Glutamatergic and Rapid-Acting Agents in Psychiatry

A high-level examination of ketamine, esketamine, the oral NMDA approaches, the neurosteroids, and the psychedelics — the drugs that broke the delayed-onset paradigm

Why This Class Exists: The Problem It Solves

The serotonergic antidepressants share a defining limitation: they take weeks to work, fail to produce durable remission in most patients on the first try, and do nothing for the patient in acute suicidal crisis who needs relief tonight, not in a month. For sixty years, the monoamine reuptake-inhibitor paradigm — serotonin and norepinephrine — defined what an antidepressant was, and that paradigm could not solve the speed problem, because (as the plasticity reframing argues) it works by slowly grinding out neuroplastic change over weeks.

The agents in this document matter because they break that constraint. Ketamine produces antidepressant effects within hours, including in patients who have failed multiple conventional drugs, and reduces suicidal ideation acutely. This single empirical fact — rapid, robust antidepressant action through a completely non-monoaminergic mechanism — did more to reshape the theory of how antidepressants work than any other discovery of the past three decades. It is the strongest evidence for the plasticity hypothesis, because ketamine appears to reach the same downstream endpoint as the SSRIs (BDNF release, synaptogenesis, network remodeling) but through a different molecular door and far faster.

This overview covers the genuinely glutamatergic agents at the core of the rapid-acting revolution (ketamine, esketamine, the oral NMDA-modulator approaches, and the developmental pipeline), plus two adjacent families united with them by the rapid-acting, plasticity-inducing theme but reaching it by other mechanisms: the neurosteroid GABAergic agents (brexanolone, zuranolone) and the serotonergic psychedelics (psilocybin and its relatives). The psychedelics are not glutamatergic — they are 5-HT2A agonists — but they belong here because they share the defining features that distinguish this whole paradigm from the SSRIs: rapid, dramatic induction of neural plasticity rather than slow accumulation of it.

The honest framing throughout: these are the most exciting developments in psychiatric pharmacology in a generation, and also the ones most freighted with hype, commercial opportunism, durability problems, abuse and safety concerns, and unknown long-term consequences. The revolution is real; so is the reason for caution.

The Glutamate System: Necessary Scaffolding

Where serotonin is a diffuse modulator released by a few thousand brainstem neurons, glutamate is the brain's principal excitatory neurotransmitter — the workhorse of fast synaptic transmission, used by the majority of central synapses, the molecular substrate of learning and memory itself. This difference in scale and function is the key to understanding why glutamatergic drugs behave so differently from monoaminergic ones: they act not on a tuning system but on the brain's core signaling and plasticity machinery directly.

The receptors. Glutamate acts on two broad classes:

• Ionotropic (ligand-gated ion channels), which mediate fast transmission: NMDA receptors (the ketamine target — calcium-permeable, voltage-dependent, requiring both glutamate and glycine/D-serine co-agonist binding, and central to synaptic plasticity and long-term potentiation), AMPA receptors (the primary mediators of fast excitatory transmission, and — critically — the receptors whose activation appears to trigger the rapid antidepressant cascade), and kainate receptors.
• Metabotropic (mGluR1–8), G-protein-coupled receptors that modulate transmission on slower timescales and represent a large, mostly still-developmental drug-target space.

The plasticity engine. NMDA-receptor-dependent long-term potentiation and depression — the strengthening and weakening of synapses based on activity — is the best-understood cellular model of learning. Because NMDA receptors sit at the heart of synaptic plasticity, drugs that modulate them are, almost by definition, drugs that modulate the brain's capacity to change itself. This is why the glutamatergic antidepressants are most coherently understood as plasticity drugs, and why their mechanism connects so directly to the network/critical-period reframing of antidepressant action.

The excitation–inhibition balance. Glutamatergic excitation is held in check by GABAergic inhibition, and much of the brain's function depends on the balance between them. As the next section shows, ketamine's antidepressant mechanism is, paradoxically, thought to begin with disinhibition — blocking NMDA receptors preferentially on inhibitory interneurons, releasing the brake on glutamate, and producing a transient glutamate surge. This is also why the GABAergic neurosteroids can be grouped with the glutamatergic agents under the rapid-acting banner: both ultimately act on the excitation–inhibition balance and the plasticity it governs, from opposite sides.

Ketamine and Esketamine: The Core of the Revolution

What they are

Ketamine is a non-competitive NMDA-receptor antagonist, in clinical use as a dissociative anesthetic since the 1960s and on the WHO essential medicines list for that purpose. Racemic ketamine is a mixture of two enantiomers; esketamine (the S-enantiomer) is the more potent NMDA blocker and the version developed as an intranasal antidepressant (Spravato), FDA-approved in 2019 for treatment-resistant depression and in 2020 for depressive symptoms with acute suicidal ideation or behavior. Racemic ketamine, given intravenously, is used widely off-label for the same indications and arguably has a comparable or stronger evidence base than esketamine — an irony driven by the economics of which formulation could be patented and run through registration trials.

What they do clinically

The clinically transformative features:

• Speed. Antidepressant effects emerge within hours of a single dose — a fundamental break from the weeks-long latency of every monoaminergic agent.
• Efficacy in treatment resistance. Robust response in patients who have failed multiple conventional antidepressants, the population with the fewest options.
• Anti-suicidal action. Rapid reduction in suicidal ideation, partly independent of the general antidepressant effect — uniquely valuable in acute crisis, where no oral antidepressant acts fast enough to matter.

The defining limitation is equally important: the effect is transient. A single dose's benefit typically fades over days to a couple of weeks, which is why protocols use repeated dosing (commonly an induction series of several treatments over weeks, then maintenance) — and why ketamine is better understood as a recurring intervention than a cure, with all the cost, access, and dependence implications that follow.

Mechanism: the disinhibition–AMPA–BDNF–mTOR cascade

This is where ketamine reorganized antidepressant theory, and where it connects to the plasticity argument running through this series. The leading mechanistic account is not that NMDA blockade is itself the therapeutic event. Rather:

1. Ketamine preferentially blocks NMDA receptors on GABAergic inhibitory interneurons (which are tonically active and thus more exposed to the open-channel blocker), removing their inhibitory brake on glutamate neurons — disinhibition.
2. This produces a transient glutamate surge in the prefrontal cortex.
3. The surge activates AMPA receptors (not NMDA — AMPA activation appears necessary for the antidepressant effect, since blocking AMPA abolishes it).
4. AMPA activation triggers BDNF release and activation of the mTOR signaling pathway.
5. These drive rapid synaptogenesis — the formation of new dendritic spines and synapses in the prefrontal cortex within hours, reversing the synaptic loss associated with chronic stress and depression.

The endpoint — BDNF, synaptic remodeling, restored prefrontal connectivity — is the same endpoint the SSRIs reach slowly. Ketamine reaches it in hours. This is the single strongest piece of evidence that antidepressant action is fundamentally about plasticity rather than monoamine levels, and it reframes the whole field: the question is not "which neurotransmitter is low" but "how do we induce the plastic, synaptogenic state that lets a depressed brain reorganize."

The metabolite controversy

A significant and unresolved complication: work by Zanos, Gould, and colleagues argued that a ketamine metabolite, (2R,6R)-hydroxynorketamine (HNK), produces antidepressant-like effects in animals independent of NMDA-receptor blockade (apparently via AMPA-receptor mechanisms), implying that the therapeutic action might be separable from the NMDA antagonism — and therefore potentially from the dissociation and abuse liability that NMDA blockade carries. If true, this would point toward non-dissociative, non-addictive rapid-acting antidepressants. The finding has been partially challenged and not cleanly replicated in all hands, and HNK-based drug development is ongoing. The honest status: a genuinely important lead that complicates the simple "NMDA antagonism = antidepressant" story and remains unsettled.

The dissociation question

A practically and theoretically charged issue: ketamine's acute dissociative and psychotomimetic effects (depersonalization, perceptual changes, the "k-hole" at higher doses) were long assumed to be side effects separable from benefit — but some analyses find dissociation correlates with antidepressant response, raising the uncomfortable possibility (paralleling the emotional-blunting question with SSRIs) that the subjective drug experience is not fully separable from the therapeutic mechanism. This matters enormously for the field: if the experience is part of the medicine, then non-dissociative rapid-acting agents may not work as well, and the line between ketamine and the psychedelics blurs. The evidence is mixed and the question is open.

The Oral NMDA-Modulator Approaches

The logistical burden of ketamine — parenteral or intranasal administration, in-clinic monitoring, the dissociation — drove efforts to capture rapid glutamatergic antidepressant action in a take-home oral pill.

Dextromethorphan–bupropion (Auvelity), FDA-approved in 2022 for major depression, is the leading example. Dextromethorphan is an NMDA-receptor antagonist and sigma-1 receptor agonist; on its own it is metabolized too fast by CYP2D6 to maintain useful levels, so it is combined with bupropion, whose primary job in the formulation is to inhibit that metabolism and raise dextromethorphan exposure (bupropion's own noradrenergic/dopaminergic antidepressant action is a secondary contributor). The pitch is a meaningfully faster oral antidepressant — separation from placebo within a week or two rather than the SSRI's month — without ketamine's administration logistics or (largely) its dissociation. Whether it delivers a clinically important speed and efficacy advantage over existing oral agents in routine practice, versus representing an incremental and heavily marketed improvement, is still being established; the registration data were positive but the comparator and durability questions are real.

Esmethadone (REL-1017, dextromethadone) is an NMDA-channel blocker (the d-enantiomer of methadone, with low opioid activity) that advanced into late-stage depression trials as a potential oral rapid-acting agent. Its trial results have been mixed — early promise followed by failed or ambiguous Phase 3 readouts — and it stands as a reminder that the glutamatergic pipeline has produced as many disappointments as successes.

The broader lesson of the oral-NMDA effort: the glutamatergic mechanism is harder to bottle in convenient form than the early ketamine enthusiasm implied, and several rational-looking candidates have underperformed.

The Developmental Graveyard (and Why It Matters)

A balanced account must include the glutamatergic agents that failed, because they temper the revolution's narrative:

• Rapastinel (GLYX-13), an NMDA-receptor glycine-site partial agonist, generated substantial excitement as a potentially non-dissociative rapid-acting antidepressant — and then failed its Phase 3 depression trials outright, a major and instructive disappointment.
• Riluzole, a glutamate-modulating agent (used in ALS), was tried in depression and anxiety on the glutamatergic rationale with unimpressive controlled results.
• Various mGluR-targeting compounds and other glycine-site and NMDA-subunit-selective approaches have moved through and largely out of development.

The pattern matters: the glutamatergic mechanism is real and ketamine works, but "target glutamate" has not been a reliable formula, several elegant mechanistic bets have failed in humans, and the field should be read with the same skepticism toward over-promising that the serotonergic document applied to its class. Ketamine's success has not yet generalized cleanly into a robust new drug class; it remains, to a substantial degree, sui generis.

The Neurosteroid Parallel: Brexanolone and Zuranolone

These are not glutamatergic — they are positive allosteric modulators of GABA-A receptors, acting on the inhibitory side of the excitation–inhibition balance — but they belong in this document because they share the rapid-acting, plasticity-relevant identity that distinguishes this paradigm from the SSRIs, and because they target a specific, mechanistically coherent population.

The neurosteroid allopregnanolone is a metabolite of progesterone and a powerful endogenous GABA-A modulator; its dramatic fall after childbirth is implicated in postpartum depression. Brexanolone (an IV allopregnanolone formulation, approved 2019) was the first drug specifically approved for postpartum depression, producing rapid improvement — but requiring a 60-hour inpatient infusion, severely limiting its use. Zuranolone (an oral neurosteroid, approved 2023 for postpartum depression) captured the rapid neurosteroid mechanism in a two-week oral course with rapid onset, a far more practical form. (Zuranolone's broader major-depression program was less successful, and it is approved specifically for the postpartum indication.)

The conceptual significance: like ketamine, the neurosteroids demonstrate rapid antidepressant action through a non-monoaminergic mechanism — here by restoring GABAergic tone in a state (the postpartum period) of acute neurosteroid withdrawal. They illustrate that the rapid-acting paradigm is not exclusively glutamatergic, that distinct mechanisms can produce the speed the SSRIs lack, and that mechanism-matched-to-pathophysiology (replacing a specific deficient neurosteroid) can work where the diffuse monoamine approach is slow. They are the cleanest current example of a targeted rapid antidepressant.

The Serotonergic Psychedelics: The Plasticity Paradigm's Other Wing

Psilocybin, LSD, DMT, and related classic psychedelics are 5-HT2A receptor agonists — mechanistically serotonergic, not glutamatergic. They are included here because they share this paradigm's defining signature: rapid, dramatic induction of neural plasticity, and a single (or few) administrations producing effects that long outlast the drug's presence — the opposite of the SSRI's chronic-dosing, slow-accumulation model.

Mechanism and the plasticity convergence

5-HT2A agonism on cortical pyramidal neurons produces, downstream, increased glutamatergic transmission, BDNF release, and — strikingly — rapid structural plasticity: psychedelics promote dendritic spine growth and synaptogenesis in prefrontal cortex (the "psychoplastogen" concept of David Olson and others). So the psychedelics and ketamine, though they enter through different receptors (5-HT2A versus NMDA), converge on the same plasticity endpoint — glutamate, BDNF, synaptogenesis — that the SSRIs reach slowly. This convergence is the strongest theoretical argument that plasticity induction, not any particular neurotransmitter, is the common currency of antidepressant action.

A vivid and influential demonstration of the "reopened critical period" idea comes from Gül Dölen's work: psychedelics appear to reopen developmental critical periods for social and other forms of learning — returning the adult brain to a juvenile-like state of heightened, experience-dependent plasticity. This dovetails precisely with Castrén's account of SSRIs reopening plasticity, and it reframes the entire class: these drugs may work not by directly improving mood but by reopening windows in which entrenched patterns become modifiable by experience — which is exactly why psychedelic treatments are delivered embedded in psychotherapy, and why "set and setting" (the psychological and environmental context) is treated as integral rather than incidental. The drug opens the window; the therapeutic context determines what comes through it.

Clinical status

Psilocybin-assisted therapy has shown promising results in treatment-resistant and major depression, end-of-life and cancer-related distress, and (with related compounds) addictions, in mostly small but increasingly rigorous trials; it holds FDA Breakthrough Therapy designation, and Phase 3 programs are ongoing. As of this writing it is not an approved general treatment. The MDMA-assisted-therapy program for PTSD (MDMA is an entactogen with mixed serotonergic-releasing mechanism, not a classic psychedelic) reached FDA review and was rejected in 2024, with the agency requesting an additional trial amid concerns about trial design, functional unblinding, and therapist-conduct issues — a cautionary episode for the whole field about the gap between enthusiasm and regulatory-grade evidence, and about the unusual difficulty of running rigorous trials of drugs whose subjective effects make blinding nearly impossible.

Problems With Their Use

The rapid-acting agents carry a distinct burden profile, and a balanced account must take it seriously against the genuine excitement.

Ketamine/esketamine

• Transient effect and the maintenance trap. Benefits fade over days to weeks, requiring ongoing repeated dosing — turning an acute intervention into a potentially indefinite, expensive, monitored treatment course with uncertain long-term consequences.
• Abuse and dependence liability. Ketamine is a recreational drug of abuse with real dependence potential; the proliferation of loosely regulated ketamine clinics and at-home telehealth ketamine programs has raised legitimate concern about a class with genuine addictive potential being dispensed with variable oversight.
• Acute dissociation and psychotomimetic effects, hemodynamic effects (transient blood pressure and heart rate increases), and acute monitoring requirements (the basis for esketamine's restrictive REMS program, requiring in-clinic administration and observation).
• Bladder and urinary tract toxicity (ketamine-induced cystitis) with chronic or heavy use — a serious, sometimes irreversible consequence documented in chronic recreational users and a real concern for long-term therapeutic dosing.
• Cognitive effects with repeated long-term use, incompletely characterized.
• Unknown long-term safety. The therapeutic use of repeated ketamine over years is too new for its long-term consequences to be known — a genuine evidence gap, not a reassurance.
• Cost, access, and equity. Clinic-based administration, REMS requirements, and inconsistent insurance coverage make these expensive and unevenly accessible, even as they are marketed aggressively.

The neurosteroids

Brexanolone's 60-hour inpatient infusion and cost; zuranolone's sedation and CNS effects, the narrow (postpartum) indication after the broader depression program underdelivered, and the usual newness/cost considerations.

The psychedelics

• Psychological risk: acute anxiety/panic ("bad trips"), and rare but serious adverse psychological reactions, particularly in vulnerable individuals; contraindication in psychosis and bipolar disorder (risk of precipitating mania or psychosis) is a hard line.
• The blinding problem: the unmistakable subjective effects make placebo-controlled trials genuinely difficult to interpret (functional unblinding inflates apparent effects), a methodological challenge the MDMA rejection foregrounded.
• The therapy-embedded model makes the "drug effect" inseparable from the substantial psychotherapeutic scaffolding around it — which is theoretically coherent (plasticity needs input) but complicates both regulation and any claim about the drug per se.
• Therapist-conduct and boundary risks in a model that induces extreme suggestibility and vulnerability — a concern made concrete by documented cases in the trial literature.
• Regulatory and legal status remains unsettled, and the gap between public enthusiasm, commercial positioning, and regulatory-grade evidence is wide.

The cross-cutting concern

As with every class in this series, the deployment problem looms: rapid-acting agents are arriving into a system primed to over-reach, with commercial incentives (ketamine clinics, psychedelic startups) running well ahead of the durability data, the long-term safety data, and the careful patient-selection these powerful tools require. The science is genuinely revolutionary; the marketing and the clinic economics are, in places, genuinely concerning.

A Theoretical Synthesis

The unifying lesson of this entire class is the plasticity hypothesis, stated more strongly than the serotonergic document could state it alone. Ketamine (via NMDA antagonism → disinhibition → glutamate → AMPA → BDNF → mTOR → synaptogenesis), the psychedelics (via 5-HT2A → glutamate → BDNF → synaptogenesis), and even the SSRIs (slowly, via the same downstream machinery) all converge on rapid or gradual induction of synaptic plasticity in prefrontal-limbic circuits. The common currency of antidepressant action is not a neurotransmitter level but the induction of a plastic, synaptogenic, reorganization-permitting brain state. The neurosteroids reach a related endpoint from the GABAergic side. The drugs differ enormously in speed (hours versus weeks), in route, and in the intensity of the subjective experience — but the convergence on plasticity is the deepest pattern in the data.

This reframing carries a profound clinical implication that runs through the whole series: if these drugs work by opening windows of plasticity, then what the patient encounters during the open window — psychotherapy, behavioral change, a supportive context, "set and setting" — is not adjunctive but central. The psychedelic field has internalized this most explicitly (drug plus therapy as a single intervention), ketamine practice is moving toward it, and it retroactively explains why combination treatment outperforms medication alone across the entire field. The next era of psychiatric pharmacology may be defined less by which receptor a drug hits than by how rapidly and how controllably it opens the brain to change — and by how well the surrounding care fills that opening.

It does not resolve the field's hard questions: whether the subjective experience (dissociation, the trip) is separable from the therapeutic effect; whether durable benefit is achievable without indefinite re-dosing; what the long-term consequences of repeated plasticity-induction are; and whether any of these can be deployed safely at scale in a commercial system poorly equipped to provide the psychosocial scaffolding the mechanism seems to require. The honest state of the field: a genuine paradigm shift in our understanding of how antidepressants work, a small number of genuinely transformative tools (ketamine above all), a pipeline that has failed as often as it has succeeded, and a deployment environment racing ahead of the evidence.

The Clinical Bottom Line

Ketamine/esketamine is a genuine advance for treatment-resistant depression and acute suicidality — situations where speed and efficacy in the previously untreatable justify the burdens — used as a monitored, time-limited or carefully-maintained intervention, with honest attention to its transient effect, dependence potential, bladder and cognitive risks, and unknown long-term safety. It is not a casual or first-line treatment, and the clinic-proliferation environment warrants real caution.

The neurosteroids (zuranolone, brexanolone) are a meaningful, mechanism-matched advance for postpartum depression, where rapid action matters acutely and the targeted mechanism fits the pathophysiology.

The oral NMDA agents (Auvelity) offer a possibly-faster oral option whose advantage over conventional antidepressants in routine practice is still being established.

The psychedelics are promising and not yet approved; they should be regarded as investigational, delivered only within rigorous therapy-embedded protocols, contraindicated in psychosis and bipolar disorder, and viewed with sober awareness of the trial-methodology and conduct concerns the MDMA rejection exposed.

Across all of them: these are plasticity drugs, they work best embedded in a psychosocial context that gives the opened window something to act on, and they are arriving into a system inclined to over-reach. Used with the right patients, for the right conditions, with the right scaffolding and honest consent, they represent the most important expansion of psychiatric pharmacology in decades. Used as the hype invites — fast, alone, commercially, without the surrounding care or the long-term data — they risk repeating, in a more powerful register, the deployment failures of the serotonergic era.

Selected References and Further Reading

1. Berman, R.M., et al. (2000). Antidepressant effects of ketamine in depressed patients. Biological Psychiatry, 47(4), 351–354.
2. Zarate, C.A., et al. (2006). A randomized trial of an N-methyl-D-aspartate antagonist in treatment-resistant major depression. Archives of General Psychiatry, 63(8), 856–864.
3. Duman, R.S., & Aghajanian, G.K. (2012). Synaptic dysfunction in depression: Potential therapeutic targets. Science, 338(6103), 68–72.
4. Li, N., et al. (2010). mTOR-dependent synapse formation underlies the rapid antidepressant effects of NMDA antagonists. Science, 329(5994), 959–964.
5. Zanos, P., et al. (2016). NMDAR inhibition-independent antidepressant actions of ketamine metabolites. Nature, 533(7604), 481–486.
6. Daly, E.J., et al. (2018/2019). Efficacy of esketamine nasal spray plus oral antidepressant in treatment-resistant depression (TRANSFORM/SUSTAIN trials). JAMA Psychiatry / American Journal of Psychiatry.
7. Wilkinson, S.T., et al. (2018). The effect of a single dose of intravenous ketamine on suicidal ideation: A systematic review and individual participant data meta-analysis. American Journal of Psychiatry, 175(2), 150–158.
8. Iosifescu, D.V., et al. (2022). Efficacy and safety of AXS-05 (dextromethorphan-bupropion) in patients with major depressive disorder (GEMINI trial). Journal of Clinical Psychiatry, 83(4).
9. Meltzer-Brody, S., et al. (2018). Brexanolone injection in post-partum depression: Two multicentre, double-blind, randomised, placebo-controlled trials. The Lancet, 392(10152), 1058–1070.
10. Deligiannidis, K.M., et al. (2021). Effect of zuranolone vs placebo in postpartum depression. JAMA Psychiatry, 78(9), 951–959.
11. Carhart-Harris, R.L., et al. (2021). Trial of psilocybin versus escitalopram for depression. New England Journal of Medicine, 384(15), 1402–1411.
12. Goodwin, G.M., et al. (2022). Single-dose psilocybin for a treatment-resistant episode of major depression. New England Journal of Medicine, 387(18), 1637–1648.
13. Olson, D.E. (2018). Psychoplastogens: A promising class of plasticity-promoting neurotherapeutics. Journal of Experimental Neuroscience, 12.
14. Nardou, R., Dölen, G., et al. (2023). Psychedelics reopen the social reward learning critical period. Nature, 618(7966), 790–798.
15. Mitchell, J.M., et al. (2021/2023). MDMA-assisted therapy for severe PTSD (MAPP1/MAPP2 trials). Nature Medicine. [FDA complete response letter, 2024.]
16. Krystal, J.H., Abdallah, C.G., Sanacora, G., Charney, D.S., & Duman, R.S. (2019). Ketamine: A paradigm shift for depression research and treatment. Neuron, 101(5), 774–778.
17. Sanacora, G., et al. (2017). A consensus statement on the use of ketamine in the treatment of mood disorders. JAMA Psychiatry, 74(4), 399–405.
18. Short, B., et al. (2018). Side effects associated with ketamine use in depression: A systematic review. Lancet Psychiatry, 5(1), 65–78.
19. Castrén, E., & Monteggia, L.M. (2021). Brain-derived neurotrophic factor signaling in depression and antidepressant action. Biological Psychiatry, 90(2), 128–136.
20. Aleksandrova, L.R., & Phillips, A.G. (2021). Neuroplasticity as a convergent mechanism of ketamine and classical psychedelics. Trends in Pharmacological Sciences, 42(11), 929–942.