Effects of pinealectomy, oestradiol and melatonin on plasma prolactin and LH secretion in ovariectomized sheep.

The study examines whether the pineal gland mediates in the seasonal rhythm in the response of the hypothalamo-pituitary axis to oestrogen in the sheep. Five groups of ewes, each comprised of five sham-pinealectomized ewes and five pinealectomized ewes, were maintained under field conditions over a 2-year period. Group I ewes were ovariectomized and treated with two oestradiol capsules in the first year and one oestradiol capsule in the second year; group 2 ewes were ovariectomized and treated with two oestradiol capsules and three melatonin sachets in the first year and one oestradiol capsule and three melatonin sachets in the second year; group 3 ewes were ovariectomized and treated with three melatonin sachets in the first year and one oestradiol capsule in the second year; group 4 ewes were ovariectomized and treated with empty implants; group 5 had intact ovaries. Blood samples were taken at weekly intervals for determination of plasma prolactin, LH and FSH levels. Cyclic ovarian activity, determined by rams fitted with a marking harness, occurred in both sham-pinealectomized and pinealectomized ewes in group 5 during December to May in both years. A circannual rhythm in plasma prolactin levels was apparent in all ten groups of animals. The levels were highest in spring (September to November) and decreased before the onset of breeding activity in midsummer (December). Ovariectomized ewes with intact pineal glands and treated with oestradiol capsules (group 1) exhibited marked changes in LH and FSH concentrations during the study.(ABSTRACT TRUNCATED AT 250 WORDS)