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

Behavioral and electrophysiological (ECoG) effects of haplophyllum robustum and TRPA1 antagonist in adult male wistar rats

This article aimed to investigate the effects of Haplophyllum robustum hydroalcoholic extract on animals' behavioral and electrocorticographic changes. This plant is mainly found in Turkey, Iran, and Central Asia, and is reported to have convulsive effects. In this article, we worked on the effects of its hydroalcoholic extract on electrocorticography (ECoG), along with changes induced by intracerebroventricular administration of GABAA antagonists. Furthermore, the effects of low doses of this extract on behavioral depression were examined. Four animal sets were used to compare ECoG in Wistar rats. A group of negative control, a group of positive control (PTZ), and two groups received an injection of plant extract (500 mg/kg, ip), with or without administration of Diazepam (5 mg/kg). Also, three sets were applied to compare receiving and not receiving intracerebroventricular (icv) injection of Transient receptor potential ankyrin 1 antagonist (HC-030031) (2 μg/kg) on plant-induced seizure delay and animal death. Two groups of control and a group with plant extract together with TRPA1 antagonist were administrated. Furthermore, in the present study, the forced swimming test (FST) was used as a model of depression. The behaviors of animals in three groups of negative control and positive control (Fluoxetine) and plant extract (200 mg/kg, ip) were compared. According to the ECoG, high doses of extract of plants led to seizures similar to PTZ, which were then reduced by diazepam injection. At this dose, injection of TRPA1 antagonist did not significantly delay the onset of seizures or the death of the animals. Further, a subconvulsive dose of hydroalcoholic plant extracts was equally effective in treating depression as Fluoxetine injections.

Molecular Mechanisms of Epilepsy: The Role of the Chloride Transporter KCC2

Epilepsy is a neurological disease characterized by abnormal or synchronous brain activity causing seizures, which may produce convulsions, minor physical signs, or a combination of symptoms. These disorders affect approximately 65 million people worldwide, from all ages and genders. Seizures apart, epileptic patients present a high risk to develop neuropsychological comorbidities such as cognitive deficits, emotional disturbance, and psychiatric disorders, which severely impair quality of life. Currently, the treatment for epilepsy includes the administration of drugs or surgery, but about 30% of the patients treated with antiepileptic drugs develop time-dependent pharmacoresistence. Therefore, further investigation about epilepsy and its causes is needed to find new pharmacological targets and innovative therapeutic strategies. Pharmacoresistance is associated to changes in neuronal plasticity and alterations of GABA_A receptor-mediated neurotransmission. The downregulation of GABA inhibitory activity may arise from a positive shift in GABA_A receptor reversal potential, due to an alteration in chloride homeostasis. In this paper, we review the contribution of K⁺-Cl^--cotransporter (KCC2) to the alterations in the Cl^- gradient observed in epileptic condition, and how these alterations are coupled to the increase in the excitability.

Metabotropic Glutamate Receptors 1 Regulates Rat Carotid Body Response to Acute Hypoxia via Presynaptic Mechanism

Background: The carotid body (CB) plays a critical role in oxygen sensing; however, the role of glutamatergic signaling in the CB response to hypoxia remains uncertain. We previously found that functional multiple glutamate transporters and inotropic glutamate receptors (iGluRs) are expressed in the CB. The aim of this present research is to investigate the expression of group I metabotropic glutamate receptors (mGluRs) (mGluR1 and 5) in the CB and its physiological function in rat CB response to acute hypoxia.

Methods: RT-PCR and immunostaining were conducted to examine the mRNA and protein expression of group I mGluRs in the human and rat CB. Immunofluorescence staining was performed to examine the cellular localization of mGluR1 in the rat CB. In vitro carotid sinus nerve (CSN) discharge recording was performed to detect the physiological function of mGluR1 in CB response to acute hypoxia.

Results: We found that (1) mRNAs of mGluR1 and 5 were both expressed in the human and rat CB. (2) mGluR1 protein rather than mGluR5 protein was present in rat CB. (3) mGluR1 was distributed in type I cells of rat CB. (4) Activation of mGluR1 inhibited the hypoxia-induced enhancement of CSN activity (CSNA), as well as prolonged the latency time of CB response to hypoxia. (5) The inhibitory effect of mGluR1 activation on rat CB response to hypoxia could be blocked by GABA_B receptor antagonist.

Conclusion: Our findings reveal that mGluR1 in CB plays a presynaptic feedback inhibition on rat CB response to hypoxia.

Expression of type I mGluRs predicts plasticity in the hippocampal stratum radiatum interneurons

Changes in synaptic strength between hippocampal CA1 pyramidal cell synapses are partly responsible for memory acquisition. This plasticity is modulated by feedforward inhibitory interneurons in the stratum radiatum. While radiatum interneurons experience either long-term depression (LTD), short-term depression (STD), or lack plasticity, it is unclear whether plasticity correlates to specific interneuron subtypes. Using whole-cell electrophysiology and real-time quantitative PCR, we characterized the plasticity expressed by different interneuron subtypes. We first analyzed calcium binding proteins and cholecystokinin mRNA expression patterns to determine cell subtype. We then assessed endocannabinoid (eCB) biosynthetic enzyme mRNA expression, including diacylglycerol lipase α, N-acyl-phosphatidylethanolamine phospholipase D, and 12-lipoxygenase, and metabotropic glutamate receptors that often mediate plasticity. Neurons exhibiting LTD tended to co-express mRNA for at least one eCB biosynthetic enzyme and the metabotropic glutamate receptor 5 (mGluR5). Conversely, mGluR5 was not expressed by neurons exhibiting STD or no plasticity. Neurons that exhibited STD tended to express mRNA for at least one eCB biosynthetic enzyme and mGluR1, but not mGluR5. This suggests that plasticity of stratum radiatum interneurons could be predicted based on type I mGluR expression.

Critical role of glutamatergic and GABAergic neurotransmission in the central mechanisms of theta-burst stimulation

Theta-burst stimulation (TBS) is a varied form of repetitive transcranial magnetic stimulation (rTMS) and has more rapid and powerful effects than rTMS. Experiments on the human motor cortex have demonstrated that intermittent TBS has facilitatory effects, whereas continuous TBS has inhibitory effects. Huang's simplified model provides a solid basis for elucidating such after-effects. However, evidence increasingly indicates that not all after-effects of TBS are as expected, and high variability among individuals has been observed. Studies have suggested that the GABAergic and glutamatergic neurotransmission play a vital role in the aforementioned after-effects, which might explain the interindividual differences in these after-effects. Herein, we reviewed the latest findings on TBS from animal and human experiments on glutamatergic and GABAergic neurotransmissions in response to TBS. Furthermore, an updated theoretical model integrating glutamatergic and GABAergic neurotransmissions is proposed.

Group I metabotropic glutamate receptors generate two types of intrinsic membrane oscillations in hippocampal oriens/alveus interneurons

GABAergic interneurons in the hippocampus are critically involved in almost all hippocampal circuit functions including coordinated network activity. Somatostatin-expressing oriens-lacunosum moleculare (O-LM) interneurons are a major subtype of dendritically projecting interneurons in hippocampal subregions (e.g., CA1), and express group I metabotropic glutamate receptors (mGluRs), specifically mGluR₁ and mGluR₅. Group I mGluRs are thought to regulate hippocampal circuit functions partially through GABAergic interneurons. Previous studies suggest that a group I/II mGluR agonist produces slow supra-threshold membrane oscillations (<0.1 Hz), which are associated with high-frequency action potential (AP) discharges in O-LM interneurons. However, the properties and underlying mechanisms of these slow oscillations remain largely unknown. We performed whole-cell patch-clamp recordings from mouse interneurons in the stratum oriens/alveus (O/A interneurons) including CA1 O-LM interneurons. Our study revealed that the selective mGluR_1/5 agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced slow membrane oscillations (<0.1 Hz), which were associated with gamma frequency APs followed by AP-free perithreshold gamma oscillations. The selective mGluR₁ antagonist (S)-(+)-α-Amino-4-carboxy-2-methylbenzeneacetic acid (LY367385) reduced the slow oscillations, and the selective mGluR₅ antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) partially blocked them. Blockade of nonselective cation-conducting transient receptor potential channels, L-type Ca²⁺ channels, or ryanodine receptors all abolished the slow oscillations, suggesting the involvement of multiple mechanisms. Our findings suggest that group I mGluR activation in O/A interneurons may play an important role in coordinated network activity, and O/A interneuron vulnerability to excitotoxicity, in disease states like seizures, is at least in part due to an excessive rise in intracellular Ca²⁺.

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.

Plasticity in Single Axon Glutamatergic Connection to GABAergic Interneurons Regulates Complex Events in the Human Neocortex

In the human neocortex, single excitatory pyramidal cells can elicit very large glutamatergic EPSPs (VLEs) in inhibitory GABAergic interneurons capable of triggering their firing with short (3–5 ms) delay. Similar strong excitatory connections between two individual neurons have not been found in nonhuman cortices, suggesting that these synapses are specific to human interneurons. The VLEs are crucial for generating neocortical complex events, observed as single pyramidal cell spike-evoked discharge of cell assemblies in the frontal and temporal cortices. However, long-term plasticity of the VLE connections and how the plasticity modulates neocortical complex events has not been studied. Using triple and dual whole-cell recordings from synaptically connected human neocortical layers 2–3 neurons, we show that VLEs in fast-spiking GABAergic interneurons exhibit robust activity-induced long-term depression (LTD). The LTD by single pyramidal cell 40 Hz spike bursts is specific to connections with VLEs, requires group I metabotropic glutamate receptors, and has a presynaptic mechanism. The LTD of VLE connections alters suprathreshold activation of interneurons in the complex events suppressing the discharge of fast-spiking GABAergic cells. The VLEs triggering the complex events may contribute to cognitive processes in the human neocortex, and their long-term plasticity can alter the discharging cortical cell assemblies by learning.

Many microscale features in the human neocortex—a part of the brain involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, and language—are closely similar to those reported in experimental animals commonly used in neuroscience, like mice. However, the human neocortical neurons also exhibit specializations only reported in our species. One such feature is the capacity of excitatory principal cells to elicit firing in local inhibitory interneurons with a single action potential via very strong excitatory synapses. It has been suggested that this feature has specifically evolved to enhance coordinated firing of neuronal ensembles in higher brain functions. However, it is unknown how these circuits are modified by learning. Therefore, we investigated how these very strong excitatory synapses are changed, and if their impact on the firing of local inhibitory neurons is altered by repetitive action potentials mimicking learning-related activity. By recording in human neocortical slices, we report that the strong excitatory synapses on interneurons exhibit robust activity-dependent long-term plasticity. The plasticity also regulates the discharge of local interneurons driven by these synapses. Although these specialized synapses have only been reported in the human neocortex, their plasticity mechanism is evolutionarily conserved. We suggest that the strong synapses with robust plasticity have evolved to enhance complex brain functions and learning.

Fmr1 KO and fenobam treatment differentially impact distinct synapse populations of mouse neocortex

Cognitive deficits in fragile X syndrome (FXS) are attributed to molecular abnormalities of the brain's vast and heterogeneous synapse populations. Unfortunately, the density of synapses coupled with their molecular heterogeneity presents formidable challenges in understanding the specific contribution of synapse changes in FXS. We demonstrate powerful new methods for the large-scale molecular analysis of individual synapses that allow quantification of numerous specific changes in synapse populations present in the cortex of a mouse model of FXS. Analysis of nearly a million individual synapses reveals distinct, quantitative changes in synaptic proteins distributed across over 6,000 pairwise metrics. Some, but not all, of these synaptic alterations are reversed by treatment with the candidate therapeutic fenobam, an mGluR5 antagonist. These patterns of widespread, but diverse synaptic protein changes in response to global perturbation suggest that FXS and its treatment must be understood as a networked system at the synapse level.

Molecular mechanisms of epilepsy

Decades of experimental work have established an imbalance of excitation and inhibition as the leading mechanism of the transition from normal brain function to seizure. In epilepsy, these transitions are rare and abrupt. Transition processes incorporating positive feedback, such as activity-dependent disinhibition, could provide these uncommon timing features. A rapidly expanding array of genetic etiologies will help delineate the molecular mechanism(s). This delineation will entail quite a bit of cell biology. The genes discovered so far are more remarkable for their diversity than their similarities.

Enhanced excitatory synaptic network activity following transient group I metabotropic glutamate activation

Prolonged activation of group I metabotropic glutamate receptors (mGluRs) using the agonist (S)-3,5-dihydroxyphenylglycine (DHPG) produces long-lasting changes in the CA3 region of the hippocampal slice. Changes in CA3 pyramidal neuron excitability that follow DHPG exposure result in abnormal network activity manifest by epileptiform activity that consists of interictal and longer lasting ictal epileptiform discharges. In this study we evaluated changes in synaptic activity of CA3 neurons in rat hippocampal slices that occurred after exposure to DHPG. Whole-cell voltage-clamp recordings were made from visually identified CA3 neurons in control artificial cerebrospinal fluid at times greater than 1h after DHPG exposure. Compared to control slices, neurons from slices exposed to DHPG showed enhanced amplitude and frequency of spontaneously occurring excitatory postsynaptic currents (EPSCs) without a concurrent change in inhibitory postsynaptic current (IPSC) amplitude or frequency. Miniature EPSCs were not affected by DHPG exposure but mIPSCs occurred less frequently and were of reduced amplitude. IPSCs recorded in the presence of ionotropic glutamate receptor blockade occurred less frequently in neurons that had been exposed to DHPG. Monosynaptic-evoked IPSPs were also reduced in amplitude in neurons that had been exposed to DHPG. Taken together, these findings demonstrated an enhanced network excitability of the CA3 region and failure of compensatory synaptic inhibition. We propose that prolonged activation of group I mGluR that may occur under conditions of pathological glutamate release results in long-lasting changes in CA3 synaptic network activity and epileptiform activity driven by excessive synaptic excitation.

Dendritic calcium nonlinearities switch the direction of synaptic plasticity in fast-spiking interneurons

Postsynaptic calcium (Ca2+) nonlinearities allow neuronal coincidence detection and site-specific plasticity. Whether such events exist in dendrites of interneurons and play a role in regulation of synaptic efficacy remains unknown. Here, we used a combination of whole-cell patch-clamp recordings and two-photon Ca2+ imaging to reveal Ca2+ nonlinearities associated with synaptic integration in dendrites of mouse hippocampal CA1 fast-spiking interneurons. Local stimulation of distal dendritic branches within stratum oriens/alveus elicited fast Ca2+ transients, which showed a steep sigmoidal relationship to stimulus intensity. Supralinear Ca2+ events required Ca2+ entry through AMPA receptors with a subsequent Ca2+ release from internal stores. To investigate the functional significance of supralinear Ca2+ signals, we examined activity-dependent fluctuations in transmission efficacy triggered by Ca2+ signals of different amplitudes at excitatory synapses of interneurons. Subthreshold theta-burst stimulation (TBS) produced small amplitude postsynaptic Ca2+ transients and triggered long-term potentiation. In contrast, the suprathreshold TBS, which was associated with the generation of supralinear Ca2+ events, triggered long-term depression. Blocking group I/II metabotropic glutamate receptors (mGluRs) during suprathreshold TBS resulted in a slight reduction of supralinear Ca2+ events and induction of short-term depression. In contrast, blocking internal stores and supralinear Ca2+ signals during suprathreshold TBS switched the direction of plasticity from depression back to potentiation. These data reveal a novel type of supralinear Ca2+ events at synapses lacking the GluA2 AMPA subtype of glutamate receptors and demonstrate a general mechanism by which Ca2+ -permeable AMPA receptors, together with internal stores and mGluRs, control the direction of plasticity at interneuron excitatory synapses.

Predicting the diagnosis of autism spectrum disorder using gene pathway analysis

Autism spectrum disorder (ASD) depends on a clinical interview with no biomarkers to aid diagnosis. The current investigation interrogated single-nucleotide polymorphisms (SNPs) of individuals with ASD from the Autism Genetic Resource Exchange (AGRE) database. SNPs were mapped to Kyoto Encyclopedia of Genes and Genomes (KEGG)-derived pathways to identify affected cellular processes and develop a diagnostic test. This test was then applied to two independent samples from the Simons Foundation Autism Research Initiative (SFARI) and Wellcome Trust 1958 normal birth cohort (WTBC) for validation. Using AGRE SNP data from a Central European (CEU) cohort, we created a genetic diagnostic classifier consisting of 237 SNPs in 146 genes that correctly predicted ASD diagnosis in 85.6% of CEU cases. This classifier also predicted 84.3% of cases in an ethnically related Tuscan cohort; however, prediction was less accurate (56.4%) in a genetically dissimilar Han Chinese cohort (HAN). Eight SNPs in three genes (KCNMB4, GNAO1, GRM5) had the largest effect in the classifier with some acting as vulnerability SNPs, whereas others were protective. Prediction accuracy diminished as the number of SNPs analyzed in the model was decreased. Our diagnostic classifier correctly predicted ASD diagnosis with an accuracy of 71.7% in CEU individuals from the SFARI (ASD) and WTBC (controls) validation data sets. In conclusion, we have developed an accurate diagnostic test for a genetically homogeneous group to aid in early detection of ASD. While SNPs differ across ethnic groups, our pathway approach identified cellular processes common to ASD across ethnicities. Our results have wide implications for detection, intervention and prevention of ASD.

Distinct increased metabotropic glutamate receptor type 5 (mGluR5) in temporal lobe epilepsy with and without hippocampal sclerosis

Metabotropic glutamate receptor type 5 (mGluR5) upregulation in temporal lobe epilepsy (TLE) and the correlation of its expression with features of hippocampal sclerosis (HS) remains unclear. Here we characterized mGluR5 immunoreactivity in hippocampus, entorhinal cortex (EC), and subiculum of TLE specimens with confirmed HS, with neocortical TLE (non-HS) and necropsy controls. We correlated mGluR5 immunoreactivity with neuronal density, mossy fiber sprouting, astrogliosis (GFAP), and dendritic alterations (MAP2). TLE specimens showed increased mGluR5 expression, which was most pronounced in the EC, subiculum, CA2, and dentate gyrus outer molecular layer. Increased mGluR5 expression was seen in hippocampal head and body segments and was independent of neuronal density, astrogliosis, or dendritic alterations. Positive correlation between mGluR5 expression with mossy fiber sprouting and with MAP2 in CA3 and CA1 was found only in HS specimens. Negative correlation between mGluR5 expression with seizure frequency and epilepsy duration was found only in non-HS cases. Specimens from HS patients without previous history of febrile seizure (FS) showed higher mGluR5 and MAP2 expression in CA2. Our study suggests that mGluR5 upregulation is part of a repertoire of post-synaptic adaptations that might control overexcitation and excessive glutamate release rather than a dysfunction that leads to seizure facilitation. That would explain why non-HS cases, on which seizures are likely to originate outside the hippocampal formation, also exhibit upregulated mGluR5. On the other hand, lower mGluR5 expression was related to increased seizure frequency. In addition to its role in hyperexcitability, mGluR5 upregulation could play a role in counterbalance mechanisms along the hyperexcitable circuitry uniquely altered in sclerotic hippocampal formation. Inefficient post-synaptic compensatory morphological (dendritic branching) and glutamatergic (mGluR5 expression) mechanisms in CA2 subfield could potentially underlie the association of FS with HS and TLE. © 2013 Wiley Periodicals, Inc.

Endocannabinoid-mediated long-term depression of afferent excitatory synapses in hippocampal pyramidal cells and GABAergic interneurons

Although endocannabinoids have emerged as essential retrograde messengers in several forms of synaptic plasticity, it remains controversial whether they mediate long-term depression (LTD) of glutamatergic synapses onto excitatory and inhibitory neurons in the hippocampus. Here, we show that parvalbumin- and somatostatin/metabotropic glutamate receptor 1(a) (mGlu(1a))-positive GABAergic interneurons express diacylglycerol lipase-α (DGL-α), a synthesizing enzyme of the endocannabinoid 2-arachidonoylglycerol (2-AG), albeit at lower levels than principal cells. Moreover, this lipase accumulates postsynaptically around afferent excitatory synapses in all three cell types. To address the role of retrograde 2-AG signaling in LTD, we investigated two forms: (1) produced by postsynaptic spiking paired with subsequent presynaptic stimulation or (2) induced by group I mGlu activation by (S)-3,5-dihydroxyphenylglycine (DHPG). Neither form of LTD was evoked in the presence of the mGlu(5) antagonist MPEP [2-methyl-6-(phenylethynyl)-pyridine], the DGL inhibitor THL [N-formyl-l-leucine (1S)-1-[[(2S,3S)-3-hexyl-4-oxo-2-oxetanyl]methyl]dodecyl ester], or the intracellularly applied Ca(2+) chelator BAPTA in CA1 pyramidal cells, fast-spiking interneurons (representing parvalbumin-containing cells) and interneurons projecting to stratum lacunosum-moleculare (representing somatostatin/mGlu(1a)-expressing interneurons). Both forms of LTD were completely absent in CB(1) cannabinoid receptor knock-out mice, whereas pharmacological blockade of CB(1) led to inconsistent results. Notably, in accordance with their lower DGL-α level, a higher stimulation frequency or higher DHPG concentration was required for LTD induction in interneurons compared with pyramidal cells. These findings demonstrate that hippocampal principal cells and interneurons produce endocannabinoids to mediate LTD in a qualitatively similar, but quantitatively different manner. The shifted induction threshold implies that endocannabinoid-LTD contributes to cortical information processing during distinct network activity patterns in a cell type-specific manner.

Neural functions of calcineurin in synaptic plasticity and memory

Major brain functions depend on neuronal processes that favor the plasticity of neuronal circuits while at the same time maintaining their stability. The mechanisms that regulate brain plasticity are complex and engage multiple cascades of molecular components that modulate synaptic efficacy. Protein kinases (PKs) and phosphatases (PPs) are among the most important of these components that act as positive and negative regulators of neuronal signaling and plasticity, respectively. In these cascades, the PP protein phosphatase 2B or calcineurin (CaN) is of particular interest because it is the only Ca(2+)-activated PP in the brain and a major regulator of key proteins essential for synaptic transmission and neuronal excitability. This review describes the primary properties of CaN and illustrates its functions and modes of action by focusing on several representative targets, in particular glutamate receptors, striatal enriched protein phosphatase (STEP), and neuromodulin (GAP43), and their functional significance for synaptic plasticity and memory.

Dendritic Signaling in Inhibitory Interneurons: Local Tuning via Group I Metabotropic Glutamate Receptors

Communication between neurons is achieved by rapid signal transduction via highly specialized structural elements known as synaptic contacts. In addition, numerous extrasynaptic mechanisms provide a flexible platform for the local regulation of synaptic signals. For example, peri- and extra-synaptic signaling through the group I metabotropic glutamate receptors (mGluRs) can be involved in the highly compartmentalized regulation of dendritic ion conductances, the induction of input-specific synaptic plasticity, and the local release of retrograde messengers. Therefore, extrasynaptic mechanisms appear to play a key role in the local tuning of dendritic computations. Here, we review recent findings on the role of group I mGluRs in the dendritic signaling of inhibitory interneurons. We propose that group I mGluRs provide a dual-mode signaling device that integrates different patterns of neural activity. By implementing distinct forms of intrinsic and synaptic regulation, group I mGluRs may be responsible for the local fine-tuning of dendritic function.

Identification of mRNA for endocannabinoid biosynthetic enzymes within hippocampal pyramidal cells and CA1 stratum radiatum interneuron subtypes using quantitative real-time polymerase chain reaction

The hippocampus is required for short-term memory and contains both excitatory pyramidal cells and inhibitory interneurons. These cells exhibit various forms of synaptic plasticity, the mechanism underlying learning and memory. More recently, endocannabinoids were identified to be involved in synaptic plasticity. Our goal was to describe the distribution of endocannabinoid biosynthetic enzymes within CA1 stratum radiatum interneurons and CA3/CA1 pyramidal cells. We extracted mRNA from single interneurons and pyramidal cells and used real-time quantitative polymerase chain reaction (RT-PCR) to detect the presence of 12-lipoxygenase, N-acyl-phosphatidylethanolamine-specific phospholipase D, diacylglycerol lipase α, and type I metabotropic glutamate receptors, all known to be involved in endocannabinoid production and plasticity. We observed that the expression of endocannabinoid biosynthetic enzyme mRNA does occur within interneurons and that it is coexpressed with type I metabotropic glutamate receptors, suggesting interneurons have the potential to produce endocannabinoids. We also identified that CA3 and CA1 pyramidal cells express endocannabinoid biosynthetic enzyme mRNA. Our data provide the first molecular biological evidence for putative endocannabinoid production in interneurons, suggesting their potential ability to regulate endocannabinoid-mediated processes, such as synaptic plasticity.

Simulation of postsynaptic glutamate receptors reveals critical features of glutamatergic transmission

Activation of several subtypes of glutamate receptors contributes to changes in postsynaptic calcium concentration at hippocampal synapses, resulting in various types of changes in synaptic strength. Thus, while activation of NMDA receptors has been shown to be critical for long-term potentiation (LTP) and long term depression (LTD) of synaptic transmission, activation of metabotropic glutamate receptors (mGluRs) has been linked to either LTP or LTD. While it is generally admitted that dynamic changes in postsynaptic calcium concentration represent the critical elements to determine the direction and amplitude of the changes in synaptic strength, it has been difficult to quantitatively estimate the relative contribution of the different types of glutamate receptors to these changes under different experimental conditions. Here we present a detailed model of a postsynaptic glutamatergic synapse that incorporates ionotropic and mGluR type I receptors, and we use this model to determine the role of the different receptors to the dynamics of postsynaptic calcium with different patterns of presynaptic activation. Our modeling framework includes glutamate vesicular release and diffusion in the cleft and a glutamate transporter that modulates extracellular glutamate concentration. Our results indicate that the contribution of mGluRs to changes in postsynaptic calcium concentration is minimal under basal stimulation conditions and becomes apparent only at high frequency of stimulation. Furthermore, the location of mGluRs in the postsynaptic membrane is also a critical factor, as activation of distant receptors contributes significantly less to calcium dynamics than more centrally located ones. These results confirm the important role of glutamate transporters and of the localization of mGluRs in postsynaptic sites in their signaling properties, and further strengthen the notion that mGluR activation significantly contributes to postsynaptic calcium dynamics only following high-frequency stimulation. They also provide a new tool to analyze the interactions between metabotropic and ionotropic glutamate receptors.

Group I mGluR agonist-evoked long-term potentiation in hippocampal oriens interneurons

Several subtypes of interneurons in the feedback circuit in stratum oriens of the hippocampus exhibit NMDA receptor-independent long-term potentiation (LTP) at glutamatergic synapses made by local pyramidal neurons. LTP has been reported with both "Hebbian" and "anti-Hebbian" induction protocols, where high-frequency presynaptic stimulation is paired with either postsynaptic depolarization or hyperpolarization. Do these phenomena represent distinct forms of plasticity, dependent on group I metabotropic receptors (mGluRs) and rectifying Ca2+ -permeable AMPA receptors, respectively? Blockade of either mGluR1 or mGluR5 prevented anti-Hebbian LTP induction in stratum oriens interneurons in rat hippocampal slices. Exogenous activation of group I mGluRs by the selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) was unable to induce LTP on its own, and instead depressed excitatory transmission. However, when paired with postsynaptic hyperpolarization, DHPG or the group I metabotropic receptor (mGluR5)-selective agonist (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG) elicited a delayed long-lasting potentiation, which was accompanied by a decrease in paired-pulse facilitation. Anti-Hebbian LTP occluded the effect of DHPG paired with hyperpolarization, implying that the induction cascades triggered by both conjunctions of stimuli converge on common expression mechanisms.

mGluRs modulate strength and timing of excitatory transmission in hippocampal area CA3

Excitatory transmission within hippocampal area CA3 stems from three major glutamatergic pathways: the perforant path formed by axons of layer II stellate cells in the entorhinal cortex, the mossy fiber axons originating from the dentate gyrus granule cells, and the recurrent axon collaterals of CA3 pyramidal cells. The synaptic communication of each of these pathways is modulated by metabotropic glutamate receptors that fine-tune the signal by affecting both the timing and strength of the connection. Within area CA3 of the hippocampus, group I mGluRs (mGluR1 and mGluR5) are expressed postsynaptically, whereas group II (mGluR2 and mGluR3) and III mGluRs (mGluR4, mGluR7, and mGluR8) are expressed presynaptically. Receptors from each group have been demonstrated to be required for different forms of pre- and postsynaptic long-term plasticity and also have been implicated in regulating short-term plasticity. A recent observation has demonstrated that a presynaptically expressed mGluR can affect the timing of action potentials elicited in the postsynaptic target. Interestingly, mGluRs can be distributed in a target-specific manner, such that synaptic input from one presynaptic neuron can be modulated by different receptors at each of its postsynaptic targets. Consequently, mGluRs provide a mechanism for synaptic specialization of glutamatergic transmission in the hippocampus. This review will highlight the variability in mGluR modulation of excitatory transmission within area CA3 with an emphasis on how these receptors contribute to the strength and timing of network activity within pyramidal cells and interneurons.