Release of pituitary gonadotrophins GtH I and GtH II in the rainbow trout (Oncorhynchus mykiss): modulation by estradiol and catecholamines

Modulation of pituitary dopamine D1 or D2 receptors and secretion of follicle stimulating hormone and luteinizing hormone during the annual reproductive cycle of female rainbow trout

The two gonadotrophins follicle stimulating hormone (FSH) and luteinizing hormone (LH) have distinct temporal expression and release profiles in fish, but little is known regarding their neuroendocrine control, especially for FSH. The present experiments were performed on previtellogenic, mature and preovulatory female trout. The catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, increased plasma LH and FSH concentrations of mature fish. The dopamine agonist apomorphine decreased and the dopamine antagonist domperidone increased plasma LH concentration of preovulatory fish and delayed ovulation, but did not modify plasma FSH concentration. The dopamine D2 agonist bromocryptine inhibited LH release in cultured gonadotrophs from mature and preovulatory fish, but not from previtellogenic fish. Bromocryptine also significantly inhibited basal and salmon gonadotrophin releasing-hormone (sGnRH)-induced FSH release from cultured gonadotrophs of mature fish, but not of preovulatory fish, and increased FSH release from gonadotrophs of previtellogenic fish. The dopamine D1 agonist SKF 38393 had no observed effect on the release of FSH and LH, at any reproductive stage studied. The D1 agonist SKF 38393, the D2 agonist bromocriptine and sGnRH had no observed effects on cell contents of FSH and LH. Taken together, these data suggest that, at the level of the pituitary, dopamine inhibits LH release as vitellogenesis proceeds, via activation of dopamine D2 receptors. We demonstrate for the first time in fish a control of FSH release (a dopamine control), especially in mature fish which have low circulating concentrations of FSH.

Effects of salmon gonadotropin-releasing hormone on follicle stimulating hormone secretion and subunit gene expression in coho salmon (Oncorhynchus kisutch)

Previous work has indicated that, during the process of gametogenesis in salmon, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are differentially synthesized and released. Although substantial information is available on the regulation of LH in many fish species, relatively little is known about the regulation of FSH biosynthesis and secretion or the regulation of two types of alpha subunit in salmon. In this study, the effects of salmon gonadotropin-releasing hormone (sGnRH) on in vitro secretion of FSH, and alpha1, alpha2, LH beta, and FSH beta subunit gene expression were investigated in coho salmon (Oncorhynchus kisutch) using primary pituitary cell cultures. To quantify FSH beta, LH beta, alpha1, and alpha2 subunit transcript levels, a multiplex RNase protection assay (RPA) was developed. Probes for the beta subunits of coho salmon FSH and LH were available from previous studies. To generate probes for the alpha subunit RPAs, alpha1 and alpha2 subunit cDNAs were cloned using reverse transcriptase PCR. Release of FSH and LH into cell culture medium was quantified by radioimmunoassays. The effects of sGnRH on gonadotropin release and gene expression were tested at two points during the spring (April and May) prior to spawning in the autumn; a period when plasma and pituitary FSH levels are increasing and females are in early stages of secondary oocyte growth. In both experiments, sGnRH increased steady-state mRNA levels of FSH beta, alpha1, and alpha2, whereas LH beta mRNA levels were not detectable. Secretion of FSH was stimulated by sGnRH in a concentration-dependent manner. Medium LH was not detectable in the first experiment (April) and was measurable only after sGnRH treatment in the second experiment (May). Control levels of medium FSH and transcripts for FSH beta and alpha1 subunits increased approximately fourfold between April and May, whereas alpha2 transcript levels remained relatively constant, suggesting that the seasonal increase in FSH release may involve increased production of alpha1. Therefore, sGnRH has direct stimulatory effects on both secretion of FSH and FSH subunit biosynthesis, most likely due to increased transcription. However, alterations in rates of transcript degradation cannot be ruled out.

Energetic dependence of NPY-induced LH secretion in a teleost fish (Dicentrarchus labrax)

The purpose of this work was to examine the role of energetic status in neuropeptide Y (NPY)-induced luteinizing hormone (LH) secretion and glucose metabolism in fish. Fasted juvenile sea bass (Dicentrarchus labrax) were injected intraperitoneally with pig (p) NPY or pNPY + glucose, whereas fed animals were injected with pNPY alone and plasma glucose, insulin, and LH levels were examined. pNPY alone or in combination with glucose was found to induce a dose-dependent increase in LH secretion in fasted animals. Similar LH responses to pNPY were observed in vitro in dispersed pituitary cells isolated from fed and fasted animals incubated in L-15 and restricted media. Injection of pNPY + glucose in fasted animals resulted in depletion of glucose. Insulin plasma levels decreased in fasted animals coinjected with pNPY + glucose but remained stable when NPY was administrated alone to fed and fasted animals. Results suggest that 1) NPY-induced LH secretion in fish is dependent on energetic status and 2) NPY is capable of modifying glucose metabolism.

Ontogeny of follicle-stimulating hormone and luteinizing hormone gene expression during pubertal development in the female striped bass, Morone saxatilis (Teleostei)

Pubertal development in teleost fish is characterized by gonadal growth that is directly stimulated by the pituitary gonadotropins, FSH and LH. We used a quantitative ribonuclease protection assay to provide, for the first time, the developmental profiles of the alpha-, betaFSH-, and betaLH-subunit gene expression in a seasonal breeding fish, the female striped bass (3-yr study, n = 207). Two-year-old females were sexually immature, although a transient rise in all gonadotropin subunit mRNAs was measured in the pituitary. Pubertal ovarian development occurred in 65% of 3-yr-old females, characterized by the appearance of lipid droplets within the oocytes. This reproductive phase, termed pubertal development, was associated with a 34-fold increase in the mRNA levels of betaFSH and a rise in the pituitary concentration of LH. The first sexual maturation took place in 4-yr-old females and coincided with a 218-fold increase in the mRNA levels of betaFSH. During this time period, the mRNA levels of the alpha and betaLH subunits increased by 11- and 8-fold, respectively. At the final stages of vitellogenic growth, mRNA levels of betaFSH declined to basal levels, whereas the mRNA levels of the alpha and betaLH subunits remained elevated. Throughout the study, pituitary LH concentration was positively correlated to the mRNA levels of betaLH, but plasma levels of LH remained low and unchanged (0.4-0.8 ng/ml) despite increasing levels of pituitary LH concentration, suggesting a regulated secretion pathway. Taken together, the data show that the profiles of betaFSH and betaLH mRNAs appear to follow an annual rhythm that is associated with developmental events in the growing oocytes. In particular, increasing levels of betaFSH mRNA appear to underlie the first sexual maturity in the female striped bass.

The brain-pituitary-gonadal axis of female rainbow trout Oncorhynchus mykiss: effects of photoperiod manipulation1

Two groups of post-spawned female rainbow trout were exposed to two different photoperiods, an ambient photoperiod (56 degrees N) and a combination of long and short photoperiods (a constant 18L:6D from February 1 until May 10, then a constant 6L:18D), which acted to advance maturation and spawning. The stimulatory long-short photoperiod advanced spawning by 3-4 months and correspondingly advanced peaks in serum levels of 17beta-estradiol, testosterone, calcium (an index of vitellogenin), and GTH II. Earlier events in gonadal recrudescence appeared to be less affected by the photoperiod. The initiation of exogenous vitellogenesis coincided with high levels of both pituitary salmon gonadotropin-releasing hormone (sGnRH) content and serum follicle-stimulating hormone (FSH, GTH I) levels. High levels of serum FSH were associated with rapid gonadal growth in the fish exposed to the stimulatory long-short photoperiod. In contrast, the fish exposed to the ambient photoperiod showed gonadal steroid production, formation of vitellogenin, and secondary oocyte growth without any detectable increase in serum FSH levels. The possible roles and interactions of sGnRH, gonadotropins, and steroids with respect to normal and artificially stimulated ovarian maturation are discussed.

Changes in the pituitary metabolism of monoamines (dopamine, norepinephrine, and serotonin) in female and male rainbow trout (Oncorhynchus mykiss) during gonadal recrudescence

The dynamics of the levels and metabolism of dopamine, norepinephrine, and serotonin were studied in pituitaries of male and female rainbow trout at different stages of gonadal development. In female rainbow trout, the turnover of dopamine (calculated using the inhibitor of tyrosine hydroxylase alpha-methyl-p-tyrosine methyl-ester HCl), serotonin metabolism, and norepinephrine levels decreased in the advanced stage of exogenous vitellogenesis with respect to the initial stage. However, data obtained in males did not show changes in either serotonergic or noradrenergic metabolism during the last stages of gonadal development. However, an increase of dopaminergic turnover was noticed in the male fish at the end of spermiation. Finally, pituitary dopaminergic activity was significantly higher in immature (prepubescent stage) than in adult fish.

Feedback control of gonadotropins in Atlantic salmon, Salmo salar, male parr.I. Castration effects in rematuring and nonrematuring fish

Sexual maturation of Atlantic salmon (Salmo salar) male parr is a seasonally recurrent "all or none" response; either a fish matures fully or it does not mature. To study whether gonadal feedback on gonadotropic hormones, GTH I and GTH II, is involved in the control of maturation, previously mature Atlantic salmon male parr were either sham-operated or castrated in spring. They were then sampled during the onset of gonadal growth (late June-early July) or shortly before the breeding season (late September). In autumn, sham-operated males separated into two groups: nonrematuring males with low pituitary and plasma levels of both GTH I and GTH II, and those rematuring with high levels of gonadotropins. Castrated males had low GTH I and GTH II plasma and pituitary levels, similar to those of the nonrematuring fish, suggesting positive feedback mechanisms, separating the sham-operated fish into low and high GTH level groups. In the summer, plasma GTH II was nondetectable in all fish. Pituitary GTH II content was lower in nonrematuring, than in rematuring males and was even lower in castrated fish. In contrast, castration increased pituitary and plasma levels of GTH I in the summer, suggesting a negative feedback at this reproductive stage. There were no significant differences in immunoassayable levels of GTH I in plasma in rematuring and nonrematuring sham-operated males at this time. The control of rematuration is complex and may involve factors other than circulating GTH I levels, possibly with differences in gonadal sensitivity to GTH I.

Feedback control of gonadotropins in Atlantic salmon, Salmo salar, male parr.II. Aromatase inhibitor and androgen effects

Both positive and negative feedback on the (hypothalmus)- pituitary-gonad axis occur in salmonids. The aim of the present study was to investigate the role of different androgens, and in particular the involvement of aromatization of androgens to estrogens in feedback mechanisms. Previously mature Atlantic salmon, Salmo salar, male parr were studied in two experiments. In the first experiment, intact fish were implanted with Silastic capsules filled with the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) in spring and sampled in the autumn when the rematuring males were starting to display running milt. In the second experiment, castrated males were implanted with capsules containing ATD, the androgens testosterone (T) and 11-ketoandrostenedione (11KA), or ATD and T combined in spring. These fish were sampled in the summer when rematuring fish were starting to show signs of gonadal growth. Pituitary and plasma gonadotropins (GTH I and GTH II) were studied using radioimmunoassay. In autumn, ATD treatment reduced pituitary and plasma GTH II levels. In summer, GTH II was consistently nondetectable in plasma. Castration diminished pituitary GTH II content. Treatment with T increased pituitary GTH II content, an effect that was attenuated when T treatment was combined with ATD. All these results are consistent with the presence of an aromatase-dependent positive feedback of T on GTH II. 11KA also had a stimulatory effect on GTH II, although weaker than that of T. Testicular size and spermiation was reduced by ATD in autumn; the latter of these results is likely to be due to the inhibitory effect of ATD on GTH II. Positive effects of ATD on plasma GTH I were found in autumn, indicative of an aromatase-dependent negative feedback in this phase. On the other hand, castration increased plasma and pituitary GTH I levels in summer, indicating that the gonads in this phase exert a predominantely negative control of GTH I. In summer, negative effects of T on GTH I pituitary levels were not suppressed, but were rather enhanced, by the combined treatment with ATD. Furthermore, plasma GTH I levels were lower after treatment with T in combination with ATD than with T or ATD separately. These negative effects of T were not diminished by ATD, so that they are nonaromatase-dependent. Furthermore, since they were actually more pronounced in the presence of ATD it is suggested that there is also a positive aromatase-dependent feedback component in this phase. In addition, 11KA had a negative effect on plasma and pituitary GTH I in castrated previously mature males. Thus, GTH I secretion is controlled by both aromatase-dependent and nonaromatase-dependent feedback effects, of which at least the former may be positive or negative depending on season. In summary, the feedback control of GTH I appears to be more complex than that of GTH II.

Growth hormone (GH) and gonadotropin subunit gene expression and pituitary and plasma changes during spermatogenesis and oogenesis in rainbow trout (Oncorhynchus mykiss)

In order to evaluate potential interactions between somatotropic and gonadotropic axes in rainbow trout (Oncorhynchus mykiss), changes in pituitary content of the specific messenger RNA of growth hormone (GH) and gonadotropin (GTH) alpha- and beta-subunits were studied during gametogenesis with respect to pituitary and plasma hormone concentrations. Quantitative analyses of mRNA and hormones were performed by dot blot hybridization and homologous RIA on individual fish according to stage of spermatogenesis and oogenesis. All transcripts were detectable in 9-month-old immature fish. GH, GTH IIbeta, and GTH alpha increased moderately throughout most of gametogenesis and then more dramatically at spermiation and during the periovulatory period. GTH Ibeta mRNA increased first from stage I to V in males and more abruptly at spermiation, while in females GTH Ibeta transcripts increased first during early vitellogenesis and again around ovulation. Pituitary GH absolute content (microgram/pituitary, not normalized with body weight) increased slowly during gametogenesis and more abruptly in males during spermiation. In the pituitary of previtellogenic females and immature males, GTH I beta peptide contents were 80- to 500-fold higher than GTH II beta peptide contents. GTH I contents rose regularly during the initial phases of vitellogenesis and spermatogenesis and then more abruptly in the final stages of gonadal maturation, while GTH II contents show a dramatic elevation during final oocyte growth and maturation, in postovulated females, and during spermiogenesis and spermiation in males. Blood plasma GTH II concentrations were undetectable in most gonadal stages, but were elevated during spermiogenesis and spermiation and during oocyte maturation and postovulation. In contrast, plasma GTH I was already high ( approximately 2 ng/ml) in fish with immature gonads, significantly increased at the beginning of spermatogonial proliferation, and then increased again between stages III and VI to reach maximal levels ( approximately 9 ng/ml) toward the end of sperm cell differentiation, but decreased at spermiation. In females, plasma GTH I rose strongly for the first time up to early exogenous vitellogenesis, decreased during most exogenous vitellogenesis, and increased again around ovulation. Our data revealed that patterns of relative abundance of GTH Ibeta mRNA and pituitary and plasma GTH I were similar, but not the GTH II patterns, suggesting differential regulation between these two hormones at the transcriptional and posttranscriptional levels. Pituitary and plasma GH changes could not be related to sexual maturation, and only a weak relationship was observed between GH and gonadotropin patterns, demonstrating that no simple connection exists between somatotropic and gonadotropic axes at the pituitary level during gametogenesis.