N-Alkylated Analogs of 4-Methylamphetamine (4-MA) Differentially Affect Monoamine Transporters and Abuse Liability

Abstract

Clandestine chemists synthesize novel stimulant drugs by exploiting structural templates known to target monoamine transporters for dopamine, norepinephrine, and serotonin (DAT, NET, and SERT, respectively). 4-Methylamphetamine (4-MA) is an emerging drug of abuse that interacts with transporters, but limited structure-activity data are available for its analogs. Here we employed uptake and release assays in rat brain synaptosomes, voltage-clamp current measurements in cells expressing transporters, and calcium flux assays in cells coexpressing transporters and calcium channels to study the effects of increasing N-alkyl chain length of 4-MA on interactions at DAT, NET, and SERT. In addition, we performed intracranial self-stimulation in rats to understand how the chemical modifications affect abuse liability. All 4-MA analogs inhibited uptake at DAT, NET, and SERT, but lengthening the amine substituent from methyl to ethyl, propyl, and butyl produced a stepwise decrease in potency. N-methyl 4-MA was an efficacious substrate-type releaser at DAT that evoked an inward depolarizing current and calcium influx, whereas other analogs did not exhibit these effects. N-methyl and N-ethyl 4-MA were substrates at NET, whereas N-propyl and N-butyl 4-MA were not. All analogs acted as SERT substrates, though N-butyl 4-MA had very weak effects. Intracranial self-stimulation in rats showed that elongating the N-alkyl chain decreased abuse-related effects in vivo that appeared to parallel reductions in DAT activity. Overall, converging lines of evidence show that lengthening the N-alkyl substituent of 4-MA reduces potency to inhibit transporters, eliminates substrate activity at DAT and NET, and decreases abuse liability of the compounds.

Effects of N-Alkyl-4-Methylamphetamine Optical Isomers on Plasma Membrane Monoamine Transporters and Abuse-Related Behavior

“…In general, within this series, increased bulk/length of the N-alkyl substituent was associated with varied potency to inhibit uptake at brain DAT, NET, and SERT in rat brain synaptosomes, decreased potency as substrates, and reduced efficacy to promote release. 15 For example, only 3 , as with 2 , was fully efficacious as a releasing agent at DAT whereas 4 and 5 were partial releasers. 15 In contrast, 3–5 were fully efficacious releasing agents at SERT.…”

“…15 For example, only 3 , as with 2 , was fully efficacious as a releasing agent at DAT whereas 4 and 5 were partial releasers. 15 In contrast, 3–5 were fully efficacious releasing agents at SERT. Compound 6 did not display releasing action at DAT, NET, or SERT, was a very weak uptake inhibitor at DAT and NET, and inactive at SERT.…”

“…1,2,14 We recently investigated the effects of increasing the N-alkyl chain length on the transporter activity of 4-MA. 15 Specifically, we examined compounds 3–6 that introduced linear bulk on the amine of 4-MA in a homologous manner. That is, all of these compounds were secondary amines, but bear amine substituents with increasing steric bulk (length).…”

“…Compound 6 did not display releasing action at DAT, NET, or SERT, was a very weak uptake inhibitor at DAT and NET, and inactive at SERT. 15 …”

“…15 As a means to investigate the mechanism of partial releasers in greater detail, we developed methods that involved co-transfection of cells with DAT, NET, and SERT along with voltage-gated calcium channels. Under these circumstances, the electrophysiological effects of transporter substrates or blockers are transduced to measurable calcium fluorescence.…”