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 inhibi tion of LH secretion during early lactation in the sow. A preliminary experiment suggested that 2 mg/kg naloxone given as a single bolus inj ection was above the threshold for antagonism of endogenous opioid act ivity 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 lactatin g 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). Ther e were no effects of the naloxone treatment at any time on FSH secreti on. Naloxone decreased (p < 0.05) plasma prolactin at Day 10 of lactat ion; but again during the first 78 h after farrowing, chronic naloxone treatment did not affect plasma prolactin. These data suggest that th e 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-indu ced suppression of LH secretion in established lactation.