Cannabinoid CB1 Receptors Are Localized in Striated Muscle Mitochondria and Regulate Mitochondrial Respiration

Abstract

The cannabinoid type 1 (CB₁) receptor is widely distributed in the brain and peripheral organs where it regulates cellular functions and metabolism. In the brain, CB₁ is mainly localized on presynaptic axon terminals but is also found on mitochondria (mtCB₁), where it regulates cellular respiration and energy production. Likewise, CB₁ 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 mtCB₁ in three different striated muscles. Immunoelectron microscopy for CB₁ was used in skeletal muscles (gastrocnemius and rectus abdominis) and myocardium from wild-type and <i>CB</i>_<i>1</i> -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 (Δ⁹-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 CB₁. Furthermore, the proportion of mtCB1 versus total CB₁ immunoparticles was about 60% in the gastrocnemius, 55% in the rectus abdominis and 78% in the myocardium. Importantly, the CB₁ immunolabeling pattern disappeared in muscles of <i>CB</i>_<i>1</i> -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 <i>CB</i>_<i>1</i> -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 <i>CB</i>_<i>1</i> -WT and <i>CB</i>_<i>1</i> -KO. <i>CB</i>_<i>1</i> -KO showed an increase in the gene expression of <i>Eno3, Pkm2</i>, and <i>Pdha1</i>, suggesting an increased production of pyruvate. In contrast, no significant difference was observed in the <i>Sdha</i> and <i>Cox4i1</i> expression, between <i>CB</i>_<i>1</i> -WT and <i>CB</i>_<i>1</i> -KO. In conclusion, CB₁ receptors in skeletal and myocardial muscles are predominantly localized in mitochondria. The activation of mtCB₁ 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.