Effect of the suckling stimulus on daily LH surges induced by chronic oestrogen treatment in ovariectomized lactating rats

Abstract

ABSTRACT Effects of the suckling stimulus on the daily LH surge induced by chronic oestrogen treatment were examined in ovariectomized lactating rats. Wistar– Imamichi strain rats were kept under 14 h light:10 h darkness (lights on at 05.00 h). Litter size was adjusted to eight on day 1 (day 0 = day of parturition) and ovariectomy performed on day 2. Lactating rats deprived of their litters on day 0 served as non-lactating controls. Silicone elastomer tubing filled with oestradiol was implanted on day 6 or 15. Blood samples were collected through an indwelling cannula at 10.00 and 17.00 h on each day after implantation to detect daily LH surges. Daily LH surges occurred in the late afternoon in both lactating and non-lactating rats implanted with oestradiol on day 6 or 15. The amplitude of daily LH surges in lactating rats implanted on day 6 declined much more rapidly than in non-lactating rats implanted on day 6, but no significant difference was found in the profile of the LH surge between lactating and non-lactating rats implanted on day 15. Pituitary LH contents just before the daily LH surge (12.00–12.30 h) 4 days after implantation in lactating rats implanted with oestradiol on day 6 were significantly less than those in non-lactating rats implanted with oestradiol on day 6 or 15 and in lactating rats implanted on day 15. These results suggest that the mechanisms responsible for the oestradiol-induced LH surge were not impaired by the suckling stimulus, and that a rapid decline of the amplitude of LH surges observed in mid-lactation could be ascribed to the small amount of LH stored in the pituitary. J. Endocr. (1988) 118, 311–316