Suppressive effect of morphine on single-unit activity of cells in Rexed lamina VII

Using an extracellular microelectrode recording technique, the effects of intravenously administered morphine sulfate upon the single-unit activities of cells in Rexed lamina VII of the lumbar spinal cord were studied in cats following decerebration and spinal cord transection at L1. These neurons responded principally to high-threshold mechanical and thermal stimuli applied to a receptive field in the ipsilateral hind limbs. Morphine sulfate, 0.25, 0.5, and 1.0 mg/kg, caused dose-related suppression of the spontaneous activities of these neurons. The firing rates at maximum suppression, observed 5-10 min after administration of morphine, were 63.9 +/- 9.2 (mean +/- 1 SE), 43.0 +/- 5.4, and 26.5 +/- 6.0 per cent of the control values, respectively. Since these cells have been shown by others to be associated with the spinothalamic and spinoreticular pathways, the results suggest that the analgesic state may result from the action of morphine on the cells of origin of these major ascending pathways in the spinal cord.