Developmental Expression of Three Forms of Gonadotropin-Releasing Hormone and Ontogeny of the Hypothalamic-Pituitary-Gonadal Axis in Gilthead Seabream (Sparus aurata)1

Abstract

To address the complexity of the origin of the GnRH system in perciforms, we investigated the ontogenic expression of three GnRHs in gilthead seabream. Using in situ hybridization, chicken (c) GnRH-II mRNA-expressing cells were detected in the hindbrain at 1.5 days postfertilization (DPF) and in the midbrain at 2 DPF and thereafter; the hindbrain signals became undetectable after 10 DPF. Salmon (s) GnRH mRNA-expressing cells were first seen in the olfactory placode at 3 DPF, started caudal migration at 14 DPF, and reached the preoptic areas at 59 DPF. Seabream (sb) GnRH mRNA-expressing cells were first detected in the terminal nerve ganglion cells (TNgc), ventral part of the ventral telencephalon, nucleus preopticus parvocellularis, and thalamus at 39 DPF, and extended to the nucleus preopticus magnocellularis at 43 DPF, ventrolateral hypothalamus at 51 DPF, and nucleus lateralis tuberis and posterior tuberculum at 59 DPF. Coexpression of sbGnRH and sGnRH transcripts was found in the TNgc. Using real-time fluorescence-based quantitative polymerase chain reaction, transcript levels of cGnRH-II and sGnRH were first detected at 1 and 1.5 DPF, respectively, and increased and remained high thereafter. Transcript levels of sbGnRH remained low after first detection at 1 DPF. Furthermore, these GnRH expression profiles were correlated with the expression profiles of reproduction-related genes in which at least four concomitant increases of GnRH, GnRH receptor, gonadotropin, gonadotropin receptor, and Vasa transcripts were found at 5, 8, 14, and 28 DPF. Our data provide an expanded view of the ontogeny of the GnRH system and reproductive axis in perciforms.

Developmental expression of the G protein-coupled receptor 54 and three GnRH mRNAs in the teleost fish cobia

“…The detection of GnRH-I mRNA expression at 1 DAH, and its lower abundance when compared with the levels of GnRH-II and GnRH-III mRNAs during the larval and juvenile periods of cobia are in agreement with similar findings in gilthead seabream (Wong et al 2004). Cells expressing GnRH-I mRNA have been detected at 30 DAH in seabass and at 39 DAH in seabream using ISH techniques (González-Martínez et al 2002, Wong et al 2004.…”

“…Cells expressing GnRH-I mRNA have been detected at 30 DAH in seabass and at 39 DAH in seabream using ISH techniques (González-Martínez et al 2002, Wong et al 2004. The inability of ISH techniques to detect very low mRNA levels at earlier stages of development appears to be responsible for the delayed detection of GnRH-I mRNA in these and other similar ISH studies.…”

“…Several studies in fish have reported the expression of multiple GnRH mRNAs during different stages of larval development using in situ hybridization (ISH) techniques (González-Martínez et al 2002, Wong et al 2004. The results of these and similar other ISH studies show that the cells expressing GnRH mRNAs can be detected in several-day-old fish larvae.…”

“…The inability of ISH techniques to detect very low mRNA levels at earlier stages of development appears to be responsible for the delayed detection of GnRH-I mRNA in these and other similar ISH studies. In addition, Wong et al (2004) have reported early detection of GnRH receptor, FSHb, FSH, and LH receptors and Vasa mRNAs during the larval development of gilthead seabream using real-time RT-PCR. Therefore, these genes of the hypothalamicpituitary-gonad (HPG) axis may potentially be active much earlier than it has been suggested based on the ISH studies (Cambré et al 1990, Parhar et al 1998.…”

“…The results of these and similar other ISH studies show that the cells expressing GnRH mRNAs can be detected in several-day-old fish larvae. On the other hand, a recent study in gilthead seabream using a real-time RT-PCR method has demonstrated the expression of three GnRH mRNAs before the second day after hatching (Wong et al 2004). These studies suggest that GnRHs may play important roles in some aspects of early larval development.…”