Effects of five natural gonadotropin-releasing hormones on cell suspensions of marine bivalve gonad: Stimulation of gonial DNA synthesis

Gonadotropin-releasing hormones (GnRH) constitute a family of neuropeptides which are important regulators of reproduction in vertebrates. The effect of mammalian GnRH (mGnRH), salmon GnRH, chicken GnRH-I, chicken GnRH-II, an d lamprey GnRH-I on [H-3]thymidine incorporation into DNA of dissociated go nadal cells of marine bivalves has been studied. The incorporation of [H-3] thymidine is linear between 1.5 and 8 h of incubation. All five GnRHs signi ficantly increased DNA synthesis in genial cells of Crassostrea gigas. The maximal activation was about of 135-140% above control. The activation is d ose dependent, over the range 10(-11) to 10(-6) M, but is modulated by the physiological condition of the cells and the stage of sexual maturity of th e gonad. mGnRH has also a mitogenic effect in dissociated mantle cells of M ytilus edulis. The effect of mGnRH is blocked by a GnRH antagonist ([D-pGlu (1),D-Phe(2),D-Trp(3,6)] GnRH, 5 x 10(-6)M) in C. gigas as well as in M. ed ulis, suggesting that the GnRH action in the gonad is mediated by specific receptors for GnRH or GnRH-like peptides. The existence of GnRH-immunoreact ive neurons and fibers in the cerebral and pedal ganglia of M. edulis was d emonstrated by immunocytochemistry. They are located principally in the ant erior internal area of the cerebral ganglia, close to the cerebral commissu re and in the posterior part of the pedal ganglia. The presence of GnRH-res ponsive cells and GnRH-like immunoreactive material suggests that peptides of the GnRH-like family are present and functional in bivalve molluscs. (C) 1999 Academic Press.