Histochemical evidence for a post-lesion reorganization of cholinergic afferents in the hippocampal formation of the mature cat

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.

Origin of spinal cord axons in the lizard regenerated tail: supernormal projections from local spinal neurons

During tail regeneration most lizards also regenerate the tail spinal cord. The regenerated spinal cord primarily contains neuroepithelium (i.e., the ependymal tube which forms the central canal) and descending axons. The present experiments identify the source of the axons in the regenerated spinal cord. Application of HRP to normal tail spinal cord resulted in labeled cells in the nucleus paraventricularis, the interstitial nucleus of the fasciculus longitudinalis medialis, the nucleus ruber, the medullary reticular formation (including raphe nuclei), as well as in vestibular nuclei. HRP applied to the regenerated spinal cord labeled only 4% of the cells seen in normal animals, and these were confined to rhombencephalic nuclei. The lack of labeling of more rostral nuclei was not due to the death of descending neurons. Application of HRP immediately rostral to the regenerated spinal cord resulted in the labeling of a normal, and in some cases, greater than normal, number of neurons. To quantify the origin of axons in the regenerated spinal cord, electron microscopic montages of the regenerated spinal cord were made and the number of axons counted, before and after various spinal lesions. Only lesions within one spinal segment of the regenerated spinal cord had a significant effect on the number of axons in the regenerated tail spinal cord. This indicated that most of the regenerated axons were of local spinal origin. A significant increase in the number of labeled local spinal neurons was revealed following application of HRP to a regenerated tail spinal cord. These results suggest that while various portions of the lizard central nervous system can grow axons into the regenerating tail spinal cord, the great majority of axons in the regenerate are of local origin and that some of these arise from neurons that do not normally possess descending projections. Finally, to test whether new neurons were participating in the regeneration process, 3H-thymidine was injected during the regrowth of the tail. No labeled spinal cord cells were conclusively identified as neurons. Thus, the regenerating lizard tail spinal cord exhibits robust axonal sprouting from neurons near the site of a spinal transection in a manner reminiscent of sprouting in the mammalian CNS. This sprouting can develop into descending spinal projections that extend for significant distances into the regenerated tail spinal cord and provides a unique model for exploring the requirements for successful axon growth in an adult vertebrate CNS.

A transcriptome database for astrocytes, neurons, and oligodendrocytes: a new resource for understanding brain development and function

Understanding the cell-cell interactions that control CNS development and function has long been limited by the lack of methods to cleanly separate neural cell types. Here we describe methods for the prospective isolation and purification of astrocytes, neurons, and oligodendrocytes from developing and mature mouse forebrain. We used FACS (fluorescent-activated cell sorting) to isolate astrocytes from transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of an S100beta promoter. Using Affymetrix GeneChip Arrays, we then created a transcriptome database of the expression levels of >20,000 genes by gene profiling these three main CNS neural cell types at various postnatal ages between postnatal day 1 (P1) and P30. This database provides a detailed global characterization and comparison of the genes expressed by acutely isolated astrocytes, neurons, and oligodendrocytes. We found that Aldh1L1 is a highly specific antigenic marker for astrocytes with a substantially broader pattern of astrocyte expression than the traditional astrocyte marker GFAP. Astrocytes were enriched in specific metabolic and lipid synthetic pathways, as well as the draper/Megf10 and Mertk/integrin alpha(v)beta5 phagocytic pathways suggesting that astrocytes are professional phagocytes. Our findings call into question the concept of a "glial" cell class as the gene profiles of astrocytes and oligodendrocytes are as dissimilar to each other as they are to neurons. This transcriptome database of acutely isolated purified astrocytes, neurons, and oligodendrocytes provides a resource to the neuroscience community by providing improved cell-type-specific markers and for better understanding of neural development, function, and disease.

Reactive plasticity in the dentate gyrus following bilateral entorhinal cortex lesions in cynomolgus monkeys

Hippocampal structural plasticity induced by entorhinal cortex (EC) lesions has been studied extensively in the rat, but little comparable research has been conducted in primates. In the current study we assessed the long-term effects of bilateral aspiration lesions of the EC on multiple markers of circuit organization in the hippocampal dentate gyrus of young adult monkeys (Macaca fascicularis). Alternate histological sections were processed for the visualization of somatostatin and vesicular acetylcholine transporter (VAChT) immunoreactivity and acetylcholinesterase histochemistry (AChE). The markers revealed the distinct laminar organization of dentate gyrus circuitry for stereology-based morphometric quantification. Consistent with findings in rats, the volume of the somatostatin-immunopositive outer molecular layer (OML), innervated by projections from the EC, was decreased by 42% relative to control values. The inner molecular layer (IML) displayed a corresponding volumetric expansion in response to denervation of the OML as measured by AChE staining, but not when visualized for quantification by VAChT immunoreactivity. Nonetheless, stereological estimation revealed a 36% increase in the total length of VAChT-positive cholinergic fibers in the IML after EC damage, along with no change in the OML. Together, these findings suggest that despite substantial species differences in the organization of hippocampal circuitry, the capacity for reactive plasticity following EC damage, previously documented in rats, is at least partly conserved in the primate dentate gyrus.

Unmasking recurrent excitation generated by mossy fiber sprouting in the epileptic dentate gyrus: an emergent property of a complex system

Seizure-induced sprouting of the mossy fiber pathway in the dentate gyrus has been observed nearly universally in experimental models of limbic epilepsy and in the epileptic human hippocampus. The observation of progressive mossy fiber sprouting induced by kindling demonstrated that even a few repeated seizures are sufficient to alter synaptic connectivity and circuit organization. As it is now recognized that seizures induce synaptic reorganization in hippocampal and cortical pathways, the implications of seizure-induced synaptic reorganization for circuit properties and function have been subjects of intense interest. Detailed anatomical characterization of the sprouted mossy fiber pathway has revealed that the overwhelming majority of sprouted synapses in the inner molecular layer of the dentate gyrus form recurrent excitatory connections, and are thus likely to contribute to recurrent excitation and potentially to enhanced susceptibility to seizures. Nevertheless, difficulties in detecting functional abnormalities in circuits reorganized by mossy fiber sprouting and the fact that some sprouted axons appear to form synapses with inhibitory interneurons have been cited as evidence that sprouting may not contribute to seizure susceptibility, but could form recurrent inhibitory circuits and be a compensatory response to prevent seizures. Quantitative analysis of the synaptic connections of the sprouted mossy fiber pathway, assessment of the functional features of sprouted circuitry using reliable physiological measures, and the perspective of complex systems analysis of neural circuits strongly support the view that the functional effects of the recurrent excitatory circuits formed by mossy fiber sprouting after seizures or injury emerge only conditionally and intermittently, as observed with spontaneous seizures in human epilepsy. The recognition that mossy fiber sprouting is induced after hippocampal injury and seizures and contributes conditionally to emergence of recurrent excitation has provided a conceptual framework for understanding how injury and seizure-induced circuit reorganization may contribute to paroxysmal network synchronization, epileptogenesis, and the consequences of repeated seizures, and thus has had a major influence on understanding of fundamental aspects of the epilepsies.

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.

Seizure-Induced Axonal Sprouting: Assessing Connections Between Injury, Local Circuits, and Epileptogenesis

Neurons and neural circuits undergo extensive structural and functional remodeling in response to seizures. Sprouting of axons in the mossy fiber pathway of the hippocampus is a prominent example of a seizure-induced structural alteration which has received particular attention because it is easily detected, is induced by intense or repeated brief seizures in focal chronic models of epilepsy, and is also observed in the human epileptic hippocampus. During the last decade the association of mossy fiber sprouting with seizures and epilepsy has been firmly established. Many anatomical features of mossy fiber sprouting have been described in considerable detail, and there is evidence that sprouting occurs in a variety of other pathways in association with seizures and injury. There is uncertainty, however, about how or when mossy fiber sprouting may contribute to hippocampal dysfunction and generation of seizures. Study of mossy fiber sprouting has provided a strong theoretical and conceptual framework for efforts to understand how seizures and injury may contribute to epileptogenesis and its consequences. It is likely that investigation of mossy fiber sprouting will continure to offer significant opportunities for insights into seizure-induced plasticity of neural circuits at molecular, cellular, and systems levels.

Seizure-Induced Axonal Sprouting: Assessing Connections between Injury, Local Circuits, and Epileptogenesis

Neurons and neural circuits undergo extensive structural and functional remodeling in response to seizures. Sprouting of axons in the mossy fiber pathway of the hippocampus is a prominent example of a seizure-induced structural alteration which has received particular attention because it is easily detected, is induced by intense or repeated brief seizures in focal chronic models of epilepsy, and is also observed in the human epileptic hippocampus. During the last decade the association of mossy fiber sprouting with seizures and epilepsy has been firmly established. Many anatomical features of mossy fiber sprouting have been described in considerable detail, and there is evidence that sprouting occurs in a variety of other pathways in association with seizures and injury. There is uncertainty, however, about how or when mossy fiber sprouting may contribute to hippocampal dysfunction and generation of seizures. Study of mossy fiber sprouting has provided a strong theoretical and conceptual framework for efforts to understand how seizures and injury may contribute to epileptogenesis and its consequences. It is likely that investigation of mossy fiber sprouting will continure to offer significant opportunities for insights into seizure-induced plasticity of neural circuits at molecular, cellular, and systems levels.

Postinjury administration of L-deprenyl improves cognitive function and enhances neuroplasticity after traumatic brain injury

The rat model of combined central fluid percussion traumatic brain injury (TBI) and bilateral entorhinal cortical lesion (BEC) produces profound, persistent cognitive deficits, sequelae associated with human TBI. In contrast to percussive TBI alone, this combined injury induces maladaptive hippocampal plasticity. Recent reports suggest a potential role for dopamine in CNS plasticity after trauma. We have examined the effect of the dopamine enhancer l-deprenyl on cognitive function and neuroplasticity following TBI. Rats received fluid percussion TBI, BEC alone, or combined TBI + BEC lesion and were treated once daily for 7 days with l-deprenyl, beginning 24 h after TBI alone and 15 min after BEC or TBI + BEC. Postinjury motor assessment showed no effect of l-deprenyl treatment. Cognitive performance was assessed on days 11-15 postinjury and brains from the same cases examined for dopamine beta-hydroxylase immunoreactivity (DBH-IR) and acetylcholinesterase (AChE) histochemistry. Significant cognitive improvement relative to untreated injured cases was observed in both TBI groups following l-deprenyl treatment; however, no drug effects were seen with BEC alone. l-Deprenyl attenuated injury-induced loss in DBH-IR over CA1 and CA3 after TBI alone. However, after combined TBI + BEC, l-deprenyl was only effective in protecting CA1 DBH-IR. AChE histostaining in CA3 was significantly elevated with l-deprenyl in both injury models. After TBI + BEC, l-deprenyl also increased AChE in the dentate molecular layer relative to untreated injured cases. These results suggest that dopaminergic/noradrenergic enhancement facilitates cognitive recovery after brain injury and that noradrenergic fiber integrity is correlated with enhanced synaptic plasticity in the injured hippocampus.

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.

Differential presynaptic and postsynaptic expression of m1-m4 muscarinic acetylcholine receptors at the perforant pathway/granule cell synapse

A family of muscarinic acetylcholine receptor proteins mediates diverse pre- and postsynaptic functions in the hippocampus. However the roles of individual receptors are not understood. The present study identified the pre- and postsynaptic muscarinic acetylcholine receptors at the perforant pathway synapses in rat brain using a combination of lesioning, immunocytochemistry and electron microscopic techniques. Entorhinal cortex lesions resulted in lamina-specific reductions of m2, m3, and m4 immunoreactivity in parallel with the degeneration of the medial and lateral perforant pathway terminals in the middle and outer thirds of the molecular layer, respectively. In contrast, granule cell lesions selectively reduced m1 and m3 receptors consistent with degeneration of postsynaptic dendrites. Direct visualization of m1-m4 by electron microscopic immunocytochemistry confirmed their differential pre- and postsynaptic localizations. Together, these findings provide strong evidence for both redundancy and spatial selectivity of presynaptic (m2, m3 and m4) and postsynaptic (m1 and m3) muscarinic acetylcholine receptors at the perforant pathway synapse.

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.

Changes in nerve growth factor immunoreactivity following entorhinal cortex lesions: possible molecular mechanism regulating cholinergic sprouting

To assess the possible role of trophic factors in lesion-induced plasticity, we have used a sensitive immunohistochemical technique to evaluate changes in nerve growth factor (NGF) staining in the hippocampal formation 3, 8, 16, and 30 days following entorhinal cortex lesions. Our results indicate that a band of NGF immunoreactivity appears in the outer molecular layer of the ipsilateral dentate gyrus following entorhinal ablation. The distribution of the NGF-immunoreactive band exactly coincides with the distribution of sprouting cholinergic terminals revealed by acetylcholinesterase histochemistry or NGF-receptor immunostaining. Increased NGF-immunoreactivity is detectable at 3 days postlesion, is most intense at 8 days, and decreases to near control levels by 30 days. Lesion-induced increases in NGF immunostaining also occur in animals in which septohippocampal fibers had been removed by prior destruction of the fimbria-fornix. Increases in NGF-immunoreactivity, however, are substantially reduced in animals receiving intraventricular injections of colchicine, which presumably blocks NGF release. These results indicate that 1) increases in NGF immunostaining, which occur following entorhinal lesions, precede any changes in cholinergic sprouting parameters and are greatest during the period of maximal cholinergic sprouting; 2) increased NGF-immunoreactivity is not due to NGF binding by septohippocampal fibers; and 3) increased NGF-immunoreactivity appears to depend on the release of NGF by neurons that produce it. We hypothesize that, following entorhinal lesions, NGF immunostaining within the hippocampal formation may represent NGF "anchored" within the tissue and that NGF accumulation by such a mechanism may direct the sprouting response of NGF-sensitive cholinergic neurons.

Distribution of acetylcholinesterase in the hippocampal region of the mouse. III. The area dentata

The distribution of acetylcholinesterase (AChE) was examined in the area dentata of the adult mouse (Mus musculus domesticus). A distinctly stratified distribution of the enzyme was observed and was compared in detail with cytoarchitectural fields and layers. In the stratum moleculare, bands of relatively high AChE activity were seen immediately beneath the pia, at the borders between the outer, middle, and deep portions of the stratum moleculare, and superficial to the granule cell layer. AChE activity was low in the intervening parts of the stratum moleculare. In contrast to the rat, three sublaminae could be discerned in the hilus of the mouse at most septotemporal levels: a limiting subzone, a hilar plexiform layer, and a deep hilar cell mass. Deep to the granule cell layer, AChE activity was high in the limiting subzone and, septally, in the hilar plexiform layer. The deep hilar cell mass stained lightly towards the septal pole of the region but darker at more temporal levels. Numerous AChE-stained cells were seen in the hilus, with the exception of the most temporal levels. A comparative analysis of the AChE pattern of the area dentata reveals that 1) AChE-intense supra- and infragranular bands are found in all mammals, whereas 2) considerable difference between various strains of mice and between species are seen in the stratum moleculare. The functional significance of the AChE pattern is discussed in relation to species differences and connectivity and also with respect to possible activities of the enzyme other than hydrolysis of ACh, which may be involved in growth-related functions and in the plastic and degenerative processes observed in Alzheimer's disease.

Lesion-induced synapse reorganization in the hippocampus of cats: sprouting of entorhinal, commissural/associational, and mossy fiber projections after unilateral entorhinal cortex lesions, with comments on the normal organization of these pathways

This study evaluates whether three forms of sprouting occur in the hippocampus of the cat following unilateral entorhinal cortex (EC) lesions: (1) sprouting of projections from the EC contralateral to the lesion; (2) sprouting of the commissural/associational system; and (3) sprouting of mossy fibers. Tract tracing techniques were used to define the normal organization of the entorhinal cortical projection system, the commissural/associational (C/A) systems, and the mossy fiber projections in normal cats. The same techniques were then used to evaluate whether there were changes in these projections in animals with long-standing unilateral EC lesions. The projections from the entorhinal cortex were evaluated autoradiographically following injections of 3H proline into the entorhinal area. The projections of the C/A system were traced using the Fink-Heimer technique after lesions of the hippocampal commissures, and by using autoradiographic techniques after injections of 3H proline into the hippocampus. The distribution of mossy fibers was evaluated using the Timm's stain. The results reveal that unilateral lesions of the EC in cats lead to the same sorts of sprouting that have been described in rats. There is: (1) an increase in the density of the crossed projection from the surviving EC to the contralateral dentate gyrus that had been deprived of its normal EC inputs; (2) an expansion of the terminal field of the C/A projection system into portions of the molecular layer of the dentate gyrus normally occupied by EC projections; and (3) an increase in supragranular mossy fibers in some animals. The mossy fiber sprouting was especially prominent when the lesions encroached upon the hippocampus. The studies also reveal additional details about the normal organization of hippocampal pathways in cats. The most important points are: (1) there is a crossed projection from the entorhinal cortex to the contralateral dentate gyrus; and (2) there is a complex laminar organization of the commissural and associational terminal fields in the molecular layer of the dentate gyrus that appears to be related to the point of origin of the projections along the septotemporal axis of the hippocampus. This heretofore unrecognized aspect of the laminar organization of C/A terminations has important implications for the temporal competition hypothesis, which has been advanced to account for the development of these afferent systems.

Axonal sprouting and frank regeneration in the lizard tail spinal cord: correlation between changes in synaptic circuitry and axonal growth

In our previous studies, we found that the number of supraspinal neurons projecting to the level of tail spinal cord increases by 74% during tail regeneration and that the number of local spinal neurons with descending projections increases 233%. However, only a small fraction of the supraspinal axons (less than 4%) and half of the local spinal axons actually enter the regenerated spinal cord. We suggested that this may be the result of "synaptic capture" in which regrowing axons make synapses on denervated targets rostral to the transection, aborting further regeneration. To examine this hypothesis, morphometric analysis of electron microscope (EM) photomontages was used to test for changes in synaptic distribution on ventral horn neurons rostral to regenerating tail spinal cord. In addition, 3H-thymidine and retrograde markers were used to determine whether the regenerate axons arose from cut axons, neurogenesis, or sprouting from uninjured neurons. 3H-thymidine injections during regeneration, combined with retrograde HRP pathway tracing, did not reveal the production of new neurons in the tail spinal cord. To test whether cut axons regenerate, fluorescein isothiocyanate conjugated latex beads were applied to the exposed end of the tail spinal cord. After tail regeneration, HRP was applied to the new spinal cord in the regenerated tail. Examination of local spinal neurons (the primary source of axons that enter the regenerated tail spinal cord) revealed that 28% of the neurons contained both labels. This indicated that cut axons successfully regrew into the new tail spinal cord. The regenerated axons that fail to enter the new tail spinal cord can be found in the normal spinal cord immediately rostral to the regenerated tail. To determine whether these axons were making synaptic contacts, lamina IX ventral horn neurons were examined. EM photomontages of the spinal cord rostral to the regenerate tail revealed the following properties: (1) neurons rostral to regenerated tails are larger in area compare to non-regenerates (mean increase = 112%); (2) axosomatic contacts cover a greater percentage of the neuronal soma following regeneration compared to normal (mean increase = 23%); and (3) this increased innervation is the result of an increase in the number of synaptic boutons rather than larger boutons. The number of synaptic contacts in regenerated lizards returned to normal following lumbar transection, indicating that supraspinal and/or long descending propriospinal afferents were the major source of the increased synaptic contacts.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic reorganization within the human central nervous system following injury

Behavioral recovery following brain injury in humans is well recognized; however, the anatomical basis for such recovery has not been demonstrated. Two cases are presented that show reorganization of synaptic connections (plasticity) in the dentate gyrus of human brain following uncal herniation. The neurohistological appearance of these cases is very similar to a well-described animal model of anatomical, physiological, and behavioral recovery following experimental surgical injury.

Reorganization of neuronal connections following CNS trauma: principles and experimental paradigms

The present review summarizes how the nervous system responds to trauma. The goal is to provide an introduction to the problems, techniques, experimental paradigms, current issues, and future promise. The review is especially designed for basic scientists and clinicians who are not currently involved in research on CNS reorganization, and for students just entering the field. The review characterizes the secondary degenerative events that occur after trauma, and the types of growth that commonly occur. A standard terminology is set forth with criteria for differentiating between related phenomena. Experimental methods are described that can be used documenting reorganization of circuitry. The principles that determine whether a given process will or will not occur are summarized, and some of the factors that may regulate the nature and extent of growth are considered. Research strategies are outlined that have been used to evaluate whether reorganization of circuitry is functionally significant. Finally, future directions in research and clinical application are discussed, focusing especially on the efforts to facilitate regeneration, and the work on transplants of CNS tissue to facilitate growth of surviving connections, and to replace tissue destroyed by trauma.

Plasticity of hippocampal circuitry in Alzheimer's disease

Two markers of neuronal plasticity were used to compare the response of the human central nervous system to neuronal loss resulting from Alzheimer's disease with the response of rats to a similar neuronal loss induced by lesions. In rats that had received lesions of the entorhinal cortex, axon sprouting of commissural and associational fibers into the denervated molecular layer of the dentate gyrus was paralleled by a spread in the distribution of tritiated kainic acid-binding sites. A similar expansion of kainic acid receptor distribution was observed in hippocampal samples obtained postmortem from patients with Alzheimer's disease. An enhancement of acetylcholinesterase activity in the dentate gyrus molecular layer, indicative of septal afferent sprouting, was also observed in those patients with a minimal loss of cholinergic neurons. These results are evidence that the central nervous system is capable of a plastic response in Alzheimer's disease. Adaptive growth responses occur along with the degenerative events.

Cell biology of synaptic plasticity

The nervous system of mammals retains throughout the animals' life-span the ability to modify the number, nature, and level of activity of its synapses. Synaptic plasticity is most evident after injury to the nervous system, and the cellular and molecular mechanisms that make it possible are beginning to be understood. Transplantation of brain tissue provides a powerful approach for studying mechanisms of synaptic plasticity. In turn, understanding the response of the central nervous system to injury can be used to optimize transplant survival and integration with the host brain.

The sprouting of septal afferents to the dentate gyrus after lesions of the entorhinal cortex in adult rats

The projection of the septum to the dentate gyrus has been demonstrated autoradiographically and the pattern of acetylcholinesterase (AChE) staining in the dentate gyrus has been mapped histochemically, in a series of normal young adult rats and in a group of animals in which the entorhinal cortex had been ablated or its efferents to the dentate gyrus interrupted, some weeks earlier. It is clear from this material that the normal disposition of the septal projection to the dentate gyrus differs significantly from the pattern of AChE staining; however, in the denervated region of the molecular layer in the experimental animals there is a marked increase in the density of the septal projection which precisely coincides with the zone of intensification of AChE staining. It follows from this that although the distribution AChE does not accurately reflect the organization of the septo-dentate projection in normal animals, the intensification of AChE staining provides a good indication of the reorganization which occurs in this pathway following entorhinal deafferentation.

Neuronal activity of the septum following various types of deafferentation

Extracellular recording of neuronal activity of septal nuclei was performed in unanesthetized chronic rabbits in the following conditions: (1) intact septal nuclei; (2) complete interruption of connections with hippocampus; (3) complete basal undercutting of septum. Additional series of records were performed in transversal guinea pig septal slices incubated in vitro. Computer analysis of activity was used. Interruption of hippocampal connections increased the mean frequency of discharges in the lateral septal nucleus 2-fold. Theta-bursts were preserved and the proportion of theta-bursting neurons increased from 28 to 42%. The mean frequency of theta-bursts was slightly augmented (mean 5.4 cps versus 5.2 cps in intact septum). Twenty-six per cent of neurons were characterized by regular, pacemaker-like discharges, while in the normal septum such units were almost completely absent. After septal undercutting the mean frequency of spontaneous activity was increased 2-fold in the medial septal nucleus. Rhythmic bursts were again preserved, the proportion of bursting neurons was close to normal (25%), but the mean frequency of bursts was significantly lower (mean 3.3 cps). Rhythmic activity was extremely regular. Neurons (21%) with pacemaker-like activity were also present. In septal slices neurons possessed relatively high spontaneous activity (mean 10.2/sec). In the majority of units (64%) activity was highly regular. Twenty-two per cent of the units possessed rhythmic bursts in their activity with frequencies of 2.0-5.1 cps. The role of intrinsic septal mechanisms and afferent influences in generation and modulation of theta-bursts is discussed.

A functional sign of reorganization in the visual system of adult cats: lateral geniculate neurons with displaced receptive fields after lesions of the nasal retina

High-intensity photocoagulator lesions placed nasally to the optic disc one eye destroyed all retinal layers and led to visual deafferentation of the lateral part of layer A in the contralateral lateral geniculate nucleus (LGN) of adult cats. The retino-geniculate topography was determined with vertical tungsten microelectrode penetrations through LGN layers A and A1 before and at different times after photocoagulation. Before the 20th day a border of light excitability was found in layer. A corresponding to the normal projection of the lesion onto the LGN. Deviations from the normal topographical organization were evident 30 days and later after photocoagulation. Cells were recorded within the deafferented part of layer A near to the border of deafferentiation which had receptive field (RF) centers displaced by more than two degrees of visual angle with respect to the normal retinotopy. During formation of the retinal scar no identifiable points on the retina were displaced in the same direction as the RF centers. The histological control of the retinal lesions proved that there was no potentially excitable tissue left to account for a spread of excitation into the lesion. Latency measurements after electrical stimulation of the optic tract and the visual cortex suggested a retinal input and a projection into the visual cortex for the LGN cells with displaced receptive field centers. RF center displacements up to five degrees at the border of the lesions indicated a lateral spread of excitation within the LGN up to 250 micron beyond normal.