Endogenous galanin is required for the full expression of central sensitization following peripheral nerve injury

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Differential gene expression profiling and biological process analysis in proximal nerve segments after sciatic nerve transection

After traumatic injury, peripheral nerves can spontaneously regenerate through highly sophisticated and dynamic processes that are regulated by multiple cellular elements and molecular factors. Despite evidence of morphological changes and of expression changes of a few regulatory genes, global knowledge of gene expression changes and related biological processes during peripheral nerve injury and regeneration is still lacking. Here we aimed to profile global mRNA expression changes in proximal nerve segments of adult rats after sciatic nerve transection. According to DNA microarray analysis, the huge number of genes was differentially expressed at different time points (0.5 h–14 d) post nerve transection, exhibiting multiple distinct temporal expression patterns. The expression changes of several genes were further validated by quantitative real-time RT-PCR analysis. The gene ontology enrichment analysis was performed to decipher the biological processes involving the differentially expressed genes. Collectively, our results highlighted the dynamic change of the important biological processes and the time-dependent expression of key regulatory genes after peripheral nerve injury. Interestingly, we, for the first time, reported the presence of olfactory receptors in sciatic nerves. Hopefully, this study may provide a useful platform for deeply studying peripheral nerve injury and regeneration from a molecular-level perspective.

Peripheral galanin receptor 2 as a target for the modulation of pain

The neuropeptide galanin is widely expressed in the nervous system and has an important role in nociception. It has been shown that galanin can facilitate and inhibit nociception in a dose-dependent manner, principally through the central nervous system, with enhanced antinociceptive actions after nerve injury. However, following nerve injury, expression of galanin within the peripheral nervous system is dramatically increased up to 120-fold. Despite this striking increase in the peripheral nervous system, few studies have investigated the role that galanin plays in modulating nociception at the primary afferent nociceptor. Here, we summarise the recent work supporting the role of peripherally expressed galanin with particular reference to the dual actions of the galanin receptor 2 in neuropathic pain highlighting this as a potential target analgesic.

Galanin acts as a trophic factor to the central and peripheral nervous systems

The neuropeptide galanin is widely, but not ubiquitously, expressed in the adult nervous system. Its expression is markedly up-regulated in many neuronal tissues after nerve injury or disease. Over the last 10 years, we have demonstrated that the peptide plays a developmental survival role to subsets of neurons in the peripheral and central nervous systems with resulting phenotypic changes in neuropathic pain and cognition. Galanin also appears to play a trophic role to adult sensory neurons following injury, via activation of GalR2, by stimulating neurite outgrowth. Furthermore, galanin also plays a neuroprotective role to the hippocampus following excitotoxic injury, again mediated by activation of GalR2. Most recently, we have shown that galanin expression is markedly up-regulated in multiple sclerosis (MS) lesions and in the experimental autoimmune encephalomyelitis (EAE) model of MS. Over-expression of galanin in transgenic mice abolishes disease in the EAE model, whilst loss-of-function mutations in galanin or GalR2 increase disease severity. In summary, these studies demonstrate that a GalR2 agonist might have clinical utility in a variety of human diseases that affect the nervous system.

Nonselective innervation of lamina I projection neurons by cocaine- and amphetamine-regulated transcript peptide (CART)-immunoreactive fibres in the rat spinal dorsal horn

Cocaine- and amphetamine-regulated transcript (CART) peptides have been implicated in spinal pain transmission. A dense plexus of CART-immunoreactive fibres has been described in the superficial laminae of the spinal cord, which are key areas in sensory information and pain processing. We demonstrated previously that the majority of these fibres originate from nociceptive primary afferents. Using tract tracing, multiple immunofluorescent labelling and electronmicroscopy we determined the proportion of peptidergic primary afferents expressing CART, looked for evidence for coexistence of CART with galanin in these afferents in lamina I and examined their targets. Almost all (97.9%) randomly selected calcitonin gene-related peptide (CGRP)-immunoreactive terminals were substance P (SP)-positive (+) and CART was detected in approximately half (48.6%) of them. Most (81.4%) of the CGRP/SPergic boutons were galanin+ and approximately half (49.0%) of these contained CART. Many (72.9%) of the CARTergic boutons which expressed CGRP were also immunoreactive for galanin, while only 8.6% of the CARTergic terminals were galanin+ without CGRP. Electron microscopy showed that most of the CART terminals formed asymmetrical synapses, mainly with dendrites. All different morphological and neurochemical subtypes of spinoparabrachial projection neurons in the lamina I received contacts from CART-immunoreactive nociceptive afferents. The innervation density from these boutons did not differ significantly between either the different neurochemical or the morphological subclasses of these cells. This suggests a nonselective innervation of lamina I projection neurons from a subpopulation of CGRP/SP afferents containing CART peptide. These results provide anatomical evidence for involvement of CART peptide in spinal pain transmission.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Peripheral nerve injury induces reorganization of galanin-containing afferents in the superficial dorsal horn of monkey spinal cord

Peripheral nerve injury-induced structural and chemical modifications of the sensory circuits in the dorsal horn of the spinal cord contribute to the mechanism of neuropathic pain. In contrast to the topographic projection of primary afferents in laminae I-IV in the rat spinal cord, the primary afferents of Macaca mulatta monkeys almost exclusively project into laminae I-II of the spinal cord. After peripheral nerve injury, up-regulation of galanin has been found in sensory neurons in both monkey and rat dorsal root ganglia. However, the nerve injury-induced ultrastructural modification of galanin-containing afferents in the monkey spinal cord remains unknown. Using immunoelectron microscopy, we found that 3 weeks after unilateral sciatic nerve transection, the number of galanin-containing afferents was increased in ipsilateral lamina II of monkey spinal cord. Branching of these galanin-positive afferents was often observed. The afferent terminals contained a large number of synaptic vesicles, peptidergic vesicles and mitochondria, whereas the number of synapses was markedly reduced. Some of the afferents-enriched microtubules were often packed into bundles. Moreover, galanin-labeling could be associated with endosomal structures in many dendrites and axonal terminals of dorsal horn neurons. These results suggest that peripheral nerve injury induces an expansion of the central projection of galanin-containing afferents in lamina II of the monkey spinal cord, not only by increasing galanin levels in primary afferents but also by triggering afferent branching.

Ovarian steroids regulate neuropeptides in the trigeminal ganglion

Women are more than three times as likely as men to experience migraine headaches and temporomandibular joint pain, and painful episodes are often linked to the menstrual cycle. To understand how hormone levels may influence head and face pain, we assessed expression of pain-associated neuropeptides and estrogen receptor alpha (ERalpha) during the natural estrous cycle in mice. Gene expression was analyzed in the trigeminal ganglia of cycling female mice at proestrus, estrus and diestrus using RT-PCR. Peptide/protein expression in trigeminal neurons was analyzed using immunohistochemistry. ERalpha mRNA was present at all stages and highest at estrus. ERalpha protein was present in the cytoplasm of medium-sized and small trigeminal neurons. ERalpha immunoreactive neurons were most common at diestrus. CGRP and ANP mRNAs did not change across the estrous cycle, while expression of galanin and NPY mRNAs were strongly linked to the estrous cycle. Galanin mRNA levels peaked at proestrus, when expression was 8.7-fold higher than the diestrus levels. Galanin immunoreactivity also peaked at proestrus. At proestrus, 7.5% of trigeminal neurons contained galanin, while at estrus, 6.2% of trigeminal neurons contained galanin, and at diestrus, 4.9% of trigeminal neurons contained galanin. NPY mRNA peaked at estrus, when levels were 4.7-fold higher than at diestrus. Our findings suggest that estrogen receptors in trigeminal neurons modulate nociceptive responses through effects on galanin and NPY. Variations in neuropeptide content in trigeminal neurons across the natural estrous cycle may contribute to increases in painful episodes at particular phases of the menstrual cycle.

Use of genetically engineered transgenic mice to investigate the role of galanin in the peripheral nervous system after injury

The neuropeptide galanin is present at high levels within the dorsal root ganglia (DRG) and spinal cord during development and after peripheral nerve damage in the adult. This pattern of expression suggests that it may play a role in the adaptive response of the peripheral nervous system (PNS) to injury. Several experimental paradigms have demonstrated that galanin modulates pain transmission, particularly after nerve injury. In our laboratory we have used a transgenic approach to further elucidate the functions of galanin within the somatosensory system. We have generated mice which over-express galanin (either inducibly after nerve injury, or constitutively), and knock-out (KO) mice, in which galanin is absent in all cells, throughout development and in the adult. Analysis of the nociceptive behaviour of the galanin over-expressing animals, before and after nerve injury, supports the view that galanin is an inhibitory neuromodulator of spinal cord transmission. In apparent contradiction to these findings, galanin KO animals fail to develop allodynia and hyperalgesia after nerve injury. However, further studies have shown that galanin is critical for the developmental survival of a subset of small diameter, unmyelinated sensory neurons that are likely to be nociceptors. This finding may well explain the lack of neuropathic pain-like behaviour after injury in the KO animals. Furthermore, the developmental survival role played by galanin is recapitulated in the adult where the peptide is required for optimal neuronal regeneration after injury, and in the hippocampus where it plays a neuroprotective role after excitotoxic injury.

Galanin increases membrane excitability and enhances Ca(2+) currents in adult, acutely dissociated dorsal root ganglion neurons

We examined the effect of galanin (10(-15) - 10(-7) M) on dispersed, mainly small-sized dorsal root ganglion (DRG) neurons in adult rats using whole-cell patch-clamp. Galanin and AR-M1896, a selective galanin type 2 receptor (GalR2) agonist, both significantly increased the number of action potentials in response to current pulses in 77% of the neurons, indicating an increase in excitability. Galanin also caused a rise in input resistance, decreased the holding current for -60 mV and depolarized the resting potential. In addition, Ca(2+) currents elicited by voltage steps were significantly increased by both galanin and AR-M1896 in nearly 70% of the cells. This enhancement was observed in 30% of the neurons in the presence of nimodipine or omega-conotoxin, but in each case approximately 60% less than without blocking either N- or L-type Ca(2+) channels, indicating modulation of both types of Ca(2+) channels. The percentage of small- and medium-sized neurons expressing GalR2 mRNA in DRGs in situ was similar to that showing increased excitability and Ca(2+) current after galanin application, i.e. approximately 70-80% of the neurons. The findings suggest that GalR2 has a role in controlling both the excitability, probably by inhibition of GIRK or leak K(+) channels, and Ca(2+) entry in a large population of presumably nociceptive neurons. The combination of the two effects, which possibly arise from separate biochemical pathways, would increase excitability and enhance intracellular Ca(2+) signalling which would enhance sensory transmission. These mechanisms involving GalR2 receptors may underlie the pronociceptive effects of galanin described in the literature.

Distribution and regulation of galanin receptor 1 messenger RNA in the forebrain of wild type and galanin-transgenic mice

To learn more about molecular alterations in the brain that occur as a consequence of either the chronic excess or absence of peptide neurotransmitters, we examined the impact of genetically manipulating the neuropeptide galanin on the expression of one of its cognate receptors, galanin receptor 1. First, we examined the distribution of galanin receptor 1 messenger RNA in the mouse forebrain, and found it to be abundantly expressed in many brain regions, including in numerous hypothalamic and other forebrain regions associated with neuroendocrine function. The distribution of galanin receptor 1 messenger RNA in the mouse was similar to previous reports in the rat, with additional expression noted in the caudate putamen and in several midbrain regions. Next, using quantitative in situ hybridization, we measured cellular levels of galanin receptor 1 messenger RNA in the brains of mice that either overexpress galanin (galanin transgenic) or lack a functional galanin gene (galanin knockout). We report that relative to wild-type controls, the expression of galanin receptor 1 messenger RNA was increased in discrete areas of the brain in galanin-transgenic mice, but that depletion of galanin/noradrenergic innervation to the hypothalamus with the neurotoxin 6-hydroxydopamine did not alter levels of galanin receptor 1 messenger RNA. We also report that levels of galanin receptor 1 messenger RNA were not different between galanin-knockout and wild-type mice. These results suggest that compensatory adjustments in the expression of cognate receptors represent one mechanism by which the developing nervous system attempts to maintain homeostasis in response to overexpression of a peptidergic transmitter. However, the lack of significant changes in galanin receptor 1 messenger RNA in galanin-knockout mice suggests that developmentally programmed levels of receptor expression are maintained even in the complete absence of ligand.

Transgenic overexpression of galanin in the dorsal root ganglia modulates pain-related behavior

The neuropeptide galanin is expressed in the dorsal root ganglia (DRG) and spinal cord and is thought to be involved in the modulation of pain processing. However, its mechanisms of action are complex and poorly understood, as both facilitatory and inhibitory effects have been described. To understand further the role played by galanin in nociception, we have generated two transgenic lines that overexpress galanin in specific populations of primary afferent DRG neurons in either an inducible or constitutive manner. In the first line, a previously defined enhancer region from the galanin locus was used to target galanin to the DRG (Gal-OE). Transgene expression recapitulates the spatial endogenous galanin distribution pattern in DRG neurons and markedly overexpresses the peptide in the DRG after nerve injury but not in the uninjured state. In the second line, an enhancer region of the c-Ret gene was used to constitutively and ectopically target galanin overexpression to the DRG (Ret-OE). The expression of this second transgene does not alter significantly after nerve injury. Here, we report that intact Ret-OE, but not Gal-OE, animals have significantly elevated mechanical and thermal thresholds. After nerve damage, using a spared nerve-injury model, mechanical allodynia is attenuated markedly in both the Gal-OE and Ret-OE mice compared with WT controls. These results support an inhibitory role for galanin in the modulation of nociception both in intact animals and in neuropathic pain states.

Progressive tactile hypersensitivity after a peripheral nerve crush: non-noxious mechanical stimulus-induced neuropathic pain

Neuropathic pain syndromes are characterized by spontaneous pain and by stimulus-evoked allodynia and hyperalgesia. Stimulus-induced pain, i.e. the capacity of external stimuli to alter sensory processing so as to generate a pain hypersensitivity that outlasts the initiating stimulus, is usually present only after intense activation of nociceptors. In abnormal pain states, however, such as after capsaicin injection or inflammation, a stimulus-induced incremental pain can be generated by repetitive light touch, termed progressive tactile hypersensitivity (PTH). In the present study, we have examined whether PTH also occurs in two experimental models of neuropathic pain: a crush injury of the sciatic nerve and the spared nerve injury (SNI) model. When applied during the first weeks after injury to the territory of the injured crushed nerve, repeated low-intensity mechanical stimulation did not change the mechanical withdrawal threshold response. However, 10 weeks and after, the same repeated stimulation induced a progressive tactile hypersensitivity that persisted after discontinuation of the tactile stimulation. Following SNI, repeated stimulation of the hypersensitive skin territory, corresponding to the intact spared sural nerve, never induced PTH. Tactile stimulation of regenerating afferents but not spared non-injured afferents, can induce, therefore, PTH and such a stimulus-induced alteration in pain processing may contribute to clinical neuropathic pain.

Replicate high-density rat genome oligonucleotide microarrays reveal hundreds of regulated genes in the dorsal root ganglion after peripheral nerve injury

BACKGROUND

Rat oligonucleotide microarrays were used to detect changes in gene expression in the dorsal root ganglion (DRG) 3 days following sciatic nerve transection (axotomy). Two comparisons were made using two sets of triplicate microarrays, naïve versus naïve and naïve versus axotomy.

RESULTS

Microarray variability was assessed using the naïve versus naïve comparison. These results support use of a P < 0.05 significance threshold for detecting regulated genes, despite the large number of hypothesis tests required. For the naïve versus axotomy comparison, a 2-fold cut off alone led to an estimated error rate of 16%; combining a >1.5-fold expression change and P < 0.05 significance reduced the estimated error to 5%. The 2-fold cut off identified 178 genes while the combined >1.5-fold and P < 0.05 criteria generated 240 putatively regulated genes, which we have listed. Many of these have not been described as regulated in the DRG by axotomy. Northern blot, quantitative slot blots and in situ hybridization verified the expression of 24 transcripts. These data showed an 83% concordance rate with the arrays; most mismatches represent genes with low expression levels reflecting limits of array sensitivity. A significant correlation was found between actual mRNA differences and relative changes between microarrays (r2 = 0.8567). Temporal patterns of individual genes regulation varied.

CONCLUSIONS

We identify parameters for microarray analysis which reduce error while identifying many putatively regulated genes. Functional classification of these genes suggest reorganization of cell structural components, activation of genes expressed by immune and inflammatory cells and down-regulation of genes involved in neurotransmission.