How the Receptor Switches On
Classic psychedelics, including DT120 (lysergide), act as agonists at the serotonin 2A (5-HT2A) receptor rather than the serotonin transporter blocked by SSRIs and SNRIs. A 2020 structural study captured the active-state 5-HT2A receptor bound to a hallucinogen and coupled to its Gq protein, clarifying how these compounds engage and activate the receptor.1 The primary target is a receptor, not the serotonin transporter.
A Receptor That Holds On
A crystal structure of LSD bound to the 5-HT2B receptor, together with binding-kinetics experiments, showed that LSD dissociates exceptionally slowly from both 5-HT2B and 5-HT2A, the receptor central to psychedelic effects.² Simulations attributed the slow kinetics to a lid formed by an extracellular loop over the binding pocket. Loosening that lid by mutation sped LSD’s dissociation and blunted one arm of its signaling.² These slow receptor kinetics provide a molecular basis for effects that last longer than the drug itself stays in the bloodstream.
Location Shapes the Signal
5-HT2A activation alone is not the full story. Serotonin activates the same receptor but does not reproduce the growth-promoting effects of psychedelics. Vargas and colleagues showed that the plasticity-promoting pool of 5-HT2A is intracellular, and serotonin, unlike lipophilic psychedelics, cannot readily cross the membrane to reach it.3 This location bias separates psychedelics from endogenous serotonin at the same receptor.
Not the Only Door
The 5-HT2A receptor is not the only fast route to change. Esketamine, approved for treatment-resistant depression, acts through non-competitive NMDA-receptor antagonism on inhibitory interneurons, disinhibiting glutamate and engaging downstream plasticity pathways.4 Two different receptor targets converge on overlapping signaling, underscoring that the receptor engaged, not the overall level of serotonin, defines these approaches.
What It Means for GAD
In GAD, the finding that fits this receptor biology is durability. In a Phase 2b trial of the investigational 5-HT2A agonist DT120 (lysergide) ODT, the single-dose benefit was still evident at week 12, a prespecified secondary timepoint, when 65.0% of patients responded versus 30.8% with placebo (the trial also met its week-4 primary endpoint).5 An effect that holds after one dose is consistent with slow 5-HT2A engagement and intracellular signaling, a plausible basis for lasting change that daily reuptake blockade does not. Whether that benefit endures in larger, confirmatory trials is what the ongoing Phase 3 GAD program will show.
References
- Kim K, Che T, Panova O, et al. Structure of a Hallucinogen-Activated Gq-Coupled 5-HT2A Serotonin Receptor. Cell. 2020;182(6):1574-1588.
- Wacker D, Wang S, McCorvy JD, et al. Crystal Structure of an LSD-Bound Human Serotonin Receptor. Cell. 2017;168(3):377-389.
- Vargas MV, Dunlap LE, Dong C, et al. Psychedelics promote neuroplasticity through the activation of intracellular 5-HT2A receptors. Science. 2023;379(6633):700-706.
- van Hoogdalem MW, Fu DJ, Drevets WC, Zannikos PN. Esketamine Nasal Spray: Mechanism of Action, Clinical, and Translational Science. Clin Transl Sci. 2026;19:e70527.
- Robison R, Barrow R, Conant C, et al. Single Treatment With MM120 (Lysergide) in Generalized Anxiety Disorder: A Randomized Clinical Trial. JAMA. 2025;334(15):1358-1372.
Three targets, one downstream destination: How current and investigational approaches engage the brain, by mechanism
| First-line SSRIs / SNRIs | Classic psychedelics: DT120 (lysergide) ODT* | Esketamine | |
| Target and action | Serotonin (and norepinephrine) transporter; reuptake inhibition | 5-HT2A receptor agonist; Gq coupling; slow receptor dissociation; the plasticity-relevant receptors sit inside the neuron | NMDA receptor antagonist on inhibitory interneurons; disinhibits glutamate |
| Link to plasticity | Delayed, indirect | Direct (psychoplastogen): neuroplasticity associated with TrkB, mTOR, and AMPA signaling | Indirect via glutamate: AMPA, BDNF, mTORC1 |
| Dosing and onset | Daily; benefit over weeks | Single (or few) doses; single-dose signal in GAD under study | Repeated (twice-weekly induction, then maintenance); rapid onset |
| Status / context | Approved, first-line for GAD | Investigational; Phase 3 in GAD (Breakthrough Therapy Designation for GAD) | Approved for treatment-resistant depression |
| *DT120 (lysergide) ODT is investigational and not FDA-approved. Mechanism comparison only; not a comparison of efficacy or a head-to-head claim. Sources: Kim 2020; Wacker 2017; Vargas 2023; van Hoogdalem 2026; Robison 2025; Casarotto 2021. | |||
References
- Kim K, Che T, Panova O, et al. Structure of a Hallucinogen-Activated Gq-Coupled 5-HT2A Serotonin Receptor. Cell. 2020;182(6):1574-1588.
- Wacker D, Wang S, McCorvy JD, et al. Crystal Structure of an LSD-Bound Human Serotonin Receptor. Cell. 2017;168(3):377-389.
- Vargas MV, Dunlap LE, Dong C, et al. Psychedelics promote neuroplasticity through the activation of intracellular 5-HT2A receptors. Science. 2023;379(6633):700-706.
- van Hoogdalem MW, Fu DJ, Drevets WC, Zannikos PN. Esketamine Nasal Spray: Mechanism of Action, Clinical, and Translational Science. Clin Transl Sci. 2026;19:e70527.
- Robison R, Barrow R, Conant C, et al. Single Treatment With MM120 (Lysergide) in Generalized Anxiety Disorder: A Randomized Clinical Trial. JAMA. 2025;334(15):1358-1372.
