Cells of the rat lateral habenula respond to high-threshold somatosensory inputs.

Extracellular electrical recordings of single units in the anaesthetized rat demonstrate that about two thirds of the neurons in the lateral habenula respond to peripheral noxious stimuli in a way similar to that of the cells in the centrum-medianum complex which can be activated (with several peaks of various latencies) or inhibited. The firing pattern of these lateral habenula cells is either excitatory (75%) or inhibitory (24%) and is related to the intensity of the stimulus; their receptive field is large and bilateral. Most of these cells also respond to other kinds of noxious but not to non-noxious stimuli. The present study strongly suggests that the lateral habenula is a central target at the upper brainstem level for nociceptive inputs. This original finding is consistent with our previous observation of naloxone reversible analgesia induced by stimulation of the habenula and suggests an involvement of this structure in the processing of noxious inputs.