Chronic effects of topical application of capsaicin to the sciatic nerve on responses of primate spinothalamic neurons

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Abstract
Onses of STT cells recorded in 3 experimental groups were compared: untreated (21 cells), vehicle-treated (40 cells), and capsaicin-treated (83 cells). The background activity of cells in the vehicle- and capsaicin-treated groups was the same as in the untreated group (that is, cells on the side contralateral to surgery). Responses to innocuous (BRUSH) and noxious (PINCH) mechanical stimuli were unchanged by vehicle or by capsaicin treatment. However, responses to other noxious (PRESSURE and SQUEEZE) mechanical stimuli were significantly increased in the vehicle-treated group. Compared with a large reference population, all experimental groups showed a significant increase in overall responsiveness to mechanical stimuli (as determined by cluster analysis), greatest in the vehicle-treated group. Responses to noxious heat stimuli were significantly reduced in the capsaicin-treated group for 45°C and 47°C stimuli. Volleys in A fibers, probably Aδ fibers, evoked prolonged responses in many STT cells of all treatment groups. Electron microscopic counts of axons in the sciatic nerves of animals treated with capsaicin showed a reduced number of C fibers but no appreciable loss of myelinated axons. This loss of unmyelinated sensory fibers was presumably responsible for the reduction in the responses of the STT cells to noxious heat stimuli. Increased responses to some noxious mechanical stimuli and to A fiber volleys may have been the consequence of several factors, including surgical manipulation, a chemical action of vehicle and a contralateral action of capsaicin treatment. ∗Correspondence to: Jin Mo Chung, Ph.D., Marine Biomedical Institute, University of Texas Medical Branch, 200 University Boulevard, Galveston, TX 77555-0843, USA. Tel.: (409) 772-2106; FAX: (409) 762-9382. Submitted July 16, 1992; revised December 11, 1992; accepted December 18, 1992. © Lippincott-Raven Publishers....