Innervation of embryonic hippocampal implants by regene-rating axons of cholinergic septal neurons in the adult rat

Slow gamma rhythms in CA3 are entrained by slow gamma activity in the dentate gyrus

In hippocampal area CA1, slow (∼25-55 Hz) and fast (∼60-100 Hz) gamma rhythms are coupled with different CA1 afferents. CA1 slow gamma is coupled to inputs from CA3, and CA1 fast gamma is coupled to inputs from the medial entorhinal cortex (Colgin LL, Denninger T, Fyhn M, Hafting T, Bonnevie T, Jensen O, Moser MB, Moser EI. Nature 462: 353-357, 2009). CA3 gives rise to highly divergent associational projections, and it is possible that reverberating activity in these connections generates slow gamma rhythms in the hippocampus. However, hippocampal gamma is maximal upstream of CA3, in the dentate gyrus (DG) region (Bragin A, Jando G, Nadasdy Z, Hetke J, Wise K, Buzsaki G. J Neurosci 15: 47-60, 1995). Thus it is possible that slow gamma in CA3 is driven by inputs from DG, yet few studies have examined slow and fast gamma rhythms in DG recordings. Here we investigated slow and fast gamma rhythms in paired recordings from DG and CA3 in freely moving rats to determine whether slow and fast gamma rhythms in CA3 are entrained by DG. We found that slow gamma rhythms, as opposed to fast gamma rhythms, were particularly prominent in DG. We investigated directional causal influences between DG and CA3 by Granger causality analysis and found that DG slow gamma influences CA3 slow gamma. Moreover, DG place cell spikes were strongly phase-locked to CA3 slow gamma rhythms, suggesting that DG excitatory projections to CA3 may underlie this directional influence. These results indicate that slow gamma rhythms do not originate in CA3 but rather slow gamma activity upstream in DG entrains slow gamma rhythms in CA3.

Propagation of Neuronal Damage to Embryonic Grafts Transplanted in the Hippocampus of Murine Models of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, characterized by the presence of two principal hallmarks-amyloid plaques and neurofibrillary tangles. The primary cause of the majority of AD cases is not known. Likewise, the mechanisms underlying the propagation of the pathology from affected tissue to neighboring healthy neurons are largely unknown, but knowledge about them could be helpful to design strategies aimed at halting the progression of the disease. To throw light on the mechanisms of propagation of neuronal damage to healthy tissue, wild-type (WT) hippocampal solid tissue chunks derived from green fluorescent protein (GFP)-positive embryos were grafted into the hippocampus of 6-month-old WT and 3xTg-AD mice, a triple-transgenic mouse model that exhibits both amyloid-beta (Aβ) and tau protein pathology. The histological and morphological alterations of the grafted tissues were assessed 3 months post-transplantation. Tissues grafted in 3xTg-AD hosts, compared to those grafted in WT recipients, presented a significant decrease in neurite outgrowth (35.4%) and dendritic spine density (41.3%), mainly due to a reduction of stubby and thin-shaped spines. Moreover, some cells of the tissue transplanted in 3xTg-AD hosts accumulated intracellular amyloid peptide deposits similar to the cells of the host. Furthermore, the immunohistochemical examination of reactive astrocytes and microglia revealed the presence of more inflammation in the grafted tissues hosted in 3xTg-AD compared to WT recipients. These results show a propagation of neuronal damage to initially healthy embryonic grafts, validating this methodology for future studies on the mechanisms of the progression of AD pathology to surrounding regions.

Effects of intranigral vs intrastriatal fetal mesencephalic neural grafts on motor behavior disorders in a rat Parkinson model

BACKGROUND

Numerous experimental and clinical studies have shown that intrastriatal fetal mesencephalic grafts grow, survive, and reinnervate host brain tissue, resulting in partial recovery of motor deficits. In addition, pharmacological evidence indicates that these grafts increase dopamine secretion in lesioned brain. However, to date, no grafting method has completely restored the nigrostriatal pathway, and there is no consensus on optimal graft numbers or locations. This study compared outcomes with multiple striatal grafts vs a single intranigral graft in a rat model of Parkinson disease.

METHODS

Forty-one female Wistar rats weighing 200 to 250 g were used. First, baseline rotational behavior testing with amphetamine injection was done to identify each animal's dominant nigrostriatal pathway (left vs right hemisphere). Some rats then received a unilateral intranigral injection of 6-hydroxydopamine (4 microL [8 microg]) to produce the Parkinson model lesion, and rotational testing was repeated. One group of the lesioned rats received a single intranigral injection of suspended fetal ventral mesencephalic cells (n = 11), and another received multiple intrastriatal grafts of the same type (n = 11).

RESULTS

Both grafted groups showed significant improvement on rotational testing with amphetamine and apomorphine at 6 weeks "postgrafting" (P < .001 for "postlesioning" vs postgrafting results in each of the 2 groups); however, the animals with multiple intrastriatal grafts showed complete recovery from motor asymmetry, whereas the rats with single intranigral grafts showed only partial improvement.

CONCLUSION

The findings indicate that multiple intrastriatal grafts result in significantly greater functional improvement than single intranigral grafts in this rat Parkinson model.

Cerebellar grafting in the oculomotor system as a model to study target influence on adult neurons

In the last decades, there have been many efforts directed to gain a better understanding on adult neuron-target cell relationships. Embryonic grafts have been used for the study of neural circuit rewiring. Thus, using several donor neuronal tissues, such as cerebellum or striatum, developing grafted cells have been shown to have the capability of substituting neural cell populations and establishing reciprocal connections with the host. In addition, different lesion paradigms have also led to a better understanding of target dependence in neuronal cells. Thus, for example, axotomy induces profound morphofunctional changes in adult neurons, including the loss of synaptic inputs and discharge alterations. These alterations are probably due to trophic factor loss in response to target disconnection. In this review, we summarize the different strategies performed to disconnect neurons from their targets, and the effects of target substitution, performed by tissue grafting, upon neural properties. Using the oculomotor system-and more precisely the abducens internuclear neurons-as a model, we describe herein the effects of disconnecting a population of central neurons from its natural target (i.e., the medial rectus motoneurons at the mesencephalic oculomotor nucleus). We also analyze target-derived influences in the structure and physiology of these neurons by using cerebellar embryonic grafts as a new target for the axotomized abducens internuclear neurons.

Pattern of long-distance projections from fetal hippocampal field CA3 and CA1 cell grafts in lesioned CA3 of adult hippocampus follows intrinsic character of respective donor cells

Fetal hippocampal grafts transplanted to the lesioned CA3 of adult hippocampus can extend axonal projections to many regions of the host brain. However, the identity of grafted cells that project to specific host regions is unknown. We hypothesize that the pattern of long-distance axonal projections from distinct fetal hippocampal cells grafted to lesioned CA3 is specified by the intrinsic nature of respective donor cells rather than characteristics of the host graft region. We grafted fetal hippocampal CA3 or CA1 cells into kainic acid lesioned CA3 of adult hippocampus at four days post-lesion. Neurons projecting to either the contralateral hippocampus or the ipsilateral septum were then measured in these grafts at four months post-grafting using Fluoro-Gold and DiI tract tracing. CA3 grafts located close to the degenerated CA3 cell layer showed a high propensity for establishing projections into the contralateral hippocampus (commissural projections) compared to similarly located CA1 grafts, which exhibited negligible commissural projections. Similar distinction was observed between the two graft types even when they were located only partially in the lesioned CA3. Among CA3 grafts, those placed near the degenerated CA3 cell layer established significantly greater commissural projections than those placed only partially in the CA3 region. Septal projections, in contrast, were robust from both CA3 and CA1 grafts. This differential projection pattern between CA3 and CA1 grafts resembles projections of CA3 and CA1 cells in intact hippocampus.These results demonstrate that the intrinsic character of grafted fetal cells determines the type of efferent projections from fetal grafts into different targets in the lesioned adult host brain. However, the extent of efferent projections from specific grafts is also influenced by the location of grafted cells within the host region. Thus, graft-mediated appropriate reconstruction of damaged circuitry in the lesioned brain may require grafting of homotopic donor cells. Further, the robust and specific projections observed from CA3 grafts is likely beneficial for functional recovery of hippocampus following CA3 injury and hence of significance towards developing a graft-mediated therapy for human temporal lobe epilepsy.

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.

Development of long-distance efferent projections from fetal hippocampal grafts depends upon pathway specificity and graft location in kainate-lesioned adult hippocampus

Fetal hippocampal cells grafted into the excitotoxically lesioned hippocampus of adult rats are capable of extending axonal projections into the host brain. We hypothesize that the axonal growth of grafted fetal cells into specific host targets, and the establishment of robust long-distance efferent graft projections, require placement of fetal cells in close proximity to appropriate axon guidance pathways. Intracerebroventricular administration of kainic acid in adult rats leads to a specific loss of hippocampal CA3 pyramidal neurons. We grafted 5'-bromodeoxyuridine-labeled embryonic day 19 hippocampal cells into adult hippocampus at four days post-kainic acid lesion, and quantitatively measured the projection of grafted cells into the contralateral hippocampus and the septum after three to four months survival using Fluoro-Gold and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (Dil) tracing. Grafts located in or near the degenerated CA3 cell layer exhibited numerous neurons which established commissural projections with the contralateral hippocampus. However, such projection did not occur in intrahippocampal grafts located away from the CA3 cell layer. In contrast, neurons in all grafts established robust projections into the septum regardless of location within hippocampus although grafts located near the degenerated CA3 cell layer displayed more neurons with such projections. Location of grafted cells clearly influences the development of efferent graft projections into distant targets in the adult host brain, particularly access to axon guidance pathways to facilitate the formation of projections. The establishment of robust long-distance commissural projections of fetal hippocampal grafts is clearly dependent on their placement in or near the degenerated CA3 cell layer, suggesting that appropriate axon guidance pathways for commissural pathways are tightly focussed near this cell layer. However, the establishment of septal projections of these grafts was not dependent on specific location within the CA3 cell layer, suggesting that axonal guidance mechanisms to the septum are more diffuse and not limited to the CA3 dendritic layers. The results underscore that fetal hippocampal grafts are capable of partly restoring lesioned hippocampal circuitry in adult animals when appropriately placed in the host hippocampus.

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.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

Neural grafting of cholinergic neurons in the hippocampal formation

The cholinergic septohippocampal system plays an important role in spatial learning and memory functions. Transections of the septohippocampal pathway have been shown to result in a near complete loss of cholinergic innervation in the hippocampus and induce severe spatial memory impairments. In this article, we have reviewed the studies which demonstrate the ability of intrahippocampal septal grafts to reinnervate the hippocampal formation and ameliorate spatial learning and memory deficits. Neuroanatomical studies suggest that grafts of cholinergic tissue can innervate the host hippocampal formation in a pattern that mimics that of the normal septohippocampal pathway. This innervation, in turn, is associated with the formation of graft-to-host synaptic connections. Neurochemical studies reveal that intrahippocampal grafts of septal cells can restore choline acetyltransferase activity, acetylcholine synthesis, and high affinity choline uptake in presynaptic terminals of grafted neurons. In addition, these grafts can normalize the upregulation of cholinergic muscarinic receptors seen postsynaptically in the hippocampus following lesions of the septohippocampal pathway. The functional nature of these grafts is also substantiated by electrophysiological recordings which demonstrate stimulus-evoked graft-to-host synaptic transmission as well as the reinstatement of EEG activity typical of septohippocampal connectivity. In addition to graft-to-host connections, behavioral and neurochemical studies also provide evidence for host-to-graft connections that can regulate the activity of grafted cholinergic neurons during the performance of specific behavioral tasks requiring spatial memory function. Together, these studies suggest that grafts of cholinergic neurons from the medial septal nucleus can become anatomically and functionally incorporated into the circuitry of the host hippocampal formation.

Differential regenerative response of Purkinje cell and inferior olivary axons confronted with embryonic grafts: environmental cues versus intrinsic neuronal determinants

Regeneration of severed central axons is supposed to depend on two factors: a permissive local environment and the particular intrinsic properties of axotomized neurones. To assess the role of each of these factors in axonal regeneration, the capability of two particular axon populations of the adult mouse cerebellum to grow into target-specific (cerebellum) and target-unspecific (neocortex) embryonic grafts was determined. Purkinje cell and inferior olivary axons were transected by passing a microscalpel through the axial white matter of the cerebellar folia, particularly those of the anterior lobe. Immediately after the injury, solid transplants were placed in the lesion cavity. Purkinje cell axons were labelled by using anticalbindin immunocytochemistry, and olivocerebellar fibres were visualized by biotinylated dextran amine anterograde axonal tracing. Following axotomy, Purkinje cell axons appeared as thickened processes ending with large terminal clubs. Their morphology and number did not change up to the longest survival time considered (2 months), thereby confirming previous demonstrations that Purkinje cells survive axon injury (I. Dusart and C. Sotelo, 1994, J. Comp. Neurol. 347:211-232). Inferior olivary axons were thinner and bore smaller terminal bulbs. When embryonic cerebellar grafts, containing cortical and deep nuclear precursors, were placed close to the injured axons, olivocerebellar fibres vigorously regenerated into the transplants and ended in new climbing fibres along the dendrites of grafted Purkinje cells. By contrast, host Purkinje cell axons never showed any outgrowth towards the graft. Similarly, these axons failed to regenerate into grafts containing solely the rostromedial portion of the cerebellar anlage, mostly consisting of deep nuclear neurones, their main targets. Comparable results were obtained by transplanting embryonic neocortical tissue: inferior olivary axons also regenerated into the grafts, although with distinct terminal arbours without the climbing fibre phenotype, whereas Purkinje cell axons always failed to grow. These results provide the first direct demonstration that severed inferior olivary axons are able to regenerate. In addition, they show that the growth-permissive/-promoting conditions created by embryonic nervous tissue are not sufficient to induce the regeneration of every axonal type and allow us to hypothesise that successful regeneration depends on the interplay between environmental cues and intrinsic properties of the axotomized neurones.

Behavioral effects of basal forebrain grafts after dorsal septo-hippocampal pathway lesions

There are many reports that basal forebrain grafts ameliorate behavioral impairments produced by dorsal septo-hippocampal pathway lesions, but several studies have either found that this recovery may be unrelated to concomitant restitution of cholinergic markers, may be modest and depend on certain experimental conditions or instead that grafts may actually exacerbate lesion-induced impairments. In this study, rats received one of three lesions of the dorsal septo-hippocampal pathways or a sham lesion, at 32 days of age, and intrahippocampal basal forebrain grafts or the vehicle control 10 days later. In grafted rats with total aspirative lesion of the fimbria-fornix, there was a substantial AChE-positive hippocampal reinnervation but no improvement of the severe lesion-induced spatial learning deficits, either reference memory or working memory, whether tested at 1 or 5 months post-grafting. In rats with bilateral medial fimbria lesions, grafts were successful, normal in appearance and produced substantial hippocampal cholinergic reinnervation; relative to non-grafted counterparts, however, grafted medial fimbria rats showed an early reference memory impairment and a persistent exacerbation of a working memory deficit. Exacerbation of learning impairments was also apparent in grafted rats with partial hippocampal denervation due to lesion of the cingulate and adjacent cortex above the fimbria-fornix. Nonetheless, basal forebrain grafts normalised general activity in these lesion groups, irrespective of whether the lesion-induced change was an increase or a decrease relative to controls. Graft-derived lesion groups, irrespective of whether the lesion-induced change was an increase or a decrease relative to controls. Graft-derived AChE-positive innervation was more marked than expected in both grafted cingulate-lesioned rats and grafted sham-lesioned rats, while control grafts of fetal cortex (above the septum) produced little or no AChE-positive innervation. Size of basal forebrain grafts, originally 3 microliters at two dorsal sites per hippocampus, increased markedly from rostral to caudal dorsal hippocampus in all groups but did not differ significantly across grafted groups, even with respect to non-lesioned rats. This study adds further evidence that basal forebrain grafts, successful with respect to cholinergic reinnervation, do not always enhance cognitive functions in rat hippocampal lesion models, and confirms that these grafts may have adverse effects after partial septo-hippocampal system lesions. It is important to attend to both the potential negative and positive effects of neural grafts.

Effects of target depletion on adult mammalian central neurons: functional correlates

The physiological signals and patterns of synaptic connectivity that CNS neurons display after the loss of their target cells were evaluated in adult cats for one year. Abducens internuclear neurons were chosen as the experimental model because of their highly specific projection onto the medial rectus motoneurons of the oculomotor nucleus. Selective death of medial rectus motoneurons was induced by the injection into the medial rectus muscle of ricin, a potent cytotoxic lectin that leaves the presynaptic axons intact. The electrical activity of antidromically identified abducens internuclear neurons was recorded in chronic alert animals, during both spontaneous and vestibularly induced eye movements, before and after target removal. During the three weeks that followed ricin injection, abducens internuclear neurons exhibited several firing-related abnormal properties. There was an overall reduction in firing rate with a corresponding increase in the eye position threshold for recruitment. In addition, neuronal sensitivities to eye position and velocity were significantly decreased with respect to control data. Bursting activity was also altered since low-frequency delayed burst accompanied the saccades in the on-direction and, occasionally, internuclear neurons exhibited low-frequency discharges associated with off-directed saccades. Intracellular recordings carried out seven and 15 days after ricin injection demonstrated no significant changes in their electrical properties, although a marked depression of synaptic transmission was evident. The amplitude of both excitatory and inhibitory postsynaptic potentials of vestibular origin was reduced by 60-85% with respect to controls. However, postsynaptic potentials recorded one month after ricin injection showed normal amplitude values which persisted unaltered one year after target loss. Recovery of synaptic transmission occurred at the same time as the re-establishment of normal eye-related signals in the discharge pattern of abducens internuclear neurons recorded in alert cats from days 25-30 post lesion. The functional restoration of firing properties was maintained in the long term (one year). Conversely, abducens motoneurons showed normal firing and synaptic patterns at all time intervals analysed. These results demonstrate that, after an initial period of altered physiological properties, abducens internuclear neurons survive the loss of their target motoneurons and regain a normal discharge pattern and afferent synaptic connections.

Temporal and spatial patterns of expression of laminin, chondroitin sulphate proteoglycan and HNK-1 immunoreactivity during regeneration in the goldfish optic nerve

Current views suggest that the extracellular environment is critically important for successful axonal regeneration in the CNS. The goldfish optic nerve readily regenerates, indicating the presence of an environment that supports regeneration. An analysis of changes that occur during regeneration in this model may help identify those molecules that contribute to a favourable environment for axonal regrowth. We examined the distribution and expression of two extracellular matrix molecules, laminin and chondroitin sulphate proteoglycan, and a carbohydrate epitope shared by a family of adhesion molecules (HNK-1), using immunocytochemical detection in sections from the normal adult goldfish optic nerve and in nerves from one hour to five months following optic nerve crush. We also used in vitro preparations to determine if neurites in retinal explants could express these same molecules. The linear distributions of laminin and chondroitin sulphate proteoglycan immunoreactivity in control optic nerves are co-extensive with the glia limitans, suggesting both are expressed by non-neuronal components surrounding the axon fascicles. Between one and three weeks postoperatively when axons elongate and reach their target, laminin and chondroitin sulphate proteoglycan immunoreactivity increases around the crush site and distally. At six weeks postoperatively the pattern of immunoreactivity has returned to normal. While the temporal pattern of changes in immunoreactivity is similar, the spatial pattern of these two extracellular proteins in the regenerating nerve differs. Chondroitin sulphate proteoglycan immunoreactivity is organized in discrete columns associated with regenerating axons while laminin immunoreactivity is more diffusely distributed. Examination of retinal explants reveals growing neurites express chondroitin sulphate proteoglycan but not laminin. Our results suggest that laminin is only associated with non-neuronal cells, while chondroitin sulphate proteoglycan is associated with axons as well as non-neuronal cells. HNK-1 immunoreactivity is co-extensive with both the glia limitans and axon fascicles and is more extensively distributed in the intact nerve than either laminin or chondroitin sulphate proteoglycan immunoreactivity. In contrast to laminin and chondroitin sulphate proteoglycan, HNK-1 immunoreactivity is substantially decreased at the crush site within one week following optic nerve crush. HNK-1 immunoreactivity reappears through the crush site during the next several weeks, although non-immunoreactive regions, co-extensive with areas predominantly containing non-neuronal cells, persist both proximal and distal to the crush, up to six weeks postoperatively. The pattern suggests that HNK-1 epitope expression by these non-neuronal cells is decreased during axonal regeneration.(ABSTRACT TRUNCATED AT 400 WORDS)

Fetal neocortical grafts implanted in adult hypertensive rats with cortical infarcts following a middle cerebral artery occlusion: ingrowth of afferent fibers from the host brain

This study is focused on the survival of fetal neocortical grafts placed in the infarcted adult host cortex of the spontaneously hypertensive rat and describes the ability of host axonal regeneration into the graft after a focal ischaemic lesion. Five to seven days following ligation of the right middle cerebral artery, dissociated neocortical primordium from fetuses of gestational age 12-18 days was implanted into the infarcted cortical area. Surviving transplants were seen in all rats, although grafts derived from gestational age 12-14 days displayed an irregular morphology rich in sinusoid-like cavities and containing fewer cells of apparently mature neuronal morphology. Grafts from older donors contained perikarya of neuronal appearance; however, they lacked normal cortical lamination. Ten days postgrafting, fibers stained by acetylcholinesterase histochemistry, dopamine-beta-hydroxylase, and 5-hydroxytryptamine immunohistochemistry were found in the grafts, and by 10-23 weeks after transplantation the fiber density had increased substantially. When the retrograde tracer Fluoro-Gold was injected into the grafted tissue, labeled cells were found in several subcortical nuclei of the host, including the nucleus basalis of Meynert, ventral pallidum, thalamus, dorsal raphe, locus coeruleus, as well as the ipsilateral and contralateral neocortex. This study shows that grafts of dissociated neocortical tissue exhibit good survival and growth potential when implanted into infarcted neocortex and that several nerve fiber systems of the adult host have a regenerative capacity sufficient to innervate the grafted tissue.

Effects of delayed transplantation of cultured Schwann cells on axonal regeneration from central nervous system cholinergic neurons

The introduction of transplants consisting of cultured Schwann cells and their associated extracellular matrix (Sc/ECM) into a central nervous system (CNS) lesion cavity facilitates axonal regeneration from injured, adult mammalian neurons with subsequent reinnervation of their appropriate target (Kromer and Cornbrooks: Proceedings of the National Academy of Sciences of the United States of America 82:6330-6334, 1985). In the present study, the effects of a delayed transplantation procedure on the time course of this regenerative response were evaluated. For these experiments, bilateral CNS lesions were created between the septum and hippocampus by removing the fimbria-fornix pathway. Lesion cavities received either no transplants, transplants of collagen, or Sc/ECM transplants at the time the lesion was created or 6 days later. When no transplants or transplants of collagen were used, axonal sprouts extended for very short distances into the lesion cavity. These axons were not preferentially associated with the collagen transplants nor maintained at long post-lesion survival times. In animals that received Sc/ECM transplants, the number of sprouting axons and the progression of axonal growth along the transplants was much more extensive than for the collagen transplants. Although more axons were detected in cavities that received transplants immediately after the fimbria-fornix lesion, axonal regeneration along the transplants was similar regardless of whether there was a delay in transplanting the Schwann cells. By using histochemical techniques to identify acetylcholinesterase (AChE), regenerating AChE-positive axons were first detected in the cavity at 3 days post-transplantation, were associated with the Sc/ECM transplants by 5 days, and crossed the cavity within 8 days post-transplantation. Regenerating, neurofilament-positive axons crossed the CNS-Sc/ECM transplant interfaces in association with laminin-positive, glial fibrillary acidic protein-positive cellular pathways. Upon reaching the caudal end of the Sc/ECM transplant, the cholinergic axons abandoned the transplant and oriented directly toward the adjacent hippocampus. Both the simultaneous and delayed transplantation paradigms demonstrated a similar reinnervation pattern of AChE-positive fibers in the hippocampus, but there was a more rapid penetration and more extensive arborization of fibers in animals receiving the delayed transplants. Cholinergic fibers initially invaded the dentate gyrus molecular layer and hilus between 8 and 14 days post-transplantation. By 45 days post-transplantation, AChE-positive axons were detected throughout the dentate gyrus and regio inferior, but few fibers were present in regio superior of the hippocampus.(ABSTRACT TRUNCATED AT 400 WORDS)

Reactive astrocytes are substrates for the growth of adult CNS axons in the presence of elevated levels of nerve growth factor

To assess the necessary parameters for the growth of axons within the adult rat CNS, we have used intracerebral grafts of primary fibroblasts genetically engineered to express nerve growth factor (NGF). Following the implantation of NGF-producing primary fibroblasts within the striatum, cholinergic axons arising from the nucleus basalis grow toward and penetrate these grafts between 1 and 8 weeks. Grafts of noninfected control cells do not elicit axon sprouting at any time. Unmyelinated axons grow into grafts of NGF-producing cells only on reactive astrocytic processes, which contribute to a surrounding glial border. From our data concerning axon growth within the adult rat CNS, we conclude that reactive astrocytes can act as conducive substrates for growing axons; and only in the presence of elevated levels of NGF will permissive substrates (e.g., astrocytes) support axon growth by NGF-sensitive neurons.

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)

Transient and long-lasting beneficial behavioral effects of grafts in the damaged hippocampus of rat

One week after receiving bilateral neurotoxic lesions of their dorsal hippocampi, adult Wistar rats were bilaterally grafted with fetal hippocampal tissue suspensions. The behavior of the animals was tested during a period of 5 months after grafting to determine changes in lesion-induced deficits. The transplants caused various behavioral effects with different time courses. Grafted animals showed an early, however, transient amelioration of behavioral deficits in a T-maze alternation task and they performed with a long-lasting improvement in the alcove-test. Transplant histology demonstrated high levels of AChE-activity in patches correlating with clusters or rudimentary layers of pyramidal neurons.

Object recognition memory in the rat: the role of the hippocampus

Object recognition memory of rats with fimbria-fornix or ventral temporal lesions was evaluated with a behavioral protocol (delayed non-matching-to-sample task with trial-unique stimuli) similar to that used to test recognition functions in primates. Animals with damage to the hippocampal system showed no evidence of lasting impairment on the object recognition task with retention intervals up to 30 s. In contrast, rats with fimbria-fornix lesions displayed severe and enduring deficits on a test of spatial memory, i.e. rewarded alternation, with but 5 s delays. These results provide further evidence that a dissociation exists between the types of memory that are and are not lost following damage to the hippocampus. Whereas the hippocampus is necessary for some types of mnemonic processes, other types of recognition functions (e.g. perceptual recognition) may be fully mediated in regions of sensory and/or association neocortex without the involvement of the hippocampus.

Regeneration of Lesioned Septohippocampal Acetylcholinesterase-positive Axons is Improved by Antibodies Against the Myelin-associated Neurite Growth Inhibitors NI-35/250

Two oligodendrocyte membrane proteins, NI-35 and NI-250, have been shown to be highly inhibitory for neurite growth. Upon neutralization of these components with the specific monoclonal antibody IN-1, lesioned corticospinal tract fibres were able to regenerate over long distances. In the present study, we have investigated the behaviour of regenerating cholinergic septohippocampal tract fibres. Large fimbria/fornix aspiration lesions were bridged by human amnion extracellular matrix material containing nerve growth factor, and the inhibitor-neutralizing antibody IN-1 or a control antibody were applied. After 3 - 5 weeks survival time, acetylcholinesterase (AchE)-positive fibres had crossed the bridge and, upon entering the hippocampus, had developed a profuse fibre plexus. In the controls (antibody against peroxidase) the fibre growth within the hippocampal tissue remained limited to maximally 1 mm in the caudal and lateral directions. In the presence of the antibody IN-1, however, AchE-positive fibres were seen to grow for 2 - 4 mm both in the caudal and lateral directions. Interestingly, the regenerated fibres preferably grew to their original terminal areas in the infra- and suprapyramidal layers of the hippocampus proper and the hilus, and in the supragranular layer of the dentate gyrus. These data show that the neurite growth inhibitors severely impede regenerative axon growth also for the cholinergic fibres in the hippocampus, and that their neutralization increases axon growth and leads to partial reconstitution of the original anatomical fibre distribution.

Proximity as a factor in the innervation of host brain regions by retinal transplants

Embryonic mouse retinae transplanted to a variety of locations within the rostral midbrain of neonatal rats exhibit selective innervation of host visual nuclei when studied at maturity. Some of these nuclei (superior colliculus, nucleus of the optic tract, dorsal terminal nucleus) usually receive extensive transplant projections, others are innervated partially (dorsal division of the lateral geniculate nucleus, olivary pretectal nucleus, medial terminal nucleus), while a few (ventral division of the lateral geniculate nucleus, suprachiasmatic nucleus, intergeniculate leaflet) are not innervated at all. The selectivity of this innervation is largely independent of the transplant's position within the rostral brainstem, while the density of innervation of individual nuclei depends in part upon the proximity of the transplant to the nucleus and upon whether the host retinal projection to that nucleus is present or absent. These findings provide a foundation for further studies of the behavioral capabilities of retinal transplants, for developmental studies of factors responsible for the establishment of normal neural projections, and for examination of the immunological consequences of transplantation.

NGF released from a polymer matrix prevents loss of ChAT expression in basal forebrain neurons following a fimbria-fornix lesion

Following a unilateral fimbria-fornix lesion, the delivery of nerve growth factor (NGF) to the ipsilateral lateral ventricle of the rat can prevent the lesion-induced loss of choline acetyltransferase (ChAT) expression in the ipsilateral medial septum and vertical diagonal band region. In the present study, the ability of polymer rods to deliver NGF and to prevent a decrease in basal forebrain ChAT expression following a fimbria-fornix lesion was assessed. NGF was loaded into an ethylene vinyl acetate copolymer (EVAc) rod, fabricated by a melt-extrusion process. NGF release was established by the ability of the rods to induce neurite extension from PC12 cells and chick E12 dorsal root ganglia. Unilateral aspirative lesions of the fimbria-fornix were performed in adult rats, followed by implantation of a polymer rod into the ipsilateral lateral ventricle. Five animals received EVAc rods containing only the carrier molecule bovine serum albumin (BSA), and six received EVAc rods containing both BSA and NGF. After 2 weeks, ChAT-positive cells were counted in the medial septum and vertical diagonal band regions. Rats with NGF-releasing rods displayed ChAT(+) cell counts ipsilateral to the lesion equal to 88% of those on the contralateral side. In contrast, ChAT(+) cell numbers were 42% in animals with rods releasing BSA only (P less than 0.001). No undue reaction to implanted rods was noted. Following a fimbria-fornix lesion, NGF released from polymer matrices effectively prevents a lesion-induced reduction in ChAT expression in basal forebrain neurons.

New method of transplanting purified glial cells into the brain

This paper describes a new transplantation method for testing the ability of purified populations of glial cells to support axonal growth in the brains of adult animals. Thin tubes, rolled from porous polycarbonate film, are coated with poly-L-lysine and filled with cultured Schwann cells. Schwann cell-filled tubes or control tubes (poly-L-lysine coated only) are then implanted into the brains of adult rats so that one end of the tube is in the thalamus and the other extends extracranially. After survival times of 4-16 weeks horseradish peroxidase (HRP) is applied to the extracranial end of the tube. One or two days later the animal is perfused and the brain is sectioned and processed histochemically. Results show that tubes containing Schwann cells are densely filled with tissue and are well vascularized. Further, neurons in the central nervous system are retrogradely labeled with HRP and most labeled cells are concentrated in regions of the diencephalon near the end of the tube. Control tubes contain very little tissue and show no evidence that they support axonal growth. These results are consistent with the hypothesis that Schwann cells can support axonal growth in the brains of adult rats.

The role of fibroblast growth factors in the central nervous system

The fibroblast growth factors are well-characterized mitogens that are found in the central nervous system (CNS). Their physiological roles are not yet known, but increasing evidence suggests their involvement in CNS development, injury responses, and possibly oncogenesis.

Thalamic retrograde degeneration following cortical injury: an excitotoxic process?

Traumatic or stroke-like injuries of the cerebral cortex result in the rapid retrograde degeneration of thalamic relay neurons that project to the damaged area. Although this phenomenon has been well documented, neither the basis for the relay neuron's extreme sensitivity to axotomy nor the mechanisms involved in the degenerative process have been clearly identified. Physiological and biochemical studies of the thalamic response to cortical ablation indicate that pathological overexcitation might contribute to the degenerative process. The responses of thalamic projection neurons, protoplasmic astrocytes, and inhibitory thalamic reticular neurons in adult mice were examined from one to 120 days following ablation of the somatosensory cortex as part of an investigation of the role of excitotoxicity in thalamic retrograde degeneration. The responses of thalamic neurons to cortical ablation were compared with those produced by intracortical injection of the convulsant excitotoxin kainic acid, since the degeneration of neurons in connected brain structures distant to the site of kainic acid injection is also thought to occur via an excitotoxic mechanism. Within two days after either type of cortical injury, protoplasmic astrocytes in affected regions of the thalamic ventrobasal complex and the medial division of the posterior thalamic nuclei became reactive and expressed increased levels of immunohistochemically detectable glial fibrillary acidic protein. Within the affected regions of the ventrobasal complex an increased intensity of puncta positive for glutamate decarboxylase immunoreactivity, presumably due to an increase in its content within the terminals of the reciprocally interconnected thalamic reticular neurons, was also evident. These immunohistochemically detectable alterations in the milieu of the damaged thalamic neurons preceded the disappearance of the affected relay neurons by at least two days following cortical ablation and by seven to 10 days following intracortical kainic acid injection. Regions of the thalamus containing reactive astrocytes corresponded very closely to the regions undergoing retrograde degeneration. Protoplasmic astrocytes in these areas remained intensely reactive up to 60 days after cortical injury. Levels of glutamate decarboxylase were only transiently elevated in the degenerating regions of the ventrobasal complex following cortical ablation and returned to normal by 14 days. Increased glutamate decarboxylase immunoreactivity was transiently seen through the entire ventrobasal complex following intracortical kainic acid injection but was markedly more intense in degenerating regions. These patterns of labeling did not return to normal until 50 days after intracortical kainic acid injection, well after the death of the relay neurons. Cortical ablation and intracortical kainic acid injection produce similar alterations in thalamic neuronal and glial populations.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Fetal brain grafts rescue adult retinal ganglion cells from axotomy-induced cell death

After intraorbital transection of the optic nerve of adult rats, 90% of the retinal ganglion cells die within 30 days. Since fetal brain extracts and cocultured fetal target regions support the survival of retinal ganglion cells in vitro (Nurcombe and Bennett: Exp. Brain Res. 44: 249-258, '81; McCaffery et al.: Exp. Brain Res. 48: 377-386, '82; Armson and Bennett: Neurosci. Lett. 38: 181-186, '83) we investigated whether cell death in the adult retina could be prevented by transplanting fetal (E16) thalamus and tectum to the proximal stump of the optic nerve of adult rats that was completely transected 2-3 mm behind the optic disc. Unoperated eyes contained 119,973 (+/- 939, SEM) retinal ganglion cells, estimated from axon counts of the intact optic nerve. Of these, 11,601 (+/- 1,857) remained in control operated eyes at 30 days postoperation while in the eyes of grafted rats, 35,086 (+/- 2,278) retinal ganglion cells were counted. Thus, 23,485 (= 22% of those normally dying after transection of the optic nerve) ganglion cells were rescued by the fetal grafts from cell death normally following axotomy. These results indicate that fetal target regions of retinal ganglion cells contain and/or produce neurotrophic molecules that promote the survival of adult axotomized retinal ganglion cells.

Regeneration of axons from the adult rat optic nerve: influence of fetal brain grafts, laminin, and artificial basement membrane

After transection of the optic nerve of adult rats, most of the axons in the proximal stump die and the surviving ones are unable to regenerate into the distal optic nerve. Since the fetal brain has an inherent capacity to regenerate axons, we investigated whether fetal (E16) target regions of optic axons (thalamus and tectum) transplanted to the completely transected optic nerve of adult rats would promote axon regeneration. In control operated rats, axon growth beyond the site of transection was restricted to a few fibers that grew irregularly within the connective tissue scar. By contrast, in grafted animals directed outgrowth of optic axons toward the transplant started at 6 days postoperation (p.o.) and reached its maximum 15 days p.o. and later, when numerous single optic fibers and small axon fascicles had grown toward and into the graft, where they formed arborizations and terminal varicosities. Regenerating optic axons were further advanced than GFAP-positive strands of astroglia that emanated from the proximal optic nerve stump. Laminin immunoreactivity appeared at 6 days p.o. in the zone of reactive astroglia in the terminal part of the optic nerve stump. Later it showed a distribution complementary to the pattern of GFAP immunoreactivity, which it seemd to circumscribe. There was no unequivocal codistribution of laminin immunoreactivity with regenerating axons. In further experiments, target regions from different ontogenetic stages (E14 to neonate and adult) and nontarget regions (E16, cerebral cortex or spinal cord) were grafted to the optic nerve stump. With the exception of the adult grafts, all transplants had effects on axon regeneration comparable to those of E16 target regions. In order to test the effects of extracellular matrix molecules on axon regeneration, a basement membrane gel reconstituted from individual components of the Engelbreth-Holm-Sarcoma (EHS) sarcoma was implanted between proximal and distal optic nerve stumps. No axons were induced to regenerate by this matrix. Likewise, laminin adsorbed to nitrocellulose paper and implanted at the lesion site did not stimulate axon growth from the proximal optic nerve stump. These results indicate that fetal brain is able to induce and direct regrowth of axons from the optic nerve toward the graft across a substrate that is not composed of astroglia or basement membrane components like laminin. The directed growth of axons in the absence of a preformed substrate implies a chemotactic growth response along a concentration gradient mediated by neurotropic molecules released from the graft.

Interactions between donor and host tissue following cross-species septohippocampal transplants

Interactions between donor and host tissues following xenogeneic transplantation were studied using the neural cell surface antigen, Thy 1.2, as a marker for the donor tissue. Dissociated septal cells from Thy 1.2-positive fetal mice were transplanted to the dentate gyrus of Thy 1.2-negative adult rats. At post-transplantation survival times between 1 and 5 months, an antibody to Thy 1.2 was used to identify donor tissue. The results demonstrate that the donor tissue was capable of migrating and developing within the host following transplantation. Thy 1.2-positive cells and processes were consistently found within the supragranular, infragranular, and molecular layers of the dentate gyrus, and occasionally within the hilus, suggesting that mechanisms existed within the host which influenced the development of the transplanted tissue. Additionally, the survival and growth of the Thy 1.2-positive neurons differed from previous reports describing the growth of acetylcholinesterase (AChE)-positive fibers from xenogeneic transplants. This finding suggested that in addition to growing within the host, xenogeneic transplants may also stimulate a compensatory sprouting response from the host.

Fetal brain grafts induce recovery of learning deficits and connectivity in rats with gustatory neocortex lesion

Three groups of rats showing disrupted taste aversion due to gustatory neocortex lesions, were studied. One group received a transplant of homotopic cortical tissue, another of heterotopic tectal tissue, obtained from 17-day-old fetuses. The third group remained without transplant as a lesioned control group. Comparisons of the taste aversion scores before and after graft, revealed that cortical grafted animals significantly improved the taste aversion, whereas those which received tectal grafts, and the cortical-lesioned controls did not. Moreover, results with horseradish peroxidase (HRP) histochemistry revealed that the homotopic, but not the heterotopic, brain transplants were able to re-establish connections with amygdala and with the ventromedial nucleus of the thalamus areas who normally kept connectivity with the gustatory neocortex. These results support the hypothesis that fetal brain transplants can reestablish cognitive functions, as well as connectivity with its host tissue.

Development of the septohippocampal projection in vitro

Slices were prepared from septal and hippocampal tissue and co-cultured for periods up to one month. The presence of cholinergic neurons within the septal slices was demonstrated by histochemical staining techniques for acetylcholinesterase or by Golgi-like immunoperoxidase techniques with antibodies raised against the enzyme choline acetyltransferase. Cholinergic fibers originating in the septal explants started to grow radially in all directions. By day 7, the first fibers were seen to reach their target, but maximal hippocampal ingrowth occurred between day 8 and 14 in vitro. Only those fibers reaching the target were maintained, whereas cholinergic fibers growing in other directions degenerated. Electrophysiological studies showed that cholinergic fibers established functional cholinergic connections with hippocampal pyramidal cells. As a result of septal stimulation, two different potassium currents were inhibited in pyramidal cells: a calcium-independent current, IM, and a calcium-dependent current, IAHP, underlying spike afterhyperpolarization. Application of nerve growth factor (NGF) strongly increased the number of cholinergic fibers which invaded the hippocampal slices and raised the activities of the cholinergic enzymes choline acetyltransferase and acetylcholinesterase, effects which were completely blocked by anti-NGF antibodies. The response of septohippocampal co-cultures to NGF depended on the time of application. During the first two weeks in vitro, NGF elicited sustained increases in enzyme activities, whereas later administration of NGF produced effects which were only maintained for several days.

Organization of Host Afferents to Cerebellar Grafts Implanted into Kainate Lesioned Cerebellum in Adult Rats

This paper examines the organization of host afferents within cerebellar grafts implanted into kainic acid lesioned cerebellum. Our selection of a cerebellum, a prime example of a 'point-to-point' system, permits precise determination of the degree and the specificity of host-graft interactions. One month after a cerebellar injection of kainic acid, the lesion produced can be divided into two concentric regions: (i) a central necrotic zone, totally depleted of neurons (zone 1), and (ii) a peripheral zone which lacks all Purkinje cells but preserves its cortical lamination (zone 2). Two months after the implantation of solid pieces of embryonic cerebellum, the graft has evolved into a minicerebellar structure, occupying most of zone 1. The grafted minicerebellum consists of a highly convoluted trilaminated cortex with a core containing deep nuclear neurons. Purkinje cells are positioned between the molecular and granular layer with their short and irregular dendrites branching within the former. Donor foetal Purkinje cells migrate into the contiguous portion of the molecular layer of the host zone 2. These embryonic neurons set up within the upper three-quarters of the host molecular layer, and develop monoplanar dendritic trees that span the whole width of the layer. The organization of host-graft interactions was studied by autoradiography of anterogradely transported tritiated leucine, injected in the host bulbar region containing the caudal half of the inferior olivary complex (origin of all vermal climbing fibres) and the dorsally adjacent paramedian reticular nucleus (origin of a few mossy fibres). Numerous labelled fibres cross the host-graft interface from the white matter of the host cerebellum, and provide innervation to the minicerebellar structure. The vast majority of these labelled axons terminate in the molecular layer, forming axonal arborizations that follow the shape of the Purkinje cell dendrites. The labelled climbing fibres are organized into uneven sagittally aligned strips, which mimic that of olivocerebellar projections in control rats. Only a small proportion of host labelled fibres end in the donor granular layer, forming typical mossy fibre rosettes. The latter are present in the region of the graft close to the host-graft interface. In addition, labelled axons are observed climbing over the dendritic trees of grafted Purkinje cells that have invaded a portion of the host molecular layer of zone 2. In all regions containing grafted Purkinje cells and labelled climbing fibres, the density of the innervation is close to normal with practically all Purkinje cells receiving a climbing fibre. The extensive integration of the grafted cells into the deficient neuronal networks of the host clearly illustrates the positive neurotropic effect exerted by immature cerebellar neurons on adult extracerebellar afferent fibres. The hodological integration, allowing a possible restoration of the impaired cerebellar circuitry, takes place respecting the specificity and topographic distribution which characterize the 'point-to-point' arrangement of normal cerebellar circuitry.

Neuronal changes in fetal cortex transplanted to ischemic adult rat cortex

Fetal cortex from 16- and 17-day-old embryonic rats was transplanted into the parietal cortex of 12 adult rats rendered ischemic by temporary intraluminal occlusion of the middle cerebral artery. Ischemic injury in the host cortex adjacent to all nine surviving transplants was demonstrated with hematoxylin and eosin and cresyl violet strains. Nicotidamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemical studies revealed a normal number of NADPH-d-positive neurons, whereas acetylcholinesterase (AChE) staining revealed many more AChE-positive neurons in the transplants compared to the host parietal cortex. This could be due to: 1) selective survival of AChE neurons in the transplants compared to the host cortex; 2) increased expression of AChE in transplanted neurons; 3) induction of AChE in normally AChE-negative neurons; or 4) decreased transport of the AChE enzyme from the perikarya to fibers in surviving transplanted neurons. Many fibers positive for AChE and NADPH-d crossed between the host and transplant, although fiber density in the transplants was less than in normal host cortex. These results should encourage future investigation of whether similar transplants improve neurological function following experimental stroke.

Ultrastructural evidence for the development of adrenal medullary grafts in the brain

This study shows that mouse mature chromaffin cells can elaborate neurite-like fibers and became integrated with the host brain. A piece of adrenal medulla, with or without attached adrenal cortical tissue, was implanted into the subarachnoid space or the hippocampal formation and examined using the electron microscopy. One week after transplantation, chromaffin cells could be observed surrounded by a basal lamina, containing many dense-cored vesicles 100-280 nm in diameter, including synaptic-like vesicles, which tended to gather in the cytoplasmic area or processes. The cells were irregularly shaped and bore cytoplasmic processes which sometimes ended with thick growth cone-like structures. The Golgi complex seemed to be well developed, suggesting the synthesis of new storage vesicles. One month after transplantation, the vast majority of chromaffin cells showed the noradrenaline phenotype typical of noradrenaline-storing cells in the normal gland, irrespective of graft components used or implantation sites. Some cells, presumably corresponding to the adrenaline phenotype, had secretory vesicles (140-210 nm in diameter) with denser cores than in the adrenaline-storing cells of normal gland. In the subarachnoid space, both types of graft had mostly cuboid chromaffin cells which bore a few, short, blung cytoplasmic processes. In the intracerebral transplants, the chromaffin cells of cortex-free adrenal medullary grafts developed processes having the characteristics of neurites extending from the chromaffin cells, in contrast to their counterparts with attached adrenocortical tissue. Thin sections through both types of graft showed isolated nerve cells, morphologically similar to sympathetic neurons, in the neighbourhood of the chromaffin cells. Reinnervation of the chromaffin cells was frequently observed in cortex-free implants. The integration of these grafts in the host brain is strongly suggested.

Time course of neocortical graft innervation by AChE-positive fibers

Acetylcholinesterase (AChE)-containing axons are the only extrinsic fibers projecting to the adult cortex that readily innervate embryonic cortical grafts up to normal densities without prior manipulation of the host brain. In the present paper we compare the time course of AChE-positive fiber innervation in the normal mouse cortex with that seen in neocortical grafts by using AChE histochemistry as a marker for presumed cholinergic fibers. Donor tissue was taken at two different stages of gestation; before (embryonic days 12-14, or E12-14) and after (E17-19) the cortical plate is formed. Three features are analyzed: 1) the distribution and density of AChE-containing fibers, 2) the presence of AChE-positive cells, and 3) the distribution of butyrylcholinesterase (BuChE)-positive elements. The modification of Koelle's method used for AChE localization showed AChE-positive fibers in developing parietal neocortex as early as E18-19. The distribution of AChE-labeled fibers in the normal cortex achieves the mature pattern by the end of the third postnatal week. The rate of innervation of transplants takes longer and depends on the age of the donor tissue. Tissue from both donor ages first showed AChE-positive fibers crossing the host-transplant interface by 7 days postsurgery. E17-19 tissue approaches the density of AChE-positive fibers in the normal adult cortex by 15 weeks after grafting, whereas the E12-14 donor tissue does not approach normal innervation densities until after 20 weeks. While the degree of innervation in the E12-14 donor tissue never equalled the surrounding adult cortex within our range of survival times, a few of the E17-19 transplants did develop densities equal to that of the host cortex. AChE-positive cells are first detectable in the normal parietal cortex on the day of birth, peak by the end of the first postnatal week, and then decline in number to the low levels of the mature cortex after the second postnatal week. Grafted cells in E12-14 tissue stain lightly for AChE by 7 days postsurgery, achieve maximal densities by 3 weeks, and become markedly reduced in number and density by 10 weeks. Cells in E17-19 tissue are lightly reactive by 7 days postsurgery, reach maximal numbers by 2 weeks postsurgery, and become similar in number and density to those seen in the mature cortex after 4 weeks. The appearance of BuChE-reactive blood vessels, neurons, and glia in both normal development and in the transplants is described and discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Alterations in excitatory and GABAergic inhibitory connections in hippocampal transplants

Solid pieces of embryonic hippocampal tissue were implanted in a cavity formed by aspiration of the fimbria-fornix and the overlying cingulate cortex in adult rats. Six to 8 months after the transplantation, chronic recording electrodes were implanted into the graft and the host hippocampi for the recording of electroencephalogram and unit activity in the freely moving animal. Irregularly occurring sharp waves or electroencephalogram spikes and concurrent synchronous discharge of large groups of neurons dominated the electrical activity of the grafts, in contrast to the situation in normal animals. Light microscopy and GABA immunocytochemistry in the grafts revealed that the three major cell types of the hippocampal formation, i.e. pyramidal neurons, dentate granule cells and GABA-immunoreactive interneurons were present in the hippocampal grafts. At the ultrastructural level, however, significant alterations in connectivity were observed. The most striking finding was the absence or sparse occurrence of synapses on the axon initial segments of pyramidal neurons. The axon initial segments are normally densely covered by GABAergic synapses derived from a specialized type of interneuron, the chandelier or axo-axonic cell. On the other hand, numerous GABA-immunoreactive terminals were found in synaptic contact with somata of pyramidal neurons, suggesting that other types of GABAergic interneurons and their efferent connections may have developed in a normal manner. The cell bodies of pyramidal neurons received, in addition, several asymmetric synapses from GABA-negative terminals. These presumably excitatory synapses are not present on the somata of pyramidal cells in the normally developing hippocampus. We hypothesize that the somatic excitatory synapses originate, at least in part, from the axon collaterals of the neighbouring pyramidal cells in the graft. We suggest that the hyperexcitability of the neuronal circuitry within the graft is due to reduced inhibition (lack of axo-axonic synapses) coupled with increased collateral excitation of the pyramidal neurons.

Fetal homotypic transplant in the excitotoxically neuron-depleted thalamus: light microscopy

One month after an in situ injection of kainic acid into the ventrobasal thalamic complex (VB), the lesioned area is totally depleted of neurons. The present study has been undertaken to determine the cytoarchitecture and connectivity of the nucleus constructed by fetal thalamic neurons implanted into the excitotoxically lesioned area. Adult rats received an injection of kainic acid inducing a total neuronal depletion of the right lateral thalamus (including both the nucleus reticularis thalami and the lateral portion of the ventrobasal complex). One month later, homotypic neurons were taken from the dorsal thalamic primordium of rat embryos (gestational age 15-16 days), dissociated, and injected into the lesioned area as a cell suspension. After 2-4-month survival, the cytoarchitecture of the neonucleus formed by the grafted neurons within the previously neuron-depleted area was analyzed. Additionally, connectivity was analyzed in seven rats in which dorsal column nuclei and/or cortical projections to the area were labeled anterogradely with either 3H-leucine or wheat-germ agglutinin conjugated to HRP, and the animals were perfused and processed following various histological procedures (Nissl staining, autoradiographic processing, and histochemistry for visualization of peroxidase). Fetal neurons grew, differentiated, and progressively occupied the previously neuron-depleted area of the adult host CNS. They organized themselves into a neonucleus with particular cytoarchitectural features including 1) the existence of two concentric zones--a central zone containing neurons and glial cells and a marginal zone only filled with a band of glial cells, 2) an increase in cellular density compared to the intact thalamus, 3) the grouping of neurons in spherical clusters, and 4) apparent polymorphism of neuronal somata. Lemniscal and corticothalamic afferents originating from the host were observed in the neonucleus when the fetal neurons had been implanted correctly into the lesioned area but not when they had been misplaced into either normal thalamic tissue or the internal capsule. The afferents labeled from either the dorsal column nuclei or the somatosensory cortex were, however, less dense in the neonucleus than in the normal thalamus. These results are discussed with regard to the normal cytoarchitecture and connectivity of the ventrobasal complex of the rat thalamus.

Cholinergic neurons within the rat hippocampus: response to fimbria-fornix transection

The distribution and morphologic characteristics of choline acetyltransferase (ChAT)-containing neurons were studied throughout the rostrocaudal extent of the rat hippocampus and in a midline area just dorsal to the dorsal hippocampus. Peroxidase reaction product was observed with the aid of immunohistochemical methods and a high-titer polyclonal antibody against ChAT, the acetylcholine biosynthetic enzyme. ChAT-positive cells in the hippocampus were characterized by small, round or oval perikarya with two or more proximal processes. They were located within the caudal and temporal hippocampal formation, predominantly within the subiculum, in the stratum lacunosum moleculare, at the border of the stratum lacunosum moleculare and the stratum radiatum, and in the molecular layer of the dentate gyrus. The cells resembled in morphology the small, bipolar and multipolar neocortical ChAT-immunoreactive cells. In addition to the hippocampus, ChAT-positive neurons were observed caudally in a region just above the dorsal hippocampal commissure and rostrally in the columns of the fornix. These cells were large with an oval perikarya and darkly labeled compared to neurons in the hippocampus. They more closely resembled the ChAT-positive neurons in the midline raphe of the medial septal nucleus. Examination of the rat hippocampus 2 and 8 weeks following unilateral lesioning of the fimbria-fornix and supracallosal striae revealed a sparse innervation of ChAT-positive fibers in the hippocampus ipsilateral to the lesion. ChAT-labeled neurons in the hippocampus did not appear to sprout in response to the lesion. In contrast, ChAT-positive cells in the midline did appear to sprout into the medial dorsal subiculum and dorsal medial hippocampus. We conclude that these two populations of cells are distinct with respect to their response to hippocampal denervation and, furthermore, that this distinction may be attributed to a differential response to nerve growth factor.

Transplants of the embryonal rat somatosensory neocortex in the barrel field of the adult rat: responses of the grafted neurons to sensory stimulation

The degree of participation of grafted neurons in sensory analysis was investigated in embryonal rat somatosensory neocortex transplanted into the cavity at the place of the barrel field in the neocortex (SI) of adult rats. The neurons were investigated extracellularly 3 to 6 months after grafting. In the majority of grafts the neurons had normal levels and patterns of spontaneous activity. Many of them (65%) responded to displacement of the whiskers with latencies insignificantly different (18 +/- 0.8 ms) from those for reactions in the intact barrel field (16 +/- 0.5 ms). The receptive fields of the grafted neurons were very large. None of the neurons responded to stimulation of a single vibrissa, as in intact cortex. As a rule, the same neuron responded to isolated deflections of several (up to 10-20) vibrissae. Many of them were responsive to stimulation of the small anterior vibrissae and tactile stimulation of nose, limbs and body surface. Nevertheless, there was some spatial gradient in the effectiveness of stimulation of the body surface at various distances from the vibrissal pad; among effective vibrissae, usually several adjacent ones (2-4) produced larger responses with shorter latencies than the other ones. All units responded to painful stimuli irrespective of their location. The data show that the grafted neurons receive and may transmit sensory signals. The grafts which were proved histologically to be isolated from the host's brain did not respond to sensory stimulation and were characterized by the presence of aperiodic hypersynchronous bursts in their background activity. Electrophysiological criteria may be used for intravital diagnosis on the degree of the graft morphofunctional integration.

Neural transplantation

Neural transplantation has recently emerged as an exciting extension of neural regeneration and plasticity studies. In this review, the roots of current attempts at autologous and heterologous grafting of neural tissue are traced. Grafts of peripheral and central nervous tissue have been shown to be viable after implantation in a variety of locations in adult animals' brain and spinal cord, and survival data are impressive. Donor tissue is optimal when harvested from fetuses, and successful growth and differentiation of neural grafts have been demonstrated in host animals in a broad age range. A variety of morphologic, physiologic, and behavioral parameters suggest a certain degree of integration of graft tissue into the host central nervous system, although technical limitations do not yet allow definitive statements regarding the extent of functional reinnervation. Perhaps the most promising and innovative of current studies are those that utilize a combination of peripheral and central neural tissue as transplant material. There are a number of possible applications of neural transplantation to clinical neurology and neurosurgery, some of which are discussed.

Examination of autologous and embryonic cortical brain tissue transplantation to adult brain cortex in rats

Autologous and embryonic cortical brain tissue was transplanted to adult rats in order to reconstruct experimentally degenerated cortical brain tissue. Rats were decapitated within 6 or 12 weeks. Viability of the graft tissues was studied by light and electron microscopy. Embryonic cortical brain tissue grafts became enlarged but adult cortical brain tissue grafts were found to be unaltered. Electron-microscopically observed mitochondria and other cell organellae and the newly vascularized areas clearly showed that the graft tissues were alive.