Basal and touch-evoked fos-like immunoreactivity during experimental inflammation in the rat

Contribution of µ Opioid Receptor-expressing Dorsal Horn Interneurons to Neuropathic Pain-like Behavior in Mice

BACKGROUND

Intersectional genetics have yielded tremendous advances in our understanding of molecularly identified subpopulations and circuits within the dorsal horn in neuropathic pain. The authors tested the hypothesis that spinal µ opioid receptor-expressing neurons (Oprm1-expressing neurons) contribute to behavioral hypersensitivity and neuronal sensitization in the spared nerve injury model in mice.

METHODS

The authors coupled the use of Oprm1Cre transgenic reporter mice with whole cell patch clamp electrophysiology in lumbar spinal cord slices to evaluate the neuronal activity of Oprm1-expressing neurons in the spared nerve injury model of neuropathic pain. The authors used a chemogenetic approach to activate or inhibit Oprm1-expressing neurons, followed by the assessment of behavioral signs of neuropathic pain.

RESULTS

The authors reveal that spared nerve injury yielded a robust neuroplasticity of Oprm1-expressing neurons. Spared nerve injury reduced Oprm1 gene expression in the dorsal horn as well as the responsiveness of Oprm1-expressing neurons to the selective µ agonist (D-Ala2, N-MePhe4, Gly-ol)-enkephalin (DAMGO). Spared nerve injury sensitized Oprm1-expressing neurons, as reflected by an increase in their intrinsic excitability (rheobase, sham 38.62 ± 25.87 pA [n = 29]; spared nerve injury, 18.33 ± 10.29 pA [n = 29], P = 0.0026) and spontaneous synaptic activity (spontaneous excitatory postsynaptic current frequency in delayed firing neurons: sham, 0.81 ± 0.67 Hz [n = 14]; spared nerve injury, 1.74 ± 1.68 Hz [n = 10], P = 0.0466), and light brush-induced coexpression of the immediate early gene product, Fos in laminae I to II (%Fos/tdTomato+: sham, 0.42 ± 0.57% [n = 3]; spared nerve injury, 28.26 ± 1.92% [n = 3], P = 0.0001). Chemogenetic activation of Oprm1-expressing neurons produced mechanical hypersensitivity in uninjured mice (saline, 2.91 ± 1.08 g [n = 6]; clozapine N-oxide, 0.65 ± 0.34 g [n = 6], P = 0.0006), while chemogenetic inhibition reduced behavioral signs of mechanical hypersensitivity (saline, 0.38 ± 0.37 g [n = 6]; clozapine N-oxide, 1.05 ± 0.42 g [n = 6], P = 0.0052) and cold hypersensitivity (saline, 6.89 ± 0.88 s [n = 5] vs. clozapine N-oxide, 2.31 ± 0.52 s [n = 5], P = 0.0017).

CONCLUSIONS

The authors conclude that nerve injury sensitizes pronociceptive µ opioid receptor-expressing neurons in mouse dorsal horn. Nonopioid strategies to inhibit these interneurons might yield new treatments for neuropathic pain.

EDITOR’S PERSPECTIVE

Peripheral and central nervous system alterations in a rat model of inflammatory arthritis

It is consistently reported that in inflammatory arthritis (IA), pain may continue despite well-controlled inflammation, most likely due to interactions between joint pathology and pain pathway alterations. Nervous system alterations have been described, but much remains to be understood about neuronal and central non-neuronal changes in IA. Using a rat model of IA induced by intra-articular complete Freund's adjuvant injection, this study includes a thorough characterization of joint pathology and objectives to identify peripheral innervation changes and alterations in the spinal dorsal horn (DH) that could alter DH excitatory balancing. Male and female rats displayed long-lasting pain-related behavior, but, in agreement with our previous studies, other pathological alterations emerged only at later times. Cartilage vascularization, thinning, and decreased proteoglycan content were not detectable in the ipsilateral cartilage until 4 weeks after complete Freund's adjuvant. Sympathetic and peptidergic nociceptive fibers invaded the ipsilateral cartilage alongside blood vessels, complex innervation changes were observed in the surrounding skin, and ipsilateral nerve growth factor protein expression was increased. In the DH, we examined innervation by peptidergic and nonpeptidergic nociceptors, inhibitory terminal density, the KCl cotransporter KCC2, microgliosis, and astrocytosis. Here, we detected the presence of microgliosis and, interestingly, an apparent loss of inhibitory terminals and decreased expression of KCC2. In conclusion, we found evidence of anatomical, inflammatory, and neuronal alterations in the peripheral and central nervous systems in a model of IA. Together, these suggest that there may be a shift in the balance between incoming and outgoing excitation, and modulatory inhibitory tone in the DH.

Cyclic nucleotide signaling in sensory neuron hyperexcitability and chronic pain after nerve injury

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Activation of cAMP-PKA and cGMP-PKG pathways contributes to injury-induced sensory neuron hyperexcitability.

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Activation of cAMP and cGMP contributes to the development of bone cancer pain.

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PAR2 activation mediates injury-induced cAMP-dependent sensory neuron hyperexcitability.

The cyclic nucleotide signaling, including cAMP-PKA and cGMP-PKG pathways, has been well known to play critical roles in regulating cellular growth, metabolism and many other intracellular processes. In recent years, more and more studies have uncovered the roles of cAMP and cGMP in the nervous system. The cAMP and cGMP signaling mediates chronic pain induced by different forms of injury and stress. Here we summarize the roles of cAMP-PKA and cGMP-PKG signaling pathways in the pathogenesis of chronic pain after nerve injury. In addition, acute dissociation and chronic compression of the dorsal root ganglion (DRG) neurons, respectively, leads to neural hyperexcitability possibly through PAR2 activation-dependent activation of cAMP-PKA pathway. Clinically, radiotherapy can effectively alleviate bone cancer pain at least partly through inhibiting the cancer cell-induced activation of cAMP-PKA pathway. Roles of cyclic nucleotide signaling in neuropathic and inflammatory pain are also seen in many other animal models and are involved in many pro-nociceptive mechanisms including the activation of hyperpolarization-activated cyclic nucleotide (HCN)-modulated ion channels and the exchange proteins directly activated by cAMP (EPAC). Further understanding the roles of cAMP and cGMP signaling in the pathogenesis of chronic pain is theoretically significant and clinically valuable for treatment of chronic pain.

Different phosphoinositide 3-kinase isoforms mediate carrageenan nociception and inflammation

PI3K-α, -δ, and -γ all participate in inflammation induction. Antagonism of only PI3K-γ blocks nociception, which is indicative of a role for this isoform within the afferent.

Phosphoinositide 3-kinases (PI3Ks) participate in signal transduction cascades that can directly activate and sensitize nociceptors and enhance pain transmission. They also play essential roles in chemotaxis and immune cell infiltration leading to inflammation. We wished to determine which PI3K isoforms were involved in each of these processes. Lightly anesthetized rats (isoflurane) were injected subcutaneously with carrageenan in their hind paws. This was preceded by a local injection of 1% DMSO vehicle or an isoform-specific antagonist to PI3K-α (compound 15-e), -β (TGX221), -δ (Cal-101), or -γ (AS252424). We measured changes in the mechanical pain threshold and spinal c-Fos expression (4 hours after injection) as indices of nociception. Paw volume, plasma extravasation (Evans blue, 0.3 hours after injection), and neutrophil (myeloperoxidase; 1 hour after injection) and macrophage (CD11b+; 4 hour after injection) infiltration into paw tissue were the measured inflammation endpoints. Only PI3K-γ antagonist before treatment reduced the carrageenan-induced pain behavior and spinal expression of c-Fos (P ≤ 0.01). In contrast, pretreatment with PI3K-α, -δ, and-γ antagonists reduced early indices of inflammation. Plasma extravasation PI3K-α (P ≤ 0.05), -δ (P ≤ 0.05), and -γ (P ≤ 0.01), early (0-2 hour) edema -α (P ≤ 0.05), -δ (P ≤ 0.001), and -γ (P ≤ 0.05), and neutrophil infiltration (all P ≤ 0.001) were all reduced compared to vehicle pretreatment. Later (2-4 hour), edema and macrophage infiltration (P ≤ 0.05) were reduced by only the PI3K-δ and -γ isoform antagonists, with the PI3K-δ antagonist having a greater effect on edema. PI3K-β antagonism was ineffective in all paradigms. These data indicate that pain and clinical inflammation are pharmacologically separable and may help to explain clinical conditions in which inflammation naturally wanes or goes into remission, but pain continues unabated.

Central Sensitization and Neuropathic Features of Ongoing Pain in a Rat Model of Advanced Osteoarthritis

Osteoarthritis (OA) pain is most commonly characterized by movement-triggered joint pain. However, in advanced disease, OA pain becomes persistent, ongoing and resistant to treatment with nonsteroidal anti-inflammatory drugs (NSAIDs). The mechanisms underlying ongoing pain in advanced OA are poorly understood. We recently showed that intra-articular (i.a.) injection of monosodium iodoacetate (MIA) into the rat knee joint produces concentration-dependent outcomes. Thus, a low dose of i.a. MIA produces NSAID-sensitive weight asymmetry without evidence of ongoing pain and a high i.a. MIA dose produces weight asymmetry and NSAID-resistant ongoing pain. In the present study, palpation of the ipsilateral hind limb of rats treated 14 days previously with high, but not low, doses of i.a. MIA produced expression of the early oncogene, FOS, in the spinal dorsal horn. Inactivation of descending pain facilitatory pathways using a microinjection of lidocaine within the rostral ventromedial medulla induced conditioned place preference selectively in rats treated with the high dose of MIA. Conditioned place preference to intra-articular lidocaine was blocked by pretreatment with duloxetine (30 mg/kg, intraperitoneally at -30 minutes). These observations are consistent with the likelihood of a neuropathic component of OA that elicits ongoing, NSAID-resistant pain and central sensitization that is mediated, in part, by descending modulatory mechanisms. This model provides a basis for exploration of underlying mechanisms promoting neuropathic components of OA pain and for the identification of mechanisms that might guide drug discovery for treatment of advanced OA pain without the need for joint replacement.

PERSPECTIVE

Difficulty in managing advanced OA pain often results in joint replacement therapy in these patients. Improved understanding of mechanisms driving NSAID-resistant ongoing OA pain might facilitate development of alternatives to joint replacement therapy. Our findings suggest that central sensitization and neuropathic features contribute to NSAID-resistant ongoing OA joint pain.

Radiotherapy Suppresses Bone Cancer Pain through Inhibiting Activation of cAMP Signaling in Rat Dorsal Root Ganglion and Spinal Cord

Radiotherapy is one of the major clinical approaches for treatment of bone cancer pain. Activation of cAMP-PKA signaling pathway plays important roles in bone cancer pain. Here, we examined the effects of radiotherapy on bone cancer pain and accompanying abnormal activation of cAMP-PKA signaling. Female Sprague-Dawley rats were used and received tumor cell implantation (TCI) in rat tibia (TCI cancer pain model). Some of the rats that previously received TCI treatment were treated with X-ray radiation (radiotherapy). Thermal hyperalgesia and mechanical allodynia were measured and used for evaluating level of pain caused by TCI treatment. PKA mRNA expression in dorsal root ganglion (DRG) was detected by RT-PCR. Concentrations of cAMP, IL-1β, and TNF-α as well as PKA activity in DRG and the spinal cord were measured by ELISA. The results showed that radiotherapy significantly suppressed TCI-induced thermal hyperalgesia and mechanical allodynia. The level of PKA mRNA in DRG, cAMP concentration and PKA activity in DRG and in the spinal cord, and concentrations of IL-1β and TNF-α in the spinal cord were significantly reduced by radiotherapy. In addition, radiotherapy also reduced TCI-induced bone loss. These findings suggest that radiotherapy may suppress bone cancer pain through inhibition of activation of cAMP-PKA signaling pathway in DRG and the spinal cord.

Differential contributions of A- and C-nociceptors to primary and secondary inflammatory hypersensitivity in the rat

There is sensitization to thermal A-nociceptor activation in arthritic secondary hyperalgesia, with enhanced activation of spinal lamina I neurons.

Primary hyperalgesia is characterized by increased responsiveness to both heat and mechanical stimulation in the area of injury. By contrast, secondary hyperalgesia is generally associated with increased responses to mechanical but not heat stimuli. We tested the hypothesis that sensitization in secondary hyperalgesia is dependent on the class of peripheral nociceptor (C- or A-nociceptor) rather than the modality of stimulation (mechanical vs heat). A- and C-nociceptors were selectively activated using contact heat ramps applied to the hind paw dorsum in animals with hind paw inflammation (primary hyperalgesia) and knee inflammatory arthritis (secondary hyperalgesia). Sensitization to A- and C-nociceptor activation in primary and secondary hyperalgesia was assessed by reflex withdrawal thresholds and by Fos immunocytochemistry in the dorsal horn of the spinal cord, as an index of neuronal activation. In primary hyperalgesia, only C-nociceptor-evoked withdrawal reflexes were sensitized. This was associated with increased spinal lamina I neuronal activation to both A- and C-nociceptor activation. Fos-like immunoreactivity (FLI) was unchanged in other dorsal horn laminae. In secondary hyperalgesia, only A-nociceptor-evoked withdrawal reflexes were sensitized, and FLI was increased in both superficial and deep dorsal laminae. Neurons in the superficial dorsal horn receive and process nociceptor inputs from the area of primary hyperalgesia, resulting in functional sensitization to C-nociceptive inputs. In inflammatory arthritis, secondary hyperalgesia is evoked by A-nociceptor thermal stimulation, suggesting that secondary hyperalgesia is A-nociceptor, rather than stimulus modality (mechanical vs thermal), dependent. Fos-like immunoreactivity evoked by A-nociceptor stimulation in secondary hyperalgesia suggests that the sensitization is underpinned by spinal neuronal sensitization in laminae I and IV/V.

Ultraviolet Radiation on the Skin: A Painful Experience?

Excessive exposure of skin to ultraviolet radiation (UVR) has dramatic clinical effects in humans, and it is a significant public health concern. Discomfort and sensory changes caused by skin sunburn are the main common features experienced by many of us, a phenomena triggered by the combination of long and short wavelengths radiation (UVA and UVB, respectively). Although the biological processes underlying UVR exposure are not fully understood, in the last few years many studies have made significant progress in characterizing sunburn at the cellular and molecular levels, making use of both humans and laboratory animal models. Here we review and reason that UVR can be used as an excellent model of sensitization and inflammation for pain research. UVR, particularly UVB, produces a controllable and sterile inflammation that causes a robust dose‐dependent hypersensitivity with minimal confounding effects. Importantly, we show that UVR animal models precisely recapitulate the sensory, cellular, and molecular changes observed in human skin, giving it great confidence as a translational model. Furthermore, in this article, we give an overview of the pharmacology underlying UVB inflammation, the latest advances in the field, and potential new targets for inflammatory pain.

Activation of spinal phosphatidylinositol 3-kinase/protein kinase B mediates pain behavior induced by plantar incision in mice

The etiology of postoperative pain may be different from antigen-induced inflammatory pain and neuropathic pain. However, central neural plasticity plays a key role in incision pain. It is also known that phosphatidylinositol 3-kinase (PI3K) and protein kinase B/Akt (PKB/Akt) are widely expressed in laminae I-IV of the spinal horn and play a critical role in spinal central sensitization. In the present study, we explored the role of PI3K and Akt in incision pain behaviors. Plantar incision induced a time-dependent activation of spinal PI3K-p110γ and Akt, while activated Akt and PI3K-p110γ were localized in spinal neurons or microglias, but not in astrocytes. Pre-treatment with PI3K inhibitors, wortmannin or LY294002 prevented the activation of Akt brought on by plantar incision in a dose-dependent manner. In addition, inhibition of spinal PI3K signaling pathway prevented pain behaviors (dose-dependent) and spinal Fos protein expression caused by plantar incision. These data demonstrated that PI3K signaling mediated pain behaviors caused by plantar incision in mice.

Loss of neurons in rostral ventromedial medulla that express neurokinin-1 receptors decreases the development of hyperalgesia

It is well known that neurons in the rostral ventromedial medulla (RVM) are involved in descending modulation of nociceptive transmission in the spinal cord. It has been shown that activation of neurokinin-1 receptors (NK-1Rs) in the RVM, which are presumably located on pain facilitating ON cells, produces hyperalgesia whereas blockade of NK-1Rs attenuates hyperalgesia. To obtain a better understanding of the functions of NK-1R expressing neurons in the RVM, we selectively ablated these neurons by injecting the stable analog of substance P (SP), Sar(9),Met(O2)(11)-Substance P, conjugated to the ribosomal toxin saporin (SSP-SAP) into the RVM. Rats received injections of SSP-SAP (1 μM) or an equal volume of 1 μM of saporin conjugated to artificial peptide (Blank-SAP). Stereological analysis of NK-1R- and NeuN-labeled neurons in the RVM was determined 21-24 days after treatment. Withdrawal responses to mechanical and heat stimuli applied to the plantar hindpaw were determined 5-28 days after treatment. Withdrawal responses were also determined before and after intraplantar injection of capsaicin (acute hyperalgesia) or complete Freund's adjuvant (CFA) (prolonged hyperalgesia). The proportion of NK-1R-labeled neurons in the RVM was 8.8 ± 1.3% in naïve rats and 8.1 ± 0.8% in rats treated with Blank-SAP. However, injection of SSP-SAP into the RVM resulted in a 90% decrease in NK-1R-labeled neurons. SSP-SAP did not alter withdrawal responses to mechanical or heat stimuli under normal conditions, and did not alter analgesia produced by morphine administered into the RVM. In contrast, the duration of nocifensive behaviors produced by capsaicin and mechanical and heat hyperalgesia produced by capsaicin and CFA were decreased in rats pretreated with SSP-SAP as compared to those that received Blank-SAP. These data support our earlier studies using NK-1R antagonists in the RVM and demonstrate that RVM neurons that possess the NK-1R do not play a significant role in modulating acute pain or morphine analgesia, but rather are involved in pain facilitation and the development and maintenance of hyperalgesia.

Dynamic genotype-selective "phenotypic switching" of CGRP expression contributes to differential neuropathic pain phenotype

Using a genetic model we demonstrate the role played by "phenotypic switching" of calcitonin gene related peptide (CGRP) expression in axotomized large Aβ afferents in the development of neuropathic pain behavior in rats. After nerve injury both substance P and CGRP are upregulated in Aβ afferents in the corresponding DRGs. It has been proposed that intraspinal release of these neurotransmitters upon gentle stroking of skin drives ascending pain signaling pathways resulting in tactile allodynia. We reported previously that in rat lines genetically selected for high (HA) vs. low (LA) pain phenotype, SP is upregulated equally in both strains, but that CGRP is upregulated exclusively in the pain prone HA line (Nitzan-Luques et al., 2011). This implicates CGRP as the principal driver of tactile allodynia. Here we confirm this conclusion by showing: 1) that the time of emergence of CGRP-IR in DRG Aβ neurons and their central terminals in HA rats matches that of pain behavior, 2) that following spinal nerve lesion (SNL) selective activation of low threshold afferents indeed drives postsynaptic pain-signaling neurons and induces central sensitization in HA rats, as monitored using c-Fos as a marker. These changes are much less prominent in LA rats, 3) that intrathecal (i.t.) administration of CGRP induces tactile allodynia in naïve rats and 4) that i.t. administration of the CGRP-receptor antagonist BIBN4096BS (Olcegepant) attenuates SNL-evoked tactile allodynia, without blocking baseline nociception. Together, these observations support the hypothesis that genotype-selective phenotypic switching of CGRP expression in Aβ afferents following nerve injury is a fundamental mechanism of neuropathic tactile allodynia.

Descending facilitation maintains long-term spontaneous neuropathic pain

Neuropathic pain is frequently characterized by spontaneous pain (ie, pain at rest) and, in some cases, by cold- and touch-induced allodynia. Mechanisms underlying the chronicity of neuropathic pain are not well understood. Rats received spinal nerve ligation (SNL) and were monitored for tactile and thermal thresholds. While heat hypersensitivity returned to baseline levels within approximately 35 to 40 days, tactile hypersensitivity was still present at 580 days after SNL. Tactile hypersensitivity at post-SNL day 60 (D60) was reversed by microinjection of 1) lidocaine; 2) a cholecystokinin 2 receptor antagonist into the rostral ventromedial medulla; or 3) dorsolateral funiculus lesion. Rostral ventromedial medulla lidocaine at D60 or spinal ondansetron, a 5-hydroxytryptamine 3 antagonist, at post-SNL D42 produced conditioned place preference selectively in SNL-treated rats, suggesting long-lasting spontaneous pain. Touch-induced FOS was increased in the spinal dorsal horn of SNL rats at D60 and prevented by prior dorsolateral funiculus lesion, suggesting that long-lasting tactile hypersensitivity depends upon spinal sensitization, which is mediated in part by descending facilitation, in spite of resolution of heat hypersensitivity.

PERSPECTIVE

These data suggest that spontaneous pain is present for an extended period of time and, consistent with likely actions of clinically effective drugs, is maintained by descending facilitation.

Sodium channel Na(v)1.7 is essential for lowering heat pain threshold after burn injury

Marked hypersensitivity to heat and mechanical (pressure) stimuli develop after a burn injury, but the neural mechanisms underlying these effects are poorly understood. In this study, we establish a new mouse model of focal second-degree burn injury to investigate the molecular and cellular basis for burn injury-induced pain. This model features robust injury-induced behavioral effects and tissue-specific altered cytokine profile, but absence of glial activation in spinal dorsal horn. Three voltage-gated sodium channels, Na(v)1.7, Na(v)1.8, and Na(v)1.9, are preferentially expressed in peripheral somatosensory neurons of the dorsal root ganglia (DRGs) and have been implicated in injury-induced neuronal hyperexcitability. Using knock-out mice, we provide evidence that Na(v)1.7 selectively contributes to burn-induced hypersensitivity to heat, but not mechanical, stimuli. After burn model injury, wild-type mice display increased sensitivity to heat stimuli, and a normally non-noxious warm stimulus induces activity-dependent Fos expression in spinal dorsal horn neurons. Strikingly, both effects are absent in Na(v)1.7 conditional knock-out (cKO) mice. Furthermore, burn injury increases density and shifts activation of tetrodotoxin-sensitive currents in a hyperpolarized direction, both pro-excitatory properties, in DRG neurons from wild-type but not Na(v)1.7 cKO mice. We propose that, in sensory neurons damaged by burn injury to the hindpaw, Na(v)1.7 currents contribute to the hyperexcitability of sensory neurons, their communication with postsynaptic spinal pain pathways, and behavioral thresholds to heat stimuli. Our results offer insights into the molecular and cellular mechanisms of modality-specific pain signaling, and suggest Na(v)1.7-blocking drugs may be effective in burn patients.

Electroacupuncture Confers Antinociceptive Effects via Inhibition of Glutamate Transporter Downregulation in Complete Freund's Adjuvant-Injected Rats

When we evaluated changes of glial fibrillary acidic protein (GFAP) and two glutamate transporter (GTs) by immunohistochemistry, expression of GFAP showed a significant increase in complete Freund's adjuvant (CFA)-injected rats; however, this expression was strongly inhibited by electroacupuncture (EA) stimulation. Robust downregulation of glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) was observed in CFA-injected rats; however, EA stimulation resulted in recovery of this expression. Double-labeling staining showed co-localization of a large proportion of GLAST or GLT-1 with GFAP. Using Western blot, we confirmed protein expression of two GTs, but no differences in the mRNA content of these GTs were observed. Because EA treatment resulted in strong inhibition of CFA-induced proteasome activities, we examined the question of whether thermal sensitivities and GTs expression could be regulated by proteasome inhibitor MG132. CFA-injected rats co-treated with EA and MG132 showed a significantly longer thermal sensitivity, compared with CFA-injected rats with or without MG132. Both EA and MG132 blocked CFA-induced GLAST and GLT-1 downregulation within the spinal cord. These results provide evidence for involvement of GLAST and GLT-1 in response to activation of spinal astrocytes in an EA antinociceptive effect. Antinociceptive effect of EA may be induced via proteasome-mediated regulation of spinal GTs.

A comparative study of the effect of 17β-estradiol and estriol on peripheral pain behavior in rats

Although estradiol has been reported to influence pain sensitivity, the role of estriol (an estradiol metabolite and another widely used female sex hormone) remains unclear. In this study, pain behavior tests, whole-cell patch clamp recording and Western blotting were used to determine whether estriol plays a role in pain signal transduction and transmission. Either systemic or local administration of 17β-estradiol produced a significant rise of mechanical pain threshold, while estriol lacked this effect in normal and ovariectomized (OVX) rats following estriol replacement. Local administration of 17β-estradiol or estriol significantly decreased ATP-induced spontaneous hind-paw withdrawal duration (PWD), which was blocked by an estrogen receptor antagonist, ICI 182, 780. However, systemic application of estriol in normal or OVX rats lacked this similar effect. In cultured dorsal root ganglion neurons, estriol attenuated α,β-methylene ATP-induced transient currents which were blocked by ICI 182, 780. In complete Freund's adjuvant treated (CFA) rats, systemic application of 17β-estradiol or estriol decreased the mechanical pain threshold significantly, but did not change the inflammatory process. Similar effects were observed after estriol replacement in OVX rats. The expression of c-fos in lumbosacral spinal cord dorsal horn (SCDH) was increased significantly by administration of 17β-estradiol but not estriol, and not by estriol replacement in OVX rats. These results suggest that 17β-estradiol but not estriol plays an anti-hyperalgesic role in physiological pain. However, both peripheral 17β-estradiol and estriol play anti-hyperalgesic roles in ATP-induced inflammatory pain. Systemic application of estriol as well as 17β-estradiol plays hyperalgesic roles in CFA-induced chronic pain.

Differential release of neurotransmitters from superficial and deep layers of the dorsal horn in response to acute noxious stimulation and inflammation of the rat paw

Experimental evidence suggests that release of neurotransmitters in response to acute noxious stimulation and inflammation can differ in superficial and deeper dorsal horn (DH) laminae. Using two different microdialysis probes, we studied changes in levels of glutamate, aspartate, arginine and GABA in dialysates collected from the surface of the spinal cord and within the DH induced by pinching the paw or paw inflammation. In penthotal anaesthetized rats, a flexible microdialysis probe was placed on the dorsal surface of the L4-L5 or L6-S2 spinal segments. In other rats, a rigid microdialysis probe was implanted within the DH of the same segments. Samples were collected every minute before, during and after pinching the hind paw (acute pain), and every half an hour after injecting either carrageenan or saline into the same paw (inflammation-induced pain). Amino acids were measured by capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIFD). Pinching the paw induced a significant but short lasting increase in extracellular glutamate and aspartate in dialysates from the surface of the DH. Carrageenan, but not saline, injected into the paw significantly increased concentrations of glutamate, aspartate and arginine both on the surface and within the DH of L4-L5 and also within the DH of the L6-S2 segments. The GABA level was significantly increased following carrageenan only within the DH. The maximum increase on the surface was detected 60-120 min after the onset of inflammation whereas the response within the DH reached a maximum between 150 and 180 min after carrageenan. These results indicate that unlike acute mechanical noxious stimulation which enhances amino acid neurotransmitters in surface dialysate, inflammation induced neurotransmitter release in all layers of the DH suggesting sensitization of the DH.

Light touch induces ERK activation in superficial dorsal horn neurons after inflammation: involvement of spinal astrocytes and JNK signaling in touch-evoked central sensitization and mechanical allodynia

Activation of extracellular signal-regulated kinase (ERK) in spinal cord neurons could serve as a marker for sensitization of dorsal horn neurons in persistent pain. ERK is normally activated by high-threshold noxious stimuli. We investigated how low-threshold mechanical stimuli could activate ERK after complete Freund's adjuvant (CFA)-induced inflammation. Unilateral injection of CFA induced ipsilateral heat hyperalgesia and bilateral mechanical allodynia. CFA-induced ERK activation in ipsilateral dorsal horn neurons declined after 2 days. Interestingly, low-threshold mechanical stimulation given by light touch either on the inflamed paw or the contralateral non-inflamed paw dramatically increased ERK phosphorylation in the dorsal horn ipsilateral to touch stimulation. Notably, light touch induced ERK phosphorylation mainly in superficial neurons in laminae I-IIo. Intrathecal administration of the astroglial toxin L-α-aminoadipate on post-CFA day 2 reversed CFA-induced bilateral mechanical allodynia but not heat hyperalgesia. Furthermore, L-α-aminoadipate, the glial inhibitor fluorocitrate, and a peptide inhibitor of c-Jun N-terminal Kinase all reduced light touch-evoked ERK activation ipsilateral to touch. Collectively, these data suggest that (i) ERK can be activated in superficial dorsal horn neurons by low-threshold mechanical stimulation under pathological condition and (ii) ERK activation by light touch is associated with mechanical allodynia and requires an astrocyte network.

Plasticity of hyperpolarization-activated and cyclic nucleotid-gated cation channel subunit 2 expression in the spinal dorsal horn in inflammatory pain

A great deal of experimental evidence has already been accumulated that hyperpolarization-activated and cyclic nucleotide-gated cation channels (HCN) expressed by peripheral nerve fibers contribute to the initiation of nerve activities leading to pain. Complementing these findings, we have recently demonstrated that HCN subunit 2 (HCN2) channel protein is also widely expressed by axon terminals of substance P (SP)-containing peptidergic nociceptive primary afferents in laminae I-IIo of the spinal dorsal horn, and postulated that they may play a role in spinal pain processing. In the present study, we investigated how the expression of HCN2 ion channels in the spinal dorsal horn may change in inflammatory pain evoked by unilateral injection of complete Freund's adjuvant (CFA) into the hind paw of rats. We found that 3 days after CFA injection, when the nociceptive responsiveness of the inflamed hind paw had substantially increased, the numbers of HCN2-immunolabeled axon terminals were also significantly augmented in laminae I-IIo of the spinal dorsal horn ipsilateral to the site of CFA injection. The elevation of HCN2 immunoreactivity was paralleled by an increase in SP immunoreactivity. In addition, similarly to control animals, the co-localization between HCN2 and SP immunoreactivity was remarkably high, suggesting that central axon terminals of nociceptive primary afferents that increased their SP expression in response to CFA injection into the hind paw also increased their HCN2 expression. The results indicate that HCN2 ion channel mechanisms may play a role in SP-mediated spinal pain processing not only in naive animals but also in chronic inflammatory pain.

Differential distribution of activated spinal neurons containing glycine and/or GABA and expressing c-fos in acute and chronic pain models

The inhibitory transmitters GABA and glycine play an important role in modulating pain transmission, both in normal and in pathological situations. In the present study we have combined in situ hybridization for identifying spinal neurons that use the transmitter(s) glycine and/or GABA (Gly/GABA neurons) with immunohistochemistry for c-fos, a marker for neuronal activation. This procedure was used with acute pain models induced by the injection of capsaicin or formalin; and chronic pain models using Complete Freund's Adjuvant (CFA, chronic inflammation), and the spared nerve injury (SNI) model (neuropathic pain). In all models Gly/GABA neurons were activated as indicated by their expression of c-fos. The pattern of Gly/GABA neuronal activation was different for every model, both anatomically and quantitatively. However, the averaged percentage of activated neurons that were Gly/GABA in the chronic phase (≥20h survival, 46%) was significantly higher than in the acute phase (≤2h survival, 34%). In addition, the total numbers of activated Gly/GABA neurons were similar in both phases, showing that the activation of non-Gly/GABA (presumed excitatory) neurons in the chronic phase decreased. Finally, morphine application equally decreased the total number of activated neurons and activated Gly/GABA neurons. This showed that morphine did not specifically activate Gly/GABA neurons to achieve nociceptive inhibition. The present study shows an increased activity of Gly/GABA neurons in acute and chronic models. This mechanism, together with mechanisms that antagonize the effects of GABA and glycine at the receptor level, may determine the sensitivity of our pain system during health and disease.

A synthetic cell-penetrating peptide antagonizing TrkA function suppresses neuropathic pain in mice

Nerve growth factor (NGF) and its high-affinity receptor, TrkA, are one of the targets in the production of new drugs for the treatment of neuropathic pain. NGF contributes to both the initiation and maintenance of sensory abnormalities after peripheral nerve injury. This study examined the effects of IPTRK3, a new synthetic cell-penetrating peptide that antagonizes TrkA function, on neuropathic pain in mice. Partial sciatic nerve ligation (PSNL) was used to generate neuropathic pain, and we injected IPTRK3 (2 or 10 mg/kg) intraperitoneally on day 7 after PSNL. Effects of the peptide on hyperalgesia, allodynia, and expression of Fos in the spinal cord were examined. Single administration of the peptide on day 7 significantly suppressed both thermal hyperalgesia and mechanical allodynia. Gentle touch stimuli-evoked Fos expression in the lumbar spinal cord was also significantly reduced. Intraperitoneal injection of a cell-penetrating peptide antagonizing TrkA function appears effective for treatment of neuropathic pain in a mouse pain model.

Treatment with ketanserin produces opioid-mediated hypoalgesia in the late phase of carrageenan-induced inflammatory hyperalgesia in rats

Both pro-nociceptive and antinociceptive mediators are released in the tissues during inflammation. Balance of these two types of mediators determines the induction and maintenance of pain or hypernociception. This study was designed to explore whether 5-HT(2A) receptors in the periphery contributed to the maintenance of carrageenan-evoked hyperalgesia. Intraplantar (i.pl.) injection of carrageenan evoked hyperalgesia detected by noxious heat stimulus. The 5-HT(2A) receptor antagonist ketanserin administered i.pl. 1 h after carrageenan dose-dependently (2-20 microg) prolonged paw withdrawal latency (PWL) during the late phase (24 h) of carrageenan-evoked inflammation. Following treatments with carrageenan and ketanserin, i.pl. injection of formalin (1%) produced significantly fewer nocifensive behaviors and expression of c-Fos protein in the spinal dorsal horn, confirming the hypoalgesic status in the inflamed site. However, injection of ketanserin in naive site failed to produce hypoalgesia. The hypoalgesia was completely abolished by local or systemic injection of naloxone methiodide. The present study suggests that 5-HT(2A) receptors were involved in the maintenance of inflammatory pain, and that 5-HT suppressed inflammation-associated endogenous opioid analgesia contributing to its pro-nociceptive actions in the periphery. It implied a possible therapeutic benefit of blockade of local 5-HT(2A) receptors in the treatment of inflammatory pain.

Effects of systemic bicuculline or morphine on formalin-evoked pain-related behaviour and c-Fos expression in trigeminal nuclei after formalin injection into the lip or tongue in rats

This study examined differences in nociceptive responses between lip and tongue. Formalin-induced pain-related behaviour and c-Fos expression in the trigeminal caudal nucleus (Vc) with/without systemic preadministration of a gamma-aminobutyric acid (GABA) type A receptor antagonist, bicuculline (2 mg/kg, i.p., 10 min before formalin injection) or a micro-opioid receptor agonist, morphine (3 mg/kg, i.p., 10 min before formalin injection) have been studied. Formalin injection into the upper lip induced an immediate pain-related behaviour, mostly face-rubbing behaviour, for 15 min (phase 1, mean +/- SEM/5 min, 81.2 +/- 30.1), followed by a more increased activity for 15 min (phase 2, 205.4 +/- 43.6) and a decline to baseline for next 15 min (phase 3, 63.9 +/- 28.0). Formalin injection into the tongue induced similar amount of pain-related behaviour at phase 1 (67.9 +/- 16.7), followed by similar activity at phase 2 (48.6 +/- 6.2), and lesser behaviour at phase 3 (20.4 +/- 7.6). The behaviour at phase 2 decreased following preadministration of bicuculline or morphine when formalin was injected into the lip (b, 62.5 +/- 14.5; m, 95.8 +/- 10.0) but not into the tongue (b, 31.0 +/- 9.2; m, 77.4 +/- 27.0). A considerable numbers of c-Fos-immunoreactive (IR) cells were induced in the caudal and inter-medio-lateral center of superficial layers of the Vc (VcI/II; mean +/- SEM/section = 225.8 +/- 12.9) and magnocellular zone of the Vc (VcIII/IV; 67.1 +/- 4.7) 2 h after formalin injection into the lip. Much smaller numbers of c-Fos-IR cells were induced in the rostral and dorso-medial one-fourth of the VcI/II (72.6 +/- 3.7) and VcIII/IV (55.6 +/- 6.6) after formalin injection into the tongue. Following preadministration with systemic bicuculline or morphine, the formalin-induced c-Fos-IR cells were decreased more in the VcI/II when formalin was injected into the lip (VcI/II, 102.4 +/- 8.0; VcIII/IV, 32.8 +/- 1.4) than into the tongue (VcI/II, 49.5 +/- 8.1; VcIII/IV, 31.7 +/- 5.3). These results show that the lip is more sensitive to formalin-induced noxious stimulation and regulated more through GABA(A) and micro-opioid receptors than the tongue.

c-Fos and pERK, which is a better marker for neuronal activation and central sensitization after noxious stimulation and tissue injury?

c-Fos, the protein of the protooncogene c-fos, has been extensively used as a marker for the activation of nociceptive neurons in the spinal cord for more than twenty years since Hunt et al. first reported that peripheral noxious stimulation to a hind paw of rats leads to a marked induction of c-Fos in superficial and deep dorsal horn neurons in 1987. In 1999, Ji et al. reported that phosphorylated extracellular signal-regulated kinase (pERK) is specifically induced by noxious stimulation in superficial dorsal horn neurons. Accumulating evidence indicates that pERK induction or ERK activation in dorsal horn neurons is essential for the development of central sensitization, increased sensitivity of dorsal horn neurons that is responsible for the generation of persistent pain. Further, molecular mechanisms underlying ERK-mediated central sensitization have been revealed. In contrast, direct evidence for c-Fos-mediated central sensitization is not sufficient. After a noxious stimulus (e.g., capsaicin injection) or tissue injury, c-Fos begins to be induced after 30-60 minutes, whereas pERK can be induced within a minute, which can correlate well with the development of pain hypersensitivity. While c-Fos is often induced in the nuclei of neurons, pERK can be induced in different subcellular structures of neurons such as nuclei, cytoplasma, axons, and dendrites. pERK can even be induced in spinal cord microglia and astrocytes after nerve injury. In summary, both c-Fos and pERK can be used as markers for neuronal activation following noxious stimulation and tissue injury, but pERK is much more dynamic and appears to be a better marker for central sensitization.

Noradrenergic neurons in the locus coeruleus contribute to neuropathic pain

Current theories of neuropathic hypersensitivity include an imbalance of supraspinal inhibition and facilitation. Our overall hypothesis is that the locus coeruleus (LC), classically interpreted as a source of pain inhibition, may paradoxically result in facilitation after tibial and common peroneal nerve transection (spared sural nerve injury--SNI). We first tested the hypothesis that non-noxious tactile hind paw stimulation of the spared sural innervation territory increases neuronal activity in the LC in male rats. We observed a bilateral increase in the stimulus-evoked expression of transcription factors Fos and phosphorylated CREB (pCREB) in LC after SNI but not sham surgery; these markers of neuronal activity correlated with the intensity of tactile allodynia. We next tested the hypothesis that noradrenergic neurons contribute to the development of neuropathic pain. To selectively destroy these neurons, we delivered antidopamine-beta-hydroxylase saporin (anti-DbetaH-saporin) into the i.c.v. space 2 weeks before SNI. We found that anti-DbetaH-saporin, but not an IgG-saporin control, reduced behavioral signs of tactile allodynia, mechanical hyperalgesia, and cold allodynia from 3 to 28 days. after SNI. Our final experiment tested the hypothesis that the LC contributes to the maintenance of neuropathic pain. We performed SNI, waited 2 weeks for maximal allodynia and hyperalgesia to develop, and then administered the local anesthetic lidocaine (4%) directly into the LC parenchyma. Lidocaine reduced all behavioral signs of neuropathic pain in a reversible manner, suggesting that the LC contributes to pain facilitation. We conclude that, in addition to its well-known inhibition of acute and inflammatory pain, the LC facilitates the development and maintenance of neuropathic pain in the SNI model. Further studies are needed to determine the facilitatory pathways emanating from the LC.

Spatiotemporal characteristics of pain-associated neuronal activities in primary somatosensory cortex induced by peripheral persistent nociception

The primary somatosensory cortex (S1 area) is one of the key brain structures for central processing of somatic noxious information to produce pain perception. However, so far, the spatiotemporal characteristics of neuronal activities associated with peripheral persistent nociception have rarely been studied. In the present report, we used c-Fos as a neuronal marker to analyze spatial and temporal patterns of pain-related neuronal activities within the S1 area of rats subjecting to subcutaneous (s.c.) injection of bee venom (BV) solution, a well-established animal model of persistent pain. In naïve and saline-treated rats, c-Fos-labeled neurons were diffusely and sparsely distributed in the hindlimb region of S1 area. Following s.c. BV injection, c-Fos-labeled neurons became densely increased in superficial layers (II-III) and less increased in deep layers (IV-VI). The mean number of c-Fos positive neurons in the layers II-III began to increase at 1h and reached a peak at 2h after BV treatment that was followed by a gradual decrease afterward. The time course of c-Fos expression in the layers IV-VI was in parallel with that of the superficial layers, but with a much lower density and magnitude. The present results demonstrated that BV-induced peripheral persistent nociception could evoke increased neuronal activities in the S1 area with predominant localization in layers II-III.

Effects of transgene-mediated endomorphin-2 in inflammatory pain

We examined the analgesic properties of endomorphin-2 expressed in DRG neurons transduced with a non-replicating herpes simplex virus (HSV)-based vector containing a synthetic endomorphin-2 gene construct. HSV-mediated endomorphin-2 expression reduced nocisponsive behaviors in response to mechanical and thermal stimuli after injection of complete Freund's adjuvant (CFA) into the paw, and reduced peripheral inflammation measured by paw swelling after injection of CFA. The analgesic effect of the vector was blocked by either intraperitoneal or intrathecal administration of naloxone methiodide, blocking peripheral and central mu opioid receptors, respectively. Endomorphin-2 vector injection also reduced spontaneous pain-related behaviors in the delayed phase of the formalin test and in both CFA and formalin models suppressed spinal c-fos expression. The magnitude of the vector-mediated analgesic effect on the delayed phase of the formalin test was similar in naïve animals and in animals with opiate tolerance induced by twice daily treatment with morphine, suggesting that there was no cross-tolerance between vector-mediated endomorphin-2 and morphine. These results suggest that transgene-mediated expression of endomorphin-2 in transduced DRG neurons in vivo acts both peripherally and centrally through mu opioid receptors to reduce pain perception.

Peripheral therapeutic ultrasound stimulation alters the distribution of spinal C-fos immunoreactivity induced by early or late phase of inflammation

The purpose of this investigation was to examine the central modulated effects of therapeutic ultrasound (US) on neuronal activity in the spinal cord on early and late phases of inflammation. In this study, induction of c-Fos protein, which reflects neuronal activation (particularly inflammatory nociception), was investigated in the lumbar spinal cord with immunohistochemistry. Inflammatory monoarthritis was induced in 20 male Wistar rats (weighing 250-300 g) via intra-articular injection of complete Freund's adjuvant (CFA) into the tibiotarsal joint. Two phases of arthritis, early phase (18 h after adjuvant injection) and late phase (7 d after adjuvant injection), were studied in the rats. Pulsed-mode US (1 MHz, the spatial average temporal average intensity [I(SATA)] = 0.5 W/cm(2), 50% duty cycle) was applied for 5 min. The effects of US and sham treatments against these phases of arthritis were demonstrated by spinal c-Fos-like immunoreactivity (c-Fos-LI). All data were evaluated statistically with the paired t-test or analysis of variance with Bonferroni corrections. c-Fos-LI neurons were abundant (average 264.2 +/- 11.9) in the L3 and L4 neurons of the spinal cord in areas ipsilateral to the CFA-induced arthritic leg in the early phase, but few were present (average 40.4 +/- 4.5) in the late phase in sham-treated animals. Bonferroni corrections to the alpha level were used to check the group differences in spinal c-Fos expression, and significance was reached when p < 0.025. In the early inflammatory phase, US treatment significantly suppressed the increased number of c-Fos-LI neurons associated with CFA-induced arthritis in superficial laminae, nucleus proprius, deep laminae and ventral horn of the spinal cord. However, during the late inflammatory phase, US significantly triggered c-Fos expression in most laminae, particularly in the nucleus proprius, deep laminae and ventral horn of the spinal cord. The results of our study suggest that administration of US causes a reduction of early nociceptive inflammatory processing, as shown by a decrease in CFA-induced c-Fos-LI neurons at the level of the spinal cord. In contrast, the US did not suppress, but rather enhanced, the number of c-Fos-LI neurons during the late inflammatory phase. The peripheral influences of US on the central modulation of the spinal nociceptive processing system is suggested and may reflect the work being done through the neuroplasticity of spinal cord in response to peripheral stimulation of US. Therefore, we propose a difference in spinal expression of c-Fos-LI neurons between effects of peripheral US stimulation in arthritic models that underlie early and late inflammatory pain. (E-mail: sherrie@sunrise.hk.edu.tw).

C-fos expression at the spinal dorsal horn of streptozotocin-induced diabetic rats

BACKGROUND

Pain during diabetic neuropathy is associated with peripheral nerve damage but recent evidences suggest the occurrence of central effects. We used the activation of the c-fos protooncogene to study the activity of spinal dorsal horn neurons in streptozotocin (STZ)-induced diabetic rats in the absence of stimulation or in response to innocuous or noxious stimuli.

METHODS

Four weeks after saline or STZ (50 mg/kg) injection, rats were anaesthetized and either not further manipulated or submitted to innocuous (gentle touch), noxious mechanical (pinching) or noxious thermal (radiant heat) stimulation of the hindlimb skin. In each experimental situation, the numbers of Fos-immunoreactive (Fos-IR) neurons occurring in the superficial (laminae I-II) or deep (laminae III-V) dorsal horn were compared.

RESULTS

In the absence of stimulation, STZ-injected rats presented significantly higher numbers of Fos-IR neurons than controls, both in the superficial and deep dorsal horn (DDH). In comparison with the respective baseline levels, innocuous stimulation did not induce a significant increase in the numbers of Fos-IR neurons in controls or STZ-rats. Noxious mechanical and noxious thermal stimuli increased the numbers of Fos-IR neurons, both in control and STZ-rats, but in a more pronounced manner when diabetic rats were subjected to noxious mechanical stimulation.

CONCLUSIONS

The present study demonstrates that the responses of spinal cord neurons are strongly affected during diabetes. The higher baseline neuronal activity probably underlies the spontaneous pain detected during diabetes since the spinal dorsal horn is the major relay station in the ascending transmission of nociceptive input to the brain.

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.

Characterisation of ultraviolet-B-induced inflammation as a model of hyperalgesia in the rat

In humans, the acute inflammatory reaction caused by ultraviolet (UV) radiation is well studied and the sensory changes that are found have been used as a model of cutaneous hyperalgesia. Similar paradigms are now emerging as rodent models of inflammatory pain. Using a narrowband UVB source, we irradiated the plantar surface of rat hind paws. This produced the classical feature of inflammation, erythema, and a significant dose-dependent reduction in both thermal and mechanical paw withdrawal thresholds. These sensory changes peaked 48h after irradiation. At this time there is a graded facilitation of noxious heat evoked (but not basal) c-fos-like immunoreactivity in the L4/5 segments of the spinal cord. We also studied the effects of established analgesic compounds on the UVB-induced hyperalgesia. Systemic as well as topical application of ibuprofen significantly reduced both thermal and mechanical hyperalgesia. Systemic morphine produced a dose-dependent and naloxone sensitive reversal of sensory changes. Similarly, the peripherally restricted opioid loperamide also had a dose-dependent anti-hyperalgesic effect, again reversed by naloxone methiodide. Sequestration of NGF, starting at the time of UVB irradiation, significantly reduced sensory changes. We conclude that UVB inflammation produces a dose-dependent hyperalgesic state sensitive to established analgesics. This suggests that UVB inflammation in the rat may represent a useful translational tool in the study of pain and the testing of analgesic agents.

Neuronal activation at the spinal cord and medullary pain control centers after joint stimulation: a c-fos study in acute and chronic articular inflammation

Chronic inflammatory pain induces short- and long-term central changes, which have been mainly studied at the spinal cord level. Supraspinal pain control centers intrinsically connected with the dorsal horn are also prone to be affected by chronic inflammatory pain. C-fos expression was used as a neuronal activation marker at spinal and supraspinal levels to i) compare acute and chronic articular inflammation, and ii) analyze the effects of brief innocuous or noxious stimulation of a chronically inflamed joint. Acute articular inflammation was induced by an inflammatory soup with prostaglandin E(2) and bradykinin, both at 10(-5) M. Chronic articular inflammation consisted of 14 days of monoarthritis. Early c-fos expression was studied 4 min after inflammatory soup injection or stimulation of the arthritic joint whereas late c-fos expression was evaluated 2 h after those stimuli. At the spinal cord, the analysis was focused on the dorsal horn (laminae I-V) and supraspinally, five major regions of the endogenous pain control system were considered: the caudal ventrolateral medulla (VLM), the dorsal reticular nucleus (DRt), the ventral reticular nucleus (VRt), the nucleus of the solitary tract (Sol) and the rostroventromedial medulla (RVM). Acute articular inflammation induced early and late increases in c-fos expression at the spinal level and late increases supraspinally whereas the effects of monoarthritis were more moderate and restricted to the spinal cord. When monoarthritic animals were subjected to gentle touch or bending of the joint, early increases in c-fos expression were detected supraspinally, but not at the spinal level. In this region, noxious mechanical stimulation induced late increases in non-inflamed animals and both early and late increases in monoarthritic rats. Supraspinally, noxious stimulation induced only late increases in c-fos expression. The present results show complex differences in the patterns of c-fos expression between the spinal cord and medullary areas of the pain control system during articular inflammation, which indicate that the somatosensory system is differentially affected by the installation of chronic pain.

Nerve injury-induced tactile allodynia is present in the absence of FOS labeling in retrogradely labeled post-synaptic dorsal column neurons

The dorsal column pathway consists of direct projections from primary afferents and of ascending fibers of the post-synaptic dorsal column (PSDC) cells. This pathway mediates touch but may also mediate allodynia after nerve injury. The role of PSDC neurons in nerve injury-induced mechanical allodynia is unknown. Repetitive gentle, tactile stimulus or noxious pinch was applied to the ipsilateral hindpaw of rats with spinal nerve ligation (SNL) or sham surgery that had previously received tetramethylrhodamine dextran in the ipsilateral n. gracilis. Both touch and noxious stimuli produced marked increases in FOS expression in other cells throughout all laminae of the ipsilateral dorsal horn after nerve injury. However, virtually none of the identified PSDC cells expressed FOS immunofluorescence in response to repetitive touch or pinch in either the nerve-injured or sham groups. In contrast, labeled PSDC cells expressed FOS in response to ureter ligation and labeled spinothalamic tract (STT) cells expressed FOS in response to noxious pinch. Identified PSDC neurons from either sham-operated or SNL rats did not express immunoreactivity to substance P, CGRP, NPY, PKCY, MOR, the NK1 and the NPY-Y1 receptor. Retrogradely labeled DRG cells of nerve injured rats were large diameter neurons, which expressed NPY, but no detectable CGRP or substance P. Spinal nerve injury sensitizes neurons in the spinal dorsal horn to repetitive light touch but PSDC neurons apparently do not participate in touch-evoked allodynia. Sensitization of these non-PSDC neurons may result in activation of projections integral to the spinal/supraspinal processing of enhanced pain states and of descending facilitation, thus priming the central nervous system to interpret tactile stimuli as being aversive.

Correlation of noxious evoked c-fos expression in areas of the somatosensory system during chronic pain: Involvement of spino-medullary and intra-medullary connections

Chronic pain induces functional alterations of the endogenous pain control system namely in the modulation of nociceptive transmission at the spinal cord. We used the c-fos expression as a tool to study correlated neuronal activation, induced by bending the inflamed paw of monoarthritic animals, between the spinal dorsal horn and medullary centers belonging to the endogenous pain control system, namely the lateralmost reticular formation of the ventrolateral medulla (VLMlat), the lateral reticular nucleus (LRt), the dorsal reticular nucleus (DRt), the nucleus tractus solitarius (Sol) and the rostroventromedial medulla (RVM). Awake monoarthritic rats were subjected to 4 min of paw bending followed by anaesthesia and perfusion either immediately or 2h later. The numbers of Fos immunoreactive neurons in the spinal dorsal horn and in the medulla oblongata were significantly correlated mainly immediately after stimulation: lamina I correlated with the VLMlat, LRt, Sol and RVM; lamina II correlated with the VLMlat, LRt and Sol; and laminae IV-V correlated with the VLMlat and LRt. Between medullary pain control centers significant correlations occurred immediately and 2h after bending at the VLMlat-Sol and LRt-Sol, at the VLMlat-LRt and VLMlat-RVM in animals perfused immediately, and at the VLMlat-DRt and LRt-RVM in animals perfused 2h later. These data demonstrate that the mobilization of a chronically inflamed paw triggers intense correlated neuronal activity in several areas of the somatosensory system, indicating functional relevant links in pain control.

Spinal NK-1 receptor expressing neurons mediate opioid-induced hyperalgesia and antinociceptive tolerance via activation of descending pathways

Opioids can induce hyperalgesia in humans and in animals. Mechanisms of opiate-induced hyperalgesia and possibly of spinal antinociceptive tolerance may be linked to pronociceptive adaptations occurring at multiple levels of the nervous system including activation of descending facilitatory influences from the brainstem, spinal neuroplasticity, and changes in primary afferent fibers. Here, the role of NK-1 receptor expressing cells in the spinal dorsal horn in morphine-induced hyperalgesia and spinal antinociceptive tolerance was assessed by ablating these cells with intrathecal injection of SP-saporin (SP-SAP). Ablation of NK-1 receptor expressing cells prevented (a) morphine-induced thermal and mechanical hypersensitivity, (b) increased touch-evoked spinal FOS expression, (c) upregulation of spinal dynorphin content and (d) the rightward displacement of the spinal morphine antinociceptive dose-response curve (i.e., tolerance). Morphine-induced hyperalgesia and antinociceptive tolerance were also blocked by spinal administration of ondansetron, a serotonergic receptor antagonist. Thus, NK-1 receptor expressing neurons play a critical role in sustained morphine-induced neuroplastic changes which underlie spinal excitability reflected as thermal and tactile hypersensitivity to peripheral stimuli, and to reduced antinociceptive actions of spinal morphine (i.e., antinociceptive tolerance). Ablation of these cells likely eliminates the ascending limb of a spinal-bulbospinal loop that engages descending facilitation and elicits subsequent spinal neuroplasticity. The data may provide a basis for understanding mechanisms of prolonged pain which can occur in the absence of tissue injury.

Selective local PMN recruitment by CXCL1 or CXCL2/3 injection does not cause inflammatory pain

Polymorphonuclear cells (PMN) are recruited in early inflammation and are believed to contribute to inflammatory pain. However, studies demonstrating a hyperalgesic role of PMN did not examine selective PMN recruitment or did not document effective PMN recruitment. We hypothesized that hyperalgesia does not develop after chemokine-induced PMN selective recruitment and is independent of PMN infiltration in complete Freund's adjuvant (CFA)-induced, local inflammation. PMN were recruited by intraplantar injection of CXC chemokine ligand 1 (CXCL1; keratinocyte-derived chemokine), CXCL2/3 (macrophage inflammatory protein-2), or CFA, with or without preceding systemic PMN depletion. Chemokine inoculation resulted in dose (0-30 microg)- and time (0-12 h)-dependent, selective recruitment of PMN as quantified by flow cytometry. CXCL2/3, but not CXCL1, was less effective at high doses, probably as a result of significant down-regulation of CXC chemokine receptor 2 expression on blood PMN. Neither chemokine caused mechanical or thermal hyperalgesia as determined by the Randall-Selitto and Hargreaves test, respectively, despite comparable expression of activation markers (i.e., CD11b, CD18, and L-selectin) on infiltrating PMN. In contrast, CFA injection induced hyperalgesia, independent of PMN recruitment. c-Fos mRNA and immunoreactivity in the spinal cord were increased significantly after inoculation of CFA-independent of PMN-migration but not of CXCL2/3. Measurement of potential hyperalgesic mediators showed that hyperalgesia correlated with local prostaglandin E2 (PGE2) but not with interleukin-1beta production. In summary, hyperalgesia, local PGE2 production, and spinal c-Fos expression occur after CFA-induced inflammation but not after CXCL1- or CXCL2/3-induced, selective PMN recruitment. Thus, PMN seem to be less important in inflammatory hyperalgesia than previously thought.

Dissociation between post-surgical pain behaviors and spinal Fos-like immunoreactivity in the rat

Previous studies have demonstrated that Fos-like immunoreactivity is increased in spinal dorsal horn neurons in several pain models, and have suggested that Fos-like immunoreactivity could be used as a marker of neurons activated by painful stimulation. In the present study, we evaluated nociceptive behaviors and spinal Fos-like immunoreactivity in a rat skin incision model of post-operative pain. In this model, evoked and non-evoked pain behaviors were observed at least for 2 days after paw surgery, an increased number of Fos-like immunoreactive neurons was observed in the spinal dorsal horn at lumbar levels 4-5 two-hour post-surgery. The number of Fos-like immunoreactive neurons was significantly greater in animals with skin-muscle incision compared to animals with skin-alone incision. Interestingly, spinal Fos-like immunoreactivity was quickly normalized in rats with paw surgery at later time points (8 and 24 h post-surgery), whereas nociceptive behaviors were still observed. Furthermore, at 24 h post-surgery, spinal Fos-like immunoreactivity induced by thermal stimulation (42, 44, 46, 48, 52 degrees C for 15 s) was not significantly different between sham animals and animals with surgery. In both groups, an increase in spinal Fos-like immunoreactive neurons was observed with increasing temperatures, with similar laminar distribution. Finally, systemic morphine reduced post-operative pain and Fos-like immunoreactivity in a naloxone reversible manner, with greater potency and efficacy on behavioral endpoints than on Fos-like immunoreactivity. These results demonstrate a different profile of nociceptive behaviors and spinal Fos-like immunoreactivity in the rat skin incision model, suggesting a limited potential of spinal Fos-like immunoreactivity to study post-surgical pain and its pharmacology.

cAMP and cGMP Contribute to Sensory Neuron Hyperexcitability and Hyperalgesia in Rats With Dorsal Root Ganglia Compression

Numerous studies have implicated the cAMP-protein kinase A (PKA) pathway in producing hyperexcitability of dorsal root ganglia (DRG) sensory neurons under conditions associated with pain. Evidence is presented for roles of both the cAMP-PKA and cGMP-protein kinase G (PKG) pathways in maintaining neuronal hyperexcitability and behavioral hyperalgesia in a neuropathic pain model: chronic compression of the DRG (CCD treatment). Lumbar DRGs were compressed by a steel rod inserted into the intervertebral foramen. Thermal hyperalgesia was revealed by shortened latencies of foot withdrawal to radiant heat. Intracellular recordings were obtained in vitro from lumbar ganglia after in vivo DRG compression. Activators of the cAMP-PKA pathway, 8-Br-cAMP and Sp-cAMPS, and of the cGMP-PKG pathway, 8-Br-cGMP and Sp-cGMPS, increased the hyperexcitability of DRG neurons already produced by CCD treatment, as shown by further decreases in action potential threshold and increased repetitive discharge during depolarization. The adenylate cyclase inhibitor, SQ22536, the PKA antagonist, Rp-cAMPS, the guanylate cyclase inhibitor, ODQ, and the PKG inhibitor, Rp-8-pCPT-cGMPS, reduced the hyperexcitability of CCD DRG neurons. In vivo application of PKA and PKG antagonists transiently depressed behavioral hyperalgesia induced by CCD treatment. Unexpectedly, application of these agonists and antagonists to ganglia of naïve, uninjured animals had little effect on electrophysiological properties of DRG neurons and no effect on foot withdrawal, suggesting that sensitizing actions of these pathways in the DRG are enabled by prior injury or stress. The only effect observed in uncompressed ganglia was modest depolarization of DRG neurons by PKA and PKG agonists. CCD treatment also depolarized DRG neurons, but CCD-induced depolarization was not affected by agonists or antagonists of these pathways.

Expression of opioid receptors and c-fos in CB1 knockout mice exposed to neuropathic pain

The development of neuropathic pain is associated with multiple changes in gene expression occurring in the dorsal root ganglia (DRG) and spinal cord. The goal of this study was to evaluate whether the disruption of CB1 cannabinoid receptor gene modulates the changes induced by neuropathic pain in the expression of mu- (MOR), delta- (DOR) and kappa-opioid receptors (KOR) mRNA levels in the DRG and spinal cord. The induction of c-fos expression in the lumbar and sacral regions of the spinal cord was also evaluated in these animals. Opioid receptors mRNA levels were determined by using real-time PCR and Fos protein levels by immunohistochemistry. Nerve injury significantly reduced the expression of MOR in the DRG and the lumbar section of the spinal cord from CB1 cannabinoid knockout (KO) mice and wild-type littermates (WT). In contrast, mRNA levels of DOR and KOR were not significantly changed in any of the different sections analysed. Furthermore, sciatic nerve injury evoked a similar increase of c-fos expression in lumbar and sacral regions of the spinal cord of both KO and WT. In all instances, no significant differences were observed between WT and KO mice. These data revealed specific changes induced by neuropathic pain in MOR expression and c-fos levels in the DRG and/or spinal cord that were not modified by the genetic disruption of CB1 cannabinoid receptors.

Effects of electroacupuncture with different frequencies on spinal ionotropic glutamate receptor expression in complete Freund's adjuvant-injected rat

We investigated expressional changes of spinal glutamate receptors by electroacupuncture (EA) in an inflammatory animal model. Inflammation was induced by an intraplantar injection of complete Freund's adjuvant (CFA) into the hindpaw of male Sprague-Dawley rats. Bilateral EA stimulation at 2, 15 and 120 Hz was applied at those acupoints corresponding to Zusanli and Sanyinjiao in man using needles with 3-day intervals for 30 days. Paw edema and mechanical thresholds were measured by a water displacement plethysmometer and Analgesy-Meter, respectively. Edema and mechanical sensitivity of the hindpaw induced by CFA-injection were strongly inhibited by EA stimulation. At 30 days after CFA-injection, effects of EA on ionotropic glutamate receptor (NR-1, NR-2A, GluR-1 and GluR-2/3) expression in association with c-fos and calcitonin gene-related peptide (CGRP) expression were observed in the dorsal horn of the spinal cord using immunohistochemistry. The number of c-fos-like immunostained cells was decreased significantly in the superficial laminae of the dorsal horn by 2Hz EA, but CGRP expression also showed a marked decrease in the same region using the other types of EA stimulation. N-methyl-D-aspartate receptor (NR-1 and NR-2A) expression was attenuated in all regions of the dorsal horn by all types of EA. Of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (GluR-1 and -2/3), only GluR-1 expression was prevented by EA treatment in the superficial laminae and the neck of the dorsal horn. It is concluded that EA treatment can attenuate inflammatory edema and mechanical thresholds in CFA-injected rats through modulating expression of ionotropic glutamate receptors, and especially N-methyl-D-aspartate receptors, in the dorsal horn of the spinal cord.

The AMPA Receptor Subunits GluR-A and GluR-B Reciprocally Modulate Spinal Synaptic Plasticity and Inflammatory Pain

Ca(2+)-permeable AMPA receptors are densely expressed in the spinal dorsal horn, but their functional significance in pain processing is not understood. By disrupting the genes encoding GluR-A or GluR-B, we generated mice exhibiting increased or decreased numbers of Ca(2+)-permeable AMPA receptors, respectively. Here, we demonstrate that AMPA receptors are critical determinants of nociceptive plasticity and inflammatory pain. A reduction in the number of Ca(2+)-permeable AMPA receptors and density of AMPA channel currents in spinal neurons of GluR-A-deficient mice is accompanied by a loss of nociceptive plasticity in vitro and a reduction in acute inflammatory hyperalgesia in vivo. In contrast, an increase in spinal Ca(2+)-permeable AMPA receptors in GluR-B-deficient mice facilitated nociceptive plasticity and enhanced long-lasting inflammatory hyperalgesia. Thus, AMPA receptors are not mere determinants of fast synaptic transmission underlying basal pain sensitivity as previously thought, but are critically involved in activity-dependent changes in synaptic processing of nociceptive inputs.