Inflammation in sympathetic ganglia proximal to sciatic nerve transection in rats

Micropatterns and peptide gradient on the inner surface of a guidance conduit synergistically promotes nerve regeneration in vivo

Both of the surface topographical features and distribution of biochemical cues can influence the cell-substrate interactions and thereby tissue regeneration in vivo. However, they have not been combined simultaneously onto a biodegradable scaffold to demonstrate the synergistic role so far. In this study, a proof-of-concept study is performed to prepare micropatterns and peptide gradient on the inner wall of a poly (D,L-lactide-co-caprolactone) (PLCL) guidance conduit and its advantages in regeneration of peripheral nerve in vivo. After linear ridges/grooves of 20/40 μm in width are created on the PLCL film, its surface is aminolyzed in a kinetically controlled manner to obtain the continuous gradient of amino groups, which are then transferred to CQAASIKVAV peptide density gradient via covalent coupling of glutaraldehyde. The Schwann cells are better aligned along with the stripes, and show a faster migration rate toward the region of higher peptide density. Implantation of the nerve guidance conduit made of the PLCL film having both the micropatterns and peptide gradient can significantly accelerate the regeneration of sciatic nerve in terms of rate, function recovery and microstructures, and reduction of fibrosis in muscle tissues. Moreover, this nerve conduit can also benefit the M2 polarization of macrophages and promote vascularization in vivo.

The Role of the IL-4 Signaling Pathway in Traumatic Nerve Injuries

Following traumatic peripheral nerve injury, adequate restoration of function remains an elusive clinical goal. Recent research highlights the complex role that the immune system plays in both nerve injury and regeneration. Pro-regenerative processes in wounded soft tissues appear to be significantly mediated by cytokines of the type 2 immune response, notably interleukin (IL)-4. While IL-4 signaling has been firmly established as a critical element in general tissue regeneration during wound healing, it has also emerged as a critical process in nerve injury and regeneration. In this context of peripheral nerve injury, endogenous IL-4 signaling has recently been confirmed to influence more than leukocytes, but including also neurons, axons, and Schwann cells. Given the role IL-4 plays in nerve injury and regeneration, exogenous IL-4 and/or compounds targeting this signaling pathway have shown encouraging preliminary results to treat nerve injury or other neuropathy in rodent models. In particular, the exogenous stimulation of the IL-4 signaling pathway appears to promote postinjury neuron survival, axonal regeneration, remyelination, and thereby improved functional recovery. These preclinical data strongly suggest that targeting IL-4 signaling pathways is a promising translational therapy to augment treatment approaches of traumatic nerve injury. However, a better understanding of the type 2 immune response and associated signaling networks functioning within the nerve injury microenvironment is still needed to fully develop this promising therapeutic avenue.

Satellite Glial Cells in Pain Research: A Targeted Viewpoint of Potential and Future Directions

Chronic pain is known to be caused by sensitization within the pain circuits. An imbalance occurs between excitatory and inhibitory transmission that enables this sensitization to form. In addition to neurons, the contribution of central glia, especially astrocytes and microglia, to the pathogenesis of pain induction and maintenance has been identified. This has led to the targeting of astrogliosis and microgliosis to restore the normal functions of astrocytes and microglia to help reverse chronic pain. Gliosis is broadly defined as a reactive response of glial cells in response to insults to the central nervous system (CNS). The role of glia in the peripheral nervous system (PNS) has been less investigated. Accumulating evidence, however, points to the contribution of satellite glial cells (SGCs) to chronic pain. Hence, understanding the potential role of these cells and their interaction with sensory neurons has become important for identifying the mechanisms underlying pain signaling. This would, in turn, provide future therapeutic options to target pain. Here, a viewpoint will be presented regarding potential future directions in pain research, with a focus on SGCs to trigger further research. Promising avenues and new directions include the potential use of cell lines, cell live imaging, computational analysis, 3D tissue prints and new markers, investigation of glia–glia and macrophage–glia interactions, the time course of glial activation under acute and chronic pathological pain compared with spontaneous pain, pharmacological and non-pharmacological responses of glia, and potential restoration of normal function of glia considering sex-related differences.

The effects of sympathetic nerve damage on satellite glial cells in the mouse superior cervical ganglion

Neurons in sensory, sympathetic, and parasympathetic ganglia are surrounded by satellite glial cell (SGCs). There is little information on the effects of nerve damage on SGCs in autonomic ganglia. We studied the consequences of damage to sympathetic nerve terminals by 6-hydroxydopamine (6-OHDA) on SGCs in the mouse superior cervical ganglia (Sup-CG). Immunostaining revealed that at 1-30 d post-6-OHDA injection, SGCs in Sup-CG were activated, as assayed by upregulation of glial fibrillary acidic protein. Intracellular labeling showed that dye coupling between SGCs around different neurons increased 4-6-fold 1-14 d after 6-OHDA injection. Behavioral testing 1-7 d post-6-OHDA showed that withdrawal threshold to tactile stimulation of the hind paws was reduced by 65-85%, consistent with hypersensitivity. A single intraperitoneal injection of the gap junction blocker carbenoxolone restored normal tactile thresholds in 6-OHDA-treated mice, suggesting a contribution of SGC gap junctions to pain. Using calcium imaging we found that after 6-OHDA treatment responses of SGCs to ATP were increased by about 30% compared with controls, but responses to ACh were reduced by 48%. The same experiments for SGCs in trigeminal ganglia from 6-OHDA injected mice showed no difference from controls, confirming that 6-OHDA acted selectively on sympathetic nerves. However, systemic inflammation induced by lipopolysaccharide did not affect SGCs of Sup-CG, but did influence SGCs in trigeminal ganglia in the same manner as 6-OHDA did on SGCs in Sup-CG. We conclude that even though SGCs in sympathetic and sensory ganglia are morphologically similar, they are quite different functionally, particularly after damage.

Macrophage biology in the peripheral nervous system after injury

Neuroinflammation has positive and negative effects. This review focuses on the roles of macrophage in the PNS. Transection of PNS axons leads to degeneration and clearance of the distal nerve and to changes in the region of the axotomized cell bodies. In both locations, resident and infiltrating macrophages are found. Macrophages enter these areas in response to expression of the chemokine CCL2 acting on the macrophage receptor CCR2. In the distal nerve, macrophages and other phagocytes are involved in clearance of axonal debris, which removes molecules that inhibit nerve regeneration. In the cell body region, macrophage trigger the conditioning lesion response, a process in which neurons increase their regeneration after a prior lesion. In mice in which the genes for CCL2 or CCR2 are deleted, neither macrophage infiltration nor the conditioning lesion response occurs in dorsal root ganglia (DRG). Macrophages exist in different phenotypes depending on their environment. These phenotypes have different effects on axonal clearance and neurite outgrowth. The mechanism by which macrophages affect neuronal cell bodies is still under study. Overexpression of CCL2 in DRG in uninjured animals leads to macrophage accumulation in the ganglia and to an increase in the growth potential of DRG neurons. This increased growth requires activation of neuronal STAT3. In contrast, in acute demyelinating neuropathies, macrophages are involved in stripping myelin from peripheral axons. The molecular mechanisms that trigger macrophage action after trauma and in autoimmune disease are receiving increased attention and should lead to avenues to promote regeneration and protect axonal integrity.

Cholinergic responses of satellite glial cells in the superior cervical ganglia

Satellite glial cells (SGCs) surround the neurons in sympathetic ganglia and are believed to make important contributions to the function of the ganglia under normal and pathological conditions. It has been proposed that SGCs communicate chemically with the neurons, but little is known about their pharmacological properties and there is no information on whether they respond to acetylcholine (ACh), which is the major neurotransmitter in these ganglia. We used calcium imaging to examine responses of SGCs in the mouse superior cervical ganglion to ACh. The SGCs responded to ACh (0.01-2 mM) with an elevation of intracellular Ca²⁺, which appeared to be due to direct action on these cells, as the response persisted in the presence of the nerve blocker tetrodotoxin (1 μM). The response was largely inhibited by atropine, indicating an action on muscarinic ACh receptors. In contrast to this, sensory ganglia (nodose and trigeminal) were not sensitive to ACh. Incubation of the ganglia in ACh (0.5 or 1 mM) increased the expression of glial fibrillay acidic protein, which is a marker for glial activation. Such incubation also increased the electrical coupling of SGCs, which is known to occur in sensory ganglia following injury. We conclude that SGCs in the superior cervical ganglia display muscarinic ACh receptors, which enable them to communicate chemically with the sympathetic neurons.

Inflammatory Changes in Paravertebral Sympathetic Ganglia in Two Rat Pain Models

Injury to peripheral nerves can lead to neuropathic pain, along with well-studied effects on sensory neurons, including hyperexcitability, abnormal spontaneous activity, and neuroinflammation in the sensory ganglia. Neuropathic pain can be enhanced by sympathetic activity. Peripheral nerve injury may also damage sympathetic axons or expose them to an inflammatory environment. In this study, we examined the lumbar sympathetic ganglion responses to two rat pain models: ligation of the L5 spinal nerve, and local inflammation of the L5 dorsal root ganglion (DRG), which does not involve axotomy. Both models resulted in neuroinflammatory changes in the sympathetic ganglia, as indicated by macrophage responses, satellite glia activation, and increased numbers of T cells, along with very modest increases in sympathetic neuron excitability (but not spontaneous activity) measured in ex vivo recordings. The spinal nerve ligation model generally caused larger responses than DRG inflammation. Plasticity of the sympathetic system should be recognized in studies of sympathetic effects on pain.

Inflammation in dorsal root ganglia after peripheral nerve injury: effects of the sympathetic innervation

Following a peripheral nerve injury, a sterile inflammation develops in sympathetic and dorsal root ganglia (DRGs) with axons that project in the damaged nerve trunk. Macrophages and T-lymphocytes invade these ganglia where they are believed to release cytokines that lead to hyperexcitability and ectopic discharge, possibly contributing to neuropathic pain. Here, we examined the role of the sympathetic innervation in the inflammation of L5 DRGs of Wistar rats following transection of the sciatic nerve, comparing the effects of specific surgical interventions 10-14 days prior to the nerve lesion with those of chronic administration of adrenoceptor antagonists. Immunohistochemistry was used to define the invading immune cell populations 7 days after sciatic transection. Removal of sympathetic activity in the hind limb by transecting the preganglionic input to the relevant lumbar sympathetic ganglia (ipsi- or bilateral decentralization) or by ipsilateral removal of these ganglia with degeneration of postganglionic axons (denervation), caused less DRG inflammation than occurred after a sham sympathectomy. By contrast, denervation of the lymph node draining the lesion site potentiated T-cell influx. Systemic treatment with antagonists of α1-adrenoceptors (prazosin) or β-adrenoceptors (propranolol) led to opposite but unexpected effects on infiltration of DRGs after sciatic transection. Prazosin potentiated the influx of macrophages and CD4(+) T-lymphocytes whereas propranolol tended to reduce immune cell invasion. These data are hard to reconcile with many in vitro studies in which catecholamines acting mainly via β2-adrenoceptors have inhibited the activation and proliferation of immune cells following an inflammatory challenge.

Transection of preganglionic axons leads to CNS neuronal plasticity followed by survival and target reinnervation

The goals of the present study were to investigate the changes in sympathetic preganglionic neurons following transection of distal axons in the cervical sympathetic trunk (CST) that innervate the superior cervical ganglion (SCG) and to assess changes in the protein expression of brain derived neurotrophic factor (BDNF) and its receptor TrkB in the thoracic spinal cord. At 1 week, a significant decrease in soma volume and reduced soma expression of choline acetyltransferase (ChAT) in the intermediolateral cell column (IML) of T1 spinal cord were observed, with both ChAT-ir and non-immunoreactive neurons expressing the injury marker activating transcription factor 3. These changes were transient, and at later time points, ChAT expression and soma volume returned to control values and the number of ATF3 neurons declined. No evidence for cell loss or neuronal apoptosis was detected at any time point. Protein levels of BDNF and/or full length TrkB in the spinal cord were increased throughout the survival period. In the SCG, both ChAT-ir axons and ChAT protein remained decreased at 16 weeks, but were increased compared to the 10 week time point. These results suggest that though IML neurons show reduced ChAT expression and cell volume at 1 week following CST transection, at later time points, the neurons recovered and exhibited no significant signs of neurodegeneration. The alterations in BDNF and/or TrkB may have contributed to the survival of the IML neurons and the recovery of ChAT expression, as well as to the reinnervation of the SCG.

Segmental spinal root avulsion in the adult rat: a model to study avulsion injury pain

Road traffic accidents are the most common cause of avulsion injury, in which spinal roots are torn from the spinal cord. Patients suffer from a loss of sensorimotor function, intractable spontaneous pain, and border-zone hypersensitivity. The neuropathic pains are particularly difficult to treat because the lack of a well-established animal model of avulsion injury prevents identifying the underlying mechanisms and hinders the development of efficacious drugs. This article describes a hindlimb model of avulsion injury in adult rats where the L5 dorsal and ventral spinal root are unilaterally avulsed (spinal root avulsion [SRA]), leaving the adjacent L4 spinal root intact. SRA produced a significant ipsilateral hypersensitivity to mechanical and thermal stimulation by 5 days compared with sham-operated or naïve rats. This hypersensitivity is maintained for up to 60 days. No autotomy was observed and locomotor deficits were minimal. The hypersensitivity to peripheral stimuli could be temporarily ameliorated by administration of amitriptyline and carbamazepine, drugs that are currently prescribed to avulsion patients. Histological assessment of the L4 ganglion cells revealed no significant alterations in calcitonin gene-related peptide (CGRP), IB4, transient receptor potential cation channel subfamily V member 1 (TrpV1), or N52 staining across groups. Immunohistochemistry of the spinal cord revealed a localized glial response, phagocyte infiltration, and neuronal loss within the ipsilateral avulsed segment. A comparable response from glia and phagocytes was also found in the intact L4 spinal cord, supporting the role for central mechanisms within the L4-5 spinal cord in contributing to the generation of the pain-related behavior. The SRA model provides a platform to investigate possible new pharmacological treatments for avulsion injuries.

Postlaminectomy stabilization of the spine in a rat model of neuropathic pain reduces pain-related behavior

STUDY DESIGN

Spine deformity and pain-related behavior after laminectomy with and without spine stabilization were investigated.

OBJECTIVE

We tested hypothesis that spine stabilization after extensive laminectomy can prevent spine deformation and consequent pain-related behavior.

SUMMARY OF BACKGROUND DATA

Various ablative procedures requiring laminectomy have been tested for prevention or reversal of pain-related behavior in studies using experimental animals. However, there is no precise description indicating how laminectomy should be performed. Lack of standardized surgical techniques makes it difficult to achieve uniformity of result reporting and to compare results of different research groups meaningfully.

METHODS

To test our hypothesis, extensive laminectomy with and without spine stabilization was performed in Sprague-Dawley rats. U-shaped surgical wire was used for stabilization of the spine. A validated test of mechanical hyperalgesia was used to test the development of neuropathic pain behavior after surgery. Deformity of the spine was evaluated by calculating deviation from the central axis on radiographs obtained in anteroposterior projection.

RESULTS

Surgical stabilization of the spine after laminectomy prevented development of spinal deformity. Laminectomy without stabilization induced hyperalgesia on the 8th and 15th days after surgery. Group with stabilized spine exhibited significant reduction in pain-related behavior on the 8th and 15th postoperative days compared with the group without stabilization.

CONCLUSION

Surgical stabilization of the spine after laminectomy prevented development of spinal deformity and pain-related behavior. Our results suggest that spine stabilization procedure should be used in all experimental pain models in which laminectomy is performed.

Transient changes in spinal cord glial cells following transection of preganglionic sympathetic axons

Following peripheral nerve injury, retrograde signals originating from the injury site may activate intrinsic factors in the injured neurons, possibly leading to regenerative growth. Retrograde influences from peripheral injury sites may lead to the activation of glial cells in the vicinity of the centrally located cell bodies of the injured neurons. Few studies have examined changes in the spinal cord intermediolateral cell column (IML), which houses sympathetic preganglionic cell bodies, following injury to distal axons in the cervical sympathetic trunk (CST). The goal of the present study was to determine if transection of the CST results in plasticity in glial cells in the IML. At 1 day following injury, changes in the expression of microglial marker Iba1 were observed and the typical oligodendrocyte-neuronal relationship was altered. By 7 days, astrogliosis, microglial aggregation, and increased numbers of oligodendrocytes, as well as enhanced glial-glial and glial-neuronal relationships were present. The majority of cases were similar to controls at 3 weeks following injury and no changes were observed in any cases at 10 weeks following the injury. These results revealed changes in astrocytes, microglia, oligodendrocytes in the spinal cord following transection of preganglionic axons comprising the CST, indicating their ability to respond to distal axonal injury.

Satellite glial cells in sympathetic and parasympathetic ganglia: in search of function

Glial cells are established as essential for many functions of the central nervous system, and this seems to hold also for glial cells in the peripheral nervous system. The main type of glial cells in most types of peripheral ganglia - sensory, sympathetic, and parasympathetic - is satellite glial cells (SGCs). These cells usually form envelopes around single neurons, which create a distinct functional unit consisting of a neuron and its attending SGCs. This review presents the knowledge on the morphology of SGCs in sympathetic and parasympathetic ganglia, and the (limited) available information on their physiology and pharmacology. It appears that SGCs carry receptors for ATP and can thus respond to the release of this neurotransmitter by the neurons. There is evidence that SGCs have an uptake mechanism for GABA, and possibly other neurotransmitters, which enables them to control the neuronal microenvironment. Damage to post- or preganglionic nerve fibers influences both the ganglionic neurons and the SGCs. One major consequence of postganglionic nerve section is the detachment of preganglionic nerve terminals, resulting in decline of synaptic transmission. It appears that, at least in sympathetic ganglia, SGCs participate in the detachment process, and possibly in the subsequent recovery of the synaptic connections. Unlike sensory neurons, neurons in autonomic ganglia receive synaptic inputs, and SGCs are in very close contact with synaptic boutons. This places the SGCs in a position to influence synaptic transmission and information processing in autonomic ganglia, but this topic requires much further work.

Microglial responses around intrinsic CNS neurons are correlated with axonal regeneration

Background

Microglia/macrophages and lymphocytes (T-cells) accumulate around motor and primary sensory neurons that are regenerating axons but there is little or no microglial activation or T-cell accumulation around axotomised intrinsic CNS neurons, which do not normally regenerate axons. We aimed to establish whether there was an inflammatory response around the perikarya of CNS neurons that were induced to regenerate axons through a peripheral nerve graft.

Results

When neurons of the thalamic reticular nucleus (TRN) and red nucleus were induced to regenerate axons along peripheral nerve grafts, a marked microglial response was found around their cell bodies, including the partial enwrapping of some regenerating neurons. T-cells were found amongst regenerating TRN neurons but not rubrospinal neurons. Axotomy alone or insertion of freeze-killed nerve grafts did not induce a similar perineuronal inflammation. Nerve grafts in the corticospinal tracts did not induce axonal regeneration or a microglial or T-cell response in the motor cortex.

Conclusions

These results strengthen the evidence that perineuronal microglial accumulation (but not T-cell accumulation) is involved in axonal regeneration by intrinsic CNS and other neurons.