Brain-derived neurotrophic factor scales presynaptic calcium transients to modulate excitatory neurotransmission.

Brain-derived neurotrophic factor (BDNF) plays a critical role in synaptic physiology, as well as mechanisms underlying various neuropsychiatric diseases and their treatment. Despite its clear physiological role and disease relevance, BDNF's function at the presynaptic terminal, a fundamental unit of neurotransmission, remains poorly understood. In this study, we evaluated single synapse dynamics using optical imaging techniques in hippocampal cell cultures. We find that exogenous BDNF selectively increases evoked excitatory neurotransmission without affecting spontaneous neurotransmission. However, acutely blocking endogenous BDNF has no effect on evoked or spontaneous release, demonstrating that different approaches to studying BDNF may yield different results. When we suppressed BDNF-Tropomyosin receptor kinase B (TrkB) activity chronically over a period of days to weeks using a mouse line enabling conditional knockout of TrkB, we found that evoked glutamate release was significantly decreased while spontaneous release remained unchanged. Moreover, chronic blockade of BDNF-TrkB activity selectively downscales evoked calcium transients without affecting spontaneous calcium events. Via pharmacological blockade by voltage-gated calcium channel (VGCC) selective blockers, we found that the changes in evoked calcium transients are mediated by the P/Q subtype of VGCCs. These results suggest that BDNF-TrkB activity increases presynaptic VGCC activity to selectively increase evoked glutamate release.

Effects of open access publishing on article metrics in Neuropsychopharmacology.

Neuropsychopharmacology (NPP) offers the option to publish articles in different tiers of an open access (OA) publishing system: Green, Bronze, or Hybrid. Green articles follow a standard access (SA) subscription model, in which readers must pay a subscription fee to access article content on the publisher's website. Bronze articles are selected at the publisher's discretion and offer free availability to readers at the same article processing charge (APC) as Green articles. Hybrid articles are fully OA, but authors pay an increased APC to ensure public access. Here, we aimed to determine whether publishing tier affect the impact and reach of scientific articles in NPP. A sample of 6000 articles published between 2001-2021 were chosen for the analysis. Articles were separated by article type and publication year. Citation counts and Altmetric scores were compared between the three tiers. Bronze articles received significantly more citations than Green and Hybrid articles overall. However, when analyzed by year, Bronze and Hybrid articles received comparable citation counts within the past decade. Altmetric scores were comparable between all tiers, although this effect varied by year. Our findings indicate that free availability of article content on the publisher's website is associated with an increase in citations of NPP articles but may only provide a moderate boost in Altmetric score. Overall, our results suggest that easily accessible article content is most often cited by readers, but that the higher APCs of Hybrid tier publishing may not guarantee increased scholarly or social impact.

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.

Ketamine and rapid antidepressant action: new treatments and novel synaptic signaling mechanisms.

Ketamine is an open channel blocker of ionotropic glutamatergic N-Methyl-D-Aspartate (NMDA) receptors. The discovery of its rapid antidepressant effects in patients with depression and treatment-resistant depression fostered novel effective treatments for mood disorders. This discovery not only provided new insight into the neurobiology of mood disorders but also uncovered fundamental synaptic plasticity mechanisms that underlie its treatment. In this review, we discuss key clinical aspects of ketamine's effect as a rapidly acting antidepressant, synaptic and circuit mechanisms underlying its action, as well as how these novel perspectives in clinical practice and synapse biology form a road map for future studies aimed at more effective treatments for neuropsychiatric disorders.

Spatially non-overlapping Ca2+ signals drive distinct forms of neurotransmission.

Calcium (Ca2+) signaling is tightly regulated within a presynaptic bouton. Here, we visualize Ca2+ signals within hippocampal presynaptic boutons using GCaMP8s tagged to synaptobrevin, a synaptic vesicle protein. We identify evoked presynaptic Ca2+ transients (ePreCTs) that derive from synchronized voltage-gated Ca2+ channel openings, spontaneous presynaptic Ca2+ transients (sPreCTs) that originate from ryanodine sensitive Ca2+ stores, and a baseline Ca2+ signal that arises from stochastic voltage-gated Ca2+ channel openings. We find that baseline Ca2+, but not sPreCTs, contributes to spontaneous glutamate release. We employ photobleaching as a use-dependent tool to probe nano-organization of Ca2+ signals and observe that all three occur in non-overlapping domains within the synapse at near-resting conditions. However, increased depolarization induces intermixing of these Ca2+ domains via both local and non-local synaptic vesicle turnover. Our findings reveal nanosegregation of Ca2+ signals within a presynaptic terminal that derive from multiple sources and in turn drive specific modes of neurotransmission.

Bridging rapid and sustained antidepressant effects of ketamine.

Acute administration of (R,S)-ketamine (ketamine) produces rapid antidepressant effects that in some patients can be sustained for several days to more than a week. Ketamine blocks N-methyl-d-asparate (NMDA) receptors (NMDARs) to elicit specific downstream signaling that induces a novel form of synaptic plasticity in the hippocampus that has been linked to the rapid antidepressant action. These signaling events lead to subsequent downstream transcriptional changes that are involved in the sustained antidepressant effects. Here we review how ketamine triggers this intracellular signaling pathway to mediate synaptic plasticity which underlies the rapid antidepressant effects and links it to downstream signaling and the sustained antidepressant effects.

Neurotransmitter release progressively desynchronizes in induced human neurons during synapse maturation and aging.

Rapid release of neurotransmitters in synchrony with action potentials is considered a key hardwired property of synapses. Here, in glutamatergic synapses formed between induced human neurons, we show that action potential-dependent neurotransmitter release becomes progressively desynchronized as synapses mature and age. In this solely excitatory network, the emergence of NMDAR-mediated transmission elicits endoplasmic reticulum (ER) stress leading to downregulation of key presynaptic molecules, synaptotagmin-1 and cysteine string protein α, that synchronize neurotransmitter release. The emergence of asynchronous release with neuronal maturity and subsequent aging is maintained by the high-affinity Ca2+ sensor synaptotagmin-7 and suppressed by the introduction of GABAergic transmission into the network, inhibition of NMDARs, and ER stress. These results suggest that long-term disruption of excitation-inhibition balance affects the synchrony of excitatory neurotransmission in human synapses.

Rapid homeostatic plasticity and neuropsychiatric therapeutics.

Neuronal and synaptic plasticity are widely used terms in the field of psychiatry. However, cellular neurophysiologists have identified two broad classes of plasticity. Hebbian forms of plasticity alter synaptic strength in a synapse specific manner in the same direction of the initial conditioning stimulation. In contrast, homeostatic plasticities act globally over longer time frames in a negative feedback manner to counter network level changes in activity or synaptic strength. Recent evidence suggests that homeostatic plasticity mechanisms can be rapidly engaged, particularly by fast-acting antidepressants such as ketamine to trigger behavioral effects. There is increasing evidence that several neuropsychoactive compounds either directly elicit changes in synaptic activity or indirectly tap into downstream signaling pathways to trigger homeostatic plasticity and subsequent behavioral effects. In this review, we discuss this recent work in the context of a wider paradigm where homeostatic synaptic plasticity mechanisms may provide novel targets for neuropsychiatric treatment advance.

MeCP2 loss-of-function dysregulates microRNAs regionally and disrupts excitatory/inhibitory synaptic transmission balance.

Rett syndrome is a leading cause of intellectual disability in females primarily caused by loss of function mutations in the transcriptional regulator MeCP2. Loss of MeCP2 leads to a host of synaptic phenotypes that are believed to underlie Rett syndrome pathophysiology. Synaptic deficits vary by brain region upon MeCP2 loss, suggesting distinct molecular alterations leading to disparate synaptic outcomes. In this study, we examined the contribution of MeCP2's newly described role in miRNA regulation to regional molecular and synaptic impairments. Two miRNAs, miR-101a and miR-203, were identified and confirmed as upregulated in MeCP2 KO mice in the hippocampus and cortex, respectively. miR-101a overexpression in hippocampal cultures led to opposing effects at excitatory and inhibitory synapses and in spontaneous and evoked neurotransmission, revealing the potential for a single miRNA to broadly regulate synapse function in the hippocampus. These results highlight the importance of regional alterations in miRNA expression and the specific impact on synaptic function with potential implications for Rett syndrome.

Optical analysis of AMPAR-mediated synaptic scaling in mouse hippocampus.

Immunolabeling of surface AMPA receptors (AMPARs) can be used for in vivo or ex vivo examination of synaptic scaling, a type of homeostatic plasticity. Here, we present a protocol to analyze changes in synaptic weights using immunohistochemistry for surface AMPARs coupled with optical imaging analysis. We detail immunostaining of AMPARs in mouse brain sections, followed by confocal imaging of surface AMPARs in dendritic region of hippocampal CA1. We then describe using Fiji/ImageJ and rank order plots for analyzing synaptic weight. For complete details on the use and execution of this protocol, please refer to Suzuki et al. (2021).

Probing the segregation of evoked and spontaneous neurotransmission via photobleaching and recovery of a fluorescent glutamate sensor.

Synapses maintain both action potential-evoked and spontaneous neurotransmitter release; however, organization of these two forms of release within an individual synapse remains unclear. Here, we used photobleaching properties of iGluSnFR, a fluorescent probe that detects glutamate, to investigate the subsynaptic organization of evoked and spontaneous release in primary hippocampal cultures. In nonneuronal cells and neuronal dendrites, iGluSnFR fluorescence is intensely photobleached and recovers via diffusion of nonphotobleached probes with a time constant of ~10 s. After photobleaching, while evoked iGluSnFR events could be rapidly suppressed, their recovery required several hours. In contrast, iGluSnFR responses to spontaneous release were comparatively resilient to photobleaching, unless the complete pool of iGluSnFR was activated by glutamate perfusion. This differential effect of photobleaching on different modes of neurotransmission is consistent with a subsynaptic organization where sites of evoked glutamate release are clustered and corresponding iGluSnFR probes are diffusion restricted, while spontaneous release sites are broadly spread across a synapse with readily diffusible iGluSnFR probes.

BDNF signaling in context: From synaptic regulation to psychiatric disorders.

Brain-derived neurotrophic factor (BDNF) is a neuropeptide that plays numerous important roles in synaptic development and plasticity. While its importance in fundamental physiology is well established, studies of BDNF often produce conflicting and unclear results, and the scope of existing research makes the prospect of setting future directions daunting. In this review, we examine the importance of spatial and temporal factors on BDNF activity, particularly in processes such as synaptogenesis, Hebbian plasticity, homeostatic plasticity, and the treatment of psychiatric disorders. Understanding the fundamental physiology of when, where, and how BDNF acts and new approaches to control BDNF signaling in time and space can contribute to improved therapeutics and patient outcomes.

Convergence of distinct signaling pathways on synaptic scaling to trigger rapid antidepressant action.

Ketamine is a noncompetitive glutamatergic N-methyl-d-aspartate receptor (NMDAR) antagonist that exerts rapid antidepressant effects. Preclinical studies identify eukaryotic elongation factor 2 kinase (eEF2K) signaling as essential for the rapid antidepressant action of ketamine. Here, we combine genetic, electrophysiological, and pharmacological strategies to investigate the role of eEF2K in synaptic function and find that acute, but not chronic, inhibition of eEF2K activity induces rapid synaptic scaling in the hippocampus. Retinoic acid (RA) signaling also elicits a similar form of rapid synaptic scaling in the hippocampus, which we observe is independent of eEF2K functioni. The RA signaling pathway is not required for ketamine-mediated antidepressant action; however, direct activation of the retinoic acid receptor α (RARα) evokes rapid antidepressant action resembling ketamine. Our findings show that ketamine and RARα activation independently elicit a similar form of multiplicative synaptic scaling that is causal for rapid antidepressant action.

A synaptic locus for TrkB signaling underlying ketamine rapid antidepressant action.

Ketamine produces rapid antidepressant action in patients with major depression or treatment-resistant depression. Studies have identified brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin receptor kinase B (TrkB), as necessary for the antidepressant effects and underlying ketamine-induced synaptic potentiation in the hippocampus. Here, we delete BDNF or TrkB in presynaptic CA3 or postsynaptic CA1 regions of the Schaffer collateral pathway to investigate the rapid antidepressant action of ketamine. The deletion of Bdnf in CA3 or CA1 blocks the ketamine-induced synaptic potentiation. In contrast, ablation of TrkB only in postsynaptic CA1 eliminates the ketamine-induced synaptic potentiation. We confirm BDNF-TrkB signaling in CA1 is required for ketamine's rapid behavioral action. Moreover, ketamine application elicits dynamin1-dependent TrkB activation and downstream signaling to trigger rapid synaptic effects. Taken together, these data demonstrate a requirement for BDNF-TrkB signaling in CA1 neurons in ketamine-induced synaptic potentiation and identify a specific synaptic locus in eliciting ketamine's rapid antidepressant effects.

A subthreshold synaptic mechanism regulating BDNF expression and resting synaptic strength.

Recent studies have demonstrated that protein translation can be regulated by spontaneous excitatory neurotransmission. However, the impact of spontaneous neurotransmitter release on gene transcription remains unclear. Here, we study the effects of the balance between inhibitory and excitatory spontaneous neurotransmission on brain-derived neurotrophic factor (BDNF) regulation and synaptic plasticity. Blockade of spontaneous inhibitory events leads to an increase in the transcription of Bdnf and Npas4 through altered synaptic calcium signaling, which can be blocked by antagonism of NMDA receptors (NMDARs) or L-type voltage-gated calcium channels (VGCCs). Transcription is bidirectionally altered by manipulating spontaneous inhibitory, but not excitatory, currents. Moreover, blocking spontaneous inhibitory events leads to multiplicative downscaling of excitatory synaptic strength in a manner that is dependent on both transcription and BDNF signaling. These results reveal a role for spontaneous inhibitory neurotransmission in BDNF signaling that sets excitatory synaptic strength at rest.

Sustained effects of rapidly acting antidepressants require BDNF-dependent MeCP2 phosphorylation.

The rapidly acting antidepressants ketamine and scopolamine exert behavioral effects that can last from several days to more than a week in some patients. The molecular mechanisms underlying the maintenance of these antidepressant effects are unknown. Here we show that methyl-CpG-binding protein 2 (MeCP2) phosphorylation at Ser421 (pMeCP2) is essential for the sustained, but not the rapid, antidepressant effects of ketamine and scopolamine in mice. Our results reveal that pMeCP2 is downstream of BDNF, a critical factor in ketamine and scopolamine antidepressant action. In addition, we show that pMeCP2 is required for the long-term regulation of synaptic strength after ketamine or scopolamine administration. These results demonstrate that pMeCP2 and associated synaptic plasticity are essential determinants of sustained antidepressant effects.

Brain-Derived Neurotrophic Factor Signaling in Depression and Antidepressant Action.

Neurotrophic factors, particularly BDNF (brain-derived neurotrophic factor), have been associated with depression and antidepressant drug action. A variety of preclinical and clinical studies have implicated impaired BDNF signaling through its receptor TrkB (neurotrophic receptor tyrosine kinase 2) in the pathophysiology of mood disorders, but many of the initial findings have not been fully supported by more recent meta-analyses, and more both basic and clinical research is needed. In contrast, increased expression and signaling of BDNF has been repeatedly implicated in the mechanisms of both typical and rapid-acting antidepressant drugs, and recent findings have started to elucidate the mechanisms through which antidepressants regulate BDNF signaling. BDNF is a critical regulator of various types of neuronal plasticities in the brain, and plasticity has increasingly been connected with antidepressant action. Although some equivocal data exist, the hypothesis of a connection between neurotrophic factors and neuronal plasticity with mood disorders and antidepressant action has recently been further strengthened by converging evidence from a variety of more recent data reviewed here.

A key requirement for synaptic Reelin signaling in ketamine-mediated behavioral and synaptic action.

Ketamine is a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist that produces rapid antidepressant action in some patients with treatment-resistant depression. However, recent data suggest that ∼50% of patients with treatment-resistant depression do not respond to ketamine. The factors that contribute to the nonresponsiveness to ketamine's antidepressant action remain unclear. Recent studies have reported a role for secreted glycoprotein Reelin in regulating pre- and postsynaptic function, which suggests that Reelin may be involved in ketamine's antidepressant action, although the premise has not been tested. Here, we investigated whether the disruption of Reelin-mediated synaptic signaling alters ketamine-triggered synaptic plasticity and behavioral effects. To this end, we used mouse models with genetic deletion of Reelin or apolipoprotein E receptor 2 (Apoer2), as well as pharmacological inhibition of their downstream effectors, Src family kinases (SFKs) or phosphoinositide 3-kinase. We found that disruption of Reelin, Apoer2, or SFKs blocks ketamine-driven behavioral changes and synaptic plasticity in the hippocampal CA1 region. Although ketamine administration did not affect tyrosine phosphorylation of DAB1, an adaptor protein linked to downstream signaling of Reelin, disruption of Apoer2 or SFKs impaired baseline NMDA receptor-mediated neurotransmission. These results suggest that maintenance of baseline NMDA receptor function by Reelin signaling may be a key permissive factor required for ketamine's antidepressant effects. Taken together, our results suggest that impairments in Reelin-Apoer2-SFK pathway components may in part underlie nonresponsiveness to ketamine's antidepressant action.