Vol. 285, Issue 3, 1023-1030, June 1998

Pharmacological Characterization of a Simple Behavioral Response Mediated Selectively by Central Adenosine A₁ Receptors, Using In Vivo and In Vitro Techniques¹

Hugh M. Marston, Keith Finlayson, Takuya Maemoto, Henry J. Olverman, Atsushi Akahane, John Sharkey and Steven P. Butcher

Fujisawa Institute of Neuroscience (H.M.M., T.M., J.S., S.P.B.), Department of Pharmacology (K.F., H.J.O.), University of Edinburgh, 1 George Square, Edinburgh, UK, and New Drug Research Laboratories (A.A.), Fujisawa Pharmaceutical Co. Ltd, Osaka, Japan

The behavioral profile of a range of adenosine receptor ligands was examined in rats using a locomotor activity model. Adenosine receptor agonists, including the selective A₁ receptor agonist, N⁶-cyclopentyladenosine (CPA) and the A_2A agonist, 2-[(2-aminoethylamino)carbonylethyl-phenylethylamino]-5'-ethylcarboxamidoadenosine (APEC), reduced spontaneous motor activity in a dose-dependent manner. CPA-induced locomotor depression was attenuated by adenosine A₁ receptor selective antagonists, such as 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)-acryloyl]-2-piperidine ethanol (FK453), and (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)-acryloyl]-piperidin-2-yl acetic acid (FK352), but not by the A_2A receptor antagonist, (E)-1,3-dipropyl-8-(3,4-dimethoxystyryl)-7-methylxanthine (KF17837). By contrast, APEC-induced hypolocomotion was attenuated by KF17837 but not by DPCPX, confirming that adenosine A₁ and A_2A receptor activation mediates locomotor output independently. It was found that two peripheral adenosine receptor antagonists, 8-(p-sulphophenyl)-1,3-dipropylxanthine (DPSPX) and 8-(p-sulphophenyl)-1,3-dimethylxanthine (8-PST), did not alter CPA-induced hypolocomotion. This confirmed that pharmacological reversal of the adenosine A₁ receptor-mediated response involved a central site of drug action. The relationship between occupancy of central adenosine A₁ receptors and behavioral effect was therefore assessed. Regression analysis on log transformed data confirmed associations between antagonist affinity for brain [³H]DPCPX binding sites and, in order of increasing significance, the equivalent behavioral dose (EBD) for reversal of CPA-induced hypolocomotion (r² = 0.32), the serum concentration of drug (r² = 0.65), and most significantly with the brain concentration of drug detected 20 min after administration of the (EBD) (r² = 0.95). These data suggest that competition between agonists and antagonists, for occupancy of central adenosine A₁ receptors, is intrinsic to the pharmacological reversal of CPA-induced hypolocomotion. The validity of the model as a simple predictive screen for the blood/brain barrier permeability of adenosine A₁ receptor antagonists was thereby confirmed.