Environmental influences on prolactin cell development in the cyprinodont fish, Cynolebias whitei

A combined immunocytochemical and morphometric study on the development of the prolactin (PRL) cells of the annual cyprinodont Cynolebias whitei, transferred as newly hatched larvae to water with different salinities and/or Ca2+-concentrations, was carried out. The percentage of the pituitary volume occupied by PRL cells and the affinity of PRL cells for immunocytochemical staining were used as criteria for their activity. Exposure of the larvae for one day to salt water (260 mOsm/kg) led to a significant reduction in the pituitary volume occupied by PRL cells, indicating an osmoregulatory function of PRL shortly after hatching. In fish reared in diluted artificial seawater (70 and 260 mOsm/kg) or Na+-enriched fresh water the development of PRL cells was significantly retarded, but such an effect was not observed in fish placed in Ca2+-enriched fresh water. These experiments show that in C. whitei the development and activity of PRL cells are influenced by changes in environmental osmolarity and not by changes in ambient Ca2+-concentration.