Adrenergic excitation of cutaneous nociceptors in chronically inflamed rats.

To probe the mechanisms underlying sympathetically maintained pain, the effects of lumbar sympathetic chain stimulation (SS) and close arterial injection of norepinephrine (NE) on cutaneous nociceptor activities were studied in adjuvant inflamed rats. Two to 28 weeks after adjuvant treatment, single nerve recordings of polymodal receptors from the saphenous nerve were performed. In treated rats but not in controls, SS and NE were excitatory in 35-40% of units. Yohimbine and CH-38083 blocked NE excitation, but prazosin did not. Our data show that chronic inflammation initiates circumstances in which alpha 2-adrenoceptor-mediated sympathetic activity excites some nociceptors.