The interactions of season, leptin and melatonin levels with thyroid hormone secretion, using an in vitro approach

Leptin's actions on the hypothalamic–pituitary–thyroid axis occur both centrally and peripherally. In addition, the thyroid activity may also be stimulated by melatonin. The mechanisms through which leptin, melatonin (MT) and the thyroid hormones co-operate in the regulation of the seasonal physiology in sheep, however, remains unclear. The aim of the study was to evaluate the interactions of season, leptin and melatonin levels with thyroid hormone secretion in sheep, using an in vitro approach. Thyroid glands from 18 ewes were collected during long-day (LD, n = 9) and short-day (SD, n = 9) photoperiods. The glands were dissected into approximately 30 mg samples. Each sample was equilibrated in RPMI 1640 (Roswell Park Memorial Institute 1640) + F12 (Nutrient Mixture F-12) medium (1/1) for 90 min, followed by an additional 3 h incubation. At the end of the equilibration period, all tissue samples were treated with a medium containing either 0 or 50 ng/ml leptin, with or without MT (40 ng/ml) and a medium containing MT (40 ng/ml) only. Secretions of thyroxine (T4) and triiodothyronine (T3) were higher (P < 0.05) in the control group during the short-day (SD) period, compared to the long-day (LD) period. Melatonin tented to reduce the T4 secretion during the LD and SD periods. However, leptin stimulated (P < 0.001) T4 secretion in both seasons. Leptin, MT, and both leptin and MT increased the secretion of T3 during LD period (P < 0.001). Results demonstrated leptin to have a stimulatory effect on T3 and T4 secretion in sheep, however the effect of melatonin is dependent on the season.