Opioidergic control of luteinizing hormone and prolactin secretion in late 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 progesterone secretion. Ten sows of mixed parity were sampled via indwelling jugular vein catheters for two periods of 12 h (0600-1800 h) on Days 107 and 108 of gestation. In a repeat measures design, all sows received naloxone on either the first or the second day of sampling at an initial dose of 2.0 mg/kg BW 6 h after sampling began, followed by two further injections of 1.0 mg/kg at hourly intervals, and acted as controls on the alternate day of sampling. Plasma LH, PRL, and progesterone concentrations were determined by RIA. For statistical analysis, each 12-h sampling block was split into 6-h pre- and posttreatment periods, 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 for LH (p < 0.03) and PRL (p < 0.015). Naloxone elevated LH concentrations 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 the within-day comparison (Period 4 vs. 3; p = 0.0067). The EOP therefore modulate LH and PRL secretion during late gestation in the sow. A daily rhythm of PRL secretion was also detected. The data were also consistent with the existence of a luteotropic complex that supports progesterone secretion at this stage of gestation.