Corticotropin-releasing factor in the dorsal raphe nucleus increases medial prefrontal cortical serotonin via type 2 receptors and median raphe nucleus activity

Dexmedetomidine Inhibits Paraventricular Corticotropin-releasing Hormone Neurons that Attenuate Acute Stress-induced Anxiety-like Behavior in Mice

BACKGROUND

Dexmedetomidine has repeatedly shown to improve anxiety, but the precise neural mechanisms underlying this effect remain incompletely understood. This study aims to explore the role of corticotropin-releasing hormone-producing hypothalamic paraventricular nucleus (CRHPVN) neurons in mediating the anxiolytic effects of dexmedetomidine.

METHODS

A social defeat stress mouse model was used to evaluate the anxiolytic effects induced by dexmedetomidine through the elevated plus maze, open-field test, and measurement of serum stress hormone levels. In vivo Ca2+ signal fiber photometry and ex vivo patch-clamp recordings were used to determine the excitability of CRHPVN neurons and investigate the specific mechanism involved. CRHPVN neuron modulation was achieved through chemogenetic activation or inhibition.

RESULTS

Compared with saline, dexmedetomidine (40 µg/kg) alleviated anxiety-like behaviors. Additionally, dexmedetomidine reduced CRHPVN neuronal excitability. Chemogenetic activation of CRHPVN neurons decreased the time spent in the open arms of the elevated plus maze and in the central area of the open-field test. Conversely, chemogenetic inhibition of CRHPVN neurons had the opposite effect. Moreover, the suppressive impact of dexmedetomidine on CRHPVN neurons was attenuated by the α2-receptor antagonist yohimbine.

CONCLUSIONS

The results indicate that the anxiety-like effects of dexmedetomidine are mediated via α2-adrenergic receptor-triggered inhibition of CRHPVN neuronal excitability in the hypothalamus.

EDITOR’S PERSPECTIVE

High-fat diet, microbiome-gut-brain axis signaling, and anxiety-like behavior in male rats

Obesity, associated with the intake of a high-fat diet (HFD), and anxiety are common among those living in modern urban societies. Recent studies suggest a role of microbiome-gut-brain axis signaling, including a role for brain serotonergic systems in the relationship between HFD and anxiety. Evidence suggests the gut microbiome and the serotonergic brain system together may play an important role in this response. Here we conducted a nine-week HFD protocol in male rats, followed by an analysis of the gut microbiome diversity and community composition, brainstem serotonergic gene expression (tph2, htr1a, and slc6a4), and anxiety-related defensive behavioral responses. We show that HFD intake decreased alpha diversity and altered the community composition of the gut microbiome in association with obesity, increased brainstem tph2, htr1a and slc6a4 mRNA expression, including in the caudal part of the dorsomedial dorsal raphe nucleus (cDRD), a subregion previously associated with stress- and anxiety-related behavioral responses, and, finally, increased anxiety-related defensive behavioral responses. The HFD increased the Firmicutes/Bacteroidetes ratio relative to control diet, as well as higher relative abundances of Blautia, and decreases in Prevotella. We found that tph2, htr1a and slc6a4 mRNA expression were increased in subregions of the dorsal raphe nucleus in the HFD, relative to control diet. Specific bacterial taxa were associated with increased serotonergic gene expression in the cDRD. Thus, we propose that HFD-induced obesity is associated with altered microbiome-gut-serotonergic brain axis signaling, leading to increased anxiety-related defensive behavioral responses in rats.

The role of brain serotonin signaling in excessive alcohol consumption and withdrawal: A call for more research in females

Alcohol Use Disorder (AUD) is a leading cause of death and disability worldwide, but current treatments are insufficient in fully addressing the symptoms that often lead to relapses in alcohol consumption. The brain's serotonin system has been implicated in AUD for decades and is a major regulator of stress-related behaviors associated with increased alcohol consumption. This review will discuss the current literature on the association between neurobiological adaptations in serotonin systems and AUD in humans as well as the effectiveness of serotonin receptor manipulations on alcohol-related behaviors like consumption and withdrawal. We will further discuss how these findings in humans relate to findings in animal models, including a comparison of systemic pharmacological manipulations modulating alcohol consumption. We next provide a detailed overview of brain region-specific roles for serotonin and serotonin receptor signaling in alcohol-related behaviors in preclinical animal models, highlighting the complexity of forming a cohesive model of serotonin function in AUD and providing possible avenues for more effective therapeutic intervention. Throughout the review, we discuss what is known about sex differences in the sequelae of AUD and the role of serotonin in these sequelae. We stress a critical need for additional studies in women and female animals so that we may build a clearer path to elucidating sex-specific serotonergic mechanisms and develop better treatments.

Chronic administration of glucocorticoid receptor ligands increases anxiety-like behavior and selectively increase serotonin transporters in the ventral hippocampus

Organic cation transporter-3 (OCT3) is widely distributed in the brain with high expression in portions of the stress axis. These high capacity, polyspecific transporters function in monoamine clearance and are sensitive to the stress hormone corticosterone. In rats, withdrawal from chronic amphetamine increases OCT3 expression in specific limbic brain regions involved anxiety and stress responses, including the ventral hippocampus, central nucleus of amygdala (CeA) and dorsomedial hypothalamus. (DMH). Previous studies show that glucocorticoid receptor (GR) agonists increase OCT1 mRNA and OCT2 mRNA expression in non-neural tissues. Thus, we hypothesized that corticosterone increases OCT3 expression in the brain by activating GRs. Male Sprague-Dawley rats were pre-treated daily with the GR antagonist mifepristone (20 mg/kg; sc.) or vehicle followed 45 min later by injections of corticosterone or vehicle for 2 weeks. Corticosterone treatment significantly increased OCT3 expression in the ventral hippocampus and increased anxiety-like behavior. However, these effects were not blocked by mifepristone. Interestingly, treatment with mifepristone alone reduced plasma corticosterone levels and increased serotonin transporter and GR expression in the ventral hippocampus but did not significantly affect OCT3 expression or behavior. No treatment effects on OCT3, serotonin transporter or GR expression were observed in the DMH, CeA or dorsal hippocampus. Our findings suggest that corticosterone increases OCT3 expression in the ventral hippocampus by a mechanism independent of GRs, and that mifepristone and corticosterone can act in an independent manner to affect HPA axis-related physiological and behavioral parameters.

Prior stress and vasopressin promote corticotropin-releasing factor inhibition of serotonin release in the central nucleus of the amygdala

Corticotropin-releasing factor (CRF) is essential for coordinating endocrine and neural responses to stress, frequently facilitated by vasopressin (AVP). Previous work has linked CRF hypersecretion, binding site changes, and dysfunctional serotonergic transmission with anxiety and affective disorders, including clinical depression. Crucially, CRF can alter serotonergic activity. In the dorsal raphé nucleus and serotonin (5-HT) terminal regions, CRF effects can be stimulatory or inhibitory, depending on the dose, site, and receptor type activated. Prior stress alters CRF neurotransmission and CRF-mediated behaviors. Lateral, medial, and ventral subdivisions of the central nucleus of the amygdala (CeA) produce CRF and coordinate stress responsiveness. The purpose of these experiments was to determine the effect of intracerebroventricular (icv) administration of CRF and AVP on extracellular 5-HT as an index of 5-HT release in the CeA, using in vivo microdialysis in freely moving rats and high performance liquid chromatography (HPLC) analysis. We also examined the effect of prior stress (1 h restraint, 24 h prior) on CRF- and AVP-mediated release of 5-HT within the CeA. Our results show that icv CRF infusion in unstressed animals had no effect on 5-HT release in the CeA. Conversely, in rats with prior stress, CRF caused a profound dose-dependent decrease in 5-HT release within the CeA. This effect was long-lasting (240 min) and was mimicked by CRF plus AVP infusion without stress. Thus, prior stress and AVP functionally alter CRF-mediated neurotransmission and sensitize CRF-induced inhibition of 5-HT release, suggesting that this is a potential mechanism underlying stress-induced affective reactivity in humans.

Corticotropin releasing factor and drug seeking in substance use disorders: Preclinical evidence and translational limitations

The neuropeptide, corticotropin releasing factor (CRF), has been an enigmatic target for the development of medications aimed at treating stress-related disorders. Despite a large body of evidence from preclinical studies in rodents demonstrating that CRF receptor antagonists prevent stressor-induced drug seeking, medications targeting the CRF-R1 have failed in clinical trials. Here, we provide an overview of the abundant findings from preclinical rodent studies suggesting that CRF signaling is involved in stressor-induced relapse. The scientific literature that has defined the receptors, mechanisms and neurocircuits through which CRF contributes to stressor-induced reinstatement of drug seeking following self-administration and conditioned place preference in rodents is reviewed. Evidence that CRF signaling is recruited with repeated drug use in a manner that heightens susceptibility to stressor-induced drug seeking in rodents is presented. Factors that may determine the influence of CRF signaling in substance use disorders, including developmental windows, biological sex, and genetics are examined. Finally, we discuss the translational failure of medications targeting CRF signaling as interventions for substance use disorders and other stress-related conditions. We conclude that new perspectives and research directions are needed to unravel the mysterious role of CRF in substance use disorders.

Anxiety and Depression: What Do We Know of Neuropeptides?

In modern society, there has been a rising trend of depression and anxiety. This trend heavily impacts the population's mental health and thus contributes significantly to morbidity and, in the worst case, to suicides. Modern medicine, with many antidepressants and anxiolytics at hand, is still unable to achieve remission in many patients. The pathophysiology of depression and anxiety is still only marginally understood, which encouraged researchers to focus on neuropeptides, as they are a vast group of signaling molecules in the nervous system. Neuropeptides are involved in the regulation of many physiological functions. Some act as neuromodulators and are often co-released with neurotransmitters that allow for reciprocal communication between the brain and the body. Most studied in the past were the antidepressant and anxiolytic effects of oxytocin, vasopressin or neuropeptide Y and S, or Substance P. However, in recent years, more and more novel neuropeptides have been added to the list, with implications for the research and development of new targets, diagnostic elements, and even therapies to treat anxiety and depressive disorders. In this review, we take a close look at all currently studied neuropeptides, their related pathways, their roles in stress adaptation, and the etiology of anxiety and depression in humans and animal models. We will focus on the latest research and information regarding these associated neuropeptides and thus picture their potential uses in the future.

Exploring the role of neuropeptides in depression and anxiety

Depression is one of the most prevalent forms of mental disorders and is the most common cause of disability in the Western world. Besides, the harmful effects of stress-related mood disorders on the patients themselves, they challenge the health care system with enormous social and economic impacts. Due to the high proportion of patients not responding to existing drugs, finding new treatment strategies has become an important topic in neurobiology, and there is much evidence that neuropeptides are not only involved in the physiology of stress but may also be clinically important. Based on preclinical trial data, new neuropharmaceutical candidates may target neuropeptides and their receptors and are expected to be essential and valuable tools in the treatment of psychiatric disorders. In the current article, we have summarized data obtained from animal models of depressive disorder and transgenic mouse models. We also focus on previously published research data of clinical studies on corticotropin-releasing hormone (CRH), galanin (GAL), neuropeptide Y (NPY), neuropeptide S (NPS), Oxytocin (OXT), vasopressin (VP), cholecystokinin (CCK), and melanin-concentrating hormone (MCH) stress research fields.

Correlation of Expression Changes between Genes Controlling 5-HT Synthesis and Genes Crh and Trh in the Midbrain Raphe Nuclei of Chronically Aggressive and Defeated Male Mice

Midbrain raphe nuclei (MRNs) contain a large number of serotonergic neurons associated with the regulation of numerous types of psychoemotional states and physiological processes. The aim of this work was to study alterations of the MRN transcriptome in mice with prolonged positive or negative fighting experience and to identify key gene networks associated with the regulation of serotonergic system functioning. Numerous genes underwent alterations of transcription in the MRNs of male mice that either manifested aggression or experienced social defeat in daily agonistic interactions. The expression of the Tph2 gene encoding the rate-limiting enzyme of the serotonin synthesis pathway correlated with the expression of many genes, 31 of which were common between aggressive and defeated mice and were downregulated in the MRNs of mice of both experimental groups. Among these common differentially expressed genes (DEGs), there were genes associated with behavior, learning, memory, and synaptic signaling. These results suggested that, in the MRNs of the mice, the transcriptome changes associated with serotonergic regulation of various processes are similar between the two groups (aggressive and defeated). In the MRNs, more DEGs correlating with Tph2 expression were found in defeated mice than in the winners, which is probably a consequence of deeper Tph2 downregulation in the losers. It was shown for the first time that, in both groups of experimental mice, the changes in the transcription of genes controlling the synthesis and transport of serotonin directly correlate with the expression of genes Crh and Trh, which control the synthesis of corticotrophin- and thyrotropin-releasing hormones. Our findings indicate that CRH and TRH locally produced in MRNs are related to serotonergic regulation of brain processes during a chronic social conflict.

Effects of Acute Stress on the Oscillatory Activity of the Hippocampus-Amygdala-Prefrontal Cortex Network

Displaying a stress response to threatening stimuli is essential for survival. These reactions must be adjusted to be adaptive. Otherwise, even mental illnesses may develop. Describing the physiological stress response may contribute to distinguishing the abnormal responses that accompany the pathology, which may help to improve the development of both diagnoses and treatments. Recent advances have elucidated many of the processes and structures involved in stress response management; however, there is still much to unravel regarding this phenomenon. The main aim of the present research is to characterize the response of three brain areas deeply involved in the stress response (i.e., to an acute stressful experience). Specifically, the electrophysiological activity of the infralimbic division of the medial prefrontal cortex (IL), the basolateral nucleus of the amygdala (BLA), and the dorsal hippocampus (dHPC) was recorded after the infusion of 0.5 µl of corticosterone-releasing factor into the dorsal raphe nucleus (DRN), a procedure which has been validated as a paradigm to cause acute stress. This procedure induced a delayed reduction in slow waves in the three structures, and an increase in faster oscillations, such as those in theta, beta, and gamma bands. The mutual information at low theta frequencies between the BLA and the IL increased, and the delta and slow wave mutual information decreased. The low theta-mid gamma phase-amplitude coupling increased within BLA, as well as between BLA and IL. This electrical pattern may facilitate the activation of these structures, in response to the stressor, and memory consolidation.

Cannabinoid 2 receptors regulate dopamine 2 receptor expression by a beta-arrestin 2 and GRK5-dependent mechanism in neuronal cells

We have previously reported that the repeated exposure to cannabinoids upregulates and enhances the activity of serotonin 2A (5-HT_2A) and dopamine 2 (D₂) receptors and facilitates the formation of D₂-5-HT_2A receptor heterodimers in the rat prefrontal cortex and two neuronal cell lines. Because the repeated exposure to cannabinoids has been associated with adverse neuropsychiatric disorders, this study investigated the mechanisms that underly the cannabinoid-mediated regulation of D₂ receptor expression in a neuronal cell model, CLU213 cells. We initially tested the effects of repeated exposure (72 h) to a non-selective cannabinoid agonist (1 nM CP55940), a selective CB₁ receptor agonist (15 nM ACEA), or a selective CB₂ receptor drug (1 nM GP1a) on the expression of postsynaptic D₂ (D₂L) receptors in CLU213 cells. Repeated CP55940, GP1a, or ACEA treatments significantly increased D₂L receptor protein levels (99 % ± 7%, 30 % ± 7%, and 39 % ± 5% increases compared with control levels, respectively). Repeated exposure to both GP1a and ACEA increased D₂L receptor protein levels by 73 % ± 8%. Interestingly, CP55940 and GP1a, but not ACEA, upregulated D₂ mRNA. Using cells that were stably transfected with short-hairpin RNA (shRNA) lentiviral particles targeting CB₂ receptors, G protein-coupled receptor kinase 5 (GRK5), and β-arrestin 2, we found that CB₂ receptors regulated D₂ expression through a mechanism that is dependent on GRK5, β-arrestin 2, and extracellular signal-related kinase (ERK)1/2. We also found that repeated exposure to either ACEA or GP1a selectively stimulated the protein and mRNA expression of GRK proteins. ACEA significantly upregulated GRK2 proteins, whereas GP1a upregulated GRK5 protein expression. Our results identified mechanisms associated with the upregulation of D₂ receptors in neuronal cells after the repeated exposure to cannabinoids. These data can shed light on the mechanisms that can be targeted to prevent potential adverse effects, while simultaneously determining the therapeutic benefits of cannabinoids.

Novel Brain-Penetrating Oxime Acetylcholinesterase Reactivators Attenuate Organophosphate-Induced Neuropathology in the Rat Hippocampus

Organophosphate (OP) anticholinesterases cause excess acetylcholine leading to seizures which, if prolonged, result in neuronal damage in the rodent brain. Novel substituted phenoxyalkyl pyridinium oximes have previously shown evidence of penetrating the rat blood-brain barrier (BBB) in in vivo tests with a sarin surrogate (nitrophenyl isopropyl methylphosphonate, NIMP) or the active metabolite of the insecticide parathion, paraoxon (PXN), by reducing the time to cessation of seizure-like behaviors and accumulation of glial fibrillary acidic protein, whereas 2-PAM did not. The neuroprotective ability of our lead oximes (15, 20, and 55) was tested using NeuN, Nissl, and Fluoro-Jade B staining in the rat hippocampus. Following lethal-level subcutaneous challenge with NIMP or PXN, rats were intramuscularly administered a novel oxime or 2-PAM plus atropine and euthanized at 4 days. There were statistically significant increases in the median damage scores of the NeuN-stained NIMP, NIMP/2-PAM, and NIMP/Oxime 15 groups compared with the control whereas the scores of the NIMP/Oxime 20 and NIMP/Oxime 55 were not significantly different from the control. The same pattern of statistical significance was observed with PXN. Nissl staining provided a similar pattern, but without statistical differences. Fluoro-Jade B indicated neuroprotection from PXN with novel oximes but not with 2-PAM. The longer blood residence times of Oximes 20 and 55 compared with Oxime 15 might have contributed to their greater efficacy. These results suggest that novel oximes 20 and 55 were able to penetrate the BBB and attenuate neuronal damage after NIMP and PXN exposure, indicating potential broad-spectrum usefulness.

Crh receptor priming in the bed nucleus of the stria terminalis (BNST) induces tph2 gene expression in the dorsomedial dorsal raphe nucleus and chronic anxiety

The bed nucleus of the stria terminalis (BNST) is a nodal structure in neural circuits controlling anxiety-related defensive behavioral responses. It contains neurons expressing the stress- and anxiety-related neuropeptide corticotropin-releasing hormone (Crh) as well as Crh receptors. Repeated daily subthreshold activation of Crh receptors in the BNST is known to induce a chronic anxiety-like state, but how this affects neurotransmitter-relevant gene expression in target regions of the BNST is still unclear. Since the BNST projects heavily to the dorsal raphe nucleus (DR), the main source of brain serotonin, we here tested the hypothesis that such repeated, anxiety-inducing activation of Crh receptors in the BNST alters the expression of serotonergic genes in the DR, including tph2, the gene encoding the rate-limiting enzyme for brain serotonin synthesis, and slc6a4, the gene encoding the serotonin transporter (SERT). For 5 days, adult male Wistar rats received daily, bilateral, intra-BNST microinjections of vehicle (1% bovine serum albumin in 0.9% saline, n = 11) or behaviorally subthreshold doses of urocortin 1 (Ucn1, n = 11), a potent Crh receptor agonist. Priming with Ucn1 increased tph2 mRNA expression selectively within the anxiety-related dorsal part of the DR (DRD) and decreased social interaction (SI) time, a measure of anxiety-related defensive behavioral responses in rodents. Decreased social interaction was strongly correlated with increased tph2 mRNA expression in the DRD. Together with previous studies, our data are consistent with the hypothesis that Crh-mediated control of the BNST/DRD-serotonergic system plays a key role in the development of chronic anxiety states, possibly also contributing to stress-induced relapses in drug abuse and addiction behavior.

Neuroprotection From Organophosphate-Induced Damage by Novel Phenoxyalkyl Pyridinium Oximes in Rat Brain

The nerve agents are extremely toxic organophosphates which lead to massive inhibition of acetylcholinesterase (AChE) in both the central and peripheral nervous systems. The currently approved pyridinium oxime reactivators of organophosphate-inhibited AChE (eg, 2-PAM in the United States) cannot penetrate the blood-brain barrier because of the permanent positive charge in the pyridinium ring. Therefore these current oximes cannot rescue inhibited AChE in the brain. Our laboratories have invented and patented a platform of substituted phenoxyalkyl pyridinium oximes that have been tested for efficacy as therapy within the brains of adult male rats which were challenged with a high sublethal dosage of highly relevant surrogates of sarin (nitrophenyl isopropyl methylphosphonate, NIMP) and VX (nitrophenyl ethyl methylphosphonate, NEMP). The histochemical astrocyte marker glial fibrillary acidic protein (GFAP) was investigated as an indication of neuropathology in two brain regions, the piriform cortex and the dentate gyrus of the hippocampus, which are regions known to be damaged by nerve agent toxicity. Rats treated with either NIMP or NEMP without therapy or with NIMP or NEMP plus 2-PAM therapy showed similar increases in GFAP compared with vehicle controls. However, the rats challenged with NIMP or NEMP plus therapy with our novel Oxime 20 (either a bromide or a mesylate salt) showed GFAP levels statistically undistinguishable from controls. These data provide highly supportive functional evidence of novel oxime entry into the brain. These novel oximes have the potential to provide central neuroprotection from organophosphate anticholinesterase-induced damage, which is a characteristic not displayed by most pyridinium oximes.

Effects of maternal separation on serotonergic systems in the dorsal and median raphe nuclei of adult male Tph2-deficient mice

Previous studies have highlighted interactions between serotonergic systems and adverse early life experience as important gene x environment determinants of risk of stress-related psychiatric disorders. Evidence suggests that mice deficient in Tph2, the rate-limiting enzyme for brain serotonin synthesis, display disruptions in behavioral phenotypes relevant to stress-related psychiatric disorders. The aim of this study was to determine how maternal separation in wild-type, heterozygous, and Tph2 knockout mice affects mRNA expression of serotonin-related genes. Serotonergic genes studied included Tph2, the high-affinity, low-capacity, sodium-dependent serotonin transporter (Slc6a4), the serotonin type 1a receptor (Htr1a), and the corticosterone-sensitive, low-affinity, high-capacity sodium-independent serotonin transporter, organic cation transporter 3 (Slc22a3). Furthermore, we studied corticotropin-releasing hormone receptors 1 (Crhr1) and 2 (Crhr2), which play important roles in controlling serotonergic neuronal activity. For this study, offspring of Tph2 heterozygous dams were exposed to daily maternal separation for the first two weeks of life. Adult, male wild-type, heterozygous, and homozygous offspring were subsequently used for molecular analysis. Maternal separation differentially altered serotonergic gene expression in a genotype- and topographically-specific manner. For example, maternal separation increased Slc6a4 mRNA expression in the dorsal part of the dorsal raphe nucleus in Tph2 heterozygous mice, but not in wild-type or knockout mice. Overall, these data are consistent with the hypothesis that gene x environment interactions, including serotonergic genes and adverse early life experience, play an important role in vulnerability to stress-related psychiatric disorders.

The Arctic/Swedish APP mutation alters the impact of chronic stress on cognition in mice

Chronic stress is a major risk factor for developing Alzheimer's disease (AD) and promotes the processing of amyloid precursor protein (APP) to β-amyloid (Aβ). However, the precise relationship of stress and disease-typical cognitive decline is presently not well understood. The aim of this study was to investigate how early life stress may affect cognition in adult mice with and without soluble Aβ pathology typical for the early stages of the disease. We focussed on sustained attention and response control, aspects of cognition mediated by the prefrontal cortex that are consistently impaired both in early AD and after chronic stress exposure. Young wild-type mice as well as transgenic arcAβ mice overexpressing the hAPParc/swe transgene were exposed to a chronic unpredictable stress paradigm (age 3-8 weeks). At 15 weeks, these mice were tested on the 5-choice serial reaction time task, a test of sustained attention and executive control. We found that, expectedly, chronic stress increased impulsive choices and impaired sustained attention in wild-type mice. However, the same treatment reduced impulsivity and did not interfere with sustained attention in arcAβ mice. These findings suggest an unexpected interaction between chronic stress and Aβ whereby Aβ-pathology caused by the hAPParc/swe mutation prevented and/or reversed stress-induced cognitive changes through mechanisms that deserve further investigation. They also indicate that Aβ, in modest amounts, may have a beneficial role for cognitive stability, for example by protecting neural networks from the impact of further physiological or behavioural stress.

Neural oscillations in the infralimbic cortex after electrical stimulation of the amygdala. Relevance to acute stress processing

The stress system coordinates the adaptive reactions of the organism to stressors. Therefore, dysfunctions in this circuit may correlate to anxiety-related disorders, including depression. Comprehending the dynamics of this network may lead to a better understanding of the mechanisms that underlie these diseases. The central nucleus of the amygdala (CeA) activates the hypothalamic-pituitary-adrenal axis and brainstem nodes by triggering endocrine, autonomic and behavioral stress responses. The medial prefrontal cortex plays a significant role in regulating reactions to stressors, and is specifically important for limiting fear responses. Brain oscillations reflect neural systems activity. Synchronous neuronal assemblies facilitate communication and synaptic plasticity, mechanisms that cooperatively support the temporal representation and long-term consolidation of information. The purpose of this article was to delve into the interactions between these structures in stress contexts by evaluating changes in oscillatory activity. We particularly analyzed the local field potential in the infralimbic region of the medial prefrontal cortex (IL) in urethane-anesthetized rats after the electrical activation of the central nucleus of the amygdala by mimicking firing rates induced by acute stress. Electrical CeA activation induced a delayed, but significant, change in the IL, with prominent slow waves accompanied by an increase in the theta and gamma activities, and spindles. The phase-amplitude coupling of both slow waves and theta oscillations significantly increased with faster oscillations, including theta-gamma coupling and the nesting of spindles, theta and gamma oscillations in the slow wave cycle. These results are further discussed in neural processing terms of the stress response and memory formation.

Corticotropin-Releasing Factor (CRF) circuit modulation of cognition and motivation

The neuropeptide, corticotropin-releasing factor (CRF), is a key modulator of physiological, endocrine, and behavioral responses during stress. Dysfunction of the CRF system has been observed in stress-related affective disorders including post-traumatic stress disorder, depression, and anxiety. Beyond affective symptoms, these disorders are also characterized by impaired cognition, for which current pharmacological treatments are lacking. Thus, there is a need for pro-cognitive treatments to improve quality of life for individuals suffering from mental illness. In this review, we highlight research demonstrating that CRF elicits potent modulatory effects on higher-order cognition via actions within the prefrontal cortex and subcortical monoaminergic and cholinergic systems. Additionally, we identify questions for future preclinical research on this topic, such as the need to investigate sex differences in the cognitive and microcircuit actions of CRF, and whether CRF may represent a pharmacological target to treat cognitive dysfunction. Addressing these questions will provide new insight into pathophysiology underlying cognitive dysfunction and may lead to improved treatments for neuropsychiatric disorders.

Current understanding of fear learning and memory in humans and animal models and the value of a linguistic approach for analyzing fear learning and memory in humans

Fear is an emotion that serves as a driving factor in how organisms move through the world. In this review, we discuss the current understandings of the subjective experience of fear and the related biological processes involved in fear learning and memory. We first provide an overview of fear learning and memory in humans and animal models, encompassing the neurocircuitry and molecular mechanisms, the influence of genetic and environmental factors, and how fear learning paradigms have contributed to treatments for fear-related disorders, such as posttraumatic stress disorder. Current treatments as well as novel strategies, such as targeting the perisynaptic environment and use of virtual reality, are addressed. We review research on the subjective experience of fear and the role of autobiographical memory in fear-related disorders. We also discuss the gaps in our understanding of fear learning and memory, and the degree of consensus in the field. Lastly, the development of linguistic tools for assessments and treatment of fear learning and memory disorders is discussed.

Cortisol Stress Response and in Vivo PET Imaging of Human Brain Serotonin 1A Receptor Binding

BACKGROUND

Abnormalities in the hypothalamic-pituitary-adrenal axis, serotonergic system, and stress response have been linked to the pathogenesis of major depressive disorder. State-dependent hyper-reactivity of the hypothalamic-pituitary-adrenal axis is seen in major depressive disorder, and higher binding to the serotonin 1A receptor is observed as a trait in both currently depressed and remitted untreated major depressive disorder. Here, we sought to examine whether a relationship exists between cortisol secretion in response to a stressor and serotonin 1A receptor binding throughout the brain, both in healthy controls and participants with major depressive disorder.

METHODS

Research participants included 42 medication-free, depressed subjects and 31 healthy volunteers. Participants were exposed to either an acute, physical stressor (radial artery catheter insertion) or a psychological stressor (Trier Social Stress Test). Levels of serotonin 1A receptor binding on positron emission tomography with [11C]WAY-100635 were also obtained from all participants. The relationship between [11C]WAY-100635 binding and cortisol was examined using mixed linear effects models with group (major depressive disorder vs control), cortisol, brain region, and their interactions as fixed effects and subject as a random effect.

RESULTS

We found a positive correlation between post-stress cortisol measures and serotonin 1A receptor ligand binding levels across multiple cortical and subcortical regions, independent of diagnosis and with both types of stress. The relationship between [11C]WAY-100635 binding and cortisol was homogenous across all a priori brain regions. In contrast, resting cortisol levels were negatively correlated with serotonin 1A receptor ligand binding levels independently of diagnosis, except in the RN. There was no significant difference in cortisol between major depressive disorder participants and healthy volunteers with either stressor. Similarly, there was no correlation between cortisol and depression severity in either stressor group.

CONCLUSIONS

This study suggests that there may be a common underlying mechanism that links abnormalities in the serotonin system and hypothalamic-pituitary-adrenal axis hyper-reactivity to stress. Future studies need to determine how hypothalamic-pituitary-adrenal axis dysfunction affects mood to increase the risk of suicide in major depression.

Serotonin actions within the prelimbic cortex induce anxiolysis mediated by serotonin 1a receptors

BACKGROUND:

Serotonin plays an important role in the regulation of anxiety, acting through complex modulatory mechanisms within distinct brain structures. Serotonin can act through complex negative feedback mechanisms controlling the neuronal activity of serotonergic circuits and downstream physiologic and behavioral responses. Administration of serotonin or the serotonin 1A receptor agonist, (±)-8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT), into the prefrontal cortex, inhibits anxiety-like responses. The prelimbic area of the prefrontal cortex regulates serotonergic neurons within the dorsal raphe nucleus and is involved in modulating anxiety-like behavioral responses.

AIMS:

This study aimed to investigate the serotonergic role within the prelimbic area on anxiety- and panic-related defensive behavioral responses.

METHODS:

We investigated the effects of serotonin within the prelimbic area on inhibitory avoidance and escape behaviors in the elevated T-maze. We also extended the investigation to serotonin 1A, 2A, and 2C receptors.

RESULTS:

Intra-prelimbic area injection of serotonin or 8-OH-DPAT induced anxiolytic effects without affecting escape behaviors. Previous administration of the serotonin 1A receptor antagonist, WAY-100635, into the prelimbic area counteracted the anxiolytic effects of serotonin. Neither the serotonin 2A nor the serotonin 2C receptor preferential agonists, (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) and 6-chloro-2-(1-piperazinyl) pyrazine (MK-212), respectively, affected behavioral responses in the elevated T-maze.

CONCLUSION:

Facilitation of serotonergic signaling within the prelimbic area of rats induced an anxiolytic effect in the elevated T-maze test, which was mediated by local serotonin 1A receptors. This inhibition of anxiety-like defensive behavioral responses may be mediated by prelimbic area projections to neural systems controlling anxiety, such as the dorsal raphe nucleus or basolateral amygdala.

The CRF Family of Neuropeptides and their Receptors - Mediators of the Central Stress Response

Background:

Dysregulated stress neurocircuits, caused by genetic and/or environmental changes, underlie the development of many neuropsychiatric disorders. Corticotropin-releasing factor (CRF) is the major physiological activator of the hypothalamic-pituitary-adrenal (HPA) axis and conse-quently a primary regulator of the mammalian stress response. Together with its three family members, urocortins (UCNs) 1, 2, and 3, CRF integrates the neuroendocrine, autonomic, metabolic and behavioral responses to stress by activating its cognate receptors CRFR1 and CRFR2.

Objective:

Here we review the past and current state of the CRF/CRFR field, ranging from pharmacologi-cal studies to genetic mouse models and virus-mediated manipulations.

Results:

Although it is well established that CRF/CRFR1 signaling mediates aversive responses, includ-ing anxiety and depression-like behaviors, a number of recent studies have challenged this viewpoint by revealing anxiolytic and appetitive properties of specific CRF/CRFR1 circuits. In contrast, the UCN/CRFR2 system is less well understood and may possibly also exert divergent functions on physiol-ogy and behavior depending on the brain region, underlying circuit, and/or experienced stress conditions.

Conclusion:

A plethora of available genetic tools, including conventional and conditional mouse mutants targeting CRF system components, has greatly advanced our understanding about the endogenous mecha-nisms underlying HPA system regulation and CRF/UCN-related neuronal circuits involved in stress-related behaviors. Yet, the detailed pathways and molecular mechanisms by which the CRF/UCN-system translates negative or positive stimuli into the final, integrated biological response are not completely un-derstood. The utilization of future complementary methodologies, such as cell-type specific Cre-driver lines, viral and optogenetic tools will help to further dissect the function of genetically defined CRF/UCN neurocircuits in the context of adaptive and maladaptive stress responses.

Negative consequences of early-life adversity on substance use as mediated by corticotropin-releasing factor modulation of serotonin activity

Early-life adversity is associated with increased risk for substance abuse in later life, with women more likely to report past and current stress as a mediating factor in their substance use and relapse as compared to men. Preclinical models of neonatal and peri-adolescent (early through late adolescence) stress all support a direct relationship between experiences of early-life adversity and adult substance-related behaviors, and provide valuable information regarding the underlying neurobiology. This review will provide an overview of these animal models and how these paradigms alter drug and alcohol consumption and/or seeking in male and female adults. An introduction to the corticotropin-releasing factor (CRF) and serotonin systems, their development and their interactions at the level of the dorsal raphe will be provided, illustrating how this particular stress system is sexually dimorphic, and is well positioned to be affected by stressors early in development and throughout maturation. A model for CRF-serotonin interactions in the dorsal raphe and how these influence dopaminergic activity within the nucleus accumbens and subsequent reward-associated behaviors will be provided, and alterations to the activity of this system following early-life adversity will be identified. Overall, converging findings suggest that early-life adversity has long-term effects on the functioning of the CRF-serotonin system, highlighting a potentially important and targetable mediator linking stress to addiction. Future work should focus on identifying the exact mechanisms that promote long-term changes to the expression and activity of CRF receptors in the dorsal raphe. Moreover, it is important to clarify whether similar neurobiological mechanisms exist for males and females, given the sexual dimorphism both in CRF receptors and serotonin indices in the dorsal raphe and in the behavioral outcomes of early-life adversity.

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Early life stress increases risk for substance abuse in adulthood.

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Stress and drugs increase CRF which alters serotonin release in the brain.

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CRF₂ receptor expression in the dorsal raphe is altered by early life stress.

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Resultant changes to serotonin output facilitates dopamine in the accumbens.

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CRF₂-sertotonin-dopamine interactions may link early life stress with substance abuse.

Two models of inescapable stress increase tph2 mRNA expression in the anxiety-related dorsomedial part of the dorsal raphe nucleus

Expression of TPH2, the rate-limiting enzyme for brain serotonin synthesis, is elevated in the dorsal raphe nucleus (DR) of depressed suicide victims. One hypothesis is that this increase in TPH2 expression is stress-induced. Here, we used an established animal model to address whether exposure to an acute stressor, inescapable tail shock (IS), increases tph2 mRNA and Tph2 protein expression, and if IS sensitizes the DR to a subsequent, heterotypic stressor. In Experiment 1, we measured tph2 mRNA expression 4 h after IS or home cage (HC) control conditions in male rats, using in situ hybridization histochemistry. In Experiment 2, we measured Tph2 protein expression 12 h or 24 h after IS using western blot. In Experiment 3, we measured tph2 mRNA expression following IS on Day 1, and cold swim stress (10 min, 15 °C) on Day 2. Inescapable tail shock was sufficient to increase tph2 mRNA expression 4 h and 28 h later, selectively in the dorsomedial DR (caudal aspect of the dorsal DR, cDRD; an area just rostral to the caudal DR, DRC) and increased Tph2 protein expression in the DRD (rostral and caudal aspects of the dorsal DR combined) 24 h later. Cold swim increased tph2 mRNA expression in the dorsomedial DR (cDRD) 4 h later. These effects were associated with increased immobility during cold swim, elevated plasma corticosterone, and a proinflammatory plasma cytokine milieu (increased interleukin (IL)-6, decreased IL-10). Our data demonstrate that two models of inescapable stress, IS and cold swim, increase tph2 mRNA expression selectively in the anxiety-related dorsomedial DR (cDRD).

Iron Oxide Nanoparticle Delivery of Peptides to the Brain: Reversal of Anxiety during Drug Withdrawal

Targeting neuropeptide systems is important for future advancements in treatment of neurological and psychiatric illnesses. However, many of the peptides and their analogs do not cross the blood-brain barrier (BBB) efficiently. Nanoparticles such as iron oxide can cross the BBB, and here we describe a novel method for the conjugation of a peptide antisauvagine-30 (ASV-30) to iron oxide nanoparticles. Previous research has shown that direct infusion of ASV-30 into the brain reduces anxiety-like behavior in animal models via actions on corticotropin releasing factor type 2 (CRF₂) receptors. Therefore, we tested whether iron oxide+ASV-30 complexes cross the BBB of rats and then determined whether iron oxide+ASV-30 nanoparticles are localized with CRF₂-expressing neurons. Finally we tested the hypothesis that systemic infusion of iron oxide+ASV-30 can reduce anxiety-like behavior. First we describe the synthesis and demonstrate the stability of iron oxide-peptide nanoparticle complexes. Next, nanoparticles (87.7 μg/kg Fe₂O₃) with or without ASV-30 (200 μg/kg, ip) were injected into male rats 30 min prior to transcardial perfusion and brain fixation for immunohistochemical analysis, or before testing on the elevated plus maze (EPM) in an amphetamine withdrawal model of anxiety. Systemically administered iron oxide+ASV-30 particles were present in the brain and associated with neurons, including those that express CRF₂ receptors, but did not localize with the iron storage protein ferritin. Furthermore, systemic administration of ironoxide+ASV-30 reduced amphetamine withdrawal-induced anxiety without affecting locomotion, suggesting that the anxiolytic effects of ASV-30 were preserved and the bioavailability of ASV-30 was sufficient. The findings demonstrate a novel approach to peptide delivery across the BBB and provide insight as to the neural distribution and efficacy of this nanotechnology.

Impact of juvenile chronic stress on adult cortico-accumbal function: Implications for cognition and addiction

Repeated exposure to stress during childhood is associated with increased risk for neuropsychiatric illness, substance use disorders and other behavioral problems in adulthood. However, it is not clear how chronic childhood stress can lead to emergence of such a wide range of symptoms and disorders in later life. One possible explanation lies in stress-induced disruption to the development of specific brain regions associated with executive function and reward processing, deficits in which are common to the disorders promoted by childhood stress. Evidence of aberrations in prefrontal cortex and nucleus accumbens function following repeated exposure of juvenile (pre- and adolescent) organisms to a variety of different stressors would account not only for the similarity in symptoms across the wide range of childhood stress-associated mental illnesses, but also their persistence into adulthood in the absence of further stress. Therefore, the goal of this review is to evaluate the current knowledge regarding disruption to executive function and reward processing in adult animals or humans exposed to chronic stress over the juvenile period, and the underlying neurobiology, with particular emphasis on the prefrontal cortex and nucleus accumbens. First, the role of these brain regions in mediating executive function and reward processing is highlighted. Second, the neurobehavioral development of these systems is discussed to illustrate how juvenile stress may exert long-lasting effects on prefrontal cortex-accumbal activity and related behavioral functions. Finally, a critical review of current animal and human findings is presented, which strongly supports the supposition that exposure to chronic stress (particularly social aggression and isolation in animal studies) in the juvenile period produces impairments in executive function in adulthood, especially in working memory and inhibitory control. Chronic juvenile stress also results in aberrations to reward processing and seeking, with increased sensitivity to drugs of abuse particularly noted in animal models, which is in line with greater incidence of substance use disorders seen in clinical studies. These consequences are potentially mediated by monoamine and glutamatergic dysfunction in the prefrontal cortex and nucleus accumbens, providing translatable therapeutic targets. However, the predominant use of male subjects and social-based stressors in preclinical studies points to a clear need for determining how both sex differences and stressor heterogeneity may differentially contribute to stress-induced changes to substrates mediating executive function and reward processing, before the impact of chronic juvenile stress in promoting adult psychopathology can be fully understood.

Region-specific roles of the corticotropin-releasing factor-urocortin system in stress

Dysregulation of the corticotropin-releasing factor (CRF)-urocortin (UCN) system has been implicated in stress-related psychopathologies such as depression and anxiety. It has been proposed that CRF-CRF receptor type 1 (CRFR1) signalling promotes the stress response and anxiety-like behaviour, whereas UCNs and CRFR2 activation mediate stress recovery and the restoration of homeostasis. Recent findings, however, provide clear evidence that this view is overly simplistic. Instead, a more complex picture has emerged that suggests that there are brain region- and cell type-specific effects of CRFR signalling that are influenced by the individual's prior experience and that shape molecular, cellular and ultimately behavioural responses to stressful challenges.

Brain serotonin 4 receptor binding is associated with the cortisol awakening response

Serotonin signalling is considered critical for an appropriate and dynamic adaptation to stress. Previously, we have shown that prefrontal serotonin transporter (SERT) binding is positively associated with the cortisol awakening response (CAR) (Frokjaer et al., 2013), which is an index of hypothalamic-pituitary-adrenal (HPA)-axis output dynamics. Here, we investigated in healthy individuals if cerebral serotonin 4 receptor (5-HT4r) binding, reported to be a proxy for serotonin levels, is associated with CAR. Thirty healthy volunteers (25 males, age range 20-56 years) underwent 5-HT4r PET imaging with [(11)C]-SB207145, genotyping of the SERT-linked polymorphic region (5-HTTLPR), and performed serial home sampling of saliva (5 time points from 0 to 60min from awakening) to assess CAR. The association between 5-HT4r binding in 4 regions of interest (prefrontal cortex, anterior cingulate cortex, pallidostriatum, and hippocampus) and CAR was tested using multiple linear regression with adjustment for age and 5-HTTLPR genotype. Finally, an exploratory voxel-based analysis of the association was performed. CAR was negatively associated with 5-HT4r binding in pallidostriatum (p=0.01), prefrontal cortex (p=0.03), and anterior cingulate cortex (p=0.002), respectively, but showed no association in hippocampus. The results remained significant when taking into account other potentially relevant covariates. In conclusion, our finding reinforces an association between HPA-axis function and serotonin signaling in vivo in humans. We suggest that higher synaptic serotonin concentration, here indexed by lower 5-HT4r binding, supports HPA-axis dynamics, which in healthy volunteers is reflected by a robust CAR.

Early life adversity and serotonin transporter gene variation interact to affect DNA methylation of the corticotropin-releasing factor gene promoter region in the adult rat brain

The interaction between childhood maltreatment and the serotonin transporter (5-HTT) gene linked polymorphic region has been associated with increased risk to develop major depression. This Gene × Environment interaction has furthermore been linked with increased levels of anxiety and glucocorticoid release upon exposure to stress. Both endophenotypes are regulated by the neuropeptide corticotropin-releasing factor (CRF) or hormone, which is expressed by the paraventricular nucleus of the hypothalamus, the bed nucleus of the stria terminalis, and the central amygdala (CeA). Therefore, we hypothesized that altered regulation of the expression of CRF in these areas represents a major neurobiological mechanism underlying the interaction of early life stress and 5-HTT gene variation. The programming of gene transcription by Gene × Environment interactions has been proposed to involve epigenetic mechanisms such as DNA methylation. In this study, we report that early life stress and 5-HTT genotype interact to affect DNA methylation of the Crf gene promoter in the CeA of adult male rats. Furthermore, we found that DNA methylation of a specific site in the Crf promoter significantly correlated with CRF mRNA levels in the CeA. Moreover, CeA CRF mRNA levels correlated with stress coping behavior in a learned helplessness paradigm. Together, our findings warrant further investigation of the link of Crf promoter methylation and CRF expression in the CeA with behavioral changes that are relevant for psychopathology.

Human fear acquisition deficits in relation to genetic variants of the corticotropin-releasing hormone receptor 1 and the serotonin transporter--revisited

We recently showed that a genetic polymorphism (rs878886) in the human corticotropin-releasing hormone receptor 1 (CRHR1) is associated with reduced fear-conditioned responses to a threat cue. This is a potentially important finding considering that the failure to acquire fear contingencies can leave an individual in a maladaptive state of more generalized anxiety. Consistent with that idea, the CRHR1-dependent fear acquisition deficit translated into heightened contextual anxiety when taking genetic variability within the serotonin transporter long polymorphic region (5-HTTLPR) into account. To replicate our previous findings, we conducted a replication study in 224 healthy medication-free human subjects using the exact same cue and context virtual reality fear-conditioning procedure as in study by Heitland et al. (2013). In the replication study, consistent with the original findings, CRHR1 rs878886 G-allele carriers showed reduced acquisition of cue-specific fear-conditioned responses compared with C/C homozygotes. Also, in this larger sample the cue acquisition deficit of G-allele carriers translated into heightened contextual anxiety, even independent of 5-HTT gene variation. In contrast to our earlier findings, there was an additional interaction effect of CRHR1 rs878886 and the triallelic 5-HTTLPR/rs25531 variant on cued fear acquisition. In summary, this study replicated the initially reported association of the CRHR1 rs878886 G-allele with cued fear acquisition deficits, albeit with a different pattern of results regarding the interaction with 5-HTT variation. This further supports the notion that the human corticotropin-releasing hormone plays a role in the acquisition of fears.

Serotonergic systems in the balance: CRHR1 and CRHR2 differentially control stress-induced serotonin synthesis

Anxiety and affective disorders are often associated with hypercortisolism and dysfunctional serotonergic systems, including increased expression of TPH2, the gene encoding the rate-limiting enzyme of neuronal serotonin synthesis. We previously reported that chronic glucocorticoid exposure is anxiogenic and increases rat Tph2 mRNA expression, but it was still unclear if this also translates to increased TPH2 protein levels and in vivo activity of the enzyme. Here, we found that adult male rats treated with corticosterone (CORT, 100 μg/ml) via the drinking water for 21 days indeed show increased TPH2 protein expression in the dorsal and ventral part of the dorsal raphe nucleus (DRD, DRV) during the light phase, abolishing the enzyme's diurnal rhythm. In a second study, we systemically blocked the conversion of 5-hydroxytryptophan (5-HTP) to serotonin immediately before rats treated with CORT or vehicle were either exposed to 30 min acoustic startle stress or home cage control conditions. This allowed us to measure 5-HTP accumulation as a direct readout of basal versus stress-induced in vivo TPH2 activity. As expected, basal TPH2 activity was elevated in the DRD, DRV and MnR of CORT-treated rats. In response to stress, a multitude of serotonergic systems reacted with increased TPH2 activity, but the stress-, anxiety-, and learned helplessness-related dorsal and caudal DR (DRD/DRC) displayed stress-induced increases in TPH2 activity only after chronic CORT-treatment. To address the mechanisms underlying this region-specific CORT-dependent sensitization, we stereotaxically implanted CORT-treated rats with cannulae targeting the DR, and pharmacologically blocked either corticotropin-releasing hormone receptor type 1 (CRHR1) or type 2 (CRHR2) 10 min prior to acoustic startle stress. CRHR2 blockade prevented stress-induced increases of TPH2 activity within the DRD/DRC, while blockade of CRHR1 potentiated stress-induced TPH2 activity in the entire DR. Stress-induced TPH2 activity in the DRD/DRC furthermore predicted TPH2 activity in the amygdala and in the caudal pontine reticular nucleus (PnC), while serotonin synthesis in the PnC was strongly correlated with the maximum startle response. Our data demonstrate that chronically elevated glucocorticoids sensitize stress- and anxiety-related serotonergic systems, and for the first time reveal competing roles of CRHR1 and CRHR2 on stress-induced in vivo serotonin synthesis.

Cognitive disruptions in stress-related psychiatric disorders: A role for corticotropin releasing factor (CRF)

This article is part of a Special Issue "SBN 2014". Stress is a potential etiology contributor to both post-traumatic stress disorders (PTSD) and major depression. One stress-related neuropeptide that is hypersecreted in these disorders is corticotropin releasing factor (CRF). Dysregulation of CRF has long been linked to the emotion and mood symptoms that characterize PTSD and depression. However, the idea that CRF also mediates the cognitive disruptions observed in patients with these disorders has received less attention. Here we review literature indicating that CRF can alter cognitive functions. Detailed are anatomical studies revealing that CRF is poised to modulate regions required for learning and memory. We also describe preclinical behavioral studies that demonstrate CRF's ability to alter fear conditioning, impair memory consolidation, and alter a number of executive functions, including attention and cognitive flexibility. The implications of these findings for the etiology and treatment of the cognitive impairments observed in stress-related psychiatric disorders are described.

Corticotropin-releasing Factor in the Rat Dorsal Raphe Nucleus Promotes Different Forms of Behavioral Flexibility Depending on Social Stress History

The stress-related neuropeptide, corticotropin-releasing factor (CRF) regulates the dorsal raphe nucleus-serotonin (DRN-5-HT) system during stress and this may underlie affective and cognitive dysfunctions that characterize stress-related psychiatric disorders. CRF acts on both CRF1 and CRF2 receptor subtypes in the DRN that exert opposing inhibitory and excitatory effects on DRN-5-HT neuronal activity and 5-HT forebrain release, respectively. The current study first assessed the cognitive effects of intra-DRN microinfusion of CRF or the selective CRF2 agonist, urocortin II in stress-naive rats on performance of an operant strategy set-shifting task that is mediated by the medial prefrontal cortex (mPFC). CRF (30 ng) facilitated strategy set-shifting performance, whereas higher doses of CRF and urocortin II that would interact with CRF2 were without effect, consistent with a CRF1-mediated action. This dose decreased 5-HT extracellular levels in the mPFC, further supporting a role for CRF1. The effects of CRF were then assessed in rats exposed to repeated social stress using the resident-intruder model. Repeated social stress shifted the CRF effect from facilitation of strategy set shifting to facilitation of reversal learning and this was most prominent in a subpopulation of rats that resist defeat. Notably, in this subpopulation of rats 5-HT neuronal responses to CRF have been demonstrated to shift from CRF1-mediated inhibition to CRF2-mediated excitation. Because 5-HT facilitates reversal learning, the present results suggest that stress-induced changes in the cellular effects of CRF in the DRN translate to changes in cognitive effects of CRF. Together, the results underscore the potential for stress history to shift cognitive processing through changes in CRF neurotransmission in the DRN and the association of this effect with coping strategy.

High fat diet decreases beneficial effects of estrogen on serotonin-related gene expression in marmosets

The administration of estradiol-17β (E) to animal models after loss of ovarian steroid production has many beneficial effects on neural functions, particularly in the serotonin system in nonhuman primates (NHPs). E also has anorexic effects, although the mechanism of action is not well defined. In the US, obesity has reached epidemic proportions, and blame is partially directed at the Western style diet, which is high in fat and sugar. This study examined the interaction of E and diet in surgically menopausal nonhuman primates with a 2×2 block design. Marmosets (Callithrix jacchus; n=4/group) were placed on control-low fat diet (LFD; 14%kcal from fat) or high fat diet (HFD; 28%kcal from fat) 1month prior to ovariectomy (Ovx). Empty (placebo) or E-filled Silastic capsules were implanted immediately following Ovx surgery. Treatments extended 6months. The established groups were: placebo+LFD, E+LFD, placebo+HFD, or E+HFD. At necropsy, the brain was flushed with saline and harvested. The midbrain was dissected and a small block containing the dorsal raphe nucleus was processed for qRT-PCR using Evagreen (Biotinum). Genes previously found to impact serotonin neural functions were examined. Results were compared with 2-way ANOVA followed by Bonferroni post-hoc tests or Cohen's D analysis. There was a significant effect of treatment on tryptophan hydroxylase 2 (TPH2) across the groups (p=0.019). E stimulated TPH2 expression and HFD prevented E-stimulated TPH2 expression (p<0.01). Treatment differentially affected monoamine oxidase B (MAO-B) across the groups (p=0.05). E increased MAO-B with LFD, and this stimulatory effect was prevented by HFD (p<0.05). There was a significant difference between treatments in corticotrophin releasing factor-receptor 2 (CRF-R2) expression (p=0.012). E increased CRF-R2 and this stimulatory effect was blocked by HFD (p<0.01). Regardless of diet, E increased Fev mRNA (p=0.028) and decreased CRF-receptor 1 (CRF-R1) mRNA (p=0.04). HFD suppressed urocortin 1 (UCN1; stresscopin) expression (p=0.045) but E treatment had no effect. Monoamine oxidase A (MAO-A) was different due to treatment across the groups (p=0.028). MAO-A was increased in the E+HFD group (p<0.01) whereas previous studies showed E suppressed MAO-A in macaques. The serotonin reuptake transporter (SERT), the serotonin 1A receptor (5HT1A), estrogen receptor beta (ERβ) and progestin receptor (PR) expressions were not different between groups. Estrogen receptor alpha (ERα) was undetectable. In summary, the data indicate that important actions of hormone therapy in the serotonin system may be lost in the context of a HFD.

Prevention of alcohol-heightened aggression by CRF-R1 antagonists in mice: critical role for DRN-PFC serotonin pathway

Alcohol can escalate aggressive behavior in a significant subgroup of rodents, humans, and nonhuman primates. The present study investigated whether blockade of corticotropin-releasing factor receptor type 1 (CRF-R1) could prevent the emergence of alcohol-heightened aggression in mice. The serotonin (5-HT) pathway from the dorsal raphe nucleus (DRN) to the medial prefrontal cortex (mPFC) by CRF-R1 was investigated as a possible target for the prevention of alcohol-heightened aggressive behavior. Male CFW mice that reliably exhibited aggressive behaviors after consuming 1 g/kg of alcohol received systemic or intra-DRN administration of CRF-R1 antagonists, CP-154,526 or MTIP, before a confrontation with a male conspecific. Blockade of DRN CRF-R1 receptors with both antagonists significantly reduced only alcohol-heightened aggression, whereas systemic administration reduced both alcohol-heightened and species-typical aggression. Next, a 5-HT1A agonist, 8-OH-DPAT, was coadministered with CP-154,526 into the DRN to temporarily disrupt 5-HT activity. This manipulation abolished the antiaggressive effects of intra-DRN CP-154,526. In the mPFC, in vivo microdialysis revealed that extracellular 5-HT levels were increased in mice that consumed alcohol and were then injected with CP-154,526, both systemically or intra-DRN. Neither alcohol nor CP-154,526 alone affected 5-HT release in the mPFC. The present results suggest the DRN as a critical site for CRF-R1 to modulate alcohol-heightened aggression via action on the serotonergic DRN-PFC pathway.

Stress physiology of migrant birds during stopover in natural and anthropogenic woodland habitats of the Northern Prairie region

Anthropogenic alterations of woodland habitat may influence stopover biology, which in turn could alter the stress physiology of migratory landbirds. Woodland stopover habitats are scarce in the Northern Prairie region of North America and consist of native riparian corridor woodlands (corridors) and smaller, more isolated woodlots of anthropogenic origin around farmsteads (woodlots). Corridor habitats have been greatly reduced since the time of European settlement, but woodlot habitats have appeared over this same time period. In this study, we compared stopover biology and stress physiology of migratory landbirds using natural and anthropogenic woodland habitats. We first tested for differences between birds in the two habitats for baseline corticosterone (CORTB) and the magnitude of the stress response for individual species, taxonomic families and foraging guilds. Plasma corticosterone increased significantly for all bird groups in both habitats following 30 min of restraint stress (CORT30), and neither CORTB nor the magnitude of the stress response (CORT30 - CORTB) differed significantly between birds in the two habitats. Secondly, because CORTB levels are often elevated and CORT secretion following a stressor is often suppressed for birds in poor body condition, we hypothesized that woodland migrants with higher fattening rates would show reduced CORTB and a robust stress response. We tested this hypothesis by assessing the relationships between plasma corticosterone and plasma metabolites associated with refuelling. We found that CORTB was negatively associated and the magnitude of the stress response positively associated with plasma triglycerides (an indicator of fat deposition), with opposite patterns for corticosterone and plasma β-hydroxybutyrate (an indicator of fat catabolism). These data suggest that both corridor and woodlot habitats serve as effective stopover habitat and that the reduction of corridor habitat and increased reliance on anthropogenic woodlots is not detrimental to the stress physiology of migrants in a region with limited woodland habitats.

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.

Central CRF2 receptor antagonism reduces anxiety states during amphetamine withdrawal

Increased depressive and anxiety-like behaviors are exhibited by rats and humans during withdrawal from psychostimulants. Anxiety-like behaviors observed during amphetamine withdrawal are mediated by increased expression and activity of corticotropin releasing factor type 2 (CRF2) receptors in the dorsal raphe nucleus (dRN). Anxiety-like behavior of rats during withdrawal can be reversed by CRF2 receptor antagonism in the dRN, but the efficacy of global central CRF2 receptor antagonism is unknown. Rats were treated with amphetamine (2.5mg/kg, ip.) or saline daily for 2 weeks, and were tested for anxiety-like behaviors during withdrawal. Rats undergoing withdrawal showed increased anxiety-like behavior, which was reduced by ventricular infusion of the CRF2 antagonist antisauvagine-30 (ASV 2 μg/2 μl). Surprisingly, ventricular ASV increased anxiety-like behavior in rats pre-treated with saline, but had an anxiolytic effect in un-treated rats. Western blots were performed to determine whether differences in CRF receptor densities could explain ASV-induced behavioral results. Saline pre-treated rats showed reduced CRF1 receptor expression in the lateral septum compared to amphetamine pre-treated and un-treated rats. Overall, these results suggest that central CRF2 antagonism reduces anxiety states during amphetamine withdrawal, and that behavioral effects may be dependent upon the balance of CRF1 and CRF2 receptor activity in anxiety-related regions.

The role of serotonin in reward, punishment and behavioural inhibition in humans: insights from studies with acute tryptophan depletion

Deakin and Graeff proposed that forebrain 5-hydroxytryptamine (5-HT) projections are activated by aversive events and mediate anticipatory coping responses including avoidance learning and suppression of the fight-flight escape/panic response. Other theories proposed 5-HT mediates aspects of behavioural inhibition or reward. Most of the evidence comes from rodent studies. We review 36 experimental studies in humans in which the technique of acute tryptophan depletion (ATD) was used to explicitly address the role of 5-HT in response inhibition, punishment and reward. ATD did not cause disinhibition of responding in the absence of rewards or punishments (9 studies). A major role for 5-HT in reward processing is unlikely but further tests are warranted by some ATD findings. Remarkably, ATD lessened the ability of punishments (losing points or notional money) to restrain behaviour without affecting reward processing in 7 studies. Two of these studies strongly indicate that ATD blocks 5-HT mediated aversively conditioned Pavlovian inhibition and this can explain a number of the behavioural effects of ATD.

In abstinent MDMA users the cortisol awakening response is off-set but associated with prefrontal serotonin transporter binding as in non-users

Serotonergic signaling is considered critical for an appropriate adaptation to stress. We have previously observed that in healthy volunteers, prefrontal serotonin transporter (SERT) binding is positively associated with hypothalamic-pituitary-adrenal (HPA)-axis output in terms of the cortisol awakening response (CAR). Here, we tested (1) if such a correlation persists in a human model of chronic serotonin depletion, namely in 3,4-Methylenedioxymethamphetamine (MDMA or 'Ecstasy') users, and (2) if CAR differed between MDMA users (N = 18) and non-using healthy volunteers (N = 32). Participants underwent SERT brain imaging with [11C]DASB-PET, and performed home-sampling of CAR, defined as the area under curve with respect to cortisol increase from awakening level. When adjusting for age and group, CAR was positively coupled to prefrontal SERT binding (p = 0.006) and MDMA users showed significantly higher CAR than the control group (p = 0.0003). In conclusion, our data confirm the recently described positive association between prefrontal SERT binding and CAR, this time in a human model of serotonin deficiency. Also, we find that CAR was higher in MDMA users relative to non-users. We suggest that the inhibitory control on HPA-axis output is less efficient in the off-balance state established by recent MDMA use, most likely through mechanisms other than those that can be compensated by lowering SERT levels.

Fibroblast growth factor 8 deficiency compromises the functional response of the serotonergic system to stress

Functionally heterogeneous populations of serotonergic neurons, located within the dorsal raphe nucleus (DR), play a role in stress-related behaviors and neuropsychiatric illnesses such as anxiety and depression. Abnormal development of these neurons may permanently alter their structure and connections, making the organism more susceptible to anxiety-related disorders. A factor that critically regulates the development of serotonergic neurons is fibroblast growth factor 8 (Fgf8). In this study, we used acute restraint stress followed by behavioral testing to examine whether Fgf8 signaling during development is important for establishing functional stress- and anxiety-related DR neurocircuits in adulthood. Wild-type and heterozygous male mice globally hypomorphic for Fgf8 were exposed to acute restraint stress and then tested for anxiety-like behavior on the elevated plus-maze. Further, we measured c-Fos immunostaining as a marker of serotonergic neuronal activation and tissue 5-hydroxyindoleacetic acid concentrations as a marker of serotonin functional output. Results showed that Fgf8 hypomorphs exhibited 1) an exaggerated response of DR anxiety-promoting circuits and 2) a blunted response of a DR panic-inhibiting circuit to stress, effects that together were associated with increased baseline anxiety-like behavior. Overall, our results provide a neural substrate upon which Fgf8 deficiency could affect stress response and support the hypothesis that developmental disruptions of serotonergic neurons affect their postnatal functional integrity.

Individual differences in the neurobiology of social stress: implications for depression-cardiovascular disease comorbidity

Stress initiates a cascade of complex neural and peripheral changes that promote healthy adaption to stress, but when unabated, leads to pathology. Fascinating individual differences arise in the ability to cope with a stressor, rendering an individual more or less likely to develop stress-induced pathologies such as depression, anxiety, and cardiovascular disease. In this review we evaluate recent findings that investigate the neural underpinnings of adopting a passive or active coping response during social defeat stress. Because passive coping is associated with vulnerability to stress-related pathologies and active coping confers resiliency, understanding neurobiological adaptations associated with these diverse coping strategies may reveal biomarkers or targets impacting stress susceptibility. The co-occurrence of stress-induced depression and cardiovascular disease is becoming increasingly clear. Therefore this review focuses on the central mechanisms capable of contributing to psychopathology and cardiovascular disease such as corticotropin releasing factor, neuropeptide Y, monoamines, cytokines and oxidative stress. The impetus for this review is to highlight neurobiological systems that warrant further evaluation for their contribution to the pathophysiology of depression-cardiovascular disease comorbidity.

CRF type 1 receptors of the medial amygdala modulate inhibitory avoidance responses in the elevated T-maze

Corticotropin-releasing factor (CRF) plays a critical role in the mediation of physiological and behavioral responses to stressors. In the present study, we investigated the role played by the CRF system within the medial amygdala (MeA) in the modulation of anxiety and fear-related responses. Male Wistar rats were bilaterally administered into the MeA with CRF (125 and 250 ng/0.2μl, experiment 1) or with the CRFR1 antagonist antalarmin (25 ng/0.2 μl, experiment 2) and 10 min later tested in the elevated T-maze (ETM) for inhibitory avoidance and escape measurements. In clinical terms, these responses have been respectively related to generalized anxiety and panic disorder. To further verify if the anxiogenic effects of CRF were mediated by CRFR1 activation, we also investigated the effects of the combined treatment with CRF (250 ng/0.2 μl) and antalarmin (25 ng/0.2 μl) (experiment 3). All animals were tested in an open field, immediately after the ETM, for locomotor activity assessment. Results showed that CRF, in the two doses administered, facilitated ETM avoidance, an anxiogenic response. Antalarmin significantly decreased avoidance latencies, an anxiolytic effect, and was able to counteract the anxiogenic effects of CRF. None of the compounds administered altered escape responses or locomotor activity measurements. These results suggest that CRF in the MeA exerts anxiogenic effects by activating type 1 receptors, which might be of relevance to the physiopathology of generalized anxiety disorder.

Increased anxiety in corticotropin-releasing factor type 2 receptor-null mice requires recent acute stress exposure and is associated with dysregulated serotonergic activity in limbic brain areas

Background

Corticotropin-releasing factor type 2 receptors (CRFR2) are suggested to facilitate successful recovery from stress to maintain mental health. They are abundant in the midbrain raphe nuclei, where they regulate serotonergic neuronal activity and have been demonstrated to mediate behavioural consequences of stress. Here, we describe behavioural and serotonergic responses consistent with maladaptive recovery from stressful challenge in CRFR2-null mice.

Results

CRFR2-null mice showed similar anxiety levels to control mice before and immediately after acute restraint stress, and also after cessation of chronic stress. However, they showed increased anxiety by 24 hours after restraint, whether or not they had been chronically stressed.

Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents were quantified and the level of 5-HIAA in the caudal dorsal raphe nucleus (DRN) was increased under basal conditions in CRFR2-null mice, indicating increased 5-HT turnover. Twenty-four hours following restraint, 5-HIAA was decreased only in CRFR2-null mice, suggesting that they had not fully recovered from the challenge. In efferent limbic structures, CRFR2-null mice showed lower levels of basal 5-HT in the lateral septum and subiculum, and again showed a differential response to restraint stress from controls.

Local cerebral glucose utilization (LCMRglu) revealed decreased neuronal activity in the DRN of CRFR2-null mice under basal conditions. Following 5-HT receptor agonist challenge, LCMRglu responses indicated that 5-HT_1A receptor responses in the DRN were attenuated in CRFR2-null mice. However, postsynaptic 5-HT receptor responses in forebrain regions were intact.

Conclusions

These results suggest that CRFR2 are required for proper functionality of 5-HT_1A receptors in the raphe nuclei, and are key to successful recovery from stress. This disrupted serotonergic function in CRFR2-null mice likely contributes to their stress-sensitive phenotype. The 5-HT content in lateral septum and subiculum was notably altered. These areas are important for anxiety, and are also implicated in reward and the pathophysiology of addiction. The role of CRFR2 in stress-related psychopathologies deserves further consideration.

Gene-environment interactions and intermediate phenotypes: early trauma and depression

This review focuses on current research developments in the study of gene by early life stress (ELS) interactions and depression. ELS refers to aversive experiences during childhood and adolescence such as sexual, physical or emotional abuse, emotional or physical neglect as well as parental loss. Previous research has focused on investigating and characterizing the specific role of ELS within the pathogenesis of depression and linking these findings to neurobiological changes of the brain, especially the stress response system. The latest findings highlight the role of genetic factors that increase vulnerability or, likewise, promote resilience to depression after childhood trauma. Considering intermediate phenotypes has further increased our understanding of the complex relationship between early trauma and depression. Recent findings with regard to epigenetic changes resulting from adverse environmental events during childhood promote current endeavors to identify specific target areas for prevention and treatment schemes regarding the long-term impact of ELS. Taken together, the latest research findings have underscored the essential role of genotypes and epigenetic processes within the development of depression after childhood trauma, thereby building the basis for future research and clinical interventions.

Anxiolytic-like effects of antisauvagine-30 in mice are not mediated by CRF2 receptors

The role of brain corticotropin-releasing factor type 2 (CRF₂) receptors in behavioral stress responses remains controversial. Conflicting findings suggest pro-stress, anti-stress or no effects of impeding CRF₂ signaling. Previous studies have used antisauvagine-30 as a selective CRF₂ antagonist. The present study tested the hypotheses that 1) potential anxiolytic-like actions of intracerebroventricular (i.c.v.) administration of antisauvagine-30 also are present in mice lacking CRF₂ receptors and 2) potential anxiolytic-like effects of antisauvagine-30 are not shared by the more selective CRF₂ antagonist astressin₂-B. Cannulated, male CRF₂ receptor knockout (n = 22) and wildtype littermate mice (n = 21) backcrossed onto a C57BL/6J genetic background were tested in the marble burying, elevated plus-maze, and shock-induced freezing tests following pretreatment (i.c.v.) with vehicle, antisauvagine-30 or astressin₂-B. Antisauvagine-30 reduced shock-induced freezing equally in wildtype and CRF₂ knockout mice. In contrast, neither astressin₂-B nor CRF₂ genotype influenced shock-induced freezing. Neither CRF antagonist nor CRF₂ genotype influenced anxiety-like behavior in the plus-maze or marble burying tests. A literature review showed that the typical antisauvagine-30 concentration infused in previous intracranial studies (∼1 mM) was 3 orders greater than its IC₅₀ to block CRF₁-mediated cAMP responses and 4 orders greater than its binding constants (K_d, K_i) for CRF₁ receptors. Thus, increasing, previously used doses of antisauvagine-30 also exert non-CRF₂-mediated effects, perhaps via CRF₁. The results do not support the hypothesis that brain CRF₂ receptors tonically promote anxiogenic-like behavior. Utilization of CRF₂ antagonists, such as astressin₂-B, at doses that are more subtype-selective, can better clarify the significance of brain CRF₂ systems in stress-related behavior.

The role of neuropeptides in suicidal behavior: a systematic review

There is a growing evidence that neuropeptides may be involved in the pathophysiology of suicidal behavior. A critical review of the literature was conducted to investigate the association between neuropeptides and suicidal behavior. Only articles from peer-reviewed journals were selected for the inclusion in the present review. Twenty-six articles were assessed for eligibility but only 22 studies were included. Most studies have documented an association between suicidality and some neuropeptides such as corticotropin-releasing factor (CRF), VGF, cholecystokinin, substance P, and neuropeptide Y (NPY), which have been demonstrated to act as key neuromodulators of emotional processing. Significant differences in neuropeptides levels have been found in those who have attempted or completed suicide compared with healthy controls or those dying from other causes. Despite cross-sectional associations between neuropeptides levels and suicidal behavior, causality may not be inferred. The implications of the mentioned studies were discussed in this review paper.