The metabolic clearance rate of estradiol-17 beta in rainbow trout, Salmo gairdneri R., estimated by both single injection and constant infusion methods: increase during oocyte maturation

Disruption of the rainbow trout reproductive endocrine axis by the polycyclic aromatic hydrocarbon benzo[a]pyrene

Successful reproduction in salmonids depends on a complex and highly regulated interplay between the pharmacokinetics and pharmacodynamics of naturally circulating sex steroids. The effects of a single intraperitoneal administration of the model PAH benzo[a]pyrene (B[a]P) on the kinetics of circulating levels of estradiol and testosterone through 7d post-injection in mature male and female rainbow trout (Oncorhynchus mykiss) in pre-spawning and spawning condition were investigated. Detailed measurements of the time course of injected E2 and excretion into the bile followed by pharmacokinetic modeling techniques were used to aid in identifying the potential mechanism of ED caused by B[a]P exposure. Plasma E2 and T concentrations were reduced significantly in both male and female trout. Administration of the GnRH analogue des-Gly(10)[D-Ala(6)]LH-RH-ethylamide, to induce spawning steroid profiles increased plasma E2 concentrations in control females, but not in B[a]P-treated fish. The mechanism underlying reductions in sex steroids in pre-spawning and spawning salmonids appears to be unrelated to the induction of P450 and related biotransformation enzymes by B[a]P. Induced biotransformation enzyme activities did not result in altered [(3)H]estradiol pharmacokinetics (e.g. terminal half-life) or elimination of steroid in bile, suggesting that B[a]P alters plasma E2 and T concentrations by other ED mechanisms in an anti-estrogenic manner.

Plasma nesfatin-1 is not affected by long-term food restriction and does not predict rematuration among iteroparous female rainbow trout (Oncorhynchus mykiss)

The metabolic peptide hormone nesfatin-1 has been linked to the reproductive axis in fishes. The purpose of this study was to determine how energy availability after spawning affects plasma levels of nesfatin-1, the metabolic peptide hormone ghrelin, and sex steroid hormones in rematuring female rainbow trout (Oncorhynchus mykiss). To limit reproductive maturation, a group of female trout was food-restricted after spawning and compared with a control group that was fed a standard broodstock ration. The experiment was conducted twice, once using two-year-old trout (second-time spawners) and once using three-year-old trout (third-time spawners). During monthly sampling, blood was collected from all fish, and a subset of fish from each treatment was sacrificed for pituitaries. Pituitary follicle-stimulating hormone-beta (fsh-β) mRNA expression was analyzed with q-RT-PCR; plasma hormone levels were quantified by radioimmunoassay (17β-estradiol and ghrelin) and enzyme-linked immunosorbent assay (11-keto-testosterone and nesfatin-1). Although plasma nesfatin-1 levels increased significantly in the months immediately after spawning within both feeding treatments, plasma nesfatin-1 did not differ significantly between the two treatments at any point. Similarly, plasma ghrelin levels did not differ significantly between the two treatments at any point. Food restriction arrested ovarian development by 15–20 weeks after spawning, shown by significantly lower plasma E2 levels among restricted-ration fish. Pituitary fsh-β mRNA levels were higher among control-ration fish than restricted-ration fish starting at 20 weeks, but did not differ significantly between treatment groups until 30 weeks after spawning. Within both treatment groups, plasma 11-KT was elevated immediately after spawning and rapidly decreased to and persisted at low levels; starting between 20 and 25 weeks after spawning, plasma 11-KT was higher among control-ration fish than restricted-ration fish. The results from these experiments do not provide support for plasma nesfatin-1 as a signal for the initiation of reproductive development in rematuring female rainbow trout.

Diversity and biological significance of sex hormone-binding globulin in fish, an evolutionary perspective

In fish, two different genes, shbga and shbgb, exist that encode for very different proteins. Shbga is the ortholog of mammalian Shbg and was found in all investigated teleosts. In contrast, Shbgb is highly divergent and appears to be a salmonid-specific protein. Here, we review existing data on fish Shbga and Shbgb that have been obtained in chondrichthyes and osteichtyes. Even though other significant expression sites exist, existing data indicate that Shbga is mainly expressed in liver and subsequently secreted into the blood as a homodimer. In contrast, Shbgb is mainly expressed in the ovary, probably secreted as a monomer, and could contribute to the regulation of local steroid action. Binding studies indicate a specialization of circulating Shbg during evolution towards the preferential binding of estradiol and testosterone in teleosts. In contrast, specific fish steroids such as 11-oxo-androgens and oocyte maturation-inducing steroids that are crucial for reproduction are poorly bound by either form of Shbg.

Effects of progesterone and estradiol on the reproductive axis in immature diploid and triploid rainbow trout

In fish species, many studies demonstrated the crucial role of estradiol (E2) in the development of the reproductive axis, but progesterone (P) has been described mainly as a precursor steroid and no clear role by itself has been reported. Moreover, a cooperative effect of P (or another progestin) and E2 in fish has never been reported to our knowledge. In the present work, we investigated the effects of P, alone or in combination with E2, on the reproductive-axis of immature rainbow trout (Oncorhynchus mykiss). Liver vitellogenin and estradiol receptor (rtER) mRNA levels increased after E2 treatment, but were unchanged by P treatments as a reflection of peripheral action of steroids. In contrast, at the pituitary level, LH contents increased after E2 and/or P treatments. Focusing on the brain level, we confirmed a clear up regulation of rtER expression by E2 in sterile triploid females, and we also demonstrated a similar stimulating effect of P alone but no cooperative effect together with E2. In conclusion, our data demonstrate that in immature trout, prior to the beginning of the first reproductive cycle, unlike E2, P is able to stimulate the reproductive brain-pituitary axis without affecting vitellogenin synthesis in the liver.

The effects of long-term estradiol-17beta treatment on the growth and physiology of female triploid brook trout (Salvelinus fontinalis)

Triploid female fish show impaired gonadal development, which results in the production of only a few oocytes relative to diploids. The rate of maturation of these few oocytes in much slower relative to those of diploids and is attributed to an insufficient estradiol-17beta (E(2)) stimulus. Female triploid brook trout, aged 2+, were fed E(2) for seven months (i.e., July 1998 to February 1999) in an attempt to increase plasma E(2) comparable to diploid levels and thereby accelerate rates of oocyte growth. Compared to triploids fed a normal diet (i.e., control-triploids), the administration of 30 mg E(2)/kg feed to triploids (i.e., E(2)-treated-triploids) led to the development of secondary sex characteristics, a significant reduction in hematocrit and total blood hemoglobin level, a decrease in plasma testosterone and no weight gain. Plasma vitellogenin was significantly higher in E(2)-treated-triploids than in control-triploids and diploids at every month except October 1999. Although greater than in the control-triploids, plasma E(2) for the E(2)-treated-triploids did not attain the high levels shown by the diploids during the months of peak vitellogenesis (i.e., September and October). Following maturation and spawning of diploids, sacrificed E(2)-treated and control-triploids showed no differences in ovarian development or liver size. The E(2) dose for the treated-triploids was increased to 80 mg E(2)/kg feed for approximately seven more weeks in a further attempt to increase plasma E(2), with no success. It is suggested that the low plasma E(2) of the treated-triploids was the result of a high metabolic clearance rate.

Uptake and clearance of exogenous estradiol-17β and testosterone during the early development of coho salmon (Oncorhynchus kisutch), including eggs, alevins and fry

The uptake and clearance of estradiol-17β (E2) and testosterone (T) were examined during the initial stages of development of coho salmon (Oncorhynchus kisutch), including eyed-eggs, newly hatched alevins and first feeding fry. Radiolabeled steroids were administered through the water in tracer amounts with or without their nonradioactive form at 400 μg l(-1). Regardless of developmental stage, saturation levels were invariably attained earlier for T than for E2, thus resulting in a higher incorporation of E2. However, both steroids had similar clearance patterns. Uptake and clearance was clearly stage-dependent, being fastest in fry, intermediate in alevins and slowest in eggs. Furthermore, combined uptake and clearance patterns showed that exposure to steroid was also higher for E2 than for T and stage-dependent, but always markedly highest in alevins. Subsequently, based on the observed elimination of the estrogen, a double immersion in E2 at 400 μg 1(-1), administered 2 days apart to maximize exposure during the alevin stage, was assayed for its effect on sex reversal and found to induce the production of 100% females. We suggest that the yolk, which is present in substantial amounts during the initial stages of development in salmonids, can retain the exogenously administered liposoluble steroids, thus providing developing embryos with an extended supply of, and exposure to, these steroids well after the treatment is finished. Together, these findings help to explain the previously observed high effectiveness of sex steroids administered during early development in regulating gonadal differentiation in salmonids, the higher effectiveness of E2 compared to T, and clarify the localization of the most sensitive period to the action of exogenous steroids at the alevin stage in the coho salmon.

Plasma kinetics of ingested tritiated estradiol and the influence on estradiol plasma levels in the cultured Siberian sturgeonAcipenser baeri

Plasma kinetics of tritiated estradiol (E2) were studied in the Siberian sturgeonAcipenser baeri in order to explain the large amount of E2 found in plasma of 4, 5 and 10-year-old cultured males. This work presents two approaches. The first is based on a single ingestion of(3)H-labelled E2, which allowed us to plot model curves of resorption and elimination processes. The second deals with chronic ingestion of(3)H-labelled E2 during a five day period, twice a day, based on a rhythm copied from the feeding practice on a fish farm. Three different doses were tested, based on the amount present in fish diets. Oral administration of E2 to sturgeon leads to E2 accumulation and a saturation of the metabolic processes generally involved in the elimination of aromatic xenobiotics. An explanation of this progressive accumulation of E2 in sturgeon plasma has to take into account the steroid binding proteins. Their synthesis could be induced by the orally administrated E2 and protect it from metabolism.

Effects of chronic LHRH-a + 17-methyltestosterone or LHRH-a + testosterone therapy on oocyte growth in the striped mullet (Mugil cephalus)

The striped mullet (Mugil cephalus) exhibits a restricted spawning season and matures only once per spawning cycle. In vivo monitoring of ovarian maturation of individual females during the season reveals that timing of full maturity varies. A small percentage of females that mature early can be induced to spawn and will remature. Such females can be spawned a second time within the same season. Chronic administration of luteinizing hormone-releasing analog (LHRH-a) + testosterone results in accelerated egg growth. The majority of females undergoing this therapy mature at least a month before control fish. When these females are induced to spawn, a higher percentage remature and can be spawned again. The double spawnings of both control and LHRH-a + testosterone-treated females occur without any appreciable loss in egg quality. Administration of LHRH-a + 17 alpha-methyltestosterone essentially inhibits egg growth. Steroid profiles from females undergoing this therapy exhibit a significant drop in circulating testosterone and estradiol 17 beta, compared to control and LHRH-a + testosterone-treated females. Hypotheses for the poor performance of this therapy are discussed.

Ovarian early postovulatory development and oestrogen production in rainbow trout (Salmo gairdneri R.) from a spring-spawning strain

Ovarian early postovulatory development and oestrogen production were studied in rainbow trout from a spring-spawning strain, 5 to 50 days postovulation (PO). Early postovulatory development was assessed by the ovarian development index, corresponding to the relative volume occupied in the ovary by oocytes in an advanced stage of differentiation (containing cortical alveoli). This index was shown to be related to the maximal oocyte diameter and to the time since ovulation, as well as to metabolic criteria. In contrast, gonadosomatic index was not correlated with any of these, confirming its unreliability for assessing the gonadal development, at least at this stage. Size of follicles and presence of vitellus (lipoprotein) in the oocytes of several animals indicate that ovarian development was more advanced in our females than in other studies. It is suggested that young follicles start developing before ovulation of the preceding batch of oocytes. Oestradiol (E2) plasma levels were low but detectable, and showed, in some females, peak values (max 5.5 ng/ml) within the first month PO. Besides, in vitro metabolism was studied using two isotopes of androstenedione as precursors. No sharp discrepancies were observed between the two types of incubation. Conjugation of the precursor or its metabolites was high but decreased with time postovulation. This suggests that it proceeds from postovulatory follicles. E2 production potential was demonstrated from the shortest time postovulation. Moreover, it increased with ovarian development, suggesting that E2 is synthesized in developing follicles.