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Influences of Season and Artificial Photoperiod on Stallions: Pituitary and Testicular Responses to Exogenous GnRH^1,2,

Colorado State University, Fort Collins 80523

Abstract

Effects of season and photoperiod on the anterior pituitary gland and testes were studied by responses to exogenous GnRH. Stallions were assigned to one of three treatments: 1) control, exposed to natural day length; 2) S-L, 8 h of light and 16 h dark (8:16) for 20 wk beginning July 16, 1982 then 16:8 from December 2, 1982 until March 5, 1984; or 3) S-S, 8:16 from July 16, 1982 until March 5, 1984. Approximately every 8 wk, stallions were administered GnRH (2 Ég/kg BW) and blood was sampled at 20-min intervals for 2 h before and 8 h after GnRH administration. Concentrations of LH, FSH and testosterone were determined. Baseline concentrations (mean of pre-GnRH samples) of all hormones fluctuated seasonally (P < .05), but only LH and testosterone displayed seasonal changes (P < .05) in maximum response to GnRH (highest concentration above baseline after GnRH). The FSH response to GnRH was not affected (P > .05) by season, photoperiod or the season x treatment interaction. Exposure of S-L stallions to 16:8 in December resulted in early recrudescence of baseline concentrations of LH, FSH and testosterone. Maximum concentration of testosterone in response to GnRH was stimulated by 16:8, but the increase in baseline LH concentrations in S-L stallions was not associated with an increase in maximum LH response to GnRH. Seasonal patterns of baseline concentrations of FSH and testosterone and maximum LH response to GnRH in S-S stallions were similar to those for control stallions. Baseline concentrations of LH and testosterone response to GnRH did not reach a maximum in S-S stallions until 4 mo after maximal breeding season concentrations were achieved in control stallions. We conclude that seasonal changes in secretion of LH in stallions may be associated with a reduction and replenishment of pituitary LH as assessed by discharge following exogenous GnRH. The primary effect of increasing day length in the spring probably is stimulation, or removal of an inhibition, of LH release.

¹ This research was supported, in part, by grants from the American Quarter Horse Assoc., Amarillo, TX 79168 and the Ed H. Honnen Scholarship fund.

² Appreciation is extended to R. K. Shideler for his attention to the medical needs of the animals in this study, to V. M. Cook, and to the numerous students who assisted in this project. Special thanks are extended to Louann Star, Jeni Eknes, Susie Hippensteel and Julie Paranka.

³ Reprint requests to: Anim. Reprod. Lab., Colorado State Univ., Fort Collins 80523.