Systemic versus tissue-level prolactin signaling in a teleost during a tidal cycle

Area of Science:

• * Physiology
• * Ecology
• * Genomics

Background:

• * Euryhaline Mozambique tilapia (Oreochromis mossambicus) inhabit estuaries with fluctuating salinities.
• * These fish experience dramatic shifts from freshwater (FW) to seawater (SW) conditions within tidal cycles.
• * Understanding their osmoregulatory mechanisms is crucial for their survival in dynamic environments.

Purpose of the Study:

• * To investigate the temporal modulation of prolactin (PRL) signaling in tilapia under simulated tidal salinity changes.
• * To examine the parallel regulation of genes involved in branchial osmoregulation.
• * To elucidate the role of local (branchial) endocrine signaling in adaptation to dynamic salinity.

Main Methods:

• * Tilapia were reared under a simulated tidal salinity regimen (FW to SW).
• * Samples were collected every 3 hours over 24 hours for physiological and gene expression analysis.
• * Measured parameters included plasma osmolality, PRL concentrations, pituitary PRL gene expression, and branchial gene expression of PRL receptors (prlr1, prlr2) and osmoregulatory transporters (ncc2, nkcc1a, nkaα1a, nkaα1b, cftr, aqp3).

Main Results:

• * Plasma osmolality varied with tidal phases (FW vs. SW), but pituitary PRL gene expression and plasma PRL levels remained stable.
• * Branchial gene expression of osmoregulatory transporters (ncc2, nkcc1a, nkaα1a, nkaα1b, cftr, aqp3) mirrored patterns seen in static SW conditions.
• * Branchial PRL receptor (prlr1, prlr2) gene expression was highly dynamic throughout the tidal cycle.

Conclusions:

• * Tilapia maintain stable systemic PRL signaling despite fluctuating environmental salinity.
• * Branchial expression of osmoregulatory genes adapts to salinity changes, similar to fish in static SW.
• * Local regulation of branchial PRL receptor expression is key to the adaptive capacity of euryhaline fish in dynamic salinity environments.