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

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.

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

Glial Cell Line-Derived Neurotrophic Factor and Somatostatin Levels in Cerebrospinal Fluid of Patients Affected by Chronic Migraine and Fibromyalgia

The aim of the present study was to verify cerebrospinal fluid (CSF) levels of glial cell line-derived neurotrophic factor (GDNF) and somatostatin, both measured by sensitive immunoassay, in: 16 chronic migraine (CM) patients, 15 patients with an antecedent history of migraine without aura diagnosed as having probable chronic migraine (PCM) and probable analgesic-abuse headache (PAAH), 20 patients affected by primary fibromyalgia syndrome (PFMS), and 20 control subjects. Significantly lower levels of GDNF and somatostatin were found in the CSF of both CM and PCM + PAAH patients compared with controls (GDNF = P < 0.001, P < 0.002; somatostatin = P < 0.002, P < 0.0003), without significant difference between the two groups. PFMS patients, with and without analgesic abuse, also had significantly lower levels of both somatostatin and GDNF ( P < 0.0002, P < 0.001), which did not differ from those of CM and PCM + PAAH patients. A significant positive correlation emerged between CSF values of GDNF and those of somatostatin in CM ( r = 0.70, P < 0.02), PCM + PAAH ( r = 0.78, P < 0.004), and PFMS patients ( r = 0.68, P < 0.008). Based on experimental findings, it can be postulated that reduced CSF levels of GDNF and somatostatin in both CM and PCM + PAAH patients can contribute to sustained central sensitization underlying chronic head pain. The abuse of simple or combination analgesics does not seem to influence the biochemical changes investigated, which appear to be more strictly related to the chronic pain state, as demonstrated also for fibromyalgia.

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.

Comparison of GDF5 and GDNF as neuroprotective factors for postnatal dopamine neurons in ventral mesencephalic cultures

Loss of dopamine neurons is associated with the motor deficits that occur in Parkinson's disease. Although many drugs have proven to be useful in the treatment of the symptoms of this disease, none has been shown to have a significant impact on the development of the disease. However, we believe that several neurotrophic factors have the potential to reduce its progression. Glial cell line-derived neurotrophic factor (GDNF), a member of the transforming growth factor-β superfamily of neurotrophic factors, has been extensively studied in this regard. Less attention has been paid to growth/differentiation factor 5 (GDF5), another member of the same superfamily. This study compares GDNF and GDF5 in dissociated cultures prepared from ventral mesencephalon and in organotypic co-cultures containing substantia nigra, striatum, and neocortex. We report that both GDNF (10-500 ng/ml) and GDF5 (100-500 ng/ml) promoted the survival of dopamine neurons from the substantia nigra of postnatal rats, although GDNF was considerably more potent than GDF5. In contrast, neither factor had any significant effect on the survival of dopamine neurons from the rat ventral tegmental area. Using organotypic co-cultures, we also compared GDF5 with GDNF as chemoattractants for the innervation of the striatum and the neocortex by dopamine neurons from the substantia nigra. The addition of either GDF5 or GDNF (100-500 ng/ml) caused innervation by dopamine neurons into the cortex as well as the striatum, which did not occur in untreated cultures. Our results are consistent with similar findings suggesting that GDF5, like GDNF, deserves attention as a possible therapeutic intervention for Parkinson's disease.

Overexpression of GDNF in the uninjured DRG exerts analgesic effects on neuropathic pain following segmental spinal nerve ligation in mice

Glial cell line-derived neurotrophic factor (GDNF), a survival-promoting factor for a subset of nociceptive small-diameter neurons, has been shown to exert analgesic effects on neuropathic pain. However, its detailed mechanisms of action are still unknown. In the present study, we investigated the site-specific analgesic effects of GDNF in the neuropathic pain state using lentiviral vector-mediated GDNF overexpression in mice with left fifth lumbar (L5) spinal nerve ligation (SNL) as a neuropathic pain model. A lentiviral vector expressing both GDNF and enhanced green fluorescent protein (EGFP) was constructed and injected into the left dorsal spinal cord, uninjured fourth lumbar (L4) dorsal root ganglion (DRG), injured L5 DRG, or plantar skin of mice. In SNL mice, injection of the GDNF-EGFP-expressing lentivirus into the dorsal spinal cord or uninjured L4 DRG partially but significantly reduced the mechanical allodynia in association with an increase in GDNF protein expression in each virus injection site, whereas injection into the injured L5 DRG or plantar skin had no effects. These results suggest that GDNF exerts its analgesic effects in the neuropathic pain state by acting on the central terminals of uninjured DRG neurons and/or on the spinal cells targeted by the uninjured DRG neurons.

PERSPECTIVE

This article shows that GDNF exerts its analgesic effects on neuropathic pain by acting on the central terminals of uninjured DRG neurons and/or on the spinal cells targeted by these neurons. Therefore, research focusing on these GDNF-dependent neurons in the uninjured DRG would provide a new strategy for treating neuropathic pain.

The somatostatin analogue octreotide inhibits capsaicin-mediated activation of nociceptive primary afferent fibres in spinal cord lamina II (substantia gelatinosa)

Somatostatin (SST) in spinal cord has been linked with the inhibition of nociceptive neurotransmission in several experimental paradigms. The SST2 receptor (SSTR2) is the main SST receptor subtype in the superficial dorsal horn (DH) and is activated, besides to the naïve peptide, by the SST synthetic analogue octreotide (OCT). In the present work, we have studied the central effects of SSTR2 activation on capsaicin (CAP)-induced glutamate release in mouse DH. In neurons of the lamina II of DH, CAP (2 μM) induced a strong increase of mEPSC frequency that was significantly reduced (70%) by OCT. SSTR2 involvement was assessed by using the specific antagonist CYN 154806. No differences were observed between frequency increase in CAP alone vs. CAP in the presence of CYN 154806+OCT. The effect of OCT was further investigated by studying c-fos expression in spinal cord slices. The CAP-induced increase in density of Fos immunoreactive nuclei in the superficial DH was strongly prevented by OCT. SSTR2a (a splicing variant of SSTR2) immunoreactivity was found in both pre- and post-synaptic compartments of laminae I-II synapses. By light and electron microscopy, SSTR2a was mainly localized onto non-peptidergic isolectin B4 (IB4)-positive primary afferent fibres (PAFs). A subset of them was also found to express the CAP receptor TRPV1. These data show that the SST analogue OCT inhibits CAP-mediated activation of non-peptidergic nociceptive PAFs in lamina II. Our data indicate that SSTR2a plays an important role in the pre-synaptic modulation of central excitatory nociceptive transmission in mouse.

Colitis induces calcitonin gene-related peptide expression and Akt activation in rat primary afferent pathways

Previous study has shown that colitis-induced increases in calcitonin gene-related peptide (CGRP) immunoreactivity in bladder afferent neurons result in sensory cross-sensitization. To further determine the effects of colitis on CGRP expression in neurons other than bladder afferents, we examined and compared the levels of CGRP mRNA and immunoreactivity in the lumbosacral dorsal root ganglia (DRG) and spinal cord before and during colitis in rats. We also examined the changes in CGRP immunoreactivity in colonic afferent neurons during colitis. Results showed increases in CGRP mRNA levels in L1 (2.5-fold, p<0.05) and S1 DRG (1.9-2.4-fold, p<0.05). However, there were no changes in CGRP mRNA levels in L1 and S1 spinal cord during colitis. CGRP protein was significantly increased in L1 (2.5-fold increase, p<0.05) but decreased in S1 (50% decrease, p<0.05) colonic afferent neurons, which may reflect CGRP release from these neurons during colitis. In L1 spinal cord, colitis caused increases in the number of CGRP nerve fibers in the deep lamina region extending to the gray commissure where the number of phospho-Akt neurons was also increased. In S1 spinal cord, colitis caused the increases in the intensity of CGRP fibers in the regions of dorso-lateral tract, and caused the increases in the level of phospho-Akt in the superficial dorsal horn of the spinal cord. In spinal cord slice culture, exogenous CGRP increased the phosphorylation level of Akt but not the phosphorylation level of extracellular-signal regulated kinase ERK1/2 even though our previous studies showed that colitis increased the phosphorylation level of ERK1/2 in L1 and S1 spinal cord. These results suggest that CGRP is synthesized in the DRG and may transport to the spinal cord where it initiates signal transduction during colitis.

Controlled release of GDNF reduces nerve root-mediated behavioral hypersensitivity

Nerve root compression produces persistent behavioral sensitivity in models of painful neck injury. This study utilized degradable poly(ethylene glycol) hydrogels to deliver glial cell line-derived neurotrophic factor (GDNF) to an injured nerve root. Hydrogels delivered approximately 98% of encapsulated GDNF over 7 days in an in vitro release assay without the presence of neurons and produced enhanced outgrowth of processes in cortical neural cell primary cultures. The efficacy of a GDNF hydrogel placed on the root immediately after injury was assessed in a rat pain model of C7 dorsal root compression. Control groups included painful injury followed by: (1) vehicle hydrogel treatment (no GDNF), (2) a bolus injection of GDNF, or (3) no treatment. After injury, mechanical allodynia (n = 6/group) was significantly decreased with GDNF delivered by the hydrogel compared to the three injury control groups (p < 0.03). The bolus GDNF treatment did not reduce allodynia at any time point. The GDNF receptor (GFRalpha-1) decreased in small, nociceptive neurons of the affected dorsal root ganglion, suggesting a decrease in receptor expression following injury. GDNF receptor immunoreactivity was significantly greater in these neurons following GDNF hydrogel treatment relative to GDNF bolus treated and untreated rats (p < 0.05). These data suggest efficacy for degradable hydrogel delivery of GDNF and support this treatment approach for nerve root-mediated pain.

Role of the immune system in chronic pain

During the past two decades, an important focus of pain research has been the study of chronic pain mechanisms, particularly the processes that lead to the abnormal sensitivity - spontaneous pain and hyperalgesia - that is associated with these states. For some time it has been recognized that inflammatory mediators released from immune cells can contribute to these persistent pain states. However, it has only recently become clear that immune cell products might have a crucial role not just in inflammatory pain, but also in neuropathic pain caused by damage to peripheral nerves or to the CNS.

Increase by FK960, a novel cognitive enhancer, in glial cell line-derived neurotrophic factor production in cultured rat astrocytes

We examined the effect of N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate (FK960), a novel anti-dementia drug, on neurotrophic factor production in cultured rat astrocytes. FK960 (100nM) increased mRNA and protein levels of glial cell line-derived neurotrophic factor (GDNF). FK960 did not affect mRNA levels of neurotrophic factors other than GDNF. The effect of FK960 was not affected by antagonists of dopamine and alpha7-nicotinic acetylcholine receptors. FK960 stimulated phosphorylation of mitogen-activated protein/extracellular signal-regulated kinase (ERK) without any effect on phosphoryolation of p38 and c-Jun N-terminal kinase. FK960 increased the levels of c-Fos and phosphorylation of cAMP responsive element binding protein (CREB). The effect of FK960 on c-Fos was inhibited by PD98059 (10microM), an ERK kinase inhibitor, and cycloheximide (1microg/ml), a transcription inhibitor, and the effect of FK960 on CREB phosphorylation was blocked by PD98059. The effect of FK960 on GDNF mRNA expression was attenuated by PD98059, curcumin (10microM), an activator protein-1 inhibitor, cycloheximide and actinomycin D (10microg/ml). These results suggest that FK960 stimulates GDNF production in c-Fos- and CREB-dependent mechanisms in cultured astrocytes and that ERK signal is responsible for both c-Fos expression and CREB phosphorylation in the cascades.

Spinal delivery of analgesics in experimental models of pain and analgesia

Systemic administration of analgesics can lead to serious adverse side effects compromising therapeutic benefit in some patients. Information coding pain transmits along an afferent neuronal network, the first synapses of which reside principally in the spinal cord. Delivery of compounds to spinal cord, the intended site of action for some analgesics, is potentially a more efficient and precise method for inhibiting the pain signal. Activation of specific proteins that reside in spinal neuronal membranes can result in hyperpolarization of secondary neurons, which can prevent transmission of the pain signal. This is one of the mechanisms by which opioids induce analgesia. The spinal cord is enriched in such molecular targets, the activation of which inhibit the transmission of the pain signal early in the afferent neuronal network. This review describes the pre-clinical models that enable new target discovery and development of novel analgesics for site-directed pain management.