Intrathecally delivered glial cell line-derived neurotrophic factor produces electrically evoked release of somatostatin in the dorsal horn of the spinal cord

Interplay of BDNF and GDNF in the Mature Spinal Somatosensory System and Its Potential Therapeutic Relevance

The growth factors BDNF and GDNF are gaining more and more attention as modulators of synaptic transmission in the mature central nervous system (CNS). The two molecules undergo a regulated secretion in neurons and may be anterogradely transported to terminals where they can positively or negatively modulate fast synaptic transmission. There is today a wide consensus on the role of BDNF as a pro-nociceptive modulator, as the neurotrophin has an important part in the initiation and maintenance of inflammatory, chronic, and/or neuropathic pain at the peripheral and central level. At the spinal level, BDNF intervenes in the regulation of chloride equilibrium potential, decreases the excitatory synaptic drive to inhibitory neurons, with complex changes in GABAergic/glycinergic synaptic transmission, and increases excitatory transmission in the superficial dorsal horn. Differently from BDNF, the role of GDNF still remains to be unraveled in full. This review resumes the current literature on the interplay between BDNF and GDNF in the regulation of nociceptive neurotransmission in the superficial dorsal horn of the spinal cord. We will first discuss the circuitries involved in such a regulation, as well as the reciprocal interactions between the two factors in nociceptive pathways. The development of small molecules specifically targeting BDNF, GDNF and/or downstream effectors is opening new perspectives for investigating these neurotrophic factors as modulators of nociceptive transmission and chronic pain. Therefore, we will finally consider the molecules of (potential) pharmacological relevance for tackling normal and pathological pain.

Botulinum toxin type A induces changes in the chemical coding of substance P-immunoreactive dorsal root ganglia sensory neurons supplying the porcine urinary bladder

Botulinum toxin (BTX) is a potent neurotoxin which blocks acetylcholine release from nerve terminals, and therefore leads to cessation of somatic motor and/or parasympathetic transmission. Recently it has been found that BTX also interferes with sensory transmission, thus, the present study was aimed at investigating the neurochemical characterization of substance P-immunoreactive (SP-IR) bladder-projecting sensory neurons (BPSN) after the toxin treatment. Investigated neurons were visualized with retrograde tracing method and their chemical profile was disclosed with double-labelling immunohistochemistry using antibodies against SP, calcitonin gene-related peptide (CGRP), pituitary adenylate cyclase activating polypeptide (PACAP), neuronal nitric oxide synthase (nNOS), galanin (GAL), calbindin (CB), and somatostatin (SOM). In the control group (n = 6), 45% of the total population of BPSN were SP-IR. Nearly half of these neurons co-expressed PACAP or CGRP (45% and 35%, respectively), while co-localization of SP with GAL, nNOS, SOM or CB was found less frequently (3.7%, 1.8%, 1.2%, and 0.7%, respectively). In BTX-treated pigs (n = 6), toxin-injections caused a decrease in the number of SP-IR cells containing CGRP, SOM or CB (16.2%, 0.5%, and 0%, respectively) and a distinct increase in these nerve cells immunopositive to GAL (27.2%). The present study demonstrates that BTX significantly modifies the chemical phenotypes of SP-IR BPSN.

Targeting the glial-derived neurotrophic factor and related molecules for controlling normal and pathologic pain

INTRODUCTION

Glial-derived neurotrophic factor (GDNF) and its family of ligands (GFLs) have several functions in the nervous system. As a survival factor for dopaminergic neurons, GDNF was used in clinical trials for Parkinson's disease. GFLs and their receptors are also potential targets for new pain-controlling drugs. Although molecules with analgesic activities in rodents mostly failed to be effective in translational studies, this potential should not be underestimated.

AREAS COVERED

The circuitry, molecular, and cellular mechanisms by which GFLs control nociception and their intervention in inflammatory and neuropathic pain are considered first. The problems related to effective GDNF delivery to the brain and the possibility to target the GFL receptor complex rather than its ligands are then discussed, also considering the use of non-peptidyl agonists.

EXPERT OPINION

In nociceptive pathways, an ideal drug should either: i) target the release of endogenous GFLs from large dense-cored vesicles (LGVs) by acting, for example, onto the phosphatidylinositol-3-phosphate [PtdIns(3)P] pool, which is sensitive to Ca(2+) modulation, or ii) target the GFL receptor complex. Besides XIB403, a tiol molecule that enhances GFRα family receptor signaling, existing drugs such as retinoic acid and amitriptyline should be considered for effective targeting of GDNF, at least in neuropathic pain. The approach of pain modeling in experimental animals is discussed.

Presynaptic modulation of spinal nociceptive transmission by glial cell line-derived neurotrophic factor (GDNF)

The role of glial cell line-derived neurotrophic factor (GDNF) in nociceptive pathways is still controversial, as both pronociceptive and antinociceptive actions have been reported. To elucidate this role in the mouse, we performed combined structural and functional studies in vivo and in acute spinal cord slices where C-fiber activation was mimicked by capsaicin challenge. Nociceptors and their terminals in superficial dorsal horn (SDH; laminae I-II) constitute two separate subpopulations: the peptidergic CGRP/somatostatin+ cells expressing GDNF and the nonpeptidergic IB4+ neurons expressing the GFRα1-RET GDNF receptor complex. Ultrastructurally the dorsal part of inner lamina II (LIIid) harbors a mix of glomeruli that either display GDNF/somatostatin (GIb)-IR or GFRα1/IB4 labeling (GIa). LIIid thus represents the preferential site for ligand-receptor interactions. Functionally, endogenous GDNF released from peptidergic CGRP/somatostatin+ nociceptors upon capsaicin stimulation exert a tonic inhibitory control on the glutamate excitatory drive of SDH neurons as measured after ERK1/2 phosphorylation assay. Real-time Ca(2+) imaging and patch-clamp experiments with bath-applied GDNF (100 nM) confirm the presynaptic inhibition of SDH neurons after stimulation of capsaicin-sensitive, nociceptive primary afferent fibers. Accordingly, the reduction of the capsaicin-evoked [Ca(2+)]i rise and of the frequency of mEPSCs in SDH neurons is specifically abolished after enzymatic ablation of GFRα1. Therefore, GDNF released from peptidergic CGRP/somatostatin+ nociceptors acutely depresses neuronal transmission in SDH signaling to nonpeptidergic IB4+ nociceptors at glomeruli in LIIid. These observations are of potential pharmacological interest as they highlight a novel modality of cross talk between nociceptors that may be relevant for discrimination of pain modalities.

Botulinum toxin type A-induced changes in the chemical coding of dorsal root ganglion neurons supplying the porcine urinary bladder

Botulinum toxin type A (BTX) is a potent neurotoxin, which in recent years has been effectively applied in experimental treatments of many neurogenic disorders of the urinary bladder. BTX is a selective, presynaptically-acting blocking agent of acetylcholine release from nerve terminals what, in turn, leads to the cessation of somatic motor and/or parasympathetic transmission. However, application of this toxin in urological practice is still in the developmental stages and the full mechanism of its action remain elusive. Thus, the present study was aimed at investigating the neurochemical characterization of dorsal root ganglion (DRG) neurons supplying the porcine urinary bladder after BTX treatment. Retrograde tracer Fast Blue (FB) was injected into the urinary bladder wall in six juvenile female pigs and three weeks later, intramural bladder injections of BTX (100 IU per animal) were carried out in all the animals. After a week, DRG from L1 to Cql were harvested from the pigs and neurochemical characterization of FB+ neurons was performed using double- labeling immunofluorescence technique on 10-microm-thick cryostat sections. BTX injections led to a significant decrease in the number of FB+ neurons containing substance P (SP), calcitonin gene-related peptide (CGRP), calbindin (CB), somatostatin (SOM) and neuronal nitric oxide synthase (nNOS) when compared with that found in the healthy animals (19% vs. 45%, 18% vs. 36%, 0.6% vs. 3%, 0.4 vs. 4% and 0.1% vs. 6%, respectively) These data demonstrated that BTX changed the chemical coding of bladder sensory neurons, and therefore this drug should be taken into consideration when it planning experimental therapy of selected neurogenic bladder disorders.

Glial cell line-derived neurotrophic factor gene transfer exerts protective effect on axons in sciatic nerve following constriction-induced peripheral nerve injury

Damage to peripheral nerves following trauma or neurodegenerative diseases often results in various sensory and motor abnormalities and chronic neuropathic pain. The loss of neurotrophic factor support has been proposed to contribute to the development of peripheral neuropathy. The main objective of this study was to investigate the protective effect of glial cell line-derived neurotrophic factor (GDNF) using peripheral gene delivery in a rat model of constriction-induced peripheral nerve injury. In this study, it was shown that mechanical and thermal hypersensitivity increased on the injured limb at day 7 after chronic constrictive injury (CCI) was induced. The neurological changes were correlated with the structural changes and loss of GDNF/Akt signaling, particularly in the distal stump of the injured sciatic nerve. Subsequently, recombinant adenovirus was employed to evaluate the potential of intramuscular GDNF gene delivery to alleviate the CCI-induced nerve degeneration ad neuropathic pain. After CCI for 3 days, intramuscular injection of adenovirus encoding GDNF (Ad-GDNF) restored the protein level and activity of GDNF/Akt signaling pathway in the sciatic nerve. This was associated with an improved myelination profile and behavioral outcomes in animals with CCI. In conclusion, the present study demonstrates the involvement of GDNF loss in the pathogenesis of CCI-induced neuropathic pain and the therapeutic potential of intramuscular GDNF gene delivery for the treatment of peripheral nerve degeneration.

The timing and location of glial cell line-derived neurotrophic factor expression determine enteric nervous system structure and function

Ret signaling is critical for formation of the enteric nervous system (ENS) because Ret activation promotes ENS precursor survival, proliferation, and migration and provides trophic support for mature enteric neurons. Although these roles are well established, we now provide evidence that increasing levels of the Ret ligand glial cell line-derived neurotrophic factor (GDNF) in mice causes alterations in ENS structure and function that are critically dependent on the time and location of increased GDNF availability. This is demonstrated using two different strains of transgenic mice and by injecting newborn mice with GDNF. Furthermore, because different subclasses of ENS precursors withdraw from the cell cycle at different times during development, increases in GDNF at specific times alter the ratio of neuronal subclasses in the mature ENS. In addition, we confirm that esophageal neurons are GDNF responsive and demonstrate that the location of GDNF production influences neuronal process projection for NADPH diaphorase-expressing, but not acetylcholinesterase-, choline acetyltransferase-, or tryptophan hydroxylase-expressing, small bowel myenteric neurons. We further demonstrate that changes in GDNF availability influence intestinal function in vitro and in vivo. Thus, changes in GDNF expression can create a wide variety of alterations in ENS structure and function and may in part contribute to human motility disorders.

Brief, low frequency stimulation of rat peripheral C-fibres evokes prolonged microglial-induced central sensitization in adults but not in neonates

The sensitization of spinal dorsal horn neurones leads to prolonged enhancement of pain behaviour and can be evoked by intense C-fibre stimulation, tissue inflammation and peripheral nerve injury. Activation of central immune cells plays a key role in establishing pain hypersensitivity but the exact nature of the afferent input that triggers the activation of microglia and other glial cells within the CNS, remains unclear. Here intense but non-damaging, electrical stimulation of intact adult rat C-fibres for 5 min at 10 Hz induced central sensitization characterized by significant decreases in mechanical withdrawal thresholds 3, 24 and 48 h later. This maintained (>3 h) hypersensitivity was not observed following topical skin application of capsaicin. C-fibre evoked sensitization was accompanied by significant microglial activation, shown by increased Iba-1 immunoreactivity throughout the dorsal horn at 24 and 48 h and significant upregulation of markers of microglial activation: IL-6 and Mcp-1 at 3 h and Mmp3, CSF-1 and CD163 at 24 and 48 h. C-fibre stimulation caused no nerve damage at ultrastructural and molecular levels. Lower intensity stimulation that did not activate C-fibres or sham stimulation did not increase Iba-1 immunoreactivity or induce behavioural sensitivity. Pre-treatment with minocycline (40 mg/kg, i.p.) prevented the C-fibre evoked sensitization and microglial activation. Identical C-fibre stimulation in 10-day old rat pups failed to activate microglia or change behaviour. These results demonstrate that a brief period of low frequency C-fibre stimulation, in the absence of nerve damage, is sufficient to activate microglia resulting in behavioural hyperalgesia.

Involvement of neural cell adhesion molecule signaling in glial cell line-derived neurotrophic factor-induced analgesia in a rat model of neuropathic pain

Since neuropathic pain is resistant to conventional analgesics such as opiates and non-steroidal anti-inflammatory drugs, the development of new types of drugs for its treatment has been awaited. Several key molecules associated with nociception have been suggested as potential targets for new analgesics. Glial cell line-derived neurotrophic factor (GDNF) has a variety of functions affecting the survival and development of specified neural cell populations, mediated via transmission of intracellular signals through binding to its high-affinity receptor, GFR*1, and subsequent activation of a tyrosine receptor kinase, RET, neural cell adhesion molecule (NCAM), or other signaling molecules. GDNF also exhibits analgesic effects in rodent models of neuropathic pain, although the underlying mechanisms are still largely unknown, including the intracellular signal transduction involved. We report here that NCAM signaling plays a role in mediating the analgesic effect of GDNF in rats with chronic constrictive injury (CCI). We found that NCAM was expressed in intrinsic neurons in the spinal dorsal horn and in dorsal root ganglion neurons with small cell bodies. Reduction of NCAM expression by NCAM antisense oligodeoxynucleotide administration to CCI rats abolished the analgesic effect of GDNF without affecting RET signaling activation. An NCAM mimetic peptide, C3d, partially reduced the chronic pain induced by CCI. These findings suggest that NCAM signaling plays a critical role in the analgesic effect of GDNF and that development of new drugs activating GDNF-NCAM signaling may represent a new strategy for the relief of intractable pain.

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.

Effects of electroacupuncture on expression of somatostatin and preprosomatostatin mRNA in dorsal root ganglions and spinal dorsal horn in neuropathic pain rats

Somatostatin (SOM) is an endogenous non-opioid neuropeptide that has analgesic effect in rodents and human beings. Previous studies indicated that SOM might be involved in the modulating effects of electroacupuncture (EA). Using immunohistochemistry and RT-PCR, the present study observed the effects of EA on the expression of SOM peptide and preprosomatostatin (ppSOM) mRNA in a rat model of neuropathic pain induced by chronic constriction injury (CCI) to the sciatic nerve. No significant change was detected in the expression of SOM and ppSOM mRNA following CCI. However, EA could significantly enhance SOM expression in dorsal root ganglion (DRG) and spinal dorsal horn as well as ppSOM mRNA level in DRG of neuropathic pain rats. The present data demonstrated that EA could activate endogenous SOM of neuropathic pain rats and this might be one of the mechanisms that underlie the effectiveness of EA in the treatment of neuropathic pain.

NGF and GDNF differentially regulate TRPV1 expression that contributes to development of inflammatory thermal hyperalgesia

The transient receptor potential ion channel, TRPV1 plays an essential role in the development of inflammatory thermal hyperalgesia. We investigated the dependence of inflammatory TRPV1 induction on neurotrophic factor. Rat dorsal root ganglia (DRG) neurons were classified according to immunostaining for trk-A and IB4 and the effects of antibodies against NGF or GDNF on TRPV1 expression within the groups were then analysed by immunohistochemical means. The data were compared with the time course of trophic factor expression and the effects of their antibodies on thermal hyperalgesia against radiant heat after inflammation. Although the levels of both NGF and GDNF were increased by inflammation, NGF rapidly and transiently increased whereas GDNF increased gradually over a period of approximately one week. TRPV1 expression was increased within both trk-A positive and IB4 positive neurons after inflammation. Increased TRPV1 expression within trk-A positive neurons was prevented by anti-NGF but not by anti-GDNF, whereas TRPV1 induction within the IB4 positive group was blocked by anti-GDNF but not by anti-NGF. Both antibodies prevented the short latency of withdrawing an inflamed paw from radiant heat. These results suggest that inflammation differentially increases both NGF and GDNF, which facilitate TRPV1 expression within distinctive neurons to induce thermal hyperalgesia.

Inflammation alters somatostatin mRNA expression in sensory neurons in the rat

Proinflammatory neuropeptides, such as substance P and calcitonin gene-related peptide, are up-regulated in primary afferent neurons in acute and chronic inflammation. While these neuropeptides have been intensively studied, potentially anti-inflammatory and/or anti-nociceptive neuropeptides such as somatostatin (SS) have been less widely investigated. Endogenous somatostatin is thought to exert a tonic antinociceptive effect. Exogenous SS is anti-inflammatory and antinociceptive and is thought to exert these actions through inhibition of proinflammatory neuropeptide release. In this study we have compared the expression of somatostatin in two inflammatory models: arthritis, a condition associated with increased nociception, and periodontitis, in which there is little evidence of altered nociceptive thresholds. In acute arthritis (< 24 h) SS mRNA was down-regulated in ipsilateral dorsal root ganglia (DRG; 52 +/- 7% of control, P < 0.05), and up-regulated in contralateral DRG (134 +/- 10% of control; P < 0.05). In chronic arthritis (14 days) this pattern of mRNA regulation was reversed, with SS being up-regulated ipsilaterally and down-regulated contralaterally. In chronic mandibular periodontitis (7-10 days), SS mRNA was up-regulated in only the mandibular division of the ipsilateral trigeminal ganglion (TG) (day 7, 219 +/- 9% and day 10, 217 +/- 12% of control; P < 0.02) but showed no change in other divisions of the trigeminal ganglion or in the mesencephalic nucleus. These data show that antinociceptive and anti-inflammatory neuropeptides are also regulated in inflammation. It is possible that the degree of inflammation and nociception seen may depend on the balance of pro- and anti-inflammatory and nociceptive peptide expression in a particular condition.

Decreased expression of glial cell line-derived neurotrophic factor signaling in rat models of neuropathic pain

1. In an attempt to clarify whether glial cell line-derived neurotrophic factor (GDNF), a survival factor for subpopulations of primary afferent neurons, is involved in the states of neuropathic pain, we observed changes in the expressions of GDNF and its signal-transducing receptor Ret after nerve injury in two rat models of neuropathic pain. 2. In the rats treated with sciatic nerve ligation (chronic constrictive injury (CCI) model) or spinal nerve ligation at L5 (SNL model), the thresholds of paw withdrawal in response to mechanical or heat stimuli began to decrease on the injured side within the first week after the operation and the decreases in the thresholds persisted for more than 2 weeks. 3. In CCI-treated rats, the GDNF contents in L4 and L5 dorsal root ganglia (DRGs) on the injured side were markedly decreased at day 7 after the operation and stayed at low levels at day 14. In SNL-treated rats, comparable reductions of GDNF levels in L4 and L5 DRGs on the injured side were observed at 14 postoperative days. 4. Significant decreases of the percentages of DRG neurons expressing Ret were also observed at L4 DRGs in CCI-treated rats at 7 and 14 postoperative days and in SNL-treated rats at 14 days. 5. In CCI- or SNL-treated rats, continuous intrathecal administration of GDNF (12 microg day-1) using an osmotic pump suppressed the increased sensitivities to nociceptive stimuli to control levels. 6. The present results suggested that the dysfunction of GDNF signaling in the nociceptive afferent system may contribute to the development and/or maintenance of neuropathic pain states.

BDNF modulates sensory neuron synaptic activity by a facilitation of GABA transmission in the dorsal horn

Topical application of brain-derived neurotrophic factor (BDNF) to the adult rat isolated dorsal horn with dorsal root attached preparation inhibited the electrically evoked release of substance P (SP) from sensory neurons. This effect of BDNF was dose dependent (EC(50) 250 pM) and reversed by the tyrosine kinase inhibitor, K-252a. BDNF-induced inhibition of SP release was blocked by the GABA(B) receptor antagonist CGP 55485 but not by naloxone. Acute application of BDNF significantly increased potassium-stimulated release of GABA in the dorsal horn isolated in vitro and this effect was blocked by K-252a. Intrathecal injection of BDNF into the rat lumbar spinal cord induced a short-lasting increase in hindpaw threshold to noxious thermal stimulation that was blocked by CGP 55485 and was associated with activation of ERK in dorsal horn. These data suggest that exogenous BDNF can indirectly modulate primary sensory neuron synaptic efficacy via facilitation of the release of GABA from dorsal horn interneurons.

A novel control mechanism based on GDNF modulation of somatostatin release from sensory neurones

Small-diameter sensory neurones found in the rat dorsal root ganglia (DRG) include cells sensitive to glial cell line-derived neurotrophic factor (GDNF), which express the inhibitory peptide somatostatin (SOM). Here we addressed the functional relationship between GDNF and sensory neurone-derived SOM. Topical application of GDNF through the rat isolated dorsal horn of the spinal cord promoted activity-induced release of SOM from central terminals of sensory neurones. Once released by sensory neurones, SOM is known to act, at least in part, by opposing the action of Substance P (SP) in neurogenic inflammation. Therefore, we evaluated GDNF ability to modulate two well-documented effects of peripherally and centrally administered SP. Local application of GDNF in the mouse air pouch reduced SP-induced leukocyte migration. This effect of GDNF was mimicked by the SOM analog octreotide (OCT) and required intact SOM neuronal pools. Intrathecal injection of GDNF activated rat lumbar dorsal horn neurones and inhibited intrathecal SP-induced thermal hypersensitivity. This effect of GDNF was reversed by the SOM antagonist c-SOM and mimicked by OCT. In conclusion we propose GDNF regulation of neuronal SOM release as a novel mechanism that, if explored, may lead to new therapeutic strategies based on local release of somatostatin.

Regulation of nociceptive neurons by nerve growth factor and glial cell line derived neurotrophic factor

Nociceptive dorsal root ganglion (DRG) cells can be divided into three main populations, namely (1) small diameter non-peptide-expressing cells, (2) small-diameter peptide-expressing (calcitonin gene related peptide (CGRP), substance P) cells, and (3) medium-diameter peptide-expressing (CGRP) cells. The properties of these cell populations will be reviewed, with a special emphasis on the expression of the vanilloid (capsaicin) receptor VR1 and its regulation by growth factors. Cells in populations 1 and 2 express VR1, a nonselective channel that transduces certain nociceptive stimuli and that is crucial to the functioning of polymodal nociceptors. Cells in population 1 can be regulated by glial cell line derived neurotrophic factor (GDNF) and those in populations 2 and 3 by nerve growth factor (NGF). In vivo, DRG cells express a range of levels of VR1 expression and VR1 is downregulated after axotomy. However, treatment with NGF or GDNF can prevent this downregulation. In vitro, DRG cells also show a range of VR1 expression levels that is NGF and (or) GDNF dependent. Functional studies indicate that freshly dissociated cells also show differences in sensitivity to capsaicin. The significance of this is not known but may indicate a difference in the physiological role of cells in populations 1 and 2.