The ovine sexually dimorphic nucleus (oSDN) of the preoptic area
My laboratory discovered a sex difference in part of the sheep brain called that's known to control sexual behaviors. The difference consists of the size of a cluster of neurons in the preoptic area, which we named the ovine sexually dimorphic nucleus or oSDN. On average the oSDN is 2 times larger in rams than in ewes and expresses high levels of the enzyme aromatase. Approximately 8% of adult rams prefer to mount other rams instead of ewes, and the oSDN in these rams is smaller than in rams that prefer females; more similar to the volume of the oSDN in ewes. The sex difference in oSDN volume is present before sheep are born and controlled by hormone exposure during a critical period in fetal development. Testosterone produced by the fetal testis is responsible for the sex difference in oSDN volume. Thus, it seems plausible to suggest that natural variations in testosterone production and/or disruptions in its actions may reduce the volume of the oSDN and constitute a biological mechanism that predisposes rams to be acquire same-sex attractions. Our studies are exploring this possibility by systematically studying the processes that control sex-specific oSDN development in sheep.
Ongoing Research Projects
Sheep are one of the few animal models in which natural variations in male sexual partner preferences have been studied experimentally. It is possible to identify rams with distinct sexual preferences by giving them choice tests – making them a valuable model to study the biological underpinnings/causes of sexual partner preferences.
To study possible neural mechanisms, we focused on the preoptic area. The preoptic area is a brain region essential for the control of male sexual behavior and sexual preferences and is characterized by marked neuroanatomical sex differences in most vertebrate species.
The ovine sexually dimorphic nucleus (oSDN) is a cluster of cells in the central portion of the medial preoptic nucleus of sheep that is larger in female-oriented rams than in male-oriented rams and ewes.
The oSDN is established before lambs are born suggesting that differences in the size of this brain area could cause animals to express specific sexual preferences when they become adults. Our studies have also now showed that the development of the oSDN is under control of testosterone secreted by the fetal testis. Exposure to testosterone early in gestation is responsible for determining genital sex, while exposure somewhat later determines the sex of the oSDN.
Although we still don't know exactly what causes some rams to be male-oriented, most of the evidence we've accumulated so far point to a strong role for the prenatal hormone environment. It's possible that natural variations among individuals or environmental influences like stress, undernutrition or other factors can lead to differences in the amount or timing of testosterone exposure during development and that this could, in turn, affect SDN development and influence brain differentiation and the expression of sexual partner preferences.