Electron microscopic study of the effect of capsaicin on the mouse chorda tympani nerves

Studies were made to determine whether this nerve contains capsaicin-sensitive fibres. Capsaicin (50 mg/kg, subcutaneously) was injected into 2 animals on day 2 after birth and into one animal on days 2 and 3 after birth. Both chorda tympani of these and 4 control mice were later excised. The constituent fibres of 3 of the capsaicin-treated and all 8 control nerves were then analysed. The myelinated fibres in 3 chorda tympani of treated and control animals were measured, and the unmyelinated axons in Schwann cells were counted from electron-micrograph montages of the entire nerve. Normal chorda tympani contained about 600 nerve fibres, 55% myelinated and 45% unmyelinated. Capsaicin-treatment did not change the constituent fibres nor the size distribution of the myelinated fibres. Thus no capsaicin-sensitive, nociceptive fibres were found in the mouse chorda tympani. Capsaicin does not destroy the neurones of the geniculate ganglion and parasympathetic, presynaptic fibres. Therefore, gustation and secretion of saliva are not influenced by capsaicin.