Intrathecally applied morphine inhibits nociceptive C fiber input to the primary somatosensory cortex (SI) of the rat

Pain. 1998 Sep;77(3):323-9. doi: 10.1016/s0304-3959(98)00115-8.


Nociceptive C fiber input to SI in the halothane-nitrous oxide anesthetized rat was assessed by recording cortical field potentials evoked by noxious thermal cutaneous stimulation with CO2-laser pulses. Morphine topically applied onto the lumbar spinal cord produced a dose-dependent inhibition of nociceptive C fiber input from the hind paw to the contralateral SI. The inhibitory effect of morphine was reversed by naloxone. Potentials evoked by CO2-laser stimulation of the forepaw were unaffected by morphine applied on the lumbar cord, indicating that the effect of morphine was exerted at the segmental level. It is concluded that input from nociceptive C fibers to SI is relayed in the spinal cord and can be inhibited by spinal opioid receptor activation. The present method offers an interesting model of ascending nociceptive transmission to the cerebral cortex.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Analgesics, Opioid / pharmacology*
  • Animals
  • Evoked Potentials, Somatosensory / drug effects
  • Injections, Spinal
  • Male
  • Morphine / pharmacology*
  • Naloxone / pharmacology
  • Narcotic Antagonists / pharmacology
  • Nerve Fibers / drug effects
  • Nerve Fibers / physiology
  • Neural Inhibition / drug effects
  • Neurons, Afferent / drug effects
  • Neurons, Afferent / physiology*
  • Neurons, Afferent / ultrastructure
  • Nociceptors / drug effects*
  • Pain / drug therapy
  • Physical Stimulation
  • Rats
  • Rats, Wistar
  • Somatosensory Cortex / cytology*
  • Somatosensory Cortex / physiology
  • Spinal Cord / cytology
  • Spinal Cord / physiology
  • Synaptic Transmission / drug effects


  • Analgesics, Opioid
  • Narcotic Antagonists
  • Naloxone
  • Morphine