Global Cerebral Ischemia in Male Long Evans Rats Impairs Dopaminergic/ΔFosB Signalling in the Mesocorticolimbic Pathway Without Altering Delay Discounting Rates

Global cerebral ischemia (GCI) in rats has been shown to promote exploration of anxiogenic zones of the Elevated-Plus Maze (EPM) and Open Field Test (OFT). This study investigated changes in impulsive choice and/or defensive responses as possible contributors of heightened anxiogenic exploration observed after ischemia. Impulsivity was assessed using delay discounting (DD) paradigms, while the Predator Odour Test (PO) served to assess changes in defensive responses towards a naturally aversive stimulus. Male Long Evans rats underwent 9 days of autoshaping training and 24 days of DD training prior to GCI or sham surgery (n = 9/group). Post-surgery, rats completed the OFT, EPM, and PO, followed by 6 days of DD sessions. Blood droplets served to evaluate corticosterone secretion associated with PO exposure. With impulsivity being regulated through mesocorticolimbic monoaminergic pathways, we also characterised post-ischemic changes in the expression of dopamine D2 receptors (DRD2), dopamine transporters (DAT), and 1FosB in the basolateral amygdala (BLA), nucleus accumbens core (NAcC) and shell (NAcS), and ventromedial prefrontal cortex (vmPFC) using immunohistofluorescence. Our findings revealed no impact of GCI on delay discounting rates, while PO approach behaviours were minimally affected. Nonetheless, GCI significantly reduced DRD2 and ΔFosB-ir in the NAcS and NAcC, respectively, while DAT-ir was diminished in both NAc subregions. Collectively, our findings refine the understanding of cognitive-behavioural and biochemical responses following stroke or cardiac arrest. They support significant alterations to the dopaminergic mesocorticolimbic pathway after ischemia, which are not associated with altered impulsive choice in a DD task but may influence locomotor exploration of the OFT and EPM.

CRF-R1 Antagonist Treatment Exacerbates Circadian Corticosterone Secretion under Chronic Stress, but Preserves HPA Feedback Sensitivity

Despite promising initial reports, corticotropin-releasing factor receptor type-1 (CRF-R1) antagonists have mostly failed to display efficacy in clinical trials for anxiety or depression. Rather than broad-spectrum antidepressant/anxiolytic-like drugs, they may represent an ‘antistress’ solution for single stressful situations or for patients with chronic stress conditions. However, the impact of prolonged CRF-R1 antagonist treatments on the hypothalamic–pituitary–adrenal (HPA) axis under chronic stress conditions remained to be characterized. Hence, our study investigated whether a chronic CRF-R1 antagonist (crinecerfont, formerly known as SSR125543, 20 mg·kg−1·day−1 ip, 5 weeks) would alter HPA axis basal circadian activity and negative feedback sensitivity in mice exposed to either control or chronic stress conditions (unpredictable chronic mild stress, UCMS, 7 weeks), through measures of fecal corticosterone metabolites, plasma corticosterone, and dexamethasone suppression test. Despite preserving HPA axis parameters in control non-stressed mice, the 5-week crinercerfont treatment improved the negative feedback sensitivity in chronically stressed mice, but paradoxically exacerbated their basal corticosterone secretion nearly all along the circadian cycle. The capacity of chronic CRF-R1 antagonists to improve the HPA negative feedback in UCMS argues in favor of a potential therapeutic benefit against stress-related conditions. However, the treatment-related overactivation of HPA circadian activity in UCMS raise questions about possible physiological outcomes with long-standing treatments under ongoing chronic stress.

Corticosterone secretion in tuatara (Sphenodon punctatus): Influential factors and conservation applications

<p>Animals are regularly exposed to environmental, social and physiological challenges. In reaction to these challenges, individuals adjust their physiology and behaviour to maintain essential processes and optimise fitness. The most widely used indicators of physiological stress in vertebrates are glucocorticoid hormones (corticosterone (CORT) or cortisol), which are commonly referred to as ‘stress hormones’. The use of CORT as a tool to understand how individuals respond to natural or human-caused challenges is central to stress physiology research. Here, I investigated intrinsic and extrinsic factors associated with CORT secretion, CORT secretion as an indicator of physiological response to challenges/stressors, and the value of CORT secretion as conservation tool in an iconic protected reptile (the tuatara, Sphenodon punctatus). A capture-restraint time series revealed a significant CORT response over a 24 h period in male and female (non-gravid and gravid) tuatara. Baseline CORT and the CORT response to capture and restraint (i.e. a standardised capture-stress protocol) were similar between sexes; however, female reproductive condition was correlated with CORT secretion in that higher baseline CORT and a lower CORT response were observed in gravid females. An observational study incorporating data across a wide range of ambient temperatures (from four island sites) confirmed that body temperature (Tb) is positively correlated with baseline CORT in gravid females only, and revealed a positive correlation between the CORT response and higher Tb in all adults. A supporting experimental study showed that acute ambient temperature increase (in which mean Tb reached 21.4±0.4°C) elicits a significant CORT response to capture-restraint in gravid females. These results confirmed that gravid females are not secreting CORT maximally during nesting, but actively modulate secretion. An inter-island comparison of CORT secretion (for four populations) revealed that baseline CORT secretion was similar among populations during the non-breeding and breeding seasons; however, the CORT response to capture-restraint varied significantly among populations. Inter-population variation in testosterone (T) was observed in males (but not females) and was positively linked with increased baseline CORT from the non-breeding season to the breeding season, suggesting male reproductive activity may drive seasonal change of baseline CORT. Significant correlations were observed between the CORT response to capture-restraint (but not baseline CORT) and habitat elements of latitude, tuatara density and seabird abundance and 2) demogenetic factors of sex ratio and genetic diversity. The measurement of CORT as a physiological monitoring tool indicated that short- and long- term dynamics of CORT secretion in tuatara are not altered through translocation to a new island, as the acute CORT response remained stable throughout exposure to cumulative stressors and long-term dynamics of CORT secretion in translocated populations simultaneously mirrored those in source populations. These findings deliver the most detailed study of CORT secretion patterns in tuatara to date. Moreover, as the first study to apply CORT secretion data as an conservation physiology monitoring tool in tuatara, these results serve as a baseline reference for future research and monitoring of conservation efforts.</p>

Corticosterone secretion in tuatara (Sphenodon punctatus): Influential factors and conservation applications

<p>Animals are regularly exposed to environmental, social and physiological challenges. In reaction to these challenges, individuals adjust their physiology and behaviour to maintain essential processes and optimise fitness. The most widely used indicators of physiological stress in vertebrates are glucocorticoid hormones (corticosterone (CORT) or cortisol), which are commonly referred to as ‘stress hormones’. The use of CORT as a tool to understand how individuals respond to natural or human-caused challenges is central to stress physiology research. Here, I investigated intrinsic and extrinsic factors associated with CORT secretion, CORT secretion as an indicator of physiological response to challenges/stressors, and the value of CORT secretion as conservation tool in an iconic protected reptile (the tuatara, Sphenodon punctatus). A capture-restraint time series revealed a significant CORT response over a 24 h period in male and female (non-gravid and gravid) tuatara. Baseline CORT and the CORT response to capture and restraint (i.e. a standardised capture-stress protocol) were similar between sexes; however, female reproductive condition was correlated with CORT secretion in that higher baseline CORT and a lower CORT response were observed in gravid females. An observational study incorporating data across a wide range of ambient temperatures (from four island sites) confirmed that body temperature (Tb) is positively correlated with baseline CORT in gravid females only, and revealed a positive correlation between the CORT response and higher Tb in all adults. A supporting experimental study showed that acute ambient temperature increase (in which mean Tb reached 21.4±0.4°C) elicits a significant CORT response to capture-restraint in gravid females. These results confirmed that gravid females are not secreting CORT maximally during nesting, but actively modulate secretion. An inter-island comparison of CORT secretion (for four populations) revealed that baseline CORT secretion was similar among populations during the non-breeding and breeding seasons; however, the CORT response to capture-restraint varied significantly among populations. Inter-population variation in testosterone (T) was observed in males (but not females) and was positively linked with increased baseline CORT from the non-breeding season to the breeding season, suggesting male reproductive activity may drive seasonal change of baseline CORT. Significant correlations were observed between the CORT response to capture-restraint (but not baseline CORT) and habitat elements of latitude, tuatara density and seabird abundance and 2) demogenetic factors of sex ratio and genetic diversity. The measurement of CORT as a physiological monitoring tool indicated that short- and long- term dynamics of CORT secretion in tuatara are not altered through translocation to a new island, as the acute CORT response remained stable throughout exposure to cumulative stressors and long-term dynamics of CORT secretion in translocated populations simultaneously mirrored those in source populations. These findings deliver the most detailed study of CORT secretion patterns in tuatara to date. Moreover, as the first study to apply CORT secretion data as an conservation physiology monitoring tool in tuatara, these results serve as a baseline reference for future research and monitoring of conservation efforts.</p>

Urocortin 3 in the posterodorsal medial amygdala mediates stress-induced suppression of LH pulsatility in female mice

Psychosocial stress disrupts reproduction and interferes with pulsatile LH secretion. The posterodorsal medial amygdala (MePD) is an upstream modulator of the reproductive axis and stress. Corticotropin-releasing factor type-2 receptors (CRFR2) are activated in the presence of psychosocial stress together with increased expression of the CRFR2 ligand Urocortin3 (Ucn3) in the MePD of rodents. We investigate whether Ucn3 signalling in the MePD is involved in mediating the suppressive effect of psychosocial stress on LH pulsatility. Firstly, we administered Ucn3 into the MePD and monitored the effect on LH pulses in ovariectomised mice. Next, we delivered Astressin2B, a selective CRFR2 antagonist, intra-MePD in the presence of predator odor, 2,4,5-Trimethylthiazole (TMT) and examined the effect on LH pulses. Subsequently, we virally infected Ucn3-cre-tdTomato mice with inhibitory DREADDs targeting MePD Ucn3 neurons while exposing mice to TMT or restraint stress and examined the effect on LH pulsatility as well as corticosterone release. Administration of Ucn3 into the MePD dose-dependently inhibited LH pulses and administration of Astressin2B blocked the suppressive effect of TMT on LH pulsatility. Additionally, DREADDs inhibition of MePD Ucn3 neurons blocked TMT and restraint stress-induced inhibition of LH pulses and corticosterone release. These results demonstrate for the first time that Ucn3 neurons in the MePD mediate psychosocial stress-induced suppression of the GnRH pulse generator and corticosterone secretion. Ucn3 signalling in the MePD plays a role in modulating the hypothalamic-pituitary-ganadal and hypothalamic-pituitary-adrenal axes, and this brain locus may represent a nodal centre in the interaction between the reproductive and stress axes.

Effects of Growth Hormone Receptor Ablation in Corticotropin-Releasing Hormone Cells

Corticotropin-releasing hormone (CRH) cells are the dominant neuronal population responsive to the growth hormone (GH) in the paraventricular nucleus of the hypothalamus (PVH). However, the physiological importance of GH receptor (GHR) signaling in CRH neurons is currently unknown. Thus, the main objective of the present study was to investigate the consequences of GHR ablation in CRH-expressing cells of male and female mice. GHR ablation in CRH cells did not cause significant changes in body weight, body composition, food intake, substrate oxidation, locomotor activity, glucose tolerance, insulin sensitivity, counterregulatory response to 2-deoxy-D-glucose and ghrelin-induced food intake. However, reduced energy expenditure was observed in female mice carrying GHR ablation in CRH cells. The absence of GHR in CRH cells did not affect anxiety, circadian glucocorticoid levels or restraint-stress-induced corticosterone secretion and activation of PVH neurons in both male and female mice. In summary, GHR ablation, specifically in CRH-expressing neurons, does not lead to major alterations in metabolism, hypothalamic–pituitary–adrenal axis, acute stress response or anxiety in mice. Considering the previous studies showing that central GHR signaling regulates homeostasis in situations of metabolic stress, future studies are still necessary to identify the potential physiological importance of GH action on CRH neurons.

Plasticity of intrinsic excitability across the estrous cycle in hypothalamic CRH neurons

Stress responses are highly plastic and vary across physiological states. The female estrous cycle is associated with a number of physiological changes including changes in stress responses, however, the mechanisms driving these changes are poorly understood. Corticotropin-releasing hormone (CRH) neurons are the primary neural population controlling the hypothalamic–pituitary–adrenal (HPA) axis and stress-evoked corticosterone secretion. Here we show that CRH neuron intrinsic excitability is regulated over the estrous cycle with a peak in proestrus and a nadir in estrus. Fast inactivating voltage-gated potassium channel (IA) currents showed the opposite relationship, with current density being lowest in proestrus compared to other cycle stages. Blocking IA currents equalized excitability across cycle stages revealing a role for IA in mediating plasticity in stress circuit function over the female estrous cycle.

Involvement of JAK-STAT-SOCS3 Pathway in the Regression of Mice X-Zone of Adrenal Cortex

Previously, we showed that the histological markers of the mice X-zone of adrenal cortex were still present in adult male and female postpartum SF1/SOCS3KO mice. Abnormal distribution of lipid droplets along the adrenal cortex and reduced ACTH-induced corticosterone secretion were observed in SF1/SOCS3KO mice (1). Here we have examined the adrenals of the SF1/SOCS3KO male and virgin female at 3, 8, 15 and 30 weeks through morphological and molecular analysis. Hematoxylin-eosin stains showed X-zone retention in the SF1/SOCS3KO mice adrenals regardless of the postnatal age analyzed. CYP17A1-positive cells were immunolocalized in the X-zone of SF1/SOCS3KO mice that were confirmed by immunoblotting. A fetal adrenal enhancer (FAdE) and Pik3c2g and 20αHSD genes expression were analyzed by RT-PCR, and these genes were present in the male SF1/SOCS3KO mice up to the age of 8 and 15 weeks, but not in the control mice. Therefore, we showed retention of X-zone in the adrenal cortex of SOCS3KO mice up to the age of 30 week, which suggest the involvement of JAK/STAT/SOCS3 signaling pathway in the differentiation process of adrenal cortex. Reference: (1) Pedroso et al., J Endocrinol.2017 Dec;235(3):207–222.

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.