Restoration of rhythmic slow activity (theta) in the subcortically denervated hippocampus by fetal CNS transplants

The medial septum controls hippocampal supra-theta oscillations

Hippocampal theta oscillations orchestrate faster beta-to-gamma oscillations facilitating the segmentation of neural representations during navigation and episodic memory. Supra-theta rhythms of hippocampal CA1 are coordinated by local interactions as well as inputs from the entorhinal cortex (EC) and CA3 inputs. However, theta-nested gamma-band activity in the medial septum (MS) suggests that the MS may control supra-theta CA1 oscillations. To address this, we performed multi-electrode recordings of MS and CA1 activity in rodents and found that MS neuron firing showed strong phase-coupling to theta-nested supra-theta episodes and predicted changes in CA1 beta-to-gamma oscillations on a cycle-by-cycle basis. Unique coupling patterns of anatomically defined MS cell types suggested that indirect MS-to-CA1 pathways via the EC and CA3 mediate distinct CA1 gamma-band oscillations. Optogenetic activation of MS parvalbumin-expressing neurons elicited theta-nested beta-to-gamma oscillations in CA1. Thus, the MS orchestrates hippocampal network activity at multiple temporal scales to mediate memory encoding and retrieval.

Theta-gamma cascades and running speed

Oscillations in the hippocampal local field potential at theta and gamma frequencies are prominent during awake behavior and have demonstrated several behavioral correlates. Both oscillations have been observed to increase in amplitude and frequency as a function of running speed. Previous investigations, however, have examined the relationship between speed and each of these oscillation bands separately. Based on energy cascade models where "…perturbations of slow frequencies cause a cascade of energy dissipation at all frequency scales" (Buzsaki G. Rhythms of the Brain, 2006), we hypothesized that cross-frequency interactions between theta and gamma should increase as a function of speed. We examined these relationships across multiple layers of the CA1 subregion, which correspond to synaptic zones receiving different afferents. Across layers, we found a reliable correlation between the power of theta and the power of gamma, indicative of an amplitude-amplitude relationship. Moreover, there was an increase in the coherence between the power of gamma and the phase of theta, demonstrating increased phase-amplitude coupling with speed. Finally, at higher velocities, phase entrainment between theta and gamma increases. These results have important implications and provide new insights regarding how theta and gamma are integrated for neuronal circuit dynamics, with coupling strength determined by the excitatory drive within the hippocampus. Specifically, rather than arguing that different frequencies can be attributed to different psychological processes, we contend that cognitive processes occur across multiple frequency bands simultaneously with organization occurring as a function of the amount of energy iteratively propagated through the brain. NEW & NOTEWORTHY Often, the theta and gamma oscillations in the hippocampus have been believed to be a consequence of two marginally overlapping phenomena. This perspective, however, runs counter to an alternative hypothesis in which a slow-frequency, high-amplitude oscillation provides energy that cascades into higher frequency, lower amplitude oscillations. We found that as running speed increases, all measures of cross-frequency theta-gamma coupling intensify, providing evidence in favor of the energy cascade hypothesis.

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.

Hippocampal sharp wave-ripple: A cognitive biomarker for episodic memory and planning

Sharp wave ripples (SPW-Rs) represent the most synchronous population pattern in the mammalian brain. Their excitatory output affects a wide area of the cortex and several subcortical nuclei. SPW-Rs occur during "off-line" states of the brain, associated with consummatory behaviors and non-REM sleep, and are influenced by numerous neurotransmitters and neuromodulators. They arise from the excitatory recurrent system of the CA3 region and the SPW-induced excitation brings about a fast network oscillation (ripple) in CA1. The spike content of SPW-Rs is temporally and spatially coordinated by a consortium of interneurons to replay fragments of waking neuronal sequences in a compressed format. SPW-Rs assist in transferring this compressed hippocampal representation to distributed circuits to support memory consolidation; selective disruption of SPW-Rs interferes with memory. Recently acquired and pre-existing information are combined during SPW-R replay to influence decisions, plan actions and, potentially, allow for creative thoughts. In addition to the widely studied contribution to memory, SPW-Rs may also affect endocrine function via activation of hypothalamic circuits. Alteration of the physiological mechanisms supporting SPW-Rs leads to their pathological conversion, "p-ripples," which are a marker of epileptogenic tissue and can be observed in rodent models of schizophrenia and Alzheimer's Disease. Mechanisms for SPW-R genesis and function are discussed in this review.

Ripples make waves: binding structured activity and plasticity in hippocampal networks

Establishing novel episodic memories and stable spatial representations depends on an exquisitely choreographed, multistage process involving the online encoding and offline consolidation of sensory information, a process that is largely dependent on the hippocampus. Each step is influenced by distinct neural network states that influence the pattern of activation across cellular assemblies. In recent years, the occurrence of hippocampal sharp wave ripple (SWR) oscillations has emerged as a potentially vital network phenomenon mediating the steps between encoding and consolidation, both at a cellular and network level by promoting the rapid replay and reactivation of recent activity patterns. Such events facilitate memory formation by optimising the conditions for synaptic plasticity to occur between contingent neural elements. In this paper, we explore the ways in which SWRs and other network events can bridge the gap between spatiomnemonic processing at cellular/synaptic and network levels in the hippocampus.

Recent advancements in stem cell and gene therapies for neurological disorders and intractable epilepsy

The potential applications of stem cell therapies for treating neurological disorders are enormous. Many laboratories are focusing on stem cell treatments for CNS diseases, including spinal cord injury, Amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease, multiple sclerosis, stroke, traumatic brain injury, and epilepsy. Among the many stem cell types under testing for neurological treatments, the most common are fetal and adult brain stem cells, embryonic stem cells, induced pluripotent stem cells, and mesenchymal stem cells. An expanding toolbox of molecular probes is now available to allow analyses of neural stem cell fates prior to and after transplantation. Concomitantly, protocols are being developed to direct the fates of stem cell-derived neural progenitors, and also to screen stem cells for tumorigenicity and aneuploidy. The rapid progress in the field suggests that novel stem cell and gene therapies for neurological disorders are in the pipeline.

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.

Fimbria-Fornix Lesions Compromise the Induction of Long-Term Potentiation at the Schaffer Collateral-CA1 Synapse in the Rat In Vivo

Although bilateral fimbria-fornix (FF) lesioning impairs spatial performance in animals, the literature is equivocal regarding its effects on hippocampal long-term potentiation (LTP). We examined the effects of FF lesioning on LTP induction in the Schaffer collateral–CA1 pathway in vivo with a protocol that delivered theta burst stimulation (TBS) trains of increasing length until a sufficient length was reached to induce LTP of the monosynaptic field excitatory postsynaptic potential (fEPSP). Experiments were performed in urethan-anesthetized Long-Evans rats either 4 or 12–16 wk after lesioning. In sham-operated controls, TBS trains ranging from 4 to 12 bursts were sufficient to induce robust LTP [170 ± 10% (mean ± SF) of control fEPSP slope; n = 8]. Four-week post -FF-lesioned animals also displayed clear LTP (167 ± 12% of control fEPSP slope; n = 4) that did not differ from the shams ( P > 0.05). In contrast, animals in the 12- to 16-wk post-lesion group showed a highly significant deficit in LTP induction (95 ± 3% of control fEPSP slope; n = 8; ≤28 burst TBS trains tested; P < 0.001 vs. sham- and 4-wk post-FF-lesion groups). Other quantitative measures of synaptic excitability (i.e., baseline fEPSP slope and input-output relation) did not differ between the sham- and the 12- to 16-wk post-FF-lesion groups. These results indicate that the FF lesion leads to an enduring defect in hippocampal long-term synaptic plasticity that may relate mechanistically to the cognitive deficits characterized in this model.

Theta oscillations in the hippocampus

Theta oscillations represent the "on-line" state of the hippocampus. The extracellular currents underlying theta waves are generated mainly by the entorhinal input, CA3 (Schaffer) collaterals, and voltage-dependent Ca(2+) currents in pyramidal cell dendrites. The rhythm is believed to be critical for temporal coding/decoding of active neuronal ensembles and the modification of synaptic weights. Nevertheless, numerous critical issues regarding both the generation of theta oscillations and their functional significance remain challenges for future research.

Effects of metrifonate on the hippocampal theta rhythm of freely moving intact and MS-lesioned mice

Changes in hippocampal electroencephalogram (EEG) have been suggested to be closely associated with spatial learning ability. Spatial learning can be improved in medial septal (MS)-lesioned mice by metrifonate, a cholinesterase inhibitor. We designed this study to investigate the effects of metrifonate on the hippocampal theta oscillation of intact and MS-lesioned mice. Intact and MS-lesioned C57BL mice were treated with acute injections of metrifonate (doses: 15, 50 and 100 mg/kg ip). These included a dose that considerably improved spatial memory of MS-lesioned mice in our earlier study. In addition, subtype selective muscarinic agents, BIBN-99, AF267B and AF150(S) were used. Recordings of hippocampal theta during movement and awake immobility revealed a dramatic reduction of theta in the lesioned animals. Metrifonate induced prominent changes in the EEG of intact mice, but not of MS-lesioned mice. The effect of metrifonate was not mimicked by two selective M(1)-agonists and was augmented by a combined injection of a selective M(2)-antagonist. These data suggest that improved spatial learning by the cholinesterase inhibitor metrifonate is unrelated to its effects on the hippocampal EEG. These two effects may be mediated through different muscarinic receptor subtypes.

Sustained activation of hippocampal pyramidal cells by 'space clamping' in a running wheel

In contrast to sensory cortical areas of the brain, the relevant physiological inputs to the hippocampus, leading to selective activation of pyramidal cells, are largely unknown. Pyramidal cells are thought to be phasically activated by spatial cues and a variety of sensory and motor stimuli. Here, we used a behavioural 'space clamp' method, which involved the confinement of the actively running animal in a defined position in space (running wheel) and kept sensory inputs constant. Twelve percent of the recorded CA1 pyramidal cells were selectively active while the rat was running in the wheel. Cell firing was specific to the direction of running and disappeared after rotating the recording apparatus. The discharge frequency of pyramidal cells and interneurons was sustained as long as the rat ran continuously in the wheel. Furthermore, the discharge frequency of pyramidal cells and interneurons increased with increasing running velocity, even though the frequency of hippocampal theta waves remained constant. The discharge frequency of some 'wheel-related' pyramidal cells could increase more than 10-fold between 10 and 100 cm/s, whereas the firing rate of 'non-wheel' cells remained constantly low. We hypothesize that: (i) a necessary condition for place-specific discharge of hippocampal pyramidal cells is the presence of theta oscillation; and (ii) relevant stimuli can tonically and selectively activate hippocampal pyramidal cells as long as theta activity is present.

Hippocampal grafts of acetylcholine-producing cells are sufficient to improve behavioural performance following a unilateral fimbria-fornix lesion

Lesions of the septohippocampal pathway produce cognitive deficits that are partially attenuated by grafts of cholinergic-rich tissue into denervated target regions or by systemic administration of cholinomimetic drugs. In the present study, fibroblasts engineered to produce acetylcholine were used to test the hypothesis that restoration of hippocampal acetylcholine in rats with septohippocampal lesions is sufficient to improve cognitive processing post-damage. Rats received unilateral grafts of acetylcholine-producing or control fibroblasts into the hippocampus immediately prior to an aspirative lesion of the ipsilateral fimbria-fornix. Some rats with fimbria-fornix lesions were implanted with acetylcholine-producing or control fibroblasts into the neocortex, another major target of the basal forebrain cholinergic system, to determine if the site of acetylcholine delivery to the damaged brain is critical for functional recovery. Rats were tested in a hidden platform water maze task, a cued water maze task and activity chambers between one and three weeks post-grafting. Compared to unoperated controls, rats with fimbria fornix lesions only were significantly impaired in hidden platform water maze performance. Hippocampal grafts of acetylcholine-producing cells reduced lesion-induced deficits in the water maze, whereas hippocampal control grafts and cortical grafts of either cell type were without effect. Locomotor activity and cued water maze performance were unaffected by the lesion or the implants. Taken together, these data indicate that water maze deficits produced by fimbria fornix lesions, which disrupt a number of hippocampal neurotransmitter systems, can be attenuated by target specific replacement of acetylcholine in the hippocampus and that this recovery occurs in the absence of circuitry repair.

Recovery of spatial performance in the Morris water maze following bilateral transection of the fimbria/fornix in rats

The present study investigated whether spatial performance in the Morris water maze (MWM) recovers after bilateral transection of the fimbria/fornix (FF) in rats, whether such recovery results from restored or residual spatial cognitive capacity, and what contribution, if any, pre-operative training makes to such recovery. Following surgery, rats were administered extensive training to a constant submerged platform location with frequent probe tests to assess performance strategies. Following the attainment of asymptotic performance levels, rats were tested for acquisition of a second platform location. FF lesions were found to produce a severe impairment both in pre-operatively trained rats (a retention or retrieval deficit) and in naive rats (an acquisition deficit) as shown by the use of indirect routes to the platform on submerged platform trials and an absence of localized searching in the platform's area on probe trials. However, with further training, performance recovered in both groups, such that they eventually used direct escape routes to the submerged platform and showed highly localized searching in its area on probe trials. When tested for acquisition of a second platform location, a substantial deficit reappeared, but was again overcome with additional training. Pre-operative training was found to attenuate the initial post-operative deficit and speed recovery of performance but did not affect asymptotic performance levels nor acquisition of the second platform location. These data show that, though spatial cognition as assessed in the MWM is impaired by FF lesions, spatial performance eventually recovers. Moreover, pre-operative training, though of some initial post-operative benefit, is not essential for this recovery. The deficit shown in acquisition of the second platform location argues against recovery of spatial cognition and suggests that the basis of recovered performance is residual spatial cognitive capacity. Several limitations of this residual capacity are apparent: (i) rate of acquisition of spatial information is reduced; (ii) utilization of spatial information stored pre-operatively is restricted; and (iii) translation of spatial information into navigational behaviour is less efficient. The neural bases of this residual system are speculated to include spared intra-hippocampal storage mechanisms and/or mechanisms involved in extra-hippocampal long-term memory consolidation while the neural bases of the FF's contribution to spatial information storage in the intact brain are speculated to involve theta synchronization of hippocampal activity and the induction and expression of hippocampal long-term potentiation.

Intra-retrosplenial cortical grafts of cholinergic neurons: functional incorporation and restoration of high affinity choline uptake

Fetal septal neurons transplanted into the deafferented retrosplenial cortex (RSC) of rats have been shown to reinnervate the host brain and ameliorate spatial memory deficits. In the present study we examined the effects of implanting cholinergic neurons on high affinity choline uptake (HACU) in the denervated RSC and the correlational relationship between this cholinergic parameter and the level of behavioral recovery. Three groups of animals were used: 1) normal control rats (NC), 2) rats with lesions of the fornix and cingulate pathways (FX), and 3) lesioned rats with fetal septal grafts in the RSC (RSCsep-TPL). We found that intra-RSC septal grafts produced significant increases in HACU, and that recovery of HACU was significantly correlated with the improvements in the performance of spatial reference memory, spatial navigation, and spatial working memory tasks. We have also investigated the ability of the host brain to modulate the activity of the implanted neurons. In particular we evaluated the effect of the animals' performance in a 6-arm radial maze task on high affinity choline uptake (HACU). Animals in each of the NC, FX, and RSCsep-TPL groups were randomly assigned one of the following subgroups: 1) rats that performed the maze task before the determination of HACU (BEH), or 2) rats that did not perform the maze task before the determination of HACU (NON-BEH). Significant increases were observed in the NC and RSCsep-TPL groups, but not in the FX animals, indicating that fetal septal grafts in the RSC can become functionally incorporated with the host neural circuitry, and that the activity of the implanted cholinergic neurons can be modulated by the host brain.

Dense GABAergic input on somata of parvalbumin-immunoreactive GABAergic neurons in the hippocampus of the mouse

GABAergic neurons in the hippocampus proper are greatly diverse in their morphological and physiological features. In the present study we examined whether or not they are also diverse regarding the density of GABAergic input on their somata. GABAergic neurons were immunocytochemically identified with antibodies against glutamic acid decarboxylase (GAD), and the densities of GAD-immunoreactive (GAD-IR) boutons that abutted on GAD-IR somata were estimated by conventional light microscopic, combined light and electron microscopic, and confocal laser scanning microscopic analyses. GAD-IR somata were apparently diverse regarding the density of GABAergic input on them, and those surrounded by higher densities of GAD-IR boutons were distributed mainly in the strata pyramidale and oriens of the CA3 and CA1 regions and could be correlated to a parvalbumin (PV)-IR subpopulation of GABAergic neurons. Quantitative analysis clearly revealed the statistically significant difference between PV-positive and PV-negative GAD-IR neurons in the densities of their somatic GAD-IR boutons. Particularly, most of PV-IR neurons in the CA3 stratum pyramidale as well as some in other layers are characterized by an exceedingly high density of perisomatic GAD-IR boutons. Furthermore, the majority of GAD-IR boutons on PV-IR somata in the stratum pyramidale were also PV-IR. Bilateral transection of the fimbria-fornix, which was supposed to remove GABAergic afferents from the septum, had only partial effects on the densities of PV-IR boutons on PV-IR somata, indicating these PV-IR boutons mainly originated from intrinsic PV-IR neurons. These observations indicate the dense mutual connection between PV-IR GABAergic neurons through perisomatic synaptic contacts, particularly in the stratum pyramidale.

Analysis of gamma rhythms in the rat hippocampus in vitro and in vivo

1. We have shown previously, with experimental and computer models, how a '40 Hz' (gamma) oscillation can arise in networks of hippocampal interneurones, involving mutual GABAA-mediated synaptic inhibition and a source of tonic excitatory input. Here, we explore implications of this model for some hippocampal network phenomena in the rat in vitro and in vivo. 2. A model network was constructed of 1024 CA3 pyramidal cells and 256 interneurones. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid), NMDA (N-methyl-D-aspartate), GABAA and GABAB receptors were simulated on pyramidal cells and on interneurones. 3. In both model and experiment, the frequency of network oscillations, in the gamma range, depended upon three parameters: GABAA conductance and decay time constant in interneurone-->interneurone connections, and the driving current to the interneurones. 4. The model of gamma rhythm predicts an average zero phase lag between firing of pyramidal cells and interneurones, as observed in the rat hippocampus in vivo. The model also reproduces a gamma rhythm whose frequency changes with time, at theta frequency (about 5 Hz). This occurs when there is 5 Hz modulation of a tonic signal to chandelier and basket cells. 5. Synchronized bursts can be produced in the model by several means, including partial blockade of GABAA receptors or of AMPA receptors on interneurones, or by augmenting AMPA-mediated EPSCs. In all of these cases, the burst can be followed by a 'tail' of transiently occurring gamma waves, a phenomenon observed in the hippocampus in vivo following sharp waves. This tail occurs in the model because of delayed excitation of the interneurones by the synchronized burst. A tail of gamma activity was found after synchronized epileptiform bursts both in the hippocampal slice (CA3 region) and in vivo. 6. Our data suggest that gamma-frequency EEG activity arises in the hippocampus when pools of interneurones receive a tonic or slowly varying excitation. The frequency of the oscillation depends upon the strength of this excitation and on the parameters regulating the inhibitory coupling between the interneurones. The interneurone network output is then imposed upon pyramidal neurones in the form of rhythmic synchronized IPSPs.

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.

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.