title: Seasonal changes in plasma adrenocorticotropic hormone and α-melanocyte-stimulating hormone in response to thyrotropin-releasing hormone in normal, aged horses.
abstract: BACKGROUND
  Results of diagnostic tests for equine pituitary pars intermedia dysfunction (PPID), including endogenous ACTH concentration and the overnight dexamethasone suppression test (DST), are affected by season. New and potentially more sensitive diagnostic tests for equine PPID, such as thyrotropin-releasing hormone (TRH)-stimulated ACTH response, have been developed, but have had limited evaluation of seasonality.
  
  
  OBJECTIVE
  Our purpose was to evaluate seasonal changes in plasma ACTH and alpha-melanocyte-stimulating hormone (α-MSH) responses to TRH administration.
  
  
  METHODS
  Nine, healthy, aged horses with normal DST results.
  
  
  METHODS
  Synthetic TRH (1 mg) was administered IV. Plasma ACTH and α-MSH concentrations were measured at 0, 5, 10, 15, 20, 25, 30, 45, 60, and 180 minutes. Testing was performed in February, July, August, September, October, and November. Mean TRH-stimulated ACTH and α-MSH concentrations were compared across months and time by repeated measures analysis of variance. Significance was set at the P < .05 level.
  
  
  RESULTS
  Concentrations of ACTH and α-MSH significantly increased after TRH administration. Endogenous and TRH-stimulated ACTH and α-MSH concentrations were significantly different across months with higher concentrations in the summer and fall compared with February.
  
  
  CONCLUSIONS
  Plasma ACTH and α-MSH responses to TRH administration experience seasonal variation, with TRH-stimulated ACTH and α-MSH concentrations increasing from summer through fall. These results support previous evidence of a seasonal influence on the equine pituitary-adrenal axis. More research is warranted with a larger number of horses to determine if seasonal reference ranges for TRH stimulation testing need to be defined.
authors: Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL 36849, USA.
authors: R A
authors: RA
authors: Funk
authors: A J
authors: AJ
authors: Stewart
authors: A A
authors: AA
authors: Wooldridge
authors: E
authors: E
authors: Kwessi
authors: R J
authors: RJ
authors: Kemppainen
authors: E N
authors: EN
authors: Behrend
authors: Q
authors: Q
authors: Zhong
authors: A K
authors: AK
authors: Johnson
mesh_terms: D000324:Adrenocorticotropic Hormone / Q000097:blood* / Q000378:metabolism; D000375:Aging / Q000097:blood / Q000502:physiology*; D000818:Animals; D005260:Female; D006736:Horses / Q000097:blood / Q000502:physiology*; D008297:Male; D012016:Reference Values; D012621:Seasons; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D013997:Time Factors; D000521:alpha-MSH / Q000097:blood* / Q000378:metabolism
chemical_list: D000521:alpha-MSH; D013973:Thyrotropin-Releasing Hormone; D000324:Adrenocorticotropic Hormone