Introduction

The cannabinoid type 2 receptor (CB2R) is a G-protein-coupled receptor that was cloned in 19931. Since then, the expression and function of CB2Rs in the brain have been debated. Early studies suggested that CB2Rs were absent in the brain because CB2 mRNA transcripts were not detected in brain tissue using various methods2,3,4,5. Based on these findings, CB2Rs have been considered the “peripheral” cannabinoid receptor1,6,7. Recently, this concept has been challenged by the identification of CB2Rs throughout the central nervous system (CNS)5,6. Compared with CB1Rs, brain CB2Rs exhibit several unique features: (1) Brain CB2Rs have lower expression levels than CB1Rs, suggesting that CB2Rs may not mediate the effect of cannabis under normal physiological conditions. (2) Brain CB2Rs are highly inducible; thus, under some pathological conditions (eg, addiction, inflammation, anxiety), CB2R expression is quickly enhanced in the brain8. This finding suggests a close relationship between the alteration of CB2R expression/function and various psychiatric and neurological diseases. (3) Brain CB2Rs have a specific distribution. Given that they are chiefly expressed in neuronal somatodendritic areas9 (postsynaptic), the activation of CB2Rs may lead to opposing effects from CB1Rs. For example, CB1Rs are predominantly expressed on neuronal terminals, especially on GABAergic terminals (presynaptic) in ventral tegmental area (VTA) dopamine (DA) neurons10. The activation of CB1Rs reduces GABA release onto DA neurons, leading to an increase in DA neuronal firing through a disinhibition mechanism. However, CB2Rs are mainly located on postsynaptic somatodendritic areas, and the activation of CB2Rs reduces VTA DA neuron firing and excitability11. Considering these characteristics, CB2Rs appear to be an important substrate for neuroprotection12, and targeting CB2Rs will likely offer a novel therapeutic strategy for treating neuropsychiatric and neurological diseases without typical CB1R-mediated side effects13. Thus, an urgent need to understand the functional effects of CB2Rs in the brain has emerged. Unfortunately, CB2R-mediated modulation of neuronal functions, including ion channels, receptors, synaptic transmission and plasticity, and neuronal networks in the CNS, has not been well investigated, and to date, studies of the functional effects of CB2Rs in neurons have ignited debate and controversy due to the following reasons: (1) a lack of highly selective CB2R antibodies14; (2) a lack of full knockout (KO) CB2R mice as the two types of CB2R KO mice that have been recently made available are partial knockouts7; and (3) under some conditions, CB1R and CB2R can form a heteromer15, which makes the identification of CB2R function even more complex and difficult. Nevertheless, by combining multiple experimental approaches, several recent papers have described CB2R expression and function in the brain neurons. Recently, we reported that functional CB2R was expressed in VTA DA neurons, and activation of VTA CB2Rs reduces neuronal excitability and cocaine-seeking behavior11. It has been reported that CB2R modulates hippocampal CA1 synaptic plasticity16 and CA3 neural plasticity and synchronization17. In addition, a recent report showed that CB2R-dependent inhibition of DA release underlies the positive allosteric modulation of the M4 muscarinic receptor in antipsychotic-like effects18. These accumulating lines of evidence suggest that brain CB2Rs are important in the modulation of brain function and disorders.

Brain CB2R expression

Although early studies were not able to detect CB2 mRNA transcripts in brain tissue using various methods2,3,4,5, recent lines of evidence show that significant CB2 mRNA has been detected by in situ hybridization (ISH) in the globus pallidus of non-human primates19. RT-PCR analysis has also been used to detect CB2 mRNA expression in various brain regions, including the retina20, cortex19,21,22,23, striatum2,23, hippocampus17,19,24, amygdala22,23, brainstem25, and cerebellum26. Furthermore, two CB2 mRNA transcripts (CB2A and CB2B) are transcribed from two independent promoters in rodent and human tissue21. Recently, we have confirmed that CB2 mRNA is relatively highly expressed in VTA DA neurons11. Moreover, immunoblot and immunohistochemical (IHC) assays have detected significant CB2-like bands or immunostaining in various brain regions25,27,28,29,30, including the VTA11. These findings suggest that CB2R expression exists not only in peripheral tissue but also in the brain, although the CB2R expression level in the brain is much lower than that of CB1R. This low-level brain CB2R expression suggests that CB2R may not participate in important brain physiology function; thus, unlike CB1Rs that mediate serious psychiatric side effects after activation, pharmacological intervention of CB2Rs has much fewer side effects. In addition to brain neurons, brain glia cells express CB2Rs31,32, where CB2Rs play an important role in the modulation of central immune function33 and neuroinflammation-associated diseases34,35,36.

Brain CB2R distribution and function

Although both CB1Rs and CB2Rs are G-protein (Gi/o)-coupled receptors, they exhibit different distributions. In general, the CB1Rs are mainly expressed in GABAergic neuronal axon terminals, including in VTA GABA neurons37,38 and hippocampal CCK-positive GABA neurons39. The activation of CB1Rs reduces presynaptic GABA release, eliminates GABAergic inhibitory control of postsynaptic neurons, and excites these postsynaptic neurons through this dis-inhibition role. However, the CB2Rs are mainly expressed in the postsynaptic cell body11,17; thus, the activation of these postsynaptic CB2Rs usually hyperpolarizes the membrane potential and inhibits postsynaptic neuronal function. Therefore, this difference in distribution results in opposite effects after CB1R and CB2R activation. Activation of CB2Rs reduces neuronal excitability through different mechanisms. In VTA DA neurons, activation of CB2Rs decreases neuronal excitability through the CB2R-associated modulation of K+ channel function11. In prefrontal cortical neurons, intracellular CB2Rs are coupled to the Gq11-PLC-IP3 pathway, which opens the Ca2+-dependent Cl- channels, hyperpolarizes the cell membrane and results in neuronal inhibition40. In hippocampal CA3/CA2 pyramidal neurons, activation of CB2Rs triggers activation of the Na+/Bicarbonate co-transporter and causes a long-term neuronal hyperpolarization. This CB2R activation occurs in a purely self-regulatory manner, robustly alters the input/output function of CA3 pyramidal cells, and modulates gamma oscillations in vivo17. The relatively high expression of CB2Rs in midbrain DA neurons suggests that they modulate a variety of important DA-associated behaviors41. It has been reported that CB2Rs modulate food intake, body weight42,43,44,45, depression46, anxiety22,47, and schizophrenia-like behavior23,48. Recent reports emerging from several labs, including ours, have shown that brain CB2Rs play a pivotal role in reducing cocaine, alcohol, and nicotine addiction49,50,51. Collectively, these lines of evidence strongly suggest an important impact of CB2Rs in the mesocorticolimbic system, as well as critical roles of CB2Rs in various brain functions, including psychiatric, cognitive, and neurobiological activity.

Inducible feature of brain CB2Rs and their modulation in neurological and psychiatric disorders

The most attractive property of the CB2R is its inducible feature. Brain CB2Rs are expressed at low levels under physiological conditions; however, in pathological conditions, such as neuropathic pain52, stroke53, traumatic brain injury54, neurodegenerative diseases55,56,57 or drug addiction58,59, CB2R expression up-regulates quickly and profoundly. This inducible feature lets CB2Rs serve as a disease-associated target, and pharmacotherapeutic manipulation of CB2Rs can treat diseases without side effects. For example, the mesocorticolimbic DA system is a key brain circuit implicated in a number of drug addictions. Alterations of the mesocorticolimbic DA circuit are the major cellular mechanisms to promote or prevent drug reward, dependence, and addiction. Emerging evidence has demonstrated that CB2Rs modulate animal drug-seeking behaviors, including cocaine, alcohol, and nicotine49,50,51, suggesting a significant impact of brain CB2Rs in animal drug reward, dependence, and addiction. Given the lack of psychoactivity demonstrated by selective CB2R agonists, CB2R ligands have been developed as new candidates for the treatment of a variety of neurological and psychiatric disorders13,60,61, including pain62,63,64,65, neuroinflammation66, stroke67,68, Alzheimer's disease69, Parkinson's disease, and Huntington's disease36,70,71,72,73,74,75. Three medicines that activate cannabinoid CB1/CB2 receptors are now used in clinics: Cesamet (nabilone), Marinol (dronabinol; Δ9-tetrahydrocannabinol [Δ9-THC]), and Sativex (Δ9-THC with cannabidiol). Additionally, a selective CB2R agonist “Resunab™” has been designated by the FDA for a fast-track development program in a Phase II human clinical trial for scleroderma. However, significant attention is currently being directed at the possibility of developing medicines from compounds that can activate CB2Rs at doses that induce little or no CB1R activation. Accumulating lines of evidence have demonstrated that many of the adverse effects induced by mixed CB1/CB2 receptor agonists result from CB1R rather than from CB2R activation and that CB2R-selective agonists have a number of important potential therapeutic applications60. Therefore, we anticipate the emergence of new drugs that modulate CB2Rs once a better understanding of the cannabinoid receptors is gained.

Limitation of brain CB2Rs as a therapeutic target

The major challenge of how to selectively target brain CB2Rs without affecting peripheral CB2Rs remains, as CB2R levels are much higher in peripheral tissues (eg, T-cells in the spleen) than in the brain. Thus, systemic exposure of CB2R ligands to activate brain CB2Rs will always activate peripheral CB2Rs. We have two thoughts regarding this challenge: 1) Brain CB2Rs are dramatically inducible, meaning they are up-regulated during disease conditions such as addiction, degeneration and inflammation. This pathology-associated increase significantly enhances the benefit to side-effect ratio76. 2) Activation of brain CB2Rs protects neurons from pathological conditions (eg, addiction, anxiety, stroke, epilepsy, pain) while also activating peripheral CB2Rs (eg, T-cells), which may cause side effects in addition to central therapeutic effects. However, peripheral CB2R activation will reduce the immune response and prevent an over-inflammatory reaction, which are beneficial for central protective effects. Therefore, the activation of peripheral CB2Rs may not always induce side effects when brain CB2Rs are activated, but rather, both central and peripheral CB2Rs may work together to protect brain neurons from pathological alterations through both neuronal and immune mechanisms. Figure 1 shows a diagram of the impact of brain CB2R distribution, function and disease.

Figure 1
figure 1

A diagram summarizing brain CB2R expression and function and the association with neuropsychiatric and neurological diseases. CB2Rs are expressed in brain neurons, where they participate in the modulation of a variety of neural functions and disorders. CB2Rs are also expressed in brain glia cells, where they modulate immune function and neuroinflammatory responses. Therefore, brain CB2Rs are an important target for the modulation of brain function and disease.

PowerPoint slide

Full size image