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High-affinity choline uptake carrier in Alzheimer's disease: implications for the cholinergic hypothesis of dementia
J Pascual1, A Fontán, J J Zarranz
1Department of Physiology and Pharmacology, University of Cantabria, Faculty of Medicine, Santander, Spain.
Insights
Alzheimer's disease brains show reduced high-affinity choline uptake (HACU) sites in the frontal cortex and hippocampus. However, other brain regions and binding affinity remain largely unchanged, suggesting complex mechanisms in AD dementia.
Area of Science:
- Neuroscience
- Neuropathology
- Biochemistry
Background:
- Alzheimer's disease (AD) is characterized by neurodegeneration.
- Cholinergic system deficits are implicated in AD pathogenesis.
- High-affinity choline uptake (HACU) is a key marker for presynaptic cholinergic terminals.
Purpose of the Study:
- To investigate the density and affinity of [3H]hemicholinium-3 ([3H]HC-3) binding sites, a marker for HACU carriers.
- To compare these markers in specific brain regions of postmortem Alzheimer's disease (AD) and control brains.
- To determine the extent of cholinergic degeneration in AD.
Main Methods:
- Postmortem brain tissue analysis from 13 AD patients and 12 controls.
- Radioligand binding assays using [3H]hemicholinium-3 ([3H]HC-3) to quantify HACU sites.
- Saturation assays to determine binding affinity in specific brain regions.
Main Results:
- Significant reductions in [3H]HC-3 binding site density were observed in the frontal cortex (-44.7%) and hippocampus (-36.5%) of AD brains compared to controls.
- No significant differences in [3H]HC-3 binding site density were found in the caudate-putamen and cerebellar cortex.
- Binding affinity of HACU sites remained unchanged in the frontal cortex and hippocampus.
Conclusions:
- Cholinergic degeneration, specifically in basal forebrain neurons projecting to the frontal cortex and hippocampus, occurs in AD.
- The observed changes suggest that HACU loss is region-specific and may not fully explain the severity of dementia in all AD patients.
- These findings imply that compensatory mechanisms might be involved and that treatments targeting acetylcholine precursors or muscarinic agonists may have limited efficacy in AD dementia.
Abstract:
We examined the density and the state of affinity of [3H]hemicholinium-3 ([3H]HC-3) binding sites, a marker of the presynaptic high-affinity choline uptake (HACU) carrier, in 4 representative regions of 13 postmortem Alzheimer's disease (AD) brains, as well as in 12 matched control brains. Significant reductions in the densities of [3H]HC-3 binding sites were found both in frontal cortex (-44.7%) and hippocampus (-36.5%) of AD brains in comparison to controls. On the other hand the densities of [3H]HC-3 binding sites in AD brains in caudate-putamen and cerebellar cortex showed no significant differences when compared to controls. No significant change in the state of affinity of these sites could be observed in the saturation assays carried out in hippocampus and frontal cortex. Our findings concur with the reported data by using other presynaptic cholinergic markers in AD and confirm that some degree of cholinergic degeneration, highly specific for the basal forebrain neurons, occurs in AD. However, these results, obtained in a group of AD brains belonging to severely demented patients, do not show a dramatic loss of the HACU in many AD brains. Although this fact could be due to the existence of a compensatory mechanism, our results probably suggest that dementia in AD cannot be explained only by the loss of neocortical cholinergic presynaptic terminals arising from the basal forebrain and also may clarify as to why the acetylcholine precursors or the muscarinic agonists are not effective in AD dementia.