A3R antagonists(National & Kapodistrian University of Athens)

Dopamine D3 receptor (D3R) antagonists may be effective medications for multiple substance use disorders (SUDs). However, no selective D3R antagonists are currently available for clinical testing. Buspirone, originally characterized as a 5-HT1A partial agonist and used as an anxiolytic, also binds to D3R and D4R with high affinity, with lower affinity to D2R, and interferes with cocaine reward. Here we used PET with [11C]PHNO (D3R-preferring radioligand), [11C]raclopride (D2R/D3R radioligand) and [11C]NNC-112 (D1R radioligand) to measure occupancy of oral and parenteral buspirone in the primate brain. Intramuscular buspirone (0.19 and 0.5 mg/kg) blocked both [11C]PHNO and [11C]raclopride binding to striatum, exhibiting high occupancy (50-85%) at 15 min and rapid wash-out over 2-6 h. In contrast, oral buspirone (3 mg/kg) significantly blocked [11C]PHNO binding in D3-rich regions (globus pallidum and midbrain) at 3 h, but had minimal effects on [11C]raclopride binding (28-37% at 1 h and 10% at 3 h). Buspirone did not block [11C]NNC-112. Our findings provide evidence that i.m. buspirone blocks D3R and D2R, whereas oral buspirone is more selective towards D3R blockade in vivo, consistent with extensive first pass metabolism and supporting the hypothesis that its metabolites (5- and 6'-hydroxybuspirone) merit evaluation for treating SUDs. They also indicate that for oral buspirone to achieve greater than 80% sustained D3R occupancy, as might be needed to treat addiction, higher doses (at least three-fold) than those used to treat anxiety (maximal 60 mg) will be required. Nonetheless, based on previous clinical studies, these doses would be safe and well tolerated.

The extension of microglial processes toward injured sites in the brain is triggered by the stimulation of the purinergic receptor P2Y12 by extracellular ATP. We recently showed that P2Y12 stimulation by ATP induces microglial process extension in collagen gels. In the present study, we found that a P2Y12 agonist, 2‐methylthio‐ADP (2MeSADP), failed to induce the process extension of microglia in collagen gels and that co‐stimulation with adenosine, a phosphohydrolytic derivative of ATP, and 2MeSADP restored the chemotactic process extension. An adenosine A3 receptor (A3R)‐selective agonist restored the chemotactic process extension, but other receptor subtype agonists did not. The removal of adenosine by adenosine deaminase and the blocking of A3R by an A3R‐selective antagonist inhibited ADP‐induced process extension. The A3R antagonist inhibited ADP‐induced microglial migration, and an A3R agonist promoted 2MeSADP‐stimulated migration. ADP and the A3R agonist activated Jun N‐terminal kinase in microglia, and a Jun N‐terminal kinase inhibitor inhibited the ADP‐induced process extension. An RT‐PCR analysis showed that A1R and A3R were expressed by microglia sorted from adult rat brains and that the A2AR expression level was very low. These results suggested that A3R signaling may be involved in the ADP‐induced process extension and migration of microglia.