Prolactin and its Receptor in Galliformes

In recent years, the range of reported actions ofhormones has considerably expanded. The adenohy-pophyseal hormone, prolactin (PRL), however, is stillunrivalled in the extraordinary diversity of biologicalactions which have been attributed to it. The first roleattributed to PRL was the induction of milk secretionin rabbits (Stricker and Greuter, 1928) and in 1935, itwas shown that PRL was a causative factor in theinduction of incubation behaviour in ‘‘broody strains’’of actively laying chickens (Riddle et al., 1935). Sincethen, the roles of PRL have considerably expanded anda recent review categorizes over 300 separate actionsinto those which modulate: (1) water and electrolytebalance, (2) growth and development, (3) endocri-nology and metabolism, (4) brain and behaviour, (5)reproduction and (6) immunoregulation and protection(Bole-Feysot et al., 1998).One of the most studied roles of PRL in avesinvolves the incubation phase of broody behaviour.Broodiness occurs during the breeding season in manyavian species and consists of 2 phases: the incubationof eggs and the raising of young. Many studies haveindicated that hyperprolactinemia is associated withthe onset and maintenance of incubation behaviour inchickens and turkeys. The role(s) that PRL may effectduring this behaviour are not known but immunolo-gical evidence (suppression of PRL levels via active orpassive immunization against PRL or its majorreleasing factor, VIP (Sharp, 1997; Criso´stomo et al.,1997, 1998) clearly suggest that high levels of PRL area requisite aspect of this behaviour. Since the expres-sion of incubation behaviour is also associated withlarge changes in physiology including gonadal involu-tion, aphagia and adipsea it may be expected that PRLmay have multiple roles in mediating adaptions to thebehaviour. How does a single hormone cause so manydifferent responses? In this review, we will brieflyconsider functional aspects of the genes encoding PRLand its receptor, changes in glycosylation patterns ofPRL and genetic variation in the genes.