Postsynaptic induction and presynaptic expression of group 1 mGluR-dependent LTD in the hippocampal CA1 region

Functional specialization of different PI3K isoforms for the control of neuronal architecture, synaptic plasticity, and cognition

Neuronal connectivity and activity-dependent synaptic plasticity are fundamental properties that support brain function and cognitive performance. Phosphatidylinositol 3-kinase (PI3K) intracellular signaling controls multiple mechanisms mediating neuronal growth, synaptic structure, and plasticity. However, it is still unclear how these pleiotropic functions are integrated at molecular and cellular levels. To address this issue, we used neuron-specific virally delivered Cre expression to delete either p110α or p110β (the two major catalytic isoforms of type I PI3K) from the hippocampus of adult mice. We found that dendritic and postsynaptic structures are almost exclusively supported by p110α activity, whereas p110β controls neurotransmitter release and metabotropic glutamate receptor-dependent long-term depression at the presynaptic terminal. In addition to these separate functions, p110α and p110β jointly contribute to N-methyl-d-aspartate receptor-dependent postsynaptic long-term potentiation. This molecular and functional specialization is reflected in different proteomes controlled by each isoform and in distinct behavioral alterations for learning/memory and sociability in mice lacking p110α or p110β.

Reduced Metabotropic Glutamate Receptor Type 5 Availability in the Epileptogenic Hippocampus: An in vitro Study

Abnormalities in the expression of metabotropic glutamate receptor type 5 (mGluR5) have been observed in the hippocampus of patients with drug-resistant mesial Temporal Lobe Epilepsy (mTLE). Ex-vivo studies in mTLE hippocampal surgical specimens have shown increased mGluR5 immunoreactivity, while in vivo whole brain imaging using positron emission tomography (PET) demonstrated reduced hippocampal mGluR5 availability. To further understand mGluR5 abnormalities in mTLE, we performed a saturation autoradiography study with [³H]ABP688 (a negative mGluR5 allosteric modulator). We aimed to evaluate receptor density (B_max) and dissociation constants (K_D) in hippocampal mTLE surgical specimens and in non-epilepsy hippocampi from necropsy controls. mTLE specimens showed a 43.4% reduction in receptor density compared to control hippocampi, which was independent of age, sex and K_D (multiple linear regression analysis). There was no significant difference in K_D between the groups, which suggests that the decreased mGluR5 availability found in vivo with PET cannot be attributed to reduced affinity between ligand and binding site. The present study supports that changes within the epileptogenic tissue include mGluR5 internalization or conformational changes that reduce [³H]ABP688 binding, as previously suggested in mTLE patients studied in vivo.

Selective Recruitment of Presynaptic and Postsynaptic Forms of mGluR-LTD

In area CA1 of the hippocampus, long-term depression (LTD) can be induced by activating group I metabotropic glutamate receptors (mGluRs), with the selective agonist DHPG. There is evidence that mGluR-LTD can be expressed by either a decrease in the probability of neurotransmitter release [P(r)] or by a change in postsynaptic AMPA receptor number. However, what determines the locus of expression is unknown. We investigated the expression mechanisms of mGluR-LTD using either a low (30 μM) or a high (100 μM) concentration of (RS)-DHPG. We found that 30 μM DHPG generated presynaptic LTD that required the co-activation of NMDA receptors, whereas 100 μM DHPG resulted in postsynaptic LTD that was independent of the activation of NMDA receptors. We found that both forms of LTD occur at the same synapses and that these may constitute the population with the lowest basal P(r). Our results reveal an unexpected complexity to mGluR-mediated synaptic plasticity in the hippocampus.

In vivo hippocampal cornu ammonis 1-3 glutamatergic abnormalities are associated with temporal lobe epilepsy surgery outcomes

OBJECTIVE

Previous positron emission tomography (PET) studies using [¹¹ C]ABP688 show reduced metabotropic glutamate receptor type 5 (mGluR5) allosteric binding site availability in the epileptogenic hippocampus of mesial temporal lobe epilepsy (MTLE) patients. However, the link between mGluR5 abnormalities and postsurgical outcomes remains unclear. Here, we test whether reduced PET [¹¹ C]ABP688 binding in cornu ammonis (CA) sectors more vulnerable to glutamatergic excitotoxicity relates to surgical outcomes.

METHODS

We obtained magnetic resonance imaging (MRI) and [¹¹ C]ABP688-PET from 31 unilateral MTLE patients and 30 healthy controls. MRI hippocampal subfields were segmented using FreeSurfer. To respect the lower PET special resolution, MRI-derived anatomical subfields were combined into CA1-3, CA4/dentate gyrus, and Subiculum. Partial volume corrected [¹¹ C]ABP688 nondisplaceable binding potential (BP_ND ) values were averaged across each subfield, and Z-scores were calculated. Subfield [¹¹ C]ABP688-BP_ND was compared between seizure-free and non-seizure-free patients. In addition, we also assessed subfield volumes and [¹⁸ F]fluorodeoxyglucose (FDG) uptake in each clinical group.

RESULTS

MTLE [¹¹ C]ABP688-BP_ND was reduced in ipsilateral (epileptogenic) CA1-3 and CA4/dentate-gyrus (p < .001, 95% confidence interval [CI] = .29-.51) compared to controls, with no difference in Subiculum. [¹¹ C]ABP688-BP_ND and subfield volumes were compared between seizure-free (Engel IA, n = 13) and non-seizure-free patients (Engel IC-III, n = 10). In ipsilateral CA1-3 only, [¹¹ C]ABP688-BP_ND was lower in seizure-free patients than in non-seizure-free patients (p = .012, 95% CI = 1.46-11.0) independently of volume. A subset analysis of 12 patients with [¹¹ C]ABP688-PET+[¹⁸ F]FDG-PET showed no between-group significant difference in [¹⁸ F]FDG uptake, whereas CA1-3 [¹¹ C]ABP688-BP_ND remained significantly lower in the seven of 12 seizure-free patients (p = .03, 95% CI = -3.13 to -.21).

SIGNIFICANCE

Reduced mGluR5 allosteric site availability in hippocampal CA1-3, measured in vivo by [¹¹ C]ABP688-PET, is associated with postsurgery seizure freedom independent of atrophy or hypometabolism. Information derived from hippocampal CA1-3 [¹¹ C]ABP688-PET is a promising imaging biomarker potentially impactful in surgical decisions for MRI-negative/PET-negative MTLE patients.

Presynaptic endoplasmic reticulum regulates short-term plasticity in hippocampal synapses

Short-term plasticity preserves a brief history of synaptic activity that is communicated to the postsynaptic neuron. This is primarily regulated by a calcium signal initiated by voltage dependent calcium channels in the presynaptic terminal. Imaging studies of CA3-CA1 synapses reveal the presence of another source of calcium, the endoplasmic reticulum (ER) in all presynaptic terminals. However, the precise role of the ER in modifying STP remains unexplored. We performed in-silico experiments in synaptic geometries based on reconstructions of the rat CA3-CA1 synapses to investigate the contribution of ER. Our model predicts that presynaptic ER is critical in generating the observed short-term plasticity profile of CA3-CA1 synapses and allows synapses with low release probability to operate more reliably. Blocking the ER lowers facilitation in a manner similar to what has been previously characterized in animal models of Alzheimer's disease and underscores the important role played by presynaptic stores in normal function.

Protrudin-deficient mice manifest depression-like behavior with abnormalities in activity, attention, and cued fear-conditioning

Protrudin is a protein that resides in the membrane of the endoplasmic reticulum and is highly expressed in the nervous system. Although mutations in the human protrudin gene (ZFYVE27, also known as SPG33) give rise to hereditary spastic paraplegia (HSP), the physiological role of the encoded protein has been largely unclear. We therefore generated mice deficient in protrudin and subjected them to a battery of behavioral tests designed to examine their intermediate phenotypes. The protrudin-deficient mice were found to have a reduced body size and to manifest pleiotropic behavioral abnormalities, including hyperactivity, depression-like behavior, and deficits in attention and fear-conditioning memory. They exhibited no signs of HSP, however, consistent with the notion that HSP-associated mutations of protrudin may elicit neural degeneration, not as a result of a loss of function, but rather as a result of a gain of toxic function. Overall, our results suggest that protrudin might play an indispensable role in normal neuronal development and behavior.

Illuminating Relationships Between the Pre- and Post-synapse

Excitatory synapses in the mammalian cortex are highly diverse, both in terms of their structure and function. However, relationships between synaptic features indicate they are highly coordinated entities. Imaging techniques, that enable physiology at the resolution of individual synapses to be investigated, have allowed the presynaptic activity level of the synapse to be related to postsynaptic function. This approach has revealed that neuronal activity induces the pre- and post-synapse to be functionally correlated and that subsets of synapses are more susceptible to certain forms of synaptic plasticity. As presynaptic function is often examined in isolation from postsynaptic properties, the effect it has on the post-synapse is not fully understood. However, since postsynaptic receptors at excitatory synapses respond to release of glutamate, it follows that they may be differentially regulated depending on the frequency of its release. Therefore, examining postsynaptic properties in the context of presynaptic function may be a useful way to approach a broad range of questions on synaptic physiology. In this review, we focus on how optophysiology tools have been utilized to study relationships between the pre- and the post-synapse. Multiple imaging techniques have revealed correlations in synaptic properties from the submicron to the dendritic level. Optical tools together with advanced imaging techniques are ideally suited to illuminate this area further, due to the spatial resolution and control they allow.

In vivo metabotropic glutamate receptor type 5 abnormalities localize the epileptogenic zone in mesial temporal lobe epilepsy

OBJECTIVE

Surgical specimens from patients with mesial temporal lobe epilepsy (MTLE) show abnormalities in tissue concentrations of metabotropic glutamate receptor type 5 (mGluR5). To clarify whether these abnormalities are specific to the epileptogenic zone (EZ), we characterized in vivo whole-brain mGluR5 availability in MTLE patients using positron emission tomography (PET) and [¹¹ C]ABP688, a radioligand that binds specifically to the mGluR5 allosteric site.

METHODS

Thirty-one unilateral MTLE patients and 30 healthy controls underwent [¹¹ C]ABP688 PET. We compared partial volume corrected [¹¹ C]ABP688 nondisplaceable binding potentials (BP_ND ) between groups using region-of-interest and whole-brain voxelwise analyses. [¹⁸ F]Fluorodeoxyglucose (FDG) PET was acquired in 15 patients, for whom we calculated asymmetry indices of [¹¹ C]ABP688 BP_ND and [¹⁸ F]FDG uptake to compare lateralization and localization differences.

RESULTS

[¹¹ C]ABP688 BP_ND was focally reduced in the epileptogenic hippocampal head and amygdala (p < 0.001). Patients with hippocampal atrophy showed more extensive abnormalities, including the ipsilateral temporal neocortex (p = 0.006). [¹¹ C]ABP688 BP_ND showed interhemispheric differences of higher magnitude and discriminated the epileptogenic structures more accurately when compared to [¹⁸ F]FDG uptake, which showed more widespread hypometabolism. Among 23 of 25 operated patients with >1 year of follow-up, 13 were seizure-free (Engel Ia) and showed significantly lower [¹¹ C]ABP688 BP_ND in the ipsilateral entorhinal cortex.

INTERPRETATION

[¹¹ C]ABP688 PET provides a focal biomarker for the EZ in MTLE with higher spatial accuracy compared to [¹⁸ F]FDG PET. Focally reduced mGluR5 availability in the EZ might reflect receptor internalization or conformational changes in response to excessive extracellular glutamate, supporting a potential role for mGluR5 as therapeutic target in human MTLE. Ann Neurol 2019; 1-11 ANN NEUROL 2019;85:218-228.

The Probability of Neurotransmitter Release Governs AMPA Receptor Trafficking via Activity-Dependent Regulation of mGluR1 Surface Expression

A major mechanism contributing to synaptic plasticity involves alterations in the number of AMPA receptors (AMPARs) expressed at synapses. Hippocampal CA1 synapses, where this process has been most extensively studied, are highly heterogeneous with respect to their probability of neurotransmitter release, P(r). It is unknown whether there is any relationship between the extent of plasticity-related AMPAR trafficking and the initial P(r) of a synapse. To address this question, we induced metabotropic glutamate receptor (mGluR) dependent long-term depression (mGluR-LTD) and assessed AMPAR trafficking and P(r) at individual synapses, using SEP-GluA2 and FM4-64, respectively. We found that either pharmacological or synaptic activation of mGluR1 reduced synaptic SEP-GluA2 in a manner that depends upon P(r); this process involved an activity-dependent reduction in surface mGluR1 that selectively protects high-P(r) synapses from synaptic weakening. Consequently, the extent of postsynaptic plasticity can be pre-tuned by presynaptic activity.

Activation of TRPC1 Channel by Metabotropic Glutamate Receptor mGluR5 Modulates Synaptic Plasticity and Spatial Working Memory

Group I metabotropic glutamate receptors, in particular mGluR5, have been implicated in various forms of synaptic plasticity that are believed to underlie declarative memory. We observed that mGluR5 specifically activated a channel containing TRPC1, an isoform of the canonical family of transient receptor potential (TRPC) channels highly expressed in CA1-3 regions of the hippocampus. TRPC1 is able to form tetrameric complexes with TRPC4 and/or TRPC5 isoforms. TRPC1/4/5 complexes have recently been involved in the efficiency of synaptic transmission in the hippocampus. We therefore used a mouse model devoid of TRPC1 expression to investigate the involvement of mGluR5-TRPC1 pathway in synaptic plasticity and memory formation. Trpc1^-/- mice showed alterations in spatial working memory and fear conditioning. Activation of mGluR increased synaptic excitability in neurons from WT but not from Trpc1^-/- mice. LTP triggered by a theta burst could not maintain over time in brain slices from Trpc1^-/- mice. mGluR-induced LTD was also impaired in these mice. Finally, acute inhibition of TRPC1 by Pico145 on isolated neurons or on brain slices mimicked the genetic depletion of Trpc1 and inhibited mGluR-induced entry of cations and subsequent effects on synaptic plasticity, excluding developmental or compensatory mechanisms in Trpc1^-/- mice. In summary, our results indicate that TRPC1 plays a role in synaptic plasticity and spatial working memory processes.

Regulation of Excitatory Amino Acid Transmission in the Retina: Studies on Neuroprotection

Excitotoxicity occurs in neurons due to the accumulation of excitatory amino acids such as glutamate in the synaptic and extrasynaptic locations. In the retina, excessive glutamate concentrations trigger a neurotoxic cascade involving several mechanisms, including the elevation of intracellular calcium (Ca²⁺⁾ and the activation of α-amino-3-hydroxy 5-methyl-4-iso-xazole-propionic acid/kainate (AMPA/KA) and N-methyl-d-aspartate (NMDA) receptors leading to retinal degeneration. Both ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) are present in the mammalian retina. Indeed, due to the abundant expression of GluRs, the mammalian retina is highly susceptible to excitotoxic neurodegeneration. Excitotoxicity has been postulated to present a common downstream mechanism for several stimuli, including hypoglycemia, hypoxia, ischemia, and chronic neurodegenerative diseases. Experimental approaches to the study of neuroprotection in the retina have utilized insults that trigger hypoxia, hypoglycemia, or excitotoxicity. Using these experimental approaches, the neuroprotective potential of GluR agents, including the NMDA receptor modulators (MK801, ifenprodil, memantine); AMPA/KA receptor antagonist (CNQX); Group II and III mGluR agonists (LY354740, quisqualate); and Ca²⁺-channel blockers (diltiazem, lomerizine, verapamil, ω-conotoxin), and others (pituitary adenylate cyclase activating polypeptide, neuropeptide Y, acetylcholine receptor agonists) have been elucidated. In addition to corroborating the exocytotic role of excitatory amino acids in retinal degeneration, these studies affirm that multiple mechanism/s contribute to the prevention of damage caused by excitotoxicity in the retina. Therefore, it is feasible that several pathways are involved in protecting the retina from toxic insults in ocular neurodegenerative conditions such as glaucoma and retinal ischemia. Furthermore, these experimental models are viable tools for evaluating therapeutic candidates in ocular neuropathies.

Molecular mechanisms of group I metabotropic glutamate receptor mediated LTP and LTD in basolateral amygdala in vitro

The roles of group I metabotropic glutamate receptors, metabotropic glutamate receptor 1 (mGluR1) and mGluR5, in regulating synaptic plasticity and metaplasticity in the basolateral amygdala (BLA) remain unclear. The present study examined mGluR1- and mGluR5-mediated synaptic plasticity in the BLA and their respective signaling mechanisms. Bath application of the group I mGluR agonist, 3,5-dihydroxyphenylglycine (DHPG) (20 μM), directly suppressed basal fEPSPs (84.5 ± 6.3% of the baseline). The suppressive effect persisted for at least 30 min after washout; it was abolished by the mGluR1 antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) but was unaffected by the mGluR5 antagonist 2-methyl-6- (phenylethynyl)-pyridine (MPEP). Interestingly, application of DHPG (at both 2 and 20 μM), regardless of the presence of CPCCOEt, could transform single theta burst stimulation (TBS)-induced short-term synaptic potentiation into a long-term potentiation (LTP). Such a facilitating effect could be blocked by the mGluR5 antagonist MPEP. Blockade of phospholipase C (PLC), the downstream enzyme of group I mGluR, with U73122, prevented both mGluR1- and mGluR5-mediated effects on synaptic plasticity. Nevertheless, blockade of protein kinase C (PKC), the downstream enzyme of PLC, with chelerythrine (5 μM) only prevented the transforming effect of DHPG on TBS-induced LTP and did not affect DHPG-induced long-term depression (LTD). These results suggest that mGluR1 activation induced LTD via a PLC-dependent and PKC-independent mechanism, while the priming action of mGluR5 receptor on the BLA LTP is both PLC and PKC dependent. The BLA metaplasticity mediated by mGluR1 and mGluR5 may provide signal switching mechanisms mediating learning and memory with emotional significance.

Differential ability of the dorsal and ventral rat hippocampus to exhibit group I metabotropic glutamate receptor-dependent synaptic and intrinsic plasticity

Background:

The hippocampus is critically involved in learning and memory processes. Although once considered a relatively homogenous structure, it is now clear that the hippocampus can be divided along its longitudinal axis into functionally distinct domains, responsible for the encoding of different types of memory or behaviour. Although differences in extrinsic connectivity are likely to contribute to this functional differentiation, emerging evidence now suggests that cellular and molecular differences at the level of local hippocampal circuits may also play a role.

Methods:

In this study, we have used extracellular field potential recordings to compare basal input/output function and group I metabotropic glutamate receptor-dependent forms of synaptic and intrinsic plasticity in area CA1 of slices taken from the dorsal and ventral sectors of the adult rat hippocampus.

Results:

Using two extracellular electrodes to simultaneously record field EPSPs and population spikes, we show that dorsal and ventral hippocampal slices differ in their basal levels of excitatory synaptic transmission, paired-pulse facilitation, and EPSP-to-Spike coupling. Furthermore, we show that slices taken from the ventral hippocampus have a greater ability than their dorsal counterparts to exhibit long-term depression of synaptic transmission and EPSP-to-Spike potentiation induced by transient application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine.

Conclusions:

Together, our results provide further evidence that the information processing properties of local hippocampal circuits differ in the dorsal and ventral hippocampal sectors, and that these differences may in turn contribute to the functional differentiation that exists along the hippocampal longitudinal axis.

Chronic but not acute immobilization stress stably enhances hippocampal CA1 metabotropic glutamate receptor dependent Long-Term Depression

Acute stress has been shown to facilitate but not increase metabotropic glutamate receptor (mGluR) mediated Long-Term Depression (LTD) in the hippocampus. However, the effect of chronic stress on mGluR dependent LTD has not been investigated. Moreover, whether stress leads to a transient modification LTD threshold or a more stable change in synaptic plasticity needs to be addressed. In the present study, we have explored the effects of both a ten-day long and a single day immobilization stress protocol on mGluR-LTD at the CA3:CA1synapse in the hippocampus of adult male Sprague-Dawley rats, a day after applying stress. Bath application of the selective group 1 mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) promoted robust LTD in hippocampal slices from control (i.e. un-stressed) animals. Administration of immobility stress for two hours per day for ten days significantly elevated this LTD to a level almost twice that of control, when observed 24h following the last stress event. Acute stress i.e. a single day of two hours of immobilization, however, failed to significantly enhance LTD, 24h later. These results demonstrate for the first time, that repeated exposure to stress, but not a single stress event, is required to bring about a stable alteration in mGluR mediated synaptic plasticity.

Protein Synthesis Inhibitors Did Not Interfere with Long-Term Depression Induced either Electrically in Juvenile Rats or Chemically in Middle-Aged Rats

In testing the hypothesis that long-term potentiation (LTP) maintenance depends on triggered protein synthesis, we found no effect of protein synthesis inhibitors (PSIs) on LTP stabilization. Similarly, some studies reported a lack of effect of PSIs on long-term depression (LTD); the lack of effect on LTD has been suggested to be resulting from the short time recordings. If this proposal were true, LTD might exhibit sensitivity to PSIs when the recording intervals were enough long. We firstly induced LTD by a standard protocol involving low frequency stimulation, which is suitable for eliciting NMDAR-LTD in CA1 area of hippocampal slices obtained from juvenile Sprague-Dawley rats. This LTD was persistent for intervals in range of 8–10 h. Treating slices with anisomycin, however, did not interfere with the magnitude and persistence of this form of LTD. The failure of anisomycin to block synaptic-LTD might be relied on the age of animal, the type of protein synthesis inhibitors and/or the inducing protocol. To verify whether those variables altogether were determinant, NMDA or DHPG was used to chemically elicit LTD recorded up to 10 h on hippocampal slices obtained from middle-aged rats. In either form of LTD, cycloheximide did not interfere with LTD stabilization. Furthermore, DHPG application did show an increase in the global protein synthesis as assayed by radiolabeled methodology indicating that though triggered protein synthesis can occur but not necessarily required for LTD expression. The findings confirm that stabilized LTD in either juvenile, or middle-aged rats can be independent of triggered protein synthesis. Although the processes responsible for the independence of LTD stabilization on the triggered protein synthesis are not yet defined, these findings raise the possibility that de novo protein synthesis is not universally necessary.

Hippocampal metabotropic glutamate receptor long-term depression in health and disease: focus on mitogen-activated protein kinase pathways

Group I metabotropic glutamate receptor (mGluR) dependent long-term depression (LTD) is a major form of synaptic plasticity underlying learning and memory. The molecular mechanisms involved in mGluR-LTD have been investigated intensively for the last two decades. In this 60th anniversary special issue article, we review the recent advances in determining the mechanisms that regulate the induction, transduction and expression of mGluR-LTD in the hippocampus, with a focus on the mitogen-activated protein kinase (MAPK) pathways. In particular we discuss the requirement of p38 MAPK and extracellular signal-regulated kinase 1/2 (ERK 1/2) activation. The recent advances in understanding the signaling cascades regulating mGluR-LTD are then related to the cognitive impairments observed in neurological disorders, such as fragile X syndrome and Alzheimer's disease. mGluR-LTD is a form of synaptic plasticity that impacts on memory formation. In the hippocampus mitogen-activated protein kinases (MAPKs) have been found to be important in mGluR-LTD. In this 60th anniversary special issue article, we review the independent and complementary roles of two classes of MAPK, p38 and ERK1/2 and link this to the aberrant mGluR-LTD that has an important role in diseases. This article is part of the 60th Anniversary special issue.

Extracellular ATP modulates synaptic plasticity induced by activation of metabotropic glutamate receptors in the hippocampus

Synaptic plasticity is believed to be a cellular mechanism for memory formation in the brain. It has been known that the metabotropic glutamate receptor (mGluR) is required for persistent forms of memory and induction of synaptic plasticity. Application of mGluR agonists induces synaptic plasticity in the absence of electrical conditioning stimulation, such as high or low frequency stimulation. The direction of the mGluR-induced synaptic plasticity, i.e., either long-term potentiation (LTP) or long-term-depression (LTD), is dependent on whether N-methyl-D-aspartate receptors (NMDARs) are co-activated with mGluRs. ATP has modulatory effects on neuronal functions and, in particular, there is increasing evidence that it plays a crucial role in synaptic plasticity. LTP can be induced by application of ATP, and this effect is inhibited by NMDAR antagonist. Although cooperative effects of NMDARs and mGluRs and of NMDARs and extracellular ATP in synaptic plasticity have been revealed, the effect of extracellular ATP on mGluR-induced synaptic plasticity is unknown. In this article, we summarize published data on mGluR- and ATP-induced synaptic plasticity, and present new data showing that extracellular ATP facilitates both the LTP and LTD induced by mGluR activation.

Retroactive modulation of spike timing-dependent plasticity by dopamine

Most reinforcement learning models assume that the reward signal arrives after the activity that led to the reward, placing constraints on the possible underlying cellular mechanisms. Here we show that dopamine, a positive reinforcement signal, can retroactively convert hippocampal timing-dependent synaptic depression into potentiation. This effect requires functional NMDA receptors and is mediated in part through the activation of the cAMP/PKA cascade. Collectively, our results support the idea that reward-related signaling can act on a pre-established synaptic eligibility trace, thereby associating specific experiences with behaviorally distant, rewarding outcomes. This finding identifies a biologically plausible mechanism for solving the 'distal reward problem'.

Activation of InsP₃ receptors is sufficient for inducing graded intrinsic plasticity in rat hippocampal pyramidal neurons

The synaptic plasticity literature has focused on establishing necessity and sufficiency as two essential and distinct features in causally relating a signaling molecule to plasticity induction, an approach that has been surprisingly lacking in the intrinsic plasticity literature. In this study, we complemented the recently established necessity of inositol trisphosphate (InsP3) receptors (InsP3R) in a form of intrinsic plasticity by asking if InsP3R activation was sufficient to induce intrinsic plasticity in hippocampal neurons. Specifically, incorporation of d-myo-InsP3 in the recording pipette reduced input resistance, maximal impedance amplitude, and temporal summation but increased resonance frequency, resonance strength, sag ratio, and impedance phase lead. Strikingly, the magnitude of plasticity in all these measurements was dependent on InsP3 concentration, emphasizing the graded dependence of such plasticity on InsP3R activation. Mechanistically, we found that this InsP3-induced plasticity depended on hyperpolarization-activated cyclic nucleotide-gated channels. Moreover, this calcium-dependent form of plasticity was critically reliant on the release of calcium through InsP3Rs, the influx of calcium through N-methyl-d-aspartate receptors and voltage-gated calcium channels, and on the protein kinase A pathway. Our results delineate a causal role for InsP3Rs in graded adaptation of neuronal response dynamics, revealing novel regulatory roles for the endoplasmic reticulum in neural coding and homeostasis.

In vivo imaging of mGluR5 changes during epileptogenesis using [11C]ABP688 PET in pilocarpine-induced epilepsy rat model

Introduction

Metabotropic glutamate receptor 5 (mGluR5) that regulates glutamatergic neurotransmission contributes to pathophysiology of epilepsy. In this study, we monitored the changes of mGluR5 in vivo using [¹¹C]ABP688 PET during the epileptogenesis in a pilocarpine-induced epilepsy rat model.

Methods

In vivo mGluR5 images were acquired using [¹¹C]ABP688 microPET/CT in pilocarpine-induced chronic epilepsy rat models and controls. We also acquired microPET/CT at acute, subacute as well as chronic periods after status epilepticus. Non-displaceable binding potential (BP_ND) of [¹¹C]ABP688 was calculated using simplified reference tissue model in a voxel-based manner. mGluR5 BP_ND of the rat brains of epilepsy models and controls were compared.

Results

Status epilepticus developed after pilocarpine administration and was followed by recurrent spontaneous seizures for more than 3 weeks. In chronic epilepsy rat model, BP_ND in hippocampus and amygdala was reduced on a voxel-based analysis. Temporal changes of mGluR5 BP_ND was also found. In acute period after status epilepticus, mGluR5 BP_ND was reduced in the whole brain. BP_ND of caudate-putamen was restored in subacute period, while BP_ND of the rest of the brain was still lower. In chronic period, global BP_ND was normalized except in hippocampus and amygdala.

Conclusions

In vivo imaging of mGluR5 using [¹¹C]ABP688 microPET/CT could successfully reveal the regional changes of mGluR5 binding potential of the rat brain in a pilocarpine-induced epilepsy model. The temporal and spatial changes in mGluR5 availability suggest [¹¹C]ABP688 PET imaging in epilepsy provide abnormal glutamatergic network during epileptogenesis.

MHC class I immune proteins are critical for hippocampus-dependent memory and gate NMDAR-dependent hippocampal long-term depression

Memory impairment is a common feature of conditions that involve changes in inflammatory signaling in the brain, including traumatic brain injury, infection, neurodegenerative disorders, and normal aging. However, the causal importance of inflammatory mediators in cognitive impairments in these conditions remains unclear. Here we show that specific immune proteins, members of the major histocompatibility complex class I (MHC class I), are essential for normal hippocampus-dependent memory, and are specifically required for NMDAR-dependent forms of long-term depression (LTD) in the healthy adult hippocampus. In β2m^−/−TAP^−/−mice, which lack stable cell-surface expression of most MHC class I proteins, NMDAR-dependent LTD in area CA1 of adult hippocampus is abolished, while NMDAR-independent forms of potentiation, facilitation, and depression are unaffected. Altered NMDAR-dependent synaptic plasticity in the hippocampus of β2m^−/−TAP^−/−mice is accompanied by pervasive deficits in hippocampus-dependent memory, including contextual fear memory, object recognition memory, and social recognition memory. Thus normal MHC class I expression is essential for NMDAR-dependent hippocampal synaptic depression and hippocampus-dependent memory. These results suggest that changes in MHC class I expression could be an unexpected cause of disrupted synaptic plasticity and cognitive deficits in the aging, damaged, and diseased brain.

Modulation of neurological deficits and expression of glutamate receptors during experimental autoimmune encephalomyelitis after treatment with selected antagonists of glutamate receptors

The aim of our investigation was to characterize the role of group I mGluRs and NMDA receptors in pathomechanisms of experimental autoimmune encephalomyelitis (EAE), the rodent model of MS. We tested the effects of LY 367385 (S-2-methyl-4-carboxyphenylglycine, a competitive antagonist of mGluR1), MPEP (2-methyl-6-(phenylethynyl)-pyridine, an antagonist of mGluR5), and the uncompetitive NMDA receptor antagonists amantadine and memantine on modulation of neurological deficits observed in rats with EAE. The neurological symptoms of EAE started at 10-11 days post-injection (d.p.i.) and peaked after 12-13 d.p.i. The protein levels of mGluRs and NMDA did not increase in early phases of EAE (4 d.p.i.), but starting from 8 d.p.i. to 25 d.p.i., we observed a significant elevation of mGluR1 and mGluR5 protein expression by about 20% and NMDA protein expression by about 10% over the control at 25 d.p.i. The changes in protein levels were accompanied by changes in mRNA expression of group I mGluRs and NMDARs. During the late disease phase (20–25 d.p.i.), the mRNA expression levels reached 300% of control values. In contrast, treatment with individual receptor antagonists resulted in a reduction of mRNA levels relative to untreated animals.

A modulatory effect of the feedback from higher visual areas to V1 in the mouse

Using a mouse brain slice preparation, we studied the modulatory effects of a feedback projection from higher visual cortical areas, mostly or exclusively area LM (or V2), on two inputs to layer 4 cells in the first visual area (V1). The two inputs to these cells were geniculocortical and an unspecified intracortical input, possibly involving layer 6 cells. We found that activation of metabotropic glutamate receptors (mGluRs) from stimulation of the feedback projection reduced the evoked excitatory postsynaptic currents of both of these inputs to layer 4 but that this modulation acts in an input-specific way. Reducing the strength of the geniculocortical input in adults involved both presynaptic and postsynaptic group I mGluRs (although in younger animals presynaptic group II mGluRs were also involved), whereas modulation of the intracortical input acted entirely via postsynaptic group II mGluRs. These results demonstrate that one of the effects of this feedback pathway is to control the gain of geniculocortical transmission.

Antagonists reversibly reverse chemical LTD induced by group I, group II and group III metabotropic glutamate receptors

Metabotropic glutamate (mGlu) receptors are implicated in many neurological and psychiatric diseases and are the targets of therapeutic agents currently in clinical development. Their activation has diverse effects in the central nervous system (CNS) that includes an involvement in synaptic plasticity. We previously reported that the brief exposure of hippocampal slices to dihydroxyphenylglycine (DHPG) can result in a long-term depression (LTD) of excitatory synaptic transmission. Surprisingly, this LTD could be fully reversed by mGlu receptor antagonists in a manner that was itself fully reversible upon washout of the antagonist. Here, 15 years after the discovery of DHPG-LTD and its reversible reversibility, we summarise these initial findings. We then present new data on DHPG-LTD, which demonstrates that evoked epileptiform activity triggered by activation of group I mGlu receptors can also be reversibly reversed by mGlu receptor antagonists. Furthermore, we show that the phenomenon of reversible reversibility is not specific to group I mGlu receptors. We report that activation of group II mGlu receptors in the temporo-ammonic pathway (TAP) and mossy fibre pathway within the hippocampus and in the cortical input to neurons of the lateral amygdala induces an LTD that is reversed by LY341495, a group II mGlu receptor antagonist. We also show that activation of group III mGlu8 receptors induces an LTD at lateral perforant path inputs to the dentate gyrus and that this LTD is reversed by MDCPG, an mGlu8 receptor antagonist. In conclusion, we have shown that activation of representative members of each of the three groups of mGlu receptors can induce forms of LTD than can be reversed by antagonists, and that in each case washout of the antagonist is associated with the re-establishment of the LTD. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Activity-dependent regulation of retinogeniculate signaling by metabotropic glutamate receptors

Thalamocortical neurons in dorsal lateral geniculate nucleus (dLGN) dynamically convey visual information from retina to the neocortex. Activation of metabotropic glutamate receptors (mGluRs) exerts multiple effects on neural integration in dLGN; however, their direct influence on the primary sensory input, namely retinogeniculate afferents, is unknown. In the present study, we found that pharmacological or synaptic activation of type 1 mGluRs (mGluR(1)s) significantly depresses glutamatergic retinogeniculate excitation in rat thalamocortical neurons. Pharmacological activation of mGluR(1)s attenuates excitatory synaptic responses in thalamocortical neurons at a magnitude sufficient to decrease suprathreshold output of these neurons. The reduction in both NMDA and AMPA receptor-dependent synaptic responses results from a presynaptic reduction in glutamate release from retinogeniculate terminals. The suppression of retinogeniculate synaptic transmission and dampening of thalamocortical output was mimicked by tetanic activation of retinogeniculate afferent in a frequency-dependent manner that activated mGluR(1)s. Retinogeniculate excitatory synaptic transmission was also suppressed by the glutamate transport blocker TBOA (dl-threo-β-benzyloxyaspartic acid), suggesting that mGluR(1)s were activated by glutamate spillover. The data indicate that presynaptic mGluR(1) contributes to an activity-dependent mechanism that regulates retinogeniculate excitation and therefore plays a significant role in the thalamic gating of visual information.

Gating of NMDA receptor-mediated hippocampal spike timing-dependent potentiation by mGluR5

Hippocampal long-term potentiation (LTP) is believed to be important for learning and memory. Experimentally, the pairing of precisely timed pre- and postsynaptic spikes within a time window of ∼10 ms can induce timing-dependent LTP (tLTP), but the requirements for induction of tLTP change with development: in young rodents single postsynaptic spikes are sufficient to induce tLTP, whereas postsynaptic burst firing appears to be required in the adult. However, hippocampal neurons in vivo show theta-modulated single spike activities also in older hippocampus. Here we investigated the conditions for single spike pairing to induce tLTP at older CA3–CA1 synapses. We found that the pairing of single pre- and postsynaptic spikes could induce tLTP in older hippocampus when the postsynaptic neuronal membrane was depolarized and the pairing frequency exceeded ∼4 Hz. The spike frequency requirement is postsynaptic, as tLTP could still be induced with presynaptic stimulation at 1 Hz as long as the postsynaptic spike frequency exceeded ∼4 Hz, suggesting that postsynaptic theta-frequency activity is required for the successful induction of tLTP at older CA3–CA1 synapses. The induction of tLTP was blocked by an NMDA receptor antagonist and by the selective mGluR5 blockers, MPEP and MTEP, whereas activation of mGluR1 and mGluR5 by DHPG relieved the postsynaptic spike frequency requirement for tLTP induction. These results suggest that activation of mGluR5 during single-spike pairing at older CA3–CA1 synapses gates NMDA receptor-dependent tLTP.

Role of presynaptic metabotropic glutamate receptors in the induction of long-term synaptic plasticity of vesicular release

While postsynaptic ionotropic and metabotropic glutamate receptors have received the lions share of attention in studies of long-term activity-dependent synaptic plasticity, it is becoming clear that presynaptic metabotropic glutamate receptors play critical roles in both short-term and long-term plasticity of vesicular transmitter release, and that they act both at the level of voltage-dependent calcium channels and directly on proteins of the vesicular release machinery. Activation of G protein-coupled receptors can transiently inhibit vesicular release through the release of Gβγ which binds to both voltage-dependent calcium channels to reduce calcium influx, and directly to the C-terminus region of the SNARE protein SNAP-25. Our recent work has revealed that the binding of Gβγ to SNAP-25 is necessary, but not sufficient, to elicit long-term depression (LTD) of vesicular glutamate release, and that the concomitant release of Gα(i) and the second messenger nitric oxide are also necessary steps in the presynaptic LTD cascade. Here, we review the current state of knowledge of the molecular steps mediating short-term and long-term plasticity of vesicular release at glutamatergic synapses, and the many gaps that remain to be addressed. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Functional coupling of the metabotropic glutamate receptor, InsP3 receptor and L-type Ca2+ channel in mouse CA1 pyramidal cells

Activity-dependent regulation of calcium dynamics in neuronal cells can play significant roles in the modulation of many cellular processes such as intracellular signalling, neuronal activity and synaptic plasticity. Among many calcium influx pathways into neurons, the voltage-dependent calcium channel (VDCC) is the major source of calcium influx, but its modulation by synaptic activity has still been under debate. While the metabotropic glutamate receptor (mGluR) is supposed to modulate L-type VDCCs (L-VDCCs), its reported actions include both facilitation and suppression, probably reflecting the uncertainty of both the molecular targets of the mGluR agonists and the source of the recorded calcium signal in previous reports. In this study, using subtype-specific knockout mice, we have shown that mGluR5 induces facilitation of the depolarization-evoked calcium current. This facilitation was not accompanied by the change in single-channel properties of the VDCC itself; instead, it required the activation of calcium-induced calcium release (CICR) that was triggered by VDCC opening, suggesting that the opening of CICR-coupled cation channels was essential for the facilitation. This facilitation was blocked or reduced by the inhibitors of both L-VDCCs and InsP3 receptors (InsP3Rs). Furthermore, L-VDCCs and mGluR5 were shown to form a complex by coimmunoprecipitation, suggesting that the specific functional coupling between mGluR5, InsP3Rs and L-VDCCs played a pivotal role in the calcium-current facilitation. Finally, we showed that mGluR5 enhanced VDCC-dependent long-term potentiation (LTP) of synaptic transmission. Our study has identified a novel mechanism of the interaction between the mGluR and calcium signalling, and suggested a contribution of mGluR5 to synaptic plasticity.

Presynaptic inhibition of gamma-aminobutyric acid release in the bed nucleus of the stria terminalis by kappa opioid receptor signaling

BACKGROUND

The kappa opioid receptor (KOR) and its endogenous agonist, the neuropeptide dynorphin, are a critical component of the central stress system. Both dynorphin and KOR are expressed in the bed nucleus of the stria terminalis (BNST), a brain region associated with anxiety and stress. This suggests that KOR activation in this region may play a role in the regulation of emotional behaviors. To date, however, there has been no investigation of the ability of KOR to modulate synaptic transmission in the BNST.

METHODS

We used whole-cell patch-clamp recordings from acutely prepared mouse brain slices to examine the actions of KOR on inhibitory transmission in the BNST. Additionally, we used neurochemical and pathway-specific optogenetic manipulations to selectively stimulate gamma-aminobutyric acid (GABA)ergic fibers from the central nucleus of the amygdala (CeA) to the BNST.

RESULTS

We found that activation of KOR reduced GABAergic transmission through a presynaptic mechanism. Furthermore, we examined the signal transduction pathways that mediate this inhibition and provide the first functional information implicating extracellular signal-regulated kinase in KOR-mediated presynaptic modulation. Moreover, we found that at KOR signaling robustly reduced inhibitory synaptic transmission in the CeA to BNST pathway.

CONCLUSIONS

Together, these results demonstrate that KOR provides important inhibitory control over presynaptic GABAergic signaling within the BNST and provides the first direct functional demonstration of KOR-sensitive long-range GABAergic connections between the CeA and the BNST.

A form of synaptically induced metabotropic glutamate receptor-dependent long-term depression that does not require postsynaptic calcium

The calcium control hypothesis posits that postsynaptic calcium increases are required to trigger synaptic plasticity, with large increases inducing LTP and small increases inducing LTD. In CA1 of the hippocampus, however, LTD induced by chemical activation of metabotropic glutamate receptors (agonist-LTD) is independent of increases in postsynaptic calcium. Here we tested whether LTD induced by pairing of presynaptic stimulation with postsynaptic depolarization (synaptic-LTD) is similarly calcium-independent. This protocol induced an NMDA-dependent LTP when paired at 0mV, which was converted to mGluR-dependent LTD when paired at -20mV. The LTD was not blocked by calcium chelation, blockers of L- or T-type voltage-dependent calcium channels, or hyperpolarization to -70mV. We conclude that synaptically induced mGluR-dependent LTD, like agonist induced mGluR LTD, does not require calcium influx for its induction.

NMDA receptors, mGluR5, and endocannabinoids are involved in a cascade leading to hippocampal long-term depression

In the CA1 region of the rat hippocampus, metabotropic glutamate receptor-5 (mGluR5) and cannabinoid-1 receptors (CB1Rs) are believed to participate in long-term synaptic depression (LTD). How mGluRs and CB1Rs interact to promote LTD remains uncertain. In this study, we examined LTD induced by CB1R agonists, mGluR5 agonists, and low-frequency electrical stimulation (LFS) of the Schaffer collateral pathway. Synthetic CB1R agonists induced robust LTD that was mimicked by the endocannabinoid (EC), noladin ether (NLDE), but not by anandamide. 2-Arachidonylglycerol (2AG) produced only a small degree of LTD. The selective mGluR5 agonist, namely (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), also induced robust LTD, and CHPG and NLDE occluded each other's effects. Similarly, CHPG and NLDE occluded LFS-induced LTD, and LTD resulting from all three treatments was blocked by a CB1R antagonist. CHPG-LTD and NLDE-LTD were insensitive to N-methyl-D-aspartate receptor (NMDAR) block, even though LFS-LTD requires NMDARs. LTD induced by LFS or CHPG, but not NLDE-LTD, was blocked by a selective mGluR5 antagonist. (RS)-3,5-dihydroxyphenylglycine (DHPG), a less selective group I mGluR agonist, also induced LTD, but its effects were not blocked by mGluR5 or CB1R antagonists. Furthermore, DHPG-LTD was additive with LFS-LTD and CHGP-LTD. These results suggest that NMDARs, mGluR5, and CB1Rs participate in a cascade that underlies LFS-LTD and that release of an EC and CB1R activation occur downstream of NMDARs and mGluR5. Furthermore, DHPG induces a form of LTD that differs mechanistically from LFS-induced depression.

Calcineurin is required for TRPV1-induced long-term depression of hippocampal interneurons

Transient receptor potential vanilloid 1 (TRPV1) mediates a novel form of presynaptic long-term depression (LTD) in hippocampal interneurons. To date, while TRPV1 is currently being heavily studied in the PNS for its anti-nociceptive and anti-inflammatory properties, much less is known regarding TRPV1 signaling and function in the CNS, including the mechanism mediating hippocampal interneuron LTD. Here we performed whole-cell voltage clamp electrophysiology experiments on CA1 hippocampal interneurons from Sprague-Dawley male rats to identify this signaling mechanism. Because calcineurin is linked to multiple synaptic plasticity types, we investigated whether TRPV1 activates presynaptic calcineurin, which in turn induces LTD. To do so we employed calcineurin inhibitors cyclosporin A or FK-506. To determine the location of the calcineurin involved we either bath applied calcineurin antagonists, blocking calcineurin activity ubiquitously in the slice, presynaptically and postsynaptically, or applied antagonists to the internal solution to block calcineurin postsynaptically. Both calcineurin antagonists applied to the bath blocked TRPV1-dependent LTD, indicating calcineurin involvement in LTD. Because calcineurin antagonist applied to the internal solution did not block TRPV1-LTD, it suggests presynaptic calcineurin is required for LTD. Finally, because high frequency stimulus used to induce LTD could potentially activate receptors besides TRPV1, we confirmed that bath, but not intracellularly applied cyclosporin A, also blocked TRPV1 agonist-induced depression of CA1 interneurons. In conclusion, these data illustrate that presynaptic calcineurin activity is required for both TRPV1-induced LTD and TRPV1 agonist-induced depression. This finding is the first to demonstrate the TRPV1-induced signaling mechanism in CA1 hippocampus.

Differential trafficking of AMPA receptors following activation of NMDA receptors and mGluRs

The removal of AMPA receptors from synapses is a major component of long-term depression (LTD). How this occurs, however, is still only partially understood. To investigate the trafficking of AMPA receptors in real-time we previously tagged the GluA2 subunit of AMPA receptors with ecliptic pHluorin and studied the effects of NMDA receptor activation. In the present study we have compared the effect of NMDA receptor and group I mGluR activation, using GluA2 tagged with super ecliptic pHluorin (SEP-GluA2) expressed in cultured hippocampal neurons. Surprisingly, agonists of the two receptors, which are both able to induce chemical forms of LTD, had clearly distinct effects on AMPA receptor trafficking. In agreement with our previous work we found that transient NMDA receptor activation results in an initial decrease in surface GluA2 from extrasynaptic sites followed by a delayed reduction in GluA2 from puncta (putative synapses). In contrast, transient activation of group I mGluRs, using DHPG, led to a pronounced but more delayed decrease in GluA2 from the dendritic shafts. Surprisingly, there was no average change in the fluorescence of the puncta. Examination of fluorescence at individual puncta, however, indicated that alterations did take place, with some puncta showing an increase and others a decrease in fluorescence. The effects of DHPG were, like DHPG-induced LTD, prevented by treatment with a protein tyrosine phosphatase (PTP) inhibitor. The electrophysiological correlate of the effects of DHPG in the SEP-GluA2 infected cultures was a reduction in mEPSC frequency with no change in amplitude. The implications of these findings for the initial mechanisms of expression of both NMDA receptor- and mGluR-induced LTD are discussed.

Distinct mechanisms contribute to agonist and synaptically induced metabotropic glutamate receptor long-term depression

Metabotropic glutamate receptor mediated long-term depression (mGlu receptor LTD) is evoked in hippocampal area CA1 chemically by the agonist 3,5-Dihydroxyphenylglycine (DHPG, agonist LTD) and synaptically by paired-pulse low frequency stimulation (PP-LFS, synaptic LTD). We tested the hypothesis that different expression mechanisms regulate mGlu receptor LTD in 15-21 day old rats through neurophysiologic recordings in CA1. Several findings, in fact, suggest that agonist and synaptic mGlu receptor LTD are expressed through different mechanisms. First, neither LTD occluded the other. Second, a low calcium solution enhanced agonist LTD but did not alter synaptic LTD. Third, application of the mGlu receptor antagonist LY341495 (2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid) reversed agonist LTD expression, but did not alter synaptic LTD. Finally, an N-type, voltage-dependent calcium channel antagonist, ω-conotoxin GVIA (CTX), reduced agonist LTD expression by 35%, but did not alter synaptic LTD. CTX also blocked the increase in the paired-pulse ratio associated with agonist LTD. This study cautions against assuming that agonist and synaptic LTD are equivalent as several components underlying their expression appear to differ. Moreover, the data suggest that agonist LTD, but not synaptic LTD, has a presynaptic, N-channel mediated component.

Therapeutic promise and principles: metabotropic glutamate receptors

For a number of disease entities, oxidative stress becomes a significant factor in the etiology and progression of cell dysfunction and injury. Therapeutic strategies that can identify novel signal transduction pathways to ameliorate the toxic effects of oxidative stress may lead to new avenues of treatment for a spectrum of disorders that include diabetes, Alzheimer's disease, Parkinson's disease and immune system dysfunction. In this respect, metabotropic glutamate receptors (mGluRs) may offer exciting prospects for several disorders since these receptors can limit or prevent apoptotic cell injury as well as impact upon cellular development and function. Yet the role of mGluRs is complex in nature and may require specific mGluR modulation for a particular disease entity to maximize clinical efficacy and limit potential disability. Here we discuss the potential clinical translation of mGluRs and highlight the role of novel signal transduction pathways in the metabotropic glutamate system that may be vital for the clinical utility of mGluRs.

Short- and long-term depression at glutamatergic synapses on hippocampal interneurons by group I mGluR activation

Group I metabotropic glutamate receptors (mGluRs) are expressed by many interneurons of the hippocampus. Although they have been implicated in short- and long-term synaptic plasticity of glutamatergic transmission, their roles in modulating transmission to interneurons are incompletely understood. The selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) acutely depressed transmission at synapses in the feed-forward inhibitory pathway made by Schaffer collaterals on interneurons in the rat hippocampal CA1 sub-field. DHPG elicited a qualitatively similar depression at synapses made by pyramidal neuron axon collaterals on interneurons in the feedback circuit in stratum oriens. Selective blockers revealed a link from mGluR1 to reversible, and mGluR5 to long-lasting, depression. The acute DHPG-induced depression was consistently accompanied by an elevation in paired-pulse ratio, implying a presynaptic decrease in release probability. However, it was also attenuated by blocking G-protein and Ca(2+) signalling within the postsynaptic neuron, arguing for a retrograde signalling cascade. The DHPG-evoked depression was unaffected by antagonists of CB1 and GABA(B) receptors but was occluded when presynaptic P/Q-type Ca(2+) channels were blocked. Finally, high-frequency stimulation delivered to an independent conditioning pathway evoked a heterosynaptic reversible depression, which was sensitive to group I mGluR antagonists. Group I mGluRs thus powerfully modulate synaptic excitation of hippocampal interneurons and mediate inter-synaptic cross-talk. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

DGKι regulates presynaptic release during mGluR-dependent LTD

Diacylglycerol (DAG) is an important lipid second messenger. DAG signalling is terminated by conversion of DAG to phosphatidic acid (PA) by diacylglycerol kinases (DGKs). The neuronal synapse is a major site of DAG production and action; however, how DGKs are targeted to subcellular sites of DAG generation is largely unknown. We report here that postsynaptic density (PSD)-95 family proteins interact with and promote synaptic localization of DGKι. In addition, we establish that DGKι acts presynaptically, a function that contrasts with the known postsynaptic function of DGKζ, a close relative of DGKι. Deficiency of DGKι in mice does not affect dendritic spines, but leads to a small increase in presynaptic release probability. In addition, DGKι-/- synapses show a reduction in metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) at neonatal (∼2 weeks) stages that involve suppression of a decrease in presynaptic release probability. Inhibition of protein kinase C normalizes presynaptic release probability and mGluR-LTD at DGKι-/- synapses. These results suggest that DGKι requires PSD-95 family proteins for synaptic localization and regulates presynaptic DAG signalling and neurotransmitter release during mGluR-LTD.

A novel non-CB1/TRPV1 endocannabinoid-mediated mechanism depresses excitatory synapses on hippocampal CA1 interneurons

Endocannabinoids (eCBs) mediate various forms of synaptic plasticity at excitatory and inhibitory synapses in the brain. The eCB anandamide binds to several receptors including the transient receptor potential vanilloid 1 (TRPV1) and cannabinoid receptor 1 (CB1). We recently identified that TRPV1 is required for long-term depression at excitatory synapses on CA1 hippocampal stratum radiatum interneurons. Here we performed whole-cell patch clamp recordings from CA1 stratum radiatum interneurons in rat brain slices to investigate the effect of the eCB anandamide on excitatory synapses as well as the involvement of Group I metabotropic glutamate receptors (mGluRs), which have been reported to produce eCBs endogenously. Application of the nonhydrolysable anandamide analog R-methanandamide depressed excitatory transmission to CA1 stratum radiatum interneurons by ∼50%. The Group I mGluR agonist DHPG also depressed excitatory glutamatergic transmission onto interneurons to a similar degree, and this depression was blocked by the mGluR5 antagonist MPEP (10 μM) but not by the mGluR1 antagonist CPCCOEt (50 μM). Interestingly, however, neither DHPG-mediated nor R-methanandamide-mediated depression was blocked by the TRPV1 antagonist capsazepine (10 μM), the CB1 antagonist AM-251 (2 μM) or a combination of both, suggesting the presence of a novel eCB receptor or anandamide target at excitatory hippocampal synapses. DHPG also occluded R-methanandamide depression, suggesting the possibility that the two drugs elicit synaptic depression via a shared signaling mechanism. Collectively, this study illustrates a novel CB1/TRPV1-independent eCB pathway present in the hippocampus that mediates depression at excitatory synapses on CA1 stratum radiatum interneurons.

Postnatal alterations in induction threshold and expression magnitude of long-term potentiation and long-term depression at hippocampal synapses

Activity-dependent synaptic plasticity refines neural networks during development and subserves information processing in adulthood. Previous research has revealed postnatal alterations in synaptic plasticity at nearly all forebrain synapses, suggesting different forms of synaptic plasticity may contribute to network development and information processing. To assess possible relationships between modifications in synaptic plasticity and maturation of cognitive ability, we examined excitatory synaptic function in area CA1 of the mouse hippocampus ∼3 weeks of age, when hippocampal-dependent learning and memory abilities first emerge. Long-term potentiation (LTP) and depression (LTD) of synaptic efficacy were observed in slices from juvenile animals younger than 3 weeks of age. Both pre- and postsynaptic mechanisms supported LTP and LTD in juveniles. After the third postnatal week, the magnitude of LTP was reduced and the threshold for postsynaptic induction was reduced, but the threshold for presynaptic induction was increased. The reduced threshold for postsynaptic LTP appeared to be due, partly, to an increase in baseline excitatory synaptic strength, which likely permitted greater postsynaptic depolarization during induction. Low frequency stimulation did not induce LTD at this more mature stage, but it blocked subsequent induction of LTP, suggesting metaplastic differences across age groups. Late postnatal modifications in activity-dependent synaptic plasticity might reflect attenuation of mechanisms more closely tied to network formation (presynaptic potentiation and pre- and postsynaptic depression) and unmasking of mechanisms underlying information processing and storage (associative postsynaptic potentiation), which likely impact the integrative capacity of the network and regulate the emergence of adult-like cognitive abilities.

Activation conditions for the induction of metabotropic glutamate receptor-dependent long-term depression in hippocampal CA1 pyramidal cells

Two forms of homosynaptic long-term depression (LTD) are distinguished in hippocampal CA1 pyramidal cells, one which is NMDA receptor dependent and the other metabotropic glutamate receptor (mGluR) dependent. Although the molecular processes involved in mGluR-LTD are well characterized, the conditions of circuit activation required for its induction remain unclear. We show that mGluR-LTD cannot be induced in synaptically coupled CA3-CA1 pyramidal cell pairs. Experiments to address the underlying mechanisms indicate that, even when glutamate transporters are blocked, one presynaptic cell releases insufficient glutamate to evoke an mGluR-mediated current in a connected CA1 cell. These findings imply that extrasynaptic diffusion is not a limiting factor and are consistent with a sparse distribution of functional mGluRs in the dendritic tree of pyramidal cells. Thus, the discharge of multiple Schaffer collaterals to a targeted cell is necessary for mGluR-LTD. Our experiments indicate that approximately eight CA3 inputs to a CA1 pyramidal cell must be activated to induce mGluR-LTD.

Endocannabinoids differentially modulate synaptic plasticity in rat hippocampal CA1 pyramidal neurons

BACKGROUND

Hippocampal CA1 pyramidal neurons receive two excitatory glutamatergic synaptic inputs: their most distal dendritic regions in the stratum lacunosum-moleculare (SLM) are innervated by the perforant path (PP), originating from layer III of the entorhinal cortex, while their more proximal regions of the apical dendrites in the stratum radiatum (SR) are innervated by the Schaffer-collaterals (SC), originating from hippocampal CA3 neurons. Endocannabinoids (eCBs) are naturally occurring mediators capable of modulating both GABAergic and glutamatergic synaptic transmission and plasticity via the CB1 receptor. Previous work on eCB modulation of excitatory synapses in the CA1 region largely focuses on the SC pathway. However, little information is available on whether and how eCBs modulate glutamatergic synaptic transmission and plasticity at PP synapses.

METHODOLOGY/PRINCIPAL FINDINGS

By employing somatic and dendritic patch-clamp recordings, Ca(2+) uncaging, and immunostaining, we demonstrate that there are significant differences in low-frequency stimulation (LFS)- or DHPG-, an agonist of group I metabotropic glutamate receptors (mGluRs), induced long-term depression (LTD) of excitatory synaptic transmission between SC and PP synapses in the same pyramidal neurons. These differences are eliminated by pharmacological inhibition with selective CB1 receptor antagonists or genetic deletion of the CB1 receptor, indicating that these differences likely result from differential modulation via a CB1 receptor-dependent mechanism. We also revealed that depolarization-induced suppression of excitation (DSE), a form of short-term synaptic plasticity, and photolysis of caged Ca(2+)-induced suppression of Excitatory postsynaptic currents (EPSCs) were less at the PP than that at the SC. In addition, application of WIN55212 (WIN) induced a more pronounced inhibition of EPSCs at the SC when compared to that at the PP.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that CB1 dependent LTD and DSE are differentially expressed at the PP versus SC synapses in the same neurons, which may have an impact on synaptic scaling, integration and plasticity of hippocampal CA1 pyramidal neurons.

Distinct forms of Gq-receptor-dependent plasticity of excitatory transmission in the BNST are differentially affected by stress

Long-term depression (LTD) is an important synaptic mechanism for limiting excitatory influence over circuits subserving cognitive and emotional behavior. A major means of LTD induction is through the recruitment of signaling via G(q)-linked receptors activated by norepinephrine (NE), acetylcholine, and glutamate. Receptors from these transmitter families have been proposed to converge on a common postsynaptic LTD maintenance mechanism, such that hetero- and homosynaptic induction produce similar alterations in glutamate synapse efficacy. We report that in the dorsolateral and ventrolateral bed nucleus of the stria terminalis (BNST), recruitment of G(q)-linked receptors by glutamate or NE initiates mechanistically distinct forms of postsynaptically maintained LTD and these LTDs are differentially regulated by stress exposure. In particular, we show that although both mGluR5- and alpha(1)-adrenergic receptor (AR)-dependent LTDs involve postsynaptic endocytosis, the alpha(1)-AR-initiated LTD exclusively involves modulation of signaling through calcium-permeable AMPA receptors. Further, alpha(1)-AR- but not mGluR5- dependent LTD is disrupted by restraint stress. alpha(1)-AR LTD is also impaired in mice chronically exposed to ethanol. These data thus suggest that in the BNST, NE- and glutamate-activated G(q)-linked signaling pathways differentially tune glutamate synapse efficacy in response to stress.

Presynaptic m1 muscarinic receptors are necessary for mGluR long-term depression in the hippocampus

To investigate the role of M1 muscarininc acetylcholine receptors (m1 receptors) in metabotropic glutamate receptor (mGluR)-mediated long-term depression (LTD), we produced mouse lines in which deletion of the m1 gene is restricted to the forebrain (FB-m1KO) or hippocampal CA3 pyramidal neurons (CA3-m1KO). Stimulation in FB-m1KO hippocampal slices resulted in excitatory postsynaptic potentials and long-term synaptic plasticity (long-term potentiation and LTD) similar to controls. The mice were deficient in (S)-3,5-dihydroxyphenylglycine hydrate (DHPG)-induced mGluR LTD, which correlated with a presynaptic increase in the release of neurotransmitters. Protein kinase C (PKC) activity, which is downstream from both mGluRs and m1 receptors, was reduced in CA3 but not in CA1. The presynaptic requirement of m1 receptors was confirmed by the lack of DHPG-induced mGluR LTD in the CA1 of slices from CA3-m1KO mice. mGluR LTD was rescued by stimulating PKC activity pharmacologically in CA3-m1KO mice. These data confirm a role for PKC activation in presynaptic induction of mGluR LTD and distinguish between the roles of mGluRs and m1 receptors.

HCN1 channels constrain DHPG-induced LTD at hippocampal Schaffer collateral-CA1 synapses

HCN channels play a fundamental role in determining resting membrane potential and regulating synaptic function. Here, we investigated the involvement of HCN channels in basal synaptic transmission and long-term depression (LTD) at the Schaffer collateral-CA1 synapse. Bath application of the HCN channel blocker ZD7288 (10 microM) caused a significant increase in synaptic transmission that was due to an enhancement in AMPA receptor-mediated excitatory postsynaptic potentials. This enhancement was accompanied by a significant decrease in the paired-pulse ratio (PPR), suggesting a presynaptic mechanism. Experiments with the irreversible use-dependent NMDA receptor blocker MK-801 showed that ZD7288 led to an increase in glutamate release probability. LTD induced by brief application of (RS)-3,5-dihydroxyphenylglycine (DHPG, 100 microM, 10 min) was significantly enhanced when HCN channels were blocked by ZD7288 (10 microM) prior to DHPG application. Moreover, the concomitant increase in PPR after DHPG-induced LTD was significantly larger than without ZD7288 bath application. Conversely, ZD7288 application after DHPG washout did not alter DHPG-LTD. A significant enhancement of DHPG-LTD was also observed in HCN1-deficient mice as compared with wild types. However, LTD induced by low-frequency stimulation (LFS) remained unaltered in HCN1-deficient mice, suggesting a differential effect of HCN1 channels on synaptic plasticity constraining DHPG-LTD, but not LFS-LTD.

Metabotropic glutamate receptor-mediated long-term depression: molecular mechanisms

The ability to modify synaptic transmission between neurons is a fundamental process of the nervous system that is involved in development, learning, and disease. Thus, synaptic plasticity is the ability to bidirectionally modify transmission, where long-term potentiation and long-term depression (LTD) represent the best characterized forms of plasticity. In the hippocampus, two main forms of LTD coexist that are mediated by activation of either N-methyl-d-aspartic acid receptors (NMDARs) or metabotropic glutamate receptors (mGluRs). Compared with NMDAR-LTD, mGluR-LTD is less well understood, but recent advances have started to delineate the underlying mechanisms. mGluR-LTD at CA3:CA1 synapses in the hippocampus can be induced either by synaptic stimulation or by bath application of the group I selective agonist (R,S)-3,5-dihydroxyphenylglycine. Multiple signaling mechanisms have been implicated in mGluR-LTD, illustrating the complexity of this form of plasticity. This review provides an overview of recent studies investigating the molecular mechanisms underlying hippocampal mGluR-LTD. It highlights the role of key molecular components and signaling pathways that are involved in the induction and expression of mGluR-LTD and considers how the different signaling pathways may work together to elicit a persistent reduction in synaptic transmission.

BC1 regulation of metabotropic glutamate receptor-mediated neuronal excitability

Regulatory RNAs have been suggested to contribute to the control of gene expression in eukaryotes. Brain cytoplasmic (BC) RNAs are regulatory RNAs that control translation initiation. We now report that neuronal BC1 RNA plays an instrumental role in the protein-synthesis-dependent implementation of neuronal excitation-repression equilibria. BC1 repression counter-regulates translational stimulation resulting from synaptic activation of group I metabotropic glutamate receptors (mGluRs). Absence of BC1 RNA precipitates plasticity dysregulation in the form of neuronal hyperexcitability, elicited by group I mGluR-stimulated translation and signaled through the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway. Dysregulation of group I mGluR function in the absence of BC1 RNA gives rise to abnormal brain function. Cortical EEG recordings from freely moving BC1(-/-) animals show that group I mGluR-mediated oscillations in the gamma frequency range are significantly elevated. When subjected to sensory stimulation, these animals display an acute group I mGluR-dependent propensity for convulsive seizures. Inadequate RNA control in neurons is thus causally linked to heightened group I mGluR-stimulated translation, neuronal hyperexcitability, heightened gamma band oscillations, and epileptogenesis. These data highlight the significance of small RNA control in neuronal plasticity.

Novel mGluR- and CB1R-independent suppression of GABA release caused by a contaminant of the group I metabotropic glutamate receptor agonist, DHPG

Background

Metabotropic glutamate receptors (mGluRs) are ubiquitous throughout the body, especially in brain, where they mediate numerous effects. MGluRs are classified into groups of which group I, comprising mGluRs 1 and 5, is especially important in neuronal communication. Group I actions are often investigated with the selective agonist, S-3,5-dihydroxyphenylglycine (DHPG). Despite the selectivity of DHPG, its use has often led to contradictory findings. We now report that a particular commercial preparation of DHPG can produce mGluR-independent effects. These findings may help reconcile some discrepant reports.

Methods

We carried out electrophysiological recordings in the rat in vitro hippocampal slice preparation, focusing mainly on pharmacologically isolated GABA_A-receptor-mediated synaptic currents. Principal Findings: While preparations of DHPG from three companies suppressed GABAergic transmission in an mGluR-dependent way, one batch had an additional, unusual effect. Even in the presence of antagonists of mGluRs, it caused a reversible, profound suppression of inhibitory transmission. This mGluR - independent action was not due to a higher potency of the compound, or its ability to cause endocannabinoid-dependent responses. Field potential recordings revealed that glutamatergic transmission was not affected, and quantal analysis of GABA transmission confirmed the unusual effect was on GABA release, and not GABA_A receptors. We have not identified the responsible factor in the DHPG preparation, but the samples were 99% pure as determined by HPLC and NMR analyses.

Conclusions

In certain respects our observations with the anomalous batch strikingly resemble some published reports of unusual DHPG effects. The present findings could therefore contribute to explaining discrepancies in the literature. DHPG is widely employed to study mGluRs in different systems, hence rigorous controls should be performed before conclusions based on its use are drawn.