Fentanyl Suppression of Nociceptive Neurons in the Superficial Dorsal Horn of the Cat

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Abstract
This study was designed to examine the influence of spinally administered fentanyl on the spontaneous and noxiously evoked activity of high threshold (HT) and wide dynamic range (WDR) neurons in the superficial layers (lamina I and II) of the dorsal horn of cats made decerebrate and in which the spinal cord had been transected. Single unit activity was recorded using extracellular microelectrode recording techniques. Neuronal activity was evoked by the presentation of noxious radiant heat (51.degree. C) to the cells'' receptive fields on the bind paws. Evoked activity of WDR neurons was monitored, both before and after the spinal administration of either 10 .mu.g (n = 9) or 25 .mu.g (n = 10) of fentanyl. HT neurons were examined before and after either 25 .mu.g (n = 7) or 50 .mu.g (n = 7) of spinally administered fentanyl. In all cases 31 min after fentanyl administration naloxone (0.1 mg) was administered intravenously (iv), and its antagonistic effect on the fentanyl suppression was determined. All doses of fentanyl tested suppressed both spontaneous and evoked activity of both types of neurons. Within 30 minutes 10 and 25 .mu.g of fentanyl reduced the mean evoked activity of WDR neurons to 61% and 19% of control values, respectively, and 25 and 50 .mu.g of fentanyl reduced the mean evoked activity of HT neurons to 70% and 47% of control values, respectively. Naloxone reversed the suppression seen in all cells studied. The results of the present study demonstrate that HT neurons are significantly less supppressed by the spinal administration of fentanyl than WDR neurons located in the same superficial layers of the dorsal horn. This difference in the sensitivity of HT neurons and WDR neurons to fentanyl suppression may reflect an important difference in the mechanism of spinal opiate analgesia.

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