The distribution of vasopressin in the brain of the garden dormouse (Eliornys quercinus L.) was examined by immunocytochemistry at different times of the year. In spring-summer, sexual dimorphism in the density of vasopressin labeling was observed in several areas of the brain. In these regions, such as the lateral septum and the nucleus of the vertical limb of the diagonal band of Broca, male garden dormice showed more vasopressin immunoreactivily than their female counterparts. In autumn-winter, at the time of hibernation, vasopressin was undetectable in the sexually dimorphically innervated brain regions in males as well as females. In early spring. there was large variation in vasopressin staining in the male brain, whereas the female brain was consistently lacking in vasopressin labeling. In brain regions that exhibited no sexually dimorphic innervation in the summer, vasopressin labeling remained the same throughout the year. The difference in the amount of vasopressin immunoreactivity in males in summer and winter correlated significantly with differences in testes weight. In early spring, because of interindividual variations in the start of sexual activation, testes size does not correlate with the density of vasopressin immunoreactivity.
Similar seasonal variations in density of vasopressin labeling in the brain were also observed in the European hamster (Cricrtus cricetus). I n this animal, central vasopressin infusion during the winter prevents hibernation. The presence of a similar pattern of changing vasopressin content in another hibernator, the garden dormouse, suggests an important role for this neuropeptide in seasonal functions.
In the garden dormouse, vasopressin fibres could be detected in a number of brain regions where, in other rodents, such fibres are undetectable, e.g., the interpeduncular complex and the lateral superior olive. I t is suggested that the central vasopressin system in the rat and European hamster is essentially the same as in the garden dormouse, but more difficult to detect immunocytochemically.