mu-Opioid agonists inhibit the enhanced intracellular Ca(2+) responses in inflammatory activated astrocytes co-cultured with brain endothelial cells

Oxidative Stress and Extracellular Matrix Remodeling Are Signature Pathways of Extracellular Vesicles Released upon Morphine Exposure on Human Brain Microvascular Endothelial Cells

Morphine, a commonly used antinociceptive drug in hospitals, is known to cross the blood-brain barrier (BBB) by first passing through brain endothelial cells. Despite its pain-relieving effect, morphine also has detrimental effects, such as the potential induction of redox imbalance in the brain. However, there is still insufficient evidence of these effects on the brain, particularly on the brain endothelial cells and the extracellular vesicles that they naturally release. Indeed, extracellular vesicles (EVs) are nanosized bioparticles produced by almost all cell types and are currently thought to reflect the physiological state of their parent cells. These vesicles have emerged as a promising source of biomarkers by indicating the functional or dysfunctional state of their parent cells and, thus, allowing a better understanding of the biological processes involved in an adverse state. However, there is very little information on the morphine effect on human brain microvascular endothelial cells (HBMECs), and even less on their released EVs. Therefore, the current study aimed at unraveling the detrimental mechanisms of morphine exposure (at 1, 10, 25, 50 and 100 µM) for 24 h on human brain microvascular endothelial cells as well as on their associated EVs. Isolation of EVs was carried out using an affinity-based method. Several orthogonal techniques (NTA, western blotting and proteomics analysis) were used to validate the EVs enrichment, quality and concentration. Data-independent mass spectrometry (DIA-MS)-based proteomics was applied in order to analyze the proteome modulations induced by morphine on HBMECs and EVs. We were able to quantify almost 5500 proteins in HBMECs and 1500 proteins in EVs, of which 256 and 148, respectively, were found to be differentially expressed in at least one condition. Pathway enrichment analysis revealed that the "cell adhesion and extracellular matrix remodeling" process and the "HIF1 pathway", a pathway related to oxidative stress responses, were significantly modulated upon morphine exposure in HBMECs and EVs. Altogether, the combination of proteomics and bioinformatics findings highlighted shared pathways between HBMECs exposed to morphine and their released EVs. These results put forward molecular signatures of morphine-induced toxicity in HBMECs that were also carried by EVs. Therefore, EVs could potentially be regarded as a useful tool to investigate brain endothelial cells dysfunction, and to a different extent, the BBB dysfunction in patient circulation using these "signature pathways".

Immune-mediated febrile response in female rats: Role of central hypothalamic mediators

Lipopolysaccharide (LPS) induces fever through cytokines like receptor-activator of nuclear factor κB ligand (RANKL), triggering mediators like prostaglandins (PG), endothelin-1 (ET-1), corticotrophin-releasing factor (CRF), substance P (SP) and endogenous opioids. LPS-induced fever is reduced in females compared with males except in ovariectomized (OVX) females which show increased fever mediated by PG. The present study aimed to identify the mediators involved in fever in intact and OVX female rats. Fever was induced with LPS (50 μg/kg) intraperitoneally or CRF (2.5 μg), ET-1 (1 pg), morphine (10 μg) and SP (500 ng) intracerebroventricularly in sham-operated and OVX rats. The role of RANKL was evaluated with osteoprotegerin (OPG, 1 μg, intracerebroventricularly). Expression of RANK, CRFI/II, ETB, μ-opioid (MOR) and NK1 receptors was evaluated by confocal microscopy. Besides LPS, only morphine induced fever in OVX rats while all mediators induced fever in sham-operated animals. OPG abolished LPS-induced fever in OVX but not sham-operated animals. Overall, fever involves similar central mediators in cycling females and males but only morphine induced fever in OVX females. Importantly, RANK/RANKL participates in LPS-induced fever in OVX females, as in males but not in cycling females.

Glial neuroimmune signaling in opioid reward

The opioid epidemic is a growing public concern affecting millions of people worldwide. Opioid-induced reward is the initial and key process leading to opioid abuse and addiction. Therefore, a better understanding of opioid reward may be helpful in developing a treatment for opioid addiction. Emerging evidence suggests that glial cells, particularly microglia and astrocytes, play an essential role in modulating opioid reward. Indeed, glial cells and their associated immune signaling actively regulate neural activity and plasticity, and directly modulate opioid-induced rewarding behaviors. In this review, we describe the neuroimmune mechanisms of how glial cells affect synaptic transmission and plasticity as well as how opioids can activate glial cells affecting the glial-neuronal interaction. Last, we summarize current attempts of applying glial modulators in treating opioid reward.

Anti-inflammatory effects induced by ultralow concentrations of bupivacaine in combination with ultralow concentrations of sildenafil (Viagra) and vitamin D3 on inflammatory reactive brain astrocytes

Network coupled cells, such as astrocytes, regulate their cellular homeostasis via Ca²⁺ signals spread between the cells through gap junctions. Intracellular Ca²⁺ release is controlled by different signaling pathways that can be stimulated by ATP, glutamate and serotonin (5-HT). Based on our findings, all these pathways are influenced by inflammatory agents and must be restored to fully recover the Ca²⁺ signaling network. An ultralow concentration of the local anesthetic agent bupivacaine reduced 5-HT-evoked intracellular Ca²⁺ release, and an ultralow concentration of the phosphodiesterase-5 inhibitor sildenafil in combination with vitamin D3 reduced ATP-evoked intracellular Ca²⁺ release. Combinations of these three substances downregulated 5-HT-, glutamate- and ATP-evoked intracellular Ca²⁺ release to a more normal Ca²⁺ signaling state. Furthermore, inflammatory Toll-like receptor 4 expression decreased with a combination of these three substances. Substance P receptor neurokinin (NK)-1 expression was reduced by ultralow concentrations of bupivacaine. Here, bupivacaine and sildenafil (at extremely low concentrations) combined with vitamin D3 have potential anti-inflammatory properties. According to the present study, drug combinations at the right concentrations, especially extremely low concentrations of bupivacaine and sildenafil, affect different cellular biochemical mechanisms and represent a potential solution for downregulating inflammatory parameters, thereby restoring cells or networks to normal physiological homeostasis.

Signalling in response to sub-picomolar concentrations of active compounds: Pushing the boundaries of GPCR sensitivity

There is evidence for ultra-sensitive responses to active compounds at concentrations below picomolar levels by proteins and receptors found in species ranging from bacteria to mammals. We have recently shown that such ultra-sensitivity is also demonstrated by a wide range of prototypical GPCRs, and we have determined the molecular mechanisms behind these responses for three family A GPCRs: the relaxin receptor, RXFP1; the β₂ -adrenoceptor; and the M₃ muscarinic ACh receptor. Interestingly, there are reports of similar ultra-sensitivity by more than 15 human GPCR families, in addition to other human receptors and channels. These occur through a diverse range of signalling pathways and produce modulation of important physiological processes, including neuronal transmission, chemotaxis, gene transcription, protein/ion uptake and secretion, muscle contraction and relaxation, and phagocytosis. Here, we summarise the accumulating evidence of ultra-sensitive receptor signalling to show that this is a common, though currently underappreciated, property of GPCRs. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Inhibitive Effects of FGF2/FGFR1 Pathway on Astrocyte-Mediated Inflammation in vivo and in vitro After Infrasound Exposure

Infrasound, a kind of ambient noise, can cause severe disorders to various human organs, specially to central nervous system (CNS). Our previous studies have shown that infrasound-induced CNS injury was closely related with astrocytes activation and astrocytes-mediated neuroinflammation, but the underlying molecular mechanisms are still largely unclear. FGF2/FGFR1 (Fibroblast growth factor 2/Fibroblast growth factor receptor 1) pathway was reported to play an important role in anti-inflammation in CNS disorders. To further study the possible roles of FGF2/FGFR1 pathway in infrasound-induced CNS injury, here we exposed Sprague-Dawley rats or cultured astrocytes to 16 Hz, 150 dB infrasound, and explored the effects of FGF2 on infrasound-induced astrocytes activation and neuroinflammation. Western blotting, immunofluorescence and liquid chip method were used in this experiment. Our results showed that after 3- or 7-day exposure (2 h/day) of rats as well as 2 h exposure of cultured astrocytes to 16 Hz, 150 dB infrasound, astrocyte-expressed FGFR1 was downregulated in vivo and in vitro. FGF2 pretreatment not only inhibited infrasound-induced astrocyte activation in rat hippocampal CA1 region, but also reduced the levels of pro-inflammatory cytokines, such as TNF-α, IL-1β, IL-18, IL-6, and IFN-γ in vitro and in vivo. However, FGF2 significantly upregulated the expression of FGFR1. Furthermore, we showed that FGF2 could attenuate IκBα phosphorylation, NF-κB p65 translocation, pro-inflammatory cytokines levels, and neuronal loss in the CA1 region induced by infrasound. On the contrary, PD173074, a special antagonist of FGFR1, could reverse the effects above in vitro and in vivo. Taken together, our findings showed that FGF2/FGFR1 pathway may exert inhibitive effects on astrocyte-mediated neuroinflammation in vitro and in vivo after infrasound exposure.

In vitro screening of nanomedicines through the blood brain barrier: A critical review

The blood-brain barrier accounts for the high attrition rate of the treatments of most brain disorders, which therefore remain one of the greatest health-care challenges of the twenty first century. Against this background of hindrance to brain delivery, nanomedicine takes advantage of the assembly at the nanoscale of available biomaterials to provide a delivery platform with potential to raising brain levels of either imaging or therapeutic agents. Nevertheless, to prevent later failure due to ineffective drug levels at the target site, researchers have been endeavoring to develop a battery of in vitro screening procedures that can predict earlier in the drug discovery process the ability of these cutting-edge drug delivery platforms to cross the blood-brain barrier for biomedical purposes. This review provides an in-depth analysis of the currently available in vitro blood-brain barrier models (both cell-based and non-cell-based) with the focus on their suitability for understanding the biological brain distribution of forthcoming nanomedicines. The relationship between experimental factors and underlying physiological assumptions that would ultimately lead to a more predictive capacity of their in vivo performance, and those methods already assayed for the evaluation of the brain distribution of nanomedicines are comprehensively discussed.

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.

Glial activation and midkine and pleiotrophin transcription in the ventral tegmental area are modulated by morphine administration

Opiates cause persistent restructuring in the mesolimbic reward system. Although a possible role for midkine and pleiotrophin cytokines in the field of synaptic plasticity has been proposed, it has not been assessed whether morphine administration regulates astrogliosis and midkine and pleiotrophin transcription. We observed that single morphine injection and chronic morphine increased glial fibrillary acidic protein expression in the ventral tegmental area (VTA). Interestingly, single morphine injection and chronic morphine increased VTA midkine and pleiotrophin mRNA expression. Given these results, we hypothesize a role for these cytokines in mediating, at least in part, acute neuroprotective effects and chronic neurotrophic adaptations that contribute to drug dependence.

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.

Blood-brain barrier models and their relevance for a successful development of CNS drug delivery systems: a review

During the research and development of new drugs directed at the central nervous system, there is a considerable attrition rate caused by their hampered access to the brain by the blood-brain barrier. Throughout the years, several in vitro models have been developed in an attempt to mimic critical functionalities of the blood-brain barrier and reliably predict the permeability of drug candidates. However, the current challenge lies in developing a model that retains fundamental blood-brain barrier characteristics and simultaneously remains compatible with the high throughput demands of pharmaceutical industries. This review firstly describes the roles of all elements of the neurovascular unit and their influence on drug brain penetration. In vitro models, including non-cell based and cell-based models, and in vivo models are herein presented, with a particular emphasis on their methodological aspects. Lastly, their contribution to the improvement of brain drug delivery strategies and drug transport across the blood-brain barrier is also discussed.

Differential expression of the alternatively spliced OPRM1 isoform μ-opioid receptor-1K in HIV-infected individuals

OBJECTIVE

We previously examined the expression of specific C-terminal μ-opioid receptor (MOR) splice variants in human central nervous system cell types and HIV-infected brain tissue from individuals with neurocognitive impairment ± HIV encephalitis (HIVE). In the present study, we examined the N-terminal splice variant MOR-1K, which mediates excitatory cellular signaling.

METHODS AND RESULTS

We found segregation of expression ranging from undetectable to seemingly exclusive across nervous system cell types compared to the pool of C-terminal MOR splice variants using the real-time polymerase chain reaction (RT-PCR). Expression of MOR-1K mRNA was also increased in HIV-infected individuals with combined neurocognitive impairment and HIVE compared with the other groups. MOR-1K expression correlated with the level of patient neurocognitive impairment, whereas the pool of C-terminal MOR splice variants did not. HIVE was also associated with increased expression of the inflammatory mediators MCP-1, MCP-2, and RANTES, but not the host HIV coreceptors CXCR4 and CCR5 or the CD4 receptor using qRT-PCR. Network analysis of microarray data from these same patients revealed filamin A (FLNA) as a possible interaction partner with MOR-1K, and FLNA gene expression was also found to be upregulated in HIVE using qRT-PCR. Overexpression of FLNA in HEK293 cells redistributed MOR-1K from intracellular compartments to the cell surface.

CONCLUSION

These results suggest that HIVE, and neurocognitive impairment depending on its severity, are associated with enhanced MOR-1K signaling through both increased expression and trafficking to the cell surface, which may alter the contribution of MOR receptor isoforms and exacerbate the effects of MOR activation in neuroAIDS.

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.

Pharmacological characterization of LPS and opioid interactions at the toll-like receptor 4

BACKGROUND AND PURPOSE

Previous work in our laboratory showed opioid agents inhibit cytokine expression in astrocytes. Recently, Watkins and colleagues hypothesized that opioid agonists activate toll-like receptor 4 (TLR4) signalling, which leads to neuroinflammation. To test this hypothesis, we characterized LPS and opioid effects on TLR4 signalling in reporter cells.

EXPERIMENTAL APPROACH

NF-κB reporter cells expressing high levels of TLR4 were used to compare LPS and opioid effects on NF-κB activation, a pathway activated by TLR4 stimulation.

KEY RESULTS

LPS increased TLR4 signalling in a concentration-dependent manner and was antagonized by LPS antagonist (LPS-RS, from Rhodobacter sphaeroides). A concentration ratio analysis showed that LPS-RS was a competitive antagonist. The opioid agonists, morphine and fentanyl, produced minor activation of TLR4 signalling when given alone. When tested following LPS stimulation, opioid agonists inhibited NF-κB activation but this inhibition was not blocked by the general opioid antagonist, naloxone, nor by the selective μ opioid receptor antagonist, β-FNA. Indeed, both naloxone and β-FNA also inhibited NF-κB activation in reporter cells. Further examination of fentanyl and β-FNA effects revealed that both opioid agents inhibited LPS signalling in a non-competitive fashion.

CONCLUSIONS AND IMPLICATIONS

These results show that LPS-RS is a competitive antagonist at the TLR4 complex, and that both opioid agonists and antagonists inhibit LPS signalling in a non-competitive fashion through a non-GPCR, opioid site(s) in the TLR4 signalling pathway. If confirmed, existing opioid agents or other drug molecules more selective at this novel site may provide a new therapeutic approach to the treatment of neuroinflammation.

Neurovascular unit in chronic pain

Chronic pain is a debilitating condition with major socioeconomic impact, whose neurobiological basis is still not clear. An involvement of the neurovascular unit (NVU) has been recently proposed. In particular, the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB), two NVU key players, may be affected during the development of chronic pain; in particular, transient permeabilization of the barrier is suggested by several inflammatory- and nerve-injury-based pain models, and we argue that the clarification of molecular BBB/BSCB permeabilization events will shed new light in understanding chronic pain mechanisms. Possible biases in experiments supporting this theory and its translational potentials are discussed. Moving beyond an exclusive focus on the role of the endothelium, we propose that our understanding of the mechanisms subserving chronic pain will benefit from the extension of research efforts to the NVU as a whole. In this view, the available evidence on the interaction between analgesic drugs and the NVU is here reviewed. Chronic pain comorbidities, such as neuroinflammatory and neurodegenerative diseases, are also discussed in view of NVU changes, together with innovative pharmacological solutions targeting NVU components in chronic pain treatment.

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.

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.

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.

Exploring the neuroimmunopharmacology of opioids: an integrative review of mechanisms of central immune signaling and their implications for opioid analgesia

Vastly stimulated by the discovery of opioid receptors in the early 1970s, preclinical and clinical research was directed at the study of stereoselective neuronal actions of opioids, especially those played in their crucial analgesic role. However, during the past decade, a new appreciation of the non-neuronal actions of opioids has emerged from preclinical research, with specific appreciation for the nonclassic and nonstereoselective sites of action. Opioid activity at Toll-like receptors, newly recognized innate immune pattern recognition receptors, adds substantially to this unfolding story. It is now apparent from molecular and rodent data that these newly identified signaling events significantly modify the pharmacodynamics of opioids by eliciting proinflammatory reactivity from glia, the immunocompetent cells of the central nervous system. These central immune signaling events, including the release of cytokines and chemokines and the associated disruption of glutamate homeostasis, cause elevated neuronal excitability, which subsequently decreases opioid analgesic efficacy and leads to heightened pain states. This review will examine the current preclinical literature of opioid-induced central immune signaling mediated by classic and nonclassic opioid receptors. A unification of the preclinical pharmacology, neuroscience, and immunology of opioids now provides new insights into common mechanisms of chronic pain, naive tolerance, analgesic tolerance, opioid-induced hyperalgesia, and allodynia. Novel pharmacological targets for future drug development are discussed in the hope that disease-modifying chronic pain treatments arising from the appreciation of opioid-induced central immune signaling may become practical.

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.

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.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

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.

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

The aim of this study was to investigate whether nicotine acetylcholine receptors (nAChRs) are expressed in a more pronounced way in astrocytes co-cultured with microvascular endothelial cells from adult rat brain, compared with monocultured astrocytes, as a sign of a more developed signal transduction system. Also investigated was whether nicotine plays a role in the control of neuroinflammatory reactivity in astrocytes. Ca(2+) imaging experiments were performed using cells loaded with the Ca(2+) indicator Fura-2/AM. Co-cultured astrocytes responded to lower concentrations of nicotine than did monocultured astrocytes, indicating that they are more sensitive to nicotine. Co-cultured astrocytes also expressed a higher selectivity for alpha7nAChR and alpha4/beta2 subunits and evoked higher Ca(2+) transients compared with monocultured astrocytes. The Ca(2+) transients referred to are activators of Ca(2+)-induced Ca(2+) release from intracellular stores, both IP(3) and ryanodine, triggered by influx through receptor channels. The nicotine-induced Ca(2+) transients were attenuated after incubation with the inflammatory mediator lipopolysaccharide (LPS), but were not attenuated after incubation with the pain-transmitting peptides substance P and calcitonin-gene-related peptide, nor with the infection and inflammation stress mediator, leptin. Furthermore, LPS-induced release of interleukin-1beta (IL-1beta) measured by enzyme-linked immunosorbent assay (ELISA) was more pronounced in co-cultured versus monocultured astrocytes. Incubation with both LPS and IL-1beta further attenuated nicotine-induced Ca(2+) response. We also found that LPS and IL-1beta induced rearrangement of the F-actin filaments, as measured with an Alexa488-conjugated phalloidin probe. The rearrangements consisted of increases in ring formations and a more dispersed appearance of the filaments. These results indicate that there is a connection between a dysfunction of nicotine Ca(2+) signaling in inflammatory reactive astrocytes and upregulation of IL-1beta and the rearrangements of actin filaments in the cells.