Presence and distribution of urocortin in the duck (Anas platyrhynchos).

Urocortin (UCN) is a 40 aminoacid peptide which belongs to corticotrophin-relasing factor (CRF) family. This family of peptides stimulates the secretion of proopiomelanocortin (POMC)-derived peptides, ACTH, b-endorphin and melanocyte-stimulating hormone in the pituitary gland. Urocortin has first been identified in rat midbrain. Two receptors of the CRF family peptide have been identified, CRF1 and CRF2 receptor (CRF-R). While CRF-R1 binds Ucn with a 6-fold higher activity than CRF, CRF-R2 binds Ucn with approximately a 40-fold higher activity than CRF. In the rat, UCN has been found to be expressed in the brain as well as in the gastrointestinal tract, testis, kidney, heart, and liver. Even though urocortin was not expressed in the neurons of the pigeon Edinger-Westphal nucleus, it was found in several neurons of the pigeon paramedian subgriseal mesencephalon in response to stress. Urocortin expression has been shown in the rat thymus, spleen, macrophages and lymphocytes and has been addressed to play an important role in the regulation of the immune functions. Up until now, no data are available regarding the presence and role of UCN in the lymphoid organs of birds. In the present study we have, therefore, investegated the presence and distribution of UCN in maturing thymus and bursa of Fabricius of the duck by means of western blotting and immunohistochemistry. In the bursa of Fabricius and thymus, western blot demonstrated the presence of a peptide having a molecular weight compatible with that of the mammalian UCN. In the bursa of Fabricius, immunoreactivity was located in the medullary epithelial cells and in the follicular associated and cortico-medullary epithelium. In the thymus, immunoreactivity was located in single epithelial cells and in interdigitating cells. Double labelling immunofluorescence studies showed that UCN immunoreactivity completely colocalised with cytokeratin immunoreactivity. In the bursa of Fabricius, a significant and progressive increase in the percentage of the immunoreactive areas was observed up until 140 days of age. After this stage a decrease was observed. In the thymus a significant difference in the Urocortin-ir was observed between the first (T10) and the other stages. The results suggest that, in birds, urocortin has a role in regulating the function of the immune system.