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The characteristics of nociceptive responses from presumed dopaminergic (DAergic) neurons in the SN were investigated in the anesthetized rat with extracellular recordings. 194 presumed DAergic neurons were recorded. A majority of these neurons (78%) were inhibited by intensive electrical stimulation performed at the tail (PNS) and 15% were excited. Both inhibitory and excitatory responses were intensity-dependent. Single shock stimulation of the lateral habenula (LHb) inhibited 89% of the tested DAergic neurons, most of which (83.8%) were also inhibited by PNS. LHb stimulation increased PNS-induced inhibition of DAergic neurons and electrical destruction of ipsilateral LHb depressed their nociceptive responses. Our results strongly suggest that DAergic neurons encode the nociceptive stimulation intensity and that the LHb shares a step in nociceptive projection to the SN.

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.