Male Reproductive Behavior: Pathways and Messengers

Rams are selected for genetic traits which a producer desires to propagate in his flock. Even though practically all sheep are naturally bred, rams are rarely evaluated for expression of sexual interest or behavior. Research at the USSES has reported numbers of rams with limited interest in ewes to be almost 30% of the total number of breeding rams, but these rams sire less than 16% of the lamb crop. While producers recognize the problem, their solution is to increase the number of rams in breeding flocks risking open ewes particularly in small and large flocks when ram resources are limited. Increased costs and a lack of genetic progress are clearly important considerations, but more intriguing, may be the biological question as to what controls sexual interest. Circulating concentrations of testosterone do not differ among sexually active and inactive rams. However, following exposure to estrous ewes (precluding mounting activity), sexually active, but not inactive, rams experience an increase in LH pulsatile activity, a biological response to sexual stimuli. This begs the question as to whether sexually inactive rams recognize sexual cues. Using c fos activity as an indicator of neural activity, differences in the olfactory pathway was determined among sexually active and inactive rams. Differences in activity were not detected in the olfactory bulb or medial amygdala. However, sexually inactive rams had lower fos activity in the central amygdala, bed nucleus of the stria terminalis, and the preoptic area of the hypothalamus following exposure to sexual olfactory stimuli. This suggests sexually inactive rams detect olfactory cues, but may have decreased vigilance to sensory stimuli and a muted response to sexual signals in areas of the brain required for the execution of sexual performance. With the amygdala receiving and integrating sensory signals from many areas of the brain, including the mid-brain ventral tegmental area, it was considered that sexually inactive rams may lack normal reward mechanisms contributing to their lack of sexual interest. Rams with limited sexual interest have decreased dopamine synthesis in the mid-brain and a lower expression of dopamine D2 receptors in the forebrain. Thus a tempered reward pathway in combination with decreased vigilance and sensory processing in the amygdala may reduce stimulation or responsiveness in areas of the brain required for expression of sexual behavior.