bluuebunny/crossref_metadata_2025_split huggingface dataset:
Withdrawal of Somatostatin Augments L‐type Ca2+ Current in Primary Cultured Rat Somatotrophs
Authors: Kato Masakatsu 
Published: September 2006

AbstractIt is known that withdrawal of somatostatin (SRIF) augments the growth hormone (GH) releasing hormone (GRF)‐induced GH secretion. To investigate the mechanism of this augmentation in GH secretion, effects of GRF and SRIF on L‐type Ca2+ current (Ba2+ was used as a charge carrier) of primary cultured rat somatotroph were studied by perforated patch clamp technique. The reason is that GRF‐induced GH secretion is thought to be causally related to the influx of Ca2+ through L‐type Ca2+ channels. 10 mM GRF augmented maximum amplitude of L‐type Ba2+ current by 12.2% (n = 12). Subsequent application of SRIF slightly suppressed the currents but the suppression never exceeded the control level of the current. Removal of SRIF, however, promptly augmented the L‐type Ba2+ current by 26.8%. Such off‐response of SRIF was not observed in cells treated overnight with 100 ng/ml pertussis toxin. Further, specific inhibitor of protein kinase A, H‐89 at 1 μM reversibly suppressed the augmentation of L‐type Ba2+ current to control level. At 10 μM, H‐89 suppressed L‐type Ba2+ current by more than 40% from control level.These results suggest that (1) L‐type Ca2+ channel of somatotroph is probably phosphorylated in a basal condition and may be slightly modulated by GRF through increased level of CAMP; (2) SRIF only slightly suppress the channel activity; (3) Withdrawal of SRIF facilitates the activity of L‐type Ca2+ channel via PTX‐sensitive G‐protein, although the precise mechanism of this facilitation is unknown. The augmentation by SRIF‐pretreatment of GRF‐induced GH secretion may be at least partly due to the facilitation of the activity of L‐type Ca2+ channel.
The Photoperiodic Response in Syrian Hamster Depends upon a Melatonin‐Driven Circadian Rhythm of Sensitivity to Melatonin
Authors: Pitrosky B. , Kirsch R. , Vivien‐Roels B. , Georg‐Bentz I. , Canguilhem B. , Pevet P. 
Published: September 2006

AbstractThe pineal gland, via the daily pattern of melatonin (MEL) secretion, is directly involved in the conduction of photoperiodic information. The duration of MEL secretion is proportional to the duration of the dark period and, whatever the photoperiod is, MEL synthesis occurs 3 or 4 h after the dark onset in Syrian hamsters. In order to determine the relative importance of the duration or the coincidence hypothesis, a daily infusion protocol was used in sexually active pinealectomized hamsters. Long duration of MEL infusion (10 h) completely inhibit testes whereas short duration infusion (5 h) had no effect. When the animals were infused twice within 2 h 30 min separated by 3 h, they presented a complete gonadal atrophy, similar to the one observed with the 10 h infusion. Measurement of plasma MEL during the infusion and seperation periods revealed that MEL reached physiological nighttime values during the infusion period and fell to daytime values 1 h after the end of an infusion period. Thus, the results could not be due to a time additive action of the two MEL pulses. An intermediate response was observed when the 2 signals were applied across the light/dark transition. Gonadal regression did not occur when the 2 periods of infusion were separated by 5 h 30 min. The efficiency of this type of infusion was not dependent on the ambiant photoperiod since similar results were obtained in long and short photoperiods. The infusion was also as effective during the day as well as during the night. These results suggest that there is a rhythm of sensitivity to MEL, based on the coincidence hypotheses, that are important for transmission of photoperiodic information. This rhythm of sensitivity to MEL seems to be entrained by MEL itself, since the efficiency of the two pulses of MEL is not dependent of time of application and/or of photoperiod.
Localization of mRNA Coding for the Three Subtypes of Atrial Natriuretic Factor (ANF) Receptors in Rat Anterior Pituitary Gland Cells
Authors: Grandclément Béatrice , Brisson Christine , Bayard Francis , Tremblay Johanne , Gossard Francis , Morel Gerard 
Published: September 2006

AbstractAtrial Natriuretic Factor (ANF) action is mediated by highly selective and specific receptors. Three subtypes have been characterized and cloned: ANF receptor‐A, ‐B and ‐C. These subtypes are all expressed in the anterior pituitary of the rat. In the present study, the mRNA for each subtype was detected by in situ hybridization. The amounts of ANFR‐A and ‐B mRNA were found to be similar, and to be twice that of ANFR‐C mRNA. At the ultrastructural level, the three types of ANFR mRNA were expressed in three anterior pituitary cell types, namely lactotrophs, corticotrophs, and gonadotrophs, identified by their hormonal content. No signal was revealed in somatotrophs or thyrotrophs. The different forms of mRNA were similar in terms of subcellular localization: in the cytoplasmic matrix and the nuclear euchromatin.These data indicate that the anterior pituitary is an important target tissue for ANF action.
Dexamethasone Regulates the Expression of Neuronal Properties of a Rat Insulinoma Cell Line
Authors: Atouf Fouad , Tazi Abdelali , Polak Michel , Czemichow Paul , Scharfmann Raphael 
Published: September 2006

AbstractInsulin producing cells of the pancreas (beta cells) and neuronal cells share a large number of similarities. For example, different molecules, thought to be specific of neuronal cells, are expressed by beta cells. The factors regulating the expression of these molecules in beta cells are poorly understood. In the present work, we have studied the effect of dexamethasone, a synthetic glucocorticoid, on the expression of three different neuronal traits expressed by INS‐1 cells, a highly differentiated beta cell line. We demonstrate that dexamethasone treatment decreases the steady state levels of mRNAs coding for both the low‐and the high‐affinity NGF receptors and of mRNA coding for NF‐H, an intermediate neurofilament specific of neurons. This effect was time‐dependent, the decrease being detectable after 4–8 h treatment. The decrease in NGF receptors mRNAs steady state levels was paralleled by a decrease in the number of NGF binding sites as demonstrated after Scatchard analysis. We further focused on the mechanisms by which dexamethasone affects the expression of the low affinity NGF receptor. The effect is countered by the glucocorticoid antagonist RU486, indicating that it is mediated by the glucocorticoid receptor. Finally, the decrease in the low‐affinity nerve growth factor receptor mRNA steady state level after dexamethasone treatment is not due to mRNA destabilization but can be rather explained through a change in gene transcription.
Effects of Growth Hormone‐Releasing Peptide‐2 (GHRP‐2) on Membrane Ca2+ Permeability in Cultured Ovine Somatotrophs
Authors: Chen Chen , Clarke Iain J. 
Published: September 2006

AbstractThe newly synthesized GH‐releasing peptide, GHRP‐2 (D‐Ala‐D‐βNal‐Ala‐Trp‐D‐Phe‐Lys‐NH2), was studied in somatotroph‐enriched populations of ovine pituitary cells in primary culture. Nystatin‐perforated whole‐cell recordings were made on identified somatotrophs after 4–14 days of culture. Using a standard bath solution (containing Na+, Ca2+) and an electrode solution containing K+ in current‐clamp recordings, GHRP‐2 (10 nM) depolarized the membrane potential of the cells triggering a burst of action potentials. Voltage‐clamp recordings indicated that GHRP‐2 produced a slowly inactivated inward current with a slight reduction in outward current. The inward current was blocked by the Ca2+ channel blocker, Co2+ (0.5 mM). Ca2+ currents were then isolated using tetraethylammonium bath solution and an electrode solution containing Cs+. Ovine somatotrophs possess transient (T type) and long lasting (L type) Ca2+ currents. The L type current was abolished by addition of nifedipine (3 μM) to the bath solution and T type current was isolated on this basis. Current‐voltage relationships indicated an increase in both T and L type Ca2+ currents in response to GHRP‐2. The voltage‐dependent inactivation curve for T type Ca2+ current was shifted towards a less negative level by the peptide. Intracellular free Ca2+ concentration ([Ca2+]i) in somatotroph‐enriched populations was specifically increased by GHRP‐2 but this effect was totally abolished by blockade of membrane Ca2+ channels. These data show that GHRP‐2 causes an influx of Ca2+ leading to an increase in [Ca2+]i in ovine somatotrophs. The Ca2+ currents were both L type and T type with a shift in the inactivation curve of the latter by the releasing peptide.
Effect of Sustained Physiological Hyperinsulinaemia on Hypothalamic Neuropeptide Y and NPY mRNA Levels in the Rat
Authors: Cusin Isabelle , Dryden Simon , Wang Qiong , Rohner‐Jeanrenaud Francoise , Jeanrenaud Bernard , Williams Gareth 
Published: September 2006

AbstractNeuropeptide Y (NPY) synthesized in the arcuato‐paraventricular projection in the rat hypothalamus is thought to play an important role in controlling energy homeostasis. The factors that regulate hypothalamic NPY are not known but, amongst others, insulin has been postulated as an inhibitory modulatory agent. To test this hypothesis, normal male rats were given either insulin (2 units/day) or saline via subcutaneous osmotic minipumps for 3 days. Euglycaemia was maintained by a concomitant glucose infusion in insulin‐infused rats which had peripheral insulin levels 5–8 times higher than saline‐infused controls. Hyperinsulinaernic rats ate 42% less than controls, but their total energy intake (food intake plus glucose infusion) was higher than that of controls, and they gained more weight than controls during the experimental period. Hyperinsulinaemia had no significant effect on hypothalamic NPY mRNA or NPY levels in the arcuate nucleus. NPY concentrations in the paraventricular nucleus were, however, significantly increased by 73% in hyperinsulinaemic rats, but were closely similar to controls in all other areas. Insulin may act as a satiety factor in that hyperinsulinaemic rats ate less, but the fact that these animals had increased total energy intake and gained excessive weight suggests that insulin may not function as an overall regulator of energy balance. In addition, physiological hyperinsulinaemia does not apparently inhibit NPY gene expression in the arcuate nucleus. Due to the lack of effect of hyperinsulinaemia on NPY synthesis in the arcuate nucleus, the elevated NPY concentrations in the paraventricular nucleus could result from a reduction of its release, which would be in keeping with the reduction in food intake.
The Neuroactive Steroid 5α‐Tetrahydrodeoxycorticosterone Increases GABAergic Postsynaptic Inhibition in Rat Neocortical Neurons in vitro
Authors: Teschemacher Anja , Zeise Marc L. , Holsboer Florian , Zieglgänsberger Walter 
Published: September 2006

AbstractThe neuroactive steroid 5α‐pregnane‐3α,21‐diol‐20‐one (5α‐tetrahydrodeoxycorticosterone; 5α‐THDOC) has been shown to potentiate GABA‐induced chloride currents in cell cultures and subcellular preparations.In this study, we recorded from pyramidal neurons in an in vitro slice preparation of the adult rat frontal neocortex using intracellular microelectrodes. 5α‐THDOC (10 μM) increased and prolonged the inhibitory postsynaptic potential (IPSP). The mean maximal synaptic conductance of the early, GABAA receptor‐mediated, IPSP was enhanced to more than 700%, the one at the maximum of the late, partially GABAA receptor‐mediated, IPSP to approximately 400%. The progesterone/glucocorticoid receptor antagonist RU 38486 did not prevent the IPSP increase. At a concentration of 1 μM 5α‐THDOC increased only the early IPSP to about 125%.Responses to the iontophoretically applied specific GABAA receptor agonist muscimol but not to the specific GABAB receptor agonist L‐baclofen were enhanced by 5α‐THDOC (10 μM). In the giga‐seal whole‐cell configuration when the GABAB receptor‐mediated IPSP component was absent due to intracellular perfusion, 5α‐THDOC (10 μM) increased IPSPs to a similar extent as in the conventional microelectrode recordings. Excitatory postsynaptic potentials, resting membrane potential, input resistance and action potential amplitude were not affected by 5α‐THDOC (10 μM).These data demonstrate that in neocortical tissue of the rat 5α‐THDOC enhances GABAergic inhibition by interacting with postsynaptic GABAA receptors while synaptic excitation and parameters of electric excitability remain unchanged.
The C‐Type Natriuretic Peptide Receptor is the Predominant Natriuretic Peptide Receptor mRNA Expressed in Rat Hypothalamus
Authors: Langub M. C. , Dolgas C. M. , Watson R. E. , Herman J. P. 
Published: September 2006

AbstractThe natriuretic peptide receptors (NPR) are membrane‐bound guanylate cyclases with extracellular binding domains specific for particular members of the natriuretic peptide family. NPR‐A binds atrial natriuretic peptide (ANP) with high affinity, whereas the NPR‐B appears to be specific for C‐type natriuretic peptide (CNP). Previous data indicating extensive overlap between localization of ANP and CNP in hypothalamic neuroendocrine circuits suggest the importance of determining whether specificity of natriuretic peptide action may be conferred via receptor type present on target cells. To address this issue, we used in situ hybridization histochemistry to localize NPR‐A and NPR‐B mRNA in the hypothalamus. NPR‐A mRNA was not found in substantial abundance in any hypothalamic nucleus; however, detectable NPR‐A signal was observed in other brain regions, including the subfornical organ and medial habenula. In contrast, NPR‐B mRNA was expressed throughout the hypothalamus, including neurons of the magnocellular and parvocellular paraventricular, the arcuate, and the supraoptic nuclei. Expression was also seen in other nuclei essential to neuroendocrine control, including the median preoptic, anteroventral periventricular, tuberomammillary, ventromedial and suprachiasmatic nuclei. NPR‐B mRNA was also observed in the neural lobe of the pituitary gland, suggesting expression by pituicytes. The results suggest that NPR‐B is the primary natriuretic peptide receptor in hypothalamus, and by inference indicate that CNP is the primary active natriuretic peptide in neuroendocrine regulation.
Influence of Environmental Colour and Diurnal Phase on MCH Gene Expression in the Trout
Authors: Suzuki Masakazu , Narnaware Y. K. , Baker Bridget I. , Levy Andy 
Published: September 2006

AbstractMelanin‐concentrating hormone (MCH) gene expression in the brain of rainbow trout, reared and maintained in either pale or black‐coloured tanks, was studied using in situ hybridization histochemistry. MCH transcripts were most prevalent in the magnocellular neurones of the nucleus lateralis tuberis (NLT), which project to the pituitary gland. They were also present, although at much lower levels, in dorsally projecting parvocellular neurones, sited more posteriorly above the lateral ventricular recess (LVR). In the NLT the most intense hybridization signal was seen over the pituitary stalk; above the LVR, the most active neurones were located caudally. In both the NLT and above the LVR, MCH hybridization signal was 4‐fold stronger in white‐reared fish than in black‐reared fish. There was also a marked diurnal variation in MCH expression in both sites, with high levels at 16.00 h and lower levels at 04.00 h. The results show that gene activity in both hormonal (NLT) and neuromodulator/neurotransmitter (LVR) MCH neurones is induced by pale environmental colour and that MCH gene activity is subject to pronounced diurnal variation.
Resistance of Growth Hormone Secretion to Hypoglycemia in the Mouse
Authors: Tamaki Miho , Sato Makoto , Niimi Michio , Takahara Jiro 
Published: September 2006

AbstractAlthough previous studies have demonstrated that acute hypoglycemia inhibits growth hormone (GH) secretion due to stimulation (hypothalamic somatostatin (SS) neurones in the rat, the effect of hypoglycemia on GH secretion has not yet been elucidated in the mouse In this study, the effects of insulin‐induced hypoglycemia on mouse GH secretion, hypothalamic c‐fos expression, GH‐releasing hormon (GRH) and SS mRNA levels were investigated in conscious male mice. Seven days after implantation of chronic atrial catheters, bloo samples were taken every 20 min from 1200–1600 h under unrestrained conditions. Insulin was administered iv every 20 min fro1 1200‐1240 h to induce moderate hypoglycemia (MH) and severe hypoglycemia (SH), respectively. Expression of hypothalamic c‐fos protei was examined 30 min and 60 min after induction of hypoglycemia by immunohistochemistry. Hypothalamic GRH and SS mRNA level were examined 1 h and 3 h after induction of hypoglycemia by Northern blot analysis. The lowest mean plasma glucose levels after insuli injections were 49.1 ± 4.1 mg/dl and 34.2 ± 5.6 mg/dl in conscious mice, respectively. However, pulsatile GH secretion was no significantly altered in either group. Although both MH and SH markedly stimulated c‐fos expression in specific hypothalamic nuclc including the paraventricular nucleus, they did not induce c‐fos protein in the periventricular nucleus. Neither MH nor SH altere hypothalamic GRH or SS mRNA levels. These results suggest that hypoglycemia does not activate SS neurons which inhibit GH secretio in the mouse.
Sex‐Steroid Control of Galanin in the Rat Hypothalamic‐Pituitary Axis
Authors: Selvais Philippe L. , Denef Jean‐François , Adam Evelyne , Maiter Dominique M. 
Published: September 2006

AbstractTo further investigate how sex steroids regulate galanin (GAL) in the rat pituitary and hypothalamus, we examined the effects of prepubertal gonadectomy (Gx) and long‐term (9 weeks) replacement with estradiol (E2) or testosterone (T) on pituitary and hypothalamic GAL concentrations in Wistar rats (5–6/group). Sham‐operated animals served as controls (CTR). Pituitary GAL concentration was markedly higher in random‐cycling CTR‐females than in CTR‐males (1391 ± 247 vs 39 ± 5 pg/mg protein, P<0.01) and decreased after Gx only in females (20 ± 3 pg/mg protein, P<0.01). E2 strongly increased pituitary GAL in Gx‐females and Gx‐males (4470 ± 365 and 3853 ± 347 pg/mg protein, P<0.01), whereas T had no effect. Inversely, hypothalamic GAL was higher in CTR males than in CTR females (5.4 ± 0.3 vs 4.0 ± 0.5 ng/mg protein, P<0.05), and decreased significantly after gonadectomy in males (3.7 ± 0.2 ng/mg protein, P<0.01). The only steroid treatment that significantly modified hypothalamic GAL in Gx animals was administration of E2 to females (5.7 ± 0.4 ng/mg protein, P<0.01 vs non‐treated Gx). We also studied in hypophysectomized (Hx) rats (8/group) the effects of sex steroids on hypothalamic GAL concentration and distribution. The low hypothalamic GAL concentration observed in male and female Hx rats (1.0 ± 0.1 ng/mg protein) was significantly increased by T in males and in females (respectively, by 40% and by 50%, P<0.02) and by E2 in males (by 60%, P<0.02). Immunohistochemistry showed that both E2 and T increased GAL labeling in fibers restricted to the lateral regions of the median eminence of male and female Hx rats, a distribution corresponding to that of GnRH‐immunoreactive fibers. In conclusion, in addition to the marked stimulatory effect of estradiol on pituitary GAL, endogenous and exogenous sex steroids also modulate hypothalamic GAL in male and female rats. Both E2 and T may exert a stimulatory influence on hypothalamic GAL concentration and histochemical analysis shows that sex steroids enhance GAL immunoreactivity in a subset of neurons that closely overlaps the distribution of GnRH neurons.
Disruption of Reproductive Rhythms and Patterns of Melatonin and Prolactin Secretion Following Bilateral Lesions of the Suprachiasmatic Nuclei in the Ewe
Authors: Scott Christopher J. , Jansen Heiko T. , Kao Chen‐Chih , Kuehl David E. , Jackson Gary L. 
Published: September 2006

AbstractTo determine whether the photoperiodic responses of reproductive and prolactin (PRL) rhythms in the ewe requires an intact suprachiasmatic nucleus (SCN) driving the pineal rhythm of melatonin secretion, four groups of ovary‐intact ewes over a 6‐year period were subjected to bilateral (n = 40) or sham lesions (n = 15) of the SCN. Animals were exposed to an alternating 90–120 day photoregimen of 9L:15D and 16L: 8D photoperiods. Blood samples taken twice weekly were assayed for prolactin and for progesterone to monitor oestrous cycles. On several occasions blood samples also were taken at hourly intervals for 24 h and analyzed for melatonin. Melatonin concentrations in sham lesioned ewes were basal during the lights‐on period and rose robustly during darkness. Those sheep bearing unilateral lesions of the SCN (n = 13) or where the lesion spared the SCN entirely (n = 8) had patterns of melatonin secretion similar to sham ewes. The remaining ewes, having complete (n = 9) or incomplete bilateral (n = 8) destruction of the SCN, with one exception, had disrupted patterns of melatonin secretion. The nature of this disruption varied from complete suppression to continuously elevated levels. In lesioned ewes where melatonin secretion was not affected the onset and cessation of ovarian cycles were similar to sham ewes; stimulation of oestrous cycles under 9L:15D and cessation of oestrous cycles under 16L:8D. In contrast, 13 of 17 ewes with disrupted melatonin secretion also exhibited disrupted patterns of ovarian activity. In these animals oestrous cycles were no longer entrained by photoperiod but still occurred in distinct clusters, that is, groups of cycles began and ended spontaneously. Sheep with normal melatonin patterns showed low levels of PRL secretion during short days and elevated PRL levels during long days. However, 8 of 13 ewes with disrupted melatonin showed patterns of PRL secretion that were no longer entrained by photoperiod. A minority of ewes with disrupted melatonin patterns still showed reproductive (n = 4) and PRL (n = 5) responses similar to those of sham‐lesioned ewes. These results show that bilateral destruction of the SCN in the ewe disrupts the circadian pattern of melatonin secretion and that this disruption usually, but not always, is associated with altered photoperiodic responses. These results strongly suggest that the SCN are important neural elements within the photoperiod time–keeping system in this species. A role for the SCN in the generation of endogenous transitions in reproductive activity (refractoriness) and prolactin secretion is not supported.