Expression of the enkephalin precursor gene in C6 rat glioma cells: regulation by beta-adrenergic agonists and glucocorticoids

Calcium modulates endopeptidase 24.15 (EC 3.4.24.15) membrane association, secondary structure and substrate specificity

The metalloendopeptidase 24.15 (EP24.15) is ubiquitously present in the extracellular environment as a secreted protein. Outside the cell, this enzyme degrades several neuropeptides containing from 5 to 17 amino acids (e.g. gonadotropin releasing hormone, bradykinin, opioids and neurotensin). The constitutive secretion of EP24.15 from glioma C6 cells was demonstrated to be stimulated linearly by reduced concentrations of extracellular calcium. In the present report we demonstrate that extracellular calcium concentration has no effect on the total amount of the extracellular (cell associated + medium) enzyme. Indeed, immuno-cytochemical analyses by confocal and electron microscopy suggested that the absence of calcium favors the enzyme shedding from the plasma membrane into the medium. Two putative calcium-binding sites on EP24.15 (D93 and D159) were altered by site-directed mutagenesis to investigate their possible contribution to binding of the enzyme at the cell surface. These mutated recombinant proteins behave similarly to the wild-type enzyme regarding enzymatic activity, secondary structure, calcium sensitivity and immunoreactivity. However, immunocytochemical analyses by confocal microscopy consistently show a reduced ability of the D93A mutant to associate with the plasma membrane of glioma C6 cells when compared with the wild-type enzyme. These data and the model of the enzyme's structure as determined by X-ray diffraction suggest that D93 is located at the enzyme surface and is consistent with membrane association of EP24.15. Moreover, calcium was also observed to induce a major change in the EP24.15 cleavage site on distinctive fluorogenic substrates. These data suggest that calcium may be an important modulator of ep24.15 cell function.

Field-specific changes in hippocampal opioid mRNA, peptides, and receptors due to prenatal morphine exposure in adult male rats

Alterations in the opioid system in the hippocampal formation and some of the possible functional consequences were investigated in adult male rats that were prenatally exposed to either saline or morphine (10 mg/kg twice daily on gestational days 11-18). In situ hybridization and Northern blots were used to measure proenkephalin and prodynorphin mRNA, and radioimmunoassays quantified proenkephalin- and prodynorphin-derived peptide levels in the dentate gyrus, CA3, and CA1 subfields of the hippocampal formation. Prenatal morphine exposure in male rats decreases proenkephalin and increases prodynorphin mRNA selectively in the granule cell layer of the dentate gyrus. Similarly, met-enkephalin peptide levels are decreased and dynorphin B peptide levels are increased in the dentate gyrus but not CA3 or CA1 of prenatally morphine-exposed males. In addition, there are decreases in dynorphin-derived peptides in the CA3 subfield. Receptor autoradiography revealed increases in the density of micro but not delta receptor labeling in discrete strata of specific hippocampal subfields in morphine-exposed males. Because alterations in the hippocampal opioid system suggest possible alterations in the excitability of the hippocampal formation, changes in opioid regulation of seizures were examined. Morphine exposure, however, does not alter the latency to onset or number of episodes of wet dog shakes or clonic seizures induced by infusion of 10 nmol [D-Ala2, MePhe4, Gly-ol5]enkephalin into the ventral hippocampal formation. Interestingly, a naloxone (5 mg/kg) injection 30 min before bicuculline administration reverses the increased latency to onset of clonic and tonic-clonic seizures in morphine-exposed males. Thus, the present study suggests that exposure of rats to morphine during early development alters the hippocampal opioid system, suggesting possible consequences for hippocampal-mediated functions.

The comparative analysis of proenkephalin mRNA expression induced by cholera toxin and pertussis toxin in primary cultured rat cortical astrocytes

In rat astrocytes, incubation with cholera toxin (CTX; 0.1 microg/ml) for 8 h increased proenkephalin (proENK) mRNA level (10-fold), which was further increased by dexamethasone (DEX; 1 microM) (2.2-fold as much as CTX alone). Although pertussis toxin (PTX; 0.1 microg/ml) did not affect the basal proENK mRNA level, DEX significantly increased proENK mRNA level in PTX-treated cells (6-fold). The inhibition of protein synthesis by cycloheximide (CHX; 15 microM) also increased proENK mRNA level in PTX-treated cells (5.2-fold), but not in CTX-stimulated cells. The treatment with CTX, but not PTX, increased c-Fos and Fra-2 protein levels as well as AP-1, CRE, or ENKCRE-2 DNA binding activity, but neither toxin affected Fra-1, c-Jun, JunB, and JunD protein levels. CHX significantly attenuated CTX-induced increase of c-Fos or Fra-2 protein level and AP-1, CRE, or ENKCRE-2 DNA binding activity, although CHX alone did not affect the basal AP-1, CRE, and ENKCRE-2 DNA binding activities. Phosphorylated CREB level was increased by both CTX and PTX, although the magnitude of phosphorylation of CREB by PTX was much less than that by CTX. In addition, CHX further or persistently increased PTX- or CTX-induced phosphorylated CREB levels in parallel with increases in proENK mRNA. However, DEX did not alter the basal or stimulated phosphorylated-CREB level. These results suggest that the elevation of phosphorylation of CREB rather than AP-1 level may be involved in CTX-induced and CHX-dependent-PTX-induced increase of proENK mRNA level. In addition, AP-1 expression or CREB phosphorylation appears not to be involved the potentiative action of DEX on proENK mRNA expression in CTX- and PTX-treated astrocytes.

The differential molecular mechanisms underlying proenkephalin mRNA expression induced by forskolin and phorbol-12-myristic-13-acetate in primary cultured astrocytes

In rat astrocytes, forskolin (FSK; 5 microM) and phorbol-12-myristic-13-acetate (PMA; 2.5 microM) increase the proenkephalin (proENK) mRNA level via different pathways. FSK-induced proENK mRNA expression is independent of protein de novo synthesis, and well correlated with CREB phosphorylation. This is in contrast to PMA-induced proENK mRNA expression that is dependent on protein de novo synthesis and is well correlated with the increase of AP-1 DNA binding activity rather than CREB phosphorylation. Differential regulation of AP-1 proteins by PMA and FSK was also observed. While c-Fos, Fra-2 and JunB were increased in response to either stimuli, only Fra-1, c-Jun and JunD were increased by PMA. The combined treatment with FSK and PMA additively increased the proENK mRNA level, which was correlated with AP-1 or ENKCRE-2 DNA binding activity, and CREB phosphorylation. Dexamethasone (DEX; 1 microM) further enhanced FSK- or PMA-induced proENK mRNA expression, which was not correlated with the activation of AP-1 expression and CREB phosphorylation, suggesting that synergistic interaction of glucocorticoid with PKA or PKC pathway for the regulation of proENK mRNA expression appears to be mediated by other pathways rather than CREB and AP-1 families.

Antidepressants: past, present and future

Since the discovery of first antidepressants in mid-1950's, the field has been intensively studied. Several new classes of compounds emerged and several hypotheses on the mechanism of their action were proposed. The novel antidepressants are either selective and reversible monoamine oxidase inhibitors, (e.g., moclobemide), or selective serotonin reuptake inhibitors (e.g., citalopram or paroxetine), or serotonin and noradrenaline reuptake inhibitors (e.g. , venlafaxine). Recently neuropeptides (e.g., thyrotropin-releasing hormone,TRH) or antagonists of neuropeptide receptors (e.g., tachykinin NK(1) receptor) undergo clinical tests. Several hypotheses proposed the predominant involvement of one or few neurotransmitter receptors in the mechanism of antidepressant action, but it is now assumed that several distinct receptor mechanisms' trigger different but converging intracellular signal cascades that activate transcription factors, which, in turn, promote the expression of genes encoding for proteins, that play a crucial role in restoring of neuronal functions involved in mood regulation.

Prostaglandin E2 increases proenkephalin mRNA level in rat astrocyte-enriched culture

The effect of prostaglandin E2 (PGE2) on proenkephalin (proENK) mRNA expression in primary cultured rat astrocytes was studied. The proENK mRNA level was significantly increased about 3.3-fold 4 h after PGE2 (10 microM) treatment and this increase was potentiated by the pre-treatment with cycloheximide (CHX; 15 microM) about 1.7-fold as much as PGE2 alone treated cells. The pretreatment with staurosporine (1 microM) completely inhibited the increase of PGE2-induced proENK mRNA level, although only a partial inhibition of PGE2-induced proENK mRNA level (approximately 1.5-fold) by H89 (10 microM) was observed. The increase of PGE2-induced proENK mRNA level was not affected by the pretreatment with PD98059 (1, 5, and 10 microM), omega-conotoxin GIVA (1 microM), nimodipine (1 microM), calmidazolium (1 microM), or KN-62 (1 microM). In addition to the proENK mRNA level, PGE2 also increased c-Fos (approximately 4.3-fold), Fra-1 ( approximately 3.8 fold), and Fra-2 (approximately 8.2-fold) protein levels at 4 h after drug treatment. However, c-Jun, JunB, and JunD protein levels were not affected by PGE2. Indeed, PGE2 failed to up-regulate c-jun mRNA expression as well as its protein product. Surprisingly, although three Jun proteins were not induced by PGE2, AP-1 and ENKCRE-2 DNA binding activities were increased by PGE2, (approximately 5 and approximately 2.8-fold, respectively) and which were effectively reduced by CHX (approximately 2.5 and 2-fold, respectively). In western blot analyses, PGE2 enhanced the phosphorylation of CREB (approximately 2.6-fold at 1 h), and CHX showed a potentiative effect on PGE2-induced CREB phosphorylation ( approximately 1.7 fold at 1 h) which is similar to the action on proENK mRNA regulation. Our results suggest that PGE2 increases proENK mRNA expression via activating serine/threonine protein kinase such as PKA, but not calcium/calmodulin dependent protein kinase and MAPK. In addition, phosphorylation of CREB rather than the increase of AP-1 may have a possible role at least early stage in PGE2-induced proENK mRNA level and CHX-evoked potentiation.

Lack of beta adrenoceptor desensitization in brain following the dual noradrenaline and serotonin reuptake inhibitor venlafaxine

Venlafaxine, a dual amine reuptake inhibitor, was utilized to delineate the role of the individual aminergic components of the 'serotonin/noradrenaline link' in modifying receptor-linked second messenger cascades. Venlafaxine (20 mg/kg i.p. bid for 10 days) failed to alter in normal animals either the density of beta adrenoceptors or the response of the beta adrenoceptor-coupled adenylate cyclase system to noradrenaline but significantly decreased the cyclic AMP response to noradrenaline in the brain of rats with selective depletion of brain serotonin by p-chlorophenylalanine. The studies provide evidence for a cross-talk between noradrenergic and serotonergic receptor cascades at the level of mechanisms involved in the desensitization of the beta adrenoceptor-coupled adenylate cyclase system.

The stimulation of rat astrocytes with phorbol-12-myristate-13-acetate increases the proenkephalin mRNA: involvement of proto-oncogenes

The effect of phorbol-12-myristate-13-acetate (PMA) on the regulation of proenkephalin (proENK) mRNA level, ENKCRE-2 or AP-1 DNA binding activity, and the mRNA and protein levels of proto-oncogenes (c-fos, fra-1, and c-jun) in primary cultured rat astrocytes were studied. The proENK mRNA level was elevated at 4 h after the treatment of PMA (2.5 microM) without altering the intracellular proENK protein level, and this increase was attenuated by pre-treatment with cycloheximide (CHX; 15 microM), a protein synthesis inhibitor. Both AP-1 and ENKCRE-2 DNA binding activities were markedly increased at 1-4 h by PMA treatment and these PMA-induced responses were inhibited by pre-treatment with CHX, showing that the increase of proENK mRNA level was well correlated with the AP-1 and ENKCRE-2 DNA binding activities. In contrast, although the phospho-CREBP level was also increased by PMA at 0.5-1 h, the pre-treatment with CHX further increased the PMA-induced phospho-CREBP level. In addition, PMA caused the induction of c-fos, c-jun and fra-1 mRNA level and, especially, PMA-induced increase of fra-1 mRNA level was further enhanced by CHX treatment at 4 h. Furthermore, western immunoblot assay showed that PMA caused induction of c-Fos, Fra-1, and c-Jun protein levels. PMA-induced increases of proto-oncoproteins levels were also inhibited by CHX treatment. The results suggest that newly synthesized AP-1 proteins, such as c-Fos, Fra-1, and c-Jun may play important roles in the regulation of PMA-induced proENK gene expression in cultured rat astrocytes. Phospho-CREB protein appears not to be involved in the regulation of PMA-induced proENK gene expression.

Evidence for Fos involvement in the regulation of proenkephalin and prodynorphin gene expression in the rat hippocampus

For a long time Fos has been proposed to play some role in regulation of the proenkephalin (PENK) and prodynorphin (PDYN) gene expression. In recent years, however, evidence has accumulated that the transcription of both genes in several brain regions in vivo is transactivated by the transcription factor CREB rather than by Fos. In the present study, involvement of Fos in the mechanism of the PENK and PDYN gene induction in the hippocampal dentate gyrus during seizures elicited by kainic acid was studied using a knock-down technique. Pretreatment with an antisense oligonucleotide complementary to c-fos mRNA did not influence the kainic acid-elicited convulsions. It inhibited, by about 50%, the induction of Fos protein in the dentate gyrus during seizures. The subsequent induction of PENK and PDYN mRNAs was reduced by more than 60% by the c-fos antisense oligonucleotide, while constitutive expression of three other genes (alpha-tubulin, NMDA receptor-1, and GS protein alpha-subunit) was not affected. The obtained results support the view that Fos may be involved in regulation of the PENK and PDYN gene expression in the dentate gyrus during seizures, which further suggests that the mechanisms triggering the up-regulation of both these genes in the dentate gyrus may differ from these working in other brain regions, such as the striatum and hypothalamus.

Activation of protein kinase A prevents the ethanol-induced up-regulation of delta-opioid receptor mRNA in NG108-15 cells

We have used a sensitive solution hybridization assay with a riboprobe transcribed from the coding sequence of the delta-opioid receptor gene (DOR) to study the up-regulation of the DOR mRNA by ethanol in NG108-15 cells. Exposure of the cells to compounds that increase cAMP levels (forskolin, forskolin + IBMX, or dibutyryl cAMP) resulted in the attenuation of ethanol-induced up-regulation of DOR mRNA. The inactive analogue of forskolin, 1,9-dideoxy forskolin had no effect. Northern blot analysis of RNA extracts from ethanol-, forskolin- or ethanol + forskolin-treated cells showed proportional changes in each of the multiple DOR mRNA bands, so that no difference was observed in the fraction of the total hybridization signal produced by each band of the DOR mRNA. In the absence of ethanol, forskolin or dibutyryl cAMP reduced the basal levels of DOR mRNA. The cAMP analogue (Rp)-cAMPS, a protein kinase A (PKA) inhibitor, increased DOR mRNA levels. However, the combination of (Rp)-cAMPS and ethanol did not further increase DOR mRNA levels compared to ethanol or (Rp)-cAMPS alone. Signaling through cAMP and PKA down-regulates DOR mRNA levels. The ethanol-induced increase in DOR mRNA levels in NG108-15 cells appears to be mediated via a reduction of PKA.

Molecular mechanisms underlying the regulation of proenkephalin gene expression in cultured spinal cord cells

The regulation of proenkephalin (proENK) mRNA levels by cAMP and protein kinase C (PKC) pathways was studied in cultured rat spinal cord cells in the present study. Spinal cord cells were cultured from 14 day (E 14) embryos of Sprague-Dawley rats. After 7 days in vitro, the spinal cord cells were incubated with either forskolin (5 microM) or phorbol-13-myristate acetate (PMA; 2.5 microM) for 1, 3, 6, 9, 12 or 24 h and total RNA and proteins were isolated for Northern and Western blot analyses. The proENK mRNA level began to increase within an hour, then reached and remained at a peak 3-12 h after stimulation by both forskolin and PMA. The increased proENK mRNA level in forskolin-treated cells was slightly decreased 24 h after the stimulation, whereas the level of proENK mRNA returned to basal levels in PMA-treated cells. A Western blot assay revealed that the intracellular level of proENK protein was not changed by treatment with either forskolin or PMA. Pretreatment of cells with cycloheximide (a protein synthesis inhibitor; 10 microM) did not affect the forskolin- or PMA-induced increase of proENK mRNA. However, pretreatment with nimodipine (an L-type Ca2+ channel blocker; 2 microM), omega-conotoxin (an N-type Ca2+ channel blocker; 1 microM), calmidazolium (a calmodulin antagonist; 1 microM) or KN-62 (a Ca2+/calmodulin-dependent protein kinase II inhibitor; 5 microM) attenuated the forskolin- or PMA-induced increase of proENK mRNA levels. Dexamethasone (1 microM) did not affect the forskolin-induced increase of proENK mRNA levels. Our results suggest that the elevation of proENK mRNA levels in the spinal cord is regulated by both cAMP and PKC pathways. Calcium influx through both L- and N-type calcium channels, calmodulin and Ca2+/calmodulin-dependent protein kinase II appear to be involved in the increase of proENK mRNA levels induced by either forskolin or PMA. Furthermore, ongoing protein synthesis is not required for forskolin- or PMA-induced alterations in proENK mRNA.

Dexamethasone and forskolin synergistically increase [Met5]enkephalin accumulation in mixed brain cell cultures

Possible synergistic effects of the glucocorticoid dexamethasone (DEX, 10(-7) M) and the adenylate cyclase agonist forskolin (FSK, 10(-5) M) on [Met5]enkephalin (ME) accumulation were examined in enriched rat glial cultures and in mixed neuronal/glial cultures. In enriched glial cultures, DEX and FSK each stimulated the accumulation of ME 2-3-fold over basal media levels, but there was little additional stimulation when these agonists were combined. In contrast, mixed neuronal/glial cultures showed only weak responses to DEX or FSK alone, but the combination of these agonists produced a pronounced synergistic effect on media ME accumulation (6-10-fold over basal levels). The DEX effect was mediated via a classical glucocorticoid receptor, since DEX was potent (acting over a concentration range of 10(-11)-10(-7) M), mimicked by corticosterone (10(-6) M), and blocked by the glucocorticoid receptor antagonist RU486. There was a pronounced time lag (2 days) for the synergistic effects of DEX + FSK to develop. In situ hybridization and immunocytochemical studies suggested that astrocytes were the major source for the increased ME production in all mixed neuronal/glial cultures examined. Creating a mixed culture by plating fetal neurons onto confluent, enriched P7 glial cultures inhibited accumulation of ME in the media. DEX + FSK, but neither agonist alone, overcame this neuronal inhibition and increased accumulation of media ME to levels identical to levels in stimulated enriched glial cultures. The net effect was a 6-fold increase in ME accumulation in the mixed neuronal/glial cultures relative to a 2.5-fold increase in the enriched glial cultures. Neuronal inhibition of basal glial ME production could explain the similar synergistic effects of DEX + FSK observed in all mixed neuronal/glial cultures examined, and may be important in suppressing ME production by astrocytes in the brain.

Molecular characterization of immune derived proenkephalin mRNA and the involvement of the adrenergic system in its expression in rat lymphoid cells

Proenkephalin (PENK), a classically defined opioid gene, was originally thought to be expressed almost exclusively in the mature nervous and neuroendocrine systems. In the last few years, it was demonstrated, however, that significant levels of PENK mRNA and PENK-derived peptides are transiently expressed in cells of the immune system. Very little is known about the molecular mechanisms regulating this transient expression. In order to investigate those mechanisms, we examined the in vivo expression of PENK mRNA in mesenteric lymph nodes after exposing rats to lipopolysaccharide. In the present study we demonstrate that: (i) promoter usage and splicing of PENK mRNA function similarly in mesenteric lymph nodes as in neural cells; (2) PENK expression in mesenteric lymph nodes is modulated by adrenaline via adrenergic receptors; and (3) the adrenergic system participates in the modulation of the LPS induced PENK mRNA expression. These results provide more evidence for the involvement of opioids in neuro-immune interactions.

Prenatal morphine exposure differentially alters expression of opioid peptides in striatum of newborns

The biochemical and cellular mechanisms involved in the development and/or maintenance of morphine tolerance remain unclear. In the adult central nervous system (CNS) results are contradictory. For the neonate, a variety of drug induced deficits have been observed following prenatal addiction to opioids, although very little work on the biochemical and molecular level has been done. Therefore, the present study was carried out to investigate the effects of prenatal morphine treatment on the levels and expression of endogenous opioid peptides in brain regions of newborns. Dams were implanted with one morphine pellet (75 mg each) 1 week prior to the birth of pups. Changes in mRNA levels for the opioid peptides were determined by Northern blot analysis. Alterations in opioid peptide levels were determined by radioimmunoassays. Prenatal morphine treatment significantly increased proenkephalin mRNA levels and decreased met-enkephalin levels in striatum of newborns. These data are in contrast to what is observed in the adult CNS. These data indicate that prenatal morphine treatment may increase met-enkephalin release and/or cause inhibition at the level of translation. In addition, increased transcription may be necessary to maintain equilibrium in the system when there is an increase in met-enkephalin release.

Effects of morphine on forskolin-stimulated pro-enkephalin mRNA levels in rat striatum: a model for acute and chronic opioid actions in brain

Opioid agonists inhibit adenylyl cyclase in brain through Gi-coupled receptors. One potential biological role for this reaction would be to decrease pro-enkephalin mRNA synthesis by decreasing intracellular cAMP levels and preventing stimulation of gene expression via the cAMP regulatory element (CRE). To determine whether such effects occur in vivo, rats were injected i.c.v. with a water-soluble analog of forskolin, 7-beta-deacetyl-7 beta[gamma-(morpholino-butyryl] butyryl] forskolin (DMB-forskolin), to stimulate the CRE. Pro-enkephalin mRNA levels were assayed in striatum by Northern blot analysis. The treatment with the forskolin analog increased striatal pro-enkephalin mRNA levels approximately 2-fold in 4 h. When rats were injected with morphine (20 mg/kg i.p.) 1 h before DMB-forskolin administration, the stimulation of pro-enkephalin mRNA levels was eliminated. This acute effect of morphine was blocked by co-administration with 10 mg/kg naloxone. When rats were chronically treated with morphine for 8 days, then injected with morphine (20 mg/kg i.p.) 1 h before DMB-forskolin administration, the inhibitory effect of morphine was lost (i.e. DMB-forskolin increased pro-enkephalin mRNA levels by 2 fold in either control and morphine-treated rats). These data not only demonstrate the in vivo relevance of opioid-inhibited adenylyl cyclase in the control of pro-enkephalin mRNA levels, but also show that this model is useful for studying how this signal transduction system is attenuated during the development of tolerance.

The transcriptional regulation of the preproenkephalin gene

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Neuron-glia interactions in the release of oxytocin and vasopressin from the rat neural lobe: the role of opioids, other neuropeptides and their receptors

The release of the neurohormones oxytocin and vasopressin from the neural lobe into the circulation is regulated in a complex manner, which has only been partly elucidated. At the level of the neural lobe, regulation of release can occur by various endogenous compounds that act on specific receptors present on the nerve terminals themselves. In addition, release may be modulated by an alternative pathway in which the local glia cells, the pituicytes, are involved. It is especially the latter pathway that is discussed in detail in this commentary.

Morphine-induced reciprocal alterations in G alpha s and opioid peptide mRNA levels in discrete brain regions

The mechanisms involved in the development of morphine tolerance and dependence are still unknown. Recently much attention has been directed toward the changes in post receptor events. Opiate receptors, like other hormone and neurotransmitter receptors, have been shown to mediate their effects through guanine nucleotide binding proteins (G-proteins). This, in turn, may cause alterations in intracellular events, one of which is transcription of specific genes. We investigated the changes in the levels of mRNA of proenkephalin (PPE) and prodynorphin (DYN) and the stimulatory G protein alpha subunit (G alpha s) in adult morphine tolerant rats. Chronic morphine treatment induced reciprocal alterations in the levels of opioid peptide mRNA and G alpha s mRNA in discrete brain regions. In striatum, PPE mRNA decreased by 49% (P < .01) and in hypothalamus, DYN mRNA showed a decrease of 21% (P < .01). In contrast, G alpha s mRNA increased 20% (P < .01) in striatum and 97% (P < .01), in hypothalamus. In hippocampus the changes were reversed: PPE mRNA increased (55%, P < .05) and G alpha s mRNA decreased (33%, P < .01). Frontal cortex exhibited a small decrease in PPE (11.5%, P < .05) without any change on G alpha s or DYN mRNA levels. These reciprocal alterations suggest an opposing mode of regulation of G alpha s and PPE/DYN gene expression in morphine tolerant animals.

The ‘serotonin/norepinephrine link’ beyond the β adrenoceptor

C6 rat glioma cells were utilized as a model system to probe the 'serotonin/norepinephrine link' at the level of preproenkephalin (PPE) gene expression. The beta adrenoceptor mediated increase in PPE mRNA was attenuated by the selective beta 1 adrenoceptor antagonist metoprolol which blocked the isoproterenol induced cyclic AMP generation by 97%. The subtype nonspecific antagonist propranolol blocked both the isoproterenol induced increase in cyclic AMP and the increase in the PPE mRNA steady-state levels. Serotonin (5-HT) had no effect on the density of beta adrenoceptors or their down-regulation by isoproterenol and did not alter the PPE gene expression in the absence of the beta signal. However, 5-HT significantly deamplified the beta signal mediated enhancement of the PPE mRNA thus indicating that the aminergic link occurs beyond the beta adrenoceptor.

Preproenkephalin RNA increases in the hypothalamus of rats stressed by social deprivation

1. Pharmacological evidence indicates that stress induced by brief (14 to 20-day) social deprivation in the rat is associated with an activation of the central preproenkephalin (ENK) opioid system. This study examines the neurochemical evidence that substantiates such an activation. 2. Using a specific ENK complementary DNA probe, ENK RNA levels were measured by dot blot and Northern blot analyses in different brain areas of socially deprived rats. Immunoreactivity to met-enkephalin-derived peptides was also evaluated by radioimmunoassay in the same brain regions. 3. Brief social deprivation increased the levels of ENK RNA and enkephalin immunoreactivity in whole hypothalamus. 4. Our data suggest that this type of stress appears to be associated to an induction of ENK gene transcription in hypothalamus.

Regulation of prodynorphin gene expression in the hippocampus by glucocorticoids

The regulation of prodynorphin gene expression by glucocorticoids in the hippocampus was examined in rats that were adrenalectomized (ADX) either 7, 30, 60 and 90 days prior to sacrifice. Peptide levels in the hippocampus of ADX rats were determined by radioimmunoassay and immunocytochemistry. Prodynorphin (PDYN) mRNA was measured by Northern blot analysis and in situ hybridization. A time-dependent decrease in dynorphin A(1-8)(DYN) levels in the hippocampus (18% at 7 days; 44% at 30 days; 58% at 60 days) of ADX rats was found, which was accompanied by a comparable decrease in the abundance of PDYN mRNA. An in situ hybridization analysis revealed that both the number of positively hybridized cells and the number of silver grains per cell were decreased in the dentate gyrus after ADX. The administration of dexamethasone after surgery reversed the peptide and mRNA attenuation induced by ADX. ADX had no effect on the expression of proenkephalin mRNA or [Met5]-enkephalin immunoreactivity in the hippocampus. Examination of thionin-counterstained tissue showed that the dentate granule cell layer was intact. The decrement of DYN expression in this system is proposed to have resulted from the removal of glucocorticoid input and not dentate granule cell loss. This study provides the strong evidence for a differential susceptibility of these two opioid peptides in the hippocampus to the removal of glucocorticoids. In addition, these data provide support for a potentially selective, glucocorticoid-permissive component in PDYN gene expression.

Glucocorticoid regulation of preproenkephalin gene expression in the rat forebrain

The effects of glucocorticoids on the levels of preproenkephalin (PPE) mRNA in the rat forebrain were analyzed with in situ hybridization and dot blots. In adrenally-intact rats, high levels of PPE mRNA, as assessed by in situ hybridization, were localized in the caudate-putamen, nucleus accumbens, central amygdala, and ventrolateral ventromedial hypothalamus (VMHVL), and low levels in the hippocampus. After adrenalectomy, the density of PPE mRNA-positive cells and the level of PPE mRNA/cell were decreased in all regions except the hippocampus. Acute treatment with corticosterone (CORT) in adrenalectomized rats increased the level of PPE mRNA/cell in the caudate-putamen and VMHVL. In intact rats, chronic treatment with CORT increased the density of PPE mRNA-positive cells in the caudate-putamen and hippocampus, and the level of PPE mRNA/cell in the caudate-putamen and nucleus accumbens. The effect of chronic CORT treatment on PPE mRNA in the striatum, amygdala, hippocampus and mediobasal hypothalamus was assessed with dot blots. Chronic CORT treatment increased PPE mRNA levels in the caudate-putamen and hippocampus. There was a good correlation between results on the effect of chronic CORT treatment on PPE mRNA levels in intact rats, obtained from dot blots and in situ hybridization. Results from this study suggest that glucocorticoids are required for the maintenance of basal PPE mRNA levels in most regions of the rat forebrain. There is, however, considerable regional heterogeneity in the effect of glucocorticoid treatment on PPE mRNA levels in adrenalectomized and intact rats. Increased PPE mRNA levels in response to high circulating levels of glucocorticoids, e.g. in stress, may have important pathophysiological consequences.

Altered genetic response to beta-adrenergic receptor activation in late passage C6 glioma cells

Previous studies have demonstrated variability in the phenotype of rat C6 glioma cells. In the present study, we compared morphology, growth rate, and beta-adrenergic regulation of gene expression in early (P39-47) and late (P55-90) passage C6 cells. Morphological changes were observed in five independently derived, late passage populations. In four of the five, the untreated cells were more polygonal than the fibroblast-like parental cells, and only a small fraction exhibited process outgrowth after dbcAMP treatment. Untreated cells from the fifth late passage population had longer cytoplasmic processes than parental cells and responded to dbcAMP with further process outgrowth. All late passage populations had shorter generation times than the parental cells. In early passage cells, treatment with the beta-adrenergic agonist, isoproterenol (IPR), resulted in an increase in c-fos mRNA and a decrease in c-jun mRNA (Gu-bits RM, Yu H: J Neurosci Res, 30:625-630, 1991). Both of these immediate early gene responses were irreversibly lost between P50 and P55. Additional differences in basal or IPR-induced mRNA levels were observed for beta-APP, GFAP, NGF, and PPE, but not for a number of other mRNAs. These results are discussed in relationship to previously described differences in the ability of early and late passage C6 cells to accumulate cAMP (Mallorga P, et al.: Biochim Biophys Acta 678:221-229, 1981).

Primary sequence of -1436 to +53 bp of the rat preproenkephalin gene putative Z-DNA and regulatory motifs

We report novel sequence data extending -1436 bases 5' of the rat proenkephalin gene start site known as E4. We noted an interesting stretch of 58 bases of alternating pyrimidines that lies immediately adjacent to 71 bases of an alternating purine-pyrimidine Z-DNA-like sequence that lies between -694 bp and -566 bp. Multiple sequence homologies to putative cis-acting regulatory factor binding sites were identified by a computer aided sequence search.

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.

Regulation of adrenomedullary preproenkephalin mRNA: effects of hypoglycemia during development

To further evaluate whether transsynaptic mechanisms account for stress-induced changes in adrenomedullary preproenkephalin mRNA (ppEnk mRNA), neonatal rats were made hypoglycemic at a time when synapses are non-functional (less than 10 days postnatal age). While ppEnk mRNA in medullae from adult rats increased as much as 60-fold in this paradigm (insulin 10 U/kg), ppEnk mRNA levels in the newborn increased only 1.6-fold (insulin 20 U/kg). To evaluate whether postsynaptic cholinergic pathways of the neonatal adrenal medulla were functional, we treated 5-day-old pups with cholinergic agonists (nicotine [1 mg/kg, s.c., q 12 h] + carbachol [1.7 mumol/kg, s.c., q 12 h x 4 days]). Combined cholinergic agonist treatment augmented enkephalin prohormone and peptide levels up to 3-fold (P less than 0.05). To determine whether the blunted response to hypoglycemia in the newborn resulted from a deficiency in functional transsynaptic activity, synapses were matured using thyroid hormone pretreatment (postnatal days 2 and 3) before hypoglycemic stress. Hypoglycemia now caused a 40-fold increase in adrenomedullary ppEnk mRNA levels only in the T3/insulin treated group. To exclude other secondary effects of hypoglycemia (eg. hormonal, or insulin treatment-dependent), intracellular glycopenia was produced in the presence of secondary hyperglycemia by injecting adult rats or pups with 2-deoxyglucose (500 mg/kg). Similar to the insulin-hypoglycemia group, a large increase in adrenomedullary ppEnk mRNA resulted in the adult but not in the 5-day-old neonatal adrenal medullae. We conclude that enkephalin biosynthesis, like co-stored catecholamines, is induced by a transsynaptic process.(ABSTRACT TRUNCATED AT 250 WORDS)

Posttranslational processing of proenkephalin in SK-N-MC cells: evidence for phosphorylation

SK-N-MC cells have recently been shown to be a rich source of proenkephalin and/or the proenkephalin-derived peptide, peptide B. We have investigated the synthesis and the posttranslational processing of proenkephalin in these cells. SK-N-MC cells retain very little of the proenkephalin synthesized; greater than 99% of the immunoreactive enkephalin synthesized within a 48-h period is secreted into the medium rather than contained intracellularly. When medium samples were subjected to gel filtration and assayed for the various enkephalins present within proenkephalin, only two major molecular-weight classes of peptides, with molecular weights and immunoreactive profiles consistent with those of proenkephalin and the 3.6-kDa carboxyl-terminal fragment peptide B, were observed. The proenkephalin-like peptide present in medium samples was shown by western blot procedures to consist of a 32-kDa protein with a slight amount of a higher-molecular-weight immunoreactive component above it. Only proenkephalin-sized peptides were present within cell extracts. Radiolabeled proenkephalin added to cell cultures was also cleaved to products similarly sized to those found in medium extracts; radiolabeled proenkephalin incubated in the absence of cells was not cleaved. Cleavage of exogenous proenkephalin thus probably at least partially occurs following secretion. Cell radiolabeling experiments with [32P]orthophosphate demonstrated that SK-N-MC proenkephalin is phosphorylated. Microheterogeneity of proenkephalin was also observed using isoelectric focusing coupled with western blotting. Our results suggest that the SK-N-MC cell line represents a useful model to study the earliest steps of the posttranslational processing of human proenkephalin in a neuronal cell type.

Glucocorticoid-mediated down regulation of c-fos mRNA in C6 glioma cells: lack of correlation with proenkephalin mRNA

We investigate the linkage between the transcriptional factor, c-fos, and expression of proenkephalin in rat C6 glioma cells. C6 cells contained abundant levels of c-fos mRNA. Treatment of cells with dexamethasone resulted in a 10-fold decline in c-fos transcripts and a small increase in proenkephalin mRNA. Combined exposure to dexamethasone and isoproterenol also induced a decrease in c-fos mRNA while proenkephalin mRNA increased 8-fold. Treatment of the C6 cells with phorbol 12-myristate 13-acetate caused a 13-fold increase in c-fos expression 0.5 h after administration and a decrease in proenkephalin mRNA. These data indicate that c-fos and proenkephalin mRNA are not regulated in a sequential, parallel manner, that newly synthesized c-fos is not the determining factor controlling proenkephalin gene regulation, and that c-fos expression is under negative control by glucocorticoids.

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)

Beta-adrenergic treatment of C6 glioma cells produces opposite changes in c-fos and c-jun mRNA levels

The AP1 transcriptional complex is a heterodimer composed of proteins encoded by the fos and jun proto-oncogene families. Changes in the concentration and composition of AP1 occur after cells are perturbed in a variety of different ways (Curran, in Reddy et al., eds. "The Oncogene Handbook," Amsterdam: Elsevier, pp 307-325, 1988; Sonnenberg et al., Neuron 3:359-365, 1989). Transient changes in AP1 content presumably result in altered expression of AP1-regulated target genes, that help to mediate the cell's long-term response to changes in its environment. One factor that may be important in determining which target genes are regulated by AP1 in a given context is the identity of the jun family member present in the complex (Chiu et al., Cell 59:979-986, 1989; Schutte et al., Cell 59:987-997, 1989). Fos induction has been demonstrated after binding of beta-adrenergic ligands to their cell surface receptors (Barka et al., Mol Cell Biol 6:2984-2989, 1986; Gubits et al., Mol Brain Res 6: 39-45, 1989; Arenander et al., J Neurosci Res 24: 107-114, 1989; Mocchetti et al., Proc Natl Acad Sci USA 86:3891-3895, 1989). However, the response of the jun gene family to this treatment has not been reported. We have therefore examined the effect of beta-adrenergic receptor activation on the expression of c-fos, c-jun, and junB mRNA levels in C6 glioma cells. Our results indicate that c-fos and junB mRNA levels are increased by 52- and 2.7-fold, respectively, after 45 min of isoproterenol (IPR) treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

cAMP-dependent regulation of proenkephalin by JunD and JunB: positive and negative effects of AP-1 proteins

We demonstrate that JunD, a component of the AP-1 transcription factor complex, activates transcription of the human proenkephalin gene in a fashion that is completely dependent upon the cAMP-dependent protein kinase, protein kinase A. Activation of proenkephalin transcription by JunD is dependent upon a previously characterized cAMP-, phorbol ester-, and Ca(2+)-inducible enhancer, and JunD is shown to bind the enhancer as a homodimer. Another component of the AP-1 transcription complex, JunB, is shown to inhibit activation mediated by JunD. As a homodimer JunB is unable to bind the enhancer; however in the presence of c-Fos, high-affinity binding is observed. Furthermore, JunD is shown to activate transcription of genes linked to both cAMP and phorbol ester response elements in a protein kinase A-dependent fashion, further blurring the distinction between these response elements. These results demonstrate that the transcriptional activity of an AP-1-related protein is regulated by the cAMP-dependent second-messenger pathway and suggest that JunD and other AP-1-related proteins may play an important role in the regulation of gene expression by cAMP-dependent intracellular signaling pathways.

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 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.

Coordinate and differential regulation of proenkephalin A and PNMT mRNA expression in cultured bovine adrenal chromaffin cells: responses to secretory stimuli

The expression of proenkephalin A (ProEnk A) mRNA and phenylethanolamine N-methyltransferase (PNMT) mRNA in response to nicotine and to a number of secretagogues was examined in cultured bovine adrenal chromaffin cells. Prolonged incubation with nicotine (10 microM) resulted in a 2-fold increase in ProEnk A mRNA but had no significant effect on the level of PNMT mRNA. Similarly, prolonged stimulation with high K+ (56 mM) induced a time-dependent elevation in the level of ProEnk A mRNA reaching 4-fold basal level after 24 h incubation. By contrast, the level of PNMT mRNA was not changed by treatment with high K+. The increase in the level of ProEnk A mRNA by high K+ was abolished by the presence of 10 microM D600, a calcium channel blocker. Unlike the effects of high K+, treatment of the cells with the sodium channel activator veratridine significantly elevated the levels of both ProEnk A and PNMT mRNA. This increase in ProEnk A and PNMT mRNA levels was however less affected by D600. Stimulation of the cells with Ba2+ (1.1 mM) also stimulated the levels of ProEnk A and PNMT mRNA and this action required the presence of extracellular Ca2+. This was in contrast to the effect of Ba2+ in stimulating catecholamine secretion, which was inhibited by Ca2+ and enhanced in Ca2(+)-free buffer. The results of the present study indicate that membrane depolarization and entry of extracellular Ca2+ play an important role on the regulation of ProEnk A and PNMT mRNAs, in addition to their well-known actions on hormone secretion. Furthermore, these results suggest that the expression of ProEnk A mRNA and PNMT mRNA are under independent regulation in response to secretory stimulation.

Cellular localization of proenkephalin mRNA and enkephalin peptide products in cultured astrocytes

To identify the possible cellular sites of opioid gene expression during ontogeny, proenkephalin mRNA and enkephalin peptide expression were examined, respectively, by in situ hybridization and immunocytochemistry in organotypic explants of rat cerebellum and in astrocyte-enriched cultures of murine cerebral hemispheres. High levels of proenkephalin mRNA and enkephalin immunoreactivity were detected in immature cells identified as astrocytes. Double-labeling studies combining in situ hybridization and immunocytochemical localization of the astrocytic marker, glial fibrillary acidic protein, provided direct evidence that proenkephalin mRNA is expressed by astrocytes in culture. Based on previous studies that Met-enkephalin can inhibit astrocyte growth in vitro, the present results suggest that proenkephalin gene expression by astrocytes is important during central nervous system maturation.

Pattern of interleukin 6 gene expression in vivo suggests a role for this cytokine in angiogenesis

Interleukin 6 (IL-6) is a cytokine that acts on various cell types. Here we show that IL-6 mRNA is produced in vivo in two self-limiting physiologic angiogenic processes: (i) the formation of the vascular system accompanying development of ovarian follicles and (ii) the formation of a capillary network in the maternal decidua following embryonic implantation. In situ and RNA blot hybridization analyses detected transient expression of IL-6 mRNA in gonadotropin-primed hyperstimulated ovaries, with maximal mRNA levels coinciding with the period of formation of a capillary network around follicles. Expression of IL-6 mRNA was detected in the vasculature extending from the ovarian medulla to the forming capillary sheath in the thecal layer of individual growing follicles. No expression was detected in more-developed preovulatory follicles once angiogenesis had been completed. IL-6 mRNA was also detected in the uterus of pregnant mice 9.5 days postcoitum, and there was no appreciable IL-6 mRNA at later stages of embryonic development. Expression in the uterus was confined to cords of endothelial cells in the process of formation of an anastomosing network that traversed the maternal decidua towards the developing embryo. The expression of IL-6 mRNA in two independent physiological angiogenic processes and the transient nature of its expression in endothelial cells suggest a role for IL-6 in angiogenesis.

Pharmacology of neuronal gene expression

Pharmacological treatments were used to estimate trans-synaptic regulation of opioid peptide gene expression occuring at specific neurotransmitter receptors. In vitro and in vivo studies have shown that different signal-transduction mechanisms regulate the transcription of proenkephalin, proopiomelanocortin and nerve growth factor mRNA. The activation of receptors coupled to adenylate cyclase elicited the increase of proenkephalin and nerve growth factor gene expression. Therefore, a cAMP-dependent mechanism was suggested to be involved in such regulation. However, the temporal delay between the elevation of the intracellular cAMP content and the increase in nerve growth factor and proenkephalin mRNAs prompted us to investigate whether additional mechanisms associated with the second messenger were operative in the regulation of the expression of these two genes. We report evidence that a protein(s), probably functioning as a trans-acting factor, might be involved in the regulation of nerve growth factor gene transcription. The characterization and isolation of these DNA regulatory proteins will provide the pharmacologist with valuable information for the development of new compounds in the therapy of mental disorders.

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

Astrocytes have many neuronal characteristics, such as neurotransmitter receptors, ion channels, and neurotransmitter uptake systems. Cultured astrocytes were shown to express certain neuropeptide genes, with specificity for both the gene expressed and the brain region from which the cells were prepared. Somatostatin messenger RNA and peptides were detected only in cerebellar astrocytes, whereas proenkephalin messenger RNA and enkephalin peptides were present in astrocytes of cortex, cerebellum, and striatum. Cholecystokinin was not expressed in any of the cells. These results support the hypothesis that peptides synthesized in astrocytes may play a role in the development of the central nervous system.

Phorbol ester regulates the abundance of enkephalin precursor mRNA but not of amyloid beta-protein precursor mRNA in rat testicular peritubular cells

Cultured peritubular cells prepared from the testes of 20-day-old rats contained both preproenkephalin (A) mRNA (1.5 kb) and amyloid beta-protein precursor mRNA (3.6 and 2.8 kb). The phorbol ester TPA and forskolin (an adenylate cyclase activator) increased the preproenkephalin mRNA abundance to 9.0 and 5.8 times the control, respectively. TPA alone had no effect on the intracellular cAMP level. A combination of TPA and forskolin elicited a synergistic increase in the ppEnk mRNA abundance over 30-fold. Dexamethasone potentiated the effect of forskolin but not of TPA. These results suggest that TPA regulates the preproenkephalin mRNA abundance through a cAMP-independent pathway. In contrast, TPA, forskolin, and dexamethasone showed little or no effect on the abundance of amyloid beta-protein precursor mRNA.

Endogenous opioids and neural cancer: an immunoelectron microscopic study

[Met5]-enkephalin, an endogenous opioid peptide derived from proenkephalin A, participates in tumorigenic events by serving as a natural trophic factor that inhibits cell replication. In order to understand how endogenous opioids function in modulating neoplasia, the present study examined the fine structural association of enkephalin with the cellular components of a tumor cell. Immunoelectron microscopic studies were undertaken using antibodies recognizing [Met5]-enkephalin-like substances, and murine S20Y neuroblastoma cells that are known to be responsive to endogenous opioid modulation. Enkephalin was found throughout the cell body and process. Immunoreactivity was associated with the plasma membrane, outer nuclear envelope, and a variety of organelles. With the exception of aggregates of immunoreactivity subjacent to the inner nuclear envelope, the nucleus was not reactive. These results establish that growth-related enkephalins are localized discretely within neuroblastoma cells. Since neuroblastoma cells produce and secrete enkephalins, and enkephalins interact with receptors to mediate actions on cell replication, this study examined enkephalins involved in two different patterns of traffic; further work will be needed to examine each aspect.

Modulation of proenkephalin A gene expression by cyclic AMP

Regulation of proenkephalin A expression was studied in the human neuroblastoma SK-N-MC cell line with respect to mRNA-level, translation, posttranslational processing of the prohormone and secretion of the processed products into the culture medium. Cells were treated with either norepinephrine (NE), dexamethasone (DEX), dibutyryl-3',5'-cyclic AMP (dbcAMP) or the combination of NE and DEX. In an additional investigation, proenkephalin A mRNA levels were determined after 9 h of treatment with dbcAMP, NE, isoproterenol, NE + propranolol and dbcAMP + DEX. NE or dbcAMP for 1-48 h transiently elevated proenkephalin A mRNA 1.5-4.5 times compared to control. The effect of NE was partially blocked by the beta-adrenoceptor antagonist propranolol and was reproduced by the beta-adrenoceptor agonist isoproterenol, suggesting involvement of the beta-adrenoceptor. DEX alone had no significant effect. However it markedly antagonized the effect of NE but not that of dbcAMP suggesting an action on the beta-adrenoceptor. The intracellular content of Met-enkephalin-Arg6,Phe7 immunoreactivity was increased during drug treatment in parallel with changes in proenkephalin A mRNA. DEX gave no effect. No significant change in the ratio of low versus high molecular weight immunoreactive material could be detected in the cell extracts as determined at different time points. Secretion of immunoreactivity into the culture medium increased 5-fold after 18 h of treatment with NE, whereas dbcAMP gave a 2-fold increase. The proportion of low-molecular weight secreted material increased markedly. DEX alone did not induce any change but inhibited the effect of NE. Apparently, regulation of gene expression, prohormone processing and secretion are coordinated by a cAMP-dependent mechanism.

Characterization of an estradiol-stimulated mRNA in the brain of adult male rats

Differential hybridization of a cDNA library from rat C6 glioma cells with cDNA probes from naive C6 glioma cells and from cells exposed to 17 beta-estradiol identified cDNAs of an mRNA stimulated by 17 beta-estradiol. This mRNA designated ESP1 mRNA, reached maximal levels after 8 h of treatment with 17 beta-estradiol. The stimulation was not suppressed by cycloheximide. Dexamethasone treatment of C6 glioma cells did not induce ESP1 mRNA. It codes for a 164 amino acids long peptide. The sequence is similar in part to that of CRIP protein, a probably member of the ferredoxin superfamily. The conservation of primary structure suggests a role of ESP1 peptide in oxygen consumption. ESP1 mRNA expression is sexually dimorphic in body tissue, whereas it is expressed to comparative levels in the brain of adult males and females. This suggests that 17 beta-estradiol stimulates the expression of the ESP1 gene in the brain of both gender.

A new species of enkephalin precursor mRNA with a distinct 5'-untranslated region in haploid germ cells

To elucidate the primary structure of preproenkephalin (A) mRNA expressed by haploid germ cells (round spermatids) in rat testis, we have screened a lambda gt11 cDNA library for preproenkephalin cDNA inserts. The largest cDNA insert contained a protein-coding sequence encoding 269 amino acid residues as well as 327 and 309 bases of the 5'- and 3'-untranslated regions, respectively. The protein-coding region plus 3'-untranslated region of the mRNA was over 99% homologous to that of brain preproenkephalin mRNA, whereas the 5'-untranslated region contained a distinct sequence including a partial sequence of intron A of the preproenkephalin gene [(1984) J. Biol. Chem. 259, 14301-14308; (1984) J. Biol. Chem. 259, 14309-14313]. Northern blot analysis using a 5'-end-specific probe showed that this type of preproenkephalin mRNA exists exclusively in the germ cells.

Expression of the preproenkephalin A gene in tumor cells and brain glioma: a northern and in situ hybridization study

Preproenkephalin A (PPA) mRNA expression was studied by Northern blot and in situ hybridization in cell lines (rat glioma C6, rat hepatoma HTC, human neuroblastoma IMR32, mouse neuroblastoma NS20Y, rat fibroblast FR3T3, human bladder carcinoma EJ, human vulva carcinoma A431, myelocytic leukemia HL60, rat adrenal carcinoma Y1) and in brain tumours (implanted C6 cells). C6 glioma in cell culture, as well as in brain tumours, expressed high levels of PPA mRNA as compared to the caudate nucleus of the rat brain. EJ and FR3T3 cell lines also expressed the PPA mRNA, which was not detectable in A431, Y1, NS20Y, IMR32, HTC, HL60 cell lines as well as in the rat liver. This observation provides an interesting model to study the mechanisms by which the malignant transformation can induce in glial cells the derepression of a gene which is usually expressed in neurons or in neuron-like cells.

Expression of the enkephalin precursor gene in rat Sertoli cells. Regulation by follicle-stimulating hormone

Searching for somatic cells expressing the preproenkephalin (A) gene in the testis, we have isolated Sertoli cells from the testes of 20-day-old rats. Cultured Sertoli cells contained a single species (about 1.5 kb) of preproenkephalin mRNA, and follicle-stimulating hormone (FSH) transiently increased the mRNA abundance to a maximum (about 30 molecules per cell) at 12 h. Various compounds that activate the cyclic AMP system in Sertoli cells similarly increased the abundance of preproenkephalin mRNA. Moreover, FSH increased intracellular Met-enkephalin immunoreactive peptides in Sertoli cells. Thus, the preproenkephalin gene expression in Sertoli cells is positively regulated by FSH through the cyclic AMP system.

Transient increases and individual variations in preprocholecystokinin gene expression in rat telencephalic regions during postnatal development: a quantitative comparison between preprocholecystokinin mRNA and preproenkephalin mRNA

Postnatal development of the tissue levels of mRNAs encoding preprocholecystokinin and preproenkephalin in rat telencephalic regions was analyzed by quantitative dot blot hybridization using synthetic RNAs as standards. Preprocholecystokinin mRNA levels per tissue weight showed distinct peaks in the frontal cortex and hippocampus on postnatal days 10 and 20, respectively, while preproenkephalin mRNA levels in these regions showed no apparent peaks. In the striatum, preproenkephalin mRNA abundance linearly increased 8-fold from day 5-43, while a transient increase in the preprocholecystokinin mRNA abundance between days 10 and 20 was noted. Furthermore, the striatal levels of preprocholecystokinin mRNA showed marked individual variations during the early period of postnatal development. These results suggest transient and variable increases in cell density and/or in gene expression of cholecystokinin-synthesizing cells in the developing telencephalon.

Expression of preproenkephalin mRNA by cultured astrocytes and neurons

Expression of preproenkephalin mRNA by developing glia and neurons was examined in cultures of embryonic and neonatal rat brain. Cultured glia from specific regions of embryonic day 17 and neonatal day 1 rat brain were identified as astrocytes on the basis of both morphology and expression of immunoreactivity for glial fibrillary acidic protein. The level of preproenkephalin mRNA in cultured neonatal hypothalamic astrocytes was comparable to levels present in cultured embryonic striatal and hypothalamic neurons. Levels of the mRNA were significantly higher in astrocytes derived from neonatal hypothalamus compared to astrocytes derived from other areas of the brain. Thus, there is heterogeneity among astrocytes with respect to preproenkephalin expression. Levels of preproenkephalin mRNA in cultured neonatal striatal astrocytes were only one-third as high as levels in embryonic striatal astrocytes; this observation suggests that glial expression of the gene may be down-regulated during development. Although cultured hypothalamic neurons contained substantial levels of prodynorphin mRNA, levels of this mRNA were not detectable in cultured astrocytes from any brain region or in cultured striatal neurons. Thus, glia do not express all opioid peptide genes during development. These observations suggest that expression of the preproenkephalin gene by astrocytes may play a role in development of the brain.

Regulation of neuropeptide gene expression by steroid hormones

Steroid hormones modify several brain functions, at least in part by altering expression of particular genes. Of interest are those genes that are involved in cell-cell communication in the brain, for instance neuropeptide genes and genes that code for enzymes involved in synthesis of neurotransmitters. Steroid regulation of mRNA levels for several genes has been reported, including the genes coding for the neuropeptides vasopressin, corticotropin releasing factor, luteinizing hormone-releasing factor, pro-opiomelanocortin; somatostatin, preproenkephalin, and the enzyme tyrosine hydroxylase. Steroid control of releasing factor genes is consistent with classical neuroendocrine concepts of negative feedback. Steroid-induced plasticity of gene expression is sometimes in evidence, with the presence or absence of a particular steroid inducing expression of a neuropeptide gene in neurons that under other conditions do not express the gene. As a means of gaining some insight into the mechanism of action of steroid hormones, several groups have determined some of the neuropeptide profiles of neurons that contain receptors for steroid hormones. Marked heterogeneity is found, in that often only a subpopulation of phenotypically-similar neurons, even within a single brain area, contains receptors for a given steroid.