SIGNAL CHANGES IN THE SPINAL-CORD OF THE RAT AFTER INJECTION OF FORMALIN INTO THE HINDPAW - CHARACTERIZATION USING FUNCTIONAL MAGNETIC-RESONANCE-IMAGING

Changes in metabolism and local circulation occur in the spinal cord d uring peripheral noxious stimulation, Evidence is presented that this stimulation also causes signal intensity alterations in functional mag netic resonance images of the spinal cord during formalin-induced pain , These results indicate the potential of functional magnetic resonanc e imaging in assessing noninvasively the extent and intensity of spina l cord excitation in this well characterized pain model, Therefore, th e aim of this study was to establish functional magnetic resonance ima ging as a noninvasive method to characterize temporal changes in the s pinal cord after a single injection of 50 mu l of formalin subcutaneou sly into the hindpaw of the anesthetized rat, This challenge produced a biphasic licking activity in the freely moving conscious animal, Ima ges of the spinal cord were acquired within 2 min, enabling monitoring of the site and the temporal evolution of the signal changes during t he development of formalin-induced hyperalgesia without the need of an y surgical procedure, The time course of changes in the spinal cord fu nctional image in the isoflurane-anesthetized animal was similar to th at obtained from behavioral experiments. Also, comparable physiologica l data, control experiments, and the inhibition of a response through application of the local anesthetic agent lidocaine indicate that the signal changes observed after formalin injection were specifically rel ated to excitability changes in the relevant segments of the lumbar sp inal cord, This approach could be useful to characterize different mod els of pain and hyperalgesia and, more importantly, to evaluate effect s of analgesic drugs.