Effect of naloxone on plasma concentrations of prolactin and LH in lactating sows.

Six lactating sows were injected through an indwelling vena cava cannula with naloxone (2.5 mg/kg body weight) on Day 15 post partum. Blood samples were collected through the cannulas at 10-min intervals for 8 h before and 10 h after naloxone administration. Plasma prolactin and LH concentrations were measured by radioimmunoassay. Naloxone caused a marked suppression of plasma prolactin concentrations lasting 4-6 h. LH concentrations were also affected by naloxone: LH rose to reach maximum values 20-50 min after naloxone treatment. Pretreatment values were recorded 200-300 min after the treatment. These results indicate that endogenous opioids are involved in causing the endocrine patterns occurring during lactation, i.e. high prolactin and low LH concentrations.