Restoration of learning ability in fornix-transected monkeys after fetal basal forebrain but not fetal hippocampal tissue transplantation

Reconstruction of brain circuitry by neural transplants generated from pluripotent stem cells

Pluripotent stem cells (embryonic stem cells, ESCs, and induced pluripotent stem cells, iPSCs) have the capacity to generate neural progenitors that are intrinsically patterned to undergo differentiation into specific neuronal subtypes and express in vivo properties that match the ones formed during normal embryonic development. Remarkable progress has been made in this field during recent years thanks to the development of more refined protocols for the generation of transplantable neuronal progenitors from pluripotent stem cells, and the access to new tools for tracing of neuronal connectivity and assessment of integration and function of grafted neurons. Recent studies in brains of neonatal mice or rats, as well as in rodent models of brain or spinal cord damage, have shown that ESC- or iPSC-derived neural progenitors can be made to survive and differentiate after transplantation, and that they possess a remarkable capacity to extend axons over long distances and become functionally integrated into host neural circuitry. Here, we summarize these recent developments in the perspective of earlier studies using intracerebral and intraspinal transplants of primary neurons derived from fetal brain, with special focus on the ability of human ESC- and iPSC-derived progenitors to reconstruct damaged neural circuitry in cortex, hippocampus, the nigrostriatal system and the spinal cord, and we discuss the intrinsic and extrinsic factors that determine the growth properties of the grafted neurons and their capacity to establish target-specific long-distance axonal connections in the damaged host brain.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Topographical memory impairments after unilateral lesions of the anterior thalamus and contralateral inferotemporal cortex

Monkeys with crossed unilateral excitotoxic lesions of the anterior thalamus and unilateral inferotemporal cortex ablation were severely impaired at learning two tasks which required the integration of information about the appearance of objects and their positions in space. The lesioned monkeys were also impaired at learning a spatial task and a task which required the integration of information about the appearance of objects and the background on which the objects were situated. Monkeys with only one of the unilateral lesions were not impaired and previous work has shown that monkeys with bilateral lesions of the anterior thalamus were not impaired on these tasks. These results indicate that the whole of the inferotemporal cortex-anterior thalamic circuit, which passes via the hippocampus, fornix, mamillary bodies and mamillothalamic tract, is essential for the topographical analysis of information about specific objects in different positions in space. Together with previous work, the results show that a unilateral lesion may affect cognition in the presence of other brain damage when an equivalent bilateral lesion alone does not. The tasks required the slow acquisition of information into long term memory and therefore assessed semantic knowledge although other research has shown impairment on topographical processing within working or episodic memory following lesions of the hippocampal-diencephalic circuit. It is argued that the hippocampal-diencephalic circuit does not have a role in a specific form of memory such as episodic memory but rather is involved in topographical analysis of the environment in perception and across all types of declarative memory.

Environment-spatial conditional learning in rats with selective lesions of medial septal cholinergic neurons

Cholinergic medial septal neurons may regulate several aspects of hippocampal function, including place field stability and spatial working memory. Monkeys with damage to septal cholinergic neurons are impaired in visual-spatial conditional learning tasks; however, this candidate function of septal cholinergic neurons has not been studied extensively in the rat. In the present study, rats with selective lesions of cholinergic neurons in the medial septum and vertical limb of the diagonal band of Broca (MS/VDB), made with 192 IgG-saporin, were tested on a conditional associative learning task. In this task, which we term "environment-spatial" conditional learning, the correct location of a spatial response depended on the array of local environmental cues. MS/VDB-lesioned rats were impaired when the two parts of the conditional problem were presented concurrently, but not when one environment had been learned before the full conditional problem was presented. Our findings suggest that cholinergic MS/VDB neurons participate in some aspects of conditional associative learning in rats. They may also shed light on the involvement of cholinergic projections to the hippocampus in modulating and remodeling hippocampal spatial representations.

CNS regeneration: clinical possibility or basic science fantasy?

Following injury to the CNS, severed axons undergo a phase of abortive sprouting in the vicinity of the wound, but do not spontaneously re-grow or regenerate. From a long history of attempts to stimulate regeneraion, a major strategy that has been developed clinically is the implantation of tissue into denervated target regions. Unfortunately trials have so far not borne out the promise that this would prove a useful therapy for disorders such as Parkinson's disease. Many strategies have also been developed to stimulate the regeneration of axons across sites of injury, particularly in the spinal cord. Animal data have demonstrated that some of these approaches hold promise and that the spinal cord has a remarkable degree of intrinsic plasticity. Attempts are now being made to utilize experimental techniques in spinal patients.

Role of the hippocampal system in associative learning beyond the spatial domain

Expert opinion remains divided on the issue of whether the hippocampal system functions exclusively in spatial information processing, e.g. in navigation or in understanding spatial relations, or whether it plays a more general role in higher brain function. Previous work on monkeys and rats has tended to support the former view, whereas observations in the clinic point to the latter, including functions as diverse as declarative knowledge, episodic memory, word learning, and understanding relations among objects. One influential theory posits a general role for the hippocampal system in associative learning, with emphasis on associations learned rapidly and recently. The results presented here are consistent with this theory, along with previous clinical and theoretical studies indicating that the hippocampal system is necessary for associative learning even if no component of the association relies on spatial information. In the study reported here, rhesus monkeys learned a series of conditional stimulus-response associations involving complex visual stimuli presented on a video monitor. Each stimulus instructed one of three responses: tapping the stimulus with the hand, steady hand contact with the stimulus for a brief period of time, or steady contact for a longer time. Fornix transection impaired the learning of these associations, even though both the stimuli and the responses were nonspatially differentiated, and this deficit persisted for at least 2 years. This finding indicates that the hippocampal system plays an important role in associative learning regardless of the relevance of spatial information to any aspect of the association. Fornix-transected monkeys were impaired in learning new stimulus-response associations even when the stimuli were highly familiar. Thus, the deficit was one of associating each stimulus with a response, as opposed to problems in distinguishing the stimuli from each other. In contrast to these effects, fornix transection did not impair performance when familiar stimuli instructed a response according to an already-learned association, which shows that the deficit was one of learning new associations rather than one of retention or retrieval of previously learned ones. Taken together, these results show that fornix transection causes a long-lasting impairment in associative learning outside of the spatial domain, in a manner consistent with theories of hippocampal-system function that stress a general role in the rapid acquisition of associative knowledge.

Learning impairments in monkeys with combined but not separate excitotoxic lesions of the anterior and mediodorsal thalamic nuclei

Clinical studies in humans and experiments in macaques suggest that damage to the anterior and the mediodorsal thalamus can induce a moderate amnesia, but a more dense impairment may result from substantial damage within the temporal lobes or their subcortical connections. Lesions of the anterior thalamus in macaques produce impairments which resemble those seen after lesions of the fornix-mamillary pathway, which carries projections from the hippocampus to the anterior thalamus, while lesions of the mediodorsal thalamus, which receives inputs from frontal and temporal cortex, produce moderate impairments on a wider range of memory tasks. In the present study, we have made bilateral excitotoxic lesions of either the anterior or the mediodorsal thalamus, or both, in marmoset monkeys. Monkeys with lesions of both thalamic nuclei were severely impaired on retention and new learning of examples of the visuospatial conditional task, a task which is specifically impaired by lesions of the fornix or hippocampus. They were not impaired on performance of a visuovisual conditional task on which monkeys with hippocampal lesions are impaired, nor were they impaired on any visual discrimination task, including the concurrent discrimination task on which monkeys with temporal neocortical ablations are impaired. Monkeys with separate lesions of either the anterior or the mediodorsal thalamus were not impaired on any of these tasks. These results suggest that the mediodorsal thalamus and the anterior thalamus are both involved in processing the output of the hippocampal-fornix-thalamic circuit. Dense amnesia may result from damage to circuits additional to the temporal lobe efferents to either the anterior or the mediodorsal nuclei.

Crossed unilateral lesions of temporal lobe structures and cholinergic cell bodies impair visual conditional and object discrimination learning in monkeys

Monkeys with excitotoxic lesions of the CA1/subiculum region in the right hemisphere and with immunotoxic lesions of the cholinergic cells of the diagonal band in the left hemisphere were impaired on a visual conditional task. In this task, correct choice of one of two objects depends on which of two background fields both objects are presented against, irrespective of the spatial positions of the objects. They were not impaired on simple object or shape discrimination tasks. The pattern of impairments is the same as that seen after bilateral excitotoxic lesions of CA1/subiculum, implying that the diagonal band lesion disables the ipsilateral CA1/subiculum. It also argues that CA1/subiculum, sustained by its cholinergic input, is necessary for some forms of nonspatial conditional learning. Addition of an inferotemporal (IT) cortical ablation to the left hemisphere did not affect simple visual discrimination learning, although all the monkeys then failed to learn a new visual conditional task. This demonstrates that intact IT cortex in only one hemisphere is sufficient to sustain simple visual discrimination learning but implies that the cholinergic input and the inferotemporal cortical input to the hippocampus both contribute to visual conditional learning. The subsequent addition of an immunotoxic lesion of the basal nucleus of Meynert in the right hemisphere resulted in an additional impairment on a difficult shape discrimination. This argues that it is the cholinergic projection to the inferotemporal cortex, rather than to the rest of the cortex, which contributes to visual discrimination learning and memory.

Non-spatial acquisition and retention deficits following small excitotoxic lesions within the hippocampus in monkeys

Marmoset monkeys with excitotoxic lesions confined to cornu ammonis subfields 1-3, subiculum and pre-subiculum, but sparing the entorhinal cortex, were impaired on retention and learning of conditional object-choice discriminations. For each of these discriminations, the monkeys were required to choose one of two objects depending on which of two patterned backgrounds was used on each trial. Two styles of order of trial presentation were used: 'random' presentation which maximised the degree of interference between trials, and 'runs' presentation which was intended to encourage the monkeys to learn each component of the discrimination separately. Before surgery monkeys found the discriminations more difficult to learn when the trials were presented in the 'runs' style than when presented in the 'random' style suggesting that the task is best learnt by applying a conditional rule. After surgery a significant 'group x style' interaction indicated that the 'runs' style was especially difficult for the lesioned monkeys. From these results we suggest that the hippocampus is involved in learning about and remembering non-spatial, conditional relations between objects.

Visual agnosia and Klüver-Bucy syndrome in marmosets (Callithrix jacchus) following ablation of inferotemporal cortex, with additional mnemonic effects of immunotoxic lesions of cholinergic projections to medial temporal areas

Inferotemporal ablations in the New World monkey, the common marmoset (Callithrix jacchus), produced a persistent impairment on visual discrimination learning and a florid, but transient, Klüver-Bucy syndrome. Monkeys with these ablations were impaired on acquisition of object discriminations to a high criterion and on concurrent discrimination learning, to a single high criterion across all trials. Neither the control monkeys nor the monkeys with inferotemporal ablations found acquisition more difficult when the component discriminations of a set were presented concurrently compared to consecutively, although the monkeys with inferotemporal ablations found acquisition under both these conditions somewhat more difficult than did control monkeys. This suggests that the severe impairment caused by inferotemporal ablations on concurrent learning measured across all trials is due to the need for sustained performance across a concurrent set rather than to the extra mnemonic demands of concurrent presentation. When immunotoxic lesions of the cholinergic projection to the hippocampal formation were added to the inferotemporal ablations, a further impairment on retention, and a differential impairment on concurrent, compared to consecutive, learning was observed. Previous studies have shown that lesions of the cholinergic projection to the hippocampus alone, or excitotoxic hippocampal lesions, do not affect simple visual discrimination learning. It is suggested that large inferotemporal ablations in monkeys produce a visual agnosia which causes severe 'psychic blindness' in the first instance, and a persistent impairment on visual discrimination learning. The hippocampus makes a contribution, which may be mnemonic, to discrimination performance after inferotemporal ablations.

Visual discrimination learning impairments produced by combined transections of the anterior temporal stem, amygdala and fornix in marmoset monkeys

Marmoset monkeys (Callithrix jacchus) with bilateral transections of the anterior temporal stem, amygdala and fornix were unable to relearn a 2-choice object discrimination first learnt prior to surgery, and were very severely impaired at relearning a concurrent object discrimination task which they had learnt and relearnt prior to surgery, indicating that they had a dense retrograde amnesia. They also had difficulty learning new visual object discriminations but were only mildly impaired on spatial learning. When tested on new learning of concurrent discriminations 8 to 10 weeks after surgery, three operated monkeys were unable to reach criterion in 400 trials while the remaining two operated monkeys performed within the normal range. The operated monkeys were subsequently shown to be impaired on acquisition of shape discriminations using black objects. These anterograde effects suggest that the impairment runs mainly in the domain of visual analysis. The monkeys also exhibited many of the features of the Klüver-Bucy syndrome. Histological analysis indicated that in addition to cutting some of the subcortical temporal lobe efferent pathways, the surgical procedures had cut the cholinergic afferents to the temporal neocortex, entorhinal cortex, and hippocampus. In a second experiment we found that treatment with the cholinergic agonist pilocarpine, which is effective in monkeys with specific cholinergic lesions, was unable to remediate the lesion-induced impairments. This suggests that transection of the non-cholinergic afferents, or the temporal lobe subcortical efferents, contributed to the behavioural syndrome and the learning and retention deficits seen in these monkeys.

Cyto- and chemoarchitecture of basal forebrain cholinergic neurons in the common marmoset (Callithrix jacchus)

The cyto- and chemoarchitecture of basal forebrain cholinergic neurons (BFCN) was investigated in the lower primate, the common marmoset (Callithrix jacchus). A large population of magnocellular, hyperchromic, and choline acetyltransferase (ChAT)-positive neurons was detected in the marmoset basal forebrain. The distribution of these neurons was similar to those in higher primates. Thus, ChAT-positive neurons were observed in the medial septum (Ch2), the vertical (Ch2) and horizontal (Ch3) limbs of the diagonal band of Broca, and the nucleus basalis of Meynert (Ch4). The Ch4 complex was relatively well differentiated and displayed distinct sectors. We detected anterior (Ch4a, with a medial and a lateral subdivision), intermediate (Ch4i, with a dorsal and a ventral subdivision), and posterior (Ch4p) sectors in the marmoset Ch4. The Ch4i was relatively small while the Ch4p was large. Similar to the rodent, the marmoset Ch1 extended quite a distance posteriorly, and the Ch4p displayed a major interstitial component distributed within the globus pallidus, its medullary laminae, and the internal capsule. Virtually all of the marmoset BFCN displayed acetylcholinesterase activity, and low affinity (p75(NTR)) and high affinity (Trk) neurotrophin receptor immunoreactivity. A majority contained immunoreactivity for calbindin-D(28K) and calretinin. Many of the Ch4 neurons also displayed tyrosine hydroxylase immunoreactivity. The BFCN lacked galanin immunoreactivity, but were innervated by galanin-positive fibers. None of the marmoset BFCN were NADPH-d-positive. Thus, the BFCN display major anatomical and biochemical differences in the marmoset when compared with higher primates. The marmoset BFCN also display many characteristics common to other primates. This fact, combined with the relatively short life span of the marmoset, indicates that this species may be ideal for studies of age-related changes in the BFCN.

Synergistic effects of unilateral immunolesions of the cholinergic projections from the basal forebrain and contralateral ablations of the inferotemporal cortex and hippocampus in monkeys

Monkeys, with unilateral immunotoxic lesions of the basal nucleus of Meynert that remove cholinergic innervation of the ipsilesional neocortex, and ablations of the contralateral inferotemporal neocortex, were impaired on retention of visual discriminations learnt before surgery and on acquisition of new discriminations. This demonstrates that the cholinergic projection from the basal nucleus supports the functions of its cortical target area. Our previous studies have shown that the impairment on discrimination performance following bilateral lesions of the basal nucleus is transient and that bilateral lesions of the diagonal band of Broca, that remove cholinergic innervation of the hippocampus, are without effect on these tasks. However, the impairment resulting from bilateral lesions of the basal nucleus plus the diagonal band, or from bilateral inferotemporal cortex ablations, is severe and persistent. Bilateral inferotemporal ablations deprive the hippocampus of much of its visual input by producing a discontinuity in cortico-cortical transmission, whereas basal nucleus lesions may merely prevent the modification of visually-derived information in the inferotemporal cortex without depriving the hippocampus of visual input. In the monkeys with crossed unilateral basal nucleus plus inferotemporal cortex lesions, the addition of a diagonal band lesion to the basal nucleus lesion produced an impairment on retention of visual discriminations and sustained the acquisition impairment. This confirms the previous finding that the basal nucleus and diagonal band act synergistically in producing a severe and permanent impairment. Further addition of an excitotoxic hippocampal lesion to the hemisphere with the inferotemporal cortex ablation did not add to the learning impairment. This supports the suggestion that the inferotemporal cortex ablation has deprived the hippocampus of its visual input.Overall, these experiments demonstrate that the cholinergic projections from the basal nucleus and diagonal band participate in the learning and memory functions of the temporal lobes.

Severe learning impairment caused by combined immunotoxic lesion of the cholinergic projections to the cortex and hippocampus in monkeys

Monkeys with immunotoxic lesions of both the basal nucleus of Meynert and the vertical limb of the diagonal band of Broca (NBM+VDB) lost cholinergic innervation throughout the cortex and hippocampus. They were impaired at learning discriminations between objects differing in either few, or many, attributes and at learning visuospatial conditional discriminations. Monkeys with immunotoxic lesions of the NBM lost cholinergic innervation of the neocortex only. Initially, they were unable to learn a simple visual discrimination where the stimuli differed in a limited number of attributes but they were unimpaired at learning discriminations between objects that differed in more attributes. They were mildly impaired at learning a visuospatial conditional task. The impairment exhibited by monkeys with lesions of the NBM alone ameliorated with time but that following NBM+VDB lesions did not. Previous experiments have shown that monkeys with immunotoxic lesions of the VDB alone are impaired at learning visuospatial conditional discriminations but are unimpaired at learning simple visual discriminations. When monkeys with NBM lesions were given excitotoxic lesions of the CA1 field of the hippocampus the learning impairment on discriminations between objects which differed in few attributes was reinstated. Pretreatment with a cholinergic agonist improved learning ability on visual discrimination learning in all monkeys but this improvement was significantly greater in monkeys with lesions of the NBM. On conditional discrimination learning, which is particularly sensitive to hippocampal damage, pilocarpine produced a significant improvement in monkeys with NBM+VDB lesions (where the hippocampal dysfunction was cholinergic) but not in monkeys with NBM+CA1 lesions (where the hippocampal damage was structural).

Estrogen and basal forebrain cholinergic neurons: implications for brain aging and Alzheimer's disease-related cognitive decline

Recent studies suggest that estrogen replacement therapy can reduce the risk and severity of Alzheimer's disease (AD)-related dementia in postmenopausal women. Many different mechanisms by which estrogen therapy may help to reduce the risk and severity of AD-related pathophysiology have been proposed. Recent animal studies suggest that one way in which estrogen replacement may help to reduce cognitive deficits associated with aging and AD is by enhancing the functional status of cholinergic projections to the hippocampus and cortex. Here we review the evidence that estrogen is important in the maintenance of cholinergic neurons projecting to the hippocampus and cortex and that estrogen replacement can enhance the functional status of these neurons, as well as reduce cognitive deficits associated with muscarinic cholinergic impairment. Based on these studies, we conclude that, in animals, short-term treatment with physiological levels of estrogen, or estrogen and progesterone, has significant positive effects on cholinergic neurons in the medial septum and nucleus basalis magnocellularis and on their projections to the hippocampus and cortex. We hypothesize that similar effects in humans may help delay the decline in basal forebrain cholinergic function associated with aging and AD and thereby reduce the risk and severity of AD-related dementia in postmenopausal women.

The role of the central cholinergic projections in cognition: implications of the effects of scopolamine on discrimination learning by monkeys

In humans, administration of the cholinergic antagonist scopolamine impairs the encoding of information into long-term memory and has effects on other cognitive processes. It has been supposed that it is inhibition of the rising cholinergic projections from the basal forebrain, specifically from the basal nucleus of Meynert (NBM) to the neocortex and from the medial septum/vertical limb of the diagonal band of Broca (MS/VDB) to the hippocampus, that results in these cognitive impairments. In this paper, we describe the effects of scopolamine treatment in monkeys on learning different sorts of visual discrimination and visuospatial conditional tasks and compare these results to the effects of lesions of the rising cholinergic projections. Experiments in rodents in which these projections have been selectively destroyed have failed to produce a consensus view of the functions of these two areas. In particular, highly specific immunotoxic lesions of the NBM have largely failed to produce changes in task performance that can be interpreted as resulting from a cognitive impairment. In monkeys, lesions of the NBM produce modest or short-lasting, impairments in visual discrimination learning, retention, and reversal, whereas lesions of the MS/VDB produce large and permanent impairments of certain types of conditional learning. Similar impairments produced by scopolamine in monkeys and additive effects of lesions of the NBM or MS/VDB with scopolamine suggest that scopolamine has these effects by acting on the rising cholinergic pathways rather than on other cholinergic systems in the brain. It is argued that the rising cholinergic projections sustain the functions of the target areas; in the case of the hippocampus in humans, the function is usually regarded as being the analysis of information in a way that is pertinent to the formation of episodic memories and in the case of the neocortex, is the analysis of information in a manner that is relevant to the cognitive processing of on-going events and the acquisition of semantic knowledge.

Learning impairments following injection of a selective cholinergic immunotoxin, ME20.4 IgG-saporin, into the basal nucleus of Meynert in monkeys

Four groups of monkeys (Callithrix jacchus) were injected with saline or increasing amounts of the immunotoxin, ME20.4 IgG-saporin, directly into the basal nucleus of Meynert via a frontal trajectory which avoided damage to the overlying basal ganglia. ME20.4 IgG binds to the primate p75 low-affinity neurotrophin receptor, when the saporin derivitized antibody is injected into the basal forebrain, it selectively destroys the magnocellular neurons of the basal nucleus of Meynert which are the cells of origin of the cholinergic projection to the neocortex. The highest dose of ME20.4 IgG-saporin produced a significant impairment on acquisition of a perceptually difficult visual discrimination. There was no significant effect on retention of tasks learnt before or after surgery, nor on concurrent acquisition of several perceptually easy discriminations or serial reversal of an easy discrimination. These results suggest that the impairment is not due to visual, motor or motivational difficulties and does not consist of difficulties with the formation of reward associations. Rather the impairment is largely confined to acquisition of perceptual discriminations. There was a significant correlation between the density of ME20.4 immunostaining in the basal nucleus of Meynert and the density of acetylcholinesterase histochemical staining in the frontal and temporal cortex and an inverse correlation between both of these and the degree of learning impairment in the animals. Lesioned animals also showed significant impairment on acquisition and reversal of perceptually easy discriminations when treated with a dose of scopolamine which did not impair performance in control animals. These results provide further evidence that cortical cholinergic neurotransmission contributes to certain forms of learning. The availability of a selective cholinergic immunotoxin effective in primates provides an important new tool for the study of cholinergic function and its involvement in ageing, Alzheimer's disease and other pathological states.

Evidence for a specific information processing deficit in monkeys with lesions of the septo-hippocampal system

Monkeys with dysfunction of the septo-hippocampal system induced by excitotoxic lesion of the CA1 region of the hippocampus, or the septal/diagonal band area (which sends cholinergic projections to the hippocampus via the fornix), or with fornix transection were impaired on conditional learning tasks (when X choose A not B, when Y choose B not A) when trials with these different contingencies were presented in pseudorandom order but they were not impaired on learning this type of task if, prior to learning with trials in the pseudorandom order, the two types of trial had been presented in a fixed number of alternating batches of each type of trial. These results suggest that the septo-hippocampal system is required to process information in a particular type of way rather than to process a particular type of information and supports the view that the amnesia which results from medial temporal lobe damage in humans comprises an impairment of encoding information into long-term memory.

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.

Effects of lesions of different parts of the septo-hippocampal system in primates on learning and retention of information acquired before or after surgery

Data from a large series of experiments on marmosets with lesions of the septal/diagonal band area (DB), fornix or CA1 area of the hippocampus are analysed in terms of retention of information learned before surgery, acquisition of new information and retention of information acquired after surgery. It is shown that although all three lesions impair acquisition of a specific type of new information, lesions of CA1 result in a severe retrograde amnesia but no forgetting of that type of information adequately acquired after surgery, whereas lesions of the DB do not cause retrograde amnesia but do result in significant forgetting. Monkeys with fornix transection occupied an intermediate position in their pattern of learning impairments; some animals showed evidence of forgetting, whereas the great majority showed retrograde amnesia. These data may be relevant to an understanding of the different extent of amnesia in patients with different pathology within the medial temporal lobe and associated subcortical structures.

Conditional learning and memory impairments following neurotoxic lesion of the CA1 field of the hippocampus

Monkeys with bilateral lesions of the CA1 field of the hippocampus produced by the injection of neurotoxin diagonally along the length of the hippocampus were found to have a severe impairment on the retention of a conditional task learnt prior to surgery and on the new acquisition of several types of this task. They were equally impaired on conditional tasks that required a spatial response or an object choice in response to either visual or spatial cues. They were not impaired on simple visual discrimination tasks, simple spatial discrimination tasks or reversal learning of these tasks. This patterns of impairment resembles that seen in the same species with neurotoxic lesions within the vertical limb of the diagonal band of Broca or transection of the fornix. Monkeys with subtotal lesions of the adjacent medial temporal area were not consistently impaired on any of these tasks. The results suggest that hippocampal lesions produce anterograde and retrograde amnesia for information other than reward association.

Trophic-factor modulation of cortical acetylcholinesterase reappearance following transection of the medial cholinergic pathway in the adult rat

Laminar patterns of cortical acetylcholinesterase (AChE) activity are reestablished in the adult, pharmacologically unmanipulated rat following axotomy of the medial cholinergic pathway. The extent to which trophic and/or growth promoting or inhibiting agents modulate AChE fiber reappearance is not fully understood. Such studies, however, would further clarify possible roles for these agents in neuronal plasticity in response to injury, as well as in plastic processes associated with normative functions. In the present experiments, we explored trophic modulation by intracortically infusing nerve growth factor (NGF) or somatostatin into cingulate cortex at a site distal to transection of the medial cholinergic pathway. Comparisons were made with sham-operated or noninfused transected controls, as well as with transected animals infused with renin or antibodies against NGF. Administration began 2 days after axotomy and continued at successive 3-day intervals for 4 weeks. It was found that, proximal to the lesion site, NGF increased and somatostatin decreased optical density of AChE; the number of AChE-containing fibers was unaltered compared to controls. Distal to the knife cut, both NGF and somatostatin increased number of AChE fibers but did not alter overall AChE optical density. Nonetheless, NGF produced an increase in the number of intensely staining puncta both proximal and distal to the cut. Neither renin nor anti-NGF antibodies produced statistically significant effects on optical density or number of fibers at any cortical locus studied. We conclude that NGF and somatostatin have opposite effects on the expression of AChE: whereas NGF increases AChE levels, somatostatin inhibits AChE accumulation in proximal fibers, perhaps by actions on synthesis or transport. Fiber proliferation, which only occurred distally, was affected positively by both NGF and somatostatin, indicating that neurite-promoting effects produced by both agents are confined to tissue regions where neurite extension is stimulated by axotomy. Increases in AChE-positive puncta produced by NGF, however, were not confined to regions of fiber proliferation.

Restoration of cognitive abilities by cholinergic grafts in cortex of monkeys with lesions of the basal nucleus of Meynert

Three groups of marmosets were trained to perform a series of visual discrimination tasks in a Wisconsin General Test Apparatus. Two groups then received bilateral lesions of the basal nucleus of Meynert using the excitotoxin N-methyl-D-aspartate and were found to be severely impaired on relearning a visual discrimination first learnt prior to surgery. One lesioned group then received grafts of acetylcholine-rich tissue dissected from the basal forebrain of fetal marmosets. Three months later the marmosets with lesion alone remained impaired on a number of retention and reversal tasks whereas the transplanted animals were no longer significantly impaired. Histological examination of the brains indicated that all lesioned animals had sustained substantial loss of the cholinergic neurons of the basal nucleus of Meynert (assessed by nerve growth factor receptor immunoreactivity) and that the lesion-alone animals showed marked loss of the cholinergic marker acetylcholinesterase in the dorsolateral frontal and parietal cortex. All transplanted animals had surviving graft tissue (visualized by Cresyl Violet staining, dense acetylcholinesterase staining and the presence of a limited number of nerve growth factor receptor-immunoreactive neurons) in the neocortex and 5/6 transplanted animals showed near complete restitution of acetylcholinesterase staining in frontal and parietal cortex. Examination of individual animal data showed that the animal without this restitution performed very poorly. The performance of the remaining transplanted animals was significantly better than that of the animals with lesion alone. There was a significant positive correlation between the degree of acetylcholinesterase staining and good performance on tasks sensitive to frontal lobe damage. These results demonstrate that acetylcholine-rich tissue transplanted into the neocortex of primates with damage to the cholinergic projections to the neocortex can produce substantial restitution of function provided that an appropriate level of interaction between graft and host tissue is achieved.

Estrogen and nerve growth factor-related systems in brain. Effects on basal forebrain cholinergic neurons and implications for learning and memory processes and aging

Estrogen replacement can significantly affect the expression of ChAT and NGF receptors in specific basal forebrain cholinergic neurons. The time-course of the effects is consistent with a direct up-regulation of ChAT followed by either direct or indirect down-regulation of p75NGFR and trkA NGF receptors, possibly due to increased cholinergic activity in the hippocampal formation and cortex and a decrease in hippocampal levels of NGF. Current evidence suggests ChAT, p75NGFR, trkA, and NGF all play a role in regulating cholinergic function in the hippocampal formation and cortex. In addition, all have been implicated in the maintenance of normal learning and memory processes as well as in changes in cognitive function associated with aging and with neurodegenerative disease. It is possible that estrogen may affect cognitive function via effects on NGF-related systems and basal forebrain cholinergic neurons. Effects of estrogen on cognitive function have been reported, as has some preliminary evidence for beneficial effects of estrogen in decreasing the prevalence of and reducing some cognitive deficits associated with Alzheimer's disease. Whether these effects are related to effects on NGF-related systems or basal forebrain cholinergic neurons is currently unknown. Indirect evidence suggests that estrogen interacts with NGF-related systems and that changes in circulating levels of estrogen can contribute to age-related changes in hippocampal levels of NGF. These findings have important implications for consideration of estrogen replacement therapy in pre- and post-menopausal women. Further studies examining effects of different regimens of estrogen replacement as well as estrogen combined with progesterone on NGF and basal forebrain cholinergic neurons in young and aged animals are required. Prospective studies correlating aging and estrogen replacement with numbers of basal forebrain cholinergic neurons and hippocampal and cortical levels of NGF also need to be performed to better assess the potential benefits of estrogen replacement in reducing age- and disease-related cognitive decline.

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

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

Behavioral assessment of the ability of intracerebral embryonic neural tissue grafts to ameliorate the effects of brain damage in marmosets

The transplantation of neuronal tissue into the brains of patients with Parkinson's disease is already being assessed as an experimental treatment for the symptoms of this disease, and the possibility of using similar graft tissue to ameliorate the symptoms of other neurodegenerative diseases is being considered. In this context, a small number of transplant experiments have been carried out in monkeys with lesions of the central dopamine and cholinergic systems. These experiments make it possible to determine the optimum methods of transplantation in an animal whose brain is structurally more closely related to the human than that of the rat and to assess the behavioral consequences of transplantation on symptoms that either resemble very closely the symptoms seen in patients, or are of a complex cognitive nature and are therefore more difficult to measure in the rat. It is intended that these experiments will contribute to the development of better treatments for the neurodegenerative diseases, either by the use of transplantation as a clinical treatment, or by contributing to a better understanding of the mechanisms that normally maintain neuronal function and that fail in these diseases.

Behavioral effects of cholinergic grafts

Experimental work in animals and, to a more limited extent, in humans, has demonstrated that the cholinergic system is involved in mechanisms which control learning and memory. Since there is cholinergic loss in a variety of dementing illnesses, any treatment designed to alleviate the mental symptoms of these diseases must address the issue of cholinergic dysfunction even if other treatments are also required to overcome other neurotransmitter imbalances. Work in rodents has demonstrated that cholinergic-rich fetal neural tissue transplants can, under certain circumstances, alleviate the behavioral effects of cholinergic lesions or of cholinergic decline associated with aging. More complex cognitive testing can be achieved using primates and, in this case, the common marmoset is a suitable species to use because its rapid and reliable reproductive rate aids the provision of appropriate transplant tissue. Marmosets with transection of the fornix are deprived of a cholinergic input into the dentate gyrus, posterior hippocampus and entorhinal cortex and are specifically impaired on learning tasks which require remembering a rule of responding (non-evaluative memory). Transplantation of cholinergic-rich fetal septal tissue into the hippocampus of such animals completely restores their ability to learn this type of task, whereas transplantation of cholinergic-poor fetal hippocampal tissue into the same area produces no such improvements. These results demonstrate that where a learning impairment is produced by a relatively simple procedure which has a major effect on one neurotransmitter, that function can be restored by transplantation of tissue containing that neurotransmitter even where the impairment consists of a very "high level" cognitive dysfunction.

Stimulus-bound perseveration after frontal ablations in marmosets

Marmosets with bilateral ablations of either the lateral or ventral surface of the frontal lobes were found to perseverate on object but not spatial serial reversal. They also perseverated on reversal of a visuospatial task where different stimuli required different spatial responses. No differences were found between the two lesion groups. Since the control animals showed mild perseveration on spatial but not object serial reversal it is argued that frontal ablations do not lead to perseveration of a natural tendency but rather that object and visuospatial perseveration are forms of stimulus-bound behaviour which do not occur when the animal is performing a spatial task in which stimulus position is irrelevant. Perseveration was reduced by pretreatment with a dopamine-blocking drug (haloperidol). It is suggested that information from temporal lobe mechanisms, involved in long-term memory, and from frontal lobe mechanisms, which exert shorter acting influences on behaviour, compete within the basal ganglia to determine stimulus choice. The ascending dopaminergic pathway may modulate the balance between these competing factors.