Astroglial Kir4.1 in the lateral habenula drives neuronal bursts in depression

Alcohol withdrawal drives depressive behaviors by activating neurons in the rostromedial tegmental nucleus

Rostromedial tegmental nucleus (RMTg) GABA neurons exert a primary inhibitory drive onto midbrain dopamine neurons and are excited by a variety of aversive stimuli. There is, however, little evidence that the RMTg-ventral tegmental area (VTA)-nucleus accumbens shell (Acb) circuit plays a role in the aversive consequences of alcohol withdrawal. This study was performed in adult male Long-Evans rats at 48-h withdrawal from chronic alcohol drinking in the intermittent schedule. These rats displayed clear anhedonia and depression-like behaviors, as measured with the sucrose preference, and forced swimming tests. These aberrant behaviors were accompanied by a substantial increase in cFos expression in the VTA-projecting RMTg neurons, identified by a combination of immunohistochemistry and retrograde-tracing techniques. Pharmacological or chemogenetic inhibition of RMTg neurons mitigated the anhedonia and depression-like behaviors. Ex vivo electrophysiological data showed that chemogenetic inactivation of RMTg neurons reduced GABA release and accelerated spontaneous firings of VTA dopamine neurons. Finally, using a functional hemispheric disconnection procedure, we demonstrated that inhibition of unilateral RMTg, when combined with activation of D1 and D2 dopamine receptors in the contralateral (but not ipsilateral) Acb, mitigated the anhedonia and depression-like behaviors in alcohol-withdrawal rats. These data show that the integrity in the RMTg-VTA-Acb pathway in a single hemisphere is sufficient to elicit depression-like behavior during ethanol-withdrawal. Overall, the present results reveal that the RMTg-VTA-Acb pathway plays a crucial role in the depression-like behavior in animals undergoing alcohol withdrawal, further advocating the RMTg as a potential therapeutic target for alcoholism.

Pharmacological stimulation of sigma-1 receptor promotes activation of astrocyte via ERK1/2 and GSK3β signaling pathway

Astrocyte is considered to be a type of passive supportive cells that preserves neuronal activity and survival. The dysfunction of astrocytes is involved in the pathological processes of major depression. Recent studies implicate sigma-1 receptors as putative therapeutic targets for current available antidepressant drugs. However, it is absent of direct evidences whether sigma-1 receptor could promote activation of astrocyte. In the present study, we took advantage of primary astrocyte culture and a highly selective agonist of sigma-1 receptor, (+)SKF-10047 to determine the effect of sigma-1 receptor on Brdu (bromodeoxyuridine) labeling positive cells, migration as well as GFAP (glial fibrillary acidic protein) expression of astrocyte. The results showed that (+)SKF-10047 notably increased the number of Brdu labeling positive cells, migration, and the expression of GFAP in primary astrocytes, which were blocked by antagonist of sigma-1 receptor. Moreover, we also found that (+)SKF-10047 increased the phosphorylation of ERK1/2 (extracellular signal-regulated kinases 1/2) and GSK3β (glycogen synthase kinase 3β) (ser 9) in the primary astrocytes. In addition, pharmacological inhibition of ERK1/2 and GSK3β (ser 9) abolished sigma-1 receptor-promoted activation of astrocyte. Therefore, sigma-1 receptor could be considerate as a new pattern for modulating astrocytic function might emerge as therapeutic strategies.

Immune mechanisms of stress susceptibility and resilience: Lessons from animal models

Stress has an impact on the brain and the body. A growing literature demonstrates that feedback between the peripheral immune system and the brain contributes to individual differences in the behavioral response to stress. Here we examine preclinical literature to demonstrate a holistic vision of risk and resilience to stress. We identify a variety of cellular, cytokine and molecular mechanisms in adult animals that act in concert to produce a stress susceptible individual response. We discuss how cross talk between immune cells in the brain and in the periphery act together to increase permeability across the blood brain barrier or block it, resulting in susceptible or stress resilient phenotype. These preclinical studies have importance for understanding how individual differences in the immune response to stress may be contributing to mood related disorders such as depression, anxiety and posttraumatic stress disorders.

Glia Accumulate Evidence that Actions Are Futile and Suppress Unsuccessful Behavior

When a behavior repeatedly fails to achieve its goal, animals often give up and become passive, which can be strategic for preserving energy or regrouping between attempts. It is unknown how the brain identifies behavioral failures and mediates this behavioral-state switch. In larval zebrafish swimming in virtual reality, visual feedback can be withheld so that swim attempts fail to trigger expected visual flow. After tens of seconds of such motor futility, animals became passive for similar durations. Whole-brain calcium imaging revealed noradrenergic neurons that responded specifically to failed swim attempts and radial astrocytes whose calcium levels accumulated with increasing numbers of failed attempts. Using cell ablation and optogenetic or chemogenetic activation, we found that noradrenergic neurons progressively activated brainstem radial astrocytes, which then suppressed swimming. Thus, radial astrocytes perform a computation critical for behavior: they accumulate evidence that current actions are ineffective and consequently drive changes in behavioral states. VIDEO ABSTRACT.

Sonic hedgehog signaling in astrocytes mediates cell type-specific synaptic organization

Astrocytes have emerged as integral partners with neurons in regulating synapse formation and function, but the mechanisms that mediate these interactions are not well understood. Here, we show that Sonic hedgehog (Shh) signaling in mature astrocytes is required for establishing structural organization and remodeling of cortical synapses in a cell type-specific manner. In the postnatal cortex, Shh signaling is active in a subpopulation of mature astrocytes localized primarily in deep cortical layers. Selective disruption of Shh signaling in astrocytes produces a dramatic increase in synapse number specifically on layer V apical dendrites that emerges during adolescence and persists into adulthood. Dynamic turnover of dendritic spines is impaired in mutant mice and is accompanied by an increase in neuronal excitability and a reduction of the glial-specific, inward-rectifying K⁺ channel Kir4.1. These data identify a critical role for Shh signaling in astrocyte-mediated modulation of neuronal activity required for sculpting synapses.

A central system of neurons in the spinal cord and brain coordinate most of our body’s actions, ranging from regulating our heart rate to controlling our movement and thoughts. As the brain develops, neurons form specialized contacts with one another known as synapses. If the number of synapses is not properly regulated this can disrupt communication between the neurons, leading to diseases like schizophrenia and autism.

As the brain develops, it first forms an excess of synapses and later eliminates unnecessary or weak connections. Various factors, such gene expression or a neuron’s level of activity, regulate this turnover process. However, neurons cannot do this alone, and rely on other types of cells to help regulate their behavior. In the central nervous system, for example, a cell called an astrocyte is known to support the formation and activity of synapses. Now, Hill and Blaeser et al. show that astrocytes also exert influence over synaptic turnover during development, leading to long lasting changes in the number of synapses.

Hill, Blaeser et al. revealed that disrupting activity of the signaling pathway known as Sonic hedgehog, or Shh for short, in the astrocytes of mice led to disordered synaptic connections. Notably, neurons produce Shh, suggesting that neurons use this signaling pathway to communicate to specific astrocyte partners. Further experiments showed that reducing astrocyte’s ability to respond to Shh impaired synaptic turnover as the brain developed, leading to an overabundance of synapses. Importantly, these effects were only found to influence neuron populations associated with astrocytes that actively use Shh signaling. This suggests that distinct populations of neurons and astrocytes interact in specialized ways to build the connections within the nervous system.

To address how astrocytes use Shh signaling to regulate synaptic turnover, Hill, Blaeser et al. examined gene expression changes in astrocytes that lack Shh signaling. Astrocytes with a reduced capacity to respond to Shh were found to have lower levels of a protein responsible for transporting potassium ions into and out of the cell. This impairs astrocyte’s ability to regulate neuronal activity, which may lead to a failure in eliminating unnecessary synapses.

Understanding how synapses are controlled and organized by astrocytes could help identify new ways to treat diseases of the developing nervous system. However, further studies would be needed to improve our understanding of how this process works.

Stress transforms lateral habenula reward responses into punishment signals

Neuronal activity in the lateral habenula (LHb), a brain region implicated in depression [C. D. Proulx, O. Hikosaka, R. Malinow, Nat. Neurosci. 17, 1146-1152 (2014)], decreases during reward and increases during punishment or reward omission [M. Matsumoto, O. Hikosaka, Nature 447, 1111-1115 (2007)]. While stress is a major risk factor for depression and strongly impacts the LHb, its effect on LHb reward signals is unknown. Here we image LHb neuronal activity in behaving mice and find that acute stress transforms LHb reward responses into punishment-like neural signals; punishment-like responses to reward omission also increase. These neural changes matched the onset of anhedonic behavior and were specific to LHb neurons that distinguished reward and its omission. Thus, stress distorts LHb responsivity to positive and negative feedback, which could bias individuals toward negative expectations, a key aspect of the proposed pathogenesis of depression [A. T. Beck, Depression: Clinical, Experimental, and Theoretical Aspects, sixth Ed (1967)].

[The roles of habenula and related neural circuits in neuropsychiatric diseases]

The habenula is a small and bilateral nucleus above dorsal thalamus, which contains several different types of neurons. The habenula has extensive connections with the forebrain, septum and monoaminergic nuclei in the midbrain and brainstem. Habenula is known as an 'anti-reward' nucleus, which can be activated by aversive stimulus and negative reward prediction errors. Accumulating researchs have implicated that the habenula is involved in several behaviors crucial to survival. Meanwhile, the roles of the habenula in neuropsychiatric diseases have received increasing attention. This review summaries the studies regarding the roles of habenula and the related circuits in neuropathic pain, depression, drug addiction and schizophrenia, and discusses the possibility to use the habenula as a treatment target.

Hyperactivity with Disrupted Attention by Activation of an Astrocyte Synaptogenic Cue

Hyperactivity and disturbances of attention are common behavioral disorders whose underlying cellular and neural circuit causes are not understood. We report the discovery that striatal astrocytes drive such phenotypes through a hitherto unknown synaptic mechanism. We found that striatal medium spiny neurons (MSNs) triggered astrocyte signaling via γ-aminobutyric acid B (GABAB) receptors. Selective chemogenetic activation of this pathway in striatal astrocytes in vivo resulted in acute behavioral hyperactivity and disrupted attention. Such responses also resulted in upregulation of the synaptogenic cue thrombospondin-1 (TSP1) in astrocytes, increased excitatory synapses, enhanced corticostriatal synaptic transmission, and increased MSN action potential firing in vivo. All of these changes were reversed by blocking TSP1 effects. Our data identify a form of bidirectional neuron-astrocyte communication and demonstrate that acute reactivation of a single latent astrocyte synaptogenic cue alters striatal circuits controlling behavior, revealing astrocytes and the TSP1 pathway as therapeutic targets in hyperactivity, attention deficit, and related psychiatric disorders.

Hypoactivity of the lateral habenula contributes to negative symptoms and cognitive dysfunction of schizophrenia in rats

Dopaminergic (DAergic) hypofunction in the medial prefrontal cortex (mPFC) has been implicated in the negative and cognitive symptoms of schizophrenia and is regulated by serotonergic (5-HTergic) neurons in the dorsal raphe nucleus (DRN). The lateral habenula (LHb) is a key element in controlling DRN 5-HT neurons. We investigated how the LHb impacts the activity of mPFC neurons and whether it mediates the involvement of DRN on development of symptoms in a pharmacological animal model of schizophrenia. We used immunohisochemistry to assess cytochrome-c oxidase (COX) activity of the LHb in MK-801 model rats and extracellular firing recording to compare firing rates in LHb neurons of acute MK-801-treated rats. The sucrose preference, social interaction, and radial arm maze tests were used to access schizophrenia-like behavior in rats with electrolytically lesioned LHb. Finally, we examined levels of the dopamine D1 receptor (D1R) and tyrosine hydroxylase (TH) in the mPFC, and tryptophan hydroxylase 2 (TPH2) in the DRN of rats with LHb lesions to determine the possible mechanism underlying the schizophrenia-like behavior associated with lesioned LHb. We found that COX levels and LHb neuron firing rates decreased significantly in MK-801-treated animals. The LHb lesions induced negative and cognitive, but not positive symptoms of schizophrenia. The D1R and TH levels decreased in the mPFC while TPH2 expression elevated in the DRN and mPFC of LHb-lesioned rats. These results suggest that LHb hypoactivity may contribute to the negative and cognitive symptoms of schizophrenia by downregulating D1R expression in the mPFC, which might be mediated by DRN 5-HT neurons.

Neuronal Dynamics Regulating Brain and Behavioral State Transitions

Prolonged behavioral challenges can cause animals to switch from active to passive coping strategies to manage effort-expenditure during stress; such normally adaptive behavioral state transitions can become maladaptive in psychiatric disorders such as depression. The underlying neuronal dynamics and brainwide interactions important for passive coping have remained unclear. Here, we develop a paradigm to study these behavioral state transitions at cellular-resolution across the entire vertebrate brain. Using brainwide imaging in zebrafish, we observed that the transition to passive coping is manifested by progressive activation of neurons in the ventral (lateral) habenula. Activation of these ventral-habenula neurons suppressed downstream neurons in the serotonergic raphe nucleus and caused behavioral passivity, whereas inhibition of these neurons prevented passivity. Data-driven recurrent neural network modeling pointed to altered intra-habenula interactions as a contributory mechanism. These results demonstrate ongoing encoding of experience features in the habenula, which guides recruitment of downstream networks and imposes a passive coping behavioral strategy.

Glial Ca2+signaling links endocytosis to K+ buffering around neuronal somas to regulate excitability

Glial-neuronal signaling at synapses is widely studied, but how glia interact with neuronal somas to regulate their activity is unclear. Drosophila cortex glia are restricted to brain regions devoid of synapses, providing an opportunity to characterize interactions with neuronal somas. Mutations in the cortex glial NCKX^zydeco elevate basal Ca²⁺, predisposing animals to seizure-like behavior. To determine how cortex glial Ca²⁺ signaling controls neuronal excitability, we performed an in vivo modifier screen of the NCKX^zydeco seizure phenotype. We show that elevation of glial Ca²⁺ causes hyperactivation of calcineurin-dependent endocytosis and accumulation of early endosomes. Knockdown of sandman, a K_2P channel, recapitulates NCKX^zydeco seizures. Indeed, sandman expression on cortex glial membranes is substantially reduced in NCKX^zydeco mutants, indicating enhanced internalization of sandman predisposes animals to seizures. These data provide an unexpected link between glial Ca²⁺ signaling and the well-known role of glia in K⁺ buffering as a key mechanism for regulating neuronal excitability.

Lateral Habenular Burst Firing as a Target of the Rapid Antidepressant Effects of Ketamine

The revolutionary discovery of the rapid antidepressant ketamine has been a milestone in psychiatry field in the last half century. Unlike conventional antidepressants that often take weeks to months to show efficacy, ketamine causes rapid antidepressant effects, emerging as early as within 1h after administration. However, how ketamine improves mood symptoms so quickly has remained elusive. Here, we first introduce the historical background of ketamine as a rapid antidepressant. We then discuss current hypotheses underlying ketamine's rapid antidepressant effects, with a focus on our latest discovery that ketamine silences NMDAR-dependent burst firing in the 'anti-reward center', the lateral habenula. While ketamine may act on many brain regions, we argue that its rapid antidepressant effects are critically dependent on ketamine's action in the lateral habenula, with this brain region acting as a primary site of action (or one among a few primary nodes). This molecular-, cellular-, and circuit-based mechanism advances our understanding of the etiology of depression and suggests a new conceptual framework for the rapid antidepressant effects of ketamine.

Astrocyte morphology: Diversity, plasticity, and role in neurological diseases

Astrocytes are the most abundant glial cells in the central nervous system (CNS) and participate in synaptic, circuit, and behavioral functions. The well‐developed protoplasmic astrocytes contain numerous processes forming well‐delineated bushy territories that overlap by as little as 5% at their boundaries. This highly complex morphology, with up to approximately 80% of the cell's membrane constituted by fine processes with dimensions on the tens of nanometer scale and high surface area to volume ratios, comes in contact with synapses, blood vessels, and other glial cells. Recent progress is challenging the conventional view that astrocytes are morphologically homogeneous throughout the brain; instead, they display circuit‐ and region‐specific morphological diversity that may contribute to the heterogeneous astrocyte‐neuron spatiotemporal interplay in different brain areas. Further, the fine structure of astrocytes is found to be highly plastic and activity‐dependent. We are beginning to understand how astrocyte structural plasticity contributes to brain functions. The change/loss of astrocyte morphology, traditionally known as a hallmark for reactive astrogliosis, is a common pathological feature in many neurological disorders. However, recent data suggest the fine structural deficits preceding reactive astrogliosis may drive disease progression. This review summarizes recent advances in astrocyte morphological diversity, plasticity, and disease‐related deficits.

Inhibition of acid-sensing ion channels reduces the hypothalamus-pituitary-adrenal axis activity and ameliorates depression-like behavior in rats

Depression is the leading cause of disability worldwide, and its treatment represents a major clinical challenge. The hypothalamus–pituitary–adrenal (HPA) axis has been known to play a crucial role in depression and serves as a target for antidepressants. Acid-sensing ion channels (ASICs) are widely expressed in the nervous system and may be implicated in depression. Whether ASICs could act on the HPA axis to affect depression-related behaviors is not fully understood. In this study, we investigated the effect of inhibition of ASICs on the HPA axis activity in chronic stress-subjected rats. We found that treatment with the ASIC selective antagonist amiloride reversed chronic stress-induced elevation of adrenocorticotropic hormone (ACTH) and corticosterone in serum, which is reflective of the HPA axis activity. In addition, amiloride also alleviated chronic stress-induced anhedonia-like behavior. These results suggest that inhibition of ASICs may act on the HPA axis to alleviate the symptoms of depression.

Depression is the leading cause of disability worldwide, and its treatment represents a major clinical challenge.

Valence-encoding in the lateral habenula arises from the entopeduncular region

Lateral habenula (LHb) neurons are activated by negative motivational stimuli and play key roles in the pathophysiology of depression. Prior reports suggested that rostral entopeduncular nucleus (rEPN) neurons drive these responses in the LHb and rostromedial tegmental nucleus (RMTg), but these influences remain untested. Using rabies viral tracers, we demonstrate disynaptic projections from the rEPN to RMTg, but not VTA, via the LHb in rats. Using in vivo electrophysiology, we find that rEPN or LHb subpopulations exhibit activation/inhibition patterns after negative/positive motivational stimuli, similar to the RMTg, while temporary inactivation of a region centered on the rEPN decreases LHb basal and burst firing, and reduces valence-related signals in LHb neurons. Additionally, excitotoxic rEPN lesions partly diminish footshock-induced cFos in the LHb and RMTg. Together, our findings indicate an important role of the rEPN, and possibly immediately adjacent hypothalamus, in driving basal activities and valence processing in LHb and RMTg neurons.

Downregulation of M-channels in lateral habenula mediates hyperalgesia during alcohol withdrawal in rats

Hyperalgesia often occurs in alcoholics, especially during abstinence, yet the underlying mechanisms remain elusive. The lateral habenula (LHb) has been implicated in the pathophysiology of pain and alcohol use disorders. Suppression of m-type potassium channels (M-channels) has been found to contribute to the hyperactivity of LHb neurons of rats withdrawn from chronic alcohol administration. Here, we provided evidence that LHb M-channels may contribute to hyperalgesia. Compared to alcohol naïve counterparts, in male Long-Evans rats at 24-hours withdrawal from alcohol administration under the intermittent access paradigm for eight weeks, hyperalgesia was evident (as measured by paw withdrawal latencies in the Hargreaves Test), which was accompanied with higher basal activities of LHb neurons in brain slices, and lower M-channel protein expression. Inhibition of LHb neurons by chemogenetics, or pharmacological activation of M-channels, as well as overexpression of M-channels' subunit KCNQ3, relieved hyperalgesia and decreased relapse-like alcohol consumption. In contrast, chemogenetic activation of LHb neurons induced hyperalgesia in alcohol-naive rats. These data reveal a central role for the LHb in hyperalgesia during alcohol withdrawal, which may be due in part to the suppression of M-channels and, thus, highlights M-channels in the LHb as a potential therapeutic target for hyperalgesia in alcoholics.

Astroglial Mechanisms of Ketamine Action Include Reduced Mobility of Kir4.1-Carrying Vesicles

The finding that ketamine, an anaesthetic, can elicit a rapid antidepressant effect at low doses that lasts for weeks in patients with depression is arguably a major achievement in psychiatry in the last decades. However, the mechanisms of action are unclear. The glutamatergic hypothesis of ketamine action posits that ketamine is a N-methyl-D-aspartate receptor (NMDAR) antagonist modulating downstream cytoplasmic events in neurons. In addition to targeting NMDARs in synaptic transmission, ketamine may modulate the function of astroglia, key homeostasis-providing cells in the central nervous system, also playing a role in many neurologic diseases including depression, which affects to 20% of the population globally. We first review studies on astroglia revealing that (sub)anaesthetic doses of ketamine attenuate stimulus-evoked calcium signalling, a process of astroglial cytoplasmic excitability, regulating the exocytotic release of gliosignalling molecules. Then we address how ketamine alters the fusion pore activity of secretory vesicles, and how ketamine affects extracellular glutamate and K⁺ homeostasis, both considered pivotal in depression. Finally, we also provide evidence indicating reduced cytoplasmic mobility of astroglial vesicles carrying the inward rectifying potassium channel (Kir4.1), which may regulate the density of Kir4.1 at the plasma membrane. These results indicate that the astroglial capacity to control extracellular K⁺ concentration may be altered by ketamine and thus indirectly affect the action potential firing of neurons, as is the case in lateral habenula in a rat disease model of depression. Hence, ketamine-altered functions of astroglia extend beyond neuronal NMDAR antagonism and provide a basis for its antidepressant action through glia.

A Visual Circuit Related to Habenula Underlies the Antidepressive Effects of Light Therapy

Light plays a pivotal role in the regulation of affective behaviors. However, the precise circuits that mediate the impact of light on depressive-like behaviors are not well understood. Here, we show that light influences depressive-like behaviors through a disynaptic circuit linking the retina and the lateral habenula (LHb). Specifically, M4-type melanopsin-expressing retinal ganglion cells (RGCs) innervate GABA neurons in the thalamic ventral lateral geniculate nucleus and intergeniculate leaflet (vLGN/IGL), which in turn inhibit CaMKIIα neurons in the LHb. Specific activation of vLGN/IGL-projecting RGCs, activation of LHb-projecting vLGN/IGL neurons, or inhibition of postsynaptic LHb neurons is sufficient to decrease the depressive-like behaviors evoked by long-term exposure to aversive stimuli or chronic social defeat stress. Furthermore, we demonstrate that the antidepressive effects of light therapy require activation of the retina-vLGN/IGL-LHb pathway. These results reveal a dedicated retina-vLGN/IGL-LHb circuit that regulates depressive-like behaviors and provide a potential mechanistic explanation for light treatment of depression.

Increased expression of inwardly rectifying Kir4.1 channel in the parietal cortex from patients with major depressive disorder

BACKGROUND

The inwardly rectifying K⁺ channel subtype Kir4.1 has been well studied in the astrocyte within brain; however, the precise role of this protein in psychiatric disorders is unknown. Kir4.1 is also known to interact with GABA_B receptors which may be implicated in psychiatric disorders. Here we studied whether expression of Kir4.1 and GABA_B receptors was altered in the postmortem brain samples (parietal cortex and cerebellum) from patients with major psychiatric disorders.

METHODS

Protein expression of Kir4.1 and GABA_B receptors in the parietal cortex and cerebellum from control, major depressive disorder (MDD), schizophrenia (SZ), and bipolar disorder (BD) groups was measured.

RESULTS

Levels of Kir4.1 in the parietal cortex from MDD group, but not SZ and BD groups, were significantly higher than the control group. Furthermore, levels of GABA_B receptor subunit 1 in the parietal cortex from MDD group and SZ group, but not BD group, were also significantly higher than the control group. Interestingly, there was a positive correlation between Kir4.1 protein and GABA_B receptor subunit 1 in the parietal cortex from control group, but not MDD group.

LIMITATIONS

The small number in each group may limit our interpretation. Only two brain regions were analyzed.

CONCLUSIONS

Abnormalities in the interaction of Kir4.1 and GABA_B receptor in the parietal cortex might play a role in the pathophysiology of MDD.

The Astrocytic cAMP Pathway in Health and Disease

Astrocytes are major glial cells that play critical roles in brain homeostasis. Abnormalities in astrocytic functions can lead to brain disorders. Astrocytes also respond to injury and disease through gliosis and immune activation, which can be both protective and detrimental. Thus, it is essential to elucidate the function of astrocytes in order to understand the physiology of the brain to develop therapeutic strategies against brain diseases. Cyclic adenosine monophosphate (cAMP) is a major second messenger that triggers various downstream cellular machinery in a wide variety of cells. The functions of astrocytes have also been suggested as being regulated by cAMP. Here, we summarize the possible roles of cAMP signaling in regulating the functions of astrocytes. Specifically, we introduce the ways in which cAMP pathways are involved in astrocyte functions, including (1) energy supply, (2) maintenance of the extracellular environment, (3) immune response, and (4) a potential role as a provider of trophic factors, and we discuss how these cAMP-regulated processes can affect brain functions in health and disease.

Decoding Depression: Insights from Glial and Ketamine Regulation of Neuronal Burst Firing in Lateral Habenula

The rapid antidepressant effect of ketamine is arguably one of the most significant advances in the mental health field in the last half century. However, its mechanism of action has remained elusive. Here, we describe our latest discovery on how ketamine blocks N-methyl-D-aspartate receptor (NMDAR)-dependent burst firing of an "antireward" center in the brain, the lateral habenula (LHb), to mediate its antidepressant effects. We also discuss a novel structure-function mechanism at the glia-neuron interface to account for the enhanced LHb bursting during depression. These results reveal new molecular targets for the therapeutic intervention of major depression.

A Major Role for the Lateral Habenula in Depressive Illness: Physiologic and Molecular Mechanisms

Emerging preclinical and clinical evidence indicate that the lateral habenula plays a major role in the pathophysiology of depressive illness. Aberrant increases in neuronal activity in the lateral habenula, an anti-reward center, signals down-regulation of brainstem dopaminergic and serotonergic firing, leading to anhedonia, helplessness, excessive focus on negative experiences, and, hence, depressive symptomatology. The lateral habenula has distinctive regulatory adaptive role to stress regulation in part due to its bidirectional connectivity with the hypothalamic-pituitary-adrenal (HPA) axis. In addition, studies show that increased lateral habenula activity affects components of sleep regulation including slow wave activity and rapid eye movement (REM), both disrupted in depressive illness. Lack of perceived reward experienced during the adverse outcomes also precipitates lateral habenula firing, while outcomes that meet or exceed expectations decrease lateral habenula firing and, in turn, increase midbrain dopaminergic and serotonergic neurotransmission. The ability to update expectations of the environment based on rewards and aversive stimuli reflects a potentially important survival mechanism relevant to the capacity to adapt to changing circumstances. What if one lives in a continuously aversive and invalidating environment or under the conditions of chronic stress? If there is a propensity of the habenula to release many burst discharges over time, an individual could habitually come to perceive the world as perpetually disappointing. Conceivably, the lateral habenula could learn to expect an adverse outcome systematically and communicate it more easily. Thus, if the lateral habenula fires more frequently, it may lead to a state of continuous disappointment and hopelessness, akin to depression. Furthermore, postmortem studies reveal that the size of the lateral habenula and total number of neurons are decreased in patients who had depressive illness. Novel research in the field shows that ketamine induces rapid and sustained antidepressant effect. Intriguingly, recent preclinical animal models show that ketamine abolishes N-methyl-D-aspartate receptor (NMDAR)-dependent lateral habenula bursting activity, leading to rapid resolution of depressive symptoms.

Ventral midbrain astrocytes display unique physiological features and sensitivity to dopamine D2 receptor signaling

Astrocytes are ubiquitous CNS cells that support tissue homeostasis through ion buffering, neurotransmitter recycling, and regulation of CNS vasculature. Yet, despite the essential functional roles they fill, very little is known about the physiology of astrocytes in the ventral midbrain, a region that houses dopamine-releasing neurons and is critical for reward learning and motivated behaviors. Here, using a combination of whole-transcriptome sequencing, histology, slice electrophysiology, and calcium imaging, we performed the first functional and molecular profiling of ventral midbrain astrocytes and observed numerous differences between these cells and their telencephalic counterparts, both in their gene expression profile and in their physiological properties. Ventral midbrain astrocytes have very low membrane resistance and inward-rectifying potassium channel-mediated current, and are extensively coupled to surrounding oligodendrocytes through gap junctions. They exhibit calcium responses to glutamate but are relatively insensitive to norepinephrine. In addition, their calcium activity can be dynamically modulated by dopamine D2 receptor signaling. Taken together, these data indicate that ventral midbrain astrocytes are physiologically distinct from astrocytes in cortex and hippocampus. This work provides new insights into the extent of functional astrocyte heterogeneity within the adult brain and establishes the foundation for examining the impact of regional astrocyte differences on dopamine neuron function and susceptibility to degeneration.

Inflaming sex differences in mood disorders

Men and women often experience different symptoms or rates of occurrence for a variety of mood disorders. Many of the symptoms of mood disorders overlap with autoimmune disorders, which also have a higher prevalence in women. There is a growing interest in exploring the immune system to provide biomarkers for diagnosis of mood disorders, along with new targets for developing treatments. This review examines known sex differences in the immune system and their relationship to mood disorders. We focus on immune alterations associated with unipolar depression, bipolar depression, and anxiety disorders. We describe work from both basic and clinical research examining potential immune mechanisms thought to contribute to stress susceptibility and associated mood disorders. We propose that sex and age are important, intertwined factors that need to be included in future experimental designs if we are going to harness the power of the immune system to develop a new wave of treatments for mood disorders.

Hippocampal CA1 βCaMKII mediates neuroinflammatory responses via COX-2/PGE2 signaling pathways in depression

Background

Neuroinflammation has recently emerged as a critical risk factor in the pathophysiology of depression. However, the underlying molecular mechanisms and the development of novel therapeutic strategies as means to target these inflammatory pathways for use in the treatment of depression remain unresolved. In the present study, we aimed to investigate the molecular events of neuroinflammation as related to its induction of depression-like behaviors.

Methods

Chronic unpredictable mild stress (CUMS) or lipopolysaccharide (LPS) was used to induce depression-like behaviors in rats. The inflammatory factors and related proteins were verified by RT-PCR, immunoblotting, and immunofluorescence assay. In vivo intracerebral injection of adenovirus-associated virus (AAV) in rats was used to overexpress or block the function of the β form of the calcium/calmodulin-dependent protein kinase type II (βCaMKII). In vivo intracerebroventricular injection of SB203580 was used to block p38 mitogen-activated protein kinase (MAPK). Finally, the prostaglandin E2 (PGE2) concentration was verified by using enzyme-linked assay kit.

Results

The expression of cyclo-oxygenase (COX)-2, which is responsible for production of the pro-inflammatory factor PGE2 and thus glial activation, was increased in the CA1 hippocampus in a rat model of depression. Further, the βCaMKII in CA1 was significantly upregulated in depressed rats, while antidepressant treatment downregulated this kinase. Overexpression of βCaMKII via infusion of a constructed AAV-βCaMKII into the CA1 region resulted in phosphorylation of the p38 MAPK and the activating transcription factor 2 (ATF2). These effects were accompanied by an enhanced activity of the COX-2/PGE2 pathway and effectively induced core symptoms of depression. Conversely, knockdown of βCaMKII within the CA1 region reversed these inflammation-related biochemical parameters and significantly rescued depression symptoms.

Conclusion

These results demonstrate that βCaMKII can act as a critical regulator in depression via activating neuroinflammatory pathways within the CA1 region. Moreover, this study provides new perspectives on molecular targets and drug therapies for future investigation in the treatment of depression.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1377-0) contains supplementary material, which is available to authorized users.

Dysregulation of the Lateral Habenula in Major Depressive Disorder

Clinical and preclinical evidence implicates hyperexcitability of the lateral habenula (LHb) in the development of psychiatric disorders including major depressive disorder (MDD). This discrete epithalamic nucleus acts as a relay hub linking forebrain limbic structures with midbrain aminergic centers. Central to reward processing, learning and goal directed behavior, the LHb has emerged as a critical regulator of the behaviors that are impaired in depression. Stress-induced activation of the LHb produces depressive- and anxiety-like behaviors, anhedonia and aversion in preclinical studies. Moreover, deep brain stimulation of the LHb in humans has been shown to alleviate chronic unremitting depression in treatment resistant depression. The diverse neurochemical processes arising in the LHb that underscore the emergence and treatment of MDD are considered in this review, including recent optogenetic studies that probe the anatomical connections of the LHb.

Inwardly Rectifying Potassium Channel Kir4.1 as a Novel Modulator of BDNF Expression in Astrocytes

Brain-derived neurotrophic factor (BDNF) is a key molecule essential for neural plasticity and development, and is implicated in the pathophysiology of various central nervous system (CNS) disorders. It is now documented that BDNF is synthesized not only in neurons, but also in astrocytes which actively regulate neuronal activities by forming tripartite synapses. Inwardly rectifying potassium (Kir) channel subunit Kir4.1, which is specifically expressed in astrocytes, constructs Kir4.1 and Kir4.1/5.1 channels, and mediates the spatial potassium (K⁺) buffering action of astrocytes. Recent evidence illustrates that Kir4.1 channels play important roles in bringing about the actions of antidepressant drugs and modulating BDNF expression in astrocytes. Although the precise mechanisms remain to be clarified, it seems likely that inhibition (down-regulation or blockade) of astrocytic Kir4.1 channels attenuates K⁺ buffering, increases neuronal excitability by elevating extracellular K⁺ and glutamate, and facilitates BDNF expression. Conversely, activation (up-regulation or opening) of Kir4.1 channels reduces neuronal excitability by lowering extracellular K⁺ and glutamate, and attenuates BDNF expression. Particularly, the former pathophysiological alterations seem to be important in epileptogenesis and pain sensitization, and the latter in the pathogenesis of depressive disorders. In this article, we review the functions of Kir4.1 channels, with a focus on their regulation of spatial K⁺ buffering and BDNF expression in astrocytes, and discuss the role of the astrocytic Kir4.1-BDNF system in modulating CNS disorders.

The roles of astrocyte in the brain pathologies following ischemic stroke

Aim: In this work, we systematically explored the physiological functions of astrocytes and their roles following ischemic stroke, additionally, the potential therapy strategy targeting the astrocytes was also discussed. Methods: This work searched the PubMed database (including MEDLINE) until 14 Feb 2018, and furthermore, the studies were identified through cross-referencing and by consulting the experts in this field. Results: This study indicated that the astrocytes can not only play harmful roles following ischemic stroke through release of inflammatory factors and formation of glial scar but also have protective effects through quenching glutamate excitotoxicity and maintaining the clearance function of glymphatic system in brain. Conclusion: Owing to their important roles in physiological functions of brain and in the pathological conditions following ischemic stroke, the astrocytes might be a potential but promising therapeutic target for treating the ischemic stroke in the future.

Glia as architects of central nervous system formation and function

Glia constitute roughly half of the cells of the central nervous system (CNS) but were long-considered to be static bystanders to its formation and function. Here we provide an overview of how the diverse and dynamic functions of glial cells orchestrate essentially all aspects of nervous system formation and function. Radial glia, astrocytes, oligodendrocyte progenitor cells, oligodendrocytes, and microglia each influence nervous system development, from neuronal birth, migration, axon specification, and growth through circuit assembly and synaptogenesis. As neural circuits mature, distinct glia fulfill key roles in synaptic communication, plasticity, homeostasis, and network-level activity through dynamic monitoring and alteration of CNS structure and function. Continued elucidation of glial cell biology, and the dynamic interactions of neurons and glia, will enrich our understanding of nervous system formation, health, and function.

Oligodendrocyte-encoded Kir4.1 function is required for axonal integrity

Glial support is critical for normal axon function and can become dysregulated in white matter (WM) disease. In humans, loss-of-function mutations of KCNJ10, which encodes the inward-rectifying potassium channel KIR4.1, causes seizures and progressive neurological decline. We investigated Kir4.1 functions in oligodendrocytes (OLs) during development, adulthood and after WM injury. We observed that Kir4.1 channels localized to perinodal areas and the inner myelin tongue, suggesting roles in juxta-axonal K⁺ removal. Conditional knockout (cKO) of OL-Kcnj10 resulted in late onset mitochondrial damage and axonal degeneration. This was accompanied by neuronal loss and neuro-axonal dysfunction in adult OL-Kcnj10 cKO mice as shown by delayed visual evoked potentials, inner retinal thinning and progressive motor deficits. Axon pathologies in OL-Kcnj10 cKO were exacerbated after WM injury in the spinal cord. Our findings point towards a critical role of OL-Kir4.1 for long-term maintenance of axonal function and integrity during adulthood and after WM injury.

Cocaine- and Amphetamine-Regulated Transcript (CART) Peptide Plays Critical Role in Psychostimulant-Induced Depression

Cocaine- and amphetamine-regulated transcript (CART) peptide is a widely distributed neurotransmitter expressed in the central nervous systems. Previously, several reports demonstrated that nucleus accumbal-injected CART peptide positively modulated behavioral sensitization induced by psychostimulants and regulated the mesocorticolimbic dopaminergic pathway. It is confirmed that CART peptide exerted inhibitory effect on psychostimulant-enhanced dopamine receptors signaling, Ca²⁺/calmodulin-dependent kinase signaling and crucial transcription factors expression. Besides modulation of dopamine receptors-related pathways, CART peptide also exhibited elaborated interactions with other neurotransmitter receptors, such as glutamate receptors and γ-aminobutyric acid receptors, which further account for attribution of CART peptide to inhibition of psychostimulant-potentiated locomotor activity. Recently, CART peptide has been shown to have anxiolytic functions on the aversive mood and uncontrolled drug-seeking behaviors following drug withdrawal. Moreover, microinjection of CART peptide has been shown to have an anti-depressant effect, which suggests its potential utility in the mood regulation and avoidance of depression-like behaviors. In this review, we discuss CART pathways in neural circuits and their interactions with neurotransmitters associated with psychostimulant-induced depression.

Distal tubule basolateral potassium channels: cellular and molecular mechanisms of regulation

PURPOSE OF REVIEW

Multiple clinical and translational evidence support benefits of high potassium diet; however, there many uncertainties underlying the molecular and cellular mechanisms determining effects of dietary potassium. Kir4.1 and Kir5.1 proteins form a functional heteromer (Kir4.1/Kir5.1), which is the primary inwardly rectifying potassium channel on the basolateral membrane of both distal convoluted tubule (DCT) and the collecting duct principal cells. The purpose of this mini-review is to summarize latest advances in our understanding of the evolution, physiological relevance and mechanisms controlling these channels.

RECENT FINDINGS

Kir4.1 and Kir5.1 channels play a critical role in determining electrolyte homeostasis in the kidney and blood pressure, respectively. It was reported that Kir4.1/Kir5.1 serves as potassium sensors in the distal nephron responding to variations in dietary intake and hormonal stimuli. Global and kidney specific knockouts of either channel resulted in hypokalemia and severe cardiorenal phenotypes. Furthermore, knock out of Kir5.1 in Dahl salt-sensitive rat background revealed the crucial role of the Kir4.1/Kir5.1 channel in salt-induced hypertension.

SUMMARY

Here, we focus on reviewing novel experimental evidence of the physiological function, expression and hormonal regulation of renal basolateral inwardly rectifying potassium channels. Further investigation of molecular and cellular mechanisms controlling Kir4.1 and Kir4.1/Kir5.1-mediating pathways and development of specific compounds targeting these channels function is essential for proper control of electrolyte homeostasis and blood pressure.

Biological and Psychological Perspectives of Resilience: Is It Possible to Improve Stress Resistance?

The term “resilience” refers to the ability to adapt successfully to stress, trauma and adversity, enabling individuals to avoid stress-induced mental disorders such as depression, posttraumatic stress disorder (PTSD) and anxiety. Here, we review evidence from both animal models and humans that is increasingly revealing the neurophysiological and neuropsychological mechanisms that underlie stress susceptibility, as well as active mechanisms underlying the resilience phenotype. Ultimately, this growing understanding of the neurobiological mechanisms of resilience should result in the development of novel interventions that specifically target neural circuitry and brain areas that enhance resilience and lead to more effective treatments for stress-induced disorders. Stress resilience can be improved, but the outcomes and effects depend on the type of intervention and the species treated.

Identifying miRNA-mRNA regulation network of major depressive disorder in ovarian cancer patients

Major depression disorder (MDD) has become increasingly common in patients with ovarian cancer, which complicates the treatment course. The microRNA (miRNA)-mRNA regulation network may help elucidate the potential mechanism of MDD in ovarian cancer. The differentially expressed microRNAs (DEmiRs) and mRNAs (DEmRNAs) were therefore identified from the GSE61741, GSE58105 and GSE9116 ovarian cancer datasets using GEO2R. The target genes of the DEmiRs were then obtained using the TargetScan, microRNAorg, microT-CDS, miRDB and miRTarBase prediction tools. The DAVID program was used to identify the KEGG pathways of target genes, and the core genes of major depressive disorder (MDD) were identified using the Kaplan-Meier Plotter for ovarian cancer. A total of 5 DEmiRs (miR-23b-3p, miR-33b-3p, miR-1265, miR-933 and miR-629-5p) were obtained from GSE61741 and GSE58105. The target genes of these DEmiRs were enriched in pathways that were considered high risk for developing MDD in ovarian cancer. A total of 11 risk genes were selected from these pathways as the core genes in the miRNA-mRNA network of MDD in ovarian cancer, and eventually identified the following 12 miRNA-mRNAs pairs: miR-629-5p-FGF1, miR-629-5p-AKT3, miR-629-5p-MAGI2, miR-933-BDNF, miR-933-MEF2A, miR-23b-3p-TJP1, miR-23b-3p-JMJD1, miR-23b-3p-APAF1, miR-23b-3p-CAB39, miR-1265-CDKN1B, miR-33b-3p-CDKN1B, and miR-33b-3p-F2R. These results may provide novel insights into the mechanisms of developing MDD in ovarian cancer patients.

Lateral Habenula Gone Awry in Depression: Bridging Cellular Adaptations With Therapeutics

Depression is a highly heterogeneous disease characterized by symptoms spanning from anhedonia and behavioral despair to social withdrawal and learning deficit. Such diversity of behavioral phenotypes suggests that discrete neural circuits may underlie precise aspects of the disease, rendering its treatment an unmet challenge for modern neuroscience. Evidence from humans and animal models indicate that the lateral habenula (LHb), an epithalamic center devoted to processing aversive stimuli, is aberrantly affected during depression. This raises the hypothesis that rescuing maladaptations within this nucleus may be a potential way to, at least partially, treat aspects of mood disorders. In this review article, we will discuss pre-clinical and clinical evidence highlighting the role of LHb and its cellular adaptations in depression. We will then describe interventional approaches aiming to rescue LHb dysfunction and ultimately ameliorate depressive symptoms. Altogether, we aim to merge the mechanistic-, circuit-, and behavioral-level knowledge obtained about LHb maladaptations in depression to build a general framework that might prove valuable for potential therapeutic interventions.

Ketamine blocks bursting in the lateral habenula to rapidly relieve depression

The N-methyl-d-aspartate receptor (NMDAR) antagonist ketamine has attracted enormous interest in mental health research owing to its rapid antidepressant actions, but its mechanism of action has remained elusive. Here we show that blockade of NMDAR-dependent bursting activity in the 'anti-reward center', the lateral habenula (LHb), mediates the rapid antidepressant actions of ketamine in rat and mouse models of depression. LHb neurons show a significant increase in burst activity and theta-band synchronization in depressive-like animals, which is reversed by ketamine. Burst-evoking photostimulation of LHb drives behavioural despair and anhedonia. Pharmacology and modelling experiments reveal that LHb bursting requires both NMDARs and low-voltage-sensitive T-type calcium channels (T-VSCCs). Furthermore, local blockade of NMDAR or T-VSCCs in the LHb is sufficient to induce rapid antidepressant effects. Our results suggest a simple model whereby ketamine quickly elevates mood by blocking NMDAR-dependent bursting activity of LHb neurons to disinhibit downstream monoaminergic reward centres, and provide a framework for developing new rapid-acting antidepressants.

Functional Astrocyte Heterogeneity and Implications for Their Role in Shaping Neurotransmission

In recent years, the role of astrocytes in shaping neuronal signaling has come to the forefront of neuroscience research. The development of genetic tools that enable targeted manipulation of astrocytes has revealed a wealth of mechanisms by which they can alter the synaptic strength and intrinsic excitability of neurons in behaviorally relevant ways. In parallel, several studies have demonstrated significant variability in the gene expression and physiology of astrocytes within and between brain regions. Thus, to form an accurate understanding of how astrocytes contribute to neuronal transmission, we must take into consideration the diversity that exists in their intrinsic properties. In this review, we will summarize recent findings on astrocyte heterogeneity and discuss the implications for their interactions with neurons and their effects on neuronal transmission.

Danshen-Honghua Ameliorates Stress-Induced Menopausal Depression in Rats

Objective

Previously, we have shown that Danshen-Honghua (DSHH) for cognitive deficits after ischemia induced impairments of the hippocampus. Here, we investigate the effects of DSHH on stress-induced depression in menopausal rats.

Methods

A rat model with menopausal depression was established with bilateral ovariectomies in female SD rats followed by chronic mild stress treatment for 21 days. 40 rats were randomly divided into the sham surgery group (sham surgery and no stress treatment), surgery group (surgery with no stress treatment), surgery/stress group (surgery and stress treatment), fluoxetine group (2.4 mg·kg⁻¹, with surgery and stress treatment), and DSHH group (35 g·kg⁻¹, with surgery and stress treatment). The rats in the last two groups were treated with stresses together with intragastric drug administration for three weeks after the surgery. Then open-field locomotor scores and sucrose intake were tested for behavior changes. Also, the levels of norepinephrine (NE), dopamine (DA), serotonin (5-HT), and cortisone were determined by high-performance liquid chromatography (HPLC). Serum estradiol (E₂), follicle-stimulating hormone (FSH), and luteinizing hormone (LH) were determined by radioimmunoassay.

Results

The results of open-field locomotor scores, sucrose intake in both the fluoxetine group and DSHH group, were significantly higher than those of the surgery/stress group (P < 0.01). Serum LH, FSH, and cortisone levels in both the DSHH group and fluoxetine group were significantly lower than those in the surgery/stress group (P < 0.01). Serum E₂ levels in these groups were slightly increased in these medicine groups (P < 0.01). The monoamine levels in the DSHH group were much higher than those in the surgery/stress group (P < 0.01).

Conclusion

DSHH can ameliorate stress-induced depressed syndromes in the surgery/stressed rats via regulating LH and FSH levels as well as monoamine levels.

Kir 4.1 inward rectifier potassium channel is upregulated in astrocytes in a murine multiple sclerosis model

Multiple sclerosis (MS) is a high prevalence degenerative disease characterized at the cellular level by glial and neuronal cell death. The causes of cell death during the disease course are not fully understood. In this work we demonstrate that in a MS model induced by Theiler's murine encephalomyelitis virus (TMEV) infection, the inward rectifier (Kir) 4.1 potassium channel subunit is overexpressed in astrocytes. In voltage clamp experiments the inward current density from TMEV-infected astrocytes was significantly larger than in mock-infected ones. The cRNA hybridization analysis from mock- and TMEV-infected cells showed an upregulation of a potassium transport channel coding sequence. We validated this mRNA increase by RT-PCR and quantitative PCR using Kir 4.1 specific primers. Western blotting experiments confirmed the upregulation of Kir 4.1, and alignment between sequences provided the demonstration that the over-expressed gene encodes for a Kir family member. Flow cytometry showed that the Kir 4.1 protein is located mainly in the cell membrane in mock and TMEV-infected astrocytes. Our results demonstrate an increase in K⁺ inward current in TMEV-infected glial cells, this increment may reduce the neuronal depolarization, contributing to cell resilience mechanisms.

Mechanisms of ketamine action as an antidepressant

Clinical studies have demonstrated that a single sub-anesthetic dose of the dissociative anesthetic ketamine induces rapid and sustained antidepressant actions. Although this finding has been met with enthusiasm, ketamine's widespread use is limited by its abuse potential and dissociative properties. Recent preclinical research has focused on unraveling the molecular mechanisms underlying the antidepressant actions of ketamine in an effort to develop novel pharmacotherapies, which will mimic ketamine's antidepressant actions but lack its undesirable effects. Here we review hypotheses for the mechanism of action of ketamine as an antidepressant, including synaptic or GluN2B-selective extra-synaptic N-methyl-D-aspartate receptor (NMDAR) inhibition, inhibition of NMDARs localized on GABAergic interneurons, inhibition of NMDAR-dependent burst firing of lateral habenula neurons, and the role of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor activation. We also discuss links between ketamine's antidepressant actions and downstream mechanisms regulating synaptic plasticity, including brain-derived neurotrophic factor (BDNF), eukaryotic elongation factor 2 (eEF2), mechanistic target of rapamycin (mTOR) and glycogen synthase kinase-3 (GSK-3). Mechanisms that do not involve direct inhibition of the NMDAR, including a role for ketamine's (R)-ketamine enantiomer and hydroxynorketamine (HNK) metabolites, specifically (2R,6R)-HNK, are also discussed. Proposed mechanisms of ketamine's action are not mutually exclusive and may act in a complementary manner to exert acute changes in synaptic plasticity, leading to sustained strengthening of excitatory synapses, which are necessary for antidepressant behavioral actions. Understanding the molecular mechanisms underpinning ketamine's antidepressant actions will be invaluable for the identification of targets, which will drive the development of novel, effective, next-generation pharmacotherapies for the treatment of depression.