Transplantation of septal cholinergic neurons to the hippocampus improves memory impairments of spatial learning in rats treated with AF64A

Behavioral Effects of Neural Transplantation

Considerable evidence suggests that transplantation of fetal neural tissue ameliorates the behavioral deficits observed in a variety of animal models of CNS disorders. However, it is also becoming increasingly clear that neural transplants do not necessarily produce behavioral recovery, and in some cases have either no beneficial effects, magnify existing behavioral abnormalities, or even produce a unique constellation of deficits. Regardless, studies demonstrating the successful use of neural transplants in reducing or eliminating behavioral deficits in these animal models has led directly to their clinical application in human neurodegenerative disorders such as Parkinson's disease. This review examines the beneficial and deleterious behavioral consequences of neural transplants in different animal models of human diseases, and discusses the possible mechanisms by which neural transplants might produce behavior recovery.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

Neural Transplantation in Animal Models of Dementia

Neural transplantation provides a powerful novel technique for investigating the neurobiological basis and potential strategies for repair of a variety of neurodegenerative conditions. The present review considers applications of this technique to dementia. After a general introduction (section 1), attempts to replace damaged neural systems by transplantation are considered in the context of distinct animal models of dementia. These include grafting into aged animals (section 2), into animals with neurotransmitter-selective lesions of subcortical nuclei, in particular involving basal forebrain cholinergic systems (section 3), and into animals with non-specific lesions of neocortical and hippocampal systems (section 4). The next section considers the alternative use of grafts as a source of growth/trophic factors to inhibit degeneration and promote regeneration in the aged brain (section 5). Finally, a number of recent studies have employed transplanted tissues to model and study the neurodegenerative processes associated with ageing and Alzheimer's disease taking place within the transplant itself (section 6). It is concluded (section 7) that although neural transplantation does not offer any immediate prospect of therapeutic repair in clinical dementia, the technique does offer a powerful neurobiological tool for studying the neuropathological processes involved in both spontaneous degeneration and specific diseases of ageing. New understandings derived from neural transplantation may be expected to lead to rational development of novel strategies to inhibit the neurodegenerative process and to promote regeneration in the aged brain.

Impairment of spatial memory in kaolin-induced hydrocephalic rats is associated with changes in the hippocampal cholinergic and noradrenergic contents

We investigated the relationship between the degree of spatial memory impairment in an 8-arm radial maze and the changes in the contents of acetylcholine (ACh) and noradrenaline (NA) in the dorsal and ventral hippocampus and the frontal cortex, along with histological changes in kaolin-induced hydrocephalic rats. Kaolin-induced hydrocephalic rats were divided into three groups (non-impaired, impaired and severely impaired) according to the degree of impairment in a radial maze. Thirty percent of the hydrocephalic rats could not solve a radial maze (severely impaired group), while the remaining hydrocephalic rats could (non-impaired rats in the standard task). Forty percent of the non-impaired rats in the standard task failed to solve the delayed-response task (impaired group), whereas the remaining rats were able to solve it (non-impaired group). A positive correlation was observed between the impairment of spatial memory and ventricular dilatation. The ACh content in the dorsal and ventral hippocampus, and the NA content in the ventral hippocampus were decreased in the severely impaired group. Moreover, the NA content in the ventral hippocampus was decreased in the impaired group. These results suggest that the impairment of spatial memory in kaolin-induced hydrocephalic rats is associated with dysfunction of the hippocampal cholinergic and noradrenergic systems.

Pig fetal septal neurons implanted into the hippocampus of aged or cholinergic deafferented rats grow axons and form cross-species synapses in appropriate target regions

The anatomical specificity of axon growth from fetal pig septal xenografts was studied by transplanting septal cells from E30-35 pig fetuses into cholinergic deafferented (192-IgG-saporin-infused) rats or into aged rats (> 18 months). Cell suspensions (100,000 cells/microl) were injected bilaterally into the dorsal and ventral hippocampus of immunosuppressed rats (10 mg/kg/day cyclosporine A). To assess axonal growth and synapse formation, acetylcholinesterase histochemistry, an antibody to choline acetyltransferase (ChAT), and three pig-positive/rat-negative antibodies: bovine 70kD neurofilament (NF70), human low-affinity NGF receptor (hNGFr), and human synaptobrevin (hSB) were used. In rats with surviving grafts at 6 months, NF70 axonal labeling was more extensive than either ChAT or hNGFr labeling. All three markers demonstrated graft axons extending selectively through the hippocampal CA fields and the molecular layer of the dentate gyrus. Graft axons did not extend into adjacent entorhinal cortex or neocortex. The distribution of pig hSB-positive synapses correlated with AChE-positive fiber outgrowth in to the host. Electron microscopic analysis of hSB-immunostained hippocampal sections revealed pig presynaptic terminals in contact with normal rat postsynaptic structures in the CA fields and the dentate gyrus. These data demonstrate target-appropriate growth of pig cholinergic axons and the formation of cross-species synapses in the deafferented or aged rat hippocampus.

Amelioration of spatial navigation and short-term memory deficits by grafts of foetal basal forebrain tissue placed into the hippocampus and cortex of rats with selective cholinergic lesions

Impairments in learning and memory, induced by surgical or excitotoxic lesions of the septo-hippocampal or basalo-cortical pathways, can be ameliorated by grafts of cholinergic-rich foetal basal forebrain tissue into the hippocampus and/or neocortex. However, the effects of such grafts have been only partial, which may be due to the non-specific nature of the lesioning procedures used in these studies, known to destroy both cholinergic and non-cholinergic neuronal projections. In the present study, we have explored the effects of cholinergic-rich grafts in rats subjected to selective cholinergic lesions, induced by intraventricular injections of the immunotoxin 192 IgG-saporin. This lesion, which selectively destroyed 85-95% of the cholinergic neurons in both the septal-diagonal band and nucleus basalis, produced a long-lasting, substantial impairment in both the acquisition of spatial reference memory in the Morris water maze task and delay-dependent short-term memory performance, as seen in a delayed matching-to-position test. Foetal cholinergic grafts (but not control grafts of cerebellar tissue) implanted at multiple sites into both the hippocampus and fronto-parietal neocortex, bilaterally, completely reversed the acquisition deficit in place navigation in the water maze, to an extent that greatly exceeded that previously seen in animals with non-selective lesions. Most notably, however, the impairment in short-term memory was only partially and inconsistently affected, and only at the longest delay times. The morphological analysis, performed at about 7 months after transplantation, showed that the grafts had re-established a close to normal cholinergic innervation in the initially denervated cortical and hippocampal territories. It is proposed that the differential effects of cholinergic-rich transplants on different aspects of cognitive performance may define intrinsic limitations to the functional capacity of the ectopically placed grafts, which may be due to incomplete integration of the grafted cholinergic neurons into functional regulatory circuitries normally available to the basal forebrain cholinergic system.

Effects of oxidative stress on the expression of limbic-specific protease neuropsin and avoidance learning in mice

We evaluated the effects of oxidative stress in mouse brain induced by the intraperitoneal injection of diethyldithiocarbamate (DDC) on gene expression of the novel serine protease, neuropsin, and on shock-avoidance learning. The level of neuropsin mRNA in the hippocampal pyramidal neurons increased at 2 h after DDC treatment and decreased thereafter. At 7 days neuropsin mRNA significantly decreased to 60% of the pretreated control level and then returned to the control level at 30 days. Genes for tissue plasminogen activator, manganese superoxide dismutase, and heat shock protein did not differ in DDC-treated mice vs. the control group at 7 and 30 days. The shuttle-box avoidance learning was retarded at 7 days after DDC administration. However, it recovered to the control level at 30 days after DDC administration. The results suggest that generation of reactive oxygen species has an important role in neuropsin transcript in the limbic areas which might be related to the disturbance in avoidance learning.

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.

Long-term spatial cognitive impairment after middle cerebral artery occlusion in rats: no involvement of the hippocampus

The behavioral and neurochemical changes in the chronic phase of permanent occlusion of the right middle cerebral artery (MCA) in rats were investigated. One month after MCA occlusion, 23 rats were unable to solve a radial eight-arm maze task during an entire 1-month period, whereas seven rats were able to solve this task. Three months after occlusion, 19 MCA-occluded rats failed to solve the task successfully again for at least 1 month (the cognitively impaired rats), whereas 11 MCA-occluded rats were able to solve it (the cognitively unimpaired rats). The rats that underwent behavioral testing were examined for any changes in the acetylcholine (ACh) levels in the hippocampus using HPLC with electrochemical detection or the formation of long-term potentiation (LTP) in the population spike of the hippocampal CA1 field. The immunohistochemical distribution of either the microtubule-associated protein 2 (MAP2) or glial fibrillary acidic protein (GFAP) in the hippocampus of the cognitively impaired rats was also studied. In the cognitively impaired rats, neither the suppression of the induction of LTP, nor the degradation of MAP2, nor the increase in the GFAP immunoreactivity was observed in the hippocampus. The levels of ACh in the hippocampus did not change significantly among the cognitively impaired, unimpaired, and the sham-operated rats. These results suggest that MCA occlusion is capable of producing long-term spatial cognitive disturbance in rats without any evidence of neurobiological damage in the hippocampus.

Long-term spatial cognitive impairment following middle cerebral artery occlusion in rats. A behavioral study

Behavioral changes in the chronic phase of permanent occlusion of the right middle cerebral artery (MCA) in rats were investigated. One month after MCA occlusion, 23 rats were unable and 7 rats were able to solve a radial 8-arm maze task during a 1-month period. Three months after occlusion, 19 MCA-occluded rats failed to solve the task successfully again during at least a 1-month period (the cognitively impaired rats), and 11 MCA-occluded rats were able to solve it (the cognitively unimpaired rats). When a delay of 60 min was imposed for this task, five cognitively unimpaired rats failed to solve it. The locomotor activity of the cognitively impaired rats increased significantly 2 months after occlusion, and this increase showed good correlation with spatial cognitive deficit. However, the mean time a rat spent at each arm remained unchanged among the cognitively impaired, unimpaired, and sham-operated rats. There was no significant difference in the ratio between the cognitively impaired and unimpaired rats for disturbed motor coordination. These results suggest that MCA occlusion is capable of producing long-term spatial cognitive disturbance in rats. In addition, this spatial cognitive deficit does not seem to be primarily due to hypermotility or a disturbance in motor coordination.

The spinal cord as an alternative model for nerve tissue graft

The spinal cord provides an alternative model for nerve tissue grafting experiments. Anatomo-functional correlations are easier to make here than in any other region of the CNS because of a direct implication of spinal cord neurons in sensorimotor activities. Lesions can be easily performed to isolate spinal cord neurons from descending inputs. The anatomy of descending monoaminergic systems is well defined and these systems offer a favourable paradigm for lesion-graft experiments.

Multiple obstacles to gene therapy in the brain

Neuwelt et al. have proposed gene-transfer experiments utilizing an animal model that offers many important advantages for investigating the feasibility of gene therapy in the human brain. A variety of tissues concerning the viral vector and mode of delivery of the corrective genes need to be resolved, however, before such therapy is scientifically supportable.

Principles of brain tissue engineering

It is often presumed that effects of neural tissue transplants are due to release of neurotransmitter. In many cases, however, effects attributed to transplants may be related to phenomena such as trophic effects mediated by glial cells or even tissue reactions to injury. Any conclusion regarding causation of graft effects must be based on the control groups or other comparisons used. In human clinical studies, for example, comparing the same subject before and after transplantation allows for many interpretations of the causes of clinical changes.

Lessons on transplant survival from a successful model system

Studies on the snailMelampusreveal that connectivity is crucial to the survival of transplanted ganglia. Transplanted CNS ganglia can innervate targets or induce supernumerary structures. Neuron survival is optimized by the neural incorporation that occurs when a transplanted ganglion is substituted for an excised ganglion. Better provision for the trophic requirements of neurons will improve the success of mammalian fetal transplants.

Repairing the brain: Trophic factor or transplant?

Three experiments on neural grafting with adult rat hosts are described. Working memory impairments were produced by lesioning the hippocampus or severing its connections with the septum by ablating the fimbria-fornix. The results suggest that the survival and growth of a neural graft, whether an autograft or a xenograft, is not a necessary condition for functional recovery on a task tapping working memory.

Will brain tissue grafts become an important therapy to restore visual function in cerebrally blind patients?

Grafting embryonic brain tissue into the brain of patients with visual field loss due to cerebral lesions may become a method to restore visual function. This method is not without risk, however, and will only be considered in cases of complete blindness after bilateral occipital lesions, when other, risk-free neuropsychological methods fail.

Difficulties inherent in the restoration of dynamically reactive brain systems

The responses displayed by an injured or diseased nervous system are complex. Some of the responses may effect a functional reorganization of the affected neural circuitry. Strategies aimed at the restoration of function, whether or not these involve transplantation, need to recognize the innate reactive capacity of the nervous system to damage. More successful strategies will probably incorporate, rather than ignore, the adaptive responses of the compromised neural systems.

Elegant studies of transplant-derived repair of cognitive performance

Cholinergic-rich grafts have been shown to be effective in restoring maze-learning deficits in rats with lesions of the forebrain cholinergic projection system. However, the relevance of those studies to developing novel therapies for Alzheimer's disease is questioned.

Neural transplants are grey matters

The lesion and transplantation data cited by Sinden et al., when considered in tandem, seem to harbor an internal inconsistency, raising questions of false localization of function. The extrapolation of such data to cognitive impairment and potential treatment strategies in Alzheimer's disease is problematic. Patients with focal basal forebrain lesions (e.g., anterior communicating artery aneurysm rupture) might be a more appropriate target population.

Immunobiology of neural transplants and functional incorporation of grafted dopamine neurons

In contrast to the views put forth by Stein & Glasier, we support the use of inbred strains of rodents in studies of the immunobiology of neural transplants. Inbred strains demonstrate homology of the major histocompatibility complex (MHC). Virtually all experimental work in transplantation immunology is performed using inbred strains, yet very few published studies of immune rejection in intracerebral grafts have used inbred animals.

Local and global gene therapy in the central nervous system

For focal neurodegenerative diseases or brain tumors, localized delivery of protein or genetic vectors may be sufficient to alleviate symptoms, halt disease progression, or even cure the disease. One may circumvent the limitation imposed by the blood-brain barrier by transplantation of genetically altered cell grafts or focal inoculation of virus or protein. However, permanent gene replacement therapy for diseases affecting the entire brain will require global delivery of genetic vectors. The neurotoxicity of currently available viral vectors and the transient nature of transgene expression invivomust be overcome before their use in human gene therapy becomes clinically applicable.

Neural grafting in human disease versus animal models: Cautionary notes

Over the past two decades, research on neural transplantation in animal models of neurodegeneration has provided provocative in sights into the therapeutic use of grafted tissue for various neurological diseases. Although great strides have been made and functional benefits gained in these animal models, much information is still needed with regard to transplantation in human patients. Several factors are unique to human disease, for example, age of the recipient, duration of disease, and drug interaction with grafted cells; these need to be explored before grafting can be considered a safe and effective therapeutic tool.

Building a rational foundation for neural transplantation

The neural transplantation research described by Sinden and colleagues provides part of the rationale for the clinical application of neural transplantation. The authors are asked to clarify their view of the role of the cholinergic system in cognition, to address extrahippocampal damage caused by transient forebrain ischemia, and to consider the effects of delayed neural degeneration in their structure-function analysis.

Intraretrosplenial grafts of cholinergic neurons and spatial memory function

The transplantation of cholinergic neurons into the hippocampal formation has been well characterized. We describe our studies on the effects of cholinergic transplants in the retrosplenial cortex. These transplants were capable of ameliorating spatial navigation deficits in rats with septohippocampal lesions. In addition, we provide evidence for the modulation of transplanted neurons by the host brain.

Gene therapy and neural grafting: Keeping the message switched on

A major problem in developing an effective gene therapy for the nervous system lies in understanding the principles that maintain or turn off the expression of genes following their transfer into the CNS.

Therapeutic neural transplantation: Boon or boondoggle?

Despite reports of recovery of function after neural transplantation, the biological interactions between transplanted neurons and the host brain that are necessary to mediate recovery are unclear at present. One source of confusion is in the variety of models and protocols used in these studies. It is suggested that multisite experimentation using standard protocols, models, and recovery criteria would be helpful in moving neural transplantation from the laboratory to the clinic.

The ethics of fetal tissue grafting should be considered along with the science

In addition to the scientific and medical issues surrounding the use of fetal tissue transplants, the ethical implications should be considered. Two major ethical issues are relevant. The first of these is whether this experimental procedure can be justified on the basis of potential benefit to the patient. The second is whether the use of tissue obtained from intentionally aborted fetuses can be justified in the context of historical and existing guidelines for the protection of human subjects. The separation of ethical decisions from medical practice and scientific research is necessary to prevent the exploitation of innocent human life.

Gene therapy for neurodegenerative disorders and malignant brain tumors

Gene therapy approaches have great promise in the treatment of neurodegenerative disorders and malignant brain tumors. Neuwelt et al. review available viral-mediated gene therapy methods and their blood-brain-barrier (BBB) disruption delivery technique, briefly mentioning nonviral mediated gene therapy methods. This commentary discussed the BBB disruption delivery technique, viral and nonviral mediated gene therapy approaches to Parkinson's disease, and the potential use of antisense oligo to suppress malignant brain tumors.

Behavioral effects of neural grafts: Action still in search of a mechanism

This commentary reviews data supporting circuitry reconstruction, replacement neurotransmitters, and trophic action as mechanisms whereby transplants promote recovery of function. Issue is taken with the thesis of Sinden et al. that adequate data exist to indicate that reconstruction of hippocampal circuitry damaged by hypoxia with CA1 transplants is a confirmed mechanism whereby these transplants produce recovery. Sinden et al.'s and Stein & Glasier's proposal that there is definitive evidence showing that all transplants produce trophic effects is also questioned.

Neural transplantation, cognitive aging and speech

Research on neural transplantation has great potential societal importance in part because of the expanding proportion of the population that is elderly. Transplantation studies can benefit from the guidance of research on cognitive aging, especially in connection with the assessment of behavioral outcomes. Speech for example, might be explored using avian models.

Pathway rewiring with neural transplantation

A lesion to the brain is not necessary for a successful neural transplantation. Embryonic Purkinje cells placed on the surface of an uninjured adult cerebellum can develop and migrate into the host molecular layer. Both the Purkinje cells that migrated into the host cerebellum and those that remained in the graft were innervated by collateral sprouting of adult intact climbing fibers.

Multiple potential mechanisms of graft action is not a new idea

It is well established that neural grafts can exert functional effects on the host animal by a multiplicity of different mechanisms – by diffuse release of trophic molecules, neurohormones, and deficient neurotransmitters, as well as by growth and reformation of neural circuits. Our challenge is to understand how these different mechanisms complement each other.

Grafts and the art of mind's reconstruction

The use of neural transplantation to alleviate cognitive deficits is still in its infancy. We have an inadequate understanding of the deficits induced by different types of brain damage and their homologies in animal models against which to assess graft-induced recovery, and of the ways in which graft growth and function are influenced by factors within the host brain and the environment in which the host is operating. Further, use of fetal tissue may only be a transitory phase in the search for appropriate donor sources. Nevertheless, findings from our laboratory and elsewhere have made aprima faciecase for successful cognitive reconstruction by graft methods.

Studying restoration of brain function with fetal tissue grafts: Optimal models

We concur that basic research on the use of CNS grafts is needed. Two important model systems for functional studies of grafts are ignored by Stein & Glasier. In the first, reproductive function is restored in hypogonadal mice by transplantation of GnRH-synthesizing neurons. In the second, circadian rhythmicity is restored by transplantation of the suprachiasmatic nucleus.

Gene replacement therapy in the CNS: A view from the retina

Gene replacement therapy holds great promise in the treatment of many genetic CNS disorders. This commentary discusses the feasibility of gene replacement therapy in the unique context of the retina, with regard to: (1) the genetics of retinal neoplasia and degeneration, (2) available gene transfer technology, and (3) potential gene delivery vehicles.

The limitations of central nervous systemdirected gene transfer

Complementation and correction of a genetic defect with CNS manifestations lags behind gene therapy for inherited disorders affecting other organ systems because of shortcomings in delivery vehicles and access to the CNS. The effects of improvements in viral and nonviral vectors, coupled with the development of delivery strategies designed to transfer genetic material thoughout the CNS are being investigated by a number of laboratories in efforts to overcome these problems.

CNS transplant utility may surive even their hasty clinical application

Neural cell transplants have been introduced in clinical practice during the last decade with mixed results, encouraged by success with simple animal models. This commentary is a reminder that although the ideas and techniques of transplantation appear simple, the variables involved in host-transplant integration still require further study. The field may benefit from a concerted, multidisciplinary approach.

Are fetal brain tissue grafts necessary for the treatment of brain damage?

Despite some clinical promise, using fetal transplants for degenerative and traumatic brain injury remains controversial and a number of issues need further attention. This response reexamines a number of questions. Issues addressed include: temporal factors relating to neural grafting, the role of behavioral experience in graft outcome, and the relationship of rebuilding of neural circuitry to functional recovery. Also discussed are organization and type of transplanted tissue, the “trophic hypothesis” of transplant viability, and whether transplants are really needed to obtain functional recovery after brain damage.

Transplantation, plasticity, and the aging host

Neural transplantation as a recovery strategy for neuro-degenerative diseases in humans has used mainly grafting following acute denervation strategies in young adult hosts. Our work in aged mice and rats demonstrates an age-related increase in susceptibility to oxidative damage from neurotoxins, a remarkably poor recovery of C57BL/6 mice from MPTP insult with transplantation and growth factors, even at 12 months of age, and diminished plasticity of host neurons. We believe that extrapolation of data from young adult animal models to aged humans without thorough investigation of transplantation and host response inagedrecipients is scientifically and ethically inappropriate.

The effects of intrahippocampal raphe and/or septal grafts in rats with fimbria-fornix lesions depend on the origin of the grafted tissue and the behavioural task used

Long-Evans female rats sustained electrolytic lesions of the fimbria and the dorsal fornix and, two weeks later, received intrahippocampal suspension grafts of fetal tissue. The grafts were prepared from regions including either the medial septum and the diagonal band of Broca (septal grafts), or the mesencephalic raphe (raphe grafts), or from both these regions together (co-grafts). All rats were submitted to a series of behavioural tests (home cage and open-field locomotion, spontaneous alternation, radial-arm maze and Morris water maze performance) run over two periods after grafting (one to nine weeks and 20-35 weeks). Two weeks after completion of behavioural testing, histological (acetylcholinesterase and Cresyl Violet staining) and/or neurochemical (choline acetyltransferase activity, high-affinity synaptosomal uptake of choline and serotonin, noradrenaline, serotonin and 5-hydroxyindolacetic acid concentrations) verifications were performed on the hippocampus. Compared to sham-operated rats, lesion-only rats exhibited hyperactivity which was transient in a familiar environment (home cage) and lasting in an unfamiliar one (open field), decreased rates of spontaneous T-maze alternation, and impaired memory performance in both the radial-arm maze and the Morris water maze. These rats also showed decreased cholinergic and serotonergic markers with a maximal depletion in the septal two-thirds of the hippocampus. Noradrenaline concentration tended to be increased in the dorsal third of the hippocampus, but was not modified in the other two-thirds. While septal grafts specifically increased the cholinergic markers and raphe grafts the serotonergic ones, neither of these grafts produced a lasting effect on any behavioural variable. Conversely, the co-grafts, which increased both the cholinergic and serotonergic markers in the septal two-thirds of the hippocampus, completely normalized the Morris water maze probe trial performance, but failed to affect any of the other behavioural variables. Our present results confirm that grafts of fetal neurons injected into the denervated hippocampus may induce a neurochemical recovery that depends on the anatomical origin of the grafted cells, and that co-grafting two fetal brain regions allows the combination of their individual neurochemical properties. Furthermore, our results show that these neurochemical effects of the co-grafts may be involved in the recovery of behavioural function observed in the water maze. However, somewhat paradoxically, those effects appear inefficient for inducing any recovery in other behavioural tasks, even in the radial-arm maze; which is assumed to measure similar spatial functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of the acute administration of a new trimethylxanthine derivative, S 9977-2, on local cerebral blood flow and glucose utilization in the rat

S 9977-2 is a new trimethylxanthine derivative with promnesic properties. Its effects on cerebral glucose utilization and blood flow were studied by means of quantitative autoradiography. S 9977-2 was injected intravenously into adult rats at doses of 0.1, 1.0 and 10 mg/kg. At 0.1 mg/kg, S 9977-2 induced a significant increase in cerebral glucose utilization over control values in two white matter areas and in the vestibular nucleus. At 1.0 mg/kg, glucose utilization was affected in 14 areas out of the 63 studied, mainly limbic regions such as the hippocampus, raphe nuclei and locus coeruleus, as well as some posterior areas. Conversely, after the injection of 10 mg/kg S 9977-2, cerebral glucose utilization was similar to that of control rats. At the three doses tested, S9977-2 did not induce any significant variation in local rates of cerebral blood flow compared to those of controls. Likewise, S 9977-2 did not change the level of coupling between cerebral blood flow and metabolism, except at 10 mg/kg, where a relative hypoperfusion at a constant metabolic level was recorded. These data show that, at 1.0 mg/kg, S 9977-2 increased glucose utilization in hippocampal areas, an effect which may be related to the promnesic properties of this compound at the same dose. Moreover, at low doses, the lack of change in the level of coupling between cerebral blood flow and metabolism is indicative of the rather selective action of this compound, compared to that of caffeine. Thus S9977-2 should have therapeutic effects, mainly via its promnesic properties, without having many side effects.

Behaviour-dependent changes in acetylcholine release in normal and graft-reinnervated hippocampus: evidence for host regulation of grafted cholinergic neurons

Grafted neurons obtained from the fetal basal forebrain can provide a functional cholinergic reinnervation of the hippocampal formation in rats with a lesion of the intrinsic septal cholinergic afferents. In the present experiments graft-derived acetylcholine release in the hippocampus was studied by microdialysis in awake rats during different types of behaviours which are known to activate the innate septohippocampal cholinergic system and during different activity periods of the day-night cycle. Two types of basal forebrain grafts were studied: cell suspensions implanted into the hippocampus in rats with an aspirative lesion of the fimbria-fornix, and grafts of solid tissue implanted as a tissue bridge into the fimbria-fornix lesion cavity. Increased acetylcholine overflow was seen in both groups with grafts during sensory stimulation (by handling). The strongest response (50% increase in acetylcholine release) was seen in rats with solid basal forebrain grafts (equivalent to two-thirds of that seen in intact rats). Immobilization stress and motor activity (swimming) also resulted in increased, but more variable, acetylcholine release (+ 30%; about one-third of the normal response). None of these effects was seen in the control rats with fimbria-fornix lesion only. The two-fold difference in hippocampal acetylcholine release in normal animals between day and night was absent in both types of grafted rats. An acute knife-cut, transecting the connections between the solid basal forebrain graft and the host hippocampus, caused an immediate 75% reduction in acetylcholine release (similar to the effect of an acute fimbria-fornix transection in the normal rats) and the response to swimming was no longer evident. The results show that grafted cholinergic neurons can be functionally integrated into the host brain, allowing the grafted neurons to be activated in the correct behavioural contexts, although the changes in acetylcholine overflow were overall smaller and more variable than normal. The ability of the host to influence cholinergic graft activity, most probably mediated via activation of afferent host-graft connections, may contribute to the efficacy of basal forebrain grafts in the amelioration of behavioural impairments in animals with lesions of the forebrain cholinergic system.

Transplantation of fetal cholinergic neurons into the hippocampus attenuates the cognitive and neurochemical deficits induced by AF64A

The present experiments examined whether transplanted fetal cholinergic neurons would attenuate the behavioral and neurochemical deficits induced by the cholinotoxin AF64A (ethylcholine aziridinium ion). Bilateral injections of AF64A (3 nmol) into the lateral ventricles produced significant learning and memory impairments together with decreases in hippocampal high-affinity choline uptake (HAChU). AF64A-treated rats were impaired on both a standard radial arm maze (RAM) task and a working memory version in which a one-hour delay was imposed between the fourth and fifth arm choices. Transplantation of embryonic day E-17 septal/diagonal band tissue into the hippocampus (HPC) promoted recovery of performance on the standard version of the RAM task. However, this recovery was not observed when the animals were tested on the more difficult delay version of the task. Neurochemical analysis indicated that AF64A produced a significant (31%) decrease in hippocampal HAChU that was attenuated (14%) by transplantation of fetal cholinergic neurons. Histological analysis revealed that the transplants survived and innervated the HPC. There was no apparent relationship between fiber ingrowth into the HPC and behavioral recovery. These data indicate that transplant-induced behavioral recovery may be related to and limited by the cognitive demands of the testing situation. Generalized increases in cholinergic activity, transplant-mediated release of trophic factors, or a combination of both may underlie the behavioral recovery observed in the present studies.