The synthesis and release of LH and FSH in the pituitary of vertebrates are differentially regulated during gonadal development and maturation. However, the underlying neuroendocrine mechanisms remain to be fully elucidated. The present study examined the possible involvement of isotocin (Ist), an oxytocin-like neuropeptide, in the regulation of Lh and Fsh in a teleost, the ricefield eel Monopterus albus. The immunoreactive isotocin receptor 2 (Istr2) was shown to be localized to Lh but not Fsh cells. In contrast, immunoreactive isotocin receptor 1 (Istr1) was not observed in either Lh or Fsh cells in the pituitary. Interestingly, Lh cells in female ricefield eels expressed Istr2 and secreted Lh in response to Ist challenge stage-dependently and in correlation with ovarian vitellogenesis. Moreover, Ist decreased Lh contents in the pituitary of female fish, indicating its stimulatory roles on Lh release in vivo. The induction of Lh release by Ist in dispersed pituitary cells was blocked by a PLC or IP3R inhibitor but not by a PKA or PKC inhibitor, indicating the involvement of the IP3/Ca²⁺ pathway. Collectively, the above results indicate that isotocin may bind to Istr2 to stimulate Lh release via the IP3/Ca²⁺ pathway, and play important roles in the ovarian maturation in ricefield eels. Furthermore, the present study suggests a novel neuroendocrine mechanism underlying the differential regulation of Lh and Fsh in vertebrates.

Summary sentence

Isotocin binds to isotocin receptor 2 on Lh cells to simulate Lh release in ricefield eels, but seems to exert no effects on Fsh cells.