Priming effect of exogenous oestradiol on luteinizing hormone secretion in prepubertal lambs

The effect of repeated administration of oestradiol-17beta on luteinizing hormone (LH) secretion was studied in 3-month old female lambs (n = 20). Animals were randomly divided in five groups and treated or not treated (group I) with 1, 2 or 3 i.m. injections of oestradiol in corn oil vehicle (1 microg/kg). Animals were slaughtered 12 (group II) and 24 h (group III) after the first injection, 24 h after the second injection (group IV) and 24 h after the third injection (group V). Animals in groups IV and V were catheterized and blood samples were collected every 4 h starting before treatment until time of sacrifice. In the rest of the groups blood samples were taken at the time of slaughter. The number of follicles > 1 mm in diameter on the ovarian surface were recorded and classified according to size. Maximum levels of oestradiol ranged from 103 to 250 pmol/l and returned to baseline within 12 to 16 h after each injection. LH secretion showed a consistent pattern in all lambs, with increases between 8 and 16 h after each oestradiol injection. The highest amplitude and longest duration (8-12 h) of LH secretion was recorded after the second oestradiol injection. Preliminary data indicated that FSH secretion resembled that of LH. There was an increase in the number of follicles with a diameter of more than 2 mm. Plasma concentrations of progesterone and cortisol were low and did not differ within groups or between treatments. The findings confirm that the pituitary LH release system in ewe lambs is sensitive to the stimulatory effects of oestradiol long before puberty. Results indicate that priming with oestradiol increases pituitary LH release to subsequent challenges of oestradiol, but long time exposure to oestradiol may have a negative effect on LH secretion. Although none of the oestradiol-treated lambs ovulated, the increase in the number of large follicles with repeated injections of oestradiol suggests that small follicles were gonadotrophin-responsive and stimulated by the gonadotrophin release.