Serotonin 1A\nReceptors Modulate Serotonin 2A Receptor-Mediated\nBehavioral Effects of 5‑Methoxy‑<i>N</i>,<i>N</i>‑dimethyltryptamine Analogs in Mice

5-methoxy-<i>N</i>,<i>N</i>-dimethyltrytpamine\n(5-MeO-DMT) analogs are used as recreational drugs, but they are also\nbeing developed as potential medicines, warranting further investigation\ninto their pharmacology. Here, we investigated the neuropharmacology\nof 5-MeO-DMT and several of its <i>N</i>-alkyl, <i>N</i>-allyl, and 2-methyl analogs, with three major aims: 1)\nto determine in vitro receptor profiles for the compounds, 2) to characterize\nin vitro functional activities at serotonin (5-HT) 2A receptors (5-HT<sub>2A</sub>) and 1A receptors (5-HT<sub>1A</sub>), and 3) to examine\nthe influence of 5-HT<sub>1A</sub> on 5-HT<sub>2A</sub>-mediated psychedelic-like\neffects in the mouse head twitch response (HTR) model. In vitro receptor\nbinding and functional assays showed that all 5-MeO-DMT analogs bind\nwith high affinity and activate multiple targets (e.g., 5-HT receptor\nsubtypes, alpha adrenergic receptors), including potent effects at\n5-HT<sub>2A</sub> and 5-HT<sub>1A</sub>. In C57Bl/6J mice, subcutaneous\ninjection of the analogs induced HTRs with varying potencies (ED<sub>50</sub> range = 0.2–1.8 mg/kg) and maximal effects (<i>E</i><sub>max</sub> range = 20–60 HTRs/30 min), while\ninducing hypothermia and hypolocomotion at higher doses (ED<sub>50</sub> range = 3.2–20.6 mg/kg). 5-HT<sub>2A</sub> antagonist pretreatment\nblocked drug-induced HTRs, whereas 5-HT<sub>1A</sub> antagonist pretreatment\nenhanced HTRs. In general, <i>N</i>,<i>N</i>-dialkyl\nand <i>N</i>-isopropyl derivatives displayed HTR activity,\nwhile the <i>N</i>-methyl, <i>N</i>-ethyl, and\n2-methyl analogs did not. Importantly, blockade of 5-HT<sub>1A</sub> unmasked latent HTR activity for the <i>N</i>-ethyl analog\nand markedly increased maximal responses for other HTR-active compounds\n(40–90 HTRs/30 min), supporting the notion that 5-HT<sub>1A</sub> agonist activity can dampen 5-HT<sub>2A</sub>-mediated HTRs. Suppression\nof 5-HT<sub>2A</sub>-mediated HTRs by 5-HT<sub>1A</sub> only occurred\nafter high 5-MeO-DMT doses, suggesting involvement of other receptors\nin modulating psychedelic-like effects. Overall, our findings provide\nkey information about the receptor target profiles for 5-MeO-DMT analogs,\nthe structure–activity relationships for inducing psychedelic-like\neffects, and the critical role of 5-HT<sub>1A</sub> agonism in modulating\nacute psychoactive effects of 5-HT<sub>2A</sub> agonists.