Loss of spinal mu-opioid receptor is associated with mechanical allodynia in a rat model of peripheral neuropathy

Epigenetic Landscapes of Pain: DNA Methylation Dynamics in Chronic Pain

Chronic pain is a prevalent condition with a multifaceted pathogenesis, where epigenetic modifications, particularly DNA methylation, might play an important role. This review delves into the intricate mechanisms by which DNA methylation and demethylation regulate genes associated with nociception and pain perception in nociceptive pathways. We explore the dynamic nature of these epigenetic processes, mediated by DNA methyltransferases (DNMTs) and ten-eleven translocation (TET) enzymes, which modulate the expression of pro- and anti-nociceptive genes. Aberrant DNA methylation profiles have been observed in patients with various chronic pain syndromes, correlating with hypersensitivity to painful stimuli, neuronal hyperexcitability, and inflammatory responses. Genome-wide analyses shed light on differentially methylated regions and genes that could serve as potential biomarkers for chronic pain in the epigenetic landscape. The transition from acute to chronic pain is marked by rapid DNA methylation reprogramming, suggesting its potential role in pain chronicity. This review highlights the importance of understanding the temporal dynamics of DNA methylation during this transition to develop targeted therapeutic interventions. Reversing pathological DNA methylation patterns through epigenetic therapies emerges as a promising strategy for pain management.

The Downregulation of Opioid Receptors and Neuropathic Pain

Neuropathic pain (NP) refers to pain caused by primary or secondary damage or dysfunction of the peripheral or central nervous system, which seriously affects the physical and mental health of 7-10% of the general population. The etiology and pathogenesis of NP are complex; as such, NP has been a hot topic in clinical medicine and basic research for a long time, with researchers aiming to find a cure by studying it. Opioids are the most commonly used painkillers in clinical practice but are regarded as third-line drugs for NP in various guidelines due to the low efficacy caused by the imbalance of opioid receptor internalization and their possible side effects. Therefore, this literature review aims to evaluate the role of the downregulation of opioid receptors in the development of NP from the perspective of dorsal root ganglion, spinal cord, and supraspinal regions. We also discuss the reasons for the poor efficacy of opioids, given the commonness of opioid tolerance caused by NP and/or repeated opioid treatments, an angle that has received little attention to date; in-depth understanding might provide a new method for the treatment of NP.

Targeting Chemokines and Chemokine GPCRs to Enhance Strong Opioid Efficacy in Neuropathic Pain

Neuropathic pain (NP) originates from an injury or disease of the somatosensory nervous system. This heterogeneous origin and the possible association with other pathologies make the management of NP a real challenge. To date, there are no satisfactory treatments for this type of chronic pain. Even strong opioids, the gold-standard analgesics for nociceptive and cancer pain, display low efficacy and the paradoxical ability to exacerbate pain sensitivity in NP patients. Mounting evidence suggests that chemokine upregulation may be a common mechanism driving NP pathophysiology and chronic opioid use-related consequences (analgesic tolerance and hyperalgesia). Here, we first review preclinical studies on the role of chemokines and chemokine receptors in the development and maintenance of NP. Second, we examine the change in chemokine expression following chronic opioid use and the crosstalk between chemokine and opioid receptors. Then, we examine the effects of inhibiting specific chemokines or chemokine receptors as a strategy to increase opioid efficacy in NP. We conclude that strong opioids, along with drugs that block specific chemokine/chemokine receptor axis, might be the right compromise for a favorable risk/benefit ratio in NP management.

Transcriptomics Analysis of Porcine Caudal Dorsal Root Ganglia in Tail Amputated Pigs Shows Long-Term Effects on Many Pain-Associated Genes

Tail amputation by tail docking or as an extreme consequence of tail biting in commercial pig production potentially has serious implications for animal welfare. Tail amputation causes peripheral nerve injury that might be associated with lasting chronic pain. The aim of this study was to investigate the short- and long-term effects of tail amputation in pigs on caudal DRG gene expression at different stages of development, particularly in relation to genes associated with nociception and pain. Microarrays were used to analyse whole DRG transcriptomes from tail amputated and sham-treated pigs 1, 8, and 16 weeks following tail treatment at either 3 or 63 days of age (8 pigs/treatment/age/time after treatment; n = 96). Tail amputation induced marked changes in gene expression (up and down) compared to sham-treated intact controls for all treatment ages and time points after tail treatment. Sustained changes in gene expression in tail amputated pigs were still evident 4 months after tail injury. Gene correlation network analysis revealed two co-expression clusters associated with amputation: Cluster A (759 down-regulated) and Cluster B (273 up-regulated) genes. Gene ontology (GO) enrichment analysis identified 124 genes in Cluster A and 61 genes in Cluster B associated with both “inflammatory pain” and “neuropathic pain.” In Cluster A, gene family members of ion channels e.g., voltage-gated potassium channels (VGPC) and receptors e.g., GABA receptors, were significantly down-regulated compared to shams, both of which are linked to increased peripheral nerve excitability after axotomy. Up-regulated gene families in Cluster B were linked to transcriptional regulation, inflammation, tissue remodeling, and regulatory neuropeptide activity. These findings, demonstrate that tail amputation causes sustained transcriptomic expression changes in caudal DRG cells involved in inflammatory and neuropathic pain pathways.

Temporal and Spatial Changes of μ-Opioid Receptors in the Brain, Spinal Cord and Dorsal Root Ganglion in a Rat Lumbar Disc Herniation Model

STUDY DESIGN

Controlled, interventional, animal study.

OBJECTIVE

To investigate the spatial and temporal changes of μ-opioid receptor (MOR) expression in a rat lumbar disc herniation (LDH) model.

SUMMARY OF BACKGROUND DATA

MORs widely express in the peripheral and central nervous systems, and opioid drugs produce an analgesic effect through their activation. However, the efficacy of opioid drugs is sometimes inadequate in several pathological conditions of pain. MORs in the brain as well as the spinal cord (SC) and dorsal root ganglion (DRG) are thought to be associated with pain-related behavior, but the underlying mechanisms are not completely understood.

METHODS

In all, 91 adult female Sprague-Dawley rats were used. Autologous nucleus pulposus (NP) was applied onto the left L5 DRG in the NP group rats. Rats were divided into two surgical groups, the NP and the sham group. The von Frey test of left hind paw was performed before surgery, and 2, 7, 14, 21 and 28 days after surgery. Immunohistochemistry and immunoblotting in the DRG, SC, Caudate putamen, nucleus accumbens (NAc) and periaqueductal grey matter were performed before surgery, and 2, 7, 14, 21 and 28 days after surgery.

RESULTS

The thresholds in the NP group were significantly lower than those in the sham group from day 2 onwards. At days 7 and 14, MOR expression in the injured-side SC and DRG were significantly lower than those in the sham group. At day 21, MOR in the NAc was significantly decreased compared to that in the sham group.

CONCLUSION

Changes of MOR expression in the NAc, SC and DRG were associated with pain-related behavior. This result might show the underling pathogenesis of the resistance to MOR agonists in the patient with LDH.

LEVEL OF EVIDENCE

N/A.

The effect of exercise frequency on neuropathic pain and pain-related cellular reactions in the spinal cord and midbrain in a rat sciatic nerve injury model

Background

Exercise regimens are established methods that can relieve neuropathic pain. However, the relationship between frequency and intensity of exercise and multiple cellular responses of exercise-induced alleviation of neuropathic pain is still unclear. We examined the influence of exercise frequency on neuropathic pain and the intracellular responses in a sciatic nerve chronic constriction injury (CCI) model.

Materials and methods

Rats were assigned to four groups as follows: CCI and high-frequency exercise (HFE group), CCI and low-frequency exercise (LFE group), CCI and no exercise (No-Ex group), and naive animals (control group). Rats ran on a treadmill, at a speed of 20 m/min, for 30 min, for 5 (HFE) or 3 (LFE) days a week, for a total of 5 weeks. The 50% withdrawal threshold was evaluated for mechanical sensitivity. The activation of glial cells (microglia and astrocytes), expression of brain-derived neurotrophic factor (BDNF) and μ-opioid receptor in the spinal dorsal horn and endogenous opioid in the midbrain were examined using immunohistochemistry. Opioid receptor antagonists (naloxone) were administered using intraperitoneal injection.

Results

The development of neuropathic pain was related to the activation of glial cells, increased BDNF expression, and downregulation of the μ-opioid receptor in the ipsilateral spinal dorsal horn. In the No-Ex group, neuropathic pain showed the highest level of mechanical hypersensitivity at 2 weeks, which improved slightly until 5 weeks after CCI. In both exercise groups, the alleviation of neuropathic pain was accelerated through the regulation of glial activation, BDNF expression, and the endogenous opioid system. The expression of BDNF and endogenous opioid in relation to exercise-induced alleviation of neuropathic pain differed in the HFE and LFE groups. The effects of exercise-induced alleviation of mechanical hypersensitivity were reversed by the administration of naloxone.

Conclusion

The LFE and HFE program reduced neuropathic pain. Our findings indicated that aerobic exercise-induced alleviated neuropathic pain through the regulation of glial cell activation, expression of BDNF in the ipsilateral spinal dorsal horn, and the endogenous opioid system.

Nerve Decompression Improves Spinal Synaptic Plasticity of Opioid Receptors for Pain Relief

Nerve decompression is an essential therapeutic strategy for pain relief clinically; however, its potential mechanism remains poorly understood. Opioid analgesics acting on opioid receptors (OR) within the various regions of the nervous system have been used widely for pain management. We therefore hypothesized that nerve decompression in a neuropathic pain model of chronic constriction injury (CCI) improves the synaptic OR plasticity in the dorsal horn, which is in response to alleviate pain hypersensitivity. After CCI, the Sprague-Dawley rats were assigned into Decompression group, in which the ligatures around the sciatic nerve were removed at post-operative week 4 (POW 4), and a CCI group, in which the ligatures remained. Pain hypersensitivity, including thermal hyperalgesia and mechanical allodynia, was entirely normalized in Decompression group within the following 4 weeks. Substantial reversal of mu- and delta-OR immunoreactive (IR) expressions in Decompression group was detected in primary afferent terminals in the dorsal horn. In Decompression group, mu-OR antagonist (CTOP, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 [Disulfide Bridge: 2-7]) and delta-OR antagonist (NTI, 17-(cyclopropylmethyl)-6,7-dehydro-4,5α-epoxy-3,14-dihydroxy-6,7-2',3'-indolomorphinan hydrochloride) re-induced pain hypersensitivity by intrathecal administration in a dose-responsive manner. Additionally, mu-OR agonist (DAMGO, [D-Ala2, NMe-Phe4, Gly-ol5]-enkephalin) and delta-OR agonist (SNC80, ((+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethyl-benzamide) were administrated intrathecally to attenuating CCI-induced chronic and acute pain hypersensitivity dose-dependently. Our current results strongly suggested that nerve decompression provides the opportunity for improving the synaptic OR plasticity in the dorsal horn and pharmacological blockade presents a novel insight into the therapeutic strategy for pain hypersensitivity.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

Dorsal root ganglion transcriptome analysis following peripheral nerve injury in mice

BACKGROUND

Peripheral nerve injury leads to changes in gene expression in primary sensory neurons of the injured dorsal root ganglia. These changes are believed to be involved in neuropathic pain genesis. Previously, these changes have been identified using gene microarrays or next generation RNA sequencing with poly-A tail selection, but these approaches cannot provide a more thorough analysis of gene expression alterations after nerve injury.

METHODS

The present study chose to eliminate mRNA poly-A tail selection and perform strand-specific next generation RNA sequencing to analyze whole transcriptomes in the injured dorsal root ganglia following spinal nerve ligation. Quantitative real-time reverse transcriptase polymerase chain reaction assay was carried out to verify the changes of some differentially expressed RNAs in the injured dorsal root ganglia after spinal nerve ligation.

RESULTS

Our results showed that more than 50 million (M) paired mapped sequences with strand information were yielded in each group (51.87 M-56.12 M in sham vs. 51.08 M-57.99 M in spinal nerve ligation). Six days after spinal nerve ligation, expression levels of 11,163 out of a total of 27,463 identified genes in the injured dorsal root ganglia significantly changed, of which 52.14% were upregulated and 47.86% downregulated. The largest transcriptional changes were observed in protein-coding genes (91.5%) followed by noncoding RNAs. Within 944 differentially expressed noncoding RNAs, the most significant changes were seen in long interspersed noncoding RNAs followed by antisense RNAs, processed transcripts, and pseudogenes. We observed a notable proportion of reads aligning to intronic regions in both groups (44.0% in sham vs. 49.6% in spinal nerve ligation). Using quantitative real-time polymerase chain reaction, we confirmed consistent differential expression of selected genes including Kcna2, Oprm1 as well as lncRNAs Gm21781 and 4732491K20Rik following spinal nerve ligation.

CONCLUSION

Our findings suggest that next generation RNA sequencing can be used as a promising approach to analyze the changes of whole transcriptomes in dorsal root ganglia following nerve injury and to possibly identify new targets for prevention and treatment of neuropathic pain.

Endogenous analgesia, dependence, and latent pain sensitization

Endogenous activation of µ-opioid receptors (MORs) provides relief from acute pain. Recent studies have established that tissue inflammation produces latent pain sensitization (LS) that is masked by spinal MOR signaling for months, even after complete recovery from injury and re-establishment of normal pain thresholds. Disruption with MOR inverse agonists reinstates pain and precipitates cellular, somatic, and aversive signs of physical withdrawal; this phenomenon requires N-methyl-D-aspartate receptor-mediated activation of calcium-sensitive adenylyl cyclase type 1 (AC1). In this review, we present a new conceptual model of the transition from acute to chronic pain, based on the delicate balance between LS and endogenous analgesia that develops after painful tissue injury. First, injury activates pain pathways. Second, the spinal cord establishes MOR constitutive activity (MORCA) as it attempts to control pain. Third, over time, the body becomes dependent on MORCA, which paradoxically sensitizes pain pathways. Stress or injury escalates opposing inhibitory and excitatory influences on nociceptive processing as a pathological consequence of increased endogenous opioid tone. Pain begets MORCA begets pain vulnerability in a vicious cycle. The final result is a silent insidious state characterized by the escalation of two opposing excitatory and inhibitory influences on pain transmission: LS mediated by AC1 (which maintains the accelerator) and pain inhibition mediated by MORCA (which maintains the brake). This raises the prospect that opposing homeostatic interactions between MORCA analgesia and latent NMDAR-AC1-mediated pain sensitization creates a lasting vulnerability to develop chronic pain. Thus, chronic pain syndromes may result from a failure in constitutive signaling of spinal MORs and a loss of endogenous analgesic control. An overarching long-term therapeutic goal of future research is to alleviate chronic pain by either (a) facilitating endogenous opioid analgesia, thus restricting LS within a state of remission, or (b) extinguishing LS altogether.

Increased methylation of the MOR gene proximal promoter in primary sensory neurons plays a crucial role in the decreased analgesic effect of opioids in neuropathic pain

BACKGROUND

The analgesic potency of opioids is reduced in neuropathic pain. However, the molecular mechanism is not well understood.

RESULTS

The present study demonstrated that increased methylation of the Mu opioid receptor (MOR) gene proximal promoter (PP) in dorsal root ganglion (DRG) plays a crucial role in the decreased morphine analgesia. Subcutaneous (s.c.), intrathecal (i.t.) and intraplantar (i.pl.), not intracerebroventricular (i.c.v.) injection of morphine, the potency of morphine analgesia was significantly reduced in nerve-injured mice compared with control sham-operated mice. After peripheral nerve injury, we observed a decreased expression of MOR protein and mRNA, accompanied by an increased methylation status of MOR gene PP, in DRG. However, peripheral nerve injury could not induce a decreased expression of MOR mRNA in the spinal cord. Treatment with 5-aza-2'-deoxycytidine (5-aza-dC), inhibited the increased methylation of MOR gene PP and prevented the decreased expression of MOR in DRG, thereby improved systemic, spinal and periphery morphine analgesia.

CONCLUSIONS

Altogether, our results demonstrate that increased methylation of the MOR gene PP in DRG is required for the decreased morphine analgesia in neuropathic pain.

Spinal μ and δ opioids inhibit both thermal and mechanical pain in rats

The expression and contribution of μ (MOPR) and δ opioid receptors (DOPR) in polymodal nociceptors have been recently challenged. Indeed, MOPR and DOPR were shown to be expressed in distinct subpopulation of nociceptors where they inhibit pain induced by noxious heat and mechanical stimuli, respectively. In the present study, we used electrophysiological measurements to assess the effect of spinal MOPR and DOPR activation on heat-induced and mechanically induced diffuse noxious inhibitory controls (DNICs). We recorded from wide dynamic range neurons in the spinal trigeminal nucleus of anesthetized rats. Trains of 105 electrical shocks were delivered to the excitatory cutaneous receptive field. DNICs were triggered either by immersion of the hindpaw in 49°C water or application of 300 g of mechanical pressure. To study the involvement of peptidergic primary afferents in the activation of DNIC by noxious heat and mechanical stimulations, substance P release was measured in the spinal cord by visualizing neurokinin type 1 receptor internalization. We found that the activation of spinal MOPR and DOPR similarly attenuates the DNIC and neurokinin type 1 receptor internalization induced either by heat or mechanical stimuli. Our results therefore reveal that the activation of spinal MOPR and DOPR relieves both heat-induced and mechanically induced pain with similar potency and suggest that these receptors are expressed on polymodal, substance P-expressing neurons.

Paeonol attenuates H₂O₂-induced NF-κB-associated amyloid precursor protein expression

Hydrogen peroxide (H₂O₂) has been shown to promote neurodegeneration by inducing the activation of nuclear factor-κB (NF-κB). In this study, NF-κB activation was induced by H₂O₂ in human neuroblastoma SH-SY5Y cells. Whether paeonol, one of the phenolic phytochemicals isolated from the Chinese herb Paeonia suffruticosa Andrews (MC), would attenuate the H₂O₂-induced NF-κB activity was investigated. Western blot results showed that paeonol inhibited the phosphorylation of IκB and the translocation of NF-κB into the nucleus. The ability of paeonol to reduce DNA binding ability and suppress the H₂O₂-induced NF-κB activation was confirmed by an electrophoretic mobility shift assay and a luciferase reporter assay. Using a microarray combined with gene set analysis, we found that the suppression of NF-κB was associated with mature T cell up-regulated genes, the c-jun N-terminal kinase pathway, and two hypoxia-related gene sets, including the hypoxia up-regulated gene set and hypoxia inducible factor 1 targets. Moreover, using network analysis to investigate genes that were altered by H₂O₂ and reversely regulated by paeonol, we found that NF-κB was the primary center of the network and amyloid precursor protein (APP) was the secondary center. Western blotting showed that paeonol inhibited APP at the protein level. In conclusion, our work suggests that paeonol down-regulates H₂O₂-induced NF-κB activity, as well as NF-κB-associated APP expression. Furthermore, the gene expression profile accompanying the suppression of NF-κB by paeonol was identified. The new gene set that can be targeted by paeonol provided a potential use for this drug and a possible pharmacological mechanism for other phenolic compounds that protect against oxidative-related injury.

Are spinal GABAergic elements related to the manifestation of neuropathic pain in rat?

Impairment in spinal inhibition caused by quantitative alteration of GABAergic elements following peripheral nerve injury has been postulated to mediate neuropathic pain. In the present study, we tested whether neuropathic pain could be induced or reversed by pharmacologically modulating spinal GABAergic activity, and whether quantitative alteration of spinal GABAergic elements after peripheral nerve injury was related to the impairment of GABAergic inhibition or neuropathic pain. To these aims, we first analyzed the pain behaviors following the spinal administration of GABA antagonists (1 microg bicuculline/rat and 5 microg phaclofen/rat), agonists (1 microg muscimol/rat and 0.5 microg baclofen/rat) or GABA transporter (GAT) inhibitors (20 microg NNC-711/rat and 1 microg SNAP-5114/rat) into naïve or neuropathic animals. Then, using Western blotting, PCR or immunohistochemistry, we compared the quantities of spinal GABA, its synthesizing enzymes (GAD65, 67) and its receptors (GABA(A) and GABA(B)) and transporters (GAT-1, and -3) between two groups of rats with different severity of neuropathic pain following partial injury of tail-innervating nerves; the allodynic and non-allodynic groups. Intrathecal administration of GABA antagonists markedly lowered tail-withdrawal threshold in naïve animals, and GABA agonists or GAT inhibitors significantly attenuated neuropathic pain in nerve-injured animals. However, any quantitative changes in spinal GABAergic elements were not observed in both the allodynic and non-allodynic groups. These results suggest that although the impairment in spinal GABAergic inhibition may play a role in mediation of neuropathic pain, it is not accomplished by the quantitative change in spinal elements for GABAergic inhibition and therefore these elements are not related to the generation of neuropathic pain following peripheral nerve injury.

Spinal inhibitory neurotransmission in neuropathic pain

Nerve injury increases the spinal cord expression and/or activity of voltage- and ligand-gated ion channels, peptide receptors, and neuroimmune factors, which then drive dorsal horn neuron hyperexcitability. The intensity and duration of this central sensitization is determined by the net activity of local excitatory and inhibitory neurotransmitter systems, together with ongoing/evoked primary afferent activity and descending supraspinal control. Spinal endogenous inhibitory systems serve as opposing compensatory influences and are gaining recognition for their powerful capacity to restrain allodynia and hyperalgesia. These include numerous G protein-coupled receptors (mu- and delta-opioid, alpha(2)-adrenergic, purinergic A1, neuropeptide Y1 and Y2, cannabinoid CB1 and CB2, muscarinic M2, gamma-amino-butyric acid type B, metabotropic glutamate type II-III, somatostatin) and perhaps nuclear receptors (peroxisome proliferator-activated receptor gamma). Excessive downregulation or defective compensatory upregulation of these systems may contribute to the maintenance of neuropathic pain. An increasing number of pharmacotherapeutic strategies for neuropathic pain are emerging that mimic and enhance inhibitory neurotransmission in the dorsal horn.

Nociceptive behaviour upon modulation of mu-opioid receptors in the ventrobasal complex of the thalamus of rats

The role of mu-opioid receptors (MORs) in the inflammatory pain processing mechanisms within the ventrobasal complex of the thalamus (VB) is not well understood. This study investigated the effect of modulating MOR activity upon nociception, by stereotaxically injecting specific ligands in the VB. Nociceptive behaviour was evaluated in two established animal models of inflammatory pain, by using the formalin (acute and tonic pain) and the ankle-bend (chronic monoarthritic pain) tests. Control (saline intra-VB injection) formalin-injected rats showed acute and tonic pain-related behaviours. In contrast, intrathalamic administration of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin acetate (DAMGO), a MOR-specific agonist, induced a statistically significant decrease of all tonic phase pain-related behaviours assessed until 30-35min after formalin hind paw injection. In the acute phase only the number of paw-jerks was affected. In monoarthritic rats, there was a noticeable antinociceptive effect with approximately 40min of duration, as denoted by the reduced ankle-bend scores observed after DAMGO injection. Intra-VB injection of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), a specific MOR antagonist, or of CTOP followed, 10min after, by DAMGO had no effects in either formalin or ankle-bend tests. Data show that DAMGO-induced MOR activation in the VB has an antinociceptive effect in the formalin test as well as in chronic pain observed in MA rats, suggesting an important and specific role for MORs in the VB processing of inflammatory pain.

Ultra-low dose naloxone restores the antinociceptive effect of morphine in pertussis toxin-treated rats by reversing the coupling of mu-opioid receptors from Gs-protein to coupling to Gi-protein

Pertussis toxin (PTX) treatment results in ADP-ribosylation of Gi-protein and thus in disruption of mu-opioid receptor signal transduction and loss of the antinociceptive effect of morphine. We have previously demonstrated that pretreatment with ultra-low dose naloxone preserves the antinociceptive effect of morphine in PTX-treated rats. The present study further examined the effect of ultra-low dose naloxone on mu-opioid receptor signaling in PTX-treated rats and the underlying mechanism. Male Wistar rats implanted with an intrathecal catheter received an intrathecal injection of saline or PTX (1 microg in 5 microl of saline), then, 4 days later, were pretreated by intrathecal injection with either saline or ultra-low dose naloxone (15 ng in 5 microl of saline), followed, 30 min later, by saline or morphine (10 microg in 5 microl of saline). Four days after PTX injection, thermal hyperalgesia was observed, together with increased coupling of excitatory Gs-protein to mu-opioid receptors in the spinal cord. Ultra-low dose naloxone pretreatment preserved the antinociceptive effect of morphine, and this effect was completely blocked by the mu-opioid receptor antagonist CTOP, but not by the kappa-opioid receptor antagonist nor-BNI or the delta-opioid receptor antagonist naltrindole. Moreover, a co-immunoprecipitation study showed that ultra-low dose naloxone restored mu-opioid receptor/Gi-protein coupling and inhibited the PTX-induced mu-opioid receptor/Gs-protein coupling. In addition to the anti-neuroinflammatory effect and glutamate transporter modulation previously observed in PTX-treated rats, the re-establishment of mu-opioid receptor Gi/Go-protein coupling is involved in the restoration of the antinociceptive effect of morphine by ultra-low dose naloxone pretreatment by normalizing the balance between the excitatory and inhibitory signaling pathways. These results show that ultra-low dose naloxone preserves the antinociceptive effect of morphine, suppresses spinal neuroinflammation, and reduces PTX-elevated excitatory Gs-coupled opioid receptors in PTX-treated rats. We suggest that ultra-low dose naloxone might be clinically valuable in pain management.

Governing role of primary afferent drive in increased excitation of spinal nociceptive neurons in a model of sciatic neuropathy

Previously we reported that the cuff model of peripheral neuropathy, in which a 2 mm polyethylene tube is implanted around the sciatic nerve, exhibits aspects of neuropathic pain behavior in rats similar to those in humans and causes robust hyperexcitation of spinal nociceptive dorsal horn neurons. The mechanisms mediating this increased excitation are not known and remain a key unresolved question in models of peripheral neuropathy. In anesthetized adult male Sprague-Dawley rats 2-6 weeks after cuff implantation we found that elevated discharge rate of single lumbar (L(3-4)) wide dynamic range (WDR) neurons persists despite acute spinal transection (T9) but is reversed by local conduction block of the cuff-implanted sciatic nerve; lidocaine applied distal to the cuff (i.e. between the cuff and the cutaneous receptive field) decreased spontaneous baseline discharge of WDR dorsal horn neurons approximately 40% (n=18) and when applied subsequently proximal to the cuff, i.e. between the cuff and the spinal cord, it further reduced spontaneous discharge by approximately 60% (n=19; P<0.05 proximal vs. distal) to a level that was not significantly different from that of naive rats. Furthermore, in cuff-implanted rats WDR neurons (n=5) responded to mechanical cutaneous stimulation with an exaggerated afterdischarge which was reversed entirely by proximal nerve conduction block. These results demonstrate that the hyperexcited state of spinal dorsal horn neurons observed in this model of peripheral neuropathy is not maintained by tonic descending facilitatory mechanisms. Rather, on-going afferent discharges originating from the sciatic nerve distal to, at, and proximal to the cuff maintain the synaptically-mediated gain in discharge of spinal dorsal horn WDR neurons and hyperresponsiveness of these neurons to cutaneous stimulation. Our findings reveal that ectopic afferent activity from multiple regions along peripheral nerves may drive CNS changes and the symptoms of pain associated with peripheral neuropathy.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

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).