Mossy fiber sprouting in the fascia dentata after unilateral entorhinal lesions: quantitative analysis using computer-assisted image processing

Multineuronal spike sequences repeat with millisecond precision

Cortical microcircuits are nonrandomly wired by neurons. As a natural consequence, spikes emitted by microcircuits are also nonrandomly patterned in time and space. One of the prominent spike organizations is a repetition of fixed patterns of spike series across multiple neurons. However, several questions remain unsolved, including how precisely spike sequences repeat, how the sequences are spatially organized, how many neurons participate in sequences, and how different sequences are functionally linked. To address these questions, we monitored spontaneous spikes of hippocampal CA3 neurons ex vivo using a high-speed functional multineuron calcium imaging (fMCI) technique that allowed us to monitor spikes with millisecond resolution and to record the location of spiking and non-spiking neurons. Multineuronal spike sequences (MSSs) were overrepresented in spontaneous activity compared to the statistical chance level. Approximately 75% of neurons participated in at least one sequence during our observation period. The participants were sparsely dispersed and did not show specific spatial organization. The number of sequences relative to the chance level decreased when larger time frames were used to detect sequences. Thus, sequences were precise at the millisecond level. Sequences often shared common spikes with other sequences; parts of sequences were subsequently relayed by following sequences, generating complex chains of multiple sequences.

The effects of aging on dentate circuitry and function

The central nervous system (CNS) undergoes a variety of anatomic, physiologic, and behavioral changes during aging. One region that has received a great deal of attention is the hippocampal formation due to the increased incidence of impaired spatial learning and memory with age. The hippocampal formation is also highly susceptible to Alzheimer's disease, ischemia/hypoxia, and seizure generation, the three most common aging-related neurological disorders. While data reveal that the dentate gyrus plays a key role in hippocampal function and dysfunction, the majority of electrophysiological studies that have examined the effects of age on the hippocampal formation have focused on CA3 and CA1. We perceive this to be an oversight and consequently will highlight data in this review which demonstrate an age-related disruption in dentate circuitry and function, and propose that these changes contribute to the decline in hippocampal-dependent behavior seen with "normal" aging.

Reduction of Zinc-Positive Terminal Fields in Striatum of Mouse after 1-Methyl-4-Phenylpyridinium Neurotoxicity

Zinc is an essential trace element in the central nervous system and is located in three distinct pools: free zinc, vesicular zinc and protein-bound zinc. Zinc may serve as an endogenous neuromodulator and has been associated with neuropathologies. This study was undertaken to determine whether levels of vesicular zinc in neuronal terminals would decrease in response to the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium ion (MPP+). Adult male C-57 black mice were injected with MPP+ (0.72 mg/kg) into their right lateral ventricle. All animals were killed at 1, 2, 24 h and 7 days after MPP+ or saline administration. The brains were stained for zinc sulfides and the density of zinc-positive terminal fields was evaluated after MPP+ administration. The relative optical density analysis of zinc-positive terminal fields showed significant decreases in the striatum at 1, 2 and 24 h (24, 18 and 14%, respectively, versus control) and ventricular epithelium (1, 2, 24 h and 7 days). The hippocampus showed increase in the stratum oriens and stratum radiatum at different times. MPP+ administration reduced dopamine levels at 24h and 7 days (36 and 40%, respectively, versus control) as a result of the neurotoxic action of MPP+. The decrease of zinc-positive neuronal terminal fields in the striatum after MPP+ administration is most likely due to a neuronal release of vesicular zinc in response to its dopaminergic neurotoxicity.

Electrophysiological evidence of monosynaptic excitatory transmission between granule cells after seizure-induced mossy fiber sprouting

Mossy fiber sprouting is a form of synaptic reorganization in the dentate gyrus that occurs in human temporal lobe epilepsy and animal models of epilepsy. The axons of dentate gyrus granule cells, called mossy fibers, develop collaterals that grow into an abnormal location, the inner third of the dentate gyrus molecular layer. Electron microscopy has shown that sprouted fibers from synapses on both spines and dendritic shafts in the inner molecular layer, which are likely to represent the dendrites of granule cells and inhibitory neurons. One of the controversies about this phenomenon is whether mossy fiber sprouting contributes to seizures by forming novel recurrent excitatory circuits among granule cells. To date, there is a great deal of indirect evidence that suggests this is the case, but there are also counterarguments. The purpose of this study was to determine whether functional monosynaptic connections exist between granule cells after mossy fiber sprouting. Using simultaneous recordings from granule cells, we obtained direct evidence that granule cells in epileptic rats have monosynaptic excitatory connections with other granule cells. Such connections were not obtained when age-matched, saline control rats were examined. The results suggest that indeed mossy fiber sprouting provides a substrate for monosynaptic recurrent excitation among granule cells in the dentate gyrus. Interestingly, the characteristics of the excitatory connections that were found indicate that the pathway is only weakly excitatory. These characteristics may contribute to the empirical observation that the sprouted dentate gyrus does not normally generate epileptiform discharges.

The effects of NMDA-induced retrohippocampal lesions on performance of four spatial memory tasks known to be sensitive to hippocampal damage in the rat

Four separate cohorts of rats were employed to examine the effects of cytotoxic retrohippocampal lesions in four spatial memory tasks which are known to be sensitive to direct hippocampal damage and/or fornix-fimbria lesions in the rat. Selective retrohippocampal lesions were made by means of multiple intracerebral infusions of NMDA centred on the entorhinal cortex bilaterally. Cell damage typically extended from the lateral entorhinal area to the distal ventral subiculum. Experiment 1 demonstrated that retrohippocampal lesions spared the acquisition of a reference memory task in the Morris water maze, in which the animals learned to escape from the water by swimming to a submerged platform in a fixed location. In the subsequent transfer test, when the escape platform was removed, rats with retrohippocampal lesions tended to spend less time searching in the appropriate quadrant compared to controls. Experiment 2 demonstrated that the lesions also spared the acquisition of a working memory version of the water maze task in which the location of the escape platform was varied between days. In experiment 3, both reference and working memory were assessed using an eight-arm radial maze in which the same four arms were constantly baited between trials. In the initial acquisition, reference memory but not working memory was affected by the lesions. During subsequent reversal learning in which previously baited arms were now no longer baited and vice versa, lesioned animals made significantly more reference memory errors as well as working memory errors. In experiment 4, spatial working memory was assessed in a delayed matching-to-position task conducted in a two-lever operant chamber. There was no evidence for any impairment in rats with retrohippocampal lesions in this task. The present study demonstrated that unlike direct hippocampal damage, retrohippocampal cell loss did not lead to a general impairment in spatial learning, implying that the integrity of the retrohippocampus and/or its interconnection with the hippocampal formation is not critical for normal hippocampal-dependent spatial learning and memory. This outcome is surprising for a number of current hippocampal theories, and suggests that other cortical as well as subcortical inputs to the hippocampus might be of more importance, and further raises the question regarding the functional significance of the retrohippocampal region.

Nonobligate role of early or sustained expression of immediate-early gene proteins c-fos, c-jun, and Zif/268 in hippocampal mossy fiber sprouting

Axon sprouting in dentate granule cells is an important model of structural plasticity in the hippocampus. Although the process can be triggered by deafferentation, intense activation of glutamate receptors, and other convulsant stimuli, the specific molecular steps required to initiate and sustain mossy fiber (MF) reorganization are unknown. The cellular immediate early genes (IEGs) c-fos, c-jun, and zif/268 are major candidates for the initial steps of this plasticity, because they encode transcription factors that may trigger cascades of activity-dependent neuronal gene expression and are strongly induced in all experimental models of MF sprouting. The mutant mouse stargazer offers an important opportunity to test the specific role of IEGs, because it displays generalized nonconvulsive epilepsy and intense MF sprouting in the absence of regional cell injury. Here we report that stargazer mice show no detectable elevations in c-Fos, c-Jun, or Zif/268 immediate early gene proteins (IEGPs) before or during MF growth. Experimental results in stargazer, including (1) a strong IEGP response to kainate-induced convulsive seizures, (2) no IEGP response after prolongation of spike-wave synchronization, (3) no IEGP increase at the developmental onset of seizures or after prolonged seizure suppression, and (4) unaltered levels of the intracellular Ca2+-buffering proteins calbindin-D28k or parvalbumin, exclude the possibility that absence of an IEGP response in stargazer is either gene-linked or suppressed by known refractory mechanisms. These data demonstrate that increased levels of these IEGPs are not an obligatory step in MF-reactive sprouting and differentiate the early downstream molecular cascades of two major seizure types.

A comparison between the effects of medial septal lesions and entorhinal cortex lesions on performance of nonspatial working memory tasks and reversal learning

Rats with either electrolytic medial septal lesions or cytotoxic entorhinal lesions were compared to unoperated controls on a series of delayed matching-to-sample (DMS) tasks. A DMS trial consisted of two runs. In the first (information) run, the subject was familiarized with a sample discriminandum. In the second (choice) run, the subject was required to discriminate the sample discriminandum from a novel one. When a set of 20 discrete complex objects were used as discriminanda and each discriminandum was used once per day, neither lesions impaired choice accuracy. However, when a single pair of simple discriminanda was employed and re-used between trials within a day, rats with medial septal lesions were severely impaired whereas rats with entorhinal lesions performed at a level comparable to unoperated controls. Next, proactive interference was demonstrated by the introduction of an extra run prior to the information run. When this extra (pre-information) run required the subjects to visit the (eventual) negative discriminandum such that correct choice had to be guided by relative familiarity judgement, choice performance was reduced. Neither lesion group was selectively affected by this manipulation. But when the relative reinforcement history of the pre-information run and the information run was manipulated, such that a correct response required the subject to approach a discriminandum that had recently been non-rewarded, rats with entorhinal lesions were selectively impaired. The effect of delay was demonstrated when a 20-s interval was imposed between information run and choice run. This reduced overall choice accuracy, and this effect appeared to be more pronounced in both lesion groups, although not significantly so. Finally, neither lesion affected the acquisition of a simple discrimination task, but reversal learning was selectively enhanced in the entorhinal lesion group.

Injury-induced physiological events that may modulate gene expression in neurons and glia

Damage to the brain triggers a host of reactive responses in neurons and glia which are seen at sites of focal injury as well as at sites that are at a distance from the injury. Although many of these responses have been studied extensively, the signals that initiate the different responses have not been fully characterized, and it is still not understood how focal injury affects neurons and glia in distant sites. The present review summarizes recent findings that suggest that physiological events that occur at the time of the injury or during the early postlesion period can play an important and variable role in modulating neuronal and glial responses to injury. We focus on the events that occur in the hippocampal formation following unilateral lesions of the entorhinal cortex - a model system that has been used extensively for studies of cellular responses following focal brain injury. This lesion destroys the cells of origin of a massive excitatory projection to the dentate gyrus and hippocampus proper. Over time, the denervated neurons in the hippocampal formation are almost completely reinnervated as a result of local sprouting of systems that survive the lesion. Thus, this model system has been useful for studying cellular responses to both denervation and reinnervation. We summarize the information that this injury triggers physiological events that can strongly modulate gene expression in neurons and glia, including episodes of spreading depression that occur at the time of the injury, seizures that occur during the early postlesion period, the loss of afferent drive which leads to decreases in postsynaptic activity, and the restoration of activity that occurs in conjunction with reinnervation. We describe recent studies which suggest that some of these physiological events occur to a variable extent in different animals, especially the episodes of spreading depression and the recurrent seizures. Thus, the spatial pattern and temporal dynamics of altered gene expression following this "model" experimental injury may vary from animal to animal. The fact that physiological events strongly modulate the reactive changes in gene expression that occur following injury has important implications for understanding the sequelae of injury, and offers new opportunities for experimental and therapeutic interventions that may improve cellular repair, regeneration, and recovery of function.

Seizure-induced damage to somatostatin-immunoreactive neurons in the rat hippocampus is regulated by fimbria-fornix transection

In both experimental and human temporal lobe epilepsy, seizures cause loss of hilar somatostatin-immunoreactive (SOM-ir) neurons and sprouting of mossy fibers. To investigate whether in rats these alterations are modulated by hippocampal input projections, we transected the fimbria-fornix or the perforant pathway bilaterally 2 days after seizures induced by systemic administration of kainic acid (9 mg/kg, i.p.). Two months later, the number of SOM-ir neurons in the hilus was counted and mossy fiber sprouting in the supragranular area and in the inner molecular layer was analyzed. In seizured rats with sham-operation, 50% of the hilar SOM-ir neurons were left in the septal end of the hippocampus and only 16% remained in the temporal end. In seizured rats with transection of the fimbria-fornix, the number of hilar SOM-ir neurons in the septal end of the hippocampus did not differ from that in controls (98% of SOM-ir neurons left). However, the temporal end was severely damaged (41% of SOM-ir neurons left). In seizured rats with transection of the perforant pathway, 61% of the hilar SOM-ir neurons were left in the septal end and 51% in the temporal end of the hippocampus. Mossy fiber sprouting was evident throughout the septotemporal axis of the hippocampus in all seizured rats. Our results suggest that in the septal end of the hippocampus the severity of neuronal damage in the hilus is modulated by mechanism(s) that are dependent on the afferent pathways entering the hippocampus via the fimbria-fornix. Transection of the fimbria-fornix, however, does not significantly modulate the severity or the target regions of seizure-induced sprouting of mossy fibers.

Correlated axonal sprouting and dendritic spine formation during kainate-induced neuronal morphogenesis in the dentate gyrus of adult mice

Several examples of structural plasticity in the adult brain have been provided in the hippocampus, among which the most striking concerns axonal remodeling of the dentate gyrus granule cells. We have recently demonstrated that a single injection of kainic acid into the dorsal hippocampus of adult mice triggers a conspicuous morphogenetic response of granule cells. Cellular labeling with biocytin 1, 2, and 4 weeks after injection of kainate revealed a progressive increase in dendritic thickness and length (up to 2.5-times), combined with an increase in the number of dendritic spines. This correlation resulted in the conservation of total spine density. No modifications of the dendritic arborization pattern were noted. In addition to dendritic changes, the number of axonal profiles observed within the hypertrophied granular layer and the inner part of the molecular layer appeared dramatically increased. Timm staining and anterograde labeling of two of the main extra-hippocampal afferent systems (i.e., septal, entorhinal) evidenced sprouting of mossy fibers and of septal afferents. Entorhinal fibers were not obviously modified. As revealed by calretinin-immunohistochemistry, commissural afferents also responded by an extensive sprouting. In addition, increases of dendritic spine number and dendritic length were noticeably greater in portions of dendrites that receive mossy fiber collaterals and septal and hypothalamic afferents, than in the external portion which receives entorhinal afferents. Although qualitative, this correlation suggests a relationship between kainate-induced structural plasticity of mature granule cells and the specific capacities of afferent systems to elaborate axon collaterals.

Effects of prenatal protein malnutrition on mossy fibers of the hippocampal formation in rats of four age groups

The present study was undertaken to investigate the effect of prenatal protein deprivation on the postnatal development of the mossy fiber plexus of the hippocampal formation on postnatal (P) days 15, 30, 90, and 220. Although there is extensive information about the effects of malnutrition on cell body and dendrite morphology, little attention has been paid to axons or axon plexuses. The mossy fiber plexus represents the dentate gyrus granule cell axonal projection to areas CA4 and CA3 of the hippocampal formation and is readily demonstrated with Timm's heavy metal stain. With the use of this stain, the plexus was measured at 13 levels throughout the hippocampal complex. There was no effect of the diet on the anatomical distribution of the plexus. The current study, however, does show significant effects of prenatal protein malnutrition on postnatal development of the mossy fiber plexus that are age dependent. The prenatally malnourished rats show significant deficits in the total rostro-caudal extent and volume of the plexus on P15, P90, and P220, with the most marked dietary effect on P220. There was no significant diet effect on P30 in either extent or volume.

Acute and chronic administration of ibogaine to the rat results in astrogliosis that is not confined to the cerebellar vermis

Acute administration of high doses of ibogaine (IBG) to the male rat results in degeneration of Purkinje cells and reactive gliosis in the cerebellar vermis. We examined whether acute and chronic administration of IBG to male and female rats results in gliosis as determined by quantification of the astroglial intermediate filament protein, glial fibrillary acidic protein (GFAP). After acute administration of IBG, rats of both sexes showed dose-related increases in GFAP that were not confined to the cerebellar vermis. After chronic administration of IBG, female, but not male rats, showed large (as much as 200% of control), dose-related increases in GFAP in hippocampus, olfactory bulbs, brain stem and striatum, but not cerebellum. In hippocampus, the cytoskeletal proteins, neurofilament 68 (NF-68) and beta-tubulin were increased in females treated chronically with IBG, findings consistent with a damage-induced sprouting response. Together, the data indicate that IBG damages areas of the brain outside the cerebellum and that the sites damaged are dependent on sex and dosage regimen.

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.

Resistance of the immature hippocampus to seizure-induced synaptic reorganization

Temporal lobe epilepsy is a common form of epilepsy in human adults and is associated with a unique pattern of damage in the hippocampus. The damage includes cell loss of the CA3 and CA4 areas and synaptic growth (sprouting) of mossy fibers in the supragranular layer of the dentate gyrus. Experimental evidence indicates that in adult rats the excitatory amino acid, kainic acid, induces a similar pattern of changes in hippocampal circuitry associated with alterations in perforant path excitation and inhibition. It has been suggested that, in humans, this type of damage may be a result of seizures early in life. In this study we examined the effects of kainic acid-induced status epilepticus on synaptic reorganization and paired-pulse electrophysiology in developing rats and adults. Kainic acid induced more severe seizures in 15-day-old rat pups than in adults. In contrast to adult rats, these seizures did not produce CA3/CA4 neuronal loss, mossy fiber sprouting or changes in paired-pulse excitation or inhibition in the hippocampus of rat pups tested 2-4 weeks after status epilepticus. Our results provide evidence that the immature hippocampus may be more resistant to seizure-induced changes than the mature hippocampus.

Using genetically-defined rodent strains for the identification of hippocampal traits relevant for two-way avoidance behavior: a non-invasive approach

Genetically-defined rodent strains permit the identification of hippocampal traits which are of functional relevance for the performance of two-way avoidance behavior. This is exemplified here by analyzing the relationship between infrapyramidal mossy fibers (a tiny projection terminating upon the basal dendrites of hippocampal pyramidal neurons) and two-way avoidance learning in about 800 animals. The necessary steps include 1) identification of structural traits sensitive to selective breeding for extremes in two-way avoidance, 2) testing the robustness of the associations found by studying individual and genetical correlations between hippocampal traits and behavior, 3) establishing causal relationships by Mendelian crossing of strains with extreme structural traits and studying the behavioral consequences of such structural 'randomization', 4) confirming causal relationships by manipulating the structural variable in inbred (isogenic) strains, thereby eliminating the possibility of genetic linkage, and 5) ruling out the possibility of spurious associations by studying the correlations between the hippocampal trait and other behaviors known to depend on hippocampal functioning. In comparison with the classical lesion approach for identifying relationships between brain and behavior, the present procedure appears to be superior in two aspects: it is non-invasive, and it focuses automatically on those brain traits which are used by natural selection to shape behaviorally-defined animal populations, i.e., it reveals the natural regulators of behavior.

Excessive intra- and supragranular mossy fibers in the dentate gyrus of tottering (tg/tg) mice

In Timm's sulfide silver preparations, intragranular and supragranular mossy fiber staining is found to be much more prevalent in the temporal dentate gyrus of the spontaneous epileptic mouse, tottering, than at matching levels in unaffected littermate controls. This aberrant distribution of mossy fibers may be due to the spontaneous seizures affecting this mutant.

Synaptic reorganization in the hippocampus induced by abnormal functional activity

Abnormal functional activity induces long-lasting physiological alterations in neural pathways that may play a role in the development of epilepsy. The cellular mechanisms of these alterations are not well understood. One hypothesis is that abnormal activity causes structural reorganization of neural pathways and promotes epileptogenesis. This report provides morphological evidence that synchronous perforant path activation and kindling of limbic pathways induce axonal growth and synaptic reorganization in the hippocampus, in the absence of overt morphological damage. The results show a previously unrecognized anatomic plasticity associated with synchronous activity and development of epileptic seizures in neural pathways.

Ultrastructural organization of normal and transplanted rat fascia dentata: I. A qualitative analysis of intracerebral and intraocular grafts

Few studies have dealt with the general ultrastructure and synaptic organization of grafted brain tissue. This study was therefore performed to extend current light microscopic observations on intracerebral and intraocular grafts of hippocampal tissue to the ultrastructural level. Blocks of tissue containing the hippocampus and fascia dentata from day 21 embryonic rats were grafted into the brain of developing and adult rats and to the anterior eye chamber of adult rats. After 100 or 200 days of survival the recipient rat brains or eyes were processed for electron microscopy. Tissue containing the graft dentate molecular layer and adjacent granule cell layer was selected for ultrastructural analysis, together with a few samples of the hilus and CA3. Normal dentate tissue was analyzed as control. At the light microscopic level most intracerebral and intraocular grafts displayed an organotypic organization with clearly recognizable cell and neuropil layers. Under the electron microscope the grafted granule cells had normal-appearing dendrites bearing the normal types of spines and forming the normal types of synapses. This was the case even in the absence of the normal major extrinsic afferents like the perforant path. The graft dentate granule cells formed axons and terminals with characteristics of the normal mossy fiber system in the hilus and CA3, in addition to aberrant supragranular mossy fiber terminals known from light microscopic studies of dentate transplants. Abnormal structures included a few dendritic growth cones and an increased occurrence of polyribosomes in spines. Their occurrence indicates ongoing dendritic plasticity even 100 days after transplantation. There was also an increased density of glial elements, particularly in the intraocular grafts. In some of these grafts the granule cells displayed immature traits in terms of nuclear indentations. Dentate interneurons of the basket cell type were present in both the intracerebral and the intraocular grafts. We conclude that grafted dentate granule cells, in different surroundings and without the normal, major perforant path input, can develop a basically normal cellular morphology, which includes the normal ultrastructural characteristics of the dendrites with spines and synapses, and the mossy fibers and its terminals.

Aberrant growth of mossy fibers and enhanced kainic acid binding sites induced in rats by early hyperthyroidism

Early hyperthyroidism induces an aberrant growth of hippocampal mossy fibers both in the fascia dentata (supragranular layer) and in Ammon's horn (infrapyramidal layer of CA3). Using a quantitative autoradiographic method, we found a corresponding increase in kainic acid binding sites supporting previous observations of a very close association between mossy fiber terminals and kainic acid binding sites.

Suppression of neurofilament degradation by protease inhibitors in experimental spinal cord injury

Intraperitoneal administration of the neutral protease inhibitors leupeptin and E-64c substantially suppressed the degradation of neurofilament proteins (NFP) at the site of mechanical insult and secondary axonal degeneration, and facilitated the recovery of motor functions in acute spinal cord injury in rats. The drug effects were assessed by sodium dodecyl sulphate polyacrylamide gel electrophoresis of NFP fractions from the injured tissue and by morphometry of degenerating axons revealed by the Fink-Heimer method in distal spinal cord segments with the aid of an automated image analyzer. The role of calcium-activated neutral proteases in acute central nervous tissue damage and potential use of protease inhibitors as therapeutic modalities are discussed.

Lesion-induced transneuronal plasticity in the adult rat hippocampus

The process of reactive synaptogenesis has been demonstrated in several areas of the central nervous system, including the hippocampal dentate gyrus. After a complete unilateral entorhinal lesion, approximately 85% of the input to the outer two-thirds of the ipsilateral dentate molecular layer is lost. Bilateral fluctuations in synaptic density within non-denervated zones of the dentate molecular layer predict further alterations in neural circuitry at sites located transneuronally to the denervated dentate granule cells. Using quantitative electron microscopy, our study demonstrates a complete cycle of synapse loss and reacquisition within the ipsilateral but not contralateral CA4/hilus region of the hippocampal formation. This area is one of the terminal fields for the dentate granule cell mossy fiber axons. In addition the granule cell mossy fiber axons sprout during the postlesion time course and form a significantly increased number of new mossy fiber terminals within the ipsilateral and contralateral CA4/hilus area. Our results indicate that responses to brain injury may no longer be confined to a local denervated site, but probably include polyneuronal circuitry loops, which may encompass one or more areas of the central nervous system. Previous difficulties in providing a close behavioral or functional correlation to localized structural events may be explained by a more global brain response to an injury.

Intracerebral transplants of the rat fascia dentata: a Golgi/electron microscope study of dentate granule cells

In the present study we describe the morphological characteristics of dentate granule cells in intracerebral allografts of the rat fascia dentata. Blocks of hippocampal tissue containing the fascia dentata were taken from late embryonic and newborn rats and transplanted to the hippocampal region of other newborn and young adult rats. After survival periods of several months the recipient brains were fixed by perfusion and serially sectioned on a Vibratome. Some sections were stained with thionin to determine the localization and general histological organization of the transplants, while others were Golgi stained with a modification of the section Golgi technique. Well-impregnated transplant granule cells were gold-toned and deimpregnated thus allowing a correlated, light and electron microscopic analysis of identified neurons to be done. At the light microscopic level the morphology of the dentate granule cells in the transplants was very similar to Golgi-impregnated, gold-toned granule cells in the fascia dentata of normal rats (controls). A few irregular, more obliquely curved dendrites occurred, but basal dendrites passing into the hilar region were never observed. Following an initial spine-free segment granule cell dendrites were densely covered with spines. The axon, the mossy fiber, originated as usual from the basal pole of the cell body. In the electron microscope, both small and larger complex spines (v and w types) were seen to emerge from the gold-toned dendrites of the identified granule cells. The thin unmyelinated granule cell axons gave rise to giant mossy fiber boutons in the dentate hilus, but in addition numerous aberrant mossy fiber terminals were found innermost in the dentate molecular layer just above the granule cell layer. The results demonstrate that dentate granule cells that have gone through the major part of their differentiation after transplantation develop characteristic dendritic and axonal elements very similar to those of granule cells in the fascia dentata in situ. The minor changes observed correspond to the redistribution of intrinsic connections that results from the absence of major extrinsic afferents.

Experimental spinal cord injury: quantitation of axonal damage by automated image analysis

The severity of acute experimental spinal cord injury in rats was assessed quantitatively with the aid of an automated image analyzer by measuring the amount of degenerating axons that had developed distal to the site of mechanical insult. Spinal cord injury was produced in adult male rats by epidural compression at T-11 with a Biemer vascular clip. On the 7th postoperative day, the animals were graded according to the degree of hindlimb motor deficit, as follows: Grade 0: normal (three rats); Grade 1: crawling with difficulty (10 rats); Grade 2: some voluntary movement (nine rats); and Grade 3: no voluntary movement (nine rats). The rats were then sacrificed. The L-6 segment was chosen for selective silver impregnation of degenerating axons by the Fink-Heimer method. Silver grains, representing degenerating axons and their terminals, were accumulated in the descending tracts and in Rexed's laminae VII and VIII. The extent of axonal damage was expressed by the percentage of the area occupied by silver grains in Rexed's lamina VIII. The area occupied by silver grains was 17.0% (mean) in Grade 0 rats, 22.3% +/- 2.63% (mean +/- standard deviation) in Grade 1 rats, 28.7% +/- 3.35% in Grade 2 rats, and 35.9% +/- 2.76% in Grade 3 rats. The severity in Grade 3 rats was close to that of rats with transected cords (37.6% +/- 0.89%). The differences among the groups were statistically significant (p less than 0.001). This method may serve as a useful tool for the objective assessment of therapeutic modalities in large series of small experimental animals.

Direct evidence for enhanced axon sprouting in adult rats exposed to ethanol in utero

The anterograde transport of horseradish peroxidase (HRP) was utilized to examine the post-lesion expansion of the commissural projection to the molecular layer of the dentate gyrus in adult rats prenatally exposed to ethanol, and in normal and pair-fed controls. Mean daily ethanol consumption by the pregnant dams was 12.0 +/- 1.6 g/kg. Similar consumption in a separate group of pregnant dams produced mean blood ethanol concentrations of 102.8 +/- 5.2 mg/dl of blood. The commissural terminal field of rats exposed to ethanol in utero and given unilateral entorhinal lesions as adults exhibited a significantly greater expansion compared to controls. There were no differences in the HRP-labeled terminal fields between normal and pair-fed animals with similar lesions, suggesting that the effect in the ethanol-exposed rats was due to ethanol teratogenicity rather than reduced caloric intake. Furthermore, the effect was not a function of altered organization of commissural and perforant path terminal fields (terminal field overlap). These data demonstrate that exposure to ethanol in utero produces long-lasting alterations in lesion-induced axon sprouting.