Highly potent cocaine analogs cause long-lasting increases in locomotor activity

Three cocaine analogs were compared with cocaine for the capacity to affect: (1) dopamine transporter binding and function; and (2) locomotor activity. RTI-55 (3 beta-[4-iodophenyl]tropane-2 beta-carboxylic acid methyl ester tartrate), RTI-121 (3 beta-[4-iodophenyl]tropan-2 beta-carboxylic acid isopropyl ester hydrochloride) and RTI-130 (3 beta-[4-chlorophenyl-2 beta-[1,2,4-oxadiazol-3-phenyl-5-yl]tropane hydrochloride) competed for [H-3]WIN 35428 binding in rat striatum in vitro, with IC50 values at least 50-fold less than that of cocaine. These analogs inhibited [H-3]dopamine transport into rat striatal synaptosomes, with IC50 values again less (at least 100-fold) than that for cocaine. Intravenous RTI-55, RTI-121 or RTI-130 injection effected dose-related increases in locomotor activity in mice, with estimated relative potencies at least 10-fold greater than that of cocaine. These increases were long lasting: whereas increased activity ceased within 2 h after cocaine administration, increased locomotion was observed at least 10 h after RTI-55, RTI-121, or RTI-130 administration. Parallel line analysis indicated that the slopes of the ascending portion of the RTI-121 and RTI-130 dose-response curves differed from that of cocaine, suggesting the involvement of mechanisms different from that of cocaine.