Spontaneous neurotransmission: an independent pathway for neuronal signaling?

Linker mutations reveal the complexity of synaptotagmin 1 action during synaptic transmission

The Ca(2+) sensor for rapid synaptic vesicle exocytosis, synaptotagmin 1 (syt), is largely composed of two Ca(2+)-sensing C2 domains, C2A and C2B. We investigated the apparent synergy between the tandem C2 domains by altering the length and rigidity of the linker that connects them. The behavior of the linker mutants revealed a correlation between the ability of the C2 domains to penetrate membranes in response to Ca(2+) and to drive evoked neurotransmitter release in cultured mouse neurons, uncovering a step in excitation-secretion coupling. Using atomic force microscopy, we found that the synergy between these C2 domains involved intra-molecular interactions between them. Thus, syt function is markedly affected by changes in the physical nature of the linker that connects its tandem C2 domains. Moreover, the linker mutations uncoupled syt-mediated regulation of evoked and spontaneous release, revealing that syt also acts as a fusion clamp before the Ca(2+) trigger.

Physiological temperatures drive glutamate release onto trigeminal superficial dorsal horn neurons

Trigeminal sensory afferent fibers terminating in nucleus caudalis (Vc) relay sensory information from craniofacial regions to the brain and are known to express transient receptor potential (TRP) ion channels. TRP channels are activated by H(+), thermal, and chemical stimuli. The present study investigated the relationships among the spontaneous release of glutamate, temperature, and TRPV1 localization at synapses in the Vc. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded from Vc neurons (n = 151) in horizontal brain-stem slices obtained from Sprague-Dawley rats. Neurons had basal sEPSC rates that fell into two distinct frequency categories: High (≥10 Hz) or Low (<10 Hz) at 35°C. Of all recorded neurons, those with High basal release rates (67%) at near-physiological temperatures greatly reduced their sEPSC rate when cooled to 30°C without amplitude changes. Such responses persisted during blockade of action potentials indicating that the High rate of glutamate release arises from presynaptic thermal mechanisms. Neurons with Low basal frequencies (33%) showed minor thermal changes in sEPSC rate that were abolished after addition of TTX, suggesting these responses were indirect and required local circuits. Activation of TRPV1 with capsaicin (100 nM) increased miniature EPSC (mEPSC) frequency in 70% of neurons, but half of these neurons had Low basal mEPSC rates and no temperature sensitivity. Our evidence indicates that normal temperatures (35-37°C) drive spontaneous excitatory synaptic activity within superficial Vc by a mechanism independent of presynaptic action potentials. Thus thermally sensitive inputs on superficial Vc neurons may tonically activate these neurons without afferent stimulation.

Modulation of neurotransmission by GPCRs is dependent upon the microarchitecture of the primed vesicle complex

G(i/o)-protein-coupled receptors (GPCRs) ubiquitously inhibit neurotransmission, principally via Gβγ, which acts via a number of possible effectors. GPCR effector specificity has traditionally been attributed to Gα, based on Gα's preferential effector targeting in vitro compared with Gβγ's promiscuous targeting of various effectors. In synapses, however, Gβγ clearly targets unique effectors in a receptor-dependent way to modulate synaptic transmission. It remains unknown whether Gβγ specificity in vivo is due to specific Gβγ isoform-receptor associations or to spatial separation of distinct Gβγ pathways through macromolecular interactions. We thus sought to determine how Gβγ signaling pathways within axons remain distinct from one another. In rat hippocampal CA1 axons, GABA(B) receptors (GABA(B)Rs) inhibit presynaptic Ca(2+) entry, and we have now demonstrated that 5-HT(1B) receptors (5-HT(1B)Rs) liberate Gβγ to interact with SNARE complex C terminals with no effect on Ca(2+) entry. Both GABA(B)Rs and 5-HT(1B)Rs inhibit Ca(2+)-evoked neurotransmitter release, but 5-HT(1B)Rs have no effect on Sr(2+)-evoked release. Sr(2+), unlike Ca(2+), does not cause synaptotagmin to compete with Gβγ binding to SNARE complexes. 5-HT(1B)Rs also fail to inhibit release following cleavage of the C terminus of the SNARE complex protein SNAP-25 with botulinum A toxin. Thus, GABA(B)Rs and 5-HT(1B)Rs both localize to presynaptic terminals, but target distinct effectors. We demonstrate that disruption of SNARE complexes and vesicle priming with botulinum C toxin eliminates this selectivity, allowing 5-HT(1B)R inhibition of Ca(2+) entry. We conclude that receptor-effector specificity requires a microarchitecture provided by the SNARE complex during vesicle priming.

Evoked and spontaneous transmission favored by distinct sets of synapses

BACKGROUND

Spontaneous "miniature" transmitter release takes place at low rates at all synapses. Long thought of as an unavoidable leak, spontaneous release has recently been suggested to be mediated by distinct pre- and postsynaptic molecular machineries and to have a specialized role in setting up and adjusting neuronal circuits. It remains unclear how spontaneous and evoked transmission are related at individual synapses, how they are distributed spatially when an axon makes multiple contacts with a target, and whether they are commonly regulated.

RESULTS

Electrophysiological recordings in the Drosophila larval neuromuscular junction, in the presence of the use-dependent glutamate receptor (GluR) blocker philanthotoxin, indicated that spontaneous and evoked transmission employ distinct sets of GluRs. In vivo imaging of transmission using synaptically targeted GCaMP3 to detect Ca(2+) influx through the GluRs revealed little spatial overlap between synapses participating in spontaneous and evoked transmission. Spontaneous and evoked transmission were oppositely correlated with presynaptic levels of the protein Brp: synapses with high Brp favored evoked transmission, whereas synapses with low Brp were more active spontaneously. High-frequency stimulation did not increase the overlap between evoked and spontaneous transmission, and instead decreased the rate of spontaneous release from synapses that were highly active in evoked transmission.

CONCLUSIONS

Although individual synapses can participate in both evoked and spontaneous transmission, highly active synapses show a preference for one mode of transmission. The presynaptic protein Brp promotes evoked transmission and suppresses spontaneous release. These findings suggest the existence of presynaptic mechanisms that promote synaptic specialization to either evoked or spontaneous transmission.

Isolation of TRPV1 independent mechanisms of spontaneous and asynchronous glutamate release at primary afferent to NTS synapses

Cranial visceral afferents contained within the solitary tract (ST) contact second-order neurons in the nucleus of the solitary tract (NTS) and release the excitatory amino acid glutamate via three distinct exocytosis pathways; synchronous, asynchronous, and spontaneous release. The presence of TRPV1 in the central terminals of a majority of ST afferents conveys activity-dependent asynchronous glutamate release and provides a temperature sensitive calcium conductance which largely determines the rate of spontaneous vesicle fusion. TRPV1 is present in unmyelinated C-fiber afferents and these facilitated forms of glutamate release may underlie the relative strength of C-fibers in activating autonomic reflex pathways. However, pharmacological blockade of TRPV1 signaling eliminates only ~50% of the asynchronous profile and attenuates the temperature sensitivity of spontaneous release indicating additional thermosensitive calcium influx pathways may exist which mediate these forms of vesicle release. In the present study we isolate the contribution of TRPV1 independent forms of glutamate release at ST-NTS synapses. We found ST afferent innervation at NTS neurons and synchronous vesicle release from TRPV1 KO mice was not different to control animals; however, only half of TRPV1 KO ST afferents completely lacked asynchronous glutamate release. Further, temperature driven spontaneous rates of vesicle release were not different from 33 to 37°C between control and TRPV1 KO afferents. These findings suggest additional temperature dependent mechanisms controlling asynchronous and thermosensitive spontaneous release at physiological temperatures, possibly mediated by additional thermosensitive TRP channels in primary afferent terminals.

Spontaneous and evoked release are independently regulated at individual active zones

Neurotransmitter release from synaptic vesicle fusion is the fundamental mechanism for neuronal communication at synapses. Evoked release following an action potential has been well characterized for its function in activating the postsynaptic cell, but the significance of spontaneous release is less clear. Using transgenic tools to image single synaptic vesicle fusion events at individual release sites (active zones) in Drosophila, we characterized the spatial and temporal dynamics of exocytotic events that occur spontaneously or in response to an action potential. We also analyzed the relationship between these two modes of fusion at single release sites. A majority of active zones participate in both modes of fusion, although release probability is not correlated between the two modes of release and is highly variable across the population. A subset of active zones is specifically dedicated to spontaneous release, indicating a population of postsynaptic receptors is uniquely activated by this mode of vesicle fusion. Imaging synaptic transmission at individual release sites also revealed general rules for spontaneous and evoked release, and indicate that active zones with similar release probability can cluster spatially within individual synaptic boutons. These findings suggest neuronal connections contain two information channels that can be spatially segregated and independently regulated to transmit evoked or spontaneous fusion signals.

Exploring the central modulation hypothesis: do ancient memory mechanisms underlie the pathophysiology of trigger points?

A myofascial trigger point (TrP) is a point of focal tenderness, associated with a taut band of muscle fibers, that can develop in any skeletal muscle. TrPs are a common source of pain and motor dysfunction in humans and other vertebrates. There is no universally accepted pathophysiology to explain the etiology, symptomatology and treatment of TrPs. This article reviews and extends the author's previously published hypothesis for the pathophysiology of TrPs, "Trigger Points and Central Modulation-A New Hypothesis." The author proposes that central nervous system-maintained global changes in α-motoneuron function, resulting from sustained plateau depolarization, rather than a local dysfunction of the motor endplate, underlie the pathogenesis of TrPs.

GluA2-dependent AMPA receptor endocytosis and the decay of early and late long-term potentiation: possible mechanisms for forgetting of short- and long-term memories

The molecular processes involved in establishing long-term potentiation (LTP) have been characterized well, but the decay of early and late LTP (E-LTP and L-LTP) is poorly understood. We review recent advances in describing the mechanisms involved in maintaining LTP and homeostatic plasticity. We discuss how these phenomena could relate to processes that might underpin the loss of synaptic potentiation over time, and how they might contribute to the forgetting of short-term and long-term memories. We propose that homeostatic downscaling mediates the loss of E-LTP, and that metaplastic parameters determine the decay rate of L-LTP, while both processes require the activity-dependent removal of postsynaptic GluA2-containing AMPA receptors.

Nanoscale scaffolding domains within the postsynaptic density concentrate synaptic AMPA receptors

Scaffolding molecules at the postsynaptic membrane form the foundation of excitatory synaptic transmission by establishing the architecture of the postsynaptic density (PSD), but the small size of the synapse has precluded measurement of PSD organization in live cells. We measured the internal structure of the PSD in live neurons at approximately 25 nm resolution using photoactivated localization microscopy (PALM). We found that four major PSD scaffold proteins were each organized in distinctive ∼80 nm ensembles able to undergo striking changes over time. Bidirectional PALM and single-molecule immunolabeling showed that dense nanodomains of PSD-95 were preferentially enriched in AMPA receptors more than NMDA receptors. Chronic suppression of activity triggered changes in PSD interior architecture that may help amplify synaptic plasticity. The observed clustered architecture of the PSD controlled the amplitude and variance of simulated postsynaptic currents, suggesting several ways in which PSD interior organization may regulate the strength and plasticity of neurotransmission.

The synaptic maintenance problem: membrane recycling, Ca2+ homeostasis and late onset degeneration

Most neurons are born with the potential to live for the entire lifespan of the organism. In addition, neurons are highly polarized cells with often long axons, extensively branched dendritic trees and many synaptic contacts. Longevity together with morphological complexity results in a formidable challenge to maintain synapses healthy and functional. This challenge is often evoked to explain adult-onset degeneration in numerous neurodegenerative disorders that result from otherwise divergent causes. However, comparably little is known about the basic cell biological mechanisms that keep normal synapses alive and functional in the first place. How the basic maintenance mechanisms are related to slow adult-onset degeneration in different diseasesis largely unclear. In this review we focus on two basic and interconnected cell biological mechanisms that are required for synaptic maintenance: endomembrane recycling and calcium (Ca²⁺) homeostasis. We propose that subtle defects in these homeostatic processes can lead to late onset synaptic degeneration. Moreover, the same basic mechanisms are hijacked, impaired or overstimulated in numerous neurodegenerative disorders. Understanding the pathogenesis of these disorders requires an understanding of both the initial cause of the disease and the on-going changes in basic maintenance mechanisms. Here we discuss the mechanisms that keep synapses functional over long periods of time with the emphasis on their role in slow adult-onset neurodegeneration.

The role of eukaryotic elongation factor 2 kinase in rapid antidepressant action of ketamine

Major depressive disorder is a devastating mental disorder. Current antidepressant medications can be effective for some patients with depression; however, these drugs exert mood-elevating effects only after prolonged administration, and a sizable fraction of the patient population fails to respond to treatment. There is an urgent need for faster-acting antidepressants with reliable treatment outcomes and sustained efficacy for individuals with depression, in particular those contemplating suicide. Recent clinical studies report that ketamine, an ionotropic glutamatergic N-methyl-D-aspartate (NMDA) receptor blocker, shows fast-acting antidepressant action, thus bringing fresh perspective into preclinical studies investigating novel antidepressant targets and treatments. Our recent studies show that the effects of ketamine are dependent on brain-derived neurotrophic factor (BDNF) and subsequent activation of the high-affinity BDNF receptor, TrkB. Our findings also suggest that the fast-acting antidepressant effects of ketamine require rapid protein translation, but not transcription, resulting in robust increases in dendritic BDNF protein levels that are important for the behavioral effect. These findings also uncover eukaryotic elongation factor 2 kinase (eEF2K), a Ca²⁺/calmodulin dependent serine/threonine kinase that phosphorylates eEF2 and regulates the elongation step of protein translation, as a major molecular substrate mediating the rapid antidepressant effect of ketamine. Our results show that ketamine-mediated suppression of resting NMDA receptor activity leads to inhibition of eEF2 kinase and subsequent dephosphorylation of eEF2 and augmentation of BDNF synthesis. This article outlines our recent studies on the synaptic mechanisms that underlie ketamine action, in particular the properties of eEF2K as a potential antidepressant target.

Synaptic ribbons influence the size and frequency of miniature-like evoked postsynaptic currents

Nonspiking cells of several sensory systems respond to stimuli with graded changes in neurotransmitter release and possess specialized synaptic ribbons. Here, we show that manipulations to synaptic ribbons caused dramatic effects on mEPSC-like (mlEPSC) amplitude and frequency. Damage to rod-bipolar cell ribbons using fluorophore-assisted light inactivation resulted in the immediate reduction of mlEPSC amplitude and frequency, whereas the first evoked response after damage remained largely intact. The reduction in amplitude could not be recovered by increasing release frequency after ribbon damage. In parallel experiments, we looked at mlEPSCs from cones of hibernating ground squirrels, which exhibit dramatically smaller ribbons than awake animals. Fewer and smaller mlEPSCs were observed postsynaptic to cones from hibernating animals, although depolarized cones were able to generate larger mlEPSCs. Our results indicate that ribbon size may influence mlEPSC frequency and support a role for ribbons in coordinating multivesicular release.

Peptide and lipid modulation of glutamatergic afferent synaptic transmission in the solitary tract nucleus

The brainstem nucleus of the solitary tract (NTS) holds the first central neurons in major homeostatic reflex pathways. These homeostatic reflexes regulate and coordinate multiple organ systems from gastrointestinal to cardiopulmonary functions. The core of many of these pathways arise from cranial visceral afferent neurons that enter the brain as the solitary tract (ST) with more than two-thirds arising from the gastrointestinal system. About one quarter of ST afferents have myelinated axons but the majority are classed as unmyelinated C-fibers. All ST afferents release the fast neurotransmitter glutamate with remarkably similar, high-probability release characteristics. Second order NTS neurons receive surprisingly limited primary afferent information with one or two individual inputs converging on single second order NTS neurons. A- and C-fiber afferents never mix at NTS second order neurons. Many transmitters modify the basic glutamatergic excitatory postsynaptic current often by reducing glutamate release or interrupting terminal depolarization. Thus, a distinguishing feature of ST transmission is presynaptic expression of G-protein coupled receptors for peptides common to peripheral or forebrain (e.g., hypothalamus) neuron sources. Presynaptic receptors for angiotensin (AT1), vasopressin (V1a), oxytocin, opioid (MOR), ghrelin (GHSR1), and cholecystokinin differentially control glutamate release on particular subsets of neurons with most other ST afferents unaffected. Lastly, lipid-like signals are transduced by two key ST presynaptic receptors, the transient receptor potential vanilloid type 1 and the cannabinoid receptor that oppositely control glutamate release. Increasing evidence suggests that peripheral nervous signaling mechanisms are repurposed at central terminals to control excitation and are major sites of signal integration of peripheral and central inputs particularly from the hypothalamus.

The unsilent majority-TRPV1 drives "spontaneous" transmission of unmyelinated primary afferents within cardiorespiratory NTS

Cranial primary afferent sensory neurons figure importantly in homeostatic control of visceral organ systems. Of the two broad classes of visceral afferents, the role of unmyelinated or C-type class remains poorly understood. This review contrasts key aspects of peripheral discharge properties of C-fiber afferents and their glutamate transmission mechanisms within the solitary tract nucleus (NTS). During normal prevailing conditions, most information arrives at the NTS through myelinated A-type nerves. However, most of visceral afferent axons (75-90%) in NTS are unmyelinated, C-type axons. Centrally, C-type solitary tract (ST) afferent terminals have presynaptic transient receptor potential vanilloid type 1 (TRPV1) receptors. Capsaicin activation of TRPV1 blocks phasic or synchronous release of glutamate but facilitates release of glutamate from a separate pool of vesicles. This TRPV1-operated pool of vesicles is active at normal temperatures and is responsible for actively driving a 10-fold higher release of glutamate at TRPV1 compared with TRPV1- terminals even in the absence of afferent action potentials. This novel TRPV1 mechanism is responsible for an additional asynchronous release of glutamate that is not present in myelinated terminals. The NTS is rich with presynaptic G protein-coupled receptors, and the implications of TRPV1-operated glutamate offer unique targets for signaling in C-type sensory afferent terminals from neuropeptides, inflammatory mediators, lipid metabolites, cytokines, and cannabinoids. From a homeostatic view, this combination could have broad implications for integration in chronic pathological disturbances in which the numeric dominance of C-type endings and TRPV1 would broadly disturb multisystem control mechanisms.

Enhanced hippocampal long-term potentiation and fear memory in Btbd9 mutant mice

Polymorphisms in BTBD9 have recently been associated with higher risk of restless legs syndrome (RLS), a neurological disorder characterized by uncomfortable sensations in the legs at rest that are relieved by movement. The BTBD9 protein contains a BTB/POZ domain and a BACK domain, but its function is unknown. To elucidate its function and potential role in the pathophysiology of RLS, we generated a line of mutant Btbd9 mice derived from a commercial gene-trap embryonic stem cell clone. Btbd9 is the mouse homolog of the human BTBD9. Proteins that contain a BTB/POZ domain have been reported to be associated with synaptic transmission and plasticity. We found that Btbd9 is naturally expressed in the hippocampus of our mutant mice, a region critical for learning and memory. As electrophysiological characteristics of CA3-CA1 synapses of the hippocampus are well characterized, we performed electrophysiological recordings in this region. The mutant mice showed normal input-output relationship, a significant impairment in pre-synaptic activity, and an enhanced long-term potentiation. We further performed an analysis of fear memory and found the mutant mice had an enhanced cued and contextual fear memory. To elucidate a possible molecular basis for these enhancements, we analyzed proteins that have been associated with synaptic plasticity. We found an elevated level of dynamin 1, an enzyme associated with endocytosis, in the mutant mice. These results suggest the first identified function of Btbd9 as being involved in regulating synaptic plasticity and memory. Recent studies have suggested that enhanced synaptic plasticity, analogous to what we have observed, in other regions of the brain could enhance sensory perception similar to what is seen in RLS patients. Further analyses of the mutant mice will help shine light on the function of BTBD9 and its role in RLS.

Tonic regulation of GABAergic synaptic activity on vasopressin neurones by cannabinoids

Synaptic activity in magnocellular neurosecretory neurones is influenced by the retrograde (i.e. somatodendritic) release of vasopressin, oxytocin and cannabinoids (CBs). For oxytocin neurones, oxytocin exerts constitutive effects on pre-synaptic activity through its ability to release CBs post-synaptically. In the present study, we examined evoked inhibitory post-synaptic currents (eIPSCs) and spontaneous inhibitory post-synaptic currents (sIPSCs) in identified vasopressin (VP) neurones in coronal slices from virgin rats to determine: (i) the extent to which CBs may also tonically modulate VP synaptic activity; and (ii) to determine whether depolarisation-induced suppression of inhibition was present in VP neurones, and if so, whether it was mediated by VP or CBs. The CB1 antagonists AM251 (1 μm) and SR14171 (1 μm) consistently increased the frequency of sIPSCs in VP neurones without affecting their amplitude, suggesting a tonic CB presence. This effect on frequency was independent of action potential activity, and blocked by chelating intracellular calcium with 10 mm ethylene glycol tetraacetic acid (EGTA). AM251 also increased the amplitude of eIPSCs and decreased the paired-pulse ratio (PPR) in VP neurones-effects that were completely blocked with even low (1 mm EGTA) internal calcium chelation. Bouts of evoked firing of VP neurones consistently suppressed sIPSCs but had no effect on eIPSCs or the PPR. This depolarisation-induced suppression of IPSCs was reduced by AM251, and was totally blocked by 10 μm of the mixed vasopressin/oxytocin antagonist, Manning compound. We then tested the effect of vasopressin on IPSCs at the same time as blocking CB1 receptors. Vasopressin (10-100 nm) inhibited sIPSC frequency but had no effect on sIPSC or eIPSC amplitudes, or on the PPR, in the presence of AM251. Taken together, these results suggest a tonic, pre-synaptic inhibitory modulation of IPSCs in VP neurones by CBs that is largely dependent on post-synaptic calcium, and an inhibitory effect of VP on IPSCs that is independent of CB release.

Spontaneous neurotransmission: a SNARE for the rest

In addition to activity-dependent neurotransmission, neurons can undergo spontaneous activity-independent neurotransmitter release with low probability. In this issue of Neuron, Ramirez et al. (2012) now identify the noncanonical endosomal SNARE Vps10p-tail-interactor1a (Vti1a) as a regulator of spontaneously fusing vesicles.

Vti1a identifies a vesicle pool that preferentially recycles at rest and maintains spontaneous neurotransmission

Recent studies suggest that synaptic vesicles (SVs) giving rise to spontaneous neurotransmission are distinct from those that carry out evoked release. However, the molecular basis of this dichotomy remains unclear. Here, we focused on two noncanonical SNARE molecules, Vps10p-tail-interactor-1a (vti1a) and VAMP7, previously shown to reside on SVs. Using simultaneous multicolor imaging at individual synapses, we could show that compared to the more abundant vesicular SNARE synaptobrevin2, both vti1a and VAMP7 were reluctantly mobilized during activity. Vti1a, but not VAMP7, showed robust trafficking under resting conditions that could be partly matched by synaptobrevin2. Furthermore, loss of vti1a function selectively reduced high-frequency spontaneous neurotransmitter release detected postsynaptically. Expression of a truncated version of vti1a augmented spontaneous release more than full-length vti1a, suggesting that an autoinhibitory process regulates vti1a function. Taken together, these results support the premise that in its native form vti1a selectively maintains spontaneous neurotransmitter release.

Inhibitory-excitatory synaptic balance is shifted toward increased excitation in magnocellular neurosecretory cells of heart failure rats

Despite the well-established contribution of neurohumoral activation to morbidity and mortality in heart failure (HF) patients, relatively little is known about the underlying central nervous system mechanisms. In this study, we aimed to determine whether changes in GABAergic inhibitory and glutamatergic excitatory synaptic function contribute to altered hypothalamic magnocellular neurosecretory cell (MNC) activity in HF rats. Patch-clamp recordings were obtained from MNCs in brain slices from sham and HF rats. Glutamate excitatory (EPSCs) and GABAergic inhibitory postsynaptic currents (IPSCs) were simultaneously recorded, and changes in their strengths, as well as their interactions, were evaluated. We found a diminished GABAergic synaptic strength in MNCs of HF rats, reflected as faster decaying IPSCs and diminished mean IPSC charge transfer. Opposite changes were observed in glutamate EPSC synaptic strength, resulting in a shift in the GABA-glutamate balance toward a relatively stronger glutamate influence in HF rats. The prolongation of glutamate EPSCs during HF was mediated, at least in part, by an enhanced contribution of AMPA receptor desensitization to the EPSC decay time course. EPSC prolongation, and consequently increased unitary strength, resulted in a stronger AMPA receptor-mediated excitatory drive to firing discharge in MNCs of HF rats. Blockade of GABA(A) synaptic activity diminished the EPSC waveform variability observed among events in sham rats, an effect that was blunted in HF rats. Together, our results suggest that opposing changes in postsynaptic properties of GABAergic and glutamatergic synaptic function contribute to enhanced magnocellular neurosecretory activity in HF rats.

NMDA receptor blockade at rest triggers rapid behavioural antidepressant responses

Clinical studies consistently demonstrate that a single sub-psychomimetic dose of ketamine, an ionotropic glutamatergic n-methyl-d-aspartate receptor (NMDAR) antagonist, produces fast-acting antidepressant responses in patients suffering from major depressive disorder (MDD), although the underlying mechanism is unclear^1-3. Depressed patients report alleviation of MDD symptoms within two hours of a single low-dose intravenous infusion of ketamine with effects lasting up to two weeks^1-3, unlike traditional antidepressants (i.e. serotonin reuptake inhibitors), which take weeks to reach efficacy. This delay is a major drawback to current MDD therapies, leaving a need for faster acting antidepressants particularly for suicide-risk patients³. Ketamine's ability to produce rapidly acting, long-lasting antidepressant responses in depressed patients provides a unique opportunity to investigate underlying cellular mechanisms. We show that ketamine and other NMDAR antagonists produce fast-acting behavioural antidepressant-like effects in mouse models that depend on rapid synthesis of brain-derived neurotrophic factor (BDNF). We find that ketamine-mediated NMDAR blockade at rest deactivates eukaryotic elongation factor 2 (eEF2) kinase (also called CaMKIII) resulting in reduced eEF2 phosphorylation and desuppression of BDNF translation. Furthermore, we find inhibitors of eEF2 kinase induce fast-acting behavioural antidepressant-like effects. Our findings suggest that protein synthesis regulation by spontaneous neurotransmission may serve as a viable therapeutic target for fast-acting antidepressant development.

Differential regulation of spontaneous and evoked neurotransmitter release at central synapses

Recent studies have begun to scrutinize the presynaptic machinery and vesicle populations that give rise to action potential evoked and spontaneous forms of neurotransmitter release. In several cases this work produced unexpected results which lend support to the notion that regulation, mechanisms, postsynaptic targets and possibly presynaptic origins of evoked and spontaneous neurotransmitter release differ. Furthermore, the list of regulatory pathways that impact spontaneous and evoked release in a divergent manner is rapidly growing. These findings challenge our classical views on the relationship between evoked and spontaneous neurotransmission. In contrast to the well-characterized neuromodulatory pathways that equally suppress or augment all forms of neurotransmitter release, molecular substrates specifically controlling spontaneous release remain unclear. In this review, we outline possible mechanisms that may underlie the differential regulation of distinct forms of neurotransmission and help demultiplex complex neuronal signals and generate parallel signaling events at their postsynaptic targets.