Glucocorticoid receptor gene expression in the embryonic rat brain

The olfactory bulb: A neuroendocrine spotlight on feeding and metabolism

Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in the external environment, before sight, taste or sound. Odour molecules activate olfactory sensory neurons that reside on the olfactory epithelium in the nasal cavity, which transmits this odour-specific information to the olfactory bulb (OB), where it is relayed to higher brain regions involved in olfactory perception and behaviour. Besides odour processing, recent studies suggest that the OB extends its function into the regulation of food intake and energy balance. Furthermore, numerous hormone receptors associated with appetite and metabolism are expressed within the OB, suggesting a neuroendocrine role outside the hypothalamus. Olfactory cues are important to promote food preparatory behaviours and consumption, such as enhancing appetite and salivation. In addition, altered metabolism or energy state (fasting, satiety and overnutrition) can change olfactory processing and perception. Similarly, various animal models and human pathologies indicate a strong link between olfactory impairment and metabolic dysfunction. Therefore, understanding the nature of this reciprocal relationship is critical to understand how olfactory or metabolic disorders arise. This present review elaborates on the connection between olfaction, feeding behaviour and metabolism and will shed light on the neuroendocrine role of the OB as an interface between the external and internal environments. Elucidating the specific mechanisms by which olfactory signals are integrated and translated into metabolic responses holds promise for the development of targeted therapeutic strategies and interventions aimed at modulating appetite and promoting metabolic health.

Infant adrenocortical reactivity and behavioral functioning: relation to early exposure to maternal intimate partner violence

Prenatal stress negatively affects fetal development, which in turn may affect infant hypothalamic-pituitary-adrenal (HPA) axis regulation and behavioral functioning. We examined effects of exposure to a traumatic stressor in families [intimate partner violence (IPV)] on both infants' HPA axis reactivity to stress and their internalizing and externalizing behaviors. Infants (n = 182, 50% girls, x age = 11.77 months) were exposed to a laboratory challenge task designed to induce frustration and anger (i.e. arm restraint). Saliva samples were taken pre-task and 20 and 40 min post-task and then assayed for cortisol. Mothers reported on their pregnancy and postpartum IPV history, current mental health, substance use and their infants' behaviors. Structural equation modeling revealed that prenatal, but not postnatal, IPV was independently associated with infant cortisol reactivity and problem behavior. Maternal mental health predicted infant behavioral functioning but not infant HPA axis reactivity. These findings are consistent with the prenatal programing hypothesis; that is, early life stress affects later risk and vulnerability for altered physiological and behavioral regulation.

Developmental neurobiology of the rat attachment system and its modulation by stress

Stress is a powerful modulator of brain structure and function. While stress is beneficial for survival, inappropriate stress dramatically increases the risk of physical and mental health problems, particularly when experienced during early developmental periods. Here we focus on the neurobiology of the infant rat's odor learning system that enables neonates to learn and approach the maternal odor and describe the unique role of the stress hormone corticosterone in modulating this odor approach learning across development. During the first nine postnatal days, this odor approach learning of infant rats is supported by a wide range of sensory stimuli and ensures attachment to the mother's odor, even when interactions with her are occasionally associated with pain. With maturation and the emergence of a stress- or pain-induced corticosterone response, this odor approach learning terminates and a more adult-like amygdala-dependent fear/avoidance learning emerges. Strikingly, the odor approach and attenuated fear learning of older pups can be re-established by the presence of the mother, due to her ability to suppress her pups' corticosterone release and amygdala activity. This suggests that developmental changes in stress responsiveness and the stimuli that produce a stress response might be critically involved in optimally adapting the pup's attachment system to its respective ecological niche.

Prenatal stress inhibits hippocampal neurogenesis but spares olfactory bulb neurogenesis

The dentate gyrus (DG) and the olfactory bulb (OB) are two regions of the adult brain in which new neurons are integrated daily in the existing networks. It is clearly established that these newborn neurons are implicated in specific functions sustained by these regions and that different factors can influence neurogenesis in both structures. Among these, life events, particularly occurring during early life, were shown to profoundly affect adult hippocampal neurogenesis and its associated functions like spatial learning, but data regarding their impact on adult bulbar neurogenesis are lacking. We hypothesized that prenatal stress could interfere with the development of the olfactory system, which takes place during the prenatal period, leading to alterations in adult bulbar neurogenesis and in olfactory capacities. To test this hypothesis we exposed pregnant C57Bl/6J mice to gestational restraint stress and evaluated behavioral and anatomic consequences in adult male offspring.

We report that prenatal stress has no impact on adult bulbar neurogenesis, and does not alter olfactory functions in adult male mice. However, it decreases cell proliferation and neurogenesis in the DG of the hippocampus, thus confirming previous reports on rats. Altogether our data support a selective and cross-species long-term impact of prenatal stress on neurogenesis.

Differential subcellular localization of the glucocorticoid receptor in distinct neural stem and progenitor populations of the mouse telencephalon in vivo

Glucocorticoids are given to pregnant women at risk for premature delivery to promote lung maturation. Despite reports of detrimental effects of glucocorticoids on telencephalic neural stem/progenitor cells (NSPCs), the regional and cellular expressions of the glucocorticoid receptor (GR) in various NSPC populations in the intact brain have not been thoroughly assessed. Therefore in this study we performed a detailed analysis of GR protein expression in the developing mouse ventral and dorsal telencephalon in vivo. At embryonic day 11.5 (E11.5), the majority of Pax6-positive radial glial cells (RGCs) and Tbr2-positive intermediate progenitor cells (IPCs) expressed nuclear GR, while a small number of RGCs on the apical ventricular zone (aVZ), expressed cytoplasmic GR. However, on E13.5, the latter population of RGCs increased in size, whereas abventricular NSPCs and especially neurons of the cortical plate, expressed nuclear GR. In IPCs, GR was always nuclear. A similar expression profile was observed throughout the ventral telencephalon, hippocampus and olfactory bulb, with NSPCs of the aVZ primarily expressing cytoplasmic GR, while abventricular NSPCs and mature cells primarily expressed nuclear GR. Close to birth, nuclear GR accumulated within specific cortical areas such as layer V, the subplate and CA1 area of the hippocampus. In summary, our data show that GR protein is present in early NSPCs of the dorsal and ventral telencephalon at E11.5 and primarily occupies the nucleus. Moreover, our study suggests that the subcellular localization of the receptor may be subjected to region and neurodevelopmental stage-specific regulation.

Role of the hypothalamic-pituitary-adrenal axis in developmental programming of health and disease

Adverse environments during the fetal and neonatal development period may permanently program physiology and metabolism, and lead to increased risk of diseases in later life. Programming of the hypothalamic-pituitary-adrenal (HPA) axis is one of the key mechanisms that contribute to altered metabolism and response to stress. Programming of the HPA axis often involves epigenetic modification of the glucocorticoid receptor (GR) gene promoter, which influences tissue-specific GR expression patterns and response to stimuli. This review summarizes the current state of research on the HPA axis and programming of health and disease in the adult, focusing on the epigenetic regulation of GR gene expression patterns in response to fetal and neonatal stress. Aberrant GR gene expression patterns in the developing brain may have a significant negative impact on protection of the immature brain against hypoxic-ischemic encephalopathy in the critical period of development during and immediately after birth.

The Long and the Short of it: Gene and Environment Interactions During Early Cortical Development and Consequences for Long-Term Neurological Disease

Cortical development is a complex amalgamation of proliferation, migration, differentiation, and circuit formation. These processes follow defined timescales and are controlled by a combination of intrinsic and extrinsic factors. It is currently unclear how robust and flexible these processes are and whether the developing brain has the capacity to recover from disruptions. What is clear is that there are a number of cognitive disorders or conditions that are elicited as a result of disrupted cortical development, although it may take a long time for the full pathophysiology of the conditions to be realized clinically. The critical window for the manifestation of a neurodevelopmental disorder is prolonged, and there is the potential for a complex interplay between genes and environment. While there have been extended investigations into the genetic basis of a number of neurological and mental disorders, limited definitive associations have been discovered. Many environmental factors, including inflammation and stress, have been linked to neurodevelopmental disorders, and it may be that a better understanding of the interplay between genes and environment will speed progress in this field. In particular, the development of the brain needs to be considered in the context of the whole materno-fetal unit as the degree of the metabolic, endocrine, or inflammatory responses, for example, will greatly influence the environment in which the brain develops. This review will emphasize the importance of extending neurodevelopmental studies to the contribution of the placenta, vasculature, cerebrospinal fluid, and to maternal and fetal immune response. These combined investigations are more likely to reveal genetic and environmental factors that influence the different stages of neuronal development and potentially lead to the better understanding of the etiology of neurological and mental disorders such as autism, epilepsy, cerebral palsy, and schizophrenia.

Dexamethasone induces apoptosis in the developing rat amygdala in an age-, region-, and sex-specific manner

Exposure to glucocorticoids (GCs) in early development can lead to long-term changes in brain function and behavior, although little is known about the underlying neural mechanisms. Perinatal exposure to GCs alters adult anxiety and neuroendocrine responses to stress. Therefore, we investigated the effects of either late gestational or neonatal exposure to the GC receptor agonist dexamethasone (DEX), on apoptosis within the amygdala, a region critical for emotional regulation. DEX was administered to timed-pregnant rat dams from gestational day 18 until parturition, or postnatal day 4-6. Offspring were sacrificed the day following the last DEX treatment, and tissue was processed for immunohistochemical detection of cleaved caspase-3, a marker for apoptotic cells. Prenatal DEX treatment significantly increased the number of cleaved caspase-3-positive cells in the amygdala of both sexes, largely due to increases within the medial and basomedial subregions. Postnatal DEX treatment also increased cleaved caspase-3 immunoreactivity within the amygdala, although effects reached significance only in the central nucleus of females. Overall, DEX induction of cleaved caspase-3 in the amygdala was greater following prenatal compared with postnatal treatment, yet in both instances, elevations in cleaved caspase-3 correlated with an increase in pro-apoptotic Bax mRNA expression. Dual-label immunohistochemistry of cleaved caspase-3 and the neuronal marker NeuN confirmed that virtually all cleaved caspase-3-positive cells in the amygdala were neurons, and a subset of these cells (primarily following postnatal treatment) expressed a GABAergic calcium-binding protein phenotype (calbindin or calretinin). Together these results indicate that early developmental GC exposure induces neuronal apoptosis within the amygdala in an age-, sex-, and region-dependent manner.

Prenatal stress induces long term stress vulnerability, compromising stress response systems in the brain and impairing extinction of conditioned fear after adult stress

Stress is a risk factor for the development of affective disorders, including depression, post-traumatic stress disorder, and other anxiety disorders. However, not all individuals who experience either chronic stress or traumatic acute stress develop such disorders. Thus, other factors must confer a vulnerability to stress, and exposure to early-life stress may be one such factor. In this study we examined prenatal stress (PNS) as a potential vulnerability factor that may produce stable changes in central stress response systems and susceptibility to develop fear- and anxiety-like behaviors after adult stress exposure. Pregnant Sprague-Dawley rats were immobilized for 1 h daily during the last week of pregnancy. Controls were unstressed. The male offspring were then studied as adults. As adults, PNS or control rats were first tested for shock-probe defensive burying behavior, then half from each group were exposed to a combined chronic plus acute prolonged stress (CAPS) treatment, consisting of chronic intermittent cold stress (4 °C, 6 h/d, 14 days) followed on day 15 by a single session of sequential acute stressors (social defeat, immobilization, cold swim). After CAPS or control treatment, different groups were tested for open field exploration, social interaction, or cued fear conditioning and extinction. Rats were sacrificed at least 5 days after behavioral testing for measurement of tyrosine hydroxylase (TH) and glucocorticoid receptor (GR) expression in specific brain regions, and plasma adrenocorticotropic hormone (ACTH) and corticosterone. Shock-probe burying, open field exploration and social interaction were unaffected by any treatment. However, PNS elevated basal corticosterone, decreased GR protein levels in hippocampus and prefrontal cortex, and decreased TH mRNA expression in noradrenergic neurons in the dorsal pons. Further, rats exposed to PNS plus CAPS showed attenuated extinction of cue-conditioned fear. These results suggest that PNS induces vulnerability to subsequent adult stress, resulting in an enhanced fear-like behavioral profile, and dysregulation of brain noradrenergic and hypothalamic-pituitary-adrenal axis (HPA) activity.

Rodent model of infant attachment learning and stress

Here we review the neurobiology of infant odor learning in rats, and discuss the unique role of the stress hormone corticosterone (CORT) in the learning necessary for the developing rat. During the first 9 postnatal (PN) days, infants readily learn odor preferences, while aversion and fear learning are attenuated. Such restricted learning may ensure that pups only approach their mother. This sensitive period of preference learning overlaps with the stress hyporesponsive period (SHRP, PN4-14) when pups have a reduced CORT response to most stressors. Neural underpinnings responsible for sensitive-period learning include increased activity within the olfactory bulb and piriform "olfactory" cortex due to heightened release of norepinephrine from the locus coeruleus. After PN10 and with the decline of the SHRP, stress-induced CORT release permits amygdala activation and facilitates learned odor aversions and fear. Remarkably, odor preference and attenuated fear learning can be reestablished in PN10-15 pups if the mother is present, an effect due to her ability to suppress pups' CORT and amygdala activity. Together, these data indicate that functional changes in infant learning are modified by a unique interaction between the developing CORT system, the amygdala, and maternal presence, providing a learning system that becomes more flexible as pups mature.

Impaired neural stem/progenitor cell proliferation in streptozotocin-induced and spontaneous diabetic mice

Diabetes mellitus is associated with adverse complications in many organ systems including the brain. Accumulating evidence indicates that diabetes, regardless of its type, impairs adult neurogenesis in the dentate gyrus (DG) of the hippocampus (HPC). However, the effects of the disease on neurogenesis in the subventricular zone (SVZ) are not well established. We induced diabetes in male NOD/SCID (non-obese diabetic/severe combined immunodeficiency) mice and C57BL/6 mice with a single intraperitoneal injection of streptozotocin (STZ). On day 7 or day 21 after STZ injection mice received the thymidine analog 5-bromo-2'-deoxyuridine (BrdU) for labeling of proliferative cells. Mice were sacrificed 24h later and brain coronal sections were stained with anti-BrdU antibodies. Neural stem/progenitor cell (NSC/NPC) proliferation, as revealed by BrdU-labeled cells, was markedly decreased in the subgranular zone of the DG in STZ-treated diabetic mice. A similar reduction of NSC/NPC proliferation was seen in the SVZ. Reduced DG and SVZ cell proliferation was also found in diabetic NOD mice, a model of spontaneous diabetes, and the reduction was attenuated by bilateral adrenalectomy (Adx). Adx did not alter blood glucose or insulin levels in either prediabetic or diabetic NOD mice, but Adx partly increased mRNA levels of hippocampal and SVZ brain-derived neurotrophic factor (BDNF), a crucial regulator of NSC/NPC proliferation. Moreover, NOD and NOD/SCID mice showed a more rapid reduction of NSC/NPC proliferation than C57BL/6 mice in response to diabetes. Thus, we conclude that diabetes inhibits cell proliferation in both the SVZ and HPC, and inhibition was associated with elevated glucocorticoid levels and reduced BDNF expression.

Culture condition and embryonic stage dependent silence of glucocorticoid receptor expression in hippocampal neurons

Glucocorticoid (GC) plays a key role in controlling numerous cellular processes during embryogenesis and fetal development. The actions of glucocorticoids are mediated by interaction with their receptors. We previously reported that hippocampal neurons from embryonic day 18 (E18) rats showed silence of glucocorticoid receptor (GR) expression when cultured in serum-free condition. In this study, using western blot, immunofluorescence staining and real-time RT-PCR, we found that while this silence occurred in hippocampal neurons isolated from E16 and E18 rats, it did not happen in those from E20 and neonatal (P0) rats. And when cultured under serum-containing condition, none of them showed GR silence anymore. Corticosterone could not rescue the expression of GR in E16 and E18 neurons in serum-free condition, whereas adding of serum could induce the re-expression of the silenced GR. The absence of GR silence in P0 neurons was not due to the perturbation during parturition. Moreover, the unique expression profile of GR in protein and mRNA level was well reflected in the changes of GR function. These results suggested that under in vitro condition, serum was critical for the maintaining of GR expression in hippocampal neurons of early embryonic stages but less important in later developmental stages. Thus, our data implied that at different developmental stages, the expression of GR in hippocampal neurons might have different susceptibilities to environment changes and there might be a critical time window for the switching of such characteristics during development.

A protective role of 27-kDa heat shock protein in glucocorticoid-evoked apoptotic cell death of hippocampal progenitor cells

Hippocampus is one of the most vulnerable tissues to glucocorticoid (GC). In the present study, we demonstrate that dexamethasone (DEX), a synthetic GC, induces apoptotic cell death in hippocampal progenitor HiB5 cells without any additional insult. Interestingly, expression of 27-kDa heat shock protein (HSP27) was markedly induced by DEX in time- and dose-dependent manners. This induction was dependent on the production of reactive oxygen species (ROS), suggesting that DEX-evoked oxidative damage to HiB5 cells is responsible for the HSP27 induction. To evaluate a possible role of HSP27, we generated two mutant HiB5 cell lines, in which expression of HSP27 was inhibited or enhanced by the over-expression of HSP27 cDNA with antisense or sense orientation (AS-HSP27 and S-HSP27, respectively). DEX-induced apoptotic cell population was significantly increased in AS-HSP27 HiB5 cells and evidently decreased in S-HSP27 cells. These results indicate that HSP27 protects hippocampal progenitor cells from GC-induced apoptotic cell death.

Disruption of rat forebrain development by glucocorticoids: critical perinatal periods for effects on neural cell acquisition and on cell signaling cascades mediating noradrenergic and cholinergic neurotransmitter/neurotrophic responses

Glucocorticoids are the consensus treatment for the prevention of respiratory distress in preterm infants, but there is evidence for increased incidence of neurodevelopmental disorders as a result of their administration. We administered dexamethasone (Dex) to developing rats at doses below or within the range of those used clinically, evaluating the effects on forebrain development with exposure in three different stages: gestational days 17-19, postnatal days 1-3, or postnatal days 7-9. At 24 h after the last dose, we evaluated biomarkers of neural cell acquisition and growth, synaptic development, neurotransmitter receptor expression, and synaptic signaling mediated by adenylyl cyclase (AC). Dex impaired the acquisition of neural cells, with a peak effect when given in the immediate postnatal period. In association with this defect, Dex also elicited biphasic effects on cholinergic presynaptic development, promoting synaptic maturation at a dose (0.05 mg/kg) well below those used therapeutically, whereas the effect was diminished or lost when doses were increased to 0.2 or 0.8 mg/kg. Dex given postnatally also disrupted the expression of adrenergic receptors known to participate in neurotrophic modeling of the developing brain and evoked massive induction of AC activity. As a consequence, disparate receptor inputs all produced cyclic AMP overproduction, a likely contributor to disrupted patterns of cell replication, differentiation, and apoptosis. Superimposed on the heterologous AC induction, Dex impaired specific receptor-mediated cholinergic and adrenergic signals. These results indicate that, during a critical developmental period, Dex administration leads to widespread interference with forebrain development, likely contributing to eventual, adverse neurobehavioral outcomes.

Corticosterone influences on Mammalian neonatal sensitive-period learning

Infant rats exhibit sensitive-period odor learning characterized by olfactory bulb neural changes and odor preference acquisitions critical for survival. This sensitive period is coincident with low endogenous corticosterone (CORT) levels and stress hyporesponsivity. The authors hypothesized that low corticosterone levels modulate sensitive-period learning. They assessed the effects of manipulating CORT levels by increasing and removing CORT during (Postnatal Day 8) and after (Postnatal Day 12) the sensitive period. Results show that (a) exogenous CORT prematurely ends sensitive-period odor-shock-induced preferences; (b) adrenalectomy developmentally extends the sensitive period as indicated by odor-shock-induced odor-preference learning in older pups, whereas CORT replacement can reinstate fear learning; and (c) CORT manipulation modulates olfactory bulb correlates of sensitive-period odor learning in a manner consistent with behavior.

Glucocorticoid receptor deficiency increases vulnerability of the nigrostriatal dopaminergic system: critical role of glial nitric oxide

Glucocorticoids (GCs) exert via glucocorticoid receptors (GRs) potent anti-inflammatory and immunosuppressive effects. Emerging evidence indicates that an inflammatory process is involved in dopaminergic nigro-striatal neuronal loss in Parkinson's disease. We here report that the GR deficiency of transgenic (Tg) mice expressing GR antisense RNA from early embryonic life has a dramatic impact in "programming" the vulnerability of dopaminergic neurons to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The GR deficiency of Tg mice exacerbates MPTP-induced toxicity to dopaminergic neurons, as revealed by both severe loss of tyrosine hydroxylase positive nigral neurons and sharp decreases in striatal levels of dopamine and its metabolites. In addition, the late increase in dopamine oxidative metabolism and ascorbic acid oxidative status in GR-deficient mice was far greater than in wild-type (Wt) mice. Inducible nitric oxide synthase (iNOS) was sharply increased in activated astrocytes, macrophages/microglia of GR-deficient as compared with Wt mice. Moreover, GR-deficient microglia produced three- to fourfold higher nitrite levels than Wt mice; these increases preceded the loss of dopaminergic function and were resistant to GR the inhibitory effect of GC, pointing to peroxynitrites as candidate neurotoxic effectors. The iNOS inhibitor N6-(1-iminoethyl)-L-lysine normalized vulnerability of Tg mice, thus establishing a novel link between genetic impairment of GR function and vulnerability to MPTP.

Regional specificity of brain glucocorticoid receptor mRNA alterations in subjects with schizophrenia and mood disorders

Glucocorticoid receptors (GR) mediate the direct effects of glucocorticoids released in response to stress and the regulation of the hypothalamic-pituitary-adrenocortical (HPA) system through a negative feedback mechanism. Individuals with major mental illness, who often exhibit hypercortisolemia, may have down-regulated levels of GR mRNA. In situ hybridization for GR mRNA was performed on post-mortem specimens from patients suffering from depression, bipolar disorder, schizophrenia and from normal controls (n = 15 per group). In frontal cortex, GR mRNA levels were decreased in layers III-VI in the subjects with depression and schizophrenia. In inferior temporal cortex, GR mRNA levels were decreased in layer IV in all three diagnostic groups. In the entorhinal cortex, GR mRNA levels were decreased in layers III and VI in the bipolar group. In hippocampus, GR mRNA levels were reduced in the dentate gyrus, CA(4), CA(3) and CA(1) in the schizophrenia group. In the subiculum, GR mRNA levels were reduced in the bipolar group. These results suggest that GR dysregulation occurs in all three major psychiatric illnesses with variability according to anatomical site. The severity and heterogeneity of this reduction may underlie some of the clinical heterogeneity seen in these disorders.

Seasonal changes in hepatic progesterone receptor mRNA, estrogen receptor mRNA, and vitellogenin mRNA in the painted turtle, Chrysemys picta

Previous studies using the fresh water turtle Chrysemys picta have demonstrated that progesterone (P) inhibits estradiol (E)-induced vitellogenin (vtg) secretion in this species. Further, there is evidence for the differential expression of the two P receptor isoforms (PRA and PRB) in the liver during the turtle seasonal cycle, correlating with hepatic vitellogenesis. In this study we report changes in the hepatic PR mPNA, ER mRNA, and vitellogenin (vtg) mRNA transcripts during the reproductive cycle of the turtle. Fragments of the turtle hepatic PR and ER cDNAs were cloned and sequenced and a previously cloned turtle vtg cDNA were used as probes in Northern blotting. No 3.7-kb PR mRNA, corresponding to the smaller PR transcript, PRA of other species was found, although, a smaller 1.8-kb transcript (putative PRC mRNA) was present. These observations suggest that the turtle as in the chicken and human, the 4.5-kb PR mRNA transcript encodes both PRA and PRB proteins. Only the larger PR mRNA transcript (4.5-kb), was found to vary significantly during the annual cycle, being highest when vitellogenesis was inhibited in winter and summer. Vtg mRNA could not be detected during the summer or winter, was highest during vitellogenesis in the spring, and reappeared during the fall period of vitellogenesis and ovarian recrudescence. ER mRNA followed a similar pattern, being highest during spring and early fall, when vtg synthesis is high. The data suggest that P/PR, as well as E/ER, may be involved in the seasonal regulation of hepatic vitellogenesis in this species.

Glucocorticoid inhibits growth factor-induced differentiation of hippocampal progenitor HiB5 cells

In the present study, we investigated the effect of glucocorticoid on neuronal differentiation of hippocampal progenitor HiB5 cells. Dexamethasone (DEX), a synthetic glucocorticoid, inhibited platelet-derived growth factor (PDGF)-induced differentiation of HiB5 cells. The inhibitory effect of DEX was antagonized by RU486, a glucocorticoid receptor (GR) antagonist, indicating the GR-mediated processes. Nestin mRNA level was decreased and midsize neurofilament (NF-M) mRNA level was increased as a function of neuronal differentiation. DEX significantly blocked PDGF-induced down-regulation of nestin mRNA level, and up-regulation of NF-M mRNA level, which were similar to those of undifferentiated cells. DEX inhibited PDGF-induced activation of cyclic AMP-responsive element binding protein (CREB) and AP-1, suggesting that glucocorticoid interfered with signal transduction cascades linking the PDGF receptor and downstream transcription factors. Indeed, DEX reduced PDGF-induced phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2). Tyrosine phosphatase inhibitor reversed the effect of DEX on ERK1/2. In accordance with this finding, blockage of ERK1/2 signaling pathway with PD098059, a potent inhibitor for Ras/ERK pathway, mimicked the inhibitory effect of DEX on differentiation processes. Taken together, these results indicate that glucocorticoid inhibits PDGF-induced differentiation of hippocampal progenitor HiB5 cells by inhibiting the ERK1/2 signaling cascade via a tyrosine phosphatase-dependent mechanism.

Development of the choroid plexus

Mammalian choroid plexuses develop at four sites in the roof of the neural tube shortly after its closure, in the order IVth, lateral, and IIIrd ventricles. Bone morphogenetic proteins and tropomyosin are involved in early specification of these sites and in early plexus growth. Four stages of lateral ventricular plexus development have been defined, based on human and sheep fetuses; these depend mainly on the appearance of epithelial cells and presence or absence of glycogen. Other plexuses and other species are probably similar, although marsupials may lack glycogen. Choroid plexuses form one of the blood-brain barrier interfaces that control the brain's internal environment. The mechanisms involved combine a structural diffusion restraint (tight junctions between the plexus epithelial cells) and specific exchange mechanisms. In this review, it is argued that barrier mechanisms in the developing brain are different in important respects from those in the adult brain, but these differences do not necessarily reflect immaturity of the system. Absence of a barrier mechanism or presence of one not found in the adult may be a specialisation that is appropriate for that stage of brain development. Emphasis is placed on determining which mechanisms are present in the immature brain and relating them to brain development. One mechanism unique to the developing brain transfers specific proteins from blood to cerebrospinal fluid (CSF), via tubulocisternal endoplasmic reticulum in plexus epithelial cells. This results in a high concentration of proteins in early CSF. These proteins do not penetrate into brain extracellular space because of "strap" junctions between adjacent neuroependymal cells, which disappear later in development, when the protein concentration in CSF is much lower. Functions of the proteins in early CSF are discussed in terms of generation of a "colloid" osmotic pressure that expands the ventricular system as the brain grows; the proteins may also act as specific carriers and growth factors in their own right. The pathway for low molecular weight compounds, which is much more permeable in the developing choroid plexuses, appears also to be a transcellular one, rather than paracellular via tight junctions. There is thus good evidence to support a novel view of the state of development and functional significance of barrier mechanisms in the immature brain. It grows in an environment that is different from that of the rest of the fetus/neonate and that is also different in some respects from that of the adult. But these differences reflect developmental specialisation rather than immaturity.

Maternal adrenalectomy at the early onset of gestation impairs the postnatal development of the rat hippocampal formation: effects on cell numbers and differentiation, connectivity and calbindin-D28k immunoreactivity

The possible role of the maternal glucocorticoids on the postnatal development of the hippocampus was tested with bilateral adrenalectomy of pregnant rats. Surgery was performed 24 hr after sperm-positiveness was determined. The offspring from adrenalectomized mothers, compared with animals from control sham-operated mothers, showed decreased body weight and increased brain weight. The CA1 field of the hippocampus of these animals showed lower number of both Nissl-stained and Calbindin-immunoreactive cells, whereas the granule cell layer of the dentate gyrus showed higher number of both populations. Both types of cell numbers were statistically similar from postnatal Day 21, however, suggesting some compensatory mechanism. The neuronal populations of adrenalectomized animals appeared with a delay in the development of their dendritic trees, cytoplasmic differentiation, and synaptic connections. In the same way, both septohippocampal and hippocamposeptal projections appeared delayed in the adrenalectomized animals with respect to control ones by several days, mainly with regard to regressive events typical of the first 8 days of age. The ultrastructural study showed that every ADX postnatal group appeared more immature than the corresponding control group. These results suggest that gestational levels of maternal glucocorticoids (that were removed by adrenalectomy) influence the normal postnatal development of the hippocampus as reflected in neuron numbers and cell maturation, as well as in the developmental timing of the pattern of connectivity, and that this effect must be accomplished both in neuroepithelium and post-mitotic cells before the endogenous fetal hormones are secreted and reach concentrations capable to produce a response.

Perinatal changes in choroidal 15-hydroxyprostaglandin dehydrogenase: implications for prostaglandin removal from brain

We have previously shown in the sheep fetus at 0.7 and 0.9 gestation that the choroid plexus, unlike brain parenchyma, catabolizes prostaglandins (PGs). Peculiarly, in the choroid plexus, PGE(2) catabolism persists throughout the neonatal period to abate in the adult, while PGF(2alpha) catabolism abates shortly after birth. To explain this differential behavior and elucidate the function of catabolic enzymes, we examined the cellular location and activity of the rate-limiting enzyme for PGE(2) and PGF(2alpha) catabolism, 15-hydroxyprostaglandin dehydrogenase (15-PGDH). Immunofluorescence histochemistry and immunogold electronmicroscopy revealed abundant 15-PGDH expression in the epithelial cytosol close to the brush-border membrane at 0.7 and 0.9 gestation. In contrast, at 5 and 15 days postnatal, 15-PGDH was found throughout the cytosol of stromal fibroblasts. No staining was observed at either location in pregnant adults. PGF(2alpha) catabolism was minimal in the total homogenate and 100000xg supernatant of the fetal choroid plexus at 0.7 and 0.9 gestation, while PGE(2) catabolism was evident at 0.7 gestation only. In contrast, both PGs were catabolized in minced specimens at either age. In conclusion, our study shows immunoreactive 15-PGDH in the choroid plexus from fetal and neonatal, but not pregnant adult, sheep. Results suggest that PGE(2) catabolism is not as critically dependent as that of PGF(2alpha) on tissue integrity and 15-PGDH location. Given the key role being assigned to the choroid plexus in PG removal from brain, we speculate that persistence of PGE(2) catabolism into the early postnatal period protects against central respiratory depression caused by the compound during this susceptible stage of development.

Antenatal glucocorticoids and programming of the developing CNS

Glucocorticoids (GCs) are essential for many aspects of normal brain development. However, there is growing evidence from a number of species that exposure of the fetal brain to excess GC, at critical stages of development, can have life-long effects on behavior and neuroendocrine function. The hypothalamo-pituitary-adrenal axis, which is central to the integration of the individual's endocrine and behavioral response to stress, appears highly sensitive to excess GC exposure during development. A number of animal studies have shown that exposure to synthetic GCs in utero results in adult offspring that exhibit hyperactivity of the hypothalamo-pituitary-adrenal axis. This will have a long-term impact on health, inasmuch as increased life-long exposure to endogenous GC has been linked to the premature onset of diseases associated with aging. The mechanisms involved in the permanent programming of hypothalamo-pituitary-adrenal function and behavior are not well understood. Synthetic GCs are used extensively to promote pulmonary maturation in fetuses at risk of being delivered before term. Therefore, it is important that we understand the potential long-term consequences of prenatal GC exposure on brain development as well as the underlying mechanisms involved. This review will explore the current state of knowledge in this rapidly expanding field.

Inhibition of 11beta-hydroxysteroid dehydrogenase, the foeto-placental barrier to maternal glucocorticoids, permanently programs amygdala GR mRNA expression and anxiety-like behaviour in the offspring

Glucocorticoids may underlie the association between prenatal stress, low birth weight and adult stress-associated disorders, e.g. hypertension and type 2 diabetes, increased hypothalamic-pituitary-adrenal (HPA) activity and affective dysfunction. Normally, 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) rapidly inactivates glucocorticoids in placenta and many foetal tissues, thus acting as a 'barrier' to maternal steroids. We investigated the effect of inhibiting foeto-placental 11beta-HSD in rats, using carbenoxolone (CBX), on subsequent HPA activity and regulation and stress-induced behaviour in adult offspring. Pregnant Wistar rats were injected with CBX (12.5 mg s.c.) or vehicle daily throughout pregnancy. CBX treatment reduced birth weight. Adult offspring of CBX-treated dams had persistently reduced body weight, increased basal corticosterone (CORT) levels, increased corticotropin-releasing hormone (CRH) and reduced glucocorticoid receptor (GR) mRNA in the hypothalamic paraventricular nucleus, though hippocampal GR and mineralocorticoid receptor (MR) mRNA expression were unaltered. In addition, these animals showed less grooming and rearing in an open field and reduced immobility in a forced swim test, and had increased GR mRNA expression in the basolateral (BLA), central (CEA) and medial (MEA) nuclei of the amygdala, with unaltered MR mRNA. These data suggest that disturbance of the foeto-placental enzymatic barrier to maternal glucocorticoids reduces birth and body weight, and produces permanent alterations of the HPA axis and anxiety-like behaviour in aversive situations. The behavioural and HPA effects may reflect GR gene programming in amygdala and hypothalamus, respectively. Foetal overexposure to endogenous glucocorticoids (prenatal stress or reduced activity of foeto-placental 11beta-HSD) may represent a common link between the prenatal environment, foetal growth and adult neuroendocrine and affective disorders.

Calbindin-D28k- and astroglial protein-immunoreactivities, and ultrastructural differentiation in the prenatal rat cerebral cortex and hippocampus are affected by maternal adrenalectomy

Maternal adrenal steroid hormones have been proven to be crucial for lung and adrenal prenatal maturation. These hormones mediate the effects of prenatal stress crossing the placenta and influencing the development of the hypothalamus-pituitary-adrenal axis of fetuses. In the present study, we have compared the prenatal development of fetuses from adrenalectomized mothers (ADX group) and from sham-operated mothers. We have used immunohistochemistry for calcium binding-protein Calbindin-D28k, astroglial proteins vimentin and glial fibrillary acidic protein (GFAP), and the ultrastructural differentiation of the cerebral cortex and hippocampus to measure putative differences. The ontogeny of the Calbindin-D28k immunoreactivity was delayed, as transient Calbindin-positive neuronal populations in the ADX group disappeared later during development as compared to that of control animals both in cerebral cortex and hippocampus; cell counts revealed that ADX animals had a significantly higher number of Calbindin-positive cells than controls in the cerebral cortex, while that number was lower in ADX fetuses' hippocampus. Cerebral cortex of ADX animals also had a scattered distribution of stained cells compared with controls, while the hippocampi of the ADX animals had an impaired migration of marginal zone interneurons. No GFAP immunoreactivity was found in the studied prenatal stages. Instead, vimentin-immunoreactivity appeared more profusely distributed throughout the cerebral cortex, in the ADX group than in control animals. At the ultrastructural level, no remarkable differences were found before E20, when a higher undifferentiation in the ADX group, in both cerebral cortex and hippocampus, was evident. The results show for the first time the vulnerability of the prenatal rat brain to maternal adrenalectomy and the necessity of maternal glucocorticoids for encephalic development.

Dynamic changes in glucocorticoid and mineralocorticoid receptor mRNA in the developing guinea pig brain

The guinea pig has a high degree of neurological maturity at birth. Since glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) are central to several aspects of brain and neuroendocrine development, we examined the hypothesis that development of central GR and MR systems takes place during fetal life, in species which give birth to mature young. Fetal guinea pigs were retrieved on gestational days (gd) 40-45, 50-55, 60-65. A group of 7-day old neonates was also euthanized. Levels of GR and MR mRNA were determined by in situ hybridization followed by computerized image analysis. MR mRNA was confined to limbic structures, and was present at high levels in the hippocampus and dentate gyrus by gd40. Hippocampal MR mRNA levels decreased with the progression of gestation. GR mRNA was more widely distributed, with highest levels being expressed in the cingulate cortex, hippocampus, amygdala and hypothalamic paraventricular nucleus (PVN). In the hippocampus, GR mRNA levels increased with progression of gestation, attaining highest levels near term. In contrast to the hippocampus, GR mRNA levels were highest in the PVN at gd40-45, but decreased dramatically in the last 25 days of gestation. In conclusion, there are dynamic site-specific changes in the expression of corticosteroid receptors in the brain of the fetal guinea pig, at the time of most rapid brain growth. The decreases in GR mRNA levels in the PVN in late gestation likely facilitate the simultaneous increases in ACTH and cortisol that occur near term, and which are critical for the delivery of viable young.

Distinct ontogeny of glucocorticoid and mineralocorticoid receptor and 11beta-hydroxysteroid dehydrogenase types I and II mRNAs in the fetal rat brain suggest a complex control of glucocorticoid actions

Glucocorticoids (GCs) act via intracellular mineralocorticoid (MR) and glucocorticoid receptors (GR). However, it has recently been recognized that GC access to receptors is determined by the presence of tissue-specific 11beta-hydroxysteroid dehydrogenases (11beta-HSDs) that catalyze the interconversion of active corticosterone and inert 11-dehydrocorticosterone. 11beta-HSD type 1 (11beta-HSD1) is a bidirectional enzyme in vitro that acts predominantly as a reductase (regenerating corticosterone) in intact neurons. In contrast, 11beta-HSD type 2 (11beta-HSD2) is a higher affinity exclusive dehydrogenase that excludes GCs from MR in the kidney, producing aldosterone-selectivity in vivo. We have examined the ontogeny of 11beta-HSD mRNAs and enzyme activity during prenatal brain development and correlated this with GR and MR mRNA development. These data reveal that (1) 11beta-HSD2 mRNA is highly expressed in all CNS regions during midgestation, but expression is dramatically reduced during the third trimester except in the thalamus and cerebellum; (2) 11beta-HSD2-like activity parallels closely the pattern of mRNA expression; (3) 11beta-HSD1 mRNA is absent from the CNS until the the third trimester, and activity is low or undectectable; and (4) GR mRNA is highly expressed throughout the brain from midgestation, but MR gene expression is absent until the last few days of gestation. High 11beta-HSD2 at midgestation may protect the developing brain from activation of GR by GCs. Late in gestation, repression of 11beta-HSD2 gene expression may allow increasing GC activation of GR and MR, permitting key GC-dependent neuronal and glial maturational events.

Prenatal dexamethasone exposure alters brain monoamine metabolism and adrenocortical response in rat offspring

In this study, it has been clearly demonstrated that prenatal dexamethasone treatment (Dex; 0.05 mg/kg on gestational days 17, 18, and 19) resulted in the significant reductions of 5-hydroxytryptamine (5-HT) turnover in four brain regions, including the neocortex, hippocampus, hypothalamus, and midbrain + pons-medulla (M + P-M) but not in the striatum in the offspring at 3 and 14 wk of life, as well as dopamine turnover in the hypothalamus. [3H]paroxetine binding densities were increased in the hypothalamus and M + P-M at 14 wk of life, which corresponded to increased 5-HT contents in both regions. On the other hand, significantly lower norepinephrine contents in the neocortex and hippocampus were observed in the Dex group compared with the control group at 14 wk of life. In addition, the exposure to new environmental condition elevated blood corticosterone levels and enhanced behavioral activities to a greater extent in the Dex group than in controls at 7 wk of life, suggesting that elevated glucocorticoid levels during the pregnancy mimicked prenatal mild stress, producing developmental alterations in brain monoamine metabolism, endocrine response, and behavior in adult offspring.

Glucocorticoid receptor gene expression during rat embryogenesis. An in situ hybridization study

Glucocorticoids play an important role in embryonic development. The existence of sufficient amounts of their receptors during rodent embryogenesis has proved to be an absolute necessity for the physiological growth of the animal. We have analyzed the pattern of glucocorticoid receptor gene expression in the rat embryo through embryonic days 12 to 17, by using in situ hybridization histochemistry. Glucocorticoid receptor mRNA is present in the rat liver on embryonic day (E) 12, and by E13 the signal can also be detected in several other tissues, such as the lung, the heart, the mesonephros, the sclerotomes, the thymus and Rathke's pouch. Glucocorticoid receptor gene expression was quite ubiquitous in tissue derivatives of all three germ layers and appeared to vary in intensity within the same tissue during embryogenesis. These variations in the level of receptor gene expression paralleled the developmental stage of each tissue: Intense labelling was detected just prior to the final differentiation step of a structure. Upon differentiation, cell populations highly expressing glucocorticoid receptor gene in the previous stage were found to have reduced amounts of the receptor mRNA. Our results support a morphogenetic role for glucocorticoids during embryogenesis.