Abstract
The brain of salmonid fishes contains two types of gonadotropin-releasing hormone (GnRH) peptide, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II). Although cDNA encoding the mammalian GnRH (mGnRH) receptor has been cloned and characterized in several mammalian species, structures of cDNAs of other GnRH receptors in non-mammalian vertebrates have not been determined yet. As a step to isolate cDNA clones for the sGnRH and the cGnRH-II receptors from masu salmon (Oncorhynchus masou) brains, we examined functional expression of these receptors electrophysiologically using Xenopus oocyte expression system. Oocytes injected with poly(A)+ RNA of the brain or the pituitary of the masu salmon responded to both sGnRH and cGnRH-II. Oocytes injected with poly(A) + RNA of pituitaries of the masu salmon responded to only sGnRH. The poly(A) + RNA of the brain was further fractionated according to the size. Injection of pooled fractions of poly(A) + RNA of a size range between 5.5 and 6.0 kb induced the oocyte response to sGnRH, but the response to cGnRH-II was not clearly induced. Injection of poly(A) + RNA of about 5.0-5.5 kb in size induced the response to cGnRH-II, but the response to sGnRH was not clearly detected. These results suggest that the sGnRH and cGnRH-II receptors are encoded by different RNA, and that the preference of the agonists of these two receptors are clearly different each other.
