Association Between Smoking and Cholinergic Basal Forebrain Volume in Healthy Aging and Prodromal and Dementia Stages of Alzheimer's Disease

Background: Smoking has been found associated with decreased cerebral volumes in healthy adults and in various neuropsychiatric disorders. Objective: We aimed to determine whether chronic nicotine exposure through smoking is associated with reduced volume of cortically projecting cholinergic basal forebrain nuclei in healthy aging, mild cognitive impairment (MCI), and dementia stages of Alzheimer's disease (AD). Methods: We retrieved cross-sectional data from the Alzheimer's Disease Neuroimaging Initiative (ADNI) database including 179 cognitively normal elderly subjects, 270 subjects with early stage MCI, 136 subjects in later, more advanced, stage of MCI, and 86 subjects in dementia stages of AD. We determined the association between past or current smoking versus lifetime non-smoker status on the volumes of the basal forebrain determined from volumetric MRI scans. Hippocampus volume was used as a control region. Significant effects were controlled for mediating or moderating effects of respiratory and cardiovascular morbidity. Results: In cognitively healthy individuals and early MCI, past or current smoking was significantly associated with smaller basal forebrain volume. This effect was independent from age, sex, or cardiovascular or respiratory morbidity. Hippocampus volume was not associated with smoking. In late MCI and AD dementia, smoking was not associated with basal forebrain or hippocampus volumes. Conclusions: Our findings suggest that chronic nicotine exposure through smoking may lead to atrophy of cholinergic input areas of the basal forebrain. This effect may account for an increased risk of AD dementia onset with smoking by exhausting the cholinergic system reserve capacity.