Substance P receptor (NK1)-immunoreactive neurons projecting to the periaqueductal gray: distribution in the spinal trigeminal nucleus and the spinal cord of the rat.

Substance P receptor (SPR)-immunoreactive neurons projecting to the periaqueductal gray (PAG) were examined in the rat spinal trigeminal nucleus and spinal cord by a retrograde tracing method combined with immunofluorescence histochemistry. After injection of Fluoro-gold (FG) into the PAG, SPR-immunoreactive neurons labeled with FG were observed mainly in the lateral spinal nucleus and lamina I of the medullary and spinal dorsal horns and additionally in laminae V and X of the spinal cord.