The Hypothalamo-Pituitary Gonadotrophic Axis of Suckled and Nonsuckled Dairy Cows Postpartum

Abstract

The objective was to determine whether the suckling-induced delay in return to estrus postpartum could be explained by changes in hypothalamic LHRH content or ability of the pituitary to release LH and FSH in response to LHRH or 59 mM K+ in vitro. In addition, serum concentrations of several other hormones were measured. Nine Holstein cows were suckled ad libitum by two calves and milked by machine twice daily and eight were milked by machine only from calving until slaughter on day 14 postpartum. On day 13 postpartum, blood was collected at 15-min intervals from 0815 to 1200 hr and from 2015 to 2400 hours. Suckled cows had lower (P less than .05) mean serum LH concentrations on day 13 postpartum than did nonsuckled controls. This decrease resulted from a 60% reduction in frequency and a 40% reduction in amplitude of episodic LH peaks. Suckling did not affect body weight change postpartum or serum concentrations of progesterone, estradiol-17 beta, total glucocorticoids, prolactin or FSH during the first 14 days postpartum. The suckling-induced decrease in serum LH was not reflected by a reduction in hypothalamic LHRH or pituitary LH on day 14 postpartum. However, pituitary explants from suckled cows on day 14 postpartum secreted 50% less (P less than .01) LH in response to LHRH (25 ng/ml for 30 min) or K+ (59 mM for 30 min) in vitro than did those from nonsuckled cows. Secretion of FSH was increased 20-fold by LHRH and K+ in vitro, but differences due to suckling treatment were not significant. Decreased frequency and amplitude of episodic LH secretion in vivo and reduced capacity of pituitaries to respond to LHRH may be the cause of suckling-induced inhibition of postpartum ovulation in cattle.

Immunocytochemical Evidence that Suckling Inhibits the Postovariectomy Depletion of Median Eminence Luteinizing Hormone Releasing Hormone

“…Since the postovariectomy elevation of plasma LH is impaired by suckling as well [13,18,19,27,38], it seems likely that suckling acts to inhibit the release of LHRH from the ME. Previous studies using only intact, lactating animals have reported a significant decrease [29,38] or, as observed in the present study, no change [7,32] in the concentration of immunoreactive LHRH in the hypothala mus as a result of the suckling stimulus. These results de monstrate that evaluation of the effects of suckling on hypo thalamic LHRH may require the use of a model (such as the ovariectomized rat) which will produce more extreme, measurable changes.…”

“…Previous studies have suggested that suckling reduces the release of LH from the pituitary by lowering the concentration of lu teinizing hormone releasing hormone (LHRH) In the hypo thalamus [29,38]. However, recently reported data in the ewe [32] and cow [7] have failed to detect a significant differ- Ovariectomy has been reported to cause a significant decrease in the concentration of LHRH present in the hypo thalamus [1, 5,8,17,23,41] while plasma LH levels are markedly increased [5,8,23,41]. The reduction in hypotha lamic LHRH content is believed to reflect an increased rate of release which exceeds the rate of LHRH synthesis.…”

Prevalence of reproductive diseases and its associated risk factors in crossbred dairy cows

“…This can be explained by the fact that suckling tends to suppress growth of follicles (Carter et al, 1980) and blocks ovulation (Spicer and Echternkamp, 1986). Moreover, the frequency and amplitude of LH peaks, together with reduced sensitivity of the pituitary to LH-releasing hormone may be the cause of inhibition of ovulation in suckled cows (Carruthers et al, 1980).…”

Evidence for Maternal Behavior as a Requisite Link in Suckling-Mediated Anovulation in Cows1

“…Episodic secretion of GnRH from the hypothalamus regulates the pulsatile pattern of LH release from the pituitary [2]. Although hypothalamic content of GnRH is not affected by suckling status [3][4][5], GnRH concentrations in the hypophyseal portal system are suppressed by suckling [6]. Therefore, even though anterior pituitary content of LH is replenished in suckled cows within 2-3 wk after parturition [7], secretory patterns of LH remain suboptimal for the development of preovulatory follicles [8,9].…”