The Volume of a Sexually Dimorphic Nucleus in the Ovine Medial Preoptic Area/Anterior Hypothalamus Varies with Sexual Partner Preference

Abstract

Sheep are one of the few animal models in which natural variations in male sexual preferences have been studied experimentally. Approximately 8% of rams exhibit sexual preferences for male partners (male-oriented rams) in contrast to most rams, which prefer female partners (female-oriented rams). We identified a cell group within the medial preoptic area/anterior hypothalamus of age-matched adult sheep that was significantly larger in adult rams than in ewes. This cell group was labeled the ovine sexually dimorphic nucleus (oSDN). In addition to a sex difference, we found that the volume of the oSDN was two times greater in female-oriented rams than in male-oriented rams. The dense cluster of neurons that comprise the oSDN express cytochrome P450 aromatase. Aromatase mRNA levels in the oSDN were significantly greater in female-oriented rams than in ewes, whereas male-oriented rams exhibited intermediate levels of expression. Because the medial preoptic area/anterior hypothalamus is known to control the expression of male sexual behaviors, these results suggest that naturally occurring variations in sexual partner preferences may be related to differences in brain anatomy and capacity for estrogen synthesis.

Neonatal exposure to 17β-estradiol down-regulates the expression of synaptogenesis related genes in selected brain regions of adult female rats

“…Studies in the rat SDN-POA inspired the examination of human POA where a nucleus was found that was smaller in women than in men and smaller in gay men than in straight men [14,23]. A similar difference was seen in sheep, POA is smaller in male sheep than prefer to mount other rams than in males that prefer to mount ewes [32]. These observations do suggest that the brain is important for determining sexual orientation and thus this study is relevant to human situation.…”

Inhibition of Steroid Receptor Coactivator-1 Blocks Estrogen and Androgen Action on Male Sex Behavior and Associated Brain Plasticity

“…We also calculated the integrated optical density (IOD) of the aromatase in the POM reflecting the aromatase content in the nucleus (Roselli et al, 2004). This was defined as the relative optical density of the aromatase immunoreactivity multiplied by the fractional area covered by the signal within the nucleus and by the total POM volume.…”

Roles of sex and gonadal steroids in mammalian pheromonal communication

“…Size of the ovine SDN in male sheep is correlated with subjects' sexual orientation: males with the largest oSDN prefer to seek out and mount an estrous female whereas other males, with significantly smaller oSDNs, showed a spontaneous preference to mount other males instead of females (Roselli et al, 2004a(Roselli et al, , 2004b. It is interesting to note that Fos immunoreactivity in the mPOA/AH was significantly higher in female-oriented than in male-oriented rams that were exposed to the sight, sound and smell of either ewes or other rams (Alexander et al, 2001;Roselli et al, 2011).…”