Kisspeptins stimulate the hypothalamus - pituitary - ovarian axis and induce final oocyte maturation and ovulation in female stinging catfish (Heteropneustes fossilis): Evidence from in vivo and in vitro studies

In the present study, effects of synthetic human Kisspeptin1 (hKiss1) and catfish Kisspeptin2 (cfKiss2) on the hypothalamic - pituitary - ovarian (HPO) axis, and induction of final oocyte maturation and ovulation were investigated in prespawning female stinging catfish (Heteropneustes fossilis) under in vivo and in vitro conditions. Gene expression was quantified by real time quantitative PCR and steroids were measured by specific enzyme-linked immunoassays. Intraperitoneal (ip) injections (1, 2, 3 ng/g body weight, BW) of synthetic hKiss1 and cfKiss2 stimulated hypothalamic, pituitary and ovarian gnrh1 and gnrh2 expression at 24 h and the effect was higher after the cfKiss2 treatment. In vitro incubation of hypothalamus, pituitary and ovary pieces with the Kiss peptides (5, 10, 20 nM) produced a similar effect. GPR54 (Kiss1 receptor) antagonist peptide234, when given ip (5, 10, 20 ng/g BW) or incubated in vitro (5, 10, 20 nM), inhibited the gnrh1 and gnrh2 expression at 24 h. The supplementation with hKiss1 or cfKiss2 restored the inhibition due to peptide234 in vivo and in vitro and the effect was higher in the cfKiss2 combination group. Both hKiss1 and cfKiss2 altered the expression of pituitary gonadotropin (Gth) subunit genes follicle-stimulating hormone β (fshβ), luteinizing hormone β (lhβ) and glycoprotein α (gpα) in vivo and in vitro. The expression of fshβ was more sensitive to the treatments than lhβ expression and the effect was greater in the cfKiss2 groups. Peptide234 in vivo and in vitro inhibited the expression of the Gth genes and the effect was reversed and restored in the hKiss1 and cfKiss2 combination groups. The Kiss peptide treatments in vivo or in vitro stimulated both plasma and ovarian levels of estradiol-17β, progesterone and 17,20β-dihydoxy-4-pregnen-3-one levels. The peptide234 treatment inhibited, or elicited a decreasing trend on the steroid levels both in vivo and in vitro, and the inhibition was reversed by the hKiss1 and cfKiss2 combination treatments. Incubation of post vitellogenic follicles with hKiss1 or cfKiss2 stimulated germinal vesicle breakdown (GVBD) and ovulation. The inhibition due to peptide234 was reversed in the combination groups. Ovulation was not elicited or unaffected in the peptide234 treated groups. The data show that the Kiss peptides act downstream the HPO axis to stimulate oocyte maturation and ovulation, and cfKiss2 peptide is functionally more effective than hKiss1.