SENSITIZATION OF SPINAL NEURONS BY NONNOXIOUS STIMULI IN THE AWAKE BUT NOT ANESTHETIZED STATE

Background: The observation that peripheral trauma causes enhanced spi nal neuronal excitability has provided the scientific rationale for th e concept of ''preemptive analgesia.'' The premise has been that only noxious stimuli cause sensitization in sensory pathways, but this prem ise has not been tested in the conscious state. Methods: Responses of single spinal neurons were recorded in instrumented sheep that were un trained and free from drugs or recent surgery, in either fully conscio us or halothane-anesthetized states, Receptive field (RF) size was mea sured before and after non-noxious mechanical conditioning stimulation . Results: Noxious conditioning stimuli in anesthetized sheep caused e nlargement of RF areas, as expected. Conditioning with nonpainful scra tching or other stimuli was without effect in anesthetized animals; in marked contrast, it caused enlargement of RF size in conscious animal s, in which 29 of 33 wide dynamic range units but only 1 of 12 low-thr eshold mechanoreceptive neurons were affected. Conclusions: Sensitizat ion of spinal sensory neurons evidently is a process that is not restr icted to pathologic pain states but rather that occurs under normal ph ysiologic conditions independent of painful stimuli, The significance of such sensitization processes therefore needs reevaluation. The sens itization triggered by non-nociceptive afferents is likely to be opioi d-resistant and therefore may contribute to the rather disappointing r esults seen in several clinical trials of ''preemptive analgesia.''