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Abstract In euryhaline fish, prolactin (Prl) plays an essential role in freshwater (FW) acclimation. In the euryhaline and eurythermal Mozambique tilapia,Oreochromis mossambicus,Prl cells are model osmoreceptors, recently described to be thermosensitive. To investigate the effects of temperature on osmoreception, we incubated Prl cells of tilapia acclimated to either FW or seawater (SW) in different combinations of temperatures (20, 26 and 32 °C) and osmolalities (280, 330 and 420 mOsm/kg) for 6 h. Release of both Prl isoforms, Prl₁₈₈and Prl₁₇₇, increased in hyposmotic media and were further augmented with a rise in temperature. Hyposmotically-induced release of Prl₁₈₈, but not Prl₁₇₇, was suppressed at 20 °C. In SW fish, mRNA expression ofprl₁₈₈increased with rising temperatures at lower osmolalities, while andprl₁₇₇decreased at 32 °C and higher osmolalities. In Prl cells of SW-acclimated tilapia incubated in hyperosmotic media, the expressions of Prl receptors,prlr1 and prlr2,and the stretch-activated Ca²⁺channel,trpv4,decreased at 32 °C, suggesting the presence of a cellular mechanism to compensate for elevated Prl release. Transcription factors,pou1f1,pou2f1b,creb3l1,cebpb,stat3,stat1aandnfat1c, known to regulateprl₁₈₈andprl₁₇₇,were also downregulated at 32 °C. Our findings provide evidence that osmoreception is modulated by temperature, and that both thermal and osmotic responses vary with acclimation salinity. 

Prolactin (PRL) cells within the rostral pars distalis (RPD) of euryhaline and eurythermal Mozambique tilapia, Oreochromis mossambicus, rapidly respond to a hyposmotic stimulus by releasing two distinct PRL isoforms, PRL 188 and PRL 177 . Here, we describe how environmentally relevant temperature changes affected mRNA levels of prl 188 and prl 177 and the release of immunoreactive prolactins from RPDs and dispersed PRL cells. When applied under isosmotic conditions (330 mosmol/kgH 2 O), a 6°C rise in temperature stimulated the release of PRL 188 and PRL 177 from both RPDs and dispersed PRL cells under perifusion. When exposed to this same change in temperature, ∼50% of dispersed PRL cells gradually increased in volume by ∼8%, a response partially inhibited by the water channel blocker, mercuric chloride. Following their response to increased temperature, PRL cells remained responsive to a hyposmotic stimulus (280 mosmol/kgH 2 O). The mRNA expression of transient potential vanilloid 4, a Ca 2+ -channel involved in hyposmotically induced PRL release, was elevated in response to a rise in temperature in dispersed PRL cells and RPDs at 6 and 24 h, respectively; prl 188 and prl 177 mRNAs were unaffected. Our findings indicate that thermosensitive PRL release is mediated, at least partially, through a cell-volume-dependent pathway similar to how osmoreceptive PRL release is achieved. 

Euryhaline teleost fish are characterized by their ability to tolerate a wide range of environmental salinities by modifying the function of osmoregulatory cells and tissues. In this study, we experimentally addressed the age-related decline in the sensitivity of osmoregulatory transcripts associated with a transfer from fresh water (FW) to seawater (SW) in the euryhaline teleost, Mozambique tilapia, Oreochromis mossambicus . The survival rates of tilapia transferred from FW to SW were inversely related with age, indicating that older fish require a longer acclimation period during a salinity challenge. The relative expression of Na + /K + /2Cl − cotransporter 1a ( nkcc1a ), which plays an important role in hyposmoregulation, was significantly upregulated in younger fish after SW transfer, indicating a clear effect of age in the sensitivity of branchial ionocytes. Prolactin (Prl), a hyperosmoregulatory hormone in O. mossambicus , is released in direct response to a fall in extracellular osmolality. Prl cells of 4-month-old tilapia were sensitive to hyposmotic stimuli, while those of >24-month-old fish did not respond. Moreover, the responsiveness of branchial ionocytes to Prl was more robust in younger fish. Taken together, multiple aspects of osmotic homeostasis, from osmoreception to hormonal and environmental control of osmoregulation, declined in older fish. This decline appears to undermine the ability of older fish to survive transfer to hyperosmotic environments.