title: Inhibition by spinal mu- and delta-opioid agonists of afferent-evoked substance P release.
abstract: Opioid mu- and delta-receptors are present on the central terminals of primary afferents, where they are thought to inhibit neurotransmitter release. This mechanism may mediate analgesia produced by spinal opiates; however, when they used neurokinin 1 receptor (NK1R) internalization as an indicator of substance P release, Trafton et al. (1999) noted that this evoked internalization was altered only modestly by morphine delivered intrathecally at spinal cord segment S1-S2. We reexamined this issue by studying the effect of opiates on NK1R internalization in spinal cord slices and in vivo. In slices, NK1R internalization evoked by dorsal root stimulation at C-fiber intensity was abolished by the mu agonist [D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO) (1 microM) and decreased by the delta agonist [D-Phe2,5]-enkephalin (DPDPE) (1 microM). In vivo, hindpaw compression induced NK1R internalization in ipsilateral laminas I-II. This evoked internalization was significantly reduced by morphine (60 nmol), DAMGO (1 nmol), and DPDPE (100 nmol), but not by the kappa agonist trans-(1S,2S)-3,4-dichloro-N-mathyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide hydrochloride (200 nmol), delivered at spinal cord segment L2 using intrathecal catheters. These doses of the mu and delta agonists were equi-analgesic as measured by a thermal escape test. Lower doses neither produced analgesia nor inhibited NK1R internalization. In contrast, morphine delivered by percutaneous injections at S1-S2 had only a modest effect on thermal escape, even at higher doses. Morphine decreased NK1R internalization after systemic delivery, but at a dose greater than that necessary to produce equivalent analgesia. All effects were reversed by naloxone. These results indicate that lumbar opiates inhibit noxious stimuli-induced neurotransmitter release from primary afferents at doses that are confirmed behaviorally as analgesic.
authors.affiliation: Department of Anesthesiology, University of California-San Diego, La Jolla, California 92093, USA.
authors.forename: Ichiro
authors.initials: I
authors.lastname: Kondo
authors.forename: Juan Carlos G
authors.initials: JC
authors.lastname: Marvizon
authors.forename: Bingbing
authors.initials: B
authors.lastname: Song
authors.forename: Frances
authors.initials: F
authors.lastname: Salgado
authors.forename: Simone
authors.initials: S
authors.lastname: Codeluppi
authors.forename: Xiao-Ying
authors.initials: XY
authors.lastname: Hua
authors.forename: Tony L
authors.initials: TL
authors.lastname: Yaksh
mesh_terms: D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer / Q000494:pharmacology; D000344:Afferent Pathways / Q000378:metabolism*; D018712:Analgesics, Non-Narcotic / Q000494:pharmacology; D000701:Analgesics, Opioid / Q000008:administration & dosage*; D000704:Analysis of Variance; D000818:Animals; D001522:Behavior, Animal; D004305:Dose-Response Relationship, Drug; D004333:Drug Administration Routes; D004347:Drug Interactions; D004558:Electric Stimulation / Q000379:methods; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D020881:Enkephalin, D-Penicillamine (2,5)- / Q000494:pharmacology; D007839:Functional Laterality; D007120:Immunochemistry / Q000379:methods; D066298:In Vitro Techniques; D008297:Male; D018613:Microscopy, Confocal / Q000379:methods; D009020:Morphine / Q000008:administration & dosage*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D010147:Pain Measurement; D010812:Physical Stimulation / Q000379:methods; D051381:Rats; D017207:Rats, Sprague-Dawley; D018040:Receptors, Neurokinin-1 / Q000378:metabolism; D013116:Spinal Cord / Q000187:drug effects* / Q000528:radiation effects; D013126:Spinal Nerve Roots / Q000528:radiation effects; D013373:Substance P / Q000378:metabolism*
chemical_list: D018712:Analgesics, Non-Narcotic; D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D018040:Receptors, Neurokinin-1; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D013373:Substance P; D009270:Naloxone; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D009020:Morphine; D020881:Enkephalin, D-Penicillamine (2,5)-