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

Involvement of D1 dopamine receptor in the nucleus of the solitary tract of rats in stress-induced hypertension and exercise

OBJECTIVE

Chronic stress can cause hypertension, whereas daily exercise promotes healthy well being through destressing. Although the nucleus of the solitary tract (NTS) is involved in the development of hypertension, the molecular and physiological mechanisms of stress and exercise remain unclear. In this study, we tested whether gene expression in the NTS is altered by stress and daily exercise and whether this is involved in cardiovascular regulation.

METHODS

We have performed RT2 Profiler PCR arrays targeting a panel of neurotransmitter receptor genes in the NTS of Wistar rats subjected to chronic restraint stress (1 h a day over 3 weeks) with or without voluntary wheel exercise. We also performed immunohistochemistry to determine whether the identified molecules were expressed at the protein level. Additionally, microinjection studies in anesthetized rats were performed to examine whether validated molecules exhibit physiological roles in cardiovascular regulation of the NTS.

RESULTS

We observed that blood pressure was significantly increased by stress and the increase was suppressed by exercise. Using PCR analysis, we determined that the expression levels of four genes in the NTS, including the dopamine receptor D1 gene (Drd1), were significantly affected by stress and suppressed by exercise. We also examined dopamine D1 receptor (D1R) expression in NTS neurons and found significantly greater expression in the stressed than nonstressed animals. Furthermore, the microinjection of a D1R agonist into the NTS in anesthetized rats induced hypotensive effects.

CONCLUSION

These results suggest that NTS D1R plays a role in the counteracting processes of stress-induced hypertension.

Research Progress of Central and Peripheral Corticotropin-Releasing Hormone in Irritable Bowel Syndrome with Comorbid Dysthymic Disorders

Irritable bowel syndrome (IBS) is considered a stress disorder characterized by psychological and gastrointestinal dysfunction. IBS patients not only suffer from intestinal symptoms such as abdominal pain, diarrhea, or constipation but also, experience dysthymic disorders such as anxiety and depression. Studies have found that corticotropin-releasing hormone plays a key role in IBS with comorbid dysthymic disorders. Next, we will summarize the effects of corticotropin-releasing hormone from the central nervous system and periphery on IBS with comorbid dysthymic disorders and relevant treatments based on published literatures in recent years.

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.

Cannabinoid and Serotonergic Systems: Unraveling the Pathogenetic Mechanisms of Stress-Induced Analgesia

The perception of "stress" triggers many physiological and behavioral responses, collectively called the stress response. Such a complex process allows for coping with stress and also triggers severe pathology. Because of the multidirectional effect of stress on the body, multiple systems participate in its pathogenesis, with the endogenous cannabinoid and the serotoninergic ones among them. These two systems also take part in the pain perception decrease, known as stress-induced analgesia (SIA), which can then be taken as an indirect indicator of the stress response. The aim of our study was to study the changes in cold SIA (c-SIA) resulting from the exogenous activation of cannabinoid receptor type 1 (CB1) and 5-hydroxytryptamine (5-HT, serotonin) receptor type 1A (5-HT1A). Various combinations of agonists and/or antagonists of CB1 and 5-HT1A, before or after 1 h of cold exposure, were applied, since we presumed that the exogenous activation of the receptors before the cold exposure would influence the pathogenesis of the stress response, while their activation after the stressful trigger would influence the later development. Our results show that the serotonergic system "maintained" c-SIA in the pre-stress treatment, while the cannabinoids' modulative effect was more prominent in the post-stress treatment. Here, we show the interactions of the two systems in the stress response. The interpretation and understanding of the mechanisms of interaction between CB1 and 5-HT1A may provide information for the prevention and control of adverse stress effects, as well as suggest interesting directions for the development of targeted interventions for the control of specific body responses.

The exploration of neuroinflammatory mechanism by which CRHR2 deficiency induced anxiety disorder

Inflammation stimulates the hypothalamic-pituitary adrenal (HPA) axis and triggers glial neuroinflammatory phenotypes, which reduces monoamine neurotransmitters by activating indoleamine 2,3-dioxygenase enzyme. These changes can induce psychiatric diseases, including anxiety. Corticotropin releasing hormone receptor 2 (CRHR2) in the HPA axis is involved in the etiology of anxiety. Omega(n)-3 polyunsaturated fatty acids (PUFAs) can attenuate anxiety through anti-inflammation and HPA axis modulation. However, the underlying molecular mechanism by CRHR2 modulates anxiety and its correlation with neuroinflammation remain unclear. Here, we first constructed a crhr2 zebrafish mutant line, and evaluated anxiety-like behaviors, gene expression associated with the HPA axis, neuroinflammatory response, neurotransmitters, and PUFAs profile in crhr2+/+ and crhr2-/- zebrafish. The crhr2 deficiency decreased cortisol levels and up-regulated crhr1 and down-regulated crhb, crhbp, ucn3l and proopiomelanocortin a (pomc a) in zebrafish. Interestingly, a significant increase in the neuroinflammatory markers, translocator protein (TSPO) and the activation of microglia M1 phenotype (CD11b) were found in crhr2-/- zebrafish. As a consequence, the expression of granulocyte-macrophage colony-stimulating factor, pro-inflammatory cytokines vascular endothelial growth factor, and astrocyte A1 phenotype c3 were up-regulated. While microglia anti-inflammatory phenotype (CD206), central anti-inflammatory cytokine interleukin-4, arginase-1, and transforming growth factor-β were downregulated. In parallel, crhr2-deficient zebrafish showed an upregulation of vdac1 protein, a TSPO ligand, and its downstream caspase-3. Furthermore, 5-HT/5-HIAA ratio was decreased and n-3 PUFAs deficiency was identified in crhr2-/- zebrafish. In conclusion, anxiety-like behavior displayed by crhr2-deficient zebrafish may be caused by the HPA axis dysfunction and enhanced neuroinflammation, which resulted in n-3 PUFAs and monoamine neurotransmitter reductions.

Evaluation of Peripheral Blood Concentrations of Phoenixin, Spexin, Nesfatin-1 and Kisspeptin as Potential Biomarkers of Bipolar Disorder in the Pediatric Population

There are some initial suggestions in the literature that phoenixin, spexin, nesfatin-1 and kisspeptin may play a role in the pathogenesis of affective disorders. Therefore, they may also be cautiously considered as potential diagnostic or predictive biomarkers of BD. This study aimed to evaluate the levels of the aforementioned neuropeptides in the peripheral blood of children and adolescents with bipolar. This study included 122 individuals: 67 persons with diagnosed bipolar disorder types I and II constituted the study group, and 55 healthy persons were included in the control group. Statistically significant differences in the concentrations of neuropeptides between the control and study groups were noted in relation to nesfatin-1 and spexin (although spexin lost statistical significance after introducing the Bonferroni correction). In a logistic regression analysis, an increased risk of bipolar disorder was noted for a decrease in nesfatin-1 concentration. Lower levels of nesfatin-1 seemed to be a significant risk factor for the development of bipolar disorder types I and II. Furthermore, the occurrence of bipolar disorder was associated with significantly elevated levels of spexin. None of the analyzed neuropeptides was significantly correlated with the number of symptoms of bipolar disorder.

One day away from mum has lifelong consequences on brain and behaviour

This chapter presents a brief overview of attachment theory and discusses the importance of the neonatal period in shaping an individual's physiological and behavioural responses to stress later in life, with a focus on the role of the parent-infant relationship, particularly in rodents. In rodents, the role of maternal behaviours goes far beyond nutrition, thermoregulation and excretion, acting as hidden regulators of the pup's physiology and development. In this review, we will discuss the inhibitory role of specific maternal behaviours on the ACTH and corticosterone (CORT) stress response. The interest of our group to explore the long-term consequences of maternal deprivation for 24 h (DEP) at different ages (3 days and 11 days) in rats was sparked by its opposite effects on ACTH and CORT levels. In early adulthood, DEP3 animals (males and females alike) show greater negative impact on affective behaviours and stress related parameters than DEP11, indicating that the latter is more resilient in tests of anxiety-like behaviour. These findings create an opportunity to explore the neurobiological underpinnings of vulnerability and resilience to stress-related disorders. The chapter also provides a brief historical overview and highlights the relevance of attachment theory, and how DEP helps to understand the effects of childhood parental loss as a risk factor for depression, schizophrenia, and PTSD in both childhood and adulthood. Furthermore, we present the concept of environmental enrichment (EE), its effects on stress responses and related behavioural changes and its benefits for rats previously subjected to DEP, along with the clinical implications of DEP and EE.

Comorbidity of Novel CRHR2 Gene Variants in Type 2 Diabetes and Depression

The corticotropin-releasing hormone receptor 2 (CRHR2) gene encodes CRHR2, contributing to the hypothalamic–pituitary–adrenal stress response and to hyperglycemia and insulin resistance. CRHR2−/− mice are hypersensitive to stress, and the CRHR2 locus has been linked to type 2 diabetes and depression. While CRHR2 variants confer risk for mood disorders, MDD, and type 2 diabetes, they have not been investigated in familial T2D and MDD. In 212 Italian families with type 2 diabetes and depression, we tested 17 CRHR2 single nucleotide polymorphisms (SNPs), using two-point parametric-linkage and linkage-disequilibrium (i.e., association) analysis (models: dominant-complete-penetrance-D1, dominant-incomplete-penetrance-D2, recessive-complete-penetrance-R1, recessive-incomplete-penetrance-R2). We detected novel linkage/linkage-disequilibrium/association to/with depression (3 SNPs/D1, 2 SNPs/D2, 3 SNPs/R1, 3 SNPs/R2) and type 2 diabetes (3 SNPs/D1, 2 SNPs/D2, 2 SNPs/R1, 1 SNP/R2). All detected risk variants are novel. Two depression-risk variants within one linkage-disequilibrium block replicate each other. Two independent novel SNPs were comorbid while the most significant conferred either depression- or type 2 diabetes-risk. Although the families were primarily ascertained for type 2 diabetes, depression-risk variants showed higher significance than type 2 diabetes-risk variants, implying CRHR2 has a stronger role in depression-risk than type 2 diabetes-risk. In silico analysis predicted variants’ dysfunction. CRHR2 is for the first time linked to/in linkage-disequilibrium/association with depression-type 2 diabetes comorbidity and may underlie the shared genetic pathogenesis via pleiotropy.

Anxiety and Depression: What Do We Know of Neuropeptides?

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

The role of the corticotropin-releasing hormone and its receptors in the regulation of stress response

Stress is an essential part of everyday life. The neuropeptide corticotropin-releasing hormone (CRH, also called CRF and corticoliberin) plays a key role in the integration of neuroendocrine, autonomic and behavioral responses to stress. The activation of the hypothalamic-pituitary-adrenal axis (HPA axis) by neurons of the paraventricular hypothalamic nucleus (PVN), the primary site of synthesis CRH, triggers stress reactions. In addition to the hypothalamus, CRH is widespread in extrahypothalamic brain structures, where it functions as a neuromodulator for coordination and interaction between the humoral and behavioral aspects of a stress response. The axons of neurons expressing CRH are directed to various structures of the brain, where the neuropeptide interacts with specific receptors (CRHR1, CRHR2) and can affect various mediator systems that work together to transmit signals to different brain regions to cause many reactions to stress. Moreover, the effect of stress on brain functions varies from behavioral adaptation to increased survival and increased risk of developing mental disorders. Disturbances of the CRH system regulation are directly related to such disorders: mental pathologies (depression, anxiety, addictions), deviations of neuroendocrinological functions, inflammation, as well as the onset and development of neurodegenerative diseases such as Alzheimer’s disease. In addition, the role of CRH as a regulator of the neurons structure in the areas of the developing and mature brain has been established. To date, studies have been conducted in which CRHR1 is a target for antidepressants, which are, in fact, antagonists of this receptor. In this regard, the study of the participation of the CRH system and its receptors in negative effects on hormone-dependent systems, as well as the possibility of preventing them, is a promising task of modern physiological genetics. In this review, attention will be paid to the role of CRH in the regulation of response to stress, as well as to the involvement of extrahypothalamic CRH in pathophysiology and the correction of mental disorders.

Gray areas: Neuropeptide circuits linking the Edinger-Westphal and Dorsal Raphe nuclei in addiction

The circuitry of addiction comprises several neural networks including the midbrain - an expansive region critically involved in the control of motivated behaviors. Midbrain nuclei like the Edinger-Westphal (EW) and dorsal raphe (DR) contain unique populations of neurons that synthesize many understudied neuroactive molecules and are encircled by the periaqueductal gray (PAG). Despite the proximity of these special neuron classes to the ventral midbrain complex and surrounding PAG, functions of the EW and DR remain substantially underinvestigated by comparison. Spanning approximately -3.0 to -5.2 mm posterior from bregma in the mouse, these various cell groups form a continuum of neurons that we refer to collectively as the subaqueductal paramedian zone. Defining how these pathways modulate affective behavioral states presents a difficult, yet conquerable challenge for today's technological advances in neuroscience. In this review, we cover the known contributions of different neuronal subtypes of the subaqueductal paramedian zone. We catalogue these cell types based on their spatial, molecular, connectivity, and functional properties and integrate this information with the existing data on the EW and DR in addiction. We next discuss evidence that links the EW and DR anatomically and functionally, highlighting the potential contributions of an EW-DR circuit to addiction-related behaviors. Overall, we aim to derive an integrated framework that emphasizes the contributions of EW and DR nuclei to addictive states and describes how these cell groups function in individuals suffering from substance use disorders. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

Novel serotonin-boosting effect of incense smoke from Kynam agarwood in mice: The involvement of multiple neuroactive pathways

ETHNOPHARMACOLOGICAL RELEVANCE

Stress is a state of feeling that inhibits one from responding properly in the face of a threat. Agarwood smoke has been used in traditional medicine as a sedative anti-anxious, and anti-restless therapy. Its scent emitted from heat induces people to enter a stable state; however, the underlying molecular effect is still unclear.

AIM OF THE STUDY

This study analyzed novel biological events and gene expression signatures induced by agarwood incense smoke in mice.

MATERIALS AND METHODS

Incense smoke was produced by heating at 150 °C for 30 min in a headspace autosampler oven. We treated mice with exposure to incense smoke from Kynam agarwood for 45 min/day for 7 consecutive days. After a 7-day inhalation period, the potent agarwood smoke affected-indicators in serum were measured, and the RNA profiles of the mouse brains were analyzed by microarray to elucidate the biological events induced by agarwood incense smoke.

RESULTS

Chemical profile analysis showed that the major component in the incense smoke of Kynam was 2-(2-phenylethyl) chromone (26.82%). Incense smoke from Kynam induced mice to enter a stable state and increased the levels of serotonin in sera. The emotion-related pathways, including dopaminergic synapse, serotonergic synapse, GABAergic synapse, long-term depression and neuroactive ligand-receptor interaction, were significantly affected by incense smoke. Moreover, the expression of Crhr2 and Chrnd genes, involved with neuroactive ligand-receptor interaction pathway, was upregulated by incense smoke.

CONCLUSIONS

By a newly-established incense smoke exposure system, we first identified that anti-anxious and anti-depressant effects of agarwood incense smoke were likely associated with the increase of serotonin levels and multiple neuroactive pathways in mice.

Effects of Immunization With the Soil-Derived Bacterium Mycobacterium vaccae on Stress Coping Behaviors and Cognitive Performance in a "Two Hit" Stressor Model

Previous studies demonstrate that Mycobacterium vaccae NCTC 11659 (M. vaccae), a soil-derived bacterium with anti-inflammatory and immunoregulatory properties, is a potentially useful countermeasure against negative outcomes to stressors. Here we used male C57BL/6NCrl mice to determine if repeated immunization with M. vaccae is an effective countermeasure in a "two hit" stress exposure model of chronic disruption of rhythms (CDR) followed by acute social defeat (SD). On day -28, mice received implants of biotelemetric recording devices to monitor 24-h rhythms of locomotor activity. Mice were subsequently treated with a heat-killed preparation of M. vaccae (0.1 mg, administered subcutaneously on days -21, -14, -7, and 27) or borate-buffered saline vehicle. Mice were then exposed to 8 consecutive weeks of either stable normal 12:12 h light:dark (LD) conditions or CDR, consisting of 12-h reversals of the LD cycle every 7 days (days 0-56). Finally, mice were exposed to either a 10-min SD or a home cage control condition on day 54. All mice were exposed to object location memory testing 24 h following SD. The gut microbiome and metabolome were assessed in fecal samples collected on days -1, 48, and 62 using 16S rRNA gene sequence and LC-MS/MS spectral data, respectively; the plasma metabolome was additionally measured on day 64. Among mice exposed to normal LD conditions, immunization with M. vaccae induced a shift toward a more proactive behavioral coping response to SD as measured by increases in scouting and avoiding an approaching male CD-1 aggressor, and decreases in submissive upright defensive postures. In the object location memory test, exposure to SD increased cognitive function in CDR mice previously immunized with M. vaccae. Immunization with M. vaccae stabilized the gut microbiome, attenuating CDR-induced reductions in alpha diversity and decreasing within-group measures of beta diversity. Immunization with M. vaccae also increased the relative abundance of 1-heptadecanoyl-sn-glycero-3-phosphocholine, a lysophospholipid, in plasma. Together, these data support the hypothesis that immunization with M. vaccae stabilizes the gut microbiome, induces a shift toward a more proactive response to stress exposure, and promotes stress resilience.

Responses to chronic corticosterone on brain glucocorticoid receptors, adrenal gland, and gut microbiota in mice lacking neuronal serotonin

Dysregulation of the stress-induced activation of the hypothalamic-pituitary-adrenocortical axis can result in disease. Bidirectional communication exists between the brain and the gut, and alterations in these interactions appear to be involved in stress regulation and in the pathogenesis of neuropsychiatric diseases, such as depression. Serotonin (5HT) plays a crucial role in the functions of these two major organs but its direct influence under stress conditions remains unclear. To investigate the role of neuronal 5HT on chronic stress responses and its influence on the gut microbiome, mice lacking the gene for tryptophan hydroxylase-2 were treated with the stress hormone corticosterone (CORT) for 21 days. The intake of fluid and food, as well as body weights were recorded daily. CORT levels, expression of glucocorticoid receptors (GR) in the brain and the size of the adrenal gland were evaluated. Caecum was used for 16S rRNA gene characterization of the gut microbiota. Results show that 5HT depletion produced an increase in food intake and a paradoxical reduction in body weight that were enhanced by CORT. Neuronal 5HT depletion impaired the feedback regulation of CORT levels but had no putative effect on the CORT-induced decrease in hippocampal GR expression and the reduction of the adrenal cortex size. Finally, the composition and structure of the gut microbiota were significantly impacted by the absence of neuronal 5HT, and these alterations were enhanced by chronic CORT treatment. Therefore, we conclude that neuronal 5HT influences the stress-related responses at different levels involving CORT levels regulation and the gut microbiome.

Serotonergic Plasticity in the Dorsal Raphe Nucleus Characterizes Susceptibility and Resilience to Anhedonia

Chronic stress induces anhedonia in susceptible but not resilient individuals, a phenomenon observed in humans as well as animal models, but the molecular mechanisms underlying susceptibility and resilience are not well understood. We hypothesized that the serotonergic system, which is implicated in stress, reward, and antidepressant therapy, may play a role. We found that plasticity of the serotonergic system contributes to the differential vulnerability to stress displayed by susceptible and resilient animals. Stress-induced anhedonia was assessed in adult male rats using social defeat and intracranial self-stimulation, while changes in serotonergic phenotype were investigated using immunohistochemistry and in situ hybridization. Susceptible, but not resilient, rats displayed an increased number of neurons expressing the biosynthetic enzyme for serotonin, tryptophan-hydroxylase-2 (TPH2), in the ventral subnucleus of the dorsal raphe nucleus (DRv). Further, a decrease in the number of DRv glutamatergic (VGLUT3+) neurons was observed in all stressed rats. This neurotransmitter plasticity is activity-dependent, as was revealed by chemogenetic manipulation of the central amygdala, a stress-sensitive nucleus that forms a major input to the DR. Activation of amygdalar corticotropin-releasing hormone (CRH)+ neurons abolished the increase in DRv TPH2+ neurons and ameliorated stress-induced anhedonia in susceptible rats. These findings show that activation of amygdalar CRH+ neurons induces resilience, and suppresses the gain of serotonergic phenotype in the DRv that is characteristic of susceptible rats. This molecular signature of vulnerability to stress-induced anhedonia and the active nature of resilience could be targeted to develop new treatments for stress-related disorders like depression.SIGNIFICANCE STATEMENT Depression and other mental disorders can be induced by chronic or traumatic stressors. However, some individuals are resilient and do not develop depression in response to chronic stress. A complete picture of the molecular differences between susceptible and resilient individuals is necessary to understand how plasticity of limbic circuits is associated with the pathophysiology of stress-related disorders. Using a rodent model, our study identifies a novel molecular marker of susceptibility to stress-induced anhedonia, a core symptom of depression, and a means to modulate it. These findings will guide further investigation into cellular and circuit mechanisms of resilience, and the development of new treatments for depression.

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

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

Divergent immune responses in behaviorally-inhibited vs. non-inhibited male rats

Stable behavioral traits (temperament, personality) often predict health outcomes. Temperament-specific differences in immune function could explain temperament-specific health outcomes, however, we have limited information on whether immune function varies by personality. In the present study, we examined the relationship between a basic behavioral trait (behavioral-inhibition vs. non-inhibition) and two immune responses (innate inflammation and delayed-type hypersensitivity, DTH) in a rodent model. In humans, behavioral inhibition (fearful temperament) is associated with altered stress physiology and allergies. In laboratory rats, the trait is associated with elevated glucocorticoid production. We hypothesized that behavioral inhibition is associated with glucocorticoid resistance and dampened T-helper 1 cell responses often associated with chronic stress and allergies. Further, this immune profile would predict poorly-regulated innate inflammation and dampened DTH. In male Sprague-Dawley rats, we quantified consistent behavioral phenotypes by measuring latency to contact two kinds of novelty (object vs. social), then measured lipopolysaccharide(LPS)-induced innate inflammation or keyhole limpet hemocyanin(KLH)-induced DTH. Behaviorally-inhibited rats had heightened glucocorticoid and interleukin-6 responses to a low/moderate dose of LPS and reduced DTH swelling to KLH re-exposure compared to non-inhibited rats. These results suggest that behavioral inhibition is associated with a glucocorticoid resistant state with poorly regulated innate inflammation and dampened cell-mediated immune responses. This immune profile may be associated with exaggerated T-helper 2 responses, which could set the stage for an allergic/asthmatic/atopic predisposition in inhibited individuals. Human and animal models of temperament-specific immune responses represent an area for further exploration of mechanisms involved in individual differences in health.

Effects of repeated voluntary or forced exercise on brainstem serotonergic systems in rats

Voluntary exercise increases stress resistance by modulating stress-responsive neurocircuitry, including brainstem serotonergic systems. However, it remains unknown how exercise produces adaptations to serotonergic systems. Recruitment of serotonergic systems during repeated, daily exercise could contribute to the adaptations in serotonergic systems following exercise, but whether repeated voluntary exercise recruits serotonergic systems is unknown. In this study, we investigated the effects of six weeks of voluntary or forced exercise on rat brain serotonergic systems. Specifically, we analyzed c-Fos and FosB/ΔFosB as markers of acute and chronic cellular activation, respectively, in combination with tryptophan hydroxylase, a marker of serotonergic neurons, within subregions of the dorsal raphe nucleus using immunohistochemical staining. Compared to sedentary controls, rats exposed to repeated forced exercise, but not repeated voluntary exercise, displayed decreased c-Fos expression in serotonergic neurons in the rostral dorsal portion of the dorsal raphe nucleus (DRD) and increased c-Fos expression in serotonergic neurons in the caudal DR (DRC), and interfascicular part of the dorsal raphe nucleus (DRI) during the active phase of the diurnal activity rhythm. Similarly, increases in c-Fos expression in serotonergic neurons in the DRC, DRI, and ventral portion of the dorsal raphe nucleus (DRV) were observed in rats exposed to repeated forced exercise, compared to rats exposed to repeated voluntary exercise. Six weeks of forced exercise, relative to the sedentary control condition, also increased FosB/ΔFosB expression in DRD, DRI, and DRV serotonergic neurons. While both voluntary and forced exercise increase stress resistance, these results suggest that repeated forced exercise, but not repeated voluntary exercise, increases activation of DRI serotonergic neurons, an effect that may contribute to the stress resistance effects of forced exercise. These results also suggest that mechanisms of exercise-induced stress resistance may differ depending on the controllability of the exercise.

Ketamine Administration Reverses Corticosterone-Induced Alterations in Excitatory and Inhibitory Transmission in the Rat Dorsal Raphe Nucleus

Ketamine, a N-methyl-D-aspartate (NMDA) receptor antagonist, exerts rapid antidepressant effects in human patients and ameliorates depressive-like behavioral effects of chronic stress in animal models. Chronic stress and elevated corticosterone levels have been shown to modify serotonin (5-HT) neurotransmission, and ketamine's antidepressant-like activity involves a 5-HT-dependent mechanism. However, it is not known if and how ketamine affects the electrophysiological characteristics of neurons and synaptic transmission within the dorsal raphe nucleus (DRN), the main source of 5-HT forebrain projections. Our study was aimed at investigating the effects of a single ketamine administration on excitatory and inhibitory transmission in the DRN of rats which had previously been administered corticosterone twice daily for 7 days. Spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) were then recorded from DRN projection cells in ex vivo slice preparations obtained 24 h after ketamine injection. Repeated corticosterone administration increased sEPSC frequency and decreased sIPSC frequency in DRN projection cells. There were no changes either in the amplitude of postsynaptic currents or in the excitability of these cells. In slices prepared from rats with ketamine administered after the end of corticosterone treatment, the frequencies of sEPSCs and sIPSCs were similar to those in control preparations. These data indicate that a single administration of ketamine reversed the effects of corticosterone on excitatory and inhibitory transmission in the DRN.

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

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

Identification of Cholecystokinin by Genome-Wide Profiling as Potential Mediator of Serotonin-Dependent Behavioral Effects of Maternal Separation in the Amygdala

Converging evidence suggests a role of serotonin (5-hydroxytryptamine, 5-HT) and tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme of 5-HT synthesis in the brain, in modulating long-term, neurobiological effects of early-life adversity. Here, we aimed at further elucidating the molecular mechanisms underlying this interaction, and its consequences for socio-emotional behaviors, with a focus on anxiety and social interaction. In this study, adult, male Tph2 null mutant (Tph2 -/-) and heterozygous (Tph2 +/-) mice, and their wildtype littermates (Tph2 +/+) were exposed to neonatal, maternal separation (MS) and screened for behavioral changes, followed by genome-wide RNA expression and DNA methylation profiling. In Tph2 -/- mice, brain 5-HT deficiency profoundly affected socio-emotional behaviors, i.e., decreased avoidance of the aversive open arms in the elevated plus-maze (EPM) as well as decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Tph2 +/- mice showed an ambiguous profile with context-dependent, behavioral responses. In the EPM they showed similar avoidance of the open arm but decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Notably, MS effects on behavior were subtle and depended on the Tph2 genotype, in particular increasing the observed avoidance of EPM open arms in wildtype and Tph2 +/- mice when compared to their Tph2 -/- littermates. On the genomic level, the interaction of Tph2 genotype with MS differentially affected the expression of numerous genes, of which a subset showed an overlap with DNA methylation profiles at corresponding loci. Remarkably, changes in methylation nearby and expression of the gene encoding cholecystokinin, which were inversely correlated to each other, were associated with variations in anxiety-related phenotypes. In conclusion, next to various behavioral alterations, we identified gene expression and DNA methylation profiles to be associated with TPH2 inactivation and its interaction with MS, suggesting a gene-by-environment interaction-dependent, modulatory function of brain 5-HT availability.

Effects of chronic caffeine exposure during adolescence and subsequent acute caffeine challenge during adulthood on rat brain serotonergic systems

Caffeine is the most commonly used drug in the world. However, animal studies suggest that chronic consumption of caffeine during adolescence can result in enhanced anxiety-like behavioral responses during adulthood. One mechanism through which chronic caffeine administration may influence subsequent anxiety-like responses is through actions on brainstem serotonergic systems. In order to explore potential effects of chronic caffeine consumption on brainstem serotonergic systems, we evaluated the effects of a 28-day exposure to chronic caffeine (0.3 g/L; postnatal day 28-56) or vehicle administration in the drinking water, followed by 24 h caffeine withdrawal, and subsequent challenge with caffeine (30 mg/kg; s.c.) or vehicle in adolescent male rats. In Experiment 1, acute caffeine challenge induced a widespread activation of serotonergic neurons throughout the dorsal raphe nucleus (DR); this effect was attenuated in rats that had been exposed to chronic caffeine consumption. In Experiment 2, acute caffeine administration profoundly decreased tph2 and slc22a3 mRNA expression throughout the DR, with no effects on htr1a or slc6a4 mRNA expression. Chronic caffeine exposure for four weeks during adolescence was sufficient to decrease tph2 mRNA expression in the DR measured 28 h after caffeine withdrawal. Chronic caffeine administration during adolescence did not impact the ability of acute caffeine to decrease tph2 or slc22a3 mRNA expression. Together, these data suggest that both chronic caffeine administration during adolescence and acute caffeine challenge during adulthood are important determinants of serotonergic function and serotonergic gene expression, effects that may contribute to chronic effects of caffeine on anxiety-like responses.

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.

The Impact of Stressor Exposure and Glucocorticoids on Anxiety and Fear

Anxiety disorders and trauma- and stressor-related disorders, such as posttraumatic stress disorder (PTSD), are common and are associated with significant economic and social burdens. Although trauma and stressor exposure are recognized as a risk factors for development of anxiety disorders and trauma or stressor exposure is recognized as essential for diagnosis of PTSD, the mechanisms through which trauma and stressor exposure lead to these disorders are not well characterized. An improved understanding of the mechanisms through which trauma or stressor exposure leads to development and persistence of anxiety disorders or PTSD may result in novel therapeutic approaches for the treatment of these disorders. Here, we review the current state-of-the-art theories, with respect to mechanisms through which stressor exposure leads to acute or chronic exaggeration of avoidance or anxiety-like defensive behavioral responses and fear, endophenotypes in both anxiety disorders and trauma- and stressor-related psychiatric disorders. In this chapter, we will explore physiological responses and neural circuits involved in the development of acute and chronic exaggeration of anxiety-like defensive behavioral responses and fear states, focusing on the role of the hypothalamic-pituitary-adrenal (HPA) axis and glucocorticoid hormones.

Social approach, anxiety, and altered tryptophan hydroxylase 2 activity in juvenile BALB/c and C57BL/6J mice

Autism spectrum disorder (ASD) is a heterogeneous and highly heritable condition with multiple aetiologies. Although the biological mechanisms underlying ASD are not fully understood, evidence suggests that dysregulation of serotonergic systems play an important role in ASD psychopathology. Preclinical models using mice with altered serotonergic neurotransmission may provide insight into the role of serotonin in behaviours relevant to clinical features of ASD. For example, BALB/c mice carry a loss-of-function single nucleotide polymorphism (SNP; C1473 G) in tryptophan hydroxylase 2 (Tph2), which encodes the brain-specific isoform of the rate-limiting enzyme for serotonin synthesis, and these mice frequently have been used to model symptoms of ASD. In this study, juvenile male BALB/c (G/G; loss-of-function variant) and C57BL/6 J (C/C; wild type variant) mice, were exposed to the three-chamber sociability test, and one week later to the elevated plus-maze (EPM). Tryptophan hydroxylase 2 (TPH2) activity was measured following injection of the aromatic amino acid decarboxylase (AADC)-inhibitor, NSD-1015, and subsequent HPLC detection of 5-hydroxytryptophan (5-HTP) within subregions of the dorsal raphe nucleus (DR) and median raphe nucleus (MnR). The BALB/c mice showed reduced social behaviour and increased anxious behaviour, as well as decreased 5-HTP accumulation in the rostral and mid-rostrocaudal DR. In the full cohort of mice, TPH2 activity in the mid-rostrocaudal DR was correlated with anxious behaviour in the EPM, however these correlations were not statistically significant within each strain, suggesting that TPH2 activity was not directly associated with either anxiety or sociability. Further research is therefore required to more fully understand how serotonergic systems are involved in mouse behaviours that resemble some of the clinical features of ASD.

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

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

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

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

Dorsal raphé nucleus glucocorticoid receptors inhibit tph2 gene expression in male C57BL/6J mice

The serotonergic dorsal raphé nucleus (DRN) expresses glucocorticoid receptors (GR), and systemic glucocorticoids have been shown to regulate expression and activity of tryptophan hydroxylase isoform 2, the rate-limiting enzyme for serotonin synthesis in brain. We have used intra-DRN injection of pseudotyped adeno-associated virus AAV2/9 transducing either green fluorescent protein (GFP control) or Cre recombinase (DRN GR deletion) in floxed GR mice to determine if DRN GR directly regulate DRN mRNA levels of tryptophan hydroxylase 2 (tph2). In a separate set of similarly-treated floxed GR mice, we also measured limbic forebrain region concentrations of serotonin (5-hydroxytryptamine; 5-HT) and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA). DRN GR deletion increased tph2 mRNA levels in the dorsal, lateral wing, and caudal parts of the DRN without altering tissue concentrations of 5-HT, 5-HIAA, or the 5-HIAA/5-HT ratio in limbic forebrain regions. We conclude that DRN GR inhibit DRN tph2 gene expression in mice without marked effects on serotonin metabolism, at least under basal conditions at the circadian nadir. These data provide the first evidence of localized control of DRN tph2 mRNA expression by DRN GR in mice.

Increased temporal discounting after chronic stress in CHL1-deficient mice is reversed by 5-HT2C agonist Ro 60-0175

Schizophrenia is a neurodevelopmental disorder in which impaired decision-making and goal-directed behaviors are core features. One of the genes associated with schizophrenia is the Close Homolog of L1 (CHL1); CHL1-deficient mice are considered a model of schizophrenia-like deficits, including sensorimotor gating, interval timing and spatial memory impairments. Here we investigated temporal discounting in CHL1-deficient (KO) mice and their wild-type littermates. Although no discounting differences were found under baseline conditions, CHL1-KO mice showed increased impulsive choice following chronic unpredictable stress (fewer % larger-later choices, and reduced area under the discounting curve). Stressed CHL1-KO mice also showed decreased neuronal activation (number of cFos positive neurons) in the discounting task in the prelimbic cortex and dorsal striatum, areas thought to be part of executive and temporal processing circuits. Impulsive choice alterations were reversed by the 5-HT2C agonist Ro 60-0175. Our results provide evidence for a gene x environment, double-hit model of stress-related decision-making impairments, and identify CHL1-deficient mice as a mouse model for these deficits in regard to schizophrenia-like phenotypes.

Clinical Phenotype of Depression Affects Interleukin-6 Synthesis

Major depressive disorder (MDD) is not a single disease, but a number of various ailments that form one entity. Psychomotor retardation, anhedonia, sleep disorders, an increased suicide risk, and anxiety are the main symptoms that often define the clinical diagnosis of depression. Interleukin-6 (IL-6), as one of the proinflammatory cytokines, seems to be overexpressed during certain mental disorders, including MDD. Overexpression of IL-6 in depression is thought to be a factor associated with bad prognosis and worse disease course. IL-6 may directly affect brain functioning and production of neurotransmitters; moreover, its concentration is correlated with certain clinical symptoms within the wide range of depressive symptomatology. Furthermore, there is a strong correlation between IL-6 synthesis and psychosomatic functioning of the patient. This article discusses potential sources and significance of IL-6 in the pathogenesis of depression.

Association of gene coding variation and resting metabolic rate in a multi-ethnic sample of children and adults

Background

Resting metabolic rates (RMR) vary across individuals. Understanding the determinants of RMR could provide biological insight into obesity and its metabolic consequences such as type 2 diabetes and cardiovascular diseases.

Methods

The present study measured RMR using reference standard indirect calorimetry and evaluated genetic variations from an exome array in a sample of children and adults (N = 262) predominantly of African and European ancestry with a wide range of ages (10 – 67 years old) and body mass indices (BMI; 16.9 – 56.3 kg/m² for adults, 15.1 – 40.6 kg/m2 for children).

Results

Single variant analysis for RMR identified suggestive loci on chromosomes 15 (rs74010762, TRPM1, p-value = 2.7 × 10−6), 1 (rs2358728 and rs2358729, SH3D21, p-values < 5.8x10−5), 17 (AX-82990792, DHX33, 5.5 × 10−5) and 5 (rs115795863 and rs35433829, C5orf33 and RANBP3L, p-values < 8.2 × 10−5). To evaluate the effect of low frequency variations with RMR, we performed gene-based association tests. Our most significant locus was SH3D21 (p-value 2.01 × 10−4), which also contained suggestive results from single-variant analyses. A further investigation of all variants within the reported genes for all obesity-related loci from the GWAS catalog found nominal evidence for association of body mass index (BMI- kg/m²)-associated loci with RMR, with the most significant p-value at rs35433754 (TNKS, p-value = 0.0017).

Conclusions

These nominal associations were robust to adjustment for BMI. The most significant variants were also evaluated using phenome-wide association to evaluate pleiotropy, and genetically predicted gene expression using the summary statistics implicated loci related to in obesity and body composition. These results merit further examination in larger cohorts of children and adults.

Electronic supplementary material

The online version of this article (doi:10.1186/s40608-017-0145-5) contains supplementary material, which is available to authorized users.