Divergent effects of acute depolarization on somatostatin release and protein synthesis in cultured fetal and neonatal rat brain cells

Insights into a role of GH secretagogues in reversing the age-related decline in the GH/IGF-I axis

Growth hormone (GH) secretion and serum insulin-like growth factor-I (IGF-I) decline with aging. This study addresses the role played by the hypothalamic regulators in the aging GH decline and investigates the mechanisms through which growth hormone secretagogues (GHS) activate GH secretion in the aging rats. Two groups of male Wistar rats were studied: young-adult (3 mo) and old (24 mo). Hypothalamic growth hormone-releasing hormone (GHRH) mRNA and immunoreactive (IR) GHRH dramatically decreased (P < 0.01 and P < 0.001) in the old rats, as did median eminence IR-GHRH. Decreases of hypothalamic IR-somatostatin (SS; P < 0.001) and SS mRNA (P < 0.01), and median eminence IR-SS were found in old rats as were GHS receptor and IGF-I mRNA (P < 0.01 and P < 0.05). Hypothalamic IGF-I receptor mRNA and protein were unmodified. Both young and old pituitary cells, cultured alone or cocultured with fetal hypothalamic cells, responded to ghrelin. Only in the presence of fetal hypothalamic cells did ghrelin elevate the age-related decrease of GH secretion to within normal adult range. In old rats, growth hormone-releasing peptide-6 returned the levels of GH and IGF-I secretion and liver IGF-I mRNA, and partially restored the lower pituitary IR-GH and GH mRNA levels to those of young untreated rats. These results suggest that the aging GH decline may result from decreased GHRH function rather than from increased SS action. The reduction of hypothalamic GHS-R gene expression might impair the action of ghrelin on GH release. The role of IGF-I is not altered. The aging GH/IGF-I axis decline could be rejuvenated by GHS treatment.

Growth hormone action on proliferation and differentiation of cerebral cortical cells from fetal rat

To define the role of GH during central nervous system development, we performed studies in cultured rat cerebral cortical cells from 14- (E14) and 17-d-old embryos (E17). The expression of GH receptor, IGF-I receptor, and IGF-I mRNAs was confirmed. In E17, GH increased total cell number (3.9-fold), [(3)H]-thymidine incorporation (3.5-fold), proliferating cell nuclear antigen levels (2.5-fold), and bromodeoxyuridine (BrdU)-positive cells (2.5-fold). GH action on nestin/BrdU-positive cells was increased in E14 cells at 3 d in vitro (80-fold) but not at 7 d in vitro. In E14 cells, GH increased (9.5-fold) beta-tubulin/BrdU cells. In E17 cells, GH induced neuronal differentiation, as indicated by the absence of beta-tubulin/BrdU-positive cells and the 5.9-fold increment of beta-tubulin protein, and increased glial fibrillary acidic protein/BrdU-positive cells (2.5-fold) and glial fibrillary acidic protein expression (4.5-fold). GH-induced proliferation and differentiation was blocked by IGF-I antiserum. GH increased IGF-binding protein-3 (IGFBP-3), IGF-I receptor protein and its phosphorylation. This study shows that GH promotes proliferation of neural precursors, neurogenesis, and gliogenesis during brain development. These responses are mediated by locally produced IGF-I. GH-induced IGFBP-3 may also have a role in these responses. Therefore, GH is able to activate the IGF-I/IGFBP-3 system in these cerebral cells and induce a physiological action of IGF-I.

Regulation of the preprosomatostatin gene by cyclic-AMP in cerebrocortical neurons

The gene coding for preprosomatostatin (ppSom), the molecular precursor of somatostatin (Som), is regulated at the level of transcription by calcium ions and cyclic-AMP [F. Baldino, S. Fitzpatrick-McElligott, T. O'Kane, I. Gozes, Hormonal regulation of somatostatin, Synapse 2 (1988) 317-325; M.R. Montminy, M.J. Low, L. Tapia-Arancibia, Cyclic AMP regulates somatostatin mRNA accumulation in primary diencephalic cultures and in transfected fibroblast cells, J. Neurosci. 6 (1986) 1171-1176.], or by agents which increase intracellular levels of cAMP directly, such as forskolin [M.R. Montminy, M.J. Low, L. Tapia-Arancibia, Cyclic AMP regulates somatostatin mRNA accumulation in primary diencephalic cultures and in transfected fibroblast cells, J. Neurosci. 6 (1986) 1171-1176.]. Transcriptional induction of the ppSom gene as examined in PC12 cells, transfected fibroblasts and primary diencephalic cultures, requires the highly conserved cAMP response element (CRE), which confers gene responsiveness to cAMP [M. Comb, N. Mermod, S.E. Hyman, Proteins bound at adjacent DNA elements act synergistically to regulate human proenkephalin cAMP inducible transcription, EMBO J. 7 (1988) 3793-3805; T. Tsukada, J.S. Fink, G. Mandel, Identification of a region in the human vasoactive intestinal polypeptide gene responsible for regulation by cyclic AMP, J. Biol. Chem. 262 (1987) 8743-8747.]. The ppSom gene is subject to stringent regulation during cerebrocortical development in vivo; however, little information is available regarding ppSom gene regulation by neurotransmitters or second-messengers in cortical neurons. We used primary cerebrocortical cell cultures from fetal mice to examine the dose-response and time-course of ppSom gene expression in response to the cyclic-AMP analogs, dibutyrl-cAMP (dbcAMP), and 8-bromo-cAMP (8-BrcAMP). We report a dose-response for both analogs in the range of 0.1-10 mM. Dose-response studies using agents which directly stimulate intracellular cAMP synthesis (forskolin) or inhibit its breakdown (3-isobutyl 1-methyl xanthine) were also performed. We observed an apparent synergistic effect on ppSom expression when used in combination. An increase in ppSom mRNA levels was observed by 4 h, with a maximal response at 12-24 h. No change in ppSom mRNA levels was observed in response to phorbol myristate acetate (PMA). Our findings confirm the specificity of ppSom gene regulation by cAMP and Ca2+ ions, and demonstrate the utility of using primary cerebrocortical cultures for the study of somatostatin gene expression by neurotransmitters and second-messengers as a model of human neurologic disorders.

Regulation of somatostatin gene expression by veratridine-induced depolarization in cultured fetal cerebrocortical cells

The stimulatory effect of veratridine (VTD) depolarization upon somatostatin mRNA (SS mRNA) levels in primary cultures of fetal cerebrocortical cells was analyzed. Depolarizing stimuli, such as 100 microM VTD exposure for 30 min, elicited an increase in immunoreactive somatostatin (IR-SS) release to the media without affecting SS mRNA levels. These levels increased when exposure to depolarization stimuli was prolonged up to 3 or more hours. At this time, veratridine acted as a secretagogue, stimulating somatostatin secretion, but was also effective in stimulating somatostatin mRNA levels. These changes were blunted by the Na+ channel blockade tetrodotoxin (TTX), and partially abolished by the Ca2+ channel antagonist, verapamil (VPM). To study whether VTD may affect mRNA stability we determine the rate of disappearance of SS mRNA after inhibition of transcription by actinomycin D and demonstrated that VTD stimulation did not stabilize the SS mRNA. These results indicate that the induction of SS mRNA expression by VTD involves the modulation of Ca2+ and Na+ channels. The time course study confirmed that the VTD-induced SS mRNA accumulation is time-dependent, and requires a prolonged exposure to stimulate SS gene expression. VTD stimulation does not modify the SS mRNA rate of degradation.

Triiodothyronine regulates somatostatin gene expression in cultured fetal rat cerebrocortical cells

The effect of triiodothyronine (T3) on somatostatin (SS) mRNA levels in cultured fetal rat cerebrocortical cells was studied. Two different experimental approaches were sought. They differed in the length of time in which cells were deprived of thyroid hormones prior to the addition of exogenous T3. When the cells were not deprived of thyroid hormones, T3 caused a dose-related decrease in SS mRNA content at all doses tested. However, when the cells were deprived of T3 for 24 h, a biphasic effect was observed. These findings suggest that T3 regulates SS gene expression in fetal cultured cerebrocortical cells.

Neurotransmitter regulation of somatostatin secretion by fetal rat cerebral cortical cells in culture

Extensive studies exploring the regulation of hypothalamic somatostatin GHRIH release have been reported, but the factors regulating GHRIH release in the cerebral cortex have not been well defined. We have studied the effects of central neurotransmitters on GHRIH secretion by cultured fetal rat cerebral cortical cells and on intracellular GHRIH levels. Cells maintained in vitro for 15-20 days were incubated with dopamine (DA), acetylcholine (ACh), gamma-aminobutyric acid (GABA), norepinephrine (NE), serotonin (SE) or histamine (His) (10(-11) M to 10(3) M) for 30 minutes. Following incubation, immunoreactive GHRIH was measured by RIA in cell extracts and incubation media. DA increased intracellular GHRIH content but have no effect on GHRIH in the media. Both media and intracellular GHRIH content were significantly reduced by GABA and SE. The effect of NE was stimulatory at low (10(-9) M) and inhibitory at high (10(-5) M to 10(-3) M) concentrations. ACh was found to increase media GHRIH and to decrease intracellular GHRIH content; 30 min exposure to His did not significantly modify either media or intracellular GHRH. Our findings with fetal rat cerebral cortical cells in culture demonstrate that endogenous neurotransmitters do have the capacity to directly influence GHRIH regulation.

Potassium-induced depolarization stimulates somatostatin gene expression in cultured fetal rat cerebrocortical cells

The stimulatory effect of potassium depolarization upon somatostatin mRNA (SS mRNA) levels in primary cultures of fetal cerebrocortical cells was analyzed. Depolarizing stimuli, such as 56 mM K+ concentration for 24 hours, elicited an increase in immunoreactive somatostatin (IR-SS) release to the media and SS mRNA levels, suggesting that somatostatin secretion can be coupled to SS mRNA accumulation. These changes were inhibited by the Ca2+ channel antagonist verapamil (VPM). In contrast, Na+ channel blockade by tetrodotoxin (TTX) did not modify the 24 h potassium-induced increase in SS mRNA. These results suggest that the induction of SS mRNA expression by K+ involves the modulation of calcium ion channels.

Regulation of somatostatin and growth hormone-releasing factor by gonadal steroids in fetal rat hypothalamic cells in culture

The mechanism underlying the sexually dimorphic pattern of growth hormone (GH) secretion in the rat has not been clearly elucidated. In the present study, we assayed the possible direct effect of gonadal steroids on both somatostatin (SS) and growth hormone-releasing factor (GRF) in fetal rat hypothalamic cells in culture. Hypothalamic cells, obtained by mechanical dispersion, were maintained as monolayer cultures in serum-supplemented medium. After 20 days in culture, cells were incubated with serum free medium containing testosterone (T, 10, 20, 40 ng/dl) or estradiol (E, 0.1, 1, 10 ng/dl) for 48 h. At the end of the experiments, immunoreactive SS (IR-SS) and immunoreactive GRF (IR-GRF) were measured by specific radioimmunoassays (RIAs) in media and cell extracts. After 48 h of incubation with testosterone, somatostatin in both media and cells was significantly reduced. On the contrary, this treatment lead to a dose-dependent increase in media and cell GRF content. When cells were incubated with estradiol for 48 h, a significant inhibition in medium SS release was observed, whereas intracellular SS slightly increased at the highest concentration of 10 ng/dl. Estradiol treatment resulted in an inconsistent decrease in media and cells IR-GRF. Our results indicate that both SS and GRF are under the influence of testosterone and estradiol acting at the hypothalamic level, and furthermore suggest that at this stage of brain development, gonadal steroids may regulate GH secretion through their ability to modulate hypothalamic SS and GRF.

Endogenous vasoactive intestinal peptide (VIP) regulates somatostatin secretion by cultured fetal rat cerebral cortical and hypothalamic cells

To determine the possible physiological role of endogenous vasoactive intestinal peptide (VIP) in the control of cerebral somatostatin (SS), we studied the effect of endogenous VIP blockade on immunoreactive SS (IR-SS) accumulation by fetal rat cerebral cortical and hypothalamic cells in culture. Cells were cultured in minimum essential medium (MEM) with 10% fetal calf serum and 10% horse serum. After 7-10 days 'in vitro' media were replaced with MEMs without sera containing anti-VIP immunoglobulins G (IgG) for 1, 3, 6, 24 or 48 h. Controls received the same amount of IgG from normal rabbit serum (NRS). In another group of experiments, cells were incubated with VIP (10(-11) M to 10(-7) M) for 1, 3, 6 or 24 h. Exposure to anti-VIP IgG resulted in a decreased accumulation of IR-SS in both cerebral cortical and hypothalamic cells, whereas the addition of VIP caused a dose-dependent increase in total IR-SS, these effects being evident after 3 h incubation. The stimulatory action VIP on IR-SS was up to 129%, this being decreased to 86% by the addition of anti-VIP to plates containing 10(-7) M VIP. Patterns of IR-SS accumulation throughout prolonged incubation periods were qualitatively similar (in both cerebrocortical and hypothalamic cells) in the presence or absence of anti-VIP IgG. However, in plates containing anti-VIP, the total amount of IR-SS was lower than in the control groups (IgG from NRS). These findings demonstrate that, at this time of brain development, somatostatinergic neurons may be under the physiological regulation of locally produced VIP.

Transient cerebral ischemia induces changes in SRIF mRNA in the fascia dentata

A transient cerebral ischemia produced in rats by 4-vessel occlusion, produces with a delay of 24 h a fall in the number of somatostatin-containing neurons. In the present study we show that this loss is preceded by a loss of somatostatin mRNA that starts as soon as 30 min after the anoxic episode. By 24 h of revascularization the surviving somatostatinergic hilar cells present a transient recovery of hybridization signal. This effect could be related to a previously reported increase in intracellular calcium.

Neurosecretory and trophic action on fetal rat neuroblasts induced by an amino acid mixture

The effects of a synthetically obtained mixture of amino acids (FACE) were investigated on the trophic and neurosecretory activity of in vitro cultures of fetal rat neuronal cells. The addition of 10(-6) M FACE to the culture medium significantly increased cell DNA content. Secretions of IR-SRIF, IR-VIP, and IR-GRF were also augmented in different proportions by the presence of FACE. Time studies demonstrated that IR-SRIF was significantly increased after 48 (P less than 0.05) and 72 (P less than 0.01) hr of exposure to FACE, and IR-VIP secretion was potentiated after only 24 hr of culture. Dose-response experiments with 10(-7) to 10(-4) M FACE indicated that concentrations of 10(-5) and 10(-4) M significantly increased both somatostatin released to the medium and cell content of IR-SRIF. FACE concentrations as low as 10(-10) M augmented the secretion of IR-GRF, and there was a dose-response correlation between 10(-10) and 10(-5) M FACE. The release and cell content of IR-VIP were also increased by FACE, with a dose-response relation at concentrations of 10(-9) to 10(-6) M. It can thus be concluded that FACE has a powerful effect on the multiplication and survival of fetal cerebrocortical cells and is also an important potentiator of IR-SRIF, IR-VIP, and IR-GRF secretion.

Influence of thyroid hormones on somatostatin processing in cultured cerebro-cortical cells

Previous data from our laboratory showed that prolonged exposure of cultural cerebral cortical cells to high potassium concentrations and veratridine resulted in the stimulation of immunoreactive somatostatin (IR-SRIF) synthesis and caused a major increase in its high molecular weight forms. Somatostatin (SRIF) synthesis by cortical and hypothalamic cells was also affected by thyroid hormone (TH). In the present work we have examined to what extent TH might also affect SRIF processing. Cerebral cortical cells maintained as monolayer culture for 7-10 days received triiodothyronine (T3) in concentrations of 10(-11) M and 10(-7) M for 48 h. We found that the total amount of IR-SRIF was increased by high T3 concentrations as reported previously. When the IR-SRIF was characterized by high pressure liquid chromatography (HPLC) or gel filtration, it was evident that thyroid hormone treatment modified the elution profile of IR-SRIF in cells and medium on Bio-Gel P-10 and HPLC, increasing somatostatin 28 (S-28) and decreasing somatostatin 14 (S-14). The results indicate that thyroid hormones affect SRIF processing, leading to a major increase in the synthesis of its high molecular weight forms.

Secretions of somatostatin and VIP in cultures of fetal rat neuroblasts increased by amino acids

Neuroblasts obtained from 17 day old rat embryos were incubated for 8 days, after which half of them were treated with 10(-6) M FACE (a mixture of amino acids high in glycine, alanine and aspartic acid), and the other half were left as controls. At the end of 20 days, levels of somatostatin (SRIF) were over 6,000 pg/plate in neuroblasts treated with FACE, versus 500 pg/plate in controls. At this time vasoactive intestinal peptide (VIP) levels were over 230 pg/plate in the FACE treated cultures, while their controls contained less than 150 pp/plate. Protein totals were similar (about 1,000 micrograms/plate) in all FACE treated cultures and controls, indicating that increases in SRIF and VIP were not determined by changes in cell population, but by their synthetic and/or secretory activities triggered by minute amounts of FACE. These results may be of interest in the understanding of Alzheimer's disease.