Ectopic expression of the CRF-binding protein: minor impact on HPA axis regulation but induction of sexually dimorphic weight gain

Corticotropin Releasing Factor Binding Protein as a Novel Target to Restore Brain Homeostasis: Lessons Learned From Alcohol Use Disorder Research

Stress is well-known to contribute to the development of many psychiatric illnesses including alcohol and substance use disorder (AUD and SUD). The deleterious effects of stress have also been implicated in the acceleration of biological age, and age-related neurodegenerative disease. The physio-pathology of stress is regulated by the corticotropin-releasing factor (CRF) system, the upstream component of the hypothalamic-pituitary-adrenal (HPA) axis. Extensive literature has shown that dysregulation of the CRF neuroendocrine system contributes to escalation of alcohol consumption and, similarly, chronic alcohol consumption contributes to disruption of the stress system. The CRF system also represents the central switchboard for regulating homeostasis, and more recent studies have found that stress and aberrations in the CRF pathway are implicated in accelerated aging and age-related neurodegenerative disease. Corticotropin releasing factor binding protein (CRFBP) is a secreted glycoprotein distributed in peripheral tissues and in specific brain regions. It neutralizes the effects of CRF by sequestering free CRF, but may also possess excitatory function by interacting with CRF receptors. CRFBP's dual role in influencing CRF bioavailability and CRF receptor signaling has been shown to have a major part in the HPA axis response. Therefore, CRFBP may represent a valuable target to treat stress-related illness, including: development of novel medications to treat AUD and restore homeostasis in the aging brain. This narrative review focuses on molecular mechanisms related to the role of CRFBP in the progression of addictive and psychiatric disorders, biological aging, and age-related neurodegenerative disease. We provide an overview of recent studies investigating modulation of this pathway as a potential therapeutic target for AUD and age-related neurodegenerative disease.

Role of corticotropin-releasing factor in alcohol and nicotine addiction

The two most prevalent substance use disorders involve alcohol and nicotine, which are often co-abused. Robust preclinical and translational evidence indicates that individuals initiate drug use for the acute rewarding effects of the substance. The development of negative emotional states is key for the transition from recreational use to substance use disorders as subjects seek the substance to obtain relief from the negative emotional states of acute withdrawal and protracted abstinence. The neuropeptide corticotropin-releasing factor (CRF) is a major regulator of the brain stress system and key in the development of negative affective states. The present review examines the role of CRF in preclinical models of alcohol and nicotine abuse and explores links between CRF and anxiety-like, dysphoria-like, and other negative affective states. Finally, the present review discusses preclinical models of nicotine and alcohol use with regard to the CRF system, advances in molecular and genetic manipulations of CRF, and the importance of examining both males and females in this field of research.

Corticotropin releasing factor-binding protein (CRF-BP) as a potential new therapeutic target in Alzheimer's disease and stress disorders

Alzheimer's disease is the most common cause of dementia and one of the most complex human neurodegenerative diseases. Numerous studies have demonstrated a critical role of the environment in the pathogenesis and pathophysiology of the disease, where daily life stress plays an important role. A lot of epigenetic studies have led to the conclusion that chronic stress and stress-related disorders play an important part in the onset of neurodegenerative disorders, and an enormous amount of research yielded valuable discoveries but has so far not led to the development of effective treatment strategies for Alzheimer's disease. Corticotropin-releasing factor (CRF) is one of the major hormones and at the same time a neuropeptide acting in stress response. Deregulation of protein levels of CRF is involved in the pathogenesis of Alzheimer's disease, but little is known about the precise roles of CRF and its binding protein, CRF-BP, in neurodegenerative diseases. In this review, we summarize the key evidence for and against the involvement of stress-associated modulation of the CRF system in the pathogenesis of Alzheimer's disease and discuss how recent findings could lead to new potential treatment possibilities in Alzheimer's disease by using CRF-BP as a therapeutic target.

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

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

Corticotropin-releasing hormone-binding protein and stress: from invertebrates to humans

Corticotropin-releasing hormone (CRH) is a key regulator of the stress response. This peptide controls the hypothalamic-pituitary-adrenal (HPA) axis as well as a variety of behavioral and autonomic stress responses via the two CRH receptors, CRH-R1 and CRH-R2. The CRH system also includes an evolutionarily conserved CRH-binding protein (CRH-BP), a secreted glycoprotein that binds CRH with subnanomolar affinity to modulate CRH receptor activity. In this review, we discuss the current literature on CRH-BP and stress across multiple species, from insects to humans. We describe the regulation of CRH-BP in response to stress, as well as genetic mouse models that have been utilized to elucidate the in vivo role(s) of CRH-BP in modulating the stress response. Finally, the role of CRH-BP in the human stress response is examined, including single nucleotide polymorphisms in the human CRHBP gene that are associated with stress-related affective disorders and addiction. Lay summary The stress response is controlled by corticotropin-releasing hormone (CRH), acting via CRH receptors. However, the CRH system also includes a unique CRH-binding protein (CRH-BP) that binds CRH with an affinity greater than the CRH receptors. In this review, we discuss the role of this highly conserved CRH-BP in regulation of the CRH-mediated stress response from invertebrates to humans.

Pituitary CRH-binding protein and stress in female mice

The CRH-binding protein (CRH-BP) binds CRH with very high affinity and inhibits CRH-mediated ACTH release from anterior pituitary cells in vitro, suggesting that the CRH-BP functions as a negative regulator of CRH activity. Our previous studies have demonstrated sexually dimorphic expression of CRH-BP in the murine pituitary. Basal CRH-BP expression is higher in the female pituitary, where CRH-BP mRNA is detected in multiple anterior pituitary cell types. In this study, we examined stress-induced changes in CRH-BP mRNA and protein expression in mouse pituitary and assessed the in vivo role of CRH-BP in modulating the stress response. Pituitary CRH-BP mRNA was greater than 200-fold more abundant in females than males, and restraint stress increased pituitary CRH-BP mRNA by 11.8-fold in females and 3.2-fold in males as assessed by qRT-PCR. In females, restraint stress increased CRH-BP mRNA levels not only in POMC-expressing cells, but also in PRL-expressing cells. The increase in female pituitary CRH-BP mRNA following stress resulted in significant increases in CRH-BP protein 4-6h after a 30-minute restraint stress as detected by [(125)I]-CRH:CRH-BP cross-linking analyses. Based on this temporal profile, the physiological role of CRH-BP was assessed using a stressor of longer duration. In lipopolysaccharide (LPS) stress studies, female CRH-BP-deficient mice showed elevated levels of stress-induced corticosterone release as compared to wild-type littermates. These studies demonstrate a role for the pituitary CRH-BP in attenuating the HPA response to stress in female mice.

Evolution and phylogeny of the corticotropin-releasing factor (CRF) family of peptides: expansion and specialization in the vertebrates

New sequence data on CRF family members from a number of genomes has led to the modification of our understanding of CRF evolution in the Metazoa. The corticotropin-releasing factor (CRF) family of peptides include four paralogous lineages in jawed vertebrates; CRF, urotensin-I/urocortin/sauvagine, urocortin 2 (Ucn2) and urocortin 3 (Ucn3). CRF and the urotensin-I/urocortin/sauvagine group represent a gene duplication from one lineage, whereas Ucns 2 and 3 are the result of a gene duplication in the other paralogous lineage. Both paralogous lineages are the result of a gene duplication from a single ancestral peptide that occurred after the divergence of the tunicates from the ancestor that led to the evolution of chordates and vertebrates. The presence of a single CRF-like peptide in tunicates and insects suggests that a single CRF-like ancestor was present before the separation of deuterostomes and protostomes. Currently there is no strong evidence that indicates that CRF-like peptides were present in metazoan taxa that evolved before this time although the structural similarity between some CRF peptides in insects, tunicates and vertebrates with the calcitonin family of peptides hints that prior to the formation of deuterostomes and protostomes the ancestral peptide possessed both CRF and calcitonin-like structural attributes. Here, we show evidences of conservation of CRF-like function dating back to early prokaryotes. This ancestral CRF-calcitonin-like peptide may have initially resulted from a horizontal gene transfer event from prokaryotes to a protistan species that later gave rise to the metazoans.

Behavioral Studies and Genetic Alterations in Corticotropin-Releasing Hormone (CRH) Neurocircuitry: Insights into Human Psychiatric Disorders

To maintain well-being, all organisms require the ability to re-establish homeostasis in the presence of adverse physiological or psychological experiences. The regulation of the hypothalamic-pituitary adrenal (HPA) axis during stress is important in preventing maladaptive responses that may increase susceptibility to affective disorders. Corticotropin-releasing hormone (CRH) is a central stress hormone in the HPA axis pathway and has been implicated in stress-induced psychiatric disorders, reproductive and cardiac function, as well as energy metabolism. In the context of psychiatric disorders, CRH dysfunction is associated with the occurrence of post-traumatic stress disorder, major depression, anorexia nervosa, and anxiety disorders. Here, we review the synthesis, molecular signaling and regulation, as well as synaptic activity of CRH. We go on to summarize studies of altered CRH signaling in mutant animal models. This assembled data demonstrate an important role for CRH in neuroendocrine, autonomic, and behavioral correlates of adaptation and maladaptation. Next, we present findings regarding human genetic polymorphisms in CRH pathway genes that are associated with stress and psychiatric disorders. Finally, we discuss a role for regulators of CRH activity as potential sites for therapeutic intervention aimed at treating maladaptive behaviors associated with stress.

The corticotropin releasing factor system in the kidney: perspectives for novel therapeutic intervention in nephrology

The adaptation to endogenous and exogenous stress stimuli is crucial for survival but also for the onset of various diseases in humans. Corticotropin releasing factor (CRF) system is the major regulator of stress response and homeostasis. The members of this family of peptides extend their actions also outside CNS to the periphery where they may affect various body systems independently, acting via vagal and/or autocrine/paracrine pathways. In search for peripheral targets, kidney has rarely been studied separately, regarding expression and action of CRF and CRF-related peptides. We reviewed the existing literature concerning expression and action of the CRF system in normal and pathological renal tissue and explored possible clinical implications in nephrology. CRF system components are expressed in the kidney of experimental animals and in humans. The intrarenal distribution is reported to be equally extensive, suggesting a physiological or pathophysiological role in renal function and in the occurrence of renal disease. Urocortins have given multiple interesting observations in experimental models of renal disease and clinical studies, showing robust effects in renal regulation mechanisms. We summarize the relevant data and put them in context, proposing applications with clinical significance in the field of hypertension, diabetic nephropathy, chronic kidney disease, cardiorenal syndrome, and peritoneal dialysis.

Neuroendocrine control of the gut during stress: corticotropin-releasing factor signaling pathways in the spotlight

Stress affects the gastrointestinal tract as part of the visceral response. Various stressors induce similar profiles of gut motor function alterations, including inhibition of gastric emptying, stimulation of colonic propulsive motility, and hypersensitivity to colorectal distension. In recent years, substantial progress has been made in our understanding of the underlying mechanisms of stress's impact on gut function. Activation of corticotropin-releasing factor (CRF) signaling pathways mediates both the inhibition of upper gastrointestinal (GI) and the stimulation of lower GI motor function through interaction with different CRF receptor subtypes. Here, we review how various stressors affect the gut, with special emphasis on the central and peripheral CRF signaling systems.

Deletion of corticotropin-releasing factor binding protein selectively impairs maternal, but not intermale aggression

Corticotropin-releasing factor (CRF) binding protein (CRF-BP) is a secreted protein that acts to bind and limit the activity of the neuropeptides, CRF and urocortin (Ucn) 1. We previously selected for high maternal defense (protection of offspring) in mice and found CRF-BP to be elevated in the CNS of selected mice. We also previously determined that both CRF and Ucn 1 are potent inhibitors of offspring protection when administered centrally. Thus, elevated CRF-BP could promote defense by limiting endogenous actions of CRF or Ucn 1. To test this hypothesis, we crossed the deletion for CRF-BP into the mice selected for high maternal defense and evaluated offspring protection and other maternal behaviors. CRF-BP knockout (KO) mice exhibited significant deficits in maternal aggression relative to wild-type (WT) mice in three different measures. Other maternal features were almost identical between groups, including dam and pup weight, litter size, nursing time, and pup retrieval. Both groups performed similarly in a forced swim stress test and aggression in both groups was reduced following the swim test. Virgin KO female mice exhibited higher levels of anxiety-like behavior in terms of decreased time in the light portion of the light/dark box test. For males, no differences in light/dark box or swim test were found. However, increased anxiety-like behavior in male KO mice was identified in terms of contact and approach to a novel object both with and without previous exposure to the swim test. No differences in isolation induced resident intruder male aggression were found between groups. Together, these results indicate that loss of CRF-BP selectively impairs maternal, but not intermale aggression and that loss of the gene induces anxiety-like behavior in males and females, but there are sex differences in terms of how that anxiety is revealed.

Distribution and regional stressor-induced regulation of corticotrophin-releasing factor binding protein in rainbow trout (Oncorhynchus mykiss)

The corticotrophin-releasing factor (CRF) system plays a key role in the co-ordination of the physiological response to stress in vertebrates. Although the binding protein (BP) for CRF-related peptides, CRF-BP, is an important player in the many functions of the CRF system, the distribution of CRF-BP and the impact of stressors on its expression in fish are poorly understood. In the present study, we describe the distribution of CRF-BP in the brain and peripheral tissues of rainbow trout (Oncorhynchus mykiss) using a combination of real-time reverse transcriptase-polymerase chain reaction, in situ hybridisation and immunohistochemistry. Our results indicate a widespread and highly localised distribution of CRF-BP in the central nervous system, but do not support a significant peripheral production of the protein. Major expression sites in the brain include the area ventralis telencephali, nucleus preopticus, anterior and lateral tuberal nuclei, and the posterior region of the pituitary pars distalis. We further characterise changes in CRF-BP gene expression in three discrete brain regions after exposure to 8 h and 24 h of social stress or hypoxia. The plasma cortisol concentration in subordinate fish was much higher than in dominant fish and controls, and was indicative of a relatively severe stressor. By contrast, the increase in plasma cortisol concentration in fish exposed to hypoxia was characteristic of the response to a mild stressor. Changes in CRF-BP gene expression were only observed after 24 h of either stressor, and were region-specific. CRF-BP mRNA in the telencephalon increased in both subordinate fish and fish exposed to hypoxia, but CRF-BP in the preoptic area only increased after 24 h of hypoxia exposure. In the hypothalamus, CRF-BP mRNA levels decreased in dominant fish relative to controls after 24 h. Taken together, our results support a diverse role for CRF-BP in the central actions of the fish CRF system, but a negligible role in the peripheral functions of circulating CRF-related peptides. Furthermore, the differential changes in forebrain CRF-BP mRNA appear to occur independently of the hypothalamic-pituitary-inter-renal axis.

The molecular mechanisms underlying the regulation of the biological activity of corticotropin-releasing hormone receptors: implications for physiology and pathophysiology

The CRH receptor (CRH-R) is a member of the secretin family of G protein-coupled receptors. Wide expression of CRH-Rs in the central nervous system and periphery ensures that their cognate agonists, the family of CRH-like peptides, are capable of exerting a wide spectrum of actions that underpin their critical role in integrating the stress response and coordinating the activity of fundamental physiological functions, such as the regulation of the cardiovascular system, energy balance, and homeostasis. Two types of mammal CRH-R exist, CRH-R1 and CRH-R2, each with unique splicing patterns and remarkably distinct pharmacological properties, but similar signaling properties, probably reflecting their distinct and sometimes contrasting biological functions. The regulation of CRH-R expression and activity is not fully elucidated, and we only now begin to fully understand the impact on mammalian pathophysiology. The focus of this review is the current and evolving understanding of the molecular mechanisms controlling CRH-R biological activity and functional flexibility. This shows notable tissue-specific characteristics, highlighted by their ability to couple to distinct G proteins and activate tissue-specific signaling cascades. The type of activating agonist, receptor, and target cell appears to play a major role in determining the overall signaling and biological responses in health and disease.

Corticotropin-releasing factor-binding protein, stress and major depression

Major depressive disorder (MDD) is characterized by a dysregulation of the stress response system. A corticotropin-releasing factor (CRF) hyperdrive is a consistent and well-documented finding. CRF-binding protein (CRF-BP) may play a role in the pathogenesis of MDD. CRF-BP reduces the availability of CRF by binding free CRF and inhibits CRF function at the pituitary level. Moreover, CRF-BP expression increases in the pituitary and amygdala in response to acute stress, providing an additional feedback mechanism to maintain the homeostasis of the stress response. There are different regulatory elements of the expression of CRF-BP gene that are implicated in the pathophysiology of MDD, including CRF, glucocorticoids, cytokines and estrogens. A specific haplotype within the CRF-BP gene has been associated with MDD, but confirmation of this finding is necessary. Currently, the possible role of CRF-BP in the pathophysiology of conditions that have been associated with a hypofunction of the CRF system and immune dysfunctions is unclear. Implications of the function of CRF-BP for therapeutic strategies in MDD are being discussed. An important advantage of ligands that target CRF-BP is that concentrations of free CRF can be altered without acting directly on the transmission of CRF through its receptor.

Dissecting the genetic effect of the CRH system on anxiety and stress-related behaviour

Corticotropin-releasing hormone (CRH) plays a central role in the adaptation of the body to stress. CRH integrates the endocrine, autonomic and behavioural responses to stress acting as a secretagogue within the line of the hypothalamic pituitary adrenocortical (HPA) system and as a neurotransmitter modulating synaptic transmission in the central nervous system. Accumulating evidence suggests that the neuroendocrine and behavioural symptoms observed in patients suffering from major depression are at least in part linked to a hyperactivity of the CRH system. Genetic modifications of the CRH system by conventional and conditional gene targeting strategies in the mouse allowed us to study the endogenous mechanisms underlying HPA system regulation and CRH-related neuronal circuitries involved in pathways mediating anxiety and stress-related behaviour.

Distribution of corticotropin-releasing factor binding protein-immunoreactivity in the rat hypothalamus: association with corticotropin-releasing factor-, urocortin 1- and vimentin-immunoreactive fibres

Corticotropin-releasing factor binding protein (CRF-BP) is a 37-kDa protein with high affinity binding sites for both corticotropin-releasing factor (CRF) and urocortin 1. Previous studies have examined the distribution of CRF-BP mRNA and peptide within the central nervous system. Due to the predominant cortical localisation, very little is known about CRF-BP in subcortical structures including the hypothalamus. The present study employed immunohistochemistry to characterise the distribution of CRF-BP-like-immunoreactive (-ir) cells and fibres in the rat hypothalamus. Bipolar and multipolar CRF-BP-ir neurones were scattered throughout the rostro-caudal extent of the hypothalamus. Distinct clusters of CRF-BP-ir neurones were identified in the anterior and posterior parvocellular and dorsal cap subdivisions of the paraventricular nucleus (PVN), as well as in the dorsal hypothalamic area, dorsomedial hypothalamic nucleus (DMN), ventral premammillary nucleus and zona incerta. CRF-BP-ir fibres extending from the third ventricle were found in the mediobasal hypothalamus and within the arcuate nucleus-median eminence region. Double immunostaining together with confocal microscopy demonstrated that the CRF-BP-immunostained fibres within the mediobasal hypothalamus coincided with vimentin immunostaining indicating that CRF-BP-ir is present within tanycytes. To define the relationship between CRF-BP-ir cells and endogenous ligands for CRF-BP, double immunohistochemistry was performed to examine possible sites within the hypothalamus where CRF- or urocortin 1-ir fibres innervate regions that contain CRF-BP-ir cell bodies. CRF-BP-ir cell bodies typically coincided with dense CRF-ir, but not urocortin 1-ir fibre innervation. CRF-ir fibre innervation was moderate to high within the anterior and posterior parvocellular subdivisions of the PVN, the dorsal cap of the PVN, DMN and the zona incerta; all regions that contained CRF-BP-ir cell populations. These studies demonstrate that, within the hypothalamus, CRF-BP-ir cells and fibres are concentrated within a circuitry known to be involved in mediating neuroendocrine and autonomic responses to stress.

Mutagenesis and knockout models: hypothalamic-pituitary-adrenocortical system

Hyperactivity of central neuropeptidergic circuits such as the corticotropin-releasing hormone (CRH) and vasopressin (AVP) neuronal systems is thought to play a causal role in the etiology and symptomatology of anxiety disorders. Indeed, there is increasing evidence from basic science that chronic stress-induced perturbation of CRH and AVP neurocircuitries may contribute to abnormal neuronal communication in conditions of pathological anxiety. Anxiety disorders aggregate in families, and accumulating evidence supports the notion that the major source of familial risk is genetic. In this context, refined molecular technologies and the creation of genetically engineered mice have allowed us to specifically target individual genes involved in the regulation of the elements of the CRH (e.g., CRH peptides, CRH-related peptides, their receptors, binding protein). During the past few years, studies performed in such mice have complemented and extended our knowledge. The cumulative evidence makes a strong case implicating dysfunction of CRH-related systems in the pathogenesis of anxiety disorders and depression and leads us beyond the monoaminergic synapse in search of eagerly anticipated strategies to discover and develop better therapies.

Listening to mutant mice: a spotlight on the role of CRF/CRF receptor systems in affective disorders

Genetically engineered mice were originally generated to delineate the role of a specific gene product in behavioral or neuroendocrine phenotypes, rather than to produce classic animal models of depression. To learn more about the neurobiological mechanisms underlying a clinical condition such as depression, it has proven worthwhile to investigate changes in behaviors characteristic of depressed humans, such as anxiety, regardless of whether or not these alterations may also occur in other disorders besides depression. The majority of patients with mood and anxiety disorders have measurable shifts in their stress hormone regulation as reflected by elevated secretion of central and peripheral stress hormones or by altered hormonal responses to neuroendocrine challenge tests. In recent years, these alterations have been increasingly translated into testable hypotheses addressing the pathogenesis of illness. Refined molecular technologies and the creation of genetically engineered mice have allowed to specifically target individual genes involved in regulation of corticotropin releasing factor (CRF) system elements (e.g. CRF and CRF-related peptides, their receptors, binding protein). Studies performed in such mice have complemented and extended our knowledge. The cumulative evidence makes a strong case implicating dysfunction of these systems in the pathogenesis of depression and leads us beyond the monoaminergic synapse in search of eagerly anticipated strategies to discover and develop better therapies for depression.

Long double-stranded RNA-mediated RNA interference as a tool to achieve site-specific silencing of hypothalamic neuropeptides

RNA interference (RNAi) has become a popular tool to silence gene expression in a variety of in vitro and in vivo systems. However, it has met with limited success in inhibiting gene expression in adult mammals. Here we demonstrate that long double-stranded RNA (dsRNA) can be used to create a "site-specific", transient knockdown of genes in a fashion that is phenotypically akin to genetically manipulated organisms. Corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) that regulate a variety of physiological processes including the hypothalamic-pituitary-adrenal axis (HPA axis), energy and water homeostasis were used as model systems. Stereotaxic injections of dsRNA against CRF and AVP in the PVN specifically abolished the expression of these genes in the PVN leaving expression in other loci intact. Control dsRNA did not affect CRF or AVP expression in any brain region, suggesting that dsRNA did not shut down global protein synthesis. ANOVA showed significant main effects of silencing of CRF on dampening of the stress-activated release of adrenocorticotrophin hormone (ACTH) (F(2,7)=4.87; p<0.047). Silencing of AVP resulted in increased water consumption, increased urine output and decreased urine osmolality as compared to control dsRNA-treated rats. Furthermore, dsRNA had no obvious deleterious effects on body weight or food consumption, variables considered essential in ruling out adverse physiologic effects in animal models. Thus, using long dsRNA, we were able to ascertain site-specific roles of CRF and AVP in adult rats without any developmental compensation and in a wild-type background.

Transgenic growth hormone mice exposed to lifetime constant illumination: gender-specific effects

Photoperiod affects most of the features altered in transgenic growth hormone (TG) mice, and laboratory rats and mice retain some sensitivity to photoperiod. We examined growth, feeding, longevity, and reproduction of TG mice and normal control mice (Mus musculus L., 1758) in 12 h light : 12 h dark (LD) and 24 h light (LL) photoperiods. Sexual dichotomy in growth and hepatic gene expression are considered to require gender-specific patterns of growth hormone secretion that are absent in TG mice. Regardless, in the LD photoperiod mature TG females were 82.8% (46.8 g) of the mass of TG males (56.5 g, p < 0.05), whereas control mice showed no size dichotomy (≈33 g). Mature masses of TG males and of control mice of either gender were unaffected by the LL photoperiod. TG females, however, reached a mature mass 92% (50.9 g) of that of mature TG males in the LL photoperiod, attenuating the sexual size dichotomy expressed in the LD photoperiod. Growth of females was slower than that of males, even in the control group. TG females in the LL photoperiod expressed faster growth, higher reproduction, and greater mean longevity than TG females in the LD photoperiod. Differences in age-related feeding associated with gender and photoperiod reflected differential growth rates. Females grew more slowly and ate more than males of similar age because they were smaller (i.e., had lower growth efficiencies). The LL photoperiod improved the energy balance of TG females. Possible mechanisms mediating such gender-specific effects are explored.

CRF and CRF receptors: role in stress responsivity and other behaviors

Since corticotropin-releasing factor (CRF) was first characterized, a growing family of ligands and receptors has evolved. The mammalian family members include CRF, urocortinI (UcnI), UcnII, and UcnIII, along with two receptors, CRFR1 and CRFR2, and a CRF binding protein. These family members differ in their tissue distribution and pharmacology. Studies have provided evidence supporting an important role of this family in regulation of the endocrine and behavioral responses to stress. Although CRF appears to play a stimulatory role in stress responsivity through activation of CRFR1, specific actions of UcnII and UcnIII on CRFR2 may be important for dampening stress sensitivity. As the only ligand with high affinity for both receptors, UcnI's role may be promiscuous. Regulation of the relative contribution of the two CRF receptors to brain CRF pathways may be essential in coordinating physiological responses to stress. The development of disorders related to heightened stress sensitivity and dysregulation of stress-coping mechanisms appears to involve regulatory mechanisms of CRF family members.

Hypothalamic control of energy balance: different peptides, different functions

Energy balance is maintained via a homeostatic system involving both the brain and the periphery. A key component of this system is the hypothalamus. Over the past two decades, major advances have been made in identifying an increasing number of peptides within the hypothalamus that contribute to the process of energy homeostasis. Under stable conditions, equilibrium exists between anabolic peptides that stimulate feeding behavior, as well as decrease energy expenditure and lipid utilization in favor of fat storage, and catabolic peptides that attenuate food intake, while stimulating sympathetic nervous system (SNS) activity and restricting fat deposition by increasing lipid metabolism. The equilibrium between these neuropeptides is dynamic in nature. It shifts across the day-night cycle and from day to day and also in response to dietary challenges as well as peripheral energy stores. These shifts occur in close relation to circulating levels of the hormones, leptin, insulin, ghrelin and corticosterone, and also the nutrients, glucose and lipids. These circulating factors together with neural processes are primary signals relaying information regarding the availability of fuels needed for current cellular demand, in addition to the level of stored fuels needed for long-term use. Together, these signals have profound impact on the expression and production of neuropeptides that, in turn, initiate the appropriate anabolic or catabolic responses for restoring equilibrium. In this review, we summarize the evidence obtained on nine peptides in the hypothalamus that have emerged as key players in this process. Data from behavioral, physiological, pharmacological and genetic studies are described and consolidated in an attempt to formulate a clear statement on the underlying function of each of these peptides and also on how they work together to create and maintain energy homeostasis.

Transgenic studies on the regulation of the anterior pituitary gland function by the hypothalamus

The anterior pituitary gland is composed of five different cell types secreting hormones whose functions include the regulation of post-natal growth (growth hormone, GH), lactation (prolactin, PRL), reproduction (luteinising hormone, LH, and follicle stimulating hormone, FSH), metabolism (thyroid stimulating hormone, TSH), and stress (adrenocorticotrophic hormone, ACTH). The synthesis and secretion of the anterior pituitary hormones is under the control of neuropeptides released from the hypothalamus into a capillary portal plexus which flows through the external zone of the median eminence to the anterior lobe. This review describes the ways that gene transfer technologies have been applied to whole animals in order to study the regulation of anterior pituitary function by the hypothalamus. The extensive studies on these neuronal systems, within the context of the physiological integrity of the intact organism, not only exemplify the successful application of transgenic technologies to neuroendocrine systems, but also illustrate the problems that have been encountered, and the challenges that lie ahead.

Targeted mutations of the corticotropin-releasing factor system: effects on physiology and behavior

Genetic modifications of the genes that encode proteins integral to the corticotropin-releasing factor (CRF) system have been employed in the creation of mutant mice that serve as tools for studying the role of this neuropeptide in regulated and dysregulated behaviors and physiology. Overexpression of the CRF peptide and CRF binding protein as well as deletion of the peptide, binding protein, and both known receptors has been achieved and these mouse models have been characterized for anatomical, neuroendocrine, and behavioral sequelae. The profile of results, consistent with current knowledge of CRF function from more traditional assays, indicates that enhancement of CRF function is associated with an activation of the hypothalamic-pituitary-adrenal axis, an anxious phenotype, alterations in cognitive performance and reductions in feeding. In general, blockade of CRF function produces the opposite effects. Genetic mouse models allow further analysis of specific elements in the CRF circuitry for which more traditional tools have not existed. These animal models are valuable for increasing our understanding of the underlying pathology associated with a variety of psychiatric and neuroendocrine disorders and for the development and testing of novel treatment agents.

Genetically engineered mice for studies of stress-related clinical conditions

Genetically engineered mice with a specific deletion of targeted genes provide a novel and useful tool to study the endogenous mechanisms underlying aberrant behaviour. In this review we take the stress hormone (hypothalamic-pituitary-adrenocortical) system as an example to demonstrate how refined molecular technologies have allowed to target individual genes involved in stress hormone regulation. We describe different gene targeting methods: the generation of "conventional" knock-out mice enables us to delete a gene of interest in every cell of the body. Equally important for the studies of gene function in the mouse is the use of tissue-specific regulatory systems that allow gene inactivation to be restricted to specific tissues and, in some cases, to specific time points during development, such as the "conditional" knock-out, or the application of antisense techniques. Importantly, deletion of individual genes is not providing animal models for certain psychiatric disorders as these are caused by a manifold of minor changes in a series of so-called susceptibility genes. However, these gene targeting methods have become valuable tools to dissect the functions of individual components of complex biological systems in behavioural neuroscience: genetically engineered animals help to unravel the complex interactions and correlations between individual genes, hormonal regulation and behaviour, the most complex form of biological organization.

The molecular neurobiology of stress--evidence from genetic and epigenetic models

Knowledge of the genetic and molecular events underlying the neuroendocrine and behavioural sequelae of the response to stress has advanced rapidly over recent years. The response of an individual to a stressful experience is a polygenic trait, but also involves non-genetic sources of variance. Using a combination of top-down (quantitative trait locus [QTL] and microarray analysis) and bottom-up (gene targeting, transgenesis, antisense technology and random mutagenesis) strategies, we are beginning to dissect the molecular players in the mediation of the stress response. Given the wealth of the data obtained from mouse mutants, this review will primarily focus on the contributions made by transgenesis and knockout studies, but the relative contribution of QTL studies and microarray studies will also be briefly addressed. From these studies it is evident that several neuroendocrine and behavioural alterations induced by stress can be modelled in mouse mutants with alterations in hypothalamic-pituitary-adrenal axis activity or other, extrahypothalamic, neurotransmitter systems known to be involved in the stress response. The relative contribution of these models to understanding the stress response and their limitations will be discussed.

Selective stimulatory actions of corticotropin-releasing factor ligands on correlates of energy balance

Acute administration of corticotropin-releasing factor (CRF) results in anorexic and sympathomimetic effects that suggest efficacy in chronic models of energy balance. The present studies employed a broad spectrum energy balance indices in lean and genetically obese Zucker rats in order to fully characterize the pharmacological efficacy of CRF and a CRF binding protein (CRF-BP) ligand inhibitor, CRF(6-33), which is thought to liberate CRF from CRF-BP. Acute administration of CRF(6-33) significantly increased CRF(2) receptor density by 10% within the ventromedial hypothalamic (VMH) nucleus of Zucker lean rats and decreased density by 10% in Zucker obese rats. A single infusion of CRF(6-33) increased nonshivering thermogenesis by 25-30% as measured by proton conductance in brown adipose tissue of both lean and obese rats. Chronic CRF(6-33) infusion suppressed body weight gain and elevated core temperature irrespective of genotype while increasing motor activity in obese rats without altering heart rate or blood pressure. Taken together, these results document strain-dependent, long-term effects of a CRF-BP ligand inhibitor on a select subset of physiological and behavioral measures of increased energy expenditure.

Corticotropin-releasing factor (CRF) or CRF binding-protein ligand inhibitor administration suppresses food intake in mice and elevates body temperature in rats

Corticotropin-releasing factor (CRF) receptor agonist and CRF binding-protein (CRF-BP) ligand inhibitor peptides both activate CRF systems but exert very distinct functional profiles in animal models of arousal, energy balance and emotionality. The present studies were designed to extend the dissimilar efficacy profiles of central administration of a CRF agonist, r/h CRF(1-41), versus a CRF-BP ligand inhibitor, r/h CRF(6-33), into mouse and rat models of energy balance in order to further explore in vivo efficacy of these ligands in two separate animal species. In CD-1 mice, food intake was significantly attenuated 3 h after acute administration of CRF(1-41) (0.007-0.2 nmol), but not CRF(6-33). In obese Ob/Ob mice, both CRF(1-41) (0.007-0.2 nmol) and CRF(6-33) (0.02-2.3 nmol) significantly attenuated basal feeding over 3 h following acute peptide administration. In rats, CRF(1-41) (1 nmol) and CRF(6-33) (1.5-7.7 nmol) infusion significantly increased rectal temperature. In studies employing a telemetry apparatus, core temperature was also increased by CRF(1-41) (1 nmol) and CRF(6-33) (1.5 nmol), whereas only CRF(1-41) increased locomotor activity and heart rate. These results suggest that CRF receptor agonist administration is capable of producing a global profile of negative energy balance by reducing food intake in mice and increasing energy expenditure in rats. In contrast, CRF-BP ligand inhibitor administration appears to suppress food intake in a mouse strain selective manner and to elevate rectal and core temperature in rats without accompanying cardiovascular activation.

Mouse models of altered CRH-binding protein expression

CRH is the key physiological mediator of the endocrine, autonomic, and behavioral responses to stress. The recent characterization of urocortin, a new mammalian CRH-like ligand, adds to the complexity of the CRH system. Both CRH and urocortin mediate their endocrine and/or synaptic effects via two classes of CRH receptors. Similarly, both CRH and urocortin bind to the CRH-binding protein (CRH-BP). This secreted binding protein is smaller than the CRH receptors, but binds CRH and urocortin with an affinity equal to or greater than that of the receptors, and blocks CRH-mediated ACTH release in vitro. Several regions of CRH-BP expression colocalize with sites of CRH synthesis or release, suggesting that this binding protein may have a profound impact on the biological activity of CRH (or urocortin). While in vitro and in vivo studies have characterized the biochemical properties and regulation of the CRH-BP, animal models of altered CRH-BP expression can provide additional information on the in vivo role of this important modulatory protein. This review focuses on three mouse models of CRH-BP overexpression or deficiency. These animal models show numerous physiological changes in the HPA axis and in energy balance, with additional alterations in anxiogenic behavior. These changes are consistent with the hypothesis that CRH-BP plays an important in vivo modulatory role by regulating levels of "free" CRH and other CRH-like peptides in the pituitary and central nervous system.

Genetic models of obesity and energy balance in the mouse

Obesity is a health problem of epidemic proportions in the industrialized world. The cloning and characterization of the genes for the five naturally occurring monogenic obesity syndromes in the mouse have led to major breakthroughs in understanding the physiology of energy balance and the contribution of genetics to obesity in the human population. However, the regulation of energy balance is an extremely complex process, and it is quickly becoming clear that hundreds of genes are involved. In this article, we review the naturally occurring monogenic and polygenic obese mouse strains, as well as the large number of transgenic and knockout mouse models currently available for the study of obesity and energy balance.

Biochemical characterization and expression analysis of the Xenopus laevis corticotropin-releasing hormone binding protein

Corticotropin-releasing hormone (CRH) plays a key role in the regulation of responses to stress. The presence of a high affinity binding protein for CRH (CRH-BP) has been reported in mammals. We have characterized the biochemical properties and expression of CRH-BP in the South African clawed frog, Xenopus laevis. Apparent inhibition constants (K(i[app])) for different ligands were determined by competitive binding assay. Xenopus CRH-BP (xCRH-BP) exhibited a high affinity for xCRH (K(i[app])=1.08 nM) and sauvagine (1.36 nM). Similar to rodent and human CRH-BPs, the frog protein binds urotensin I and urocortin with high affinity, and ovine CRH with low affinity. RT-PCR analysis showed that xCRH-BP is expressed in brain, pituitary, liver, tail, and intestine. Brain xCRH-BP mRNA is expressed at a relatively constant level throughout metamorphosis and increases slightly in the metamorphic frog. By contrast, the gene is strongly upregulated in the tail at metamorphic climax. Thus, regulation of xCRH-BP gene expression is tissue specific. Because xCRH-BP binds CRH-like peptides with high affinity the protein may regulated, the bioavailability of CRH in amphibia as it does in mammals.

Pharmacological characterization of recombinant rat corticotropin releasing factor binding protein using different sauvagine analogs

Little is known on the structural ligand requirements for corticotropin-releasing factor binding protein (CRFBP) of the rat used as an important experimental animal. To obtain such information recombinant rat CRFBP was produced in stably transfected HEK 293 cells. The primary structure and posttranslational processing of purified rat CRFBP was established by peptide mapping using HPLC combined with mass spectrometric analysis. Rat CRFBP was pharmacologically characterized employing a competition binding assay with tritium-labeled rat urocortin. The rank order of declining affinity of various CRF analogs was urotensin-I, human/rat CRF (h/rCRF), rat urocortin, sauvagine (Svg), and ovine CRF in agreement with the rank order found for human CRFBP. In contrast to astressin, the CRF receptor 2-selective antagonist anti-sauvagine-30 did not show any detectable specific binding to rat CRFBP. The significance of residues 10 to 12 and 21 to 24 of Svg for its low affinity binding was established by changing these residues of Svg to those of h/rCRF. The corresponding residues 22 to 25 of h/rCRF represented the ARAE motif determined to be crucial for binding in agreement with reported data on human CRFBP. Residues 11 to 13 of CRF introduced into Svg also enhanced the affinity to rat CRFBP.

Corticotropin-releasing hormone and animal models of anxiety: gene-environment interactions

The study of the neural substrates underlying stress and anxiety has in recent years been enriched by a burgeoning pool of genetic information gathered from rodent studies. Two general approaches have been used to characterize the interaction of genetic and environmental factors in stress regulation: the evaluation of stress-related behavioral and endocrine responses in animals with targeted deletion or overexpression of specific genes and the evaluation of changes in central nervous system gene expression in response to environmental perturbations. We review recent studies that have used molecular biology and genetic engineering techniques such as in situ hybridization, transgenic animal, and antisense oligonucleotide gene-targeting methodologies to characterize the function of corticotropin-releasing hormone (CRH) system genes in stress. The effects of genetic manipulations of each element of the CRH system (CRH, its two receptors, and its binding protein) on stress-related responses are summarized. In addition, the effects of stress (acute, repeated, or developmental) on CRH system gene expression are described. The results from these studies indicate that experimentally engineered or stress-induced dysregulation of gene expression within the CRH system is associated with aberrant responses to environmental contingencies. These results are discussed in the context of how CRH system dysfunction might contribute to stress-related psychopathology and are presented in conjunction with clinical findings of CRH system dysregulation in psychiatric illness. Finally, future research strategies (i.e., high-throughput gene screening and novel gene-targeting methodologies) that may be used to gain a fuller understanding of how CRH system gene expression affects stress-related functioning are discussed.

Transgenic study of energy homeostasis equation: implications and confounding influences

Recently novel molecular mediators and regulatory pathways for feeding and body weight regulation have been identified in the brain and the periphery. Mice lacking or overexpressing these mediators or receptors have been produced by molecular genetic techniques, and observations on mutant mice have shed new light on the role of each element in the homeostatic loop of body weight regulation. However, the interpretation of the phenotype is under the potential influence of developmental compensation and other genetic and environmental confounds. Specific alterations of the mediators and the consequences of the altered expression patterns are reviewed here and discussed in the context of their functions as suggested from conventional pharmacological studies. Advanced gene targeting strategies in which genes can be turned on or off at desired tissues and times would undoubtedly lead to a better understanding of the highly integrated and redundant systems for energy homeostasis equation.

Molecular integration of hypothalamo-pituitary-adrenal axis-related neurohormones on the GnRH neuron

Gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus is pivotal to the regulation of reproductive physiology in vertebrates. GnRH and the reproductive axis, in general, can be inhibited during periods of stress or injury. Stress, in the form of mechanical, psychological or immunological insult to an organism results in the activation of the hypothalamo-pituitary-adrenal (HPA) axis initiated by the hypothalamic release of corticotropin-releasing factor (CRF). Recent studies indicate that CRF may act either directly on the GnRH neuron to down-regulate GnRH synthesis, or indirectly via a β-endorphin-mediated pathway. Moreover, in vitro studies suggest that CRF-related peptides can increase the sensitivity of the GnRH neuron to prolactin by increasing the synthesis of the prolactin receptor.

Corticotropin-releasing factor antagonists, binding-protein and receptors: implications for central nervous system disorders

Corticotrophin-releasing factor (CRF; interchangeable with corticotrophin-releasing hormone, CRH) is a neurohormone family of peptides which implements endocrine, physiological and behavioural responses to stressor exposure. Built-in biological diversity and selectivity of CRF system function is provided by multiple endogenous ligands and receptors which are heterogeneously distributed in both brain and peripheral tissues across species. At present, there are at least five distinct targets for CRF with unique cDNA sequences, pharmacology and localization. These fall into three distinct classes, encoded by three different genes and have been termed the CRF1 and CRF2 receptors and the CRF-binding protein. Significant gains in knowledge about the physiological role of CRF binding sites in brain have emerged recently due to the proliferation of novel, high-affinity, receptor-selective pharmacological tools as well as multiple knock-out and knock-in mutant mouse models. These results support a role for CRF binding sites in co-ordinating stress reactivity, emotionality and energy balance over the life-span of the organism.

Altered anxiety and weight gain in corticotropin-releasing hormone-binding protein-deficient mice

Corticotropin-releasing hormone (CRH) is widely recognized as the primary mediator of the neuroendocrine and behavioral responses to stress, including stress-induced anxiety. The biological activity of CRH and other mammalian CRH-like peptides, such as urocortin, may be modulated by CRH-binding protein (CRH-BP). To assess directly the CRH-BP function, we created a mouse model of CRH-BP deficiency by gene targeting. Basal adrenocorticotropic hormone and corticosterone levels are unchanged in the CRH-BP-deficient mice, and the animals demonstrate a normal increase in adrenocorticotropic hormone and corticosterone after restraint stress. In contrast, adult male CRH-BP-deficient mice show significantly reduced body weight when compared with wild-type controls. CRH-BP-deficient mice also exhibit a significant increase in anxiogenic-like behavior as assessed by the elevated plus maze and defensive withdrawal tests. The increased anorectic and anxiogenic-like behavior most likely is caused by increased "free" CRH and/or urocortin levels in the brain of CRH-BP-deficient animals, suggesting an important role for CRH-BP in maintaining appropriate levels of these peptides in the central nervous system.

Evolution and physiology of the corticotropin-releasing factor (CRF) family of neuropeptides in vertebrates

Corticotropin-releasing factor (CRF), urotensin-I, urocortin and sauvagine belong to a family of related neuropeptides found throughout chordate taxa and likely stem from an ancestral peptide precursor early in metazoan ancestry. In vertebrates, current evidence suggests that CRF on one hand, and urotensin-I, urocortin and sauvagine, on the other, form paralogous lineages. Urocortin and sauvagine appear to represent tetrapod orthologues of fish urotensin-I. Sauvagine's unique structure may reflect the distinctly derived evolutionary history of the anura and the amphibia in general. The physiological actions of these peptides are mediated by at least two receptor subtypes and a soluble binding protein. Although the earliest functions of these peptides may have been associated with osmoregulation and diuresis, a constellation of physiological effects associated with stress and anxiety, vasoregulation, thermoregulation, growth and metabolism, metamorphosis and reproduction have been identified in various vertebrate species. The elaboration of neural circuitry for each of the two paralogous neuropeptide systems appears to have followed distinct pathways in the actinopterygian and sarcopterygian lineages of vertebrates. A comparision of the functional differences between these two lineages predicts additional functions of these peptides.

Animal models of CRH deficiency

Corticotropin-releasing hormone (CRH), the major regulator of hypothalamic-pituitary-adrenal (HPA) axis, was first isolated due to its ability to stimulate the release of adrenocorticotropic hormone from the anterior pituitary. Later, it was also found to have also a wide spectrum of actions within the central nervous system and the periphery. Studies with pharmacological administration of this peptide and/or antagonists and antibody neutralization techniques have yielded important information concerning the physiological relevance of CRH. The development of CRH knockout mice (CRH KO) has been an important tool for addressing the physiologic and pathologic roles of CRH. This review describes the phenotype of CRH-deficient mice, as well as the use of this model to study the roles of CRH on fetal development and postnatal life. The role of CRH in prenatal development and postnatal regulation of the HPA axis, in activation of the reproductive system during stress, and in modulation of the immune function will be discussed. The review concludes with a comparison of CRH KO mice with other models of CRH deficiency.

Cloning and characterization of corticotropin-releasing factor and urocortin in Syrian hamster (Mesocricetus auratus)

Corticotropin-releasing factor and urocortin belong to a superfamily of neuropeptides that includes the urotensins-I in fishes and the insect diuretic peptides. Sequence analysis suggests that urocortin is the mammalian ortholog of urotensin-I, although the physiological role for this peptide in mammals is not known. Within the Rodentia, hamsters belong to a phylogenetically older lineage than that of mice and rats and possess significant differences in hypothalamic organization. We have, therefore, cloned the coding region of the Syrian hamster (Mesocricetus auratus) corticotropin-releasing factor and urocortin mature peptide by polymerase chain reaction. Hamster urocortin was prepared by solid-phase synthesis, and its pharmacological actions on human corticotropin-releasing factor R1 and R2 receptors were investigated. The deduced hamster corticotropin-releasing factor amino acid sequence and cleavage site is identical to that in rat, whereas the urocortin sequence is unique among the urocortin/urotensin-I/sauvagine family in possessing asparagine and alanine in positions 38 and 39, respectively. The hamster urocortin carboxy terminus sequence bears greater structural similarity to the insect diuretic peptide family, suggesting either retrogressive mutational changes within the mature peptide or convergent sequence evolution. Despite these changes, human and hamster urocortin are generally equipotent at cAMP activation, neuronal acidification rate, and R1/R2 receptor affinities.