Glial cell activation in the spinal cord and dorsal root ganglia induced by surgery in mice

Widespread hyperexcitability of nociceptor somata outlasts enhanced avoidance behavior after incision injury

ABSTRACT

Nociceptors with somata in dorsal root ganglia (DRGs) readily switch from an electrically silent state to a hyperactive state of tonic, nonaccommodating, low-frequency, irregular discharge of action potentials (APs). Spontaneous activity (SA) during this state is present in vivo in rats months after spinal cord injury (SCI) and has been causally linked to SCI pain. Intrinsically generated SA and, more generally, ongoing activity (OA) are induced by various neuropathic conditions in rats, mice, and humans and are retained in nociceptor somata after dissociation and culturing, providing a powerful tool for investigating its mechanisms and functions. The present study shows that long-lasting hyperexcitability that can generate OA during modest depolarization in probable nociceptors dissociated from DRGs of male and female rats is induced by plantar incision injury. OA occurred when the soma was artificially depolarized to a level within the normal range of membrane potentials where large, transient depolarizing spontaneous fluctuations (DSFs) can approach AP threshold. This hyperexcitability persisted for at least 3 weeks, whereas behavioral indicators of affective pain-hind paw guarding and increased avoidance of a noxious substrate in an operant conflict test-persisted for 1 week or less. The most consistent electrophysiological alteration associated with OA was enhancement of DSFs. An unexpected discovery after plantar incisions was hyperexcitability in neurons from thoracic DRGs that innervate dermatomes distant from the injured tissue. Potential in vivo functions of widespread, low-frequency nociceptor OA consistent with these and other findings are to contribute to hyperalgesic priming and to drive anxiety-related hypervigilance.

Widespread latent hyperactivity of nociceptors outlasts enhanced avoidance behavior following incision injury

Nociceptors with somata in dorsal root ganglia (DRGs) exhibit an unusual readiness to switch from an electrically silent state to a hyperactive state of tonic, nonaccommodating, low-frequency, irregular discharge of action potentials (APs). Ongoing activity (OA) during this state is present in vivo in rats months after spinal cord injury (SCI), and has been causally linked to SCI pain. OA induced by various neuropathic conditions in rats, mice, and humans is retained in nociceptor somata after dissociation and culturing, providing a powerful tool for investigating its mechanisms and functions. An important question is whether similar nociceptor OA is induced by painful conditions other than neuropathy. The present study shows that probable nociceptors dissociated from DRGs of rats subjected to postsurgical pain (induced by plantar incision) exhibit OA. The OA was most apparent when the soma was artificially depolarized to a level within the normal range of membrane potentials where large, transient depolarizing spontaneous fluctuations (DSFs) can approach AP threshold. This latent hyperactivity persisted for at least 3 weeks, whereas behavioral indicators of affective pain - hindpaw guarding and increased avoidance of a noxious substrate in an operant conflict test - persisted for 1 week or less. An unexpected discovery was latent OA in neurons from thoracic DRGs that innervate dermatomes distant from the injured tissue. The most consistent electrophysiological alteration associated with OA was enhancement of DSFs. Potential in vivo functions of widespread, low-frequency nociceptor OA consistent with these and other findings are to amplify hyperalgesic priming and to drive anxiety-related hypervigilance.

Remifentanil does not affect human microglial immune activation in response to pro-inflammatory cytokines

Remifentanil is a potent ultra-short acting μ-opioid analgesic drug, frequently used in anaesthesia due to its favorable pharmacodynamic and pharmacokinetic profile. It may be associated with the occurrence of hyperalgesia. Preclinical studies suggest a potential role of microglia, although the molecular mechanisms have not been fully elucidated. Considering the role of microglia in brain inflammation and the relevant differences among species, the effects of remifentanil were studied on the human microglial C20 cells. The drug was tested at clinically relevant concentrations under basal and inflammatory conditions. In the C20 cells, the expression and secretion of interleukin 6, interleukin 8 and the monocyte chemotactic protein 1 were rapidly induced by a mixture of pro-inflammatory cytokines. This stimulatory effect was sustained up to 24 h. Remifentanil did not exert any toxic effect nor modify the production of these inflammatory mediators, thus suggesting the lack of direct immune modulatory actions on human microglia.

Effects of propofol on macrophage activation and function in diseases

Macrophages work with monocytes and dendritic cells to form a monocyte immune system, which constitutes a powerful cornerstone of the immune system with their powerful antigen presentation and phagocytosis. Macrophages play an essential role in infection, inflammation, tumors and other pathological conditions, but these cells also have non-immune functions, such as regulating lipid metabolism and maintaining homeostasis. Propofol is a commonly used intravenous anesthetic in the clinic. Propofol has sedative, hypnotic, anti-inflammatory and anti-oxidation effects, and it participates in the body’s immunity. The regulation of propofol on immune cells, especially macrophages, has a profound effect on the occurrence and development of human diseases. We summarized the effects of propofol on macrophage migration, recruitment, differentiation, polarization, and pyroptosis, and the regulation of these propofol-regulated macrophage functions in inflammation, infection, tumor, and organ reperfusion injury. The influence of propofol on pathology and prognosis via macrophage regulation is also discussed. A better understanding of the effects of propofol on macrophage activation and function in human diseases will provide a new strategy for the application of clinical narcotic drugs and the treatment of diseases.

How Is Peripheral Injury Signaled to Satellite Glial Cells in Sensory Ganglia?

Injury or inflammation in the peripheral branches of neurons of sensory ganglia causes changes in neuronal properties, including excessive firing, which may underlie chronic pain. The main types of glial cell in these ganglia are satellite glial cells (SGCs), which completely surround neuronal somata. SGCs undergo activation following peripheral lesions, which can enhance neuronal firing. How neuronal injury induces SGC activation has been an open question. Moreover, the mechanisms by which the injury is signaled from the periphery to the ganglia are obscure and may include electrical conduction, axonal and humoral transport, and transmission at the spinal level. We found that peripheral inflammation induced SGC activation and that the messenger between injured neurons and SGCs was nitric oxide (NO), acting by elevating cyclic guanosine monophosphate (cGMP) in SGCs. These results, together with work from other laboratories, indicate that a plausible (but not exclusive) mechanism for neuron-SGCs interactions can be formulated as follows: Firing due to peripheral injury induces NO formation in neuronal somata, which diffuses to SGCs. This stimulates cGMP synthesis in SGCs, leading to their activation and to other changes, which contribute to neuronal hyperexcitability and pain. Other mediators such as proinflammatory cytokines probably also contribute to neuron-SGC communications.

Phenotypical changes of satellite glial cells in a murine model of GM1 -gangliosidosis

Satellite glial cells (SGCs) of dorsal root ganglia (DRG) react in response to various injuries in the nervous system. This study investigates reactive changes within SGCs in a murine model for G_M1‐gangliosidosis (G_M1). DRG of homozygous β‐galactosidase‐knockout mice and homozygous C57BL/6 wild‐type mice were investigated performing immunostaining on formalin‐fixed, paraffin‐embedded tissue. A marked upregulation of glial fibrillary acidic protein (GFAP), the progenitor marker nestin and Ki67 within SGCs of diseased mice, starting after 4 months at the earliest GFAP, along with intracytoplasmic accumulation of ganglioside within neurons and deterioration of clinical signs was identified. Interestingly, nestin‐positive SGCs were detected after 8 months only. No changes regarding inwardly rectifying potassium channel 4.1, 2, 3‐cyclic nucleotide 3‐phosphodiesterase, Sox2, doublecortin, periaxin and caspase3 were observed in SGCs. Iba1 was only detected in close vicinity of SGCs indicating infiltrating or tissue‐resident macrophages. These results indicate that SGCs of DRG show phenotypical changes during the course of G_M1, characterized by GFAP upregulation, proliferation and expression of a neural progenitor marker at a late time point. This points towards an important role of SGCs during neurodegenerative disorders and supports that SGCs represent a multipotent glial precursor cell line with high plasticity and functionality.

Behavioral characterization, potential clinical relevance and mechanisms of latent pain sensitization

Chronic pain is a debilitating disorder that can occur as painful episodes that alternates with bouts of remission and occurs despite healing of the primary insult. Those episodes are often triggered by stressful events. In the last decades, a similar situation has been evidenced in a wide variety of rodent models (including inflammatory pain, neuropathy and opioid-induced hyperalgesia) where animals develop a chronic latent hyperalgesia that silently persists after behavioral signs of pain resolution. This state, referred as latent pain sensitization, is due to the compensatory activation of antinociceptive systems, such as the opioid system or NPY and its receptors. A transitory phase of hyperalgesia can then be reinstated by pharmacological or genetic blockade of these antinociceptive systems or by submitting animals to acute stress. Those observations reveal that there is a constant endogenous analgesia responsible for chronic pain inhibition that might paradoxically contribute to maintain this maladaptive state and could then participate to the transition from acute to chronic pain. Thus, demonstration of the existence of this phenomenon in humans and a better understanding of the mechanisms by which latent pain sensitization develops and maintains over long periods of time will be of particular interest to help identifying new therapeutic strategies and targets for chronic pain treatment. The present review aims to recapitulate behavioral expression, potential clinical relevance, cellular mechanisms and intracellular signaling pathways involved so far in latent pain sensitization.

Phenotypical peculiarities and species-specific differences of canine and murine satellite glial cells of spinal ganglia

Satellite glial cells (SGCs) are located in the spinal ganglia (SG) of the peripheral nervous system and tightly envelop each neuron. They preserve tissue homeostasis, protect neurons and react in response to injury. This study comparatively characterizes the phenotype of murine (mSGCs) and canine SGCs (cSGCs). Immunohistochemistry and immunofluorescence as well as 2D and 3D imaging techniques were performed to describe a SGC-specific marker panel, identify potential functional subsets and other phenotypical, species-specific peculiarities. Glutamine synthetase (GS) and the potassium channel Kir 4.1 are SGC-specific markers in murine and canine SG. Furthermore, a subset of mSGCs showed CD45 immunoreactivity and the majority of mSGCs were immunopositive for neural/glial antigen 2 (NG2), indicating an immune and a progenitor cell character. The majority of cSGCs were immunopositive for glial fibrillary acidic protein (GFAP), 2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNPase) and Sox2. Therefore, cSGCs resemble central nervous system glial cells and progenitor cells. SGCs lacked expression of macrophage markers CD107b, Iba1 and CD204. Double labelling with GS/Kir 4.1 highlights the unique anatomy of SGC-neuron units and emphasizes the indispensability of further staining and imaging techniques for closer insights into the specific distribution of markers and potential colocalizations.

Emerging importance of satellite glia in nervous system function and dysfunction

Satellite glial cells (SGCs) closely envelop cell bodies of neurons in sensory, sympathetic and parasympathetic ganglia. This unique organization is not found elsewhere in the nervous system. SGCs in sensory ganglia are activated by numerous types of nerve injury and inflammation. The activation includes upregulation of glial fibrillary acidic protein, stronger gap junction-mediated SGC-SGC and neuron-SGC coupling, increased sensitivity to ATP, downregulation of Kir4.1 potassium channels and increased cytokine synthesis and release. There is evidence that these changes in SGCs contribute to chronic pain by augmenting neuronal activity and that these changes are consistent in various rodent pain models and likely also in human pain. Therefore, understanding these changes and the resulting abnormal interactions of SGCs with sensory neurons could provide a mechanistic approach that might be exploited therapeutically in alleviation and prevention of pain. We describe how SGCs are altered in rodent models of four common types of pain: systemic inflammation (sickness behaviour), post-surgical pain, diabetic neuropathic pain and post-herpetic pain.

Colchicine alleviates acute postoperative pain but delays wound repair in mice: roles of neutrophils and macrophages

Background

Acute postoperative pain is induced by most incisional surgeries and usually resolves with wound repair. However, many patients experience moderate to severe pain despite receiving currently available postoperative pain relief. Accumulating evidence suggests that inflammatory cells, neutrophils, and macrophages infiltrating the wound site contribute to the acute inflammation, pain, and subsequent wound repair. Colchicine is commonly used to relieve pain in gout by inhibiting the infiltration of granulocytes and other motile cells. In this study, we examined the effects of colchicine on acute postoperative pain and wound repair by correlating the infiltration of neutrophils and macrophages in a mouse model of postoperative pain induced by plantar incision. Furthermore, these effects of colchicine were compared with clodronate liposomes, which selectively deplete circulating macrophages.

Results

Plantar incision induced mechanical hypersensitivity in the ipsilateral hind paw that peaked one day and lasted for three days after the surgery. Treatment with colchicine significantly attenuated the early infiltration of Gr1-positive cells (neutrophils) around the incision site and mechanical hypersensitivity, which was accompanied with inhibition of the subsequent infiltration of Iba1-positive cells (macrophages) and macrophage polarization toward the proinflammatory M1 phenotype. By contrast, an intravenous injection of clodronate liposomes significantly inhibited the infiltration of macrophages around the incision site but had little effect on the infiltration of neutrophils or mechanical hypersensitivity. Importantly, colchicine treatment significantly delayed wound closure after the incisional surgery, whereas clodronate liposome administration had no effect on wound closure.

Conclusion

These results suggest that colchicine can alleviate acute postoperative pain and also enhance the risk of delayed wound repair, which are associated with the suppression of neutrophil and subsequent proinflammatory M1 macrophage infiltration around the incision site, while the involvement of macrophages may be limited.

Persistent Postsurgical Pain

The development of chronic pain is considered a major complication after surgery. Basic science research in animal models helps us understand the transition from acute to chronic pain by identifying the numerous molecular and cellular changes that occur in the peripheral and central nervous systems. It is now well recognized that inflammation and nerve injury lead to long-term synaptic plasticity that amplifies and also maintains pain signaling, a phenomenon referred to as pain sensitization. In the context of surgery in humans, pain sensitization is both responsible for an increase in postoperative pain via the expression of wound hyperalgesia and considered a critical factor for the development of persistent postsurgical pain. Using specific drugs that block the processes of pain sensitization reduces postoperative pain and prevents the development of persistent postoperative pain. This narrative review of the literature describes clinical investigations evaluating different preventative pharmacologic strategies that are routinely used by anesthesiologists in their daily clinical practices for preventing persistent postoperative pain. Nevertheless, further efforts are needed in both basic and clinical science research to identify preclinical models and novel therapeutics targets. There remains a need for more patient numbers in clinical research, for more reliable data, and for the development of the safest and the most effective strategies to limit the incidence of persistent postoperative pain.

Analgesic effects and pharmacologic mechanisms of the Gelsemium alkaloid koumine on a rat model of postoperative pain

Postoperative pain (POP) of various durations is a common complication of surgical procedures. POP is caused by nerve damage and inflammatory responses that are difficult to treat. The neuroinflammation-glia-steroid network is known to be important in POP. It has been reported that the Gelsemium alkaloid koumine possesses analgesic, anti-inflammatory and neurosteroid modulating activities. This study was undertaken to test the analgesic effects of koumine against POP and explore the underlying pharmacologic mechanisms. Our results showed that microglia and astroglia were activated in the spinal dorsal horn post-incision, along with an increase of proinflammatory cytokines (interleukin 1β, interleukin 6, and tumor necrosis factor α). Both subcutaneous and intrathecal (i.t.) koumine treatment after incision significantly prevented mechanical allodynia and thermal hyperalgesia, inhibited microglial and astroglial activation, and suppressed expression of proinflammatory cytokines. Moreover, the analgesic effects of koumine were antagonized by i.t. administration of translocator protein (18 kDa) (TSPO) antagonist PK11195 and GABAA receptor antagonist bicuculline. Together, koumine prevented mechanical allodynia and thermal hyperalgesia caused by POP. The pharmacologic mechanism of koumine-mediated analgesia might involve inhibition of spinal neuroinflammation and activation of TSPO. These data suggested that koumine might be a potential pharmacotherapy for the management of POP.

Transplantation of wild-type mouse hematopoietic stem and progenitor cells ameliorates deficits in a mouse model of Friedreich's ataxia

Friedreich's ataxia (FRDA) is an incurable autosomal recessive neurodegenerative disease caused by reduced expression of the mitochondrial protein frataxin due to an intronic GAA-repeat expansion in the FXN gene. We report the therapeutic efficacy of transplanting wild-type mouse hematopoietic stem and progenitor cells (HSPCs) into the YG8R mouse model of FRDA. In the HSPC-transplanted YG8R mice, development of muscle weakness and locomotor deficits was abrogated as was degeneration of large sensory neurons in the dorsal root ganglia (DRGs) and mitochondrial capacity was improved in brain, skeletal muscle, and heart. Transplanted HSPCs engrafted and then differentiated into microglia in the brain and spinal cord and into macrophages in the DRGs, heart, and muscle of YG8R FRDA mice. We observed the transfer of wild-type frataxin and Cox8 mitochondrial proteins from HSPC-derived microglia/macrophages to FRDA mouse neurons and muscle myocytes in vivo. Our results show the HSPC-mediated phenotypic rescue of FRDA in YG8R mice and suggest that this approach should be investigated further as a strategy for treating FRDA.

Antihyperalgesic effects of dexketoprofen and tramadol in a model of postoperative pain in mice - effects on glial cell activation

OBJECTIVES

To define likely targets (i.e. glia) and protocols (analgesic combinations) to improve postoperative pain outcomes and reduce chronic pain after surgery. Specifically, to assess the antihyperalgesic effects of the dexketoprofen : tramadol (DEX : TRM) combination, exploring the implication of glial activation.

METHODS

In a mouse model of postincisional pain, we evaluated mechanical nociceptive thresholds (Von Frey) for 21 days postoperatively. We assessed DEX and TRM alone and combined (1 : 1 ratio) on postoperative hyperalgesia (POH, day 1) and delayed latent pain sensitisation (substantiated by a naloxone challenge; PS, day 21). The interactions were analysed using isobolograms, and concomitant changes in spinal glial cell activation were measured.

KEY FINDINGS

On day 1, DEX completely blocked POH, whereas TRM induced 32% inhibition. TRM, but not DEX, partially (47%) protected against PS, at 21 days. Co-administration of DEX : TRM (1 : 1 ratio) showed additivity for antihyperalgesia. Both drugs and their combination totally inhibited surgery-induced microglia activation on day 1, but had no effect on surgery-induced astrocyte activation (1 day) or re-activation after naloxone (21 days).

CONCLUSIONS

The DEX : TRM combination could have clinical advantages: a complete prevention of POH after surgery, together with a substantial (48%) inhibition of the development of PS by TRM. Microglia, but not astrocyte activation, could play a relevant role in the development of postoperative pain hypersensitivity.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Inhibition of N-myc Downstream–regulated Gene-2 Is Involved in an Astrocyte-specific Neuroprotection Induced by Sevoflurane Preconditioning

Background:

Mechanism of sevoflurane preconditioning–induced cerebral ischemic tolerance is unclear. This study investigates the role of N-myc downstream–regulated gene-2 (NDRG2) in the neuroprotection of sevoflurane preconditioning in ischemic model both in vivo and in vitro.

Methods:

At 2 h after sevoflurane (2%) preconditioning for 1 h, rats were subjected to middle cerebral artery occlusion for 120 min. Neurobehavioral scores (n = 10), infarct volumes (n = 10), cellular apoptosis (n = 6), and NDRG2 expression (n = 6) were determined at 24 h after reperfusion. In vitro, cultural astrocytes were exposed to oxygen–glucose deprivation for 4 h. Cellular viability, cytotoxicity, apoptosis, and NDRG2 expression (n = 6) were evaluated in the presence or absence of NDRG2-specific small interfering RNA or NDRG2 overexpression plasmid.

Results:

Sevoflurane preconditioning decreased apoptosis (terminal deoxynucleotidyl transferase–mediated 2’-deoxyuridine 5’-triphosphate nick-end labeling–positive cells reduced to 31.2 ± 5.3% and cleaved Caspase-3 reduced to 1.42 ± 0.21 fold) and inhibited NDRG2 expression (1.28 ± 0.15 fold) and nuclear translocation (2.21 ± 0.29 fold) in ischemic penumbra. Similar effects were observed in cultural astrocytes exposed to oxygen–glucose deprivation. NDRG2 knockdown by small interfering RNA attenuated oxygen–glucose deprivation–induced injury (cell viability increased to 80.5 ± 4.1%; lactate dehydrogenase release reduced to 30.5 ± 4.0%) and cellular apoptosis (cleaved Caspase-3 reduced to 1.55 ± 0.21 fold; terminal deoxynucleotidyl transferase–mediated 2’-deoxyuridine 5’-triphosphate nick-end labeling–positive cells reduced to 18.2 ± 4.3%), whereas NDRG2 overexpression reversed the protective effects of sevoflurane preconditioning. All the data are presented as mean ± SD.

Conclusion:

Sevoflurane preconditioning inhibits NDRG2 up-regulation and nuclear translocation in astrocytes to induce cerebral ischemic tolerance via antiapoptosis, which represents one new mechanism of sevoflurane preconditioning and provides a novel target for neuroprotection.

Endogenous PI3K/Akt and NMDAR act independently in the regulation of CREB activity in lumbosacral spinal cord in cystitis

The integral interaction of signaling components in the regulation of visceral inflammation-induced central sensitization in the spinal cord has not been well studied. Here we report that phosphoinositide 3-kinase (PI3K)-dependent Akt activation and N-methyl-d-aspartic acid receptor (NMDAR) in lumbosacral spinal cord independently regulate the activation of cAMP response element-binding protein (CREB) in vivo in a rat visceral pain model of cystitis induced by intraperitoneal injection of cyclophosphamide (CYP). We demonstrate that suppression of endogenous PI3K/Akt activity with a potent PI3K inhibitor LY294002 reverses CYP-induced phosphorylation of CREB, however, it has no effect on CYP-induced phosphorylation of NR1 at Ser(897) and Ser(896); conversely, inhibition of NMDAR in vivo with MK801 fails to block CYP-induced Akt activation but significantly attenuates CYP-induced CREB phosphorylation in lumbosacral spinal cord. This novel interrelationship of PI3K/Akt, NMDAR, and CREB activation in lumbosacral spinal cord is further confirmed in an ex vivo spinal slice culture system exposed to an excitatory neurotransmitter calcitonin gene-related peptide (CGRP). Consistently we found that CGRP-triggered CREB activation can be blocked by both PI3K inhibitor LY294002 and NMDAR antagonists MK801 and D-AP5. However, CGRP-triggered Akt activation cannot be blocked by MK801 or D-AP5; vice versa, LY294002 pretreatment that suppresses the Akt activity fails to reverse CGRP-elicited NR1 phosphorylation. These results suggest that PI3K/Akt and NMDAR independently regulate spinal plasticity in visceral pain model, and target of a single pathway is necessary but not sufficient in treatment of visceral hypersensitivity.