Molecular Signaling and Translational Significance of the Corticotropin Releasing Factor System

Prior stress and vasopressin promote corticotropin-releasing factor inhibition of serotonin release in the central nucleus of the amygdala

Corticotropin-releasing factor (CRF) is essential for coordinating endocrine and neural responses to stress, frequently facilitated by vasopressin (AVP). Previous work has linked CRF hypersecretion, binding site changes, and dysfunctional serotonergic transmission with anxiety and affective disorders, including clinical depression. Crucially, CRF can alter serotonergic activity. In the dorsal raphé nucleus and serotonin (5-HT) terminal regions, CRF effects can be stimulatory or inhibitory, depending on the dose, site, and receptor type activated. Prior stress alters CRF neurotransmission and CRF-mediated behaviors. Lateral, medial, and ventral subdivisions of the central nucleus of the amygdala (CeA) produce CRF and coordinate stress responsiveness. The purpose of these experiments was to determine the effect of intracerebroventricular (icv) administration of CRF and AVP on extracellular 5-HT as an index of 5-HT release in the CeA, using in vivo microdialysis in freely moving rats and high performance liquid chromatography (HPLC) analysis. We also examined the effect of prior stress (1 h restraint, 24 h prior) on CRF- and AVP-mediated release of 5-HT within the CeA. Our results show that icv CRF infusion in unstressed animals had no effect on 5-HT release in the CeA. Conversely, in rats with prior stress, CRF caused a profound dose-dependent decrease in 5-HT release within the CeA. This effect was long-lasting (240 min) and was mimicked by CRF plus AVP infusion without stress. Thus, prior stress and AVP functionally alter CRF-mediated neurotransmission and sensitize CRF-induced inhibition of 5-HT release, suggesting that this is a potential mechanism underlying stress-induced affective reactivity in humans.

Early life stress and susceptibility to addiction in adolescence

Early life stress (ELS) is a risk factor for developing a host of psychiatric disorders. Adolescence is a particularly vulnerable period for the onset of these disorders and substance use disorders (SUDs). Here we discuss ELS and its effects in adolescence, especially SUDs, and their correlates with molecular changes to signaling systems in reward and stress neurocircuits. Using a maternal separation (MS) model of neonatal ELS, we studied a range of behaviors that comprise a "drug-seeking" phenotype. We then investigated potential mechanisms underlying the development of this phenotype. Corticotropin releasing factor (CRF) and serotonin (5-HT) are widely believed to be involved in "stress-induced" disorders, including addiction. Here, we show that ELS leads to the development of a drug-seeking phenotype indicative of increased susceptibility to addiction and concomitant sex-dependent upregulation of CRF and 5-HT system components throughout extended brain reward/stress neurocircuits.

Cardinal role of the environment in stress induced changes across life stages and generations

The stress response in rodents and humans is exquisitely dependent on the environmental context. The interactive element of the environment is typically studied by creating laboratory models of stress-induced plasticity manifested in behavior or the underlying neuroendocrine mediators of the behavior. Here, we discuss three representative sets of studies where the role of the environment in mediating stress sensitivity or stress resilience is considered across varying windows of time. Collectively, these studies testify that environmental variation at an earlier time point modifies the relationship between stressor and stress response at a later stage. The metaplastic effects of the environment on the stress response remain possible across various endpoints, including behavior, neuroendocrine regulation, region-specific neural plasticity, and regulation of receptors. The timescale of such variation spans adulthood, across stages of life history and generational boundaries. Thus, environmental variables are powerful determinants of the observed diversity in stress response. The predominant role of the environment suggests that it is possible to promote stress resilience through purposeful modification of the environment.

Calcineurin signaling as a target for the treatment of alcohol abuse and neuroinflammatory disorders

Converging lines of evidence point to a significant role of neuroinflammation in a host of psychiatric conditions, including alcohol use disorder, TBI, and PTSD. A complex interaction of both peripheral and central signaling underlies processes involved in neuroinflammation. Calcineurin is a molecule that sits at the nexus of these processes and has been clearly linked to a number of psychiatric disorders including alcohol use disorder (AUD). Like its role in regulating peripheral immune cells, calcineurin (CN) plays an integral role in processes regulating neuroimmune function and neuroinflammatory processes. Targeting CN or elements of its signaling pathways at critical points may aid in the functional recovery from neuroinflammatory related disorders. In this review we will highlight the role of neuroinflammation and calcineurin signaling in AUD, TBI and stress-induced disorders and discuss recent findings demonstrating a therapeutic effect of immunosuppressant-induced calcineurin inhibition in a pre-clinical model of binge alcohol drinking.

A Molecular Dynamics Study of Vasoactive Intestinal Peptide Receptor 1 and the Basis of Its Therapeutic Antagonism

Vasoactive intestinal peptide receptor 1 (VPAC1) is a member of a secretin-like subfamily of G protein-coupled receptors. Its endogenous neuropeptide (VIP), secreted by neurons and immune cells, modulates various physiological functions such as exocrine and endocrine secretions, immune response, smooth muscles relaxation, vasodilation, and fetal development. As a drug target, VPAC1 has been selected for therapy of inflammatory diseases but drug discovery is still hampered by lack of its crystal structure. In this study we presented the homology model of this receptor constructed with the well-known web service GPCRM. The VPAC1 model is composed of extracellular and transmembrane domains that form a complex with an endogenous hormone VIP. Using the homology model of VPAC1 the mechanism of action of potential drug candidates for VPAC1 was described. Only two series of small-molecule antagonists of confirmed biological activity for VPAC1 have been described thus far. Molecular docking and a series of molecular dynamics simulations were performed to elucidate their binding to VPAC1 and resulting antagonist effect. The presented work provides the basis for the possible binding mode of VPAC1 antagonists and determinants of their molecular recognition in the context of other class B GPCRs. Until the crystal structure of VPAC1 will be released, the presented homology model of VPAC1 can serve as a scaffold for drug discovery studies and is available from the author upon request.

High-Fat Feeding Improves Anxiety-Type Behavior Induced by Ovariectomy in Rats

Menopause-induced changes may include increased incidence of both depression/anxiety and obesity. We hypothesized that behavioral changes that may develop after ovarian failure could be related to neurochemical and metabolic aspects affected by this condition and that high-fat intake may influence these associations. The present study investigated in rats the effects of ovariectomy, either alone or combined with high-fat diets enriched with either lard or fish-oil, on metabolic, behavioral and monoaminergic statuses, and on gene expression of neuropeptides and receptors involved in energy balance and mood regulation. Female rats had their ovaries removed and received either standard chow (OvxC) or high-fat diets enriched with either lard (OvxL) or fish-oil (OvxF) for 8 weeks. The Sham group received only chow diet. Ovariectomy increased feed efficiency and body weight gain and impaired glucose homeostasis and serotonin-induced hypophagia, effects either maintained or even accentuated by the lard diet but counteracted by the fish diet. The OvxL group developed obesity and hyperleptinemia. Regarding components of hypothalamic serotonergic system, both ovariectomy alone or combined with the fish diet increased 5-HT2C expression while the lard diet reduced 5-HT1B mRNA. Ovariectomy increased the anxiety index, as derived from the elevated plus maze test, while both high-fat groups showed normalization of this index. In the forced swimming test, ovariectomy allied to high-lard diet, but not to fish-oil diet, reduced the latency to immobility, indicating vulnerability to a depressive-like state. Linear regression analysis showed hippocampal AgRP to be negatively associated with the anxiety index and hypothalamic AgRP to be positively associated with the latency to immobility. These AgRp gene expression associations are indicative of a beneficial involvement of this neuropeptide on both depression and anxiety measures. The present findings demonstrate metabolic, neurochemical and behavioral alterations after ovaries removal and highlight a positive effect of high-fat feeding on the anxiety-like behavior shown by ovariectomized animals. Since the polyunsaturated ômega-3 intake (fish diet), unlike the saturated fat intake (lard diet), failed to induce deleterious metabolic or neurochemical consequences, further studies are needed focusing on the potential of this dietary component as an adjuvant anxiolytic agent after menopause.

The neuroanatomic complexity of the CRF and DA systems and their interface: What we still don't know

Corticotropin-releasing factor (CRF) is a neuropeptide that mediates the stress response. Long known to contribute to regulation of the adrenal stress response initiated in the hypothalamic-pituitary axis (HPA), a complex pattern of extrahypothalamic CRF expression is also described in rodents and primates. Cross-talk between the CRF and midbrain dopamine (DA) systems links the stress response to DA regulation. Classically CRF + cells in the extended amygdala and paraventricular nucleus (PVN) are considered the main source of this input, principally targeting the ventral tegmental area (VTA). However, the anatomic complexity of both the DA and CRF system has been increasingly elaborated in the last decade. The DA neurons are now recognized as having diverse molecular, connectional and physiologic properties, predicted by their anatomic location. At the same time, the broad distribution of CRF cells in the brain has been increasingly delineated using different species and techniques. Here, we review updated information on both CRF localization and newer conceptualizations of the DA system to reconsider the CRF-DA interface.

Tectal corticotropin-releasing factor (CRF) neurons respond to fasting and a reactive stressor in the African Clawed Frog, Xenopus laevis

It is well established that hypothalamic neurons producing the peptide corticotropin-releasing factor (CRF) play a key role in stress adaptation, including reduction of food intake when a threat or stressor is present. We have previously reported on the presence of an intrinsic CRF signaling system within the optic tectum (OT), a brain area that plays a key role in visually guided prey capture/predator avoidance decisions. To better understand the potential role of tectal CRF neurons in regulating adaptive behavior and energy balance during stress we examined evidence for modulation of tectal CRF neuronal activity after stressor exposure and food deprivation in the African clawed frog Xenopus laevis. We tested two predictions, 1) that exposure to categorically distinct stressors (ether vapors and shaking) will reduce food intake and modulate the activity of tectal CRF cells, and 2) that food deprivation will modulate the activity of tectal CRF cells. Exposure to ether increased tectal content of CRF and CRF transcript, but lowed CRFR1 transcript abundance. Two weeks of food deprivation reduced total fat stores in frogs and decreased tectal content of CRF content while having no effect on CRF and CRFR1 transcript abundance. Our data are consistent with a role for tectal CRF neurons in modulating food intake in response to certain stressors.

Central Administration of Cyclosporine A Decreases Ethanol Drinking

Aims

Abstinence among alcohol dependent liver graft recipients is remarkably high. The routine use of anti-immune agents in these patients led to rodent studies showing that immunosuppressants acting through inhibition of calcineurin (CLN) are highly effective in decreasing alcohol consumption. It remained unclear, however, whether the decreased alcohol consumption in rodent models is mediated through peripheral suppression of immune response or centrally through direct inhibition of cyclophilin-CLN in the brain. We tested the hypothesis that direct brain inhibition of CLN with intracerebroventricular (ICV) injections of the immunosuppressant cyclosporine A (CsA) is sufficient to decrease ethanol consumption in a rodent model of binge-like drinking.

Methods

Male C57BL/6NHsd mice were put through a modified 'drinking in the dark' (DID) paradigm. Effects of both peripheral (IP) and central (ICV) injections of CsA on ethanol consumption were assessed.

Results

Here, as in earlier work, IP CsA administration significantly decreased alcohol consumption. Supporting our hypothesis, central administration of CsA was sufficient to decrease alcohol consumption in a dose-dependent manner. There was no significant effect of CsA on water or sucrose consumption.

Conclusions

These results clearly implicate a CLN-mediated mechanism in brain in the inhibitory effects of CsA on ethanol consumption and provide novel targets for investigation of treatment for Alcohol Use Disorders (AUD). These results also add to the growing body of literature implicating neuroimmune mechanisms in the etiology, pathophysiology and behaviors driving AUD.

Short Summary

The unusually high abstinence rate and routine use of immunosuppressants in AUD liver graft recipients led us to rodent studies showing that immunosuppressants acting through inhibition of calcineurin (CLN) are highly effective in decreasing drinking. Here we demonstrate that this effect is mediated by brain rather than peripheral immune mechanisms.

Food intake inhibition in rainbow trout induced by activation of serotonin 5-HT2C receptors is associated with increases in POMC, CART and CRF mRNA abundance in hypothalamus

In rainbow trout, the food intake inhibition induced by serotonin occurs through 5-HT2C and 5-HT1A receptors, though the mechanisms involved are still unknown. Therefore, we assessed if a direct stimulation of 5-HT2C and 5-HT1A serotonin receptors (resulting in decreased food intake in rainbow trout), affects gene expression of neuropeptides involved in the control of food intake, such as pro-opiomelanocortin (POMC), cocaine- and amphetamine-regulated transcript (CART), corticotrophin releasing factor (CRF), and agouti-related peptide (AgRP). In a first set of experiments, the injection of the 5-HT2C receptor agonists MK212 (60 μg kg(-1) icv) and WAY 161503 (1 mg kg(-1) ip), and of the 5-HT1A receptor agonist 8-OH-DPAT (1 mg kg(-1) ip and 30 μg kg(-1) icv) induced food intake inhibition. In a second set of experiments, we observed that the injection of MK212 or WAY 161503 (1 and 3 mg kg(-1)) significantly increased hypothalamic POMC mRNA abundance. CART mRNA abundance in hypothalamus was enhanced by treatment with MK212 and unaffected by WAY 161503. The administration of the 5-HT1A receptor agonist 8-OH-DPAT did not induce any significant variation in the hypothalamic POMC or CART mRNA levels. CRF mRNA abundance was only affected by MK212 that increased hypothalamic values. Finally, hypothalamic AgRP mRNA abundance was only evaluated with the agonist 5-HT2C MK212 resulting in no significant effects. The results show that the reduction in food intake mediated by 5-HT2C receptors is associated with increases in hypothalamic POMC, CART and CRF mRNA abundance.

Molecular Mechanisms of Cannabis Signaling in the Brain

Cannabis has been cultivated and used by humans for thousands of years. Research for decades was focused on understanding the mechanisms of an illegal/addictive drug. This led to the discovery of the vast endocannabinoid system. Research has now shifted to understanding fundamental biological questions related to one of the most widespread signaling systems in both the brain and the body. Our understanding of cannabinoid signaling has advanced significantly in the last two decades. In this review, we discuss the state of knowledge on mechanisms of Cannabis signaling in the brain and the modulation of key brain neurotransmitter systems involved in both brain reward/addiction and psychiatric disorders. It is highly probable that various cannabinoids will be found to be efficacious in the treatment of a number of psychiatric disorders. However, while there is clearly much potential, marijuana has not been properly vetted by the medical-scientific evaluation process and there are clearly a range of potentially adverse side-effects-including addiction. We are at crossroads for research on endocannabinoid function and therapeutics (including the use of exogenous treatments such as Cannabis). With over 100 cannabinoid constituents, the majority of which have not been studied, there is much Cannabis research yet to be done. With more states legalizing both the medicinal and recreational use of marijuana the rigorous scientific investigation into cannabinoid signaling is imperative.

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

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

Mouse models of fear-related disorders: Cell-type-specific manipulations in amygdala

Fear conditioning is a model system used to study threat responses, fear memory and their dysregulation in a variety of organisms. Newly developed tools such as optogenetics, Cre recombinase and DREADD technologies have allowed researchers to manipulate anatomically or molecularly defined cell subtypes with a high degree of temporal control and determine the effect of this manipulation on behavior. These targeted molecular techniques have opened up a new appreciation for the critical contributions different subpopulations of cells make to fear behavior and potentially to treatment of fear and anxiety disorders. Here we review progress to date across a variety of techniques to understand fear-related behavior through the manipulation of different cell subtypes within the amygdala.

Synthesis, F-18 radiolabeling, and microPET evaluation of 3-(2,4-dichlorophenyl)-N-alkyl-N-fluoroalkyl-2,5-dimethylpyrazolo[1,5-a]pyrimidin-7-amines as ligands of the corticotropin-releasing factor type-1 (CRF1) receptor

A series of 3-(2,4-dichlorophenyl)-N-alkyl-N-fluoroalkyl-2,5-dimethylpyrazolo[1,5-a]pyrimidin-7-amines were synthesized and evaluated as potential positron emission tomography (PET) tracers for the corticotropin-releasing factor type-1 (CRF1) receptor. Compounds 27, 28, 29, and 30 all displayed high binding affinity (⩽1.2 nM) to the CRF1 receptor when assessed by in vitro competition binding assays at 23 °C, whereas a decrease in affinity (⩾10-fold) was observed with compound 26. The logP7.4 values of [(18)F]26-[(18)F]29 were in the range of ∼2.2-2.8 and microPET evaluation of [(18)F]26-[(18)F]29 in an anesthetized male cynomolgus monkey demonstrated brain penetrance, but specific binding was not sufficient enough to differentiate regions of high CRF1 receptor density from regions of low CRF1 receptor density. Radioactivity uptake in the skull, and sphenoid bone and/or sphenoid sinus during studies with [(18)F]28, [(18)F]28-d8, and [(18)F]29 was attributed to a combination of [(18)F]fluoride generated by metabolic defluorination of the radiotracer and binding of intact radiotracer to CRF1 receptors expressed on mast cells in the bone marrow. Uptake of [(18)F]26 and [(18)F]27 in the skull and sphenoid region was rapid but then steadily washed out which suggests that this behavior was the result of binding to CRF1 receptors expressed on mast cells in the bone marrow with no contribution from [(18)F]fluoride.

Direct targeting of peptidergic amygdalar neurons by noradrenergic afferents: linking stress-integrative circuitry

Amygdalar norepinephrine (NE) plays a key role in regulating neural responses to emotionally arousing stimuli and is involved in memory consolidation of emotionally charged events. Corticotropin-releasing factor (CRF) and dynorphin (DYN), two neuropeptides that mediate the physiological and behavioral responses to stress, are abundant in the central nucleus of the amygdala (CeA), and directly innervate brainstem noradrenergic locus coeruleus (LC) neurons. Whether the CRF- and DYN-containing amygdalar neurons receive direct noradrenergic innervation has not yet been elucidated. The present study sought to define cellular substrates underlying noradrenergic modulation of CRF- and DYN-containing neurons in the CeA using immunohistochemistry and electron microscopy. Ultrastructural analysis revealed that NE-labeled axon terminals form synapses with CRF- and DYN-containing neurons in the CeA. Semi-quantitative analysis showed that approximately 31 % of NET-labeled axon terminals targeted CeA neurons that co-expressed DYN and CRF. As a major source of CRF innervation to the LC, it is also not known whether CRF-containing CeA neurons are directly targeted by noradrenergic afferents. To test this, retrograde tract tracing using FluoroGold from the LC was combined with immunocytochemical detection of CRF and NET in the CeA. Our results revealed a population of LC-projecting CRF-containing CeA neurons that are directly innervated by NE afferents. Analysis showed that approximately 34 % of NET-labeled axon terminals targeted LC-projecting CeA neurons that contain CRF. Taken together, these results indicate significant interactions between NE, CRF and DYN in this critical limbic region and reveal direct synaptic interactions of NE with amygdalar CRF that influence the LC-NE arousal system.

Inhibition of corticotropin releasing factor expression in the central nucleus of the amygdala attenuates stress-induced behavioral and endocrine responses

Corticotropin releasing factor (CRF) is a primary mediator of endocrine, autonomic and behavioral stress responses. Studies in both humans and animal models have implicated CRF in a wide-variety of psychiatric conditions including anxiety disorders such as post-traumatic stress disorder (PTSD), depression, sleep disorders and addiction among others. The central nucleus of the amygdala (CeA), a key limbic structure with one of the highest concentrations of CRF-producing cells outside of the hypothalamus, has been implicated in anxiety-like behavior and a number of stress-induced disorders. This study investigated the specific role of CRF in the CeA on both endocrine and behavioral responses to stress. We used RNA Interference (RNAi) techniques to locally and specifically knockdown CRF expression in CeA. Behavior was assessed using the elevated plus maze (EPM) and open field test (OF). Knocking down CRF expression in the CeA had no significant effect on measures of anxiety-like behavior in these tests. However, it did have an effect on grooming behavior, a CRF-induced behavior. Prior exposure to a stressor sensitized an amygdalar CRF effect on stress-induced HPA activation. In these stress-challenged animals silencing CRF in the CeA significantly attenuated corticosterone responses to a subsequent behavioral stressor. Thus, it appears that while CRF projecting from the CeA does not play a significant role in the expression stress-induced anxiety-like behaviors on the EPM and OF it does play a critical role in stress-induced HPA activation.

Central corticotropin releasing factor and social stress

Social interactions are a main source of stress in vertebrates. Social stressors, as well as other stressors, activate the hypothalamic–pituitary–adrenal (HPA) axis resulting in glucocorticoid release. One of the main components of the HPA axis is corticotropin releasing factor (CRF). The neuropeptide CRF is part of a peptide family including CRF, urocortin 1–3, urotensin 1–3, and sauvagine. The actions of the CRF family are mediated by at least two different receptors with different anatomical distribution and affinities for the peptides. The CRF peptides affect several behavioral and physiological responses to stress including aggression, feeding, and locomotor activity. This review will summarize recent research in vertebrates concerning how social stress interacts with components of the CRF system. Consideration will be taken to the different models used for social stress ranging from social isolation, dyadic interactions, to group dominance hierarchies. Further, the temporal effect of social stressor from acute, intermittent, to chronic will be considered. Finally, strains selected for specific behavior or physiology linked to social stress will also be discussed.

CRF-like diuretic hormone negatively affects both feeding and reproduction in the desert locust, Schistocerca gregaria

Diuretic hormones (DH) related to the vertebrate Corticotropin Releasing Factor (CRF) have been identified in diverse insect species. In the migratory locust, Locusta migratoria, the CRF-like DH (CRF/DH) is localized in the same neurosecretory cells as the Ovary Maturating Parsin (OMP), a neurohormone that stimulates oocyte growth, vitellogenesis and hemolymph ecdysteroid levels in adult female locusts. In this study, we investigated whether CRF-like DH can influence feeding and reproduction in the desert locust, Schistocerca gregaria. We identified two highly similar S. gregaria CRF-like DH precursor cDNAs, each of which also encodes an OMP isoform. Alignment with other insect CRF-like DH precursors shows relatively high conservation of the CRF/DH sequence while the precursor region corresponding to OMP is not well conserved. Quantitative real-time RT-PCR revealed that the precursor transcripts mainly occur in the central nervous system and their highest expression level was observed in the brain. Injection of locust CRF/DH caused a significantly reduced food intake, while RNAi knockdown stimulated food intake. Therefore, our data indicate that CRF-like DH induces satiety. Furthermore, injection of CRF/DH in adult females retarded oocyte growth and caused lower ecdysteroid titers in hemolymph and ovaries, while RNAi knockdown resulted in opposite effects. The observed effects of CRF/DH may be part of a wider repertoire of neurohormonal activities, constituting an integrating control system that affects food intake and excretion, as well as anabolic processes like oocyte growth and ecdysteroidogenesis, following a meal. Our discussion about the functional relationship between CRF/DH and OMP led to the hypothesis that OMP may possibly act as a monitoring peptide that can elicit negative feedback effects.