Ketamine and Esketamine: Rapid-Acting Antidepressant Pharmacology

Glutamate is the primary excitatory neurotransmitter in the brain. The N-methyl-D-aspartate receptor is a glutamate-gated ion channel that plays a central role in synaptic plasticity, learning, and the regulation of neuronal circuits relevant to mood. It requires both glutamate binding and membrane depolarization to open fully, and when activated it conducts calcium ions into the neuron — the calcium influx that drives downstream signaling for synaptic strengthening and neuroplasticity.

Ketamine is an open-channel blocker of the N-methyl-D-aspartate receptor — it enters and binds within the channel pore when the channel is open, physically blocking ion flow. This mechanism is dose-dependent and use-dependent: channel opening is required for ketamine to gain access to its binding site. At the doses used for antidepressant treatment (substantially below anesthetic doses), ketamine selectively blocks N-methyl-D-aspartate receptors in circuits relevant to mood without producing full anesthesia.

The speed of ketamine's antidepressant action is explained by the downstream consequences of N-methyl-D-aspartate blockade rather than by the blockade itself. When ketamine blocks N-methyl-D-aspartate receptors on inhibitory interneurons in the prefrontal cortex, it releases pyramidal neurons from inhibitory control, producing a rapid burst of glutamate release. This glutamate burst activates another receptor type — the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor — which triggers the release of brain-derived neurotrophic factor and activation of the tropomyosin receptor kinase B receptor pathway. This cascade drives rapid synaptogenesis — the formation of new synaptic connections in prefrontal circuits — within hours. The new synapses reverse the synaptic loss caused by chronic stress and depression and represent the structural correlate of the antidepressant effect.

The antidepressant effect outlasts the drug itself by days: ketamine is eliminated within a few hours, but the synaptic changes it triggers persist for three to seven days following a single dose. This is the pharmacological basis for the multi-day antidepressant response seen clinically even after a single infusion.

Ketamine exists as two mirror-image forms (enantiomers). Racemic ketamine — the form used for intravenous infusion — contains equal proportions of both enantiomers. Esketamine is the S-enantiomer alone and has approximately three to four times greater affinity for the N-methyl-D-aspartate receptor than the R-enantiomer, making it the more pharmacologically potent form. Esketamine also produces more pronounced dissociative effects per unit dose for the same reason. The intranasal formulation of esketamine (Spravato) is the only Food and Drug Administration-approved product for antidepressant use; racemic ketamine given intravenously is used off-label.

Dissociation is the most consistent acute effect of both intravenous ketamine and intranasal esketamine at antidepressant doses. It is characterized by perceptual distortions, feelings of unreality, depersonalization, altered sense of time, and sometimes visual misperceptions — effects that do not reach the intensity of frank psychosis at therapeutic doses. These effects begin within minutes of intravenous infusion or within 10 to 20 minutes of intranasal administration, peak near the end of the infusion or 30 to 40 minutes after nasal dosing, and resolve within 60 to 90 minutes in most patients.

Dissociation is expected and dose-dependent — it is a pharmacological effect, not a sign of adverse reaction, at typical antidepressant doses. Patients should be warned before the first treatment that perceptual changes will occur and will resolve. Ketamine and esketamine should be used with extreme caution or avoided in patients with a personal or family history of schizophrenia or other primary psychotic disorders, where dissociative states may precipitate or worsen psychosis.

Ketamine produces transient increases in heart rate and blood pressure through sympathomimetic activity — it inhibits catecholamine reuptake, raising sympathetic tone. At antidepressant doses, blood pressure increases of 10 to 20 mmHg systolic are typical, peaking during infusion or shortly after nasal dosing and returning to baseline within one to two hours. Patients with baseline hypertension, significant cardiac disease, or a history of hypertensive emergency require pre-treatment blood pressure optimization and close monitoring during the session. Vital signs are measured before administration, during the monitoring period at regular intervals, and before discharge.

Ketamine is a Schedule III controlled substance under the United States Controlled Substances Act, reflecting established potential for misuse at higher doses and frequencies than those used in antidepressant treatment. At recreational doses, ketamine produces intense dissociation and euphoria. Chronic high-dose recreational use is associated with ketamine-induced uropathy — a serious and potentially permanent bladder condition — and with cognitive impairment. Esketamine carries the same Schedule III classification. The Risk Evaluation and Mitigation Strategy program for esketamine directly addresses the abuse risk by requiring supervised in-clinic administration and prohibiting take-home dispensing.

Nausea and vomiting occur in approximately 20 to 30 percent of patients. Pre-treatment with antiemetics is standard practice at most ketamine treatment centers and reduces the frequency and severity of nausea. Headache and sedation are common after esketamine — sedation is the pharmacological basis for the two-hour post-dose monitoring requirement and the no-driving restriction.

Absolute contraindications to esketamine and intravenous ketamine in the antidepressant setting include: history of aneurysmal vascular disease or arteriovenous malformation; intracerebral hemorrhage; hypersensitivity to ketamine. Uncontrolled hypertension is a relative contraindication requiring pre-treatment blood pressure management. Active psychotic disorders including schizophrenia are clinical contraindications.