Serotonin 1A Receptors Modulate Serotonin 2A Receptor-Mediated Behavioral Effects of 5-Methoxy-N,N-dimethyltryptamine...

5-methoxy-N,N-dimethyltrytpamine (5-MeO-DMT) analogs are used as recreational drugs, but they are also being developed as potential medicines, warranting further investigation into their pharmacology. Here, we investigated the neuropharmacology of 5-MeO-DMT and several of its N-alkyl, N-allyl, and 2-methyl analogs, with three major aims: 1) to determine in vitro receptor profiles for the compounds, 2) to characterize in vitro functional activities at serotonin (5-HT) 2A receptors (5-HT_2A) and 1A receptors (5-HT_1A), and 3) to examine the influence of 5-HT_1A on 5-HT_2A-mediated psychedelic-like effects in the mouse head twitch response (HTR) model. In vitro receptor binding and functional assays showed that all 5-MeO-DMT analogs bind with high affinity and activate multiple targets (e.g., 5-HT receptor subtypes, alpha adrenergic receptors), including potent effects at 5-HT_2A and 5-HT_1A. In C57Bl/6J mice, subcutaneous injection of the analogs induced HTRs with varying potencies (ED₅₀ range = 0.2-1.8 mg/kg) and maximal effects (E_max range = 20-60 HTRs/30 min), while inducing hypothermia and hypolocomotion at higher doses (ED₅₀ range = 3.2-20.6 mg/kg). 5-HT_2A antagonist pretreatment blocked drug-induced HTRs, whereas 5-HT_1A antagonist pretreatment enhanced HTRs. In general, N,N-dialkyl and N-isopropyl derivatives displayed HTR activity, while the N-methyl, N-ethyl, and 2-methyl analogs did not. Importantly, blockade of 5-HT_1A unmasked latent HTR activity for the N-ethyl analog and markedly increased maximal responses for other HTR-active compounds (40-90 HTRs/30 min), supporting the notion that 5-HT_1A agonist activity can dampen 5-HT_2A-mediated HTRs. Suppression of 5-HT_2A-mediated HTRs by 5-HT_1A only occurred after high 5-MeO-DMT doses, suggesting involvement of other receptors in modulating psychedelic-like effects. Overall, our findings provide key information about the receptor target profiles for 5-MeO-DMT analogs, the structure-activity relationships for inducing psychedelic-like effects, and the critical role of 5-HT_1A agonism in modulating acute psychoactive effects of 5-HT_2A agonists.