Brain region and gene specificity of neuropeptide gene expression in cultured astrocytes

Immunoelectron microscopic demonstration of oxytocin and vasopressin in pituicytes and in nerve terminals forming synaptoid contacts with pituicytes in the rat neural lobe

A pre-embedding immunoelectron microscopic technique was used to obtain morphological evidence for a role of oxytocin and vasopressin in the regulation of their own or each others release from the neural lobe. No synaptoid contacts of oxytocin- or vasopressin-containing axons with other neuronal structures were observed. However, synaptoid contacts of oxytocin- and vasopressin-containing nerve terminals and Herring bodies with pituicytes were frequently observed. These findings suggest that the pituicyte may participate in auto- and/or cross-regulation of oxytocin and vasopressin release. Moreover, oxytocin and vasopressin precursor-derived peptides were found in the cytoplasm of some pituicytes, an unexpected finding that will be discussed.

Differential effects of a phorbol ester on carboxypeptidase E in cultured astrocytes and AtT-20 cells, a neuroendocrine cell line

Cultured astrocytes have been shown to secrete various neuropeptides and the neuropeptide processing enzyme, carboxypeptidase E (CPE). The secretion of CPE enzymatic activity from astrocytes has been shown previously to be increased approximately twofold by treatment with tetradecanoylphorbol 13-acetate (TPA), a phorbol ester. In this study, metabolic labeling with [35S]Met was utilized to examine the effect of TPA on the biosynthesis of CPE protein in cultured astrocytes and in AtT-20 cells, a pituitary-derived cell line. Treatment of astrocytes with 0.1 micrograms/ml TPA for 24 h caused an 80% increase in the level of radiolabeled CPE in both the media and the cells, indicating that the synthesis of CPE was stimulated by the TPA. AtT-20 cells also secreted more radiolabeled CPE in response to TPA, but this increase was offset by a proportional decrease in the cellular level of radiolabeled CPE, and synthesis of CPE was not stimulated in this cell line. Northern blot analysis demonstrated that 0.1 micrograms/ml TPA elevated CPE mRNA by approximately 50% in cultured astrocytes but not in AtT-20 cells. Quantitative in situ hybridization studies demonstrated that the TPA-induced increase in CPE mRNA expression was largely due to increases in the number of cells expressing CPE mRNA, although for astrocytes from some brain regions the average level of CPE mRNA per cell was also elevated by TPA. These results suggest that astrocytes can be induced to express CPE, which is consistent with a role for astrocytes in intercellular signaling.

Astrocyte subtypes in the rat olfactory bulb: morphological heterogeneity and differential laminar distribution

Despite increased recognition of the importance and heterogeneity of astrocyte functions throughout the central nervous system (CNS) relatively little attention has been paid to morphological diversity among astrocytes. Recent studies have indicated that subsets of astrocytes are involved in glial-axonal interactions critical to both development and reinnervation of the rat olfactory bulb. Here, we have characterized the morphologies and distribution of astrocytes within anatomically and functionally distinct layers of the adult main olfactory bulb (MOB). Using a known immunohistochemical marker for astrocytes, glial fibrillary acidic protein (GFAP), and the classic gold sublimate method, we identified six astrocyte subtypes based on their morphology and distribution: (1) unipolar, (2) irregular, (3) wedge-shape, (4) circular, (5) semicircular, and (6) elongate. Unipolar, irregular and wedge-shape astrocytes have not been previously described in the CNS. The unipolar and irregular types are located exclusively in the olfactory nerve layer. Wedge-shape astrocytes are unique to, and are the major subtype in, the glomerular layer. These three morphologically unique astrocyte subtypes may correspond to olfactory nerve layer (ONL) and glomerular layer (GL) astrocytes, which express molecules that regulate axonal growth or synaptogenesis during development and/or regeneration of the olfactory nerve. In the glomerular layer, astrocytes are highly organized with respect to the glomeruli. Individual astrocytes are loyal to a single glomerulus. In the external plexiform layer, astrocytes are spaced relatively uniformly. In the granule cell layer, astrocytes appear to compartmentalize granule cell aggregates, recently shown to be coupled by tight junctions. The distribution and patterns of astrocyte processes and the density of GFAP immunoreactivity are distinctive for each of the layers of the olfactory bulb. The spacing of astrocytes and the organization of their processes may be important to compartmentalization of neuronal functions. High levels of GFAP immunoreactivity correlated with layers of high neuronal plasticity. The morphological diversity and differential distribution of astrocytes in the olfactory bulb reported here support growing evidence for functional diversity of astrocytes and important interactions among specific astrocyte and neuron subtypes. It is reasonable to hypothesize, therefore, that as for neurons, morphologically distinctive astrocyte subtypes may correspond to functionally specific classes.

Cultured astrocytes express mRNA for peptidylglycine-alpha-amidating monooxygenase, a neuropeptide processing enzyme

Cultured astrocytes have been previously found to express several neuropeptides, as well as the neuropeptide processing enzyme carboxypeptidase E (CPE). To investigate whether cultured astrocytes contain additional peptide-processing enzymes, Northern blots were screened for peptidylglycine-alpha-amidating monooxygenase (PAM) mRNA. PAM is involved with the formation of amide groups on the C-terminus of numerous peptide hormones and neurotransmitters. Primary cultures of astrocytes contain moderate levels of PAM mRNA, as determined by Northern blot analysis. The level of PAM mRNA in cultured hypothalamic astrocytes is similar to the level expressed in cultured hypothalamic neurons. The relative abundance of PAM mRNA differs up to 6-fold between astrocytes cultured from various brain regions. Astrocytes cultured from hypothalamus have high levels of PAM mRNA, those cultured from striatum, frontal cortex, and hippocampus have moderate levels, and those cultured from cerebellum have low levels. To investigate whether all cultured astrocytes express PAM mRNA, in situ hybridization analysis of cultured astrocytes was performed. Interestingly, virtually all of the astrocytes cultured from either hypothalamus or cerebellum express PAM mRNA, in contrast to a previous finding that only 20-40% of similarly cultured astrocytes express CPE. The presence of PAM mRNA in cultured astrocytes suggests that these cells have the capacity to produce amidated neuropeptides.

Characterization of endozepine-related peptides in the central nervous system and in peripheral tissues of the rat

Endozepines represent a novel family of regulatory peptides that have been isolated by their ability to displace benzodiazepines from their binding sites. All endozepines derive from an 86 amino acid precursor polypeptide called diazepam binding inhibitor (DBI), which generates, through proteolytic cleavage, several biologically active endozepines. The aim of the present study was to compare the molecular forms of endozepines present in different regions of the rat brain and in various peripheral organs using an antiserum raised against the central (biologically active) region of DBI. Combination of HPLC analysis and RIA detection revealed the existence of two major forms (peaks I and II) of endozepine-immunoreactive peptides. The retention times of the two peaks (36 and 39 min, respectively) were identical in all tissues or organs tested. Western blotting analysis of cerebral cortex extracts confirmed the existence of two immunoreactive species with apparent molecular weights 4000 and 6000 Da, which respectively correspond to peaks I and II. Tryptic digestion of peaks I and II generated a single immunoreactive peptide that coeluted with the synthetic octadecaneuropeptide ODN [DBI(33-50)]. These results show that, in different parts of the brain and in various peripheral organs, DBI is rapidly processed to generate two peptides of apparent molecular weight of 4000 and 6000 Da, which both possess the biologically active determinant of endozepines.

Evidence for upregulation of galanin synthesis in rat glial cells in vivo after colchicine treatment

The localization of galanin (GAL) and GAL mRNA was studied in rat brain after colchicine or vinblastine treatment using in situ hybridization and immunohistochemistry. GAL-like immunoreactivity was found in glial cells, presumably activated microglia, in the cortex, caudate nucleus and septum, mainly on the injection side. GAL mRNA expression was found in small cells in the same areas with an overlapping distribution, including the adjacent white matter. The results suggest that the glial cells initiate synthesis of the peptide GAL in response to intraventricular injection of high doses of the 2 mitosis inhibitors.

Transforming growth factor-alpha gene expression in the hypothalamus is developmentally regulated and linked to sexual maturation

Hypothalamic injury causes female sexual precocity by activating luteinizing hormone-releasing hormone (LHRH) neurons, which control sexual development. Transforming growth factor-alpha (TGF-alpha) has been implicated in this process, but its involvement in normal sexual maturation is unknown. The present study addresses this issue. TGF-alpha mRNA and protein were found mostly in astroglia, in regions of the hypothalamus concerned with LHRH control. Hypothalamic TGF-alpha mRNA levels increased at times when secretion of pituitary gonadotropins--an LHRH-dependent event--was elevated, particularly at the time of puberty. Gonadal steroids involved in the control of LHRH secretion increased TGF-alpha mRNA levels. Blockade of TGF-alpha action in the median eminence, a site of glial-LHRH nerve terminal association, delayed puberty. These results suggest that TGF-alpha of glial origin is a component of the developmental program by which the brain controls mammalian sexual maturation.

Brain corticotropin-releasing hormone receptors on neurons and astrocytes

Corticotropin-releasing hormone (CRH) exerts many potent effects within brain and is considered an important brain neuroregulator. CRH acts via receptors that are widely distributed throughout brain which exhibits highest CRH receptor concentrations in extrahypothalamic regions. We have previously characterized CRH receptors in heterogeneous extrahypothalamic forebrain cell cultures consisting of neurons and glia, and have shown them to exhibit similar kinetic and pharmacological characteristics as CRH receptors in pituitary and in situ brain. However, it is not known whether CRH receptors are present on neurons, glia or both. We tested the hypothesis that CRH receptors are present on neurons in extrahypothalamic forebrain cell cultures derived from day 17-18 fetal rats by characterizing receptors in predominantly neuronal (N), glial/astrocytic (G) cultures and mixed (M) cultures. Mean CRH receptor concentrations (fmol/mg protein) in N (10.4), G (9.4), and M (9.8) cultures were similar. Following Scatchard analyses derived from competition curves, all cell populations exhibited similar mean high-affinity/low-capacity (Kd = 1.0-1.9 nM; Bmax = 183-388 fmol/mg protein) and low-affinity/high-capacity (Kd = 92-104 nM; Bmax = 2034-5008 fmol/mg protein) classes of binding sites.

IN CONCLUSION

(1) Neurons and astrocytes in fetal extrahypothalamic brain cell cultures contain CRH receptors which exhibit similar concentrations and similar kinetic characteristics. (2) These observations suggest that biological effects of CRH in brain could be mediated via actions on neurons and/or glial astrocytes.

In vivo effects of beta-amyloid implants in rodents: lack of potentiation of damage associated with transient global forebrain ischemia

Recent studies have shown that the principal component of the senile plaque in Alzheimer's disease (AD), beta-amyloid protein (beta AP) can exert direct and indirect neurotoxicity in vitro. Because of the studies that demonstrated potentiation of excitatory amino acid toxicity by beta AP, we decided to test whether beta AP was able to potentiate damage in an in vivo model where excitotoxic damage is thought to be important. The present study evaluated the in vivo effects of beta AP implants in the brain of rats before and after being subjected to 10 min of transient global forebrain ischemia by 4-vessel occlusion (4-VO). Implants of either synthetic beta AP or prolactin (PRL), which was used as a control protein, were made into the striatum and the hippocampus of either the left (beta AP) or the right (PRL) cerebral hemisphere. The implants were made in a lipophilic, non-toxic vehicle so as to try and achieve sustained beta AP exposure. One group of animals was evaluated for direct in vivo effects within 1 week following implantation; the other group was subjected to 4-VO 3-4 days post-implantation for evaluation of potential indirect effects. This latter group was compared to the histopathology of animals subjected to 4-VO without prior implantation. In the group of animals evaluated for direct effects, no evidence of neurotoxicity was observed. Bielschowsky silver staining and immunostaining for ubiquitin were unremarkable in all lesions. beta AP was detected by immunocytochemistry in the parenchymal tissue that received beta AP implants. Marked glial activation was observed to be associated with experimental and control implants. Under the experimental conditions employed in this study, significant protection from ischemia rather than potentiation of damage was observed. These results suggest that beta AP may not be neurotoxic in rodents in vivo and that the lesions and/or trauma produced by the implantation procedure 3-4 days prior to 4-VO may have induced factors that were protective against ischemia-induced damage.

Directed establishment of rat brain cell lines with the phenotypic characteristics of type 1 astrocytes

Interest in obtaining cell lines for use in studies on the development and biochemistry of the central nervous system has motivated efforts to establish cells from primary brain cultures by the use of oncogene-transfer techniques. In previous reports, cell lines derived from astrocytes in this way have had immature or abnormal phenotypes. We have explored the possibility of specifically "targeting" expression of exogenous oncogenes to differentiated astrocytes by using the promoter of the gene encoding glial fibrillary acidic protein, which is expressed almost exclusively in such cells. We report here that cell lines displaying the phenotypic characteristics of type 1 astrocytes can be established reproducibly in this manner. Given the heterogeneity of primary cultures, the availability of clonal cell lines displaying characteristics of type 1 astrocytes should greatly facilitate our understanding of the biology of these cells.

Transforming growth factor-beta 1 differentially regulates proliferation and MHC class-II antigen expression in forebrain and brainstem astrocyte primary cultures

To facilitate investigation of cytokine regulation of reactive astrogliosis, primary astrocyte cultures from neonatal murine forebrain and brainstem were established. Forebrain and brainstem astrocytes proliferated at a similar rate under basal culture conditions, and both were growth-inhibited by treatment with recombinant murine interferon-gamma. The growth of cultured brainstem astrocytes was significantly enhanced by exposure to recombinant human transforming growth factor-beta 1. In contrast, proliferation of forebrain astrocytes was not significantly affected by transforming growth factor-beta 1. The disparate responses of brainstem and forebrain astrocytes to transforming growth factor-beta 1 treatment were not limited to effects on cell growth, since transforming growth factor-beta 1 could block interferon-gamma-induced MHC class-II antigen expression on cultured brainstem astrocytes but not on forebrain cells. Results could not be attributed to use of an heterologous cytokine/cellular target system, since similar variability in transforming growth factor-beta 1 modulation of major histocompatibility complex antigen expression could be demonstrated using two human astrocytoma cell lines. This report is the first to document mitogenic response to transforming growth factor-beta 1 for neuroepithelial cells. The role of transforming growth factor-beta 1 in regulating aspects of reactive astrogliosis, particularly in the context of inflammatory demyelination, requires further investigation. Furthermore, these studies may provide insight into regional variability in the sequelae of inflammation within the central nervous system.

Developmental expression of the proenkephalin and prosomatostatin genes in cultured cortical and cerebellar astrocytes

Astrocytes were prepared from rats of 4 ages, embryonic day 20, postnatal days 3 and 8, and adult, in order to study the developmental time course of expression of enkephalin and somatostatin (SS). Glial fibrillary acidic protein (GFAP) content was constant in both cortical and cerebellar astrocytes prepared from all ages. SS mRNA and peptide decreased over this developmental time course in cerebellar astrocytes; the time course of changes in SS mRNA paralleled that for rat cerebellum. Proenkephalin (PE) mRNA increased about 3-fold in cerebellar astrocytes from embryonic day 20 to adult but remained constant in cortical astrocytes; in contrast, PE mRNA showed a 10- to 12-fold increase in rat cerebellum and cortex developmentally. For both cerebellar and cortical astrocytes, free met-enkephalin decreased from embryonic day 20 to adult, whereas total met-enkephalin (measured following trypsin-carboxypeptidase B digestion of the extracts) increased. These results suggest (1) that there is a developmental regulation of the expression of both enkephalin and SS peptides in astrocytes, and (2) that the regulation occurs at the level of transcription for SS but at the level of precursor processing for PE. Possible trophic functions for astrocyte-derived peptides early in CNS development are discussed.

Secretion of carboxypeptidase E from cultured astrocytes and from AtT-20 cells, a neuroendocrine cell line: implications for neuropeptide biosynthesis

Cultured astrocytes have recently been shown to produce certain neuropeptides, as well as neuropeptide processing enzymes. To characterize the secretory pathway in cultured astrocytes, we used the neuropeptide processing enzyme carboxypeptidase E (CPE) as a marker for neuropeptide secretion. Cultured astrocytes and AtT-20 cells, a mouse pituitary-derived neuroendocrine cell line, were labeled with [35S]Met for 15 min and then chased with unlabeled Met. CPE was isolated from either medium or cell extracts using a substrate affinity column. The time course of secretion of radiolabeled CPE was significantly different for cultured astrocytes as compared with AtT-20 cells. CPE was rapidly secreted from the astrocytes after a 30-min lag time, presumably reflecting transport through the endoplasmic reticulum and Golgi apparatus, followed by constitutive secretion. The secretion of radiolabeled CPE was essentially complete by 2 h. In contrast, only a portion of the radiolabeled CPE was secreted from AtT-20 cells over a 2-3-h period, indicating that the majority of newly synthesized CPE is stored, presumably in secretory granules within the AtT-20 cells. The regulation of CPE secretion from astrocytes was also examined. CPE secretion is stimulated two- to threefold by prolonged treatment (3-48 h) with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) but not by treatment with other secretagogues that stimulate CPE secretion from AtT-20 cells (forskolin, isoproterenol, A23187, and vasoactive intestinal peptide) or short (less than 3 h) exposure to TPA. Taken together, these results indicate that the secretory pathway for CPE, and presumably neuropeptides, is substantially different in astrocytes than the secretory pathway for CPE in neuroendocrine cells.

The semi-quantitative distribution and cellular localization of angiotensinogen mRNA in the rat brain

The present study describes the regional distribution and cellular localization of angiotensinogen-mRNA in the rat brain as investigated by means of in situ hybridization also in combination with immunocytochemistry for glial fibrillary acidic protein. The angiotensinogen gene expression seemed to be restricted to astroglia and showed marked regional differences. In some areas angiotensinogen-mRNA was present in almost all astrocytes with a strong signal (e.g. hypoglossal nucleus), whereas in other areas the angiotensinogen gene was expressed only in a certain population of glial cells. Some areas like the lateral septum were devoid of any detectable angiotensinogen-mRNA. A semi-quantitative atlas of the regional distribution of brain angiotensinogen-mRNA was obtained by using computer-assisted microdensitometry and revealed considerable rostro-caudal fluctuations of the angiotensinogen-mRNA content of certain regions (e.g. the subfornical organ). Furthermore, a semi-quantitative analysis on the cellular level of angiotensinogen gene expression was performed showing a correlation of the angiotensinogen gene expression to the glia content of the regions examined. It was also demonstrated that the angiotensinogen gene expression had its highest levels in several distinct areas of the brain (e.g. the preoptic region and the hypothalamus), whereas other areas showed only low to moderate levels (e.g. the thalamus). The expression of the angiotensinogen gene in the rat brain was not only restricted to areas involved in cardiovascular and neuroendocrine control, but was also present in functionally different regions. Our data thus indicate that, based on the regional distribution of angiotensinogen-mRNA, angiotensin peptides may have other functions besides participation in cardiovascular and neuroendocrine control.

Neuronal activity triggers calcium waves in hippocampal astrocyte networks

The recent discovery that the neurotransmitter glutamate can trigger actively propagating Ca2+ waves in the cytoplasm of cultured astrocytes suggests the possibility that synaptically released glutamate may trigger similar Ca2+ waves in brain astrocytes in situ. To explore this possibility, we used confocal microscopy and the Ca2+ indicator fluo-3 to study organotypically cultured slices of rat hippocampus, where astrocytic and neuronal networks are intermingled in their normal tissue relationships. We find that astrocytic Ca2+ waves are present under these circumstances and that these waves can be triggered by the firing of glutamatergic neuronal afferents with latencies as short as 2 s. The Ca2+ waves closely resemble those previously observed in cultured astrocytes: they propagate both within and between astrocytes at velocities of 7-27 microns/s at 21 degrees C. The ability of tissue astrocyte networks to respond to neuronal network activity suggests that astrocytes may have a much more dynamic and active role in brain function than has been generally recognized.

Heterogeneous expression of carboxypeptidase E and proenkephalin mRNAs by cultured astrocytes

Cultured astrocytes have been found to express neuropeptides, neuropeptide processing enzymes and their mRNAs. Although astrocytes were shown to display regional variation in their expression of these mRNAs, it was unclear whether all astrocytes cultured from the same brain region express similar mRNA levels or if this expression is heterogeneous. We examined the individual heterogeneity of astrocytes cultured from several brain regions by in situ hybridization. Astrocytes derived from the frontal cortex, hypothalamus and cerebellum of neonatal rat brains were cultured for 3 weeks and then analyzed by in situ hybridization using 35S-labeled cRNA probes to carboxypeptidase E (CPE), proenkephalin (PE), and cyclophilin (1B15) mRNAs and an oligomeric probe complimentary to the first 45 bases of rat 18S rRNA. Frequency histograms generated by counting the grains produced over emulsion-coated cells demonstrated populations of astrocytes expressing high levels of mRNA for CPE and PE and a population expressing low to background levels of these mRNAs. In contrast, all cultured astrocytes expressed high levels of 1B15 mRNA and 18S rRNA. The percentage of cultured astrocytes expressing high levels of CPE mRNA was 42% for frontal cortex astrocytes and 23% for cerebellar astrocytes. While the percentages of cultured astrocytes expressing high levels of PE mRNA varied slightly between brain region (24-28%), the level of PE mRNA expression per cell showed greater variation between regions. The effect of culture density on the expression of PE mRNA was also examined. Approximately 55% of the cells in low density cultures expressed PE mRNA, while only 5-10% of the cells in high density cultures expressed this mRNA. These results indicate that cultured astrocytes display individual heterogeneity with regard to neuropeptide biosynthesis and that the expression of neuropeptides by these cells is regulated.

Uptake of [3H]serotonin and [3H]glutamate by primary astrocyte cultures. II. Differences in cultures prepared from different brain regions

Regional astrocyte cultures were derived by dissecting six regions; brain stem, cerebellum, mesencephalon, basal ganglia plus diencephalon, cerebral cortex, and hippocampus, from 3 to 4-day-old neonatal rat brains. Glial fibrillary acidic protein (GFAP) immunocytochemistry was used to confirm the astrocyte composition of the cultures. The percentage of GFAP (+) cells between regions varied from 75% to 100%. Once confluent these cultures were incubated with radiolabeled serotonin or glutamate for uptake and autoradiographic studies. For the different brain regions Na(+)-dependent, [3H] L-glutamate, and fluoxetine-sensitive [3H] 5-HT uptake varied markedly. The relative order of uptake for [3H] 5-HT was MS (mesencephalon) greater than CC (cerebral cortex) greater than BG + DI (basal ganglia + diencephalon) greater than HP (hippocampus) greater than BS (brain stem) greater than CB (cerebellum). For [3H] L-glutamate the order was HP greater than CC greater than BG + DI greater than MS = BS greater than CB. For [3H] 5-HT this essentially corresponds to the reported order of binding in situ of the [3H] 5-HT-specific uptake ligand [3H] citalopram. For [3H] L-glutamate regional variation of the uptake for the different cultures corresponds to the regional uptake reported for different regions of rat brain. Double-label studies with GFAP and radiolabeled neurotransmitters were also used to study uptake into GFAP(+) astrocytes by autoradiography. Flat GFAP cells with or without processes comprised 65-98% of the cultures and represented most of the uptake. The percentage of all GFAP(+) cells that were positive for uptake of ARG varied from 50% to 90% and also showed differences in grain density both intra- and inter-regionally. These differences in transmitter uptake by GFAP(+) astrocytes in primary culture, which are dependent on the region of origin and correspond to regional differences in situ, suggest that such uptake in vitro may reflect uptake by astrocytes in vivo. Implied in this is that uptake by astrocytes represents a significant component of serotonin uptake in vivo.

Heterogeneity in gap junction expression in astrocytes cultured from different brain regions

Heterogeneity among astrocytes suggests that their role in the central nervous system is more complex than is commonly recognized. This paper describes just such a functional difference, comparing gap junctions in astrocytes derived from two brain regions. Astrocytes, both in situ and in culture, employ gap junctions as a means of intercellular communication. Recent evidence utilizing cultured rat cortical and striatal astrocytes has shown that these channels consist of subunits of connexin 43, the same protein as that composing cardiac gap junctions. Here we report that astrocytes cultured from neonatal rat hypothalamus contain a greater number of functional channels than astrocytes from the striatum, a difference reflected in both connexin 43 protein and mRNA. Specifically, in hypothalamic astrocytes the level of connexin 43 protein was approximately four times that found in comparable cultures from the striatum, as determined by immunoblotting. Complementary results from immunocytochemical experiments using an antibody specific for connexin 43 reveal significantly greater fluorescence in astrocytes cultured from the hypothalamus as compared to those from the striatum. Northern blot analysis showed that connexin 43 mRNA levels were also approximately 4-fold greater in the hypothalamic cultures, consistent with the difference seen by immunoblotting. Finally, dye coupling studies using confluent cultures consistently showed that within 1 min Lucifer Yellow injected into striatal astrocytes spread to immediately surrounding cells while in hypothalamic astrocytes dye often spread to apparent third or fourth order neighbors within the same time period. Thus, the higher level of connexin 43 expression seen in hypothalamic astrocytes results in cells with greater numbers of functional channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Interleukin-1 beta regulates proenkephalin gene expression in astrocytes cultured from rat cortex

Glial cells execute essential functions in central nervous system (CNS) development and are also believed to play important roles during gliosis in response to trauma or disease. These developmental and pathological states have also been associated with elevated expression of opioid genes. Because levels of the cytokine interleukin-1 beta (IL-1 beta) increase following CNS lesions, we examined the possible influence of IL-1 beta on the expression of opioid genes in astrocytes cultured from rat cortex. Proenkephalin mRNA expression was stimulated by IL-1 beta in a time- and concentration-dependent manner, being maximal with 5 U/ml IL-1 beta at 4 h. Although the beta-adrenergic agonist isoproterenol was also active, interferon, glutamate, and carbachol were not. Unlike isoproterenol, the actions of IL-1 beta were not associated with a cyclic adenosine monophosphate (AMP)-dependent pathway. Interleukin-1 beta also regulated a proenkephalin-chloramphenicol acetyltransferase fusion gene transiently transfected into astrocytes, with a dose-response similar to that active in proenkephalin mRNA. These effects of IL-1 beta were region-specific, not being observed with either cerebellar or hippocampal astrocytes; however, isoproterenol was active in the latter cell populations. Proenkephalin mRNA in cortical astrocytes was stimulated following a temperature stress. These results suggest that enhanced proenkephalin gene expression in astrocytes by IL-1 beta may be important in neuroimmune interactions and in trauma-induced CNS injury or stress.

Synthesis and release of neuroactive substances by glial cells

Glia contain, synthesize, or release more than 20 neuroactive compounds including neuropeptides, amino acid transmitters, eicosanoids, steroids, and growth factors. The stimuli that elicit release differ among compounds but include neuropeptides, neurotransmitters, receptor agonists, and elevated external [K+]. The mechanisms of release are poorly understood in most cases. Many of the neuroactive compounds are localized in discrete subpopulations of glia. Thus, glia are equipped to send as well as receive chemical messages and appear to be present as classes of cells with differing abilities to communicate chemically. It is possible that glia are as diverse as neurons in their functional characteristics.

Serotonin promotes region-specific glial influences on cultured serotonin and dopamine neurons

To test the hypothesis that glia mediate interactions between embryonic serotonergic (5-HT) neurons and dopamine neurons, we studied the effects of 5-HT in co-cultures of E14 raphe neurons of mesencephalic dopamine neurons and radial glia/astrocytes derived from the same (homotypic) or opposite (heterotypic) brain region using a dose (10(-5) M) that would produce 5-HT uptake into glial cells as well as activate 5-HT receptors. Morphometric analysis of 5-HT and tyrosine hydroxylase (TH) immunoreactive neurons revealed regional differences in the effects of 5-HT (and nialamide) on survival, cell soma size, and dendrite-like neurite outgrowth in neuronal-glial co-cultures. In general, 5-HT had more significant effects on both types of monoamine neuron when they were cultured with mesencephalic glia (GSN). Stimulatory effects of 5-HT on growth of TH neurons in GSN cultures suggest that developing raphe axons, which reach the mesencephalon during the early differentiation of these neurons, may enhance the influence of local glial-derived trophic factors. Likewise, the promotion of 5-HT neuronal survival in these cultures suggests that glial factors in the mesencephalon may contribute to the support of 5-HT neurons in addition to the influences of raphe glia. The inhibitory effects of 5-HT on neurite outgrowth by raphe neurons in GSN co-cultures indicates enhanced sensitivity of these neurons to the inhibitory effects of 5-HT in the presence of mesencephalic glia. The region-specific effects of 5-HT and nialamide in glial co-cultures suggest that raphe and mesencephalic glia may express different capacities for 5-HT uptake, receptors, and/or monoamine oxidase (MAO) activities. These characteristics could be important for the specificity of growth-regulatory influences of glial cells on the development of brain monoamine neurons.

Neurotransmitters as Neurotrophic Factors: a New Set of Functions

At the start of this review, factors were deemed trophic if they stimulated mitosis, permitted neural cell survival, promoted neurite sprouting and growth cone motility, or turned on a specific neuronal phenotype. The in vitro evidence from cell cultures is overwhelming that both neurotransmitters and neuropeptides can have such actions. Furthermore, the same chemical can exert several of these effects, either on the same or on different cell populations. Perhaps the most striking example is that of VIP, which can stimulate not only mitosis, but also survival and neurite sprouting of sympathetic ganglion neuroblasts (Pincus et al., 1990a,b). The in vivo data to support the in vitro experiments are starting to appear. A role for VIP in neurodevelopment is supported by in vivo studies that show behavioral deficits produced in neonatal rats by treatment with a VIP antagonist (Hill et al., 1991). The work of Shatz' laboratory (Chun et al., 1987; Ghosh et al., 1990) suggests that neuropeptide-containing neurons, transiently present, serve as guideposts for thalamocortical axons coming in to innervate specific cortical areas. Along similar lines, Wolff et al. (1979) demonstrated gamma-aminobutyric acid-accumulating glia in embryonic cortex that appeared to form axoglial synapses and suggested the possibility that gamma-aminobutyric acid released from the glia might play a role in synaptogenesis by increasing the number of postsynaptic thickenings. Meshul et al. (1987) have provided evidence that astrocytes can regulate synaptic density in the developing cerebellum. The work of Zagon and McLaughlin (1986a,b, 1987) has shown that naltrexone, an antagonist of the endogenous opioid peptides, affects both cell number and neuronal sprouting. Lauder's laboratory (Lauder et al., 1982) has shown a role for 5-HT in regulation of the proliferation of numerous cell types. These studies illustrate another important point, that neurotransmitters and neuropeptides function in communication not only between neurons, but also between neurons and glial cells, and between glial cells. Given that astrocytes can express virtually all of the neural receptors and can produce at least some of the neurotransmitters and neuropeptides, they must now be considered equal partners in the processes of intercellular communication in the nervous system, including the trophic responses. The actions of neurotransmitters and neuropeptides have to be considered in terms of a broad spectrum of actions that range from the trophic actions described in this review, to the classic transmitter actions, to potential roles in neurotoxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of proenkephalin mRNA in developing cerebellar cortex of the rat: expression levels coincide with maturational gradients in Purkinje cells

The cellular localization of proenkephalin (PE) mRNA expression was systematically examined in midsagittal (vermal) sections of the developing rat cerebellar cortex by in situ hybridization. PE mRNA was initially detected in Golgi cells of postnatal day 7 (PND 7) rats and in each group thereafter. Moreover, PND 7 rats also displayed an intense layer of PE mRNA hybridization signal over the Purkinje cell layer. By PND 14, distinct cellular labeling was observed in a subpopulation of Purkinje cells in all lobules of the vermis except lobule III. At PND 7 and 14, the area and level of intensity of Purkinje cell associated PE mRNA hybridization signal followed a gradient that was most intense caudally but then decreased rostrally. At PND 21, the proportion of labeled Purkinje cells and the intensity of PE hybridization signal was evenly dispersed between the anterior and posterior lobules of the cerebellar vermis. PE hybridization signal was not detected in the developing neural cells of the external granular layer or the interneurons of the molecular layer in the vermis. These results indicate that the ontogeny of PE mRNA expression in Purkinje cells is developmentally regulated since levels of expression closely follow the chronological order of settling and maturation of these neurons. Based on prior evidence that endogenous opioids inhibit the growth of Purkinje cell dendrites and dendritic spines, PE expression is likely to be important for Purkinje cell maturation.

Immunohistochemistry of primitive neuroectodermal tumors in infants with special emphasis on cytokeratin expression

Eleven primitive neuroectodermal tumor (PNET) biopsies from infants under the age of 3 years were studied for the presence of various differentiation markers for neuroectodermal stem cells. Special emphasis was placed on the expression of cytokeratin proteins. The tumor cells expressed different cytokeratin proteins (CK8, CK13, CK18, CK19, KL1, AE1/AE3, MNF16) in 3 of 11 cases. These cases were furthermore characterized by a strong expression of glial fibrillary acidic protein, S-100 protein and vimentin. Vimentin and cytokeratin proteins were co-expressed; cross-reactivity between these two intermediate filaments could be excluded by immunoblotting. It is noteworthy that the three positive tumors were all from infants in their 1st year. We assume that PNETs in early infancy are characterized by a particularly wide range of differentiation patterns. The presence of cytokeratin proteins in these cases seems to be associated with the expression of vimentin and must be regarded as an indicator of an early developmental stage of the tumor cells.

Cultured astrocytes release proenkephalin

Astrocytes as well as neurons express the mRNA encoding the opioid peptide precursor, proenkephalin. In neurons proenkephalin is cleaved intracellularly to yield smaller, bioactive peptides such as Met-enkephalin and Leu-enkephalin. By contrast, utilizing a combination of radioimmunoassay and chromatographic analysis, we report here that astrocytes cultured from neonatal rat brain contain primarily unprocessed proenkephalin and only small amounts of Met-enkephalin. Further, similar experiments with and without the inclusion of several peptidase inhibitors indicate that cultured astrocytes release proenkephalin itself into the medium where it may be subsequently cleaved to smaller peptide products. The release of intact proenkephalin by astrocytes suggests that the glial propeptide subserves a different function than neuronal proenkephalin and that opioid peptides may play novel roles in the central nervous system.

Region-specific immunolocalization of atrial natriuretic peptide in mixed fetal rat brain cell cultures

In adult rodent CNS, atrial natriuretic peptide (ANP) has been localized by immunolabeling and nucleic acid hybridization techniques primarily to hypothalamic neurons and, to a lesser extent, to neurons of the telencephalon and brainstem. In canine brain, ANP immunoreactivity has been reported in neocortical and brainstem astrocytes. Yet, in a recent study using fetal rat tissue, ANP was detected by radioimmunoassay in predominantly neuronal, but not glial, cultures. In the present study ANP was detected by radioimmunoassay on in vitro day 8 in media derived from fetal rat diencephalic and rhombencephalic, but not telencephalic, cultures and in cell homogenates from all 3 regions surveyed. Using indirect immunofluorescence less than one cell per 400x field stained for ANP in the telencephalic cultures and ANP immunopositivity colocalized exclusively to neurons by concomitant anti-neurofilament immunolabeling. In diencephalic monolayers, approximately 1-3 cells per 400x field were ANP-positive; although most of these cells were neurons, small numbers of ANP-positive astrocytes were also detected using anti-glial fibrillary acidic protein (GFAP) immunolabeling. ANP-positive cells were most numerous in rhombencephalic cultures (5-10 cells per 400x field); as in diencephalic cultures, ANP immunoreactivity colocalized to both neurons and astrocytes in this region. In diencephalon and rhombencephalon, less than 1% of all ANP-positive cells were astrocytes and less than 1% of GFAP-positive astrocytes exhibited immunoreactive ANP. In fetal brain, neuronal and astrocytic ANP may play a role in fluid and electrolyte homeostasis. Alternatively, fetal brain cell ANP may subserve entirely different functions (e.g. as trophic factors) as has been suggested for other neuropeptides in the developing nervous system.

Characterization of opioid-dependent glial development in dissociated and organotypic cultures of mouse central nervous system: critical periods and target specificity

Opioid-dependent changes in glial growth were characterized in primary dissociated and organotypic explant cultures of the developing mouse central nervous system (CNS) continuously grown in the presence of an endogenous opioid, [Met5]enkephalin, or an opiate drug, morphine. The glia in dissociated, astrocyte-enriched cultures derived from the cerebra of postnatal day 1, 3, or 5 mice, respectively, displayed age-dependent reductions in glial numbers that occurred at 3, 7, or 9 days in vitro (DIV) in response to continuous [Met5]enkephalin (10(-6) M) exposure. In contrast, in cultures derived from gestational day 19 mice, glial numbers were not reduced following continuous exposure to 10(-6) M [Met5]enkephalin during the first 7 days in vitro. An examination of [3H]thymidine incorporation by glial fibrillary acidic protein-(GFAP) immunoreactive astrocytes with flat (type 1) morphology in dissociated cultures derived from postnatal day 1 mice revealed that the reduction in glial numbers at 3 DIV was not immediately preceded by a reduction in the rate of [3H]thymidine incorporation at 2 DIV, although previous studies have shown that opioids inhibit the rate of [3H]thymidine incorporation by more mature astrocytes at 4 or 6 DIV. Early (i.e., at 2 to 3 DIV) changes in glial numbers may result from an inhibition of the proliferative rate of non-GFAP-containing glia or astrocyte precursors, or an enhanced rate of glial death. The rate of [3H]thymidine incorporation by GFAP-immunoreactive astrocytes with process-bearing (type 2) morphology was unchanged by opioid treatment. In separate experiments, a comparison of the area of growth of GFAP-immunoreactive astrocytes in paired symmetrical (right vs left) organotypic explant cultures demonstrated that opiates (i.e., 10(-5) M morphine) can inhibit astrocyte growth when the normal histiotypic organization of neurons and glia are maintained, and that there are regional differences in astrocyte responsiveness. Opioid-dependent alterations in astrocyte growth were mediated through specific opioid receptors since they were prevented by simultaneous treatment with (-)naloxone. The results suggest that the ability of opioids to modify glial growth is highly selective and varies depending on astrocyte type, as well as temporal and regional factors. Spatial and temporal differences in the response of developing glia to opioids may determine critical periods of CNS vulnerability to opioids in the maturing brain.

Characterization of Xenopus laevis proenkephalin gene

Enkephalins are opiate peptides found in a variety of tissues including brain and pituitary. In brain, they function as neurotransmitters, neuromodulators and neurohormones. Recent studies show that proenkephalin mRNA is expressed early in development both in mammals and the amphibian, suggesting that enkephalins may play a unique role in embryogenesis. In order to characterize factors which regulate the onset and patterning of expression of this gene in adult and developing frog embryos, the proenkephalin A gene was cloned from Xenopus laevis. The clones have been characterized by DNA sequencing and restriction endonuclease mapping. The gene is made up of three exons which span approximately 12 kb. Exon I encodes the 5' untranslated region of the mRNA. Exon II contains the signal peptide and the N terminus of the mature protein. Biologically active opioid peptides are generated from exon III. Comparison to mammalian proenkephalin genomic sequence indicated that nucleotide sequences of the 5' flanking region, noncoding exon I and exon II were not well conserved but exon III was highly conserved. Primer extension and RNase protection assay analyses of the RNA transcripts revealed two major 5' ends. The putative TATA box, CAAT box, CRE and Pit 1 elements have been identified on this gene by sequence homology to published consensus sequences. To assay for sequences that could potentially regulate Xenopus proenkephalin expression, we transfected constructs that contained upstream genomic sequences linked to the CAT reporter gene into various eukaryotic cell lines. The expression of the fusion gene constructs were detected and could be induced 10- to 30-fold upon treatment with forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)

Migration patterns of donor astrocytes after reciprocal striatum-cerebellum transplantation into newborn hosts

Fragments of striatum or cerebellum from E 25 rabbit embryo were implanted into either the striatum or the mesencephalon of newborn mice. Implanted rabbit astrocytes were selectively identified by monoclonal antibodies to the GFAP which are unable to combine with mouse GFAP. Previous investigations had shown that xenogenic astrocytes have the capacity to migrate in host CNS. The purpose of this study was to compare the patterns of migration of transplant-derived astroglial cells according to the topographic origin of the transplant and location of the grafting site. We found that the migration pattern of the grafted cells from any of both selected sites of implantation was independent from the topographic origin of the transplant. The routes as well as the distances of migration were similar after homo- or heterotopic transplantation. We conclude that astroglial cells or their precursors do not express information which would direct them to move specifically toward a defined region in the host brain according to the region of origin in the donor.

Region-specific regulation of preproenkephalin mRNA in cultured astrocytes

Regulation of preproenkephalin (PPE) mRNA was examined in astrocytes cultured from several regions of the neonatal rat brain. Astrocytes from these regions expressed differing levels of PPE mRNA, with higher levels in astrocytes from the hypothalamus followed by frontal cortex and striatum. Further, PPE mRNA was regulated differently in hypothalamic than in striatal glia. Treatment of striatal astrocytes with the beta-adrenergic agonist, isoproterenol, or with agents which directly increased intracellular cAMP (forskolin or 8-bromo-cAMP) elevated levels of PPE mRNA. By contrast, none of these treatments altered levels of PPE mRNA in hypothalamic astrocytes despite increasing cAMP levels 60-fold. These observations indicate that there is striking regional heterogeneity in the expression and regulation of PPE mRNA by astrocytes, suggesting that proenkephalin or its derived peptides help to mediate region-specific brain functions.

Desipramine modulation of sigma and opioid peptide receptor expression in glial cells

Exposure of C6 glial cell cultures to desipramine induced the appearance of opioid receptors and up-regulated sigma receptors. Opioid binding was demonstrated with 3H-etorphine and 3H-dihydromorphine (DHM), but was not observed with the mu, delta and kappa ligands 3H-DAMGE, 3H-DADLE or 3H-(-)ethylketocyclazocine in the presence of specific blockers, respectively. Competition experiments with 3H-DHM and either (-)naloxone or (+)naloxone indicated the presence of authentic opioid receptors. In similar studies with beta-endorphin, its truncated form (1-27) or their N-acetyl derivatives, beta-endorphin proved to have the highest affinity. Opioid receptors in glial cell aggregates were primarily kappa, with few mu and delta sites. Desipramine increased Bmax values for kappa but not mu and delta.

Postmortem changes in rat brain extracellular opioid peptides revealed by microdialysis

Microdialysis combined with a solid-phase radioimmunoassay was used to monitor changes in extracellular opioid peptide levels in the rat globus pallidus/ventral pallidum as a result of terminal brain ischemia. Ischemia was induced by anesthetic overdose or by severance of blood vessels supplying the brain. In control animals the recovered immunoreactivity increased an average of 13-fold in the 30-min sample following anesthetic overdose. Perfusion of a calcium-free, 10 mM EGTA-containing medium through the dialysis probe significantly attenuated the amplitude of this response, with the average increase being only threefold. Shorter sampling intervals (5 min) indicated that release of opioid peptide material into the extracellular environment occurs within the first 5 min of ischemia resulting from severance of the blood supply to the brain. HPLC analysis identified the majority of the postmortem-induced immunoreactive material as Met- and Leu-enkephalin.

Morphine alters astrocyte growth in primary cultures of mouse glial cells: evidence for a direct effect of opiates on neural maturation

To determine whether exogenous opiate drugs with abuse liability directly modify neural growth, the present study investigated the effects of morphine on astrocyte proliferation and differentiation in primary cultures of murine glial cells. The results indicate that morphine decreases glial cell production in a dose-dependent, naloxone-reversible manner. Most notably, gliogenesis virtually ceased in the presence of 10(-6) M morphine during the first week in culture, whereas 10(-8) M or 10(-10) M morphine caused an intermediate suppression of growth compared to control or 10(-6) M morphine treated cultures. Moreover, morphine treatment inhibited [3H]thymidine incorporation by glial fibrillary acidic protein (GFAP) immunoreactive, flat (type 1) astrocytes, suggesting that the decrease in glial cell production was due in part to an inhibition of astrocyte proliferation. Morphine also caused significant increases in both cytoplasmic area and process elaboration in flat (type 1) astrocytes indicating greater morphologic differentiation. In the above experiments, morphine-dependent alterations in astrocyte growth were antagonized by naloxone, indicating that morphine action was mediated by specific opioid receptors. These observations suggest that opiate drugs can directly modify neural growth by influencing two critical developmental events in astrocytes, i.e., inhibiting proliferation and inducing morphologic differentiation.

D2 dopamine receptor gene expression in the rat striatum during ontogeny: an in situ hybridization study

D2 dopamine receptor (D2R) gene expression in the rat striatum was studied by in situ hybridization throughout the pre- and the postnatal period from gestational day 12 to postnatal day 8. D2R mRNA was detected with 35S-labelled oligonucleotide probes, one that hybridized equally to the two isoforms of the D2R mRNA (D2(415) and D2(444)) and the other that hybridized specifically to the large isoform (D2(444)). D2R mRNA was first detected in the striatal primordium at day 14 of gestation with the probe that recognizes indifferently the two isoforms and with the probe specific for the D2(444) mRNA. At day 16, D2R mRNA was present in the lateral part of the striatum and in the germinal ventricular zone lining the lateral ventricle. At day 18, D2R mRNA was found in neurons of the caudate-putamen, the nucleus accumbens, the olfactory tubercle and the subependymal zone lining the lateral ventricle. The microautoradiographic analysis demonstrated that the labelled cells have a neuroblastic and immature aspect before birth. After birth the topography and aspect of labelled cells was similar to the one observed in the adult animals. D2R mRNA was present in neurons of the caudate-putamen, the nucleus accumbens and the olfactory tubercle. In the caudate-putamen there was a latero-medial gradient of labelling. From postnatal day 2 onward the D2R gene was expressed in two striatal cell types, small neurons probably enkephalinergic, and large-sized neurons with prominent cytoplasm, most probably cholinergic.(ABSTRACT TRUNCATED AT 250 WORDS)

Retrovirus-induced spongiform myeloencephalopathy in mice: regional distribution of infected target cells and neuronal loss occurring in the absence of viral expression in neurons

The Cas-Br-E murine leukemia virus (MuLV) induces a spongiform myeloencephalopathy resulting in a progressive hindlimb paralysis. We have used in situ hybridization with a Cas-Br-E MuLV-specific probe to study viral expression in the central nervous system. Infected cells were concentrated in regions where spongiform lesions and gliosis are detected (lumbosacral spinal cord, brainstem, deep cerebellar regions), suggesting a causative link between the level of virus expression and the degree of pathological changes in this disease. However, viral expression was not in itself sufficient to cause disease, since significant viral expression was observed in regions that did not exhibit pathological changes (cerebellar cortex, hippocampus, corpus callosum, peripheral nervous system). In both diseased and nondiseased regions, endothelial and glial cells were identified as the main target cells. Neurons in diseased regions did not show viral expression. The regional distribution of the spongiform changes appears to be laid down very early following infection, since expression could be detected at 10 days postinfection in regions that become diseased. These results indicate that nonneuronal cells have distinct properties in various regions of the central nervous system and suggest an indirect mechanism of neuronal loss consequent to viral expression in nonneuronal cells.

Co-localization of proenkephalin mRNA using cRNA probes and a cell-type-specific immunocytochemical marker for intact astrocytes in vitro

To determine whether cultured astrocytes express opioid gene mRNA, a method was developed for co-localizing a cell-type specific immunocytochemical marker for astrocytes, glial fibrillary acidic protein (GFAP), and proenkephalin mRNA in situ hybridization signal using high affinity cRNA probes. GFAP immunoreactivity and proenkephalin mRNA hybridization reaction were examined in intact glial cell preparations from neonatal mice that were cultured for 4-6 days prior to fixation. The double labelling method described herein permits the unambiguous identification of mRNA expression in specific populations of intact cultured cells using cell type-specific markers.

Retinal neurite growth on astrocytes is not modified by extracellular matrix, anti-L1 antibody, or oligodendrocytes

Two factors that may influence the course of axonal regeneration in the central nervous system (CNS) are extracellular matrix (ECM) and cell surface molecules that may enhance or inhibit neurite outgrowth. Whereas cultured astrocytes have been reported to be a good substratum for neurite outgrowth, there is recent evidence that cultured oligodendrocytes are inhibitory. To test the influences of 1) ECM components, 2) the L1 adhesion molecule, and 3) the inhibitory potential of mature oligodendrocytes in the astrocytic environment, we have utilized a culture system in which neurites from embryonic rat retina grow vigorously on astrocyte monolayers. The major ECM components were assembled in neonatal rat cortical astrocyte-retina co-cultures only when the medium contained serum. In electron microscopic studies of serum containing cultures, retinal neurites were seen to be related to astrocyte surfaces but rarely were found in contact with ECM; in serum-free medium the association between neurites and astrocytes was similar. In addition, the growth of neurites was vigorous whether ECM was present or absent. Presence of antibodies against the cell surface adhesion molecule L1 did not inhibit retinal neurite elongation on glial fibrillary acidic protein-positive astrocytes. When oligodendrocytes from adult rat spinal cord were combined with the astrocytes, retinal neurites grew as well on the mixed glial population as on astrocytes alone. Immunostaining for galactocerebroside showed many oligodendrocyte processes to be aligned in the direction of neurite growth, suggesting association between the two cell types. This association was verified by electron microscopy. Furthermore, retinal explants extended neurites among myelin basic protein-positive oligodendrocytes cultured without astrocytes. Thus, the astrocyte surface is a strong promoter of neurite growth from embryonic rat retina. This growth did not depend upon either ECM or the L1 adhesion molecule. Because neurites grew on astrocytes in the presence of mature oligodendrocytes or among oligodendrocytes alone, we conclude that oligodendrocytes do not inhibit neurite growth under certain conditions.

Immunocytochemical distribution of glial fibrillary acidic protein in the central nervous system of the Japanese quail (Coturnix coturnix japonica)

In the present study we detailed the distribution of GFAP-immunopositive structures within the central nervous system of the Japanese quail. Different fixation and embedding procedures were applied. The best results were obtained on frozen cryostatic sections from freshly dissected brains subsequently fixed by a short immersion in cold acetone. Immunopositive structures were observed both with immunofluorescence, and with immunoperoxidase methods. Immunoreactive cell bodies and processes were observed within the whole central nervous system, and different cell types can be identified on the basis of their topographical location and morphology. A first class of astrocytes is composed of intensely stained unipolar cells lining the inner surface of the pia mater and the large blood vessels. A second type is represented by multipolar astrocytes of variable size, provided with an irregular cell body. The last type is represented by similar elements, showing an immunonegative cell body, that can be identified only by the presence of converging processes. These three types of cells, and several isolated processes, show a differential distribution within the quail central nervous system, both in the grey and in the white matter. Present results suggest that GFAP may represent a good marker for at least part of the astroglial population in quail.

Calcium-dependent regulation of the enkephalin phenotype by neuronal activity during early ontogeny

Genetic components of the neuronal phenotype are regulated by epigenetic factors--trophic molecules and neuronal activity--during neurodifferentiation. Developing neurons in dissociated cultures of embryonic mouse spinal cord show spontaneous electrical activity after one week in culture. We now report that the blockade of this spontaneous electrical activity for two days with tetrodotoxin (TTX) causes virtually complete down-regulation of preproenkephalin A gene transcripts in embryonic spinal cord cultures. This TTX-induced down-regulation is fully reversed upon reinitiation of neuronal activity (removal of TTX from cultures). This reversible, tetrodotoxin-induced down-regulation of enkephalin mRNA is confined to a restricted period of early neurodevelopment (days 7 to 21 in culture). Since depolarization triggers calcium entry through voltage-activated calcium channels, we have investigated the involvement of calcium in the mechanism of this activity- and age-dependent regulation of preproenkephalin A expression. The selective activation of the L-type of voltage-sensitive calcium channels by a dihydropyridine derivative [(+) 202-791] prevented this TTX-induced down-regulation without reducing methionine enkephalin secretion. This effect was observed only when the drug was applied to electrically active cultures, prior to the addition of TTX. Simultaneous application of (+) 202-791 and TTX, or pretreatment with TTX, failed to prevent TTX-induced down-regulation. Thus, activity-dependent phenotypic plasticity of met-enkephalinergic neurons in spinal cord is: 1) maximum at an early age of neuronal development (less than 10 days in culture) and becomes less apparent in old cultures (greater than 30 days); 2) reversible throughout; and 3) mediated by calcium entry through L-type channels.

Striatal proenkephalin turnover and gene transcription are regulated by cyclic AMP and protein kinase C-related pathways

Preproenkephalin metabolism, in the rat, was studied in primary striatal neurons maintained in a chemically defined medium. Acute treatment (30 min) with forskolin (10(-5) M) or phorbol 12 myristate 13 acetate (10(-7) M) resulted, respectively, in a two- and seven-fold increase in methionine-enkephalin secretion. Chronic treatment with forskolin or phorbol 12 myristate 13 acetate (24 h) induced a 100% increase in methionine-enkephalin content (forskolin) and on the other hand a 50% decrease in methionine-enkephalin (phorbol 12 myristate 13 acetate). Both treatments increased preproenkephalin mRNA levels in a time-dependent manner, this augmentation being observable after 180 min by Northern blot analysis and in situ hybridization. These data indicate that under chronic stimulation, with either forskolin or phorbol 12 myristate 13 acetate, proenkephalin turnover is accelerated. However, after stimulation with phorbol 12 myristate 13 acetate, the more potent methionine-enkephalin secretagogue, increased peptide synthesis is not sufficient to replenish methionine-enkephalin intracellular stores. Preproenkephalin gene transcription was analysed by introducing the preproenkephalin gene promoter fused to the bacterial acetyl chloramphenicol transferase reporter gene into primary neurons. Chronic stimulation (48 h) by forskolin (10(-5) M) or phorbol 12 myristate 13 acetate (10(-7) M) of striatal neurons transfected with this fusion gene increased chloramphenicol acetyltransferase activity six-fold and the two effects were additive. These data suggest that the cyclic AMP and the protein kinase C pathways directly activate preproenkephalin gene transcription.

Elevation of Met-enkephalin-like immunoreactivity in the rat striatum and globus pallidus following the focal injection of excitotoxins

The present study examined the effects of excitotoxins which activate distinct excitatory amino acid (EAA) receptor subtypes on the levels of Methionine-enkephalin-like immunoreactivity (ME-i.r.) in the striatum and globus pallidus, with a view to developing a model of the striatopallidal enkephalin deficit that prevails in Huntington's disease (HD). Each of the 4 excitotoxins, N-methyl-D-aspartate (NMDA, 50-150 nmol), quisqualate (QUIS, 26.5-102 nmol), kainate (KA, 0.5-7 nmol) and quinolinate (QUIN, 18-288 nmol), were unilaterally infused into the right striatum under halothane anaesthesia. Seven days after the injection, levels of ME-i.r. in the ipsilateral and contralateral striatum or globus pallidus were measured by radioimmunoassay (RIA). Injection of each of the 4 excitotoxins produced dose-related and bilateral elevations in ME-i.r. in both brain regions. Generally, the excitotoxin-induced contralateral response mirrored that on the ipsilateral side and the globus pallidus showed a greater change in ME-i.r. levels than did the striatum. The rank order of apparent efficacy for these 4 agents, based on the magnitude of the maximal effect produced by the excitotoxin, was QUIN = KA greater than NMDA = QUIS. In contrast, the rank order of apparent potency, based on the doses producing a maximal effect, was KA greater than QUIS greater than QUIN greater than NMDA. Histological examination of brain sections revealed that in all cases of excitotoxin injection, the dose producing a maximal increase in ME-i.r. was associated with tissue damage in the injection area. However, no tissue damage was apparent in the globus pallidus or the contralateral striatum. To determine the involvement of EAA receptors in the observed elevations of ME-i.r., the action of 3 EAA antagonists was evaluated in co-injection experiments. Kynurenate (KYN), but not CNQX, antagonized the actions of QUIS on pallidal ME-i.r. levels. Both KYN and CPP, a potent NMDA receptor antagonist, blocked the effect of QUIN. The possibility that contralateral changes in the striatum or globus pallidus were due to mobilization of an endogenous EAA was investigated by injection of CPP into the striatum contralateral to the QUIN infusion. This injection of CPP (1.8-3.6 nmol) did not block the QUIN-induced contralateral response, but reduced the elevation in ME-i.r. in the ipsilateral pallidum. Although the excitotoxin-induced changes in ME-i.r. levels do not appear to correspond to the enkephalin deficit seen in HD, such a deficit may be discernible in different parameters of enkephalinergic cell function.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term evaluation of hemiparkinsonian monkeys after adrenal autografting or cavitation alone

Autografts of adrenal medulla were implanted into preformed cavities in the caudate nuclei of four rhesus monkeys with hemiparkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Five other hemiparkinsonian monkeys underwent caudate cavitation, but received no tissue implant. All of the animals had marked bradykinesia of the affected arm and stable apomorphine-induced turning before cavitation or implantation. Moderate behavioral recovery was seen in all five monkeys with cavitation and two of the three monkey with long-term adrenal autografts (the fourth adrenal recipient was sacrificed 10 days after grafting). The improvement occurred months after the procedure and was not as early or as complete as that seen after fetal dopaminergic grafts. Surviving adrenal tissue was found only in the animal that showed no behavioral recovery. The other two adrenal autograft recipients (with no surviving adrenal medulla) and all of the animals with cavitation had ingrowth of dopaminergic fibers from the area olfactoria and nucleus accumbens into the caudate, oriented toward the cavity. These findings show that the mechanism of improvement after adrenal medullary implants for parkinsonism is not dopamine secretion by chromaffin cells, but may be related to the sprouted host fibers. The results also indicate that the limited recovery after adrenal implants in parkinsonian patients may be a result of the cavitation, and not necessarily the result of tissue implantation.

ATP-evoked arachidonic acid mobilization in astrocytes is via a P2Y-purinergic receptor

To reveal more of the mechanism whereby ATP induces arachidonic acid (AA) mobilization in astrocytes, primary cell cultures prelabeled with [3H]AA were exposed to ATP and various analogs. Release of 3H was dose and time dependent and was inhibited by blocking ATP binding. The potencies of a range of ATP analogs in mobilizing AA were consistent with that predicted for the involvement of a P2Y-purinergic receptor. Mobilization of AA was not due to non-specific cell permeabilization, as assessed by leakage of cytoplasmic lactate dehydrogenase. AA mobilization by ATP was reduced when mobilization of intracellular calcium was inhibited and in the absence of extracellular calcium. Thapsigargin, which induces release of intracellular calcium, evoked mobilization of AA and thromboxane formation, findings similar to the effects of ATP. These results suggest that ATP stimulates AA mobilization via a P2Y-purinergic receptor and that, although extracellular calcium is involved, mobilization of intracellular calcium activates phospholipase A2.

Detection of vasopressin mRNA in the neurointermediate lobe of the rat pituitary

A messenger ribonucleic acid (mRNA) homologous to the transcript that encodes vasopressin (VP) was detected in the neurointermediate lobe (NIL) of the rat pituitary. The abundance of this transcript is approximately 1/100th the amount detected in the hypothalamus. In rats drinking 2% NaCl-water for 0,2,4, or 10 days, or for 10 days and then tap water for 14 days, the levels of VP mRNA in the NIL were altered in a fashion that paralleled changes in the hypothalamus.

Expression of proenkephalin A mRNA and enkephalin-containing peptides in cultured fibroblasts

Proenkephalin A (PEA) gene was found to be expressed in primary, secondary and tertiary cultures of rat fibroblasts. The 1.4 kb PEA mRNA was detected by Northern blot analysis. The same cultures do not express detectable amounts of proenkephalin B (prodynorphin) or (POMC) mRNAs. Acidic cell extracts were purified on a C18 octadecyl Amprep column and analysed with a specific methionine enkephalin radioimmunoassay to detect whether PEA mRNA is translated. A significant amount of enkephalin immunoreactivity (178-185 fmol/mg protein) was observed upon trypsin and carboxypeptidase B digestion of fibroblast cell extracts, whereas only 3-5% of this amount was free enkephalin. It is therefore indicated that the PEA mRNA expressed in fibroblasts is indeed translated to the proenkephalin precursor protein, but the cells accumulate only a small quantity of the processed pentapeptides. The implication of these observations to the possible developmental role of PEA in various non-neuronal cells, including mesodermal lineages, is discussed.