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"Off" and "on" cells of the medulla oblongata as possible mediators of analgesia produced by mesencephalic and diencephalic stimulation in rats.

The present study shows the effects of electrical' stimulation of mesencephalic and diencephalic structures on the activity of two types of neuron from the rostral ventromedial medulla, which are related to the nocifensive reflex known as tail flick response (TF). One type of neuron, the off-cell, abruptly stops firing immediately before the tail is flicked, while the other type, the on-cell, increases firing just before the flick. When electrical stimulation was applied to mesencephalic and diencephalic structures the TF was inhibited and, simultaneously, both kinds of cells showed increments in their activities. On average, this increment was 81 +/- 22.09% for the off-cells, and 1563 +/- 257.66% for the on-cells. Quantitative analysis showed a directly proportional relationship between the activation of the firing rate of both kinds of cells and the intensity of the stimulation currents. Also, when the stimulation electrode was positioned more rostrally in the brain, greater currents were needed to reach the threshold of analgesia. The present work contributes to a large body of evidence indicating that off-cells, on-cells, or both, are involved in the complex mechanism of the control of nociceptive transmission and nocifensive reflexes.