Circulating prolactin levels at the end of the photophase and the end of the scotophase throughout the reproductive cycle of the turkey hen

Abstract

Plasma prolactin (PRL) levels rise following long daily photostimulation and increase dramatically at the onset of incubation behavior. Previous work has shown that a daily rhythm in PRL secretion may occur, with the lowest PRL levels found prior to lights out and the highest levels found prior to lights on. It has been suggested that an early event in the onset of incubation behavior may be an increase in nocturnal PRL levels. A retrospective study was conducted to contrast the morning and evening PRL secretion patterns at weekly intervals throughout the reproductive cycle in birds that exhibited: 1) incubation behavior; 2) high egg production and low nesting frequency for the last 16 wk of a 21-wk reproductive cycle; 3) high egg production while nesting frequently; or 4) photorefractoriness (defined by a cessation of egg production without incubation behavior). All hens showed an increase in PRL levels following photostimulation. When day and night PRL levels for each hen were compared over the entire reproductive cycle, more than 50% of those studied had significantly higher PRL levels at the end of the scotophase than at the end of the photophase. Circulating PRL levels increased greatly with the onset of incubation behavior, but morning and evening PRL levels changed in parallel. Good layers had moderate PRL levels throughout egg production, but PRL levels did not differ among laying hens with high or low nesting frequency. Plasma PRL levels declined to low levels in photorefractory hens. These results show that daytime PRL measurements accurately reflect reproductive state, and that moderate PRL levels seem consistent with optimum egg production.