CHARACTERIZATION OF (2-BENZOTHIAZOLYL)THIO-2-PROPYL]-2-CHLOROADENOSINE ([H-3]-NNC-21-0136) BINDING TO RAT-BRAIN - PROFILE OF A NOVEL SELECTIVE AGONIST FOR ADENOSINE A(1) RECEPTORS

(2-Benzothiazolyl)thio-2-propyl]-2-chloroadenosine (NNC21-0136) is a n ovel adenosine agonist with neuroprotectant properties in rodent model s of focal and global ischemia that exhibits diminished cardiovascular side effects when compared to reference A(1) agonists [Sheardown et a l., 1995]. NNC 21-0136 is shown to be a potent agonist of adenosine A( 1) receptors showing around 60- and 30-fold selectivity over adenosine A(2A) and A(3) receptors, respectively. In order to further character ize the central nervous system molecular target for NNC 21-0136, the c ompound has been radiolabeled and utilized in receptor binding experim ents. In vitro receptor autoradiography with [H-3]-NNC 21-0136 reveale d high levels of receptors in the CA1 region of the hippocampus, the g ranule cell layer of the cerebellar cortex, and moderate uniform level s throughout the cerebral cortex. [H-3]-NNC 21-0136 binding to rat cer ebral cortical membranes was reversible and saturable (B-max = 0.98 +/ - 0.04 pmol/mg protein) to a high affinity site (K-d = 1.16 +/- 0.06 n M) as determined by saturation binding experiments. [H-3]-NNC 21-0136 binding was sensitive to guanosine-5'-O-(3-thio)triphosphate-gamma-S a nd enhanced by the presence of divalent cations (Ca2+ and Mg2+). A hig hly significant correlation between the affinities of several compound s to displace [H-3]-NNC 21-0136 binding as compared to [H-3]-N-R-(2-ph enylisopropyl)adenosine ([H-3]-R-PIA) binding to adenosine A(1) recept ors in rat cortical membranes was observed. We conclude that [H-3]-NNC 21-0136 is a new, selective radioligand for adenosine Al receptors. ( C) 1997 Wiley-Liss, Inc.