Suppressive effect of spinal dorsal-horn neuronal activity by local spinal-cord cooling is reversed by naloxone in cats

Purpose. The purpose of this study was to assess the effect of local spinal cord cooling on spinal dorsal-horn neuronal activity, with special emphasis on the role of endogenous opioid. Methods. Decerebrate, spinal-cord-transected cats (n = 30) were subjected to local spinal-cord irrigation, using 0.9N saline solution (15°C; n = 15, and 35°C; n = 15) for 90 min. The extracellular, single-cell activity of spinal dorsal-horn neurons responding to noxious stimulation was recorded. Sixty-one minutes after induction of local spinal-cord irrigation, naloxone (0.1 mg·kg^-1) was administered intravenously. Local spinal-cord blood flow was measured using the hydrogen clearance technique. Results. Local spinal cord cooling produced significant suppression of both spontaneous and evoked activity (33.1 ± 7.7% and 31.4 ± 5.5%, respectively; mean ± SE). Naloxone reversed this suppression immediately. Local spinal-cord blood flow was significantly reduced during spinal-cord cooling, but naloxone did not change local spinal-cord blood flow. Conclusion. The results demonstrate that endogenous opioids may play an important role in dorsal-horn neuronal suppression induced by local spinal-cord cooling.