The effects of morphine, naloxone and capsaicin on C-fiber reflex and heat evoked discharge in acute spinal cats.

The electrophysiological effects of morphine, naloxone and capsaicin were studied on c fiber reflex and heat evoked discharge in adult spinal cats. C fiber reflex was recorded from the L7 Ventral root at the time of superficial peroneal nerve electric stimulation. Heat evoked discharge was recorded from the L7 ventral root at the time of hind paw heating. The drugs were injected slowly into the right cephalic vein. Our results can be summarized as follows: morphine depressed the excitatory neuronal activity of capsaicin; morphine followed by capsaicin was antagonized by 0.4 mg/Kg of naloxone; at the c fiber reflex, 2 mg/Kg of morphine depressed the neuronal activity; and at heat evoked discharge, 4 mg/Kg of morphine depressed the neuronal activity. These findings suggest that morphine depressed the effect of capsaicin and this effect was antagonized by naloxone. At the c fiber reflex, morphine was more sensitive than at heat evoked discharge.