The Highly conserved gonadotropin-releasing hormone-2 form acts as a melatonin-releasing factor in the pineal of a teleost fish, the european sea bass Dicentrarchus labrax.
With the exception of modern mammals, most vertebrate species possess two GnRH genes, GnRH-1 and GnRH-2. In addition, in many teleost fish, there is a third gene called GnRH-3. If the main function of GnRH-1 is unambiguously to stimulate gonadotropin release, the other two GnRH forms still lack clear functions. This is particularly true for the highly conserved GnRH-2 that encodes chicken GnRH-II. This GnRH variant is consistently expressed in neurons of the dorsal synencephalon in most vertebrate groups but still has no clear functions supported by anatomical, pharmacological, and physiological data. In this study performed on a perciform fish, the European sea bass, we show for the first time that the pineal organ receives GnRH-2-immunoreactive fibers originating from the synencephalic GnRH-2 neurons. This was shown through a combination of retrograde tracing and immunohistochemistry, using highly specific antibodies. Supporting the presence of GnRH-2 functional targets, RT-PCR data together with the in situ hybridization studies showed that the sea bass pineal gland strongly expressed a GnRH receptor (dlGnRHR-II-2b) with clear selectivity for GnRH-2 and, to a lesser extent, the dlGnRHR-II-1a subtype. Finally, in vitro and in vivo experiments demonstrate stimulatory effects of GnRH-2 on nocturnal melatonin secretion by the sea bass pineal organ. Altogether, these data provide, for the first time in a vertebrate species, converging evidence supporting a role of GnRH-2 in the modulation of fish pineal functions.
Servili
Arianna
A
Departamento de Biología, Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz, Puerto Real, Spain.
Lethimonier
Christèle
C
Lareyre
Jean-Jacques
JJ
López-Olmeda
José Fernando
JF
Sánchez-Vázquez
Francisco Javier
FJ
Kah
Olivier
O
Muñoz-Cueto
José Antonio
JA
D000818:Animals; D001921:Brain / Q000166:cytology / Q000378:metabolism; D019556:COS Cells; D002522:Chlorocebus aethiops; D029941:Fish Proteins / Q000235:genetics / Q000378:metabolism*; D020869:Gene Expression Profiling; D007987:Gonadotropin-Releasing Hormone / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology*; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D007295:Inositol Phosphates / Q000378:metabolism; D008297:Male; D008550:Melatonin / Q000097:blood / Q000378:metabolism*; D009474:Neurons / Q000166:cytology / Q000378:metabolism; D010870:Pineal Gland / Q000187:drug effects* / Q000378:metabolism; D011966:Receptors, LHRH / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014162:Transfection
D029941:Fish Proteins; D007295:Inositol Phosphates; D011966:Receptors, LHRH; D007987:Gonadotropin-Releasing Hormone; D008550:Melatonin