The presence and function of cannabinoid CB2 receptors in the brain have been subject to debate. We report here that systemic, intranasal or intra-accumbens local administration of JWH133, a selective CB2 receptor agonist, dose-dependently inhibits intravenous cocaine self-administration, cocaine-enhanced locomotion, and cocaine-enhanced accumbens dopamine (DA) in wild-type (WT) and CB1 receptor-knockout (CB1−/−), but not CB2−/−, mice. This inhibition is mimicked by GW405833, another CB2 receptor agonist with a different chemical structure, and is blocked by AM630, a selective CB2 receptor antagonist. Intra-accumbens JWH133 alone dose-dependently decreases, while intra-accumbens AM630 elevates, extracellular DA and locomotion in WT and CB1−/− mice, but not in CB2−/− mice. Intra-accumbens AM630 also blocks the reduction in cocaine self-administration and extracellular DA produced by systemic administration of JWH133. These findings, for the first time, suggest that brain CB2 receptors modulate cocaine’s rewarding and locomotor-stimulating effects, likely by a DA-dependent mechanism.
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...
SUMMARY Microglia play critical roles in tissue homeostasis and can also modulate neuronal function and synaptic connectivity. In contrast to astrocytes and oligodendrocytes, which arise from multiple progenitor pools, microglia arise from yolk sac progenitors and are widely considered to be equivalent throughout the CNS. However, little is known about basic properties of deep brain microglia, such as those within the basal ganglia (BG). Here, we show that microglial anatomical features, lysosome content, membrane properties, and transcriptomes differ significantly across BG nuclei. Region-specific phenotypes of BG microglia emerged during the second postnatal week and were re-established following genetic or pharmacological microglial ablation and repopulation in the adult, indicating that local cues play an ongoing role in shaping microglial diversity. These findings demonstrate that microglia in the healthy brain exhibit a spectrum of distinct functional states and provide a critical foundation for defining microglial contributions to BG circuit function.
SUMMARY Endocannabinoids (eCBs) exert major control over neuronal activity by activating cannabinoid receptors (CBRs). The functionality of the eCB system is primarily ascribed to the well-documented retrograde activation of presynaptic CB1Rs. We find that action potential-driven eCB release leads to a long-lasting membrane potential hyperpolarization in hippocampal principal cells that is independent of CB1Rs. The hyperpolarization, which is specific to CA3 and CA2 pyramidal cells (PCs), depends on the activation of neuronal CB2Rs, as shown by a combined pharmacogenetic and immunohistochemical approach. Upon activation, they modulate the activity of the sodium-bicarbonate co-transporter, leading to a hyperpolarization of the neuron. CB2R activation occurred in a purely self-regulatory manner, robustly altered the input/output function of CA3 PCs, and modulated gamma oscillations in vivo. To conclude, we describe a cell type-specific plasticity mechanism in the hippocampus that provides evidence for the neuronal expression of CB2Rs and emphasizes their importance in basic neuronal transmission.
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