[I-125] 4-AMINOBENZYL-5'-N-METHYLCARBOXAMIDOADENOSINE ([I-125]AB-MECA) LABELS MULTIPLE ADENOSINE RECEPTOR SUBTYPES IN RAT-BRAIN

Adenosine modulates neuronal activity and neurotransmitter release thr ough interaction with cell surface receptors. Four adenosine receptor subtypes, A(1), A(2A), A(2B), and A(3) receptors, have been cloned and characterized. The agonist ligand, [I-125]AB-MECA 125]4-aminobenzyl-5 '-N-methylcarboxamidoadenosine) has high affinity for recombinant A(1) and A(3) receptors [Olah et al., Mel. Pharmacol., 45 (1994 978-982]. Rodent A(3) receptors are relatively insensitive to xanthines; inhibit ion of A(1) receptors with xanthines allows selective detection of A(3 ) receptors despite the lack of selectivity of the ligand. We studied whether [I-125]AB-MECA is useful for localization and characterization of A(3) receptors in rat brain. The autoradiographic distribution of total [I-125]AB-MECA (400 pM) binding closely resembled the pattern of A(1) receptor binding, with highest levels in cerebellum hippocampus, and thalamus, and moderate levels in cortex and striatum. Drug compet ition studies confirmed that almost all [I-125]AB-MECA binding could b e attributed to labeling of A(1) receptors. Xanthine amine congener (1 mu M) reduced specific [I-125]AB-MECA binding by >95%, indicating tha t xanthine-resistant A(3) receptors represent a quantitatively minor s ubtype. Despite the use of a radioligand with high affinity and high s pecific activity, the low density of A(3) receptors in rat brain appea rs insufficient to allow localization, or even consistent detection, o f this receptor subtype. In the presence of DPCPX (50 nM, to block A(1 ) receptors), residual [I-125]AB-MECA binding to A(2A) receptors was o bserved in the striatum. Thus, [I-125]AB-MECA labels primarily A(1) an d A(2A) adenosine receptors in rat brain.