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.

GHSR controls food deprivation-induced activation of CRF neurons of the hypothalamic paraventricular nucleus in a LEAP2-dependent manner.

OBJECTIVE: Prolonged fasting is a major challenge for living organisms. An appropriate metabolic response to food deprivation requires the activation of the corticotropin-releasing factor-producing neurons of the hypothalamic paraventricular nucleus (PVHCRF neurons), which are a part of the hypothalamic-pituitary-adrenal axis (HPA), as well as the growth hormone secretagogue receptor (GHSR) signaling, whose activity is up- or down-regulated, respectively, by the hormones ghrelin and the liver-expressed antimicrobial peptide 2 (LEAP2). Since ghrelin treatment potently up-regulates the HPA axis, we studied the role of GHSR in mediating food deprivation-induced activation of the PVHCRF neurons in mice. METHODS: We estimated the activation of the PVHCRF neurons, using immuno-staining against CRF and the marker of neuronal activation c-Fos in brain sections, and assessed plasma levels of corticosterone and glucose in different pharmacologically or genetically manipulated mouse models exposed, or not, to a 2-day food deprivation protocol. In particular, we investigated ad libitum fed or food-deprived male mice that: (1) lacked GHSR gene expression, (2) had genetic deletion of the ghrelin gene, (3) displayed neurotoxic ablation of the hypothalamic arcuate nucleus, (4) were centrally treated with an anti-ghrelin antibody to block central ghrelin action, (5) were centrally treated with a GHSR ligand that blocks ghrelin-evoked and constitutive GHSR activities, or (6) received a continuous systemic infusion of LEAP2(1-12). RESULTS: We found that food deprivation results in the activation of the PVHCRF neurons and in a rise of the ghrelin/LEAP2 molar ratio. Food deprivation-induced activation of PVHCRF neurons required the presence and the signaling of GHSR at hypothalamic level, but not of ghrelin. Finally, we found that preventing the food deprivation-induced fall of LEAP2 reverses the activation of the PVHCRF neurons in food-deprived mice, although it has no effect on body weight or blood glucose. CONCLUSION: Food deprivation-induced activation of the PVHCRF neurons involves ghrelin-independent actions of GHSR at hypothalamic level and requires a decrease of plasma LEAP2 levels. We propose that the up-regulation of the actions of GHSR associated to the fall of plasma LEAP2 level are physiologically relevant neuroendocrine signals during a prolonged fasting.

Reduced Numbers of Corticotropin-Releasing Hormone Neurons in Narcolepsy Type 1.

Narcolepsy type 1 (NT1) is a chronic sleep disorder correlated with loss of hypocretin(orexin). In NT1 post-mortem brains, we observed 88% reduction in corticotropin-releasing hormone (CRH)-positive neurons in the paraventricular nucleus (PVN) and significantly less CRH-positive fibers in the median eminence, whereas CRH-neurons in the locus coeruleus and thalamus, and other PVN neuronal populations were spared: that is, vasopressin, oxytocin, tyrosine hydroxylase, and thyrotropin releasing hormone-expressing neurons. Other hypothalamic cell groups, that is, the suprachiasmatic, ventrolateral preoptic, infundibular, and supraoptic nuclei and nucleus basalis of Meynert, were unaffected. The surprising selective decrease in CRH-neurons provide novel targets for diagnostics and therapeutic interventions. ANN NEUROL 2022;91:282-288.

Gαi/o-coupled Htr2c in the paraventricular nucleus of the hypothalamus antagonizes the anorectic effect of serotonin agents.

The anorexigenic effect of serotonergic compounds has largely been attributed to activation of serotonin 2C receptors (Htr2cs). Using mouse genetic models in which Htr2c can be selectively deleted or restored (in Htr2c-null mice), we investigate the role of Htr2c in forebrain Sim1 neurons. Unexpectedly, we find that Htr2c acts in these neurons to promote food intake and counteract the anorectic effect of serotonergic appetite suppressants. Furthermore, Htr2c marks a subset of Sim1 neurons in the paraventricular nucleus of the hypothalamus (PVH). Chemogenetic activation of these neurons in adult mice suppresses hunger, whereas their silencing promotes feeding. In support of an orexigenic role of PVH Htr2c, whole-cell patch-clamp experiments demonstrate that activation of Htr2c inhibits PVH neurons. Intriguingly, this inhibition is due to Gαi/o-dependent activation of ATP-sensitive K+ conductance, a mechanism of action not identified previously in the mammalian nervous system.

CB1 receptors in corticotropin-releasing factor neurons selectively control the acoustic startle response in male mice.

The endocannabinoid system is an important regulator of the hormonal and behavioral stress responses, which critically involve corticotropin-releasing factor (CRF) and its receptors. While it has been shown that CRF and the cannabinoid type 1 (CB1) receptor are co-localized in several brain regions, the physiological relevance of this co-expression remains unclear. Using double in situ hybridization, we confirmed co-localization in the piriform cortex, the lateral hypothalamic area, the paraventricular nucleus, and the Barrington's nucleus, albeit at low levels. To study the behavioral and physiological implications of this co-expression, we generated a conditional knockout mouse line that selectively lacks the expression of CB1 receptors in CRF neurons. We found no effects on fear and anxiety-related behaviors under basal conditions nor after a traumatic experience. Additionally, plasma corticosterone levels were unaffected at baseline and after restraint stress. Only acoustic startle responses were significantly enhanced in male, but not female, knockout mice. Taken together, the consequences of depleting CB1 in CRF-positive neurons caused a confined hyperarousal phenotype in a sex-dependent manner. The current results suggest that the important interplay between the central endocannabinoid and CRF systems in regulating the organism's stress response is predominantly taking place at the level of CRF receptor-expressing neurons.

Arginine vasopressin in the medial amygdala causes greater post-stress recruitment of hypothalamic vasopressin neurons.

Arginine vasopressin (AVP) is expressed in both hypothalamic and extra-hypothalamic neurons. The expression and role of AVP exhibit remarkable divergence between these two neuronal populations. Polysynaptic pathways enable these neuronal groups to regulate each other. AVP neurons in the paraventricular nucleus of the hypothalamus increase the production of adrenal stress hormones by stimulating the hypothalamic-pituitary-adrenal axis. Outside the hypothalamus, the medial amygdala also contains robust amounts of AVP. Contrary to the hypothalamic counterpart, the expression of extra-hypothalamic medial amygdala AVP is sexually dimorphic, in that it is preferentially transcribed in males in response to the continual presence of testosterone. Male gonadal hormones typically generate a negative feedback on the neuroendocrine stress axis. Here, we investigated whether testosterone-responsive medial amygdala AVP neurons provide negative feedback to hypothalamic AVP, thereby providing a feedback loop to suppress stress endocrine response during periods of high testosterone secretion. Contrary to our expectation, we found that AVP overexpression within the posterodorsal medial amygdala increased the recruitment of hypothalamic AVP neurons during stress, without affecting the total number of AVP neurons or the number of recently activated neurons following stress. These observations suggest that the effects of testosterone on extra-hypothalamic AVP facilitate stress responsiveness through permissive influence on the recruitment of hypothalamic AVP neurons.

Oxytocin system driven by experiences modifies social recognition and neuron morphology in female BALB/c mice.

The oxytocin (OT) system, affected by life experiences, modulates neuron morphology in a sex-specific manner, leading to sex differences in social interactions. To date, few studies have focused on the OT system and social interactions of female mice. In this study, we used maternal deprivation (MD) and its possible treatment, environmental enrichment (EE), to affect social recognition in female BALB/c mice. We checked neuron morphology, synaptic connections, oxytocinergic (OTergic) neurons in the hypothalamus paraventricular nucleus (PVH), and OT receptor (OTR) in the basolateral amygdala (BLA) and layer II/III of the prelimbic cortex (PL). Our results showed that MD induced social recognition impairments, increased OTR levels in the BLA, and, meanwhile, reduced OTergic neurons in the magnocellular region of the PVH (mPVH). Decreased Nissl bodies, increased cell nuclei, and increased dendrites of projection neurons paralleled the increased OTR levels in the BLA of MD mice. EE restored MD-induced the impairments of novel object recognition and sociability; this effect paralleled a decrease in cell density in the PL and an increase in OTergic neurons in the parvocellular regions of the PVH and synaptic connections in the BLA and layer II/III of the PL. Our findings indicate that early life stress such as MD impairs social recognition, and meanwhile, remodels neuron morphology region-specifically in the female brain, apparently in the BLA but slightly in the PL; and EE could partially restore the deficits induced by MD. The results provide new insights into sex differences in the prevalence of psychological development disorders.

Oxytocin neurons enable social transmission of maternal behaviour.

Maternal care, including by non-biological parents, is important for offspring survival1-8. Oxytocin1,2,9-15, which is released by the hypothalamic paraventricular nucleus (PVN), is a critical maternal hormone. In mice, oxytocin enables neuroplasticity in the auditory cortex for maternal recognition of pup distress15. However, it is unclear how initial parental experience promotes hypothalamic signalling and cortical plasticity for reliable maternal care. Here we continuously monitored the behaviour of female virgin mice co-housed with an experienced mother and litter. This documentary approach was synchronized with neural recordings from the virgin PVN, including oxytocin neurons. These cells were activated as virgins were enlisted in maternal care by experienced mothers, who shepherded virgins into the nest and demonstrated pup retrieval. Virgins visually observed maternal retrieval, which activated PVN oxytocin neurons and promoted alloparenting. Thus rodents can acquire maternal behaviour by social transmission, providing a mechanism for adapting the brains of adult caregivers to infant needs via endogenous oxytocin.

Dedicated C-fiber vagal sensory afferent pathways to the paraventricular nucleus of the hypothalamus.

The nucleus of the solitary tract (NTS) receives viscerosensory information from the vagus nerve to regulate diverse homeostatic reflex functions. The NTS projects to a wide network of other brain regions, including the paraventricular nucleus of the hypothalamus (PVN). Here we examined the synaptic characteristics of primary afferent pathways to PVN-projecting NTS neurons in rat brainstem slices.Expression of the Transient Receptor Potential Vanilloid receptor (TRPV1+ ) distinguishes C-fiber afferents within the solitary tract (ST) from A-fibers (TRPV1-). We used resiniferatoxin (RTX), a TRPV1 agonist, to differentiate the two. The variability in the latency (jitter) of evoked excitatory postsynaptic currents (ST-EPSCs) distinguished monosynaptic from polysynaptic ST-EPSCs. Rhodamine injected into PVN was retrogradely transported to identify PVN-projecting NTS neurons within brainstem slices. Graded shocks to the ST elicited all-or-none EPSCs in rhodamine-positive NTS neurons with latencies that had either low jitter (<200 µs - monosynaptic), high jitter (>200 µs - polysynaptic inputs) or both. RTX blocked ST-evoked TRPV1 + EPSCs whether mono- or polysynaptic. Most PVN-projecting NTS neurons (17/21 neurons) had at least one input polysynaptically connected to the ST. Compared to unlabeled NTS neurons, PVN-projecting NTS neurons were more likely to receive indirect inputs and be higher order. Surprisingly, sEPSC rates for PVN-projecting neurons were double that of unlabeled NTS neurons. The ST synaptic responses for PVN-projecting NTS neurons were either all TRPV1+ or all TRPV1-, including neurons that received both direct and indirect inputs. Overall, PVN-projecting NTS neurons received direct and indirect vagal afferent information with strict segregation regarding TRPV1 expression.

Dendritic osmosensors modulate activity-induced calcium influx in oxytocinergic magnocellular neurons of the mouse PVN.

Hypothalamic oxytocinergic magnocellular neurons have a fascinating ability to release peptide from both their axon terminals and from their dendrites. Existing data indicates that the relationship between somatic activity and dendritic release is not constant, but the mechanisms through which this relationship can be modulated are not completely understood. Here, we use a combination of electrical and optical recording techniques to quantify activity-induced calcium influx in proximal vs. distal dendrites of oxytocinergic magnocellular neurons located in the paraventricular nucleus of the hypothalamus (OT-MCNs). Results reveal that the dendrites of OT-MCNs are weak conductors of somatic voltage changes; however, activity-induced dendritic calcium influx can be robustly regulated by both osmosensitive and non-osmosensitive ion channels located along the dendritic membrane. Overall, this study reveals that dendritic conductivity is a dynamic and endogenously regulated feature of OT-MCNs that is likely to have substantial functional impact on central oxytocin release.

Oxytocin is often referred to as a 'love hormone' because it can be released during activities such as hugging, snuggling, or sex. Reality, of course, can be a bit more complicated. In the brain, oxytocin can have powerful and diverse effects on mood, stress, anxiety, and social interactions. In the body it helps regulate fluid balance, promotes contractions during childbirth, and stimulates the letdown of milk during breastfeeding. Much of the oxytocin produced in both humans and rodents comes from oxytocin-synthetizing magnocellular neurons located in an area of the brain called the hypothalamus. These very specialized neurons have separate, but overlapping, mechanisms for releasing oxytocin into the brain and into the rest of the body. This means that while certain signals cause the neurons to release oxytocin into the body and the brain at the same time, others can cause them to release the hormone preferentially into the body or the brain. Sheng et al. wanted to better understand how these different release mechanisms work, and, in particular, to learn more about how release of oxytocin into the brain is regulated. This is important, because when oxytocin is given as a medicine, much of it fails to reach the brain. A lot of the oxytocin that acts in the brain is released from a specific part of the oxytocin-synthesizing magnocellular neurons called the dendrites. When these neurons are stimulated, calcium enters the dendrites, triggering the release of oxytocin directly into the brain. Sheng et al. used electrical and optical tools on brain tissue extracted from mice to measure how different signals change the amount of calcium that enters the dendrites of oxytocin-synthesizing magnocellular neurons in response to a consistent stimulus. The results showed that increasing the osmolarity, the amount of water-soluble particles that cannot spontaneously cross the cell membrane, in the liquid surrounding the neurons reduced the amount of calcium that flowed into the dendrites during stimulation. Meanwhile, decreasing osmolarity had the opposite effect. Sheng et al. also found that the influx of calcium induced by stimulating the neurons can be strongly regulated by activating receptors in the dendrites that detect a common molecule in the brain called GABA. This occurs even absent a change in osmolarity. These results shed light on some of the physiological processes that control the release of oxytocin into the brain. Understanding these processes is a necessary step towards developing new drugs intended to regulate levels of oxytocin in the brain. Such drugs could be useful in the treatment of several types of mental health disorders.

In Vivo Electrophysiology of Peptidergic Neurons in Deep Layers of the Lumbar Spinal Cord after Optogenetic Stimulation of Hypothalamic Paraventricular Oxytocin Neurons in Rats.

The spinal ejaculation generator (SEG) is located in the central gray (lamina X) of the rat lumbar spinal cord and plays a pivotal role in the ejaculatory reflex. We recently reported that SEG neurons express the oxytocin receptor and are activated by oxytocin projections from the paraventricular nucleus of hypothalamus (PVH). However, it is unknown whether the SEG responds to oxytocin in vivo. In this study, we analyzed the characteristics of the brain-spinal cord neural circuit that controls male sexual function using a newly developed in vivo electrophysiological technique. Optogenetic stimulation of the PVH of rats expressing channel rhodopsin under the oxytocin receptor promoter increased the spontaneous firing of most lamina X SEG neurons. This is the first demonstration of the in vivo electrical response from the deeper (lamina X) neurons in the spinal cord. Furthermore, we succeeded in the in vivo whole-cell recordings of lamina X neurons. In vivo whole-cell recordings may reveal the features of lamina X SEG neurons, including differences in neurotransmitters and response to stimulation. Taken together, these results suggest that in vivo electrophysiological stimulation can elucidate the neurophysiological response of a variety of spinal neurons during male sexual behavior.

A subpopulation of embryonic microglia respond to maternal stress and influence nearby neural progenitors.

The interplay between hypothalamic neurons and microglia as they integrate stressors to regulate homeostasis is of growing interest. We asked if microglia in the embryonic hypothalamus were likewise stress responsive and, if so, whether their precocious activation perturbs nearby neural stem cell (NSC) programs. We performed single-cell transcriptomics to define embryonic hypothalamic microglia heterogeneity and identified four microglial subsets, including a subpopulation adjacent to NSCs that was responsive to gestational cold stress. Stress exposure elevated CCL3 and CCL4 secretion, but only in male brains, and ex vivo CCL4 treatment of hypothalamic NSCs altered proliferation and differentiation. Concomitantly, gestational stress decreased PVN oxytocin neurons only in male embryos, which was reversed by microglia depletion. Adult offspring exposed to gestational stress displayed altered social behaviors, which was likewise microglia dependent, but only in males. Collectively, immature hypothalamic microglia play an unappreciated role in translating maternal stressors to sexually dimorphic perturbation of neurodevelopmental programs.

The effect of amisulpride, olanzapine, quetiapine, and aripiprazole single administration on c-Fos expression in vasopressinergic and oxytocinergic neurons of the rat hypothalamic paraventricular nucleus.

Antipsychotics, including amisulpride (AMI), quetiapine (QUE), aripiprazole (ARI), and olanzapine (OLA), are used to treat mental illnesses associated with psychotic symptoms. The effect of these drugs on c-Fos expression in vasopressinergic (AVP) and oxytocinergic (OXY) neurons was studied in the hypothalamic paraventricular nucleus (PVN) of rats. The presence of c-Fos in AVP and OXY perikarya was investigated in seven PVN cells segregations: the anterior (Ant), dorsal cup (Dc), wing-shaped (Wi), periventricular zone (Pe), circle-shaped core (Co) and shell of core (Sh), and the posterior (pPVN) after an acute treatment with AMI-20 mg/kg, QUE-15 mg/kg, ARI-10 mg/kg, and OLA-5 mg/kg/bw in rats. Ninety min after treatments, the animals were sacrificed by transcardial perfusion with fixative and the PVN area sliced into 35 µm thick coronal sections for immunohistochemistry. The c-Fos was processed by avidin-biotin-peroxidase complex intensified with nickel-enhanced 3,3'-diaminobenzidine tetrahydrochloride. Visualization of AVP- and OXY-synthesizing neurons was achieved by a fluorescent marker Alexa Flour 568. The c-Fos-AVP and c-Fos-OXY colocalizations were evaluated from c-Fos stained sections merged with AVP or OXY ones. AMI, QUE, ARI, and OLA, single administration distinctly increased the c-Fos expression in each of the PVN cells segregations. QUE induced the highest magnitude of activation of AVP and OXY neurons, while OLA and AMI had only moderate effects. Incontestable variabilities detected in c-Fos expression in PVN AVP and OXY neurons extend the knowledge of selected antipsychotics extra-striatal actions and may also be helpful in a presumption of their possible functional impact.

Network dynamics of hypothalamic feeding neurons.

Mutations in the melanocortin 4 receptor (MC4R) result in hyperphagia and obesity and are the most common cause of monogenic obesity in humans. Preclinical rodent studies have determined that the critical role of the MC4R in controlling feeding can be mapped in part to its expression in the paraventricular nucleus of the hypothalamus (paraventricular nucleus [PVN]), where it regulates the activity of anorexic neural circuits. Despite the critical role of PVN MC4R neurons in regulating feeding, the in vivo neuronal activity of these cells remains largely unstudied, and the network activity of PVN MC4R neurons has not been determined. Here, we utilize in vivo single-cell endomicroscopic and mathematical approaches to determine the activity and network dynamics of PVN MC4R neurons in response to changes in energy state and pharmacological manipulation of central melanocortin receptors. We determine that PVN MC4R neurons exhibit both quantitative and qualitative changes in response to fasting and refeeding. Pharmacological stimulation of MC4R with the therapeutic MC4R agonist setmelanotide rapidly increases basal PVN MC4R activity, while stimulation of melanocortin 3 receptor (MC3R) inhibits PVN MC4R activity. Finally, we find that distinct PVN MC4R neuronal ensembles encode energy deficit and energy surfeit and that energy surfeit is associated with enhanced network connections within PVN MC4R neurons. These findings provide valuable insight into the neural dynamics underlying hunger and energy surfeit.

Fasting reduces the number of TRH immunoreactive neurons in the hypothalamic paraventricular nucleus of male rats, but not in mice.

During fasting or weight loss, the fall in leptin levels leads to suppression of thyrotropin-releasing hormone (TRH) expression in the paraventricular nucleus of the hypothalamus (PVH) and, consequently, inhibition of the hypothalamic-pituitary-thyroid (HPT) axis. However, differently than rats, just few PVHTRH neurons express the leptin receptor in mice. In the present study, male adult rats and mice were submitted to 48 -h fasting to evaluate the consequences on proTRH peptide expression at the PVH level. Additionally, the proTRH peptide expression was also assessed in the brains of leptin-deficient (Lepob/ob) mice. We observed that approximately 50 % of PVHTRH neurons of leptin-injected rats exhibited phosphorylation of the signal transducer and activator of transcription 3 (pSTAT3), a marker of leptin receptor activation. In contrast, very few PVHTRH neurons of leptin-injected mice exhibited pSTAT3. Rats submitted to 48 -h fasting showed a significant reduction in the number of PVHTRH immunoreactive neurons, as compared to fed rats. On the other hand, no changes in the number of PVHTRH immunoreactive neurons were observed between fasted and fed mice. Next, the number of TRH immunoreactive cells was determined in the PVH, dorsomedial nucleus of the hypothalamus and nucleus raphe pallidus of Lepob/ob and wild-type mice and no significant differences were observed, despite reduced plasma T4 levels in Lepob/ob mice. Taken together, these findings provide additional evidence of the important species-specific differences in the mechanisms used by fasting and/or leptin to regulate the HPT axis.

Prepubertal and adult male rats differ in the degree and pattern of stress reactive neurons in brain regions that project to the paraventricular nucleus of the hypothalamus.

The hormonal stress response, mediated by the hypothalamic-pituitary-adrenal (HPA) axis, shows greater responsiveness to various stressors in prepubertal compared to adult animals. Though the implications of this age-related change are unclear, this heightened reactivity might contribute to the increase in stress-related dysfunctions observed during adolescence. Interestingly, prepubertal animals show greater stress-induced neural activation compared to adults in the paraventricular nucleus of the hypothalamus (PVN), the area responsible for initiating the hormonal stress response. Thus, it is possible that direct afferents to the PVN, such as the anterior bed nucleus of the stria terminalis (aBST), nucleus of the solitary tract (NTS), posterior BST (pBST), medial preoptic area (MPOA), and dorsomedial nucleus (DMN), contribute to this age-dependent change in reactivity. To investigate these possibilities, two separate experiments were conducted in prepubertal (30 days old) and adult (70 days old) male rats using the retrograde tracer, Fluoro-Gold (FG), and FOS immunohistochemistry to study neural connectivity and activation, respectively. Though there was no difference in the number or size of FG-positive cells in the PVN afferents we examined, we found a significantly greater number of stress-induced FOS-like-positive cells in the aBST and significantly fewer in the DMN in prepubertal compared to adult animals. Together these data suggest that functional, instead of structural, changes in nuclei that project to the PVN may lead to the greater PVN stress responsiveness observed prior to adolescence. Furthermore, these data indicate that nuclei known to directly modulate HPA stress responsiveness show differential activation patterns before and after adolescent development.

Discrete TrkB-expressing neurons of the dorsomedial hypothalamus regulate feeding and thermogenesis.

Mutations in the TrkB neurotrophin receptor lead to profound obesity in humans, and expression of TrkB in the dorsomedial hypothalamus (DMH) is critical for maintaining energy homeostasis. However, the functional implications of TrkB-fexpressing neurons in the DMH (DMHTrkB) on energy expenditure are unclear. Additionally, the neurocircuitry underlying the effect of DMHTrkB neurons on energy homeostasis has not been explored. In this study, we show that activation of DMHTrkB neurons leads to a robust increase in adaptive thermogenesis and energy expenditure without altering heart rate or blood pressure, while silencing DMHTrkB neurons impairs thermogenesis. Furthermore, we reveal neuroanatomically and functionally distinct populations of DMHTrkB neurons that regulate food intake or thermogenesis. Activation of DMHTrkB neurons projecting to the raphe pallidus (RPa) stimulates thermogenesis and increased energy expenditure, whereas DMHTrkB neurons that send collaterals to the paraventricular hypothalamus (PVH) and preoptic area (POA) inhibit feeding. Together, our findings provide evidence that DMHTrkB neuronal activity plays an important role in regulating energy expenditure and delineate distinct neurocircuits that underly the separate effects of DMHTrkB neuronal activity on food intake and thermogenesis.

Mast cells in the paraventricular nucleus participate in visceral hypersensitivity induced by neonatal maternal separation.

Early-life stress (ELS) is a high-risk factor for the development of chronic visceral pain in adulthood. Emerging evidence suggests that mast cells play a key role in the development of visceral hypersensitivity through interaction with neurons. The sensitization of corticotropin-releasing factor (CRF) neurons in the hypothalamic paraventricular nucleus (PVN) plays a pivotal role in the pathogenesis of visceral pain. However, the precise mechanism by which mast cells and CRF neurons interact in the PVN in the pathogenesis of visceral hypersensitivity remains elusive. In the present study, we used neonatal maternal separation (MS), an ELS model, and observed that neonatal MS induced visceral hypersensitivity and triggered PVN mast cell activation in adult rats, which was repressed by intra-PVN infusion of the mast cell stabilizer disodium cromoglycate (cromolyn). Wild-type (WT) mice but not mast cell-deficient KitW-sh/W-sh mice that had experienced neonatal MS exhibited chronic visceral hypersensitivity. MS was associated with an increase in the expression of proinflammatory mediators, the number of CRF+ cells and CRF protein in the PVN, which was prevented by intra-PVN infusion of cromolyn. Furthermore, we demonstrated that intra-PVN infusion of the mast degranulator compound 48/80 significantly induced mast cell activation, resulting in proinflammatory mediator release, CRF neuronal sensitization, and visceral hypersensitivity, which was suppressed by cromolyn. Overall, our findings demonstrated that neonatal MS induces the activation of PVN mast cells, which secrete numerous proinflammatory mediators that may participate in neighboring CRF neuronal activity, ultimately directly inducing visceral hypersensitivity in adulthood.

Characterization of Kisspeptin Neurons in the Human Rostral Hypothalamus.

BACKGROUND: Kisspeptin (KP) neurons in the rostral periventricular region of the 3rd ventricle (RP3V) of female rodents mediate positive estrogen feedback to gonadotropin-releasing hormone neurons and, thus, play a fundamental role in the mid-cycle luteinizing hormone (LH) surge. The RP3V is sexually dimorphic, and male rodents with lower KP cell numbers are unable to mount estrogen-induced LH surges. OBJECTIVE: To find and characterize the homologous KP neurons in the human brain, we studied formalin-fixed post-mortem hypothalami. METHODS: Immunohistochemical techniques were used. RESULTS: The distribution of KP neurons in the rostral hypothalamus overlapped with distinct subdivisions of the paraventricular nucleus. The cell numbers decreased after menopause, indicating that estrogens positively regulate KP gene expression in the rostral hypothalamus in humans, similarly to several other species. Young adult women and men had similar cell numbers, as opposed to rodents reported to have more KP neurons in the RP3V of females. Human KP neurons differed from the homologous rodent cells as well, in that they were devoid of enkephalins, galanin and tyrosine hydroxylase. Further, they did not contain known KP neuron markers of the human infundibular nucleus, neurokinin B, substance P and cocaine- and amphetamine-regulated transcript, while they received afferent input from these KP neurons. CONCLUSIONS: The identification and positive estrogenic regulation of KP neurons in the human rostral hypothalamus challenge the long-held view that positive estrogen feedback may be restricted to the mediobasal part of the hypothalamus in primates and point to the need of further anatomical, molecular and functional studies of rostral hypothalamic KP neurons.

Neurochemical phenotype of growth hormone-responsive cells in the mouse paraventricular nucleus of the hypothalamus.

Multiple neuroendocrine, autonomic and behavioral responses are regulated by the paraventricular nucleus of the hypothalamus (PVH). Previous studies have shown that PVH neurons express the growth hormone (GH) receptor (GHR), although the role of GH signaling on PVH neurons is still unknown. Given the great heterogeneity of cell types located in the PVH, we performed a detailed analysis of the neurochemical identity of GH-responsive cells to understand the possible physiological importance of GH action on PVH neurons. GH-responsive cells were detected via the phosphorylated form of the signal transducer and activator of transcription-5 (pSTAT5) in adult male mice that received an intraperitoneal GH injection. Approximately 51% of GH-responsive cells in the PVH co-localized with the vesicular glutamate transporter 2. Rare co-localizations between pSTAT5 and vesicular GABA transporter or vasopressin were observed, whereas approximately 20% and 38% of oxytocin and tyrosine hydroxylase (TH) cells, respectively, were responsive to GH in the PVH. Approximately 55%, 35% and 63% of somatostatin, thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH) neurons expressed GH-induced pSTAT5, respectively. Additionally, 8%, 49% and 75% of neuroendocrine TH, TRH and CRH neurons, and 67%, 32% and 74% of nonneuroendocrine TH, TRH and CRH neurons were responsive to GH in the PVH of Fluoro-Gold-injected mice. Our findings suggest that GH action on PVH neurons is involved in the regulation of the thyroid, somatotropic and adrenal endocrine axes, possibly influencing homeostatic and stress responses.