Exp Neurobiol 1:13-20, 1992

Effect of Neonatal Capsaicin on the Calcitonin Gene-related Peptide (CGRP) Immunoreactive Nerve Fibers in the Dorsal Horn of the Rat.

Won Taek Lee, Jong Eun Lee, Chang-Hyun Yang, and Kyung Ah Park

Department of Anatomy, Yonsei University College of Medicine

The effect of the capsaicin, a major pungent ingredient of the plant genus Capsicum (hot pepper), administration to the neonatal rat results in life-long insensitivity to the noxious stimuli accompanied by destruction of the unmyelinated and lightly myelinated fibers as well as small B type dorsal root ganglion neurons.

The purpose of this study is to determine the effect of capsaicin to the dorsal horn of the rat by densitometric measurement of the primary afferent calcitonin gene-related peptide (CGRP) labelled dorsal horn area and to find the morphological changes at the ultrastructural level using electron microscopic immunocytochemical method.

Area of the CGRP-labelled dorsal horn decreased after neonatal capsaicin administration (38.1% decrease in the area of CGRP-labelled dorsal horn and 20.4% decrease of gray level). These result suggests that the destructive effect of capsaicin was rather limited because many primary afferent CGRP immunoreactive axons had been survived. But, in the ultrastructural level, most of the surviving CGRP-labelled fibers were unmyelinated fibers and shows some abnormalities.

These result raises some possibility that the life-long insensitivity to noxious stimuli after neonatal capsaicin was not the result from the total destruction of the pain conducting fibers alone, but resulted from combined effect of destruction of axon and functional changes of survived axons.

Key words: spinal cord, immunohistochemistry, ultrastructure, primary afferent