The suppression of pulsatile luteinizing hormone secretion during lactation in the rat

In the intact rat, the pattern of LH secretion during lactation was uniformly nonpulsatile, despite significant differences between animals suckling two and eight pups in pituitary responsiveness to GnRH. In intact rats suckling two pups during day 10 of lactation, significant LH secretion was stimulated by 0.4-ng pulses of GnRH every 50 min, while animals with eight pups secreted little LH in response to the same stimulus. It was concluded that a two-pup suckling stimulus was sufficient to completely suppress pulsatile GnRH release without affecting pituitary function, whereas an eight-pup suckling stimulus also depressed pituitary sensitivity to GnRH. In ovariectomized (ovx) rats suckling two pups, seven of nine animals showed no postcastration rise in LH secretion or evidence of pulsatile LH secretion during day 5 of lactation. In the remaining two animals, a castrate pattern of pulsatile LH secretion was observed, with a LH interpulse interval of 31 ± 6 min. By day 10 of lactation, all animals suckling two pups had castration patterns of LH secretion, with a LH interpulse interval of 35 ± 2 min, which was significantly different from the LH interpulse interval of 26 ± 1 min observed in ovx animals without pups. Therefore, a two-pup suckling stimulus is capable of retarding the increase in LH pulse frequency characteristically seen in the rat after castration. In ovx rats suckling eight pups, the postcastration rise in LH secretion was completely inhibited in all animals examined on days 5 and 10 of lactation, and the pattern of LH secretion was uniformly nonpulsatile. A consistent pattern of pulsatile LH secretion was not reinitiated until 72 h after removal of the suckling stimulus (LH interpulse interval, 31 ± 2 min). These data demonstrate that the suppression of LH secretion during lactation is the result of 1) a profound suppression of GnRH release from the hypothalamus wrought by the suckling stimulus itself and by an ovarian factor and 2) a decrease in pituitary responsiveness to GnRH stimulation in the presence of a large suckling stimulus.