Serotonin and substance P afferents to parafascicular and central medial nuclei.

Potent analgesia is elicited by electrical stimulation of the periaqueductal gray (PAG), dorsal raphe nucleus (DRN) and intralaminar thalamus. Horseradish peroxidase conjugated wheat germ agglutinin (HRP-WGA) was stereotaxically pressure injected into the parafascicular (PF) or central medial (CM) nucleus to identify brainstem afferents to the intralaminar thalamus. WGA-immunoreactive (-ir) neurons were identified in the DRN, PAG and lateral dorsal tegmentum (LDTg) after PF and CM injections. Many retrogradely labeled cells in the DRN and ventral PAG were also serotonin-ir, and a portion of WGA-ir cells in the LDTg were substance P-ir. These results substantiate previous studies implicating the intralaminar thalamus and periaqueductal region, as well as serotonin and substance P, in antinociception.