PROLONGED AGONIST EXPOSURE INDUCES IMBALANCE OF A1 AND A2 RECEPTOR-MEDIATED FUNCTIONS IN RAT-BRAIN SLICES

Agonist-induced desensitization of A1 and A2 adenosine receptors was studied in brain slices obtained from either rat striatum or cortex, exposed to the adenosine analogue N6-cyclopentyl adenosine (CPA) for selected time periods (15-60 min), and repeatedly washed at the end of agonist exposure. Adenosine receptor function in control and agonist-exposed slices was evaluated by measuring coupling with the cAMP producing system in striatum and with membrane phosphoinositide metabolism in cortex. In rat striatum, exposure to the adenosine analog resulted in a rapid, time- and dose-dependent desensitization of cyclase-linked A1 receptors, as shown by a gradual loss of A1 receptor-mediated inhibition of basal cAMP formation. Binding studies with [H-3]cyclo-hexyl-adenosine (CHA) showed no changes of A1 receptors for the shortest agonist-exposure periods utilized (i.e., 15 and 30 min), indeed suggesting an initial uncoupling of receptors from their transduction system. After a 60 min exposure to the adenosine analogue, a significant change of [H-3]CHA higher affinity component was instead detected, probably reflecting later agonist-induced receptor changes related to irreversible receptor loss. The initial agonist-induced loss of A1 receptor function was also confirmed in rat cortex, where A1 receptor-mediated modulation of phosphoinositide breakdown was dose-dependently impaired by pre-exposure of slices to the adenosine analogue. A2 receptors seemed resistant to agonist-induced desensitization, at least for the exposure periods utilized. In striatum, changes of A1 receptor function were indeed accompanied by a concomitant increase of A2 receptor mediated stimulation of adenylate cyclase activity, suggesting an ''unmasking'' of A2 stimulatory receptors as a consequence of the loss of inhibitory receptors. These results therefore indicate that the over-exposure to adenosine analogues can result in a functional unbalance between Al and A2 adenosine receptors, which might be at the bases of changes of adenosine neuroprotective activity in brain pathological conditions characterized by an increased release of the nucleoside.