Effect of a graded single constriction of the rat sciatic nerve on pain behavior and expression of immunoreactive NPY and NPY Y1 receptor in DRG neurons and spinal cord

Description of Novel Molecular Factors in Lumbar DRGs and Spinal Cord Factors Underlying Development of Neuropathic Pain Component in the Animal Model of Osteoarthritis

Osteoarthritis (OA) is one of the most common joint disorder, with pain accompanied by functional impairment, as the most pronounced clinical symptom. Currently used pharmacotherapy involves symptomatic treatment that do not always provide adequate pain relief. This may be due to concomitance of central sensitization and development of neuropathic features in OA patients. Here we performed studies in the animal model of OA to investigate of the neuropathic component. Intraarticular injection of monoiodoacetate (MIA, 1 mg) was used to induce OA in Wistar male rats. Development of pain phenotype was assessed by behavioral testing (PAM test and von Frey's test), while corresponding changes in dorsal root ganglia (DRGs L3-L5) and spinal cord (SC) gene expression were assessed by means of qRT-PCR technique. We also performed microtomography of OA-affected knee joints to correlate the level of bone degradation with observed behavioral and molecular changes. We observed gradually developing remote allodynia after MIA treatment, indicating the presence of neuropathic component. Our results showed that, among DRGs innervating knee joint, development of central sensitization is most likely due to peripheral input of stimuli through DRG L5. In SC, development of secondary hypersensitivity correlated with increased expression of TAC1 and NPY. Our studies provided molecular records on abnormal activation of pain transmission markers in DRG and SC during development of OA that are responsible for the manifestation of neuropathic features. The obtained results increase insight into molecular changes occurring in the neuronal tissue during OA development and may contribute to readdressing treatment paradigms.

Molecular Changes in the Dorsal Root Ganglion during the Late Phase of Peripheral Nerve Injury-induced Pain in Rodents: A Systematic Review

The dorsal root ganglion is widely recognized as a potential target to treat chronic pain. A fundamental understanding of quantitative molecular and genomic changes during the late phase of pain is therefore indispensable. The authors performed a systematic literature review on injury-induced pain in rodent dorsal root ganglions at minimally 3 weeks after injury. So far, slightly more than 300 molecules were quantified on the protein or messenger RNA level, of which about 60 were in more than one study. Only nine individual sequencing studies were performed in which the most up- or downregulated genes varied due to heterogeneity in study design. Neuropeptide Y and galanin were found to be consistently upregulated on both the gene and protein levels. The current knowledge regarding molecular changes in the dorsal root ganglion during the late phase of pain is limited. General conclusions are difficult to draw, making it hard to select specific molecules as a focus for treatment.

Nociceptive mechanisms driving pain in a post-traumatic osteoarthritis mouse model

In osteoarthritis (OA), pain is the dominant clinical symptom, yet the therapeutic approaches remain inadequate. The knowledge of the nociceptive mechanisms in OA, which will allow to develop effective therapies for OA pain, is of utmost need. In this study, we investigated the nociceptive mechanisms involved in post-traumatic OA pain, using the destabilization of the medial meniscus (DMM) mouse model. Our results revealed the development of peripheral pain sensitization, reflected by augmented mechanical allodynia. Along with the development of pain behaviour, we observed an increase in the expression of calcitonin gene-related peptide (CGRP) in both the sensory nerve fibers of the periosteum and the dorsal root ganglia. Interestingly, we also observed that other nociceptive mechanisms commonly described in non-traumatic OA phenotypes, such as infiltration of the synovium by immune cells, neuropathic mechanisms and also central sensitization were not present. Overall, our results suggest that CGRP in the sensory nervous system is underlying the peripheral sensitization observed after traumatic knee injury in the DMM model, highlighting the CGRP as a putative therapeutic target to treat pain in post-traumatic OA. Moreover, our findings suggest that the nociceptive mechanisms involved in driving pain in post-traumatic OA are considerably different from those in non-traumatic OA.

Targeting spinal neuropeptide Y1 receptor-expressing interneurons to alleviate chronic pain and itch

An accelerating basic science literature is providing key insights into the mechanisms by which spinal neuropeptide Y (NPY) inhibits chronic pain. A key target of pain inhibition is the G_i-coupled neuropeptide Y1 receptor (Y1). Y1 is located in key sites of pain transmission, including the peptidergic subpopulation of primary afferent neurons and a dense subpopulation of small, excitatory, glutamatergic/somatostatinergic interneurons (Y1-INs) that are densely expressed in the dorsal horn, particularly in superficial lamina I-II. Selective ablation of spinal Y1-INs with an NPY-conjugated saporin neurotoxin attenuates the development of peripheral nerve injury-induced mechanical and cold hypersensitivity. Conversely, conditional knockdown of NPY expression or intrathecal administration of Y1 antagonists reinstates hypersensitivity in models of chronic latent pain sensitization. These and other results indicate that spinal NPY release and the consequent inhibition of pain facilitatory Y1-INs represent an important mechanism of endogenous analgesia. This mechanism can be mimicked with exogenous pharmacological approaches (e.g. intrathecal administration of Y1 agonists) to inhibit mechanical and thermal hypersensitivity and spinal neuron activity in rodent models of neuropathic, inflammatory, and postoperative pain. Pharmacological activation of Y1 also inhibits mechanical- and histamine-induced itch. These immunohistochemical, pharmacological, and cell type-directed lesioning data, in combination with recent transcriptomic findings, point to Y1-INs as a promising therapeutic target for the development of spinally directed NPY-Y1 agonists to treat both chronic pain and itch.

Neuropeptide Y and cannabinoids interaction in the amygdala after exposure to shock and reminders model of PTSD

Modulation of cannabinoid and neuropeptide Y (NPY) receptors may offer therapeutic benefits for post-traumatic stress disorder (PTSD). In this study, we aimed to investigate the functional interaction between these systems in the basolateral amygdala (BLA) in a rat model of PTSD. Rats were exposed to the shock and reminders model of PTSD and tested for hyper arousal/PTSD- and depression-like behaviors 3 weeks later. Immediately after shock exposure rats were microinjected into the BLA with URB597, a selective inhibitor of fatty acid amide hydrolase (FAAH) that increases the levels of the endocannabinoid anandamide or with the NPY1 receptor agonist Leu³¹,Pro³⁴-NPY (Leu). Intra-BLA URB597 prevented the shock/reminders-induced PTSD- behaviors (extinction, startle) and depression-behaviors (despair, social impairments). These preventing effects of URB597 on PTSD- and depression-like behaviors were shown to be mostly mediated by cannabinoid CB1 and NPY1 receptors, as they were blocked when URB597 was co-administered with a low dose of a CB1 or NPY1 receptor antagonist. Similarly, intra-BLA Leu prevented development of all the behaviors. Interestingly, a CB1 antagonist prevented the effects of Leu on despair and social behavior, but not the effects on extinction and startle. Moreover, exposure to shock and reminders upregulated CB1 and NPY1 receptors in the BLA and infralimbic prefrontal cortex and this upregulation was restored to normal with intra-BLA URB597 or Leu. The findings suggest that the functional interaction between the eCB and NPY1 systems is complex and provide a rationale for exploring novel therapeutic strategies that target the cannabinoid and NPY systems for stress-related diseases.

Facilitation of neuropathic pain by the NPY Y1 receptor-expressing subpopulation of excitatory interneurons in the dorsal horn

Endogenous neuropeptide Y (NPY) exerts long-lasting spinal inhibitory control of neuropathic pain, but its mechanism of action is complicated by the expression of its receptors at multiple sites in the dorsal horn: NPY Y1 receptors (Y1Rs) on post-synaptic neurons and both Y1Rs and Y2Rs at the central terminals of primary afferents. We found that Y1R-expressing spinal neurons contain multiple markers of excitatory but not inhibitory interneurons in the rat superficial dorsal horn. To test the relevance of this spinal population to the development and/or maintenance of acute and neuropathic pain, we selectively ablated Y1R-expressing interneurons with intrathecal administration of an NPY-conjugated saporin ribosomal neurotoxin that spares the central terminals of primary afferents. NPY-saporin decreased spinal Y1R immunoreactivity but did not change the primary afferent terminal markers isolectin B4 or calcitonin-gene-related peptide immunoreactivity. In the spared nerve injury (SNI) model of neuropathic pain, NPY-saporin decreased mechanical and cold hypersensitivity, but disrupted neither normal mechanical or thermal thresholds, motor coordination, nor locomotor activity. We conclude that Y1R-expressing excitatory dorsal horn interneurons facilitate neuropathic pain hypersensitivity. Furthermore, this neuronal population remains sensitive to intrathecal NPY after nerve injury. This neuroanatomical and behavioral characterization of Y1R-expressing excitatory interneurons provides compelling evidence for the development of spinally-directed Y1R agonists to reduce chronic neuropathic pain.

Subclinical lipopolysaccharide from Salmonella Enteritidis induces neuropeptide dysregulation in the spinal cord and the dorsal root ganglia

Background

Despite increasing evidence that lipopolysaccharide (LPS) affects the biological active substances of dorsal root ganglia (DRG) we have limited knowledge of the influence of a single low dose of LPS, which does not result in any clinical symptoms of disease (subclinical LPS) on neuropeptides connected with the sensory pathway. Accordingly, in this work, we investigated the influence of subclinical LPS from Salmonella Enteritidis on selected neuropeptides: substance P (SP), galanin (GAL), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) and somatostatin (SOM) in the cervical, thoracic, lumbar and sacral regions of the DRG and spinal cord.

Methods

This study was performed on immature female pigs of the Pietrain × Duroc breed. Seven days after the intravenous injection of saline solution for control animals (n = 5) and 5 μg/kg b.w. LPS from S. Enteritidis for the experimental group (n = 5), the DRG and the spinal cord were collected to extract the neuropeptides using solid-phase extraction technology.

Results

Our results demonstrated that subclinical LPS in DRG was able to change the levels of all studied neuropeptides except SOM, whereas in the spinal cord it down-regulated all studied neuropeptides in the sacral spinal cord, maintaining the concentration of all studied neuropeptides in other regions similar to that observed in the control animals. The significant differences in the intensity and character of observed changes between particular regions of the DRG suggest that the exact functions of the studied neuropeptides and mechanisms of responses to subclinical LPS action depend on specific characteristics and functions of each examination region of DRG.

Conclusions

The mechanisms of observed changes are not fully understood and require further study of the molecular interactions between subclinical LPS from S. Enteritidis and neuronal and non-neuronal cells of DRG and spinal cord. The peripheral and central pain pathways must be analysed with the aspect of unknown long-term consequences of the influence of subclinical LPS from S. Enteritidis on neuropeptides in the spinal cord and the dorsal root ganglia.

Comorbidity of Alcohol Use Disorder and Chronic Pain: Genetic Influences on Brain Reward and Stress Systems

Alcohol use disorder (AUD) is highly comorbid with chronic pain (CP). Evidence has suggested that neuroadaptive processes characterized by reward deficit and stress surfeit are involved in the development of AUD and pain chronification. Neurological data suggest that shared genetic architecture associated with the reward and stress systems may contribute to the comorbidity of AUD and CP. This monograph first delineates the prevailing theories of the development of AUD and pain chronification focusing on the reward and stress systems. It then provides a brief summary of relevant neurological findings followed by an evaluation of evidence documented by molecular genetic studies. Candidate gene association studies have provided some initial support for the genetic overlap between AUD and CP; however, these results must be interpreted with caution until studies with sufficient statistical power are conducted and replications obtained. Genomewide association studies have suggested a number of genes (e.g., TBX19, HTR7, and ADRA1A) that are either directly or indirectly related to the reward and stress systems in the AUD and CP literature. Evidence reviewed in this monograph suggests that shared genetic liability underlying the comorbidity between AUD and CP, if present, is likely to be complex. As the advancement in molecular genetic methods continues, future studies may show broader central nervous system involvement in AUD-CP comorbidity.

Differential expression of Cathepsin E in transthyretin amyloidosis: from neuropathology to the immune system

Background

Increasing evidence supports a key role for inflammation in the neurodegenerative process of familial amyloidotic polyneuropathy (FAP). While there seems to be an overactivation of the neuronal interleukin-1 signaling pathway, the immune response is apparently compromised in FAP. Accordingly, little immune cell infiltration is observed around pre-fibrillar or fibrillar amyloid deposits, with the underlying mechanism for this phenomenon remaining poorly understood. Cathepsin E (CtsE) is an important intermediate for antigen presentation and chemotaxis, but its role in the pathogenesis of FAP disease remains unknown.

Methods

In this study, we used both mouse primary macrophages and in vivo studies based on transgenic models of FAP and human samples to characterize CtsE expression in different physiological systems.

Results

We show that CtsE is critically decreased in bone marrow-derived macrophages from a FAP mouse model, possibly contributing for cell function impairment. Compromised levels of CtsE were also found in injured nerves of transgenic mice and, most importantly, in naïve peripheral nerves, sensory ganglia, murine stomach, and sural nerve biopsies derived from FAP patients. Expression of CtsE in tissues was associated with transthyretin (TTR) deposition and differentially regulated accordingly with the physiological system under study. Preventing deposition with a TTR small interfering RNA rescued CtsE in the peripheral nervous system (PNS). In contrast, the expression of CtsE increased in splenic cells (mainly monocytes) or peritoneal macrophages, indicating a differential macrophage phenotype.

Conclusion

Altogether, our data highlights the potential of CtsE as a novel FAP biomarker and a possible modulator for innate immune cell chemotaxis to the disease most affected tissues—the peripheral nerve and the gastrointestinal tract.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0891-9) contains supplementary material, which is available to authorized users.

Spinal activation of the NPY Y1 receptor reduces mechanical and cold allodynia in rats with chronic constriction injury

Neuropeptide tyrosine (NPY) and its associated receptors Y1R and Y2R have been previously implicated in the spinal modulation of neuropathic pain induced by total or partial sectioning of the sciatic nerve. However, their role in chronic constrictive injuries of the sciatic nerve has not yet been described. In the present study, we analyzed the consequences of pharmacological activation of spinal Y1R, by using the specific Y1R agonist Leu³¹Pro³⁴-NPY, in rats with chronic constriction injury (CCI). CCI and sham-injury rats were implanted with a permanent intrathecal catheter (at day 7 after injury), and their response to the administration of different doses (2.5, 5, 7, 10 or 20μg) of Leu³¹Pro³⁴-NPY (at a volume of 10μl) through the implanted catheter, recorded 14days after injury. Mechanical allodynia was tested by means of the up-and-down method, using von Frey filaments. Cold allodynia was tested by application of an acetone drop to the affected hindpaw. Intrathecal Leu³¹Pro³⁴-NPY induced an increase of mechanical thresholds in rats with CCI, starting at doses of 5μg and becoming stronger with higher doses. Intrathecal Leu³¹Pro³⁴ also resulted in reductions in the frequency of withdrawal to cold stimuli, although the effect was somewhat more moderate and mostly observed for doses of 7μg and higher. We thus show that spinal activation of the Y1R is able to reduce neuropathic pain due to a chronic constrictive injury and, together with other studies, support the use of a spinal Y1R agonist as a therapeutic agent against chronic pain induced by peripheral neuropathy.

Overexpression of Protocadherin-10 in Transthyretin-Related Familial Amyloidotic Polyneuropathy

Overwhelming data suggest that oncogenic and neurodegenerative pathways share several altered cellular responses to insults such as oxidative stress, extracellular matrix remodeling, inflammation, or cell dyscommunication. Protocadherin-10 (Pcdh10) is an adhesion molecule found to protect against tumorigenesis and essential for axonal elongation and actin dynamics during development. Here, by using genome microarrays we identified for the first time Pcdh10 up-regulation in tissues from transgenic mouse models, cultured Schwann cells, and human samples from a familial form of peripheral neuropathy (familial amyloidotic polyneuropathy). Familial amyloidotic polyneuropathy is characterized by poor functional recovery and impaired nerve regenerative response after misfolding and deposition in the peripheral nervous system of mutant transthyretin. Not only increased transcriptional and translational Pcdh10 levels occurred in axons and Schwann cells of nerves with deposited transthyretin aggregates but the pattern also extended to associated cues of axon guidance like neuropilin-1 and F-actin. These findings suggest that Pcdh10 may influence subcellular actin cytoskeletal organization and axon-axon interactions in the course of familial amyloidotic polyneuropathy. Moreover, when preventing nonfibrillar transthyretin deposition with anakinra or transthyretin siRNA, Pcdh10 protein levels were reduced, highlighting its potential as a novel disease biomarker. Whether Pcdh10 overexpression in familial amyloidotic polyneuropathy represents a protective or deleterious response, enhancing survival or promoting cell death will need further investigation.

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.

Bone Injury and Repair Trigger Central and Peripheral NPY Neuronal Pathways

Bone repair is a specialized type of wound repair controlled by complex multi-factorial events. The nervous system is recognized as one of the key regulators of bone mass, thereby suggesting a role for neuronal pathways in bone homeostasis. However, in the context of bone injury and repair, little is known on the interplay between the nervous system and bone. Here, we addressed the neuropeptide Y (NPY) neuronal arm during the initial stages of bone repair encompassing the inflammatory response and ossification phases in femoral-defect mouse model. Spatial and temporal analysis of transcriptional and protein levels of NPY and its receptors, Y1R and Y2R, reported to be involved in bone homeostasis, was performed in bone, dorsal root ganglia (DRG) and hypothalamus after femoral injury. The results showed that NPY system activity is increased in a time- and space-dependent manner during bone repair. Y1R expression was trigged in both bone and DRG throughout the inflammatory phase, while a Y2R response was restricted to the hypothalamus and at a later stage, during the ossification step. Our results provide new insights into the involvement of NPY neuronal pathways in bone repair.

Cutaneous tissue damage induces long-lasting nociceptive sensitization and regulation of cellular stress- and nerve injury-associated genes in sensory neurons

Tissue damage is one of the major etiological factors in the emergence of chronic/persistent pain, although mechanisms remain enigmatic. Using incision of the back skin of adult rats as a model for tissue damage, we observed sensitization in a nociceptive reflex enduring to 28days post-incision (DPI). To determine if the enduring behavioral changes corresponded with a long-term impact of tissue damage on sensory neurons, we examined the temporal expression profile of injury-regulated genes and the electrophysiological properties of traced dorsal root ganglion (DRG) sensory neurons. The mRNA for the injury/stress-hub gene Activating Transcription Factor 3 (ATF3) was upregulated and peaked within 4 DPI, after which levels declined but remained significantly elevated out to 28 DPI, a time when the initial incision appears healed and tissue-inflammation largely resolved. Accordingly, stereological image analysis indicated that some neurons expressed ATF3 only transiently (mostly medium-large neurons), while in others it was sustained (mostly small neurons), suggesting cell-type-specific responses. In retrogradely-traced ATF3-expressing neurons, Calcium/calmodulin-dependent protein kinase type IV (CAMK4) protein levels and isolectin-B4 (IB4)-binding were suppressed whereas Growth Associated Protein-43 (GAP-43) and Neuropeptide Y (NPY) protein levels were enhanced. Electrophysiological recordings from DiI-traced sensory neurons 28 DPI showed a significant sensitization limited to ATF3-expressing neurons. Thus, ATF3 expression is revealed as a strong predictor of single cells displaying enduring pain-related electrophysiological properties. The cellular injury/stress response induced in sensory neurons by tissue damage and indicated by ATF3 expression is positioned to contribute to pain which can occur after tissue damage.

Fracture pain-Traveling unknown pathways

An increase of fracture incidence is expected for the next decades, mostly due to the undeniable increase of osteoporotic fractures, associated with the rapid population ageing. The rise in sports-related fractures affecting the young and active population also contributes to this increased fracture incidence, and further amplifies the economical burden of fractures. Fracture often results in severe pain, which is a primary symptom to be treated, not only to guarantee individual's wellbeing, but also because an efficient management of fracture pain is mandatory to ensure proper bone healing. Here, we review the available data on bone innervation and its response to fracture, and discuss putative mechanisms of fracture pain signaling. In addition, the common therapeutic approaches to treat fracture pain are discussed. Although there is still much to learn, research in fracture pain has allowed an initial insight into the mechanisms involved. During the inflammatory response to fracture, several mediators are released and will putatively activate and sensitize primary sensory neurons, in parallel, intense nerve sprouting that occurs in the fracture callus area is also suggested to be involved in pain signaling. The establishment of hyperalgesia and allodynia after fracture indicates the development of peripheral and central sensitization, still, the underlying mechanisms are largely unknown. A major concern during the treatment of fracture pain needs to be the preservation of proper bone healing. However, the most common therapeutic agents, NSAIDS and opiates, can cause significant side effects that include fracture repair impairment. The understanding of the mechanisms of fracture pain signaling will allow the development of mechanisms-based therapies to effectively and safely manage fracture pain.

Injury of primary afferent neurons may contribute to osteoarthritis induced pain: an experimental study using the collagenase model in rats

OBJECTIVE

Pain exacerbated by movement and loading on the joint is the major symptom of osteoarthritis (OA), but the mechanisms of chronic pain in this pathology are still poorly understood. Using the intra-articular (i.a.) injection of collagenase in the knee of rats as a model of OA, we aimed at evaluating whether injury of sensory neurons may contribute to the development of OA-associated nociception.

DESIGN

OA was induced by i.a. injection of collagenase into the left knee joint of adult male Wistar rats. Histopathological changes and movement and loading-induced nociception were assessed for 6 weeks. A time-course analysis of the expression of the neuronal injury markers activating transcription factor-3 (ATF-3) and neuropeptide Y (NPY) and of the neuropeptide SP in the dorsal root ganglion (DRG) was performed. Gabapentin's effect on nociception was evaluated, as well as the expression of the α2δ-1 voltage-gated calcium channel subunit.

RESULTS

Collagenase induced the development of OA-like histopathological changes and of movement-induced nociception. Altered expression of ATF-3, NPY and SP was observed in the DRG, correlating with the degree of articular degeneration after 6 weeks of disease progression. Repeated administration of gabapentin reversed the nociceptive responses 6 weeks after the induction of OA. α2δ-1 was upregulated in the DRG.

CONCLUSION

By inducing nociceptive behaviours associated with relevant joint structural changes, the i.a. injection of collagenase presents itself as a pertinent model for the study of OA pain. The findings in this study support the hypothesis that injury of sensory neurons innervating OA joints may be a significant element in the mechanisms of OA-associated pain.

Novel expression pattern of neuropeptide Y immunoreactivity in the peripheral nervous system in a rat model of neuropathic pain

Background

Neuropeptide Y (NPY) has been implicated in the modulation of pain. Under normal conditions, NPY is found in interneurons in the dorsal horn of the spinal cord and in sympathetic postganglionic neurons but is absent from the cell bodies of sensory neurons. Following peripheral nerve injury NPY is dramatically upregulated in the sensory ganglia. How NPY expression is altered in the peripheral nervous system, distal to a site of nerve lesion, remains unknown. To address this question, NPY expression was investigated using immunohistochemistry at the level of the trigeminal ganglion, the mental nerve and in the skin of the lower lip in relation to markers of sensory and sympathetic fibers in a rat model of trigeminal neuropathic pain.

Results

At 2 and 6 weeks after chronic constriction injury (CCI) of the mental nerve, de novo expression of NPY was seen in the trigeminal ganglia, in axons in the mental nerve, and in fibers in the upper dermis of the skin. In lesioned animals, NPY immunoreactivity was expressed primarily by large diameter mental nerve sensory neurons retrogradely labelled with Fluorogold. Many axons transported this de novo NPY to the periphery as NPY-immunoreactive (IR) fibers were seen in the mental nerve both proximal and distal to the CCI. Some of these NPY-IR axons co-expressed Neurofilament 200 (NF200), a marker for myelinated sensory fibers, and occasionally colocalization was seen in their terminals in the skin. Peptidergic and non-peptidergic C fibers expressing calcitonin gene-related peptide (CGRP) or binding isolectin B4 (IB4), respectively, never expressed NPY. CCI caused a significant de novo sprouting of sympathetic fibers into the upper dermis of the skin, and most, but not all of these fibers, expressed NPY.

Conclusions

This is the first study to provide a comprehensive description of changes in NPY expression in the periphery after nerve injury. Novel expression of NPY in the skin comes mostly from sprouted sympathetic fibers. This information is fundamental in order to understand where endogenous NPY is expressed, and how it might be acting to modulate pain in the periphery.

Protective role of anakinra against transthyretin-mediated axonal loss and cell death in a mouse model of familial amyloidotic polyneuropathy

Familial amyloidotic polyneuropathy (FAP) is characterized by a length-dependent axonal loss in the peripheral nervous system that results from deposition of extracellular prefibrillar transthyretin (TTR) and amyloid fibrils. We have previously shown that an inflammatory stimulus in the peripheral nerve in a mouse model of FAP triggers local TTR expression and deposition, leading to poor regeneration. We also demonstrated that blocking interleukin-1 (IL-1) signaling by the IL-1 receptor antagonist anakinra is beneficial in preventing nerve TTR deposition and associated toxicity. Here, we investigated whether IL-1 signaling influences TTR biology after an injury stimulus in a V30M FAP mouse model. Animals were treated with anakinra 48 hours before sciatic nerve ligation; the nerves were analyzed 7 days postlesion. Anakinra decreased TTR expression by Schwann cells and TTR extracellular deposition after nerve injury, which resulted in improved regeneration. Moreover, treated mice had less apoptotic cell death. In wild-type mice, inflammation is important for regeneration but, in the FAP model mice, an altered threshold of the inflammatory response differentially regulates TTR. Taken together, our results show that anakinra administration before injury can modulate TTR-induced peripheral nervous system pathology, thereby corroborating the protective interference of this drug in a FAP preclinical model.

Axotomy of tributaries of the pelvic and pudendal nerves induces changes in the neurochemistry of mouse dorsal root ganglion neurons and the spinal cord

Using immunohistochemical techniques, we characterized changes in the expression of several neurochemical markers in lumbar 4-sacral 2 (L4-S2) dorsal root ganglion (DRG) neuron profiles (NPs) and the spinal cord of BALB/c mice after axotomy of the L6 and S1 spinal nerves, major tributaries of the pelvic (targeting pelvic visceral organs) and pudendal (targeting perineum and genitalia) nerves. Sham animals were included. Expression of cyclic AMP-dependent transcription factor 3 (ATF3), calcitonin gene-related peptide (CGRP), transient receptor potential cation channel subfamily V, member 1 (TRPV1), tyrosine hydroxylase (TH) and vesicular glutamate transporters (VGLUT) types 1 and -2 was analysed seven days after injury. L6-S1 axotomy induced dramatic de novo expression of ATF3 in many L6-S1 DRG NPs, and parallel significant downregulations in the percentage of CGRP-, TRPV1-, TH- and VGLUT2-immunoreactive (IR) DRG NPs, as compared to their expression in uninjured DRGs (contralateral L6-S1-AXO; sham mice); VGLUT1 expression remained unaltered. Sham L6-S1 DRGs only showed a small ipsilateral increase in ATF3-IR NPs (other markers were unchanged). L6-S1-AXO induced de novo expression of ATF3 in several lumbosacral spinal cord motoneurons and parasympathetic preganglionic neurons; in sham mice the effect was limited to a few motoneurons. Finally, a moderate decrease in CGRP- and TRPV1-like-immunoreactivities was observed in the ipsilateral superficial dorsal horn neuropil. In conclusion, injury of a mixed visceral/non-visceral nerve leads to considerable neurochemical alterations in DRGs matched, to some extent, in the spinal cord. Changes in these and potentially other nociception-related molecules could contribute to pain due to injury of nerves in the abdominopelvic cavity.

Potential role of stem cells for neuropathic pain disorders

Chronic neuropathic pain is a debilitating disease process associated with several medical disorders. Different from pain caused by inflammation, neuropathic pain is a diffuse pain disorder often found to be recalcitrant to the limited medical treatments available. Intractable nerve pain may benefit from other therapies capable of longer-lasting pain coverage or greater efficacy. A growing number of reports have emerged suggesting a role for stem cells as a cellular delivery source with neuroprotective agents opposing the effects of nerve damage. Here, the authors review the current experimental therapies examining the use of stem cells for the treatment of neuropathic pain disorders.

VGLUTs in Peripheral Neurons and the Spinal Cord: Time for a Review

Vesicular glutamate transporters (VGLUTs) are key molecules for the incorporation of glutamate in synaptic vesicles across the nervous system, and since their discovery in the early 1990s, research on these transporters has been intense and productive. This review will focus on several aspects of VGLUTs research on neurons in the periphery and the spinal cord. Firstly, it will begin with a historical account on the evolution of the morphological analysis of glutamatergic systems and the pivotal role played by the discovery of VGLUTs. Secondly, and in order to provide an appropriate framework, there will be a synthetic description of the neuroanatomy and neurochemistry of peripheral neurons and the spinal cord. This will be followed by a succinct description of the current knowledge on the expression of VGLUTs in peripheral sensory and autonomic neurons and neurons in the spinal cord. Finally, this review will address the modulation of VGLUTs expression after nerve and tissue insult, their physiological relevance in relation to sensation, pain, and neuroprotection, and their potential pharmacological usefulness.

Neuropeptide Y and posttraumatic stress disorder

Resiliency to the adverse effects of extraordinary emotional trauma on the brain varies within the human population. Accordingly, some people cope better than others with traumatic stress. Neuropeptide Y (NPY) is a 36-amino-acid peptide transmitter abundantly expressed in forebrain limbic and brain stem areas that regulate stress and emotional behaviors. Studies largely in rodents demonstrate a role for NPY in promoting coping with stress. Moreover, accruing data from the genetic to the physiological implicate NPY as a potential 'resilience-to-stress' factor in humans. Here, we consolidate findings from preclinical and clinical studies of NPY that are of relevance to stress-associated syndromes, most prototypically posttraumatic stress disorder (PTSD). Collectively, these data suggest that reduced central nervous system (CNS) NPY concentrations or function may be associated with PTSD. We also link specific symptoms of human PTSD with extant findings in the NPY field to reveal potential physiological contributions of the neuropeptide to the disorder. In pursuit of understanding the physiological basis and treatment of PTSD, the NPY system is an attractive target.

Some lumbar sympathetic neurons develop a glutamatergic phenotype after peripheral axotomy with a note on VGLUT₂-positive perineuronal baskets

Glutamate is the main excitatory neurotransmitter in the nervous system, including in primary afferent neurons. However, to date a glutamatergic phenotype of autonomic neurons has not been described. Therefore, we explored the expression of vesicular glutamate transporter (VGLUT) types 1, 2 and 3 in lumbar sympathetic chain (LSC) and major pelvic ganglion (MPG) of naïve BALB/C mice, as well as after pelvic nerve axotomy (PNA), using immunohistochemistry and in situ hybridization. Colocalization with activating transcription factor-3 (ATF-3), tyrosine hydroxylase (TH), vesicular acetylcholine transporter (VAChT) and calcitonin gene-related peptide was also examined. Sham-PNA, sciatic nerve axotomy (SNA) or naïve mice were included. In naïve mice, VGLUT(2)-like immunoreactivity (LI) was only detected in fibers and varicosities in LSC and MPG; no ATF-3-immunoreactive (IR) neurons were visible. In contrast, PNA induced upregulation of VGLUT(2) protein and transcript, as well as of ATF-3-LI in subpopulations of LSC neurons. Interestingly, VGLUT(2)-IR LSC neurons coexpressed ATF-3, and often lacked the noradrenergic marker TH. SNA only increased VGLUT(2) protein and transcript in scattered LSC neurons. Neither PNA nor SNA upregulated VGLUT(2) in MPG neurons. We also found perineuronal baskets immunoreactive either for VGLUT(2) or the acetylcholinergic marker VAChT in non-PNA MPGs, usually around TH-IR neurons. VGLUT(1)-LI was restricted to some varicosities in MPGs, was absent in LSCs, and remained largely unaffected by PNA or SNA. This was confirmed by the lack of expression of VGLUT(1) or VGLUT(3) mRNAs in LSCs, even after PNA or SNA. Taken together, axotomy of visceral and non-visceral nerves results in a glutamatergic phenotype of some LSC neurons. In addition, we show previously non-described MPG perineuronal glutamatergic baskets.

Tonic inhibition of chronic pain by neuropeptide Y

Dramatically up-regulated in the dorsal horn of the mammalian spinal cord following inflammation or nerve injury, neuropeptide Y (NPY) is poised to regulate the transmission of sensory signals. We found that doxycycline-induced conditional in vivo (Npy(tet/tet)) knockdown of NPY produced rapid, reversible, and repeatable increases in the intensity and duration of tactile and thermal hypersensitivity. Remarkably, when allowed to resolve for several weeks, behavioral hypersensitivity could be dramatically reinstated with NPY knockdown or intrathecal administration of Y1 or Y2 receptor antagonists. In addition, Y2 antagonism increased dorsal horn expression of Fos and phosphorylated form of extracellular signal-related kinase. Taken together, these data establish spinal NPY receptor systems as an endogenous braking mechanism that exerts a tonic, long-lasting, broad-spectrum inhibitory control of spinal nociceptive transmission, thus impeding the transition from acute to chronic pain. NPY and its receptors appear to be part of a mechanism whereby mammals naturally recover from the hyperalgesia associated with inflammation or nerve injury.

Topography and time course of changes in spinal neuropeptide Y immunoreactivity after spared nerve injury

We used a new computer-assisted method to precisely localize and efficiently quantify increases in neuropeptide Y immunoreactivity (NPY-ir) along the mediolateral axis of the L4 dorsal horn (DH) following transection of either the tibial and common peroneal nerves (thus sparing the sural branch, spared nerve injury (SNI)), the tibial nerve, or the common peroneal and sural nerves. Two weeks after SNI, NPY-ir increased within the tibial and peroneal innervation territories; however, NPY-ir in the central-lateral region (innervated by the spared sural nerve) was indistinguishable from that of sham. Conversely, transection of the sural and common peroneal nerves induced an increase in NPY-ir in the central-lateral region, while leaving the medial region (innervated by the tibial nerve) unaffected. All nerve injuries increased NPY-ir in dorsal root ganglia (DRG) and nucleus gracilis (NG). By 24 weeks, both NPY-ir upregulation in the DH and hyper-responsivity to cold and noxious mechanical stimuli had resolved. Conversely, NPY-ir in DRG and NG, and hypersensitivity to non-noxious static mechanical stimuli, did not resolve within 24 weeks. Over this time course, the average cross-sectional area of NPY-immunoreactive DRG neurons increased by 151 mum(2). We conclude that the upregulation of NPY after SNI is restricted to medial zones of the DH, and therefore cannot act directly upon synapses within the more lateral (sural) zones to control sural nerve hypersensitivity. Instead, we suggest that NPY in the medial DH tonically inhibits hypersensitivity by interrupting mechanisms of central sensitization and integration of sensory signals at the spinal and supraspinal levels.

Bone marrow stromal cells attenuate injury-induced changes in galanin, NPY and NPY Y1-receptor expression after a sciatic nerve constriction

Single ligature nerve constriction (SLNC) of the rat sciatic nerve triggers neuropathic pain-related behaviors and induces changes in neuropeptide expression in primary afferent neurons. Bone marrow stromal cells (MSCs) injected into the lumbar 4 (L4) dorsal root ganglia (DRGs) of animals subjected to a sciatic nerve SLNC selectively migrate to the other ipsilateral lumbar DRGs (L3, L5 and L6) and prevent mechanical and thermal allodynia. In this study, we have evaluated the effect of MSC administration on the expression of the neuropeptides galanin and NPY, as well as the NPY Y(1)-receptor (Y(1)R) in DRG neurons. Animals were subjected to a sciatic nerve SLNC either alone or followed by the administration of MSCs, phosphate-buffered saline (PBS) or bone marrow non-adherent mononuclear cells (BNMCs), directly into the ipsilateral L4 DRG. Seven days after injury, the ipsilateral and contralateral L4-5 DRGs were dissected out and processed for standard immunohistochemistry, using specific antibodies. As previously reported, SLNC induced an ipsilateral increase in the number of galanin and NPY immunoreactive neurons and a decrease in Y(1)R-positive DRG neurons. The intraganglionic injection of PBS or BNMCs did not modify this pattern of expression. In contrast, MSC administration partially prevented the injury-induced changes in galanin, NPY and Y(1)R expression. The large number of Y(1)R-immunoreactive neurons together with high levels of NPY expression in animals injected with MSCs could explain, at least in part, the analgesic effects exerted by these cells. Our results support MSC participation in the modulation of neuropathic pain and give insight into one of the possible mechanisms involved.

Differential galanin upregulation in dorsal root ganglia and spinal cord after graded single ligature nerve constriction of the rat sciatic nerve

Single ligature nerve constriction (SLNC) is a newly developed animal model for the study of neuropathic pain. SLNC of the rat sciatic nerve induces pain-related behaviors, as well as changes in the expression of neuropeptide tyrosine and the Y(1) receptor in lumbar dorsal root ganglia (DRGs) and spinal cord. In the present study, we have analyzed the expression of another neuropeptide, galanin, in lumbar DRGs and spinal cord after different degrees of constriction of the rat sciatic nerve. The nerve was ligated and reduced to 10-30, 40-80 or 90% of its original diameter (light, medium or strong SLNCs). At different times after injury (7, 14, 30, 60 days), lumbar 4 and 5 DRGs and the corresponding levels of the spinal cord were dissected out and processed for galanin-immunohistochemistry. In DRGs, SLNC induced a gradual increase in the number of galanin-immunoreactive (IR) neurons, in direct correlation with the degree of constriction. Thus, after light SLNC, a modest upregulation of galanin was observed, mainly in small-sized neurons. However, following medium or strong SLNCs, there was a more drastic increase in the number of galanin-IR neurons, involving also medium and large-sized cells. The highest numbers of galanin-IR neurons were detected 14 days after injury. In the dorsal horn of the spinal cord, medium and strong SLNCs induced a marked ipsilateral increase in galanin-like immunoreactivity in laminae I-II. These results show that galanin expression in DRGs and spinal cord is differentially regulated by different degrees of nerve constriction and further support its modulatory role on neuropathic pain.

Bone marrow stromal cells induce changes in pain behavior after sciatic nerve constriction

Peripheral nerve injury, i.e. a single ligature nerve constriction (SLNC), triggers neuropathic pain. Bone marrow stromal cells (MSCs) have been observed to migrate to the injured tissues and mediate functional recovery following brain, spinal cord and peripheral nerve lesions. We have recently shown MSC selective migration to the ipsilateral lumbar (L3-6) dorsal root ganglia (DRGs) after a sciatic nerve SLNC. In this study, we have analyzed the thermal and mechanical sensitivities of animals subjected to a SLNC of the sciatic nerve and an ipsilateral intraganglionic MSC injection, using the von Frey and Choi tests. Control animals were subjected to the nerve lesion either alone or followed by the administration of phosphate-buffered saline (PBS) or bone marrow non-adherent mononuclear cells (BNMCs). All the animals were tested both before surgery and after 1, 3, 7, 14, 21, 28 and 56 days. Animals subjected to the sciatic nerve constriction developed ipsilateral mechanical and thermal allodynia already 3 days after the lesion. The allodynic responses were maintained even after 56 days. MSC administration prevented the generation of mechanical allodynia and reduced the number of allodynic responses to cold stimuli. On the contrary, the injection of either PBS or BNMCs could not counteract allodynia. These results suggest that MSCs may modulate pain generation after sciatic nerve constriction. The underlying mechanisms by which MSCs exert their actions on pain behavior need to be clarified.

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).

Modification of classical neurochemical markers in identified primary afferent neurons with Aβ-, Aδ-, and C-fibers after chronic constriction injury in mice

It is functionally important to differentiate between primary afferent neurons with A-fibers, which are nociceptive or nonnociceptive, and C-fibers, which are mainly nociceptive. Neurochemical markers such as neurofilament 200 (NF200), substance P (SP), and isolectin B4 (IB4) have been useful to distinguish between A- and C-fiber neurons. However, the expression patterns of these markers change after peripheral nerve injury, so that it is not clear whether they still distinguish between fiber types in models of neuropathic pain. We identified neurons with Abeta-, Adelta-, and C-fibers by their conduction velocity (corrected for utilization time) in dorsal root ganglia taken from mice after a chronic constriction injury (CCI) of the sciatic nerve and control mice, and later stained them for IB4, SP, calcitonin gene-related peptide (CGRP), NF200, and neuropeptide Y (NPY). NF200 remained a good marker for A-fiber neurons, and IB4 and SP remained good markers for C-fiber neurons after CCI. NPY was absent in controls but was expressed in A-fiber neurons after CCI. After CCI, a group of C-fiber neurons emerged that expressed none of the tested markers. The size distribution of the markers was investigated in larger samples of unidentified dorsal root ganglion neurons and, together with the results from the identified neurons, provided only limited evidence for the expression of SP in Abeta-fiber neurons after CCI. The extent of up-regulation of NPY showed a strong inverse correlation with the degree of heat hyperalgesia.

Selective migration and engraftment of bone marrow mesenchymal stem cells in rat lumbar dorsal root ganglia after sciatic nerve constriction

Bone marrow mesenchymal stem cells (MSCs) preferentially migrate to the injured hemisphere when administered intravenously to rats with traumatic or ischemic brain injuries. In this study, we have investigated the localization of MSCs injected into the lumbar-4 dorsal root ganglion (L4-DRG) of rats with a sciatic nerve single ligature nerve constriction (SLNC). MSCs were isolated by their adherence to plastic, cultured until confluence and labelled with Hoechst. Animals with a unilateral injection of MSCs were subjected to an ipsilateral, bilateral or contralateral SLNC. After 9 days, they were perfused and the lumbar DRGs were dissected out, cut in a cryostat and observed with a fluorescence microscope. Large numbers of Hoechst-positive cells were observed in the injected L4-DRG, distributed around primary afferent neurons, resembling the anatomical localization of glial cells. In animals with an ipsilateral SLNC, some cells were detected in the ipsilateral L3, L5 or L6-DRGs but not in the contralateral ganglia. In animals with a bilateral lesion, MSCs migrated to both the ipsilateral and contralateral DRGs whereas in animals with a contralateral ligature, MSCs migrated to the contralateral DRGs. These results suggest that MSCs preferentially engraft in DRGs hosting primary sensory neurons affected by a lesion of their peripheral branches. Further studies should be carried out in order to elucidate the molecular mechanisms involved in this migration and homing, in order to evaluate the possible use of MSCs as a new therapeutic strategy for the treatment of peripheral nerve neuropathies.

Spinal estrogen attenuates the exercise pressor reflex but has little effect on the expression of genes regulating neurotransmitters in the dorsal root ganglia

Previously, our laboratory showed that estrogen, topically applied to the spinal cord, attenuated the exercise pressor reflex in female cats (Schmitt PM and Kaufman MP. J Appl Physiol 95: 1418-1424, 2003; 98: 633-639, 2005). The attenuation was gender specific and was in part opioid dependent. Our finding that the mu- and delta-opioid antagonist naloxone was only able to partially restore estrogen's attenuating effect on the pressor response to static contraction suggested that estrogen affected an additional pathway, involving the dorsal root ganglion (DRG). Estrogen has been described to stimulate transcription within 10 min of its application to the DRG, raising the possibility that rapid genomic effects on neurotransmitter production may have contributed to estrogen's effect on the exercise pressor reflex. This prompted us to test the hypothesis that estrogen modulated the pressor response to static contraction by influencing gene expression of the neurotransmitters released by the thin-fiber muscle afferents that evoke the exercise pressor reflex. We confirmed in decerebrated female rats that topical application of estrogen (0.01 microg/ml) to the lumbosacral spinal cord attenuated the pressor response to static muscle contraction (from 10+/-3 to 1+/-1 mmHg; P<0.05). DRG were then harvested postmortem, and changes in mRNA expression were analyzed. GeneChip analysis revealed that neither estrogen nor contraction alone changed the mRNA expression of substance P, the neurokinin-1 receptor, CGRP, NGF, the P2X3 receptor, GABAA and GABAB, the 5-HT3A and 5-HT3B receptor, N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, opioid receptors, and opioid-like receptor. Surprisingly, however, contraction stimulated the expression of neuropeptide Y in the DRG in the presence and absence of estrogen. We conclude that estrogen does not attenuate the exercise pressor reflex through a genomic effect in the DRG.

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.

Neuropeptide Y2 receptor protein is present in peptidergic and nonpeptidergic primary sensory neurons of the mouse

The localization of the neuropeptide tyrosine (NPY) Y2 receptor (Y2R) protein was studied in mouse dorsal root ganglia (DRGs) and spinal cord, by using a recently developed rabbit anti-Y2R antibody and a sensitive immunohistochemical method. Y2R-like immunoreactivity (-LI) was observed in about 10% of the small/medium-sized lumbar DRG neurons. Among these, about 44% were calcitonin gene-related peptide-immunoreactive, and about 38% bound isolectin B4. In the dorsal horn of the spinal cord, an intense Y2R-LI was seen in the most superficial layers, mostly restricted to laminae I-II. This immunoreactivity was completely abolished by dorsal rhizotomy. Y2R-L1 was also detected on the skin, more abundantly in hairy than glabrous skin. Specificity experiments showed complete disappearance of the Y2R-LI described above after incubation with antibody preadsorbed with the immunogenic peptide. Furthermore, Y2R-LI was also absent in a Y2R knockout mouse. These results demonstrate that the NPY Y2R is associated mainly with both peptidergic and nonpeptidergic small, presumably nociceptive, neurons projecting to the superficial layers of the dorsal horn. The results also support a role for this receptor and NPY in pain mechanisms.