Unexpected effects of metyrapone on corticosteroid receptor interaction with the genome and subsequent gene transcription in the hippocampus of male rats

Effects of the combination of chronic unpredictable stress and environmental enrichment on anxiety-like behavior assessed using the elevated plus maze in Swiss male mice: Hypothalamus-Pituitary-Adrenal Axis-mediated mechanisms

Environmental enrichment (EE) is a paradigm that offers the animal a plethora of stimuli, including physical, cognitive, sensory, and social enrichment. Exposure to EE can modulate both anxiety responses and plasma corticosterone. In this study, our objective was to explore how chronic unpredictable stress (CUS) impacts anxiety-related behaviors in male Swiss mice raised in EE conditions. Additionally, we investigated corticosterone and adrenocorticotropic hormone (ACTH) levels to assess the involvement of the hypothalamic-pituitary-adrenal (HPA) axis in mediating these responses. Mice were housed under either EE or standard housing conditions for 21 days. Afterward, they were exposed to 11 days of CUS while still reared in their distinct housing conditions, with half of the mice receiving daily pretreatment with the vehicle and the other half receiving daily metyrapone (MET) injections, an inhibitor of steroid synthesis, 30 mins before CUS exposure. Blood samples were obtained to assess plasma corticosterone and ACTH levels. The 11-day CUS protocol induced anxiety-like phenotype and elevated ACTH levels in EE mice. Chronic MET pretreatment prevented anxiety-like behavior in the EE-CUS groups, by mechanisms involving increased plasma corticosterone levels and decreased ACTH. These results suggest a role of the HPA axis in the mechanism underlying the anxiogenic phenotype induced by CUS in EE mice and shed light on the complex interplay between environmental factors, stress, and the HPA axis in anxiety regulation.

Does the Glucocorticoid Stress Response Make Toads More Toxic? An Experimental Study on the Regulation of Bufadienolide Toxin Synthesis

Chemical defense is a crucial component of fitness in many organisms, yet the physiological regulation of defensive toxin synthesis is poorly understood, especially in vertebrates. Bufadienolides, the main defensive compounds of toads, are toxic to many predators and other natural enemies, and their synthesis can be upregulated by stressors, including predation risk, high conspecific density, and pollutants. Thus, higher toxin content may be the consequence of a general endocrine stress response in toads. Therefore, we hypothesized that bufadienolide synthesis may be stimulated by elevated levels of corticosterone (CORT), the main glucocorticoid hormone of amphibians, or by upstream regulators that stimulate CORT production. To test these alternatives, we treated common toad tadpoles with exogenous CORT (exoCORT) or metyrapone (MTP, a CORT-synthesis inhibitor that stimulates upstream regulators of CORT by negative feedback) in the presence or absence of predation cues for 2 or 6 days, and subsequently measured their CORT release rates and bufadienolide content. We found that CORT release rates were elevated by exoCORT, and to a lesser extent also by MTP, regardless of treatment length. Bufadienolide content was significantly decreased by treatment with exoCORT for 6 days but was unaffected by exposure to exoCORT for 2 days or to MTP for either 6 or 2 days. The presence or absence of predation cues affected neither CORT release rate nor bufadienolide content. Our results suggest that changes in bufadienolide synthesis in response to environmental challenges are not driven by CORT but may rather be regulated by upstream hormones of the stress response.

From Ligands to Behavioral Outcomes: Understanding the Role of Mineralocorticoid Receptors in Brain Function

Stress is a normal response to situational pressures or demands. Exposure to stress activates the hypothalamic-pituitary-adrenal (HPA) axis and leads to the release of corticosteroids, which act in the brain via two distinct receptors: mineralocorticoid receptors (MR) and glucocorticoid receptors (GR). Persistent HPA axis overactivation or dysregulation can disrupt an individual's homeostasis, thereby contributing to an increased risk for mental illness. On the other hand, successful coping with stressful events involves adaptive and cognitive processes in the brain that render individuals more resilient to similar stressors in the future. Here we review the role of the MR in these processes, starting with an overview of the physiological structure, ligand binding, and expression of MR, and further summarizing its role in the brain, its relevance to psychiatric disorders, and related rodent studies. Given the central role of MR in cognitive and emotional functioning, and its importance as a target for promoting resilience, future research should investigate how MR modulation can be used to alleviate disturbances in emotion and behavior, as well as cognitive impairment, in patients with stress-related psychiatric disorders.

Future directions in urban endocrinology - The effects of endocrine plasticity on urban tolerance

After twenty years of studies of endocrine traits in animals living in cities, the field of urban endocrinology has built a robust literature including numerous studies looking for signatures of the effects of urban living, usually in mean circulating hormone concentrations. The findings of this past research have primarily demonstrated the absence of any generalizable endocrine responses to city life. In this opinion paper, I suggest that a strong route forward would include investigations of the role of variation in endocrine plasticity in determining the degree to which organisms tolerate urban challenges (i.e., urban tolerance). Achieving this research aim will require creative experimental and comparative studies, consideration of alternative study systems, and teasing apart of sources of variation in plastic phenotypes (plasticity, sorting, and contemporary evolution). Insight into the role of endocrine plasticity in influencing urban tolerance could help us better understand and predict impacts of expanding urbanization on biodiversity across the globe.

Transcriptional Profiling of Rat Prefrontal Cortex after Acute Inescapable Footshock Stress

Stress is a primary risk factor for psychiatric disorders such as Major Depressive Disorder (MDD) and Post Traumatic Stress Disorder (PTSD). The response to stress involves the regulation of transcriptional programs, which is supposed to play a role in coping with stress. To evaluate transcriptional processes implemented after exposure to unavoidable traumatic stress, we applied microarray expression analysis to the PFC of rats exposed to acute footshock (FS) stress that were sacrificed immediately after the 40 min session or 2 h or 24 h after. While no substantial changes were observed at the single gene level immediately after the stress session, gene set enrichment analysis showed alterations in neuronal pathways associated with glia development, glia-neuron networking, and synaptic function. Furthermore, we found alterations in the expression of gene sets regulated by specific transcription factors that could represent master regulators of the acute stress response. Of note, these pathways and transcriptional programs are activated during the early stress response (immediately after FS) and are already turned off after 2 h-while at 24 h, the transcriptional profile is largely unaffected. Overall, our analysis provided a transcriptional landscape of the early changes triggered by acute unavoidable FS stress in the PFC of rats, suggesting that the transcriptional wave is fast and mild, but probably enough to activate a cellular response to acute stress.

Genomic regulation of Krüppel-like-factor family members by corticosteroid receptors in the rat brain

Hippocampal mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs) mediate glucocorticoid hormone (GC) action in the hippocampus. These receptors bind to glucocorticoid responsive elements (GREs) within target genes, eliciting transcriptional effects in response to stress and circadian variation. Until recently, little was known about the genome-wide targets of hippocampal MRs and GRs under physiological conditions. Following on from our genome-wide MR and GR ChIP-seq and Ribo-Zero RNA-seq studies on rat hippocampus, we investigated the Krüppel-like factors (KLFs) as targets of MRs and GRs throughout the brain under circadian variation and after acute stress. In particular, Klf2, Klf9 and Klf15 are known to be stress and/or GC responsive and play a role in neurobiological processes including synaptic plasticity and neuronal differentiation. We found increased binding of MR and GR to GREs within Klf2, Klf9 and Klf15 in the hippocampus, amygdala, prefrontal cortex, and neocortex after acute stress and resulting from circadian variation, which was accompanied by upregulation of corresponding hnRNA and mRNA levels. Adrenalectomy abolished transcriptional upregulation of specific Klf genes. These results show that MRs and GRs regulate Klf gene expression throughout the brain following exposure to acute stress or in response to circadian variation, likely alongside other transcription factors.

Cell type specificity of glucocorticoid signaling in the adult mouse hippocampus

Glucocorticoid stress hormones are powerful modulators of brain function and can affect mood and cognitive processes. The hippocampus is a prominent glucocorticoid target and expresses both the glucocorticoid receptor (GR: Nr3c1) and the mineralocorticoid receptor (MR: Nr3c2). These nuclear steroid receptors act as ligand-dependent transcription factors. Transcriptional effects of glucocorticoids have often been deduced from bulk mRNA measurements or spatially informed individual gene expression. However, only sparse data exists allowing insights on glucocorticoid-driven gene transcription at the cell type level. Here, we used publicly available single-cell RNA sequencing data to assess the cell-type specificity of GR and MR signaling in the adult mouse hippocampus. The data confirmed that Nr3c1 and Nr3c2 expression differs across neuronal and non-neuronal cell populations. We analyzed co-expression with sex hormones receptors, transcriptional coregulators, and receptors for neurotransmitters and neuropeptides. Our results provide insights in the cellular basis of previous bulk mRNA results and allow the formulation of more defined hypotheses on the effects of glucocorticoids on hippocampal function.

Acute pre-learning stress selectively impairs hippocampus-dependent fear memory consolidation: Behavioral and molecular evidence

Despite compelling evidence that stress or stress-related hormones influence fear memory consolidation processes, the understanding of molecular mechanisms underlying the effects of stress is still fragmentary. The release of corticosterone in response to pre-learning stress exposure has been demonstrated to modulate positively or negatively memory encoding and/or consolidation according to many variables such as stress intensity, the emotional valence of the learned material or the interval between stressful episode and learning experience. Here, we report that contextual but not cued fear memory consolidation was selectively impaired in male mice exposed to a 50 min-period of restraint stress just before the unpaired fear conditioning session. In addition to behavioral impairment, acute stress down-regulated activated/phosphorylated ERK1/2 (pERK1/2) in dorsal hippocampal area CA1 in mice sacrificed 60 min and 9 h after unpaired conditioning. In lateral amygdala, although acute stress by itself increased the level of pERK1/2 it nevertheless blocked the peak of pERK1/2 that was normally observed 15 min after unpaired conditioning. To examine whether stress-induced corticosterone overflow was responsible of these detrimental effects, the corticosterone synthesis inhibitor, metyrapone, was administered 30 min before stress exposure. Metyrapone abrogated the stress-induced contextual fear memory deficits but did not alleviate the effects of stress on pERK1/2 and its downstream target phosphorylated CREB (pCREB) in hippocampus CA1 and lateral amygdala. Collectively, our observations suggest that consolidation of hippocampus-dependent memory and the associated signaling pathway are particularly sensitive to stress. However, behavioral normalization by preventive metyrapone treatment was not accompanied by renormalization of the canonical signaling pathway. A new avenue would be to consider surrogate mechanisms involving proper metyrapone influence on both nongenomic and genomic actions of glucocorticoid receptors.

Corticosteroid-binding-globulin (CBG)-deficient mice show high pY216-GSK3β and phosphorylated-Tau levels in the hippocampus

Corticosteroid-binding globulin (CBG) is the specific carrier of circulating glucocorticoids, but evidence suggests that it also plays an active role in modulating tissue glucocorticoid activity. CBG polymorphisms affecting its expression or affinity for glucocorticoids are associated with chronic pain, chronic fatigue, headaches, depression, hypotension, and obesity with an altered hypothalamic pituitary adrenal axis. CBG has been localized in hippocampus of humans and rodents, a brain area where glucocorticoids have an important regulatory role. However, the specific CBG function in the hippocampus is yet to be established. The aim of this study was to investigate the effect of the absence of CBG on hippocampal glucocorticoid levels and determine whether pathways regulated by glucocorticoids would be altered. We used cbg^-/- mice, which display low total-corticosterone and high free-corticosterone blood levels at the nadir of corticosterone secretion (morning) and at rest to evaluate the hippocampus for total- and free-corticosterone levels; 11β-hydroxysteroid dehydrogenase expression and activity; the expression of key proteins involved in glucocorticoid activity and insulin signaling; microtubule-associated protein tau phosphorylation, and neuronal and synaptic function markers. Our results revealed that at the nadir of corticosterone secretion in the resting state the cbg^-/- mouse hippocampus exhibited slightly elevated levels of free-corticosterone, diminished FK506 binding protein 5 expression, increased corticosterone downstream effectors and altered MAPK and PI3K pathway with increased pY216-GSK3β and phosphorylated tau. Taken together, these results indicate that CBG deficiency triggers metabolic imbalance which could lead to damage and long-term neurological pathologies.