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Department of Physiology and Pharmacology, Oregon Health & Science University (C.E.R., K.L., J.A.R.), Portland, Oregon 97239; Department of Animal Sciences, Oregon State University (F.S.), Corvallis, Oregon 97331; and Agricultural Research Service, United States Sheep Experiment Station (J.N.S.), Dubois, Idaho 83423
Address all correspondence and requests for reprints to: Dr. Charles E. Roselli, Department of Physiology and Pharmacology L334, Oregon Health & Science University, 3181 S.W. Sam Jackson Park Road, Portland, Oregon 97201-3098. E-mail: rosellic{at}ohsu.edu.
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.
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