Is ethylcholine mustard aziridinium ion a specific cholinergic neurotoxin?

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.

Depletion of cholinergic amacrine cells by a novel immunotoxin does not perturb the formation of segregated on and off cone bipolar cell projections

Cone bipolar cells are the first retinal neurons that respond in a differential manner to light onset and offset. In the mature retina, the terminal arbors of On and Off cone bipolar cells terminate in different sublaminas of the inner plexiform layer (IPL) where they form synapses with the dendrites of On and Off retinal ganglion cells and with the stratified processes of cholinergic amacrine cells. Here we first show that cholinergic processes within the On and Off sublaminas of the IPL are present early in development, being evident in the rat on the day of birth, approximately 10 d before the formation of segregated cone bipolar cell axons. This temporal sequence, as well as our previous finding that the segregation of On and Off cone bipolar cell inputs occurs in the absence of retinal ganglion cells, suggested that cholinergic amacrine cells could provide a scaffold for the subsequent in-growth of bipolar cell axons. To test this hypothesis directly, a new cholinergic cell immunotoxin was constructed by conjugating saporin, the ribosome-inactivating protein toxin, to an antibody against the vesicular acetylcholine transporter. A single intraocular injection of the immunotoxin caused a rapid, complete, and selective loss of cholinergic amacrine cells from the developing rat retina. On and Off cone bipolar cells were visualized using an antibody against recoverin, the calcium-binding protein that labels the soma and processes of these interneurons. After complete depletion of cholinergic amacrine cells, cone bipolar cell axon terminals still formed their two characteristic strata within the IPL. These findings demonstrate that the presence of cholinergic amacrine cells is not required for the segregation of recoverin-positive On and Off cone bipolar cell projections.

Influences of cholinergic neurotoxin ethylcholine aziridinium ion on circadian rhythms in rats

To investigate whether damages of cholinergic neurons in the brain produce aging-like changes in circadian rhythms, we examined the influences of intracerebroventricular injection of cholinergic neurotoxin ethylcholine aziridinium ion (AF64A, 5 nmol/5 microl) on circadian rhythms in rats, by measuring locomotor activity and body temperature with the automatic behavioral measurement system combined with the telemetry. Daily rhythms in locomotor activity and body temperature were observed in AF64A-treated rats under a 12:12 h light:dark (LD) cycle, however, in AF64A-treated rats, the amplitude of activity and temperature rhythms was significantly decreased, the phase of the both rhythms was advanced and the amount of activity was decreased, compared with control rats. Locomotor activity and body temperature also showed a circadian rhythm in AF64A-treated rats under the constant dark condition with the period similar to that in the control rats. The present findings are in accordance with the observation in aged animals in which cholinergic hypofunction are often observed, suggesting that hypofunctions of the cholinergic systems in the brain might be involved in aging-like changes in the circadian rhythms.

Glucocorticoid hypersecretion following intracerebroventricular injection of ethylcholine mustard aziridinium ion in rats

To investigate whether cholinergic hypofunctions in the brain influence hypothalamic-pituitary-adrenal activity, we examined the effects of cholinergic neurotoxin ethylcholine mustard aziridinium ion on basal and stress-induced levels of corticosterone in rats. Blood sampling from rats following intracerebroventricular injection of saline (5 microl, as a control) or this neurotoxin (5 nmol/5 microl) was performed over a day in one series, and was taken before, during and after an immobilization stress exposure in another series. Plasma levels of corticosterone and adrenocorticotropin were determined by the radioimmunoassay. The basal levels of plasma corticosterone and adrenocorticotropin over a day were significantly higher in the neurotoxin-treated rats, compared with the control rats (corticosterone, P<0.001; adrenocorticotropin, P<0.05). Further, relative adrenal gland weight of the neurotoxin-treated rats was significantly greater than that of the control rats (P<0.05). However, responses in plasma corticosterone level caused by the immobilization stress in the neurotoxin-treated rats were not different from those in the control rats. The present study demonstrated that damage to the cholinergic neurons in the brain increased hypothalamic-pituitary-adrenal activity over a day, probably due to freedom from inhibitory influences of the hippocampal cholinergic system, but that this damage did not influence stress-induced changes in plasma glucocorticoid level.

Neurochemical determination of the location of NMDA and GABA receptors on rat striatal cholinergic neurons

This paper reports on the protocol for neurochemical determination of the location of various receptors on cholinergic neurons in various brain regions. We applied this protocol to investigate whether NMDA and GABA receptors are located on rat striatal cholinergic neurons. When striatal cholinergic neurons were selectively destroyed by intrastriatal injection of cholinergic neurotoxin, ethylcholine mustard aziridinium ion (AF64A), the number of NMDA and GABA(A) receptors decreased. However, no significant changes were observed on the number of GABA(B) receptors. These results suggest that NMDA and GABA(A), but not GABA(B) receptors are located on cholinergic neurons in the striatum. These results also indicate the usefulness and scientific applicability of the present protocol.

Modulation of acetylcholine release via GABAA and GABAB receptors in rat striatum

In order to investigate whether changes in acetylcholine (ACh) release induced by GABA receptors are due to a direct or indirect effect on cholinergic neurons in the striatum, GABAA and GABAB receptor bindings were assayed in the striatum microinjected with ethylcholine mustard aziridinium ion (AF64A), a cholinergic neurotoxin. Intra-striatal injection of a selective concentration of AF64A (10 nmol) reduced GABAA receptor binding without significantly altering GABAB receptor binding. Treatment with a higher, less selective concentration of AF64A (20 nmol) reduced all markers examined. These results suggest that GABAA, but not GABAB receptors, are located on cholinergic neurons in the striatum, and that GABA can directly modulate ACh release through stimulation of GABAA receptors. Findings further suggest that GABA can also indirectly modulate ACh release through stimulation of GABAB receptors located on non-cholinergic neuronal elements in the striatum.

Effects of ethylcholine mustard azirinium ion (AF64A) on the choline acetyltransferase and nitric oxide synthase activities in mesopontine cholinergic neurons of the rat

The choline analogue, ethylcholine mustard azirinium ion (AF64A), has been proposed as a selective neurotoxin that produces degeneration of central cholinergic neurons. However, the mechanisms of action and the specificity or non-specificity of this toxin are still undefined. In this study, we have investigated the effects of AF64A, in comparison with kainic acid, on cholinergic neurons of the mesopontine formation (pedunculopontine and laterodorsal tegmental nuclei), a neuronal population also expressing nitric oxide synthase, the enzyme responsible for the synthesis of nitric oxide. We used choline acetyltransferase immunohistochemistry as a marker of acetylcholine activity, and nitric oxide synthase immunohistochemistry and NADPH-diaphorase histochemistry as markers of nitric oxide synthase activity. Our results show that the injection of low doses of AF64A produces: (1) an area of cavitation in the injection site of pedunculopontine tegmental nucleus (local non-specific effect), and (2) a transient decrease in choline acetyltransferase immunoreactivity in choline acetyltransferase-nitric oxide synthase neurons in both the ipsilateral laterodorsal tegmental nucleus and the perilesional area of the pedunculopontine tegmental nucleus, while their morphology and nitric oxide synthase immunoreactivity remain unaltered (post-diffusion specific effect). These findings indicate that the loss of choline-related enzymatic activity is not necessarily associated with degeneration of cholinergic neurons, and that the recovery of choline acetyltransferase immunoreactivity may arise from neurons whose activity is diminished during the first postinjection weeks. Taking into account that AF64A is a suitable tool to develop a reversible model of neurological disorders related to cholinergic deficit, further efforts should be directed toward elimination of its local non-specific effect.

Involvement of cholinergic neurons in the release of dopamine elicited by stimulation of mu-opioid receptors in striatum

The involvement of striatal cholinergic neurons in the release of dopamine (DA) elicited by the mu-opioid receptor agonist DAGO ([D-Ala2, NMePhe4-Gly5(ol)]enkephalin) was explored. The striatal release of DA was measured by microdialysis in rats anesthetized with chloral hydrate. When infused in the striatum, through the microdialysis probe, DAGO increased the extracellular levels of DA. The previous injection in striatum of AF 64-A, a toxin for cholinergic neurons, or the concomitant infusion of the M2-muscarinic antagonist methoctramine abolished the effect of DAGO on the DA release. It is concluded that stimulation of mu-opioid receptors, by inhibiting the acetylcholine release which stimulates tonically M2-muscarinic receptors likely associated with dopaminergic nerve endings, indirectly increases the striatal DA release.

Immunolesion by 192IgG-saporin of rat basal forebrain cholinergic system: a useful tool to produce cortical cholinergic dysfunction

Cholinergic lesion paradigms have been used to study the role of the cholinergic system in cortical arousal and cognitive function, and its implication in cognitive deficits that occur in Alzheimer's disease. In the last few years an increasing number of studies have applied neurotoxins including excitotoxins or cholinotoxins (e.g. AF64A) by stereotaxic injection into the Nbm to produce reductions in cortical cholinergic activity. One of the most serious limitations of these lesion paradigms is the fact that basal forebrain cholinergic neurons are always intermingled with populations of noncholinergic cells and that the cytotoxins used are far from being selective to cholinergic cells. Excitoxins when infused directly into the Nbm destroy non-specifically cell bodies but spare axons passing the injection site, whereas the specificity of AF64A to destroy cholinergic neurons depends on both the dosage applied and the site of injection. Recently, a monoclonal antibody to the low-affinity nerve growth factor (NGF) receptor, 192IgG, coupled to a cytotoxin, saporin, has been described as an efficient and selective immunotoxin for the NGF-receptor bearing cholinergic neurons in rat basal forebrain. Intraventricular administration of the 192IgG-saporin conjugate appears to induce a nearly complete and specific lesion of neocortical and hippocampal cholinergic afferents. Other neuronal systems in the basal forebrain are spared by the immunotoxin. Electrolytic, ibotenic acid, and cholinergic immunotoxic lesions of cholinergic basal forebrain nuclei resulted in slightly different effects on cortical cholinergic markers: Electrolytic lesion of the Nbm did not change M1-mAChR but resulted in reduced M2-mAChR in frontal and parietal cortices 1 week after lesion. Ibotenic acid lesion of the nucleus basalis did not alter M1-mAChR in any cortical region but led to enhanced M2-mAChR binding in the parietal cortex only. When applying the cholinergic immunotoxin 192IgG-saporin, both M1- and M2-mAChR binding sites were increased in a number of cortical areas 1 week after lesion. This comparison suggests that possibly the destruction of non-cholinergic basal forebrain cells by ibotenic acid and electrolytic lesion, might partly contribute to these different cortical effects. NMDA receptor binding was markedly reduced and AMPA, kainate, and GABAA receptor binding has been significantly increased in cortical regions displaying a reduced activity of AChE and decreased levels of high-affinity choline uptake sites due to immunolesion of the basal forebrain cholinergic system. Equivalent changes in cortical glutamate and GABA receptor subtype levels have been observed 7 days after electrolytic or ibotenic acid lesion of the Nbm. The data suggest that cholinergic immunolesion by 192IgG-saporin exhibits a valuable tool to produce specific cholinergic deficits in rats, which can be used as a model to study the effect of treatment with various drugs for compensating the impaired cortical cholinergic input.

Behavioral and neurobiological alterations induced by the immunotoxin 192-IgG-saporin: cholinergic and non-cholinergic effects following i.c.v. injection

192-IgG-Saporin is an anti-neuronal immunotoxin that combines the 192 monoclonal antibody to the p75 neurotrophin receptor found on terminals and cell bodies of neurons in the cholinergic basal forebrain with the ribosome-inactivating protein saporin. Bilateral intraventricular injection of the 192-saporin produced a variety of dose-related behavioral, neurochemical, and histological alterations in adult male rats. While both the 2 micrograms and 4 micrograms dose produced comparable cholinergic hypofunction only the high dose produced behavioral changes. Behavioral deficits induced by the 4 micrograms dose of 192-saporin induced alterations in rotorod performance and reactivity on the hot-plate which recovered over 8 weeks. In addition, the 4 micrograms dose produced a persistent impairment in the acquisition and performance of standard Morris water maze task as well as a cued version of the task. The neurobiological alterations induced by 192-saporin involved both cholinergic and non-cholinergic systems. Both doses of 192-saporin produced a 60-80% decrease in high affinity choline transport in the hippocampus and cortex without altering this parameter in the striatum. In addition, there was a significant dose-related decrease of norepinephrine in the hippocampus in the high dose group. 192-saporin did not alter the content of dopamine, serotonin, or their metabolites in any region examined. 192-saporin also produced a loss of Purkinje cells in the cerebellum. This cell type also expresses the p75 receptor and appears to be a target for intraventricular 192-saporin. This complex interplay of factors makes the i.c.v. model of 192-saporin very problematic for studying the functional properties of the cholinergic basal forebrain. However, recent data suggest that injection of 192-saporin directly into components of the cholinergic basal forebrain can be used to further elaborate the function of this brain system and to model disorders of cholinergic hypofunction such as Alzheimer's disease.

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.

Selective lesioning of the basal forebrain cholinergic system by intraventricular 192 IgG-saporin: behavioural, biochemical and stereological studies in the rat

The elucidation of the functional role of the basal forebrain cholinergic system will require access to a highly specific and efficient cholinergic neurotoxin. Recently, selective depletion of the nerve growth factor (NGF) receptor-bearing cholinergic neurons in the rat basal forebrain and a dramatic loss of cholinergic innervation in the related cortical regions have been obtained following intraventricular injection of a newly introduced immunotoxin, 192 IgG-saporin. Here we extend these initial findings and report that administration of increasing doses (1.25, 2.5, 5.0 or 10 micrograms) of the 192 IgG-saporin conjugate into the lateral ventricles of adult rats induced dose-dependent impairments in the water maze task and passive avoidance retention, but only weak and inconsistent effects on locomotor activity. These behavioural changes were paralleled by a reduction in choline acetyltransferase activity in hippocampus and several cortical areas (up to 97%) and selective depletions of NGF receptor-positive cholinergic neurons in the septal-diagonal band area and nucleus basalis magnocellularis (up to 99%). By contrast, the non-cholinergic parvalbumin-containing neurons in the septum were completely spared, and other cholinergic projection systems (such as in the striatum, thalamus, brainstem and spinal cord) were unaffected even at the highest dose. The observed changes in the water maze and passive avoidance tasks, as well as the cholinergic cell loss, were maintained up to at least 8 months following the intraventricular injection of a single dose (5 micrograms) of the immunotoxin. The results confirm the usefulness of the 192 IgG-saporin toxin for selective and profound lesions of the basal forebrain cholinergic neurons and provide further support for a role of the basal forebrain cholinergic system in cognitive functions.

Lesion with the neurotoxin AF64A alters hippocampal cholinergic receptor function

The effect of selective lesion of cholinergic inputs to the hippocampus on the function of hippocampal cholinergic receptors was examined. Hippocampal cholinergic neurons were lesioned in the rat by administration of the selective cholinergic neurotoxin AF64A (ethylcholine mustard aziridinium). Cholinergic receptor function was examined by assessing the ability of cholinergic agonists and antagonists to modulate the evoked release of radiolabelled acetylcholine (ACh) from hippocampal slices. Nicotine enhanced release, with a bell-shaped dose-response curve. The dose-response curve and EC50 for nicotine was shifted 10-fold to the left in lesioned rats, suggesting an increased sensitivity to nicotine. However, there were no differences in either the number of affinity of nicotinic receptors as determined with binding studies. The muscarinic agonist oxotremorine inhibited the evoked release of ACh in control tissues, but had much less effect in AF64A-lesioned tissues. Binding to the M1 receptor subtype was not changed. However, the Kd for binding to the high affinity subtype of the M2 receptor was increased 10-fold, suggesting that the receptor has become less sensitive to stimulation. Loss of M2 function may allow an increase in the effect of stimulating nicotinic receptors that modulate ACh release.

Hemicholinium-3 (HC3) blocks the effects of ethylcholine mustard aziridinium (AF64A) in the developing rat

Two- to 3-day-old rat pups received bilateral intracerebroventricular (i.c.v.) injections of 2.0 nmol/microliters AF64A or vehicle. Half of the pups had been preinjected i.c.v. with hemicholinium-3 (HC3) and the other half with saline. The administration of AF64A impaired spatial learning/memory and caused brain damage characterized by marked loss of forebrain cortical/subcortical tissue and ventricular hypertrophy when these were assessed in adulthood. Neither the behavioral nor the histopathological effects of AF64A were observed in rats that had been pretreated with HC3. Since HC3 is a potent and relatively selective inhibitor of high affinity choline uptake (HACU), the results indicate that the toxic effects of AF64A in the neonatal rat are dependent upon its uptake via the HACU site. If as other research suggests, this site is primarily on Ach neurons in the neonatal rat, then the consequences of neonatal damage to cholinergic neurons are severe for forebrain development.

Cortical cholinergic deafferentation following the intracortical infusion of 192 IgG-saporin: a quantitative histochemical study

The immunotoxin 192 IgG-saporin has been hypothesized to selectively lesion cholinergic neurons that bear the low-affinity p75 nerve growth factor (NGF) receptor. To evaluate the usefulness of this toxin in studies intended to determine the functions of cholinergic afferents of cortical areas, relatively small concentrations and volumes of the immunotoxin (0.01-0.05 micrograms/0.5-1.0 microliters) were infused into cortical areas of one hemisphere of rats, while the vehicle was infused into homologous areas of the contralateral hemisphere. The effects of these infusions on the density of cortical acetylcholinesterase (AChE)-positive fibers and of normal fibers (as revealed by a reduced silver stain) were quantified. The infusion of the immunotoxin did not produce local gliosis in excess of the gliosis resulting from the infusion of vehicle. When compared with the frontoparietal cortex of the intact hemisphere, the number of cortical AChE-positive fibers was reduced by 36-39% and the density of the silver-stained fibers was decreased by 20-25%. While the loss of AChE-positive fibers and silver-stained fibers correlated significantly in layers V/VI, a linear regression analysis suggested that the magnitude of the loss of AChE-positive fibers was greater than would be predicted on the basis of the residual density of normal fibers. Thus, the data suggest that infusions of 192 IgG-saporin into the cortex did not result in the loss of non-cholinergic afferents. Intracortical infusions of relatively small concentrations and volumes of 192 IgG-saporin appear to provide a useful approach for the examination of the functions of cholinergic inputs to specific cortical regions.