Gene expression in the amygdala, bed nucleus of the stria terminalis and main olfactory bulb in low sexually-performing rams: Are rams practicing abstinence

Sheep producers depend on high sexually-performing rams to produce maximal numbers of desirable offspring. However, approximately 15--30% of rams exhibit poor mating behaviors. Male sexual interest is initially activated by chemosensory stimuli emitted by estrous females. Olfactory odorants stimulate brain activity as measured by c-fos expression in the main olfactory bulb (MOB), amygdala (AMYG) and bed nucleus of the stria terminalis (BNST) leading to modulation of hypothalamic function. Vasopressin, oxytocin and their receptors, possibly through interaction with steroid hormone receptors, are important in social recognition and social memory behaviors modulating male sexual behavior. The objective of this study was to determine if gene expression in the MOB, AMYG and BNST is associated with sexual interest in rams. Sexual behavior was individually evaluated by exposing rams to stimulus ewes for 30 minutes. Rams not exhibiting sexual behavior towards estrous ewes were evaluated for male-oriented behavior. Rams were further evaluated for male-oriented behavior in their home pen using marking harnesses. Rams not displaying anticipatory or consummatory behavior towards males or females were classified as low sexually-performing (LP) rams (n = 6). Rams were classified as high sexually-performing (HP) rams (n = 6) if ewes were mounted within 5 min and achieved >10 mounts during the 30 min test period. Prior to tissue collection, rams were fence-line exposed to estrous ewes for 45 min for short exposure (LP = 3, HP = 3) and 5 h for long exposure (LP = 3, HP = 3). Tissue was collected immediately following short exposure and 18 h after long exposure. Serum concentrations of testosterone averaged 1.48 +/- 0.95 ng/mL before fence-line exposure and 2.73 +/- 1.59 ng/mL after fence-line exposure, and were similar (P > 0.18) among rams. A bovine Affymetrix microarray indicated changes ( P < 0.05) in 159 genes (59 up-regulated; 100 down-regulated) in the AMYG of short exposure LP rams compared to HP rams. Selected ( P < 0.01) microarray genes; tumor protein D52-like1 (TPD52L1), dimeric dihydrodiol dehydrogenase (DHDH), sine oculis binding protein homolog (SOBP), and 5-hydroxytryptamine (serotonin) receptor 1F (HTR1F) were not confirmed (P > 0.26) by qPCR analysis in LP rams after short or long exposure. Furthermore, expression of vasopressin (AVP), androgen receptor (AR), estrogen receptor alpha (ERalpha), and estrogen receptor beta (ERbeta) genes in the AMYG and BNST did not differ (P > 0.15) in LP rams after short or long exposure. Oxytocin receptor (OXTR) mRNA levels did not differ ( P > 0.53) in the AMYG of LP rams after short or long exposure. Abundance of type 1 vomeronasal receptor 1 (V1R1), AVP receptor 1b (AVPR1b) and ERalpha mRNA did not differ (P > 0.11) in the MOB of LP rams after short or long exposure. Interestingly, gene expression of AVP, AVP receptor 1a (AVPR1a) and ERbeta tended to differ (P < 0.07) in the MOB of LP rams after short exposure, while there was no difference ( P > 0.13) in LP rams after long exposure. However, there was a time by treatment effect (P < 0.05) on gene expression of AVP and AVPR1a. Tendencies observed in the MOB of LP rams after short exposure may indicate a difference in social recognition memory of LP rams in response to female stimuli.