Bed nuclei of the stria terminalis modulate memory consolidation via glucocorticoid-dependent and -independent circuits

Neural circuits regulating visceral pain

Visceral hypersensitivity, a common clinical manifestation of irritable bowel syndrome, may contribute to the development of chronic visceral pain, which is a major challenge for both patients and health providers. Neural circuits in the brain encode, store, and transfer pain information across brain regions. In this review, we focus on the anterior cingulate cortex and paraventricular nucleus of the hypothalamus to highlight the progress in identifying the neural circuits involved in visceral pain. We also discuss several neural circuit mechanisms and emphasize the importance of cross-species, multiangle approaches and the identification of specific neurons in determining the neural circuits that control visceral pain.

Relationships between trace elements and cognitive and depressive behaviors in sprague dawley and wistar albino rats

Introduction: This study investigates the effects of social isolation on mental health and cognitive functions in Sprague Dawley (SD) and Wistar Albino (WIS) rat strains, prompted by the heightened awareness of such impacts amid the COVID-19 pandemic. This study aims to explore the impact of social isolation on memory, learning, and behavioral changes in middle-aged SD and WIS rat strains and to investigate cortical trace element levels, seeking potential correlations between these levels and the observed behavioral responses to social isolation. Methods: Four groups of 14-month-old male rats were established: control and isolated SDs and WIS rats (CONT-SD, ISO-SD, CONT-WIS, ISO-WIS). Morris Water Maze and Porsolt Forced Swimming tests were conducted for behavioral assessment. Following behavioral tests, rats were sacrificed under general anesthesia, and cortices were isolated for analysis of macro and trace element levels (ICP/MS). Results: In behavioral tests, CONT-SD rats exhibited superior performance in the Morris Water Maze test compared to CONT-WIS rats, but displayed increased depressive behaviors following social isolation, as evident in the Porsolt Forced Swimming test (p < 0.05). ISO-SD rats showed elevated levels of Co and Cu, along with reduced levels of Cs and As, compared to ISO-WIS rats. Moreover, isolation resulted in decreased Cu and Mo levels but increased Rb levels in WIS rats. Comparison of trace element levels in naïve groups from different strains revealed lower Zn levels in the WIS group compared to SD rats. Discussion: The findings suggest that the SD strain learns faster, but is more susceptible to depression after isolation compared to the WIS strain. Increased Co and Cu levels in ISO-SD align with previous findings, indicating potential trace element involvement in stress responses. Understanding these mechanisms could pave the way for preventive treatment strategies or therapeutic targets against the consequences of stressors, contributing to research and measures promoting a balanced diet to mitigate neurobehavioral abnormalities associated with social isolation in the future.

The cortisol switch between vulnerability and resilience

In concert with neuropeptides and transmitters, the end products of the hypothalamus-pituitary-adrenal (HPA) axis, the glucocorticoid hormones cortisol and corticosterone (CORT), promote resilience: i.e., the ability to cope with threats, adversity, and trauma. To exert this protective action, CORT activates mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) that operate in a complementary manner -as an on/off switch- to coordinate circadian events, stress-coping, and adaptation. The evolutionary older limbic MR facilitates contextual memory retrieval and supports an on-switch in the selection of stress-coping styles at a low cost. The rise in circulating CORT concentration after stress subsequently activates a GR-mediated off-switch underlying recovery of homeostasis by providing the energy for restraining the primary stress reactions and promoting cognitive control over emotional reactivity. GR activation facilitates contextual memory storage of the experience to enable future stress-coping. Such complementary MR-GR-mediated actions involve rapid non-genomic and slower gene-mediated mechanisms; they are time-dependent, conditional, and sexually dimorphic, and depend on genetic background and prior experience. If coping fails, GR activation impairs cognitive control and promotes emotional arousal which eventually may compromise resilience. Such breakdown of resilience involves a transition to a chronic stress construct, where information processing is crashed; it leads to an imbalanced MR-GR switch and hence increased vulnerability. Novel MR-GR modulators are becoming available that may reset a dysregulated stress response system to reinstate the cognitive flexibility required for resilience.

Glucocorticoid feedback paradox: a homage to Mary Dallman

As the end product of the hypothalamus-pituitary-adrenal (HPA) axis, the glucocorticoid hormones cortisol and corticosterone coordinate circadian activities, stress-coping, and adaptation to change. For this purpose, the hormone promotes energy metabolism and controls defense reactions in the body and brain. This life-sustaining action exerted by glucocorticoids occurs in concert with the autonomic nervous and immune systems, transmitters, growth factors/cytokines, and neuropeptides. The current contribution will focus on the glucocorticoid feedback paradox in the HPA-axis: the phenomenon that stress responsivity remains resilient if preceded by stress-induced secretion of glucocorticoid hormone, but not if this hormone is previously administered. Furthermore, in animal studies, the mixed progesterone/glucocorticoid antagonist RU486 or mifepristone switches to an apparent partial agonist upon repeated administration. To address these enigmas several interesting phenomena are highlighted. These include the conditional nature of the excitation/inhibition balance in feedback regulation, the role of glucose as a determinant of stress responsivity, and the potential of glucocorticoids in resetting the stress response system. The analysis of the feedback paradox provides also a golden opportunity to review the progress in understanding the role of glucocorticoid hormone in resilience and vulnerability during stress, the science that was burned deeply in Mary Dallman's emotions.

Increased hippocampal CREB phosphorylation after retrieval of remote contextual fear memories in Carioca high-conditioned freezing rats

The participation of the hippocampal formation in consolidation and reconsolidation of contextual fear memories has been widely recognized and known to be dependent on the activation of the cAMP response element (CRE) binding protein (CREB) pathway. Recent findings have challenged the prevailing view that over time contextual fear memories migrate to neocortical circuits and no longer require the hippocampus for retrieval of remote fearful memories. It has also recently been found that this brain structure is important for the maintenance and recall of remote fear memories associated with aversive events, a common trait in stress-related disorders such as generalized anxiety disorder (GAD), major depression, and post-traumatic stress disorder. In view of these findings, here we examined the putative role of CREB in the hippocampus of an animal model of GAD during the retrieval of remote contextual fear memories. Specifically, we evaluated CREB phosphorylation in the hippocampus of male Carioca High- and Low-conditioned Freezing rats (CHF and CLF, respectively) upon re-exposure of animals to contextual cues associated to footshocks weeks after fear conditioning. Age-matched male rats from a randomized crossbreeding population served as controls (CTL). Adrenal catecholamine levels were also measured as a biological marker of stress response. Seven weeks after contextual fear conditioning, half of the sample of CHF (n = 9), CLF (n = 10) and CTL (n = 10) rats were randomly assigned to return to the same context chamber where footshocks were previously administrated (Context condition), while the remaining animals were individually placed in standard housing cages (Control condition). Western blot results indicated that pCREB levels were significantly increased in the hippocampus of CHF rats for both Context and Control conditions when compared to the other experimental groups. CHF rats in the Context condition also exhibited significant more freezing than that observed for both CLF and CTL rats. Lastly, CHF animals in the Context condition displayed significantly higher adrenal catecholamine levels than those in the Control condition, whereas no differences in catecholamine levels were observed between Context and Control conditions for CLF and CTL rats. These findings are discussed from a perspective in which the hippocampus plays a role in the maintenance and recall of remote contextual fear memories via the CREB pathway.

Bed nuclei of the stria terminalis: A key hub in the modulation of anxiety

The bed nuclei of the stria terminalis (BST) is recognised as a pivotal integrative centre for monitoring emotional valence. It is implicated in the regulation of diverse affective states and motivated behaviours, and decades of research have firmly established its critical role in anxiety-related behavioural processes. Researchers have recently intricately dissected the BST's dynamic activities, its connection patterns and its functions with respect to specific cell types using multiple techniques such as optogenetics, in vivo calcium imaging and transgenic tools to unmask the complex circuitry mechanisms that underlie anxiety. In this review, we principally focus on studies of anxiety-involved neuromodulators within the BST and provide a comprehensive architecture of the anxiety network-highlighting the BST as a key hub in orchestrating anxiety-like behaviour. We posit that these promising efforts will contribute to the identification of an accurate roadmap for future treatment of anxiety disorders.

What the hippocampus tells the HPA axis: Hippocampal output attenuates acute stress responses via disynaptic inhibition of CRF+ PVN neurons

The hippocampus exerts inhibitory feedback on the release of glucocorticoids. Because the major hippocampal efferent projections are excitatory, it has been hypothesized that this inhibition is mediated by populations of inhibitory neurons in the hypothalamus or elsewhere. These regions would be excited by hippocampal efferents and project to corticotropin-releasing factor (CRF) cells in the paraventricular nucleus of the hypothalamus (PVN). A direct demonstration of the synaptic responses elicited by hippocampal outputs in PVN cells or upstream GABAergic interneurons has not been provided previously. Here, we used viral vectors to express channelrhodopsin (ChR) and enhanced yellow fluorescent protein (EYFP) in pyramidal cells in the ventral hippocampus (vHip) in mice expressing tdTomato in GABA- or CRF-expressing neurons. We observed dense innervation of the bed nucleus of the stria terminalis (BNST) by labeled vHip axons and sparse labeling within the PVN. Using whole-cell voltage-clamp recording in parasagittal brain slices containing the BNST and PVN, photostimulation of vHip terminals elicited rapid excitatory postsynaptic currents (EPSCs) and longer-latency inhibitory postsynaptic currents (IPSCs) in both CRF+ and GAD + cells. The ratio of synaptic excitation and inhibition was maintained in CRF + cells during 20 Hz stimulus trains. Photostimulation of hippocampal afferents to the BNST and PVN in vivo inhibited the rise in blood glucocorticoid levels produced by acute restraint stress. We thus provide functional evidence suggesting that hippocampal output to the BNST contributes to a net inhibition of the hypothalamic-pituitary axis, providing further mechanistic insights into this process using methods with enhanced spatial and temporal resolution.

Brain mineralocorticoid and glucocorticoid receptor balance in neuroendocrine regulation and stress-related psychiatric etiopathologies

Cortisol and corticosterone (CORT) coordinate circadian events and manage the stress response by differential activation of two complementary brain receptor systems, i.e., the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR), which mediate rapid non-genomic and slow genomic actions. Several recent discoveries are highlighted from molecular fine-tuning of the MR/GR balance by FKBP5 to CORTs role in neural network regulation underlying stress adaptation in emotional, cognitive, and social domains of behavior. The data suggest that MR mediates CORT action on risk assessment, social interaction, and response selection, while GR activation promotes memory consolidation and behavioral adaptation; there are also sex differences in CORT action. New evidence suggests that targeting the MR/GR balance resets a dysregulated stress response system and promotes resilience.

Mineralocorticoid receptor and glucocorticoid receptor work alone and together in cell-type-specific manner: Implications for resilience prediction and targeted therapy

‘You can't roll the clock back and reverse the effects of experiences' Bruce McEwen used to say when explaining how allostasis labels the adaptive process. Here we will for once roll the clock back to the times that the science of the glucocorticoid hormone was honored with a Nobel prize and highlight the discovery of their receptors in the hippocampus as inroad to its current status as master regulator in control of stress coping and adaptation. Glucocorticoids operate in concert with numerous neurotransmitters, neuropeptides, and other hormones with the aim to facilitate processing of information in the neurocircuitry of stress, from anticipation and perception of a novel experience to behavioral adaptation and memory storage. This action, exerted by the glucocorticoids, is guided by two complementary receptor systems, mineralocorticoid receptors (MR) and glucocorticoid receptors (GR), that need to be balanced for a healthy stress response pattern. Here we discuss the cellular, neuroendocrine, and behavioral studies underlying the MR:GR balance concept, highlight the relevance of hypothalamic-pituitary-adrenal (HPA) -axis patterns and note the limited understanding yet of sexual dimorphism in glucocorticoid actions. We conclude with the prospect that (i) genetically and epigenetically regulated receptor variants dictate cell-type-specific transcriptome signatures of stress-related neuropsychiatric symptoms and (ii) selective receptor modulators are becoming available for more targeted treatment. These two new developments may help to ‘restart the clock’ with the prospect to support resilience.

Extended amygdala, conditioned withdrawal and memory consolidation

Opioid withdrawal can be associated to environmental cues through classical conditioning. Exposure to these cues can precipitate a state of conditioned withdrawal in abstinent subjects, and there are suggestions that conditioned withdrawal can perpetuate the addiction cycle in part by promoting the storage of memories. This review discusses evidence supporting the hypothesis that conditioned withdrawal facilitates memory consolidation by activating a neurocircuitry that involves the extended amygdala. Specifically, the central amygdala, the bed nucleus of the stria terminalis, and the nucleus accumbens shell interact functionally during withdrawal, mediate expression of conditioned responses, and are implicated in memory consolidation. From this perspective, the extended amygdala could be a neural pathway by which drug-seeking behaviour performed during a state of conditioned withdrawal is more likely to become habitual and persistent.

Forced swim stressor: Trends in usage and mechanistic consideration

The acquired immobility response during the “forced swim test (FST)” is not a rodent model of depression, but the test has some validity in predicting a compound's antidepressant potential. Nevertheless, 60% of the about 600 papers that were published annually the past 2 years label the rodent's immobility response as depression‐like behaviour, but the relative contribution per country is changing. When the Editors‐in‐Chief of 5 journals publishing most FST papers were asked for their point of view on labelling immobility as depression‐like behaviour and despair, they responded that they primarily rely on the reviewers regarding scientific merit of the submission. One Editor informs authors of the recent NIMH notice (https://grants.nih.gov/grants/guide/notice‐files/NOT‐MH‐19‐053.html) which encourages investigators to use animal models “for” addressing neurobiological questions rather than as model “of” specific mental disorders. The neurobiological questions raised by use of the FST fall in two categories. First, research on the role of endocrine and metabolic factors, with roots in the 1980s, and with focus on the bottom‐up action of glucocorticoids on circuits processing salient information, executive control and memory consolidation. Second, recent findings using novel technological and computational advances that have allowed great progress in charting top‐down control in the switch from active to passive coping with the inescapable stressor executed by neuronal ensembles of the medial prefrontal cortex via the peri‐aquaductal grey. It is expected that combining neural top‐down and endocrine bottom‐up approaches will provide new insights in the role of stress‐coping and adaptation in pathogenesis of mental disorders.

Somatostatin Neurons of the Bed Nucleus of Stria Terminalis Enhance Associative Fear Memory Consolidation in Mice

Excessive fear learning and generalized, extinction-resistant fear memories are core symptoms of anxiety and trauma-related disorders. Despite significant evidence from clinical studies reporting hyperactivity of the bed nucleus of stria terminalis (BNST) under these conditions, the role of BNST in fear learning and expression is still not clarified. Here, we tested how BNST modulates fear learning in male mice using a chemogenetic approach. Activation of GABAergic neurons of BNST during fear conditioning or memory consolidation resulted in enhanced cue-related fear recall. Importantly, BNST activation had no acute impact on fear expression during conditioning or recalls, but it enhanced cue-related fear recall subsequently, potentially via altered activity of downstream regions. Enhanced fear memory consolidation could be replicated by selectively activating somatostatin (SOM), but not corticotropin-releasing factor (CRF), neurons of the BNST, which was accompanied by increased fear generalization. Our findings suggest the significant modulation of fear memory strength by specific circuits of the BNST.SIGNIFICANCE STATEMENT The bed nucleus of stria terminalis (BNST) mediates different defensive behaviors, and its connections implicate its integrative modulatory role in fear memory formation; however, the involvement of BNST in fear learning has yet to be elucidated in detail. Our data highlight that BNST stimulation enhances fear memory formation without direct effects on fear expression. Our study identified somatostatin (SOM) cells within the extended amygdala as specific neurons promoting fear memory formation. These data underline the importance of anxiety circuits in maladaptive fear memory formation, indicating elevated BNST activity as a potential vulnerability factor to anxiety and trauma-related disorders.