The interpeduncular nucleus excites the on-cells and inhibits the off-cells of the nucleus raphe magnus.

The interpeduncular nucleus (IPN) was stimulated electrically while single-cell activity was recorded in the nucleus raphe magnus (NRM) of rats under pentobarbital anesthesia. Two classes of NRM cell were examined, those inhibited (off-cells) and those excited (on-cells) by noxious mechanical skin stimulation. Off-cells (92%) were found to be inhibited by IPN stimulation, whereas on-cells (50%) were excited. Based on previously suggested roles for the NRM neurons, we argue that both effects are hyperalgetic.