Differential appearance of the subunits of glycoprotein hormones (LH, FSH, and TSH) in the pituitary of bullfrog (Rana catesbeiana) larvae during metamorphosis

Expression profiles of LHbeta, FSHbeta and their gonadal receptor mRNAs during sexual differentiation of Xenopus laevis tadpoles

The gonadotropins, luteinising hormone (LH) and follicle stimulating hormone (FSH), are important hormones regulating reproductive biology in vertebrates, especially the processes of steroidogenesis and gamete maturation. Despite the role of gonadotropins during the reproductive cycle in amphibians is well established, much less is known about the functional maturation of the hypothalamus-pituitary-gonad axis during larval development. Therefore, the present study aimed to analyze the expression profiles of hypophyseal LHbeta and FSHbeta mRNA and of their corresponding gonadal receptors (LH-R, FSH-R) in Xenopus laevis tadpoles during their ontogeny and sexual differentiation. The first significant elevation of LHbeta and FSHbeta mRNA was observed at late premetamorphosis. A clear raise of LHbeta mRNA was present during prometamorphic stages especially in males, while the LH-R only slowly increased during ontogeny with highest levels during metamorphic climax. In contrast, FSHbeta mRNA expression only slightly increased during ontogeny, however in both sexes the FSH-R mRNA was considerably elevated at prometamorphosis and further at metamorphic climax. Our results suggest that LHbeta and LH-R mRNA expression might be involved in initial maturation events of gametes, at least in males, while the gradually increase of FSH-R mRNA coincided with the advancing process of gamete maturation in both sexes. The present study provides for the first time evidence based on expression of gonadotropins and their corresponding gonadal receptors that the hypothalamus-pituitary-gonad axis evolves already at early stages of ontogeny and sexual differentiation in amphibians.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Metamorphosis of the amphibian tadpole is a thyroid hormone (TH)-dependent developmental process. For this reason, the tadpole is considered to be an ideal bioassay system to identify disruption of thyroid function by environmental contaminants. Here we provide an in-depth review of the amphibian thyroid system with particular focus on the role that TH plays in metamorphosis. The amphibian thyroid system is similar to that of mammals and other tetrapods. We review the amphibian hypothalamic-pituitary-thyroid (HPT) axis, focusing on thyroid hormone synthesis, transport, and metabolism. We also discuss the molecular mechanisms of TH action, including the role of TH receptors, the actions of TH on organogenesis, and the mechanisms that underlie the pleiotropic actions of THs. Finally, we discuss methods for evaluating thyroid disruption in frogs, including potential sites of action, relevant endpoints, candidate protocols for measuring thyroid axis disruption, and current gaps in our knowledge. The utility of amphibian metamorphosis as a model for evaluating thyroid axis disruption has recently led to the development of a bioassay using Xenopus laevis.

Thyroid hormones inhibit frog corticotropin-releasing factor-induced thyrotropin release from the bullfrog pituitary in vitro

Due to the lack of a radioimmunoassay (RIA) system for amphibian thyrotropin (TSH), no direct evidence that thyroid hormone suppresses the release of TSH from the amphibian pituitary has been obtained. However, we recently developed an RIA for bullfrog (Rana catesbeiana) TSH and thus were able to study the effect of thyroid hormone on the release of TSH from the bullfrog pituitary. Enzymatically dispersed pituitary cells of larval, juvenile, and adult bullfrogs were cultured in the absence or presence of 100 nM corticotropin-releasing factor of bullfrog origin (fCRF), which is known to be a potent stimulator of the release of TSH. The amount of spontaneously released TSH was higher in late prometamorphic and climactic tadpoles than in early prometamorphic larvae and juvenile and adult frogs. Pituitary cells from tadpoles at metamorphic climax responded to fCRF to release much more TSH than those from early and late prometamorphic tadpoles and juvenile and adult frogs. In all cases, the fCRF (100 nM)-induced, but not the basal, release of TSH was significantly suppressed by 1 nM triiodothyronine (T(3)) and 1000 nM thyroxine (T(4)), when examined using adult pituitary cells. The suppressive effect of thyroid hormones was revealed to be dependent on their concentrations.

Sequence analysis and expressional regulation of messenger RNAs encoding beta subunits of follicle-stimulating hormone and luteinizing hormone in the red-bellied newt, Cynops pyrrhogaster

Two distinct cDNAs encoding beta subunits of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were cloned from the cDNA library constructed for the pituitary of the red-bellied newt, Cynops pyrrhogaster, and sequenced. The newt FSHbeta and LHbeta cDNAs encode polypeptides of 129 and 131 amino acids, including signal peptides of 20 and 19 amino acids, respectively. The number and position of cysteine and N-glycosylation in each of the beta subunits of FSH and LH, which are considered essential for assembly of the alpha subunit, are well conserved between the newt and other tetrapods. The high homology (41.6%) between the beta subunits of newt FSH and LH imply less specificity of FSH and LH in gonadal function. One cDNA encoding the common polypeptide chain alpha subunit of FSH and LH was also isolated from the newt pituitary gland. The mRNAs of FSHbeta, LHbeta, and the alpha subunit were expressed only in the pituitary gland among various newt tissues. Double-staining with in situ hybridization and immunohistochemistry revealed coexpression of FSHbeta and LHbeta in the same newt pituitary cells. Ovariectomy induced a significant increase in FSHbeta mRNA levels, but there was no significant change in LHbeta or alpha subunit mRNA levels compared with those in control animals. Taken together, these data suggest that two kinds of gonadotropins, namely FSH and LH, are expressed in the same gonadotropin-producing cells in the pars distalis of the newt as well as in other tetrapods and that the expression of FSHbeta is negatively regulated by the ovaries.

Bullfrog ghrelin is modified by n-octanoic acid at its third threonine residue

We have identified the amphibian ghrelin from the stomach of the bullfrog. We also examined growth hormone (GH)-releasing activity of this novel peptide in both the rat and bullfrog. The three forms of ghrelin identified, each comprised of 27 or 28 amino acids, possessed 29% sequence identity to the mammalian ghrelins. A unique threonine at amino acid position 3 (Thr(3)) in bullfrog ghrelin differs from the serine present in the mammalian ghrelins; this Thr(3) is acylated by either n-octanoic or n-decanoic acid. The frog ghrelin-28 has a complete structure of GLT (O-n-octanoyl)FLSPADMQKIAERQSQNKLRHGNM; the structure of frog ghrelin-27 was determined to be GLT(O-n-octanoyl)FLSPADMQKIAERQSQNKLRHGN; frog ghelin-27-C10 possessed a structure of GLT(O-n-decanoyl)FLSPADMQKIAERQSQNKLRHGN. Northern blot analysis demonstrated that ghrelin mRNA is predominantly expressed in the stomach. Low levels of gene expression were observed in the heart, lung, small intestine, gall bladder, pancreas, and testes, as revealed by reverse transcription polymerase chain reaction analysis. Bullfrog ghrelin stimulated the secretion of both GH and prolactin in dispersed bullfrog pituitary cells with potency 2-3 orders of magnitude greater than that of rat ghrelin. Bullfrog ghrelin, however, was only minimally effective in elevating plasma GH levels following intravenous injection into rats. These results indicate that although the regulatory mechanism of ghrelin to induce GH secretion is evolutionary conserved, the structural changes in the different ghrelins result in species-specific receptor binding.

Steroid hormones stimulate gonadotrophs in juvenile male African catfish (Clarias gariepinus)

In juvenile African catfish (Clarias gariepinus), the pituitary LH content strongly increased after the beginning of spermatogonial proliferation. We hypothesized that a signal of testicular origin is involved in stimulating the gonadotrophs. We investigated the effects of castration and sex steroid treatment on gonadotrophs in juvenile males by quantifying LH production and release and LH subunit transcript levels and by examining gonadotroph morphology and proliferation. Castration reduced but did not abolish the maturation-associated elevation in pituitary LH content. Treatment with testosterone but not with 11-ketotestosterone, an otherwise potent androgen in fish, reversed the castration-induced decrease of pituitary LH levels. An increased pituitary LH content was accompanied by an increased number of cytologically mature gonadotrophs. However, no evidence was found for gonadotroph proliferation, so that quiescent gonadotrophs may have become activated. Although 11-ketotestosterone treatments had no effect in castrated males, this androgen attenuated gonadotroph activation in intact males. Because androgen production in juvenile catfish is downregulated by treatment with 11-ketotestosterone, its inhibitory effects on gonadotrophs in gonad-intact males may be due to suppression of Leydig cell testosterone production, which appears to be a limiting factor for the activation of catfish gonadotrophs. Aromatizable androgens may have opposite effects on fish (stimulatory) and mammalian (inhibitory) gonadotrophs.

Ontogenetic profile of FSH and LH in Rana esculenta

Circulating levels and pituitary content of FSH and LH were determined by specific radioimmunoassays in Rana esculenta starting a few days after hatching until the completion of metamorphosis. Both gonadotropins were found in the pituitary as well as in the blood plasma at all stages of development examined here. The plasma concentrations of FSH and LH were more or less uniform during pre- and prometamorphosis, but increased significantly at the onset of metamorphic climax. The plasma levels of FSH and LH remained high at the completion of metamorphosis. The pituitary content of FSH and LH was low in early premetamorphosis. It increased slightly through prometamorphosis and metamorphic climax, following which a highly significant increase occurred. Whereas plasma concentrations of FSH and LH were essentially similar within a single stage of development, the pituitary FSH content was severalfold higher than pituitary LH. The significance of these results is discussed in relation to the functional maturation of the brain-pituitary-gonadal axis in the frog.

Immunochemical identification of thyrotropes and gonadotropes in the pars distalis and pars tuberalis of the toad (Bufo boreas) with reference to ontogenic changes

Morphologically distinct secretory cells in the pituitary pars distalis and pars tuberalis of larval and adult toads (Bufo boreas) immunoreactive cells in the pars distalis. Thyrotropin immunoactivity appears in pars tuberalis and pars distalis before gonadotropin immunoreactivity during early development. Antisera which distinguish gonadotropes (stained with human and sea turtle LH beta) and thyrotropes (stained with human TSH beta) as separate cell types in the pars distalis of the adult toad immunoreact with the same single type of cell in the pars distalis of the tadpole up through metamorphosis, suggesting the existence of a single pluripotent, glycoprotein-producing precursor cell early in development. Gonadotropin antisera do not react with the pars tuberalis in tadpoles or adults.

Enhancement by prolactin of the GnRH-induced release of LH from dispersed anterior pituitary cells of the bullfrog (Rana catesbeiana)

The response of enzymatically dispersed anterior pituitary cells of the bullfrog (Rana catesbeiana) to gonadotropin-releasing hormone (GnRH) was studied by monitoring the release of luteinizing hormone (LH) into the culture medium. The cells responded to GnRH by releasing LH according to the incubation time and to the GnRH concentration. The responsiveness to GnRH became less conspicuous as the cell density was reduced. Addition of prolactin (PRL) to the medium enhanced the responsiveness to the secretagogue, and addition of antiserum against PRL lowered the responsiveness to a certain extent. Immunohistochemical studies of sectioned pituitaries revealed that PRL cells most frequently located in contact with LH cells. The possibility that PRL acts directly on gonadotrophs to enhance their responsiveness to GnRH was suggested.

Immunohistochemical studies on the development of the hypothalamo-hypophysial system in Xenopus laevis

BACKGROUND

Few attempts have been made to clarify the relational development of the hypothalamo-adenohypophysial and -neurohypophysial systems in species higher than amphibians.

METHODS

The appearance and topographical distribution of endocrine and neuroendocrine cells and fibers in these systems were immunohistochemically examined in the larvae of Xenopus laevis from immediately before hatching (stage 32, Nieuwkoop and Faber's classification) to the end of metamorphosis (stage 66).

RESULTS

(1) Each endocrine cell differentiated until the middle premetamorphic period. MSH cells initially appeared in the posterior half of the pituitary anlage at stage 35/36, followed by the differentiation of GH cells at stage 39 in the middle part, PRL cells at stage 46 in the anterior half of the pituitary anlage, and LH cells at stage 50 in the posterior two thirds of the pars distalis. With the progression of development, the cells which differentiated at early stages shifted from their initial positions; MSH cells, to the pars intermedia; and GH cells, to the posterior half of the pars distalis. 2) Oxytocin and vasopressin fibers were observed at stage 47/48 in the median eminence, and converged to the pars nervosa at later stages. 3) Neuroendocrine fibers innervated the median eminence during the middle premetamorphic to prometamorphic period: SOM fibers, at stage 45; CRH, 47/48; GRH, 48; dopamine, 58; and LHRH, 60. The cells containing these hormones were observed in the (presumptive) preoptic and/or infundibular nuclei.

CONCLUSION

These results suggest the following three chronological steps in the development of hypothalamo-hypophysial systems and their target organs: independent development of target organs at early developmental stages; appearance of hypophysial hormones to control the development of target organs at middle developmental stages; appearance of hypothalamic hormones to control the function or maturation of the hypophysis at late developmental stages.

A morphometric analysis of the subcellular distribution of LH beta and FSH beta in secretory granules in the pituitary gonadotrophs of the frog (Rana japonica)

Immunohistochemical localization of lutropin beta (LH beta) and follitropin beta (FSH beta) in the pituitary gland of the frog Rana japonica was studied by the peroxidase-anti-peroxidase method and the two-face, double-labeling method with different-sized gold particles at the light- and electron-microscopic levels, respectively, using monoclonal antibodies against bullfrog LH beta and FSH beta. Light-microscopic immunohistochemistry indicated that approximately 66.0% of all the gonadotrophs in the pituitary contained both LH beta and FSH beta, whereas 33.4% of gonadotrophs contained only LH beta, and 0.6% contained only FSH beta. The staining intensity of LH beta and FSH beta varied from cell to cell. The gonadotrophs were classified into four types (Types I-IV) in terms of their ultrastructural and immunolabeling characteristics. Moreover, several secretory granule types were recognized according to differences in their shape and electron density. In all the cell types, both LH beta and FSH beta were often seen in the same secretory granules, but the proportion of granules bearing both hormones ranged from 5.5% in Type I to 32.7% in Type IV. Most secretory granules in Types I and II were immunolabeled with LH beta alone, whereas a small number of granules were immunolabeled with FSH beta alone. More immunolabeled FSH beta granules were present in Types III and IV than in Types I and II.

The alpha-subunit of glycoprotein hormones exists in the prolactin secretory granules of the bullfrog (Rana catesbeiana) pituitary gland

Our recent finding that the number of immunoreactive alpha-subunit cells was invariably greater than the total number of immunoreactive gonadotropin (GTH) and thyrotropin (TSH) cells in the bullfrog (Rana catesbeiana) pituitary gland raises the possibility that the alpha-subunit also exists in pituitary cells other than GTH and TSH cells. The present study demonstrates that there are a considerable number of immunoreactive prolactin (PRL) cells that are also stained with antibody against the alpha-subunit when adjacent sections are immunocytochemically examined. Neither immunoreactive growth hormone nor adrenocorticotropin cells are stained with the antibody against the alpha-subunit. The specificity of the antibody against the alpha-subunit and of that against PRL was demonstrated by preabsorption test, non-competitive binding test, and immunoblot analysis. Double-immunolabeling with gold particles of different sizes for the alpha-subunit and PRL revealed that most of the immunolabeled PRL-secretory granules are also labeled with the alpha-subunit antibody. The gold particles indicating the presence of the alpha-subunit were mostly found in the peripheral zone of the secretory granules.