CHRONIC EFFECTS OF TOPICAL APPLICATION OF CAPSAICIN TO THE SCIATIC-NERVE ON RESPONSES OF PRIMATE SPINOTHALAMIC NEURONS

The responses of 144 spinothalamic tract (STT) cells were recorded in 15 anesthetized macaque monkeys (Macaca fascicularis). Three to 4 week s prior to the acute experiment, the sciatic nerve was surgically expo sed on one or both sides so that capsaicin or vehicle could be applied . Responses of STT cells recorded in 3 experimental groups were compar ed: untreated (21 cells), vehicle-treated (40 cells), and capsaicin-tr eated (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 innocuo us (BRUSH) and noxious (PINCH) mechanical stimuli were unchanged by ve hicle 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 populati on, all experimental groups showed a significant increase in overall r esponsiveness to mechanical stimuli (as determined by cluster analysis ), greatest in the vehicle-treated group. Responses to noxious heat st imuli were significantly reduced in the capsaicin-treated group for 45 -degrees-C and 47-degrees-C stimuli. Volleys in A fibers, probably Ade lta fibers, evoked prolonged responses in many STT cells of all treatm ent 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 unmyelinate d sensory fibers was presumably responsible for the reduction in the r esponses of the STT cells to noxious heat stimuli. Increased responses to some noxious mechanical stimuli and to A fiber volleys may have be en the consequence of several factors, including surgical manipulation , a chemical action of vehicle and a contralateral action of capsaicin treatment.