Potential involvement of serotonergic signaling in ketamine’s antidepressant actions: A critical review

Interest in the glutamatergic system in major depressive disorder (MDD) increased after reports that a single subanesthetic intravenous infusion of ketamine, an NMDA receptor antagonist, produces rapid antidepressant effects within hours and remission in up to ~70% of treatment-resistant patients for periods lasting more than a week. Most mechanistic work has focussed on ketamine's effects on glutamate signalling and consequent synaptic plasticity, but ketamine has a complex pharmacological profile and may engage non-glutamatergic systems. Given longstanding evidence linking reduced central serotonergic tone to MDD and to responsiveness to conventional antidepressants, recent preclinical studies have explored whether serotonin (5-hydroxytryptamine; 5-HT) contributes to ketamine’s antidepressant-like actions. Gaarn Du Jardin and colleagues set out to critically review the available evidence for involvement of serotonergic signalling in ketamine’s antidepressant effects. They examine ketamine’s pharmacological targets and metabolites, summarise major mechanistic hypotheses for ketamine’s effects, outline points of interaction between glutamatergic and serotonergic systems at intracellular, synaptic and circuitry levels, and review preclinical and imaging data bearing on 5-HT tone, 5-HT release, and specific 5-HT receptor subtypes in relation to ketamine’s behavioural and neurochemical effects.

This article is a narrative critical review rather than a systematic review or meta-analysis; the extracted text does not report a formal search strategy, inclusion criteria, or methods for study selection. The authors synthesize pharmacological data, preclinical experimental findings (behavioural tests, microdialysis, electrophysiology, imaging, and receptor binding studies), and mechanistic hypotheses from the literature to evaluate serotonergic involvement in ketamine’s antidepressant-like effects. Topics brought together include: ketamine’s enantiomers and metabolites and their receptor activities; three prominent mechanistic hypotheses for ketamine’s antidepressant action (mTOR activation following a glutamate surge and BDNF release; desuppression of BDNF release via resting NMDA blockade and eEF2 pathways; and anti-inflammatory effects); known intracellular and synaptic crosstalk between glutamatergic and serotonergic signalling; circuit-level interactions focusing on prefrontal cortex, hippocampus and raphe nuclei; experimental manipulations of 5-HT tone (tryptophan hydroxylase inhibition and lesion models); measures of 5-HT reuptake, efflux and raphe neuronal firing; and pharmacological probing of specific 5-HT receptor subtypes (notably 5-HT1A and 5-HT1B, but also 5-HT2A and 5-HT3). Where available, the authors integrate results from microdialysis, PET imaging, electrophysiology, and multiple rodent behavioural paradigms (forced swim test, novelty suppressed feeding, tail suspension test, novelty suppressed feeding and social interaction after stress).

Ketamine and its metabolites interact with multiple targets beyond NMDA receptors. The racemate contains R- and S-enantiomers with differing NMDA potencies, and metabolites such as (R,S)-norketamine retain NMDA antagonism while hydroxynorketamines (notably (2R,6R)-hydroxynorketamine) have been reported to potentiate AMPA receptor activation. Some metabolites act as negative allosteric modulators at α7-nicotinic receptors and inhibit α3β4-nicotinic receptors at higher concentrations. Clinically relevant subanesthetic i.v. dosing (~0.5 mg/kg; peak plasma concentrations ~1 μM) overlaps with reported potencies for several non-NMDA targets, but the precise contribution of these targets to clinical antidepressant efficacy remains unresolved. Preclinical evidence indicates ketamine can increase extracellular 5-HT in prefrontal cortex. Microdialysis studies in rats and primates show a transient rise in prefrontal 5-HT lasting roughly 1–2 hours after systemic ketamine at antidepressant-like doses, whereas local ketamine application to prefrontal cortex or dorsal raphe nuclei does not reproduce this effect. Pharmacological work suggests the increase is mediated through AMPA receptor mechanisms, likely involving a circuit in which ventral hippocampal NMDA antagonism disinhibits prefrontal pyramidal cells, activating descending glutamatergic inputs to the dorsal raphe and thereby increasing raphe-driven cortical 5-HT release. One study reported more sustained enhancement of cortical 5-HT responses at 24 h after dosing together with increased spine density; this sustained effect was blocked by the mTOR inhibitor rapamycin. Manipulations of 5-HT synthesis and tone produce mixed but largely supportive results. Depletion of 5-HT with the tryptophan hydroxylase inhibitor 4-chloro-DL-phenylalanine reduced cortical 5-HT by ~70% in one study and prevented ketamine’s antidepressant-like effect at 24 h but not at 1 h. Gaarn Du Jardin and colleagues report that more extensive depletion (~94% in Flinders Sensitive Line rats) abolished ketamine’s antidepressant-like effects at both acute (1 h) and sustained (48 h) time points. Other groups found intermediate depletion (~74%) suppressed acute behavioural effects in mice. Differences in models, depletion magnitude, dosing and behavioural tests likely underlie discrepancies. The authors note potential off-target monoamine depletion at very high doses of the inhibitor and recommend alternative depletion methods to confirm findings. Electrophysiological and imaging data further implicate serotonergic mechanisms. PET in non-human primates reported reduced SERT binding after ketamine at an antidepressant dose, but in vitro potency for SERT inhibition occurs at concentrations far above clinical levels, so direct SERT blockade is unlikely to explain in vivo effects. Ketamine increased c-fos expression in dorsal raphe serotonergic neurons when injected into medial prefrontal cortex and systemic increases in raphe activity were blocked by prefrontal AMPA antagonism. One study did not find altered dorsal raphe firing ~2 h after dosing, but timing and anaesthetic choice may have limited detection. Region specificity is evident: ketamine prevented stress-induced increases in basolateral amygdala 5-HT and rescued stress-related social deficits in one model via prelimbic prefrontal to raphe projections. Receptor-specific findings point to 5-HT1-family involvement. The selective 5-HT1A antagonist WAY100635 dose-dependently attenuated ketamine’s acute antidepressant-like effect in the novelty suppressed feeding test, suggesting that 5-HT1A receptor agonism is necessary in that paradigm. PET studies showed increased 5-HT1B receptor binding in primate striatal regions after ketamine, an effect blocked by NBQX; in behavioural work, a selective 5-HT1B agonist (CP94253) rescued ketamine’s effect in 5-HT-depleted rats when present at testing. By contrast, blockade of 5-HT2A/2C with ritanserin did not alter ketamine’s effects in one test, and data on 5-HT3 receptors are mixed: a 5-HT3 antagonist combined with sub-threshold ketamine produced an antidepressant-like effect in one mouse study, whereas ketamine has been reported to enhance 5-HT3 receptor currents in vitro. Overall, data most consistently implicate 5-HT1A and 5-HT1B receptor mechanisms, but systematic evaluation of the full 5-HT receptor repertoire is lacking.

The authors conclude that serotonergic signalling is a plausible contributor to ketamine’s antidepressant-like effects, and that this idea is compatible with existing glutamate-centred hypotheses rather than mutually exclusive. Two principal possibilities are proposed: first, endogenous activation of specific 5-HT receptors may be required as an essential, regulatory step within intracellular or extracellular cascades (for example, mTOR or eEF2 phosphorylation and BDNF secretion) that mediate ketamine’s downstream therapeutic effects; second, ketamine may exert antidepressant effects by directly modulating serotonergic neurotransmission in key brain regions. Gaarn Du Jardin and colleagues highlight converging preclinical evidence that ketamine can augment cortical 5-HT release transiently and can induce more sustained enhancements of serotonergic responsiveness that correlate with synaptic strengthening; AMPA receptor-dependent activation of dorsal raphe circuits and mTOR signalling appear integral to these effects. Behavioural pharmacology and imaging findings predominantly support roles for 5-HT1A and 5-HT1B receptors, though data are not comprehensive across receptor subtypes. The authors caution that many findings are preclinical, region specific, and sometimes inconsistent across models, dosing regimens and depletion methods. They note unresolved issues such as the incomplete characterisation of ketamine’s 5-HT receptor profile, discrepant results regarding acute versus sustained 5-HT dependence, and the need to disentangle direct receptor interactions from circuit-level indirect effects. Implications emphasised by the authors include the need for more targeted studies: systematic exploration of individual 5-HT receptor subtypes in ketamine’s actions; use of complementary depletion or lesion methods to confirm serotonergic dependence; and experiments that combine established ketamine mechanistic readouts (for example, eEF2 and mTOR phosphorylation and BDNF secretion) with manipulations of 5-HT signalling. Such work could clarify whether serotonergic mechanisms are required upstream, downstream or in parallel with glutamatergic processes. The authors also acknowledge that clinical translation requires further investigation because much of the current evidence derives from animal models and imaging studies rather than controlled clinical trials.

Ketamine’s pharmacology encompasses multiple targets, including significant affinity at 5-HT2A receptors and modulatory effects on serotonergic neurotransmission, creating a mechanistic space in which serotonergic signalling could contribute to its antidepressant response. Gaarn Du Jardin and colleagues summarise preclinical evidence suggesting ketamine both modulates 5-HT release in stress- and region-dependent ways and depends, at least in part, on endogenous 5-HT receptor activation for its antidepressant-like effects. Recent data particularly implicate 5-HT1A and 5-HT1B receptors, but the authors stress that the role of serotonergic signalling is not yet well defined and call for further studies to map specific 5-HT pathways and receptors and to relate serotonergic manipulations to canonical ketamine endpoints such as mTOR and BDNF signalling.