Serotonin May Improve Stallion Fertility

Horses remain resistant to certain assisted reproductive techniques, such as in vitro fertilization, that help preserve desirable genetic traits and minimize economic losses associated with reproductive failure.

Roadblocks to reproductive success that specifically involve stallions include:

• Difficulties in semen preservation for artificial insemination;
• Decreased viability of sperm following cooling or freezing;
• Low success rates of in vitro fertilization procedures; and
• Intrinsic seasonality of semen quantity and quality.

These barriers have led veterinarians and researchers to find alternate ways to improve reproductive parameters in horses. One idea involves serotonin, a neurohormone integral to reproduction.

“Based on research involving species other than horses, primarily humans, serotonin influences the regulation of testicular blood flow and the secretion of testosterone. Imbalances in serotonin can negatively affect the reproductive status of mammals, potentially resulting in infertility,” explained Laura Petroski, B.V.M.S, a veterinarian for Kentucky Equine Research.

Additional research shows that semen contains serotonin receptors, and that several reproductive organs produce the neurohormone, including the testes, epididymis, deferent ducts, and prostate.

“Based on serotonin research in humans, one recently published article in equine medicine* states that serotonin could enhance capacitation—the processes a single sperm must go through to fertilize an egg—and sperm hyperactivity to improve conception rates in stallions,” shared Petroski.

In that study, Jiménez-Trejo and colleagues demonstrated for the first time that equine semen, just like human semen, contains a variety of serotonin receptors, transporters, markers, and enzymes.

“Our results suggest that [a] serotoninergic system is present in stallion sperm, which could be a pharmacological target to increase the conception rates in domestic horses,” summarized the researchers.

“This finding is exciting and offers new avenues for reaping the benefits associated with assisted reproductive techniques in horses,” Petroski said. “To ensure soundness and health, stallions should be fed an appropriate diet, maintained in moderate body condition, and receive a fish oil supplement containing DHA and EPA such as EO-3.”

*Jiménez-Trejo, F., I. Coronado-Mares, M. Boeta, et al. Identification of serotoninergic system components in stallion sperm. Histology and Histopathology. In press.