Distribution of corticotropin-releasing factor receptor 1 in the developing mouse forebrain: A novel sex difference revealed in the rostral periventricular hypothalamus

Role of corticotropin-releasing factor neurotransmission in the lateral hypothalamus on baroreflex impairment evoked by chronic variable stress in rats

Despite the importance of physiological responses to stress in a short-term, chronically these adjustments may be harmful and lead to diseases, including cardiovascular diseases. The lateral hypothalamus (LH) has been reported to be involved in expression of physiological and behavioral responses to stress, but the local neurochemical mechanisms involved are not completely described. The corticotropin-releasing factor (CRF) neurotransmission is a prominent brain neurochemical system implicated in the physiological and behavioral changes induced by aversive threats. Furthermore, chronic exposure to aversive situations affects the CRF neurotransmission in brain regions involved in stress responses. Therefore, in this study, we evaluated the influence of CRF neurotransmission in the LH on changes in cardiovascular function and baroreflex activity induced by chronic variable stress (CVS). We identified that CVS enhanced baseline arterial pressure and impaired baroreflex function, which were followed by increased expression of CRF2, but not CRF1, receptor expression within the LH. Local microinjection of either CRF1 or CRF2 receptor antagonist within the LH inhibited the baroreflex impairment caused by CVS, but without affecting the mild hypertension. Taken together, the findings documented in this study suggest that LH CRF neurotransmission participates in the baroreflex impairment related to chronic stress exposure.

Sex Differences in Stress Response: Classical Mechanisms and Beyond

Neuropsychiatric disorders, which are associated with stress hormone dysregulation, occur at different rates in men and women. Moreover, nowadays, preclinical and clinical evidence demonstrates that sex and gender can lead to differences in stress responses that predispose males and females to different expressions of similar pathologies. In this curated review, we focus on what is known about sex differences in classic mechanisms of stress response, such as glucocorticoid hormones and corticotrophin-releasing factor (CRF), which are components of the hypothalamicpituitary- adrenal (HPA) axis. Then, we present sex differences in neurotransmitter levels, such as serotonin, dopamine, glutamate and GABA, as well as indices of neurodegeneration, such as amyloid β and Tau. Gonadal hormone effects, such as estrogens and testosterone, are also discussed throughout the review. We also review in detail preclinical data investigating sex differences caused by recentlyrecognized regulators of stress and disease, such as the immune system, genetic and epigenetic mechanisms, as well neurosteroids. Finally, we discuss how understanding sex differences in stress responses, as well as in pharmacology, can be leveraged into novel, more efficacious therapeutics for all. Based on the supporting evidence, it is obvious that incorporating sex as a biological variable into preclinical research is imperative for the understanding and treatment of stress-related neuropsychiatric disorders, such as depression, anxiety and Alzheimer's disease.

Sex differences in androgen receptor, estrogen receptor alpha, and c-Fos co-expression with corticotropin releasing factor expressing neurons in restrained adult mice

Gonadal hormone actions through androgen receptor (AR) and estrogen receptor alpha (ERα) regulate sex differences in hypothalamic-pituitary-adrenal (HPA) axis responsivity and stress-related behaviors. Here we tested whether corticotropin releasing factor (CRF) expressing neurons, which are widely known to regulate neuroendocrine and behavioral stress responses, co-express AR and ERα as a potential mechanism for gonadal hormone regulation of these responses. Using Crh-IRES-Cre::Ai9 reporter mice we report high co-localization of AR in CRF neurons within the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BST), medial amygdala (MeA), and ventromedial hypothalamus (VMH), moderate levels within the central amygdala (CeA) and low levels in the paraventricular hypothalamus (PVN). Sex differences in CRF/AR co-expression were found in the principal nucleus of the BST (BSTmpl), CeA, MeA, and VMH (males>females). CRF co-localization with ERα was generally lower relative to AR co-localization. However, high co-expression was found within the MPOA, AVPV, and VMH, with moderate co-expression in the arcuate nucleus (ARC), BST, and MeA and low levels in the PVN and CeA. Sex differences in CRF/ERα co-localization were found in the BSTmpl and PVN (males>females). Finally, we assessed neural activation of CRF neurons in restraint-stressed mice and found greater CRF/c-Fos co-expression in females in the BSTmpl and periaqueductal gray, while co-expression was higher in males within the ARC and dorsal CA1. Given the known role of CRF in regulating behavioral stress responses and the HPA axis, AR/ERα co-expression and sex-specific activation of CRF cell groups indicate potential mechanisms for modulating sex differences in these functions.

Sexually dimorphic oxytocin receptor-expressing (OXTR) neurons in the anteroventral periventricular nucleus (AVPV) in the postpartum female mouse are involved in maternal behavior

Maternal care is crucial for the survival and development of offspring. Oxytocin modulates maternal behavior by binding to oxytocin receptors (OXTRs) in various parts of the brain. Previously, we showed that OXTRs are expressed in the anteroventral periventricular nucleus (AVPV) of female, but not male mice. Because the AVPV is involved in the regulation of maternal behavior and oxytocin enhances its induction, this finding leads to the hypothesis that the female specific population of OXTR neurons in the AVPV regulates maternal behavior. To address this hypothesis, OXTR-Venus reporter mice were used to assess if expression levels of OXTR in the AVPV are changed during the postpartum period. The total number of OXTR-Venus neurons was significantly greater in postpartum dams compared to virgin females. To assess efferent projections of the AVPV-OXTR neurons, a Cre-dependent fluorescent protein (tdTomato) expressing a viral vector was injected into one side of the AVPV of female OXTR-Cre mice. Fibers expressing tdTomato were found in hypothalamic areas containing oxytocin neurons (the supraoptic and paraventricular nuclei) and the midbrain areas (the ventral tegmental area and periaqueductal gray) that are involved in the regulation of maternal motivation. To assess if activity of the AVPV-OXTR neurons is involved in the regulation of maternal behaviors, a chemogenetic approach was employed. Specific inhibition of activity of AVPV-OXTR neurons completely abolished pup retrieval and nest building behaviors. Collectively, these findings demonstrate that AVPV-OXTR neurons in postpartum female mice constitute an important node in the neural circuitry that regulates maternal behavior.

Changes in Corticotropin-Releasing Factor Receptor Type 1, Co-Expression with Tyrosine Hydroxylase and Oxytocin Neurons, and Anxiety-Like Behaviors across the Postpartum Period in Mice

INTRODUCTION

Corticotropin-releasing factor and its primary receptor (CRFR1) are critical regulators of behavioral and neuroendocrine stress responses. CRFR1 has also been associated with stress-related behavioral changes in postpartum mice. Our previous studies indicate dynamic changes in CRFR1 levels and coupling of CRFR1 with tyrosine hydroxylase (TH) and oxytocin (OT) neurons in postpartum mice. In this study, we aimed to determine the time course of these changes during the postpartum period.

METHODS

Using a CRFR1-GFP reporter mouse line, we compared postpartum mice at five time points with nulliparous mice. We performed immunohistochemistry to assess changes in CRFR1 levels and changes in co-expression of TH/CRFR1-GFP and OT/CRFR1-GFP across the postpartum period. Mice were also assessed for behavioral stress responses in the open field test.

RESULTS

Relative to nulliparous mice, CRFR1 levels were elevated in the anteroventral periventricular nucleus (AVPV/PeN) but were decreased in the medial preoptic area from postpartum day 1 (P1) through P28. In the paraventricular hypothalamus (PVN), there is a transient decline in CRFR1 mid-postpartum with a nadir at P7. Co-localization of CRFR1 with TH-expressing neurons was also altered with a transient decrease found in the AVPV/PeN at P7 and P14. Co-expression of CRFR1 and OT neurons of the PVN and supraoptic nucleus was dramatically altered with virtually no co-expression found in nulliparous mice, but levels increased shortly after parturition and peaked near P21. A transient decrease in open field center time was found at P7, indicating elevated anxiety-like behavior.

CONCLUSION

This study revealed various changes in CRFR1 across the postpartum period, which may contribute to stress-related behavior changes in postpartum mice.

Roles of corticotropin-releasing factor signaling in the lateral habenula in anxiety-like and alcohol drinking behaviors in male rats

Corticotropin-releasing factor (CRF) signaling in the mesocorticolimbic system is known to modulate anxiety-like behavior and alcohol consumption, behaviors that also have been associated with the hyper-glutamatergic state of the lateral habenula (LHb) neurons in rats. However, the role of CRF signaling in the LHb on the glutamate transmission, anxiety-like behaviors and alcohol consumption is unknown. Here, we used male rats that had been consuming alcohol for three months to address this gap in the literature. First, using electrophysiological techniques, we evaluated CRF's effects on the glutamate transmission in LHb neurons in brain slices. CRF facilitated glutamate transmission. The facilitation was greater in neurons of alcohol-withdrawing rats than in those of naïve rats. The facilitation was mimicked by the activation of CRF receptor 1 (CRF1R) but attenuated by the activation of CRF receptor 2 (CRF2R). This facilitation was mediated by upregulating CRF1R-protein kinase A signaling. Conversely, protein kinase C blockade attenuated CRF's facilitation in neurons of naïve rats but promoted it in neurons of alcohol-withdrawing rats. Next, using site-direct pharmacology, we evaluated the role of CRF signaling in the LHb on anxiety-like behaviors and alcohol consumption. Intra-LHb inhibition of CRF1R or activation of CRF2R ameliorated the anxiety-like behaviors in alcohol-withdrawing rats and reduced their alcohol intake when drinking was resumed. These observations provide the first direct behavioral pharmacological and cellular evidence that CRF signaling in the LHb modulates glutamate transmission, anxiety-like behaviors and alcohol consumption, and that adaptation occurs in CRF signaling in the LHb after chronic alcohol consumption.

•

CRF regulates glutamate transmission in the lateral habenula of male rats.

•

CRF1R blockage or CRF2R activation in the LHb reduces anxiety in male rats.

•

CRF1R blockage/CRF2R activation in the LHb reduces alcohol consumption in male rats.

•

Acute ethanol facilitates LHb glutamate transmission involving CRF signaling.

Alterations in corticotropin-releasing factor receptor type 1 in the preoptic area and hypothalamus in mice during the postpartum period

Corticotropin-releasing factor (CRF) signaling through CRF receptor 1 (CRFR1) regulates autonomic, endocrine, and behavioral responses to stress, as well as behavioral changes during the maternal period. Previous work in our lab reported higher levels of CRFR1 in female, compared to male, mice within the rostral anteroventral periventricular nucleus (AVPV/PeN), a brain region involved in maternal behaviors. In this study, we used CRFR1-GFP reporter mice to investigate whether the reproductive status (postpartum vs. nulliparous) of acutely stressed females affects levels of CRFR1 in the AVPV/PeN and other regions involved in maternal functions. Compared to nulliparous, postpartum day 14 females showed increased AVPV/PeN CRFR1-GFP immunoreactivity and an elevated number of restraint stress-activated AVPV/PeN CRFR1 cells as assessed by immunohistochemical co-localization of CRFR1-GFP and phosphorylated CREB (pCREB). The medial preoptic area (MPOA) and paraventricular hypothalamus (PVN) of postpartum mice showed modest decreases in CRFR1-GFP immunoreactivity, while increased CRFR1-GFP/pCREB co-expressing cells were found in the PVN following restraint stress relative to nulliparous mice. Tyrosine hydroxylase (TH) and CRFR1-GFP co-localization was also assessed in the AVPV/PeN and other regions and revealed a decrease in co-localized neurons in the AVPV/PeN and ventral tegmental area of postpartum mice. Corticosterone analysis of restrained mice revealed blunted peak, but elevated recovery, levels in postpartum compared to nulliparous mice. Finally, we investigated projection patterns of AVPV/PeN CRFR1 neurons using female CRFR1-Cre mice and revealed dense efferent projections to several preoptic, hypothalamic, and hindbrain regions known to control stress-associated and maternal functions. Together, these findings contribute to our understanding of the neurobiology that might underlie changes in stress-related functions during the postpartum period.

Sex differences in CRF1, CRF, and CRFBP expression in C57BL/6J mouse brain across the lifespan and in response to acute stress

Signaling pathways mediated by corticotropin-releasing factor and its receptor 1 (CRF1) play a central role in stress responses. Dysfunction of the CRF system has been associated with neuropsychiatric disorders. However, dynamic changes in the CRF system during brain development and aging are not well investigated. In this study, we characterized CRF1, CRF, and corticotropin-releasing factor binding protein (CRFBP) expression in different brain regions in both male and female C57BL/6J mice from 1 to 18 months of age under basal conditions as well as after an acute 2-hr-restraint stress. We found that CRF and CRF1 levels tended to increase in the hippocampus and hypothalamus, and to decrease in the prefrontal cortex with aging, especially at 18 months of age, whereas CRFBP expression followed an opposite direction in these brain areas. We also observed area-specific sex differences in the expression of these three proteins. For example, CRF expression was lower in females than in males in all the brain regions examined except the prefrontal cortex. After acute stress, CRF and CRF1 were up-regulated at 1, 6, and 12 months of age, and down-regulated at 18 months of age. Females showed more robust changes compared to males of the same age. CRFBP expression either decreased or remained unchanged in most of the brain areas following acute stress. Our findings suggest that brain CRF1, CRF, and CRFBP expression changes dynamically across the lifespan and under stress condition in a sex- and regional-specific manner. Sex differences in the CRF system in response to stress may contribute to the etiology of stress-related neuropsychiatric disorders.

Corticotropin-releasing factor neurotransmission in the lateral hypothalamus modulates the tachycardiac response during acute emotional stress in rats

The lateral hypothalamus (LH) is implicated in the physiological and behavioral responses during stressful events. However, the local neurochemical mechanisms related to control of stress responses by this hypothalamic area are not completely understood. Therefore, in this study we evaluated the involvement of CRFergic neurotransmission acting through the CRF₁ receptor within the LH in cardiovascular responses evoked by an acute session of restraint stress in rats. For this, we investigated the effect of bilateral microinjection of different doses (0.01, 0.1 and 1 nmol/100 nL) of the selective CRF₁ receptor antagonist CP376395 into the LH on arterial pressure and heart rate increases and decrease in tail skin temperature evoked by acute restraint stress. We found that all doses of the CRF₁ receptor antagonist microinjected into the LH decreased the restraint-evoked tachycardia, but without affecting the arterial pressure and tail skin temperature responses. Additionally, treatment of the LH with CP376395 at the doses of 0.1 and 1 nmol/100 nL increased the basal values of both heart rate and arterial pressure, whereas the dose of 0.1 nmol/100 nL decreased the skin temperature. Taken together, these findings indicate that CRFergic neurotransmission in the LH, acting through activation of local CRF₁ receptors, plays a facilitatory role in the tachycardia observed during aversive threats, but without affecting the pressor and tail skin temperature responses. Our results also provide evidence that LH CRFergic neurotransmission in involved in tonic maintenance of cardiovascular function.

CRF1 and CRF2 receptors in the lateral hypothalamus differently modulate the baroreflex function in unanesthetized rats

The lateral hypothalamus (LH) is a diencephalic structure that has been considered part of the central circuitry regulating the baroreflex function. However, the local neurochemical mechanisms involved in baroreflex control by this hypothalamic area are poorly understood. Therefore, in the present study we investigated the role of corticotropin-releasing factor (CRF) neurotransmission within the LH acting via local CRF₁ and CRF₂ receptors in cardiac baroreflex responses in unanesthetized rats. For this, the baroreflex activity was assessed using two approaches: i) the pharmacological approach via intravenous infusion of vasoactive agents, and ii) the sequence analysis technique that evaluates reflex responses during spontaneous arterial pressure variations. The sequence analysis technique indicated that LH treatment with the selective CRF₁ receptor antagonist CP376395 decreased the baroreflex effectiveness index, whereas the selective CRF₂ receptor antagonist antisauvagine-30 increased the reflex shortening of pulse interval during spontaneous arterial pressure decreases. However, the pharmacological approach did not indicate effect of the bilateral microinjection of either CP376395 or antisauvagine-30 into the LH in the tachycardia evoked by blood pressure decrease or the reflex bradycardia caused by blood pressure increase. Overall, these findings indicate that CRF neurotransmission within the LH controls baroreflex function during a narrow range of physiological arterial pressure variations. Besides, results provide evidence that CRF₁ and CRF₂ receptors in the LH oppositely modulate the spontaneous baroreflex activity through different mechanisms.

Sex-dependent effects of chronic variable stress on discrete corticotropin-releasing factor receptor 1 cell populations

Anxiety and depression are strikingly more prevalent in women compared with men. Dysregulation of corticotropin-releasing factor (CRF) binding to its cognate receptor (CRFR1) is thought to play a critical role in the etiology of these disorders. In the present study, we investigated whether there were sex differences in the effects of chronic variable stress (CVS) on CRFR1 cells using CRFR1-GFP reporter mice experiencing a 9-day CVS paradigm. Brains were collected from CVS and stress naïve female and male mice following exposure to the open field test. This CVS paradigm effectively increased anxiety-like behavior in female and male mice. In addition, we assessed changes in activation of CRFR1 cells (co-localization with c-Fos and phosphorylated CREB (pCREB)) in stress associated brain structures, including two sexually dimorphic CRFR1 cell groups in the anteroventral periventricular nucleus (AVPV/PeN; F>M) and paraventricular hypothalamus (PVN; M>F). CVS increased CRFR1-GFP cell number as well as the number of CRFR1/pCREB co-expressing cells in the female but not male AVPV/PeN. In the PVN, the number of CRFR1/pCREB co-expressing cells was overall greater in males regardless of treatment and CVS resulted in a male-specific reduction of CRFR1/c-Fos cells. In addition, CVS induced a female-specific reduction in CRFR1/c-Fos cells within the anteroventral bed nucleus of the stria terminalis and both sexes exhibited a reduction in CRFR1/c-Fos co-expressing cells following CVS within the ventral basolateral amygdala. Overall, these sex-specific effects of CVS on CRFR1 populations may have implications for sex differences in stress-induction of mood disorders.

CRF signaling between neurons in the paraventricular nucleus of the hypothalamus (PVN) coordinates stress responses

The importance of a precisely coordinated neuroendocrine, autonomic, and behavioral stress response was a primary theme at the Stress Neurobiology Workshop 2018, held in the beautiful setting of Banff Provincial Park in Alberta, Canada. Much of the research featured at this meeting reinforced the importance of appropriately responding to stress in order to avoid various neuropsychiatric pathologies, including Post-Traumatic Stress Disorder (PTSD), depression, and addiction. Corticotropin-Releasing Factor (CRF) neurons in the paraventricular nucleus of the hypothalamus (PVN) are central players in the stress response, integrating both external and visceral stress-relevant information, then directing neuroendocrine, autonomic and behavioral adaptations via endocrine and neural outputs of the PVN. The PVN contains a densely packed array of neuron types that respond to stress, including CRF neurons that activate the Hypothalamic-Pituitary-Adrenal (HPA) axis. Recently, identification of a new population of neurons in the PVN that express CRF Receptor 1 (CRFR1) has suggested that CRF release in the PVN signals to neighboring CRF responsive neurons, potentially functioning in HPA axis feedback, neuroendocrine coordination, and autonomic signaling. Here, we review our recent work characterizing an intra-PVN microcircuit in which locally released CRF release activates CRFR1+ neurons that make recurrent inhibitory GABAergic synapses onto CRF neurons to dampen excitability , therebylimiting HPA axis hyperactivity in response to stress and promoting stress recovery, which we presented in a poster session at the conference. We then discuss questions that have arisen following publication of our initial characterization of the microcircuit, regarding specific features of intra-PVN CRF signaling and its potential role in coordinating neuroendocrine, autonomic, and behavioral outputs of the PVN. Our presented work, as well as many of the presentations at the Stress Neurobiology Workshop 2018 together establish intra-PVN signaling as an important regulatory node in stress response pathways, which are central to the pathogenesis of neuropsychiatric disorders.

Stress-induced neural activation is altered during early withdrawal from chronic methamphetamine

Chronic methamphetamine (MA) use can lead to increased symptoms of depression and anxiety during abstinence. Less is known about the specific brain regions that are altered following repeated MA that may be associated with these behavioral perturbations. Furthermore, MA has been reported to recruit and activate microglia in the brain, which may exacerbate stress-associated behavioral changes. In the present study, male and female mice were injected with MA (5 mg/kg) or saline once daily for 10 days, and during early withdrawal were assessed for alterations in immediate early gene (c-Fos) responses to a forced swim stressor. Chronic MA exposure increased floating and decreased swim time in the forced swim test in male and female mice tested 48 h after the final dose, indicating elevated depressive-like behavior. Furthermore, assessment of nest building, a measure of distress or despair-like behavior, revealed a sex-specific effect with only MA-treated females showing impairments. The c-Fos response to forced swim was attenuated by prior MA exposure in the central amygdala, CA3 hippocampal region, prefrontal cortex, and bed nucleus of the stria terminalis (BST). In the BST this attenuation occurred only in males. Neither the total number of microglia or activated microglia were altered by chronic MA exposure in regions examined. The primary findings indicate that chronic MA exposure attenuates activation of select stress-associated brain regions, a dysregulation that might contribute to alterations in mood-related behaviors.

A sexually dimorphic distribution of corticotropin-releasing factor receptor 1 in the paraventricular hypothalamus

Sex differences in neural structures are generally believed to underlie sex differences reported in anxiety, depression, and the hypothalamic-pituitary-adrenal axis, although the specific circuitry involved is largely unclear. Using a corticotropin-releasing factor receptor 1 (CRFR1) reporter mouse line, we report a sexually dimorphic distribution of CRFR1 expressing cells within the paraventricular hypothalamus (PVN; males > females). Relative to adult levels, PVN CRFR1-expressing cells are sparse and not sexually dimorphic at postnatal days 0, 4, or 21. This suggests that PVN cells might recruit CRFR1 during puberty or early adulthood in a sex-specific manner. The adult sex difference in PVN CRFR1 persists in old mice (20-24 months). Adult gonadectomy (6 weeks) resulted in a significant decrease in CRFR1-immunoreactive cells in the male but not female PVN. CRFR1 cells show moderate co-expression with estrogen receptor alpha (ERα) and high co-expression with androgen receptor, indicating potential mechanisms through which circulating gonadal hormones might regulate CRFR1 expression and function. Finally, we demonstrate that a psychological stressor, restraint stress, induces a sexually dimorphic pattern of neural activation in PVN CRFR1 cells (males >females) as assessed by co-localization with the transcription/neural activation marker phosphorylated CREB. Given the known role of CRFR1 in regulating stress-associated behaviors and hormonal responses, this CRFR1 PVN sex difference might contribute to sex differences in these functions.

Characterization and gonadal hormone regulation of a sexually dimorphic corticotropin-releasing factor receptor 1 cell group

Corticotropin-releasing factor binds with high affinity to CRF receptor 1 (CRFR1) and is implicated in stress-related mood disorders such as anxiety and depression. Using a validated CRFR1-green fluorescent protein (GFP) reporter mouse, our laboratory recently discovered a nucleus of CRFR1 expressing cells that is prominent in the female rostral anteroventral periventricular nucleus (AVPV/PeN), but largely absent in males. This sex difference is present in the early postnatal period and remains dimorphic into adulthood. The present investigation sought to characterize the chemical composition and gonadal hormone regulation of these sexually dimorphic CRFR1 cells using immunohistochemical procedures. We report that CRFR1-GFP-ir cells within the female AVPV/PeN are largely distinct from other dimorphic cell populations (kisspeptin, tyrosine hydroxylase). However, CRFR1-GFP-ir cells within the AVPV/PeN highly co-express estrogen receptor alpha as well as glucocorticoid receptor. A single injection of testosterone propionate or estradiol benzoate on the day of birth completely eliminates the AVPV/PeN sex difference, whereas adult gonadectomy has no effect on CRFR1-GFP cell number. These results indicate that the AVPV/PeN CRFR1 is regulated by perinatal but not adult gonadal hormones. Finally, female AVPV/PeN CRFR1-GFP-ir cells are activated following an acute 30-min restraint stress, as assessed by co-localization of CRFR1-GFP cells with phosphorylated (p) CREB. CRFR1-GFP/pCREB cells were largely absent in the male AVPV/PeN. Together, these data indicate a stress and gonadal hormone responsive nucleus that is unique to females and may contribute to sex-specific stress responses.

Dysregulation of the Lateral Habenula in Major Depressive Disorder

Clinical and preclinical evidence implicates hyperexcitability of the lateral habenula (LHb) in the development of psychiatric disorders including major depressive disorder (MDD). This discrete epithalamic nucleus acts as a relay hub linking forebrain limbic structures with midbrain aminergic centers. Central to reward processing, learning and goal directed behavior, the LHb has emerged as a critical regulator of the behaviors that are impaired in depression. Stress-induced activation of the LHb produces depressive- and anxiety-like behaviors, anhedonia and aversion in preclinical studies. Moreover, deep brain stimulation of the LHb in humans has been shown to alleviate chronic unremitting depression in treatment resistant depression. The diverse neurochemical processes arising in the LHb that underscore the emergence and treatment of MDD are considered in this review, including recent optogenetic studies that probe the anatomical connections of the LHb.

The Corticotropin-Releasing Factor Family: Physiology of the Stress Response

The physiological stress response is responsible for the maintenance of homeostasis in the presence of real or perceived challenges. In this function, the brain activates adaptive responses that involve numerous neural circuits and effector molecules to adapt to the current and future demands. A maladaptive stress response has been linked to the etiology of a variety of disorders, such as anxiety and mood disorders, eating disorders, and the metabolic syndrome. The neuropeptide corticotropin-releasing factor (CRF) and its relatives, the urocortins 1–3, in concert with their receptors (CRFR1, CRFR2), have emerged as central components of the physiological stress response. This central peptidergic system impinges on a broad spectrum of physiological processes that are the basis for successful adaptation and concomitantly integrate autonomic, neuroendocrine, and behavioral stress responses. This review focuses on the physiology of CRF-related peptides and their cognate receptors with the aim of providing a comprehensive up-to-date overview of the field. We describe the major molecular features covering aspects of gene expression and regulation, structural properties, and molecular interactions, as well as mechanisms of signal transduction and their surveillance. In addition, we discuss the large body of published experimental studies focusing on state-of-the-art genetic approaches with high temporal and spatial precision, which collectively aimed to dissect the contribution of CRF-related ligands and receptors to different levels of the stress response. We discuss the controversies in the field and unravel knowledge gaps that might pave the way for future research directions and open up novel opportunities for therapeutic intervention.

Corticotropin-Releasing Factor Receptor 1 in the Anterior Cingulate Cortex Mediates Maternal Absence-Induced Attenuation of Transport Response in Mouse Pups

A human infant initially shows non-selective sociality, and gradually develops selective attachment toward its caregiver, manifested as "separation anxiety." It was unclear whether such sophistication of attachment system occurs in non-human mammals. To seek a mouse model of separation anxiety, we utilized a primitive attachment behavior, the Transport Response, in that both human and mouse newborns immediately stop crying and stay immobile to cooperate with maternal carrying. We examined the mouse Transport Response in three social contexts: 30-min isolation in a novel environment, 30-min maternal absence experienced with littermates in the home cage, and the control home-cage condition with the mother and littermates. The pups after postnatal day (PND) 13 attenuated their Transport Response not only in complete isolation but also by maternal absence, and activated several brain areas including the periventricular nucleus of the hypothalamus, suggesting that 30-min maternal absence was perceived as a social stress by mouse pups after PND13. This attenuation of Transport Response by maternal absence was independent with plasma corticosterone, but was diminished by prior administration of a corticotropin-releasing factor receptor 1 (CRFR1) antagonist. Among 18 brain areas examined, only neurons in the anterior cingulate cortex (ACC) co-express c-fos mRNA and CRFR1 after maternal absence. Consistently, excitotoxic ACC lesions inhibited the maternal absence-induced attenuation of Transport Response. These data indicate that the expression of mouse Transport Response is influenced not only by social isolation but also by maternal absence even in their home cage with littermates after PND13, at least partly via CRF-CRFR1 signaling in the ACC.

Chronic Methamphetamine Exposure Attenuates Neural Activation in Hypothalamic-Pituitary-Adrenal Axis-Associated Brain Regions in a Sex-specific Manner

Sex differences in methamphetamine (MA) abuse and consequences of MA have been reported with females showing an increased addiction phenotype and withdrawal symptoms. One mechanism through which these effects might occur is via sex-specific alterations in the hypothalamic-pituitary-adrenal (HPA) axis and its associated brain regions. In this study, mice were administered MA (5 mg/kg) or saline for 10 consecutive days. During early withdrawal, anxiety-like behaviors were assessed in the open field, light/dark box, and elevated plus maze. At ten days of withdrawal, mice were injected with a final dose of MA (5 mg/kg) or saline. Chronic MA did not alter anxiety-like behaviors or corticosterone responses to a final dose of MA, although females showed elevated corticosterone responses compared to males. Chronic MA attenuated final MA-induced c-Fos in both sexes in the paraventricular hypothalamus (PVH), bed nucleus of the stria terminalis (BNST), cingulate cortex, central and basolateral amygdala. In CA1 and CA3 hippocampal areas, c-Fos attenuation by chronic MA occurred only in females. Within the PVH, final MA injection increased c-Fos to a greater extent in females compared to males regardless of prior MA exposure. Dual-labeling of c-Fos with glucocorticoid receptor revealed a specific attenuation of neural activation within this cell type in the PVH, central and basolateral amygdala, and BNST. Together these findings demonstrate that chronic MA can suppress subsequent activation of HPA axis-associated brain regions and cell phenotypes. Further, in select regions this reduction is sex-specific. These changes may contribute to reported sex differences in MA abuse patterns.