Both A1 and A2a Purine Receptors Regulate Striatal Acetylcholine Release

Abstract

The receptors responsible for the adenosine-mediated control of acetylcholine release from immunoaffinitypurified rat striatal cholinergic nerve terminals have been characterized. The relative affinities of three analogues for the inhibitory receptor were (R)-phenylisopropyladenosine > cyclohexyladenosine > N-ethylcarboxamidoadenosine (NECA), with binding being dependent of the presence of Mg²⁺ and inhibited by 5′-guanylylimidodiphosphate [Gpp(NH)p] and adenosine receptor antagonists. Adenosine A₁ receptor agonists inhibited forskolin-stimulated cholinergic adenylate cyclase activity, with an IC₅₀ of 0.5 nM for (R)-phenylisopropyladenosine and 500 nM for (S)-phenylisopropyladenosine. A₁ agonists inhibited acetylcholine release at concentrations approximately 10% of those required to inhibit the cholinergic adenylate cyclase. High concentrations (1 μM) of adenosine A₁ agonists were less effective in inhibiting both adenylate cyclase and acetylcholine release, due to the presence of a lower affinity stimulatory A₂ receptor. Blockade of the A₁ receptor with 8-cyclopentyl-1,3-dipropylxanthine revealed a half-maximal stimulation by NECA of the adenylate cyclase at 10 nM, and of acetylcholine release at approximately 100 nM. NECA-stimulated adenylate cyclase activity copurified with choline acetyltransferase in the preparation of the cholinergic nerve terminals, suggesting that the striatal A₂ receptor is localized to cholinergic neurones. The possible role of feedback inhibitory and stimulatory receptors on cholinergic nerve terminals is discussed.