title: Differential presynaptic effects of opioid agonists on Adelta- and C-afferent glutamatergic transmission to the spinal dorsal horn.
abstract: BACKGROUND
  Although intrathecal administration of opioids produces antinociceptive effects in the spinal cord, it has not been established whether intrathecal opioid application more effectively terminates C fiber-mediated pain than A fiber-mediated pain. Here, the authors focus on the differences in opioid actions on Adelta- and C-afferent responses.
  
  
  METHODS
  Using the whole cell patch clamp technique, the authors investigated the presynaptic inhibitory actions of micro-, delta-, and kappa-opioid receptor agonists on primary afferent-evoked excitatory postsynaptic currents (EPSCs) in substantia gelatinosa neurons of adult rat spinal cord slices.
  
  
  RESULTS
  The micro agonist DAMGO (0.1, 1 microM) reduced the amplitude of glutamatergic monosynaptic Adelta- or C fiber-evoked EPSCs. C fiber-evoked EPSCs were inhibited to a greater extent than Adelta fiber-evoked EPSCs. The delta agonist DPDPE (1, 10 microM) produced modest inhibition of Adelta- or C fiber-evoked EPSCs. In contrast, the kappa agonist U69593 (1 microM) did not affect the amplitude of either Adelta or C fiber-evoked EPSCs.
  
  
  CONCLUSIONS
  These results indicate that opioids suppress excitatory synaptic transmission mainly through activation of micro receptors on primary afferent C fibers. Given that the substantia gelatinosa is the main termination of Adelta and C fibers transmitting nociceptive information, the current findings may partially explain the different potency of opioid agonists.
authors: Division of Anesthesiology, Niigata University Graduate School of Medical and Dental Sciences Niigata, Japan.
authors: Miho
authors: M
authors: Ikoma
authors: Tatsuro
authors: T
authors: Kohno
authors: Hiroshi
authors: H
authors: Baba
mesh_terms: D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000494:pharmacology; D000818:Animals; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000008:administration & dosage / Q000494:pharmacology*; D018683:Excitatory Amino Acid Agents / Q000008:administration & dosage / Q000494:pharmacology*; D019706:Excitatory Postsynaptic Potentials; D018698:Glutamic Acid; D066298:In Vitro Techniques; D009412:Nerve Fibers / Q000187:drug effects*; D009413:Nerve Fibers, Myelinated / Q000187:drug effects; D036421:Nerve Fibers, Unmyelinated; D018408:Patch-Clamp Techniques; D020671:Posterior Horn Cells / Q000187:drug effects*; D017729:Presynaptic Terminals / Q000187:drug effects*; D051381:Rats; D013131:Spine / Q000187:drug effects; D013376:Substantia Gelatinosa / Q000187:drug effects; D009435:Synaptic Transmission / Q000187:drug effects*
chemical_list: D000701:Analgesics, Opioid; D018683:Excitatory Amino Acid Agents; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D018698:Glutamic Acid