Neurotrophic agents may exacerbate the pathologic cascade of Alzheimer's disease

Thyroid hormone modulates the development of cholinergic terminal fields in the rat forebrain: relation to nerve growth factor receptor

Hyperthyroidism, induced in rat pups by the daily intraperitoneal administration of 1 microgram/g body weight triiodothyronine, facilitated the development of ChAT fiber plexuses in brain regions innervated by basal forebrain cholinergic neurons, leading to an earlier and increased expression of cholinergic markers in those fibers in the cortex, hippocampus and amygdala. A similar enhancement was seen in the caudate-putamen complex. This histochemical profile was correlated with an accelerated appearance of ChAT-positive telencephalic puncta, as well as with a larger total number of cholinergic terminals expressed, which persisted throughout the eight postnatal week, the longest time examined in the present study. Hypothyroidism was produced in rat pups by adding 0.5% propylthiouracil to the dams' diet beginning the day after birth. This dietary manipulation resulted in the diminished expression of ChAT in forebrain fibers and terminals. Hypothyroid treatment also reduced the quantity of ChAT puncta present during postnatal weeks 2 and 3, and, from week 4 and continuing through week 6, the number of ChAT-positive terminals in the telencephalic regions examined was actually less than the amount extant during the former developmental epoch. Immunostaining for nerve growth factor receptor (NGF-R), which is associated almost exclusively with ChAT-positive somata and fibers in the basal forebrain, demonstrated a different time course of postnatal development. Forebrain fibers and terminals demonstrating NGF-R were maximally visualized 1 week postnatally, a time at which these same neuronal elements evinced minimal ChAT-like immunopositivity. Thereafter and correlated with increased immunoreactivity for ChAT, fine details of NGF-R stained fibers were observed less frequently. Although propylthiouracil administration decreased NGF-R immunodensity, no alteration in the development of that receptor was observed as a function of triiodothyronine treatment. Cholinergic terminals in the ventrobasal thalamus, which derive from ChAT-positive neurons in the pedunculopontine and laterodorsal tegmental nucleus, were unaffected by either hyperthyroid or hypothyroid conditions. These cells also do not demonstrate NGF-R. We conclude from these experiments (1) that cholinergic fiber plexuses eventually exhibiting ChAT positivity in the telencephalon demonstrate NGF-R prior to the cholinergic synthetic enzyme, (2) that susceptibility to thyroid hormone manipulations may involve sensitivity to NGF, at least in some forebrain cholinergic systems and (3) that the effects of thyroid hormone imbalances on brain cholinergic neurons are regionally selective.

Alpha 1-antichymotrypsin supports short-term survival of cerebral neurons in culture

alpha 1-Antichymotrypsin (ACT) is a component of the amyloid deposits in Alzheimer's disease. To elucidate its biological activities in the central nervous system, dissociated cultures of rat cerebral neurons were grown in a serum-free medium containing ACT. The addition of ACT (0.1-1 microgram/ml) enhanced survival of microtubule-associated protein 2-immunoreactive neurons under the culture conditions examined.

Involvement of microtubules in modifications associated with cellular aging

Microtubules are ubiquitous cellular components involved in the control of cell structure and functions, such as cell division, regulation of shape and polarity, intracellular transport, etc. Consequently, any alteration affecting them in structure or function has a good chance of affecting the cell and generally leads to cell dysfunctions. This has been shown for instance, after treatment with microtubule-interacting drugs. Cellular aging is also characterized by the appearance of various cell dysfunctions, but the possible involvement of the microtubules in the aging process, although a rather tempting hypothesis, has not yet been extensively investigated. In this paper, I will first rapidly review the different components that build, organize and control the microtubules in normal cells, independently of the aging process. I will then consider the possible involvement of the microtubules in the aging process, more particularly in models of cells aging in vitro and in aging neuronal cells, which have been the most extensively investigated. There is some evidence for alterations in the microtubule organization both in cells aging in vitro and in the aging brain. But the interpretation of these data awaits further experiments, taking into account the latest progress in tubulin genetics and in microtubule biochemistry. Microtubules could also represent one of the cellular targets affected after signal transduction and could thus be involved in the resulting cellular responses. This hypothesis will be discussed, as it offers new insights into the regulation of microtubule organization, dynamics and functions in normal cells, which will be worthwhile to investigate during the aging process.

Cytoskeletal alterations might account for the phylogenetic vulnerability of the human brain to Alzheimer's disease

A theory is presented here in the attempt to explain why Alzheimer's disease (AD) primarily affects areas of the human brain that have been acquired recently in phylogenesis. Disturbances in cytoskeletal function are proposed to play a fundamental role in triggering the sequence of pathologic events leading to the occurrence of AD-related histopathological markers and to the degeneration and death of neurons. These deficits are supposed to occur more likely in neuronal populations that possess a high degree of plasticity, the substrate of memory functions, and that constitute, in fact, the phylogenetically new telencephalic regions of the human brain.

Cholinergic fiber perturbations and neuritic outgrowth produced by intrafimbrial infusion of the neurofilament-disrupting agent 2,5-hexanedione

The compound 2,5-hexanedione (HD) produces axonopathies in peripheral nerves characterized by selective accumulation of neurofilaments. Its direct actions on neurotransmitter-specific neurons in the brain are unknown. In an attempt to address this latter issue, we infused HD into the fimbria and evaluated histochemically and immunohistochemically possible structural alterations in cholinergic neurons projecting from the basal nuclear complex to the hippocampus. Putative cholinergic fibers expressing nerve growth factor receptor and acetylcholinesterase showed increases in caliber and perturbations in trajectories 2-4 days following HD treatment. Similar morphologic changes were observed in neuronal elements processed for the 68 kDa neurofilament protein. At 7 days, short collateral ramifications appeared in many cholinergic axons that were suggestive of neurite outgrowth. Correlated with these fiber alterations was a transient reduction in the number of medial septal and diagonal band somata expressing choline acetyltransferase, which returned to control levels within 6 weeks following HD treatment. These data support the view that neurofilaments play an important, perhaps cytoarchitecturally stabilizing, role in regulating axonal morphology in certain populations of cholinergic neurons.

Cell loss and nuclear hypertrophy in topographical subdivisions of the nucleus basalis of Meynert in Alzheimer's disease

The nucleus basalis of Meynert was examined in six patients with Alzheimer's disease and five age-matched controls. A cytoarchitectonic study was followed by quantitative analysis of the population of neurons and by the determination of their nuclear area. Confirming previous neuropathological observations in Alzheimer's disease, a neuronal loss of 43% in the anterior, 25% in the intermediate and 30.5% in the posterior subdivisions of the nucleus basalis of Meynert was observed. Numerous surviving cells showed neurofibrillary tangles. In addition, we found that the nuclear area of the remaining nucleus basalis of Meynert neurons was significantly increased in all three subdivisions by at least 16%. The combined observation of cell loss and nuclear hypertrophy suggests that both regenerative and degenerative changes co-exist in the nucleus basalis in Alzheimer's disease.

Cholinergic systems in the rat brain: IV. Descending projections of the pontomesencephalic tegmentum

Descending projections from cholinergic neurons in the pedunculopontine and laterodorsal tegmental nuclei, collectively referred to as the pontomesencephalotegmental (PMT) cholinergic complex, were studied by use of the fluorescent retrograde tracers fluorogold, true blue, or Evans Blue in combination with choline acetyltransferase (ChAT) immunohistochemistry of acetylcholinesterase (AChE) pharmacohistochemistry. Pedunculopontine somata positive for ChAT or staining intensely for AChE were retrogradely labeled with fluorescent tracers following infusions into the motor nuclei of cranial nerves 5, 7, and 12. ChAT-positive cells in both the pedunculopontine and laterodorsal tegmental nuclei demonstrated projections to the vestibular nuclei, the spinal nucleus of the 5th cranial nerve, deep cerebellar nuclei, pontine nuclei, locus ceruleus, raphe magnus nucleus, dorsal raphe nucleus, median raphe nucleus, the medullary reticular nuclei, and the oral and caudal pontine reticular nuclei. Fluorescent tracers used in combination with AChE pharmacohistochemistry corroborated these projections and, in addition, provided evidence for cholinergic pontomesencephalic projections to the lateral reticular nucleus and inferior olive. The majority of retrogradely labeled neurons demonstrating ChAT-like immunoreactivity were found ipsilateral to the injection site, but, in all cases, tracer-containing cholinergic cells contralateral to the infused side of the brain were detected also. More retrogradely labeled cells containing ChAT were observed in the pedunculopontine tegmental than in the laterodorsal tegmental nucleus following tracer injections at all sites with the exceptions of the locus ceruleus and dorsal raphe nucleus where the converse profile was observed. None of the pedunculopontine or laterodorsal tegmental cells immunopositive for ChAT or stained intensely for AChE contained retrogradely transported tracers following dye infusions into the cerebellar cortex or cervical spinal cord. Triple-label experiments using two tracers infused into different sites in the same animal revealed that individual ChAT-immunoreactive cells in the PMT cholinergic complex projected to more than one hindbrain site in some cases and had ascending projections as well. Certain ChAT-positive somata in the pedunculopontine and laterodorsal tegmental nuclei were found in close association with several fiber tracts, including the superior cerebellar peduncle, lateral lemniscus, dorsal tegmental tract, and medial longitudinal fasciculus.

Plasticity, pathology, and Alzheimer's disease

Much basic data support the notion that growth factors are important in the maintenance and repair of the CNS. Although local increases in growth factors might contribute to aberrant growth and senile plaque formation in Alzheimer's disease (AD), the administration of growth factor inhibitors as a therapeutic intervention would be predicted to decrease the vulnerability of remaining healthy neurons. The administration of neurotrophic agents in AD may help to protect and maintain neurons, and thereby minimize neuronal death and the resultant sprouting response.

Trophic factors and Alzheimer's disease

The paper by Butcher and Woolf presents a troubling hypothesis, that neurotrophic factors may be contributory to the pathology of Alzheimer's disease rather than potentially serving as therapeutic agents. Much more experimental work, especially in primates, will be required to tease out the positive and/or negative influences these factors may have on the pathogenesis of the neurodegenerative diseases causing dementias.

Nerve growth factor in Alzheimer's disease: to treat or not to treat?

Several hypotheses can be proposed to link neurotrophic factors with neurodegenerative diseases. Not surprisingly, different hypotheses suggest completely different approaches to therapy; some would suggest use of neurotrophic factors, while others would propose that the actions of these factors be blocked. It has been suggested that NGF be used to prevent the loss of basal forebrain cholinergic neurons in Alzheimer's disease (AD). At this time it is not possible to conclude whether or not NGF is implicated in the causation or progression of this disorder. Nevertheless, experimental studies in animals have given a strong rationale for its use. Given the lack of an effective treatment for this disorder, the careful approach to NGF trials outlined by an ad hoc committee of the National Institute on Aging should be pursued.