The mammalian suprachiasmatic nucleus (SCN) is considered to be a major component of the biological clock implicated in the temporal organization of a variety of physiological, endocrine, and behavioral processes. There is now a great deal of evidence indicating that many of these rhythms are progressively disturbed during senescence. The present study was aimed at investigating the influence of aging on the seasonal rhythm of the vasopressin (VP)-expressing neurons in the human SCN. To that end, brains obtained at autopsy of 48 human subjects, ranging in age from 6 to 91 years, were studied. Subjects were divided into two age groups, viz. ''young subjects'' (up to 50 years) acid ''elderly subjects'' (over 50 years). It is shown that the number of VP-immunoreactive neurons in the human SCN exhibits a marked annual oscillation in young but not in elderly people. Whereas in young subjects low VP-immunoreactive neuron numbers were found during the summer (May-July) and peak values in autumn (September-November), the SCN of elderly people showed a disrupted annual cycle with a reduced amplitude. These data suggest that the biosynthesis of vasopressin in the human SCN exhibits a seasonal rhythm that becomes disturbed later in life.
