Habenula lesions attenuate lateral hypothalamic analgesia in the formalin test.

The purpose of the experiment was to determine the role of the habenula complex (Hb) in lateral hypothalamic (LH) analgesia for tonic pain. The results demonstrate that lesions of the Hb attenuated LH analgesia in the formalin test. The result of Hb lesions cannot be attributed to non-specific brain damage since bilateral caudate nucleus lesions of similar extent to the Hb complex damage failed to modify the analgesic effect of LH stimulation. The critical damage was probably to the lateral habenula nucleus since it receives a major input from the LH. In addition to the primary finding, we observed that lesions of the Hb had no effect on the baseline response to formalin.