Gonadal and steroid feedback regulation of the hypothalamus-pituitary axis in striped bass (Morone saxatilis)
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The objective of the present study was to expand our understanding of the mechanisms of gonadal steroid feedback regulation of the hypothalamus-pituitary (HP) axis during several reproductive stages (juvenile, pubertal, adult) throughout the life cycle of the striped bass. Towards this end, we investigated effects of bilateral gonadectomy and steroid replacement on the endocrine correlates of the HP axis in vivo. We also developed a brain-slice culture method and utilized pituitary cell cultures to investigate direct effects of estrogen on these correlates at the level of the brain and the pituitary in vitro. Our findings indicate that: 1) During their development, the gonads play an important role in providing feedback to the HP axis. These feedback patterns change during the transformation from the juvenile to the adult and throughout the adult reproductive cycle. The pathways involved use both non-steroidal and steroidal pathways as regulatory mechanisms. 2) Gonads, through their steroids, become more involved in regulating the HP axis during reproductive development and their main feedback target appears to be gene transcription in the pituitary. 3) The observed changes in gonadal feedback throughout the adult reproductive cycle probably reflect the physiological requirements of gametogenesis. 4) The responsiveness of the HP axis towards steroids initially appears during puberty and further increases in adult females. In adults, steroids solely affect the pituitary in early stages of gametogenesis, while in later stages GnRH expression in the brain is also regulated by steroids. However, the nature of the feedback is dependent on estrogenic and/or androgenic pathways. 5) Our in vitro studies showed that estrogens act directly at the levels of the brain and the pituitary in female adult fish. Based on these findings, it appears that the activity along the endocrine reproductive web of striped bass intensifies with age, and that prior cycles of oocyte development may prime the HP axis to respond faster and more vigorously in subsequent years. This study has provided an improved resolution and a broader perspective on mechanisms involved in gonadal steroid feedback regulation of GnRH neural activity and its targets at the level of the pituitary in striped bass.