Effects of corticotropin-releasing factor on neuronal activity in the serotonergic dorsal raphe nucleus

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.

Peri-ictal activation of dorsomedial dorsal raphe serotonin neurons reduces mortality associated with maximal electroshock seizures

Over one-third of patients with epilepsy will develop refractory epilepsy and continue to experience seizures despite medical treatment. These patients are at the greatest risk for sudden unexpected death in epilepsy. The precise mechanisms underlying sudden unexpected death in epilepsy are unknown, but cardiorespiratory dysfunction and arousal impairment have been implicated. Substantial circumstantial evidence suggests serotonin is relevant to sudden unexpected death in epilepsy as it modulates sleep/wake regulation, breathing and arousal. The dorsal raphe nucleus is a major serotonergic center and a component of the ascending arousal system. Seizures disrupt the firing of dorsal raphe neurons, which may contribute to reduced responsiveness. However, the relevance of the dorsal raphe nucleus and its subnuclei to sudden unexpected death in epilepsy remains unclear. The dorsomedial dorsal raphe may be a salient target due to its role in stress and its connections with structures implicated in sudden unexpected death in epilepsy. We hypothesized that optogenetic activation of dorsomedial dorsal raphe serotonin neurons in TPH2-ChR2-YFP (n = 26) mice and wild-type (n = 27) littermates before induction of a maximal electroshock seizure would reduce mortality. In this study, pre-seizure activation of dorsal raphe nucleus serotonin neurons reduced mortality in TPH2-ChR2-YFP mice with implants aimed at the dorsomedial dorsal raphe. These results implicate the dorsomedial dorsal raphe in this novel circuit influencing seizure-induced mortality. It is our hope that these results and future experiments will define circuit mechanisms that could ultimately reduce sudden unexpected death in epilepsy.

Alteration of serotonin release response in the central nucleus of the amygdala to noxious and non-noxious mechanical stimulation in a neuropathic pain model rat

Previously, we found that serotonin (5-HT) release in the central nucleus of the amygdala (CeA) of anesthetized rats decreases in response to innocuous stroking of the skin, irrespective of stimulus laterality, but increases in response to noxious pinching applied to a hindlimb contralateral to the 5-HT measurement site. The aim of the present study was to determine whether intra-CeA 5-HT release responses to cutaneous stimulation were altered in an animal model of neuropathic pain induced by ligation of the left L5 spinal nerve. In anesthetized neuropathic pain model rats, stroking of the left hindlimb increased 5-HT release in the CeA, whereas stroking of the right hindlimb decreased it. Meanwhile, pinching of the left hindlimb increased intra-CeA 5-HT release irrespective of stimulus laterality. In conclusion, the present study demonstrated that intra-CeA 5-HT release responses to cutaneous stimulation are altered in an animal model of neuropathic pain.

Association between RMTg Neuropeptide Genes and Negative Effect during Alcohol Withdrawal in Mice

Alcohol use disorders (AUDs) frequently co-occur with negative mood disorders, such as anxiety and depression, exacerbating relapse through dopaminergic dysfunction. Stress-related neuropeptides play a crucial role in AUD pathophysiology by modulating dopamine (DA) function. The rostromedial tegmental nucleus (RMTg), which inhibits midbrain dopamine neurons and signals aversion, has been shown to increase ethanol consumption and negative emotional states during abstinence. Despite some stress-related neuropeptides acting through the RMTg to affect addiction behaviors, their specific roles in alcohol-induced contexts remain underexplored. This study utilized an intermittent voluntary drinking model in mice to induce negative effect behavior 24 h into ethanol (EtOH) abstinence (post-EtOH). It examined changes in pro-stress (Pnoc, Oxt, Npy) and anti-stress (Crf, Pomc, Avp, Orx, Pdyn) neuropeptide-coding genes and analyzed their correlations with aversive behaviors. We observed that adult male C57BL/6J mice displayed evident anxiety, anhedonia, and depression-like symptoms at 24 h post-EtOH. The laser-capture microdissection technique, coupled with or without retrograde tracing, was used to harvest total ventral tegmental area (VTA)-projecting neurons or the intact RMTg area. The findings revealed that post-EtOH consistently reduced Pnoc and Orx levels while elevating Crf levels in these neuronal populations. Notably, RMTg Pnoc and Npy levels counteracted ethanol consumption and depression severity, while Crf levels were indicative of the mice's anxiety levels. Together, these results underscore the potential role of stress-related neuropeptides in the RMTg in regulating the negative emotions related to AUDs, offering novel insights for future research.

Musclin prevents depression-like behavior in male mice by activating urocortin 2 signaling in the hypothalamus

Introduction

Physical activity is recommended as an alternative treatment for depression. Myokines, which are secreted from skeletal muscles during physical activity, play an important role in the skeletal muscle-brain axis. Musclin, a newly discovered myokine, exerts physical endurance, however, the effects of musclin on emotional behaviors, such as depression, have not been evaluated. This study aimed to access the anti-depressive effect of musclin and clarify the connection between depression-like behavior and hypothalamic neuropeptides in mice.

Methods

We measured the immobility time in the forced swim (FS) test, the time spent in open arm in the elevated-plus maze (EPM) test, the mRNA levels of hypothalamic neuropeptides, and enumerated the c-Fos-positive cells in the paraventricular nucleus (PVN), arcuate nucleus (ARC), and nucleus tractus solitarii (NTS) in mice with the intraperitoneal (i.p.) administration of musclin. Next, we evaluated the effects of a selective corticotropin-releasing factor (CRF) type 1 receptor antagonist, selective CRF type 2 receptor antagonist, melanocortin receptor (MCR) agonist, and selective melanocortin 4 receptor (MC4R) agonist on changes in behaviors induced by musclin. Finally we evaluated the antidepressant effect of musclin using mice exposed to repeated water immersion (WI) stress.

Results

We found that the i.p. and i.c.v. administration of musclin decreased the immobility time and relative time in the open arms (open %) in mice and increased urocortin 2 (Ucn 2) levels but decreased proopiomelanocortin levels in the hypothalamus. The numbers of c-Fos-positive cells were increased in the PVN and NTS but decreased in the ARC of mice with i.p. administration of musclin. The c-Fos-positive cells in the PVN were also found to be Ucn 2-positive. The antidepressant and anxiogenic effects of musclin were blocked by central administration of a CRF type 2 receptor antagonist and a melanocortin 4 receptor agonist, respectively. Peripheral administration of musclin also prevented depression-like behavior and the decrease in levels of hypothalamic Ucn 2 induced by repeated WI stress.

Discussion

These data identify the antidepressant effects of musclin through the activation of central Ucn 2 signaling and suggest that musclin and Ucn 2 can be new therapeutic targets and endogenous peptides mediating the muscle-brain axis.

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.

Progress in Personalized Psychiatric Therapy with the Example of Using Intranasal Oxytocin in PTSD Treatment

Post-traumatic stress disorder (PTSD) is a severe mental disorder that results in the frequent coexistence of other diseases, lowers patients’ quality of life, and has a high annual cost of treatment. However, despite the variety of therapeutic approaches that exist, some patients still do not achieve the desired results. In addition, we may soon face an increase in the number of new PTSD cases because of the current global situation—both the COVID-19 pandemic and the ongoing armed conflicts. Hence, in recent years, many publications have sought a new, more personalized treatment approach. One such approach is the administration of intranasal oxytocin (INOXT), which, due to its pleiotropic effects, seems to be a promising therapeutic option. However, the current findings suggest that it might only be helpful for a limited, strictly selected group of patients.

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.

The hypothalamus and its role in hypertension

For the majority of hypertensive patients, the etiology of their disease is unknown. The hypothalamus is a central structure of the brain which provides an adaptive, integrative, autonomic, and neuroendocrine response to any fluctuations in physiological conditions of the external or internal environment. Hypothalamic insufficiency leads to severe metabolic and functional disorders, including persistent increase in blood pressure. Here, we discuss alterations in the neurochemical organization of the paraventricular and suprachiasmatic nucleus in the hypothalamus of patients who suffered from essential hypertension and died suddenly due to acute coronary failure. The changes observed are hypothesized to contribute to the pathogenesis of disease.

CRF-5-HT interactions in the dorsal raphe nucleus and motivation for stress-induced opioid reinstatement

RATIONALE

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Our previous data show that stressors can inhibit 5-HT neuronal activity and release by stimulating the release of the stress neurohormone corticotropin-releasing factor (CRF) within the serotonergic dorsal raphe nucleus (DRN). The inhibitory effects of CRF on 5-HT DRN neurons are indirect, mediated by CRF-R1 receptors located on GABAergic afferents.

OBJECTIVES

We tested the hypothesis that DRN CRF-R1 receptors contribute to stress-induced reinstatement of morphine-conditioned place preference (CPP). We also examined the role of this circuitry in stress-induced negative affective state with 22-kHz distress ultrasonic vocalizations (USVs), which are naturally emitted by rats in response to environmental challenges such as pain, stress, and drug withdrawal.

METHODS

First, we tested if activation of CRF-R1 receptors in the DRN with the CRF-R1-preferring agonist ovine CRF (oCRF) would reinstate morphine CPP and then if blockade of CRF-R1 receptors in the DRN with the CRF-R1 antagonist NBI 35965 would attenuate swim stress-induced reinstatement of morphine CPP. Second, we tested if intra-DRN pretreatment with NBI 35965 would attenuate foot shock stress-induced 22-kHz USVs.

RESULTS

Intra-DRN injection of oCRF reinstated morphine CPP, while intra-DRN injection of NBI 35965 attenuated swim stress-induced reinstatement. Moreover, intra-DRN pretreatment with NBI 35965 significantly reduced 22-kHz distress calls induced by foot shock.

CONCLUSIONS

These data provide evidence that stress-induced negative affective state is mediated by DRN CRF-R1 receptors and may contribute to reinstatement of morphine CPP.

Protagonist Role of Opioidergic System on Post-Traumatic Stress Disorder and Associated Pain

OBJECTIVE

Post-traumatic stress disorder (PTSD) and chronic pain often co-occur. Studies have shown an interaction between pain and PTSD. In this narrative review, we aim to support conducting comprehensive studies by describing PTSD, pain and determining whether opioidergic system, its agonist and antagonist manipulation could positively or negatively affect PTSD symptoms and concurrent pain.

METHODS

Term searches was done in Google Scholar, Scopus, ScienceDirect, Web of Science and PubMed databases as well as hand searching in key resource journals from 1979-2019.

RESULTS

There are a lot of contradictions and disputes when endogenous opioidergic system and opioidergic antagonist system are studied in PTSD patients. Exogenous morphine administration in PTSD patients can decrease the symptoms of PTSD but it doesn't have a pain reduction effect to an acceptable level. Beta-endorphin as an endogenous opioid is effective in pain reduction in the moment of events but after minutes to hours, the endorphins withdrawal syndrome leads to exaggerated intrusive thoughts and flashbacks of PTSD, which exacerbate the pain. It has also been shown that naloxone, as an opioidergic antagonist, can reduce or increase the PTSD symptoms and its associated pain.

CONCLUSION

Data suggest different roles of opioidergic system and their antagonist in pain control and mood in PTSD. However, further investigations need to be done in order to reveal the role of endogenous opioidergic system and opioidergic antagonist system as a mediator in PTSD patients suffering from acute or chronic pain.

Embracing diversity in the 5-HT neuronal system

Neurons that synthesize and release 5-hydroxytryptamine (5-HT; serotonin) express a core set of genes that establish and maintain this neurotransmitter phenotype and distinguish these neurons from other brain cells. Beyond a shared 5-HTergic phenotype, these neurons display divergent cellular properties in relation to anatomy, morphology, hodology, electrophysiology and gene expression, including differential expression of molecules supporting co-transmission of additional neurotransmitters. This diversity suggests that functionally heterogeneous subtypes of 5-HT neurons exist, but linking subsets of these neurons to particular functions has been technically challenging. We discuss recent data from molecular genetic, genomic and functional methods that, when coupled with classical findings, yield a reframing of the 5-HT neuronal system as a conglomeration of diverse subsystems with potential to inspire novel, more targeted therapies for clinically distinct 5-HT-related disorders.

GABAergic modulation of serotonergic neurons in the dorsal raphe nucleus

The dorsal raphe nucleus (DRN), located in the brainstem, is involved in several functions such as sleep, temperature regulation, stress responses, and anxiety behaviors. This nucleus contains the largest population of serotonin expressing neurons in the brain. Serotonergic DRN neurons receive tonic γ-aminobutyric acid (GABA)inhibitory inputs from several brain areas, as well as from interneurons within the same nucleus. Serotonergic and GABAergic neurons in the DRN can be distinguished by their size, location, pharmacological responses, and electrophysiological properties. GABAergic neurons regulate the excitability of DRN serotonergic neurons and the serotonin release in different brain areas. Also, it has been shown that GABAergic neurons can synchronize the activity of serotonergic neurons across functions such as sleep or alertness. Moreover, dysregulation of GABA signaling in the DRN has been linked to psychiatric disorders such as anxiety and depression. This review focuses on GABAergic transmission in the DRN. The interaction between GABAergic and serotonergic neurons is discussed considering some physiological implications. Also, the main electrophysiological and morphological characteristics of serotonergic and GABAergic neurons are described.

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.

The hypothalamic-pituitary-adrenal axis in PTSD: Pathophysiology and treatment interventions

Questions of how altered functioning of the hypothalamic pituitary adrenal (HPA) axis contribute to the development and maintenance of posttraumatic stress disorder (PTSD) have been the focus of extensive animal and human research. As a rule, results have been inconsistent across studies, likely due to a variety of confounding variables that have received inadequate attention. Important confounding factors include the effects of early life stress, biological sex, and the glucocorticoid used for interventions. In this manuscript we review: 1) the literature on identified abnormalities of HPA axis function in PTSD, both in terms of basal functioning and as part of challenge paradigms; 2) the role of HPA axis function pre- and immediately post-trauma as a risk factor for PTSD development; 3) the impact of HPA axis genes' allelic variants and epigenetic modifications on PTSD risk; 4) the contributions of HPA axis components to fear learning and extinction; and 5) therapeutic manipulations of the HPA axis to both prevent and treat PTSD, including the role of glucocorticoids as part of medication enhanced psychotherapy.

Evidence that preimmunization with a heat-killed preparation of Mycobacterium vaccae reduces corticotropin-releasing hormone mRNA expression in the extended amygdala in a fear-potentiated startle paradigm

Posttraumatic stress disorder (PTSD) is a trauma and stressor-related disorder that is characterized by dysregulation of glucocorticoid signaling, chronic low-grade inflammation, and impairment in the ability to extinguish learned fear. Corticotropin-releasing hormone (Crh) is a stress- and immune-responsive neuropeptide secreted from the paraventricular nucleus of the hypothalamus (PVN) to stimulate the hypothalamic-pituitary-adrenal (HPA) axis; however, extra-hypothalamic sources of Crh from the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST) govern specific fear- and anxiety-related defensive behavioral responses. We previously reported that preimmunization with a heat-killed preparation of the immunoregulatory environmental bacterium Mycobacterium vaccae NCTC 11659 enhances fear extinction in a fear-potentiated startle (FPS) paradigm. In this follow-up study, we utilized an in situ hybridization histochemistry technique to investigate Crh, Crhr1, and Crhr2 mRNA expression in the CeA, BNST, and PVN of the same rats from the original study [Fox et al., 2017, Brain, Behavior, and Immunity, 66: 70-84]. Here, we demonstrate that preimmunization with M. vaccae NCTC 11659 decreases Crh mRNA expression in the CeA and BNST of rats exposed to the FPS paradigm, and, further, that Crh mRNA expression in these regions is correlated with fear behavior during extinction training. These data are consistent with the hypothesis that M. vaccae promotes stress-resilience by attenuating Crh production in fear- and anxiety-related circuits. These data suggest that immunization with M. vaccae may be an effective strategy for prevention of fear- and anxiety-related disorders.

CRF modulation of central monoaminergic function: Implications for sex differences in alcohol drinking and anxiety

Decades of research have described the importance of corticotropin-releasing factor (CRF) signaling in alcohol addiction, as well as in commonly co-expressed neuropsychiatric diseases, including anxiety and mood disorders. However, CRF signaling can also acutely regulate binge alcohol consumption, anxiety, and affect in non-dependent animals, possibly via modulation of central monoaminergic signaling. We hypothesize that basal CRF tone is particularly high in animals and humans with an inherent propensity for high anxiety and alcohol consumption, and thus these individuals are at increased risk for the development of alcohol use disorder and comorbid neuropsychiatric diseases. The current review focuses on extrahypothalamic CRF circuits, particularly those stemming from the bed nucleus of the stria terminalis (BNST), found to play a role in basal phenotypes, and examines whether the intrinsic hyperactivity of these circuits is sufficient to escalate the expression of these behaviors and steepen the trajectory of development of disease states. We focus our efforts on describing CRF modulation of biogenic amine neuron populations that have widespread projections to the forebrain to modulate behaviors, including alcohol and drug intake, stress reactivity, and anxiety. Further, we review the known sex differences and estradiol modulation of these neuron populations and CRF signaling at their synapses to address the question of whether females are more susceptible to the development of comorbid addiction and stress-related neuropsychiatric diseases because of hyperactive extrahypothalamic CRF circuits compared to males.

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.

MicroRNA-34 Contributes to the Stress-related Behavior and Affects 5-HT Prefrontal/GABA Amygdalar System through Regulation of Corticotropin-releasing Factor Receptor 1

Recent studies show that microRNA-34 (miR-34) family is critical in the regulation of stress response also suggesting that it may contribute to the individual responsiveness to stress. We have recently demonstrated that mice carrying a genetic deletion of all miR-34 isoforms (triple knockout, TKO) lack the stress-induced serotonin (5-HT) and GABA release in the medial prefrontal cortex (mpFC) and basolateral amygdala (BLA), respectively. Here, we evaluated if the absence of miR-34 was also able to modify the stress-coping strategy in the forced swimming test. We found that the blunted neurochemical response to stress was associated with lower levels of immobility (index of active coping behavior) in TKO compared to WT mice. Interestingly, among the brain regions mostly involved in the stress-related behaviors, the miR-34 displayed the strongest expression in the dorsal raphe nuclei (DRN) of wild-type (WT) mice. In the DRN, the corticotropin-releasing factor receptors (CRFR) 1 and 2, contribute to determine the stress-coping style and the CRFR1 is a target of miR-34. Thus, we hypothesized that the miR-34-dependent modulation of CRFR1 expression may be involved in the DRN regulation of stress-coping strategies. In line with this hypothesis, we found increased CRFR1 levels in the DNR of TKO compared to WT mice. Moreover, infusion of CRFR1 antagonist in the DRN of TKO mice reverted their behavioral and neurochemical phenotype. We propose that miR-34 modulate the mpFC 5-HT/BLA GABA response to stress acting on CRFR1 in the DRN and that this mechanism could contribute to determine individual stress-coping strategy.

Corticotropin-Releasing Factor Receptor 1 Antagonism Is Ineffective for Women With Posttraumatic Stress Disorder

BACKGROUND

Medication and psychotherapy treatments for posttraumatic stress disorder (PTSD) provide insufficient benefit for many patients. Substantial preclinical and clinical data indicate abnormalities in the hypothalamic-pituitary-adrenal axis, including signaling by corticotropin-releasing factor, in the pathophysiology of PTSD.

METHODS

We conducted a double-blind, placebo-controlled, randomized, fixed-dose clinical trial evaluating the efficacy of GSK561679, a corticotropin-releasing factor receptor 1 (CRF₁ receptor) antagonist in adult women with PTSD. The trial randomized 128 participants, of whom 96 completed the 6-week treatment period.

RESULTS

In both the intent-to-treat and completer samples, GSK561679 failed to show superiority over placebo on the primary outcome of change in Clinician-Administered PTSD Scale total score. Adverse event frequencies did not significantly differ between GSK561679- and placebo-treated subjects. Exploration of the CRF₁ receptor single nucleotide polymorphism rs110402 found that response to GSK561679 and placebo did not significantly differ by genotype alone. However, subjects who had experienced a moderate or severe history of childhood abuse and who were also GG homozygotes for rs110402 showed significant improvement after treatment with GSK561679 (n = 6) but not with placebo (n = 7) on the PTSD Symptom Scale-Self-Report.

CONCLUSIONS

The results of this trial, the first evaluating a CRF₁ receptor antagonist for the treatment of PTSD, combined with other negative trials of CRF₁ receptor antagonists for major depressive disorder, generalized anxiety disorder, and social anxiety disorder, suggest that CRF₁ receptor antagonists lack efficacy as monotherapy agents for these conditions.

Somatosensory regulation of serotonin release in the central nucleus of the amygdala is mediated via corticotropin releasing factor and gamma-aminobutyric acid in the dorsal raphe nucleus

Noxious cutaneous stimulation increases, whereas innocuous cutaneous stimulation decreases serotonin (5-HT) release in the central nucleus of the amygdala (CeA) in anesthetized rats. In the present study, we investigated the contribution of corticotropin releasing factor (CRF) receptors and gamma-aminobutyric acid (GABA) receptors in the dorsal raphe nucleus (DRN) to those responses. Release of 5-HT in the CeA was monitored by microdialysis before and after 10-min stimulation by pinching or stroking. Increased 5-HT release in the CeA in response to pinching was abolished by CRF2 receptor antagonism in the DRN. Decreased 5-HT release in the CeA in response to stroking was abolished by either CRF1 receptor antagonism or GABAA receptor antagonism in the DRN. These results suggest that opposite responses of 5-HT release in the CeA to noxious versus innocuous stimulation of the skin are due to separate contributions of CRF2, CRF1 and GABAA receptors in the DRN.

Escalated cocaine "binges" in rats: enduring effects of social defeat stress or intra-VTA CRF

RATIONALE

Exposure to intermittent social defeat stress elicits corticotropin releasing factor (CRF) release into the VTA and induces long-term modulation of mesocorticolimbic dopamine activity in rats. These adaptations are associated with an intense cocaine-taking phenotype, which is prevented by CRF receptor antagonists.

OBJECTIVE

The present studies examine whether infusion of CRF into the VTA is sufficient to escalate cocaine-taking behavior, in the absence of social defeat experience. Additionally, we aimed to characterize changes in cocaine valuation that may promote binge-like cocaine intake.

METHODS

Male Long-Evans rats were microinjected into the VTA with CRF (50 or 500 ng/side), vehicle, or subjected to social defeat stress, intermittently over 10 days. Animals were then trained to self-administer IV cocaine (FR5). Economic demand for cocaine was evaluated using a within-session behavioral-economics threshold procedure, which was followed by a 24-h extended access "binge."

RESULTS

Rats that experienced social defeat or received intra-VTA CRF microinfusions (50 ng) both took significantly more cocaine than controls over the 24-h binge but showed distinct patterns of intake. Behavioral economic analysis revealed that individual demand for cocaine strongly predicts binge-like consumption, and demand elasticity (i.e. α) is augmented by intra-VTA CRF, but not by social defeat. The effects of CRF on cocaine-taking were also prevented by intra-VTA pretreatment with CP376395, but not Astressin-2B.

CONCLUSIONS

Repeated infusion of CRF into the VTA persistently alters cocaine valuation and intensifies binge-like drug intake in a CRF-R1-dependent manner. Conversely, the persistent pattern of cocaine bingeing induced by social defeat stress may suggest impaired inhibitory control, independent of reward valuation.

Pain Relief in Depressive Disorders: A Meta-Analysis of the Effects of Antidepressants

BACKGROUND

Pain is a common symptom in patients with depressive disorders, which, if present, worsens the prognosis. However, there is little empirical knowledge of the therapeutic effects of antidepressants on painful physical symptoms of patients with depressive disorders. Furthermore, tricyclic/tetracyclic antidepressants (TCAs) have not yet been included in existing meta-analyses.

METHODS

A broad, systematic search of PubMed literature on antidepressant drug treatment of patients with depressive disorders with comorbid pain symptoms was carried out. A random-effects meta-analysis has been performed among 3 different groups of drugs for the 2 end points: pain and depression.

RESULTS

Fourteen placebo-controlled studies with selective serotonin-noradrenaline reuptake inhibitors (SSNRIs) could be included, with 3 of them also investigating selective serotonin reuptake inhibitors (SSRIs). Three further placebo-controlled SSRI studies were identified, but only 2 placebo-controlled TCA studies.Both SSNRIs and SSRIs, but not TCAs, were significantly superior to placebo as regards their analgesic effects. However, all effects were small. For SSNRIs, there was a strong positive correlation between their effectiveness for pain relief and their positive effect on the mood of the patients.

DISCUSSION

The analgesic effects of SSNRIs and SSRIs in patients with primary depressive disorders can be interpreted as largely equivalent. Because of a lack of placebo-controlled TCA studies, the results for TCAs would be comparable only to those of SSRIs and SSNRIs, if non-placebo-controlled TCA studies were included. The positive correlation found indicates a close relationship of pain relief and antidepressant treatment effects. These results refer merely to patients with primary depressive disorders, not to patients with primary pain disorders. Further studies comparing the effects of different types of antidepressant drugs on pain in depressive patients are warranted.

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.

Early adverse experience and substance addiction: dopamine, oxytocin, and glucocorticoid pathways

Substance addiction may follow a chronic, relapsing course and critically undermine the physical and psychological well-being of the affected individual and the social units of which the individual is a member. Despite the public health burden associated with substance addiction, treatment options remain suboptimal, with relapses often seen. The present review synthesizes growing insights from animal and human research to shed light upon developmental and neurobiological pathways that may increase susceptibility to addiction. We examine the dopamine system, the oxytocin system, and the glucocorticoid system, as they are particularly relevant to substance addiction. Our aim is to delineate how early adverse experience may induce long-lasting alterations in each of these systems at molecular, neuroendocrine, and behavioral levels and ultimately lead to heightened vulnerability to substance addiction. We further discuss how substance addiction in adulthood may increase the risk of suboptimal caregiving for the next generation, perpetuating the intergenerational cycle of early adverse experiences and addiction.

Alterations in the corticotropin-releasing hormone (CRH) neurocircuitry: Insights into post stroke functional impairments

Although it is well accepted that changes in the regulation of the hypothalamic-pituitary adrenal (HPA) axis may increase susceptibility to affective disorders in the general population, this link has been less examined in stroke patients. Yet, the bidirectional association between depression and cardiovascular disease is strong, and stress increases vulnerability to stroke. Corticotropin-releasing hormone (CRH) is the central stress hormone of the HPA axis pathway and acts by binding to CRH receptors (CRHR) 1 and 2, which are located in several stress-related brain regions. Evidence from clinical and animal studies suggests a role for CRH in the neurobiological basis of depression and ischemic brain injury. Given its importance in the regulation of the neuroendocrine, autonomic, and behavioral correlates of adaptation and maladaptation to stress, CRH is likely associated in the pathophysiology of post stroke emotional impairments. The goals of this review article are to examine the clinical and experimental data describing (1) that CRH regulates the molecular signaling brain circuit underlying anxiety- and depression-like behaviors, (2) the influence of CRH and other stress markers in the pathophysiology of post stroke emotional and cognitive impairments, and (3) context and site specific interactions of CRH and BDNF as a basis for the development of novel therapeutic targets. This review addresses how the production and release of the neuropeptide CRH within the various regions of the mesocorticolimbic system influences emotional and cognitive behaviors with a look into its role in psychiatric disorders post stroke.

Serotonergic Neuroplasticity in Alcohol Addiction

Alcohol addiction is a debilitating disorder producing maladaptive changes in the brain, leading drinkers to become more sensitive to stress and anxiety. These changes are key factors contributing to alcohol craving and maintaining a persistent vulnerability to relapse.

Serotonin (5-Hydroxytryptamine, 5-HT) is a monoamine neurotransmitter widely expressed in the central nervous system where it plays an important role in the regulation of mood. The serotonin system has been extensively implicated in the regulation of stress and anxiety, as well as the reinforcing properties of all of the major classes of drugs of abuse, including alcohol. Dysregulation within the 5-HT system has been postulated to underlie the negative mood states associated with alcohol use disorders.

This review will describe the serotonergic (5-HTergic) neuroplastic changes observed in animal models throughout the alcohol addiction cycle, from prenatal to adulthood exposure. The first section will focus on alcohol-induced 5-HTergic neuroadaptations in offspring prenatally exposed to alcohol and the consequences on the regulation of stress/anxiety. The second section will compare alterations in 5-HT signalling induced by acute or chronic alcohol exposure during adulthood and following alcohol withdrawal, highlighting the impact on the regulation of stress/anxiety signalling pathways. The third section will outline 5-HTergic neuroadaptations observed in various genetically-selected ethanol preferring rat lines. Finally, we will discuss the pharmacological manipulation of the 5-HTergic system on ethanol- and anxiety/stress-related behaviours demonstrated by clinical trials, with an emphasis on current and potential treatments.

Sex-dependent programming effects of prenatal glucocorticoid treatment on the developing serotonin system and stress-related behaviors in adulthood

Prenatal stress and overexposure to glucocorticoids (GC) during development may be associated with an increased susceptibility to a number of diseases in adulthood including neuropsychiatric disorders, such as depression and anxiety. In animal models, prenatal overexposure to GC results in hyper-responsiveness to stress in adulthood, and females appear to be more susceptible than males. Here, we tested the hypothesis that overexposure to GC during fetal development has sex-specific programming effects on the brain, resulting in altered behaviors in adulthood. We examined the effects of dexamethasone (DEX; a synthetic GC) during prenatal life on stress-related behaviors in adulthood and on the tryptophan hydroxylase-2 (TpH2) gene expression in the adult dorsal raphe nucleus (DRN). TpH2 is the rate-limiting enzyme for serotonin (5-HT) synthesis and has been implicated in the etiology of human affective disorders. Timed-pregnant rats were treated with DEX from gestational days 18-22. Male and female offspring were sacrificed on the day of birth (postnatal day 0; P0), P7, and in adulthood (P80-84) and brains were examined for changes in TpH2 mRNA expression. Adult animals were also tested for anxiety- and depressive- like behaviors. In adulthood, prenatal DEX increased anxiety- and depressive- like behaviors selectively in females, as measured by decreased time spent in the center of the open field and increased time spent immobile in the forced swim test, respectively. Prenatal DEX increased TpH2 mRNA selectively in the female caudal DRN at P7, whereas it decreased TpH2 mRNA selectively in the female caudal DRN in adulthood. In animals challenged with restraint stress in adulthood, TpH2 mRNA was significantly lower in rostral DRN of prenatal DEX-treated females compared to vehicle-treated females. These data demonstrated that prenatal overexposure to GC alters the development of TpH2 gene expression and these alterations correlated with lasting behavioral changes found in adult female offspring.

Stress-Induced Reinstatement of Drug Seeking: 20 Years of Progress

In human addicts, drug relapse and craving are often provoked by stress. Since 1995, this clinical scenario has been studied using a rat model of stress-induced reinstatement of drug seeking. Here, we first discuss the generality of stress-induced reinstatement to different drugs of abuse, different stressors, and different behavioral procedures. We also discuss neuropharmacological mechanisms, and brain areas and circuits controlling stress-induced reinstatement of drug seeking. We conclude by discussing results from translational human laboratory studies and clinical trials that were inspired by results from rat studies on stress-induced reinstatement. Our main conclusions are (1) The phenomenon of stress-induced reinstatement, first shown with an intermittent footshock stressor in rats trained to self-administer heroin, generalizes to other abused drugs, including cocaine, methamphetamine, nicotine, and alcohol, and is also observed in the conditioned place preference model in rats and mice. This phenomenon, however, is stressor specific and not all stressors induce reinstatement of drug seeking. (2) Neuropharmacological studies indicate the involvement of corticotropin-releasing factor (CRF), noradrenaline, dopamine, glutamate, kappa/dynorphin, and several other peptide and neurotransmitter systems in stress-induced reinstatement. Neuropharmacology and circuitry studies indicate the involvement of CRF and noradrenaline transmission in bed nucleus of stria terminalis and central amygdala, and dopamine, CRF, kappa/dynorphin, and glutamate transmission in other components of the mesocorticolimbic dopamine system (ventral tegmental area, medial prefrontal cortex, orbitofrontal cortex, and nucleus accumbens). (3) Translational human laboratory studies and a recent clinical trial study show the efficacy of alpha-2 adrenoceptor agonists in decreasing stress-induced drug craving and stress-induced initial heroin lapse.

Dissociation of μ-opioid receptor and CRF-R1 antagonist effects on escalated ethanol consumption and mPFC serotonin in C57BL/6J mice

Both the opioid antagonist naltrexone and corticotropin-releasing factor type-1 receptor (CRF-R1) antagonists have been investigated for the treatment of alcoholism. The current study examines the combination of naltrexone and CP154526 to reduce intermittent access ethanol drinking [intermittent access to alcohol (IAA)] in C57BL/6J male mice, and if these compounds reduce drinking via serotonergic mechanisms in the dorsal raphe nucleus (DRN). Systemic injections and chronic intracerebroventricular infusions of naltrexone, CP154526 or CP376395 transiently decreased IAA drinking. Immunohistochemistry revealed CRF-R1 or μ-opioid receptor immunoreactivity was co-localized in tryptophan hydroxylase (TPH)-immunoreactive neurons as well as non-TPH neurons in the DRN. Mice with a history of IAA or continuous access to alcohol were microinjected with artificial cerebral spinal fluid, naltrexone, CP154526 or the combination into the DRN or the median raphe nucleus (MRN). Either intra-DRN naltrexone or CP154526 reduced IAA in the initial 2 hours of fluid access, but the combination did not additively suppress IAA, suggesting a common mechanism via which these two compounds affect intermittent drinking. These alcohol-reducing effects were localized to the DRN of IAA drinkers, as intra-MRN injections only significantly suppressed water drinking, and continuous access drinkers were not affected by CRF-R1 antagonism. Extracellular serotonin was measured in the medial prefrontal cortex (mPFC) using in vivo microdialysis after intra-DRN microinjections in another group of mice. Intra-DRN CP154526 increased serotonin impulse flow to the mPFC while naltrexone did not. This suggests the mPFC may not be an essential location to intermittent drinking, as evidenced by different effects on serotonin signaling to the forebrain yet similar behavioral findings.

Effects of cocaine history on postsynaptic GABA receptors on dorsal raphe serotonin neurons in a stress-induced relapse model in rats

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Stressors and stress hormones can inhibit the dorsal raphe nucleus (DRN)-5-HT system, which composes the majority of forebrain-projecting 5-HT. This inhibition is mediated via stimulation of GABA synaptic activity at DRN-5-HT neurons. Using swim stress-induced reinstatement of morphine conditioned place-preference, recent data from our laboratory indicate that morphine history sensitizes DRN-5-HT neurons to GABAergic inhibitory effects of stress. Moreover, GABAA receptor-mediated inhibition of the serotonergic DRN is required for this reinstatement. In our current experiment, we tested the hypothesis that GABAergic sensitization of DRN-5-HT neurons is a neuroadaptation elicited by multiple classes of abused drugs across multiple models of stress-induced relapse by applying a chemical stressor (yohimbine) to induce reinstatement of previously extinguished cocaine self-administration in Sprague-Dawley rats. Whole-cell patch-clamp recordings of GABA synaptic activity in DRN-5-HT neurons were conducted after the reinstatement. Behavioral data indicate that yohimbine triggered reinstatement of cocaine self-administration. Electrophysiology data indicate that 5-HT neurons in the cocaine group exposed to yohimbine had increased amplitude of inhibitory postsynaptic currents compared to yoked-saline controls exposed to yohimbine or unstressed animals in both drug groups. These data, together with previous findings, indicate that interaction between psychostimulant or opioid history and chemical or physical stressors may increase postsynaptic GABA receptor density and/or sensitivity in DRN-5-HT neurons. Such mechanisms may result in serotonergic hypofunction and consequent dysphoric mood states which confer vulnerability to stress-induced drug reinstatement.

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.

Effects of acute treadmill running at different intensities on activities of serotonin and corticotropin-releasing factor neurons, and anxiety- and depressive-like behaviors in rats

Accumulating evidence suggests that physical exercise can reduce and prevent the incidence of stress-related psychiatric disorders, including depression and anxiety. Activation of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) is implicated in antidepressant/anxiolytic properties. In addition, the incidence and symptoms of these disorders may involve dysregulation of the hypothalamic-pituitary-adrenal axis that is initiated by corticotropin-releasing factor (CRF) neurons in the hypothalamic paraventricular nucleus (PVN). Thus, it is possible that physical exercise produces its antidepressant/anxiolytic effects by affecting these neuronal activities. However, the effects of acute physical exercise at different intensities on these neuronal activation and behavioral changes are still unclear. Here, we examined the activities of 5-HT neurons in the DRN and CRF neurons in the PVN during 30 min of treadmill running at different speeds (high speed, 25 m/min; low speed, 15m/min; control, only sitting on the treadmill) in male Wistar rats, using c-Fos/5-HT or CRF immunohistochemistry. We also performed the elevated plus maze test and the forced swim test to assess anxiety- and depressive-like behaviors, respectively. Acute treadmill running at low speed, but not high speed, significantly increased c-Fos expression in 5-HT neurons in the DRN compared to the control, whereas high-speed running significantly enhanced c-Fos expression in CRF neurons in the PVN compared with the control and low-speed running. Furthermore, low-speed running resulted in decreased anxiety- and depressive-like behaviors compared with high-speed running. These results suggest that acute physical exercise with mild and low stress can efficiently induce optimal neuronal activation that is involved in the antidepressant/anxiolytic effects.

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.

Lifelong disturbance of serotonin transporter functioning results in fear learning deficits: Reversal by blockade of CRF1 receptors

The inability to associate aversive events with relevant cues (i.e. fear learning) may lead to maladaptive anxiety. To further study the role of the serotonin transporter (SERT) in fear learning, classical fear conditioning was studied in SERT knockout rats (SERT(-/-)) using fear potentiation of the startle reflex. Next, fear acquisition and concomitant development of contextual conditioned fear were monitored during training. To differentiate between developmental and direct effects of reduced SERT functioning, effects of acute and chronic SSRI treatment were studied in adult rats. Considering the known interactions between serotonin and corticotropin-releasing factor (CRF), we studied the effect of the CRFR1 antagonist CP154,526 on behavioral changes observed and determined CRF1 receptor levels in SERT(-/-) rats. SERT(-/-) showed blunted fear potentiation and enhanced contextual fear, which resulted from a deficit in fear acquisition. Paroxetine treatment did not affect acquisition or expression of fear-potentiated startle, suggesting that disturbed fear learning in SERT(-/-) results from developmental changes and not from reduced SERT functioning. Although CRF1 receptor levels did not differ significantly between genotypes, CP154,526 treatment normalized both cue- and contextual fear in SERT(-/-) during acquisition, but not expression of fear-potentiated startle. The disrupted fear acquisition and concomitant increase in contextual conditioned fear-potentiated startle fear in SERT(-/-) resembles the associative learning deficit seen in patients with panic disorder and suggests that normal SERT functioning is crucial for the development of an adequate fear neuro-circuitry. Moreover, the normalization of fear acquisition by CP154,526 suggests a role for central CRF signaling in the generalization of fear.

Sex differences and stress across the lifespan

Sex differences in stress responses can be found at all stages of life and are related to both the organizational and activational effects of gonadal hormones and to genes on the sex chromosomes. As stress dysregulation is the most common feature across neuropsychiatric diseases, sex differences in how these pathways develop and mature may predict sex-specific periods of vulnerability to disruption and increased disease risk or resilience across the lifespan. The aging brain is also at risk to the effects of stress, where the rapid decline of gonadal hormones in women combined with cellular aging processes promote sex biases in stress dysregulation. In this Review, we discuss potential underlying mechanisms driving sex differences in stress responses and their relevance to disease. Although stress is involved in a much broader range of diseases than neuropsychiatric ones, we highlight here this area and its examples across the lifespan.

Angiotensin type 1a receptors on corticotropin-releasing factor neurons contribute to the expression of conditioned fear

Although generally associated with cardiovascular regulation, angiotensin II receptor type 1a (AT1a R) blockade in mouse models and humans has also been associated with enhanced fear extinction and decreased post-traumatic stress disorder (PTSD) symptom severity, respectively. The mechanisms mediating these effects remain unknown, but may involve alterations in the activities of corticotropin-releasing factor (CRF)-expressing cells, which are known to be involved in fear regulation. To test the hypothesis that AT1a R signaling in CRFergic neurons is involved in conditioned fear expression, we generated and characterized a conditional knockout mouse strain with a deletion of the AT1a R gene from its CRF-releasing cells (CRF-AT1a R((-/-)) ). These mice exhibit normal baseline heart rate, blood pressure, anxiety and locomotion, and freeze at normal levels during acquisition of auditory fear conditioning. However, CRF-AT1a R((-/-)) mice exhibit less freezing than wild-type mice during tests of conditioned fear expression-an effect that may be caused by a decrease in the consolidation of fear memory. These results suggest that central AT1a R activity in CRF-expressing cells plays a role in the expression of conditioned fear, and identify CRFergic cells as a population on which AT1 R antagonists may act to modulate fear extinction.

Evidence for the role of corticotropin-releasing factor in major depressive disorder

Major depressive disorder (MDD) is a devastating disease affecting over 300 million people worldwide, and costing an estimated 380 billion Euros in lost productivity and health care in the European Union alone. Although a wealth of research has been directed toward understanding and treating MDD, still no therapy has proved to be consistently and reliably effective in interrupting the symptoms of this disease. Recent clinical and preclinical studies, using genetic screening and transgenic rodents, respectively, suggest a major role of the CRF1 gene, and the central expression of CRF1 receptor protein in determining an individual's risk of developing MDD. This gene is widely expressed in brain tissue, and regulates an organism's immediate and long-term responses to social and environmental stressors, which are primary contributors to MDD. This review presents the current state of knowledge on CRF physiology, and how it may influence the occurrence of symptoms associated with MDD. Additionally, this review presents findings from multiple laboratories that were presented as part of a symposium on this topic at the annual 2014 meeting of the International Behavioral Neuroscience Society (IBNS). The ideas and data presented in this review demonstrate the great progress that has been made over the past few decades in our understanding of MDD, and provide a pathway forward toward developing novel treatments and detection methods for this disorder.

Serotonergic Systems in the Pathophysiology of Ethanol Dependence: Relevance to Clinical Alcoholism

Alcoholism is a progressive brain disorder that is marked by increased sensitivity to the positive and negative reinforcing properties of ethanol, compulsive and habitual use despite negative consequences, and chronic relapse to alcohol drinking despite repeated attempts to reduce intake or abstain from alcohol. Emerging evidence from preclinical and clinical studies implicates serotonin (5-hydroxytryptamine; 5-HT) systems in the pathophysiology of alcohol dependence, suggesting that drugs targeting 5-HT systems may have utility in the treatment of alcohol use disorders. In this Review, we discuss the role of 5-HT systems in alcohol dependence with a focus on 5-HT interactions with neural circuits that govern all three stages of the addiction cycle. We attempt to clarify how 5-HT influences circuit function at these different stages with the goal of identifying neural targets for pharmacological treatment of this debilitating disorder.

Suppression of piriform cortex activity in rat by corticotropin-releasing factor 1 and serotonin 2A/C receptors

The piriform cortex (PC) is richly innervated by corticotropin-releasing factor (CRF) and serotonin (5-HT) containing axons arising from central amygdala and Raphe nucleus. CRFR₁ and 5-HT_2A/2CRs have been shown to interact in manner where CRFR activation subsequently potentiates the activity of 5-HT_2A/2CRs. The purpose of this study was to determine how the activation of CRFR₁ and/or 5-HT₂Rs modulates PC activity at both the circuit and cellular level. Voltage sensitive dye imaging showed that CRF acting through CRFR₁ dampened activation of the Layer II of PC and interneurons of endopiriform nucleus. Application of the selective 5-HT_2A/CR agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) following CRFR₁ activation potentiated this effect. Blocking the interaction between CRFR₁ and 5-HT₂R with a Tat-CRFR₁-CT peptide abolished this potentiation. Application of forskolin did not mimic CRFR₁ activity but instead blocked it, while a protein kinase A antagonist had no effect. However, activation and antagonism of protein kinase C (PKC) either mimicked or blocked CRF modulation, respectively. DOI had no effect when applied alone indicating that the prior activation of CRFR₁ receptors was critical for DOI to show significant effects similar to CRF. Patch clamp recordings showed that both CRF and DOI reduced the synaptic responsiveness of Layer II pyramidal neurons. CRF had highly variable effects on interneurons within Layer III, both increasing and decreasing their excitability, but DOI had no effect on the excitability of this group of neurons. These data show that CRF and 5-HT, acting through both CRFR₁ and 5-HT_2A/CRs, reduce the activation of the PC. This modulation may be an important blunting mechanism of stressor behaviors mediated through the olfactory cortex.

Sex dependent reduction by prenatal stress of the expression of 5HT1A receptors in the prefrontal cortex and CRF type 2 receptors in the raphe nucleus in rats: reversal by citalopram

RATIONALE

Alterations in the serotonergic transmission and activity of corticotropin-releasing factor (CRF) family may underlie anxiety and depressive disorders. These could be corrected by treatment with SSRIs.

OBJECTIVES

The objective of the current study is to determine whether the increased anxiety of prenatally stressed (PS) rats of both sexes is associated with changes in 5HT1A and CRF type 2 receptors (5HT1AR and CRFR2) in the prefrontal cortex (PFC)-dorsal raphe nuclei (DRN) axis, and how these are affected by chronic treatment with citalopram (10 mg/kg/day). We focussed on GABAergic cells that co-express parvalbumin and/or neuropeptide Y, and 5HT1AR in the medial prefrontal cortex (mPFC) and on cells that express 5HT, parvalbumin, 5HT1AR or CRFR2 in the DRN.

RESULTS

Immunohistochemistry with fluorescent antibodies demonstrated sex differences in the expression of 5HT1AR and CRFR2 protein. Prenatal stress selectively reduced the expression of 5HT1AR on GABAergic cells in the mPFC in males and that of CRFR2 in the DRN of females. Citalopram treatment for 5 weeks abolished the increase in anxiety in both sexes, restored the intensity of expression of 5HT1AR in the mPFC in males and increased their expression in the mPFC and DRN in females. Citalopram reduced CRFR2 expression in control and PS males but increased it in PS females.

CONCLUSIONS

Male and female rats show differences in the expression of 5HT1AR and CRFR2 protein that are selectively reduced by prenatal stress. Reversal by citalopram of the changes in the expression of these receptors induced by prenatal stress support their role in the aetiology of anxiety.

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.

Effect of the fragrance inhalation of essential oil from Asarum heterotropoides on depression-like behaviors in mice

Background

Psychological stressors may cause affective disorders, such as depression and anxiety, by altering expressions of corticotropin releasing factor (CRF), serotonin (5-HT), and tyrosine hydroxylase (TH) in the brain. This study investigated the effects of essential oil from Asarum heterotropoides (EOAH) on depression-like behaviors and brain expressions of CRF, 5-HT, and TH in mice challenged with stress.

Methods

Male ICR mice received fragrance inhalation of EOAH (0.25, 0.5, 1.0, and 2.0 g) for 3 h in the special cage capped with a filter paper before start of the forced swimming test (FST) and tail suspension test (TST). The duration of immobility was measured for the determination of depression-like behavior in the FST and TST. The selective serotonin reuptake inhibitor fluoxetine as positive control was administered at a dose of 15 mg/kg (i.p.) 30 min before start of behavioral testing. Immunoreactivities of CRF, 5-HT, and TH in the brain were also measured using separate groups of mice subjected to the FST.

Results

EOAH at higher doses (1.0 and 2.0 g) reduced immobility time in the FST and TST. In addition, EOAH at a dose of 1.0 g significantly reduced the expected increases in the expression of CRF positive neurons in the paraventricular nucleus and the expression of TH positive neurons in the locus coeruleus, and the expected decreases of the 5-HT positive neurons in the dorsal raphe nucleus.

Conclusion

These results provide strong evidence that EOAH effectively inhibits depression-like behavioral responses, brain CRF and TH expression increases, and brain 5-HT expression decreases in mice challenged with stress.