Ketamine Mechanism of Action for Treatment-Resistant Depression: The BDNF/mTOR Story

Medically reviewed by Dr. Ben Soffer, DO. Dr. Soffer is a board-certified physician licensed in Florida and New Jersey. He personally evaluates and treats every patient in the Discreet Ketamine program, from the first eligibility review through dose selection, monitoring, and follow-up. Last reviewed January 2026.

What you need to know first

Treatment-resistant depression (TRD) is the clinical term for major depressive disorder that has not responded adequately to at least two different antidepressants, each taken at a therapeutic dose for a full six to eight week trial. Roughly 30 to 40 percent of people with major depression fall into this group. It is common, it is not a personal failing, and it is the population in which ketamine has the strongest evidence base.

Ketamine is usually not a first step. Standard practice in the United States is to try two or more adequate antidepressant trials before considering it, because SSRIs have a longer track record, lower cost, and a favorable first-line safety profile. Ketamine makes the most clinical sense once those options have been exhausted, or when depression is severe enough (for example, active suicidal ideation) that waiting weeks for an SSRI to work is not safe.

Who qualifies, in practice: adults with a diagnosis of depression (and often co-occurring anxiety, PTSD, or chronic pain) who have tried and not adequately responded to standard antidepressants, who do not have uncontrolled blood pressure or active psychosis, and who can be monitored during treatment. A brief medical screen sorts most of this out.

What it costs: prices vary widely by format. In-clinic intravenous (IV) ketamine infusions are commonly reported in the range of about $400 to $800 per session, with an induction course of roughly six sessions. Spravato (esketamine), the FDA-approved intranasal form, is given in a certified clinic under monitoring and is often partially covered by insurance, but still carries meaningful facility and monitoring costs. At-home compounded oral or sublingual ketamine programs are generally the lowest-cost option because they remove the clinic-visit overhead.

That is the practical frame. The rest of this article is the mechanism: what ketamine is actually doing at the synaptic level that lets it work when serotonin drugs have not.

This is the mechanism article. It is longer and more technical than our plain-language explainer. The focus is narrow: what ketamine does at the synaptic level that allows it to produce antidepressant response in patients for whom serotonin-modulating drugs have not. If you have already tried two or more antidepressants, you have earned the right to be skeptical of anything else marketed as a breakthrough, so here is the actual biology.

Why SSRIs don't always work

Depression isn't one disease. The term groups together a set of clinical pictures that share symptoms (persistent low mood, anhedonia, sleep and appetite changes, cognitive slowing) but that can arise from very different underlying neurobiology. Current psychiatric practice starts with SSRIs or SNRIs because they work for a substantial fraction of patients, they're well-tolerated at a population level, and we don't yet have good biomarkers to predict which patient will respond to which mechanism.

Roughly 30-40% of patients with major depressive disorder don't respond adequately to first-line antidepressants. That's treatment-resistant depression. The current thinking, based on a growing body of neuroimaging and molecular work, is that TRD involves more than just serotonin signaling: glutamatergic dysregulation, HPA-axis dysfunction, and deficits in synaptic plasticity all appear to play larger roles than they do in SSRI-responsive depression.

This is why a drug that acts on a completely different pathway (glutamate, synaptogenesis, BDNF) can produce response in patients who've failed multiple serotonergic agents. The tool is matching a different part of the problem.

The molecular cascade, in order

Ketamine's antidepressant mechanism involves several linked events that happen on different timescales. Here they are in order.

1. NMDA receptor blockade on GABAergic interneurons (minutes)

Ketamine binds to NMDA (N-methyl-D-aspartate) glutamate receptors, preferentially blocking them on inhibitory GABAergic interneurons in the prefrontal cortex. These interneurons normally dampen the activity of nearby pyramidal neurons. Blocking their NMDA receptors disinhibits the pyramidal neurons; in plain terms, the brakes come off.

This is the "disinhibition hypothesis," developed primarily by Ronald Duman's lab at Yale and supported by subsequent work from multiple groups (reviewed in Zanos and Gould, 2018). The key insight is that ketamine's therapeutic effect doesn't come from blocking glutamate signaling; it comes from unblocking it, by removing the inhibition that was suppressing it.

2. Glutamate surge in prefrontal cortex (30-60 minutes)

Once the inhibitory brake is released, pyramidal neurons fire more actively, releasing glutamate into synapses across prefrontal cortex. This glutamate surge is measurable on MR spectroscopy and correlates with the onset of antidepressant response. Patients whose prefrontal glutamate doesn't rise tend not to respond; patients whose glutamate rises robustly tend to respond better and sooner.

Crucially, the glutamate surge activates AMPA receptors (a different class of glutamate receptor from NMDA). AMPA activation is what drives the downstream cascade. Blocking AMPA with pharmacological inhibitors abolishes ketamine's antidepressant effect in preclinical models. This is one of the cleanest mechanism proofs we have.

3. BDNF release and TrkB activation (hours)

The AMPA-driven pyramidal activity triggers release of BDNF (brain-derived neurotrophic factor), a neurotrophin that binds tropomyosin receptor kinase B (TrkB) on dendrites. BDNF-TrkB signaling is the master regulator of synaptic growth and remodeling in the adult brain. Chronic stress and depression suppress BDNF signaling; ketamine acutely restores and amplifies it, and animal work shows that this BDNF release is required for the rapid antidepressant response (Autry et al., 2011).

This step is where the treatment effect becomes lasting. A transient block of NMDA receptors produces, several steps downstream, a sustained increase in the growth signal for new synaptic connections.

4. mTORC1 activation and protein synthesis (hours)

TrkB activation engages the mTORC1 (mechanistic target of rapamycin complex 1) signaling pathway, which controls protein synthesis in neurons. mTORC1 turns on the translation of synaptic proteins, specifically the proteins needed to build and strengthen new dendritic spines. Rapamycin, an mTORC1 inhibitor, blocks ketamine's antidepressant effect in animal models, which is another strong piece of mechanism evidence (Li et al., 2010).

The mTORC1 activation is what translates the transient neurochemical event into durable structural change.

5. Synaptogenesis and spine density recovery (24-72 hours)

The proteins produced downstream of mTORC1 are used to build new dendritic spines and strengthen existing ones. Post-mortem and neuroimaging studies consistently show that chronic stress and depression reduce dendritic spine density in prefrontal cortex and hippocampus. Ketamine administration reverses this: new spines form, spine density recovers, and the architectural deficit that characterizes depressed brains begins to repair (Li et al., 2010; Moda-Sava et al., 2019).

This is the part that's genuinely remarkable. Most psychiatric medications work on transmitter balance; ketamine works by regrowing the structural connections between neurons. The mechanism is more akin to what BDNF-driven processes do after injury or during learning.

6. Behavioral and mood response (hours to days)

The molecular cascade above produces measurable antidepressant effect within hours, often faster than patients expect. Formal rating-scale response (MADRS, HAM-D) typically shows significant reduction at 24 hours post-infusion in clinical trials. The behavioral effect is downstream of and proportional to the synaptic changes.

What the clinical trials have shown

Ketamine's antidepressant effect is not a recent or fringe observation. The first controlled demonstration came from a small double-blind, placebo-controlled study in which depressed patients given a single subanesthetic infusion improved within hours, an effect no standard antidepressant produces (Berman et al., 2000). Six years later, a randomized, placebo-controlled trial at the National Institute of Mental Health tested a single dose specifically in treatment-resistant major depression and found rapid, significant improvement within 24 hours, with benefit that outlasted the drug itself (Zarate et al., 2006). A larger two-site randomized controlled trial then confirmed the finding against an active comparator (midazolam), showing the response was not simply a reaction to feeling the drug's acute effects (Murrough et al., 2013).

The evidence is strongest, and most time-sensitive, in patients with suicidal ideation. A systematic review and meta-analysis of individual-patient data found that a single dose of intravenous ketamine reduced suicidal thoughts within hours, with effects detectable for up to a week (Wilkinson et al., 2018). That rapid anti-suicidal signal is part of why ketamine has drawn so much clinical interest for severe, acute depression.

In 2019, the FDA approved esketamine (Spravato), the S-enantiomer of ketamine delivered as a nasal spray, for treatment-resistant depression, after trials showed it improved symptoms when added to a standard oral antidepressant (Daly et al., 2018; FDA). It was the first genuinely new mechanism approved for depression in decades.

Why this mechanism helps with TRD specifically

Three properties of this mechanism are directly relevant to treatment-resistant cases.

It bypasses serotonin entirely. If a patient's depression doesn't respond to serotonergic manipulation, ketamine reaches for a different lever. There's no reason to expect that five failed SSRI trials should predict failure on a glutamatergic intervention.

It addresses the structural deficit. The dendritic-spine pruning seen in chronic depression is a maintenance problem: the brain can't get out of the depressed state partly because the physical connectivity that would support a healthier state has atrophied. Ketamine drives regrowth of that connectivity. SSRIs don't do this; their effect is almost entirely on neurotransmitter balance.

It's rapid. For severe depression, especially depression with suicidal ideation, a six-week delay for an SSRI to maybe work is often not acceptable. Ketamine's hours-to-days response window is clinically meaningful, not just convenient (Wilkinson et al., 2018).

Why the effect isn't permanent from a single session

A single ketamine session produces a burst of BDNF signaling, a wave of mTORC1 activation, and a period of spine regrowth. But the brain's normal plasticity mechanisms (pruning, stabilization, adjustment to ongoing input) continue operating. Without reinforcement, the newly grown spines can be pruned again within weeks if the usage patterns that drove the depression (rumination, social isolation, inactivity, poor sleep) haven't changed.

This is why:

• Response from single sessions typically lasts 1-2 weeks.
• Induction courses (10 or more sessions over 4-8 weeks) produce more durable response than single sessions.
• Patients who combine ketamine with psychotherapy, sleep restoration, exercise, and real integration work do substantially better than patients who treat sessions as isolated drug events.

The neuroplasticity window is a tool, not a cure. What you do inside it determines what persists. Patients often describe this as slow, deliberate work rather than an overnight switch:

"Ketamine is helping me confront and address previously buried and inaccessible layers of deep pain and rage and sadness in a safe environment over time. Dr. Ben's expertise and own personal experience has been an invaluable resource on my ongoing journey to live with less suffering."

Moshe, verified patient (Google review)

How ketamine differs from other rapid-acting options

Spravato (esketamine). Same general mechanism (NMDA antagonism to disinhibition to BDNF cascade), delivered as intranasal S-ketamine under a REMS program. FDA-approved for TRD (Daly et al., 2018; FDA). See our R vs. S ketamine piece for the enantiomer specifics.

TMS (transcranial magnetic stimulation). Induces activity in prefrontal circuits through an entirely different mechanism: electromagnetic stimulation of cortical neurons. Effective for some TRD patients; produces response over weeks rather than hours.

ECT (electroconvulsive therapy). Still the gold standard for severe TRD, especially with psychotic features. Produces strong and rapid response. The side-effect profile (cognitive effects) is the main reason patients prefer alternatives when available.

Next-generation NMDA modulators. Compounds like aptinurin and members of the rapastinel family are in various stages of development and aim to preserve the antidepressant effect while minimizing the dissociative experience. None currently FDA-approved.

Response rates and what to expect

Meta-analyses and expert evidence syntheses of ketamine for TRD consistently report response rates (defined as ≥50% reduction in depression scores) in the 60-75% range within 24-72 hours of a single dose. Remission rates (full symptom resolution) are lower, in the 30-50% range (McIntyre et al., 2021; Bahji et al., 2021). Response rates with a proper induction course (10 or more sessions over 4-8 weeks) and integration work are higher.

These numbers are for TRD populations specifically: patients who have failed at least two prior adequate antidepressant trials. Response rates in SSRI-naive depression are similar to or somewhat higher than with SSRIs, but ketamine is rarely first-line in that population; the drug makes most clinical sense when other options have been exhausted.

Frequently Asked Questions

Why does ketamine work when SSRIs have failed?

SSRIs work by raising serotonin availability and slowly remodeling brain function over weeks. Ketamine works through an entirely different system: it transiently blocks NMDA receptors on inhibitory interneurons, which disinhibits prefrontal pyramidal neurons, drives a glutamate surge, triggers BDNF release and mTORC1 activation, and rebuilds the dendritic spine connectivity that chronic depression atrophies. If your depression is the kind that doesn't respond to serotonergic manipulation (and roughly 30-40% of patients with major depression are in that group), reaching for a different lever can produce response where SSRIs cannot.

How fast does ketamine produce antidepressant effect in treatment-resistant depression?

Within hours. Formal rating-scale response (MADRS, HAM-D) typically shows significant reduction at 24 hours after a session in clinical trials. Most patients describe noticing some shift (sleep, mood, anxiety, the texture of internal monologue) within the first 24 to 72 hours. The mechanistic reason is that the molecular cascade (NMDA blockade, glutamate surge, BDNF release, synaptogenesis) operates on hours-to-days timescales, not the weeks that serotonergic drugs require.

What is the BDNF cascade in plain language?

BDNF (brain-derived neurotrophic factor) is a protein the brain releases that tells neurons to grow, strengthen connections, and form new synaptic spines. Chronic stress and depression suppress BDNF signaling, which is one reason depressed brains show reduced dendritic spine density. Ketamine acutely restores and amplifies BDNF release, which engages the mTORC1 pathway, which translates the proteins needed to rebuild the synaptic connections that depression has thinned out. The cascade in shorthand: NMDA blockade to glutamate surge to AMPA activation to BDNF release to TrkB activation to mTORC1 to new synaptic proteins to spine regrowth.

How long does the neuroplasticity window last after a session?

The acute molecular cascade plays out over the first 24 to 72 hours, with a tapering window of heightened plasticity that lasts roughly a week. This is the therapeutic window: the period when the brain is most receptive to new patterns, learning, and behavioral change. What happens inside the window matters as much as the dose itself; sleep, integration work, therapy, exercise, and reduced rumination all support the consolidation of new circuits.

Is response durable from a single session, or does it require an induction course?

Single sessions typically produce 1 to 2 weeks of antidepressant response. Induction courses of 10 or more sessions over 4 to 8 weeks produce more durable response and higher remission rates. The reason is that newly grown synaptic spines need reinforcement to stabilize; a single burst of plasticity is often pruned back if the underlying patterns (rumination, isolation, poor sleep) haven't shifted. Repeated dosing across an induction course gives the brain enough cycles of regrowth to consolidate new circuits.

Can ketamine work for someone who has only tried one antidepressant?

Clinically, yes, but it is rarely the first move. Standard practice in the United States is to try two or more adequate antidepressant trials (each at therapeutic dose for 6 to 8 weeks) before reaching for ketamine, because SSRIs have a much longer track record, lower cost, and a favorable safety profile for first-line use. Exceptions include severe depression with active suicidality, where the days-to-weeks delay of an SSRI trial is not safe, or patients whose first SSRI caused intolerable side effects.

Why is therapy or integration work important alongside ketamine?

Because the medication opens a window of heightened neuroplasticity, but does not decide what gets built inside it. Patients who pair ketamine with structured psychotherapy, deliberate integration practice (journaling, talk therapy, behavioral change work), restored sleep, and reduced rumination tend to do substantially better than patients who treat each session as an isolated drug event. The molecular mechanism rebuilds the brain's capacity to change; the behavioral work directs that change toward something useful.

Why patients choose Discreet Ketamine

Everything above is the science. Here is how our program applies it. Discreet Ketamine provides physician-supervised, at-home ketamine therapy to residents of Florida and New Jersey. Every patient is evaluated and treated personally by Dr. Ben Soffer, a board-certified physician, from the first eligibility review through dose selection, monitoring, and follow-up. Treatment happens in your own home, with real medical oversight, rather than in a clinic infusion chair.

"Have to give Dr. Ben Soffer an A+. He's a very caring individual who is focused on me and my well being as a patient when we meet via the internet. Very professional and good with communicating about how I'm doing, side effects if they should occur."

Jack Webb, verified patient (Google review)

"Very helpful and efficient. Feel supported in my experience."

Jane Seales, verified patient (Google review)

Our at-home programs range from $250 for a one-month plan to $2,200 for a full year, which is generally lower than a comparable course of in-clinic infusions because there is no per-visit facility cost. Compounded racemic ketamine is prescribed off-label; it is not FDA-approved for depression, and it is not right for everyone. The eligibility screen is designed to tell you honestly whether it fits your situation.

References

1. Berman RM, Cappiello A, Anand A, et al. Antidepressant effects of ketamine in depressed patients. Biological Psychiatry. 2000.
2. Zarate CA, Singh JB, Carlson PJ, et al. A randomized trial of an NMDA antagonist in treatment-resistant major depression. Archives of General Psychiatry. 2006.
3. Murrough JW, Iosifescu DV, Chang LC, et al. Antidepressant efficacy of ketamine in treatment-resistant major depression: a two-site randomized controlled trial. American Journal of Psychiatry. 2013.
4. Daly EJ, Singh JB, Fedgchin M, et al. Efficacy and safety of intranasal esketamine adjunctive to oral antidepressant therapy in treatment-resistant depression. JAMA Psychiatry. 2018.
5. Wilkinson ST, Ballard ED, Bloch MH, et al. The effect of a single dose of intravenous ketamine on suicidal ideation: a systematic review and meta-analysis. American Journal of Psychiatry. 2018.
6. U.S. Food and Drug Administration. FDA approves new nasal spray medication for treatment-resistant depression (Spravato, esketamine): prescribing information and approval.
7. Li N, Lee B, Liu RJ, et al. mTOR-dependent synapse formation underlies the rapid antidepressant effects of NMDA antagonists. Science. 2010;329(5994):959-964. PMID: 20724638.
8. Autry AE, Adachi M, Nosyreva E, et al. NMDA receptor blockade at rest triggers rapid behavioural antidepressant responses. Nature. 2011;475(7354):91-95. PMID: 21677641.
9. Zanos P, Gould TD. Mechanisms of ketamine action as an antidepressant. Molecular Psychiatry. 2018;23(4):801-811. PMID: 29532791.
10. Moda-Sava RN, Murdock MH, Parekh PK, et al. Sustained rescue of prefrontal circuit dysfunction by antidepressant-induced spine formation. Science. 2019;364(6436):eaat8078. PMID: 30975859.
11. McIntyre RS, Rosenblat JD, Nemeroff CB, et al. Synthesizing the Evidence for Ketamine and Esketamine in Treatment-Resistant Depression: An International Expert Opinion on the Available Evidence and Implementation. American Journal of Psychiatry. 2021;178(5):383-399. PMID: 33726522.
12. Bahji A, Vazquez GH, Zarate CA. Comparative efficacy of racemic ketamine and esketamine for depression: A systematic review and meta-analysis. Journal of Affective Disorders. 2021. PMID: 33022440.

Related reading

• Why ketamine works when 2+ antidepressants have failed: the clinical picture behind the neurobiology.
• Understanding treatment-resistant depression: diagnostic context.
• Ketamine vs. TMS for treatment-resistant depression: mechanism, response rate, and access compared against the other major non-medication TRD treatment.
• The integration process: how to actually use the neuroplasticity window ketamine opens.
• Ketamine-assisted psychotherapy: the structured version of integration work.

Ready to start?

If you've been through several antidepressants without adequate response and this mechanism-level explanation fits your clinical picture, the five-minute eligibility check will tell you whether our at-home program is a reasonable next step.

Discreet Ketamine provides at-home ketamine therapy supervised by Dr. Ben Soffer, a board-certified physician, to residents of Florida and New Jersey. Compounded racemic ketamine is prescribed off-label for depression, anxiety, PTSD, and chronic pain; it is not FDA-approved for these indications.