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

Spinal cord injury in small animals. 1. Mechanisms of spontaneous recovery

Spinal cord injury causes obvious clinical deficits early in the course of lesion evolution, but it is commonly observed that recovery can occur spontaneously during a period of days, weeks or even months afterwards. Spinal cord dysfunction arises after injury because of a combination of reversible alterations in the concentration of intra- and extracellular ionic constituents and irreversible tissue destruction. Recovery can therefore occur through re-establishment of the normal microenvironment of the spinal cord, which occurs soon after injury induction, and also by formation of new patterns of central nervous system circuitry. Alterations in circuitry, termed 'plasticity', can occur during the immediate period after injury but apparently continue for many weeks or months. There are differences in the extent and nature of recovery between complete and incomplete experimental spinal cord injuries that illustrate the roles played by reorganisation of intra- and suprasegmental circuitry. Information that is available on mechanisms of spontaneous recovery may aid development of novel therapies for clinical spinal cord injury.

Posttraumatic syringomyelia: profound neuronal loss, yet preserved function

Posttraumatic syrinxes may extend many cord segments rostral to a spinal cord injury (SCI) and significantly dilate the spinal cord, yet few neurologic deficits may be noted. Careful physical examination may reveal ascending loss of pain and temperature without evident functional motor decline. We present a 49-year-old man with T4 paraplegia and a large posttraumatic syrinx who died 3 weeks after syringoperitoneal shunting. Neuropathologic study revealed a large bilateral syrinx cavity from T1 to C6 that tapered to a small unilateral syrinx at C2. Light microscopy of sections from T1 to C2 showed massive loss of intermediate to intermedio-lateral gray neurons and moderate reduction of motoneurons at T1 to C6 levels. Despite these findings, manual muscle testing results remained normal for wrist extensors and elbow extensors, and the patient continued to perform independent sliding board transfers. We conclude that this large progressive syrinx did not merely dissect neural elements apart but caused extensive neuronal damage. Loss of interneurons was evident in spinal segments with preserved strength and function. Possible mechanisms to explain the relatively minimal clinical deficits in view of the neuronal loss are discussed.

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

Genetic influences on cellular reactions to brain injury: activation of microglia in denervated neuropil in mice carrying a mutation (Wld(S)) that causes delayed Wallerian degeneration

This study examines the relationship between the appearance of degenerative changes in synaptic terminals and axons and the activation of microglia in denervated neuropil regions of normal mice of the C57BL/6 strain and mutant mice (Wld(S)), in which Wallerian degeneration is substantially delayed. The time course of degenerative changes in synaptic terminals and axons was assessed using selective silver staining. Microglial cells were identified by immunostaining for Mac-1, a monoclonal antibody to the CR3 complement receptor, and by histochemical staining for nucleoside diphosphatase (NDPase). Increased argyrophilia, indicative of degenerative changes, was evident as early as 1 day postlesion in normal mice, but was not seen until 6-8 days in mice with the Wld(S) mutation. Microglial activation in normal C57BL/6 mice was evident by 24 hours postlesion, as evidenced by increased immunostaining for Mac-1, increased histochemical staining for NDPase, and morphological changes indicative of an activated phenotype (short, thick processes). Quantitative evaluation of immunostaining for Mac-1 revealed that peak activation occurred between 2 and 6 days postlesion with a return to a quiescent phenotype by 12 days. In contrast, the microglial response was significantly delayed and prolonged in mice bearing the Wld(S) mutation. Activated microglia were not seen within the deafferented area until 6 to 8 days postlesion and peak activation occurred between 12 and 20 days postlesion. These data suggest that the response of microglia in denervated neuropil zones is triggered by the same types of degenerative changes that cause increased argyrophilia as detected by selective silver staining methods.

Genetic influences on cellular reactions to CNS injury: the reactive response of astrocytes in denervated neuropil regions in mice carrying a mutation (Wld(S)) that causes delayed Wallerian degeneration

This study compares the reactive changes in astrocytes in denervated neuropil regions in normal mice and in mice carrying the Wld(S) mutation which leads to delayed Wallerian degeneration. In situ hybridization and immunocytochemical techniques were used to define the time course of changes in the levels of glial fibrillary acidic protein (GFAP) and GFAP mRNA in the denervated neuropil of the hippocampus after unilateral aspiration lesions of the entorhinal cortex. In control mice, GFAP mRNA levels increased rapidly in the denervated neuropil to a peak that was about tenfold higher than control at 2-4 days, decreased between 6 and 8 days postlesion, and then increased again to a second peak at 10 days postlesion. Increases in immunostaining for GFAP were evident by 2 days, remained elevated until 12 days postlesion and then decreased slowly. In mice carrying the Wld(S) mutation, the upregulation of GFAP mRNA levels in the denervated laminae was substantially delayed. Strikingly absent was the dramatic increase in labeling at 2-4 days postlesion which was such a prominent feature of the response in control animals. Peak labeling in the denervated laminae was not seen until 10-12 days postlesion. The development of a well-defined band of intensely immunostained and hypertrophied astrocytes in the denervated zone was also delayed in the Wld(S) animals, although there were modest increases in immunostaining as early as 2 days postlesion that were seen throughout the hippocampus ipsilateral to the lesion. These results suggest that degenerative changes in axons and synaptic terminals are the principal trigger for upregulating GFAP expression in the denervated neuropil, although other signals also play a role in the early postlesion response.

The effect of combined fluid percussion and entorhinal cortical lesions on long-term potentiation

Among the pathological processes initiated by traumatic brain injury are excessive neuroexcitation and target cell deafferentation. The current study examines the contribution of these injury components, separately as well as their combined effect, on postinjury alterations in the capacity for long-term potentiation and the immunolocalization of N-methyl-D-aspartate receptors and GABA. Adult rats underwent central fluid percussion traumatic brain injury, electrolytic bilateral entorhinal cortex lesions, or a combined injury of both procedures separated by 24 h. At two or 15 days postinjury, the capacity for long-term potentiation of the Schaffer collateral-commissural input to CA1 was measured in acute electrophysiological recordings. Entorhinal cortical lesions resulted in time-dependent increases in the effectiveness of tetanic stimulation to elevate population postsynaptic potentials and population spike amplitudes. These lesions also resulted in a marked intensification in the density of N-methyl-D-aspartate receptors in the CA1 stratum lacunosum-moleculare. All injury conditions that included fluid percussion as a component (alone or in combined injuries) produced a persistent impairment in long-term potentiation of the evoked population postsynaptic potentials. Thus, in combined injuries, the presence of concussion-induced neuroexcitation attenuated deafferentation-induced response increases. Both N-methyl-D-aspartate receptor and GABA immunobinding following combined injuries were also reduced relative to those observed following entorhinal lesions alone. The present results suggest that a process of receptor plasticity, possibly involving reactive synaptogenesis, may contribute to postdeafferentation enhancements of long-term potentiation, and that a traumatic brain insult will attenuate these enhancements. This interaction of different injury components suggests that recovery of function following brain injury may be enhanced by pharmacological reduction of neuroexcitation during postinjury intervals of reactive receptor plasticity.

Progressive entorhinal cortex lesions accelerate hippocampal sprouting and spare spatial memory in rats

Accelerating hippocampal sprouting by making unilateral progressive lesions of the entorhinal cortex spared the spatial memory of rats tested for retention of a learned alternation task. Subsequent transection of the sprouted crossed temporodentate pathway (CTD), as well as a simultaneous CTD transection and progressive entorhinal lesion, produced a persistent deficit on the memory task. These results suggest that CTD sprouting, which is homologous to the original perforant path input to the dentate gyrus of the hippocampus, is behaviorally significant and can ameliorate at least some of the memory deficits associated with hippocampal deafferentation.

Brain imaging and behavioral outcome in traumatic brain injury

Brain imaging studies have become an essential diagnostic assessment procedure in evaluating the effects of traumatic brain injury (TBI). Such imaging studies provide a wealth of information about structural and functional deficits following TBI. But how pathologic changes identified by brain imaging methods relate to neurobehavioral outcome is not as well known. Thus, the focus of this article is on brain imaging findings and outcome following TBI. The article starts with an overview of current research dealing with the cellular pathology associated with TBI. Understanding the cellular elements of pathology permits extrapolation to what is observed with brain imaging. Next, this article reviews the relationship of brain imaging findings to underlying pathology and how that pathology relates to neurobehavioral outcome. The brain imaging techniques of magnetic resonance imaging, computerized tomography, and single photon emission computed tomography are reviewed. Various image analysis procedures, and how such findings relate to neuropsychological testing, are discussed. The importance of brain imaging in evaluating neurobehavioral deficits following brain injury is stressed.

Central nervous system plasticity after spinal cord injury in man: interlimb reflexes and the influence of cutaneous stimulation

In persons who have sustained severe injuries to the cervical spinal cord, electrical stimulation of mixed peripheral nerves in a lower limb can evoke short-latency, bilateral motor responses in muscles of the distal upper limbs; such motor responses have been termed interlimb reflexes. In the present study, we investigated the role that cutaneous stimulation plays in evoking interlimb reflexes. Fifteen subjects with chronic injury (> 1 year) to the cervical spinal cord were investigated. Single motor unit activity was recorded from a number of distal upper limb muscles. The lower limb cutaneous area within which stimulation recruited a given motor unit of the upper limb was defined as that motor unit's 'receptive field'. Activity from a total of 48 single motor units was analyzed. The majority of motor units responded to light touch, individual hair movement, and thermal (hot and cold) stimulation. Excitatory responses were observed bilaterally, although contralateral responses predominated. Stimulation occasionally resulted in inhibition of a spontaneously active motor unit. Receptive fields varied a great deal in size, with proximal locations being larger than those encountered in more distal lower limb locations (i.e. the toes). The spinocervical tract is a possible candidate for mediating some portion of these interlimb reflexes, the origin of which may be due to new growth (regenerative sprouting) in the spinal cord caudal to a severe injury.

Selective upregulation of T alpha 1 alpha-tubulin and neuropeptide Y mRNAs after intermittent excitatory stimulation in adult rat hippocampus in vivo

Adult central neurons exhibit significant structural and molecular changes in epilepsy. We have examined changes in two markers of morphological and physiological plasticity, T alpha 1 alpha-tubulin (T alpha 1) and neuropeptide Y (NPY) mRNAs, in response to intermittent (20 Hz, 10 seconds, 1 minute-1) stimulation of the rat perforant path in vivo. Stimulus trains elicited brief (0.5-3 seconds) afterdischarges in the ipsilateral dentate gyrus (DG). Four hours of stimulation caused no significant loss of inhibition in the DG 40-48 hours after stimulation ceased. However, it did lead to an increase in NPY mRNA in neurons of the ipsilateral and, to a lesser extent, contralateral DGs and Ammon's Horn. Many of these were presumably interneurons that normally express NPY. However, dentate granule cells (DGCs), which do not normally express this peptide, also expressed robust levels of NPY mRNA bilaterally. NPY mRNA levels peaked at 4-24 hours and returned to baseline by 48 hours poststimulation. Although 24 hours of stimulation induced a similar increase in interneurons, DGCs showed no detectable NPY mRNA. Afterdischarges were necessary to elevate NPY mRNA expression. Four hours of stimulation elevated T alpha 1 mRNA expression in both ipsilateral and, to a lesser extent, contralateral DGCs; this elevation peaked at 24 hours poststimulation and declined to baseline by 72 hours. Stimulation for 24 hours caused broader changes in T alpha 1 mRNA expression, with increases in DGCs and in CA3 pyramidal cells bilaterally. Acute denervation of the DG did not affect T alpha 1 mRNA level in the hippocampal formation. Elevated synaptic input resulting in afterdischarges, but not necessarily in excitability changes in the DG, led to alterations in the expression of molecular markers of plasticity. These changes may reflect adaptive responses to physiological activation.

The pathobiology of traumatically induced axonal injury in animals and humans: a review of current thoughts

This manuscript provides a review of those factors involved in the pathogenesis of traumatically induced axonal injury in both animals and man. The review comments on the issue of primary versus secondary, or delayed, axotomy, pointing to the fact that in cases of experimental traumatic brain injury, secondary, or delayed, axotomy predominates. This review links the process of secondary axotomy to an impairment of axoplasmic transport which is initiated, depending upon the severity of the injury, by either focal cytoskeletal. misalignment or axolemmal permeability change with concomitant cytoskeletal. collapse. Data are provided to show that these focal axonal changes are related to the focal impairment of axoplasmic transport which, in turn, triggers the progression of reactive axonal change, leading to disconnection. In the context of experimental studies, evidence is also provided to explain the damaging consequences of diffuse axonal injury. The implications of diffuse axonal injury and its attendant deafferentation are considered by noting that with mild injury such deafferentation may lead to an adaptive neuroplastic recovery, whereas in more severe injury a disordered and/or maladaptive neuroplastic re-organization occurs, consistent with the enduring morbidity associated with severe injury. In closing, the review focuses on the implications of the findings made in experimental animals for our understanding of those events ongoing in traumatically brain-injured humans. It is noted that the findings made in experimental animals have been confirmed, in large part, in humans, suggesting the relevance of animal models for continued study of human traumatically induced axonal injury.

Focal brain injury and upregulation of a developmentally regulated extracellular matrix protein

Tenascin is an extracellular matrix glycoprotein expressed during both normal development and neoplastic growth in both neural and nonneural tissues. During development of the central nervous system (CNS), tenascin is synthesized by glial cells, in particular by immature astrocytes, and is concentrated in transient boundaries around emerging groups of functionally distinct neurons. In the mature CNS, only low levels of the glycoprotein can be detected. The present study demonstrates that following trauma to the adult human cerebral cortex, discrete populations of reactive astrocytes upregulate their expression of tenascin and dramatically increase their transcription of the tenascin gene. The enhanced expression of tenascin may be involved in CNS wound healing, and may also affect neurite growth within and around a brain lesion.

Combined fluid percussion brain injury and entorhinal cortical lesion: a model for assessing the interaction between neuroexcitation and deafferentation

Laboratory studies suggest that excessive neuroexcitation and deafferentation contribute to long-term morbidity following human head injury. Because no current animal model of traumatic brain injury (TBI) has been shown to combine excessive neuroexcitation and significant levels of deafferentation, we developed a rat model combining the neuroexcitation of fluid percussion TBI with subsequent entorhinal cortical (EC) deafferentation. In this paradigm, moderate fluid percussion TBI was induced in each rat, followed 24 h later by bilateral EC lesion (BEC). Six conditions were examined: (1) fluid percussion TBI followed 24 h later by bilateral EC lesion (TBEC), (2) fluid percussion TBI (TBI), (3) bilateral EC lesion (BEC), (4) sham fluid percussion TBI (SHAM), (5) TBI followed 24 h later by unilateral EC lesion (TUEC), and (6) unilateral EC lesion (UEC). The first four groups were assessed for motor (with beam-balance and beam-walk testing) and cognitive deficits (with the Morris water maze) and hippocampal morphology (with immunocytochemistry and electron microscopy). The TUEC and UEC groups were assessed for cognitive deficits alone. Motor deficits were greater in the TBEC injury than in TBI or sham alone; however, no significant difference was observed between the TBEC and BEC conditions in motor performance. Cognitive deficits were of a greater magnitude in the combined TBEC injury model relative to each individual insult. These cognitive deficits appeared to be additive for the two experimental injuries, BEC deafferentation producing deficits intermediate between TBI and TBEC insults. Morphologic analysis of the dentate gyrus molecular layer at 15 days after TBEC showed that the distribution of synaptophysin-positive presynaptic terminals was distinct from that observed after either TBI or BEC alone. Specifically, the laminar pattern of presynaptic rearrangement induced by BEC lesion did not occur after TBEC injury. The present results show that axonal injury and its attendant deafferentation, when coupled with traumatically induced neuroexcitation, produce an enhancement of the morbidity associated with TBI. Moreover, they indicate that this model can effectively be used to study the interaction between neuroexcitation and synaptic plasticity.

Expression of GAP43 mRNA in normally developing and transplanted neurons from the rat ventral mesencephalon

These experiments were designed to determine whether the neuronal growth-related protein GAP43 is expressed at high levels by neurons that collateralize extensively or have long periods of synaptogenesis. We also evaluated the effects of target availability on GAP43 expression. Dopaminergic neurons of the rat ventral mesencephalon (VM) were chosen for investigation because they undergo extensive collateralization and synaptogenesis during postnatal development. Double label in situ hybridization histochemistry (ISHH) and immunocytochemistry (ICC) were used to measure changes in GAP43 mRNA levels within tyrosine hydroxylase (TH)-immunoreactive and -nonimmunoreactive neurons of the VM during postnatal development (p5-adult). TH neurons show higher levels of GAP43 mRNA than do non-TH neurons throughout normal postnatal development and in the adult. This result may be due to more extensive axonal arborization and synaptic remodeling on the part of TH neurons as they innervate the striatum. To test the effects of target availability on GAP43 utilization, grafts of embryonic (e15) VM were placed within previously 6-hydroxydopamine (6-OHDA)-lesioned striata and allowed to develop for 10-28 days. Levels of GAP43 mRNA in grafted TH neurons were reduced at all time points. The short distance to target in the graft paradigm may shorten the overall axonal process length, resulting in lower requirements for growth-related proteins such as GAP43. However, grafted non-TH neurons had elevated levels of GAP43 mRNA, perhaps attributable to prolonged target seeking by neurons that have been isolated from their normal targets.

Sprouting of CA3 pyramidal neurons to the dentate gyrus in rat hippocampal organotypic cultures

The understanding of the mechanisms of a functional synaptic plasticity in the hippocampus has been expanded greatly by the use of in vitro slice preparations. The question addressed in the present study was whether morphological plasticity observed in vivo can also be reproduced in hippocampal slices. In vivo, hippocampal commissural and association fibers are known to sprout and occupy synaptic sites vacated by deafferentation of the dentate gyrus (DG). In hippocampal slice preparations, the major input to the DG is eliminated, so that the DG is deafferented. Might intrinsic neurons sprout to the DG if the slice preparation is maintained for weeks? In this study hippocampal slices obtained from 6-day-old rats were cultured. Stimulation of the dentate stratum moleculare produced antidromic field potentials in the CA3 of the slices cultivated for more than 1 week. The antidromic response was not observed in CA1 pyramidal neurons. The CA3 to DG projection response was also observed in a CA3 mini-slice placed near a co-cultured whole hippocampal slice, when the DG in the latter was stimulated. Moreover, stimulation of the CA3 mini-slice induced synaptic responses in the DG of the whole-slice. The conclusion drawn is that deafferentation could induce axonal sprouting in a neuron-specific manner in hippocampal organotypic culture. This preparation would be potentially useful for the screening of chemical factors that influence sprouting of central neurons.

Depression following traumatic brain injury: a 1 year longitudinal study

A group of 66 patients hospitalized for the treatment of closed head injury, were assessed for the presence of mood disorders during their hospital admission and at 3, 6 and 12 months follow-up. A total 28 patients met DSM-III-R diagnostic criteria for major depression at some time during the study (17 in the acute stage, 11 during follow-up). The mean duration of major depression was 4.7 months. However, there appeared to be a group of transiently depressed patients (41%) who where depressed inhospital but were no longer depressed at 3 months follow-up. Throughout the follow-up period, major depression showed a strong relationship with poor social functioning. There was not, however, a consistent relationship between depression and quantitative measures of either physical or cognitive impairment. Location of the brain lesion was associated with the development of major depression only in the acute stage. Transient depressive syndromes were associated with left dorsolateral frontal and/or left basal ganglia lesions.

Signals that induce sprouting in the central nervous system: sprouting is delayed in a strain of mouse exhibiting delayed axonal degeneration

This study evaluates whether CNS sprouting is initiated by signals related to the degeneration of presynaptic axons. We evaluate the time course of sprouting of cholinergic septohippocampal fibers after unilateral entorhinal cortex (EC) lesions in a substrain of mice carrying a mutation which leads to a substantial delay in the onset of Wallerian degeneration. We first verified that axonal degeneration resulting from EC lesions was delayed in mutant mice using silver-staining techniques (the Fink-Heimer method). Cholinergic sprouting was then evaluated using a histochemical technique for acetylcholinesterase (AChE) in mutant mice and normal controls. In normal control mice, both axonal degeneration and cholinergic sprouting occurred with a time course that was comparable to that described in rats. Argyrophilic degeneration debris was prominent by 4 days postlesion, and increases in AChE staining in the molecular layer of the dentate gyrus were well developed by 10 days. In mice carrying the "Ola" mutation, however, argyrophilic degeneration debris was not detectable at 4 or 6 days postlesion, began to appear in the dentate gyrus by 8 days postlesion, but did not become prominent until 12 days. Increases in AChE staining in the molecular layer of the dentate gyrus were not detectable even at 12 days postlesion, but developed gradually after 14 days. These results demonstrate that the signals which initiate at least one form of CNS sprouting are related to the degeneration of presynaptic axons.

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.

GAP-43 and 5B4-CAM immunoreactivity during the development of transplanted fetal mesencephalic neurons

Developing neurons contain high levels of several proteins which are absent or relatively scarce in mature neurons. GAP-43 is a cytoplasmic protein primarily found within neurons; high levels of this protein are correlated with axonal elongation or regeneration. 5B4-CAM, a glycosylated transmembrane protein, is a member of the NCAM family present in growth cones and in plastic CNS structures. Antibodies directed against these two developmentally regulated proteins were used to characterize the time-course of maturation of transplanted fetal mesencephalic neurons. For our experiments unilateral injections of 6-hydroxydopamine were made into the nigrostriatal bundle in Sprague-Dawley rats. The effectiveness of the lesion was verified by apomorphine-induced rotation and by postmortem examination of the substantia nigra. Following behavioral testing, pieces of ventral mesencephalon obtained from E15 fetuses were transplanted into the caudoputamen ipsilateral to the lesion. Immunocytochemistry revealed high levels of GAP-43 and 5B4-CAM at 5, 11, and 15 days post-transplant but relatively lower levels by 3 weeks. At 13 weeks the immunoreactivity present within the transplant tissue was approximately equal to that found within the host striatal neuropil. This time-course of higher GAP-43 and 5B4-CAM immunoreactivities coincides with the time-course of neuritic outgrowth of dopamine containing cell populations within the ventral mesencephalon in situ as well as within ventral mesencephalic transplants. This implies that axon elongation occurs over a period similar to that which occurs during normal development. These data suggest that the effects of transplantation surgery and the altered environment of the host striatum do not significantly affect the time-course of development of ventral mesencephalic neurons.

Exo-focal postischemic neuronal death in the rat brain

We describe delayed neuronal damage in ipsilateral areas remote from the ischemic area of rat brain after transient focal ischemia induced by embolization of the right middle cerebral artery (MCA). After 15, 30, 60 and 90 min of MCA occlusion, recirculation was achieved by removal of the embolus. Chronological changes in the distribution of the neuronal damage were determined by using the 45Ca autoradiographic technique and the histological method, and the mechanism involved was investigated by measuring local cerebral glucose metabolism. Depending on the duration of ischemia, 45Ca accumulation extended to the lateral segment of the caudate putamen and to the cerebral cortex, both supplied by the occluded MCA. Moreover, 3 days after ischemic insult, 45Ca had accumulated in the ipsilateral substantia nigra and ventral posterior nucleus of the thalamus. Histological examination revealed that the neurons in both areas suffered damage and were selectively reduced in number. Cerebral glucose utilization decreased in the thalamus, but increased approximately 30% (P less than 0.01) in the substantia nigra compared with the value in the corresponding contralateral area. Both areas lie outside the ischemic area, but have transsynaptic connections with the ischemic focus. Based on the present study, we suggest that the mechanisms of delayed neuronal death in these two remote areas may not be identical, but that this phenomenon may be caused by a transsynaptic process associated with the ischemic focus.

Neural damage in the rat thalamus after cortical infarcts

Histopathologic changes in the thalamus of 23 rats after somatosensory cortical infarction produced by middle cerebral artery occlusion were examined using the Fink-Heimer silver staining method, immunohistochemistry with antibodies against glial fibrillary acidic protein and laminin, and conventional stains. Middle cerebral artery occlusion produced cortical infarcts in the lateral parietal region, with variable involvement of the frontoparietal parasagittal sensorimotor cortex. Within 3 days after occlusion, massive terminal degeneration but no neuronal changes were apparent in the ipsilateral thalamus. By 1 week after occlusion, abnormal neurons with darkly stained, shrunken nuclei and atrophic perikarya were present in the ipsilateral thalamic nuclei. These neurons were densely argyrophilic in Fink-Heimer sections. Rats with small lateral parietal cortical lesions had degenerating neurons limited to the medial ventroposteromedial nucleus. Large lesions involving the parasagittal sensorimotor cortex resulted in widespread neuronal damage in the ventroposteromedial, ventroposterolateral, intralaminar, and posterior nuclear regions but nowhere else. Immunoreactivity to laminin antibody decreased, and astrocytic proliferation was abundant in affected thalamic areas. These findings are consistent with retrograde neuronal degeneration due to thalamocortical fiber damage in ischemic cortical regions. Such lesions remote from the infarct may influence functional recovery in patients with stroke.