MAINTENANCE AT ELEVATED-TEMPERATURE DELAYS THE STEROIDOGENIC AND OVULATORY RESPONSIVENESS OF RAINBOW-TROUT ONCORHYNCHUS-MYKISS TO LUTEINIZING-HORMONE-RELEASING HORMONE ANALOG

Vitellogenic rainbow trout were held at 12 or 18 degrees C for 83 days ; then injected with either saline or luteinizing hormone releasing ho rmone analogue (LHRHA). Repeat blood samples were taken over the 96 h following injection, and fish were monitored for ovulation for up to 4 4 days post-treatment. Most fish held at 12 degrees C ovulated in resp onse to LHRHA within 8 days of injection, whereas controls began to ov ulate 19 days after injection and all had undergone ovulation by day 4 4. In contrast, no control or LHRHA-injected fish held at 18 degrees C underwent ovulation for 23 days after injection. Fish held at 18 degr ees C were reinjected with saline or LHRHA 25 days after first treatme nt, and were now responsive to LHRHA, with six out of eight fish ovula ting within 11 days of injection. Spontaneous ovulation also occurred in about half of control fish at 18 degrees C over the 20 days followi ng the second injection. Eggs from all ovulations were incubated at th e same holding temperature as the adults. Eggs held at 12 degrees C sh owed high survival to neural streak (> 90%) and eyed stages (> 80%), w hereas incubation at 18 degrees C resulted in low survival to equivale nt stages (< 60 and < 50%, respectively). LHRHA-injected fish at 12 de grees C showed a significant fall in plasma 17 beta-oestradiol (E-2) a t 72 and 96 h post-injection (pi), a transient increase in plasma test osterone (T) at 24 h pi, and a marked elevation in 17, 20 beta-dihydro xy-4-preenen-3-one (17,20 beta P) at 48, 72 and 96 h pi relative to co ntrols. At 18 degrees C (first injection) LHRHA-injected fish had elev ated plasma T at 48 h pi, but no changes in plasma levels of E-2 or 17 ,20 beta P. In contrast, the plasma steroid profile of fish at 18 degr ees C after the second injection of LHRHA was very similar to that sho wn by fish at 12 degrees C at first injection. Ovarian follicles from fish held at 12 degrees C were unresponsive to in vitro treatment with human chorionic gonadotropin (hCG), whereas follicles from fish held at 18 degrees C produced increased amounts of E-2 in response to hCG. This was consistent with the low aromatase activity seen in vivo in fi sh held at 12 degrees C. The results show that maintenance at elevated temperature retards steroidogenic responsiveness to LHRHA, either by delaying pituitary responsiveness to LHRHA, or the shift ill ovarian s teroid secretion from E-2 to 17,20 beta P. (C) 1998 Elsevier Science B .V. All rights reserved.