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 a nd the opioid antagonist naloxone during lactation, and between these chara cteristics 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 la ctation 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 slaugh ter 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 af ter weaning was established. Although there were differences (P < 0.001) be tween sows in follicular fluid volume, there were no correlations between m ean follicular fluid volume and mean LH concentrations after GnRH or after weaning. These data indicate that the LH response to GnRH during late lacta tion 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 th erefore still needed for afunctional link between differences in LH in lact ation and differences in fertility after weaning. (C) 1999 Elsevier Science B.V. All rights reserved.