Suppression of noxiously evoked WDR dorsal horn neuronal activity by spinally administered morphine

The present study was carried out in order to examine the ability of spinally administered morphine to suppress noxiously evoked activity of wide-dynamic-range (WDR) neurons in the dorsal horn of the spinal cord in decerebrate, spinal cord-transected cats. All cells (n = 25) responded maximally to high-intensity noxious heat stimulation (51 degrees C) and were classified as wide dynamic range neurons. The spinal administration of 0.1 mg of morphine caused a significant reduction of noxiously evoked activity but did not significantly change spontaneous activity. The 0.25-mg dose caused a significant reduction of both types of activity. Thirty minutes after spinal administration, 0.1 mg of morphine caused a 27% reduction of spontaneous activity and a 43% reduction of noxiously evoked activity. The 0.25-mg dose reduced spontaneous activity by 44% and the evoked activity by 70%. Naloxone partially reversed the morphine-induced neuronal suppression. In addition, in the four neurons in which it was tried, spinally administered epinephrine was found to further suppress the remaining neuronal activity following the spinal morphine effect. These results demonstrate for the first time that spinally administered morphine is capable of suppressing noxiously evoked activity of wide-dynamic-range neurons in the dorsal horn of the spinal cord. They also demonstrate the dose-dependent nature of this effect and the potential importance of the interaction between morphine and adrenergic agonists in blocking information about noxious events. This information provides a probable mechanism of action for spinal opiate analgesia.