Is the primitive regulation of pituitary prolactin (tPRL₁₇₇ and tPRL₁₈₈) 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 (tPRL₁₇₇ and tPRL₁₈₈) and levels of pituitary tPRL₁₇₇ and tPRL₁₈₈ 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 recovery in 1/4 seawater, tilapia were transferred to fresh water (FW), 1/4 seawater (SW) or SW. Serum tPRL₁₇₇ and tPRL₁₈₈ levels in sham- operated and RPD-autotransplanted fish were highest in FW and decreased as salinity was increased. tPRL₁₇₇ and tPRL₁₈₈ mRNA levels in RPD implants as well as in pituitaries from the sham, operated fish were also highest in FW and decreased with increasing salinity. Serum osmolality increased with salinity, with the highest levels occurring in the seawater groups. We conclude that some plasma factor (probably plasma osmolality), in the absence of hypothalamic innervation, exerts a direct regulatory action on prolactin release and gene expression in the pituitary of O. mossambicus. This regulation is in accord with the actions of the two prolactins in the freshwater osmoregulation of the tilapia.