Corticosterone modulates fear responses and the expression of glucocorticoid receptors in the brain of high-anxiety rats

Two Hits of EDCs Three Generations Apart: Evaluating Multigenerational Anxiety-Like Behavioral Phenotypes in Female Rats Exposed to Aroclor 1221 and Vinclozolin

BACKGROUND

Endocrine-disrupting chemicals (EDCs) are exogenous chemical compounds that interfere with the normal function of the endocrine system and are linked to direct and inherited adverse effects in both humans and wildlife. Legacy EDCs such as polychlorinated biphenyls (PCBs) are no longer used yet remain detectable in biological specimens around the world; concurrently, we are exposed to newer EDCs like the fungicide vinclozolin (VIN). This combination of individuals' direct environmental chemical exposures and any heritable changes caused by their ancestors' chemical exposures leads to a layered pattern of both direct and ancestrally inherited exposures that might have cumulative effects over generations.

OBJECTIVES

We assessed consequences of both direct and ancestral exposure to EDCs over six generations, examining anxiety-like behaviors in maternal and paternal lines of female rats. We used the "two hits, three generations apart" multigenerational exposure model to explore how two distinct EDCs-the weakly estrogenic PCB mixture Aroclor 1221 (A1221) and the antiandrogenic VIN-interact on behavior across generations. We also explored serum hormones as a potential mechanism.

METHODS

Rats were prenatally exposed to A1221, VIN, or vehicle (DMSO) in the F1 generation, and a second exposure (same or different) was administered to the F4 generation. Anxiety-like behavior was measured in the Open Field test, Light:Dark box, and Elevated Plus Maze in the F1, F3, F4, and F6 generations. Serum concentrations of estradiol and corticosterone were analyzed.

RESULTS

Behavioral effects were not detectable in the F1 generation but emerged and became more robust across generations. Rats with ancestral VIN exposure demonstrated less anxiety-like behavior in the F3 paternal line in comparison with controls. Rats exposed to ancestral then prenatal A1221/VIN and VIN/A1221 had more anxiety-like behavior in the F4 maternal line, and those with two ancestral hits of VIN/VIN had more anxiety in the F6 paternal line, in comparison with controls.

DISCUSSION

Our findings suggest that anxiety-like behavioral phenotypes can manifest in rats following germline exposure to EDCs and that subsequent exposures across generations can intensify these effects in a lineage-dependent manner. https://doi.org/10.1289/EHP15621.

Two Hits of EDCs Three Generations Apart: Evaluating Multigenerational Anxiety-Like Behavioral Phenotypes in Male Rats Exposed to Aroclor 1221 and Vinclozolin

BACKGROUND

Increasing evidence supports an association of endocrine-disrupting chemical (EDC) exposures with adverse biological effects in humans and wildlife. Recent studies reveal that health consequences of environmental exposures may persist or emerge across generations. This creates a dual conundrum: that we are exposed to contemporary environmental chemicals overlaid upon the inheritance of our ancestors' exposure profiles. Even when legacy EDCs are phased out, they may remain relevant due to persistence in the environment together with intergenerational inheritance of their adverse biological effects. Thus, we all possess a body burden of legacy contaminants, and we are also increasingly exposed to new generations of EDCs.

OBJECTIVES

We assessed the effects of direct and ancestral exposures to EDCs across six generations on anxiety-like behaviors in male rats using our "two hits, three generations apart" multigenerational EDC exposure experimental model. We investigated two classes of EDCs with distinct hormonal actions and historical use-the weakly estrogenic polychlorinated biphenyl (PCB) mixture Aroclor 1221 (A1221) and the anti-androgenic fungicide vinclozolin (VIN)-in both the maternal and paternal line. We also determined if a hormonal mechanism drives these effects across generations.

METHODS

Rats were gestationally exposed to A1221, VIN, or vehicle [dimethyl sulfoxide (DMSO)] in the F1 generation. Three generations later, the F4 generation was given the same or a different exposure. Anxiety-like behavior was measured in the open field test, light:dark box, and elevated plus maze across generations. Serum was collected at the end of the experiment, and concentrations of estradiol and corticosterone were analyzed.

RESULTS

Although direct exposure did not affect behavior in F1 males, ancestral exposure to VIN decreased anxiety-like behavior in the F3 paternal line compared to vehicle. In the F4 paternal line, ancestral A1221 followed by direct exposure to VIN increased anxiety-like behavior compared to controls. In the F6 maternal line, relative to vehicle, the double ancestral hits of A1221/VIN decreased anxiety-like behavior. Serum hormones weakly predicted behavioral changes in the F4 paternal line and were modestly affected in the F4 and F6 maternal lines.

DISCUSSION

Our data suggest that anxiety-like behavioral phenotypes emerge transgenerationally in male rats in response to EDC exposure and that multiple hits of either the same or a different EDC can increase the impact in a lineage-specific manner. https://doi.org/10.1289/EHP15684.

Global cerebral ischemia in adult female rats interrupts estrous cyclicity and induces lasting changes in hypothalamic-pituitary-gonadal axis signaling peptides

Persistent post-ischemic alterations to the hypothalamic-pituitary-adrenal (HPA) axis occur following global cerebral ischemia (GCI) in rodents. However, similar effects on hypothalamic-pituitary-gonadal (HPG) axis activation remain to be determined. Therefore, this study evaluated the effects of GCI in adult female rats (via four-vessel occlusion) on the regularity of the estrous cycle for 24-days post ischemia. A second objective aimed to assess persistent alterations of HPG axis activation through determination of the expression of estrogen receptor alpha (ERα), kisspeptin (Kiss1), and gonadotropin-inhibitory hormone (GnIH/RFamide-related peptide; RFRP3) in the medial preoptic area (POA), arcuate nucleus (ARC), dorsomedial nucleus (DMH) of the hypothalamus, and CA1 of the hippocampus 25 days post ischemia. Expression of glucocorticoid receptors (GR) in the paraventricular nucleus of the hypothalamus (PVN) and CA1 served as a proxy of altered HPA axis activation. Our findings demonstrated interruption of the estrous cycle in 87.5 % of ischemic rats, marked by persistent diestrus, lasting on average 11.86 days. Moreover, compared to sham-operated controls, ischemic female rats showed reduced Kiss1 expression in the hypothalamic ARC and POA, concomitant with elevated ERα in the ARC and increased GnIH in the DMH and CA1. Reduced GR expression in the CA1 was associated with increased GR-immunoreactivity in the PVN, indicative of lasting dysregulation of HPA axis activation. Together, these findings demonstrate GCI disruption of female rats' estrous cycle over multiple days, with a lasting impact on HPG axis regulators within the reproductive axis.

Contributions of glucocorticoid receptors in cortical astrocytes to memory recall

Dysfunctions in memory recall lead to pathological fear; a hallmark of trauma-related disorders, like posttraumatic stress disorder (PTSD). Both, heightened recall of an association between a cue and trauma, as well as impoverished recall that a previously trauma-related cue is no longer a threat, result in a debilitating fear toward the cue. Glucocorticoid-mediated action via the glucocorticoid receptor (GR) influences memory recall. This literature has primarily focused on GRs expressed in neurons or ignored cell-type specific contributions. To ask how GR action in nonneuronal cells influences memory recall, we combined auditory fear conditioning in mice and the knockout of GRs in astrocytes in the prefrontal cortex (PFC), a brain region implicated in memory recall. We found that knocking out GRs in astrocytes of the PFC disrupted memory recall. Specifically, we found that knocking out GRs in astrocytes in the PFC (AstroGRKO) after fear conditioning resulted in higher levels of freezing to the CS+ tone when compared with controls (AstroGRintact). While we did not find any differences in extinction of fear toward the CS+ between these groups, AstroGRKO female but not male mice showed impaired recall of extinction training. These results suggest that GRs in cortical astrocytes contribute to memory recall. These data demonstrate the need to examine GR action in cortical astrocytes to elucidate the basic neurobiology underlying memory recall and potential mechanisms that underlie female-specific biases in the incidence of PTSD.

Differential impact of stress and environmental enrichment on corticolimbic circuits

Stress exposure can produce profound changes in physiology and behavior that can impair health and well-being. Of note, stress exposure is linked to anxiety disorders and depression in humans. The widespread impact of these disorders warrants investigation into treatments to mitigate the harmful effects of stress. Pharmacological treatments fail to help many with these disorders, so recent work has focused on non-pharmacological alternatives. One of the most promising of these alternatives is environmental enrichment (EE). In rodents, EE includes social, physical, and cognitive stimulation for the animal, in the form of larger cages, running wheels, and toys. EE successfully reduces the maladaptive effects of various stressors, both as treatment and prophylaxis. While we know that EE can have beneficial effects under stress conditions, the morphological and molecular mechanisms underlying these behavioral effects are still not well understood. EE is known to alter neurogenesis, dendrite development, and expression of neurotrophic growth factors, effects that vary by type of enrichment, age, and sex. To add to this complexity, EE has differential effects in different brain regions. Understanding how EE exerts its protective effects on morphological and molecular levels could hold the key to developing more targeted pharmacological treatments. In this review, we summarize the literature on the morphological and molecular consequences of EE and stress in key emotional regulatory pathways in the brain, the hippocampus, prefrontal cortex, and amygdala. The similarities and differences among these regions provide some insight into stress-EE interaction that may be exploited in future efforts toward prevention of, and intervention in, stress-related diseases.

Distinct patterns of brain Fos expression in Carioca High- and Low-conditioned Freezing Rats

BACKGROUND

The bidirectional selection of high and low anxiety-like behavior is a valuable tool for understanding the neurocircuits that are responsible for anxiety disorders. Our group developed two breeding lines of rats, known as Carioca High- and Low-conditioned Freezing (CHF and CLF), based on defensive freezing in the contextual fear conditioning paradigm. A random selected line was employed as a control (CTL) comparison group for both CHF and CLF lines of animals. The present study performed Fos immunochemistry to investigate changes in neural activity in different brain structures among CHF and CLF rats when they were exposed to contextual cues that were previously associated with footshock.

RESULTS

The study indicated that CHF rats expressed high Fos expression in the locus coeruleus, periventricular nucleus of the hypothalamus (PVN), and lateral portion of the septal area and low Fos expression in the medial portion of the septal area, dentate gyrus, and prelimbic cortex (PL) compared to CTL animals. CLF rats exhibited a decrease in Fos expression in the PVN, PL, and basolateral nucleus of the amygdala and increase in the cingulate and perirhinal cortices compared to CTL animals.

CONCLUSIONS

Both CHF and CLF rats displayed Fos expression changes key regions of the anxiety brain circuitry. The two bidirectional lines exhibit different pattern of neural activation and inhibition with opposing influences on the PVN, the main structure involved in regulating the hypothalamic-pituitary-adrenal neuroendocrine responses observed in anxiety disorders.

Environmental enrichment ameliorates chronic immobilisation stress-induced spatial learning deficits and restores the expression of BDNF, VEGF, GFAP and glucocorticoid receptors

Severe and prolonged stress is the main environmental factor that precipitates depression, anxiety and cognitive dysfunctions. On the other hand, exposure to environmental enrichment (EE) has been shown to induce progressive plasticity in the brain and improve learning and memory in various neurological and psychiatric disorders. It is not known whether exposure to enriched environment could ameliorate chronic immobilisation stress-induced cognitive deficits and altered molecular markers. Hence, in the present study we aimed to evaluate the effect of enriched environment on chronic immobilisation stress (CIS) associated changes in spatial learning and memory, behavioural measures of anxiety, depression and molecular markers as well as structural alterations. Male Wistar rats were subjected to chronic immobilisation stress for 2h/day/10days followed by 2weeks of exposure to EE. CIS resulted in weight loss, anhedonia, increased immobility, spatial learning and memory impairment, enhanced anxiety, and reduced expression of BDNF, VEGF, GFAP and glucocorticoid receptors (GR) in discrete brain regions. Interestingly, stressed rats exposed to enrichment ameliorated behavioural depression, spatial learning and memory impairment and reduced anxiety behaviour. In addition, EE restored BDNF, VEGF, GFAP and GR expression and normalized hypotrophy of dentate gyrus and hippocampus in CIS rats. In contrast, EE did not restore hypertrophy of the amygdalar complex. Thus, EE ameliorates stress-induced cognitive deficits by modulating the neurotrophic factors, astrocytes and glucocorticoid receptors in the hippocampus, frontal cortex and amygdala.

Tactile stimulation during different developmental periods modifies hippocampal BDNF and GR, affecting memory and behavior in adult rats

Recent studies have shown that tactile stimulation (TS) in pups is able to prevent and/or minimize fear, anxiety behaviors, and addiction to psychostimulant drugs in adult rats. In these studies, animals have been exposed to handling from postnatal day (PND) 1-21. This study was designed to precisely establish which period of preweaning development has a greater influence of TS on neuronal development. After birth, male pups were exposed to TS from PND1-7, PND8-14, and PND15-21. In adulthood, the different periods of postnatal TS were assessed through behavioral, biochemical, and molecular assessments. Animals that received TS from PND8-14 showed lower anxiety-like symptoms, as observed by decreased anxiety index in elevated plus maze. This same TS period was able to improve rats' working memory by increasing the percentage of alternation rate in Y-maze, and induce better ability to cope with stressful situations, as showed in the defensive burying test by a reduced time of burying behavior. On the other hand, animals receiving TS in the first week of life showed longest cumulative burying time, which is directly related to increased anxiety-like behavior. Moreover, TS from PND8-14 showed lower corticosterone levels and better oxidative status, as observed by decreased lipid peroxidation and increased catalase activity in the hippocampus. Brain-derived neurotrophic factor (BDNF) immunocontent was increased in the hippocampus of animals receiving TS from PND8-14, while glucocorticoid receptors immunocontent was decreased in both TS_1-7 and TS_15-21 , but not TS_8-14 . To the best of our knowledge, this study is the first to show TS can be more efficient if applied over a focused period of neonatal development (PND8-14) and this beneficial influence can be reflected on reduced emotionality and increased ability to address stressful situations in adulthood. © 2016 Wiley Periodicals, Inc.

Childhood adversity and epigenetic regulation of glucocorticoid signaling genes: Associations in children and adults

Early childhood experiences have lasting effects on development, including the risk for psychiatric disorders. Research examining the biologic underpinnings of these associations has revealed the impact of childhood maltreatment on the physiologic stress response and activity of the hypothalamus-pituitary-adrenal axis. A growing body of literature supports the hypothesis that environmental exposures mediate their biological effects via epigenetic mechanisms. Methylation, which is thought to be the most stable form of epigenetic change, is a likely mechanism by which early life exposures have lasting effects. We present recent evidence related to epigenetic regulation of genes involved in hypothalamus-pituitary-adrenal axis regulation, namely, the glucocorticoid receptor gene (nuclear receptor subfamily 3, group C, member 1 [NR3C1]) and FK506 binding protein 51 gene (FKBP5), after childhood adversity and associations with risk for psychiatric disorders. Implications for the development of interventions and future research are discussed.

Environmental Enrichment Increases Glucocorticoid Receptors and Decreases GluA2 and Protein Kinase M Zeta (PKMζ) Trafficking During Chronic Stress: A Protective Mechanism?

Environmental enrichment (EE) housing paradigms have long been shown beneficial for brain function involving neural growth and activity, learning and memory capacity, and for developing stress resiliency. The expression of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluA2, which is important for synaptic plasticity and memory, is increased with corticosterone (CORT), undermining synaptic plasticity and memory. Thus, we determined the effect of EE and stress on modulating GluA2 expression in Sprague-Dawley male rats. Several markers were evaluated which include: plasma CORT, the glucocorticoid receptor (GR), GluA2, and the atypical protein kinase M zeta (PKMζ). For 1 week standard-(ST) or EE-housed animals were treated with one of the following four conditions: (1) no stress; (2) acute stress (forced swim test, FST; on day 7); (3) chronic restraint stress (6 h/day for 7 days); and (4) chronic + acute stress (restraint stress 6 h/day for 7 days + FST on day 7). Hippocampi were collected on day 7. Our results show that EE animals had reduced time immobile on the FST across all conditions. After chronic + acute stress EE animals showed increased GR levels with no change in synaptic GluA2/PKMζ. ST-housed animals showed the reverse pattern with decreased GR levels and a significant increase in synaptic GluA2/PKMζ. These results suggest that EE produces an adaptive response to chronic stress allowing for increased GR levels, which lowers neuronal excitability reducing GluA2/PKMζ trafficking. We discuss this EE adaptive response to stress as a potential underlying mechanism that is protective for retaining synaptic plasticity and memory function.

Rapid Activation of Glucocorticoid Receptors in the Prefrontal Cortex Mediates the Expression of Contextual Conditioned Fear in Rats

The aim of the present study was to investigate the role of glucocorticoids in medial prefrontal cortex (mPFC) activity and the expression of contextual conditioned fear (freezing). Rats were pretreated with vehicle or metyrapone, a corticosterone synthesis blocker, and exposed to a context previously paired with footshocks. Freezing and Fos-protein expression in different mPFC regions were assessed. Exposure to the aversive context led to increased freezing and Fos expression in the prelimbic (PrL), anterior cingulate areas 1 and 2 (Cg1/Cg2). Pretreatment with metyrapone decreased freezing and Fos expression in these areas. Administration of spironolactone, an MR antagonist, in the PrL before the test decreased freezing. Pretreatment with RU38486, a glucocorticoid receptor (GR) antagonist, reduced this effect of spironolactone, suggesting that the effects of this MR antagonist may be attributable to a redirection of endogenous corticosterone actions to GRs. Consistent with this result, the decrease in freezing that was induced by intra-PrL injections of corticosterone was attenuated by pretreatment with RU38486 but not spironolactone. These findings indicate that corticosterone release during aversive conditioning influences mPFC activity and the retrieval of conditioned fear memory indicating the importance of balance between MR:GR-mediated effects in this brain region in this process.