Is the primitive regulation of pituitary prolactin (tPRL(177) and tPRL(188)) secretion and gene expression in the euryhaline tilapia (Oreochromis mossambicus) hypothalamic or environmental?

We examined the effects of environmental salinity on circulating levels of the two prolactins (tPRL(177) and tPRL(188)) and levels of pituitary tPRL(1 77) and tPRL(188) mRNA in the euryhaline tilapia, Oreochromis mossambicus. Fish were sham-operated or hypophysectomized and the rostral pars distalis (RPD) autotransplanted onto the optic nerve. Following post-operative recov ery in 1/4 seawater, tilapia were transferred to fresh water (FW), 1/4 seaw ater (SW) or SW. Serum tPRL(177) and tPRL(188) levels in sham-operated and RPD-autotransplanted fish were highest in FW and decreased as salinity was increased. tPRL(177) and tPRL(188) mRNA levels in RPD implants as well as i n pituitaries from the sham-operated fish were also highest in FW and decre ased with increasing salinity. Serum osmolality increased with salinity, wi th the highest levels occurring in die seawater groups. We conclude that so me plasma factor (probably plasma osmolality), in the absence of hypothalam ic innervation, exerts a direct regulatory action on prolactin release and gene expression in the pituitary of O. mossambicus. This regulation is in a ccord with the actions of the two prolactins in the freshwater osmoregulati on of the tilapia.