Evidence for adenosine A2 receptor involvement in the hypomobility effects of adenosine analogues in mice

The hypomobility induced by a series of adenosine analogues was investigated using a holeboard test and their behavioral potencies correlated to their reported adenosine A1 and A2 receptor affinities. All of the adenosine analogues dose dependently inhibited both exploratory behavior (head dipping) and locomotor activity. The rank order of potency 5'-ethylcarboxamido adenosine (NECA) greater than 5'-methylcarboxamido adenosine (MECA) greater than N6-[(R)-1-methyl-2-phenylethyl]adenosine (R-PIA) greater than N6-cyclohexyladenosine (CHA) = N6-cyclopentyladenosine (CPA) greater than N6-[(S)-1-methyl-2-phenylethylladenosine (S-PIA) greater than N6-(2-hydroxyethyl)adenosine (2-OH-ethyl) was observed for inhibiting both activities. The behavioral potency of these adenosine analogues correlates extremely well with their reported A2 receptor affinity (r2 = 0.93, P less than 0.01 and r2 = 0.86, P less than 0.05 for locomotor and head dipping activity respectively), but very poorly with their reported A1 receptor affinity (r2 less than 0.02, P greater than 0.50 for both activities). These results suggest that adenosine A2 receptors, but not A1 receptors, may be involved in the hypomobility induced by adenosine analogues.