Interaction of psychoactive tryptamines with biogenic amine transporters and serotonin receptor subtypes

Synthetic hallucinogenic tryptamines — many first described by Alexander Shulgin — remain used recreationally and are chemically related to endogenous serotonin and classical psychedelics such as LSD and psilocybin. Prior work has established that agonism at the serotonin 2A (5-HT2A) receptor is central to psychedelic effects, but not all 5-HT2A agonists produce hallucinations, and evidence for functional selectivity and involvement of other targets (for example SERT, 5-HT1A, 5-HT2C, trace amine and dopamine receptors) suggests multiple neurochemical pathways may shape subjective effects. Structure–activity observations have also suggested that some tryptamines may act at the serotonin transporter (SERT) either as uptake inhibitors or as transporter substrates that induce 5-HT release, analogous to the distinction between MDMA (a releaser) and LSD (a 5-HT2A agonist) in therapeutic contexts. Blough and colleagues set out to characterise a panel of 21 synthetic tryptamines across biogenic amine transporters (DAT, NET, SERT) and serotonin receptor subtypes implicated in psychedelic pharmacology (human 5-HT2A and 5-HT1A). The study aimed to determine which compounds act as SERT substrates (releasers) versus SERT uptake inhibitors, to profile 5-HT2A and 5-HT1A agonist activity including β-arrestin recruitment, and to relate these in vitro pharmacologies to structural features of the molecules.

The investigators evaluated 21 tryptamines in transporter assays using rat brain synaptosomes and in human receptor functional assays using transfected cell lines. Synaptosomes for dopamine transporter (DAT) assays were prepared from rat striatum; synaptosomes for norepinephrine transporter (NET) and SERT assays were prepared from whole brain minus striatum and cerebellum. Radiolabelled uptake assays used low-nanomolar concentrations of tracer substrate (the extracted text reports approximately 5 nM for DA and 5-HT, and 10 nM for NE) and included selective unlabeled blockers to optimise selectivity for each transporter. Compounds were characterised as uptake inhibitors or as transporter substrates/releasers, with release activity confirmed by substrate reversal (a method that distinguishes releasers from blockers). Receptor activity was measured with human receptors in cell-based functional assays. 5-HT2A-mediated Gq/11 signalling was assessed by calcium mobilisation in HEK293 cells stably expressing the human 5-HT2A receptor. 5-HT1A activity was measured by calcium mobilisation in Galpha16-CHO cells stably expressing the human 5-HT1A receptor. β-arrestin recruitment at 5-HT2A was quantified using a DiscoveRx PathHunter assay in CHO-K1 cells expressing the 5-HT2A receptor fused to a ProLink fragment; cells were plated at 15,000 cells/well, compounds incubated, and luminescence measured to derive EC50 and percent Emax values. EC50 and Emax values were obtained from three independent experiments performed in duplicate unless otherwise noted, using three-parameter logistic curve fitting. Chemical sourcing and preparation were reported briefly: many compounds were synthesised following Shulgin-style methods, some were procured from the National Institute on Drug Abuse Drug Supply Program, and others were purchased commercially. Structural variation among the panel included differing N-alkyl substitutions (from primary amines to N,N-diisopropyl) and indole ring substituents (unsubstituted, 5-methoxy, 5-hydroxy, or 4-hydroxy), allowing exploration of structure–activity relationships for transporter and receptor interactions.

Twenty-one tryptamines were profiled for transporter and serotonin receptor activity. The compounds segregated into two functional groups at SERT: eight acted as SERT-mediated releasers (substrates) and the remaining 13 were either SERT uptake inhibitors or inactive in the transporter assays (compounds 2 and 23 reported as inactive). Smaller, less sterically hindered molecules (for example primary and simple secondary amines) tended to be releasers, while larger N-alkyl substituents generally produced uptake inhibition, consistent with the hypothesis that translocation through the transporter favours smaller substrates. Quantitatively, unsubstituted indole releasers displayed SERT-mediated release potencies in the low tens of nanomolar (reported EC50 range ~18.6 to 32.6 nM). N-ethyltryptamine (NETP, compound 15) was the most potent and selective 5-HT releaser in the series, with an EC50 for SERT-mediated release of 18.6 nM and relatively little activity at DAT or NET. By contrast, 5-methoxy substitutions generally reduced SERT potency: 5-MeO-DMT (compound 7) acted as a weak SERT uptake inhibitor (IC50 reported as 2,184 nM), whereas the 5-hydroxy analog bufotenin (5-OH-DMT, compound 16) was a potent SERT-mediated releaser (EC50 30.5 nM). Psilocin (N,N-dimethyl-4-hydroxytryptamine, compound 24) functioned as a moderate SERT uptake inhibitor (IC50 662 nM). Examples illustrating how small positional changes altered mechanism included the transposition of a hydroxyl from 5- to 4-position converting a substrate into an uptake inhibitor. Several examples illustrate the effect of N-substitution size: DET (compound 19) was a relatively potent SERT uptake inhibitor (IC50 258 nM) whereas its 5-methoxy analog had far lower potency (IC50 2,184 nM). Diisopropyl derivatives (DIPT and 5-MeO-DIPT) showed improved SERT inhibition relative to their monosubstituted counterparts, with reported IC50s of about 288 and 646 nM, respectively. Some N,N-dialkyl tertiary amines, notably DMT and N,N-dimethyl-5-hydroxytryptamine, were unexpectedly SERT-selective releasers (EC50s reported as ~114 nM and 30.5 nM), challenging the prevailing expectation that releasers are limited to primary or secondary amines. At the receptor level, all compounds acted as 5-HT2A agonists in calcium mobilisation assays, with a wide span of potencies and efficacies. Methoxy-substituted analogs tended to display low-nanomolar potency at 5-HT2A and were frequently active in β-arrestin recruitment, whereas many unsubstituted compounds were weaker or inactive in the β-arrestin assay. Notable receptor values included 5-methoxy-N-ethyltryptamine (compound 18) with a 5-HT2A EC50 of 1.9 nM and 112% efficacy, and an unnamed compound (25 in the text) reported as particularly potent (EC50 0.5 nM, 119% efficacy). Only a minority of compounds were 5-HT1A agonists and those that were showed partial efficacies (reported 43–95%). Most compounds that elicited Gq/11-mediated responses also recruited β-arrestin, with exceptions such as N-methyltryptamine (NMT) and DMT, which were inactive in the β-arrestin assay at the top tested concentration (the extracted text reports 10 µM). The authors report no clear correlation between the in vitro transporter or receptor profiles and the heterogeneous psychoactive effects collated from human reports; metabolic factors, pharmacokinetics and interactions with other receptors likely modulate in vivo outcomes.

Blough and colleagues interpret their data as broadly consistent with a serotonergic basis for the pharmacology of these tryptamines: every compound acted as a 5-HT2A agonist, and many also interacted with SERT either as substrates (releasers) or as uptake inhibitors. The investigators emphasise structure–activity patterns in which sterically small compounds tend to be transporter substrates able to induce SERT-mediated 5-HT release, whereas bulkier N-alkyl substitutions reduce substrate translocation and favour uptake inhibition. The finding that certain tertiary amines (for example DMT and an N,N-dimethyl-5-hydroxy analog) behave as SERT substrates was highlighted as surprising and noteworthy because transporter substrates have typically been primary or secondary amines in prior experience. Differences in indole substitution also modulated activity: 5-methoxy substitution commonly reduced SERT releaser potency but increased 5-HT2A potency and β-arrestin recruitment in several cases, while moving a hydroxy group from the 5- to the 4-position could switch a compound from a releaser to an uptake inhibitor. These observations illustrate how small structural changes can alter both transporter mechanism and receptor functional outcomes. The authors note evidence of functional selectivity at 5-HT2A — for example variation between calcium mobilisation (Gq/11) and β-arrestin recruitment — and suggest that such biased signalling could conceivably influence behavioural effects, although further pathway-specific studies would be required to validate this. Several limitations and caveats are acknowledged. The extracted text emphasises that in vitro pharmacology does not necessarily predict human psychoactivity and that other factors — metabolic stability, monoamine oxidase metabolism, pharmacokinetic profiles, and activity at additional targets such as dopamine, trace-amine, cannabinoid, sigma-1 and kappa opioid receptors — could importantly shape in vivo effects. The authors therefore recommend broader pharmacological screening and further studies of downstream signalling pathways and in vivo pharmacology to better understand how transporter and receptor activities map onto subjective and behavioural outcomes. The overall conclusion is that while 5-HT2A agonism is a common feature of psychoactive tryptamines, SERT activity (either release or blockade) and receptor signalling bias are additional dimensions that contribute to the compounds' pharmacological diversity.