Distribution of vesicular glutamate transporter mRNA in rat hypothalamus

Prenatal exposure to the mineralocorticoid receptor antagonist spironolactone disrupts hippocampal area CA2 connectivity and alters behavior in mice

In the brain, the hippocampus is enriched with mineralocorticoid receptors (MR; Nr3c2), a ligand-dependent transcription factor stimulated by the stress hormone corticosterone in rodents. Recently, we discovered that MR is required for the acquisition and maintenance of many features of mouse area CA2 neurons. Notably, we observed that immunofluorescence for the vesicular glutamate transporter 2 (vGluT2), likely representing afferents from the supramammillary nucleus (SuM), was disrupted in the embryonic, but not postnatal, MR knockout mouse CA2. To test whether pharmacological perturbation of MR activity in utero similarly disrupts CA2 connectivity, we implanted slow-release pellets containing the MR antagonist spironolactone in mouse dams during mid-gestation. After confirming that at least one likely active metabolite crossed from the dams' serum into the embryonic brains, we found that spironolactone treatment caused a significant reduction of CA2 axon fluorescence intensity in the CA1 stratum oriens, where CA2 axons preferentially project, and that vGluT2 staining was significantly decreased in both CA2 and dentate gyrus in spironolactone-treated animals. We also found that spironolactone-treated animals exhibited increased reactivity to novel objects, an effect similar to what is seen with embryonic or postnatal CA2-targeted MR knockout. However, we found no difference in preference for social novelty between the treatment groups. We infer these results to suggest that persistent or more severe disruptions in MR function may be required to interfere with this type of social behavior. These findings do indicate, though, that developmental disruption in MR signaling can have persistent effects on hippocampal circuitry and behavior.

Dopaminergic dominance in the ventral medial hypothalamus: A pivotal regulator for methamphetamine-induced pathological aggression

Methamphetamine (METH) abuse is associated with a spectrum of behavioral consequences, among which heightened aggression presents a significant challenge. However, the causal role of METH's impact in aggression and its target circuit mechanisms remains largely unknown. We established an acute METH exposure-aggression mouse model to investigate the role of ventral tegmental area (VTA) dopaminergic neurons and ventral medial hypothalamus VMH glutamatergic neuron. Our findings revealed that METH-induced VTA dopamine excitability activates the ventromedial hypothalamus (VMH) glutamatergic neurons, contributing to pathological aggression. Notably, we uncovered a dopaminergic transmission within the VTA-VMH circuit that exclusively functioned under METH influence. This dopaminergic pathway emerged as a potential key player in enabling dopamine-related pathological aggression, with heightened dopaminergic excitability implicated in various psychiatric symptoms. Also, the modulatory function of this pathway opens new possibilities for targeted therapeutic strategies for intervention to improve treatment in METH abuse and may have broader implications for addressing pathological aggression syndromes.

Sex-specific prefrontal-hypothalamic control of behavior and stress responding

Depression and cardiovascular disease are both augmented by daily life stress. Yet, the biological mechanisms that translate psychological stress into affective and physiological outcomes are unknown. Previously, we demonstrated that stimulation of the ventromedial prefrontal cortex (vmPFC) has sexually divergent outcomes on behavior and physiology. Importantly, the vmPFC does not innervate the brain regions that initiate autonomic or neuroendocrine stress responses; thus, we hypothesized that intermediate synapses integrate cortical information to regulate stress responding. The posterior hypothalamus (PH) directly innervates stress-effector regions and receives substantial innervation from the vmPFC. In the current studies, circuit-specific approaches examined whether vmPFC synapses in the PH coordinate stress responding. Here we tested the effects of optogenetic vmPFC-PH circuit stimulation in male and female rats on social and motivational behaviors as well as physiological stress responses. Additionally, an intersectional genetic approach was used to knock down synaptobrevin in PH-projecting vmPFC neurons. Our collective results indicate that male vmPFC-PH circuitry promotes positive motivational valence and is both sufficient and necessary to reduce sympathetic-mediated stress responses. In females, the vmPFC-PH circuit does not affect social or preference behaviors but is sufficient and necessary to elevate neuroendocrine stress responses. Altogether, these data suggest cortical regulation of stress reactivity and behavior is mediated, in part, by projections to the hypothalamus that function in a sex-specific manner.

Single-cell genomics reveals region-specific developmental trajectories underlying neuronal diversity in the human hypothalamus

The development and diversity of neuronal subtypes in the human hypothalamus has been insufficiently characterized. To address this, we integrated transcriptomic data from 241,096 cells (126,840 newly generated) in the prenatal and adult human hypothalamus to reveal a temporal trajectory from proliferative stem cell populations to mature hypothalamic cell types. Iterative clustering of the adult neurons identified 108 robust transcriptionally distinct neuronal subtypes representing 10 hypothalamic nuclei. Pseudotime trajectories provided insights into the genes driving formation of these nuclei. Comparisons to single-cell transcriptomic data from the mouse hypothalamus suggested extensive conservation of neuronal subtypes despite certain differences in species-enriched gene expression. The uniqueness of hypothalamic neuronal lineages was examined developmentally by comparing excitatory lineages present in cortex and inhibitory lineages in ganglionic eminence, revealing both distinct and shared drivers of neuronal maturation across the human forebrain. These results provide a comprehensive transcriptomic view of human hypothalamus development through gestation and adulthood at cellular resolution.

The Co-Expression Pattern of Calcium-Binding Proteins with γ-Aminobutyric Acid and Glutamate Transporters in the Amygdala of the Guinea Pig: Evidence for Glutamatergic Subpopulations

The amygdala has large populations of neurons utilizing specific calcium-binding proteins such as parvalbumin (PV), calbindin (CB), or calretinin (CR). They are considered specialized subsets of γ-aminobutyric acid (GABA) interneurons; however, many of these cells are devoid of GABA or glutamate decarboxylase. The neurotransmitters used by GABA-immunonegative cells are still unknown, but it is suggested that a part may use glutamate. Thus, this study investigates in the amygdala of the guinea pig relationships between PV, CB, or CR-containing cells and GABA transporter (VGAT) or glutamate transporter type 2 (VGLUT2), markers of GABAergic and glutamatergic neurons, respectively. The results show that although most neurons using PV, CB, and CR co-expressed VGAT, each of these populations also had a fraction of VGLUT2 co-expressing cells. For almost all neurons using PV (~90%) co-expressed VGAT, while ~1.5% of them had VGLUT2. The proportion of neurons using CB and VGAT was smaller than that for PV (~80%), while the percentage of cells with VGLUT2 was larger (~4.5%). Finally, only half of the neurons using CR (~53%) co-expressed VGAT, while ~3.5% of them had VGLUT2. In conclusion, the populations of neurons co-expressing PV, CB, and CR are in the amygdala, primarily GABAergic. However, at least a fraction of neurons in each of them co-express VGLUT2, suggesting that these cells may use glutamate. Moreover, the number of PV-, CB-, and CR-containing neurons that may use glutamate is probably larger as they can utilize VGLUT1 or VGLUT3, which are also present in the amygdala.

Neural basis for fasting activation of the hypothalamic-pituitary-adrenal axis

Fasting initiates a multitude of adaptations to allow survival. Activation of the hypothalamic-pituitary-adrenal (HPA) axis and subsequent release of glucocorticoid hormones is a key response that mobilizes fuel stores to meet energy demands1-5. Despite the importance of the HPA axis response, the neural mechanisms that drive its activation during energy deficit are unknown. Here, we show that fasting-activated hypothalamic agouti-related peptide (AgRP)-expressing neurons trigger and are essential for fasting-induced HPA axis activation. AgRP neurons do so through projections to the paraventricular hypothalamus (PVH), where, in a mechanism not previously described for AgRP neurons, they presynaptically inhibit the terminals of tonically active GABAergic afferents from the bed nucleus of the stria terminalis (BNST) that otherwise restrain activity of corticotrophin-releasing hormone (CRH)-expressing neurons. This disinhibition of PVHCrh neurons requires γ-aminobutyric acid (GABA)/GABA-B receptor signalling and potently activates the HPA axis. Notably, stimulation of the HPA axis by AgRP neurons is independent of their induction of hunger, showing that these canonical 'hunger neurons' drive many distinctly different adaptations to the fasted state. Together, our findings identify the neural basis for fasting-induced HPA axis activation and uncover a unique means by which AgRP neurons activate downstream neurons: through presynaptic inhibition of GABAergic afferents. Given the potency of this disinhibition of tonically active BNST afferents, other activators of the HPA axis, such as psychological stress, may also work by reducing BNST inhibitory tone onto PVHCrh neurons.

Estrogen-responsive neural circuits governing male and female mating behavior in mice

Decades of knockout analyses have highlighted the crucial involvement of estrogen receptors and downstream genes in controlling mating behaviors. More recently, advancements in neural circuit research have unveiled a distributed subcortical network comprising estrogen-receptor or estrogen-synthesis-enzyme-expressing cells that transforms sensory inputs into sex-specific mating actions. This review provides an overview of the latest discoveries on estrogen-responsive neurons in various brain regions and the associated neural circuits that govern different aspects of male and female mating actions in mice. By contextualizing these findings within previous knockout studies of estrogen receptors, we emphasize the emerging field of "circuit genetics", where identifying mating behavior-related neural circuits may allow for a more precise evaluation of gene functions within these circuits. Such investigations will enable a deeper understanding of how hormone fluctuation, acting through estrogen receptors and downstream genes, influences the connectivity and activity of neural circuits, ultimately impacting the manifestation of innate mating actions.

Sex-specific prefrontal-hypothalamic control of behavior and stress responding

Depression and cardiovascular disease are both augmented by daily life stress. Yet, the biological mechanisms that translate psychological stress into affective and physiological outcomes are unknown. Previously, we demonstrated that stimulation of the ventromedial prefrontal cortex (vmPFC) has sexually divergent outcomes on behavior and physiology. Importantly, the vmPFC does not innervate the brain regions that initiate autonomic or neuroendocrine stress responses; thus, we hypothesized that intermediate synapses integrate cortical information to regulate stress responding. The posterior hypothalamus (PH) directly innervates stress-effector regions and receives substantial innervation from the vmPFC. In the current studies, circuit-specific approaches examined whether vmPFC synapses in the PH coordinate stress responding. Here we tested the effects of optogenetic vmPFC-PH circuit stimulation in male and female rats on social and motivational behaviors as well as physiological stress responses. Additionally, an intersectional genetic approach was used to knock down synaptobrevin in PH-projecting vmPFC neurons. Our collective results indicate that male vmPFC-PH circuitry promotes positive motivational valence and is both sufficient and necessary to reduce sympathetic-mediated stress responses. In females, the vmPFC-PH circuit does not affect social or preference behaviors but is sufficient and necessary to elevate neuroendocrine stress responses. Altogether, these data suggest cortical regulation of stress reactivity and behavior is mediated, in part, by projections to the hypothalamus that function in a sex-specific manner.

Neurochemistry of the mammillary body

The mammillary body (MB) is a component of the extended hippocampal system and many studies have shown that its functions are vital for mnemonic processes. Together with other subcortical structures, such as the anterior thalamic nuclei and tegmental nuclei of Gudden, the MB plays a crucial role in the processing of spatial and working memory, as well as navigation in rats. The aim of this paper is to review the distribution of various substances in the MB of the rat, with a description of their possible physiological roles. The following groups of substances are reviewed: (1) classical neurotransmitters (glutamate and other excitatory transmitters, gamma-aminobutyric acid, acetylcholine, serotonin, and dopamine), (2) neuropeptides (enkephalins, substance P, cocaine- and amphetamine-regulated transcript, neurotensin, neuropeptide Y, somatostatin, orexins, and galanin), and (3) other substances (calcium-binding proteins and calcium sensor proteins). This detailed description of the chemical parcellation may facilitate a better understanding of the MB functions and its complex relations with other structures of the extended hippocampal system.

Distinct populations of corticotropin-releasing factor (CRF) neurons mediate divergent yet complementary defensive behaviors in response to a threat

Defensive behaviors in response to a threat are shared across the animal kingdom. Active (fleeing, sheltering) or passive (freezing, avoiding) defensive responses are adaptive and facilitate survival. Selecting appropriate defensive strategy depends on intensity, proximity, temporal threat threshold, and past experiences. Hypothalamic corticotropin-releasing factor (CRF) is a major driver of an acute stress response, whereas extrahypothalamic CRF mediates stress-related affective behaviors. In this review, we shift the focus from a monolithic role of CRF as an anxiogenic peptide to comprehensively dissecting contributions of distinct populations of CRF neurons in mediating defensive behaviors. Direct interrogation of CRF neurons of the central amygdala (CeA) or the bed nucleus of the stria terminalis (BNST) show they drive unconditioned defensive responses, such as vigilance and avoidance of open spaces. Although both populations also contribute to learned fear responses in familiar, threatening contexts, CeA-CRF neurons are particularly attuned to the ever-changing environment. Depending on threat intensities, they facilitate discrimination of salient stimuli predicting manageable threats, and prevent their generalization. Finally, hypothalamic CRF neurons mediate initial threat assessment and active defense such as escape to shelter. Overall, these three major populations of CRF neurons demonstrate divergent, yet complementary contributions to the versatile defense system: heightened vigilance, discriminating salient threats, and active escape, representing three legs of the defense tripod. Despite the 'CRF exhaustion' in the field of affective neuroscience, understanding contributions of specific CRF neurons during adaptive defensive behaviors is needed in order to understand the implications of their dysregulation in fear- and anxiety-related psychiatric disorders. This article is part of the Special Issue on "Fear, Anxiety and PTSD".

Nucleus accumbens circuit disinhibits lateral hypothalamus glutamatergic neurons contributing to morphine withdrawal memory in male mice

The lateral hypothalamus (LH) is physiologically critical in brain functions. The LH also plays an important role in drug addiction. However, neural circuits underlying LH involvement of drug addiction remain obscure. In the present study,our results showed that in male mice, during context-induced expression of morphine withdrawal memory, LH glutamatergic neurons played an important role; dopamine D1 receptor-expressing medium spiny neurons (D1-MSNs) projecting from the core of nucleus accumbens (NAcC) to the LH were an important upstream circuit to activate LH glutamatergic neurons; D1-MSNs projecting from the NAcC to the LH activated LH glutamatergic neurons through inhibiting LH local gamma-aminobutyric acid (GABA) neurons. These results suggest that disinhibited LH glutamatergic neurons by neural circuits from the NAcC importantly contribute to context-induced the expression of morphine withdrawal memory.

Populational heterogeneity and partial migratory origin of the ventromedial hypothalamic nucleus: genoarchitectonic analysis in the mouse

The ventromedial hypothalamic nucleus (VMH) is one of the most distinctive hypothalamic tuberal structures, subject of numerous classic and modern functional studies. Commonly, the adult VMH has been divided in several portions, attending to differences in cell aggregation, cell type, connectivity, and function. Consensus VMH partitions in the literature comprise the dorsomedial (VMHdm), and ventrolateral (VMHvl) subnuclei, which are separated by an intermediate or central (VMHc) population (topographic names based on the columnar axis). However, some recent transcriptome analyses have identified a higher number of different cell types in the VMH, suggesting additional subdivisions, as well as the possibility of separate origins. We offer a topologic and genoarchitectonic developmental study of the mouse VMH complex using the prosomeric axis as a reference. We analyzed genes labeling specific VMH subpopulations, with particular focus upon the Nkx2.2 transcription factor, a marker of the alar-basal boundary territory of the prosencephalon, from where some cells seem to migrate dorsoventrally into VMH. We also identified separate neuroepithelial origins of a Nr2f1-positive subpopulation, and a new Six3-positive component, as well as subtle differences in origin of Nr5a1 positive versus Nkx2.2-positive cell populations entering dorsoventrally the VMH. Several of these migrating cell types are born in the dorsal tuberal domain and translocate ventralwards to reach the intermediate tuberal domain, where the adult VMH mass is located in the adult. This work provides a more detailed area map on the intrinsic organization of the postmigratory VMH complex, helpful for deeper functional studies of this basal hypothalamic entity.

Single-cell multiplex qPCR evidence for sex-dimorphic glutamate decarboxylase, estrogen receptor, and 5'-AMP-activated protein kinase alpha subunit mRNA expression by ventromedial hypothalamic nucleus GABAergic neurons

The inhibitory amino acid transmitter γ-aminobutryic acid (GABA) acts within the ventromedial hypothalamus to regulate systemic glucose homeostasis, but the issue of whether this neurochemical signal originates locally or is supplied by afferent innervation remains controversial. Here, combinatory in situ immunocytochemistry/laser-catapult microdissection/single-cell multiplex qPCR techniques were used to investigate the premise that ventromedial hypothalamic nucleus ventrolateral (VMNvl) and/or dorsomedial (VMNdm) division neurons contain mRNAs that encode glutamate decarboxylase (GAD)65 or GAD67 and metabolic-sensory biomarkers, and that expression of these genes is sex-dimorphic. In male and female rats, GAD65 mRNA was elevated in VMNvl versus VMNdm GAD65/67-immunopositive (-ir) neurons, yet the female exhibited higher GAD67 transcript content in VMNdm versus VMNvl GABAergic nerve cells. Estrogen receptor (ER)-alpha transcripts were lower in female versus male GABA neurons from either VMN division; ER-beta and G-protein-coupled ER-1 mRNA expression profiles were also comparatively reduced in cells from female versus male VMNvl. VMNvl and VMNdm GAD65/67-ir-positive neurons showed equivalent levels of glucokinase and sulfonylurea receptor-1 mRNA between sexes. 5'-AMP-activated protein kinase-alpha 1 (AMPKα1) and -alpha 2 (AMPKα2) transcripts were lower in female versus male VMNdm GABAergic neurons, yet AMPKα2 mRNA levels were higher in cells acquired from female versus male VMNvl. Current studies document GAD65 and -67 gene expression in VMNvl and VMNdm GAD65/67-ir-positive neurons in each sex. Results infer that GABAergic neurons in each division may exhibit sex differences in receptiveness to estradiol. Outcomes also support the prospect that energy sensory function by this neurotransmitter cell type may predominate in the VMNvl in female versus VMNdm in the male.

Origin of thyrotropin-releasing hormone neurons that innervate the tuberomammillary nuclei

Hypophysiotropic thyrotropin-releasing hormone (TRH) neurons function as metabolic sensors that regulate the thyroid axis and energy homeostasis. Less is known about the role of other hypothalamic TRH neurons. As central administration of TRH decreases food intake and increases histamine in the tuberomammillary nuclei (TMN), and TMN histamine neurons are densely innervated by TRH fibers from an unknown origin, we mapped the location of TRH neurons that project to the TMN. The retrograde tracer, cholera toxin B subunit (CTB), was injected into the TMN E1-E2, E4-E5 subdivisions of adult Sprague-Dawley male rats. TMN projecting neurons were observed in the septum, preoptic area, bed nucleus of the stria terminalis (BNST), perifornical area, anterior paraventricular nucleus, peduncular and tuberal lateral hypothalamus (TuLH), suprachiasmatic nucleus and medial amygdala. However, CTB/pro-TRH178-199 double-labeled cells were only found in the TuLH. The specificity of the retrograde tract-tracing result was confirmed by administering the anterograde tracer, Phaseolus vulgaris leuco-agglutinin (PHAL) into the TuLH. Double-labeled PHAL-pro-TRH boutons were identified in all subdivisions of the TMN. TMN neurons double-labeled for histidine decarboxylase (Hdc)/PHAL, Hdc/Trh receptor (Trhr), and Hdc/Trh. Further confirmation of a TuLH-TRH neuronal projection to the TMN was established in a transgenic mouse that expresses Cre recombinase in TRH-producing cells following microinjection of a Cre recombinase-dependent AAV that expresses mCherry into the TuLH. We conclude that, in rodents, the TRH innervation of TMN originates in part from TRH neurons in the TuLH, and that this TRH population may contribute to regulate energy homeostasis through histamine Trhr-positive neurons of the TMN.

Neuropeptides and small-molecule amine transmitters: cooperative signaling in the nervous system

Neuropeptides are expressed in cell-specific patterns throughout mammalian brain. Neuropeptide gene expression has been useful for clustering neurons by phenotype, based on single-cell transcriptomics, and for defining specific functional circuits throughout the brain. How neuropeptides function as first messengers in inter-neuronal communication, in cooperation with classical small-molecule amine transmitters (SMATs) is a current topic of systems neurobiology. Questions include how neuropeptides and SMATs cooperate in neurotransmission at the molecular, cellular and circuit levels; whether neuropeptides and SMATs always co-exist in neurons; where neuropeptides and SMATs are stored in the neuron, released from the neuron and acting, and at which receptors, after release; and how neuropeptides affect 'classical' transmitter function, both directly upon co-release, and indirectly, via long-term regulation of gene transcription and neuronal plasticity. Here, we review an extensive body of data about the distribution of neuropeptides and their receptors, their actions after neuronal release, and their function based on pharmacological and genetic loss- and gain-of-function experiments, that addresses these questions, fundamental to understanding brain function, and development of neuropeptide-based, and potentially combinatorial peptide/SMAT-based, neurotherapeutics.

In Vivo Photometry Reveals Insulin and 2-Deoxyglucose Maintain Prolonged Inhibition of VMH Vglut2 Neurons in Male Mice

The ventromedial hypothalamic (VMH) nucleus is a well-established hub for energy and glucose homeostasis. In particular, VMH neurons are thought to be important for initiating the counterregulatory response to hypoglycemia, and ex vivo electrophysiology and immunohistochemistry data indicate a clear role for VMH neurons in sensing glucose concentration. However, the temporal response of VMH neurons to physiologically relevant changes in glucose availability in vivo has been hampered by a lack of available tools for measuring neuronal activity over time. Since the majority of neurons within the VMH are glutamatergic and can be targeted using the vesicular glutamate transporter Vglut2, we expressed cre-dependent GCaMP7s in Vglut2 cre mice and examined the response profile of VMH to intraperitoneal injections of glucose, insulin, and 2-deoxyglucose (2DG). We show that reduced available glucose via insulin-induced hypoglycemia and 2DG-induced glucoprivation, but not hyperglycemia induced by glucose injection, inhibits VMH Vglut2 neuronal population activity in vivo. Surprisingly, this inhibition was maintained for at least 45 minutes despite prolonged hypoglycemia and initiation of a counterregulatory response. Thus, although VMH stimulation, via pharmacological, electrical, or optogenetic approaches, is sufficient to drive a counterregulatory response, our data suggest VMH Vglut2 neurons are not the main drivers required to do so, since VMH Vglut2 neuronal population activity remains suppressed during hypoglycemia and glucoprivation.

GABAergic and Glutamatergic Phenotypes of Neurons Expressing Calcium-Binding Proteins in the Preoptic Area of the Guinea Pig

The mammalian preoptic area (POA) has large populations of calbindin (CB), calretinin (CR) and parvalbumin (PV) neurons, but phenotypes of these cells are unknown. Therefore, the question is whether neurons expressing CB, CR, and/or PV are GABAergic or glutamatergic. Double-immunofluorescence staining followed by epifluorescence and confocal microscopy was used to determine the coexpression patterns of CB, CR and PV expressing neurons with vesicular GABA transporters (VGAT) as specific markers of GABAergic neurons and vesicular glutamate transporters (VGLUT 2) as specific markers of glutamatergic neurons. The guinea pig was adopted as, like humans, it has a reproductive cycle with a true luteal phase and a long gestation period. The results demonstrated that in the guinea pig POA of both sexes, ~80% of CB+ and ~90% of CR+ neurons coexpress VGAT; however, one-fifth of CB+ neurons and one-third of CR+ cells coexpress VGLUT. About two-thirds of PV+ neurons express VGAT, and similar proportion of them coexpress VGLUT. Thus, many CB+, CR+ and PV+ neurons may be exclusively GABAergic (VGAT-expressing cells) or glutamatergic (VGLUT-expressing cells); however, at least a small fraction of CR+ cells and at least one-third of PV+ cells are likely neurons with a dual GABA/glutamate phenotype that may coexpress both transporters.

Hippocampus-Anterior Hypothalamic Circuit Modulates Stress-Induced Endocrine and Behavioral Response

Hippocampal input to the hypothalamus is known to be critically involved in mediating the negative feedback inhibition of stress response. However, the underlying neural circuitry has not been fully elucidated. Using a combination of rabies tracing, pathway-specific optogenetic inhibition, and cell-type specific synaptic silencing, the present study examined the role of hippocampal input to the hypothalamus in modulating neuroendocrine and behavioral responses to stress in mice. Transsynaptic rabies tracing revealed that the ventral hippocampus (vHPC) is monosynaptically connected to inhibitory cells in the anterior hypothalamic nucleus (AHN-GABA cells). Optogenetic inhibition of the vHPC→AHN pathway during a restraint stress resulted in a prolonged and exaggerated release of corticosterone, accompanied by an increase in stress-induced anxiety behaviors. Consistently, tetanus toxin-mediated synaptic inhibition in AHN-GABA cells produced a remarkably similar effect on the corticosterone release profile, corroborating the role of HPC→AHN pathway in mediating the hippocampal control of stress responses. Lastly, we found that chronic inhibition of AHN-GABA cells leads to cognitive impairments in both object and social recognition memory. Together, our data present a novel hypothalamic circuit for the modulation of adaptive stress responses, the dysfunction of which has been implicated in various affective disorders.

Cortical–Hypothalamic Integration of Autonomic and Endocrine Stress Responses

The prevalence and severity of cardiovascular disease (CVD) are exacerbated by chronic stress exposure. While stress-induced sympathetic activity and elevated glucocorticoid secretion impair cardiovascular health, the mechanisms by which stress-responsive brain regions integrate autonomic and endocrine stress responses remain unclear. This review covers emerging literature on how specific cortical and hypothalamic nuclei regulate cardiovascular and neuroendocrine stress responses. We will also discuss the current understanding of the cellular and circuit mechanisms mediating physiological stress responses. Altogether, the reviewed literature highlights the current state of stress integration research, as well unanswered questions about the brain basis of CVD risk.

Layer-Specific Vesicular Glutamate Transporter 1 Immunofluorescence Levels Delineate All Layers of the Human Hippocampus Including the Stratum lucidum

The hippocampal formation consists of the Ammon’s horn (cornu Ammonis with its regions CA1-4), dentate gyrus, subiculum, and the entorhinal cortex. The rough extension of the regions CA1-3 is typically defined based on the density and size of the pyramidal neurons without clear-cut boundaries. Here, we propose the vesicular glutamate transporter 1 (VGLUT1) as a molecular marker for the CA3 region. This is based on its strong labeling of the stratum lucidum (SL) in fluorescently stained human hippocampus sections. VGLUT1 puncta of the intense SL band co-localize with synaptoporin (SPO), a protein enriched in mossy fibers (MFs). Owing to its specific intensity profile throughout all hippocampal layers, VGLUT1 could be implemented as a pendant to Nissl-staining in fluorescent approaches with the additional demarcation of the SL. Furthermore, by high-resolution confocal microscopy, we detected VGLUT2 in the human hippocampus, thus reconciling two previous studies. Finally, by VGLUT1/SPO co-staining, we provide evidence for the existence of infrapyramidal MFs in the human hippocampus and we show that SPO expression is not restricted to MF synapses as demonstrated for rodent tissue.

Ventromedial Nucleus of the Hypothalamus Neurons Under the Magnifying Glass

The ventromedial nucleus of the hypothalamus (VMH) is a complex brain structure that is integral to many neuroendocrine functions, including glucose regulation, thermogenesis, and appetitive, social, and sexual behaviors. As such, it is of little surprise that the nucleus is under intensive investigation to decipher the mechanisms which underlie these diverse roles. Developments in genetic and investigative tools, for example the targeting of steroidogenic factor-1-expressing neurons, have allowed us to take a closer look at the VMH, its connections, and how it affects competing behaviors. In the current review, we aim to integrate recent findings into the literature and contemplate the conclusions that can be drawn.

Immunoelectron Microscopic Characterization of Vasopressin-Producing Neurons in the Hypothalamo-Pituitary Axis of Non-Human Primates by Use of Formaldehyde-Fixed Tissues Stored at -25 °C for Several Years

Translational research often requires the testing of experimental therapies in primates, but research in non-human primates is now stringently controlled by law around the world. Tissues fixed in formaldehyde without glutaraldehyde have been thought to be inappropriate for use in electron microscopic analysis, particularly those of the brain. Here we report the immunoelectron microscopic characterization of arginine vasopressin (AVP)-producing neurons in macaque hypothalamo-pituitary axis tissues fixed by perfusion with 4% formaldehyde and stored at −25 °C for several years (4–6 years). The size difference of dense-cored vesicles between magnocellular and parvocellular AVP neurons was detectable in their cell bodies and perivascular nerve endings located, respectively, in the posterior pituitary and median eminence. Furthermore, glutamate and the vesicular glutamate transporter 2 could be colocalized with AVP in perivascular nerve endings of both the posterior pituitary and the external layer of the median eminence, suggesting that both magnocellular and parvocellular AVP neurons are glutamatergic in primates. Both ultrastructure and immunoreactivity can therefore be sufficiently preserved in macaque brain tissues stored long-term, initially for light microscopy. Taken together, these results suggest that this methodology could be applied to the human post-mortem brain and be very useful in translational research.

The Structural and Electrophysiological Properties of Progesterone Receptor-Expressing Neurons Vary along the Anterior-Posterior Axis of the Ventromedial Hypothalamus and Undergo Local Changes across the Reproductive Cycle

Sex hormone levels continuously fluctuate across the reproductive cycle, changing the activity of neuronal circuits to coordinate female behavior and reproductive capacity. The ventrolateral division of the ventromedial hypothalamus (VMHvl) contains neurons expressing receptors for sex hormones and its function is intimately linked to female sexual receptivity. However, recent findings suggest that the VMHvl is functionally heterogeneous. Here, we used whole recordings and intracellular labeling to characterize the electrophysiological and morphologic properties of individual VMHvl neurons in naturally cycling females and report the existence of multiple electrophysiological phenotypes within the VMHvl. We found that the properties of progesterone receptor expressing (PR+) neurons, but not PR– neurons, depended systematically on the neuron’s location along the anterior-posterior (AP) axis of the VMHvl and the phase within the reproductive cycle. Prominent among this, the resting membrane potential of anterior PR+ neurons decreased during the receptive phase, while the excitability of medial PR+ neurons increased during the non-receptive phase. During the receptive phase of the cycle, posterior PR+ neurons simultaneously showed an increase in dendritic complexity and a decrease in spine density. These findings reveal an extensive diversity of local rules driving structural and physiological changes in response to fluctuating levels of sex hormones, supporting the anatomic and functional subdivision of the VMHvl and its possible role in the orchestration of different aspects of female socio-sexual behavior.

Tangential Intrahypothalamic Migration of the Mouse Ventral Premamillary Nucleus and Fgf8 Signaling

The tuberal hypothalamic ventral premamillary nucleus (VPM) described in mammals links olfactory and metabolic cues with mating behavior and is involved in the onset of puberty. We offer here descriptive and experimental evidence on a migratory phase in the development of this structure in mice at E12.5–E13.5. Its cells originate at the retromamillary area (RM) and then migrate tangentially rostralward, eschewing the mamillary body, and crossing the molecularly distinct perimamillary band, until they reach a definitive relatively superficial ventral tuberal location. Corroborating recent transcriptomic studies reporting a variety of adult glutamatergic cell types in the VPM, and different projections in the adult, we found that part of this population heterogeneity emerges already early in development, during tangential migration, in the form of differential gene expression properties of at least 2–3 mixed populations possibly derived from subtly different parts of the RM. These partly distribute differentially in the core and shell parts of the final VPM. Since there is a neighboring acroterminal source of Fgf8, and Fgfr2 is expressed at the early RM, we evaluated a possible influence of Fgf8 signal on VPM development using hypomorphic Fgf8^neo/null embryos. These results suggested a trophic role of Fgf8 on RM and all cells migrating tangentially out of this area (VPM and the subthalamic nucleus), leading in hypomorphs to reduced cellularity after E15.5 without alteration of the migrations proper.

Decoding neuronal composition and ontogeny of individual hypothalamic nuclei

The hypothalamus plays crucial roles in regulating endocrine, autonomic, and behavioral functions via its diverse nuclei and neuronal subtypes. The developmental mechanisms underlying ontogenetic establishment of different hypothalamic nuclei and generation of neuronal diversity remain largely unknown. Here, we show that combinatorial T-box 3 (TBX3), orthopedia homeobox (OTP), and distal-less homeobox (DLX) expression delineates all arcuate nucleus (Arc) neurons and defines four distinct subpopulations, whereas combinatorial NKX2.1/SF1 and OTP/DLX expression identifies ventromedial hypothalamus (VMH) and tuberal nucleus (TuN) neuronal subpopulations, respectively. Developmental analysis indicates that all four Arc subpopulations are mosaically and simultaneously generated from embryonic Arc progenitors, whereas glutamatergic VMH neurons and GABAergic TuN neurons are sequentially generated from common embryonic VMH progenitors. Moreover, clonal lineage-tracing analysis reveals that diverse lineages from multipotent radial glia progenitors orchestrate Arc and VMH-TuN establishment. Together, our study reveals cellular mechanisms underlying generation and organization of diverse neuronal subtypes and ontogenetic establishment of individual nuclei in the mammalian hypothalamus.

Effect of relative social rank within a social hierarchy on neural activation in response to familiar or unfamiliar social signals

Competent social functioning of group-living species relies on the ability of individuals to detect and utilize conspecific social cues to guide behavior. Previous studies have identified numerous brain regions involved in processing these external cues, collectively referred to as the Social Decision-Making Network. However, how the brain encodes social information with respect to an individual's social status has not been thoroughly examined. In mice, cues about an individual's identity, including social status, are conveyed through urinary proteins. In this study, we assessed the neural cFos immunoreactivity in dominant and subordinate male mice exposed to familiar and unfamiliar dominant and subordinate male urine. The posteroventral medial amygdala was the only brain region that responded exclusively to dominant compared to subordinate male urine. In all other brain regions, including the VMH, PMv, and vlPAG, activity is modulated by a combination of odor familiarity and the social status of both the urine donor and the subject receiving the cue. We show that dominant subjects exhibit robust differential activity across different types of cues compared to subordinate subjects, suggesting that individuals perceive social cues differently depending on social experience. These data inform further investigation of neurobiological mechanisms underlying social-status related brain differences and behavior.

Infralimbic cortical glutamate output is necessary for the neural and behavioral consequences of chronic stress

Exposure to prolonged stress is a major risk-factor for psychiatric disorders such as generalized anxiety and major depressive disorder. Human imaging studies have identified structural and functional abnormalities in the prefrontal cortex of subjects with depression and anxiety disorders, particularly Brodmann's area 25 (BA25). Further, deep brain stimulation of BA25 reduces symptoms of treatment-resistant depression. The rat homolog of BA25 is the infralimbic cortex (IL), which is critical for cognitive appraisal, executive function, and physiological stress reactivity. Previous studies indicate that the IL undergoes stress-induced changes in excitatory/inhibitory balance culminating in reduced activity of glutamate output neurons. However, the regulatory role of IL glutamate output in mood-related behaviors after chronic variable stress (CVS) is unknown. Here, we utilized a lentiviral-packaged small-interfering RNA to reduce translation of vesicular glutamate transporter 1 (vGluT1 siRNA), thereby constraining IL glutamate output. This viral-mediated gene transfer was used in conjunction with a quantitative anatomical analysis of cells expressing the stable immediate-early gene product FosB/ΔFosB, which accumulates in response to repeated neural activation. Through assessment of FosB/ΔFosB-expressing neurons across the frontal lobe in adult male rats, we mapped regions altered by chronic stress and determined the coordinating role of the IL in frontal cortical plasticity. Specifically, CVS-exposed rats had increased density of FosB/ΔFosB-expressing cells in the IL and decreased density in the insula. The latter effect was dependent on IL glutamate output. Next, we examined the interaction of CVS and reduced IL glutamate output in behavioral assays examining coping, anxiety-like behavior, associative learning, and nociception. IL glutamate knockdown decreased immobility during the forced swim test compared to GFP controls, both in rats exposed to CVS as well as rats without previous stress exposure. Further, vGluT1 siRNA prevented CVS-induced avoidance behaviors, while also reducing risk aversion and passive coping. Ultimately, this study identifies the necessity of IL glutamatergic output for regulating frontal cortical neural activity and behavior following chronic stress. These findings also highlight how disruption of excitatory/inhibitory balance within specific frontal cortical cell populations may impact neurobehavioral adaptation and lead to stress-related disorders.

Multiple convergent hypothalamus-brainstem circuits drive defensive behavior

The hypothalamus is composed of many neuropeptidergic cell populations and directs multiple survival behaviors, including defensive responses to threats. However, the relationship between the peptidergic identity of neurons and their roles in behavior remains unclear. Here, we address this issue by studying the function of multiple neuronal populations in the zebrafish hypothalamus during defensive responses to a variety of homeostatic threats. Cellular registration of large-scale neural activity imaging to multiplexed in situ gene expression revealed that neuronal populations encoding behavioral features encompass multiple overlapping sets of neuropeptidergic cell classes. Manipulations of different cell populations showed that multiple sets of peptidergic neurons play similar behavioral roles in this fast-timescale behavior through glutamate co-release and convergent output to spinal-projecting premotor neurons in the brainstem. Our findings demonstrate that homeostatic threats recruit neurons across multiple hypothalamic cell populations, which cooperatively drive robust defensive behaviors.

Amyloid-beta1-42 induced glutamatergic receptor and transporter expression changes in the mouse hippocampus

Alzheimer's disease (AD) is the leading type of dementia worldwide. With an increasing burden of an aging population coupled with the lack of any foreseeable cure, AD warrants the current intense research effort on the toxic effects of an increased concentration of beta-amyloid (Aβ) in the brain. Glutamate is the main excitatory brain neurotransmitter and it plays an essential role in the function and health of neurons and neuronal excitability. While previous studies have shown alterations in expression of glutamatergic signaling components in AD, the underlying mechanisms of these changes are not well understood. This is the first comprehensive anatomical study to characterize the subregion- and cell layer-specific long-term effect of Aβ_1-42 on the expression of specific glutamate receptors and transporters in the mouse hippocampus, using immunohistochemistry with confocal microscopy. Outcomes are examined 30 days after Aβ_1-42 stereotactic injection in aged male C57BL/6 mice. We report significant decreases in density of the glutamate receptor subunit GluA1 and the vesicular glutamate transporter (VGluT) 1 in the conus ammonis 1 region of the hippocampus in the Aβ_1-42 injected mice compared with artificial cerebrospinal fluid injected and naïve controls, notably in the stratum oriens and stratum radiatum. GluA1 subunit density also decreased within the dentate gyrus dorsal stratum moleculare in Aβ_1-42 injected mice compared with artificial cerebrospinal fluid injected controls. These changes are consistent with findings previously reported in the human AD hippocampus. By contrast, glutamate receptor subunits GluA2, GluN1, GluN2A, and VGluT2 showed no changes in expression. These findings indicate that Aβ_1-42 induces brain region and layer specific expression changes of the glutamatergic receptors and transporters, suggesting complex and spatial vulnerability of this pathway during development of AD neuropathology. Read the Editorial Highlight for this article on page 7. Cover Image for this issue: https://doi.org/10.1111/jnc.14763.

Maternal Obesity Modulates Expression of Satb2 in Hypothalamic VMN of Female Offspring

Maternal obesity during pregnancy is associated with a greater risk of poor health outcomes in offspring, including obesity, metabolic disorders, and anxiety, however the incidence of these diseases differs for males and females. Similarly, animal models of maternal obesity have reported sex differences in offspring, for both metabolic outcomes and anxiety-like behaviors. The ventromedial nucleus of the hypothalamus (VMN) is a brain region known to be involved in the regulation of both metabolism and anxiety, and is well documented to be sexually dimorphic. As the VMN is largely composed of glutamatergic neurons, which are important for its functions in modulating metabolism and anxiety, we hypothesized that maternal obesity may alter the number of glutamatergic neurons in the offspring VMN. We used a mouse model of a maternal high-fat diet (mHFD), to examine mRNA expression of the glutamatergic neuronal marker Satb2 in the mediobasal hypothalamus of control and mHFD offspring at GD17.5. We found sex differences in Satb2 expression, with mHFD-induced upregulation of Satb2 mRNA in the mediobasal hypothalamus of female offspring, compared to controls, but not males. Using immunohistochemistry, we found an increase in the number of SATB2-positive cells in female mHFD offspring VMN, compared to controls, which was localized to the rostral region of the nucleus. These data provide evidence that maternal nutrition during gestation alters the developing VMN, possibly increasing its glutamatergic drive of offspring in a sex-specific manner, which may contribute to sexual dimorphism in offspring health outcomes later in life.

VGLUT-VGAT expression delineates functionally specialised populations of vasopressin-containing neurones including a glutamatergic perforant path-projecting cell group to the hippocampus in rat and mouse brain

The origin and functional significance of vasopressin (AVP)-containing fibres in limbic regions has been an ongoing subject of investigation for several years. We have previously identified AVP-magnocellular neurones of rat hypothalamus that provide glutamatergic projections to the hippocampus, amygdala, lateral habenula and locus coeruleus. However, we also reported AVP-immunopositive fibres in those regions that are thin and make Gray type II synapses, which are unlikely to be of magnocellular origin. Therefore, in the present study, we characterised AVP mRNA co-expression with expression of mRNAs marking glutamatergic (vesicular glutamate transporter [VGLUT]) and GABAergic (vesicular GABA transporter [VGAT]) neuronal traits in rat and mouse brain, using high-resolution in situ hybridisation methods, including a radio-ribonucleotide and RNAscope 2.5 HD duplex assay, with Slc17a7, Slc17a6, Slc32a1 and Avp probes corresponding to mRNAs of VGLUT1, VGLUT2, VGAT and AVP, respectively. We located 18 cell groups expressing Avp and identified their molecular signatures for VGLUT and VGAT mRNA expression. Avp cell groups of hypothalamus and midbrain are mainly VGLUT mRNA-expressing, whereas those in regions derived from cerebral nuclei are mainly VGAT mRNA-expressing, suggesting a functional segregation of glutamate/GABA co-transmission with AVP. A newly identified Slc17a7 and Slc17a6 (but not Slc32a1) expressing vasopressinergic cell group was found in layer II-III neurones of the central entorhinal cortex, which projects to the hippocampus. These data support the notion of a complex role for AVP with respect to modulating multiple central circuits controlling behaviour in specific ways depending on co-transmission with glutamate or GABA, potentially giving rise to a functional classification of AVPergic neurones in the central nervous system.

Regulation and neurochemical identity of melanin-concentrating hormone neurones in the preoptic area of lactating mice

Neurones expressing the melanin-concentrating hormone (MCH) can be found in the medial preoptic area (mPOA) and ventral aspects of the periventricular preoptic nucleus of rats by mid-to-late lactation and this expression disappears after weaning. The transitory expression of MCH in the preoptic area suggests a role for these neurones in the control of the end of lactation. However, the neurochemical identity of mPOA MCH neurones and the regulatory factors that control the transient MCH expression remain largely unknown, especially in the mouse. In the present study, we showed that mice also present the transitory expression of MCH in the mPOA at late lactation. mPOA MCH cells did not colocalise significantly with markers of GABAergic (VGAT), glutamatergic (VGLUT2 and VGLUT3) or dopaminergic (tyrosine hydroxylase) neurones. mPOA MCH cells also did not express Kiss1 or oxytocin. By contrast, approximately 70% and 90% of mPOA MCH neurones colocalised with oestrogen receptor α and prolactin-induced phosphorylated signal transducer and activator of transcription 5 (STAT5), respectively. Finally, we demonstrated that the number of MCH neurones in the mPOA is significantly higher in females during the first lactation, compared to mice on the second lactation or pregnant mice during the first lactation or brain-specific STAT5 knockout mice during the first lactation. In summary, our findings indicate that MCH neurones in the mPOA of lactating mice are sensitive to oestrogens and prolactin. Thus, mPOA MCH expression is possibly influenced by hormonal variations. Furthermore, the STAT5 signalling pathway is likely involved in the regulation of MCH expression in the mPOA of lactating mice.

Prefrontal Cortex Regulates Chronic Stress-Induced Cardiovascular Susceptibility

Background The medial prefrontal cortex is necessary for appropriate appraisal of stressful information, as well as coordinating visceral and behavioral processes. However, prolonged stress impairs medial prefrontal cortex function and prefrontal-dependent behaviors. Additionally, chronic stress induces sympathetic predominance, contributing to health detriments associated with autonomic imbalance. Previous studies identified a subregion of rodent prefrontal cortex, infralimbic cortex (IL), as a key regulator of neuroendocrine-autonomic integration after chronic stress, suggesting that IL output may prevent chronic stress-induced autonomic imbalance. In the current study, we tested the hypothesis that the IL regulates hemodynamic, vascular, and cardiac responses to chronic stress. Methods and Results A viral-packaged small interfering RNA construct was used to knockdown vesicular glutamate transporter 1 (vGluT1) and reduce glutamate packaging and release from IL projection neurons. Male rats were injected with a vGluT1 small interfering RNA-expressing construct or GFP (green fluorescent protein) control into the IL and then remained as unstressed controls or were exposed to chronic variable stress. IL vGluT1 knockdown increased heart rate and mean arterial pressure reactivity, while chronic variable stress increased chronic mean arterial pressure only in small interfering RNA-treated rats. In another cohort, chronic variable stress and vGluT1 knockdown interacted to impair both endothelial-dependent and endothelial-independent vasoreactivity ex vivo. Furthermore, vGluT1 knockdown and chronic variable stress increased histological markers of fibrosis and hypertrophy. Conclusions Knockdown of glutamate release from IL projection neurons indicates that these cells are necessary to prevent the enhanced physiological responses to stress that promote susceptibility to cardiovascular pathophysiology. Ultimately, these findings provide evidence for a neurobiological mechanism mediating the relationship between stress and poor cardiovascular health outcomes.

Off-Target Effects in Transgenic Mice: Characterization of Dopamine Transporter (DAT)-Cre Transgenic Mouse Lines Exposes Multiple Non-Dopaminergic Neuronal Clusters Available for Selective Targeting within Limbic Neurocircuitry

Transgenic mouse lines are instrumental in our attempt to understand brain function. Promoters driving transgenic expression of the gene encoding Cre recombinase are crucial to ensure selectivity in Cre-mediated targeting of floxed alleles using the Cre-Lox system. For the study of dopamine (DA) neurons, promoter sequences driving expression of the Dopamine transporter (Dat) gene are often implemented and several DAT-Cre transgenic mouse lines have been found to faithfully direct Cre activity to DA neurons. While evaluating an established DAT-Cre mouse line, reporter gene expression was unexpectedly identified in cell somas within the amygdala. To indiscriminately explore Cre activity in DAT-Cre transgenic lines, systematic whole-brain analysis of two DAT-Cre mouse lines was performed upon recombination with different types of floxed reporter alleles. Results were compared with data available from the Allen Institute for Brain Science. The results identified restricted DAT-Cre-driven reporter gene expression in cell clusters within several limbic areas, including amygdaloid and mammillary subnuclei, septum and habenula, areas classically associated with glutamatergic and GABAergic neurotransmission. While no Dat gene expression was detected, ample co-localization between DAT-Cre-driven reporter and markers for glutamatergic and GABAergic neurons was found. Upon viral injection of a fluorescent reporter into the amygdala and habenula, distinct projections from non-dopaminergic DAT-Cre neurons could be distinguished. The study demonstrates that DAT-Cre transgenic mice, beyond their usefulness in recombination of floxed alleles in DA neurons, could be implemented as tools to achieve selective targeting in restricted excitatory and inhibitory neuronal populations within the limbic neurocircuitry.

Differential Effects of Dietary MSG on Hippocampal Dependent Memory Are Mediated by Diet

Introduction

Free glutamate is a common dietary flavor enhancer and is also an important excitatory neurotransmitter in the body. A good number of food additives which contain glutamate are found in the Western Diet, and this diet has also been linked to increased risk of cognitive dysfunction.

Objective

To examine the effects of dietary glutamate on hippocampal and non-hippocampal memory performance, and whether consuming a diet high in fat/sugar could influence any observed associations.

Methods

Sixty-four adult male Sprague-Dawley rats were trained concurrently on two different discrimination problems: (1) Pavlovian serial feature negative (sFN) discrimination, in which a brief tone stimulus was reinforced with sucrose pellets when it was presented alone (T+ trials) and non-reinforced on trials when it was preceded by the presentation of a brief light (LT− trials); and (2) a simple discrimination (SD) problem in which a white noise (WN+) cue was reinforced with sucrose pellets and a clicker (C-) stimulus was not reinforced. Previous research has shown that sFN, but not SD performance, depends on the functional integrity of the hippocampus. After solving both problems, the rats were assigned to one of four ad libitum-fed diet groups, matched on weight and discrimination performance: (1) high fat, high sugar western-style diet (WD), (2) standard laboratory rodent chow diet (chow), (3) WD + monosodium glutamate (MSG), or (4) chow + MSG.

Results

After 14 weeks, rats fed WD had higher adiposity than rats fed chow. Consistent with previous findings, rats fed WD exhibited impaired performance on the sFN problem, but not on the SD, relative to rats fed chow. Adding MSG to WD abolished this impairment, whereas rats fed chow + MSG had impaired sFN performance compared to rats fed chow alone. No differences in performance on the SD task were observed.

Conclusion

This study demonstrates differing effects of dietary glutamate on hippocampal dependent memory function, with MSG impairing hippocampal function in animals receiving chow, while improving hippocampal function in animals receiving a Western-type diet, high in fat and sugar. More research will be needed to explore the cause of these differential effects.

Activation of a lateral hypothalamic-ventral tegmental circuit gates motivation

Across species, motivated states such as food-seeking and consumption are essential for survival. The lateral hypothalamus (LH) is known to play a fundamental role in regulating feeding and reward-related behaviors. However, the contributions of neuronal subpopulations in the LH have not been thoroughly identified. Here we examine how lateral hypothalamic leptin receptor-expressing (LH^LEPR) neurons, a subset of GABAergic cells, regulate motivation in mice. We find that LH^LEPR neuronal activation significantly increases progressive ratio (PR) performance, while inhibition decreases responding. Moreover, we mapped LH^LEPR axonal projections and demonstrated that they target the ventral tegmental area (VTA), form functional inhibitory synapses with non-dopaminergic VTA neurons, and their activation promotes motivation for food. Finally, we find that LH^LEPR neurons also regulate motivation to obtain water, suggesting that they may play a generalized role in motivation. Together, these results identify LH^LEPR neurons as modulators within a hypothalamic-ventral tegmental circuit that gates motivation.

Mom doesn't care: When increased brain CRF system activity leads to maternal neglect in rodents

Mothers are the primary caregivers in mammals, ensuring their offspring's survival. This strongly depends on the adequate expression of maternal behavior, which is the result of a concerted action of "pro-maternal" versus "anti-maternal" neuromodulators such as the oxytocin and corticotropin-releasing factor (CRF) systems, respectively. When essential peripartum adaptations fail, the CRF system has negative physiological, emotional and behavioral consequences for both mother and offspring often resulting in maternal neglect. Here, we provide an elaborate and unprecedented review on the implications of the CRF system in the maternal brain. Studies in rodents have advanced our understanding of the specific roles of brain regions such as the limbic bed nucleus of the stria terminalis, medial preoptic area and lateral septum even in a CRF receptor subtype-specific manner. Furthermore, we discuss potential interactions of the CRF system with other neurotransmitters like oxytocin and noradrenaline, and present valuable translational aspects of the recent research.

Perineuronal nets in the lateral hypothalamus area regulate cue-induced reinstatement of cocaine-seeking behavior

We previously reported that a small, circumscribed region of the lateral hypothalamus, the anterior dorsal region (LHAad), stains heavily for PNNs and dense extracellular matrix (PNNs/ECM) with Wisteria floribunda agglutinin (WFA), and critically contributes to the acquisition of cocaine-induced conditioned place preference and cocaine self-administration. Here we tested the role of LHAad PNNs/ECM in cue-induced reinstatement in cocaine self-administering (SA) rats and identified how it is embedded in the circuitry of motivated behavior and drug reward. Degradation of PNNs/ECM in the LHAad using chondroitinase ABC (Ch-ABC) blocked the expression of cue-induced reinstatement of cocaine- but not sucrose-seeking behavior. We also identified for the first time the phenotype of LHAad PNN/ECM-surrounded neurons. LHAad neurons co-localized mainly with parvalbumin (PV+) and GABA. Predominant co-localization of WFA with VGLUT2 and GABA but not with GAD65/67 or glutamate indicates that the PNN/ECM-rich LHAad is predominantly GABAergic and receives dense glutamatergic input. The LHAad did not express significant amounts of melanin-concentrating hormone (MCH), orexin, or galanin; neuropeptides that regulate both food-induced and cocaine-induced behavior. In addition, retrobead injections demonstrated that the LHAad receives robust prelimbic prefrontal cortex (PFC) input and provides moderate input to the prelimbic PFC and ventral tegmental area (VTA), with no apparent input to the nucleus accumbens. In summary, the dense PNN/ECM zone in the LHAad embedded within the circuitry associated with reward pinpoints a novel region that controls the expression of cocaine-seeking behavior.

Macroscale intrinsic network architecture of the hypothalamus

Control of multiple life-critical physiological and behavioral functions requires the hypothalamus. Here, we provide a comprehensive description and rigorous analysis of mammalian intrahypothalamic network architecture. To achieve this at the gray matter region (macroscale) level, macroscale connection (macroconnection) data for the rat hypothalamus were extracted from the primary literature. The dataset indicated the existence of 7,982 (of 16,770 possible) intrahypothalamic macroconnections. Network analysis revealed that the intrahypothalamic macroconnection network (its macroscale subconnectome) is divided into two identical top-level subsystems (or subnetworks), each composed of two nested second-level subsystems. At the top-level, this suggests a deeply integrated network; however, regional grouping of the two second-level subsystems suggested a partial separation between control of physiological functions and behavioral functions. Furthermore, inclusion of four candidate hubs (dominant network nodes) in the second-level subsystem that is associated prominently with physiological control suggests network primacy with respect to this function. In addition, comparison of network analysis with expression of gene markers associated with inhibitory (GAD65) and excitatory (VGLUT2) neurotransmission revealed a significant positive correlation between measures of network centrality (dominance) and the inhibitory marker. We discuss these results in relation to previous understandings of hypothalamic organization and provide, and selectively interrogate, an updated hypothalamus structure-function network model to encourage future hypothesis-driven investigations of identified hypothalamic subsystems.

A Synaptically Connected Hypothalamic Magnocellular Vasopressin-Locus Coeruleus Neuronal Circuit and Its Plasticity in Response to Emotional and Physiological Stress

The locus coeruleus (LC)-norepinephrine (NE) system modulates a range of salient brain functions, including memory and response to stress. The LC-NE system is regulated by neurochemically diverse inputs, including a range of neuropeptides such as arginine-vasopressin (AVP). Whilst the origins of many of these LC inputs, their synaptic connectivity with LC neurons, and their contribution to LC-mediated brain functions, have been well characterized, this is not the case for the AVP-LC system. Therefore, our aims were to define the types of synapses formed by AVP+ fibers with LC neurons using immunohistochemistry together with confocal and transmission electron microscopy (TEM), the origins of such inputs, using retrograde tracers, and the plasticity of the LC AVP system in response to stress and spatial learning, using the maternal separation (MS) and Morris water maze (MWM) paradigms, respectively, in rat. Confocal microscopy revealed that AVP+ fibers contacting tyrosine hydroxylase (TH)+ LC neurons were also immunopositive for vesicular glutamate transporter 2, a marker of presynaptic glutamatergic axons. TEM confirmed that AVP+ axons formed Gray type I (asymmetric) synapses with TH+ dendrites thus confirming excitatory synaptic connections between these systems. Retrograde tracing revealed that these LC AVP+ fibers originate from hypothalamic vasopressinergic magnocellular neurosecretory neurons (AVPMNNs). MS induced a significant increase in the density of LC AVP+ fibers. Finally, AVPMNN circuit upregulation by water-deprivation improved MWM performance while increased Fos expression was found in LC and efferent regions such as hippocampus and prefrontal cortex, suggesting that AVPMMN projections to LC could integrate homeostatic responses modifying neuroplasticity.

Defining the caudal hypothalamic arcuate nucleus with a focus on anorexic excitatory neurons

KEY POINTS

Excitatory glutamate neurons are sparse in the rostral hypothalamic arcuate nucleus (ARC), the subregion that has received the most attention in the past. In striking contrast, excitatory neurons are far more common (by a factor of 10) in the caudal ARC, an area which has received relatively little attention. These glutamate cells may play a negative role in energy balance and food intake. They can show an increase in phosphorylated Stat-3 in the presence of leptin, are electrically excited by the anorectic neuromodulator cholecystokinin, and inhibited by orexigenic neuromodulators neuropeptide Y, met-enkephalin, dynorphin and the catecholamine dopamine. The neurons project local axonal connections that excite other ARC neurons including proopiomelanocortin neurons that can play an important role in obesity. These data are consistent with models suggesting that the ARC glutamatergic neurons may play both a rapid and a slower role in acting as anorectic neurons in CNS control of food intake and energy homeostasis.

ABSTRACT

Here we interrogate a unique class of excitatory neurons in the hypothalamic arcuate nucleus (ARC) that utilizes glutamate as a fast neurotransmitter using mice expressing GFP under control of the vesicular glutamate transporter 2 (vGluT2) promoter. These neurons show a unique distribution, synaptic characterization, cellular physiology and response to neuropeptides involved in energy homeostasis. Although apparently not previously appreciated, the caudal ARC showed a far greater density of vGluT2 cells than the rostral ARC, as seen in transgenic vGluT2-GFP mice and mRNA analysis. After food deprivation, leptin induced an increase in phosphorylated Stat-3 in vGluT2-positive neurons, indicating a response to hormonal cues of energy state. Based on whole-cell recording electrophysiology in brain slices, vGluT2 neurons were spontaneously active with a spike frequency around 2 Hz. vGluT2 cells were responsive to a number of neuropeptides related to energy homeostasis; they were excited by the anorectic peptide cholecystokinin, but inhibited by orexigenic neuropeptide Y, dynorphin and met-enkephalin, consistent with an anorexic role in energy homeostasis. Dopamine, associated with the hedonic aspect of enhancing food intake, inhibited vGluT2 neurons. Optogenetic excitation of vGluT2 cells evoked EPSCs in neighbouring neurons, indicating local synaptic excitation of other ARC neurons. Microdrop excitation of ARC glutamate cells in brain slices rapidly increased excitatory synaptic activity in anorexigenic proopiomelanocortin neurons. Together these data support the perspective that vGluT2 cells may be more prevalent in the ARC than previously appreciated, and play predominantly an anorectic role in energy metabolism.

Repeated low level domoic acid exposure increases CA1 VGluT1 levels, but not bouton density, VGluT2 or VGAT levels in the hippocampus of adult mice

Domoic acid (DA) is a neurotoxin produced during harmful algal blooms that accumulates in marine organisms that serve as food resources for humans. While acute DA neurotoxicity can cause seizures and hippocampal lesions, less is known regarding how chronic, subacute DA exposure in adulthood impacts the hippocampus. With more frequent occurrences of harmful algal blooms, it is important to understand the potential impact of repeated, low-level DA exposure on human health. To model repeated, low-dose DA exposure, adult mice received a single low-dose (0.75 ± 0.05 μg/g) of DA or vehicle weekly for 22 consecutive weeks. Quantitative immunohistochemistry was performed to assess the effects of repeated, low-level DA exposure on hippocampal cells and synapses. Vesicular glutamate transporter 1 (VGluT1) immunoreactivity within excitatory boutons in CA1 of DA-exposed mice was increased. Levels of other vesicular transporter proteins (i.e., VGluT2 and the vesicular GABA transporter (VGAT)) within boutons, and corresponding bouton densities, were not significantly altered in CA1, CA3, or dentate gyrus. There were no significant changes in neuron density or glial fibrillary acidic protein (GFAP) immunoreactivity following chronic, low-dose exposure. This suggests that repeated low doses of DA, unlike high doses of DA, do not cause neuronal loss or astrocyte activation in hippocampus in adult mice. Instead, these findings demonstrate that repeated exposure to low levels of DA leads to subtle changes in VGluT1 expression within CA1 excitatory boutons, which may alter glutamatergic transmission in CA1 and disrupt behaviors dependent on spatial memory.

Neurobiological characteristics underlying metabolic differences between males and females

The hypothalamus is the main integrating center for metabolic control. Our understanding of how hypothalamic circuits function to control appetite and energy expenditure has increased dramatically in recent years, due to the rapid rise in the incidence of obesity and the search for effective treatments. Increasing evidence indicates that these treatments will most likely differ between males and females. Indeed, sex differences in metabolism have been demonstrated at various levels, including in two of the most studied neuronal populations involved in metabolic control: the anorexigenic proopiomelanocortin neurons and the orexigenic neuropeptide Y/Agouti-related protein neurons. Here we review what is known to date regarding the sex differences in these two neuronal populations, as well as other neuronal populations involved in metabolic control and glial cells.

Gene expression profiling of the SCN in young and old rhesus macaques

In mammals, the suprachiasmatic nucleus (SCN) is the location of a master circadian pacemaker. It receives photic signals from the environment via the retinal hypothalamic tract, which play a key role in synchronizing the body's endogenously generated circadian rhythms with the 24-h rhythm of the environment. Therefore, it is plausible that age-related changes within the SCN contribute to the etiology of perturbed activity-rest cycles that become prevalent in humans during aging. To test this hypothesis, we used gene arrays and quantitative RT-PCR to profile age-related gene expression changes within the SCN of male rhesus macaques - a pragmatic translational animal model of human aging, which similarly displays an age-related attenuation of daytime activity levels. As expected, the SCN showed high expression of arginine vasopressin, vasoactive intestinal polypeptide, calbindin and nuclear receptor subfamily 1, group D, member 1 (NR1D1) (also known as reverse strand of ERBA (REV-ERBα), both at the mRNA and protein level. However, no obvious difference was detected between the SCNs of young (7-12 years) and old animals (21-26 years), in terms of the expression of core clock genes or genes associated with SCN signaling and neurotransmission. These data demonstrate the resilience of the primate SCN to normal aging, at least at the transcriptional level and, at least in males, suggest that age-related disruption of activity-rest cycles in humans may instead stem from changes within other components of the circadian system, such as desynchronization of subordinate oscillators in other parts of the body.

The Presynaptic Protein Mover Is Differentially Expressed Across Brain Areas and Synapse Types

The assembly and function of presynaptic nerve terminals relies on evolutionarily conserved proteins. A small number of presynaptic proteins occurs only in vertebrates. These proteins may add specialized functions to certain synapses, thus increasing synaptic heterogeneity. Here, we show that the vertebrate-specific synaptic vesicle (SV) protein mover is differentially distributed in the forebrain and cerebellum of the adult mouse. Using a quantitative immunofluorescence approach, we compare the expression of mover to the expression of the general SV marker synaptophysin in 16 brain areas. We find that mover is particularly abundant in the septal nuclei (SNu), ventral pallidum (VPa), amygdala and hippocampus. Within the hippocampus, mover is predominantly associated with excitatory synapses. Its levels are low in layers that receive afferent input from the entorhinal cortex, and high in layers harboring intra-hippocampal circuits. In contrast, mover levels are high in all nuclei of the amygdala, and mover is associated with inhibitory synapses in the medioposterior amygdala. Our data reveal a striking heterogeneity in the abundance of mover on three levels, i.e., between brain areas, within individual brain areas and between synapse types. This distribution suggests a role for mover in providing specialization to subsets of synapses, thereby contributing to the functional diversity of brain areas.

VGAT and VGLUT2 expression in MCH and orexin neurons in double transgenic reporter mice

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MCH neurons contain neither VGAT nor VGLUT2.

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Majority of orexin neurons contain VGLUT2.

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MCH neurons do not contain orexin.

The neuropeptides orexin and melanin-concentrating hormone (MCH), as well as the neurotransmitters GABA (γ-Aminobutyric acid) and glutamate are chief modulators of the sleep-wake states in the posterior hypothalamus. To investigate co-expression of vesicular GABA transporter (VGAT, a marker of GABA neurons) and the vesicular glutamate transporter-2 (VGLUT2, a marker of glutamate neurons) in orexin and MCH neurons, we generated two transgenic mouse lines. One line selectively expressed the reporter gene EYFP in VGAT+ neurons, whereas the other line expressed reporter gene tdTomato in VGLUT2+ neurons. Co-localization between these genetic reporters and orexin or MCH immunofluorescent tags was determined using 3D computer reconstructions of Z stacks that were acquired using a multiphoton laser confocal microscope. Our results demonstrated that MCH neurons expressed neither VGAT nor VGLUT2, suggesting MCH neurons are a separate cluster of cells from VGAT+ GABAergic neurons and VGLUT2+ glutamatergic neurons. Moreover, most orexin neurons expressed VGLUT2, indicating these neurons are glutamatergic. Our data suggested that in the posterior hypothalamus there are four major distinct groups of neurons: VGAT+, orexin+/VGLUT2+, orexin-/VGLUT2+, and MCH neurons. This study facilitated our understanding of the role of these neurotransmitters and neuropeptides in relation to sleep/wake regulation.

Hindbrain 5'-Adenosine Monophosphate-activated Protein Kinase Mediates Short-term Food Deprivation Inhibition of the Gonadotropin-releasing Hormone-Luteinizing Hormone Axis: Role of Nitric Oxide

Hindbrain-derived stimuli restrain the gonadotropin-releasing hormone (GnRH)-pituitary luteinizing hormone (LH) reproductive neuroendocrine axis during energy insufficiency. Interruption of food intake, planned or unplanned, is emblematic of modern life. This study investigated the premise that the hindbrain energy sensor 5'-adenosine monophosphate-activated protein kinase (AMPK) inhibits reproductive neuroendocrine function in short term, e.g. 18-h food-deprived (FD) estradiol (E)-implanted ovariectomized female rats. Intra-caudal fourth ventricular administration of the AMPK inhibitor Compound C (Cc) reversed FD-induced inhibition of rostral preoptic (rPO) GnRH protein expression and LH release in animals given E to replicate proestrus (high-E dose-, but not metestrus (low-E dose)-stage plasma steroid levels. FD caused Cc-reversible augmentation or diminution of preoptic norepinephrine (NE) activity in high- versus low-E rats, respectively, and AMPK-independent reductions in hypothalamic NE accumulation in the latter. Nitric oxide (NO) and kisspeptin are key stimulatory signals for the preovulatory LH surge. Here, FD inhibited rPO neuronal nitric oxide synthase protein expression in high-, but not low-E-dosed animals. Lateral ventricular delivery of the NO donor 3-morpholinosydnonimine (SIN-1) reversed inhibitory GnRH and LH responses to FD in high-E rats, and normalized rPO Vglut2, anteroventral periventricular KiSS1, and dorsomedial hypothalamic RFRP-3 mRNA and/or protein profiles. Data show that FD curtails reproductive neuroendocrine outflow by hindbrain AMPK-dependent mechanisms in the presence of peak estrous cycle E levels. Results indicate that neural networks linking this sensor to GnRH neurons likely involve NO signaling, which may function upstream of one or more neurotransmitters identified here by SIN-1-reversible inhibitory responses to FD.

Loss of dorsomedial hypothalamic GLP-1 signaling reduces BAT thermogenesis and increases adiposity

Objective

Glucagon-like peptide-1 (GLP-1) neurons in the hindbrain densely innervate the dorsomedial hypothalamus (DMH), a nucleus strongly implicated in body weight regulation and the sympathetic control of brown adipose tissue (BAT) thermogenesis. Therefore, DMH GLP-1 receptors (GLP-1R) are well placed to regulate energy balance by controlling sympathetic outflow and BAT function.

Methods

We investigate this possibility in adult male rats by using direct administration of GLP-1 (0.5 ug) into the DMH, knocking down DMH GLP-1R mRNA with viral-mediated RNA interference, and by examining the neurochemical phenotype of GLP-1R expressing cells in the DMH using in situ hybridization.

Results

GLP-1 administered into the DMH increased BAT thermogenesis and hepatic triglyceride (TG) mobilization. On the other hand, Glp1r knockdown (KD) in the DMH increased body weight gain and adiposity, with a concomitant reduction in energy expenditure (EE), BAT temperature, and uncoupling protein 1 (UCP1) expression. Moreover, DMH Glp1r KD induced hepatic steatosis, increased plasma TG, and elevated liver specific de-novo lipogenesis, effects that collectively contributed to insulin resistance. Interestingly, DMH Glp1r KD increased neuropeptide Y (NPY) mRNA expression in the DMH. GLP-1R mRNA in the DMH, however, was found in GABAergic not NPY neurons, consistent with a GLP-1R-dependent inhibition of NPY neurons that is mediated by local GABAergic neurons. Finally, DMH Glp1r KD attenuated the anorexigenic effects of the GLP-1R agonist exendin-4, highlighting an important role of DMH GLP-1R signaling in GLP-1-based therapies.

Conclusions

Collectively, our data show that DMH GLP-1R signaling plays a key role for BAT thermogenesis and adiposity.

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DMH GLP-1R stimulation acutely increases BAT thermogenesis.

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DMH GLP-1R mRNA knockdown decreases EE and BAT thermogenesis.

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DMH GLP-1R mRNA knockdown impairs lipid and glucose metabolism.

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Reduced DMH GLP-1R signaling blunts the anorexigenic responses to Ex-4.

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DMH GLP-1R signaling indirectly regulates NPY gene expression.

Hypoglycemia: Role of Hypothalamic Glucose-Inhibited (GI) Neurons in Detection and Correction

Hypoglycemia is a profound threat to the brain since glucose is its primary fuel. As a result, glucose sensors are widely located in the central nervous system and periphery. In this perspective we will focus on the role of hypothalamic glucose-inhibited (GI) neurons in sensing and correcting hypoglycemia. In particular, we will discuss GI neurons in the ventromedial hypothalamus (VMH) which express neuronal nitric oxide synthase (nNOS) and in the perifornical hypothalamus (PFH) which express orexin. The ability of VMH nNOS-GI neurons to depolarize in low glucose closely parallels the hormonal response to hypoglycemia which stimulates gluconeogenesis. We have found that nitric oxide (NO) production in low glucose is dependent on oxidative status. In this perspective we will discuss the potential relevance of our work showing that enhancing the glutathione antioxidant system prevents hypoglycemia associated autonomic failure (HAAF) in non-diabetic rats whereas VMH overexpression of the thioredoxin antioxidant system restores hypoglycemia counterregulation in rats with type 1 diabetes.We will also address the potential role of the orexin-GI neurons in the arousal response needed for hypoglycemia awareness which leads to behavioral correction (e.g., food intake, glucose administration). The potential relationship between the hypothalamic sensors and the neurocircuitry in the hindbrain and portal mesenteric vein which is critical for hypoglycemia correction will then be discussed.