Seasonal changes in plasma adrenocorticotropic hormone and α-melanocyte-stimulating hormone in response to thyrotropin-releasing hormone in normal, aged horses.

The researchers investigated how the levels of two specific hormones, ACTH and α-MSH, change throughout different seasons in normal, aging horses, following administration of thyrotropin-releasing hormone. The results suggested that both of these hormones significantly increase after the administration of thyrotropin-releasing hormone.

Objective and Purpose of Research

• The study aimed to assess the seasonal fluctuations in two key hormones, plasma ACTH and alpha-melanocyte-stimulating hormone (α-MSH), in response to the administration of thyrotropin-releasing hormone (TRH) in older, healthy horses.
• The relevance of such an evaluation arises from the potential diagnostic application of these tests for a condition known as equine pituitary pars intermedia dysfunction (PPID). Previously conducted tests, including the measurement of naturally occurring ACTH levels and the overnight dexamethasone suppression test (DST), showed variations depending on the season.

Methods of Research

• The study involved a sample of nine healthy, aged horses with normal DST results.
• The ACTH and α-MSH hormone concentrations were measured at various intervals before and after the administration of synthetic TRH. These measurements were conducted in six different months of the year, spread across different seasons.
• The measured ACTH and α-MSH concentrations after TRH stimulation were compared over the different months using repeated measures analysis of variance, a statistical method designed for comparisons of measurements taken at different times.

Results of Research

• The research found significant increases in ACTH and α-MSH concentrations following TRH administration. These elevations were markedly higher in the summer and fall seasons compared to February.
• The study’s results illustrate that TRH-stimulated release of ACTH and α-MSH experiences seasonal variation.

Conclusions and Future Research

• The observations support earlier evidence hinting towards a seasonal influence on the equine pituitary-adrenal axis.
• The researchers suggest further studies involving more horses to establish whether seasonal boundaries for TRH stimulation testing need to be created. This could help improve and fine-tune the diagnosis of PPID in horses.