Hormonal actions of relaxin-like gonad-stimulating peptide in starfish

Oocyte maturation is a process that occurs in the ovaries, where immature oocytes resume meiosis to attain competence for normal fertilization after ovulation/spawning. Gonadotropic hormone (GTH) is the initial trigger of this process. Relaxin-like gonad-stimulating peptide (RGP) in starfish is the first neuropeptide hormone with GTH-like activity identified in invertebrates. RGP is a heterodimeric peptide consisting of A- and B-chains, belonging to the insulin superfamily, particularly the relaxin group. This review describes the mechanism of action of RGP from oocyte maturation to ovulation in starfish. Recently, RGP has been found to be distributed in the nerve plexus around gonoducts connecting gonads and gonopores. This suggests that RGP secreted from the gonoducts stimulates ovarian follicle cells and testicular interstitial cells to produce the maturation-inducing hormone, 1-methyladenine (1-MeAde). RGP acts on receptors, G proteins, and adenylyl cyclase in these target cells, increasing intracellular cyclic AMP levels and resulting in 1-MeAde production. The RGP receptor is a rhodopsin-like G protein-coupled receptor which is homologous to human relaxin family peptide receptor and Drosophila leucine-rich repeat G protein-coupled receptor 3. Interestingly, the Gαs protein, which is important for RGP signal transduction, is absent from follicle cells surrounding the small oocytes before the breeding season, but is expressed during the breeding season. Neither RGP nor 1-MeAde is directly involved in the gonadal contraction required for ovulation. While some neuropeptides may be involved in gonadal contraction and relaxation, 1-MeAde likely induces the secretion of non-neuronal acetylcholine in the ovaries and testes, resulting in muscle contraction of the gonadal wall to release mature oocytes and sperm.