DIURNAL VARIATIONS OF PLASMA PROLACTIN IN GROWING SHEEP UNDER TWO LIGHTING RÉGIMES AND THE EFFECT OF PINEALECTOMY

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Abstract
The role of the pineal gland in mediating the stimulatory effect of long daylength on blood levels of prolactin was investigated in 24 growing castrated sheep. Eight were pinealectomized, eight were sham-pinealectomized and eight remained as unoperated control animals. After a preliminary period with 12 h light:12 h darkness (12L: 12D) pairs of similar sheep were allocated to photoperiods of 16L: 8D or 8L: 16D for 15 weeks. Each animal was individually fed at the same level as its partner exposed to the other daylength. Blood samples were taken weekly by jugular venepuncture. On two occasions, once in the preliminary period (12L: 12D) and again 41 days after the start of the subsequent photoperiods, samples were taken through indwelling jugular catheters at frequent intervals for 24 h. The plasma samples were assayed for prolactin by radioimmunoassay and concentrations were higher in samples taken by jugular puncture. There was a consistent positive effect of daylength on plasma prolactin in sheep with pineal glands that did not occur in pinealectomized sheep. In contrast, pinealectomy did not block the marked rise in prolactin that occurred at dusk. It appears that the effect of the photoperiod on levels of plasma prolactin throughout the day and night is not mediated by the same mechanism as that which controls the surge of prolactin at dusk in the sheep.