Presence of a non-neuronal cholinergic system and occurrence of up- and down-regulation in expression of M2 muscarinic acetylcholine receptors: new aspects of importance regarding Achilles tendon tendinosis (tendinopathy)

Abstract

Limited information is available concerning the existence of a cholinergic system in the human Achilles tendon. We have studied pain-free normal Achilles tendons and chronically painful Achilles tendinosis tendons with regard to immunohistochemical expression patterns of the M(2) muscarinic acetylcholine receptor (M(2)R), choline acetyltransferase (ChAT), and vesicular acetylcholine transporter (VAChT). M(2)R immunoreactivity was detected in the walls of blood vessels. As evidenced via parallel staining for CD31 and alpha-smooth muscle actin, most M(2)R immunoreactivity was present in the endothelium. M(2)R immunoreactivity also occured in tenocytes, which regularly immunoreact for vimentin. The degree of M(2)R immunoreactivity was highly variable, tendinosis tendons that exhibit hypercellularity and hypervascularity showing the highest levels of immunostaining. Immunoreaction for ChAT and VAChT was detected in tenocytes in tendinosis specimens, particularly in aberrant cells. In situ hybridization revealed that mRNA for ChAT is present in tenocytes in tendinosis specimens. Our results suggest that autocrine/paracrine effects occur concerning the tenocytes in tendinosis. Up-regulation/down-regulation in the levels of M(2)R immunoreactivity possibly take place in tenocytes and blood vessel cells during the various stages of tendinosis. The presumed local production of acetylcholine (ACh), as evidenced by immunoreactivity for ChAT and VAChT and the detection of ChAT mRNA, appears to evolve in response to tendinosis. These observations are of importance because of the well-known vasoactive, trophic, and pain-modulating effects that ACh is known to have and do unexpectedly establish the presence of a non-neuronal cholinergic system in the Achilles tendon.

The Achilles Tendinopathy: Pathogenesis Review

“…Scarring in turn may lead to impaired circulation with mechanical constriction until the obliteration of neovessels: in 20-50% of long-lasting tendinopathy patients no neovascularization is found [128,129] . This reduction in blood flow may also be explained by the fact that pathological tendons exhibit a decreased occurrence of sympathetic nerve fibres: the reduction in vasoregulatory noradrenalin causes vasoconstriction [105] . Nerve however will survive causing persistence of pain.…”

“…Nowadays, however, it is conceivable that inflammation and degeneration are not mutually exclusive: tendons respond to repetitive overload beyond physiological threshold by either inflammation of their sheath, degeneration of their body, or a combination of both [102,103] ; inflammation has been shown to play its role mainly in early human tendinopathy, while in the late phase of tendinopathy degeneration gradually occurs. Over time, possibly due to the lack of blood vessels within the tendon, instead of a chemical, a neurogenic inflammatory process takes place [104,105] .…”

“…The "neurogenic hypothesis" Substance P and other neuropeptides have been detected in tendon and in fluids collected from around painful tendons [104,105] . Substance P has been shown to modulate the expression of several tendon matrix genes and enzymes (in rabbit models [106] ), possibly causing some of the morphologic changes in Achilles tendinopathy.…”

Neuronal pathways in tendon healing and tendinopathy - update

“…Neuronal immunoreaction to choline acetyltransferase, vesicular acetylcholine transporter and acetylcholinesterase in tendons is reported to be limited compared to other tissues investigated. However, whether cholinergic innervation in tendinopathy is significantly lower than that of healthy tendons is still unclear (66,67,140).…”

“…Immunohistochemical studies have identified activated markers of cellular acetylcholine production in human tendinopathy, most prominently seen in morphologically altered tenocytes (66,67,140). Immunoreaction to choline acetyltransferase and vesicular acetylcholine transporter has recently been detected in normal Achilles tenocytes (141).…”