Research paperHypothalamus-pituitary-gonad axis of rainbow trout (Oncorhynchus mykiss) during early ovarian development and under dense rearing condition

- Hypothalamic sgnrh-1 and cgnrh-2 displayed opposite expression patterns.
- Dopamine levels in high density were significantly higher.
- Ovarian fshr mRNA expression and E2 were in accordance with pituitary fsh.

The objective of this study was to determine the hypothalamus-pituitary-gonad (HPG) axis of female rainbow trout (Oncorhynchus mykiss) during early ovarian development and under high rearing density. Trouts were sampled from 240 (ovarian stage II) to 540 (ovarian stage IV) days following hatching (DFH) as control group (Ctrl, 4.6-31.1 kg/m3) to determine HPG axis during early ovarian development. Trouts from the same batch of fertilized eggs were reared in two higher densities during 240-540 DFH as stocking density 1 and 2 (SD1, 6.6-40.6 kg/m3; SD2, 8.6-49.3 kg/m3) to elucidate effects of high density on reproductive parameters. Dopamine, E2 (estradiol), 17α,20β-P (17α,20β-dihydroxy4-pregnen-3-one) and P4 (progesterone) were evaluated by radioimmunoassay or ELISA. mRNA expression of hypothalamic gnrh-1, -2 (gonadotropin-releasing hormone-1, -2), pituitary gonadotropins (fsh/lh, follicle-stimulating hormone/luteinizing hormone) and their cognate receptors (fshr/lhr) in ovaries were examined by qRT-PCR. Our findings demonstrated mRNA expression of hypothalamic sgnrh-1, pituitary fsh and ovarian fshr increased in early ovarian development (360 DFH). Serum 17α,20β-P and pituitary lh mRNA expression first increased when trouts were in ovarian stage III (420 DFH). Ovaries were at different stages when reared in different densities. Long-term high density treatment (over 31.7 kg/m3) resulted in decreased hypothalamic sgnrh-1, pituitary fsh, ovarian fshr, serum E2, and increased hypothalamus gnrh-2 and serum dopamine during vitellogenin synthesis, suggesting HPG of rainbow trout might be retarded under dense rearing condition.