The role of orexin-A in the regulation of prolactin secretion in lambs born in different photoperiods was determined. The experiment was conducted with 30 female lambs. Pituitary glands were collected from 40-day-old lambs in three different seasons (n = 10 per each season) and photoperiods (light (L) hours to darkness (D) hours): long (May – L 14 : D 10), intermediate (August – L 13 : D 11), and short (December – L 10 : D 14). At 40 days of age, the lambs were decapitated, and their pituitaries were dissected and cut in half along the longitudinal fissure so that each half contained both glandular and nervous parts. The in vitro incubation of the glands was performed for 3 h in Parker’s medium at 37°C. In each season, the control pituitary glands (K1, K2, K3) were incubated in Parker’s medium, while the experimental pituitary glands (Ox1, Ox2, Ox3) were incubated in medium containing 1000 ng/ml of exogenous orexin. The administration of orexin-A during the long photoperiod (May) caused a significant (P ≤ 0.01) increase in prolactin secretion during the first hour of incubation (Ox1: 47.56 ± 8.4 ng/ml vs K1: 36.08 ± 7.8 ng/ml). During the intermediate photoperiod (August), a significantly (P ≤ 0.01) higher concentration of prolactin was observed in the first hour of incubation in the Ox2 group than in the control group (K2). During the first hour of incubation in December, the prolactin concentrations were significantly (P ≤ 0.01) higher in the Ox3 group than in the K3 group. Orexin-A increased prolactin secretion from pituitary explants collected from lambs born under all investigated time-points. The study results indicate that orexin-A is a stimulatory factor of prolactin secretion in sheep.
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