G-PROTEIN-DEPENDENT ACTIVATION OF PHOSPHOLIPASE-C BY ADENOSINE A(3) RECEPTORS IN RAT-BRAIN

The recently cloned G protein-coupled adenosine A(3) receptor has been proposed to play a role in the pathophysiology of cerebral ischemia. Because phospholipase C activation occurs as a very early response to brain ischemia, we evaluated the ability of A(3)-selective and nonsele ctive adenosine analogues to elicit phosphoinositide hydrolysis. In my o-[H-3]inositol-labeled rat striatal and hippocampal slices, A(3) agon ists stimulated formation of [H-3]inositol phosphates in a concentrati on-dependent manner. In striatum, the potency order was -N-6-(3-iodobe nzyl)-adenosine-5'-N-methyluronamide greater than or equal to N-6-(3-i odobenzyl)-adenosine-5'-N-methyluronamide >> hyl-1,3-di-n-butylxanthin e-7-beta-D-ribofuronamide greater than or equal to 5'-N-ethylcarboxami doadenosine greater than or equal to N-6-2-(4-aminophenyl)-ethyladenos ine > N-6-(p-sulfophenyl)-adenosine = 1,3-dibutylxanthine-7-riboside, which is identical to the potency order in binding studies at cloned r at A(3) receptors. Stimulation of phospholipase C activity was abolish ed by guanosine-5'-O-(2-thiodiphosphate), confirming the involvement o f a G protein-coupled receptor. Activation of phospholipase C was high er in the striatum than in the hippocampus, consistent with A(3) recep tor densities. Stimulation of phospholipase C activity by adenosine an alogues was only modestly antagonized by xanthine derivatives and at m uch higher concentrations than needed for blocking adenosine A(1), A(2 A), and A(2B) receptors. In the presence of an A(1)/A(2) antagonist, a selective A(3) agonist only weakly inhibited forskolin-stimulated ade nylyl cyclase activity in rat striatum. Thus, stimulation of phospholi pase C activity represents a principal transduction mechanism for A(3) receptors in mammalian brain, and perhaps A, receptor-mediated increa ses of inositol phosphates in the ischemic brain contribute to neurode generation by raising intracellular calcium levels.