Cloning, tissue distribution, and central expression of the gonadotropin-releasing hormone receptor in the rainbow trout (Oncorhynchus mykiss)

A full-length cDNA encoding a GnRH receptor (GnRH-R) has been obtained from the brain of rainbow trout. This cDNA encodes a protein of 386 amino acids (aa) exhibiting the typical arrangement of the C-protein-coupled receptors in seven transmembrane domains. However, a second ATC could give rise to a receptor with a 30-aa longer extracellular domain. As already shown in oth er fish and Xenopus, this protein possesses an intracellular domain, in con trast with its mammalian counterparts. In the case of rainbow trout, this i ntracellular carboxy-terminal tail consists of 58 residues. Northern blotti ng experiments carried out in the brain, the pituitary, and the liver only resulted in a single band of 1.9-2 kilobases in the pituitary, although rev erse transcription-polymerase chain reaction amplification products were fo und in the brain, the pituitary, the retina, and the ovary. In situ hybridi zation using a probe corresponding to the full-length coding region of the receptor was performed on vitellogenic or ovulating females and allowed to detect a weak but specific signal in the proximal pars distalis of the pitu itary, the preoptic region, the mediobasal hypothalamus, and the optic tect um. However, the strongest signal was consistently detected in a mesencepha lic structure, the nucleus lateralis valvulae, the significance of which is presently open to speculation.