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Keywords:

  • capsaicin;
  • knockout mice;
  • morphine;
  • NMDA;
  • spinal cord

Abstract

Nociceptive primary afferents have the capacity to induce a state of increased excitability in dorsal horn neurons of the spinal cord or central sensitization causing thermal hyperalgesia and touch-evoked pain (allodynia). It is believed that primary afferent C-fibres become hypersensitive and induce hyperalgesia and that low-threshold Aβ-fibres are responsible for induction of allodynia, the mechanisms of which remain elusive. We previously showed that intrathecal administration of prostaglandin E2 (PGE2) and prostaglandin F (PGF) induce allodynia in conscious mice. Here we demonstrated that selective elimination of C-fibres by neonatal capsaicin treatment resulted in the disappearance of allodynia induced by PGE2, but not that by PGF. PGE2-induced allodynia was not observed in N-methyl-D-aspartate (NMDA) receptor ε1 subunit knockout mice and was sensitive to morphine. In contrast, PGF-induced allodynia was not observed in NMDA ε4 subunit knockout mice and was insensitive to morphine. Furthermore, while PGF showed a capsaicin-insensitive feeble facilitatory action on evoked excitatory postsynaptic currents in dorsal horn neurons, PGE2 induced a long-lasting facilitation of evoked excitatory postsynaptic currents in a capsaicin-sensitive manner. Taken together, the present study demonstrates that there are two pathways for induction of allodynia and that capsaicin-sensitive C-fibres may participate in PGE2-induced allodynia.