NMDA receptor activation enhances inhibitory GABAergic transmission onto hippocampal pyramidal neurons via presynaptic and postsynaptic mechanisms

Regulation of GABAergic neurotransmission by purinergic receptors in brain physiology and disease

Purinergic receptors regulate the processing of neural information in the hippocampus and cerebral cortex, structures related to cognitive functions. These receptors are activated when astrocytic and neuronal populations release adenosine triphosphate (ATP) in an autocrine and paracrine manner, following sustained patterns of neuronal activity. The modulation by these receptors of GABAergic transmission has only recently been studied. Through their ramifications, astrocytes and GABAergic interneurons reach large groups of excitatory pyramidal neurons. Their inhibitory effect establishes different synchronization patterns that determine gamma frequency rhythms, which characterize neural activities related to cognitive processes. During early life, GABAergic-mediated synchronization of excitatory signals directs the experience-driven maturation of cognitive development, and dysfunctions concerning this process have been associated with neurological and neuropsychiatric diseases. Purinergic receptors timely modulate GABAergic control over ongoing neural activity and deeply affect neural processing in the hippocampal and neocortical circuitry. Stimulation of A2 receptors increases GABA release from presynaptic terminals, leading to a considerable reduction in neuronal firing of pyramidal neurons. A1 receptors inhibit GABAergic activity but only act in the early postnatal period when GABA produces excitatory signals. P2X and P2Y receptors expressed in pyramidal neurons reduce the inhibitory tone by blocking GABAA receptors. Finally, P2Y receptor activation elicits depolarization of GABAergic neurons and increases GABA release, thus favoring the emergence of gamma oscillations. The present review provides an overall picture of purinergic influence on GABAergic transmission and its consequences on neural processing, extending the discussion to receptor subtypes and their involvement in the onset of brain disorders, including epilepsy and Alzheimer's disease.

Pyramidal and parvalbumin neurons modulate the process of electroacupuncture stimulation for stroke rehabilitation

Electroacupuncture (EA) stimulation has been shown to be beneficial in stroke rehabilitation; however, little is known about the neurological mechanism by which this peripheral stimulation approach treats for stroke. This study showed that both pyramidal and parvalbumin (PV) neuronal activity increased in the contralesional primary motor cortex forelimb motor area (M1FL) after ischemic stroke induced by focal unilateral occlusion in the M1FL. EA stimulation reduced pyramidal neuronal activity and increased PV neuronal activity. These results were obtained by a combination of fiber photometry recordings, in vivo and in vitro electrophysiological recordings, and immunofluorescence. Moreover, EA was found to regulate the expression/function of N-methyl-D-aspartate receptors (NMDARs) altered by stroke pathology. In summary, our findings suggest that EA could restore disturbed neuronal activity through the regulation of the activity of pyramidal and PV neurons. Furthermore, NMDARs we shown to play an important role in EA-mediated improvements in sensorimotor ability during stroke rehabilitation.

Altered resting-state brain oscillation and the associated cognitive impairments in late-life depression with different depressive severity: An EEG power spectrum and functional connectivity study

OBJECTIVE

Cognitive impairments are prevalent in late-life depression (LLD). However, it remains unclear whether there are concurrent brain oscillation alterations in resting condition across varying level of depression severity. This cross-sectional study aims to investigate the characteristics of altered resting-state oscillations, including power spectrum and functional connectivity, and their association with the cognitive impairments in LLD with different depression severity.

METHODS

A total of 65 patients with LLD and 40 elder participants without depression were recruited. Global cognition and subtle cognitive domains were evaluated. A five-minute resting-state electroencephalography (EEG) was conducted under eyes-closed conditions. Measurements included the ln-transformed absolute power for power spectrum analysis and the weighted phase lag index (wPLI) for functional connectivity analysis.

RESULTS

Attentional and executive dysfunction were exhibited in Moderate-Severe LLD group. Enhanced posterior upper gamma power was observed in both LLD groups. Additionally, enhanced parietal and fronto-parietal/occipital theta connectivity were observed in Moderate-Severe LLD group, which were associated with the attentional impairment.

LIMITATIONS

Limitations include a small sample size, concomitant medication use, and a relatively higher proportion of females.

CONCLUSIONS

Current study observed aberrant brain activity patterns in LLD across different levels of depression severity, which were linked to cognitive impairments. The altered posterior brain oscillations may be trait marker of LLD. Moreover, cognitive impairments and associated connectivity alterations were exhibited in moderate-severe group, which may be a state-like marker of moderate-to severe LLD. The study deepens understanding of cognitive impairments with the associated oscillation changes, carrying implications for neuromodulation targets in LLD.

P2X receptor- and postsynaptic NMDA receptor-mediated long-lasting facilitation of inhibitory synapses in the rat insular cortex

Adenosine triphosphate (ATP) changes the efficacy of synaptic transmission. Despite recent progress in terms of the roles of purinergic receptors in cerebrocortical excitatory synaptic transmission, their contribution to inhibitory synaptic transmission is unknown. To elucidate the effects of α,β-methylene ATP (αβ-mATP), a selective agonist of P2X receptors (P2XRs), on inhibitory synaptic transmission in the insular cortex (IC), we performed whole-cell patch-clamp recording from IC pyramidal neurons (PNs) and fast-spiking neurons (FSNs) in either sex of VGAT-Venus transgenic rats. αβ-mATP increased the amplitude of miniature IPSCs (mIPSCs) under conditions in which NMDA receptors (NMDARs) are recruitable. αβ-mATP-induced facilitation of mIPSCs was sustained even after the washout of αβ-mATP, which was blocked by preincubation with fluorocitrate. The preapplication of NF023 (a P2X1 receptor antagonist) or AF-353 (a P2X3 receptor antagonist) blocked αβ-mATP-induced mIPSC facilitation. Intracellular application of the NMDAR antagonist MK801 blocked the facilitation. d-serine, which is an intrinsic agonist of NMDARs, mimicked αβ-mATP-induced mIPSC facilitation. The intracellular application of BAPTA a Ca2+ chelator, or the bath application of KN-62, a CaMKII inhibitor, blocked αβ-mATP-induced mIPSC facilitation, thus indicating that mIPSC facilitation by αβ-mATP required postsynaptic [Ca2+]i elevation through NMDAR activation. Paired whole-cell patch-clamp recordings from FSNs and PNs demonstrated that αβ-mATP increased the amplitude of unitary IPSCs without changing the paired-pulse ratio. These results suggest that αβ-mATP-induced IPSC facilitation is mediated by postsynaptic NMDAR activations through d-serine released from astrocytes. Subsequent [Ca2+]i increase and postsynaptic CaMKII activation may release retrograde messengers that upregulate GABA release from presynaptic inhibitory neurons, including FSNs. (250/250 words).

Developmental Inhibitory Changes in the Primary Somatosensory Cortex of the Stargazer Mouse Model of Absence Epilepsy

Childhood absence epilepsy seizures arise in the cortico-thalamocortical network due to multiple cellular and molecular mechanisms, which are still under investigation. Understanding the precise mechanisms is imperative given that treatment fails in ~30% of patients while adverse neurological sequelae remain common. Impaired GABAergic neurotransmission is commonly reported in research models investigating these mechanisms. Recently, we reported a region-specific reduction in the whole-tissue and synaptic GABA_A receptor (GABA_AR) α1 subunit and an increase in whole-tissue GAD65 in the primary somatosensory cortex (SoCx) of the adult epileptic stargazer mouse compared with its non-epileptic (NE) littermate. The current study investigated whether these changes occurred prior to the onset of seizures on postnatal days (PN) 17-18, suggesting a causative role. Synaptic and cytosolic fractions were biochemically isolated from primary SoCx lysates followed by semiquantitative Western blot analyses for GABA_AR α1 and GAD65. We found no significant changes in synaptic GABA_AR α1 and cytosolic GAD65 in the primary SoCx of the stargazer mice at the critical developmental stages of PN 7-9, 13-15, and 17-18. This indicates that altered levels of GABA_AR α1 and GAD65 in adult mice do not directly contribute to the initial onset of absence seizures but are a later consequence of seizure activity.

Ketamine administration in idiopathic epileptic and healthy control dogs: Can we detect differences in brain metabolite response with spectroscopy?

Introduction

In recent years ketamine has increasingly become the focus of multimodal emergency management for epileptic seizures. However, little is known about the effect of ketamine on brain metabolites in epileptic patients. Magnetic resonance spectroscopy (MRS) is a non-invasive technique to estimate brain metabolites in vivo. Our aim was to measure the effect of ketamine on thalamic metabolites in idiopathic epileptic (IE) dogs using 3 Tesla MRS. We hypothesized that ketamine would increase the glutamine-glutamate (GLX)/creatine ratio in epileptic dogs with and without antiseizure drug treatment, but not in control dogs. Furthermore, we hypothesized that no different responses after ketamine administration in other measured brain metabolite ratios between the different groups would be detected.

Methods

In this controlled prospective experimental trial IE dogs with or without antiseizure drug treatment and healthy client-owned relatives of the breeds Border Collie and Greater Swiss Mountain Dog, were included. After sedation with butorphanol, induction with propofol and maintenance with sevoflurane in oxygen and air, a single voxel MRS at the level of the thalamus was performed before and 2 min after intravenous administration of 1 mg/kg ketamine. An automated data processing spectral fitting linear combination model algorithm was used to estimate all commonly measured metabolite ratios. A mixed ANOVA with the independent variables ketamine administration and group allocation was performed for all measured metabolites. A p < 0.05 was considered statistically significant.

Results

Twelve healthy control dogs, 10 untreated IE and 12 treated IE dogs were included. No significant effects for GLX/creatine were found. However, increased glucose/creatine ratios were found (p < 0.001) with no effect of group allocation. Furthermore, increases in the GABA/creatine ratio were found in IEU dogs.

Discussion

MRS was able to detect changes in metabolite/creatine ratios after intravenous administration of 1 mg/kg ketamine in dogs and no evidence was found that excitatory effects are induced in the thalamus. Although it is beyond the scope of this study to investigate the antiseizure potential of ketamine in dogs, results of this research suggest that the effect of ketamine on the brain metabolites could be dependent on the concentrations of brain metabolites before administration.

Early Developmental PMCA2b Expression Protects From Ketamine-Induced Apoptosis and GABA Impairments in Differentiating Hippocampal Progenitor Cells

PMCA2 is not expressed until the late embryonic state when the control of subtle Ca²⁺ fluxes becomes important for neuronal specialization. During this period, immature neurons are especially vulnerable to degenerative insults induced by the N-methyl-D-aspartate (NMDA) receptor blocker, ketamine. As H19-7 hippocampal progenitor cells isolated from E17 do not express the PMCA2 isoform, they constitute a valuable model for studying its role in neuronal development. In this study, we demonstrated that heterologous expression of PMCA2b enhanced the differentiation of H19-7 cells and protected from ketamine-induced death. PMCA2b did not affect resting [Ca²⁺]_c in the presence or absence of ketamine and had no effect on the rate of Ca²⁺ clearance following membrane depolarization in the presence of the drug. The upregulation of endogenous PMCA1 demonstrated in response to PMCA2b expression as well as ketamine-induced PMCA4 depletion were indifferent to the rate of Ca²⁺ clearance in the presence of ketamine. Yet, co-expression of PMCA4b and PMCA2b was able to partially restore Ca²⁺ extrusion diminished by ketamine. The profiling of NMDA receptor expression showed upregulation of the NMDAR1 subunit in PMCA2b-expressing cells and increased co-immunoprecipitation of both proteins following ketamine treatment. Further microarray screening demonstrated a significant influence of PMCA2b on GABA signaling in differentiating progenitor cells, manifested by the unique regulation of several genes key to the GABAergic transmission. The overall activity of glutamate decarboxylase remained unchanged, but Ca²⁺-induced GABA release was inhibited in the presence of ketamine. Interestingly, PMCA2b expression was able to reverse this effect. The mechanism of GABA secretion normalization in the presence of ketamine may involve PMCA2b-mediated inhibition of GABA transaminase, thus shifting GABA utilization from energetic purposes to neurosecretion. In this study, we show for the first time that developmentally controlled PMCA expression may dictate the pattern of differentiation of hippocampal progenitor cells. Moreover, the appearance of PMCA2 early in development has long-standing consequences for GABA metabolism with yet an unpredictable influence on GABAergic neurotransmission during later stages of brain maturation. In contrast, the presence of PMCA2b seems to be protective for differentiating progenitor cells from ketamine-induced apoptotic death.

The Yin and Yang of GABAergic and Glutamatergic Synaptic Plasticity: Opposites in Balance by Crosstalking Mechanisms

Synaptic plasticity is a critical process that regulates neuronal activity by allowing neurons to adjust their synaptic strength in response to changes in activity. Despite the high proximity of excitatory glutamatergic and inhibitory GABAergic postsynaptic zones and their functional integration within dendritic regions, concurrent plasticity has historically been underassessed. Growing evidence for pathological disruptions in the excitation and inhibition (E/I) balance in neurological and neurodevelopmental disorders indicates the need for an improved, more "holistic" understanding of synaptic interplay. There continues to be a long-standing focus on the persistent strengthening of excitation (excitatory long-term potentiation; eLTP) and its role in learning and memory, although the importance of inhibitory long-term potentiation (iLTP) and depression (iLTD) has become increasingly apparent. Emerging evidence further points to a dynamic dialogue between excitatory and inhibitory synapses, but much remains to be understood regarding the mechanisms and extent of this exchange. In this mini-review, we explore the role calcium signaling and synaptic crosstalk play in regulating postsynaptic plasticity and neuronal excitability. We examine current knowledge on GABAergic and glutamatergic synapse responses to perturbances in activity, with a focus on postsynaptic plasticity induced by short-term pharmacological treatments which act to either enhance or reduce neuronal excitability via ionotropic receptor regulation in neuronal culture. To delve deeper into potential mechanisms of synaptic crosstalk, we discuss the influence of synaptic activity on key regulatory proteins, including kinases, phosphatases, and synaptic structural/scaffolding proteins. Finally, we briefly suggest avenues for future research to better understand the crosstalk between glutamatergic and GABAergic synapses.

Spatial regulation of coordinated excitatory and inhibitory synaptic plasticity at dendritic synapses

The induction of synaptic plasticity at an individual dendritic glutamatergic spine can affect neighboring spines. This local modulation generates dendritic plasticity microdomains believed to expand the neuronal computational capacity. Here, we investigate whether local modulation of plasticity can also occur between glutamatergic synapses and adjacent GABAergic synapses. We find that the induction of long-term potentiation at an individual glutamatergic spine causes the depression of nearby GABAergic inhibitory synapses (within 3 μm), whereas more distant ones are potentiated. Notably, L-type calcium channels and calpain are required for this plasticity spreading. Overall, our data support a model whereby input-specific glutamatergic postsynaptic potentiation induces a spatially regulated rearrangement of inhibitory synaptic strength in the surrounding area through short-range heterosynaptic interactions. Such local coordination of excitatory and inhibitory synaptic plasticity is expected to influence dendritic information processing and integration.

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LTP of individual dendritic spines causes iLTD at neighboring GABAergic synapses

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Interaction between single-spine LTP and iLTD occurs in the spatial range of ±3 μm

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This iLTD depends on the local dendritic calcium increase and calpain activation

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iLTD is associated with reduced gephyrin clustering and increased GABAAR mobility

Subjective tinnitus: lesion-induced pathological central homeostasis remodeling

Subjective tinnitus is the most common type of tinnitus, which is the manifestation of pathological activities in the brain. It happens in a substantial portion of the general population and brings significant burden to the society. Severe subjective tinnitus can lead to depression and insomnia and severely affects patients' quality of life. However, due to poor understanding of its etiology and pathogenesis, treatment of subjective tinnitus remains challenging. In recent decades, a growing number of studies have shown that subjective tinnitus is related to lesion-induced neural plasticity of auditory and non-auditory central systems. This article reviews cellular mechanisms of neural plasticity in subjective tinnitus to provide further understanding of its pathogenesis.

Change in gene expression levels of GABA, glutamate and neurosteroid pathways due to acoustic trauma in the cochlea

The characteristic feature of noise-induced hearing loss (NIHL) is the loss or malfunction of the outer hair cells (OHC) and the inner hair cells (IHC) of the cochlea. 90-95% of the spiral ganglion neurons, forming the cell bodies of cochlear nerve, synapse with the IHCs. Glutamate is the most potent excitatory neurotransmitter for IHC-auditory nerve synapses. Excessive release of glutamate in response to acoustic trauma (AT), may cause excitotoxicity by causing damage to the spiral ganglion neurons (SGN) or loss of the spiral ganglion dendrites, post-synaptic to the IHCs. Another neurotransmitter, GABA, plays an important role in the processing of acoustic stimuli and central regulation after peripheral injury, so it is potentially related to the regulation of hearing function and sensitivity after noise. The aim of this study is to evaluate the effect of AT on the expressions of glutamate excitotoxicity, GABA inhibition and neurosteroid synthesis genes.We exposed 24 BALB/c mice to AT. Controls were sacrificed without exposure to noise, Post-AT(1) and Post-AT(15) were sacrificed on the 1st and 15th day, respectively, after noise exposure. The expressions of various genes playing roles in glutamate, GABA and neurosteroid pathways were compared between groups by real-time PCR.Expressions of Cyp11a1, Gls, Gabra1, Grin2b, Sult1a1, Gad1, and Slc1a2 genes in Post-AT(15) mice were significantly decreased in comparison to control and Post-AT(1) mice. No significant differences in the expression of Slc6a1 and Slc17a8 genes was detected.These findings support the possible role of balance between glutamate excitotoxicity and GABA inhibition is disturbed during the post AT days and also the synthesis of some neurosteroids such as pregnenolone sulfate may be important in this balance.

Early life stress dysregulates kappa opioid receptor signaling within the lateral habenula

The lateral habenula (LHb) is an epithalamic brain region associated with value-based decision making and stress evasion through its modulation of dopamine (DA)-mediated reward circuitry. Specifically, increased activity of the LHb is associated with drug addiction, schizophrenia and stress-related disorders such as depression, anxiety and posttraumatic stress disorder. Dynorphin (Dyn)/Kappa opioid receptor (KOR) signaling is a mediator of stress response in reward circuitry. Previously, we have shown that maternal deprivation (MD), a severe early life stress, increases LHb spontaneous neuronal activity and intrinsic excitability while blunting the response of LHb neurons to extrahypothalamic corticotropin-releasing factor (CRF) signaling, another stress mediator. CRF pathways also interact with Dyn/KOR signaling. Surprisingly, there has been little study of direct KOR regulation of the LHb despite its distinct role in stress, reward and aversion processing. To test the functional role of Dyn/KOR signaling in the LHb, we utilized ex-vivo electrophysiology combined with pharmacological tools in rat LHb slices. We show that activation of KORs by a KOR agonist (U50,488) exerted differential effects on the excitability of two distinct sub-populations of LHb neurons that differed in their expression of hyperpolarization-activated cation currents (HCN, Ih). Specifically, KOR stimulation increased neuronal excitability in LHb neurons with large Ih currents (Ih+) while decreasing neuronal excitability in small/negative Ih (Ih-) neurons. We found that an intact fast-synaptic transmission was required for the effects of U50,488 on the excitability of both Ih- and Ih+ LHb neuronal subpopulations. While AMPAR-, GABAAR-, or NMDAR-mediated synaptic transmission alone was sufficient to mediate the effects of U50,488 on excitability of Ih- neurons, either GABAAR- or NMDAR-mediated synaptic transmission could mediate these effects in Ih+ neurons. Consistently, KOR activation also altered both glutamatergic and GABAergic synaptic transmission where stimulation of presynaptic KORs uniformly suppressed glutamate release onto LHb neurons while primarily decreased or in some cases increased GABA release. We also found that MD significantly increased immunolabeled Dyn (the endogenous KOR agonist) labeling in neuronal fibers in LHb while significantly decreasing mRNA levels of KORs in LHb tissues compared to those from non-maternally deprived (non-MD) control rats. Moreover, the U50,488-mediated increase in LHb neuronal firing observed in non-MD rats was absent following MD. Altogether, this is the first demonstration of the existence of functional Dyn/KOR signaling in the LHb that can be modulated in response to severe early life stressors such as MD.

Datumetine exposure alters hippocampal neurotransmitters system in C57BL/6 mice

Our previous study showed that datumetine modulates NMDAR activity with long term exposure leading to memory deficit and altered NMDAR signaling. We aim to explore the neurotransmitters perturbations of acute datumetine-NMDAR interaction. Fifteen C57/BL6 mice were used for the study, they are divided into three groups of 5 animals each. Animals were administered DMSO (DMSO/Control), 0.25 mg/kg body weight of datumetine (0.25 Datumetine) and 1 mg/kg bodyweight of datumetine (1.0 Datumetine) intraperitoneally for 14 days. At the end of treatment, animals were euthanized in isofluorane chamber, perfused transcardially with 1XPBS followed by PFA. Immunofluorescence procedure was done to check the distribution of neurons, astrocytes, microglia and major neuronal subtypes in the hippocampus. Expansion and electron microscopy techniques were used to assess the condition of the synapses. Quantitative data were expressed as mean ± SEM and analyzed using ANOVA with Tukey post hoc using p < 0.05 as significant. Datumetine increased the expression of CD11b, GFAP, vGlut1, GABA, CHRNA7 and TH while expression of TrPH and NeuN were reduced in the hippocampus compared to control animals. Synaptic loss was evident in datumetine exposed animals with reduced synaptic vesicles accompanied by a thickness of postsynaptic density than that of control animals. This study concludes that acute datumetine exposure alters hippocampal neurotransmitter systems.

Simulations reveal how M-currents and memory-based inputs from CA3 enable single neuron mismatch detection for EC3 inputs to the CA1 subfield of hippocampus

Significant evidence has accumulated to support the hypothesis that hippocampal region CA1 operates as an associative mismatch detector (e.g., Hasselmo ME, Schnell E, Barkai E. J Neurosci 15: 5249-5262, 1995; Duncan K, Curtis C, Davachi L. J Neurosci 29: 131-139, 2009; Kumaran D, Maguire EA. J Neurosci 27: 8517-8524, 2007; Lisman JE, Grace AA. Neuron 46: 703-713, 2005; Lisman JE, Otmakhova NA. Hippocampus 11: 551-568 2001; Lörincz A, Buzsáki G. Ann N Y Acad Sci 911: 83-111, 2000; Meeter M, Murre JMJ, Talamini LM. Hippocampus 14: 722-741, 2004; Schiffer AM, Ahlheim C, Wurm MF, Schubotz RI. PLoS One 7: e36445, 2012; Vinogradova OS. Hippocampus 11: 578-598 2001). CA1 compares predictive synaptic signals from CA3 with synaptic signals from EC3, which reflect actual sensory inputs. The new CA1 pyramidal model presented here shows that the distal-proximal segregation of synaptic inputs from EC3 versus CA3, along with other biophysical features, enable such pyramids to serve as comparators that switch output encoding from a brief burst, for a match, to prolonged tonic spiking, for a mismatch. By including often-overlooked features of CA1 pyramidal neurons, this new model allows simulation of pharmacological effects that can eliminate either the match (phasic mode) response or the mismatch (tonic mode) response. These simulations reveal that dysfunctions can arise from either too much or too little ACh stimulation of the muscarinic receptors that control KCNQ channels. Additionally, a dysfunction caused by administration of an N-methyl-d-aspartate antagonist could be rescued by simultaneous administration of a KCNQ channel agonist, such as retigabine.NEW & NOTEWORTHY Hippocampal region CA1 operates as an associative mismatch detector, comparing predictive signals from CA3 with signals from EC3 reflecting sensory inputs. This new CA1 pyramidal model shows that biophysical features enable these comparators to switch output between brief bursts for matches and tonic spiking for mismatches. This suggests that cognitive learning models (e.g., predictive coding) may require much less match/mismatch circuitry than commonly assumed. Additional simulations illuminate deficits seen in psychiatric disorders and drug-induced states.

Temporal Changes on 123I-Iomazenil and Cerebral Blood Flow Single-photon Emission Computed Tomography in a Patient with Anti-N-methyl-D-aspartate Receptor Encephalitis

A 45-year-old man was admitted due to tonic seizures, aphasia, disturbance of consciousness, and abnormal behavior. Because cerebral magnetic resonance imaging findings were normal and mild cerebrospinal fluid (CSF) pleocytosis was observed, autoimmune encephalitis was suspected. The presence of anti-N-methyl-D-aspartate (NMDA) receptor antibodies in the CSF was subsequently confirmed. ¹²³I-Iomazenil and cerebral blood flow single photon emission computed tomography (SPECT) revealed an abnormal uptake in the left frontotemporal region. Multimodal immunotherapy was administered, which remarkably improved the level of consciousness. Progressive reversibility of SPECT findings with clinical improvement suggested that the disorder-related functional deficits had been caused by anti-NMDA receptor antibodies.

A Cooperative Mechanism Involving Ca2+-Permeable AMPA Receptors and Retrograde Activation of GABAB Receptors in Interpeduncular Nucleus Plasticity

The medial habenula-interpeduncular nucleus (MHb-IPN) pathway, which connects the limbic forebrain to the midbrain, has recently been implicated in aversive behaviors. The MHb-IPN circuit is characterized by a unique topographical organization, an excitatory role of GABA, and a prominent co-release of neurotransmitters and neuropeptides. However, little is known about synaptic plasticity in this pathway. An application of a high-frequency stimulation resulted in a long-lasting potentiation of glutamate release in IPN neurons. Our experiments reveal that a Ca²⁺-permeable AMPA receptor (CPAR)-dependent release of GABA from IPN neurons and a retrograde activation of GABA_B receptors on MHb terminals result in a long-lasting enhancement of glutamate release. Strikingly, adolescent IPN neurons lacked CPARs and exhibited an inability to undergo plasticity. In addition, fear conditioning suppressed an activity-dependent potentiation of MHb-IPN synapses, whereas fear extinction reversed this plasticity deficit, suggesting a role of the MHb-IPN synaptic plasticity in the regulation of aversive behaviors.

NMDA receptor-dependent presynaptic inhibition at the calyx of Held synapse of rat pups

N-Methyl-d-aspartate receptors (NMDARs) play diverse roles in synaptic transmission, synaptic plasticity, neuronal development and neurological diseases. In addition to their postsynaptic expression, NMDARs are also expressed in presynaptic terminals at some central synapses, and their activation modulates transmitter release. However, the regulatory mechanisms of NMDAR-dependent synaptic transmission remain largely unknown. In the present study, we demonstrated that activation of NMDARs in a nerve terminal at a central glutamatergic synapse inhibits presynaptic Ca²⁺ currents (I_Ca) in a GluN2C/2D subunit-dependent manner, thereby decreasing nerve-evoked excitatory postsynaptic currents. Neither presynaptically loaded fast Ca²⁺ chelator BAPTA nor non-hydrolysable GTP analogue GTPγS affected NMDAR-mediated I_Ca inhibition. In the presence of a glutamate uptake blocker, the decline in I_Ca amplitude evoked by repetitive depolarizing pulses at 20 Hz was attenuated by an NMDAR competitive antagonist, suggesting that endogenous glutamate has a potential to activate presynaptic NMDARs. Moreover, NMDA-induced inward currents at a negative holding potential (−80 mV) were abolished by intra-terminal loading of the NMDAR open channel blocker MK-801, indicating functional expression of presynaptic NMDARs. We conclude that presynaptic NMDARs can attenuate glutamate release by inhibiting voltage-gated Ca²⁺ channels at a relay synapse in the immature rat auditory brainstem.

Pursuit of Neurotransmitter Functions: Being Attracted with Fascination of the Synapse

In the beginning of the 1970s, only two chemical substances, acetylcholine and γ-aminobutyric acid (GABA), had been definitely established as neurotransmitters. Under such circumstances, I started my scientific career in Professor Masanori Otsuka's lab searching for the transmitter of primary sensory neurons. Until 1976, lines of evidence had accumulated indicating that the undecapeptide substance P could be released as a transmitter from primary afferent fibers into spinal synapses, although the substance P-mediated synaptic response had yet to be identified. Peripheral synapses could serve as a good model and thus, it was demonstrated in the prevertebral sympathetic ganglia by1985 that substance P released from axon collaterals of primary sensory neurons acts as the transmitter mediating non-cholinergic slow excitatory postsynaptic potential (EPSP). At that time, we also found that autonomic synapses were useful to uncover the transmitter role of the opioid peptide enkephalins, whose functions had been unknown since their discovery in 1975. Accordingly, enkephalins were found to serve a transmitter role in mediating presynaptic inhibition of cholinergic fast and non-cholinergic slow transmission in the prevertebral sympathetic ganglia. In 1990s, we attempted to devise a combined technique of brain slices and patch-clamp recordings. We applied it to study the regulatory mechanisms that operate around cerebellar GABAergic inhibitory synapses, because most of the studies then had centered on excitatory synapses and because inhibitory synapses are crucially involved in brain functions and disorders. Consequently, we discovered novel forms of heterosynaptic interactions, dual actions of a single transmitter, and receptor crosstalk, the details of which are described in this review.

Late Effect of Developmental Exposure to 3,3'-Iminodipropionitrile on Neurogenesis in the Hippocampal Dentate Gyrus of Mice

The effects of developmental exposure to 3,3'-iminodipropionitrile (IDPN), a neurotoxicant that causes proximal axonopathy, on mouse hippocampal neurogenesis was examined. Pregnant mice were exposed to IDPN at 0, 600, or 1200 ppm in their drinking water from gestational day 6 to postnatal day (PND) 21. On PND 21, male offspring showed increased postmitotic neuron-specific NeuN-immunoreactive⁽⁺⁾ granule cell numbers in the dentate subgranular zone (SGZ) and granule cell layer (GCL) and decreased glutamate receptor gene Grin2d levels in the dentate gyrus at 1200 ppm. On PND 77, decreased numbers were observed for TBR2⁺ progenitor cells in the SGZ at ≥600 ppm and GFAP⁺ stem cells, DCX⁺ progenitor cells and immature granule cells, NeuN⁺ immature and mature granule cells, PCNA⁺ proliferating cells in the SGZ and/or GCL, and immunoreactive cells for ARC or FOS, immediate-early gene products related to neuronal and synaptic plasticity, in the GCL at 1200 ppm. Additionally, at 1200 ppm of IDPN, downregulation of Kit, the gene encoding the stem cell factor (SCF) receptor, and upregulation of Kitl, encoding SCF, were observed in the dentate gyrus. Therefore, maternal IDPN exposure in mice affects neurogenesis involving glutamatergic signals at the end of developmental exposure, with late effects suppressing SGZ cell proliferation, reducing the broad range of granule cell lineage population, which may be responsible for SCF receptor downregulation. The upregulated SCF was likely a feedback response to the decreased receptor level. These results suggest that reduced SCF signaling may cause suppressed neuronal and synaptic plasticity.

Voluntary exercise rescues sevoflurane-induced memory impairment in aged male mice

Postoperative cognitive impairment is especially common in older patients following major surgery. Although exposure to sevoflurane is known to cause memory deficits, few studies have examined the putative approaches to reduce such impairments. This study tested the hypotheses that sevoflurane exposure can decrease NR2B subunit-containing NMDA receptor activity in hippocampus of aged mice, and voluntary exercise may counteract the declining hippocampal functions. We found that long exposure (3 h/day for 3 days), but not short exposure (1 h/day for 3 days), to 3 % sevoflurane produced a long-lasting spatial memory deficits up to 3 weeks in aged mice, and such an effect was not due to the neuronal loss in the hippocampus, but was correlated with a long-term decrease in Fyn kinase expression and NR2B subunit phosphorylation in the hippocampus. Furthermore, voluntary exercise rescued sevoflurane-induced spatial memory deficits in aged mice and restored Fyn kinase expression and NR2B subunit phosphorylation in the hippocampus to a level comparable to control animals. Generally, our results suggested that Fyn-mediated NR2B subunit phosphorylation may play a critical role in sevoflurane-induced impairment in cognitive functions in aged animals, and voluntary exercise might be an important non-pharmacological approach to treatment of inhaled anesthetics-induced postoperative cognitive impairment in clinical settings.

GluN2D-Containing N-methyl-d-Aspartate Receptors Mediate Synaptic Transmission in Hippocampal Interneurons and Regulate Interneuron Activity

N-methyl-d-aspartate receptors (NMDARs) are ionotropic glutamatergic receptors that have been implicated in learning, development, and neuropathological conditions. They are typically composed of GluN1 and GluN2A-D subunits. Whereas a great deal is known about the role of GluN2A- and GluN2B-containing NMDARs, much less is known about GluN2D-containing NMDARs. Here we explore the subunit composition of synaptic NMDARs on hippocampal interneurons. GluN2D mRNA was detected by single-cell PCR and in situ hybridization in diverse interneuron subtypes in the CA1 region of the hippocampus. The GluN2D subunit was detectable by immunoblotting and immunohistochemistry in all subfields of the hippocampus in young and adult mice. In whole-cell patch-clamp recordings from acute hippocampal slices, (+)-CIQ, the active enantiomer of the positive allosteric modulator CIQ, significantly enhanced the amplitude of the NMDAR component of miniature excitatory postsynaptic currents (mEPSCs) in CA1 interneurons but not in pyramidal cells. (+)-CIQ had no effect in slices from Grin2d-/- mice, suggesting that GluN2D-containing NMDARs participate in excitatory synaptic transmission onto hippocampal interneurons. The time course of the NMDAR component of the mEPSC was unaffected by (+)-CIQ potentiation and was not accelerated in slices from Grin2d-/- mice compared with wild-type, suggesting that GluN2D does not detectably slow the NMDAR EPSC time course at this age. (+)-CIQ increased the activity of CA1 interneurons as detected by the rate and net charge transfer of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from CA1 pyramidal cells. These data provide evidence that interneurons contain synaptic NMDARs possessing a GluN2D subunit, which can influence interneuron function and signal processing.

Acute Ethanol Inhibition of γ Oscillations Is Mediated by Akt and GSK3β

Hippocampal network oscillations at gamma band frequency (γ, 30-80 Hz) are closely associated with higher brain functions such as learning and memory. Acute ethanol exposure at intoxicating concentrations (≥50 mM) impairs cognitive function. This study aimed to determine the effects and the mechanisms of acute ethanol exposure on γ oscillations in an in vitro model. Ethanol (25-100 mM) suppressed kainate-induced γ oscillations in CA3 area of the rat hippocampal slices, in a concentration-dependent, reversible manner. The ethanol-induced suppression was reduced by the D1R antagonist SCH23390 or the PKA inhibitor H89, was prevented by the Akt inhibitor triciribine or the GSk3β inhibitor SB415286, was enhanced by the NMDA receptor antagonist D-AP5, but was not affected by the MAPK inhibitor U0126 or PI3K inhibitor wortmanin. Our results indicate that the intracellular kinases Akt and GSk3β play a critical role in the ethanol-induced suppression of γ oscillations and reveal new cellular pathways involved in the ethanol-induced cognitive impairment.

FUS-linked essential tremor associated with motor dysfunction in Drosophila

Essential tremor (ET) is one of the most common adult-onset neurological disorders which produce motor and non-motor symptoms. To date, there are no gold standard pathological hallmarks of ET, and despite a strong genetic contribution toward ET development, only a few pathogenic mutations have been identified. Recently, a pathogenic FUS-Q290X mutation has been reported in a large ET-affected family; however, the pathophysiologic mechanism underlying FUS-linked ET is unknown. Here, we generated transgenic Drosophila expressing hFUS-WT and hFUS-Q290X and targeted their expression in different tissues. We found that the targeted expression of hFUS-Q290X in the dopaminergic and the serotonergic neurons did not cause obvious neuronal degeneration, but it resulted in motor dysfunction which was accompanied by impairment in the GABAergic pathway. The involvement of the GABAergic pathway was supported by rescue of motor symptoms with gabapentin. Interestingly, we observed gender specific downregulation of GABA-R and NMDA-R expression and reduction in serotonin level. Overexpression of hFUS-Q290X also caused an increase in longevity and this was accompanied by downregulation of the IIS/TOR signalling pathway. Our in vivo studies of the hFUS-Q290X mutation in Drosophila link motor dysfunction to impairment in the GABAergic pathway. Our findings would facilitate further efforts in unravelling the pathophysiology of ET.

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.

Supplementation of Antipsychotic Treatment with the Amino Acid Sarcosine Influences Proton Magnetic Resonance Spectroscopy Parameters in Left Frontal White Matter in Patients with Schizophrenia

Dysfunction of the glutamatergic system, the main stimulating system in the brain, has a major role in pathogenesis of schizophrenia. The frontal white matter (WM) is partially composed of axons from glutamatergic pyramidal neurons and glia with glutamatergic receptors. The natural amino acid sarcosine, a component of a normal diet, inhibits the glycine type 1 transporter, increasing the glycine level. Thus, it modulates glutamatergic transmission through the glutamatergic ionotropic NMDA (N-methyl-d-aspartate) receptor, which requires glycine as a co-agonist. To evaluate the concentrations of brain metabolites (NAA, N-acetylaspartate; Glx, complex of glutamate, glutamine, and γ-aminobutyric acid (GABA); mI, myo-inositol; Cr, creatine; Cho, choline) in the left frontal WM, Proton Nuclear Magnetic Resonance (¹H-NMR) spectroscopy was used. Twenty-five patients randomly chosen from a group of fifty with stable schizophrenia (DSM-IV-TR) and dominant negative symptoms, who were receiving antipsychotic therapy, were administered 2 g of sarcosine daily for six months. The remaining 25 patients received placebo. Assignment was double blinded. ¹H-NMR spectroscopy (1.5 T) was performed twice: before and after the intervention. NAA, Glx and mI were evaluated as Cr and Cho ratios. All patients were also assessed twice with the Positive and Negative Syndrome Scale (PANSS). Results were compared between groups and in two time points in each group. The sarcosine group demonstrated a significant decrease in WM Glx/Cr and Glx/Cho ratios compared to controls after six months of therapy. In the experimental group, the final NAA/Cr ratio significantly increased and Glx/Cr ratio significantly decreased compared to baseline values. Improvement in the PANSS scores was significant only in the sarcosine group. In patients with schizophrenia, sarcosine augmentation can reverse the negative effect of glutamatergic system overstimulation, with a simultaneous beneficial increase of NAA/Cr ratio in the WM of the left frontal lobe. Our results further support the glutamatergic hypothesis of schizophrenia.

Stimulating somatosensory psychophysics: a double-blind, sham-controlled study of the neurobiological mechanisms of tDCS

The neuromodulation technique transcranial direct current stimulation (tDCS) is thought to produce its effects on behavior by altering cortical excitability. Although the mechanisms underlying the observed effects are thought to rely on the balance of excitatory and inhibitory neurotransmission, the physiological principles of the technique are not completely understood. In this study, we examine the influence of tDCS on vibrotactile adaptation, using a simple amplitude discrimination paradigm that has been shown to exhibit modifications in performance due to changes in inhibitory neurotransmission. Double-blind tDCS (Anodal/Sham) of 1 mA was delivered for 600 s to electrodes positioned in a somatosensory/contralateral orbit montage. Stimulation was applied as part of a pre/post design, between blocks of the behavioral tasks. In accordance with previous work, results obtained before the application of tDCS indicated that amplitude discrimination thresholds were significantly worsened during adaptation trials, compared to those achieved at baseline. However, tDCS failed to modify amplitude discrimination performance. Using a Bayesian approach, this finding was revealed to constitute substantial evidence for the null hypothesis. The failure of DC stimulation to alter vibrotactile adaptation thresholds is discussed in the context of several factors that may have confounded the induction of changes in cortical plasticity.

Cyclodextrin Alters GABAergic Input to CA1 Pyramidal Cells in Wild-Type But Not in NPC1-Deficient Mice

Niemann–Pick type C1 disease (NPC1) is a neurodegenerative disorder caused by mutations in the NPC1 gene. Actual, no causative treatment for NPC1 is available, although some drugs have been proven to be beneficial to patients, for example, 2-hydroxypropyl-β-cyclodextrin (CDX). In this study, we used the BALB/c_Nctr-Npc1m1N/-J mouse strain to study the effect of CDX, which is described to prolong the life span and to alleviate the pathogenic phenotype. By means of patch clamp recordings, we measured inhibitory postsynaptic currents (IPSCs) of CA1 pyramidal cells of CDX-treated and -untreated animals to elucidate the influence of CDX on the synaptic transmission. Surprisingly, CDX induced a significantly higher GABAergic IPSC frequency in wild-type mice than in NPC1^−/− mice. Although the IPSCs were mainly GABAergic, we observed a significant reduction of the IPSC frequency in the presence of the glycine receptor antagonist strychnine. The effect of strychnine did not differ in untreated and treated animals, indicating that the effect of CDX was most likely not based on an interaction with glycinergic transmission machinery. However, the unexpected effect of CDX on the GABAergic synaptic transmission is of special interest as a disturbance plays, for example, a crucial role in epilepsy and, moreover, as CDX is currently under investigation as a treatment for NPC1 in humans.

Two cellular hypotheses explaining the initiation of ketamine's antidepressant actions: Direct inhibition and disinhibition

A single, low dose of ketamine evokes antidepressant actions in depressed patients and in patients with treatment-resistant depression (TRD). Unlike classic antidepressants, which regulate monoamine neurotransmitter systems, ketamine is an antagonist of the N-methyl-D-aspartate (NMDA) family of glutamate receptors. The effectiveness of NMDAR antagonists in TRD unveils a new set of targets for therapeutic intervention in major depressive disorder (MDD) and TRD. However, a better understanding of the cellular mechanisms underlying these effects is required for guiding future therapeutic strategies, in order to minimize side effects and prolong duration of efficacy. Here we review the evidence for and against two hypotheses that have been proposed to explain how NMDAR antagonism initiates protein synthesis and increases excitatory synaptic drive in corticolimbic brain regions, either through selective antagonism of inhibitory interneurons and cortical disinhibition, or by direct inhibition of cortical pyramidal neurons. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Immunogold characteristics of VGLUT3-positive GABAergic nerve terminals suggest corelease of glutamate

There is compelling evidence that glutamate can act as a cotransmitter in the mammalian brain. Interestingly, the third vesicular glutamate transporter (VGLUT3) is primarily found in neurons that were anticipated to be nonglutamatergic. Whereas the function of VGLUT3 in acetylcholinergic and serotoninergic neurons has been elucidated, the role of VGLUT3 in neurons releasing gamma-aminobutyric acid (GABA) is not settled. We have previously shown that VGLUT3 is found together with the vesicular GABA transporter (VIAAT) on synaptic vesicle membranes in the hippocampus. Now we provide novel electron microscopic data from the rat hippocampus suggesting that glutamate is enriched in inhibitory nerve terminals containing VGLUT3 compared to those lacking VGLUT3. The opposite was found for GABA; VGLUT3-positive inhibitory terminals contained lower density of GABA labeling compared to VGLUT3-negative inhibitory terminals. In addition, semiquantitative confocal immunofluorescence showed that N-methyl-D-aspartate (NMDA)-receptor labeling was present more frequently in VGLUT3-positive/VIAAT-positive synapses versus in VGLUT3-negative/VIAAT-positive synapses. Electron microscopic immunogold data further suggest that NMDA receptors are enriched in VGLUT3 containing inhibitory terminals. Our data reveal new chemical characteristics of a subset of GABAergic interneurons in the hippocampus. The analyses suggest that glutamate is coreleased with GABA from hippocampal basket cell-synapses to act on NMDA receptors.

Pyramidal cell selective ablation of N-methyl-D-aspartate receptor 1 causes increase in cellular and network excitability

BACKGROUND

Neuronal activity at gamma frequency is impaired in schizophrenia (SZ) and is considered critical for cognitive performance. Such impairments are thought to be due to reduced N-methyl-D-aspartate receptor (NMDAR)-mediated inhibition from parvalbumin interneurons, rather than a direct role of impaired NMDAR signaling on pyramidal neurons. However, recent studies suggest a direct role of pyramidal neurons in regulating gamma oscillations. In particular, a computational model has been proposed in which phasic currents from pyramidal cells could drive synchronized feedback inhibition from interneurons. As such, impairments in pyramidal neuron activity could lead to abnormal gamma oscillations. However, this computational model has not been tested experimentally and the molecular mechanisms underlying pyramidal neuron dysfunction in SZ remain unclear.

METHODS

In the present study, we tested the hypothesis that SZ-related phenotypes could arise from reduced NMDAR signaling in pyramidal neurons using forebrain pyramidal neuron specific NMDA receptor 1 knockout mice.

RESULTS

The mice displayed increased baseline gamma power, as well as sociocognitive impairments. These phenotypes were associated with increased pyramidal cell excitability due to changes in inherent membrane properties. Interestingly, mutant mice showed decreased expression of GIRK2 channels, which has been linked to increased neuronal excitability.

CONCLUSIONS

Our data demonstrate for the first time that NMDAR hypofunction in pyramidal cells is sufficient to cause electrophysiological, molecular, neuropathological, and behavioral changes related to SZ.

Theta-burst stimulation: a new form of TMS treatment for depression?

Major depressive disorder (MDD) is a common debilitating condition where only one third of patients achieve remission after the first antidepressant treatment. Inadequate efficacy and adverse effects of current treatment strategies call for more effective and tolerable treatment options. Transcranial magnetic stimulation (TMS) is a noninvasive approach to manipulate brain activity and alter cortical excitability. There has been more than 15 years of research on the use of repetitive form of TMS (rTMS) for the treatment of patients with depression, which has shown it to be an effective antidepressant treatment. Even though rTMS treatment has shown efficacy in treating depression, there is a high degree of interindividual variability in response. A newer form of rTMS protocol, known as theta-burst stimulation (TBS), has been shown to produce similar if not greater effects on brain activity than standard rTMS. TBS protocols have a major advantage over standard rTMS approaches in their reduced administration duration. Conventional rTMS procedures last between 20 and 45 min, as compared to TBS paradigms that require 1 to 3 min of stimulation. Recently, a small number of studies have suggested that TBS has similar or better efficacy in treating depression compared to rTMS. Optimization, identification of response predictors, and clarification of neurobiological mechanisms of TBS is required if it is to be further developed as a less time intensive, safe, and effective treatment for MDD.

Long-term depression is differentially expressed in distinct lamina of hippocampal CA1 dendrites

Information storage in CA1 hippocampal pyramidal neurons is compartmentalized in proximal vs. distal apical dendrites, cell bodies, and basal dendrites. This compartmentalization is thought to be essential for synaptic integration. Differences in the expression of long-term potentiation (LTP) in each of these compartments have been described, but less is known regarding potential differences in long-term depression (LTD). Here, to directly compare LTD expression in each compartment and to bypass possible differences in input-specificity and stimulation of presynaptic inputs, we used global application of NMDA to induce LTD. We then examined LTD expression in each dendritic sub-region—proximal and distal apical, and basal dendrites—and in cell bodies. Interestingly, we found that distal apical dendrites exhibited the greatest magnitude of LTD of all areas tested and this LTD was maintained, whereas LTD in proximal apical dendrites was not maintained. In basal dendrites, LTD was also maintained, but the magnitude of LTD was less than in distal apical dendrites. Blockade of inhibition blocked LTD maintenance in both distal apical and basal dendrites. Population spikes recorded from the cell body layer correlated with apical dendrite field EPSP (fEPSP), where LTD was maintained in distal dendrites and decayed in proximal dendrites. On the other hand, LTD of basal dendrite fEPSPs was maintained but population spike responses were not. Thus E-S coupling was distinct in basal and apical dendrites. Our data demonstrate cell autonomous differential information processing in somas and dendritic sub-regions of CA1 pyramidal neurons in the hippocampus, where LTD expression is intrinsic to distinct dendritic regions, and does not depend on the nature of stimulation and input specificity.

Enhancement of tonic and phasic GABAergic currents following nitric oxide synthase inhibition in hippocampal CA1 pyramidal neurons

Nitric oxide (NO) is involved in synaptic plasticity in the hippocampus through different presynaptic and postsynaptic mechanisms that include the modulation of the GABAergic neurotransmission. Inhibitory synapses on hippocampal pyramidal neurons are known to possess the molecular machinery for retrograde NO-signaling, but the modulation of GABAARs function by NO in these neurons and the mechanisms of action involved have not been fully characterized. Here we show that suppression of the endogenous NO generation by the nitric oxide synthase (NOS) inhibitor L-NAME produces significant and reversible increases in the magnitude of both tonic and phasic GABAergic currents in CA1 hippocampal pyramidal neurons. GABA-evoked chloride currents were measured in the presence or absence of L-NAME using whole-cell patch-clamp recordings in acute hippocampal slices from young adult mice. Enhancement of the tonic GABA responses induced by L-NAME was insensitive to TTX and decreased by co-incubation with the NO donor DEA/NO. Applications of DEA/NO alone did not produce significant effects on tonic GABA responses. L-NAME treatment also increased the amplitude of phasic GABAergic currents evoked by GABA-puffs. Our results indicate that the extent of tonic and phasic inhibition mediated by GABAA receptors in CA1 hippocampal pyramidal neurons is affected by endogenous NO production.

Metabolomic Analysis Provides Insights on Paraquat-Induced Parkinson-Like Symptoms in Drosophila melanogaster

Paraquat (PQ) exposure causes degeneration of the dopaminergic neurons in an exposed organism while altered metabolism has a role in various neurodegenerative disorders. Therefore, the study presented here was conceived to depict the role of altered metabolism in PQ-induced Parkinson-like symptoms and to explore Drosophila as a potential model organism for such studies. Metabolic profile was generated in control and in flies that were fed PQ (5, 10, and 20 mM) in the diet for 12 and 24 h concurrent with assessment of indices of oxidative stress, dopaminergic neurodegeneration, and behavioral alteration. PQ was found to significantly alter 24 metabolites belonging to different biological pathways along with significant alterations in the above indices. In addition, PQ attenuated brain dopamine content in the exposed organism. The study demonstrates that PQ-induced alteration in the metabolites leads to oxidative stress and neurodegeneration in the exposed organism along with movement disorder, a phenotype typical of Parkinson-like symptoms. The study is relevant in the context of Drosophila and humans because similar alteration in the metabolic pathways has been observed in both PQ-exposed Drosophila and in postmortem samples of patients with Parkinsonism. Furthermore, this study provides advocacy towards the applicability of Drosophila as an alternate model organism for pre-screening of environmental chemicals for their neurodegenerative potential with altered metabolism.

PACAP induces plasticity at autonomic synapses by nAChR-dependent NOS1 activation and AKAP-mediated PKA targeting

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide found at synapses throughout the central and autonomic nervous system. We previously found that PACAP engages a selective G-protein coupled receptor (PAC1R) on ciliary ganglion neurons to rapidly enhance quantal acetylcholine (ACh) release from presynaptic terminals via neuronal nitric oxide synthase (NOS1) and cyclic AMP/protein kinase A (PKA) dependent processes. Here, we examined how PACAP stimulates NO production and targets resultant outcomes to synapses. Scavenging extracellular NO blocked PACAP-induced plasticity supporting a retrograde (post- to presynaptic) NO action on ACh release. Live-cell imaging revealed that PACAP stimulates NO production by mechanisms requiring NOS1, PKA and Ca(2+) influx. Ca(2+)-permeable nicotinic ACh receptors composed of α7 subunits (α7-nAChRs) are potentiated by PKA-dependent PACAP/PAC1R signaling and were required for PACAP-induced NO production and synaptic plasticity since both outcomes were drastically reduced following their selective inhibition. Co-precipitation experiments showed that NOS1 associates with α7-nAChRs, many of which are perisynaptic, as well as with heteromeric α3*-nAChRs that generate the bulk of synaptic activity. NOS1-nAChR physical association could facilitate NO production at perisynaptic and adjacent postsynaptic sites to enhance focal ACh release from juxtaposed presynaptic terminals. The synaptic outcomes of PACAP/PAC1R signaling are localized by PKA anchoring proteins (AKAPs). PKA regulatory-subunit overlay assays identified five AKAPs in ganglion lysates, including a prominent neuronal subtype. Moreover, PACAP-induced synaptic plasticity was selectively blocked when PKA regulatory-subunit binding to AKAPs was inhibited. Taken together, our findings indicate that PACAP/PAC1R signaling coordinates nAChR, NOS1 and AKAP activities to induce targeted, retrograde plasticity at autonomic synapses. Such coordination has broad relevance for understanding the control of autonomic synapses and consequent visceral functions.

NO regulates the strength of synaptic inputs onto hippocampal CA1 neurons via NO-GC1/cGMP signalling

GABAergic interneurons are the predominant source of inhibition in the brain that coordinate the level of excitation and synchronization in neuronal circuitries. However, the underlying cellular mechanisms are still not fully understood. Here we report nitric oxide (NO)/NO-GC1 signalling as an important regulatory mechanism of GABAergic and glutamatergic synaptic transmission in the hippocampal CA1 region. Deletion of the NO receptor NO-GC1 induced functional alterations, indicated by a strong reduction of spontaneous and evoked inhibitory postsynaptic currents (IPSCs), which could be compensated by application of the missing second messenger cGMP. Moreover, we found a general impairment in the strength of inhibitory and excitatory synaptic inputs onto CA1 pyramidal neurons deriving from NO-GC1KO mice. Finally, we disclosed one subpopulation of GABAergic interneurons, fast-spiking interneurons, that receive less excitatory synaptic input and consequently respond with less spike output after blockage of the NO/cGMP signalling pathway. On the basis of these and previous findings, we propose NO-GC1 as the major NO receptor which transduces the NO signal into cGMP at presynaptic terminals of different neuronal subtypes in the hippocampal CA1 region. Furthermore, we suggest NO-GC1-mediated cGMP signalling as a mechanism which regulates the strength of synaptic transmission, hence being important in gating information processing between hippocampal CA3 and CA1 region.

Glycine transporters as novel therapeutic targets in schizophrenia, alcohol dependence and pain

Glycine transporters are endogenous regulators of the dual functions of glycine, which acts as a classical inhibitory neurotransmitter at glycinergic synapses and as a modulator of neuronal excitation mediated by NMDA (N-methyl-D-aspartate) receptors at glutamatergic synapses. The two major subtypes of glycine transporters, GlyT1 and GlyT2, have been linked to the pathogenesis and/or treatment of central and peripheral nervous system disorders, including schizophrenia and related affective and cognitive disturbances, alcohol dependence, pain, epilepsy, breathing disorders and startle disease (also known as hyperekplexia). This Review examines the rationale for the therapeutic potential of GlyT1 and GlyT2 inhibition, and surveys the latest advances in the biology of glycine reuptake and transport as well as the drug discovery and clinical development of compounds that block glycine transporters.

Modulation of human corticospinal excitability by paired associative stimulation

Paired Associative Stimulation (PAS) has come to prominence as a potential therapeutic intervention for the treatment of brain injury/disease, and as an experimental method with which to investigate Hebbian principles of neural plasticity in humans. Prototypically, a single electrical stimulus is directed to a peripheral nerve in advance of transcranial magnetic stimulation (TMS) delivered to the contralateral primary motor cortex (M1). Repeated pairing of the stimuli (i.e., association) over an extended period may increase or decrease the excitability of corticospinal projections from M1, in manner that depends on the interstimulus interval (ISI). It has been suggested that these effects represent a form of associative long-term potentiation (LTP) and depression (LTD) that bears resemblance to spike-timing dependent plasticity (STDP) as it has been elaborated in animal models. With a large body of empirical evidence having emerged since the cardinal features of PAS were first described, and in light of the variations from the original protocols that have been implemented, it is opportune to consider whether the phenomenology of PAS remains consistent with the characteristic features that were initially disclosed. This assessment necessarily has bearing upon interpretation of the effects of PAS in relation to the specific cellular pathways that are putatively engaged, including those that adhere to the rules of STDP. The balance of evidence suggests that the mechanisms that contribute to the LTP- and LTD-type responses to PAS differ depending on the precise nature of the induction protocol that is used. In addition to emphasizing the requirement for additional explanatory models, in the present analysis we highlight the key features of the PAS phenomenology that require interpretation.

Role of a hippocampal SRC-family kinase-mediated glutamatergic mechanism in drug context-induced cocaine seeking

Glutamatergic neurotransmission in the dorsal hippocampus (DH) is necessary for drug context-induced reinstatement of cocaine-seeking behavior in an animal model of drug relapse. Furthermore, in vitro studies suggest that the Src family of tyrosine kinases critically regulates glutamatergic cellular functions within the DH. Thus, Src-family kinases in the DH may similarly control contextual cocaine-seeking behavior. To test this hypothesis, rats were trained to lever press for un-signaled cocaine infusions in a distinct context followed by extinction training in a different context. Cocaine-seeking behavior (non-reinforced active lever pressing) was then assessed in the previously cocaine-paired and extinction contexts after AP5 (N-methyl-D-aspartate glutamate (NMDA) receptor (NMDAR) antagonist; 0.25 or 2.5 μg/0.5 μl/hemisphere), PP2 (Src-family kinase inhibitor; 6.25 or 62.5 ng/0.5 μl/hemisphere), Ro25-6981 (NR2B subunit-containing NMDAR antagonist; 0.2 or 2 μg/0.5 μl/hemisphere), or vehicle administration into the DH. Administration of AP5, PP2, or Ro25-6981 into the DH dose-dependently impaired drug context-induced reinstatement of cocaine-seeking behavior relative to vehicle, without altering instrumental behavior in the extinction context or food-reinforced instrumental responding and general motor activity in control experiments. Cocaine-seeking behavior during the first 20 min of the test session in the cocaine-paired context was associated with an increase in NR2B subunit activation, and intra-DH PP2 pretreatment disrupted this relationship. Together, these findings suggest that Src-family kinase activation, NMDAR stimulation, and likely Src-family kinase-mediated NR2B subunit-containing NMDAR activation in the DH are necessary for incentive motivational and/or memory processes that promote contextual cocaine-seeking behavior.

The role of nitric oxide in pre-synaptic plasticity and homeostasis

Since the observation that nitric oxide (NO) can act as an intercellular messenger in the brain, the past 25 years have witnessed the steady accumulation of evidence that it acts pre-synaptically at both glutamatergic and GABAergic synapses to alter release-probability in synaptic plasticity. NO does so by acting on the synaptic machinery involved in transmitter release and, in a coordinated fashion, on vesicular recycling mechanisms. In this review, we examine the body of evidence for NO acting as a retrograde factor at synapses, and the evidence from in vivo and in vitro studies that specifically establish NOS1 (neuronal nitric oxide synthase) as the important isoform of NO synthase in this process. The NOS1 isoform is found at two very different locations and at two different spatial scales both in the cortex and hippocampus. On the one hand it is located diffusely in the cytoplasm of a small population of GABAergic neurons and on the other hand the alpha isoform is located discretely at the post-synaptic density (PSD) in spines of pyramidal cells. The present evidence is that the number of NOS1 molecules that exist at the PSD are so low that a spine can only give rise to modest concentrations of NO and therefore only exert a very local action. The NO receptor guanylate cyclase is located both pre- and post-synaptically and this suggests a role for NO in the coordination of local pre- and post-synaptic function during plasticity at individual synapses. Recent evidence shows that NOS1 is also located post-synaptic to GABAergic synapses and plays a pre-synaptic role in GABAergic plasticity as well as glutamatergic plasticity. Studies on the function of NO in plasticity at the cellular level are corroborated by evidence that NO is also involved in experience-dependent plasticity in the cerebral cortex.

Differential modulation of corticospinal excitability by different current densities of anodal transcranial direct current stimulation

Background

Novel non-invasive brain stimulation techniques such as transcranial direct current stimulation (tDCS) have been developed in recent years. TDCS-induced corticospinal excitability changes depend on two important factors current intensity and stimulation duration. Despite clinical success with existing tDCS parameters, optimal protocols are still not entirely set.

Objective/hypothesis

The current study aimed to investigate the effects of four different anodal tDCS (a-tDCS) current densities on corticospinal excitability.

Methods

Four current intensities of 0.3, 0.7, 1.4 and 2 mA resulting in current densities (CDs) of 0.013, 0.029, 0.058 and 0.083 mA/cm² were applied on twelve right-handed (mean age 34.5±10.32 yrs) healthy individuals in different sessions at least 48 hours apart. a-tDCS was applied continuously for 10 minute, with constant active and reference electrode sizes of 24 and 35 cm² respectively. The corticospinal excitability of the extensor carpi radialis muscle (ECR) was measured before and immediately after the intervention and at 10, 20 and 30 minutes thereafter.

Results

Post hoc comparisons showed significant differences in corticospinal excitability changes for CDs of 0.013 mA/cm² and 0.029 mA/cm² (P = 0.003). There were no significant differences between excitability changes for the 0.013 mA/cm² and 0.058 mA/cm² (P = 0.080) or 0.013 mA/cm² and 0.083 mA/cm² (P = 0.484) conditions.

Conclusion

This study found that a-tDCS with a current density of 0.013 mA/cm² induces significantly larger corticospinal excitability changes than CDs of 0.029 mA/cm². The implication is that might help to avoid applying unwanted amount of current to the cortical areas.

Neonatal NMDA receptor blockade alters anxiety- and depression-related behaviors in a sex-dependent manner in mice

There is increasing evidence that N-methyl-D-aspartate (NMDA) receptor blockade in the neonatal period has a long-lasting influence on brain and behavior development and has been linked to an increased risk for neuropsychiatric disorders in later life. We sought to determine whether postnatal NMDA receptor blockade can affect normal development of body weight, corticosterone levels, anxiety- and depression-related behaviors in male and female mice in adulthood. For this purpose, male and female NMRI mice were treated with either saline or phencyclidine (PCP; 5 and 10 mg/kg, s.c.) on postnatal days (PND) 7, 9, and 11, and then subjected to different behavioral tests, including open field, elevated plus-maze, elevated zero-maze, light-dark box, tail suspension test and forced swimming test in adulthood. The results indicated that neonatal PCP treatment reduced body weight during neonatal and adulthood periods, and did not alter baseline corticosterone levels in both male and female mice. Moreover, this study obtained some experimental evidence showing the PCP at dose of 10 mg/kg increases stress-induced corticosterone levels, anxiety- and depression-related behaviors in males, while decreasing levels of anxiety without any significant effect on depression in female mice in adulthood. These data support the argument that neonatal NMDA receptor blockade can lead to behavioral abnormalities and psychiatric diseases in adulthood. Collectively, our findings suggest that neonatal exposure to PCP may have profound effects on the development of anxiety- and depression-related behaviors in a sex- and dose-dependent manner in mice.

Allopregnanolone and puberty: modulatory effect on glutamate and GABA release and expression of 3α-hydroxysteroid oxidoreductase in the hypothalamus of female rats

The hypothalamic release of glutamate and GABA regulates neurosecretory functions that may control the onset of puberty. This release may be influenced by neurosteroids such as allopregnanolone. Using superfusion experiments we examined the role of allopregnanolone on the K(+)-evoked and basal [(3)H]-glutamate and [(3)H]-GABA release from mediobasal hypothalamus and anterior preoptic area in prepubertal, vaginal opening and pubertal (P) rats and evaluated its modulatory effect on GABAA and NMDA (N-methyl-d-aspartic acid) receptors. Also, we examined the hypothalamic activity and mRNA expression of 3α-hydroxysteroid oxidoreductase (3α-HSOR) - enzyme that synthesizes allopregnanolone - using a spectrophotometric method and RT-PCR, respectively. Allopregnanolone increased both the K(+)-evoked [(3)H]-glutamate and [(3)H]-GABA release in P rats, being the former effect mediated by the modulation of NMDA receptors - as was reverted by Mg(2+) and by the NMDA receptor antagonist AP-7 and the latter by the modulation of NMDA and GABAA receptors - as was reverted by Mg(2+) and the GABAA receptor antagonist bicuculline. The neurosteroid also increased the basal release of [(3)H]-glutamate in VO rats in an effect that was dependent on the modulation of NMDA receptors as was reverted by Mg(2+). On the other hand we show that allopregnanolone reduced the basal release of [(3)H]-GABA in P rats although we cannot elucidate the precise mechanism by which the neurosteroid exerted this latter effect. The enzymatic activity and the mRNA expression of 3α-HSOR were both increased in P rats regarding the other two studied stages of sexual development. These results suggest an important physiological function of allopregnanolone in the hypothalamus of the P rat where it might be involved in the 'fine tuning' of neurosecretory functions related to the biology of reproduction of the female rats.