Changes in the pulsatile secretion of LH after the removal of and subsequent resuckling by pups in ovariectomized lactating rats.

Changes in the pulsatile secretion of LH after removal of pups and subsequent resuckling were examined in ovariectomized lactating rats, and the change after removal of pups was compared with that after the removal of ovaries in cyclic female rats. The day of parturition was designated day 0 of lactation. All lactating rats were ovariectomized on day 2 of lactation. They were deprived of their pups for 6, 12, 18, 24 or 45 h before blood sampling on day 8 of lactation, or were resuckled by their pups for 1, 4, 7 or 12 h before blood collection after separation from pups for 24 h. Cyclic female rats were ovariectomized on the day of dioestrus and blood samples were taken 12, 18, 24 or 48 h or 6 days after ovariectomy. Typical LH pulses appeared in some animals from 12 h after the removal of pups. The mean LH level and the frequency and amplitude of LH pulses gradually increased after removal of pups, until after 45 h of separation the frequency reached the high level observed 6 days after ovariectomy in cyclic rats. The subsequent resuckling by pups after a 24-h separation decreased these three parameters of LH pulses rapidly. In contrast, the frequency of LH pulses was unchanged after ovariectomy in cyclic rats, although the mean LH level and the amplitude of LH pulses increased. These results suggest that the suckling stimulus suppresses pulsatile LH secretion in a different manner from that of ovarian steroids.