Inhibition of evoked C-fibre responses in the dorsal horn after contralateral intramuscular injection of capsaicin involves activation of descending pathways

In this study extracellular recordings of nociceptive dorsal horn neurones driven by electrical stimulation of the sciatic nerve were performed in int act urethane-anaesthetized Sprague-Dawley rats. Spikes 0-40, 40-250 and 250 -800 ms after stimulus were defined as A and C-fibre responses and post-dis charge, respectively, and the effect of 200 mu g capsaicin (8-methyl-N-vani llyl-6-noneamide) injected into the contralateral gastrocnemius-soleus musc le was investigated. In most cells rested, regardless of the size or locati on of their receptive fields, the injection of capsaicin caused a clear inh ibition of the electrically evoked C-fibre responses. In animals with intac t descending pathways the mean C-fibre response was inhibited to 51% of bas eline 15 min after injection of capsaicin. In contrast, when capsaicin was given during cold block of the spinal cord between the brainstem and the si te of recording in the dorsal horn, the same response was inhibited to 91% of baseline. A significant interaction between cold block and capsaicin was detected. We conclude that stimulation of capsaicin-sensitive afferents in the deep tissue in the hind limb can inhibit the electrically evoked C-fib re responses in the dorsal horn by activating inhibitory descending project ions from higher centres. The model presented here may be an important tool for further investigations of the endogenous descending antinociceptive sy stem. (C) 1999 International Association for the Study of Pain. Published b y Elsevier Science B.V.