Inhibition of spinal nociceptive responses after intramuscular injection of capsaicin involves activation of noradrenergic and opioid systems

Extracellular recordings of wide dynamic range neurones in the dorsal horn driven by electrical stimulation of the sciatic nerve were performed in intact urethane-anaesthetized Sprague–Dawley rats. The electrically evoked neuronal responses were defined as A- and C-fibres responses according to latencies, and the effect of a deep nociceptive conditioning stimulus induced by 200 μg capsaicin (8-methyl- N-vanillyl-6-noneamide) injected into the contralateral gastrocnemius–soleus muscle was studied for at least 30 min. Independent of the size and location of the receptive field of the neurone under study, a clear inhibition of the neuronal responses was observed. The electrically evoked C-fibre responses were inhibited to 53% of baseline 15–30 min after injection of capsaicin. This inhibition was only slightly attenuated by 125 nmol of the α-adrenoceptor antagonist phentolamine or 250 nmol of the opioid receptor antagonist naloxone applied directly onto the spinal cord when the two compounds were administered separately 5 min before capsaicin. In contrast, when a mixture of the two compounds was given 5 min before capsaicin, the effect of capsaicin was completely abolished. These results indicate that activation of the capsaicin-sensitive afferents in the gastrocnemius–soleus muscle inhibits the electrically evoked C-fibre responses in the dorsal horn by activating noradrenergic and opioidergic inhibitory systems. Moreover, our data indicate that the activation of these two systems following injection of capsaicin has a sub-additive inhibitory effect on the wide dynamic range neurones in the spinal cord. We conclude that only one of these systems is sufficient for the inhibition to occur.