Introduction: Tendinopathy entails pain and degenerative tissue proliferation such as tenocyte transformation and increased numbers of sensory nerves and microvessels. Pain and tissue proliferation are suggested to be modulated via nerve transmitters, including substance P (SP) and glutamate, both detected in tendinopathy. Substance P and glutamate are known to activate glutamate receptors in a variety of pain conditions and additionally to be implicated in cell transformation. However, the presence of different glutamate receptors, eg. ionotropic (NMDA) and metabotropic (mGlu), and whether they are up- or downregulated in tendinopathy is still unknown. In this study we assessed the

presence,

Methods: All procedures were conducted with local ethical committee approval and patient consent. Human patellar tendon biopsies of tendinopathic patients (n=10) and controls (n=8) were single- and double-stained immunohistochemically for glutamate, glutamate receptors NMDAR1, mGluR1, mGluR5 and mGluR6,7, the nerve marker PGP9.5 and SP and assessed subjectively and semi-quantitatively with image analysis. Images were taken using an epifluorescence microscope with camera and were subjectively assessed by two independent observers blinded with regard to the identity of the slides. Tenocyte density and morphologic characteristics were assessed. Non-parametric Mann-Whitney U-tests for independent samples were used, and the level for significance was set at p< 0.05.

Results: Of the glutamate receptors tested all except mGluR1 was identified in the tendons, however only NMDAR1 was found significantly altered between both groups. The chronic painful tendons exhibited a significant elevation of NMDAR1 (9-fold) and also of glutamate (10-fold). This up-regulation of NMDAR1 and glutamate was found to be co-localized on sensory nerve fibers, blood vessels as well as on transformed tenocytes. None of the controls exhibited neuronal co-existence of glutamate with NMDAR1.

Conclusions: This study establishes for the first time that patients with tendinopathy exhibit an elevation of peripheral glutamate receptor NMDAR1, morphologically co-localized with increased glutamate expression. The up-regulated NMDAR1/glutamate system may represent hyper-excitability of the cells – leading to cell proliferative effects observed as angiogenesis, tenocyte transformation, and nerve sprouting. Moreover, the neuronal co-existence of glutamate and NMDAR1 observed in painful tendinosis, but not seen in any of the controls, strongly suggests a role in pain signalling. Future studies will focuse on interventional approaches to investigate if modulation of NMDAR1 pathways can ameliorate the symptoms of tendinopathic patients.