Prolactin-releasing peptide receptor expressed in the pituitary in Mozambique tilapia Oreochromis mossambicus: an aspect of prolactin regulatory mechanisms.

Prolactin (PRL)-releasing peptide (PrRP) has been reported as a strong candidate for a stimulating factor of both PRL secretion and expression in teleost species; however, there is no information available on its receptor. Here we report cDNA cloning and characterization of PrRP receptor expressed in the pituitary of Mozambique tilapia Oreochromis mossambicus. The deduced amino acid sequence of cDNA for tilapia PrRP receptor shared 50-83% homology with other vertebrate homologs. Intracellular calcium mobilization assay revealed that PrRP receptor responded to as low as 1nM order of tilapia PrRP, indicating its high affinity to PrRP. The expression of PrRP receptor was detected in the brain, pituitary, heart, spleen, kidney and rectum of freshwater (FW)- and seawater (SW)-adapted fish. There was no significant difference between FW and SW fish in transcription levels of PrRP receptor in the rostral pars distalis (RPD) of the pituitary. Similarly, the PrRP expression level in the whole brain was not changed by environmental salinity. Immunohistochemistry with a specific antibody showed that PrRP receptor was mainly localized in the cells of the RPD and neurohypophysis in the pituitary of both FW and SW tilapia. We also examined the effects of PrRP on PRL expression in primary-incubated PRL cells of FW tilapia; PrRP failed to stimulate PRL expression in PRL cells in vitro. These results suggest that in vivo stimulatory effects of PrRP on PRL gene expression reported in teleosts are presumably mediated by an unknown regulator secreted from the neurohypophysis expressing PrRP receptor.