OPIOIDERGIC CONTROL OF LUTEINIZING-HORMONE AND PROLACTIN SECRETION INLATE-GESTATION IN THE SOW

This study determined effects of treatment with the endogenous opioid peptide (EOP) antagonist naloxone on LH and prolactin (PRL) secretion in late gestation, as well as possible relationships between LH and pr ogesterone secretion. Ten sows of mixed parity were sampled via indwel ling jugular vein catheters for two periods of 12 h (0600-7800 h) on D ays 107 and 108 of gestation. In a repeat measures design, all sows re ceived naloxone on either the first or the second day of sampling at a n initial dose of 2.0 mg/kg BW 6 h after sampling began, followed by t wo further injections of 1.0 mg/kg at hourly intervals, and acted as c ontrols on the alternate day of sampling. Plasma LH, PRL, and progeste rone concentrations were determined by RIA. For statistical analysis, each 12-h sampling block was split into 6-h pre- and posttreatment per iods, designated as Periods 1 and 2 on control days and Periods 3 and 4 on naloxone days. There was a significant period x day interaction f or LH (p < 0.03) and PRL (p < 0.015). Naloxone elevated LH concentrati ons whether compared across days (Period 4 vs. 2; p = 0.003) or within days (Period 4 vs. 3; p = 0.007) and decreased PRL concentration in t he within-day comparison (Period 4 vs. 3; p = 0.0067). The EOP therefo re modulate LH and PRL secretion during late gestation in the sow. A d aily rhythm of PRL secretion was also detected. The data were also con sistent with the existence of a luteotropic complex that supports prog esterone secretion at this stage of gestation.