Inhibition by Spinal μ- and δ-Opioid Agonists of Afferent-Evoked Substance P Release

Authors:
  • JMJuan Carlos G. Marvizon
    Juan Carlos G. Marvizon
  • BSBingbing Song
    Bingbing Song
  • FSFrances Salgado
    Frances Salgado
  • XHXiao-Ying Hua
    Xiao-Ying Hua
  • TYTony L. Yaksh
    Tony L. Yaksh
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Abstract
Opioid μ- and δ-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 andin vivo. In slices, NK1R internalization evoked by dorsal root stimulation at C-fiber intensity was abolished by the μ agonist [d-Ala2,N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO) (1 μm) and decreased by the δ agonist [d-Phe2,5]-enkephalin (DPDPE) (1 μm).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 κ agonisttrans-(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 μ and δ 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.

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