The Experts below are selected from a list of 207 Experts worldwide ranked by ideXlab platform
Peter J. Morgan - One of the best experts on this subject based on the ideXlab platform.
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Sensitization: a mechanism for melatonin action in the Pars tuberalis.
Journal of neuroendocrinology, 2003Co-Authors: Perry Barrett, Carole Schuster, Julian G. Mercer, Peter J. MorganAbstract:Sensitization of adenylate cyclase is a recently discovered phenomenon. Melatonin can induce a sensitized response of adenylate cyclase in ovine Pars tuberalis cells where the receptor for melatonin is endogenously expressed. Although the mechanism is not fully understood, sensitization of adenylate cyclase may be an important part of the mechanism by which melatonin encodes daylength in the Pars tuberalis of sheep and other animals. We used this as a hypothesis to search for a natural ligand that would activate adenylate cyclase in ovine Pars tuberalis cells. The approach revealed pituitary adenylate cyclase activating polypeptide to be an indirect activator of adenylate cyclase in the ovine Pars tuberalis. We discuss this in relation to the mechanism and importance of sensitization to the function to the Pars tuberalis.
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Evidence for the biosynthesis of a prolactin-releasing factor from the ovine Pars tuberalis, which is distinct from thyrotropin-releasing hormone.
Journal of neuroendocrinology, 2002Co-Authors: E. Scott Graham, David G. Hazlerigg, Catriona A. Webster, Peter J. MorganAbstract:This study demonstrates the presence of two prolactin-releasing (PR) factors in media conditioned by primary Pars tuberalis cells prepared from dispersed Pars tuberalis tissue. One factor was identified as thyrotropin-releasing hormone (TRH) on the basis of immunoreactivity and following purification by high-performance liquid chromatography and mass spectrometry. The origin of TRH in the Pars tuberalis conditioned media was investigated by measuring the expression of glutaminyl-cyclase (QC) by in situ hybridization. QC expression was not detected in Pars tuberalis-specific cells, but was relatively abundant in cells in the Pars distalis and hypothalamic paraventricular nucleus. These data suggest that TRH is not synthesized by the ovine Pars tuberalis and more likely originated from the hypothalamic neuronal processes from the paraventricular nucleus that terminate in the median eminence. The second component of the conditioned media PR bioactivity was insensitive to the TRH-antiserum, less than 1 kDa and was not retained by the C18 reverse-phase column. The biosynthesis of the PR bioactivity by Pars tuberalis cells was investigated using cycloheximide, forskolin and melatonin. Cycloheximide reduced the level of PR bioactivity produced by the Pars tuberalis cells. Melatonin inhibited the increased level of PR bioactivity stimulated by forskolin. Collectively, these data demonstrate the synthesis of at least one regulator of prolactin secretion by ovine Pars tuberalis-specific cells.
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Photoperiodic regulation of prolactin gene expression in the Syrian hamster by a Pars tuberalis-derived factor.
Journal of neuroendocrinology, 2001Co-Authors: J. A. Stirland, Peter J. Morgan, Jonathan D. Johnston, Felino R. Cagampang, Maria G. Castro, Michael R. H. White, Julian R. E. Davis, Andrew S. I. LoudonAbstract:Syrian hamsters exhibit a marked seasonal variation in prolactin secretion. The aim of this study was to analyse the nature of the photoperiodic regulation of prolactin gene expression, and to define the role of melatonin and the Pars tuberalis of the anterior pituitary in this process. Pituitary prolactin gene expression, restricted to the Pars distalis, was increased in hamsters maintained in long daylengths (16 h : 8 h, light : dark) compared to hamsters exposed to short daylengths (8 h : 16 h, light : dark) for 8–12 weeks. Analysis of single cells by in situ hybridization showed that photoperiod had no effect on the percentage of Pars distalis cells expressing prolactin mRNA, but shifted the frequency distribution of prolactin mRNA expression per cell, such that in long photoperiods a greater proportion of cells were recruited to a higher expressing population. In vitro coculture of hamster Pars tuberalis fragments increased prolactin promoter-driven luciferase activity in stably transfected GH3 cells in a dose- and duration-dependent manner. Conditioned medium from hamster and ovine Pars tuberalis also activated the prolactin promoter. Furthermore, basal and forskolin-stimulated conditioned medium from hamster Pars tuberalis increased prolactin mRNA expression in primary cultures of Pars distalis cells. Melatonin attenuated the activity of Pars tuberalis-conditioned medium but had no direct effect on either prolactin mRNA expression or secretion in Pars distalis cell cultures. Finally, Pars tuberalis fragments from long photoperiod hamsters stimulated prolactin gene promoter activity to a greater extent than those from short photoperiod hamsters. In conclusion, this study provides the first evidence in a seasonal mammal that the synthesis of prolactin depends on photoperiodic modulation of a Pars tuberalis-derived factor. Our data support further the hypothesis that seasonal modulation of prolactin gene expression depends upon a melatonin-dependent paracrine action of the Pars tuberalis on Pars distalis lactotrophic cells.
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Decoding Photoperiodic Time and Melatonin in Mammals: What Can We Learn from the Pars tuberalis?:
Journal of biological rhythms, 2001Co-Authors: David G. Hazlerigg, Peter J. Morgan, Sophie MessagerAbstract:The cellular and molecular mechanisms through which the melatonin signal is decoded to drive/synchronize photoperiodic responses remain unclear. Much of our current understanding of the processes involved in this readout derives from studies of melatonin action in the Pars tuberalis of the anterior pituitary. Here, the authors review current knowledge and highlight critical gaps in our present understanding.
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The Pars tuberalis of the pituitary: a gateway for neuroendocrine output.
Reviews of reproduction, 1996Co-Authors: Peter J. Morgan, Lynda WilliamsAbstract:The Pars tuberalis is a structurally distinct region of the adenohypophysis, the function of which has been unclear for decades. Recent studies, which demonstrate the localization of melatonin receptors on the Pars tuberalis, suggest a photoperiodic function. The principal cell type of the Pars tuberalis is morphologically distinct from others in the pituitary and is thought to secrete a specific product. In support of this, evidence is emerging that ovine Pars tuberalis cells secrete a factor ('tuberalin') that exerts hormonal control over both gene expression and prolactin release from the Pars distalis lactotrophs. These data in conjunction with physiological studies, which show that photoperiodically driven cycles in prolactin secretion can occur in the absence of an intact hypothalamic-pituitary axis, suggest that the function of the Pars tuberalis is to act as an endocrine intermediate in the photoperiodic effects of melatonin on prolactin secretion. Studies of the cellular biochemistry of the ovine Pars tuberalis suggest that the main function of melatonin is to prevent or terminate transcriptional and translational activation by an unidentified factor (Stim X). On the basis of these physiological and biochemical studies, a hypothetical model is proposed to account for the mechanism of photoperiodic regulation of prolactin secretion by melatonin.
Jean Pelletier - One of the best experts on this subject based on the ideXlab platform.
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Changes in LHβ-gene and FSHβ-gene expression in the ram Pars tuberalis according to season and castration
Cell and tissue research, 1995Co-Authors: Jean Pelletier, R. Counis, M. De Reviers, M. Moumni, Yves TilletAbstract:Luteinizing hormone beta (LHβ) and follicle stimulating hormone beta (FSHβ) subunits and their mRNAs were studied in the ram Pars tuberalis following different seasonal (winter vs summer) and experimental (intact vs castrated animals) conditions. Hormone-containing cells were identified by immunohistochemistry, and mRNAs for LHβ and FSHβ by in situ hybridization using homologous double-stranded 35S-cDNAs. The labelling was quantified by image analysis. Immunohistochemical staining showed that cells containing LHβ and FSHβ were localized mainly in the ventral part of the Pars tuberalis but that, in the summer, additional LHβ-containing cells were present in the dorsal part in intact rams. On the other hand, LHβ-mRNA labelling was found in the whole Pars tuberalis in wethers but only in the ventral part in intact rams. The magnitude of LHβ-mRNA labelling was significantly greater in summer than in winter rams, and in castrated than in intact animals (P
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melatonin receptors in the lamb Pars tuberalis median eminence throughout the day
Neuroendocrinology, 1993Co-Authors: Vincent Piketty, Jean PelletierAbstract:After validation of the methodology, melatonin receptors have been measured by binding of (125I)-melatonin to membranes of individual Pars tuberalis/median eminence of lambs at different ti
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Melatonin Receptors in the Lamb Pars tuberalis/Median Eminence throughout the Day
Neuroendocrinology, 1993Co-Authors: Vincent Piketty, Jean PelletierAbstract:After validation of the methodology, melatonin receptors have been measured by binding of (125I)-melatonin to membranes of individual Pars tuberalis/median eminence of lambs at different ti
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Localization of luteinizing hormone β-mRNA by in situ hybridization in the sheep Pars tuberalis
Cell and tissue research, 1992Co-Authors: Jean Pelletier, R. Counis, M. De Reviers, Yves TilletAbstract:The localization of luteinizing hormone beta (LHβ)-mRNA was studied by in situ hybridization in the Pars tuberalis of sheep using a homologous sheep double-stranded 32P-or 35S-cDNA. The labelled cDNA probe detected one mRNA sequence in the Pars tuberalis by Northern blot analysis; this sequence was similar to that detected in the pituitary. In situ, the labelling of LHβ-mRNA in the horizontal and sagittal tissue sections was found throughout the Pars tuberalis. This labelling was prevented by adding an excess of cold probe or treating the sections by ribonuclease before in situ hybridization. Controls showed a labelling in the Pars distalis, but not in the median eminence, hypothalamus, cerebral cortex and liver sections. Double labelling by using a specific LHβ-antiserum indicated that the labelling of LHβ-mRNA appeared more intense in LH-containing cells that were found only in the ventral part of the Pars tuberalis. These results suggest that the entire Pars tuberalis is able to produce the LHβ subunit, but that the level of translation greatly varies according to the location of the cells.
W. Wittkowski - One of the best experts on this subject based on the ideXlab platform.
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The Pars tuberalis of the monkey (Macaca fascicularis) hypophysis: cell types and hormone expression.
Cells tissues organs, 2001Co-Authors: N. Bock, Jürgen Bockmann, Tobias M. Böckers, P. Nowak, E. Buse, W. WittkowskiAbstract:The Pars tuberalis (Pt) of most mammalian species contains specific cells which are structurally and functionally different from the Pars distalis (Pd) cells. Pt-specific cells possess melatonin recep
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CELL AND MOLECULAR BIOLOGY OF THE Pars tuberalis OF THE PITUITARY
International review of cytology, 1999Co-Authors: W. Wittkowski, Jürgen Bockmann, Michael R. Kreutz, Tobias M. BöckersAbstract:The Pars tuberalis of the adenohypophysis is mainly composed of a special type of endocrine cells, Pars tuberalis-specific cells, lining the primary capillary plexus of the hypophysial portal system. Dense expression of melatonin receptors and marked changes in morphological appearance, production pattern, and secretory activity during annual cycle show that these cells are highly sensitive to changes in photoperiod. This leads to the hypothesis that the Pars tuberalis is involved in the transmission of photoperiodic stimuli to endocrine targets. Several investigations support the theory that Pars tuberalis-specific cells are multipotential cells exerting a modulatory influence on the secretory activity of the Pars distalis. Specifically, there is accumulating evidence that seasonal modulation of prolactin secretion, independent of hypothalamic input, is due to melatonin-regulated activity of Pars tuberalis-specific cells. The exact nature of secretory products and their effects within neuroendocrine regulation, however, remain rather enigmatic. Accordingly, molecular mechanisms regulating gene expression under the influence of photoperiod, respectively, circulating melatonin levels are still incomplete. Recent cloning of melatonin receptor genes and new data on intracellular signal transduction will probably lead to new insights on melatonin action and Pars tuberalis-specific cell physiology.
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TSH Expression in Murine Hypophyseal Pars tuberalis-Specific Cells
Acta anatomica, 1997Co-Authors: Jürgen Bockmann, W. Wittkowski, Michael R. Kreutz, Tobias M. BöckersAbstract:Several experiments in photoperiod-dependent species suggest that the hypophyseal Pars tuberalis (PT) plays a key role in transducing light/dark information to the endocrine system. In rat and hamster
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Pars tuberalis-specific cells in the ovine pituitary do express the common α-chain of glycoprotein hormones: an in situ hybridization and immunocytochemical study
European journal of endocrinology, 1994Co-Authors: Tobias M. Böckers, Jürgen Bockmann, Jan-dirk Fauteck, M. R. Kreutz, Bock R, W. WittkowskiAbstract:The ovine pituitary Pars tuberalis was investigated by electron microscopy, immunocytochemistry and non-radioactive in situ hybridization in order to characterize further the Pars tuberalis-specific cells, whose functional role within the endocrine system is still enigmatic. Ultrastructural analysis revealed that, besides gonadotropic cells, the vast majority of cells in the ovine Pars tuberalis show the typical characteristics of Pars tuberalis-specific cells with clear signs of secretory activity. Immunocytochemical staining with a polyclonal antibody directed against the alpha-subunit of ovine glycoprotein hormones and in situ hybridization with an antisense oligonucleotide complementary to the alpha-subunit mRNA showed that the common alpha-chain of glycoprotein hormones is expressed in Pars tuberalis-specific cells. Antibodies against the beta-subunits failed to detect any of the known beta-chains of pituitary glycoprotein hormones in these cells. The demonstration of the glycoprotein alpha-subunit in Pars tuberalis-specific cells of the adult sheep supports previously existing evidence that these cells do secrete one or more glycoprotein hormones. With respect to the thyrotropin-beta-like immunoreactivity in rats and hamsters, one might speculate that Pars tuberalis-specific cells are a pluripotent cell type with a low secretory activity under basal conditions. Further studies should prove the hypothesis that a pre-existing mRNA pool in these cells can be used for sustained translation of glycoprotein hormones after physiological or pharmacological stimulation.
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Photoperiod-dependent Changes in Exocytotic Activity in the Hypophyseal Pars tuberalis of the Djungarian Hamster, Phodopus Sungorus
Cell and tissue research, 1993Co-Authors: T. Merks, A. Schulze-bonhage, W. WittkowskiAbstract:The Pars tuberalis of the hypophysis of the Djungarian hamster, Phodopus sungorus, was investigated with regard to secretory activity by applying the tannic acid-Ringer perfusion technique. Two groups were maintained under long photoperiods (16 h light: 8 h dark) or short photoperiods (8 h light: 16 h dark), respectively. Perfusion with tannic acid showed that specific Pars tuberalis cells release some of their secretory granules as indicated by typical exocytotic figures. The percentage of cells displaying exocytotic activity was significantly higher in the Pars tuberalis of hamsters kept under long photoperiods. The number of exocytotic figures per single cell was not increased. These results provide further evidence for a secretory activity of the Pars tuberalis and support the hypothesis of its involvement as a mediator between photoperiodic stimuli and the endocrine system.
David G. Hazlerigg - One of the best experts on this subject based on the ideXlab platform.
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clock genes and the long term regulation of prolactin secretion evidence for a photoperiod circannual timer in the Pars tuberalis
Journal of Neuroendocrinology, 2003Co-Authors: G. A. Lincoln, Håkan Andersson, David G. HazleriggAbstract:Prolactin secretion is regulated by photoperiod through changes in the 24-h melatonin profile and displays circannual rhythmicity under constant photoperiod. These two processes appear to occur principally within the pituitary gland, controlled by the Pars tuberalis. This is evident because: (i) hypothalamic-pituitary disconnected (HPD) sheep show marked changes in prolactin secretion in response to switches in photoperiod and manipulations of melatonin, similar to brain-intact controls; (ii) HPD sheep also show photoperiod-specific, long-term cycles in prolactin secretion under constant long or short days, with the timing maintained even when prolactin secretion is blocked for 2-3 months; and (iii) Pars tuberalis cells, but not lactotrophs, express high concentrations of melatonin (MT1) receptor, and exhibit a duration-sensitive, cAMP-dependant, inhibitory response to physiological concentrations of melatonin. This suggests the existence of an intrinsic, reversible photoperiod-circannual timer in Pars tuberalis cells. A full complement of clock genes (Bmal1, Clock, Per1, Per2, Cry1 and Cry2) are expressed in the ovine Pars tuberalis, and undergo 24-h cyclical expression as observed in a cell autonomous, circadian clock. Activation of Per genes occurs in the early day (melatonin off-set), while activation of Cry genes occurs in the early night (melatonin on-set). This temporal association is evident under both long and short days, thus the Per-Cry interval varies directly with photoperiod. Because, PER : CRY, protein : protein interactions affect stability, nuclear entry and gene transcription based on rodent data, the change in phasing of Per/Cry expression provides a potential mechanism for decoding the long day/short day melatonin signal. A speculative, but testable, extension of this hypothesis is that intrinsically regulated changes in the phase of Per/Cry rhythms, regulates both photorefractoriness and the generation of circannual rhythms in prolactin secretion.
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Clock genes and the long-term regulation of prolactin secretion: evidence for a photoperiod/circannual timer in the Pars tuberalis.
Journal of neuroendocrinology, 2003Co-Authors: G. A. Lincoln, Håkan Andersson, David G. HazleriggAbstract:Prolactin secretion is regulated by photoperiod through changes in the 24-h melatonin profile and displays circannual rhythmicity under constant photoperiod. These two processes appear to occur principally within the pituitary gland, controlled by the Pars tuberalis. This is evident because: (i) hypothalamic-pituitary disconnected (HPD) sheep show marked changes in prolactin secretion in response to switches in photoperiod and manipulations of melatonin, similar to brain-intact controls; (ii) HPD sheep also show photoperiod-specific, long-term cycles in prolactin secretion under constant long or short days, with the timing maintained even when prolactin secretion is blocked for 2-3 months; and (iii) Pars tuberalis cells, but not lactotrophs, express high concentrations of melatonin (MT1) receptor, and exhibit a duration-sensitive, cAMP-dependant, inhibitory response to physiological concentrations of melatonin. This suggests the existence of an intrinsic, reversible photoperiod-circannual timer in Pars tuberalis cells. A full complement of clock genes (Bmal1, Clock, Per1, Per2, Cry1 and Cry2) are expressed in the ovine Pars tuberalis, and undergo 24-h cyclical expression as observed in a cell autonomous, circadian clock. Activation of Per genes occurs in the early day (melatonin off-set), while activation of Cry genes occurs in the early night (melatonin on-set). This temporal association is evident under both long and short days, thus the Per-Cry interval varies directly with photoperiod. Because, PER : CRY, protein : protein interactions affect stability, nuclear entry and gene transcription based on rodent data, the change in phasing of Per/Cry expression provides a potential mechanism for decoding the long day/short day melatonin signal. A speculative, but testable, extension of this hypothesis is that intrinsically regulated changes in the phase of Per/Cry rhythms, regulates both photorefractoriness and the generation of circannual rhythms in prolactin secretion.
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Evidence for the biosynthesis of a prolactin-releasing factor from the ovine Pars tuberalis, which is distinct from thyrotropin-releasing hormone.
Journal of neuroendocrinology, 2002Co-Authors: E. Scott Graham, David G. Hazlerigg, Catriona A. Webster, Peter J. MorganAbstract:This study demonstrates the presence of two prolactin-releasing (PR) factors in media conditioned by primary Pars tuberalis cells prepared from dispersed Pars tuberalis tissue. One factor was identified as thyrotropin-releasing hormone (TRH) on the basis of immunoreactivity and following purification by high-performance liquid chromatography and mass spectrometry. The origin of TRH in the Pars tuberalis conditioned media was investigated by measuring the expression of glutaminyl-cyclase (QC) by in situ hybridization. QC expression was not detected in Pars tuberalis-specific cells, but was relatively abundant in cells in the Pars distalis and hypothalamic paraventricular nucleus. These data suggest that TRH is not synthesized by the ovine Pars tuberalis and more likely originated from the hypothalamic neuronal processes from the paraventricular nucleus that terminate in the median eminence. The second component of the conditioned media PR bioactivity was insensitive to the TRH-antiserum, less than 1 kDa and was not retained by the C18 reverse-phase column. The biosynthesis of the PR bioactivity by Pars tuberalis cells was investigated using cycloheximide, forskolin and melatonin. Cycloheximide reduced the level of PR bioactivity produced by the Pars tuberalis cells. Melatonin inhibited the increased level of PR bioactivity stimulated by forskolin. Collectively, these data demonstrate the synthesis of at least one regulator of prolactin secretion by ovine Pars tuberalis-specific cells.
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Decoding Photoperiodic Time and Melatonin in Mammals: What Can We Learn from the Pars tuberalis?:
Journal of biological rhythms, 2001Co-Authors: David G. Hazlerigg, Peter J. Morgan, Sophie MessagerAbstract:The cellular and molecular mechanisms through which the melatonin signal is decoded to drive/synchronize photoperiodic responses remain unclear. Much of our current understanding of the processes involved in this readout derives from studies of melatonin action in the Pars tuberalis of the anterior pituitary. Here, the authors review current knowledge and highlight critical gaps in our present understanding.
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p72, a marker protein for melatonin action in ovine Pars tuberalis cells: its regulation by protein kinase A and protein kinase C and differential secretion relative to prolactin.
Neuroendocrinology, 1994Co-Authors: Peter J. Morgan, Perry Barrett, W. Lawson, Gary Davidson, David G. HazleriggAbstract:The function of the Pars tuberalis as a mediator of the action of melatonin remains elusive. As a direct method of assessing the potential role of secretory proteins, ovine Pars tuberalis cells have b
Yoko Kameda - One of the best experts on this subject based on the ideXlab platform.
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Hes1 regulates formations of the hypophyseal Pars tuberalis and the hypothalamus
Cell and tissue research, 2010Co-Authors: Minekatsu Akimoto, Toshiyuki Nishimaki, Yuta Arai, Eiju Uchinuma, Hiroshi Yamauchi, Yoko KamedaAbstract:The hypophyseal Pars tuberalis surrounds the median eminence and infundibular stalk of the hypothalamus as thin layers of cells. The Pars tuberalis expresses MT1 melatonin receptor and participates in mediating the photoperiodic secretion of pituitary hormones. Both the rostral tip of Rathke’s pouch (Pars tuberalis primordium) and the Pars tuberalis expressed αGSU mRNA, and were immunoreactive for LH, chromogranin A, and TSHβ in mice. Hes genes control progenitor cell differentiation in many embryonic tissues and play a crucial role for neurulation in the central nervous system. We investigated the Hes1 function in outgrowth and differentiation of the Pars tuberalis by using the markers for the Pars tuberalis. In homozygous Hes1 null mutant embryos, the rostral tip was formed in the basal-ventral part of Rathke’s pouch at embryonic day (E)11.5 as well as in wild-type embryos. In contrast to the wild-type, the rostral tip of null mutants could not extend rostrally with age; it remained in the low extremity of Rathke’s pouch during E12.5–E13.5 and disappeared at E14.5, resulting in lack of the Pars tuberalis. Development of the ventral diencephalon was impaired in the null mutants at early stages. Rathke’s pouch, therefore, could not link with the nervous tissue and failed to receive inductive signals from the diencephalon. In a very few mutant mice in which the ventral diencephalon was partially sustained, some Pars tuberalis cells were distributed around the hypoplastic infundibulum. Thus, Hes1 is required for development of the Pars tuberalis and its growth is dependent on the ventral diencephalon.
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Effect of pinealectomy on the photoperiod-dependent changes of the specific secretory cells and α-subunit mRNA level in the chicken Pars tuberalis
Cell and tissue research, 2002Co-Authors: Yoko Kameda, Masaaki Miura, Shigeyoshi MaruyamaAbstract:The effect of pinealectomy on the synthesis of the common alpha-subunit of glycoprotein hormones by the specific secretory cells of the Pars tuberalis (PT-specific cells) was examined in male chicks. Expression of melatonin receptor (Mel(1c)) mRNA was demonstrated in the chick Pars tuberalis by reverse transcription-polymerase chain reaction analysis. Northern blot analyses revealed that, after pinealectomy, common alpha-subunit mRNA levels were increased in the Pars tuberalis of chicks kept under normal lighting (L:D=12 h:12 h), indicating that melatonin inhibits the synthesis of the alpha-subunit of PT-specific cells. Furthermore, alpha-subunit mRNA levels in the Pars tuberalis were shown to display similar photoperiod-dependent changes in pinealectomized chicks as in intact animals. Levels of alpha-subunit mRNA in the Pars tuberalis were decreased in both pinealectomized and control chicks kept under continuous light (L:D=24 h:0), whereas the levels were enhanced in pinealectomized chicks kept under extended darkness (L:D=1 h:23 h) and under normal lighting. Thus, pinealectomy did not affect the inhibition or stimulation of the alpha-subunit synthesis in the chicken Pars tuberalis elicited by continuous light or extended darkness, respectively. Quantitative electron-microscopic analyses showed that, after exposure to continuous light for 30 days, many PT-specific cells were filled with enlarged secretory granules in both pinealectomized and control chicks. Exposure to extended darkness for 30 days caused an increase in the cytoplasmic and nuclear areas of the PT-specific cells. Secretory granules were however larger in pinealectomized than in intact control chicks. These results suggest that the activity of PT-specific cells is mainly regulated by photoperiod.
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Effect of different photoperiods on the ultrastructure of the specific secretory cells and α-subunit mRNA level in the chicken Pars tuberalis
Cell and tissue research, 2001Co-Authors: Yoko Kameda, Masaaki Miura, Toshiyuki NishimakiAbstract:The effect of different photoperiods on the specific secretory cells of the Pars tuberalis was examined in male chicks. Animals were placed in one of three different photoperiod regimens: (1) normal control (light:dark = 12 h:12 h), (2) continuous light (L:D = 24 h:0), and (3) extended darkness (L:D = 1 h:23 h). The levels of common alpha-subunit mRNA in the Pars tuberalis were examined by Northern blot analysis and compared with those in the Pars distalis. In chicks exposed to continuous light for 1 week, alpha-subunit mRNA level in the Pars tuberalis was decreased, although the level in the Pars distalis was increased. Exposure to continuous light for 30 days also induced a decrease in alpha-subunit mRNA level in the Pars tuberalis. On the other hand, in chicks exposed to extended darkness for 1 week, the alpha-subunit mRNA level of the Pars tuberalis was markedly increased. In situ hybridization with digoxigenin-labeled common alpha-subunit cRNA probe also showed that the hybridization signals for alpha-subunit mRNA in the Pars tuberalis cells become weak under continuous light for 30 days but they are very intense under extended darkness. Thus, the synthesis of alpha-subunits in the chick Pars tuberalis was inhibited by continuous light but stimulated by extended darkness. These results were confirmed by semiquantitative electron-microscopic analyses. After exposure to continuous light for 30 days, many Pars tuberalis (PT)-specific cells were filled with enlarged secretory granules, showing the reduction of secretory activity. On the contrary, extended darkness for 30 days induced hypertrophy of the PT-specific cells; the areas of cytoplasm and nucleus were significantly increased. In addition, secretory granules became small in size and exocytotic features were more frequent. Mitochondria and lysosomes were also increased in number. Thus, the synthetic and secretory activities of the PT-specific cells were increased under extended darkness. The data indicate that the specific cells of the Pars tuberalis are responsive to photoperiodic changes in the chick.
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Expression of the common α-subunit mRNA of glycoprotein hormones during the chick pituitary organogenesis, with special reference to the Pars tuberalis
Cell and tissue research, 1999Co-Authors: Yoko Kameda, Masaaki Miura, Sae OhnoAbstract:The expression of a common α-subunit mRNA of glycoprotein hormones was examined in the pituitary of chick embryos at various stages of development by in situ hybridization with a digoxigenin-labeled quail α-subunit cRNA probe. As a comparison with the expression of α-subunit mRNA, the onset of luteinizing hormone (LH) immunoreactivity was examined by immunohistochemical staining with a chicken LH antiserum. Both α-subunit mRNA and LH immunoreactivity began to appear in the basal-posterior region of the Rathke's pouch at embryonic day (E) 3.5. At E4.5 when the cephalic and caudal lobes of the Pars distalis could be distinguished in the Rathke's pouch, intense signal for α-subunit mRNA was restricted to the cephalic lobe, consisting of a high columnar epithelium. At E6, gonadotrophs that were ovoid in shape, expressed intense signal for α-subunit mRNA, and revealed intense immunoreactivity for LH, were first detected in the cephalic lobe. At this stage, α-subunit mRNA expression became weak in the undifferentiated columnar cells of the cephalic lobe. At E8, the Pars tuberalis primordium located close to the median eminence was formed at the lateral-apical end of the cephalic lobe. The primordium expressed intense signal for α-subunit mRNA. Gonadotrophs showing immunoreactivity for LH were densely distributed throughout the cephalic and caudal lobes in 8-day-old embryos. The Pars tuberalis primordium expressing α-subunit mRNA progressively extended along the median eminence with embryonal age and reached the rostoral end by E14. Thus, both primordia of the Pars distalis and Pars tuberalis expressed intense signal for the common α-subunit mRNA. This subunit may play a role in the cytodifferentiation of the adenohypophysis.
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Localization and development of chromogranin A and luteinizing hormone immunoreactivities in the secretory-specific cells of the hypophyseal Pars tuberalis of the chicken.
Histochemistry and cell biology, 1998Co-Authors: Yoko Kameda, Masaaki Miura, Sae OhnoAbstract:The Pars tuberalis mainly consists of the secretory cells specific to this portion of the pituitary. We examined the localization and development of luteinizing hormone (LH) and chromogranin A in the chicken Pars tuberalis by immunohistochemistry. The vast majority of the chicken Pars tuberalis was occupied by cells immunoreactive for both LH and chromogranin A. Furthermore, immunoblot analysis of chicken Pars tuberalis extracts with LH antiserum demonstrated that two bands, the large α-subunit and small β-subunit of the LH molecule, were expressed in this tissue as well as in the Pars distalis. A band for chromogranin A was also detected in Pars tuberalis extracts with chromogranin A antiserum. In contrast to the cells of mammalian species that contain only a few small secretory granules, the specific cells of the chicken Pars tuberalis were characterized by the presence of many secretory granules ranging from 90 to 400 nm in diameter. Postembedding immunogold labeling showed that gold particles representing immunoreactivity for LH were densely located on all secretory granules of the secretory-specific cells. Many secretory granules, especially the large ones, of the cells were also loaded with immunogold particles for chromogranin A. Double immunogold labeling confirmed that LH and chromogranin A were colocalized on the same secretory granules. During embryonic development, the primordium of the Pars tuberalis was first detected at 8 days of incubation as a small group of cells containing LH- and chromogranin-immunoreactive cells. In the Pars distalis, the onset of LH and chromogranin expression occurred earlier, at 6 days of incubation. At 10 days of incubation, the Pars tuberalis primordium became large cell masses consisting of LH- and chromogranin-immunoreactive cells, which were located close to the median eminence. Subsequently, the primordium extended along the median eminence progressively with age. At 14 days of incubation, it reached to the rostral end and surrounded the median eminence as slender cell cords. These results indicate that specific cells of the chicken Pars tuberalis synthesize a glycoprotein hormone related to the LH molecule, which is stored in the secretory granules together with chromogranin A. The Pars tuberalis may be involved in the regulation of gonadal function in a different way from that of the Pars distalis.