MRS 1191 is putative A3 adenosine receptor antagonist, highly selective for human A3 receptor vs human A1 receptor. MRS 1067, MRS 1191 and MRS 1220 were found to be competitive in saturation binding studies using the agonist radioligand [125I]AB-MECA at cloned human brain A3 receptors expressed in HEK-293 cells. Antagonism was demonstrated in functional assays consisting of agonist-induced inhibition of adenylate cyclase and the stimulation of binding of [35S]guanosine 5′-O-(3-thiotriphosphate) ([35S]GTP-gamma-S) to the associated G-proteins. Activation of the human A3 receptor in A3R-CHO results in markedly impaired cell cycle progression, suggesting an important role for this adenosine receptor subtype in cell cycle regulation and cell growth. Activation of adenosine A3 receptors by Cl-IBMECA (100 nM) increased the magnitude of theta-burst induced LTP (from 1.2+/-0.6% in the control solution to 25.5+/-0.8% in the presence of Cl-IBMECA) and attenuated LTD (from 30.0+/-5.5% decrease in the control solution to 13.6+/-6.6% decrease in the presence of Cl-IBMECA). The selective adenosine A3 receptor antagonist, MRS 1191 (5-10 μM), prevented the effects of Cl-IBMECA. These findings indicate a functional role for adenosine A3 receptors in the modulation of synaptic plasticity.
