Autoradiographic characterization of high-affinity adenosine A2 receptors in the rat brain

Abstract Binding of the non-selective adenosine receptor agonist, [ 3 H]NECA (5′-N-Ethylcar☐amidoadenosine) was evaluated in sections of rat brain using quantitative receptor autoradiography. [ 3 H]NECA bound specifically to a variety of different brain regions including striatum, cerebellum and thalamus. In the presence of the selective adenosine A 1 receptor agonist, cyclopentyladenosine (CPA; 50 nM), [ 3 H]NECA binding was exclusively localized to the striatum and olfactory tubercle. Binding in rat striatum occured at a single site (K d =9nM) with limited capacity (apparent B max = 230fmol/mg tissue). Competition experiments in both striatum and olfactory tubercle with various adenosine agonists and antagonists indicated that the sites labeled by [ 3 H]NECA in the presence of 50 nM CPA were A 2 in nature, the rank order of activity for agonists being NECA> 2-chloroadenosine(2-CADO), >R- N 6 -phenylisopropyladenosine(R-PIA) >CPA>S- N 6 -phenylisopropyladenosine(S-PIA). For xanthine antagonists the order was 1,3-dipropyl-8(2-amino-4-chloro)phenylxanthine(PACPX) >xanthine amino acid congener(XAC) >xanthine car☐ylic acid congener(XCC) > 1,3-diethyl-8-phenylxanthine(DPX). The localization of A 2 receptors to discrete regions of rat brain indicates that the purine may have a selective role in modulating basal ganglia function.