat by acting in the arcuate hypothalamus

A reduction in food intake is commonly observed after bacterial infection, a phenomenon that can be reproduced by peripheral administration of Gram-negative bacterial lipopolysaccharide (LPS) or interleukin-1beta (IL-1β), a pro-inflammatory cytokine released by LPS-activated macrophages. The arcuate nucleus of the hypothalamus (ARH) plays a major role in food intake regulation and expresses IL-1 type 1 receptor (IL-1R1) mRNA. In the present work, we tested the hypothesis that IL-1R1 expressing cells in the ARH mediate IL-1β and/or LPS-induced hypophagia in the rat. To do so, we developed an IL-1β-saporin conjugate, which eliminated IL-R1-expressing neurons in the hippocampus, and micro-injected it into the ARH prior to systemic IL-1β and LPS administration. ARH IL-1β-saporin injection resulted in loss of neuropeptide Y-containing cells and attenuated hypophagia and weight loss after intraperitoneal IL-1β, but not LPS, administration. In conclusion, the present study shows that ARH NPY-containing neurons express functional IL-1R1s that mediate peripheral IL-1β-, but not LPS-, induced hypophagia. Our present and previous findings indicate that the reduction of food intake after IL-1β and LPS are mediated by different neural pathways.

Sweet and bitter taste stimuli activate VTA projection neurons in the parabrachial nucleus

This study investigated neural projections from the parabrachial nucleus (PBN), a gustatory and visceral processing area in the brainstem, to the ventral tegmental area (VTA) in the midbrain. The VTA contains a large population of dopaminergic neurons that have been shown to play a role in reward processing. Anterograde neural tracing methods were first used to confirm that a robust projection from the caudal PBN terminates in the dorsal VTA; this projection was larger on the contralateral side. In the next experiment, we combined dual retrograde tracing from the VTA and the gustatory ventral posteromedial thalamus (VPMpc) with taste-evoked Fos protein expression, which labels activated neurons. Mice were stimulated through an intraoral cannula with sucrose, quinine, or water, and PBN sections were processed for immunofluorescent detection of Fos and retrograde tracers. The distribution of tracer-labeled PBN neurons demonstrated that the populations of cells projecting to the VTA or VPMpc are largely independent. Quantification of cells double labeled for Fos and either tracer demonstrated that sucrose and quinine were effective in activating both pathways. These results indicate that information about both appetitive and aversive tastes is delivered to a key midbrain reward interface via direct projections from the PBN.

Morphofunctional Organization of the Connections From the Medial and Intermediate Parts of the Central Nucleus of the Amygdala Into Distinct Divisions of the Lateral Hypothalamic Area in the Rat

Projections from the central nucleus of the amygdala (CEA) into the lateral hypothalamic area (LHA) show a very complex pattern. After injection of an anterograde tracer (Phaseolus vulgaris leucoagglutinin—PHAL) into the medial and intermediate parts of the CEA, we observed that labeled axons converged onto the caudal lateral LHA but provided distinct patterns in rostral tuberal regions. These projections were compared to that of neurons containing the peptides “melanin-concentrating hormone” (MCH) or hypocretin (Hcrt). Because the distribution of these neurons is stereotyped, it was possible to characterize distinct divisions into the LHA. Some of them in the rostral tuberal LHA [the dorsal (LHAd) and suprafornical regions (LHAs)] received a distinct innervation by projections that originated from neurons in respectively anterior or posterior regions of the medial part (CEAm) or from the intermediate part (CEAi) of the central nucleus. Therefore, this work illustrates that projections from the CEAm and CEAi converge into the caudal lateral LHA but diverge into the rostral tuberal LHA.

Contextual fear retrieval-induced Fos expression across early development in the rat: An analysis using established nervous system nomenclature ontology

The neural circuits underlying the acquisition, retention and retrieval of contextual fear conditioning have been well characterized in the adult animal. A growing body of work in younger rodents indicates that context-mediated fear expression may vary across development. However, it remains unclear how this expression may be defined across the full range of key developmental ages. Nor is it fully clear whether the structure of the adult context fear network generalizes to earlier ages. In this study, we compared context fear retrieval-induced behavior and neuroanatomically constrained immediate early-gene expression across infant (P19), early and late juvenile (P24 and P35), and adult (P90) male Long-Evans rats. We focused our analysis on neuroanatomically defined subregions and nuclei of the basolateral complex of the amygdala (BLA complex), dorsal and ventral portions of the hippocampus and the subregions of the medial prefrontal cortex as defined by the nomenclature of the Swanson (2004) adult rat brain atlas. Relative to controls and across all ages tested, there were greater numbers of Fos immunoreactive (Fos-ir) neurons in the posterior part of the basolateral amygdalar nuclei (BLAp) following context fear retrieval that correlated statistically with the expression of freezing. However, Fos-ir within regions having known connections with the BLA complex was differentially constrained by developmental age: early juvenile, but not adult rats exhibited an increase of context fear-dependent Fos-ir neurons in prelimbic and infralimbic areas, while adult, but not juvenile rats displayed increases in Fos-ir neurons within the ventral CA1 hippocampus. These results suggest that juvenile and adult rodents may recruit developmentally unique pathways in the acquisition and retrieval of contextual fear. This study extends prior work by providing a broader set of developmental ages and a rigorously defined neuroanatomical ontology within which the contextual fear network can be studied further.

The Parabrachial Nucleus: CGRP Neurons Function as a General Alarm

The parabrachial nucleus (PBN), which is located in the pons and is dissected by one of the major cerebellar output tracks, is known to relay sensory information (visceral malaise, taste, temperature, pain, itch) to forebrain structures including the thalamus, hypothalamus, and extended amygdala. The availability of mouse lines expressing Cre recombinase selectively in subsets of PBN neurons and viruses for Cre-dependent gene expression is beginning to reveal the connectivity and functions of PBN component neurons. This review focuses on PBN neurons expressing calcitonin gene-related peptide (CGRPPBN) that play a major role in regulating appetite and transmitting real or potential threat signals to the extended amygdala. The functions of other specific PBN neuronal populations are also discussed. This review aims to encourage investigation of the numerous unanswered questions that are becoming accessible.

Encoding of danger by parabrachial CGRP neurons

Animals must respond to various threats to survive. Neurons that express calcitonin gene-related peptide (CGRP) in the parabrachial nucleus (PBN) relay sensory signals that contribute to satiation and pain-induced fear behavior, but it is unknown how they encode these distinct processes. By recording calcium transients in vivo from individual CGRP^PBN neurons, we reveal that most neurons are activated by noxious cutaneous (shock, heat, itch) and visceral stimuli (lipopolysaccharide). These same neurons are inhibited during feeding, but become activated during satiation, consistent with evidence that CGRP^PBN neurons prevent overeating. CGRP^PBN neurons are also activated during consumption of novel food or by an auditory cue that was previously paired with electrical foot shocks. Correspondingly, silencing CGRP^PBN neurons attenuates expression of food neophobia and conditioned fear responses. Therefore, in addition to transducing primary sensory danger signals, CGRP^PBN neurons promote affective-behavioral states that limit harm in response to potential threats.

Mapping Molecular Datasets Back to the Brain Regions They are Extracted from: Remembering the Native Countries of Hypothalamic Expatriates and Refugees

This article focuses on approaches to link transcriptomic, proteomic, and peptidomic datasets mined from brain tissue to the original locations within the brain that they are derived from using digital atlas mapping techniques. We use, as an example, the transcriptomic, proteomic and peptidomic analyses conducted in the mammalian hypothalamus. Following a brief historical overview, we highlight studies that have mined biochemical and molecular information from the hypothalamus and then lay out a strategy for how these data can be linked spatially to the mapped locations in a canonical brain atlas where the data come from, thereby allowing researchers to integrate these data with other datasets across multiple scales. A key methodology that enables atlas-based mapping of extracted datasets-laser-capture microdissection-is discussed in detail, with a view of how this technology is a bridge between systems biology and systems neuroscience.

Neuronal connections of the central amygdalar nucleus with refeeding-activated brain areas in rats

Following fasting, satiety is accompanied by neuronal activation in brain areas including the central amygdalar nucleus (CEA). Since CEA is known to inhibit food intake, we hypothesized that CEA contributes to the termination of meal during refeeding. To better understand the organization of this satiety-related circuit, the interconnections of the CEA with refeeding-activated neuronal groups were elucidated using retrograde (cholera toxin-β subunit, CTB) and anterograde (phaseolus vulgaris leucoagglutinin, PHA-L) tracers in male rats. C-Fos-immunoreactivity was used as marker of neuronal activation. The refeeding-activated input of the CEA primarily originated from the paraventricular thalamic, parasubthalamic and parabrachial nuclei. Few CTB-c-Fos double-labeled neurons were detected in the prefrontal cortex, lateral hypothalamic area, nucleus of the solitary tract (NTS) and the bed nuclei of the stria terminalis (BNST). Only few refeeding-activated proopiomelanocortin-producing neurons of the arcuate nucleus projected to the CEA. Anterograde tract tracing revealed a high density of PHAL-labeled axons contacted with refeeding-activated neurons in the BNST, lateral hypothalamic area, parasubthalamic, paraventricular thalamic and parabrachial nuclei and NTS; a low density of labeled axons was found in the paraventricular hypothalamic nucleus. Chemogenetic activation of the medial CEA (CEAm) inhibited food intake during the first hour of refeeding, while activation of lateral CEA had no effect. These data demonstrate the existence of reciprocal connections between the CEA and distinct refeeding-activated hypothalamic, thalamic and brainstem nuclei, suggesting the importance of short feedback loops in the regulation of satiety and importance of the CEAm in the regulation of food intake during refeeding.

Bombesin receptor subtype-3-expressing neurons regulate energy homeostasis through a novel neuronal pathway in the hypothalamus

OBJECTIVES

Bombesin receptor subtype-3 (BRS-3) has been suggested to play a potential role in energy homeostasis. However, the physiological mechanism of BRS-3 on energy homeostasis remains unknown. Thus, we investigated the BRS-3-mediated neuronal pathway involved in food intake and energy expenditure.

MATERIALS AND METHODS

Expression of BRS-3 in the rat brain was histologically examined. The BRS-3 neurons activated by refeeding-induced satiety or a BRS-3 agonist were identified by c-Fos immunostaining. We also analyzed expression changes in feeding-relating peptides in the brain of fasted rats administered with the BRS-3 agonist.

RESULTS

In the paraventricular hypothalamic nucleus (PVH), dorsomedial hypothalamic nucleus (DMH), and medial preoptic area (MPA), strong c-Fos induction was observed in the BRS-3 neurons especially in PVH after refeeding. However, the BRS-3 neurons in the PVH did not express feeding-regulating peptides, while the BRS-3 agonist administration induced c-Fos expression in the DMH and MPA, which were not refeeding-sensitive, as well as in the PVH. The BRS-3 agonist administration changed the Pomc and Cart mRNA level in several brain regions of fasted rats.

CONCLUSION

These results suggest that BRS-3 neurons in the PVH are a novel functional subdivision in the PVH that regulates feeding behavior. As the MPA and DMH are reportedly involved in thermoregulation and energy metabolism, the BRS-3 neurons in the MPA/DMH might mediate the energy expenditure control. POMC and CART may contribute to BRS-3 neuron-mediated energy homeostasis regulation. In summary, BRS-3-expressing neurons could regulate energy homeostasis through a novel neuronal pathway.

Parasubthalamic and calbindin nuclei in the posterior lateral hypothalamus are the major hypothalamic targets for projections from the central and anterior basomedial nuclei of the amygdala

The parasubthalamic nucleus (PSTN) and the ventrally adjacent calbindin nucleus (CbN) form a nuclear complex in the posterior lateral hypothalamic area (LHA), recently characterized as connected with the central nucleus of the amygdala (CEA). The aim of the present work is to analyze in detail the projections from the amygdala into the PSTN/CbN, also focusing on pathways into the LHA. After fluorogold injections into the PSTN/CbN, the medial part of the CEA (CEAm) appears to be the main supplier of projections from the CEA. Other amygdalar nuclei contribute to the innervation of the PSTN/CbN complex, including the anterior part of the basomedial nucleus (BMAa). Injections of the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHAL), into the CEAm and BMAa revealed that projections from the CEAm follow two pathways into the LHA: a dorsal pathway formed by axons that also innervate the paraventricular hypothalamic nucleus, the anterior perifornical LHA and the PSTN, and a ventral pathway that runs laterally adjacent to the ventrolateral hypothalamic tract (vlt) and ends in the CbN. By contrast, the BMAa and other telencephalic structures, such as the fundus striatum project to the CbN via the ventral pathway. Confirming the microscopic observation, a semi-quantitative analysis of the density of these projections showed that the PSTN and the CbN are the major hypothalamic targets for the projections from the CEAm and the BMAa, respectively. PSTN and CbN receive these projections through distinct dorsal and ventral routes in the LHA. The ventral pathway forms a differentiated tract, named here the ventrolateral amygdalo-hypothalamic tract (vlah), that is distinct from, but runs adjacent to, the vlt. Both the vlt and the vlah had been previously described as forming an olfactory path into the LHA. These results help to better characterize the CbN within the PSTN/CbN complex and are discussed in terms of the functional organization of the network involving the PSTN and the CbN as well as the CEA and the BMAa.

Suckling induced activation pattern in the brain of rat pups

OBJECTIVES

The aim of the study was to understand the effects of suckling on the brain of the pups by mapping their brain activation pattern in response to suckling.

METHODS

The c-fos method was applied to identify activated neurons. Fasted rat pups were returned to their mothers for suckling and sacrificed 2 hours later for Fos immunohistochemistry. Double labeling was also performed to characterize some of the activated neurons. For comparison, another group of fasted pups were given dry food before Fos mapping.

RESULTS

After suckling, we found an increase in the number of Fos-immunoreactive neurons in the insular and somatosensory cortices, central amygdaloid nucleus (CAm), paraventricular (PVN) and supraoptic hypothalamic nuclei, lateral parabrachial nucleus (LPB), nucleus of the solitary tract (NTS), and the area postrema. Double labeling experiments demonstrated the activation of calcitonin gene-related peptide-ir (CGRP-ir) neurons in the LPB, corticotropin-releasing hormone-ir (CRH-ir) but not oxytocin-ir neurons in the PVN, and noradrenergic neurons in the NTS. In the CAm, Fos-ir neurons did not contain CRH but were apposed to CGRP-ir fiber terminals. Refeeding with dry food-induced Fos activation in all brain areas activated by suckling. The degree of activation was higher following dry food consumption than suckling in the insular cortex, and lower in the supraoptic nucleus and the NTS. Furthermore, the accumbens, arcuate, and dorsomedial hypothalamic nuclei, and the lateral hypothalamic area, which were not activated by suckling, showed activation by dry food.

DISCUSSION

Neurons in a number of brain areas are activated during suckling, and may participate in the signaling of satiety, taste perception, reward, food, and salt balance regulation.