Neuronal transplants used in the repair of acute ischemic injury in the central nervous system

Seizures and Epilepsy After Ischemic Stroke

Background— Although a long-recognized clinical phenomenon, there remain many questions regarding the epidemiology of seizures and epilepsy after ischemic stroke, their effect on outcome, and their treatment.

Summary of Review— Interpretation of the various studies that have been conducted of postischemic stroke seizures and epilepsy are complicated by their heterogeneous designs, inconsistent uses of terminology, small sample sizes, different periods of follow-up, and ambiguities in seizure identification and classification. Estimates of the rate of early postischemic stroke seizures range from 2% to 33%. The rates of late seizures vary from 3% to 67%. The rate of postischemic stroke epilepsy is ≈2% to 4% and is higher in those who have a late seizure. Data reflecting seizure subtypes are limited. Aside from cortical location and, possibly, stroke severity, no other risk factors for postischemic stroke seizures have been consistently demonstrated. Results regarding the impact of postischemic stroke seizures on outcome are inconsistent.

Conclusions— Much additional work is needed to better understand the epidemiology and social impact of postischemic stroke seizures and epilepsy, their prevention, and optimal management.

Restorative neurosurgery: opportunities for restoration of function in acquired, degenerative, and idiopathic neurological diseases

Historically, neurosurgery has improved the environment of the nervous system to promote maximal spontaneous recovery of function. The population of patients whom we treat at present is a small portion of those who suffer from disabling neurological illnesses. Based on a combination of new technology, and advances in neuroscience, restorative neurosurgery is advancing the frontiers of our specialty, and providing the potential to restore lost function. Significant advancements in gene therapy, the discovery and delivery of neurotrophic factors, and cell transplantation now require neurosurgeons to broaden the scope of our practice so that it includes the restoration of function in an enormous number of patients with acquired, degenerative and idiopathic neurological diseases. In order to meet the present challenge, neurosurgeons must broaden our vision, our role, and our future educational goals. In this review, we summarize the landmark advances in the basic and clinical neurosciences and the results of clinical trials that are driving our evolution from passive reaction to disease to active attempts to restore lost central nervous system function.

Embryonic transplantation and ischemic memory deficit

Transient forebrain ischemia is associated with selective neuronal vulnerability and persistent memory deficit. This study compares functional outcome and morphological changes in rats subjected to post-ischemic CA1 or hilus/dentate gyrus region hippocampal fetal transplantation. Ischemia was produced by bilateral common carotid artery occlusion with hypotension. Fetal hippocampal neurons were transplanted into both sides of the CA1 or hilus/dentate gyrus region of the dorsal hippocampus, 1 week post-ischemia. Four weeks post transplantation, the rats underwent behavioral testing for 5 consecutive days using the water maze trial. All animals were perfusion fixed for morphological studies. Transplants in the CA1 region of the dorsal hippocampus were associated with memory and morphological recovery, while grafts placed into the hilus/dentate gyrus region of the dorsal hippocampus were not. Similarly, neurons transplanted in the CA1 region of the dorsal hippocampus were morphologically similar to CA1 pyramidal cell neurons and stained positive with calbindin D(28k). In contrast the grafts transplanted into the hilus/dentate gyrus region of the dorsal hippocampus were morphologically heterogeneous and staining with calbindin D(28k) was not as robust. Post-ischemic transplantation in the CA1 region of the dorsal hippocampus is effective in improving memory and morphological function.

Development of fetal hippocampal grafts in intact and lesioned hippocampus

Functional recovery observed in Parkinson's disease patients following grafting of fetal substantia nigra has encouraged the development of similar grafting therapy for other neurological disorders. Fetal hippocampal grafting paradigms are of considerable significance because of their potential to treat neurological disorders affecting primarily hippocampus, including temporal lobe epilepsy, cerebral ischemia, stroke, and head injury. Since many recent studies of hippocampal transplants were carried out with an aim of laying the foundation for future clinical applications, an overview of the development of fetal hippocampal transplants, and their capability for inducing functional recovery under different host conditions is timely. In this review, we will summarize recent developments in hippocampal transplants, especially the anatomical and/or functional integration of grafts within the host brain under specific host conditions, including a comparison of intact hippocampus with various types of hippocampal lesions or injury. Improvements in grafting techniques, methods for analysis of graft integration and graft function will be summarized, in addition to critical factors which enhance the survival and integration of grafted cells and alternative sources of donor cells currently being tested or considered for hippocampal transplantation. Viewed collectively, hippocampal grafting studies show that fetal hippocampal tissue/cells survive grafting, establish both afferent and efferent connections with the host brain, and are also capable of ameliorating certain learning and memory deficits in some models. However, the efficacy of intracerebral fetal hippocampal grafts varies considerably in different animal models, depending on several factors: the mode of donor tissue preparation, the method of grafting, the state of host hippocampus at the time of grafting, and the placement of grafts within the hippocampus. Functional improvement in many models appeared to be caused partially by re-establishment of damaged circuitry and partially by a trophic action of grafts. However, exact mechanisms of graft-mediated behavioral recovery remain to be clarified due to the lack of correlative analysis in the same animal between the degree of graft integration and behavioral recovery. Issues of mechanisms of action, degree of restoration of host circuitry and amelioration of host pathological conditions will need to be sorted out clearly prior to clinical use of fetal hippocampal transplants for susceptible neurological conditions.

Ischemia and lesion induced imbalances in cortical function

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.

Ischaemia-induced long-term hyperexcitability in rat neocortex

The long-term structural and functional consequences of transient forebrain ischaemia were studied with morphological, immunohistochemical and in vitro electrophysiological techniques in the primary somatosensory cortex of Wistar rats. After survival times of 10-17 months postischaemia, neocortical slices obtained from ischaemic animals were characterized by a pronounced neuronal hyperexcitability in comparison with untreated age-matched controls. Extra- and intracellular recordings in supragranular layers revealed all-or-none long-latency recurrent responses to orthodromic synaptic stimulation of the afferent pathway. These responses were characterized by durations up to 1.7 s, by multiple components and by repetitive synaptic burst discharges. The reversible blockade of this late activity by DL-amino-phosphonovaleric acid (APV) suggested that this activity was mediated by N-methyl-D-aspartate (NMDA) receptors. The peak conductance of inhibitory postsynaptic potentials was significantly smaller in neurons recorded in neocortical slices obtained from ischaemic animals than those from the controls. However, the average number of parvalbumin (PV)-labelled neurons per mm3, indicative of a subpopulation of GABAergic interneurons, and the average number and length of dendritic processes arising from PV-containing cells was not significantly different between ischaemic and control cortex. The prominent dysfunction of the inhibitory system in ischaemic animals occurred without obvious structural alterations in PV-labelled cells, indicating that this subpopulation of GABAergic interneurons is not principally affected by ischaemia. Our data suggest a long-term down-regulation of inhibitory function and a concurrent NMDA receptor-mediated hyperexcitability in ischaemic neocortex. These alterations may result from structural and/or functional properties of inhibitory non-PV-positive neurons or permanent functional modifications on the subcellular molecular level, i.e. alterations in the phosphorylation status of GABA and/or NMDA receptors. The net result of these long-term changes is an imbalance between the excitatory and inhibitory systems in the ischaemic cortex with the subsequent expression and manifestation of intracortical hyperexcitability.

New aspects of neurotransplantation

We have examined the possibility of promoting axonal regeneration within lesioned neural tissue using grafted artificial gel matrices. Polymeric matrices which feature a three-dimensional crosslinked macromolecular network were implanted into preformed lesions of the central nervous system (CNS). The host response consisted of matrix invasion by glial elements and the deposition of newly synthesized extracellular molecules. This rearrangement of the brain scarring process into an organized cellular coating promoted axonal regeneration into the gels. Entrapment of embryonic neurons and embryonal carcinoma (EC)-derived neurons, within the gels, was performed to explore the possibility of using polymer brain implants as neural graft microcarriers. Our results suggest that this approach will be useful for the delivery of cells and the promotion of axonal elongation required for successful neurotransplantation.

A glycine antagonist reduces ischemia-induced CA1 cell loss in vivo

Excessive activation of the N-methyl-D-aspartate (NMDA) receptor-channel complex has been implicated as one of the mechanisms by which ischemia-induced neuronal damage is mediated. Elevated glycine levels during ischemia may contribute to damage mediated by the NMDA receptor as glycine binding potentiates NMDA responses, and may be necessary for channel opening. We investigated the protective effects of 7-chlorokynurenic acid--a competitive antagonist at the glycine binding site associated with the NMDA receptor--against hippocampal CA1 cell loss induced by transient forebrain ischemia in rats. Intraventricular administration of the drug immediately before the onset of ischemia significantly attenuated neuronal loss compared to vehicle-treated animals.

Extracellular dopamine in the rat striatum during ischemia and reperfusion as measured by in vivo electrochemistry and in vivo microdialysis

The effects of transient global forebrain ischemia and reperfusion on striatal extracellular dopamine levels were analyzed using both in vivo electrochemistry and in vivo microdialysis in urethane-anesthetized rats. Electrochemical records showed that extracellular dopamine levels increased once during the period of ischemia, and a second time during reperfusion. This biphasic pattern was not detected by microdialysis, probably because of the relatively low time resolution of this technique. Microdialysis provided evidence that the voltammetric signal was a measure of dopamine, and also allowed measurement of the metabolites dihydroxyphenylacetic acid and homovanillic acid, both of which decreased during ischemia. The biphasic dopamine pattern seen in rats is similar to that reported previously in gerbils, suggesting that it is a phenomenon common to transient ischemia and reperfusion across different species and models of transient global ischemia. This phenomenon may have important implications for therapeutic intervention in cerebral ischemia.

Anatomical and physiological localization of prelabeled grafts in rat hippocampus

Dissociated rat fetal hippocampal cells were grafted into normal adult rats. The fetal cells were incubated with one of a number of fluorescent compounds at the time of the dissociation to facilitate identification of the individual grafted cells. The fluorescent labels which were analyzed for this purpose included rhodamine latex microspheres, Cascade blue latex beads, rhodamine-dextran-amine, DiI, and carboxyfluorescein ester. The labeled cells were stereotaxically placed as a suspension into normal rat host hippocampi. The rats were sacrificed 2 to 6 weeks after the grafting for in vitro physiological recordings, and the prelabeled grafts were located using fluorescence optics. During intracellular recordings neurons within the prelabeled grafts were injected with Lucifer yellow to visualize the morphology and integration of neuronal processes within the host. Following the recordings the host slices with the grafts were fixed in 4% paraformaldehyde for anatomical analysis. The ability to prelabel cellular grafts allows subsequent anatomical and physiological analysis of the integration of grafted neurons at the resolution of a single neuron. Such an analysis will improve our understanding of the survival, differentiation, migration, and integration of the grafted neurons and their potential to replace lost function in the lesioned hippocampus.

Hippocampal grafts into the intact brain induce epileptic patterns

Spontaneous hippocampal EEG activity and evoked field potentials were investigated in intact rats and in animals with fetal hippocampal grafts. Pieces of hippocampal grafts, derived from 15- to 16-day-old fetuses, were used to prepare cell suspensions and grafted directly into the intact hippocampus. Control animals received suspension grafts of the cerebellum derived from fetuses of identical age. Host hippocampal electrical patterns were monitored with chronic single electrodes or with a 16-microelectrode probe from 7 to 10 months after grafting. In contrast to previously reported high survival rates of fetal grafts in studies with damage to the host brain prior to grafting, survival of both hippocampal (60%) and cerebellar grafts (20%) was very poor in the intact hippocampus. In animals with cerebellar transplants or without surviving grafted neurons the electrical activity of the host hippocampus was indistinguishable from normal controls. In rats with hippocampal grafts short duration, large amplitude EEG spikes (up to 10 mV) were recorded, predominantly during immobility. When the EEG spikes (putative interictal spikes) were of large amplitude and contained population spikes, test evoked responses delivered to the perforant path were suppressed after the spontaneous events. In contrast, evoked responses were facilitated by interictal spikes without population spikes. The threshold of electrically induced afterdischarges did not differ significantly between groups of intact rats and animals with or without hippocampal grafts. However, in three rats with hippocampal grafts the evoked afterdischarges were associated with behavioral seizures. In two of these rats spontaneously occurring seizures were also observed. Synaptophysin-immunoreactivity demonstrated growth of the host mossy fibers into the graft.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged electrophysiological maturation of transplanted hippocampal neurons

The CA1 region of the rat hippocampal formation was lesioned by transient forebrain ischemia and subsequently repopulated with dispersed fetal hippocampal neurons. Using the hippocampal slice preparation, 7-12 months post-transplantation, intracellular recordings were made during synaptic activation and extracellular calcium measurements were made during iontophoretic application of excitatory amino acids. The data indicate that the developmental period of the transplanted neurons is prolonged with respect to N-methyl-D-aspartate (NMDA) receptor-mediated responses and may be involved in maintaining calcium dependent developmental processes such as fiber outgrowth.

Effects of transient forebrain ischemia and pargyline on extracellular concentrations of dopamine, serotonin, and their metabolites in the rat striatum as determined by in vivo microdialysis

Striatal microdialysis was performed in rats subjected to 20 min of transient forebrain ischemia produced by occlusion of the carotid arteries during hemorrhagic hypotension. Extracellular changes of dopamine, serotonin, and their metabolites were monitored before, during, and after the ischemic insult at 10-min intervals by on-line HPLC analysis. During ischemia, extracellular dopamine increased dramatically (156 times baseline), as did 3-methoxytyramine (3-MT), whereas 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) decreased (15-25% of baseline). Upon reperfusion, dopamine was cleared from the extracellular fluid within 40 min and reached a stable level (70% of baseline). DOPAC and HVA increased (250-330%) transiently and reached their maximum 1 h following reperfusion, whereas 3-MT decreased to undetectable levels within 20 min. Although baseline levels of serotonin were not detectable, serotonin and 5-hydroxyindoleacetic acid showed a qualitatively similar temporal pattern to dopamine and its acid metabolites. Killing rats by cervical dislocation produced changes in extracellular dopamine, serotonin, and their metabolites that were almost identical to those seen during ischemia. Pargyline pretreatment 2 h before ischemia had marginal effects on the postischemic clearing of dopamine. The pargyline pretreatment, however, did increase the survival rate of rats subjected to ischemia, and this protective effect might be due to the pargyline-induced blockade of the post-ischemic monoamine oxidase-mediated increase in dopamine metabolism and the concurrent production of the potentially neurotoxic molecule, hydrogen peroxide.

Grafting of fetal CA3 neurons to excitotoxic, axon-sparing lesions of the hippocampal CA3 area in adult rats

Hippocampal CA3 neurons from fetal rats were grafted to excitotoxic lesions in the CA3 subfield of the adult rat hippocampus and the formation of graft-host brain nerve connections examined. The excitotoxic lesions were induced by localized, stereotaxic injection of ibotenic acid (IA), a glutamic acid agonist, into CA3 of the dorsal hippocampus. The result was a so-called axon-sparing lesion with localized degeneration of nerve cells, but preservation of the extrinsic afferent fibers, now deprived of their targets. One week after the lesion a suspension of embryonic (E18-20) CA3 cells was grafted to the lesion site. Six weeks or more later the recipient brains were processed and analyzed by ordinary cell stains, histochemistry for acetylcholinesterase (AChE) and heavy metals (Timm staining), immunohistochemistry for the neuropeptides cholecystokinin and somatostatin and glial fibrillary acidic protein (GFAP) for astroglia, electron microscopy, and axonal tracing with retrogradely axonal transported fluorescent dyes or lesion-induced, anterograde degeneration combined with silver staining or electron microscopy. More than 90% of the grafts survived. They contained the normal types of CA3 neurons, which are mainly pyramidal cells, in addition to some normal, peptidergic, cholecystokinin- and somatostatin-reactive neurons. The grafts were innervated by AChE-positive, host cholinergic fibers, Timm-positive mossy fiber terminals from the host fascia dentata, and host commissural fibers traced by axonal degeneration. Efferent transplant projections were traced to the ipsilateral host CA1 (Schaffer collaterals) and the contralateral host hippocampus by retrograde axonal transport of fluorochromes injected into these host brain areas. All grafts analyzed by electron microscopy contained axonal varicosities resembling axonal growth cones even after long survival times. The results demonstrate that fetal rat hippocampal neurons, grafted to excitotoxic, axon-sparing lesions in the adult brain, can become both structurally and connectively well incorporated in the mature host central nervous system.

The grafted hippocampus: an epileptic focus

Field potentials and unitary activity were investigated in the grafted and the host hippocampi in freely moving rats and in vitro. The subcortical afferents and efferents of the hippocampus (fimbria-fornix, FF) were removed by aspiration. Solid pieces of hippocampal grafts derived from 15- to 16-day-old fetuses were placed in the lesion cavity in rats with unilateral FF lesions, and cell suspensions prepared from fetal hippocampi were grafted directly into the host hippocampi in animals with bilateral FF lesions. Reciprocal communication between the grafted and the host hippocampi was monitored with a 16-microelectrode probe from 7 to 10 months after grafting. The fluorescent retrograde tracer, Fluorogold, was used to examine graft-host projections and acetylcholinesterase staining to reveal host-derived fibers in the graft. The most typical neuronal pattern of the hippocampal graft was a highly synchronous population burst with concurrent EEG spike. The speed of propagation of the EEG spike within the graft and across the graft-host interface was either fast (greater than 3 m/s) or slow (less than 0.5 m/s). Large amplitude, short duration EEG spikes usually propagated with a high speed, while smaller amplitude, wider spikes with broad population bursts spread at a lower velocity. The direction of propagation was usually uniform indicating that the population burst was triggered by a localized subgroup of highly excitable neurons ("focus"). Spontaneous seizures were also present in the solid graft which frequently invaded the host hippocampus. The incidence of EEG spikes was three times higher in rats with bilateral suspension grafts than in animals with FF lesion only. In about half of the grafted rats spontaneous behavioral seizures were also observed. Intracellular recordings from putative pyramidal cells in the graft and in the host revealed large amplitude (10-12 mV), spontaneously occurring EPSPs. IPSPs were difficult to detect even during depolarizations of up to 20 mV from rest. We suggest that the increased excitability of the hippocampal graft is due to the high incidence of recurrent excitatory collaterals terminating on or close to the somata of pyramidal neurons. Population bursts may spread fast via extensively arborizing axon collaterals or slowly by successively activating new sets of neighboring neurons. Spontaneous behavioral convulsions are explained by assuming that the grafted hippocampus serves as an epileptic focus which is capable of kindling the host brain by repeated seizure induction.(ABSTRACT TRUNCATED AT 400 WORDS)