Immunohistochemical Localization of Gonadotropin-Releasing Hormone and Somatolactin During Sexual Maturation and Spawning of Lates niloticus

The present study investigated the concomitant changes of mGnRH and SL immunoreactivities within the brain and the pituitary gland at different stages of the gonadal cycle of Lates niloticus. mGnRH cell bodies were observed in the mediobasal hypothalamus within the Nucleus Lateralis Tuberis Pars posterior (NLTP). Cells with immunoreactivity for SL were detected in the Pars Intermedia (PI) of the pituitary gland. The synthetic and secretory activity of both mGnRH-ir neurons and SL-IR cells were increased during sexual maturation and spawning. Both mGnRH-ir neurons and SL-ir cells were small with moderate immunoreactivity in immature fish. However, the synthetic activity of these cells was increased as indicated by the increased numbers and sizes of cells with more immunoreactivity during gonadal development. In late stages of maturation (prespawning), the synthetic activity of mGnRH-ir neurons and SL-ir cells reached their maximum in ripe fish. Finally, during spawning, mGnRH-ir neurons and SL cells showed an increase in the secretory activity in spent fish as reflected by their small sizes, vacuolated appearance and weak immunoreactivity. Furthermore, mGnRH-ir fibers, which are in close association with SL-ir cells, showed similar seasonal changes in the density and immunoreactivity as for mGnRH-ir neurons and SL cells. The dominance of mGnRH in the hypothalamus and its close contact with SL-ir cells in the pituitary gland, together with the concurrent changes of GnRH and SL immunoreactivity in relation to seasonal changes in the reproductive conditions suggest that mGnRH plays an important role for SL release to regulate some biological events related to the reproduction in L. niloticus.