Involvement of highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive granule cells in the amygdaloid-kindling-induced sprouting of a hippocampal mossy fiber trajectory

Effect of low-frequency stimulation on kindling induced changes in rat dentate gyrus: an ultrastructural study

It has been shown that low-frequency stimulation (LFS) can induce anticonvulsant effects. In this study, the effect of different LFS frequencies on kindling induced behavioral and ultrastructural changes was investigated. For induction of kindled seizures in rats, stimulating and recording electrodes were implanted in perforant path and dentate gyrus, respectively. Animals were stimulated in a rapid kindling manner. Different groups of animals received LFS at different frequencies (0.5, 1 and 5 Hz) following kindling stimulations and their effects on kindling rate were determined using behavioral and ultrastructural studies. Kindling stimulations were applied for 7 days. Then, the animals were sacrificed and their dentate gyrus was sampled for ultrastructural studies under electron microscopy. All three used LFS frequencies (0.5, 1 and 5 Hz) had a significant inhibitory effect on kindling rate and decreased afterdischarge duration and the number of stimulations to achieve stage 4 and 5 seizures significantly. In addition, application of LFS prevented the increase in the post-synaptic density and induction of concave synaptic vesicles following kindling. There was no significant change between anticonvulsant effects of LFS at different frequencies. Obtained results show that LFS application can prevent the neuronal hyperexcitability by preventing the ultrastructural changes during kindling and this may be one of the mechanisms of LFS anticonvulsant effects.

The effect of PTZ-induced epileptic seizures on hippocampal expression of PSA-NCAM in offspring born to kindled rats

BACKGROUND

Maternal epileptic seizures during pregnancy can affect the hippocampal neurons in the offspring. The polysialylated neural cell adhesion molecule (PSA-NCAM), which is expressed in the developing central nervous system, may play important roles in neuronal migration, synaptogenesis, and axonal outgrowth. This study was designed to assess the effects of kindling either with or without maternal seizures on hippocampal PSA-NCAM expression in rat offspring.

METHODS

Forty timed-pregnant Wistar rats were divided into four groups: A) Kind+/Seiz+, pregnant kindled (induced two weeks prior to pregnancy) rats that received repeated intraperitoneal (i.p.) pentylenetetrazol, PTZ injections on gestational days (GD) 14-19; B) Kind-/Seiz+, pregnant non-kindled rats that received PTZ injections on GD14-GD19; C) Kind+/Seiz-, pregnant kindled rats that did not receive any PTZ injections; and D) Kind-/Seiz-, the sham controls. Following birth, the pups were sacrificed on PD1 and PD14, and PSA-NCAM expression and localization in neonates' hippocampi were analyzed by Western blots and immunohistochemistry.

RESULTS

Our data show a significant down regulation of hippocampal PSA-NCAM expression in the offspring of Kind+/Seiz+ (p = 0.001) and Kind-/Seiz+ (p = 0.001) groups compared to the sham control group. The PSA-NCAM immunoreactivity was markedly decreased in all parts of the hippocampus, especially in the CA3 region, in Kind+/Seiz+ (p = 0.007) and Kind-/Seiz+ (p = 0.007) group's newborns on both PD1 and 14.

CONCLUSION

Our findings demonstrate that maternal seizures but not kindling influence the expression of PSA-NCAM in the offspring's hippocampi, which may be considered as a factor for learning/memory and cognitive impairments reported in children born to epileptic mothers.

Polysialic acid is required for dopamine D2 receptor-mediated plasticity involving inhibitory circuits of the rat medial prefrontal cortex

Decreased expression of dopamine D2 receptors (D2R), dysfunction of inhibitory neurotransmission and impairments in the structure and connectivity of neurons in the medial prefrontal cortex (mPFC) are involved in the pathogenesis of schizophrenia and major depression, but the relationship between these changes remains unclear. The polysialylated form of the neural cell adhesion molecule (PSA-NCAM), a plasticity-related molecule, may serve as a link. This molecule is expressed in cortical interneurons and dopamine, via D2R, modulates its expression in parallel to that of proteins related to synapses and inhibitory neurotransmission, suggesting that D2R-targeted antipsychotics/antidepressants may act by affecting the plasticity of mPFC inhibitory circuits. To understand the role of PSA-NCAM in this plasticity, rats were chronically treated with a D2R agonist (PPHT) after cortical PSA depletion. PPHT-induced increases in GAD67 and synaptophysin (SYN) neuropil expression were blocked when PSA was previously removed, indicating a role for PSA-NCAM in this plasticity. The number of PSA-NCAM expressing interneuron somata also increased after PPHT treatment, but the percentages of these cells belonging to different interneuronal subpopulations did not change. Cortical pyramidal neurons did not express PSA-NCAM, but puncta co-expressing this molecule and parvalbumin could be found surrounding their somata. PPHT treatment increased the number of PSA-NCAM and parvalbumin expressing perisomatic puncta, but decreased the percentage of parvalbumin puncta that co-expressed SYN. PSA depletion did not block these effects on the perisomatic region, but increased further the number of parvalbumin expressing puncta and increased the percentage of puncta co-expressing SYN and parvalbumin, suggesting that the polysialylation of NCAM may regulate perisomatic inhibition of mPFC principal neurons. Summarizing, the present results indicate that dopamine acting on D2R influences structural plasticity of mPFC interneurons and point to PSA-NCAM as a key player in this remodeling.

Morphometrical study of polysialylated neural cell adhesion molecule positive cells in rat pups hippocampus following induction of seizure during pregnancy

BACKGROUND

The polysialylated neural cell adhesion molecule (PSA-NCAM) is expressed in developing brain. Fetal brain damage is caused by different conditions such as seizure and hypoxia. The present study was designed to investigate the effect of maternal seizures on the number of PSA-NCAM positive cells in pup's hippocampus.

METHODS

Female Wistar rats were divided into four groups: (a) kindled rats which received PTZ (40 mg/kg, i.p.) during pregnancy from embryonic day 14-19 (E14-E19) every 48 h, (b) kindled rats which did not receive PTZ during pregnancy, (c) non-kindle, pregnant rats which received PTZ injection (40 mg/kg, i.p.) during pregnancy from E14 to E19 every 48 h, and (d) non-kindle, pregnant rats which received injection with an equal volume of normal saline as sham controls. At postnatal day 14 (PD14), rat pups were perfused, and their brain were fixed, embedded and coronal sections stained by immunohistochemistry method. The number of PSA-NCAM positive cells per unit area in the pup's hippocampus was counted.

RESULTS

The number of PSA-NCAM positive cells in the CA1, CA3, and DG fields of pup's hippocampus, which was obtained from mothers who experienced PTZ injection during pregnancy, was decreased approximately 2.6 (P = 0.001), 2 (P = 0.001), and 2.1 (P = 0.001) times compared with non-PTZ treated maternal groups, respectively.

CONCLUSIONS

Our study showed that maternal seizures reduced the number of neurons and also PSA-NCAM positive cells per unit area in the offspring hippocampus that it may cause impairment in hippocampal functions.

Extracellular matrix protein SC1/hevin in the hippocampus following pilocarpine-induced status epilepticus

Pilocarpine-induced status epilepticus (SE) mimics many features of temporal lobe epilepsy and is a useful model to study neural changes that result from prolonged seizure activity. In this study, distribution of the anti-adhesive extracellular matrix protein SC1 was examined in the rat hippocampus following SE. Western blotting showed decreased levels of SC1 protein in the week following SE. Immunohistochemistry demonstrated that the decrease in overall SC1 protein levels was reflected by a reduction of SC1 signal in granule cells of the dentate gyrus. Interestingly, levels of SC1 protein in neurons of the seizure-resistant CA2 sector of the hippocampus did not change throughout the seizure time course. However, at 1 day post-SE, a subset of neurons of the hippocampal CA1, CA3, and hilar regions, which are noted for extensive neuronal degeneration after SE, exhibited a transient increase in SC1 signal. Neurons exhibiting enhanced SC1 signal were not detected at 7 days post-SE. The cellular stress response was also examined. A prominent induction of heat-shock protein (Hsp70) and Hsp27 was detected following SE, while levels of constitutively expressed Hsp40, Hsp90, Hsp110, and Hsc70 showed little change at the time points examined. The subset of neurons that demonstrated a transient increase in SC1 colocalized with the cellular stress marker Hsp70, the degeneration marker Fluoro-Jade B, and the neuron activity marker activity-regulated cytoskeleton-associated protein (Arc). Taken together, these findings suggest that SC1 may be a component of the 'matrix response' involved in remodeling events associated with neuronal degeneration following neural injury.

Polysialic acid–neural cell adhesion molecule in brain plasticity: From synapses to integration of new neurons

Isoforms of the neuronal cell adhesion molecule (NCAM) carrying the linear homopolymer of alpha 2,8-linked sialic acid (polysialic acid, PSA) have emerged as particularly attractive candidates for promoting plasticity in the nervous system. The large negatively charged PSA chain of NCAM is postulated to be a spacer that reduces adhesion forces between cells allowing dynamic changes in membrane contacts. Accumulating evidence also suggests that PSA-NCAM-mediated interactions lead to activation of intracellular signaling cascades that are fundamental to the biological functions of the molecule. An important role of PSA-NCAM appears to be during development, when its expression level is high and where it contributes to the regulation of cell shape, growth or migration. However, PSA-NCAM does persist in adult brain structures such as the hippocampus that display a high degree of plasticity where it is involved in activity-induced synaptic plasticity. Recent advances in the field of PSA-NCAM research have not only consolidated the importance of this molecule in plasticity processes but also suggest a role for PSA-NCAM in the regulation of higher cognitive functions and psychiatric disorders. In this review, we discuss the role and mode of actions of PSA-NCAM in structural plasticity as well as its potential link to cognitive processes.

Development of neurons expressing estrogen receptor α transiently in facial nucleus of prenatal and postnatal rat brains

The transient expression of estrogen receptor alpha (ERalpha) in the facial nucleus of rats during development was already reported. However, how and whether the receptor functions physiologically in the nucleus of developing rats are as yet unclear. In this study, we applied a retrograde tracer into one of the possible target muscles of the motoneurons in the nucleus, that is, the transverse auricular muscle (Mta), and examined whether ERalpha-immunopositive neurons take up the tracer. Because it is probable that neurogenesis, apoptosis, and maturation may be associated with the transient expression of ERalpha, we attempted to analyze the neurons expressing the receptor in the nucleus. We found that ERalpha-immunopositive neurons in the medial facial subnucleus innervate mostly the Mta. Quantitative analyses showed that the number of motoneurons projecting to the Mta remained the same throughout the ages examined, whereas that of ERalpha-immunopositive neurons decreased between postnatal days 6 and 11. Apoptosis and neurogenesis in the nucleus were not affected by the expression of ERalpha during development. ERalpha expression coincided with the maturation of neurons in the nucleus. Thus, it is possible that ERalpha expression in the facial nucleus during development plays important roles in the development of motoneurons and/or external pinna muscles.

Impact of the PSA-NCAM system on pathophysiology in a chronic rodent model of temporal lobe epilepsy

Polysialylation is a posttranslational modification of the neural cell adhesion molecule (NCAM). In the adult brain, polysialylated NCAM (PSA-NCAM) is restricted to regions of neurogenesis and neuroplasticity, where PSA promotes plastic changes. Because a variety of plastic changes including neurogenesis have been suggested to be functionally involved in the pathophysiology of epilepsies, it is of specific interest to define the impact of the PSA-NCAM system on development and progression of this disease and associated comorbidities. Here, we studied the impact of transient enzymatic depolysialylation of NCAM on the pathophysiology in the amygdala kindling model, a chronic rodent model of temporal lobe epilepsy. The investigations focused on seizure-induced neurogenesis, seizure progression, and on the development of kindling-associated changes in behavior and cognition. Loss of PSA decreased the number of hippocampal newborn cells that incorporated BrdU during the kindling process and the number of new neurons that were ectopically located in the hilus. The persistence of basal dendrites has been suggested to be a hallmark of newborn granule cells in the epileptic brain. Loss of PSA increased the number of cells with persistent basal dendrites. The modification of the hippocampal cell proliferation rate and the fate of newborn neurons which occurred as a consequence of PSA removal did not affect the generation of a hyperexcitable kindled network or associated behavioral changes. Kindling progression was comparable in rats with and without removal of PSA. In contrast, loss of PSA increased acute seizure susceptibility as indicated by reduced seizure thresholds before kindling. The data indicate that hippocampal proliferation rates and ectoptic hilar newborn neurons are less critical for epileptic network generation. The PSA-NCAM system was not substantiated as a target for antiepileptogenic strategies. However, its impact on ectopic newborn neurons gives evidence that modulation of PSA-NCAM function may be a strategy to promote neuroregeneration in different central nervous system insults.

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.

alpha-Tocopherol affects neuronal plasticity in adult rat dentate gyrus: the possible role of PKCdelta

Hippocampus dentate gyrus (DG) is characterized by neuronal plasticity processes in adulthood, and polysialylation of NCAM promotes neuronal plasticity. In previous investigations we found that alpha-tocopherol increased the PSA-NCAM-positive granule cell number in adult rat DG, suggesting that alpha-tocopherol may enhance neuronal plasticity. To verify this hypothesis, in the present study, structural remodeling in adult rat DG was investigated under alpha-tocopherol supplementation conditions. PSA-NCAM expression was evaluated by Western blotting, evaluation of PSA-NCAM-positive granule cell density, and morphometric analysis of PSA-NCAM-positive processes. In addition, the optical density of synaptophysin immunoreactivity and the synaptic profile density, examined by electron microscopy, were evaluated. Moreover, considering that PSA-NCAM expression has been found to be related to PKCdelta activity and alpha-tocopherol has been shown to inhibit PKC activity in vitro, Western blotting and immunohistochemistry followed by densitometry were used to analyze PKC. Our results demonstrated that an increase in PSA-NCAM expression and optical density of DG molecular layer synaptophysin immunoreactivity occurred in alpha-tocopherol-treated rats. Electron microscopy analysis showed that the increase in synaptophysin expression was related to an increase in synaptic profile density. In addition, Western blotting revealed a decrease in phospho-PKC Pan and phospho-PKCdelta, demonstrating that alpha-tocopherol is also able to inhibit PKC activity in vivo. Likewise, immunoreactivity for the active form of PKCdelta was lower in alpha-tocopherol-treated rats than in controls, while no changes were found in PKCdelta expression. These results demonstrate that alpha-tocopherol is an exogenous factor affecting neuronal plasticity in adult rat DG, possibly through PKCdelta inhibition.

Pax6 is required for production and maintenance of progenitor cells in postnatal hippocampal neurogenesis

Neurogenesis is crucial for brain formation and continues to take place in certain regions of the postnatal brain including the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). Pax6 transcription factor is a key player for patterning the brain and promoting embryonic neurogenesis, and is also expressed in the SGZ. In the DG of wild-type rats, more than 90% of total BrdU-incorporated cells expressed Pax6 at 30 min time point after BrdU injection. Moreover, approximately 60% of Pax6+ cells in the SGZ exhibited as GFAP+ cells with a radial glial phenotype and about 30% of Pax6+ cells exhibited as PSA-NCAM+ cells in clusters. From BrdU labeling for 3 days, we found that cell proliferation was 30% decreased at postnatal stages in Pax6-deficient rSey2/+ rat. BrdU pulse/chase experiments combined with marker staining revealed that PSA-NCAM+ late progenitor cells increased at the expense of GFAP+ early progenitors in rSey2/+ rat. Furthermore, expression of Wnt ligands in the SGZ was markedly reduced in rSey2/+ rat. Taken all together, an appropriate dosage of Pax6 is essential for production and maintenance of the GFAP+ early progenitor cells in the postnatal hippocampal neurogenesis.

Ischemia-stimulated neurogenesis is regulated by proliferation, migration, differentiation and caspase activation of hippocampal precursor cells

A brief ischemic injury to the gerbil forebrain that caused selective damage in the CA1 region of the hippocampus also enhanced the production of new cells in the hippocampal neurogenic area. When evaluated 1 week after bromodeoxyuridine (BrdU) injection, approximately ten times more labeled cells were detected in the hippocampal dentate gyrus in ischemic animals than controls, indicating a stimulation of mitotic activity. To assess the temporal course of the survival and fate of these newborn cells, we monitored BrdU labeling and cell marker expression up to 60 days after ischemia (DAI). Loss of BrdU-positive cells was observed from both control and ischemic animals, but at 30 DAI and afterward, the ischemic group maintained more than 3 times as many BrdU-positive cells as the control group. In addition, ischemic injury also fostered the neuronal differentiation of these cells beyond the capacity observed in control animals and facilitated the migration of developing neurons to a neuronal cellular layer. The establishment of a temporal correlation between differentiation and migration provides evidence of the functional maturation of these cells. Surprisingly, we found that ischemic injury induced activation of caspase-3, not only in the CA1 region as expected, but also in the dentate subgranular zone (SGZ). Active caspase-3 immunoreactivity in the subgranular layer was co-localized with an early neuronal marker, suggesting that caspase-mediated apoptosis could mediate the loss of neurogenic cells in the SGZ. Inhibiting caspase-3 in the context of ischemia-induced neurogenesis might provide an opportunity for functional repair and a therapeutic outcome in the wake of ischemic injury.

Cell Proliferation in the Adult Hippocampal Formation of Rodents and its Modulation by Entorhinal and Fimbria–Fornix Afferents

New granule neurons are produced in the dentate gyrus (DG) of rodents throughout adult life. Recent studies have also reported adult neurogenesis in the cerebral cortex in normal animals or after brain injury. However, few of these studies focused on the hippocampal formation (HF), a cortical area involved in learning and memory in which extensive cell death occurs in neurodegenerative diseases. Thus, we studied cell proliferation in the HF of rodents and the intrinsic putative neurogenic potential of entorhinal cortex (EC) progenitors. We show that only the DG generates new neurons in the HF. In addition, neurospheres from the EC differentiate into neurons and glia in vitro and after transplantation in the adult DG. We also analyzed whether the absence of the synaptic input from the main hippocampal afferents induces neuronal generation in the HF outside the DG and/or regulates the proliferation of DG neuroprogenitors. We show that the denervation of the hippocampus does not induce neurogenesis in HF regions other than the DG. However, neuroprogenitor proliferation in the DG is reduced after fimbria-fornix lesions but not after entorhinal deafferentation, which supports the view that neuroprogenitor proliferation and/or differentiation in the DG are controlled from basal forebrain/septal neurons.

Calbindin-D28k content and firing pattern of hippocampal granule cells in amygdala-kindled rats: a perforated patch-clamp study

The dentate gyrus is believed to play an important pathophysiological role during experimentally induced kindling. In this study, we investigated whether an altered content of the calcium binding protein calbindin-D(28k) or an increased intrinsic excitability of hippocampal granule cells contribute to the induction of the kindling phenomenon. We determined the firing pattern of granule cells in hippocampal slices using perforated patch-clamp recordings in current clamp mode. The expression of calbindin-D(28k) and glutamic acid decarboxylase (GAD(67)) by granule cells was analyzed immunohistochemically. Rats developed secondarily generalized limbic seizures within approximately 11 days of twice-daily stimulation of the amygdala. As reported for other kindling paradigms, this protocol induced a clear up-regulation of GAD(67) in granule cells, indicating their involvement in the induced neuronal activity. However, when comparing kindled and control rats, we could not detect any differences in intrinsic excitability: Firing frequency, after-hyperpolarisations, action potentials, input resistance and membrane potentials were nearly identical between both groups. Furthermore, we did not observe any differences in the calbindin-D(28k) immunoreactivity between groups. In every slice, virtually all granule cells were found to be strongly calbindin-D(28k) positive, and there was no apparent reduction in the general level of calbindin-D(28k) expression. We conclude that changes in intrinsic membrane properties or in the calbindin-D(28k) content of granule cells are not necessary for the development of amygdala kindling.