The supraoptic nucleus of the adult rat hypothalamus displays marked sexual dimorphism which is dependent on body weight.

The neurons of the supraoptic nucleus in the rat hypothalamus are reported not to possess receptors for gonadal steroids and sexual dimorphism has not previously been described in this nucleus. We have analysed this nucleus in groups of Sprague-Dawley rats (six males or six females per group), one, two, six, 12 and 18 months after birth. Body and brain weights were recorded, the volume of the nucleus was determined from the right hemisphere and all other quantitative parameters were determined from the left nucleus. In addition, different groups of four male and four female rats aged two and 18 months were analysed after immunocytochemical staining to distinguish between vasopressin and oxytocin neurons. The total number of neurons was constant in all groups studied, despite which the volume of the supraoptic nucleus increased progressively with age in both males and females. The cross-sectional areas and volumes of supraoptic neurons also increased with age. The volume density of the neuropil remained constant in all groups and there was a progressive decrease with age in the numerical density of neurons. Immunocytochemistry revealed that the age-dependent increases in the size of the neurons involved primarily the vasopressin neurons. The age-related changes were much greater in males than in females, resulting in significant differences between the sexes at two, six, 12 and 18 months with respect to the volume of the supraoptic nucleus, the cross-sectional areas of neuronal somata and nuclei, and the volume of supraoptic neurons. Thus the supraoptic nucleus and its vasopressin neurons are larger in adult males than in age-matched females. Since we have also shown that body weight is very closely correlated with changes in the size of supraoptic neurons, and adult male rats are heavier than females of the same age, we suggest that these size changes reflect adaptation of the vasopressin neurons of the supraoptic nucleus to increasing functional demands associated with the regulation of water balance in bodies of increasing size.