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K. K. Schillo, M. A. Green, and S. H. Hayes

Summary. Finnish Landrace × Southdown ewes were ovariectomized (OVX) and subjected to daily photoperiods of 16L:8D (Group I) or 8L:16D (Group II) for 84 days. Ewes were then either adrenalectomized (ADX) (N = 5 for Group I; N = 4 for Group II) or sham ADX (N = 6 for Groups I + II). After surgery, ewes in Group I were subjected to 8L:16D for 91 days and 16L:8D for 91 days whereas ewes in Group II were exposed to 16L:8D for 91 days and 8L:16D for 91 days. Oestradiol implants were inserted into all ewes on Day 148. Sequential blood samples were taken at 28, 56, 91, 119, 147 and 168 days after surgery to determine secretory profiles of LH and prolactin. Photoperiod did not influence LH release in Group I in the absence of oestradiol. Although photoperiod influenced frequency and amplitude of LH pulses in Group II before oestradiol treatment, adrenalectomy did not prevent these changes in patterns of LH release. However, in Group II the increase in LH pulse amplitude during exposure to long days was greater (P < 0·01) in adrenalectomized ewes than in sham-operated ewes. Mean concentrations of LH increased in ADX ewes on Days 91 (P = 0·07) and 119 (P < 0·05). Adrenalectomy failed to influence photoperiod-induced changes in mean concentrations of LH, amplitude of LH pulses and frequency of LH pulses in the presence of oestradiol. Concentrations of prolactin were influenced by photoperiod. In Groups I and II concentrations of prolactin increased (P < 0·01) after adrenalectomy, but the magnitude of this effect decreased over time. These results suggest that the effects of photoperiod on patterns of LH release are mediated in part by a mechanism that is not dependent on sex steroids and that the adrenal may influence release of prolactin.

Keywords: photoperiod; LH; prolactin; adrenalectomy; sheep