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The annual cycle of changing day length (photoperiod) is widely used by animals to synchronise their biology to environmental seasonality. In mammals, melatonin is the key hormonal relay for the photoperiodic message, governing thyroid-stimulating hormone (TSH) production in the pars tuberalis (PT) of the pituitary stalk. TSH acts on neighbouring hypothalamic cells known as tanycytes, which in turn control hypothalamic function through effects on thyroid hormone (TH) signalling, mediated by changes in expression of the type II and III deiodinases (Dio2 and Dio3, respectively). Among seasonally breeding rodents, voles of the genus Microtus are notable for a high degree of sensitivity to nutritional and social cues, which act in concert with photoperiod to control reproductive status. In the present study, we investigated whether the TSH/Dio2/Dio3 signalling pathway of female common voles (Microtus arvalis) shows a similar degree of photoperiodic sensitivity to that described in other seasonal mammal species. Additionally, we sought to determine whether the plant metabolite 6-methoxy-2-benzoxazolinone (6-MBOA), described previously as promoting reproductive activation in voles, had any influence on the TSH/Dio2/Dio3 system. Our data demonstrate a high degree of photoperiodic sensitivity in this species, with no observable effects of 6-MBOA on upstream pituitary/hypothalamic gene expression. Further studies are required to characterise how photoperiodic and nutritional signals interact to modulate hypothalamic TH signalling pathways in mammals., (Copyright © 2012 Elsevier Inc. All rights reserved.)

We used ex vivo and in vivo experiments with Xenopus laevis tadpoles to examine the hypothesis that the set-point for negative feedback on pituitary thyroid-stimulating hormone (TSH) synthesis and secretion by thyroid hormones (THs) increases as metamorphosis progresses to allow for the previously documented concomitant increase in serum TH concentrations and pituitary TSH mRNA expression during this transformative process. First, pituitaries from climactic tadpoles were cultured for up to 96 h to characterize the ability of pituitary explants to synthesize and secrete TSHβ in the absence of hypothalamic and circulating hormones. Next, pituitary explants from tadpoles NF stages 54-66 were exposed to physiologically-relevant concentrations of THs to determine whether stage-specific differences exist in pituitary sensitivity to negative feedback by THs. Finally, in vivo exposures of tadpoles to THs were conducted to confirm the results of the ex vivo experiments. When pituitaries from climactic tadpoles were removed from the influence of endogenous hormones, TSHβ mRNA expression increased late or not at all whereas the rate of TSHβ secreted into media increased dramatically, suggesting that TSH secretion, but not TSH mRNA expression, is under the negative regulation of an endogenous signal during the climactic stages of metamorphosis. Pituitaries from pre- and prometamorphic tadpoles were more sensitive to TH-induced inhibition of TSHβ mRNA expression and secretion than pituitaries from climactic tadpoles. The observed decrease in sensitivity of pituitary TSHβ mRNA expression to negative feedback by THs from premetamorphosis to metamorphic climax was confirmed by in vivo experiments in which tadpoles were reared in water containing THs. Based on the results of this study, a model is proposed to explain the seemingly paradoxical, concurrent rise in serum TH concentrations and pituitary TSH mRNA expression during metamorphosis in larval anurans., (Published by Elsevier Inc.)

Seasonal changes in the mRNA levels of glycoprotein alpha, gonadotropin (GTH) Ibeta and IIbeta, and thyrotropin (thyroid-stimulating hormone (TSH)) beta subunits in the pituitary of goldfish were quantified by Northern blot analysis and laser densitometry. Reproductive development and thyroid activity were monitored by measuring gonadosomatic index, plasma GTH II, testosterone (T), estradiol, 11-ketotestosterone, and thyroid hormones (T4 and T3). Plasma GTH II and steroids showed characteristic increases, while plasma thyroid hormones levels, in general, decreased in association with the reproductive period. In females, the mRNA levels of the alpha, GTH Ibeta, and GTH IIbeta subunits increased synchronously during early spawning period (April) and then decreased during ovarian regression (August). In males, the levels of the alpha and GTH IIbeta subunits showed changes similar to those in females, but the GTH Ibeta mRNA levels showed only a small increase during the late spawning period (May). In both sexes, TSHbeta mRNA levels were high during winter to early spring (February and April) and low during late spring to summer (May and August). These results suggest that in goldfish the gonadotropins may be synthesized synchronously in order for asynchronous gametogenesis to take place. Additionally, the data suggest a negative feedback relationship between synthesis of the TSHbeta subunit and the thyroid hormones., (Copyright 1999 Academic Press.)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a multisite carcinogen. Although the hepatocarcinogenic actions of TCDD have received the most attention, it has been demonstrated in several rodent carcinogenicity bioassays that TCDD causes a dose-related increase in thyroid follicular cell adenomas and carcinomas. The purpose of the present experiment was to investigate the dose-response relationship for thyroid function alterations in female Sprague-Dawley rats following chronic treatment with TCDD. TCDD was administered via oral gavage biweekly for 30 weeks at average daily equivalent doses of 0.1-125 ng/kg/day, thereby more than encompassing the dose range historically used in previous TCDD rodent bioassays. The endpoints examined include serum levels of thyroxine (T4), triiodothyronine (T3), and thyroid-stimulating hormone (TSH). In addition, the induction of the dioxin-responsive genes UDP-glucuronosyltransferase-1 (UGT1) and cytochrome P450 1A1 (CYP1A1) in liver were measured using reverse-transcriptase-polymerase chain reaction (RT-PCR). In agreement with previous hypotheses, TCDD appears to alter thyroid function via a secondary mechanism, namely increased excretion of T4-glucuronide resulting from TCDD induction of UGT1. The observed follicular cell hyperplasia and hypertrophy are consistent with the observed elevated TSH levels and may represent the early stages in the progression of thyroid carcinogenesis. Therefore, TCDD induces alterations in thyroid hormone function, probably as a result of chronic perturbations of liver-pituitary-thyroid axis.

The first appearance of adenohypophysial cells in the developing rainbow trout embryo was studied by immunocytochemistry. Antibodies generated against the beta subunits of coho salmon gonadotropins (GTH I and GTH II), the beta subunit of human thyroid stimulating hormone (TSH), chum salmon prolactin (PRL), chum salmon growth hormone (GH), and synthetic alpha-MSH were used as immunocytochemical probes. The pituitary anlage was first recognized at developmental stage 21 (18 days postfertilization). At this stage, immunoreactive (ir) PRL and alpha-MSH cells were identified in the rostral and caudal regions of the pituitary, respectively. Cells containing ir-GH and ir-adrenocorticotropin (ACTH) appeared at stage 28 (35 days postfertilization). At the last stage before hatching (stage 29, 42 days postfertilization) ir-TSH cells were identified. Ir-GTH I cells were first observed in the pituitary at stage 32 (15 days after hatching), when mitosis of gonadal germ cells also was observed. At stage 35 (35 days after hatching), meiotic figures were first observed in a few germ cells of some fish indicating that gonadal sex differentiation probably begins at about this time. Cells containing ir-GTH II were not apparent in the pituitary at any stage of embryonic or larval development (up to stage 35) and were not present in pituitaries of trout at 6 months of age when the ovaries were in the perinucleolus stage and when only spermatogonia, but no spermatocytes were present in the testis. These observations suggest that GTH I, but not GTH II, may regulate initial gonadal growth and development in the embryonic and larval rainbow trout.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormones suppress the synthesis and release of thyrotropin from thyrotropes in the anterior pituitary gland, a feature that is critical in the classic negative-feedback loop of the pituitary-thyroid endocrine axis. The major effect of thyroid hormones in this system is exerted at the transcriptional level. The molecular mechanisms by which there is negative regulation of TSH subunit gene expression by thyroid hormone have been elucidated. The TSH subunit genes have isolated and characterized. Structure-function analyses using fusion genes and DNA transfection approaches have defined the putative negative TREs among the promoters of the rat, mouse, and human alpha and TSH beta genes. These sequences are either largely overlapping direct TRE half-sites, TRE half-sites as direct repeats gapped by two nucleotides, or single TRE half-sites. These arrangements are distinct from those seen in positive TREs. Recent knowledge regarding the molecular mechanisms of thyroid action in general forces consideration of multiple TR isoforms, TR heterodimer partners (TRAPs), and thyroid hormones in the ultimate mechanisms of negative action. Several models have been proposed, but none has yet been proved. In addition, the role of thyroid hormone in the regulation of gene expression at the posttranscriptional level is beginning to be addressed. Future work should continue to illuminate these important facets of gene regulation.

Hypophysial prolactin (PRL) and thyrotropin (TSH) cells of Rana perezi were examined after treatment in vitro with synthetic thyrotropin-releasing hormone (TRH). Ultrastructural morphometry applied to PRL and TSH immunoidentified cells estimated the volume density (Vv) of the secretory granules (SG), rough endoplasmic reticulum (ER), and Golgi complex (GC) as well as the numerical density (Nv) of the granules. Hemipituitaries were cultured in a superfusion system with or without 100 ng TRH/ml for 4, 14, or 24 hr. PRL cells showed significant degranulation (42%) of medium size SG after 14 hr of stimulation, whereas the biosynthetic machinery (ER and GC) was significantly developed after 4 hr of TRH culture (increase of Vv of GC, 1.5-fold, and ER, 1.3-fold, in comparison to the control). Most of these changes remained after 24 hr of TRH treatment. Compared with controls, TRH-treated TSH cells differed only after 24 hr when SG showed degranulation (40%), mainly of the medium size ones, and Vv of GC (1.6-fold) and ER (1.3-fold) increased. These results suggest that TRH acts directly on amphibian PRL and TSH cells stimulating hormonal synthesis and release. The time courses of responses to TRH differed in that PRL cells gave an immediate response while TSH cells gave a more delayed reaction.

The adenohypophysial cell types in three species of sciaenid teleosts were identified using a combination of classical histochemical techniques and immunocytochemistry with antisera raised against piscine and human pituitary hormones. In general greater specificity for the different cell types was observed with antisera to the piscine pituitary hormones than with antisera raised against the human hormones. The distribution of the cell types did not differ significantly among the three species, Atlantic croaker (Micropogonias undulatus), spotted seatrout (Cynoscion nebulosus), and red drum (Sciaenops ocellatus), and was similar to that reported in several other teleosts. A major portion of the rostral pars distalis was composed of acidophilic prolactin cells which showed specific binding to antisera to carp, coho salmon, and chinook salmon prolactins. The acidophilic corticotrops formed cords of cells which bordered the prolactin cells and showed strong immunoreactivity with anti-human ACTH. This antiserum also cross-reacted with the lead hematoxylin-positive cells in the pars intermedia. The acidophilic growth hormone cells were restricted to the dorsal part of the proximal pars distalis in close contact with the neurohypophysis and were specifically bound by anti-carp and anti-chum salmon growth hormone. The basophilic thyrotrops were located in the dorsal-anterior part of the proximal pars distalis and showed strong immunoreactivity to antiserum to the human thyrotropin beta subunit. This antiserum also bound weakly to the basophilic gonadotrops which were concentrated in the ventral part of the proximal pars distalis and also extended posteriorly to form a border around the pars intermedia. Antiserum to croaker maturational gonadotropin, which recognizes two distinct gonadotropins in sciaendis, bound strongly and specifically to the gonadotrops. The gonadotrops were also bound specifically by antisera to the alpha and beta subunits of human luteinizing hormone. Antisera to the beta subunits of two recently isolated coho salmon gonadotropins, GTH-I and GTH-II, failed to bind the gonadotrops strongly or differentially in the three sciaenid species. Consequently, the number of gonadotropin cell types in sciaenid fishes remains unresolved.

Immunocharacteristics of the pituitary pars distalis cell types of the musk shrew, Suncus murinus, were studied by the unlabeled antibody enzyme technique, using peroxidase-antiperoxidase or avidin-biotin-peroxidase complex. The thyrotropin (TSH)-, gonadotropin (GTH)-, corticotropin (ACTH)-, prolactin (PRL)-, and growth hormone (GH)-secreting cells of the PD were identified on the basis of their immunoreactivity with different heterologous antisera. The TSH cells showed specific immunoreactivity with antisera against human (h) TSH beta and rat (r) TSH beta. Cells showing immunoreactivity with the antisera against hLH beta and ovine (o) LH beta were designated as GTH cells as no immunoreactivity was observed with antisera against hFSH beta and oFSH beta. The ACTH cells as well as the cells of the pars intermedia were revealed by anti-ACTH1-24 and anti-ACTH1-10 sera. Whereas the PRL cells were recognized by their immunoreactivity with antisera against hPRL and oPRL, the GH cells were identified with anti-hGH, anti-oGH, and anti-bovine (b) GH sera. TSH and GTH, TSH and ACTH, GTH and ACTH, ACTH and GH, ACTH and PRL, and GH and PRL cells were visualized in the same section using the dual immunoperoxidase technique. Comparison of the immunohistochemically identified cells with those described histochemically reveals several discrepancies, which expose the limitations of the latter techniques identifying adenohypophysial cells.

Immunocytochemistry and in situ hybridization were used to demonstrate beta thyroid-stimulating hormone (TSH) immunoreactivity and beta TSH messenger RNA (mRNA) in the same tissue section of rat pituitary. Sections (10 micronsP of 4% paraformaldehyde-fixed pituitaries from surgically thyroidectomized and intact male rats were first hybridized with a 32P-labelled complementary RNA beta TSH probe and were then immunostained for beta TSH. In both sets of animals beta TSH mRNA and beta TSH immunoreactivity were simultaneously localized to many thyrotrophs, although there was considerable heterogeneity in mRNA labelling intensity between individual cells. In hypothyroid rats more cells were positive with both probe and antiserum, and the intensity of mRNA labelling in most quantified by direct apposition of hybridized sections to autoradiography film, and the image grey levels were measured using an image analyser to convert them into equivalent amounts of radioactivity. In hypothyroid rats this amount was 22 times greater than in controls (control 4.77 +/- 0.84 attomoles mm-2; hypothyroid 107.06 +/- 14.2 attomoles mm-2; mean +/- SEM p less than 0.001). In situ hybridization has been used in combination with immunocytochemistry, to demonstrate beta TSH and mRNA and immunoreactivity at a cellular level in rats with different thyroid status.

The ultrastructure of each adenohypophysial secretory cell type was examined in pituitaries of adult female mollies (Poecilia latipinna) after various periods in vitro, and with varied medium osmotic pressure (OP), Na+, and Ca2+ concentrations. Prolactin (PRL) cells were markedly activated by 18 hr, and after 7 or 14 days were almost totally degranulated, with massive arrays of Golgi and RER. Reduction in OP, but not Na+ or Ca2+, caused an additional activation of PRL cells after periods of 18 hr or longer. Corticotroph (ACTH) cells became noticeably activated by 4 hr, and were possibly affected by OP, but not Na+ or Ca2+. Growth hormone (GH) cells were activated by 6 hr, and after 18 hr were quite degranulated with extensive arrays of RER. OP had no effect on GH cells before 3 days, when reduced OP appeared to cause an additional activation, with the appearance of large irregular secretory granule (SG)-like inclusions. Na+ and Ca2+ again had no effect. Gonadotrophic (GtH) cells appeared to be little affected by in vitro incubation; however, the very active cells from vitellogenic fish underwent a reduction in dilated RER after prolonged culture. Thyrotrophic (TSH) cells gradually became activated in vitro, but the response again varied with the sexual condition of the fish. Neither GtH nor TSH cells were affected by OP, Na+, or Ca2+. The findings are discussed in relation to hypothalamic control, via releasing/inhibiting factors, of adenohypophysial cell activity.