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

The role of sodium channels in neuropathic pain

Our knowledge of the ion channels, receptors and signalling mechanisms involved in pain pathophysiology, and which specific channels play a role in subtypes of pain such as neuropathic and inflammatory pain, has expanded considerably in recent years. It is now clear that in the neuropathic state the expression of certain channels is modified, and that these changes underlie the plasticity of responses that occur to generate inappropriate pain signals from normally trivial inputs. Pain is modulated by a subset of the voltage-gated sodium channels, including Nav1.3, Nav1.7, Nav1.8 and Nav1.9. These isoforms display unique expression patterns within specific tissues, and are either up- or down-regulated upon injury to the nervous system. Here we describe our current understanding of the roles of sodium channels in pain and nociceptive information processing, with a particular emphasis on neuropathic pain and drugs useful for the treatment of neuropathic pain that act through mechanisms involving block of sodium channels. One of the future challenges in the development of novel sodium channel blockers is to design and synthesise isoform-selective channel inhibitors. This should provide substantial benefits over existing pain treatments.

MCP-1 enhances excitability of nociceptive neurons in chronically compressed dorsal root ganglia

Previous experimental results from our laboratory demonstrated that monocyte chemoattractant protein-1 (MCP-1) depolarizes or increases the excitability of nociceptive neurons in the intact dorsal root ganglion (DRG) after a chronic compression of the DRG (CCD), an injury that upregulates neuronal expression of both MCP-1 and mRNA for its receptor CCR2. We presently explore the ionic mechanisms underlying the excitatory effects of MCP-1. MCP-1 (100 nM) was applied, after CCD, to acutely dissociated small DRG neurons with nociceptive properties. Under current clamp, the proportion of neurons depolarized was similar to that previously observed for CCD-treated neurons in the intact ganglion, although the magnitude of depolarization was greater. MCP-1 induced a decrease in rheobase by 44 +/- 10% and some cells became spontaneously active at resting potential. Action potential width at a voltage equal to 10% of the peak height was increased from 4.94 +/- 0.23 to 5.90 +/- 0.47 ms. In voltage clamp, MCP-1 induced an inward current in 27 of 50 neurons held at -60 mV, which increased with concentration over the range of 3 to 300 nM (EC(50) = 45 nM). The MCP-1-induced current was not voltage dependent and had an estimated reversal potential of -27 mV. In addition, MCP-1 inhibited a voltage-dependent, noninactivating outward current, presumably a delayed rectifier type K(+) conductance. We conclude that MCP-1 enhances excitability in CCD neurons by, at least, two mechanisms: 1) activation of a nonvoltage-dependent depolarizing current with characteristics similar to a nonselective cation conductance and 2) inhibition of a voltage-dependent outward current.

The subventricular zone releases factors which can be protective in oxygen/glucose deprivation-induced cortical damage: an organotypic study

A number of studies have already established the role of the subventricular zone in sustaining adult neurogenesis in different brain regions and under different pathological conditions, but nothing is reported about the role of this germinal area in preserving cell viability. In this work, we developed an organotypic culture model of the forebrain structures that comprise the neocortex, striatum, subventricular zone, and corpus callosum. With this model, we investigated the role of the subventricular zone in modulating cell viability in the cortex after oxygen/glucose deprivation. Here we have demonstrated that soluble heat-labile factors released by the subventricular zone in the media can lead to protection specifically in the cortical area. No protection was observed when medium, conditioned with factors released during the insult was administered to the hippocampal slices. Moreover, the use of different modifications of the slice cultures showed that the removal of the subventricular zone increased the cellular damage induced by oxygen/glucose deprivation. Furthermore, by using pharmacological experiments to investigate the possible mechanisms that regulate this subventricular function, we found evidence of purinergic involvement. We postulate that extracellular ATP signaling in the subventricular zone exacerbates cortical damage induced by hypoxia/hypoglycemia. For the first time, we demonstrate in vitro that the germinal subventricular zone can release factors that can be protective after exposure to a metabolic stressor. These released factors are not yet characterized but we identified in the extracellular ATP a factor that may interfere with the protective role of the subventricular zone during metabolic cortical damage.

Exposure of the nucleus pulposus to the outside of the anulus fibrosus induces nerve injury and regeneration of the afferent fibers innervating the lumbar intervertebral discs in rats

STUDY DESIGN

Using a retrograde tracing method and immunohistochemistry, we assessed the expression of activating transcription factor 3 (ATF3), a marker of nerve injury, and growth-associated protein 43 (GAP-43), a marker of axonal growth, in dorsal root ganglion (DRG) neurons innervating the lumbar intervertebral discs in rats.

OBJECTIVES

To investigate ATF3 and GAP-43 expression in DRGs innervating the intervertebral discs after exposure of the nucleus pulposus to the outside of the anulus fibrosus.

SUMMARY OF BACKGROUND DATA

Degeneration of lumbar intervertebral discs is considered as a cause of low back pain. We speculated that exposure of the nucleus pulposus to the outside of the anulus fibrosus may induce nerve injury and ingrowth into the disc.

METHODS

A neurotracer, Fluoro-Gold (F-G), was applied to the ventral aspect of L5-L6 intervertebral discs in 20 rats. The rats were classified into 2 groups: an NP group whose disc was punctured to expose the nucleus pulposus (n = 10) and a sham-operated group whose anulus fibrosus surface was scratched superficially (n = 10). Ten days after surgery, bilateral L1-L5 DRGs were processed for staining of ATF3 and GAP-43.

RESULTS

In the NP group, 13.9% +/- 2.9% of the F-G-labeled neurons innervating the discs were positive for ATF3, while 19.3% +/- 2.7% were positive for GAP-43. In contrast, in the sham-operated group, only 0.8% +/- 0.4% of the F-G-labeled neurons were positive for ATF3 while 7.4% +/- 1.7% were positive for GAP-43. The percentage of both ATF3-immunoreactive (IR) and GAP-43-IR neurons in the NP group was significantly higher than in the sham-operated group (P < 0.05).

CONCLUSIONS

ATF3-IR and GAP-43-IR neurons were significantly increased in the NP group. These results suggested that exposure of the nucleus pulposus to the outside of the anulus fibrosus induced nerve injury and in growth into the discs. These findings may explain discogenic lower back pain in patients with lumbar disc degeneration.

Purinergic P2 receptors as targets for novel analgesics

Following hints in the early literature about adenosine 5'-triphosphate (ATP) injections producing pain, an ion-channel nucleotide receptor was cloned in 1995, P2X3 subtype, which was shown to be localized predominantly on small nociceptive sensory nerves. Since then, there has been an increasing number of papers exploring the role of P2X3 homomultimer and P2X2/3 heteromultimer receptors on sensory nerves in a wide range of organs, including skin, tongue, tooth pulp, intestine, bladder, and ureter that mediate the initiation of pain. Purinergic mechanosensory transduction has been proposed for visceral pain, where ATP released from epithelial cells lining the bladder, ureter, and intestine during distension acts on P2X3 and P2X2/3, and possibly P2Y, receptors on subepithelial sensory nerve fibers to send messages to the pain centers in the brain as well as initiating local reflexes. P1, P2X, and P2Y receptors also appear to be involved in nociceptive neural pathways in the spinal cord. P2X4 receptors on spinal microglia have been implicated in allodynia. The involvement of purinergic signaling in long-term neuropathic pain and inflammation as well as acute pain is discussed as well as the development of P2 receptor antagonists as novel analgesics.

Reversal of neurochemical changes and pain-related behavior in a model of neuropathic pain using modified lentiviral vectors expressing GDNF

In this study, we evaluated the possible use of lentiviral vectors in the treatment of neuropathic pain. We chose to administer GDNF-expressing vectors because of the known beneficial effect of this trophic factor in alleviation of neuropathic pain in adult rodents. Lentiviral vectors expressing either GDNF or control, green fluorescent protein or beta-galactosidase, were injected unilaterally into the spinal dorsal horn 5 weeks before a spinal nerve ligation was induced (or sham surgery for the controls). We observed that intraspinally administered lentiviral vectors resulted in a large and sustained expression of transgenes in both neurons and glial cells. Injection of GDNF-expressing viral vectors induced a significant reduction of ATF-3 up-regulation and IB4 down-regulation in damaged DRG neurons. In addition, it produced a partial but significant reversal of thermal and mechanical hyperalgesia observed following the spinal nerve ligation. In conclusion, our study suggests that lentiviral vectors are efficient tools to induce a marked and sustained expression of trophic factors in specific areas of the CNS and can, even if with some limitations, be efficient in the treatment of neuropathic pain.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

ATF3 expression in L4 dorsal root ganglion neurons after L5 spinal nerve transection

Activating transcription factor 3 (ATF3) is a widely used marker of damaged primary sensory neurons that is induced in essentially all dorsal root ganglion (DRG) neurons by spinal nerve axotomy. Whether such injuries induce its expression in neurons of adjacent DRGs remains unknown. Following L5 spinal nerve ligation, experimental but not sham-operated rats develop thermal and mechanical hypersensitivity. In the L4 DRG, 11-12% of neurons were ATF3 positive by 1 day post-surgery, and numbers remain unchanged at 2 weeks. Importantly, sham exposure of the L5 spinal nerve produced a nearly identical number of ATF3-positive neurons in the L4 DRG and also a substantial increase in the L5 DRG, with a similar time-course to experimental animals. There was no correlation between behaviour and magnitude of ATF3 expression. Co-localization studies with the DRG injury markers galanin, neuropeptide Y and nitric oxide synthase (NOS) showed that approximately 75, 50 and 25%, respectively, of L4 ATF3-positive neurons co-expressed these markers after L5 transection or sham surgery. Additionally, increases in galanin and NOS were seen in ATF3-negative neurons in L4. Our results strongly suggest that the surgical exposure of spinal nerves induces ATF3 in the L4-5 DRG, irrespective of whether the L5 nerve is subsequently cut. This probably reflects minor damage to the neurons or their axons but nevertheless is sufficient to induce phenotypic plasticity. Caution is therefore warranted when interpreting the phenotypic plasticity of DRG neurons in adjacent ganglia in the absence of positive evidence that they are not damaged.

GDNF family ligand receptor components Ret and GFRalpha-1 in the human trigeminal ganglion and sensory nuclei

The occurrence of Ret and GFRalpha-1 receptors is shown by immunohistochemistry in the human trigeminal sensory system at pre-, postnatal and adult age. Receptor-labeled neurons occur in both trigeminal ganglion and mesencephalic nucleus. In adult trigeminal ganglion, about 75% of Ret- and 65% of GFRalpha-1-labeled neurons are small- and medium-sized. The proportion of Ret+ and GFRalpha-1+ trigeminal ganglion neurons in the adult is about 25 and 60%, respectively. The majority of Ret+ are double labeled for GFRalpha-1 and glial cell line-derived neurotrophic factor (GDNF). Most of the GFRalpha-1+ cells contain GDNF and about 50% of them contain Ret. Triple labeling shows many Ret+/GDNF+/GFRalpha-1+ neurons, but also a number of Ret-/GDNF+/GFRalpha-1+ and Ret+/GDNF-/GFRalpha-1+ cells. Both Ret+ and GFRalpha-1+ neuronal subpopulations overlap with that containing calcitonin gene-related peptide. Ret+ pericellular basket-like nerve fibers occur in the adult trigeminal ganglion. Centrally, immunoreactivity is restricted to the spinal nucleus pars caudalis and pars interpolaris and to the mesencephalic nucleus. In adult specimens, Ret+ nerve fibers and puncta gather in the inner substantia gelatinosa. Ret+ neurons occur in the spinal nucleus and are more frequent in newborn than in adult subjects. Central GFRalpha-1+-labeled neurons and punctate elements are sparse. These findings support the involvement of GDNF and possibly other cognate ligands in the trophism of human trigeminal primary sensory neurons from prenatal life to adulthood, indicating a selective commitment to cells devoted to protopathic and proprioceptive sensory transmission. They also support the possibility that receptor molecules other than Ret could be active in transducing the ligand signal.

Astrocyte-derived transgene GDNF promotes complete and long-term survival of adult facial motoneurons following avulsion and differentially regulates the expression of transcription factors of AP-1 and ATF/CREB families

Glial-cell-line-derived neurotrophic factor (GDNF) is a potent survival factor for motoneurons (MNs). We have previously demonstrated that overexpression of GDNF in astrocytes of GFAP-GDNF mice promotes long-term survival of neonatal MNs after facial nerve axotomy. In the present study, we investigated whether astrocyte-derived GDNF could also have a neuroprotective effect on adult MNs following facial nerve avulsion. We also examined avulsion- and GDNF-induced changes in the expression pattern of several members of the AP-1 and ATF/CREB families of transcription factors, which are involved in the fate determination of neurons following injury. We demonstrated that GDNF promotes complete rescue of avulsed MNs for at least 4 months post-injury. Transgene GDNF significantly upregulates c-Jun expression in naive MNs, further upregulates injury-induced c-Jun expression in facial MNs, and results in its activation in most surviving MNs. No significant changes were found in c-Fos expression. We found that GDNF has an opposing effect on ATF2 and ATF3 expression. It dramatically downregulates increased levels of ATF3 in response to injury, whereas the expression of ATF2, which is normally reduced after injury, is completely preserved in GFAP-GDNF mice. Our data suggest that maintenance of high levels of ATF2 in injured MNs could be crucial in modulating c-Jun function, and c-Jun/ATF2 signaling could be involved in GDNF-mediated survival of mature MNs.

Spinal-supraspinal serotonergic circuits regulating neuropathic pain and its treatment with gabapentin

Not all neuropathic pain patients gain relief from current therapies that include the anticonvulsant, gabapentin, thought to modulate calcium channel function. We report a neural circuit that is permissive for the effectiveness of gabapentin. Substance P-saporin (SP-SAP) was used to selectively ablate superficial dorsal horn neurons expressing the neurokinin-1 receptor for substance P. These neurons project to the brain as shown by retrograde labelling and engage descending brainstem serotonergic influences that enhance spinal excitability via a facilitatory action on 5HT(3) receptors. We show the integrity of this pathway following nerve injury contributes to the behavioural allodynia, neuronal plasticity of deep dorsal horn neurons and the injury-specific actions of gabapentin. Thus SP-SAP attenuated the tactile and cold hypersensitivity and abnormal neuronal coding (including spontaneous activity, expansion of receptive field size) seen after spinal nerve ligation. Furthermore the powerful actions of gabapentin after neuropathy were blocked by either ablation of NK-1 expressing neurones or 5HT(3) receptor antagonism using ondansetron. Remarkably, 5HT(3) receptor activation provided a state-dependency (independent of that produced by neuropathy) allowing GBP to powerfully inhibit in normal uninjured animals. This circuit is therefore a crucial determinant of the abnormal neuronal and behavioural manifestations of neuropathy and importantly, the efficacy of gabapentin. As this spino-bulbo-spinal circuit contacts areas of the brain implicated in the affective components of pain, this loop may represent a route by which emotions can influence the degree of pain in a patient, as well as the effectiveness of the drug treatment. These hypotheses are testable in patients.

Immune and inflammatory mechanisms in neuropathic pain

Tissue damage, inflammation or injury of the nervous system may result in chronic neuropathic pain characterised by increased sensitivity to painful stimuli (hyperalgesia), the perception of innocuous stimuli as painful (allodynia) and spontaneous pain. Neuropathic pain has been described in about 1% of the US population, is often severely debilitating and largely resistant to treatment. Animal models of peripheral neuropathic pain are now available in which the mechanisms underlying hyperalgesia and allodynia due to nerve injury or nerve inflammation can be analysed. Recently, it has become clear that inflammatory and immune mechanisms both in the periphery and the central nervous system play an important role in neuropathic pain. Infiltration of inflammatory cells, as well as activation of resident immune cells in response to nervous system damage, leads to subsequent production and secretion of various inflammatory mediators. These mediators promote neuroimmune activation and can sensitise primary afferent neurones and contribute to pain hypersensitivity. Inflammatory cells such as mast cells, neutrophils, macrophages and T lymphocytes have all been implicated, as have immune-like glial cells such as microglia and astrocytes. In addition, the immune response plays an important role in demyelinating neuropathies such as multiple sclerosis (MS), in which pain is a common symptom, and an animal model of MS-related pain has recently been demonstrated. Here, we will briefly review some of the milestones in research that have led to an increased awareness of the contribution of immune and inflammatory systems to neuropathic pain and then review in more detail the role of immune cells and inflammatory mediators.

Mechanisms underlying enhanced P2X receptor-mediated responses in the neuropathic pain state

P2X3 and P2X2/3 receptors in dorsal root ganglia (DRG) appear to participate in producing nociceptive responses after nerve injury. However, the mechanisms underlying the receptor-mediated nociception in the neuropathic state remain unclear. Using spared nerve injury (SNI) rats, we found that allodynic and nocifensive (flinch) behavioral responses developed after injury can be reversed by P2X receptor antagonists, indicating an involvement of P2X receptors. Immunocytochemical studies revealed that P2X3 receptors are expressed in small and medium but rarely in large DRG neurons of both normal and SNI rats. Thus, contrary to the conventional view that only large A beta cells mediate allodynia, small and medium cells are intimately involved in P2X3 receptor-mediated allodynia. Measuring ATP levels in the subcutaneous space of the rat paw, we showed that ATP release does not change after SNI. On the other hand, the P2X receptor agonist, alpha beta-methylene ATP produces 3.5-fold larger flinch responses at a 8.0-fold lower dose. Thus, sensitization of P2X3 receptors rather than a change in ATP release is responsible for the neuropathic pain behaviors. We further demonstrated that sensitization of P2X3 receptors arises from an increase in receptor function. ATP-induced P2X3 receptor-mediated currents in DRG neurons is 2.5-fold larger after SNI. The expression of P2X3 receptors on the cell membrane is significantly enhanced while the total expression of P2X3 receptors remained unchanged. Thus, the enhancement of trafficking of P2X3 receptors is likely an important mechanism contributing to the increase in receptor function after nerve injury.

Down-regulation of GFRalpha-1 expression by antisense oligodeoxynucleotide attenuates electroacupuncture analgesia on heat hyperalgesia in a rat model of neuropathic pain

Glial cell line-derived neurotrophic factor (GDNF) has been proved to play an important role in the modulation of nociceptive transmission especially during neuropathic pain. It was reported that electroacupuncture (EA) had potent analgesic effect on neuropathic pain and our previous studies indicated that EA could activate endogenous GDNF signaling system (GDNF and its receptor GFRalpha-1) in dorsal root ganglions (DRGs) of neuropathic pain rats. In order to investigate whether GDNF signaling system was involved in EA analgesia on neuropathic pain, which was induced by chronic constriction injury (CCI) of the sciatic nerve in rats, antisense oligodeoxynucleotide (ODN) specifically against GFRalpha-1 was used in the present study to result in down-regulation of GFRalpha-1 expression. The results showed that: (1) cumulative EA had potent analgesic effect on neuropathic pain in rats; (2) the expression of GFRalpha-1 in DRGs was down-regulated by intrathecal delivery of antisense ODN, but not by normal saline (NS) or mismatch ODN; (3) EA analgesia was significantly attenuated by antisense ODN treatment. The present study demonstrated that endogenous GDNF signaling system was involved in EA analgesia on neuropathic pain in rats, which would deepen our realization of the mechanism of EA analgesia.

Role of the peripheral benzodiazepine receptor in sensory neuron regeneration

Peripheral benzodiazepine receptor (PBR) expression increases in small dorsal root ganglion (DRG) sensory neurons after peripheral nerve injury. To determine the functional significance of this induction, we evaluated the effects of PBR ligands on rodent sensory axon outgrowth. In vitro, Ro5-4864, a PBR agonist, enhanced outgrowth only of small peripherin-positive DRG neurons. When DRG cells were preconditioned into an active growth state by a prior peripheral nerve injury Ro5-4864 augmented and PK 11195, a PBR antagonist, blocked the injury-induced increased outgrowth. In vivo, Ro5-4864 increased the initiation of regeneration after a sciatic nerve crush injury and the number of GAP-43-positive axons in the distal nerve while PK 11195 inhibited the enhanced growth produced by a preconditioning lesion. These results show that PBR has a role in the early regenerative response of small caliber sensory axons, the preconditioning effect, and that PBR agonists enhance sensory axon regeneration.

Down-regulation of GFRalpha-1 expression by antisense oligodeoxynucleotide aggravates thermal hyperalgesia in a rat model of neuropathic pain

Glial cell line-derived neurotrophic factor (GDNF) has been hypothesized to play an important role in the modulation of nociceptive signals especially during neuropathic pain. The present study examined the expression of GDNF and GFRalpha-1 (the high-affinity receptor of GDNF) in dorsal root ganglions (DRG) in a rat model of neuropathic pain induced by chronic constriction injury (CCI) to the sciatic nerve. In order to address the role of GDNF and GFRalpha-1 in neuropathic pain, antisense oligodeoxynucleotide (ODN) specifically against GFRalpha-1 was intrathecally administered to result in down-regulation of GFRalpha-1 expression. The results showed that both the protein and mRNA levels of GDNF and GFRalpha-1 were significantly increased after CCI, while the thermal hyperalgesia of neuropathic pain rats could be significantly aggravated by antisense ODN treatment, but not by normal saline (NS) or mismatch ODN treatment. The present study demonstrated that endogenous GDNF and GFRalpha-1 might play an anti-hyperalgesic role in neuropathic pain of rats. In addition, we found a down-regulation of somatostatin (SOM) in DRG and spinal dorsal horn after expression of GFRalpha-1 was knocked down, which suggested the possible relationship between the anti-hyperalgesic effect of GDNF and GFRalpha-1 on neuropathic pain and endogenous SOM.

alpha2-adrenoceptors inhibit the intracellular Ca2+ response to electrical stimulation in normal and injured sensory neurons, with increased inhibition of calcitonin gene-related peptide expressing neurons after injury

Nerve injury resulting in chronic pain is associated with novel excitatory effects of norepinephrine on injured peripheral nerve terminals and their cell bodies, due to actions on alpha2-adrenoceptors. Paradoxically, alpha2-adrenoceptor agonists administered near peripheral terminals or their cell bodies results in analgesia, not pain. This study tested, using intracellular Ca2+ response to stimulation, the effects of alpha2-adrenoceptor agonists on injured sensory neurons and classified their neuronal phenotype. Dorsal root ganglion cells from normal and spinal nerve-ligated rats were dissociated and activated twice with electrical field stimulation, while measuring Fura-2 fluorescence. Cells were perfused between stimulations with vehicle or alpha2-adrenoceptor agonists alone or with antagonists. Cells were considered inhibited if the ratio of their peak Ca2+ response to the second stimulus divided by the first was less than the 2.5th percentile for vehicle controls. alpha2-, But not alpha1-adrenoceptor agonists inhibited the Ca2+ response in a concentration related fashion, and this inhibition was blocked by alpha2-adrenoceptor antagonists. Clonidine inhibited a similar percentage of cells in the normal and spinal nerve-ligated group. In both groups, the large majority of clonidine-inhibited cells stained for isolectin B4. Spinal nerve ligation resulted in a 4-10-fold increase in the percentage of clonidine inhibited cells which immunostained for calcitonin gene-related peptide. These data are consistent with the known inhibition of Ca2+ currents by alpha2-adrenoceptors and suggest that, at the level of intracellular Ca2+, the key determinant of neurotransmitter release, alpha2-adrenoceptors are inhibitory after nerve injury, not excitatory. There is a shift in phenotype of sensory neurons which are inhibited by clonidine after nerve injury, which may explain clonidine's increased potency in the treatment of neuropathic compared with acute pain.

Changes of expression of glial cell line-derived neurotrophic factor and its receptor in dorsal root ganglions and spinal dorsal horn during electroacupuncture treatment in neuropathic pain rats

Injury to the nervous system occasionally leads to intense and persistent neuropathic pain, which is resistant to conventional analgesic methods. It was reported that electroacupuncture (EA) had potent analgesic effect on neuropathic pain by activating various endogenous transmitters such as the opioid peptides. Glial cell line-derived neurotrophic factor (GDNF) has been hypothesized to play an important role in modulation of nociceptive signals especially during neuropathic pain state. Using immunohistochemistry, Western blot, and RT-PCR analysis techniques, the present study observed the effects of EA on the expression of GDNF and GDNF family receptor alpha-1 (GFRalpha-1, the high-affinity receptor of GDNF) in neuropathic pain rats. The results showed that both protein and mRNA levels of GDNF and GFRalpha-1 in the dorsal root ganglions (DRG), as well as GDNF protein in the spinal dorsal horn, were significantly increased after chronic constriction injury (CCI) of the rats' sciatic nerve and could be further enhanced by EA treatment. The present data demonstrated that EA could activate endogenous GDNF and GFRalpha-1 system of neuropathic pain rats and this might underlie the effectiveness of EA in the treatment of neuropathic pain.

Identification of versican as an isolectin B4-binding glycoprotein from mammalian spinal cord tissue

Nociceptors are specialized nerve fibers that transmit noxious pain stimuli to the dorsal horn of the spinal cord. A subset of nociceptors, the nonpeptidergic C-fibers, is characterized by its reactivity for the plant isolectin B4 (IB4) from Griffonia simplicifolia. The molecular nature of the IB4-reactive glycoconjugate, although used as a neuroanatomical marker for more than a decade, has remained unknown. We here present data which strongly suggest that a splice variant of the extracellular matrix proteoglycan versican is the IB4-reactive glycoconjugate associated with these nociceptors. We isolated (by subcellular fractionation and IB4 affinity chromatography) a glycoconjugate from porcine spinal cord tissue that migrated in SDS/PAGE as a single distinct protein band at an apparent molecular mass of > 250 kDa. By using MALDI-TOF/TOF MS, we identified this glycoconjugate unambiguously as a V2-like variant of versican. Moreover, we demonstrate that the IB4-reactive glycoconjugate and the versican variant can be co-released from spinal cord membranes by hyaluronidase, and that the IB4-reactive glycoconjugate and the versican variant can be co-precipitated by an anti-versican immunoglobulin and perfectly co-migrate in SDS/PAGE. Our findings shed new light on the role of the extracellular matrix, which is thought to be involved in plastic changes underlying pain-related phenomena such as hyperalgesia and allodynia.

Medium and large injured dorsal root ganglion cells increase TRPV-1, accompanied by increased α2C-adrenoceptor co-expression and functional inhibition by clonidine

Some electrophysiologic studies demonstrate new, excitatory alpha2-adrenoceptors on peripheral nociceptors and their dorsal root ganglion (DRG) cell bodies after nerve injury, yet administration of alpha2-adrenoceptor agonists at these sites reduces hypersensitivity rather than worsens it. Since TRPV-1 expressing nociceptor afferents are important in many pain states, we examined the expression of this channel and its co-expression with alpha2C-adrenoceptors in injured DRG cell bodies and the ability of alpha2-adrenoceptors to inhibit responses to stimulation. Rats underwent tight ligation of the left L5 and L6 spinal nerves, followed by behavioral testing, removal of L5 and L6 DRGs, and either immunostaining for TRPV-1 channels and alpha2C-adrenoceptors or intracellular calcium videomicroscopy in response to electrical field stimulation before and after perfusion with clonidine and capsaicin. Spinal nerve ligation produced tactile allodynia. In normal and sham controls, about one-third of DRG neurons were TRPV-1-immunoreactive (IR), one half were alpha2C-adrenoceptor-IR and one-fourth co-expressed both. After nerve ligation there was a reduction in the number of small, strongly TRPV-1-IR or alpha2C-adrenoceptor-IR neurons, but an increase in medium and large, lightly stained cells and in their co-expression. The proportion of clonidine inhibited cells which responded to capsaicin increased 5 fold after injury. We conclude that TRPV-1 and alpha2C-adrenoceptors are up-regulated in some injured medium and large size neurons after nerve ligation. Increased co-expression by immunocytochemistry, and increased proportion of cells inhibited by clonidine and expressing functional TRPV-1 channels suggest that these cells may play an important role in the analgesic effects of alpha2-adrenoceptor agonists in neuropathic pain.

The behavioral and neuroanatomical effects of IB4-saporin treatment in rat models of nociceptive and neuropathic pain

One distinguishing feature of primary afferent neurons is their ability to bind the lectin IB(4). Previous work suggested that neurons in the inner part of lamina II (IIi), onto which IB(4)-positive sensory neurons project, facilitate nociceptive transmission following tissue or nerve injury. Using an IB(4)-saporin conjugate (IB(4)-SAP), we examined the contribution of IB(4)-positive neurons to nociceptive processing in rats with and without nerve injury. Intrasciatic injection of IB(4)-SAP (5 mug/5 mul) significantly decreased IB(4)-labeling and immunoreactive P(2)X(3) in the spinal cord and delayed the behavioral and neuroanatomical consequences of L5 spinal nerve ligation (SNL) injury. In the absence of injury, thermal and mechanical nociceptive thresholds increased 2 weeks post-treatment only in IB(4)-SAP-treated, but not control (saline or saporin only), rats. Acute NGF-induced hyperalgesia was also attenuated following IB(4)-SAP treatment. In the SNL model, mechanical allodynia failed to develop 1 and 2 weeks post-injury, but was fully established by 4 weeks. Moreover, neuropeptide Y immunoreactivity (NPY-ir), which increases in the spinal cord after nerve injury, was unchanged in IB(4)-SAP-treated animals whereas immunoreactive PKCgamma decreased 2, but not 4, weeks post-injury. Quantitative RT-PCR revealed a reduction in P(2)X(3) mRNA in L4 DRG of IB(4)-SAP-treated animals, but no change in TrkA expression. Our results suggest that IB(4)-positive neurons in L4 are required for the full expression of NGF-induced hyperalgesia and participate in the behavioral and anatomical consequences that follow injury to the L5 spinal nerve.

Differential pH and capsaicin responses of Griffonia simplicifolia IB4 (IB4)-positive and IB4-negative small sensory neurons

Protons play a key role in nociception caused by inflammation and ischaemia, but little is known about the relative sensitivities of different dorsal root ganglion (DRG) neurons. We have therefore examined the responses in vitro of rat DRG cells classified according to whether or not they bind Griffonia simplicifolia IB4 (IB4), a lectin which is widely used to distinguish between two major populations of small diameter neurons. Under voltage-clamp conditions, proton-activated inward currents were found in approximately 90% of small DRG neurons and showed one of three waveforms: transient, sustained or mixed. The majority of IB4-positive (IB4+) neurons (63%) gave rise to sustained inward currents that were sensitive to capsazepine. In contrast, the most prevalent waveform in small IB4-negative (IB4-) neurons (69%) was a mixed response containing transient and sustained components. The transient component was inhibited by amiloride whilst the sustained component showed a variable sensitivity to capsazepine. We also found that more IB4+ cells responded to capsaicin and, on average, gave rise to a larger magnitude of response than small IB4- neurons, consistent with their higher prevalence and greater amplitude of vanilloid receptor 1 (TRPV1)-like acid responses. The increase in intracellular Ca(2+) induced by capsaicin was also slightly greater in IB4+ neurons and in these cells its magnitude correlated with the level of TRPV1 immunoreactivity. Our data suggest that acid-sensing ion channels (ASICs) and TRPV1 are the major acid-sensitive receptors in small IB4- neurons, whilst TRPV1 is the predominant one in IB4+ neurons. Because ASIC-like responses were approximately 10-fold more sensitive to changes in H(+) than TRPV1-like responses, we speculate that small IB4- rather than IB4+ neurons play an essential role in sensing acid. Our results also highlight differences in capsaicin responses between IB4+ and IB4- small neurons and reveal the close link between capsaicin responses and levels of TRPV1 expression.

Cell signaling and the genesis of neuropathic pain

Damage to the nervous system can cause neuropathic pain, which is in general poorly treated and involves mechanisms that are incompletely known. Currently available animal models for neuropathic pain mainly involve partial injury of peripheral nerves. Multiple inflammatory mediators released from damaged tissue not only acutely excite primary sensory neurons in the peripheral nervous system, producing ectopic discharge, but also lead to a sustained increase in their excitability. Hyperexcitability also develops in the central nervous system (for instance, in dorsal horn neurons), and both peripheral and spinal elements contribute to neuropathic pain, so that spontaneous pain may occur or normally innocuous stimuli may produce pain. Inflammatory mediators and aberrant neuronal activity activate several signaling pathways [including protein kinases A and C, calcium/calmodulin-dependent protein kinase, and mitogen-activated protein kinases (MAPKs)] in primary sensory and dorsal horn neurons that mediate the induction and maintenance of neuropathic pain through both posttranslational and transcriptional mechanisms. In particular, peripheral nerve lesions result in activation of MAPKs (p38, extracellular signal-regulated kinase, and c-Jun N-terminal kinase) in microglia or astrocytes in the spinal cord, or both, leading to the production of inflammatory mediators that sensitize dorsal horn neurons. Activity of dorsal horn neurons, in turn, enhances activation of spinal glia. This neuron-glia interaction involves positive feedback mechanisms and is likely to enhance and prolong neuropathic pain even in the absence of ongoing peripheral external stimulation or injury. The goal of this review is to present evidence for signaling cascades in these cell types that not only will deepen our understanding of the genesis of neuropathic pain but also may help to identify new targets for pharmacological intervention.

Voltage-gated sodium channels and hyperalgesia

Physiological and pharmacological evidence both have demonstrated a critical role for voltage-gated sodium channels (VGSCs) in many types of chronic pain syndromes because these channels play a fundamental role in the excitability of neurons in the central and peripheral nervous systems. Alterations in function of these channels appear to be intimately linked to hyperexcitability of neurons. Many types of pain appear to reflect neuronal hyperexcitability, and importantly, use-dependent sodium channel blockers are effective in the treatment of many types of chronic pain. This review focuses on the role of VGSCs in the hyperexcitability of sensory primary afferent neurons and their contribution to the inflammatory or neuropathic pain states. The discrete localization of the tetrodotoxin (TTX)-resistant channels, in particular NaV1.8, in the peripheral nerves may provide a novel opportunity for the development of a drug targeted at these channels to achieve efficacious pain relief with an acceptable safety profile.

NGF and GDNF ameliorate the increase in ATF3 expression which occurs in dorsal root ganglion cells in response to peripheral nerve injury

Activating transcription factor-3 (ATF3) is a member of the ATF/CREB transcription factor superfamily and is induced in dorsal root ganglion (DRG) cells after nerve injury. In order to study the regulation of ATF3, we have examined the effect of nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) on ATF3 expression. In untreated rats, sciatic nerve transection induced ATF3 immunoreactivity in 82% of L4 DRG cells at 14 days after axotomy. Intrathecal delivery of NGF or GDNF for 2 weeks commencing immediately after injury reduced the ATF3 expression to 35 and 23% of DRG cells, respectively. Cell size analysis indicated that NGF had protected a population of mainly small- to medium-sized cells, but that the GDNF had protected a population of both small and large cells. This effect was confirmed by double labelling for P2X(3), CGRP and 200 kDa neurofilament, markers for small peptide-poor cells, peptide-rich cells and large cells, respectively. Thus GDNF reduced the percentage of ATF3-immunoreactive P2X(3) cells from 70 to 4%, and the percentage of ATF3-immunoreactive neurofilament cells from 63 to 24%. NGF was less effective than GDNF in reducing ATF3 expression in these cell types, but reduced the percentage of ATF3-immunoreactive CGRP cells from 10% to < 1%. These results show that ATF3 expression in specific populations of DRG cells can be modulated by exogenous supplementation of specific trophic factors, and suggest that ATF3 expression may normally be induced by the loss of target-derived NGF and GDNF.

Effects of spinal nerve ligation on immunohistochemically identified neurons in the L4 and L5 dorsal root ganglia of the rat

This study examined the effect of spinal nerve ligation on different populations of immunohistochemically identified neurons in the dorsal root ganglia (DRG) of the rat. The optical fractionator method was used to count neurons in the ipsilateral L4 and L5 DRG 1-20 weeks after ligation of the L5 and L6 spinal nerves, sham surgery, or no surgery. One week after ligation, neurons in the L5 DRG that were labeled by IB4, a marker of unmyelinated primary afferent neurons, were largely absent. The numbers of IB4-labeled neurons then progressively increased to reach control values by 20 weeks. A smaller, sustained decrease occurred in the number of small-, medium- and large-sized neurons immunoreactive for calcitonin gene-related peptide (CGRP), a marker for peptidergic primary afferents, in the L5 DRG. There was a proportionately greater decrease in the numbers of medium- to large-sized CGRP-like immunoreactive neurons. The number of myelinated afferents in the L5 DRG, identified by their staining for neurofilament protein (N52), did not change after ligation. However, closer examination revealed a significant decrease in the numbers of large-sized neurons, coupled with an increase in the numbers of small- to medium-sized neurons, and the appearance of a novel population of very small-sized neurons labeled by N52. The numbers and cell size distributions of IB4-labeled, CGRP-like immunoreactive, and N52-labeled neurons were unchanged in the adjacent L4 DRG. Unlike the L5 DRG, injury-induced changes in the expression of various receptors, neurotransmitters and neurotrophic factors in the L4 DRG are not confounded by a change in the immunohistochemical phenotype of primary afferent neurons.