Cell type-specific modifications of corticotropin-releasing factor (CRF) and its type 1 receptor (CRF1) on startle behavior and sensorimotor gating

CB1 receptors in corticotropin-releasing factor neurons selectively control the acoustic startle response in male mice

The endocannabinoid system is an important regulator of the hormonal and behavioral stress responses, which critically involve corticotropin-releasing factor (CRF) and its receptors. While it has been shown that CRF and the cannabinoid type 1 (CB1) receptor are co-localized in several brain regions, the physiological relevance of this co-expression remains unclear. Using double in situ hybridization, we confirmed co-localization in the piriform cortex, the lateral hypothalamic area, the paraventricular nucleus, and the Barrington's nucleus, albeit at low levels. To study the behavioral and physiological implications of this co-expression, we generated a conditional knockout mouse line that selectively lacks the expression of CB1 receptors in CRF neurons. We found no effects on fear and anxiety-related behaviors under basal conditions nor after a traumatic experience. Additionally, plasma corticosterone levels were unaffected at baseline and after restraint stress. Only acoustic startle responses were significantly enhanced in male, but not female, knockout mice. Taken together, the consequences of depleting CB1 in CRF-positive neurons caused a confined hyperarousal phenotype in a sex-dependent manner. The current results suggest that the important interplay between the central endocannabinoid and CRF systems in regulating the organism's stress response is predominantly taking place at the level of CRF receptor-expressing neurons.

Long-term molecular differences between resilient and susceptible mice after a single traumatic exposure

BACKGROUND AND PURPOSE

PTSD is a heterogeneous disorder induced by trauma, resulting in severe long-term impairments of an individual's mental health. Interestingly, PTSD does not develop in every individual; thus, some individuals are more resilient than others. However, the underlying molecular mechanisms are poorly understood. Here, we aimed at shedding light on these processes.

EXPERIMENTAL APPROACH

We used a single-trauma PTSD model in mice to induce long-term maladaptive behaviours and profiled the mice four weeks post-trauma into resilient or susceptible individuals. The phenotype's classification was based on their individual responses in different behavioural experiments. We analysed microbiome, circulating endocannabinoids, and long-term changes in brain phospholipid and transcript levels.

KEY RESULTS

We found a plethora of molecular differences between resilient and susceptible individuals across multiple molecular domains, including lipidome, transcriptome, and gut microbiome. Some of these differences were stable even several weeks after the trauma, indicating the long-term impact of traumatic stimuli on the organism's physiology. Furthermore, the integration of these multi-layered molecular data revealed that resilient and susceptible individuals have very distinct molecular signatures across various physiological systems.

CONCLUSIONS AND IMPLICATIONS

We showed that trauma induces individual-specific behavioural responses that, in combination with a longitudinal characterization of mice, can be used to identify distinct sub-phenotypes within the trauma-exposed group. These groups differ significantly not only in their behaviour but also in specific molecular aspects across a variety of tissues and brain regions. This approach may reveal new targets and predictive biomarkers for the pharmacological treatment and prognosis of stress-related disorders.

The implications of hypothalamic abnormalities for schizophrenia

Until a few years ago, the hypothalamus was believed to play only a marginal role in schizophrenia pathophysiology. However, recent findings show that this rather small brain region involved in many pathways found disrupted-in schizophrenia. Gross anatomic abnormalities (volume changes of the third ventricle, the hypothalamus, and its individual nuclei) as well as alterations at the cellular level (circumscribed loss of neurons) can be observed. Further, increased or decreased expression of hypothalamic peptides such as oxytocin, vasopressin, several factors involved in the regulation of appetite and satiety, endogenous opiates, products of schizophrenia susceptibility genes as well as of enzymes involved in neurotransmitter and neuropeptide metabolism have been reported in schizophrenia and/or animal models of the disease. Remarkably, although profound disturbances of the hypothalamus-pituitary-adrenal axis, hypothalamus-pituitary-thyroid axis, and the hypothalamus-pituitary-gonadal axis are typical signs of schizophrenia, there is currently no evidence for alterations in the expression of hypothalamic-releasing and inhibiting factors that control these hormonal axes. Finally, the implications of hypothalamus for disease-related disturbances of the sleep-wakefulness cycle and neuroimmune dysfunctions in schizophrenia are outlined.

Vesicular monoamine transporter 2 mediates fear behavior in mice

A subset of people exposed to a traumatic event develops post-traumatic stress disorder (PTSD), which is associated with dysregulated fear behavior. Genetic variation in SLC18A2, the gene that encodes vesicular monoamine transporter 2 (VMAT2), has been reported to affect risk for the development of PTSD in humans. Here, we use transgenic mice that express either 5% (VMAT2-LO mice) or 200% (VMAT2-HI mice) of wild-type levels of VMAT2 protein. We report that VMAT2-LO mice have reduced VMAT2 protein in the hippocampus and amygdala, impaired monoaminergic vesicular storage capacity in both the striatum and frontal cortex, decreased monoamine metabolite abundance and a greatly reduced capacity to release dopamine upon stimulation. Furthermore, VMAT2-LO mice showed exaggerated cued and contextual fear expression, altered fear habituation, inability to discriminate threat from safety cues, altered startle response compared with wild-type mice and an anxiogenic-like phenotype, but displayed no deficits in social function. By contrast, VMAT2-HI mice exhibited increased VMAT2 protein throughout the brain, higher vesicular storage capacity and greater dopamine release upon stimulation compared with wild-type controls. Behaviorally, VMAT2-HI mice were similar to wild-type mice in most assays, with some evidence of a reduced anxiety-like responses. Together, these data show that presynaptic monoamine function mediates PTSD-like outcomes in our mouse model, and suggest a causal link between reduced VMAT2 expression and fear behavior, consistent with the correlational relationship between VMAT2 genotype and PTSD risk in humans. Targeting this system is a potential strategy for the development of pharmacotherapies for disorders like PTSD.

Deletion of CRH From GABAergic Forebrain Neurons Promotes Stress Resilience and Dampens Stress-Induced Changes in Neuronal Activity

Dysregulation of the corticotropin-releasing hormone (CRH) system has been implicated in stress-related psychopathologies such as depression and anxiety. Although most studies have linked CRH/CRH receptor 1 signaling to aversive, stress-like behavior, recent work has revealed a crucial role for distinct CRH circuits in maintaining positive emotional valence and appetitive responses under baseline conditions. Here we addressed whether deletion of CRH, specifically from GABAergic forebrain neurons (Crh ^CKO-GABA mice) differentially affects general behavior under baseline and chronic stress conditions. Expression mapping in Crh ^{CK O-GABA} mice revealed absence of Crh in GABAergic neurons of the cortex and limbic regions including the hippocampus, central nucleus of the amygdala and the bed nucleus of the stria terminals, but not in the paraventricular nucleus of hypothalamus. Consequently, conditional CRH knockout animals exhibited no alterations in circadian and stress-induced corticosterone release compared to controls. Under baseline conditions, absence of Crh from forebrain GABAergic neurons resulted in social interaction deficits but had no effect on other behavioral measures including locomotion, anxiety, immobility in the forced swim test, acoustic startle response and fear conditioning. Interestingly, following exposure to chronic social defeat stress, Crh ^CKO-GABA mice displayed a resilient phenotype, which was accompanied by a dampened, stress-induced expression of immediate early genes c-fos and zif268 in several brain regions. Collectively our data reveals the requirement of GABAergic CRH circuits in maintaining appropriate social behavior in naïve animals and further supports the ability of CRH to promote divergent behavioral states under baseline and severe stress conditions.

Recent advances in the neurobiology of posttraumatic stress disorder: A review of possible mechanisms underlying an effective pharmacotherapy

Recent progress in the field of neurobiology supported by clinical evidence gradually reveals the mystery of human brain functioning. So far, many psychiatric disorders have been described in great detail, although there are still plenty of cases that are misunderstood. These include posttraumatic stress disorder (PTSD), which is a unique disease that combines a wide range of neurobiological changes, which involve disturbances of the hypothalamic-pituitary-adrenal gland axis, hyperactivation of the amygdala complex, and attenuation of some hippocampal and cortical functions. Such multiplicity results in differential symptomatology, including elevated anxiety, nightmares, fear retrieval episodes that may trigger delusions and hallucinations, sleep disturbances, and many others that strongly interfere with the quality of the patient's life. Because of widespread neurological changes and the disease manifestation, the pharmacotherapy of PTSD remains unclear and requires a multidimensional approach and involvement of polypharmacotherapy. Hopefully, more and more neuroscientists and clinicians will study PTSD, which will provide us with new information that would possibly accelerate establishment of well-tolerated and effective pharmacotherapy. In this review, we have focused on neurobiological changes regarding PTSD, addressing the most disturbed brain structures and neurotransmissions, as well as discussing in detail the recently taken and novel therapeutic paths.

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.

Role of environmental stressors in determining the developmental outcome of neonatal anesthesia

BACKGROUND

The majority of studies evaluating neurocognition in humans who had procedures under anesthesia early in life found long-term deficits even though the typical anesthesia duration normalized to the human life span is much shorter than that shown to induce developmental abnormalities in rodents. Therefore, we studied whether subsequent environmental stressors contribute to deficiencies programmed by a brief neonatal etomidate exposure.

METHODS

Postnatal days (P) 4, 5, or 6, Sprague-Dawley rats, pretreated with vehicle or the Na+-K+-2Cl- (NKCC1) inhibitor, bumetanide, received two injections of etomidate resulting in anesthesia for 2h. To simulate stress after anesthesia, the animals were exposed to a single maternal separation for 3h at P10. 3-7days after exposure to etomidate the rats had increased hypothalamic NKCC1 mRNA and corticotropin releasing hormone (CRH) mRNA and decreased K+-2Cl- (KCC2) mRNA levels with greater changes in males. In rats neonatally exposed to both etomidate and maternal separation, these abnormalities persisted into adulthood. These animals also exhibited extended corticosterone responses to restraint stress with increases in total plasma corticosterone more robust in males, as well as behavioral abnormalities. Pretreatment with the NKCC1 inhibitor ameliorated most of these effects.

CONCLUSIONS

Post-anesthesia stressors may exacerbate/unmask neurodevelopmental abnormalities even after a relatively short anesthetic with etomidate, leading to dysregulated stress response systems and neurobehavioral deficiencies in adulthood. Amelioration by bumetanide suggests a mechanistic role for etomidate-enhanced gamma-aminobutyric acid type A receptor-mediated depolarization in initiating long-lasting alterations in gene expression that are further potentiated by subsequent maternal separation.

Relationship between Toxoplasma gondii seropositivity and acoustic startle response in an inner-city population

Toxoplasma gondii (TOXO) is a neuroinvasive protozoan parasite that induces the formation of persistent cysts in mammalian brains. It infects approximately 1.1million people in the United States annually. Latent TOXO infection is implicated in the etiology of psychiatric disorders, especially schizophrenia (SCZ), and has been correlated with modestly impaired cognition. The acoustic startle response (ASR) is a reflex seen in all mammals. It is mediated by a simple subcortical circuit, and provides an indicator of neural function. We previously reported the association of TOXO with slowed acoustic startle latency, an index of neural processing speed, in a sample of schizophrenia and healthy control subjects. The alterations in neurobiology with TOXO latent infection may not be specific to schizophrenia. Therefore we examined TOXO in relation to acoustic startle in an urban, predominately African American, population with mixed psychiatric diagnoses, and healthy controls. Physiological and diagnostic data along with blood samples were collected from 364 outpatients treated at an inner-city hospital. TOXO status was determined with an ELISA assay for TOXO-specific IgG. A discrete titer was calculated based on standard cut-points as an indicator of seropositivity, and the TOXO-specific IgG concentration served as serointensity. A series of linear regression models were used to assess the association of TOXO seropositivity and serointensity with ASR magnitude and latency in models adjusting for demographics and psychiatric diagnoses (PTSD, major depression, schizophrenia, psychosis, substance abuse). ASR magnitude was 11.5% higher in TOXO seropositive subjects compared to seronegative individuals (p=0.01). This effect was more pronounced in models with TOXO serointensity that adjusted for sociodemographic covariates (F=7.41, p=0.0068; F=10.05, p=0.0017), and remained significant when psychiatric diagnoses were stepped into the models. TOXO showed no association with startle latency (t=0.49, p=0.63) in an unadjusted model, nor was TOXO associated with latency in models that included demographic factors. After stepping in individual psychiatric disorders, we found a significant association of latency with a diagnosis of PTSD (F=5.15, p=0.024), but no other psychiatric diagnoses, such that subjects with PTSD had longer startle latency. The mechanism by which TOXO infection is associated with high startle magnitude is not known, but possible mechanisms include TOXO cyst burden in the brain, parasite recrudescence, or molecular mimicry of a host epitope by TOXO. Future studies will focus on the neurobiology underlying the effects of latent TOXO infection as a potential inroad to the development of novel treatment targets for psychiatric disease.

HIGH AND LOW THRESHOLD FOR STARTLE REACTIVITY ASSOCIATED WITH PTSD SYMPTOMS BUT NOT PTSD RISK: EVIDENCE FROM A PROSPECTIVE STUDY OF ACTIVE DUTY MARINES

BACKGROUND

Heightened startle response is a symptom of PTSD, but evidence for exaggerated startle in PTSD is inconsistent. This prospective study aimed to clarify whether altered startle reactivity represents a trait risk-factor for developing PTSD or a marker of current PTSD symptoms.

METHODS

Marines and Navy Corpsmen were assessed before (n = 2,571) and after (n = 1,632) deployments to Iraq or Afghanistan with the Clinician-Administered PTSD Scale (CAPS). A predeployment startle-threshold task was completed with startle probes presented over 80-114 dB[A] levels. Latent class mixture modeling identified three growth classes of startle performance: "high," "low," and "moderate" threshold classes. Zero-inflated negative binomial regression was used to assess relationships between predeployment startle threshold and pre- and postdeployment psychiatric symptoms.

RESULTS

At predeployment, the low-threshold class had higher PTSD symptom scores. Relative to the moderate-threshold class, low-threshold class membership was associated with decreased likelihood of being symptom-free at predeployment, based on CAPS, with particular associations with numbing and hyperarousal subscales, whereas high-threshold class membership was associated with more severe predeployment PTSD symptoms, in particular avoidance. Associations between low-threshold membership and CAPS symptoms were independent from measures of trauma burden, whereas associations between high-threshold membership and CAPS were not. Predeployment startle threshold did not predict postdeployment symptoms.

CONCLUSIONS

This study found that both low startle threshold (heightened reactivity) and high startle threshold (blunted reactivity) were associated with greater current PTSD symptomatology, suggesting that startle reactivity is associated with current PTSD rather than a risk marker for developing PTSD.

Supraphysiological hormonal status, anxiety disorders, and COMT Val/Val genotype are associated with reduced sensorimotor gating in women

Pregnancy is a period characterized by a supraphysiological hormonal status, and greater anxiety proneness, which can lead to peripartum affective symptoms with dramatic consequences not only for the woman but also for the child. Clinical psychiatry is heavily hampered by the paucity of objective and biology-based intermediate phenotypes. Prepulse inhibition (PPI) of the startle response, a neurophysiological measure of sensorimotor gating, has been poorly investigated in relation to anxiety and in pregnant women. In the present study, the PPI of healthy non-pregnant women (n = 82) and late pregnant women (n = 217) was investigated. Age, BMI, depression and anxiety symptoms, tobacco use, and antidepressant medication were considered. We investigated and provided evidence of lower PPI: (i) in healthy pregnant women compared to healthy non-pregnant controls, (ii) in pregnant women with anxiety disorders compared to healthy pregnant women, (iii) in pregnant women with anxiety disorders using SSRI compared to un-medicated pregnant women with anxiety disorders, and (iv) in healthy pregnant women carrying the COMT Val158Met Val/Val genotype compared to Met carriers. Altogether, a reduced sensorimotor gating as an effect of supraphysiological hormonal status, anxiety disorders, SSRIs, and catecholaminergic genotype, implicate the putative relevance of lower PPI as an objective biological correlate of anxiety proneness in pregnant women. These findings call for prospective studies to dissect the multifactorial influences on PPI in relation to mental health of pregnant women.