Mapping of the distribution of polysialylated neural cell adhesion molecule throughout the central nervous system of the adult rat: an immunohistochemical study

Role of the cholinergic system in regulating survival of newborn neurons in the adult mouse dentate gyrus and olfactory bulb

Neurogenesis in the subgranular zone of the hippocampal dentate gyrus and olfactory bulbs continues into adulthood and has been implicated in the cognitive function of the adult brain. The basal forebrain cholinergic system has been suggested to play a role in regulating neurogenesis as well as learning and memory in these regions. Herein, we report that highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive immature cells as well as neuronal nuclei (NeuN)-positive mature neurons in the dentate gyrus and olfactory bulb express multiple acetylcholine receptor subunits and make contact with cholinergic fibers. To examine the function of acetylcholine in neurogenesis, we used donepezil (Aricept), a potent and selective acetylcholinesterase inhibitor that improves cognitive impairment in Alzheimer's disease. Intraperitoneal administrations of donepezil significantly enhanced the survival of newborn neurons, but not proliferation of neural progenitor cells in the subgranular zone or the subventricular zone of normal mice. Moreover, donepezil treatment reversed the chronic stress-induced decrease in neurogenesis. Taken together, these results suggest that activation of the cholinergic system promotes survival of newborn neurons in the adult dentate gyrus and olfactory bulb under both normal and stressed conditions.

PSA-NCAM in mammalian structural plasticity and neurogenesis

Polysialic acid (PSA) is a linear homopolymer of alpha2-8-N acetylneuraminic acid whose major carrier in vertebrates is the neural cell adhesion molecule (NCAM). PSA serves as a potent negative regulator of cell interactions via its unusual biophysical properties. PSA on NCAM is developmentally regulated thus playing a prominent role in different forms of neural plasticity spanning from embryonic to adult nervous system, including axonal growth, outgrowth and fasciculation, cell migration, synaptic plasticity, activity-induced plasticity, neuronal-glial plasticity, embryonic and adult neurogenesis. The cellular distribution, developmental changes and possible function(s) of PSA-NCAM in the central nervous system of mammals here are reviewed, along with recent findings and theories about the relationships between NCAM protein and PSA as well as the role of different polysialyltransferases. Particular attention is focused on postnatal/adult neurogenesis, an issue which has been deeply investigated in the last decade as an example of persisting structural plasticity with potential implications for brain repair strategies. Adult neurogenic sites, although harbouring all subsequent steps of cell differentiation, from stem cell division to cell replacement, do not faithfully recapitulate development. After birth, they undergo morphological and molecular modifications allowing structural plasticity to adapt to the non-permissive environment of the mature nervous tissue, that are paralled by changes in the expression of PSA-NCAM. The use of PSA-NCAM as a marker for exploring differences in structural plasticity and neurogenesis among mammalian species is also discussed.

Anatomical perspectives on adult neural stem cells

The concept of stem cells within the adult brain is not new. However, only recently have scientific techniques become sufficiently advanced to identify them although this remains problematic and the technology is still developing. Nevertheless, it is now generally recognized that stem cells are restricted to two germinal regions within the intact brain. From here they can migrate to specific destinations where they integrate with existing circuitry. Their identity remains controversial but a growing body of evidence suggests it may have an astrocytic phenotype. Within the germinal regions the stem cells are confined to a niche environment and are capable of responding to environmental signals generated locally in an autocrine or paracrine fashion. The niche environment is also modulated by more generalized systemic and physiological activity. These observations are exciting in their own right and form the basis of this review. They are also beginning to alter how we think about neural injury and disease and to impact on the development of novel therapies.

Reorganization of the rat cerebellar cortex during postnatal development following cisplatin treatment

We examined the effects of the antitumor agent cisplatin on the development and plasticity of cerebellar cytoarchitecture. Since knowledge of the parallel and climbing fiber-Purkinje cell system is important in order to determine the architectural basis of cerebellar function, we used immunofluorescence for vesicular glutamate transporters (VGluT1 and VGluT2) to evaluate the trend of synaptogenesis of parallel and climbing fibers on Purkinje cells in the cerebellum vermis after a single injection of cisplatin to 10-day-old rats, i.e., during a crucial period of cerebellar development. The temporal and spatial patterns of VGluT1 and VGluT2 immunoreactivity after the early cisplatin injury provided evidence that remodeling of excitatory afferents and Purkinje cell dendrites occurs. After an early slow down of Purkinje cell dendrite growth, 7 days following the treatment, the extension of the molecular layer was reduced, as was parallel fiber innervation, but VGluT1 immunoreactive fibers contacted Purkinje cell dendrite branches extending within the external granular layer. VGluT2 immunopositive climbing fiber varicosities were still largely present on the soma and stem dendrites of Purkinje cells. Twenty days after the cisplatin injection, the thickness of the VGluT1 immunopositive molecular layer was reduced. VGluT2 climbing fiber varicosities were found on the remodeled Purkinje cell dendrites, as in controls, although at a lower density. Alterations in the immunoreactivity for polysialic acid neural cell adhesion molecule (PSA-NCAM) during the recovery phase suggest that this molecule plays a fundamental role not only during development, but also in the reorganization of neuroarchitecture. The changes were restricted to the neocerebellar vermis and were likely dependent on the different timing of lobule formation. The results of these investigations reveal the existence of vulnerability windows of the cerebellum to exposure to experimental or environmental cytotoxic agents during a critical period in development.

Remodeling of astrocytes, a prerequisite for synapse turnover in the adult brain? Insights from the oxytocin system of the hypothalamus

Neurons, including their synapses, are generally ensheathed by fine processes of astrocytes, but this glial coverage can be altered under different physiological conditions that modify neuronal activity. Changes in synaptic connectivity accompany astrocytic transformations so that an increased number of synapses are associated with reduced astrocytic coverage of postsynaptic elements, whereas synaptic numbers are reduced on reestablishment of glial coverage. A system that exemplifies activity-dependent structural synaptic plasticity in the adult brain is the hypothalamo-neurohypophysial system, and in particular, its oxytocin component. Under strong, prolonged activation (parturition, lactation, chronic dehydration), extensive portions of somatic and dendritic surfaces of magnocellular oxytocin neurons are freed of intervening astrocytic processes and become directly juxtaposed. Concurrently, they are contacted by an increased number of inhibitory and excitatory synapses. Once stimulation is over, astrocytic processes again cover oxytocinergic surfaces and synaptic numbers return to baseline levels. Such observations indicate that glial ensheathment of neurons is of consequence to neuronal function, not only directly, for example by modifying synaptic transmission, but indirectly as well, by preparing neuronal surfaces for synapse turnover.

A subpial, transitory germinal zone forms chains of neuronal precursors in the rabbit cerebellum

Protracted neurogenesis occurs at different postnatal stages in different brain locations, whereby leading to site-specific adult neurogenesis in some cases. No spontaneous genesis of neurons occurs in the cerebellum after the postnatal genesis of granule cells from the external germinal layer (EGL), a transitory actively proliferating zone which is thought to be exhausted before puberty. Here, we show the protracted genesis of newly generated neuronal precursors in the cerebellar cortex of young rabbits, persisting beyond puberty. Neuroblasts generated within an actively proliferating subpial layer thus extending the postnatal EGL are arranged to form thousands of tangential chains reminiscent of those responsible for cell migration in the forebrain subventricular zone. These subpial chains cover the whole cerebellar surface from the 2nd to the 5th month of life, then disappearing after puberty. In addition, we describe the appearance of similar groups of cells at the end of granule cell genesis in the mouse cerebellum, here limited to the short period of EGL exhaustion (4-5 days). These results show common features do exist in the postnatal reorganization of secondary germinal layers of brain and cerebellum at specific stages, parallel to differences in the slowing down of cerebellar neurogenesis among mammalian species.

Neurogenesis in the caudate nucleus of the adult rabbit

Stem cells with the potential to give rise to new neurons reside in different regions of the adult rodents CNS, but in vivo only the hippocampal dentate gyrus and the subventricular zone-olfactory bulb system are neurogenic under physiological condition. Comparative analyses have shown that vast species differences exist in the way the mammalian brain is organized and in its neurogenic capacity. Accordingly, we have demonstrated recently that, in the adult rabbit brain, striking structural plasticity persists in several cortical and subcortical areas. Here, by using markers for immature and mature neuronal and glial cell types, endogenous and exogenously administered cell-proliferation markers, intraventricular cell tracer injections coupled to confocal analysis, three-dimensional reconstructions, and in vitro tissue cultures, we demonstrate the existence of newly formed neurons in the caudate nucleus of normal, untreated, adult rabbit. Our results suggest that neurogenesis in the caudate nucleus is a phenomenon independent from that occurring in the adjacent subventricular zone, mostly attributable to the activity of clusters of proliferating cells located within the parenchyma of this nucleus. These clusters originate chains of neuroblasts that ultimately differentiate into mature neurons, which represent only a small percentage of the total neuronal precursors. These results indicate that striatum of rabbit represents a favorable environment for genesis rather than survival of newly formed neurons.

A study of the role of neuro-glial remodeling in the oxytocin system at lactation

Under conditions of strong secretion of neurohypophysial hormone, such as during parturition, lactation and dehydration, the hypothalamic oxytocin-system displays a remarkable morphological plasticity such that astrocytic coverage of its neurones diminishes, their surfaces become directly juxtaposed and contacted by an increased number of synapses. A growing body of evidence indicates that these anatomical changes have an impact on glutamatergic neurotransmission in the supraoptic nucleus, and may be therefore of physiological consequence. We here evaluated the consequences of the inhibition of such plasticity on the overall activity of the oxytocin system during lactation. Remodeling was prevented by performing hypothalamic microinjections in gestating rats of endoneuraminidase, an enzyme that removes polysialic acid from the neural cell adhesion molecule. Our earlier studies established that the presence of polysialic acid is a prerequisite for remodeling of the oxytocin system in the supraoptic and paraventricular nuclei. In dams in which polysialic acid was absent in all magnocellular nuclei after bilateral endoneuraminidase injections, parturition was normal and neither the frequency nor the amplitude of suckling-induced reflex milk ejections was different from vehicle-treated dams. The weight gain of pups was also normal as was water intake by the dams. We then assessed the electrical activity of antidromically identified magnocellular neurones in the polysialic acid-free supraoptic nucleus of isoflurane-anesthetized lactating rats. Basal and bursting activity characteristic of oxytocin neurones before each reflex milk ejection was not significantly different from that recorded in the supraoptic nucleus of rats with normal levels of polysialic acid. Our results indicate that neuro-glial remodeling, despite its role on fine modulation of oxytocin neuronal activity, is not essential to parturition and lactation.

Opiates, psychostimulants, and adult hippocampal neurogenesis: Insights for addiction and stem cell biology

Once thought to produce global, nonspecific brain injury, drugs of abuse are now known to produce selective neuro-adaptations in particular brain regions. These neuro-adaptations are being closely examined for clues to the development, maintenance, and treatment of addiction. The hippocampus is an area of particular interest, as it is central to many aspects of the addictive process, including relapse to drug taking. A recently appreciated hippocampal neuro-adaptation produced by drugs as diverse as opiates and psychostimulants is decreased neurogenesis in the sub-granular zone (SGZ). While the role of adult-generated neurons is not clear, their functional integration into hippocampal circuitry raises the possibility that decreased adult SGZ neurogenesis may alter hippocampal function in such a way as to maintain addictive behavior or contribute to relapse. Here, we review the impact of opiates and psychostimulants on the different stages of cell development in the adult brain, as well as the different stages of the addictive process. We discuss how examination of drug-induced alterations of adult neurogenesis advances our understanding of the complex mechanisms by which opiates and psychostimulants affect brain function while also opening avenues for novel ways of assessing the functional role of adult-generated neurons. In addition, we highlight key discrepancies in the field and underscore the necessity to move "beyond BrdU"--beyond merely counting new hippocampal cells labeled with the S phase marker bromodeoxyuridine--so as to probe mechanistic questions about how drug-induced alterations in adult hippocampal neurogenesis occur and what the functional ramifications of alterations in neurogenesis are for addiction.

Gangliogenesis in the enteric nervous system: Roles of the polysialylation of the neural cell adhesion molecule and its regulation by bone morphogenetic protein-4

The neural crest-derived cells that colonize the fetal bowel become patterned into two ganglionated plexuses. The hypothesis that bone morphogenetic proteins (BMPs) promote ganglionation by regulating neural cell adhesion molecule (NCAM) polysialylation was tested. Transcripts encoding the sialyltransferases, ST8Sia IV (PST) and ST8Sia II (STX), which polysialylate NCAM, were detectable in fetal rat gut by E12 but were downregulated postnatally. PSA-NCAM-immunoreactive neuron numbers, but not those of NCAM, were developmentally regulated similarly. Circular smooth muscle was transiently (E16-20) PSA-NCAM-immunoreactive when it is traversed by migrating precursors of submucosal neurons. Neurons developing in vitro from crest-derived cells immunoselected at E12 with antibodies to p75(NTR) expressed NCAM and PSA-NCAM. BMP-4 promoted neuronal NCAM polysialylation and clustering. N-butanoylmannosamine, which blocks NCAM polysialylation, but not N-propanoylmannosamine, which does not, interfered with BMP-4-induced neuronal clustering. Observations suggest that BMP signaling enhances NCAM polysialylation, which allows precursors to migrate and form ganglionic aggregates during the remodeling of the developing ENS.

A conserved mRNA expression profile of SREB2 (GPR85) in adult human, monkey, and rat forebrain

SREB is a subfamily of G-protein-coupled receptors, which consists of SREB1 (GPR27), SREB2 (GPR85), and SREB3 (GPR173). Its high evolutionary conservation and predominant expression in the CNS suggest that SREB family members and their undiscovered ligand(s) may have significant functions in the nervous system. SREB2 is the most conserved receptor throughout vertebrate evolution. As a first step in understanding the function of the SREB family, we have determined the anatomical gene expression profile of SREB2 in adult human, monkey, and rat forebrain using in situ hybridization histochemistry. The expression pattern of SREB2 mRNA was well conserved across three mammalian species. SREB2 mRNA was expressed in neurons throughout the brain and the most abundant expression was detected in the hippocampal dentate gyrus in all species examined. The areas expressing high levels of SREB2 mRNA overlap with brain structures known to possess high levels of plasticity, namely, the hippocampal formation, olfactory system, and supraoptic and paraventricular nuclei. Further, the anatomical expression of SREB1 and SREB3 overlapped with that of SREB2 in the adult monkey brain. Together, these data suggest a possible link between SREB family and neural plasticity, which may explain its extremely high conservation throughout vertebrate evolution.

Polysialylated neural cell adhesion molecule is necessary for selective targeting of regenerating motor neurons

It is well established that peripheral nerves regenerate after injury. Therefore, incomplete functional recovery usually results from misguided axons rather than a lack of regeneration per se. Despite this knowledge very little is known about the molecular mechanisms regulating axon guidance during regeneration. In the developing neuromuscular system the neural cell adhesion molecule (NCAM) and its polysialic acid (PSA) moiety are essential for proper motor axon guidance. In this study we used a well established model of nerve transection and repair to examine whether NCAM and/or PSA promotes selective regeneration of femoral motor nerves in wild-type and NCAM (-/-) mice. We found that regenerating axons innervating the muscle pathway and, to a lesser extent, cutaneous axons in the sensory pathway reexpress high levels of PSA during the time when the cut axons are crossing the lesion site. Second, we found that motor neurons in wild-type mice preferentially reinnervated muscle pathways, whereas motor neurons in NCAM (-/-) mice reinnervated muscle and cutaneous pathways with equal preference. Preferential regeneration was not observed in wild-type mice when PSA was removed enzymatically from the regenerating nerve, indicating that this form of selective motor axon targeting requires PSA. Finally, transgenic mice were used to show that the number of collateral sprouts, their field of arborization, and the withdrawal of misprojected axons were all attenuated significantly in mice lacking PSA. These results indicate that regenerating motor axons must express polysialylated NCAM, which reduces axon-axon adhesion and enables motor neurons to reinnervate their appropriate muscle targets selectively.

Chain formation and glial tube assembly in the shift from neonatal to adult subventricular zone of the rodent forebrain

The subventricular zone (SVZ) is regarded as an embryonic germinal layer persisting at the end of cerebral cortex neurogenesis and capable of generating neuronal precursors throughout life. The two distinct compartments of the adult rodent forebrain SVZ, astrocytic glial tubes and chains of migrating cells, are not distinguishable in the embryonic and early postnatal counterpart. In this study we analyzed the SVZ of mice and rats around birth and throughout different postnatal stages, describing molecular and morphological changes which lead to the typical structural arrangement of adult SVZ. In both species studied, most changes occurred during the first month of life, the transition being slightly delayed in mice, in spite of their earlier development. Important modifications affected the glial cells, eventually leading to glial tube assembly. These changes involved an overall reorganization of glial processes and their mutual relationships, as well as gliogenesis occurring within the SVZ which gives rise to glial cell subpopulations. The neuroblast cell population remained qualitatively quite homogeneous throughout all the stages investigated, changes being restricted to the relationships among cells and consequent formation of chains at about the third postnatal week. Electron microscopy showed that chain formation is not directly linked to glial tube assembly, generally preceding the occurrence of complete glial ensheathment. Moreover, chain and glial tube formation is asymmetric in the medial/lateral aspect of the SVZ, being inversely related. The attainment of an adult SVZ compartmentalization, on the other hand, seems linked to the pattern of expression of adhesion and extracellular matrix molecules.

Polysialic acid-induced plasticity reduces neuropathic insult to the central nervous system

Under chronic conditions of neuropathic pain, nociceptive C terminals are lost from their target region in spinal lamina II, leading to reduced thermal hyperalgesia. This region of the spinal cord expresses high levels of polysialic acid (PSA), a cell surface carbohydrate known to weaken cell-cell interactions and promote plasticity. Experimental removal of PSA from the spinal cord exacerbates hyperalgesia and results in retention of C terminals, whereas it has no effect on plasticity of touch Abeta fibers and allodynia. We propose that expression of PSA at this stress pathway relay point could serve to protect central circuitry from chronic sensory overload.

Intrathecal injection of GDNF and BDNF induces immediate early gene expression in rat spinal dorsal horn

Glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) are potent trophic factors for dorsal root ganglion cells. In addition, these factors are produced in subsets of dorsal root ganglion cells and transported anterogradely to their terminals in the superficial dorsal horn of the spinal cord, where they constitute the only source of GDNF and BDNF. We investigated the effect of 10 mug GDNF and BDNF injected by lumbar puncture on the expression of the immediate early gene (IEG) products c-Fos, c-Jun, and Krox-24 in the adult rat dorsal horn. In the dorsal horn of S1 spinal segments, GDNF and BDNF induced a strong increase in IEG expression, which was most pronounced in laminae I and II (2.9- to 4.5-fold). More distal from the injection site, in the dorsal horn of L1/L2 spinal segments, the increase in IEG expression was less pronounced, suggesting a concentration-dependent effect. In order to explain the effects of intrathecally injected GDNF, we investigated whether lumbo-sacral dorsal horn neurons expressed RET protein, the signal-transducing element of the receptor complex for GDNF. It was found that several of these neurons contained RET immunoreactivity and that some of the RET-labeled neurons had the appearance of nociceptive-specific cells, confirming their presumed role in pain transmission. Additionally, using double-labeling immunofluorescence combined with confocal microscopy, it was found that after intrathecal GDNF injection 35% of c-Fos-labeled cells were also labeled for RET. These results demonstrate that intrathecally administered GDNF and BDNF induce IEG expression in dorsal horn neurons in the adult rat, supposedly by way of their cognate receptors, which are present on these neurons. We further suggest that the endogenous release of GDNF and BDNF, triggered by nociceptive stimuli, is involved in the induction of changes in spinal nociceptive transmission as in various pain states.

In vivo neurogenesis in the dorsal vagal complex of the adult rat brainstem

The dorsal vagal complex (DVC) encompasses the nucleus tractus solitarii (NTS), the dorsal motor nucleus of the vagus nerve (DMX) and the area postrema (AP), that altogether provide the major integrative center for the mammalian autonomic nervous system. The adult rat DVC has been reported to contain afferent-dependent concentration of the plasticity-promoting polysialylated form of neural cell adhesion molecule [J Neurosci 21 (2001) 4721; Eur J Neurosci 14 (2001) 1194]. This prompted us to assess the occurrence of neurogenesis in the DVC of adult rats. Cumulative in vivo labeling of cell proliferation with i.p. bromodeoxyuridine (BrdU) injections was combined with phenotypic markers and confocal microscopy on serial brainstem sections throughout the DVC extent. In basal condition, sparse BrdU+ nuclei were selectively detected in the DVC according to a discrete and reproducible pattern. Some of them were found to colocalize with the neuronal markers doublecortin, HuC/D, or neuronal-specific antigen (NeuN), demonstrating that neurogenesis does occur within the DVC of adult rat. In the NTS, 10% of the BrdU+ nuclei were also NeuN+. A comparable proportion of astrogliogenesis was found in the DVC. Nestin immunohistochemistry yielded a highly specific labeling pattern at the border between AP and NTS. These data may relate to the neural stem cells that have been reported in the floor of the IVth ventricle [J Neurosci 16 (1996) 7599]. In order to assess a possible modulation of neurogenesis by afferent input in vivo, unilateral vagotomy was performed prior to cumulative BrdU treatment. Such DVC deafferentation triggered a large increase of BrdU incorporation in the ipsilateral DVC, which was associated with microglial proliferation in the DMX and with increased genesis of neurons and astrocytes in the NTS. These findings establish DVC as a novel model of adult neurogenesis that is reactive to deafferentation.

Cell types, lineage, and architecture of the germinal zone in the adult dentate gyrus

New neurons continue to be born in the subgranular zone (SGZ) of the dentate gyrus in the hippocampus of adult mammals, including humans. Previous work has shown that astrocytes function as the progenitors of these new neurons through immature intermediate D cells. In the first part of the present study, we determined the structure of each of these progenitors and how they are organized in three dimensions. Serial-section reconstructions of the SGZ, using confocal and electron microscopy demonstrate that SGZ astrocytes form baskets that hold clusters of D cells, largely insulating them from the hilus. Two types of glial fibrillary acidic protein-expressing astrocytes (radial and horizontal) and three classes of doublecortin and PSA-NCAM-positive D cells (D1, D2, D3) were observed. Radial astrocytes appear to interact closely with clusters of D cells forming radial proliferative units. In the second part of this study, we show that retrovirally labeled radial astrocytes give rise to granule neurons. We also used bromodeoxyuridine and [3H]thymidine labeling to study the sequence of appearance of the different D cells after a 7-day treatment with anti-mitotics. This analysis, together with retroviral labeling data, suggest that radial astrocytes divide to generate D1 cells, which in turn divide once to form postmitotic D2 cells. D2 cells mature through a D3 stage to form new granule neurons. These observations provide a model of how the germinal zone of the adult hippocampus is organized and suggest a sequence of cellular stages in the generation of new granule neurons.

Oxytocin neuron activation in NCAM-deficient mice: anatomical and functional consequences

During stimulated neurosecretion in the rat, oxytocin neurons display a reduced glial coverage and receive an increased number of synapses, changes that are reversed on arrest of stimulation. We identified polysialic acid on the neural cell adhesion molecule (NCAM) as an important mediator of such plasticity. To investigate further the role of this cell surface glycoprotein, we examined the oxytocin system in mice genetically deficient in NCAM. First, ultrastructural analyses revealed that in wild-type mice, the supraoptic nucleus (SON) underwent the same remodelling as in the rat because oxytocin neurons had a diminished astrocytic coverage and increased synaptic input during lactation or chronic salt loading. Surprisingly, the SON displayed this morphology in NCAM-deficient mice as well, whether they were nongestating and hydrated, lactating or dehydrated. The oxytocin system in NCAM-deficient mice was abnormally hyperactive, as illustrated by enhanced plasma and intranuclear concentrations of oxytocin and reduced anxiety-related behaviour. Plasma oxytocin concentrations were also high in lactating NCAM-deficient dams but certain parameters of lactation and maternal behaviour were impaired. NCAM-deficient mice survived ingestion of 2% saline for 7 days and had increased plasma oxytocin but they did not cope with more severe osmotic challenges. Our observations highlight further the remarkable capacity of the adult oxytocin system to undergo neuronal and glial remodelling whenever it is activated. That lack of NCAM did not prevent remodelling indicates that NCAM can be substituted by other molecular mechanisms. Finally, while NCAM deficiency greatly enhanced oxytocin release, it led to impaired oxytocin-dependent physiological and behavioural responses.

Neuronal, glial and synaptic remodeling in the adult hypothalamus: functional consequences and role of cell surface and extracellular matrix adhesion molecules

The adult hypothalamo-neurohypophysial system (HNS) undergoes activity-dependent morphological plasticity which modifies astrocytic coverage of its oxytocinergic neurons and their synaptic inputs. Thus, during physiological conditions that enhance central and peripheral release of oxytocin (OT), adjacent somata and dendrites of OT neurons become extensively juxtaposed, without intervening astrocytic processes and receive an increased number of synapses. The morphological changes occur within a few hours and are reversible with termination of stimulation. The reduced astrocytic coverage has direct functional consequences since it modifies extracellular ionic homeostasis, synaptic transmission, and the size and geometry of the extracellular space. It also contributes indirectly to neuronal function by permitting formation of synapses on neuronal surfaces freed of astrocytic processes. Overall, such remodeling is expected to potentiate activated neuronal firing, especially in clusters of tightly packed neurons, an anatomical arrangement characterizing OT neurons. This plasticity connotes dynamic cell interactions that must bring into play cell surface and extracellular matrix adhesive proteins like those intervening in developing neuronal systems undergoing neuronal-glial and synaptogenic transformations. It is worth noting, therefore, that adult HNS neurons and glia continue to express such molecules, including polysialic acid (PSA)-enriched neural cell adhesion molecule (PSA-NCAM) and the glycoprotein, tenascin-C. PSA is a large, complex sugar on the extracellular domain of NCAM considered a negative regulator of adhesion; it occurs in large amounts on the surfaces of HNS neurons and astrocytes. Tenascin-C, on the other hand, possesses adhesive and repulsive properties; it is secreted by HNS astrocytes and occurs in extracellular spaces and on cell surfaces after interaction with appropriate ligands. These molecules have been considered permissive factors for morphological plasticity. However, because of their localization and inherent properties, they may also serve to modulate the extracellular environment and in consequence, synaptic and volume transmission in a system in which the extracellular compartment is constantly being modified.

Blockade of N-methyl-D-aspartate receptors by phencyclidine causes the loss of corticostriatal neurons

Perinatal administration of the N-methyl-Dd-aspartate (NMDA) receptor antagonist phencyclidine (PCP) has been reported to produce regionally selective apoptotic cell death in the frontal cortex. The development of certain behavioral abnormalities following PCP treatment suggested that extracortical regions such as the striatum also could be affected. In this study, perinatal PCP treatment caused a marked reduction in striatal, but not hippocampal, staining for polysialic acid-neural cell adhesion molecule (PSA-NCAM), an NMDA-regulated molecule important in synaptogenesis. In order to isolate striatal influences to the cortex, this investigation was continued in vitro using corticostriatal slices. For these experiments we cultured coronal corticostriatal slices from postnatal day 7 rats. After 4 days in vitro, PCP was added for 48 h and then washed out for 24 h before harvesting the tissue. Similar to what was observed in vivo, we found that PCP treatment results in a marked reduction in striatal staining for PSA-NCAM. No change was observed in the mature form of NCAM. In striatal synaptoneurosomes, immunoblot analysis confirmed that the levels of PSA-NCAM and synaptophysin, a molecule often used as a marker of synaptogenesis, were substantially down-regulated by PCP. These effects were prevented by M40403, a superoxide dismutase mimetic that also prevented the PCP-induced terminal dUTP nick-end labeling of DNA fragments that was observed selectively in the cortex. These data suggest that PCP causes cell death by apoptosis selectively in the cortex, but not in the striatum, following either in vivo treatment of perinatal rat pups or in vitro treatment of corticostriatal slices. Further, cortical apoptosis induced by PCP negatively impacts striatal synaptogenesis, a process important in normal neural development. This deficit is probably caused by a reduction in corticostriatal neurotransmission. It is possible that the dysregulation of striatal synaptogenesis contributes to the behavioral abnormalities observed following perinatal PCP administration in vivo.

Clinical evaluation of a group B meningococcal N-propionylated polysaccharide conjugate vaccine in adult, male volunteers

The safety and immunogenicity of a group B meningococcal vaccine, consisting of N-propionylated (NPr) B capsular polysaccharide conjugated to tetanus toxoid, was tested for the first time, in 17 healthy male volunteers aged between 18 and 40 years. Four escalating dosages of vaccine were tested and each was given as three intramuscular injections at 4-week intervals. The vaccine was well tolerated and induced only mild and transient, dose-dependent, injection-site reactions. One month after the last injection, there was no evidence of the production of autoantibodies or antibodies binding to PSA-NCAM. The vaccine induced an increase in the pre-existing titres of IgM specific to B polysaccharide and NPr B polysaccharide. Moreover, it induced IgG antibodies specific to NPr B polysaccharide, which were undetectable before vaccination. However, no functional activity of vaccine-induced antibodies was demonstrated in bactericidal assays, opsonophagocytic tests or passive protection tests.

Dynamic neuronal-glial interactions: an overview 20 years later

After commenting on some perceived reasons why our review may have been relatively frequently cited, a brief overview is presented that first summarizes what we knew 25 years ago about the dynamic neuronal-astroglial interactions that occur in response to changes in the physiological state of the animal. The brain system in which these dynamic interactions were studied was the magnocellular hypothalamo-neurohypophysial system (mHNS) of the rat. The mHNS developed as and continues to be the model system yielding the most coherent picture of dynamic morphological changes and insights into their functional consequences. Many other brain areas, however, have more recently come under scrutiny in the search for glial-neuronal dynamisms. Outlined next are some of the questions concerning this phenomenon that led to the research efforts immediately following the initial discoveries, along with the answers, both complete and incomplete, obtained to those research questions. The basis for this first wave of follow-up research can be characterized by the phrase "what we knew we didn't know at that time." The final section is an update and brief overview of highlights of both "what we know now" and "what we now know that we don't know" about dynamic neuronal-astroglial interactions in the mHNS.

Olfactory ensheathing cells (OECs) and the treatment of CNS injury: advantages and possible caveats

One of the main research strategies to improve treatment for spinal cord injury involves the use of cell transplantation. This review looks at the advantages and possible caveats of using glial cells from the olfactory system in transplant-mediated repair. These glial cells, termed olfactory ensheathing cells (OECs), ensheath the axons of the olfactory receptor neurons. The primary olfactory system is an unusual tissue in that it can support neurogenesis throughout life. In addition, newly generated olfactory receptor neurons are able to grow into the CNS environment of the olfactory bulb tissue and reform synapses. It is thought that this unique regenerative property depends in part on the presence of OECs. OECs share some of the properties of both astrocytes and Schwann cells but appear to have advantages over these and other glial cells for CNS repair. In particular, OECs are less likely to induce hypertrophy of CNS astrocytes. As well as remyelinating demyelinated axons, OEC grafts appear to promote the restoration of functions lost following a spinal cord lesion. However, much of the evidence for this is based on behavioural tests, and the mechanisms that underlie their potential benefits in transplant-mediated repair remain to be clarified.

Distribution of polysialylated neural cell adhesion molecule in rat septal nuclei and septohippocampal pathway: Transient increase of polysialylated interneurons in the subtriangular septal zone during memory consolidation

During memory consolidation neuroplastic events in the mediotemporal corticohippocampal pathway are accompanied by transient increases in the frequency of neurons expressing polysialylated neural cell adhesion molecule (NCAM PSA), a posttranslational modification associated with morphofunctional change. As a bidirectional pathway between the hippocampus and the septal nuclei also influences memory processing, we have determined the distribution of NCAM PSA within this system before and after learning in the adult Wistar rat. The most intense NCAM PSA immunoreactivity was observed in the medial and triangular septal nuclei, regions that regulate hippocampal theta rhythm during memory consolidation. Within the fimbria, NCAM PSA was expressed only in a subpopulation of fibres, most likely cholinergic projections from the medial septum to the hippocampus. Grey level analysis or direct cell counting revealed no learning-specific change in NCAM PSA expression in these septal subregions after avoidance conditioning or spatial training. A population of discrete polysialylated neurons in the subtriangular septal zone, however, exhibited a transient twofold frequency increase at 12 hr after training in either task. Immunohistochemical analysis revealed these cells to be gamma-aminobutyric acid (GABAergic) interneurons co-expressing vasoactive intestinal peptide. The unique location of these interneurons is proposed to provide a natural plexus by which bidirectional communication between the septum and hippocampus may be modified during memory consolidation.

Prenatal development of the rodent rostral migratory stream

The aim of this study was to elucidate the embryological origins of the unique neuronal progenitor cells that form the rostral migratory stream (RMS), the path traversed by cells from the anterior part of the forebrain subventricular zone (SVZa) en route to the olfactory bulb. To determine when and where cells constituting the RMS initially exhibit their characteristic neuronal phenotype and high mitotic capacity, we analyzed the cells of the rat forebrain between embryonic day 14 (E14) and postnatal day 2 (P2). At E14, cells with a neuronal phenotype were observed within the ventricular zone in close proximity to the mantle layer of the future olfactory bulb. By E15, cells expressing neuronal markers are also PSA-NCAM immunoreactive and become aligned in chains of similarly oriented cells, a hallmark of the postnatal RMS. The cells that form chains organize into a patch that enlarges in the anterior-posterior and medial-lateral dimensions from E16 to E22 (birth). In comparing the forebrain cytoarchitecture to the pattern of cell type-specific staining, the patch constitutes only the central part of the proximal RMS. Early during development, the region of the RMS surrounding the patch expresses low levels of PSA-NCAM and neuron-specific markers. The proliferative activity of cells forming the patch vs. nonpatch regions of the RMS was analyzed following a short bromodeoxyuridine (BrdU) exposure. Between E15 and E22, the patch can be recognized by the mitotic activity of its cells; the cells of the patch incorporate less BrdU than the nonpatch portion of the RMS. The time course of appearance of cells forming the RMS indicates that the RMS arises in advance and independently of the cortical SVZ. Although the patch and the nonpatch regions of the embryonic RMS appear to merge postnatally, the two regions may originate separately under the influence of distinct intrinsic and extrinsic factors.

The minimal structural domains required for neural cell adhesion molecule polysialylation by PST/ST8Sia IV and STX/ST8Sia II

A limited number of mammalian proteins are modified by polysialic acid, with the neural cell adhesion molecule (NCAM) being the most abundant of these. We hypothesize that polysialylation is a protein-specific glycosylation event and that an initial protein-protein interaction between polysialyltransferases and glycoprotein substrates mediates this specificity. To evaluate the regions of NCAM required for recognition and polysialylation by PST/ST8Sia IV and STX/ST8Sia II, a series of domain deletion proteins were generated, co-expressed with each enzyme, and their polysialylation analyzed. A protein consisting of the fifth immunoglobulin-like domain (Ig5), which contains the reported sites of polysialylation, and the first fibronectin type III repeat (FN1) was polysialylated by both enzymes, whereas a protein consisting of Ig5 alone was not polysialylated by either enzyme. This demonstrates that the Ig5 domain of NCAM and FN1 are sufficient for polysialylation, and suggests that the FN1 may constitute an enzyme recognition and docking site. Two other NCAM mutants, NCAM-6 (Ig1-5) and NCAM-7 (FN1-FN2), were weakly polysialylated by PST/ST8Sia IV, suggesting that a weaker enzyme recognition site may exist within the Ig domains, and that glycans in the FN region are polysialylated. Further analysis indicated that O-linked oligosaccharides in NCAM-7, and O-linked and N-linked glycans in full-length NCAM, are polysialylated when these proteins are co-expressed with the polysialyltransferases in COS-1 cells. Our data support a model in which the polysialyltransferases bind to the FN1 of NCAM to polymerize polysialic acid chains on appropriately presented glycans in adjacent regions.

Deprivation of sensory inputs to the olfactory bulb up-regulates cell death and proliferation in the subventricular zone of adult mice

The main olfactory bulb (MOB) is the first relay on the olfactory sensory pathway and the target of the neural progenitor cells generated in the subventricular zone (SVZ) lining the lateral ventricles and which migrate along the rostral extension of the SVZ, also called the rostral migratory stream (RMS). Within the MOB, the neuroblasts differentiate into granular and periglomerular interneurons. A reduction in the number of granule cells during sensory deprivation suggests that neurogenesis may be influenced by afferent activity. Here, we show that unilateral sensory deafferentation of the MOB by axotomy of the olfactory receptor neurons increases apoptotic cell death in the SVZ and along the rostro-caudal extent of the RMS. The vast majority of dying cells in the RMS are migrating neuroblasts as indicated by double Terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick-end labeling/PSA-NCAM labeling. Counting bromodeoxyuridine-labeled cells in animals killed immediately or 4 days after tracer administration showed a bilateral increase in proliferation in the SVZ and RMS which was balanced by cell death on the operated side. These data suggest that olfactory inputs are required for the survival of newborn neural progenitors. The greatest enhancement in proliferation occurred in the extension of the RMS located in the MOB, revealing a population of local precursors mitotically stimulated following axotomy. Together, these findings indicate that olfactory inputs may strongly modulate the balance between neurogenesis and apoptosis in the SVZ and RMS and provide a model for further investigation of the underlying molecular mechanisms of this activity-dependent neuronal plasticity.

Autocrine/paracrine activation of the GABA(A) receptor inhibits the proliferation of neurogenic polysialylated neural cell adhesion molecule-positive (PSA-NCAM+) precursor cells from postnatal striatum

GABA and its type A receptor (GABA(A)R) are present in the immature CNS and may function as growth-regulatory signals during the development of embryonic neural precursor cells. In the present study, on the basis of their isopycnic properties in a buoyant density gradient, we developed an isolation procedure that allowed us to purify proliferative neural precursor cells from early postnatal rat striatum, which expressed the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). These postnatal striatal PSA-NCAM+ cells were shown to proliferate in the presence of epidermal growth factor (EGF) and formed spheres that preferentially generated neurons in vitro. We demonstrated that PSA-NCAM+ neuronal precursors from postnatal striatum expressed GABA(A)R subunits in vitro and in situ. GABA elicited chloride currents in PSA-NCAM+ cells by activation of functional GABA(A)R that displayed a typical pharmacological profile. GABA(A)R activation in PSA-NCAM+ cells triggered a complex intracellular signaling combining a tonic inhibition of the mitogen-activated protein kinase cascade and an increase of intracellular calcium concentration by opening of voltage-gated calcium channels. We observed that the activation of GABA(A)R in PSA-NCAM+ neuronal precursors from postnatal striatum inhibited cell cycle progression both in neurospheres and in organotypic slices. Furthermore, postnatal PSA-NCAM+ striatal cells synthesized and released GABA, thus creating an autocrine/paracrine mechanism that controls their proliferation. We showed that EGF modulated this autocrine/paracrine loop by decreasing GABA production in PSA-NCAM+ cells. This demonstration of GABA synthesis and GABA(A)R function in striatal PSA-NCAM+ cells may shed new light on the understanding of key extrinsic cues that regulate the developmental potential of postnatal neuronal precursors in the CNS.

Dynamic regulation of polysialylated neural cell adhesion molecule in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) prominently expresses polysialic acid (PSA), a carbohydrate polymer that is attached to neural cell adhesion molecule (NCAM) and promotes changes in cell interactions. Previous studies have shown that expression of PSA is important for circadian rhythm stability under constant darkness, and for photic entrainment of the SCN circadian clock. In the present study, immunoblot analyses of the Syrian hamster SCN revealed marked diurnal fluctuations in PSA under a 24-h light/dark cycle. PSA levels were reduced by >90% during the mid-to-late dark phase, and were elevated to maximal daytime levels approximately 1 h after lights-on. A similar pattern of PSA fluctuation persisted under constant darkness. Exposure of animals under a 24-h light/dark cycle to a 30-min light pulse during the late dark phase dramatically increased SCN contents of PSA within 60 min, and these values returned to basal levels 1-2 h later. There was no effect of light-on expression of PSA in the hippocampus. Parallel studies revealed changes in the NCAM-180 isoform that carries PSA in the brain, suggesting that regulation of PSA may include protein as well as carbohydrate-associated mechanisms. Immunohistological analysis revealed light-induced enhancement of PSA in the SCN subregion containing calbindin D(28K) cells. PSA staining was also closely associated with the majority of SCN cells expressing light-inducible Fos protein. This rhythmic, light-inducible expression of PSA within the SCN suggests that dynamic cell interactions are important for the photic regulation of circadian clock phase.

Removal of PSA from NCAM affects the survival of magnocellular vasopressin- and oxytocin-producing neurons in organotypic cultures of the paraventricular nucleus

The expression of the polysialic acid neural cell adhesion molecule (PSA-NCAM) in the hypothalamo-neurohypophyseal system has been correlated with morphofunctional plasticity. In this study, we investigated the role of PSA-NCAM in the survival of oxytocin (OT)- and vasopressin (VP)-producing magnocellular cells of this system. We used a recently developed organotypic slice culture model of the rat hypothalamic paraventricular nucleus (PVN) in which ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF) are potent survival factors for magnocellular neurons. We demonstrate by means of confocal microscopy that cultured magnocellular VP and OT neurons express strong immunoreactivity for PSA-NCAM. Removal of PSA from NCAM by the enzyme Endo N leads to a significant loss of both VP and OT neurons in the presence of low concentrations of CNTF. Endo N treatment did not change cell survival in the presence of LIF. These results suggest that, in addition to its role in neuro-glial plasticity, PSA-NCAM might also influence the trophic factor responsiveness of hypothalamic VP and OT neurosecretory cells.

Intrinsic role of polysialylated neural cell adhesion molecule in photic phase resetting of the Mammalian circadian clock

The suprachiasmatic nuclei (SCN), the location of the mammalian circadian clock, are one of the few adult brain regions that express the highly polysialylated form of neural cell adhesion molecule (PSA-NCAM). A role for the polysialic acid (PSA) component of PSA-NCAM, which is known to promote tissue plasticity, has been reported for photic entrainment of circadian rhythmicity in vivo. The in vivo results, however, do not discriminate between PSA acting upstream or downstream of the glutamatergic synapses that convey photic information to the SCN. To address this key issue, we exploited an in vitro rat brain slice preparation that retains robust circadian function. As in the intact SCN, PSA levels in the isolated SCN are rhythmic, with higher levels during the early subjective day and lower levels during subjective night. Importantly, bath application of glutamate to SCN slices rapidly and transiently increases PSA levels during both the subjective day and night. Pretreating the slices with endoneuraminidase, which selectively removes PSA from NCAM and thereby prevents this increase, abolishes glutamate- and optic chiasm stimulation-induced phase delays of the SCN circadian neuronal activity rhythm. These results support the hypothesis that PSA expression in the SCN is controlled by both the circadian clock and photic input to the clock and that expression of PSA in the SCN is critical for photic-like phase shifts of the clock. Together, these results establish that such actions of PSA are manifested downstream from presynaptic retinohypothalamic terminals and therefore are intrinsic to the SCN itself.

Exercise increases mRNA levels for adhesion molecules N-CAM and L1 correlating with BDNF response

In situ hybridization was used to evaluate whether long-term moderate locomotor exercise, which up-regulates BDNF and TrkB levels in the spinal gray matter of the adult rat, similarly influences the expression of the cell adhesion molecules N-CAM and L1. Exercise doubled the level of N-CAM mRNA hybridization signal in the lumbar spinal gray. The increase in L1 mRNA was less consistent. N-CAM mRNA levels slightly increased in the white matter. BDNF mRNA levels also increased in cells of the ventral horn and the white matter due to the exercise. These results suggest that exercise-induced rearrangements of the spinal network involve N-CAM, L1 and BDNF, crucial in different aspects of synaptic plasticity and synapse formation.

Glycobiology of the synapse: the role of glycans in the formation, maturation, and modulation of synapses

Synapses, which are the fundamental functional unit of the nervous system, are considered to be highly specialized cell adhesion structures. Studies since the 1960s demonstrated that various carbohydrates and glycoproteins are expressed in synapses in the central and peripheral nervous system. Although the functional roles of these synaptic carbohydrates and glycoproteins remain to be determined, rapidly accumulating data suggest that they may play critical roles in the formation, maturation, and functional modulation of synapses.

Transneuronal induction of the highly sialylated isoform of the neural cell adhesion molecule following nerve injury

The polysialylated, embryonic form of the neuronal cell adhesion molecule (PSA-NCAM) is known to participate in a whole series of synaptic rearrangements even in adult animals. The possible role of this molecule in neuroplastic changes of the adult rat somatosensory cortex induced by unilateral transection of the infraorbital branch of the trigeminal nerve was studied with PSA-NCAM immunostaining at light microscopic level. Two- and three-month-old CFY albino rats were sacrificied on days 1, 4, 6, 14 and 21 following operation and PSA-NCAM immunoreaction was examined at three levels of the vibrissa-cortex neuraxis, namely, in the principal nucleus of the trigeminal nerve, in the ventral posteromedial nucleus of the thalamus and in the somatosensory cortex. The lower levels of the neuraxis remained free of PSA-NCAM labeling, similarly to control, intact animals. However, a large number of scattered small neurons became PSA-NCAM immunoreactive in layers IV-VI on both ipsi- and contralateral sides of the somatosensory cortex from day 6 onwards, suggesting a possible transynaptic regulation of NCAM sialylation state.

Localization of phosphorylated cAMP response element-binding protein in immature neurons of adult hippocampus

Neurogenesis continues to occur in the adult hippocampus, although many of the newborn cells degenerate 1-2 weeks after birth. The number and survival of newborn cells are regulated by a variety of environmental stimuli, but very little is known about the intracellular signal transduction pathways that control adult neurogenesis. In the present study, we examine the expression of the phosphorylated cAMP response element-binding protein (pCREB) in immature neurons in adult hippocampus and the role of the cAMP cascade in the survival of new neurons. The results demonstrate that virtually all immature neurons, identified by triple immunohistochemistry for bromodeoxyuridine (BrdU) and polysialic acid-neural cell adhesion molecule (PSA-NCAM), are also positive for pCREB. In addition, upregulation of cAMP (via pharmacological inhibition of cAMP breakdown or by antidepressant treatment) increases the survival of BrdU-positive cells. A possible role for pCREB in the regulation of PSA-NCAM, a marker of immature neurons involved in neuronal remodeling and neurite outgrowth, is supported by cell culture studies demonstrating that the cAMP-CREB pathway regulates the expression of a rate-limiting enzyme responsible for the synthesis of PSA-NCAM. These findings indicate that the cAMP-CREB pathway regulates the survival, and possibly the differentiation and function, of newborn neurons.

PSA-NCAM immunocytochemistry in the cerebral cortex and other telencephalic areas of the lizard Podarcis hispanica: differential expression during medial cortex neuronal regeneration

The lizard medial cortex, a region homologous to the mammalian dentate gyrus, shows postnatal neurogenesis and the surprising ability to replace its neurons after being lesioned specifically with the neurotoxin 3-acetylpyridine. As the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) is expressed during neuronal migration and differentiation, we have studied its distribution in adult lizards and also during the lesion-regeneration process. In the medial cortex of control animals, many labeled fusiform somata, presumably corresponding to migratory neuroblasts, appeared in the inner plexiform layer. There were also scattered immunoreactive granule neurons in the cell layer. Double immunocytochemistry with 5'-bromodeoxyuridine revealed that some of the PSA-NCAM-expressing cells in the inner plexiform and cell layers were generated recently. PSA-NCAM immunoreactivity was also present in the dorsomedial, dorsal, and lateral cortices, as well as in the dorsal ventricular ridge, the nucleus accumbens, and the nucleus sphericus. Twelve hours after the injection of 3-acetylpyridine, some medial cortex granule neurons appeared degenerated, although some of them still expressed PSA-NCAM. One to 2 days after the injection, most granule neurons appeared degenerated and no PSA-NCAM immunoreactivity was detected in the medial cortex cell layer. Four to 7 days after treatment, abundant labeled fusiform cells populated the inner plexiform layer and some immunoreactive somata were seen in the cell layer. Fifteen to 30 days after the neurotoxin injection, the number of PSA-NCAM expressing granule neurons augmented considerably and the level was still above control levels in lizards that survived 42 days. Our results show for the first time the expression of PSA-NCAM in a reptile brain, where it appears to participate in the migration and differentiation of granule neurons during adult neurogenesis and regeneration.

Removal of polysialic acid from the SCN potentiates nonphotic circadian phase resetting

The adult suprachiasmatic nucleus (SCN) expresses a polysialylated form of neural cell adhesion molecule (PSA-NCAM) that modulates cell interactions. Previous studies have shown that PSA is important for photic entrainment of the SCN circadian clock, suggesting that changes in cell-cell interactions may contribute to the phase-resetting capacity of this system. A possible role for PSA in nonphotic circadian phase resetting was evaluated using the enzyme endoneuraminidase (endo N) to selectively remove PSA from the SCN. Pretreatment of rat brain slices containing the SCN with endo N enhanced the daytime phase-advancing effects of the serotonin agonist, 8-hydroxy-2-dipropylaminotetralin [8-OH-DPAT] (41% greater than inactivated enzyme controls; P<.05). Similarly, removal of PSA from the Syrian hamster SCN in vivo potentiated the daytime phase-advancing effects of behavioral arousal (sleep deprivation) and of systemic application of 8-OH-DPAT (61% and 220% greater, respectively, than inactivated enzyme controls; both P<.05). While endo N perturbation of both photic and nonphotic phase resetting suggests that PSA plays a central role in clock regulation, it is striking that the effects on the two inputs are opposite in direction. This difference could reflect the antagonistic relationship between photic and nonphotic signaling pathways. It could also be explained by a permissive role of PSA common to both inputs, which in and of itself would not specify direction of response. Such a bidirectional control mechanism, based on PSA's attenuation of cell-cell interactions, is well documented in the developing nervous system, and may be retained in plastic regions of the adult brain.

Neuroendocrine plasticity in GnRH release during rat estrous cycle: correlation with molecular markers of synaptic remodeling

Morphological changes in the gonadotropin releasing hormone (GnRH) neurons in the preoptic area (POA) and their terminals in the median eminence-arcuate (ME-ARC) region are reported to occur during ovarian cycle that may be involved in the GnRH release into the portal blood during preovulatory surge. However, the neuronal substrates participating in altered GnRH neuronal plasticity are poorly understood. The present study was designed to determine whether morphological changes occurring in the GnRH neuron cell bodies in the POA and their terminals in the ME-ARC region of hypothalamus with pulsatile GnRH release in cycling female rats are associated with expression of intrinsic determinants of neuronal plasticity. The plasticity markers studied are polysialylated neural cell adhesion molecule (PSA-NCAM), high molecular weight isoforms of NCAM, growth associated protein (GAP-43), glial fibrillary acidic protein (GFAP) and synaptophysin. Regularly cycling female rats were sacrificed at diestrous, i.e., when GnRH release is low, and at proestrous, i.e., when preovulatory GnRH surge occurs, using perfusion fixation method for immunohistochemical staining of GnRH cells. GnRH cell bodies and their terminals from the POA and ME-ARC region respectively, were localized using immunohistochemical technique in proestrous and diestrous phase of estrous cycle and our results showed a marked increase in the GnRH nerve terminals length and immunoreactivity in the ME-ARC region from proestrous phase rats as compared to diestrous rats. Immunoblot analyses of the POA and ME-ARC region of the hypothalamus revealed a significant increase in the content of PSA-NCAM, NCAM-180, NCAM-140, GAP-43 and synaptophysin from proestrous phase rats as compared to diestrous phase rats. The ME-ARC region showed more pronounced increase in the protein expression of these markers of neuronal plasticity as compared to the POA, whereas, hippocampal region did not show any significant change in the content of these markers showing specificity of the changes to the GnRH system. GFAP content was significantly decreased in the POA with a marginal increase in the GFAP level from the ME-ARC region. These results demonstrate the involvement of synaptic proteins in the dynamic plasticity of the ME-ARC region of hypothalamus, allowing GnRH nerve terminals to release the neurohormone into the pituitary portal blood on the day of proestrous.

Contribution of the neural cell adhesion molecule to neuronal and synaptic plasticity

The neural cell adhesion molecule (NCAM) and its polysialylated form PSA-NCAM contribute to many aspects of the development and plasticity of the central nervous system. This includes mechanisms of cell differentiation and migration, neurite outgrowth, establishment of specific patterns of synaptic connections, synaptic plasticity and long-term potentiation. How NCAM and PSA-NCAM contribute to regulate all these different mechanisms remains essentially unknown. Adhesive properties appear to be important, but recent studies also point to possible interactions between NCAM and PSA-NCAM with intracellular signalling cascades that are essential to biological functions. Some of these mechanisms are discussed and a hypothesis is proposed based on the existence of cross-talk between these molecules and signalling pathways mediated by growth factors.

Neuronal-glial remodeling: a structural basis for neuronal-glial interactions in the adult hypothalamus

Increasing evidence is establishing that adult neurons and their associated glia can undergo state-dependent changes in their morphology and in consequence, in their relationships and functional interactions. A neuronal system that illustrates this kind of neuronal-glial plasticity in an exemplary fashion is that responsible for the secretion of the neurohormone oxytocin (OT). As shown by comparative ultrastructural analysis, during physiological conditions like lactation and dehydration, which result in enhanced peripheral and central release of the peptide, astrocytic coverage of OT neurons is markedly reduced and their surfaces are left directly juxtaposed. Such reduced glial coverage is of consequence to neuronal activity since it modifies extracellular ionic homeostasis and glutamate neurotransmission. In addition, it is probably prerequisite to the synaptic remodeling that occurs concurrently, and results in an enhanced number of inhibitory (GABAergic) and excitatory (glutamatergic, noradrenergic) synapses, thus further affecting neuronal function. The neuronal-glial and synaptic changes occur rapidly, within a matter of hours, and are reversible with termination of stimulation. The adult OT system retains many juvenile molecular features that may allow such plasticity, including expression of cell adhesion molecules implicated in neuronal-glial interactions during development, like polysialylated NCAM, F3/contactin and its ligand, the matrix glycoprotein, tenascin-C. On the other hand, OT itself can induce the changes since in vivo (ventricular microinfusion) or in vitro (on acute hypothalamic slices) application leads to glial and neuronal transformations similar to those induced by physiological stimuli.

Non-granule PSA-NCAM immunoreactive neurons in the rat hippocampus

The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) continues to be expressed in the adult hippocampus, mainly in a subset of neurons located in the innermost portion of the granule cell layer. PSA-NCAM immunoreactive neurons have also been described outside this layer in humans, where they are severely reduced in schizophrenic brains. Given this important clinical implication, we were interested in finding whether similar neurons existed in the adult rat hippocampus and to characterize their distribution, morphology and phenotype. PSA-NCAM immunocytochemistry reveals labeled neurons in the subiculum, fimbria, alveus, hilus, and stratum oriens, lucidum and radiatum of CA3 and CA1. They are mainly distributed in the ventral hippocampus, and have polygonal or fusiform somata with multipolar or bipolar morphology. These neurons show long straight dendrites, which reach several strata and even enter the fimbria and the alveus. These dendrites are often varicose, appear devoid of excrescences and apparently do not show spines. Most of these neurons display GABA immunoreactivity and further analysis has shown that a subpopulation expresses calretinin, but not somatostatin, neuropeptide Y, parvalbumin, calbindin or NADPH diaphorase. Our study demonstrates that there is an important subpopulation of PSA-NCAM immunoreactive neurons, many of which can be considered interneurons, outside the rat granule cell layer, probably homologous to those described in the human hippocampus. The presence of the polysialylated form of NCAM in these neurons could indicate that they are undergoing continuous remodeling during adulthood and may have an important role in hippocampal structural plasticity.

Adult neural stem cells: plasticity and developmental potential

Stem cells play an essential role during the processes of embryonic tissue formation and development and in the maintenance of tissue integrity and renewal throughout adulthood. The differentiation potential of stem cells in adult tissues has been thought to be limited to cell lineages present in the organ from which they derive, but there is evidence that somatic stem cells may display a broader differentiation repertoire. This has been documented for bone marrow stem cells (which can give rise to muscle, hepatic and brain cells) and for muscle precursors, which can turn into blood cells. The adult central nervous system (CNS) has long been considered incapable of cell renewal and structural remodeling. Recent findings indicate that, even in postnatal and adult mammals, neurogenesis does occur in different brain regions and that these regions actually contain adult stem cells. These cells can be expanded both in vivo and ex vivo by exposure to different combinations of growth factors and subsequently give rise to a differentiated progeny comprising the major cell types of the CNS. Almost paradoxically, adult neural stem cells display a multipotency much broader than expected, since they can differentiate into non-CNS mesodermal-derivatives, such as blood cells and skeletal muscle cells. We review the recent findings documenting this unforeseen plasticity and unexpected developmental potential of somatic stem cells in general and of neural stem cells in particular. To better introduce these concepts, some basic notions on the functional properties of adult neural stem cells will also be discussed, particularly focusing on the emerging role of the microenvironment in determining and maintaining their peculiar characteristics.

Oxytocin-secreting neurons: A physiological model of morphological neuronal and glial plasticity in the adult hypothalamus

Oxytocin-secreting neurons of the hypothalamoneurohypophysial system undergo reversible morphological changes whenever they are strongly stimulated. In the hypothalamus, such structural plasticity is represented by modifications in the size and shape of their somata and dendrites, in the extent to which their surfaces are covered by glia, and in the density of their synapses. In the neurohypophysis, there is a parallel reduction in glial (pituicyte) coverage of their axons together, with retraction of pituicyte processes from the perivascular basal lamina and an increase in the number and size of their terminals. These changes occur rapidly, within a few hours. On the other hand, the system returns to its prestimulated condition on arrest of stimulation at a rate that depends on the length of time it has remained activated. Such neuronal-glial changes have several functional consequences. In the hypothalamic nuclei, reduction in astrocytic coverage of oxytocinergic neurons and their synapses modifies extracellular ionic homeostasis and glutamate clearance and, therefore, their overall excitability. Since it results in extensive dendritic bundling, it may also lead to ephaptic interactions and may facilitate dendritic electrotonic coupling. A most important indirect effect may be to permit synaptic remodeling that occurs concomitantly and that results in significant increases in the number of excitatory and inhibitory synapses driving their activity. In the stimulated neurohypophysis, glial retraction results in increased levels of extracellular K+ which can enhance neurohormone release while an enlarged neurovascular contact zone may facilitate diffusion of neurohormone into the circulation. Ongoing work aims to unravel the cell mechanisms and factors underlying such plasticity and has revealed that neurons and glia of the hypothalamoneurohypophysial system continue to express juvenile molecular features associated with similar neuronglial interactions and synaptic events during development and regeneration. They include strong expression of cell surface adhesion molecules like F3/contactin and polysialylated neural cell adhesion molecule, extracellular matrix glycoproteins like tenascin C, and cytoskeletal proteins like vimentin and microtubule-associated protein 1D. Some of these molecules reach the cell surface constitutively while others follow the activity-dependent regulated pathway. We consider many of these molecular features permissive, allowing oxytocin neurons and their glia to undergo morphological remodeling throughout life, provided the proper stimulus intervenes. In the hypothalamic nuclei, one such stimulus is centrally released oxytocin; in the neurohypophysis, an adrenergic, cAMP-mediated mechanism appears responsible.

Maternity leads to morphological synaptic plasticity in the oxytocin system

The oxytocinergic system, which plays a major role in the control of different aspects of maternity, undergoes extensive synaptic and neuronal-glial remodelling during parturition and lactation and has thus become a remarkable example of activity-dependent morphological synaptic plasticity in the adult mammalian brain. The use of different comparative ultrastructural analyses on the rat supraoptic and paraventricular nuclei, together with identification of pre- and post-synaptic elements, has allowed us to show that there is a significant increase in the number of GABAergic, glutamatergic and noradrenergic synapses impinging on oxytocin neurons, concomitant with a reduction of glial coverage of the neurons. This synaptic plasticity involves axo-dendritic and axo-somatic contacts originating from terminals making one or several synaptic contacts in one plane of section. While noradrenergic afferents arise from medullary catecholaminergic neurons, our recent in vitro observations indicate that GABAergic and glutamatergic afferents derive, at least partly, from local intrahypothalamic neurons, in close proximity to oxytocin neurons. The cellular mechanisms underlying this morphological synaptic plasticity remain to be determined but it is highly likely that they depend on increased activity in both pre- and post-synaptic elements. Moreover, the oxytocin system continues to express 'embryonic' molecular features that may allow the morphological plasticity to occur. In particular, it expresses high levels of cell surface adhesion molecules currently thought to intervene in synaptic remodelling in the developing and lesioned central nervous system, including the weakly adhesive polysialylated isoform of the Neural Cell Adhesion Molecule, the axonal glycoprotein F3 and its ligand, the extracellular matrix glycoprotein, tenascin-C.

Neurogenesis in the subventricular zone and rostral migratory stream of the neonatal and adult primate forebrain

Throughout life, the anterior part of the postnatal rodent subventricular zone (SVZa), surrounding the lateral ventricles, contains a prolific source of neuronal progenitor cells that retain their capacity to concurrently generate neurons and migrate along the rostral migratory stream (RMS) to the olfactory bulb, where they differentiate into interneurons. This study was designed to determine whether the SVZ and RMS of the postnatal primate also harbor a specialized population of neuronal progenitors with the capacity to divide while they migrate. In order to reveal the spatial-temporal changes in the distribution and composition of the neuronal progenitor cells in the primate SVZ and RMS, seven rhesus monkeys, ranging in age from 2 days to 8 years, were given a single injection of the cell proliferation marker bromodeoxyuridine (BrdU) 3 h before they were perfused. The phenotypic identity of the BrdU(+) cells was revealed by double labeling sagittal sections with cell type-specific markers. From birth onward the distribution of BrdU(+) cells with a neuronal phenotype is extensive and largely overlapping with that of the rodent. Similar to the rodent brain the neuronal progenitors are most numerous in neonates. The BrdU(+) neurons in the primate forebrain extend lateral and ventral to the lateral ventricle and all along the RMS. The cytoarchitectonic arrangement and appearance of the neuronal progenitor cells is quite varied in the primate compared to the rodent; in some locations the cells are aligned in parallel arrays resembling the neuronal chains of the adult rodent RMS, whereas in other positions the cells have a homogeneous "honeycomb" arrangement. The chains are progressively more pervasive in older primates. Akin to the RMS of adult rodents, in the primate SVZ and RMS the astrocytes often form long tubes enveloping the chains of neuronal progenitors. Our study demonstrates that the primate forebrain, similar to the rodent forebrain, harbors a specialized population of mitotically active neuronal progenitor cells that undergo extensive rearrangements while continuing to proliferate throughout life.

Activity and cross-reactivity of antibodies induced in mice by immunization with a group B meningococcal conjugate

The capsular polysaccharide of group B Neisseria meningitidis is composed of a linear homopolymer of alpha(2-8) N-acetyl neuraminic acid or polysialic acid (PSA) that is also carried by isoforms of the mammalian neural cell adhesion molecule (NCAM), which is especially expressed on brain cells during development. Here we analyzed the ability of antibodies induced by the candidate vaccine N-propionyl polysaccharide tetanus toxoid conjugate to recognize PSA-NCAM. We hyperimmunized mice to produce a pool of antisera and a series of immunoglobulin G monoclonal antibodies and evaluated their self-reactivity profile by using a battery of tests (immunoprecipitation, immunoblotting, and immunofluorescence detection on live cells and human tissue sections) chosen for their sensitivity and specificity to detect PSA-NCAM in various environments. We also searched for the effects of the vaccine-induced antibodies in two functional assays involving cell lysis or cell migration. Although they were highly bactericidal, all the antibodies tested showed very low or no recognition of PSA-NCAM, in contrast to PSA-specific monoclonal antibodies used as controls. Different patterns of cross-reactions were revealed by the tests used, likely due to affinity and specificity differences among the populations of induced antibodies. Furthermore, neither cell lysis nor perturbation of migration was observed in the presence of the tested antibodies. Importantly, we showed that whereas enzymatic removal of PSA groups from the surfaces of live cells perturbed their migration, blocking them with PSA-specific antibodies was not functionally detrimental. Taken together, our data indicated that this candidate vaccine induced antibodies that could not demonstrate an immunopathologic effect.

Dual effects of NMDA receptor activation on polysialylated neural cell adhesion molecule expression during brainstem postnatal development

Here we show a dual role of N-methyl-d-aspartate receptor (NMDAR) activation in controlling polysialylated neural cell adhesion molecule (PSA-NCAM) dynamic expression in the dorsal vagal complex (DVC), a gateway for many primary afferent fibres. In this structure the overall expression of PSA-NCAM decreases during the first 2 weeks after birth to persist only at synapses in the adult. Electrical stimulation of the vagal afferents causes a rapid increase of PSA-NCAM expression both in vivo and in acute slices before postnatal day (P) 14 whereas a similar stimulation induces a decrease after P15. Inhibition of NMDAR activity in vitro completely prevented these changes. These regulations depend on calmodulin activation and cGMP production at all stages. By contrast, blockade of neuronal nitric oxide synthase (nNOS) prevented these changes only after P10 in agreement with its late expression in the DVC. The pivotal role of NMDAR is also supported by the observation that chronic blockade induces a dramatic decrease in PSA-NCAM expression.