Basal forebrain lesions produce a dissociation of trial-dependent and trial-independent memory performance

The behavioral effects of lesions in the basal forebrain (BF) of rats were evaluated using two tasks. The BF lesions included both the nucleus basalis magnocellularis (NBM) and the medial septal area (MSA). The first task was a Stone maze, which has 14 consecutive choice points and is a task of complex, trial-independent memory. BF lesions did not impair choice accuracy in this task. The second task was a win-shift spatial discrimination in a radial arm maze, which requires trial-dependent memory. BF lesions produced a significant decrease in choice accuracy in this task. These results demonstrate that BF lesions impair trial-dependent (working) memory but not trial-independent reference memory, and that task difficulty is not the sole factor determining whether BF lesions produce behavioral impairments.     

Selective lesions of basal forebrain cholinergic neurons produce anterograde and retrograde deficits in a social transmission of food preference task in rats

We examined the performance of Long-Evans rats with 192 IgG-saporin lesions of the medial septum/vertical limb of the diagonal band (MS/VDB) or nucleus basalis magnocellularis/substantia innominata (NBM/SI), which removed cholinergic projections mainly to hippocampus or neocortex, respectively. We studied the effects of these lesions on anterograde and retrograde memory for a natural form of hippocampal-dependent associative memory, the social transmission of food preference. In a study of anterograde memory, MS/VDB lesions did not affect the immediate, 24-h or 3-week retention of the task. In contrast, NBM/SI lesions severely impaired immediate and 24-h retention. In a study of retrograde memory in which rats acquired the food preference 5 days or 1 day before surgery and they were tested 10-11 days after surgery, MS/VDB-lesioned rats showed striking memory deficits for the preference acquired at a long delay (5 days) before surgery, although all lesioned rats exhibited poorer retention on both retest sessions than on their pretest performance. Subsequent testing of new anterograde learning in these rats revealed no disrupting effects of lesions on a standard two-choice test. When rats were administered a three-choice test, in which the target food was presented along with two more options, NBM/SI-lesioned rats were somewhat impaired on a 24-h retention test. These results provide evidence that NBM/SI and MS/VDB cholinergic neurons are differentially involved in a social memory task that uses olfactory cues, suggesting a role for these neurons in acquisition and consolidation/retrieval of nonspatial declarative memory.     

Prefrontal cortical projections to the cholinergic neurons in the basal forebrain

The prefrontal cortex (PFC) projections to the basal forebrain cholinergic cell groups in the medial septum (MS), vertical and horizontal limbs of the diagonal band of Broca (VDB and HDB), and the magnocellular basal nucleus (MBN) in the rat were investigated by anterograde transport of Phaseolus vulgaris leuco-agglutinin (PHA-L) combined with acetylcholinesterase (AChE) histochemistry or choline acetyltransferase (ChAT) immunocytochemistry. The experiments revealed rich PHA-L-labeled projections to discrete parts of the basal forebrain cholinergic system (BFChS) essentially originating from all prefrontal areas investigated. The PFC afferents to the BFChS display a topographic organization, such that medial prefrontal areas project to the MS, VDB, and the medial part of the HDB, whereas the orbital and agranular insular areas predominantly innervate the HDB and MBN, respectively. Since the recurrent BFChS projection to the prefrontal cortex is arranged according to a similar topography, the relationship between the BFChS and the prefrontal cortex is characterized by reciprocal connections. Furthermore, tracer injections in the PFC resulted in anterograde labeling of numerous "en passant" and terminal boutons apposing perikarya and proximal dendrites of neurons in the basal forebrain, which were stained for the cholinergic marker enzymes. These results indicate that prefrontal cortical afferents make direct synaptic contacts upon the cholinergic neurons in the basal forebrain, although further analysis at the electron microscopic level will be needed to provide conclusive evidence.     

Mnemonic deficits in animals depend upon the degree of cholinergic deficit and task complexity

The role of cholinergic basal forebrain (CBF) neurons in mnemonic behaviors was investigated using the immunotoxin 192IgG-saporin. We assessed two routes of immunotoxin administration: intracerebroventricular (ICV) and intraparenchymal (INTRA). INTRA lesions of the medial septum (MS) and/or the nucleus basalis magnocellularis (NBM) were compared with ICV-lesions, INTRA-phosphate-buffered saline injected, and naive controls. The INTRA-NBM/MS and ICV NBM/MS lesions produced a similar depletion of choline acetyltransferase activity of 80% across all CBF projections. Water maze performance was similarly impaired for ICV- and INTRA-NBM/MS animals during various phases of testing, whereas animals with individual lesions of the NBM or MS performed at the level of controls. In contrast to the allocentric demands of water maze performance, the egocentric-based T-maze task revealed a vast group difference between the ICV- and the INTRA-NBM/MS animals. INTRA-NBM/MS animals showed a severe deficit in the non-match- and match-to-position version, whereas again, animals with single lesions were unimpaired. In addition, a dichotomy between animals with complete cholinergic deafferentation was observed in the inhibitory avoidance task. ICV-NBM/MS showed a diminished retention for the aversive stimulus while the INTRA-NBM/MS animals remembered well. During plus maze testing, only the INTRA-NBM/MS animals had a reduced level of anxiety. Although non-CBF regions may have been differently affected by the two routes of immunotoxin administration, global measures of arousal, motivation, and motor initiation did not reveal a different behavioral pattern. Our findings suggest that a dynamic interplay exists between the degree of cholinergic deficit and task demands revealing different types of mnemonic impairments.     

Behavior alters the uptake of [3h]choline into acetylcholinergic neurons of the nucleus basalis magnocellularis and medial septal area

Behavioral experience changed sodium-dependent high affinity choline uptake (SDHACU) in the hippocampus and frontal cortex. Rats were trained on various behavioral tasks and sacrificed after testing. SDHACU was determined in frontal cortex and hippocampus, areas that receive cholinergic innervation from the nucleus basalis magnocellularis (NBM) and the medial septal area (MSA), respectively. Untrained rats taken directly from their home cages had fairly consistent levels of SDHACU in the hippocampus (1.76 ± 0.45, X ± S.E.) and frontal cortex (1.46 ± 0.37). In the hippocampus of rats performing in a radial maze and T-maze and in rats that surpassed a criterion level in an active avoidance task, SDHACU increased significantly above Cage (untrained) group levels. In the cortex of rats performing the radial maze task, SDHACU decreased slightly. There were no other changes in frontal cortical SDHACU. After behavioral testing ceased, SDHACU in rats performing the radial maze task remained elevated above Control and Treadmill group levels for 20 days, but returned to near control levels 40 days later. Our data demonstrate that a functional differentiation exists between the MSA and NBM cholinergic systems, and that the measurement of SDHACU in central cholinergic neurons is a useful tool to identify the influences of behavior and environment upon changes in neurochemical events and neuronal activity.     

Heterogeneity and selectivity of the degeneration of cholinergic neurons in the basal forebrain of patients with Alzheimer's disease

Cholinergic neurons were studied by immunohistochemistry, with an antiserum against choline acetyltransferase (ChAT), in the basal forebrain (Ch1 to Ch4) of four patients with Alzheimer's disease (AD) and four control subjects. ChAT-positive cell bodies were mapped and counted in Ch1 (medial septal nucleus), Ch2 (vertical nucleus of the diagonal band), Ch3 (horizontal nucleus of the diagonal band) and Ch4 (nucleus basalis of Meynert). Compared to controls, the number of cholinergic neurons in AD patients was reduced by 50% on average. The interindividual variations in cholinergic cell loss were high, neuronal loss ranging from moderate (27%) to severe (63%). Despite the small number of brains studied, a significant correlation was found between the cholinergic cell loss and the degree of intellectual impairment. To determine the selectivity of cholinergic neuronal loss in the basal forebrain of AD patients, NPY-immunoreactive neurons were also investigated. The number of NPY-positive cell bodies was the same in controls and AD patients. The results (1) confirm cholinergic neuron degeneration in the basal forebrain in AD and the relative sparing of these neurons in some patients, (2) indicate that degneration of cholinergic neurons in the basal forebrain contributes to intellectual decline, and (3) show that, in AD, such cholinergic cell loss is selective, since NPY-positive neurons are preserved in the basal forebrain.     

Disruption of central cholinergic systems in the rat by basal forebrain lesions or atropine: effects on feeding, sensorimotor behaviour, locomotor activity and spatial navigation

Rats with ibotenic acid lesions of the nucleus basalis magnocellularis, the origin of the extrinsic cholinergic innervation of the cortex, were examined for changes in feeding, sensorimotor behaviour, nocturnal locomotor activity, and place navigation in the Morris swimming pool task, in comparison with control rats and rats receiving the muscarinic antagonist, atropine. The lesions produced acute feeding impairments, marked by weight loss and vigorous active rejection of food and water lasting 2-4 days, sensorimotor impairments in placing and orienting, and overnight hyperactivity. A similar hyperactivity was induced by atropine, lasting approximately 6 h following the injection. Rats with lesions or receiving atropine were similarly impaired in the acquisition of the spatial navigation task, they failed to reach control levels of efficiency even once they had acquired the task, and they showed small but significant retention impairments when pretrained in the absence of either treatment. The results are discussed in terms of the lesions producing a disruption of cortical cholinergic systems, with implications for the clinical disorder of senile dementia of the Alzheimer type, and in terms of possible associated disruption to non-cholinergic systems.     

Mapping of the basal forebrain cholinergic system of the dog: A choline acetyltransferase immunohistochemical study

In an effort to produce a canine model of basal forebrain ischemia with memory deficits, we have shown that dogs possess a medial striate artery that perfuses basal forebrain territory, homologous to the human recurrent artery of Heubner. In the present study, we set out to delineate the precise topography of the cholinergic neurons in the canine forebrain, a neuronal system implicated in cognitive and memory functions. Floating coronal sections, derived from the head of the caudate nucleus to the rostral border of the hippocampus, were stained for choline acetyltransferase using a monoclonal antibody. Representative sections from one dog brain were drawn. These outlines were used for measurement of cell density, cell size, number of processes, and cell roundness. Choline acetyltransferase-positive neurons constituted four major subdivisions within the basal forebrain. A relatively dense population of cholinergic neurons was present in the medial septal nucleus (Ch1). A continuum of densely packed cells was also delineated within the vertical (Ch2) and horizontal (Ch3) nuclei of the diagonal band of Broca. A fourth group of heterogeneously packed cholinergic neurons represented the nucleus basalis magnocellularis (Ch4). Except for the caudal component of the Ch4 population, the forebrain cholinergic corticopetal system was located within the perfusion territory of the medial striate arteries. The Ch4 cell group in dogs is better defined than that of rodents but is not as sharply demarcated as in human and nonhuman primates. Our findings indicate that the dog may serve as an excellent model for assessing neurological and memory deficits, which, in humans, results from hypoperfusion of the recurrent artery of Heubner. © 1996 Wiley-Liss, Inc.     

Impairment of semantic memory after basal forebrain and fornix lesion

Semantic memory impairment is classically associated with lesion of the anterior temporal lobe. We report the case of a patient with severe semantic knowledge impairment and anterograde amnesia after bilateral ischemic lesion of the fornix and of the basal forebrain following surgical clipping of an aneurysm of the anterior communicating artery. Fluorodeoxyglucose positron emission tomography (FDG-PET) showed a temporal hypometabolism. Severe semantic impairment is a rare complication after rupture of an anterior communicating artery aneurysm and may result from disconnection of the temporal lobe.     

Transplantation of embryonic ventral forebrain grafts to the neocortex of rats with bilateral lesions of nucleus basalis magnocellularis ameliorates a lesion-induced deficit in spatial memory

Embryonic ventral forebrain grafts containing developing cholinergic cells were transplanted to the neocortex of rats with bilateral quisqualic acid lesions of the nucleus basalis magnocellularis. A lesion-induced deficit on performance of a spatial alternation test of memory was reduced by such transplants. When the same animals were treated with the acetylcholinesterase inhibitor physostigmine (0.05 mg/kg), however, performance on the behavioral task was not further promoted, and therefore, under these conditions, the cholinergic cortical transplants appear not to be subject to modulation by anticholinesterase drugs.     

Alterations in dendritic morphology of frontal cortical neurons after basal forebrain lesions in adult and aged rats

The nucleus basalis magnocellularis (NBM) is the major cholinergic projection to neocortex in the rat and plays a role in the modulation of cortical activity. Lesions of the NBM decrease thickness of lamina II–III of frontal cortex and decrease soma size of lamina II–III neurons. Additionally, aging produces changes in neuron size and numbers in the basal forebrain and frontal cortex of rats. We assessed dendritic changes in neurons from lamina II–III of frontal cortex in adult, middle-aged, and aged rats three months after unilateral lesions of the NBM. While lesions did not affect dendritic morphology in young adult rats, they decreased total dendritic length in middle-aged and aged rats, with dendritic alterations most pronounced in middle-aged rats. In middle-aged rats, lesion-induced changes in basilar arbor were apparently due to decreased dendritic branching: lesions markedly decreased the number of first-, second-, and third-order branches, but did not affect higher-order branching. In aged rats, lesions resulted in a small decrease in dendritic material proximal to the soma and a pronounced decrease in dendritic material distal to the soma, apparently due to a decrease in the length of terminal branches. These results suggest that the plasticity of neocortical neurons in the basalocortical system changes with age, and that early in aging this system may be particularly vulnerable to neural damage.     

Radial-maze performance in the rat following lesions of posterior neocortex

The present experiment was designed to investigate the role of posterior neocortex (areas 17, 18 and 18a) in the maintenance of performance on the radial maze. Following training to criterion on the 8-arm radial maze, rats received either sham operations, bilateral eye enucleations, lesions of posterior neocortex, or combined enucleations and lesions of posterior neocortex. While the enucleated animals with intact brains showed a slight, but significant performance decrement relative to the sham-operated group, the other two groups, with lesions of areas 17, 18 and 18a, each showed a massive deficit. This large deficit was observed even in the group in which both the eyes and neocortex had been removed. These results suggest that the visual projection areas of cortex not only play an important role in the maintenance of accurate radial-maze performance in sighted animals, but that the integrity of these areas is necessary for the maintenance of criterion performance in blind animals.     

Impairment in T-maze reinforced alternation performance following nucleus basalis magnocellularis lesions in rats

Rats were trained on a reinforced alternation paradigm using an elevated T-maze. After pre-surgical training subjects received either ibotenic acid (4 micrograms/0.4 microliter) or vehicle (pH 7.4, 0.4 microliter) bilaterally into the region of the nucleus basalis magnocellularis--an important source of neocortical acetylcholine projections. Acetylcholinesterase staining of sectioned brains revealed a loss of neocortical, but not hippocampal staining in lesioned animals. On the T-maze task, lesioned rats showed significantly impaired choice performance relative to controls. They also demonstrated significant side biases, the degree of which was correlated with choice performance deficit.     

Profound disturbances of spontaneous and learned behaviors following lesions of the nucleus basalis magnocellularis in the rat

It has been shown that a marked decline in the cortical activity of the cholinergic synthesizing enzyme choline-acetyltransferase (ChAT), accompanied by a severe neuronal loss in the nucleus basalis magnocellularis of Meynert occurs in the brains of patients with senile dementia of the Alzheimer type. However, the functional role of these neurons is largely unknown. In fact, very few studies have been done in animals. In this paper we report the behavioral effects of the lesion of the nucleus basalis magnocellularis in the rat either by radiofrequency current or by ibotenic acid injection at the level of the cell bodies. The two kinds of lesion lead to a profound disturbance of spontaneous and learned behaviors. There is a complete disorganization of behavior which is evidenced by an enhanced locomotor activity, an alteration in alimentary and hoarding behavior. In addition, we observed a deterioration of spatial memory and an incapacity to reverse a previously learned response. Biochemical assay showed that radiofrequency and ibotenic acid lesions produced a decrease of ChAT activity in the prefrontal and sensorimotor cortices and in amygdala without affecting the hippocampus or striatum. Ibotenic acid lesions seem to specifically destroy the cell bodies of the nucleus basalis magnocellularis since the dopaminergic and noradrenergic fibers of passage remained intact as measured by the unchanged level of endogenous catecholamine concentration in the terminal region in the prefrontal cortex. Presently, it cannot be said that the behavioral syndrome results solely from the lesion of the cholinergic neurons. Also, it is likely that the lesion of the nucleus basalis magnocellularis in the rat does not exactly reproduce the behavioral syndrome observed in Alzheimer's disease in man. However, this experimental approach in leading to a better knowledge of the functioning of these neurones could improve our understanding of this disease.     

Characteristics of memory impairment following lesioning of the basal forebrain and medial septal nucleus in rats

Memory impairment in rats with lesions of the basal forebrain (BF) and medial septal nucleus (MS) including cell bodies of the cortical and septohippocampal cholinergic systems, respectively, were compared in order to evaluate the functional contribution of the two cholinergic systems to memory. Biochemical assay revealed that lesioning of the BF and MS resulted in marked and selective decreases in both choline acetyltransferase and acetylcholinesterase activities in the cerebral cortex and hippocampus, respectively. Rats with BF lesions exhibited a severe deficit in a passive avoidance task; acquisition of passive avoidance by repeated training was sluggish, and the acquired response was rapidly eliminated in a subsequent extinction test. However, only slight impairment of passive avoidance was observed in rats with MS lesions. Memory impairment in rats with BF or MS lesions was also investigated using two spatial localization tasks, the Morris water task and the 8-arm radial maze task. Both BF and MS lesions elicited a significant impairment in the Morris water task that required reference memory, as demonstrated by an apparent increase in the latency to escape onto a hidden platform in a large water tank. The impairment was much more obvious in the BF-lesioned rats. In contrast, in the radial maze task primarily requiring working memory, rats with lesions of the MS showed severe disruption, exhibiting a marked increase in total errors, a decrease in the number of initial correct responses, and an apparent change in the strategy pattern. However, corresponding changes in the rats with BF lesions were slight. These results suggest that BF lesions may lead to substantial long-term memory impairment while MS lesions may primarily produce short-term or working memory impairment, indicating a qualitatively different contribution of the two cholinergic systems to memory. It is also suggested that these two experimental animal models may be useful for evaluation of therapeutic drugs for senile dementia of the Alzheimer type.     

Lack of correlation between cortical levels of choline acetyltransferase and learning scores in rats with globus pallidus lesions

Previous work has shown that rats with lesions of the globus pallidus (GP) exhibit a generalized learning impairment. Data are presented suggesting that this impairment is not due to inadvertent damage to the nucleus basalis magnocellularis. Rats with GP lesions evidenced a significant visual discrimination learning loss and a significant reduction in cortical choline acetyltransferase (ChAT) activity. However, there was no significant correlation between the severity of the learning loss and the amount of reduction of cortical ChAT activity.     

Galanin-immunoreactive synaptic terminals on basal forebrain cholinergic neurons in the rat

Previous studies have indicated that galanin is one of the most abundant peptides in the basal forebrain and that it has a significant modulatory influence on cholinergic transmission. The aim of the present study was to use a light electron microscopic correlation technique to determine whether galanin-immunoreactive terminals form synaptic contacts with basal forebrain cholinergic cells of the rat. Sections from fixed-perfused brains were stained at the light and electron microscopic levels for galanin and choline acetyltransferase immunoreactivity in the same section by using a dual-colour immunohistochemical method. The results showed that galanin-immunoreactive axonal terminals are unevenly distributed in the medial septal nucleus, the diagonal band, and the nucleus basalis. Galanin-positive synapses were most prominent on choline acetyltransferase-positive neurons in the lateral parts of the nucleus of the diagonal band and in the posterior half of the nucleus basalis, which is where there was the greatest overlap between the distribution of galanin-immunoreactive terminals and choline acetyltransferase-positive neurons. The origins of these galanin-positive terminals are not known, but the results confirm that the basal forebrain galaninergic system has a synaptic influence on basal forebrain cholinergic neurons in the rat. J. Comp. Neurol. 383:82–93, 1997. © 1997 Wiley-Liss, Inc.     

The role of two major cholinergic systems in memory acquistion and retention in the eight-arm radial maze

The role of the nucleus basalis magnocellularis (NBM) and the medial septum (MS) in postoperative memory acquisition and postoperative memory retention was assessed in the eight-arm radial maze task (Olton). NBM lesions and MS lesion significantly decreased choline acetyltransferase (CAT) activity in the innervated areas. Animals with MS lesion, but not NBM lesions, showed significant impairment of memory acquisition. NBM lesions induced a unique strategic pattern, but the development of strategic pattern with training was comparable to the sham-injected group. Animals with MS lesion did not develop strategic pattern with training. NBM and MS lesions did not influence either memory retention or developed strategic pattern. These results suggest that (1) MS contributes to the radial maze task; (2) the two systems have qualitatively different contributions in the radial maze task; (3) the two systems do not contribute to memory retention; (4) the two major cholinergic systems have different functions in the pathophysiology of Alzheimer's disease.     

Maternal choline supplementation differentially alters the basal forebrain cholinergic system of young‐adult Ts65Dn and disomic mice

Down syndrome (DS), trisomy 21, is a multifaceted condition marked by intellectual disability and early presentation of Alzheimer's disease (AD) neuropathological lesions including degeneration of the basal forebrain cholinergic neuron (BFCN) system. Although DS is diagnosable during gestation, there is no treatment option for expectant mothers or DS individuals. Using the Ts65Dn mouse model of DS that displays age‐related degeneration of the BFCN system, we investigated the effects of maternal choline supplementation on the BFCN system in adult Ts65Dn mice and disomic (2N) littermates at 4.3–7.5 months of age. Ts65Dn dams were maintained on a choline‐supplemented diet (5.1 g/kg choline chloride) or a control, unsupplemented diet with adequate amounts of choline (1 g/kg choline chloride) from conception until weaning of offspring; post weaning, offspring were fed the control diet. Mice were transcardially perfused with paraformaldehyde, and brains were sectioned and immunolabeled for choline acetyltransferase (ChAT) or p75‐neurotrophin receptor (p75^NTR). BFCN number and size, the area of the regions, and the intensity of hippocampal labeling were determined. Ts65Dn‐unsupplemented mice displayed region‐ and immunolabel‐dependent increased BFCN number, larger areas, smaller BFCNs, and overall increased hippocampal ChAT intensity compared with 2N unsupplemented mice. These effects were partially normalized by maternal choline supplementation. Taken together, the results suggest a developmental imbalance in the Ts65Dn BFCN system. Early maternal‐diet choline supplementation attenuates some of the genotype‐dependent alterations in the BFCN system, suggesting this naturally occurring nutrient as a treatment option for pregnant mothers with knowledge that their offspring is trisomy 21. J. Comp. Neurol. 522:1390–1410, 2014. © 2013 Wiley Periodicals, Inc.     

Memory deficits in advanced Parkinson's disease

Summary Apart from global dementia various isolated cognitive deficits have been described in Parkinson's disease (PD). We investigated 31 non-demented Parkinsonian patients in their late stages of disease and 50 control subjects with regard to verbal memory. Eleven patients suffered from an isolated verbal memory deficit as defined by two list learning tasks using the Buschke selective reminding procedure. The isolated memory impairment did not depend on depression but was associated with longer duration of PD. Twelve, demented PD patients were comparable to PD patients with isolated memory impairment with regard to age at onset and duration of PD. We speculate that the isolated memory impairment in PD is associated with isolated neuronal loss in the nucleus basalis of Meynert, without cortical or limbic pathology of the Alzheimer's type.