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

MGE-Like Neural Progenitor Cell Survival and Expression of Parvalbumin and Proenkephalin in a Jaundiced Rat Model of Kernicterus

Kernicterus is a permanent condition caused by brain damage from bilirubin toxicity. Dystonia is one of the most debilitating symptoms of kernicterus and results from damage to the globus pallidus (GP). One potential therapeutic strategy to treat dystonia in kernicterus is to replace lost GP neurons and restore basal ganglia circuits through stem cell transplantation. Toward this end, we differentiated human embryonic stem cells (hESCs) into medial ganglion eminence (MGE; the embryological origin of most of the GP neurons)-like neural precursor cells (NPCs). We determined neurochemical phenotype in cell culture and after transplanting into the GP of jaundiced Gunn rats. We also determined grafted cell survival as well as migration, distribution, and morphology after transplantation. As in the GP, most cultured MGE-like NPCs expressed γ-aminobutyric acid (GABA), with some co-expressing markers for parvalbumin (PV) and others expressing markers for pro-enkephalin (PENK). MGE-like NPCs survived in brains at least 7 weeks after transplantation, with most aggregating near the injection site. Grafted cells expressed GABA and PV or PENK as in the normal GP. Although survival was low and the maturity of grafted cells varied, many cells produced neurite outgrowth. While promising, our results suggest the need to further optimize the differentiation protocol for MGE-like NPC for potential use in treating dystonia in kernicterus.

Peripherally Acting Opioids in Orofacial Pain

The activation of opioid receptors by exogenous or endogenous opioids can produce significant analgesic effects in peripheral tissues. Numerous researchers have demonstrated the expression of peripheral opioid receptors (PORs) and endogenous opioid peptides (EOPs) in the orofacial region. Growing evidence has shown the involvement of PORs and immune cell-derived EOPs in the modulation of orofacial pain. In this review, we discuss the role of PORs and EOPs in orofacial pain and the possible cellular mechanisms involved. Furthermore, the potential development of therapeutic strategies for orofacial pain is also summarized.

Opioid and neuroHIV Comorbidity - Current and Future Perspectives

With the current national opioid crisis, it is critical to examine the mechanisms underlying pathophysiologic interactions between human immunodeficiency virus (HIV) and opioids in the central nervous system (CNS). Recent advances in experimental models, methodology, and our understanding of disease processes at the molecular and cellular levels reveal opioid-HIV interactions with increasing clarity. However, despite the substantial new insight, the unique impact of opioids on the severity, progression, and prognosis of neuroHIV and HIV-associated neurocognitive disorders (HAND) are not fully understood. In this review, we explore, in detail, what is currently known about mechanisms underlying opioid interactions with HIV, with emphasis on individual HIV-1-expressed gene products at the molecular, cellular and systems levels. Furthermore, we review preclinical and clinical studies with a focus on key considerations when addressing questions of whether opioid-HIV interactive pathogenesis results in unique structural or functional deficits not seen with either disease alone. These considerations include, understanding the combined consequences of HIV-1 genetic variants, host variants, and μ-opioid receptor (MOR) and HIV chemokine co-receptor interactions on the comorbidity. Lastly, we present topics that need to be considered in the future to better understand the unique contributions of opioids to the pathophysiology of neuroHIV.

Graphical Abstract

Short Term Development and Fate of MGE-Like Neural Progenitor Cells in Jaundiced and Non-Jaundiced Rat Brain

Neonatal hyperbilirubinemia targets specific brain regions and can lead to kernicterus. One of the most debilitating symptoms of kernicterus is dystonia, which results from bilirubin toxicity to the globus pallidus (GP). Stem cell transplantation into the GP to replace lost neurons and restore basal ganglia circuits function is a potential therapeutic strategy to treat dystonia in kernicterus. In this study we transplanted human medial ganglionic eminence (MGE)-like neural progenitor cells (NPCs) that we differentiated into a primarily gamma-aminobutyric acid (GABA)ergic phenotype, into the GP of non-immunosuppressed jaundiced (jj) and non-jaundiced (Nj) rats. We assessed the survival and development of graft cells at three time-points post-transplantation. While grafted MGE-like NPCs survived and generated abundant fibers in both jj and Nj brains, NPC survival was greater in the jj brain. These results were consistent with our previous finding that excitatory spinal interneuron-like NPCs exhibited a higher survival rate in the jj brain than in the Nj brain. Our findings further support our hypothesis that slightly elevated bilirubin levels in the jj brain served as an antioxidant and immunosuppressant to protect the transplanted cells. We also identified graft fibers growing toward brain regions that receive projections from the GP, as well as host fibers extending toward the graft. These promising findings suggest that MGE-like NPCs may have the capacity to restore the circuits connecting GP and other nuclei.

p38β Mitogen-Activated Protein Kinase Signaling Mediates Exenatide-Stimulated Microglial β-Endorphin Expression

Recent discoveries established that activation of glucagon-like peptide-1 receptors (GLP-1Rs) mediates neuroprotection and antinociception through microglial β-endorphin expression. This study aimed to explore the underlying signaling mechanisms of microglial β-endorphin. GLP-1Rs and β-endorphin were coexpressed in primary cultures of microglia. Treatment with the GLP-1R agonist exenatide concentration-dependently stimulated microglial expression of the β-endorphin precursor gene proopiomelanocortin (POMC) and peptides, with EC₅₀ values of 4.1 and 7.5 nM, respectively. Exenatide also significantly increased intracellular cAMP levels and expression of p-protein kinase A (PKA), p-p38, and p-cAMP response element binding protein (CREB) in cultured primary microglia. Furthermore, exenatide-induced microglial expression of POMC was completely blocked by reagents that specifically inhibit adenylyl cyclase and activation of PKA, p38, and CREB. In addition, knockdown of p38β (but not p38α) using short interfering RNA (siRNA) eliminated exenatide-induced microglial p38 phosphorylation and POMC expression. In contrast, lipopolysaccharide increased microglial activation of p38, and knockdown of p38α (but not p38β) partially suppressed expression of proinflammatory factors (including tumor necrosis factor-α, interleukin-1β, and interleukin-6). Exenatide-induced phosphorylation of p38 and CREB was also totally blocked by the PKA inhibitor and siRNA/p38β, but not by siRNA/p38α Seven-day intrathecal injections of siRNA/p38β (but not siRNA/p38α) completely blocked exenatide-induced spinal p38 activation, β-endorphin expression, and mechanical antiallodynia in rats with established neuropathy, although siRNA/p38β and siRNA/p38α were not antiallodynic. To our knowledge, our results are the first to show a causal relationship between the PKA-dependent p38β mitogen-activated protein kinase/CREB signal cascade and GLP-1R agonism-mediated microglial β-endorphin expression. The differential role of p38α and p38β activation in inflammation and nociception was also highlighted.

Opiate Drugs with Abuse Liability Hijack the Endogenous Opioid System to Disrupt Neuronal and Glial Maturation in the Central Nervous System

The endogenous opioid system, comprised of multiple opioid neuropeptide and receptor gene families, is highly expressed by developing neural cells and can significantly influence neuronal and glial maturation. In many central nervous system (CNS) regions, the expression of opioid peptides and receptors occurs only transiently during development, effectively disappearing with subsequent maturation only to reemerge under pathologic conditions, such as with inflammation or injury. Opiate drugs with abuse liability act to modify growth and development by mimicking the actions of endogenous opioids. Although typically mediated by μ-opioid receptors, opiate drugs can also act through δ- and κ-opioid receptors to modulate growth in a cell-type, region-specific, and developmentally regulated manner. Opioids act as biological response modifiers and their actions are highly contextual, plastic, modifiable, and influenced by other physiological processes or pathophysiological conditions, such as neuro-acquired immunodeficiency syndrome. To date, most studies have considered the acute effects of opiates on cellular maturation. For example, activating opioid receptors typically results in acute growth inhibition in both neurons and glia. However, with sustained opioid exposure, compensatory factors become operative, a concept that has been largely overlooked during CNS maturation. Accordingly, this article surveys prior studies on the effects of opiates on CNS maturation, and also suggests new directions for future research in this area. Identifying the cellular and molecular mechanisms underlying the adaptive responses to chronic opiate exposure (e.g., tolerance) during maturation is crucial toward understanding the consequences of perinatal opiate exposure on the CNS.

Prenatal morphine exposure reduces pyramidal neurons in CA1, CA2 and CA3 subfields of mice hippocampus

OBJECTIVE(S)

This study was carried out to evaluate the effect of maternal morphine exposure during gestational and lactation period on pyramidal neurons of hippocampus in 18 and 32 day mice offspring.

MATERIALS AND METHODS

Thirty female mice were randomly allocated into cases and controls. In case group, animals received morphine sulfate 10 mg/kg.body weight intraperitoneally during 7 days before mating, gestational period (GD 0-21), 18 and 32 days after delivery in the experimental groups. The control animals received an equivalent volume of normal saline. Cerebrum of six offsprings in each group was removed and stained with cresyl violet and a monoclonal antibody NeuN for immunohistochemical detection of surviving pyramidal neurons. Quantitative computer-assisted morphometric study was done on hippocampus.

RESULTS

The number of pyramidal neurons in CA1, CA2 and CA3 in treated groups was significantly reduced in postnatal day 18 and 32 (P18, P32) compared to control groups (P<0.05). The mean thickness of the stratum pyramidal layer was decreased in the treated groups in comparison with controls (P<0.05), whereas the mean thickness of the stratum oriens, stratum radiatum and stratum lacunosum-moleculare in CA1 field and stratum oriens, stratum lucidum, stratum radiatum and stratum lacunosum-moleculare in CA3 were significantly increased in morphine treated group in comparison with controls (P<0.05).

CONCLUSION

Morphine administration before and during pregnancy and during lactation period causes pyramidal neurons loss in 18 and 32 days old infant mice.

Acute and chronic mu opioids differentially regulate thrombospondins 1 and 2 isoforms in astrocytes

Chronic opioids induce synaptic plasticity, a major neuronal adaptation. Astrocyte activation in synaptogenesis may play a critical role in opioid tolerance, withdrawal, and dependence. Thrombospondins 1 and 2 (TSP1/2) are astrocyte-secreted matricellular glycoproteins that promote neurite outgrowth as well as dendritic spine and synapse formation, all of which are inhibited by chronic μ opioids. In prior studies, we discovered that the mechanism of TSP1 regulation by μ opioids in astrocytes involves crosstalk between three different classes of receptors, μ opioid receptor, EGFR and TGFβR. Moreover, TGFβ1 stimulated TSP1 expression via EGFR and ERK/MAPK activation, indicating that EGFR is a signaling hub for opioid and TGFβ1 actions. Using various selective antagonists, and inhibitors, here we compared the mechanisms of chronic opioid regulation of TSP1/2 isoform expression in vivo and in immortalized rat cortical astrocytes. TSP1/2 release from astrocytes was also monitored. Acute and chronic μ opioids, morphine, and the prototypic μ ligand, DAMGO, modulated TSP2 protein levels. TSP2 but not TSP1 protein content was up-regulated by acute (3 h) morphine or DAMGO by an ERK/MAPK dependent mechanism. Paradoxically, TSP2 protein levels were altered neither by TGFβ1 nor by astrocytic neurotrophic factors, EGF, CNTF, and BMP4. TSP1/2 immunofluorescence was increased in astrocytes subjected to scratch-wounding, suggesting TSPs may be useful markers for the "reactive" state of these cells and potentially for different types of injury. Previously, we determined that chronic morphine attenuated both neurite outgrowth and synapse formation in cocultures of primary astrocytes and neurons under similar temporal conditions that μ opioids reduced TSP1 protein levels in astrocytes. Here we found that, after the same 8 day treatment, morphine or DAMGO diminished TSP2 protein levels in astrocytes. Therefore, μ opioids may deter synaptogenesis via both TSP1/2 isoforms, but by distinct mechanisms.

Interactions of HIV and drugs of abuse: the importance of glia, neural progenitors, and host genetic factors

Considerable insight has been gained into the comorbid, interactive effects of HIV and drug abuse in the brain using experimental models. This review, which considers opiates, methamphetamine, and cocaine, emphasizes the importance of host genetics and glial plasticity in driving the pathogenic neuron remodeling underlying neuro-acquired immunodeficiency syndrome and drug abuse comorbidity. Clinical findings are less concordant than experimental work, and the response of individuals to HIV and to drug abuse can vary tremendously. Host-genetic variability is important in determining viral tropism, neuropathogenesis, drug responses, and addictive behavior. However, genetic differences alone cannot account for individual variability in the brain "connectome." Environment and experience are critical determinants in the evolution of synaptic circuitry throughout life. Neurons and glia both exercise control over determinants of synaptic plasticity that are disrupted by HIV and drug abuse. Perivascular macrophages, microglia, and to a lesser extent astroglia can harbor the infection. Uninfected bystanders, especially astroglia, propagate and amplify inflammatory signals. Drug abuse by itself derails neuronal and glial function, and the outcome of chronic exposure is maladaptive plasticity. The negative consequences of coexposure to HIV and drug abuse are determined by numerous factors including genetics, sex, age, and multidrug exposure. Glia and some neurons are generated throughout life, and their progenitors appear to be targets of HIV and opiates/psychostimulants. The chronic nature of HIV and drug abuse appears to result in sustained alterations in the maturation and fate of neural progenitors, which may affect the balance of glial populations within multiple brain regions.

Opiate drug use and the pathophysiology of neuroAIDS

Opiate abuse and HIV-1 have been described as interrelated epidemics, and even in the advent of combined anti-retroviral therapy, the additional abuse of opiates appears to result in greater neurologic and cognitive deficits. The central nervous system (CNS) is particularly vulnerable to interactive opiate-HIV-1 effects, in part because of the unique responses of microglia and astroglia. Although neurons are principally responsible for behavior and cognition, HIV-1 infection and replication in the brain is largely limited to microglia, while astroglia and perhaps glial progenitors can be latently infected. Thus, neuronal dysfunction and injury result from cellular and viral toxins originating from HIV-1 infected/exposed glia. Importantly, subsets of glial cells including oligodendrocytes, as well as neurons, express µ-opioid receptors and therefore can be direct targets for heroin and morphine (the major metabolite of heroin in the CNS), which preferentially activate µ-opioid receptors. This review highlights findings that neuroAIDS is a glially driven disease, and that opiate abuse may act at multiple glial-cell types to further compromise neuron function and survival. The ongoing, reactive cross-talk between opiate drug and HIV-1 co-exposed microglia and astroglia appears to exacerbate critical proinflammatory and excitotoxic events leading to neuron dysfunction, injury, and potentially death. Opiates enhance synaptodendritic damage and a loss of synaptic connectivity, which is viewed as the substrate of cognitive deficits. We especially emphasize that opioid signaling and interactions with HIV-1 are contextual, differing among cell types, and even within subsets of the same cell type. For example, astroglia even within a single brain region are heterogeneous in their expression of µ-, δ-, and κ-opioid receptors, as well as CXCR4 and CCR5, and Toll-like receptors. Thus, defining the distinct targets engaged by opiates in each cell type, and among brain regions, is critical to an understanding of how opiate abuse exacerbates neuroAIDS.

Purkinje cells loss in off spring due to maternal morphine sulfate exposure: a morphometric study

The toxic effects of morphine sulfate in the adult cerebral cortex and one-day neonatal cerebellum have been studied. This study was carried out to evaluate the effect of maternal morphine exposure during gestational and lactation period on the Purkinje cells and cerebellar cortical layer in 18- and 32-day-old mice offspring. Thirty female mice were randomly allocated into cases and controls. In cases, animals received morphine sulfate (10 mg/kg/body weight intraperitoneally) during the 7 days before mating, gestational day (GD 0-21) 18 or 32. The controls received an equivalent volume of saline. The cerebellum of six infants for each group was removed and each was stained with cresyl violet. Quantitative computer-assisted morphometric study was done on cerebellar cortex. The linear Purkinje cell density in both experimental groups (postnatal day [P]18, 23.40±0.5; P32, 23.45±1.4) were significantly reduced in comparison with the control groups (P18, 28.70±0.9; P32, 28.95±0.4) (P<0.05). Purkinje cell area, perimeter and diameter at apex and depth of simple lobules in the experimental groups were significantly reduced compared to the controls (P<0.05). The thickness of the Purkinje layer of the cerebellar cortex was significantly reduced in morphine treated groups (P<0.05). This study reveals that morphine administration before pregnancy, during pregnancy and during the lactation period causes Purkinje cells loss and Purkinje cell size reduction in 18- and 32-day-old infant mice.

Morphine and gp120 toxic interactions in striatal neurons are dependent on HIV-1 strain

A rigorously controlled, cell culture paradigm was used to assess the role of HIV-1 gp120 ± morphine in mediating opioid-HIV interactive toxicity in striatal neurons. Computerized time-lapse microscopy tracked the fate of individual neurons co-cultured with mixed-glia from mouse striata during opioid and gp120 exposure. Subpopulations of neurons and astroglia displayed μ-opioid receptor, CXCR4, and CCR5 immunoreactivity. While gp120 alone was or tended to be neurotoxic irrespective of whether X4-tropic gp120(IIIB), R5-tropic gp120(ADA), or dual-tropic gp120(MN) was administered, interactive toxicity with morphine differed depending on HIV-1 strain. For example, morphine only transiently exacerbated gp120(IIIB)-induced neuronal death; however, in combination with gp120(MN), morphine caused sustained increases in the rate of neuronal death compared to gp120(MN) alone that were prevented by naloxone. Alternatively, gp120(ADA) significantly increased the rate of neuron death, but gp120(ADA) toxicity was unaffected by morphine. The transient neurotoxic interactions between morphine and gp120(IIIB) were abrogated in the absence of glia suggesting that glia contribute significantly to the interactive pathology with chronic opiate abuse and neuroAIDS. To assess how mixed-glia might contribute to the neurotoxicity, the effects of morphine and/or gp120 on the production of reactive oxygen species (ROS) and on glutamate buffering were examined. All gp120 variants, and to a lesser extent morphine, increased ROS and/or decreased glutamate buffering, but together failed to show any interaction with morphine. Our findings indicate that HIV-1 strain-specific differences in gp120 are critical determinants in shaping both the timing and pattern of neurotoxic interactions with opioid drugs.

Inhibition of EGF-induced ERK/MAP kinase-mediated astrocyte proliferation by mu opioids: integration of G protein and beta-arrestin 2-dependent pathways

Although micro, kappa, and delta opioids activate extracellular signal-regulated kinase (ERK)/mitogen-activated protein (MAP) kinase, the mechanisms involved in their signaling pathways and the cellular responses that ensue differ. Here we focused on the mechanisms by which micro opioids rapidly (min) activate ERK and their slower (h) actions to inhibit epidermal growth factor (EGF)-induced ERK-mediated astrocyte proliferation. The micro-opioid agonists ([d-ala(2), mephe(4), gly-ol(5)] enkephalin and morphine) promoted the phosphorylation of ERK/MAP kinase within 5 min via G(i/o) protein, calmodulin (CaM), and beta-arrestin2-dependent signaling pathways in immortalized and primary astrocytes. This was based on the attenuation of the micro-opioid activation of ERK by pertussis toxin (PTX), the CaM antagonist, W-7, and siRNA silencing of beta-arrestin2. All three pathways were shown to activate ERK via an EGF receptor transactivation-mediated mechanism. This was disclosed by abolishment of micro-opioid-induced ERK phosphorylation with the EGF receptor-specific tyrosine phosphorylation inhibitor, AG1478, and micro-opioid-induced reduction of EGF receptor tyrosine phosphorylation by PTX, and beta-arrestin2 targeting siRNA in the present studies and formerly by CaM antisense. Long-term (h) treatment of primary astrocytes with [d-ala(2),mephe(4),gly-ol(5)] enkephalin or morphine, attenuated EGF-induced ERK phosphorylation and proliferation (as measured by 5'-bromo-2'-deoxy-uridine labeling). PTX and beta-arrestin2 siRNA but not W-7 reversed the micro-opioid inhibition. Unexpectedly, beta-arrestin-2 siRNA diminished both EGF-induced ERK activation and primary astrocyte proliferation suggesting that this adaptor protein plays a novel role in EGF signaling as well as in the opioid receptor phase of this pathway. The results lend insight into the integration of the different micro-opioid signaling pathways to ERK and their cellular responses.

Trafficking and fusion of neuropeptide Y-containing dense-core granules in astrocytes

It is becoming clear that astrocytes are active participants in synaptic functioning and exhibit properties, such as the secretion of classical transmitters, previously thought to be exclusively neuronal. Whether these similarities extend to the release of neuropeptides, the other major class of transmitters, is less clear. Here we show that cortical astrocytes can synthesize both native and foreign neuropeptides and can secrete them in a stimulation-dependent manner. Reverse transcription-PCR and mass spectrometry indicate that cortical astrocytes contain neuropeptide Y (NPY), a widespread neuronal transmitter. Immunocytochemical studies reveal NPY-immunoreactive (IR) puncta that colocalize with markers of the regulated secretory pathway. These NPY-IR puncta are distinct from the synaptic-like vesicles that contain classical transmitters, and the two types of organelles are differentially distributed. After activation of metabotropic glutamate receptors and the release of calcium from intracellular stores, the NPY-IR puncta fuse with the cell membrane, and the peptide-containing dense cores are displayed. To determine whether peptide secretion subsequently occurred, exocytosis was monitored from astrocytes expressing NPY-red fluorescent protein (RFP). In live cells, after activation of glutamate receptors, the intensity of the NPY-RFP-labeled puncta declined in a step-like manner indicating a regulated release of the granular contents. Because NPY is a widespread and potent regulator of synaptic transmission, these results suggest that astrocytes could play a role in the peptidergic modulation of synaptic signaling in the CNS.

The OGF-OGFr axis utilizes the p16INK4a and p21WAF1/CIP1 pathways to restrict normal cell proliferation

Opioid growth factor (OGF) is an endogenous opioid peptide ([Met(5)]enkephalin) that interacts with the OGF receptor (OGFr) and serves as a tonically active negative growth factor in cell proliferation of normal cells. To clarify the mechanism by which OGF inhibits cell replication in normal cells, we investigated the effect of the OGF-OGFr axis on cell cycle activity in human umbilical vein endothelial cells (HUVECs) and human epidermal keratinocytes (NHEKs). OGF markedly depressed cell proliferation of both cell lines by up to 40% of sterile water controls. Peptide treatment induced cyclin-dependent kinase inhibitor (CKI) p16(INK4a) protein expression and p21(WAF1/CIP1) protein expression in HUVECs and NHEKs, but had no effect on p15, p18, p19, or p27 protein expression in either cell type. Inhibition of either p16(INK4a) or p21(WAF1/CIP1) activation by specific siRNAs blocked OGF inhibitory action. Human dermal fibroblasts and mesenchymal stem cells also showed a similar dependence of OGF action on p16(INK4a) and p21(WAF1/CIP1). Collectively, these results indicate that both p16(INK4a) and p21(WAF1/CIP1) are required for the OGF-OGFr axis to inhibit cell proliferation in normal cells.

‘Neuro’-peptides in glia: Focus on NPY and galanin

The term neuropeptide was advanced by de Wied and collaborators in the early seventies. At that time, they defined neuropeptides as endogenous substances synthesized in nerve cells and involved in nervous system functions. Since then, several studies have revealed that the very same 'neuropeptides' are also expressed in non-neuronal cells. It is therefore generally accepted that the original definition of these peptides was too limited and, consequently, it has recently been revised. Among the non-neuronal cells that synthesize neuropeptides are several glial cell types.

Selective vulnerability of cerebellar granule neuroblasts and their progeny to drugs with abuse liability

Cerebellar development is shaped by the interplay of genetic and numerous environmental factors. Recent evidence suggests that cerebellar maturation is acutely sensitive to substances with abuse liability including alcohol, opioids, and nicotine. Assuming substance abuse disrupts cerebellar maturation, a central question is: what are the basic mechanisms underlying potential drug-induced developmental defects? Evidence reviewed herein suggests that the maturation of granule neurons and their progeny are intrinsically affected by several classes of substances with abuse liability. Although drug abuse is also likely to target directly other cerebellar neuron and glial types, such as Purkinje cells and Bergmann glia, findings in isolated granule neurons suggest that they are often the principle target for drug actions. Developmental events that are selectively disrupted by drug abuse in granule neurons and/or their neuroblast precursors include proliferation, migration, differentiation (including neurite elaboration and synapse formation), and programmed cell death. Moreover, different classes of drugs act through distinct molecular mechanisms thereby disrupting unique aspects of development. For example, drug-induced perturbations in: (i) neurotransmitter biogenesis; (ii) ligand and ion-gated receptor function and their coupling to intracellular effectors; (iii) neurotrophic factor biogenesis and signaling; and (iv) intercellular adhesion are all likely to have significant effects in shaping developmental outcome. In addition to identifying therapeutic strategies for drug abuse intervention, understanding the mechanisms by which drugs affect cellular maturation is likely to provide a better understanding of the neurochemical events that normally shape central nervous system development.

Endogenous opioids and oligodendroglial function: possible autocrine/paracrine effects on cell survival and development

Previous work has shown that oligodendrocytes (OLs) express both micro- and kappa-opioid receptors. In developing OLs, micro receptor activation increases OL proliferation, while the kappa-antagonist nor-binaltorphimine (NorBNI) affects OL differentiation. Because exogenous opioids were not present in our defined culture medium, we hypothesized that NorBNI blocked endogenous opioids produced by the OLs themselves. To test this, intact and partially processed proenkephalin and prodynorphin-derived peptides were assessed in OLs using immunocytochemistry or Western blot analysis, or both. Immature OLs possessed large amounts of intact and partially processed proenkephalin precursors, as well as posttranslational products of prodynorphin including dynorphin A (1-17). With maturation, however, intact or partially processed proenkephalin was expressed by only about 50% of OLs, while dynorphin A (1-17) was undetectable. To assess the function of OL-derived opioids, the effect of kappa-agonists/antagonists on OL differentiation and death was explored. kappa-Agonists alone had no effect. In contrast, NorBNI significantly increased OL death. Additive OL losses were evident when NorBNI was paired with toxic levels of glutamate, suggesting that kappa-receptor blockade alone is sufficient to induce OL death. Thus, the results indicate that OLs express proenkephalin and prodynorphin peptides in a developmentally regulated manner, and further suggest that opioids produced by OLs modulate OL maturation and survival through local (i.e., autocrine and/or paracrine) mechanisms.

Expression of preproenkephalin mRNA and production and secretion of enkephalins by human thymocytes

Human thymocytes were tested for the capacity to express the preproenkephalin (PPE) gene and for production of the end product metenkephalin (MENK). It is shown here for the first time that the cytokines IL-2, IL-4, IL-6, and TGF-beta are capable of inducing PPE mRNA expression. Moreover, a culture of thymocytes with the cytokines results in intracellular expression of MENK as determined by immunohistochemistry. Thymocytes do not secrete detectable amounts of MENK, however, but only the larger MENK-containing peptides or proteins. Cytokines IL-1 beta and IL-10 increase the expression of PPE mRNA in 50% of the thymuses tested, whereas IFN-gamma does not induce changes in PPE mRNA expression.

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.

Kappa opioid receptors are expressed by interneurons in the CA1 area of the rat hippocampus: a correlated light and electron microscopic immunocytochemical study

A local GABA-system is known to have a mediatory function between several afferents and the principal cells of the hippocampus. This study examines the distribution and fine structure of kappa opioid receptor-immunoreactive elements in the CA1 subfield and reveals some new aspects concerning the structural basis of opioid-GABA interaction in the rat hippocampal formation. Kappa receptors were visualized immunocytochemically with a previously produced and characterized monoclonal antibody, the mAb KA8 (Maderspach, K., Németh, K., Simon, J., Benyhe, S., Szûcs, M., Wollemann, M., 1991. A monoclonal antibody recognizing kappa-, but not mu- and delta-opioid receptors. J. Neurochem. 56, 1897-1904). The antibody selectively recognizes the kappa opioid receptor with preference to the kappa(2) subtype. Neuronal cell bodies, proximal dendrites and occasionally glial processes surrounding neuronal perikarya were labelled in the CA1 area. The immunopositive cells were present mainly in the stratum oriens, followed by the stratum pyramidale in a rostrocaudally increasing number. Their shape was fusiform, or multipolar. Occasionally kappa receptor-immunoreactive boutons surrounding weakly immunopositive somata were also observed. Electron microscopy of immunopositive neurons showed that the DAB labelling was intensive in the perinuclear cytoplasm. The widths and electron densities of the postsynaptic densities of some axosomatic synapses were remarkably increased. Similar increase of postsynaptic densities were observable at some axodendritic and axospinous synapses. On the basis of their location and fine structural properties the labelled cells are suggested to be GABAergic inhibitory interneurons, probably belonging to the somatostatinergic sub-population. The axons of these inhibitory interneurons are known to arborize in the stratum lacunosum-moleculare where the entorhinal afferents terminate. A modulatory effect of opioids on the entorhinal input, mediated by somatostatinergic interneurons is suggested

A regulated secretory pathway in cultured hippocampal astrocytes

Glial cells have been reported to express molecules originally discovered in neuronal and neuroendocrine cells, such as neuropeptides, neuropeptide processing enzymes, and ionic channels. To verify whether astrocytes may have regulated secretory vesicles, the primary cultures prepared from hippocampi of embryonic and neonatal rats were used to investigate the subcellular localization and secretory pathway followed by secretogranin II, a well known marker for dense-core granules. By indirect immunofluorescence, SgII was detected in a large number of cultured hippocampal astrocytes. Immunoreactivity for the granin was detected in the Golgi complex and in a population of dense-core vesicles stored in the cells. Subcellular fractionation experiments revealed that SgII was stored in a vesicle population with a density identical to that of the dense-core secretory granules present in rat pheochromocytoma cells. In line with these data, biochemical results indicated that 40-50% of secretogranin II synthesized during 18-h labeling was retained intracellularly over a 4-h chase period and released after treatment with different secretagogues. The most effective stimulus appeared to be phorbol ester in combination with ionomycin in the presence of extracellular Ca(2+), a treatment that was found to produce a large and sustained increase in intracellular calcium [Ca(2+)](i) transients. Our findings indicate that a regulated secretory pathway characterized by (i) the expression and stimulated exocytosis of a typical marker for regulated secretory granules, (ii) the presence of dense-core vesicles, and (iii) the ability to undergo [Ca(2+)](i) increase upon specific stimuli is present in cultured hippocampal astrocytes.

Delta-opioid receptor immunoreactivity on astrocytes is upregulated during mitosis

Endogenous opioid peptides and opioid receptors are expressed by brain cells early during normal development, and exogenous opiate exposure in this period is known to affect brain cell proliferation and maturation. Despite the abundant evidence that opioids affect brain development, little is known about the mechanisms involved. In this study cortical astrocytes in primary culture were examined immunohistochemically by using antibodies against the opioid receptors. The immunoreactivity for delta-opioid receptors was strongly upregulated during mitosis with an increase in immunostaining that started in early prophase and lasted through the M-phase to cytokinesis. Similar effects could not be observed when antibodies against the mu- or kappa-opioid receptor subtypes were used. Cultured neurons and microglia presented a strong and homogenous immunostaining for the delta-opioid receptor and no further upregulation of immunoreactivity could be detected in these cells. The presence of functional delta-opioid receptors on the mitotic astrocytes was verified by using microspectrofluorometry for detection of delta-opioid agonist induced changes in intracellular free calcium concentrations ([Ca2+]i). In these experiments fluo-3/AM incubated cells showed a rapidly induced delta-opioid agonist (DPDPE, 10(-6) M) evoked increase in [Ca2+]i. These results suggest an upregulation of the delta-opioid receptors that could represent a mechanism involved in the response to opioids in the developing brain.

Opioids disrupt Ca2+ homeostasis and induce carbonyl oxyradical production in mouse astrocytes in vitro: transient increases and adaptation to sustained exposure

Pharmacologically distinct subpopulations of astroglia express mu, delta, and/or kappa opioid receptors. Activation of mu, delta, or kappa opioid receptors can destabilize intracellular calcium ([Ca2+]i) in astrocytes leading to cellular hypertrophy and reactive injury. To assess whether acute or sustained opioid exposure might adversely affect astroglial function by disrupting Ca2+ homeostasis or by producing reactive oxygen species, fura-2 and a novel fluorescent-tagged biotin-4-amidobenzoic hydrazide reagent, respectively, were used to detect [Ca2+]i and carbonyl oxidation products within individual murine astrocytes. Acute (3 h) exposure to mu; (H-Tyr-Pro-Phe (N-Me) -D-Pro-NH2; PLO17), delta ([D-Pen2, D-Pen5]-enkephalin), and kappa (trans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrr olidinyl) cyclohexyl] benzeneacetamide methanesulfonate; U50,488H) opioid agonists caused significant mean increases in [Ca2+]i and in the levels of oxidative products in astrocytes. In contrast, following 72 h of continuous opioid exposure, [Ca2+]i and carbonyl levels returned to normal, irrespective of opioid treatment. These preliminary findings indicate that opioids initially destabilize [Ca2+]i and increase reactive oxygen species in astrocytes; however, astrocytes later recover and adapt to sustained opioid exposure.

Opioid receptor and peptide mRNA expression in proliferative zones of fetal rat central nervous system

There is increasing evidence to suggest that opioid peptides may have widespread effects as regulators of growth. To evaluate the hypothesis that endogenous opioids control cellular proliferation during neural development, we have used in situ hybridization to examine opioid peptide and receptor mRNA expression in neuroepithelial zones of fetal rat brain and spinal cord. Our data show that proenkephalin mRNA is widely expressed in forebrain germinal zones and choroid plexus during the second half of gestation. In contrast, prodynorphin mRNA expression is restricted to the periventricular region of the ventral spinal cord. Little µ or delta receptor mRNA expression was detected in any regions of neuronal proliferation prior to birth. However, kappa receptor mRNA is widely expressed in hindbrain germinal zones during the 3rd week of gestation. Our present findings support the hypothesis that endogenous opioids may regulate proliferation of both neuronal and non-neuronal cells during central nervous system development. Given the segregated expression of proenkephalin mRNA in forebrain neuroepithelium and kappa receptor mRNA within hindbrain, different opioid mechanisms may regulate cell division in rostral and caudal brain regions.Key words: enkephalin, dynorphin, ontogeny, neurogenesis.

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.

Regional, developmental, and cell cycle-dependent differences in mu, delta, and kappa-opioid receptor expression among cultured mouse astrocytes

The diversity of opioid receptor expression was examined in astrocytes in low-density and non-dividing (confluent) cultures from the cerebral cortex, hippocampus, cerebellum, and striatum of 1-day-old mice. Mu, delta, and kappa opioid receptor expression was assessed in individual cells immunocytochemically, by using flow cytometry, and functionally by examining agonist-induced changes in intracellular calcium ([Ca2+]i). Significant spatial and temporal differences were evident in the pattern of expression of mu, delta, and kappa receptors among astrocytes. In low-density cultures, greater proportions of astrocytes expressed mu-opioid receptor immunoreactivity in the cerebral cortex and hippocampus (26-34%) than in the cerebellum or striatum (7-12%). At confluence, a greater percentage of astrocytes in cerebellar (26%) and striatal (30%) cultures expressed mu-immunoreactivity. Fewer astrocytes possessed delta-immunoreactivity in low-density striatal cultures (8%) compared to other regions (16-22%). The proportion of delta receptor-expressing astrocytes declined in the cerebellum but increased in the hippocampus. Kappa-opioid receptors were uniformly expressed by 27-34% of astrocytes from all regions, except in cortical cultures, where the proportion of kappa expressing cells was 38% at low-density and decreased to 22% at confluence. Selective mu (PLO 17; H-Tyr-Pro-Phe (N-Me) -D-Pro-NH2, delta ([D-Pen2, D-Pen5] enkephalin), or kappa (U50,488H; trans-(+/-)-3,4-Dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeneacetamide methanesulfonate) opioid receptor agonists increased [Ca2+]i in subpopulations of astrocytes indicating the presence of functional receptors. Lastly, opioid receptor immunofluorescence varied during the cell division cycle. A greater proportion of astrocytes in the G2/M phase of the cell cycle were mu or delta receptor immunofluorescent than at G0/G1. When astrocytes were reversibly arrested in G1, significantly fewer cells expressed delta receptor immunofluorescence; however, upon reentry into the cell cycle immunofluorescent cells reappeared. In conclusion, opioid phenotype varies considerably among individual cultured astrocytes, and this diversity was determined by regional and developmental (age and cell cycle dependent) differences in the brain. These in vitro findings suggest astroglia contribute to regional and developmental idiosyncrasies in opioid function within the brain.

Endogenous opioid system in developing normal and jimpy oligodendrocytes: mu and kappa opioid receptors mediate differential mitogenic and growth responses

The early development of both neurons and neuroglia may be modulated by signaling through opioid mediated pathways. Neurons and astroglia not only express specific types of opiate receptors, but also respond functionally to opioids with altered rates of proliferation and growth. The present study was undertaken to determine if opioids also modulate development of the other major CNS macroglial cell, the oligodendrocyte (OL). Using well-characterized polyclonal antibodies specific for delta-, kappa-, and mu-opiate receptors, OLs grown in vitro were shown to express mu-receptors at a very immature stage prior to expression of kappa-receptors. This developmentally regulated sequence differs from the pattern of expression in neurons and astroglia. delta-receptors are apparently absent from cultured OLs. OLs also have physiologic responses to selective mu- and kappa-receptor agonists and antagonists. Exposure of relatively immature O4+ OLs to the mu-receptor agonist PL017 [H-Tyr-Pro-Phe(N-Me)-D-Pro-NH2] resulted in a significant enhancement in the rate of DNA synthesis. This effect, which was not observed in more mature MBP+ OLs, was entirely blocked by the antagonist naloxone. Although the kappa-receptor pathway appeared to be uninvolved in controlling proliferation, the kappa-receptor antagonist nor-binaltorphimine significantly increased the size of myelin-like membranes produced by the cultured OLs. Interestingly, OLs derived from the jimpy mouse, a mutant characterized by an almost complete lack of CNS myelin and premature death of OLs, were found to be deficient in kappa-opiate receptors. Our findings clearly show that OLs not only express specific opiate receptors, but also respond to changes in their level of stimulation in ways that could profoundly impact nervous system morphology and function. If opiate receptors are expressed by OLs in vivo, their pharmacological manipulation might provide a novel pathway for modulating OL and myelin production both during development and in demyelinated conditions.

Peptide receptors on astrocytes

In recent years, it has become apparent that astrocytes (at least in vitro) harbor functional receptors to almost all possible neurotransmitters (with the potential noticeable exception of acetylcholine nicotinic receptors). Peptides are no exception, since receptors to all neuropeptides known to be produced in the CNS have been found on cultured astrocytes, and the presence of many of these has been confirmed on astrocytes in vivo. A variety of methodologies have been used to detect peptide receptors on astrocytes, as summarized in the current review. Special emphasis is also put on the possible roles that peptides may play in the regulation of astrocyte functions. These include proliferation, morphology, release of eicosanoids and arachidonic acid, induction of calcium transients and calcium waves, and control of internal pH, glucose uptake, glycogen metabolism, and gap junctional conductance. Recent data concerning the effects of natriuretic peptides on astrocytes are reviewed, and why these peptides may constitute priviledged tools to test the effects of peptides on astrocyte-neuron interactions is also discussed.

The interleukin-1beta-mediated regulation of proenkephalin and opioid receptor messenger RNA in primary astrocyte-enriched cultures

Opioids have been found to modulate the function of the immune system by regulating the biochemical and proliferative properties of its cellular components. The interaction of opioid and immune systems, however, is not unidirectional, but rather, bidirectional in nature. In the CNS, one cellular target of immune system activation is the astrocytes, glial cells known to synthesize proenkephalin. We have recently shown that these cells also express the messenger RNA transcripts for the opioid receptors mu, delta and kappa, raising the question of the functional significance of this opioid peptide and the related receptors in the astrocytes. That is, why do astrocytes express proenkephalin and opioid receptors, and are these molecules responsive to a factor to which the astrocytes could be exposed in vivo? Furthermore, do these molecules respond to this factor in a region-specific fashion? In the present study, in order to characterize the astrocytic opioid response to an immune factor, we examined the concomitant regulation of mu, delta, kappa and proenkephalin messenger RNAs by interleukin-1beta (1 ng/ml=60 pM, 24 h) in primary astrocyte-enriched cultures derived from the rat (post-natal day 1-2) cortex, striatum, cerebellum, hippocampus and hypothalamus. Interleukin-1beta treatment was found to increase by 55-75% the level of mu receptor messenger RNA in striatal, cerebellar and hippocampal cultures, but not in cultures derived from the cortex or hypothalamus. However, the cytokine had no effect on the level of delta receptor messenger RNA in any of the five cultures examined. In marked contrast to its stimulatory effects on mu receptor messenger RNA levels and its lack of an effect on 6 receptor messenger RNA expression, interleukin-1beta reduced to 10-30% of control levels the kappa receptor messenger RNA levels in all cultures. Interleukin-1beta had no effect on the level of proenkephalin messenger RNA in cortical, striatal, cerebellar and hypothalamic cultures, but did significantly decrease the expression of proenkephalin messenger RNA in hippocampal cultures to 40% of the control level. Therefore, interleukin-1beta differentially regulated opioid receptor messenger RNA in astrocyte-enriched cultures in a manner dependent upon both the receptor type and the brain region from which the culture was derived. The cytokine also differentially regulated proenkephalin messenger RNA in a region-dependent fashion. These findings suggest a capacity for astrocytes to differentially regulate opioid peptide and receptor messenger RNAs in response to an immune factor, supporting the potential existence of a novel immune-opioid system interaction in the CNS.

Subcellular analysis of Tyr-aminopeptidase activities in the developing rat cerebellum

The endogenous opioid system seems to play important roles in the developing cerebellum. The first opioid peptide isolated, Met-enkephalin, is expressed transiently in this brain area. In the present study, several enzyme activities capable of hydrolyzing enkephalins are measured during the first month of cerebellar development, using Tyr-beta-naphthylamyde as substrate and puromycin as inhibitor of one of the membrane-bound aminopeptidases. Puromycin-sensitive soluble and membrane-bound aminopeptidase activities decrease in the synaptosomal and mitochondrial fractions at the end of the first month of life, just when enkephalin-like immunoreactivity decreases in the cerebellum. Membrane-bound enzyme also decreases in the myelinic fraction. Synaptosomal activity increases after birth, coinciding with decreases in the activity in the microsomal fraction. Puromycin-insensitive and membrane-bound aminopeptidase shows less significant developmental changes and they occur mainly in the first week of life, coinciding with the axonal and dendrite growth. These results could suggest a possible role of these enzymes, together with the rest of the opioid system, in cerebellar development.

kappa-opioid receptor expression defines a phenotypically distinct subpopulation of astroglia: relationship to Ca2+ mobilization, development, and the antiproliferative effect of opioids

To assess the role of kappa-opioid receptors in astrocyte development, the effect of kappa-agonists on the growth of astroglia derived from 1-2-day-old mouse cerebra was examined in vitro. kappa-Opioid receptor expression was assessed immunocytochemically (using KA8 and KOR1 antibodies), as well as functionally by examining the effect of kappa-receptor activation on intracellular calcium ([Ca2+]i) homeostasis and DNA synthesis. On days 6-7, as many as 50% of the astrocytes displayed kappa-receptor (KA8) immunoreactivity or exhibited increases in [Ca2+]i in response to kappa-agonist treatment (U69,593 or U50,488H). Exposure to U69,593 (100 nM) for 72 h caused a significant reduction in number and proportion of glial fibrillary acidic protein-immunoreactive astrocytes incorporating bromodeoxyuridine (BrdU) that could be prevented by co-administering the kappa-antagonist, nor-binaltorphimine (300 nM). In contrast, on day 14, only 5 or 14%, respectively, of the astrocytes were kappa-opioid receptor (KA8) immunoreactive or displayed functional increases in [Ca2+]i. Furthermore, U69,593 (100 nM) treatment failed to inhibit BrdU incorporation at 9 days in vitro. Experimental manipulations showed that kappa-receptor activation increases astroglial [Ca2+]i both through influx via L-type channels and through mobilization of intracellular stores (which is an important Ca2+ signaling pathway in cell division). Collectively, these results indicate that a subpopulation of developing astrocytes express kappa-opioid receptors in vitro, and suggest that the activation of kappa-receptors mobilizes [Ca2+]i and inhibits cell proliferation. Moreover, the proportion of astrocytes expressing kappa-receptors was greatest during a period of rapid cell growth suggesting that they are preferentially expressed by proliferating astrocytes.

mu-Opioid receptor-induced Ca2+ mobilization and astroglial development: morphine inhibits DNA synthesis and stimulates cellular hypertrophy through a Ca(2+)-dependent mechanism

Morphine, a preferential mu-opioid receptor agonist, alters astroglial development by inhibiting cell proliferation and by promoting cellular differentiation. Although morphine affects cellular differentiation through a Ca(2+)-dependent mechanism, few studies have examined whether Ca2+ mediates the effect of opioids on cell proliferation, or whether a particular Ca2+ signal transduction pathway mediates opioid actions. Moreover, it is uncertain whether one or more opioid receptor types mediates the developmental effects of opioids. To address these questions, the present study examined the role of mu-opioid receptors and Ca2+ mobilization in morphine-induced astrocyte development. Morphine (1 microM) and non-morphine exposed cultures enriched in murine astrocytes were incubated in Ca(2+)-free media supplemented with < 0.005, 0.3, 1.0, or 3.0 mM Ca2+ ([Ca2+]o), or in unmodified media containing Ca2+ ionophore (A23187), nifedipine (1 microM), dantrolene (10 microM), thapsigargin (100 nM), or L-glutamate (100 microM) for 0-72 h. mu-Opioid receptor expression was examined immunocytochemically using specific (MOR1) antibodies. Intracellular Ca2+ ([Ca2+]i) was measured by microfluorometric analysis using fura-2. Astrocyte morphology and bromodeoxyuridine (BrdU) incorporation (DNA synthesis) were assessed in glial fibrillary acidic protein (GFAP) immunoreactive astrocytes. The results showed that morphine inhibited astroglial growth by activating mu-opioid receptors. Astrocytes expressed MOR1 immunoreactivity and morphine's actions were mimicked by the selective mu agonist PL017. In addition, morphine inhibited DNA synthesis by mobilizing [Ca2+]i in developing astroglia. At normal [Ca2+]o, morphine attenuated DNA synthesis by increasing [Ca2+]i; low [Ca2+]o (0.3 mM) blocked this effect, while treatment with Ca2+ ionophore or glutamate mimicked morphine's actions. At extremely low [Ca2+]o (< 0.005 mM), morphine paradoxically increased BrdU incorporation. Although opioids can increase [Ca2+]i in astrocytes through several pathways, not all affect DNA synthesis or cellular morphology. Nifedipine (which blocks L-type Ca2+ channels) did not prevent morphine-induced reductions in BrdU incorporation or cellular differentiation, while thapsigargin (which depletes IP3-sensitive Ca2+ stores) severely affected inhibited DNA synthesis and cellular differentiation-irrespective of morphine treatment. However, dantrolene (an inhibitor of Ca(2+)-dependent Ca2+ release) selectively blocked the effects of morphine. Collectively, the findings suggest that opioids suppress astroglial DNA synthesis and promote cellular hypertrophy by inhibiting Ca(2+)-dependent Ca2+ release from dantrolene-sensitive intracellular stores. This implies a fundamental mechanism by which opioids affect central nervous system maturation.

Primary astroglial cultures derived from several rat brain regions differentially express mu, delta and kappa opioid receptor mRNA

The existence of opioid receptors within glial cell membranes has been proposed by several laboratories based on biochemical and radioligand binding data. The recent cloning of the mu, delta and kappa receptors has enabled us to directly examine the issue of opioid receptor expression in rat brain astroglia by using solution hybridization/ribonuclease protection assays to analyze the total RNA obtained from primary cultures of cortical, striatal, cerebellar, hippocampal and hypothalamic astrocytes. The results indicate that all five glial cultures expressed mu, delta and kappa receptor mRNA. The rank order of receptor mRNA abundance, expressed collectively across all five cultures, was determined to be delta > or = kappa >> mu. An analysis of the glial distribution profile for each receptor type revealed that mu receptor mRNA levels were the most abundantly expressed in cortical cultures, while the greatest levels of delta receptor mRNA were found in the cortical and hypothalamic cultures, and significant kappa receptor mRNA levels were produced by the cortical, hypothalamic and cerebellar cultures. Furthermore, the five glial cultures each expressed different levels of total opioid receptor (mu + delta + kappa) mRNA. The rank order of total opioid receptor mRNA expression across different astroglial cultures was found to be cortex > hypothalamus > cerebellum = hippocampus > striatum. An analysis of the relative expression profiles for mu, delta and kappa receptor mRNA within each culture revealed that all cultures manifested relatively high levels of delta and kappa receptor mRNA, but relatively low levels of mu receptor mRNA. Generally, cortical, hippocampal and hypothalamic cultures were characterized by comparable levels of delta and kappa receptor mRNA, and little, if any, mu receptor mRNA. However, striatal cultures were characterized by a high level of delta receptor mRNA which was approximately twice and four times that of the kappa and mu receptor mRNA, respectively. In contrast, cerebellar cultures expressed predominantly kappa receptor mRNA at a level which was almost twice that of the delta receptor mRNA, and expressed very little mu receptor mRNA. These data show that primary astroglial cultures not only express mu, delta and kappa receptor mRNAs, but they do so in a manner dependent upon receptor type and brain region. This suggests a regional heterogeneity of astrocytes with respect to opioid receptor expression, a characteristic previously described only for neurons. Furthermore, it suggests the existence of an additional anatomical component in CNS opioid systems.

Localization of vasopressin mRNA and immunoreactivity in pituicytes of pituitary stalk-transected rats after osmotic stimulation

The presence of [arginine] vasopressin (AVP) mRNA and AVP immunoreactivity in pituicytes of the neural lobe (NL) of intact and pituitary stalk-transected rats, with and without osmotic stimulation, was examined. AVP mRNA was analyzed by Northern blotting, as well as by in situ hybridization in combination with immunocytochemistry using anti-glial fibrillary acidic protein (GFAP) as a marker for pituicytes. In intact rats, a poly(A) tail-truncated 0.62-kb AVP mRNA was detected in the NL and was found to increase 10-fold with 7 days of continuous salt loading. Morphological analysis of the NL of 7-day salt-loaded rats revealed the presence of AVP mRNA in a significant number of GFAP-positive pituicytes in the NL and in areas most probably containing nerve fibers. Eight days after pituitary stalk transection the NL AVP mRNA diminished in animals given water to drink, whereas in those given 2% saline for 18 h followed by 6 h of water, a treatment repeated on 6 successive days beginning 2 days after surgery, the 0.62-kb AVP mRNA was present. The AVP mRNA in the pituitary stalk-transected, salt-loaded rats showed an exclusive cellular distribution in the NL, indicative of localization in pituicytes. Immunoelectron microscopy showed the presence of AVP immunoreactivity in a subpopulation of pituicytes 7 and 10 days after pituitary stalk transection in salt-loaded animals, when almost all AVP fibers had disappeared from the NL. These data show that a subset of pituicytes in the NL is activated to synthesize AVP mRNA and AVP in response to osmotic stimulation.

The transcriptional regulation of the preproenkephalin gene

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Sex dimorphic alterations in postnatal brain catecholamines after gestational morphine

The concentration of brain catecholamines was measured in the hypothalamus, preoptic area (POA), frontal cortex, cerebellum, and striatum of rats exposed in utero to morphine (5-10 mg/kg/twice daily) during gestation days 11-18. Prenatal morphine induced regionally specific, sexually dimorphic alterations in male and female norepinephrine (NE), and dopamine (DA) content at different postnatal ages. Prenatal morphine significantly increased NE content in the hypothalamus of both sexes at postnatal day (PND) 23. In the POA, on the other hand, morphine increased NE content in exposed males at PND 23 and in females at PND 33. In the cerebellum, the NE content of both sexes was significantly elevated at PND 45. In the striatum, NE content was increased by the prenatal morphine only in females at PND 16. The concentration of DA was also affected in a sexually dimorphic manner. At PND 16, prenatal morphine increased the levels of hypothalamic DA only in males, and it reduced the content of DA in female but not male POA. At PND 45, prenatal morphine increased DA in the hypothalamus of females and decreased it in males. In the cerebellum of 16-day-old morphine-exposed animals, DA levels were increased only in males; at PND 45, the levels of DA were still increased in males but had not changed in females. In the striatum, the DA content was reduced only in males at PND 16. Thus, prenatal morphine alters the development of both NE and DA neurotransmitter systems in the hypothalamus, POA, striatum, and cerebellum in a sexually dimorphic manner.

Morphine inhibits Purkinje cell survival and dendritic differentiation in organotypic cultures of the mouse cerebellum

The effects of morphine on the morphogenesis and survival of calbindin-D28k-immunoreactive Purkinje cells were studied in organotypic explant cultures isolated from 1- or 7-day-old mouse cerebella. To reduce experimental variability, bilaterally matched pairs of organotypic cultures were used to compare the effects of opiate drug treatment. One explant within each pair was untreated, while the remaining explant was continuously treated for 7 to 10 days with morphine, morphine plus naloxone, or naloxone alone. In explants derived from 1-day-old mice, morphine treatment significantly reduced Purkinje cell dendritic length compared to symmetrically matched untreated control explants. The concentration of morphine estimated to cause a half-maximal reduction (EC50) in dendritic length was 4.9 x 10(-8) M. At higher concentrations (EC50 = 3.6 x 10(-6) M), morphine also significantly decreased the number of Purkinje cells in explants from 1-day-old mice compared to untreated explants. Electron microscopy identified increased numbers of degenerating Purkinje cells in explants derived from 1-day-old mice. This showed that high concentrations (10(-5) M) of morphine reduced Purkinje cell numbers by decreasing their rate of survival. In explants derived from 7-day-old mice, morphine (10(-5) M) neither affected Purkinje cell dendritic length nor cell numbers compared to symmetrically matched untreated (control) explants. Collectively, these findings suggest that morphine per se, through a direct action on the cerebellum, can affect Purkinje cell differentiation and survival. The results additionally suggest that there is a critical period during development when Purkinje cells are especially vulnerable to the effects of morphine.

Ontogeny and effect of activity on proenkephalin mRNA expression during development of the chick spinal cord

Numerous studies have shown in the adult nervous system that mRNA expression can be regulated by neuronal activity. To examine the effect of activity during embryogenesis, the ontogeny of proenkephalin mRNA expression and expression following activity blockade was investigated during development of chick spinal cord. A cDNA fragment (ca. 0.5 kb) coding for chick proenkephalin was cloned and sequenced. With this cDNA, a cRNA probe was made to examine proenkephalin mRNA expression in the spinal cord during embryogenesis. Proenkephalin mRNA was expressed in spinal cord in clusters of cells located in the developing dorsal horn and intermediate lamina at the earliest stages examined (stage 22; E4). Proenkephalin-positive cells in the intermediate lamina were located immediately adjacent to the ventricular zone. At stage 28 (E6) an additional cluster of proenkephalin mRNA-positive cells was seen at the lateral border of the developing intermediate lamina. At stage 33 (E7.5-5-8) the pattern of hybridization positive cells was similar to earlier stages, but individual cells could be identified. At stage 39 (E13) densely labeled cells were seen throughout the dorsal horn and intermediate laminae including the column of Terni. To determine whether neural activity affects proenkephalin mRNA expression, d-tubocurarine (an inhibitor of neural activity) was injected into developing embryos. Following administration of d-tubocurarine a dramatic decrease was seen in proenkephalin mRNA hybridization in the dorsal horn and intermediate lamina of the spinal cord. This study demonstrates in vivo that changes in the level of neural activity can alter gene expression during embryogenesis and suggests that activity is required for expression of nervous system-specific genes.

Receptor-mediated regulation of neuropeptide gene expression in astrocytes

One of the functions of glial receptors is to regulate synthesis and release of a variety of neuropeptides and growth factor peptides, which in turn act on neurons or other glia. Because of the potential importance of these interactions in injured brain, we have examined the role of two different receptors in the regulation of astrocyte neuropeptide synthesis. Stimulation of beta-adrenergic receptors on type 1 astrocytes resulted in increased mRNA and protein for the proenkephalin (PE) and somatostatin genes. This receptor also increased expression of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). The potential role of opiate receptors was examined in several ways. Treatment of newborn rats for 7 days with the opiate antagonist naltrexone, prior to preparation of astrocytes, had no effect on PE mRNA or met-enkephalin content but resulted in a significant increase in NGF content. However, treatment of astrocytes in culture with met-enkephalin, morphine, or naltrexone had no effect on any of these parameters. No opiate binding could be detected, using either etorphine or bremazocine, to membranes of astrocytes prepared from cortex, cerebellum, striatum, or hippocampus of 1-day, 7-day, or 14-day postnatal rats. Thus we conclude that type 1 astrocytes do not express opiate receptors and that the in vivo effects of naltrexone are mediated indirectly via some other cell type/receptor.

Evidence for dopamine D2 receptor mRNA expression by striatal astrocytes in culture: in situ hybridization and polymerase chain reaction studies

The expression of dopamine D2 receptor mRNA in cultured rat striatal and cerebellar astrocytes was examined by in situ hybridization (ISH) and polymerase chain reaction (PCR). Cells double-labelled for glial fibrillary acidic protein (GFAP) immuno-histochemistry and dopamine D2 receptor mRNA (ISH) provide evidence that striatal but not cerebellar astrocytes express the dopamine D2 gene in vitro. These results were confirmed by polymerase chain reaction studies. As judged by GFAP immunostaining and morphology of the cells, this gene is almost exclusively expressed by astrocytes type 1. The expression of dopamine D2 receptor mRNA by striatal astrocytes in vitro, as found in this study, brings thus evidences for the existence of dopamine D2 receptors in such glial cells. This had been previously suggested from ligand binding studies but the typical dopaminergic nature of the binding to striatal astrocytes was left questionable. Our results with molecular biological techniques thus suggest that striatal dopamine might modulate the functions of striatal astrocytes.

Preproenkephalin mRNA expression in the developing and adult rat brain

[Met5]-Enkephalin is derived from the protein precursor, proenkephalin A, which in turn is encoded by the preproenkephalin (PPE) gene. [Met5]-Enkephalin is not only a putative neuromodulatory substance, but also serves as a growth factor (= opioid growth factor, OGF). OGF exerts an inhibitory influence on the developing nervous system and is especially targeted to cell proliferative and differentiative events. This study examined the relationship of PPE mRNA expression to late prenatal and postnatal rat brain development. Northern blot analysis of the whole brain and cerebellum showed that message is present in the fetal nervous system on prenatal day 15 (the earliest timepoint examined), is expressed at relatively similar levels within each tissue during the first 2 postnatal weeks, and reaches adult levels by the beginning of the 3rd postnatal week. In situ hybridization methodology revealed that PPE mRNA was prominent in areas associated with cell generation. Message was found in sites of primary (i.e., ventricular region) and secondary (e.g., external germinal layer of the cerebellum) cellular replication, as well as in discrete foci of cell proliferation (e.g., medullary layer of the cerebellum). PPE mRNA was also present for varying periods of time in postmitotic cells. During development, a number of patterns (decrease, increase, and no perceptible change) of PPE mRNA could be detected in relationship to the fetal/neonatal period. Given the strong evidence (e.g., regulation of cell proliferation and differentiation, temporal and spatial patterns of peptide and zeta opioid receptor) that enkephalin immunoreactivity is associated with proliferating and differentiating neurons and glia, these results suggest that the source of [Met5]-enkephalin is both autocrine and paracrine in nature.

Opiates selectively increase intracellular calcium in developing type-1 astrocytes: role of calcium in morphine-induced morphologic differentiation

Endogenous opioids and opiate drugs inhibit nervous system maturation, in part, by affecting the growth of astrocytes. Opiates inhibit astrocyte proliferation and cause premature differentiation. The emerging importance of Ca2+ in astrocyte function prompted us to explore whether opiates might affect astrocyte development by altering Ca2+ homeostasis. Astrocyte-enriched cultures were derived from newborn ICR mouse cerebra. Quantitative fluorescent measurements of intracellular free Ca2+ ([Ca2+]i) using Fura-2 as well as fluo-3 and computer-aided image analysis showed that 1 microM morphine significantly increased [Ca2+]i in flat, polyhedral, glial fibrillary acidic protein (GFAP) immunoreactive astrocytes at 2 and 6 min, and at 72 h. Co-administration of 3 microM naloxone blocked morphine-dependent increases in [Ca2+]i. Treatment with 1 microM concentrations of the kappa-opioid receptor agonist, U69,593, but not equimolar amounts of mu ([D-Ala2,MePhe4,Gly(ol)5]enkephalin)- or delta ([D-Pen2,D-Pen5]enkephalin)-opioid receptor agonists, significantly increased [Ca2+]i in astrocytes. To assess the role of Ca2+ in morphine-induced astrocyte differentiation, untreated and 1 microM morphine-treated astrocyte cultures were incubated for 5 days in < 0.01, 0.3, 1.0, or 3.0 mM extracellular Ca2+ ([Ca2+]o), or incubated with 1.0 mM [Ca2+]o in the presence of 1 microM of the Ca2+ ionophore, A23187. The areas of single astrocytes were measured and there was a positive correlation between astrocyte area and [Ca2+]o. Morphine had an additive effect on area and form factor measures when [Ca2+]o was 1.0 mM. High [Ca2+]o (3.0 mM) alone mimicked the action of morphine. Morphine alone had no effect on astrocyte area in the presence of 3.0 mM Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of proenkephalin mRNA and enkephalin peptide expression in the cerebellar cortex of the rat: spatial and temporal patterns of expression follow maturational gradients in the external granular layer and in Purkinje cells

Proenkephalin mRNA and peptide products were examined in developing cells of the postnatal rat cerebellar cortex using in situ hybridization and immunocytochemistry. On day 7, proenkephalin mRNA was first detected as discrete cellular labeling in Golgi cells and as a diffuse hybridization signal over the Purkinje cell layer. On day 14, proenkephalin mRNA and peptide products primarily appeared in distinct subpopulations of Purkinje cells present in the posterior and lateral cerebellum. Similarly, in the external granular layer (EGL), enkephalin immunoreactivity was present only in the posterior and lateral portions of the cerebellum on day 14. However, proenkephalin mRNA was not detected in enkephalin-immunoreactive EGL cells. On day 21, the subset of Purkinje cells that expressed proenkephalin mRNA and peptides were distributed more uniformly throughout the cerebellum. On day 28, a few enkephalin-immunoreactive Purkinje cells were uniformly present throughout the cerebellum, but proenkephalin mRNA was not detected in most of these cells. The spatial gradients in proenkephalin mRNA expression evident in the Purkinje cells of younger rats were no longer present in 28-day-old rats. These findings are important, because endogenous opioids such as enkephalin have been previously shown to inhibit the growth of Purkinje cell dendrites and dendritic spines, and inhibit the rate of mitosis in EGL neuroblasts. Cells do not develop at uniform rates within the cerebellum. There are regional differences in the timing of the formation of the EGL, and in the morphogenesis of Purkinje cells. In conjunction with previous work, the present findings suggest that during development, the pattern of enkephalin immunoreactivity in Purkinje and EGL cells closely follows the spatial and temporal gradients of maturation in both these cell types. The emergence and disappearance of enkephalin immunoreactivity in Purkinje and EGL cells is spatially and temporally related, and coincides with proenkephalin mRNA expression in Purkinje cells. Thus, the transient and coordinated appearance of enkephalin in cerebellar Purkinje and EGL cells may contribute to regional differences in the rate of cerebellar maturation, and may help synchronize the developmental interactions between these two cell types.

Immunocytochemical visualization of kappa-opioid receptors on chick embryonic neurons differentiating in vitro

The present paper is the first immunocytochemical demonstration of kappa-opioid receptors in neurons isolated from seven-day-old chick embryonic forebrains and cultivated for one to seven days. The monoclonal antibody KA8 (IgG1-k) [Maderspach et al. (1991) J. Neurochem. 56, 1897-1904] was raised against the frog brain kappa-opioid receptor as an antigen and recognizes an epitope in or near the ligand binding site. The KA8 immunostaining of the neurons displayed individual variations and changed with the in vitro differentiation. Receptors often appeared at the pole of the primary outgrowing process, later on in the whole soma and finally on the branched processes. Specific radioligand binding and KA8 immunocytochemistry both presented an increase in the receptor concentration with development. The equilibrium binding values that were measured at 1 nM [3H]naloxone concentration were 2.9 and 6.1 fmol/10(6) cells on the first and sixth cultivation days, respectively. Neurons were treated with 10(-7) M bremazocine or dynorphine (agonists with relative specificity to kappa-opioid receptors) on the second and third cultivation days. The agonist promoted the morphological differentiation which was already visible within 24 h. It also promoted the expression of the 200,000 mol. wt neurofilament protein, this became pronounced after two to three days. The changes provoked by the agonist were reduced by the opioid antagonist norbinaltorphimine (10(-7) M) or naloxone (10(-5) M) indicating that the effect was receptor-mediated. The hypothesis that kappa-opioid agonists through their receptors may function as regulatory signals in the early neuronal differentiation is discussed.