Identification of the somatostatin receptor subtypes involved in regulation of growth hormone secretion in chickens

SSTR2 Mediates the Inhibitory Effect of SST/CST on Lipolysis in Chicken Adipose Tissue

Somatostatin shows an anti-lipolytic effect in both chickens and ducks. However, its molecular mediator remains to be identified. Here, we report that somatostatin type 2 receptor (SSTR2) is expressed at a high level in chicken adipose tissue. In cultured chicken adipose tissue, the inhibition of glucagon-stimulated lipolysis by somatostatin was blocked by an SSTR2 antagonist (CYN-154086), supporting an SSTR2-mediated anti-lipolytic effect. Furthermore, a significant pro-proliferative effect was detected in SST28-treated immortalized chicken preadipocytes (ICP-1), and this cell proliferative effect may be mediated through the MAPK/ERK signaling pathway activated by SSTR2. In summary, our results demonstrate that SSTR2 may regulate adipose tissue development by affecting the number and volume of adipocytes in chickens.

Melatonin mediates monochromatic light-induced expression of somatostatin in the hypothalamus and pituitary of chicks

Melatonin (MEL) plays an important role in regulating growth and development of organisms and the cellular metabolism. This study was conducted to explore the role of MEL in mediating monochromatic light-induced secretion of somatostatin (SST) in the hypothalamus and pituitary in chicks. Pinealectomy models of newly hatched broilers were exposed to white (WL), red (RL), green (GL), and blue (BL) lights. The results showed that SST immunoreactive neurons and fibers were distributed in the hypothalamus. SST and SST receptor 2 (SSTR2) mRNA and protein levels in the hypothalamus and pituitary were higher in chicks exposed to RL than in chicks exposed to GL and BL. However, after pinealectomy, the mRNA and protein levels of SST and SSTR2 in the hypothalamus and pituitary in the different light groups were increased, and the differences between the groups disapeared. The expression trend of SSTR5 mRNA in the pituitary was the idential to that of SSTR2 mRNA in the pituitary. In vitro, exogenous SST inhibited growth hormone (GH) secretion, and selective antogonists of SSTR2 and SSTR5 promoted GH secretion. Selective antogonists of the melatonin receptor 1b (Mel1b) and Mel1c increased the relative concentrations of SST in the adenohypophysis cells. These results indicated that monochromatic light affects the expression of SST in chick hypothalamus and pituitary. MEL, via Mel1b and Mel1c, decreased SST secretion under GL, which was associated with the inhibition of SST, SSTR2, and SSTR5 in adenohypophysis cells.

In ovo green light photostimulation during the late incubation stage affects somatotropic axis activity

Targeted green light photostimulation during the last stage of broiler incubation increases expression of the somatotropic axis. The purpose of this study was to further shorten the in ovo green light photostimulation and determine the critical age for photostimulation in broilers embryos, as a future strategy for broiler incubation. Fertile broilers eggs (n = 420) were divided into 5 treatment groups. The first group was incubated under standard conditions (in the dark) as the negative control group. The second was incubated under intermittent monochromatic green light using light-emitting diode lamps with an intensity of 0.1 W/m² at shell level from embryonic day (ED) 0 of incubation until hatch, as a positive control. The third, fourth, and fifth groups were incubated under intermittent monochromatic green light from ED 15, 16, and 18 of incubation, respectively, until hatch. All treatment groups showed elevated somatotropic axis expression compared with the negative control, with the group incubated under monochromatic green light from ED 18 until hatch showing results closest to the positive control. This suggests that broiler embryos can be exposed to in ovo green light photostimulation from a late stage of incubation (when transferring the eggs to the hatchery) and exhibit essentially the same outcome as obtained by photostimulation during the entire incubation period.

Thyrotropin-Releasing Hormone (TRH) and Somatostatin (SST), but not Growth Hormone-Releasing Hormone (GHRH) nor Ghrelin (GHRL), Regulate Expression and Release of Immune Growth Hormone (GH) from Chicken Bursal B-Lymphocyte Cultures

It is known that growth hormone (GH) is expressed in immune cells, where it exerts immunomodulatory effects. However, the mechanisms of expression and release of GH in the immune system remain unclear. We analyzed the effect of growth hormone-releasing hormone (GHRH), thyrotropin-releasing hormone (TRH), ghrelin (GHRL), and somatostatin (SST) upon GH mRNA expression, intracellular and released GH, Ser133-phosphorylation of CREB (pCREB^S133), intracellular Ca²⁺ levels, as well as B-cell activating factor (BAFF) mRNA expression in bursal B-lymphocytes (BBLs) cell cultures since several GH secretagogues, as well as their corresponding receptors (-R), are expressed in B-lymphocytes of several species. The expression of TRH/TRH-R, ghrelin/GHS-R1a, and SST/SST-Rs (Subtypes 1 to 5) was observed in BBLs by RT-PCR and immunocytochemistry (ICC), whereas GHRH/GHRH-R were absent in these cells. We found that TRH treatment significantly increased local GH mRNA expression and CREB phosphorylation. Conversely, SST decreased GH mRNA expression. Additionally, when added together, SST prevented TRH-induced GH mRNA expression, but no changes were observed in pCREB^S133 levels. Furthermore, TRH stimulated GH release to the culture media, while SST increased the intracellular content of this hormone. Interestingly, SST inhibited TRH-induced GH release in a dose-dependent manner. The coaddition of TRH and SST decreased the intracellular content of GH. After 10 min. of incubation with either TRH or SST, the intracellular calcium levels significantly decreased, but they were increased at 60 min. However, the combined treatment with both peptides maintained the Ca²⁺ levels reduced up to 60-min. of incubation. On the other hand, BAFF cytokine mRNA expression was significantly increased by TRH administration. Altogether, our results suggest that TRH and SST are implicated in the regulation of GH expression and release in BBL cultures, which also involve changes in pCREB^S133 and intracellular Ca²⁺ concentration. It is likely that TRH, SST, and GH exert autocrine/paracrine immunomodulatory actions and participate in the maturation of chicken BBLs.

In-ovo green light photostimulation during different embryonic stages affect somatotropic axis

Previous studies demonstrated that in-ovo photostimulation with monochromatic green light increased the somatotropic axis expression in broilers embryos. The objective of the current study was to detect the critical period for in-ovo GL photostimulation, in order to find the optimal targeted photostimulation period during the incubation process. Three hundred thirty-six fertile broiler eggs were divided into 4 groups. The first group was incubated under dark conditions as a negative control. The second incubated under intermittent monochromatic green light using light-emitting diode (LED) lamps with an intensity of 0.1 W\m2 at shell level from d 0 of the incubation as a positive control. The third group incubated under intermittent monochromatic green light from d 10 of the incubation. The last group incubated under intermittent monochromatic green light from d 15 of the incubation. In-ovo green light photostimulation from embryonic d 0 (ED0) increased plasma growth hormone (GH), as well as hypothalamic growth hormone releasing hormone (GHRH) and liver growth hormone receptor (GHR) and insulin-like growth factor-1 (IGF-1) mRNA levels. In-ovo green light photostimulation from ED10 increased the GH plasma levels compared to the negative control group, without affecting somatotropic axis mRNA genes expressions of GHRH, GHR, and IGF-1. In-ovo green light photostimulation from ED15 caused an increase in both the plasma GH levels and the somatotropic axis mRNA genes expressions of GHRH, GHR, and IGF-1, compared to the negative control group. These results suggest that the critical period of somatotropic axis acceleration by GL photostimulation start at 15 d of incubation.

Characterization of Neuropeptide B (NPB), Neuropeptide W (NPW), and Their Receptors in Chickens: Evidence for NPW Being a Novel Inhibitor of Pituitary GH and Prolactin Secretion

The 2 structurally and functionally related peptides, neuropeptide B (NPB) and neuropeptide W (NPW), together with their receptor(s) (NPBWR1/NPBWR2) constitute the NPB/NPW system, which acts mainly on the central nervous system to regulate many physiological processes in mammals. However, little is known about this NPB/NPW system in nonmammalian vertebrates. In this study, the functionality and expression of this NPB/NPW system and its actions on the pituitary were investigated in chickens. The results showed that: 1) chicken NPB/NPW system comprises an NPB peptide of 28 amino acids (cNPB28), an NPW peptide of 23 or 30 amino acids (cNPW23/cNPW30), and their 2 receptors (cNPBWR1 and cNPBWR2), which are highly homologous to their human counterparts. 2) Using a pGL3-CRE-luciferase reporter system, we demonstrated that cNPBWR2 expressed in Chinese hamster ovary cells can be potently activated by cNPW23 (not cNPB28), and its activation inhibits the intracellular cAMP signaling pathway, whereas cNPBWR1 shows no response to peptide treatment, suggesting a crucial role of cNPBWR2 in mediating cNPW/cNPB actions. 3) Quantitative real-time PCR revealed that cNPW and cNPB are widely expressed in chicken tissues, including hypothalamus, whereas cNPBWR1 and cNPBWR2 are mainly expressed in brain or pituitary. 4) In accordance with abundant cNPBWR2 expression in pituitary, cNPW23 could dose dependently inhibit GH and prolactin secretion induced by GHRH and vasoactive intestinal polypeptide, respectively, in cultured chick pituitary cells, as monitored by Western blotting. Collectively, our data reveal a functional NPB/NPW system in birds and offer the first proof that NPW can act directly on pituitary to inhibit GH/prolactin secretion in vertebrates.

Identification of the receptors for somatostatin (SST) and cortistatin (CST) in chickens and investigation of the roles of cSST28, cSST14, and cCST14 in inhibiting cGHRH1-27NH2-induced growth hormone secretion in cultured chicken pituitary cells

Somatostatin receptors (SSTRs) are proposed to mediate the actions of somatostatin (SST) and its related peptide, cortistatin (CST), in vertebrates. However, the identity, functionality, and tissue expression of these receptors remain largely unknown in most non-mammalian vertebrates including birds. In this study, five SSTRs (named cSSTR1, cSSTR2, cSSTR3, cSSTR4, cSSTR5) were cloned from chicken brain by RT-PCR. Using a pGL3-CRE-luciferase reporter system, we demonstrated that activation of each cSSTR expressed in CHO cells by cSST28, cSST14 and cCST14 treatment could inhibit forskolin-induced luciferase activity of CHO cells, indicating the functional coupling of all cSSTRs to Gi protein(s). Interestingly, cSSTR1-4 expressed in CHO cells could be activated by cSST28, cSST14 and cCST14 with high potencies, suggesting that they may function as the receptors common for these peptides. In contrast, cSSTR5 could be potently activated by cSST28 only, indicating that it is a cSST28-specific receptor. Using RT-PCR, wide expression of cSSTRs was detected in chicken tissues including pituitary. In accordance with their expression in pituitary, cSST28, cSST14, and cCST14 were demonstrated to inhibit basal and novel cGHRH1-27NH2-induced GH secretion in cultured chicken pituitary cells dose-dependently (0-10nM) by Western blot analysis, suggesting the involvement of cSSTR(s) common for these peptides in mediating their inhibitory actions. Collectively, our study establishes a molecular basis to elucidate the roles of SST/CST in birds and provide insights into the roles of SST/CST in vertebrates, such as their conserved actions on pituitary.

The evolution of vertebrate somatostatin receptors and their gene regions involves extensive chromosomal rearrangements

Background

Somatostatin and its related neuroendocrine peptides have a wide variety of physiological functions that are mediated by five somatostatin receptors with gene names SSTR1-5 in mammals. To resolve their evolution in vertebrates we have investigated the SSTR genes and a large number of adjacent gene families by phylogeny and conserved synteny analyses in a broad range of vertebrate species.

Results

We find that the SSTRs form two families that belong to distinct paralogons. We observe not only chromosomal similarities reflecting the paralogy relationships between the SSTR-bearing chromosome regions, but also extensive rearrangements between these regions in teleost fish genomes, including fusions and translocations followed by reshuffling through intrachromosomal rearrangements. These events obscure the paralogy relationships but are still tractable thanks to the many genomes now available. We have identified a previously unrecognized SSTR subtype, SSTR6, previously misidentified as either SSTR1 or SSTR4.

Conclusions

Two ancestral SSTR-bearing chromosome regions were duplicated in the two basal vertebrate tetraploidizations (2R). One of these ancestral SSTR genes generated SSTR2, -3 and -5, the other gave rise to SSTR1, -4 and -6. Subsequently SSTR6 was lost in tetrapods and SSTR4 in teleosts. Our study shows that extensive chromosomal rearrangements have taken place between related chromosome regions in teleosts, but that these events can be resolved by investigating several distantly related species.

Somatostatin dramatically stimulates growth hormone release from primate somatotrophs acting at low doses via somatostatin receptor 5 and cyclic AMP

Somatostatin and cortistatin have been shown to act directly on pituitary somatotrophs to inhibit growth hormone (GH) release. However, previous results from nonprimate species indicate that these peptides can also directly stimulate GH secretion, at low concentrations. The relevance of this phenomenon in a nonhuman primate model was investigated in the present study by testing the impact of somatostatin/cortistatin on GH release in primary pituitary cell cultures from baboons. High doses (> 10(-10) m) of somatostatin/cortistatin did not alter basal GH secretion but blocked GH-releasing hormone (GHRH)- and ghrelin-induced GH release. However, at low concentrations (10(-17)-10(-13) m), somatostatin/cortistatin dramatically stimulated GH release to levels comparable to those evoked by GHRH or ghrelin. Use of somatostatin receptor (sst) specific agonists/antagonists, and signal transduction blockers indicated that sst2 and sst1 activation via intact adenylate cylcase and mitogen-activated protein kinase systems mediated the inhibitory actions of high-concentration somatostatin. By contrast, the stimulatory actions of low-dose somatostatin on GH release were mediated by sst5 signalling through adenylate cylcase/cAMP/protein kinase A and intracellular Ca(2+) pathways, and were additive with ghrelin (not GHRH). Notably, low-concentrations of somatostatin, similar to sst5-agonists, inhibited prolactin release. These results clearly demonstrate that the ultimate impact of somatostatin/cortistatin on hormone release is dose-dependent, cell type-selective and receptor-specific, where the stimulatory effects of low-concentration somatostatin/cortistatin on GH release extend to primates, thereby supporting the notion that this action is relevant in regulating GH secretion in humans.

Nanobiology and physiology of growth hormone secretion

Growth hormone (GH) secretion is controlled by hypothalamic releasing hormones from the median eminence together with hormones and neuropeptides produced by peripheral organs. Secretion of GH involves movement of secretory vesicles along microtubules, transient ‘docking’ with the porosome in the cell membrane and subsequent release of GH. Release of GH is stimulated by GH releasing hormone (GHRH) and inhibited by somatostatin (SRIF). Ghrelin may be functioning to stimulate GH release from somatotropes acting via the GH secretagogue (GHS) receptor (GHSR). However, recent physiological studies militate against this. In addition, ghrelin does influence GH release acting within the hypothalamus. Release of GH from the somatotropes involves the GH-containing secretory granules moving close to the cell surface followed by transitory fusion of the secretory granules with the porosomes located in multiple secretory pits in the cell membrane. Other peptides/proteins can influence GH secretion, particularly in species of non-mammalian vertebrates.

Ontogenic characterization of gene expression in the developing neuroendocrine system of the chick

The neuroendocrine system consists of five major hypothalamic-pituitary hormone axes that regulate several important metabolic processes, and it develops in all vertebrates during embryogenesis. In order to define initiation and establishment of these five axes, mRNA expression profiles of hypothalamic releasing and release-inhibiting factors, their pituitary receptors, and pituitary hormones were characterized during the second half of embryogenesis and first week post-hatch in the chick. Axis initiation was defined as the age when pituitary hormone mRNA levels began to increase substantially, and establishment was defined as the age when mRNA for all components had reached maximum expression levels. The adrenocorticotropic axis appears established by e12, as there were no major increases in gene expression after that age. Hypothalamic thyrotropin-releasing hormone and pituitary thyroid-stimulating hormone β-subunit increased between e10 and e18, indicating establishment of the thyrotropic axis during this period. Pituitary growth hormone substantially increased on e16, and hypothalamic growth hormone-releasing hormone did not increase until e20, indicating that somatotropic axis activity is established late in embryonic development. Lactotropic axis initiation is evident just prior to hatch, as pituitary prolactin and vasoactive intestinal peptide receptor 1 did not increase until e18 and e20, respectively. Hypothalamic gonadotropin-releasing hormone 1 increased after hatch, and pituitary luteinizing hormone β-subunit expression remained low until d3, indicating the gonadotropic axis is not fully functional until after hatching. This study is the first to characterize major hypothalamic and pituitary components of all five neuroendocrine axes simultaneously and considerably increases our understanding of neuroendocrine system establishment during development.

Role of endogenous somatostatin in regulating GH output under basal conditions and in response to metabolic extremes

Somatostatin (SST) was first described over 30 years ago as a hypothalamic neuropeptide which inhibits GH release. Since that time a large body of literature has accumulated describing how endogenous SST mediates its effects on GH-axis function under normal conditions and in response to metabolic extremes. This review serves to summarize the key findings in this field with a focus on recent progress, much of which has been made possible by the availability of genetically engineered mouse models and SST receptor-specific agonists.

Expression and localization of growth hormone and its receptors in the chicken ovary during sexual maturation

Roles of pituitary growth hormone (GH) in female reproduction are well established. Autocrine and/or paracrine actions of GH in the mammalian ovary have additionally been proposed, although whether the ovary is an extra-pituitary site of GH expression in the laying hen is uncertain. This possibility has therefore been assessed in the ovaries of Hy-Line hens before (between 10-16 weeks of age) and after (week 17) the onset of egg laying. Reverse transcription/polymerase chain reaction (RT-PCR) analysis has consistently detected a full-length (690 bp) pituitary GH cDNA in ovarian stroma from 10 weeks of age, although GH expression is far lower than that in the pituitary gland or hypothalamus. GH mRNA is also present in small (>1-4 mm diameter) follicles after their ontogenetic appearance at 14 weeks of age and in all other developing follicles after 16 weeks of age (>4-30 mm diameter). Immunoreactivity for GH is similarly present in the ovarian stroma from 10 weeks of age and in small (<4 mm diameter) and large (>4-30 mm) follicles from 14 and 16 weeks of age, respectively. The relative intensity of GH staining in the ovarian follicles is consistently greater in the granulosa cells than in the thecal cells and is comparable with that in the follicular epithelium. A 321-bp fragment of GH receptor (GHR) cDNA, coding for the intracellular domain of the receptor, has also been detected by RT-PCR in the ovary and is present in stromal tissue by 10 weeks of age, in small follicles (<4 mm diameter) by 14 weeks of age, and in larger follicles (>4-30 mm diameter) from 16 weeks. GHR immunoreactivity has similarly been detected, like GH, in the developing ovary and in all follicles and is more intense in granulosa cells than in the theca interna or externa. The expression and location of the GH gene therefore parallels that of the GHR gene during ovarian development in the laying hen, as does the appearance of GH and GHR immunoreactivity. These results support the possibility that GH has autocrine and/or paracrine actions in ovarian function prior to and after the onset of lay in hens.

Functional analysis of neuropeptides in avian thymocyte development

The function of lymphoid organs and immune cells is often modulated by peptides and hormones produced by the neuroendocrine and immune systems. We have previously reported the intrathymic expression of neuropeptides in the thymus of different species and that neuropeptides can influence murine thymocyte development in vitro. To further explore the evolutionary nature of neuroendocrine interactions in the thymus, we identified the expression of calcitonin-gene-related peptide, neuropeptide Y, somatostatin (SOM), substance P and vasointestinal polypeptide, as well as their receptors on chicken thymic epithelial cells (TEC) and thymocytes by immunofluorescence and reverse transcription polymerase chain reaction (RT-PCR). All the studied neuropeptides and their receptors were found to be expressed in both TEC and thymocytes, suggesting that intrathymic neuroendocrine interactions may take place within the avian thymus. In order to elucidate whether such interactions play a role in avian thymocyte development, neuropeptides and their antagonists were added to embryonic thymus organ cultures and found to influence chicken thymopoiesis. In particular, an antagonist of SOM increased the proportion of double-positive thymocytes, while SOM itself appeared to inhibit the early stages of thymocyte development. Taken together, these data provide further evidence to suggest that neuropeptides play a conserved role in vertebrate thymocyte development.

Feedback control of thyrotropin secretion in the chicken: thyroid hormones increase the expression of hypophyseal somatostatin receptor types 2 and 5

We have studied the involvement of the somatostatin receptor type 5 (SSTR5) in the control of thyrotropin (TSH) release in the chicken. Hypothalamic somatostatin (SS-14) is known to inhibit both thyrotropin-releasing hormone (TRH)- and corticotropin-releasing hormone (CRH)-induced TSH secretion. Studies using receptor-specific agonists have indicated that the inhibitory effect of SS-14 on TRH-induced TSH release is mediated by SSTR2 and SSTR5. Using the same agonists, we were able to demonstrate the involvement of SSTR5 in the inhibition of the in vitro CRH-induced TSH secretion by SS-14. Subsequently, we determined hypophyseal SSTR5 mRNA expression during the last week of embryonic development using real-time PCR. SSTR5 mRNA levels were low until day 19 of incubation, but between day 19 and hatching SSTR5 mRNA expression increased 3-fold. Since this increase coincides with the increasing plasma T(3) levels towards hatching, and a similar ontogenetic expression pattern was found for SSTR2, we quantified hypophyseal SSTR2 and SSTR5 mRNA expression levels in chicken embryos treated with thyroid hormones. Injection of thyroid hormones was indeed found to increase the expression of both mRNAs significantly. We hypothesize that the negative feedback exerted by the increasing plasma T(3) levels towards hatching is at least in part mediated by an increased expression of SSTR2 and SSTR5.

Porcine somatostatin receptor 2 displays typical pharmacological sst2 features but unique dynamics of homodimerization and internalization

Somatostatin (SRIF) exerts its multiple actions, including inhibition of GH secretion and of tumoral growth, through a family of five receptor subtypes (sst1-sst5). We recently reported that an sst2-selective agonist markedly decreases GH release from pig somatotropes, suggesting important roles for this scarcely explored receptor, psst2. Here, functional expression of psst2 in Chinese hamster ovary-K1 and human embryonic kidney-293-AD cell lines was employed to determine its pharmacological features and functional ability to reduce cAMP, and to examine its homodimerization and internalization dynamics in real time in single living cells. Results show that psst2 is a high-affinity receptor (dissociation constant = 0.27 nM) displaying a typical sst2 profile (nM affinity for SRIF-14> or =SRIF-28>cortistatin>MK678>octreotide) and high selectivity (EC(50) = 1.1 nM) for the sst2 agonist l-779,976, but millimolar or undetectable affinity to other sst-specific agonists (sst3>sst1>sst5>>>sst4). Accordingly, SRIF dose-dependently inhibited forskolin-stimulated cAMP with high potency (EC(50) = 6.55 pm) and modest efficacy (maximum 29.1%) via psst2. Cotransfection of human embryonic kidney-293 and Chinese hamster ovary-K1 cells with two receptor constructs modified with distinct fluorescent tags (psst2-YFP/psst2-CFP) enabled fluorescence resonance energy transfer measurement of physical interaction between psst2 receptors and also receptor internalization in single living cells. This revealed that under basal conditions, psst2 forms constitutive homodimers/homomultimers, which dissociate immediately (11 sec) upon SRIF binding. Interestingly, contrary to human sst2, psst2 rapidly reassociates (110.5 sec) during a subsequent process that temporally overlaps with receptor internalization (half-maximal = 95.1 sec). Therefore, psst2 is a potent inhibitory receptor displaying a unique set of interrelated dynamic features of agonist-dependent dimerization, dissociation, internalization, and reassociation, a cascade of events that might be critical for receptor function.

Identification of the chicken growth hormone-releasing hormone receptor (GHRH-R) mRNA and gene: regulation of anterior pituitary GHRH-R mRNA levels by homologous and heterologous hormones

GHRH stimulates GH secretion in chickens as in mammals. However, nothing is known about the chicken GHRH receptor (GHRH-R). Here we report the cDNA sequence of chicken GHRH-R. Comparison of the cDNA sequence with the chicken genome localized the GHRH-R gene to chicken chromosome 2 and indicated that the chicken GHRH-R gene consists of 13 exons. Expression of all exons was confirmed by RT-PCR amplification of pituitary mRNA. The amino acid sequence predicted by the GHRH-R cDNA is homologous to that in other vertebrates and contains seven transmembrane domains and a conserved hormone-binding domain. The predicted size of the GHRH-R protein (48.9 kDa) was confirmed by binding of (125)I-GHRH to chicken pituitary membranes and SDS-PAGE. GHRH-R mRNA was readily detected by RT-PCR in the pituitary but not in the hypothalamus, total brain, lung, adrenal, ovary, or pineal gland. Effects of corticosterone (CORT), GHRH, ghrelin, pituitary adenylate cyclase-activating peptide, somatostatin (SRIF), and TRH on GHRH-R and GH gene expression were determined in cultures of chicken anterior pituitary cells. GHRH-R and GH mRNA levels were determined by quantitative real-time RT-PCR. Whereas all treatments affected levels of GH mRNA, only CORT, GHRH, and SRIF significantly altered GHRH-R mRNA levels. GHRH-R gene expression was modestly increased by GHRH and suppressed by SRIF at 4 h, and CORT dramatically decreased levels of GHRH-R mRNA at 72 h. We conclude that adrenal glucocorticoids may substantially impact pituitary GH responses to GHRH in the chicken through modulation of GHRH-R gene expression.

Hypothalamic control of the thyroidal axis in the chicken: over the boundaries of the classical hormonal axes

The pituitary gland, occupying a central position in the hypothalamo-pituitary thyroidal axis, produces thyrotropin (TSH), which is known to stimulate the thyroid gland to synthetize and release its products, thyroid hormones. TSH is produced by a specific cell population in the pituitary, the so-called thyrotropes. Their secretory activity is controlled by the hypothalamus, releasing both stimulatory and inhibitory factors that reach the pituitary through a portal system of blood vessels. Based on early experiments in mammals, thyrotropin-releasing hormone (TRH) is generally mentioned as the main stimulator of the thyrotropes. During the past few decades, it has become clear that the hypophysiotropic function of the hypothalamus is more complex, with different hormonal axes interacting with each other. In the chicken, it was found that not only TRH, but also corticotropin-releasing hormone (CRH), the main stimulator of corticotropin release, is a potent stimulator of TSH secretion. Somatostatin (SRIH), a hypothalamic factor known for its inhibitory effect on growth hormone secretion, was demonstrated to blunt the TSH response to TRH and CRH. In this review we summarize the latest studies concerning the "interaxial" hypothalamic control of TSH release in the chicken, with a special emphasis on the molecular components of these control mechanisms. It remains to be demonstrated if these findings could also be extrapolated to other species or classes of vertebrates.

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.

Different somatostatin receptor subtypes are operating in the brain of the teleost fish,Coris julis

Characterization of somatostatinergic (sst) neuronal activity through the application of nonpeptidyl agonists L-779,976 and L-817,818 which are highly specific for the sst receptors (sstr) sstr(2) and sstr(5), respectively, shows for the first time that sstr2, 5-like subtypes are the two major sstr subtypes operating in the brain of the teleost sea wrasse, Coris julis. A somewhat high but heterogeneous distribution pattern (> 30 < 180 fmol/mg wet tissue weight) of neurons expressing sstr2, 5 was reported in the different diencephalic regions plus in mesencephalon and telencephalon while low values were obtained in the cerebellum. Application of the above nonpeptidyl agonists permitted us to identify sstr2-like as the predominant subtype in telencephalic areas such as the entopeduncular nucleus (E) and postcommissural nucleus of the ventral telencephalon (Vp) as well as in hypothalamic and thalamic areas. At the same time high levels of neurons expressing sstr5-like, that greatly overlap those of sstr2-like in the diencephalic areas such as the anteroventral part of the preoptic nucleus (NPOav), the dorsal habenular nucleus (NHd) and the ventrolateral thalamic nucleus (VL), indicate that sstr2-like is very likely not the only sstr subtype acting in this fish brain. The predominance of sstr5-like in other brain areas is confirmed by the high quantities of this subtype in mesencephalic areas such as the torus longitudinalis (TLo). Overall, the discriminately differing densities of neurons expressing both subtypes seem to point to this system as a key molecular basis accounting for the distinct neurophysiological and behavioral sst-dependent activities in Coris julis.

Immunolocalisation of serotonin, gastrin, somatostatin and glucagon in entero-endocrine cells of the goose (Anser anser)

The processes of digestion in the avian gastrointestinal tract depend on sophisticated control systems that co-ordinate secretion of digestive juices and movement of the luminal contents. In the current study, the distribution of serotonin-, gastrin-, glucagon- and somatostatin-immunoreactive endocrine cells was investigated by immunocytochemical methods in the intestinal tract of the goose. The number of cells immunoreactive for each antiserum was evaluated in different regions of the intestinal tract. Serotonin-, glucagon- and somatostatin-immunoreactive endocrine cells were seen throughout the intestinal tract, but somatostatin-immunoreactive cells were not detected in the colon of the goose. Gastrin-immunoreactive cells were detected only in the duodenum, jejunum and colon mucosa. It is concluded that the distribution pattern of the entero-endocrine cells in the goose is similar to that of most of the mammalian and other poultry species.

Involvement of thyrotropin-releasing hormone receptor, somatostatin receptor subtype 2 and corticotropin-releasing hormone receptor type 1 in the control of chicken thyrotropin secretion

Thyrotropin or thyroid-stimulating hormone (TSH) secretion in the chicken is controlled by several hypothalamic hormones. It is stimulated by thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH), whereas somatostatin (SRIH) exerts an inhibitory effect. In order to determine the mechanism by which these hypothalamic hormones modulate chicken TSH release, we examined the cellular localization of TRH receptors (TRH-R), CRH receptors type 1 (CRH-R1) and somatostatin subtype 2 receptors (SSTR2) in the chicken pars distalis by in situ hybridization (ISH), combined with immunological staining of thyrotropes. We show that thyrotropes express TRH-Rs and SSTR2s, allowing a direct action of TRH and SRIH at the level of the thyrotropes. CRH-R1 expression is virtually confined to corticotropes, suggesting that CRH-induced adrenocorticotropin release is the result of a direct stimulation of corticotropes, whereas CRH-stimulated TSH release is not directly mediated by the known chicken CRH-R1. Possibly CRH-induced TSH secretion is mediated by a yet unknown type of CRH-R in the chicken. Alternatively, a pro-opiomelanocortin (POMC)-derived peptide, secreted by the corticotropes following CRH stimulation, could act as an activator of TSH secretion in a paracrine way.

Biological activity of somatostatin receptors in GC rat tumour somatotrophs: evidence with sst1-sst5 receptor-selective nonpeptidyl agonists

The physiological actions of somatostatin-14 (SRIF: somatotrophin release inhibitory factor) receptor subtypes (sst(1)-sst(5)), which are endogenously expressed in growth cells (GC cells), have not yet been elucidated, although there is evidence that sst(2) receptors are negatively coupled to cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and adenosine 3,5'-cyclic monophosphate (cAMP) accumulation. In addition, both sst(1) and sst(2) receptors are negatively coupled to growth hormone (GH) secretion in GC cells. Here we report on studies concerning the expression, the pharmacology and the functional role of native SRIF receptors in GC cells with the use of five nonpeptidyl agonists, highly selective for each of the SRIF receptors. Radioligand binding studies show that sst(2) and sst(5) receptors are present at different relative densities, while the presence of sst(3) and sst(4) receptors appears to be negligible. The absence of sst(1) receptor binding was unexpected in view of sst(1) receptor functional effects on GH secretion. This suggests very efficient receptor-effector coupling of a low-density population of sst(1) receptors. Functionally, only sst(2) receptors are coupled to the inhibition of [Ca(2+)](i) and cAMP accumulation and the selective activation of sst(5) receptors facilitates the stimulation of adenylyl cyclase activity through G(i/o) proteins. This effect was not observed when sst(2) and sst(5) receptors were simultaneously activated, suggesting that there is a functional interaction between sst(2) and sst(5) receptors. In addition, sst(1), sst(2) and sst(5) receptor activation inhibits GH release, further indicating that SRIF can modulate GH secretion in GC cells through mechanisms both dependent and independent on [Ca(2+)](i) and cAMP-dependent pathways. The present data suggest SRIF-mediated functional effects in GC cells to be very diverse and provides compelling arguments to propose that multiple native SRIF receptors expressed in the same cells are not simply redundant, but contribute to marked signalling diversity.

Somatostatin-like receptors in goldfish: cloning of four new receptors

In this study, four somatostatin-like receptor (Sst) cDNAs were identified from goldfish pituitary, using RT-PCR screening and rapid amplification of cDNA ends (RACE) strategies. These include two type-five like Sst (Sst(5B) and Sst(5C)) and two type-three like Sst receptors (Sst(3A) and Sst(3B)), designated based on their amino acid sequence similarities to the known mammalian and fish Sst(5) and Sst(3). Both Sst(5C) and Sst(3A) mRNAs are widely expressed in all brain regions and pituitary; however, Sst(3B) expression is restricted to forebrain and Sst(5B) expression is mainly detected in pituitary and spinal cord.