Memory impairments following basal forebrain lesions

The functional contribution of the nucleus basalis magnocellularis (NBM) and medial septal area (MSA) to memory was evaluated in 4 behavioral tasks. The tasks were postoperative acquisition of a win-stay spatial discrimination in a T-maze, a win-shift spatial discrimination on a radial arm maze, active avoidance in a shuttle box, and passive avoidance in a shuttle box. Bilateral lesions were made by injecting ibotenic acid (IBO) into the NBM or MSA. Control rats received operations in which no neurotoxin was injected. When compared to controls, rats with lesions in either the NBM or MSA had significantly impaired choice accuracy in the T-maze and radial maze tasks, took significantly fewer trials to reach criterion in the acquisition, but not the retention of an active avoidance task, and significantly more trials to reach criterion in the passive avoidance task. The results show that equivalent behavioral changes are obtained from lesions in the NBM and MSA in tasks that vary in their type of motivation, reinforcement, response-reinforcement contingency, and response. These behavioral changes suggest that the NBM and MSA may both be involved in memory.     

Behavioural, histological and immunocytochemical consequences following 192 IgG-saporin immunolesions of the basal forebrain cholinergic system

Use of the selective immunotoxin; 192 IgG-saporin, is helping to elucidate the role of the cholinergic system in cognition by overcoming the problems of interpretation associated with the use of non-specific lesioning agents. In separate studies, we have compared the long- and short-term effects of single site and combined saporin lesions of the nucleus basalis magnocellularis and medial septal area, on spatial learning and memory in radial arm and water maze tasks. At 11 months, only rats with combined lesions showed deficits in both radial and water maze tasks, although terminal cholinergic deafferentation was substantial and extensive tissue loss was seen at the injection sites in both single and combined lesions. However, the extensive tissue loss with long-term lesions suggested that behavioural deficits were not solely attributable to cholinergic deafferentation. In contrast, when rats with combined lesions were tested 5 months after lesioning, no deficits were apparent, although there was almost complete loss of choline acetyltransferase- and nerve growth factor receptor-immunoreactivity in the basal forebrain with no tissue damage at the injection sites. This study supports existing literature that selective loss of cholinergic neurons in the basal forebrain does not produce behavioural impairments in standard tasks of learning and memory, but deficits are apparent when damage is non-selective as occurs late after lesioning, confounding interpretation of behavioural data. It further highlights potential problems with this immunotoxin in long-term studies.     

The basal forebrain cholinergic system and object memory in the rat

Rats with near complete destruction of basal forebrain cholinergic neurons from intracerebroventricular injections of 192 IgG-saporin were trained on object discrimination problems and then retrained two weeks later to measure retention. Despite dramatic reductions of acetylcholinesterase-positive fibers in hippocampus and neocortex, these animals did not differ from controls on an analysis of savings scores. Thus, the basal forebrain cholinergic system may serve functions that support non-spatial memory but are not specifically mnemonic in nature.     

The cholinergic basal forebrain system during development and its influence on cognitive processes: important questions and potential answers

This review seeks to address, though perhaps not answer fully, four important questions about the cholinergic basal forebrain (BF) system in developing mammals. First, what role does the cholinergic basal forebrain system play in the development of cognitive functions? Second, does the cholinergic BF system play a fundamentally similar role in development vs. adulthood? Third, does sexual dimorphism of the developing cholinergic BF system influence cognition differently in the two sexes? Finally, what role does the developing cholinergic BF system play in developmental disorders such as Down syndrome and Rett syndrome? Examples from the literature, primarily studies in mice and rats, are given in an attempt to answer these important questions.     

Spatial learning impairments in rats with selective immunolesion of the forebrain cholinergic system.

A monoclonal antibody to the low-affinity NGF receptor, 192 IgG, coupled to a cytotoxin, saporin, was recently introduced as an efficient selective neurotoxin for the NGFr-bearing cholinergic neurones in the rat basal forebrain. In the present study we report that an intracerebroventricular injection of this 192 IgG-saporin conjugate induces a severe, long-lasting spatial learning impairment, as assessed in the Morris water-maze task. This behavioural impairment was associated with 65-90% depletion of choline acetyltransferase activity (ChAT) in the hippocampus and cortex. ChAT activity associated with other cholinergic neurone systems in the brain (striatum, mesencephalon, spinal cord), was left virtually unaffected. This new immunotoxin holds great promise as a tool for selective and efficient lesions of the forebrain cholinergic system in functional and behavioural studies.     

The effects of neonatal basal forebrain lesions oncognition : towards understanding the developmental roleof the cholinergic basal forebrain

Abnormal development of the cholinergic basal forebrain has been implicated innumerous developmental disabilities such as Rett Syndrome and Down Syndrome. This reviewsummarizes recent data using two rodent animal models that involve interrupting cholinergic basalforebrain projections on postnatal day 1 and postnatal day 7 when basal forebrain fibers arebeginning to innervate their neocortical and hippocampal targets, respectively. In one model,electrolytic lesions in mice aimed at the basal forebrain on postnatal day 1 transiently reducecholinergic markers in neocortex which induce permanent alterations in neocortical anatomy thatcorrelate with impairments on cognitive tasks. Furthermore, the lesion effects are sex dependent.In another model, 192 IgG saporin lesions in rats on postnatal day 7 permanently reducecholinergic markers in neocortex and hippocampus, and result in mild impairments in spatialprocessing, acquisition and exploratory activities. These data suggest that during the firstpostnatal week of development the cholinergic basal forebrain system is critical for normalneocortical differentiation and, possibly synaptogenesis in neural circuits that will be important forspatial memory and acquisition of spatial data. During the second postnatal week of development,the cholinergic basal forebrain system appears to take on a role largely similar to its adult role inselective attention and processing of new information. These studies also suggest strongly thatinterrupting cholinergic basal forebrain innervation of neocortex and hippocampus leads toanatomical and neurochemical abnormalities that may serve as neural substrates for some of thecognitive deficits seen in disorders such as Rett Syndrome and Down Syndrome.     

Multimodal amnesic syndrome following bilateral temporal and basal forebrain damage

A 55-year-old right-handed man (patient DRB) had a major amnesic syndrome following extensive bilateral damage to the temporal lobe and basal forebrain, caused by herpes simplex encephalitis. His amnesia was both anterograde and retrograde. The retrograde amnesia spanned the five decades of his life, sparing only generic (semantic) material and shreds of previous experiences devoid of appropriate temporal and spatial placement. The anterograde amnesia encompassed both generic (semantic) and contextual (episodic) material. With the exception of preserved learning of a visuomotor skill, the patient did not show acquisition of any new information since his illness in 1975. Elementary perceptual, intellectual, and linguistic abilities remained intact. Because several anatomic and behavioral characteristics of this case are different from those of previously reported cases of amnesia, they may provide new insight into the neuroanatomic substrate of human 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.     

Aging, Alzheimer's disease, and the cholinergic system of the basal forebrain

All giant neurons of the medial basal forebrain stained for choline acetyltransferase (ChAT). Cell numbers declined from 400,000 to 475,000 in young controls to approximately 140,000 in elderly controls. Five senile dementia cases had counts ranging from 45,000 to 100,000 cells. ChAT levels in control frontal cortex decreased from 1.2 mumol/hr/100 mg protein at age 40 to 0.5 at age 95. Five senile dementia cases had levels ranging from 0.04 to 0.30. When the cholinergic cell count in the basal forebrain drops below about 100,000 cells, the level of cortical ChAT may be so low that clinical dementia appears.     

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.     

Prior pathology in the basal forebrain cholinergic system predisposes to inflammation-induced working memory deficits: reconciling inflammatory and cholinergic hypotheses of delirium

Delirium is a profound, acute confusional state that leads to long-term cognitive decline. Increased anticholinergic medications and prior dementia, in which basal forebrain cholinergic degeneration is a prominent feature, both predict delirium. Thus, cholinergic hypoactivity is thought to be important in cognitive dysfunction during delirium, and acute systemic inflammation is a major trigger for this dysfunction. Here, we hypothesize that decreased cholinergic function confers increased susceptibility to acute inflammation-induced cognitive deficits. We used the murine-p75-saporin immunotoxin (mu-p75-sap) to induce selective lesions of the basal forebrain cholinergic system in mice, mimicking early dementia-associated cholinergic loss, and superimposed systemic inflammation using low-dose bacterial lipopolysaccharide (LPS). Intracerebroventricular injection of mu-p75-sap produced depletion of cholinergic neurons in the basal forebrain and decreased innervation of the hippocampus, but left performance on hippocampal-dependent reference and working memory tasks relatively intact. However, systemic LPS (100 μg/kg) induced acute and transient working memory deficits in lesioned animals without effect in unlesioned controls. CNS inflammatory responses were similar in normal and lesioned animals and the acetylcholinesterase inhibitor, donepezil (1 mg/kg), protected against the acute cognitive deficits in this cholinergic-dependent task. Thus, cholinergic depletion predisposes to development of acute cognitive deficits upon subsequent systemic inflammatory insult. These data provide a useful model for examining interactions between acute systemic inflammation and chronic cholinergic hypofunction in delirium and have implications for the recent trial of rivastigmine in sepsis-associated delirium.     

Developing cholinergic basal forebrain neurons are sensitive to thyroid hormone

The influence of thyroid hormone on the development of cholinergic neurons in nucleus basalis was assessed in hyperthyroid, hypothyroid, and euthyroid rats by use of CAT immunohistochemistry and single-section Golgi-impregnation histology. Animals were made either hyperthyroid by daily injections of 1.0 micrograms/gm body weight triiodothyronine starting at postnatal day (P) 3 or hypothyroid by providing 0.4% propylthiouracil in the diet of dams from P2. Compared to developing control rats, increased exposure to thyroid hormone resulted in accelerated expression of CAT in nucleus basalis neurons. Overshoot in cell body size, a normal developmental phenomenon of cholinergic neurons in the basal nuclear complex, occurred earlier in hyperthyroid brains and was of a greater magnitude than in controls. Furthermore, increased numbers of primary dendrites and dendritic branchpoints accompanied by dendritic and perisomal filopodia-like structures were observed for nucleus basalis neurons in hyperthyroid rats. These dendritic changes persisted throughout the second postnatal month. After the fifth postnatal week, cell body sizes of these hyperthyroid CAT-positive neurons began to decrease and by P50 were significantly less than controls or similarly treated animals at earlier ages. By P64, the number of cholinergic neurons in nucleus basalis was appreciably less than in age-matched controls. Hypothyroidism resulted in a delay of normal CAT expression that persisted throughout the third postnatal week. After this time, CAT staining increased until normal immunoreactivity was attained in cell bodies, fibers, and terminal regions by P35. A deficit in thyroid hormone during development prevented overshoot in perikaryal size and resulted in diminished cross-sectional areas throughout the cholinergic nucleus basalis at all ages examined. Hypothyroidism also prevented the normal overproduction of dendrites in those cells and produced stunted dendritic trees at all ages examined. These morphological abnormalities persisted throughout the second postnatal month. The effects of thyroid hormone on cholinergic projection neurons in the rat brain appeared relatively selective for cells in the basal nuclear complex because neither hypothyroid nor hyperthyroid treatment produced changes in the cell body areas of the phenotypically similar CAT-positive neurons of the pontomesencephalotegmental complex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neurogenesis of basal forebrain cholinergic neurons in rat

The basal forebrain cholinergic system embodies a heterogeneous group of neurons distributed in the basal telencephalon that project topographically to the cortical mantle. We sought to examine the generation of these neurons to determine whether basal forebrain neurons have unique patterns of neurogenesis or, if, in contrast, they are born along general neurogenic gradients. The techniques of tritiated thymidine autoradiography and choline acetyltransferase (ChAT) immunocytochemistry were combined to determine the birthdays and neurogenic gradients of cholinergic cells in this region of rat brain. Cholinergic neurogenesis throughout the basal forebrain ranged from embryonic days 12 to 17 (E12-17). Neurogenesis in the nucleus basalis magnocellularis occurred over E12-16, with a peak day of generation on E13. The horizontal limb nucleus of the diagonal band which is located rostral to the nucleus basalis was generated over E12-17, with the majority of cells arising on E14-15. The rostral-most nuclei of the basal forebrain cholinergic system, the vertical limb of the diagonal band and the medial septum, were generated between E13-17, with peak days of neurogenesis on E15 and E15-16, respectively. These results were evaluated quantitatively and demonstrated that the basal forebrain cholinergic neurons were generated along the general caudal-to-rostral gradient previously described for all neurons in this brain region. The results of this study, in combination with those of similar investigations, emphasize that position-dependent epigenetic factors appear to be more potent determinants of the time of neuronal origin than factors which influence a cell's transmitter phenotype.     

Ganglioside AGF2 prevents the cognitive impairments and cholinergic cell loss following intraventricular colchicine

Ganglioside AGF2 prevented the cognitive and locomotor alterations induced by intraventricular colchicine. Adult male rats were initially trained to perform a standard radial arm maze (RAM) task. Following training, they were injected intraperitoneally with 10 mg/kg AGF2 (COL/AGF2), cerebrospinal fluid (CSF/AGF2) or the saline vehicle (COL/SAL, CSF/SAL) for 3 days prior to and for 14 days following the bilateral injection of colchicine (7 micrograms/0.5 microliters) or artificial CSF into the lateral ventricles. Colchicine (COL/SAL) impaired performance of the standard RAM task as well as a working memory version of the task in which various delays were imposed between the fourth and fifth arm choices. Colchicine also produced a transient hyperactivity which subsided within 10 weeks following surgery. In contrast, AGF2 (COL/AGF2) prevented the impairments in RAM performance and the alterations in locomotor behavior. Colchicine also produced significant decreases in hippocampal ChAT activity and high affinity choline uptake that were prevented by prior treatment with AGF2. Finally, colchicine produced a 35% decrease in the number of acetylcholinesterase-positive (cholinergic) neurons in the medial septum and vertical limb of the diagonal band (MS/VLDB) which was also prevented by AGF2. Thus, the behavioral and neurochemical protection afforded by AGF2 was paralleled by a prevention of the loss of hippocampal cholinergic parameters and cholinergic neurons in the MS/VLDB.     

Postnatal development of cholinergic system in mouse basal forebrain: acetylcholinesterase histochemistry and choline-acetyltransferase immunoreactivity

The distribution of acetylcholinesterase histochemistry and choline-O-acetyltransferase immunohistochemistry in the basal forebrain was studied in newborn mice (P0) and until 60 days of postnatal life (P60). A weak acetylcholinesterase activity was found at P0 and P2 in the anterior and intermediate parts of the basal forebrain, and higher in the posterior region. The intensity of labeling, neuronal size and dendritic growth seems to increase progressively in all regions of basal forebrain from P4 to P10. The AChE+ cell count shows that in the anterior portion of the magnocellular basal nucleus the number of cells does not vary significantly from birth to the second month of postnatal life. However, in the intermediate and posterior portions of the nucleus the mean number of labeled cells increases significantly from birth to the end of the second week of postnatal life (P13). The choline-acetyltransferase immunoreactivity appears only detectable at the end of the first week (P6) as a slight immunoreaction, which increases progressively in intensity at P8, and at P10 seems to attain the same intensity of labeling found at P60. These results seem to indicate that the acetylcholinesterase could have a non-classic cholinergic role in the first stages of postnatal development, acting as a growth and cellular differentiation factor.     

The cortical relationships of certain basal ganglia and the cholinergic basal forebrain nuclei

Retrograde cellular degeneration has been found in the basal nucleus of Meynert in macaque monkeys after large lesions of the neocortex, and in the human brain after either hemidecortication or leucotomy. These observations may be relevant to the interpretation of the cellular degeneration in the basal nucleus in Alzheimer's disease.     

Specific contributions of the basal forebrain corticopetal cholinergic system to electroencephalographic activity and sleep/waking behaviour

The present study examined the role of the basal forebrain corticopetal cholinergic projection in the regulation of cortical electroencephalographic activity across sleep/wake states in rats. Selective lesions of this projection were effected by local intraparenchymal infusions of the immunotoxin 192 IgG-saporin. Lesions spared the septo-hippocampal cholinergic system, as well as p75-receptor-bearing noncholinergic neurons in the suprachiasmatic nucleus. Relative to sham-lesioned control animals, rats with lesions of basal forebrain cholinergic neurons displayed a significant reduction in high frequency EEG activity, characterized especially by a reduction in gamma EEG power. Lesions did not significantly alter the overall proportion of sleeping and waking states as defined behaviourally, but the attenuation of high frequency EEG activity was apparent across all stages, including REM-like periods. Results are consistent with the view that the basal forebrain corticopetal cholinergic system exerts a general activational effect on the cortical mantle. Although this system may not be essential for sleep/wake stage-switching, it does impact on the cortical states associated with those stages.     

Fiber pathways of basal forebrain cholinergic neurons in monkeys

In rhesus monkeys, autoradiographic tracing methods, complemented by immunocytochemical and histochemical techniques, were used to delineate pathways by which cholinergic neurons of the nucleus basalis of Meynert (nbM) and nucleus of the diagonal band of Broca (ndbB) project to forebrain targets. Following injections of [3H]amino acids into these nuclei, 5 major fiber pathways were identified: axons of the nbM and ndbB project medially, principally within the cingulum bundle, to dorsomedial portions of the hemispheres; nbM and ndbB fibers exit laterally beneath the pallidum and striatum, enter the external and extreme capsules, and pass within the corona radiata to terminate in lateral and caudal regions of neocortex; axons coursing ventrally from the nbM project to portions of the temporal lobe, including the amygdala; some fibers pass through the fibrae pass orbitofrontales to the orbitofrontal cortex; and, finally axons of the nbM/ndbB project via the fimbria/rornix and a ventral pathway to the hippocampus. The presence of these 5 radiolabeled pathways arising from basal forebrain cholinergic neurons was confirmed by acetylcholinesterase histochemistry and choline acetyltransferase immunocytochemistry.     

GABAergic control of basal forebrain cholinergic neurons and memory

The involvement of the GABAergic innervation of basal forebrain neurons in the rats' conditional visual discrimination performance was examined. Performance in such a task is based on the subjects's ability to retrieve information about response rules, and previous experiments have demonstrated that basal forebrain lesions interfere with this ability. Following the acquisition of the task, chronic guide cannulae were stereotaxically implanted into the substantia innominata of both hemispheres, and the animals were retrained. Administration of the GABAA-agonist muscimol into the substantia innominata (0, 25, 50 ng/0.5 microliters/hemisphere) dose-dependently decreased the number of correct responses, increased the number of errors of omission, increased response latency, but did not affect side bias. Systemic co-administration of the cholinesterase inhibitor physostigmine (0, 0.1, 0.2 mg/kg; i.p.) exclusively interacted with the effects of muscimol on correct responding. Specifically, physostigmine dose-dependently intensified and attenuated the muscimol-induced reduction in correct responding. Although it cannot be excluded that alternative neuronal mechanisms were involved in the mediation of the effects of muscimol and their interaction with physostigmine, these findings support previous evidence indicating that the activity of basal forebrain cholinergic neurons is controlled by a GABAergic input, and that this neuronal link is involved in mnemonic processing.