On the presence of mononuclear leucocytes in dorsal root ganglia following transection of the sciatic nerve

Neuroimmune interactions and immunoengineering strategies in peripheral nerve repair

Peripheral nerve injuries result in disrupted cellular communication between the central nervous system and somatic distal end targets. The peripheral nervous system is capable of independent and extensive regeneration; however, meaningful target muscle reinnervation and functional recovery remain limited and may result in chronic neuropathic pain and diminished quality of life. Macrophages, the primary innate immune cells of the body, are critical contributors to regeneration of the injured peripheral nervous system. However, in some clinical scenarios, macrophages may fail to provide adequate support with optimal timing, duration, and location. Here, we review the history of immunosuppressive and immunomodulatory strategies to treat nerve injuries. Thereafter, we enumerate the ways in which macrophages contribute to successful nerve regeneration. We argue that implementing macrophage-based immunomodulatory therapies is a promising treatment strategy for nerve injuries across a wide range of clinical presentations.

Sarm1/Myd88-5 Regulates Neuronal Intrinsic Immune Response to Traumatic Axonal Injuries

Traumatic injuries can trigger inflammatory reactions, leading to profound neuropathological consequences. However, the immune capacity of neurons, distinct from that of immune cells or glial cells, in response to traumatic insults remains to be fully characterized. In this study, we demonstrate that neurons can detect, cell autonomously, distant axonal damage, resulting in rapid production of a specific collection of cytokines and chemokines. This neuronal immune response appears spatially and temporally separated from injury-induced axon degeneration. We then identify through the genetic screen that this immune response is regulated by TIR-domain adaptor Sarm1/Myd88-5. We further show that Sarm1 functions through the downstream Jnk-c-Jun signal, and blockage of this Sarm1-Jnk-c-Jun pathway effectively abolishes the recruitment of immune cells to injury-afflicted neural tissues. We therefore uncover the key function of the Sarm1 signaling pathway, independent of its known role in axon degeneration, in the neuronal intrinsic immune response to traumatic axonal 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.

Trigeminal injury causes kappa opioid-dependent allodynic, glial and immune cell responses in mice

BACKGROUND

The dynorphin-kappa opioid receptor (KOR) system regulates glial proliferation after sciatic nerve injury. Here, we investigated its role in cell proliferation following partial ligation of infraorbital nerve (pIONL), a model for trigeminal neuropathic pain. Mechanical allodynia was enhanced in KOR gene deleted mice (KOR-/-) compared to wild type mice. Using bromodeoxyuridine (BrdU) as a mitotic marker, we assessed cell proliferation in three different areas of the trigeminal afferent pathway: trigeminal nucleus principalis (Vp), trigeminal root entry zone (TREZ), and trigeminal ganglion (TG).

RESULTS

In KOR-/- mice or norBNI-treated mice, the number of proliferating cells in the Vp was significantly less than in WT mice, whereas cell proliferation was enhanced in TREZ and TG. The majority of the proliferating cells were nestin positive stem cells or CD11b positive microglia in the Vp and macrophages in the TG. GFAP-positive astrocytes made a clear borderline between the CNS and the PNS in TREZ, and phosphorylated KOR staining (KOR-p) was detectable only in the astrocytes in CNS in WT mice but not in KOR-/- or norBNI-treated mice.

CONCLUSIONS

These results show that kappa opioid receptor system has different effects after pIONL in CNS and PNS: KOR activation promotes CNS astrocytosis and microglial or stem cell proliferation but inhibits macrophage proliferation in PNS. The trigeminal central root has a key role in the etiology and treatment of trigeminal neuralgia, and these newly identified responses may provide new targets for developing pain therapies.

Distinct functional types of macrophage in dorsal root ganglia and spinal nerves proximal to sciatic and spinal nerve transections in the rat

Inflammation proximal to a peripheral nerve injury may be responsible for ectopic discharge and/or death of sensory neurones, factors thought to contribute to the development and/or maintenance of neuropathic pain. Here, ED1+, ED2+ and major histocompatibility complex class II (MHC II)+ macrophages in dorsal root ganglia (DRGs) and spinal nerve roots have been compared quantitatively in adult rats following transection of one sciatic or one spinal nerve, using double labelling immunohistochemistry. In control DRGs, all ED2+ cells expressed ED1 and some also MHC II. One week after either lesion, the ED2+ cells changed negligibly, except that all expressed MHC II. ED1+ and MHC II+ cell density increased markedly, with cells expressing MHC II alone (the majority), ED1/MHC II or rarely ED1 alone. In the spinal roots, ED1+ and MHC II+ cell density increased less after sciatic than after spinal nerve transection when ED1+ foamy cells were prominent. All ED2- macrophages were aggregated with T lymphocytes around blood vessels at 1 week or around isolated somata at later stages. ED1+ cell density declined more rapidly than MHC II+ cell density. Within the DRG, the debris of retrogradely labelled neurones appeared in ED2+ cells and a small proportion of MHC II+ cells that contained ED1. The data suggest that (i) resident ED2+ macrophages do not proliferate but are phagocytic and (ii) of ED1+ and MHC+ monocytes invading from the blood, only ED1+/MHC II+ cells are phagocytic. Four functional subtypes of macrophage within the DRGs were distinct from ED1+ foamy cells that phagocytosed myelin after spinal nerve transection.

A comparison of the changes in the non-neuronal cell populations of the superior cervical ganglia following decentralization and axotomy

Transecting the axons of neurons in the adult superior cervical ganglion (SCG; axotomy) results in the survival of most postganglionic neurons, the influx of circulating monocytes, proliferation of satellite cells, and changes in neuronal gene expression. In contrast, transecting the afferent input to the SCG (decentralization) results in nerve terminal degeneration and elicits a different pattern of gene expression. We examined the effects of decentralization on macrophages in the SCG and compared the results to those previously obtained after axotomy. Monoclonal antibodies were used to identify infiltrating (ED1+) and resident (ED2+) macrophages, as well as macrophages expressing MHC class II molecules (OX6+). Normal ganglia contained ED2+ cells and OX6+ cells, but few infiltrating macrophages. After decentralization, the number of infiltrating ED1+ cells increased in the SCG to a density about twofold greater than that previously seen after axotomy. Both the densities of ED2+ and OX6+ cells were essentially unchanged after decentralization, though a large increase in OX6+ cells occurred after axotomy. Proliferation among the ganglion's total non-neuronal cell population was examined and found to increase about twofold after decentralization and about fourfold after axotomy. Double-labeling experiments indicated that some of these proliferating cells were macrophages. After both surgical procedures, the percentage of proliferating ED2+ macrophages increased, while neither procedure altered the proliferation of ED1+ macrophages. Axotomy, though not decentralization, increased the proliferation of OX6+ cells. Future studies must address what role(s) infiltrating and/or resident macrophages play in regions of decentralized and axotomized neurons and, if both are involved, whether they play distinct roles.

Macrophage and lymphocyte invasion of dorsal root ganglia after peripheral nerve lesions in the rat

The distribution of major histocompatibility complex class II (MHC II)-positive non-neuronal cells and T-lymphocytes was examined immunohistochemically in dorsal root ganglia (DRGs) up to 12 weeks following transection of one sciatic or lumbar spinal nerve in adult rats. Unlike within the brain, MHC II immunopositive (+) and T-cells are normally present within DRGs. After nerve transection, MHC II+ cell density increased (by about four times after each lesion) in DRGs projecting into lesioned nerves. Subsequently the number declined after sciatic but not spinal nerve transection. MHC II+ cells did not contain glial markers, even when these were up-regulated after the lesions. Initially, MHC II+ cells lay outside the satellite glia but, by 11 weeks, they had moved through them to lie against the somata. T-cells invaded the lesioned DRGs earlier than the MHC II+ cells. They achieved greater numbers after spinal (30 x control) than after sciatic (12 x control) nerve transection. They also increased in undamaged ganglia adjacent to the spinal nerve injury. T-cell density progressively declined after spinal but not sciatic nerve transection. Both cell types appeared to invade the DRGs initially through blood vessels and the meninges, particularly near the subarachnoid angle. At later stages, occasional neurones had dense aggregations of T-cell receptor+ and MHC II+ cells associated with them. We conclude that the magnitude and time course of changes in MHC II expression and T-cell numbers in lesioned DRGs differ from the responses within motor nuclei after axotomy. The influx of inflammatory cells may contribute to neurone survival in the short term. Their long-term presence has implications for patients. These cells have the potential to release excitatory cytokines that may generate ectopic impulse activity in sensory neurones after nerve injury and so may play a role in the generation of chronic neuropathic pain.

Causes and consequences of sympathetic basket formation in dorsal root ganglia

Injury to peripheral nerves can result in severe and intractable neuropathic pain, and in some cases the symptoms are sympathetically maintained. In recent years much effort has been put into elucidating the anatomical nature of nerve injury-induced sympathetic-sensory coupling. The demonstration of sympathetic sprouting into dorsal root ganglia (DRG) of nerve-injured rats has led to the suggestion that this phenomenon might underlie sympathetically-maintained pain. As a result, several studies have been undertaken to determine what factor or factors are responsible for the sprouting, and for the formation of abnormal sympathetic terminal arborizations or 'baskets' around some DRG neurons. In this review we examine in particular the roles of nerve growth factor (NGF) and the cytokines leukemia inhibitory factor (LIF) and interleukin-6 (IL-6), as these have all been shown to contribute to sympathetic sprouting. We also stress the role of satellite cells within axotomized DRG, as these have been shown to express not only neurotrophin mRNA, but also the low-affinity neurotrophin receptor p75. We propose a mechanism for sympathetic sprouting in the DRG involving; (i) the activation of satellite cells on the DRG by a factor such as LIF or IL-6, and (ii) the generation of a sympathetic axon-guiding gradient by p75-bound neurotrophins on the activated satellite cells. We also highlight the possibility that a sympathetic sprouting signal may be derived from the periphery, as NGF, LIF and IL-6 are all produced as a result of Wallerian degeneration, and can be retrograde transported to the DRG. The possible relevance of sympathetic sprouting in the DRG to neuropathic pain is also discussed.

Changes in the macrophage population of the rat superior cervical ganglion after postganglionic nerve injury

Following peripheral nerve transection, a series of biochemical changes occurs in axons and Schwann cells both at the site of the lesion and distal to it. Macrophages differentiated from monocytes that invade the area in response to transection (elicited macrophages) and, perhaps, also macrophages normally present in the tissue (resident macrophages) play important roles in these changes. In addition, nerve transection produces changes in the cell bodies of axotomized neurons and their surrounding glial cells, located at some distance from the lesion. To determine whether macrophages might play a role in the changes occurring in the superior cervical ganglion (SCG) after axotomy, we examined the presence of macrophages before and after axonal damage. The monoclonal antibodies ED1, ED2, and OX6 were used, each of which recognizes a somewhat different population of macrophages. Ganglia from normal rats contained a population of resident cells that were ED2+ but very few that were ED1+. Within 2 days after the post-ganglionic nerves were transected, the number of ED1+ cells increased substantially, with little change in immunostaining for ED2. These data, in combination with published studies on other tissues, suggest that ED1 in the SCG is selective for elicited macrophages and ED2 for resident macrophages. OX6 immunostaining was prominent in normal ganglia but also increased significantly after axotomy, suggesting that it reflects both macrophage populations. Systemic administration of 6-hydroxydopamine, a neurotoxin that causes the destruction of sympathetic nerve endings, also produced an increase in ED1 immunostaining. Thus, the change in ED1 immunostaining in the SCG does not require surgery, with the attendant severing of local blood vessels and connective tissue, but rather only the disconnection of sympathetic neurons from their end organs. The time course of the invasion of monocytes after axotomy indicates that this process is not required to trigger the biochemical changes occurring in the ganglion within the first 24 h. On the other hand, the existence of a resident population of macrophages raises the possibility that changes in those cells might be involved.

Inflammation and axonal regeneration

Inflammatory cells and their products contribute to neuronal survival and axonal regeneration after injury. Following sciatic nerve transection in rats, macrophages accumulate in the corresponding dorsal root ganglion, potentially supplying neurotrophic support to nerve cell bodies, and enhancing axonal regeneration. Growth factors characterized for their functions in the haematopoietic and immune systems also act on neurons and vice versa, by sharing common subunits among receptors for cytokines and neurotrophic factors.

Responses of macrophages in rat dorsal root ganglia following peripheral nerve injury

Immunohistochemical studies with monoclonal antibodies to macrophage antigens were performed on sections of rat lumbar dorsal root ganglia. In confirmation of previous observations, cells with macrophage antigenicity were detected in normal ganglia. Many of these presumptive macrophages were perineuronal in contact with the neuron/satellite cell complex, a few were perivascular, and others were in interstitial position not in apparent contact with either blood vessels or neurons. The number of macrophages in lumbar dorsal root ganglia started to increase 2-4 days after sciatic nerve transection and remained elevated for four weeks. Perineuronal macrophages resembled satellite glial cells in light microscope appearance but were distinguished from glial cells by their lack of S-100 immunoreactivity. Following this sciatic nerve injury, macrophage counts were modestly increased in contralateral lumbar dorsal root ganglia but not in cervical dorsal root ganglia. Thus peripheral nerve injury induces a recruitment and/or proliferation of macrophages in the corresponding dorsal root ganglion. Although the functions of these macrophages are unclear, those in perineuronal position could contribute to the survival or regeneration of axotomized neurons.

Structure and nature of macrophages participating in the wallerian degeneration of nerve fibers

The posttraumatic processes of Wallerian degeneration of nerves have been illuminated in detail. The dynamics of the breakdown of axons and the myelin sheaths of nerve fibers has been established, as have been the periods of the changes in the composition of myelin, and the reactive changes in the Schwann cells and the connective tissue structures in the makeup of the nerve as well as the formation of "foam" cells have been described. The controversial questions which have been raised in these studies regarding the role of the cellular elements (the Schwann cells, the endoneurial fibroblasts, the cells of the epi- and perineurium) during Wallerian degeneration remain unresolved until the present time. In particular, the question as to which cells participate in the cleanup of the products of the breakdown of the myelin sheaths, and as to the character of the inflammatory infiltration in Wallerian degeneration and the degree of the participation of the various cellular elements in the destructive and reparative processes, has not been elucidated. Some investigators believe that the Schwann cells accomplish the cleanup of the products of the breakdown of the myelin sheaths. There are also data suggesting that the macrophages are of considerable significance in the cleanup of the products of the breakdown of nerve fibers of both the PNS and the CNS following their injury. It has been demonstrated by means of monoclonal antibodies to macrophages, radioautography, and immunocytochemical methods that these macrophages have a hematogenous origin.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensory afferent impulses originate from dorsal root ganglia as well as from the periphery in normal and nerve injured rats

Single units were recorded in dorsal roots or in the sciatic nerve of anaesthetised rats. It was shown by making sections, by stimulation and by collision that some ongoing nerve impulses were originating from the dorsal root ganglia and not from the central or peripheral ends of the axons. In a sample of 2731 intact or acutely sectioned myelinated sensory fibres, 4.75% +/- 3.7% contained impulses generated within the dorsal root ganglia. In 2555 axons sectioned in the periphery 2-109 days before, this percentage rose to 8.6% +/- 4.8%. There was a considerable variation between animals; 0-14% in intact and acutely sectioned nerves and 1-21% in chronically sectioned nerves. The conduction velocity of the active fibres did not differ significantly from the conduction velocity of unselected fibres. The common pattern of ongoing activity from the ganglion was irregular and with a low frequency (about 4 Hz) in contrast to the pattern of impulses originating in a neuroma which usually have a higher frequency with regular intervals. Slight mechanical pressure on the dorsal root ganglion increased the frequency of impulses. Unmyelinated fibres were also found to contain impulses originating in the dorsal root ganglion. In intact or acutely sectioned unmyelinated axons, the percentage of active fibres 4.4% +/- 3.5% was approximately the same as in myelinated fibres but there were no signs of an increase following chronic section. Fine filament dissection of dorsal roots and of peripheral nerves and collision experiments showed that impulses originating in dorsal root ganglia were propagated both orthodromically into the root and antidromically into the peripheral nerve. It was also shown that the same axon could contain two different alternating sites of origin of nerve impulses: one in the neuroma or sensory ending and one in the ganglion. These observations suggest that the dorsal root ganglion with its ongoing activity and mechanical sensitivity could be a source of pain producing impulses and could particularly contribute to pain in those conditions of peripheral nerve damage where pain persists after peripheral anaesthesia or where vertebral manipulation is painful.

Development of mouse dorsal root ganglia: an autoradiographic and quantitative study

Pulse labelling with tritiated thymidine was used to determine the cell birthdays of dorsal root ganglion (DRG) neurons in foetal mice. The peak number of cell birthdays occurred at 11.5 days foetal age in cervical DRGs, and at 12.5 days in lumbar DRGs. The satellite cells were becoming heavily labelled by day 13.5 in lumbar and some hours earlier in cervical regions. A very sharp peak of satellite cell labelling was seen at 13 days in the lumbar region. Evidence for the existence of more than one neuronal cell type is presented. The earliest cells to stop dividing were part of a widely spread distribution which included all the large neurons. The birthdays of the population of small neurons began later and continued for at least 48 h after division of the large cells had ceased.

Fine structure of reactive cells in injured nervous tissue labeled with 3H-thymidine injected before injury

To examine the fine structure of blood mononuclear cells in injured nervous tissue, mice were given repeated injections of 3H-thymidine with the last injection at least 16 hours before injury. Under ether anesthesia the animals either were given a stab wound to the spinal cord or had their left hypoglossal nerve transected. The animals were killed at 2, 4, 8, or 16 days after injury. Tissue sections containing the spinal cord wound or both hypoglossal nuclei were prepared for electron microscopic radioautography, and all labeled cells were photographed. About half the labeled cells in the injured spinal cords and almost all the labeled cells in the nuclei of the injured hypoglossal nerves had nuclei with dark staining peripheral heterochromatin, dark cytoplasm with long cisternae of granular endoplasmic reticulum, and other ultrastructural features characteristic of the cells usually identified as microglia. The remaining labeled cells in the injured spinal cords were macrophages, fibroblasts, cells with pale nuclei, some of which contained cytoplasmic filaments, and vascular cells. Since uninjured nervous tissue has extremely few labeled cells and since 3H-thymidine should be available for only a short time following injection, most of the labeled cells in this experiment should be derived from blood mononuclear cells. However, the possibility is discussed that some or all of the labeled cells may be intrinsic cells proliferating in response to the injury and labeled through reutilization of labeled DNA precursor material.

Nerve cell degeneration and death in the trigeminal ganglion of the adult rat following peripheral nerve transection

Trigeminal ganglia of normal rats and of adult rats subjected to unilateral transection of the infraorbital nerve were studied by light and electron microscopy. Counts of ganglion cells in ganglia on operated and unoperated sides were made following long postoperative survival times. The ultrastructural changes in ganglia of the operated side were studied from 3 to 70 days postoperatively. The quantitative observations show that a considerable loss of ganglion cells takes place on the operated side. The ultrastructural observations demonstrate the occurrence of ganglion cell degeneration, nerve fibre degeneration and phagocytosis by satellite and Schwann cells. The results are compatible with the view that degeneration of trigeminal afferents in the brain stem following lesions of peripheral trigeminal nerve branches is related to retrograde degeneration of trigeminal ganglion cells.