Effects of repeated tianeptine treatment on CRF mRNA expression in non-stressed and chronic mild stress-exposed rats

Distinct populations of corticotropin-releasing factor (CRF) neurons mediate divergent yet complementary defensive behaviors in response to a threat

Defensive behaviors in response to a threat are shared across the animal kingdom. Active (fleeing, sheltering) or passive (freezing, avoiding) defensive responses are adaptive and facilitate survival. Selecting appropriate defensive strategy depends on intensity, proximity, temporal threat threshold, and past experiences. Hypothalamic corticotropin-releasing factor (CRF) is a major driver of an acute stress response, whereas extrahypothalamic CRF mediates stress-related affective behaviors. In this review, we shift the focus from a monolithic role of CRF as an anxiogenic peptide to comprehensively dissecting contributions of distinct populations of CRF neurons in mediating defensive behaviors. Direct interrogation of CRF neurons of the central amygdala (CeA) or the bed nucleus of the stria terminalis (BNST) show they drive unconditioned defensive responses, such as vigilance and avoidance of open spaces. Although both populations also contribute to learned fear responses in familiar, threatening contexts, CeA-CRF neurons are particularly attuned to the ever-changing environment. Depending on threat intensities, they facilitate discrimination of salient stimuli predicting manageable threats, and prevent their generalization. Finally, hypothalamic CRF neurons mediate initial threat assessment and active defense such as escape to shelter. Overall, these three major populations of CRF neurons demonstrate divergent, yet complementary contributions to the versatile defense system: heightened vigilance, discriminating salient threats, and active escape, representing three legs of the defense tripod. Despite the 'CRF exhaustion' in the field of affective neuroscience, understanding contributions of specific CRF neurons during adaptive defensive behaviors is needed in order to understand the implications of their dysregulation in fear- and anxiety-related psychiatric disorders. This article is part of the Special Issue on "Fear, Anxiety and PTSD".

Corticotropin releasing factor and norepinephrine related circuitry changes in the bed nucleus of the stria terminalis in stress and alcohol and substance use disorders

Alcohol Use Disorder (AUD) affects around 14.5 million individuals in the United States, with Substance Use Disorder (SUD) affecting an additional 8.3 million individuals. Relapse is a major barrier to effective long-term treatment of this illness with stress often described as a key trigger for a person with AUD or SUD to relapse during a period of abstinence. Two signaling molecules, norepinephrine (NE) and corticotropin releasing factor (CRF), are released during the stress response, and also play important roles in reward behaviors and the addiction process. Within the addiction literature, one brain region in which there has been increasing research focus in recent years is the bed nucleus of the stria terminalis (BNST). The BNST is a limbic structure with numerous cytoarchitecturally and functionally different subregions that has been implicated in drug-seeking behaviors and stress responses. This review focuses on drug and stress-related neurocircuitry changes in the BNST, particularly within the CRF and NE systems, with an emphasis on differences and similarities between the major dorsal and ventral BNST subregions.

Extended amygdala circuits are differentially activated by context fear conditioning in male and female rats

As the incidence of anxiety disorders is more prevalent in females, comparing the neural underpinnings of anxiety in males and females is imperative. The bed nucleus of the stria terminalis (BNST) contributes to long-lasting, anxiety-like states including the expression of context fear conditioning. Currently, there is conflicting evidence as to which nuclei of the BNST contribute to these behaviors. The anterolateral portion of the BNST (BNST-AL) located dorsal to the anterior commissure and lateral to the stria terminalis sends robust projections to the central nucleus of the amygdala (CE). Here we asked whether the BNST-AL is active during the expression of context fear conditioning in both male and female rats. At the cellular level, the expression of context fear produced upregulation of the immediate-early gene ARC in the BNST-AL as well as an upregulation of ARC specifically in neurons projecting to the CE, as labeled by the retrograde tracer Fluorogold infused into the CE. However, this pattern of ARC expression was observed in male rats only. Excitotoxic lesions of the BNST reduced context fear expression in both sexes, suggesting that a different set of BNST subnuclei may be recruited by the expression of fear and anxiety-like behaviors in females. Overall, our data highlight the involvement of the BNST-AL in fear expression in males, and suggest that subnuclei of the BNST may be functionally different in male and female rats.

Both CRF1 and CRF2 receptors in the bed nucleus of stria terminalis are involved in baroreflex impairment evoked by chronic stress in rats

Chronic exposure to adverse events has been proposed as a prominent factor involved in etiology and progression of cardiovascular dysfunctions in humans and animals. However, the neurobiological mechanisms involved are still poorly understood. In this sense, chronic stress has been reported to evoke neuroplasticity in corticotropin-releasing factor (CRF) neurotransmission in several limbic structures, including the bed nucleus of the stria terminalis. However, a possible involvement of BNST CRF neurotransmission in cardiovascular dysfunctions evoked by chronic stress has never been reported. Thus, this study investigated the involvement of CRF₁ and CRF₂ receptors within the BNST in cardiovascular changes evoked by chronic stress in rats. We identified that exposure to a 10-day chronic variable stress (CVS) protocol decreased expression of both CRF₁ and CRF₂ receptors within the BNST. These effects were followed by increased arterial pressure and impairment of baroreflex function, but without changes on heart rate. Bilateral microinjection of either the selective CRF₁ receptor antagonist CP376395 or the selective CRF₂ receptor antagonist antisauvagine-30 into the BNST did not affect CVS-evoked arterial pressure increase. Nevertheless, BNST treatment with CP376395 decreased both tachycardic and bradycardic responses of the baroreflex in non-stressed rats; but these effects were not identified in chronically stressed animals. BNST pharmacological treatment with antisauvagine-30 decreased the reflex tachycardia in control animals, whereas reflex bradycardic response was increased in CVS animals. Altogether, the results reported in the present study indicate that down regulation of both CRF₁ and CRF₂ receptors within the BNST is involved in baroreflex impairment evoked by chronic stress.

Chronic Stress Induces Maladaptive Behaviors by Activating Corticotropin-Releasing Hormone Signaling in the Mouse Oval Bed Nucleus of the Stria Terminalis

The bed nucleus of the stria terminalis (BNST) is a forebrain region highly responsive to stress that expresses corticotropin-releasing hormone (CRH) and is implicated in mood disorders, such as anxiety. However, the exact mechanism by which chronic stress induces CRH-mediated dysfunction in BNST and maladaptive behaviors remains unclear. Here, we first confirmed that selective acute optogenetic activation of the oval nucleus BNST (ovBNST) increases maladaptive avoidance behaviors in male mice. Next, we found that a 6 week chronic variable mild stress (CVMS) paradigm resulted in maladaptive behaviors and increased cellular excitability of ovBNST CRH neurons by potentiating mEPSC amplitude, altering the resting membrane potential, and diminishing M-currents (a voltage-gated K⁺ current that stabilizes membrane potential) in ex vivo slices. CVMS also increased c-fos⁺ cells in ovBNST following handling. We next investigated potential molecular mechanism underlying the electrophysiological effects and observed that CVMS increased CRH⁺ and pituitary adenylate cyclase-activating polypeptide⁺ (PACAP; a CRH upstream regulator) cells but decreased striatal-enriched protein tyrosine phosphatase⁺ (a STEP CRH inhibitor) cells in ovBNST. Interestingly, the electrophysiological effects of CVMS were reversed by CRHR1-selective antagonist R121919 application. CVMS also activated protein kinase A (PKA) in BNST, and chronic infusion of the PKA-selective antagonist H89 into ovBNST reversed the effects of CVMS. Coadministration of the PKA agonist forskolin prevented the beneficial effects of R121919. Finally, CVMS induced an increase in surface expression of phosphorylated GluR1 (S845) in BNST. Collectively, these findings highlight a novel and indispensable stress-induced role for PKA-dependent CRHR1 signaling in activating BNST CRH neurons and mediating maladaptive behaviors.SIGNIFICANCE STATEMENT Chronic stress and acute activation of oval bed nucleus of the stria terminalis (ovBNST) induces maladaptive behaviors in rodents. However, the precise molecular and electrophysiological mechanisms underlying these effects remain unclear. Here, we demonstrate that chronic variable mild stress activates corticotropin-releasing hormone (CRH)-associated stress signaling and CRH neurons in ovBNST by potentiating mEPSC amplitude and decreasing M-current in male mice. These electrophysiological alterations and maladaptive behaviors were mediated by BNST protein kinase A-dependent CRHR1 signaling. Our results thus highlight the importance of BNST CRH dysfunction in chronic stress-induced disorders.

The intersection of stress and reward: BNST modulation of aversive and appetitive states

The bed nucleus of the stria terminalis (BNST) is widely acknowledged as a brain structure that regulates stress and anxiety states, as well as aversive and appetitive behaviours. The diverse roles of the BNST are afforded by its highly modular organisation, neurochemical heterogeneity, and complex intrinsic and extrinsic circuitry. There has been growing interest in the BNST in relation to psychopathologies such as anxiety and addiction. Although research on the human BNST is still in its infancy, there have been extensive preclinical studies examining the molecular signature and hodology of the BNST and their involvement in stress and reward seeking behaviour. This review examines the neurochemical phenotype and connectivity of the BNST, as well as electrophysiological correlates of plasticity in the BNST mediated by stress and/or drugs of abuse.

Maternal separation induces long-term changes in mineralocorticoid receptor in rats subjected to chronic stress and treated with tianeptine

PURPOSE

The aim of this study was to analyze whether early maternal separation would result in long-term, persistent alterations in stress response in adulthood, altering mineralocorticoid receptor immunoreactivity (MR-ir) in the dorsal hippocampal areas [CA1, CA2, CA3 and dentate gyrus (DG)], paraventricular nucleus of the hypothalamus and medial and central nucleus of the amygdala, key structures involved in stress response regulation. We also analyzed whether chronic treatment with the antidepressant tianeptine reverses these possible changes.

MATERIAL AND METHODS

Male Wistar rats were subjected to daily maternal separation for 4.5 h during 3 weeks or left undisturbed. As adults, they were exposed to chronic stress during 24 days or left undisturbed, and they were also daily treated with tianeptine (10 mg/kg i.p.) or isotonic solution.

RESULTS

In the CA2 and DG areas of the dorsal hippocampus, there was an increase in MR-ir in non-maternally separated and chronic stressed groups. Tianeptine raised MR-ir in the CA3. In the DG, control and maternally separated + chronic stress groups treated with tianeptine showed more MR-ir than their respective vehicle groups. In the paraventricular nucleus, tianeptine decreased MR-ir in non-separated groups, but not in maternally separated rats.

CONCLUSIONS

Our results support findings that early-life events induce long-term changes in stress response regulation, persistent into adulthood, which are manifested during challenges in later life, and that treatment with tianeptine, which tends to attenuate the hypothalamus-pituitary-adrenal axis dysregulation, depends on the individual experience of each rat.

Oxytocin facilitates adaptive fear and attenuates anxiety responses in animal models and human studies-potential interaction with the corticotropin-releasing factor (CRF) system in the bed nucleus of the stria terminalis (BNST)

Despite its relatively well-understood role as a reproductive and pro-social peptide, oxytocin (OT) tells a more convoluted story in terms of its modulation of fear and anxiety. This nuanced story has been obscured by a great deal of research into the therapeutic applications of exogenous OT, driving more than 400 ongoing clinical trials. Drawing from animal models and human studies, we review the complex evidence concerning OT's role in fear learning and anxiety, clarifying the existing confusion about modulation of fear versus anxiety. We discuss animal models and human studies demonstrating the prevailing role of OT in strengthening fear memory to a discrete signal or cue, which allows accurate and rapid threat detection that facilitates survival. We also review ostensibly contrasting behavioral studies that nonetheless provide compelling evidence of OT attenuating sustained contextual fear and anxiety-like behavior, arguing that these OT effects on the modulation of fear vs. anxiety are not mutually exclusive. To disambiguate how endogenous OT modulates fear and anxiety, an understudied area compared to exogenous OT, we survey behavioral studies utilizing OT receptor (OTR) antagonists. Based on emerging evidence about the role of OTR in rat dorsolateral bed nucleus of stria terminalis (BNST) and elsewhere, we postulate that OT plays a critical role in facilitating accurate discrimination between stimuli representing threat and safety. Supported by human studies, we demonstrate that OT uniquely facilitates adaptive fear but reduces maladaptive anxiety. Last, we explore the limited literature on endogenous OT and its interaction with corticotropin-releasing factor (CRF) with a special emphasis on the dorsolateral BNST, which may hold the key to the neurobiology of phasic fear and sustained anxiety.

How does early maternal separation and chronic stress in adult rats affect the immunoreactivity of serotonergic neurons within the dorsal raphe nucleus?

Vulnerability to emotional disorders like depression derives from interactions between early and late environments, including stressful conditions. The serotonin (5HT) system is strongly affected by stress and chronic unpredictable stress can alter the 5HT system. We evaluated the distribution of active serotonergic neurons in the dorsal raphe nucleus (DR) through immunohistochemistry in maternally separated and chronically stressed rats treated with an antidepressant, tianeptine, whose mechanism of action is still under review. Male Wistar rats were subjected to daily maternal separation (MS) for 4.5 h between postnatal days (PND) 1-21, or to animal facility rearing (AFR). Between (PND) days 50-74, rats were exposed to chronic unpredictable stress and were treated daily with tianeptine (10 mg/kg) or vehicle. We found an interaction between the effects of MS and chronic unpredictable stress on Fos-5HT immunoreactive cells at mid-caudal level of the DR. MS-chronically stressed rats showed an increase of Fos-5HT immunoreactive cells compared with AFR-chronically stressed rats. The ventrolateral (DRL/VLPAG) and dorsal (DRD) subdivisions of the DR were significantly more active than the ventral part (DRV). At the rostral level of the DR, tianeptine decreased the number of Fos-5HT cells in DR in the AFR groups, both unstressed and stressed. Overall, our results support the idea of a match in phenotype exhibited when the early and the adult environment correspond.

Maternal separation in early life modifies anxious behavior and Fos and glucocorticoid receptor expression in limbic neurons after chronic stress in rats: effects of tianeptine

Early-life adversity can lead to long-term consequence persisting into adulthood. Here, we assess the implications of an adverse early environment on vulnerability to stress during adulthood. We hypothesized that the interplay between early and late stress would result in a differential phenotype regarding the number of neurons immunoreactive for glucocorticoid receptor (GR-ir) and neuronal activity as assessed by Fos immunoreactivity (Fos-ir) in brain areas related to stress responses and anxiety-like behavior. We also expected that the antidepressant tianeptine could correct some of the alterations induced in our model. Male Wistar rats were subjected to daily maternal separation (MS) for 4.5 h during the first 3 weeks of life. As adults, the rats were exposed to chronic stress for 24 d and they were treated daily with tianeptine (10 mg/kg intraperitoneal) or vehicle (isotonic saline). Fos-ir was increased by MS in all structures analyzed. Chronic stress reduced Fos-ir in the hippocampus, but increased it in the paraventricular nucleus. Furthermore, chronic stress increased GR-ir in hippocampus (CA1) and amygdala in control non-MS rats. By contrast, when MS and chronic stress were combined, GR-ir was decreased in these structures. Additionally, whereas tianeptine did not affect Fos-ir, it regulated GR-ir in a region-dependent manner, in hippocampus and amygdala opposing in some cases the stress or MS effects. Furthermore, tianeptine reversed the MS- or stress-induced anxious behavior. The interplay between MS and chronic stress observed indicates that MS rats have a modified phenotype, which is expressed when they are challenged by stress in later life.

Neurobiological Interactions Between Stress and the Endocannabinoid System

Stress affects a constellation of physiological systems in the body and evokes a rapid shift in many neurobehavioral processes. A growing body of work indicates that the endocannabinoid (eCB) system is an integral regulator of the stress response. In the current review, we discuss the evidence to date that demonstrates stress-induced regulation of eCB signaling and the consequential role changes in eCB signaling have with respect to many of the effects of stress. Across a wide array of stress paradigms, studies have generally shown that stress evokes bidirectional changes in the two eCB molecules, anandamide (AEA) and 2-arachidonoyl glycerol (2-AG), with stress exposure reducing AEA levels and increasing 2-AG levels. Additionally, in almost every brain region examined, exposure to chronic stress reliably causes a downregulation or loss of cannabinoid type 1 (CB1) receptors. With respect to the functional role of changes in eCB signaling during stress, studies have demonstrated that the decline in AEA appears to contribute to the manifestation of the stress response, including activation of the hypothalamic-pituitary-adrenal (HPA) axis and increases in anxiety behavior, while the increased 2-AG signaling contributes to termination and adaptation of the HPA axis, as well as potentially contributing to changes in pain perception, memory and synaptic plasticity. More so, translational studies have shown that eCB signaling in humans regulates many of the same domains and appears to be a critical component of stress regulation, and impairments in this system may be involved in the vulnerability to stress-related psychiatric conditions, such as depression and posttraumatic stress disorder. Collectively, these data create a compelling argument that eCB signaling is an important regulatory system in the brain that largely functions to buffer against many of the effects of stress and that dynamic changes in this system contribute to different aspects of the stress response.

Stress Modulation of Opposing Circuits in the Bed Nucleus of the Stria Terminalis

The anterior bed nucleus of the stria terminalis (BNST) has been recognized as a critical structure in regulating trait anxiety, contextual fear memory, and appetitive behavior, and is known to be sensitive to stress manipulations. As one of the most complex structures in the central nervous system, the intrinsic circuitry of the BNST is largely unknown; however, recent technological developments have allowed researchers to begin to untangle the internal connections of the nucleus. This research has revealed the possibility of two opposing circuits, one anxiolytic and one anxiogenic, within the BNST, the relative strength of which determines the behavioral outcome. The balance of these pathways is critical in maintaining a normal physiological and behavioral state; however, stress and drugs of abuse can differentially affect the opposing circuitry within the nucleus to shift the balance to a pathological state. In this review, we will examine how stress interacts with the neuromodulators, corticotropin-releasing factor, norepinephrine, dopamine, and serotonin to affect the circuitry of the BNST as well as how synaptic plasticity in the BNST is modulated by stress, resulting in long-lasting changes in the circuit and behavioral state.

GABA and NMDA receptors in CRF neurons have opposing effects in fear acquisition and anxiety in central amygdala vs. bed nucleus of the stria terminalis

This article is part of a Special Issue "SBN 2014". Beginning with Vale and Colleagues in 1981, corticotropin releasing factor (CRF) also called corticotropin releasing hormone (CRH) has repeatedly been identified as an important contributor to fear and anxiety behavior. These findings have proven useful to further our understanding of disorders that have significant fear-dysregulation, such as post-traumatic stress, as well as other stress- and anxiety-related disorders. Unfortunately, the data are not all in agreement. In particular the role of CRF in fear learning is controversial, with studies pointing to contradictory effects from CRF manipulation even within the same brain structure. Further, very few studies address the potentially promising role of CRF manipulation in fear extinction behavior. Here, we briefly review the role of CRF in anxiety, fear learning and extinction, focusing on recent cell-type and neurotransmitter-specific studies in the amygdala and bed nucleus of the stria terminalis (BNST) that may help to synthesize the available data on the role of CRF in fear and anxiety-related behaviors.

The Deakin/Graeff hypothesis: focus on serotonergic inhibition of panic

The Deakin/Graeff hypothesis proposes that different subpopulations of serotonergic neurons through topographically organized projections to forebrain and brainstem structures modulate the response to acute and chronic stressors, and that dysfunction of these neurons increases vulnerability to affective and anxiety disorders, including panic disorder. We outline evidence supporting the existence of a serotonergic system originally discussed by Deakin/Graeff that is implicated in the inhibition of panic-like behavioral and physiological responses. Evidence supporting this panic inhibition system comes from the following observations: (1) serotonergic neurons located in the 'ventrolateral dorsal raphe nucleus' (DRVL) as well as the ventrolateral periaqueductal gray (VLPAG) inhibit dorsal periaqueductal gray-elicited panic-like responses; (2) chronic, but not acute, antidepressant treatment potentiates serotonin's panicolytic effect; (3) contextual fear activates a central nucleus of the amygdala-DRVL/VLPAG circuit implicated in mediating freezing and inhibiting panic-like escape behaviors; (4) DRVL/VLPAG serotonergic neurons are central chemoreceptors and modulate the behavioral and cardiorespiratory response to panicogenic agents such as sodium lactate and CO2. Implications of the panic inhibition system are discussed.

Striatal-enriched protein tyrosine phosphatase-STEPs toward understanding chronic stress-induced activation of corticotrophin releasing factor neurons in the rat bed nucleus of the stria terminalis

BACKGROUND

Striatal-enriched protein tyrosine phosphatase (STEP) is a brain-specific protein tyrosine phosphatase that opposes the development of synaptic strengthening and the consolidation of fear memories. In contrast, stress facilitates fear memory formation, potentially by activating corticotrophin releasing factor (CRF) neurons in the anterolateral cell group of the bed nucleus of the stria terminalis (BNSTALG).

METHODS

Here, using dual-immunofluorescence, single-cell reverse transcriptase polymerase chain reaction, quantitative reverse transcriptase polymerase chain reaction, Western blot, and whole-cell patch-clamp electrophysiology, we examined the expression and role of STEP in regulating synaptic plasticity in rat BNSTALG neurons and its modulation by stress.

RESULTS

Striatal-enriched protein tyrosine phosphatase was selectively expressed in CRF neurons in the oval nucleus of the BNSTALG. Following repeated restraint stress (RRS), animals displayed a significant increase in anxiety-like behavior, which was associated with a downregulation of STEP messenger RNA and protein expression in the BNSTALG, as well as selectively enhancing the magnitude of long-term potentiation (LTP) induced in Type III, putative CRF neurons. To determine if the changes in STEP expression following RRS were mechanistically related to LTP facilitation, we examined the effects of intracellular application of STEP on the induction of LTP. STEP completely blocked the RRS-induced facilitation of LTP in BNSTALG neurons.

CONCLUSIONS

Hence, STEP acts to buffer CRF neurons against excessive activation, while downregulation of STEP after chronic stress may result in pathologic activation of CRF neurons in the BNSTALG and contribute to prolonged states of anxiety. Thus, targeted manipulations of STEP activity might represent a novel treatment strategy for stress-induced anxiety disorders.

Optogenetic strategies to investigate neural circuitry engaged by stress

Optogenetic techniques have given researchers unprecedented access to the function of discrete neural circuit elements and have been instrumental in the identification of novel brain pathways that become dysregulated in neuropsychiatric diseases. For example, stress is integrally linked to the manifestation and pathophysiology of neuropsychiatric illness, including anxiety, addiction and depression. Due to the heterogeneous populations of genetically and neurochemically distinct neurons in areas such as the bed nucleus of the stria terminalis (BNST), as well as their substantial number of projections, our understanding of how neural circuits become disturbed after stress has been limited. Using optogenetic tools, we are now able to selectively isolate distinct neural circuits that contribute to these disorders and perturb these circuits in vivo, which in turn may lead to the normalization of maladaptive behavior. This review will focus on current optogenetic strategies to identify, manipulate, and record from discrete neural circuit elements in vivo as well as highlight recent optogenetic studies that have been utilized to parcel out BNST function.

Central CRF neurons are not created equal: phenotypic differences in CRF-containing neurons of the rat paraventricular hypothalamus and the bed nucleus of the stria terminalis

Corticotrophin-releasing factor (CRF) plays a key role in initiating many of the endocrine, autonomic, and behavioral responses to stress. CRF-containing neurons of the paraventricular nucleus of the hypothalamus (PVN) are classically involved in regulating endocrine function through activation of the stress axis. However, CRF is also thought to play a critical role in mediating anxiety-like responses to environmental stressors, and dysfunction of the CRF system in extra-hypothalamic brain regions, like the bed nucleus of stria terminalis (BNST), has been linked to the etiology of many psychiatric disorders including anxiety and depression. Thus, although CRF neurons of the PVN and BNST share a common neuropeptide phenotype, they may represent two functionally diverse neuronal populations. Here, we employed dual-immunofluorescence, single-cell RT-PCR, and electrophysiological techniques to further examine this question and report that CRF neurons of the PVN and BNST are fundamentally different such that PVN CRF neurons are glutamatergic, whereas BNST CRF neurons are GABAergic. Moreover, these two neuronal populations can be further distinguished based on their electrophysiological properties, their co-expression of peptide neurotransmitters such as oxytocin and arginine-vasopressin, and their cognate receptors. Our results suggest that CRF neurons in the PVN and the BNST would not only differ in their response to local neurotransmitter release, but also in their action on downstream target structures.

Sensitization of restraint-induced corticosterone secretion after chronic restraint in rats: involvement of 5-HT₇ receptors

Serotonin (5-HT) modulates the hypothalamic-pituitary-adrenal (HPA) axis response to stress. We examined the effect of chronic restraint stress (CRS; 20 min/day) as compared to control (CTRL) conditions for 14 days, on: 1) restraint-induced ACTH and corticosterone (CORT) secretion in rats pretreated with vehicle or SB-656104 (a 5-HT₇ receptor antagonist); 2) 5-HT₇ receptor-like immunoreactivity (5-HT₇-LI) and protein in the hypothalamic paraventricular nucleus (PVN) and adrenal glands (AG); 3) baseline levels of 5-HT and 5-hydroxyindolacetic acid (5-HIAA), and 5-HIAA/5-HT ratio in PVN and AG; and 4) 5-HT-like immunoreactivity (5-HT-LI) in AG and tryptophan hydroxylase (TPH) protein in PVN and AG. On day 15, animals were subdivided into Treatment and No treatment groups. Treatment animals received an i.p. injection of vehicle or SB-656104; No Treatment animals received no injection. Sixty min later, Treatment animals were either decapitated with no further stress (0 min) or submitted to acute restraint (10, 30, 60 or 120 min); hormone serum levels were measured. No Treatment animals were employed for the rest of measurements. CRS decreased body weight gain and increased adrenal weight. In CTRL animals, acute restraint increased ACTH and CORT secretion in a time of restraint-dependent manner; both responses were inhibited by SB-656104. Exposure to CRS abolished ACTH but magnified CORT responses to restraint as compared to CTRL conditions; SB-656104 had no effect on ACTH levels but significantly inhibited sensitized CORT responses. In CTRL animals, 5-HT₇-LI was detected in magnocellular and parvocellular subdivisions of PVN and sparsely in adrenal cortex. Exposure to CRS decreased 5-HT₇-LI and protein in the PVN, but increased 5-HT₇-LI in the adrenal cortex and protein in whole AG. Higher 5-HT and 5-HIAA levels were detected in PVN and AG from CRS animals but 5-HIAA/5-HT ratio increased in AG only. Finally, whereas 5-HT-LI was sparsely observed in the adrenal cortex of CTRL animals, it strongly increased in the adrenal cortex of CRS animals. No TPH protein was detected in AG from both animal groups. Results suggest that CRS promotes endocrine disruption involving decreased ACTH and sensitized CORT responses to acute restraint. This phenomenon may be associated with increased function and expression of 5-HT₇ receptors as well as 5-HT turnover in AG.

Effects of continuously enhanced corticotropin releasing factor expression within the bed nucleus of the stria terminalis on conditioned and unconditioned anxiety

The lateral division of the bed nucleus of the stria terminalis (BNST), which forms part of the circuitry regulating fear and anxiety, contains a large number of neurons expressing corticotropin releasing factor (CRF), a neuropeptide that has a prominent role in the etiology of fear- and anxiety-related psychopathologies. Stress increases CRF expression within BNST neurons, implicating these cells in stress- and anxiety-related behaviors. These experiments examined the effect of chronically enhanced CRF expression within BNST neurons on conditioned and unconditioned anxiety-related behavior by using a lentiviral vector containing a promoter that targets CRF gene overexpression (OE) to CRFergic cells. We found that BNST CRF-OE did not affect unconditioned anxiety-like responses in the elevated plus maze or basal acoustic startle amplitude. CRF-OE induced before training weakened sustained fear (conditioned anxiety); when induced after conditioning, CRF-OE increased expression of the conditioned emotional memory. Increased BNST CRF expression did not affect plasma corticosterone concentration but did decrease CRFR1 receptor density within the BNST and CRFR2 receptor density within the dorsal portion of the caudal dorsal raphe nucleus. These data raise the possibility that the observed behavioral effects may be mediated by enhanced CRF receptor signaling or compensatory changes in CRF receptor density within these structures. Together, these studies demonstrate that CRF neurons within the lateral BNST modulate conditioned anxiety-like behaviors and also suggest that enhanced CRF expression within these neurons may contribute to inappropriate regulation of emotional memories.

Effects of fluoxetine on CRF and CRF1 expression in rats exposed to the learned helplessness paradigm

RATIONALE

Stress is a common antecedent reported by people suffering major depression. In these patients, extrahypothalamic brain areas, like the hippocampus and basolateral amygdala (BLA), have been found to be affected. The BLA synthesizes CRF, a mediator of the stress response, and projects to hippocampus. The main hippocampal target for this peptide is the CRF subtype 1 receptor (CRF1). Evidence points to a relationship between dysregulation of CRF/CRF1 extrahypothalamic signaling and depression.

OBJECTIVE

Because selective serotonin reuptake inhibitors (SSRIs) are the first-line pharmacological treatment for depression, we investigated the effect of chronic treatment with the SSRI fluoxetine on long-term changes in CRF/CRF1 signaling in animals showing a depressive-like behavior.

METHODS

Male Wistar rats were exposed to the learned helplessness paradigm (LH). After evaluation of behavioral impairment, the animals were treated with fluoxetine (10 mg/kg i.p.) or saline for 21 days. We measured BLA CRF expression with RT-PCR and CRF1 expression in CA3 and the dentate gyrus of the hippocampus with in situ hybridization. We also studied the activation of one of CRF1's major intracellular signaling targets, the extracellular signal-related kinases 1 and 2 (ERK1/2) in CA3.

RESULTS

In saline-treated LH animals, CRF expression in the BLA increased, while hippocampal CRF1 expression and ERK1/2 activation decreased. Treatment with fluoxetine reversed the changes in CRF and CRF1 expressions, but not in ERK1/2 activation.

CONCLUSION

In animals exposed to the learned helplessness paradigm, there are long-term changes in CRF and CRF1 expression that are restored with a behaviorally effective antidepressant treatment.

The same antidepressant elicits contrasting patterns of synaptic changes in the amygdala vs hippocampus

As depression-like symptoms are often precipitated by some form of stress, animal models of stress have been used extensively to investigate cellular mechanisms of depression. Despite being implicated in the emotional symptoms of depression, the amygdala has received little attention compared to the hippocampus in the past studies of antidepressant action. Further, these investigations have not taken into account the contrasting effects of chronic stress on the hippocampus vs amygdala. If an antidepressant is to be equally effective in countering the differential effects of stress on both brain areas, then it is faced with the challenge of eliciting contrasting effects in these two structures. We tested this prediction by examining the impact of tianeptine, an antidepressant with proven clinical efficacy, on neurons of the lateral amygdala (LA) and hippocampal area CA1. Tianeptine reduces N-methyl-D-aspartate (NMDA)-receptor-mediated synaptic currents, without affecting α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) currents, in LA neurons. By contrast, tianeptine enhances both NMDA and AMPA currents in area CA1. Tianeptine also lowers action potential firing in LA neurons. As tianeptine modulates cellular metrics that, in addition to mediating amygdalar behavioral output, are also affected by stress, we tested if tianeptine succeeds in countering stress effects in the intact animal. We find that tianeptine prevents two important functional consequences of chronic stress-induced plasticity in the amygdala--dendritic growth and enhanced anxiety-like behavior. These results provide evidence for antidepressant action on amygdalar neurons that are not only distinct from the hippocampus, but also protect against the debilitating impact of stress on amygdalar structure and function.

Neurobiology of chronic mild stress: parallels to major depression

The chronic mild (or unpredictable/variable) stress (CMS) model was developed as an animal model of depression more than 20 years ago. The foundation of this model was that following long-term exposure to a series of mild, but unpredictable stressors, animals would develop a state of impaired reward salience that was akin to the anhedonia observed in major depressive disorder. In the time since its inception, this model has also been used for a variety of studies examining neurobiological variables that are associated with depression, despite the fact that this model has never been critically examined to validate that the neurobiological changes induced by CMS are parallel to those documented in depressive disorder. The aim of the current review is to summarize the current state of knowledge regarding the effects of chronic mild stress on neurobiological variables, such as neurochemistry, neurochemical receptor expression and functionality, neurotrophin expression and cellular plasticity. These findings are then compared to those of clinical research examining common variables in populations with depressive disorders to determine if the changes observed following chronic mild stress are in fact consistent with those observed in major depression. We conclude that the chronic mild stress paradigm: (1) evokes an array of neurobiological changes that mirror those seen in depressive disorders and (2) may be a suitable tool to investigate novel systems that could be disturbed in depression, and thus aid in the development of novel targets for the treatment of depression.

Chronic stress induces sex-specific alterations in methylation and expression of corticotropin-releasing factor gene in the rat

BACKGROUND

Although the higher prevalence of depression in women than in men is well known, the neuronal basis of this sex difference is largely elusive.

METHODS

Male and female rats were exposed to chronic variable mild stress (CVMS) after which immediate early gene products, corticotropin-releasing factor (CRF) mRNA and peptide, various epigenetic-associated enzymes and DNA methylation of the Crf gene were determined in the hypothalamic paraventricular nucleus (PVN), oval (BSTov) and fusiform (BSTfu) parts of the bed nucleus of the stria terminalis, and central amygdala (CeA).

RESULTS

CVMS induced site-specific changes in Crf gene methylation in all brain centers studied in female rats and in the male BST and CeA, whereas the histone acetyltransferase, CREB-binding protein was increased in the female BST and the histone-deacetylase-5 decreased in the male CeA. These changes were accompanied by an increased amount of c-Fos in the PVN, BSTfu and CeA in males, and of FosB in the PVN of both sexes and in the male BSTov and BSTfu. In the PVN, CVMS increased CRF mRNA in males and CRF peptide decreased in females.

CONCLUSIONS

The data confirm our hypothesis that chronic stress affects gene expression and CRF transcriptional, translational and secretory activities in the PVN, BSTov, BSTfu and CeA, in a brain center-specific and sex-specific manner. Brain region-specific and sex-specific changes in epigenetic activity and neuronal activation may play, too, an important role in the sex specificity of the stress response and the susceptibility to depression.

Prolonged and site-specific over-expression of corticotropin-releasing factor reveals differential roles for extended amygdala nuclei in emotional regulation

Corticotropin-releasing factor (CRF) has a key role in the central stress response, and altered levels of this neuropeptide are linked to stress-related psychopathologies such as anxiety and depression. These disorders are associated with the inability to properly regulate stress response, specifically following exposure to prolonged stressful stimuli. Therefore, the current study assessed the effects of prolonged and site-specific over-expression of CRF, which mimics the state of chronic production, in extended amygdala nuclei that are known to be involved in mediating anxiety-like states. We first constructed and generated lentiviruses that overexpress (OE) CRF in a robust and stable manner, and then generated two male mouse models continuously over-expressing CRF, either at the central nucleus of the amygdala (CeA), or at the dorsolateral subdivision of the bed nucleus of the stria terminalis (BNSTdl). After 4 months, behavioral assessments were conducted for anxiety and depressive indices on these mice. Surprisingly, prolonged CRF OE at the CeA attenuated stress-induced anxiety-like behaviors, whereas prolonged CRF OE in the BNSTdl increased depressive-like behaviors, without affecting anxiety levels. These results show possible differential roles for CRF expressed by distinct loci of the extended amygdala, in mediating stress-induced emotional behaviors.

Beneficial Effects of Tianeptine on Hippocampus-Dependent Long-Term Memory and Stress-Induced Alterations of Brain Structure and Function

Tianeptine is a well-described antidepressant which has been shown to prevent stress from producing deleterious effects on brain structure and function. Preclinical studies have shown that tianeptine blocks stress-induced alterations of neuronal morphology and synaptic plasticity. Moreover, tianeptine prevents stress from impairing learning and memory, and, importantly, demonstrates memory-enhancing properties in the absence of stress. Recent research has indicated that tianeptine works by normalizing glutamatergic neurotransmission, a mechanism of action that may underlie its effectiveness as an antidepressant. These findings emphasize the value in focusing on the mechanisms of action of tianeptine, and specifically, the glutamatergic system, in the development of novel pharmacotherapeutic strategies in the treatment of depression.

Corticotrophin-releasing factor and stress-induced inhibition of the gonadotrophin-releasing hormone pulse generator in the female

It is well established that stress activates the hypothalamo-pituitary-adrenal (HPA) axis and suppresses the hypothalamo-pituitary-gonadal (HPG) axis. A large literature dealing with various stressors that regulate gonadotrophin-releasing hormone (GnRH) secretion in a variety of species (including nonhuman primates, sheep, and rats) provides evidence that stress modulates GnRH secretion by activating the corticotrophin-releasing factor (CRF) system and sympathoadrenal pathways, as well as the limbic brain. Different stressors may suppress the HPG axis by activating or inhibiting various pathways in the CNS. In addition to CRF being the principal hypophysiotropic factor driving the HPA axis, it is a potent inhibitor of the GnRH pulse generator. The suppression of the GnRH pulse generator by a variety of stressful stimuli can be blocked by CRF antagonists, suggesting a pivotal role for endogenous CRF. The differential roles for CRF receptor type 1 (CRF-R1) and CRF-R2 in stress-induced suppression of the GnRH pulse generator add to the complexity of CRF regulation of the HPG axis. Although the precise sites and mechanisms of action remain to be elucidated, noradrenergic and gamma-amino-butyric acid (GABA) neurones are implicated in the system's regulation, and opioids and kisspeptin in the medial preoptic area (mPOA) and hypothalamic arcuate nucleus (ARC) may operate downstream of the CRF neuronal system.

Tianeptine: an antidepressant with memory-protective properties

The development of effective pharmacotherapy for major depression is important because it is such a widespread and debilitating mental disorder. Here, we have reviewed preclinical and clinical studies on tianeptine, an atypical antidepressant which ameliorates the adverse effects of stress on brain and memory. In animal studies, tianeptine has been shown to prevent stress-induced morphological sequelae in the hippocampus and amygdala, as well as to prevent stress from impairing synaptic plasticity in the prefrontal cortex and hippocampus. Tianeptine also has memory-protective characteristics, as it blocks the adverse effects of stress on hippocampus-dependent learning and memory. We have further extended the findings on stress, memory and tianeptine here with two novel observations: 1) stress impairs spatial memory in adrenalectomized (ADX), thereby corticosterone-depleted, rats; and 2) the stress-induced impairment of memory in ADX rats is blocked by tianeptine. These findings are consistent with previous research which indicates that tianeptine produces anti-stress and memory-protective properties without altering the response of the hypothalamic-pituitary-adrenal axis to stress. We conclude with a discussion of findings which indicate that tianeptine accomplishes its anti-stress effects by normalizing stress-induced increases in glutamate in the hippocampus and amygdala. This finding is potentially relevant to recent research which indicates that abnormalities in glutamatergic neurotransmission are involved in the pathogenesis of depression. Ultimately, tianeptine’s prevention of depression-induced sequelae in the brain is likely to be a primary factor in its effectiveness as a pharmacological treatment for depression.

Stress-related endocrinological and psychopathological effects of short- and long-term 50Hz electromagnetic field exposure in rats

It is believed that different electromagnetic fields do have beneficial and harmful biological effects. The aim of the present work was to study the long-term consequences of 50 Hz electromagnetic field (ELF-EMF) exposure with special focus on the development of chronic stress and stress-induced psychopathology. Adult male Sprague-Dawley rats were exposed to ELF-EMF (50 Hz, 0.5 mT) for 5 days, 8h daily (short) or for 4-6 weeks, 24h daily (long). Anxiety was studied in elevated plus maze test, whereas depression-like behavior of the long-treated group was examined in the forced swim test. Some days after behavioral examination, the animals were decapitated among resting conditions and organ weights, blood hormone levels as well as proopiomelanocortin mRNA level from the anterior lobe of the pituitary gland were measured. Both treatments were ineffective on somatic parameters, namely none of the changes characteristic to chronic stress (body weight reduction, thymus involution and adrenal gland hypertrophy) were present. An enhanced blood glucose level was found after prolonged ELF-EMF exposure (p=0.013). The hormonal stress reaction was similar in control and short-term exposed rats, but significant proopiomelanocortin elevation (p<0.000) and depressive-like behavior (enhanced floating time; p=0.006) were found following long-term ELF-EMF exposure. Taken together, long and continuous exposure to relatively high intensity electromagnetic field may count as a mild stress situation and could be a factor in the development of depressive state or metabolic disturbances. Although we should stress that the average intensity of the human exposure is normally much smaller than in the present experiment.

Chronic psychosocial stress affects corticotropin-releasing factor in the paraventricular nucleus and central extended amygdala as well as urocortin 1 in the non-preganglionic Edinger-Westphal nucleus of the tree shrew

Stressful stimuli evoke neuronal and neuroendocrine responses helping an organism to adapt to changed environmental conditions. Chronic stressors may induce maladaptive responses leading to psychiatric diseases, such as anxiety and major depression. A suitable animal model to unravel mechanisms involved in the control of adaptation to chronic stress is the psychological subordination stress in the male tree shrew. Subordinate male tree shrews exhibit chronic hypothalamo-pituitary-adrenal (HPA) activation as reflected in continuously elevated cortisol secretion, and structural changes in the hippocampal formation. Corticotropin-releasing factor (CRF) is the major peptide released upon activation of the HPA axis in response to stress. Recent evidence suggests that besides CRF, urocortin 1 (Ucn1) also plays a role in stress adaptation. We have tested the significance of CRF and Ucn1 in adaptation to chronic psychosocial stress in male tree shrews exposed for 35 days to daily psychosocial conflict, by performing semi-quantitative immunocytochemistry for CRF in the parvocellular hypothalamic paraventricular nucleus (pPVN), extended amygdala, viz. central extended amygdala (CeA) and dorsolateral nucleus of the bed nucleus of the stria terminalis (BNSTdl) as well as that for Ucn1 in the non-preganglionic Edinger-Westphal nucleus (npEW). Compared to unstressed animals, psychosocial stress resulted in an immediate and sustained activation of the HPA axis and sympathetic tone as well as reduced testosterone concentration and decreased body and testis weights vs. non-stressed tree shrews. In the pPVN, the number of CRF-immunoreactive neurons and the specific signal density of CRF-immunoreactive fiber terminals in the CeA were strongly reduced (-300 and -40%, respectively; P<0.05), whereas no significant difference in CRF fiber density was found in BNSTdl. The npEW revealed 4 times less Ucn1-immunoreactive neurons (P<0.05). These clear effects on both Ucn1- and CRF-neuropeptide contents may reflect a crucial mechanism enabling the animal to adapt successfully to the stressors, and point to the significance of the pPVN, CeA and npEW in stress-induced brain diseases.

Characterization of central and peripheral components of the hypothalamus-pituitary-adrenal axis in the inbred Roman rat strains

Several studies performed in outbred Roman high- and low-avoidance lines (RHA and RLA, respectively) have demonstrated that the more anxious line (RLA) is characterized by a higher hypothalamic-pituitary-adrenal (HPA) response to certain stressors than the less anxious one (RHA). However, inconsistent results have also been reported. Taking advantage of the generation of an inbred colony of RLA and RHA rats (RHA-I and RLA-I, respectively), we have characterized in the two strains not only resting and stress levels of peripheral HPA hormones but also central components of the HPA axis, including CRF gene expression in extra-hypothalamic areas. Whereas resting levels of ACTH and corticosterone did not differ between the strains, a greater response to a novel environment was found in RLA-I as compared to RHA-I rats. RLA-I rats showed enhanced CRF gene expression in the paraventricular nucleus (PVN) of the hypothalamus, with normal arginin-vasopressin gene expression in both parvocellular and magnocellular regions of the PVN. This enhanced CRF gene expression is not apparently related to altered negative corticosteroid feedback as similar levels of expression of brain glucorticoid and mineralocorticoid receptors were found in the two rat strains. CRF gene expression tended to be higher in the central amygdala and it was significantly higher in the dorsal region of the bed nucleus of stria terminalis (BNST) of RLA-I rats, while no differences appeared in the ventral region of BNST. Considering the involvement of CRF and the BNST in anxiety and stress-related behavioral alterations, the present data suggest that the CRF system may be a critical neurobiological substrate underlying differences between the two rat strains.

Projections from the paraventricular nucleus of the thalamus to the forebrain, with special emphasis on the extended amygdala

The paraventricular nucleus of the thalamus (PVT) is part of a group of midline and intralaminar thalamic nuclei implicated in arousal and attention. This study examined the connections between the PVT and the forebrain by using the retrograde tracer cholera toxin B (CTb) and the anterograde tracer biotin dextran amine (BDA). The anterior and posterior regions of the PVT were found to send a dense projection to the nucleus accumbens. The posterior PVT was also found to provide a strong projection to the lateral bed nucleus of the stria terminalis (BST), interstitial nucleus of the posterior limb of the anterior commissure (IPAC), and central nucleus of the amygdala (CeA), regions associated with the extended amygdala. In contrast, the anterior PVT was found to send a weaker projection to the extended amygdala. The basolateral nucleus of the amygdala and the medial prefrontal cortex were found to receive a relatively weak projection from the PVT, and other regions of the BST and amygdala were found to be poorly innervated by the PVT. In addition, the PVT was found to innervate regions in the extended amygdala that contained corticotropin-releasing factor (CRF) neurons, many of which were found to receive apparent contacts from PVT fibers. The projection from the PVT to the nucleus accumbens and extended amygdala places the PVT in a key anatomical position to influence adaptive behaviors as well as the physiological and neuroendocrine responses associated with these behaviors.

Methods to produce hyperthermia-induced brain dysfunction

The recent increase in the frequency and intensity of killer heat waves across the globe has aroused worldwide medical attention to exploring therapeutic strategies to attenuate heat-related morbidity and/or mortality. Death due to heat-related illnesses often exceeds >50% of heat victims. Those who survive are crippled with lifetime disabilities and exhibit profound cognitive, sensory, and motor dysfunction akin to premature neurodegeneration. Although more than 50% of the world populations are exposed to summer heat waves; our understanding of detailed underlying mechanisms and the suitable therapeutic strategies have still not been worked out. One of the basic reasons behind this is the lack of a reliable experimental model to simulate clinical hyperthermia. This chapter describes a suitable animal model to induce hyperthermia in rats (or mice) comparable to the clinical situation. The model appears to be useful for studying the effects of heat-related illnesses on changes in various organs and systems, including the central nervous system (CNS). Since hyperthermia is often associated with profound brain dysfunction, additional methods to examine some crucial parameters of brain injury, e.g., blood-brain barrier (BBB) breakdown and brain edema formation, are also described.