Concurrent effect of morphine on thalamic nociceptive neurons and medullary on- and off-cells.
On- and off-cells of the rostral ventromedial medulla project to the spinal cord, where off-cells are postulated ot inhibit, and on-cells to facilitate, nociceptive transmission. In the present experiments, unitary recordings were made from ventrobasal (VB) thalamic neurons and, simultaneously, on- or off-cells in lightly anesthetized rats. When the tail was heated (45-55 degrees C) off-cells stopped firing (pause) and on-cells began to fire (burst) 0.5-0.6s before VB neurons responded to the tail stimulus. Administration of morphine sulfate (2 mg/kg i.v.) concurrently prevented the off-cell pause, the on-cell burst an the VB neuronal response. Naloxone (1 mg/kg i.v.) reversed these effects. The results are consistent with the notion that prevention of the off-cell pause and the on-cell burst by morphine is responsible for the lack of nociceptive information flow towards the thalamus.
D000818:Animals; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects / Q000502:physiology; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009474:Neurons / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D009619:Nociceptors / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time; D013788:Thalamus / Q000187:drug effects*
D009270:Naloxone; D009020:Morphine