Chronic stress dampens excitatory synaptic gain in the paraventricular nucleus of the hypothalamus

Modulating reward and aversion: Insights into addiction from the paraventricular nucleus

BACKGROUND

Drug addiction, characterized by compulsive drug use and high relapse rates, arises from complex interactions between reward and aversion systems in the brain. The paraventricular nucleus (PVN), located in the anterior hypothalamus, serves as a neuroendocrine center and is a key component of the hypothalamic-pituitary-adrenal axis.

OBJECTIVE

This review aimed to explore how the PVN impacts reward and aversion in drug addiction through stress responses and emotional regulation and to evaluate the potential of PVN as a therapeutic target for drug addiction.

METHODS

We review the current literature, focusing on three main neuron types in the PVN-corticotropin-releasing factor, oxytocin, and arginine vasopressin neurons-as well as other related neurons, to understand their roles in modulating addiction.

RESULTS

Existing studies highlight the PVN as a key mediator in addiction, playing a dual role in reward and aversion systems. These findings are crucial for understanding addiction mechanisms and developing targeted therapies.

CONCLUSION

The role of PVN in stress response and emotional regulation suggests its potential as a therapeutic target in drug addiction, offering new insights for addiction treatment.

Windows into stress: a glimpse at emerging roles for CRHPVN neurons

The corticotropin-releasing hormone cells in the paraventricular nucleus of the hypothalamus (CRH^PVN) control the slow endocrine response to stress. The synapses on these cells are exquisitely sensitive to acute stress, leveraging local signals to leave a lasting imprint on this system. Additionally, recent work indicates that these cells also play key roles in the control of distinct stress and survival behaviors. Here we review these observations and provide a perspective on the role of CRH^PVN neurons as integrative and malleable hubs for behavioral, physiological, and endocrine responses to stress.

Increased GABAergic projections in the paraventricular nucleus regulate colonic hypersensitivity via oxytocin in a rat model of irritable bowel syndrome

Irritable bowel syndrome (IBS) is characterized by gastrointestinal dysmotility and visceral hyperalgesia, and the impaired brain-gut axis is accepted as a crucial cause for the onset of IBS. The objective of this study is to investigate the effects of the adaptive changes in the central neural system induced by stress on IBS-like syndromes in rats. Long-term water avoidance stress (WAS) was used to prepare IBS animals. The changes in neuronal excitation and GABA expression were shown by immunohistochemistry. The mRNA and protein expressions of neurotransmitters were detected with Quantitative reverse-transcription PCR (qRT-PCR) and Enzyme-linked immunosorbent assay (ELISA). The intestinal transit time, fecal moisture content, and abdominal withdrawal reflex scores of rats were recorded to monitor intestinal motility and visceral hyperalgesia. In the WAS-treated rats with enhanced intestinal motility and visceral hypersensitivity, more GABAergic projections were found in the paraventricular nucleus (PVN) of the hypothalamus, which inhibited the firing rate of neurons and decreased the expression of oxytocin. Exogenous oxytocin improved gut motility and decreased AWR scores. The inhibition of oxytocin by the adaptive GABAergic projection in the PVN might be an important mediator of IBS, which indicates a potential novel therapeutic target.

State-dependent activity dynamics of hypothalamic stress effector neurons

The stress response necessitates an immediate boost in vital physiological functions from their homeostatic operation to an elevated emergency response. However, the neural mechanisms underlying this state-dependent change remain largely unknown. Using a combination of in vivo and ex vivo electrophysiology with computational modeling, we report that corticotropin releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN), the effector neurons of hormonal stress response, rapidly transition between distinct activity states through recurrent inhibition. Specifically, in vivo optrode recording shows that under non-stress conditions, CRH_PVN neurons often fire with rhythmic brief bursts (RB), which, somewhat counterintuitively, constrains firing rate due to long (~2 s) interburst intervals. Stressful stimuli rapidly switch RB to continuous single spiking (SS), permitting a large increase in firing rate. A spiking network model shows that recurrent inhibition can control this activity-state switch, and more broadly the gain of spiking responses to excitatory inputs. In biological CRH_PVN neurons ex vivo, the injection of whole-cell currents derived from our computational model recreates the in vivo-like switch between RB and SS, providing direct evidence that physiologically relevant network inputs enable state-dependent computation in single neurons. Together, we present a novel mechanism for state-dependent activity dynamics in CRH_PVN neurons.

Cannabinoid and vanilloid pathways mediate opposing forms of synaptic plasticity in corticotropin-releasing hormone neurons

Activity-dependent release of retrograde signaling molecules form micro-feedback loops to regulate synaptic function in neural circuits. Single neurons can release multiple forms of these signaling molecules, including endocannabinoids and endovanilloids, which act via cannabinoid (CB) receptors and transient receptor potential vanilloid 1 (TRPV1) receptors. In hypothalamic corticotrophin-releasing hormone (CRH) neurons, endocannabinoids acting via CB1 receptors have been shown to play an important role in regulating excitability and hence stress hormone secretion. However, the importance of endovanilloid signaling in CRH neurons is currently unclear. Here, we show that, in response to postsynaptic depolarization, CRH neurons release endocannabinoid/endovanilloid molecules that can activate CB1 and TRPV1 receptors. Activation of CB1 receptors suppresses glutamate neurotransmission whereas activation of TRPV1 enhances spontaneous glutamate transmission. However, the excitatory effects of TRPV1 are normally masked by the inhibitory effects of CB1. When the degradation of the endocannabinoid 2-arachidonoylglycerol (2-AG) was inhibited, this revealed tonic activation of CB1 receptors, suggesting tonic endocannabinoid release. However, we found no evidence for tonic activation of TRPV1 receptors under similar conditions. These findings show that activation of CRH neurons can drive the release of signaling molecules that activate parallel endocannabinoid and endovanilloid receptor pathways to mediate opposing forms of synaptic plasticity.

Physical restraint mouse models to assess immune responses under stress with or without habituation

Physical confinement, or restraint, is a psychological stressor used in rodent studies. A single restraint episode elevates blood corticosterone levels, a hallmark of stress responses. Repeated restraint results in habituation (or desensitization), whereas chronic exposure to unpredictable stressors fails to induce habituation. Here, we provide our protocols and guidelines in using three mouse restraint models, namely prolonged restraint stress, repeated restraint stress, and chronic variable stress, to examine immunological homeostasis/competence, or lack thereof, under stress with or without habituation.

For complete information on the generation and use of these protocols, please refer to Rudak et al. (2021).

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Three physical restraint mouse models to study the impact of long-term stress on immunity

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A model of prolonged restraint stress altering immune homeostasis/competence

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A model of repeated daily restraint stress resulting in habituation in animals

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An optimized protocol for chronic variable stress circumventing habituation

Impact of chronic variable stress on neuroendocrine hypothalamus and pituitary in male and female C57BL/6J mice

Chronic stress exerts multiple negative effects on the physiology and health of an individual. In the present study, we examined hypothalamic, pituitary and endocrine responses to 14 days of chronic variable stress (CVS) in male and female C57BL/6J mice. In both sexes, CVS induced a significant decrease in body weight and enhanced the acute corticosterone stress response, which was accompanied by a reduction in thymus weight only in females. However, single-point blood measurements of basal prolactin, thyroid-stimulating hormone, luteinising hormone, growth hormone and corticosterone levels taken at the end of the CVS were not different from those of controls. Similarly, pituitary mRNA expression of Fshb, Lhb, Prl and Gh was unchanged by CVS, although Pomc and Tsh were significantly elevated. Within the adrenal medulla, mRNA for Th, Vip and Gal were elevated following CVS. Avp transcript levels within the paraventricular nucleus of the hypothalamus were increased by CVS; however, levels of Gnrh1, Crh, Oxt, Sst, Trh, Ghrh, Th and Kiss1 remained unchanged. Oestrous cycles were lengthened slightly by CVS and ovarian histology revealed a reduction in the number of preovulatory follicles and corpora lutea. Taken together, these observations indicate that 14 days of CVS induces an up-regulation of the neuroendocrine stress axis and creates a mild disruption of female reproductive function. However, the lack of changes in other neuroendocrine axes controlling anterior and posterior pituitary secretion suggest that most neuroendocrine axes are relatively resilient to CVS.

Morphohistochemical alterations of neurons of the supraoptic nucleus of the rat hypothalamus at different durations of the photoperiod and melatonin administration

We studied the morphologic and histochemical organization of neurons of the hypothalamic supraoptic nucleus in rats exposed to different durations of photoperiod and injection of melatonin. Morphometric and histochemical analyses of neurons were performed after staining brain histological sections for RNA. Prolonged illumination leads to more pronounced changes in the parameters of hypothalamic structures at 2 a.m. than at 2 p.m., particularly decreasing the concentration of RNA in the cell nuclei. The use of exogenous melatonin does not normalize the revealed changes in the parameters of the studied structures of the neurons of the supraoptic nucleus of the hypothalamus caused by the prolonged stay of rats under conditions of constant illumination.

Neuronal hypertrophy dampens neuronal intrinsic excitability and stress responsiveness during chronic stress

KEY POINTS

The hypothalamic-pituitary-adrenal (HPA) axis habituates to repeated stress exposure. We studied hypothalamic corticotropin-releasing hormone (CRH) neurons that form the apex of the HPA axis in a mouse model of stress habituation using repeated restraint. The intrinsic excitability of CRH neurons decreased after repeated stress in a time course that coincided with the development of HPA axis habituation. This intrinsic excitability plasticity co-developed with an expansion of surface membrane area, which increased a passive electric load and dampened membrane depolarization in response to the influx of positive charge. We report a novel structure-function relationship for intrinsic excitability plasticity as a neural correlate for HPA axis habituation.

ABSTRACT

Encountering a stressor immediately activates the hypothalamic-pituitary-adrenal (HPA) axis, but this stereotypic stress response also undergoes experience-dependent adaptation. Despite the biological and clinical importance, how the brain adjusts stress responsiveness in the long term remains poorly understood. We studied hypothalamic corticotropin-releasing hormone neurons that form the apex of the HPA axis in a mouse model of stress habituation using repeated restraint. Using patch-clamp electrophysiology in acute slices, we found that the intrinsic excitability of these neurons substantially decreased after daily repeated stress in a time course that coincided with their loss of stress responsiveness in vivo. This intrinsic excitability plasticity co-developed with an expansion of surface membrane area, which increased a passive electric load, and dampened membrane depolarization in response to the influx of positive charge. Multiphoton imaging and electron microscopy revealed that repeated stress augmented ruffling of the plasma membrane, suggesting an ultrastructural plasticity that may efficiently accommodate the membrane area expansion. Overall, we report a novel structure-function relationship for intrinsic excitability plasticity as a neural correlate for adaptation of the neuroendocrine stress response.

Profiling of key brain nuclei involved in CNS control of stress and glucose homeostasis

Previous work have shown several key brain nuclei involved in acute psychological stress and glucose homeostasis. Acute stress influences glucose metabolism via released stress hormones by activating the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system. Little is known about the brain nuclei which response to peripheral glucose alteration are either abundant with glucosesensing neurons or the activations are secondary to stress. Here we profile and compare the brain nuclei that response to stress and glucose homeostasis in mouse models of acute restraint stress, glucose and 2-DG injections respectively. Our present work provide a comprehensive depiction on key brain nuclei involved in CNS control of stress and glucose homeostasis, which gives clue for functional identification of brain nuclei that regulate glucose homeostasis under stress.

An enhanced expression of hypothalamic neuronal nitric oxide synthase in a rat model of simulated transport stress

BACKGROUND

Transport stress not only causes physiological changes but also induces behavioral responses, including anxiety-like and depression-like behavioral responses in animals. The neuronal nitric oxide synthase (nNOS) plays a pivotal role in transport stress. This study aimed to investigate the effects of acute transport stress on the expression of nNOS and the distribution of nNOS-positive neurons in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus in rats and to explore the neuroendocrine mechanism of transport stress.

RESULTS

In this study, for the first time, we investigated the effects of transport stress on nitric oxide (NO)-NOS in the hypothalamus. After simulated stress, rats exhibited behavioral changes in the open field test (OFT), increased serum corticosterone (CORT) and norepinephrine (NE) levels, and increased NO content in the hypothalamus. In addition, nNOS expression in the hypothalamic PVN was upregulated, and its distribution was altered in stressed rats compared with that of unstressed rats.

CONCLUSIONS

Our findings indicate that simulated transport stress increases nNOS expression and alters its distribution in the PVN of the rat hypothalamus.

Contribution of DNA methylation in chronic stress-induced cardiac remodeling and arrhythmias in mice

It is recognized that stress can induce cardiac dysfunction, but the underlying mechanisms are not well understood. The present study aimed to test the hypothesis that chronic negative stress leads to alterations in DNA methylation of certain cardiac genes, which in turn contribute to pathologic remodeling of the heart. We found that mice that were exposed to chronic restraint stress (CRS) for 4 wk exhibited cardiac remodeling toward heart failure, as characterized by ventricular chamber dilatation, wall thinning, and decreased contractility. CRS also induced cardiac arrhythmias, including intermittent sinus tachycardia and bradycardia, frequent premature ventricular contraction, and sporadic atrioventricular conduction block. Circulating levels of stress hormones were elevated, and the cardiac expression of tyrosine hydroxylase, a marker of sympathetic innervation, was increased in CRS mice. Using reduced representation bisulfite sequencing, we found that although CRS did not lead to global changes in DNA methylation in the murine heart, it nevertheless altered methylation at specific genes that are associated with the dilated cardiomyopathy (DCM) (e.g., desmin) and adrenergic signaling of cardiomyocytes (ASPC) (e.g., adrenergic receptor-α1) pathways. We conclude that CRS induces cardiac remodeling and arrhythmias, potentially through altered methylation of myocardial genes associated with the DCM and ASPC pathways.-Zhang, P., Li, T., Liu, Y.-Q., Zhang, H., Xue, S.-M., Li, G., Cheng, H.-Y.M., Cao, J.-M. Contribution of DNA methylation in chronic stress-induced cardiac remodeling and arrhythmias in mice.

Stress-elicited glucocorticoid receptor signaling upregulates TIGIT in innate-like invariant T lymphocytes

Stress is known to impede certain host defense mechanisms, including those governed by conventional T lymphocytes. However, whether innate-like T lymphocytes, such as invariant natural killer T (iNKT) and mucosa-associated invariant T (MAIT) cells, are impacted by stress is unclear. Herein, we report that prolonged psychological stress caused by physical confinement results in robust upregulation of T cell immunoreceptor with immunoglobulin and ITIM domains (TIGIT), an immune checkpoint receptor that controls antitumor and antiviral immune responses. Elevated TIGIT expression was found not only on NK and conventional T cells, but also on iNKT and MAIT cells. Stress-provoked TIGIT upregulation was reversed through treatment with the glucocorticoid receptor (GR) antagonist RU486, but not with 6-hydroxydopamine that induces chemical sympathectomy. A Cre/Lox gene targeting model in which GR was ablated in cells expressing Lck under its proximal promoter revealed that TIGIT upregulation in stressed animals stems from direct GR signaling in T and iNKT cells. In fact, long-term oral administration of exogenous corticosterone (CS) to wild-type C57BL/6 (B6) mice was sufficient to increase TIGIT expression levels on T and iNKT cells. In vitro treatment with CS also potently and selectively upregulated TIGIT, but not CTLA-4 or LAG-3, on mouse iNKT and MAIT hybridomas. These results were recapitulated using primary hepatic iNKT and MAIT cells from wild-type B6 and B6.MAIT^CAST mice, respectively. Subjecting B6.MAIT^CAST mice to physical restraint also raised the frequency of TIGIT⁺ cells among hepatic MAIT cells in a GR-dependent manner. Finally, we found that TIGIT is similarly upregulated in a chronic variable stress model in which animals are exposed to unpredictable heterotypic stressors without developing habituation. Taken together, our findings link, for the first time to our knowledge, GR signaling to TIGIT expression. We propose that glucocorticoid hormones dampen immune responses, in part, by enhancing TIGIT expression across multiple critical subsets of effector lymphocytes, including innate-like T cells. Therefore, TIGIT may constitute an attractive target in immune-enhancing interventions for sustained physiological stress.

A Nitric Oxide-Dependent Presynaptic LTP at Glutamatergic Synapses of the PVN Magnocellular Neurosecretory Cells in vitro in Rats

The magnocellular neurosecretory cells (MNCs) of the hypothalamic paraventricular nucleus (PVN) integrate incoming signals to secrete oxytocin (OT), and vasopressin (VP) from their nerve terminals in the posterior pituitary gland. In the absence of gamma-aminobutyric acid A (GABA_A) and cannabinoids 1 (CB1) receptor activity, we used whole-cell patch-clamp recording, single-cell reverse transcription-multiplex polymerase chain reaction (SC-RT-mPCR), biocytin histochemistry and pharmacological methods to examine the mechanism of high frequency stimulus (HFS, 100 Hz)-induced long-term potentiation (LTP) at glutamatergic synapses in the PVN MNCs of juvenile male rats. Our results showed that HFS-induced LTP at glutamatergic synapses was accompanied by a decrease in the paired-pulse ratio (PPR) of the PVN MNCs. In these MNCs, HFS-induced LTP persisted in the presence of a group 1 metabotropic glutamate receptor (mGluR1) antagonist; however, it was abolished by an N-methyl-D-aspartic acid (NMDA) receptor blocker. Notably, HFS-induced LTP in the PVN MNCs was completely prevented by a nitric oxide synthase (NOS) inhibitor. The application of an NO donor not only induced the LTP of excitatory glutamatergic inputs in the PVN MNCs, but also occluded the HFS-induced LTP in these MNCs. Moreover, HFS-induced LTP in the PVN MNCs was also abolished by a specific protein kinase A (PKA) inhibitor, KT5720. SC-RT-mPCR analysis revealed that 64.5% (62/96) of MNCs expressed OT mRNA. Our results indicate that a HFS can induce an NMDA receptor and NO cascades dependent on presynaptic glutamatergic LTP in the PVN MNCs via a PKA signaling pathway.

Corticosterone mediated functional and structural plasticity in corticotropin-releasing hormone neurons

Corticosteroid stress hormones drive a multitude of adaptations in the brain. Hypothalamic corticotropin-releasing hormone (CRH) neurons control the circulating levels of corticosteroid stress hormones in the body and are themselves highly sensitive to corticosteroids. CRH neurons have been shown to undergo various adaptions in response to acute stress hormone elevations. However, their structural and physiological changes under chronically elevated corticosterone are less clear. To address this, we determined the structural and functional changes in CRH neurons in the paraventricular nucleus of the hypothalamus following 14 days of corticosterone treatment. We find that prolonged corticosterone elevation reduces CRH neuron intrinsic excitability as measured by summation of subthreshold postsynaptic depolarisations and spiking output. We find that under normal conditions, CRH neurons have a relatively compact and simple dendritic arbor, with a low density of somatic and dendritic spines. Interestingly, the axon originated from a proximal dendrite close to the soma in approximately half of the CRH neurons reconstructed. While prolonged elevation in corticosterone levels did not result in any changes to gross dendritic morphology, it induced a significant reduction in both somatic and dendritic spine density. Together these data reveal the morphological features of hypothalamic CRH neurons and highlight their capacity to undergo functional and morphological plasticity in response to chronic corticosterone elevations. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Heterosynaptic modulation in the paraventricular nucleus of the hypothalamus

The stress response-originally described by Hans Selye as "the nonspecific response of the body to any demand made upon it"-is chiefly mediated by the hypothalamic-pituitary-adrenal (HPA) axis and is activated by diverse sensory stimuli that inform threats to homeostasis. The diversity of signals regulating the HPA axis is partly achieved by the complexity of afferent inputs that converge at the apex of the HPA axis: this apex is formed by a group of neurosecretory neurons that synthesize corticotropin-releasing hormone (CRH) in the paraventricular nucleus of the hypothalamus (PVN). The afferent synaptic inputs onto these PVN-CRH neurons originate from a number of brain areas, and PVN-CRH neurons respond to a long list of neurotransmitters/neuropeptides. Considering this complexity, an important question is how these diverse afferent signals independently and/or in concert influence the excitability of PVN-CRH neurons. While many of these inputs directly act on the postsynaptic PVN-CRH neurons for the summation of signals, accumulating data indicates that they also modulate each other's transmission in the PVN. This mode of transmission, termed heterosynaptic modulation, points to mechanisms through which the activity of a specific modulatory input (conveying a specific sensory signal) can up- or down-regulate the efficacy of other afferent synapses (mediating other stress modalities) depending on receptor expression for and spatial proximity to the heterosynaptic signals. Here, we review examples of heterosynaptic modulation in the PVN and discuss its potential role in the regulation of PVN-CRH neurons' excitability and resulting HPA axis activity. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.