ANNUAL VARIATIONS IN THE VASOPRESSIN NEURON POPULATION OF THE HUMAN SUPRACHIASMATIC NUCLEUS

The mammalian suprachiasmatic nucleus is considered to be the major co mponent of the biological clock, involved in the temporal organization of a wide variety of physiological and behavioral processes. The pres ent study was conducted to investigate whether there are diurnal or an nual variations in the morphology of the vasopressin-containing neuron population of the suprachiasmatic nucleus in human beings. To that en d, the brains of 48 human subjects were investigated. A marked annual variation was observed in the volume and vasopressin cell number of th e human suprachiasmatic nucleus: the volume of the vasopressin cell po pulation was, on average, 2.5 times larger in October November than in May June and contained 2.7 times as many vasopressin-immunoreactive n eurons. In general, the annual cycle of the human suprachiasmatic nucl eus showed a non-sinusoidal pattern with a maximum in early autumn, a lower plateau in winter and a deep trough in late spring and early sum mer. In contrast, no such seasonal variations could be detected in sup rachiasmatic nucleus vasopressin numerical cell density or cell-nuclea r diameter. The number of vasopressin-immunoreactive neurons in the pa raventricular nucleus of the hypothalamus, on the other hand, did not show any significant periodic changes over the year, indicating the sp ecificity of the suprachiasmatic nucleus rhythm. In contrast with the annual cycle of the suprachiasmatic nucleus, no significant diurnal va riations were observed in any of these parameters. In conclusion, the findings indicate that photoperiod may be considered a potential envir onmental factor controlling the activity of the vasopressin system of the human suprachiasmatic nucleus.