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

Using immunohistochemistry and in situ hybridization technique, the distrib ution of substance P (Sf) and SP receptors was studied in the dorsal horn o f the rat spinal cord after neonatal capsaicin treatment. Sprague-Dawley ra ts administered 100 mg/kg of capsaicin subcutaneously within 24 h after bir th were examined at 8 weeks of age. In the capsaicin administered rats, sli ght reduction of SP immunoreactivities in lamina I, and severe decrease in lamina II were observed. In the control group, SP receptor-mRNA was observe d in all laminae, and SP receptor-immunoreactivites 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 immun oreactivities decreased in laminae III-V. Furthermore, the density of the S P receptor immunoreactivities was considerably decreased in the nerve cells of lamina III. We concluded that elevation of the threshold to painful sti mulation in rats was as a result of the decrease in SP immunoreactive affer ent fibers in laminae I and II, decrease of the SP receptor-mRNA in the lam inae II-V, or the decrease in SP receptor immunoreactive neurons in laminae III-V. (C) 2000 Elsevier Science Ireland Ltd and the Japan Neuroscience So ciety. All rights reserved.