The Journal of Neuroscience. June 1993. 13(6): 2689-2702 Patterns of Increased Brain Activity Indicative of Pain in a Rat Model of Peripheral Mononeuropathy Jianren Mao, David J. Mayer, and Donald D. Price Department of Anesthesiology, Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298 Regional changes in brain neural activity were examined in haviors in CCI rats and clinical symptoms in neuropathic pain rats with painful peripheral mononeuropathy (chronic con- patients. strictive injury, Ccl) by using the fully quantitative W-2- [Key words: neuropathic pain, P-deoxyglucose, hyperal- deoxyglucose (2-DG) autoradiographic technique to mea- gesia, spontaneous pain, nerve injury, brainstem, cerebral sure local glucose utilization rate. CCI rats used in the cortex, thalamusj experiment exhibited demonstrable thermal hyperalgesia and spontaneous pain behaviors 10 d after sciatic nerve ligation when the 2-DG experiment was carried out. In the absence Peripheral nerve injury sometimes results in a chronic neuro- of overt peripheral stimulation, reliable increases in 2-DG pathic pain syndrome characterized by hyperalgesia, sponta- metabolic activity were observed in CCI rats as compared neouspain, radiation of pain, and nociceptive responsesto nor- to sham-operated rats within extensive brain regions that mally innocuous stimulation (allodynia) (Bon@ 1979; Thomas, have been implicated in supraspinal nociceptive processing. 1984; Price et al., 1989). Most of these symptoms have been These brain regions included cortical somatosensory areas, recently observed in a rodent model of painful peripheral mon- clngulate cortex, amygdala, ventral posterolateral thalamic oneuropathy induced by loose ligation of the rat’s common nucleus, posterior thalamic nucleus, hypothalamic arcuate sciatic nerve(chronicconstrictive injury, CCI) (Bennett and Xie, nucleus, central gray matter, deep layers of superior collic- 1988). For example, one prominent change seen in CC1 rats is ulus, pontine reticular nuclei, locus coeruleus, parabrachial the appearanceof ongoing behaviors such as typical hind paw nucleus, gigantocellular reticular nucleus, and paragigan- guarding positions following sciatic nerve ligation, suggesting tocellular nucleus. The increase in 2-DG metabolic activity the presenceof persistent spontaneouspain (Bennett and Xie, was bilateral in most brain regions of CCI rats. However, 1988; Attai et al., 1990; Mao et al., 1992a,b). However, based somatosensory regions within the thalamus and the cerebral on these behaviors alone, it is difficult to determine whether cortex were activated in CCI rats. High levels of 2-DG met- CCI rats truly experience spontaneous pain or these ongoing abolic activity were obsenred within the cortical hind limb behaviors simply occur as a defense against pains evoked by area, ventral posterolateral thalamic nucleus, and posterior mechanical or thermal stimuli from the affected area. Some thalamic nucleus contralateral to the ligated sciatic nerve, evidence indicating the presence of spontaneouspain in CC1 and these levels were higher than ipsilateral corresponding rats derives from recent W-2-deoxyglucose (2-DG) autoradio- regions in CCI rats. In addition, patterns of increased neural graphic studies that show that spinal cord neural activity (in- activity found in the brain of CCI rats showed some similar- ferred from increased local glucose utilization rate) increases ities and differences to those found in the brain of rats ex- substantially in the absenceof overt peripheral stimulation in posed to acute nociceptton induced by noxious heat or for- CC1 rats 10 d after sciatic nerve ligation (Price et al., 1991; Mao malin stimulation. Thus, these CCLinduced spontaneous et al., 1992a). Consistent with spontaneousincreases in spinal increases in neural activity within extensive brain regions of cord neural activity in CC1 rats, electrophysiological studies CCI rats previously implicated in sensory-discriminative and have indicated increasesin spontaneous neuronal discharges affective-motivational dimensions of pain as well as cen- both in spinothalamic tract neurons (Palecek et al., 1991) and trifugal modulation of pain are likely to reflect brain neural in the ventrobasal thalamic complex (Guilbaud et al., 1990) of processing of spontaneous pain. Implications of increased CC1 rats 2-3 weeksafter peripheral nerve injury. In view of the brain neural activity in mechanisms of neuropathic pain are critical roles of supraspinalstructures in nociceptive processing, discussed with emphasis on correlations between spatial more conclusive evidence for the existenceof spontaneouspain patterns of altered brain neural activity and pain-related be- in CC1rats should be provided if increasedneural activity occurs in brain regions that have been implicated in pain. Therefore, it would be of great interest to examine the spatial Received Aug. 24, 1992; revised Nov. 10, 1992; accepted Jan. 12, 1993. distribution of neural activity throughout the entire brain of We thank Dr. John G. McHaRie and Juan Lu for the technical assistance. We also thank Dr. Robert C. Coghill for his assistance with the 2-DG technique. This CC1 rats, examining brain structures implicated in supraspinal work was supported by U.S. Public Health Service Grant NS-24009. nociceptive processing.The advantage of usinga neural imaging Correspondence should be addressed to Jianren Mao, M.D., Ph.D., Department of Anesthesiology, Medical College of Virginia, P.O. Box 516, Richmond, VA approach such as the 2-DG technique is that it allows simul- 23298. taneous examination of multiple CNS areasaffected by a stim- Copyright 0 1993 Society for Neuroscience 0270-6474/93/132689-14$05.00/O ulus or behavioral condition. As reported previously, increases 2690 Mao et al. l Patterns of Increased Brain Activity Indicative of Pain in spinal cord neural activity in CC1rats exhibit specific patterns lowski and Marshall, 1983; Di Rocco ct al., 1989; Coghill et al., with respect to the spatial distribution of spontaneously in- 1991; Price et al., 1991; Mao et al., 1992a). creasedneural activity (Price et al., 1991; Mao et al., 1992a). These patterns of increased neural activity can serve as the Materials and Methods guidelines for examining spatial patterns of increasesin neural Subjccfs. Adult male Sprague-Dawley rats (Hilltop) weighing 300-350 activity in specificbrain regionsof CC1rats. For instance, spinal gm were used. Animals were individually housed in cages with water cord regions showing increasedneural activity in theseCC1 rats and food pellets available ad libitum. The animal room was artificially include superficial laminae I-II, deep dorsal horn laminae V- illuminated from 0700 to 1900 hr. All experimental procedures were VI, and the ventral horn lamina VII (Price et al., 199 1; Mao et approved by our Institutional Animal Care and Use Committee. Lum- bar spinal cords of the rats used in the present study have been analyzed al., 1992a),regions highly implicated in spinal cord nociceptivc in our previous 2-DC studies (Price et al., 199 1; Mao et al., 1992a). processingand representing the main origin of ascending spi- Surgical preparation of CC1 rats. Rats in both the ligation and the nothalamic, spinoreticular,and spinomesencephalictracts. These sham operation group (n = 6/group) were anesthetized with 50 mg/kg pathways transmit somatosensoryinformation to many brain pentobarbital. For sciatic nerve ligation, the right sciatic nerve was structures involved in supraspinalnociccptive processing(Wil- exposed and loosely ligated with four ligatures (4-O chromic gut) ac- cording to the method of Bennett and Xie (1988). For sham operation, lis, 1985; Price, 1988). the nerve was exposed as above but not ligated. All animals received Given the bilateral pattern and the regional specificity (lam- intramuscular potassium penicillin (30,000 W/rat) postoperatively. inae I-IV, V-VI, and VII) ofincreased spinalcord neural activity Behavioral assessmcnfs. Thermal hyperalgesia and spontaneous pain in CC1 rats (seeresults in Price et al., 199 1; Mao et al., 1992a), behaviors were examined in both ligated and sham-operated rats on days 0 (baseline), 5, 7, and IO postsurgety. Thermal hyperalgesia to several hypotheses may be made with respect to the possible radiant heat was assessed by using a foot withdrawal test. Foot with- spatial distribution of increasesin brain neural activity. First, drawal latency was defined as the time elapsed from the onset of radiant increasesin neural activity should occur bilaterally in broad heat to the hind paw withdrawal. Since the foot withdrawal latency supraspinal regions of CC1 rats in association with ascending taken from the hind paw directly touching the ground differs from that nociceptive pathways (including the spinothalamic tract), par- slightly elevated from the ground, a condition sometimes seen in the nerve-ligated hind paw of CC1 rats, we tested CC1 rats only during the ticularly those in connection with spinorcticular and spinomes- period when their affected hind paws were in contact with the ground. encephalic tracts, such as brainstem reticular formation and Foot withdrawal latency difference scores (contralateral side minus ip- central gray matter. Second, since at the spinal level maximal silateral side) were used to determine the degree of hyperalgesia as increasesin neural activity occur in somatotopically appropriate described in detail elsewhere (Mao et al., 1992a,b). Spontaneous pain behaviors