Characterization of [3H]forskolin binding sites in the human caudate nucleus

[3H]Forskolin binds to a site in rat and bovine brain probably reflecting the coupling of the alpha subunit of the stimulatory guanine nucleotide binding protein with adenylate cyclase. The purpose of this study was to pharmacologically characterize high-affinity binding of [3H]forskolin in the human caudate nucleus. The dissociation constant (Kd) of [3H]forskolin binding was 102 nM and the Bmax was 628 fmol/mg of protein. Guanylyl-5'-imidodiphosphate significantly increased the number (Bmax) of [3H]forskolin binding sites in the caudate nucleus (EC50 = 3.1 +/- 0.05 microM). [3H]Forskolin binding in the human caudate may provide insights into the coupling of a stimulatory guanine nucleotide binding protein with adenylate cyclase.