Quantitative in situ hybridization of three gonadotropin-releasing hormone-encoding mRNAs in castrated and progesterone-treated male tilapia

We investigated the effects of castration and progesterone administration o n the three gonadotropin-releasing hormone (GnRH)-encoding mRNAs in sexuall y mature male tilapia Oreochromis niloticus. In situ hybridization histoche mistry was performed using S-35-labeled antisense oligonucleotide probes co mplementary to salmon-, seabream-, and chicken II-GnRH cDNAs to quantify ce llular GnRH mRNA expression in the terminal nerve ganglia (nucleus olfactor etinalis), preoptic area, and midbrain tegmentum of animals castrated for 2 weeks and injected intraperitoneally with sesame oil or progesterone. Cast ration significantly elevated salmon-GnRH mRNA but not seabream- or chicken II-GnRH mRNA levels. Progesterone treatment had no effect on salmon-, seab ream-, or chicken II-GnRH mRNA levels. Comparisons between intact, castrate d, and progesterone-treated animals showed no change in the total volume of nucleus olfactoretinalis, cell sizes, and total numbers of cells expressin g GnRH mRNA within the midbrain and preoptic area. These results demonstrat e that salmon-GnRH but not seabream- or chicken II-GnRH-synthesizing neuron s are under a gonadal steroid negative feedback control and that progestero ne might not be the main hormone regulating the three GnRH-encoding mRNAs i n the male tilapia. (C) 1998 Academic Press.