Activation of extrasynaptic NMDA receptors induces a PKC-dependent switch in AMPA receptor subtypes in mouse cerebellar stellate cells

Neural circuitry for maternal oxytocin release induced by infant cries

Oxytocin is a neuropeptide that is important for maternal physiology and childcare, including parturition and milk ejection during nursing1-6. Suckling triggers the release of oxytocin, but other sensory cues-specifically, infant cries-can increase the levels of oxytocin in new human mothers7, which indicates that cries can activate hypothalamic oxytocin neurons. Here we describe a neural circuit that routes auditory information about infant vocalizations to mouse oxytocin neurons. We performed in vivo electrophysiological recordings and photometry from identified oxytocin neurons in awake maternal mice that were presented with pup calls. We found that oxytocin neurons responded to pup vocalizations, but not to pure tones, through input from the posterior intralaminar thalamus, and that repetitive thalamic stimulation induced lasting disinhibition of oxytocin neurons. This circuit gates central oxytocin release and maternal behaviour in response to calls, providing a mechanism for the integration of sensory cues from the offspring in maternal endocrine networks to ensure modulation of brain state for efficient parenting.

Epileptiform activity promotes decreasing of Ca2+ conductivity of NMDARs, AMPARs, KARs, and voltage-gated calcium channels in Mg2+-free model

NMDA, AMPA, and kainate receptors are the principal excitatory receptors in the brain. These receptors have been considered as the main targets in the treatment of epilepsy in recent years. This work aimed to determine how the Ca²⁺ conductivity of ionotropic glutamate receptors and voltage-gated Ca²⁺ channels changes in an in vitro model of epilepsy. For induction of epileptiform activity, hippocampal neurons were exposed to Mg²⁺-free medium. It has been shown that removal of Mg²⁺ from the medium not only removes the block from the NMDA receptors but also stimulates the release of glutamate in a way that is independent of the NMDA receptors. Under these conditions, the structure of the bursts significantly differs from the spontaneous bursts arising in mature hippocampal cultures. We have demonstrated that the frequency and amplitude of Mg²⁺-free medium-induced Ca²⁺ oscillations decrease after the 60-min exposure. Besides, the Ca²⁺ conductivity of ionotropic glutamate receptors and voltage-gated calcium channels significantly reduces. Thus, the decrease of Ca²⁺ conductivity can be considered as one of the mechanisms of adaptation during epilepsy.

Vascular endothelial growth factor increases the function of calcium-impermeable AMPA receptor GluA2 subunit in astrocytes via activation of protein kinase C signaling pathway

Astrocytic calcium signaling plays pivotal roles in the maintenance of neural functions and neurovascular coupling in the brain. Vascular endothelial growth factor (VEGF), an original biological substance of vessels, regulates the movement of calcium and potassium ions across neuronal membrane. In this study, we investigated whether and how VEGF regulates glutamate-induced calcium influx in astrocytes. We used cultured astrocytes combined with living cell imaging to detect the calcium influx induced by glutamate. We found that VEGF quickly inhibited the glutamate/hypoxia-induced calcium influx, which was blocked by an AMPA receptor antagonist CNQX, but not D-AP5 or UBP310, NMDA and kainate receptor antagonist, respectively. VEGF increased phosphorylation of PKCα and AMPA receptor subunit GluA2 in astrocytes, and these effects were diminished by SU1498 or calphostin C, a PKC inhibitor. With the pHluorin assay, we observed that VEGF significantly increased membrane insertion and expression of GluA2, but not GluA1, in astrocytes. Moreover, siRNA-produced knockdown of GluA2 expression in astrocytes reversed the inhibitory effect of VEGF on glutamate-induced calcium influx. Together, our results suggest that VEGF reduces glutamate-induced calcium influx in astrocytes via enhancing PKCα-mediated GluA2 phosphorylation, which in turn promotes the membrane insertion and expression of GluA2 and causes AMPA receptors to switch from calcium-permeable to calcium-impermeable receptors, thereby inhibiting astrocytic calcium influx. The present study reveals that excitatory neurotransmitter glutamate-mediated astrocytic calcium influx can be regulated by vascular biological factor via activation of AMPA receptor GluA2 subunit and uncovers a novel coupling mechanism between astrocytes and endothelial cells within the neurovascular unit.

Emergence of Endocytosis-Dependent mGlu1 LTD at Nucleus Accumbens Synapses After Withdrawal From Cocaine Self-Administration

Extended-access cocaine self-administration induces a progressive intensification of cue-induced drug craving during withdrawal termed “incubation of cocaine craving”. Rats evaluated after >1 month of withdrawal (when incubation of craving is robust) display alterations in excitatory synapses onto medium spiny neurons (MSNs) of the nucleus accumbens (NAc), including elevated levels of Ca²⁺-permeable AMPA receptors (CP-AMPAR) and a transition from group I metabotropic glutamate receptor (mGluR) mGlu5- to mGlu1-mediated synaptic depression. It is important to further characterize the emergent form of mGlu1-mediated synaptic depression because it has been demonstrated that mGlu1 stimulation, by normalizing CP-AMPAR transmission, reduces cue-induced cocaine craving. In the present study, we conducted whole-cell patch-clamp recordings in NAc core MSNs, comparing rats that underwent >35 days of withdrawal from cocaine self-administration to control rats that had self-administered saline. Bath application of the nonselective group I mGluR agonist dihydroxyphenylglycine (DHPG) produced a transient mGlu5-mediated synaptic depression in saline controls, whereas a persistent mGlu1-mediated synaptic depression emerged in cocaine rats. This form of long-term depression (LTD) was abolished by the inclusion of dynamin inhibitory peptide (DIP) in the recording electrode, indicating that it is mediated by removal of CP-AMPARs through a dynamin-dependent endocytosis mechanism. We further showed that CP-AMPAR endocytosis is normally coupled to the PICK1-mediated insertion of Ca²⁺-impermeable AMPARs (CI-AMPAR). Interestingly, this coupling is not obligatory because disruption of PICK1-mediated CI-AMPAR insertion with pep2-EVKI spared mGlu1-mediated CP-AMPAR endocytosis. Collectively, these results reveal similarities but also differences from mGlu1-LTD observed in other brain regions, and further our understanding of a form of plasticity that may be targeted to reduce cue-induced craving for cocaine and methamphetamine.

Fast changes of NMDA and AMPA receptor activity under acute hyperammonemia in vitro

It was established in experiments on cell cultures of neurons and astrocytes that ammonium ions at concentrations of 4-8 mM cause hyperexcitation of the neuronal network, as a result of which there is a disturbance of calcium homeostasis, which can lead to the death of neurons. In the present study, we investigated the effect of toxic doses of ammonium (8 mM NH₄Cl) on the activity of NMDA and AMPA receptors and the role of these receptors in spontaneous synchronous activity (SSA). In a control experiment in the absence of NH₄Cl, SSA is not suppressed by NMDA receptor inhibitors, but is suppressed by AMPA receptor antagonists. In the presence of toxic doses of NH₄Cl, SSA is completely inhibited by NMDA receptor inhibitors in 63% of neurons and by AMPA receptor inhibitors in 33% of neurons. After short-term applications of toxic doses of ammonium, the amplitude of the Ca²⁺ response to 10 μM NMDA increases, and decreases in response to 500 nM FW (agonist of AMPA receptors). NMDA receptor blocker MK-801 (20 μM), competitive antagonist D-AP5 (10 μM) and competitive AMPA receptor antagonist NBQX (2 μM) abolished the activating ammonium mediated effect on the NMDA receptors while only MK-801, but not NBQX, abolished the inhibiting ammonium mediated effect on AMPA receptors. These data indicate that under acute hyperammonemia, the activity of NMDA receptors increases, while the activity of AMPA receptors decreases. This phenomenon could explain such a wide range of toxic effects of ammonium ions mediated by NMDA receptors.

Topological Regulation of Synaptic AMPA Receptor Expression by the RNA-Binding Protein CPEB3

Synaptic receptors gate the neuronal response to incoming signals, but they are not homogeneously distributed on dendrites. A spatially defined receptor distribution can preferentially amplify certain synaptic inputs, resize receptive fields of neurons, and optimize information processing within a neuronal circuit. Thus, a longstanding question is how the spatial organization of synaptic receptors is achieved. Here, we find that action potentials provide local signals that influence the distribution of synaptic AMPA receptors along dendrites in mouse cerebellar stellate cells. Graded dendritic depolarizations elevate CPEB3 protein at proximal dendrites, where we suggest that CPEB3 binds to GluA2 mRNA, suppressing GluA2 protein synthesis leading to a distance-dependent increase in synaptic GluA2 AMPARs. The activity-induced expression of CPEB3 requires increased Ca(2+) and PKC activation. Our results suggest a cell-autonomous mechanism where sustained postsynaptic firing drives graded local protein synthesis, thus directing the spatial organization of synaptic AMPARs.

Knockdown of Myo-Inositol Transporter SMIT1 Normalizes Cholinergic and Glutamatergic Function in an Immortalized Cell Line Established from the Cerebral Cortex of a Trisomy 16 Fetal Mouse, an Animal Model of Human Trisomy 21 (Down Syndrome)

The Na⁺/myo-inositol cotransporter (SMIT1) is overexpressed in human Down syndrome (DS) and in trisomy 16 fetal mice (Ts16), an animal model of the human condition. SMIT1 overexpression determines increased levels of intracellular myo-inositol, a precursor of phophoinositide synthesis. SMIT1 is overexpressed in CTb cells, an immortalized cell line established from the cerebral cortex of a Ts16 mouse fetus. CTb cells exhibit impaired cytosolic Ca²⁺ signals in response to glutamatergic and cholinergic stimuli (increased amplitude and delayed time-dependent kinetics in the decay post-stimulation), compared to our CNh cell line, derived from the cerebral cortex of a euploid animal. Considering the role of myo-inositol in intracellular signaling, we normalized SMIT1 expression in CTb cells using specific mRNA antisenses. Forty-eight hours post-transfection, SMIT1 levels in CTb cells reached values comparable to those of CNh cells. At this time, decay kinetics of Ca²⁺ signals induced by either glutamate, nicotine, or muscarine were accelerated in transfected CTb cells, to values similar to those of CNh cells. The amplitude of glutamate-induced cytosolic Ca²⁺ signals in CTb cells was also normalized. The results suggest that SMIT1 overexpression contributes to abnormal cholinergic and glutamatergic Ca²⁺ signals in the trisomic condition, and knockdown of DS-related genes in our Ts16-derived cell line could constitute a relevant tool to study DS-related neuronal dysfunction.

Neuroligins Are Selectively Essential for NMDAR Signaling in Cerebellar Stellate Interneurons

Neuroligins are postsynaptic cell-adhesion molecules that contribute to synapse specification. However, many other postsynaptic cell-adhesion molecules are known and the relative contributions of neuroligins versus other such molecules in different types of synapses and neurons remains largely unknown. Here, we have studied the role of neuroligins in cerebellar stellate interneurons that participate in a well defined circuit that converges on Purkinje cells as the major output neurons of cerebellar cortex. By crossing triple conditional knock-out (cKO) mice targeting all three major neuroligins [neuroligin-1 to neuroligin-3 (NL123)] with parvalbumin-Cre (PV-Cre) transgenic mice, we deleted neuroligins from inhibitory cerebellar interneurons and Purkinje cells, allowing us to study the effects of neuroligin deletions on cerebellar stellate cell synapses by electrophysiology in acute slices. PV-Cre/NL123 cKO mice did not exhibit gross alterations of cerebellar structure or cerebellar interneuron morphology. Strikingly, electrophysiological recordings in stellate cells from these PV-Cre/NL123 cKO mice revealed a large decrease in NMDAR-mediated excitatory synaptic responses, which, in stellate cells, are largely extrasynaptic, without a change in AMPA-receptor-mediated responses. Parallel analyses in PV-Cre/NL1 mice that are single NL1 cKO mice uncovered the same phenotype, demonstrating that NL1 is responsible for recruiting extrasynaptic NMDARs. Moreover, we observed only a modest impairment in inhibitory synaptic responses in stellate cells lacking NL123 despite a nearly complete suppression of inhibitory synaptic transmission in Purkinje cells by the same genetic manipulation. Our results suggest that, unlike other types of neurons investigated, neuroligins are selectively essential in cerebellar stellate interneurons for enabling the function of extrasynaptic NMDARs.

SIGNIFICANCE STATEMENT

Neuroligins are postsynaptic cell-adhesion molecules genetically linked to autism. However, the contributions of neuroligins to interneuron functions remain largely unknown. Here, we analyzed the role of neuroligins in cerebellar stellate interneurons. We deleted neuroligin-1, neuroligin-2, and neuroligin-3, the major cerebellar neuroligin isoforms, from stellate cells in triple NL123 conditional knock-out mice and analyzed synaptic responses by acute slice electrophysiology. We find that neuroligins are selectively essential for extrasynaptic NMDAR-mediated signaling, but dispensable for both AMPAR-mediated and inhibitory synaptic transmission. Our results reveal a critical and selective role for neuroligins in the regulation of NMDAR responses in cerebellar stellate interneurons.

Presynaptic GluN2D receptors detect glutamate spillover and regulate cerebellar GABA release

Glutamate directly activates N-methyl-d-aspartate (NMDA) receptors on presynaptic inhibitory interneurons and enhances GABA release, altering the excitatory-inhibitory balance within a neuronal circuit. However, which class of NMDA receptors is involved in the detection of glutamate spillover is not known. GluN2D subunit-containing NMDA receptors are ideal candidates as they exhibit a high affinity for glutamate. We now show that cerebellar stellate cells express both GluN2B and GluN2D NMDA receptor subunits. Genetic deletion of GluN2D subunits prevented a physiologically relevant, stimulation-induced, lasting increase in GABA release from stellate cells [long-term potentiation of inhibitory transmission (I-LTP)]. NMDA receptors are tetramers composed of two GluN1 subunits associated to either two identical subunits (di-heteromeric receptors) or to two different subunits (tri-heteromeric receptors). To determine whether tri-heteromeric GluN2B/2D NMDA receptors mediate I-LTP, we tested the prediction that deletion of GluN2D converts tri-heteromeric GluN2B/2D to di-heteromeric GluN2B NMDA receptors. We find that prolonged stimulation rescued I-LTP in GluN2D knockout mice, and this was abolished by GluN2B receptor blockers that failed to prevent I-LTP in wild-type mice. Therefore, NMDA receptors that contain both GluN2D and GluN2B mediate the induction of I-LTP. Because these receptors are not present in the soma and dendrites, presynaptic tri-heteromeric GluN2B/2D NMDA receptors in inhibitory interneurons are likely to mediate the cross talk between excitatory and inhibitory transmission.

A Model of In vitro Plasticity at the Parallel Fiber-Molecular Layer Interneuron Synapses

Theoretical and computational models of the cerebellum typically focus on the role of parallel fiber (PF)—Purkinje cell (PKJ) synapses for learned behavior, but few emphasize the role of the molecular layer interneurons (MLIs)—the stellate and basket cells. A number of recent experimental results suggest the role of MLIs is more important than previous models put forth. We investigate learning at PF—MLI synapses and propose a mathematical model to describe plasticity at this synapse. We perform computer simulations with this form of learning using a spiking neuron model of the MLI and show that it reproduces six in vitro experimental results in addition to simulating four novel protocols. Further, we show how this plasticity model can predict the results of other experimental protocols that are not simulated. Finally, we hypothesize what the biological mechanisms are for changes in synaptic efficacy that embody the phenomenological model proposed here.

Synaptic Multivesicular Release in the Cerebellar Cortex: Its Mechanism and Role in Neural Encoding and Processing

The number of synaptic vesicles released during fast release plays a major role in determining the strength of postsynaptic response. However, it remains unresolved how the number of vesicles released in response to action potentials is controlled at a single synapse. Recent findings suggest that the Cav2.1 subtype (P/Q-type) of voltage-gated calcium channels is responsible for inducing presynaptic multivesicular release (MVR) at rat cerebellar glutamatergic synapses from granule cells to molecular layer interneurons. The topographical distance from Cav2.1 channels to exocytotic Ca(2+) sensors is a critical determinant of MVR. In physiological trains of presynaptic neurons, MVR significantly impacts the excitability of postsynaptic neurons, not only by increasing peak amplitude but also by prolonging decay time of the postsynaptic currents. Therefore, MVR contributes additional complexity to neural encoding and processing in the cerebellar cortex.

Integrated plasticity at inhibitory and excitatory synapses in the cerebellar circuit

The way long-term potentiation (LTP) and depression (LTD) are integrated within the different synapses of brain neuronal circuits is poorly understood. In order to progress beyond the identification of specific molecular mechanisms, a system in which multiple forms of plasticity can be correlated with large-scale neural processing is required. In this paper we take as an example the cerebellar network, in which extensive investigations have revealed LTP and LTD at several excitatory and inhibitory synapses. Cerebellar LTP and LTD occur in all three main cerebellar subcircuits (granular layer, molecular layer, deep cerebellar nuclei) and correspondingly regulate the function of their three main neurons: granule cells (GrCs), Purkinje cells (PCs) and deep cerebellar nuclear (DCN) cells. All these neurons, in addition to be excited, are reached by feed-forward and feed-back inhibitory connections, in which LTP and LTD may either operate synergistically or homeostatically in order to control information flow through the circuit. Although the investigation of individual synaptic plasticities in vitro is essential to prove their existence and mechanisms, it is insufficient to generate a coherent view of their impact on network functioning in vivo. Recent computational models and cell-specific genetic mutations in mice are shedding light on how plasticity at multiple excitatory and inhibitory synapses might regulate neuronal activities in the cerebellar circuit and contribute to learning and memory and behavioral control.

A functional coupling between extrasynaptic NMDA receptors and A-type K+ channels under astrocyte control regulates hypothalamic neurosecretory neuronal activity

Neuronal activity is controlled by a fine-tuned balance between intrinsic properties and extrinsic synaptic inputs. Moreover, neighbouring astrocytes are now recognized to influence a wide spectrum of neuronal functions. Yet, how these three key factors act in concert to modulate and fine-tune neuronal output is not well understood. Here, we show that in rat hypothalamic magnocellular neurosecretory cells (MNCs), glutamate NMDA receptors (NMDARs) are negatively coupled to the transient, voltage-gated A-type K(+) current (IA). We found that activation of NMDARs by extracellular glutamate levels influenced by astrocyte glutamate transporters resulted in a significant inhibition of IA. The NMDAR-IA functional coupling resulted from activation of extrasynaptic NMDARs, was calcium- and protein kinase C-dependent, and involved enhanced steady-state, voltage-dependent inactivation of IA. The NMDAR-IA coupling diminished the latency to the first evoked spike in response to membrane depolarization and increased the total number of evoked action potentials, thus strengthening the neuronal input/output function. Finally, we found a blunted NMDA-mediated inhibition of IA in dehydrated rats. Together, our findings support a novel signalling mechanism that involves a functional coupling between extrasynaptic NMDARs and A-type K(+) channels, which is influenced by local astrocytes. We show this signalling complex to play an important role in modulating hypothalamic neuronal excitability, which may contribute to adaptive responses during a sustained osmotic challenge such as dehydration.

Activation of extrasynaptic NMDARs at individual parallel fiber-molecular layer interneuron synapses in cerebellum

NMDA receptors (NMDARs) expressed by cerebellar molecular layer interneurons (MLIs) are not activated by single exocytotic events but can respond to glutamate spillover following coactivation of adjacent parallel fibers (PFs), indicating that NMDARs are perisynaptic. Several types of synaptic plasticity rely on these receptors but whether they are activated at isolated synapses is not known. Using a combination of electrophysiological and optical recording techniques in acute slices of rat cerebellum, along with modeling, we find that repetitive activation of single PF-MLI synapses can activate NMDARs in MLIs. High-frequency stimulation, multivesicular release (MVR), or asynchronous release can each activate NMDARs. Frequency facilitation was found at all PF-MLI synapses but, while some showed robust MVR with increased release probability, most were limited to univesicular release. Together, these results reveal a functional diversity of PF synapses, which use different mechanisms to activate NMDARs.

Norepinephrine enhances a discrete form of long-term depression during fear memory storage

Amygdala excitatory synaptic strengthening is thought to contribute to both conditioned fear and anxiety. Thus, one basis for behavioral flexibility could allow these pathways to be weakened and corresponding emotion to be attenuated. However, synaptic depression within the context of amygdala-dependent behavior remains poorly understood. Previous work identified lateral amygdala (LA) calcium-permeable AMPA receptors (CP-AMPARs) as a key target for synaptic removal in long-term depression (LTD) and persistent fear attenuation. Here we demonstrate that LA neurons express two equally potent forms of LTD with contrasting requirements for protein kinase and phosphatase activity and differential impact on CP-AMPAR trafficking. Selective removal of CP-AMPARs from synapses is contingent on group 1 metabotropic glutamate receptor (mGluR1) and PKC signaling, in contrast to an alternate LTD pathway that nonselectively removes AMPARs and requires calcineurin (PP2b). Intriguingly, the balance between these forms of LTD is shifted by posttraining activation of β-adrenergic receptors in fear conditioned mice, resulting in selective augmentation of mGluR-dependent depression. These results highlight the complexity of core mechanisms in LTD and suggest that norepinephrine exposure mediates a form of synaptic metaplasticity that recalibrates fear memory processing.

Ketamine, magnesium and major depression--from pharmacology to pathophysiology and back

The glutamatergic mechanism of antidepressant treatments is now in the center of research to overcome the limitations of monoamine-based approaches. There are several unresolved issues. For the action of the model compound, ketamine, NMDA-receptor block, AMPA-receptor activation and BDNF release appear to be involved in a mechanism, which leads to synaptic sprouting and strengthened synaptic connections. The link to the pathophysiology of depression is not clear. An overlooked connection is the role of magnesium, which acts as physiological NMDA-receptor antagonist: 1. There is overlap between the actions of ketamine with that of high doses of magnesium in animal models, finally leading to synaptic sprouting. 2. Magnesium and ketamine lead to synaptic strengthening, as measured by an increase in slow wave sleep in humans. 3. Pathophysiological mechanisms, which have been identified as risk factors for depression, lead to a reduction of (intracellular) magnesium. These are neuroendocrine changes (increased cortisol and aldosterone) and diabetes mellitus as well as Mg(2+) deficiency. 4. Patients with therapy refractory depression appear to have lower CNS Mg(2+) levels in comparison to health controls. 5. Experimental Mg(2+) depletion leads to depression- and anxiety like behavior in animal models. 6. Ketamine, directly or indirectly via non-NMDA glutamate receptor activation, acts to increase brain Mg(2+) levels. Similar effects have been observed with other classes of antidepressants. 7. Depressed patients with low Mg(2+) levels tend to be therapy refractory. Accordingly, administration of Mg(2+) either alone or in combination with standard antidepressants acts synergistically on depression like behavior in animal models.

CONCLUSION

On the basis of the potential pathophysiological role of Mg(2+)-regulation, it may be possible to predict the action of ketamine and of related compounds based on Mg(2+) levels. Furthermore, screening for compounds to increase neuronal Mg(2+) concentration could be a promising instrument to identify new classes of antidepressants. Overall, any discussion of the glutamatergic system in affective disorders should consider the role of Mg(2+).

A role of TARPs in the expression and plasticity of calcium-permeable AMPARs: evidence from cerebellar neurons and glia

The inclusion of GluA2 subunits has a profound impact on the channel properties of AMPA receptors (AMPARs), in particular rendering them impermeable to calcium. While GluA2-containing AMPARs are the most abundant in the central nervous system, GluA2-lacking calcium-permeable AMPARs are also expressed in wide variety of neurons and glia. Accumulating evidence suggests that the dynamic control of the GluA2 content of AMPARs plays a critical role in development, synaptic plasticity, and diverse neurological conditions ranging from ischemia-induced brain damage to drug addiction. It is thus important to understand the molecular mechanisms involved in regulating the balance of AMPAR subtypes, particularly the role of their co-assembled auxiliary subunits. The discovery of transmembrane AMPAR regulatory proteins (TARPs), initially within the cerebellum, has transformed the field of AMPAR research. It is now clear that these auxiliary subunits play a key role in multiple aspects of AMPAR trafficking and function in the brain. Yet, their precise role in AMPAR subtype-specific regulation has only recently received particular attention. Here we review recent findings on the differential regulation of calcium-permeable (CP-) and -impermeable (CI-) AMPARs in cerebellar neurons and glial cells, and discuss the critical involvement of TARPs in this process.

This article is part of the Special Issue entitled ‘Glutamate Receptor-Dependent Synaptic Plasticity’.

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Calcium-permeable AMPARs are present in various cerebellar neurons and glial cells.

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The contribution of calcium-permeable AMPARs to transmission is dynamically regulated.

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TARPs influence the relative expression of AMPAR subtypes.

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Evidence suggests that TARPs play a role in calcium-permeable AMPAR plasticity.

TRPM4 cation channel mediates axonal and neuronal degeneration in experimental autoimmune encephalomyelitis and multiple sclerosis

In multiple sclerosis, an inflammatory disease of the central nervous system (CNS), axonal and neuronal loss are major causes for irreversible neurological disability. However, which molecules contribute to axonal and neuronal injury under inflammatory conditions remains largely unknown. Here we show that the transient receptor potential melastatin 4 (TRPM4) cation channel is crucial in this process. TRPM4 is expressed in mouse and human neuronal somata, but it is also expressed in axons in inflammatory CNS lesions in experimental autoimmune encephalomyelitis (EAE) in mice and in human multiple sclerosis tissue. Deficiency or pharmacological inhibition of TRPM4 using the antidiabetic drug glibenclamide resulted in reduced axonal and neuronal degeneration and attenuated clinical disease scores in EAE, but this occurred without altering EAE-relevant immune function. Furthermore, Trpm4(-/-) mouse neurons were protected against inflammatory effector mechanisms such as excitotoxic stress and energy deficiency in vitro. Electrophysiological recordings revealed TRPM4-dependent neuronal ion influx and oncotic cell swelling upon excitotoxic stimulation. Therefore, interference with TRPM4 could translate into a new neuroprotective treatment strategy.

Light-induced plasticity of synaptic AMPA receptor composition in retinal ganglion cells

Light-evoked responses of all three major classes of retinal ganglion cells (RGCs) are mediated by NMDA receptors (NMDARs) and AMPA receptors (AMPARs). Although synaptic activity at RGC synapses is highly dynamic, synaptic plasticity has not been observed in adult RGCs. Here, using patch-clamp recordings in dark-adapted mouse retina, we report a retina-specific form of AMPAR plasticity. Both chemical and light activation of NMDARs caused the selective endocytosis of GluA2-containing, Ca(2+)-impermeable AMPARs on RGCs and replacement with GluA2-lacking, Ca(2+)-permeable AMPARs. The plasticity was expressed in ON but not OFF RGCs and was restricted solely to the ON responses in ON-OFF RGCs. Finally, the plasticity resulted in a shift in the light responsiveness of ON RGCs. Thus, physiologically relevant light stimuli can induce a change in synaptic receptor composition of ON RGCs, providing a mechanism by which the sensitivity of RGC responses may be modified under scotopic conditions.

Distributed synergistic plasticity and cerebellar learning

Studies on synaptic plasticity in the context of learning have been dominated by the view that a single, particular type of plasticity forms the underlying mechanism for a particular type of learning. However, emerging evidence shows that many forms of synaptic and intrinsic plasticity at different sites are induced conjunctively during procedural memory formation in the cerebellum. Here, we review the main forms of long-term plasticity in the cerebellar cortex that underlie motor learning. We propose that the different forms of plasticity in the granular layer and the molecular layer operate synergistically in a temporally and spatially distributed manner, so as to ultimately create optimal output for behaviour.

Channel properties reveal differential expression of TARPed and TARPless AMPARs in stargazer neurons

Dynamic regulation of calcium-permeable AMPA receptors (CP-AMPARs) is important for normal synaptic transmission, plasticity and pathological changes. Although the involvement of transmembrane AMPAR regulatory proteins (TARPs) in trafficking of calcium-impermeable AMPARs (CI-AMPARs) has been extensively studied, their role in the surface expression and function of CP-AMPARs remains unclear. We examined AMPAR-mediated currents in cerebellar stellate cells from stargazer mice, which lack the prototypical TARP stargazin (g-2). We found a marked increase in the contribution of CP-AMPARs to synaptic responses, indicating that, unlike CI-AMPARs, these can localize at synapses in the absence of g-2. In contrast with CP-AMPARs in extrasynaptic regions, synaptic CP-AMPARs displayed an unexpectedly low channel conductance and strong block by intracellular spermine, suggesting that they were ‘TARPless’. As a proof of principle that TARP association is not an absolute requirement for AMPAR clustering at synapses, miniature excitatory postsynaptic currents mediated by TARPless AMPARs were readily detected in stargazer granule cells following knockdown of their only other TARP, g-7.

Activation of protein kinase C isozymes for the treatment of dementias

Memories are much more easily impaired than improved. Dementias, a lasting impairment of memory function, occur in a variety of cognitive disorders and become more clinically dominant as the population ages. Protein kinase C is one of the "cognitive kinases," and plays an essential role in both memory acquisition and maintenance. Deficits in protein kinase C (PKC) signal cascades in neurons represent one of the earliest changes in the brains of patients with Alzheimer's disease (AD) and other types of memory impairment, including those related to cerebral ischemia and ischemic stroke. Inhibition or impairment of PKC activity results in compromised learning and memory, whereas an appropriate activation of certain PKC isozymes leads to an enhancement of learning and memory and/or antidementic effects. In preclinical studies, PKC activators have been shown to increase the expression and activity of PKC isozymes, thereby restoring PKC signaling and downstream activity, including stimulation of neurotrophic activity, synaptic/structural remodeling, and synaptogenesis in the hippocampus and related cortical areas. PKC activators also reduce the accumulation of neurotoxic amyloid and tau protein hyperphosphorylation and support anti-apoptotic processes in the brain. These observations strongly suggest that PKC pharmacology may represent an attractive area for the development of effective cognition-enhancing therapeutics for the treatment of dementias.

Redefining the classification of AMPA-selective ionotropic glutamate receptors

AMPA-type ionotropic glutamate receptors (iGluRs) represent the major excitatory neurotransmitter receptor in the developing and adult vertebrate CNS. They are crucial for the normal hardwiring of glutamatergic circuits but also fine tune synaptic strength by cycling into and out of synapses during periods of sustained patterned activity or altered homeostasis. AMPARs are grouped into two functionally distinct tetrameric assemblies based on the inclusion or exclusion of the GluA2 receptor subunit. GluA2-containing receptors are thought to be the most abundant AMPAR in the CNS, typified by their small unitary events, Ca(2+) impermeability and insensitivity to polyamine block. In contrast, GluA2-lacking AMPARs exhibit large unitary conductance, marked divalent permeability and nano- to micromolar polyamine affinity. Here, I review evidence for the existence of a third class of AMPAR which, though similarly Ca(2+) permeable, is characterized by its near-insensitivity to internal and external channel block by polyamines. This novel class of AMPAR is most notably found at multivesicular release synapses found in the avian auditory brainstem and mammalian retina. Curiously, these synapses lack NMDA-type iGluRs, which are conventionally associated with controlling AMPAR insertion. The lack of NMDARs suggests that a different set of rules may govern AMPAR cycling at these synapses. AMPARs with similar functional profiles are also found on some glial cells suggesting they may have a more widespread distribution in the mammalian CNS. I conclude by noting that modest changes to the ion-permeation pathway might be sufficient to retain divalent permeability whilst eliminating polyamine sensitivity. Consequently, this emerging AMPAR subclass need not be assembled from novel subunits, yet to be cloned, but could simply occur by varying the stoichiometry of existing proteins.

Computational investigation of the changing patterns of subtype specific NMDA receptor activation during physiological glutamatergic neurotransmission

NMDA receptors (NMDARs) are the major mediator of the postsynaptic response during synaptic neurotransmission. The diversity of roles for NMDARs in influencing synaptic plasticity and neuronal survival is often linked to selective activation of multiple NMDAR subtypes (NR1/NR2A-NMDARs, NR1/NR2B-NMDARs, and triheteromeric NR1/NR2A/NR2B-NMDARs). However, the lack of available pharmacological tools to block specific NMDAR populations leads to debates on the potential role for each NMDAR subtype in physiological signaling, including different models of synaptic plasticity. Here, we developed a computational model of glutamatergic signaling at a prototypical dendritic spine to examine the patterns of NMDAR subtype activation at temporal and spatial resolutions that are difficult to obtain experimentally. We demonstrate that NMDAR subtypes have different dynamic ranges of activation, with NR1/NR2A-NMDAR activation sensitive at univesicular glutamate release conditions, and NR2B containing NMDARs contributing at conditions of multivesicular release. We further show that NR1/NR2A-NMDAR signaling dominates in conditions simulating long-term depression (LTD), while the contribution of NR2B containing NMDAR significantly increases for stimulation frequencies that approximate long-term potentiation (LTP). Finally, we show that NR1/NR2A-NMDAR content significantly enhances response magnitude and fidelity at single synapses during chemical LTP and spike timed dependent plasticity induction, pointing out an important developmental switch in synaptic maturation. Together, our model suggests that NMDAR subtypes are differentially activated during different types of physiological glutamatergic signaling, enhancing the ability for individual spines to produce unique responses to these different inputs.

Release of glutamate from one neuron onto glutamate receptors on adjacent neurons serves as the primary basis for neuronal communication. Further, different types of glutamate signals produce unique responses within the neuronal network, providing the ability for glutamate receptors to discriminate between alternative types of signaling. The NMDA receptor (NMDAR) is a glutamate receptor that mediates a variety of physiological functions, including the molecular basis for learning and memory. These receptors exist as a variety of subtypes, and this molecular heterogeneity is used to explain the diversity in signaling initiated by NMDARs. However, the lack of reliable experimental tools to control the activation of each subtype has led to debate over the subtype specific roles of the NMDAR. We have developed a stochastic model of glutamate receptor activation at a single synapse and find that NMDAR subtypes detect different types of glutamate signals. Moreover, the presence of multiple populations of NMDAR subtypes on a given neuron allows for differential patterns of NMDAR activation in response to varied glutamate inputs. This model demonstrates how NMDAR subtypes enable effective and reliable communication within neuronal networks and can be used as a tool to examine specific roles of NMDAR subtypes in neuronal function.

Inhibition of Ca2+-activated large-conductance K+ channel activity alters synaptic AMPA receptor phenotype in mouse cerebellar stellate cells

Many fast-spiking inhibitory interneurons, including cerebellar stellate cells, fire brief action potentials and express α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors (AMPAR) that are permeable to Ca(2+) and do not contain the GluR2 subunit. In a recent study, we found that increasing action potential duration promotes GluR2 gene transcription in stellate cells. We have now tested the prediction that activation of potassium channels that control the duration of action potentials can suppress the expression of GluR2-containing AMPARs at stellate cell synapses. We find that large-conductance Ca(2+)-activated potassium (BK) channels mediate a large proportion of the depolarization-evoked noninactivating potassium current in stellate cells. Pharmacological blockade of BK channels prolonged the action potential duration in postsynaptic stellate cells and altered synaptic AMPAR subtype from GluR2-lacking to GluR2-containing Ca(2+)-impermeable AMPARs. An L-type channel blocker abolished an increase in Ca(2+) entry that was associated with spike broadening and also prevented the BK channel blocker-induced switch in AMPAR phenotype. Thus blocking BK potassium channels prolongs the action potential duration and increases the expression of GluR2-containing receptors at the synapse by enhancing Ca(2+) entry in cerebellar stellate cells.

Stargazin (TARP gamma-2) is required for compartment-specific AMPA receptor trafficking and synaptic plasticity in cerebellar stellate cells

In the cerebellar cortex, parallel fiber-to-stellate cell (PF-SC) synapses exhibit a form of synaptic plasticity manifested as a switch in the subunit composition of synaptic AMPA receptors (AMPARs) from calcium-permeable, GluA2-lacking to calcium-impermeable, GluA2-containing receptors. Here, we examine the role of stargazin (γ-2), canonical member of the transmembrane AMPAR regulatory protein (TARP) family, in the regulation of GluA2-lacking AMPARs and synaptic plasticity in SCs from epileptic and ataxic stargazer mutant mice. We found that AMPAR-mediated synaptic transmission is severely diminished in stargazer SCs, and that the rectification index (RI) of AMPAR current is reduced. Activity-dependent plasticity in the rectification of synaptic AMPARs is also impaired in stargazer SCs. Despite the dramatic loss in synaptic AMPARs, extrasynaptic AMPARs are preserved. We then examined the role of stargazin in regulating the rectification of extrasynaptic AMPARs in nucleated patches and found, in contrast to previous reports, that wild-type extrasynaptic AMPARs have moderate RI values (average RI = 0.38), while those in stargazer SCs are low (average RI = 0.24). The GluA2-lacking AMPAR blocker, philanthotoxin-433 (PhTx-433), was used as an alternative measure of GluA2 content in wild-type and stargazer SCs. Despite the difference in RI, PhTx-433 sensitivity of both synaptic and extrasynaptic AMPARs remains unchanged, suggesting that the dramatic changes in RI and the impairment in synaptic plasticity observed in the stargazer mouse are not the result of a specific impairment in GluA2 trafficking. Together, these data suggest that stargazin regulates compartment-specific AMPAR trafficking, as well as activity-dependent plasticity in synaptic AMPAR rectification at cerebellar PF-SC synapses.

Remodeling of synaptic AMPA receptor subtype alters the probability and pattern of action potential firing

Changes in the subunit composition of postsynaptic AMPA-type glutamate receptors can be induced at CNS synapses by neural activity and under certain pathological conditions. Fear-induced incorporation of GluR2-containing receptors at cerebellar synapses selectively prolongs the decay time of synaptic currents, whereas a switch from GluR2-lacking to GluR2-containing receptors induced by parallel fiber stimulation reduces the amplitude in addition to lengthening the duration of EPSCs. Although it is often assumed that these two forms of synaptic plasticity will alter action potential (AP) firing in the postsynaptic neuron, this has not been directly tested. Using a dynamic current-clamp approach, we now show that the fear-induced increase in EPSC duration increases the size of EPSPs and thereby markedly enhances the AP firing probability. In contrast, the parallel fiber stimulation-triggered switch in GluR2 expression reduces the EPSP-AP coupling because of the decrease in the synaptic current amplitude. The switch also abolished the paired-pulse facilitation that arose from an activity and spermine-dependent unblock of GluR2-lacking receptors and hence reduced the ability of paired stimuli to evoke two consecutive APs. Therefore, fear-induced incorporation of GluR2 receptors enhances the EPSP-AP coupling, but the parallel fiber stimulation-triggered switch reduces both the EPSP-AP coupling and evoked AP doublets. In contrast to long-term potentiation and depression, which modify the amplitude of synaptic currents, this activity-induced change in AMPA receptor phenotype alters synaptic conductance waveform and postsynaptic short-term plasticity. These changes modulate both the probability and pattern of evoked AP firing via a fundamentally different mechanism from long-term potentiation and long-term depression.

A single fear-inducing stimulus induces a transcription-dependent switch in synaptic AMPAR phenotype

Changes in emotional state are known to alter neuronal excitability and can modify learning and memory formation. Such experience–dependent neuronal plasticity can be long-lasting and is thought to involve the regulation of gene transcription. Here we show that a single fear-inducing stimulus increases GluR2 mRNA abundance and promotes synaptic incorporation of GluR2-containing AMPA receptors (AMPARs) in mouse cerebellar stellate cells. The switch in synaptic AMPAR phenotype is mediated by noradrenaline and action potential prolongation. The subsequent rise in intracellular Ca²⁺ and activation of Ca²⁺-sensitive ERK /MAPK signaling trigger new GluR2 gene transcription and a switch in the synaptic AMPAR phenotype from GluR2-lacking, Ca²⁺-permeable, to GluR2-containing Ca²⁺-impermeable receptors on the order of hours. The change in glutamate receptor phenotype alters synaptic efficacy in cerebellar stellate cells. Thus, a single fear-inducing stimulus can induce a long-term change in synaptic receptor phenotype and may alter the activity of an inhibitory neural network.

Spine microdomains for postsynaptic signaling and plasticity

Changes in the molecular composition and signaling properties of excitatory glutamatergic synapses onto dendritic spines mediate learning-related plasticity in the mammalian brain. This molecular adaptation serves as the most celebrated cell biological model for learning and memory. Within their micron-sized dimensions, dendritic spines restrict the diffusion of signaling molecules and spatially confine the activation of signal transduction pathways. Much of this local regulation occurs by spatial compartmentalization of glutamate receptors. Here, we review recently identified cell biological mechanisms regulating glutamate receptor mobility within individual dendritic spines. We discuss the emerging functions of glutamate receptors residing within sub-spine microdomains and propose a model for distinct signaling platforms with specialized functions in synaptic plasticity.

Glutamatergic modulation of cerebellar interneuron activity is mediated by an enhancement of GABA release and requires protein kinase A/RIM1alpha signaling

Information processing in the CNS is controlled by the activity of neuronal networks composed of principal neurons and interneurons. Activity-dependent modification of synaptic transmission onto principal neurons is well studied, but little is known about the modulation of inhibitory transmission between interneurons. However, synaptic plasticity at this level has clear implications for the generation of synchronized activity. We investigated the molecular mechanism(s) and functional consequences of an activity-induced lasting increase in GABA release that occurs between inhibitory interneurons (stellate cells) in the cerebellum. Using whole-cell recording and cerebellar slices, we found that stimulation of glutamatergic inputs (parallel fibers) with a physiological-like pattern of activity triggered a lasting increase in GABA release from stellate cells. This activity also potentiated inhibitory transmission between synaptically connected interneurons. Extracellular recording revealed that the enhanced inhibitory transmission reduced the firing frequency and altered the pattern of action potential activity in stellate cells. The induction of the sustained increase in GABA release required activation of NMDA receptors. Using pharmacological and genetic approaches, we found that presynaptic cAMP/PKA (protein kinase A) signaling and RIM1alpha, an active zone protein, is the critical pathway that is required for the lasting enhancement of GABA release. Thus, a common mechanism can underlie presynaptic plasticity of both excitatory and inhibitory transmission. This activity-dependent regulation of synaptic transmission between inhibitory interneurons may serve as an important mechanism for interneuronal network plasticity.

Light-induced rescue of breathing after spinal cord injury

Paralysis is a major consequence of spinal cord injury (SCI). After cervical SCI, respiratory deficits can result through interruption of descending presynaptic inputs to respiratory motor neurons in the spinal cord. Expression of channelrhodopsin-2 (ChR2) and photostimulation in neurons affects neuronal excitability and produces action potentials without any kind of presynaptic inputs. We hypothesized that after transducing spinal neurons in and around the phrenic motor pool to express ChR2, photostimulation would restore respiratory motor function in cervical SCI adult animals. Here we show that light activation of ChR2-expressing animals was sufficient to bring about recovery of respiratory diaphragmatic motor activity. Furthermore, robust rhythmic activity persisted long after photostimulation had ceased. This recovery was accomplished through a form of respiratory plasticity and spinal adaptation which is NMDA receptor dependent. These data suggest a novel, minimally invasive therapeutic avenue to exercise denervated circuitry and/or restore motor function after SCI.

Calcium-permeable presynaptic AMPA receptors in cerebellar molecular layer interneurones

Axons of cerebellar molecular layer interneurones (MLIs) bear ionotropic glutamate receptors. Here, we show that these receptors elicit cytosolic [Ca2+] transients in axonal varicosities following glutamate spillover induced by stimulation of parallel fibres (PFs). A spatial profile analysis indicates that these transients occur at the same locations when induced by PF stimulation or trains of action potentials. They are not affected by the NMDAR antagonist AP-V, but are abolished by the AMPAR inhibitor GYKI-53655. Mimicking glutamate spillover by a puff of AMPA triggers axonal [Ca2+]i transients even in the presence of TTX. Addition of specific voltage-dependent Ca2+ channel (VDCC) blockers such as omega-AGAIVA and omega-conotoxin GVIA or broad range inhibitors such as Cd2+ did not significantly inhibit the signal indicating the involvement of Ca2+-permeable AMPARs. This hypothesis is further supported by the finding that the subunit specific AMPAR antagonist IEM-1460 blocks 75% of the signal. Bath application of AMPA increases the frequency and mean peak amplitude of GABAergic mIPSCs, an effect that is blocked by philanthotoxin-433 (PhTx) and reinforced by facilitating concentrations of ryanodine. By contrast, a high concentration of ryanodine or dantrolene reduced the effects of AMPA on mIPSCs. Single-cell RT-PCR experiments show that all GluR1-4 subunits are potentially expressed in MLI. Taken together, the results suggest that Ca2+-permeable AMPARs are colocalized with VDCCs in axonal varicosities and can be activated by glutamate spillover through PF stimulation. The AMPAR-mediated Ca2+ signal is amplified by Ca2+-induced Ca2+ release from intracellular stores, leading to GABA release by MLIs.

Surface trafficking of N-methyl-D-aspartate receptors: physiological and pathological perspectives

The N-methyl-D-aspartate receptor (NMDAR) plays a crucial role in shaping the strength of synaptic connections. Over the last decades, extensive studies have defined the cellular and molecular mechanisms by which synaptic NMDARs control the maturation and plasticity of synaptic transmission, and how altered synaptic NMDAR signaling is implicated in neurodegenerative and psychiatric disorders. It is now clear that activation of synaptic or extrasynaptic NMDARs produces different signaling cascades and thus neuronal functions. Our current understanding of NMDAR surface distribution and trafficking is only emerging. Exchange of NMDARs between synaptic and extrasynaptic areas through surface diffusion is a highly dynamic and regulated process. The aim of this review is to describe the identified mechanisms that regulate surface NMDAR behaviors and discuss the impact of this new trafficking pathway on the well-established NMDAR-dependent physiological and pathophysiological processes.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.

Long-term synaptic plasticity in cerebellar stellate cells

Inhibitory transmission controls the action potential firing rate and pattern of Purkinje cell activity in the cerebellum. A long-term change in inhibitory transmission is likely to have a profound effect on the activity of cerebellar neuronal circuits. However, little is known about how neuronal activity regulates synaptic transmission in GABAergic inhibitory interneurons (stellate/basket cells) in the cerebellar cortex. We have examined how glutamate released from parallel fibers (the axons of granule cells) influences postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in stellate cells and modulates gamma-aminobutyric acid (GABA) release from these neurons. First, we found that burst stimulation of presynaptic parallel fibers changes the subunit composition of post-synaptic AMPA receptors from GluR2-lacking to GluR2-containing receptors. This switch reduces the Ca(2+) permeability of AMPA receptors and the excitatory postsynaptic potential amplitude and prolongs the duration of the synaptic current, producing a qualitative change in synaptic transmission. This switch in AMPA receptor phenotype can be induced by activation of extrasynaptic N-methyl-D: -aspartate (NMDA) receptors and involves PICK1 and the activation of protein kinase C. Second, activation of presynaptic NMDA receptors triggers a lasting increase in GABA release from stellate cells. These changes may provide a cellular mechanism underlying associative learning involving the cerebellum.

Output-based comparison of alternative kinetic schemes for the NMDA receptor within a glutamate spillover model

Recent experimental and theoretical work continues to explore the mechanisms and implications of neurotransmitter spillover. Here we examine N-methyl-D-aspartate receptor (NMDA-R) kinetics to determine their implication(s) in glutamate spillover by comparing two mechanistically different NMDA-R models, the 5-state Lester and Jahr (LJ) model and the 8-state Banke and Traynelis (BT) model, within the context of a glutamate spillover model. We employ a search-survey-and-summarize strategy to analyze the relationships within model behavior (model relational analysis) and form a model output landscape. Our results indicate that model relational analysis can reveal differences in models whose outputs would be considered the same. The analysis reveals that the BT model, with its more complex kinetics, is less reliant on diffusion compared to the LJ version, resulting in differences in the relationships between open probability and glutamate concentration despite the fact that both model versions were able to produce the same target output values. Additionally, model relational analysis is able to distinguish between the BT and LJ NMDA-R model versions even though factor analysis indicates that the overall model output space dimensions are the same for both NMDA-R models. Furthermore, the work presented here suggests that model relational analysis may be broadly applicable as a means to examine the complex interactions hidden within overall model behavior.