Identification of an opioid peptide secreted by rat embryonic mixed brain cells as a promoter of macrophage migration

Model infant biscuits release the opioid-acting peptides milk β-casomorphins and gluten exorphins after in vitro gastrointestinal digestion

Infant biscuits (IBs) are commonly used during the complementary feeding of infants from the 6th month of life. They contain wheat flour and dairy ingredients, which can release the opioid-acting peptides β-casomorphins (BCMs) and gluten exorphins (GEs) after gastrointestinal digestion. In the present study, five model IBs were prepared with or without gluten and different powdered milk derivatives in the formulations. IBs were digested simulating an in vitro static gastrointestinal digestion for infants aged 6-12 months. BCMs and GEs were identified and quantified by UPLC/HR-MS. The amounts of BCM7 and the GE A5 were related to the β-CN and gluten content of the formulations. To date, levels of BCMs and GEs in digests of IBs have not been reported in literature. This work represents an in vitro investigation regarding the release of opioid-acting peptides in IBs. It could add additional knowledge on complementary foods for infant health.

Food-Derived Opioid Peptides in Human Health: A Review

World Health Organization data suggest that stress, depression, and anxiety have a noticeable prevalence and are becoming some of the most common causes of disability in the Western world. Stress-related disorders are considered to be a challenge for the healthcare system with their great economic and social impact. The knowledge on these conditions is not very clear among many people, as a high proportion of patients do not respond to the currently available medications for targeting the monoaminergic system. In addition, the use of clinical drugs is also associated with various side effects such as vomiting, dizziness, sedation, nausea, constipation, and many more, which prevents their effective use. Therefore, opioid peptides derived from food sources are becoming one of the safe and natural alternatives because of their production from natural sources such as animals and plant proteins. The requirement for screening and considering dietary proteins as a source of bioactive peptides is highlighted to understand their potential roles in stress-related disorders as a part of a diet or as a drug complementing therapeutic prescription. In this review, we discussed current knowledge on opioid endogenous and exogenous peptides concentrating on their production, purification, and related studies. To fully understand their potential in stress-related conditions, either as a drug or as a therapeutic part of a diet prescription, the need to screen more dietary proteins as a source of novel opioid peptides is emphasized.

Identification and Detection of Bioactive Peptides in Milk and Dairy Products: Remarks about Agro-Foods

Food-based components represent major sources of functional bioactive compounds. Milk is a rich source of multiple bioactive peptides that not only help to fulfill consumers 'nutritional requirements but also play a significant role in preventing several health disorders. Understanding the chemical composition of milk and its products is critical for producing consistent and high-quality dairy products and functional dairy ingredients. Over the last two decades, peptides have gained significant attention by scientific evidence for its beneficial health impacts besides their established nutrient value. Increasing awareness of essential milk proteins has facilitated the development of novel milk protein products that are progressively required for nutritional benefits. The need to better understand the beneficial effects of milk-protein derived peptides has, therefore, led to the development of analytical approaches for the isolation, separation and identification of bioactive peptides in complex dairy products. Continuous emphasis is on the biological function and nutritional characteristics of milk constituents using several powerful techniques, namely omics, model cell lines, gut microbiome analysis and imaging techniques. This review briefly describes the state-of-the-art approach of peptidomics and lipidomics profiling approaches for the identification and detection of milk-derived bioactive peptides while taking into account recent progress in their analysis and emphasizing the difficulty of analysis of these functional and endogenous peptides.

Astroglial Mechanisms of Ketamine Action Include Reduced Mobility of Kir4.1-Carrying Vesicles

The finding that ketamine, an anaesthetic, can elicit a rapid antidepressant effect at low doses that lasts for weeks in patients with depression is arguably a major achievement in psychiatry in the last decades. However, the mechanisms of action are unclear. The glutamatergic hypothesis of ketamine action posits that ketamine is a N-methyl-D-aspartate receptor (NMDAR) antagonist modulating downstream cytoplasmic events in neurons. In addition to targeting NMDARs in synaptic transmission, ketamine may modulate the function of astroglia, key homeostasis-providing cells in the central nervous system, also playing a role in many neurologic diseases including depression, which affects to 20% of the population globally. We first review studies on astroglia revealing that (sub)anaesthetic doses of ketamine attenuate stimulus-evoked calcium signalling, a process of astroglial cytoplasmic excitability, regulating the exocytotic release of gliosignalling molecules. Then we address how ketamine alters the fusion pore activity of secretory vesicles, and how ketamine affects extracellular glutamate and K⁺ homeostasis, both considered pivotal in depression. Finally, we also provide evidence indicating reduced cytoplasmic mobility of astroglial vesicles carrying the inward rectifying potassium channel (Kir4.1), which may regulate the density of Kir4.1 at the plasma membrane. These results indicate that the astroglial capacity to control extracellular K⁺ concentration may be altered by ketamine and thus indirectly affect the action potential firing of neurons, as is the case in lateral habenula in a rat disease model of depression. Hence, ketamine-altered functions of astroglia extend beyond neuronal NMDAR antagonism and provide a basis for its antidepressant action through glia.

Microglia Express Mu Opioid Receptor: Insights From Transcriptomics and Fluorescent Reporter Mice

Background: Microglia activation contributes to chronic pain and to the adverse effects of opiate use such as analgesic tolerance and opioid-induced hyperalgesia. Both mu opioid receptor (MOR) encoded by Oprm1/OPRM1 gene and toll like receptor 4 (TLR4) have been reported to mediate these morphine effects and a current question is whether microglia express the Oprm1 transcript and MOR protein. The aim of this study was to characterize Oprm1-MOR expression in naive murine and human microglia, combining transcriptomics datasets previously published by other groups with our own imaging study using the Cx3cr1-eGFP-MOR-mCherry reporter mouse line. Methods: We analyzed microglial Oprm1/OPRM1 expression obtained from transcriptomics datasets, focusing on ex vivo studies from adult wild-type animals and adult post-mortem human cerebral cortex. Oprm1, as well as co-regulated gene sets were examined. The expression of MOR in microglia was also investigated using our novel fluorescent Cx3cr1-eGFP-MOR-mcherry reporter mouse line. We determined whether CX3cR1-eGFP positive microglial cells expressed MOR-mCherry protein by imaging various brain areas including the Frontal Cortex, Nucleus Accumbens, Ventral Tegmental Area, Central Amygdala, and Periaqueductal Gray matter, as well as spinal cord. Results: Oprm1 expression was found in all 12 microglia datasets from mouse whole brain, in 7 out of 8 from cerebral cortex, 3 out of 4 from hippocampus, 1 out of 1 from striatum, and 4 out of 5 from mouse or rat spinal cord. OPRM1 was expressed in 16 out of 17 microglia transcriptomes from human cerebral cortex. In Cx3cr1-eGFP-MOR-mCherry mice, the percentage of MOR-positive microglial cells ranged between 35.4 and 51.6% in the different brain areas, and between 36.8 and 42.4% in the spinal cord. Conclusion: The comparative analysis of the microglia transcriptomes indicates that Oprm1/OPRM1 transcripts are expressed in microglia. The investigation of Cx3cr1-eGFP-MOR-mCherry mice also shows microglial expression of MOR proteinin the brain and spine. These results corroborate functional studies showing the actions of MOR agonists on microglia and suppression of these effects by MOR-selective antagonists or MOR knockdown.

The protective effect of hydromorphone to ischemia in rat glial cells

BACKGROUND

Ischemic insults during operation can cause ischemic-reperfusion injuries in brain as well as memory impairments. Total intravenous anesthesia (TIVA) is the preferred anesthetic method in brain surgery, as it utilizes motor evoked potential monitoring. And the use of opioids is common in TIVA. However there are few studies about ischemic protective effect of opioids to glial cells.

METHODS

We used mixed cultures of rat glial cells, which were harvested from the brain of 1-day old rat. We divided the experimental groups according to their hydromorphone conditioning period: (a) pre-culture, (b) per-culture, or (c) pre- and per-culture. We measured the levels of the reactive oxygen species (ROS) induced by tert-butyl hydroperoxide (TBH) using flow cytometry. The ROS levels in the glial cells were also measured after the administration of 100 nM hydromorphone and selective opioid receptor antagonists.

RESULTS

The ROS levels were reduced in the hydromorphone-treated group, as compared to the control group (only TBH treated). There were no differences between pre-conditioned and per-conditioned groups. However, the ROS levels were more reduced in pre- and per-conditioned group compared to pre-conditioned or per-conditioned only groups. Furthermore, selective antagonists for the delta, kappa, or mu opioid receptor partially negated the hydromorphone effect.

CONCLUSION

This study demonstrated that hydromorphone can have additional protective effects on oxidative stress when pre- and per-conditioning is combined. Furthermore we proved that μ, δ, κ opioid receptors participate in protective mechanism of hydromorphone to glial cells.

Milk derived bioactive peptides and their impact on human health - A review

Milk-derived bioactive peptides have been identified as potential ingredients of health-promoting functional foods. These bioactive peptides are targeted at diet-related chronic diseases especially the non-communicable diseases viz., obesity, cardiovascular diseases and diabetes. Peptides derived from the milk of cow, goat, sheep, buffalo and camel exert multifunctional properties, including anti-microbial, immune modulatory, anti-oxidant, inhibitory effect on enzymes, anti-thrombotic, and antagonistic activities against various toxic agents. Majority of those regulate immunological, gastrointestinal, hormonal and neurological responses, thereby playing a vital role in the prevention of cancer, osteoporosis, hypertension and other disorders as discussed in this review. For the commercial production of such novel bioactive peptides large scale technologies based on membrane separation and ion exchange chromatography methods have been developed. Separation and identification of those peptides and their pharmacodynamic parameters are necessary to transfer their potent functional properties into food applications. The present review summarizes the preliminary classes of bioactive milk-derived peptides along with their physiological functions, general characteristics and potential applications in health-care.

Opioid-induced central immune signaling: implications for opioid analgesia

Despite being the mainstay of pain management, opioids are limited in their clinical utility by adverse effects, such as tolerance and paradoxical hyperalgesia. Research of the past 15 years has extended beyond neurons, to implicate central nervous system immune signaling in these adverse effects. This article will provide an overview of these central immune mechanisms in opioid tolerance and paradoxical hyperalgesia, including those mediated by Toll-like receptor 4, purinergic, ceramide, and chemokine signaling. Challenges for the future, as well as new lines of investigation will be highlighted.

Inhibition of microglial P2X4 receptors attenuates morphine tolerance, Iba1, GFAP and mu opioid receptor protein expression while enhancing perivascular microglial ED2

Anti-nociceptive tolerance to opioids is a well-described phenomenon, which severely limits the clinical efficacy of opioids for the treatment of chronic pain syndromes. The mechanisms that drive anti-nociceptive tolerance, however, are less well understood. We have previously shown that glia have a central role in the development of morphine tolerance and that administration of a glial modulating agent attenuated tolerance formation. Recently, we have demonstrated that morphine enhances microglial Iba1 expression and P2X4 receptor-mediated microglial migration via direct mu opioid receptor signaling in in vitro microglial cultures. We hypothesize that P2X4 receptors drive morphine tolerance and modulate morphine-induced spinal glial reactivity. Additionally, we hypothesize that perivascular microglia play a role in morphine tolerance and that P2X4 receptor expression regulates perivascular microglia ED2 expression. To test these hypotheses, rats were implanted with osmotic minipumps releasing morphine or saline subcutaneously for seven days. Beginning three days prior to morphine treatment, P2X4 receptor antisense oligonucleotide (asODN) was injected intrathecally daily, to selectively inhibit P2X4 receptor expression. P2X4 receptor asODN treatment inhibited morphine-induced P2X4 receptor expression and blocked anti-nociceptive tolerance to systemically administered morphine. P2X4 receptor asODN treatment also attenuated the morphine-dependent increase of spinal ionized calcium binding protein (Iba1), glial fibrillary acidic protein (GFAP) and mu opioid receptor protein expression. Chronic morphine also decreased perivascular microglial ED2 expression, which was reversed by P2X4 receptor asODN. Together, these data suggest that the modulation of P2X4 receptor expression on microglia and perivascular microglia may prove an attractive target for adjuvant therapy to attenuate opioid-induced anti-nociceptive tolerance.

Spinal microglial motility is independent of neuronal activity and plasticity in adult mice

Microglia are the resident macrophages in the central nervous system. In the spinal cord dorsal horn, microglia stay in resting condition during physiological sensory processing, and are activated under pathological conditions such as peripheral nerve injury. In cases such as this, the nearby resting microglia increase their motility and accumulate at the site of injury. However, direct evidence to support that nerve activity can enhance the motility of microglia has not yet to be reported. In this study we investigated whether the activation of spinal microglia under in vivo nerve injury may be mimicked by neuronal activity in the spinal cord slice preparation. We found that local application of spinal excitatory neurotransmitters, such as glutamate and substance P did not cause any change in the motility of microglial cells in the spinal cord dorsal horn. The motility of microglial cells is unlikely modulated by other transmitters, neuromodulators and chemokines, because similar applications such as GABA, serotonin, noradrenaline, carbachol, fractalkine or interleukin did not produce any obvious effect. Furthermore, low or high frequency stimulation of spinal dorsal root fibers at noxious intensities failed to cause any enhanced extension or retraction of the microglia processes. By contrast, focal application of ATP triggered rapid and robust activation of microglial cells in the spinal dorsal horn. Our results provide the first evidence that the activation of microglia in the spinal cord after nerve injury is unlikely due solely to neuronal activity, non-neuronal factors are likely responsible for the activation of nerve injury-related microglial cells in the spinal dorsal horn.

Cannabinoids prevent the opposite regulation of astroglial connexin43 hemichannels and gap junction channels induced by pro-inflammatory treatments

Brain injuries as well as neurodegenerative diseases, are associated with neuro-inflammation characterized by astroglial and microglial activation and/or proliferation. Recently, we reported that lipopolysaccharide (LPS)-activation of microglia inhibits junctional channels and promotes hemichannels, two connexin43 functions in astrocytes. This opposite regulation is mediated by two pro-inflammatory cytokines, interleukin-1 beta and tumor necrosis factor-alpha, released from activated microglia. Because cannabinoids (CBs) have anti-inflammatory properties and their receptors are expressed by glial cells, we investigated on primary cortical cultures the effects of CB agonists, methanandamide and synthetic CBs on (i) cytokines released from LPS-activated microglia and (ii) connexin43 functions in astrocytes subjected to pro-inflammatory treatments. We observed that CBs inhibited the LPS-induced release of interleukin-1 beta and tumor necrosis factor-alpha from microglia. Moreover, the connexin43 dual regulation evoked by the pro-inflammatory treatments, was prevented by CB treatments. Pharmacological characterizations of CB actions on astrocytic connexin43 channels revealed that these effects were mainly mediated through CB1 receptors activation, although non-CB1/CB2 receptors seemed to mediate the action of the methanandamide. Altogether these data demonstrate that in inflammatory situations CBs exert, through the activation of different sub-types of glial CB receptors, a regulation on two functions of connexin43 channels in astrocytes known to be involved in neuron survival.

Morphine enhances microglial migration through modulation of P2X4 receptor signaling

Opioids, although fundamental to the treatment of pain, are limited in efficacy by side effects including tolerance and hyperalgesia. Using an in vitro culture system, we report that morphine increased microglial migration via a novel interaction between mu-opioid and P2X(4) receptors, which is dependent upon PI3K/Akt pathway activation. Morphine at 100 nm enhanced migration of primary microglial cells toward adenosine diphosphate by 257, 247, 301, 394, and 345% following 2, 6, 12, 24, and 48 h of stimulation, respectively. This opioid-dependent migration effect was inhibited by naloxone and confirmed to be mu-opioid receptor-dependent through the use of selective agonists and antagonists. PPADS [pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid)], a P2X(1-3,5-7) antagonist, had no effect on microglial migration; however, TNP-ATP [2',3'-O-(2,4,6-trinitrophenyl)-ATP], a P2X(1-7) antagonist, inhibited morphine-induced migration, suggesting a P2X(4) receptor-mediated effect. The PI3K inhibitors wortmannin and LY294002 decreased morphine-induced microglial migration. Iba1 protein, a microglial marker, and P2X(4) receptor expression were significantly increased after 6, 12, 24, and 48 h of morphine stimulation. Together, these results provide evidence for two phases of morphine effects on microglia. The initial phase takes place in minutes, involves PI3K/Akt pathway activation and leads to acutely enhanced migration. The longer-term phase occurs on the order of hours and involves increased expression of Iba1 and P2X(4) receptor protein, which imparts a promigratory phenotype and is correlated with even greater migration. These data provide the first necessary step in supporting microglial migration as an attractive target for the prevention or attenuation of morphine-induced side effects including tolerance and hyperalgesia.

Cx43 hemichannels and gap junction channels in astrocytes are regulated oppositely by proinflammatory cytokines released from activated microglia

Astrocytes have a role in maintaining normal neuronal functions, some of which depend on connexins, protein subunits of gap junction channels and hemichannels. Under inflammatory conditions, microglia release cytokines, including interleukin-1beta and tumor necrosis factor-alpha, that reduce intercellular communication via gap junctions. Now, we demonstrate that either conditioned medium harvested from activated microglia or a mixture of these two cytokines enhances the cellular exchange with the extracellular milieu via Cx43 hemichannels. These changes in membrane permeability were not detected in astrocytes cultured from Cx43 knock-out mice and were abrogated by connexin hemichannel blockers, including La3+, mimetic peptides, and niflumic acid. Both the reduction in gap junctional communication and the increase in membrane permeability were mediated by a p38 mitogen-activated protein kinase-dependent pathway. However, the increase in membrane permeability, but not the gap junction inhibition, was rapidly reversed by the sulfhydryl reducing agent dithiothreitol, indicating that final regulatory mechanisms are different. Treatment with proinflammatory cytokines reduced the total and cell surface Cx43 levels, suggesting that the increase in membrane permeability was attributable to an increase in hemichannels activity. Indeed, unitary events of approximately 220 pS corresponding to Cx43 hemichannels were much more frequent in astrocytes treated with microglia conditioned medium than under control conditions. Finally, the effect of cytokines enhanced the uptake and reduced the intercellular diffusion of glucose, which might explain changes in the metabolic status of astrocytes under inflammatory conditions. Accordingly, this opposite regulation may affect glucose trafficking and certainly will modify the metabolic status of astrocytes involved in brain inflammation.

Proinflammatory cytokines released from microglia inhibit gap junctions in astrocytes: potentiation by beta-amyloid

Brain inflammation is characterized by a reactive gliosis involving the activation of astrocytes and microglia. This process, common to many brain injuries and diseases, underlies important phenotypic changes in these two glial cell types. One characteristic feature of astrocytes is their high level of intercellular communication mediated by gap junctions. Previously, we have reported that astrocyte gap junctional communication (AGJC) and the expression of connexin 43 (Cx43), the main constitutive protein of gap junctions, are inhibited in microglia (MG)-astrocyte cocultures. Here, we report that bacterial lipopolysaccharide activation of microglia increases their inhibitory effect on Cx43 expression and AGJC. This inhibition is mimicked by treating astrocyte cultures with conditioned medium harvested from activated microglia. Interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) were identified as being the main factors responsible for this conditioned medium-mediated activity. Interestingly, an inflammatory response characterized by MG activation and reactive astrocytes occurs in Alzheimer's disease, at sites of beta-amyloid (Abeta) deposits. We found that this peptide potentiates the inhibitory effect of a conditioned medium diluted at a concentration that is not effective per se. This potentiation is prevented by treating astrocytes with specific blockers of IL-1beta and TNF-alpha activities. Thus, the suppression of communication between astrocytes, induced by activated MG could contribute to the proposed role of reactive gliosis in this neurodegenerative disease.

κ-opioids induce a reversible inhibition of CFU-GM from CD133+ cord blood cells

BACKGROUND

Opioid agonists have been shown to exert an inhibitory action on a number of malignant and non-malignant cell types. However, there are no reports dealing with their effect on hemopoietic progenitors. Based upon our previous experience of opioid agonists we examined whether opioids could interfere with the growth of CFU-GM from CD133(+) cord blood cells.

METHODS

Cord blood samples were subjected to CD133(+) column selection, with subsequent exposure to opioid agonists and antagonists or both, in semi-solid cultures for CFU-GM growth. Colonies of day 7 of culture were replated in fresh medium in the absence of opioids. The colonies were evaluated at 7 and 14 days of culture. RT-PCR was performed for the detection of opioid and somatostatin receptors. Apoptosis tests and immunophenotypic evaluations were employed in liquid cultures in conditions identical to those of the semi-solid ones.

RESULTS AND DISCUSSION

Our results suggest that opioids can induce a significant inhibition of CFU-GM growth, which is reversible and not mediated through opioid or somatostatin receptors, while apoptosis is not implicated. Whether this finding could be used for clinical intervention remains to be examined.

A pro- and an anti-inflammatory cytokine are synthesised in distinct brain macrophage cells during innate activation

Brain macrophages are known to exert dual and opposing functions on neuronal survival, which can be either beneficial or detrimental. The rationale of our study is that this duality could arise from an exclusive secretion of either pro- or anti-inflammatory cytokine by distinct cell subsets, cytokines that could respectively mediate neurotoxic or neurotrophic effects. Innate immune response was induced in macrophage cultures prepared from embryonic-day-16 to postnatal-day-8 mouse brains. By immunofluorescent detection of intracellular cytokines, we have assessed the occurrence of TNFalpha or IL10 synthesis at single cell level and observed distinct secretory patterns that include cells producing exclusively TNFalpha or IL10, cells producing both cytokines and non-producer cells. These secretory patterns are differentially regulated by MAP-kinase inhibitors. Altogether, these results demonstrate that synthesis of either a pro- or an anti-inflammatory cytokine can segregate distinct brain macrophages and suggests a functional cell-subset-specialisation.

Morphine-induced chemotaxis and brain-derived neurotrophic factor expression in microglia

The addition of morphine at 1 mum induced morphological changes of cultured microglia such that they changed from having globular or bipolar rod-like shapes to being flat and lamellipodial, with membrane ruffling at the edge, which was stained with phalloidin. The membrane ruffling was clearly colocalized with Rac. Morphine also induced chemotaxis in Boyden chamber analysis at concentrations of 1 mum or more in microglia and the microglial cell line EOC 2. All of these changes were abolishable by naloxone, antisense oligodeoxynucleotide for mu-opioid receptor (MOR), pertussis toxin (PTx), and wortmannin, but not genistein or 1,10-phenanthroline. The addition of morphine to microglia stimulated the gene expression of brain-derived neurotrophic factor (BDNF) as early as the 1 hr point, and this lasted for >12 hr. Morphine induced BDNF gene expression and ERK1/2 (extracellular signal-regulated kinase 1/2) phosphorylation, and these were abolishable by naloxone, wortmannin, PD98059, genistein, and 1,10-phenanthroline. The addition of conditioned medium derived from the culture of morphine-treated microglia also increased the phosphorylation of ERK1/2. All of these findings suggest that morphine induces significant changes in both morphology and gene expression at relatively high concentrations, but the underlying signaling pathways downstream of MOR and G(i/o) appear to be different from each other. Phosphoinositide 3-kinase gamma activation and Rac activation are involved in chemotaxis, whereas indirect pathways through ERK1/2 phosphorylation induced by unknown growth factors generated through an MOR-mediated metalloprotease activation are linked to the enhanced BDNF gene expression.

Albumin stimulates monocyte chemotactic protein-1 expression in rat embryonic mixed brain cells

Albumin, a blood protein absent from the adult brain in physiological situations, can be brought into contact with brain cells during development or, in adult, following breakdown of the blood-brain barrier occurring as a result of local inflammation. In the present study, we show that ovalbumin and albumin induce the release of monocyte chemotactic protein 1 (MCP-1/CCL2) from rat embryonic mixed brain cells. A short-term exposure to ovalbumin during the cell dissociation procedure is sufficient to generate MCP-1 mRNA. A comparable effect is observed when the cells are incubated for 4 hr with ovalbumin or rat albumin, while MCP-1 messengers are barely detectable following bovine albumin exposure. The amount of MCP-1 protein measured in 4 hr-supernatants of albumin-treated cells followed the same albumin-inducing pattern as that of MCP-1 mRNA, while all albumins tested induced MCP-1 protein after a 17 hr-incubation period. The albumin-induced MCP-1 production is significantly inhibited in calphostin C-treated cells, suggesting the implication of a protein kinase C-dependent signaling pathway. This MCP-1-inducing activity is maintained after a lipid extraction procedure but abolished by proteinase K or trypsin treatments of albumin. The MCP-1 secretion following albumin contact with nervous cells could thus interfere, by chemotactic gradient formation, with the brain infiltration program of blood-derived cells during development or brain injury.

Hydrogen peroxide increases gap junctional communication and induces astrocyte toxicity: regulation by brain macrophages

Cultured astrocytes are highly coupled by gap junction channels mainly constituted by connexin 43. We have previously shown that gap junctional communication (GJC) represents a functional property of astrocytes that is a target for their interaction with other brain cell types, including neurons and brain macrophages. In pathological situations, neurons as well as brain macrophages produce superoxide ions leading to the formation of hydrogen peroxide (H2O2) that can be cytotoxic. We report here that 10-min exposure to 100 microM H2O2 increases GJC in astrocytes. Moreover, 30-min exposure to 100 microM H2O2 induces, 24 h later, an astrocyte cell death by both apoptosis and necrosis. This H2O2-induced astrocyte cell death is not affected when gap junctions are inhibited by several uncoupling agents, including 18alpha-glycyrrhetinic acid, halothane, heptanol, and endothelin-1, indicating that the proportion of cell death is not related to the level of GJC. The effect of H2O2 on gap junction channels does not result from the production of free radicals but is rather linked to modification of the redox equilibrium in astrocytes. Indeed, an oxidative agent reproduces the H2O2-evoked response while reducing agents prevent the effect of H2O2. Finally, when astrocytes are cocultured with brain macrophages, the effects of H2O2 on both GJC and toxicity are not observed, revealing a new protective role of brain macrophages during oxidative stress.

Heparan Sulfate Proteoglycans Are Involved in Opiate Receptor-Mediated Cell Migration

Opioid receptors are expressed in cells of the immune system, and potent immunomodulatory effects of their natural and synthetic ligands have been reported. In some studies, the opiate receptor antagonist naloxone itself displayed immunomodulatory actions. We investigated effects of naloxone on leukocyte chemotaxis. Cell migration was tested in micropore filter assays using modified Boyden chambers, and receptor expression was investigated using radiolabel binding assays. Naloxone induced peripheral blood nonadherent mononuclear cell and neutrophil chemotaxis at nanomolar concentrations and deactivated their migration toward beta-endorphin, angiotensin II, somatostatin, or interleukin-8 but not toward RANTES, vasoactive intestinal peptide, or substance P. Ligand binding studies showed no alteration in the binding of interleukin-8 to neutrophils by naloxone. Cleavage of heparan sulfate from proteoglycans on the cells' surface completely inhibited chemotactic and deactivating properties of naloxone but not other attractants. Chemotactic properties were abolished by pretreating cells with heparinase, chondroitinase, sodium chlorate, and anti-syndecan-4 antibodies, indicating the involvement of syndecan-4. The extent of migration toward naloxone was diminished by pretreatment with dimethylsphingosine, a specific sphingosine kinase inhibitor. As syndecan-4 signaling in leukocyte chemotaxis involves activation of sphingosine kinase, results indicate that naloxone interacts with syndecan-4 function in cell migration and suggest a role for heparan sulfate proteoglycans as coreceptors to members of the delta-opiate receptor family.

Regulating factors for microglial activation

Microglia, residential macrophages in the central nervous system, can release a variety of factors including cytokines, chemokines, etc. to regulate the communication among neuronal and other types of glial cells. Microglia play immunological roles in mechanisms underlying the phagocytosis of invading microorganisms and removal of dead or damaged cells. When microglia are hyperactivated due to a certain pathological imbalance, they may cause neuronal degeneration. Pathological activation of microglia has been reported in a wide range of conditions such as cerebral ischemia, Alzheimer's disease, prion diseases, multiple sclerosis, AIDS dementia, and others. Nearly 5000 papers on microglia can be retrieved on the Web site PubMed at present (November 2001) and half of them were published within the past 5 years. Although it is not possible to read each paper in detail, as many factors as possible affecting microglial functions in in vitro culture systems are presented in this review. The factors are separated into "activators" and "inhibitors," although it is difficult to classify many of them. An overview on these factors may help in the development of a new strategy for the treatment of various neurodegenerative diseases.

Brain macrophages inhibit gap junctional communication and downregulate connexin 43 expression in cultured astrocytes

Astrocytes are typically interconnected by gap junction channels that allow, in vitro as well as in vivo, a high degree of intercellular communication between these glial cells. Using cocultures of astrocytes and neurons, we have demonstrated that gap junctional communication (GJC) and connexin 43 (Cx43) expression, the major junctional protein in astrocytes, are controlled by neuronal activity. Moreover, neuronal death downregulates these two parameters. Because in several brain pathologies neuronal loss is associated with an increase in brain macrophage (BM) density, we have now investigated whether coculture with BM affects astrocyte gap junctions. We report here that addition of BM for 24 h decreases the expression of GJC and Cx43 in astrocytes in a density-dependent manner. In contrast, Cx43 is not detected in BM and no heterotypic coupling is observed between the two cell types. A soluble factor does not seem to be involved in these inhibitions because they are not observed either in the presence of BM conditioned media or in the absence of direct contact between the two cell types by using inserts. These observations could have pathophysiological relevance as neuronal death, microglial proliferation and astrocytic reactions occur in brain injuries and pathologies. Because astrocyte interactions with BM and dying neurons both result in the downregulation of Cx43 expression and in the inhibition of GJC, a critical consequence on astrocytic phenotype in those situations could be the inhibition of gap junctions.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.