Effects of level of food intake of ewes on the secretion of LH and FSH and on the pituitary response to gonadotrophin-releasing hormone in ovariectomized ewes

The effect of level of food intake on LH and FSH profiles and pituitary sensitivity to gonadotrophin-releasing hormone (GnRH) was investigated in two groups of 12 ovariectomized ewes. Ewes with a high intake (group H) had a mean daily intake (± S.E.M.) of 1.99 ± 0.075 kg dry matter (DM)/head per day while ewes with a moderate intake (group M) consumed a mean of 1.02 ± 0.021 kg DM/head per day. Ovaries were surgically removed from six ewes of each group on day 11 of the luteal phase and from the remainder 30 h after an injection of 100 μg prostaglandin analogue given on day 11 to induce luteolysis. During both the luteal phase and follicular phase, mean LH and FSH concentrations and LH pulse frequencies and amplitudes were unaffected by the levels of intake but mean plasma prolactin concentrations were higher (P < 0.05) in group H than in group M ewes in the follicular phase. Mean LH and FSH concentrations at day 2 ovariectomy were unaffected by treatment while mean prolactin concentrations were higher (P < 0.05) in group H than in group M ewes. At day 7 after ovariectomy, mean LH and FSH concentrations were lower (P < 0.05) in group H than in group M ewes although mean LH pulse frequencies and pulse amplitudes were not significantly affected by the level of intake at either time. The level of food intake and the stage of the oestrous cycle at the time of ovariectomy did not affect the amount of LH released in response to a bolus injection of GnRH (10 μg, i.v.) but the FSH response was significantly (P < 0.05) greater in group M than in group H ewes. It is concluded that the pituitary glands of ovariectomized ewes with moderate levels of intake are more responsive to GnRH than those of ewes with a high intake and that hypothalamic activity and GnRH secretion are not affected by the level of food intake.