Effects of sciatic nerve injuries on delta -opioid receptor and substance P immunoreactivities in the superficial dorsal horn of the rat

Dimerization with cannabinoid receptors allosterically modulates delta opioid receptor activity during neuropathic pain

The diversity of receptor signaling is increased by receptor heteromerization leading to dynamic regulation of receptor function. While a number of studies have demonstrated that family A G-protein-coupled receptors are capable of forming heteromers in vitro, the role of these heteromers in normal physiology and disease has been poorly explored. In this study, direct interactions between CB(1) cannabinoid and delta opioid receptors in the brain were examined. Additionally, regulation of heteromer levels and signaling in a rodent model of neuropathic pain was explored. First we examined changes in the expression, function and interaction of these receptors in the cerebral cortex of rats with a peripheral nerve lesion that resulted in neuropathic pain. We found that, following the peripheral nerve lesion, the expression of both cannabinoid type 1 receptor (CB(1)R) and the delta opioid receptor (DOR) are increased in select brain regions. Concomitantly, an increase in CB(1)R activity and decrease in DOR activity was observed. We hypothesize that this decrease in DOR activity could be due to heteromeric interactions between these two receptors. Using a CB(1)R-DOR heteromer-specific antibody, we found increased levels of CB(1)R-DOR heteromer protein in the cortex of neuropathic animals. We subsequently examined the functionality of these heteromers by testing whether low, non-signaling doses of CB(1)R ligands influenced DOR signaling in the cortex. We found that, in cortical membranes from animals that experienced neuropathic pain, non-signaling doses of CB(1)R ligands significantly enhanced DOR activity. Moreover, this activity is selectively blocked by a heteromer-specific antibody. Together, these results demonstrate an important role for CB(1)R-DOR heteromers in altered cortical function of DOR during neuropathic pain. Moreover, they suggest the possibility that a novel heteromer-directed therapeutic strategy for enhancing DOR activity, could potentially be employed to reduce anxiety associated with chronic pain.

Analgesic properties of loperamide differ following systemic and local administration to rats after spinal nerve injury

BACKGROUND

The analgesic properties and mechanisms of loperamide hydrochloride, a peripherally acting opioid receptor agonist, in neuropathic pain warrant further investigation.

METHODS

We examined the effects of systemic or local administration of loperamide on heat and mechanical hyperalgesia in rats after an L5 spinal nerve ligation (SNL).

RESULTS

 (1) Systemic loperamide (0.3-10 mg/kg, subcutaneous in the back) dose dependently reversed heat hyperalgesia in SNL rats, but did not produce thermal analgesia. Systemic loperamide (3 mg/kg) did not induce thermal antinociception in naïve rats; (2) systemic loperamide-induced anti-heat hyperalgesia was blocked by pretreatment with intraperitoneal naloxone methiodide (5 mg/kg), but not by intraperitoneal naltrindole (5 mg/kg) or intrathecal naltrexone (20 μg/10 μL); (3) local administration of loperamide (150 μg), but not vehicle, into plantar or dorsal hind paw tissue induced thermal analgesia in SNL rats and thermal antinociception in naïve rats; (4) the analgesic effect of intraplantar loperamide (150 μg/15 μL) in SNL rats at 45 min, but not 10 min, post-injection was blocked by pretreatment with an intraplantar injection of naltrexone (75 μg/10 μL); (5) systemic (3.0 mg/kg) and local (150 μg) loperamide reduced the exaggerated duration of hind paw elevation to noxious pinprick stimuli in SNL rats. Intraplantar injection of loperamide also decreased the frequency of pinprick-evoked response in naïve rats.

CONCLUSIONS

These findings suggest that both systemic and local administration of loperamide induce an opioid receptor-dependent inhibition of heat and mechanical hyperalgesia in nerve-injured rats, but that local paw administration of loperamide also induces thermal and mechanical antinociception.

Cannabinoid-opioid interactions during neuropathic pain and analgesia

Opiates and exogenous cannabinoids, both potent analgesics used for the treatment of patients with neuropathic pain, bind to and activate class A G-protein-coupled receptors (GPCRs). Several lines of evidence have recently suggested that opioid and cannabinoid receptors can functionally interact in the central nervous system (CNS). These interactions may be direct, such as through receptor heteromerization, or indirect, such as through signaling cross-talk that includes agonist-mediated release and/or synthesis of endogenous ligands that can activate downstream receptors. Interactions between opioid and cannabinoid receptors may mediate many of the behavioral phenomena associated with the use of these drugs, including the production of acute antinociception and the development of tolerance and cross-tolerance to the antinociceptive effects of opioid and cannabinoid-specific ligands. This review summarizes behavioral, anatomical, and molecular data characterizing these interactions during the development of neuropathic pain and during antinociceptive treatment with these drugs alone or in combination. These studies are critical for understanding how the receptor systems involved in pain relief are altered during acute or chronic pain, and for designing better antinociceptive drug therapies, such as the combined use of opioid and cannabinoid receptor agonists or selective activation of receptor heteromers, that directly target the altered neurophysiology of patients experiencing pain.

Decreased substance P and NK1 receptor immunoreactivity and function in the spinal cord dorsal horn of morphine-treated neonatal rats

Opiate analgesic tolerance is defined as a need for higher doses of opiates to maintain pain relief after prolonged opiate exposure. Though changes in the opioid receptor undoubtedly occur during conditions of opiate tolerance, there is increasing evidence that opiate analgesic tolerance is also caused by pronociceptive adaptations in the spinal cord. We have previously observed increased glutamate release in the spinal cord dorsal horn of neonatal rats made tolerant to the opiate morphine. In this study, we investigate whether spinal substance P (SP) and its receptor, the neurokinin 1 (NK1) receptor, are also modulated by prolonged morphine exposure. Immunocytochemical studies show decreased SP- and NK1-immunoreactivity in the dorsal horn of morphine-treated rats, whereas SP mRNA in the dorsal root ganglia is not changed. Electrophysiological studies show that SP fails to activate the NK1 receptor in the morphine-treated rat. Taken together, the data indicate that chronic morphine treatment in the neonatal rat is characterized by a loss of SP effects on the NK1 receptor in lamina I of the neonatal spinal cord dorsal horn. The results are discussed in terms of compensatory spinal cord processes that may contribute to opiate analgesic tolerance.

PERSPECTIVE

This article describes anatomical and physiological changes that occur in the spinal cord dorsal horn of neonatal rats after chronic morphine treatment. These changes may represent an additional compensatory process of morphine tolerance and may represent an additional therapeutic target for the retention and restoration of pain relief with prolonged morphine treatment.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

Anti-allodynic effects of peripheral delta opioid receptors in neuropathic pain

The analgesic effects of local administration of opioid agonists into peripheral tissues in alleviating pain have been well documented in both clinical and preclinical studies, although few studies have examined their effects in neuropathic pain. In this study, we investigated the anti-allodynic effects of peripherally acting delta opioid receptor (DOR) agonists in a rat model of neuropathic pain. Peripheral nerve injury (PNI) produced a time-dependent decrease in mechanical withdrawal thresholds that was attenuated by local administration into the hind paw of either morphine or the DOR agonist deltorphin II. Using Western blotting techniques, no change in DOR protein expression was detected in DRG ipsilateral to the site of injury compared to contralateral. However, an up-regulation of DOR protein was found in neuropathic DRG compared to sham, suggesting that there may be a bilateral increase in the expression of DOR following PNI. Results obtained from immunohistochemical studies confirmed up-regulation in small and large DRG neurons in neuropathic compared to sham animals. Additionally, there was an increase in DOR protein within the ipsilateral sciatic nerve of neuropathic animals compared to sham and contralateral neuropathic conditions indicating the occurrence of receptor trafficking to the site of injury. Taken together, our findings suggest that functional peripheral DORs are present in sensory neurons following PNI and validate the development of selective DOR agonists for alleviating neuropathic pain.

Antihyperalgesic effects of loperamide in a model of rat neuropathic pain are mediated by peripheral delta-opioid receptors

The possible antihyperalgesic and antiallodynic activity of loperamide, an opioid agonist which does not readily penetrate the blood-brain barrier, were examined in the spinal nerve ligation model of experimental neuropathic pain. Intraperitoneal (i.p.) injection of loperamide effectively reversed thermal hyperalgesia. In contrast, loperamide had minimal effects on cold allodynia and no effects on mechanical allodynia. The antihyperalgesic action of loperamide against noxious heat was antagonized by naltrindole, a delta-opioid receptor selective antagonist, but not by pretreatment with beta-funaltrexamine, a mu-opioid receptor selective antagonist, or administration of nor-binaltorphimine, a kappa-opioid receptor selective antagonist. Furthermore, i.p. injection of [d-Ala(2), Glu(4)]-deltorphin II, a delta-opioid receptor selective peptide agonist, also reversed thermal hyperalgesia. The present results suggest that thermal hyperalgesia in experimental neuropathic pain can be reduced through activation of peripheral delta-opioid receptors. The data suggest the possible application of peripherally restricted and delta-opioid receptor selective agonists in the treatment of some aspects of neuropathic pain without many of the side effects associated with centrally acting opioids and without the peripheral side effects of opioid agonists acting at mu-receptors.

Morphine and pain-related stimuli enhance cell surface availability of somatic delta-opioid receptors in rat dorsal root ganglia

The present study demonstrates that perikaryaldelta-opioid receptors (deltaORs) in rat dorsal root ganglion (DRG) neurons bind and internalize opioid ligands circulating in the CSF. Using confocal and electron microscopy, we found that prolonged morphine treatment increased the cell surface density of these perikaryal deltaORs and, by way of consequence, receptor-mediated internalization of the fluorescent deltorphin (DLT) analog omega-Bodipy 576/589 deltorphin-I 5-aminopentylamide (Fluo-DLT) in all three types of DRG neurons (small, medium, and large). In contrast, chronic inflammatory pain induced by the injection of complete Freund's adjuvant (CFA) into one hindpaw selectively increased Fluo-DLT internalization in small and medium-sized DRG neurons ipsilateral to the inflammation. Based on our previous studies in the spinal cord of mu-opioid receptor (muOR) knock-out mice, it may be assumed that the enhanced membrane recruitment of deltaORs observed after sustained morphine is attributable to stimulation of muORs. However, the selectivity of the effect induced by inflammatory pain suggests that it involves a different mechanism, namely a modality-specific and pain-related activation of C and Adelta fibers. Indeed, stimulation by capsaicin of transient receptor potential vanilloid 1 receptors, which are selectively expressed by small diameter (< 600 microm2) DRG neurons, increased Fluo-DLT internalization exclusively in this cell population. The present results, therefore, demonstrate that DRG neurons express perikaryal deltaORs accessible to CSF-circulating ligands and that the density and, hence, presumably also the responsiveness, of these receptors may be modulated by both pain-related stimuli and sustained exposure to muOR agonists.

Expression of opioid receptors and c-fos in CB1 knockout mice exposed to neuropathic pain

The development of neuropathic pain is associated with multiple changes in gene expression occurring in the dorsal root ganglia (DRG) and spinal cord. The goal of this study was to evaluate whether the disruption of CB1 cannabinoid receptor gene modulates the changes induced by neuropathic pain in the expression of mu- (MOR), delta- (DOR) and kappa-opioid receptors (KOR) mRNA levels in the DRG and spinal cord. The induction of c-fos expression in the lumbar and sacral regions of the spinal cord was also evaluated in these animals. Opioid receptors mRNA levels were determined by using real-time PCR and Fos protein levels by immunohistochemistry. Nerve injury significantly reduced the expression of MOR in the DRG and the lumbar section of the spinal cord from CB1 cannabinoid knockout (KO) mice and wild-type littermates (WT). In contrast, mRNA levels of DOR and KOR were not significantly changed in any of the different sections analysed. Furthermore, sciatic nerve injury evoked a similar increase of c-fos expression in lumbar and sacral regions of the spinal cord of both KO and WT. In all instances, no significant differences were observed between WT and KO mice. These data revealed specific changes induced by neuropathic pain in MOR expression and c-fos levels in the DRG and/or spinal cord that were not modified by the genetic disruption of CB1 cannabinoid receptors.

Interactions between delta opioid receptors and alpha-adrenoceptors

1. Several studies have reported functional interactions between different subtypes of opioid and alpha2A-adrenoceptors in the induction of spinal cord analgesia. The mechanisms underlying this phenomenon are not well characterized. We propose that direct receptor-receptor associations could account for some of the observed functional interactions. In the present study, we examined the presence of delta opioid receptors and alpha2A-adrenoceptors in interacting complexes and the functional implications of such interactions on receptor activity. 2. Using the proximity based bioluminescence resonance energy transfer (BRET) assay, we found that the delta opioid receptors and alpha2A-adrenoceptors are in close enough proximity (< 100 A) in live cells that can foster physical interactions. 3. Using coimmunoprecipitation of differentially epitope-tagged receptors, we found that delta opiate receptors exist in interacting complexes with alpha2A-adrenoceptors in heterologous cells. 4. Finally, using receptor activity mediated neurite outgrowth in Neuro 2A cells as a physiological readout, we found that interactions between delta opiate receptors and alpha2A-adrenoceptors have functional consequences. The expression of alpha2A-adrenoceptors is sufficient to promote delta opiate receptor-mediated neurite outgrowth, suggesting that the presence of inactive alpha2A-adrenoceptors can enhance delta opiate receptor-mediated signalling. 5. Taken together, these findings suggest that modulation of receptor function as a result of physical associations between delta opiate receptors and alpha2A-adrenoceptors may account for the observed synergy between opiate and adrenergic agonists in spinal analgesia.

Contralateral, ipsilateral and bilateral treatments with the kappa-opioid receptor agonist U-50,488H in mononeuropathic rats

The effect of repeated contralateral administration of the kappa-opioid receptor agonist U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate) on nociceptive behaviour was investigated and compared with ipsilateral and bilateral treatments in a rat model of peripheral unilateral neuropathy (chronic constriction of the common sciatic nerve). Administration of 0.3 mg U-50,488H into the contralateral hindpaw on days 6 and 10 after induction of mononeuropathy increased hindpaw withdrawal latency to mechanical but not to thermal stimulation compared to saline-treated rats. No difference in pain-related behaviour was found between different peripheral (contralateral, ipsilateral and bilateral) treatments with 0.3 mg U-50,488H. Autotomy behaviour was reduced for 6 weeks after sciatic nerve ligation in rats treated contralaterally with the opioid receptor agonist. Antinociceptive effects of contralaterally administered U-50,488H were abolished by the peripherally acting opioid receptor antagonist naloxone methiodide. Our findings indicate that contralateral treatment with U-50,488H attenuates nociceptive behaviour in mononeuropathic rats. These antinociceptive effects are mediated via peripheral opioid receptors.

Morphine-induced changes in delta opioid receptor trafficking are linked to somatosensory processing in the rat spinal cord

An in vivo fluorescent deltorphin (Fluo-DLT) internalization assay was used to assess the distribution and regulation of pharmacologically available delta opioid receptors (deltaORs) in the rat lumbar (L4-5) spinal cord. Under basal conditions, intrathecal injection of Fluo-DLT resulted in the labeling of numerous deltaOR-internalizing neurons throughout dorsal and ventral horns. The distribution and number of Fluo-DLT-labeled perikaryal profiles were consistent with that of deltaOR-expressing neurons, as revealed by in situ hybridization and immunohistochemistry, suggesting that a large proportion of these cells was responsive to intrathecally administered deltaOR agonists. Pretreatment of rats with morphine for 48 hr resulted in a selective increase in Fluo-DLT-labeled perikaryal profiles within the dorsal horn. These changes were not accompanied by corresponding augmentations in either deltaOR mRNA or (125)I-deltorphin-II binding levels, suggesting that they were attributable to higher densities of cell surface deltaOR available for internalization rather than to enhanced production of the receptor. Unilateral dorsal rhizotomy also resulted in increased Fluo-DLT internalization in the ipsilateral dorsal horn when compared with the side contralateral to the deafferentation or to non-deafferented controls, suggesting that deltaOR trafficking in dorsal horn neurons may be regulated by afferent inputs. Furthermore, morphine treatment no longer increased Fluo-DLT internalization on either side of the spinal cord after unilateral dorsal rhizotomy, indicating that microOR-induced changes in the cell surface availability of deltaOR depend on the integrity of primary afferent inputs. Together, these results suggest that regulation of deltaOR responsiveness through microOR activation in this region is linked to somatosensory information processing.

Effects of peripheral nerve injury on delta opioid receptor (DOR) immunoreactivity in the rat spinal cord

Morphine and other opioids have direct analgesic actions in the spinal cord and chronic spinal administration of opioid agonists is used clinically in patients suffering from severe, chronic pain. Neuropathic pain resulting from peripheral nerve injury is often less sensitive to opioid therapy than other forms of chronic pain in both humans and animal models. Changes in spinal mu-opioid receptor (MOR) expression have been demonstrated in animal models of neuropathic pain. However, these changes alone fail to account for the attenuation of opioid activity. Reduced expression of delta-opioid receptors (DOR) following peripheral nerve injury has been reported but most of these reports are limited to subjective observation. The magnitude and consistency of these changes is therefore unclear. In addition, previous studies did not evaluate the effects of nerve injury on behavioral measures to confirm induction of aberrant pain symptoms. We therefore performed quantitative image analysis to evaluate the effect of peripheral nerve injury on DOR-immunoreactivity in spinal cord sections from rats previously characterized for sensory responsiveness. We observed statistically significant decreases ipsilateral to nerve injury in all three models tested: sciatic nerve transection, chronic constriction injury of the sciatic nerve and L5/L6 spinal nerve ligation. These results suggest that decreases in the expression of DOR are a common feature of peripheral nerve injury.

Phylogenetic changes in the expression of delta opioid receptors in spinal cord and dorsal root ganglia

To assess the validity of rodent models for investigating the role of delta opioid receptors (DOR) in analgesia, the distribution of DOR binding and mRNA were compared between rodent and primate spinal cord and dorsal root ganglia (DRG), using receptor autoradiography and in situ hybridization, respectively. In mouse and rat spinal cord, [(125)I]-deltorphin-labeled DOR binding sites were detected throughout the gray matter. In contrast, in primate and particularly in human spinal cord, DOR binding was mainly present in laminae I-II, with little to no binding in deeper layers. Accordingly, in rodent spinal cord, DOR mRNA was expressed by a large number of neurons distributed throughout the ventral and dorsal horns, whereas in the primate, DOR expression was significantly lower, as evidenced by a moderate number of labeled cells throughout the gray matter in monkey and by only few labeled cells in human, mainly in Clarke's column and lamina IX. Major species differences in DOR expression were also observed in primary afferent cells bodies. In rat DRG, intense DOR mRNA hybridization was primarily observed over large ganglion cells immunopositive for neurofilament 200. In contrast, in monkey and human DRG, DOR mRNA was primarily detected over small and medium-sized ganglion cells. These results demonstrate major differences in the expression and distribution of DOR in the spinal cord and DRG between mammalian species. Specifically, they point to a progressive specialization of DOR toward the regulation of primary somatosensory, namely nociceptive, inputs during phylogeny and suggest that the effects of DOR agonists in rodents may not be entirely predictive of their action in humans.

Substance P release in the spinal dorsal horn following peripheral nerve injury

Spinal microdialysis was used to study the potassium induced in vivo release of substance P (SP) in the rat dorsal horn at different time points (3, 14, and 60 days) following partial sciatic nerve ligation (PNL) or sciatic nerve axotomy. The withdrawal threshold to innocuous mechanical stimuli was investigated with von Frey filaments in the PNL rats prior to microdialysis. The release of SP was significantly elevated at 60 days following PNL but not following complete nerve injury. However, the PNL rats in all time groups displayed mechanical hypersensitivity, which implies that this late change in SP release seems to be unrelated to the development of neuropathy. The present results indicate that there is an increase of the releasable pool of SP in the dorsal horn at late post-operative times after PNL. This change in SP release may reflect an altered sensory processing or may instead relate to adaptive responses to promote recovery.

Opioids in chronic pain

The advance in our understanding of the biogenesis of various endogenous opioid peptides, their anatomical distribution, and the characteristics of the multiple receptors with which they interact open a new avenue for understanding the role of opioid peptide systems in chronic pain. The main groups of opioid peptides: enkephalins, dynorphins and beta-endorphin derive from proenkephalin, prodynorphin and proopiomelanocortin, respectively. Recently, a novel group of peptides has been discovered in the brain and named endomorphins, endomorphin-1 and -2. They are unique in comparison with other opioid peptides by atypical structure and high selectivity towards the mu-opioid receptor. Another group, which joined the endogenous opioid peptide family in the last few years is the pronociceptin system comprising the peptides derived from this prohormone, acting at ORL1 receptors. Three members of the opioid receptor family were cloned in the early 1990s, beginning with the mouse delta-opioid receptor (DOR1) and followed by cloning of mu-opioid receptor (MOR1) and kappa-opioid receptor (KOR1). These three receptors belong to the family of seven transmembrane G-protein coupled receptors, and share extensive structural homologies. These opioid receptor and peptide systems are significantly implicated in antinociceptive processes. They were found to be represented in the regions involved in nociception and pain. The effects of opioids in animal models of inflammatory pain have been studied in great detail. Inflammation in the periphery influences the central sites and changes the opioid action. Inflammation increased spinal potency of various opioid receptor agonists. In general, the antinociceptive potency of opioids is greater against various noxious stimuli in animals with peripheral inflammation than in control animals. Inflammation-induced enhancement of opioid antinociceptive potency is characteristic predominantly for mu opioid receptors, since morphine elicits a greater increase in spinal potency of mu- than of delta- and kappa-opioid receptor agonists. Enhancement of the potency of mu-opioid receptor agonists during inflammation could arise from the changes occurring in opioid receptors, predominantly in affinity or number of the mu-opioid receptors. Inflammation has been shown to alter the expression of several genes in the spinal cord dorsal horn. Several studies have demonstrated profound alterations in the spinal PDYN system when there is peripheral inflammation or chronic arthritis. Endogenous dynorphin biosynthesis also increases under various conditions associated with neuropathic pain following damage to the spinal cord and injury of peripheral nerves. Interestingly, morphine lacks potent analgesic efficacy in neuropathic pain. A vast body of clinical evidence suggests that neuropathic pain is not opioid-resistant but only that reduced sensitivity to systemic opioids is observed in this condition, and an increase in their dose is necessary in order to obtain adequate analgesia. Reduction of morphine antinociceptive potency was postulated to be due to the fact that nerve injury reduced the activity of spinal opioid receptors or opioid signal transduction. Our recent study with endogenous ligands of the mu-opioid receptor, endomorphins, further complicates the issue, since endomorphins appear to be effective in neuropathic pain. Identification of the involved differences may be of importance to the understanding of the molecular mechanism of opioid action in neuropathic pain, as well as to the development of better and more effective drugs for the treatment of neuropathic pain in humans.

Delayed administration of NGF reverses nerve injury induced central alterations of primary afferents

We have examined whether delayed exogenous NGF administered to an axotomised peripheral nerve reverses the increased transganglionic choleragenoid (CTB) labelling in lamina II. Two, four, eight or 18 weeks after bilateral sciatic nerve section, NGF was applied unilaterally for an additional 2-week period to the transected nerve stump. The transganglionic choleragenoid labelling and substance P (SP) expression were determined and compared to the contralateral axotomised side in the spinal cord dorsal horn. Delayed NGF administration reversed the transganglionic choleragenoid labelling in lamina II when administered 2 or 18 weeks after the sciatic nerve lesion, but not at 4 or 8 weeks. There was also a clear recovery of SP on the axotomised, NGF-treated side 2 or 18 weeks after the sciatic nerve lesion, but not at the intermediate survival times. At the longer survival time, however, there was a recovery of SP regardless of NGF treatment. These results suggest that there is a critical window as to when NGF administration can be effective in reversing axotomy-induced changes in the spinal cord.