Chondroitinase ABC promotes functional recovery after spinal cord injury

The inability of axons to regenerate after a spinal cord injury in the adult mammalian central nervous system (CNS) can lead to permanent paralysis. At sites of CNS injury, a glial scar develops, containing extracellular matrix molecules including chondroitin sulphate proteoglycans (CSPGs). CSPGs are inhibitory to axon growth in vitro, and regenerating axons stop at CSPG-rich regions in vivo. Removing CSPG glycosaminoglycan (GAG) chains attenuates CSPG inhibitory activity. To test the functional effects of degrading chondroitin sulphate (CS)-GAG after spinal cord injury, we delivered chondroitinase ABC (ChABC) to the lesioned dorsal columns of adult rats. We show that intrathecal treatment with ChABC degraded CS-GAG at the injury site, upregulated a regeneration-associated protein in injured neurons, and promoted regeneration of both ascending sensory projections and descending corticospinal tract axons. ChABC treatment also restored post-synaptic activity below the lesion after electrical stimulation of corticospinal neurons, and promoted functional recovery of locomotor and proprioceptive behaviours. Our results demonstrate that CSPGs are important inhibitory molecules in vivo and suggest that their manipulation will be useful for treatment of human spinal injuries.

Injury type-specific calcium channel alpha 2 delta-1 subunit up-regulation in rat neuropathic pain models correlates with antiallodynic effects of gabapentin

The calcium channel alpha2delta-1 subunit is a structural subunit important for functional calcium channel assembly. In vitro studies have shown that this subunit is the binding site for gabapentin, an anticonvulsant that exerts antihyperalgesic effects by unknown mechanisms. Increased expression of this subunit in the spinal cord and dorsal root ganglia (DRG) has been suggested to play a role in enhanced nociceptive responses of spinal nerve-injured rats to innocuous mechanical stimulation (allodynia). To investigate whether a common mechanism underlies allodynic states derived from different etiologies, and if so, whether similar alpha2delta-1 subunit up-regulation correlates with these allodynic states, we compared DRG and spinal cord alpha2delta-1 subunit levels and gabapentin sensitivity in allodynic rats with mechanical nerve injuries (sciatic nerve chronic constriction injury, spinal nerve transection, or ligation), a metabolic disorder (diabetes), or chemical neuropathy (vincristine neurotoxicity). Our data indicated that even though allodynia occurred in all types of nerve injury investigated, DRG and/or spinal cord alpha2delta-1 subunit up-regulation and gabapentin sensitivity only coexisted in the mechanical and diabetic neuropathies. Thus, induction of the alpha2delta-1 subunit in the DRG and spinal cord is likely regulated by factors that are specific for individual neuropathies and may contribute to gabapentin-sensitive allodynia. However, the calcium channel alpha2delta-1 subunit is not the sole molecular change that uniformly characterizes the neuropathic pain states.

VR1 protein expression increases in undamaged DRG neurons after partial nerve injury

Changes in phenotype or connectivity of primary afferent neurons following peripheral nerve injury may contribute to the hyperalgesia and allodynia associated with neuropathic pain conditions. Although earlier studies using partial nerve injury models have focused on the role of damaged fibres in the generation of ectopic discharges and pain, it is now thought that remaining undamaged fibres may be equally important. We have examined the expression of the sensory neuron-specific cation channel Vanilloid Receptor 1 (VR1), an important transducer of noxious stimuli, in three models of nerve injury in the rat, using anatomical separation or fluorescent retrograde tracers to identify damaged or undamaged sensory neurons. After total or partial sciatic nerve transection, or spinal nerve ligation, VR1-immunoreactivity (IR) was significantly reduced in the somata of all damaged dorsal root ganglion (DRG) neuronal profiles, compared to controls. However, after partial transection or spinal nerve ligation, VR1 expression was greater in the undamaged DRG somata than in controls. Unexpectedly, after L5 spinal nerve ligation, VR1-IR of the A-fibre somata increased approximately 3-fold in the uninjured L4 DRG compared to controls; a much greater increase than seen in the somata with C-fibres. Furthermore, we found that VR1-IR persisted in the transected sciatic nerve proximal to the lesion, despite its down-regulation in the damaged neuronal somata. This persistence in the nerve proximal to the lesion after nerve section, together with increased VR1 in DRG neurons left undamaged after partial nerve injury, may be crucial to the development or maintenance of neuropathic pain.

Tumor necrosis factor alpha and nucleus-pulposus-induced nerve root injury

STUDY DESIGN

The effects of nucleus pulposus and various treatments to block tumor necrosis factor alpha activity were evaluated in an experimental set-up using immunohistochemistry and nerve conduction velocity recordings.

OBJECTIVES

To assess the presence of tumor necrosis factor alpha in pig nucleus pulposus cells, and to see if block of tumor necrosis factor alpha also blocks the nucleus-pulposus-induced reduction of nerve root conduction velocity.

SUMMARY AND BACKGROUND DATA

A meta-analysis of observed effects induced by nucleus pulposus revealed that these effects might relate to one specific cytokine-tumor necrosis factor alpha.

METHODS

Series-1: Cultured nucleus pulposus cells were stained immunohistologically with a monoclonal antibody for tumor necrosis factor alpha. Series-2: Nucleus pulposus was harvested from lumbar discs and applied to the sacrococcygeal cauda equina in 13 pigs autologously. Four pigs received 100 mg of doxycycline intravenously; five pigs had a blocking monoclonal antibody to tumor necrosis factor alpha applied locally in the nucleus pulposus, and four pigs remained nontreated, forming a control group. Three days after the application, the nerve root conduction velocity was determined over the application zone by local electrical stimulation.

RESULTS

Series-1: Tumor necrosis factor alpha was found to be present in the nucleus pulposus cells. Series-2: The selective antibody to tumor necrosis factor alpha limited the reduction of nerve conduction velocity, although in comparison with the control group this was not statistically significant. However, treatment with doxycycline significantly blocked the nucleus-pulposus-induced reduction of conduction velocity.

CONCLUSION

For the first time, a specific substance, tumor necrosis factor alpha, has been linked to the nucleus-pulposus-induced effects of nerve roots after local application. Although the effects of this substance may be synergistic with those of other similar substances, the data of the current study may be of significant importance for the continued understanding of nucleus pulposus' biologic activity, and of possible potential use for future strategies in managing sciatica.

Mechanical and thermal hyperalgesia and ectopic neuronal discharge after chronic compression of dorsal root ganglia

Chronic compression of the dorsal root ganglion (CCD) was produced in adult rats by implanting a stainless steel rod unilaterally into the intervertebral foramen, one rod at L(4) and another at L(5). Two additional groups of rats received either a sham surgery or an acute injury consisting of a transient compression of the ganglion. Withdrawal of the hindpaw was used as evidence of a nocifensive response to mechanical and thermal stimulation of the plantar surface. In addition, extracellular electrophysiological recordings of spontaneous discharges were obtained from dorsal root fibers of formerly compressed ganglia using an in vitro nerve-DRG-dorsal root preparation. The mean threshold force of punctate indentation and the mean threshold temperature of heating required to elicit a 50% incidence of foot withdrawal ipsilateral to the CCD were significantly lower than preoperative values throughout the 35 days of postoperative testing. The number of foot withdrawals ipsilateral to the CCD during a 20-min contact with a temperature-controlled floor was significantly increased over preoperative values throughout postoperative testing when the floor was 4 degrees C (hyperalgesia) and, to a lesser extent, when it was 30 degrees C (spontaneous pain). Stroking the foot with a cotton wisp never elicited a reflex withdrawal before surgery but did so in most rats tested ipsilateral to the CCD during the first 2 postoperative weeks. In contrast, the CCD produced no changes in responses to mechanical or thermal stimuli on the contralateral foot. The sham operation and acute injury produced no change in behavior other than slight, mechanical hyperalgesia for approximately 1 day, ipsilateral to the acute injury. Ectopic spontaneous discharges generated within the chronically compressed ganglion and, occurring in the absence of blood-borne chemicals and without an intact sympathetic nervous system, were recorded from neurons with intact, conducting, myelinated or unmyelinated peripheral nerve fibers. The incidence of spontaneously active myelinated fibers was 8.61% for CCD rats versus 0.96% for previously nonsurgical rats. We hypothesize that a chronic compression of the dorsal root ganglion after certain injuries or diseases of the spine may produce, in neurons with intact axons, abnormal ectopic discharges that originate from the ganglion and potentially contribute to low back pain, sciatica, hyperalgesia, and tactile allodynia.

Spinal glial activation and cytokine expression after lumbar root injury in the rat

STUDY DESIGN

This study was designed to examine the behaviorial immunohistochemical changes of spinal glial cells and spinal Interleukin (IL)-1beta expression after various nerve root injuries used as models of lumbar radiculopathy.

OBJECTIVES

In order to better understand the role of central inflammation in the pathophysiologic mechanisms that give rise to pain associated with lumbar radiculopathy, this research studied the relationship between pain-related behavior associated with spinal glial activation and IL-1beta expression generated by three types of nerve root injury: loose ligation with chromic gut, loose ligation with silk, and tight ligation with silk.

SUMMARY OF BACKGROUND DATA

An animal model of lumbar radiculopathy originally described by Kawakami and Weinstein involved loose ligation of unilateral L4-L6 nerve roots with chromic gut. Characterization and establishment of such an animal model of low back pain enables further investigation of the nature of the pathophysiologic mechanisms associated with lumbar radiculopathy in humans.

METHODS

Seventy-three rats were divided into four treatment groups. Chromic group (n = 25): The L5 nerve roots (dorsal and ventral) were exposed by hemilaminectomy and loosely ligated with chromic gut. Tight silk group (n = 18): The exposed L5 nerve roots were tightly ligated extradurally with 5-0 silk suture. Loose silk group (n = 15): two loose ligatures of 5-0 silk were placed around the exposed L5 nerve roots. Sham group (n = 15): the rats were subjected to laminectomy alone for exposing nerve roots. Following surgery, thermal hyperalgesia and mechanical allodynia was assessed time-dependently up to 42 days post operatively. At 1, 3, 7, 14, and 42 days postoperatively, the rats in each group were perfused with fixative. The L5 spinal cord segments was harvested and cryosectioned for glial and cytokine immunohistochemistry.

RESULTS

In the chromic and the tight silk group, an immediate and sustained mechanical allodynia was observed in the ipsilateral hind paw up to 35 days postoperatively. The loose silk group also showed an immediate mechanical allodynia that subsided by 14 days postoperatively. Sham-treated animals exhibited mild mechanicalallodynia for the initial 7 days after the surgery. Thermalhyperalgesia was evident in the three primary treatment groups, but not in the sham-treated rats. OX-42 expression was elevated in the gray matter of the L5 spinal section by 3 days in the chromic, the tight silk, and the loose silk groups as compared to the sham group. Astrocytic activation increased over time in all groups except the sham group. There was no direct correlation between degree of microglial response and severity of pain behaviors. In contrast, astrocytic activation demonstrated a direct relationship with the elevation of mechanical allodynia for the first 7 days. In addition, spinal IL-1beta protein expression was increased bilaterally in the superficial layer of the dorsal horn and cell nuclei of the ventral horns in the ligature treated groups as compared with the sham group.

CONCLUSION

Direct mechanical and/or chemical injury to lumbar roots in the rat gives rise to pain behavior suggestive of lumbar radiculopathy. The finding that glial activation and enhanced IL-1beta expression are observed in the spinal cord after root injury supports a central, neuroimmune component in the generation of lumbar radiculopathy. A further understanding of the immunologic consequences of root injury may lead to further development and the novel use of selective cytokine-inflammatory inhibitors for the treatment of low back pain associated with radiculopathy.

Stimulation of nicotinamide adenine dinucleotide biosynthetic pathways delays axonal degeneration after axotomy

Axonal degeneration occurs in many neurodegenerative diseases and after traumatic injury and is a self-destructive program independent from programmed cell death. Previous studies demonstrated that overexpression of nicotinamide mononucleotide adenylyltransferase 1 (Nmnat1) or exogenous application of nicotinamide adenine dinucleotide (NAD) can protect axons of cultured dorsal root ganglion (DRG) neurons from degeneration caused by mechanical or neurotoxic injury. In mammalian cells, NAD can be synthesized from multiple precursors, including tryptophan, nicotinic acid, nicotinamide, and nicotinamide riboside (NmR), via multiple enzymatic steps. To determine whether other components of these NAD biosynthetic pathways are capable of delaying axonal degeneration, we overexpressed each of the enzymes involved in each pathway and/or exogenously administered their respective substrates in DRG cultures and assessed their capacity to protect axons after axotomy. Among the enzymes tested, Nmnat1 had the strongest protective effects, whereas nicotinamide phosphoribosyl transferase and nicotinic acid phosphoribosyl transferase showed moderate protective activity in the presence of their substrates. Strong axonal protection was also provided by Nmnat3, which is predominantly located in mitochondria, and an Nmnat1 mutant localized to the cytoplasm, indicating that the subcellular location of NAD production is not crucial for protective activity. In addition, we showed that exogenous application of the NAD precursors that are the substrates of these enzymes, including nicotinic acid mononucleotide, nicotinamide mononucleotide, and NmR, can also delay axonal degeneration. These results indicate that stimulation of NAD biosynthetic pathways via a variety of interventions may be useful in preventing or delaying axonal degeneration.

Chronic neuropathic pain is accompanied by global changes in gene expression and shares pathobiology with neurodegenerative diseases

Neuropathic pain is induced by injury or disease of the nervous system. Studies aimed at understanding the molecular pathophysiology of neuropathic pain have so far focused on a few known molecules and signaling pathways in neurons. However, the pathophysiology of neuropathic pain appears to be very complex and remains poorly understood. A global understanding of the molecular mechanisms involved in neuropathic pain is needed for a better understanding of the pathophysiology and treatment of neuropathic pain. Towards this end, we examined global gene expression changes as well as the pathobiology at the cellular level in a spinal nerve ligation neuropathic pain model using DNA microarray, quantitative real-time PCR and immunohistochemistry. We found that the behavioral hypersensitivity that is manifested in the persistent pain state is accompanied by previously undescribed changes in gene expression. In the DRG, we found regulation of: (1) immediate early genes; (2) genes such as ion channels and signaling molecules that contribute to the excitability of neurons; and (3) genes that are indicative of secondary events such as neuroinflammation. In addition, we studied gene regulation in both injured and uninjured DRG by quantitative PCR, and observed differential gene regulation in these two populations of DRGs. Furthermore, we demonstrated unexpected co-regulation of many genes, especially the activation of neuroinflammation markers in both the PNS and CNS. The results of our study provide a new picture of the molecular mechanisms that underlie the complexity of neuropathic pain and suggest that chronic pain shares common pathobiology with progressive neurodegenerative disease.

Contribution of injured and uninjured dorsal root ganglion neurons to pain behavior and the changes in gene expression following chronic constriction injury of the sciatic nerve in rats

Neuropathic pain models, such as the chronic constriction injury (CCI) model, are partial nerve injury models where there exist both intact and injured peripheral axons. Recent studies suggested that dorsal root ganglion (DRG) neurons with intact axons also show the alteration of excitability and gene expression and might have some role in the pathophysiological mechanisms of neuropathic pain. The incidence of pain-related behavior after the CCI is unstable and variable. In the present study, we used activating transcription factor 3 (ATF3) expression as a neuronal injury marker, and analyzed a relationship between the number of axotomized neurons and the incidence of pain-related behavior. We divided all rats into three groups according to the percentage of ATF3-immunoreactive (IR) neurons, group 1 (<12.5%), group 2 (12.5-25%), and group 3 (>25%). We found that rats in groups 2 and 3 showed thermal hyperalgesia, whereas only the rats in group 2 developed tactile allodynia from the third day to the fourteenth day after surgery. Rats in group 1 did not show thermal hyperalgesia or tactile allodynia. The DRG neurons in group 2 contained ATF3-IR neurons mainly in medium- and large-sized neurons. In order to investigate brain-derived neurotrophic factor (BDNF) and gamma-aminobutyric acid(A)-receptor (GABA(A)-R) regulation in both intact and injured primary afferent neurons after the CCI, we used a double-labeling method with immunohistochemistry and in situ hybridization, as well as double immunofluorescent staining. The CCI induced an increased number of BDNF-labeled neurons in the ipsilateral DRG and the increase in BDNF expression was observed mainly in small- and medium-sized neurons that were mainly ATF3-negative. On the other hand, the number of GABA(A)-Rgamma2 subunit mRNA-positive neurons decreased in the ipsilateral DRG and GABA(A)-R- and ATF3-labeled neurons rarely overlapped. These changes in molecular phenotype in intact and injured primary afferents may be involved in the pathophysiological mechanisms of neuropathic pain produced by partial nerve injury.

Locally translated mTOR controls axonal local translation in nerve injury

How is protein synthesis initiated locally in neurons? We found that mTOR (mechanistic target of rapamycin) was activated and then up-regulated in injured axons, owing to local translation of mTOR messenger RNA (mRNA). This mRNA was transported into axons by the cell size-regulating RNA-binding protein nucleolin. Furthermore, mTOR controlled local translation in injured axons. This included regulation of its own translation and that of retrograde injury signaling molecules such as importin β1 and STAT3 (signal transducer and activator of transcription 3). Deletion of the mTOR 3' untranslated region (3'UTR) in mice reduced mTOR in axons and decreased local translation after nerve injury. Both pharmacological inhibition of mTOR in axons and deletion of the mTOR 3'UTR decreased proprioceptive neuronal survival after nerve injury. Thus, mRNA localization enables spatiotemporal control of mTOR pathways regulating local translation and long-range intracellular signaling.

The transcription factor ATF-3 promotes neurite outgrowth

Dorsal root ganglion (DRG) neurons regenerate after a peripheral nerve injury but not after injury to their axons in the spinal cord. A key question is which transcription factors drive the changes in gene expression that increase the intrinsic growth state of peripherally injured sensory neurons? A prime candidate is activating transcription factor-3 (ATF-3), a transcription factor that we find is induced in all DRG neurons after peripheral, but not central axonal injury. Moreover, we show in adult DRG neurons that a preconditioning peripheral, but not central axonal injury, increases their growth, correlating closely with the pattern of ATF-3 induction. Using viral vectors, we delivered ATF-3 to cultured adult DRG neurons and find that ATF-3 enhances neurite outgrowth. Furthermore, ATF-3 promotes long sparsely branched neurites. ATF-3 overexpression did not increase c-Jun expression. ATF-3 may contribute, therefore, to neurite outgrowth by orchestrating the gene expression responses in injured neurons.

Immunolocalization of SNS/PN3 and NaN/SNS2 sodium channels in human pain states

The tetrodotoxin-resistant (TTX-R) voltage-gated sodium channel SNS/PN3 and the newly discovered NaN/SNS2 are expressed in sensory neurones, particularly in nociceptors. Using specific antibodies, we have studied, for the first time in humans, the presence of SNS/PN3 and NaN/SNS2 in peripheral nerves, including tissues from patients with chronic neurogenic pain. In brachial plexus injury patients, there was an acute decrease of SNS/PN3- and NaN/SNS2-like immunoreactivity in sensory cell bodies of cervical dorsal root ganglia (DRG) whose central axons had been avulsed from spinal cord, with gradual return of the immunoreactivity to control levels over months. In contrast, there was increased intensity of immunoreactivity to both channels in some peripheral nerve fibers just proximal to the site of injury in brachial plexus trunks, and in neuromas. These findings suggest that the expression of these sodium channels in neuronal cell bodies is reduced after spinal cord root avulsion injury in man, but that pre-synthesized channel proteins may undergo translocation with accumulation at sites of nerve injury, as in animal models of peripheral axotomy. The latter may contribute to positive symptoms, as our patients all showed a positive Tinel's sign. Nerve terminals in distal limb neuromas and skin from patients with chronic local hyperalgesia and allodynia all showed marked increases of SNS/PN3-immunoreactive fibers, but little or no NaN/SNS2-immunoreactivity, suggesting that the former may be related to the persistent hypersensitive state. Axonal immunoreactivity to both channels was similar to control nerves in sural nerve biopsies in a selection of neuropathies, irrespective of nerve inflammation, demyelination or spontaneous pain, including a patient with congenital insensitivity to pain. Our studies suggest that the best target for SNS/PN3 blocking agents is likely to be chronic local hypersensitivity.

BDNF prevents and reverses adult rat motor neuron degeneration and induces axonal outgrowth

To assess the therapeutic potential of brain-derived neurotrophic factor (BDNF) in clinics, we extensively investigated the effects of BDNF on adult motor neurons in a rat spinal root avulsion model. Intrathecal administration of BDNF immediately after the spinal root avulsion greatly protected against the motor neuron cell death. BDNF also showed a protective effect on the atrophy of soma and on the reduction of transmitter-related enzymes such as choline acetyl transferase and acetylcholine esterase. Very interestingly, BDNF induced axonal outgrowth of severely damaged motor neurons at the avulsion site. The BDNF administration following 2-week treatment with phosphate-buffered saline after avulsion prevented further augmentation of cell death and reversed cholinergic transmitter-related enzyme deficiency. BDNF was demonstrated to possess a wide variety of biological effects on survival, soma size, cholinergic enzymes, and axonal outgrowth of adult motor neurons. These results provide a rationale for BDNF treatment in motor neuron diseases such as spinal cord injury and amyotrophic lateral sclerosis.

Endogenous interleukin-6 contributes to hypersensitivity to cutaneous stimuli and changes in neuropeptides associated with chronic nerve constriction in mice

Partial nerve injury is a potential cause of distressing chronic pain for which conventional analgesic treatment with opiates or anti-inflammatory agents is not very effective. Constriction nerve injury, widely used to study neuropathic pain, was shown here to induce interleukin-6 (IL-6) mRNA in a subset of rat primary sensory neurons. When we inflicted chronic nerve constriction on mice with null mutation of the IL-6 gene, the hypersensitivity to cutaneous heat and pressure that is induced in wild-type mice was not evident, the loss of substance P in sensory neurons was excessive and the induction of galanin in central sensory projections was reduced. In additional experiments, intrathecal infusion of IL-6 in rats was shown to stimulate synthesis of galanin in approximately one-third of lumbar dorsal root ganglion neurons. The results of these experiments indicate that endogenous IL-6 mediates some of the hypersensitive responses that characterize peripheral neuropathic pain, and influences two neuropeptides that have been implicated in pain transmission.

Primary sensory neurons and satellite cells after peripheral axotomy in the adult rat: timecourse of cell death and elimination

The timecourse of cell death in adult dorsal root ganglia after peripheral axotomy has not been fully characterised. It is not clear whether neuronal death begins within 1 week of axotomy or continues beyond 2 months after axotomy. Similarly, neither the timecourse of satellite cell death in the adult, nor the effect of nerve repair has been described. L4 and L5 dorsal root ganglia were harvested at 1-14 days, 1-6 months after sciatic nerve division in the adult rat, in accordance with the Animals (Scientific Procedures) Act 1986. In separate groups the nerve was repaired either immediately or following a 1-week delay, and the ganglia were harvested 2 weeks after the initial transection. Microwave permeabilisation and triple staining enabled combined TUNEL staining, morphological examination and neuron counting by the stereological optical dissector technique. TUNEL-positive neurons, exhibiting a range of morphologies, were seen at all timepoints (peak 25 cells/group 2 weeks after axotomy) in axotomised ganglia only. TUNEL-positive satellite cell numbers peaked 2 months after axotomy and were more numerous in axotomised than control ganglia. L4 control ganglia contained 13,983 (SD 568) neurons and L5, 16,285 (SD 1,313). Neuron loss was greater in L5 than L4 axotomised ganglia, began at 1 week (15%, P=0.045) post-axotomy, reached 35% at 2 months ( P<0.001) and was not significantly greater at 4 months or 6 months. Volume of axotomised ganglia fell to 19% of control by 6 months ( P<0.001). In animals that underwent nerve repair, both the number of TUNEL-positive neurons and neuron loss were reduced. Immediate repair was more protective than repair after a 1-week delay. Thus TUNEL positivity precedes actual neuron loss, reflecting the time taken to complete cell death and elimination. Neuronal death begins within 1 day of peripheral axotomy, the majority occurs within the first 2 months, and limited death is still occurring at 6 months. Neuronal death is modulated by peripheral nerve repair and by its timing after axotomy. Secondary satellite cell death also occurs, peaking 2 months after axotomy. These results provide a logical framework for future research into neuronal and satellite cell death within the dorsal root ganglia and provide further insight into the process of axotomy induced neuronal death.

Glial cell plasticity in sensory ganglia induced by nerve damage

Numerous studies have been done on the effect of nerve injury on neurons of sensory ganglia but little is known about the contribution of satellite glial cells (SCs) in these ganglia to post-injury events. We investigated cell-to-cell coupling and ultrastructure of SCs in mouse dorsal root ganglia after nerve injury (axotomy). Under control conditions SCs were mutually coupled, but mainly to other SCs around a given neuron. After axotomy SCs became extensively coupled to SCs that enveloped other neurons, apparently by gap junctions. Serial section electron microscopy showed that after axotomy SC sheaths enveloping neighboring neurons formed connections with each other. Such connections were absent in control ganglia. The number of gap junctions between SCs increased 6.5-fold after axotomy. We propose that axotomy induces growth of perineuronal SC sheaths, leading to contacts between SCs enveloping adjacent neurons and to formation of new gap junctions between SCs. These changes may be an important mode of glial plasticity and can contribute to neuropathic pain.

Expression of caveolin-1 and -2 in differentiating PC12 cells and dorsal root ganglion neurons: caveolin-2 is up-regulated in response to cell injury

Caveolae are cholesterol/sphingolipid-rich microdomains of the plasma membrane that have been implicated in signal transduction and vesicular trafficking. Caveolins are a family of caveolae-associated integral membrane proteins. Caveolin-1 and -2 show the widest range of expression, whereas caveolin-3 expression is restricted to muscle cell types. It has been previously reported that little or no caveolin mRNA species are detectable in the brain by Northern blot analyses or in neuroblastoma cell lines. However, it remains unknown whether caveolins are expressed within neuronal cells. Here we demonstrate the expression of caveolin-1 and -2 in differentiating PC12 cells and dorsal root ganglion (DRG) neurons by using mono-specific antibody probes. In PC12 cells, caveolin-1 expression is up-regulated on day 4 of nerve growth factor (NGF) treatment, whereas caveolin-2 expression is transiently up-regulated early in the differentiation program and then rapidly down-regulated. Interestingly, caveolin-2 is up-regulated in response to the mechanical injury of differentiated PC12 cells; up-regulation of caveolin-2 under these conditions is strictly dependent on continued treatment with NGF. Robust expression of caveolin-1 and -2 is also observed along the entire cell surface of DRG neurons, including high levels on growth cones. These findings demonstrate that neuronal cells express caveolins.

Glial cell line-derived neurotrophic factor normalizes neurochemical changes in injured dorsal root ganglion neurons and prevents the expression of experimental neuropathic pain

Glial cell line-derived neurotrophic factor (GDNF) is necessary for the development of sensory neurons, and appears to be critical for the survival of dorsal root ganglion (DRG) cells that bind the lectin IB4. Intrathecal infusion of GDNF has been shown to prevent and reverse the behavioral expression of experimental neuropathic pain arising from injury to spinal nerves. This effect of GDNF has been attributed to a blockade of the expression of the voltage gated, tetrodotoxin-sensitive sodium channel subtype, Na(V)1.3, in the injured DRG. Here we report that GDNF given intrathecally via osmotic-pump to nerve-injured rats (L5/L6 spinal nerve ligation) prevented the changes in a variety of neurochemical markers in the DRG upon injury. They include a loss of binding of IB4, downregulation of the purinergic receptor P2X(3), upregulation of galanin and neuropeptide Y immunoreactivity in large diameter DRG cells, and expression of the transcription factor ATF3. GDNF infusion concomitantly prevented the development of spinal nerve ligation-induced tactile hypersensitivity and thermal hyperalgesia. These observations suggest that high dose, exogenous GDNF has a broad neuroprotective role in injured primary afferent. The receptor(s) that mediates these effects of GDNF is not known. GDNF's ability to block neuropathic pain states is not likely to be specific to Na(V)1.3 expression.

Neuronal-Specific TUBB3 Is Not Required for Normal Neuronal Function but Is Essential for Timely Axon Regeneration

We generated a knockout mouse for the neuronal-specific β-tubulin isoform Tubb3 to investigate its role in nervous system formation and maintenance. Tubb3-/- mice have no detectable neurobehavioral or neuropathological deficits, and upregulation of mRNA and protein of the remaining β-tubulin isotypes results in equivalent total β-tubulin levels in Tubb3-/- and wild-type mice. Despite similar levels of total β-tubulin, adult dorsal root ganglia lacking TUBB3 have decreased growth cone microtubule dynamics and a decreased neurite outgrowth rate of 22% in vitro and in vivo. The effect of the 22% slower growth rate is exacerbated for sensory recovery, where fibers must reinnervate the full volume of the skin to recover touch function. Overall, these data reveal that, while TUBB3 is not required for formation of the nervous system, it has a specific role in the rate of peripheral axon regeneration that cannot be replaced by other β-tubulins.

Peripheral nerve injury fails to induce growth of lesioned ascending dorsal column axons into spinal cord scar tissue expressing the axon repellent Semaphorin3A

We have investigated the hypothesis that the chemorepellent Semaphorin3A may be involved in the failure of axonal regeneration after injury to the ascending dorsal columns of adult rats. Following transection of the thoracic dorsal columns, fibroblasts in the dorsolateral parts of the lesion site showed robust expression of Semaphorin3A mRNA. In addition, dorsal root ganglion (DRG) neurons with projections through the dorsal columns to the injury site persistently expressed both Semaphorin3A receptor components, neuropilin-1 and plexin-A1. These ascending DRG collaterals failed to invade scar regions occupied by Semaphorin3A-positive fibroblasts, even in animals which had received conditioning lesions of the sciatic nerve to enhance regeneration. Other axon populations in the dorsal spinal cord were similarly unable to penetrate Semaphorin3A-positive scar tissue. These data suggest that Semaphorin3A may create an exclusion zone for regenerating dorsal column fibres and that enhancing the intrinsic regenerative response of DRG neurons has only limited effects on axonal regrowth. Tenascin-C and chondroitin sulphate proteoglycans were also detected at the injury site, which was largely devoid of central nervous system (CNS) myelin, showing that several classes of inhibitory factors, including semaphorins, with only partially overlapping spatial and temporal patterns of expression are in a position to participate in preventing regenerative axonal growth in the injured dorsal columns. Interestingly, conditioning nerve injuries enabled numerous ascending DRG axons to regrow across areas of strong tenascin-C and chondroitin sulphate proteoglycan expression, while areas containing Semaphorin3A and CNS myelin were selectively avoided by (pre)primed axonal sprouts.

Signaling the pathway to regeneration

Robust axon regeneration occurs after peripheral nerve injury through coordinated activation of a genetic program and local intracellular signaling cascades. Although regeneration-associated genes are being identified with increasing frequency, most aspects of regeneration-associated intracellular signaling remain poorly understood. Two independent studies now report that upregulation of cAMP is a component of the PNS regeneration program that can be exploited to enhance axon regeneration through the normally inhibitory CNS environment.

Retinoic acid receptor β2 promotes functional regeneration of sensory axons in the spinal cord

The embryonic CNS readily undergoes regeneration, unlike the adult CNS, which has limited axonal repair after injury. Here we tested the hypothesis that retinoic acid receptor beta2 (RARbeta2), critical in development for neuronal growth, may enable adult neurons to grow in an inhibitory environment. Overexpression of RARbeta2 in adult rat dorsal root ganglion cultures increased intracellular levels of cyclic AMP and stimulated neurite outgrowth. Stable RARbeta2 expression in DRG neurons in vitro and in vivo enabled their axons to regenerate across the inhibitory dorsal root entry zone and project into the gray matter of the spinal cord. The regenerated neurons enhanced second-order neuronal activity in the spinal cord, and RARbeta2-treated rats showed highly significant improvement in sensorimotor tasks. These findings show that RARbeta2 induces axonal regeneration programs within injured neurons and may thus offer new therapeutic opportunities for CNS regeneration.

Permeability of injured and intact peripheral nerves and dorsal root ganglia

BACKGROUND

Nerve injury that produces behavioral changes of allodynia and hyperalgesia in animals is associated with electrophysiologic changes in dorsal root ganglion (DRG) cells. The introduction of drugs into the DRG or the peripheral nerve that alter calcium, sodium, or potassium channel activity may be of therapeutic benefit after nerve injury. For this reason, the authors sought to determine whether drugs that do not ordinarily cross the blood-nerve barrier will enter the DRG after intravenous or regional injection and to determine whether nerve injury alters drug access to DRGs or peripheral nerves.

METHODS

Both intact and spinal nerve-ligated rats were injected with sodium fluorescein by intravenous, intrathecal, peri-DRG, perisciatic, and epidural routes. DRG, sciatic nerve, and spinal cord tissues were harvested and frozen, and histologic sections were analyzed quantitatively for tissue fluorescence.

RESULTS

In both intact and nerve-injured animals, fluorescein accumulated in DRGs after intravenous, peri-DRG, and epidural injection. There was accumulation in the proximal portion of the ganglion after intrathecal injection. Minimal amounts of fluorescein were found in the sciatic nerve in intact animals after intravenous or perineural injection, but substantial amounts were found in some nerve fascicles in nerve-injured animals after both intravenous and perineural injection. There was almost no fluorescein found in the spinal cord except after intrathecal administration.

CONCLUSIONS

In both intact and nerve-injured animals, fluorescein accumulates freely in the DRG after intravenous, epidural, or paravertebral injection. The sciatic nerve is relatively impermeable to fluorescein, but access by either systemic or regional injection is enhanced after nerve injury.

Increased TRPA1, TRPM8, and TRPV2 expression in dorsal root ganglia by nerve injury

Thermosensitive TRP channels display unique thermal responses, suggesting distinct roles mediating sensory transmission of temperature. However, whether relative expression of these channels in dorsal root ganglia (DRG) is altered in nerve injury is unknown. We developed a multiplex ribonuclease protection assay (RPA) to quantify rat TRPV1, TRPV2, TRPV3, TRPV4, TRPA1, and TRPM8 RNA levels in DRG. We used the multiplex RPA to measure thermosensitive TRP channel RNA levels in DRG from RTX-treated rats (300 microg/kg) or rats with unilateral sciatic nerve chronic constriction injury (CCI). TRPV1 and TRPA1 RNA were significantly decreased in DRG from RTX-treated rats, indicating functional colocalization of TRPA1 and TRPV1 in sensory nociceptors. In DRG from CCI rats, TRPA1, TRPV2, and TRPM8 RNA showed slight but significant increases ipsilateral to peripheral nerve injury. Our findings support the hypothesis that increased TRP channel expression in sensory neurons may contribute to mechanical and cold hypersensitivity.

cAMP and cGMP Contribute to Sensory Neuron Hyperexcitability and Hyperalgesia in Rats With Dorsal Root Ganglia Compression

Numerous studies have implicated the cAMP-protein kinase A (PKA) pathway in producing hyperexcitability of dorsal root ganglia (DRG) sensory neurons under conditions associated with pain. Evidence is presented for roles of both the cAMP-PKA and cGMP-protein kinase G (PKG) pathways in maintaining neuronal hyperexcitability and behavioral hyperalgesia in a neuropathic pain model: chronic compression of the DRG (CCD treatment). Lumbar DRGs were compressed by a steel rod inserted into the intervertebral foramen. Thermal hyperalgesia was revealed by shortened latencies of foot withdrawal to radiant heat. Intracellular recordings were obtained in vitro from lumbar ganglia after in vivo DRG compression. Activators of the cAMP-PKA pathway, 8-Br-cAMP and Sp-cAMPS, and of the cGMP-PKG pathway, 8-Br-cGMP and Sp-cGMPS, increased the hyperexcitability of DRG neurons already produced by CCD treatment, as shown by further decreases in action potential threshold and increased repetitive discharge during depolarization. The adenylate cyclase inhibitor, SQ22536, the PKA antagonist, Rp-cAMPS, the guanylate cyclase inhibitor, ODQ, and the PKG inhibitor, Rp-8-pCPT-cGMPS, reduced the hyperexcitability of CCD DRG neurons. In vivo application of PKA and PKG antagonists transiently depressed behavioral hyperalgesia induced by CCD treatment. Unexpectedly, application of these agonists and antagonists to ganglia of naïve, uninjured animals had little effect on electrophysiological properties of DRG neurons and no effect on foot withdrawal, suggesting that sensitizing actions of these pathways in the DRG are enabled by prior injury or stress. The only effect observed in uncompressed ganglia was modest depolarization of DRG neurons by PKA and PKG agonists. CCD treatment also depolarized DRG neurons, but CCD-induced depolarization was not affected by agonists or antagonists of these pathways.