Substance P receptor-expressing neurons in the medullary and spinal dorsal horns projecting to the nucleus of the solitary tract in the rat.

By using substance P receptor (SPR) immunofluorescence histochemistry combined with fluorescent retrograde labeling, we examined the distribution of the trigeminal and spinal neurons with SPR-like immunoreactivity (-LI) projecting to the nucleus of the solitary tract in the rat. After injection of Fluoro-Gold (FG) into the nucleus of the solitary tract, FG-labeled neurons showing SPR-LI were mainly seen in lamina I of the medullary and spinal dorsal horns, lamina V and the lateral spinal nucleus of the spinal cord. The present results suggest that the trigeminal and spinal neurons with SPR-LI, especially those in lamina I may be involved in the transmission of somatic and/or visceral nociceptive information from the medullary and spinal dorsal horns to the nucleus of the solitary tract.