The effect of elevated oocyte triiodothyronine content on development of rainbow trout embryos and expression of mRNA encoding for thyroid hormone receptors

The ability of developing rainbow trout Oncorhynchus mykiss embryos to compensate for elevated oocyte triiodothyronine (T 3 ) content and whether elevation of oocyte T 3 content within a physiologically meaningful range affects growth rates of the embryo or the expression of genes encoding for thyroid hormone receptors α (TRα) and β (TRβ) were examined. Oocytes were immersed in ovarian fluid alone (control) or T 3 -enriched ovarian fluid prior to fertilization and water hardening, to induce a dose-dependant increase in oocyte T 3 content of c. 3 (control). c. 30 (LT 3 ) or c. 110 ng egg -1 (HT 3 ). To examine the interaction of embryo somatic growth with altered thyroid state more effectively, the embryos were reared at two ambient temperatures (8.5 and 5.5°C) to induce different growth rates. A significant decline in whole embryo T 3 content was measured in the T 3 -treatment groups reared at both water temperatures by 3 weeks post-fertilization (dpf), and may have reflected the action of outer ring monodeiodinase, which was present in microsomes prepared from embryos 23 dpf. Whole embryo T 3 levels in the HT 3 group, however, remained higher than controls until phase 2 of development [the onset of endogenous thyroid hormone (TH) release]. This suggested that the embryos exerted some control over their response to exogenous TH, but that there was a limit to the level of control exerted by the embryonic tissues. Reverse transcriptase-polymerase chain reaction (RT-PCR) revealed the presence of mRNA encoding for the two TR isoforms as early as 26dpf, and quantitative real-time RT-PCR (qPCR) was used to examine the effect of elevated oocyte T 3 content on the expression of these TR genes in embryos raised at 8.5 and 5.5°C, and sampled at similar developmental stages prior to the onset of embryonic TH synthesis, to ensure that the oocyte T 3 was the only source of TH exposure to the embryo. There was a suppression of the TRa gene expression in the control 5.5°C group relative to the control 8.5°C group. In addition, both TRa and TRβ mRNA accumulation was lower, relative to the controls, in the LT 3 treatment group reared at 8.5°C suggesting a suppressive effect of the lower level of T 3 treatment on the TR gene expression. Conversely, there were no differences from controls in the HT 3 treatment group, possibly indicating that this level of exposure overrides the down-regulating capacity of the embryo. Similar patterns were seen for TRa and TRβ mRNA accumulation in embryos reared at 5.5°C, but because of the temperature suppressed level of TRa mRNA in the controls, significant affects of the LT 3 treatment were only found for TRβ. There were no measurable effects of T 3 treatment on oocyte fertility or embryo somatic growth for either temperature treatment group, nor was somatic growth hormone content (measured only in the 8.5°C treatment group) apparently related to in ovo T 3 levels. The results suggest that altered in ovo T 3 levels, within the ranges used here, do not induce marked affects on embryo development, probably because of the ability of the embryo to maintain the integrity of its TH milieu.