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

5-methoxy-,-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 -alkyl, -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) and 1A receptors (5-HT), and 3) to examine the influence of 5-HT on 5-HT-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 and 5-HT. In C57Bl/6J mice, subcutaneous injection of the analogs induced HTRs with varying potencies (ED range = 0.2-1.8 mg/kg) and maximal effects ( range = 20-60 HTRs/30 min), while inducing hypothermia and hypolocomotion at higher doses (ED range = 3.2-20.6 mg/kg). 5-HT antagonist pretreatment blocked drug-induced HTRs, whereas 5-HT antagonist pretreatment enhanced HTRs. In general, ,-dialkyl and -isopropyl derivatives displayed HTR activity, while the -methyl, -ethyl, and 2-methyl analogs did not. Importantly, blockade of 5-HT unmasked latent HTR activity for the -ethyl analog and markedly increased maximal responses for other HTR-active compounds (40-90 HTRs/30 min), supporting the notion that 5-HT agonist activity can dampen 5-HT-mediated HTRs. Suppression of 5-HT-mediated HTRs by 5-HT 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 agonism in modulating acute psychoactive effects of 5-HT agonists.