Effect of active immunization against recombinant-derived chicken prolactin fusion protein on the onset of broodiness and photoinduced egg laying in bantam hens

The hypothesis that the onset of incubation behaviour (broodiness) in the domestic hen is induced by an increase in prolactin secretion was investigated by actively immunizing bantam hens against recombinant-derived chicken prolactin. A second objective was to establish whether active immunization against prolactin affects photoinduced onset of egg laying and the rate of egg production. The immunogen was a fusion protein (βgals–prolactin, 23 kDa) produced in Escherichia coli, comprising chicken prolactin (without the nine amino-terminal amino acids) fused to 18 amino acids of E. coli β-galactosidase. A control immunogen was produced in the same strain of E. coli harbouring the same plasmid vector used to produce βgals–prolactin minus the prolactin gene sequence. Hens were immunized i.m. with 1 mg of protein containing 0.8–0.9 mg of fusion protein in Freund's incomplete adjuvant at 4–8 week intervals beginning before or after egg laying, which was induced by increasing the daily photoperiod. The βgals–prolactin immunogen, but not the control immunogen, stimulated the production of antibodies to chicken prolactin. In Expts 1, 2 and 3, hens were placed in floor pens with nest boxes after photostimulation to induce broodiness. In these experiments, immunization with βgals–prolactin reduced the incidence or delayed the development of broodiness. This effect was more pronounced if immunization was initiated before, rather than after, the onset of egg laying. In Expts 1 and 2 hens were immunized with βgals–prolactin before photostimulation. The presence of antibodies to prolactin in their blood did not affect photoinduced onset of egg laying. In Expt 4, hens were maintained in individual cages to suppress the development of broodiness to measure the rate of egg laying. In these conditions, hens immunized with βgals–prolactin laid eggs at the same rate as did control hens. It is concluded that in the domestic hen increased prolactin secretion plays an important role in the initiation of incubation behaviour, and that active immunization against prolactin does not affect photoinduced onset or subsequent rate of egg laying.