Different effects of sympathetic stimulation on evoked and spontaneous discharges of the polymodal nociceptors in the rat

In the present study the tail nerve in rat was dissected of small bundles offibers and unit discharges of afferent C fibers evoked by noxious stimuli such asmechanical pressure and direct current potassium iontophoresis and heat wererecorded. The unit discharges were used as indices of activity of the polymodalnociceptors, the specialized pain receptors in skin, to investigate the modulatingeffects of sympathetic nerve on peripheral sensation process of pain. 57 units ofthe polymodal nociceptors were studied, the following two main facts were found. (1) Stimulation of the peripheral end of the cat lumbosacral sympathetictrunk could inhibit markedly the unit discharges of the polymodal nociceptorsevoked by noxious stimuli. The inhibitory effect occurred quickly and caused the number of unit dischargesto decrease about one-third. The after effect lasted for more than ten min. In-jection of norepinepherine into the tail artery had a similar inhibitory effect. Thisfinding confirmed our previous report that the sympathetic nerve possessed an in-hibitory effect on pain receptors. (2) The modulating effect of sympathetic stimulation on the activity of partsof the polymodal nociceptors showed a dual nature, namely, it inhibited the evokeddischarges but facilitated the spontaneous discharges of the same unit. Based on these results, it was suggested that abnomality of the dual effect ofthe sympathetic nerve on the polymodal nociceptors might be one of the causesproducing causalgia and an assumption that acupuncture might modulate theperipheral sensation process of pain via the sympathetic nerve was discussed.