Increased expression of cannabinoid CB₁ receptors in Achilles tendinosis

Abstract

Background: The endogenous cannabinoid system is involved in the control of pain. However, little is known as to the integrity of the cannabinoid system in human pain syndromes. Here we investigate the expression of the cannabinoid receptor 1 (CB₁) in human Achilles tendons from healthy volunteers and from patients with Achilles tendinosis.

Methodology: Cannabinoid CB₁ receptor immunoreactivity (CB₁IR) was evaluated in formalin-fixed biopsies from individuals suffering from painful Achilles tendinosis in comparison with healthy human Achilles tendons.

Principal findings: CB₁IR was seen as a granular pattern in the tenocytes. CB₁IR was also observed in the blood vessel wall and in the perineurium of the nerve. Quantification of the immunoreactivity in tenocytes showed an increase of CB₁ receptor expression in tendinosis tissue compared to control tissue.

Conclusion: Expression of cannabinoid receptor 1 is increased in human Achilles tendinosis suggesting that the cannabinoid system may be dysregulated in this disorder.

Figure 1. Immunofluorescence for CB₁ in normal Achilles tendon and in reference tissue.

Panels show sections processed for CB₁ (a,c,e) and with CB₁ antibody preabsorbed with CB₁ antigen (b,d,f). Immunoreactions are shown in the elongated tenocytes in (a) but not in (b). Arrows indicate tenocytes. Immunoreactions are also seen in cells of the mucosa and submucosa (arrows) and the epithelial layer (arrowheads) of human colon in (c) but not in (d), and in cell bodies of a rat dorsal root ganglion in (e) but not in (f) (arrows). Asterisks in similar region in (c) and (d). Original magnification ×40.

Figure 2. Immunofluorescence for CB₁ in Achilles tendinosis.

Panels show sections of Achilles tendinosis tendons processed for demonstration of CB₁ (a,c,d) and of CB₁ after preabsorption with the immunogenic peptide (b). Numerous tenocytes are seen in low magnification in (a) and (b) (arrows at tenocytes). They show specific immunoreactions in (a) but not in (b). In panel c and d in which the tenocytes are shown in high magnification, punctuate immunoreactions in tenocytes are shown (arrows). Original magnification ×20 (a,b), ×63 (c,d).

Figure 3. Immunofluorescence for CB₁ in blood vessel and nerve fasicle.

Panels show sections of Achilles tendinosis tissue showing a small blood vessel (a,b) and a part of a nerve fascicle (c,d) stained with htx-eosin (a), processed for CB₁IR (b,d), and for PGP9.5 (c). Immunoreactions (arrows) are seen in the blood vessel wall (b) and in the perineurium of the nerve fascicle (arrows) (d). Original magnification ×20 (a), ×40 (c,d), ×63 (b). Asterisks in similar region in (a) and (b) and in the perineurium in (c) and (d).

Figure 4. CB₁IR scores for biopsy samples from controls and patients with Achilles tendinopathy.

Shown is a box and whiskers plot of CB₁ immunoreactivity in pain-free Achilles tendons (controls, n = 7) vs. tendons from patients with Achilles tendinopathy (n = 17). *p = <0.05, two-tailed Mann-Whitney U test. Values are mean of the scoring made by the two investigators.