Vasopressin response to dehydration in Alzheimer's disease.

Alzheimer's disease is a progressive deterioration of neuropsychological functioning. One of the main neuropathological correlates of the disease is a drop-out of cholinergic neurons within the central nervous system. The neuropeptide that is responsible for water homeostasis and defense against dehydration, vasopressin, is also under direct cholinergic control. Several studies have suggested that in Alzheimer's disease there has been a trend toward lower vasopressin levels than in age-matched controls. In order to improve discrimination of normal from diminished vasopressin levels, nine subjects with Alzheimer's disease (mean age 65 +/- 2 years) and nine age- and sex-matched controls (68 +/- 3 years) underwent a mild provocative challenge of overnight fluid restriction. Individuals with Alzheimer's disease had a greater degree of dehydration, with overnight serum osmolality of 313 +/- 4 vs 300 +/- 3 Mosmol/kg, P = .01, and diminished "thirst" as measured by water ingested in one hour of ad libitum water intake. Eight of the nine with Alzheimer's disease had levels of vasopressin which, by extrapolation, appear to be subnormal for their serum osmolalities, whereas seven of the nine control subjects has vasopressin levels within or above the reference range (P less than .05). Elderly individuals with Alzheimer's disease may be at increased risk of dehydration during periods of fluid restriction due to the loss of normal physiological responses of "thirst" and vasopressin secretion.