Habenular stimulation produces analgesia in the formalin test

Electrical stimulation of the habenula produces a striking reduction of continuous, formalin-induced pain in the rat. The analgesia occurs at current levels which do not appear aversive and persists for variable durations, ranging from 1 to 21 min in this experimental situation. The effect is not blocked by subcutaneous administration of a large dose of naloxone prior to the stimulation, indicating that it is not dependent on an opiate-sensitive system. Stimulation of the adjacent paraventricular nucleus of the thalamus was either aversive or had no effect on the pain scores. The anatomical connections of the habenula suggest that it may mediate the interaction of limbic forebrain structures with midbrain structures known to play a role in pain and analgesia.