Age-Dependent Decline in Salinity Tolerance in a Euryhaline Fish

Neuregulin 1 mitigated prolactin deficiency through enhancing TRPM8 signaling under the influence of melatonin in senescent pituitary lactotrophs

The age-related alterations in pituitary function, including changes in prolactin (PRL) production contributes to the systemic susceptibility to age-related diseases. Our previous research has shown the involvement of Nrg1 in regulating the expression and secretion of PRL. However, the precise role of Nrg1 in mitigating the senescence of pituitary lactotrophs and the underlying mechanisms are yet to be comprehended. Here, data from the GEPIA database was used to evaluate the association between transient receptor potential cation channel subfamily M member 8 (TRPM8) and PRL in normal human pituitary tissues, followed by immunofluorescence verification using a human pituitary tissue microarray. TRPM8 levels showed a significant positive association with PRL expression in normal human pituitary tissues, and both TRPM8 and PRL levels declined during aging, suggesting that TRPM8 may regulate pituitary aging by affecting PRL production. It was also found that treatment with exogenous neuregulin 1 (Nrg1) markedly delayed the senescence of GH3 cells (rat lactotroph cell line) generated by D-galactose (D-gal). In addition, melatonin reduced the levels of senescence-related markers in senescent pituitary cells by promoting Nrg1 / ErbB4 signaling, stimulating PRL expression and secretion. Further investigation showed that Nrg1 attenuated senescence in pituitary cells by increasing TRPM8 expression. Downregulation of TRPM8 activation eliminated Nrg1-mediated amelioration of pituitary cell senescence. These findings demonstrate the critical function of Nrg1 / ErbB signaling in delaying pituitary lactotroph cell senescence and enhancing PRL production via promoting TRPM8 expression under the modulation of melatonin.

Changes in cortisol and corticosteroid receptors during dynamic salinity challenges in Mozambique tilapia

In estuarine environments, euryhaline fish maintain a narrow range of internal osmolality despite daily changes in environmental salinity that can range from fresh water (FW) to seawater (SW). The capacity of euryhaline fish to maintain homeostasis in a range of environmental salinities is primarily facilitated by the neuroendocrine system. One such system, the hypothalamic-pituitary-interrenal (HPI) axis, culminates in the release of corticosteroids such as cortisol into circulation. Cortisol functions as both a mineralocorticoid and glucocorticoid in fish because of its roles in osmoregulation and metabolism, respectively. The gill, a key site for osmoregulation, and the liver, the primary storage site for glucose, are known targets of cortisol's actions during salinity stress. While cortisol facilitates acclimation to SW environments, less is known on its role during FW adaptation. In this study, we characterized the responses of plasma cortisol, mRNA expression of pituitary pro-opiomelanocortin (pomc), and mRNA expression of liver and gill corticosteroid receptors (gr1, gr2, and mr) in the euryhaline Mozambique tilapia (Oreochromis mossambicus) under salinity challenges. Specifically, tilapia were subjected to salinity transfer regimes from steady-state FW to SW, SW to FW (experiment 1) or steady state FW or SW to tidal regimen (TR, experiment 2). In experiment 1, fish were sampled at 0 h, 6 h, 1, 2, and 7 d post transfer; while in experiment 2, fish were sampled at day 0 and day 15. We found a rise in pituitary pomc expression and plasma cortisol following transfer to SW while branchial corticosteroid receptors were immediately downregulated after transfer to FW. Moreover, branchial expression of corticosteroid receptors changed with each salinity phase of the TR, suggesting rapid environmental modulation of corticosteorid action. Together, these results support the role of the HPI-axis in promoting salinity acclimation, including in dynamically-changing environments.

Endocrine and osmoregulatory responses to tidally-changing salinities in fishes

Salinity is one of the main physical properties that govern the distribution of fishes across aquatic habitats. In order to maintain their body fluids near osmotic set points in the face of salinity changes, euryhaline fishes rely upon tissue-level osmotically-induced responses and systemic endocrine signaling to direct adaptive ion-transport processes in the gill and other critical osmoregulatory organs. Some euryhaline teleosts inhabit tidally influenced waters such as estuaries where salinity can vary between fresh water (FW) and seawater (SW). The physiological adaptations that underlie euryhalinity in teleosts have been traditionally identified in fish held under steady-state conditions or following unidirectional transfers between FW and SW. Far fewer studies have employed salinity regimes that simulate the tidal cycles that some euryhaline fishes may experience in their native habitats. With an emphasis on prolactin (Prl) signaling and branchial ionocytes, this mini-review contrasts the physiological responses between euryhaline fish responding to tidal versus unidirectional changes in salinity. Three patterns that emerged from studying Mozambique tilapia (Oreochromis mossambicus) subjected to tidally-changing salinities include, 1) fish can compensate for continuous and marked changes in external salinity to maintain osmoregulatory parameters within narrow ranges, 2) tilapia maintain branchial ionocyte populations in a fashion similar to SW-acclimated fish, and 3) there is a shift from systemic to local modulation of Prl signaling.