Multiple mechanisms of microglia: a gatekeeper's contribution to pain states

Microglia are gatekeepers in the CNS for a wide range of pathological stimuli and they blow the whistle when things go wrong. Collectively, microglia form a CNS tissue alarm system (Kreutzberg's "sensor of pathology"), and their involvement in physiological pain is in line with this function. However, pathological neuropathic pain is characterized by microglial activation that is unwanted and considered to contribute to or even cause tactile allodynia, hyperalgesia and spontaneous pain. Such abnormal microglial behavior seems likely due to an as yet ill-understood disturbance of microglial functions unrelated to inflammation. The idea that microglia have roles in the CNS that differ from those of peripheral macrophages has gained momentum with the discovery of their separate, pre-hematopoietic lineage during embryonic development and their direct interactions with synapses.     

Scratching activates microglia in the mouse spinal cord

This study tested the hypothesis that repetitive scratching provoked by two known pruritogens, compound 48/80 and 5′‐guanidinonaltrindole (GNTI), is accompanied by activation of microglial cells in the mouse spinal cord. Immunohistochemical studies revealed that the complement receptor 3, also known as cluster determinant 11b (CD11b), a cell surface marker of microglial cells, was upregulated in the spinal cord 10–30 min after a subcutaneous (s.c.) injection of compound 48/80 (50 μg/100 μl) or GNTI (0.3 mg/kg) to the back of the mouse neck. Numerous intensely labeled CD11b‐immunoreactive (CD11b‐ir) cells, with the appearance of hypertrophic reactive microglia, were distributed throughout the gray and white matter. In contrast, weakly labeled CD11b‐ir cells were distributed in the spinal cord from mice injected with saline. Western blots showed that CD11b expression levels were significantly increased in spinal cords of mice injected s.c. with either pruritogen, reached a peak response in about 30 min, and declined to about the basal level in the ensuing 60 min. In addition, phospho‐p38 (p‐p38) but not p38 levels were upregulated in spinal cords from mice injected with compound 48/80 or GNTI, with a time course parallel to that of CD11b expression. Pretreatment of the mice with nalfurafine (20 µg/kg; s.c.), a κ‐opioid receptor agonist that has been shown to suppress scratching, reduced CD11b and p‐p38 expression induced by either pruritogen. The results demonstrate, for the first time, that scratch behavior induced by the pruritogens GNTI and compound 48/80 is accompanied by a parallel activation of microglial cells in the spinal cord. © 2014 Wiley Periodicals, Inc.     

Plastic responses to spinal cord injury

The commonly accepted view of a highly rigid, hardwired central nervous system has evolved considerably over the past few decades. The adult central nervous system is now known to be capable of significant functional reorganization, frequently referred to as plasticity, in order to adapt to a changing environment or to a change in the CNS hardware itself, for example after trauma. Focusing on the motor system, I will discuss the cellular responses that occur after spinal cord injury and that illustrate distinct plastic events occurring at the lesion site as well as at other levels of the neuraxis. These plastic responses are not restricted to neuronal cells and to the formation of new circuitries, but can also be illustrated by the change of morphology, fate and biochemical properties of non-neuronal cells (i.e. astrocytes and neural precursor cells). These cellular responses interact with one another and contribute, together, to the tissue remodeling and to the sparing or recovery of some motor functions. Understanding these cellular responses as well as their interrelations is necessary to find appropriate approaches to manipulate one with no detrimental effects to the others. This is a prerequisite for the development of new and effective therapeutic strategies for the treatment of patients with CNS injuries.     

Localization of microglia in the human fetal cervical spinal cord.

Differential morphologic subtypes of microglia have been identified in the human fetal frontal cerebrum using a lectin, Ricinus communis agglutinin 1 (RCA-1), and a monoclonal antibody, EBM-11. In this report, microglia were characterized in the human fetal cervical spinal cord. RCA-1-positive microglia were ramified in the developing gray matter while in the developing white matter they had a less differentiated (ameboid) appearance. EBM-11, a monoclonal antibody that recognizes CD68 on human macrophages, and microglia labeled only ameboid-type microglia in the developing white matter. This suggests that distinct subpopulations of microglia exist, which may represent different stages in microglial development, and that CD68 may be a differentiation marker for less mature forms. Therefore, cytologically less differentiated forms of microglia appear to be associated with myelination.     

Reaction of spinal cord central canal cells to cord transection and their contribution to cord regeneration

After transection, the spinal cord of the eel Anguilla quickly regrows and reconnects, and function recovers. We describe here the changes in the central canal region that accompany this regeneration by using serial semithin plastic sections and immunohistochemistry. The progress of axonal regrowth was followed in material labeled with DiI. The canal of the uninjured cord is surrounded by four cell types: S-100-immunopositive ependymocytes, S-100- and glial fibrillary acidic protein (GFAP)-immunopositive tanycytes, vimentin-immunopositive dorsally located cells, and lateral and ventral liquor-contacting neurons, which label for either gamma-aminobutyric acid (GABA) or tyrosine hydroxylase (TH). After cord transection, a new central canal forms rapidly as small groups of cells at the leading edges of the transection create flat "plates" that serve as templates for subsequent formation of the lateral and dorsal walls. Profile counts and 5-bromo-2'-deoxyuridine immunohistochemistry indicate that these cells are dividing rapidly during the first 20 days of the repair process. The newly formed canal, which bridges the transection by day 10 but is not complete until about day 20, is greatly enlarged (相似文献   

Chromaffin allografts into arachnoid of spinal cord reduce basal pain responses in rats.

In this present study, behavioral responses to a subcutaneous formalin test for pain are evaluated in rats that previously received an allograft of adrenal chromaffin tissue into arachnoid of the dorsal spinal cord and in control animals. In the group of rats with grafts, a significant basal analgesia, reversed by the opioid antagonist naloxone, is found. These findings suggest that the grafts secrete some substance that reduces the response to painful stimulation and whose action is blocked by naloxone.     

Lumbar spinal cord responses to limb vein distention

The purpose of this study was to determine if central neural responses were elicited by distention of limb veins, and to compare the pattern of these response to those produced in previous studies using electrical stimulation to excite limb venous afferent fibers. Spinal evoked potentials were measured in response to stretch of the wall of a segment of the femoral-saphenous vein by perfusion-distention or by mechanical stretch. These studies revealed that spinal cord evoked potentials were elicited by these procedures, and that the activated venous afferent fibers coursed through the saphenous nerve and entered the sixth lumber spinal cord segment. The minimum stretches which were required to elicit spinal evoked potentials were produced by perfusion pressures starting at 2-3 mm Hg, or by mechanical stretch of the wall of 5 micron/mm. A vein wall proprioceptor hypothesis is proposed and discussed in the light of these findings. In addition to the cord dorsum evoked potentials, distention or stretch of the vein wall elicited ventral root potentials (excitatory postsynaptic population potentials) which are known to be produced by excitatory inputs to motoneurons. A venous afferent mediated muscle-tonus venopressor mechanism hypothesis is proposed and discussed in the light of these and previous findings.     

Experimental autoimmune neuritis induces differential microglia activation in the rat spinal cord

The reactive spatial and temporal activation pattern of parenchymal spinal cord microglia was analyzed in rat experimental autoimmune neuritis (EAN). We observed a differential activation of spinal cord microglial cells. A significant increase in ED1⁺ microglia predominantly located in the dorsal horn grey matter of lumbar and thoracic spinal cord levels was observed on Day 12. As revealed by morphological criteria and by staining with further activation markers [allograft inflammatory factor 1 (AIF-1), EMAPII, OX6, P2X₄R], reactive microglia did not reach a macrophage-like state of full activation. On Day 12, a significant proliferative response could be observed, affecting all spinal cord areas and including ED1⁺ microglial cells and a wide range of putative progenitor cells. Thus, in rat EAN, a reactive localized and distinct microglial activation correlating with a generalized proliferative response could be observed.     

Return to work after spinal cord injury: the potential contribution of physical fitness.

The history, physical characteristics and fitness status of 60 persons who had sustained a spinal cord injury at least 3 years previously were considered in relation to current occupation. All subjects had completed their education, 39 being gainfully employed and 21 unemployed. The general characteristics of the sample, mainly beneficiaries of the Quebec Automobile Insurance Plan, were typical of spinal cord injured individuals in North America. The working group had a significantly higher current level of education than those who were unemployed (p less than .01). In terms of physical fitness, the workers were lighter, with a lower body mass index and a higher aerobic power (p less than .05). Isokinetic testing suggested a trend toward a higher peak torque in the workers. The total work performed (Nm.kg-1) during an isokinetic endurance test (25 biphasic contraction at 180 degrees.sec-1) was significantly higher in the workers, suggesting that such muscular endurance might be even more useful than greater peak isokinetic strength during vocational activities. However, the likelihood of employment was unrelated to habitual patterns of either aerobic exercise or overall physical activity. No significant differences of physical fitness or physical activity habits were found between workers holding sedentary versus physically demanding jobs. The results verified the positive relationship between physical fitness (body composition, aerobic power, muscular endurance) and the gainful employment of paraplegics, but failed to show any significant relationship between physical fitness and the acceptance of physically demanding work by such individuals.     

Diagnostic spinal anaesthesia in chronic spinal cord injury pain.

In a double blind study, 21 patients with chronic spinal cord injury (SCI) pain underwent placement of a lumbar subarachnoid catheter and injection of placebo and lidocaine. The effects on pain intensity, distribution, altered sensations and sensory level of anaesthesia were monitored. Four patients responded briefly to placebo, while 13 demonstrated a mean reduction of pain intensity of 37.8 +/- 37% for a mean duration of 123.1 +/- 95.3 minutes in response to lidocaine. The pain response to subarachnoid lidocaine differed significantly (p less than 0.01) from placebo. Spinal anaesthesia was also associated with changes in pain distribution and altered sensation. A spinal anaesthetic-induced sensory level could not be achieved cephalad to the sensory level of neurological injury in 5 patients who presented with spinal canal obstruction. This study has demonstrated that response to diagnostic spinal anaesthesia in chronic SCI pain is complex, requiring individual interpretation in each patient and consideration of the following factors; symptomatology, etiology, pain perception, spinal canal anatomy, CSF chemistry and local anaesthetic pharmacology.     

Psychological approach to the rehabilitation of the spinal cord injured: the contribution of relaxation techniques.

We analyse the benefit of learning relaxation techniques as an essential coping strategy in the behavioural medicine field. This has proved useful as a part of the newly spinal cord injured rehabilitation treatment or concerning later problems if there is readmission. We report the changes we have made in the relaxation standard methods to be used in spinal cord injured patients as well as the timing in the rehabilitation process when these techniques were applied.     

Mechanisms of spinal cord stimulation in neuropathic pain

The understanding of the mode of action of spinal cord stimulation (SCS) as treatment of neuropathic pain is still fragmentary. SCS evolved from the gate-control theory postulating a spinal modulation of noxious inflow, but there is little evidence that SCS influences nociceptive pain; pain relief in peripheral vascular disease and angina pectoris is presumably secondary to other SCS effects. In man, SCS may effectively abolish both continuous and evoked pain (tactile/thermal allodynia) whereas induced, acute nociceptive pain is unaffected. Recent SCS studies performed on rat models of mononeuropathy have demonstrated a preferential effect on A beta fiber mediated functions, and the hyperexcitability of wide-dynamic-range dorsal horn neurons was attenuated. These effects were coupled to increased release of GABA and reduced glutamate and aspartate release in the dorsal horn. Intrathecal administration of GABA, baclofen and adenosine enhanced the SCS effect on tactile allodynia even in previously non-responsive rats. Preliminary results indicate that gabapentin may have a similar effect. GABAergic and adenosine-related mechanisms conceivably represent only examples of a number of putative receptor systems involved in SCS. Clinical trials have been initiated exploring the possibility to improve the efficacy of SCS by concomitant pharmacotherapy.     

Neurogenic pain and steroid synthesis in the spinal cord

The spinal cord (SC) is a biosynthetic center for neurosteroids, including pregnenolone (PREG), progesterone (PROG), and 3alpha/5alpha-tetrahydroprogesterone (3alpha/5alpha-THP). In particular, an active form of cytochrome P450 sidechain cleavage (P450scc) has been localized in sensory networks of the rat SC dorsal horn (DH). P450scc is the key enzyme catalyzing the conversion of cholesterol (CHOL) into PREG, the rate-limiting step in the biosynthesis of all classes of steroids. To determine whether neurosteroidogenesis might be involved in the pivotal role played by the DH in nociception, effects of neurogenic pain provoked by sciatic nerve ligature were investigated on P450scc expression, cellular distribution, and activity in the SC. P450scc mRNA concentration was threefold higher in the DH of neuropathic rats than in controls. The nerve ligature also increased the density of P450sccpositive neuronal perykarya and fibers in the ipsilateral DH. Incubation of spinal tissue homogenates with [3H]CHOL revealed that the amount of newly synthesized [3H]PREG from [3H]CHOLwas 80% higher in the DH of neuropathic rats. Radioimmunoassays showed an increase of PREG and 3alpha/5alpha-THP concentrations in neuropathic rat DH. The upregulation of PREG and 3alpha/5alpha-THP biosynthesis might be involved in endogenous mechanisms triggered by neuropathic rats to cope with the chronic pain state. 3alpha/5alpha-THP formation from PREG can also generate PROG, which decreases sensitivity to pain and protects nerve cells against degeneration. Because apoptotic cell death has been demonstrated in the DH during neuropathic pain, activation of neurosteroidogenesis in spinal tissues might also be correlated to the neuroprotective role of steroids in the SC.     

Mechanisms of spinal cord stimulation in neuropathic pain

Abstract

The understanding of the mode of action of spinal cord stimulation (SCS) as treatment of neuropathic pain is still fragmentary. SCS evolved from the gate-control theory postulating a spinal modulation of noxious inflow, but there is little evidence that SCS influences nociceptive pain; pain relief in peripheral vascular disease and angina pectoris is presumably secondary to other SCS effects. In man, SCS may effectively abolish both continuous and evoked pain (tactile/thermal allodynia) whereas induced, acute nociceptive pain is unaffected. Recent SCS studies performed on rat models of mononeuropathy have demonstrated a preferential effect on AB fiber mediated functions, and the hyperexcitability of wide-dynamic-range dorsal horn neurons was attenuated. These effects were coupled to increased release of CABA and reduced glutamate and aspartate release in the dorsal horn. Intrathecal administration of GABA, baclofen and adenosine enhanced the SCS effect on tactile allodynia even in previously non-responsive rats. Preliminary results indicate that gabapentin may have a similar effect. GABAergic and adenosine-related mechanisms conceivably represent only examples of a number of putative receptor systems involved in SCS. Clinical trials have been initiated exploring the possibility to improve the efficacy of SCS by concomitant pharmacotherapy. [Neurol Res 2000; 22: 28S-292]     

Function of microglia and macrophages in secondary damage after spinal cord injury

Spinal cord injury （SCI） is a devastating type of neurological trauma with limited therapeutic op- portunities. The pathophysiology of SCI involves primary and secondary mechanisms of injury. Among all the secondary injury mechanisms, the inflammatory response is the major contrib- utor and results in expansion of the lesion and further loss of neurologic function. Meanwhile, the inflammation directly and indirectly dominates the outcomes of SCI, including not only pain and motor dysfunction, but also preventingneuronal regeneration. Microglia and macrophages play very important roles in secondary injury. Microglia reside in spinal parenchyma and survey the microenvironment through the signals of injury or infection. Macrophages are derived from monocytes recruited to injured sites from the peripheral circulation. Activated resident microglia and monocyte-derived macrophages induce and magnify immune and inflammatory responses not only by means of their secretory moleculesand phagocytosis, but also through their influence on astrocytes, oligodendrocytes and demyelination. In this review, we focus on the roles of mi- croglia and macrophages in secondary injury and how they contribute to the sequelae of SCI.     

Recruitment of activated microglia cells in the spinal cord of mice by ALS IgG

Mice were injected i.p. with IgG samples of different patients to test whether IgG from amyotrophic lateral sclerosis (ALS) can initiate an immune/inflammatory reaction targeting motor neurons. All IgG samples of five ALS patients and none of the disease controls recruited activated microglia cells in the ventral horn of the spinal cord. CD3 lymphocytes were not accumulated in the same tissue. Similar reaction was evoked by injection of IgG from guinea pigs with experimental autoimmune gray matter disease (EAGMD) induced by immunization with the homogenate of the ventral horn of bovine spinal cord. The results indicate that ALS IgG and anti-motoneuron IgG induce microglia reaction targeting motor neurons without initiating T cell response in the recipient mice.     

Changes in vascular permeability in the spinal cord contribute to chemotherapy-induced neuropathic pain

Chemotherapy-induced neuropathic pain is a dose-limiting side effect of many cancer therapies due to their propensity to accumulate in peripheral nerves, which is facilitated by the permeability of the blood-nerve barrier. Preclinically, the chemotherapy agent vincristine (VCR) activates endothelial cells in the murine peripheral nervous system and in doing so allows the infiltration of monocytes into nerve tissue where they orchestrate the development of VCR-induced nociceptive hypersensitivity. In this study we demonstrate that VCR also activates endothelial cells in the murine central nervous system, increases paracellular permeability and decreases trans endothelial resistance. In in vivo imaging studies in mice, VCR administration results in trafficking of inflammatory monocytes through the endothelium. Indeed, VCR treatment affects the integrity of the blood-spinal cord-barrier as indicated by Evans Blue extravasation, disrupts tight junction coupling and is accompanied by the presence of monocytes in the spinal cord. Such inflammatory monocytes (Iba-1⁺ CCR₂⁺ Ly6C⁺ TMEM119^- cells) that infiltrate the spinal cord also express the pro-nociceptive cysteine protease Cathepsin S. Systemic treatment with a CNS-penetrant, but not a peripherally-restricted, inhibitor of Cathepsin S prevents the development of VCR-induced hypersensitivity, suggesting that infiltrating monocytes play a functional role in sensitising spinal cord nociceptive neurons. Our findings guide us towards a better understanding of central mechanisms of pain associated with VCR treatment and thus pave the way for the development of innovative antinociceptive strategies.