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To the Editor:

Duo cum faciunt idem, non est idem can be translated to “when two do the same, it isn't the same.” This quotation from Adelphoe by the Roman playwright Terence is well suited to describe the discrepancies between results found in a recent study by Ghilardi and colleagues (1) and those found by others (2–5). In inflamed tissue, Ghilardi et al found high sympathetic nerve fiber density, while other groups found low sympathetic nerve fiber density.

Ghilardi et al investigated innervation of synovial tissue from knee joints of young adult mice 28 days after intraarticular injection of Freund's complete adjuvant (CFA), which is known to elicit a typical inflammatory pain response (1). No adaptive immune response is expected in this model of a chronic inflammatory condition. Along with an increased density of CD31+ vessels and CD68+ macrophages, they found an increased density of calcitonin gene-related peptide–positive nerve fibers (sensory) and tyrosine hydroxylase–positive sympathetic nerve fibers (1).

Sprouting of nerve fibers was described in the 19th century as a general phenomenon of nerve fiber repair (6). Furthermore, it was linked to the phenomenon of peripheral sensitization of painful stimuli and neurogenic inflammation (7). In Ghilardi and colleagues' study, tissue sections were scanned at low power to identify areas with the highest capillary or nerve fiber density in the synovium. These areas were called hot spots, and there were many of them adjacent to the meniscus (1). In the hot spots, densities of sensory and sympathetic nerve fibers were increased in CFA-injected animals as compared to those in control animals, in which the synovial–meniscal interface was used to study a comparable region.

In studies of human and rodent innervation of inflamed tissue performed by my research group, we counted the number of nerve fibers per high-power field expressed as number/mm2 and did not focus on hot spots. In addition, we usually did not examine the meniscus. Sometimes, we also found hot spots in the proximity of newly formed vascular networks, but, in contrast to our usual practice of counting nerve fibers in inflamed tissue, we omitted counting nerve fibers in these areas (4, 5). We ignored these areas because counting the number of nerve fibers per high-power field in a hot spot was impossible (exact separation of fibers was not possible). Ghilardi et al used a different technique, meticulously measuring nerve fiber length in a given tissue volume, where nerve fibers were manually traced and density was expressed as mm/mm3 (1). Thus, with their technique, the density per volume in a hot spot near the meniscus is ascertained, while our work ascertained the average density per area outside hot spots in the entire synovium. Figure 1 summarizes these findings.

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Figure 1. Density of tyrosine hydroxylase–positive sympathetic nerve fibers in normal and inflamed synovial tissue. Left, In the synovial–meniscal border area, hot spots that demonstrated sprouting of highly disorganized sympathetic nerve fibers were identified by Ghilardi et al. Under normal conditions a linear morphology was dominant. In our studies, in tissue remote from the meniscus, average sympathetic nerve fiber density was lower in inflamed tissue compared to normal tissue. Right, In the synovial lining area, average sympathetic nerve fiber density was low in inflamed tissue compared to normal tissue in our studies, and typical hot spots were not demonstrated. The Greek letters α and β indicate the dominating influence of sympathetic nerve fibers via α- and β-adrenergic receptors.

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The role of hot spots is presently not known. While nerve growth factor seems to be an important stimulus of hot spot generation (1), we reasoned that specific nerve-repellent factors of sympathetic nerve fibers are responsible for overall lower synovial tissue density under inflammatory conditions (8, 9). A recent study introduced interleukin-17A as a possible factor in the growth of sympathetic nerve fibers, but the authors of that article did not study tissue innervation, and the discussion is unbalanced (10). Ghilardi et al interpreted sympathetic hot spots as a possible causal factor in adrenergically supported pain. However, whether hot spots perform this function remains in question.

Noradrenaline has a higher affinity for α-adrenoceptors (∼10−8M) as compared to β-adrenoceptors (∼10−6M) (11). It has been reported that noradrenaline concentrations in the proximity of nerve terminals can be as high as 10−5M (12). Thus, in hot spots concentrations that are relevant for β-adrenoceptor binding would be expected (Figure 1). Since mainly α-adrenergic signaling was linked to peripheral sensitization of sensory nerve fiber endings in sympathetic-sensory coupling (13), concentrations of noradrenaline might be too high for sensory nerve fiber sensitization in hot spots. Thus, α-adrenergic signaling outside hot spots and β-adrenergic signaling inside hot spots would be expected (Figure 1).

It can be hypothesized that hot spots might be zones of little proinflammatory activity since β-adrenergic signaling has many antiinflammatory effects on innate and adaptive immune cells. The finding of a higher density of CD68+ macrophages in these zones (1) does not contradict this hypothesis, because this type of macrophage can be an alternatively activated M2-type macrophage.

Although the study by Ghilardi and colleagues provides important new ideas for understanding sympathetic tissue innervation under normal conditions and inflammation, a more balanced discussion would have been desirable.

  • 1
    Ghilardi JR, Freeman KT, Jimenez-Andrade JM, Coughlin K, Kaczmarska MJ, Castaneda-Corral G, et al. Neuroplasticity of sensory and sympathetic nerve fibers in a mouse model of a painful arthritic joint. Arthritis Rheum 2012; 64: 222332.
  • 2
    Pereira da Silva JA, Carmo-Fonseca M. Peptide containing nerves in human synovium: immunohistochemical evidence for decreased innervation in rheumatoid arthritis. J Rheumatol 1990; 17: 15929.
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    Mapp PI, Kidd BL, Gibson SJ, Terry JM, Revell PA, Ibrahim NB, et al. Substance P-, calcitonin gene-related peptide- and C-flanking peptide of neuropeptide Y-immunoreactive fibres are present in normal synovium but depleted in patients with rheumatoid arthritis. Neuroscience 1990; 37: 14353.
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    Miller LE, Justen HP, Scholmerich J, Straub RH. The loss of sympathetic nerve fibers in the synovial tissue of patients with rheumatoid arthritis is accompanied by increased norepinephrine release from synovial macrophages. FASEB J 2000; 14: 2097107.
  • 5
    Harle P, Mobius D, Carr DJ, Scholmerich J, Straub RH. An opposing time-dependent immune-modulating effect of the sympathetic nervous system conferred by altering the cytokine profile in the local lymph nodes and spleen of mice with type II collagen–induced arthritis. Arthritis Rheum 2005; 52: 130513.
  • 6
    Howell WH, Huber GC. A physiological, histological and clinical study of the degeneration and regeneration in peripheral nerve fibres after severance of their connections with the nerve centres. J Physiol 1892; 13: 335406.
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    Miller LE, Weidler C, Falk W, Angele P, Schaumburger J, Scholmerich J, et al. Increased prevalence of semaphorin 3C, a repellent of sympathetic nerve fibers, in the synovial tissue of patients with rheumatoid arthritis. Arthritis Rheum 2004; 50: 115663.
  • 9
    Fassold A, Falk W, Anders S, Hirsch T, Mirsky VM, Straub RH. Soluble neuropilin-2, a nerve repellent receptor, is increased in rheumatoid arthritis synovium and aggravates sympathetic fiber repulsion and arthritis. Arthritis Rheum 2009; 60: 2892901.
  • 10
    Chisholm SP, Cervi AL, Nagpal S, Lomax AE. Interleukin-17A increases neurite outgrowth from adult postganglionic sympathetic neurons. J Neurosci 2012; 32: 114655.
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    Brunton LL, Lazo JS, Parker KL, Gilman AG, Goodman LS, editors. Goodman & Gilman's the pharmacological basis of therapeutics. New York: McGraw-Hill; 2006.
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    Bevan JA. Some functional consequences of variation in adrenergic synaptic cleft width and in nerve density and distribution. Fed Proc 1977; 36: 243943.
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Rainer H. Straub MD, PhD*, * University Hospital Regensburg, Regensburg, Germany.