Embodied pain in fibromyalgia: Disturbed somatorepresentations and increased plasticity of the body schema.

Fibromyalgia syndrome (FMS) is a highly prevalent, chronic musculoskeletal condition characterized by widespread pain and evoked pain at tender points. This study evaluated various aspects of body awareness in a sample of 14 women with FMS and 13 healthy controls, such as plasticity of the body schema, body esteem, and interoceptive awareness. To this end, the Rubber Hand Illusion (RHI), the Body Esteem Scale (BES), and the Body Perception Questionnaire (BPQ) were used, respectively. Consistent with increased plasticity of the body schema, FMS patients scored higher, with large or very large effect sizes, across all three domains evaluated in the RHI paradigm, namely proprioceptive drift and perceived ownership and motor control over the rubber hand. Scores on all items addressed by the BES were consistently lower among FMS subjects (2.52, SEM .19 vs 3.89, SEM .16, respectively, p < .01, Cohen's d = .38-.66). In the FMS sample, BES scores assigned to most painful regions also were lower than those assigned to the remaining body sites (1.58, SEM .19 vs 2.87, SEM .18, respectively, p < .01). Significantly higher scores (p < .01, Cohen's d = .51-.87) were found in the FMS sample across awareness (3.57 SEM .15 vs 1.87 SEM .11), stress response (3.76 SEM .11 vs 1.78 SEM .11), autonomic nervous system reactivity (2.59 SEM .17 vs 1.35 SEM .07), and stress style 2 (2.73 SEM .27 vs 1.13 SEM .04) subscales of the BPQ. Intensity of ongoing clinical pain was found to be strongly correlated with interoceptive awareness (r = .75, p = .002). The results suggest a disturbed embodiment in FMS, characterized by instability of the body schema, negatively biased cognitions regarding one's own body, and increased vigilance to internal bodily cues. These manifestations may be interpreted as related with the inability of incoming sensory inputs to adequately update negatively biased off-line somatorepresentations stored as long-term memory.

Characterization of functional µ opioid receptor turnover in rat locus coeruleus: an electrophysiological and immunocytochemical study.

BACKGROUND AND PURPOSE: Regulation of µ receptor dynamics such as its trafficking is a possible mechanism underlying opioid tolerance that contributes to inefficient recycling of opioid responses. We aimed to characterize the functional turnover of µ receptors in the noradrenergic nucleus locus coeruleus (LC). EXPERIMENTAL APPROACH: We measured opioid effect by single-unit extracellular recordings of LC neurons from rat brain slices. Immunocytochemical techniques were used to evaluate µ receptor trafficking. KEY RESULTS: After near-complete, irreversible µ receptor inactivation with ß-funaltrexamine (ß-FNA), opioid effect spontaneously recovered in a rapid and efficacious manner. In contrast, α2 -adrenoceptor-mediated effect hardly recovered after receptor inactivation with the irreversible antagonist EEDQ. When the recovery of opioid effect was tested after various inactivating time schedules, we found that the longer the ß-FNA pre-exposure, the less efficient and slower the functional µ receptor turnover became. Interestingly, µ receptor turnover was slower when ß-FNA challenge was repeated in the same cell, indicating constitutive µ receptor recycling by trafficking from a depletable pool. Double immunocytochemistry confirmed the constitutive nature of µ receptor trafficking from a cytoplasmic compartment. The µ receptor turnover was slowed down when LC neuron calcium- or firing-dependent processes were prevented or vesicular protein trafficking was blocked by a low temperature or transport inhibitor. CONCLUSIONS AND IMPLICATIONS: Constitutive trafficking of µ receptors from a depletable intracellular pool (endosome) may account for its rapid and efficient functional turnover in the LC. A finely-tuned regulation of µ receptor trafficking and endosomes could explain neuroadaptive plasticity to opioids in the LC.

Regulation of Nociceptive Plasticity Threshold and DARPP-32 Phosphorylation in Spinal Dorsal Horn Neurons by Convergent Dopamine and Glutamate Inputs.

Dopamine can influence NMDA receptor function and regulate glutamate-triggered long-term changes in synaptic strength in several regions of the CNS. In spinal cord, regulation of the threshold of synaptic plasticity may determine the proneness to undergo sensitization and hyperresponsiveness to noxious input. In the current study, we increased endogenous dopamine levels in the dorsal horn by using re-uptake inhibitor GBR 12935. During the so-induced hyperdopaminergic transmission, conditioning low-frequency (1 Hz) stimulation (LFS) to the sciatic nerve induced long-term potentiation (LTP) of C-fiber-evoked potentials in dorsal horn neurons. The magnitude of LTP was attenuated by blockade of either dopamine D1-like receptors (D1LRs) by with SCH 23390 or NMDA receptor subunit NR2B with antagonist Ro25-6981. Conditioning LFS during GBR 12935 administration increased phosphorylation of dopamine- and cAMP-regulated phosphoprotein of Mr 32kDa (DARPP-32) at threonine 34 residue in synaptosomal (P3) fraction of dorsal horn homogenates, as assessed by Western blot analysis, which was partially prevented by NR2B blockade prior to conditioning stimulation. Conditioning LFS also was followed by higher co-localization of phosphorylated form of NR2B at tyrosine 1472 and pDARPP-32Thr34- with postsynaptic marker PSD-95 in transverse L5 dorsal horn sections. Such increase could be significantly attenuated by D1LR blockade with SCH 23390. The current results support that coincidental endogenous recruitment of D1LRs and NR2B in dorsal horn synapses plays a role in regulating afferent-induced nociceptive plasticity. Parallel increases in DARPP-32 phosphorylation upon LTP induction suggests a role for this phosphoprotein as intracellular detector of convergent D1L- and NMDA receptor activation.

Dopamine D1-like Receptors Regulate Constitutive, µ-Opioid Receptor-Mediated Repression of Use-Dependent Synaptic Plasticity in Dorsal Horn Neurons: More Harm than Good?

UNLABELLED: The current study reports on a synaptic mechanism through which D1-like receptors (D1LRs) modulate spinal nociception and plasticity by regulating activation of the µ-opioid receptor (MOR).D1LR stimulation with agonist SKF 38393 concentration-dependently depressed C-fiber-evoked potentials in rats receiving spinal nerve ligation (SNL), but not in uninjured rats. Depression was prevented by MOR- but not GABA-receptor blockade. Neurons expressing the D1 subtype were immunopositive for met-enkephalin and vesicular glutamate transporter VGLUT2, but not for GABAergic marker vGAT.Nerve ligation was followed by increased immunoreactivity for D1 in synaptic compartment (P3) in dorsal horn homogenates and presynaptic met-enkephalin-containing boutons. SNL led to increased immunoreactivity for met-enkephalin in dorsal horn homogenates, which was dose-dependently attenuated by selective D1LR antagonist SCH 23390. During blockade of either D1R or MOR, low-frequency (0.2 or 3 Hz) stimulation (LFS) to the sciatic nerve induced long-term potentiation (LTP) of C-fiber-evoked potentials, revealing a constituent role of both receptors in repressing afferent-induced synaptic plasticity. LFS consistently induced NMDA receptor-dependent LTP in nerve-injured rats. The ability of MOR both to prevent LTP and to modulate mechanical and thermal pain thresholds in behavioral tests was preserved in nerve-ligated rats that were postoperatively treated with SCH 23390. D1LR priming for 30 min sufficed to disrupt MOR function in otherwise naive rats via a mechanism involving receptor overuse.The current data support that, whereas D1LR-modulated MOR activation is instrumental in antinociception and endogenous repression of synaptic plasticity, this mechanism deteriorates rapidly by sustained use, generating increased vulnerability to afferent input. SIGNIFICANCE STATEMENT: The current study shows that dopamine D1-like receptors (D1LRs) and µ-opioid receptors (MOR) in the spinal dorsal horn constitutively repress the expression of synaptic long-term potentiation (LTP) of C-fiber-evoked potentials. Anatomical data are provided supporting that the D1 subtype regulates MOR function by modulating met-enkephalin release. Sustained neuropathic pain induced by spinal nerve ligation is accompanied by D1R and met-enkephalin upregulation, acquired D1LR-mediated antinociception, and a loss of endogenous repression of further synaptic plasticity. We show that the ability of MOR to oppose LTP is rapidly impaired by sustained D1LR activation via a mechanism involving sustained MOR activation.

Spinal D1-like dopamine receptors modulate NMDA receptor-induced hyperexcitability and NR1 subunit phosphorylation at serine 889.

Activation of the N-methyl-d-aspartate receptor (NMDAR) in dorsal horn neurons is recognized as a fundamental mechanism of central sensitization and pathologic pain. This study assessed the influence of dopaminergic, D1-like receptor-mediated input to the spinal dorsal horn on NMDAR function. Spinal superfusion with selective NMDAR agonist cis-ACPD significantly increased C-fiber-evoked field potentials in rats subjected to spinal nerve ligation (SNL), but not in sham-operated rats. Simultaneous application of D1LR antagonist SCH 23390 dramatically reduced hyperexcitability induced by cis-ACPD. Furthermore, cis-ACPD-induced hyperexcitability seen in nerve-ligated rats could be mimicked in unin-jured rats during stimulation of D1LRs by agonist SKF 38393 at subthreshold concentration. Phosphorylation of NMDAR subunit NR1 at serine 889 at postsynaptic sites was found to be increased in dorsal horn neurons 90 min after SNL, as assessed by increased co-localization with postsynaptic marker PSD-95. Increased NR1 phosphorylation was attenuated in the presence of SCH 23390 in the spinal superfusate. The present results support that D1LRs regulate most basic determinants of NMDAR function in dorsal horn neurons, suggesting a potential mechanism whereby dopaminergic input to the dorsal horn can modulate central sensitization and pathologic pain.

Striatal-enriched protein tyrosine phosphatase modulates nociception: evidence from genetic deletion and pharmacological inhibition.

The information from nociceptors is processed in the dorsal horn of the spinal cord by complex circuits involving excitatory and inhibitory interneurons. It is well documented that GluN2B and ERK1/2 phosphorylation contributes to central sensitization. Striatal-enriched protein tyrosine phosphatase (STEP) dephosphorylates GluN2B and ERK1/2, promoting internalization of GluN2B and inactivation of ERK1/2. The activity of STEP was modulated by genetic (STEP knockout mice) and pharmacological (recently synthesized STEP inhibitor, TC-2153) approaches. STEP(61) protein levels in the lumbar spinal cord were determined in male and female mice of different ages. Inflammatory pain was induced by complete Freund's adjuvant injection. Behavioral tests, immunoblotting, and electrophysiology were used to analyze the effect of STEP on nociception. Our results show that both genetic deletion and pharmacological inhibition of STEP induced thermal hyperalgesia and mechanical allodynia, which were accompanied by increased pGluN2B(Tyr1472) and pERK1/2(Thr202/Tyr204)levels in the lumbar spinal cord. Striatal-enriched protein tyrosine phosphatase heterozygous and knockout mice presented a similar phenotype. Furthermore, electrophysiological experiments showed that TC-2153 increased C fiber-evoked spinal field potentials. Interestingly, we found that STEP(61) protein levels in the lumbar spinal cord inversely correlated with thermal hyperalgesia associated with age and female gender in mice. Consistently, STEP knockout mice failed to show age-related thermal hyperalgesia, although gender-related differences were preserved. Moreover, in a model of inflammatory pain, hyperalgesia was associated with increased phosphorylation-mediated STEP(61) inactivation and increased pGluN2B(Tyr1472) and pERK1/2(Thr202/Tyr204)levels in the lumbar spinal cord. Collectively, the present results underscore an important role of spinal STEP activity in the modulation of nociception.

Plasticity of α2-adrenergic spinal antinociception following nerve injury: selective, bidirectional interaction with the delta opioid receptor.

Interactions of opioid receptors with other receptor families can be made use of to improve analgesia and reduce adverse effects of opioid analgesics. We investigated interactions of the α2-adrenergic receptor (α2AR) with opioid receptors of the mu (MOR) and delta (DOR) types in the spinal dorsal horn in an animal model of neuropathic pain induced by spinal nerve ligation. Nine days after nerve injury, immunoreactivity for the α2AR subtype A (α2AAR) was increased both in tissue homogenates and at pre- and post-synaptic sites in transverse sections. The efficacy of spinally administered α2AAR agonist guanfacine at reducing C-fiber-evoked field potentials was increased in nerve-ligated rats. This reducing effect was impaired by simultaneous administration of DOR antagonist naltrindole, but not MOR antagonist CTOP, suggesting that concurrent DOR activation was required for α2AAR-mediated inhibition. While DOR agonist deltorphin II and MOR agonist DAMGO both effectively depressed C-fiber-evoked spinal field potentials, DOR- but not MOR-mediated depression was enhanced by subclinical guanfacine. In conscious, nerve-ligated rats, chronically administered deltorphin II produced stable thermal and mechanical antinociception over the 9 following days after nerve injury without apparent signs of habituation. Such an effect was dramatically enhanced by co-administration of a low dose of guanfacine, which reversed thermal and mechanical thresholds to levels near those prior to injury. The results suggest that spinal, α2AAR-mediated antinociception is increased after nerve injury and based on DOR co-activation. We demonstrate in vivo that α2AAR/DOR interaction can be exploited to provide effective behavioral antinociception during neuropathic pain.

Synaptic upregulation and superadditive interaction of dopamine D2- and µ-opioid receptors after peripheral nerve injury.

A sound strategy for improving the clinical efficacy of opioids involves exploiting positive interactions with drugs directed at other targets in pain pathways. The current study investigated the role of dopamine receptor D2 (D2R) in modulation of spinal dorsal horn excitability to noxious input, and interactions therein with µ-opioid receptor (MOR) in an animal model of neuropathic pain induced by spinal nerve ligation (SNL). C-fiber-evoked field potentials in the spinal dorsal horn were depressed concentration dependently by spinal superfusion with the D2R agonist quinpirole both in nerve-injured and sham-operated (control) rats. However, quinpirole-induced depression was significant at 10 µmol/L after SNL but only at 100 µmol/L in control rats. This quinpirole effect was completely abolished by MOR antagonist CTOP at subclinical concentration (1 µmol/L) in nerve-injured rats, but was unaltered in sham-operated rats. Nine days after SNL, D2R was upregulated to both presynaptic and postsynaptic locations in dorsal horn neurons, as revealed by double confocal immunofluorescence stainings for synaptophysin and PSD-95. In addition, D2R/MOR co-localization was increased after SNL. Co-administration of 1 µmol/L quinpirole, insufficient per se to alter evoked potentials, dramatically enhanced inhibition of evoked potentials by MOR agonist DAMGO, reducing the IC50 value of DAMGO by 2 orders of magnitude. The present data provide evidence of profound functional and subcellular changes in D2R-mediated modulation of noxious input after nerve injury, including positive interactions with spinal MOR. These results suggest D2R co-stimulation as a potential avenue to improve MOR analgesia in sustained pain states involving peripheral nerve injury.

Coupling of serotonergic input to NMDA receptor-phosphorylation following peripheral nerve injury via rapid, synaptic up-regulation of ND2.

Evidence implicates serotonergic input to spinal dorsal horn neurons in shifting the NMDA receptor (NMDAR) into a high functional output profile after spinal nerve ligation (SNL). We investigated the involvement of adaptor protein NADH dehydrogenase subunit 2 (ND2) in NMDAR-phosphorylation and spinal hyperexcitability secondary to peripheral nerve injury. Immunofluorescence for ND2 was found in dorsal horn neurons immunopositive for NMDAR subunit NR1. Co-localization of ND2 with postsynaptic marker PSD-95 was significantly increased 60min after SNL (Rr 0.77 vs Rr 0.06 in sham controls; z=-242.85; p<0.01 at Fisher's exact test). Western blot analyses confirmed ND2 up-regulation both in cytoplasmic (S2) and synaptic (P3) compartments (p<0.01 at the Student's t test). SNL was followed by increased co-localization of ND2 with the phosphorylated form (serine 896) of NR1 (pNMDA). Spinal superfusion with ND2 inhibitor rotenone prevented up-regulation of ND2 (Rr 0.06 after rotenone vs Rr 0.78 in vehicle-treated controls, z=-253.22, p<0.01) and pNR1 in P3. C fiber-evoked dorsal horn field potentials were increased 60min after SNL by superfusion with NMDA agonist cis-ACPD at 100nM (p<0.01 at the Bonferroni test), however cis-ACPD was effective only at 10µM following prior administration of rotenone. Rotenone also abolished enhancement of evoked potentials induced by simultaneous stimulation of NMDA and 5-HR2B receptors in uninjured rats. Increased postsynaptic up-regulation of ND2/pNMDAR 60min after SNL was prevented by prior administration of selective 5-HT2B antagonist SB204741. These results support a pivotal role for ND2 in coupling serotonergic input to NMDAR-activation during neuropathic pain.

Transient, 5-HT2B receptor-mediated facilitation in neuropathic pain: Up-regulation of PKCγ and engagement of the NMDA receptor in dorsal horn neurons.

Spinal nociception can be facilitated by 5-HT2 receptors in neuropathic pain. We investigated the involvement of glutamate receptors in dorsal neuron hyperexcitation that is promoted by 5-HT2B receptor (5-HT2BR) after spinal nerve ligation (SNL) in the rat. Augmentation of C-fiber-evoked potentials by spinal superfusion with 5-HT2BR agonist BW 723C86 in nerve-ligated rats was impeded by co-administration of NMDA receptor (NMDAR) antagonist D-AP5, but not by mGluR1/5 antagonist AIDA or mGluR2/3 antagonist LY 341495. Evoked potentials were increased by cis-ACPD in nerve-injured rats, irrespective of simultaneous 5-HT2BR blockade by SB204741. In uninjured rats, NMDAR agonist cis-ACPD enhanced evoked potentials in the presence of BW 723C86 but not if administered alone or during exposure to protein kinase C γ (PKCγ) inhibitor peptide. Triple immunofluorescence labelings revealed co-localization of NMDAR and 5-HT2BR in PKCγ-expressing perikarya in lamina II neurons. As a result of SNL, PKCγ was transiently and bilaterally up-regulated in synaptic fraction from dorsal horn homogenates, peaking at day 2 and returning to basal levels by day 9. Chronic blockade of 5-HT2BR with selective antagonist SB 204741 after SNL bilaterally decreased the following: (i) PKCγ up-regulation in synaptic fraction, (ii) phosphorylation of NMDAR subunit NR1 (serine 889) in synaptic fraction, and (iii) co-localization of both PKCγ and phosphorylated NR1 with postsynaptic marker PSD-95. Chronic delivery of SB 204741 bilaterally attenuated thermal and mechanical allodynia occurring after SNL, particularly at day 2 post injury. These findings suggest that transient activation of the PKCγ/NMDAR pathway is critically involved in 5-HT2BR-mediated facilitation in the SNL model of neuropathic pain.

Time-dependent cross talk between spinal serotonin 5-HT2A receptor and mGluR1 subserves spinal hyperexcitability and neuropathic pain after nerve injury.

Emerging evidence implicates serotonergic descending facilitatory pathways from the brainstem to the spinal cord in the maintenance of pathologic pain. Upregulation of the serotonin receptor 2A (5-HT(2A)R) in dorsal horn neurons promotes spinal hyperexcitation and impairs spinal µ-opioid mechanisms during neuropathic pain. We investigated the involvement of spinal glutamate receptors, including metabotropic receptors (mGluRs) and NMDA, in 5-HT(2A)R-induced hyperexcitability after spinal nerve ligation (SNL) in rat. High-affinity 5-HT(2A)R agonist (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2) enhanced C-fiber-evoked dorsal horn potentials after SNL, which was prevented by mGluR1 antagonist AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] but not by group II mGluR antagonist LY 341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid] or NMDA antagonist d-AP5 [D-(-)-2-amino-5-phosphonopentanoic acid]. 5-HT(2A)R and mGluR1 were found to be coexpressed in postsynaptic densities in dorsal horn neurons. In the absence of SNL, pharmacological stimulation of 5-HT(2A)R with TCB-2 both induced rapid bilateral upregulation of mGluR1 expression in cytoplasmic and synaptic fractions of spinal cord homogenates, which was attenuated by PKC inhibitor chelerythrine, and enhanced evoked potentials during costimulation of mGluR1 with 3,5-DHPG [(RS)-3,5-dihydroxyphenylglycine]. SNL was followed by bilateral upregulation of mGluR1 in 5-HT(2A)R-containing postsynaptic densities. Upregulation of mGluR1 in synaptic compartments was partially prevented by chronic administration of selective 5-HT(2A)R antagonist M100907 [(R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipidinemethanol], confirming 5-HT(2A)R-mediated control of mGluR1 upregulation triggered by SNL. Changes in thermal and mechanical pain thresholds following SNL were increasingly reversed over the days after injury by chronic 5-HT(2A)R blockade. These results emphasize a role for 5-HT(2A)R in hyperexcitation and pain after nerve injury and support mGluR1 upregulation as a novel feedforward activation mechanism contributing to 5-HT(2A)R-mediated facilitation.

Selective impairment of spinal mu-opioid receptor mechanism by plasticity of serotonergic facilitation mediated by 5-HT2A and 5-HT2B receptors.

Opioid analgesia is compromised by intracellular mediators such as protein kinase C (PKC). The phosphatidylinositol hydrolysis-coupled serotonin receptor 5-HT2 is ideally suited to promote PKC activation. We test the hypothesis that 5-HT2A and 5-HT2B receptors, which have been previously shown to become pro-excitatory after spinal nerve ligation (SNL), can negatively influence the ability of opioids to depress spinal excitation evoked by noxious input. Spinal superfusion with (100 nM) mu-opioid receptor (MOR)-agonist DAMGO significantly depressed C fiber-evoked spinal field potentials. Simultaneous administration of subclinical 5-HT2AR antagonist 4F 4PP (100 nM) or 5-HT2BR antagonist SB 204741 (100 nM) significantly reduced the IC50 value for DAMGO in nerve-ligated rats (97.56 nM ± 1.51 and 1.20 nM ± 1.28 respectively, relative to 104 nM ± 1.08 at the baseline condition), but not in sham-operated rats. Both antagonists failed to alter depression induced by delta-opioid receptor (DOR)-agonist D-ala2-deltorphin II after SNL as well as in the sham condition. Western blot analysis of dorsal horn homogenates revealed bilateral upregulation of 5-HT2AR and 5-HT2BR protein band densities after SNL. As assessed from double immunofluorescence labeling for confocal laser scanning microscopy, scarce dorsal horn cell processes showed co-localization color overlay for 5-HT2AR/MOR, 5-HT2BR/MOR, 5-HT2AR/DOR, or 5-HT2BR/DOR in sham-operated rats. Intensity correlation-based analyses showed significant increases in 5-HT2AR/MOR and 5-HT2BR/MOR co-localizations after SNL. These results indicate that plasticity of spinal serotonergic neurotransmission can selectively reduce spinal MOR mechanisms via 5-HT2A and 5-HT2B receptors, including upregulation of the latter and increased expression in dorsal horn neurons containing MOR.

Is psychological distress intrinsic to fibromyalgia syndrome? Cross-sectional analysis in two clinical presentations.

Clinical presentation of fibromyalgia syndrome (FMS) is heterogeneous and often involves psychological comorbidities. Clinical subgrouping of FMS patients has been proposed as a strategy to improve patients' long-term outcomes by helping identify specific treatment needs. Using the 90 Symptom Checklist Revised (SCL-90-R), we have assessed emotional distress in two FMS patient subpopulations discriminated on the basis of their differences in scores on specific items of the Fibromyalgia Impact Questionnaire (FIQ). Subjects classed as type II exhibited high emotional distress on all ten dimensions studied, which included somatization, obsessive-compulsive, interpersonal sensitivity, depression, anxiety, hostility, phobic anxiety, paranoid ideation, psychoticism, and additional items subscales, as well as on the global severity index (GSI), positive symptom total (PST), and positive symptom distress index (PDSI). T-scores in these patients were above diagnostic cutoff level of 60 on somatization, obsessive-compulsive, and depression subscales. In contrast, the profile exhibited by type I subjects fell entirely within normal values for nonpsychiatric population. Emotional status was significantly inversely correlated with present clinical pain in type I-, but not in type II-fibromyalgia patients. Regression analysis revealed a model based on phobic anxiety, paranoid ideation, and depression subscales as best contributing to classification. The present data suggest that associated psychological distress and maladaptive emotional responses that are commonly attributed to the general FMS population may be largely a distinguishing feature of one subset of patients.

Depression of C fibre-evoked spinal field potentials by the spinal delta opioid receptor is enhanced in the spinal nerve ligation model of neuropathic pain: involvement of the mu-subtype.

The depression rate of C fibre-evoked spinal field potentials by spinally applied morphine is increased in two states of spinal hyperexcitation, namely the spinal ligation model (SNL) of neuropathic pain and long-term potentiation (LTP) of C fibre-evoked spinal field potentials. This present work sought to determine opioid receptor subtypes involved in such increase in the SNL model. We recorded spinal field potentials during spinal superfusion with increasing, cumulative concentrations of selective subtype-specific agonists in rats subjected to SNL, as well as in non-ligated animals. The mu opioid receptor (MOR) agonist DAMGO significantly depressed field potentials evoked by C (100 nM) or Adelta fibres (1 microM) both in neuropathic and non-ligated rats, whereas the kappa receptor opioid (KOR) agonist +/-U-50488 was ineffective. The delta opioid receptor (DOR) (D-Ala2)-Deltorphin II was more effective in reducing C fibre-evoked spinal field potentials in rats subjected to SNL (100 nM) than in non-ligated rats (100 microM). Subclinical MOR activation (10 nM DAMGO) produced a leftward shift in (D-Ala2)-Deltorphin II dose-response curve in non-ligated rats (IC50 16.59 +/- 0.99 microM vs 120.3 +/- 1.0 microM in the absence of DAMGO), and isobolar analysis revealed synergistic interaction (interaction index 0.25). MOR blockade (100 microM CTOP) disinhibited C fibre-evoked potentials in neuropathic, but not in basal animals, and partially impeded DOR depression in both groups. DOR blockade (1 mM naltrindole) was ineffective in either group. We show that DOR-mediated depression of spinal responses to peripheral unmyelinated fibre-input is increased in the SNL model, an increase that is contributed to by positive interaction with the spinal MOR.