Christian Zöllner

Rapid upregulation of μ opioid receptor mrna in dorsal root ganglia in response to peripheral inflammation depends on neuronal conduction

S.c. painful inflammation leads to an increase in axonal transport of opioid receptors from dorsal root ganglia (DRG) toward the periphery, thus causing a higher receptor density and enhanced opioid analgesia at the injured site. To examine whether this increase is related to transcription, the mRNA of Delta- (DOR) and mu-opioid receptor (MOR) in lumbar DRG was quantified by real time Light Cycler polymerase chain reaction (LC-PCR), and correlated to ligand binding in DRG and sciatic nerve. In normal DRG, DOR mRNA was seven times less abundantly expressed than MOR mRNA. After induction of unilateral paw inflammation, mRNA content for DOR remained unchanged, but a bi-phasic upregulation for MOR mRNA with an early peak at 1-2 h and a late increase at 96 h was found in ipsilateral DRG. As no changes were observed in DRG of the non-inflamed side, this effect was apparently not systemically mediated. A significant increase in binding of the MOR ligand DAMGO was detected after 24 h in DRG, and after early and late ligation in the sciatic nerve, indicating an enhanced axonal transport of MOR in response to inflammation. The early increase in MOR mRNA could be completely prevented by local anesthetic blockade of neuronal conduction in sciatic nerve. These data suggest that mRNA of the two opioid receptors DOR and MOR is differentially regulated in DRG during peripheral painful inflammation. The apparently increased axonal transport of MOR in response to this inflammation is preceded by upregulated mRNA-transcription, which is dependent on neuronal electrical activity.

Chronic morphine use does not induce peripheral tolerance in a rat model of inflammatory pain

Although opioids are highly effective analgesics, they are also known to induce cellular adaptations resulting in tolerance. Experimental studies are often performed in the absence of painful tissue injury, which precludes extrapolation to the clinical situation. Here we show that rats with chronic morphine treatment do not develop signs of tolerance at peripheral mu-opioid receptors (micro-receptors) in the presence of painful CFA-induced paw inflammation. In sensory neurons of these animals, internalization of mu-receptors was significantly increased and G protein coupling of mu-receptors as well as inhibition of cAMP accumulation were preserved. Opioid receptor trafficking and signaling were reduced, and tolerance was restored when endogenous opioid peptides in inflamed tissue were removed by antibodies or by depleting opioid-producing granulocytes, monocytes, and lymphocytes with cyclophosphamide (CTX). Our data indicate that the continuous availability of endogenous opioids in inflamed tissue increases recycling and preserves signaling of mu-receptors in sensory neurons, thereby counteracting the development of peripheral opioid tolerance. These findings infer that the use of peripherally acting opioids for the prolonged treatment of inflammatory pain associated with diseases such as chronic arthritis, inflammatory neuropathy, or cancer, is not necessarily accompanied by opioid tolerance.

μ-Opioid Receptor Activation Modulates Transient Receptor Potential Vanilloid 1 (TRPV1) Currents in Sensory Neurons in A Model of Inflammatory Pain

Current therapy for inflammatory pain includes the peripheral application of opioid receptor agonists. Activation of opioid receptors modulates voltage-gated ion channels, but it is unclear whether opioids can also influence ligand-gated ion channels [e.g., the transient receptor potential vanilloid type 1 (TRPV1)]. TRPV1 channels are involved in the development of thermal hypersensitivity associated with tissue inflammation. In this study, we investigated μ-opioid receptor and TRPV1 expression in primary afferent neurons in the dorsal root ganglion (DRG) in complete Freunds adjuvant (CFA)-induced paw inflammation. In addition, the present study examined whether the activity of TRPV1 in DRG neurons can be inhibited by μ-opioid receptor (μ-receptor) ligands and whether this inhibition is increased after CFA inflammation. Immunohistochemistry demonstrated colocalization of TRPV1 and μ-receptors in DRG neurons. CFA-induced inflammation increased significantly the number of TRPV1- and μ-receptor-positive DRG neurons, as well as TRPV1 binding sites. In whole-cell patch clamp studies, opioids significantly decreased capsaicin-induced TRPV1 currents in a naloxone- and pertussis toxinsensitive manner. The inhibitory effect of morphine on TRPV1 was abolished by forskolin and 8-bromo-cAMP. During inflammation, an increase in TRPV1 is apparently rivaled by an increase of μ-receptors. However, in single dissociated DRG neurons, the inhibitory effects of morphine are not different between animals with and without CFA inflammation. In in vivo experiments, we found that locally applied morphine reduced capsaicin-induced thermal allodynia. In summary, our results indicate that μ-receptor activation can inhibit the activity of TRPV1 via Gi/o proteins and the cAMP pathway. These observations demonstrate an important new mechanism underlying the analgesic efficacy of peripherally acting μ-receptor ligands in inflammatory pain.

Endogenous peripheral antinociception in early inflammation is not limited by the number of opioid-containing leukocytes but by opioid receptor expression

&NA; Endogenous inhibition of inflammatory pain is mediated by leukocytes that secrete opioid peptides upon exposure to stress (cold water swim stress, CWS) or after local injection of corticotropin releasing factor (CRF). Since in early inflammation few opioid‐containing leukocytes are detected and since peripheral opioid‐mediated antinociception is low we examined whether antinociception could be augmented by increased recruitment of opioid‐containing polymorphonuclear cells (PMN). Rats were intraplantarly (i.pl.) injected with Freunds complete adjuvant (FCA) and with the PMN‐recruiting chemokine macrophage inflammatory protein‐2 (MIP‐2, 1–10 &mgr;g; control: saline) for 2 h. Intraplantar leukocytes were quantified by flow cytometry. Paw pressure threshold (PPT) was determined before and after exposure to CWS, i.pl. injection of CRF and opioid peptides. Opioid receptors (OR) were measured by binding studies in dorsal root ganglia (DRG) and by immunohistochemistry in the paw. Our studies showed that (i) MIP‐2 injection dose‐dependently augmented recruitment of PMN and opioid‐containing leukocytes (5‐fold increase in cells/paw, P<0.05), (ii) PPT was not different between groups at baseline and after CWS or CRF (maximum MPE: 20±2.3–29±7.2%, P<0.05), (iii) injection of opioid peptides dose‐dependently increased the PPT (P<0.05, maximum MPE: and 18±2.6–21±3.6%), (iv) MOR (&mgr; OR, MOP) binding sites in the ipsilateral DRG were unchanged (24±2–22±1.2 fmol/mg protein, P>0.05, ANOVA) and (v) the number of MOR and DOR (&dgr; OR, DOP) stained nerve fibers in peripheral tissue were unaltered (both P>0.05, t‐test). In summary, antinociception during early inflammation is apparently not limited by the number of opioid‐containing leukocytes but by OR availability.

Opioids and Sensory Nerves

This chapter reviews the expression and regulation of opioid receptors in sensory neurons and the interactions of these receptors with endogenous and exogenous opioid ligands. Inflammation of peripheral tissues leads to increased synthesis and axonal transport of opioid receptors in dorsal root ganglion neurons. This results in opioid receptor upregulation and enhanced G protein coupling at peripheral sensory nerve terminals. These events are dependent on neuronal electrical activity, and on production of proinflammatory cytokines and nerve growth factor within the inflamed tissue. Together with the disruption of the perineurial barrier, these factors lead to an enhanced analgesic efficacy of peripherally active opioids. The major local source of endogenous opioid ligands (e.g. beta-endorphin) is leukocytes. These cells contain and upregulate signal-sequence-encoding messenger RNA of the beta-endorphin precursor proopiomelanocortin and the entire enzymatic machinery necessary for its processing into the functionally active peptide. Opioid-containing immune cells extravasate using adhesion molecules and chemokines to accumulate in inflamed tissues. Upon stressful stimuli or in response to releasing agents such as corticotropin-releasing factor, cytokines, chemokines, and catecholamines, leukocytes secrete opioids. Depending on the cell type, this release is contingent on extracellular Ca(2+) or on inositol triphosphate receptor triggered release of Ca(2+) from endoplasmic reticulum. Once secreted, opioid peptides activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of proinflammatory neuropeptides. These effects occur without central untoward side effects such as depression of breathing, clouding of consciousness, or addiction. Future aims include the development of peripherally restricted opioid agonists, selective targeting of opioid-containing leukocytes to sites of painful injury, and the augmentation of peripheral opioid peptide and receptor synthesis.

Peripheral Antinociceptive Effects of Exogenous and Immune Cell-Derived Endomorphins in Prolonged Inflammatory Pain

Endomorphins (EMs) are endogenous selective μ-opioid receptor agonists. Their role in inflammatory pain has not been fully elucidated. Here we examine peripheral antinociception elicited by exogenously applied EM-1 and EM-2 and the contribution of EM-containing leukocytes to stress- and corticotropin-releasing factor (CRF)-induced antinociception. To this end, we applied behavioral (paw pressure) testing, radioligand binding, immunohistochemistry, and flow cytometry in rats with unilateral hindpaw inflammation induced with Freund’s adjuvant. EMs injected directly into both hindpaws produced antinociception exclusively in inflamed paws. This was blocked by locally applied μ-receptor-selective (d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2) but not κ-receptor-selective (nor-binaltorphimine) antagonists. δ-Receptor antagonists (naltrindole and N,N-diallyl-Tyr-Aib-Aib-Phe-Leu) did not influence EM-1-induced but dose-dependently decreased EM-2-induced antinociception. Antibodies against β-endorphin, methionine-enkephalin, or leucine-enkephalin did not significantly change EM-2-induced antinociception. Both EMs displaced binding of [3H]-[d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin to μ-receptors in dorsal root ganglia (DRG). Using [3H]-naltrindole or [125I]-[d-Pen2,5]-enkephalin, no detectable δ-binding was found in DRG of inflamed hindlimbs. Numerous β-endorphin-containing and fewer EM-1- and EM-2-containing leukocytes were detected in subcutaneous tissue of inflamed paws. Leukocyte-depleting serum decreased the number of immigrating opioid-containing immune cells and attenuated swim stress- and CRF-induced antinociception in inflamed paws. Both forms of antinociception were strongly attenuated by anti-β-endorphin and to a lesser degree by anti-EM-1 and anti-EM-2 antibodies injected into inflamed paws. Together, exogenously applied and immune cell-derived EMs alleviate prolonged inflammatory pain through selective activation of peripheral opioid receptors. Exogenous EM-2 in addition to μ-receptors also activates peripheral δ-receptors, which does not involve actions via other opioid peptides.

Endothelin potentiates TRPV1 via ETA receptor-mediated activation of protein kinase C

BackgroundEndothelin-1 (ET-1) both stimulates nociceptors and sensitizes them to noxious stimuli, an effect probably mediated by the ETA receptor (ETAR) expressed in sensory neurons. The cellular mechanisms of this ET-1-mediated effect are only poorly understood. TRPV1, the heat-, pH- and capsaicin-sensitive cation channel already known to be modulated by a number of cellular mediators released in response to noxious stimuli and during inflammation, is a potential target for the action of ET-1.ResultsWe studied the effects of ET-1 on TRPV1 in sensory neurons from the dorsal root ganglion (DRG) and in HEK293 cells coexpressing TRPV1 and the ETAR. Specific 125I-ET-1 binding sites (817 ± 92 fmol/mg) were detected in membrane preparations of DRG with an ETAR/ETBR ratio of 60:40. In an immunofluorescence analysis, coexpression of TRPV1 and the ETAR was found in a subpopulation of primary sensory neurons. ET-1 strongly potentiated capsaicin-induced TRPV1 currents in some neurons, and in HEK293 cells co-expressing TRPV1 and the ETAR. Weaker potentiation was observed in HEK293 cells coexpressing TRPV1 and the ETBR. ETAR activation also increased responses to low pH and heat. In HEK293 cells, strong potentiation of TRPV1 like that induced by ET-1 via the ETAR could be induced by PKC activation, but not with activators of the adenylyl cyclase or the PKA pathway. Furthermore, inhibition of PKC with bisindolylmaleimide X (BIM X) or mutation of the PKC phosphorylation site S800 completely prevented ETAR-mediated potentiation.ConclusionWe conclude that ET-1 potentiates TRPV1 by a PKC-dependent mechanism and that this could play a major role in the algogenic and hyperalgesic effects of ET-1 described in previous studies.

Modulation of Transient Receptor Vanilloid 1 Activity by Transient Receptor Potential Ankyrin 1

Transient receptor potential vanilloid 1 (TRPV1) is a nonselective ligand-gated cation channel responding to noxious heat, protons, and chemicals such as capsaicin. TRPV1 is expressed in sensory neurons and plays a critical role in pain associated with tissue injury, inflammation, and nerve lesions. Transient receptor potential ankyrin 1 (TRPA1) is coexpressed with TRPV1. It is activated by compounds that cause a burning sensation (e.g., mustard oil) and, indirectly, by components of the inflammatory milieu eliciting nociceptor excitation and pain hypersensitivity. Previous studies indicate an interaction of TRPV1 and TRPA1 signaling pathways. Here we sought to examine the molecular mechanisms underlying such interactions in nociceptive neurons. We first excluded physical interactions of both channels using radioligand binding studies. By microfluorimetry, electrophysiological experiments, cAMP measurements, and site-directed mutagenesis we found a sensitization of TRPV1 after TRPA1 stimulation with mustard oil in a calcium and cAMP/protein kinase A (PKA)–dependent manner. TRPA1 stimulation enhanced TRPV1 phosphorylation via the putative PKA phosphorylation site serine 116. We also detected calcium-sensitive increased TRPV1 activity after TRPA1 activation in dorsal root ganglion neurons. The inhibition of TRPA1 by HC-030031 (1,2,3,6-tetrahydro-1,3-dimethyl-N-[4-(1-methylethyl)phenyl]-2,6-dioxo-7H-purine-7-acetamide, 2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)-N-(4-isopropylphenyl)acetamide) after its initial stimulation (and the calcium-insensitive TRPA1 mutant D477A) still showed increased capsaicin-induced TRPV1 activity. This excludes a calcium-induced additive TRPA1 current after TRPV1 stimulation. Our study shows sensitization of TRPV1 via activation of TRPA1, which involves adenylyl cyclase, increased cAMP, subsequent translocation and activation of PKA, and phosphorylation of TRPV1 at PKA phosphorylation residues. This suggests that cross-sensitization of TRP channels contributes to enhanced pain sensitivity in inflamed tissues.

Opioid withdrawal increases transient receptor potential vanilloid 1 activity in a protein kinase A-dependent manner.

Summary Opioid withdrawal‐induced increased pain sensation is mediated via cAMP/protein kinase A‐dependent sensitization of peripheral transient receptor potential vanilloid 1 channels. Abstract Hyperalgesia is a cardinal symptom of opioid withdrawal. The transient receptor potential vanilloid 1 (TRPV1) is a ligand‐gated ion channel expressed on sensory neurons responding to noxious heat, protons, and chemical stimuli such as capsaicin. TRPV1 can be inhibited via μ‐opioid receptor (MOR)‐mediated reduced activity of adenylyl cyclases (ACs) and decreased cyclic adenosine monophosphate (cAMP) levels. In contrast, opioid withdrawal following chronic activation of MOR uncovers AC superactivation and subsequent increases in cAMP and protein kinase A (PKA) activity. Here we investigated (1) whether an increase in cAMP during opioid withdrawal increases the activity of TRPV1 and (2) how opioid withdrawal modulates capsaicin‐induced nocifensive behavior in rats. We applied whole‐cell patch clamp, microfluorimetry, cAMP assays, radioligand binding, site‐directed mutagenesis, and behavioral experiments. Opioid withdrawal significantly increased cAMP levels and capsaicin‐induced TRPV1 activity in both transfected human embryonic kidney 293 cells and dissociated dorsal root ganglion (DRG) neurons. Inhibition of AC and PKA, as well as mutations of the PKA phosphorylation sites threonine 144 and serine 774, prevented the enhanced TRPV1 activity. Finally, capsaicin‐induced nocifensive behavior was increased during opioid withdrawal in vivo. In summary, our results demonstrate an increased activity of TRPV1 in DRG neurons as a new mechanism contributing to opioid withdrawal‐induced hyperalgesia.

Endothelin-1 potentiates capsaicin-induced TRPV1 currents via the endothelin A receptor.

Endothelin-1 (ET-1) both stimulates nociceptors and sensitizes them to painful stimuli. The cellular mechanisms of the ET-1–mediated effects are only poorly understood. TRPV1, the heat-, proton-, and capsaicin-sensitive cation channel already known to be modulated by a number of cellular mediators released by painful stimuli and during inflammation, is a potential target for the action of ET-1. In immunocytochemistry of rat lumbar dorsal root ganglion using TRPV1- and ETA receptor–specific antibodies, both proteins were found to be co-expressed in small sensory neurons. To provide evidence that ET-1 can modulate TRPV1 activity via the ETA receptor, we used HEK 293 cells transiently co-expressing a fusion protein of TRPV1 and the yellow fluorescent protein (TRPV1-YFP) and the ETA receptor. In whole-cell patch clamp recordings of HEK293 cells co-expressing TRPV1-YFP and the ETA receptor, capsaicin (10 nM) elicited small currents, which were markedly potentiated when capsaicin (10 nM) and ET-1 (100 nM) were applied simultaneously. The data indicate that ET-1 potentiates TRPV1 activity via the ETA receptor and that this process is likely to play a crucial role in the pain-producing and pain-potentiating effects of ET-1. Thus, ETA receptor antagonists may be of importance in painful states with increased circulating ET-1 levels, as found in cancer and in chronic inflammation.