Priming effect of exogenous oestradiol on luteinizing hormone secretion inprepubertal lambs

The effect of repeated administration of oestradiol-17 beta on luteinizing hormone (LH) secretion was studied in S-month old female lambs (n = 20). An imals were randomly divided in five groups and treated or not treated (grou p I) with 1, 2 or 3 i.m, injections of oestradiol in corn oil vehicle (I mu g/kg). Animals were slaughtered 12 (group II) and 24 h (group III) after t he first injection, 24 h after the second injection (group IV) and 24 h aft er the third injection (group V). Animals in groups IV and V were catheteri zed and blood samples were collected every 4 h starting before treatment un til time of sacrifice. In the rest of the groups blood samples were taken a t the time of slaughter. The number of follicles >1 mm in diameter on the o varian surface were recorded and classified according to size. Maximum leve ls of oestradiol ranged from 103 to 250 pmol/l and returned to baseline wit hin 12 to 16 h after each injection. LH secretion showed a consistent patte rn in all lambs, with increases between 8 and 16 h after each oestradiol in jection. The highest amplitude and longest duration (8-12 h) of LH secretio n was recorded after the second oestradiol injection. Preliminary data indi cated 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 concentratio ns of progesterone and cortisol were low and did not differ within groups o r between treatments. The findings confirm that the pituitary LH release sy stem in ewe lambs is sensitive to the stimulatory effects of oestradiol lon g before puberty. Results indicate that priming with oestradiol increases p ituitary LH release to subsequent challenges of oestradiol, but long time e xposure to oestradiol may have a negative effect on LH secretion. Although none of the oestradiol-treated lambs ovulated, the increase in the number o f large follicles with repeated injections of oestradiol suggests that smal l follicles were gonadotrophin-responsive and stimulated by the gonadotroph in release. (C) 1998 Published by Elsevier Science B.V. All rights reserved .