Prolactin response to thyrotrophin-releasing hormone stimulation in normal and agalactic mares.

The researchers conducted a study which found that the hormone, thyrotrophin-releasing hormone (TRH), can stimulate the release of the hormone prolactin in pregnant and lactating mares (female horses). However, mares that weren’t lactating, termed as agalactic, showed differing prolactin levels hinting towards possible resistance.

Prolactin Levels After TRH Administration

• The researchers examined the serum prolactin concentration in horses at different stages: 10 months pregnant, 2 and 4 months after giving birth and a period of 21 days around childbirth. This gave a wide range of hormonal states to examine.
• After giving the horses TRH, they found a significant increase in prolactin levels 15, 30 and 60 minutes later. This occurred regardless of the time of the year, pregnancy or lactation status, demonstrating that TRH consistently stimulates prolactin release.
• Differently, during the 21 days around childbirth, the starting prolactin levels were four times higher and TRH only caused a slight increase. This suggests that the hormonal shifts around childbirth may impact how much additional prolactin can be stimulated by TRH.

Agalactic Mares and Prolactin

• Furthermore, the researchers encountered nine mares that were not producing milk (agalactic). In six of these, the starting prolactin levels were lower than average.
• The remaining three showed normal or raised starting levels, which coupled with their lack of lactation, implied they may be resistant to the effects of prolactin. This means that even with enough prolactin in their systems, the hormone didn’t stimulate milk production.
• This results provide valuable insights towards understanding prolactin resistance and could be used to develop solutions for lactation issues in equines.