Genetically engineered mice for studies of stress-related clinical conditions

Development of the Neuroendocrine Hypothalamus

The neuroendocrine hypothalamus is composed of the tuberal and anterodorsal hypothalamus, together with the median eminence/neurohypophysis. It centrally governs wide-ranging physiological processes, including homeostasis of energy balance, circadian rhythms and stress responses, as well as growth and reproductive behaviours. Homeostasis is maintained by integrating sensory inputs and effecting responses via autonomic, endocrine and behavioural outputs, over diverse time-scales and throughout the lifecourse of an individual. Here, we summarize studies that begin to reveal how different territories and cell types within the neuroendocrine hypothalamus are assembled in an integrated manner to enable function, thus supporting the organism's ability to survive and thrive. We discuss how signaling pathways and transcription factors dictate the appearance and regionalization of the hypothalamic primordium, the maintenance of progenitor cells, and their specification and differentiation into neurons. We comment on recent studies that harness such programmes for the directed differentiation of human ES/iPS cells. We summarize how developmental plasticity is maintained even into adulthood and how integration between the hypothalamus and peripheral body is established in the median eminence and neurohypophysis. Analysis of model organisms, including mouse, chick and zebrafish, provides a picture of how complex, yet elegantly coordinated, developmental programmes build glial and neuronal cells around the third ventricle of the brain. Such conserved processes enable the hypothalamus to mediate its function as a central integrating and response-control mediator for the homeostatic processes that are critical to life. Early indications suggest that deregulation of these events may underlie multifaceted pathological conditions and dysfunctional physiology in humans, such as obesity.

A single episode of restraint stress regulates central corticotrophin- releasing hormone receptor expression and binding in specific areas of the mouse brain

The importance of restraining stress-induced activation of the hypothalamic-pituitary-adrenocortical (HPA) system within tolerable limits requires efficient mechanisms for feedback inhibition. Recently, central corticotrophin-releasing hormone (CRH) receptor type 1 (CRHR1) has been shown to mediate HPA system feedback inhibition. To date, most of the data regarding stress-associated expression changes of CRHR1 and CRHR2 mRNA and their ligand CRH have been generated in rats. Taken considerable species differences into consideration, and with the growing importance of transgenic mice, a systematic analysis of the time course of expression changes of CRH and its two receptors in the mouse brain is needed to provide more insight into the regulation of the HPA system, both under physiological and pathophysiological conditions in this species. We analysed in detail the time course of expression changes of CRH, CRHR1 and CRHR2 mRNA after of restraint stress in mice in stress-relevant brain regions (paraventricular nucleus, hippocampus, neocortex). We could show a rapid, strong and long-lasting decrease in cortical and hippocampal CRHR1 mRNA expression after stress, whereas CRHR2 mRNA increased in the same neuroanatomical areas. In situ hybridisation analyses could be further confirmed at the protein level by CRH receptor autoradiography with changes in CRH binding that persisted even 7 days after a single episode of restraint stress. Our observation that stress has opposing effects on CRHR1 and CRHR2 neuronal systems supports the idea that regulation of the relative contribution of the two CRH receptors to brain CRH pathways may be essential in coordinating physiological responses to stress. We further hypothesise that the sustained alteration of CRH receptor expression and binding after a single episode of stress could mediate the long-term effects of stress on neuroendocrine function and emotional regulation.

Chronic low dose ovine corticotropin releasing factor or urocortin II into the rostral dorsal raphe alters exploratory behavior and serotonergic gene expression in specific subregions of the dorsal raphe

Corticotropin releasing factor (CRF) family peptides play key roles in integrating neural responses to stress. Both major CRF receptors have been pharmacologically identified in the dorsal raphe nucleus (DRN), a stress sensitive and internally heterogeneous nucleus supplying many forebrain regions with serotonergic input. Despite the involvement of chronic stress and serotonergic dysfunction in human mood and anxiety disorders, little is known about the effects of chronic CRF receptor activation on the DRN. We infused ovine CRF (1 ng/h), urocortin II (UCNII, 1 ng/h), or vehicle alone into rat DRN over 6 days. During infusion, animals were allowed to freely explore an open field for 15 min on each of 2 days, with the addition of a novel object on the second day. Following behavioral testing, 5-HT1A, 5-HT1B, 5-HT transporter (SERT), and tryptophan hydroxylase-2 (Tph2) expression was examined through the DRN by in situ hybridization. Ovine CRF infusion resulted in significantly decreased novel object touches, climbs, as well as increased latency to first novel object contact. UCNII had a similar but less dramatic effect, decreasing only climbing behavior. Both ovine CRF and UCNII blunted the decrease in corner time expected on re-exposure to the open field. Both peptides also produced regionally specific changes in gene expression: 5-HT1A expression was increased 30% in the mid-rostral ventromedial DRN, while SERT was decreased by 30% in the mid-caudal shell dorsomedial DRN. There also appeared to be a shift in the relative level of Tph2 expression between the ventromedial and core dorsomedial DRN at the mid-rostral level. Changes in 5-HT1A, SERT, and relative Tph2 mRNA abundance were correlated with novel object exploration. These findings suggest chronic intra-DRN administration of CRF agonists decreases exploratory behavior, while producing subregionally limited changes in serotonergic gene expression. These studies may be relevant to mechanisms underlying behavioral changes after chronic stress.

The corticotropin-releasing factor receptor: a novel target for the treatment of depression and anxiety-related disorders

The treatment of mood disorders has been the subject of intense study for more than half a century and has resulted in the discovery and availability of a number of compounds that have seen tremendous success in the management of major depression and anxiety-related disorders. In spite of this success, these drugs have not provided a complete therapeutic solution for all patients and this has revitalised the need for a greater understanding of the underlying molecular mechanisms and targets involved in these disorders. Elucidation of these novel targets will enable the development of a better class of compounds which could benefit a greater majority of the patient population and be devoid of the current side effect liabilities. Towards that end, this review examines, in detail, the prospect of one such target, the corticotropin-releasing factor system, as having an enhanced therapeutic profile with the potential of a broader range of efficacy with reduced side effect liabilities.

Differential disinhibition of the neonatal hypothalamic- pituitary-adrenal axis in brain-specific CRH receptor 1-knockout mice

In the adult, corticotropin-releasing hormone (CRH) is the key mediator for the behavioural and neuroendocrine response to stress. It has also been hypothesized that, during postnatal development of the stress system, CRH controls the activity of the HPA axis and mediates the effects of early disturbances, e.g. 24 h of maternal deprivation. In the current study we investigated the function of specific brain corticotropin-releasing hormone receptor type 1 (CRHR1) subpopulations in the control of the HPA axis during postnatal development under basal conditions as well as after 24 h of maternal deprivation. We used two conditional CRHR1-deficient mouse lines which lack this receptor, either specifically in forebrain and limbic structures (Cam-CRHR1) or in all neurons (Nes-CRHR1). Basal circulating corticosterone was increased in Nes-CRHR1 mice compared to controls. Corticosterone response to maternal deprivation was significantly increased in both CRHR1-deficient lines. In the paraventricular nucleus, Cam-CRHR1 animals displayed enhanced CRH and decreased vasopressin expression levels. In contrast, gene expression in Nes-CRHR1 pups was strikingly similar to that in maternally deprived control pups. Furthermore, maternal deprivation resulted in an enhanced response of Cam-CRHR1 pups in the brain, while expression levels in Nes-CRHR1 mouse pups were mostly unchanged. Our results demonstrate that brainstem and/or hypothalamic CRHR1 contribute to the suppression of basal corticosterone secretion in the neonate, while limbic and/or forebrain CRHR1 dampen the activation of the neonatal HPA axis induced by maternal deprivation.

Mice with mutations in the HPA-system as models for symptoms of depression

Genetically engineered mice hold promise to help us understand the effects of enhanced or reduced gene activity upon behavior and metabolism. Because many basic and clinical studies suggest that alterations of the hypothalamic pituitary adrenocortical (HPA) system are involved in the development and course of depression, mouse mutants with genetic modifications of genes regulating the HPA system were generated. This review summarizes these effects and concludes that advanced technologies allowing for regional overexpression or inactivation of genes or introduction of polymorphisms into the mouse genome are well suited to explain individual symptoms or symptom patterns prevalent among depressives. However, as depression is a complex disorder in which minor changes of many genes as well as environmental factors (including epigenetic programming) play a causal role and determine the phenotype, the use of mice with single gene mutations needs to be critically discussed when attempting to create a genetic animal model of depression.

Norepinephrine transporter-deficient mice respond to anxiety producing and fearful environments with bradycardia and hypotension

The study of anxiety and fear involves complex interrelationships between psychiatry and the autonomic nervous system. Altered noradrenergic signaling is linked to certain types of depression and anxiety disorders, and treatment often includes specific transporter blockade. The norepinephrine transporter is crucial in limiting catecholaminergic signaling. Norepinephrine transporter-deficient mice have increased circulating catecholamines and elevated heart rate and blood pressure. We hypothesized, therefore, that reduced norepinephrine clearance would heighten the autonomic cardiovascular response to anxiety and fear. In separate experiments, norepinephrine transporter-deficient (norepinephrine transporter-/-) mice underwent tactile startle and trace fear conditioning to measure hemodynamic responses. A dramatic tachycardia was observed in norepinephrine transporter-/- mice compared with controls following both airpuff or footshock stimuli, and pressure changes were also greater. Interestingly, in contrast to normally elevated home cage levels in norepinephrine transporter-deficient mice, prestimulus heart rate and blood pressure were actually higher in norepinephrine transporter+/+ animals throughout behavioral testing. Upon placement in the behavioral chamber, norepinephrine transporter-deficient mice demonstrated a notable bradycardia and depressor effect that was more pronounced in females. Power spectral analysis indicated an increase in low frequency oscillations of heart rate variability; in mice, suggesting increased parasympathetic tone. Finally, norepinephrine transporter-/- mice exhibited sexual dimorphism in freeze behavior, which was greatest in females. Therefore, while reduced catecholamine clearance amplifies immediate cardiovascular responses to anxiety- or fear-inducing stimuli in norepinephrine transporter-/- mice, norepinephrine transporter deficiency apparently prevents protracted hemodynamic escalation in a fearful environment. Conceivably, chronic norepinephrine transporter blockade with transporter-specific drugs might attenuate recognition of autonomic and somatic distress signals in individuals with anxiety disorders, possibly lessening their behavioral reactivity, and reducing the cardiovascular risk factors associated with persistent emotional arousal.

The corticotropin-releasing factor (CRF)-system and monoaminergic afferents in the central amygdala: investigations in different mouse strains and comparison with the rat

Corticotropin-releasing-factor (CRF) containing systems and monoaminergic afferents of the central amygdaloid nucleus (Ce) are crucial players in central nervous stress responses. For functional analyses of specific roles of these systems, numerous mouse models have been generated which lack or overexpress individual signal transduction components. Since data concerning system morphologies in murine brain are rarely available, mouse studies are usually designed and interpreted based on previous findings in rats, although interspecies differences are frequent. In the present study, in situ hybridization for CRF mRNA and correlative immunocytochemistry for CRF and monoaminergic afferents revealed numerous CRF mRNA-reactive neurons in the lateral Ce subnucleus (CeL) codistributed with dense dopaminergic fiber plexus in mice as has been demonstrated in rats. However, while in rats the lateral capsular Ce (CeLc) displays only scarce CRF immunoreactive (CRF-ir) innervation, particularly dense CRF-ir fiber plexus were observed in the CeLc in mice, with differences in labeling densities between different strains. CRF-ir terminal fibers overlap with the moderate serotonergic innervation of this subnucleus in mice. Additionally, CRF mRNA-reactive neurons were found immediately dorsal to the amygdala in the region of the interstitial nucleus of the posterior limb of the anterior commissure/amygdalostriatal transition area in both species. In mice, this region displayed dense CRF-ir fiber plexus, with variations between the strains. The results indicate that in mice and rats dopaminergic afferents represent the primary monoaminergic input to the CRF neurons in the CeL. In mice only, CRF-ir afferents provide dense innervation of CeLc neurons. Since the CeLc lacks dopaminergic input in both species but possesses moderate serotonergic afferents, CRF/serotonin interactions may occur selectively in mouse CeLc. The observed interspecies and interstrain differences in CRF input and CRF/monoaminergic interactions may influence the interpretation of findings concerning Ce functions in stress and fear in mouse models.

Getting closer to affective disorders: the role of CRH receptor systems

Depressive disorders are a leading cause of morbidity and mortality worldwide. Current antidepressant drugs targeting monoamine neurotransmitter systems have a delayed onset of action, and fewer than 50% of the patients attain complete remission after therapy with a single antidepressant. A large body of preclinical and clinical evidence points to a key role of the corticotropin-releasing hormone (CRH) receptor 1 subtype (CRHR1) in mediating CRH-elicited effects in anxiety, depressive disorders and stress-associated pathologies. Genetic modification of CRHR1 function in mice by the use of conventional and conditional knockout strategies enables further analysis of specific elements in the CRH circuitry. The recent characterisation of several selective small-molecule CRHR1 antagonists offers new possibilities for the treatment of anxiety and depression.

Animal models of anxiety

Animal models for anxiety-related behavior are based on the assumption that anxiety in animals is comparable to anxiety in humans. Being anxious is an adaptive response to an unfamiliar environment, especially when confronted with danger or threat. However, pathological variants of anxiety can strongly impede the daily life of those affected. To unravel neurobiological mechanisms underlying normal anxiety as well as its pathologi- cal variations, animal models are indispensable tools. What are the characteristics of an ideal animal model? First, it should display reduced anxiety when treated with anxiolytics (predictive validity). Second, the behavioral response of an animal model to a threatening stimulus should be comparable to the response known for humans (face validity). And third, the mechanisms underlying anxiety as well as the psychological causes should be identical (construct validity). Meeting these three requirements is difficult for any animal model. Since both the physiological and the behavioral response to aversive (threatening) stimuli are similar in humans and animals, it can be assumed that animal models can serve at least two distinct purposes: as (1) behavioral tests to screen for potential anxiolytic and antidepressant effects of new drugs and (2) tools to investigate specific pathogenetic aspects of cardinal symptoms of anxiety disorders. The examples presented in this chapter have been selected to illustrate the potential as well as the caveats of current models and the emerging possibilities offered by gene technology. The main concepts in generating animal models for anxiety-that is, selective breeding of rat lines, experience-related models, genetically engineered mice, and phenotype-driven approaches-are concisely introduced and discussed. Independent of the animal model used, one major challenge remains, which is to reliably identify animal behavioral characteristics. Therefore, a description of behavioral expressions of anxiety in rodents as well as tests assays to measure anxiety-related behavior in these animals is also included in this chapter.

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.

Technical Assessment of the First 20 Years of Research Using Mouse Embryonic Stem Cell Lines

This review assesses the effect that mouse embryonic stem (ES) cells have had on biomedical research during the 20 years that followed their isolation in 1981. Notable scientific discoveries enabled by these cell lines--including insights into cell cycle regulation, spatial and temporal relationships during development, and the roles of transcription factors and homeobox genes in developmental pathways--are discussed. The acceleration of basic discovery of gene function and the genetic basis of disease using a breakthrough technology (homologous recombination between modified gene constructs and the ES cell genome) became the principal enabling method to establish transgenic laboratory animals with single targeted genetic change. This review also examines the widespread influence of mouse ES cells as an enabling technology by highlighting their effect on drug development paradigms, directed differentiation to treat specific diseases, nuclear transfer protocols used in cloning, and establishment of methodologies for isolating non-rodent ES cells. This review concludes with a brief analysis of the most influential mouse ES cell lines of the first 20 years as viewed within the twin contexts of human disease application and contributions to the primary literature.

Mouse mutants for the study of corticotropin-releasing hormone receptor function: development of novel treatment strategies for mood disorders

In today's psychiatry there is a great deal of interest in the development of compounds with a novel mechanism of action that diverge from the classical catecholaminergic neurotransmitter system targets. Within the last few years, it has become increasingly evident that the neuroendocrine and behavioral phenotypes of mood and anxiety disorders are at least in part mediated by modulation of corticotropin-releasing hormone (CRH) neurocircuitries and that normalization of an altered neurotransmission after treatment may lead to restoration of disease-related changes. Although this concept was originally derived from peripheral hypothalamic-pituitary-adrenocortical (HPA) assessments in depressed patients, central CRH neuropeptidergic circuits other than those driving the peripherally accessible HPA system may be overactive and could be therapeutic targets of antagonist actions. Genetically engineered mice provide a novel and useful tool to study the endogenous mechanisms underlying aberrant behavior and CRH neurocircuitry regulation. The results obtained from conventional and conditional mutant mice indicate that CRH type 1 receptors may be the primary target to which to direct selective nonpeptide compounds. Moreover, beyond the encouraging preclinical studies, the first clinical open trial supports the notion that CRH type 1 receptors can be safely and effectively antagonized.

Maternal inheritance of Cre activity in a Sox2Cre deleter strain

The Sox2 gene is expressed in several undifferentiated cell types. In an earlier study we described a Sox2Cre transgene that mediates epiblast-specific Cre-mediated modification of gene activity in the embryo. Here we report that this transgene is active in the female germline. Consequently, all offspring that arise from female mice heterozygous for the Sox2Cre transgene have demonstrable Cre activity irrespective of whether they inherit the transgene itself. Maternal inheritance of Cre activity allows the efficient modification of gene activity for functional analysis.

Spermidine/spermine N1-acetyltransferase overexpression in mice induces hypoactivity and spatial learning impairment

The present work addresses the role of polyamines in learning and general behavior by subjecting transgenic mice overexpressing polyamine catabolic enzyme, spermidine/spermine N(1)-acetyltransferase (SSAT) and their syngenic littermates to neurobehavioral profiling assessment (SHIRPA) and to radial eight-arm maze. The general health and physiological conditions as well as the entire behavioral battery comprising of 34 parameters were recorded. The eight-arm radial maze (8-RAM) task included an initial acquisition task for 9 days followed by a 2-day retention test after a 2-week break. In addition, blood samples were taken for hormone analysis. Transgenic mice, which showed reduced motor activity, aggression and muscle tone, spent more time in the radial maze during initial acquisition and retention tasks as compared with syngenic mice. Moreover, the learning performance of transgenic females was significantly inferior to syngenic females. Interestingly, the levels of several hormones were significantly altered in SSAT transgenic mice; circulating adrenocorticotropic hormone (ACTH) and corticosterone levels were markedly increased while testosterone and thyroidal hormone levels were decreased. These changes may be related to the dramatic increase in brain putrescine levels in SSAT-overexpressing (SSAT-OE) mice, but it is likewise possible that the behavioral changes and learning impairment are attributable to more peripheral mechanisms (such as alterations in steroid hormone metabolism), which in turn, could be a consequence of the disturbed polyamine homeostasis.

Gonadal hormones humour the brain

The relationship between the brain and the endocrine system is now seen to extend far beyond the regulation of somatic hormone production by the hypothalamus and pituitary: the brain itself can be considered both as an endocrine organ, producing hormones that act both within and outside the central nervous system, and as a target for hormones. The current extent of this concept with respect to the gonadal hormones was explored at a recent meeting ('Hormones and the Brain', Third Endocrinology Colloquium of the Fondation Ipsen, Paris, December 8, 2003). The discussion, reviewed in this article, ranged from intracellular signalling pathways and intercellular networks regulating hormone production and action in the central nervous system to hormone involvement in the generation of sexual behaviour and in development, plasticity, neuroprotection and repair. The hormonal contribution to psychiatric and neurodegenerative illnesses was also examined. The picture presented is complex, with layers of controls and with hormones that have diverse actions at different sites in the central nervous system. This richness of actions and functions is providing some interesting leads for developing new therapeutics.

Hypothalamic-pituitary-adrenocortical system and mood disorders: highlights from mutant mice

In recent years, refined molecular technologies and the generation of genetically engineered mice have allowed to specifically target individual genes involved in the regulation of the hypothalamic-pituitary-adrenocortical (HPA) system. Given the fundamental role of the corticotropin-releasing hormone (CRH) system in anxiety, stress-associated pathologies, and mood disorders, we describe genetic modifications of the genes that encode proteins integral to the CRH/CRH receptor system with particular emphasis on conditional gene-targeting strategies. The profile of results, consistent with current knowledge of CRH function from more traditional assays, indicates that enhancement of the CRH function is associated with an activation of the HPA system, an anxious phenotype, alterations in cognitive performance, reductions in food intake, and disturbances of autonomic functions. In general, blockade of CRH activity produces the opposite effects, namely an anxiety-reduced phenotype. Molecular genetic strategies for conditional inactivation or overexpression of the glucocorticoid receptor contribute to our understanding of the genetics of endocrine activity and behavior, the most complex form of biological organization. In addition, we introduce mice with a genetic manipulation in the function of the blood-brain barrier as an animal model for the study of neuroendocrine regulation and, in particular, of HPA system activity. By use of mice deficient for abcb1- (also called multidrug resistance gene 1, mdr1-) type P glycoproteins, it was shown most recently that abcb1-type P glycoproteins control the access of endogenous glucocorticoids into the central nervous system. Thus, the ABCB1-type P glycoprotein function exerts a profound influence on activity and regulation of the HPA system under both basal conditions and during stress. Taken together, these genetically engineered mice are valuable tools for increasing our understanding of HPA system dysregulation in anxiety and stress-related pathologies, including human affective disorders. The identification and detailed characterization of these molecular pathways will ultimately lead to the development of novel neuropharmacological intervention strategies.

Behavioural screening in mutagenised mice—in search for novel animal models of psychiatric disorders

Complementary to the 'gene-driven' analysis of gene function, 'phenotype-driven' approaches can be performed and may be equally important. Despite the current availability of a long list of mouse mutants, there remains an appreciable need for behavioural phenotypes in mouse models permitting to learn more about the aetiology of psychiatric disorders. This lack can be compensated by phenotype-driven ethyl-nitrosourea (ENU)-mutagenesis programs which aim at identifying novel phenotypes without any a priori assumptions, thus, representing a unique possibility to create novel animal models which approximate the underlying genetic aetiology. The power of mouse mutagenesis critically depends on the phenotyping procedures performed. In the case of ENU-mutants, behavioural phenotyping is especially challenging, as behavioural profiles have to be identified in single individuals. For high-throughput screening, approaches have been made to establish standardised screening protocols including a combination of well-validated, easy to perform behavioural tests. Different strategies are being introduced, which are used in ENU-mutagenesis screens to identify behavioural mutants representing possible endophenotypes of psychiatric diseases.

The postnatal development of the hypothalamic-pituitary-adrenal axis in the mouse

The main characteristic of the postnatal development of the stress system in the rat is the so-called stress hypo-responsive period (SHRP). Lasting from postnatal day (pnd) 4 to pnd 14, this period is characterized by very low basal corticosterone levels and an inability of mild stressors to induce an enhanced ACTH and corticosterone release. During the last years, the mouse has become a generally used animal in stress research, also due to the wide availability of genetically modified mouse strains. However, very few data are available on the ontogeny of the stress system in the mouse. This study therefore describes the postnatal ontogeny of peripheral and central aspects of the hypothalamic-pituitary-adrenal (HPA) axis in the mouse. We measured ACTH and corticosterone in blood and CRH, urocortin 3 (UCN3), mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) transcripts in the brain at postnatal days 1, 2, 4, 6, 9, 12, 14 and 16. Our results show that we can subdivide the postnatal development of the HPA axis in the mouse in two phases. The first phase corresponds to the SHRP in the rat and lasts from right after birth (pnd 1) until pnd 12. Basal corticosterone levels were low and novelty exposure did not enhance corticosterone or ACTH levels. This period is further characterized by a high expression of CRH in the paraventricular nucleus (PVN) of the hypothalamus. Expression levels of GR in the hippocampus and UCN3 in the perifornical area are low at birth but increase significantly during the SHRP, both reaching the highest expression level at pnd 12. In the second phase, the mice have developed past the SHRP and were now exhibiting enhanced corticosterone basal levels and a response of ACTH and corticosterone to mild novelty stress. CRH expression was decreased significantly, while expression of UCN3 and GR remained high, with a small decrease at pnd 16. The expression of MR in the hippocampus was very dynamic throughout the postnatal development of the HPA axis and changed in a time and subregion specific manner. These results demonstrate for the first time the correlation between the postnatal endocrine development of the mouse and gene expression changes of central regulators of HPA axis function.

Mouse models for psychiatric disorders

Genes involved in psychiatric disorders are difficult to identify, and those that have been proposed so far remain ambiguous. As it is unrealistic to expect the development of, say, a 'schizophrenic' or 'autistic' mouse, mice are unlikely to have the same role in gene identification in psychiatry as circling mice did in the discovery of human deafness genes. However, many psychiatric disorders are associated with intermediate phenotypes that can be modeled and studied in mice, including physiological or anatomical brain changes and behavioral traits. Mouse models help to evaluate the effect of a human candidate gene mutation on an intermediate trait, and to identify new candidate genes. Once a gene or pathway has been identified, mice are also used to study the interplay of different genes in that system.