Inhibitory Transmission Research Articles

Protein kinase C mediates neuropeptide Y-induced reduction in inhibitory neurotransmission in the lateral habenula

Neuropeptide Y (NPY) is one of peptide neuromodulators, well known for orexigenic, anxiolytic and antidepressant effects. We previously reported that NPY decreases GABAergic transmission in the lateral habenula (LHb). In the current study, we aim to investigate the underlying signaling pathways that mediate inhibitory action of NPY in the LHb by employing whole-cell patch clamp recording with pharmacological interventions. Here, we revealed that Y1 receptors (Y1Rs) but not Y2Rs mediate NPY-induced decrease of GABAergic transmission in the LHb. Surprisingly, NPY-induced decrease of inhibitory transmission in the LHb was not dependent on adenylyl cyclase (AC)/protein kinase A (PKA)-dependent pathway as reported in other brain areas. Instead, pharmacological blockade of phospholipase C (PLC) or protein kinase C (PKC) activity abolished the decrease of GABAergic transmission by NPY in the LHb. Our findings suggest that Y1Rs in the LHb may trigger the activation of PLC/PKC-dependent pathway but not the classical AC/PKA-dependent pathway to decrease inhibitory transmission of the LHb.

Inhibitory actions of borneol on the substantia gelatinosa neurons of the trigeminal subnucleus caudalis in mice.

The substantia gelatinosa (SG) of the trigeminal subnucleus caudalis (Vc) is the first relay site for the orofacial nociceptive inputs via the thin myelinated Aδ and unmyelinated C primary afferent fibers. Borneol, one of the valuable time-honored herbal ingredients in traditional Chinese medicine, is a popular treatment for anxiety, anesthesia, and antinociception. However, to date, little is known as to how borneol acts on the SG neurons of the Vc. To close this gap, the whole-cell patch-clamp technique was applied to elucidate the antinociceptive mechanism responding for the actions of borneol on the SG neurons of the Vc in mice. In the voltage-clamp mode, holding at –60 mV, the borneol-induced non-desensitizing inward currents were not affected by tetrodotoxin, a voltage-gated Na+ channel blocker, 6-cyano-7-nitro-quinoxaline-2,3-dione, a non-N-methyl-d-aspartate (NMDA) glutamate receptor antagonist and DL-2-amino-5-phosphonopentanoic acid, an NMDA receptor antagonist. However, borneol-induced inward currents were partially decreased in the presence of picrotoxin, a γ-aminobutyric acid (GABA)A receptor antagonist, or strychnine, a glycine receptor antagonist, and was almost suppressed in the presence of picrotoxin and strychnine. Though borneol did not show any effect on the glycine-induced inward currents, borneol enhanced GABA-mediated responses. Beside, borneol enhanced the GABA-induced hyperpolarization under the current-clamp mode. Altogether, we suggest that borneol contributes in part toward mediating the inhibitory GABA and glycine transmission on the SG neurons of the Vc and may serve as an herbal therapeutic for orofacial pain ailments.

TORC1 selectively regulates synaptic maturation and input convergence in the developing visual system.

Newly synthesized proteins support the development of functional neural circuits and previous work has suggested that dysregulated translation mediates certain forms of autism spectrum disorder (ASD). Here, we investigated the role of Target of Rapamycin Complex 1 (TORC1) in synaptic and dendritic development in vivo in the retinotectal system of Xenopus laevis tadpoles. We found that TORC1 signaling regulates dendritic growth and branching and that acute over-activation of TORC1 by Rheb overexpression drove enhanced maturation of excitatory synapses by recruiting AMPA receptors. Interestingly, TORC1 over-activation did not affect inhibitory transmission, resulting in a significant imbalance in the excitatory-to-inhibitory ratio. Rheb overexpression also enlarged excitatory visual input fields in tectal neurons, consistent with dysregulation of retinotopic input refinement and integration of the cell into the circuit. In contrast to other reports that mainly found impairments in synaptic inhibition using broad systemic deletion or mutation of TORC1 regulatory proteins, our findings from acute, local manipulation of TORC1 reveal its critical role in selectively regulating the number and maturity of excitatory, but not inhibitory, synapses in the developing brain.

David Roderick Curtis. 3 June 1927—11 December 2017

David Curtis was a pioneer in the identification of excitatory and inhibitory transmitters released at synapses in the central nervous system. He made major contributions to the identification of gamma-amino butyric acid (GABA) and glycine as inhibitory transmitters released at inhibitory synapses. His work laid the foundation for the subsequent acceptance that l -glutamate was the major excitatory transmitter. David's scientific work led to him receiving many accolades and honours, including fellowships of the Australian Academy of Sciences and the Royal Society and a Companion of the Order of Australia. Note: This memoir was commissioned by the Historical Records of Australian Science and is published here with minor amendments. It was published in June 2020 and is available at https://doi.org/10.1071/HR19016 .

Transient activation of the Notch-her15.1 axis plays an important role in the maturation of V2b interneurons.

In the vertebrate ventral spinal cord, p2 progenitors give rise to two interneuron subtypes: excitatory V2a interneurons and inhibitory V2b interneurons. In the differentiation of V2a and V2b cells, Notch signaling promotes V2b fate at the expense of V2a fate. Later, V2b cells extend axons along the ipsilateral side of the spinal cord and express the inhibitory transmitter GABA. Notch signaling has been reported to inhibit the axonal outgrowth of mature neurons of the central nervous system; however, it remains unknown how Notch signaling modulates V2b neurite outgrowth and maturation into GABAergic neurons. Here, we have investigated neuron-specific Notch functions regarding V2b axon growth and maturation into zebrafish GABAergic neurons. We found that continuous neuron-specific Notch activation enhanced V2b fate determination but inhibited V2b axonal outgrowth and maturation into GABAergic neurons. These results suggest that Notch signaling activation is required for V2b fate determination, whereas its downregulation at a later stage is essential for V2b maturation. Accordingly, we found that a Notch signaling downstream gene, her15.1, showed biased expression in V2 linage cells and downregulated expression during the maturation of V2b cells, and continuous expression of her15.1 repressed V2b axogenesis. Our data suggest that spatiotemporal control of Notch signaling activity is required for V2b fate determination, maturation and axogenesis.

Postnatal Overnutrition Induces Changes in Synaptic Transmission to Leptin Receptor-Expressing Neurons in the Arcuate Nucleus of Female Mice.

The adipocyte-derived hormone leptin is a potent neurotrophic factor that contributes to the neural plasticity and development of feeding circuitry, particularly in the arcuate nucleus of the hypothalamus (ARH). Postnatal overnutrition affects leptin secretion and sensitivity, but whether postnatal overnutrition produces changes in the development of the synaptic transmission to ARH neurons is currently unknown. We evaluated the excitatory and inhibitory currents to ARH leptin receptor (LepR)-expressing neurons in prepubertal, pubertal and adult female mice. The effects of postnatal overnutrition in the expression of genes that code ion channels subunits in the ARH were also evaluated. We observed that the transition from prepubertal to pubertal stage is characterized by a rise in both excitatory and inhibitory transmission to ARH LepR-expressing neurons in control mice. Postnatal overnutrition induces a further increase in the excitatory synaptic transmission in pubertal and adult animals, whereas the amplitude of inhibitory currents to ARH LepR-expressing cells was reduced. Postnatal overnutrition also contributes to the modulation of gene expression of N-methyl-D-aspartate, GABAB and ATP-sensitive potassium channel subunits in ARH. In summary, the synaptic transmission to ARH cells is profoundly influenced by postnatal overnutrition. Thus, increased adiposity during early postnatal period induces long-lasting effects on ARH cellular excitability.

The ins and outs of inhibitory synaptic plasticity: Neuron types, molecular mechanisms and functional roles

GABAergic interneurons are highly diverse, and their synaptic outputs express various forms of plasticity. Compelling evidence indicates that activity-dependent changes of inhibitory synaptic transmission play a significant role in regulating neural circuits critically involved in learning and memory and circuit refinement. Here, we provide an updated overview of inhibitory synaptic plasticity with a focus on the hippocampus and neocortex. To illustrate the diversity of inhibitory interneurons, we discuss the case of two highly divergent interneuron types, parvalbumin-expressing basket cells and neurogliaform cells, which support unique roles on circuit dynamics. We also present recent progress on the molecular mechanisms underlying long-term, activity-dependent plasticity of fast inhibitory transmission. Lastly, we discuss the role of inhibitory synaptic plasticity in neuronal circuits' function. The emerging picture is that inhibitory synaptic transmission in the CNS is extremely diverse, undergoes various mechanistically distinct forms of plasticity and contributes to a much more refined computational role than initially thought. Both the remarkable diversity of inhibitory interneurons and the various forms of plasticity expressed by GABAergic synapses provide an amazingly rich inhibitory repertoire that is central to a variety of complex neural circuit functions, including memory.

Prenatal stress-related alterations in synaptic transmission and 5-HT7 receptor-mediated effects in the rat dorsal raphe nucleus are ameliorated by the 5-HT7 receptor antagonist SB 269970.

Early life adversity exerts a detrimental influence on developing brain neuronal networks and its consequences may include mental health disorders. In rats, prenatal stress may lead to anxiety and depressive-like behavior in the offspring. Several lines of evidence implicated an involvement of prenatal stress in alterations of the brain serotonergic system functions, but the effects of prenatal stress on its core, the dorsal raphe nucleus (DRN), still remain incompletely understood. The present study was aimed at finding whether prenatal stress induces modifications in the glutamatergic and GABAergic inputs to DRN projection cells and whether it affects DRN 5-HT7 receptors, which modulate activity of these synapses. Prenatal stress resulted in an increase in basal frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and in a decrease in basal frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from putative projection neurons in DRN slices ex vivo. While there were no changes in the excitability of DRN projection neurons, the 5-HT7 receptor-mediated reduction in the sEPSC frequency and rise in the sIPSC frequency, seen in control rats, were largely absent in slices obtained from prenatally stressed rats. Repeated administration of SB 269970, a 5-HT7 receptor antagonist, resulted in a reversal of prenatal stress-induced alterations in 5-HT7 receptor-mediated effects on the sEPSC/sIPSC frequency. Moreover, the treatment reversed prenatal stress-induced alterations in basal excitatory transmission and partially reversed the effect of stress on basal inhibitory transmission in the DRN.

Obesity-Induced Astrocyte Dysfunction Impairs Heterosynaptic Plasticity in the Orbitofrontal Cortex

Overconsumption of palatable, energy dense food is considered a key driver of the obesity pandemic. The orbitofrontal cortex (OFC) is critical for reward valuation of gustatory signals, yet how the OFC adapts to obesogenic diets is poorly understood. Here, we show that extended access to a cafeteria diet impairs astrocyte glutamate clearance, which leads to a heterosynaptic depression of GABA transmission onto pyramidal neurons of the OFC. This decrease in GABA tone is due to an increase in extrasynaptic glutamate, which acts via metabotropic glutamate receptors to liberate endocannabinoids. This impaired the induction of endocannabinoid-mediated long-term plasticity. In obese rats, this cascade of synaptic impairments was rescued by restoring astrocyte glutamate transport with the nutritional supplement, N-acetylcysteine. Together our findings indicate that obesity targets astrocytes to disrupt the delicate balance between excitatory and inhibitory transmission in the OFC.

Increasing the interval between repeated anesthetic exposures reduces long-lasting synaptic changes in late post-natal mice.

While recent studies strongly suggest that a single, short anesthetic exposure does not affect neurodevelopment, the effects of multiple exposures remain unclear. Unfortunately, studying "multiple exposures" is challenging as it is an extremely heterogeneous descriptor comprising diverse factors. One potentially important, but unrecognized factor is the interval between anesthetic exposures. In order to evaluate the significance of interval, we exposed post-natal day 16, 17 mice to three sevoflurane exposures (2.5%, 1hr) with short (2hr) or long (24hr) intervals. Changes in synaptic transmission, plasticity, protein expression, and behavior were assessed in male and female mice. We discovered that short-interval exposures induced a female-dependent decrease in miniature inhibitory post-synaptic current (mIPSC) frequency 5days after the last exposure (control: 18.44±2.86Hz, sevoflurane:14.65±4.54Hz). Short-interval sevoflurane exposed mice also displayed long-term behavioral deficits at adult age (hypoactivity, anxiety). These behavioral changes were consistent with the sex-dependent changes in inhibitory transmission, as they were more robust in female mice. Although there was no change in learning and memory, short-interval sevoflurane exposures also impaired LTP in a non-sex-dependent manner (control: 171.10±26.90%, sevoflurane: 149.80±26.48 %). Most importantly, we were unable to find long-lasting consequences in mice that received long-interval sevoflurane exposures. Our study provides novel insights regarding the significance of the interval between multiple exposures, and also suggests that the neurotoxic effects of multiple anesthetic exposures may be reduced by simply increasing the interval between each exposure.

A capital role for the brain’s insula in the diverse fibromyalgia-associated symptoms

Unexplained yet persisting general and widespread non-articular musculoskeletal pain and the associated complaints, known as fibromyalgia (FM), is a common disorder with major social and economic impact. We postulate that in FM disturbance of neurotransmitter balances at the brain’s insula not only leads to aberrant pain processing but could also govern other associated symptoms. Symptoms might arise from central nervous system dysregulation mediated through an imbalance between the excitatory neurotransmitter glutamate and the inhibitory transmitter gamma-amino butyric acid. The insula could also have a leading role in the dysregulation of heart rate and blood pressure, bladder and bowel symptoms, and anxiety and sleep disturbances which are experienced by many FM patients. The presented hypothesis explains how the diverse FM-associated symptoms could be linked, and puts the brain’s insula forward as a possible therapeutic target to be further explored for FM.

Effect of psychotropic drugs on cortical excitability of patients with major depressive disorders: A transcranial magnetic stimulation study

Transcranial magnetic stimulation (TMS) can be used to evaluate the effects of pharmacological interventions. The aim of this study was to assess the impact of the selective serotonin reuptake inhibitor, sertraline, and the atypical antipsychotic drugs quetiapine and olanzapine, on cortical excitability in unmedicated patients with major depressive disorder (MDD). The study included 45 medication-free MDD patients diagnosed according to DSM V. They were divided randomly into three groups who received a single oral dose of one of the three drugs sertraline (50 mg), quetiapine (100 mg) and olanzapine (10 mg). Psychological evaluation was conducted using the Mini-Mental State Examination (MMSE) and Beck Depression Inventory Scale (BDI). Resting and active motor thresholds (rMT and aMT) together with contralateral and ipsilateral cortical silent periods (cSP, and iSP) were measured for each participant before and at the time of maximum concentration of drug intake. There was significant increase in excitability of motor cortex after sertraline without changes in GABAB neurotransmission. Quetiapine and olanzapine potentiated inhibitory GABAB neurotransmission (prolongation of cSP); olanzapine additionally prolonged the iSP. Thus TMS can differentiate between the impact of different psychotropic drugs on excitatory and inhibitory transmission in motor cortex.

Effects Of Aerobic Exercise On The Nicotine Addiction Induced Inhibitory Synaptic Plasticity In The Vta

PURPOSE: Disinhibition of the ventral tegmental area (VTA) dopamine neurons has been implicated in nicotine addiction. This study aimed to investigate the effect of aerobic exercise on the nicotine addiction behavior in mice, and the effect on GABAergic transmission and dopaminergic activity in the VTA. METHODS: 2 months old male C57BL/6J mice were randomly divided into sedentary + saline group (SS), sedentary + nicotine group (SN), exercise + saline group (ES) and exercise + nicotine group (EN) respectively. The ES and EN groups were made to run on the treadmill for 1 hour per one day, five times a week, for 12 weeks. The exercise workload consisted of running at a speed of 12 m/min for the first 10 min, 15 m/min for the last 50 min, with 0% grade of inclination. The conditioned place preference (CPP) assay was used to evaluate nicotine addiction related behavior. In the CPP assay, SN and EN mice were given an intraperitoneal (i.p.) injection of nicotine (0.5 mg/kg) while SS and ES mice were given an injection of saline. Patch clamp was used to investigate the dopamine neuron excitability and GABAergic transmission in the VTA. Immunofluorescence was used to detect the expression of tyrosine hydroxylase (TH) in the VTA. RESULTS: 8 weeks of treadmill exercise decreased nicotine exposure induced CPP expression (CPP score, SS, -33.69±16.57; SN, 121.51±14.53; ES, -21.72±15.12; EN, 66.34±15.12; SS vs. SN, p<0.001; EN vs. SN, p<0.05, two-way ANOVA). Exercise decreased nicotine CPP induced dopamine neuron hyperexcitability (SS, 3.36±0.45; SN, 6.63±0.56; ES, 3.22±0.48; EN, 4.60±0.18; SS vs. SN, p<0.001; EN vs. SN, p<0.01, two-way ANOVA). Exercise decreased nicotine CPP induced increase of TH expression in the VTA (relative fluorescence intensity, SS 1.00; SN, 2.03±0.15; ES, 1.38±0.06; EN, 1.04±0.07; SS vs. SN, p<0.001; EN vs. SN, p<0.001, two-way ANOVA). Exercise restored nicotine CPP induced impairment of GABA transmission in the VTA (sIPSCs frequency, % as baseline, SS, 154.62±6.86%; SN, 102.19±6.20; ES, 141.36±5.84; EN, 121.82±4.41; SS vs. SN, p<0.001; EN vs. SN, p<0.05, two-way ANOVA). CONCLUSIONS: Aerobic exercise restores nicotine addiction induced VTA dopamine neuron hyperexcitability by enhancing the inhibitory transmission. Supported by the China Postdoctoral Science Foundation (2018M641260, 2019T120067).

Roscovitine differentially facilitates cerebellar glutamatergic and GABAergic neurotransmission by enhancing Cav 2.1 channel-mediated multivesicular release.

Synaptic vesicle exocytosis is triggered by Ca2+ influx through several subtypes of voltage-gated calcium channels in the presynaptic terminal. We previously reported that paired-pulse stimulation at brief intervals increases Cav 2.1 (P/Q-type) channel-mediated multivesicular release (MVR) at glutamatergic synapses between granule cells (GCs) and molecular layer interneurons (MLIs) in rat cerebellar slices. However, it has yet to be determined how Cav 2 channel subtypes take part in MVR in single axon terminal. This study therefore aimed at examining the effects of roscovitine on different types of cerebellar synapses that make contacts with Purkinje cells (PCs), because this compound has been shown to enhance Cav 2.1 channel-mediated MVR at GC-MLI synapses. Bath application of roscovitine profoundly increased the amplitude of excitatory postsynaptic currents (EPSCs) at GC-PC synapses by a presynaptic mechanism as previously observed at GC-MLI synapses, whereas it caused a marginal effect on climbing fiber-mediated EPSCs in PCs. At MLI-PC synapses, roscovitine increased both the amplitude and decay time of inhibitory postsynaptic currents (IPSCs) by enhancing multivesicular GABA release. When extracellular Ca2+ concentration ([Ca2+ ]e ) decreased, roscovitine became less effective in increasing GC-PC EPSCs. By contrast, roscovitine was able to augment MLI-PC IPSCs in the low [Ca2+ ]e . The Cav 2.1 channel blocker ω-agatoxin IVA suppressed the roscovitine-induced facilitatory actions on both GC-PC EPSCs and MLI-PC IPSCs. These results demonstrate that roscovitine enhances MVR at the GC-PC excitatory synapses in a manner dependent on the driving force of Cav 2.1 channel-mediated Ca2+ influx into the nerve terminal, while it also facilitates MLI-PC inhibitory transmission via Ca2+ -insensitive mechanisms.

Diminished enteric neuromuscular transmission in the distal colon following experimental spinal cord injury

Neurogenic bowel following spinal cord injury (SCI) leads to decreased colonic motility, remodeling of the neuromuscular compartment and results in chronic evacuation difficulties. The distal colon of the rat serves a dual role for fluid absorption and storage that is homologous to the descending colon of humans. Dysmotility of the descending colon is one component of neurogenic bowel. We investigated the integrity of the enteric neuromuscular transmission responsible for the generation of excitatory and inhibitory junction potentials (EJPs and IJPs, respectively) in the distal colon of rats. We previously demonstrated a chronic reduction in colonic enteric neurons from rats with acute and chronic high-thoracic (T3) SCI and hypothesized that neurogenic bowel following T3-SCI results from diminished enteric neuromuscular transmission.Immunohistochemical labeling for myenteric neuronal nitric oxide synthase (nNOS) and choline acetyltransferase (ChAT) neurons demonstrated a significant loss of presumptive nitric oxide (NO) and acetylcholine (ACh) immunoreactive neurons in both 3-day and 3-week injured animals. Colonic neuromuscular transmission in response to transmural electrical stimulation of the colon was significantly reduced 3-days and 3-weeks following SCI in male rats. Specifically, cholinergic-mediated excitatory junction potentials (EJPs) and nitrergic-mediated slow inhibitory junction potentials (IJPs) were significantly reduced while ATP-mediated fast IJPs remained unaffected.We conclude that a reduction in excitatory and inhibitory enteric neuromuscular transmission contributes to neurogenic bowel observed following SCI, and that these loss-of-function changes involve enteric-mediated cholinergic and nitrergic pathways.

M140. WHAT HAPPENS IN THE BRAIN A FEW SECONDS BEFORE THE ONSET AND OFFSET OF AN HALLUCINATORY EPISODE?

BackgroundAuditory verbal hallucinations (AVH), the sense of “hearing voices” in the absence of a corresponding auditory source, are a major symptom in schizophrenia. Hemodynamic and electrophysiology studies have shown aberrant activation in fronto-temporal speech and language areas (Curcic-Blake et al., 2017 for review) – indicating neurobiological mediators relating to onset and duration of AVH episodes. However, unanswered is the question of specific neuronal precursors to the spontaneous cessation of an AVH episode. We therefore studied what happens in the brain a few seconds before both the onset and offset of an AVH episode.MethodsStructural and functional MRI data were collected from 44 patients diagnosed with a schizophrenia spectrum disorder, at three sites (University of Bergen, Norway, n=12; Medical University of Plovdiv, Bulgaria, n=13; Utrecht University Medical Center, Netherlands, n=19, total 25m, 19f, age 37.9+/-13.2 years). Symptom severity was assessed with the PANSS; mean total (+/-SD) 64.9+/-16.9; P3 hallucinatory behaviour 4.6+/-1.1. Additional assessments (not reported), scanner hardware and particulars of the MR acquisition varied between sites.Functional data were collected with a “symptom-capture” paradigm (Sommer et al., 2008), where subjects were instructed to press a button when a hallucinatory episode began, and another when the episode ended. Functional data were processed using an FSL FEAT pipeline, with additional filtering using the ICA_AROMA method. Data were then subjected to statistical analysis, contrasting hallucinatory with non-hallucinatory periods; higher-level analysis allowed comparison with findings from previous meta-analyses (Kompus et al. 2011, Jardri et al., 2011).A number of regions of interest were selected based on an initial PCA, in addition to regions nominated in the meta-analyses. For each region, for each start- and end-of-hallucination event, a time-course was extracted for the interval t=-10 to +15sec with respect to button-press onset. Time-courses were aligned and group-averaged (rejecting outliers iteratively) to yield a model time-course for start- and end- hallucinatory events in each region. Permutation analysis (n=5000) was performed to identify differential effects between start and end events.ResultsFunctional block-analysis revealed several clusters of activation, including among others the left fronto-temporal language areas (superior temporal gyrus and Broca’s area). Similar patterns were present on a per-site basis.Time-course analysis demonstrated a differential effect in the left paracingulate sulcus, characterized by a significant dip in activation (Δ=-158iu, p=0.021) at the end-of-hallucination events, in contrast to a significant peak (Δ=35iu, p=0.0142) at the start-of-hallucination events. This activity preceded the motor response for the same events.DiscussionClusters identified in the block-analysis at group-level were consistent with findings from meta-analyses; this attests to the validity of the source data and initial processing. The novel finding of anticipatory neuronal activity in the paracingulate region, differentiated between onset and offset of AVH- events, complements recent findings of reduced functional connectivity (Alonso-Solís et al., 2015) and morphological differences (Garrison et al., 2019) in the same region. This suggests that the paracingulate region may be crucial in both the initiation and the cessation of an AVH-episode and speaks to a kind of regulatory role for the region. This may in turn point to differential action of excitatory and inhibitory transmitters at a lower level of explanation.

MeCP2 in cholinergic interneurons of nucleus accumbens regulates fear learning.

Methyl-CpG-binding protein 2 (MeCP2) encoded by the MECP2 gene is a transcriptional regulator whose mutations cause Rett syndrome (RTT). Mecp2-deficient mice show fear regulation impairment; however, the cellular and molecular mechanisms underlying this abnormal behavior are largely uncharacterized. Here, we showed that Mecp2 gene deficiency in cholinergic interneurons of the nucleus accumbens (NAc) dramatically impaired fear learning. We further found that spontaneous activity of cholinergic interneurons in Mecp2-deficient mice decreased, mediated by enhanced inhibitory transmission via α2-containing GABAA receptors. With MeCP2 restoration, opto- and chemo-genetic activation, and RNA interference in ChAT-expressing interneurons of the NAc, impaired fear retrieval was rescued. Taken together, these results reveal a previously unknown role of MeCP2 in NAc cholinergic interneurons in fear regulation, suggesting that modulation of neurons in the NAc may ameliorate fear-related disorders.

SAT-595 Early Weight Gain Impact onto the Arcuate Leptin Receptor Expressing Neurons Activity During Development

The arcuate nucleus (ARH) is considered the main mediator of the effects of leptin on energy homeostasis, as well, part of the hypothalamic circuitry through which the sex steroids mediate the ovulatory cycle and therefore reproduction. While it is known that aging influences synaptic plasticity of ARH neurons, the effects of increased weight gain early in life onto in the activity of ARH neurons are still poorly understood. In order to demonstrate whether prepubertal adiposity gain is able to modulate the synaptic transmission in leptin receptor (LepR)-expressing cells of the ARH, we employed a postnatal over-nutrition model by raising mice in small litters (SL), while control mice were maintained in regular litters (6-8 pups per litter). The spontaneous currents of LepR-expressing neurons in the ARH were measured by whole-cell patch-clamp recordings in hypothalamic slices obtained from prepubertal, pubertal and adult female LepR-reporter mice. As expected, mice raised in SL exhibited increased body weight compared to control mice at prepubertal and pubertal stage (prepubertal, t(59) = 6.9, P < 0.001; pubertal, t(59) = 5.3, P < 0.001), despite no difference at adult age (t(59) = 1.5, P = 0.4). By evaluating ARH neuronal activity we observed an increased average of excitatory and inhibitory currents frequency during development, in both control females (sEPSC: F(2, 46)= 17.76, P< 0.0001, sIPSC: F(2, 21)= 4.064, P= 0.0322) and in mice raised in SL (sEPSC: F(2, 49)= 27.76, P< 0.0001, sIPSC: F(2, 19)= 6.714, P= 0.0062). However, SL and control mice exhibited similar sEPSC amplitudes at all stages of development. A significant interaction between litter size and excitatory transmission frequency onto LepR-expressing cells were noted at the pubertal and adult stages (F(2, 95)= 3.164, P= 0.046), despite no changes in the amplitudes of these signals (interaction: F(2, 95) = 0.1516, P= 0.8596; age: F(2, 95) = 0.9961, P= 0.3731; litter size: F(1, 95) = 0.076, P= 0.7832). By evaluating the inhibitory transmission to ARH LepR-expressing neurons, no significant interaction between litter size and inhibitory transmission frequency were observed (F(2, 40) = 0.09271, P= 0.9117). However, the average sIPSC amplitude were significantly reduced in the ARH cells recorded from mice raised in SL, when compared to the control group (F(2, 42) = 22.86, P< 0.0001), despite no identifiable interaction between litter size and inhibitory transmission (F(2, 42) = 0.6619, P= 0.5212). Taken together our results suggest that early weight gain influences the excitatory transmission pattern in LepR-expressing neurons by increasing presynaptic excitatory inputs and suppressing postsynaptically transmission to ARH neurons.

Blockade Of γ-Aminobutyric Acid Transporters In Brain Synapses Protects Against Hyperbaric Oxygen-Induced Convulsions

Respiration of hyperbaric oxygen (HBO2) suppresses the synthesis of γ-aminobutyric acid (GABA) in the brain, leading to weakening of inhibitory GABAergic neurotransmission and the development of convulsive syndrome of the epileptic seizure type. We report here our testing of the hypothesis that inhibition of GABA transporters might compensate for insuffi ciency of inhibitory transmitter synthesis, strengthen GABAergic transmission, and weaken or prevent the development of oxygen-induced convulsions. The development of convulsions was studied in conscious rats in a barochamber containing oxygen at a pressure of 5 atm (absolute atmospheres) after prior intracerebroventricular administration of drugs inhibiting selective neuronal (NO-711) or nonselective neuronal/glial GABA transporters (nipecotic acid). Studies in a separate group of rats measured GABA in the striatum by microdialysis with liquid chromatography. These experiments showed that inhibition of neuronal and glial GABA transporters increases the level of GABA in the brain and weakens the development of oxygen-induced convulsions. A more effective anticonvulsant effect was seen after intracerebroventricular administration of the nonselective inhibitor of GABA transporters. These data provide evidence that blockade of the functions of neuronal and glial GABA transporters increases the GABA level in the brain and weakens the development of convulsive syndrome in HBO2. The anticonvulsant effects of the inhibitors used here appear to result from strengthening of GABA-mediated synaptic and extrasynaptic neurotransmission by hyperbaric hyperoxia. Inhibition of GABA transporters may constitute a potential direction for the development of effective methods of preventing oxygen-induced convulsions.

Effects of Antagonists of Different Types of NMDA Receptor on Evoked Responses of Pyramidal Neurons in Rat Cerebral Cortex Slices

Despite the extensive pharmacology of NMDA receptors, the use of drugs able to decrease their overactivation in CNS pathologies is limited. One reason for this may be lack of knowledge of the effects of NMDA receptor antagonists on the functional properties of neurons, as a significant quantity of data has been obtained in in vitro models, which do not completely reproduce real pathophysiological processes. The present study addressed the effects of NMDA receptor antagonists with different mechanisms of action on the properties of pyramidal neuron responses in the prefrontal cortex of the rat brain. The competitive antagonist APV (50 μM) and the ion channel blocker memantine (100 μM) were inactive against evoked EPSP. In conditions of suppression of inhibitory transmission with picrotoxin (50 μM), extracellular stimulation evoked epileptiform responses with prolonged membrane depolarization and generation of a burst of action potentials. APV limited the duration and amplitude of these responses. Memantine acquired activity only when magnesium was excluded from the extracellular medium. Memantine’s lack of effect can be explained in terms of competition for the magnesium ion binding site in the channel cavity. These studies show that prediction of the systemic effects of blockers on neuron function and excitability requires use of in vitro models in which NMDA receptors are activated in the presence of a physiological magnesium concentration and without clamping of the membrane potential.