Intensity-dependent nociceptive responses from presumed dopaminergic neurons of the substantia nigra, pars compacta in the rat and their modification by lateral habenula inputs.

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.