Complex regulation of the expression of the polysialylated form of the neuronal cell adhesion molecule by glucocorticoids in the rat hippocampus

Fetal adaptation to stress: Part II. Evolutionary aspects; stress-induced hippocampal damage; long-term effects on behavior; consequences on adult health

Humans share adaptative capacities to stress with other species, as demonstrated on amphibians: the physiological response to experimental water volume and food deprivation results in the activation of the endocrine axes that drive metamorphosis, in particular the neuroendocrine stress system. Unfavorable effects may, however, occur, probably due to inappropriate timing and/or duration of stress: recent experiments are converging to show a profound impairment of hippocampal functioning in the offspring of mothers exposed to prenatal stress. Moreover, fetal changes are likely one of the risk factors for a number of diseases in adulthood.

Sialyltransferase ST8Sia-II Assembles a Subset of Polysialic Acid That Directs Hippocampal Axonal Targeting and Promotes Fear Behavior

Polysialic acid (PSA) is a post-translational protein modification that is widely expressed among neural cell types during development. Found predominantly on the neural cell adhesion molecule (NCAM), PSA becomes restricted to regions of neurogenesis and neuroplasticity in the adult. In the mammalian genome, two polysialyltransferases termed ST8Sia-II and ST8Sia-IV have been hypothesized to be responsible for the production of PSA in vivo. Approaches to discover PSA function have involved the application of endoneuraminidase-N to remove PSA and genetic manipulations in the mouse to deplete either NCAM or ST8Sia-IV. Here we report the production and characterization of mice deficient in the ST8Sia-II polysialyltransferase. We observed alterations in brain PSA expression unlike those observed in mice lacking ST8Sia-IV. This included a PSA deficit in regions of neurogenesis but without changes in the frequency of mitotic neural progenitor cells. In further contrast with ST8Sia-IV deficiency, loss of ST8Sia-II did not impair hippocampal synaptic plasticity but instead resulted in the misguidance of infrapyramidal mossy fibers and the formation of ectopic synapses in the hippocampus. Consistent with studies of animal models bearing these morphological changes, ST8Sia-II-deficient mice exhibited higher exploratory drive and reduced behavioral responses to Pavlovian fear conditioning. PSA produced by the ST8Sia-II polysialyltransferase modifies memory and behavior processes that are distinct from the neural roles reported for ST8Sia-IV. This genetic partitioning of PSA formation engenders discrete neurological processes and reveals that this post-translational modification forms the predominant basis for the multiple functions attributed to the NCAM glycoprotein.

Blockade of CRF(1) or V(1b) receptors reverses stress-induced suppression of neurogenesis in a mouse model of depression

Repeated exposure to stress is known to induce structural remodelling and reduction of neurogenesis in the dentate gyrus. Corticotrophin-releasing factor (CRF) and vasopressin (AVP) are key regulators of the stress response via activation of CRF(1) and V(1b) receptors, respectively. The blockade of these receptors has been proposed as an innovative approach for the treatment of affective disorders. The present study aimed at determining whether the CRF(1) receptor antagonist SSR125543A, the V(1b) receptor antagonist SSR149415, and the clinically effective antidepressant fluoxetine may influence newborn cell proliferation and differentiation in the dentate gyrus of mice subjected to the chronic mild stress (CMS) procedure, a model of depression with predictive validity. Repeated administration of SSR125543A (30 mg/kg i.p.), SSR149415 (30 mg/kg i.p.), and fluoxetine (10 mg/kg i.p.) for 28 days, starting 3 weeks after the beginning of the stress procedure, significantly reversed the reduction of cell proliferation produced by CMS, an effect which was paralleled by a marked improvement of the physical state of the coat of stressed mice. Moreover, mice subjected to stress exhibited a 53% reduction of granule cell neurogenesis 30 days after the end of the 7-week stress period, an effect which was prevented by all drug treatments. Collectively, these results point to an important role of CRF and AVP in the regulation of dentate neurogenesis, and suggest that CRF(1) and V(1b) receptor antagonists may affect plasticity changes in the hippocampal formation, as do clinically effective antidepressants.

Environmentally induced long-term structural changes: cues for functional orientation and vulnerabilities

Environmental challenges profoundly modify phenotypes and disrupt inherent developmental programs both at functional and structural levels. As an example, we have studied the impact of these environmental influences on adult neurogenesis in the dentate gyrus. Neurogenesis results from an inherent program, participates to hippocampal network organization and, as a consequence, to the various functional abilities depending on this region, including memories. In preclinical studies of aging we have shown that phenotypes vulnerable to the development of spatial memory disorders are characterized by lower hippocampal neurogenesis. We have hypothesized that these interindividual variations in functional expression of neurogenesis in senescent subjects could be predicted early in life. Indeed, a behavioral response (novelty-induced locomotor reactivity) and a biological trait (hypothalamo-pituitary-adrenal axis activity), which are predictive of cognitive impairments later in life, are related to neurogenesis in young adult rats. This suggests that subjects starting off with an impaired neurogenesis, here rats that are high reactive to stress, are predisposed for the development of age-related cognitive disorders. We have further shown that these inter-individual differences result from early deleterious life events. Indeed, prenatal stress orients neurogenesis in pathological ways for the entire life, and precipitates age-related cognitive impairments. Altogether these data suggest first that hippocampal neurogenesis plays a pivotal role in environmentally-induced vulnerability to the development of pathological aging, and second that environmental challenges and life events orient structural developments, leading to different phenotypes.

Chronic restraint stress and chronic corticosterone treatment modulate differentially the expression of molecules related to structural plasticity in the adult rat piriform cortex

Stress and stress-related hormones induce structural changes in neurons of the adult CNS. Neurons in the hippocampus, the amygdala and the prefrontal cortex undergo neurite remodeling after chronic stress. In the hippocampus some of these effects can be mimicked with chronic administration of adrenal steroids. These changes in neuronal structure may be mediated by certain molecules related to plastic events such as the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). The expression of PSA-NCAM persists in the adult hippocampus and it is up-regulated after chronic stress. The piriform cortex also displays considerable levels of PSA-NCAM during adulthood and indirect evidence suggests that it may also be the target of stress and stress related-hormones. Using immunohistochemistry we have studied the expression of PSA-NCAM and doublecortin (DCX; another protein implicated in neuronal structural plasticity) in the piriform cortex of adult rats subjected either to 21 days of chronic restraint stress or to oral corticosterone administration during the same period. Our results indicate that chronic stress and chronic corticosterone administration have differential effects on the expression of PSA-NCAM and DCX. While chronic stress increases the number of PSA-NCAM- and DCX-immunoreactive cells in the piriform cortex layer II, chronic corticosterone administration decreases these numbers. These findings indicate that stress and adrenal steroids affect the piriform cortex and suggest that in this region, as in the hippocampus, they may induce structural changes. This is a potential mechanism by which stress and corticosterone modulate functions of this limbic region, such as its participation in olfactory memory.

Plasticity following Injury to the Adult Central Nervous System: Is Recapitulation of a Developmental State Worth Promoting?

The adult central nervous system (CNS) appears to initiate a transient increase in plasticity following injury, including increases in growth-related proteins and generation of new cells. Recent evidence is reviewed that the injured adult CNS exhibits events and patterns of gene expression that are also observed during development and during regeneration following damage to the mature peripheral nervous system (PNS). The growth of neurons during development or regeneration is correlated, in part, with a coordinated expression of growth-related proteins, such as growth-associated-protein-43 (GAP-43), microtubule-associated-protein-1B (MAP1B), and polysialylated-neural-cell-adhesion-molecule (PSA-NCAM). For each of these proteins, evidence is discussed regarding its specific role in neuronal development, signals that modify its expression, and reappearance following injury. The rate of adult hippocampal neurogenesis is also affected by numerous endogenous and exogenous factors including injury. The continuing study of developmental neurobiology will likely provide further gene and protein targets for increasing plasticity and regeneration in the mature adult CNS.

Implication of corticosteroid receptors in the regulation of hippocampal structural plasticity

The dentate gyrus is one of the few areas of the adult brain that continues to produce neurons and to express the embryonic polysialylated isoforms of neuronal cell adhesion molecules (PSA-NCAM). The stress hormone corticosterone exerts a complex modulation on neurogenesis and PSA-NCAM, and previous studies have shown that mature granule cells require corticosterone for their survival. Thus, the aim of our work was to investigate the respective role of the different corticosteroid receptors on these three parameters in adrenalectomized rats. It was found that treatment with a low dose of the mineralocorticoid receptor agonist, aldosterone, prevents only the adrenalectomy-induced increase in cell death. Treatment with a higher dose of aldosterone normalized cell proliferation whereas PSA-NCAM expression was normalized only by treatment with the glucocorticoid receptor agonist, RU 28362. It is concluded that stimulation of the mineralocorticoid receptor is sufficient to mediate the effects of corticosterone on neurogenesis and to protect mature cells from cell death whereas stimulation of the glucocorticoid receptor is necessary to modulate PSA-NCAM expression.

Modulation of hippocampal NCAM polysialylation and spatial memory consolidation by fear conditioning

BACKGROUND

Cell adhesion molecule function is involved in hippocampal synaptic plasticity and associated with memory consolidation. At the infragranular zone of the dentate gyrus, neurons expressing the polysialylated form of the neural cell adhesion molecule (NCAM PSA) transiently increase their frequency 12 hours after training in different tasks.

METHODS

Using immunohistochemical procedures, we investigated NCAM polysialylation following training in a contextual fear conditioning paradigm that employed increasing shock intensities to separately model stressful and traumatic experiences in adult male Wistar rats.

RESULTS

Fear conditioning with a stressful.4-mA stimulus resulted in an increased frequency of dentate polysialylated neurons, the magnitude of which was indistinguishable from that observed following water maze training. By contrast, training with a traumatic 1-mA stimulus resulted in a significant decrease in the frequency of polysialylated neurons at the 12 hours posttraining time. Whereas sequential training in the water maze paradigm followed by fear conditioning resulted in potentiated consolidation of spatial information when conditioning involved a.4-mA stimulus, amnesia for spatial learning occurred when conditioning was performed with a 1-mA stimulus.

CONCLUSIONS

These results suggest traumatic fear conditioning suppresses NCAM-PSA-mediated plasticity and the concomitant inability to store the trace of recently acquired information.

Extra-adrenal production of corticosteroids

1. The major corticosteroids aldosterone and cortisol (corticosterone in rodents) are secreted from the adrenal cortex under the regulation of the renin-angiotensin system and the hypothalamic-pituitary-adrenal axis. 2. In addition to their accepted roles in such processes as blood pressure regulation, glycogenesis, hepatic glyconeogenesis and immunosuppression, the corticosteroids have been implicated in the development of cardiac fibrosis, modulation of hippocampal neuron excitability, memory formation and neurodegeneration. 3. The advent of sensitive molecular biological techniques has produced a wealth of evidence to support the existence of extra-adrenal corticosteroidogenic systems. Most attention has been paid to the cardiovascular system and the central nervous system, where the full array of enzymes required for the de novo synthesis of corticosteroids from cholesterol has been identified. 4. Although the evidence for local corticosteroid production is strong, the quantities of steroid would be small compared with adrenal production. Therefore, it is still a matter of debate as to whether extra-adrenal corticosteroids are of any physiological significance. This will depend on factors such as local concentration, proximity to target cells and, possibly, to tissue-specific control mechanisms.

Repeated restraint stress suppresses neurogenesis and induces biphasic PSA-NCAM expression in the adult rat dentate gyrus

Chronic restraint stress has been shown to induce structural remodelling throughout the interconnected dentate gyrus-CA3 fields. To find out how this stressor affects the rate of adult hippocampal neurogenesis, we subjected rats to acute or chronic restraint stress and assessed the proliferation, survival and differentiation of newly born cells in the dentate gyrus. We also examined polysialylated neural cell adhesion molecule expression, a molecule normally expressed in immature neurons and important for morphological plasticity. The results show that acute restraint stress did not change either the proliferation of dentate gyrus precursor cells or the expression of polysialylated neural cell adhesion molecule, whereas 3 weeks of chronic restraint stress suppressed proliferation by 24% and increased polysialylated neural cell adhesion molecule expression by 40%. The study was extended for an additional 3 weeks to trace the survival and development of the cells born after the initial 3 weeks of restraint. Rats subjected to 6 weeks of daily restraint stress exhibited suppressed cell proliferation and attenuated survival of the recently born cells after the extended time course, resulting in a 47% reduction of granule cell neurogenesis. Furthermore, 6 weeks of chronic stress significantly reduced the total number of granule cells by 13% and the granule cell layer volume by 5%. Expression of polysialylated neural cell adhesion molecule followed a biphasic time course, displaying a significant up-regulation after 3 weeks of daily restraint stress that was lost after 6 weeks of stress. These studies may help us understand the basis for hippocampal shrinkage and raise questions about the ultimate reversibility of the effects of chronic stress.

Persistently high corticosterone levels but not normal circadian fluctuations of the hormone affect cell proliferation in the adult rat dentate gyrus

The dentate gyrus is one of the few brain structures where new neurons are added throughout adulthood in several mammalian species, including humans. Production of new neurons can be regulated by factors which influence cell proliferation or newborn cell survival. Supplementation or deprivation of glucocorticoids, adrenal hormones involved in the response to stress, affect cell proliferation, leading to a decrease or an increase, respectively, in the number of newborn cells. Glucocorticoid secretion under physiological conditions follows a circadian pattern. We thus investigated a possible relationship between cell proliferation and circadian oscillations of corticosterone secretion in the adult rat dentate gyrus. Corticosterone is the species-specific glucocorticoid hormone of the rat. 5-Bromo-2'-deoxyuridine was used to evaluate cell proliferation at 4 different time points in the light-dark cycle. No correlation was found between corticosterone circadian oscillations and cell proliferation in the adult dentate gyrus. In contrast, constantly high corticosterone levels, obtained by implanting corticosterone pellets, decreased cell proliferation in particular zones of the dentate gyrus, i.e. the hilus and the superior blade of the granule cell layer. These findings show that a short, physiologically occurring exposure to high corticosterone levels does not influence cell proliferation, whereas a lengthy exposure to this hormone does induce anatomically localized proliferative changes.

Glucocorticoids, hypothalamo-pituitary-adrenal (HPA) development, and life after birth

Approximately 10% of women in North America are treated with synthetic glucocorticoid (sGC) between 24 and 32 weeks of pregnancy (term approximately 40 weeks), to promote lung maturation in fetuses at risk of preterm delivery. Such therapy is highly effective in reducing the frequency of respiratory complications, and as a result, repeated course treatment has become widespread. Nothing is known about the impact of repeated sGC treatment on neuroendocrine development in the human, or if specific time windows of increased sensitivity exist. Glucocorticoids 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 glucocorticoid can have life-long effects on behaviour and neuroendocrine function. We have shown that exposure of fetuses to sGC in late gestation permanently alters HPA function in pre-pubertal, post-pubertal, and aging offspring, in a sex-dependent manner. These effects are linked to changes in central glucocorticoid feedback. Prenatal glucocorticoid exposure also leads to modification of HPA-associated behaviours and organ morphology, as well as altered regulation of other neuroendocrine systems. Permanent changes in HPA function will have a long-term impact on health, since elevated cumulative exposure to endogenous glucocorticoid has been linked to the premature onset of pathologies associated with aging.

Nicotine self-administration impairs hippocampal plasticity

Nicotine, the neuroactive compound responsible for tobacco addiction, is primarily believed to have beneficial effects on the adult brain. However, in heavy smokers, abstinence from nicotine is accompanied by cognitive impairments that suggest adverse effects of nicotine on brain plasticity. For this reason, we studied changes in plasticity-related processes in the dentate gyrus (DG) of the hippocampal formation of animals trained to self-administer nicotine. The DG was chosen because it undergoes profound plastic rearrangements, many of which have been related to memory and learning performances. In this region, we examined the expression of the polysialylated (PSA) forms of neural cell adhesion molecule (NCAM), PSA-NCAM, neurogenesis, and cell death by measuring the number of pyknotic cells. It was found that nicotine self-administration profoundly decreased, in a dose-dependent manner, the expression of PSA-NCAM in the DG; a significant effect was observed at all the doses tested (0.02, 0.04, and 0.08 mg/kg per infusion). Neurogenesis was also decreased in the DG, but a significant effect was observed only for the two highest doses of nicotine. Finally, the same doses that decreased neurogenesis also increased cell death. These results raise an important additional concern for the health consequences of nicotine abuse and open new insight on the possible neural mechanisms of tobacco addiction.

The common properties of neurogenesis in the adult brain: from invertebrates to vertebrates

Until recently, it was believed that adult brains were unable to generate any new neurons. However, it is now commonly known that stem cells remain in the adult central nervous system and that adult vertebrates as well as adult invertebrates are currently adding new neurons in some specialized structures of their central nervous system. In vertebrates, the subventricular zone and the dentate gyrus of the hippocampus are the sites of neuronal precursor proliferation. In some insects, persistent neurogenesis occurs in the mushroom bodies, which are brain structures involved in learning and memory and considered as functional analogues of the hippocampus. In both vertebrates and invertebrates, secondary neurogenesis (including neuroblast proliferation and neuron differentiation) appears to be regulated by hormones, transmitters, growth factors and environmental cues. The functional implications of adult neurogenesis have not yet been clearly demonstrated and comparative study of the various model systems could contribute to better understand this phenomenon. Here, we review and discuss the common characteristics of adult neurogenesis in the various animal models studied so far.

Antenatal glucocorticoids and the developing brain: mechanisms of action

Glucocorticoids are critical for normal brain development. There is no doubt that prenatal treatment with synthetic glucocorticoid affords great benefit to the preterm infant. However, animal studies, now carried out in many species, indicate that there may be some long-term physiological costs of early exposure to excess glucocorticoid, and that these appear sex-dependent. Further, the effects may not become apparent until later life. Given the dynamics of corticosteroid receptor systems in late gestation, it is likely that there are critical windows of development when specific regions of the brain are more sensitive to the influence of synthetic glucocorticoid. Once such windows have been identified it will be possible to target prenatal treatments, so as to maximize benefit and reduce risk of long-term effects. Notwithstanding, the data reviewed below indicate that caution should be exercised in the use of multiple course glucocorticoid therapy during pregnancy.

Induction of highly polysialylated neural cell adhesion molecule (PSA-NCAM) in postischemic gerbil hippocampus mainly dissociated with neural stem cell proliferation

We investigated a possible expression of highly polysialylated neural cell adhesion molecule (PSA-NCAM) in gerbil hippocampus after 5 min of transient global ischemia in association to the proliferation of neural stem cell labeled with bromodeoxyuridine (BrdU). The number of PSA-NCAM positive cells increased in the granule cell layer (GCL) of dentate gyrus (DG) by 1.9 to 2.7-fold at 10 and 20 days after the reperfusion. The number of BrdU-labeled cells increased mainly in the subgranular zone of DG by 7.2 to 8.0-fold at 5 and 10 days after the reperfusion. Immunofluorescence for PSA-NCAM and BrdU showed that the majority of DG cells were not double labeled, while one or two cells per section were double labeled in the deepest portion of the GCL only at 10 days after the reperfusion. These results suggest different predominant spatial distribution and chronological change of PSA-NCAM positive and BrdU-labeled cells in DG after transient ischemia.

Prenatal stress produces learning deficits associated with an inhibition of neurogenesis in the hippocampus

Early experiences such as prenatal stress significantly influence the development of the brain and the organization of behavior. In particular, prenatal stress impairs memory processes but the mechanism for this effect is not known. Hippocampal granule neurons are generated throughout life and are involved in hippocampal-dependent learning. Here, we report that prenatal stress in rats induced lifespan reduction of neurogenesis in the dentate gyrus and produced impairment in hippocampal-related spatial tasks. Prenatal stress blocked the increase of learning-induced neurogenesis. These data strengthen pathophysiological hypotheses that propose an early neurodevelopmental origin for psychopathological vulnerabilities in aging.

Regulation of hippocampal cell adhesion molecules NCAM and L1 by contextual fear conditioning is dependent upon time and stressor intensity

Cell adhesion molecules (CAMs) of the immunoglobulin superfamily, NCAM and L1, as well as the post-translational addition of alpha-2, 8-linked polysialic acid (PSA) homopolymers to NCAM (PSA-NCAM), have been implicated in the neural mechanisms underlying memory formation. Given that the degree of stress elicited by the training situation is one of the key factors that influence consolidation processes, this study questioned whether training rats under different stressor intensities (0.2, 0.4, or 1 mA shock intensity) in a contextual fear conditioning task might regulate subsequent expression of NCAM, PSA-NCAM and L1 in the hippocampus, as evaluated immediately after testing rats for conditioning at 12 and 24 h after training. Behavioural inhibition (evaluated as a 'freezing' index) at testing and post-testing plasma corticosterone levels were also assessed. The results showed that 12 h post-training, conditioned animals displayed reduced NCAM, but increased L1, expression. At this time point, the group trained at the highest shock intensity (1 mA) also presented decreased PSA-NCAM expression. Analyses performed 24 h post-training indicated that the 1 mA group exhibited increased NCAM and L1 expression, but decreased expression of PSA-NCAM levels. In addition, L1 values that presented a shock intensity-dependent U-shaped pattern were also increased in the group trained at the lowest shock condition (0.2 mA) and remained unchanged in the intermediate shock condition (0.4 mA). Freezing and corticosterone values at both testing times were positively related with shock intensity experienced at training. Therefore, our results show a complex regulation of CAMs of the immunoglobulin superfamily in the hippocampus that depends upon stressor intensity and time factors. In addition, the pattern of CAMs expression found in the 1 mA group (which is the one that shows higher post-training corticosterone levels and develops the stronger and longer-lasting levels of fear conditioning) supports the view that, after a first phase of synaptic de-adherence during consolidation, NCAM and L1 might participate in the stabilization of selected synapses underlying the establishment of long-term memory for contextual fear conditioning, and suggests that glucocorticoids might play a role in the observed regulation of CAMs.

Opiates inhibit neurogenesis in the adult rat hippocampus

Recent work implicates regulation of neurogenesis as a form of plasticity in the adult rat hippocampus. Given the known effects of opiates such as morphine and heroin on hippocampal function, we examined opiate regulation of neurogenesis in this brain region. Chronic administration of morphine decreased neurogenesis by 42% in the adult rat hippocampal granule cell layer. A similar effect was seen in rats after chronic self-administration of heroin. Opiate regulation of neurogenesis was not mediated by changes in circulating levels of glucocorticoids, because similar effects were seen in rats that received adrenalectomy and corticosterone replacement. These findings suggest that opiate regulation of neurogenesis in the adult rat hippocampus may be one mechanism by which drug exposure influences hippocampal function.

Regulation of neuronal cell adhesion molecule expression by NF-kappa B

The neuronal cell adhesion molecule (NCAM) is a key mediator of structural plasticity in the central nervous system, but the mechanisms that control its expression are unknown. Equally, although the transcription factor NF-kappaB is present in the brain, few NF-kappaB-regulated genes relevant for central nervous system function have been identified. We have previously demonstrated that NF-kappaB is activated in neuronal cultures treated with kainic acid or nitric oxide. We show here that kainic acid or nitric oxide also increase the levels of NCAM mRNA and protein in neurons and that this induction of NCAM expression is sensitive to dexamethasone and to antisense, but not missense, oligonucleotides designed to suppress NF-kappaB synthesis. Nitric oxide also stimulates protein binding to an NF-kappaB site in the promoter of the NCAM gene. This indicates that NF-kappaB, which has recently been implicated in synaptic plasticity and also in the etiology of neurodegenerative disease, plays a crucial role in the activity-dependent regulation of NCAM gene expression. In addition, since both NCAM and NF-kappaB are present in the post-synaptic density, this represents a route allowing direct communication between the synapse and the nucleus.

PSA-NCAM: an important regulator of hippocampal plasticity

The Neural Cell Adhesion Molecule (NCAM) serves as a temporally and spatially regulated modulator of a variety of cell-cell interactions. This review summarizes recent results of studies aimed at understanding its regulation of expression and biological function, thereby focussing on its polysialylated isoforms (PSA-NCAM). The detailed analysis of the expression of PSA and NCAM in the hippocampal mossy fiber system and the morphological consequences of PSA-NCAM deficiency in mice support the notion that the levels of expression of NCAM are important not only for the regulation and maintenance of structural changes, such as migration, axonal growth and fasciculation, but also for activity-induced plasticity. There is evidence that PSA-NCAM can specifically contribute to a presynaptic form of plasticity, namely long-term potentiation at hippocampal mossy fiber synapses. This is consistent with previous observations that NCAM-deficient mice show deficits in spatial learning and exploratory behavior. Furthermore, our data points to an important role of the hypothalamic-pituitary-adrenal axis, which is the principle adaptive response of the organism to environmental challenges, in the control of PSA-NCAM expression in the hippocampal formation. In particular, we evidence an inhibitory influence of corticosterone on PSA-NCAM expression.

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.

Glucocorticoids up-regulate the expression of glial fibrillary acidic protein in the rat suprachiasmatic nucleus

Immunoreactivity against glial fibrillary acidic protein (GFAP) was used as a dynamic index in adrenalectomized rats subjected or not to corticosterone replacement to investigate whether glucocorticoids may interact with astrocytes in the suprachiasmatic nucleus (SCN), the master component of the central circadian clock. GFAP staining in the SCN was significantly higher in rats having received implants that restored physiological plasma levels of corticosterone within diurnal or nocturnal limits than in non-normalized rats. The effects of corticosterone were similar in the parvocellular portion of the paraventricular nucleus but were opposite in the hippocampus, another major site of negative feed-back regulation of the hypothalamic-pituitary-adrenal axis, where a decreased GFAP staining was observed in discrete regions of the dentate gyrus. This indicates that glucocorticoids may positively or negatively regulate GFAP, depending on the target brain structure. In the SCN, that contains only few if any glucocorticoid receptors, indirect mechanisms that may involve serotoninergic neurons are probably responsible for the effects of corticosterone level. It is proposed that the corticosterone-induced increase in GFAP staining in that nucleus accounts for dynamic changes in neurone-astrocyte interactions that might occur in relation with natural fluctuations of glucocorticoids over the 24 h period.

Overexpression of the alpha2,6 (N) sialyltransferase enzyme in human and rat neural cell lines is associated with increased expression of the polysialic acid epitope

The function of the neural cell adhesion molecule, NCAM, is modulated by the expression of the N-linked polysialic acid (PSA) oligosaccharide chain, with PSA serving to decrease the adhesive potential of the protein backbone. In this study, we have generated clonal cells of the rat B104 and human SH-SY5Y neuroblastoma cell lines that over-express the alpha2,6(N) sialyltransferase (ST6N) enzyme in order to investigate the role of this enzyme in PSA biosynthesis. The clonal cells exhibited ST enzyme activities of up to 20-times control levels, which remained stable throughout the duration of the study. The increase in enzyme activity paralleled an increase in enzyme protein levels, as determined by Western blot analysis, and immunocytochemical analysis confirmed the Golgi localisation of the enzyme. The induction of PSA-NCAM expression in the cells expressing high levels of ST6N was confirmed both by using anti-PSA antisera and by specific digestion with endo-N-acetylneuraminidase E, whose actions are specific for alpha2, 8-linked PSA chains. These results demonstrate that the cellular ST6N activity serves to positively influence the expression of PSA in neuronal cells.

Behavioural trait of reactivity to novelty is related to hippocampal neurogenesis

The hippocampal formation is one of the brain areas where neurogenesis persists during adulthood, with new neurons being continuously added to the population of dentate granule cells. However, the functional implications of this neurogenesis are unknown. On the other hand, the hippocampal formation is particularly concerned with the detection of novelty, and there are indications that dentate granule cells play a significant role in this function. Recently, the existence of inter-individual differences in behavioural reactivity to novelty has been evidenced, related to differences in the reactivity of the hypothalamic-pituitary-adrenal axis (HPA). Rats that are highly reactive to novelty (HR) exhibit a prolonged corticosterone secretion in response to novelty and to stress when compared with low reactive rats (LR). Taking advantage of the existence of these inter-individual differences, we investigated whether neurogenesis in the dentate gyrus is correlated with the behavioural trait of reactivity to novelty. Rats were first selected according to their locomotor reactivity to a novel environment. Two weeks later, cell proliferation, evaluated by the incorporation of 5-bromo-2'-deoxyuridine (BrdU) in progenitors, was studied by immunohistochemistry. We found that cell proliferation in the dentate gyrus was negatively correlated with locomotor reactivity to novelty. Indeed, cell proliferation in LR rats was twice that observed in HR rats. In contrast, survival of nascent neurons was not influenced by the behavioural trait of reactivity to novelty. Using an unbiased stereology, we show that LR rats had more cells within the granule cell layer of the dentate gyrus than did HR rats. These results demonstrate the existence of inter-individual differences in neurogenesis and total granule cell number within the dentate gyrus. These differences in hippocampal plasticity can be predicted by the behavioural trait of reactivity to novelty.

Temporal and spacial relationships between PSA-NCAM-expressing, newly generated granule cells, and radial glia-like cells in the adult dentate gyrus

The granule cell layer of the adult dentate gyrus possesses two characteristics of an immature nervous system. The first is that granule cells continue to be generated in the innermost region of the granule cell layer, and newly generated and developing granule cells in the adult express highly polysialylated neural cell adhesion molecule (PSA-NCAM). PSA-NCAM-expressing apical dendrites have dynamically unstable processes such as irregular shafts and many stick-like or fan-shaped fine processes. The second is that radial glia-like cells expressing glial fibrillary acidic protein (GFAP) remain in a similar region of the granular layer. The numbers of PSA-NCAM-expressing granule cells and GFAP-expressing radial glia-like cells show a parallel age-dependent decrease during aging. Moreover, by using confocal laser scanning microscopy and immunoelectron microscopy, we demonstrated that PSA-NCAM-expressing dendrites and GFAP-expressing radial processes are partly in contact with each other, and occasionally the radial glial processes envelop the PSA-NCAM-positive dendritic processes. The temporal and spatial relationship between the two immature elements suggests that the processes of the radial glia-like cells are closely associated with the dendritic growth of the newly generated granule cells in the adult dentate gyrus and that these two immature features of neurons and glia in the dentate gyrus diminish with age.

Different polysialic acid-neural cell adhesion molecule expression patterns in distinct types of mossy fiber boutons in the adult hippocampus

Dentate granule cells continue to be generated in the adult hippocampus, and the newly generated granule cells express the highly polysialylated neural cell adhesion molecule (PSA-NCAM), which has been shown to be important in neural development and plasticity. In the present study, the PSA-NCAM expression pattern in morphologically distinct types of mossy fiber boutons in adult rats was examined by immunoelectron and confocal laser scanning microscopy. Although many unmyelinated axons within the mossy fiber bundles expressed PSA-NCAM, most of the mature type of mossy fiber boutons were negative for polysialic acid (PSA) but positive for NCAM peptides, suggesting that NCAM is less polysialylated in the mature mossy fiber boutons. On the other hand, PSA was expressed by small round varicosities, irregularly shaped boutons, and the presumptive immature type of mossy fiber boutons. The PSA-positive small boutons were found to make synaptic contacts with CA3 pyramidal cells and nonpyramidal cells. The PSA-expressing presumptive immature boutons contained fewer clear synaptic vesicles and mitochondria, and, in some instances, they were invaginated by the PSA-positive, finger-like dendritic outgrowths arising from the dendritic shafts of the pyramidal cells, which are known to develop into a mossy fiber bouton-thorny excrescence complex. These findings indicate that distinct types of the mossy fiber boutons possess different PSA expression patterns in the adult hippocampus, and they also imply that PSA expression allows the mossy fibers to have the ability to regulate the bouton formation and remodeling that accompany synapse formation at the contact sites with pyramidal cells and nonpyramidal cells.

Adrenalectomy increases neurogenesis but not PSA-NCAM expression in aged dentate gyrus

Ageing is accompanied by a decline in neurogenesis and in polysialylated isoforms of neural cell adhesion molecule (PSA-NCAM) expression within the hippocampus and by elevated basal levels of circulating corticosterone. In a companion study, we demonstrated that suppression of corticosterone by adrenalectomy increased neurogenesis and PSA-NCAM expression in the dentate gyrus of adult rats. Here we show that adrenalectomy increased neurogenesis in this structure in old rats, as measured by the incorporation of 5-bromo-2'-deoxyuridine in neuronal progenitors. This effect was prevented by corticosterone replacement. In contrast, PSA-NCAM expression remained unchanged in comparison with controls. Thus, in the aged brain, stem cells are still present and able to enter the cell cycle. This may point to ways of protecting or treating age-related cognitive impairments.