Functional magnetic resonance imaging in rats subjected to intense electrical and noxious chemical stimulation of the forepaw

We examined whether cerebral activation to two different intense and painfu l stimuli could be detected using functional magnetic resonance imaging (fM RI) in alpha-chloralose anesthetized rats. Experiments were performed using a 9.4 T magnet and a surface coil centered over the forebrain, A set of gr adient echo images were acquired and analyzed using our software based on f uzzy cluster analysis (Evident(TM)). Following the injection of 50 mu l of formalin (5%) into the forepaw we observed a regional increase in signal in tensity in the MR images in all animals. Anterior cingulate cortex, frontal cortex and sensory-motor cortex were some of the regions that activated fr equently and often bilaterally. Surprisingly, activation appeared sequentia lly, often occurring first in either the right or the left hemisphere with a separation of seconds to minutes between peak activations. Morphine pre-t reatment (1 mg/kg, i.v.) delayed and/or reduced the intensity of the activa tion resulting in a decrease in the overall response. Following episodes of intense electrical stimulation, produced by two brief stimulations (15 V, 0.3 ms, 3 Hz) of the forepaw, activation was observed consistently in the s ensory-motor cortex contralateral to the stimulation. Activation also occur red frequently in the anterior cingulate cortex, ipsilateral sensory-motor cortex and frontal cortical regions. All these regions of activation were m arkedly reduced during nitrous oxide inhalation. Treatment with morphine re sulted in an inhibition of the activation response to electrical stimulatio n in most regions except for sensory-motor cortex. Thus, electrical and che mical noxious stimuli activated regions that are known to be involved in th e central processing of pain and morphine modified the activation observed. fMRI combined with appropriate exploratory data analysis tools could provi de an effective new tool with which to study novel analgesics and their eff ects on the CNS processing of pain in animal models. (C) 2000 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.