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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.