title: Resistance to morphine analgesic tolerance in rats with deleted transient receptor potential vanilloid type 1-expressing sensory neurons.
abstract: Deletion of transient receptor potential vanilloid type 1 (TRPV1)-expressing afferent neurons reduces presynaptic mu opioid receptors but paradoxically potentiates the analgesic efficacy of mu opioid agonists. In this study, we determined if removal of TRPV1-expressing afferent neurons by resiniferatoxin (RTX), an ultrapotent capsaicin analog, influences the development of opioid analgesic tolerance. Morphine tolerance was induced by daily intrathecal injections of 10 microg of morphine for 14 consecutive days or by daily i.p. injections of 10 mg/kg of morphine for 10 days. In vehicle-treated rats, the effect of intrathecal or systemic morphine on the mechanical withdrawal threshold was gradually diminished within 7 days. However, the analgesic effect of intrathecal and systemic morphine was sustained in RTX-treated rats at the time the morphine effect was lost in the vehicle group. Furthermore, the mu opioid receptor-G protein coupling in the spinal cord was significantly decreased ( approximately 22%) in vehicle-treated morphine tolerant rats, but was not significantly altered in RTX-treated rats receiving the same treatment with morphine. Additionally, there was a large reduction in protein kinase Cgamma-immunoreactive afferent terminals in the spinal dorsal horn of RTX-treated rats. These findings suggest that loss of TRPV1-expressing sensory neurons attenuates the development of morphine analgesic tolerance possibly by reducing mu opioid receptor desensitization through protein kinase Cgamma in the spinal cord. These data also suggest that the function of presynaptic mu opioid receptors on TRPV1-expressing sensory neurons is particularly sensitive to down-regulation by mu opioid agonists during opioid tolerance development.
authors.affiliation: Department of Anesthesiology and Pain Medicine, Unit 409, The University of Texas M. D. Anderson Cancer Center, 1400 Holcombe Boulevard, Houston, TX 77030-4009, USA.
authors.forename: S-R
authors.initials: SR
authors.lastname: Chen
authors.forename: A
authors.initials: A
authors.lastname: Prunean
authors.forename: H-M
authors.initials: HM
authors.lastname: Pan
authors.forename: K L
authors.initials: KL
authors.lastname: Welker
authors.forename: H-L
authors.initials: HL
authors.lastname: Pan
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D004224:Diterpenes / Q000633:toxicity; D015536:Down-Regulation / Q000187:drug effects / Q000502:physiology; D004361:Drug Tolerance / Q000502:physiology*; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009410:Nerve Degeneration / Q000139:chemically induced; D009475:Neurons, Afferent / Q000187:drug effects / Q000378:metabolism*; D009619:Nociceptors / Q000187:drug effects / Q000378:metabolism; D010146:Pain / Q000188:drug therapy / Q000378:metabolism* / Q000503:physiopathology; D017288:Pain Threshold / Q000187:drug effects / Q000502:physiology; D020671:Posterior Horn Cells / Q000187:drug effects / Q000378:metabolism*; D011493:Protein Kinase C / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D043562:Receptors, G-Protein-Coupled / Q000187:drug effects / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000187:drug effects / Q000378:metabolism; D050916:TRPV Cation Channels / Q000235:genetics / Q000378:metabolism*
chemical_list: D000701:Analgesics, Opioid; D004224:Diterpenes; D043562:Receptors, G-Protein-Coupled; D017450:Receptors, Opioid, mu; D050916:TRPV Cation Channels; C493922:Trpv1 protein, rat; D009020:Morphine; C024353:resiniferatoxin; C087857:protein kinase C gamma; D011493:Protein Kinase C