Impaired type II glucocorticoid-receptor function in mice bearing antisense RNA transgene

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.

Cloning and in vitro characterization of alpha 1(I)-collagen 11 beta-hydroxysteroid dehydrogenase type 2 transgenes as models for osteoblast-selective inactivation of natural glucocorticoids

The NAD-dependent enzyme, 11beta-hydroxysteroid dehydrogenase type II (11 beta HSD2), catalyzes the unidirectional conversion of biologically active glucocorticoids to inactive metabolites. In vivo, 11 beta HSD2 protects the mineralocorticoid receptor from activation by glucocorticoids in mineralocorticoid target tissues such as kidney. The goal of the present study was to use targeted overexpression of 11 beta HSD2 as a novel means of disrupting glucocorticoid signaling in osteoblastic cells. Rat 11 beta HSD2 complementary DNA was cloned downstream of a 2.3- and 3.6-kb alpha 1(I)-collagen (Col1a1) promoter fragment to produce the expression plasmids Col2.3-HSD2 and Col3.6-HSD2, respectively, which were transiently and/or stably transfected in osteoblastic ROS 17/2.8 and MC3T3-E1 cells. Transgene messenger RNA and protein were detected in transfected cells by Northern blot analysis and immunostaining, respectively. Transfection of 11 beta HSD2 led to higher rates of conversion of [(3)H]corticosterone to [(3)H]dehydrocorticosterone and reduced glucocorticoid-dependent regulation of a mouse mammary tumor virus promoter-reporter construct, cell growth, and messenger RNA markers compared with transfection of a control vector. Expression of 11 beta HSD2 under the control of Col1a1 promoter fragments may provide a novel model to study the role of glucocorticoid signaling in osteoblastic cells.

Conditioned activity to amphetamine in transgenic mice expressing an antisense RNA against the glucocorticoid receptor

Glucocorticoids enhance the locomotion-stimulating and the rewarding properties of stimulant drugs. Amphetamine-induced conditioned activity was investigated in B6C3F1 (controls) and antisense transgenic mice. The latter expresses a neurofilament-promotor-driven antisense RNA complementary to a fragment of cDNA that codes for the mouse glucocorticoid receptor. This gene expression leads to approximately a 50% reduction in glucocorticoid receptor mRNA in the brain. Transgenic mice showed an increased novelty response when tested in an open field, in terms of both distance traveled and number of rearings. Moreover, they displayed enhanced amphetamine-induced conditioned activity. Behavioral sensitization was observed in controls, whereas behavioral tolerance developed in transgenic mice. These data support the concept of an enhanced stress response in these transgenic mice, rather than a general downregulation of the stress response because of impaired glucocorticoid receptor function.

Intracellular regeneration of glucocorticoids by 11beta-hydroxysteroid dehydrogenase (11beta-HSD)-1 plays a key role in regulation of the hypothalamic-pituitary-adrenal axis: analysis of 11beta-HSD-1-deficient mice

11beta-Hydroxysteroid dehydrogenases (11beta-HSDs) catalyze interconversion of active corticosterone and inert 11-dehydrocorticosterone, thus regulating glucocorticoid access to intracellular receptors in vivo. 11beta-HSD type 1 is a reductase, locally regenerating active glucocorticoids. To explore the role of this isozyme in the brain, we examined hypothalamic-pituitary-adrenal axis (HPA) regulation in mice homozygous for a targeted disruption of the 11beta-HSD-1 gene. 11beta-HSD-1-deficient mice showed elevated plasma corticosterone and ACTH levels at the diurnal nadir, with a prolonged corticosterone peak, suggesting abnormal HPA control and enhanced circadian HPA drive. Despite elevated corticosterone levels, several hippocampal and hypothalamic glucocorticoid-sensitive messenger RNAs were normally expressed in 11beta-HSD-1-deficient mice, implying reduced effective glucocorticoid activity within neurons. 11beta-HSD-1-deficient mice showed exaggerated ACTH and corticosterone responses to restraint stress, with a delayed fall after stress, suggesting diminished glucocorticoid feedback. Indeed, 11beta-HSD-1-deficient mice were less sensitive to exogenous cortisol suppression of HPA activation. Thus 11beta-HSD-1 amplifies glucocorticoid feedback on the HPA axis and is an important regulator of neuronal glucocorticoid exposure under both basal and stress conditions in vivo.

Central 5-hydroxytryptamine-2A receptor expression in transgenic mice bearing a glucocorticoid receptor antisense

Transgenic mice bearing a transgene coding for a glucocorticoid receptor antisense mRNA that partially blocks glucocorticoid receptor expression were used to investigate the long-term effect of hypothalamic-pituitary-adrenal dysfunction on brain 5-hydroxytryptamine-2A (5-HT2A) receptor expression. The brain 5-HT2A receptor mRNA levels in transgenic mice were measured by in situ hybridization and compared to those in control mice. We also studied the effect of a 3-week treatment with fluoxetine on brain 5-HT2A receptor expression in the transgenic mice. No difference in 5-HT2A mRNA levels was observed between transgenic and control mice in cortical or striatal regions, and fluoxetine treatment was without effect. No difference in hypothalamic 5-HT2A mRNA levels was observed between transgenic and control mice, while fluoxetine treatment increased these levels in both transgenic as well as in the hypothalamic ventromedial and paraventricular nuclei of control mice. 5-HT2A receptor mRNA levels were similar in hippocampal CA1 and CA2 subregions of control and transgenic, but were lower in the CA3 and CA4 subregions of transgenic mice. Fluoxetine had no effect on 5-HT2A mRNA levels of transgenic mice but reduced control mouse 5-HT2A receptor mRNA levels in the CA3 subregion. These results suggest that impaired glucocorticoid receptor function can affect hippocampal 5-HT2A receptor expression in transgenic mice and that this is not corrected by fluoxetine treatment.

Effects of the monoamine oxidase A inhibitor moclobemide on hippocampal plasticity in GR-impaired transgenic mice

A reduction in glucocorticoid receptor (GR) function leads to hippocampus-dependent allocentric spatial learning deficits, altered novelty exploration and disrupted hippocampal long-term potentiation (LTP) in transgenic mice expressing a GR antisense construct. After continuous long-term treatment of these mice with moclobemide (a reversible inhibitor of monoamine oxidase A), spatial navigation performance but not accuracy improved during initial acquisition. These changes were associated with a shift of the threshold for the induction of hippocampal LTP at low stimulation frequencies. Moreover, novel object exploration increased in both control and transgenic animals following long-term treatment with moclobemide. These findings open the possibility that antidepressants might improve hippocampal function under conditions of impaired stress hormone regulation, and that these drugs might in part act through this mechanism to attenuate cognitive deficiency in disorders such as depression.

Stress-free T-cell development: glucocorticoids are not obligatory

A role for glucocorticoids in thymopoiesis has been suggested by studies using glucocorticoid receptor (GR) anti-sense transgenic mice, glucocorticoid synthesis inhibitors and GR antagonists. Unfortunately, no consensus has been reached on exactly how glucocorticoids influence T-cell development. The most recent approach, using GR knockout (GR(-/-)) mice, indicates that GR signaling is, in fact, dispensable in this entire process.

Cellular mechanisms in the vulnerability to depression and response to antidepressants

As a testable heuristic, the concept of stress response and adaptation is highly appealing, and the support for the concept is strong. This explanatory model of depression may account for hitherto apparently discordant facts--contradictory symptoms, antidepressant drugs that act on differing systems, facilitation of antidepressant response by augmentation, and response to psychotherapy and pharmacotherapy. This article has focused narrowly on specific cellular elements of the stress-adaptational mechanisms, including the AC-PKA and PLC-PKC transductional cascades, together with specific response elements, such as the HPA axis, BDNF, and NMDA receptors; however, other important mechanisms, including specific receptor subtypes (e.g., 5-HT1A and NE alpha 2), transmitter systems (e.g., acetylcholine and depamine), and hormones (e.g., thyroid and growth hormones and prolactin), which may be important, have not been discussed. As the complex interactions of these systems gradually yield to investigation, not only will new treatments be developed, but better matching of treatment to patient may become an achievable goal.

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.

5-HT-HPA interactions in two models of transgenic mice relevant to major depression

Reciprocal interactions between central 5-HT system and hypothalamo-pituitary-adrenal (HPA) axis are of particular relevance with regard to depression, in which alterations of both systems have been evidenced. In order to further explore these interactions, two models of mutant mice have been used. They consisted of knock-out mice lacking the 5-HT transporter (5-HTT-/-) and of transgenic mice with impaired glucocorticoid receptor (GR-i) expression. Under control conditions. the functional properties of 5-HT(1A) autoreceptors in GR-i mice were as in their paired wild-type. However, both chronic stress and long term treatment with fluoxetine induced abnormal adaptive changes in 5-HT(1A) autoreceptor functioning in GR-i mice. On the other hand, a marked desensitization of 5-HT(1A) autoreceptors was found in 5-HTT-/- mice as compared with paired wild-type animals, and this phenomenon was further enhanced by exposure to stressful conditions. These data show that alterations of HPA axis at the gene level has consequences on 5-HT neurotransmission, and reciprocally, that 5-HTT knock-out affects HPA-dependent responses to stress.

Neuroendocrine control of thymus physiology

The thymus gland is a central lymphoid organ in which bone marrow-derived T cell precursors undergo differentiation, eventually leading to migration of positively selected thymocytes to the peripheral lymphoid organs. This differentiation occurs along with cell migration in the context of the thymic microenvironment, formed of epithelial cells, macrophages, dendritic cells, fibroblasts, and extracellular matrix components. Various interactions occurring between microenvironmental cells and differentiating thymocytes are under neuroendocrine control. In this review, we summarize data showing that thymus physiology is pleiotropically influenced by hormones and neuropeptides. These molecules modulate the expression of major histocompatibility complex gene products by microenvironmental cells and the extracellular matrix-mediated interactions, leading to enhanced thymocyte adhesion to thymic epithelial cells. Cytokine production and thymic endocrine function (herein exemplified by thymulin production) are also hormonally controlled, and, interestingly in this latter case, a bidirectional circuitry seems to exist since thymic-derived peptides also modulate hormonal production. In addition to their role in thymic cell proliferation and apoptosis, hormones and neuropeptides also modulate intrathymic T cell differentiation, influencing the generation of the T cell repertoire. Finally, neuroendocrine control of the thymus appears extremely complex, with possible influence of biological circuitry involving the intrathymic production of a variety of hormones and neuropeptides and the expression of their respective receptors by thymic cells.

Glucocorticoids in T cell development and function*

Glucocorticoids are small lipophilic compounds that mediate their many biological effects by binding an intracellular receptor (GR) that, in turn, translocates to the nucleus and directly or indirectly regulates gene transcription. Perhaps the most recognized biologic effect of glucocorticoids on peripheral T cells is immunosuppression, which is due to inhibition of expression of a wide variety of activationinduced gene products. Glucocorticoids have also been implicated in Th lineage development (favoring the generation of Th2 cells) and, by virtue of their downregulation of fasL expression, the inhibition of activation-induced T cell apoptosis. Glucocorticoids are also potent inducers of apoptosis, and even glucocorticoid concentrations achieved during a stress response can cause the death of CD4(+)CD8(+ )thymocytes. Perhaps surprisingly, thymic epithelial cells produce glucocorticoids, and based upon in vitro and in vivo studies of T cell development it has been proposed that these locally produced glucocorticoids participate in antigen-specific thymocyte development by inhibiting activation-induced gene transcription and thus increasing the TCR signaling thresholds required to promote positive and negative selection. It is anticipated that studies in animals with tissue-specific GR-deficiency will further elucide how glucocorticoids affect T cell development and function.

Glucocorticoid receptor impairment enhances impulsive responding in transgenic mice performing on a simultaneous visual discrimination task

Transgenic mice with impaired glucocorticoid receptor (GR) function were tested for their ability to learn and perform a series of simultaneous visual discriminations which allowed a dissociation between accuracy of discrimination from those of motivation and behavioural disinhibition. Animals were first trained on an operant five-choice simultaneous discrimination autoshaping procedure, followed by a continuous reinforcement schedule on that task. Subsequently, the number of choices was limited to two and data were analysed according to the mathematical methods of signal detection theory (SDT). The effects of GR-antisense expression on accuracy when different rates of responding were required were studied under different fixed ratio response requirements (FR1-FR10). Autoshaping was retarded in transgenic animals and accuracy was impaired in both the five-choice and the two-choice discrimination tasks, although transgenic mice showed clear evidence for learning. Under conditions of low response requirements, transgenic mice showed increased response and cognitive biases, but reduced perceptual bias, and a behavioural disinhibition, characterized by a reduction in errors of omission, decreased response latencies and increased number of responses during the inter-trial interval. Increasing the response requirement improved performance in transgenic animals as reflected by enhanced accuracy. Moreover, transgenics were less susceptible to the deleterious effects of higher response requirements, as indicated by relatively unaffected bias measures in this group, while bias increased in controls. These results indicate that altered performance in GR-antisense transgenic animals cannot simply be interpreted as a mnemonic deficit, but that altered motivation and enhanced impulsive responding may account for some of these impairments.

Central 5-HT(1) and 5-HT(2) binding sites in transgenic mice with reduced glucocorticoid receptor number

Transgenic mice bearing a transgene coding for a glucocorticoid receptor antisense mRNA, which partially blocks glucocorticoid receptor expression, were used in order to clarify the role of glucocorticoid receptors in the regulation of 5-HT(1A), 5-HT(1nonA) and 5-HT(2) binding sites labelled by quantitative autoradiography in the frontal and prefrontal cortex, striatum, hypothalamus, amygdala and raphe nuclei. We found that 1 nM [3H]8-hydroxy-2-[di-N-propylamino]tetralin ([3H]8-OH-DPAT) binding to 5-HT(1A) sites was decreased in strata oriens (-15.1+/-3.5%) and radiatum-lacunosum-moleculare (-13.3+/-4.3%) of the hippocampal CA(3) area, and 2 nM [3H]5-hydroxytryptamine binding to 5-HT(1nonA) sites in the presence of 100 nM 8-OH-DPAT and mesulergine was decreased in the dorsal subiculum (-17.8+/-6.9%). By contrast, 5-HT(2) sites labelled by 0.5 nM of (+/-)-1-(2, 5-dimethoxy-4-[125I]iodophenyl)-2-aminopropane was increased in the dorsal subiculum (+35.2+/-11.5%) and CA(2) area (+29.2+/-11.3%). The observed differences in binding to 5-HT(1) and 5-HT(2) sites were all located in areas of the hippocampus that contain both gluco- and mineralo-corticoid receptors, and no difference was observed in anatomical structures which contain only glucocorticoid receptors. Therefore, it seems that the important factor for the regulation of these 5-HT receptors is the interaction between gluco- and mineralo-corticoid receptors rather than the absolute density of glucocorticoid receptors. These results suggest that some of the alterations of the serotonergic neurotransmission observed in depressed patients might be secondary to an altered glucocorticoid receptor function.

Transgenic animal models for neuropharmacology

The establishment of novel animal models using gene targeting and transgenic technology has opened a new area of neuropharmacological research. For the first time, it became possible to alter the expression of a gene in a specific cell type of an intact animal by either overexpression, inhibition or ablation. This review describes the technology and lists the relevant tools, such as reporter genes, suicide genes, immortalizing genes, and promoters, necessary for the targeted expression of these and other genes in specific cells of the central nervous system. In addition, the problem is discussed that the mouse is the species in which this technology is by far the most developed, while the rat has been used as the model species for neuropharmacology during the last century.

5'-heterogeneity of glucocorticoid receptor messenger RNA is tissue specific: differential regulation of variant transcripts by early-life events

Glucocorticoid receptor (GR) gene expression is regulated in a complex tissue-specific manner, notably by early-life environmental events that program tissue GR levels. We have identified and characterized several new rat GR mRNAs. All encode a common protein, but differ in their 5'-leader sequences as a consequence of alternate splicing of, potentially, 11 different exon 1 sequences. Most are located in a 3-kb CpG island, upstream of exon 2, that exhibits substantial promoter activity in transfected cells. Ribonuclease (RNase) protection analysis demonstrated significant levels of six alternate exons 1 in vivo in rat, with differences between liver, hippocampus, and thymus reflecting tissue-specific differences in promoter activity. Two of the alternate exons 1 (exons 1(6) and 1(10)) were expressed in all tissues examined, together present in 77-87% of total GR mRNA. The remaining GR transcripts contained tissue-specific alternate first exons. Importantly, tissue-specific first exon usage was altered by perinatal environmental manipulations. Postnatal handling, which permanently increases GR in the hippocampus, causing attenuation of stress responses, selectively elevated GR mRNA containing the hippocampus-specific exon 1(7). Prenatal glucocorticoid exposure, which increases hepatic GR expression and produces adult hyperglycemia, decreased the proportion of hepatic GR mRNA containing the predominant exon 1(10), suggesting an increase in a minor exon 1 variant. Such tissue specificity of promoter usage allows differential GR regulation and programming.

Dexamethasone suppression of corticosteroid secretion: evaluation of the site of action by receptor measures and functional studies

A dose of dexamethasone was determined in rats (50 micrograms/kg s.c.) that suppressed the corticosterone response to restraint stress by 80%. Corticosteroid receptor occupancy estimates found that the 50 micrograms/kg s.c. dose of dexamethasone had no significant effect on available glucocorticoid receptor (GR) or mineralocorticoid receptor (MR) binding in brain regions (hypothalamus, hippocampus and cortex); on the other hand dexamethasone produced a selective and significant decrease in available GR in peripheral tissues (pituitary and spleen). Functional studies showed that the 50 micrograms/kg s.c. dose of dexamethasone completely blocked the effects of corticotropin-releasing hormone (CRH; 0.3-3.0 micrograms/kg i.p.) on corticosterone secretion, but did not inhibit the corticosterone response to an adrenocorticotropin hormone (ACTH; 2.5 I.U./kg i.p.) challenge. These studies indicate that this dose of dexamethasone exerts its inhibitory effects on the HPA axis primarily by acting at GR in the pituitary. The plasma dexamethasone levels produced by this dose of dexamethasone are similar to those present in humans the afternoon after an oral dexamethasone suppression test (DST), a time at which many depressed patients escape from dexamethasone suppression. These results support and extend other studies which suggest that the DST provides a direct test of the effects of increased GR activation in the pituitary on ACTH and cortisol secretion.

How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions

The secretion of glucocorticoids (GCs) is a classic endocrine response to stress. Despite that, it remains controversial as to what purpose GCs serve at such times. One view, stretching back to the time of Hans Selye, posits that GCs help mediate the ongoing or pending stress response, either via basal levels of GCs permitting other facets of the stress response to emerge efficaciously, and/or by stress levels of GCs actively stimulating the stress response. In contrast, a revisionist viewpoint posits that GCs suppress the stress response, preventing it from being pathologically overactivated. In this review, we consider recent findings regarding GC action and, based on them, generate criteria for determining whether a particular GC action permits, stimulates, or suppresses an ongoing stress-response or, as an additional category, is preparative for a subsequent stressor. We apply these GC actions to the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology. We find that GC actions fall into markedly different categories, depending on the physiological endpoint in question, with evidence for mediating effects in some cases, and suppressive or preparative in others. We then attempt to assimilate these heterogeneous GC actions into a physiological whole.

Effect of lithium carbonate on the enhancement of serotonin 2A receptor elicited by dexamethasone

The purpose of this study was to investigate whether chronic dexamethasone (Dex) administration induces serotonin (5-HT) 2A receptor supersensitivity and if chronic lithium carbonate (Li) administration contributes to the normalization of 5-HT2A receptor supersensitivity induced by Dex in rat brain. We investigated the effects of a 14-day administration of Dex and/or Li on changes in body weight (BW), on plasma corticosterone levels, on plasma lithium levels, on 5-HT2A receptor binding sites, and on (+/-)-1-(2, 5-dimethoxy-4-iodophenyl)-2 aminopropane (DOI)-induced wet dog shake (WDS), which is mediated by 5-HT2A receptor in rats. Dex significantly reduced the BW of rats. Li did not have any effect on BW gain and did not prevent the BW loss induced by Dex. The plasma corticosterone levels of rats treated with Dex were too low to be detected. Li did not have any effect on corticosterone levels and did not prevent the decrease in the corticosterone levels induced by Dex. Six hours after the last treatment, the plasma lithium levels of rats treated with Li were significantly higher than those of rats treated with Dex/Li. Chronic Dex administration resulted in a significant increase in the density (B(max)) of the 5-HT2A receptor without a significant change in the affinity (K(d)). The increase in the B(max) induced by Dex was not prevented by chronic combined treatment with Dex and Li. Chronic Dex administration potentiated the WDS, and this increase was prevented by chronic combined treatment with Dex and Li. Chronic Li administration did not have any effect on WDS. These results indicate that chronic Li administration may improve the supersensitivity of the 5-HT2A receptor elicited by chronic Dex administration without decreasing the density of the 5-HT2A receptor, and the effect of Li was also independent of hypothalamo-pituitary-adrenal axis function.

Glucocorticoid receptor impairment alters CNS responses to a psychological stressor: an in vivo microdialysis study in transgenic mice

To study the consequences of impaired functioning of the glucocorticoid receptor (GR) for behavioural, neuroendocrine and neurochemical responses to a psychological stressor, a transgenic mouse expressing antisense RNA against GR was used. Previous studies on these transgenic mice have shown that impairment of GR evolves in disturbed neuroendocrine regulation and certain behavioural responses to stress. Here we investigated putative disturbances on the level of brain neurotransmission in GR-impaired (GR-i) mice using an in vivo microdialysis method. Through a microdialysis probe in the hippocampus, serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and free corticosterone [as an index of hypothalamic-pituitary-adrenocortical (HPA) axis activity] were monitored. Moreover, specific behaviours (e.g. grooming, eating/drinking, sniffing, nest building and locomotion) displayed by the mice during collection of the dialysates were scored. Measurement of dialysate concentrations of corticosterone on days 1 and 3 after insertion of the microdialysis probe showed that the free levels of this glucocorticoid were significantly lower in GR-i mice toward the evening. On day 2 after insertion of the microdialysis probe, baseline values of dialysate corticosterone, 5-HT and 5-HIAA were assessed, after which mice were exposed to a rat placed into their home cage. The rat and mouse were separated by a Plexiglas wall. A positive correlation between baseline hippocampal extracellular levels of 5-HT and 5-HIAA and the time spent performing active behaviours was observed in both genotypes. The main active behaviour performed at the baseline was grooming behaviour. During the rat exposure period, control mice remained mostly sitting and/or lying with their eyes fixed on the rat. Moreover, they showed a profound rise in free corticosterone levels. In contrast, GR-i mice displayed significantly more activities along the separation wall and a trend toward more grooming behaviour, but no increase of free corticosterone. In both mouse lines, exposure to a rat increased hippocampal extracellular levels of 5-HT and 5-HIAA. The rise in 5-HT was, however, more pronounced in the GR-i mice. From these data it may be concluded that life-long GR impairment has profound consequences for behavioural and neuroendocrine responses to a psychological stressor. Moreover, long-term impaired functioning of GR evolves in hyper-responsiveness of the raphe-hippocampal serotonergic system.

Glucocorticoids and depression

Depression has been associated with impaired mineralocorticoid receptor function, restrained glucocorticoid receptor feedback at the level of the hypothalamic-pituitary-adrenal (HPA) axis, raised cortisol level and increased corticotropin-releasing factor activity, which may act in concert to induce the signs and symptoms of the disorder. Pre-clinical and clinical evidence suggests that both genetic and environmental factors contribute to the development of these HPA axis abnormalities in depressed patients. Support for this view derives from models using genetically modified animals and/or chronic stress exposure at different developmental stages, although all of the current approaches have to be viewed within their limitations to model the disease. However, both animal and human studies challenging the HPA system show at least some neuroendocrine and behavioural changes comparable to those seen in depression, suggesting that some of the depressive symptoms can be attributed to HPA axis hyperactivity. Moreover, normalization of the neuroendocrine function following chronic antidepressant drug treatment seems to be a prerequisite for stable remission of depressive psychopathology, i.e. that normalization of HPA function is critical for relief of the clinical symptomatology of this disorder.

Regional serotonin metabolism under basal and restraint stress conditions in the brain of transgenic mice with impaired glucocorticoid receptor function

Transgenic (TG) mice deficient in glucocorticoid receptors (GR) were used in order to study the effects of a reduced GR function on adrenocorticotropin hormone and corticosterone plasma levels and on serotonin metabolism in different brain areas under basal resting conditions, after a 30-min restraint stress and 60 min after the end of the restraint stress. There was no difference in basal or stress-induced levels of either adrenocorticotropin hormone or corticosterone in control and TG mice, but the return of adrenocorticotropin hormone to basal values after the end of the stress was delayed in TG mice. Under basal conditions, the ratio 5-hydroxyindoleacetic acid/5-hydroxytryptamine was decreased only in the hippocampus of TG mice compared to controls. In the brain stem, the ratio 5-hydroxyindoleacetic acid/5-hydroxytryptamine increased compared to basal values after a 30-min restraint stress and values were still high 60 min after the end of the restraint stress in both control and TG mice. In the hippocampus, the ratio 5-hydroxyindoleacetic acid/5-hydroxytryptamine increased at the end of the stress and returned to basal levels 60 min later in control mice, whereas there was no change at the end of the stress but an increase 60 min later in TG mice. Finally there was no change in serotonin metabolism in the cortex, striatum or hypothalamus in either group or situation. Our results support the hypothesis of a tonic activation of serotonin turnover by corticosterone through GR in the mouse hippocampus. Moreover, stress-induced stimulation of serotonin metabolism in the brain stem and hippocampus appears to be delayed in TG mice compared to control mice. These results are particularly relevant for mood disorders such as depression where alterations of serotoninergic transmission might be secondary to an impairment of GR functions.

Genome-wide search for linkage of bipolar affective disorders in a very large pedigree derived from a homogeneous population in quebec points to a locus of major effect on chromosome 12q23-q24

We completed a genome-wide scan for susceptibility loci for bipolar affective disorders in families derived from a rather homogeneous population in the Province of Québec. The genetic homogeneity of this population stems from the migration of founding families into this relatively isolated area of Québec in the 1830s. A possible founder effect, combined with a prevalence of very large families, makes this population ideal for linkage studies. Genealogies for probands can be readily constructed from a population database of acts of baptism and marriage from the early 1830s up to the present time (the BALSAC register). We chose probands with a DSM III diagnosis of bipolar affective disorder and who may be grouped within large families having genealogical origins with the founding population of the Saguenay-Lac-St-Jean area. Living members (n approximately 120) of a very large pedigree were interviewed using the Structured Clinical Interview for DSM III (SCID I), SCID II, and with a family history questionnaire. A diagnostic panel evaluated multisource information (interview, medical records, family history) and pronounced best-estimate consensus diagnoses on all family members. Linkage, SimAPM, SimIBD, and sib-pair analyses have been performed with 332 microsatellite probes covering the entire genome at an average spacing of 11 cM. GENEHUNTER and haplotype analyses were performed on regions of interest. Analysis of a second large pedigree in the same regions of interest permitted confirmation of presumed linkages found in the region of chromosome 12q23-q24.

Disruption of the glucocorticoid receptor gene in the nervous system results in reduced anxiety

The glucocorticoid receptor (Gr, encoded by the gene Grl1) controls transcription of target genes both directly by interaction with DNA regulatory elements and indirectly by cross-talk with other transcription factors. In response to various stimuli, including stress, glucocorticoids coordinate metabolic, endocrine, immune and nervous system responses and ensure an adequate profile of transcription. In the brain, Gr has been proposed to modulate emotional behaviour, cognitive functions and addictive states. Previously, these aspects were not studied in the absence of functional Gr because inactivation of Grl1 in mice causes lethality at birth (F.T., C.K. and G.S., unpublished data). Therefore, we generated tissue-specific mutations of this gene using the Cre/loxP -recombination system. This allowed us to generate viable adult mice with loss of Gr function in selected tissues. Loss of Gr function in the nervous system impairs hypothalamus-pituitary-adrenal (HPA)-axis regulation, resulting in increased glucocorticoid (GC) levels that lead to symptoms reminiscent of those observed in Cushing syndrome. Conditional mutagenesis of Gr in the nervous system provides genetic evidence for the importance of Gr signalling in emotional behaviour because mutant animals show an impaired behavioural response to stress and display reduced anxiety.

Glutamatergic synaptic responses and long-term potentiation are impaired in the CA1 hippocampal area of calbindin D(28k)-deficient mice

The contribution of the cytosolic calcium binding protein calbindin D(28K) (CaBP) to glutamatergic neurotransmission and synaptic plasticity was investigated in hippocampal CA1 area of wild-type and antisense transgenic CaBP-deficient mice, with the use of extracellular recordings in the ex vivo slice preparation. The amplitude of non-N-methyl-D-aspartate receptor (non-NMDAr)-mediated extracellular field excitatory postsynaptic potentials (fEPSPs) recorded in control medium was significantly greater in CaBP-deficient mice, whereas the afferent fiber volley was not affected. In contrast, the amplitude of NMDAr-mediated fEPSPs isolated in a magnesium-free medium after blockade of non-NMDAr and GABAergic receptors was significantly depressed in these animals. No alteration in the magnitude of paired-pulse facilitation was found, indicating that the presynaptic calcium mechanisms controlling glutamate release were not altered in CaBP-deficient mice. The magnitude and time course of the short-term potentiation (STP) of fEPSPs induced by a 30 Hz conditioning stimulation, which was blocked by the NMDAr antagonist 2-amino-5-phosphonovalerate acid (2-APV), was not impaired in the transgenic mice, whereas long-term potentiation (LTP) induced by a 100 Hz tetanus was not maintained. The long-term depression (LTD) induced by low-frequency stimulation (1 Hz, 15 min) in the presence of the GABA antagonist bicuculline was not altered. These results argue for a contribution of CaBP to the mechanisms responsible for the maintenance of long-term synaptic potentiation, at least in part by modulating the activation of NMDA receptors.

Partial blockade of T-cell differentiation during ontogeny and marked alterations of the thymic microenvironment in transgenic mice with impaired glucocorticoid receptor function

Glucocorticoids (GCs) are widely known to be potent modulators of the immune system. The role of GCs in thymopoiesis as well as the integration of the thymus with the neuroendocrine system is, however, poorly understood. In the present work, we have studied, in transgenic mice with an impaired GC function, the alterations which occur in both T-cell differentiation and thymic stroma maturation, throughout ontogeny as well as in adult condition, analyzing their possible rebounding on the status of adult splenic T lymphocyte populations. These transgenic mice have been described to present a significant decrease (60-70%) of thymic and splenic GC receptor binding capacity but maintain normal their basal plasma ACTH and corticosterone levels. The animals showed a partial blockade of T-cell differentiation and decreased percentages of apoptotic cells during fetal development but not in adult life, when thymic cellularity was significantly increased although thymocyte apoptosis response was not affected. In contrast, thymic stroma was profoundly altered from early fetal stages and large epithelium-free areas appeared in adult thymus. On the other hand, our study revealed a reduction of the splenic TcRalphabeta population accompanied by an increase in the CD4/CD8 ratio. The analysis of different adhesion molecules as well as activation markers demonstrated that most of them (CD5, CD11a, CD11b, CD69 and MHC Class II) were normally expressed in transgenic lymphocytes, whereas CD44 and CD62L expression was altered indicating the existence of an increased proportion of primed T-cells in these animals. In view of the mutual interdependence of thymic stroma and thymocyte maturation, the partial blockade of T-cell differentiation during ontogeny and the profound alterations of the stromal cell compartment in transgenic mice with impaired GR function suggest a key role for GCs in coordinating the physiological dialogue between the developing thymocytes and their microenvironment.

Enhanced conditioned approach responses in transgenic mice with impaired glucocorticoid receptor function

The long-term consequences of impaired glucocorticoid receptor (GR) function on reward-related learning were studied in transgenic mice with impaired GR function in a series of experiments taxing conditioned and unconditioned approach responses to stimuli predictive of food. There was a double-dissociation in that transgenic mice with impaired GR activity showed enhanced conditioned exploration in situations when stimuli predicted reward, while free-feeding food consumption over 24 h was reduced. Previous experiments have shown altered accumbens dopaminergic activity in these animals. In line with these findings, we observed an enhanced behavioural stimulation of transgenic mice following administration of d-amphetamine (2 mg/kg). This suggests that the increase in preparatory responses in transgenic mice may be mediated via an enhanced accumbens dopaminergic activity, possibly secondary to alterations in other brain systems.

Antisense transgenics in animals

Antisense transgenesis provides a methodology for ablating gene expression in targeted tissues through the use of tissue-specific or controllable promoters. The two major features to be considered in the design of a construct for injection are (1) the target sequence and (2) the promoter to be used. Information is provided to help the investigator make decisions in these regards. The standard methodology of making transgenics is not replicated but some hints as how best to use a transgenic facility are provided. An overview of methods for studying transgene expression and target gene suppression is given. In summary, antisense transgenesis may provide a more readily achievable method for tissue-specific ablation of a gene's function than controllable knockouts provide.

The rationale for corticotropin-releasing hormone receptor (CRH-R) antagonists to treat depression and anxiety

Neuroendocrine studies strongly suggest that dysregulation of the hypothalamic pituitary-adrenocortical (HPA) system plays a causal role in the development and course of depression. Whereas the initial mechanism resulting in HPA hyperdrive remains to be elucidated, evidence has emerged that corticosteroid receptor function is impaired in many patients with depression and in many healthy individuals at increased genetic risk for an depressive disorder. Assuming such impaired receptor function, then central secretion of CRH would be enhanced in many brain areas, which would account for a variety of depressive symptoms. As shown in rats and also in transgenic mice with impaired glucocorticoid receptor function, antidepressants enhance the signaling through corticosteroid receptors. This mechanism of action can be amplified through blocking central mechanisms that drive the HPA system. Animal experiments using antisense oligodeoxynucleotides directed against the mRNA of both CRH receptor subtypes identified the CRH1 receptor as the mediator of the anxiogenic effects of CRH. Studies in mouse mutants in which this receptor subtype had been deleted extended these findings as the animals were less anxious than wild-type mice when experimentally stressed. Thus, patients with clinical conditions that are causally related to HPA hyperactivity may profit from treatment with a CRH1 receptor antagonist.

Disrupted allocentric but preserved egocentric spatial learning in transgenic mice with impaired glucocorticoid receptor function

Spatial and non-spatial learning of mice with an incorporated antisense RNA complementary to a fragment of cDNA coding for the glucocorticoid receptor (GR) were evaluated in allocentric and egocentric radial maze and water maze tasks, and in spontaneous object recognition and sensorimotor learning paradigms. Mice with impaired GR function did not acquire two maze paradigms based on allocentric spatial navigation, radial maze non-matching to position and water maze spatial discrimination learning. Comparison of performance in spaced and massed trials indicated that this may be due to a general inability to store information into allocentric reference memory or in retrieval processes. However, both groups of animals learned the rules of an egocentric radial maze task at similar rates and there was no difference in their ability to recognise objects once animals had equal opportunity to explore the sample objects. Sensorimotor performance was impaired in transgenic animals, but it is suggested that this is due to non-specific factors rather than to disrupted sensorimotor learning per se. These results are consistent with a disruption of hippocampal function. Histological examination of the hippocampus revealed no obvious structural abnormalities in transgenic animals. Therefore, the data suggest that functional underactivity of GRs at the level of the hippocampus induces a deficit in allocentric navigation while sparing egocentric navigation and object recognition.

Blood pressure reduction and diabetes insipidus in transgenic rats deficient in brain angiotensinogen

Angiotensin produced systemically or locally in tissues such as the brain plays an important role in the regulation of blood pressure and in the development of hypertension. We have established transgenic rats [TGR(ASrAOGEN)] expressing an antisense RNA against angiotensinogen mRNA specifically in the brain. In these animals, the brain angiotensinogen level is reduced by more than 90% and the drinking response to intracerebroventricular renin infusions is decreased markedly compared with control rats. Blood pressure of transgenic rats is lowered by 8 mmHg (1 mmHg = 133 Pa) compared with control rats. Crossbreeding of TGR(ASrAOGEN) with a hypertensive transgenic rat strain exhibiting elevated angiotensin II levels in tissues results in a marked attenuation of the hypertensive phenotype. Moreover, TGR(ASrAOGEN) exhibit a diabetes insipidus-like syndrome producing an increased amount of urine with decreased osmolarity. The observed reduction in plasma vasopressin by 35% may mediate these phenotypes of TGR(ASrAOGEN). This new animal model presenting long-term and tissue-specific down-regulation of angiotensinogen corroborates the functional significance of local angiotensin production in the brain for the central regulation of blood pressure and for the pathogenesis of hypertension.

Impaired glucocorticoid receptor function evolves in aberrant physiological responses to bacterial endotoxin

The consequences of glucocorticoid receptor (GR) dysfunction for neuroimmunoendocrine responses to an inflammatory challenge were studied in transgenic mice expressing antisense RNA directed against the GR [GR-impaired (GR-i) mice]. Mice were implanted intraperitoneally with a biotelemetry transmitter to monitor body temperature and locomotion. GR-i mice showed decreased locomotion and body temperature during the dark phase of the diurnal cycle. Intraperitoneal administration of saline caused a rapid increase in body temperature in control mice, which was terminated within 90 min. In GR-i mice, however, body temperature remained elevated for about 6 h. Intraperitoneal injection of endotoxin (10 micrograms/mouse) produced a biphasic fever in control mice. However, in endotoxin-injected GR-i mice, body temperature was not significantly different from their saline-injected controls during the first 6 h. Body temperature then increased and remained elevated during the night period. Both strains showed hypolocomotion after endotoxin. In a second experiment, mice were injected intraperitoneally with saline or endotoxin and killed after 1, 3, 6 or 24 h. In GR-i mice, endotoxin caused an augmented rise in plasma ACTH, but not in corticosterone levels. The endotoxin-induced increase in serum levels of interleukin-1 beta and interleukin-6 was not different between the strains. However, whereas in control mice tumour necrosis factor-alpha levels were below detection at the time points studied, substantial levels of this cytokine were found in the serum of GR-i mice 1 h after endotoxin administration. It may be concluded that life-long impairment of GR evolves in aberrant physiological and humoral responses to an acute inflammatory challenge. These findings expand our understanding about the neuroendocrine and physiological disturbances associated with stress-related disorders.

Genetic dissection of glucocorticoid receptor function in mice

Upon hormone binding, the activated glucocorticoid receptor (GR) functions as a transcription factor via different modes of action to control gene expression. Recent gene-targeting studies in mice provide new insight into the role of GR in vivo and are helping decipher the molecular mechanisms underlying its actions.

Transgenic animal models for the functional analysis of vasoactive peptides

The interplay of vasoactive peptide systems is an essential determinant of blood pressure regulation in mammals. While the endothelin and the renin-angiotensin systems raise blood pressure by inducing vasoconstriction and sodium retention, the kallikrein-kinin and the natriuretic-peptide systems reduce arterial pressure by eliciting vasodilatation and natriuresis. Transgenic technology has proven to be very useful for the functional analysis of vasoactive peptide systems. As an outstanding example, transgenic rats overexpressing the mouse Ren-2 renin gene in several tissues become extremely hypertensive. Several other transgenic rat and mouse strains with genetic modifications of components of the renin-angiotensin system have been developed in the past decade. Moreover, in recent years gene-targeting technology was employed to produce mouse strains lacking these proteins. The established animal models as well as the main insights gained by their analysis are summarized in this review.

Enhanced morphine-induced behavioural effects and dopamine release in the nucleus accumbens in a transgenic mouse model of impaired glucocorticoid (type II) receptor function: influence of long-term treatment with the antidepressant moclobemide

In vivo microdialysis experiments were conducted in transgenic mice with impaired glucocorticoid receptor function resulting from expression of antisense directed against glucocorticoid receptor messenger RNA. Basal corticosterone and serotonin levels in the nucleus accumbens of untreated transgenic mice were enhanced compared to control mice (B6C3F1). Following a systemic morphine injection (15 mg/kg) mesolimbic dopamine and serotonin release was markedly increased in transgenic mice compared to control mice and in parallel enhanced behavioural stimulation was observed in these animals. After pretreatment with the antidepressant moclobemide over a time period of eight weeks (15 mg/kg/day) elevated basal levels of both corticosterone and serotonin were normalized in transgenic mice. Furthermore, morphine-induced dopamine and serotonin release as well as behavioral stimulation were suppressed in transgenic mice and similar to that in control mice. The results indicate that impaired glucocorticoid receptor function influences the basal release of serotonin in the nucleus accumbens. This alteration has no effect on basal but on morphine-stimulated release of dopamine in the mesolimbic system. An enhanced sensitivity to the effects of morphine is apparently related to elevated brain corticosterone and serotonin levels and can be normalized by long-term antidepressant treatment.

Age- and stimulus-dependent changes in anxiety-related behaviour of transgenic mice with GR dysfunction

Transgenic mice expressing antisense directed against glucocorticoid receptor (GR) mRNA were used for a longitudinal study on the effects of hypothalamic-pituitary-adrenocortical dysfunction on anxiety-related behaviour and locomotor activity. Compared with age-matched controls and 5-week-old transgenic animals, 13- and 36-week-old transgenic mice made significantly more entries into and spent significantly more time on the open arms of the elevated plus-maze thereby indicating reduced basal anxiety. In contrast, time spent in the central area of the open field was significantly reduced in transgenic animals indicating an enhanced anxiety under conditions of increased stress. Locomotor activity did not differ between transgenic and control animals. Our results indicate that behavioural changes in transgenic mice differ according to the age of the animals and the behavioural paradigm used. Transgenic animals with an impaired GR function may be of help in studying the interaction of ageing, GR function and the development of behavioural abnormalities.

Psychoneuroendocrinology of depression. Hypothalamic-pituitary-adrenal axis

Among the more consistent observations in patients with major depression is dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis presenting as elevation of basal cortisol, dexamethasone-mediated negative feedback resistance, increased cerebrospinal fluid levels of corticotropin-releasing factor (CRF), and a blunted adrenocorticotropic hormone (ACTH) response to challenge with exogenous CRF. These features appear to be state, rather than trait markers, and are normalized upon successful treatment. These pathophysiologic adaptations may arise from defects in central drive to the neuroendocrine hypothalamus, disruption of normal adrenocortical hormone receptor function or a modification of HPA axis function at any level. Functional assessment of the HPA axis is thought to provide a window into central nervous system operation that may be of diagnostic value in this and other affective disorders regardless of whether CRF and glucocorticoids are directly involved in the origin of major depression or merely exacerbate the consequences of other primary defects.

Reduced activity of hypothalamic corticotropin-releasing hormone neurons in transgenic mice with impaired glucocorticoid receptor function

Loss of central glucocorticoid receptor (GR) function is thought to be involved in the development of neuroendocrine and psychiatric disorders associated with corticotropin-releasing hormone (CRH) hyperactivity. The possible causal relationship between defective GR function and altered activity of CRH neurons was studied in transgenic mice (TG) expressing antisense RNA against GR. Immunocytochemical studies showed significant reductions in CRH immunoreactive neurons in the paraventricular nucleus (PVN) and in CRH and vasopressin (AVP) stores in the external zone of the median eminence. Concomitantly, stimulus-evoked CRH secretion from mediobasal hypothalami of TG mice in vitro was reduced significantly. However, CRH mRNA levels in the PVN of TG mice were marginally lower than those in wild-type (WT) mice. 125I-CRH binding autoradiography revealed no differences between WT and TG animals in any of the brain regions that were studied. Basal plasma corticosterone (cort) levels and 125I-CRH binding, CRH-R1 mRNA, POMC mRNA, and POMC hnRNA levels in the anterior pituitary gland were similar in WT and TG mice. Intraperitoneal injection of interleukin-1beta (IL-1beta) increased plasma cort levels, CRH mRNA in the PVN, and anterior pituitary POMC hnRNA similarly in WT and TG mice. The injection of saline significantly reduced anterior pituitary CRH-R1 mRNA levels in WT mice, but not in TG mice, whereas IL-1beta produced a decrease in these mRNA levels in both strains. The data show that long-term GR dysfunction can be associated with reduced activity of CRH neurons in the PVN and decreased sensitivity of pituitary CRH-R1 mRNA to stimulus-induced downregulation. Moreover, the hypothalamic changes observed in this model suggest that impaired GR function, at least if present since early embryonic life, does not necessarily result in CRH hyperexpression characteristics of disorders such as major depression.

A positive role for thymus-derived steroids in formation of the T-cell repertoire

T cells undergo rigorous selection processes in the thymus that are necessary to prevent T cells with either autoreactive or nonfunctional T-cell receptors (TCRs) from entering the periphery. Although both positive and negative selection depend on TCR-mediated signals, the means by which a thymocyte interprets these signals to result in survival or death is not understood. Glucocorticoids are known to induce thymocyte apoptosis at high concentrations, but at lower concentrations glucocorticoids can antagonize TCR-mediated deletional signals and allow survival of thymocytes and T cell hybridomas. Interestingly, transgenic mice in which the expression of the glucocorticoid receptor has been downmodulated specifically in thymocytes have abnormal thymocyte differentiation, indicating that glucocorticoids play a significant role in T-cell development. Furthermore, we have demonstrated the presence of steroidogenic enzymes in the thymic epithelium and can show that, in vitro, these cells readily synthesize pregnenolone, the first product in the steroidogenic pathway, and deoxycorticosterone. Inhibition of local glucocorticoid biosynthesis in thymi from TCR transgenic mice during fetal thymic organ culture (FTOC) revealed significant alterations in the process of thymocyte selection. These data suggest that glucocorticoids do not simply suppress the immune system but rather are necessary for thymocyte survival and differentiation.

Transgenic and transcriptional studies on neurosecretory cell gene expression

1. Studies of the regulation of neurosecretory cell gene expression suffer from the lack of suitable cell lines. Two approaches have been used to overcome this deficit: transfection of neuropeptide genes into heterologous cell lines and generation of transgenic animals. 2. Studies with heterologous cell lines have revealed the potential involvement of nuclear hormone receptors, POU proteins, and fos/jun/ATF family members in the regulation of the vasopressin and oxytocin genes. Although limited in their scope, these studies have contributed greatly to the dissection of basic properties of elements in the vasopressin and oxytocin gene promoters. 3. Transgenic mice, and more recently rats, have been used to elucidate genomic regions governing cell specificity and physiological regulation of neurosecretory gene expression. The genes encoding the neuropeptides vasopressin and oxytocin have been used in many transgenic studies, due to the well-defined expression patterns and physiology of the endogenous neuropeptides. Cell-specific and physiologically regulated expression of these transgenes has been achieved, demonstrating the action of putative repressor elements and regulation of the expression of one gene by sequences present in the other gene. 4. Appropriate expression and translation of transgenes have resulted in the production of several useful systems. Expression of oncogene sequences in gonadotropin-releasing hormone neurons has allowed the development of cell lines from the resulting tumors, overproduction of corticotropin-releasing factor has produced animal models of anxiety and obesity, and directed ectopic expression of growth hormone has generated a potentially useful rat model of dwarfism. These and other animal models of human disease will provide important avenues for the development of therapeutic strategies.

Agonistic behavior during stress prevents the development of learned helplessness in rats

Male Wistar rats were subjected to unavoidable electrical pain stimulation either in individual cages or in pairs. During the procedure, rats stressed in pairs fought. After 48 h, rats were tested in a shuttle box for the ability to develop active avoidance responses. The proportion of escape and avoidance responses was significantly lower in rats stressed individually as compared with intact animals and animals stressed in pairs. Plasma corticosterone was assayed one day later, at rest and after dexamethasone administration. There were no significant differences in resting corticosterone level between groups of animals. Administration of dexamethasone significantly reduced the plasma corticosterone level in intact rats and in animals stressed in pairs, but not in rats stressed individually. Thus, agonistic behavior during unavoidable stress prevents the development of pathological changes in adaptive behavior and the endocrine system.

Regulation by glucocorticoids of angiotensinogen gene expression and secretion in adipose cells

Adipose cells are an important source of angiotensinogen (AT). Its activation product, angiotensin II, stimulates in vitro and in vivo the production and release of prostacyclin which acts as a potent adipogenic signal in promoting the terminal differentiation of preadipocytes to adipocytes. Since glucocorticoids are known to promote adipose cell differentiation in vitro as well as in vivo, their role in the regulation of AT gene expression and secretion has been investigated in cultured Ob1771 mouse adipose cells. In contrast with liver cells, which are the major source of AT and the target of several hormones for the regulation of its expression, adipose cells are only responsive to glucocorticoids, which are able to up-regulate AT gene expression and AT secretion rapidly and dose-dependently. On exposure to glucocorticoids, accumulation of AT mRNA appears primarily to be due to transcriptional activation of the gene and is parallelled by secretion of the protein. Similar results on AT mRNA expression and AT secretion were obtained using explants of rat adipose tissue ex vivo demonstrating a major if not exclusive mechanism of regulation of AT production by glucocorticoids in mature adipose cells. Together these results provide a potential link between glucocorticoids, AT, the growth of adipose tissue and increased blood pressure.

Spatial learning deficits in mice with a targeted glucocorticoid receptor gene disruption

Previous studies in rats using the Morris water maze suggested that the processing of spatial information is modulated by corticosteroid hormones through mineralocorticoid and glucocorticoid receptors in the hippocampus. Mineralocorticoid receptors appear to be involved in the modulation of explorative behaviour, while additional activation of glucocorticoid receptors facilitates the storage of information. In the present study we used the water maze task to examine spatial learning and memory in mice homozygous and heterozygous for a targeted disruption of the glucocorticoid receptor gene. Compared with wild-type controls, homozygous and heterozygous mice were impaired in the processing of spatial but not visual information. Homozygous mutants performed variably during training, without specific platform-directed search strategies. The spatial learning disability was partly compensated for by increased motor activity. The deficits were indicative of a dysfunction of glucocorticoid receptors as well as of mineralocorticoid receptors. Although the heterozygous mice performed similarly to wild-type mice with respect to latency to find the platform, their strategy was more similar to that of the homozygous mice. Glucocorticoid receptor-related long-term spatial memory was impaired. The increased behavioural reactivity of the heterozygous mice in the open field points to a more prominent mineralocorticoid receptor-mediated function. The findings indicate that (i) the glucocorticoid receptor is of critical importance for the control of spatial behavioural functions, and (ii) mineralocorticoid receptor-mediated effects on this behaviour require interaction with functional glucocorticoid receptors. Until the development of site-specific, inducible glucocorticoid receptor mutants, glucocorticoid receptor-knockout mice present the only animal model for the study of corticosteroid-mediated effects in the complete absence of a functional receptor.

Pancreatic beta cells are important targets for the diabetogenic effects of glucocorticoids

Abnormalities contributing to the pathogenesis of non-insulin-dependent diabetes mellitus include impaired beta cell function, peripheral insulin resistance, and increased hepatic glucose production. Glucocorticoids are diabetogenic hormones because they decrease glucose uptake and increase hepatic glucose production. In addition, they may directly inhibit insulin release. To evaluate that possible role of glucocorticoids in beta cell function independent of their other effects, transgenic mice with an increased glucocorticoid sensitivity restricted to their beta cells were generated by overexpressing the glucocorticoid receptor (GR) under the control of the insulin promoter. Intravenous glucose tolerance tests showed that the GR transgenic mice had normal fasting and postabsorptive blood glucose levels but exhibited a reduced glucose tolerance compared with their control littermates. Measurement of plasma insulin levels 5 min after intravenous glucose load demonstrated a dramatic decrease in acute insulin response in the GR transgenic mice. These results show that glucocorticoids directly inhibit insulin release in vivo and identify the pancreatic beta cell as an important target for the diabetogenic action of glucocorticoids.