Midbrain stimulation inhibits tail-flick only at currents sufficient to excite rostral medullary neurons.

The effects of midbrain electrical stimulation on the activity of tail-flick (TF) related neurons in the rostral ventromedial medulla (RVM) were studied. Neurons whose activity either decreased (off-cells) or increased (on-cells) immediately prior to TF were examined. Of 31 off- and on-cells, 26 (84%) showed increased activity during midbrain stimulation sufficient to suppress the TF. Furthermore, in 21 of these cells, the threshold for activation was identical to the threshold for TF suppression, and in the other 5 cells the threshold difference was less than or equal to 5 microA. This study provides evidence that off-and on-cells in the RVM mediate the antinociceptive actions of midbrain stimulation.