Induction of somatotroph differentiation in vivo by corticosterone administration during chicken embryonic development

Covariation between glucocorticoid levels and receptor expression modulates embryo development and postnatal phenotypes in gulls

The hypothalamic-pituitary-adrenocortical axis can translate, through glucocorticoid secretion, the prenatal environment to development to produce phenotypes that match prevailing environmental conditions. However, whether developmental plasticity is modulated by the interaction between circulating glucocorticoids and receptor expression remains unclear. Here, we tested whether covariation between plasma corticosterone (CORT) and glucocorticoid receptor gene (Nr3c1) expression in blood underlies embryonic developmental programming in yellow-legged gulls (Larus michahellis). We examined variations in circulating levels of CORT and the expression and DNA methylation patterns of Nr3c1 in response to two ecologically relevant prenatal factors: adult alarm calls (a cue of predator presence) and changes in prenatal light environment (a cue of competitive disadvantage). We then determined whether embryonic development and postnatal phenotypes were associated with CORT levels and Nr3c1 expression, and explored direct and indirect relationships between the prenatal environment, hormone-receptor covariation, and postnatal phenotypes. Prenatal exposure to alarm calls increased CORT levels and up-regulated Nr3c1 expression in gull chicks, while exposure to light cues reduced both hormone levels and receptor expression. Chicks prenatally exposed to alarm calls showed altered DNA methylation profiles in the Nr3c1 regulatory region, but patterns varied throughout the breeding season and between years. Moreover, our results suggest a negative relationship between DNA methylation and expression in Nr3c1 , at least at specific CpG sites. The interplay between circulating CORT and Nr3c1 expression affected embryo developmental timing and vocalizations, as well as hatchling mass and fitness-relevant behaviours. These findings provide a link between prenatal inputs, glucocorticoid function and phenotypic outcomes, suggesting that hormone-receptor interaction may underlie developmental programming in free-living animals.

Glucocorticoid receptor involvement in goose (Anas cygnoides) pituitary somatotroph differentiation induced by glucocorticoids during embryonic development

1. In this study, geese (Anas cygnoides) embryonic pituitary cells were cultured in vitro to determine if glucocorticoids could induce growth hormone (GH) expression and to investigate the molecular mechanisms involved in this process. 2. On embryonic day 15 (e15) and e20 the pituitary cells were treated with corticosterone (CORT), membrane impermeable bovine serum albumin-conjugate corticosterone (CORT-BSA), dexamethasone (DEX), and a glucocorticoid receptor (GR) antagonist (RU486) to detect responsiveness of somatotrophs to glucocorticoids. 3. Treatment with CORT, CORT-BSA, and DEX for as little as 6 h increased the percentage of GH-positive cells (P < 0.01) and increased GH mRNA expression (P < 0.01) in e15 goose pituitary cells compared to the control. CORT significantly increased the level of GH protein secreted from cultured e15 goose embryonic pituitary cells, and CORT-BSA increased GH secretion from e20 goose embryonic pituitary cells. 4. A significant increase was observed in the glucocorticoid receptor in GR transcription levels (P < 0.01) with CORT, CORT-BSA, and DEX treatment. Furthermore, the CORT-stimulated GH mRNA expression was completely negated by pre-treatment with RU486. 5. These findings demonstrate that glucocorticoids can stimulate somatotroph differentiation in vitro, as characterised by enhanced GH protein secretion andmRNA expression in cultured geese embryonic pituitary cells. The membrane GR was involved in pituitary somatotroph differentiation induced by glucocorticoids during the embryonic development of geese.

Corticosterone induces growth hormone expression in pituitary somatotrophs during goose embryonic development

Treatment of fetal rat and embryonic chicken with exogenous glucocorticoids induces premature differentiation of growth hormone (GH) secreting cells. The effect of corticosterone (CORT) on somatotroph differentiation was mostly studied in pituitary cells in vitro. Currently, there is no evidence for glucocorticoid-mediated induction of somatotroph differentiation during pituitary development in bird species other than chicken. In this study, we sought to find out if in ovo injection of corticosterone into developing goose embryos could induce premature increase of GH in somatotrophs. On embryonic day (e) 15, the albumen of fertile goose eggs was injected with 300 μl of 0.9% saline, 300 μl 5 × 10^-8M CORT, or 300 μl 5 × 10^-6 M CORT. Embryos were allowed to develop until e20 and e28 and isolated pituitaries were subjected to quantitative real-time PCR and immunocytochemistry to detect GH mRNA and protein, respectively. At e20 and e28, blood from chorioallantoic vessels was subjected to radioimmunoassay for analysis of plasma GH protein. In ovo administration of exogenous corticosterone brought about a 2.5-fold increase in the expression of GH mRNA and increased the in situ expression of GH protein in goose pituitary cells, and enhanced plasma GH levels compared to that of the respective controls at e20. These findings prove that administration of glucocorticoid could stimulate the expression of GH in somatotrophs during goose embryonic development. Our results suggest the probable involvement of membrane glucocorticoid receptor in the corticosterone mediated expression of GH. Together with previously published data, our results suggest that corticosterone mediated induction of GH expression during embryonic development is relatively conserved among different vertebrates.

Corticosterone induces growth hormone expression in pituitary somatotrophs during goose embryonic development

Treatment of fetal rat and embryonic chicken with exogenous glucocorticoids induces premature differentiation of growth hormone (GH) secreting cells. The effect of corticosterone (CORT) on somatotroph differentiation was mostly studied in pituitary cells in vitro. Currently, there is no evidence for glucocorticoid-mediated induction of somatotroph differentiation during pituitary development in bird species other than chicken. In this study, we sought to find out if in ovo injection of corticosterone into developing goose embryos could induce premature increase of GH in somatotrophs. On embryonic day (e) 15, the albumen of fertile goose eggs was injected with 300 μl of 0.9% saline, 300 μl 5 × 10^-8M CORT, or 300 μl 5 × 10^-6 M CORT. Embryos were allowed to develop until e20 and e28 and isolated pituitaries were subjected to quantitative real-time PCR and immunocytochemistry to detect GH mRNA and protein, respectively. At e20 and e28, blood from chorioallantoic vessels was subjected to radioimmunoassay for analysis of plasma GH protein. In ovo administration of exogenous corticosterone brought about a 2.5-fold increase in the expression of GH mRNA and increased the in situ expression of GH protein in goose pituitary cells, and enhanced plasma GH levels compared to that of the respective controls at e20. These findings prove that administration of glucocorticoid could stimulate the expression of GH in somatotrophs during goose embryonic development. Our results suggest the probable involvement of membrane glucocorticoid receptor in the corticosterone mediated expression of GH. Together with previously published data, our results suggest that corticosterone mediated induction of GH expression during embryonic development is relatively conserved among different vertebrates.

Glucocorticoid-induced osteoporosis: pathophysiological role of GH/IGF-I and PTH/VITAMIN D axes, treatment options and guidelines

Glucocorticoid-induced osteoporosis is the most frequent form of secondary osteoporosis caused by chronic exposure to glucocorticoid excess. Pathogenesis of glucocorticoid-induced osteoporosis is multifactorial including direct effects of glucocorticoids on bone cells and indirect effects of glucocorticoids on several neuroendocrine and metabolic pathways. Fragility fractures occur early in glucocorticoid-induced osteoporosis and anti-osteoporotic drugs along with calcium and vitamin D should be started soon after exposure to glucocorticoid excess. This paper summarizes some of the main topics discussed during the 9th Glucocorticoid-Induced Osteoporosis Meeting (Rome, April 2016) with a specific focus on the role of growth hormone/insulin-like growth factor-1 and parathyroid hormone/vitamin D axes in the pathogenesis of glucocorticoid-induced osteoporosis and the controversial aspects concerning therapeutic approach to skeletal fragility in this clinical setting.

Mechanisms involved in glucocorticoid induction of pituitary GH expression during embryonic development

Glucocorticoid hormones are involved in functional differentiation of GH-producing somatotrophs. Glucocorticoid treatment prematurely induces GH expression in mammals and birds in a process requiring protein synthesis and Rat sarcoma (Ras) signaling. The objective of this study was to investigate mechanisms through which glucocorticoids initiate GH expression during embryogenesis, taking advantage of the unique properties of chicken embryos as a developmental model. We determined that stimulation of GH expression occurred through transcriptional activation of GH, rather than enhancement of mRNA stability, and this process requires histone deacetylase activity. Through pharmacological inhibition, we identified the ERK1/2 pathway as a likely downstream Ras effector necessary for glucocorticoid stimulation of GH. However, we also found that chronic activation of ERK1/2 activity with a constitutively active mutant or stimulatory ligand reduced initiation of GH expression by glucocorticoid treatment. Corticosterone treatment of cultured embryonic pituitary cells increased ERK1/2 activity in an apparent cyclical manner, with a rapid increase within 5 minutes, followed by a reduction to near-basal levels at 3 hours, and a subsequent increase again at 6 hours. Therefore, we conclude that ERK1/2 signaling must be strictly controlled for maximal glucocorticoid induction of GH to occur. These results are the first in any species to demonstrate that Ras- and ERK1/2-mediated transcriptional events requiring histone deacetylase activity are involved in glucocorticoid induction of pituitary GH during embryonic development. This report increases our understanding of the molecular mechanisms underlying glucocorticoid recruitment of somatotrophs during embryogenesis and should provide insight into glucocorticoid-induced developmental changes in other tissues and cell types.

Developmental programming: the role of growth hormone

Developmental programming of the fetus has consequences for physiologic responses in the offspring as an adult and, more recently, is implicated in the expression of altered phenotypes of future generations. Some phenotypes, such as fertility, bone strength, and adiposity are highly relevant to food animal production and in utero factors that impinge on those traits are vital to understand. A key systemic regulatory hormone is growth hormone (GH), which has a developmental role in virtually all tissues and organs. This review catalogs the impact of GH on tissue programming and how perturbations early in development influence GH function.

Glucocorticoid-induced changes in gene expression in embryonic anterior pituitary cells

Within the anterior pituitary gland, glucocorticoids such as corticosterone (CORT) provide negative feedback to inhibit adrenocorticotropic hormone secretion and act to regulate production of other hormones including growth hormone (GH). The ontogeny of GH production during chicken embryonic and rat fetal development is controlled by glucocorticoids. The present study was conducted to characterize effects of glucocorticoids on gene expression within embryonic pituitary cells and to identify genes that are rapidly and directly regulated by glucocorticoids. Chicken embryonic pituitary cells were cultured with CORT for 1.5, 3, 6, 12, and 24 h in the absence and presence of cycloheximide (CHX) to inhibit protein synthesis. RNA was analyzed with custom microarrays containing 14,053 chicken cDNAs, and results for selected genes were confirmed by quantitative reverse transcription real-time PCR (qRT-PCR). Levels of GH mRNA were maximally induced by 6 h of CORT treatment, and this response was blocked by CHX. Expression of 396 genes was affected by CORT, and of these, mRNA levels for 46 genes were induced or repressed within 6 h. Pathway analysis of genes regulated by CORT in the absence of CHX revealed networks of genes associated with endocrine system development and cellular development. Eleven genes that were induced within 6 h in the absence and presence of CHX were identified, and eight were confirmed by qRT-PCR. The expression profiles and canonical pathways defined in this study will be useful for future analyses of glucocorticoid action and regulation of pituitary function.

Glucocorticoids and the regulation of growth hormone secretion

Glucocorticoids modulate the secretion of growth hormone (GH) by various and competing effects on the hypothalamus and pituitary gland. The final effects of this modulation depend on hormone concentrations and the duration of exposure. The traditional hypothesis is that chronically raised levels of glucocorticoids suppress the secretion of GH. However, a functional impairment of the GH reserve might also be observed in patients with low levels of glucocorticoids, such as those with secondary hypoadrenalism, which is consistent with the model of biphasic dose-dependent effects of glucocorticoids on the somatotropic axis. This Review updates our current understanding of the mechanisms underlying the effects of glucocorticoids on the secretion of GH and the clinical implications of the dual action of glucocorticoids on the GH reserve in humans. This Review will also address the potential diagnostic and therapeutic implications of GH for patients with a deficiency or excess of glucocorticoids.

Prenatal exposure to corticosterone impairs embryonic development and increases fluctuating asymmetry in chickens (Gallus gallus domesticus)

1. The level of corticosterone in fertilised eggs from hens (Gallus gallus domesticus) was manipulated experimentally to elucidate whether stress in laying hens is harmful to the chicks, as manifested by impaired survival and reduced growth, and whether bilateral asymmetry may represent an indicator of environmental stress in poultry. 2. Three hundred and fifty eggs were randomly divided into 4 groups; 1. untreated, 2. control, 3. 10 ng corticosterone/ml and 4. 20 ng corticosterone/ml. Each of the eggs in groups 2, 3 and 4 were injected with 100 microl ethanol-saline solution (25% ethanol in saline) containing 0, 0.6 and 1.2 microg corticosterone, respectively. After the injections, the final concentration of ethanol in the egg (albumen and yolk) was 0.03%, and the concentration of added corticosterone was 0, 10 and 20 ng/ml, respectively, in groups 2, 3 and 4. All the eggs were treated on developmental d 1. 3. Corticosterone injections resulted in greater embryonic mortality, earlier termination of foetal development and reduced growth. Moreover, chicks developing in eggs with an elevated concentration of corticosterone displayed reduced developmental stability as evidenced by increased fluctuating asymmetry (FA) in tarsus length. 4. In conclusion, an increased concentration of corticosterone in the egg was detrimental to survival and growth of the chicks. Prenatal stress also generated bilateral asymmetry, and illustrates the potential application of FA as an indicator of environmental stress in poultry.

Somatotropin response in vitro to corticosterone and triiodothyronine during chick embryonic development: Involvement of type I and type II glucocorticoid receptors

Corticosterone (CORT) can stimulate growth hormone (GH) secretion on embryonic day (e) 12 in the chicken. However, CORT failed to induce GH secretion on e20 in a single report, suggesting that regulation of GH production changes during embryonic development. Secretion in response to CORT during embryonic development is modulated by the thyroid hormones triiodothyronine (T(3)) and thyroxine (T(4)). Growth hormone responses on e12 involve both glucocorticoid (GR) and mineralocorticoid receptors (MR); however, involvement of MR has not been evaluated past e12. To further define changes in somatotroph responsiveness to CORT, pituitary cells obtained on e12-e20 were cultured with CORT alone and in combination with T(3) and GH-releasing hormone (GHRH). Growth hormone mRNA levels and protein secretion were quantified by quantitative real-time polymerase chain reaction (qRT-PCR) and radioimmunoassay (RIA), respectively. Corticosterone significantly increased GH mRNA and protein secretion on e12; however, mRNA concentration and protein secretion were unaffected on e20. Contributions of GR and MR in CORT responses were evaluated using GR and MR antagonists. Treatment with a GR-specific antagonist effectively blocked the CORT-induced increase in GH secretion on e12. The same treatment on e20 had no effect on GH secretion. These findings demonstrate that GR is directly involved in glucocorticoid stimulation of GH secretion at the time of somatotroph differentiation but is not regulatory at the end of embryonic development. We conclude that positive somatotroph responses to CORT are lost during chicken embryonic development and that GR is the primary regulator of CORT-induced GH secretion.

Hypophyseal corticosteroids stimulate somatotrope differentiation in the embryonic chicken pituitary gland

Although it is known that glucocorticoids induce differentiation of growth hormone (GH)-producing cells in rodents and birds, the effect of mineralocorticoids on GH mRNA expression and the origin of corticosteroids affecting somatotrope differentiation have not been elucidated. In this study, we therefore carried out experiments to determine the effect of mineralocorticoids on GH mRNA expression in the chicken anterior pituitary gland in vitro and to determine whether corticosteroids are synthesized in the chicken embryonic pituitary gland. In a pituitary culture experiment with E11 embryos, both corticosterone and aldosterone stimulated GH mRNA expression and increased the number of GH cells in both lobes of the pituitary gland in a dose-dependent manner. These effects of the corticosteroids were significantly reversed by pretreatment with mifepristone, a glucocorticoid receptor (GR) antagonist, or spironolactone, a mineralocorticoid receptor (MR) antagonist. Interestingly, an in vitro serum-free culture experiment with an E11 pituitary gland showed that the GH mRNA level spontaneously increased during cultivation for 2 days without any extra stimulation, and this increase in GH mRNA level was completely suppressed by metyrapone, a corticosterone-producing enzyme P450C11 inhibitor. Moreover, progesterone, the corticosterone precursor, also stimulated GH mRNA expression in the cultured chicken pituitary gland, and this effect was blocked by pretreatment with metyrapone. We also detected mRNA expression of enzymes of cytochrome P450 cholesterol side chain cleavage (P450scc) and 3beta-hydroxysteroid dehydrogenase1 (3beta-HSD1) in the developmental chicken pituitary gland from E14 and E18, respectively. These results suggest that mineralocorticoids as well as glucocorticoids can stimulate GH mRNA expression and that corticosteroids generated in the embryonic pituitary gland by intrinsic steroidogenic enzymes stimulate somatotrope differentiation.

Administration of adrenocorticotropic hormone during chicken embryonic development prematurely induces pituitary growth hormone cells

Treatment of fetal rats and embryonic chickens with exogenous glucocorticoids induces premature GH cell differentiation. However, it is unknown whether the developing adrenal gland is capable of mounting this response autonomously. The present study determined whether stimulation of the adrenal gland in developing chicken embryos through administration of ACTH could induce a premature increase in GH cells. We found that plasma corticosterone and ACTH levels increased between embryonic day (e) 11 and e17, consistent with GH cell (somatotroph) ontogeny. Injection of ACTH into eggs on e9, e10, or e11 increased somatotrophs on e14. In contrast, thyroid-stimulating hormone, CRH, alpha-MSH, GHRH, and TRH were ineffective. Culture of e11 pituitary cells with ACTH failed to induce somatotrophs, suggesting an indirect action of ACTH on GH cells in vivo. Intravenous administration of ACTH dramatically increased plasma levels of corticosterone within 1 h and increased the percentage of pituitary somatotrophs within 24 h. Although ACTH administration increased the relative abundance of pituitary GH cells, there was no effect on plasma levels of GH, IGF-I, or IGF-II, or in hepatic expression of IGF-I or IGF-II mRNA. We conclude that ACTH administration can increase the population of GH cells in the embryonic pituitary. However, this treatment alone does not lead to downstream activation of hepatic IGF production. These findings indicate that the embryonic adrenal gland, and ultimately anterior pituitary corticotrophs, may function to regulate pituitary GH cell differentiation during embryonic development.

Expression profiles of growth hormone-releasing hormone and growth hormone-releasing hormone receptor during chicken embryonic pituitary development

Growth hormone-releasing hormone (GHRH) and its receptor (GHRHR) have long been regarded as the critical molecules for the stimulation of growth hormone (GH) synthesis and release, as well as the regulation of pituitary somatotroph expansion in vertebrates. However, little is known about their expression in the embryonic pituitaries of birds. In this study, the full-length cDNA for chicken GHRHR was cloned from the chicken pituitary. It encodes 419 amino acids and shares high homology with that of the human, rat, and mouse. As in those in mammals, chicken GHRHR is predominantly expressed in the pituitary and weakly expressed in several extra-pituitary tissues including brain, pancreas, testis, and kidney, among 12 tissues examined. Using semiquantitative reverse transcription-PCR, we further examined the expression of GH, GHRH, and GHRHR during embryonic pituitary development. The expression of GHRHR on embryonic d 8 was much lower, but abundant expression was noticed as early as embryonic d 12. In contrast, the level of pituitary GHRH mRNA peaked on d 8 and declined sharply afterwards. Interestingly, unlike those of pituitary GHRH and GHRHR, the higher expression levels of GH appeared much later (from d 16 to 20). The differential expressions of GHRH, GHRHR, and GH in the developing embryonic pituitaries not only imply that pituitary-derived GHRH (or pituitary adenylate cyclase-activating polypeptide) and GHRHR may have a paracrine/autocrine role in the expansion of undifferentiated somatotroph precursor cells, but also suggest that GHRHR is likely to be involved in the somatotroph differentiation occurring at the later developmental stages.

Regulation of pituitary somatotroph differentiation by hormones of peripheral endocrine glands

Anterior pituitary somatotroph differentiation occurs during chick embryonic and rat fetal development. A number of findings support the hypothesis that differentiation of these growth hormone (GH) producing cells in the chick and the rat is regulated by adrenal glucocorticoids and thyroid hormones. Somatotroph differentiation can be induced in cultures of chick embryonic and rat fetal pituitary cells with adrenal glucocorticoids and this effect can be modulated by concomitant treatment with thyroid hormones. Plasma levels of thyroid hormones, corticosterone and adrenocorticotropic hormone increase during development, consistent with the ontogeny of somatotrophs. Treatment of chick embryos or rat fetuses with glucocorticoids in vivo induces premature somatotroph differentiation, indicating that the adrenal gland, and ultimately anterior pituitary corticotrophs, may function to regulate pituitary GH cell differentiation during development. Administration of thyroid hormones in vivo also increases somatotrophs prematurely, and administration of the thyroid hormone synthesis inhibitor methimazole inhibits somatotroph differentiation in vivo, suggesting that endogenous thyroid hormone synthesis contributes to normal somatotroph differentiation. Our working model for the regulation of somatotroph differentiation during normal development includes modulation by elements of the hypothalamo-pituitary-adrenal and hypothalamo-pituitary-thyroid axes. Additional research is reviewed defining the mechanism of action for these peripheral hormones in induction of pituitary GH gene expression during development.

Pituitary expression of type I and type II glucocorticoid receptors during chicken embryonic development and their involvement in growth hormone cell differentiation

Glucocorticoids can induce somatotroph differentiation in vitro and in vivo during chick embryonic and rat fetal development. In the present study, we identified the nuclear receptors involved in somatotroph differentiation and examined their ontogeny and cellular distribution during pituitary development in the chicken embryo. Several steroids were tested for their ability to induce GH cell differentiation. Only glucocorticoids and aldosterone were effective at low nanomolar concentrations, suggesting involvement of both type I (mineralocorticoid) and type II (glucocorticoid) receptors (MR and GR, respectively). ZK98299 and spironolactone (GR and MR antagonists, respectively) when used alone were unable to block corticosterone or aldosterone (2 nm)-induced somatotroph differentiation. However, ZK98299 and spironolactone in combination abolished corticosterone or aldosterone (2 nm)-induced somatotroph differentiation. When used separately, both antagonists attenuated induction of GH mRNA by corticosterone. Spironolactone alone blocked somatotroph differentiation induced by 0.2 nm corticosterone or aldosterone, indicating that corticosteroids at subnanomolar concentrations act only through the MR. GR protein was detected in pituitary extracts as early as embryonic d 8, whereas MR protein was readily detectable only around d 12. GR were expressed in greater than 95% of all pituitary cells, whereas MR were expressed in about 40% of all pituitary cells. Dual-label immunofluorescence revealed that the majority of somatotrophs on d 12 expressed MR. Given the high affinity of corticosteroids for MR and that corticosteroid concentrations during embryonic development are in the subnanomolar range, expression of MR may constitute a significant developmental event during somatotroph differentiation.

Ontogeny of the hypothalamo-pituitary-adrenocortical axis in the chicken embryo: a review

The embryo of the domestic fowl (Gallus domesticus) tenders one distinctive advantage over general mammalian models for investigating the development of the hypothalamo-pituitary-adrenocortical (HPA) axis. This is the relative simplicity with which the embryonic endocrine environment can be influenced without confounding maternal influences. The ease of direct manipulation of the embryonic endocrine system has facilitated analysis of the development and function of the HPA axis in the chick embryo. As the chick embryo develops, functional activation of the adrenal gland is regulated at three different levels: the adrenal gland itself, the anterior pituitary, and the hypothalamus. The adrenal gland appears capable of independent secretion of glucocorticoids from day 8 until shortly after day 14 of embryonic development, at which point the pituitary influences adrenocortical activity. Around the same age, the hypothalamic level of control also begins. The information covered in this review will describe the major steps in the development of the HPA axis in the chicken embryo and show that the chicken has an emblematic HPA neuroendocrine axis.

Thyroid hormones interact with glucocorticoids to affect somatotroph abundance in chicken embryonic pituitary cells in vitro

Our laboratory has reported that somatotroph differentiation occurs between d 14 and d 16 of chicken embryonic development and that corticosterone (CORT) can induce somatotroph differentiation at an earlier age in vitro and in vivo. The objective of the present study was to test for thyroid hormone-CORT interactions on somatotroph differentiation in vitro. Pituitary cells from d 11 chicken embryos were treated with CORT and thyroid hormones, and GH-producing somatotrophs were detected by reverse hemolytic plaque assays and immunocytochemistry. We found that thyroid hormones can act synergistically with CORT to further augment the abundance of somatotrophs in vitro but have little to no effect on their own. Both T(4) and T(3) could act synergistically with CORT to increase somatotroph abundance, but the effects of T(3) were biphasic, inhibiting CORT actions at higher concentrations. The monodeiodination inhibitor iopanoic acid inhibited the synergistic effect of T(4) on CORT induction of GH cells in vitro but not the synergistic effect of CORT and T(3) or the effect of CORT alone. Furthermore, T(3) treatment overcame the iopanoic acid-induced reduction in the T(4)-CORT effect. Our findings indicate that thyroid hormones act synergistically with CORT to further augment the abundance of somatotrophs in vitro and that conversion of T(4) to T(3) within the pituitary is involved in T(4) modulation of somatotroph abundance. Somatotroph differentiation during normal development may be regulated by complex interactions of hormones produced by the embryonic thyroid and adrenal glands.

Developmental changes in the expression of growth hormone-releasing hormone and pituitary adenylate cyclase-activating polypeptide in zebrafish

Growth hormone-releasing hormone (GHRH) and pituitary adenylate cyclase-activating polypeptide (PACAP) are structurally and functionally related members of the glucagon superfamily, a group of hormones important in development, growth, and metabolism. Our objectives were to determine the developmental expression pattern of the ghrh-pacap1 gene using the zebrafish model. The temporal and spatial expression pattern of the ghrh-pacap1 gene was examined by RT-PCR and in situ hybridization. In zebrafish, the ghrh-pacap1 mRNA transcript was expressed throughout development beginning at the transition between the blastula and gastrula periods. During midgastrulation, alternative splicing resulted in the generation of a novel transcript lacking the cryptic peptide. During the segmentation period, expression was localized to the neural tube, developing eye, and neural crest; strong expression was found in the developing cerebellum. Later in development, expression was localized in the hatching gland and developing pharyngeal arches. The temporal and spatial expression pattern of the ghrh-pacap1 transcript suggests that these hormones may modulate patterning during development.

Regulation of GHRH receptor gene expression in the neonatal and adult rat pituitary

The growth hormone releasing hormone (GHRH) receptor gene is essential for normal growth, and its expression is developmentally regulated. The factors that control GHRH receptor expression in the neonatal pituitary are not well understood. This study focuses on the regulation of GHRH receptor gene expression by thyroid hormone, glucocorticoids, insulin-like growth factor-I (IGF-I) and IGF-II in rat pituitary cell cultures. In newborn pituitaries, both T3 and hydrocortisone (24 h) caused a dose-dependent increase in GHRH receptor mRNA abundance, reaching levels 4.8-fold (P<0.001) and 6.5-fold (P<0.001) over corresponding controls. T3 and hydrocortisone also stimulated GHRH receptor expression in adult (70 day) pituitary cell cultures, consistent with our earlier findings. IGF-I treatment suppressed the inductive effects of T3 (P<0.02) and hydrocortisone (P<0.03) on GHRH receptor expression in adult pituitaries but not in neonatal pituitaries. Unlike IGF-I, IGF-II treatment had no effect on T3-induced or hydrocortisone-induced GHRH receptor expression in either neonates or adults. Taken together, these results indicate that (1) thyroid hormone and hydrocortisone act directly at the neonatal pituitary as potent stimulators of GHRH receptor gene expression, (2) IGF-I, but not IGF-II, acts at the pituitary to suppress GHRH receptor mRNA expression and (3) the effects of IGF-I on GHRH receptor gene expression are developmentally determined.

Regulation of chicken embryonic growth hormone secretion by corticosterone and triiodothyronine: evidence for a negative synergistic response

We reported that growth hormone (GH)-secreting cells differentiated by d 16 of chick embryonic development and that these somatotrophs were responsive to GH-releasing hormone and thyrotropin-releasing hormone. The present experiments evaluated effects of corticosterone and triiodothyronine (T3) on embryonic GH secretion. Anterior pituitary cells from embryonic day (e) 16, e18, and e20 were subjected to reverse hemolytic plaque assays (RHPAs) for GH in the absence or presence of corticosterone or T3. Corticosterone increased GH secretion from embryonic somatotrophs, an effect particularly evident on e16 and e18. T3 decreased GH secretion on e16, while no effect of T3 was significant on e18 or e20. Next, pituitary cells were subjected to RHPAs with T3 and corticosterone alone or in combination. Combined treatment with these hormones suppressed GH secretion from e16, e18, and e20 somatotrophs to levels below those found under basal conditions. We conclude that corticosterone can stimulate GH secretion in vitro at all embryonic ages tested. Furthermore, T3 can suppress basal GH secretion on e16, and the combination of T3 and corticosterone can suppress GH secretion at all ages. These findings indicate that GH secretion during the end of chicken embryonic development may be regulated by the interactions of endogenous glucocorticoids and thyroid hormones that increase prior to hatching.

Avian somatotrophs: differentiation, morphology, distribution, and regulation

Growth hormone (GH) is primarily synthesized, stored, and released by pituitary somatotrophs. These cells comprise a highly labile population that continuously undergoes proliferation, differentiation, and morphogenesis in response to changing physiological stimuli. They are also functionally and morphologically heterogeneous with distinct spatial and temporal distribution within the pituitary gland. The characteristics of these cells are discussed in this brief review.Key words: somatotroph, aves, growth hormone, adenohypophysis.