The influence of immature gonads on onset of gonadotropic hormone accumulation in the juvenile rainbow trout pituitary gland

Fish reproductive biology - Reflecting on five decades of fundamental and translational research

Driven by the broad diversity of species and physiologies and by reproduction-related bottlenecks in aquaculture, the field of fish reproductive biology has rapidly grown over the last five decades. This review provides my perspective on the field during this period, integrating fundamental and applied developments and milestones. Our basic understanding of the brain-pituitary-gonadal axis led to overcoming the failure of farmed fish to ovulate and spawn in captivity, allowing us to close the fish life cycle and establish a predictable, year-round production of eggs. Dissecting the molecular and hormonal mechanisms associated with sex determination and differentiation drove technologies for producing better performing mono-sex and reproductively-sterile fish. The growing contingent of passionate fish biologists, together with the availability of innovative platforms such as transgenesis and gene editing, as well as new models such as the zebrafish and medaka, have generated many discoveries, also leading to new insights of reproductive biology in higher vertebrates including humans. Consequently, fish have now been widely accepted as vertebrate reproductive models. Perhaps the best testament of the progress in our discipline is demonstrated at the International Symposia on Reproductive Physiology of Fish (ISRPF), at which our scientific family has convened every four years since the grandfather of the field, the late Ronald Billard, organized the inaugural 1977 meeting in Paimpont, France. As the one person who has been fortunate enough to attend all of these meetings since their inception, I have witnessed first-hand the astounding evolution of our field as we capitalized on the molecular and biotechnological revolutions in the life sciences, which enabled us to provide a higher resolution of fish reproductive and endocrine processes, answer more questions, and dive into deeper comprehension. Undoubtedly, the next (five) decades will be similarly exciting as we continue to integrate physiology with genomics, basic and translational research, and the small fish models with the aquacultured species.

Effects of gonadectomy on plasma gonadotropins I and II in coho salmon, Oncorhynchus kisutch

Studies of both salmon and trout have indicated that the levels of gonadotropins, GTH I and GTH II, in the pituitary and peripheral circulation vary during the reproductive cycle. To evaluate the possible feedback relationship between the gonads and pituitary GTH secretion, we studied the effects of bilateral gonadectomy on plasma levels of GTH I and GTH II in coho salmon, Oncorhynchus kisutch. During late spermatogenesis in males and late vitellogenesis in females, plasma GTH I levels increased significantly after gonadectomy, approximately 6- and 5-fold over presurgery levels at 3 and 14 days after surgery, respectively, and then declined to near presurgery levels by Day 17. No change in GTH I levels occurred in sham-operated fish. In all groups, GTH II levels were nondetectable and did not change significantly up to 17 days postsurgery. In males gonadectomized during spermiation, plasma GTH I levels increased significantly, approximately 10-fold over presurgery levels by 7 days postsurgery, and remained elevated thereafter. In contrast to the males in late spermatogenesis, the spermiating fish had detectable levels of GTH II (2-3 ng/ml), and significant elevations in plasma GTH II levels (approximately 60-fold) were observed 7 days after gonadectomy. These experiments demonstrate that the gonads exert negative feedback effects on secretion of both GTH I and GTH II, but the effect varies seasonally and the nature of the specific factor(s) from the gonads that inhibits and/or stimulates GTH production and secretion remains to be clarified.

Synthesis and regulation of 17α-hydroxy-20β-dihydroprogesterone in immature males of Oncorhynchus mykiss

Three experimental approaches were chosen to study the question if the progestin 17α-hydroxy-20β-dihydroprogesterone (17α20βOHP) is synthesised in testes of young Oncorhynchus mykiss, in which the absence of spermatozoa was verified histologically: first, in order to detect 20β-hydroxysteroid dehydrogenase activity (20βHSD), testes homogenates were incubated with (3)H-labeled 17αOHP.Metabolites were analysed by TLC, HPLC, and repeated crystallization to constant isotope ratios. One of the metabolites was identified as 17α20βOHP-(3)H, indicating that already immature testes contain 20βHSD activity and are able to produce 20β-reduced steroids. Second, 17α20βOHP was quantified by radioimmunoassay in incubates of testes fragments. The sensitivity of the gonads to gonadotropin II (GtH II) became evident when comparing incubations in the absence and presence of GtH II. Third, plasma levels of 17α20βOHP were significantly higher in animals injected with partially purified salmon gonadotropin, compared to controls. Thus, for the first time, it could be shown that 20βHSD is present in testicular cells other than spermatozoa. Furthermore, 17α20βOHP is indeed secreted at a very early stage of testicular development; 17α20βOHP secretion is also responsive to GtH II. Future studies will have to show if the functions of this progestin include the stimulation of spermatogenesis.

Effects of castration on pituitary gonadotropic cells of the male three-spined stickleback, Gasterosteus aculeatus L., under long photoperiod in winter: indications for a positive feedback

Male three-spined sticklebacks caught in winter were castrated or sham-operated and subsequently kept under long photoperiod at about 20 degrees for a month. With this treatment the sham-operated fish attained breeding condition. The gonadotropic cells of the sham-operated fish contained significantly more dilated endoplasmic reticulum and fewer granules than those of the castrated fish, indicating a higher secretory activity of the gonadotrops in the sham-operated fish. These findings suggest the existence of a physiological positive feedback within the gonadal-pituitary axis of the male stickleback when stimulated into its breeding condition by long photoperiod.

Gonadotropins I and II in juvenile coho salmon

The present study was designed to obtain basic endocrine information on GTH I and GTH II in previtellogenic and prespermatogenic coho salmon (immature). Levels of GTH II in pituitary extracts were 6.5 ± 2.0 and 6.7 ± 2.0 pg/μg pituitary protein in male and female fish, respectively. In contrast, the pituitary content of GTH I was approximately 100-fold higher than GTH II (1.302 ± .22 and 1.173 ± .21 ng/μg pituitary protein in male and female fish, respectively). Plasma levels of GTH II in immature salmon were not detectable by RIA whereas plasma GTH I levels were approximately 0.62 ± 0.12 and 0.78 ± 0.13 ng/ml in male and female fish, respectively. Highly purified coho salmon GTH I and GTH II stimulated testicular testosterone production and ovarian estradiol productionin vitro in a similar manner, though GTH II appeared more potent than GTH I. Therefore, it appears that although the salmon pituitary contains predominantly GTH I prior to puberty, the gonad can respond to both GTH I and GTH II.

Positive feedback control by the gonads on gonadotropin (GTH) and gonadoliberin (GnRH) levels in experimentally matured female silver eels,Anguilla anguilla

Treatment of sham-operated female silver eels with carp pituitary extract stimulated ovarian development and induced increases in pituitary gonadotropin (GTH) and gonadoliberin (GnRH) contents. Both effects of carp pituitary extract were abolished in ovariectomized eels, indicating the involvement of the gonads. Endogenous sexual steroids, the secretion of which was increased during sexual maturation, should be responsible for the stimulation of GTH and GnRH levels. Ovariectomy itself had no significant effect on pituitary GTH and GnRH contents, reflecting the fact that, at the silver stage, sexual steroid levels are too low to exert any significant effect on pituitary GTH and GnRH. The positive feedback control exerted by the gonads on GTH and GnRH levels during sexual maturation, in the eel as well as in some other teleosts, would produce an amplification of the pubertal stimulation of the hypothalamo-pituitary-gonadal axis.

Sexual dimorphism of plasma sex steroid levels in juvenile coho salmon, Oncorhynchus kisutch, during smoltification

Concentrations of plasma sex steroids, cortisol, and thyroxine were measured by radioimmunoassay in hatchery coho salmon (Oncorhynchus kisutch during winter and early spring. Mean plasma 11-ketotestosterone (11-KT) and estradiol levels fell into two distinct categories: 11-KT was 181-373% higher in males than in females, and estradiol was 109-143% higher in females than in males. No changes in plasma levels of estradiol in fish of both sexes, or in levels of 11-KT in males, were evident during spring when plasma thyroxine and cortisol were markedly elevated, indicating that the fish were undergoing smoltification. Although plasma 11-KT in females appeared to be lower in late April than in February, it showed no correlation with plasma thyroxine or cortisol in these individuals. Our finding of sexual dimorphism in 17 alpha-20 beta-dihydroxy-4-pregnen-3-one was inconsistent between stocks of fish and among sampling dates, thus making interpretation of the results difficult. However, no relationship between this steroid and plasma thyroxine or cortisol was observed. Therefore, plasma levels of sex steroids do not seem to be related to the changes in plasma thyroxine or cortisol observed during smoltification of coho salmon.

Influence of testosterone on precocious sexual development in immature rainbow trout

The influence of testosterone on plasma and pituitary levels of gonadotrophin (GTH) as well as on gonadal development was studied in immature rainbow trout. Among the animals receiving a testosterone-cocoa butter implant (200 micrograms) at the age of 5 months, gonadal puberty occurred 8 months later in half of the males (opposite to the controls which remained immature) and the beginning of oocyte maturation was observed in only one female. These animals were characterized by a higher pituitary GTH level. Owing to the multivariate statistical analyses made, it was possible to provide evidence for the presence of two populations with different reactions to the same steroid treatment. They also confirmed the existence of a positive testosterone feedback, in the male, leading to a precocious gonadal development. The pituitary GTH load obtained with 200 micrograms of testosterone seemed to be related to the age of first maturation. The secretion of an appropriate level of GTH resulting in the stimulation of gametogenesis required the availability of a relatively large pituitary GTH level and seemed to be possible because the animals were already in the pubertal period. The fact that the highest pituitary GTH level of the treated lot was found in the only female showing a beginning of sexual maturation suggests that testosterone may also act in females.

The brain-pituitary-gonadal axis in the rainbow trout, Salmo gairdneri. III. Absence of an inhibiting action of testosterone on gonadotrophin release in juveniles

In juvenile rainbow trout the effects of exogenous testosterone and of a synthetic gonadotrophin-releasing hormone (GnRH) on the secretion of gonadotrophin (GTH) were investigated. Treatment with implanted testosterone resulted in an accumulation of GTH in the pituitary, but did not affect the concentration of GTH in the plasma. After the testosterone implants were removed, the levels of testosterone in the circulation dropped to undetectable or control values, but the concentration of GTH in the plasma did not increase. These results indicate that testosterone stimulated the synthesis and storage of GTH, and did not prevent the release of this hormone. The synthetic GnRH, des-Gly10[D-Ala6]LH-RH ethylamide (LH-RHa) stimulated the release of GTH when injected into testosterone-pretreated fish, indicating that accumulated GTH is present in a releasable pool.

Effects of steroid hormones on pituitary immunoreactive gonadotropin in European freshwater eel, Anguilla anguilla L

The low pituitary immunoreactive gonadotropin (IR GTH) contents of immature female or male freshwater eels were strongly increased after intraperitoneal injections of estradiol. Using female eels, the effect was shown to be dose dependent. In neither sex was modification of pituitary IR GTH observed after progesterone treatment; a very small increase (significant in females but not in males) was produced by cortisol. An important sexual difference occurred with testosterone, which strongly stimulated pituitary IR GTH level in male but not in female eels. The possibility of a low central aromatase activity responsible for the lack of effect of testosterone in the female eel is discussed. Circulating IR GTH levels were very low in steroid-treated eels, as in controls, indicating a weak GTH release. Thus, as in juvenile, immature fish from other species, a sexual steroid stimulated eel pituitary GTH content; the peculiarity of the apparent "blockade" of eel sexual development appears not related to a lack of sensitivity of gonadotropin synthesis to the positive feedback of a sexual steroid.