Immunohistochemical changes in production of pituitary hormones during artificial maturation of female Japanese eel Anguilla japonica

Specific antibodies against follicle-stimulating hormone β subunit (FSHβ), prolactin (PRL), and somatolactin (SL) of the Japanese eel Anguilla japonica were produced. These antibodies, as well as antibodies against luteinizing hormone β subunit (LHβ) and growth hormone (GH) produced previously, were used to examine changes in the production of pituitary hormones in female eels during maturation induced by salmon pituitary homogenate (SPH) injection. Immunohistochemical observations showed a decrease in FSH production after SPH injection, suggesting that SPH inhibits FSH production. In contrast, LH production increased markedly with maturation. The number of GH producing cells decreased gradually during maturation, possibly because of inhibition by exogenous GH present in the SPH and/or endogenous insulin-like growth factor-I produced by the stimulation of salmon GH. Although changes in the number of PRL producing cells with maturation were not evident, the number of SL producing cells showed a peak at the late vitellogenic stages, and thereafter decreased to the migratory nucleus stage. These results suggest that GH and SL are involved in sexual maturation in SPH injected eels, as in other fishes.