Cannabinoid CB(1) Receptors Are Localized in Striated Muscle Mitochondria and Regulate Mitochondrial Respiration

The cannabinoid type 1 (CB(1)) receptor is widely distributed in the brain and peripheral organs where it regulates cellular functions and metabolism. In the brain, CB(1) is mainly localized on presynaptic axon terminals but is also found on mitochondria (mtCB(1)), where it regulates cellular respiration and energy production. Likewise, CB(1) is localized on muscle mitochondria, but very little is known about it. The aim of this study was to further investigate in detail the distribution and functional role of mtCB(1) in three different striated muscles. Immunoelectron microscopy for CB(1) was used in skeletal muscles (gastrocnemius and rectus abdominis) and myocardium from wild-type and CB(1)-KO mice. Functional assessments were performed in mitochondria purified from the heart of the mice and the mitochondrial oxygen consumption upon application of different acute delta-9-tetrahydrocannabinol (Δ(9)-THC) concentrations (100 nM or 200 nM) was monitored. About 26% of the mitochondrial profiles in gastrocnemius, 22% in the rectus abdominis and 17% in the myocardium expressed CB(1). Furthermore, the proportion of mtCB1 versus total CB(1) immunoparticles was about 60% in the gastrocnemius, 55% in the rectus abdominis and 78% in the myocardium. Importantly, the CB(1) immunolabeling pattern disappeared in muscles of CB(1)-KO mice. Functionally, acute 100 nM or 200 nM THC treatment specifically decreased mitochondria coupled respiration between 12 and 15% in wild-type isolated mitochondria of myocardial muscles but no significant difference was noticed between THC treated and vehicle in mitochondria isolated from CB(1)-KO heart. Furthermore, gene expression of key enzymes involved in pyruvate synthesis, tricarboxylic acid (TCA) cycle and mitochondrial respiratory chain was evaluated in the striated muscle of CB(1)-WT and CB(1)-KO. CB(1)-KO showed an increase in the gene expression of Eno3, Pkm2, and Pdha1, suggesting an increased production of pyruvate. In contrast, no significant difference was observed in the Sdha and Cox4i1 expression, between CB(1)-WT and CB(1)-KO. In conclusion, CB(1) receptors in skeletal and myocardial muscles are predominantly localized in mitochondria. The activation of mtCB(1) receptors may participate in the mitochondrial regulation of the oxidative activity probably through the relevant enzymes implicated in the pyruvate metabolism, a main substrate for TCA activity.