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Daily Rhythm of Cortisol, and Evidence for a Photo-Inducible Phase for Prolactin Secretion in Nonpregnant Mares Housed under Non-Interrupted and Skeleton Photoperiods^1,2,

Rutgers, The State University⁴, New Brunswick, NJ 08903

³ To whom correspondence should be directed.

Abstract

Studies were conducted in anestrous mares to characterize daily rhythms of cortisol in non-interrupted [ambient and 16 h light (L): 8 h dark (D)] and skeleton (10L:4D:2L:8D, 10L:6D:2L:6D and 10L:8D:2L:4D) photoperiods, and to determine if there exists a photosensitive phase for the secretion of prolactin. Neither peak or nadir concentrations of cortisol, nor the time of peak or nadir concentrations differed among photoperiod treatments. Highest concentrations (66 ± 4.4 ng/ml, mean ± SE) occurred between 0700 and 0900, whereas lowest concentrations (31 ± 3.6 ng/ml) were found from 1900 to 2300. Mean daily concentrations of serum prolactin were significantly higher in mares housed under the 16L:8D and the 10L:8D:2L:4D photoperiods as compared with the remaining photoperiod treatments, and were lowest in the ambient photoperiod treatment. The mean daily concentration of prolactin in February among photoperiod treatments was inversely related to the number of days (from December 1) to first seasonal ovulation (r = –.92, P = .027). The results were interpreted to: 1) suggest that mares in the 10L:8D:2L:4D skeleton photoperiod do not phase-shift to interpret the 2-h light pulse as the beginning of their subjective day; and 2) provide further evidence that the photo-inducible phase for both prolactin secretion and the stimulation of seasonal reproductive activity occurs 8 to 10 h following the onset of the dark period (scotophase).

¹ Publication D-06106-02-85 of the New Jersey Agr. Exp. Sta. This work was supported in part by the Biomed. Support Grant (PHS RR 07058-17) to A. L. Johnson.

² The authors thank C. Brown and S. Becker for excellent technical assistance, and C. J. Uckele for typing the manuscript.

⁴ Dept. of Anim. Sci.