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Concurrent effect of morphine on thalamic nociceptive neurons and medullary on- and off-cells
1Centro de Biofísica y Bioquímica, Instituto Venezolano de Investigaciones Científicas (IVIC), Caracas, Venezuela.
Abstract:
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.
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