Introduction
Eurasian perch, Perca fluviatilis L. is a promising candidate for the freshwater aquaculture and is mainly cultivated in intensive monoculture in recirculating aquaculture systems (RAS)
. Such systems depend on the artificial control of both photoperiod and temperature to induce synchronous spawning. However, the production of eggs with optimal quality under such artificial conditions proved to be challenging
. Such deterioration in reproduction might be accredited to four major corticosteroids (11-deoxycorticosterone, 11- deoxycortisol, corticosterone and cortisol) . Up to date , neither the basic roles nor the kinetics of these four corticosteroids during the whole reproductive cycle of female perch has been well defined. In p revious reports on yellow perch, the involvement of the four main corticosteroids in Final O ocyte Maturation (FOM) as maturation inducing hormones (MIH) had been suggested following some in vitro assays
. However, we recently eliminated their involvement in FOM as MIHs using in vitro culture assays. In fact, we confirmed using both in vitro and in vivo injection assays that 17α,20β-dihydroxy-4-pregnen-3-one (DHP) is the MIH in female perch
. The latter finding still cannot entirely eliminate their involvement in FOM mechanisms . In this study, we therefore further investigated other possibilities for their involvement in FOM using in vitro assays as well. Additionally, we used in vivo assays to better spot their involvement and monitor their kinetics during the whole reproductive cycle. Through both assays, we anticipate to better define the role of the four corticosteroids in female perch as an attempt to better understand the reproductive drawbacks in RAS systems.
Materials and Methods
In vitro assay: the activity of the four corticosteroids, at three different doses, was tested during FOM in combination with the recently realized MIH (DHP) at two different doses . The hormonal combinations were dissolved in Cortland culture medium into which mature female Eurasian perch follicles, at the start of the FOM, were added. The follicles were subjected to the treatment for a 62hrs incubation period .
In vivo assay: d omesticated female Eurasian perch were kept in RAS system under strictly controlled photothermal conditions used to initiate and derive the reproductive cycle until spawning . The fish were blood sampled periodically and the plasma levels of corticosteroids were measured for each sampling time.
Results
The results of our in vitro combination experiment revealed that we cannot eliminate the possibility of an effect for corticosteroids during FOM where most of corticosteroids revealed a slight inhibitory effect of the MIH activity of DHP suggesting a negative effect for these steroids on the FOM achievement. Additionally, the in vivo plasma kinetics for most of the corticosteroids revealed some elevations in their plasma levels around ovulation and spawning. It also revealed some high levels at the initiation of the reproductive cycle. Therefore, our results are in favor of the possibility for the involvement of corticosteroids in FOM and the initial stages of the reproductive cycle . Through becoming closer to defining the roles of these corticosteroids, we believe we would better combat the reproductive drawbacks in perch maintained in RAS systems.
References
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