ABSTRACT
On-and off-cells of the rostral ventromedial medulla project to the spinal cord, where off-cells are postulated to inhibit, and on-cell facilitate, nociceptive transmission. In the present experiments, unitary recordings were made from ventrobasal (VB) thalamic neurons and, simultaneously, on-or off-cells in lighhtly anesthetized rats. When the tail was heated (45-55ºC) off-cells stopped firing (pause) and on-cells began to fire (burst) 0.5-06s before VB neurons responded to the tail stimulus. Administration of morphine sulfate (2mg/kg i.v) concurrently prevented the off-cell pause, the on-cell burts an the VB neuronal response. Naloxone (1mg/kg i.v) recersed 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