Mediation of cannabidiol anti-inflammation in the retina by equilibrative nucleoside transporter and A_2A adenosine receptor

PURPOSE. Cannabidiol (CBD), a nonpsychotropic, nontoxic compound has been shown to block diabetes- and endotoxin-induced retinal damage. However, the protective mechanism of this anti-inflammatory cannabinoid is not completely understood. The goal of this study is to determine the role of adenosine signaling in retinal inflammation and its potential modulation by CBD. METHODS. The adenosine receptor (AR) subtypes expressed in rat retinal microglial cells were assessed by quantitative real-time RT-PCR. AR function was determined via in vitro and in vivo inflammatory models. Microglial cells or rats were treated with or without lipopolysaccharide (LPS) in the presence or absence of adenosine, adenosine receptor agonists/antagonists, or CBD. Adenosine uptake and tumor necrosis factor (TNF)-α release in cells or in retinas were determined. RESULTS. The results showed that A_2AARs are abundantly expressed in rat retinal microglial cells. When the cells or rats were treated with LPS, activation of the A_2AAR was the most efficient in mediating AR agonist- or CBD-induced TNF-α inhibition. CBD inhibited adenosine uptake via equilibrative nucleoside transporter 1 and synergistically enhanced adenosine's TNF-α suppression after treatment with LPS. CONCLUSIONS. These results suggest that the activated A_2AAR in the retinal microglial cells plays a major anti-inflammatory role in the retina and that CBD's anti-inflammatory effects are linked to the inhibition of adenosine uptake.