Characterization and distribution of binding sites for [H-3]-SR 141716A, a selective brain (CB1) cannabinoid receptor antagonist, in rodent brain

SR 141716A belongs to a new class of compounds (diarylpyrazole) that inhibits brain cannabinoid receptors (CB1) in vitro and in vivo. The present study showed that [H-3]-SR 141716A binds with high affinity (K-d = 0.61 +/- 0.06 nM) to a homogenous population of binding sites (B-max = 0.72 +/- 0.05 pmol/mg of protein) in rat whole brain (minus cerebellum) synaptosomes. This specific binding was displaced by known cannabinoid receptor ligands with the following rank order of potency SR 141716A > CP 55,940 > WIN 55212-2 = Delta(9)-THC > anandamide. Apart from anandamide, all these compounds were found to interact competitively with the binding sites labeled by [H-3]-SR 141716A. On the other hand, agents lacking affinity for cannabinoid receptors were unable to displace [SH]-SR 141716A from its binding sites (IC50 > 10 mu M) In addition, the binding of [H-3]-SR 141716A was insensitive to guanyl nucleotides. Regional rat brain distribution of CB1 cannabinoid receptors detected by [H-3]-SR 141716A saturation binding and autoradiographic studies, showed that this distribution was very similar to that found for [H-3]-CP 55,940. In vivo, the [H-3]-SR 141716A binding was displaced by SR 141716A with ED(50) values of 0.39 +/- 0.07 and 1.43 +/- 0.29 mg/kg following intraperitoneal and oral administration, respectively. Finally, the [H-3]-SR 141716A binding sites remained significantly occupied for at least 12 hr following oral administration of 3 mg/kg SR 141716A. Taken together, these results suggest that SR 141716A in its tritiated form is a useful research tool for labeling brain cannabinoid receptors (CB1) in vitro and in vivo.