Quantitative estimation of the effects of capsaicin on the mouse primary sensory neurons

Studies were made on the quantitative effects of capsaicin on the L4 dorsal roots and their ganglia (DRG) in mice. Capsaicin at a dose of 50 mg/kg was injected s.c. 1-3 times into neonatal mice, while other mice received only vehicle as controls. After capsaicin treatment unmyelinated fibers decreased by 41.3-75.3% compared to controls. After 2 or 3 capsaicin injections there was a greater decrease in unmyelinated fibers than after a single treatment. Capsaicin had most effect on Schwann cell subunit size of more than 13 axons. Reduction of myelinated fibers ranged from 6.1 to 11.6% (mean: 7.9) in 4 mice. A beta and A delta fibers of 4-5 microns diameter were significantly reduced, whereas A delta fibers of less than 2.0 microns diameter were slightly increased. Small DRG neurons (50-500 microns 2 area) decreased by 50.6-77.4% (mean: 69.6) compared to controls. In addition, capsaicin caused a decrease of 14.2-51.6% (mean: 35.2, n = 4) in large-sized neurons (500-1950 microns 2 area). Thus, capsaicin widely affects small and certain types of large neurons as well as unmyelinated and certain myelinated fibers.