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Submitted on August 22, 2003
Accepted on September 23, 2003
1 Department of Physiology and Pharmacology Oregon Health and Science University, Portland, OR 97239; Department of Animal Sciences, Oregon State University, Corvallis, OR 97331; Agricultural Research Service, United States Sheep Experiment Station, Dubois, ID 83423.
* To whom correspondence should be addressed. E-mail: rosellic{at}ohsu.edu.
Sheep are one of 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 MPOA/AH 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 oSDN were significantly greater in female-oriented rams than in ewes, while male-oriented rams exhibited intermediate levels of expression. Since the MPOA/AH 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.
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