Neonatal capsaicin treatment decreased substance P receptor immunoreactivity in lamina III neurons of the dorsal horn

Using immunohistochemistry and in situ hybridization technique, the distribution of substance P (SP) and SP receptors was studied in the dorsal horn of the rat spinal cord after neonatal capsaicin treatment. Sprague-Dawley rats administered 100 mg/kg of capsaicin subcutaneously within 24 h after birth were examined at 8 weeks of age. In the capsaicin administered rats, slight reduction of SP immunoreactivities in lamina I, and severe decrease in lamina II were observed. In the control group, SP receptor-mRNA was observed in all laminae, and SP receptor-immunoreactivities were seen to be intense in laminae I and III. In contrast, in the capsaicin administered rats, the SP receptor-mRNA expression was low in laminae II-V, and SP receptor immunoreactivities decreased in laminae III-V. Furthermore, the density of the SP receptor immunoreactivities was considerably decreased in the nerve cells of lamina III. We concluded that elevation of the threshold to painful stimulation in rats was as a result of the decrease in SP immunoreactive afferent fibers in laminae I and II, decrease of the SP receptor-mRNA in the laminae II-V, or the decrease in SP receptor immunoreactive neurons in laminae III-V.