Galanin is contained in GABAergic neurons in the rat spinal dorsal horn

Synaptic Targets of Glycinergic Neurons in Laminae I-III of the Spinal Dorsal Horn

A great deal of evidence supports the inevitable importance of spinal glycinergic inhibition in the development of chronic pain conditions. However, it remains unclear how glycinergic neurons contribute to the formation of spinal neural circuits underlying pain-related information processing. Thus, we intended to explore the synaptic targets of spinal glycinergic neurons in the pain processing region (laminae I-III) of the spinal dorsal horn by combining transgenic technology with immunocytochemistry and in situ hybridization accompanied by light and electron microscopy. First, our results suggest that, in addition to neurons in laminae I-III, glycinergic neurons with cell bodies in lamina IV may contribute substantially to spinal pain processing. On the one hand, we show that glycine transporter 2 immunostained glycinergic axon terminals target almost all types of excitatory and inhibitory interneurons identified by their neuronal markers in laminae I-III. Thus, glycinergic postsynaptic inhibition, including glycinergic inhibition of inhibitory interneurons, must be a common functional mechanism of spinal pain processing. On the other hand, our results demonstrate that glycine transporter 2 containing axon terminals target only specific subsets of axon terminals in laminae I-III, including nonpeptidergic nociceptive C fibers binding IB4 and nonnociceptive myelinated A fibers immunoreactive for type 1 vesicular glutamate transporter, indicating that glycinergic presynaptic inhibition may be important for targeting functionally specific subpopulations of primary afferent inputs.

Morphological and neurochemical characterization of glycinergic neurons in laminae I-IV of the mouse spinal dorsal horn

A growing body of experimental evidence shows that glycinergic inhibition plays vital roles in spinal pain processing. In spite of this, however, our knowledge about the morphology, neurochemical characteristics, and synaptic relations of glycinergic neurons in the spinal dorsal horn is very limited. The lack of this knowledge makes our understanding about the specific contribution of glycinergic neurons to spinal pain processing quite vague. Here we investigated the morphology and neurochemical characteristics of glycinergic neurons in laminae I-IV of the spinal dorsal horn using a GlyT2::CreERT2-tdTomato transgenic mouse line. Confirming previous reports, we show that glycinergic neurons are sparsely distributed in laminae I-II, but their densities are much higher in lamina III and especially in lamina IV. First in the literature, we provide experimental evidence indicating that in addition to neurons in which glycine colocalizes with GABA, there are glycinergic neurons in laminae I-II that do not express GABA and can thus be referred to as glycine-only neurons. According to the shape and size of cell bodies and dendritic morphology, we divided the tdTomato-labeled glycinergic neurons into three and six morphological groups in laminae I-II and laminae III-IV, respectively. We also demonstrate that most of the glycinergic neurons co-express neuronal nitric oxide synthase, parvalbumin, the receptor tyrosine kinase RET, and the retinoic acid-related orphan nuclear receptor β (RORβ), but there might be others that need further neurochemical characterization. The present findings may foster our understanding about the contribution of glycinergic inhibition to spinal pain processing.

Spinal Inhibitory Interneurons: Gatekeepers of Sensorimotor Pathways

The ability to sense and move within an environment are complex functions necessary for the survival of nearly all species. The spinal cord is both the initial entry site for peripheral information and the final output site for motor response, placing spinal circuits as paramount in mediating sensory responses and coordinating movement. This is partly accomplished through the activation of complex spinal microcircuits that gate afferent signals to filter extraneous stimuli from various sensory modalities and determine which signals are transmitted to higher order structures in the CNS and to spinal motor pathways. A mechanistic understanding of how inhibitory interneurons are organized and employed within the spinal cord will provide potential access points for therapeutics targeting inhibitory deficits underlying various pathologies including sensory and movement disorders. Recent studies using transgenic manipulations, neurochemical profiling, and single-cell transcriptomics have identified distinct populations of inhibitory interneurons which express an array of genetic and/or neurochemical markers that constitute functional microcircuits. In this review, we provide an overview of identified neural components that make up inhibitory microcircuits within the dorsal and ventral spinal cord and highlight the importance of inhibitory control of sensorimotor pathways at the spinal level.

Spinal GABAergic neurons are under feed-forward inhibitory control driven by A and C fibers in Gad2 td-Tomato mice

BACKGROUND

Spinal GABAergic neurons act as a critical modulator in sensory transmission like pain or itch. The monosynaptic or polysynaptic primary afferent inputs onto GABAergic neurons, along with other interneurons or projection neurons make up the direct and feed-forward inhibitory neural circuits. Previous research indicates that spinal GABAergic neurons mainly receive excitatory inputs from Aδ and C fibers. However, whether they are controlled by other inhibitory sending signals is not well understood.

METHODS

We applied a transgenic mouse line in which neurons co-expressed the GABA-synthesizing enzyme Gad65 and the enhanced red fluorescence (td-Tomato) to characterize the features of morphology and electrophysiology of GABAergic neurons. Patch-clamp whole cell recordings were used to record the evoked postsynaptic potentials of fluorescent neurons in spinal slices in response to dorsal root stimulation.

RESULTS

We demonstrated that GABAergic neurons not only received excitatory drive from peripheral Aβ, Aδ and C fibers, but also received inhibitory inputs driven by Aδ and C fibers. The evoked inhibitory postsynaptic potentials (eIPSPs) mediated by C fibers were mainly Glycinergic (66.7%) as well as GABAergic mixed with Glycinergic (33.3%), whereas the inhibition mediated by Aδ fibers was predominately both GABA and Glycine-dominant (57.1%), and the rest of which was purely Glycine-dominant (42.9%).

CONCLUSION

These results indicated that spinal GABAergic inhibitory neurons are under feedforward inhibitory control driven by primary C and Aδ fibers, suggesting that this feed-forward inhibitory pathway may play an important role in balancing the excitability of GABAergic neurons in spinal dorsal horn.

Recent advances in our understanding of the organization of dorsal horn neuron populations and their contribution to cutaneous mechanical allodynia

The dorsal horns of the spinal cord and the trigeminal nuclei in the brainstem contain neuron populations that are critical to process sensory information. Neurons in these areas are highly heterogeneous in their morphology, molecular phenotype and intrinsic properties, making it difficult to identify functionally distinct cell populations, and to determine how these are engaged in pathophysiological conditions. There is a growing consensus concerning the classification of neuron populations, based on transcriptomic and transductomic analyses of the dorsal horn. These approaches have led to the discovery of several molecularly defined cell types that have been implicated in cutaneous mechanical allodynia, a highly prevalent and difficult-to-treat symptom of chronic pain, in which touch becomes painful. The main objective of this review is to provide a contemporary view of dorsal horn neuronal populations, and describe recent advances in our understanding of on how they participate in cutaneous mechanical allodynia.

A preliminary study on DRGs and spinal cord of a galanin receptor 2-EGFP transgenic mouse

The neuropeptide galanin functions via three G-protein coupled receptors, Gal_1-3-R. Both Gal₁-R and ₂-R are involved in pain signaling at the spinal level. Here a Gal₂-R-EGFP transgenic (TG) mouse was generated and studied in pain tests and by characterizing Gal₂-R expression in both sensory ganglia and spinal cord. After peripheral spared nerve injury, mechanical allodynia developed and was ipsilaterally similar between wild type (WT) and TG mice. A Gal₂-R-EGFP-positive signal was primarily observed in small and medium-sized dorsal root ganglion (DRG) neurons and in spinal interneurons and processes. No significant difference in size distribution of DRG neuronal profiles was found between TG and WT mice. Both percentage and fluorescence intensity of Gal₂-R-EGFP-positive neuronal profiles were overall significantly upregulated in ipsilateral DRGs as compared to contralateral DRGs. There was an ipsilateral reduction in substance P-positive and calcitonin gene-related peptide (CGRP)-positive neuronal profiles, and this reduction was more pronounced in TG as compared to WT mice. Moreover, Gal₂-R-EGFP partly co-localized with three pain-related neuropeptides, CGRP, neuropeptide Y and galanin, both in intact and injured DRGs, and with galanin also in local neurons in the superficial dorsal horn. Taken together, the present results provide novel information on the localization and phenotype of DRG and spinal neurons expressing the second galanin receptor, Gal₂-R, and on phenotypic changes following peripheral nerve injury. Gal2-R may also be involved in autoreceptor signaling.

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.

Circuit dissection of the role of somatostatin in itch and pain

Stimuli that elicit itch are detected by sensory neurons that innervate the skin. This information is processed by the spinal cord; however, the way in which this occurs is still poorly understood. Here we investigated the neuronal pathways for itch neurotransmission, particularly the contribution of the neuropeptide somatostatin. We find that in the periphery, somatostatin is exclusively expressed in Nppb+ neurons, and we demonstrate that Nppb+somatostatin+ cells function as pruriceptors. Employing chemogenetics, pharmacology and cell-specific ablation methods, we demonstrate that somatostatin potentiates itch by inhibiting inhibitory dynorphin neurons, which results in disinhibition of GRPR+ neurons. Furthermore, elimination of somatostatin from primary afferents and/or from spinal interneurons demonstrates differential involvement of the peptide released from these sources in itch and pain. Our results define the neural circuit underlying somatostatin-induced itch and characterize a contrasting antinociceptive role for the peptide.

A quantitative study of neurochemically defined populations of inhibitory interneurons in the superficial dorsal horn of the mouse spinal cord

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Neurochemistry of lamina I–II inhibitory neurons in mouse is similar to that in rat.

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Five neurochemical classes account for all lamina I–II inhibitory neurons in mouse.

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Excitatory dynorphin cells are largely restricted to glabrous skin territory.

Around a quarter of neurons in laminae I–II of the dorsal horn are inhibitory interneurons. These play an important role in modulating somatosensory information, including that perceived as pain or itch. Previous studies in rat identified four largely non-overlapping neurochemical populations among these cells, defined by expression of galanin, neuropeptide Y (NPY), neuronal nitric oxide synthase (nNOS) or parvalbumin. The galanin cells were subsequently shown to coexpress dynorphin. Several recent studies have used genetically modified mice to investigate the function of different interneuron populations, and it is therefore important to determine whether the same pattern applies in mouse, and to estimate the relative sizes of these populations. We show that the neurochemical organization of inhibitory interneurons in mouse superficial dorsal horn is similar to that in the rat, although a larger proportion of these neurons (33%) express NPY. Between them, these four populations account for ∼75% of inhibitory cells in laminae I–II. Since ∼25% of inhibitory interneurons in this region belong to a novel calretinin-expressing type, our results suggest that virtually all inhibitory interneurons in superficial dorsal horn can be assigned to one of these five neurochemical populations. Although our main focus was inhibitory neurons, we also identified a population of excitatory dynorphin-expressing cells in laminae I–II that are largely restricted to the medial part of the mid-lumbar dorsal horn, corresponding to glabrous skin territory. These findings are important for interpretation of studies using molecular-genetic techniques to manipulate the functions of interneuron populations to investigate their roles in somatosensory processing.

Presynaptic inhibition of nociceptive neurotransmission by somatosensory neuron-secreted suppressors

Noxious stimuli cause pain by activating cutaneous nociceptors. The Aδ- and C-fibers of dorsal root ganglion (DRG) neurons convey the nociceptive signals to the laminae I-II of spinal cord. In the dorsal horn of spinal cord, the excitatory afferent synaptic transmission is regulated by the inhibitory neurotransmitter γ-aminobutyric acid and modulators such as opioid peptides released from the spinal interneurons, and by serotonin, norepinepherine and dopamine from the descending inhibitory system. In contrast to the accumulated evidence for these central inhibitors and their neural circuits in the dorsal spinal cord, the knowledge about the endogenous suppressive mechanisms in nociceptive DRG neurons remains very limited. In this review, we summarize our recent findings of the presynaptic suppressive mechanisms in nociceptive neurons, the BNP/NPR-A/PKG/BKCa channel pathway, the FSTL1/α1Na⁺-K⁺ ATPase pathway and the activin C/ERK pathway. These endogenous suppressive systems in the mechanoheat nociceptors may also contribute differentially to the mechanisms of nerve injury-induced neuropathic pain or inflammation-induced pain.

Spinal neuropeptide expression and neuropathic behavior in the acute and chronic phases after spinal cord injury: Effects of progesterone administration

Patients with spinal cord injury (SCI) develop chronic pain that severely compromises their quality of life. We have previously reported that progesterone (PG), a neuroprotective steroid, could offer a promising therapeutic strategy for neuropathic pain. In the present study, we explored temporal changes in the expression of the neuropeptides galanin and tyrosine (NPY) and their receptors (GalR1 and GalR2; Y1R and Y2R, respectively) in the injured spinal cord and evaluated the impact of PG administration on both neuropeptide systems and neuropathic behavior. Male rats were subjected to spinal cord hemisection at T13 level, received daily subcutaneous injections of PG or vehicle, and were evaluated for signs of mechanical and thermal allodynia. Real time PCR was used to determine relative mRNA levels of neuropeptides and receptors, both in the acute (1day) and chronic (28days) phases after injury. A significant increase in Y1R and Y2R expression, as well as a significant downregulation in GalR2 mRNA levels, was observed 1day after SCI. Interestingly, PG early treatment prevented Y1R upregulation and resulted in lower NPY, Y2R and GalR1 mRNA levels. In the chronic phase, injured rats showed well-established mechanical and cold allodynia and significant increases in galanin, NPY, GalR1 and Y1R mRNAs, while maintaining reduced GalR2 expression. Animals receiving PG treatment showed basal expression levels of galanin, NPY, GalR1 and Y1R, and reduced Y2R mRNA levels. Also, and in line with previously published observations, PG-treated animals did not develop mechanical allodynia and showed reduced sensitivity to cold stimulation. Altogether, we show that SCI leads to considerable changes in the spinal expression of galanin, NPY and their associated receptors, and that early and sustained PG administration prevents them. Moreover, our data suggest the participation of galaninergic and NPYergic systems in the plastic changes associated with SCI-induced neuropathic pain, and further supports the therapeutic potential of PG- or neuropeptide-based therapies to prevent and/or treat chronic pain after central injuries.

Identifying functional populations among the interneurons in laminae I-III of the spinal dorsal horn

The spinal dorsal horn receives input from primary afferent axons, which terminate in a modality-specific fashion in different laminae. The incoming somatosensory information is processed through complex synaptic circuits involving excitatory and inhibitory interneurons, before being transmitted to the brain via projection neurons for conscious perception. The dorsal horn is important, firstly because changes in this region contribute to chronic pain states, and secondly because it contains potential targets for the development of new treatments for pain. However, at present, we have only a limited understanding of the neuronal circuitry within this region, and this is largely because of the difficulty in defining functional populations among the excitatory and inhibitory interneurons. The recent discovery of specific neurochemically defined interneuron populations, together with the development of molecular genetic techniques for altering neuronal function in vivo, are resulting in a dramatic improvement in our understanding of somatosensory processing at the spinal level.

Targeted ablation, silencing, and activation establish glycinergic dorsal horn neurons as key components of a spinal gate for pain and itch

The gate control theory of pain proposes that inhibitory neurons of the spinal dorsal horn exert critical control over the relay of nociceptive signals to higher brain areas. Here we investigated how the glycinergic subpopulation of these neurons contributes to modality-specific pain and itch processing. We generated a GlyT2::Cre transgenic mouse line suitable for virus-mediated retrograde tracing studies and for spatially precise ablation, silencing, and activation of glycinergic neurons. We found that these neurons receive sensory input mainly from myelinated primary sensory neurons and that their local toxin-mediated ablation or silencing induces localized mechanical, heat, and cold hyperalgesia; spontaneous flinching behavior; and excessive licking and biting directed toward the corresponding skin territory. Conversely, local pharmacogenetic activation of the same neurons alleviated neuropathic hyperalgesia and chloroquine- and histamine-induced itch. These results establish glycinergic neurons of the spinal dorsal horn as key elements of an inhibitory pain and itch control circuit.

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Glycinergic dorsal horn neurons exert segmental control over pain and itch

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Their local inhibition causes hyperalgesia and signs of spontaneous discomfort

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Local activation reduces acute pain, neuropathic hyperalgesia, and chemical itch

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.

Expression of gastrin-releasing peptide by excitatory interneurons in the mouse superficial dorsal horn

Background

Gastrin-releasing peptide (GRP) and its receptor have been shown to play an important role in the sensation of itch. However, although GRP immunoreactivity has been detected in the spinal dorsal horn, there is debate about whether this originates from primary afferents or local excitatory interneurons. We therefore examined the relation of GRP immunoreactivity to that seen with antibodies that label primary afferent or excitatory interneuron terminals. We tested the specificity of the GRP antibody by preincubating with peptides with which it could potentially cross-react. We also examined tissue from a mouse line in which enhanced green fluorescent protein (EGFP) is expressed under control of the GRP promoter.

Results

GRP immunoreactivity was seen in both primary afferent and non-primary glutamatergic axon terminals in the superficial dorsal horn. However, immunostaining was blocked by pre-incubation of the antibody with substance P, which is present at high levels in many nociceptive primary afferents. EGFP⁺ cells in the GRP-EGFP mouse did not express Pax2, and their axons contained the vesicular glutamate transporter 2 (VGLUT2), indicating that they are excitatory interneurons. In most cases, their axons were also GRP-immunoreactive. Multiple-labelling immunocytochemical studies indicated that these cells did not express either of the preprotachykinin peptides, and that they generally lacked protein kinase Cγ, which is expressed by a subset of the excitatory interneurons in this region.

Conclusions

These results show that GRP is expressed by a distinct population of excitatory interneurons in laminae I-II that are likely to be involved in the itch pathway. They also suggest that the GRP immunoreactivity seen in primary afferents in previous studies may have resulted from cross-reaction of the GRP antibody with substance P or the closely related peptide neurokinin A.

Dynorphin acts as a neuromodulator to inhibit itch in the dorsal horn of the spinal cord

Menthol and other counterstimuli relieve itch, resulting in an antipruritic state that persists for minutes to hours. However, the neural basis for this effect is unclear, and the underlying neuromodulatory mechanisms are unknown. Previous studies revealed that Bhlhb5^−/− mice, which lack a specific population of spinal inhibitory interneurons (B5-I neurons), develop pathological itch. Here we characterize B5-I neurons and show that they belong to a neurochemically distinct subset. We provide cause-and-effect evidence that B5-I neurons inhibit itch and show that dynorphin, which is released from B5-I neurons, is a key neuromodulator of pruritus. Finally, we show that B5-I neurons are innervated by menthol-, capsaicin-, and mustard oil-responsive sensory neurons and are required for the inhibition of itch by menthol. These findings provide a cellular basis for the inhibition of itch by chemical counterstimuli and suggest that kappa opioids may be a broadly effective therapy for pathological itch.

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Spinal B5-I interneurons function to inhibit itch

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B5-I neurons release the kappa opioid dynorphin

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Kappa opioid signaling bidirectionally modulates itch within the spinal cord

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Spinal B5-I interneurons mediate the inhibition of itch by menthol

Neurochemical characterisation of lamina II inhibitory interneurons that express GFP in the PrP-GFP mouse

Background

Inhibitory interneurons in the superficial dorsal horn play important roles in modulating sensory transmission, and these roles are thought to be performed by distinct functional populations. We have identified 4 non-overlapping classes among the inhibitory interneurons in the rat, defined by the presence of galanin, neuropeptide Y, neuronal nitric oxide synthase (nNOS) and parvalbumin. The somatostatin receptor sst_2A is expressed by ~50% of the inhibitory interneurons in this region, and is particularly associated with nNOS- and galanin-expressing cells. The main aim of the present study was to test whether a genetically-defined population of inhibitory interneurons, those expressing green fluorescent protein (GFP) in the PrP-GFP mouse, belonged to one or more of the neurochemical classes identified in the rat.

Results

The expression of sst_2A and its relation to other neurochemical markers in the mouse was similar to that in the rat, except that a significant number of cells co-expressed nNOS and galanin. The PrP-GFP cells were entirely contained within the set of inhibitory interneurons that possessed sst_2A receptors, and virtually all expressed nNOS and/or galanin. GFP was present in ~3-4% of neurons in the superficial dorsal horn, corresponding to ~16% of the inhibitory interneurons in this region. Consistent with their sst_2A-immunoreactivity, all of the GFP cells were hyperpolarised by somatostatin, and this was prevented by administration of a selective sst₂ receptor antagonist or a blocker of G-protein-coupled inwardly rectifying K⁺ channels.

Conclusions

These findings support the view that neurochemistry provides a valuable way of classifying inhibitory interneurons in the superficial laminae. Together with previous evidence that the PrP-GFP cells form a relatively homogeneous population in terms of their physiological properties, they suggest that these neurons have specific roles in processing sensory information in the dorsal horn.

GABAergic terminals are a source of galanin to modulate cholinergic neuron development in the neonatal forebrain

The distribution and (patho-)physiological role of neuropeptides in the adult and aging brain have been extensively studied. Galanin is an inhibitory neuropeptide that can coexist with γ-aminobutyric acid (GABA) in the adult forebrain. However, galanin's expression sites, mode of signaling, impact on neuronal morphology, and colocalization with amino acid neurotransmitters during brain development are less well understood. Here, we show that galaninergic innervation of cholinergic projection neurons, which preferentially express galanin receptor 2 (GalR2) in the neonatal mouse basal forebrain, develops by birth. Nerve growth factor (NGF), known to modulate cholinergic morphogenesis, increases GalR2 expression. GalR2 antagonism (M871) in neonates reduces the in vivo expression and axonal targeting of the vesicular acetylcholine transporter (VAChT), indispensable for cholinergic neurotransmission. During cholinergic neuritogenesis in vitro, GalR2 can recruit Rho-family GTPases to induce the extension of a VAChT-containing primary neurite, the prospective axon. In doing so, GalR2 signaling dose-dependently modulates directional filopodial growth and antagonizes NGF-induced growth cone differentiation. Galanin accumulates in GABA-containing nerve terminals in the neonatal basal forebrain, suggesting its contribution to activity-driven cholinergic development during the perinatal period. Overall, our data define the cellular specificity and molecular complexity of galanin action in the developing basal forebrain.

Functional differences between neurochemically defined populations of inhibitory interneurons in the rat spinal dorsal horn

In order to understand how nociceptive information is processed in the spinal dorsal horn we need to unravel the complex synaptic circuits involving interneurons, which constitute the vast majority of the neurons in laminae I-III. The main limitation has been the difficulty in defining functional populations among these cells. We have recently identified 4 non-overlapping classes of inhibitory interneuron, defined by expression of galanin, neuropeptide Y (NPY), neuronal nitric oxide synthase (nNOS) and parvalbumin, in the rat spinal cord. In this study we demonstrate that these form distinct functional populations that differ in terms of sst(2A) receptor expression and in their responses to painful stimulation. The sst(2A) receptor was expressed by nearly all of the nNOS- and galanin-containing inhibitory interneurons but by few of those with NPY and none of the parvalbumin cells. Many galanin- and NPY-containing cells exhibited phosphorylated extracellular signal-regulated kinases (pERK) after mechanical, thermal or chemical noxious stimuli, but very few nNOS-containing cells expressed pERK after any of these stimuli. However, many nNOS-positive inhibitory interneurons up-regulated Fos after noxious thermal stimulation or injection of formalin, but not after capsaicin injection. Parvalbumin cells did not express either activity-dependent marker following any of these stimuli. These results suggest that interneurons belonging to the NPY, nNOS and galanin populations are involved in attenuating pain, and for NPY and nNOS cells this is likely to result from direct inhibition of nociceptive projection neurons. They also suggest that the nociceptive inputs to the nNOS cells differ from those to the galanin and NPY populations.

Tlx1/3 and Ptf1a control the expression of distinct sets of transmitter and peptide receptor genes in the developing dorsal spinal cord

Establishing the pattern of expression of transmitters and peptides as well as their receptors in different neuronal types is crucial for understanding the circuitry in various regions of the brain. Previous studies have demonstrated that the transmitter and peptide phenotypes in mouse dorsal spinal cord neurons are determined by the transcription factors Tlx1/3 and Ptf1a. Here we show that these transcription factors also determine the expression of two distinct sets of transmitter and peptide receptor genes in this region. We have screened the expression of 78 receptor genes in the spinal dorsal horn by in situ hybridization. We found that receptor genes Gabra1, Gabra5, Gabrb2, Gria3, Grin3a, Grin3b, Galr1, and Npy1r were preferentially expressed in Tlx3-expressing glutamatergic neurons and their derivatives, and deletion of Tlx1 and Tlx3 resulted in the loss of expression of these receptor genes. Furthermore, we obtained genetic evidence that Tlx3 uses distinct pathways to control the expression of receptor genes. We also found that receptor genes Grm3, Grm4, Grm5, Grik1, Grik2, Grik3, and Sstr2 were mainly expressed in Pax2-expressing GABAergic neurons in the spinal dorsal horn, and their expression in this region was abolished or markedly reduced in Ptf1a and Pax2 deletion mutant mice. Together, our studies indicate that Tlx1/3 and Ptf1a, the key transcription factors for fate determination of glutamatergic and GABAergic neurons in the dorsal spinal cord, are also responsible for controlling the expression of two distinct sets of transmitter and peptide receptor genes.

Fast synaptic inhibition in spinal sensory processing and pain control

The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

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 receptor-expressing dorsal horn neurons: role in nociception

Galanin, along with enkephalins and neuropeptide Y, has been hypothesized to negatively modulate nociception in the superficial dorsal horn of the spinal cord. In the present study, we sought to determine the role of presumably excitatory dorsal horn galanin receptor-expressing neurons in nociception by selectively destroying GalR1-expressing superficial dorsal horn interneurons using lumbar intrathecal injections of the targeted cytotoxin, galanin-saporin (Gal-sap). Lumbar intrathecal injection of Gal-sap (500 ng) reduced immunoperoxidase staining for GalR1 in the superficial dorsal horn without affecting primary afferent neurons in lumbar dorsal root ganglia. Lumbar intrathecal Gal-sap also: 1--reduced nocifensive reflex responding on the thermal plate at 0.3 °C, 44 °C, and 47 °C; 2--increased hot side occupancy in a thermal preference task (15 °C vs 45 °C); and, 3--decreased escape from 44 °C and 47 °C, but not 20 °C. Thus, similar to lesions of mu opiate receptor-expressing dorsal horn interneurons, selective destruction of GalR1-expressing superficial dorsal horn neurons produces heat hypo-algesia, likely due to loss of GalR1-expressing excitatory interneurons leading to reduced activation of nociceptive projection neurons in response to aversive heat. These results are different than those seen with intrathecal neuropeptide Y-saporin and suggest the potential value of selectively targeting GalR1-expressing dorsal horn neurons to control pain.

Dynorphin is expressed primarily by GABAergic neurons that contain galanin in the rat dorsal horn

Background

The opioid peptide dynorphin is expressed by certain neurons in the superficial dorsal horn of the spinal cord, but little is known about the types of cell that contain dynorphin. In this study, we have used an antibody against the dynorphin precursor preprodynorphin (PPD), to reveal the cell bodies and axons of dynorphin-expressing neurons in the rat spinal cord. The main aims were to estimate the proportion of neurons in each of laminae I-III that express dynorphin and to determine whether they are excitatory or inhibitory neurons.

Results

PPD-immunoreactive cells were concentrated in lamina I and the outer part of lamina II (IIo), where they constituted 17% and 8%, respectively, of all neurons. Around half of those in lamina I and 80% of those in lamina II were GABA-immunoreactive. We have previously identified four non-overlapping neurochemical populations of inhibitory interneurons in this region, defined by the presence of neuropeptide Y, galanin, parvalbumin and neuronal nitric oxide synthase. PPD co-localised extensively with galanin in both cell bodies and axons, but rarely or not at all with the other three markers. PPD was present in around 4% of GABAergic boutons (identified by the presence of the vesicular GABA transporter) in laminae I-II.

Conclusions

These results show that most dynorphin-expressing cells in the superficial dorsal horn are inhibitory interneurons, and that they largely correspond to the population that is defined by the presence of galanin. We estimate that dynorphin is present in ~32% of inhibitory interneurons in lamina I and 11% of those in lamina II. Since the proportion of GABAergic boutons that contain PPD in these laminae was considerably lower than this, our findings suggest that these neurons may generate relatively small axonal arborisations.

Galanin-immunoreactivity identifies a distinct population of inhibitory interneurons in laminae I-III of the rat spinal cord

Background

Inhibitory interneurons constitute 30-40% of neurons in laminae I-III and have an important anti-nociceptive role. However, because of the difficulty in classifying them we know little about their organisation. Previous studies have identified 3 non-overlapping groups of inhibitory interneuron, which contain neuropeptide Y (NPY), neuronal nitric oxide synthase (nNOS) or parvalbumin, and have shown that these differ in postsynaptic targets. Some inhibitory interneurons contain galanin and the first aim of this study was to determine whether these form a different population from those containing NPY, nNOS or parvalbumin. We also estimated the proportion of neurons and GABAergic axons that contain galanin in laminae I-III.

Results

Galanin cells were concentrated in laminae I-IIo, with few in laminae IIi-III. Galanin showed minimal co-localisation with NPY, nNOS or parvalbumin in laminae I-II, but most galanin-containing cells in lamina III were nNOS-positive. Galanin cells constituted ~7%, 3% and 2% of all neurons in laminae I, II and III, and we estimate that this corresponds to 26%, 10% and 5% of the GABAergic neurons in these laminae. However, galanin was only found in ~6% of GABAergic boutons in laminae I-IIo, and ~1% of those in laminae IIi-III.

Conclusions

These results show that galanin, NPY, nNOS and parvalbumin can be used to define four distinct neurochemical populations of inhibitory interneurons. Together with results of a recent study, they suggest that the galanin and NPY populations account for around half of the inhibitory interneurons in lamina I and a quarter of those in lamina II.

Galanin and spinal pain mechanisms: past, present, and future

Since the discovery of galanin in 1983, one of the most frequently suggested physiological function for this peptide is pain modulation at the level of the spinal cord. This notion, initially based on the preferential distribution of galanin in dorsal spinal cord, has been supported by results from a large number of morphological, molecular, and functional studies. It is generally agreed that spinally applied galanin produces a biphasic, dose-dependent effect on spinal nociception through activation of GalR1 (inhibitory) or GalR2 (excitatory) receptors. Galanin also appears to have an endogenous inhibitory role, particularly after peripheral nerve injury when the synthesis of galanin is increased in sensory neurons. In recent years, small molecule ligands of galanin receptors have been developed, which may lead to the development of analgesic drugs, which affects the galanin system at the spinal cord level.

Inhibition of inflammatory pain by activating B-type natriuretic peptide signal pathway in nociceptive sensory neurons

B-type natriuretic peptide (BNP) has been known to be secreted from cardiac myocytes and activate its receptor, natriuretic peptide receptor-A (NPR-A), to reduce ventricular fibrosis. However, the function of BNP/NPR-A pathway in the somatic sensory system has been unknown. In the present study, we report a novel function of BNP in pain modulation. Using microarray and immunoblot analyses, we found that BNP and NPR-A were expressed in the dorsal root ganglion (DRG) of rats and upregulated after intraplantar injection of complete Freund's adjuvant (CFA). Immunohistochemistry showed that BNP was expressed in calcitonin gene-related peptide (CGRP)-containing small neurons and IB4 (isolectin B4)-positive neurons, whereas NPR-A was present in CGRP-containing neurons. Application of BNP reduced the firing frequency of small DRG neurons in the presence of glutamate through opening large-conductance Ca2+-activated K+ channels (BKCa channels). Furthermore, intrathecal injection of BNP yielded inhibitory effects on formalin-induced flinching behavior and CFA-induced thermal hyperalgesia in rats. Blockade of BNP signaling by BNP antibodies or cGMP-dependent protein kinase (PKG) inhibitor KT5823 [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester] impaired the recovery from CFA-induced thermal hyperalgesia. Thus, BNP negatively regulates nociceptive transmission through presynaptic receptor NPR-A, and activation of the BNP/NPR-A/PKG/BKCa channel pathway in nociceptive afferent neurons could be a potential strategy for inflammatory pain therapy.

Subarachnoid transplantation of immortalized galanin-overexpressing astrocytes attenuates chronic neuropathic pain

Treatment of chronic neuropathic pain resulted from peripheral nerve injury is one of the most difficult problems in modern clinical practice. The use of cell lines as biologic "minipumps" to chronically deliver anti-nociceptive molecules into the pain-processing centers of spinal cord is a newly developing technique for the treatment of pain. Moreover, spinal administration of exogenous galanin (GAL) is a useful target for the treatment of chronic pain after nerve injury. Because of better histocompatibility, lower immunogenicity and reproducibility, immortalized astrocytes (IAST) have been served as a promising cellular vehicle to deliver therapeutic molecules into CNS. In this study, the rat IAST was transfected with rat preprogalanin cDNA and the galanin-synthesizing and secreting cell line, IAST/GAL, was isolated. After cells were transplanted into the subarachnoid space of rats with chronic neuropathic pain induced by spared nerve injury (SNI) of sciatic nerve, their analgesic potential was evaluated by behavioral tests. The results showed that IAST/GAL transfected with preprogalanin gene could express and secrete significantly higher level of GAL protein in vitro and in vivo as compared with control cells. In addition, the pain-related behaviors, thermal hyperalgesia and mechanical allodynia were significantly alleviated during the 1-7 weeks after grafts of IAST/GAL cells, which could be reversed by galanin receptor antagonist M35 temporarily. Taken together, these data suggest that subarachnoid transplant of immortalized galanin-overexpressing astrocytes near the pain-processing centers was able to reverse the development of chronic neuropathic pain, which offers an adjunct approach to currently used therapies for the pain management.

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.

A transcriptional network coordinately determines transmitter and peptidergic fate in the dorsal spinal cord

Dorsal horn neurons express many different neuropeptides that modulate sensory perception like the sensation of pain. Inhibitory neurons of the dorsal horn derive from postmitotic neurons that express Pax2, Lbx1 and Lhx1/5, and diversify during maturation. In particular, fractions of maturing inhibitory neurons express various neuropeptides. We demonstrate here that a coordinate molecular mechanism determines inhibitory and peptidergic fate in the developing dorsal horn. A bHLH factor complex that contains Ptf1a acts as upstream regulator and initiates the expression of several downstream transcription factors in the future inhibitory neurons, of which Pax2 is known to determine the neurotransmitter phenotype. We demonstrate here that dynorphin, galanin, NPY, nociceptin and enkephalin expression depends on Ptf1a, indicating that these neuropeptides are expressed in inhibitory neurons. Furthermore, we show that Neurod1/2/6 and Lhx1/5, which act downstream of Ptf1a, control distinct aspects of peptidergic differentiation. In particular, the Neurod1/2/6 factors are essential for dynorphin and galanin expression, whereas the Lhx1/5 factors are essential for NPY expression. We conclude that a transcriptional network operates in maturing dorsal horn neurons that coordinately determines transmitter and peptidergic fate.

Ptf1a, Lbx1 and Pax2 coordinate glycinergic and peptidergic transmitter phenotypes in dorsal spinal inhibitory neurons

Inhibitory neurons in the dorsal horn synthesize a variety of neurotransmitters, including GABA, glycine and a set of peptides. Here we show that three transcription factors, Ptf1a, Pax2, and Lbx1, which have been reported to promote a GABAergic cell fate, also specify glycinergic and peptidergic transmitter phenotypes. First, Ptf1a appears to be a master regulator, as indicated by a requirement of Ptf1a for the expression of glycinergic marker GlyT2 and a set of peptides, including neuropeptide Y (NPY), nociceptin/orphanin FQ (N/OFQ), somatostatin (SOM), enkephalin (ENK), dynorphin (DYN) and galanin (GAL). Second, Pax2 is a downstream target of Ptf1a and controls subsets of transmitter phenotypes, including the expression of GlyT2, NPY, N/OFQ, DYN, and GAL, but is dispensable for SOM or ENK expression. Third, for Lbx1, due to neuronal cell loss at late stages, our analyses focused on early embryonic stages, and we found that Lbx1 is required for the expression of GlyT2, NPY, N/OFQ and is partially responsible for SOM expression. Our studies therefore suggest a coordinated and hierarchical specification of a variety of neurotransmitters in dorsal spinal inhibitory neurons.

Selective stimulation of GalR1 and GalR2 in rat substantia gelatinosa reveals a cellular basis for the anti- and pro-nociceptive actions of galanin

Galanin modulates spinal nociceptive processing by interacting with two receptors, GalR1 and GalR2. The underlying neurophysiological mechanisms were examined by whole-cell recording from identified neurons in the substantia gelatinosa of young adult rats. GalR1 was activated with a 'cocktail' containing the GalR1/2 agonist, AR-M 961 (0.5 microM), in the presence of the GalR2 antagonist, M871 (1.0-2.5 microM). GalR2 was activated with the selective agonist, AR-M 1896 (0.5-1.0 microM). Application of the 'GalR1 agonist cocktail' often activated an inwardly-rectifying conductance in delay firing (excitatory) and tonically firing (inhibitory) neurons. This conductance was not activated by AR-M 1896 which instead decreased or increased an outwardly-rectifying conductance at voltages positive to -70 mV. Despite this variability in its actions on current-voltage relationships, AR-M 1896 very consistently decreased membrane excitability, as measured by cumulative action potential latency in response to a depolarizing current ramp. This strong GalR2-mediated effect was seen in neurons where membrane conductance was decreased, and where membrane excitability might be predicted to increase. GalR2 was also located presynaptically, as AR-M 1896 increased the interevent interval of spontaneous EPSCs in both delay and tonic cells. By contrast, the 'GalR1 agonist cocktail' had little effect on spontaneous EPSCs, suggesting that presynaptic terminals do not express GalR1. These diverse actions of GalR1 and GalR2 activation on both inhibitory and excitatory neurons are discussed in relation to the known spinal antinociceptive and pro-nociceptive actions of galanin, to the possible association of GalR1 with the inhibitory G-protein, G(i/o) and to report that GalR2 activation suppresses Ca2+ channel currents.

Neuropeptide tyrosine and pain

Research during the past two decades supports a complex role for neuropeptide tyrosine (NPY) and two of its associated receptors, the Y1 receptor and the Y2 receptor, in the modulation of pain, in addition to regeneration and survival mechanisms at the spinal level. Thus, NPY has been shown to both cause and reduce pain, in addition to having biphasic effects. Recent research has focused on the distribution of the spinal NPY-mediated system. Here, we propose various possible scenarios for the role of NPY in pain processing, based on its actions at different sites (axon versus cell body), through different receptors (Y1 receptor versus Y2 receptor) and/or types of neuron (ganglion neurons and intraganglionic cross-excitation versus interneurons versus projection neurons).

Galanin receptor 1 is expressed in a subpopulation of glutamatergic interneurons in the dorsal horn of the rat spinal cord

The 29/30 amino acid neuropeptide galanin has been implicated in pain processing at the spinal level and local dorsal horn neurons expressing the Gal(1) receptor may play a critical role. In order to determine the transmitter identity of these neurons, we used immunohistochemistry and antibodies against the Gal(1) receptor and the three vesicular glutamate transporters (VGLUTs), as well as in situ hybridization, to explore a possible glutamatergic phenotype. Gal(1) protein, which could not be demonstrated in Gal(1) knockout mice, colocalized with VGLUT2 protein, but not with glutamate decarboxylase, in many nerve endings in lamina II. Moreover, Gal(1) and VGLUT2 transcripts were often found in the same cell bodies in laminae I-IV. Gal(1)-protein and galanin-peptide showed an overlapping distribution but were not colocalized. Gal(1) staining did not appear to be affected by dorsal rhizotomy. Taken together, these findings provide strong evidence that Gal(1) is a heteroreceptor expressed on excitatory glutamatergic dorsal horn interneurons. Activation of such Gal(1) receptors may thus decrease the inhibitory tone in the superficial dorsal horn, and possibly cause antinociception.

Neuropeptides as synaptic transmitters

Neuropeptides are small protein molecules (composed of 3-100 amino-acid residues) that have been localized to discrete cell populations of central and peripheral neurons. In most instances, they coexist with low-molecular-weight neurotransmitters within the same neurons. At the subcellular level, neuropeptides are selectively stored, singularly or more frequently in combinations, within large granular vesicles. Release occurs through mechanisms different from classical calcium-dependent exocytosis at the synaptic cleft, and thus they account for slow synaptic and/or non-synaptic communication in neurons. Neuropeptide co-storage and coexistence can be observed throughout the central nervous system and are responsible for a series of functional interactions that occur at both pre- and post-synaptic levels. Thus, the subcellular site(s) of storage and sorting mechanisms into different neuronal compartments are crucial to the mode of release and the function of neuropeptides as neuronal messengers.

Phenotyping of sensory and sympathetic ganglion neurons of a galanin-overexpressing mouse--possible implications for pain processing

The distribution of galanin was studied in the lumbar 5 dorsal root ganglia (DRGs) and spinal cord, superior cervical ganglia (SCGs), and skin of transgenic mice overexpressing galanin under the dopamine beta-hydroxylase (DBH) promoter (GalOE-DBH mice) and in wild type (WT) mice. The DRGs and spinal cord were analysed before and after a unilateral, complete transection (axotomy) of the sciatic nerve and after dorsal rhizotomy. Both galanin protein and transcript were studied by, respectively, immunohistochemistry and in situ hybridization. Increased galanin expression was observed in several small, medium-sized and large DRG neuron profiles (NPs) in the naïve transgenic mouse, frequently in neurons lacking calcitonin gene-related peptide (CGRP) and isolectin B4-binding. This lack of coexistence was particularly evident in the medium-sized/large NPs. In the dorsal horn of the spinal cord, no differences were detected between GalOE-DBH and WT mice, both displaying a strong galanin-positive neuropil in the superficial laminae of the dorsal horn, but the transgenic mice showed a more abundant galanin-positive innervation of the ventral horn. A 12-day dorsal rhizotomy, surprisingly, failed to alter the galanin staining patterns in the dorsal (and ventral) dorsal horn. Unilateral axotomy induced upregulation of galanin in DRG NPs of all sizes in both types of mouse. In the hindpaw skin, a profuse galanin-positive fiber plexus was observed in sweat glands and around blood vessels of the transgenic mice, being much more restricted in WT mice. Finally, GalOE mice exhibited a strong galanin-like immunoreactivity in most SCG NPs. The overexpression of the peptide in DRGs and SCGs was paralleled by increased mRNA levels. The present results show that overexpression of galanin under the control of the DBH promoter does not only occur, as expected in these mice, in noradrenline/adrenaline neurons but also in DRG neurons, particularly in large and medium-sized NPs. To what extent and how this overexpression pattern is related to the previously shown elevated pain threshold under normal and lesion conditions is discussed [Grass, S., Crawley, J.N., Xu, X.J., Wiesenfeld-Hallin, Z., 2003a. Reduced spinal cord sensitization to C-fibre stimulation in mice over-expressing galanin. Eur. J. Neurosci. 17, 1829-1832; Hygge-Blakeman, K., Brumovsky, P., Hao, J.X., Xu, X.J., Hökfelt, T., Crawley, J.N., Wiesenfeld-Hallin, Z., 2004. Galanin over-expression decreases the development of neuropathic pain-like behaviour in mice after partial sciatic nerve injury. Brain Res. 1025, 152-158].

Sensory neuronal phenotype in galanin receptor 2 knockout mice: focus on dorsal root ganglion neurone development and pain behaviour

Galanin is a 29-amino-acid peptide expressed in dorsal root ganglion (DRG) neurones and spinal dorsal horn neurones. It affects pain threshold and has developmental and trophic effects. Galanin acts at three G-protein-coupled receptors, galanin receptors (GalR1-3), each expressed in the DRGs as suggested by in situ hybridization and/or reverse transcriptase-polymerase chain reaction. The GalR2 knockout (-/-) mice permit studies on the contributions of this receptor subtype to the role of galanin at the spinal level. At 1 week after sciatic nerve transection (axotomy), there were 16-20% fewer neurones in intact and contralateral DRGs of -/- mice as compared with wild-type (WT) mice. In addition, a significant neurone loss (26% reduction) was found in the ipsilateral DRGs of WT mice, whereas no further neurone loss was seen in -/- mice. Expression of several peptides has been examined after axotomy, including galanin, neuropeptide Y and two of its receptors as well as substance P, and no significant differences were found between -/- and WT mice in either ipsi- or contralateral DRGs, respectively. After thermal injury and spinal nerve ligation, onset and duration of hyperalgesia in the injured paw were similar in GalR2-/- and WT animals. Recovery from spinal nerve ligation-caused allodynia had the same kinetics in -/- and WT animals. These data are in line with earlier observations from the peripheral and central nervous system, suggesting that galanin actions mediated by GalR2 subtype are of importance in neurodevelopment and neuroprotection.

Mechanisms of antinociception of spinal galanin: how does galanin inhibit spinal sensitization?

Galanin by a spinal action has been shown to have an antihyperalgesic action. Thus, in rats with lumbar intrathecal (IT) catheters, the thermal hyperalgesia evoked by carrageenan paw injection was blocked by IT delivery of galanin(1-29) (Gal(1-29)) and galanin(2-11) (Gal(2-11)) with the rank order of activity being Gal(1-29)>Gal(2-11). We sought to determine whether this spinal action reflects an effect upon afferent transmitter release, e.g., substance P (SP), and/or on secondary neurons, e.g., signaling postsynaptic to neurokinin 1 (NK1) receptor activation. To address the question on afferent release, we investigated the effect of IT administration of galanin on tissue injury-induced spinal NK1 internalization (an indicator of SP release). Noxious stimulation (paw compression) produced an increase in NK1 internalization in dorsal horn lamina I. IT pretreatment of rats with Gal(1-29) and Gal(2-11) significantly attenuated the evoked NK1 internalization, with the rank order of activity being Gal(1-29)>Gal(2-11)>saline. To address the question of postsynaptic action, we examined the effects of IT galanin upon IT SP-induced thermal hyperalgesia and spinal PGE2 release. Application of SP (30 nmol) directly to spinal cord led to a decrease in thermal thresholds and a profound increase in PGE(2) concentration in spinal dialysates. Both phenomena were reversed by Gal(1-29) and Gal(2-11) (10nmol, IT). These findings suggest that the antihyperalgesic effect of spinal galanin is due to its action on sites both presynaptic (inhibition of SP release) and postsynaptic (blockade of SP-evoked hyperalgesia and PGE2 production) to the primary afferents.

Galaninergic mechanisms at the spinal level: focus on histochemical phenotyping

The 29/30 amino acid neuropeptide galanin is present in a small population of DRG neurons under normal condition but is strongly upregulated after nerve injury. There is evidence that this upregulated galanin has trophic actions, for example promoting neurite outgrowth as well as influencing pain processing. In fact, both pro- and antinociceptive effects have been reported, probably relating to activation of different receptors. It has been proposed that presynaptic GalR2 receptors are pro-nociceptive by enhancing release of excitatory transmitters in the dorsal horn, and anti-nociceptive via an action on GalR1-positive interneurons. These neurons have recently been shown to be glutamatergic. Several other peptides and molecules are also regulated by nerve injury. Here we focus on neuropeptide tyrosine (NPY), which is upregulated in parallel with galanin. We review data reporting on coexistence between galanin and NPY and between these two peptides and the two NPY receptors Y1 and Y2. The data show considerable overlap, and it will be an important task to analyse how cross-talk between these neuropeptides can influence pain processing. It is proposed that such cross-talk can occur by release of peptides from DRGs neuron somata within dorsal root ganglia. To what extent these mechanisms shown to exist in rodents also occur in human is important, if one wants to discuss novel strategies for pain treatment on the basis of these findings. So far information is limited, but it has been demonstrated that galanin is expressed in DRGs and possibly also regulated.

Galanin acts at GalR1 receptors in spinal antinociception: synergy with morphine and AP-5

The neuropeptide galanin (Gal) and its receptors (GalR1, GalR2, and GalR3) are expressed in spinal cord. We have characterized the pharmacology of the antinociceptive effects of intrathecally (i.t.) administered galanin and its analogs in the formalin test in rats, using an automated flinch detection system. Intrathecal injection of rat galanin (Gal(1-29)) or human galanin (Gal(1-30)) produced a dose-dependent inhibition of formalin-evoked flinching in phase 2, but not in phase 1. Relative potency of galanin homologs is Gal(1-29) >or= Gal(1-30) > galanin-like peptide(1-24) >or= Gal(2-11) = Gal (3-29) (an inactive analog). Galanin(1-29) and Gal(1-30) are both high-affinity agonists to GalR1/R2, whereas Gal(2-11) is a GalR2 receptor agonist. Our data suggest that i.t. galanin-produced antinociception is mediated by activation of GalR1 receptors. When comparing antinociceptive effects of i.t. Gal(1-29) to morphine and to 2-amino-5-phosphonopentanoic acid (AP-5, an N-methyl-d-aspartate antagonist), Gal(1-29) is of intermediate potency between these two analgesic agents based on the ED(50) values. An isobolographic analysis showed synergy between Gal(1-29) and morphine and between Gal(1-29) and AP-5 on the second phase. Fixed ratio dose combinations of morphine and Gal(1-29), or AP-5 and Gal(1-29) produced significantly greater antinociception than predicted from simple additivity. In summary, the present findings reveal that 1) spinal galanin produces a reliable inhibition of formalin-induced facilitated nociceptive processing, an effect possibly mediated by GalR1 receptors; and 2) galanin potentiates i.t. morphine and AP-5-induced antinociception.

The participation of galanin in pain processing at the spinal level

Galanin, a 29-amino-acid peptide expressed in dorsal root ganglia (DRG) and spinal dorsal horn interneurones, is regulated by nerve injury and peripheral inflammation. The functional significance of such regulation has been subject to intense studies, including the analysis of galanin null mice, with the production of apparently conflicting results. Here, we suggest that upregulation of galanin in DRG neurones following nerve injury results in antinociception via stimulation of galanin GAL1 receptors on dorsal horn neurones, and that the pro-nociceptive effect of galanin is related to presynaptic galanin GAL2 receptors on primary afferents. A selective GAL1 receptor agonist could therefore be valuable for the treatment of neuropathic pain.

Nerve injury induces plasticity that results in spinal inhibitory effects of galanin

Galanin is a 29-amino-acid neuropeptide that has been implicated in the processes of nociception. This study examines the effect of exogenous galanin on dorsal horn neurone activity in vivo in the spinal nerve ligation (SNL) model of neuropathic pain. SNL rats but not naive or sham-operated rats exhibited behaviour indicative of allodynia. In anaesthetized rats, extracellular recordings were made from individual convergent dorsal horn neurones following stimulation of peripheral receptive fields electrically or with natural (innocuous mechanical, noxious mechanical and noxious thermal) stimuli. Spinal administration of galanin (0.5-50 microg) caused a slight facilitation of the neuronal responses to natural and electrical stimuli in naive rats and up to a 65% inhibition of neuronal responses in sham-operated rats following 50 microg galanin. In contrast, there was a marked inhibition of up to 80% of responses to both natural and electrical stimuli in SNL rats following spinal galanin administration. These results suggest that following peripheral nerve injury, there is plasticity in the levels of galanin and/or its receptors at spinal cord level so that the effect of exogenous galanin favours inhibitory function.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Costorage and coexistence of neuropeptides in the mammalian CNS

The term neuropeptides commonly refers to a relatively large number of biologically active molecules that have been localized to discrete cell populations of central and peripheral neurons. I review here the most important histological and functional findings on neuropeptide distribution in the central nervous system (CNS), in relation to their role in the exchange of information between the nerve cells. Under this perspective, peptide costorage (presence of two or more peptides within the same subcellular compartment) and coexistence (concurrent presence of peptides and other messenger molecules within single nerve cells) are discussed in detail. In particular, the subcellular site(s) of storage and sorting mechanisms within neurons are thoroughly examined in the view of the mode of release and action of neuropeptides as neuronal messengers. Moreover, the relationship of neuropeptides and other molecules implicated in neural transmission is discussed in functional terms, also referring to the interactions with novel unconventional transmitters and trophic factors. Finally, a brief account is given on the presence of neuropeptides in glial cells.

Neuropeptides in neuropathic and inflammatory pain with special emphasis on cholecystokinin and galanin

Neuropeptides present in primary afferents and the dorsal horn of the spinal cord have an important role in the mediation of nociceptive input under normal conditions. Under pathological conditions, such as chronic inflammation or following peripheral nerve injury, the production of peptides and peptide receptors is dramatically altered, leading to a number of functional consequences. In this review, the role of two neuropeptides that undergo such altered expression under pathological conditions, cholecystokinin (CKK) and galanin, is reviewed.

Endogenous galanin potentiates spinal nociceptive processing following inflammation

We have undertaken a series of experiments using galanin null mutant mice to better define the role of endogenous galanin in spinal excitability following inflammation and in response to centrally sensitizing stimuli. We have employed a behavioural paradigm, the formalin test, as a model of tonic nociception in both galanin knock-out (gal-/-) and wild-type (gal+/+) mice. In this model, we find that gal-/- mice are markedly hypo-responsive, especially in the second phase response. Additionally, we have examined the thermal hyperalgesia which develops following peripheral injection of carrageenan into the plantar surface of one hindpaw. In this inflammatory paradigm, thermal hyperalgesia is markedly attenuated in gal-/- mice. These behavioural findings suggest that endogenous galanin contributes to nociceptive processing. We have tested this hypothesis further by employing an electrophysiological measure of spinal excitability, the flexor withdrawal reflex in gal-/- and gal+/+ mice. We found no differences in acute reflex responses to single stimuli at C-fibre strength or in the time course and magnitude of wind-up induced by a short conditioning train between non-inflamed gal+/+ and gal-/- mice. However, the long-lasting post-conditioning enhancement of reflex excitability was only seen in gal+/+ mice. Moreover, following carrageenan inflammation, there was a marked increase in spinal nociceptive reflex excitability in the inflamed gal+/+ mice, but this enhanced excitability was absent in gal-/- animals. These findings illustrate that endogenous galanin is necessary for the full expression of central sensitization, and as such, plays a critical role in the development of hyperalgesia following peripheral tissue injury.

Galanin and spinal nociceptive mechanisms: recent advances and therapeutic implications

Galanin is a peptide consisting of 29 or 30 (in humans) amino acids that is present in sensory and spinal dorsal horn neurons. Endogenous galanin may have an important modulatory function on nociceptive input at the spinal level. In addition, exogenously administered galanin exerts complex effects on spinal nociceptive transmission, where inhibitory action appears to predominate. Peripheral nerve injury and inflammation, conditions associated with chronic pain, upregulate the synthesis of galanin in sensory neurons and spinal cord neurons, respectively. Hence, the sensory effect of galanin may be increased under these conditions, raising the possibility that modulation of the activity of the galanin system may produce antinociception.

Galanin knockout mice reveal nociceptive deficits following peripheral nerve injury

The neuropeptide galanin has been identified as a potential neurotransmitter/neuromodulator within the central nervous system. In the present study, the role of endogenous galanin in nociceptive processing in the nervous system has been analysed by using mice carrying a targeted mutation in the galanin gene. Supporting this, the effect of chronic administration of exogenous galanin on nociceptive sensory inputs has been assayed in adult rats. In the absence of peripheral nerve injury, the sensitivity to threshold noxious stimuli is significantly higher in galanin mutant mice than wild-type controls. Following peripheral nerve injury, in conditions under which endogenous galanin levels are elevated, spontaneous and evoked neuropathic pain behaviours are compromised in mutant mice. Conversely, chronic intrathecal delivery of exogenous galanin to nerve-intact adult rats is associated with persistent behavioural hypersensitivity, a significant increase in c-fos expression and an increase in PKCgamma immunoreactivity within the spinal cord dorsal horn. The present results demonstrate that a relationship exists between the degree of nerve injury-induced galanin expression and the degree of behavioural hypersensitivity, and show that galanin may play a role in nociceptive processing in the spinal cord, with interrelated inhibitory and excitatory effects.