Effect of the suckling stimulus on secretion of prolactin and luteinizing hormone in conscious and anaesthetized rats.

Luteinizing hormone and prolactin were measured in plasma samples taken from conscious and anaesthetized, lactating, suckled rats after overnight separation from their litters except for one pup. In the conscious rats, there was a marked fall in plasma LH concentration (P < 0.05), and a large increase in that of prolactin (P < 0.01) within 30 min of returning the pups to the mothers. When the mothers were anaesthetized with urethane and then suckled by their litters, there was no significant change in plasma LH values, but prolactin values rose significantly (P < 0.05) after 2.5 h of suckling. However, suckling caused no significant change in plasma concentrations of LH or prolactin when the milk-ejection frequency was doubled (P < 0.001) by administering xylazine to urethane-anaesthetized mothers. It was concluded that the conscious lactating rat, separated from her litter overnight, is a very sensitive model for studying the neural events controlling LH and prolactin secretion during suckling. Anaesthesia by urethane blocks this sensitized response, but the blockade is not associated with interference in the milk-ejection reflex.