Basal expression, subcellular distribution, and up-regulation of the proto-oncogene c-JUN in the rat dentate gyrus after unilateral entorhinal cortex lesion

Lesion-Induced Axonal Sprouting in the Central Nervous System

Injury or neuronal death often come about as a result of brain disorders. Inasmuch as the damaged nerve cells are interconnected via projections to other regions of the brain, such lesions lead to axonal loss in distal target areas. The central nervous system responds to deafferentation by means of plastic remodeling processes, in particular by inducing outgrowth of new axon collaterals from surviving neurons (collateral sprouting). These sprouting processes result in a partial reinnervation, new circuitry, and functional changes within the deafferented brain regions. Lesioning of the entorhinal cortex is an established model system for studying the phenomenon of axonal sprouting. Using this model system, it could be shown that the sprouting process respects the pre-existing lamination pattern of the deafferented fascia dentata, i. e., it is layer-specific. A variety of different molecules are involved in regulating this reorganization process (extracellular matrix molecules, cell adhesion molecules, transcription factors, neurotrophic factors, growth-associated proteins). It is proposed here that molecules of the extracellular matrix define the boundaries of the laminae following entorhinal lesioning and in so doing limit the sprouting process to the deafferented zone. To illustrate the role of axonal sprouting in disease processes, special attention is given to its significance for neurodegenerative disorders, particularly Alzheimer's disease (AD), and temporal lobe epilepsy. Finally, we discuss both the beneficial as well as disadvantageous functional implications of axonal sprouting for the injured organism in question.

Parallel activation of field CA2 and dentate gyrus by synaptically elicited perforant path volleys

Previous studies showed that dorsal psalterium (PSD) volleys to the entorhinal cortex (ENT) activated in layer II perforant path neurons projecting to the dentate gyrus. The discharge of layer II neurons was followed by the sequential activation of the dentate gyrus (DG), field CA3, field CA1. The aim of the present study was to ascertain whether in this experimental model field, CA2, a largely ignored sector, is activated either directly by perforant path volleys and/or indirectly by recurrent hippocampal projections. Field potentials evoked by single-shock PSD stimulation were recorded in anesthetized guinea pigs from ENT, DG, fields CA2, CA1, and CA3. Current source-density (CSD) analysis was used to localize the input/s to field CA2. The results showed the presence in field CA2 of an early population spike superimposed on a slow wave (early response) and of a late and smaller population spike, superimposed on a slow wave (late response). CSD analysis during the early CA2 response showed a current sink in stratum lacunosum-moleculare, followed by a sink moving from stratum radiatum to stratum pyramidale, suggesting that this response represented the activation and discharge of CA2 pyramidal neurons, mediated by perforant path fibers to this field. CSD analysis during the late response showed a current sink in middle stratum radiatum of CA2 followed by a sink moving from inner stratum radiatum to stratum pyramidale, suggesting that this response was mediated by Schaffer collaterals from field CA3. No early population spike was evoked in CA3. However, an early current sink of small magnitude was evoked in stratum lacunosum-moleculare of CA3, suggesting the presence of synaptic currents mediated by perforant path fibers to this field. The results provide novel information about the perforant path system, by showing that dorsal psalterium volleys to the entorhinal cortex activate perforant path neurons that evoke the parallel discharge of granule cells and CA2 pyramidal neurons and depolarization, but no discharge of CA3 pyramidal neurons. Consequently, field CA2 may mediate the direct transfer of ENT signals to hippocampal and extrahippocampal structures in parallel with the DG-CA3-CA1 system and may provide a security factor in situations in which the latter is disrupted.

Effects of early environment on field CA2 pyramidal neurons in the guinea-pig

Previous work showed that isolation rearing produces remarkable changes in the dendritic pattern and soma of the principal neurons in the dentate gyrus and hippocampal fields CA3 and CA1 of the guinea-pig. The aim of the present study was to obtain information about the effects of early postnatal isolation on neuron morphology in field CA2, the "resistant sector" of the hippocampal formation. Male and female guinea-pigs were assigned at 6-7 days of age to either a control (social) or an isolated environment where they remained for 80-90 days. The apical and basal dendritic trees and the soma of CA2 pyramidal neurons were analyzed and quantified in Golgi-stained brains. The results showed that in both males and females early isolation caused no effects on the length and dendritic branching density of the apical tree of field CA2 pyramidal neurons. In males but not in females isolation caused a spine density reduction in the inner apical tree. Isolation notably influenced the morphology of the basal tree, but in males only. Isolated males exhibited a significant reduction in the length of the basal tree and number of dendritic branches accompanied by a reduction in spine density. The comparison of animals reared in the same environment showed that in the control environment males had more apical and basal dendritic branches and a larger neuron soma than females. In the isolated environment the sex differences in the apical tree disappeared and those in the basal tree changed direction.The results demonstrate structural changes in field CA2 pyramidal neurons following neonatal isolation, with a specific reactivity to environment of the basal tree of males. The dendritic atrophy in field CA2 of isolated males is in line with previous evidence that males react to isolation mainly with dendritic atrophy, though field CA2 neurons appear to be less damaged than those of the other hippocampal fields. This is in line with the resistance of this field to neurodegeneration. The absence of structural changes in field CA2 of isolated females confirms, once again, that males are more liable to be endangered by early isolation than females.

Matrix metalloproteinase inhibition alters functional and structural correlates of deafferentation-induced sprouting in the dentate gyrus

Molecules comprising the extracellular matrix (ECM), and the family of matrix metalloproteinases (MMPs) that regulate them, perform essential functions during neuroplasticity in both developing and adult nervous systems, including substrate guidance during neuritogenesis and the establishment of boundaries for axonal terminal fields. MMP proteolysis of ECM molecules may perform a permissive or inductive role in fiber remodeling and synaptogenesis initiated by deafferentation. This study examined functional and structural effects of MMP inhibition during the early phases of deafferentation-induced sprouting, characterizing components of the degeneration/proliferation cycle that may be dependent on MMP activity. Adult rats received unilateral lesions of the entorhinal cortex to induce collateral sprouting of the crossed temporodentate fiber pathway. This was followed by intraventricular infusion of the MMP inhibitor FN-439 (2.9 mg/kg) or saline vehicle. After 7 d postlesion, rats underwent in vivo electrophysiological recording or histological processing for electron microscopic analysis. Lesioned rats receiving vehicle exhibited normal sprouting and synaptogenesis, with the emergence of the capacity for long-term potentiation (LTP) within the sprouting pathway, and the successful clearance of degenerating terminals with subsequent synaptic proliferation. In contrast, lesioned rats receiving the MMP inhibitor failed to develop the capacity for LTP and showed persistent cellular debris. Current source density analysis also revealed an FN-439-induced disruption of the current sink, normally localized to the middle region of the granule cell dendrites, corresponding to the terminal field of the crossed temporodentate fibers. These results establish a role for MMP-dependent processes in the deafferentation/sprouting cycle.

Molecules involved in reactive sprouting in the hippocampus

Denervation of the hippocampus triggers reactive responses in neurons and glial cells in their affected strata in a temporally ordered fashion. Many of these responses have been studied extensively, focusing on the one hand on glial initiation and clearing responses during the degeneration phase and, on the other, on transneuronal reorganization and the newly adjusted physiological balance. We used the entorhinal cortex lesion (ECL) as a model system to study the cues that underlie the layer-specific sprouting response. This lesion destroys the perforant path, which is a massive excitatory projection to the dentate gyrus and hippocampus proper. In the deafferented zones of the hippocampus, sprouting of the remaining unlesioned fibers occurs, which replaces the lost afferences of the perforant path. We focus on candidate molecules which govern the layer-specific sprouting of the remaining axons and, in particular, on membrane-bound cues. The fact that layer-specific sprouting occurs even in the adult central nervous system (CNS) provides a valuable model for understanding the mechanisms of reactive neuronal growth and reorganization in the adult CNS. Isolation and analysis of the molecules involved in these mechanisms are important steps in understanding the potential and limitations of regeneration in the CNS.

Reactive astrocytes upregulate Fas (CD95) and Fas ligand (CD95L) expression but do not undergo programmed cell death during the course of anterograde degeneration

Tissue homeostasis is determined by a balance between proliferation and apoptosis. Various lesions in the brain are accompanied by proliferation and subsequent death of glial cells, but the mechanisms that limit this expansion of glial populations remains unknown. One possible candidate is the death ligand, FasL, and its receptor Fas, because the expression of both proteins was reported on glial cells. To elucidate the expression and putative function of Fas and FasL on proliferative glial cells, we performed stereotactic lesion of the entorhinal cortex of adult rats. Such lesions induce proliferation of astrocytes and microglial cells in the hippocampal fields of anterograde degeneration. Subsequently, the total number of both cell types returns to pre-lesion counts. We found that Fas and FasL is strongly upregulated on astrocytes in the zone of anterograde degeneration with a peak 5 days postlesion (dpl) and a return to control levels at 10 dpl. However, evidence for astrocytic cell death was neither detected by TUNEL staining, immunocytochemistry for c-Jun, and apoptosis-specific protein (ASP), nor by staining for morphologic hallmarks of apoptotic or necrotic cell death at the light and electron microscopic level. Thus, increased expression of Fas and FasL is not accompanied by cell death of reactive astrocytes during anterograde degeneration.

Odor regulates the expression of the mitogen-activated protein kinase phosphatase gene hVH-5 in bilateral entorhinal cortex-lesioned rats

Since it is known that several immediate early genes are induced by olfactory stimuli, we determined whether an olfactory stimulus also induces the expression of the mitogen-activated protein kinase (MAPK) phosphatase gene hVH-5 (homologue of vaccinia virus H1 phosphatase gene, clone 5), a member of a novel class of immediate early genes encoding dual-specificity protein phosphatases. The expression was studied by in situ hybridization in different brain structures involved in odor processing, in control and bilateral entorhinal cortex (EC) lesioned rats. EC-lesion did not significantly affect hVH-5 gene expression in the glomerular cell layer of the olfactory bulb (OB), while odor stimulation induced it in both control and EC-lesioned groups. In contrast, odor-induced expression of hVH-5 gene in mitral/granular cell layers was only evident after lesion of the EC. Similar results were obtained in the piriform cortex (PCx), a structure intimately connected to the mitral cell layer. In the CA1 hippocampal subfield, odor stimulation induced hVH-5 gene expression in both control and EC-lesioned animals, the increase being potentiated in lesioned rats. CA3 and dentate gyrus exhibited a similar pattern of gene expression, the odor stimulating gene expression in both control and lesioned groups. The amygdala (Am) displayed no significant change. It appears that through the induction of a MAPK phosphatase, the EC controls MAPK activities differently after odor stimulation in OB, PCx and hippocampus (Hip). The results illustrate the notion that odor representation in the brain requires plastic modifications at both anatomical and functional levels.

Differential induction of c-Fos, c-Jun and Jun B in the rat central nervous system following unilateral entorhinal cortex lesion

In order to identify some of the molecular mechanisms that occur after a central nervous system trauma, the immediate early gene encoded proteins c-Fos, c-Jun and Jun B were analysed by immunocytochemistry following unilateral entorhinal cortex lesion (controls, 30 min, 2, 5, 12 and 24 h, two, six, 10 and 14 days, four weeks and six months postlesion). In the dentate gyrus, c-Fos was induced in some supragranular neurons (30 min), massively expressed in granule cells ipsilaterally to the lesion (2 h), expressed in hilar neurons (5 h and two days) and was absent at all later stages. A basal expression of c-Jun was found in dentate granule cells of controls, which was strongly increased on the lesion side (2 h) and on the side contralateral to the lesion (12 h). c-Jun expression returned to control levels by 24 h. Jun B was induced in granule cells ipsilateral to the lesion within 2 h and was back to control levels by 5 h. In the lateral septal area, c-Fos and c-Jun were induced 30 min postlesion and decreased rapidly thereafter. In the cerebral cortex, a widespread induction of c-Fos and c-Jun occurred within 30 min after entorhinal cortex lesion and this up-regulation lasted until two days postlesion. These data indicate that electrolytic lesion of the entorhinal cortex leads to a rapid and widespread induction of c-Fos, c-Jun and Jun B. Within the denervated fascia dentata, some of these changes may be linked to the reorganization processes following the lesion. Alternatively, the alterations in immediate early gene expression reported here may be due to changes in synaptic activity or postlesional seizures which occur in this lesioning paradigm.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.