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Effect of deafferentation on the levels of uric acid in the spinal cord of the rat
Neuroscience Letters, 99 (1989) 181-186 Elsevier Scientific Publishers Ireland Ltd.
181
NSL 05983
Effect of deafferentation on the levels of uric acid in the spinal cord of the rat J. Weil-Fugazza, F. G o d e f r o y a n d A.I. Basbaum* Unitk de Recherches de Physiopharmacologie du Syst~me Nerveux, INSERM, Paris (France) (Received 11 July 1988; Revised version received 28 October 1988; Accepted 9 December 1988)
Key words:
Rat spinal cord; Rhizotomy; Uric acid; High-pressure liquid chromatography~lectrochemical detection
In previous studies we reported that the rat spinal cord contains relatively high levels of uric acid and that the levels increase in a rat model of bilateral chronic pain, experimental adjuvant arthritis. In this report we evaluate the changes in U A in the unilaterally deafferented rat, a preparation which has also been used to study chronic pain. Uric acid was measured by high-pressure liquid chromatography with electrochemical detection in the spinal cord of rats that underwent unilateral, multiple cervical dorsal rhizotomy. Compared to control and sham-operated rats, there was a significant increase in the level of uric acid in the dorsal quadrant of the spinal cord ipsilateral to the dorsal rhizotomy. This increase was present at 1 and 4 weeks after surgery. At 1 week, we also observed a small but statistically insignificant increase in uric acid levels in the dorsal quadrant contralateral to the deafferentation and in sham-operated rats. Four weeks after surgery the levels of U A in all regions except for the deafferented dorsal quadrant returned to normal. The possibility was raised that the changes in uric acid reflect an increase in purinergic metabolism in the spinal cord secondary to the increased activity of the dorsal horn neurons that occurs with deafferentation.
Several recent studies have reported the presence of the purine metabolite uric acid (UA) in the CNS. Although it has been considered that it could originate in the periphery and diffuse into the CNS [10], there is now considerable evidence for UA formation in brain or spinal cord parenchyma. For example, intracranial microdialysis or differential pulse voltammetry studies have demonstrated UA release from several brain regions [6, 19, 20, 22, 23, 26] and from the spinal cord [24]. Microinjection of allopurinol, an inhibitor of xanthine oxidase, decreased the levels of UA release [20]. In contrast, microinjection of xanthine oxidase [20] or adenosine deaminase [22] increased the levels of UA release. Taken together these data indicate that UA reflects purine metabolism in the CNS. We have detected significant amounts of UA in the rat spinal cord [2, 27]. Of particular interest was our observation that the spinal cord levels of UA increase signifi*Present address: Department of Anatomy, University of California, San Francisco, CA 94143, U.S.A. Correspondence: J. Weil-Fugazza, INSERM, U 161, 2 rue d'Al6sia, 75014 Paris, France. 0304-3940/89/$ 03.50/~) 1989 Elsevier Scientific Publishers Ireland Ltd.
182 cantly in a rat model of chronic pain, the experimental adjuvant arthritic rat [27]. These data raised the possibility that the levels of UA might provide an index of painrelated changes in purine metabolism in the spinal cord of the rat. To further address that possibility, we have reexamined the levels of UA in an experimental model of chronic deafferentation pain. This model is produced by multiple dorsal rhizotomy [l, 16]. Both adjuvant-induced and deafferentation pain are associated with increased spontaneous activity of dorsal horn neurons [3, i 5, 18]. An advantage of the deafferentation model is that the pain, in contrast to experimental arthritis, is unilateral (and specifically located in one limb). Furthermore, since recent studies implicated ATP and adenosine as transmitters in fine as well as large diameter primary afferent fibers [7, 11,21,25] by studying the deafferented rat, we were also in a position to characterize the relationship of UA to primary afferent fibers. Specifically, a decrease in the levels of UA secondary to dorsal rhizotomy would be consistent with a contribution of purines to primary afferent neurotransmission. We measured UA levels in 3 groups of adult male rats: controls, sham-operated and deafferented. Rats were anesthetized with Nembutal (50 mg/kg) and a laminectomy performed to expose the cervical enlargement. After the dura was incised, dorsal roots C5 Ti on the right side of the cord were cut with iridectomy scissors. Care was taken to avoid radicular arteries that often course within the rootlet. After the rhizotomy was completed, a strip of moistened Gelfilm was placed over the exposed cord, the overlying muscle and skin sutured and the animals returned to their cages. To prevent autotomy of the deafferented limb, all rats were housed with a female in the same cage [5]. Sham-operated rats underwent the same procedure, including dural incision, but rhizotomy was not performed. We studied the effect of rhizotomy at two different time points: 1 week and 4 weeks after surgery. The early point was chosen to reduce the possibility that any observed changes might be due to immediate postoperative surgical trauma and edema. The 30 day time point was chosen because the electrophysiological changes which have been reported alter dorsal rhizotomy typically stabilize within I month [3]. At the appropriate time, the rats were killed by decapitation and the spinal cord rapidly removed. Left and right dorsal quadrants were dissected from segments of the cervical (C4 C6) and lumbar (L 4 ]-,6) levels. U A measurements were performed by highpressure liquid chromatography (HPLC) with electrochemical detection as described elsewhere [27]. The statistical analyses were carried out using analysis of variance. When P values were greater than 0.05, differences were not considered to be significant. Fig. 1 illustrates the changes in UA levels in deafferented rats. At both I and 4 weeks after surgery, there was a significant increase in the level of U A in the dorsal quadrant of the cervical spinal cord ipsilateral to the dorsal rhizotomy. No changes were detected in the ventral cord (data not shown). At one week after surgery, we detected a small increase in UA levels in the dorsal horn of sham-operated rats and in the dorsal quadrant contralateral to the deafferentation in the rhizotomized rat, however, those changes were not statistically significant. Four weeks after surgery, the levels of UA in all regions, except for the deafferented dorsal quadrant returned
183 A-
cervical cord /t
10-
o
_a 50q(
o_ n-
O-
8 D A Y S AFTER R H I Z O T O M Y lumbar cord
41 right
left
left
right --]control
B_
rats
3 0 D A Y S AFTER R H I Z O T O M Y
cervical cord
lumbar cord
10-
]
sham-operated rats
[]
right cervical dorsal rhizotomy
/r /r
"6
g _a 5-
0
_o er
left
right
left
right
Fig. I. Effect of deafferentation on the levels of uric acid in the spinal cord of the rat. Dorsal roots (Cs-TI) on the right side of the cord were cut. Animals were killed 8 (A) or 30 (B) days later. Segments of the left and right dorsal cord were taken at cervical (C~C6) and lumbar (L4-L6)levels. Each column represents the mean + S.E.M. of 6 measurements. The results of ANOVA are as follows: (A) right cervical cord, F2 ~7 = 5.44, P<0.05; left cervical cord, F2-17 = 1 . 5 8 , P>0.05; (B) right cervical cord, /'2_17 = I0.63, P<0.01. *P<0.05: **P<0.01.
to n o r m a l . N o changes in U A levels were detected in the l u m b a r c o r d , at 1 o r 4 weeks postoperative. The fact that the s h a m - o p e r a t e d rats showed an increase o f U A , c o m p a r e d to the u n o p e r a t e d controls, suggests t h a t surgical m a n i p u l a t i o n , because o f tissue d a m a g e o r b l o o d b r a i n b a r r i e r i m p a i r m e n t , m a y have p a r t l y c o n t r i b u t e d to the changes observed. The slight increase in U A levels c o n t r a l a t e r a l to the d e a f f e r e n t a t i o n m a y also have resulted f r o m the surgical p r o c e d u r e itself. In fact there is evidence that ischemia can significantly increase p u r i n e m e t a b o l i s m in the C N S [9]; focal ischemia can p r o v o k e a local increase in U A [13]. M a s t cells that infiltrate the injured a r e a m a y also have c o n t r i b u t e d to the rise in U A [17]. We, however, d o n o t believe that the surgical p r o c e d u r e , r a t h e r t h a n deafferentation, is the p r i m a r y cause o f the in-
184 crease in UA. First, the rhizotomy was performed with minimal, and usually, no damage to radicular vessels. Second, although it has been proposed that disruption of the blood brain barrier could lead to a rise in parenchymal UA [10], this would not explain the unilateral increase that we observed. Third, the increase in UA observed in the sham-operated rats and in the dorsal quadrant contralateral to the deafferentation 1 week after surgery was n o t statistically significant, and also was not observed 1 month after surgery. Finally, we also monitored the levels of norepinephrine, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid (data not shown) which derive from descending pathways and which would be changed if the surgery was not limited to the dorsal roots [8, 14]. Consistent with a previous report [8] we did not observe changes in the levels of these monoaminergic compounds after rhizotomy. In a previous report [2] we found a relatively uniform gray matter distribution of UA in the rat spinal cord. Although that distribution did not rule out a primary afferent origin of the UA, it indicated that other (supraspinal and intrinsic) sources are likely. The fact that UA levels increased after rhizotomy is consistent with that hypothesis. Cervical deafferentation, however, only increased UA in the ipsilateral cervical enlargement, i.e. the increase is topographically related to the site of injury, and thus cannot be attributed to a non-specific increase secondary to stress and/or trauma. It is, of course, possible that the increase of UA in the deafferented cord resulted from the breakdown of purines in degenerating primary afferents. It seems unlikely, however, that such an increase would persist for one month. We favor the hypothesis that the increase in UA resulted from an increased metabolism in the deafferented dorsal cord. This may be related to the increased spontaneous activity of dorsal horn neurons observed after deafferentation [3, 15]. Since deafferentation is thought to produce a pain syndrome in rats [1, 5, 16] our results are consistent with our earlier finding that UA levels increase in the chronic pain model of experimental arthritis. Since, in the present experiment, the deafferented rats were housed with females, autotomy (the presumed manifestation of chronic pain in this model), was prevented [5]. Thus, although the increase in UA follows nerve injury, it might not correlate with the level of chronic pain experienced. On the other hand, there is evidence that peripheral nerve injury can induce abnormal pain sensations independent of the occurrence of autotomy [4, 12]. Studies directed at the particular cells which are the source of the inflammation and/or nerve injuryevoked increase in UA will hopefully provide information about the contribution of this change to pain behavior. We thank Dr. J.M. Besson for helpful discussion, Val6rie Manceau for excellent technical assistance and Eric Dehausse for the photography. This work was supported by I N S E R M and by N I H Grants 14627 and 21445.
1 Basbaum, A.I., Effectsof central lesions on disorders produced by multiple rhizotomy in the rat, Exp. Neurol., 42 (1974) 490 501.
185 2 Basbaum, A.I., Godefroy, F. and Weil-Fugazza, J., A new microdissection technique for regional biochemical analysis of the rat spinal cord: serotonin, norepinephrine, dopamine and uric acid, Brain Res., 419 (1987) 229-238. 3 Basbaum, A.I. and Wall, P.D., Chronic changes in the response of cells in adult cat dorsal horn following partial deafferentation: the appearance of responding cells in a previously non-responsive area, Exp. Neurol., 116 (1976) 121-204. 4 Bennet, G.J. and Xie, Y.K., A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man, Pain, 33 (1988) 87-107. 5 Berman, D. and Rodin, B.E., The influence of housing condition on autotomy following dorsal rhizotomy in rats, Pain, 13 (1982) 307-311. 6 Crespi, F., Sharp, T., Maidment, N. and Marsden, C., Differential pulse voltammetry in vivo - evidence that uric acid contributes to the indole oxydation peak, Neurosci. Lett., 43 (1983) 203 207. 7 Fyffe, R.E.W. and Perl, E.R., Is ATP a central synaptic mediator for certain primary afferent fibres from mammalian brain?, Proc. Natl. Acad. Sci. U.S.A., 81 (1984) 689(~6893. 8 Hadjiconstantinou, M., Panula, P., Lackovic, Z. and Neff, N.H., Spinal cord serotonin: a biochemical and immunohistochemical study following transection, Brain Res., 322 (1984) 245-254. 9 Hisanaga, K., Onodera, H. and Kogure, K., Changes in levels of purine and pyrimidine nucleotides during acute hypoxia and recovery in neonatal brain, J. Neurochem., 47 (1986) 1344~1350. 10 Honegger, C.G., Krenger, W. and Langemann, H., Increased concentrations of uric acid in the spinal cord of rats with chronic relapsing experimental allergic encephalomyelitis, Neurosci. Lett., 69 (1986) 109 114. 11 Jahr, C.E. and Jessel, T.M., ATP excites a subpopulation of rat dorsal horn neurons, Nature (Lond.) 304 (1983) 230-233. 12 Jazat, F., Attal, N., Gautron, M. and Guilbaud, G., Behavioural evidence that experimental neuropathy in rat induces abnormal pain sensation, Abstracts of the 1lth Annual Meeting of the European Neuroscience Association, Eur. J Neurosci., Suppl. (1988) 46.24. 13 Kanemitsu, H., Tamura, A., Sano, K,, Iwamoto, T., Yoshiura, M. and Iriyama, K., Changes of uric acid level in rat brain after focal ischemia, J. Neurochem., 46 (1986) 851-853. 14 Kurihara, M., Role of monoamines in experimental spinal cord injury in rats, J. Neurosurg., 62 (1985) 743 749. 15 Lombard, M.C. and Larabi, Y.L., Electrophysiological study of cervical dorsal horn cells in partially deafferented rats. In J. Bonica, U. Lindblom and A. Iggo (Eds) Advances in Pain Research and Therapy, Vol. 5, Raven, New York, 1983, pp. 147-153. 16 Lombard, M.C., Nashold Jr., B.S., Albe-Fessard, D., Salman, N. and Sakr, C., Deafferentation hypersensitivity in the rat after dorsal rhizotomy: a possible animal model of chronic pain, Pain, 6 (1979) 163-174. 17 Marquardt, D.L., Gruber, H.E. and Wasserman, S.I., Adenosine release from stimulated mast cells, Proc. Natl. Acad. Sci. U.S.A., 81 (1984) 6192~6196. 18 Menetrey, D. and Besson, J.M., Electrophysiological characteristics of dorsal horn cells in rats with cutaneous inflammation resulting from chronic arthritis, Pain, 13 (1982) 343-364. 19 Mueller, K. and Haskett, C., Effects of haloperidol on amphetamine-induced increases in ascorbic acid and uric acid as determined by voltammetry in vivo, Pharmacol. Biochem. Behav., 27 (1987) 231-234. 20 Mueller, K., Palmour, R., Andrews, C.D. and Knott, P.J., In vivo voltammetric evidence of production of uric acid by rat caudate, Brain Res., 335 (1985) 231 235. 21 Nagy, J.I. and Daddona, P.E., Anatomical and cytochemical relationships of adenosine deaminasecontaining primary afferent neurons in the rat, Neuroscience, 15 (1985) 799-813. 22 O'Neill, R.D., Adenosine modulation of striatal neurotransmitter release monitored in vivo using voltammetry, Neurosci. Lett., 63 (1986) 11 16. 23 O'Neill, R.D., Fillenz, M., Griinewald, R.A., Bloomfield, M.R., Albery, W.J., Jamieson, C.M., Williams, J.H. and Gray, J.A., Voltammetric carbon paste electrodes monitor uric acid and not 5-HIAA at the 5-hydroxyindole potential in the rat brain, Neurosci. Lett., 45 (1984) 39-46.
186 24 Rivot, J.P., Noret, E., Ory-Lavollee, L. and Besson, J.M., In vivo electrochemical detection of 5-hydroxyindoles in the dorsal horn of the spinal cord: the contribution of uric acid to the voltammograms, Brain Res., 419 (1987) 201 207. 25 Salter, M.W. and Henry, J.L., Evidence that adenosine mediates the depression of spinal dorsal horn neurons induced by peripheral vibration in the cat, Neuroscience, 22 (1987) 631 650. 26 Ungerstedt, U., Measurement of neurotransmitter release by intracranial dialysis. In ('.A. Marsden (Ed.), Measurement of Neurotransmitter Release in Vivo, Wiley, New York, 1984, pp. 81 t05. 27 WeiI-Fugazza, J., Godefroy, F., Manceau, V. and Besson, J.M., Increased norepinephrine and uric acid levels in the spinal cord of arthritic rats, Brain Res., 374 (1986) 190 194.
181
NSL 05983
Effect of deafferentation on the levels of uric acid in the spinal cord of the rat J. Weil-Fugazza, F. G o d e f r o y a n d A.I. Basbaum* Unitk de Recherches de Physiopharmacologie du Syst~me Nerveux, INSERM, Paris (France) (Received 11 July 1988; Revised version received 28 October 1988; Accepted 9 December 1988)
Key words:
Rat spinal cord; Rhizotomy; Uric acid; High-pressure liquid chromatography~lectrochemical detection
In previous studies we reported that the rat spinal cord contains relatively high levels of uric acid and that the levels increase in a rat model of bilateral chronic pain, experimental adjuvant arthritis. In this report we evaluate the changes in U A in the unilaterally deafferented rat, a preparation which has also been used to study chronic pain. Uric acid was measured by high-pressure liquid chromatography with electrochemical detection in the spinal cord of rats that underwent unilateral, multiple cervical dorsal rhizotomy. Compared to control and sham-operated rats, there was a significant increase in the level of uric acid in the dorsal quadrant of the spinal cord ipsilateral to the dorsal rhizotomy. This increase was present at 1 and 4 weeks after surgery. At 1 week, we also observed a small but statistically insignificant increase in uric acid levels in the dorsal quadrant contralateral to the deafferentation and in sham-operated rats. Four weeks after surgery the levels of U A in all regions except for the deafferented dorsal quadrant returned to normal. The possibility was raised that the changes in uric acid reflect an increase in purinergic metabolism in the spinal cord secondary to the increased activity of the dorsal horn neurons that occurs with deafferentation.
Several recent studies have reported the presence of the purine metabolite uric acid (UA) in the CNS. Although it has been considered that it could originate in the periphery and diffuse into the CNS [10], there is now considerable evidence for UA formation in brain or spinal cord parenchyma. For example, intracranial microdialysis or differential pulse voltammetry studies have demonstrated UA release from several brain regions [6, 19, 20, 22, 23, 26] and from the spinal cord [24]. Microinjection of allopurinol, an inhibitor of xanthine oxidase, decreased the levels of UA release [20]. In contrast, microinjection of xanthine oxidase [20] or adenosine deaminase [22] increased the levels of UA release. Taken together these data indicate that UA reflects purine metabolism in the CNS. We have detected significant amounts of UA in the rat spinal cord [2, 27]. Of particular interest was our observation that the spinal cord levels of UA increase signifi*Present address: Department of Anatomy, University of California, San Francisco, CA 94143, U.S.A. Correspondence: J. Weil-Fugazza, INSERM, U 161, 2 rue d'Al6sia, 75014 Paris, France. 0304-3940/89/$ 03.50/~) 1989 Elsevier Scientific Publishers Ireland Ltd.
182 cantly in a rat model of chronic pain, the experimental adjuvant arthritic rat [27]. These data raised the possibility that the levels of UA might provide an index of painrelated changes in purine metabolism in the spinal cord of the rat. To further address that possibility, we have reexamined the levels of UA in an experimental model of chronic deafferentation pain. This model is produced by multiple dorsal rhizotomy [l, 16]. Both adjuvant-induced and deafferentation pain are associated with increased spontaneous activity of dorsal horn neurons [3, i 5, 18]. An advantage of the deafferentation model is that the pain, in contrast to experimental arthritis, is unilateral (and specifically located in one limb). Furthermore, since recent studies implicated ATP and adenosine as transmitters in fine as well as large diameter primary afferent fibers [7, 11,21,25] by studying the deafferented rat, we were also in a position to characterize the relationship of UA to primary afferent fibers. Specifically, a decrease in the levels of UA secondary to dorsal rhizotomy would be consistent with a contribution of purines to primary afferent neurotransmission. We measured UA levels in 3 groups of adult male rats: controls, sham-operated and deafferented. Rats were anesthetized with Nembutal (50 mg/kg) and a laminectomy performed to expose the cervical enlargement. After the dura was incised, dorsal roots C5 Ti on the right side of the cord were cut with iridectomy scissors. Care was taken to avoid radicular arteries that often course within the rootlet. After the rhizotomy was completed, a strip of moistened Gelfilm was placed over the exposed cord, the overlying muscle and skin sutured and the animals returned to their cages. To prevent autotomy of the deafferented limb, all rats were housed with a female in the same cage [5]. Sham-operated rats underwent the same procedure, including dural incision, but rhizotomy was not performed. We studied the effect of rhizotomy at two different time points: 1 week and 4 weeks after surgery. The early point was chosen to reduce the possibility that any observed changes might be due to immediate postoperative surgical trauma and edema. The 30 day time point was chosen because the electrophysiological changes which have been reported alter dorsal rhizotomy typically stabilize within I month [3]. At the appropriate time, the rats were killed by decapitation and the spinal cord rapidly removed. Left and right dorsal quadrants were dissected from segments of the cervical (C4 C6) and lumbar (L 4 ]-,6) levels. U A measurements were performed by highpressure liquid chromatography (HPLC) with electrochemical detection as described elsewhere [27]. The statistical analyses were carried out using analysis of variance. When P values were greater than 0.05, differences were not considered to be significant. Fig. 1 illustrates the changes in UA levels in deafferented rats. At both I and 4 weeks after surgery, there was a significant increase in the level of U A in the dorsal quadrant of the cervical spinal cord ipsilateral to the dorsal rhizotomy. No changes were detected in the ventral cord (data not shown). At one week after surgery, we detected a small increase in UA levels in the dorsal horn of sham-operated rats and in the dorsal quadrant contralateral to the deafferentation in the rhizotomized rat, however, those changes were not statistically significant. Four weeks after surgery, the levels of UA in all regions, except for the deafferented dorsal quadrant returned
183 A-
cervical cord /t
10-
o
_a 50q(
o_ n-
O-
8 D A Y S AFTER R H I Z O T O M Y lumbar cord
41 right
left
left
right --]control
B_
rats
3 0 D A Y S AFTER R H I Z O T O M Y
cervical cord
lumbar cord
10-
]
sham-operated rats
[]
right cervical dorsal rhizotomy
/r /r
"6
g _a 5-
0
_o er
left
right
left
right
Fig. I. Effect of deafferentation on the levels of uric acid in the spinal cord of the rat. Dorsal roots (Cs-TI) on the right side of the cord were cut. Animals were killed 8 (A) or 30 (B) days later. Segments of the left and right dorsal cord were taken at cervical (C~C6) and lumbar (L4-L6)levels. Each column represents the mean + S.E.M. of 6 measurements. The results of ANOVA are as follows: (A) right cervical cord, F2 ~7 = 5.44, P<0.05; left cervical cord, F2-17 = 1 . 5 8 , P>0.05; (B) right cervical cord, /'2_17 = I0.63, P<0.01. *P<0.05: **P<0.01.
to n o r m a l . N o changes in U A levels were detected in the l u m b a r c o r d , at 1 o r 4 weeks postoperative. The fact that the s h a m - o p e r a t e d rats showed an increase o f U A , c o m p a r e d to the u n o p e r a t e d controls, suggests t h a t surgical m a n i p u l a t i o n , because o f tissue d a m a g e o r b l o o d b r a i n b a r r i e r i m p a i r m e n t , m a y have p a r t l y c o n t r i b u t e d to the changes observed. The slight increase in U A levels c o n t r a l a t e r a l to the d e a f f e r e n t a t i o n m a y also have resulted f r o m the surgical p r o c e d u r e itself. In fact there is evidence that ischemia can significantly increase p u r i n e m e t a b o l i s m in the C N S [9]; focal ischemia can p r o v o k e a local increase in U A [13]. M a s t cells that infiltrate the injured a r e a m a y also have c o n t r i b u t e d to the rise in U A [17]. We, however, d o n o t believe that the surgical p r o c e d u r e , r a t h e r t h a n deafferentation, is the p r i m a r y cause o f the in-
184 crease in UA. First, the rhizotomy was performed with minimal, and usually, no damage to radicular vessels. Second, although it has been proposed that disruption of the blood brain barrier could lead to a rise in parenchymal UA [10], this would not explain the unilateral increase that we observed. Third, the increase in UA observed in the sham-operated rats and in the dorsal quadrant contralateral to the deafferentation 1 week after surgery was n o t statistically significant, and also was not observed 1 month after surgery. Finally, we also monitored the levels of norepinephrine, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid (data not shown) which derive from descending pathways and which would be changed if the surgery was not limited to the dorsal roots [8, 14]. Consistent with a previous report [8] we did not observe changes in the levels of these monoaminergic compounds after rhizotomy. In a previous report [2] we found a relatively uniform gray matter distribution of UA in the rat spinal cord. Although that distribution did not rule out a primary afferent origin of the UA, it indicated that other (supraspinal and intrinsic) sources are likely. The fact that UA levels increased after rhizotomy is consistent with that hypothesis. Cervical deafferentation, however, only increased UA in the ipsilateral cervical enlargement, i.e. the increase is topographically related to the site of injury, and thus cannot be attributed to a non-specific increase secondary to stress and/or trauma. It is, of course, possible that the increase of UA in the deafferented cord resulted from the breakdown of purines in degenerating primary afferents. It seems unlikely, however, that such an increase would persist for one month. We favor the hypothesis that the increase in UA resulted from an increased metabolism in the deafferented dorsal cord. This may be related to the increased spontaneous activity of dorsal horn neurons observed after deafferentation [3, 15]. Since deafferentation is thought to produce a pain syndrome in rats [1, 5, 16] our results are consistent with our earlier finding that UA levels increase in the chronic pain model of experimental arthritis. Since, in the present experiment, the deafferented rats were housed with females, autotomy (the presumed manifestation of chronic pain in this model), was prevented [5]. Thus, although the increase in UA follows nerve injury, it might not correlate with the level of chronic pain experienced. On the other hand, there is evidence that peripheral nerve injury can induce abnormal pain sensations independent of the occurrence of autotomy [4, 12]. Studies directed at the particular cells which are the source of the inflammation and/or nerve injuryevoked increase in UA will hopefully provide information about the contribution of this change to pain behavior. We thank Dr. J.M. Besson for helpful discussion, Val6rie Manceau for excellent technical assistance and Eric Dehausse for the photography. This work was supported by I N S E R M and by N I H Grants 14627 and 21445.
1 Basbaum, A.I., Effectsof central lesions on disorders produced by multiple rhizotomy in the rat, Exp. Neurol., 42 (1974) 490 501.
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