FIBER LOSS IN PAINFUL NEUROPATHIES
Griffin J.W., Herrmann D., Scott L., Cornblath D.R., Hauer P., McArthur J.C. Johns Hopkins University School of Medicine, Baltimore, MD 21287 U.S.A.
Spontaneous neuropathic pain (SNP) in the feet characterizes several peripheral neuropathies. An association of SNP with small fiber involvement has been widely recognized clinically, but not confirmed by prior studies of neuropathology. We examined skin biopsies, stained for axons, and sural nerve biopsies, asking what fiber classes were involved. The skin biopsies (115 cases) came from patients with idiopathic small fiber sensory neuropathy (ISN), sensory neuropathy of AIDS (AIDS SN), Fabry disease, painful diabetic neuropathy, dominantly inherited late onset heritable painful neuropathy, and amyloid neuropathy. Sural biopsies were studied by light and EM morphometry and compared to 12 painless neuropathies. In all of the painful disorders, skin biopsies demonstrated distally predominant loss of epidermal sensory fibers with predegenerative axonal swellings at more proximal levels. The sural nerve biopsies identified selective small fiber loss in ISN, prominent small fiber loss in Fabry's disease, DM, and amyloid, and proportionate loss of large and small fibers in AIDS SN. In all of these disorders the loss was greatest distally, suggesting distal axonal degeneration of small sensory fibers. In an important subgroup the sural nerves at the ankles were entirely normal, including unmyelinated fiber content, but the skin at that level was markedly depleted of epidermal fibers. We conclude that small sensory fiber loss is a common feature of neuropathies with SNP, and the small sensory fiber loss is greatest distally, sometimes distal to the sural nerve.
QUANTITATIVE SUDOMOTOR TESTING AND EPIDERMAL NERVE FIBER DENSITY IN THE ASSESSMENT OF PERIPHERAL NEUROPATHY.
Spies J.M., Sheikh K.A., Gordon V.M., Singer W., Hauer P. Low P.A., McArthur J.C., Griffin J.W. Autonomic Disorders Center, Mayo Clinic, Rochester, MN and Cutaneous Nerve Laboratory, Johns Hopkins University, Baltimore, MD
We undertook to compare the diagnostic yield of quantitative sudomotor axon reflex testing (QSART) and determination of epidermal nerve fiber density (ENFD) in patients with peripheral neuropathy. Both these investigations can be used to diagnose and monitor the progression of peripheral neuropathy and are particularly useful in small fiber neuropathies as routine electrophysiological studies are often normal in these patients. We performed QSART and skin biopsy in 15 patients being investigated for peripheral neuropathy and in 10 normal controls. The patients with peripheral neuropathy fell into one of three categories: diabetic neuropathy, large fiber non-diabetic neuropathy and small fiber non-diabetic neuropathy, the latter group included patients with typical clinical features of idiopathic small fiber neuropathy (painful, burning feet) and patients with autonomic neuropathy. The tests correlated well in control subjects. In most patients QSART and skin biopsy yielded similar results. Both tests were abnormal in all patients with diabetic neuropathy and those with autonomic neuropathy. QSART was abnormal in all 4 patients with idiopathic small fiber neuropathy and ENFD was reduced in 3 of 4. In the 4 patients with non-diabetic large fiber neuropathy, both tests were abnormal in 2, both were normal in 1 and 1 patient had a normal QSART but reduced ENFD. In summary, although in most instances the test results were concordant, on occasion each test failed to detect an abnormality which was evident on the other. The correlation was best in small fiber and diabetic neuropathies and worst in the non-diabetic large fiber neuropathies. Our findings indicate that most small fiber neuropathies affect both somatic sensory and autonomic fibers but in some patients specific fiber populations are involved, suggesting that the two investigations may be complementary.
BIOLOGICAL RATIONALE FOR DISTAL THERAPIES FOR PAIN
Campbell J.N., Ali Z., Ringkamp M., Hartke T.V., Chien H.F., Flavahan N.A., Meyer R.A. Johns Hopkins School of Medicine, Baltimore, MD, USA
Local anesthetics and capsaicin applied topically to hyperalgesic areas may relieve pain in patients with neuropathic pain. Given that the painful skin is remote to the presumed pain generator (the locus of nerve injury), the efficacy of topical therapies is surprising. We investigated the hypothesis that nerve injury invokes changes in the putatively normal nociceptors that serve the partially denervated skin. Partial denervation of the dorsum of the foot in monkey was induced by tightly ligating spinal nerve L6. Recordings were obtained two to three weeks after injury from 32 uninjured C-fiber nociceptors that reached the skin from intact adjacent roots using an in vitro preparation. Control recordings were also obtained from 29 C-fiber nociceptors in unlesioned monkeys. Phenylephrine, a selective a1-adrenergic agonist, and LJK14304 (UK), a selective a2-adrenergic agonist, were applied to the receptive field for 5 min in increasing concentrations from 0.1 to 100 mM. Two abnormalities were evident in the nociceptors from nerve lesioned monkeys: 1) a significantly higher incidence of spontaneous activity, 2) a significantly higher incidence of response to phenylephrine. Staining with PGP 9.5 revealed a 55% reduction in the number of unmyelinated terminals in the epidermis of the lesioned limb compared to the contralateral limb. Thus, nerve injury induces changes in intact nociceptors that share the same innervation territory. Trophic mechanisms generated by denervation may induce these changes. Thus distal therapies are biologically plausible as strategies to treat pain from nerve injury and other neuropathies.
INFLAMMATION INDUCES INCREASED EXPRESSION OF THE PN1 SODIUM CHANNEL IN SENSORY NEURONS
England J.D., Gould H.J., Liu Z.P., Koszowski A.G., Levinson S.R. Louisiana State University School of Medicine, New Orleans, LA 70112 USA.
Inflammation can induce a chronic pain state, but the mechanisms responsible for this are poorly understood. Complete Freund's Adjuvant (CFA) 0.5 ml was injected SC into, one hindpaw of each of 20 albino rats. A control group of rats received 0.5 ml normal saline SC. Rats were sacri-ficed at various times postinjection (PI) and dorsal root ganglia (DRG) from lumbosacral segments ipsilateral and contralateral to the injection were examined immunocytochemically using anti-sodium channel antibodies. Control and contralateral DRG showed only low level cytoplasmic sodium channel expression. In contrast, DRG ipsilateral and segmentally associated with CFA injection showed intense cytoplasmic sodium channel staining by 24 hrs PI. By 72 hrs a clear heterogeneity of sodium channel expression was obvious with small neurons (< 30 microns) exhibiting the most intense staining. This abnormal staining persisted for at least three months. At all times, most of the abnormal sodium channel expression was the PN1 isoform. Thus, sodium channels, especially PN1, are rapidly upregulated in primary sensory neurons after peripheral inflammation and may play a role in inflammation-induced hyperalgesia.
NERVE MICROENVIRONMENTAL ALTERATIONS IN CCI MODEL.
Low P.A., Sasaki H., Okamoto K., Mitsui Y., Martin D. Departments of Neurology & Anesthesiology, Mayo Foundation, Rochester MN 55905.
Experimental nerve injury, such as the chronic constriction injury (CCI) model in the rat is associated with altered sensory thresholds and behavioral manifestations suggestive of a painful state. We evaluated nerve microenvironmental alterations in the CCI model, focusing on alterations in sciatic nerve and L5 dorsal root ganglion (DRG). We measured nerve blood flow (NBF), nerve pathology, calcitonin gene-related peptide (CGRP), norepinephrine (NE) and cytokine mRNA (IL1b, TNFa, IL6 IL10). NBF was measured using microelectrode hydrogen polarography, pathologic alterations using standard light and electron microscopy, NE using HPLC, NPY and CGRP by RIA and cytokine mRNA by RT-PCR. The lesioned paw had reduced sensory thresholds. NBF was significantly increased within the lesion, but reduced by >70% at the lesional edge. NE underwent a large increase (x6) between the ligatures at day 5, and normal distal to the ligature. By day 14, there was a mild (40%) reduction in NE distal to the ligatures. Glyoxylic acid fluorescence confirmed increased noradrenergic fluorescence within the ligatures. Morphometric analysis showed that >75% fibers had undergone axonal degeneration at the lesion, affecting mainly myelinated fibers. All cytokines were increased by day 3, peaking at day 7 for IL-1b and IL-6 declining over the next 45 days, while TNFa, IL-10 mRNA persisted. At L5 DRG, CGRP, NE and NPY underwent an increase on the lesioned side. We posit that the ligatures create a dam where vasoactive (NE, CGRP, NPY) and inflammatory agents accumulate, increasing NBF and nociceptor sensitization. A second site of sensitization could occur at DRG, involving a-adrenoreceptors and CGRP.
ANTI-TNF-NEUTRALIZING ANTIBODIES REDUCE PAIN-RELATED BEHAVIOR IN TWO MODELS OF PAINFUL MONONEUROPATRY
Sommer C., Lindenlaub T., Teuteberg P., Hartung T.(1), Toyka K.V. Neurologische Klinik der Universität, Würzburg, Germany and (1)Fakultät für Biologie, Universität Konstanz, Germany
Tumor-necrosis-factor-a (TNF) is a mediator of inflammatory and neuropathic pain, the pro-hyperalgesic effect being mediated by the TNF-receptor 1. Here we investigated the duration of the anti-hyperalgesic effect of anti-TNF- antibodies (AB) in chronic constrictive sciatic nerve injury (CCI). In addition, the effect of anti-TNF-AB was assessed in the model of partial sciatic nerve transection (PST). Whereas CCI involves an inflammatory component due to the epineurial tissue reaction around the ligatures, PST is a pure nerve-injury model. 50 ml of AB (1.2mg) were administered onto the lesioned nerve intraoperatively in C57B1/6 mice after placement of three loosely constrictive ligatures around one sciatic nerve (CCI) and after transection of 1/3 of the proximal sciatic nerve (PST) respectively. A separate group of mice with CCI received the AB by epineurial injection only on day 4 post surgery, when pain related behaviors were already present. In CCI, a single AB injection intraoperatively as well as on day 4 reduced thermal hyperalgesia significantly until day 21. For mechanical allodynia, the efficacy of the injection on day 4 was less than that of the intraoperative injection. In PST, both thermal hyperalgesia and mechanical allodynia were reduced by intraoperative AB-injection. In conclusion, the early administration of neutralizing anti-TNF-AB has a prolonged effect in reducing pain-related behaviors after nerve injury. Moreover, since AB are equally effective in CCI and in PST, the effect is not dependent on epineurial inflammation.