Alzheimer's disease and Down's syndrome

The nucleus basalis of Meynert in neurological disease: a quantitative morphological study

The recent finding of neuronal degeneration in the nucleus basalis of Meynert (nbM) in Alzheimer's disease suggests a possible role of this nucleus and the cholinergic system in dementing illness. We assessed neuronal population and density in two anatomical levels of the nbM in 38 coded brains from patients with a broad variety of neurological disorders. We found the most striking nbM degeneration in Alzheimer's and Parkinson's diseases. An intermediate loss of neurons was noted in the transmissible dementia Creutzfeldt-Jakob disease and in progressive supranuclear palsy. The usually nondementing diseases amyotrophic lateral sclerosis and multiple sclerosis showed cell counts comparable to those in a control group of nondemented patients with cerebrovascular disease. Characteristic neuropathological changes of Alzheimer's, Parkinson's, and Creutzfeldt-Jakob diseases, progressive supranuclear palsy, and subacute sclerosing panencephalitis were also found in the nbM. Most previous studies of the nbM have been based on small numbers of cases compared with controls. In our large series we used consistent methodology and a multiple-comparison statistical procedure to avoid overreporting of statistical significance. Our comparison of the nbM not only confirms the marked degeneration of the nbM in Alzheimer's disease, but also places such degeneration into perspective in a spectrum of dementing and nondementing neurological diseases. Larger case studies, using appropriate statistical techniques for multiple group comparisons, are needed to establish the significance of nbM degeneration in neurological disease.

AF64A, a cholinergic neurotoxin, selectively depletes acetylcholine in hippocampus and cortex, and produces long-term passive avoidance and radial-arm maze deficits in the rat

The behavioral and biochemical effects of AF64A, a presynaptic cholinergic neurotoxin, were investigated. Bilateral administration of this compound into the lateral cerebral ventricles produced transient and dose-related effects on sensorimotor function and long-term impairments of cognitive behavior. Male Fischer-F344 rats dosed with either 15 or 30 nmol of AF64A reacted 29-62% faster than CSF-injected controls in a hot-plate test 14 (but not 1, 7, 21 or 28) days following dosing. The group administered 15 nmol of AF64A was also significantly more active (41%) than controls 28 days following dosing. The activity level of this group was comparable to that of controls at other times and hyperactivity was never observed in the 30 nmol group. Retention of a step-through passive avoidance task, assessed 35 days after dosing, was impaired in both the 15 and the 30 nmol groups. Their step-through latencies were significantly shorter than the control latencies, and they exhibited more partial entries during the 24-h retention test. Radial-arm maze performance, measured 60-80 days following treatment, was markedly impaired in the treated groups. Animals treated with AF64A made fewer correct responses in their first 8 choices, required more total selections to complete the task, and had an altered pattern of spatial responding in the maze. The neurochemical changes produced by AF64A, determined 120 days after dosing, were specific to the cholinergic system and consisted of decreases of ACh in both the hippocampus (15 and 30 nmol groups) and the frontal cortex (30 nmol group). The concentrations of catecholamines, indoleamines, their metabolites and choline in various brain regions were not affected by AF64A. Furthermore, histological analysis revealed that the doses of AF64A used in the present study did not damage the hippocampus, the fimbria-fornix, the septum or the caudate nucleus. These data support the contention that cholinergic processes in the hippocampus and/or frontal cortex play an important role in learning and memory processes. Furthermore, based upon the behavioral and biochemical data presented, it is suggested that AF64A could be a useful pharmacological tool for examining the neurobiological substrates of putative cholinergic disorders such as senile dementia of the Alzheimer's type.

Alzheimer's disease: clinical features, pathogenesis, and treatment

Alzheimer's disease is an insidious degenerative disease of the brain and is the leading cause of dementia in the U.S. Numerous etiologies have been postulated, including a large body of evidence suggesting a slow viral infection, possibly in genetically predisposed individuals, but this remains to be proven. Differential diagnosis is based primarily on exclusion of other treatable forms of dementia. Neurochemical studies suggest a cholinergic deficit; thus primary emphasis in treatment has been directed at enhancing cholinergic activity. Choline and lecithin supplementation generally has been ineffective. Results with physostigmine are encouraging and further studies with this drug prototype are needed. Physostigmine's clinical usefulness is limited, however, due to peripheral side effects and its short duration of action. Other pharmacological approaches, such as naloxone, neural metabolic enhancers, stimulants, and vasopressin analogs, have been investigated. The clinical features and pathology of the disease are reviewed.

Acetylcholinesterase-immunoreactive axonal network in monkey visual cortex

Immunocytochemistry with antibodies to human erythrocyte acetylcholinesterase (AChE) was used to demonstrate axons containing this enzyme in the visual cortex (area 17) of macaques. AChE-positive fibers were most dense in layers I, IIIB, IV, and VIB of Hassler and Wagner (I, IVA, IVC, and VIB of Brodmann) with three strata of more sparsely distributed fibers (layers II-IIIA, IIIC, and V-VIA of Hassler and Wagner [II-III, IVB, and V-VIA of Brodmann]). Layer I contained the most dense plexus of AChE-positive axons, oriented horizontally. Horizontally oriented axons were also located in layers IIIB, IIIC, IV, and V of Hassler and Wagner (IVA, IVB, IVC, and V of Brodmann). Layer VIB contained axons of variable orientation, apparently afferent to cortex. Moderately stained neuronal perikarya were occasionally encountered in layer VIB and superficial white matter, but no intensely stained neurons were seen.

Alzheimer's presenile dementia, senile dementia of Alzheimer type and Down's syndrome in middle age form an age related continuum of pathological changes

A loss of nerve cells from the nucleus basalis of Meynert and the locus caeruleus together with a reduction in nucleolar volume in surviving cells was measured in twenty-two patients with Alzheimer's disease who ranged in age from 48-92 years, and in six patients over 50 years of age with Down's syndrome who also showed extensive formation of senile plaques and neurofibrillary tangles within their cerebral cortex. When compared with age matched controls the severity of these changes was greatest in the younger patients with Alzheimer's disease, but this fell with age such that by 90 years the level of change in Alzheimer's disease approached that in old age alone. There were only slight differences in the extent of these pathological changes in those patients with Down's syndrome when compared with others of similar age with Alzheimer's disease. It is concluded that the presenile dementia of Alzheimer's disease, the senile dementia of Alzheimer type and Down's syndrome in middle age all form an age-related continuum of pathological change.

Relationship between pigment accumulation and age in Alzheimer's disease and Down syndrome

The amount of lipofuscin pigment within nerve cells of the nucleus basalis of Meynert and that of melanin pigment with nerve cells of locus ceruleus was measured in seven patients with Down syndrome, in 22 patients with Alzheimer's disease and in 18 controls ranging from 30 to 88 years of age. No significant differences in amount of either pigment in these cells at any age were noted between the patient groups and their age controls.

Neurotransmitter specific alterations in dementing disorders: insights from animal models

Recent years have witnessed considerable change in the conceptualization of the pathophysiology of the cognitive impairments in dementing disorders, as a result of synaptic neurochemical analyses. Profound reductions in the forebrain cholinergic projections occur in Alzheimer's disease. In GM1 gangliosidosis, variable alterations in neurotransmitter related processes that are located in synaptic membranes have been described. Exploitation of animal models of human disorders resulting in dementia may further clarify the dynamic alterations in the biochemical processes required for effective neurotransmission in cortex.

Immunocytochemical identification of cholinergic neurons in the monkey central nervous system using monoclonal antibodies against choline acetyltransferase

A monoclonal antibody directed against rat brain choline acetyltransferase (ChAT) was used to stain cholinergic nerve cells within the brain and spinal cord of macaques. ChAT immunoreactivity was seen in motor neurons of the brainstem and spinal cord, in large neurons of the striatum, and in large neurons in the basal forebrain (medial septum--diagonal band--nucleus basalis complex); each of these groups of neurons is believed to be cholinergic. The ability to visualize cholinergic neurons in the nervous system of primates provides a new approach to the study of cholinergic systems in health and disease.

Alzheimer's disease: a disorder of cortical cholinergic innervation

Great emphasis is being placed on identification of neurotransmitter systems involved in the symptomatic manifestations of neurological and psychiatric disorders. In the case of Alzheimer's disease, which now seems to be one of the most common causes of mental deterioration in the elderly, compelling evidence has been developed that acetylcholine-releasing neurons, whose cell bodies lie in the basal forebrain, selectively degenerate. These cholinergic neurons provide widespread innervation of the cerebral cortex and related structures and appear to play an important role in cognitive functions, especially memory. These advances reflect a close interaction between experimental and clinical neuroscientists in which information derived from basic neurobiology is rapidly utilized to analyze disorders of the human brain.

Cholinergic innervation in neuritic plaques

Although several studies of Alzheimer's disease suggest that the frequency of neuritic plaques in the cerebral cortex is correlated with the severity of dementia and with reduction in presynaptic cholinergic markers in the cortex, the relationship between cholinergic cortical innervation and the pathogenesis of plaques is unknown. The hypothesis was tested that the neurites in the plaque consist, in part, of presynaptic cholinergic axons, many of which arise from neurons in the basal forebrain. This hypothesis was tested by analyzing the character and distribution of plaques in monkeys, aged 4 to 31 years, with staining for acetylcholin-esterase and also with Congo red and silver stains. Immature and mature plaques were rich in acetylcholinesterase. As the plaques matured, the amount of amyloid increased, and the number of neurites and the activity of acetylcholinesterase decreased. End-stage amyloid-rich plaques lacked acetylcholinesterase. These observations indicate that changes in cortical cholinergic innervation are an important feature in the pathogenesis and evolution of the neuritic plaque.