Neuroanatomical effects of capsaicin on the primary afferent neurons

Studies by N. JANCSO and his associates in the 1970's established that capsaicin in paprika exerts selective damage on nociceptive primary sensory neurons. The physiological and pharmacological aspects of capsaicin's effect have been repeatedly reviewed, but no report seems available concerning the neuroanatomical changes caused by capsaicin. This paper first reviews the neuroanatomical aspect of the lesion caused by capsaicin. Special attention is paid to quantitative estimations made by our group and others on the loss of dorsal root ganglion (DRG) cells, dorsal root nerve fibers, the saphenous nerve, chorda tympani nerve, and pulp nerves after neonatal treatment with capsaicin. The degenerating process of DRG cells induced by capsaicin is discussed with respect to necrosis and apoptosis. The capsaicin receptors found recently are concisely introduced with reference to their action. A discrepancy between a marked loss of dorsal root C-fibers and an unexpectedly intact response to noxious heat in mice treated with capsaicin at neonate is discussed, and attension is given to nerves sprouting from capsaicin-resistant DRG cells in the superficial dorsal horn. In addition, the architecture of the synapses between the central endings of the capsaicin-sensitive primary afferent neurons and the intrinsic inhibitory interneurons is described and its possible significance considered in terms of the transmission of nociceptive information.