A tale of two methods: Identifying neuronal CB1 receptors

Cannabinoid Receptors and the Endocannabinoid System: Signaling and Function in the Central Nervous System

The biological effects of cannabinoids, the major constituents of the ancient medicinal plant Cannabis sativa (marijuana) are mediated by two members of the G-protein coupled receptor family, cannabinoid receptors 1 (CB1R) and 2. The CB1R is the prominent subtype in the central nervous system (CNS) and has drawn great attention as a potential therapeutic avenue in several pathological conditions, including neuropsychological disorders and neurodegenerative diseases. Furthermore, cannabinoids also modulate signal transduction pathways and exert profound effects at peripheral sites. Although cannabinoids have therapeutic potential, their psychoactive effects have largely limited their use in clinical practice. In this review, we briefly summarized our knowledge of cannabinoids and the endocannabinoid system, focusing on the CB1R and the CNS, with emphasis on recent breakthroughs in the field. We aim to define several potential roles of cannabinoid receptors in the modulation of signaling pathways and in association with several pathophysiological conditions. We believe that the therapeutic significance of cannabinoids is masked by the adverse effects and here alternative strategies are discussed to take therapeutic advantage of cannabinoids.

Cannabinoid type 1 receptor-containing axons innervate NPY/AgRP neurons in the mouse arcuate nucleus

Objectives

Phytocannabinoids, such as THC and endocannabinoids, are well known to promote feeding behavior and to control energy metabolism through cannabinoid type 1 receptors (CB₁R). However, the underlying mechanisms are not fully understood. Generally, cannabinoid-conducted retrograde dis-inhibition of hunger-promoting neurons has been suggested to promote food intake, but so far it has not been demonstrated due to technical limitations.

Methods

We applied immunohistochemical labeling of CB₁R for light microscopy and electron microscopy combined with three-dimensional reconstruction from serial sections in CB₁R-expressing and CB₁R-null mice, which served as a negative control. Hunger-promoting neurons expressing Agouti-related protein and neuropeptide Y (AgRP/NPY) in the hypothalamic arcuate nucleus were identified in NPY-GFP and NPY-hrGFP mice.

Results

Using three-dimensional reconstruction from serial sections we demonstrated numerous discontinuous segments of anti-CB₁R labeling in the synaptic boutons and axonal shafts in the arcuate nucleus. We observed CB₁R in the symmetric, presumed GABAergic, synaptic boutons innervating AgRP/NPY neurons. We also detected CB₁R-containing axons producing symmetric and asymmetric synapses onto AgRP/NPY-negative neurons. Furthermore, we identified CB₁R in close apposition to the endocannabinoid (2-arachidonoylglycerol)-synthesizing enzyme diacylglycerol lipase-alpha at AgRP/NPY neurons.

Conclusions

Our immunohistochemical and ultrastructural study demonstrates the morphological substrate for cannabinoid-conducted feeding behavior via retrograde dis-inhibition of hunger-promoting AgRP/NPY neurons.

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3D electron microscopy displays CB₁R-immunopositive axons in the hypothalamus.

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CB₁R-expressing inhibitory synapses innervate hunger-promoting AgRP/NPY neurons.

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Pre-synaptic CB₁R and post-synaptic DAGL are co-localized at AgRP/NPY neurons.

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

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 CB₁-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 CB₁-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 CB₁-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 CB₁-WT and CB₁-KO. CB₁-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 CB₁-WT and CB₁-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.

Alteration of SLP2-like immunolabeling in mitochondria signifies early cellular damage in developing and adult mouse brain

Mitochondria play a critical role in various pathways of regulated cell death. Here we propose a novel method for detection of initial derangement of mitochondria in degenerating and dying neuronal cells. The method is based on our recent finding that antibodies directed against the cannabinoid type 1 receptor (CB1) also bind the mitochondrial stomatin-like protein 2 (SLP2) that belongs to an inner mitochondrial membrane protein complex. It is well established that SLP2 regulates mitochondrial biogenesis and respiratory functions. We now show that anti-CB1 antibodies recognize conformational epitopes but not the linear amino acid sequence of SLP2. In addition we found that anti-CB1 serum mostly labels swollen mitochondria with early or advanced stages of pathology in mouse brain while other proteins of the complex may mask epitopes of SLP2 in the normal mitochondria. Although neurons and endothelial cells in healthy brains contain occasional immunopositive mitochondria detectable with anti-CB1 serum, their numbers increase significantly after hypoxic insults in parallel with signs of cellular damage. Moreover, use of electron microscopy suggests relocation of SLP2 from its normal functional position in the inner mitochondrial membrane into the mitochondrial matrix in pathological cells. Thus, SLP2-like immunolabeling serves as an in situ histochemical target detecting early derangement of mitochondria. Anti-CB1 serum is crucial for this purpose because available anti-SLP2 antibodies do not provide selective labeling of mitochondria in the fixed tissue. This new method of detecting mitochondrial dysfunction can benefit the in vitro research of human diseases and developmental disorders by enabling analysis in live animal models.

Hypothalamic POMC neurons promote cannabinoid-induced feeding

Hypothalamic pro-opiomelanocortin (POMC) neurons promote satiety. Cannabinoid receptor 1 (CB1R) is critical for the central regulation of food intake. Here we test whether CB1R-controlled feeding in sated mice is paralleled by decreased activity of POMC neurons. We show that chemical promotion of CB1R activity increases feeding, and notably, CB1R activation also promotes neuronal activity of POMC cells. This paradoxical increase in POMC activity was crucial for CB1R-induced feeding, because designer-receptors-exclusively-activated-by-designer-drugs (DREADD)-mediated inhibition of POMC neurons diminishes, whereas DREADD-mediated activation of POMC neurons enhances CB1R-driven feeding. The Pomc gene encodes both the anorexigenic peptide α-melanocyte-stimulating hormone, and the opioid peptide β-endorphin. CB1R activation selectively increases β-endorphin but not α-melanocyte-stimulating hormone release in the hypothalamus, and systemic or hypothalamic administration of the opioid receptor antagonist naloxone blocks acute CB1R-induced feeding. These processes involve mitochondrial adaptations that, when blocked, abolish CB1R-induced cellular responses and feeding. Together, these results uncover a previously unsuspected role of POMC neurons in the promotion of feeding by cannabinoids.

Cannabinoid-induced changes in respiration of brain mitochondria

Cannabinoids exert various biological effects that are either receptor-mediated or independent of receptor signaling. Mitochondrial effects of cannabinoids were interpreted either as non-receptor-mediated alteration of mitochondrial membranes, or as indirect consequences of activation of plasma membrane type 1 cannabinoid receptors (CB1). Recently, CB1 receptors were confirmed to be localized to the membranes of neuronal mitochondria, where their activation directly regulates respiration and energy production. Here, we performed in-depth analysis of cannabinoid-induced changes of mitochondrial respiration using both an antagonist/inverse agonist of CB1 receptors, AM251 and the cannabinoid receptor agonists, Δ(9)-tetrahydrocannabinol (THC), cannabidiol, anandamide, and WIN 55,212-2. Relationships were determined between cannabinoid concentration and respiratory rate driven by substrates of complex I, II or IV in pig brain mitochondria. Either full or partial inhibition of respiratory rate was found for the tested drugs, with an IC50 in the micromolar range, which verified the significant role of non-receptor-mediated mechanism in inhibiting mitochondrial respiration. Effect of stepwise application of THC and AM251 evidenced protective role of AM251 and corroborated the participation of CB1 receptor activation in the inhibition of mitochondrial respiration. We proposed a model, which includes both receptor- and non-receptor-mediated mechanisms of cannabinoid action on mitochondrial respiration. This model explains both the inhibitory effect of cannabinoids and the protective effect of the CB1 receptor inverse agonist.