Sex differences among oxytocin-immunoreactive neuronal systems in the mouse hypothalamus

Serial frontal sections of male and female mouse hypothalamus were immunostained with an antiserum to oxytocin, in order to study the topographical distribution of oxytocinergic perikarya and processes. Numbers of immunostained perikarya were counted in various hypothalamic regions. The oxytocin content of microdissected hypothalamic tissue samples was measured in radioimmunoassays. While the overall topographical distribution of oxytocin neurons in the classical magnocellular nuclei was similar in both genders, quantitative differences could be observed. The numbers of immunostained perikarya and the amounts of oxytocin found in females exceeded by far the numbers and amounts found in males. Male mice had fewer oxytocin-immunostained axons, projecting within the brain, than females. This was especially apparent in parts of the limbic system. Oxytocin-immunostained neurons in the perifornical region, the lateral hypothalamus and the ventral ansa lenticularis were mostly absent in males. It is possible that the observed sex differences in oxytocin immunoreactive brain architecture are due to the different hormonal conditions in males and females.