In inflammatory reactive astrocytes co-cultured with brain endothelial cells nicotine-evoked Ca(2+) transients are attenuated due to interleukin-1beta release and rearrangement of actin filaments

Expression and function of nicotinic acetylcholine receptors in specific neuronal populations: Focus on striatal and prefrontal circuits

Nicotinic acetylcholine receptors (nAChRs) are widely expressed in the central nervous system and play an important role in the control of neural functions including neuronal activity, transmitter release and synaptic plasticity. Although the common subtypes of nAChRs are abundantly expressed throughout the brain, their expression in different brain regions and by individual neuronal types is not homogeneous or incidental. In recent years, several studies have emerged showing that particular subtypes of nAChRs are expressed by specific neuronal populations in which they have major influence on the activity of local circuits and behavior. It has been demonstrated that even nAChRs expressed by relatively rare neuronal types can induce significant changes in behavior and contribute to pathological processes. Depending on the identity and connectivity of the particular nAChRs-expressing neuronal populations, the activation of nAChRs can have distinct or even opposing effects on local neuronal signaling. In this review, we will summarize the available literature describing the expression of individual nicotinic subunits by different neuronal types in two crucial brain regions, the striatum and the prefrontal cortex. The review will also briefly discuss nicotinic expression in non-neuronal, glial cells, as they cannot be ignored as potential targets of nAChRs-modulating drugs. The final section will discuss options that could allow us to target nAChRs in a neuronal-type-specific manner, not only in the experimental field, but also eventually in clinical practice.

Astrocytes Downregulate Inflammation in Lipopolysaccharide-Induced Acute Respiratory Distress Syndrome: Applicability to COVID-19

Background: An acute respiratory distress syndrome (ARDS) is caused by the increased amounts of pro-inflammatory cytokines and neutrophil-mediated tissue injury. To date, there is no effective treatment for the ARDS available, while the need for one is growing due to the most severe complications of the current coronavirus disease-2019 (COVID-19) pandemic. The human astrocytes (AstroRx) have shown immunomodulatory properties in the central nervous system (CNS). This study aimed to evaluate the capacity of astrocytes to decrease lung inflammation and to be applied as a treatment therapy in ARDS.

Methods: First, we assessed the ability of clinical-grade AstroRx to suppress T-cell proliferation in a mixed lymphocyte reaction test. Next, we tested the therapeutical potential of AstroRx cells in a lipopolysaccharide (LPS)-based ARDS mouse model by injecting AstroRx intravenously (i.v). We determined the degree of lung injury by using a severity scoring scale of 0–2, based on the American Thoracic Society. The scoring measured the presence of neutrophils, fibrin deposits, and the thickening of alveolar walls. The state of inflammation was further assessed by quantifying the immune-cell infiltration to the bronchoalveolar lavage fluid (BALF) and by the presence of proinflammatory cytokines and chemokines in the BALF and serum.

Results: We detected that AstroRx cells were capable to suppress T-cell proliferation in vitro after exposure to the mitogen concanavalin A (ConA). In vivo, AstroRx cells were able to lower the degree of lung injury in LPS-treated animals compared with the sham injected animals (P = 0.039). In this study, 30% of AstroRx treated mice showed no lung lesions (responder mice), these mice presented a steady number of eosinophils, T cells, and neutrophils comparable with the level of naïve control mice. The inflammatory cytokines and chemokines, such as TNFα, IL1b, IL-6, and CXCL1, were also kept in check in responder AstroRx-treated mice and were not upregulated as in the sham-injected mice (P < 0.05). As a result, the LPS-treated ARDS mice had a higher survival rate when they were treated with AstroRx.

Conclusions: Our results demonstrate that the immunosuppressive activity of AstroRx cells support the application of AstroRx cells as a cell therapy treatment for ARDS. The immunoregulatory activity may also be a part of the mechanism of action of AstroRx reported in the amyotrophic lateral sclerosis (ALS) neurodegenerative disease.

Volumetric histological characterization of optic nerve degeneration using tissue clearing: literature review and practical study

Tissue clearing technologies can greatly improve the depth and accuracy with which the three-dimensional structure of tissues, especially those of the nervous system, can be visualized. A review of the present literature suggests that the growing diversity and sophistication of various approaches have contributed to the expansion of this method to a greater variety of tissue types, experimental conditions, and imaging modalities. In the proof-of-concept study presented in this paper, a simplified and modified version of the tissue clearing method CUBIC (clear, unobstructed brain imaging cocktails and computational analysis) was used in conjunction with fluorescent staining and immunohistochemistry to illustrate the three-dimensional structure and molecular characteristics of inflammatory and degenerative activity in the mouse optic nerve. Based on the studies summarized in this mini-review, and our impression from using the mCUBIC method, it appears that tissue clearing could be a viable approach revealing three-dimensional histological features of myelin-rich tissues under normal conditions and after injury.

Ibuprofen attenuates interleukin-1β-induced inflammation and actin reorganization via modulation of RhoA signaling in rabbit chondrocytes

Ibuprofen, a medication in the nonsteroidal anti-inflammatory drug class, is widely used for treating inflammatory diseases such as osteoarthritis. It has been shown in recent years that ibuprofen has a strong effect on Ras homolog gene family, member A (RhoA) inhibition in multiple cell types. Our previous finding also demonstrated that interleukin-1β (IL-1β) increases filamentous actin (F-actin) of chondrocytes via RhoA pathway. Therefore, we hypothesized that ibuprofen may suppress the IL-1β-induced F-actin upregulation in chondrocytes by inhibiting RhoA pathway. To this end, in this study, articular chondrocytes from New Zealand White rabbits were pretreated with 500 μM ibuprofen for 2 h, then with 10 ng/ml IL-1β for 24 h. Results showed that pretreatment with ibuprofen inhibited the IL-1β-induced nitric oxide (NO) and prostaglandin E2 (PGE2) production, protected the chondrocyte phenotype from IL-1β stimulation, and inhibited the IL-1β-induced actin remodeling via RhoA signaling modulation. In conclusion, ibuprofen showed not only anti-inflammatory function, but also RhoA inhibition in articular chondrocytes.

Complex Control of Striatal Neurotransmission by Nicotinic Acetylcholine Receptors via Excitatory Inputs onto Medium Spiny Neurons

The prevalence of nicotine dependence is higher than that for any other substance abuse disorder; still, the underlying mechanisms are not fully established. To this end, we studied acute effects by nicotine on neurotransmission in the dorsolateral striatum, a key brain region with respect to the formation of habits. Electrophysiological recordings in acutely isolated brain slices from rodent showed that nicotine (10 nm to 10 μm) produced an LTD of evoked field potentials. Current-clamp recordings revealed no significant effect by nicotine on membrane voltage or action potential frequency, indicating that the effect by nicotine is primarily synaptic. Nicotine did not modulate sIPSCs, or the connectivity between fast-spiking interneurons and medium spiny neurons, as assessed by whole-cell recordings combined with optogenetics. However, the frequency of sEPSCs was significantly depressed by nicotine. The effect by nicotine was mimicked by agonists targeting α7- or α4-containing nAChRs and blocked in slices pretreated with a mixture of antagonists targeting these receptor subtypes. Nicotine-induced LTD was furthermore inhibited by dopamine D2 receptor antagonist and occluded by D2 receptor agonist. In addition, modulation of cholinergic neurotransmission suppressed the responding to nicotine, which might reflect upon the postulated role for nAChRs as a presynaptic filter to differentially govern dopamine release depending on neuronal activity. Nicotine-induced suppression of excitatory inputs onto medium spiny neurons may promote nicotine-induced locomotor stimulation and putatively initiate neuroadaptations that could contribute to the transition toward compulsive drug taking.SIGNIFICANCE STATEMENT To decrease smoking, prevalence factors that may contribute to the development of nicotine addiction need to be identified. The data presented here show that nicotine suppresses striatal neurotransmission by selectively reducing the frequency of excitatory inputs to medium spiny neurons (MSNs) while rendering excitability, inhibitory neurotransmission, and fast-spiking interneuron-MSN connectivity unaltered. In addition, we show that the effect displayed by nicotine outlasts the presence of the drug, which could be fundamental for the addictive properties of nicotine. Considering the inhibitory tone displayed by MSNs on dopaminergic cell bodies and local terminals, nicotine-induced long-lasting depression of striatal output could play a role in behavioral transformations associated with nicotine use, and putatively elicit neuroadaptations underlying compulsive drug-seeking habits.

Biochemical alterations in inflammatory reactive chondrocytes: evidence for intercellular network communication

Chondrocytes are effectively involved in the pathophysiological processes of inflammation in joints. They form cellular processes in the superficial layer of the articular cartilage and form gap junction coupled syncytium to facilitate cell-to-cell communication. However, very little is known about their physiological cellular identity and communication. The aim with the present work is to evaluate the physiological behavior after stimulation with the inflammatory inducers interleukin-1β and lipopolysaccharide. The cytoskeleton integrity and intracellular Ca²⁺ release were assessed as indicators of inflammatory state. Cytoskeleton integrity was analyzed through cartilage oligomeric matrix protein and actin labeling with an Alexa 488-conjugated phalloidin probe. Ca²⁺ responses were assessed through the Ca²⁺ sensitive fluorophore Fura-2/AM. Western blot analyses of several inflammatory markers were performed. The results show reorganization of the actin filaments. Glutamate, 5-hydoxytryptamine, and ATP evoked intracellular Ca²⁺ release changed from single peaks to oscillations after inflammatory induction in the chondrocytes. The expression of toll-like receptor 4, the glutamate transporters GLAST and GLT-1, and the matrix metalloproteinase-13 increased. This work demonstrates that chondrocytes are a key part in conditions that lead to inflammation in the cartilage. The inflammatory inducers modulate the cytoskeleton, the Ca²⁺ signaling, and several inflammatory parameters. In conclusion, our data show that the cellular responses to inflammatory insults from healthy and inflammatory chondrocytes resemble those previously observed in astrocyte and cardiac fibroblasts networks.

Inflammatory activation of human cardiac fibroblasts leads to altered calcium signaling, decreased connexin 43 expression and increased glutamate secretion

Cardiac fibroblasts, which are abundant in heart tissue, are involved not only in extracellular matrix homeostasis and repair, but also in cardiac remodeling after a myocardial infarction that, in turn, can lead to loss of cardiac function and heart failure. Ca²⁺ signaling is functionally important in many cell types, but the roles of fibroblast signaling and inflammation in the pathogenesis of heart disease are unclear. Here, we tested the hypothesis that inflammatory activation affects cardiac fibroblasts, both in terms of Ca²⁺ signaling and their capacity for intercellular communication through the gap junction channel protein connexin 43 (Cx43). We examined Ca²⁺ responses induced by known modulators of cardiac function such as glutamate, ATP and 5-hydroxytryptamine (5-HT) in human cardiac fibroblasts, under normal and inflammatory conditions. We showed that activation of human cardiac fibroblasts by lipopolysaccharide (LPS) for 24 h altered Ca²⁺ signaling, increased TLR4 and decreased Cx43 expression. In the fibroblasts, LPS treatment increased glutamate-evoked and decreased 5-HT-evoked Ca²⁺ signals. LPS activation also induced increased secretion of glutamate and proinflammatory cytokines from these cells. In summary, we propose that inflammatory stimuli can affect intracellular Ca²⁺ release, Cx43 expression, glutamate release and cytokine secretion in human cardiac fibroblasts. Inflammatory conditions may, therefore, impair intercellular network communication between fibroblasts and cardiomyocytes potentially contributing to cardiac dysfunction.

Gliotransmitter Release from Astrocytes: Functional, Developmental, and Pathological Implications in the Brain

Astrocytes comprise a large population of cells in the brain and are important partners to neighboring neurons, vascular cells, and other glial cells. Astrocytes not only form a scaffold for other cells, but also extend foot processes around the capillaries to maintain the blood–brain barrier. Thus, environmental chemicals that exist in the blood stream could have potentially harmful effects on the physiological function of astrocytes. Although astrocytes are not electrically excitable, they have been shown to function as active participants in the development of neural circuits and synaptic activity. Astrocytes respond to neurotransmitters and contribute to synaptic information processing by releasing chemical transmitters called “gliotransmitters.” State-of-the-art optical imaging techniques enable us to clarify how neurotransmitters elicit the release of various gliotransmitters, including glutamate, D-serine, and ATP. Moreover, recent studies have demonstrated that the disruption of gliotransmission results in neuronal dysfunction and abnormal behaviors in animal models. In this review, we focus on the latest technical approaches to clarify the molecular mechanisms of gliotransmitter exocytosis, and discuss the possibility that exposure to environmental chemicals could alter gliotransmission and cause neurodevelopmental disorders.

Glial dysfunction and persistent neuropathic postsurgical pain

BACKGROUND

Acute pain in response to injury is an important mechanism that serves to protect living beings from harm. However, persistent pain remaining long after the injury has healed serves no useful purpose and is a disabling condition. Persistent postsurgical pain, which is pain that lasts more than 3 months after surgery, affects 10-50% of patients undergoing elective surgery. Many of these patients are affected by neuropathic pain which is characterised as a pain caused by lesion or disease in the somatosensory nervous system. When established, this type of pain is difficult to treat and new approaches for prevention and treatment are needed. A possible contributing mechanism for the transition from acute physiological pain to persistent pain involves low-grade inflammation in the central nervous system (CNS), glial dysfunction and subsequently an imbalance in the neuron-glial interaction that causes enhanced and prolonged pain transmission.

AIM

This topical review aims to highlight the contribution that inflammatory activated glial cell dysfunction may have for the development of persistent pain.

METHOD

Relevant literature was searched for in PubMed.

RESULTS

Immediately after an injury to a nerve ending in the periphery such as in surgery, the inflammatory cascade is activated and immunocompetent cells migrate to the site of injury. Macrophages infiltrate the injured nerve and cause an inflammatory reaction in the nerve cell. This reaction leads to microglia activation in the central nervous system and the release of pro-inflammatory cytokines that activate and alter astrocyte function. Once the astrocytes and microglia have become activated, they participate in the development, spread, and potentiation of low-grade neuroinflammation. The inflammatory activated glial cells exhibit cellular changes, and their communication to each other and to neurons is altered. This renders neurons more excitable and pain transmission is enhanced and prolonged. Astrocyte dysfunction can be experimentally restored using the combined actions of a μ-opioid receptor agonist, a μ-opioid receptor antagonist, and an anti-epileptic agent. To find these agents we searched the literature for substances with possible anti-inflammatory properties that are usually used for other purposes in medicine. Inflammatory induced glial cell dysfunction is restorable in vitro by a combination of endomorphine-1, ultralow doses of naloxone and levetiracetam. Restoring inflammatory-activated glial cells, thereby restoring astrocyte-neuron interaction has the potential to affect pain transmission in neurons.

CONCLUSION

Surgery causes inflammation at the site of injury. Peripheral nerve injury can cause low-grade inflammation in the CNS known as neuroinflammation. Low-grade neuroinflammation can cause an imbalance in the glial-neuron interaction and communication. This renders neurons more excitable and pain transmission is enhanced and prolonged. Astrocytic dysfunction can be restored in vitro by a combination of endomorphin-1, ultralow doses of naloxone and levetiracetam. This restoration is essential for the interaction between astrocytes and neurons and hence also for modulation of synaptic pain transmission.

IMPLICATIONS

Larger studies in clinical settings are needed before these findings can be applied in a clinical context. Potentially, by targeting inflammatory activated glial cells and not only neurons, a new arena for development of pharmacological agents for persistent pain is opened.

Coupled cell networks are target cells of inflammation, which can spread between different body organs and develop into systemic chronic inflammation

Several organs in the body comprise cells coupled into networks. These cells have in common that they are excitable but do not express action potentials. Furthermore, they are equipped with Ca²⁺ signaling systems, which can be intercellular and/or extracellular. The transport of small molecules between the cells occurs through gap junctions comprising connexin 43. Examples of cells coupled into networks include astrocytes, keratinocytes, chondrocytes, synovial fibroblasts, osteoblasts, connective tissue cells, cardiac and corneal fibroblasts, myofibroblasts, hepatocytes, and different types of glandular cells. These cells are targets for inflammation, which can be initiated after injury or in disease. If the inflammation reaches the CNS, it develops into neuroinflammation and can be of importance in the development of systemic chronic inflammation, which can manifest as pain and result in changes in the expression and structure of cellular components. Biochemical parameters of importance for cellular functions are described in this review.

Drug abuse and the neurovascular unit

Drug abuse continues to create a major international epidemic affecting society. A great majority of past drug abuse research has focused mostly on the mechanisms of addiction and the specific effects of substance use disorders on brain circuits and pathways that modulate reward, motivation, craving, and decision making. Few studies have focused on the neurobiology of acute and chronic substance abuse as it relates to the neurovascular unit (brain endothelial cell, neuron, astrocyte, microglia, and pericyte). Increasing research indicates that all cellular components of the neurovascular unit play a pivotal role in both the process of addiction and how drug abuse affects the brain response to diseases. This review will focus on the specific effects of opioids, amphetamines, alcohol, and nicotine on the neurovascular unit and its role in addiction and adaption to brain diseases. Elucidation of the role of the neurovascular unit on the neurobiology associated with drug addiction will help to facilitate the development of better therapeutic approaches for drug-dependent individuals.

Actin filament reorganization in astrocyte networks is a key functional step in neuroinflammation resulting in persistent pain: novel findings on network restoration

In recent years, the importance of glial cell activation in the generation and maintenance of long-term pain has been investigated. One novel mechanism underlying long-lasting pain is injury-induced inflammation in the periphery, followed by microglial activation in the dorsal horn of the spinal cord, which results in local neuroinflammation. An increase in neuronal excitability may follow, with intense signaling along the pain tracts to the thalamus and the parietal cortex along with other cortical regions for the identification and recognition of the injury. If the local neuroinflammation develops into a pathological state, then the astrocytes become activated. Previous studies in which lipopolysaccharide (LPS) was used to induce inflammation have shown that in a dysfunctional astrocyte network, the actin cytoskeleton is reorganized from the normally occurring F-actin stress fibers into the more diffusible, disorganized, ring-form globular G-actin. In addition, Ca(2+) signaling systems are altered, Na(+)- and glutamate transporters are downregulated, and pro-inflammatory cytokines, particularly IL-1β, are released in dysfunctional astrocyte networks. In a series of experiments, we have demonstrated that these LPS-induced changes in astrocyte function can be restored by stimulation of Gi/o and inhibition of Gs with a combination of a μ-receptor agonist and ultralow concentrations of a μ-receptor antagonist and by inhibition of cytokine release, particularly IL-1β, by the antiepileptic drug levetiracetam. These findings could be of clinical significance and indicate a novel treatment for long-term pain.

Ultralow concentrations of bupivacaine exert anti-inflammatory effects on inflammation-reactive astrocytes

Bupivacaine is a widely used, local anesthetic agent that blocks voltage-gated Na(+) channels when used for neuro-axial blockades. Much lower concentrations of bupivacaine than in normal clinical use, < 10(-8)  m, evoked Ca(2+) transients in astrocytes from rat cerebral cortex, that were inositol trisphosphate receptor-dependent. We investigated whether bupivacaine exerts an influence on the Ca(2+) signaling and interleukin-1β (IL-1β) secretion in inflammation-reactive astrocytes when used at ultralow concentrations, < 10(-8)  m. Furthermore, we wanted to determine if bupivacaine interacts with the opioid-, 5-hydroxytryptamine- (5-HT) and glutamate-receptor systems. With respect to the μ-opioid- and 5-HT-receptor systems, bupivacaine restored the inflammation-reactive astrocytes to their normal non-inflammatory levels. With respect to the glutamate-receptor system, bupivacaine, in combination with an ultralow concentration of the μ-opioid receptor antagonist naloxone and μ-opioid receptor agonists, restored the inflammation-reactive astrocytes to their normal non-inflammatory levels. Ultralow concentrations of bupivacaine attenuated the inflammation-induced upregulation of IL-1β secretion. The results indicate that bupivacaine interacts with the opioid-, 5-HT- and glutamate-receptor systems by affecting Ca(2+) signaling and IL-1β release in inflammation-reactive astrocytes. These results suggest that bupivacaine may be used at ultralow concentrations as an anti-inflammatory drug, either alone or in combination with opioid agonists and ultralow concentrations of an opioid antagonist.

Long-term nicotine treatment suppresses IL-1β release and attenuates substance P- and 5-HT-evoked Ca²⁺ responses in astrocytes

The aim of this study was to investigate whether short- or long-term nicotine treatment, had an influence on Ca(2+)-induced intracellular Ca(2+) release in astrocytes co-cultured with microvascular endothelial cells, and if the release of interleukin-1β (IL-1β) changed during this treatment. We found that nicotine-evoked Ca(2+) transients were not attenuated up to 10d of incubation with nicotine, neither was the α7-nicotine acetylcholine receptor (α7-nAChR) protein. After 10d the IL-1β release was decreased. Furthermore, substance P- and 5-hydroxytryptamine (5-HT)-evoked Ca(2+) transients were attenuated after 10d of nicotine treatment, but glial cell line-derived neurotrophic factor (GDNF) had no effect on these transients. The results show that long-term nicotine treatment had no influence on nicotine-evoked Ca(2+) transients or protein expression of the α7-nAChR, but had with a decreased IL-1β release. The Gq protein and inositoltrisphosphate system seems to be influenced by the attenuation of Ca(2+)-evoked intracellular Ca(2+) release after stimulation with substance P and 5-HT.

Inflammatory mediators alter the astrocyte transcriptome and calcium signaling elicited by multiple G-protein-coupled receptors

Inflammation features in CNS disorders such as stroke, trauma, neurodegeneration, infection, and autoimmunity in which astrocytes play critical roles. To elucidate how inflammatory mediators alter astrocyte functions, we examined effects of transforming growth factor-β1 (TGF-β1), lipopolysaccharide (LPS), and interferon-gamma (IFNγ), alone and in combination, on purified, mouse primary cortical astrocyte cultures. We used microarrays to conduct whole-genome expression profiling, and measured calcium signaling, which is implicated in mediating dynamic astrocyte functions. Combinatorial exposure to TGF-β1, LPS, and IFNγ significantly modulated astrocyte expression of >6800 gene probes, including >380 synergistic changes not predicted by summing individual treatment effects. Bioinformatic analyses revealed significantly and markedly upregulated molecular networks and pathways associated in particular with immune signaling and regulation of cell injury, death, growth, and proliferation. Highly regulated genes included chemokines, growth factors, enzymes, channels, transporters, and intercellular and intracellular signal transducers. Notably, numerous genes for G-protein-coupled receptors (GPCRs) and G-protein effectors involved in calcium signaling were significantly regulated, mostly down (for example, Cxcr4, Adra2a, Ednra, P2ry1, Gnao1, Gng7), but some up (for example, P2ry14, P2ry6, Ccrl2, Gnb4). We tested selected cases and found that changes in GPCR gene expression were accompanied by significant, parallel changes in astrocyte calcium signaling evoked by corresponding GPCR-specific ligands. These findings identify pronounced changes in the astrocyte transcriptome induced by TGF-β1, LPS, and IFNγ, and show that these inflammatory stimuli upregulate astrocyte molecular networks associated with immune- and injury-related functions and significantly alter astrocyte calcium signaling stimulated by multiple GPCRs.

Nicotine uses neuron-glia communication to enhance hippocampal synaptic transmission and long-term memory

Nicotine enhances synaptic transmission and facilitates long-term memory. Now it is known that bi-directional glia-neuron interactions play important roles in the physiology of the brain. However, the involvement of glial cells in the effects of nicotine has not been considered until now. In particular, the gliotransmitter D-serine, an endogenous co-agonist of NMDA receptors, enables different types of synaptic plasticity and memory in the hippocampus. Here, we report that hippocampal long-term synaptic plasticity induced by nicotine was annulled by an enzyme that degrades endogenous D-serine, or by an NMDA receptor antagonist that acts at the D-serine binding site. Accordingly, both effects of nicotine: the enhancement of synaptic transmission and facilitation of long-term memory were eliminated by impairing glial cells with fluoroacetate, and were restored with exogenous D-serine. Together, these results show that glial D-serine is essential for the long-term effects of nicotine on synaptic plasticity and memory, and they highlight the roles of glial cells as key participants in brain functions.

Inflammatory activation enhances NMDA-triggered Ca2+ signalling and IL-1β secretion in primary cultures of rat astrocytes

The aim of the present study was to measure the effects of NMDA receptor antagonists on rat astroglial-enriched primary cultures after incubation with lipopolysaccharide (LPS), with a view to explaining the role of NMDA receptors in the inflammatory activation of astrocytes. First, the presence of NMDA receptor subunits was confirmed at the protein level by immunocytochemical methods. The presence of functional NMDA receptors containing GluN2B subunits was then established by ratiometric fluorescent Ca(2+) imaging which revealed transient NMDA-triggered Ca(2+) responses. These responses could be blocked by the competitive antagonist 2-amino-5-phosphonopentoate (APV) and the non-competitive GluN2B subunit-selective antagonist ifenprodil. The NMDA-evoked Ca(2+) transients were dependent on Ca(2+) release from intracellular stores via interaction with InsP3-sensitive receptors as they were blocked by thapsigargin or xestospongin C. Following 24h incubation with LPS, astroglial inflammatory activation increased IL-1β secretion and NMDA-triggered Ca(2+) transients. The addition of APV or ifenprodil inhibited these enhanced responses, suggesting that LPS exposure stimulates IL-1β release from astrocytes through a mechanism that requires NMDA receptor stimulation.

Glutamate potentiates lipopolysaccharide-stimulated interleukin-10 release from neonatal rat spinal cord astrocytes

Interleukin-10 (IL-10) has important anti-inflammatory effects and can be protective in inflammatory conditions, such as chronic pain and infection. Exploring factors that modulate IL-10 levels may provide insight into pathomechanisms of inflammatory conditions and may provide a method of neuroprotection during these conditions. Lipopolysaccharide (LPS) stimulation of astrocytes is a source of IL-10; hence, it is of interest to investigate factors that modulate this process. Glutamate is present in increased concentrations in inflammatory conditions, and astrocytes also express glutamate receptors. The present study, therefore, investigated whether glutamate modulates LPS stimulation of IL-10 release from neonatal spinal cord astrocytes. Enzyme-linked immunosorbent assays (ELISAs) were used to quantify IL-10 release from cultured neonatal spinal cord astrocytes, and reverse transcriptase-polymerase chain reaction (RT-PCR) was used to measure IL-10 mRNA expression. Glutamate (1 mM) significantly increased LPS (1 μg/ml)-stimulated IL-10 release from astrocytes by 166% and significantly upregulated IL-10 mRNA levels. Glutamate synergistically signaled through metabotropic glutamate receptor subgroups and the phospholipase C signaling pathway. Spinal cord astrocytes may, therefore, play a larger anti-inflammatory role than first thought in situations where glutamate and a high concentration of Toll-like receptor 4 (TLR4) agonists are present.

Naloxone in ultralow concentration restores endomorphin-1-evoked Ca²⁺ signaling in lipopolysaccharide pretreated astrocytes

Long-term pain is a disabling condition that affects thousands of people. Pain may be sustained for a long time even after the physiological trigger has resolved. Possible mechanisms for this phenomenon include low-grade inflammation in the CNS. Astrocytes respond to inflammatory stimuli and may play an important role as modulators of the inflammatory response in the nervous system. This study aimed first to assess how astrocytes in a primary culture behave when exposed to the endogenous μ-opioid receptor agonist endomorphin-1 (EM-1), in a concentration-dependent manner, concerning intracellular Ca²⁺ responses. EM-1 stimulated the μ-opioid receptor from 10⁻¹⁵ M up to 10⁻⁴ M with increasing intensity, usually reflected as one peak at low concentrations and two peaks at higher concentrations. Naloxone, pertussis toxin (PTX), or the μ-opioid receptor antagonists CTOP did not totally block the EM-1-evoked Ca²⁺ responses. However, a combination of ultralow concentration naloxone (10⁻¹² M) and PTX (100 ng/ml) totally blocked the EM-1-evoked Ca²⁺ responses. This suggests that ultralow (picomolar) concentrations of naloxone should block the μ-opioid receptor coupled G(s) protein, and that PTX should block the μ-opioid receptor coupled G(i/o) protein. The second aim was to investigate exposure of astrocytes with the inflammatory agent lipopolysaccharide (LPS). After 4 h of LPS incubation, the EM-1-evoked Ca²⁺ transients were attenuated, and after 24 h of LPS incubation, the EM-1-evoked Ca²⁺ transients were oscillated. To restore the EM-1-evoked Ca²⁺ transients, naloxone was assessed as a proposed anti-inflammatory substance. In ultralow picomolar concentration, naloxone demonstrated the ability to restore the Ca²⁺ transients.

Acetylcholine α7 nicotinic and dopamine D2 receptors are targeted to many of the same postsynaptic dendrites and astrocytes in the rodent prefrontal cortex

The alpha-7 nicotinic acetylcholine receptor (α7nAChR) and the dopamine D(2) receptor (D(2) R) are both implicated in attentional processes and cognition, mediated in part through the prefrontal cortex (PFC). We examined the dual electron microscopic immunolabeling of α7nAChR and either D(2) R or the vesicular acetylcholine transporter (VAChT) in rodent PFC to assess convergent functional activation sites. Immunoreactivity (ir) for α7nAChR and/or D(2) R was seen in the same as well as separate neuronal and glial profiles. At least half of the dually labeled profiles were somata and dendrites, while most labeled axon terminals expressed only D(2) R-ir. The D(2) R-labeled terminals were without synaptic specializations or formed inhibitory or excitatory-type synapses with somatodendritic profiles, some of which expressed the α7nAChR and/or D(2) R. Astrocytic glial processes comprised the majority of nonsomatodendritic α7nAChR or α7nAChR and D(2) R-labeled profiles. Glial processes containing α7nAChR-ir were frequently located near VAChT-labeled terminals and also showed perisynaptic and perivascular associations. We conclude that in rodent PFC α7nACh and D(2) R activation can dually modulate (1) postsynaptic dendritic responses within the same or separate but synaptically linked neurons in which the D(2) R has the predominately presynaptic distribution, and (2) astrocytic signaling that may be crucial for synaptic transmission and functional hyperemia.

Ifenprodil restores GDNF-evoked Ca(2+) signalling and Na(+)/K(+) -ATPase expression in inflammation-pretreated astrocytes

Glial cell line-derived neurotrophic factor (GDNF) plays an important role in neuroinflammatory and neuropathic pain conditions. Astrocytes produce and secrete GDNF, which interacts with its receptors to induce Ca(2+) transients. This study aimed first to assess intracellular Ca(2+) responses of astrocytes in primary culture when exposed to the neuroprotective and anti-inflammatory peptide GDNF. Furthermore, incubation with the inflammatory inducers lipopolysaccharide (LPS), NMDA, or interleukin 1-β (IL-1β) attenuated the GDNF-induced Ca(2+) transients. The next aim was to try to restore the suppressed GDNF responses induced by inflammatory changes in the astrocytes with an anti-inflammatory substance. Ifenprodil, an NMDA receptor antagonist at the NR2B subunit, was tested. It was shown to restore the GDNF-evoked Ca(2+) transients and increased the Na(+)/K(+) -ATPase expression. Ifenprodil seems to be a potent anti-inflammatory substance for astrocytes which have been pre-activated by inflammatory stimuli.

Naloxone and ouabain in ultralow concentrations restore Na+/K+-ATPase and cytoskeleton in lipopolysaccharide-treated astrocytes

Astrocytes respond to inflammatory stimuli and may be important modulators of the inflammatory response in the nervous system. This study aimed first to assess how astrocytes in primary culture behave in response to inflammatory stimuli concerning intracellular Ca(2+) responses, expression of Toll-like receptor 4 (TLR4), Na(+)/K(+)-ATPase, actin filament organization, and expression of cytokines. In a cell culture model with lipopolysaccharide (LPS), astrocyte response was assessed first in the acute phase and then after incubation with LPS for 1-48 h. The concentration curve for LPS-stimulated Ca(2+) responses was bell-shaped, and the astrocytes expressed TLR4, which detects LPS and evokes intracellular Ca(2+) transients. After a long incubation with LPS, TLR4 was up-regulated, LPS-evoked Ca(2+) transients were expressed as oscillations, Na(+)/K(+)-ATPase was down-regulated, and the actin filaments were disorganized. Interleukin-1β (IL-1β) release was increased after 24 h in LPS. A second aim was to try to restore the LPS-induced changes in astrocytes with substances that may have dose-dependent anti-inflammatory properties. Naloxone and ouabain were tested separately in ultralow or high concentrations. Both substances evoked intracellular Ca(2+) transients for all of the concentrations from 10(-15) up to 10(-4) M. Neither substance blocked the TLR4-evoked Ca(2+) responses. Naloxone and ouabain prevented the LPS-induced down-regulation of Na(+)/K(+)-ATPase and restored the actin filaments. Ouabain, in addition, reduced the IL-1β release from reactive astrocytes. Notably, ultralow concentrations (10(-12) M) of naloxone and ouabain showed these qualities. Ouabain seems to be more potent in these effects of the two tested substances.

Nicotinic receptor agonists decrease L-dopa-induced dyskinesias most effectively in partially lesioned parkinsonian rats

L-dopa therapy for Parkinson's disease leads to dyskinesias or abnormal involuntary movement (AIMs) for which there are few treatment options. Our previous data showed that nicotine administration reduced L-dopa-induced AIMs in parkinsonian monkeys and rats. To further understand how nicotine mediates its antidyskinetic action, we investigated the effect of nicotinic receptor (nAChR) agonists in unilateral 6-OHDA-lesioned rats with varying striatal damage. We first tested the drugs in L-dopa-treated rats with a near-complete striatal dopamine lesion (>99%), the standard rodent dyskinesia model. Varenicline, an agonist that interacts with multiple nAChRs, did not significantly reduce L-dopa-induced AIMs, while 5-iodo-A-85380 (A-85380), which acts selectively at α4β2* and α6β2* subtypes, reduced AIMs by 20%. By contrast, both varenicline and A-85380 reduced L-dopa-induced AIMs by 40-50% in rats with a partial striatal dopamine lesion. Neither drug worsened the antiparkinsonian action of L-dopa. The results show that selective nicotinic agonists reduce dyskinesias, and that they are optimally effective in animals with partial striatal dopamine damage. These findings suggest that presynaptic dopamine terminal α4β2* and α6β2* nAChRs are critical for nicotine's antidyskinetic action. The current data have important implications for the use of nicotinic receptor-directed drugs for L-dopa-induced dyskinesias, a debilitating motor complication of dopamine replacement therapy for Parkinson's disease.

Long-term pain, neuroinflammation and glial activation

Nociceptive and neuropathic pain signals are known to result from noxious stimuli, which are converted into electrical impulses within tissue nociceptors. There is a complex equilibrium of pain-signalling and pain-relieving pathways connecting PNS and CNS. Drugs against long-term pain are today directed against increased neuronal excitability, mostly with less success. An injury often starts with acute physiological pain, which becomes inflammatory, nociceptive, or neuropathic, and may be transferred into long-term pain. Recently a low-grade inflammation was identified in the spinal cord and along the pain pathways to thalamus and the parietal cortex. This neuroinflammation is due to activation of glial cells, especially microglia, with production of cytokines and other inflammatory mediators within the CNS. Additionally, substances released to the blood from the injured region influence the blood-brain barrier, and give rise to an increased permeability of the tight junctions of the capillary endothelial cells, leading to passage of blood cells into the CNS. These cells are transformed into reactive microglia. If the inflammation turns into a pathological state the astrocytes will be activated. They are coupled into networks and respond to substances released by the capillary endothelial cells, to cytokines released from microglia, and to neurotransmitters and peptides released from neurons. As the astrocytes occupy a strategic position between the vasculature and synapses, they monitor the neuronal activity and transmitter release. Increased release of glutamate and ATP leads to disturbances in Ca2+ signalling, increased production of cytokines and free radicals, attenuation of the astrocyte glutamate transport capacity, and conformational changes in the astrocytic cytoskeleton, the actin filaments, which can lead to formation and rebuilding of new synapses. New neuronal contacts are established for maintaining and spreading pain sensation with the astrocytic networks as bridges. Thereby the glial cells can maintain the pain sensation even after the original injury has healed, and convert the pain into long-term by altering neuronal excitability. It can even be experienced from other parts of the body. As astrocytes are intimate co-players with neurons in the CNS, more knowledge on astrocyte responses to inflammatory activators may give new insight in our understanding of mechanisms of low-grade inflammation underlying long-term pain states and pain spreading. Novel treatment strategies would be to restore glial cell function and thereby attenuate the neuroinflammation.

Differential vesicular distribution and trafficking of MMP-2, MMP-9, and their inhibitors in astrocytes

Astrocytes play an active role in the central nervous system and are critically involved in astrogliosis, a homotypic response of these cells to disease, injury, and associated neuroinflammation. Among the numerous molecules involved in these processes are the matrix metalloproteinases (MMPs), a family of zinc-dependent endopeptidases, secreted or membrane-bound, that regulate by proteolytic cleavage the extracellular matrix, cytokines, chemokines, cell adhesion molecules, and plasma membrane receptors. MMP activity is tightly regulated by the tissue inhibitors of MMPs (TIMPs), a family of secreted multifunctional proteins. Astrogliosis in vivo and astrocyte reactivity induced in vitro by proinflammatory cues are associated with modulation of expression and/or activity of members of the MMP/TIMP system. However, nothing is known concerning the intracellular distribution and secretory pathways of MMPs and TIMPs in astrocytes. Using a combination of cell biology, biochemistry, fluorescence and electron microscopy approaches, we investigated in cultured reactive astrocytes the intracellular distribution, transport, and secretion of MMP-2, MMP-9, TIMP-1, and TIMP-2. MMP-2 and MMP-9 demonstrate nuclear localization, differential intracellular vesicular distribution relative to the myosin V and kinesin molecular motors, and LAMP-2-labeled lysosomal compartment, and we show vesicular secretion for MMP-2, MMP-9, and their inhibitors. Our results suggest that these proteinases and their inhibitors use different pathways for trafficking and secretion for distinct astrocytic functions.

Effects of immunomodulatory and organism-associated molecules on the permeability of an in vitro blood-brain barrier model to amphotericin B and fluconazole

Amphotericin B (AMB) is used to treat fungal infections of the central nervous system (CNS). However, AMB shows poor penetration into the CNS and little is known about the factors affecting its permeation through the blood-brain barrier (BBB). Therefore, we studied immunomodulatory and organism-associated molecules affecting the permeability of an in vitro BBB model to AMB. We examined the effects of interleukin-1 beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), lipopolysaccharide (LPS), lipoteichoic acid (LTA), zymosan (ZYM), dexamethasone (DEX), cyclosporine, and tacrolimus on transendothelial electrical resistance (TEER); endothelial tight junctions; filamentous actin; and permeability to deoxycholate AMB (DAMB), liposomal AMB (LAMB), and fluconazole. Proinflammatory cytokines and organism-associated molecules significantly decreased the mean TEER by 40.7 to 100% (P < or = 0.004). DEX increased the mean TEER by 18.2 to 26.4% (P < or = 0.04). TNF-alpha and LPS increased the permeability to AMB by 8.2 to 14.5% compared to that for the controls (1.1 to 2.4%) (P < or = 0.04). None of the other molecules affected the model's permeability to AMB. By comparison, the BBB model's permeability to fluconazole was >78% under all conditions studied, without significant differences between the controls and the experimental groups. LPS and TNF-alpha decreased tight-junction protein zona occludens 1 (ZO-1) between endothelial cells. In conclusion, IL-1beta, ZYM, and LTA increased the permeability of the BBB to small ions but not to AMB, whereas TNF-alpha and LPS, which disrupted the endothelial layer integrity, increased the permeability to AMB.

PACAP attenuates 5-HT, histamine, and ATP-evoked Ca2+ transients in astrocytes

Pituitary adenylate cyclase-activating polypeptide (PACAP) has neuroprotective properties and plays an important role in neuroinflammation. PACAP38 interacts with its receptors, PAC1, and VPAC, on astrocytes at 10(-8) M to induce biphasic Ca2+ transients, which were reduced to a single transient by the PAC1-blocking PACAP antagonist PACAP6-38. At 10(-12) M even the single transient, corresponding to PAC1 was blocked. PACAP-induced Ca2+ transients were more pronounced in astrocytes cocultured with brain endothelial cells than in monocultured astrocytes, indicating that astrocytes that receive signals from microvessels develop more sensitive signal transduction systems for Ca. In this sensitive system, PACAP38 attenuated 5-HT, histamine, and ATP-evoked Ca2+ transients, showing the anti-inflammatory properties of PACAP.