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. 2018 Feb 1:257:168-176.
doi: 10.1016/j.ygcen.2017.06.018. Epub 2017 Jun 23.

Acute salinity tolerance and the control of two prolactins and their receptors in the Nile tilapia (Oreochromis niloticus) and Mozambique tilapia (O. mossambicus): A comparative study

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Acute salinity tolerance and the control of two prolactins and their receptors in the Nile tilapia (Oreochromis niloticus) and Mozambique tilapia (O. mossambicus): A comparative study

Yoko Yamaguchi et al. Gen Comp Endocrinol. .

Abstract

Osmoregulation in vertebrates is largely controlled by the neuroendocrine system. Prolactin (PRL) is critical for the survival of euryhaline teleosts in fresh water by promoting ion retention. In the euryhaline Mozambique tilapia (Oreochromis mossambicus), pituitary PRL cells release two PRL isoforms, PRL188 and PRL177, in response to a fall in extracellular osmolality. Both PRLs function via two PRL receptors (PRLRs) denoted PRLR1 and PRLR2. We conducted a comparative study using the Nile tilapia (O. niloticus), a close relative of Mozambique tilapia that is less tolerant to increases in environmental salinity, to investigate the regulation of PRLs and PRLRs upon acute hyperosmotic challenges in vivo and in vitro. We hypothesized that differences in the regulation of PRLs and PRLRs underlie the variation in salinity tolerance of tilapias within the genus Oreochromis. When transferred from fresh water to brackish water (20‰), Nile tilapia increased plasma osmolality and decreased circulating PRLs, especially PRL177, to a greater extent than Mozambique tilapia. In dispersed PRL cell incubations, the release of both PRLs was less sensitive to variations in medium osmolality in Nile tilapia than in Mozambique tilapia. By contrast, increases in pituitary and branchial prlr2 gene expression in response to a rise in extracellular osmolality were more pronounced in Nile tilapia relative to its congener, both in vitro and in vivo. Together, these results support the conclusion that inter-specific differences in salinity tolerance between the two tilapia congeners are tied, at least in part, to the distinct responses of both PRLs and their receptors to osmotic stimuli.

Keywords: Osmoregulation; Osmosensitivity; Prolactin; Salinity tolerance; Tilapia.

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Figures

Figure 1
Figure 1
Plasma osmolalities at 0, 6 and 24 h after transfer from FW to FW (open symbols) or BW (20‰; filled symbols). Circles connected with a solid line depict O. niloticus, and triangles connected with a broken line depict O. mossambicus. Values represent means ± S.E.M (n=8). ***Significantly different from 0 h at P < 0.001. †††Significantly different from FW at P < 0.001, within each species. §,§§,§§§Significantly different from time-matched O. niloticus counterparts at P < 0.05, 0.01 and 0.001, respectively. Three-way ANOVA followed by Fisher's LSD test.
Figure 2
Figure 2
Plasma PRL188 (A) and PRL177 (B) at 0, 6 and 24 h after transfer from FW to FW (open symbols) or BW (20‰; filled symbols). Circles connected with a solid line depict O. niloticus, and triangles connected with a broken line depict O. mossambicus. Values represent means ± S.E.M (n=8). *Significantly different from 0 h at P < 0.05. Significantly different from FW at P < 0.05, within each species. §,§§Significantly different from time-matched O. niloticus counterparts at P < 0.05 and 0.01, respectively. Three-way ANOVA followed by Fisher's LSD test.
Figure 3
Figure 3
Pituitary gene expression of prl188 (A) and prl177 (B) at 0, 6 and 24 h after transfer from FW to FW (solid bars) or BW (20‰; hatched bars). Open-solid and open-hatched bars depict O. niloticus, while gray-solid and gray-hatched bars depict O. mossambicus. Data are normalized by ef1a and shown as fold-changes from FW groups at 0 h (solid bars at 0 h), within each species. Values represent means ± S.E.M (n=8). *,**,**Significantly different from 0 h at P < 0.05, 0.01 and 0.001, respectively. †,††,†††Significantly different from FW at P < 0.05, 0.01 and 0.001, respectively, within each species. §,§§Significantly different from time-matched O. niloticus counterparts at P < 0.05 and 0.01, respectively. Three-way ANOVA followed by Fisher's LSD test.
Figure 4
Figure 4
Pituitary and branchial gene expression of prlr1 and prlr2 at 0, 6 and 24 h after transfer from FW to FW (solid bars) or BW (20‰; hatched bars). Pituitary prlr1 (A) and prlr2 (B) mRNA levels. Branchial prlr1 (C) and prlr2 (D) mRNA levels. Open-solid and open-hatched bars depict O. niloticus, while gray-solid and gray-hatched bars depict O. mossambicus. Data are normalized by ef1a and shown as fold-changes from FW groups at 0 h (solid bars at 0 h), within each species. *,**,**Significantly different from 0 h at P < 0.05, 0.01 and 0.001, respectively. †,††,†††Significantly different from FW at P < 0.05, 0.01 and 0.001, respectively, within each species. §,§§Significantly different from time-matched O. niloticus counterparts at P < 0.05 and 0.01, respectively. Three-way ANOVA followed by Fisher's LSD test.
Fig. 5
Fig. 5
PRL188 (A) and PRL177 (B) release and prlr1 (C) and prlr2 (D) gene expression in dispersed PRL cells exposed to different medium osmolalities for 6 h. Data are expressed as fold-changes from 330 mOsm/kg (symbols for O. niloticus and O. mossambicus at 330 mOsm/kg overlap each other) and fitted by either linear (A-C) or exponential (D) regressions. Each data point represents mean ± S.E.M (n=8). Open circles and solid lines depict O. niloticus, and open triangles and broken lines depict O. mossambicus. **,***Significant regression at P < 0.01 and 0.001, respectively. †††Significant difference in regression slopes/curves from those of O. niloticus at P < 0.001, respectively. (A-C, ANCOVA; D, F test)

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