Reproductive Biology of Salmoniform and Pleuronectiform Fishes with Special Reference to Gonadotropin-Releasing Hormone (GnRH)

Abstract

Keywords • GnRH AbstractA salmonid fish, masu salmon Oncorhynchus masou, has salmon gonadotropin-releasing hormone (sGnRH) and chicken GnRH-II (cGnRH-II), while a pleuronectiform fish, barfin flounder Verasper moseri, has sGnRH, cGnRH-II and seabream GnRH (sbGnRH). In masu salmon, sGnRH-immunoreactive (ir) cell bodies are scattered from the olfactory nerve through the ventral telencephalon (VT) and the preoptic area (POA). cGnRH-II-ir cell bodies are located in the midbrain tegmentum (MT). sGnRH but not cGnRH-II is detected in the pituitary. sGnRH peptide levels and sGnRH mRNA levels in the VT and the POA increased during gonadal maturation. sGnRH neurons are derived from the olfactory epithelium and migrate into the brain. In barfin flounder, sGnRH-ir, cGnRH-II-ir, and sbGnRH-ir cell bodies are located in the olfactory bulbs and the terminal nerve ganglion (TN), the MT, and the POA, respectively, and these neurons do not migrate in the brain. sbGnRH is detected in the pituitary. sbGnRH mRNA levels in the brain increased during gonadal maturation. Although three GnRH systems exist in the barfin flounder, anatomical distinction between the TN-and the POA-GnRH systems is not clear in masu salmon. Thus, it is suggested that sGnRH neurons in masu salmon play different roles according to the location in the brain. General introductionGonadotropin-releasing hormone (GnRH) is a decapeptide originally isolated from pig and sheep hypothalami as a physiologic regulator of luteinizing hormone (LH) release from the pituitary (Matsuo et al. 1971;Burgus et al. 1972). At present, it is generally accepted that GnRH regulates synthesis and release of pituitary gonadotropin (GTH) (see King and Millar 1992;Sherwood et al. 1993). It has been shown that two or three molecular forms of GnRH exist, even within the same species (Oka 1997;Okuzawa and Kobayashi 1999;Okubo and Nagahama 2008). In addition, GnRH can act as a neuromodulator and has also been implicated in reproductive behavior in many species including teleost fish such as dwarf gourami Colisa lalia (Yamamoto et al. 1997) and goldfish Carassius auratus (Volkoff and Peter 1999).To date, 15 forms of GnRHs have been identified based on their primary structure or complementary DNAs (cDNAs) in vertebrates, as shown in Fig. 1 (Okubo and Nagahama 2008;Kavanaugh et al. 2008). GnRH forms are traditionally named after the species from which they were first identified. In addition to vertebrate species, GnRH was isolated in invertebrates: e.g., in the protochordate Ciona intestinalis (Powell et al. 1996;Adams et al. 2003), octopus Octopus vulgaris (Iwakoshi et al. 2002) and the sea hare Aplysia californica (Zhang et al. 2008).In mammalian, avian, reptilian, and amphibian animals, GnRH is conveyed to the pituitary via the hypothalamo-hypophyseal portal vessels ( Fig. 2A). In mammals, pulsatile release of mammalian GnRH (mGnRH) by hypothalamic neurons stimulates GTH secretion from the pituitary. However, teleost fishes lack the median eminence. Instead, GnRH neurons are found to directly...

Increased expression of kisspeptin and GnRH forms in the brain of scombroid fish during final ovarian maturation and ovulation

“…In several teleosts, either mRNA or peptide levels of the hypophysiotropic GnRH form, i.e., GnRH1 in teleosts expressing three GnRH forms, and mainly GnRH3 in the case of those expressing two forms, have been shown to fluctuate significantly during ovarian maturation or spawning season [ 10 , 53 ]. However, in the brain of gilthead seabream expressing three GnRH forms and showing asynchronous type of ovarian development as that of chub mackerel, elevation in the levels of all three GnRH mRNAs and plasma LH were found 8h before spawning, when the germinal vesicle was located next to the micropyle of the oocyte [ 21 ].…”