Effects of Shortened Photoperiod on Gonadotropin-Releasing Hormone, Gonadotropin, and Vitellogenin Gene Expression Associated with Ovarian Maturation in Rainbow Trout

Abstract

Reproductive activities of salmonids are synchronized by changes in photoperiod, which control the endocrine system via the brain-pituitary-gonadal axis. Gonadotropin-releasing hormone (GnRH) in the brain regulates synthesis and release of the pituitary gonadotropins (GTHs; FSH and LH). FSH and LH in turn stimulate the production of sex steroids for oocyte growth and maturation-Inducing steroid hormones for oocyte maturation and ovulation, respectively, in female salmonids. To clarify effects of long-term photoperiod manipulations on the reproductive activity of salmonids from early recrudescence to ovulation, we Investigated the gene expression profiles of GnRH, GTHs, and vitellogenin (VTG), and plasma sex steroids in female rainbow trout (Oncorhynchus mykiss). In addition, the percentages of eyed embryos and hatched alevins were examined together with the number of ovulated eggs to evaluate the effects of photoperiod regimes on egg quality. During late summer, the mRNA levels of GnRHs, GTHalpha, and LHbeta, and the plasma level of a maturational steroid (17alpha,20beta-dihydroxy-4-pregnen-3-one; 17,20beta-P) were significantly elevated by a gradually shortened photoperiod under constant temperature, in accordance with accelerated sexual maturation. The percentages of eyed embryos and hatched alevins from fish ovulated in August were comparable to those of control fish observed in December. These results clearly indicate that syntheses of GnRHs, LH, VTG, and 17,20beta-P are effectively accelerated by a programmed long-short photoperiod regime in early recrudescent female rainbow trout, without a marked deterioration in egg quality.

Modeling the endocrine control of vitellogenin production in female rainbow trout

“…This treatment was selected because it is well established that changing day length is the primary environmental factor that regulates the reproductive cycle in trout [5]. Unnatural photoperiod regimes combining rapid increases in day length with subsequent gradual or abrupt changes to shortened day length stimulate the synthesis of hormones in the hypothalamus and pituitary that are involved in the endocrine control of reproduction [2, 6]. In our study, it was anticipated that many of the upstream variables influencing E2 synthesis and in turn, Vg synthesis would be accelerated relative to trout reared under a natural photoperiod.…”

Lunar Phase-Dependent Expression of Cryptochrome and a Photoperiodic Mechanism for Lunar Phase-Recognition in a Reef Fish, Goldlined Spinefoot

“…In the MBH of long-day responsive quails, day length is measured according to the light conditions surrounding a specific phase in the early night called the “photo-inducible phase” [39], [40]. Previous studies have shown that photoperiodism also induces gonadal maturation in fish such as masu salmon ( Oncorhynchus masou , [41]), rainbow trout ( O. mykiss , [42]), and gilthead seabream ( Sparus aurata , [43]). Moonlight is seen to periodically change both in intensity and duration (timing of moonrise and moonset) in accordance with the moon phase (Figure 5A).…”

Conservation of the photoperiodic neuroendocrine axis among vertebrates: Evidence from the teleost fish, Gasterosteus aculeatus

“…In general, plasma T3 increases during sexual maturation in photoperiodic fishes [7, 21, 49], but its effects on GnRH in photoperiodic fishes are unknown. Photoperiodic manipulation stimulates GnRH, LH and FSH production [3, 19, 31, 43]. LH and FSH stimulate the gonads to produce sex hormones [8].…”