Benedict J. Kolber

Forebrain Glucocorticoid Receptors Modulate Anxiety-Associated Locomotor Activation and Adrenal Responsiveness

Stress potently modulates anxiety- and depression-related behaviors. In response to stressors, the hypothalamic-pituitary-adrenal (HPA) axis is activated, resulting in the release of glucocorticoids from the adrenal cortex. These hormones act peripherally to restore homeostasis but also feed back to the CNS to control the intensity and duration of the stress response. Glucocorticoids act in limbic areas of the CNS to mediate the psychological and behavioral effects of stress. In this study, we investigate the effect of forebrain-specific disruption of the glucocorticoid receptor (GR) on stress- and anxiety-related behaviors. We demonstrate that mice with disruption of forebrain GR show alterations in stress-induced locomotor activation in a number of anxiety-related behavioral paradigms. These changes are associated with alterations in stress-induced HPA axis activation and, importantly, are not attenuated by chronic treatment with the tricyclic antidepressant imipramine. These data demonstrate the importance of forebrain GR in regulation of physiological and behavioral stress reactivity and suggest that distinct pathways regulate despair- and anxiety-related behaviors.

Central amygdala glucocorticoid receptor action promotes fear-associated CRH activation and conditioning

The amygdala is a key limbic area involved in fear responses and pavlovian conditioning with the potential to directly respond to endocrine signals associated with fear or stress. To gain insights into the molecular mechanisms and subregional specificity of fear conditioning, we disrupted type II glucocorticoid receptors (GRs) in the central nucleus of the amygdala (CeA) by delivering lentiviral vectors containing Cre-recombinase into floxed-GR mice. GR deletion in the CeA (CeAGRKO mice) prevented conditioned fear behavior. In contrast, forebrain disruption of GRs excluding the CeA did not. The conditioned fear deficit in CeAGRKO mice was associated with decreases in cFos and corticotropin-releasing hormone (CRH) expression. Moreover, intracerebroventricular delivery of CRH rescued the conditioned fear deficit in CeAGRKO mice. We conclude that fear conditioning involves a neuroendocrine circuit by using GR activation in the CeA for acute CRH induction and long-lasting behavioral modulation.

Activation of metabotropic glutamate receptor 5 in the amygdala modulates pain-like behavior

The central nucleus of the amygdala (CeA) has been identified as a site of nociceptive processing important for sensitization induced by peripheral injury. However, the cellular signaling components underlying this function remain unknown. Here, we identify metabotropic glutamate receptor 5 (mGluR5) as an integral component of nociceptive processing in the CeA. Pharmacological activation of mGluRs with (R,S)-3,5-dihydroxyphenylglycine (DHPG) in the CeA of mice is sufficient to induce peripheral hypersensitivity in the absence of injury. DHPG-induced peripheral hypersensitivity is reduced via pharmacological blockade of mGluR5 or genetic disruption of mGluR5. Furthermore, pharmacological blockade or conditional deletion of mGluR5 in the CeA abrogates inflammation-induced hypersensitivity, demonstrating the necessity of mGluR5 in CeA-mediated pain modulation. Moreover, we demonstrate that phosphorylation of extracellular-signal regulated kinase 1/2 (ERK1/2) is downstream of mGluR5 activation in the CeA and is necessary for the full expression of peripheral inflammation-induced behavioral sensitization. Finally, we present evidence of right hemispheric lateralization of mGluR5 modulation of amygdalar nociceptive processing. We demonstrate that unilateral pharmacological activation of mGluR5 in the CeA produces distinct behavioral responses depending on whether the right or left amygdala is injected. We also demonstrate significantly higher levels of mGluR5 expression in the right amygdala compared with the left under baseline conditions, suggesting a potential mechanism for right hemispheric lateralization of amygdala function in pain processing. Together, these results establish an integral role for mGluR5 and ERK1/2 in nociceptive processing in the CeA.

Hypothalamic-pituitary-adrenal axis dysregulation and behavioral analysis of mouse mutants with altered glucocorticoid or mineralocorticoid receptor function

Corticosteroid receptors are critical for the maintenance of homeostasis after both psychological and physiological stress. To understand the different roles and interactions of the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) during stress, it is necessary to dissect the role of corticosteroid signaling at both the system and sub-system level. A variety of GR transgenic mouse lines have recently been used to characterize the role of GR in the CNS as a whole and particularly in the forebrain. We will describe both the behavioral and cellular/molecular implications of disrupting GR function in these animal models and describe the implications of this data for our understanding of normal endocrine function and stress adaptation. MRs in tight epithelia have a long established role in sodium homeostasis. Recently however, evidence has suggested that MRs in the limbic brain also play an important role in psychological stress. Just as with GR, targeted mutations in MR induce a variety of behavioral changes associated with stress adaptation. In this review, we will discuss the implications of this work on MR. Finally, we will discuss the possible interaction between MR and GR and how future work using double mutants (through conventional means or virus based gene alteration) will be needed to more fully understand how signaling through these two steroid receptors provides the adaptive mechanisms to deal with a variety of stressors.

Transient early-life forebrain corticotropin-releasing hormone elevation causes long-lasting anxiogenic and despair-like changes in mice.

During development, early-life stress, such as abuse or trauma, induces long-lasting changes that are linked to adult anxiety and depressive behavior. It has been postulated that altered expression of corticotropin-releasing hormone (CRH) can at least partially account for the various effects of stress on behavior. In accord with this hypothesis, evidence from pharmacological and genetic studies has indicated the capacity of differing levels of CRH activity in different brain areas to produce behavioral changes. Furthermore, stress during early life or adulthood causes an increase in CRH release in a variety of neural sites. To evaluate the temporal and spatial specificity of the effect of early-life CRH exposure on adult behavior, the tetracycline-off system was used to produce mice with forebrain-restricted inducible expression of CRH. After transient elevation of CRH during development only, behavioral testing in adult mice revealed a persistent anxiogenic and despair-like phenotype. These behavioral changes were not associated with alterations in adult circadian or stress-induced corticosterone release but were associated with changes in CRH receptor type 1 expression. Furthermore, the despair-like changes were normalized with antidepressant treatment. Overall, these studies suggest that forebrain-restricted CRH signaling during development can permanently alter stress adaptation leading to increases in maladaptive behavior in adulthood.

Central Amygdala Metabotropic Glutamate Receptor 5 in the Modulation of Visceral Pain

Painful bladder syndrome is a debilitating condition that affects 3–6% of women in the United States. Multiple lines of evidence suggest that changes in CNS processing are key to the development of chronic bladder pain conditions but little is known regarding the underlying cellular, molecular, and neuronal mechanisms. Using a mouse model of distention-induced bladder pain, we found that the central nucleus of the amygdala (CeA) is a critical site of neuromodulation for processing of bladder nociception. Furthermore, we demonstrate that metabotropic glutamate receptor 5 (mGluR5) activation in the CeA induces bladder pain sensitization by increasing CeA output. Thus, pharmacological activation of mGluR5 in the CeA is sufficient to increase the response to bladder distention. Additionally, pharmacological blockade or virally mediated conditional deletion of mGluR5 in the CeA reduced responses to bladder distention suggesting that mGluR5 in the CeA is also necessary for these responses. Finally, we used optogenetic activation of the CeA and demonstrated that this caused a robust increase in the visceral pain response. The CeA-localized effects on responses to bladder distention are associated with changes in extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation in the spinal cord. Overall, these data demonstrate that mGluR5 activation leads to increased CeA output that drives bladder pain sensitization.

Lipoprotein receptor LRP1 regulates leptin signaling and energy homeostasis in the adult central nervous system.

Lipoprotein receptor LRP1 play critical roles in lipid metabolism, and this study reveals a novel role for LRP1 in controlling food intake and obesity in the central nervous system of the adult mouse.

Behavioral insights from mouse models of forebrain- and amygdala-specific glucocorticoid receptor genetic disruption

Genetic modulation of glucocorticoid receptor (GR) function in the brain using transgenic and gene knockout mice has yielded important insights into many aspects of GR effects on behavior and neuroendocrine responses, but significant limitations regarding interpretation of region-specific and temporal requirements remain. Here, we summarize the behavioral phenotype associated with two knockout mouse models to define the role of GRs specifically within the forebrain and amygdala. We report that forebrain-specific GR knockout mice exhibit impaired negative feedback regulation of the hypothalamic-pituitary-adrenal (HPA) axis and increased despair- and anxiety-like behaviors. In addition, mice with a disruption of GR specifically within the central nucleus of the amygdala (CeA) are deficient in conditioned fear behavior. Overall, these models serve as beneficial tools to better understand the biology of GR signaling in the normal stress response and in mood disorders.

Corticotrophin Releasing Factor Accelerates Neuropathology and Cognitive Decline in a Mouse Model of Alzheimer's Disease

Chronic stress has been suggested to influence the pathogenesis of Alzheimers disease (AD); however, the mechanism underlying this influence remains unknown. In this study, we created a triple transgenic mouse model that overexpresses corticotrophin-releasing factor (CRF) and human amyloid-β protein precursor (AβPP), to investigate whether increases in the expression of CRF can mimic the effects of stress on amyloid metabolism and the neurodegeneration. Tg2576 mice that overexpresses human AβPP gene were crossbreed with Tetop-CRF (CRF) mice and CaMKII-tTA (tTA) mice to create a novel triple transgenic mouse model that conditioned overexpresses CRF in forebrain and overexpresses human AβPP (called AβPP+/CRF+/tTA+, or TT mice). Then we evaluated serial neuro-anatomical and behavioral phenotypes on TT mice using histological, biochemical, and behavioral assays. TT mice showed a Cushingoid-like phenotype starting at 3 months of age. At 6 months of age, these mice demonstrated increases in tissue-soluble amyloid-β (Aβ) and Aβ plaques in the cortex and hippocampus, as compared to control mice. Moreover, TT mice characterized substantial decreases in dendritic branching and dendritic spine density in pyramidal neurons in layer 4 of the frontal cortex and CA1 of the hippocampus. Finally, TT mice showed significantly impaired working memory and contextual memory, with a modest increase in anxiety-like behavior. Our results suggested genetic increases in the brain of CRF expression mimicked chronic stress on the effects of amyloid deposition, neurodegeneration, and behavioral deficits. The novel transgenic mouse model will provide a unique tool to further investigate the mechanisms between stress and AD.

Defining brain region-specific glucocorticoid action during stress by conditional gene disruption in mice

The ability of an organism to adapt during stress has a significant impact on long-term survival and health. Maladaptive responses to stress have been associated with susceptibility to the development of mood disorders, including major depressive disorder (MDD) and generalized anxiety disorder. Importantly, dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis, the endocrine stress response, has been linked to these diseases. Here, we review recent data on the region-specific role of glucocorticoid receptor (GR) signaling in the behavioral, molecular and endocrine response to stress. Using a conditional deletion approach, we have shown that disruption of GR function in the forebrain of mice induces alterations in despair-like behavior and HPA axis function, reminiscent of MDD. Furthermore, in an effort to explore the sub-regional specificity of GR activity, we have developed a model to disrupt GR in the central nucleus of the amygdala. In our initial efforts to characterize these mice, we have demonstrated a critical role for GR in the formation of fear memory.