What Neurotransmitters Does Ketamine Affect?

Ketamine touches a surprisingly wide range of neurotransmitter systems, far more than most people realize. It is best known for blocking glutamate receptors, specifically the NMDA type, but that is only the starting point. The drug also shifts activity in GABA, dopamine, serotonin, norepinephrine, acetylcholine, opioid, and endocannabinoid pathways, and it triggers downstream molecular cascades that reshape synapses. This multi-system reach helps explain why ketamine can serve as an anesthetic, a pain reliever, and a rapid-acting antidepressant all at once, and why its pharmacology has kept researchers busy for decades.

Glutamate and the NMDA Receptor

The most firmly established action of ketamine is blocking NMDA receptors, a type of glutamate receptor found throughout the brain. Glutamate is the brain’s main excitatory chemical messenger, and NMDA receptors play central roles in learning, memory, and synaptic plasticity. Ketamine lodges inside the receptor’s ion channel after it opens, physically plugging it so ions cannot flow through. Structural work has pinpointed the binding pocket: it sits in a central vestibule between the channel gate and the selectivity filter, and two specific amino acid residues form the key contacts that hold ketamine in place.¹ This blockade is “uncompetitive,” meaning ketamine can only enter the channel once glutamate has already activated it. In pain states where nerve circuits are overexcited, that property makes the drug especially effective at calming hyperactive signaling.²

Yet NMDA blockade alone does not explain everything ketamine does. Recent work has questioned whether blocking these receptors is even critical for the drug’s mood-enhancing effects, because other NMDA blockers do not reliably produce the same rapid antidepressant response.³ That observation pushed researchers to look harder at what else ketamine does, and the list keeps growing.

AMPA Receptors and the Glutamate Surge

One of the most important downstream consequences of NMDA blockade is a burst of glutamate release, particularly in the prefrontal cortex. This surge activates a different class of glutamate receptor called AMPA receptors, and a large body of evidence points to AMPA activation as a pivotal step in ketamine’s antidepressant effect. When researchers give an AMPA-blocking drug alongside ketamine, the antidepressant-like behavioral response disappears. Conversely, drugs that boost AMPA activity can mimic some of ketamine’s benefits on their own. Ketamine also increases the production of AMPA receptor subunit messenger RNA roughly one and a half to two-fold, suggesting it ramps up AMPA signaling at the gene-expression level as well.⁴ In dopaminergic neurons grown in culture, ketamine’s structural remodeling effects were abolished by AMPA receptor blockers and reproduced by an AMPA-boosting compound, further establishing AMPA receptors as central mediators.⁵

GABA and the Disinhibition Hypothesis

The glutamate surge described above does not happen randomly. The leading explanation is that ketamine preferentially blocks NMDA receptors on a particular class of inhibitory brain cells called GABA interneurons. Under normal conditions these interneurons keep excitatory pyramidal neurons in check. When ketamine silences the interneurons, the pyramidal neurons are released from that brake, firing more freely and flooding the surrounding area with glutamate. This sequence is known as the “disinhibition hypothesis.”⁶

Recordings from the medial prefrontal cortex in rodents show the process unfolding in real time: GABA neuron activity drops within minutes after ketamine injection, excitatory-to-inhibitory balance tips upward, and pyramidal neuron firing stays elevated for hours afterward.⁷ Identifying GABA interneurons as the initial cellular trigger for this cascade was a significant step, because it explained why a drug that blocks an excitatory receptor paradoxically produces more excitation in certain circuits.⁸

Dopamine

Ketamine raises dopamine levels in the brain, and this effect has been confirmed across many animal studies. A meta-analysis pooling data from rodent experiments found a significant increase in cortical dopamine after acute ketamine dosing, and a separate pooled analysis showed elevated dopamine in the broader brain as well.⁹ The regional pattern is not uniform, though. Some work in rats found that ketamine raised dopamine in the striatum in a dose-dependent way during place-preference conditioning, while prefrontal cortex dopamine did not always change in parallel.¹⁰ Earlier microdialysis studies went further, showing that ketamine could actually decrease dopamine release in the striatum under some conditions, a pattern quite different from its close chemical relative phencyclidine (PCP), which raises dopamine more broadly.¹¹

Ketamine also has a direct binding affinity for dopamine D2 receptors. One study measured its affinity for the high-affinity state of D2 receptors at roughly 0.5 µM, which is in the same range as its affinity for NMDA receptors, suggesting D2 interactions are not negligible side noise but a pharmacologically meaningful part of the picture.¹² This direct D2 engagement likely contributes to the dissociative and psychotomimetic side effects people sometimes experience during or shortly after ketamine administration.

Serotonin and Norepinephrine

Ketamine interacts with the serotonin system, though not as cleanly as selective serotonin drugs do. Animal studies have reported that NMDA antagonists like ketamine elevate extracellular serotonin in the brain. Ketamine also appears to enhance the function of 5-HT2A serotonin receptors, a subtype implicated in psychedelic-like experiences and potentially in mood regulation. Experiments with both ketamine and MK-801 showed they potentiated 5-HT2A receptor-mediated responses in a way that depended on membrane potential, pointing to a mechanism that involves ion channel blockade rather than direct binding to the serotonin receptor itself.¹³ Direct binding to 5-HT2 receptors does occur, but with a somewhat lower affinity (around 15 µM) compared to ketamine’s NMDA and D2 affinities.¹²

Norepinephrine is another monoamine that ketamine modulates. Ketamine inhibits the norepinephrine transporter (NET), both in lab experiments and in living animals and humans. In a placebo-controlled brain-imaging study, ketamine reduced the functional connectivity between the locus coeruleus, the brain’s main norepinephrine hub, and the thalamus, and the degree of that reduction depended on a person’s genetic variant of the norepinephrine transporter gene.¹⁴ That genetic dependence is a reminder that people’s neurochemical responses to ketamine are not identical.

Opioid Receptors

One of the more contentious questions in ketamine research is whether the opioid system contributes to the drug’s antidepressant action. Clinical observations that the opioid antagonist naltrexone can block ketamine’s antidepressant effect in humans raised the possibility early on.¹⁵ Most attention initially focused on mu opioid receptors, largely because naltrexone blocks them. But newer rodent work has shown that a selective kappa opioid receptor blocker also prevents ketamine’s antidepressant-like behavior, suggesting kappa receptors may be just as important.¹⁶

A metabolite of ketamine called (2R,6R)-hydroxynorketamine (HNK) adds another layer. In lab assays, HNK acted as an inverse agonist at both mu and kappa opioid receptors, meaning it reduced baseline receptor activity rather than stimulating it. It did this at very low concentrations, with an IC50 of about 0.56 nM at mu receptors.¹⁷ This is a genuinely unusual pharmacological profile and it complicates the narrative that ketamine works through a single receptor.

Acetylcholine

Ketamine inhibits both main branches of the acetylcholine receptor family. On the muscarinic side, ketamine blocks signaling with complete inhibition at around 200 µM and a half-maximal inhibition concentration well within the range reached during clinical use. That muscarinic blockade helps explain several of ketamine’s anticholinergic side effects: the sympathetic activation, the bronchodilation that makes it useful in emergency airway situations, and the pupil dilation (mydriasis) clinicians routinely observe.¹⁸

On the nicotinic side, both enantiomers of ketamine inhibit neuronal nicotinic acetylcholine receptors in a dose-dependent way, and interestingly this inhibition is not stereoselective, meaning the S(+) and R(−) forms are roughly equally potent against nicotinic receptors.¹⁹ Ketamine also inhibits nicotinic responses in cardiac ganglia, which may contribute to cardiovascular effects during anesthesia.²⁰ Age changes the acetylcholine story as well: in old rats, ketamine produced a much larger boost in hippocampal acetylcholine release than it did in young rats, even at the same dose.²¹

BDNF, mTOR, and Synaptic Remodeling

Many of ketamine’s neurotransmitter effects converge on a downstream signaling cascade that remodels synapses. The sequence, simplified, runs from AMPA receptor activation through the release of brain-derived neurotrophic factor (BDNF) and phosphorylation of the mTOR signaling pathway, ending with new protein synthesis and the growth of new dendritic spines. In mouse offspring exposed to prenatal stress, S-ketamine significantly increased both BDNF protein and the phosphorylation ratios of AKT and mTOR in the hippocampus, restoring levels that had been depressed by the stress exposure.²² In dopaminergic neurons from both mice and human stem-cell cultures, ketamine increased dendritic branching and cell body size within 72 hours, and the mTOR inhibitor rapamycin blocked those changes entirely.⁵

This BDNF-mTOR cascade is widely considered the bridge between ketamine’s acute neurotransmitter actions and the sustained antidepressant effect that can last days or even weeks after a single dose. Traditional antidepressants eventually promote BDNF too, but the process takes weeks. Ketamine’s ability to trigger it within hours is one reason the drug attracted so much psychiatric interest in the first place.²³

The Hydroxynorketamine Metabolite

Ketamine is metabolized in the body into several breakdown products, and one of them, (2R,6R)-hydroxynorketamine (HNK), turns out to have its own significant neurotransmitter activity. In mice, a single injection of HNK produced antidepressant-like behavioral and cellular effects at a dose where it could not meaningfully block NMDA receptors. The brain concentrations achieved by that dose topped out at about 8 µM, far below what would be needed for NMDA inhibition.²⁴ Instead, HNK enhances glutamate release through an NMDA receptor-independent, synapse-selective presynaptic mechanism in the hippocampus.²⁵

This finding reshapes the question of what ketamine “really” does at the neurotransmitter level. If a breakdown product that cannot block NMDA receptors still mimics many of ketamine’s antidepressant effects, then the parent drug’s NMDA blockade may be necessary to produce the metabolite and set off the initial glutamate surge, but the sustained benefits could depend on HNK working through entirely different molecular targets, including the opioid receptors discussed earlier.

Enantiomers and Why the Mirror Image Matters

Ketamine is a 50-50 mix of two mirror-image molecules: esketamine (the S-form) and arketamine (the R-form). Esketamine binds NMDA receptors roughly four to five times more tightly than arketamine, and it is the version approved as a nasal spray for treatment-resistant depression. Yet multiple animal studies have found that arketamine produces stronger and longer-lasting antidepressant effects despite weaker NMDA binding.²⁶ That paradox underscores a running theme: NMDA blockade alone does not account for the full picture. Arketamine’s additional actions on other neurotransmitter systems and signaling pathways likely explain its surprisingly strong behavioral effects, and clinical trials of arketamine as a standalone antidepressant are underway.

Less Familiar Targets

Beyond the major neurotransmitter systems, ketamine engages several lesser-known molecular players. It binds sigma receptors, a class of protein targets being explored for a potential new generation of antidepressants. In cell culture, sigma receptor antagonists blocked ketamine’s ability to stimulate neurite outgrowth, suggesting sigma-mediated neuronal remodeling may be part of its antidepressant mechanism, even though sigma antagonists did not block ketamine’s effect in a commonly used behavioral test (the forced swim test).²⁷

Adenosine signaling has recently emerged as another piece of the puzzle. A 2025 study in Nature reported that ketamine’s antidepressant effects, both at one hour and at 24 hours after dosing, were abolished in mice genetically lacking either the A1 or A2A adenosine receptors.²⁸ That is a striking result, because adenosine is not typically on anyone’s shortlist when discussing ketamine’s pharmacology. If it holds up, it could open an entirely new avenue of drug development.

Ketamine also recruits the endocannabinoid system. In the presence of a pain stimulus, ketamine triggers the release of the endocannabinoid anandamide (AEA) at the peripheral level, and blocking CB1 cannabinoid receptors prevents ketamine-induced pain relief in a dose-dependent fashion.²⁹ Separate experiments confirmed that endocannabinoids and CB1 receptors are involved in ketamine’s pain-relieving actions in the central nervous system as well.³⁰ Early evidence also hints at an interaction between the cannabinoid system and ketamine’s antidepressant-like effects, though the details remain preliminary.³¹

Neuroinflammation

Ketamine’s reach extends beyond classical neurotransmitter receptors into the immune cells of the brain. Microglia, the brain’s resident immune cells, become overactive in chronic stress and depression, releasing inflammatory molecules that can damage neurons. Ketamine dampens this process. It reduces microglial activation and lowers levels of inflammatory cytokines like IL-6 and TNF-α.³² In a mouse model of inflammation-driven depression, ketamine also reduced the infiltration of peripheral immune cells (monocytes) into the brain while promoting regulatory immune cell populations in the periphery.³³ These anti-inflammatory effects operate alongside the neurotransmitter changes and may be especially relevant for the subset of depressed patients whose condition appears to be driven by chronic low-grade inflammation.

What Happens with Chronic Exposure

The neurotransmitter picture looks quite different after repeated or prolonged ketamine use. Chronic administration in mice for 28 days led to decreased expression of multiple glutamate receptor subunits (both AMPA and NMDA types), reduced levels of synaptic scaffolding proteins, lower dendritic spine density, impaired long-term potentiation in the hippocampus, and measurable deterioration in learning and memory.³⁴ Rats that self-administered ketamine showed a similar broad downregulation of glutamate receptor expression that varied by brain region, with NMDA receptor subunits falling in the prefrontal cortex and AMPA receptor levels dropping in the hippocampus.³⁵

Dopamine receptors shift too. Brain imaging of chronic recreational ketamine users revealed that D1 dopamine receptor availability in the dorsolateral prefrontal cortex was significantly higher than in non-users, and the degree of upregulation correlated with how much ketamine a person used per week.³⁶ This kind of receptor upregulation is typically the brain’s attempt to compensate for altered dopamine signaling and is a hallmark of neuroadaptation. The contrast between the acute effects (glutamate surge, new synapse formation, BDNF release) and the chronic effects (receptor downregulation, spine loss, cognitive impairment) is a useful reminder that the neurotransmitter profile of a single therapeutic dose and the profile of repeated recreational use are essentially different pharmacological stories.

Gene Expression and Molecular Fingerprinting

Researchers have also tried to map ketamine’s full molecular fingerprint by looking at which genes it turns on and off across the brain. When compared against dozens of other psychotropic drugs using gene-expression profiling, ketamine’s pattern clustered most closely with the other NMDA antagonists memantine and PCP, but it also shared features with fluoxetine (a serotonin-focused antidepressant), the atypical antidepressant tianeptine, opioids, and even ethanol.³⁷ That kind of molecular overlap with such a diverse group of drugs reinforces the point that ketamine is not a single-target medication. Its pharmacological identity sits at a crossroads of glutamatergic, monoaminergic, and opioidergic action, which is partly why it resists being neatly slotted into any one drug category.