62 Neuroseience Letters, 115 (1990) 62-67 Elsevier Scientific Publishers Ireland Ltd. NSL 06976 A-fibers mediate mechanical hyperesthesia and allodynia and C-fibers mediate thermal hyperalgesia in a new model of causalgiform pain disorders in rats Yoram Shir and Ze'ev Seltzer Departments of Anaesthesiology and Pain Clinic, Hadassah University Hospital, Jerusalem (Israel) and Physiology Branch, Faculty o/Dental Medicine, Hebrew University of Jerusalem (Israel) (Received 17 January 1990; Revised version received 9 March 1990; Accepted 12 March 1990) Key words: Causalgia; Partial nerve injury; Myelinated afferent fibers; Unmyelinated afferent fibers; Allodynia; Hyperalgesia; Capsaicin; Rat Unilateral tight tigation of about half of the sciatic nerve in rats rapidly produces sympathetically depen- dent neuropathic pain which lasts many months and resembles causalgia in humans. The sensory abnor- malities detected at the plantar side of the hindpaws include: (1) nocifensive responses to repetitive light touch (allodynia); (2) bilateral reduction in withdrawal thresholds to repetitive von-Frey hair stimulation (mechanical hyperesthesia); (3) bilateral reduction in withdrawal thresholds to CO2 laser heat pulses; and (4) unilateral increase in response duration to an intense laser heat pulse (thermal hyperalgesia). Using neonatal capsaicin treatment, we determined the type of afferent fiber remaining in the partially injured nerve, which mediates these disorders. Capsaicin, which destroys most C- and some A6-fibers in peripheral nerves, had no effect on the touch-evoked allodynia and mechanical hyperesthesia that are produced by partial sciatic nerve injury. These disorders were, therefore, mediated by myelinated fibers. In contrast, thermal hyperalgesia failed to develop in capsaicin-treated rats following partial nerve injury. Thus, ther- mal hyperalgesia produced by partial nerve injury appears to be mediated by heat-nociceptive C-fibers. In previous reports we described a novel behavioral model of causalgiform pain in rats, produced by a unilateral ligation of about half of the sciatic nerve high in the thigh [13, 14, 16]. At the point cut, the nerve is not yet fasciculated [12] and there- fore, this injury eliminates fibers evenly across the innervation of the hindpaw. We could not detect hypoesthetic skin areas. Starting hours after the operation and last- ing for several months thereafter, rats developed guarding behavior of the partially denervated hindpaw and often licked it. These signs suggest the presence of sponta- neous pain. In response to repetitive von-Frey hair stimulation there was a bilateral sharp decrease in withdrawal thresholds ('touch-evoked hyperesthesia'). When the Correspondence." Z. Seltzer, Physiology Branch, Faculty of Dental Medicine, Hebrew University of Jerusa- lem. POB 1172, 91010 Jerusalem, Israel. 0304-3940/90/$ 03.50 (t') 1990 Elsevier Scientific Publishers Ireland Ltd.