Absence of nocturnal autonomic prolactin secretion during lactation in the rat.

During pseudopregnancy in the rat prolactin is secreted autonomously in a characteristic pattern of two daily surges, one nocturnal and one diurnal. The present study investigated whether, during lactation, prolactin secretion occurred in an autonomous nightly surge pattern, in addition to secretion resulting from suckling stimulation. Nocturnal levels of prolactin were measured daily for 2 weeks in lactating rats which were separated from their young from 21.00 to 06.00 h. It was found that no nocturnal prolactin surges were present throughout the 2 weeks of lactation. In addition, it was observed that after pups were removed for 3 h prolactin levels were lower during the first week of lactation than during the second week. It was concluded that lactational dioestrus is distinguished from pseudopregnancy by the absence of autonomous secretion of prolactin in the pattern of a nocturnal surge.