Prolonged in vitro exposure of rat brain slices to adenosine analogues: Selective desensitization of adenosine A 1 but not A 2 receptors

Abstract

Agonist-induced desensitization of adenosine A 1 and A 2 receptors was studied in rat striatum slices maintained in carbo-oxygenated Krebs buffer. Slices were exposed to adenosine analogues (either cyclo-penlyl-adenosine or N-ethyl-carboxamido-adenosine) for selected time periods (15–60 min) and repeatedly washed at the end of agonist exposure. Agonist-induced changes of adenosine receptors were then evaluated in P2 fractions prepared from slices by measuring A 1 and A 2 rcccptor-regulated adenylatc cyclase. A 1 receptors were rapidly desensitized by agonist exposure, as shown by a gradual loss of A 1 receptor-mediated inhibition of basal cyclase activity and cAMP formation, which was evident within 15–30 min after addition of the adenosine analogue. Agonist-induced desensitization of A 1 receptors was dose- and time-dependent, and seemed quicker in onset with cyclo-pentyl-adenosine, according to the higher A 1 selectivity of this receptor agonist, with respect to N-ethyl-carboxamido-adenosine. Binding of the A 1-sclective agonist [ 3H]cyclo-hexyl-adenosine was unaffected by the desensitization procedure at any of the exposure periods utilized, suggesting that an uncoupling of A 1 receptors from their transduction system is indeed responsible for the loss of functional activity. Loss of A 1 receptor function was accompanied by a time-dependent amplification of A 2 receptor-mediated stimulation of adenylete cyciase activity, likely due to an ‘unmasking’ of A 2 stimulatory receptor function as a consequence of the desensitizalion of A 1 inhibitory receptors. All these effects could be completely counteracted by the concomitant exposure to an adenosine receptor antagonist, and specifically involved the coupling mechanisms of adenosine receptors with their effector system. It is therefore concluded that the prolonged exposure of brain slices to adenosine analogues rapidly and selectively desensitizes A 14 receptors in the absence of any loss of A2 receptor function. These receptor adaptive changes might be relevant to brain pathological conditions characterized by over-exposure to endogenous adenosine.