Prolactin in the Horse: Historical Perspective, Actions and Reactions, and Its Role in Reproduction

Prolactin is a hormone with diverse biological effects in various species. The secretion of prolactin in horses is affected by season, thyrotropin-releasing hormone, dopaminergic and antidopaminergic agents, exercise and stressful stimuli, meal feeding, estrogen treatment, and antiopioidergic agents. The need of prolactin for mammary growth and lactation in mares has been elucidated from research on endophyte-infected fescue grazing and its associated problems in late gestation. This has led to the development of treatments for fescue toxicity and protocols for inducing lactation in nonpregnant mares. Treatment with prolactin has demonstrated that it is involved with the shedding of the winter coat in spring (increasing concentrations) and likely with the growth of the winter coat in the fall (decreasing concentrations). Prolactin secretion is highly correlated with the photoperiod and is low in winter and high in summer. The coincidence of rising prolactin concentrations in blood with the onset of ovarian activity during the spring transition period in mares led to research showing that prolactin treatment, or inducement of high prolactin secretion by means of antidopaminergic agents, in winter can induce ovarian activity and ovulation in seasonally anovulatory mares. The combination of a small amount of estrogen in addition to an antidopaminergic agent has been shown to produce a synergy resulting in very high prolactin concentrations in blood. The results of 39 years of research on equine prolactin illustrate nicely how the gradual accumulation of knowledge derived from basic research questions can generate applied solutions to real-world problems.