C. J. Maclean

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, Alzheimers disease and other pathological states.

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.

The 5-HT1A antagonist, WAY 100635, ameliorates the cognitive impairment induced by fornix transection in the marmoset

Abstract Fornix transection in the marmoset produces a specific pattern of cognitive deficits, notably a lack of ability to recall visuospatial tasks learnt preoperatively, and a deficit in acquiring new visuospatial tasks following transection. Previous work has shown that this learning impairment can be ameliorated by cholinergic agonists, suggesting that it occurs as a consequence of destroying the cholinergic projection from the vertical limb of the diagonal band to the hippocampus which runs through the fornix. We have now shown that this deficit in new learning can be significantly alleviated by the 5-HT1A antagonist, WAY 100635. This result supports the suggestion that 5-HT1A projections are inhibitory on the same target cells for which cholinergic projections are excitatory, and that loss of function in the target cells caused by loss of excitatory tone can be compensated by blockade of inhibitory tone. Since cholinergic loss in the hippocampus (and neocortex) occurs in association with cognitive decline in Alzheimer’s disease, these results suggest that 5-HT1A antagonists may have a role in the treatment of some of the cognitive symptoms of dementia.

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).

Naturally occurring and experimentally induced β-amyloid deposits in the brains of marmosets (Callithrix jacchus)

Summary. Cerebral β-amyloid occurs in elderly animals of some species and in Alzheimers disease. Previously, we injected 3 young marmosets intracerebrally with brain tissue from a patient with Alzheimers disease. Six years later, when the monkeys were middle aged, we found moderate numbers of intracerebral plaques and cerebrovascular deposits containing β-amyloid. We have re-examined these brains and those of 10 other marmosets injected with brain homogenate containing β-amyloid, and have found some β-amyloid in animals injected more than 4 years previously. We have found β-amyloid in 4 of 26 elderly control marmosets, but not in 9 young to middle-aged control marmosets. The β-amyloid found in middle aged marmosets injected with Alzheimer brain tissue was, therefore, not a consequence of their age. Deposits in large cerebral vessels in elderly marmosets, and in marmosets previously injected with brain tissue containing β-amyloid, reacted with antibodies to Aβ and Aβ1-40; plaques and microvessel deposits reacted with antibodies to Aβ and Aβ1-42.

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.

Learning impairment induced by lesion of the CA1 field of the primate hippocampus: attempts to ameliorate the impairment by transplantation of fetal CA1 tissue

Abstract Monkeys with bilateral excitotoxic lesion of the CA1 field of the hippocampus were severely impaired at learning visuospatial conditional tasks. This was not a general spatial impairment, because the animals were not impaired on serial spatial reversal, which requires response flexibility in the spatial domain; they were not impaired at learning to choose the position furthest away from a single stimulus, which requires analysis of spatial layout of the test area, and they were not impaired at discriminating between two patterns that differed only in orientation. CA1-lesioned monkeys were impaired at learning a visuospatial conditional task when trials of the two component types ”if AA go left” and ”if BB go right” were presented according to either a pseudorandom or alternating schedule; but they were not impaired if one component type of trial was presented until three consecutive correct responses were made, followed by the other type of trial, to three consecutive correct responses. In all cases testing continued until a criterion of 27 of 30 consecutive correct responses across both types of trial was achieved. Although this suggests that CA1-lesioned animals are particularly prone to interference effects, they had no difficulty in learning ten concurrent visual discriminations presented against either a uniform background or with each discrimination presented against its own distinctive background, a condition that might reduce interference in unoperated monkeys. Interference following hippocampal damage might occur at a deeper level than stimulus identification such that animals with hippocampal damage may be able to learn about many aspects of different stimuli in parallel but may be unable to learn about multiple related aspects of the same subject matter. Monkeys with grafts of fetal CA1 tissue in the lesioned CA1 field showed significant improvement relative to CA1-lesioned animals on those tasks on which CA1-lesioned animals were impaired, although they remained impaired relative to control animals. This suggests that the grafts had produced some improvement in performance. Grafted monkeys did not differ from unoperated control monkeys or from CA1-lesioned monkeys on those tasks that were not sensitive to CA1 damage. This demonstrates that the grafts did not have an additional deleterious effect on cognitive performance.

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.

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.

Unilateral hippocampal and inferotemporal cortex lesions in opposite hemispheres impair learning of single-pair visual discriminations as well as visuovisual conditional tasks in monkeys

Monkeys with unilateral ablations of the inferotemporal (IT) cortex were not impaired on learning or retention of single-pair object discriminations or visuovisual conditional tasks. Addition of an excitotoxic hippocampal lesion to the hemisphere opposite to the IT ablation impaired retention and acquisition of single-pair object discriminations and visuovisual conditional tasks. Histology revealed no areas of bilaterally symmetrical damage. Previous experiments have shown that bilateral excitotoxic hippocampal lesions do not impair single-pair object discriminations although they do produce a substantial impairment on visuovisual conditional tasks. Bilateral IT ablations produce impairment on single-pair object discrimination tasks. It is argued that the hippocampus in the hemisphere with the IT ablation is deprived of feed-forward visual input and that this, in addition to the contralateral hippocampal lesion, accounts for the impairment on the visuovisual conditional tasks. It is also argued that feed-back projections from the hippocampus to the IT cortex influence the learning of single-pair object discriminations. This influence may be difficult to demonstrate by the addition of hippocampal lesions to IT lesions because of the substantial effect of the IT lesion alone. It may be difficult to demonstrate by bilateral hippocampal lesions alone since the effect may be below that which generates an observable impairment. Nonetheless, an effect may be seen when a hippocampal lesion is made in monkeys with some IT damage, as in this experiment, as well as by the general observation that large lesions of the temporal lobes produce larger perceptuo-mnemonic impairments than lesions confined to the hippocampus or temporal neocortex in monkeys and man.