Intercorrelations of regional cerebral glucose metabolic rates in Alzheimer's disease

Is Alzheimer's disease an anterograde degeneration, originating in the brainstem, and disrupting metabolic and functional interactions between neurons and glial cells?

A novel hypothesis is suggested for the pathogenesis of Alzheimer's disease, i.e. that a degeneration of adrenergic neurons in locus coeruleus and/or of serotonergic neurons in the raphe nuclei leads to impairment in metabolic and functional interactions between neurons and astrocytes (in the cerebral cortex and hippocampus as well as in nucleus basalis magnocellularis), and that a resulting deficient supply of substrates and failing energy metabolism in both neurons and astrocytes causes neuronal cell death in these areas and thus interference with additional transmitter systems. The hypothesis is based on (1) the topographical distribution of ascending pathways from locus coeruleus and the raphe nuclei; (2) the peculiar termination of many of these fibres in varicosities, from which released transmitter molecules reach their targets by diffusion, rather than in genuine synapses, suggesting a partly non-neuronal target; (3) the effects of locus coeruleus lesions in experimental animals; (4) the emergence of new knowledge in cellular neurobiology, indicating profound metabolic and functional interactions between neurons and astrocytes; and (5) the effects of adrenergic and serotonergic agonists upon metabolism and function in rodent astrocytes and neurons. These compounds influence energy metabolism, membrane transport of potassium and production of growth factors in astrocytes, and glutamate release from glutamatergic neurons. They thus influence essential metabolic interactions between neurons and astrocytes, as well as neuronal-astrocytic interactions in potassium homeostasis at the cellular level. Obviously, neither the individual findings alone, nor their combination into a conceptual framework, prove the correctness of the hypothesis. However, they do provide a basis for further experimental work, using postmortem brain tissue from Alzheimer's patients and lesion studies in rodents, which can confirm or refute the hypothesis.

The role of calcium regulation in brain aging: reexamination of a hypothesis

Studies of the central nervous system have a long history; however, it is only recently that we have begun to understand brain function in health and disease states. And, the topic of the aging brain has become a subject of intense study for a short period. At present, the process of normal aging is relatively poorly understood. Although there are a number of theories of aging, no single theory appears to account for most age-dependent brain changes. This review provides a re-evaluation of the "Calcium Hypothesis of Brain Aging" in light of new evidence which supports the proposition that cellular mechanisms, which maintain the homeostasis of cytosol Ca2+ concentration, play a key role in brain aging; and that sustained changes in [Ca2+]i homeostasis provide the final common pathway for age-associated brain changes. This revision of the calcium hypothesis suggests that there is a complex interaction between the amount of [Ca2+]i perturbation and the duration of such deregulation of Ca2+ homeostasis and it proposes that a small disturbance in Ca2+ homeostasis with a sustained increase in [Ca+]i over a long period has similar cell injuring consequences as that produced by a large increase in [Ca2+]i over a shorter period. Although there are several alternative mechanisms through which the regulation of cytosol [Ca2+]i can be disrupted (such as changes in ion channels, extrusion pumps, and sequestration), this review focuses on disruptions in energy metabolism and changes in the structure and function of membranes as the most likely antecedent events which lead to disruption of Ca2+ homeostasis. The principle purpose of this review is to identify scientific opportunities and stimulate further research into cellular mechanisms of brain aging.

Cortical blood flow increases induced by stimulation of the substantia innominata in the unanesthetized rat

The possible implication of projections from the substantia innominata (SI) to the cerebral cortex in the control of local cortical blood flow (CoBF) was studied in adult Fischer rats. Local blood flow (by helium clearance) and tissue gas partial pressures (pO2, pCO2) as metabolic indices, were measured in the frontal and parietal cortices in unanesthetized animals via chronically implanted probes connected to a mass spectrometer. Stimulating electrodes, also implanted chronically, were placed in the region of the SI. Out of 37 correctly located sites, 28 gave rise to cerebrovascular responses without significant hypertension or agitation. Both frontal (+114%) and parietal CoBF (+28%) increased significantly during ipsilateral 50 microA stimulation, but did not further significantly increase at 100 microA. Contralateral stimulation induced only small, non-significant effects. SI stimulation simultaneously increased cortical pO2 and decreased cortical pCO2, significantly more so in the frontal compared to the parietal cortex, and ipsilaterally compared to contralaterally. Both the CoBF and the tissue gas changes induced by SI stimulation were strongly potentiated by infusion of 0.15 mg/kg/h of the cholinomimetic agent physostigmine. The electrocorticogram (ECoG) was not systematically activated during the SI stimulation. The evidence presented favors a role for the cholinergic projections of the SI in control of CoBF (particularly frontal cortex), especially since the flow changes observed showed no obvious dependence on changes in local pCO2 or on paCO2, and could not be attributed to hypertension or behavioral changes.

Hypothesis: Alzheimer's disease is a phylogenetic disease

It is hypothesized that Alzheimer's disease is a human phylogenetic disease which has a common multifactorial pathogenesis in sporadic and familial cases and in Down syndrome, related to a genomic character function G(x). Increments in G(x) accompanied the increased gene expression that sustained brain growth and differentiation during hominid evolution, particularly of the regions liable to Alzheimer pathology, and further occur in Down syndrome [suggesting that genes on chromosome 21 are included in G(x)]. If genes which promoted human brain evolution contribute to the value of G(x), a better understanding of the genomic events which promoted this evolution, using molecular biological techniques, should elucidate the genetic basis of Alzheimer's disease, and vice versa.

Longitudinal study of the early neuropsychological and cerebral metabolic changes in dementia of the Alzheimer type

To examine the progression of neuropsychologic and metabolic changes in the early stages of dementia of the Alzheimer type (DAT), we studied 11 midly demented patients longitudinally. Three aspects of neuropsychological function were measured: memory, attention to complex sets and abstract reasoning, and lateralized functions, i.e., language and visuoconstruction. Regional cerebral metabolic rates for glucose were measured in frontal, parietal, and temporal association cortices. Our results show that, in general, memory deficits are the first neuropsychological impairments to occur in DAT, followed by problems with attention to complex cognitive sets and abstract reasoning, which are followed in turn by deficits in language and visuospatial abilities. In addition, neocortical metabolic abnormalities usually precede impairment of neocortically mediated attention and abstract reasoning by 8 to 16 months, and precede impairment of neocortically mediated language and visuospatial function by 12 to 37 months. These findings suggest that the first nonmnestic neuropsychological consequence of neocortical physiological dysfunction in DAT is a loss of attentional capacity. Since neocortical metabolic changes generally precede the appearance of neocortically mediated neuropsychological dysfunction, physiologic dysfunction may exist for some time before cognition is affected.

Possible neurotransmitter basis of behavioral changes in Alzheimer's disease

Serotonin, 5-hydroxyindoleacetic acid, and homovanillic acid concentrations have been determined in 10 areas of the cerebral cortex from 17 subjects with Alzheimer's disease and 18 control subjects. The dopamine metabolite was not reduced in any area, whereas both indoleamines were reduced in the superior frontal, inferior temporal, and fusiform gyri, and the temporal pole. These areas and areas of the parietal cortex, where there were no changes in concentration, have not previously been reported on. We argue that the large loss of indoleamines from the frontal lobe (to 50-63% of control values) is rather unexpected based on other biochemical measurements and may relate to behavioral changes.

High metabolic activity in the visual cortex of early blind human subjects

Glucose metabolism has been studied in the visual cortex of early blind human subjects. In the forebrain of these subjects, regional glucose utilization was the highest in the striate and prestriate cortical areas. Furthermore, this activity was higher than in blindfolded sighted subjects, whether at rest or during an auditory or tactile task. These observations raise the question of the functionality of the blind's visual cortex.

Biochemical measurements in Alzheimer's disease reveal a necessity for improved neuroimaging techniques to study metabolism

A series of Alzheimer's disease and control brains were dissected to determine the extent of atrophy (based on total protein content) and loss of choline acetyltransferase activity in the cerebral cortex from the entire surface of the diseased brains. The distribution of intensity of pathology so determined is strikingly similar to the degree of hypometabolism as shown by positron emission tomography. It is argued that the hypometabolism can be explained (at least in part) by focal areas of atrophy.

Measurement of disease progression in Alzheimer's disease

Riege and Metter's comprehensive review deals with many issues pertaining to Alzheimer's disease research. We agree with the authors that early diagnosis and evaluation of disease progression represent key points that must be addressed in an integrated manner. Brain imaging holds great promise for the resolution of both issues.