Neuronal vs. glial somatostatin in the hypothalamus: a cell culture study of the ontogenesis of cellular location, content and release

Changes of gene expression profiles during neuronal differentiation of central nervous system precursors treated with ascorbic acid

Ascorbic acid (AA) has been shown to increase the yield of dopaminergic (DA) neurons derived from basic fibroblast growth factor (bFGF)-expanded mesencephalic precursors. To understand the molecular mechanisms underlying this phenomenon, we used cDNA microarray analysis to examine differential expression of neuronal genes following AA treatment. The putative precursor cells were isolated from E13 rat ventral mesencephalons and expanded in the presence of bFGF. Cells were incubated in mitogen-free media supplemented with 200 microM AA or were left untreated as a control, and total RNA was isolated at different time points (expansion stage and 1, 3, and 6 days after induction of differentiation) and subjected to cDNA microarray analysis. Differentiation was evaluated by Western blot analysis and immunocytochemistry of neuron-specific markers. AA treatment of the mesencephalic precursors increased the expression of neuronal (MAP2) and astrocytic (glial fibrillary acidic protein) markers and the percentage of tyrosine hydroxylase (TH)-positive cells. The microarray analysis revealed that 12 known genes were up-regulated and 20 known genes were down-regulated in expansion-stage AA-treated cells. Six days after the induction of differentiation, AA-treated cells showed up-regulation of 48 known genes and down-regulation of 5 known genes. Our results identified several proteins, such as transferrin, S-100, and somatostatin, as being differentially regulated in AA-treated mesencephalic precursors. This novel result may lead to a better understanding of the molecular mechanisms underlying the AA-induced differentiation of mesencephalic precursors into DA neurons and may form the basis for improved DA neuronal production for treatment of Parkinson's disease patients.

Somatostatin in the brain of the cave salamander,Hydromantes genei (Amphibia, Plethodontidae): Immunohistochemical localization and biochemical characterization

The distribution of somatostatin-like immunoreactivity in the brain of the cave salamander Hydromantes genei (Amphibia, Plethodontidae) was investigated by using two distinct antisera raised against somatostatin-14. Most somatostatin-positive cells were detected in the ependymal cell layer surrounding the ventricles. These cells possessed the typical morphological characteristics of tanycytes or radial glial cells. Double-labeling with an antiserum against somatostatin and a monoclonal antibody against glial fibrillary acidic protein showed that somatostatin-immunoreactive cells lining the ventricles also exhibited GFAP-like immunoreactivity. Injection of the neurotracer biocytin into the lateral ventricle revealed that neurons lining the ventricles did not contain somatostatin-like immunoreactivity. In the telencephalon, somatostatin-like immunoreactivity was confined to radial glial cells. In the diencephalon, in addition to somatostatin-immunoreactive cells in the ependyma, positive cell bodies were also found in the periventricular preoptic nucleus, the infundibular nucleus, the epiphysis, and the subcommissural organ. In the metencephalon, positive cell bodies were found in the auricula cerebelli, whereas in the rhombencephalon numerous somatostatin-immunoreactive cells were seen lining the ventricular cavity. Immunoreactive nerve fibers were observed in the hypothalamus-median eminence complex. In the pituitary, a discrete group of somatostatin-positive cells was found in the pars distalis. High-performance liquid chromatography analysis of brain extracts revealed that the immunoreactive material coeluted with somatostatin-14. The present results show that the somatostatin peptidergic system in the brain of the cave salamander has a more simple organization than those described in the brain of frog and other vertebrates. This feature is probably related to the expression of high pedomorphic characters in plethodontids. The distribution of somatostatin-like immunoreactivity suggests that, in the cave salamander, somatostatin may act as a neurotransmitter and/or neuromodulator, a central regulator of fluid homeostasis, and a hypophysiotropic neurohormone.

Glial somatostatin-14 expression in the rat pituitary intermediate lobe: a possible neurotrophic function during development?

Somatostatin-14 was first detected on gestational day 17 in radially-oriented, bipolar cells spanning the width of the intermediate lobe of the rat pituitary. Cells were prominent, and constituted approximately 50% of the lobe area. The presence of vimentin, the cellular shape, and the localization identified these cells as glia. At postnatal day 6, somatostatin-14 and vimentin staining appeared in stellate-shaped cells. This is in agreement with the change from bipolar to stellate shape these glia undergo after the onset of innervation ([13] Gary et al. Int. J. Devl. Neurosci. 13, 555-565, 1995). Glia were more abundant, relative to melanotropes, throughout embryonic and early postnatal development compared to adulthood. Reverse transcription-polymerase chain reaction data showed a high level of prosomatostatin mRNA in the intermediate lobe, compared to the anterior and neural lobes from postnatal day 2 animals, and a significant drop in intermediate lobe content in the adult. The correlation between the number of glia and high expression of somatostatin in neonatal relative to adult tissue, together with the close apposition of incoming axons to the abundant, radially oriented glia during innervation of the lobe, support a neurotrophic function of glia-derived somatostatin.

Up-regulation of somatostatin after lesions in the cerebellum of the teleost fish Apteronotus leptorhynchus

Following application of mechanical lesions to the corpus cerebelli, a cerebellar subdivision, in adult individuals of the teleost fish Apteronotus leptorhynchus, the pattern of expression of the neuropeptide somatostatin was examined by employing immunohistochemical techniques. In the intact corpus cerebelli, only a very few cells displayed somatostatin-like immunoreactivity. This number dramatically increased in the area of the lesion within the granule cell layer 1 day following the injury and peaked after 2 days, when the normalized total number of somatostatin-positive cells was approximately 50 times higher than the mean number of labeled cells found after 3, 6, and 12 h of survival. Between 5 and 10 days of post-lesioning survival time, this number abruptly declined and returned to background levels at 17 and 25 days. Confocal microscopy revealed three cell types, presumably corresponding to granule cell neurons, astrocytes and microglia, which all displayed a similar temporal pattern of somatostatin expression. It is hypothesized that somatostatin is involved in regulation of the genesis and/or development of new neurons which are produced in response to injuries and which replace damaged cells at the site of the lesion.

Ontogeny and sexual differentiation of somatostatin biosynthesis and secretion in the hypothalamic periventricular-median eminence pathway

The biosynthesis of somatostatin (SRIH) in the hypothalamic periventricular nucleus (PeN) is sexually differentiated in neonatal and adult rats by virtue of the organizational and activational actions, respectively, of sex steroid hormones. Little information exists, however, on the normal pattern of maturation of these neurones or on how the sexually differentiated biosynthesis may relate to ontogenetic changes in somatostatin secretion during the neonatal and pubertal periods of development. Hence in the present study we determined the postnatal developmental profile of SRIH mRNA and peptide levels in the PeN-median eminence (ME) pathway as well as SRIH secretion, using an acute explant preparation, from the day of birth, through puberty and into adulthood in male and female rats. The results demonstrate that: (1) developmental sex differences in SRIH biosynthesis in PeN neurones occurred in an orderly cascade with differences observed for mRNA expression at postnatal day 5, for peptide content in the perikarya at postnatal day 10 and for peptide content in the nerve terminal (ME) by postnatal day 25; (2) sex differences in SRIH release were not evident prior to postnatal day 40; and (3) the developmental profile of SRIH biosynthesis in PeN neurones is unique compared with other hypothalamic (ventromedial nucleus) and extrahypothalamic (parietal cortex) populations. Specific developmental changes in the biosynthetic and secretory activity of the hypothalamic SRIH PeN-ME pathway may have a functional importance in the maturation of hypothalamic SRIH pathways involved in the regulation of GH secretion.

Differential regulation of basal and cyclic adenosine 3',5'-monophosphate-induced somatostatin gene transcription in neural cells by DNA control elements that bind homeodomain proteins

A number of genes encoding neuropeptides are expressed in the peripheral and central nervous systems, in different endocrine organs, and in specialized cells distributed along the gastrointestinal tract. Whether expression of the same neuropeptide gene in different tissues is regulated by similar transcriptional mechanisms or by mechanisms that differ in a cell-specific manner remains unclear. We report on promoter studies on the regulation of the somatostatin gene in immortalized neural precursor cells derived from developing rat forebrain. Expression of the somatostatin gene in these cells was determined by RT-PCR/Southern blot analysis, by immunocytochemistry, and by RIA. We show that in cerebrocortical and hippocampal cells, expression of the somatostatin gene is regulated by several negative and positive DNA cis-regulatory elements located throughout the promoter region. The somatostatin cAMP-response element appears to play a prominent role in neural somatostatin gene expression by acting as a strong enhancer even in the absence of cAMP stimulation. Site-directed mutagenesis followed by transient transfection assays indicated that SMS-TAAT1, SMS-TAAT2, and SMS-UE, three previously identified homeodomain protein-binding regulatory elements that enhance transcription in pancreatic cells, act as repressors of transcription in neural cells. Electrophoretic mobility shifts assays indicate that those elements bind protein complexes that differ between neural and pancreatic cells. Our results support the notion that expression of the somatostatin gene in neural cells occurs via transcriptional mechanisms that are different from those regulating expression of the same gene in pancreatic cells.

Temporal changes in gene expression following cryogenic rat brain injury

Expression of 18 genes was examined at 8 different time points between 1 h and 28 days following cryogenic rat brain injury. The genes include thymidine kinase (TK), p53 tumor suppressor, c-fos, renin, myelin basic protein (MBP), proteolipid protein (PLP), transferrin, transferrin receptor, platelet-derived growth factor A (PDGF A), platelet-derived growth factor B (PDGF B), platelet-derived growth factor receptor alpha (PDGF alpha receptor), platelet-derived growth factor receptor beta (PDGF beta receptor), glial fibrillary acidic protein (GFAP), transforming growth factor-beta 1 (TGF-beta 1), basic fibroblast growth factor (bFGF), fibroblast growth factor receptor-1 (FGF-R1), insulin-like growth factor-1 (IGF-1), and somatostatin. Time courses of gene expression were determined for RNAs derived from hippocampus and cortex. Genes were divided into categories based upon those in which statistically significant changes in expression were first observed at or before 24 h (early genes) and those in which changes were first observed at or after 72 h (late genes). In the present model, many genes demonstrate elevated RNA levels in the cortex prior to hippocampus, following injury. RNAs transcribed from late genes tend to be elevated concurrently in cortex and hippocampus.

Transforming growth factor-betas inhibit somatostatin messenger ribonucleic acid levels and somatostatin secretion in hypothalamic cells in culture

Treatment of hypothalamic cells in monolayer culture with transforming growth factor-beta1 (TGFbeta1) significantly reduced both basal and cAMP-induced somatostatin messenger RNA (mRNA) levels and somatostatin secretion. This inhibitory effect was dose- and time-dependent and not mediated by glial cells, as it was also observed in glial-free hypothalamic cell cultures treated with cytosine arabinonucleoside. TGFbeta2 and -beta3 mimicked the actions of TGFbeta1, which indicated that the three isoforms of the TGFbeta family expressed in the central nervous system displayed similar effects on the somatostatinergic neurons. The blockade of synthesis of proteins with either cycloheximide or puromycin for 24 h prevented the inhibitory effect of TGFbeta1 on somatostatin mRNA. This implied that the reduction of this mRNA by TGFbeta1 required de novo protein synthesis. We next studied whether TGFbeta1 acted at the transcriptional or posttranscriptional level by altering the stability of somatostatin mRNA. Examination of the rate of disappearance of somatostatin mRNA by Northern blot, after inhibition of mRNA transcription with either actinomycin D (AcD) or 5,6-dichloro-1beta-ribofuranosyl benzimidazole revealed that TGFbeta1 did reduce the stability of somatostatin mRNA. This effect was observed when we pretreated the cultures with TGFbeta1 4 h before the addition of AcD, but not when we administered TGFbeta1 simultaneously with AcD or 5,6-dichloro-1beta-ribofuranosyl benzimidazole. Altogether these results demonstrated that the treatment of hypothalamic cells in culture with TGFbeta1, TGFbeta2, or TGFbeta3 resulted in a decrease in somatostatin mRNA levels and somatostatin secretion. TGFbeta1 reduced the steady state levels of somatostatin mRNA by inducing the synthesis of a protein (s), that appears to accelerate the degradation of the mRNA of somatostatin. Whether TGFbeta1 has additional effects on the transcription of the somatostatin gene will require further study.

Somatostatin: the neuroendocrine story

Since its original discovery as the neuroendocrine hormone responsible for inhibiting growth hormone (GH) secretion, our understanding of the functions of somatostatin [or somatotrophin release inhibitory hormone (SRIH)], both in the periphery and the CNS, has grown enormously. With the cloning of five SRIH receptors, much interest has centred recently on the potential use of SRIH analogues in the treatment of clinical conditions ranging from human cancers to Alzheimer's and Parkinson's diseases. There is a growing recognition that the physiological functions of GH also need to be extended beyond its role in growth control, e.g. to a role in the maintenance of normal immune, cardiovascular and reproductive functions. Here, Glenda Gillies addresses the importance of somatostatinergic systems in regulating the sexually dimorphic patterns of GH secretion as well as their influence on other endocrine hormones. She also considers the neurotransmitter/neuromodulator actions of SRIH within the hypothalamus, where it is involved in the neural control and integration of many aspects of endocrine function, as well as its potential role in the maturation of the hypothalamus during the critical perinatal period.

Somatostatin and leu-enkephalin in the rat auditory brainstem during fetal and postnatal development

A transient expression of the neuropeptide somatostatin has been described in several brain areas during early ontogeny and several opioid peptides, such as leu-enkephalin, have also been found in the brain at this stage in development. It is therefore believed that somatostatin and leu-enkephalin may play a role in neural maturation. The aim of the present study was to describe the spatiotemporal pattern of somatostatin and leu-enkephalin immunoreactivity in the auditory brainstem nuclei of the developing rat and to correlate it with other developmental events. In order to achieve this goal, we applied peroxidase-antiperoxidase immunocytochemistry to rat brains between embryonic day (E) 17 and adulthood. Somatostatin immunoreactivity (SIR) was found in all nuclei of the auditory brainstem, yet it was temporally restricted in most nuclei. SIR appeared prenatally and reached maximum levels around postnatal day (P) 7, when great numbers of immunoreactive neurons were present in the ventral cochlear nucleus (VCN) and in the lateral lemniscus. At that time relatively low numbers of cells were labeled in the dorsal cochlear nucleus, the lateral superior olive (LSO), and the inferior colliculus (IC). During the same period, when somata in the VCN were somatostatin-immunoreactive (SIR), a dense network of labeled fibers was also present in the LSO, the medial superior olive (MSO), and the medial nucleus of the trapezoid body (MNTB). As these nuclei receive direct input from VCN neurons, and as the distribution and morphology of the somatostatinergic fibers in the superior olivary complex (SOC) was like that of axons from VCN neurons, these findings suggest a transient somatostatinergic connection within the auditory system. Aside from the LSO, MSO, and MNTB, labeled fibers were found to a smaller extent in all other auditory brainstem nuclei. After P7, the SIR decreased and only a few immunoreactive elements were found in the adult auditory brainstem nuclei, indicating that somatostatin is transiently expressed in the rat auditory brainstem. Leu-enkephalin immunoreactivity showed a lower number and weaker intensity of labeled structures as compared to SIR, with E18 being the earliest day at which labeled fibers appeared in the SOC. At birth, immunoreactive fibers were also present in the cochlear nuclear complex and in the IC. Leu-enkephalin immunoreactive somata were found only after P12 in the CN and after P16 in the IC. Leu-enkephalin immunoreactivity was not transient, but increased progressively with age until about P21, when the adult levels were reached.(ABSTRACT TRUNCATED AT 400 WORDS)