The highly sialylated isoform of the neural cell adhesion molecule is required for estradiol-induced morphological synaptic plasticity in the adult arcuate nucleus

Expression of polysialyltransferases (STX and PST) in adult rat olfactory bulb after an olfactory associative discrimination task

Neuronal plasticity and neurogenesis occur in the adult hippocampus and in other brain structures such as the olfactory bulb and often involve the neural cell adhesion molecule NCAM. During an olfactory associative discrimination learning task, NCAM polysialylation triggers neuronal plasticity in the adult hippocampus. The PST enzyme likely modulates this polysialylation, but not STX, a second sialyltransferase. How the two polysialyltransferases are involved in the adult olfactory bulb remains unknown. We addressed this question by investigating the effect of olfactory associative learning on plasticity and neurogenesis. After a hippocampo-dependent olfactory associative task learning, we measured the expression of both PST and STX polysialyltransferases in the olfactory bulbs of adult rats using quantitative PCR. In parallel, immunohistochemistry was used to evaluate both NCAM polysialylation level and newly-born cells, with or without learning. After learning, no changes were observed neither in the expression level of PST and NCAM polysialylation, nor in STX gene expression level and newly-born cells number in the olfactory bulb.

PSA-NCAM in the posterodorsal medial amygdala is necessary for the pubertal emergence of attraction to female odors in male hamsters

During puberty, attention turns away from same-sex socialization to focus on the opposite sex. How the brain mediates this change in perception and motivation is unknown. Polysialylated neural cell adhesion molecule (PSA-NCAM) virtually disappears from most of the central nervous system after embryogenesis, but it remains elevated in discrete regions of the adult brain. One such brain area is the posterodorsal subnucleus of the medial amygdala (MePD). The MePD has been implicated in male sexual attraction, measured here as the preference to investigate female odors. We hypothesize that PSA-NCAM gates hormone-dependent plasticity necessary for the emergence of males' attraction to females. To evaluate this idea, we first measured PSA-NCAM levels across puberty in several brain regions, and identified when female odor preference normally emerges in male Syrian hamsters. We found that MePD PSA-NCAM staining peaks shortly before the surge of pubertal androgen and the emergence of preference. To test the necessity of PSA-NCAM for female odor preference, we infused endo-neuraminidase-N into the MePD to deplete it of PSAs before female odor preference normally appears. This blocked female odor preference, which suggests that PSA-NCAM facilitates behaviorally relevant, hormone-driven plasticity.

Second generation anti-epileptic drugs adversely affect reproductive functions in young non-epileptic female rats

Reproductive endocrine disturbances are a major health concern in women with epilepsy due to their long term use of antiepileptic drugs (AEDs). Second generation AEDs such as topiramate (TPM) and gabapentin are frequently used for the treatment of epilepsy as well as migraine, bipolar disorder etc. Despite the widespread clinical complications, however the definitive mechanism(s) mediating the side effects of TPM and gabapentin remain obscure. The present study was aimed to evaluate the long term effects of TPM and gabapentin on reproductive functions in young female Wistar rats. Estrous cyclicity, ovarian histology as well as estradiol, LH, leptin and insulin hormones level were studied to elucidate the long-term effect of these AEDs monotherapy on reproductive functions in non-epileptic animals. Further to explore the effects on gonadotropin releasing hormone (GnRH) neuroendocrine plasticity, the expression of GnRH, gamma-amino butyric acid (GABA), glutamic acid decarboxylase (GAD), glial fibrilliary acidic protein (GFAP) and polysialylated form of neural cell adhesion molecule (PSA-NCAM) was studied in median eminence (ME) region of these animals by immunohistochemistry, Western blot hybridization and RT-PCR. Our results demonstrate that TPM and gabapentin treatment for 8 weeks cause reproductive dysfunction as ascertained by disturbed hormonal levels and estrous cyclicity as well as alterations in GABAergic system and GnRH neuronal-glial plasticity. Our findings suggest that treatment with TPM and gabapentin disrupts the complete hypothalamo-hypophyseal-gonadal axis (HPG) through GnRH pulse generator in hypothalamus.

The histone acetyltransferase MOF activates hypothalamic polysialylation to prevent diet-induced obesity in mice

Overfeeding causes rapid synaptic remodeling in hypothalamus feeding circuits. Polysialylation of cell surface molecules is a key step in this neuronal rewiring and allows normalization of food intake. Here we examined the role of hypothalamic polysialylation in the long-term maintenance of body weight, and deciphered the molecular sequence underlying its nutritional regulation. We found that upon high fat diet (HFD), reduced hypothalamic polysialylation exacerbated the diet-induced obese phenotype in mice. Upon HFD, the histone acetyltransferase MOF was rapidly recruited on the St8sia4 polysialyltransferase-encoding gene. Mof silencing in the mediobasal hypothalamus of adult mice prevented activation of the St8sia4 gene transcription, reduced polysialylation, altered the acute homeostatic feeding response to HFD and increased the body weight gain. These findings indicate that impaired hypothalamic polysialylation contribute to the development of obesity, and establish a role for MOF in the brain control of energy balance.

Sialic acids in the brain: gangliosides and polysialic acid in nervous system development, stability, disease, and regeneration

Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

NCAM function in the adult brain: lessons from mimetic peptides and therapeutic potential

Neural cell adhesion molecules (NCAMs) are complexes of transmembranal proteins critical for cell-cell interactions. Initially recognized as key players in the orchestration of developmental processes involving cell migration, cell survival, axon guidance, and synaptic targeting, they have been shown to retain these functions in the mature adult brain, in relation to plastic processes and cognitive abilities. NCAMs are able to interact among themselves (homophilic binding) as well as with other molecules (heterophilic binding). Furthermore, they are the sole molecule of the central nervous system undergoing polysialylation. Most interestingly polysialylated and non-polysialylated NCAMs display opposite properties. The precise contributions each of these characteristics brings in the regulations of synaptic and cellular plasticity in relation to cognitive processes in the adult brain are not yet fully understood. With the aim of deciphering the specific involvement of each interaction, recent developments led to the generation of NCAM mimetic peptides that recapitulate identified binding properties of NCAM. The present review focuses on the information such advances have provided in the understanding of NCAM contribution to cognitive function.

Depletion of polysialic acid from neural cell adhesion molecule (PSA-NCAM) increases CA3 dendritic arborization and increases vulnerability to excitotoxicity

Chronic immobilization stress (CIS) shortens apical dendritic trees of CA3 pyramidal neurons in the hippocampus of the male rat, and dendritic length may be a determinant of vulnerability to stress. Expression of the polysialylated form of neural cell adhesion molecule (PSA-NCAM) in the hippocampal formation is increased by stress, while PSA removal by Endo-neuraminidase-N (endo-N) is known to cause the mossy fibers to defasciculate and synapse ectopically in their CA3 target area. We show here that enzymatic removal of PSA produced a remarkable expansion of dendritic arbors of CA3 pyramidal neurons, with a lesser effect in CA1. This expansion eclipsed the CIS-induced shortening of CA3 dendrites, with the expanded dendrites of both no-stress-endo-N and CIS-endo-N rats being longer than those in no-stress-control rats and much longer than those in CIS-control rats. As predicted by the hypothesis that endo-N-induced dendritic expansion might increase vulnerability to excitotoxic challenge, systemic injection with kainic acid, showed markedly increased neuronal degeneration, as assessed by fluorojade B histochemistry, in rats that had been treated with endo-N compared to vehicle-treated rats throughout the entire hippocampal formation. PSA removal also exacerbated the CIS-induced reduction in body weight and abolished effects of CIS on NPY and NR2B mRNA levels. These findings support the hypothesis that CA3 arbor plasticity plays a protective role during prolonged stress and clarify the role of PSA-NCAM in stress-induced dendritic plasticity.

Hypothalamic control of energy balance: insights into the role of synaptic plasticity

The past 20 years witnessed an enormous leap in understanding of the central regulation of whole-body energy metabolism. Genetic tools have enabled identification of the region-specific expression of peripheral metabolic hormone receptors and have identified neuronal circuits that mediate the action of these hormones on behavior and peripheral tissue functions. One of the surprising findings of recent years is the observation that brain circuits involved in metabolism regulation remain plastic through adulthood. In this review, we discuss these findings and focus on the role of neurons and glial cells in the dynamic process of plasticity, which is fundamental to the regulation of physiological and pathological metabolic events.

Involvement of doublecortin-expressing cells in the arcuate nucleus in body weight regulation

Hypothalamic functions, including feeding behavior, show a high degree of plasticity throughout life. Doublecortin (DCX) is a marker of plasticity and neuronal migration expressed in the hypothalamus. Therefore, we wanted to map the fate of DCX(+) cells in the arcuate nucleus (ARC) of the hypothalamus. For this purpose, we generated a BAC transgenic mouse line that expresses the inducible recombinase CreER(T2) under control of the DCX locus. Crossing this line with the Rosa26 or Ai14 reporter mouse lines, we found reporter(+) cells in the ARC upon tamoxifen treatment. They were born prenatally and expressed both DCX and the plasticity marker TUC-4. Immediately after labeling, reporter(+) cells had an enlarged soma that normalized over time, suggesting morphological remodeling. Reporter(+) cells expressed β-endorphin and BSX, neuronal markers of the feeding circuit. Furthermore, leptin treatment led to phosphorylation of STAT3 in reporter(+) cells in accordance with the concept that they are part of the feeding circuits. Indeed, we found a negative correlation between the number of reporter(+) cells and body weight and epididymal fat pads. Our data suggest that DCX(+) cells in the ARC represent a cellular correlate of plasticity that is involved in controlling energy balance in adult mice.

Enzymatic removal of polysialic acid from neural cell adhesion molecule interrupts gonadotropin releasing hormone (GnRH) neuron-glial remodeling

There is abundant evidence to prove that the astrocytes are highly dynamic cell type in CNS and under physiological conditions such as reproduction, these cells display a remarkable structural plasticity especially at the level of their distal processes ensheathing the gonadotropin releasing hormone (GnRH) axon terminals. The morphology of GnRH axon terminals and astrocytes in the median eminence region of hypothalamus show activity dependent structural plasticity during different phases of estrous cycle. In the current study, we have assessed the functional contribution of ∞-2,8-linked polysialic acid (PSA) on neural cell adhesion molecule (PSA-NCAM) in this neuronal-glial plasticity using both in vitro and in vivo model systems. In vivo experiments were carried out after stereotaxic injection of endoneuraminidase enzyme (endo-N) near median eminence region of hypothalamus to specifically remove PSA residues on NCAM followed by localization of GnRH, PSA-NCAM and glial fibrillary acidic protein (GFAP) by immunostaining. Using in vitro model, structural remodeling of GnV-3 cells, (a conditionally immortalized GnRH cell line) co-cultured with primary astrocytes was studied after treating the cells with endo-N. Marked morphological changes were observed in GnRH axon terminals in proestrous phase rats and control GnV-3 cells as compared to endo-N treatment i.e. after removal of PSA. The specificity of endo-N treatment was also confirmed by studying the expression of PSA-NCAM by Western blotting in cultures treated with and without endo-N. Removal of PSA from surfaces with endo-N prevented stimulation associated remodeling of GnRH axon terminals as well as their associated glial cells under both in vivo and in vitro conditions. The current data confirms the permissive role of PSA to promote dynamic remodeling of GnRH axon terminals and their associated glia during reproductive cycle in rats.

[Structural plasticity of the adult central nervous system: insights from the neuroendocrine hypothalamus]

Accumulating evidence renders the dogma obsolete according to which the structural organization of the brain would remain essentially stable in adulthood, changing only in response to a need for compensatory processes during increasing age and degeneration. It has indeed become clear from investigations on various models that the adult nervous system can adapt to physiological demands by altering reversibly its synaptic circuits. This potential for structural and functional modifications results not only from the plastic properties of neurons but also from the inherent capacity of the glial cellular components to undergo remodeling as well. This is currently known for astrocytes, the major glial cells in brain which are well-recognized as dynamic partners in the mechanisms of synaptic transmission, and for the tanycytes and pituicytes which contribute to the regulation of neurosecretory processes in neurohemal regions of the hypothalamus. Studies on the neuroendocrine hypothalamus, whose role is central in homeostatic regulations, have gained good insights into the spectacular neuronal-glial rearrangements that may subserve functional plasticity in the adult brain. Following pioneering works on the morphological reorganizations taking place in the hypothalamo-neurohypophyseal system under certain physiological conditions such as dehydration and lactation, studies on the gonadotropic system that orchestrates reproductive functions have re-emphasized the dynamic interplay between neurons and glia in brain structural plasticity processes. This review summarizes the major contributions provided by these researches in the field and also addresses the question of the morphological rearrangements that occur on a 24-h basis in the central component of the circadian clock responsible for the temporal aspects of endocrine regulations. Taken together, the reviewed data highlight the close cooperation between neurons and glia in developing strategies for functional adaptation of the brain to the changing conditions of the internal and external environment.

Melatonin controls photoperiodic changes in tanycyte vimentin and neural cell adhesion molecule expression in the Djungarian hamster (Phodopus sungorus)

The Djungarian hamster displays photoperiodic variations in gonadal size synchronized to the seasons by the nightly secretion of the pineal hormone melatonin. In short photoperiod (SP), the gonads regress in size, and circulating sex steroids levels decline. Thus, the brain is subject to seasonal variations of both melatonin and sex steroids. Tanycytes are specialized glial cells located in the ependymal lining of the third ventricle. They send processes either to the meninges or to blood vessels of the medio-basal hypothalamus. Furthermore, they are known to locally modulate GnRH release in the median eminence and to display seasonal structural changes. Seasonal changes in tanycyte morphology might be mediated either through melatonin or sex steroids. Therefore, we analyzed the effects of photoperiod, melatonin, and sex steroids 1) on tanycyte vimentin expression by immunohistochemistry and 2) on the expression of the neural cell adhesion molecule (NCAM) and polysialic acid as markers of brain plasticity. Vimentin immunostaining was reduced in tanycyte cell bodies and processes in SP. Similarly, tanycytes and their processes contained lower amounts of NCAM in SP. These changes induced by SP exposure could not be restored to long photoperiod (LP) levels by testosterone supplementation. Likewise, castration in LP did not affect tanycyte vimentin or NCAM expression. By contrast, late afternoon melatonin injections mimicking a SP-like melatonin peak in LP hamsters reduced vimentin and NCAM expression. Thus, the seasonal changes in vimentin and NCAM expression in tanycytes are regulated by melatonin independently of seasonal sex steroid changes.

Hypogonadism predisposes males to the development of behavioural and neuroplastic depressive phenotypes

The incidence of depression is 2-3× higher in women particularly during the reproductive years, an occurrence that has been associated with levels of sex hormones. The age-related decline of testosterone levels in men corresponds with the increased acquisition of depressive symptoms, and hormone replacement therapy can be efficacious in treating depression in hypogonadal men. Although it is not possible to model depression in rodents, it is possible to model some of the symptoms of depression including a dysregulated stress response and altered neuroplasticity. Among animal models of depression, chronic mild unpredictable stress (CMS) is a common paradigm used to induce depressive-like behaviours in rodents, disrupt the hypothalamic-pituitary adrenal axis and decrease hippocampal neuroplasticity. The purpose of this study was to assess the effect of hypogonadism, produced by gonadectomy, on the acquisition of depressive-like behaviours and changes in hippocampal neuroplasticity in adult male Sprague-Dawley rats. A 21-day unpredictable CMS protocol was used on gonadectomised (GDX) and sham-operated males which produced an attenuation of weight gain in the GDX males receiving CMS treatment (GDX-CMS). Behavioural analysis was carried out to assess anxiety- and depressive-like behaviours. The combination of GDX and CMS produced greater passive behaviours within the forced swim test than CMS exposure alone. Similarly, hippocampal cell proliferation, neurogenesis and the expression of the neuroplastic protein polysialated neural cell adhesion molecule (PSA-NCAM) were all significantly reduced in the GDX-CMS group compared to all other treatment groups. These findings indicate that testicular hormones confer resiliency to chronic stress in males therefore reducing the likelihood of developing putative physiological, behavioural or neurological depressive-like phenotypes.

Extrinsic and intrinsic factors controlling axonal regeneration after spinal cord injury

Spinal cord injury is one of the most devastating conditions that affects the central nervous system. It can lead to permanent disability and there are around two million people affected worldwide. After injury, accumulation of myelin debris and formation of an inhibitory glial scar at the site of injury leads to a physical and chemical barrier that blocks axonal growth and regeneration. The mammalian central nervous system thus has a limited intrinsic ability to repair itself after injury. To improve axonal outgrowth and promote functional recovery, it is essential to identify the various intrinsic and extrinsic factors controlling regeneration and navigation of axons within the inhibitory environment of the central nervous system. Recent advances in spinal cord research have opened new avenues for the exploration of potential targets for repairing the cord and improving functional recovery after trauma. Here, we discuss some of the important key molecules that could be harnessed for repairing spinal cord injury.

Role of polysialylated neural cell adhesion molecule in rapid eye movement sleep regulation in rats

Recent evidence suggests that synaptic plasticity occurs during homeostatic processes, including sleep-wakefulness regulation, although the underlying mechanisms are not well understood. Polysialylated neural cell adhesion molecule (PSA NCAM) is a transmembrane protein that has been implicated in various forms of plasticity. To investigate whether PSA NCAM is involved in the neuronal plasticity associated with spontaneous sleep-wakefulness regulation and sleep homeostasis, four studies were conducted using rats. First, we showed that PSA NCAM immunoreactivity is present in close proximity to key neurons in several nuclei of the sleep-wakefulness system, including the tuberomammillary hypothalamic nucleus, dorsal raphe nucleus, and locus coeruleus. Second, using western blot analysis and densitometric image analysis of immunoreactivity, we found that 6 h of sleep deprivation changed neither the levels nor the general location of PSA NCAM in the sleep-wakefulness system. Finally, we injected endoneuraminidase (Endo N) intracerebroventricularly to examine the effects of polysialic acid removal on sleep-wakefulness states and electroencephalogram (EEG) slow waves at both baseline and during recovery from 6 h of sleep deprivation. Endo N-treated rats showed a small but significant decrease in baseline rapid eye movement (REM) sleep selectively in the late light phase, and a facilitated REM sleep rebound after sleep deprivation, as compared with saline-injected controls. Non-REM sleep and wakefulness were unaffected by Endo N. These results suggest that PSA NCAM is not particularly involved in the regulation of wakefulness or non-REM sleep, but plays a role in the diurnal pattern of REM sleep as well as in some aspects of REM sleep homeostasis.

H3 receptor antagonism enhances NCAM PSA-mediated plasticity and improves memory consolidation in odor discrimination and delayed match-to-position paradigms

To further understand the procognitive actions of GSK189254, a histamine H(3) receptor antagonist, we determined its influence on the modulation of hippocampal neural cell adhesion molecule (NCAM) polysialylation (PSA) state, a necessary neuroplastic mechanism for learning and memory consolidation. A 4-day treatment with GSK189254 significantly increased basal expression of dentate polysialylated cells in rats with the maximal effect being observed at 0.03-0.3 mg/kg. At the optimal dose (0.3 mg/kg), GSK189254 enhanced water maze learning and the associated transient increase in NCAM-polysialylated cells. The increase in dentate polysialylated cell frequency induced by GSK189254 was not attributable to enhanced neurogenesis, although it did induce a small, but significant, increase in the survival of these newborn cells. GSK189254 (0.3 mg/kg) was without effect on polysialylated cell frequency in the entorhinal and perirhinal cortex, but significantly increased the diffuse PSA staining observed in the anterior, ventromedial, and dorsomedial aspects of the hypothalamus. Consistent with its ability to enhance the learning-associated, post-training increases in NCAM PSA state, GSK189254 (0.3 mg/kg) reversed the amnesia induced by scopolamine given in the 6-h post-training period after training in an odor discrimination paradigm. Moreover, GSK189254 significantly improved the performance accuracy of a delayed match-to-position paradigm, a task dependent on the prefrontal cortex and degree of cortical arousal, the latter may be related to enhanced NCAM PSA-associated plasticity in the hypothalamus. The procognitive actions of H3 antagonism combined with increased NCAM PSA expression may exert a disease-modifying action in conditions harboring fundamental deficits in NCAM-mediated neuroplasticity, such as schizophrenia and Alzheimer's disease.

Estrogens regulate posttranslational modification of neural cell adhesion molecule during the estrogen-induced gonadotropin surge

Estrogen-induced synaptic plasticity (EISP) in the periventricular area (PVA) of the hypothalamus is necessary for the preovulatory gonadotropin surge. Because in situ enzymatic desialization of hypothalamic polysialylated (PSA) neural cell adhesion molecule (NCAM) blocked EISP, we examined the presence and amount of NCAM isotopes, PSA-NCAM, and sialylation enzymes in microdissected mouse hypothalamus tissues from proestrous afternoon [peak of estrogens and nadir of arcuate nucleus (AN) synapses] and metestrous morning (nadir of estrogens and highest AN synapses). Immunohistochemistry confirmed immunoreactive (ir) PSA-NCAM staining in the perineural spaces of the PVA. The extent of staining was cycle dependent, with more dense and complete profiles of individual neurons limned by the ir-PSA-NCAM staining on proestrus and less on metestrus. Western blots showed that high levels of ir-PSA-NCAM on proestrus are accompanied by diminished ir-NCAM-140 and -180 but not ir-NCAM-120 and the reverse on metestrus (P < 0.05). To evaluate the increase of sialylated NCAM at the expense of desialylated protein, expression of the responsible polysialyltransferase enzymes polysialyltransferase (ST8Sia IV) and sialyltransferase (ST8Sia II) mRNA levels were measured using RT-PCR. Both polysialyltransferase and sialyltransferase mRNA are more abundant on proestrus than metestrus (P < 0.05), indicating that these enzymes are regulated by estrogens. These results support estrogen-regulated formation and extrusion of hydrophilic PSA-NCAM into perineural spaces in the PVA as part of the mechanism of EISP.

Polysialic acid and activity-dependent synapse remodeling

Polysialic acid (PSA) is a large carbohydrate added post-translationally to the extracellular domain of the Neural Cell Adhesion Molecule (NCAM) that influences its adhesive and other functional properties. PSA-NCAM is widely distributed in the developing nervous system where it promotes dynamic cell interactions, like those responsible for axonal growth, terminal sprouting and target innervation. Its expression becomes restricted in the adult nervous system where it is thought to contribute to various forms of neuronal and glial plasticity. We here review evidence, obtained mainly from hypothalamic neuroendocrine centers and the olfactory system, that it intervenes in structural synaptic plasticity and accompanying neuronal-glial transformations, making possible the formation and elimination of synapses that occur under particular physiological conditions. While the mechanism of action of this complex sugar is unknown, it is now clear that it is a necessary molecular component of various cell transformations, including those responsible for activity-dependent synaptic remodeling.

Curcumin-induced degradation of PKC delta is associated with enhanced dentate NCAM PSA expression and spatial learning in adult and aged Wistar rats

Polysialylation of the neural cell adhesion molecule (NCAM PSA) is necessary for the consolidation processes of hippocampus-based learning. Previously, we have found inhibition of protein kinase C delta (PKCdelta) to be associated with increased polysialyltransferase (PST) activity, suggesting inhibitors of this kinase might ameliorate cognitive deficits. Using a rottlerin template, a drug previously considered an inhibitor of PKCdelta, we searched the Compounds Available for Purchase (CAP) database with the Accelrys((R)) Catalyst programme for structurally similar molecules and, using the available crystal structure of the phorbol-binding domain of PKCdelta, found that diferuloylmethane (curcumin) docked effectively into the phorbol site. Curcumin increased NCAM PSA expression in cultured neuro-2A neuroblastoma cells and this was inversely related to PKCdelta protein expression. Curcumin did not directly inhibit PKCdelta activity but formed a tight complex with the enzyme. With increasing doses of curcumin, the Tyr(131) residue of PKCdelta, which is known to direct its degradation, became progressively phosphorylated and this was associated with numerous Tyr(131)-phospho-PKCdelta fragments. Chronic administration of curcumin in vivo also increased the frequency of polysialylated cells in the dentate infragranular zone and significantly improved the acquisition and consolidation of a water maze spatial learning paradigm in both adult and aged cohorts of Wistar rats. These results further confirm the role of PKCdelta in regulating PST and NCAM PSA expression and provide evidence that drug modulation of this system enhances the process of memory consolidation.

The role of glia in the hypothalamus: implications for gonadal steroid feedback and reproductive neuroendocrine output

Neuron-to-glia, glia-to-neuron, and glia-to-glia communication are implicated in the modulation of neuronal activity and synaptic transmission relevant to reproduction. Glial cells play an important role in neuroendocrine regulation and participate in the sexual differentiation of neuronal connectivity of brain regions involved in the control of reproductive neuroendocrine output. During puberty, modifications in the morphology and chemistry of astrocytes and tanycytes in the hypothalamus and median eminence influence the maturation of the neuronal circuits controlling the secretion of GnRH. During adult reproductive life, the glial cells participate in the transient remodeling of neuronal connectivity in the preoptic area, the arcuate nucleus, the median eminence, and other brain regions involved in the control of reproduction. Gonadal hormones regulate glial plasticity by direct and indirect effects and regulate various other endocrine signals, local soluble factors and adhesion molecules that also affect glial function and glia-to-neuron communication. The glial cells, therefore, are central to the coordination of endocrine and local inputs that bring about neural plasticity and adapt reproductive capacity to homeostatic signals.

Activity-Dependent Structural and Functional Plasticity of Astrocyte-Neuron Interactions

Observations from different brain areas have established that the adult nervous system can undergo significant experience-related structural changes throughout life. Less familiar is the notion that morphological plasticity affects not only neurons but glial cells as well. Yet there is abundant evidence showing that astrocytes, the most numerous cells in the mammalian brain, are highly mobile. Under physiological conditions as different as reproduction, sensory stimulation, and learning, they display a remarkable structural plasticity, particularly conspicuous at the level of their lamellate distal processes that normally ensheath all portions of neurons. Distal astrocytic processes can undergo morphological changes in a matter of minutes, a remodeling that modifies the geometry and diffusion properties of the extracellular space and relationships with adjacent neuronal elements, especially synapses. Astrocytes respond to neuronal activity via ion channels, neurotransmitter receptors, and transporters on their processes; they transmit information via release of neuroactive substances. Where astrocytic processes are mobile then, astrocytic-neuronal interactions become highly dynamic, a plasticity that has important functional consequences since it modifies extracellular ionic homeostasis, neurotransmission, gliotransmission, and ultimately neuronal function at the cellular and system levels. Although a complete picture of intervening cellular mechanisms is lacking, some have been identified, notably certain permissive molecular factors common to systems capable of remodeling (cell surface and extracellular matrix adhesion molecules, cytoskeletal proteins) and molecules that appear specific to each system (neuropeptides, neurotransmitters, steroids, growth factors) that trigger or reverse the morphological changes.

Potential of PSA-NCAM in neuron-glial plasticity in the adult hypothalamus: role of noradrenergic and GABAergic neurotransmitters

The present study was designed to establish the dynamic regulation of polysialylated form of neural cell adhesion molecule (PSA-NCAM) expression by neurotransmitters controlling gonadotropin releasing hormone (GnRH) secretion. The expression of PSA-NCAM and glial fibrillary acidic protein (GFAP) on GnRH cell bodies and axon terminals in the medial preoptic area (mPOA) and median eminence-arcuate (ME-ARC) region of hypothalamus was studied in the proestrous phase of cycling rats treated with alpha-adrenergic receptor blocker phenoxybenzamine (PBZ) and gamma-aminobutyric acid (GABA) by using dual immunohistofluorescent staining and Western blot hybridization. To further elucidate whether activity mediated regulation of PSA-NCAM in GnRH neuron is via regulation of PSA biosynthesis by polysialytransferase (PST) enzyme, the expression of PST-1 enzyme was studied by using fluorescent in situ hybridization (FISH). Both GnRH and PSA-NCAM immunostaining was much higher in the mPOA and ME-ARC region from proestrous phase rats, whereas, PBZ and GABA treatments significantly reduced their expression, GFAP-ir and its content were increased in the PBZ and GABA treated proestrous rats. Taken together, our observations add to the growing evidence that PSA-NCAM plays permissive role for neuronal-glial remodeling and further suggest a functional link between activity dependent structural remodeling in GnRH neurons. Further, enhanced mRNA expression of PST suggests that the biosynthesis of PSA on NCAM is regulated at the transcriptional level.

Injury to retinal ganglion cell axons increases polysialylated neural cell adhesion molecule (PSA-NCAM) in the adult rodent superior colliculus

The adult mammalian central nervous system (CNS) exhibits a limited regenerative response to injury. It is well established that polysialylated neural cell adhesion molecule (PSA-NCAM) contributes to nervous system plasticity. In the visual system, PSA-NCAM participates in retinal ganglion cell (RGC) axon growth during development and specifically influences RGC innervation of its principle target tissue, the superior colliculus (SC). The goals of this study were to determine whether PSA-NCAM is expressed in the normal adult mouse SC and to evaluate PSA-NCAM expression following RGC injury. In the normal rostral, but not caudal, SC we find that PSA-NCAM is present in the retinorecipient layers; however, PSA-NCAM and RGC axons do not co-localize. In the deeper collicular layers, PSA-NCAM is observed as several distinct patches that occur at the same depth along the medial-lateral axis throughout the colliculus. RGC axotomy denervates predominantly the contralateral colliculus, where increased PSA-NCAM levels are seen at 7 and 10 days after the injury. Further evaluation of the retinorecipient layers of the partially denervated SC reveals that some intact CTB-traced RGC axons (less than 5%) labeled from the ipsilateral eye do co-localize with PSA-NCAM. This study is the first characterization of PSA-NCAM expression in the normal and partially denervated adult SC and may indicate that PSA-NCAM is involved in attempted visual system remodeling after injury.

Fluctuation of synapse density in the arcuate nucleus during the estrous cycle

The hypothalamic arcuate nucleus integrates different hormonal and neural signals to control neuroendocrine events, feeding, energy balance and reproduction. Previous studies have shown that in adult female rats the arcuate nucleus undergoes a cyclic fluctuation in the number of axo-somatic synapses during the estrous cycle, in parallel to the variation of ovarian hormone levels in plasma. In the present study we have used an unbiased stereological analysis in conjunction with postembedding immunocytochemistry to assess whether the synaptic remodeling during the estrous cycle in rats is specific for certain types of synapses. Our findings indicate that there is a significant decrease in the number of GABAergic axo-somatic synapses on proestrus afternoon and estrus day compared with other days of the estrous cycle. This decrease in GABAergic synapses is accompanied by an increase in the number of dendritic spine synapses. The synaptic density appears to cycle back to proestrus morning values on metestrus day. In contrast, the number of synapses on dendritic shafts does not change during the cycle. These results indicate that a rapid and selective synaptic turnover of arcuate synapses occurs in physiological circumstances.

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.

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.

Complex-environment rearing prevents prenatal hypoxia-induced deficits in hippocampal cellular mechanisms necessary for memory consolidation in the adult Wistar rat

Hypoxic episodes in utero can result in enduring and debilitating neurological sequelae that include nonprogressive motor disorders and/or significant learning deficits. The extent of long-term disruption of synaptic function following prenatal hypoxia and its subsequent effect on learning ability, however, remain to be established. Polysialylation of the neural cell adhesion molecule, a cellular event integral to the consolidation of diverse learning paradigms, was used to correlate cellular end points with learning deficits as a consequence of prenatal hypoxia. Pregnant Wistar dams exposed to hypobaric hypoxia during gestational days 10-20 had significantly reduced litter sizes, but the lack of effect on subsequent pup weight gain suggested no gross developmental deficit. By contrast, adult animals with prior in utero hypoxia exhibited significant learning difficulties in both acquisition of a water maze spatial learning task and recall of a passive avoidance paradigm. Learning deficits correlated with a significant reduction in the frequency of polysialylated neurons in the dentate infragranular zone and a blunting of their transient activation 12 hr following task acquisition. Rearing animals with prior prenatal hypoxia in a complex environment, however, eliminated the task acquisition and recall deficits and restored dentate polysialylated cell frequency and their transient posttraining increase.

Plasticity in the injured spinal cord: can we use it to advantage to reestablish effective bladder voiding and continence?

Micturition is coordinated at the level of the spinal cord and the brainstem. Spinal cord injury therefore directly interrupts spinal neuronal pathways to the brainstem and results in bladder areflexia. Some time after injury, however, dyssynergic bladder and sphincter function emerges. The changes mediating the appearance of bladder function after spinal cord injury are currently unknown. Primary afferent neurons have been shown to sprout in response to spinal cord injury. Sprouting primary afferents have been linked to the pathophysiology of centrally manifested disorders, such as autonomic dysreflexia and neuropathic pain. It is proposed that sprouting of bladder primary afferents contributes to disordered bladder functioning after spinal cord injury. During development of the central nervous system, the levels of specific neuronal growth-promoting and guidance molecules are high. After spinal cord injury, some of these molecules are upregulated in the bladder and spinal cord, suggesting that axonal outgrowth is occurring. Sprouting in lumbosacral spinal cord is likely not restricted to neurons involved in the micturition reflex. Furthermore, sprouting of some afferents may be contributing to bladder function after injury, whereas sprouting of others might be hindering emergence of function. Thus selective manipulation of sprouting targeting afferents that are contributing to emergence of bladder function after injury is critical. Further research regarding the role that neuronal sprouting plays in the emergence of bladder function may contribute to improved treatment of bladder dyssynergia after spinal cord injury.

Chronic exposure of rats to cognition enhancing drugs produces a neuroplastic response identical to that obtained by complex environment rearing

Recent data suggest that Alzheimer's patients who discontinue treatment with cholinesterase inhibitors have a significantly delayed cognitive decline as compared to patients receiving placebo. Such observations suggest cholinesterase inhibitors to provide a disease-modifying effect as well as symptomatic relief and, moreover, that this benefit remains after drug withdrawal. Consistent with this suggestion, we now demonstrate that chronic administration of tacrine, nefiracetam, and deprenyl, drugs that augment cholinergic function, increases the basal frequency of dentate polysialylated neurons in a manner similar to the enhanced neuroplasticity achieved through complex environment rearing. While both drug-treated and complex environment reared animals continue to exhibit memory-associated activation of hippocampal polysialylated neurons, the magnitude is significantly reduced suggesting that such interventions induce a more robust memory pathway that can acquire and consolidate new information more efficiently. This hypothesis is supported by our findings of improved learning behavior and enhanced resistance to cholinergic deficits seen following either intervention. Furthermore, the level of enhancement of basal neuroplastic status achieved by either drug or environmental intervention correlates directly with improved spatial learning ability. As a combination of both interventions failed to further increase basal polysialylated cell frequency, complex environment rearing and chronic drug regimens most likely enhanced cognitive performance by the same mechanism(s). These findings suggest that improved memory-associated synaptic plasticity may be the fundamental mechanism underlying the disease modifying action of drugs such as cholinesterase inhibitors. Moreover, the molecular and cellular events underpinning neuroplastic responses are identified as novel targets in the search for interventive drug strategies for the treatment of neurodegenerative and neuropsychiatric disorders.

Amyloid precursor protein expression in the rat hippocampal dentate gyrus modulates during memory consolidation

Despite advances in our understanding of the basic biology of amyloid precursor protein (APP), the normal physiological function(s) of APP in learning and memory remains unclear. Here we show increased APP degradation in the hippocampus to be associated with the consolidation of a passive avoidance response. Neurone-specific APP695 expression became transiently reduced 2-4 h post-training through association with endosomal adaptin proteins and enhanced internalization. By contrast, internalization of glial-associated APP containing a Kunitz protease inhibitor-like domain (APP-KPI) was dependent on the low-density lipoprotein receptor-related protein (LRP). In addition, LRP expression and association with apolipoprotein E increased in the 2-4 h post-training period. The LRP antagonist receptor-associated protein prevented the APP-KPI internalization and LRP-apolipoprotein E association and this resulted in amnesia. Degradation of APP695 and APP-KPI did not appear to be related to alpha-secretase activity, as no learning-associated increase of secreted APP was observed in the CSF. Moreover, as internalization of APP isoforms was observed only in dentate gyrus, it probably relates to the learning-associated restructuring of the perforant path terminals. Memory-associated APP processing in both neuronal and glial compartments points to a role for glial unsheathing of synaptic connections, an event required for the synaptic restructuring that accompanies memory consolidation. These observations may have a direct relevance to understanding the pathophysiology of Alzheimer's disease as beta/gamma-secretase-derived beta-amyloid is formed following internalization of cell surface APP into the endosomal compartment.

Synaptic remodeling induced by gonadal hormones: neuronal plasticity as a mediator of neuroendocrine and behavioral responses to steroids

During recent decades, it has become a generally accepted view that structural neuroplasticity is remarkably involved in the functional adaptation of the CNS. Thus, cellular morphology in the brain is in continuous transition throughout the life span, as a response to environmental stimuli. The effects of the environment on neuroplasticity are mediated by, to some extent, the changing levels of circulating gonadal steroid hormones. Today, it is clear that the function of gonadal steroids in the brain extends beyond simply regulating reproductive and/or neuroendocrine events. In addition, or even more importantly, gonadal steroids participate in the shaping of the developing brain, while their actions during adult life are implicated in higher brain functions such as cognition, mood and memory. A large body of evidence indicates that gonadal steroid-induced functional changes are accompanied by alterations in neuron and synapse numbers, as well as in dendritic and synaptic morphology. These structural modifications are believed to serve as a morphological basis for changes in behavior and cellular activity. Due to their growing functional and clinical significance, the specificity, timeframe, as well as the molecular and cellular mechanisms of hormone-induced neuroplasticity have become the focus of many studies. In this review, we briefly summarize current knowledge and the most significant recent discoveries from our laboratories on estrogen- and dehydroepiandrosterone-induced synaptic remodeling in the hypothalamus and hippocampus, two important brain areas heavily involved in autonomic and cognitive operations, respectively.

Polysialylated neural cell adhesion molecule promotes remodeling and formation of hippocampal synapses

Expression of the neural cell adhesion molecule (NCAM) has been shown to promote long-term potentiation (LTP) and stabilization of synapses during early synaptogenesis. Here, we searched for the mechanisms of synaptogenic activity of NCAM, focusing on the role of polysialic acid (PSA), an unusual carbohydrate preferentially associated with NCAM. We show that enzymatic removal of PSA with endoneuraminidase-N (endo-N) abolished preferential formation of synapses on NCAM-expressing cells in heterogenotypic cocultures of wild-type and NCAM-deficient hippocampal neurons. Transfection of NCAM-deficient neurons with either of three major NCAM isoforms (different in intracellular domains but identical in extracellular domains and carrying PSA) stimulated preferential synapse formation on NCAM isoform-expressing neurons. Enzymatic removal of heparan sulfates from cultured neurons and a mutation in the heparin-binding domain of NCAM diminished synaptogenic activity of neuronally expressed PSA-NCAM, suggesting that interaction of NCAM with heparan sulfate proteoglycans mediates this activity. PSA-NCAM-driven synaptogenesis was also blocked by antagonists to fibroblast growth factor receptor and NMDA subtype of glutamate receptors but not by blockers of non-NMDA glutamate receptors and voltage-dependent Na+ channels. Enzymatic removal of PSA and heparan sulfates also blocked the increase in the number of perforated spine synapses associated with NMDA receptor-dependent LTP in the CA1 region of organotypic hippocampal cultures. Thus, neuronal PSA-NCAM in complex with heparan sulfate proteoglycans promotes synaptogenesis and activity-dependent remodeling of synapses.

Olfactory learning-related NCAM expression is state, time, and location specific and is correlated with individual learning capabilities

The notion that long-term synaptic plasticity is generated by activity-induced molecular modifications is widely accepted. It is well established that neural cell adhesion molecule (NCAM) is one of the prominent modulators of synaptic plasticity. NCAM can be polysialylated (PSA-NCAM), a reaction that provides it with anti-adhesion properties. In this study we have focused on NCAM and on its polysialylated state, and their relation to learning of an olfactory discrimination task, which depends on both the piriform (olfactory) cortex and hippocampus. We trained rats to distinguish between pairs of odors until rule learning was achieved, a process that normally lasts 6-8 days. At four time points, during training and after training completion, synaptic NCAM and PSA-NCAM expression were assessed in the piriform cortex and hippocampus. We report that NCAM modulation is specific to PSA-NCAM, which is upregulated in the hippocampus one day after training completion. We also report a correlation between the performance of individual rats in an early training stage and their NCAM expression, both in the piriform cortex and hippocampus. Since individual early performance in our odor discrimination task is correlated with the performance throughout the training period, we conclude that early NCAM expression is associated with odor learning capability. We therefore suggest that early synaptic NCAM expression may be one of the factors determining the capability of rats to learn.

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.

Pentyl-4-yn-valproic acid reverses age-associated memory impairment in the Wistar rat

Pentyl-4-yn-valproic acid (VPA), a cognition-enhancing agent whose mode of action has been attributed to cell adhesion molecule-mediated neuritogenesis, has been shown to enhance hippocampus-dependent spatial learning. Here, we investigated its potential to reverse age-related memory impairment that relates mainly to declarative memory. Aged spatial learning deficits in the water maze paradigm were demonstrated by swim angle analysis, the angle between axes of start-to-platform and start-to-animal position, and latency to reach a submerged platform. Chronic pentyl-4-yn-VPA administration mediated a significant improvement in both search strategy and latency to find the submerged platform in aged animals. Pentyl-4-yn-VPA also facilitated task recall in aged animals as evidenced by increased time in the target quadrant during a probe trial 3 days following the final training session. The action of pentyl-4-yn-VPA on platform latency, search strategy and task recall suggests that this agent may have great benefit in the treatment of age-dependent cognitive decline.

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.

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.

Seasonal plasticity within the gonadotropin-releasing hormone (GnRH) system of the ewe: changes in identified GnRH inputs and glial association

The annual reproductive cycle in sheep may reflect a functional remodeling within the GnRH system. Specifically, changes in total synaptic input and association with the polysialylated form of neural cell adhesion molecule have been observed. Whether seasonal changes in a specific subset(s) of GnRH inputs occur or whether glial cells specifically play a role in this remodeling is not clear. We therefore examined GnRH neurons of breeding season (BS) and nonbreeding season (anestrus) ewes and tested the hypotheses that specific (i.e. gamma-aminobutyric acid, catecholamine, neuropeptide Y, or beta-endorphin) inputs to GnRH neurons change seasonally, and concomitant with any changes in neural inputs is a change in glial apposition. Using triple-label immunofluorescent visualization of GnRH, glial acidic fibrillary protein and neuromodulator/neural terminal markers combined with confocal microscopy and optical sectioning techniques, we confirmed that total numbers of neural inputs to GnRH neurons vary with season and demonstrated that specific inputs contribute to these overall changes. Specifically, neuropeptide Y and gamma-aminobutyric acid inputs to GnRH neurons increased during BS and beta-endorphin inputs were greater during either anestrus (GnRH somas) or BS (GnRH dendrites). Associated with the changes in GnRH inputs were seasonal changes in glial apposition, glial acidic fibrillary protein density, and the thickness of glial fibrils. These findings are interpreted to suggest an increase in net stimulatory inputs to GnRH neurons during the BS contributes to the seasonal changes in GnRH neurosecretion and that this increased innervation is perhaps stabilized by glial processes.

Axonal sprouting of a brainstem-spinal pathway after estrogen administration in the adult female rhesus monkey

The nucleus retroambiguus (NRA) is located in the caudal medulla oblongata and contains premotor neurons that project to motoneuronal cell groups in the brainstem and spinal cord. NRA projections to the lumbosacral cord are species specific and might be involved in mating behavior. In the female cat, this behavior is estrogen dependent, and estrogen induces axonal sprouting in the NRA-lumbosacral pathway. Because female receptive behavior in primates is not fully dependent on estrogen, the question arises as to whether the capacity of estrogen-induced sprouting is preserved in primates. The effect of estrogen was studied on the NRA-lumbosacral projection with the use of wheat germ agglutinin conjugated to horseradish peroxidase as a tracer in six adult ovariectomized rhesus monkeys with or without estrogen priming (three controls and three treated with 20 microg/day of estradiol benzoate subcutaneously for 14 days). Light microscopy showed that the density of arborizing labeled NRA axons in the lumbosacral cord was greater in estrogen-treated than in control animals. Ultrastructurally, labeled NRA terminal profiles were quantified in motoneuron pools that supply muscles of the abdominal wall, axial, and pelvic floor. After estrogen treatment, the average number of labeled terminal profiles per area of the abdominal wall, axial, and pelvic floor motoneuron pool increased 1.5-, 3.3-, and 2.8-fold, respectively. In the estrogen-treated cases, 8.9% of labeled terminal profiles showed characteristics of growth cones. In controls, such profiles were rarely observed. The results showed that estrogen induces axonal sprouting in a brainstem-spinal pathway in the adult female rhesus monkey. These findings supported the concept that the NRA-lumbosacral pathway may be involved in sexual behavior. Moreover, they demonstrated that a long descending brainstem-spinal tract in adult nonhuman primates retains the capacity for axonal sprouting.

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.

Steroid modulation of astrocytes in the neonatal brain: implications for adult reproductive function

There is a growing appreciation for the importance of astrocytes, a type of nonneuronal glial cell, to overall brain functioning. The ability of astrocytes to respond to gonadal steroid hormones with changes in morphology has been well documented in the adult brain. It is also apparent that astrocytes of the developing brain are permanently differentiated by the neonatal hormonal milieu, in particular by estradiol, resulting in sexually dimorphic cell morphology, synaptic patterning, and density in males and females. The mechanisms of hormonally mediated astrocyte differentiation are likely to be region specific. In the arcuate nucleus of the hypothalamus, neuron-to-astrocyte signaling appears to play a critical role in estradiol-induced astrocyte differentiation during the first few days of life. Gamma aminobutyric acid (GABA) is an amino acid neurotransmitter that is synthesized and released exclusively by neurons. The levels of GABA are increased in the arcuate nucleus of neonatal males versus females. Preventing the increase in males or mimicking GABA action in females modulates astrocytes accordingly. Speculation about and evidence in support of the functional significance of this dimorphism to adult reproductive functioning is the topic of this review.