Antihyperalgesic effects of intrathecal neuropeptide Y during inflammation are mediated by Y1 receptors

Cracking the Code: The Role of Peripheral Nervous System Signaling in Fracture Repair

PURPOSE OF REVIEW

The traditionally understated role of neural regulation in fracture healing is gaining prominence, as recent findings underscore the peripheral nervous system's critical contribution to bone repair. Indeed, it is becoming more evident that the nervous system modulates every stage of fracture healing, from the onset of inflammation to repair and eventual remodeling.

RECENT FINDINGS

Essential to this process are neurotrophins and neuropeptides, such as substance P, calcitonin gene-related peptide, and neuropeptide Y. These molecules fulfill key roles in promoting osteogenesis, influencing inflammation, and mediating pain. The sympathetic nervous system also plays an important role in the healing process: while local sympathectomies may improve fracture healing, systemic sympathetic denervation impairs fracture healing. Furthermore, chronic activation of the sympathetic nervous system, often triggered by stress, is a potential impediment to effective fracture healing, marking an important area for further investigation. The potential to manipulate aspects of the nervous system offers promising therapeutic possibilities for improving outcomes in fracture healing. This review article is part of a series of multiple manuscripts designed to determine the utility of using artificial intelligence for writing scientific reviews.

Unloading-Induced Skeletal Interoception Alters Hypothalamic Signaling to Promote Bone Loss and Fat Metabolism

Microgravity is the primary factor that affects human physiology in spaceflight, particularly bone loss and disturbances of the central nervous system. However, little is known about the cellular and molecular mechanisms of these effects. Here, it is reported that in mice hindlimb unloading stimulates expression of neuropeptide Y (NPY) and tyrosine hydroxylase (TH) in the hypothalamus, resulting in bone loss and altered fat metabolism. Enhanced expression of TH and NPY in the hypothalamus occurs downstream of a reduced prostaglandin E2 (PGE2)-mediated ascending interoceptive signaling of the skeletal interoception. Sympathetic antagonist propranolol or deletion of Adrb2 in osteocytes rescue bone loss in the unloading model. Moreover, depletion of TH+ sympathetic nerves or inhibition of norepinephrine release ameliorated bone resorption. Stereotactic inhibition of NPY expression in the hypothalamic neurons reduces the food intake with altered energy expenditure with a limited effect on bone, indicating hypothalamic neuroendocrine factor NPY in the facilitation of bone formation by sympathetic TH activity. These findings suggest that reduced PGE2-mediated interoceptive signaling in response to microgravity or unloading has impacts on the skeletal and central nervous systems that are reciprocally regulated.

Simultaneous Administration of Hyperbaric Oxygen Therapy and Antioxidant Supplementation with Filipendula ulmaria Extract in the Treatment of Thermal Skin Injuries Alters Nociceptive Signalling and Wound Healing

Background and Objectives: Thermal skin injuries are a prevalent cause of skin damage, potentially leading to severe morbidity and significant mortality. In this study, we intended to estimate the effects of HBO (hyperbaric oxygen treatment) and antioxidant supplementation with Filipendula ulmaria extract, individually and simultaneously, in the treatment of thermal skin injuries. Materials and Methods: As a thermal skin injury experimental model, we used two-month-old male Wistar albino rats. Thermal injuries were made with a solid aluminium bar at a constant temperature of 75 °C for 15 s. Hyperbaric oxygen treatment was performed in a specially constructed hyperbaric chamber for rats (HYB-C 300) for seven consecutive days (100% O2 at 2.5 ATA for 60 min). Antioxidant supplementation was performed with oral administration of Filipendula ulmaria extract dissolved in tap water to reach a final concentration of 100 mg/kg b.w. for seven consecutive days. Results: Simultaneous administration of hyperbaric oxygen therapy and antioxidant supplementation with Filipendula ulmaria extract significantly ameliorated the macroscopic and histopathological characteristics of the wound area and healing. Also, this therapeutic approach decreased the local expression of genes for proinflammatory mediators and increased the expression of the μ-opioid receptor and the MT1 and MT2 receptors in the wound area and spinal cord, with a consequent increase in reaction times in behavioural testing. Conclusions: In conclusion, the presented results of our study allow evidence for the advantages of the simultaneous employment of HBO and antioxidant supplementation in the treatment of thermal skin injuries, with special reference to the attenuation of painful sensations accompanied by this type of trauma.

mTOR-neuropeptide Y signaling sensitizes nociceptors to drive neuropathic pain

Neuropathic pain is a refractory condition that involves de novo protein synthesis in the nociceptive pathway. The mTOR is a master regulator of protein translation; however, mechanisms underlying its role in neuropathic pain remain elusive. Using the spared nerve injury-induced neuropathic pain model, we found that mTOR was preferentially activated in large-diameter dorsal root ganglion (DRG) neurons and spinal microglia. However, selective ablation of mTOR in DRG neurons, rather than microglia, alleviated acute neuropathic pain in mice. We show that injury-induced mTOR activation promoted the transcriptional induction of neuropeptide Y (Npy), likely via signal transducer and activator of transcription 3 phosphorylation. NPY further acted primarily on Y2 receptors (Y2R) to enhance neuronal excitability. Peripheral replenishment of NPY reversed pain alleviation upon mTOR removal, whereas Y2R antagonists prevented pain restoration. Our findings reveal an unexpected link between mTOR and NPY/Y2R in promoting nociceptor sensitization and neuropathic pain.

Spinal neuropeptide Y Y1 receptor-expressing neurons are a pharmacotherapeutic target for the alleviation of neuropathic pain

Peripheral nerve injury sensitizes a complex network of spinal cord dorsal horn (DH) neurons to produce allodynia and neuropathic pain. The identification of a druggable target within this network has remained elusive, but a promising candidate is the neuropeptide Y (NPY) Y1 receptor-expressing interneuron (Y1-IN) population. We report that spared nerve injury (SNI) enhanced the excitability of Y1-INs and elicited allodynia (mechanical and cold hypersensitivity) and affective pain. Similarly, chemogenetic or optogenetic activation of Y1-INs in uninjured mice elicited behavioral signs of spontaneous, allodynic, and affective pain. SNI-induced allodynia was reduced by chemogenetic inhibition of Y1-INs, or intrathecal administration of a Y1-selective agonist. Conditional deletion of Npy1r in DH neurons, but not peripheral afferent neurons prevented the anti-hyperalgesic effects of the intrathecal Y1 agonist. We conclude that spinal Y1-INs are necessary and sufficient for the behavioral symptoms of neuropathic pain and represent a promising target for future pharmacotherapeutic development of Y1 agonists.

Transcription factor mesenchyme homeobox protein 2 (MEOX2) modulates nociceptor function

Mesenchyme homeobox protein 2 (MEOX2) is a transcription factor involved in mesoderm differentiation, including development of bones, muscles, vasculature and dermatomes. We have previously identified dysregulation of MEOX2 in fibroblasts from Congenital Insensitivity to Pain patients, and confirmed that btn, the Drosophila homologue of MEOX2, plays a role in nocifensive responses to noxious heat stimuli. To determine the importance of MEOX2 in the mammalian peripheral nervous system, we used a Meox2 heterozygous (Meox2^+/−) mouse model to characterise its function in the sensory nervous system, and more specifically, in nociception. MEOX2 is expressed in the mouse dorsal root ganglia (DRG) and spinal cord, and localises in the nuclei of a subset of sensory neurons. Functional studies of the mouse model, including behavioural, cellular and electrophysiological analyses, showed altered nociception encompassing impaired action potential initiation upon depolarisation. Mechanistically, we noted decreased expression of Scn9a and Scn11a genes encoding Na_v1.7 and Na_v1.9 voltage‐gated sodium channels respectively, that are crucial in subthreshold amplification and action potential initiation in nociceptors. Further transcriptomic analyses of Meox2^+/− DRG revealed downregulation of a specific subset of genes including those previously associated with pain perception, such as PENK and NPY. Based on these observations, we propose a novel role of MEOX2 in primary afferent nociceptor neurons for the maintenance of a transcriptional programme required for proper perception of acute and inflammatory noxious stimuli.

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.

Intra-articular injection of 2-pyridylethylamine produces spinal NPY-mediated antinociception in the formalin-induced rat knee-joint pain model

Low doses of histamine or H1R agonist 2-pyridylethylamine (2-PEA) into the knee-joint were found to decrease formalin-induced articular nociception in rats. In this study, we evaluated the participation of spinal NPY in the antinociceptive effect produced by 2-PEA. Injection of formalin (1.5%) into one of the knee-joints causes the limping of the respective limb due to nociception, which was registered each 5 min over 60 min. Neuropeptide Y1 receptor (Y1R) content in the spinal cord was evaluated by western-blotting. Intrathecal (i.t.) injection of Y1R agonist Leu31, Pro34-NPY (0.7-7 µmol) decreased nociception, while injection of the antagonist BIBO 3304 (4 μmol), increased nociception. Antinociception produced by 2-PEA was reversed by a sub-effective i.t. dose of the Y1R antagonist. Similarly, this antinociceptive effect was prevented by i.t. pretreatment with the neurotoxin NPY-saporin (750 ng), which also reduced immunoblotting for Y1R in spinal cord homogenates. These data support the idea that antinociception induced by H1R agonists in the knee-joint of rats may be mediated by the spinal release of NPY, and this peptide seems to be acting via Y1R.

Cellular Mechanisms for Antinociception Produced by Oxytocin and Orexins in the Rat Spinal Lamina II-Comparison with Those of Other Endogenous Pain Modulators

Much evidence indicates that hypothalamus-derived neuropeptides, oxytocin, orexins A and B, inhibit nociceptive transmission in the rat spinal dorsal horn. In order to unveil cellular mechanisms for this antinociception, the effects of the neuropeptides on synaptic transmission were examined in spinal lamina II neurons that play a crucial role in antinociception produced by various analgesics by using the whole-cell patch-clamp technique and adult rat spinal cord slices. Oxytocin had no effect on glutamatergic excitatory transmission while producing a membrane depolarization, γ-aminobutyric acid (GABA)-ergic and glycinergic spontaneous inhibitory transmission enhancement. On the other hand, orexins A and B produced a membrane depolarization and/or a presynaptic spontaneous excitatory transmission enhancement. Like oxytocin, orexin A enhanced both GABAergic and glycinergic transmission, whereas orexin B facilitated glycinergic but not GABAergic transmission. These inhibitory transmission enhancements were due to action potential production. Oxytocin, orexins A and B activities were mediated by oxytocin, orexin-1 and orexin-2 receptors, respectively. This review article will mention cellular mechanisms for antinociception produced by oxytocin, orexins A and B, and discuss similarity and difference in antinociceptive mechanisms among the hypothalamic neuropeptides and other endogenous pain modulators (opioids, nociceptin, adenosine, adenosine 5’-triphosphate (ATP), noradrenaline, serotonin, dopamine, somatostatin, cannabinoids, galanin, substance P, bradykinin, neuropeptide Y and acetylcholine) exhibiting a change in membrane potential, excitatory or inhibitory transmission in the spinal lamina II neurons.

Neuropeptide Y release in the rat spinal cord measured with Y1 receptor internalization is increased after nerve injury

Neuropeptide Y (NPY) modulates nociception in the spinal cord, but little is known about its mechanisms of release. We measured NPY release in situ using the internalization of its Y1 receptor in dorsal horn neurons. Y1 receptor immunoreactivity was normally localized to the cell surface, but addition of NPY to spinal cord slices increased the number of neurons with Y1 internalization in a biphasic fashion (EC₅₀s of 1 nM and 1 μM). Depolarization with KCl, capsaicin, or the protein kinase A activator 6-benzoyl-cAMP also induced Y1 receptor internalization, presumably by releasing NPY. NMDA receptor activation in the presence of BVT948, an inhibitor of protein tyrosine phosphatases, also released NPY. Electrical stimulation of the dorsal horn frequency-dependently induced NPY release; and this was decreased by the Y1 antagonist BIBO3304, the Nav channel blocker lidocaine, or the Cav2 channel blocker ω-conotoxin MVIIC. Dorsal root immersion in capsaicin, but not its electrical stimulation, also induced NPY release. This was blocked by CNQX, suggesting that part of the NPY released by capsaicin was from dorsal horn neurons receiving synapses from primary afferents and not from the afferent themselves. Mechanical stimulation in vivo, with rub or clamp of the hindpaw, elicited robust Y1 receptor internalization in rats with spared nerve injury but not sham surgery. In summary, NPY is released from dorsal horn interneurons or primary afferent terminals by electrical stimulation and by activation of TRPV1, PKA or NMDA receptors in. Furthermore, NPY release evoked by noxious and tactile stimuli increases after peripheral nerve injury.

Spinal Neuropeptide Y1 Receptor-Expressing Neurons Form an Essential Excitatory Pathway for Mechanical Itch

Acute itch can be generated by either chemical or mechanical stimuli, which activate separate pathways in the periphery and spinal cord. While substantial progress has been made in mapping the transmission pathway for chemical itch, the central pathway for mechanical itch remains obscure. Using complementary genetic and pharmacological manipulations, we show that excitatory neurons marked by the expression of the neuropeptide Y1 receptor (Y1Cre neurons) form an essential pathway in the dorsal spinal cord for the transmission of mechanical but not chemical itch. Ablating or silencing the Y1Cre neurons abrogates mechanical itch, while chemogenetic activation induces scratching. Moreover, using Y1 conditional knockout mice, we demonstrate that endogenous neuropeptide Y (NPY) acts via dorsal-horn Y1-expressing neurons to suppress light punctate touch and mechanical itch stimuli. NPY-Y1 signaling thus regulates the transmission of innocuous tactile information by establishing biologically relevant thresholds for touch discrimination and mechanical itch reflexes.

Similarity and dissimilarity in antinociceptive effects of dipeptidyl-peptidase 4 inhibitors, Diprotin A and vildagliptin in rat inflammatory pain models following spinal administration

Dipeptidyl-peptidase 4 (DPP4) enzyme is involved in the degradation of many biologically active peptides including opioids. Its role in pain transmission is poorly elucidated. Recently we reported on the spinal antihyperalgesic effects of DPP4 inhibitors, Ile-Pro-Ile (Diprotin A) and vildagliptin in carrageenan-evoked acute inflammatory pain in rats. The present study investigated the effects of intrathecal (it.) diprotin A and vildagliptin in Complete Freund's Adjuvant- (CFA) and formalin induced pain in rats. The former assay can model the subchronic inflammatory pain condition and the later one reflects both acute tonic and inflammatory pain conditions. The involvement of opioid receptor (OR) subtypes, Y1-, and GLP1 receptors were also investigated. In CFA pain model it. diprotin A or vildagliptin dose-dependently inhibits hyperalgesia in ipsilateral while has no effect in contralateral paws. The peak effect was achieved 30 min following drug administration which was used for further analysis. Both compounds showed naltrexone reversible antihyperalgesia. Co-administration of OR-subtype-selective antagonists with diprotin A and vildagliptin revealed involvement of μ and δ > μ opioid receptors, respectively. Co-administered Y1 but not GLP1 receptor antagonists reversed the antihyperalgesic action of both DPP4 inhibitors. In touch-hypersensitivity both compounds were ineffective. In formalin test only diprotin A showed μ and δ OR-mediated antinociception and only in the 2nd phase. This effect was Y1 or GLP-1 receptor antagonist insensitive. In conclusion, diprotin A and vildagliptin display antinociception of different mechanisms of action in subchronic inflammatory pain. Furthermore, the spinal pain relay points of inflammatory pain of acute or subchronic conditions were more effectively affected by diprotin A than vildagliptin which needs future elucidation.

Involvement of Neuropeptide Y in Post-Incisional Nociception in Rats

BACKGROUND

Neuropeptide Y (NPY) is abundantly distributed in the mammalian nervous system. Its role in nociception arising from inflammatory and neuropathic pain conditions has been elucidated. However, its involvement in post-incisional nociception, particularly at the spinal cord level, is relatively unknown.

PURPOSE

Management of postoperative pain is suboptimal. Evaluation of changes at the spinal level could facilitate better understanding of neural mechanisms underlying this type of pain.

METHODS

Rats were subjected to hind paw incision and spatiotemporal pattern of NPY expression in the dorsal horn was investigated by immunohistochemistry. Next, rats were implanted with intrathecal catheters using previously standardized procedure. NPY was injected into the intrathecal space by an indwelling catheter and behavioral assessment of nociception was performed.

RESULTS

Higher expression of NPY was observed in the superficial laminae of the dorsal horn. After incision, specific changes were observed like an abrupt decrease at 3 h after incision, which could be correlated with the intense nociception at this time. In contrast to morphine administration, which attenuated all 3 behavioral parameters of nociception, NPY decreased guarding behavior and thermal hyperalgesia during the acute phase.

CONCLUSIONS

NPY is extensively expressed in the superficial laminae of the spinal cord and exhibit marked changes after incision. Nociception is also decreased after its administration. Hence, it is likely involved in post-incisional nociception. This information could have clinical relevance.

Targeted Orexin and Hypothalamic Neuropeptides for Migraine

The hypothalamus is involved in the regulation of homeostatic mechanisms and migraine-related trigeminal nociception and as such has been hypothesized to play a central role in the migraine syndrome from the earliest stages of the attack. The hypothalamus hosts many key neuropeptide systems that have been postulated to play a role in this pathophysiology. Such neuropeptides include but are not exclusive too orexins, oxytocin, neuropeptide Y, and pituitary adenylate cyclase activating protein, which will be the focus of this review. Each of these peptides has its own unique physiological role and as such many preclinical studies have been conducted targeting these peptide systems with evidence supporting their role in migraine pathophysiology. Preclinical studies have also begun to explore potential therapeutic compounds targeting these systems with some success in all cases. Clinical efficacy of dual orexin receptor antagonists and intranasal oxytocin have been tested; however, both have yet to demonstrate clinical effect. Despite this, there were limitations in these cases and strong arguments can be made for the further development of intranasal oxytocin for migraine prophylaxis. Regarding neuropeptide Y, work has yet to begun in a clinical setting, and clinical trials for pituitary adenylate cyclase activating protein are just beginning to be established with much optimism. Regardless, it is becoming increasingly clear the prominent role that the hypothalamus and its peptide systems have in migraine pathophysiology. Much work is required to better understand this system and the early stages of the attack to develop more targeted and effective therapies aimed at reducing attack susceptibility with the potential to prevent the attack all together.

W. A. H, Lewis RJ, Vetter I. Transcriptomics in pain research: insights from new and old technologies

Physiological and pathological pain involves a complex interplay of multiple cell types and signaling pathways.

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.

Potential neurobiological benefits of exercise in chronic pain and posttraumatic stress disorder: Pilot study

This pilot study assessed the effects of cardiopulmonary exercise testing and cardiorespiratory fitness on plasma neuropeptide Y (NPY), allopregnanolone and pregnanolone (ALLO), cortisol, and dehydroepiandrosterone (DHEA), and their association with pain sensitivity. Medication-free trauma-exposed participants were either healthy (n = 7) or experiencing comorbid chronic pain/posttraumatic stress disorder (PTSD) (n = 5). Peak oxygen consumption (VO2) during exercise testing was used to characterize cardiorespiratory fitness. Peak VO2 correlated with baseline and peak NPY levels (r = 0.66, p < 0.05 and r = 0.69, p < 0.05, respectively), as well as exercise-induced changes in ALLO (r = 0.89, p < 0.001) and peak ALLO levels (r = 0.71, p < 0.01). NPY levels at the peak of exercise correlated with pain threshold 30 min after exercise (r = 0.65, p < 0.05), while exercise-induced increases in ALLO correlated with pain tolerance 30 min after exercise (r = 0.64, p < 0.05). In contrast, exercise-induced changes in cortisol and DHEA levels were inversely correlated with pain tolerance after exercise (r = -0.69, p < 0.05 and r = -0.58, p < 0.05, respectively). These data suggest that cardiorespiratory fitness is associated with higher plasma NPY levels and increased ALLO responses to exercise, which in turn relate to pain sensitivity. Future work will examine whether progressive exercise training increases cardiorespiratory fitness in association with increases in NPY and ALLO and reductions in pain sensitivity in chronic pain patients with PTSD.

A combined electrophysiological and morphological study of neuropeptide Y-expressing inhibitory interneurons in the spinal dorsal horn of the mouse

The spinal dorsal horn contains numerous inhibitory interneurons that control transmission of somatosensory information. Although these cells have important roles in modulating pain, we still have limited information about how they are incorporated into neuronal circuits, and this is partly due to difficulty in assigning them to functional populations. Around 15% of inhibitory interneurons in laminae I-III express neuropeptide Y (NPY), but little is known about this population. We therefore used a combined electrophysiological/morphological approach to investigate these cells in mice that express green fluorescent protein (GFP) under control of the NPY promoter. We show that GFP is largely restricted to NPY-immunoreactive cells, although it is only expressed by a third of those in lamina I-II. Reconstructions of recorded neurons revealed that they were morphologically heterogeneous, but never islet cells. Many NPY-GFP cells (including cells in lamina III) appeared to be innervated by C fibres that lack transient receptor potential vanilloid-1, and consistent with this, we found that some lamina III NPY-immunoreactive cells were activated by mechanical noxious stimuli. Projection neurons in lamina III are densely innervated by NPY-containing axons. Our results suggest that this input originates from a small subset of NPY-expressing interneurons, with the projection cells representing only a minority of their output. Taken together with results of previous studies, our findings indicate that somatodendritic morphology is of limited value in classifying functional populations among inhibitory interneurons in the dorsal horn. Because many NPY-expressing cells respond to noxious stimuli, these are likely to have a role in attenuating pain and limiting its spread.

Behavioral and pharmacological characteristics of bortezomib-induced peripheral neuropathy in rats

Bortezomib, an effective anticancer drug for multiple myeloma, often causes peripheral neuropathy which is mainly characterized by numbness and painful paresthesia. Nevertheless, there is no effective strategy to escape or treat bortezomib-induced peripheral neuropathy (BIPN), because we have understood few mechanism of this side effect. In this study, we evaluated behavioral and pathological characteristics of BIPN, and investigated pharmacological efficacy of various analgesic drugs and adjuvants on mechanical allodynia induced by bortezomib treatment in rats. The repeated administration of bortezomib induced mechanical and cold allodynia. There was axonal degeneration of sciatic nerve behind these neuropathic symptoms. Furthermore, the exposure to bortezomib shortened neurite length in PC12 cells. Finally, the result of evaluation of anti-allodynic potency, oral administration of tramadol (10 mg/kg), pregabalin (3 mg/kg), duloxetine (30 mg/kg) or mexiletine (100 mg/kg), but not amitriptyline or diclofenac, transiently relieved the mechanical allodynia induced by bortezomib. These results suggest that axonal degeneration of the sciatic nerve is involved in BIPN and that some analgesic drugs and adjuvants are effective in the relief of painful neuropathy.

A potential anti-allodynic mechanism of GDNF following L5 spinal nerve ligation; Mitigation of NPY up-regulation in the touch sense pathway

Intrathecal delivery of glial cell line-derived neurotrophic factor (GDNF) reverses mechanical allodynia after 5th lumbar (L5) spinal nerve ligation (SNL). However, the molecular mechanism behind this process is not fully understood. Following sciatic nerve injury, primary afferent neurons in the injured dorsal root ganglion (DRG) begin to express neuropeptide Y (NPY) that is absent in normal DRG. The aim of the current study was to determine the relationship of this de novo expression of NPY and the anti-allodynic effect of GDNF. Following L5 SNL, 73% of neurons began to express NPY mRNA in the ipsilateral L5 DRG and robust NPY-immunoreactive fibers appeared in the ipsilateral GN where the touch-sense mediating A-fiber primary afferents from the hindpaw terminate. Seven-daylong intrathecal infusion of GDNF at the L5 DRG level, starting on day three when mechanical allodynia had fully developed, reversed once-established these changes. The GN neurons normally expressed NPY Y1 receptor, but not Y2, Y4, or Y5 receptors, and L5 SNL did not change the expression pattern. Bolus intracisternal injection of BIBP3226, a Y1 receptor antagonist, dose-dependently reversed mechanical allodynia. We demonstrated that GDNF reversed once-established mechanical allodynia as well as NPY induction in the touch-sense processing pathway. NPY could facilitate touch-sense processing by Y1 receptor in the gracile nucleus after peripheral nerve injury. GDNF may exert anti-allodynic effects through mitigation of this NPY up-regulation. The effectiveness of delayed treatment further indicates the therapeutic potential of GDNF on neuropathic pain.

Neuropeptides: metabolism to bioactive fragments and the pharmacology of their receptors

The proteolytic processing of neuropeptides has an important regulatory function and the peptide fragments resulting from the enzymatic degradation often exert essential physiological roles. The proteolytic processing generates, not only biologically inactive fragments, but also bioactive fragments that modulate or even counteract the response of their parent peptides. Frequently, these peptide fragments interact with receptors that are not recognized by the parent peptides. This review discusses tachykinins, opioid peptides, angiotensins, bradykinins, and neuropeptide Y that are present in the central nervous system and their processing to bioactive degradation products. These well-known neuropeptide systems have been selected since they provide illustrative examples that proteolytic degradation of parent peptides can lead to bioactive metabolites with different biological activities as compared to their parent peptides. For example, substance P, dynorphin A, angiotensin I and II, bradykinin, and neuropeptide Y are all degraded to bioactive fragments with pharmacological profiles that differ considerably from those of the parent peptides. The review discusses a selection of the large number of drug-like molecules that act as agonists or antagonists at receptors of neuropeptides. It focuses in particular on the efforts to identify selective drug-like agonists and antagonists mimicking the effects of the endogenous peptide fragments formed. As exemplified in this review, many common neuropeptides are degraded to a variety of smaller fragments but many of the fragments generated have not yet been examined in detail with regard to their potential biological activities. Since these bioactive fragments contain a small number of amino acid residues, they provide an ideal starting point for the development of drug-like substances with ability to mimic the effects of the degradation products. Thus, these substances could provide a rich source of new pharmaceuticals. However, as discussed herein relatively few examples have so far been disclosed of successful attempts to create bioavailable, drug-like agonists or antagonists, starting from the structure of endogenous peptide fragments and applying procedures relying on stepwise manipulations and simplifications of the peptide structures.

Levels of neuropeptide Y in synovial fluid relate to pain in patients with knee osteoarthritis

Background

The precise etiology of knee osteoarthritis (KOA) pain remains highly controversial and there is no known effective treatment. Due to the known and suggested effects of neuropeptide Y (NPY) on pain, we have sought to investigate the relationship between the concentration of NPY in synovial fluid of knee, pain of KOA, and structural severity of KOA.

Methods

One hundred KOA patients and twenty healthy participants (control group) were recruited. The pain and the radiographic grade of KOA were assessed separately by Hideo Watanabe’s pain score and Tomihisa Koshino’s scoring system. Synovial fluid of knee from all participants was collected with arthrocentesis. Radioimmunoassay was used to examine the concentration of NPY in synovial fluid of knee.

Results

Concentrations of NPY in synovial fluid were significantly higher in KOA patients (124.7 ± 33.4 pg/mL) compared with controls (64.8 ± 26.3 pg/mL) (p = 0.0297). According to Hideo Watanabe’s pain score, 100 KOA patients were divided into 5 subgroups: no pain (n = 12), mild pain (n = 25), moderate pain (n = 37), strong pain (n = 19) and severe pain (n = 7). Within the KOA group, significantly higher concentrations of NPY were found in each subgroup as pain intensified (no pain 81.4 ± 11.7 pg/mL, mild pain 99.1 ± 23.2 pg/mL, moderate pain 119.9 ± 31.5 pg/mL, strong pain 171.2 ± 37.3 pg/mL and severe pain 197.3 ± 41.9 pg/mL). Meanwhile, according to Tomihisa Koshino’s scoring system, 100 KOA patients were divided into 3 subgroups: early stage (n = 30), middle stage (n = 53), advanced stage (n = 17). Concentrations of NPY in middle and advanced stage groups of KOA patients were significant higher than early stage group of KOA patients (early stage 96.4 ± 27.1 pg/mL, middle stage 153.3 ± 16.9 pg/mL, advanced stage 149.5 ± 36.7 pg/mL) (p = 0.0163, p = 0.0352). Concentrations of NPY in advanced stage group of KOA patients has no significant difference compare with middle stage group of KOA patients (p = 0. 2175).

Conclusions

This study demonstrated the presence and variation of concentrations of NPY in the KOA joint fluid, suggesting a role for NPY as a putative regulator of pain transmission and perception in KOA pain.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2474-15-319) contains supplementary material, which is available to authorized users.

Somatostatin and its 2A receptor in dorsal root ganglia and dorsal horn of mouse and human: expression, trafficking and possible role in pain

Background

Somatostatin (SST) and some of its receptor subtypes have been implicated in pain signaling at the spinal level. In this study we have investigated the role of SST and its sst2A receptor (sst2A) in dorsal root ganglia (DRGs) and spinal cord.

Results

SST and sst2A protein and sst2 transcript were found in both mouse and human DRGs, sst2A-immunoreactive (IR) cell bodies and processes in lamina II in mouse and human spinal dorsal horn, and sst2A-IR nerve terminals in mouse skin. The receptor protein was associated with the cell membrane. Following peripheral nerve injury sst2A-like immunoreactivity (LI) was decreased, and SST-LI increased in DRGs. sst2A-LI accumulated on the proximal and, more strongly, on the distal side of a sciatic nerve ligation. Fluorescence-labeled SST administered to a hind paw was internalized and retrogradely transported, indicating that a SST-sst2A complex may represent a retrograde signal. Internalization of sst2A was seen in DRG neurons after systemic treatment with the sst2 agonist octreotide (Oct), and in dorsal horn and DRG neurons after intrathecal administration. Some DRG neurons co-expressed sst2A and the neuropeptide Y Y1 receptor on the cell membrane, and systemic Oct caused co-internalization, hypothetically a sign of receptor heterodimerization. Oct treatment attenuated the reduction of pain threshold in a neuropathic pain model, in parallel suppressing the activation of p38 MAPK in the DRGs

Conclusions

The findings highlight a significant and complex role of the SST system in pain signaling. The fact that the sst2A system is found also in human DRGs and spinal cord, suggests that sst2A may represent a potential pharmacologic target for treatment of neuropathic pain.

Inflammation enhances Y1 receptor signaling, neuropeptide Y-mediated inhibition of hyperalgesia, and substance P release from primary afferent neurons

Neuropeptide Y (NPY) is present in the superficial laminae of the dorsal horn and inhibits spinal nociceptive processing, but the mechanisms underlying its anti-hyperalgesic actions are unclear. We hypothesized that NPY acts at neuropeptide Y1 receptors in the dorsal horn to decrease nociception by inhibiting substance P (SP) release, and that these effects are enhanced by inflammation. To evaluate SP release, we used microdialysis and neurokinin 1 receptor (NK1R) internalization in rat. NPY decreased capsaicin-evoked SP-like immunoreactivity in the microdialysate of the dorsal horn. NPY also decreased non-noxious stimulus (paw brush)-evoked NK1R internalization (as well as mechanical hyperalgesia and mechanical and cold allodynia) after intraplantar injection of carrageenan. Similarly, in rat spinal cord slices with dorsal root attached, [Leu(31), Pro(34)]-NPY inhibited dorsal root stimulus-evoked NK1R internalization. In rat dorsal root ganglion neurons, Y1 receptors colocalized extensively with calcitonin gene-related peptide (CGRP). In dorsal horn neurons, Y1 receptors were extensively expressed and this may have masked the detection of terminal co-localization with CGRP or SP. To determine whether the pain inhibitory actions of Y1 receptors are enhanced by inflammation, we administered [Leu(31), Pro(34)]-NPY after intraplantar injection of complete Freund's adjuvant (CFA) in rat. We found that [Leu(31), Pro(34)]-NPY reduced paw clamp-induced NK1R internalization in CFA rats but not uninjured controls. To determine the contribution of increased Y1 receptor-G protein coupling, we measured [(35)S]GTPγS binding simulated by [Leu(31), Pro(34)]-NPY in mouse dorsal horn. CFA inflammation increased the affinity of Y1 receptor G-protein coupling. We conclude that Y1 receptors contribute to the anti-hyperalgesic effects of NPY by mediating the inhibition of SP release, and that Y1 receptor signaling in the dorsal horn is enhanced during inflammatory nociception.

Neuropeptide Y is analgesic in rats after plantar incision

Previous work has demonstrated that neuropeptide tyrosine (NPY), Y(1) receptor and Y(2) receptor are critical in modulation of pain after nerve injury. We hypothesized that NPY was important for nociception after surgical incision. As a model of postoperative pain, rats underwent a plantar incision in one hindpaw. Western blots were used to quantify changes in protein expression of NPY, Y(1) receptor and Y(2) receptor after incision in skin, muscle, and dorsal root ganglion (DRG). Pain-related behaviors were tested after incision in rats treated with intrathecal NPY, Y(1) receptor antagonist (BIBO3304--Chemical Name: N-[(1R)-1-[[[[4-[[(Aminocarbonyl)amino]methyl]phenyl]methyl]amino]carbonyl]-4-[(aminoiminomethyl)amino]butyl]-α-phenyl-benzeneacetamide ditrifluoroacetate), Y(2) receptor antagonist (BIIE0246--Chemical Name: N-[(1S)-4-[(Aminoiminomethyl)amino]-1-[[[2-(3,5-dioxo-1,2-diphenyl-1,2,4-triazolidin-4-yl)ethyl]amino]carbonyl]butyl]-1-[2-[4-(6,11-dihydro-6-oxo-5H-dibenz[b,e]azepin-11-yl)-1-piperazinyl]-2-oxoethyl]-cyclopentaneacetamide), combined NPY+antagonists, morphine, or vehicle. Pain behaviors were tested after incision in rats treated with locally applied intraplantar injections of NPY, Y(1) receptor and Y(2) receptor antagonists or vehicle. NPY protein expression was significantly downregulated in muscle for two days after incision. In contrast, Y(1) receptor and Y(2) receptor protein expression was upregulated in both skin and muscle. A single intrathecal injection of NPY reduced cumulative guarding pain scores, as did morphine. The intrathecal administration of Y(2) receptor antagonist also reduced pain scores; findings that were not observed when drugs were administered locally. Intrathecal Y(2) receptor antagonists and NPY improved mechanical threshold and heat withdrawal latency 2h after incision. Intrathecal administration of NPY and/or central blockade of Y(2) receptor attenuated pain behaviors early after incision (postoperative day (POD) 1-2). Y(1) receptor antagonist administration blocked the anti-hyperalgesic effect of NPY. Together these data suggest a role for spinal NPY in postoperative pain.

Neuropeptide Y receptor-expressing dorsal horn neurons: role in nocifensive reflex and operant responses to aversive cold after CFA inflammation

The spinal Neuropeptide Y (NPY) system is a potential target for development of new pain therapeutics. NPY and two of its receptors (Y1 and Y2) are found in the superficial dorsal horn of the spinal cord, a key area of nociceptive gating and modulation. Lumbar intrathecal injection of (NPY) is antinociceptive, reducing hyper-reflexia to thermal and mechanical stimulation, particularly after nerve injury and inflammation. We have also shown that intrathecal injection of the targeted cytotoxin, Neuropeptide Y-sap (NPY-sap), is also antinociceptive, reducing nocifensive reflex responses to noxious heat and formalin. In the present study, we sought to determine the role of dorsal horn Y1R-expressing neurons in pain by destroying them with NPY-sap and testing the rats on three operant tasks. Lumbar intrathecal NPY-sap (1) reduced Complete Freund's Adjuvant (CFA)-induced hyper-reflexia on the 10°C cold plate, (2) reduced cold aversion on the thermal preference and escape tasks, (3) was analgesic to noxious heat on the escape task, (4) reduced the CFA-induced allodynia to cold temperatures experienced on the thermal preference, feeding interference, and escape tasks, and (5) did not inhibit or interfere with morphine analgesia.

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.

Attenuation of pain-related behavior evoked by injury through blockade of neuropeptide Y Y2 receptor

Neuropeptide Y (NPY) has an important but still insufficiently defined role in pain modulation. We therefore examined the ability of NPY to modulate experimentally induced neuropathic pain by injecting it directly into dorsal root ganglion (DRG) immediately following spinal nerve ligation (SNL) injury. We have found that this application exacerbates pain-related behavior induced by SNL in a modality-specific fashion. When saline was injected after SNL, the expected increase in hyperalgesia responses to needle stimulation was present on the 8th postoperative day. When we injected NPY, hyperalgesic responses were increased in a manner similar to the SNL/saline group. To characterize NPY action, specific Y1 and Y2 antagonists were also delivered directly to DRG, which revealed that behavioral actions of NPY were abolished by Y2 receptor antagonist. We tested whether NPY effects were the result of its role in immunity by immunohistochemical staining for glial fibrillary acidic protein, in order to identify activation of DRG satellite cells and dorsal horn astrocytes. Exacerbation of pain-related behavior following NPY injection was accompanied by astrocyte activation in ipsilateral dorsal horn and with satellite cells activation in the DRG proximal to injury. This activation was reduced following Y2 receptor antagonist application. These findings indicate an important link between pain-related behavior and neuroimmune activation by NPY through its Y2 receptor.

Behavioral and anatomical characterization of the bilateral sciatic nerve chronic constriction (bCCI) injury: correlation of anatomic changes and responses to cold stimuli

Background

Unilateral constrictive sciatic nerve injury (uCCI) is a common neuropathic pain model. However, the bilateral constrictive injury (bCCI) model is less well studied, and shows unique characteristics. In the present study, we sought to correlate effects of bCCI on nocifensive responses to cold and mechanical stimuli with selected dorsal horn anatomic markers. bCCI or sham ligation of both rat sciatic nerves were followed up to 90 days of behavioural testing. Additional rats sacrificed at 15, 30 and 90 days were used for anatomic analyses. Behavioural tests included hindpaw withdrawal responses to topical acetone, cold plate testing, an operant thermal preference task and hindpaw withdrawal thresholds to mechanical probing.

Results

All nocifensive responses to cold increased and remained enhanced for >45 days. Mechanical withdrawal thresholds decreased for 25 days only. Densitometric analyses of immunoperoxidase staining in the superficial dorsal horn at L4-5 revealed decreased cholecystokinin (CCK) staining at all times after bCCI, decreased mu opiate receptor (MOR) staining, maximal at 15 days, increased neuropeptide Y (NPY) staining only at days 15 and 30, and increased neurokinin-1 receptor (NK-1R) staining at all time points, maximal at 15 days. Correlation analyses at 45 days post-bCCI, were significant for individual rat nocifensive responses in each cold test and CCK and NK-1R, but not for MOR or NPY.

Conclusions

These results confirm the usefulness of cold testing in bCCI rats, a new approach using CCI to model neuropathic pain, and suggest a potential value of studying the roles of dorsal horn CCK and substance P in chronic neuropathic pain. Compared to human subjects with neuropathic pain, responses to cold stimuli in rats with bCCI may be a useful model of neuropathic pain.

Involvement of neuropeptide Y in the acute, chronic and withdrawal responses of morphine in nociception in neuropathic rats: behavioral and neuroanatomical correlates

Although morphine is a potent antinociceptive agent, its chronic use developed tolerance in neuropathic pain (NP). Furthermore, opioid antagonist naloxone attenuated the antinociceptive effect of neuropeptide Y (NPY). The present study investigated the role of NPY and NPY Y1/Y5 receptors in acute and chronic actions of morphine in neuropathic rats using thermal paw withdrawal test and immunocytochemistry. In acute study, intracerebroventricular (icv) administration of morphine, NPY or NPY Y1/Y5 receptors agonist [Leu(31),Pro(34)]-NPY produced antinociception, whereas selective NPY Y1 receptors antagonist BIBP3226 caused hyperalgesia. While NPY or [Leu(31),Pro(34)]-NPY potentiated, BIBP3226 attenuated morphine induced antinociception. Chronic icv infusion of morphine via osmotic minipumps developed tolerance to its antinociceptive effect, and produced hyperalgesia following withdrawal. However, co-administration of NPY or [Leu(31),Pro(34)]-NPY prevented the development of tolerance and withdrawal hyperalgesia. Sciatic nerve ligation resulted in significant increase in the NPY-immunoreactive (NPY-ir) fibers in ventrolateral periaqueductal gray (VLPAG) and locus coeruleus (LC); fibers in the dorsal part of dorsal raphe nucleus (DRD) did not respond. While chronic morphine treatment significantly reduced NPY-ir fibers in VLPAG and DRD, morphine withdrawal triggered significant augmentation in NPY-immunoreactivity in the VLPAG. NPY-immunoreactivity profile of LC remained unchanged in all the morphine treatment conditions. Furthermore, removal of sciatic nerve ligation reversed the effects of NP, increased pain threshold and restored NPY-ir fiber population in VLPAG. NPY, perhaps acting via Y1/Y5 receptors, might profoundly influence the processing of NP information and interact with the endogenous opioid system primarily within the framework of the VLPAG.

Y1 receptor knockout increases nociception and prevents the anti-allodynic actions of NPY

OBJECTIVE

Recent pharmacologic studies in our laboratory have suggested that the spinal neuropeptide Y (NPY) Y1 receptor contributes to pain inhibition and to the analgesic effects of NPY. To rule out off-target effects, the present study used Y1-receptor-deficient (-/-) mice to further explore the contribution of Y1 receptors to pain modulation.

METHODS AND RESULTS

Y1(-/-) mice exhibited reduced latency in the hotplate test of acute pain and a longer-lasting heat allodynia in the complete Freund's adjuvant (CFA) model of inflammatory pain. Y1 deletion did not change CFA-induced inflammation. Upon targeting the spinal NPY systems with intrathecal drug delivery, NPY reduced tactile and heat allodynia in the CFA model and the partial sciatic nerve ligation model of neuropathic pain. Importantly, we show for the first time that NPY does not exert these anti-allodynic effects in Y1(-/-) mice. Furthermore, in nerve-injured CD1 mice, concomitant injection of the potent Y1 antagonist BIBO3304 prevented the anti-allodynic actions of NPY. Neither NPY nor BIBO3304 altered performance on the Rotorod test, arguing against an indirect effect of motor function.

CONCLUSION

The Y1 receptor contributes to pain inhibition and to the analgesic effects of NPY.

Neuropeptide Y receptor-expressing dorsal horn neurons: role in nocifensive reflex responses to heat and formalin

Lumbar intrathecal injection of neuropeptide Y (NPY) is antinociceptive, particularly in models of nerve injury and inflammation. Intrathecal NPY does not alter nociception in mice null for the Y1 neuropeptide Y receptor (Y1R) and these mice show enhanced nocifensive reflex responses to aversive thermal, mechanical, visceral and chemical stimuli. Y1R and NPY receptor type 2 (Y2R) are present in the spinal dorsal horn presynaptically on primary afferent, and possibly interneuron terminals, but only Y1R is found postsynaptically on dorsal horn neurons. In the present study, we sought to assess the anatomic effects of lumbar intrathecal disulfide conjugate of neuropeptide Y and saporin (NPY-sap) and to determine the role of Y1R-expressing dorsal horn neurons in nocifensive responses to aversive thermal and chemical stimulation. Lumbar intrathecal injection of NPY-sap was used to selectively destroy Y1R-expressing lumbar dorsal horn neurons followed by testing nocifensive reflex responses on the hotplate and after hind-paw formalin injection. NPY-saporin decreased superficial dorsal horn staining for Y1R, but not neurokinin-1 receptor, mu opiate receptor or NPY peptide, and had no effect on Y1R cell counts in fourth lumbar spinal segment dorsal root ganglia. Loss of Y1R-expressing dorsal horn neurons was associated with increased first response latencies on the 44 degrees C hotplate and reduced total time rats spent licking and guarding hind paws during 600 s trials at 44 degrees C or 200 s trials at 47 degrees C. First hind-paw response latencies to high intensity phasic stimulation at 52 degrees C were unaffected. NPY-sap also reduced formalin-induced nocifensive behaviors during both interphase and phase II. These data demonstrate that selective destruction of Y1R-expressing superficial dorsal horn neurons, probably excitatory interneurons and/or projection neurons, reduces nocifensive reflex responses, particularly to activation of C nociceptors, and suggest a possible role for Y1R-expressing dorsal horn neurons in pain.

Intrathecal rosiglitazone acts at peroxisome proliferator-activated receptor-gamma to rapidly inhibit neuropathic pain in rats

In this report, we demonstrate the transcription, expression, and DNA-binding properties of the peroxisome proliferator-activated receptor (PPAR)-gamma subtype of the peroxisome proliferator-activated nuclear receptor family to the spinal cord with real-time PCR, Western blot, and electrophoretic mobility shift assay. To test the hypothesis that activation of spinal PPAR-gamma decreases nerve injury-induced allodynia, we intrathecally administered PPAR-gamma agonists and/or antagonists in rats after transection of the tibial and common peroneal branches of the sciatic nerve. Single injection of either a natural (15-deoxy-prostaglandin J2, 15d-PGJ2) or synthetic (rosiglitazone) PPAR-gamma agonist dose-dependently decreased mechanical and cold hypersensitivity. These effects were maximal at a dose of 100 microg and peaked at approximately 60 minutes after injection, a rapid time course suggestive of transcription-independent mechanisms of action. Concurrent administration of a PPAR-gamma antagonist (bisphenol A diglycidyl ether, BADGE) reversed the effects of 15d-PGJ2 and rosiglitazone, further indicating a receptor-mediated effect. In animals without nerve injury, rosiglitazone did not alter motor coordination, von Frey threshold, or withdrawal response to a cool stimulus. Intraperitoneal and intracerebroventricular administration of PPAR-gamma agonists (100 microg) did not decrease mechanical and cold hypersensitivity, arguing against effects subsequent to diffusion from the intrathecal space. We conclude that ligand-induced activation of spinal PPAR-gamma rapidly reverses nerve injury-induced mechanical allodynia. New or currently available drugs targeted at spinal PPAR-gamma may yield important therapeutic effects for the management of neuropathic pain.

PERSPECTIVE

PPAR-gamma receptor agonists such as rosiglitazone and pioglitazone are approved as insulin sensitizers by the United States Food and Drug Administration. We demonstrate PPAR-gamma expression in the spinal cord and report that activation of these receptors inhibits allodynia. BBB-permeant PPAR-gamma agonists may yield important therapeutic effects for the management of neuropathic pain.

Intrathecal neuropeptide Y reduces behavioral and molecular markers of inflammatory or neuropathic pain

Our previous work indicates that the intrathecal administration of neuropeptide Y (NPY) acts at its cognate receptors to reduce behavioral signs of nociception in several models of inflammatory pain, including the formalin test. The present study extends these findings to a rat model of peripheral neuropathic pain, and then evaluates the hypothesis that NPY inhibits inflammation- and nerve injury-induced activation of spinal nociceptive transmission. Here we show that NPY dose-dependently reduced behavioral signs of mechanical and cold hypersensitivity in the spared nerve injury (SNI) model. Intrathecal administration of either a Y1 (BIBO3304) or a Y2 (BIIE0246) receptor antagonist dose-dependently reversed the anti-allodynic actions of NPY. To monitor the effects of NPY on the stimulus-induced activation of spinal nociresponsive neurons, we quantified protein expression of the immediate-early gene c-fos in lamina I-VI of the L4-L5 dorsal horn, with special attention to the mediolateral pattern of Fos immunohistochemical staining after SNI. Either tactile stimulation of the hindpaw ipsilateral to nerve injury, or intraplantar injection of noxious formalin, increased the number of Fos-like immunoreactive profiles. Tactile stimulation evoked a mediolateral pattern of Fos expression corresponding to the innervation territory of the uninjured (sural) nerve. We found that intrathecal NPY reduced both formalin- and SNI-induced Fos expression. NPY inhibition of SNI-induced Fos expression was localized to the sural (uninjured) innervation territory, and could be blocked by intrathecal BIBO3304 and BIIE0246. We conclude that NPY acts at spinal Y1 and Y2 receptors to reduce spinal neuron activity and behavioral signs of inflammatory or neuropathic pain.

Involvement of Y5 receptors in neuropeptide Y agonist-induced analgesic-like effect in the rat hot plate test

Neuropeptide Y (NPY) induces analgesic-like effects after central administration across diverse pain models in rodents. In spinal pain models, previous studies indicate a prominent role for Y(1) receptors at mediating this effect of NPY. In supraspinal pain models like the hot plate test, the NPY receptors involved have not been thoroughly explored. By intracerebroventricular (i.c.v.) administration of selective NPY receptor ligands, the possible involvement of Y(5) receptors in analgesic-like mechanisms was investigated using the hot plate test in rats. Both NPY and selective Y(5) agonists induced analgesic-like effects as revealed by prolonged hot plate latencies. Further consistent with a role for Y(5) receptors, pretreatment with a selective Y(5) receptor antagonist blocked the Y(5) agonist-induced analgesic-like effect. The present study indicates involvement of Y(5) receptors probably at the supraspinal level in mediation of NPY agonist-induced analgesic-like effects in the hot plate test.

Neuropeptide Y acts at Y1 receptors in the rostral ventral medulla to inhibit neuropathic pain

Brain microinjection studies in the rat using local anesthetics suggest that the rostral ventral medulla (RVM) contributes to the facilitation of neuropathic pain. However, these studies were restricted to a single model of neuropathic pain (the spinal nerve ligation model) and to just two stimulus modalities (non-noxious tactile stimulus and heat). Also, few neurotransmitter systems have been shown to modulate descending facilitation. After either partial sciatic nerve ligation (PSNL) or spared nerve injury (SNI), we found that unilateral or bilateral microinjection of lidocaine into the RVM reduced not only mechanical allodynia (decreased threshold to von Frey hairs and/or an automated device) and mechanical hyperalgesia (increased paw lifting in response to a noxious pin), but also cold hypersensitivity (increased lifting in response to the hindpaw application of a drop of acetone). Application of a drop of water did not elicit paw withdrawal, indicating that the acetone test is indeed a measure of cold hypersensitivity. We found significant neuropeptide Y Y1-like immunoreactivity within, and lateral to, the midline RVM. Intra-RVM injection of neuropeptide Y (NPY) dose-dependently inhibited the mechanical and cold hypersensitivity associated with PSNL or SNI, an effect that could be blocked by the Y1 receptor antagonist BIBO 3304. We conclude that medullary facilitation spans multiple behavioral signs of allodynia and hyperalgesia in multiple models of neuropathic pain. Furthermore, NPY inhibits behavioral signs of neuropathic pain, possibly by acting at Y1 receptors in the RVM.

NPY and pain as seen from the histochemical side

The expression of neuropeptide tyrosine (NPY) and two of its receptors (Y1- and Y2Rs) in different types of rodent dorsal root ganglion (DRG) and spinal cord neurons, and their regulation by peripheral nerve injury, have suggested a role in neuropathic pain. Here we present the spinal NPYergic system from an immunohistochemical perspective based on recent studies using two specific antibodies recognizing the Y1- and Y2Rs, respectively, as well as on data from a study on a Y1R knock-out mouse. We have, for example, defined seven different neuron populations of Y1R-expressing neurons in the rat spinal cord, representing multiple targets for spinally released NPY. The differential distribution of NPY receptors probably explains both the pro- and antinociceptive effects of NPY previously reported in the literature. One system possibly responsible for antinociception is a group of Y1R-positive, presumably glutamatergic interneurons in the superficial dorsal horn laminae. We also discuss the possibility that NPY released within DRGs can act in a paracrine fashion on NPY receptors on adjacent neurons, perhaps contributing to the so-called cross excitation, a concept advanced by Devor, Amir and collaborators. Taken together with behavioral and electrophysiological results summarized by Smith et al. in this volume, histochemical analyses have advanced the knowledge on the role of NPY in pain processing.

Spinal mechanisms of NPY analgesia

We review previously published data, and present some new data, indicating that spinal application of neuropeptide Y (NPY) reduces behavioral and neurophysiological signs of acute and chronic pain. In models of acute pain, early behavioral studies showed that spinal (intrathecal) administration of NPY and Y2 receptor agonists decrease thermal nociception. Subsequent neurophysiological studies indicated that Y2-mediated inhibition of excitatory neurotransmitter release from primary afferent terminals in the substantia gelatinosa may contribute to the antinociceptive actions of NPY. As with acute pain, NPY reduced behavioral signs of inflammatory pain such as mechanical allodynia and thermal hyperalgesia; however, receptor antagonist studies indicate an important contribution of spinal Y1 rather than Y2 receptors. Interestingly, Y1 agonists suppress inhibitory synaptic events in dorsal horn neurons (indeed, well known mu-opioid analgesic drugs produce similar cellular actions). To resolve the behavioral and neurophysiological data, we propose that NPY/Y1 inhibits the spinal release of inhibitory neurotransmitters (GABA and glycine) onto inhibitory neurons, e.g. disinhibition of pain inhibition, resulting in hyporeflexia. The above mechanisms of Y1- and Y2-mediated analgesia may also operate in the setting of peripheral nerve injury, and new data indicate that NPY dose-dependently inhibits behavioral signs of neuropathic pain. Indeed, neurophysiological studies indicate that Y2-mediated inhibition of Ca(2+) channel currents in dorsal root ganglion neurons is actually increased after axotomy. We conclude that spinal delivery of Y1 agonists may be of use in the treatment of chronic inflammatory pain, and that the use of Y1 and Y2 agonists in neuropathic pain warrants further consideration.

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.