Correlation between LH response to challenges with GNRH and naloxone during lactation, and LH secretion and follicular development after weaning in the sows

The aim of this study was of establishing a correlation between endogenous LH secretion and the magnitude of the LH response to challenges with GnRH and the opioid antagonist naloxone during lactation, and between these characteristics and LH secretion and follicular development after weaning. Sows (n = 9) were sampled for 6 h at day 2 post-partum, for 12 h on day 26 of lactation and for 6 h immediately after weaning at day 27 of lactation. Four hours after the beginning of sampling at day 26 of lactation all sows were injected with 2 mg/kg i.v. of naloxone hydrochloride and 5 h later with 100 mu g/sow of GnRH. Follicular development was studied in all sows at slaughter the day after weaning. There was an effect of time (sampling period; P < 0.001) on mean plasma LH, with an increase (P < 0.05) in LH the day after weaning compared to mean LH concentrations during lactation. Naloxone and GnRH treatment both increased (P < 0.05) mean LH concentrations. A positive relationship (r = 0.58, P < 0.01) between mean plasma LH after GnRH and after weaning was established. Although there were differences (P < 0.001) between sows in follicular fluid volume, there were no correlations between mean follicular fluid volume and mean LH concentrations after GnRH or after weaning. These data indicate that the LH response to GnRH during late lactation could be useful predictor of LH activity after weaning. However, none of the measures of endogenous or induced LH secretion were associated with differences in ovarian follicular size after weaning. Direct evidence is therefore still needed for afunctional link between differences in LH in lactation and differences in fertility after weaning

SUCKLING-INDUCED INHIBITION OF LUTEINIZING-HORMONE SECRETION AND FOLLICULAR DEVELOPMENT IN THE EARLY POSTPARTUM SOW

The endocrine basis of lactational anestrus, the causes of reproductive dysfunction after early weaning, and the relationships among LH, FSH, and prolactin (PRL) secretion and follicular development were evaluated in sows weaned 6 h after farrowing (zero-weaned, n = 8) and in normally lactating sows (n = 9). An irregular, high-frequency episodic-type pattern of LH secretion was present in the early postpartum period, irrespective of treatment, and in a proportion of sows this pattern was associated with a marked elevation of baseline LH concentrations. This pattern of LH secretion was maintained in the zero-weaned sows but LH secretion was inhibited in lactating sows, resulting in a difference (p < 0.05) in mean plasma LH between groups 72-78 h postpartum. There were no differences in FSH between groups in any period of sampling. Variable but elevated plasma PRL was observed in suckled sows but declined (p < 0.05) to basal levels within 12 h of zero-weaning. Follicular development measured at laparotomy or slaughter 96 h postpartum was greater (p < 0.05) in zero-weaned than in control sows. The development of lactational anestrus in the sow therefore requires suckling-induced inhibition of LH secretion by 78 h postpartum. This inhibition of LH release does not appear to be causally related to short-term changes in PRL secretion

LUTEINIZING-HORMONE AND PROLACTIN RESPONSES TO NALOXONE VARY WITH STAGE OF LACTATION IN THE SOW

The principal aim of this study was to investigate the involvement of an opioidergic mechanism in the development of suckling-induced inhibition of LH secretion during early lactation in the sow. A preliminary experiment suggested that 2 mg/kg naloxone given as a single bolus injection was above the threshold for antagonism of endogenous opioid activity in early lactation. In the main experiment, injection of 2 mg/kg naloxone 39 h postpartum, followed by injections of 1 mg/kg naloxone at 3-h intervals from 42 h to 78 h postpartum, was not able to prevent the gradual inhibition of LH secretion observed in untreated lactating sows. In contrast, a single injection of 2 mg/kg naloxone at Day 10 of lactation was effective in increasing LH secretion (p < 0.05). There were no effects of the naloxone treatment at any time on FSH secretion. Naloxone decreased (p < 0.05) plasma prolactin at Day 10 of lactation; but again during the first 78 h after farrowing, chronic naloxone treatment did not affect plasma prolactin. These data suggest that the development of the inhibitory effect of suckling on LH secretion in the sow may not be opioid dependent. In contrast, an opiate-dependent mechanism was confirmed as an important component of the suckling-induced suppression of LH secretion in established lactation