Melatonin is a multifunctional molecule that mediates several circadian and seasonal
reproductive processes. The exact role of melatonin in modulating reproduction, however,
is not fully understood-especially its effects on the ovarian follicles and oocytes.
This study was conducted to investigate the expressions of the ASMT and melatonin-receptor
MTNR1A and MTNR1B genes in bovine oocytes and their cumulus cells, as well as the
effects of melatonin on oocyte nuclear and cytoplasmic maturation in vitro. Cumulus-oocyte
complexes (COCs) from abattoir ovaries were cultured in TCM-199 supplemented with
melatonin at concentrations of 0, 10, 50, and 100 ng/ml. The expression of ASMT, MTNR1A,
and MTNR1B genes was evaluated by RT-PCR. Moreover, the effects of melatonin on cumulus
cell expansion, nuclear maturation, mitochondrial characteristics and COCs steroidogenesis
were investigated. Furthermore, the level of reactive oxygen species (ROS) was evaluated
in denuded oocytes. Our study revealed that ASMT and MTNR1A genes were expressed in
COCs, while the MTNR1B gene was expressed only in oocytes. Additionally, melatonin
supplementation at 10 and 50 ng/ml to in vitro maturation medium significantly enhanced
oocyte nuclear maturation, cumulus cell expansion and altered the mitochondrial distribution
patterns, but had no effects on oocyte mitochondrial activity and COCs steroidogenesis.
Melatonin-treated oocytes had a significantly lower level of ROS than controls. The
presence of melatonin receptors in COCs and its promoting effects on oocyte nuclear
and cytoplasmic events, indicate the potentially important roles of this hormone in
regulating bovine oocyte maturation. Moreover, the presence of ASMT transcript in
COCs suggests the possible involvement of these cells in melatonin biosynthesis.