Cannabinoid CB ₂ receptors modulate midbrain dopamine neuronal activity and dopamine-related behavior in mice

Abstract

Cannabinoid CB 2 receptors (CB 2 Rs) have been recently reported to modulate brain dopamine (DA)-related behaviors; however, the cellular mechanisms underlying these actions are unclear. Here we report that CB 2 Rs are expressed in ventral tegmental area (VTA) DA neurons and functionally modulate DA neuronal excitability and DA-related behavior. In situ hybridization and immunohistochemical assays detected CB 2 mRNA and CB 2 R immunostaining in VTA DA neurons. Electrophysiological studies demonstrated that activation of CB 2 Rs by JWH133 or other CB 2 R agonists inhibited VTA DA neuronal firing in vivo and ex vivo, whereas microinjections of JWH133 into the VTA inhibited cocaine self-administration. Importantly, all of the above findings observed in WT or CB 1 −/− mice are blocked by CB 2 R antagonist and absent in CB 2 −/− mice. These data suggest that CB 2 R-mediated reduction of VTA DA neuronal activity may underlie JWH133's modulation of DA-regulated behaviors.T he presence of functional cannabinoid CB 2 receptors (CB 2 Rs) in the brain has been controversial. When CB 2 Rs were first cloned, in situ hybridization (ISH) failed to detect CB 2 mRNA in brain (1). Similarly, Northern blot and polymerase chain reaction (PCR) assays failed to detect CB 2 mRNA in brain (2-5). Therefore, CB 2 Rs were considered "peripheral cannabinoid receptors" (1, 6).In contrast, other studies using ISH and radioligand binding assays detected CB 2 mRNA and receptor binding in rat retina (7), mouse cerebral cortex (8), and hippocampus and striatum of nonhuman primates (9). More recent studies using RT-PCR also detected CB 2 mRNA in the cortex, striatum, hippocampus, amygdala, and brainstem (9-14). Immunoblot and immunohistochemistry (IHC) assays detected CB 2 R immunoreactivity or immunostaining in various brain regions (13,(15)(16)(17)(18)(19)(20). The specificities of the detected CB 2 R protein and CB 2 -mRNA remain questionable, however, owing to a lack of controls using CB 1 −/− and CB 2 −/− mice in most previous studies (21). A currently accepted view is that brain CB 2 Rs are expressed predominantly in activated microglia during neuroinflammation, whereas brain neurons, except for a very small number in the brainstem, lack CB 2 R expression (21).On the other hand, we recently reported that brain CB 2 Rs modulate cocaine self-administration and cocaine-induced increases in locomotion and extracellular dopamine (DA) in the nucleus accumbens in mice (22). This finding is supported by recent studies demonstrating that systemic administration of the CB 2 R agonist O-1966 inhibited cocaine-induced conditioned place preference in WT mice, but not in CB 2 −/− mice (23), and that increased CB 2 R expression in mouse brain attenuates cocaine self-administration and cocaine-enhanced locomotion (19). In addition, brain CB 2 Rs may be involved in several DA-related CNS disorders, such as Parkinson's disease (24), schizophrenia (25), anxiety (26), and depression (27). The cellular mechanisms underlying CB 2 R modulation of DA-related behav...

A new designer drug 5F-ADB activates midbrain dopaminergic neurons but not serotonergic neurons

“…Furthermore, according to a previous in vitro assay, 5F-ADB displayed agonist activity at both CB 1 and CB 2 receptors (Banister et al, 2016). Considering that CB 2 receptors are expressed on dopaminergic neurons and decrease dopaminergic-neuron activity (Zhang et al, 2014), our results suggest the possibility that 5F-ADB has dual effects on dopaminergic neurons -i.e., potent activation via CB 1 receptors and moderate inactivation via CB 2 receptors. Additionally, this idea is supported by the fact that ADB-induced increases in dopaminergic firing activity were relatively weak compared to the expected response from in vitro CB 1 affinity for 5F-ADB (Banister et al, 2016).…”

Early Phase in the Development of Cannabidiol as a Treatment for Addiction: Opioid Relapse Takes Initial Center Stage

“…For example, type 1 cannabinoid receptors (CB 1 R) are co-localized with opioid μ opioid receptors (which mediate the actions of opioid drugs) in striatal output projection neurons of the nucleus accumbens and dorsal striatum that modulate reward, goal-directed behavior, and habit formation relevant to addiction [22]. Type 2 cannabinoid receptors (CB 2 R) have very low expression in the brain generally, but recently they have been shown to be expressed in dopamine neurons of the midbrain ventral tegmental area and modulate the functional excitability of dopamine neurons central to addiction related behaviors such as drug reinforcement [23]. Stimulation of CB 2 R in mice models has an inhibitory influence on cocaine and alcohol selfadministration and related conditioned place preference, as well as nicotine place preference behavior [23,24].…”

“…Type 2 cannabinoid receptors (CB 2 R) have very low expression in the brain generally, but recently they have been shown to be expressed in dopamine neurons of the midbrain ventral tegmental area and modulate the functional excitability of dopamine neurons central to addiction related behaviors such as drug reinforcement [23]. Stimulation of CB 2 R in mice models has an inhibitory influence on cocaine and alcohol selfadministration and related conditioned place preference, as well as nicotine place preference behavior [23,24].…”

Addictive neurons

“…Note that the retrograde neurotransmission is shown for CB 1 , since CB 2 cannabinoid receptors are prevailingly established in the periphery, but not in CNS. Zhang and colleagues [70] delivered a strong message that not only CB 1 , but also CB 2 are present and may modulate brain area involved in addiction. Some of data are presented in perhaps unfortunate manner and diminishing their importance.…”

“…Some of data are presented in perhaps unfortunate manner and diminishing their importance. Since representative experiments are 'illustrating that CB 2 R staining was detected in VTA DA neurons in WT and CB1 −/− mice, but barely detectable in CB2 −/− mice', yet average values are 'illustrating a significant reduction in the density' [70] . Density of ~8 vs. 19 is not after all 'barely'.…”