Vasopressin innervation of sexually dimorphic structures of the gerbil forebrain under various hormonal conditions.

The distribution of vasopressin-immunoreactive fibers in the forebrain of male and female gerbils was studied, focusing on the lateral septum and the sexually dimorphic area (SDA) found at the border between the medial preoptic area and the anterior hypothalamus. To study hormonal influences on the densities of these fibers, some animals of each sex were gonadectomized or gonadectomized and given testosterone. Others were given sham operations. High densities of vasopressin-immunoreactive fibers were found in the lateral septum. In the SDA, the densities of these fibers varied considerably. Many were found in the medial half of the medial SDA, but few in the lateral SDA. Vasopressin-immunoreactive fibers were also sparse in the lateral half of the medial SDA, except for a dense cluster in the SDA pars compacta of males. Similar but smaller clusters were seen in the same location in females although the SDA pars compacta could not be detected in Nissl-stained sections from the female brains. Fiber densities in two areas, the lateral septum and the lateral SDA, were sensitive to gonadal steroids. In both cases, castration reduced fiber density and testosterone enhanced it. In addition, fiber densities in two areas, the lateral septum and the medial SDA, were sexually dimorphic. In each case, fiber density was greater in males. There was no hormonal effect, however, on the fiber densities in the medial SDA. The fact that the fiber plexuses in the lateral septum and the medial SDA respond differently to gonadal steroids suggests that they arise from different cells and possibly from different areas of the brain. The vasopressin-immunoreactive fibers in the lateral septum probably come from steroid-sensitive vasopressin neurons in the bed nucleus of the stria terminalis. Those in the medial SDA may originate in the dorsal aspect of the suprachiasmatic nucleus where vasopressin-immunoreactive cell bodies were seen.