Characterization of the locomotor depression produced by an A2-selective adenosine agonist

Adenosine analogs, such as N6-cyclohexyladenosine (CHA) that are selective for A1-adenosine receptors, and analogs, such as 5'-N-ethylcarboxamidoadenosine (NECA) that are active at both A1 and A2 receptors, cause a profound depression of locomotor activity in mice via a central mechanism. The depression is effectively reversed by non-selective adenosine antagonists such as theophylline. We report that 2-([2-aminoethylamino) carbonylethylphenylethylamino]-5'-N-ethylcarboxamidoadenosine (APEC), an amine derivative of the A2-selective agonist, CGS21680, is a potent locomotor depressant in mice. The in vivo pharmacology is consistent with A2-selectivity at a central site of action. Two parameters indicative of locomotor activity, horizontal activity and total distance travelled, were measured using a computerized activity monitor. From dose-response curves it was found that APEC (ED50 16 micrograms/kg) is more potent than CHA (ED50 60 micrograms/kg) and less potent than NECA (ED50 2 micrograms/kg). The locomotor depression by APEC was reversible by theophylline, but not by the A1-selective antagonists 8-cyclopentyltheophylline (CPT) and 8-cyclopentyl-1, 3-dipropyl-2-thioxanthine, nor by the peripheral antagonists 8-p-sulfophenyltheophylline (8-PST) and 1,3-dipropyl-8-p-sulfophenylxanthine. The locomotor activity depression elicited by NECA and CHA was reversed by A1-selective antagonists. These results suggest that the effects of APEC are due to stimulation of A2 adenosine receptors in the brain.