R(-)-2,5-Dimethoxy-4-⁷⁷bromoamphetamine [⁷⁷Br-R(-)DOB]: a novel radioligand which labels a 5-HT binding site subtype

R(-)-2,5-Dimethoxy-4-⁷⁷Br-amphetamine [⁷⁷Br-R(-)DOB], a radioligand of high specific activity (1500±200 Ci/mmol), was used to label membrane-associated recognition sites in rat brain. ⁷⁷Br-R(-)DOB sites were of high affinity (K_D=0.19 nM) but low density (B_max=0.32 pmol/g tissue) in rat brain preparations. Competition experiments show that both 5-hydroxytryptamine (5-HT) and 5-HT₂ antagonists display nanomolar potency for these sites. We conclude that ⁷⁷Br-R(-)DOB labels 5-HT recognition sites in rat brain which do not fit into current classifications of 5-HT binding subtypes. This finding may be of aid in deciphering the mechanism of action of hallucinogens in man.