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

Evidence for functional interaction between the CB1 and the mGlu7 receptors mediated signaling in modulation of anxiety behavior and cognition

Anxiety is a severe social problem. It is a disease entity that occurs alone or accompanies other diseases such as depression, phobia, or post-traumatic stress disorder. Our earlier studies demonstrated that blockage of arachidonic acid (AA) pathway via inhibition of cyclooxygenase-2 (COX-2) enzyme can modulate mGluRs-induced anxiety-like behavior. Here, we hypothesized that modulation of 2-arachidoglycerol (2-AG), a component of the AA pathway, concomitantly with modulation of mGluR7 signaling, should be adequate to trigger a similar response from the test organism. Since 2-AG is an endogenous agonist for CB1 receptors, we used a CB1/GPR55/μ-opioid receptor antagonist (AM251) alone and in combination with mGluR7 allosteric agonist (AMN082). Stress-induced hyperthermia (SIH) test was performed as a behavioral readout. AM251 has a dual mode on AMN082-mediated effects in SIH in CD-1 mice. Furthermore, the CB1 receptor ligand influenced adaptation to stress in repeated SIH procedures and learning possibilities of mice in the Barnes maze. We also found changes in mGluR7 protein expression levels in the prefrontal cortex (PFC) after mice were exposed to AM251, which showed the potential to attenuate the AMN082-induced decline in mGluR7 levels. The changes induced by AM251 on AMN082-mediated behavioral and biochemical effects were confirmed in electrophysiological experiments in which AM251 abolished AMN082-mediated LTP escalation in PFC. The mGluR7 overexpressed cell line was used to exclude the direct involvement of mGluR7 in AM251 activity. All the above results and the co-localization of CB1 and mGlu7 receptors detected in specific brain regions strongly suggest the specific interaction between CB1 and mGlu7 receptors and their signaling.

Interactions between metabotropic glutamate and CB1 receptors: implications for mood, cognition, and synaptic signaling based on data from mGluR and CB1R-targeting drugs

Metabotropic glutamate receptors (mGluRs) are part of the G protein-coupled receptors (GPCRs) family. They are coupled to Gαq (group I) or Gi/o (groups II and III) proteins, which result in the generation of diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3) or the inhibition of adenylyl cyclase, respectively. mGluRs have been implicated in anxiety, depression, learning, and synaptic plasticity. Similarly, CB1 cannabinoid receptors (CB1Rs), also GPCRs, play roles in cognitive function and mood regulation through Gαi/o-mediated inhibition of adenylyl cyclase. Both mGluRs and CB1Rs exhibit surface labeling and undergo endocytosis. Given the similar cellular distribution and mechanisms of action, this review complies with fundamental data on the potential interactions and mutual regulation of mGluRs and CB1Rs in the context of depression, anxiety, and cognition, providing pioneering insights into their interplay.

Cells and Molecules Underpinning Cannabis-Related Variations in Cortical Thickness during Adolescence

During adolescence, cannabis experimentation is common, and its association with interindividual variations in brain maturation well studied. Cellular and molecular underpinnings of these system-level relationships are, however, unclear. We thus conducted a three-step study. First, we exposed adolescent male mice to Δ-9-tetrahydrocannabinol (THC) or a synthetic cannabinoid WIN 55,212-2 (WIN) and assessed differentially expressed genes (DEGs), spine numbers, and dendritic complexity in their frontal cortex. Second, in human (male) adolescents, we examined group differences in cortical thickness in 34 brain regions, using magnetic resonance imaging, between those who experimented with cannabis before age 16 (n = 140) and those who did not (n = 327). Finally, we correlated spatially these group differences with gene expression of human homologs of mouse-identified DEGs. The spatial expression of 13 THC-related human homologs of DEGs correlated with cannabis-related variations in cortical thickness, and virtual histology revealed coexpression patterns of these 13 genes with cell-specific markers of astrocytes, microglia, and a type of pyramidal cells enriched in dendrite-regulating genes. Similarly, the spatial expression of 18 WIN-related human homologs of DEGs correlated with group differences in cortical thickness and showed coexpression patterns with the same three cell types. Gene ontology analysis indicated that 37 THC-related human homologs are enriched in neuron projection development, while 33 WIN-related homologs are enriched in processes associated with learning and memory. In mice, we observed spine loss and lower dendritic complexity in pyramidal cells of THC-exposed animals (vs controls). Experimentation with cannabis during adolescence may influence cortical thickness by impacting glutamatergic synapses and dendritic arborization.

Inhibition of Acute mGluR5-Dependent Depression in Hippocampal CA1 by High-Frequency Magnetic Stimulation

High-frequency magnetic stimulation (HFMS) applied directly to the hippocampal slice preparation in vitro induces activity-dependent synaptic plasticity and metaplasticity. In addition, changes in synaptic transmission following HFMS involve the activation of N-methyl-D-aspartate and metabotropic glutamate receptors (mGluR). Here, we asked whether a short period of HFMS (5 × 10 delta-burst trains, duration of ~1 min) could alter mGluR5-mediated depression at Schaffer collateral-CA1 synapses in the acute brain slice preparation at 30 min after HFMS. To this end, we obtained field excitatory postsynaptic potential (fEPSP) slopes from Schaffer collateral-CA1 synapses after HFMS or control. First, we demonstrated that activity-dependent plasticity following HFMS depends on the slice orientation towards the magnetic coil indicating specific ion fluxes induced by magnetic fields. Second, we found that the mGluR5-specific agonist (RS)-2-chloro-5-hydroxyphenylglycine reduced the field excitatory postsynaptic potential (fEPSP) slopes in control slices but rather enhanced them in HFMS-treated slices. In contrast, the compound (S)-3,5-dihydroxyphenylglycine acting at both mGluR1 and mGluR5 reduced fEPSP slopes in both control and HFMS-treated slices. Importantly, the mGluR-dependent effects were independent from the slice-to-coil orientation indicating that asynchronous glutamate release could play a role. We conclude that a short period of HFMS inhibits subsequently evoked mGluR5-dependent depression at Schaffer collateral-CA1 synapses. This could be relevant for repetitive transcranial magnetic stimulation in psychiatric disorders such as major depression.

ABHD6 drives endocytosis of AMPA receptors to regulate synaptic plasticity and learning flexibility

Trafficking of α-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors (AMPARs), mediated by AMPAR interacting proteins, enabled neurons to maintain tuning capabilities at rest or active state. α/β-Hydrolase domain-containing 6 (ABHD6), an endocannabinoid hydrolase, was an AMPAR auxiliary subunit found to negatively regulate the surface delivery of AMPARs. While ABHD6 was found to prevent AMPAR tetramerization in endoplasmic reticulum, ABHD6 was also reported to localize at postsynaptic site. Yet, the role of ABHD6 interacting with AMPAR at postsynaptic site, and the physiological significance of ABHD6 regulating AMPAR trafficking remains elusive. Here, we generated the ABHD6 knockout (ABHD6KO) mice and found that deletion of ABHD6 selectively enhanced AMPAR-mediated basal synaptic responses and the surface expression of postsynaptic AMPARs. Furthermore, we found that loss of ABHD6 impaired hippocampal long-term depression (LTD) and synaptic downscaling in hippocampal synapses. AMPAR internalization assays revealed that ABHD6 was essential for neuronal activity-dependent endocytosis of surface AMPARs, which is independent of ABHD6's hydrolase activity. The defects of AMPAR endocytosis and LTD are expressed as deficits in learning flexibility in ABHD6KO mice. Collectively, we demonstrated that ABHD6 is an endocytic accessory protein promoting AMPAR endocytosis, thereby contributes to the formation of LTD, synaptic downscaling and reversal learning.

Synaptic effects of ethanol on striatal circuitry: therapeutic implications for dystonia

Alcohol consumption affects motor behavior and motor control. Both acute and chronic alcohol abuse have been extensively investigated; however, the therapeutic efficacy of alcohol on some movement disorders, such as myoclonus-dystonia or essential tremor, still does not have a plausible mechanistic explanation. Yet, there are surprisingly few systematic trials with known GABAergic drugs mimicking the effect of alcohol on neurotransmission. In this brief survey, we aim to summarize the effects of EtOH on striatal function, providing an overview of its cellular and synaptic actions in a 'circuit-centered' view. In addition, we will review both experimental and clinical evidence, in the attempt to provide a plausible mechanistic explanation for alcohol-responsive movement disorders, with particular emphasis on dystonia. Different hypotheses emerge, which may provide a rationale for the utilization of drugs that mimic alcohol effects, predicting potential drug repositioning.

Agmatine relieves behavioral impairments in Fragile X mice model

BACKGROUND

Fragile X syndrome (FXS) is the most common heritable form of neurodevelopmental disorder, which is caused by the loss of fragile X mental retardation protein (FMRP) expression. Despite the unceasing efforts to develop therapeutic agents against FXS based on the pathophysiological changes observed in animal models of FXS and human patients, therapeutic candidates including mGluR signaling modulators have failed to provide sufficient effects. Based on the recent successful demonstration of an endogenous polyamine, agmatine, to improve the autism-like symptoms in the valproic acid animal model of autism, we investigated the effects of agmatine against FXS symptoms using Fmr1 knockout (KO) mice.

METHODS

We used male Fmr1 KO mice for behavioral tests such as marble burying, open-field test, memory tasks, social interaction tests and startle response to confirm the symptoms of FXS. We also checked the electrophysiological profile of neural activity in agmatine-treated Fmr1 KO mice.

RESULTS

Agmatine reversed the compulsion, learning and memory deficits, hyperactivity, aberrant social interaction, and communication deficit in Fmr1 KO mice while it normalized the aberrant LTP and LTD in the hippocampus.

CONCLUSIONS

The results highlight the potential of agmatine's novel disease-ameliorating effects in FXS, which warrants further studies to ascertain whether these findings translate into clinical effects in FXS patients.

Group I metabotropic glutamate receptors contribute to the antiepileptic effect of electrical stimulation in hippocampal CA1 pyramidal neurons

Low-frequency deep brain stimulation (LFS) inhibits neuronal hyperexcitability during epilepsy. Accordingly, the use of LFS as a treatment method for patients with drug-resistant epilepsy has been proposed. However, the LFS antiepileptic mechanisms are not fully understood. Here, the role of metabotropic glutamate receptors group I (mGluR I) in LFS inhibitory action on epileptiform activity (EA) was investigated. EA was induced by increasing the K⁺ concentration in artificial cerebrospinal fluid (ACSF) up to 12 mM in hippocampal slices of male Wistar rats. LFS (1 Hz, 900 pulses) was delivered to the bundles of Schaffer collaterals at the beginning of EA. The excitability of CA1 pyramidal neurons was assayed by intracellular whole-cell recording. Applying LFS reduced the firing frequency during EA and substantially moved the membrane potential toward repolarization after a high-K⁺ ACSF washout. In addition, LFS attenuated the EA-generated neuronal hyperexcitability. A blockade of both mGluR 1 and mGluR 5 prevented the inhibitory action of LFS on EA-generated neuronal hyperexcitability. Activation of mGluR I mimicked the LFS effects and had similar inhibitory action on excitability of CA1 pyramidal neurons following EA. However, mGluR I agonist's antiepileptic action was not as strong as LFS. The observed LFS effects were significantly attenuated in the presence of a PKC inhibitor. Altogether, the LFS' inhibitory action on neuronal hyperexcitability following EA relies, in part, on the activity of mGluR I and a PKC-related signaling pathway.

Low-Frequency Stimulation Prevents Kindling-Induced Impairment through the Activation of the Endocannabinoid System

Background

Cannabinoid system affects memory and has anticonvulsant effects in epileptic models. In the current study, the role of cannabinoid 1 (CB1) receptors was investigated in amelioration of the effects of low-frequency stimulation (LFS) on learning and memory impairments in kindled rats.

Methods

Electrical stimulation of the hippocampal CA1 area was employed to kindle the animals. LFS was applied to the CA1 area in four trials following the last kindling stimulation. One group of animals received intraperitoneal injection of AM251 (0.1 μg/rat), a CB1 receptor antagonist, before the LFS application. Similarly, CB1 agonist WIN55-212-2 (WIN) was administrated to another group prior to LFS. The Morris water maze (MWM) and the novel object recognition (NOR) tests were executed 48 h after the last kindling stimulation to assess learning and memory.

Results

Applying LFS in the kindled+LFS group restored learning and memory impairments in the kindled rats. There was a significant difference between the kindled and the kindled+LFS groups in learning and memory. The application of AM251 reduced the LFS effects significantly. Adversely, WIN acted similarly to LFS and alleviated learning and memory deficits in the kindled+WIN group. In addition, WIN did not counteract the LFS enhancing effects in the KLFS+WIN group.

Conclusions

Improving effects of LFS on learning and memory impairments are mediated through the activation of the endocannabinoid (ECB) system.

The neuropharmacology of cannabinoid receptor ligands in central signaling pathways

The endocannabinoid system is a complex neuronal system involved in a number of biological functions, like attention, anxiety, mood, memory, appetite, reward, and immune responses. It is at the centre of scientific interest, which is driven by therapeutic promise of certain cannabinoid ligands and the changing legalization of herbal cannabis in many countries. The endocannabinoid system is a modulatory system, with endocannabinoids as retrograde neurotransmitters rather than direct neurotransmitters. Neuropharmacology of cannabinoid ligands in the brain can therefore be understood in terms of their modulatory actions through other neurotransmitter systems. The CB₁ receptor is chiefly responsible for effects of endocannabinoids and analogous ligands in the brain. An overview of the neuropharmacology of several cannabinoid receptor ligands, including endocannabinoids, herbal cannabis and synthetic cannabinoid receptor ligands is given in this review. Their mechanism of action at the endocannabinoid system is described, mainly in the brain. In addition, effects of cannabinoid ligands on other neurotransmitter systems will also be described, such as dopamine, serotonin, glutamate, noradrenaline, opioid, and GABA. In light of this, therapeutic potential and adverse effects of cannabinoid receptor ligands will also be discussed.

Effects of CYP46A1 Inhibition on Long-Term-Depression in Hippocampal Slices ex vivo and 24S-Hydroxycholesterol Levels in Mice in vivo

The manipulation of cholesterol and its metabolites has been hypothesized to be therapeutically beneficial for mood disorders, neurodegenerative disorders, and epilepsies. A major regulator of cholesterol clearance and turnover in the central nervous system is CYP46A1, a brain enriched enzyme responsible for metabolism of cholesterol into 24S-hydroxycholesterol. Inhibition of this enzyme may negatively modulate NMDARs as 24S-hydroxycholesterol was shown to enhance NMDAR function. In addition, alterations of local cholesterol or other changes mediated by CYP46A1 activity could have important influences on central nervous system function. Here we demonstrate that humans and mice display brain region specific and similar CYP46A1 and 24S-hydroxycholesterol distribution. Treatment with distinct classes of CYP46A1 inhibitors led to central 24S-hydroxycholesterol reduction in vivo and ablation of long term depression in hippocampal slices. Our results suggest that rodents show similarity to humans for studying the impact of CYP46A1 inhibitors and that rapid, local modulation of oxysterols can be achieved through CYP46A1 inhibition.

Neuromodulators and Long-Term Synaptic Plasticity in Learning and Memory: A Steered-Glutamatergic Perspective

The molecular pathways underlying the induction and maintenance of long-term synaptic plasticity have been extensively investigated revealing various mechanisms by which neurons control their synaptic strength. The dynamic nature of neuronal connections combined with plasticity-mediated long-lasting structural and functional alterations provide valuable insights into neuronal encoding processes as molecular substrates of not only learning and memory but potentially other sensory, motor and behavioural functions that reflect previous experience. However, one key element receiving little attention in the study of synaptic plasticity is the role of neuromodulators, which are known to orchestrate neuronal activity on brain-wide, network and synaptic scales. We aim to review current evidence on the mechanisms by which certain modulators, namely dopamine, acetylcholine, noradrenaline and serotonin, control synaptic plasticity induction through corresponding metabotropic receptors in a pathway-specific manner. Lastly, we propose that neuromodulators control plasticity outcomes through steering glutamatergic transmission, thereby gating its induction and maintenance.

Impaired anandamide/palmitoylethanolamide signaling in hippocampal glutamatergic neurons alters synaptic plasticity, learning, and emotional responses

Endocannabinoid signaling via anandamide (AEA) is implicated in a variety of neuronal functions and considered a promising therapeutic target for numerous emotion-related disorders. The major AEA degrading enzyme is fatty acid amide hydrolase (FAAH). Genetic deletion and pharmacological inhibition of FAAH reduce anxiety and improve emotional responses and memory in rodents and humans. Complementarily, the mechanisms and impact of decreased AEA signaling remain to be delineated in detail. In the present study, using the Cre/loxP system combined with an adeno-associated virus (AAV)-mediated delivery system, FAAH was selectively overexpressed in hippocampal CA1-CA3 glutamatergic neurons of adult mice. This approach led to specific FAAH overexpression at the postsynaptic site of CA1-CA3 neurons, to increased FAAH enzymatic activity, and, in consequence, to decreased hippocampal levels of AEA and palmitoylethanolamide (PEA), but the levels of the second major endocannabinoid 2-arachidonoyl glycerol (2-AG) and of oleoylethanolamide (OEA) were unchanged. Electrophysiological recordings revealed an enhancement of both excitatory and inhibitory synaptic activity and of long-term potentiation (LTP). In contrast, excitatory and inhibitory long-term depression (LTD) and short-term synaptic plasticity, apparent as depolarization-induced suppression of excitation (DSE) and inhibition (DSI), remained unaltered. These changes in hippocampal synaptic activity were associated with an increase in anxiety-like behavior, and a deficit in object recognition memory and in extinction of aversive memory. This study indicates that AEA is not involved in hippocampal short-term plasticity, or eLTD and iLTD, but modulates glutamatergic transmission most likely via presynaptic sites, and that disturbances in this process impair learning and emotional responses.

Activity-dependent modulation of hippocampal synaptic plasticity via PirB and endocannabinoids

The threshold for Hebbian synaptic plasticity in the CNS is modulated by prior synaptic activity. At adult CA3-CA1 synapses, endocannabinoids play a role in this process, but how activity engages and maintains this retrograde signaling system is not well understood. Here we show that conditional deletion of Paired Immunoglobulin-like receptor B (PirB) from pyramidal neurons in adult mouse hippocampus results in deficient LTD at CA3-CA1 synapses over a range of stimulation frequencies, accompanied by an increase in LTP. This finding can be fully explained by the disengagement of retrograde endocannabinoid signaling selectively at excitatory synapses. In the absence of PirB, the NMDAR-dependent regulation of endocannabinoid signaling is lost, while CB1R-dependent and group I mGluR-dependent regulation are intact. Moreover, mEPSC frequency in mutant CA1 pyramidal cells is elevated, consistent with a higher density of excitatory synapses and altered synapse pruning. Mice lacking PirB also perform better than WT in learning and memory tasks. These observations suggest that PirB is an integral part of an NMDA receptor-mediated synaptic mechanism that maintains bidirectional Hebbian plasticity and learning via activity-dependent endocannabinoid signaling.

Endocannabinoid CB1 receptors are involved in antiepileptogenic effect of low frequency electrical stimulation during perforant path kindling in rats

INTRODUCTION

Administration of low-frequency electrical stimulation (LFS) at the kindling site has an antiepileptogenic effect. In the present study, we investigated the role of cannabinoid receptors type 1 (CB1) in mediating the inhibitory effects of LFS on the development of perforant path kindled seizures.

METHODS

For seizure generation, rats were kindled by electrical stimulation of perforant path in semi-rapid kindling manner (12 stimulations per day at 10 min intervals at afterdischarge threshold intensity).To determine the effect of LFS (0.1 ms pulse duration at 1 Hz, 800 pulses) on seizure generation, LFS was applied to the perforant path 5 min after the last kindling stimulation daily. AM281, a CB1 receptor antagonist, was microinjected into the lateral ventricle immediately after the last kindling stimulation (before LFS application) at the doses of 0.5 and 2 μg/μl during kindling procedure. The expression of cannabinoid receptors in the dentate gyrus was also investigated using immunohistochemistry.

RESULTS

Application of LFS had inhibitory effect on development of kindled seizures (kindling rate). Microinjection of AM281 (0.5 μg/μl) immediately after the last kindling stimulation (before LFS application) reduced the inhibitory effect of LFS on the kindling rate and suppressed the effects of LFS on potentiation (increasing the magnitude) of both population spike amplitude and population excitatory postsynaptic potential slope during kindling acquisition. AM281 pretreatment also prevented the effects of LFS on kindling-induced increase in early and late paired pulse depression. The higher dose of AM281 (2 μg/μl) failed to exert the effects observed with its lower dose (0.5 μg/μl). In addition, there was a decreased CB1 receptors immunostaining in kindled animals compared to control. However, application of LFS following kindling stimulations led to overexpression of CB1 receptors in the dentate gyrus.

CONCLUSION

Obtained results showed that activation of overexpressed cannabinoid CB1 receptors by endogenous cannabinoids may have a role in mediating the inhibitory effect of LFS on perforant path kindled seizures.

Physiological activation of mGlu5 receptors supports the ion channel function of NMDA receptors in hippocampal LTD induction in vivo

Synaptic long-term depression (LTD) is believed to underlie critical mnemonic processes in the adult hippocampus. The roles of the metabotropic and ionotropic actions of glutamate in the induction of synaptic LTD by electrical low-frequency stimulation (LFS) in the living adult animal is poorly understood. Here we examined the requirement for metabotropic glutamate (mGlu) and NMDA receptors in LTD induction in anaesthetized adult rats. LTD induction was primarily dependent on NMDA receptors and required the involvement of both the ion channel function and GluN2B subunit of the receptor. Endogenous mGlu5 receptor activation necessitated the local application of relatively high doses of either competitive or non-competitive NMDA receptor antagonists to block LTD induction. Moreover, boosting endogenous glutamate activation of mGlu5 receptors with a positive allosteric modulator lowered the threshold for NMDA receptor-dependent LTD induction by weak LFS. The present data provide support in the living animal that NMDA receptor-dependent LTD is boosted by endogenously released glutamate activation of mGlu5 receptors. Given the predominant perisynaptic location of mGlu5 receptors, the present findings emphasize the need to further evaluate the contribution and mechanisms of these receptors in NMDA receptor-dependent synaptic plasticity in the adult hippocampus in vivo.

Alcohol and the Brain: Neuronal Molecular Targets, Synapses, and Circuits

Ethanol is one of the most commonly abused drugs. Although environmental and genetic factors contribute to the etiology of alcohol use disorders, it is ethanol's actions in the brain that explain (1) acute ethanol-related behavioral changes, such as stimulant followed by depressant effects, and (2) chronic changes in behavior, including escalated use, tolerance, compulsive seeking, and dependence. Our knowledge of ethanol use and abuse thus relies on understanding its effects on the brain. Scientists have employed both bottom-up and top-down approaches, building from molecular targets to behavioral analyses and vice versa, respectively. This review highlights current progress in the field, focusing on recent and emerging molecular, cellular, and circuit effects of the drug that impact ethanol-related behaviors. The focus of the field is now on pinpointing which molecular effects in specific neurons within a brain region contribute to behavioral changes across the course of acute and chronic ethanol exposure.

Cannabinoid Modulation of the Stressed Hippocampus

Exposure to stressful situations is one of the risk factors for the precipitation of several psychiatric disorders, including Major Depressive Disorder, Posttraumatic Stress Disorder and Schizophrenia. The hippocampal formation is a forebrain structure highly associated with emotional, learning and memory processes; being particularly vulnerable to stress. Exposure to stressful stimuli leads to neuroplastic changes and imbalance between inhibitory/excitatory networks. These changes have been associated with an impaired hippocampal function. Endocannabinoids (eCB) are one of the main systems controlling both excitatory and inhibitory neurotransmission, as well as neuroplasticity within the hippocampus. Cannabinoids receptors are highly expressed in the hippocampus, and several lines of evidence suggest that facilitation of cannabinoid signaling within this brain region prevents stress-induced behavioral changes. Also, chronic stress modulates hippocampal CB₁ receptors expression and endocannabinoid levels. Moreover, cannabinoids participate in mechanisms related to synaptic plasticity and adult neurogenesis. Here, we discussed the main findings supporting the involvement of hippocampal cannabinoid neurotransmission in stress-induced behavioral and neuroplastic changes.

2-Arachidonoylglycerol endocannabinoid signaling coupled to metabotropic glutamate receptor type-5 modulates anxiety-like behavior in the rat ventromedial prefrontal cortex

2-Arachidonoylglycerol and anandamide are the main endocannabinoids, which act through cannabinoid type-1 and type-2 receptors. Among its many functions, anandamide modulates anxiety-like behaviors in the ventromedial prefrontal cortex. The role of 2-arachidonoylglycerol in this region, however, has remained unclear. Here, we verified whether intra- ventromedial prefrontal cortex injection of 2-arachidonoylglycerol or URB602, a monoacylglycerol lipase inhibitor (responsible for 2-arachidonoylglycerol hydrolysis), induce anxiolytic-like effects in Wistar rats. Since activation of metabotropic glutamate receptor type 5 promotes diacylglycerol lipase-α-mediated 2-arachidonoylglycerol synthesis, we also verified if the blockade of this receptor impairs the anxiolytic-like effect induced by URB 602. 2-Arachidonoylglycerol reduced anxiety-like response in rats exposed to the Elevated Plus Maze test, an effect mimicked by URB602. Cannabinoid type-1 and type-2 receptor antagonists prevented these effects. The pre-treatment with an ineffective dose of MPEP, a metabotropic glutamate receptor type 5 antagonist, also attenuated the anxiolytic-like effect of URB602. Moreover, immunofluorescence microscopy revealed co-expression of metabotropic glutamate receptor type 5 and diacylglycerol lipase-α in several neurons in slices from the ventromedial prefrontal cortex. Altogether, our results implicate 2-arachidonoylglycerol and both cannabinoid receptors on anxiety-related behaviors mediated by ventromedial prefrontal cortex. Further, these data support a role for the coupling between metabotropic glutamate receptor type 5 activation and 2-arachidonoylglycerol signalling as a mechanism modulating aversive responses.

Dissecting the Signaling Pathways Involved in the Crosstalk between Metabotropic Glutamate 5 and Cannabinoid Type 1 Receptors

The metabotropic glutamate 5 receptor and the cannabinoid type 1 receptor are G protein-coupled receptors that are widely expressed in the central nervous system. Metabotropic glutamate 5 receptors, present at the postsynaptic site, are coupled to Gαq/11 proteins and display an excitatory response upon activation, whereas the cannabinoid type 1 receptor, mainly present at presynaptic terminals, is coupled to the Gi/o protein and triggers an inhibitory response. Recent studies suggest that the glutamatergic and endocannabinoid systems exhibit a functional interaction to modulate several neural processes. In this review, we discuss possible mechanisms involved in this crosstalk and its relationship with physiologic and pathologic conditions, including nociception, addiction, and fragile X syndrome.

Contribution of synaptic plasticity in the insular cortex to chronic pain

Animal and human studies have consistently demonstrated that cortical regions are important for pain perception and pain-related emotional changes. Studies of the anterior cingulate cortex (ACC) have shown that adult cortical synapses can be modified after peripheral injuries, and long-term changes at synaptic level may contribute to long-lasting suffering in patients. It also explains why chronic pain is resistant to conventional analgesics that act by inhibiting synaptic transmission. Insular cortex (IC), another critical cortical area, is found to be highly plastic and can undergo long-term potentiation (LTP) after injury. Inhibiting IC LTP reduces behavioral sensitization caused by injury. LTP of glutamatergic transmission in pain related cortical areas serves as a key mechanism for chronic pain.

GABA and Endocannabinoids Mediate Depotentiation of Schaffer Collateral Synapses Induced by Stimulation of Temperoammonic Inputs

Long-term potentiation (LTP) of Schaffer collateral (SC) synapses in the hippocampus is thought to play a key role in episodic memory formation. Because the hippocampus is a shorter-term, limited capacity storage system, repeated bouts of learning and synaptic plasticity require that SC synapses reset to baseline at some point following LTP. We previously showed that repeated low frequency activation of temperoammonic (TA) inputs to the CA1 region depotentiates SC LTP without persistently altering basal transmission. This heterosynaptic depotentiation involves adenosine A1 receptors but not N-methyl-D-aspartate receptors, metabotropic glutamate receptors or L-type calcium channels. In the present study, we used rat hippocampal slices to explore other messengers contributing to TA-induced SC depotentiation, and provide evidence for the involvement of cannabinoid-1 and γ-aminobutyric acid (GABA) type-A receptors as more proximal signaling events leading to synaptic resetting, with A1 receptor activation serving as a downstream event. Surprisingly, we found that TA-induced SC depotentiation is independent of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate glutamate receptors. We also examined the involvement of mitogen-activated protein kinases (MAPKs), and found a role for extracellular-signal related kinase 1/2 and p38 MAPK, but not c-Jun-N-terminal kinase. These results indicate that low frequency stimulation of TA inputs to CA1 activates a complex signaling network that instructs SC synaptic resetting. The involvement of GABA and endocannabinoids suggest mechanisms that could contribute to cognitive dysfunction associated with substance abuse and neuropsychiatric disorders.

Major dorsoventral differences in the modulation of the local CA1 hippocampal network by NMDA, mGlu5, adenosine A2A and cannabinoid CB1 receptors

Recent research points to diversification in the local neuronal circuitry between dorsal (DH) and ventral (VH) hippocampus that may be involved in the large-scale functional segregation along the long axis of the hippocampus. Here, using CA1 field recordings from rat hippocampal slices, we show that activation of N-methyl-d-aspartate receptors (NMDARs) reduced excitatory transmission more in VH than in DH, with an adenosine A1 receptor-independent mechanism, and reduced inhibition and enhanced postsynaptic excitability only in DH. Strikingly, co-activation of metabotropic glutamate receptor-5 (mGluR5) with NMDAR, by CHPG and NMDA respectively, strongly potentiated the effects of NMDAR in DH but had not any potentiating effect in VH. Furthermore, the synergistic actions in DH were occluded by blockade of adenosine A2A receptors (A2ARs) by their antagonist ZM 241385 demonstrating a tonic action of these receptors in DH. Exogenous activation of A2ARs by 4-[2-[[6-amino-9-(N-ethyl-β-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride (CGS 21680) did not change the effects of mGluR5-NMDAR co-activation in either hippocampal pole. Importantly, blockade of cannabinoid CB1 receptors (CB1Rs) by their antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM 281) restricted the synergistic actions of mGluR5-NMDARs on excitatory synaptic transmission and postsynaptic excitability and abolished their effect on inhibition. Furthermore, AM 281 increased the excitatory transmission only in DH indicating that CB1Rs were tonically active in DH but not VH. Removing the magnesium ions from the perfusion medium neither stimulated the interaction between mGluR5 and NMDAR in VH nor augmented the synergy of the two receptors in DH. These findings show that the NMDAR-dependent modulation of fundamental parameters of the local neuronal network, by mGluR5, A2AR and CB1R, markedly differs between DH and VH. We propose that the higher modulatory role of A2AR and mGluR5, in combination with the role of CB1Rs, provide DH with higher functional flexibility of its NMDARs, compared with VH.

Sex Differences in Molecular Signaling at Inhibitory Synapses in the Hippocampus

The possibility that mechanisms of synaptic modulation differ between males and females has far-reaching implications for understanding brain disorders that vary between the sexes. We found recently that 17β-estradiol (E2) acutely suppresses GABAergic inhibition in the hippocampus of female rats through a sex-specific estrogen receptor α (ERα), mGluR, and endocannabinoid-dependent mechanism. Here, we define the intracellular signaling that links ERα, mGluRs, and endocannabinoids in females and identify where in this pathway males and females differ. Using a combination of whole-cell patch-clamp recording and biochemical analyses in hippocampal slices from young adult rats, we show that E2 acutely suppresses inhibition in females through mGluR1 stimulation of phospholipase C, leading to inositol triphosphate (IP3) generation, activation of the IP3 receptor (IP3R), and postsynaptic endocannabinoid release, likely of anandamide. Analysis of sex differences in this pathway showed that E2 stimulates a much greater increase in IP3 levels in females than males, whereas the group I mGluR agonist DHPG increases IP3 levels equivalently in each sex. Coimmunoprecipitation showed that ERα-mGluR1 and mGluR1-IP3R complexes exist in both sexes but are regulated by E2 only in females. Independently of E2, a fatty acid amide hydrolase inhibitor, which blocks breakdown of anandamide, suppressed >50% of inhibitory synapses in females with no effect in males, indicating tonic endocannabinoid release in females that is absent in males. Together, these studies demonstrate sex differences in both E2-dependent and E2-independent regulation of the endocannabinoid system and suggest that manipulation of endocannabinoids in vivo could affect physiological and behavioral responses differently in each sex.

SIGNIFICANCE STATEMENT

Many brain disorders vary between the sexes, yet the degree to which this variation arises from differential experience versus intrinsic biological sex differences is unclear. In this study, we demonstrate intrinsic sex differences in molecular regulation of a key neuromodulatory system, the endocannabinoid system, in the hippocampus. Endocannabinoids are involved in diverse aspects of physiology and behavior that involve the hippocampus, including cognitive and motivational state, responses to stress, and neurological disorders such as epilepsy. Our finding that molecular regulation of the endocannabinoid system differs between the sexes suggests mechanisms through which experiences or therapeutics that engage endocannabinoids could affect males and females differently.

Nampt is required for long-term depression and the function of GluN2B subunit-containing NMDA receptors

Nicotinamide adenine dinucleotide (NAD(+)) is an essential coenzyme/cosubstrate for many biological processes in cellular metabolism. The rate-limiting step in the major pathway of mammalian NAD(+) biosynthesis is mediated by nicotinamide phosphoribosyltransferase (Nampt). Previously, we showed that mice lacking Nampt in forebrain excitatory neurons (CamKIIαNampt(-/-) mice) exhibited hyperactivity, impaired learning and memory, and reduced anxiety-like behaviors. However, it remained unclear if these functional effects were accompanied by synaptic changes. Here, we show that CamKIIαNampt(-/-) mice have impaired induction of long-term depression (LTD) in the Schaffer collateral pathway, but normal induction of long-term potentiation (LTP), at postnatal day 30. Pharmacological assessments demonstrated that CamKIIαNampt(-/-) mice also display dysfunction of synaptic GluN2B (NR2B)-containing N-methyl-d-aspartate receptors (NMDARs) prior to changes in NMDAR subunit expression. These results support a novel, important role for Nampt-mediated NAD(+) biosynthesis in LTD and in the function of GluN2B-containing NMDARs.

Monoglyceride lipase: Structure and inhibitors

Monoglyceride lipase (MGL), the main enzyme responsible for the hydrolytic deactivation of the endocannabinoid 2-arachidonoyl-sn-glycerol (2-AG), is an intracellular serine hydrolase that plays critical roles in many physiological and pathological processes, such as pain, inflammation, neuroprotection and cancer. The crystal structures of MGL that are currently available provide valuable information about how this enzyme might function and interact with site-directed small-molecule inhibitors. On the other hand, its conformational equilibria and the contribution of regulatory cysteine residues present within the substrate-binding pocket or on protein surface remain open issues. Several classes of MGL inhibitors have been developed, from early reversible ones, such as URB602 and pristimerin, to carbamoylating agents that react with the catalytic serine, such as JZL184 and more recent O-hexafluoroisopropyl carbamates. Other inhibitors that modulate MGL activity by interacting with conserved regulatory cysteines act through mechanisms that deserve to be more thoroughly investigated.

Sensitivity of N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potentials and synaptic plasticity to TCN 201 and TCN 213 in rat hippocampal slices

Whereas ifenprodil has been used as a selective GluN1/GluN2B (NR1/NR2B, B-type) receptor antagonist to distinguish between GluN2B (NR2B) and GluN2A (NR2A)-containing N-methyl-d-aspartate receptors (NMDARs), TCN 201 (3-chloro-4-fluoro-N-[4-[[2-(phenylcarbonyl)hydrazino]carbonyl]benzyl]benzenesulphonamide) and TCN 213 [N-(cyclohexylmethyl)-2-[{5-[(phenylmethyl)amino]-1,3,4-thiadiazol-2-yl}thio]acetamide] have been found to be selective GluN1/GluN2A (NR1/NR2A, A-type) antagonists. Based on the premise that A- and B-types are major synaptic NMDARs, we examined whether inhibition of NMDAR excitatory postsynaptic potentials (EPSPs) by the TCN compounds and ifenprodil are complementary. Contrary to this prediction, inhibition of NMDAR EPSPs by the TCN compounds and ifenprodil were largely overlapping in the CA1 region of hippocampal slices from 30-day-old rats. After partial inhibition by ifenprodil, TCN compounds produced little further suppression of NMDAR EPSPs. Similarly, after partial inhibition by TCN compounds ifenprodil failed to further suppress NMDAR EPSPs. However, low micromolar d-2-amino-5-phosphonovalerate, a competitive NMDAR antagonist, which alone only partially inhibits NMDAR EPSPs, markedly suppresses residual NMDAR responses in the presence of ifenprodil or the TCNs, suggesting that low 2-amino-5-phosphonovalerate antagonizes both ifenprodil- and TCN-insensitive synaptic NMDARs. These observations can be most readily interpreted if ifenprodil and TCNs act on a similar population of synaptic NMDARs. Recent lines of evidence suggest that the majority of hippocampal synaptic NMDARs are triheteromers. If so, modulation of GluN2A, and not just GluN2B NMDARs, could dampen long-term depression (LTD). Indeed, both TCNs, like ifenprodil, blocked LTD, suggesting the involvement of ifenprodil- and TCN-sensitive NMDARs in LTD induction. However, the TCNs plus ifenprodil failed to inhibit long-term potentiation (LTP), suggesting that neither ifenprodil- nor TCN-sensitive NMDARs are essential for LTP induction.

Metaplastic effects of subanesthetic ketamine on CA1 hippocampal function

Ketamine is a non-competitive N-methyl-d-aspartate receptor (NMDAR) antagonist of interest in neuropsychiatry. In the present studies, we examined the effects of subanesthetic, low micromolar ketamine on excitatory postsynaptic potentials (EPSPs), population spikes (PSs) and synaptic plasticity in the CA1 region of rat hippocampal slices. Ketamine acutely inhibited NMDAR-mediated synaptic responses with half-maximal effects near 10 μM. When administered for 15-30 min at 1-10 μM, ketamine had no effect on baseline dendritic AMPA receptor-mediated EPSPs, but persistently enhanced somatic EPSPs in the pyramidal cell body layer and augmented PS firing. Acute low micromolar ketamine also had no effect on the induction of long-term potentiation (LTP) but blocked long-term depression (LTD). Following 30 min administration of 1-10 μM ketamine, however, a slowly developing and persistent form of LTP inhibition was observed that took two hours following ketamine washout to become manifest. This LTP inhibition did not result from prolonged or enhanced NMDAR inhibition during drug washout. Effects of low ketamine on somatic EPSPs and LTP were not mimicked by a high ketamine concentration that completely inhibited NMDARs, and both of these effects were blocked by co-administration of low ketamine with a low concentration of the competitive NMDAR antagonist, 2-amino-5-phosphonovalerate or inhibitors of nitric oxide synthase. These results indicate that concentrations of ketamine relevant to psychotropic and psychotomimetic effects have complex metaplastic effects on hippocampal function that involve activation of unblocked NMDARs during ketamine exposure.

Activation of mGluR5 attenuates NMDA-induced neurotoxicity through disruption of the NMDAR-PSD-95 complex and preservation of mitochondrial function in differentiated PC12 cells

Glutamate-mediated toxicity is implicated in various neuropathologic conditions, and activation of ionotropic and metabotropic glutamate receptors is considered to be the most important mechanism. It has been reported that pharmacological saturation of metabotropic glutamate receptors (mGluRs) can facilitate N-methyl-d-aspartate receptor (NMDAR) related signaling cascades, but the mechanism leading to mGluR-NMDAR interactions in excitotoxic neuronal injury has remained unidentified. In the present study, we investigated the role of mGluR5 in the regulation of N-methyl-d-aspartate (NMDA)-induced excitotoxicity in differentiated PC12 cells. We found that activation of mGluR5 with the specific agonist R,S-2-chloro-5-hydroxyphenylglycine (CHPG) increased cell viability and inhibited lactate dehydrogenase (LDH) release in a dose-dependent manner. CHPG also inhibited an increase in the Bax/Bcl-2 ratio, attenuated cleavage of caspase-9 and caspase-3, and reduced apoptotic cell death after NMDA treatment. The NMDA-induced mitochondrial dysfunction, as indicated by mitochondrial reactive oxygen species (ROS) generation, collapse of mitochondrial membrane potential (MMP), and cytochrome c release, was also partly prevented by CHPG treatment. Furthermore, CHPG blocked the NMDA-induced interaction of NMDAR with postsynaptic density protein-95 (PSD-95), but had no effects on intracellular calcium concentrations. All these results indicated that activation of mGluR5 protects differentiated PC12 cells from NMDA-induced neuronal excitotoxicity by disrupting NMDAR-PSD-95 interaction, which might be an ideal target for investigating therapeutic strategies in various neurological diseases where excitotoxicity may contribute to their pathology.

Intracellular mGluR5 can mediate synaptic plasticity in the hippocampus

Metabotropic glutamate receptor 5 (mGluR5) is widely expressed throughout the CNS and participates in regulating neuronal function and synaptic transmission. Recent work in the striatum led to the groundbreaking discovery that intracellular mGluR5 activation drives unique signaling pathways, including upregulation of ERK1/2, Elk-1 (Jong et al., 2009) and Arc (Kumar et al., 2012). To determine whether mGluR5 signals from intracellular membranes of other cell types, such as excitatory pyramidal neurons in the hippocampus, we used dissociated rat CA1 hippocampal cultures and slice preparations to localize and characterize endogenous receptors. As in the striatum, CA1 neurons exhibited an abundance of mGluR5 both on the cell surface and intracellular membranes, including the endoplasmic reticulum and the nucleus where it colocalized with the sodium-dependent excitatory amino acid transporter, EAAT3. Inhibition of EAAT3 or sodium-free buffer conditions prevented accumulations of radiolabeled agonist. Using a pharmacological approach to isolate different pools of mGluR5, both intracellular and cell surface receptors induced oscillatory Ca(2+) responses in dissociated CA1 neurons; however, only intracellular mGluR5 activation triggered sustained high amplitude Ca(2+) rises in dendrites. Consistent with the notion that mGluR5 can signal from intracellular membranes, uncaging glutamate on a CA1 dendrite led to a local Ca(2+) rise, even in the presence of ionotropic and cell surface metabotropic receptor inhibitors. Finally, activation of intracellular mGluR5 alone mediated both electrically induced and chemically induced long-term depression, but not long-term potentiation, in acute hippocampal slices. These data suggest a physiologically relevant and important role for intracellular mGluR5 in hippocampal synaptic plasticity.

CB1 augments mGluR5 function in medial prefrontal cortical neurons to inhibit amygdala hyperactivity in an arthritis pain model

The medial prefrontal cortex (mPFC) serves executive control functions and forms direct connections with subcortical areas such as the amygdala. Our previous work showed abnormal inhibition of mPFC pyramidal cells and hyperactivity of amygdala output neurons in an arthritis pain model. To restore mPFC activity and hence control pain-related amygdala hyperactivity this study focused on CB1 and mGluR5 receptors, which are important modulators of cortical functions. Extracellular single-unit recordings of infralimbic mPFC pyramidal cells and of amygdala output neurons in the laterocapsular division of the central nucleus (CeLC) were made in anesthetised adult male rats. mPFC neurons were classified as 'excited' or 'inhibited' based on their response to brief innocuous and noxious test stimuli. After arthritis pain induction, background activity and evoked responses of excited neurons and background activity and inhibition of inhibited neurons decreased. Stereotaxic application of an mGluR5-positive allosteric modulator (N-cyclobutyl-6-((3-fluorophenyl)ethynyl) nicotinamide hydrochloride, VU0360172) into the mPFC increased background and evoked activity of excited, but not inhibited, mPFC neurons under normal conditions but not in arthritis. A selective CB1 receptor agonist (arachidonyl-2-chloroethylamide) alone had no effect but restored the facilitatory effects of VU0360172 in the pain model. Coactivation of CB1 and mGluR5 in the mPFC inhibited the pain-related activity increase of CeLC neurons but had no effect under normal conditions. The data suggest that excited mPFC neurons are inversely linked to amygdala output (CeLC) and that CB1 can increase mGluR5 function in this subset of mPFC neurons to engage cortical control of abnormally enhanced amygdala output in pain.

Acute and chronic effects of ethanol on learning-related synaptic plasticity

Alcoholism is associated with acute and long-term cognitive dysfunction including memory impairment, resulting in substantial disability and cost to society. Thus, understanding how ethanol impairs cognition is essential for developing treatment strategies to dampen its adverse impact. Memory processing is thought to involve persistent, use-dependent changes in synaptic transmission, and ethanol alters the activity of multiple signaling molecules involved in synaptic processing, including modulation of the glutamate and gamma-aminobutyric acid (GABA) transmitter systems that mediate most fast excitatory and inhibitory transmission in the brain. Effects on glutamate and GABA receptors contribute to ethanol-induced changes in long-term potentiation (LTP) and long-term depression (LTD), forms of synaptic plasticity thought to underlie memory acquisition. In this paper, we review the effects of ethanol on learning-related forms of synaptic plasticity with emphasis on changes observed in the hippocampus, a brain region that is critical for encoding contextual and episodic memories. We also include studies in other brain regions as they pertain to altered cognitive and mental function. Comparison of effects in the hippocampus to other brain regions is instructive for understanding the complexities of ethanol's acute and long-term pharmacological consequences.

Interaction of DHPG-LTD and synaptic-LTD at senescent CA3-CA1 hippocampal synapses

The susceptibility, but not the magnitude, of long-term depression (LTD) induced by hippocampal CA3-CA1 synaptic activity (synaptic-LTD) increases with advanced age. In contrast, the magnitude of LTD induced by pharmacological activation of CA3-CA1 group I metabotropic glutamate receptors (mGluRs) increases during aging. This study examined the signaling pathways involved in induction of LTD and the interaction between paired-pulse low frequency stimulation-induced synaptic-LTD and group I mGluR selective agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG, 100 µM)-induced DHPG-LTD in hippocampal slices obtained from aged (22-24 months) male Fischer 344 rats. Prior induction of synaptic-LTD did not affect induction of DHPG-LTD; however, prior induction of the DHPG-LTD occluded synaptic-LTD suggesting that expression of DHPG-LTD may incorporate synaptic-LTD mechanisms. Application of individual antagonist for the group I mGluR (AIDA), the N-methyl-d-aspartate receptor (NMDAR) (AP-5), or L-type voltage-dependent Ca(2+) channel (VDCC) (nifedipine) failed to block synaptic-LTD and any two antagonists severely impaired synaptic-LTD induction, indicating that activation of any two mechanisms is sufficient to induce synaptic-LTD in aged animals. For DHPG-LTD, AIDA blocked DHPG-LTD and individually applied NMDAR or VDCC attenuated but did not block DHPG-LTD, indicating that the magnitude of DHPG-LTD depends on all three mechanisms.

Adenosine, caffeine, and performance: from cognitive neuroscience of sleep to sleep pharmacogenetics

An intricate interplay between circadian and sleep-wake homeostatic processes regulate cognitive performance on specific tasks, and individual differences in circadian preference and sleep pressure may contribute to individual differences in distinct neurocognitive functions. Attentional performance appears to be particularly sensitive to time of day modulations and the effects of sleep deprivation. Consistent with the notion that the neuromodulator, adenosine , plays an important role in regulating sleep pressure, pharmacologic and genetic data in animals and humans demonstrate that differences in adenosinergic tone affect sleepiness, arousal and vigilant attention in rested and sleep-deprived states. Caffeine--the most often consumed stimulant in the world--blocks adenosine receptors and normally attenuates the consequences of sleep deprivation on arousal, vigilance, and attention. Nevertheless, caffeine cannot substitute for sleep, and is virtually ineffective in mitigating the impact of severe sleep loss on higher-order cognitive functions. Thus, the available evidence suggests that adenosinergic mechanisms, in particular adenosine A2A receptor-mediated signal transduction, contribute to waking-induced impairments of attentional processes, whereas additional mechanisms must be involved in higher-order cognitive consequences of sleep deprivation. Future investigations should further clarify the exact types of cognitive processes affected by inappropriate sleep. This research will aid in the quest to better understand the role of different brain systems (e.g., adenosine and adenosine receptors) in regulating sleep, and sleep-related subjective state, and cognitive processes. Furthermore, it will provide more detail on the underlying mechanisms of the detrimental effects of extended wakefulness, as well as lead to the development of effective, evidence-based countermeasures against the health consequences of circadian misalignment and chronic sleep restriction.

Molecular mechanisms underlying neuronal synaptic plasticity: systems biology meets computational neuroscience in the wilds of synaptic plasticity

Interactions among signaling pathways that are activated by transmembrane receptors produce complex networks and emergent dynamical behaviors that are implicated in synaptic plasticity. Temporal dynamics and spatial aspects are critical determinants of cell responses such as synaptic plasticity, although the mapping between spatiotemporal activity pattern and direction of synaptic plasticity is not completely understood. Computational modeling of neuronal signaling pathways has significantly contributed to understanding signaling pathways underlying synaptic plasticity. Spatial models of signaling pathways in hippocampal neurons have revealed mechanisms underlying the spatial distribution of extracellular signal-related kinase (ERK) activation in hippocampal neurons. Other spatial models have demonstrated that the major role of anchoring proteins in striatal and hippocampal synaptic plasticity is to place molecules near their activators. Simulations of yet other models have revealed that the spatial distribution of synaptic plasticity may differ for potentiation versus depression. In general, the most significant advances have been made by interactive modeling and experiments; thus, an interdisciplinary approach should be applied to investigate critical issues in neuronal signaling pathways. These issues include identifying which transmembrane receptors are key for activating ERK in neurons, and the crucial targets of kinases that produce long-lasting synaptic plasticity. Although the number of computer programs for computationally efficient simulation of large reaction-diffusion networks is increasing, parameter estimation and sensitivity analysis in these spatial models remain more difficult than in single compartment models. Advances in live cell imaging coupled with further software development will continue to accelerate the development of spatial models of synaptic plasticity.

CA1 pyramidal cell theta-burst firing triggers endocannabinoid-mediated long-term depression at both somatic and dendritic inhibitory synapses

Endocannabinoids (eCBs) are retrograde lipid messengers that, by targeting presynaptic type 1 cannabinoid receptors (CB1Rs), mediate short- and long-term synaptic depression of neurotransmitter release throughout the brain. Short-term depression is typically triggered by postsynaptic, depolarization-induced calcium rises, whereas long-term depression is induced by synaptic activation of Gq/11 protein-coupled receptors. Here we report that a physiologically relevant pattern of postsynaptic activity, in the form of theta-burst firing (TBF) of hippocampal CA1 pyramidal neurons, can trigger long-term depression of inhibitory transmission (iLTD) in rat hippocampal slices. Paired recordings between CA1 interneurons and pyramidal cells, followed by post hoc morphological reconstructions of the interneurons' axon, revealed that somatic and dendritic inhibitory synaptic inputs equally expressed TBF-induced iLTD. Simultaneous recordings from neighboring pyramidal cells demonstrated that eCB signaling triggered by TBF was highly restricted to only a single, active cell. Furthermore, pairing submaximal endogenous activation of metabotropic glutamate or muscarinic acetylcholine receptors with submaximal TBF unmasked associative iLTD. Although CB1Rs are also expressed at Schaffer-collateral excitatory terminals, long-term plasticity under various recording conditions was spared at these synapses. Consistent with this observation, TBF also shifted the balance of excitation and inhibition in favor of excitatory throughput, thereby altering information flow through the CA1 circuit. Given the near ubiquity of burst-firing activity patterns and CB1R expression in the brain, the properties described here may be a general means by which neurons fine tune the strength of their inputs in a cell-wide and cell-specific manner.

Metaplastic LTP inhibition after LTD induction in CA1 hippocampal slices involves NMDA Receptor-mediated Neurosteroidogenesis

Long-term depression (LTD) induced by low-frequency electrical stimulation (LFS) in the CA1 region of the hippocampus is a form of synaptic plasticity thought to contribute to learning and memory and to the pathophysiology of neuropsychiatric disorders. In naïve hippocampal slices from juvenile rats, we previously found that LTD induction can impair subsequent induction of long-term potentiation (LTP) via a form of N-methyl-d-aspartate receptor (NMDAR)-dependent metaplasticity, and have recently observed that pharmacologically induced NMDAR-dependent LTP inhibition involves 5α-reduced neurosteroids that augment the actions of γ-aminobutyric acid (GABA). In this study, we found that both LFS-induced LTD and subsequent inhibition of LTP induction involve neurosteroid synthesis via NMDAR activation. Furthermore, the timing of 5α-reductase inhibition relative to LFS can dissociate effects on LTD and metaplastic LTP inhibition. These findings indicate that 5α-reduced neurosteroids play an important role in synaptic plasticity and synaptic modulation in the hippocampus.

Anandamide, cannabinoid type 1 receptor, and NMDA receptor activation mediate non-Hebbian presynaptically expressed long-term depression at the first central synapse for visceral afferent fibers

Presynaptic long-term depression (LTD) of synapse efficacy generally requires coordinated activity between presynaptic and postsynaptic neurons and a retrograde signal synthesized by the postsynaptic cell in an activity-dependent manner. In this study, we examined LTD in the rat nucleus tractus solitarii (NTS), a brainstem nucleus that relays homeostatic information from the internal body to the brain. We found that coactivation of N-methyl-D-aspartate receptors (NMDARs) and type 1 cannabinoid receptors (CB1Rs) induces LTD at the first central excitatory synapse between visceral fibers and NTS neurons. This LTD is presynaptically expressed. However, neither postsynaptic activation of NMDARs nor postsynaptic calcium influx are required for its induction. Direct activation of NMDARs triggers cannabinoid-dependent LTD. In addition, LTD is unaffected by blocking 2-arachidonyl-glycerol synthesis, but its induction threshold is lowered by preventing fatty acid degradation. Altogether, our data suggest that LTD in NTS neurons may be entirely expressed at the presynaptic level by local anandamide synthesis.

Long-term depression of synaptic transmission in the adult mouse insular cortex in vitro

The insular cortex (IC) is known to play important roles in higher brain functions such as memory and pain. Activity-dependent long-term depression (LTD) is a major form of synaptic plasticity related to memory and chronic pain. Previous studies of LTD have mainly focused on the hippocampus, and no study in the IC has been reported. In this study, using a 64-channel recording system, we show for the first time that repetitive low-frequency stimulation (LFS) can elicit frequency-dependent LTD of glutamate receptor-mediated excitatory synaptic transmission in both superficial and deep layers of the IC of adult mice. The induction of LTD in the IC required activation of the N-methyl-d-aspartate (NMDA) receptor, metabotropic glutamate receptor (mGluR)5, and L-type voltage-gated calcium channel. Protein phosphatase 1/2A and endocannabinoid signaling are also critical for the induction of LTD. In contrast, inhibiting protein kinase C, protein kinase A, protein kinase Mζ or calcium/calmodulin-dependent protein kinase II did not affect LFS-evoked LTD in the IC. Bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine produced another form of LTD in the IC, which was NMDA receptor-independent and could not be occluded by LFS-induced LTD. Our studies have characterised the basic mechanisms of LTD in the IC at the network level, and suggest that two different forms of LTD may co-exist in the same population of IC synapses.

Synaptic plasticity in depression: molecular, cellular and functional correlates

Synaptic plasticity confers environmental adaptability through modification of the connectivity between neurons and neuronal circuits. This is achieved through changes to synapse-associated signaling systems and supported by complementary changes to cellular morphology and metabolism within the tripartite synapse. Mounting evidence suggests region-specific changes to synaptic form and function occur as a result of chronic stress and in depression. Within subregions of the prefrontal cortex (PFC) and hippocampus structural and synapse-related findings seem consistent with a deficit in long-term potentiation (LTP) and facilitation of long-term depression (LTD), particularly at excitatory pyramidal synapses. Other brain regions are less well-studied; however the amygdala may feature a somewhat opposite synaptic pathology including reduced inhibitory tone. Changes to synaptic plasticity in stress and depression may correlate those to several signal transduction pathways (e.g. NOS-NO, cAMP-PKA, Ras-ERK, PI3K-Akt, GSK-3, mTOR and CREB) and upstream receptors (e.g. NMDAR, TrkB and p75NTR). Deficits in synaptic plasticity may further correlate disrupted brain redox and bioenergetics. Finally, at a functional level region-specific changes to synaptic plasticity in depression may relate to maladapted neurocircuitry and parallel reduced cognitive control over negative emotion.

Increased metabotropic glutamate receptor subtype 5 availability in human brain after one night without sleep

BACKGROUND

Sleep deprivation (wake therapy) provides rapid clinical relief in many patients with major depressive disorder (MDD). Changes in glutamatergic neurotransmission may contribute to the antidepressant response, yet the exact underlying mechanisms are unknown. Metabotropic glutamate receptors of subtype 5 (mGluR5) are importantly involved in modulating glutamatergic neurotransmission and neuronal plasticity. The density of these receptors is reduced in the brain of patients with MDD, particularly in brain structures involved in regulating wakefulness and sleep. We hypothesized that prolonged wakefulness would increase mGluR5 availability in human brain.

METHODS

Metabotropic glutamate receptor subtype 5 binding was quantified with positron emission tomography in 22 young healthy men who completed two experimental blocks separated by 1 week. Two positron emission tomography examinations were conducted in randomized, crossover fashion with the highly selective radioligand, ¹¹C-ABP688, once after 9 hours (sleep control) and once after 33 hours (sleep deprivation) of controlled wakefulness. ¹¹C-ABP688 uptake was quantified in 13 volumes of interest with high mGluR5 expression and presumed involvement in sleep-wake regulation.

RESULTS

Sleep deprivation induced a global increase in mGluR5 binding when compared with sleep control (p<.006). In anterior cingulate cortex, insula, medial temporal lobe, parahippocampal gyrus, striatum, and amygdala, this increase correlated significantly with the sleep deprivation-induced increase in subjective sleepiness.

CONCLUSIONS

This molecular imaging study demonstrates that cerebral functional mGluR5 availability is increased after a single night without sleep. Given that mGluR5 density is reduced in MDD, further research is warranted to examine whether this mechanism is involved in the potent antidepressant effect of wake therapy.

Group 1 metabotropic glutamate receptor function and its regulation of learning and memory in the aging brain

Normal aging is generally characterized by a slow decline of cognitive abilities albeit with marked individual differences. Several animal models have been studied to explore the molecular and cellular mechanisms underlying this phenomenon. The excitatory neurotransmitter glutamate and its receptors have been closely linked to spatial learning and hippocampus-dependent memory processes. For decades, ionotropic glutamate receptors have been known to play a critical role in synaptic plasticity, a form of adaptation regulating memory formation. Over the past 10 years, several groups have shown the importance of group 1 metabotropic glutamate receptor (mGluR) in successful cognitive aging. These G-protein-coupled receptors are enriched in the hippocampal formation and interact physically with other proteins in the membrane including glutamate ionotropic receptors. Synaptic plasticity is crucial to maintain cognitive abilities and long-term depression (LTD) induced by group 1 mGluR activation, which has been linked to memory in the aging brain. The translation and synthesis of proteins by mGluR-LTD modulate ionotropic receptor trafficking and expression of immediate early genes related to cognition. Fragile X syndrome, a genetic form of autism characterized by memory deficits, has been associated to mGluR receptor malfunction and aberrant activation of its downstream signaling pathways. Dysfunction of mGluR could also be involved in neurodegenerative disorders like Alzheimer’s disease (AD). Indeed, beta-amyloid, the main component of insoluble senile plaques and one of the hallmarks of AD, occludes mGluR-dependent LTD leading to diminished functional synapses. This review highlights recent findings regarding mGluR signaling, related synaptic plasticity, and their potential involvement in normal aging and neurological disorders.

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.