A fMRI study of brain activations during non-noxious and noxious electrical stimulation of the sciatic nerve of rats

An acute pain animal model for fMRI study would provide useful spatial and temporal information for studying the supraspinal nociceptive neuronal resp onses. The aim of the present study was to investigate whether the nocicept ive responses in different brain areas can be differentiated by using funct ional magnetic resonance imaging (fMRI) in anesthetized rats. Functional ch anges in brain regions activated by noxious or non-noxious stimuli of the s ciatic nerve were investigated using fMRI in a 4.7 T MR system in alpha -ch loralose anaesthetized rats. To determine the electrical intensity for noxi ous and non-noxious stimuli, compound action potential recording was employ ed to reveal the type of fibers activated by graded electrical stimulation of sciatic nerve. It showed that innocuous A-beta fibers were excited by tw o times the muscle twitch threshold and nociceptive A-delta and C fibers we re recruited and excited by 10 and 20 times threshold, respectively. A seri es of four-slice gradient echo images were acquired during innocuous (two t imes threshold) and noxious (10 and 20 times threshold) stimuli in a 4.7 T MR system. Contralateral somatosensory cortex was the most prominent brain area activated by innocuous stimuli. Both signal intensity and activated ar eas were significantly increased in the somatosensory cortex, cingulate cor tex, medial thalamus and hypothalamus during noxious stimuli. These four br ain areas activated by noxious stimuli were significantly suppressed by pri or intravenous injection of morphine (5 mg/kg). The present findings demons trated that the difference of the innocuous and nociceptive responses in th e brain could be detected and localized by an in vivo spatial map using fMR I. Results suggest that fMRI may be an invaluable tool for studying pain in anesthetized animals. (C) 2001 Elsevier Science B.V. All rights reserved.