Sprouting of crossed entorhinodentate fibers after a unilateral entorhinal lesion: anterograde tracing of fiber reorganization with Phaseolus vulgaris-leucoagglutinin (PHAL)

Maturation-Dependent Differences in the Re-innervation of the Denervated Dentate Gyrus by Sprouting Associational and Commissural Mossy Cell Axons in Organotypic Tissue Cultures of Entorhinal Cortex and Hippocampus

Sprouting of surviving axons is one of the major reorganization mechanisms of the injured brain contributing to a partial restoration of function. Of note, sprouting is maturation as well as age-dependent and strong in juvenile brains, moderate in adult and weak in aged brains. We have established a model system of complex organotypic tissue cultures to study sprouting in the dentate gyrus following entorhinal denervation. Entorhinal denervation performed after 2 weeks postnatally resulted in a robust, rapid, and very extensive sprouting response of commissural/associational fibers, which could be visualized using calretinin as an axonal marker. In the present study, we analyzed the effect of maturation on this form of sprouting and compared cultures denervated at 2 weeks postnatally with cultures denervated at 4 weeks postnatally. Calretinin immunofluorescence labeling as well as time-lapse imaging of virally-labeled (AAV2-hSyn1-GFP) commissural axons was employed to study the sprouting response in aged cultures. Compared to the young cultures commissural/associational sprouting was attenuated and showed a pattern similar to the one following entorhinal denervation in adult animals in vivo. We conclude that a maturation-dependent attenuation of sprouting occurs also in vitro, which now offers the chance to study, understand and influence maturation-dependent differences in brain repair in these culture preparations.

Crossed Entorhino-Dentate Projections Form and Terminate With Correct Layer-Specificity in Organotypic Slice Cultures of the Mouse Hippocampus

The entorhino-dentate projection, i.e., the perforant pathway, terminates in a highly ordered and laminated fashion in the rodent dentate gyrus (DG): fibers arising from the medial entorhinal cortex (MEC) terminate in the middle molecular layer, whereas fibers arising from the lateral entorhinal cortex (LEC) terminate in the outer molecular layer of the DG. In rats and rabbits, a crossed entorhino-dentate projection exists, which originates from the entorhinal cortex (EC) and terminates in the contralateral DG. In contrast, in mice, such a crossed projection is reportedly absent. Using single and double mouse organotypic entorhino-hippocampal slice cultures, we studied the ipsi- and crossed entorhino-dentate projections. Viral tracing revealed that entorhino-dentate projections terminate with a high degree of lamina-specificity in single as well as in double cultures. Furthermore, in double cultures, entorhinal axons arising from one slice freely intermingled with entorhinal axons originating from the other slice. In single as well as in double cultures, entorhinal axons exhibited a correct topographical projection to the DG: medial entorhinal axons terminated in the middle and lateral entorhinal axons terminated in the outer molecular layer. Finally, entorhinal neurons were virally transduced with Channelrhodopsin2-YFP and stimulated with light, revealing functional connections between the EC and dentate granule cells. We conclude from our findings that entorhino-dentate projections form bilaterally in the mouse hippocampus in vitro and that the mouse DG provides a permissive environment for crossed entorhinal fibers.

NMDA-receptor inhibition increases spine stability of denervated mouse dentate granule cells and accelerates spine density recovery following entorhinal denervation in vitro

Neuronal networks are reorganized following brain injury. At the structural level this is in part reflected by changes in the spine turnover of the denervated neurons. Using the entorhinal cortex lesion in vitro model, we recently showed that mouse dentate granule cells respond to entorhinal denervation with coordinated functional and structural changes: During the early phase after denervation spine density decreases, while excitatory synaptic strength increases in a homeostatic manner. At later stages spine density increases again, and synaptic strength decreases back to baseline. In the present study, we have addressed the question of whether the denervation-induced homeostatic strengthening of excitatory synapses could not only be a result of the deafferentation, but could, in turn, affect the dynamics of the spine reorganization process following entorhinal denervation in vitro. Using a computational approach, time-lapse imaging of neurons in organotypic slice cultures prepared from Thy1-GFP mice, and patch-clamp recordings we provide experimental evidence which suggests that the strengthening of surviving synapses can lead to the destabilization of spines formed after denervation. This activity-dependent pruning of newly formed spines requires the activation of N-methyl-d-aspartate receptors (NMDA-Rs), since pharmacological inhibition of NMDA-Rs resulted in a stabilization of spines and in an accelerated spine density recovery after denervation. Thus, NMDA-R inhibitors may restore the ability of neurons to form new stable synaptic contacts under conditions of denervation-induced homeostatic synaptic up-scaling, which may contribute to their beneficial effect seen in the context of some neurological diseases.

Structural plasticity in the dentate gyrus- revisiting a classic injury model

The adult brain is in a continuous state of remodeling. This is nowhere more true than in the dentate gyrus, where competing forces such as neurodegeneration and neurogenesis dynamically modify neuronal connectivity, and can occur simultaneously. This plasticity of the adult nervous system is particularly important in the context of traumatic brain injury or deafferentation. In this review, we summarize a classic injury model, lesioning of the perforant path, which removes the main extrahippocampal input to the dentate gyrus. Early studies revealed that in response to deafferentation, axons of remaining fiber systems and dendrites of mature granule cells undergo lamina-specific changes, providing one of the first examples of structural plasticity in the adult brain. Given the increasing role of adult-generated new neurons in the function of the dentate gyrus, we also compare the response of newborn and mature granule cells following lesioning of the perforant path. These studies provide insights not only to plasticity in the dentate gyrus, but also to the response of neural circuits to brain injury.

Intensive protein synthesis in neurons and phosphorylation of beta-amyloid precursor protein and tau-protein are triggering factors of neuronal amyloidosis and Alzheimer's disease

Recently the studies of Alzheimer’s disease have become particularly actual and have attracted scientists from all over the world to this problem as a result of dissemination of this dangerous disorder. The reason for such pathogenesis is not known, but the final image, for the first time obtained on microscopic brain sections from patients with this disease more than a hundred years ago, is well known to clinicists. This is the deposition of Ab amyloid in the brain tissue of senile plaques and fibrils. Many authors suppose that the deposition of beta-amyloid provokes secondary neuronal changes which are the reason of neuron death. Other authors associate the death of neurons with hyperphosphorylation of tau-proteins which form neurofibrillar coils inside nerve cells and lead to their death. For creation of methods of preclinical diagnostics and effective treatment of Alzheimer’s disease novel knowledge is required on the nature of triggering factors of sporadic isoforms of Alzheimer’s disease, on cause-effect relationships of phosphorylation of amyloid precursor protein with formation of pathogenic beta-amyloids, on the relationship with these factors of hyperphosphorylation of tau-protein and neuron death. In this review we analyze the papers describing the increasing of intensity of biosynthesis in neurons in normal conditions and under the stress, the possibility of development of energetic unbalanced neurons and activation of their protective systems. Phosphorylation and hyperphosphorylation of tau-proteins is also tightly connected with protective mechanisms of cells and with processes of evacuation of phosphates, adenosine mono-phosphates and pyrophosphates from the region of protein synthesis. Upon long and high intensity of protein synthesis the protective mechanisms are overloaded and the complementarity of metabolitic processes is disturbed. This results in dysfunction of neurons, transport collapse, and neuron death.

Time-lapse imaging of granule cells in mouse entorhino-hippocampal slice cultures reveals changes in spine stability after entorhinal denervation

Principal neurons that are partially denervated after brain injury remodel their synaptic connections and show biphasic changes in their dendritic spine density: during an early phase after denervation spine density decreases and during a late phase spine density recovers again. It has been hypothesized that these changes in spine density are caused by a period of increased spine loss followed by a period of increased spine formation. We have tested this hypothesis, which is based on data from fixed tissues, by using time-lapse imaging of denervated dentate granule cells in organotypic entorhino-hippocampal slice cultures of Thy1-GFP mice. Our data show that nondenervated granule cells turn over spines spontaneously while keeping their spine density constant. Denervation influenced this equilibrium and induced biphasic changes in the spine loss rate but not in the rate of spine formation: during the early phase after denervation the spine loss rate was increased and during the late phase after denervation the spine loss rate was decreased compared with nondenervated control cultures. In line with these observations, time-lapse imaging of identified spines formed after the lesion revealed that the stability of these spines was decreased during the early phase and increased during the late phase after the lesion. We conclude that biphasic changes in spine loss rate and spine stability but not in the rate of spine formation play a central role in the reorganization of dentate granule cells after entorhinal denervation in vitro.

Angiotensin II Type 1 receptor (AT1) signaling in astrocytes regulates synaptic degeneration-induced leukocyte entry to the central nervous system

Astrocytes are the major cellular component of the blood-brain barrier glia limitans and act as regulators of leukocyte infiltration via chemokine expression. We have studied angiotensin-II receptor Type 1 (AT1) and related NF-κB signaling in astrocytes. Angiotensin II derives from cleavage of angiotensin I by angiotensin converting enzyme (ACE), angiotensin I deriving from angiotensinogen via cleavage by renin. Level of expression of ACE was slightly increased in transgenic mice that express dominant-negative IκBα in astrocytes (GFAP-IκBα-dn mice), whereas angiotensinogen and renin, also constitutively expressed in the CNS, were unaffected by NF-κB inhibition. Leukocytes infiltrate the hippocampus of mice after unilateral stereotactic lesion of afferent perforant path axons in the entorhinal cortex. Upregulation of the chemokine CXCL10 that normally occurs in response to synaptic degeneration in the dentate gyrus following axonal transection was totally abrogated in GFAP-IκBα-dn mice. Whereas angiotensin II was upregulated in microglia and astrocytes in the dentate gyrus post-lesion, AT1 was exclusively expressed on astrocytes. Blocking AT1 with Candesartan led to significant increase in numbers of infiltrating macrophages in the hippocampus 2days post-lesion. Lesion-induced increases in T-cell infiltration and morphologic glial response were unaffected, and the blood-brain barrier remained intact to horseradish peroxidase. These findings show that angiotensin II signaling to astrocytes via AT1 plays an important role in regulation of leukocyte infiltration to the CNS in response to a neurodegenerative stimulus, and identify potential targets for therapies directed at adaptive immune responses in the CNS.

Unilateral entorhinal denervation leads to long-lasting dendritic alterations of mouse hippocampal granule cells

Following brain injury, neurons efferently connected from the lesion site are denervated and remodel their dendritic tree. Denervation-induced dendritic reorganization of granule cells was investigated in the dentate gyrus of the Thy1-GFP mouse. After mechanical transection of the perforant path, single granule cells were 3D-reconstructed at different time points post-lesion (3d, 7d, 10d, 30 d, 90 d and 180 d) and their soma size, total dendritic length, number of dendritic segments and dendritic branch orders were studied. Changes in spine densities were determined using 3D-analysis of individual dendritic segments. Following entorhinal denervation the granule cell arbor progressively atrophied until 90 d post-lesion (reduction of total dendritic length to ~50% of control). Dendritic alterations occurred selectively in the denervated outer molecular layer, where a loss of distal dendritic segments and a reduction of mean segment length were seen. At 180 d post-lesion total dendritic length partially recovered (~70% of control). This recovery appeared to be the result of a re-elongation of surviving dendrites rather than dendritic re-branching, since the number of dendritic segments did not recover. In contrast to the protracted dendritic changes, spine density changes followed a faster time course. In the denervated layer spine densities dropped to ~65% of control values and fully recovered by 30 d post-lesion. We conclude that entorhinal denervation in mouse causes protracted and long-term structural alterations of the granule cell dendritic tree. Spontaneously occurring reinnervation processes, such as the sprouting of surviving afferent fibers, are insufficient to maintain the granule cell dendritic arbor.

Macrophage-independent T cell infiltration to the site of injury-induced brain inflammation

We have addressed the role of macrophages in glial response and T cell entry to the CNS after axonal injury, by using intravenous injection of clodronate-loaded mannosylated liposomes, in C57BL6 mice. As expected, clodronate-liposome treatment resulted in depletion of peripheral macrophages which was confirmed by F4/80- and MOMA-1(-) stainings in spleen. Sequential clodronate-liposome treatment 4, 2 and 0 days before axotomy resulted in significant reduction of infiltrating CD45(high) CD11b+ macrophages in the hippocampus at 1, 2 and 3 days post-lesion, measured by flow cytometry. There was a slight delay in the expansion of CD45(dim) CD11+ microglia in clodronate-liposome treated mice, but macrophage depletion had no effect on the percentage of infiltrating T cells in the lesion-reactive hippocampus. Lesion-induced TNFalpha mRNA expression was not affected by macrophage depletion, suggesting that activated glial cells are the primary source of this cytokine in the axonal injury-reactive brain. This identifies a potentially important distinction from inflammatory autoimmune infiltration in EAE, where macrophages are a prominent source of TNFalpha and their depletion prevents parenchymal T cell infiltration and disease.

NF-kappaB-driven STAT2 and CCL2 expression in astrocytes in response to brain injury

Tissue response to injury includes expression of genes encoding cytokines and chemokines. These regulate entry of immune cells to the injured tissue. The synthesis of many cytokines and chemokines involves NF-kappaB and signal transducers and activators of transcription (STAT). Injury to the CNS induces glial response. Astrocytes are the major glial population in the CNS. We examined expression of STATs and the chemokine CCL2 and their relationship to astroglial NF-kappaB signaling in the CNS following axonal transection. Double labeling with Mac-1/CD11b and glial fibrillary acidic protein revealed that STAT2 up-regulation and phosphorylation colocalized exclusively to astrocytes, suggesting the involvement of STAT2 activating signals selectively in astroglial response to injury. STAT1 was also up-regulated and phosphorylated but not exclusively in astrocytes. Both STAT2 up-regulation and phosphorylation were NF-kappaB -dependent since they did not occur in the lesion-reactive hippocampus of transgenic mice with specific inhibition of NF-kappaB activation in astrocytes. We further showed that lack of NF-kappaB signaling significantly reduced injury-induced CCL2 expression as well as leukocyte infiltration. Our results suggest that NF-kappaB signaling in astrocytes controls expression of both STAT2 and CCL2, and thus regulates infiltration of leukocytes into lesion-reactive hippocampus after axonal injury. Taken together, these findings indicate a central role for astrocytes in directing immune-glial interaction in the CNS injury response.

Differential regulation of chondroitin sulfate proteoglycan mRNAs in the denervated rat fascia dentata after unilateral entorhinal cortex lesion

Following brain trauma, chondroitin sulphate proteoglycans (CSPGs) are enriched at injury sites and in denervated areas. At injury sites, CSPGs are regarded as inhibitors of axonal regeneration because of their growth inhibitory properties. In areas of denervation their role is less clear, since they are enriched in zones of sprouting, i.e. zones of axonal growth. To identify CSPGs expressed in a denervated brain area and to quantify changes in their mRNA expression, neurocan, brevican, NG2, phosphacan and aggrecan mRNA were analyzed in the rat fascia dentata following entorhinal denervation. Laser microdissection was combined with quantitative RT-PCR to measure mRNA changes specifically within the denervated portion of the molecular layer (1h, 6h, 10h, 12h, 1d, 2d, 3d, 4d, 7d and 14d post-lesion). Changes in glial fibrillary protein mRNA were measured at the same time points and used as lesion control. This approach revealed a differential regulation of CSPG mRNAs in the denervated zone: neurocan, brevican and NG2 mRNA were upregulated with a maximum around 2 days post-lesion. In contrast, aggrecan mRNA levels reached a maximum 7 days post-lesion and phosphacan mRNA levels were not significantly altered. Taken together, our data reveal a temporal pattern in CSPG mRNA expression in the denervated fascia dentata. This suggests specific biological functions for CSPGs during the denervation-induced reorganization process: whereas the early increase in CSPGs in the denervated zone could influence the pattern of sprouting, the late increase of aggrecan mRNA suggests a different role during the late phase of reorganization.

Structural reorganization of the dentate gyrus following entorhinal denervation: species differences between rat and mouse

Deafferentation of the dentate gyrus by unilateral entorhinal cortex lesion or unilateral perforant pathway transection is a classical model to study the response of the central nervous system (CNS) to denervation. This model has been extensively characterized in the rat to clarify mechanisms underlying denervation-induced gliosis, transneuronal degeneration of denervated neurons, and collateral sprouting of surviving axons. As a result, candidate molecules have been identified which could regulate these changes, but a causal link between these molecules and the postlesional changes has not yet been demonstrated. To this end, mutant mice are currently studied by many groups. A tacit assumption is that data from the rat can be generalized to the mouse, and fundamental species differences in hippocampal architecture and the fiber systems involved in sprouting are often ignored. In this review, we will (1) provide an overview of some of the basics and technical aspects of the entorhinal denervation model, (2) identify anatomical species differences between rats and mice and will point out their relevance for the axonal reorganization process, (3) describe glial and local inflammatory changes, (4) consider transneuronal changes of denervated dentate neurons and the potential role of reactive glia in this context, and (5) summarize the differences in the reorganization of the dentate gyrus between the two species. Finally, we will discuss the use of the entorhinal denervation model in mutant mice.

Toll-like receptors in brain development and homeostasis

Toll-like receptors (TLRs) are best known as initiators of the innate immune response to pathogens. Recent reports now reveal intriguing roles for TLRs in the central nervous system (CNS). These include the regulation of neuroinflammation and of neurite outgrowth. The archetypal Toll protein in Drosophila melanogaster was implicated in the development of the nervous system. Now similar functions have been uncovered for the mammalian orthologs, the TLRs. TLRs expressed on CNS glia and neurons may recognize endogenous ligands and participate both in development and in responses associated with CNS injury.

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.

Plasticity of synaptopodin and the spine apparatus organelle in the rat fascia dentata following entorhinal cortex lesion

Synaptopodin is an actin-associated molecule essential for the formation of a spine apparatus in telencephalic spines. To study whether synaptopodin and the spine apparatus organelle are regulated under conditions of lesion-induced plasticity, synaptopodin and the spine apparatus were analyzed in granule cells of the rat fascia dentata following entorhinal denervation. Confocal microscopy was employed to quantify layer-specific changes in synaptopodin-immunoreactive puncta densities. Electron microscopy was used to quantify layer-specific changes in spine apparatus organelles. Within the denervated middle and outer molecular layers, the layers of deafferentation-induced spine loss, synaptogenesis, and spinogenesis, the density of synaptopodin puncta and the number of spine apparatuses decreased by 4 days postlesion and slowly recovered in parallel with spinogenesis by 180 days postlesion. Within the nondenervated inner molecular layer, the zone without deafferentation-induced spine loss, a rapid loss of synaptopodin puncta and spine apparatuses was also observed. In this layer, spine apparatus densities recovered by 14 days postlesion, in parallel with plastic remodeling at the synaptic level and the postlesional recovery of granule cell activity. These data demonstrate layer-specific changes in the distribution of synaptopodin and the spine apparatus organelle following partial denervation of granule cells: in the layer of spine loss, spine apparatus densities follow spine densities; in the layer of spine maintenance, however, spine apparatus densities appear to be regulated by other signals.

NG2 upregulation in the denervated rat fascia dentata following unilateral entorhinal cortex lesion

The chondroitin sulfate proteoglycan NG2 is a component of the glial scar following brain injury. Because of its growth inhibiting properties, it has been suggested to impede axonal regeneration. To study whether NG2 could also regulate axonal growth in denervated brain areas, changes in NG2 were studied in the rat fascia dentata following entorhinal deafferentation and were correlated with the post-lesional sprouting response. Laser microdissection was employed to selectively harvest the denervated molecular layer and combined with quantitative RT-PCR to measure changes in NG2 mRNA (6 h, 12 h, 2 days, 4 days, 7 days post-lesion). This revealed increases of NG2 mRNA at day 2 (2.5-fold) and day 4 (2-fold) post-lesion. Immunocytochemistry was used to detect changes in NG2 protein (1 days, 4 days, 7 days, 10 days, 14 days, 30 days, 6 months post-lesion). NG2 staining was increased in the denervated outer molecular layer at day 1 post-lesion, reached a maximum 10 days post-lesion, and returned to control levels thereafter. Electron microscopy revealed NG2 immunoprecipitate on glial surfaces and in the extracellular matrix around neuronal profiles, indicating that NG2 is secreted following denervation. Double labeling of NG2-immunopositive cells with markers for astrocytes, microglia/macrophages, and mature oligodendrocytes suggested that NG2 cells are a distinct glial subpopulation before and after entorhinal deafferentation. BrdU labeling revealed that some of the NG2-positive cells are generated post-lesion. Taken together, our data revealed a layer-specific upregulation of NG2 in the denervated fascia dentata that coincides with the sprouting response. This suggests that NG2 could regulate lesion-induced axonal growth in denervated areas of the brain.

Induction of ephrin-B1 and EphB receptors during denervation-induced plasticity in the adult mouse hippocampus

Abstract It has been widely demonstrated that Eph receptors and their ephrin ligands play multiple pivotal roles in the development of the nervous system. However, less is known about their roles in the adult brain. Here we reported the expression of ephrin-B1 and its cognate EphB receptors in the adult mouse hippocampus at 3, 7, 15, 30 and 60 days after transections of the entorhinal afferents. In situ hybridization and immunohistochemistry showed the time-dependent up-regulation of ephrin-B1 in the denervated areas of the hippocampus, which initiated at 3 days postlesion (dpl), reached maximal levels at 7-15 dpl, remained slightly elevated at 30 dpl and recovered to normal levels by 60 dpl. Double labeling of ephrin-B1 and glial fibrillary acidic protein revealed that ephrin-B1-expressing cells in the denervated areas were reactive astrocytes. Furthermore, a ligand-binding assay using ephrin-B1/Fc chimera protein also displayed the up-regulation of EphB receptors in the denervated areas of the hippocampus in a similar manner to that of ephrin-B1. Within the first week postlesion, the EphB receptors were expressed by reactive astrocytes. After 7 dpl, however, EphB receptors were expressed not only by reactive astrocytes but also first by sprouting axons and later by regrowing dendrites. These results suggest that the ephrin-B1/EphB system may participate in the lesion-induced plasticity processes in the adult mouse hippocampus.

Microglial cell population dynamics in the injured adult central nervous system

Reactive microgliosis is characteristic of trauma and stroke as well as inflammatory and chronic neurodegenerative disease. A conspicuous feature of the microglial reaction to acute neural injury is a massive expansion of the microglial cell population which peaks a few days following injury. New data based on the use of radiation bone marrow-chimeric mice suggest this expansion also involves recruitment of bone marrow-derived cells, which migrate into the neural parenchyma and differentiate into microglia. Here, we discuss the contribution of bone marrow-derived cells to the injury-induced expansion of the microglial cell population, seen in the dentate gyrus with ongoing anterograde axonal and terminal synaptic degeneration, subsequent to transection of the entorhino-dentate perforant path projection. In this paradigm of minor brain injury, the bone marrow-derived cells are grossly outnumbered by activated resident microglia, which express the stem cell antigen CD34 concurrent to a marked capacity for self-renewal. The observation of a mixed origin of lesion-reactive microglia, consisting of a smaller subpopulation of exogenous bone marrow-derived microglia, and a larger population of activated resident microglia, the majority of which express CD34 and undergo proliferation, suggests that lesion-reactive microglia consist of functionally distinct cell populations. The demonstration of an injury-enhanced recruitment of bone marrow-derived cells into the perforant path-denervated dentate gyrus, raises the possibility of using genetically manipulated cells as vectors for lesion-site-specific gene therapy even in minimally injured areas of the central nervous system.

Stereological analysis of the reorganization of the dentate gyrus following entorhinal cortex lesion in mice

Denervation of the dentate gyrus by entorhinal cortex lesion has been widely used to study the reorganization of neuronal circuits following central nervous system lesion. Expansion of the non-denervated inner molecular layer (commissural/associational zone) of the dentate gyrus and increased acetylcholinesterase-positive fibre density in the denervated outer molecular layer have commonly been regarded as markers for sprouting following entorhinal cortex lesion. However, because this lesion extensively denervates the outer molecular layer and causes tissue shrinkage, stereological analysis is required for an accurate evaluation of sprouting. To this end we have performed unilateral entorhinal cortex lesions in adult C57BL/6J mice and have assessed atrophy and sprouting in the dentate gyrus using modern unbiased stereological techniques. Results revealed the expected increases in commissural/associational zone width and density of acetylcholinesterase-positive fibres on single brain sections. Yet, stereological analysis failed to demonstrate concomitant increases in layer volume or total acetylcholinesterase-positive fibre length. Interestingly, calretinin-positive fibres did grow beyond the border of the commissural/associational zone into the denervated layer and were regarded as sprouting axons. Thus, our data suggest that in C57BL/6J mice shrinkage of the hippocampus rather than growth of fibres underlies the two morphological phenomena most often cited as evidence of regenerative sprouting following entorhinal cortex lesion. Moreover, our data suggest that regenerative axonal sprouting in the mouse dentate gyrus following entorhinal cortex lesion may be best assessed at the single-fibre level.

Comparison of commissural sprouting in the mouse and rat fascia dentata after entorhinal cortex lesion

Reactive axonal sprouting occurs in the fascia dentata after entorhinal cortex lesion. This sprouting process has been described extensively in the rat, and plasticity-associated molecules have been identified that might be involved in its regulation. To demonstrate causal relationships between these candidate molecules and the axonal reorganization process, it is reasonable to analyze knockout and transgenic animals after entorhinal cortex lesion, and because gene knockouts are primarily generated in mice, it is necessary to characterize the sprouting response after entorhinal cortex lesion in this species. In the present study, Phaseolus vulgaris-leucoagglutinin (PHAL) tracing was used to analyze the commissural projection to the inner molecular layer in mice with longstanding entorhinal lesions. Because the commissural projection to the fascia dentata is neurochemically heterogeneous, PHAL tracing was combined with immunocytochemistry for calretinin, a marker for commissural/associational mossy cell axons. Using both techniques singly as well as in combination (double-immunofluorescence) at the light or electron microscopic level, it could be shown that in response to entorhinal lesion mossy cell axons leave the main commissural fiber plexus, invade the denervated middle molecular layer, and form asymmetric synapses within the denervated zone. Thus, the commissural sprouting response in mice has a considerable translaminar component. This is in contrast to the layer-specific commissural sprouting observed in rats, in which the overwhelming majority of mossy cell axons remain within their home territory. These data demonstrate an important species difference in the commissural/associational sprouting response between rats and mice that needs to be taken into account in future studies.

Associational sprouting in the mouse fascia dentata after entorhinal lesion in vitro

Collateral sprouting is a form of neuronal plasticity observed in brain following injury. In order to establish an in vitro model of collateral sprouting, entorhino-hippocampal slice cultures were prepared from brain of C57BL/6 mouse pups (P1-4) and incubated for 14-16 days in vitro. Thereafter, entorhino-hippocampal fibers were cut and the outer molecular layer of the fascia dentata was denervated. At this age, entorhino-hippocampal fibers do not regenerate, as could be shown using anterograde tracing with Miniruby. Sprouting of associational mossy cell axons was monitored using calretinin-immunocytochemistry. Control and lesioned entorhino-hippocampal slices were studied at 1, 5, and 10 days postlesion. Whereas only the inner portion of the molecular layer was occupied by calretinin-positive mossy cell axons in controls and after 1 and 5 days postlesion, the entire width of the molecular layer was occupied by associational fibers by 10 days postlesion. Thus, robust sprouting of associational mossy cell axons occurs in response to entorhinal denervation in vitro. Using organotypic entorhino-hippocampal slices of genetically engineered mice, this sprouting model can be used to identify molecules involved in the regulation of sprouting following brain injury.

Entorhinal axons exhibit sprouting in CA1 subfield of the adult hippocampus in a rat model of temporal lobe epilepsy

Intracerebroventricular kainic acid administration in rat, a model of temporal lobe epilepsy, results in CA3 pyramidal neuron degeneration leading to deafferentation of CA1 pyramidal neurons. Denervation in CA1 shows a near-complete recovery of synaptic density over 2-3 months, but the source of axons participating in the reinnervation is not clear. This study investigated the contribution of the entorhinal cortex in this reinnervation by comparing the distribution of the entorhinal axons in the CA1 subfield between the intact hippocampus and the CA3-lesioned hippocampus at 3 months after administration of kainic acid. Entorhinal axons were visualized by anterograde tracing using injections of the biotinylated dextran amine into the entorhinal cortex. In the CA1 subfield of the intact hippocampus, entorhinal axons were conspicuous in the alveus and the stratum lacunosum moleculare. The distribution in the strata oriens, pyramidale, and radiatum was sparse and was characterized by isolated entorhinal fibers of the alvear pathway crossing these strata to the stratum lacunosum moleculare. However, after kainic acid-induced CA3 lesion, the density of entorhinal axons increased significantly in the CA1 stratum radiatum (375% of the intact hippocampus), as a large number of axons emanating from the entorhinal fiber plexus in the stratum lacunosum moleculare invaded the stratum radiatum. The stratum radiatum also exhibited wavy entorhinal axons filled with boutons and oriented parallel to the stratum pyramidale, suggesting collateral sprouting from entorhinal axons traversing the stratum radiatum. Thus, a significant aberrant sprouting of entorhinal axons occurs into the CA1 stratum radiatum after CA3 lesion. The sprouted fibers appear to come from both entorhinal fiber plexus in the stratum lacunosum moleculare (translaminar sprouting) and entorhinal axons traversing the stratum radiatum (intralaminar sprouting). However, the major contribution appears to be from the entorhinal plexus in the stratum lacunosum moleculare. This aberrant sprouting may lead to altered afferent excitatory connectivity in the CA1 subfield and contribute to the persistent CA1 hyperexcitability that occurs after the CA3 lesion.

Reorganization of the rat fascia dentata after a unilateral entorhinal cortex lesion. Role of the extracellular matrix

Entorhinal cortex lesion (ECL) partially denervates the fascia dentata of the hippocampus. This is said to induce the sprouting of intact fibers from neighboring layers that invade the zone of the degenerating axons. However, recent studies using anterograde tracing failed to demonstrate sprouting across laminar boundaries. Sprouting does occur, but it mainly involves unlesioned fiber systems terminating within the layer of fiber degeneration. It is now of interest to identify the cues that could underlie this layer-specific sprouting response. Since extracellular matrix (ECM) molecules delineate boundaries of axonal growth during development, it was tested whether these molecules play a similar role during the sprouting process following ECL. After ECL, reactive astrocytes rapidly synthesize and secrete growth-inhibiting ECM molecules, such as tenascin-C and the chondroitin sulfate proteoglycan neurocan, into the ECM of the outer molecular layer. These molecules form a sharp border against the nondenervated inner molecular layer. This pattern of ECM molecule expression may contribute to the layer-specific sprouting response of surviving afferents after ECL: axons trying to grow into the denervated outer molecular layer, for example, from the inner molecular layer, would be deflected by a growth-inhibiting ECM barrier.

Involvement of non-neuronal cells in entorhinal-hippocampal reorganization following lesions

Entorhinal lesion leads to anterograde degeneration of perforant path fibers in their main hippocampal termination zones. Subsequently, remaining fibers sprout and form new synapses on the denervated dendrites. This degeneration and reorganization is accompanied by sequential changes in glial morphology and function. Within a few hours following the lesion, amoeboid microglia migrate into the zone of denervation. Some hours later, signs of activation can be seen on astrocytes in the zone of denervation, where both cell types proliferate and remain in an activated state for more than two weeks. These activated glial cells might be involved in lesion-induced plasticity in at least two ways: (1) by releasing cytokines and growth factors which regulate layer-specific sprouting and (2) by phagocytosis of axonal debris, because myelin sheaths act as obstacles for sprouting fibers in the central nervous system. Whereas direct evidence for the former is still missing, the latter was investigated using phagocytosis-dependent labeling techniques. Both microglial cells and astrocytes incorporate axonal debris. Phagocytosing microglial cells develop the immune phenotype of antigen-presenting cells, whereas astrocytes strongly express FasL (CD95L), which induces apoptosis of activated lymphocytes. Thus, the interaction of glial cells with immune cells might be another, previously underestimated, aspect of reorganization following entorhinal lesion.

Axonal sprouting regulates myelin basic protein gene expression in denervated mouse hippocampus

The regulation of oligodendrocyte gene expression and myelination in vivo in the normal and injured adult CNS is still poorly understood. We have analyzed the effects of axotomy-induced axonal sprouting and microglial activation, on oligodendrocyte myelin basic protein (MBP) gene expression from 2 to 35 days after transection of the entorhino-hippocampal perforant path axonal projection. In situ hybridization analysis showed that anterograde axonal and terminal degeneration lead to upregulated oligodendrocyte MBP mRNA expression starting between day 2 and day 4, in (1) the deep part of stratum radiatum of CA3 and the dentate hilus, which display axonal sprouting but no degenerative changes or microglial activation, and (2) the outer part of the molecular layer of the fascia dentata, and in stratum moleculare of CA3 and stratum lacunosum-moleculare of CA1, areas that display dense anterograde axonal and terminal degeneration, myelin degenerative changes, microglial activation and axotomi-induced axonal sprouting. Oligodendrocyte MBP mRNA expression reached maximum in both these areas at day 7. MBP gene transcription remained constant in stratum radiatum, stratum pyramidale and stratum oriens of CA1, areas that were unaffected by perforant path transection. These results provide strong evidence that oligodendrocyte MBP gene expression can be regulated by axonal sprouting independently of microglial activation in the injured adult CNS.

Quantitative evidence for increase in galanin-immunoreactive terminals in the hippocampal formation following entorhinal cortex lesions in the adult rat

The projection from the entorhinal cortex to the dentate gyrus and hippocampus is severely affected in Alzheimer's disease and there is a depletion of cholinergic terminals but an upregulation of the neuropeptide galanin, which inhibits the release of acetylcholine. Evidence for changes to galanin-immunoreactive terminals in the hippocampal formation was therefore examined after unilateral entorhinal cortex lesions in the adult rat. An increase in the density of galanin-immunoreactive terminals on the lesioned side was evident in the stratum lacunosum moleculare of the hippocampus and the outer molecular layer of the dentate gyrus at 17 days post-lesion, and it increased gradually until the last time point examined, at 40 days post-lesion. Thus we demonstrate that there is an increase in galanin-immunoreactive terminals in the hippocampal formation following entorhinal cortex lesions.

Endogenous FGF-2 is important for cholinergic sprouting in the denervated hippocampus

To investigate the molecular mechanisms of cholinergic sprouting in the hippocampus after removal of entorhinal cortical inputs, we evaluated trophic factor gene expression in the denervated hippocampus. Despite the proposed role for nerve growth factor (NGF) in this sprouting, we observed no change in NGF mRNA or protein at several postlesion time points. In contrast, FGF-2 mRNA was increased within 16 hr. FGF-2 immunoreactivity was localized within GFAP-positive hypertrophic astrocytes distributed specifically within the denervated outer molecular layer after the lesion. To address the functional significance of this increase in FGF-2, we assessed the magnitude of cholinergic sprouting in animals receiving chronic intracerebroventricular infusions of neutralizing antibodies specific for FGF-2 and compared it with that observed in lesioned animals receiving infusate controls. Animals given FGF-2 antibodies displayed a marked reduction in cholinergic sprouting as compared with controls. In fact, many of these animals exhibited virtually no sprouting at all despite histological verification of complete lesions. These results suggest that endogenous FGF-2 promotes cholinergic axonal sprouting in the injured adult brain. Furthermore, immunocytochemical localization of receptors for FGF-2 (i.e., FGFR1) on projecting basal forebrain cholinergic neurons suggests that FGF-2 acts directly on these neurons to induce the lesion-induced sprouting response.

Chronic changes in synaptic responses of entorhinal and hippocampal neurons after amino-oxyacetic acid (AOAA)-induced entorhinal cortical neuron loss

Chronic changes in synaptic responses of entorhinal and hippocampal neurons after amino-oxyacetic acid (AOAA)-induced entorhinal neuron loss. J. Neurophysiol. 80: 3031-3046, 1998. Synaptic responses of entorhinal cortical and hippocampal neurons were examined in vivo and in vitro, 1 mo to 1.5 yr after a unilateral entorhinal lesion caused by a focal injection of amino-oxyacetic acid (AOAA). It has been shown previously that injection of AOAA into the medial entorhinal cortex produces cell loss in layer III preferentially. Although behavioral seizures stopped approximately 2 h after AOAA treatment, abnormal evoked responses were recorded as long as 1.5 yr later in the entorhinal cortex and hippocampus. In the majority of slices from AOAA-treated rats, responses recorded in the superficial layers of the medial entorhinal cortex to white matter, presubiculum, or parasubiculum stimulation were abnormal. Extracellularly recorded responses to white matter stimulation were prolonged and repetitive in the superficial layers. Intracellular recordings showed that residual principal cells in superficial layers produced prolonged, repetitive excitatory postsynaptic potentials (EPSPs) and discharges in response to white matter stimulation compared with brief EPSPs and a single discharge in controls. Responses of deep layer neurons of AOAA-treated rats did not differ from controls in their initial synaptic response. However, in a some of these neurons, additional periods of excitatory activity occurred after a delay. Abnormal responses were recorded from slices ipsilateral as well as contralateral to the lesioned hemisphere. Recordings from the entorhinal cortex in vivo were abnormal also, as demonstrated by prolonged and repetitive responses to stimulation of the area CA1/subiculum border. Evoked responses of hippocampal neurons, recorded in vitro or in vivo, demonstrated abnormalities in selected pathways, such as responses of CA3 neurons to hilar stimulation in vitro. There was a deficit in the duration of potentiation of CA1 population spikes in response to repetitive CA3 stimulation in AOAA-treated rats. Theta activity was reduced in amplitude in area CA1 and the dentate gyrus of AOAA-treated rats, although evoked responses to angular bundle stimulation could not be distinguished from controls. The results demonstrate that a preferential lesion of layer III of the entorhinal cortex produces a long-lasting change in evoked and spontaneous activity in parts of the entorhinal cortex and hippocampus. Given the similarity of the lesion produced by AOAA and entorhinal lesions in temporal lobe epileptics, these data support the hypothesis that preferential damage to the entorhinal cortex contributes to long-lasting changes in excitability, which could be relevant to the etiology of temporal lobe epilepsy.

Morphological features of the entorhinal-hippocampal connection

The goal of this review in an overview of the structural elements of the entorhinal-hippocampal connection. The development of the dendrites of hippocampal neurons will be outlined in relation to afferent pathway specificity and the mature dendritic structure compared. Interneurons will be contrasted to pyramidal cells in terms of processing of physiological signals and convergence and divergence in control of hippocampal circuits. Mechanisms of axonal guidance and target recognition, target structures, the involvement of receptor distribution on hippocampal dendrites and the involvement of non-neuronal cellular elements in the establishment of specific connections will be presented. Mechanisms relevant for the maintenance of shape and morphological specializations of hippocampal dendrites will be reviewed. One of the significant contexts in which to view these structural elements is the degree of plasticity in which they participate, during development and origination of dendrites, mature synaptic plasticity and after lesions, when the cells must continue to maintain and reconstitute function, to remain part of the circuitry in the hippocampus. This review will be presented in four main sections: (1) interneurons-development, role in synchronizing influence and hippocampal network functioning; (2) principal cells in CA1, CA3 and dentate gyrus regions-their development, function in terms of synaptic integration, differentiating structure and alterations with lesions; (3) glia and glia/neuronal interactions-response to lesions and developmental guidance mechanisms; and (4) network and circuit aspects of hippocampal morphology and functioning. Finally, the interwoven role of these various elements participating in hippocampal network function will be discussed.

Density of choline acetyltransferase-immunoreactive terminals in the rat dentate gyrus after entorhinal cortex lesions: a quantitative light microscope study

Lesion of the entorhinal cortex in the adult rat is a model for Alzheimer's disease and produces a marked increase in acetylcholinesterase (AChE) activity in the outer molecular layer (OML) of the dentate gyrus. This has been attributed to the sprouting of cholinergic axons terminals in response to denervation of the OML. The aim of this study was to investigate the density changes of cholinergic terminals in the OML at the light microscope level by using choline acetyltransferase (ChAT) immunohistochemistry and quantitative analysis. The results showed that between days 10 and 33 after an entorhinal cortex lesion, there was a measurable increase in the density of ChAT-positive boutons in the OML of the ipsilateral dentate gyrus (x1.2-1.6 of contralateral). However, when shrinkage of the ipsilateral OML (x0.5-0.75 of contralateral) was taken into account, the apparent increase in ChAT terminal density was entirely accounted for by shrinkage of the OML. Thus ChAT immunohistochemistry at the light microscope level provides no positive evidence for a proliferation of cholinergic terminals in the entorhinal cortex lesion model. This is in agreement with previous biochemical assays that have shown no change of total ChAT activity in the dentate gyrus after entorhinal cortex lesions.

Up-regulation of astrocyte-derived tenascin-C correlates with neurite outgrowth in the rat dentate gyrus after unilateral entorhinal cortex lesion

The extracellular matrix protein tenascin-C has been implicated in the regulation of axonal growth. Using unilateral entorhinal cortex lesions, which induce a massive sprouting response in the denervated outer molecular layer of the rat fascia dentata, the role of tenascin-C for axonal growth was investigated in vivo. Monoclonal antibodies against the neurite outgrowth and anti-adhesive domains of the molecule were employed. Immunostaining was increased throughout the denervated outer molecular layer by day 2, reached a maximum around day 10, and was back to control levels by four weeks post lesion. Growth cone deflecting as well as neurite outgrowth promoting isoforms of tenascin-C were up-regulated after the lesion. Using electron microscopy, single intensely tenascin-C immunoreactive cells were identified as reactive astrocytes that phagocytose degenerated terminals. In situ hybridization histochemistry for tenascin-C messenger RNA revealed numerous cellular profiles in the denervated outer molecular layer of the ipsilateral and contralateral dentate gyrus two days post lesion. Tenascin-C messenger RNA-positive cells in the outer molecular layer were identified as astrocytes using double-labelling for tenascin-C messenger RNA and glial fibrillary acidic protein immunohistochemistry. Thus, a tenascin-C-rich substrate is present in the outer molecular layer during the time of sprouting and a sharp boundary is formed against the inner molecular layer. This pattern may contribute to the layer-specific sprouting response of surviving afferents after entorhinal lesion. Neurite outgrowth may be promoted within the denervated zone, whereas axons trying to grow into the denervated outer molecular layer, for example from the inner molecular layer, would be deflected by a tenascin-C-rich barrier.

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.

Cholinergic sprouting in the rat fascia dentata after entorhinal lesion is not linked to early changes in neurotrophin messenger RNA expression

After unilateral entorhinal cortex lesion cholinergic septohippocampal fibres sprout in the denervated fascia dentata. This process is dependent on neurotrophin changes following the lesion. Thus, there is an up-regulation of nerve growth factor and brain-derived neurotrophic factor messenger RNA expression in the denervated granule cells which is detectable 4 h postlesion and returns to control levels by 24 h. Here, using a competitive polymerase chain reaction and in situ hybridization, a transient neurotropin messenger RNA increase could be demonstrated bilaterally following unilateral electrolytic entorhinal cortex lesion. Treatment of the animals with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate blocked this messenger RNA increase, suggesting an involvement of this receptor type in the neurotrophin changes. However, in spite of this blockade, the typical cholinergic sprouting response as visualized with acetylcholinesterase histochemistry was present in animals four weeks after entorhinal cortex lesion. These data suggest that brief initial changes in neurotrophin messenger RNA expression in dentate granule cells are not responsible for the induction of the cholinergic sprouting. Changes in neurotrophin messenger RNA expression occurring immediately postlesion may be linked to glutamate release from entorhinal terminals resulting from the electrolytic lesion of the projection cells in the entorhinal cortex. We hypothesize that later changes in neurotrophin expression, for example in glial cells, are more likely to be related to the cholinergic sprouting process.

Astrocytes and microglial cells incorporate degenerating fibers following entorhinal lesion: a light, confocal, and electron microscopical study using a phagocytosis-dependent labeling technique

Entorhinal lesion leads to anterograde degeneration of perforant path fibers in their main termination zone in the outer molecular layers of the dentate gyrus. Concomitantly, astrocytes become hypertrophic, and microglial cells alter their phenotype, suggesting participation in anterograde degeneration. This study analyzes the involvement of these lesion-induced activated glial cells in the process of phagocytosis of degenerated axonal debris. We established a phagocytosis-dependent labeling technique that allows for direct and simultaneous visualization of both labeled incorporated axonal debris and incorporating glial cells. Stereotaxic application of small crystals of the biotin- and rhodamine-conjugated dextran amine Mini Ruby (MR) into the entorhinal cortex led to strong and stable axonal staining of perforant path axons. Following entorhinal lesion, labeled terminals and fibers condensed and formed small granules. Incorporation of these rhodamine-fluorescent granules resulted in a phagocytosis-dependent cell labeling. During the first 3 days, we were able to identify these cells as microglia by using double-fluorescence and confocal microscopy. The first unequivocally double-labeled astrocytes were found 6 days post lesion (dpl). Whereas in all stages a subpopulation of microglial cells remained devoid of MR-labeled granules, all astrocytes in the middle molecular layer were double-labeled after long survival times (20 dpl). On the ultrastructural level, labeled granules appeared to be perforant path axons containing the tracer. Both terminals and myelinated fibers could be seen inside the cytoplasm of microglial cells and astrocytes. Thus, anterograde degeneration is a sufficient stimulus to induce axon incorporation by both astrocytes and a subpopulation of microglial cells.

192 IgG-saporin-induced loss of cholinergic neurons in the septum abolishes cholinergic sprouting after unilateral entorhinal lesion in the rat

After unilateral lesion of the entorhinal cortex, cholinergic septohippocampal fibres are believed to sprout in the denervated outer molecular layer of the rat dentate gyrus. This cholinergic sprouting has been demonstrated by acetylcholinesterase (AChE) histochemistry, a method said selectively to label cholinergic septohippocampal fibres in the hippocampus. However, a recent report has questioned this concept, suggesting that AChE may not be an adequate marker to monitor cholinergic sprouting and that other, non-cholinergic axons sprouting after entorhinal cortex lesion cause the dense AChE-positive band in the denervated outer molecular layer. In order to determine the contribution of cholinergic septohippocampal fibres to the dense AChE band appearing after entorhinal cortex lesion, the neurotoxin 192 IgG-saporin, known to destroy cholinergic neurons in the basal forebrain selectively, was used. Rats received bilateral injections of 192 IgG-saporin into the lateral ventricles 3 weeks before entorhinal cortex lesion, simultaneously with entorhinal cortex lesion, or 8 weeks after entorhinal cortex lesion. Immunocytochemistry for choline acetyltransferase (ChAT) and in situ hybridization for ChAT mRNA demonstrated the loss of cholinergic neurons in the medial septum and diagonal band after 192 IgG-saporin treatment. The cholinergic sprouting response in the molecular layer, as visualized with AChE histochemistry, was abolished in all animals treated with immunotoxin. These data indicate that the dense AChE band forming after entorhinal cortex lesion represents the sprouting of cholinergic septohippocampal fibres.

Sprouting in the hippocampus is layer-specific

Partial removal of layer-specific afferents of the hippocampus is said to induce sprouting of intact fibers from neighboring layers that invade the zone of the degenerating axons. However, recent in vivo and in vitro studies using sensitive anterograde tracers have failed to demonstrate sprouting across laminar boundaries. Sprouting does occur; but, it mainly involves unlesioned fiber systems terminating within the layer of fiber degeneration in addition to the degenerating afferents. These findings point to rigid laminar cues attracting certain fiber systems while repelling others in normal development and after partial deafferentation.

Differential regulation of ciliary neurotrophic factor (CNTF) and CNTF receptor alpha expression in astrocytes and neurons of the fascia dentata after entorhinal cortex lesion

Neurotrophic factors have been implicated in reactive processes occurring in response to CNS lesions. Ciliary neurotrophic factor (CNTF), in particular, has been shown to ameliorate axotomy-induced degeneration of CNS neurons and to be upregulated at wound sites in the brain. To investigate a potential role of CNTF in lesion-induced degeneration and reorganization, we have analyzed the expression of CNTF protein and CNTF receptor alpha (CNTFR alpha) mRNA in the rat dentate gyrus after unilateral entorhinal cortex lesions (ECLs), using immunocytochemistry and nonradioactive in situ hybridization, respectively. In sham-operated as in normal animals, CNTF protein was not detectable by immunocytochemistry. Starting at 3 d after ECL, upregulation of CNTF expression was observed in the ipsilateral outer molecular layer (OML). Expression was maximal at around day 7, and at this stage immunoreactivity could be specifically localized to astrocytes in the ipsilateral OML. By day 14 postlesion, CNTF immunoreactivity had returned to control levels. CNTFR alpha mRNA was restricted to neurons of the granule cell layer in controls. Three days postlesion, prominent CNTFR alpha expression was observed in the deafferented OML. A similar but less prominent response was noticed in the contralateral OML. After 10 d, CNTFR alpha expression had returned to control levels. Double labeling for CNTFR alpha mRNA and glial fibrillary acidic protein (GFAP) showed that upregulation of CNTFR alpha occurred in reactive, GFAP-immunopositive astrocytes of the OML. A substantial reduction of CNTFR alpha expression in the deafferented granule cells was transiently observed at 7 and 10 d postlesion. Our results suggest a paracrine or autocrine function of CNTF in the regulation of astrocytic and neuronal responses after brain injury.

A novel entorhinal projection to the rat dentate gyrus: direct innervation of proximal dendrites and cell bodies of granule cells and GABAergic neurons

Entorhinal fibers to the fascia dentata originating from layer II stellate neurons are known to terminate exclusively in the outer two thirds of the molecular layer, where they innervate distal dendritic portions of dentate neurons. Using anterograde tracing with Phaseolus vulgaris leucoagglutinin, we unraveled a previously unknown entorhinal projection that directly innervates proximal dendritic portions and somata of granule cells and GABAergic neurons. This projection originates from neurons located in entorhinal layers IV-VI of the medial entorhinal area. These fibers enter the outer two thirds of the molecular layer, traverse the inner molecular layer (IML) and granule cell layer, where they form numerous boutons, and finally arborize subjacent to the granule cells. Correlated light and electron microscopy revealed that the boutons formed by these fibers establish asymmetric synapses on dendrites in the IML, on spines and somata of granule cells, and on spineless dendrites subjacent to the granule cell layer. Postembedding immunogold staining indicates that this entorhino-dentate projection is not GABAergic and that it also terminates on GABAergic inhibitory neurons. These data demonstrate that not all entorhino-dentate fibers display a similar high laminar specificity for the outer molecular layer (OML). Although fibers from the superficial layers of the entorhinal cortex terminate exclusively in the OML, entorhinal fibers arising from deeper layers are not confined to laminar boundaries. Finally, the possibility that these supposedly excitatory entorhinal afferents may monosynaptically activate proximal dendrites and somata of dentate neurons needs to be incorporated into contemporary concepts of the hippocampal network.

Organization of identified fiber tracts in the rat fimbria-fornix: an anterograde tracing and electron microscopic study

The fimbria is a major route for afferent and efferent fibers of the hippocampal formation. However, little is known about the intrinsic organization of the fimbria-fornix complex. In this study, the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHAL) was used to analyze the ultrastructure and topography of identified fiber tracts within the fimbria-fornix. Septo-hippocampal fibers are loosely distributed throughout the fimbria-fornix. Commissural fibers cross the midline in the ventral hippocampal commissure and form a tight fiber bundle in the fimbria. Crossed entorhino-hippocampal fibers cross the midline in the ventral hippocampal commissure rostral to the commissural fiber bundle, and crossed entorhino-entorhinal fibers pass through the dorsal hippocampal commissure. This suggests a topographical organization of fiber tracts within the fimbria-fornix that reflects the laminar organization of the hippocampal target structure: fibers of the diffusely terminating septohippocampal projection are loosely distributed throughout the fimbria-fornix, while those projections that are known to terminate in specific laminae of the hippocampal formation (commissural projection, crossed entorhino-hippocampal projection) form fiber bundles within the fimbria and the ventral hippocampal commissure.