Prolactin and heat stress; focus on domestic ruminants.

Prolactin has traditionally been associated with milk production, but recent studies identify prolactin as having many other functions. These include a role in pelage growth, sweating, immune function, metabolism, and water/electrolyte balance. A signature of heat stress HS is a rise in prolactin concentrations so the question arises as to whether this hormone has a particular function in relation to response to or mitigation of HS. Thus, prolactin plays a multifaceted role in the physiological and behavioral responses of livestock to HS, contributing to their ability to cope with warmer temperatures and maintain homeostasis. A major advance in recent years is the identification of the SLICK gene in cattle, being a mutation in the prolactin receptor. It is responsible for a phenotype of short, shiny coat. SLICK confers heat resilience and offers a realistic means of mitigating HS by introgression into cattle without the mutation. The purpose of this article is to ascertain what functions prolactin may have in the response to HS. It appears that prolactin may be involved in many of the physiological processes that are affected by HS, but it is clear that definitive evidence of cause/effect is yet to be discerned.