Research reportImpairment of cholinergic neurotransmission in adult and aged transgenic Tg2576 mouse brain expressing the Swedish mutation of human β-amyloid precursor protein
Introduction
Alzheimer’s disease, the most common neurodegenerative disorder in senile dementia, is characterized by two major morpho-pathological hallmarks. The deposition of extracellular neuritic, β-amyloid peptide-containing plaques (senile plaques) in hippocampal and cerebral cortical regions of Alzheimer patients is accompanied by the presence of intracellular neurofibrillary tangles which occupy much of the cytoplasm of selected pyramidal neurons. Besides the cortical depositions of β-amyloid plaques and the accumulation of neurofibrillary tangles, central cholinergic deficits are also one of the primary features of Alzheimer’s disease. Clinical and post mortem studies suggested the involvement of central cholinergic transmission in the cognitive deterioration of Alzheimer’s disease and supported the idea that a deficit in acetylcholine level may be responsible for the initiation of Alzheimer’s disease. Cholinergic innervation in Alzheimer’s disease has been found to be reduced in areas of the brain important for processing of information, caused by the progressive loss of basal forebrain cholinergic cells [12]. This cell loss is paralleled by reductions in a number of cholinergic markers such as choline acetyltransferase (ChAT) and acetylcholine receptor binding and as well as levels of acetylcholine (for review, see Ref. [30]). Decreased levels of ChAT, the enzyme responsible for the synthesis of acetylcholine, correlated with the number of β-amyloid senile plaques and cognitive dysfunction in Alzheimer patients [7]. Consequently, several strategies to enhance the efficiency of cholinergic neurotransmission have been developed, including the use of acetylcholine precursors, enhancer of acetylcholine release, acetylcholinesterase (AChE) inhibitors and cholinergic receptor agonists.
The abilities of β-amyloid to self aggregate in solution, to generate free radicals, to induce intracellular reactive oxygen species formation, to cause protein oxidation and lipid peroxidation, and to compromise cell membrane functions [5], [10], [17], [19], [46] have been assumed to represent fundamental processes by which the cytotoxicity of β-amyloid is mediated in Alzheimer’s disease. However, β-amyloid is also physiologically produced and secreted in the brain as a normal soluble peptide [15], [37] which circulates in human body fluids [46], which raised the possibility that β-amyloid may also play a physiological role. Recent reports using in vitro approaches have suggested that β-amyloid in nanomolar concentrations can potently inhibit various cholinergic neurotransmitter functions independently of its apparent neurotoxicity (for review, see Ref. [3]). Therefore, some of the cholinergic deficits produced in Alzheimer’s disease could be attributed to the suppression of cholinergic markers by β-amyloid peptides in the absence of cell death [8]. This is supported by findings in Alzheimer patients, that the cholinergic deficits coincide with the deposition of β-amyloid plaques at the earliest histopathological stages, before the onset of clinical symptoms [4].
The in vivo mechanisms by which β-amyloid may induce pathological changes in neurotransmission, particularly in the central cholinergic system, are still unclear and require appropriate model systems. The availability of a transgenic mouse (Tg2576) that produces human β-amyloid peptides from birth and develops β-amyloid plaques in the aged brain [22] should represent a unique experimental approach to address the question of whether β-amyloid peptides also affect cholinergic neurotransmission in vivo. Brain tissue from transgenic Tg2576 mice at ages ranging from 7 to 24 months were examined by immuno- and histochemical staining for ChAT and AChE, by assaying cholinergic enzyme activities and high-affinity choline uptake as well as by quantitative neurotransmitter receptor autoradiography to screen for β-amyloid-induced changes in cholinergic receptor subtypes. The data provide in vivo evidence of a modulatory role of β-amyloid on central cholinergic neurotransmission.
Section snippets
Materials
[acetyl-3H]Acetylcholine iodide (specific activity 2.5 GBq/mmol), [methyl-3H]choline chloride (3.03 TBq/mmol), [N-methyl-3H]pirenzepine (3.03 TBq/mmol), [2,3-dipropylamino-3H]AF-DX384 (4.44 TBq/mmol), [3,5-3H(N)]cytisine (1.16 TBq/mmol), and [acetyl-3H]acetyl coenzyme A (spec. act. 7.4 TBq/mmol) were purchased from NEN Life Science Products, Köln, Germany. Atropine (Sigma) and l-nicotine (Sigma) were used for non-specific binding; all other chemicals used were commercial products of highest
ChAT immunocytochemistry
To determine whether β-amyloid deposits induce abnormalities in cholinergic neurons and fibres, immunocytochemistry for ChAT, a specific marker for the cholinergic system, was performed in brain sections of 8-, 16- and 24-month-old transgenic Tg2576 mice and non-transgenic littermates. Microscopic inspection of ChAT immunocytochemistry revealed the presence of ChAT-expressing neurons as well as ChAT-immunoreactive lattice-like fibre network in neocortical tissue of non-transgenic mice
Discussion
The mechanisms underlying the specific degeneration of basal forebrain cholinergic neurons in Alzheimer’s disease are still largely unknown. Besides the hypothesis of a failure in NGF signaling [41], progressive basal forebrain cholinergic cell loss may also be related to the extensive β-amyloid deposition that occurs within the cholinergic cortical and hippocampal basal forebrain projection systems [29]. Indeed, infusion of various β-amyloid peptides into the rat nucleus basalis produced a
Acknowledgements
The authors gratefully acknowledge the expert technical assistance of Renate Jendrek. This study was supported by the Deutsche Forschungsgemeinschaft, grant no. Schl 363/3-3, and the Interdisziplinäres Zentrum für Klinische Forschung (IZKF) Leipzig, proj-no. TP C18, to R.S. A.K. gratefully acknowledges the receipt of a 2-month-stipend from Deutscher Akademischer Austauschdienst (DAAD). The authors would like to express their gratitude to Dr Karen Hsiao Ashe, Department of Neurology, University
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