Investigations of the protein synthesis dependency of mGluR-induced long-term depression in the dentate gyrus of freely moving rats

Differential effects of group III metabotropic glutamate receptors on spontaneous inhibitory synaptic currents in spine-innervating double bouquet and parvalbumin-expressing dendrite-targeting GABAergic interneurons in human neocortex

Diverse neocortical GABAergic neurons specialize in synaptic targeting and their effects are modulated by presynaptic metabotropic glutamate receptors (mGluRs) suppressing neurotransmitter release in rodents, but their effects in human neocortex are unknown. We tested whether activation of group III mGluRs by L-AP4 changes GABAA receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in 2 distinct dendritic spine-innervating GABAergic interneurons recorded in vitro in human neocortex. Calbindin-positive double bouquet cells (DBCs) had columnar "horsetail" axons descending through layers II-V innervating dendritic spines (48%) and shafts, but not somata of pyramidal and nonpyramidal neurons. Parvalbumin-expressing dendrite-targeting cell (PV-DTC) axons extended in all directions innervating dendritic spines (22%), shafts (65%), and somata (13%). As measured, 20% of GABAergic neuropil synapses innervate spines, hence DBCs, but not PV-DTCs, preferentially select spine targets. Group III mGluR activation paradoxically increased the frequency of sIPSCs in DBCs (to median 137% of baseline) but suppressed it in PV-DTCs (median 92%), leaving the amplitude unchanged. The facilitation of sIPSCs in DBCs may result from their unique GABAergic input being disinhibited via network effect. We conclude that dendritic spines receive specialized, diverse GABAergic inputs, and group III mGluRs differentially regulate GABAergic synaptic transmission to distinct GABAergic cell types in human cortex.

Long-term plasticity in the hippocampus: maintaining within and 'tagging' between synapses

Synapses between neurons are malleable biochemical structures, strengthening and diminishing over time dependent on the type of information they receive. This phenomenon known as synaptic plasticity underlies learning and memory, and its different forms, long-term potentiation (LTP) and long-term depression (LTD), perform varied cognitive roles in reinforcement, relearning and associating memories. Moreover, both LTP and LTD can exist in an early transient form (early-LTP/LTD) or a late persistent form (late-LTP/LTD), which are triggered by different induction protocols, and also differ in their dependence on protein synthesis and the involvement of key molecular players. Beyond homosynaptic modifications, synapses can also interact with one another. This is encapsulated in the synaptic tagging and capture hypothesis (STC), where synapses expressing early-LTP/LTD present a 'tag' that can capture the protein synthesis products generated during a temporally proximal late-LTP/LTD induction. This 'tagging' phenomenon forms the framework of synaptic interactions in various conditions and accounts for the cellular basis of the time-dependent associativity of short-lasting and long-lasting memories. All these synaptic modifications take place under controlled neuronal conditions, regulated by subcellular elements such as epigenetic regulation, proteasomal degradation and neuromodulatory signals. Here, we review current understanding of the different forms of synaptic plasticity and its regulatory mechanisms in the hippocampus, a brain region critical for memory formation. We also discuss expression of plasticity in hippocampal CA2 area, a long-overlooked narrow hippocampal subfield and the behavioural correlate of STC. Lastly, we put forth perspectives for an integrated view of memory representation in synapses.

G9a/GLP Complex Acts as a Bidirectional Switch to Regulate Metabotropic Glutamate Receptor-Dependent Plasticity in Hippocampal CA1 Pyramidal Neurons

Metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) is conventionally considered to be solely dependent on local protein synthesis. Given the impact of epigenetics on memory, the intriguing question is whether epigenetic regulation influences mGluR-LTD as well. G9a/GLP histone lysine methyltransferase complex is crucial for brain development and goal-directed learning as well as for drug-addiction. In this study, we analyzed whether the epigenetic regulation by G9a/GLP complex affects mGluR-LTD in CA1 hippocampal pyramidal neurons of 5-7 weeks old male Wistar rats. In hippocampal slices with intact CA1 dendritic regions, inhibition of G9a/GLP activity abolished mGluR-LTD. The inhibition of this complex upregulated the expression of plasticity proteins like PKMζ, which mediated the prevention of mGluR-LTD expression by regulating the NSF-GluA2-mediated trafficking of AMPA receptors towards the postsynaptic site. G9a/GLP inhibition during the induction of mGluR-LTD also downregulated the protein levels of phosphorylated-GluA2 and Arc. Interestingly, G9a/GLP inhibition could not impede the mGluR-LTD when the cell-body was severed. Our study highlights the role of G9a/GLP complex in intact neuronal network as a bidirectional switch; when turned on, it facilitates the expression of mGluR-LTD, and when turned off, it promotes the expression of long-term potentiation.

Intracellular GPCRs Play Key Roles in Synaptic Plasticity

The trillions of synaptic connections within the human brain are shaped by experience and neuronal activity, both of which underlie synaptic plasticity and ultimately learning and memory. G protein-coupled receptors (GPCRs) play key roles in synaptic plasticity by strengthening or weakening synapses and/or shaping dendritic spines. While most studies of synaptic plasticity have focused on cell surface receptors and their downstream signaling partners, emerging data point to a critical new role for the very same receptors to signal from inside the cell. Intracellular receptors have been localized to the nucleus, endoplasmic reticulum, lysosome, and mitochondria. From these intracellular positions, such receptors may couple to different signaling systems, display unique desensitization patterns, and/or show distinct patterns of subcellular distribution. Intracellular GPCRs can be activated at the cell surface, endocytosed, and transported to an intracellular site or simply activated in situ by de novo ligand synthesis, diffusion of permeable ligands, or active transport of non-permeable ligands. Current findings reinforce the notion that intracellular GPCRs play a dynamic role in synaptic plasticity and learning and memory. As new intracellular GPCR roles are defined, the need to selectively tailor agonists and/or antagonists to both intracellular and cell surface receptors may lead to the development of more effective therapeutic tools.

Updating temporal expectancy of an aversive event engages striatal plasticity under amygdala control

Pavlovian aversive conditioning requires learning of the association between a conditioned stimulus (CS) and an unconditioned, aversive stimulus (US) but also involves encoding the time interval between the two stimuli. The neurobiological bases of this time interval learning are unknown. Here, we show that in rats, the dorsal striatum and basal amygdala belong to a common functional network underlying temporal expectancy and learning of a CS-US interval. Importantly, changes in coherence between striatum and amygdala local field potentials (LFPs) were found to couple these structures during interval estimation within the lower range of the theta rhythm (3-6 Hz). Strikingly, we also show that a change to the CS-US time interval results in long-term changes in cortico-striatal synaptic efficacy under the control of the amygdala. Collectively, this study reveals physiological correlates of plasticity mechanisms of interval timing that take place in the striatum and are regulated by the amygdala.

Group III metabotropic glutamate receptors: pharmacology, physiology and therapeutic potential

Glutamate, the primary excitatory neurotransmitter in the central nervous system (CNS), exerts neuromodulatory actions via the activation of metabotropic glutamate (mGlu) receptors. There are eight known mGlu receptor subtypes (mGlu1-8), which are widely expressed throughout the brain, and are divided into three groups (I-III), based on signalling pathways and pharmacological profiles. Group III mGlu receptors (mGlu4/6/7/8) are primarily, although not exclusively, localised on presynaptic terminals, where they act as both auto- and hetero-receptors, inhibiting the release of neurotransmitter. Until recently, our understanding of the role of individual group III mGlu receptor subtypes was hindered by a lack of subtype-selective pharmacological tools. Recent advances in the development of both orthosteric and allosteric group III-targeting compounds, however, have prompted detailed investigations into the possible functional role of these receptors within the CNS, and revealed their involvement in a number of pathological conditions, such as epilepsy, anxiety and Parkinson's disease. The heterogeneous expression of group III mGlu receptor subtypes throughout the brain, as well as their distinct distribution at glutamatergic and GABAergic synapses, makes them ideal targets for therapeutic intervention. This review summarises the advances in subtype-selective pharmacology, and discusses the individual roles of group III mGlu receptors in physiology, and their potential involvement in disease.

Dysregulation of synaptic plasticity precedes appearance of morphological defects in a Pten conditional knockout mouse model of autism

The phosphoinositide signaling system is a crucial regulator of neural development, cell survival, and plasticity. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) negatively regulates phosphatidylinositol 3-kinase signaling and downstream targets. Nse-Cre Pten conditional knockout mice, in which Pten is ablated in granule cells of the dentate gyrus and pyramidal neurons of the hippocampal CA3, but not CA1, recapitulate many of the symptoms of humans with inactivating PTEN mutations, including progressive hypertrophy of the dentate gyrus and deficits in hippocampus-based social and cognitive behaviors. However, the impact of Pten loss on activity-dependent synaptic plasticity in this clinically relevant mouse model of Pten inactivation remains unclear. Here, we show that two phosphatidylinositol 3-kinase- and protein synthesis-dependent forms of synaptic plasticity, theta burst-induced long-term potentiation and metabotropic glutamate receptor (mGluR)-dependent long-term depression, are dysregulated at medial perforant path-to-dentate gyrus synapses of young Nse-Cre Pten conditional knockout mice before the onset of visible morphological abnormalities. In contrast, long-term potentiation and mGluR-dependent long-term depression are normal at CA3-CA1 pyramidal cell synapses at this age. Our results reveal that deletion of Pten in dentate granule cells dysregulates synaptic plasticity, a defect that may underlie abnormal social and cognitive behaviors observed in humans with Pten inactivating mutations and potentially other autism spectrum disorders.

Variations in postnatal maternal care and the epigenetic regulation of metabotropic glutamate receptor 1 expression and hippocampal function in the rat

Variations in maternal care in the rat affect hippocampal morphology and function as well as performance on hippocampal-dependent tests of learning and memory in the offspring. Preliminary genome-wide analyses of gene transcription and DNA methylation of the molecular basis for such maternal effects suggested differences in the epigenetic state and transcriptional activity of the Grm1 gene in the rat as a function of maternal care. Grm1 encodes the type I metabotropic glutamate receptor (mGluR1), and we found increased mGluR1 mRNA and protein in hippocampus from the adult offspring of mothers showing an increased frequency of pup licking/grooming (i.e., high-LG mothers) that was associated with a decrease in the methylation of Grm1. ChIP assays showed increased levels of histone 3 lysine 9 acetylation and histone 3 lysine 4 trimethylation of Grm1 in hippocampus from the adult offspring of high-LG compared with low-LG mothers. These histone posttranslational modifications were highly correlated, and both associate inversely with DNA methylation and positively with transcription. Studies of mGluR1 function showed increased hippocampal mGluR1-induced long-term depression in the adult offspring of high-LG compared with low-LG mothers, as well as increased paired-pulse depression (PPD). PPD is an inhibitory feedback mechanism that prevents excessive glutamate release during high-frequency stimulation. The maternal effects on both long-term depression and PPD were eliminated by treatment with an mGluR1-selective antagonist. These findings suggest that variations in maternal care can influence hippocampal function and cognitive performance through the epigenetic regulation of genes implicated in glutamatergic synaptic signaling.

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.

A mismatch-based model for memory reconsolidation and extinction in attractor networks

The processes of memory reconsolidation and extinction have received increasing attention in recent experimental research, as their potential clinical applications begin to be uncovered. A number of studies suggest that amnestic drugs injected after reexposure to a learning context can disrupt either of the two processes, depending on the behavioral protocol employed. Hypothesizing that reconsolidation represents updating of a memory trace in the hippocampus, while extinction represents formation of a new trace, we have built a neural network model in which either simple retrieval, reconsolidation or extinction of a stored attractor can occur upon contextual reexposure, depending on the similarity between the representations of the original learning and reexposure sessions. This is achieved by assuming that independent mechanisms mediate Hebbian-like synaptic strengthening and mismatch-driven labilization of synaptic changes, with protein synthesis inhibition preferentially affecting the former. Our framework provides a unified mechanistic explanation for experimental data showing (a) the effect of reexposure duration on the occurrence of reconsolidation or extinction and (b) the requirement of memory updating during reexposure to drive reconsolidation.

Group 1 mGluR-dependent synaptic long-term depression: mechanisms and implications for circuitry and disease

Many excitatory synapses express Group 1, or Gq coupled, metabotropic glutamate receptors (Gp1 mGluRs) at the periphery of their postsynaptic density. Activation of Gp1 mGluRs typically occurs in response to strong activity and triggers long-term plasticity of synaptic transmission in many brain regions, including the neocortex, hippocampus, midbrain, striatum, and cerebellum. Here we focus on mGluR-induced long-term synaptic depression (LTD) and review the literature that implicates Gp1 mGluRs in the plasticity of behavior, learning, and memory. Moreover, recent studies investigating the molecular mechanisms of mGluR-LTD have discovered links to mental retardation, autism, Alzheimer's disease, Parkinson's disease, and drug addiction. We discuss how mGluRs lead to plasticity of neural circuits and how the understanding of the molecular mechanisms of mGluR plasticity provides insight into brain disease.

Protein translation in synaptic plasticity: mGluR-LTD, Fragile X

Synaptically activated, rapid and dendritic synthesis of new proteins has long been proposed to mediate long-lasting changes at the synapse [Steward O, Schuman EM: Protein synthesis at synaptic sites on dendrites.Annu Rev Neurosci 2001, 24:299-325]. Studies of group 1 metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) have provided new insight into dendritic or local translation and plasticity. Here we highlight these exciting results and discuss how synaptic activity controls local translation, the proteins that are synthesized in dendrites, how they affect synaptic function and how altered local translational control contributes to a form of human mental retardation, Fragile X Syndrome.

Modulation by group 1 metabotropic glutamate receptors of depotentiation in the dentate gyrus of freely moving rats

In the hippocampus, synaptic depression of potentiated synapses in the form of depotentiation, or of naive synapses in the form of long-term depression (LTD) is mediated by distinct molecular mechanisms. Activation of group 1 metabotropic glutamate receptors (mGluRs) is critically required for both hippocampal long-term potentiation (LTP) and LTD in vivo, but their involvement in depotentiation is unclear. In this study, we investigated whether this class of mGluRs contributes to depotentiation in freely moving rats. Male adult Wistar rats underwent chronic implantation of stimulating and recording electrodes in the perforant path and dentate gyrus granule cell layer, respectively, as well as an injection cannula in the ipsilateral cerebral ventricle. Robust LTP which endured for over 24 h, was induced by high frequency tetanization (HFT, 200 Hz). Depotentiation was induced with LFS (5 Hz, 600 pulses) given 5 min after the LTP-inducing tetanus was applied. The selective group 1 mGluR antagonists, (S)-4-carboxyphenylglycine and (R,S)-1-aminoindan-1,5-dicarboxylic acid significantly inhibited both depotentiation and LTP. Activation of group I mGluRs leads to changes in postsynaptic intracellular calcium levels. These findings suggest that activation of group I mGluRs mediate thresholds for depotentiation and for persistent LTP. Effects may be linked to the intensity and duration of the calcium signal elicited by LFS and HFT.

Role for metabotropic glutamate receptor 5 (mGluR5) in the pathogenesis of fragile X syndrome

Metabotropic glutamate receptors (mGluRs) have been implicated in a diverse variety of neuronal functions. Studies reviewed here indicate that exaggerated signalling through mGluR5 can account for multiple cognitive and syndromic features of fragile X syndrome, the most common inherited form of mental retardation and autism. Since a reduction of mGluR5 signalling can reverse fragile X phenotypes, these studies provide a compelling rationale for the use of mGluR5 antagonists for the treatment of fragile X and related disorders.

Muscarinic receptor dependent long-term depression in rat visual cortex is PKC independent but requires ERK1/2 activation and protein synthesis

Intact cholinergic innervation of visual cortex is critical for normal processing of visual information and for spatial memory acquisition and retention. However, a complete description of the mechanisms by which the cholinergic system modifies synaptic function in visual cortex is lacking. Previously it was shown that activation of the m1 subtype of muscarinic receptor induces an activity-dependent and partially N-methyl-d-aspartate receptor (NMDAR)-dependent long-term depression (LTD) at layer 4-layer 2/3 synapses in rat visual cortex slices in vitro. The cellular mechanisms downstream of the Galphaq coupled m1 receptor required for induction of this LTD (which we term mLTD) are currently unknown. Here, we confirm a role for m1 receptors in mLTD induction and use a series of pharmacological tools to study the signaling molecules downstream of m1 receptor activation in mLTD induction. We found that mLTD is prevented by inhibitors of L-type Ca(2+) channels, the Src kinase family, and the mitogen-activated kinase/extracellular kinase. mLTD is also partially dependent on phospholipase C but is unaffected by blocking protein kinase C. mLTD expression can be long-lasting (>2 h) and its long-term maintenance requires translation. Thus we report the signaling mechanisms underlying induction of an m1 receptor-dependent LTD in visual cortex and the requirement of protein synthesis for long-term expression. This plasticity could be a mechanism by which the cholinergic system modifies glutamatergic synapse function to permit normal visual system processing required for cognition.

Group I metabotropic glutamate receptors enable two distinct forms of long-term depression in the rat dentate gyrus in vivo

The existence of long-term depression (LTD) in the dentate gyrus of freely moving rats, as well as the contribution of different types of metabotropic glutamate receptors (mGluRs) to this form of plasticity, has been the subject of much debate. Here, we describe two distinct forms of mGluR-dependent hippocampal LTD in the dentate gyrus of freely moving adult rats. LTD, induced by low-frequency stimulation (LFS) of the medial perforant path (LFS-LTD), was prevented by antagonism of the phospholipase C-coupled receptors, mGluR1 but not mGluR5. Chemical LTD, induced by intracerebral application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine, was blocked by antagonism of both mGluR5 and mGluR1. Selective activation of mGluR5, using (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG), also led to chemical LTD. To test whether LFS-LTD and chemical LTD share common induction mechanisms, we applied LFS following the induction of chemical LTD by CHPG (CHPG-LTD). Surprisingly, LFS impaired CHPG-LTD. Further analysis revealed that induction of CHPG-LTD led to altered calcium dynamics sufficient for its reversal by LFS. We found that LTD induced by (R,S)-3,5-dihydroxyphenylglycine, but not by CHPG, is impaired by N-methyl-d-aspartate receptor antagonism. Both forms of chemical LTD strongly require calcium influx through L-type voltage-gated calcium channels. This contrasts with previous findings that LFS-LTD in the dentate gyrus is both N-methyl-d-aspartate receptor and voltage-gated calcium channel independent. LFS-LTD and LTD induced by group I mGluR agonists thus appear to comprise distinct forms of LTD that require the activation of specific group I mGluRs and recruit calcium from different sources.

Comparison of cellular mechanisms of long-term depression of synaptic strength at perforant path-granule cell and Schaffer collateral-CA1 synapses

This chapter compares the cellular mechanisms that have been implicated in the induction and expression of long-term depression (LTD) at Schaffer collateral-CA1 synapses to perforant path-dentate gyrus (DG) synapses. In general, Schaffer collateral LTD and long-term potentiation (LTP) both appear to be a complex combination of many alterations in synaptic transmission that occur at both presynaptic and postsynaptic sites, while at perforant path synapses, most evidence has focused on postsynaptic long-term alterations. Within the DG, the medial perforant path is far more studied than lateral perforant path synapses, where most evidence relates to the induction of heterosynaptic LTD at lateral perforant path synapses when LTP is induced in the medial perforant path. Of course, there remain many other classes of synapses in the DG where synaptic plasticity, including LTD, have been largely neglected. It is clear that a better understanding of the range of DG loci where long-lasting activity-dependent plasticity, both LTD and LTP, are expressed will be essential to improve our understanding of the cognitive roles of such DG plasticity.

NMDA-dependent, but not group I metabotropic glutamate receptor-dependent, long-term depression at Schaffer collateral-CA1 synapses is associated with long-term reduction of release from the rapidly recycling presynaptic vesicle pool

Postsynaptic alterations have been suggested to account for NMDA receptor (NMDAR)-dependent long-term depression (LTD) and long-term potentiation of synaptic strength, although there is substantial evidence supporting changes in presynaptic release. Direct chemical activation of either NMDA or group I metabotropic glutamate receptor (mGluR1) elicits LTD of similar magnitudes, but it is unknown whether they share common expression mechanisms. Using dual-photon laser-scanning microscopy of FM1-43 [N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide] to directly visualize presynaptic vesicular release from the rapidly recycling vesicle pool (RRP) at Schaffer collateral terminals in field CA1 of rat hippocampal slices, we found that a persistent reduction in vesicular release from the RRP is induced by NMDA-LTD but not by mGluR1-LTD. Variance-mean analyses of Schaffer collateral release probability (P(r)) at varying extracellular calcium concentrations confirmed that NMDA-LTD was associated with reduced P(r), whereas mGluR1-LTD was not. Pharmacological isolation of NMDAR-dependent and mGluR-dependent forms of stimulus-evoked LTD revealed that both are composed of a combination of presynaptic and postsynaptic alterations. However, when group I mGluR-dependent LTD was isolated by combining an NMDAR blocker with a group II mGluR antagonist, this form of LTD was purely postsynaptic. The nitric oxide synthase inhibitor N omega-nitro-L-arginine blocked the induction of NMDA-LTD but did not alter mGluR-LTD, consistent with a selective role for nitric oxide as a retrograde messenger mediating NMDA-LTD. These data demonstrate that single synapses can express multiple forms of LTD with different sites of expression, that NMDA-LTD is a combination of presynaptic and postsynaptic alterations, but that group I mGluR-LTD appears to be expressed entirely postsynaptically.

Persistent (>24h) long-term depression in the dentate gyrus of freely moving rats is not dependent on activation of NMDA receptors, L-type voltage-gated calcium channels or protein synthesis

Hippocampal long-term depression (LTD) comprises a persistent reduction of synaptic strength that is typically induced by low frequency stimulation (LFS). Although LTD has been described for the dentate gyrus in vitro, this phenomenon in the dentate gyrus of the intact animal is less well understood. In the current study, we investigated the contribution of NMDA receptors, L-type voltage gated calcium channels and protein synthesis to LFS-induced LTD in the dentate gyrus of freely moving rats. Animals were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. LTD persisted for at least 24h, and was unaffected by prior treatment with the NMDA receptor antagonists AP5 or ifenprodil, which, in contrast, prevented LTP. Neither the L-type voltage-gated calcium channel antagonist, methoxyverapamil, nor the protein translation inhibitors, anisomycin or emetine had an effect on the profile of LTD. Our results suggest that NMDA receptors and L-type voltage-gated calcium channels are not involved in the induction of LTD in the dentate gyrus in vivo. Intriguingly, persistent LTD can be established without the synthesis of new proteins, suggesting that in the dentate gyrus, alternative mechanisms exist for the sustainment of enduring LTD.

The adult rat hippocampal slice revisited with multi-electrode arrays

The multi-electrode arrays (MEA) technology for the recording of brain slices is available for more than 10 years. However, despite its relative simplicity, this recording technique is not widely used in academic or pharmaceutical research laboratories. We illustrate here that MEA provide multiple possibilities to investigate some network physiological properties as well as to evaluate the pharmacological effects of compounds. We first document that MEA allow to trigger and to record conventional FP which are inhibited by the block of action potential propagation (with 500 nM TTX). FP recorded with MEA are sensitive to ionic substitutions, to ionotropic glutamate receptor antagonists (CNQX or NBQX) and to energetic failure. Second, we illustrate that different "classical" protocols (paired-pulse, LTP, chemical LTD), revealing synaptic plasticity mechanisms, could be performed. Third, we document that MEA allow spatial and temporal discriminations for the effects of known pharmacological compounds such as competitive antagonist (gabazine, bicuculline) and allosteric modulators (steroids) of GABA(A) receptors. In conclusion, we illustrate that MEA recordings of adult rat hippocampal slices constitute a powerful and sensitive system to evaluate the effect of molecules on basic synaptic propagation/transmission and on synaptic plasticity processes.

Group II mGluR-induced long term depression in the dentate gyrus in vivo is NMDA receptor-independent and does not require protein synthesis

Long term depression (LTD) can be induced by low frequency stimulation (LFS) as well as by agonist activation of neurotransmitter receptors. Group II metabotropic glutamate receptors (mGluRs) play an essential role in the regulation of electrically-induced LTD in the hippocampus in vivo: LTD is inhibited by antagonists, and enhanced by agonists of group II mGluRs. Here we investigated induction of LTD by activation of group II mGluRs as well as the cellular mechanisms which might mediate group II mGluR-induced LTD. Rats were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. Drug application was made through a cannula implanted into the ipsilateral cerebral ventricle. LTD could be induced by agonist activation of either group II mGluRs, or the group II mGluR subtype, mGluR3. Both, group II mGluR-induced LTD and mGluR3-induced LTD were not abolished by mRNA/protein synthesis inhibition. Furthermore, mGluR3-induced LTD was not inhibited by NMDA receptor antagonists or altered by L-type voltage-gated calcium channel blockers. Our data suggest that sole activation of group II mGluRs can mediate LTD in vivo. Intriguingly, this form of LTD is not dependent on protein synthesis or activation of NMDA receptors.