Spinally Administered Epinephrine Suppresses Noxiously Evoked Activity of WDR Neurons in the Dorsal Horn of the Spinal Cord

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Abstract
This study determined if spinally administered epinephrine [EP] is capable of suppressing noxiously evoked activity of wide dynamic range (WDR) neurons in the dorsal horn of the spinal cord. Extracellular activity was recorded from single WDR neurons in the dorsal horn of decerebrate, spinal cord-transected (T-12) cats. Activity was evoked by the presentation of a noxious radiant heat stimulus (51.degree. C) to the cells'' receptive fields on the hind paws. Evoked activity was monitored both before and after the spinal administration of either 50 .mu.g (n = 6) or 100 .mu.g (n = 6) EP. Both doses of EP produced a significant suppression of noxiously evoked activity, which was dose-dependent. The 100-.mu.g dose produced a suppression that was of longer duration than that seen following the 50-.mu.g dose. Recovery from suppression was recorded following both the 50- and 100-.mu.g dose. Evidently, spinally administered EP is capable of suppressing noxiously evoked activity of WDR neurons in the dorsal horn of the spinal cord. Since WDR neurons are cells of origin for the spinothalamic tract, such an action may block the central transmission of afferent pain information. This may be a mechanism by which spinally administered EP enhances the duration of intensity of spinal anesthesia produced by local anesthetics and may also explain spinal analgesia resulting from the spinal administration of adrenergic agonists. Interactions between spinally administered EP and spinally administered opioids also were studied. Following spinal fentanyl administration, 10 .mu.g of spinally administered EP produced significant suppression of noxiously evoked activity within 6 to 9 min (n = 3); 10 .mu.g of spinally administered EP by itself produced no significant suppression of noxiously evoked activity (n = 4). Adrenergic agonists may act in a multiplicative fashion with spinally administered opiates to produce a profound suppression of noxiously evoked activity.