Systemic morphine reduces the wind-up of trigeminal nociceptive neurons.
We assessed the effects of intravenous morphine on the wind-up of nociceptive neurons of the spinal trigeminal nucleus oralis (Sp5O). Extracellular recordings of Sp5O nociceptive convergent neurons were performed in intact halothane-anesthetized rats. Wind-up of C-fiber-evoked responses was elicited by repetitive electrical stimulation (train of 16 shocks, 0.66 Hz) of their receptive field at C-fiber intensity (3 times the threshold). Wind-up was tested for its sensitivity to morphine (6 mg/kg,i.v.), and the specificity of the effects was verified with naloxone (0.4 mg/kg, i.v.). Nineteen convergent neurons displaying wind-up were recorded. Morphine reduced the wind-up of all but one. In five cases, notwithstanding a reduced wind-up, the neuronal response evoked by the first stimulus in the train (initial input) was unexpectedly increased. Naloxone always antagonized morphine inhibitory effects on the wind-up. When administered systemically, morphine reduced the wind-up of trigeminal nociceptive neurons. This inhibitory effect occurred independently of morphine's ability to affect the initial C-fiber-evoked input. Our findings support the idea that systemic morphine probably blocks wind-up by acting at opioid receptors located postsynaptically to nociceptive primary afferents.
Dualé
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Laboratoire de Physiologie Oro-Faciale, Faculté de Chirurgie Dentaire, 11 Bd Charles de Gaulle, 63000 Clermont-Ferrand, France.
Raboisson
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Molat
J L
JL
Dallel
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R
D000200:Action Potentials / Q000187:drug effects* / Q000502:physiology; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004558:Electric Stimulation; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009412:Nerve Fibers / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects* / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D014279:Trigeminal Nucleus, Spinal / Q000187:drug effects* / Q000502:physiology*
D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D009270:Naloxone; D009020:Morphine