[Pharmacology of spinal facilitation: the basis for preventive analgesia]

Repeated C fiber stimulation results in increased spinal afferent processing leading to exaggerated response to a painful stimulus (hyperalgesia) and anomalous pain behavior in response to an otherwise innocuous stimulus (allodynia). Evidence points to a complex scenario in which continued C fiber stimulation leads to local spinal release of glutamate, which activates spinal N-methyl-D-aspartate receptors. Their activation then leads to increased intracellular Ca and later to spinal formation of several intermediaries, including prostanoids and nitric oxide. Both serve to facilitate spinal processing directly, by increasing the release of spinal neurotransmitters. Conversely, spinal delivery of antagonists of such processes prevents a hyperalgesixc state from developing. These observations are clearly relevant to the study and management of pain following injury in humans.