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 (KD=0.19 nM) but low density (Bmax=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.
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