Substance P receptor-like immunoreactive neurons in the caudal spinal trigeminal nucleus send axons to the gelatinosus thalamic nucleus in the rat.

By means of substance P receptor (SPR) immunofluorescence histochemistry combined with Fluoro-Gold (FG) fluorescent retrograde labeling, SPR-like immunoreactive neurons in the caudal spinal trigeminal nucleus of the rat were observed to send their axons to the gelatinosus thalamic nucleus with a clear ipsilateral dominance. FG/SPR double-labeled neurons were distributed mainly in the ventral part of lamina I at the rostral level of the caudal spinal trigeminal nucleus. The percentages of FG/SPR-LI neurons in the total number of SPR-LI neurons and FG-labeled neurons are 10.5% and 31.1%, respectively. The present results suggest that trigemino-gelatinosus thalamic projection neurons with SPR-LI in the caudal spinal trigeminal nucleus might receive SP-containing, nociceptive primary afferent fibers from the orofacial region and transmit nociception to the gelatinosus thalamic nucleus.