Maintenance at elevated temperature delays the steroidogenic and ovulatory responsiveness of rainbow trout Oncorhynchus mykiss to luteinizing hormone releasing hormone analogue

Vitellogenic rainbow trout were held at 12 or 18°C for 83 days; then injected with either saline or luteinizing hormone releasing hormone analogue (LHRHA). Repeat blood samples were taken over the 96 h following injection, and fish were monitored for ovulation for up to 44 days post-treatment. Most fish held at 12°C ovulated in response to LHRHA within 8 days of injection, whereas controls began to ovulate 19 days after injection and all had undergone ovulation by day 44. In contrast, no control or LHRHA-injected fish held at 18°C underwent ovulation for 23 days after injection. Fish held at 18°C were reinjected with saline or LHRHA 25 days after first treatment, and were now responsive to LHRHA, with six out of eight fish ovulating within 11 days of injection. Spontaneous ovulation also occurred in about half of control fish at 18°C over the 20 days following the second injection. Eggs from all ovulations were incubated at the same holding temperature as the adults. Eggs held at 12°C showed high survival to neural streak (> 90%) and eyed stages (> 80%), whereas incubation at 18°C resulted in low survival to equivalent stages (< 60 and < 50%, respectively). LHRHA-injected fish at 12°C showed a significant fall in plasma 17β-oestradiol (E2) at 72 and 96 h post-injection (pi), a transient increase in plasma testosterone (T) at 24 h pi, and a marked elevation in 17, 20β-dihydroxy-4-pregnen-3-one (17,20βP) at 48, 72 and 96 h pi relative to controls. At 18°C (first injection) LHRHA-injected fish had elevated plasma T at 48 h pi, but no changes in plasma levels of E2 or 17,20βP. In contrast, the plasma steroid profile of fish at 18°C after the second injection of LHRHA was very similar to that shown by fish at 12°C at first injection. Ovarian follicles from fish held at 12°C were unresponsive to in vitro treatment with human chorionic gonadotropin (hCG), whereas follicles from fish held at 18°C produced increased amounts of E2 in response to hCG. This was consistent with the low aromatase activity seen in vivo in fish held at 12°C. The results show that maintenance at elevated temperature retards steroidogenic responsiveness to LHRHA, either by delaying pituitary responsiveness to LHRHA, or the shift in ovarian steroid secretion from E2 to 17,20βP.