Expression of the orexigenic peptide ghrelin and the type 1a growth hormone secretagogue receptor in sheep oocytes and pre-implantation embryos produced in vitro.

Recent studies have implicated the peripheral actions of ghrelin in reproductive tissues. Here, we provide evidence that both ghrelin and its receptor GHSR-1a (the type 1a growth hormone secretagogue receptor) are expressed at both mRNA and protein levels in sheep oocytes and pre-implantation embryos produced in vitro. Real-time RT-PCR experiments confirmed that ghrelin mRNA levels varied depending on developmental stage, with the highest expression in metaphase II (MII) oocytes, higher expression at the 2-cell stage, and minimal expression in germinal vesicle (GV) oocytes, 4- and 8-cell stages, and in the blastocyst. The levels of GHSR-1a mRNA decreased from GV to MII, increased immediately at the 2-cell stage and then remained stable until the blastocyst stage. Ghrelin protein was detected mainly in the cytoplasm close to the plasma membrane in both inner cell mass and trophectoderm cells, while GHSR-1a protein was most abundant in the plasma membrane. In conclusion, the presence of the ghrelin signalling system within the sheep oocytes and pre-implantation embryos opens up the possibility of a potential regulatory role of this novel molecule in reproductive function.