Regressive events in neurogenesis

Developmental changes of synaptojanin expression in the human cerebrum and cerebellum

Synaptojanin is a highly abundant polyphosphoinositide phosphatase in nerve terminals, and has been thought to play roles in clathrin-mediated synaptic vesicle endocytosis and signaling. In order to determine the broader role of synaptojanin in the central nervous system, we examined synaptojanin expression in the cerebrum and cerebellum from the fetal to the adult period by means of immunohistochemical and Western blot analyses. Immunohistochemistry consistently revealed the localization of synaptojanin in Cajal--Retzius cells, cortical plate neurons, subplate neurons, intermediate neurons, germinal matrix cells and the ventricular neuroepithelium of the fetal cerebrum. In the fetal cerebellum, synaptojanin immunoreactivity was localized in the external granular cell layer, Purkinje cell layer neuropil, cytoplasm of Purkinje cells and internal granular cells. The immunoreactivity in these structures was decreased around birth. After birth, the synaptojanin immunoreactivity of cortical neurons in the cerebrum, Purkinje cell layer neuropil, and internal granular cells and Purkinje cells in the cerebellum increased and reached a plateau after 11 years of age. These results were consistent with the intensity observed on Western blot analysis. These developmental changes of synaptojanin suggest a broader role in not only synaptic vesicle recycling, but also the regulation of neuronal migration and synaptogenesis in the fetal cerebrum and cerebellum.

Neural plasticity and cognitive development

It has been well documented that the effects of early occurring brain injury are often attenuated relative to later occurring injury. The traditional neuropsychological account of these observations is that, although the developing neural system normally proceeds along a well-specified maturational course, it has a transient capacity for plastic reorganization that can be recruited in the wake of injury. This characterization of early neural plasticity is limited and fails to capture the much more pervasive role of plasticity in development. This article examines the role of neural plasticity in development and learning. Data from both animal and human studies show that plasticity plays a central role in the normal development of neural systems allowing for adaptation and response to both exogenous and endogenous input. The capacity for reorganization and change is a critical feature of neural development, particularly in the postnatal period. Subtractive processes play a major role in the shaping and sculpting of neural organization. However, plasticity is neither transient nor unique to developing organisms. With development, neural systems stabilize and optimal patterns of functioning are achieved. Stabilization reduces, but does not eliminate, the capacity of the system to adapt. As the system stabilizes, plasticity becomes a less prominent feature of neural functioning, but it is not absent from the adult system. The implications of this broader view of plasticity for our understanding of development following early brain damage are discussed.

The role of nitric oxide in development of topographic precision in the retinotectal projection of chick

The axonal projection from the retina to the tectum exhibits a precise topographic order in the mature chick such that neighboring ganglion cells send axons to neighboring termination zones in the contralateral tectum. The initial pattern formed during development is much less organized and is refined to the adult pattern during a discrete period of development. Refinement includes elimination of radically aberrant projections, such as those from the temporal side of the retina to posterior regions of the tectum, as well as a more subtle improvement in the topographic precision of the projection. The enzyme that synthesizes nitric oxide is expressed at high levels in the tectum during the developmental period in which the topography improves. Pharmacological blockade of nitric oxide synthesis during this period prevented elimination of topographically inappropriate retinotectal projections in a dose-dependent manner. This effect could not be duplicated by treatment of embryos with a vasoconstrictor, indicating that vascular changes were not a factor. These results show that nitric oxide is involved in refinement of the topography of the retinotectal projection as well as in other aspects of refinement of this projection in developing chick.

Math5 is required for retinal ganglion cell and optic nerve formation

The vertebrate retina contains seven major neuronal and glial cell types in an interconnected network that collects, processes and sends visual signals through the optic nerve to the brain. Retinal neuron differentiation is thought to require both intrinsic and extrinsic factors, yet few intrinsic gene products have been identified that direct this process. Math5 (Atoh7) encodes a basic helix-loop-helix (bHLH) transcription factor that is specifically expressed by mouse retinal progenitors. Math5 is highly homologous to atonal, which is critically required for R8 neuron formation during Drosophila eye development. Like R8 cells in the fly eye, retinal ganglion cells (RGCs) are the first neurons in the vertebrate eye. Here we show that Math5 mutant mice are fully viable, yet lack RGCs and optic nerves. Thus, two evolutionarily diverse eye types require atonal gene family function for the earliest stages of retinal neuron formation. At the same time, the abundance of cone photoreceptors is significantly increased in Math5−/− retinae, suggesting a binary change in cell fate from RGCs to cones. A small number of nascent RGCs are detected during embryogenesis, but these fail to develop further, suggesting that committed RGCs may also require Math5 function.

Effects of early visual experience and diurnal rhythms on BDNF mRNA and protein levels in the visual system, hippocampus, and cerebellum

The expression of brain-derived neurotrophic factor (BDNF) mRNA and the secretion of BDNF protein are tightly regulated by neuronal activity. Thus, BDNF has been proposed as a mediator of activity-dependent neural plasticity. Previous studies showed that dark rearing (DR) reduces BDNF mRNA levels in the primary visual cortex (V1), but the effects of visual experience on BDNF protein levels are unknown. We report that rearing in constant light or DR alters BDNF mRNA and protein levels in the retina, superior colliculus (SC), V1, hippocampus (HIPP), and cerebellum (CBL), although the changes in mRNA and protein are not always correlated. Most notably, DR increases BDNF protein levels in V1 although BDNF mRNA is decreased. BDNF protein levels also undergo diurnal changes. In the retina, V1, and SC, BDNF protein levels are higher during the light phase of the circadian cycle than during the dark phase. By contrast, in HIPP and CBL, the tissue concentration of BDNF protein is higher during the dark phase. The discrepancies between the experience-dependent changes in BDNF mRNA and protein suggest that via its effects on neuronal activity, early sensory experience alters the trafficking, as well as the synthesis, of BDNF protein. The circadian changes in BDNF protein suggest that BDNF could cause the diurnal modulation of synaptic efficacy in some neural circuits. The fluctuations in BDNF levels in nonvisual structures suggest a potential role of BDNF in mediating plasticity induced by hormones or motor activity.

Caspase-3 is actively involved in okadaic acid-induced lens epithelial cell apoptosis

Phosphorylation and dephosphorylation are important cellular events regulating major metabolic activities such as signal transduction, gene expression, cell cycle progression, and apoptosis. It is well documented that okadaic acid, a potent inhibitor of protein phosphatase-1 (PP-1) and -2A (PP-2A), can induce apoptosis in a variety of cell lines. Our recent studies have revealed that in the immortal rabbit lens epithelial cell line, N/N1003A, inhibition of PP-1, but not PP-2A, leads to rapid apoptosis of the lens epithelial cells. This induction of cell death is associated with up-regulated expression of a set of genes, including the tumor-suppressor gene, p53, and the proapoptotic gene, bax. In the present study, we demonstrate that inhibition of PP-1 by okadaic acid in the primary cultures of rat lens epithelial cells also leads to apoptotic death. Moreover, we show that the cysteine protease, caspase-3, is important in the execution of okadaic acid-induced apoptosis. Treatment of the primary cultures of rat lens epithelial cells with 100 nM okadaic acid up-regulates expression of caspase-3 at the mRNA, protein, and enzyme activity levels. Inhibition of the caspase-3 activity with a chemically synthesized inhibitor prevents okadaic acid-induced apoptosis in rat lens epithelial cells. Similar results are also observed in the immortal cell line N/N1003A. Furthermore, stable expression of the mouse gene encoding lens alphaB crystallin inhibits okadaic acid-induced apoptosis, and this inhibition is associated with repression of the okadaic acid-induced up-regulation of caspase-3 activity. Taken together, these results demonstrate that caspase-3 is actively involved in okadaic acid-induced lens epithelial cell apoptosis.

Attraction Exerted in Vivo by Grafts of Embryonic Neocortex on Developing Thalamic Axons

In a previous study we provided evidence that embryonic (E) day 16 frontal cortical cells grafted into the occipital cortex of newborn rats receive inputs from the ventrolateral (VL) and ventromedial (VM) thalamic nuclei which, normally, project to the frontal cortex (25). The present study was designed to examine further the conditions of development of the thalamic innervation of heterotopic neocortical grafts. We demonstrate that VL/VM axons do not provide transitory aberrant input to the occipital cortex either in intact newborn animals or in rats having received neonatal occipital lesion and subsequent graft of E16 occipital cells. These findings indicate, therefore, that the VL/VM projection to the graft does not result from the stabilization of an initial widespread cortical projection from these thalamic nuclei occurring either spontaneously or in response to the lesion and homotopic transplantation procedures. We also show that the VL/VM projection to frontal-to-occipital grafts develops within a few days posttransplantation and is maintained in adulthood. Finally, this study establishes that most VL/VM axons which enter the grafts are not collaterals of thalamofrontal axons. After having reached the cortex, they proceed caudally primarily within the infragranular layers. The findings of this and previous (25) in vivo studies for the first time provide evidence that developing thalamic axons have the capacity to respond to signals from grafts of E16 cortical cells and are capable of deviating their trajectory to establish contact with the grafts. Only those axons arising from thalamic nuclei appropriate for the cortical locus of origin of the grafted cells respond to the guidance signals. The mechanisms by which the thalamic axons find their way to the graft probably rely on cell-contact signaling and/or long-range attraction exerted by diffusible molecules.

Right amygdala volume in adolescent and young adult offspring from families at high risk for developing alcoholism

BACKGROUND

Neurobiological factors have been implicated in the increased susceptibility for developing alcohol dependence that offspring from alcoholic families exhibit. The P300 component of the event-related potential shows developmental changes during childhood and adolescence that appear to be related to risk status. The underlying structural changes that accompany these neurophysiological changes are not well understood.

METHODS

Magnetic resonance imaging was used to measure cerebral, amygdala, and hippocampal volumes in 17 high-risk adolescent and young adult offspring from multiplex alcoholism families and 17 age-, gender-, and IQ-matched control subjects without a family history for alcoholism or other substance dependence. Twenty-two of the subjects are part of a longitudinal prospective study and have been followed an average of 7.3 years, making it possible to relate P300 developmental trajectories to structural volumes.

RESULTS

High-risk adolescents and young adults showed reduced right amygdala volume in comparison with control subjects. Right amygdala volume was significantly correlated with visual P300 amplitude.

CONCLUSIONS

Offspring from families having a high density of alcoholism differ in both neurophysiological and neuroanatomical characteristics that could not be explained by personal drinking history or particular childhood and adolescent psychopathology. Because the amygdala tends to increase in volume during childhood and adolescence, smaller volumes in high-risk children may indicate a developmental delay that parallels delays seen in visual P300 amplitude.

Apoptosis in human embryo development: 2. Cerebellum

We analysed the spatial and temporal distribution of apoptosis in human cerebellum development, during embryonic and fetal periods. Cerebella excised from two human embryos (8 weeks old) and eight fetuses (12-22 weeks old), were paraffin embedded and serially sectioned. Apoptotic cells were identified by propidium iodide staining, and TUNEL. In addition, immunohistochemistry for suicide receptor Fas(APO-1/CD95) was performed. We determined the distribution and percentage of apoptotic cells as well as Fas(APO-1/CD95)-positive cells in different regions and stages of development. Apoptotic cells were seen in both proliferative zones and postmitotic regions along the migratory pathways as well as in the developing cerebellar cortex in all examined stages. The Fas(APO-1/CD95) immunoreactivity was present in all examined stages in a small population of apoptotic cells: either neuroblasts or differentiated cells in postmitotic zones. These findings suggest that apoptosis drives the selection of the cells which are committed to differentiate during the early stages of cerebellar development. The differences between apoptotic cells distribution and Fas receptor expression suggest that cell selection is driven by different apoptotic pathways.

Developmental regulation of tryptophan hydroxylase messenger RNA expression and enzyme activity in the raphe and its target fields

Tryptophan hydroxylase is the rate-limiting enzyme in the synthesis of serotonin and during development, brain serotonin levels and tryptophan hydroxylase activities increase. Increased tryptophan hydroxylase activity could result from alterations in tryptophan hydroxylase messenger RNA levels, translation, and/or post-translational regulation. Tryptophan hydroxylase messenger RNA levels in the dorsal raphe nucleus increased 35-fold between embryonic day 18 and postnatal day 22, measured by quantitative in situ hybridization, then decreased by 40% between postnatal days 22 and 61. These changes correlated with tryptophan hydroxylase enzyme activities in the raphe nuclei as expected, but not in cortical or hippocampal targets. Tryptophan hydroxylase messenger RNA expression in the nucleus raphe obscuris increased 2.5-fold between postnatal days 8 and 22 but did not correlate with enzyme activity in the spinal cord. Using an in vitro model of serotonergic raphe neuron differentiation, serotonergic differentiation was associated with an increase in both tryptophan hydroxylase promoter activity and protein expression. In vivo, tryptophan hydroxylase messenger RNA levels per single cell and per brain section were correlated during development up to postnatal day 22, but not beyond for both the dorsal raphe nucleus and nucleus raphe obscuris. Between postnatal days 22 and 61 single cell levels of tryptophan hydroxylase messenger RNA in the dorsal raphe nucleus did not change yet the levels per brain section significantly decreased by 40%. During the same period in the nucleus raphe obscuris, tryptophan hydroxylase messenger RNA levels per single cell signifcantly increased by 30% yet levels per brain section did not change. Comparison of tryptophan hydroxylase messenger RNA levels per cell and per brain section indicated a serotonergic loss between postnatal days 22 and 61 in both the dorsal raphe nucleus and nucleus raphe obscuris and may reflect either a loss of neurotransmitter phenotype or cell death. This study is the first to characterize the expression of brain tryptophan hydroxylase messenger RNA during rat development. In addition, this study is the first to report the activity of tryptophan hydroxylase in the spinal cord and hippocampus in the embryonic and neonatal rat. Together, the data provide a better understanding of the intricate relationship between patterns of tryptophan hydroxylase messenger RNA expression and enzyme activity.

Resistance to induced apoptosis in the human neuroblastoma cell line SK-N-SH in relation to neuronal differentiation. Role of Bcl-2 protein family

Much evidence suggests that apoptosis plays a crucial role in cell population homeostasis that depends on the expression of various genes implicated in the control of cell life and death. The sensitivity of human neuroblastoma cells SK-N-SH to undergo apoptosis induced by thapsigargin was examined. SK-N-SH were previously differentiated into neuronal cells by treatments with retinoic acid (RA), 4 beta-phorbol 12-myristate 13-acetate (PMA) which increases protein kinase C (PKC) activity, and staurosporine which decreases PKC activity. Neuronal differentiation was evaluated by gamma-enolase, microtubule associated protein 2 (MAP2) and synaptophysin immunocytochemistry. The sensitivity of the cells to thapsigargin-induced apoptosis was evaluated by cell viability and nuclear fragmentation (Hoechst 33258) and compared with pro-(Bcl-2, Bcl-x(L)) and anti-apoptotic (Bax, Bak) protein expression of the Bcl-2 family. Cells treated with RA and PMA were more resistant to apoptosis than controls. Conversely, the cells treated with staurosporine were more susceptible to apoptosis. In parallel with morphological modifications, the expression of inhibitors and activators of apoptosis was directly dependent upon the differentiating agent used. Bcl-2 expression was strongly increased by PMA and drastically decreased by staurosporine as was Bcl-x(L) expression. Bax and Bak expression were not significantly modified. These results demonstrate that drugs that modulate PKC activity may induce a modification of Bcl-2 expression as well as resistance to the apoptotic process. Furthermore, the expression of Bcl-2 was reduced by toxin B from Clostridium difficile and, to a lesser extent, by wortmannin suggesting a role of small G-protein RhoA and PtdIns3 kinase in the control of Bcl-2 expression. Our data demonstrate a relationship between the continuous activation of PKC, the expression of Bcl-2 protein family and the resistance of differentiated SK-N-SH to apoptosis.

Bax-induced apoptosis not demonstrated in the congenital toxoplasmosis in mice

A prominent neuropathological change observed in a murine model of congenital toxoplasmosis is cerebral cortical hypoplasia. In the early embryonic life of toxoplasmosis mice, the number of apoptotic cell observed in cerebral cortex is increased, indicating that increased number of apoptotic cells might relate to the pathogenetic mechanism of the cortical hypoplasia. Immunohistochemical expression of apoptosis-related factors, Bcl-2 and Bax has been studied in fetal murine brains infected with toxoplasma and in controls. Paraffin sections of the fetal brains on embryonic day (ED) 10, 12, 14, 16 and 18 were applied for the immunostains of Bcl-2 and Bax. Totally, 47 experimental animals (ED10: n=8, ED12: n=6, ED14: n=12, ED16: n=6, ED18: n=15) and 48 control animals (ED10: n=6, ED12: n=8, ED14: n=9, ED16: n=9, ED18: n=16) were examined. Bcl-2 positive cells were detected on ED10, whereas Bax positive cells appeared on ED14. No difference of Bcl-2 and Bax expression between toxoplasmosis and control groups was detected, suggesting that there is no clear relation between Bax-induced apoptosis and cortical dysplasia in congenital toxoplasmosis.

Insulin-like growth factor-I and over-expression of Bcl-xL prevent glucose-mediated apoptosis in Schwann cells

Schwann cells (SCs), the myelinating cells of the peripheral nervous system, are lost or damaged in patients suffering from diabetic neuropathy. In the current study, 2 model systems are used to study the mechanism of SC damage in diabetic neuropathy: the streptozotocin (STZ)-treated diabetic rat and cultures of purified SCs in vitro. Electron microscopy of dorsal root ganglia from STZ-treated rats reveals classic ultrastructural features of apoptosis in SCs, including chromatin clumping and prominent vacuolation. Bisbenzamide staining of SCs cultured in hyperglycemic defined media shows nuclear blebbing of apoptotic cells. Insulin-like growth factor-I (IGF-I) is protective. LY294002, a phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor, blocks the effect of IGF-I. High glucose induces caspase cleavage in apoptotic SCs--an effect that is blocked by bok-asp-fmk (BAF), a caspase inhibitor. Although Bcl-xL expression remains unchanged in experimental conditions, over-expression of Bcl-xL protects SCs from apoptosis. In summary, hyperglycemia induces caspase activation and morphologic changes in SCs consistent with apoptotic death, both in vivo and in vitro. Over-expression of Bcl-xL, or IGF-I, signaling via PI 3-kinase, protects SCs from glucose-mediated apoptosis in vitro. IGF-I may be useful in preventing hyperglycemia-induced damage to SCs in patients suffering from diabetic neuropathy.

Nitric oxide, impulse activity, and neurotrophins in visual system development(1)

Topographic refinement of synaptic connections within the developing visual system involves a variety of molecules which interact with impulse activity in order to produce the precise retinotopic maps found in the adult brain. Nitric oxide (NO) has been implicated in this process, as have various growth factors. Within the subcortical visual system, we have recently shown that nitric oxide contributes to pathway refinement in the superior colliculus (SC). Long-term potentiation (LTP) and long-term depression (LTD) are also expressed in SC during the time that this pathway undergoes refinement. The role of NO has been demonstrated by showing that refinement of ipsilateral fibers in the retinocollicular pathway is significantly delayed in gene knockout mice in which both the endothelial and neuronal isoforms of nitric oxide synthase (NOS) have been disrupted. The effect also depends upon Ca(2+) channels because refinement of both the ipsilateral retinocollicular and retinogeniculate pathways is disrupted in genetic mutants in which the beta3 subunit of the Ca(2+) channel has been deleted. LTD may also be involved in this process, because the time course of its expression correlates with that of pathway refinement and LTD magnitude is depressed by nitrendipine, an L-type Ca(2+) channel blocker. LTP is also expressed during early postnatal development in the LGN and SC and may contribute to synaptic stabilization. The role of neurotrophins in pathway refinement in the visual system is also reviewed.

Early loss of synaptic protein PSD-95 from rod terminals of rhodopsin P347L transgenic porcine retina

Retinitis pigmentosa (RP), a type of retinal degeneration involving first rod and then slow cone photoreceptor degeneration, can be caused by any of a number of mutations in different genes. In the cases of mutations affecting rod-specific genes such as rhodopsin, it is unclear how the mutations may cause degeneration of cones. We have used the porcine retina, which is rod-dominated and has an abundance of cones, to study the mutation-induced changes in both rod and cone photoreceptors. Like patients with the same mutation, rhodopsin P347L transgenic swine manifest rod-cone degeneration. In addition, the rod bipolar cells fail to form synaptic connections with rods; instead, they form ectopic synapses with cones. The mechanisms that prevent the formation of the rod-rod bipolar cell synaptic connection are not known. We used specific antibodies and immunocytochemistry to show that the synaptic protein, PSD-95, is present in both normal and transgenic porcine retinas. During neonatal development, however, PSD-95 is lost from rod terminals in the transgenic swine. This loss is virtually complete (90%) by postnatal day 5, at a time when greater than 80% of rod cell bodies still remain. Furthermore, the remaining rods retain their outer segments and their gross morphology appears relatively normal. In contrast, PSD-95 expression continues in cone terminals, even in 10-month-old transgenic swine, where the rods have all disappeared and the cones show signs of severe degeneration. These results suggest that loss of PSD-95 may not be a general consequence of the deteriorating cell. Rather, the very early and selective loss of PSD-95 from the rod terminals may be causally related to the absence of rod-rod bipolar cell synapses in the rhodopsin P347L transgenic retina.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.

Ectopic synaptogenesis in the mammalian retina caused by rod photoreceptor-specific mutations

In addition to rod photoreceptor loss, many mutations in rod photoreceptor-specific genes cause degeneration of other neuronal types. Identifying mechanisms of cell-cell interactions initiated by rod-specific mutations and affecting other retinal cells is important for understanding the pathogenesis and progression of retinal degeneration. Here we show in animals with rod and cone degeneration due to mutations in the genes encoding rhodopsin and cGMP phosphodiesterase beta-subunit (PDE-beta) respectively, that rod bipolar cells received ectopic synapses from cones in the absence of rods. Thus, synaptic plasticity links certain rod-specific mutations to retina-wide structural alterations that involve different types of neurons.

Do subplate neurons comprise a transient population of cells in developing neocortex of rats?

Studies were undertaken to determine whether neurons of the subplate layer represent a transient or stable population of cells in developing neocortex of rat. The first set of studies sought to determine the fraction of subplate neurons that is lost during early postnatal development. The optical dissector method was used to analyze fluorescently stained material in animals the age of postnatal day 0 (P0) to P40. These results demonstrate a reduction of slightly less than half of the total number of subplate neurons from P0 to P40. Counts of labeled cells in littermates at varied ages after [(3)H]thymidine or BRDU treatment on gestational day 14 (G14 - birthdate of occipital subplate neurons) or G18 (birthdate of layers III-IV neurons) demonstrate loss of approximately 50% of neurons in the subplate layer between P0 and P40, somewhat greater than the loss of neurons from cortical layers III-IV. The second set of studies investigated whether subplate neurons display cellular atrophy during postnatal development. Analysis of subplate neurons injected intracellularly with Lucifer yellow in fixed slice preparations indicates no reduction in soma size, number of dendrites, or extent of dendritic fields of subplate neurons taken from animals age P0 to P60. The third set of studies investigated whether functional markers of subplate neurons are reduced during postnatal development. Analysis of tissue stained histochemically for cytochrome oxidase or acetylcholinesterase, or stained immunocytochemically for GABA, somatostatin, or neuropeptide Y, demonstrate a remarkable loss of expression of staining patterns from late gestational ages to P20. These data demonstrate that, although subplate neurons seem not to be a transient population of cells in the usual sense of being eliminated by cell death or structural atrophy, the loss of histochemical and immunocytochemical markers indicates that they may be a functionally transient population of cells.

Normal development of the ipsilateral retinocollicular pathway and its disruption in double endothelial and neuronal nitric oxide synthase gene knockout mice

The development of the ipsilateral retinocollicular pathway involves activity-dependent refinement in which misdirected axons retract to form a precise retinotopic map in adults. This refinement is altered by disruption of genes for the endothelial and neuronal isoforms of nitric oxide synthase (e,nNOS), but the extent of disruption during early development is not known. Therefore, we studied the refinement of this pathway in normal C57/BL6 and e,nNOS double knockouts from P4 to P21 and in adults. Anterograde tracers were injected into one eye to localize the ipsilateral retinal projection (IRP) within the superior colliculus (SC). At P4, the IRP in normal mice was distributed throughout the dorsoventral extent of the superficial gray layer (SGL) across most of the rostrocaudal axis of SC. Between P4 and P9, the pathway retracted to the rostromedial SC, and retracted further between P15 and P21, such that multiple patches of label were seen only in the rostral 200-300 microm. Refinement also began to occur between P4 and P9 in e,nNOS double knockout mice, but labeling was more extensive in P9, P15, and P21 knockout animals. This delay in refinement was confirmed quantitatively at P15 where differences in the area occupied by the pathway were statistically significant. The refinement process is therefore in progress in both normal and e,nNOS knockout mice before eye opening but is significantly delayed in the double knockouts. The IRP in normal mice is also more exuberant at early ages, and the process of refinement more protracted than has been previously reported, suggesting that there is a prolonged critical period of synaptic plasticity.

Expression of bcl-2, bax, and caspase-3 in the brain of the developing rat

Naturally occurring neuronal death (NOND) is generally considered to be apoptotic. Apoptosis is an active form of cell death in which the regulation of specific proteins produces anti- or pro-apoptotic signals. Two of the protein families involved in this regulation are the bcl proteins and caspases. A quantitative immunoblotting technique was used to examine the temporal expression of bcl-2, bax, and two isoforms of caspase 3 (an active 20 kDa isoform and the inactive 32 kDa precursor) throughout the developing neuraxis. Long-Evans rat fetuses were collected on gestational day (G) 16 and G19, and pups were harvested on postnatal day (P) 0, P3, P6, P12, P21, and P30. Brains were divided into five segments: cortex, thalamus, midbrain, medulla/pons, and cerebellum. In general, the expression of bax increased and the ratio of bcl-2 expression to bax expression decreased concurrent with published data on the onset of NOND in a given area. The timing of these events was paralleled by an increase in the expression of active caspase 3. Unlike the bcl proteins, caspase 3 expression returned toward fetal levels as the brain matured. The timing of the changes in bcl protein and caspase expression show that both protein families are involved in promoting neuronal death. Reductions in caspase expression (and not bcl-2 and bax expression) are key to ending the period of NOND.

Regulation of the L1 cell adhesion molecule by thyroid hormone in the developing brain

Thyroid hormone is essential for brain maturation, regulating neuronal differentiation and migration, myelination, and synaptogenesis. Mutations in the cell adhesion molecule L1 cause severe neurological abnormalities in humans. We studied the effect of thyroid hormone deprivation and administration on L1 expression. Northern and in situ hybridization studies showed that hypothyroidism induces a marked increase in L1 mRNA levels in the caudate putamen, cerebral cortex, amygdala, and some thalamic nuclei. L1 protein was overexpressed in embryonic and newborn hypothyroid rats in the caudate putamen, internal capsule, habenula, and neocortex. Later in development, an abnormally high L1 expression was found in the cortical and cerebellar white matter, corpus callosum, anterior commissure, thalamocortical projections, and striatal fiber tracts of hypothyroid animals. Thyroid hormone administration reversed the upregulation of L1 expression in vivo and in cultured cells. Thus, alterations of L1 expression may contribute to the profound abnormalities caused by hypothyroidism in the developing brain.

Cell death in early neural development: beyond the neurotrophic theory

The important effect of cell death on projecting neurons during development is well established. However, this mainstream research might have diverted recognition of the cell death that occurs at earlier stages of neural development, affecting proliferating neural precursor cells and young neuroblasts. In this article, we briefly present observations supporting the occurrence of programmed cell death during early neural development in a regulated fashion that to some extent parallels the death of projecting neurons lacking neurotrophic support. These findings raise new questions, in particular the magnitude and the role of this early neural cell death.

Apoptosis in human embryo development: 1. Cerebral cortex

We investigated the apoptosis at the beginning of human cerebral cortex development, in the 6th week of embryogenesis, Carnegie stages 16 and 17. Attention was focused on the dorsal wall of the telencephalon to the ventricular zone of proliferation and to the postmitotic zone with beginning of neuronal migration. We identified apoptotic cells in tissue sections by propidium iodide staining, TUNEL and immunohistochemistry for Fas(APO-1/CD95). We determined the distribution and the percentage (reported to the propidium iodide stained nuclei) of apoptotic TUNEL-positive and Fas(APO-1/CD95)-positive cells. TUNEL-positive apoptotic cells in the proliferative zone were 20% in stage 16 and 60% in stage 17. TUNEL-positive apoptotic cells in the postmitotic zone were 8% in stage16 and 30% in stage 17. CD95-positive apoptotic cells in the proliferative zone were 5% in stage 16 and 2% in stage 17. There were no CD95-positive cells in the postmitotic zone. We evidentiated the presence of the suicide receptor Fas(APO-1/CD95) only on a small population of apoptotic neuroblasts in the proliferative zone. The differences between apoptotic distribution and receptors in early corticogenesis suggest that different apoptotic pathways drive the selection of neuronal populations.

Arsenite-induced apoptosis in cortical neurons is mediated by c-Jun N-terminal protein kinase 3 and p38 mitogen-activated protein kinase

c-Jun N-terminal protein kinase (JNK) and p38 mitogen-activated protein kinase are activated by stress and are implicated in regulation of apoptosis in several tissues. However, their contribution to stress-induced apoptosis in CNS neurons is not well defined. Here we investigated the role of JNK and p38 in cortical neuron apoptosis caused by sodium arsenite treatment. Sodium arsenite is an environmental toxicant that causes developmental defects in the CNS. Treatment of cortical neurons with sodium arsenite activated p38 and JNK3 but not JNK1 or JNK2. It also induced c-Jun phosphorylation. Furthermore, sodium arsenite induced cortical neuron apoptosis. This apoptosis was attenuated by SB203580, an inhibitor of p38, and by CEP-1347, an inhibitor of JNK activation. Expression of dominant-interfering mutants of the JNK or p38 pathways inhibited apoptosis induced by arsenite, whereas expression of constitutive active mutants for either pathway induced apoptosis. Moreover, the caspase inhibitor zVAD-fluoromethylketone as well as expression of bcl-2 or bcl-xL inhibited cortical neuron apoptosis induced by arsenite or by constitutive activation of JNK or p38. These data indicate that both JNK and p38 contribute to arsenite-induced apoptosis in primary CNS neurons, and this apoptosis requires the bcl-2-caspase pathway. This is the first evidence that a specific JNK isoform is differentially activated by stress and contributes to neuronal apoptosis.

The relationship between neuronal survival and regeneration

The ability of peripheral nervous system (PNS) but not central nervous system (CNS) neurons to regenerate their axons is a striking peculiarity of higher vertebrates. Much research has focused on the inhibitory signals produced by CNS glia that thwart regenerating axons. Less attention has been paid to the injury-induced loss of trophic stimuli needed to promote the survival and regeneration of axotomized neurons. Could differences in the mechanisms that control CNS and PNS neuronal survival and growth also contribute to the disparity in regenerative capacity? Here we review recent studies concerning the nature of the signals necessary to promote neuronal survival and growth, with an emphasis on their significance to regeneration after CNS injury.

Apoptosis in neural development and disease

Cell death via apoptosis is a prominent feature in mammalian neural development. Recent studies into the basic mechanism of apoptosis have revealed biochemical pathways that control and execute apoptosis in mammalian cells. Protein factors in these pathways play important roles during development in regulating the balance between neuronal life and death. Additionally, mounting evidence indicates such pathways may also be activated during several neurodegenerative diseases, resulting in improper loss of neurons.

Developmental cell death in dopaminergic neurons of the substantia nigra of mice

Dopaminergic neurons in the substantia nigra pars compacta (SNpc) undergo natural cell death during development in rats. Controversy exists as to the occurrence of this phenomenon in SNpc dopaminergic neurons in the developing mouse. Herein, by using an array of morphologic techniques, we show that many SNpc neurons fulfill the criteria for apoptosis and that the number of apoptotic neurons in the SNpc vary in a time-dependent manner from postnatal day 2 to 32. These dying neurons also show evidence of DNA fragmentation, of activated caspase-3, and of cleavage of beta-actin. Some, but not all of the SNpc apoptotic neurons still express their phenotypic marker tyrosine hydroxylase, confirming their dopaminergic nature. Consistent with the importance of target-derived trophic support in modulating developmental cell death, we demonstrate that destruction of intrinsic striatal neurons by a local injection of quinolinic acid (QA) dramatically enhances the magnitude of SNpc apoptosis and results in a lower number of adult SNpc dopaminergic neurons. Strengthening the apoptotic nature of the observed SNpc developmental cell death, we demonstrate that overexpression of the anti-apoptotic protein Bcl-2 attenuates both natural and QA-induced SNpc apoptosis. The present study provides compelling evidence that developmental neuronal death with a morphology of apoptosis does occur in the SNpc of mice and that this process plays a critical role in regulating the adult number of dopaminergic neurons in the SNpc.

Involvement of cystatin C in oxidative stress-induced apoptosis of cultured rat CNS neurons

Oxidative stress is involved in neuronal degeneration in cerebrovascular injury, neuropathology and aging. When rat CNS neurons were cultured in a high (50%) oxygen atmosphere, the neurons died. This high oxygen-induced cell death showed features of apoptotic cell death, characterized by DNA fragmentation, and was blocked by inhibitor of protein synthesis. We found that cystatin C and HuC mRNA, the products of which are an inhibitor of cysteine proteases and an RNA binding protein, respectively, were up-regulated in neurons cultured in the high oxygen atmosphere. In the present study, we focused on cystatin C. Cystatin C protein levels were also increased in neurons cultured in the high oxygen atmosphere. In situ hybridization with an RNA probe for rat cystatin C and immunocytochemistry with anti-human cystatin C antibody showed that microtubule-associated protein 2 (MAP2)-positive neurons expressed cystatin C mRNA and protein, respectively, in the high oxygen atmosphere. These results indicated that oxidative stress stimulates an increase in cystatin C expression in cultured neurons, and that cystatin C might have important roles in regulation of apoptosis elicited by oxidative stress.

Programmed cell death in the developing human telencephalon

Programmed cell death (PCD) in the form of apoptosis is recognized as one of the central events in the development of the central nervous system. To study the time of onset, extent and distribution of PCD in the human telencephalon, embryos and fetuses from 4.5 to 27 gestational weeks (g.w.) were examined using the TUNEL (TdT-mediated dUTP-biotin nick-end labelling) in situ method. At 4.5 g.w. sparse TUNEL(+) nuclei were observed in the ventricular zone of the neural tube. With the formation of the cortical plate at 7-8 g.w. , TUNEL(+) nuclei were seen in all developmental layers of the cortical anlage, as well as in the subcortical regions such as the ganglionic eminence and the internal capsule. The proliferative zones (the ventricular zone, the subventricular zone and the ganglionic eminence) contained the majority of all apoptotic nuclei observed in each specimen. However, the apoptotic index was highest in the subplate zone and in layer I. Double-labelling experiments suggested that neuronal precursors were the main population of cells undergoing PCD in the first trimester of gestation, whereas glial cells probably start dying around midgestation. The onset of labelling of microglial cells and apoptotic nuclei were synchronous, indicating the involvement of microglia in PCD. In conclusion, two distinct types of PCD were observed during human telencephalic development: embryonic apoptosis, which was synchronous with proliferation and migration of neuronal cells and probably not related to establishment of neuronal circuitry, and fetal apoptosis, which coincided with differentiation and synaptogenesis, and therefore may be related to the development of axonal-target connectivity.

Upregulation of ciliary neurotrophic factor in reactive Müller cells in the rat retina following optic nerve transection

We have investigated the expression and cellular localization of ciliary neurotrophic factor (CNTF) in the rat retina following optic nerve transection. In the normal retina, CNTF immunoreactivity was restricted to profiles in the ganglion cell layer. Following optic nerve transection, immunoreactivity appeared in Müller cell somata and processes and its intensity increased between three and seven days post-lesion. Quantitative evaluation by immunoblotting confirmed that CNTF expression increased continuously up to 7 days after optic nerve transection (to 430% of control levels), but decreased again to 250% of controls at 4 weeks post-lesion. Our findings suggest that CNTF supplied by Müller cells may play a protective role for axotomized ganglion cells in the rat retina.

Influence of the target on distribution and functioning of the varicosities of Helix pomatia metacerebral cell C1 in dissociated cell culture

The serotonergic metacerebral giant cell (C1) of Helix pomatia was isolated with its bifurcate axon and plated in culture under five conditions: (i) with no target; (ii) with the appropriate target B2 near the stump of the bigger branch (CBC); (iii) with B2 near the stump of the smaller branch (CC); (iv) with a wrong target (C3) near the stump of the CBC branch and (v) with B2 and C3 positioned near the CBC and CC stump, respectively. The counting of anti-serotonin antibody-labelled varicosities of the C1 neuron showed that the presence of the appropriate target in either axonal domain both down-regulated the number of varicosities of the contralateral neuritic field, and increased their average size, whereas the wrong target induced an overall reduction of the number of C1 neuron varicosities, and inhibited the evoked transmitter release. The action potential-evoked calcium concentration increase in the neuritic terminals of the C1 neuron cultured alone, or in presence of the appropriate target, reached a value significantly higher than that reached in presence of the wrong target. These results provide evidence that the postsynaptic neuron regulates both morphological and functional development of presynaptic terminals.

Cell loss in isolated human islets occurs by apoptosis

Purified islet allografts have largely failed to maintain long-term glucose homeostasis in human recipients, and the reasons for this are unclear. It is noteworthy, however, that islet isolation destroys or removes cellular and noncellular elements of the pancreas that could play an important role in supporting islet survival. The purpose of this study was to determine whether human islet isolation leads to the induction of programmed cell death. Human islets were enzymatically isolated from cadaveric donor pancreata using Liberase or Collagenase P, purified over a discontinuous BSA gradient, then cultured in RPMI 1640 at 37 degrees C in 5% CO2 for < or = 7 days. Islets were examined daily by routine histology and immunocytochemistry for islet hormones, DNA fragmentation [cell death; enzyme-linked immunosorbent assay (ELISA) and TUNEL assay] and for transglutaminase (TG) activity, two indicators of apoptosis. TG activity and DNA fragmentation increased by 1,000% and 1,890%, respectively (p < 0.05) This corresponded to the appearance of pyknotic nuclei on light microscopy, the presence of apoptotic bodies on electron microscopy, and the demonstration of TUNEL-positive cells. These were present primarily in a distribution that corresponded to the insulin-immunoreactive cells. At 5 days, 31.4 +/- 2.2% of islet cells were TUNEL positive. In summary, apoptosis of islet cells appears soon after islet isolation, and involves primarily the beta cell. This is the first report of apoptosis of islet cells after human islet isolation. The loss of beta-cell mass could be implicated in the failure of islet transplantation and merits further investigation.

Identification of the novel developmentally regulated gene, Bdm2, which is highly expressed in fetal rat brain

Most of the neurogenesis take place during the embryonic stage; the genes expressed predominantly in this stage may play important roles in the control of development of the central nervous system. Using a differential display method, we identified the novel rat gene, brain development-related molecule 2 (Bdm2), that is expressed more abundantly in the embryonic brain than in the adult brain. Full-length Bdm2 cDNA consists of 1842 base pairs (bp) and contains an open reading frame of 1260 bp. Northern blot analysis demonstrated that Bdm2 was strongly expressed in the late embryonic brain and was still detected at lower levels in an early postnatal period; in adults, Bdm2 mRNA was decreased to an undetectable level in brain, though the expression of this mRNA was revealed in other tissues. Level of Bdm2 mRNA was maintained during neuronal differentiation of mouse embryonal carcinoma cell P19, but decreased during the differentiation to glial and unidentified non-neuronal cells. In situ hybridization study demonstrated the wide distribution of Bdm2 mRNA in the embryonic brain; in the adult brain, the hybridization signals became more restricted to the hippocampus, olfactory bulb, cerebellum, and neocortex, almost coinciding with the regions where nascent and immature neurons are present. Thus, it appears likely that Bdm2 encodes a protein that is involved in both the regulation of growth of undifferentiated neural cells and the terminal differentiation of neuronal cells.

Winged helix transcription factor Foxb1 is essential for access of mammillothalamic axons to the thalamus

Our aim was to study the mechanisms of brain histogenesis. As a model, we have used the role of winged helix transcription factor gene Foxb1 in the emergence of a very specific morphological trait of the diencephalon, the mammillary axonal complex. Foxb1 is expressed in a large hypothalamic neuronal group (the mammillary body), which gives origin to a major axonal bundle with branches to thalamus, tectum and tegmentum. We have generated mice carrying a targeted mutation of Foxb1 plus the tau-lacZ reporter. In these mutants, a subpopulation of dorsal thalamic ventricular cells “thalamic palisade” show abnormal persistence of Foxb1 transcriptional activity; the thalamic branch of the mammillary axonal complex is not able to grow past these cells and enter the thalamus. The other two branches of the mammillary axonal complex (to tectum and tegmentum) are unaffected by the mutation. Most of the neurons that originate the mammillothalamic axons suffer apoptosis after navigational failure. Analysis of chimeric brains with wild-type and Foxb1 mutant cells suggests that correct expression of Foxb1 in the thalamic palisade is sufficient to rescue the normal phenotype. Our results indicate that Foxb1 is essential for diencephalic histogenesis and that it exerts its effects by controlling access to the target by one particular axonal branch.

Stabilization of growing retinal axons by the combined signaling of nitric oxide and brain-derived neurotrophic factor

The pattern of axonal projections early in the development of the nervous system lacks the precision present in the adult. During a developmental process of refinement, mistargeted projections are eliminated while correct projections are retained. Previous studies suggest that during development nitric oxide (NO) is involved in the elimination of mistargeted retinal axons, whereas brain-derived neurotrophic factor (BDNF) may stabilize retinal axon arbors. It is unclear whether these neuromodulators interact. This study showed that NO induced growth cone collapse and retraction of developing retinal axons. This effect was not attributable to NO-induced neurotoxicity. BDNF protected growth cones and axons from the effects of NO. This effect was specific to BDNF, because neither nerve growth factor (NGF) nor neurotrophin-3 (NT-3) prevented NO-induced growth cone collapse and axon retraction. Exposure to both BDNF and NO, but not either factor alone, stabilized growth cones and axons. Stabilized axons exhibited minimal retraction or extension. This response appears to be a new axon "state" and not simply a partial amelioration of the effect of NO, because lower doses of BDNF or NO allowed axon extension. Furthermore, BDNF/NO-induced growth cone stabilization correlated with the appearance of a cytochalasin D-resistant population of actin filaments. BDNF protection from NO likely was mediated locally at the level of the growth cone, because growth cones or individual filopodia in contact with BDNF-coated beads were protected from NO-induced collapse. These findings suggest a cellular mechanism by which some axonal connections are stabilized and some are eliminated during development.

Evidence that Wallerian degeneration and localized axon degeneration induced by local neurotrophin deprivation do not involve caspases

The selective degeneration of an axon, without the death of the parent neuron, can occur in response to injury, in a variety of metabolic, toxic, and inflammatory disorders, and during normal development. Recent evidence suggests that some forms of axon degeneration involve an active and regulated program of self-destruction rather than a passive "wasting away" and in this respect and others resemble apoptosis. Here we investigate whether selective axon degeneration depends on some of the molecular machinery that mediates apoptosis, namely, the caspase family of cysteine proteases. We focus on two models of selective axon degeneration: Wallerian degeneration of transected axons and localized axon degeneration induced by local deprivation of neurotrophin. We show that caspase-3 is not activated in the axon during either form of degeneration, although it is activated in the dying cell body of the same neurons. Moreover, caspase inhibitors do not inhibit or retard either form of axon degeneration, although they inhibit apoptosis of the same neurons. Finally, we cannot detect cleaved substrates of caspase-3 and its close relatives immunocytochemically or caspase activity biochemically in axons undergoing Wallerian degeneration. Our results suggest that a neuron contains at least two molecularly distinct self-destruction programs, one for caspase-dependent apoptosis and another for selective axon degeneration.

NOS inhibition during postnatal development leads to increased ipsilateral retinocollicular and retinogeniculate projections in rats

Synthesis of nitric oxide (NO) occurs downstream from activation of N-methyl-D-aspartate (NMDA) receptors; NO reportedly acts as a retrograde messenger, influencing the refinement and stabilization of coactive afferent terminals. Cells and neuropil in the rat superior colliculus (SC) and lateral geniculate body (LGB) show intense, developmentally regulated activity for NO synthase (NOS). To study the role of NO in the development of retinogeniculate and retinotectal axon arbors, we examined primary visual projections of rats that had received intraperitoneal injections of Nomega-nitro-L-arginine (L-NoArg, an NOS inhibitor) on postnatal day 0, and daily thereafter for 4-6 weeks. Treated rats showed significant alterations in ipsilateral retinotectal projections, in the mediolateral and anteroposterior axes; there was an increase in the density of fibres entering the SC, in branch length, and in the numbers of boutons on retinotectal arbors in the treated group. Ipsilaterally projecting retinal axons also showed an increase in density and distribution in the dorsal nucleus of the LGB. If animals were allowed to survive for several months after stopping treatment, similar changes were also noted, but these were much less striking. Our results support the hypothesis that, in the mammalian visual system, NO released from target neurons in the SC and LGB serves as a retrograde signal which feeds back on retinal afferents, influencing their growth. The effects of NOS inhibition are partially reversed after treatment is stopped, indicating that lack of NO synthesis delays the maturation of retinofugal connections, and also that NO plays a constitutive role in their development.

Neuronal degeneration and reorganization: a mutual principle in pathological and in healthy interactions of limbic and prefrontal circuits

Based on developmental principles and insights from animal research about neuroplasticity in cell assemblies, this article is to propose a view of plasticity that promotes a link between hippocampal and prefrontal structure and function. Both the mitotic activity (counting of BrdU-labeled cells) in hippocampal dentatus and the maturation of dopamine fibres (quantitative immunochemistry of mesoprefrontal projection) in the prefrontal cortex proved to be a measurable combination for investigating the complex chain of events that relate activity dependent neuroplasticity to normal as well as to pathological maturational processes. With our animal model we demonstrate that both rearing conditions and neuroactive substances can effectively interfere with developmental plasticity and induce a malfunctional adaptation of prefrontal structures and neurotransmitter systems (dopamine, GABA). In the hippocampal dentatus, where ontogenetic plasticity proved to be preserved by continued neuro- and synaptogenesis, serious damage can be internalized without simultaneous disruption of neural dynamics offering an approach to reverse dysfunctional reorganization in the prefrontal cortex.

Formation of cortical fields on a reduced cortical sheet

Theories of both cortical field development and cortical evolution propose that thalamocortical projections play a critical role in the differentiation of cortical fields (; ). In the present study, we examined how changing the size of the immature neocortex before the establishment of thalamocortical connections affects the subsequent development and organization of the adult neocortex. This alteration in cortex is consistent with one of the most profound changes made to the mammalian neocortex throughout evolution: cortical size. Removing the caudal one-third to three-fourths of the cortical neuroepithelial sheet unilaterally at an early stage of development in marsupials resulted in normal spatial relationships between visual, somatosensory, and auditory cortical fields on the remaining cortical sheet. Injections of neuroanatomical tracers into the reduced cortex revealed in an altered distribution of thalamocortical axons; this alteration allowed the maintenance of their original anteroposterior distribution. These results demonstrate the capacity of the cortical neuroepithelium to accommodate different cortical fields at early stages of development, although the anteroposterior and mediolateral relationships between cortical fields appear to be invariant. The shifting of afferents and efferents with cortical reduction or expansion at very early stages of development may have occurred naturally in different lineages over time and may be sufficient to explain much of the phenotypic variation in cortical field number and organization in different mammals.

Selective inhibition of spinal cord neurite outgrowth and cell survival by the Eph family ligand ephrin-A5

The Eph family tyrosine kinase receptors and their ligands, the ephrins, have been shown to play critical roles in cell migration, tissue morphogenesis, and axonal guidance in many different systems. However, their function in the spinal cord has not been examined carefully. We showed in this study that several Eph receptors, including EphA3, Eph A4, and Eph A5, are expressed in the ventral spinal cord in partially overlapping patterns, with EphA5 exhibiting the most widespread transcription in the entire ventral spinal cord during early development. Complementary to the receptor expression, a ligand of these receptors, ephrin-A5, is transcribed in the dorsal half of the spinal cord. Consistent with the spatial location of receptor expression, the ligand selectively inhibits neurite outgrowth and induces cell death of the ventral, but not the dorsal, spinal cord neurons. These observations suggest that interactions between the Eph family receptors and ligands exerts negative influences on ventral spinal cord neurons and thus may play important roles in regulating morphogenesis and axon guidance in the spinal cord.

Molecular mechanisms of cell death in the ear

Damage to sensory cells and neurones of the inner ear, induced by ototoxic drugs, physical stimulation, or even normal aging, occurs by mechanisms that are not yet fully elucidated. However, for the individual the consequent loss of hearing can be devastating. This the concept that, in some instances, this brain cell death may be preventable has provided an exciting and novel route to thwart the degenerative process. Signals that trigger cell death can be as diverse as the removal of essential growth factors, damage by exogenous toxins (including ototoxic drugs), free radicals, and excitotoxins. An important facet to the newly discovered death cascade is that it can be halted, and such interventions may rescue the dying cell. The question now remains whether any of the cell death observed on ototoxic or noise-induced hearing loss occurs by an "active" programmed mechanism (apoptosis), as physiological cell death does in the developing ear, and if so, whether it can be prevented.

Apoptotic cells associated with Wallerian degeneration after experimental spinal cord injury: a possible mechanism of oligodendroglial death

We have investigated the temporal and spatial profiles of apoptotic cells in an experimental transection spinal cord injury by the terminal deoxynucleotidyl transferase-mediated biotin-16-2'-deoxyuridine-5'-triphosphate nick-end labeling (TUNEL) method. Twenty-four hours postinjury, a numerous TUNEL-positive cells appeared both rostrally and caudally to the transection site. Those positive cells, however, gradually diminished in number by several days postinjury. In contrast, other TUNEL-positive cells were found scattered within the white matter remote from the lesion by the third day postinjury. These cells were typically embedded in or among vacuolated fibers, where they were identified in close proximity to the vacuolated space enclosed by myelin basic protein (MBP)-positive structures confirmed by TUNEL-MBP double staining. Because of their linear arrangement, these TUNEL-positive cells were considered interfascicular oligodendrocytes, a fact that was confirmed by the finding that some TUNEL-positive cells were also stained with CCI, a cell marker for oligodendrocyte. Electron microscopic studies revealed that the cells expressing apoptotic morphology were invariably encased in a space formed by myelin splitting. Although the biological significance of apoptotic interfascicular oligodendrocytes in the process of wallerian degeneration is yet to be determined, the finding of such profiles localized within degenerating myelin structures suggests that; oligodendrocytes may be "trapped" within rapidly swollen and disintegrating myelin lamellae, which isolates and perhaps predisposes them to death.

The effects of early prenatal monocular enucleation on the routing of uncrossed retinofugal axons and the cellular environment at the chiasm of mouse embryos

Whereas it has been shown that early monocular enucleations produce a reduction in the uncrossed pathway from the surviving eye in rats and ferrets, similar evidence for binocular interactions in the development of the uncrossed component in mice is currently open to question. Using retrograde tracing, we have investigated the time course of changes in the uncrossed retinofugal pathway immediately after the early prenatal monocular enucleation in mouse embryos. Removal of one eye from C57 pigmented mice at embryonic day (E) 13 does not cause a reduction of the earliest uncrossed component from the central retina examined 1 day later at E14. However, a substantial reduction of the uncrossed pathway is seen at E15, the time when the major uncrossed projection first arises from the ventral temporal retina. This reduction is greater in E16 one-eyed embryos, indicating that most retinal axons from the ventral temporal retina rely on a binocular interaction for their turning at the chiasm. Further, early removal of one eye at E13 does not produce any obvious changes in the cytoarchitecture of RC-2-immunopositive radial glia at the chiasm, nor of the stage-specific antigen-1 (SSEA-1) -expressing neurons. This lack of changes in the cellular organization at the chiasm indicates that the reduction of the uncrossed pathway is probably produced by an elimination of binocular fibre interactions at the chiasm, rather than through a degenerative change of cellular elements at the chiasm as a consequence of the eye removal procedure.

The hippocampus and delayed recall: bigger is not necessarily better?

Healthy young female participants were tested on a measure of delayed verbal recall and then received volumetric Magnetic Resonance Imaging (MRI) scans. The analysis of the MRI scans focused on the volume of the hippocampus. Left hippocampal volume was negatively associated with the level of delayed verbal recall performance. This relationship was confirmed in further testing. This finding is consistent with a previous report of a similar relationship in healthy elderly individuals, but not in patients with Alzheimer's disease, in whom the opposite relationship was observed. An explanation of these findings in terms of impaired neural pruning of the hippocampus is advanced, whereby insufficient pruning of the hippocampus during childhood and adolescence (following adequate growth) may lead to reduced mnemonic efficiency.