A.E. Bennett Research Award. Anatomic basis for differential regulation of the rostrolateral peri-locus coeruleus region by limbic afferents

A Mechanistic Analysis of the Neural Modulation of the Inflammatory System Through Vagus Nerve Stimulation: A Systematic Review and Meta-analysis

OBJECTIVE

We aimed to conduct a systematic review and meta-analysis assessing the antiinflammatory effects of various VNS methods while exploring multiple antiinflammatory pathways.

MATERIALS AND METHODS

We included clinical trials that used electrical stimulation of the vagus nerve and assessed inflammatory markers up to October 2022. We excluded studies lacking control groups, those with combined interventions, or abstracts without full text. We adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines and the Cochrane Handbook for Systematic Reviews. For each inflammatory marker, a random-effects meta-analysis using the inverse variance method was performed. Methods used include transcutaneous auricular VNS (taVNS), transcutaneous cervical VNS (tcVNS), invasive cervical VNS (iVNS), and electroacupuncture VNS (eaVNS). Main reported outcomes included tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-1ß, C-reactive protein (CRP), and IL-10. Risk of bias was evaluated using the Cochrane Collaboration Tool (RoB 2.0).

RESULTS

This review included 15 studies, involving 597 patients. No statistically significant general VNS effect was observed on TNF-α, IL-6, and IL-1ß. However, CRP, IL-10, and interferon (IFN)-γ were significantly modulated by VNS across all methods. Subgroup analysis revealed specific stimulation techniques producing significant results, such as taVNS effects in IL-1ß and IL-10, and iVNS in IL-6, whereas tcVNS and eaVNS did not convey significant pooled results individually. Cumulative exposure to VNS, higher risk of bias, study design, and pulse width were identified as effect size predictors in our meta-regression models.

CONCLUSIONS

Pooling all VNS techniques indicated the ability of VNS to modulate inflammatory markers such as CRP, IL-10, and IFN-γ. Individually, methods such as taVNS were effective in modulating IL-1ß and IL-10, whereas iVNS modulated IL-6. However, different VNS techniques should be separately analyzed in larger, homogeneous, and powerful studies to achieve a clearer and more consistent understanding of the effect of each VNS method on the inflammatory system.

Transcutaneous vagus nerve stimulation modulates depression-like phenotype induced by high-fat diet via P2X7R/NLRP3/IL-1β in the prefrontal cortex

BACKGROUND

Depression is a common psychiatric disorder in diabetic patients. Depressive mood associated with obesity/metabolic disorders is related to the inflammatory response caused by long-term consumption of high-fat diets, but its molecular mechanism is unclear. In this study, we investigated whether the antidepressant effect of transcutaneous auricular vagus nerve stimulation (taVNS) in high-fat diet rats works through the P2X7R/NLRP3/IL-1β pathway.

METHODS

We first used 16S rRNA gene sequencing analysis and LC-MS metabolomics assays in Zucker diabetic fatty (ZDF) rats with long-term high-fat diet (Purina #5008) induced significant depression-like behaviors. Next, the forced swimming test (FST) and open field test (OFT) were measured to evaluate the antidepressive effect of taVNS. Immunofluorescence and western blotting (WB) were used to measure the microglia state and the expression of P2X7R, NLRP3, and IL-1β in PFC.

RESULTS

Purina#5008 diet induced significant depression-like behaviors in ZDF rats and was closely related to purine and inflammatory metabolites. Consecutive taVNS increased plasma insulin concentration, reduced glycated hemoglobin and glucagon content in ZDF rats, significantly improved the depressive-like phenotype in ZDF rats through reducing the microglia activity, and increased the expression of P2X7R, NLRP3, and IL-1β in the prefrontal cortex (PFC).

CONCLUSION

The P2X7R/NLRP3/IL-1β signaling pathway may play an important role in the antidepressant-like behavior of taVNS, which provides a promising mechanism for taVNS clinical treatment of diabetes combined with depression.

Chemogenetic activation of CRF neurons as a model of chronic stress produces sex-specific physiological and behavioral effects

Trauma and chronic stress exposure are the strongest predictors of lifetime neuropsychiatric disease presentation. These disorders often have significant sex biases, with females having higher incidences of affective disorders such as major depression, anxiety, and PTSD. Understanding the mechanisms by which stress exposure heightens disease vulnerability is essential for developing novel interventions. Current rodent stress models consist of a battery of sensory, homeostatic, and psychological stressors that are ultimately integrated by corticotropin-releasing factor (CRF) neurons to trigger corticosteroid release. These stress paradigms, however, often differ between research groups in the type, timing, and duration of stressors utilized. These inconsistencies, along with the variability of individual animals' perception and response to each stressor, present challenges for reproducibility and translational relevance. Here, we hypothesized that a more direct approach using chemogenetic activation of CRF neurons would recapitulate the effects of traditional stress paradigms and provide a high-throughput method for examining stress-relevant phenotypes. Using a transgenic approach to express the Gq-coupled Designer Receptor Exclusively Activated by Designer Drugs (DREADD) receptor hM3Dq in CRF-neurons, we found that the DREADD ligand clozapine-N-oxide (CNO) produced an acute and robust activation of the hypothalamic-pituitary-adrenal (HPA) axis, as predicted. Interestingly, chronic treatment with this method of direct CRF activation uncovered a novel sex-specific dissociation of glucocorticoid levels with stress-related outcomes. Despite hM3Dq-expressing females producing greater corticosterone levels in response to CNO than males, hM3Dq-expressing males showed significant typical physiological stress sensitivity with reductions in body and thymus weights. hM3Dq-expressing females while resistant to the physiological effects of chronic CRF activation, showed significant increases in baseline and fear-conditioned freezing behaviors. These data establish a novel mouse model for interrogating stress-relevant phenotypes and highlight sex-specific stress circuitry distinct for physiological and limbic control that may underlie disease risk.

Corticotropin-releasing hormone neurons in the central nucleus of amygdala are required for chronic stress-induced hypertension

AIMS

Chronic stress is a well-known risk factor for the development of hypertension. However, the underlying mechanisms remain unclear. Corticotropin-releasing hormone (CRH) neurons in the central nucleus of the amygdala (CeA) are involved in the autonomic responses to chronic stress. Here, we determined the role of CeA-CRH neurons in chronic stress-induced hypertension.

METHODS AND RESULTS

Borderline hypertensive rats (BHRs) and Wistar-Kyoto (WKY) rats were subjected to chronic unpredictable stress (CUS). Firing activity and M-currents of CeA-CRH neurons were assessed, and a CRH-Cre-directed chemogenetic approach was used to suppress CeA-CRH neurons. CUS induced a sustained elevation of arterial blood pressure (ABP) and heart rate (HR) in BHRs, while in WKY rats, CUS-induced increases in ABP and HR quickly returned to baseline levels after CUS ended. CeA-CRH neurons displayed significantly higher firing activities in CUS-treated BHRs than unstressed BHRs. Selectively suppressing CeA-CRH neurons by chemogenetic approach attenuated CUS-induced hypertension and decreased elevated sympathetic outflow in CUS-treated BHRs. Also, CUS significantly decreased protein and mRNA levels of Kv7.2 and Kv7.3 channels in the CeA of BHRs. M-currents in CeA-CRH neurons were significantly decreased in CUS-treated BHRs compared with unstressed BHRs. Blocking Kv7 channel with its blocker XE-991 increased the excitability of CeA-CRH neurons in unstressed BHRs but not in CUS-treated BHRs. Microinjection of XE-991 into the CeA increased sympathetic outflow and ABP in unstressed BHRs but not in CUS-treated BHRs.

CONCLUSIONS

CeA-CRH neurons are required for chronic stress-induced sustained hypertension. The hyperactivity of CeA-CRH neurons may be due to impaired Kv7 channel activity, which represents a new mechanism involved in chronic stress-induced hypertension.

Probing the structure and function of locus coeruleus projections to CNS motor centers

The brainstem nucleus locus coeruleus (LC) sends projections to the forebrain, brainstem, cerebellum and spinal cord and is a source of the neurotransmitter norepinephrine (NE) in these areas. For more than 50 years, LC was considered to be homogeneous in structure and function such that NE would be released uniformly and act simultaneously on the cells and circuits that receive LC projections. However, recent studies have provided evidence that LC is modular in design, with segregated output channels and the potential for differential release and action of NE in its projection fields. These new findings have prompted a radical shift in our thinking about LC operations and demand revision of theoretical constructs regarding impact of the LC-NE system on behavioral outcomes in health and disease. Within this context, a major gap in our knowledge is the relationship between the LC-NE system and CNS motor control centers. While we know much about the organization of the LC-NE system with respect to sensory and cognitive circuitries and the impact of LC output on sensory guided behaviors and executive function, much less is known about the role of the LC-NE pathway in motor network operations and movement control. As a starting point for closing this gap in understanding, we propose using an intersectional recombinase-based viral-genetic strategy TrAC (Tracing Axon Collaterals) as well as established ex vivo electrophysiological assays to characterize efferent connectivity and physiological attributes of mouse LC-motor network projection neurons. The novel hypothesis to be tested is that LC cells with projections to CNS motor centers are scattered throughout the rostral-caudal extent of the nucleus but collectively display a common set of electrophysiological properties. Additionally, we expect to find these LC projection neurons maintain an organized network of axon collaterals capable of supporting selective, synchronous release of NE in motor circuitries for the purpose of coordinately regulating operations across networks that are responsible for balance and movement dynamics. Investigation of this hypothesis will advance our knowledge of the role of the LC-NE system in motor control and provide a basis for treating movement disorders resulting from disease, injury, or normal aging.

Vagus nerve stimulation for conservative therapy-refractive epilepsy and depression

Numerous studies confirm that the vagus nerve stimulation (VNS) is an efficient, indirect neuromodulatory therapy with electrically induced current for epilepsy that cannot be treated by epilepsy surgery and is therapy-refractory and for drug therapy-refractory depression. VNS is an established, evidence-based and in the long-term cost-effective therapy in an interdisciplinary overall concept.Long-term data on the safety and tolerance of the method are available despite the heterogeneity of the patient populations. Stimulation-related side effects like hoarseness, paresthesia, cough or dyspnea depend on the stimulation strength and often decrease with continuing therapy duration in the following years. Stimulation-related side effects of VNS can be well influenced by modifying the stimulation parameters. Overall, the invasive vagus nerve stimulation may be considered as a safe and well-tolerated therapy option.For invasive and transcutaneous vagus nerve stimulation, antiepileptic and antidepressant as well as positive cognitive effects could be proven. In contrast to drugs, VNS has no negative effect on cognition. In many cases, an improvement of the quality of life is possible.iVNS therapy has a low probability of complete seizure-freedom in cases of focal and genetically generalized epilepsy. It must be considered as palliative therapy, which means that it does not lead to healing and requires the continuation of specific medication. The functional principle is a general reduction of the neuronal excitability. This effect is achieved by a slow increase of the effectiveness sometimes over several years. Responders are those patients who experience a 50% reduction of the seizure incidence. Some studies even reveal seizure-freedom in 20% of the cases. Currently, it is not possible to differentiate between potential responders and non-responders before therapy/implantation.The current technical developments of the iVNS generators of the new generation like closed-loop system (cardiac-based seizure detection, CBSD) reduce also the risk for SUDEP (sudden unexpected death in epilepsy patients), a very rare, lethal complication of epilepsies, beside the seizure severity.iVNS may deteriorate an existing sleep apnea syndrome and therefore requires possible therapy interruption during nighttime (day-night programming or magnet use) beside the close cooperation with sleep physicians.The evaluation of the numerous iVNS trials of the past two decades showed multiple positive effects on other immunological, cardiological, and gastroenterological diseases so that additional therapy indications may be expected depending on future study results. Currently, the vagus nerve stimulation is in the focus of research in the disciplines of psychology, immunology, cardiology as well as pain and plasticity research with the desired potential of future medical application.Beside invasive vagus nerve stimulation with implantation of an IPG and an electrode, also devices for transdermal and thus non-invasive vagus nerve stimulation have been developed during the last years. According to the data that are currently available, they are less effective with regard to the reduction of the seizure severity and duration in cases of therapy-refractory epilepsy and slightly less effective regarding the improvement of depression symptoms. In this context, studies are missing that confirm high evidence of effectiveness. The same is true for the other indications that have been mentioned like tinnitus, cephalgia, gastrointestinal complaints etc. Another disadvantage of transcutaneous vagus nerve stimulation is that the stimulators have to be applied actively by the patients and are not permanently active, in contrast to implanted iVNS therapy systems. So they are only intermittently active; furthermore, the therapy adherence is uncertain.

Effect of Vagus Nerve Stimulation on Attention and Working Memory in Neuropsychiatric Disorders: A Systematic Review

BACKGROUND

It has been suggested that vagus nerve stimulation (VNS) may enhance attention and working memory. The neuromodulator effects of VNS are thought to activate the release of neurotransmitters involving cognition and to promote neuronal plasticity. Therefore, VNS has been studied for its effects on attention and working memory impairment in neuropsychiatric disorders.

OBJECTIVES

This study aimed to assess the effects of VNS on attention and working memory among patients with neuropsychiatric disorders, examine stimulation parameters, provide mechanistic hypotheses, and propose future studies using VNS.

MATERIALS AND METHODS

We conducted a systematic review using electronic databases MEDLINE (Ovid), Embase (Ovid), Cochrane library, and PsycINFO (Ovid). Narrative analysis was used to describe the therapeutic effects of VNS on attention and working memory, describe stimulation parameters, and propose explanatory mechanisms.

RESULTS

We identified 20 studies reporting VNS effects on attention and working memory in patients with epilepsy or mood disorders. For epilepsy, there was one randomized controlled trial from all 18 studies. It demonstrated no statistically significant differences in the cognitive tasks between active and control VNS. From a within-subject experimental design, significant improvement of working memory after VNS was demonstrated. One of three nonrandomized controlled trials found significantly improved attentional performance after VNS. The cohort studies compared VNS and surgery and found attentional improvement in both groups. Nine of 12 pretest-posttest studies showed improvement of attention or working memory after VNS. For mood disorders, although one study showed significant improvement of attention following VNS, the other did not.

CONCLUSIONS

This review suggests that, although we identified some positive results from eligible studies, there is insufficient good-quality evidence to establish VNS as an effective intervention to enhance attention and working memory in persons with neuropsychiatric disorders. Further studies assessing the efficacy of such intervention are needed.

Neurophysiologic Effects of Transcutaneous Auricular Vagus Nerve Stimulation (taVNS) via Electrical Stimulation of the Tragus: A Concurrent taVNS/fMRI Study and Review

(Appeared originally in Brain Stimulation 2018; 11:492-500) Reprinted with permission from Elsevier.

Effect of Short-Term Transcutaneous Vagus Nerve Stimulation (tVNS) on Brain Processing of Food Cues: An Electrophysiological Study

Background: The vagus nerve plays an important role in the regulation of food intake. Modulating vagal activity via electrical stimulation (VNS) in patients and animal studies caused changes in food intake, energy metabolism, and body weight. However, the moderating impact of cognitive processes on VNS effects on eating behavior has not been investigated so far.

Hypothesis: We hypothesized that transcutaneous VNS (tVNS) affects food intake by altering cognitive functions relevant to the processing of food-related information.

Methods: Using a repeated-measurement design, we applied tVNS and a sham stimulation for 2 h on two different days in normal-weight subjects. We recorded standard scalp EEG while subjects watched food and object pictures presented in an oddball task. We analyzed the event-related potentials (ERPs) P1, P2, N2, and LPP and also examined the amount of consumed food and eating duration in a free-choice test meal.

Results: Significant differences between stimulations were observed for the P1, P2, and N2 amplitudes. However, we found no tVNS-dependent modulation of food intake nor a specific food-related stimulation effect on the ERPs. Further analyses revealed a negative relationship between P2 amplitude and food intake for the sham stimulation. Significant effects are additionally confirmed by Bayesian statistics.

Conclusion: Our study demonstrates tVNS’ impact on visual processing. Since the effects were similar between food and object stimuli, a general effect on visual perceptual processing can be assumed. More detailed investigations of these effects and their relationship with food intake and metabolism seem reasonable for future studies.

Sex differences in the effect of cannabinoid type 1 receptor deletion on locus coeruleus-norepinephrine neurons and corticotropin releasing factor-mediated responses

Cannabinoids are capable of modulating mood, arousal, cognition and behavior, in part via their effects on the noradrenergic nucleus locus coeruleus (LC). Dysregulation of LC signaling and norepinephrine (NE) efflux in the medial prefrontal cortex (mPFC) can lead to the development of psychiatric disorders, and CB1r deletion results in alterations of α2- and β1-adrenoceptors in the mPFC, suggestive of increased LC activity. To determine how CB1r deletion alters LC signaling, whole-cell patch-clamp electrophysiology was conducted in LC-NE neurons of male and female wild type (WT) and CB1r-knock out (KO) mice. CB1r deletion caused a significant increase in LC-NE excitability and input resistance in male but not female mice when compared to WT. CB1r deletion also caused adaptations in several indices of noradrenergic function. CB1r/CB2r-KO male mice had a significant increase in cortical NE levels and tyrosine hydroxylase and CRF levels in the LC compared to WT males. CB1r/CB2r-KO female mice showed a significant increase in LC α2-AR levels compared to WT females. To further probe actions of the endocannabinoid system as an anti-stress neuromediator, the effect of CB1r deletion on CRF-induced responses in the LC was investigated. The increase in LC-NE excitability observed in male and female WT mice following CRF (300 nM) bath application was not observed in CB1r-KO mice. These results indicate that cellular adaptations following CB1r deletion cause a disruption in LC-NE signaling in males but not females, suggesting underlying sex differences in compensatory mechanisms in KO mice as well as basal endocannabinoid regulation of LC-NE activity.

Corticotropin-Releasing Factor-Producing Cells in the Paraventricular Nucleus of the Hypothalamus and Extended Amygdala Show Age-Dependent FOS and FOSB/DeltaFOSB Immunoreactivity in Acute and Chronic Stress Models in the Rat

Corticotropin-releasing factor (CRF) immunoreactive (ir) neurons of the paraventricular nucleus of the hypothalamus (PVN) play pivotal role in the coordination of stress response. CRF-producing cells in the central nucleus of amygdala (CeA) and oval division of the bed nucleus of stria terminalis (BNSTov) are also involved in stress adaptation and mood control. Immediate early gene products, subunits of the transcription factor activator protein 1 (AP1) are commonly used as acute (FOS) and/or chronic (FOSB/deltaFOSB) markers for the neuronal activity in stress research. It is well known that the course of aging affects stress adaptation, but little is known about the aging-related stress sensitivity of CRF neurons. To the best of our knowledge, the stress-induced neuronal activity of CRF neurons in the course of aging in acute and chronic stress models was not studied systematically yet. Therefore, the aim of the present study was to quantify the acute restraint stress (ARS) and chronic variable mild stress (CVMS) evoked neuronal activity in CRF cells of the PVN, CeA, and BNSTov using triple-label immunofluorescence throughout the whole lifespan in the rat. We hypothesized that the FOS and FOSB content of CRF cells upon ARS or CVMS decreases with age. Our results showed that the FOS and FOSB response to ARS declined with age in the PVN-CRF cells. BNSTov and CeA CRF cells did not show remarkable stress-induced elevation of these markers neither in ARS, nor in CVMS. Exposure to CVMS resulted in an age-independent significant increase of FOSB/delta FOSB immunosignal in PVN-CRF neurons. Unexpectedly, we detected a remarkable stress-independent FOSB/deltaFOSB signal in CeA- and BNSTov-CRF cells that declined with the course of aging. In summary, PVN-CRF cells show decreasing acute stress sensitivity (i.e., FOS and FOSB immunoreactivity) with the course of aging, while their (FOSB/deltaFOSB) responsivity to chronic challenge is maintained till senescence. Stress exposure does not affect the occurrence of the examined Fos gene products in CeA- and BNSTov-CRF cells remarkably suggesting that their contribution to stress adaptation response does not require AP1-controlled transcriptional changes.

Sex differences in stress reactivity in arousal and attention systems

Women are more likely than men to suffer from psychiatric disorders with hyperarousal symptoms, including posttraumatic stress disorder (PTSD) and major depression. In contrast, women are less likely than men to be diagnosed with schizophrenia and attention deficit hyperactivity disorder (ADHD), which share attentional impairments as a feature. Stressful events exacerbate symptoms of the aforementioned disorders. Thus, researchers are examining whether sex differences in stress responses bias women and men towards different psychopathology. Here we review the preclinical literature suggesting that, compared to males, females are more vulnerable to stress-induced hyperarousal, while they are more resilient to stress-induced attention deficits. Specifically described are sex differences in receptors for the stress neuropeptide, corticotropin-releasing factor (CRF), that render the locus coeruleus arousal system of females more vulnerable to stress and less adaptable to CRF hypersecretion, a condition found in patients with PTSD and depression. Studies on the protective effects of ovarian hormones against CRF-induced deficits in sustained attention are also detailed. Importantly, we highlight how comparing males and females in preclinical studies can lead to the development of novel therapeutics to improve treatments for psychiatric disorders in both women and men.

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

Background:

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

Objective:

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

Results:

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

Conclusion:

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

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

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

The Central Amygdala Corticotropin-releasing hormone (CRH) Neurons Modulation of Anxiety-like Behavior and Hippocampus-dependent Memory in Mice

The encoding, consolidation and retrieval of memories is a multifaceted process that depends strongly on the optimal level of arousal but high levels of arousal may trigger anxiety, which negatively impacts the memory processing by the brain. We investigated the role of CRH neurons in the central amygdala (CeA) for their capacity to modulate both, the anxiety-like behavior and hippocampus-dependent memory. First, we activated the CRH neurons in CeA using cre-dependent AAV-DREADD in CRH-cre mice. The activation of CeA CRH neurons increased the anxiety-like behavior in Elevated-O maze (O-maze) and Light-Dark box (LDB). The activation of the CeA CRH also decreased Y-maze memory performance and the discrimination index in novel object recognition test (NOR). The inhibition of CeA CRH neurons with AAV-DREADD had the opposite effects on the anxiety-like behavior and the memory tests. Next, we used a combination of retrograde cre virus injected into locus ceruleus (LC) and cre-dependent AAV-DREADD injected into the CeA. While the excitation of the CeA neurons that project to LC increased the anxiety-like behavior, it also led to a better performance on the memory tests. The behavioral and memory effects were accompanied by increased c-Fos expression in the LC region. Pretreatment with CRH1 receptor antagonist antalarmin hydrochloride blocked the effects that were observed after the activation of the CeA projections to LC. Our findings highlight the role of CeA CRH neuronal population not only as a generator of anxiety but also demonstrate their role in the control of hippocampus-dependent memory.

A Comprehensive Overview on Stress Neurobiology: Basic Concepts and Clinical Implications

Stress is recognized as an important issue in basic and clinical neuroscience research, based upon the founding historical studies by Walter Canon and Hans Selye in the past century, when the concept of stress emerged in a biological and adaptive perspective. A lot of research after that period has expanded the knowledge in the stress field. Since then, it was discovered that the response to stressful stimuli is elaborated and triggered by the, now known, stress system, which integrates a wide diversity of brain structures that, collectively, are able to detect events and interpret them as real or potential threats. However, different types of stressors engage different brain networks, requiring a fine-tuned functional neuroanatomical processing. This integration of information from the stressor itself may result in a rapid activation of the Sympathetic-Adreno-Medullar (SAM) axis and the Hypothalamus-Pituitary-Adrenal (HPA) axis, the two major components involved in the stress response. The complexity of the stress response is not restricted to neuroanatomy or to SAM and HPA axes mediators, but also diverge according to timing and duration of stressor exposure, as well as its short- and/or long-term consequences. The identification of neuronal circuits of stress, as well as their interaction with mediator molecules over time is critical, not only for understanding the physiological stress responses, but also to understand their implications on mental health.

Neurophysiologic effects of transcutaneous auricular vagus nerve stimulation (taVNS) via electrical stimulation of the tragus: A concurrent taVNS/fMRI study and review

BACKGROUND

Electrical stimulation of the auricular branch of the vagus nerve (ABVN) via transcutaneous auricular vagus nerve stimulation (taVNS) may influence afferent vagal networks. There have been 5 prior taVNS/fMRI studies, with inconsistent findings due to variability in stimulation targets and parameters.

OBJECTIVE

We developed a taVNS/fMRI system to enable concurrent electrical stimulation and fMRI acquisition to compare the effects of taVNS in relation to control stimulation.

METHODS

We enrolled 17 healthy adults in this single-blind, crossover taVNS/fMRI trial. Based on parameters shown to affect heart rate in healthy volunteers, participants received either left tragus (active) or earlobe (control) stimulation at 500 μs 25 HZ for 60 s (repeated 3 times over 6 min). Whole brain fMRI analysis was performed exploring the effect of: active stimulation, control stimulation, and the comparison. Region of interest analysis of the midbrain and brainstem was also conducted.

RESULTS

Active stimulation produced significant increased BOLD signal in the contralateral postcentral gyrus, bilateral insula, frontal cortex, right operculum, and left cerebellum. Control stimulation produced BOLD signal activation in the contralateral postcentral gyrus. In the active vs. control contrast, tragus stimulation produced significantly greater BOLD increases in the right caudate, bilateral anterior cingulate, cerebellum, left prefrontal cortex, and mid-cingulate.

CONCLUSION

Stimulation of the tragus activates the cerebral afferents of the vagal pathway and combined with our review of the literature suggest that taVNS is a promising form of VNS. Future taVNS/fMRI studies should systematically explore various parameters and alternative stimulation targets aimed to optimize this novel form of neuromodulation.

Vagal nerve stimulation triggers widespread responses and alters large-scale functional connectivity in the rat brain

Vagus nerve stimulation (VNS) is a therapy for epilepsy and depression. However, its efficacy varies and its mechanism remains unclear. Prior studies have used functional magnetic resonance imaging (fMRI) to map brain activations with VNS in human brains, but have reported inconsistent findings. The source of inconsistency is likely attributable to the complex temporal characteristics of VNS-evoked fMRI responses that cannot be fully explained by simplified response models in the conventional model-based analysis for activation mapping. To address this issue, we acquired 7-Tesla blood oxygenation level dependent fMRI data from anesthetized Sprague-Dawley rats receiving electrical stimulation at the left cervical vagus nerve. Using spatially independent component analysis, we identified 20 functional brain networks and detected the network-wise activations with VNS in a data-driven manner. Our results showed that VNS activated 15 out of 20 brain networks, and the activated regions covered >76% of the brain volume. The time course of the evoked response was complex and distinct across regions and networks. In addition, VNS altered the strengths and patterns of correlations among brain networks relative to those in the resting state. The most notable changes in network-network interactions were related to the limbic system. Together, such profound and widespread effects of VNS may underlie its unique potential for a wide range of therapeutics to relieve central or peripheral conditions.

Monoaminergic Modulation of Motor Cortex Function

Elaboration of appropriate responses to behavioral situations rests on the ability of selecting appropriate motor outcomes in accordance to specific environmental inputs. To this end, the primary motor cortex (M1) is a key structure for the control of voluntary movements and motor skills learning. Subcortical loops regulate the activity of the motor cortex and thus contribute to the selection of appropriate motor plans. Monoamines are key mediators of arousal, attention and motivation. Their firing pattern enables a direct encoding of different states thus promoting or repressing the selection of actions adapted to the behavioral context. Monoaminergic modulation of motor systems has been extensively studied in subcortical circuits. Despite evidence of converging projections of multiple neurotransmitters systems in the motor cortex pointing to a direct modulation of local circuits, their contribution to the execution and learning of motor skills is still poorly understood. Monoaminergic dysregulation leads to impaired plasticity and motor function in several neurological and psychiatric conditions, thus it is critical to better understand how monoamines modulate neural activity in the motor cortex. This review aims to provide an update of our current understanding on the monoaminergic modulation of the motor cortex with an emphasis on motor skill learning and execution under physiological conditions.

Sex differences in stress regulation of arousal and cognition

There are sex differences in the prevalence and presentation of many psychiatric disorders. For example, posttraumatic stress disorder (PTSD) and major depression are more common in women than men, and women with these disorders present with more hyperarousal symptoms than men. In contrast, attention deficit hyperactivity disorder (ADHD) and schizophrenia are more common in men than women, and men with these disorders have increased cognitive deficits compared to women. A shared feature of the aforementioned psychiatric disorders is the contribution of stressful events to their onset and/or severity. Here we propose that sex differences in stress responses bias females towards hyperarousal and males towards cognitive deficits. Evidence from clinical and preclinical studies is detailed. We also describe underlying neurobiological mechanisms. For example, sex differences in stress receptor signaling and trafficking in the locus coeruleus-arousal center are detailed. In learning circuits, evidence for sex differences in dendritic morphology is provided. Finally, we describe how evaluating sex-specific mechanisms for responding to stress in female and male rodents can lead to better treatments for stress-related psychiatric disorders.

Co-localization of the cannabinoid type 1 receptor with corticotropin-releasing factor-containing afferents in the noradrenergic nucleus locus coeruleus: implications for the cognitive limb of the stress response

The noradrenergic system has been shown to play a key role in the regulation of stress responses, arousal, mood, and emotional states. Corticotropin-releasing factor (CRF) is a primary mediator of stress-induced activation of noradrenergic neurons in the nucleus locus coeruleus (LC). The endocannabinoid (eCB) system also plays a key role in modulating stress responses, acting as an "anti-stress" neuro-mediator. In the present study, we investigated the cellular sites for interactions between the cannabinoid receptor type 1 (CB1r) and CRF in the LC. Immunofluorescence and high-resolution immunoelectron microscopy showed co-localization of CB1r and CRF in both the core and peri-LC areas. Semi-quantitative analysis revealed that 44% (208/468) of CRF-containing axon terminals in the core and 35% (104/294) in the peri-LC expressed CB1r, while 18% (85/468) of CRF-containing axon terminals in the core and 6.5% (19/294) in the peri-LC were presynaptic to CB1r-containing dendrites. In the LC core, CB1r + CRF axon terminals were more frequently of the symmetric (inhibitory) type; while in the peri-LC, a majority were of the asymmetric (excitatory) type. Triple label immunofluorescence results supported the ultrastructural analysis indicating that CB1r + CRF axon terminals contained either gamma amino butyric acid or glutamate. Finally, anterograde transport from the central nucleus of the amygdala revealed that CRF-amygdalar afferents projecting to the LC contain CB1r. Taken together, these results indicate that the eCB system is poised to directly modulate stress-integrative heterogeneous CRF afferents in the LC, some of which arise from limbic sources.

Neuroplasticity of Supraspinal Structures Associated with Pathological Pain

Peripheral nerve and spinal cord injuries, along with other painful syndromes such as fibromyalgia, diabetic neuropathy, chemotherapeutic neuropathy, trigeminal neuralgia, complex regional pain syndrome, and/or irritable bowel syndrome, cause several neuroplasticity changes in the nervous system along its entire axis affecting the different neuronal nuclei. This paper reviews these changes, focusing on the supraspinal structures that are involved in the modulation and processing of pain, including the periaqueductal gray matter, red nucleus, locus coeruleus, rostral ventromedial medulla, thalamus, hypothalamus, basal ganglia, cerebellum, habenula, primary, and secondary somatosensory cortex, motor cortex, mammillary bodies, hippocampus, septum, amygdala, cingulated, and prefrontal cortex. Hyperexcitability caused by the modification of postsynaptic receptor expression, central sensitization, and potentiation of presynaptic delivery of neurotransmitters, as well as the reduction of inhibitory inputs, changes in dendritic spine, neural circuit remodeling, alteration of gray matter, and upregulation of proinflammatory mediators (e.g., cytokines) by reactivation of astrocytes and microglial cells are the main functional, structural, and molecular neuroplasticity changes observed in the above supraspinal structures, associated with pathological pain. Studying these changes in greater depth may lead to the implementation and improvement of new therapeutic strategies against pathological pain. Anat Rec, 300:1481-1501, 2017. © 2017 Wiley Periodicals, Inc.

Amygdalar Gating of Early Sensory Processing through Interactions with Locus Coeruleus

Fear- and stress-induced activity in the amygdala has been hypothesized to influence sensory brain regions through the influence of the amygdala on neuromodulatory centers. To directly examine this relationship, we used optical imaging to observe odor-evoked activity in populations of olfactory bulb inhibitory interneurons and of synaptic terminals of olfactory sensory neurons (the primary sensory neurons of the olfactory system, which provide the initial olfactory input to the brain) during pharmacological inactivation of amygdala and locus coeruleus (LC) in mice. Although the amygdala does not directly project to the olfactory bulb, joint pharmacological inactivation of the central, basolateral, and lateral nuclei of the amygdala nonetheless strongly suppressed odor-evoked activity in GABAergic inhibitory interneuron populations in the OB. This suppression was prevented by inactivation of LC or pretreatment of the olfactory bulb with a broad-spectrum noradrenergic receptor antagonist. Visualization of synaptic output from olfactory sensory neuron terminals into the olfactory bulb of the brain revealed that amygdalar inactivation preferentially strengthened the odor-evoked synaptic output of weakly activated populations of sensory afferents from the nose, thus demonstrating a change in sensory gating potentially mediated by local inhibition of olfactory sensory neuron terminals. We conclude that amygdalar activity influences olfactory processing as early as the primary sensory input to the brain by modulating norepinephrine release from the locus coeruleus into the olfactory bulb. These findings show that the amygdala and LC state actively determines which sensory signals are selected for processing in sensory brain regions. Similar local circuitry operates in the olfactory, visual, and auditory systems, suggesting a potentially shared mechanism across modalities.SIGNIFICANCE STATEMENT The affective state is increasingly understood to influence early neural processing of sensory stimuli, not just the behavioral response to those stimuli. The present study elucidates one circuit by which the amygdala, a critical structure for emotional learning, valence coding, and stress, can shape sensory input to the brain and early sensory processing through its connections to the locus coeruleus. One function of this interaction appears to be sensory gating, because inactivating the central, basolateral, and lateral nuclei of the amygdala selectively strengthened the weakest olfactory inputs to the brain. This linkage of amygdalar and LC output to primary sensory signaling may have implications for affective disorders that include sensory dysfunctions like hypervigilance, attentional bias, and impaired sensory gating.

Corticotropin releasing factor dose-dependently modulates excitatory synaptic transmission in the noradrenergic nucleus locus coeruleus

The noradrenergic nucleus locus coeruleus (LC) is critically involved in the stress response and receives afferent input from a number of corticotropin releasing factor (CRF) containing structures. Several in vivo and in vitro studies in rat have shown that CRF robustly increases the firing rate of LC neurons in a dose-dependent manner. While it is known that these increases are dependent on CRF receptor subtype 1 and mediated by effects of cAMP intracellular signaling cascades on potassium conductance, the impact of CRF on synaptic transmission within LC has not been clarified. In the present study, we used whole-cell patch clamp electrophysiology to assess how varying concentrations of bath-applied CRF affect AMPA-receptor dependent spontaneous excitatory post-synaptic currents (sEPSCs). Compared to vehicle, 10, 25, and 100 nm CRF had no significant effects on any sEPSC parameters. Fifty nanomolar CRF, however, significantly increased sEPSC amplitude, half-width, and charge transfer, while these measures were significantly decreased by 200 nm CRF. These observations suggest that stress may differentially affect ongoing excitatory synaptic transmission in LC depending on how much CRF is released from presynaptic terminals. Combined with the well-documented effects of CRF on membrane properties and spontaneous LC discharge, these observations may help explain how stress and CRF release are able to modulate the signal to noise ratio of LC neurons. These findings have implications for how stress affects the fidelity of signal transmission and information flow through LC and how it might impact norepinephrine release in the CNS.

Individual differences in the locus coeruleus-norepinephrine system: Relevance to stress-induced cardiovascular vulnerability

Repeated exposure to psychosocial stress is a robust sympathomimetic stressor and as such has adverse effects on cardiovascular health. While the neurocircuitry involved remains unclear, the physiological and anatomical characteristics of the locus coeruleus (LC)-norepinephrine (NE) system suggest that it is poised to contribute to stress-induced cardiovascular vulnerability. A major theme throughout is to review studies that shed light on the role that the LC may play in individual differences in vulnerability to social stress-induced cardiovascular dysfunction. Recent findings are discussed that support a unique plasticity in afferent regulation of the LC, resulting in either excitatory or inhibitory input to the LC during establishment of different stress coping strategies. This contrasting regulation of the LC by either afferent regulation, or distinct differences in stress-induced neuroinflammation would translate to differences in cardiovascular regulation and may serve as the basis for individual differences in the cardiopathological consequences of social stress. The goal of this review is to highlight recent developments in the interplay between the LC-NE and cardiovascular systems during repeated stress in an effort to advance therapeutic treatments for the development of stress-induced cardiovascular vulnerability.

Evidence for a specialized role of the locus coeruleus noradrenergic system in cortical circuitries and behavioral operations

The brainstem nucleus locus coeruleus (LC) innervates the entire central nervous system and is the primary source of norepinephrine (NE) to the neocortex. While classically considered a homogenous modulator of forebrain activity by virtue of highly widespread and divergent axons, recent behavioral and pharmacological evidence suggest this nucleus may execute distinct operations within functionally distinct terminal fields. Summarized in this review are the anatomical and physiological properties of the nucleus within a historical context that led to the interpretation of the nucleus as a homogeneous entity with uniform and simultaneous actions throughout its terminal fields. Also included are findings from several laboratories which point to a more nuanced model of LC/NE function that parallels that seen in other forebrain-projecting monoaminergic nuclei. Such compartmentalized models of the nucleus promote the idea that specific LC circuits are involved in discrete behavioral operations, and therefore, by identifying the networks that are engaged by LC, the substrates for these behaviors can be identified and manipulated. Perturbations in the functional anatomy and physiology of this system may be related to neuropsychiatric conditions associated with dysregulation of the LC-noradrenergic system such as attention deficit hyperactivity disorder. Recent findings regarding the organization and operation of the LC/NE system collectively challenge the classical view of the nucleus as a relatively homogenous modulator of forebrain activity and provide the basis for a renewed scientific interest in this region of the brain. This article is part of a Special Issue entitled SI: Noradrenergic System.

The brain norepinephrine system, stress and cardiovascular vulnerability

Chronic exposure to psychosocial stress has adverse effects on cardiovascular health, however the stress-sensitive neurocircuitry involved remains to be elucidated. The anatomical and physiological characteristics of the locus coeruleus (LC)-norepinephrine (NE) system position it to contribute to stress-induced cardiovascular disease. This review focuses on cardiovascular dysfunction produced by social stress and a major theme highlighted is that differences in coping strategy determine individual differences in social stress-induced cardiovascular vulnerability. The establishment of different coping strategies and cardiovascular vulnerability during repeated social stress has recently been shown to parallel a unique plasticity in LC afferent regulation, resulting in either excitatory or inhibitory input to the LC. This contrasting regulation of the LC would translate to differences in cardiovascular regulation and may serve as the basis for individual differences in the cardiopathological consequences of social stress. The advances described suggest new directions for developing treatments and/or strategies for decreasing stress-induced cardiovascular vulnerability.

Dynorphin-Dependent Reduction of Excitability and Attenuation of Inhibitory Afferents of NPS Neurons in the Pericoerulear Region of Mice

The Neuropeptide S system, consisting of the 20-amino acid peptide neuropeptide S (NPS) and its G-protein coupled receptor (NPSR), modulates arousal, wakefulness, anxiety, and fear-extinction in mice. In addition, recent evidence indicates that the NPS system attenuates stress-dependent impairment of fear extinction, and that NPS-expressing neurons in close proximity to the locus coeruleus region (LC; pericoerulear, periLC) are activated by stress. Furthermore, periLC NPS neurons receive afferents from neurons of the centrolateral nucleus of the amygdala (CeL), of which a substantial population expresses the kappa opioid receptor (KOR) ligand precursor prodynorphin. This study aims to identify the effect of the dynorphinergic system on NPS neurons in the periLC via pre- and postsynaptic mechanisms. Using electrophysiological recordings in mouse brain slices, we provide evidence that NPS neurons in the periLC region are directly inhibited by dynorphin A (DynA) via activation of κ-opioid receptor 1 (KOR1) and a subsequent increase of potassium conductances. Thus, the dynorphinergic system is suited to inactivate NPS neurons in the periLC. In addition to this direct, somatic effect, DynA reduces the efficacy of GABAergic synapses on NPS neurons via KOR1 and KOR2. In conclusion, the present study provides evidence for the interaction of the NPS and the kappa opioid system in the periLC. Therefore, the endogenous opioid dynorphin is suited to inhibit NPS neurons with a subsequent decrease in NPS release in putative target regions leading to a variety of physiological consequences such as increased anxiety or vulnerability to stress exposure.

Stress Modulation of Opposing Circuits in the Bed Nucleus of the Stria Terminalis

The anterior bed nucleus of the stria terminalis (BNST) has been recognized as a critical structure in regulating trait anxiety, contextual fear memory, and appetitive behavior, and is known to be sensitive to stress manipulations. As one of the most complex structures in the central nervous system, the intrinsic circuitry of the BNST is largely unknown; however, recent technological developments have allowed researchers to begin to untangle the internal connections of the nucleus. This research has revealed the possibility of two opposing circuits, one anxiolytic and one anxiogenic, within the BNST, the relative strength of which determines the behavioral outcome. The balance of these pathways is critical in maintaining a normal physiological and behavioral state; however, stress and drugs of abuse can differentially affect the opposing circuitry within the nucleus to shift the balance to a pathological state. In this review, we will examine how stress interacts with the neuromodulators, corticotropin-releasing factor, norepinephrine, dopamine, and serotonin to affect the circuitry of the BNST as well as how synaptic plasticity in the BNST is modulated by stress, resulting in long-lasting changes in the circuit and behavioral state.

Corticotropin releasing factor impairs sustained attention in male and female rats

Stressful life events and stress-related psychiatric disorders impair sustained attention, the ability to monitor rare and unpredictable stimulus events over prolonged periods of time. Despite the link between stress and attentional disruptions, the neurobiological basis for stress regulation of attention systems remains underexplored. Here we examined whether corticotropin releasing factor (CRF), which orchestrates stress responses and is hypersecreted in patients with stress-related psychiatric disorders, impairs sustained attention. To this end, male and female rats received central infusions of CRF prior to testing on an operant sustained attention task (SAT), where rats were trained to discriminate signaled from non-signaled events. CRF caused a dose-dependent decrease in SAT performance in both male and female rats. Females were more impaired than males following a moderate dose of CRF, particularly during the middle part of the session. This sex difference was moderated by ovarian hormones. Females in the estrous cycle stage characterized by lower ovarian hormones had a greater CRF-induced attentional impairment than males and females in other cycle stages. Collectively, these studies highlight CRF as a critical stress-related factor that can regulate attentional performance. As sustained attention subserves other cognitive processes, these studies suggest that mitigating high levels of CRF in patients with stress-related psychiatric disorders may ameliorate their cognitive deficits.

Corticotropin releasing factor up-regulates the expression and function of norepinephrine transporter in SK-N-BE (2) M17 cells

Corticotropin releasing factor (CRF) has been implicated to act as a neurotransmitter or modulator in central nervous activation during stress. In this study, we examined the regulatory effect of CRF on the expression and function of the norepinephrine transporter (NET) in vitro. SK-N-BE (2) M17 cells were exposed to different concentrations of CRF for different periods. Results showed that exposure of cells to CRF significantly increased mRNA and protein levels of NET in a concentration- and time-dependent manner. The CRF-induced increase in NET expression was mimicked by agonists of either CRF receptor 1 or 2. Furthermore, similar CRF treatments induced a parallel increase in the uptake of [(3) H] norepinephrine. Both increased expression and function of NET caused by CRF were abolished by simultaneous administration of CRF receptor antagonists, indicating a mediation by CRF receptors. However, there was no additive effect for the combination of both receptor antagonists. Chromatin immunoprecipitation assays confirm an increased acetylation of histone H3 on the NET promoter following treatment with CRF. Taken together, this study demonstrates that CRF up-regulates the expression and function of NET in vitro. This regulation is mediated through CRF receptors and an epigenetic mechanism related to histone acetylation may be involved. This CRF-induced regulation on NET expression and function may play a role in development of stress-related depression and anxiety. This study demonstrated that corticotropin release factor (CRF) up-regulated the expression and function of norepinephrine transporter (NET) in a concentration- and time-dependent manner, through activation of CRF receptors and possible histone acetylation in NET promoter. The results indicate that their interaction may play an important role in stress-related physiological and pathological status.

Endogenous Opioids: The Downside of Opposing Stress

Our dynamic environment regularly exposes us to potentially life-threatening challenges or stressors. To answer these challenges and maintain homeostasis, the stress response, an innate coordinated engagement of central and peripheral neural systems is initiated. Although essential for survival, the inappropriate initiation of the stress response or its continuation after the stressor is terminated has pathological consequences that have been linked to diverse neuropsychiatric and medical diseases. Substantial individual variability exists in the pathological consequences of stressors. A theme of this Special Issue is that elucidating the basis of individual differences in resilience or its flipside, vulnerability, will greatly advance our ability to prevent and treat stress-related diseases. This can be approached by studying individual differences in "pro-stress" mediators such as corticosteroids or the hypothalamic orchestrator of the stress response, corticotropin-releasing factor. More recently, the recognition of endogenous neuromodulators with "anti-stress" activity that have opposing actions or that restrain stress-response systems suggests additional bases for individual differences in stress pathology. These "anti-stress" neuromodulators offer alternative strategies for manipulating the stress response and its pathological consequences. This review uses the major brain norepinephrine system as a model stress-response system to demonstrate how co-regulation by opposing pro-stress (corticotropin-releasing factor) and anti-stress (enkephalin) neuromodulators must be fine-tuned to produce an adaptive response to stress. The clinical consequences of tipping this fine-tuned balance in the direction of either the pro- or anti-stress systems are emphasized. Finally, that each system provides multiple points at which individual differences could confer stress vulnerability or resilience is discussed.

Sex differences in stress-related psychiatric disorders: neurobiological perspectives

Stress is associated with the onset and severity of several psychiatric disorders that occur more frequently in women than men, including posttraumatic stress disorder (PTSD) and depression. Patients with these disorders present with dysregulation of several stress response systems, including the neuroendocrine response to stress, corticolimbic responses to negatively valenced stimuli, and hyperarousal. Thus, sex differences within their underlying circuitry may explain sex biases in disease prevalence. This review describes clinical studies that identify sex differences within the activity of these circuits, as well as preclinical studies that demonstrate cellular and molecular sex differences in stress responses systems. These studies reveal sex differences from the molecular to the systems level that increase endocrine, emotional, and arousal responses to stress in females. Exploring these sex differences is critical because this research can reveal the neurobiological underpinnings of vulnerability to stress-related psychiatric disorders and guide the development of novel pharmacotherapies.

Vagus nerve stimulation for the treatment of depression and other neuropsychiatric disorders

Vagus nerve stimulation is an interesting new approach to treating neuropsychiatric diseases within the class of brain-stimulation devices sometimes labeled 'neuromodulators'. With vagus nerve stimulation, a battery-powered generator implanted in the chest wall connects to a wire wrapped around the vagus nerve in the neck, and sends intermittent pulses of electricity along the nerve directly into the brain. This mechanism takes advantage of the natural role of the vagus nerve in conveying information into the brain concerning homeostatic information (e.g., hunger, chest pain and respirations). Vagus nerve stimulation therapy is US FDA approved for the adjunctive treatment of epilepsy and has recently been FDA approved for the treatment of medication-resistant depression. Owing to its novel route into the brain, it has no drug-drug interactions or systemic side effects. This treatment also appears to have high long-term tolerability in patients, with low rates of patients relapsing on vagus nerve stimulation or becoming tolerant. However, alongside the excitement and enthusiasm for this new treatment, a lack of Class I evidence of efficacy in treating depression is currently slowing down adoption by psychiatrists. Much more research is needed regarding exactly how to refine and deliver the electrical pulses and how this differentially affects brain function in health and disease.

Forebrain GABAergic projections to locus coeruleus in mouse

The noradrenergic locus coeruleus (LC) regulates arousal, memory, sympathetic nervous system activity, and pain. Forebrain projections to LC have been characterized in rat, cat, and primates, but not systematically in mouse. We surveyed mouse forebrain LC-projecting neurons by examining retrogradely labeled cells following LC iontophoresis of Fluoro-Gold and anterograde LC labeling after forebrain injection of biotinylated dextran amine or viral tracer. Similar to other species, the central amygdalar nucleus (CAmy), anterior hypothalamus, paraventricular nucleus, and posterior lateral hypothalamic area (PLH) provide major LC inputs. By using mice expressing green fluorescent protein in γ-aminobutyric acid (GABA)ergic neurons, we found that more than one-third of LC-projecting CAmy and PLH neurons are GABAergic. LC colocalization of biotinylated dextran amine, following CAmy or PLH injection, with either green fluorescent protein or glutamic acid decarboxylase (GAD)65/67 immunoreactivity confirmed these GABAergic projections. CAmy injection of adeno-associated virus encoding channelrhodopsin-2-Venus showed similar fiber labeling and association with GAD65/67-immunoreactive (ir) and tyrosine hydroxylase (TH)-ir neurons. CAmy and PLH projections were densest in a pericoerulear zone, but many fibers entered the LC proper. Close apposition between CAmy GABAergic projections and TH-ir processes suggests that CAmy GABAergic neurons may directly inhibit noradrenergic principal neurons. Direct LC neuron targeting was confirmed by anterograde transneuronal labeling of LC TH-ir neurons following CAmy or PLH injection of a herpes virus that expresses red fluorescent protein following activation by Cre recombinase in mice that express Cre recombinase in GABAergic neurons. This description of GABAergic projections from the CAmy and PLH to the LC clarifies important forebrain sources of inhibitory control of central nervous system noradrenergic activity.

Locus coeruleus kappa-opioid receptors modulate reinstatement of cocaine place preference through a noradrenergic mechanism

Activation of kappa-opioid receptors (KORs) in monoamine circuits results in dysphoria-like behaviors and stress-induced reinstatement of drug seeking in both conditioned place preference (CPP) and self-administration models. Noradrenergic (NA) receptor systems have also been implicated in similar behaviors. Dynorphinergic projections terminate within the locus coeruleus (LC), a primary source of norepinephrine in the forebrain, suggesting a possible link between the NA and dynorphin/kappa opioid systems, yet the implications of these putative interactions have not been investigated. We isolated the necessity of KORs in the LC in kappa opioid agonist (U50,488)-induced reinstatement of cocaine CPP by blocking KORs in the LC with NorBNI (KOR antagonist). KOR-induced reinstatement was significantly attenuated in mice injected with NorBNI in the LC. To determine the sufficiency of KORs in the LC on U50,488-induced reinstatement of cocaine CPP, we virally re-expressed KORs in the LC of KOR knockout mice. We found that KORs expression in the LC alone was sufficient to partially rescue KOR-induced reinstatement. Next we assessed the role of NA signaling in KOR-induced reinstatement of cocaine CPP in the presence and absence of a α2-agonist (clonidine), β-adrenergic receptor antagonist (propranolol), and β(1)- and β(2)-antagonist (betaxolol and ICI-118,551 HCl). Both the blockade of postsynaptic β(1)-adrenergic receptors and the activation of presynaptic inhibitory adrenergic autoreceptors selectively potentiated the magnitude of KOR-induced reinstatement of cocaine CPP but not cocaine-primed CPP reinstatement. Finally, viral restoration of KORs in the LC together with β-adrenergic receptor blockade did not potentiate KOR-induced reinstatement to cocaine CPP, suggesting that adrenergic receptor interactions occur at KOR-expressing regions external to the LC. These results identify a previously unknown interaction between KORs and NA systems and suggest a NA regulation of KOR-dependent reinstatement of cocaine CPP.

Coordinate regulation of noradrenergic and serotonergic brain regions by amygdalar neurons

Based on the importance of the locus coeruleus-norepinephrine (LC-NE) system and the dorsal raphe nucleus-serotonergic (DRN-5-HT) system in stress-related pathologies, additional understanding of brain regions coordinating their activity is of particular interest. One such candidate is the amygdalar complex, and specifically, the central nucleus (CeA), which has been implicated in emotional arousal and is known to send monosynaptic afferent projections to both these regions. Our present data using dual retrograde tract tracing is the first to demonstrate a population of amygdalar neurons that project in a collateralized manner to the LC and DRN, indicating that amygdalar neurons are positioned to coordinately regulate the LC and DRN, and links these brain regions by virtue of a common set of afferents. Further, we have also characterized the phenotype of a population of these collaterally projecting neurons from the amygdala as containing corticotropin releasing factor or dynorphin, two peptides heavily implicated in the stress response. Understanding the co-regulatory influences of this limbic region on 5HT and NE regions may help fill a gap in our knowledge regarding neural circuits impacting these systems and their adaptations in stress.

CNS BOLD fMRI effects of sham-controlled transcutaneous electrical nerve stimulation in the left outer auditory canal - a pilot study

BACKGROUND

It has recently been shown that electrical stimulation of sensory afferents within the outer auditory canal may facilitate a transcutaneous form of central nervous system stimulation. Functional magnetic resonance imaging (fMRI) blood oxygenation level dependent (BOLD) effects in limbic and temporal structures have been detected in two independent studies. In the present study, we investigated BOLD fMRI effects in response to transcutaneous electrical stimulation of two different zones in the left outer auditory canal. It is hypothesized that different central nervous system (CNS) activation patterns might help to localize and specifically stimulate auricular cutaneous vagal afferents.

METHODOLOGY

16 healthy subjects aged between 20 and 37 years were divided into two groups. 8 subjects were stimulated in the anterior wall, the other 8 persons received transcutaneous vagus nervous stimulation (tVNS) at the posterior side of their left outer auditory canal. For sham control, both groups were also stimulated in an alternating manner on their corresponding ear lobe, which is generally known to be free of cutaneous vagal innervation. Functional MR data from the cortex and brain stem level were collected and a group analysis was performed.

RESULTS

In most cortical areas, BOLD changes were in the opposite direction when comparing anterior vs. posterior stimulation of the left auditory canal. The only exception was in the insular cortex, where both stimulation types evoked positive BOLD changes. Prominent decreases of the BOLD signals were detected in the parahippocampal gyrus, posterior cingulate cortex and right thalamus (pulvinar) following anterior stimulation. In subcortical areas at brain stem level, a stronger BOLD decrease as compared with sham stimulation was found in the locus coeruleus and the solitary tract only during stimulation of the anterior part of the auditory canal.

CONCLUSIONS

The results of the study are in line with previous fMRI studies showing robust BOLD signal decreases in limbic structures and the brain stem during electrical stimulation of the left anterior auditory canal. BOLD signal decreases in the area of the nuclei of the vagus nerve may indicate an effective stimulation of vagal afferences. In contrast, stimulation at the posterior wall seems to lead to unspecific changes of the BOLD signal within the solitary tract, which is a key relay station of vagal neurotransmission. The results of the study show promise for a specific novel method of cranial nerve stimulation and provide a basis for further developments and applications of non-invasive transcutaneous vagus stimulation in psychiatric patients.

Increased vulnerability of the brain norepinephrine system of females to corticotropin-releasing factor overexpression

Stress-related psychiatric disorders are more prevalent in women than men. As hypersecretion of the stress neuromediator, corticotropin-releasing factor (CRF) has been implicated in these disorders, sex differences in CRF sensitivity could underlie this disparity. Hyperarousal is a core symptom that is shared by stress-related disorders and this has been attributed to CRF regulation of the locus ceruleus (LC)-norepinephrine arousal system. We recently identified sex differences in CRF(1) receptor (CRF(1)) signaling and trafficking that render LC neurons of female rats more sensitive to CRF and potentially less able to adapt to excess CRF compared with male rats. The present study used a genetic model of CRF overexpression to test the hypothesis that females would be more vulnerable to LC dysregulation by conditions of excess CRF. In both male and female CRF overexpressing (CRF-OE) mice, the LC was more densely innervated by CRF compared with wild-type controls. Despite the equally dense CRF innervation of the LC in male and female CRF-OE mice, LC discharge rates recorded in slices in vitro were selectively elevated in female CRF-OE mice. Immunoelectron microscopy revealed that this sex difference resulted from differential CRF(1) trafficking. In male CRF-OE mice, CRF(1) immunolabeling was prominent in the cytoplasm of LC neurons, indicative of internalization, a process that would protect cells from excessive CRF. However, in female CRF-OE mice, CRF(1) labeling was more prominent on the plasma membrane, suggesting that the compensatory response of internalization was compromised. Together, the findings suggest that the LC-norepinephrine system of females will be particularly affected by conditions resulting in elevated CRF because of differences in receptor trafficking. As excessive LC activation has been implicated in the arousal components of stress-related psychiatric disorders, this may be a cellular mechanism that contributes to the increased incidence of these disorders in females.

Stress regulation of kisspeptin in the modulation of reproductive function

Stressful stimuli abound in modern society and have shaped evolution through altering reproductive development, behavior, and physiology. The recent identification of kisspeptin as an important component of the hypothalamic regulatory circuits involved in reproductive homeostasis sparked a great deal of research interest that subsequently implicated kisspeptin signaling in the relay of metabolic, environmental, and physiological cues to the hypothalamo-pituitary-gonadal axis. However, although it is widely recognized that exposure to stress profoundly impacts on reproductive function, the roles of kisspeptin within the complex mechanisms underlying stress regulation of reproduction remain poorly understood. We and others have recently demonstrated that a variety of experimental stress paradigms downregulate the expression of kisspeptin ligand and receptor within the reproductive brain. Coincidently, these stressors also inhibit gonadotropin secretion and delay pubertal onset-processes that rely on kisspeptin signaling. However, a modest literature is inconsistent with an exclusively suppressive influence of stress on the reproductive axis and suggests that complicated neural interactions and signaling mechanisms translate the stress response into reproductive perturbations. The purpose of this chapter is to review the evidence for a novel role of kisspeptin signaling in the modulation of reproductive function by stress and to broaden the understanding of this timely phenomenon.

Neuropeptide regulation of the locus coeruleus and opiate-induced plasticity of stress responses

Stress has been implicated as a risk factor in vulnerability to the initiation and maintenance of opiate abuse and is thought to play an important role in relapse in subjects with a history of abuse. Conversely, chronic opiate use and withdrawal are stressors and can potentially predispose individuals to stress-related psychiatric disorders. Because the interaction of opiates with stress response systems has potentially widespread clinical consequences, it is important to delineate how specific substrates of the stress response and endogenous opioid systems interact and the specific points at which stress circuits and endogenous opioid systems intersect. The purpose of this review is to present and discuss the results of studies that have unveiled the complex circuitry by which stress-related neuropeptides and endogenous opioids coregulate activity of the locus coeruleus (LC)-norepinephrine (NE) system and how chronic morphine, or stress, disturbs this regulation.

Sex-biased stress signaling: the corticotropin-releasing factor receptor as a model

Sex differences in the prevalence or severity of many diseases and in the response to pharmacological agents are well recognized. Elucidating the biologic bases of these differences can advance our understanding of the pathophysiology of disease and facilitate the development of treatments. Despite the importance to medicine, this has been an area of limited research. Here, we review physiologic, cellular, and molecular findings supporting the idea that there are sex differences in receptor signaling and trafficking that can be determinants of pathology. The focus is on the receptor for corticotropin-releasing factor (CRF), the orchestrator of the stress response, which has been implicated in diverse stress-related diseases that show a female prevalence. Data are reviewed that show sex differences in the association of the CRF receptor (CRF1) with the Gs protein and β-arrestin 2 that would render females more responsive to acute stress and less able to adapt to chronic stress as a result of compromised CRF1 internalization. Because β-arrestin 2 serves to link CRF1 to Gs-independent signaling pathways, this sex-biased signaling is proposed to result in distinct cellular responses to stress that are translated to different physiologic and behavioral coping mechanisms and that can have different pathologic consequences. Because stress has been implicated in diverse medical and psychiatric diseases, these sex differences in CRF1 signaling could explain sex differences in a multitude of disorders. The possibility that analogous sex differences may occur with other G-protein-coupled receptors underscores the impact of this effect and is discussed.

Divergent roles of the CRH receptors in the control of gonadotropin secretion induced by acute restraint stress at proestrus

CRH has been implicated as a mediator of stress-induced effects on the hypothalamus-pituitary-gonad axis, acting via CRH receptors in various brain regions. We investigated whether the effects of restraint stress on the secretion of gonadotropins on the morning of proestrus are mediated by the CRH-R1 or CRH-R2 receptors in the oval subdivision of the anterolateral BST, the central amygdala, the locus coeruleus (LC), or the A1 and A2 neuron groups in the medulla. At proestrus morning, rats were injected with antalarmin (a CRH-R1 antagonist), asstressin2-B (a CRH-R2 antagonist) or vehicles. Thirty minutes after the injection, the animals were placed into restraints for 30 min, and blood was sampled for 2 h. At the end of the experiment, the brains were removed for immunofluorescence analyses. Restraint stress increased the levels of FSH and LH. Antalarmin blocked the stress-induced increases in FSH and LH secretion, but astressin2-B only blocked the increase in FSH secretion. LC showed intense stress-induced neuronal activity. FOS/tyrosine-hydroxylase coexpression in LC was reduced by antalarmin, but not astressin2-B. The CRH-R1 receptor, more than CRH-R2 receptor, appears to be essential for the stimulation of the hypothalamus-pituitary-gonad axis by acute stress; this response is likely mediated in part by noradrenergic neurons in the LC. We postulate that the stress-induced facilitation of reproductive function is mediated, at least in part, by CRH action through CRH-R1 on noradrenaline neurons residing in the LC that trigger GnRH discharge and gonadotropin secretion.

Sex differences in molecular and cellular substrates of stress

Women are twice as likely as men to suffer from stress-related psychiatric disorders, like unipolar depression and post-traumatic stress disorder. Although the underlying neural mechanisms are not well characterized, the pivotal role of stress in the onset and severity of these diseases has led to the idea that sex differences in stress responses account for this sex bias. Corticotropin-releasing factor (CRF) orchestrates stress responses by acting both as a neurohormone to initiate the hypothalamic-pituitary-adrenal (HPA) axis and as a neuromodulator in the brain. One target of CRF modulation is the locus coeruleus (LC)-norepinephrine system, which coordinates arousal components of the stress response. Hypersecretion of CRF and dysregulation of targets downstream from CRF, such as the HPA axis and LC-norepinephrine system, are characteristic features of many stress-related psychiatric diseases, suggesting a causal role for CRF and its targets in the development of these disorders. This review will describe sex differences in CRF and the LC-norepinephrine system that can increase stress sensitivity in females, making them vulnerable to stress-related disorders. Evidence for gonadal hormone regulation of hypothalamic CRF is discussed as an effect that can lead to increased HPA axis activity in females. Sex differences in the structure of LC neurons that create the potential for hyperarousal in response to emotional stimuli are described. Finally, sex differences at the molecular level of the CRF(1) receptor that make the LC-norepinephrine system more reactive in females are reviewed. The implications of these sex differences for the treatment of stress-related psychiatric disorders also will be discussed.

Corticotropin-releasing factor antagonist reduces activation of noradrenalin and serotonin neurons in the locus coeruleus and dorsal raphe in the arousal response accompanied by yawning behavior in rats

We previously reported that intracerebroventricular (icv) administration of corticotropin-releasing factor (CRF) antagonist attenuates the arousal response during yawning behavior in rats. However, the CRF-related pathway involved in the arousal response during yawning is still unclear. In the present study, we assessed the involvement of the CRF-containing pathway from the hypothalamic paraventricular nucleus (PVN) to the locus coeruleus (LC) and the dorsal raphe nucleus (DRN) in the arousal response during frequent spontaneous yawning, which was induced by several microinjections of l-glutamate into the PVN in anesthetized rats, using c-Fos immunohistochemistry. The PVN stimulation showed significant increases in activation of PVN CRF neurons, LC noradrenalin (NA) neurons and DRN serotonin (5-HT) neurons as well as arousal response during yawning. But icv administration of a CRF receptor antagonist, α-helical CRF (9-41), significantly inhibited the activation of both LC NA neurons and DRN 5-HT neurons except the activation of CRF neurons in the PVN, and significantly suppressed the arousal response during yawning. These results suggest that the CRF-containing pathway from PVN CRF neurons to LC NA neurons and DRN 5-HT neurons can be involved in the arousal response during yawning behavior.

Amygdalar peptidergic circuits regulating noradrenergic locus coeruleus neurons: linking limbic and arousal centers

The endogenous opioid peptides, met- or leu-enkephalin, and corticotropin-releasing factor (CRF) regulate noradrenergic neurons in the locus coeruleus (LC) in a convergent manner via projections from distinct brain areas. In contrast, the opioid peptide dynorphin (DYN) has been shown to serve as a co-transmitter with CRF in afferents to the LC. To further define anatomical substrates targeting noradrenergic neurons by DYN afferents originating from limbic sources, anterograde tract-tracing of biotinylated dextran amine (BDA) from the central amygdaloid complex was combined with immunocytochemical detection of DYN and tyrosine hydroxylase (TH) in the same section of tissue. Triple labeling immunocytochemistry was combined with electron microscopy in the LC where BDA was identified using an immunoperoxidase marker, and DYN and TH were distinguished by the use of sequential immunogold labeling and silver enhancement to produce different sized gold particles. Results show direct evidence of a monosynaptic pathway linking amygdalar DYN afferents with LC neurons. To determine whether DYN-containing amygdalar LC-projecting neurons colocalize CRF, retrograde tract-tracing using fluorescent latex microspheres injected into the LC was combined with immunocytochemical detection of DYN and CRF in single sections in the central amygdala. Retrogradely labeled neurons from the LC were distributed throughout the rostro-caudal extent of the central nucleus of the amygdala (CeA) as previously described. Cell counts showed that approximately 42% of LC-projecting neurons in the CeA contained both DYN and CRF. Taken with our previous studies showing monosynaptic projections from amygdalar CRF neurons to noradrenergic LC cells, the present study extends this by showing that DYN and CRF are co-transmitters in monosynaptic projections to the LC and are poised to coordinately impact LC neuronal activity.

Idiopathic cystitis in domestic cats--beyond the lower urinary tract

Signs of lower urinary tract (LUT) disease in domestic cats can be acute or chronic, and can result from variable combinations of abnormalities within the lumen of the LUT, the parenchyma of the LUT itself, or other organ system(s) that then lead to LUT dysfunction. In the majority of cats with chronic signs of LUT dysfunction, no specific underlying cause can be confirmed after standard clinical evaluation of the LUT, so these cats typically are classified as having idiopathic cystitis. A syndrome in human beings commonly known as interstitial cystitis (IC) shares many features in common with these cats, permitting comparisons between the two species. A wide range of similarities in abnormalities has been identified between these syndromes outside as well as inside the LUT. A variety of potential familial and developmental risk factors also have been identified. These results have permitted generation of the hypothesis that some of these people have a disorder affecting the LUT rather than a disorder of the LUT. This perspective has suggested alternative diagnostic strategies and novel approaches to treatment, at least in cats. The purpose of this review is to summarize research investigations into the various abnormalities present in cats, to compare some of these findings with those identified in human beings, and to discuss how they might modify perceptions about the etiopathogenesis, diagnosis, and treatment of cats with this disease. Dedication: I dedicate this contribution to Professor Dennis J. Chew, whose collaboration, patience, and support made it all possible.