Alpha-melanocyte stimulating hormone release in response to thyrotropin releasing hormone in healthy horses, horses with pituitary pars intermedia dysfunction and equine pars intermedia explants.

Thyrotropin releasing hormone (TRH) stimulates an increase in plasma cortisol in horses with pars intermedia dysfunction (PPID, Cushing's disease). A similar phenomenon is observed in humans with Cushing's disease or Nelson's syndrome. The mechanism of the response in humans is not known, but an alteration in receptor expression, selectivity or responsiveness in abnormal corticotropes has been proposed. Horses with PPID, unlike humans, almost exclusively have adenomas of pars intermedia (PI) rather than pars distalis (PD) origin. Therefore, the mechanism responsible for the TRH response observed in horses likely differs. We proposed that TRH directly stimulates the PI in normal and PPID-affected horses to release proopiomelanocortin (POMC) derived peptides. Using alpha-melanocyte stimulating hormone (alpha-MSH) as a marker of a PI response and ACTH as a marker of a PD response, we were able to demonstrate a marked increase in plasma concentration of alpha-MSH and a modest, but significant increase in ACTH after TRH treatment in normal horses. The ability of TRH to directly stimulate release of POMC peptides was confirmed using PI and PD tissue explants. The presence of TRH receptor mRNA in PI tissue from both normal and PPID horses was confirmed using reverse transcriptase polymerase chain reaction. We conclude that TRH triggers the release of POMC-derived peptides from the PI through the direct stimulation of TRH receptors normally expressed on melanotropes. The increase in plasma cortisol following TRH in horses with PPID is likely attributable to the release of ACTH from the hyperplastic PI.