Effects of long-term theophylline treatment on adenosine A1-receptors in rat brain: autoradiographic evidence for increased receptor number and altered coupling to G-proteins.

The effects of 2 weeks' treatment with theophylline (20 mg/kg i.p.) on the binding to adenosine A1-receptors was studied by autoradiography using the agonist [3H]N6-cyclohexyladenosine ([3H]CHA) and the antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) as ligands. A significant increase (10%) in [3H]CHA binding was measured only in the frontoparietal cortex. However, if the brain sections were incubated in the presence of 5 microM guanosine-5'-triphosphate (GTP), which by itself decreased binding by between 15 and 80% depending on the region, the increase in the frontoparietal cortex was larger (30%) and significant increases of the same magnitude were also seen in several other structures, e.g. the caudate putamen and the central gray matter of the midbrain. In some regions, for example the hippocampus and the cerebellar cortex, small or no increases were seen. GTP 100 microM practically eliminated [3H]CHA binding in both control and treated animals, suggesting that these receptors are all coupled to G-proteins. The binding of [3H]DPCPX was increased significantly only in the frontoparietal and striate cortex (5-10%). These results suggest that the theophylline treatment had little on the total receptor number but may have altered the coupling between A1-receptors and regulatory GTP-binding proteins.