Inhibition by etorphine of the discharge of dorsal horn neurons: Effects on the neuronal response to both high- and low-threshold sensory input in the decerebrate spinal cat

Abstract

The effect of etorphine on the receptive field characteristics of neurons sensitive to brush-touch-pinch and brush-touch was examined. It was found that, aside from the depression of the neuronal response to pinch and the depression of the Ad- and C-fiber responses evoked by receptive field stimulation, etorphine at the same doses attenuated the response to brush-touch as well in about 20% of the cells. Close examination of this phenomenon revealed, first, that in these cells there was a small but detectable reduction in the extent of the brush-touch field. Second, it was found that the response to A-fiber volleys produced by stimulating beyond the edge of the natural brush-touch receptive field produced an evoked discharge which was strongly inhibited by etorphine, unlike the A-fiber input, which derived from electrical stimulation in the center of the brush-touch field. The lack of effect on A-fiber input derived from the center of the brush-touch field was not due to stimulus strength, as no effect of etorphine was observed at threshold currents. In any case, the inhibition of either high- or low-thresh-old input was pharmacologically specific, i.e., it was dose dependent, antagonized by naloxone, produced by two opiate agonists (morphine and etorphine), and not produced by dextrophan. These data suggest that, although the most prominent effect on cellular discharge is that related to high-threshold stimulation and small-fiber input, a measurable degree of depression could be detected at low doses of etorphine in cells which were responsive only to innocuous stimuli.