Abstract
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 Freund's 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.
Footnotes
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This work was supported by Deutsche Forschungsgemeinschaft (DFG) grant KFO 100/2 and a European Society of Anaesthesiology research grant.
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Parts of this work have been published previously in abstract form [Endres-Becker J, Heppenstall PA, Mousa S, Labuz D, Schäfer M, Stein C, Zöllner C. Opioids modulate the transient receptor potential vanilloid 1 (TRPV1) ion channel. International Narcotic Research Conference; July 10-15, 2005; Annapolis, MD].
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ABBREVIATIONS: DRG, dorsal root ganglion; TRPV1, transient receptor potential vanilloid type 1; μ-receptor, μ-opioid receptor; PKA, protein kinase A; PCR, polymerase chain reaction; CFA, complete Freund's adjuvant; RTX, resiniferatoxin; PBS, phosphate-buffered saline; MEM, minimal essential medium; PTX, pertussis toxin; DAMGO, [d-Ala2, N-Me-Phe4,Gly5-ol]-enkephalin; FSK, forskolin; 8-Br-cAMP, 8-bromo-cAMP; PWL, paw withdrawal latency; ANOVA, analysis of variance.
- Received May 29, 2006.
- Accepted September 27, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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