Effects of septal and/or raphe cell suspension grafts on hippocampal choline acetyltransferase activity, high affinity synaptosomal uptake of choline and serotonin, and behavior in rats with extensive septohippocampal lesions

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.

Compensatory mechanisms enhance hippocampal acetylcholine release in transgenic mice expressing human acetylcholinesterase

Central cholinergic neurotransmission was studied in learning-impaired transgenic mice expressing human acetylcholinesterase (hAChE-Tg). Total catalytic activity of AChE was approximately twofold higher in synaptosomes from hippocampus, striatum and cortex of hAChE-Tg mice as compared with controls (FVB/N mice). Extracellular acetylcholine (ACh) levels in the hippocampus, monitored by microdialysis in the absence or presence of 10(-8)-10(-3) M neostigmine in the perfusion fluid, were indistinguishable in freely moving control and hAChE-Tg mice. Muscarinic receptor functions were unchanged as indicated by similar effects of scopolamine on ACh release and of carbachol on inositol phosphate formation. However, when the mice were anaesthetized with halothane (0.8 vol. %), hippocampal ACh reached significantly lower levels in AChE-Tg mice as compared with controls. Also, the high-affinity choline uptake (HACU) in hippocampal synaptosomes from awake hAChE-Tg mice was accelerated but was reduced by halothane anaesthesia. Moreover, hAChE-Tg mice displayed increased motor activity in novel but not in familiar environment and presented reduced anxiety in the elevated plus-maze test. Systemic application of a low dose of physostigmine (100 microgram/kg i.p.) normalized all of the enhanced parameters in hAChE-Tg mice: spontaneous motor activity, hippocampal ACh efflux and hippocampal HACU, attributing these parameters to the hypocholinergic state due to excessive AChE activity. We conclude that, in hAChE-Tg mice, hippocampal ACh release is up-regulated in response to external stimuli thereby facilitating cholinergic neurotransmission. Such compensatory phenomena most likely play important roles in counteracting functional deficits in mammals with central cholinergic dysfunctions.

Serotonin release from mesencephalic raphe neurons grafted to the 5,7-dihydroxytryptamine-lesioned rat hippocampus: effects of behavioral activation and stress

Transplants of fetal midbrain raphe neurons into the adult brain have been shown to promote recovery of complex behavioral deficits in several experimental models, but the mechanisms underlying these effects are only partially understood. In the present study, we have used a well-characterized model system to ascertain whether midbrain raphe graft can display behaviorally relevant changes in transmitter release and/or metabolism. Fetal mesencephalic raphe neurons were grafted unilaterally into the hippocampus previously deprived of its innate serotonergic innervation by intraventricular injections of 5,7-dihydroxytryptamine. The contralateral hippocampus remained as a nongrafted, lesioned control. Microdialysis probes were implanted in the hippocampus 5-7 months postgrafting. Under baseline conditions, extracellular levels of serotonin were similar to normal in the grafted hippocampi, but undetectable on the contralateral, nongrafted side. Levels of the serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA), were markedly higher than normal in the grafted hippocampi, but dramatically reduced on the contralateral nongrafted side. Handling stimulation (gentle stroking of a rat's fur and tail for 15 min) induced a 64% increase in serotonin output in the intact rats and a small but significant 12% increase in the grafted animals. Non-noxious tail-pinch (15 min) enhanced serotonin release by 86% in the intact rats and 28% in the grafted ones. Extracellular 5-HIAA levels remained unchanged during both handling and tail-pinch in both the intact and the grafted rats. Forced immobilization of the rats for 15 min induced a transient 124% increase in extracellular serotonin levels in the intact rats and a significant 19% increase in the grafted animals, whereas swimming in temperate water (25-30 degrees C; 15 min) induced no detectable changes in serotonin output in any of the groups. 5-HIAA levels remained unchanged during forced immobilization, but were significantly reduced during the swimming session in both the intact (-38%) and grafted (-15%) animals. The present results indicate that median raphe grafts can become functionally integrated in the denervated host hippocampus and respond by altered indole output when the animal is exposed to different types of environmental challenges.

Modulation of acetylcholine and 5-hydroxytryptamine release in hippocampal slices of rats with fimbria-fornix lesions and intrahippocampal grafts containing cholinergic and/or serotonergic neurons

Three-month-old Long-Evans female rats sustained aspirative lesions of the dorsal septohippocampal pathways and, 2 weeks later, received intrahippocampal suspension grafts containing fetal cells from the mesencephalic raphe (rich in serotonergic neurons; RAPHE), the medial septum and the diagonal band of Broca (rich in cholinergic neurons; SEPT), or a mixture of both (COTR). Lesion-only (LES) and sham-operated rats (SHAM) were used as controls. Hippocampal slices of these rats (5-9 month after surgery) were preincubated with [3H]choline or [3H]5-HT, superfused continuously (in the presence of hemicholinium-3 or fluvoxamine) and stimulated electrically (360 pulses, 2 ms, 3 Hz, 26-28 mA) in order to study the presynaptic modulation of acetylcholine (ACh) and serotonin (5-HT) release. The accumulation of [3H]choline and the evoked overflow of [3H]ACh were significantly reduced in slices from LES and RAPHE rats, but reached a close-to-normal level in SEPT and COTR rats. As to accumulation and overflow of [3H]5-HT, the lesion-induced reduction was compensated for only in RAPHE and COTR rats. The relative amount of evoked [3H]5-HT release (in % of tissue-3H) was significantly increased in LES and SEPT rats. Only slight differences (group LES) were found in the sensitivity of muscarinic and serotonergic autoreceptors towards oxotremorine and CP 93,129, respectively. Moreover, CP 93,129 induced a significantly weaker inhibition of ACh release in slices of COTR rats than in all other groups. Using the 5-HT1A receptor agonist 8-OH-DPAT and antagonist Way 100,635, no evidence for a modulatory influence of 5-HT1A receptors was found in RAPHE and COTR rats. It is concluded that despite substantial lesion- and graft-induced changes in the amount of ACh and 5-HT released by hippocampal slices of lesion-only or grafted rats, the presynaptic modulation of these transmitters is only slightly affected by changes in the neuronal environment.

Intrahippocampal grafts containing cholinergic and serotonergic fetal neurons ameliorate spatial reference but not working memory in rats with fimbria-fornix/cingular bundle lesions

Three-month-old Long-Evans female rats sustained aspirative lesions of the dorsal septohippocampal pathways and, 2 weeks later, received intrahippocampal suspension grafts containing cells from the mesencephalic raphe, cells from the medial septum and the diagonal band of Broca, or a mixture of both. Lesion-only and sham-operated rats were used as controls. All rats were tested for locomotor activity 1 week, 3 and 5 months after lesion surgery, for spatial working memory in a radial maze from 5 to 9 months, and for reference and working memory in a water tank during the 9th month after lesioning. Determination of hippocampal concentration of acetylcholine, noradrenaline, and serotonin was made after completion of behavioral testing. Compared to sham-operated rats, all rats with lesions, whether grafted or not, exhibited increased levels of locomotor activity and made more errors in the radial maze. The lesioned rats were also impaired in the probe trial (30 first seconds) of the water-tank test made according to a protocol requiring intact reference memory capabilities. While rats with septal or raphe grafts were also impaired, the rats with co-grafts showed performances not significantly different from those of sham-operated rats. With a protocol requiring intact working memory capabilities, all lesioned rats, whether grafted or not, were impaired in the water-tank test. In the dorsal hippocampus of lesion-only rats, the concentration of acetylcholine and serotonin was significantly reduced. In rats with septal grafts or co-grafts, the concentration of acetylcholine was close to normal, as was that of serotonin in rats with raphe grafts or co-grafts. These results confirm previous findings showing that co-grafts enabled the neurochemical properties of single grafts to be combined. Data from the water-tank test suggest that cholinergic and serotonergic hippocampal reinnervations by fetal cell grafts may induce partial recovery of spatial reference, but not working memory capabilities in rats.

The potentiation of amphetamine-induced hyperlocomotion by fimbria-fornix lesions in rats is abolished by intrahippocampal grafts rich in serotonergic neurons

Three-month old Long-Evans female rats were submitted to aspirative lesions of the fimbria-fornix and intrahippocampal grafts of a cell suspension prepared from a region of the fetal brain including the septum and the diagonal band of Broca (rich in cholinergic neurons) or the raphe (rich in serotonergic neurons). A group of lesioned rats was grafted with both suspensions mixed. Lesion-only and sham-operated rats served as controls. Four months after the lesions, all rats were tested daily for locomotor activity in their home cage, 1 day without being injected, 2 days with an injection of NaCl and 5 days with an injection of 1 mg/kg (i.p.) d-amphetamine. The effects of the lesions and grafts were assessed by measuring the accumulation of [3H]-choline or [3H]-5-hydroxytryptamine (5-HT) by hippocampal slices, and the electrically-evoked release of tritium. Amphetamine injections produced hyperlocomotion which was potentiated by the lesion. This lesion-induced potentiation was also found in rats with septal grafts, but not in those with raphe or co-grafts. The uptake and electrically-evoked release of [3H]-acetylcholine or [3H]-5-HT were reduced in hippocampal slices from lesion-only rats. In rats which received grafts of septal cells or co-grafts, but not in those with raphe grafts, uptake and release of [3H]-acetylcholine were close to normal. Uptake and release of [3H]-5-HT were close to normal in rats with raphe grafts or with co-grafts, but not in those with septal grafts. Altogether, these data suggest that damage to the serotonergic afferents of the hippocampus might play some role in the potentiation of amphetamine-induced hyperlocomotion associated with fimbria-fornix lesions.

Intracerebral fetal raphé implants normalize hippocampal function but not cerebrovascular control in serotonin-depleted adult rat brain

The effects of hypercapnia upon local cerebral blood flow and local cerebral glucose utilization were measured by quantitative autoradiography in parallel groups of rats (six per group) which 14-16 weeks previously had been treated with the serotonergic neurotoxin, methylenedioxymethamphetamine, followed by implantation of fetal raphé or basal forebrain tissues. Following the experiments, transplants were visualized by acetylcholinesterase histochemistry, and serotonergic reinnervation assessed using [3H]paroxetine binding to serotonin reuptake sites. In methylenedioxymethamphetamine-treated rats, contralateral to the implants, [3H]paroxetine binding was reduced by between 50 and 90% in the neocortex and hippocampus. Hippocampal glucose utilization was significantly increased in these rats, and the normal increase in flow which accompanies hypercapnia was also significantly enhanced. High levels of [3H]paroxetine binding were found within the raphé transplants (308 +/- 13 fmol/mg tissue). In host brain adjacent to the implant, binding levels were normalized, and in these same areas glucose utilization was also normalized. Basal forebrain implants had no effect upon either [3H]paroxetine binding or glucose utilization. Raphé transplants did not, however, alter the enhanced cerebrovascular response to hypercapnia induced by methylenedioxymethamphetamine, even in those areas where there was evidence of serotonergic reinnervation. The transplants also showed the same enhanced response. In conclusion, intracerebral fetal raphé implants normalize hippocampal function but not cerebrovascular control in serotonin-depleted adult rat brain, and despite not sharing the serotonergic deficit, blood flow in the implants follows that of the dysfunctional host.

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.

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.

Serotonin, aging and cognitive functions of the hippocampus

The cholinergic hypothesis of senile dementia /18/ does not provide a sufficient explanation for age-dependent spatial learning deficits; these are observed before an appreciable reduction of cholinergic markers can be detected. Behavioral deficits similar to those observed in old rats cannot be induced in young rats by comparable cholinergic lesions but do occur following combined cholinergic/serotonergic lesions. Serotonergic raphe grafts in the hippocampus (but not in the entorhinal cortex or hypothalamus) prevent such combined lesion-induced spatial learning deficits. The behavioral deficits are associated with a reduction of hippocampal commissure feed-forward inhibition. Similar reduced inhibition is found in old rats, deficient in their performance of a spatial learning water-maze task. Finally, treating old rats with the serotonergic precursor 5-hydroxytryptophan (5-HTP) reduces the age-dependent spatial learning deficits and restores hippocampal commissure feed-forward inhibition. Serotonin may act in parallel to the cholinergic innervation of the hippocampus by affecting inhibitory interneurons but in addition it may act by modulating acetylcholine release. Acetylcholine release is modulated by serotonin and the enhancing effects of serotonin releasing drugs on dentate granule cell excitability are mediated by acetylcholine. We thus propose that a reduction of serotonergic modulation of hippocampal interneuron activity and impaired modulation of cholinergic effects in the hippocampus contribute to age-dependent cognitive deficits.

A comparison of the effects of two fimbria-fornix lesion techniques on beam-walking performance in the rat: aspiration versus electrolysis

This experiment was aimed at comparing the sensorimotor correlates of fimbria-fornix lesions made with either a classical aspiration technique that also removes part of the overlying cortical structures, or an electrolytic one that does not encroach upon these cortical structures. About 4 months after lesion surgery, Long-Evans female rats which had sustained an aspiration or an electrolytic fimbria-fornix lesion at the age of 90 days were tested to measure their beam-walking performance as an index for their sensorimotor capabilities. We found that after an aspiration lesion, the rats presented sensorimotor deficits which did not occur after an electrolytic lesion. After having found that electrolytic lesions of the fimbria and the fornix produced neurochemical deficits (in the dorsal hippocampus) and cognitive alterations close to those resulting from aspiration lesions, it is concluded from the present experiment that the electrolytic lesion technique is an interesting alternative to an aspiration technique, essentially because the former does not induce the sensorimotor deficits due to the partial damage that an aspiration technique produces in the medial parietal cortex. As the electrolytic lesion technique may minimize the risk of introducing a sensorimotor bias in the accuracy of cognitive evaluations, the present result might be of interest to neuroscientist using a fimbria-fornix lesion paradigm in order to investigate the efficacy of drugs, grafts or other treatments on the recovery from cognitive deficits.

Downregulation of muscarinic- and 5-HT1B-mediated modulation of [3H]acetylcholine release in hippocampal slices of rats with fimbria-fornix lesions and intrahippocampal grafts of septal origin

Adult Long-Evans female rats sustained electrolytic fimbria-fornix lesions and, two weeks later, received intrahippocampal suspension grafts of fetal septal tissue. Sham-operated and lesion-only rats served as controls. Between 6.5 and 8 months after grafting, both the [3H]choline accumulation and the electrically evoked [3H]acetylcholine ([3H]ACh) release were assessed in hippocampal slices. The release of [3H]ACh was measured in presence of atropine (muscarinic antagonist, 1 microM), physostigmine (acetylcholinesterase inhibitor, 0.1 microM), oxotremorine (muscarinic agonist, 0.01 microM-10 microM), mecamylamine (nicotinic antagonist, 10 microM), methiothepin (mixed 5-HT1/5-HT2 antagonist, 10 microM), 8-OH-DPAT (5-HT1A agonist, 1 microM), 2-methyl-serotonin (5-HT3 agonist, 1 microM) and CP 93129 (5-HT1B agonist, 0.1 microM-100 microM), or without any drug application as a control. In lesion-only rats, the specific accumulation of [3H]choline was reduced to 46% of normal and the release of [3H]ACh to 32% (nCi) and 43% (% of tissue tritium content). In the grafted rats, these parameters were significantly increased to 63%, 98% and 116% of control, respectively. Physostigmine reduced the evoked [3H]ACh release and was significantly more effective in grafted (-70%) than in sham-operated (-56%) or lesion-only (-54%) rats. When physostigmine was superfused throughout, mecamylamine had no effect. Conversely, atropine induced a significant increase of [3H]ACh release in all groups, but this increase was significantly larger in sham-operated rats (+209%) than in the other groups (lesioned: +80%; grafted: +117%). Oxotremorine dose-dependently decreased the [3H]ACh release, but in lesion-only rats, this effect was significantly lower than in sham-operated rats. Whatever group was considered, 8-OH-DPAT, methiothepin and 2-methyl-serotonin failed to induce any significant effect on [3H]ACh release. In contrast, CP 93129 dose-dependently decreased [3H]ACh release. This effect was significantly weaker in grafted rats than in the rats of the two other groups. Our data confirm that cholinergic terminals in the intact hippocampus possess inhibitory muscarinic autoreceptors and serotonin heteroreceptors of the 5-HT1B subtype. They also show that both types of receptors are still operative in the cholinergic terminals which survived the lesions and in the grafted cholinergic neurons. However, the muscarinic receptors in both lesioned and grafted rats, as well as the 5-HT1B receptors in grafted rats show a sensitivity which seems to be downregulated in comparison to that found in sham-operated rats. In the grafted rats, both types of downregulations might contribute to (or reflect) an increased cholinergic function that results from a reduction of the inhibitory tonus which ACh and serotonin exert at the level of the cholinergic terminal.

Serotonergic modulation of cholinergic function in the central nervous system: cognitive implications

Accumulating evidence suggests that serotonin may modulate cholinergic function in several regions of the mammalian brain and that these serotonergic/cholinergic interactions influence cognition. The first part of this review is an overview of histological, electrophysiological and pharmacological (in vitro, in vivo) data indicating that, in several brain regions (e.g., hippocampus, cortex and striatum), there are neuroanatomical substrates for a serotonergic/cholinergic interaction, and that alterations in serotonergic activity may induce functional changes in cholinergic neurons. In the second part, the review focuses on experimental approaches showing or suggesting that central cholinergic and serotonergic mechanisms are cooperating in the regulation of cognitive functions. These arguments are based on lesion, intracerebral grafting and pharmacological techniques. It is concluded that not all mnesic perturbations induced by concurrent manipulations of the serotonergic and cholinergic systems can be attributed to a serotonergic modification of the cholinergic system. The cognitive faculties of an organism arise from interactions among several neurotransmitter systems within brain structures such as, for instance, the hippocampus or the cortex, but also from influences on memory of other general functions that may involve cerebral substrates different from those classically related to mnesic functions (e.g., attention, arousal, sensory accuracy, etc.).

Cholinergic lesions by 192 IgG-saporin and short-term recognition memory: role of the septohippocampal projection

Two experiments examined the effects of cholinergic basal forebrain lesions by intraventricular and intrahippocampal infusions of the immunotoxin 192 IgG-saporin on recognition memory in an operant delayed-non-matching-to-position task in rats. Intraventricular infusions produced extensive reductions in cortical and hippocampal choline acetyltransferase activity in the first experiment. Behaviourally, a mixed delay-dependent/independent accuracy deficit and increased biased responding was observed post-lesioning. Thus, both mnemonic as well as non-mnemonic processes were affected by the lesion. This performance deficit was indistinguishable from the impairment induced by acute intraventricular injections of the choline uptake inhibitor hemicholinium-3, which suggests that cholinergic damage induced by 192 IgG-saporin disrupted performance. In the second experiment more discrete intrahippocampal 192 IgG-saporin lesions were made, which reduced hippocampal choline acetyltransferase activity about 57%, although this reduction was not as extensive as following intraventricular injections. Although intrahippocampal lesions also impaired non-matching accuracy, this effect failed to reach significance during most stages of the experiment. Scopolamine just failed to significantly impair (P = 0.053) performance in hippocampal lesioned rats more than in controls. The nicotinic antagonist mecamylamine did not affect the lesion-induced changes in performance. These results suggest that the cholinergic basal forebrain, including the septohippocampal system, is important for the mediation of recognition memory, and muscarinic receptor-mediated mechanisms may be of greater importance than alterations of nicotinic receptor-mediated processes in the septohippocampal system.

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.

The role of serotonergic-cholinergic interactions in the mediation of cognitive behaviour

Cholinergic systems have been linked to cognitive processes such as attention, learning and mnemonic function. However, other neurotransmitter systems, such as the serotonergic one, which may have only minor effects on cognitive function on their own, interact with cholinergic function and their combined effects may have marked behavioural actions. Some studies have dealt with serotonergic-cholinergic interactions, but it is unclear whether both systems affect cognition directly or whether interactions at a behavioural level result from additional alterations in non-cognitive factors. This distinction is difficult, since it is possible that the diverse cholinergic and serotonergic systems serve different roles in the mediation of cognitive processes, both at the neuroanatomical and neurochemical level. Nevertheless, it is possible that cholinergic systems primarily alter accuracy in cognitive tasks, whereas serotonergic neurotransmission modulates behaviour by altering bias (motivation, motor processes). Whether serotonin alters accuracy or bias, however, may also depend on the cognitive process under investigation: it is suggested that attention, stimulus processing and/or arousal can be influenced by both cholinergic and serotonergic systems independently from each other. Cholinergic and serotonergic projections to cortex and thalamus may be of importance in the mediation of these cognitive processes. Serotonergic-cholinergic interactions could also be of importance in the mediation of learning processes and trial-by-trial working memory. The data available do not allow an unambiguous conclusion about the role of these interactive processes in the mediation of long-term reference memory. These processes may rely on serotonergic-cholinergic interactions at the hippocampal level. It is concluded that serotonergic-cholinergic interactions play an important role in the mediation of behavioural, including cognitive, performance, but that further studies are necessary in order to elucidate the exact nature of these interactions.

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.