Role of Serotonin and Noradrenaline in the Rapid Antidepressant Action of Ketamine

The role of serotonin in depression-A historical roundup and future directions

Depression is one of the most common psychiatric disorders worldwide, affecting approximately 280 million people, with probably much higher unrecorded cases. Depression is associated with symptoms such as anhedonia, feelings of hopelessness, sleep disturbances, and even suicidal thoughts. Tragically, more than 700 000 people commit suicide each year. Although depression has been studied for many decades, the exact mechanisms that lead to depression are still unknown, and available treatments only help a fraction of patients. In the late 1960s, the serotonin hypothesis was published, suggesting that serotonin is the key player in depressive disorders. However, this hypothesis is being increasingly doubted as there is evidence for the influence of other neurotransmitters, such as noradrenaline, glutamate, and dopamine, as well as larger systemic causes such as altered activity in the limbic network or inflammatory processes. In this narrative review, we aim to contribute to the ongoing debate on the involvement of serotonin in depression. We will review the evolution of antidepressant treatments, systemic research on depression over the years, and future research applications that will help to bridge the gap between systemic research and neurotransmitter dynamics using biosensors. These new tools in combination with systemic applications, will in the future provide a deeper understanding of the serotonergic dynamics in depression.

Ketamine causes poor maternal care in rats with postpartum depression and leads to few behavioral and neurochemical alterations on male offspring

Ketamine is an anesthetic drug that also has antidepressant properties, with quick action. Despite the great number of studies showing its effectiveness as a treatment for major depression, there is little information about its effects on postpartum depression, as pharmacological treatments bring risks to the health of both mother and child. Thus, this study aimed to evaluate the effects of prolonged treatment with subanesthetic doses of ketamine in a rat model of postpartum depression. Female dams were induced to postpartum depression by the maternal separation model from lactating day (LD) 2-12. They were divided into four groups: one control and three experimental groups, which were treated with different doses of ketamine (5, 10 or 20 mg/kg) from LD 2-21 i.p. Maternal studies were conducted from LD5 to LD21 and the offspring studies from postnatal day 2 through 90. Ketamine causes poor maternal care, with few neurochemical alterations. However, the highest dose used in this study had an antidepressant effect. Regarding the male offspring, indirect exposure to ketamine through breast milk caused few behavioral changes during infancy, but they were not permanent, as they faded in adulthood. Nevertheless, this exposure was able to cause alterations in their monoaminergic neurotransmission systems that were found in both infancy and adulthood periods.

Ketamine: Mechanisms and Relevance to Treatment of Depression

Major depressive disorder (MDD) is a leading cause of suicide in the world. Monoamine-based antidepressant drugs are a primary line of treatment for this mental disorder, although the delayed response and incomplete efficacy in some patients highlight the need for improved therapeutic approaches. Over the past two decades, ketamine has shown rapid onset with sustained (up to several days) antidepressant effects in patients whose MDD has not responded to conventional antidepressant drugs. Recent preclinical studies have started to elucidate the underlying mechanisms of ketamine's antidepressant properties. Herein, we describe and compare recent clinical and preclinical findings to provide a broad perspective of the relevant mechanisms for the antidepressant action of ketamine.

Serotonin is a common thread linking different classes of antidepressants

Depression pathology remains elusive. The monoamine hypothesis has placed much focus on serotonin, but due to the variable clinical efficacy of monoamine reuptake inhibitors, the community is looking for alternative therapies such as ketamine (neurogenesis theory of antidepressant action). There is evidence that different classes of antidepressants may affect serotonin levels; a notion we test here. We measure hippocampal serotonin in mice with voltammetry and study the effects of acute challenges of escitalopram, fluoxetine, reboxetine, and ketamine. We find that pseudo-equivalent doses of these drugs similarly raise ambient serotonin levels, despite their differing pharmacodynamics because of differences in Uptake 1 and 2, rapid SERT trafficking, and modulation of serotonin by histamine. These antidepressants have different pharmacodynamics but have strikingly similar effects on extracellular serotonin. Our findings suggest that serotonin is a common thread that links clinically effective antidepressants, synergizing different theories of depression (synaptic plasticity, neurogenesis, and the monoamine hypothesis).

Pharmacological evidence for glutamatergic pathway involvement in the antidepressant-like effects of 2-phenyl-3-(phenylselanyl)benzofuran in male Swiss mice

Depression is a multifactorial and heterogeneous disease with several neurobiological mechanisms underlying its pathophysiology, including dysfunctional glutamatergic neurotransmission, which makes the exploration of the glutamate pathway an interesting strategy for developing novel rapid-acting antidepressant treatments. In the present study, we aimed to evaluate the possible glutamatergic pathway relation in the antidepressant-like action of 2-phenyl-3-(phenylselanyl)benzofuran (SeBZF1) in Swiss mice employing the tail suspension test (TST). Male Swiss mice received drugs targeting glutamate receptors before acute SeBZF1 administration at effective (50 mg/kg) or subeffective (1 mg/kg) doses by intragastric route (ig). TST and the open-field test (OFT) were employed in all behavioral experiments. The pretreatment of mice with N-methyl-D-aspartate (NMDA) (0.1 pmol/site, intracerebroventricular, icv, a selective agonist of the NMDA receptors), D-serine (30 µg/site, icv, a co-agonist at the NMDA receptor), arcaine (1 mg/kg, intraperitoneal, ip, an antagonist of the polyamine-binding site at the NMDA receptor), and 6,7-dinitroquinoxaline-2,3-dione (DNQX) (2,5 µg/site, icv, an antagonist of the AMPA/kainate type of glutamate receptors) inhibited the antidepressant-like effects of SeBZF1 (50 mg/kg, ig) in the TST. Coadministration of a subeffective dose of SeBZF1 with low doses of MK-801 (0.001 mg/kg, ip, a non-competitive NMDA receptor antagonist) or ketamine (0.1 mg/kg, ip, a non-selective antagonist of the NMDA receptors) produced significant antidepressant-like effects (synergistic action). These findings suggest the involvement of the glutamatergic system, probably through modulation of ionotropic glutamate receptors, in the antidepressant-like action of SeBZF1 in mice and contribute to a better understanding of the mechanisms underlying its pharmacological effects.

Ketamine promptly normalizes excess norepinephrine and enhances dopamine neuronal activity in Wistar Kyoto rats

Ketamine acts primarily by blocking the N-methyl-D-aspartate (NMDA) receptor at the phencyclidine site. The rapid antidepressant properties of ketamine were demonstrated in the clinic and several behavioral models of depression in rodents. We hypothesized that the normalization of abnormal activity of monoamine neurons in Wistar Kyoto (WKY) rats contributes to the rapid antidepressant effects of ketamine. A single administration of ketamine (10 mg/kg, i. p) or saline was administered to anesthetized WKY rats before in vivo electrophysiological recordings of dorsal raphe nucleus (DRN) serotonin (5-HT), locus coeruleus (LC) norepinephrine (NE) and ventral tegmental area (VTA) dopamine (DA) neuronal activity. Pyramidal neurons from the medial prefrontal cortex (mPFC) were also recorded before and after a ketamine injection. In the VTA, ketamine elicited a significant increase in the population activity of DA neurons. This enhancement was consistent with findings in other depression-like models in which such a decreased population activity was observed. In the LC, ketamine normalized increased NE neuron burst activity found in WKY rats. In the DRN, ketamine did not significantly reverse 5-HT neuronal activity in WKY rats, which is dampened compared to Wistar rats. Ketamine did not significantly alter the neuronal activity of mPFC pyramidal neurons. These findings demonstrate that ketamine normalized NE neuronal activity and enhanced DA neuronal activity in WKY rats, which may contribute to its rapid antidepressant effect.

Molecular mechanisms of rapid-acting antidepressants: New perspectives for developing antidepressants

Major depressive disorder (MDD) is a chronic relapsing psychiatric disorder. Conventional antidepressants usually require several weeks of continuous administration to exert clinically significant therapeutic effects, while about two-thirds of the patients are prone to relapse of symptoms or are completely ineffective in antidepressant treatment. The recent success of the N-methyl-D-aspartic acid (NMDA) receptor antagonist ketamine as a rapid-acting antidepressant has propelled extensive research on the action mechanism of antidepressants, especially in relation to its role in synaptic targets. Studies have revealed that the mechanism of antidepressant action of ketamine is not limited to antagonism of postsynaptic NMDA receptors or GABA interneurons. Ketamine produces powerful and rapid antidepressant effects by affecting α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors, adenosine A1 receptors, and the L-type calcium channels, among others in the synapse. More interestingly, the 5-HT2A receptor agonist psilocybin has demonstrated potential for rapid antidepressant effects in depressed mouse models and clinical studies. This article focuses on a review of new pharmacological target studies of emerging rapid-acting antidepressant drugs such as ketamine and hallucinogens (e.g., psilocybin) and briefly discusses the possible strategies for new targets of antidepressants, with a view to shed light on the direction of future antidepressant research.

Suicide prevention and ketamine: insights from computational modeling

Suicide is a pressing public health issue, with over 700,000 individuals dying each year. Ketamine has emerged as a promising treatment for suicidal thoughts and behaviors (STBs), yet the complex mechanisms underlying ketamine's anti-suicidal effect are not fully understood. Computational psychiatry provides a promising framework for exploring the dynamic interactions underlying suicidality and ketamine's therapeutic action, offering insight into potential biomarkers, treatment targets, and the underlying mechanisms of both. This paper provides an overview of current computational theories of suicidality and ketamine's mechanism of action, and discusses various computational modeling approaches that attempt to explain ketamine's anti-suicidal effect. More specifically, the therapeutic potential of ketamine is explored in the context of the mismatch negativity and the predictive coding framework, by considering neurocircuits involved in learning and decision-making, and investigating altered connectivity strengths and receptor densities targeted by ketamine. Theory-driven computational models offer a promising approach to integrate existing knowledge of suicidality and ketamine, and for the extraction of model-derived mechanistic parameters that can be used to identify patient subgroups and personalized treatment approaches. Future computational studies on ketamine's mechanism of action should optimize task design and modeling approaches to ensure parameter reliability, and external factors such as set and setting, as well as psychedelic-assisted therapy should be evaluated for their additional therapeutic value.

Ketamine and serotonergic psychedelics: An update on the mechanisms and biosignatures underlying rapid-acting antidepressant treatment

The discovery of ketamine as a rapid-acting antidepressant spurred significant research to understand its underlying mechanisms of action and to identify other novel compounds that may act similarly. Serotonergic psychedelics (SPs) have shown initial promise in treating depression, though the challenge of conducting randomized controlled trials with SPs and the necessity of long-term clinical observation are important limitations. This review summarizes the similarities and differences between the psychoactive effects associated with both ketamine and SPs and the mechanisms of action of these compounds, with a focus on the monoaminergic, glutamatergic, gamma-aminobutyric acid (GABA)-ergic, opioid, and inflammatory systems. Both molecular and neuroimaging aspects are considered. While their main mechanisms of action differ-SPs increase serotonergic signaling while ketamine is a glutamatergic modulator-evidence suggests that the downstream mechanisms of action of both ketamine and SPs include mechanistic target of rapamycin complex 1 (mTORC1) signaling and downstream GABAA receptor activity. The similarities in downstream mechanisms may explain why ketamine, and potentially SPs, exert rapid-acting antidepressant effects. However, research on SPs is still in its infancy compared to the ongoing research that has been conducted with ketamine. For both therapeutics, issues with regulation and proper controls should be addressed before more widespread implementation. This article is part of the Special Issue on "Ketamine and its Metabolites".

Novel ketamine and zinc treatment for anorexia nervosa and the potential beneficial interactions with the gut microbiome

Anorexia nervosa (AN) is a severe illness with diverse aetiological and maintaining contributors including neurobiological, metabolic, psychological, and social determining factors. In addition to nutritional recovery, multiple psychological and pharmacological therapies and brain-based stimulations have been explored; however, existing treatments have limited efficacy. This paper outlines a neurobiological model of glutamatergic and γ-aminobutyric acid (GABA)-ergic dysfunction, exacerbated by chronic gut microbiome dysbiosis and zinc depletion at a brain and gut level. The gut microbiome is established early in development, and early exposure to stress and adversity contribute to gut microbial disturbance in AN, early dysregulation to glutamatergic and GABAergic networks, interoceptive impairment, and inhibited caloric harvest from food (e.g., zinc malabsorption, competition for zinc ions between gut bacteria and host). Zinc is a key part of glutamatergic and GABAergic networks, and also affects leptin and gut microbial function; systems dysregulated in AN. Low doses of ketamine in conjunction with zinc, could provide an efficacious combination to act on NMDA receptors and normalise glutamatergic, GABAergic and gut function in AN.

Fast-Specific Fluorescent Probes to Visualize Norepinephrine Signaling Pathways and Its Flux in the Epileptic Mice Brain

Norepinephrine (NE) is synthesized in the locus coeruleus and widely projected throughout the brain and spinal cord. It regulates various actions and consciousness linked to a variety of neurological diseases. A "hunting-shooting" strategy was proposed in this work to improve the specificity and response rate of an NE fluorescent probe: 2-(cyclohex-2-en-1-ylidene)malononitrile derivatives were chosen as a fluorophore. To create a dual-site probe, an aldehyde group was added to the ortho of the ester group (or benzene sulfonate). Because of its excellent electrophilic activity, the aldehyde group could rapidly "hunt" the amino group and then form an intramolecular five-membered ring via the nucleophilic reaction with the β-hydroxyl group. The -NH- in the five-membered ring "shoots" the adjacent ester group, releasing the fluorophore and allowing for rapid and specific NE detection. The NE release and reuptake ″emetic″-″swallow″ transient process is captured and visualized under the action of the primary NE receptor drug. Furthermore, by introducing halogen into the fluorophore to lengthen the absorption wavelength, improve lipid solubility, and adjust the pK_a appropriately, the probe successfully penetrated the blood-brain barrier (BBB). In situ synchronous probe imaging was used to detect the NE level in the brains of epileptic and normal mice, and abnormal expression of NE in the brain was discovered during epilepsy. Brain anatomy was used to examine the distribution and level changes of NE in various brain regions before and after epilepsy. This research provides useful tools and a theoretical foundation for diagnosing and treating central nervous system diseases early.

Roles of the monoaminergic system in the antidepressant effects of ketamine and its metabolites

While the molecular target of (R,S)-ketamine (ketamine) is thought to be the NMDA receptor, subanesthetic doses of ketamine have been known to modulate monoaminergic neurotransmission in the central nervous system. Although the involvement of the serotonergic system in the antidepressant effects of ketamine has been reported in most studies of this topic, some recent studies have reported that the dopaminergic system plays a key role in the effects of ketamine. Additionally, several lines of evidence suggest that the antidepressant-like effects of (R)-ketamine might be independent of the monoaminergic system. Ketamine metabolites also differ considerably in their ability to regulate monoamine neurotransmitters relative to (S)-ketamine and (R)-ketamine, while (2R,6R)-hydroxynorketamine might share common serotonergic signaling mechanisms with ketamine. In the current review, we summarize the effects of ketamine and its metabolites on monoamine neurotransmission in the brain and discuss the potential roles of the monoaminergic system in the mechanism of action of ketamine.

(2R,6R)-hydroxynorketamine rapidly potentiates optically-evoked Schaffer collateral synaptic activity

(2R,6R)-hydroxynorketamine (HNK) is a metabolite of ketamine that exerts rapid and sustained antidepressant-like effects in preclinical studies. We hypothesize that the rapid antidepressant actions of (2R,6R)-HNK involve an acute increase in glutamate release at Schaffer collateral synapses. Here, we used an optogenetic approach to assess whether (2R,6R)-HNK promotes glutamate release at CA1-projecting Schaffer collateral terminals in response to select optical excitation of CA3 afferents. The red-shifted channelrhodopsin, ChrimsonR, was expressed in dorsal CA3 neurons of adult male Sprague Dawley rats. Transverse slices were collected four weeks later to determine ChrimsonR expression and to assess the acute synaptic effects of an antidepressant-relevant concentration of (2R,6R)-HNK (10 μM). (2R,6R)-HNK led to a rapid potentiation of CA1 field excitatory postsynaptic potentials evoked by recurrent optical stimulation of ChrimsonR-expressing CA3 afferents. This potentiation is mediated in part by an increase in glutamate release probability, as (2R,6R)-HNK suppressed paired-pulse facilitation at CA3 projections, an effect that correlated with the magnitude of the (2R,6R)-HNK-induced potentiation of CA1 activity. These results demonstrate that (2R,6R)-HNK increases the probability of glutamate release at CA1-projecting Schaffer collateral afferents, which may be involved in the antidepressant-relevant behavioral adaptations conferred by (2R,6R)-HNK in vivo. The current study also establishes proof-of-principle that genetically-encoded light-sensitive proteins can be used to investigate the synaptic plasticity induced by novel antidepressant compounds in neuronal subcircuits.

Ketamine use disorder: preclinical, clinical, and neuroimaging evidence to support proposed mechanisms of actions

Ketamine, a noncompetitive NMDA receptor antagonist, has been exclusively used as an anesthetic in medicine and has led to new insights into the pathophysiology of neuropsychiatric disorders. Clinical studies have shown that low subanesthetic doses of ketamine produce antidepressant effects for individuals with depression. However, its use as a treatment for psychiatric disorders has been limited due to its reinforcing effects and high potential for diversion and misuse. Preclinical studies have focused on understanding the molecular mechanisms underlying ketamine's antidepressant effects, but a precise mechanism had yet to be elucidated. Here we review different hypotheses for ketamine's mechanism of action including the direct inhibition and disinhibition of NMDA receptors, AMPAR activation, and heightened activation of monoaminergic systems. The proposed mechanisms are not mutually exclusive, and their combined influence may exert the observed structural and functional neural impairments. Long term use of ketamine induces brain structural, functional impairments, and neurodevelopmental effects in both rodents and humans. Its misuse has increased rapidly in the past 20 years and is one of the most common addictive drugs used in Asia. The proposed mechanisms of action and supporting neuroimaging data allow for the development of tools to identify 'biotypes' of ketamine use disorder (KUD) using machine learning approaches, which could inform intervention and treatment.

Response Flexibility: The Role of the Lateral Habenula

The ability to make appropriate decisions that result in an optimal outcome is critical for survival. This process involves assessing the environment as well as integrating prior knowledge about the environment with information about one’s current internal state. There are many neural structures that play critical roles in mediating these processes, but it is not yet known how such information coalesces to influence behavioral output. The lateral habenula (LHb) has often been cited as a structure critical for adaptive and flexible responding when environmental contexts and internal state changes. A challenge, however, has been understanding how LHb promotes response flexibility. In this review, we hypothesize that the LHb enables flexible responding following the integration of context memory and internal state information by signaling downstream brainstem structures known to drive hippocampal theta. In this way, animals respond more flexibly in a task situation not because the LHb selects a particular action, but rather because LHb enhances a hippocampal neural state that is often associated with greater attention, arousal, and exploration. In freely navigating animals, these are essential conditions that are needed to discover and implement appropriate alternative choices and behaviors. As a corollary to our hypothesis, we describe short- and intermediate-term functions of the LHb. Finally, we discuss the effects on the behavior of LHb dysfunction in short- and intermediate-timescales, and then suggest that new therapies may act on the LHb to alleviate the behavioral impairments following long-term LHb disruption.

Opioids Safety in Pediatric Procedural Sedation with Ketamine

OBJECTIVE

To evaluate the effects of pre- and intraprocedural opioids on adverse events in children undergoing procedural sedation with ketamine in the emergency department (ED).

STUDY DESIGN

We conducted a retrospective cohort study of all children aged 0-18 years who underwent procedural sedation with intravenous ketamine alone, or in combination with an opioid, at a tertiary-care pediatric ED between June 1, 2018, and August 31, 2020. We explored predictors of serious adverse events (SAEs), desaturation or respiratory intervention, and vomiting.

RESULTS

Of 1164 included children (694 male, 59.6%; median age 5.0 years [IQR 2.0-8.0]), 80 (6.8%) vomited, 63 (5.4%) had a desaturation or required respiratory interventions, and 6 (0.5%) had SAEs. Pre- and intraprocedural opioids were not independent predictors of sedation-related adverse events. A concurrent respiratory illness (aOR 3.73; 95% CI 1.31-10.60, P = .01), dental procedure (aOR 3.05; 95% CI 1.25-7.21, P = .01), and a greater total ketamine dose (aOR 1.75; 95% CI 1.21-2.54, P = .003) were independent predictors of desaturation or respiratory interventions. A greater total ketamine dose (aOR 1.86; 95% CI 1.16-2.98, P = .01) and older age (aOR 1.15; 95% CI 1.07-1.24, P < .001), were independent predictors of vomiting.

CONCLUSIONS

Pre- and intraprocedural opioids do not increase the likelihood of sedation-related adverse events. SAEs are rare during pediatric procedural sedation with ketamine in the ED.

Molecular Signaling Mechanisms for the Antidepressant Effects of NLX-101, a Selective Cortical 5-HT1A Receptor Biased Agonist

Depression is the most prevalent of the mental illnesses and serotonin (5-hydroxytryptamine, 5-HT) is considered to be the major neurotransmitter involved in its etiology and treatment. In this context, 5-HT_1A receptors have attracted interest as targets for therapeutic intervention. Notably the activation of presynaptic 5-HT_1A autoreceptors delays antidepressant effects whereas the stimulation of postsynaptic 5-HT_1A heteroreceptors is needed for an antidepressant action. NLX-101 (also known as F15599) is a selective biased agonist which exhibits preferred activation of cortical over brain stem 5-HT_1A receptors. Here, we used behavioral, neurochemical and molecular methods to examine the antidepressant-like effects in rats of a single dose of NLX-101 (0.16 mg/kg, i.p.). NLX-101 reduced immobility in the forced swim test when measured 30 min but not 24 h after drug administration. NLX-101 increased extracellular concentrations of glutamate and dopamine in the medial prefrontal cortex, but no changes were detected in the efflux of noradrenaline or 5-HT. NLX-101 also produced an increase in the activation of pmTOR, pERK1/2 and pAkt, and the expression of PSD95 and GluA1, which may contribute to its rapid antidepressant action.

Ketamine and sleep modulate neural complexity dynamics in cats

There is increasing evidence that the level of consciousness can be captured by neural informational complexity: for instance, complexity, as measured by the Lempel Ziv (LZ) compression algorithm, decreases during anaesthesia and non‐rapid eye movement (NREM) sleep in humans and rats, when compared with LZ in awake and REM sleep. In contrast, LZ is higher in humans under the effect of psychedelics, including subanaesthetic doses of ketamine. However, it is both unclear how this result would be modulated by varying ketamine doses, and whether it would extend to other species. Here, we studied LZ with and without auditory stimulation during wakefulness and different sleep stages in five cats implanted with intracranial electrodes, as well as under subanaesthetic doses of ketamine (5, 10, and 15 mg/kg i.m.). In line with previous results, LZ was lowest in NREM sleep, but similar in REM and wakefulness. Furthermore, we found an inverted U‐shaped curve following different levels of ketamine doses in a subset of electrodes, primarily in prefrontal cortex. However, it is worth noting that the variability in the ketamine dose–response curve across cats and cortices was larger than that in the sleep‐stage data, highlighting the differential local dynamics created by two different ways of modulating conscious state. These results replicate previous findings, both in humans and other species, demonstrating that neural complexity is highly sensitive to capture state changes between wake and sleep stages while adding a local cortical description. Finally, this study describes the differential effects of ketamine doses, replicating a rise in complexity for low doses, and further fall as doses approach anaesthetic levels in a differential manner depending on the cortex.

Mechanisms of ketamine and its metabolites as antidepressants

Treating major depression is a medical need that remains unmet by monoaminergic therapeutic strategies that commonly fail to achieve symptom remission. A breakthrough in the treatment of depression was the discovery that the anesthetic (R,S)-ketamine (ketamine), when administered at sub-anesthetic doses, elicits rapid (sometimes within hours) antidepressant effects in humans that are otherwise resistant to monoaminergic-acting therapies. While this finding was revolutionary and led to the FDA approval of (S)-ketamine (esketamine) for use in adults with treatment-resistant depression and suicidal ideation, the mechanisms underlying how ketamine or esketamine elicit their effects are still under active investigation. An emerging view is that metabolism of ketamine may be a crucial step in its mechanism of action, as several metabolites of ketamine have neuroactive effects of their own and may be leveraged as therapeutics. For example, (2R,6R)-hydroxynorketamine (HNK), is readily observed in humans following ketamine treatment and has shown therapeutic potential in preclinical tests of antidepressant efficacy and synaptic potentiation while being devoid of the negative adverse effects of ketamine, including its dissociative properties and abuse potential. We discuss preclinical and clinical studies pertaining to how ketamine and its metabolites produce antidepressant effects. Specifically, we explore effects on glutamate neurotransmission through N-methyl D-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), synaptic structural changes via brain derived neurotrophic factor (BDNF) signaling, interactions with opioid receptors, and the enhancement of serotonin, norepinephrine, and dopamine signaling. Strategic targeting of these mechanisms may result in novel rapid-acting antidepressants with fewer undesirable side effects compared to ketamine.

Esketamine Increases Neurotransmitter Releases but Simplifies Neurotransmitter Networks in Mouse Prefrontal Cortex

General anesthesia induces a profound but reversible unconscious state, which is accompanied by changes in various neurotransmitters in the cortex. Unlike the "brain silencing" effect of γ-aminobutyric acid (GABA) receptor potentiator anesthesia, ketamine anesthesia leads the brain to a paradoxical active state with higher cortical activity, which is manifested as dissociative anesthesia. However, how the overall neurotransmitter network evolves across conscious states after ketamine administration remains unclear. Using in vivo microdialysis, high-performance liquid chromatography-mass spectrometry (HPLC-MS) analysis, and electroencephalogram (EEG) recording technique, we continuously measured the concentrations of six neurotransmitters and the EEG signals during anesthesia with esketamine, an S-enantiomer of ketamine racemate. We found that there was an increase in the release of five cortical neurotransmitters after the administration of esketamine. The correlation of cortical neurotransmitters was dynamically simplified along with behavioral changes until full recovery after anesthesia. The esketamine-increased gamma oscillation power was positively correlated only with the concentration of 5-hydroxytryptamine (5-HT) in the medial prefrontal cortex. This study suggests that the transformation of the neurotransmitter network rather than the concentrations of neurotransmitters could be more indicative of the consciousness shift during esketamine anesthesia.

Novel Insights Into the Neurobiology of the Antidepressant Response From Ketamine Research: A Mini Review

The serendipitous discovery of ketamine’s antidepressant effects represents one of the major landmarks in neuropsychopharmacological research of the last 50 years. Ketamine provides an exciting challenge to traditional concepts of antidepressant drug therapy, producing rapid antidepressant effects seemingly without targeting monoaminergic pathways in the conventional way. In consequence, the advent of ketamine has spawned a plethora of neurobiological research into its putative mechanisms. Here, we provide a brief overview of current theories of antidepressant drug action including monoaminergic signaling, disinhibition of glutamatergic neurotransmission, neurotrophic and neuroplastic effects, and how these might relate to ketamine. Given that research into ketamine has not yet yielded new therapies beyond ketamine itself, current knowledge gaps and limitations of available studies are also discussed.

New insights into methoxetamine mechanisms of action: Focus on serotonergic 5-HT2 receptors in pharmacological and behavioral effects in the rat

Methoxetamine (MXE) is a dissociative substance of the arylcyclohexylamine class that has been present on the designer drug market as a ketamine-substitute since 2010. We have previously shown that MXE (i) possesses ketamine-like discriminative and positive rewarding effects in rats, (ii) affects brain processing involved in cognition and emotional responses, (iii) causes long-lasting behavioral abnormalities and neurotoxicity in rats and (iv) induces neurological, sensorimotor and cardiorespiratory alterations in mice. To shed light on the mechanisms through which MXE exerts its effects, we conducted a multidisciplinary study to evaluate the various neurotransmitter systems presumably involved in its actions on the brain. In vivo microdialysis study first showed that a single administration of MXE (0.25 and 0.5 mg/kg, i.v.) is able to significantly alter serotonin levels in the rat medial prefrontal cortex (mPFC) and nucleus accumbens. Then, we observed that blockade of the serotonin 5-HT₂ receptors through two selective antagonists, ketanserin (0.1 mg/kg, i.p.) and MDL 100907 (0.03 mg/kg, i.p.), at doses not affecting animals behavior per se, attenuated the facilitatory motor effect and the inhibition on visual sensory responses induced by MXE (3 mg/kg, i.p.) and ketamine (3 mg/kg, i.p.), and prevented MXE-induced reduction of the prepulse inhibition in rats, pointing to the 5-HT₂ receptors as a key target for the recently described MXE-induced sensorimotor effects. Finally, in-vitro electrophysiological studies revealed that the GABAergic and glutamatergic systems are also likely involved in the mechanisms through which MXE exerts its central effects since MXE inhibits, in a concentration-dependent manner, NMDA-mediated field postsynaptic potentials and GABA-mediated spontaneous currents. Conversely, MXE failed to alter both the AMPA component of field potentials and presynaptic glutamate release, and seems not to interfere with the endocannabinoid-mediated effects on mPFC GABAergic synapses. Altogether, our results support the notion of MXE as a NMDA receptor antagonist and shed further lights into the central mechanisms of action of this ketamine-substitute by pointing to serotonin 5-HT₂ receptors as crucial players in the expression of its sensorimotor altering effects and to the NMDA and GABA receptors as potential further important targets of action.

Structural connectivity and subcellular changes after antidepressant doses of ketamine and Ro 25-6981 in the rat: an MRI and immuno-labeling study

Ketamine has rapid and robust antidepressant effects. However, unwanted psychotomimetic effects limit its widespread use. Hence, several studies examined whether GluN2B-subunit selective NMDA antagonists would exhibit a better therapeutic profile. Although preclinical work has revealed some of the mechanisms of action of ketamine at cellular and molecular levels, the impact on brain circuitry is poorly understood. Several neuroimaging studies have examined the functional changes in the brain induced by acute administration of ketamine and Ro 25-6981 (a GluN2B-subunit selective antagonist), but the changes in the microstructure of gray and white matter have received less attention. Here, the effects of ketamine and Ro 25-6981 on gray and white matter integrity in male Sprague-Dawley rats were determined using diffusion-weighted magnetic resonance imaging (DWI). In addition, DWI-based structural brain networks were estimated and connectivity metrics were computed at the regional level. Immunohistochemical analyses were also performed to determine whether changes in myelin basic protein (MBP) and neurofilament heavy-chain protein (NF200) may underlie connectivity changes. In general, ketamine and Ro 25-6981 showed some opposite structural alterations, but both compounds coincided only in increasing the fractional anisotropy in infralimbic prefrontal cortex and dorsal raphe nucleus. These changes were associated with increments of NF200 in deep layers of the infralimbic cortex (together with increased MBP) and the dorsal raphe nucleus. Our results suggest that the synthesis of NF200 and MBP may contribute to the formation of new dendritic spines and myelination, respectively. We also suggest that the increase of fractional anisotropy of the infralimbic and dorsal raphe nucleus areas could represent a biomarker of a rapid antidepressant response.

Context-Specific Tolerance and Pharmacological Changes in the Infralimbic Cortex-Nucleus Accumbens Shell Pathway Evoked by Ketamine

Like other drugs, ketamine is abused due to its ability to act as a positive reinforcer in the control of behavior, just as natural reinforcers do. Besides, through Pavlovian conditioning, tolerance to drug effects can become conditioned to specific contextual cues showing that environmental stimuli can act as powerful mediators of craving and relapse. In the present study, we shall investigate the effects of long-term ketamine administration and withdrawal on behavioral measures and emotionality, the drug-context-specific influence on the tolerance to the sedative effects of an anesthetic dose of ketamine, and the neuropharmacological events underlying this phenomenon, in rats conditioned with 10 mg/kg of ketamine and later challenged with a dose of ketamine of 80 mg/kg in a familiar and non-familiar environment. Variations in dopamine and serotonin efflux in the infralimbic cortex-nucleus accumbens shell circuitry (IL-NAcSh) was further recorded in the same conditions. Our results highlight that besides its well-known reinforcing properties, ketamine also shares the ability to induce behavioral and pharmacological conditioned tolerance, associated with increases in cortical (IL), and decreases in striatal (NAcSh) dopamine release. To our knowledge, we are presenting the first set of behavioral and neurochemical data showing that, like other drugs of abuse, ketamine can induce learned context-specific tolerance.

Perioperative Methadone and Ketamine for Postoperative Pain Control in Spinal Surgical Patients: A Randomized, Double-blind, Placebo-controlled Trial

BACKGROUND

Despite application of multimodal pain management strategies, patients undergoing spinal fusion surgery frequently report severe postoperative pain. Methadone and ketamine, which are N-methyl-d-aspartate receptor antagonists, have been documented to facilitate postoperative pain control. This study therefore tested the primary hypothesis that patients recovering from spinal fusion surgery who are given ketamine and methadone use less hydromorphone on the first postoperative day than those give methadone alone.

METHODS

In this randomized, double-blind, placebo-controlled trial, 130 spinal surgery patients were randomized to receive either methadone at 0.2 mg/kg (ideal body weight) intraoperatively and a 5% dextrose in water infusion for 48 h postoperatively (methadone group) or 0.2 mg/kg methadone intraoperatively and a ketamine infusion (0.3 mg · kg-1 · h-1 infusion [no bolus] intraoperatively and then 0.1 mg · kg-1 · h-1 for next 48 h [both medications dosed at ideal body weight]; methadone/ketamine group). Anesthetic care was standardized in all patients. Intravenous hydromorphone use on postoperative day 1 was the primary outcome. Pain scores, intravenous and oral opioid requirements, and patient satisfaction with pain management were assessed for the first 3 postoperative days.

RESULTS

Median (interquartile range) intravenous hydromorphone requirements were lower in the methadone/ketamine group on postoperative day 1 (2.0 [1.0 to 3.0] vs. 4.6 [3.2 to 6.6] mg in the methadone group, median difference [95% CI] 2.5 [1.8 to 3.3] mg; P < 0.0001) and postoperative day 2. In addition, fewer oral opioid tablets were needed in the methadone/ketamine group on postoperative day 1 (2 [0 to 3] vs. 4 [0 to 8] in the methadone group; P = 0.001) and postoperative day 3. Pain scores at rest, with coughing, and with movement were lower in the methadone/ketamine group at 23 of the 24 assessment times. Patient-reported satisfaction scores were high in both study groups.

CONCLUSIONS

Postoperative analgesia was enhanced by the combination of methadone and ketamine, which act on both N-methyl-d-aspartate and μ-opioid receptors. The combination could be considered in patients having spine surgery.

EDITOR’S PERSPECTIVE

Sublingual AKBA Exerts Antidepressant Effects in the Aβ-Treated Mouse Model

The 3-O-acetyl-11-keto-β-boswellic acid (AKBA) is the most active compound of Boswellia serrata proposed for treating neurodegenerative disorders, including Alzheimer’s disease (AD), characterized in its early phase by alteration in mood. Accordingly, we have previously demonstrated that an intracerebroventricular injection of soluble amyloid beta _1-42 (Aβ) peptide evokes a depressive-like phenotype in rats. We tested the protective effects of AKBA in the mouse model of an Aβ-induced depressive-like phenotype. We evaluated the depressive-like behavior by using the tail suspension test (TST) and the splash test (ST). Behavioral analyses were accompanied by neurochemical quantifications, such as glutamate (GLU), kynurenine (KYN) and monoamines, and by biochemical measurements, such as glial fibrillary acid protein (GFAP), CD11b and nuclear factor kappa B (NF-kB), in mice prefrontal cortex (PFC) and hippocampus (HIPP). AKBA prevented the depressive-like behaviors induced by Aβ administration, since we recorded a reduction in latency to initiate self-care and total time spent to perform self-care in the ST and reduced time of immobility in the TST. Likewise, the increase in GLU and KYN levels in PFC and HIPP induced by the peptide injection were reverted by AKBA administration, as well as the displayed increase in levels of GFAP and NF-kB in both PFC and HIPP, but not in CD11b. Therefore, AKBA might represent a food supplement suitable as an adjuvant for therapy of depression in early-stage AD.

Repeated but Not Single Administration of Ketamine Prolongs Increases of the Firing Activity of Norepinephrine and Dopamine Neurons

BACKGROUND

Clinical studies have shown that the rapid antidepressant effect of the glutamate N-methyl-D-aspartate receptor antagonist ketamine generally disappears within 1 week but can be maintained by repeated administration. Preclinical studies showed that a single ketamine injection immediately increases the firing and burst activity of norepinephrine (NE) neurons, but not that of serotonin (5-HT) neurons. It also enhances the population activity of dopamine (DA) neurons. In the present study, we investigated whether such alterations of monoamine neuronal firing are still present 1 day after a single injection, and whether they can be maintained by repeated injections.

METHODS

Rats received a single ketamine injection or 6 over 2 weeks and the firing activity of dorsal raphe nucleus 5-HT, locus coeruleus NE, and ventral tegmental area DA neurons was assessed.

RESULTS

One day following a single injection of ketamine, there was no change in the firing activity of 5-HT, NE, or DA neurons. One day after repeated ketamine administration, however, there was a robust increase of the firing activity of NE neurons and an enhancement of burst and population activities of DA neurons, but still no change in firing parameters of 5-HT neurons. The increased activity of NE neurons was no longer present 3 days after the last injection, whereas that of DA neurons was still present. DA neurons were firing normally 7 days after repeated injections.

CONCLUSION

These results imply that the enhanced activity of NE and DA neurons may play a significant role in the maintenance of the antidepressant action of ketamine.

Ketamine reversed short-term memory impairment and depressive-like behavior in animal model of Parkinson's disease

The most common features of Parkinson's disease (PD) are motor impairments, but many patients also present depression and memory impairment. Ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, has been shown to be effective in patients with treatment-resistant major depression. Thus, the present study evaluated the action of ketamine on memory impairment and depressive-like behavior in an animal model of PD. Male Wistar rats received a bilateral infusion of 6 μg/side 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta (SNc). Short-term memory was evaluated by the social recognition test, and depressive-like behaviors were evaluated by the sucrose preference and forced swimming tests (FST). Drug treatments included vehicle (i.p., once a week); ketamine (5, 10 and 15 mg/kg, i.p., once a week); and imipramine (20 mg/kg, i.p., daily). The treatments were administered 21 days after the SNc lesion and lasted for 28 days. The SNc lesion impaired short-term social memory, and all ketamine doses reversed the memory impairment and anhedonia (reduction of sucrose preference) induced by 6-OHDA. In the FST, 6-OHDA increased immobility, and all doses of ketamine and imipramine reversed this effect. The anti-immobility effect of ketamine was associated with an increase in swimming but not in climbing, suggesting a serotonergic effect. Ketamine and imipramine did not reverse the 6-OHDA-induced reduction in tyrosine hydroxylase immunohistochemistry in the SNc. In conclusion, ketamine reversed depressive-like behaviors and short-term memory impairment in rats with SNc bilateral lesions, indicating a promising profile for its use in PD patients.

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Green sonochemical synthesis and fabrication of cubic MnFe2O4 electrocatalyst decorated carbon nitride nanohybrid for neurotransmitter detection in serum samples

The binary nanomaterials and graphitic carbon based hybrid has been developed as an important porous nanomaterial for fabricating electrode with applications in non-enzymatic (bio) sensors. We report a fast synthesis of bimetal oxide particles of nano-sized manganese ferrite (MnFe2O4) decorated on graphitic carbon nitride (GCN) via a high-intensity ultrasonic irradiation method for C (30 kHz and 70 W/cm2). The nanocomposites were analyzed by powder X-ray diffraction, XPS, EDS, TEM to ascertain the effects of synthesis parameters on structure, and morphology. The MnFe2O4/GCN modified electrode demonstrated superior electrocatalytic activity toward the neurotransmitter (5-hydroxytryptamine) detection with a high peak intensity at +0.21 V. The appealing application of the MnFe2O4/GCN/GCE as neurotransmitter sensors is presented and a possible sensing mechanism is analyzed. The constructed electrochemical sensor for the detection of 5-hydroxytryptamine (STN) showed a wide working range (0.1-522.6 μM), high sensitivity (19.377 μA μM-1 cm-2), and nano-molar detection limit (3.1 nM). Moreover, it is worth noting that the MnFe2O4/GCN not only enhanced activity and also promoted the electron transfer rate towards STN detection. The proposed sensor was analyzed for its real-time applications to the detection of STN in rat brain serum, and human blood serum in good satisfactory results was obtained. The results showed promising reproducibility, repeatability, and high stability for neurotransmitter detection in biological samples.

Psychedelics in Psychiatry: Neuroplastic, Immunomodulatory, and Neurotransmitter Mechanisms

Mounting evidence suggests safety and efficacy of psychedelic compounds as potential novel therapeutics in psychiatry. Ketamine has been approved by the Food and Drug Administration in a new class of antidepressants, and 3,4-methylenedioxymethamphetamine (MDMA) is undergoing phase III clinical trials for post-traumatic stress disorder. Psilocybin and lysergic acid diethylamide (LSD) are being investigated in several phase II and phase I clinical trials. Hence, the concept of psychedelics as therapeutics may be incorporated into modern society. Here, we discuss the main known neurobiological therapeutic mechanisms of psychedelics, which are thought to be mediated by the effects of these compounds on the serotonergic (via 5-HT_2A and 5-HT_1A receptors) and glutamatergic [via N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors] systems. We focus on 1) neuroplasticity mediated by the modulation of mammalian target of rapamycin-, brain-derived neurotrophic factor-, and early growth response-related pathways; 2) immunomodulation via effects on the hypothalamic-pituitary-adrenal axis, nuclear factor ĸB, and cytokines such as tumor necrosis factor-α and interleukin 1, 6, and 10 production and release; and 3) modulation of serotonergic, dopaminergic, glutamatergic, GABAergic, and norepinephrinergic receptors, transporters, and turnover systems. We discuss arising concerns and ways to assess potential neurobiological changes, dependence, and immunosuppression. Although larger cohorts are required to corroborate preliminary findings, the results obtained so far are promising and represent a critical opportunity for improvement of pharmacotherapies in psychiatry, an area that has seen limited therapeutic advancement in the last 20 years. Studies are underway that are trying to decouple the psychedelic effects from the therapeutic effects of these compounds. SIGNIFICANCE STATEMENT: Psychedelic compounds are emerging as potential novel therapeutics in psychiatry. However, understanding of molecular mechanisms mediating improvement remains limited. This paper reviews the available evidence concerning the effects of psychedelic compounds on pathways that modulate neuroplasticity, immunity, and neurotransmitter systems. This work aims to be a reference for psychiatrists who may soon be faced with the possibility of prescribing psychedelic compounds as medications, helping them assess which compound(s) and regimen could be most useful for decreasing specific psychiatric symptoms.

Involvement of NMDA receptors containing the GluN2C subunit in the psychotomimetic and antidepressant-like effects of ketamine

Acute ketamine administration evokes rapid and sustained antidepressant effects in treatment-resistant patients. However, ketamine also produces transient perceptual disturbances similarly to those evoked by other non-competitive NMDA-R antagonists like phencyclidine (PCP). Although the brain networks involved in both ketamine actions are not fully understood, PCP and ketamine activate thalamo-cortical networks after NMDA-R blockade in GABAergic neurons of the reticular thalamic nucleus (RtN). Given the involvement of thalamo-cortical networks in processing sensory information, these networks may underlie psychotomimetic action. Since the GluN2C subunit is densely expressed in the thalamus, including the RtN, we examined the dependence of psychotomimetic and antidepressant-like actions of ketamine on the presence of GluN2C subunits, using wild-type and GluN2C knockout (GluN2CKO) mice. Likewise, since few studies have investigated ketamine's effects in females, we used mice of both sexes. GluN2C deletion dramatically reduced stereotyped (circling) behavior induced by ketamine in male and female mice, while the antidepressant-like effect was fully preserved in both genotypes and sexes. Despite ketamine appeared to induce similar effects in both sexes, some neurobiological differences were observed between male and female mice regarding c-fos expression in thalamic nuclei and cerebellum, and glutamate surge in prefrontal cortex. In conclusion, the GluN2C subunit may discriminate between antidepressant-like and psychotomimetic actions of ketamine. Further, the abundant presence of GluN2C subunits in the cerebellum and the improved motor coordination of GluN2CKO mice after ketamine treatment suggest the involvement of cerebellar NMDA-Rs in some behavioral actions of ketamine.

Serotonin Transporter and Plasma Membrane Monoamine Transporter Are Necessary for the Antidepressant-Like Effects of Ketamine in Mice

Major depressive disorder is typically treated with selective serotonin reuptake inhibitors (SSRIs), however, SSRIs take approximately six weeks to produce therapeutic effects, if any. Not surprisingly, there has been great interest in findings that low doses of ketamine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, produce rapid and long-lasting antidepressant effects. Preclinical studies show that the antidepressant-like effects of ketamine are dependent upon availability of serotonin, and that ketamine increases extracellular serotonin, yet the mechanism by which this occurs is unknown. Here we examined the role of the high-affinity, low-capacity serotonin transporter (SERT), and the plasma membrane monoamine transporter (PMAT), a low-affinity, high-capacity transporter for serotonin, as mechanisms contributing to ketamine’s ability to increase extracellular serotonin and produce antidepressant-like effects. Using high-speed chronoamperometry to measure real-time clearance of serotonin from CA3 region of hippocampus in vivo, we found ketamine robustly inhibited serotonin clearance in wild-type mice, an effect that was lost in mice constitutively lacking SERT or PMAT. As expected, in wild-type mice, ketamine produced antidepressant-like effects in the forced swim test. Mapping onto our neurochemical findings, the antidepressant-like effects of ketamine were lost in mice lacking SERT or PMAT. Future research is needed to understand how constitutive loss of either SERT or PMAT, and compensation that occurs in other systems, is sufficient to void ketamine of its ability to inhibit serotonin clearance and produce antidepressant-like effects. Taken together with existing literature, a critical role for serotonin, and its inhibition of uptake via SERT and PMAT, cannot be ruled out as important contributing factors to ketamine’s antidepressant mechanism of action. Combined with what is already known about ketamine’s action at NMDA receptors, these studies help lead the way to the development of drugs that lack ketamine’s abuse potential but have superior efficacy in treating depression.

Brain NMDA Receptors in Schizophrenia and Depression

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP), dizocilpine (MK-801) and ketamine have long been considered a model of schizophrenia, both in animals and humans. However, ketamine has been recently approved for treatment-resistant depression, although with severe restrictions. Interestingly, the dosage in both conditions is similar, and positive symptoms of schizophrenia appear before antidepressant effects emerge. Here, we describe the temporal mechanisms implicated in schizophrenia-like and antidepressant-like effects of NMDA blockade in rats, and postulate that such effects may indicate that NMDA receptor antagonists induce similar mechanistic effects, and only the basal pre-drug state of the organism delimitates the overall outcome. Hence, blockade of NMDA receptors in depressive-like status can lead to amelioration or remission of symptoms, whereas healthy individuals develop psychotic symptoms and schizophrenia patients show an exacerbation of these symptoms after the administration of NMDA receptor antagonists.

A randomized placebo-controlled PET study of ketamine´s effect on serotonin(1B) receptor binding in patients with SSRI-resistant depression

The glutamate N-methyl-D-aspartate receptor antagonist ketamine has a rapid antidepressant effect. Despite large research efforts, ketamine's mechanism of action in major depressive disorder (MDD) has still not been determined. In rodents, the antidepressant properties of ketamine were found to be dependent on both the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and the serotonin (5-HT)_1B receptor. Low 5-HT_1B receptor binding in limbic brain regions is a replicated finding in MDD. In non-human primates, AMPA-dependent increase in 5-HT_1B receptor binding in the ventral striatum (VST) has been demonstrated after ketamine infusion. Thirty selective serotonin reuptake inhibitor-resistant MDD patients were recruited via advertisement and randomized to double-blind monotherapy with 0.5 mg/kg ketamine or placebo infusion. The patients were examined with the 5-HT_1B receptor selective radioligand [¹¹C]AZ10419369 and positron emission tomography (PET) before and 24-72 h after treatment. 5-HT_1B receptor binding did not significantly alter in patients treated with ketamine compared with placebo. An increase in 5-HT_1B receptor binding with 16.7 % (p = 0.036) was found in the hippocampus after one ketamine treatment. 5-HT_1B receptor binding in VST at baseline correlated with MDD symptom ratings (r = -0.426, p = 0.019) and with reduction of depressive symptoms with ketamine (r = -0.644, p = 0.002). In conclusion, reduction of depressive symptoms in MDD patients after ketamine treatment is correlated inversely with baseline 5-HT_1B receptor binding in VST. Further studies examining the role of 5-HT_1B receptors in the antidepressant mechanism of action of ketamine should be conducted, homing in on the 5-HT_1B receptor as an MDD treatment response marker.

Ketamine: The final frontier or another depressing end?

Two decades ago, the observation of a rapid and sustained antidepressant response after ketamine administration provided an exciting new avenue in the search for more effective therapeutics for the treatment of clinical depression. Research elucidating the mechanism(s) underlying ketamine's antidepressant properties has led to the development of several hypotheses, including that of disinhibition of excitatory glutamate neurons via blockade of N-methyl-d-aspartate (NMDA) receptors. Although the prominent understanding has been that ketamine's mode of action is mediated solely via the NMDA receptor, this view has been challenged by reports implicating other glutamate receptors such as AMPA, and other neurotransmitter systems such as serotonin and opioids in the antidepressant response. The recent approval of esketamine (Spravato™) for the treatment of depression has sparked a resurgence of interest for a deeper understanding of the mechanism(s) underlying ketamine's actions and safe therapeutic use. This review aims to present our current knowledge on both NMDA and non-NMDA mechanisms implicated in ketamine's response, and addresses the controversy surrounding the antidepressant role and potency of its stereoisomers and metabolites. There is much that remains to be known about our understanding of ketamine's antidepressant properties; and although the arrival of esketamine has been received with great enthusiasm, it is now more important than ever that its mechanisms of action be fully delineated, and both the short- and long-term neurobiological/functional consequences of its treatment be thoroughly characterized.