Paired-pulse and frequency facilitation in the CA1 region of the in vitro rat hippocampus

Faecal Microbiota Transplantation Modulates Morphine Addictive-Like Behaviours Through Hippocampal Metaplasticity

The microbiota-gut-brain axis has been implicated in the pathology of substance use disorders (SUDs). In light of the brain's capability to reorganize itself in response to intrinsic and extrinsic stimuli, opioid-induced dysbiosis is likely to contribute to addictive behaviour through modulating neuroplasticity. In this study, a faecal microbiota transplantation (FMT) from a saline-donor was performed on morphine-treated rats to evaluate the effects of gut microbiota on morphine-induced metaplasticity and addictive behaviours. Male Wistar rats were treated with subcutaneous injections of 10 mg/kg morphine sulphate every 12 h for 9 days in an effort to induce dependence. The withdrawal syndrome was precipitated by injecting naloxone (1.5 mg/kg, ip) after the final dose of morphine. The tolerance was induced by repeated morphine injections over a period of 7 days (10 mg/kg, once a day, ip). FMT was applied daily through gavage of processed faeces 1 week before and during the morphine treatment. Field potential recordings (i.e., fEPSP) were carried out to assess short-term and long-term synaptic plasticity in the CA1 area of the hippocampus following Schaffer-collateral stimulation. Animals subjected to FMT exhibited significant reductions in naloxone-precipitated withdrawal syndrome (one-way ANOVA, p < 0.05). Tolerance to the analgesic effects of morphine was not affected by FMT (two-way ANOVA, p > 0.05). Following high-frequency stimulation (HFS) to induce long-term potentiation (LTP), a greater fEPSP slope was observed in morphine-treated animals (unpaired t test, p < 0.05). FMT from saline-donor rats diminished morphine-induced augmented LTP (unpaired t test, p < 0.05). These results highlighted the alleviating effects of FMT from saline-donors on morphine-induced metaplasticity and dependence potentially by modulating the dysbiosis of gut microbiota.

Two-Terminal Neuromorphic Devices for Spiking Neural Networks: Neurons, Synapses, and Array Integration

The ever-increasing volume of complex data poses significant challenges to conventional sequential global processing methods, highlighting their inherent limitations. This computational burden has catalyzed interest in neuromorphic computing, particularly within artificial neural networks (ANNs). In pursuit of advancing neuromorphic hardware, researchers are focusing on developing computation strategies and constructing high-density crossbar arrays utilizing history-dependent, multistate nonvolatile memories tailored for multiply-accumulate (MAC) operations. However, the real-time collection and processing of massive, dynamic data sets require an innovative computational paradigm akin to that of the human brain. Spiking neural networks (SNNs), representing the third generation of ANNs, are emerging as a promising solution for real-time spatiotemporal information processing due to their event-based spatiotemporal capabilities. The ideal hardware supporting SNN operations comprises artificial neurons, artificial synapses, and their integrated arrays. Currently, the structural complexity of SNNs and spike-based methodologies requires hardware components with biomimetic behaviors that are distinct from those of conventional memristors used in deep neural networks. These distinctive characteristics required for neuron and synapses devices pose significant challenges. Developing effective building blocks for SNNs, therefore, necessitates leveraging the intrinsic properties of the materials constituting each unit and overcoming the integration barriers. This review focuses on the progress toward memristor-based spiking neural network neuromorphic hardware, emphasizing the role of individual components such as memristor-based neurons, synapses, and array integration along with relevant biological insights. We aim to provide valuable perspectives to researchers working on the next generation of brain-like computing systems based on these foundational elements.

Muscarinic Modulation of Synaptic Transmission and Short-Term Plasticity in the Dorsal and Ventral Hippocampus

Muscarinic neurotransmission is fundamentally involved in supporting several brain functions by modulating flow of information in brain neural circuits including the hippocampus which displays a remarkable functional segregation along its longitudinal axis. However, how muscarinic neuromodulation contributes to the functional segregation along the hippocampus remains unclear. In this study we show that the nonselective muscarinic receptor agonist carbachol similarly suppresses basal synaptic transmission in the dorsal and ventral CA1 hippocampal field, in a concentration-depended manner. Furthermore, using a ten-pulse stimulation train of varying frequency we found that carbachol changes the frequency filtering properties more in ventral than dorsal hippocampus by facilitating synaptic inputs at a wide range of input frequencies in the ventral compared with dorsal hippocampus. Using the M2 receptor antagonist gallamine and the M4 receptor antagonist tropicamide, we found that M2 receptors are involved in controlling basal synaptic transmission and short-term synaptic plasticity (STSP) in the ventral but not the dorsal hippocampus, while M4 receptors participate in modulating basal synaptic transmission and STSP in both segments of the hippocampus. These results were corroborated by the higher protein expression levels of M2 receptors in the ventral compared with dorsal hippocampus. We conclude that muscarinic transmission modulates excitatory synaptic transmission and short-term synaptic plasticity along the entire rat hippocampus by acting through M4 receptors and recruiting M2 receptors only in the ventral hippocampus. Furthermore, M4 receptors appear to exert a permissive role on the actions of M2 receptors on STSP in the ventral hippocampus. This dorsoventral differentiation of muscarinic modulation is expected to have important implications in information processing along the endogenous hippocampal circuitry.

The Paraventricular Thalamic Nucleus and Its Projections in Regulating Reward and Context Associations

The paraventricular thalamic nucleus (PVT) is a brain region that mediates aversive and reward-related behaviors as shown in animals exposed to fear conditioning, natural rewards, or drugs of abuse. However, it is unknown whether manipulations of the PVT, in the absence of external factors or stimuli (e.g., fear, natural rewards, or drugs of abuse), are sufficient to drive reward-related behaviors. Additionally, it is unknown whether drugs of abuse administered directly into the PVT are sufficient to drive reward-related behaviors. Here, using behavioral as well as pathway and cell-type specific approaches, we manipulate PVT activity as well as the PVT-to-nucleus accumbens shell (NAcSh) neurocircuit to explore reward phenotypes. First, we show that bath perfusion of morphine (10 µM) caused hyperpolarization of the resting membrane potential, increased rheobase, and decreased intrinsic membrane excitability in PVT neurons that project to the NAcSh. Additionally, we found that direct injections of morphine (50 ng) in the PVT of mice were sufficient to generate conditioned place preference (CPP) for the morphine-paired chamber. Mimicking the inhibitory effect of morphine, we employed a chemogenetic approach to inhibit PVT neurons that projected to the NAcSh and found that pairing the inhibition of these PVT neurons with a specific context evoked the acquisition of CPP. Lastly, using brain slice electrophysiology, we found that bath-perfused morphine (10 µM) significantly reduced PVT excitatory synaptic transmission on both dopamine D1 and D2 receptor-expressing medium spiny neurons in the NAcSh, but that inhibiting PVT afferents in the NAcSh was not sufficient to evoke CPP.

MiR-130/SNAP-25 axis regulate presynaptic alteration in anterior cingulate cortex involved in lead induced attention deficits

Brain volume decrease in the anterior cingulate cortex (ACC) after lead (Pb) exposure has been linked to persistent impairment of attention behavior. However, the precise structural change and molecular mechanism for the Pb-induced ACC alteration and its contribution to inattention have yet to be fully characterized. The present study determined the role of miRNA regulated synaptic structural and functional impairment in the ACC and its relationship to attention deficit disorder in Pb exposed mice. Results showed that Pb exposure induced presynaptic impairment and structural alterations in the ACC. Furthermore, we screened for critical miRNA targets responsible for the synaptic alteration. We found that miR-130, which regulates presynaptic vesicle releasing protein SNAP-25, was responsible for the presynaptic impairment in the ACC and attention deficits in mice. Blocking miR-130 function reversed the Pb-induced decrease in the expression of its presynaptic target SNAP-25, leading to the redistribution of presynaptic vesicles, as well as improved presynaptic function and attention in Pb exposed mice. We report, for the first time, that miR-130 regulating SNAP-25 mediates Pb-induced presynaptic structural and functional impairment in the ACC along with attention deficit disorder in mice.

Improved NMDA Receptor Activation by the Secreted Amyloid-Protein Precursor-α in Healthy Aging: A Role for D-Serine?

Impaired activation of the N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) by D-serine is linked to cognitive aging. Whether this deregulation may be used to initiate pharmacological strategies has yet to be considered. To this end, we performed electrophysiological extracellular recordings at CA3/CA1 synapses in hippocampal slices from young and aged mice. We show that 0.1 nM of the soluble N-terminal recombinant fragment of the secreted amyloid-protein precursor-α (sAPPα) added in the bath significantly increased NMDAR activation in aged but not adult mice without impacting basal synaptic transmission. In addition, sAPPα rescued the age-related deficit of theta-burst-induced long-term potentiation. Significant NMDAR improvement occurred in adult mice when sAPPα was raised to 1 nM, and this effect was drastically reduced in transgenic mice deprived of D-serine through genetic deletion of the synthesizing enzyme serine racemase. Altogether, these results emphasize the interest to consider sAPPα treatment targeting D-serine-dependent NMDAR deregulation to alleviate cognitive aging.

(2R,6R)-hydroxynorketamine rapidly potentiates optically-evoked Schaffer collateral synaptic activity

(2R,6R)-hydroxynorketamine (HNK) is a metabolite of ketamine that exerts rapid and sustained antidepressant-like effects in preclinical studies. We hypothesize that the rapid antidepressant actions of (2R,6R)-HNK involve an acute increase in glutamate release at Schaffer collateral synapses. Here, we used an optogenetic approach to assess whether (2R,6R)-HNK promotes glutamate release at CA1-projecting Schaffer collateral terminals in response to select optical excitation of CA3 afferents. The red-shifted channelrhodopsin, ChrimsonR, was expressed in dorsal CA3 neurons of adult male Sprague Dawley rats. Transverse slices were collected four weeks later to determine ChrimsonR expression and to assess the acute synaptic effects of an antidepressant-relevant concentration of (2R,6R)-HNK (10 μM). (2R,6R)-HNK led to a rapid potentiation of CA1 field excitatory postsynaptic potentials evoked by recurrent optical stimulation of ChrimsonR-expressing CA3 afferents. This potentiation is mediated in part by an increase in glutamate release probability, as (2R,6R)-HNK suppressed paired-pulse facilitation at CA3 projections, an effect that correlated with the magnitude of the (2R,6R)-HNK-induced potentiation of CA1 activity. These results demonstrate that (2R,6R)-HNK increases the probability of glutamate release at CA1-projecting Schaffer collateral afferents, which may be involved in the antidepressant-relevant behavioral adaptations conferred by (2R,6R)-HNK in vivo. The current study also establishes proof-of-principle that genetically-encoded light-sensitive proteins can be used to investigate the synaptic plasticity induced by novel antidepressant compounds in neuronal subcircuits.

Septotemporal variation in modulation of synaptic transmission, paired-pulse ratio and frequency facilitation/depression by adenosine and GABAB receptors in the rat hippocampus

Short-term synaptic plasticity represents a fundamental mechanism in neural information processing and is regulated by neuromodulators. Here, using field recordings from the CA1 region of adult rat hippocampal slices, we show that excitatory synaptic transmission is suppressed by strong but not moderate activation of adenosine A₁ receptors by 2-Chloro-N⁶-cyclopentyladenosine (CCPA) more in the dorsal than the ventral hippocampus; in contrast, both mild and strong activation of GABA_B receptors by baclofen (1 μM, 10 μM) suppress synaptic transmission more in the ventral than the dorsal hippocampus. Using a 10-pulse stimulation train of variable frequency, we found that CCPA modulates short-term synaptic plasticity independently of the suppression of synaptic transmission in both segments of the hippocampus and at stimulation frequencies greater than 10 Hz. However, specifically regarding the paired-pulse ratio (PPR) and frequency facilitation/depression (FF/D) we found significant drug action before but not after adjusting conditioning responses to control levels. Activation of GABA_BRs by baclofen suppressed synaptic transmission more in the ventral than the dorsal hippocampus. Furthermore, relatively high (10 μM) but not low (1 μM) baclofen concentration enhanced both PPR and FF in both hippocampal segments at stimulation frequencies greater than 1 Hz, independently of the suppression of synaptic transmission by baclofen. These results show that A₁Rs and GABA_BRs control synaptic transmission more effectively in the dorsal and the ventral hippocampus, respectively, and suggest that these receptors modulate PPR and FF/D at different frequency bands of afferent input, in both segments of the hippocampus.

Endocannabinoid and Nitric Oxide-Dependent IGF-I-Mediated Synaptic Plasticity at Mice Barrel Cortex

Insulin-like growth factor-I (IGF-I) signaling plays a key role in learning and memory. IGF-I increases the spiking and induces synaptic plasticity in the mice barrel cortex (Noriega-Prieto et al., 2021), favoring the induction of the long-term potentiation (LTP) by Spike Timing-Dependent Protocols (STDP) (Noriega-Prieto et al., 2021). Here, we studied whether these IGF-I effects depend on endocannabinoids (eCBs) and nitric oxide (NO). We recorded both excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) evoked by stimulation of the basal dendrites of layer II/III pyramidal neurons of the Barrel Cortex and analyzed the effect of IGF-I in the presence of a CB₁R antagonist, AM251, and inhibitor of the NO synthesis, L-NAME, to prevent the eCBs and the NO-mediated signaling. Interestingly, L-NAME abolished any modulatory effect of the IGF-I-induced excitatory and inhibitory transmission changes, suggesting the essential role of NO. Surprisingly, the inhibition of CB1Rs did not only block the potentiation of EPSCs but reversed to a depression, highlighting the remarkable functions of the eCB system. In conclusion, eCBs and NO play a vital role in deciding the sign of the effects induced by IGF-I in the neocortex, suggesting a neuromodulatory interplay among IGF-I, NO, and eCBs.

Flexible and transparent memristive synapse based on polyvinylpyrrolidone/N-doped carbon quantum dot nanocomposites for neuromorphic computing

Memristive devices are widely recognized as promising hardware implementations of neuromorphic computing. Herein, a flexible and transparent memristive synapse based on polyvinylpyrrolidone (PVP)/N-doped carbon quantum dot (NCQD) nanocomposites through regulating the NCQD doping concentration is reported. In situ Kelvin probe force microscopy showed that the trapping/detrapping of space charge can account for the memristive mechanism of the device. Diverse synaptic functions, including excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), spike-timing-dependent plasticity (STDP), and the transition from short-term plasticity (STP) to long-term plasticity (LTP), are emulated, enabling the PVP-NCQD hybrid system to be a valuable candidate for the design of novel artificial neural architectures. In addition, the synaptic device showed excellent flexibility against mechanical strain after repeated bending tests. This work provides a new approach to develop flexible and transparent organic artificial synapses for future wearable neuromorphic computing systems.

Astrocytic IGF-IRs Induce Adenosine-Mediated Inhibitory Downregulation and Improve Sensory Discrimination

Insulin-like growth factor-I (IGF-I) signaling plays a key role in learning and memory processes. While the effects of IGF-I on neurons have been studied extensively, the involvement of astrocytes in IGF-I signaling and the consequences on synaptic plasticity and animal behavior remain unknown. We have found that IGF-I induces long-term potentiation (LTP_IGFI) of the postsynaptic potentials that is caused by a long-term depression of inhibitory synaptic transmission in mice. We have demonstrated that this long-lasting decrease in the inhibitory synaptic transmission is evoked by astrocytic activation through its IGF-I receptors (IGF-IRs). We show that LTP_IGFI not only increases the output of pyramidal neurons, but also favors the NMDAR-dependent LTP, resulting in the crucial information processing at the barrel cortex since specific deletion of IGF-IR in cortical astrocytes impairs the whisker discrimination task. Our work reveals a novel mechanism and functional consequences of IGF-I signaling on cortical inhibitory synaptic plasticity and animal behavior, revealing that astrocytes are key elements in these processes.SIGNIFICANCE STATEMENT Insulin-like growth factor-I (IGF-I) signaling plays key regulatory roles in multiple processes of brain physiology, such as learning and memory. Yet, the underlying mechanisms remain largely undefined. Here we demonstrate that astrocytes respond to IGF-I signaling, elevating their intracellular Ca²⁺ and stimulating the release of ATP/adenosine, which triggers the LTD of cortical inhibitory synapses, thus regulating the behavioral task performance related to cortical sensory information processing. Therefore, the present work represents a major conceptual advance in our knowledge of the cellular basis of IGF-I signaling in brain function, by including for the first time astrocytes as key mediators of IGF-I actions on synaptic plasticity, cortical sensory information discrimination and animal behavior.

IGF-1 facilitates extinction of conditioned fear

Insulin-like growth factor-1 (IGF-1) plays a key role in synaptic plasticity, spatial learning, and anxiety-like behavioral processes. While IGF-1 regulates neuronal firing and synaptic transmission in many areas of the central nervous system, its signaling and consequences on excitability, synaptic plasticity, and animal behavior dependent on the prefrontal cortex remain unexplored. Here, we show that IGF-1 induces a long-lasting depression of the medium and slow post-spike afterhyperpolarization (mAHP and sAHP), increasing the excitability of layer 5 pyramidal neurons of the rat infralimbic cortex. Besides, IGF-1 mediates a presynaptic long-term depression of both inhibitory and excitatory synaptic transmission in these neurons. The net effect of this IGF-1-mediated synaptic plasticity is a long-term potentiation of the postsynaptic potentials. Moreover, we demonstrate that IGF-1 favors the fear extinction memory. These results show novel functional consequences of IGF-1 signaling, revealing IGF-1 as a key element in the control of the fear extinction memory.

Zeolite-Based Memristive Synapse with Ultralow Sub-10-fJ Energy Consumption for Neuromorphic Computation

The development of neuromorphic computation faces the appreciable challenge of implementing hardware with energy consumption on the level of a femtojoule per synaptic event to be comparable with the energy consumption of human brain. Controllable ultrathin conductive filaments are needed to achieve such extremely low energy consumption in memristive synapses but their formation is difficult to control owing to their stochastic morphology and unexpected overgrowth. Herein, a zeolite-based memristive synapse is demonstrated for the first time, in which Ag exchange in the sub-nanometer pore closely resembles synaptic Ca²⁺ dynamics across biomembrane channel. Particularly, the confined ultrasmall pore and low Ag ion migration barrier give the zeolite-based memristive synapse ultralow energy consumption below 10 fJ per synaptic spike, on par with the biological counterpart. Experimental results reveal that the gradual memristive effect is attributed to the dimension modulation of Ag clusters. In addition to emulating inherent cognitive functions through electrical stimulations, the experience-dependent transition of short-term plasticity to long-term plasticity using a chemical modulation method is achieved by treating the initial Ag quantity as a learning experience. The proposed memristors can be used to develop highly efficient memristive neural networks and are considered as a candidate for application in neuromorphic computation.

Microbial memories: Sex-dependent impact of the gut microbiome on hippocampal plasticity

Germ‐free rodents, raised in the absence of a measurable gut microbiome, have been a key model to study the microbiome‐gut‐brain axis. Germ‐free mice exhibit marked behavioural and neurochemical differences to their conventionally raised counterparts. It is as yet unclear how these neurochemical differences lead to the behavioural differences. Here, we test the electrophysiological properties of hippocampal plasticity in adult germ‐free mice and compare them to conventionally raised counterparts. Whilst basal synaptic efficacy and pre‐synaptic short‐term plasticity appear normal, we find a striking alteration of hippocampal long‐term potentiation specifically in male germ‐free slices. However, the spike output of these neurons remains normal along with altered input‐output coupling, potentially indicating homeostatic compensatory mechanisms, or an altered excitation/inhibition balance. To our knowledge this is the first time the electrophysiological properties of the hippocampus have been assessed in a microbiome deficient animal. Our data indicate that the absence of a microbiome alters integration of dendritic signalling in the CA1 region in mice.

The inhibition of evoked excitatory postsynaptic potentials produced by ammonium chloride in rat hippocampal CA1 neurons

The depression of evoked fast excitatory postsynaptic potentials (EPSPs) following superfusion with various concentrations (3 μM-5 mM) of ammonium chloride (NH₄Cl) were investigated in rat hippocampal CA1 neurons. The amplitude of the evoked fast EPSPs decreased by NH₄Cl in a concentration-dependent manner. The half-maximal inhibitory concentration for the inhibition of evoked fast EPSPs was 198 ± 125 μM (n = 8). The facilitation of a pair of field EPSPs elicited by paired-pulse stimulation (40-ms interval) (paired-pulse facilitation, PPF) was recorded following superfusion with NH₄Cl (200 μM and 3 mM). The PPF ratio increased to 180 ± 23% (n = 9) in the presence of 200 μM NH₄Cl compared with that in the absence of NH₄Cl (142 ± 24%, n = 9). In the presence of 3 mM NH₄Cl, the PPF ratio increased to 172 ± 30% (n = 7) compared with that in the absence of NH₄Cl (126 ± 13%, n = 7). This implies that NH₄Cl suppressed the presynaptic release of glutamate. Exogenous glutamate- or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced depolarization elicited by using pressure application did not reduce following superfusion with 200 μM or 5 mM NH₄Cl in the presence of 0.3 μM tetrodotoxin, suggesting that NH₄Cl did not affect the postsynaptic glutamate response. Action potentials elicited by rectangular outward current injection from CA3 neurons projecting to CA1 neurons were persistent at 200 μM NH₄Cl but disappeared at 5 mM NH₄Cl. The abolishment of action potentials in the presence of 5 mM NH₄Cl was released by increasing the amplitude of the injection current. These results suggest that NH₄Cl depresses evoked fast EPSPs mainly via a presynaptic mechanism at low NH₄Cl concentrations, and the failure of action potential propagation through the excitatory nerve may also contribute to the depression of evoked fast EPSPs at high NH₄Cl concentrations.

Caffeine has a dual influence on NMDA receptor-mediated glutamatergic transmission at the hippocampus

Caffeine, a stimulant largely consumed around the world, is a non-selective adenosine receptor antagonist, and therefore caffeine actions at synapses usually, but not always, mirror those of adenosine. Importantly, different adenosine receptors with opposing regulatory actions co-exist at synapses. Through both inhibitory and excitatory high-affinity receptors (A₁R and A₂R, respectively), adenosine affects NMDA receptor (NMDAR) function at the hippocampus, but surprisingly, there is a lack of knowledge on the effects of caffeine upon this ionotropic glutamatergic receptor deeply involved in both positive (plasticity) and negative (excitotoxicity) synaptic actions. We thus aimed to elucidate the effects of caffeine upon NMDAR-mediated excitatory post-synaptic currents (NMDAR-EPSCs), and its implications upon neuronal Ca²⁺ homeostasis. We found that caffeine (30-200 μM) facilitates NMDAR-EPSCs on pyramidal CA1 neurons from Balbc/ByJ male mice, an action mimicked, as well as occluded, by 1,3-dipropyl-cyclopentylxantine (DPCPX, 50 nM), thus likely mediated by blockade of inhibitory A₁Rs. This action of caffeine cannot be attributed to a pre-synaptic facilitation of transmission because caffeine even increased paired-pulse facilitation of NMDA-EPSCs, indicative of an inhibition of neurotransmitter release. Adenosine A_2ARs are involved in this likely pre-synaptic action since the effect of caffeine was mimicked by the A_2AR antagonist, SCH58261 (50 nM). Furthermore, caffeine increased the frequency of Ca²⁺ transients in neuronal cell culture, an action mimicked by the A₁R antagonist, DPCPX, and prevented by NMDAR blockade with AP5 (50 μM). Altogether, these results show for the first time an influence of caffeine on NMDA receptor activity at the hippocampus, with impact in neuronal Ca²⁺ homeostasis.

Morphine selectively disinhibits glutamatergic input from mPFC onto dopamine neurons of VTA, inducing reward

Ventral tegmental area (VTA) dopamine (DA) neurons presynaptic glutamate release plays a very important role in the mechanism of morphine. Previously, a study from our lab found that morphine disinhibited glutamatergic input onto the VTA-DA neurons, which was an important mechanism for the morphine-induced increase in the VTA-DA neuron firing and related behaviors (Chen et al., 2015). However, what source of glutamatergic inputs is disinhibited by morphine remains to be elucidated. Using optogenetic strategy combined with whole-cell patch-clamp, qRT-PCR, immunofluorescence and chemical genetic approach combined with behavioral methods, our results show that: 1) morphine promotes glutamate release from glutamatergic terminals of medial prefrontal cortex (mPFC) neurons projecting to VTA-DA neurons but does not on those from glutamatergic terminals of the lateral hypothalamus (LH) neurons projecting to VTA-DA neurons; 2) different response of glutamatergic neurons projecting to VTA-DA neurons from the mPFC or the LH to morphine is related to the expression of GABA_B receptors at terminals of these neurons; 3) inhibition of projection neurons from the mPFC to the VTA significantly reduces morphine-induced locomotor activity increase and conditioned place preference but inhibition of projection neurons from the LH to the VTA does not. These results suggest that morphine selectively promotes glutamate release of the glutamatergic input from mPFC onto VTA-DA neurons by removing the inhibition of the GABA_B receptors in this glutamatergic input from mPFC.

Adenosine A1 Receptor-Mediated Synaptic Depression in the Developing Hippocampal Area CA2

Immunolabeling for adenosine A1 receptors (A1Rs) is high in hippocampal area CA2 in adult rats, and the potentiating effects of caffeine or other A1R-selective antagonists on synaptic responses are particularly robust at Schaffer collateral synapses in CA2. Interestingly, the pronounced staining for A1Rs in CA2 is not apparent until rats are 4 weeks old, suggesting that developmental changes other than receptor distribution underlie the sensitivity of CA2 synapses to A1R antagonists in young animals. To evaluate the role of A1R-mediated postsynaptic signals at these synapses, we tested whether A1R agonists regulate synaptic transmission at Schaffer collateral inputs to CA2 and CA1. We found that the selective A1R agonist CCPA caused a lasting depression of synaptic responses in both CA2 and CA1 neurons in slices obtained from juvenile rats (P14), but that the effect was observed only in CA2 in slices prepared from adult animals (~P70). Interestingly, blocking phosphodiesterase activity with rolipram inhibited the CCPA-induced depression in CA1, but not in CA2, indicative of robust phosphodiesterase activity in CA1 neurons. Likewise, synaptic responses in CA2 and CA1 differed in their sensitivity to the adenylyl cyclase activator, forskolin, in that it increased synaptic transmission in CA2, but had little effect in CA1. These findings suggest that the A1R-mediated synaptic depression tracks the postnatal development of immunolabeling for A1Rs and that the enhanced sensitivity to antagonists in CA2 at young ages is likely due to robust adenylyl cyclase activity and weak phosphodiesterase activity rather than to enrichment of A1Rs.

Endocannabinoid-Dependent Long-Term Potentiation of Synaptic Transmission at Rat Barrel Cortex

Brain-derived neurotrophic factor (BDNF) plays a critical role in modulating plasticity in sensory cortices. Indeed, a BDNF-dependent long-term potentiation (LTP) at distal basal excitatory synapses of Layer 5 pyramidal neurons (L5PNs) has been demonstrated in disinhibited rat barrel cortex slices. Although it is well established that this LTP requires the pairing of excitatory postsynaptic potentials (PSPs) with Ca2+ spikes, its induction when synaptic inhibition is working remains unexplored. Here we show that low-frequency stimulation at basal dendrites of L5PNs is able to trigger a PSP followed by an action potential (AP) and a slow depolarization (termed PSP-Ca2+ response) in thalamocortical slices without blocking synaptic inhibition. We demonstrate that AP barrage-mediated release of endocannabinoids (eCBs) from the recorded L5PNs induces PSP-Ca2+ response facilitation and BDNF-dependent LTP. Indeed, this LTP requires the type 1 cannabinoid receptors activation, is prevented by postsynaptic intracellular 1,2-bis(2-aminophenoxy) ethane-N,N,N,N'-tetraacetic acid (BAPTA) or the anandamide membrane transporter inhibitor AM404, and only occurs in L5PNs neurons showing depolarization-induced suppression of inhibition. Additionally, electrical stimulation at the posteromedial thalamic nucleus induced similar response and LTP. These results reveal a novel form of eCB-dependent LTP at L5PNs that could be relevant in the processing of sensory information in the barrel cortex.

Retrograde activation of CB1R by muscarinic receptors protects against central organophosphorus toxicity

The acute toxicity of organophosphorus-based compounds is primarily a result of acetylcholinesterase inhibition in the central and peripheral nervous systems. The resulting cholinergic crisis manifests as seizure, paralysis, respiratory failure and neurotoxicity. Though overstimulation of muscarinic receptors is the mechanistic basis of central organophosphorus (OP) toxicities, short-term changes in synapse physiology that precede OP-induced seizures have not been investigated in detail. To study acute effects of OP exposure on synaptic function, field excitatory postsynaptic potentials (fEPSPs) were recorded from Schaffer collateral synapses in the mouse hippocampus CA1 stratum radiatum during perfusion with various OP compounds. Administration of the OPs paraoxon, soman or VX rapidly and stably depressed fEPSPs via a presynaptic mechanism, while the non-OP proconvulsant tetramethylenedisulfotetramine had no effect on fEPSP amplitudes. OP-induced presynaptic long-term depression manifested prior to interictal spiking, occurred independent of recurrent firing, and did not require NMDA receptor currents, suggesting that it was not mediated by activity-dependent calcium uptake. Pharmacological dissection revealed that the presynaptic endocannabinoid type 1 receptor (CB1R) as well as postsynaptic M₁ and M₃ muscarinic acetylcholine receptors were necessary for OP-LTD. Administration of CB1R antagonists significantly reduced survival in mice after a soman challenge, revealing an acute protective role for endogenous CB1R signaling during OP exposure. Collectively these data demonstrate that the endocannabinoid system alters glutamatergic synaptic function during the acute response to OP acetylcholinesterase inhibitors.

Intravenous self-administration of benzydamine, a non-steroidal anti-inflammatory drug with a central cannabinoidergic mechanism of action

Benzydamine (BZY) is a non-steroidal anti-inflammatory drug used for the topical treatment of inflammations of the oral and vaginal mucosae. Virtually nothing is known about the central pharmacological actions of BZY. Yet there are reports of voluntary systemic overdosage of BZY in drug addicts, resulting in a euphoric, hallucinatory state. In the present study, we investigated the reinforcing properties of BZY in a rat self-administration paradigm. We found that BZY has a powerful reinforcing effect and that this effect is greatly facilitated in animals that already had substance experience, having previously self-administered heroin and cocaine, indicating cross sensitization between BZY and other common drugs of abuse. We then assessed the effect of BZY on prelimbic cortex-to-nucleus accumbens glutamatergic transmission, using field recordings in rat parasagittal brain slices. BZY dose-dependently reduced both field excitatory post synaptic potential amplitude and paired pulse ratio, suggesting a presynaptic mechanism of action. Similarly to the in vivo paradigm, also the electrophysiological effects of BZY were potentiated in slices from animals that had undergone cocaine and heroin self-administration. Furthermore, BZY-induced Long Term Depression (LTD)-like responses in the prelimbic cortex-to-nucleus accumbens circuitry were significantly reduced in the presence of the CB1 receptor antagonist AM251. These findings provide firm evidence of the abuse liability of BZY and suggest a possible cannabinoidergic mechanism of action. Further research is needed in order to give insights into the molecular mechanism underlying BZY psychoactive and reinforcing effects, to better understand its abuse potential.

Caffeine-mediated BDNF release regulates long-term synaptic plasticity through activation of IRS2 signaling

Caffeine has cognitive‐enhancing properties with effects on learning and memory, concentration, arousal and mood. These effects imply changes at circuital and synaptic level, but the mechanism by which caffeine modifies synaptic plasticity remains elusive. Here we report that caffeine, at concentrations representing moderate to high levels of consumption in humans, induces an NMDA receptor‐independent form of LTP (_CAFLTP) in the CA1 region of the hippocampus by promoting calcium‐dependent secretion of BDNF, which subsequently activates TrkB‐mediated signaling required for the expression of _CAFLTP. Our data include the novel observation that insulin receptor substrate 2 (IRS2) is phosphorylated during induction of _CAFLTP, a process that requires cytosolic free Ca²⁺. Consistent with the involvement of IRS2 signals in caffeine‐mediated synaptic plasticity, phosphorylation of Akt (Ser473) in response to LTP induction is defective in Irs2^−/− mice, demonstrating that these plasticity changes are associated with downstream targets of the phosphoinositide 3‐kinase (PI3K) pathway. These findings indicate that TrkB‐IRS2 signals are essential for activation of PI3K during the induction of LTP by caffeine.

Concussion Induces Hippocampal Circuitry Disruption in Swine

Hippocampal-dependent deficits in learning and memory formation are a prominent feature of traumatic brain injury (TBI); however, the role of the hippocampus in cognitive dysfunction after concussion (mild TBI) is unknown. We therefore investigated functional and structural changes in the swine hippocampus following TBI using a model of head rotational acceleration that closely replicates the biomechanics and neuropathology of closed-head TBI in humans. We examined neurophysiological changes using a novel ex vivo hippocampal slice paradigm with extracellular stimulation and recording in the dentate gyrus and CA1 occurring at 7 days following non-impact inertial TBI in swine. Hippocampal neurophysiology post-injury revealed reduced axonal function, synaptic dysfunction, and regional hyperexcitability at one week following even "mild" injury levels. Moreover, these neurophysiological changes occurred in the apparent absence of intra-hippocampal neuronal or axonal degeneration. Input-output curves demonstrated an elevated excitatory post-synaptic potential (EPSP) output for a given fiber volley input in injured versus sham animals, suggesting a form of homeostatic plasticity that manifested as a compensatory response to decreased axonal function in post-synaptic regions. These data indicate that closed-head rotational acceleration-induced TBI, the common cause of concussion in humans, may induce significant alterations in hippocampal circuitry function that have not resolved at 7 days post-injury. This circuitry dysfunction may underlie some of the post-concussion symptomatology associated with the hippocampus, such as post-traumatic amnesia and ongoing cognitive deficits.

Interaction of nucleus reuniens and entorhinal cortex projections in hippocampal field CA1 of the rat

The nucleus reuniens (RE) and entorhinal cortex (EC) provide monosynaptic excitatory inputs to the apical dendrites of pyramidal cells and to interneurons with dendrites in stratum lacunosum moleculare (LM) of hippocampal field CA1. However, whether the RE and EC inputs interact at the cellular level is unknown. In this electrophysiological in vivo study, low-frequency stimulation was used to selectively activate each projection at its origin; field excitatory postsynaptic potentials (fEPSPs) were recorded in CA1. We applied (1) paired pulses to RE or EC, (2) combined paired pulses to RE and EC, and (3) simultaneously paired pulses to RE/EC. The main findings are that: (a) stimulation of either RE- or EC-evoked subthreshold fEPSPs, displaying paired pulse facilitation (PPF), (b) subthreshold fEPSPs evoked by combined stimulation did not display heterosynaptic PPF, and (c) simultaneous stimulation of RE/EC resulted in enhanced subthreshold fEPSPs in proximal LM displaying a nonlinear interaction. CSD analyses of RE/EC-evoked depth profiles revealed a nonlinear enlargement of the 'LM sink-radiatum source' configuration and the appearance of an additional small sink-source pair close to stratum pyramidale, likely reflecting (peri)somatic inhibition. The nonlinear interaction between both inputs indicates that RE and EC axons form synapses, at least partly, onto the same dendritic compartments of CA1 pyramidal cells. We propose that low-frequency activation of the RE-CA1 input facilitates the entorhinal-hippocampal dialogue, and may synchronize the neocortical-hippocampal slow oscillation which is relevant for hippocampal-dependent memory consolidation.

Altered responsiveness of BNST and amygdala neurons in trauma-induced anxiety

A highly conserved network of brain structures regulates the expression of fear and anxiety in mammals. Many of these structures display abnormal activity levels in post-traumatic stress disorder (PTSD). However, some of them, like the bed nucleus of the stria terminalis (BNST) and amygdala, are comprised of several small sub-regions or nuclei that cannot be resolved with human neuroimaging techniques. Therefore, we used a well-characterized rat model of PTSD to compare neuronal properties in resilient vs PTSD-like rats using patch recordings obtained from different BNST and amygdala regions in vitro. In this model, a persistent state of extreme anxiety is induced in a subset of susceptible rats following predatory threat. Previous animal studies have revealed that the central amygdala (CeA) and BNST are differentially involved in the genesis of fear and anxiety-like states, respectively. Consistent with these earlier findings, we found that between resilient and PTSD-like rats were marked differences in the synaptic responsiveness of neurons in different sectors of BNST and CeA, but whose polarity was region specific. In light of prior data about the role of these regions, our results suggest that control of fear/anxiety expression is altered in PTSD-like rats such that the influence of CeA is minimized whereas that of BNST is enhanced. A model of the amygdalo-BNST interactions supporting the PTSD-like state is proposed.

KV1 and KV3 Potassium Channels Identified at Presynaptic Terminals of the Corticostriatal Synapses in Rat

In the last years it has been increasingly clear that K_V-channel activity modulates neurotransmitter release. The subcellular localization and composition of potassium channels are crucial to understanding its influence on neurotransmitter release. To investigate the role of K_V in corticostriatal synapses modulation, we combined extracellular recording of population-spike and pharmacological blockage with specific and nonspecific blockers to identify several families of K_V channels. We induced paired-pulse facilitation (PPF) and studied the changes in paired-pulse ratio (PPR) before and after the addition of specific K_V blockers to determine whether particular K_V subtypes were located pre- or postsynaptically. Initially, the presence of K_V channels was tested by exposing brain slices to tetraethylammonium or 4-aminopyridine; in both cases we observed a decrease in PPR that was dose dependent. Further experiments with tityustoxin, margatoxin, hongotoxin, agitoxin, dendrotoxin, and BDS-I toxins all rendered a reduction in PPR. In contrast heteropodatoxin and phrixotoxin had no effect. Our results reveal that corticostriatal presynaptic K_V channels have a complex stoichiometry, including heterologous combinations K_V1.1, K_V1.2, K_V1.3, and K_V1.6 isoforms, as well as K_V3.4, but not K_V4 channels. The variety of K_V channels offers a wide spectrum of possibilities to regulate neurotransmitter release, providing fine-tuning mechanisms to modulate synaptic strength.

Paired Burst Stimulation Causes GABAA Receptor-Dependent Spike Firing Facilitation in CA1 of Rat Hippocampal Slices

The theta oscillation (4–8 Hz) is a pivotal form of oscillatory activity in the hippocampus that is intermittently concurrent with gamma (25–100 Hz) burst events. In in vitro preparation, a stimulation protocol that mimics the theta oscillation, theta burst stimulation (TBS), is used to induce long-term potentiation. Thus, TBS is thought to have a distinct role in the neural network of the hippocampal slice preparation. However, the specific mechanisms that make TBS induce such neural circuit modifications are still unknown. Using electrophysiology and voltage-sensitive dye imaging (VSDI), we have found that TBS induces augmentation of spike firing. The augmentation was apparent in the first couple of brief burst stimulation (100 Hz four pulses) on a TBS-train in a presence of NMDA receptor blocker (APV 50 μM). In this study, we focused on the characterizes of the NMDA independent augmentation caused by a pair of the brief burst stimulation (the first pair of the TBS; paired burst stimulation-PBS). We found that PBS enhanced membrane potential responses on VSDI signal and intracellular recordings while it was absent in the current recording under whole-cell clamp condition. The enhancement of the response accompanied the augmentation of excitatory postsynaptic potential (EPSP) to spike firing (E-S) coupling. The paired burst facilitation (PBF) reached a plateau when the number of the first burst stimulation (priming burst) exceeds three. The interval between the bursts of 150 ms resulted in the maximum PBF. Gabazine (a GABA_A receptor antagonist) abolished PBF. The threshold for spike generation of the postsynaptic cells measured with a current injection to cells was not lowered by the priming burst of PBS. These results indicate that PBS activates the GABAergic system to cause short-term E-S augmentation without raising postsynaptic excitability. We propose that a GABAergic system of area CA1 of the hippocampus produce the short-term E-S plasticity that could cause exaggerated spike-firing upon a theta-gamma activity distinctively, thus making the neural circuit of the CA1 act as a specific amplifier of the oscillation signal.

DAMGO depresses inhibitory synaptic transmission via different downstream pathways of μ opioid receptors in ventral tegmental area and periaqueductal gray

Opioid-induced rewarding and motorstimulant effects are mediated by an increased activity of the ventral tegmental area (VTA) dopamine (DA) neurons. The excitatory mechanism of opioids on VTA-DA neurons has been proposed to be due to the depression of GABAergic synaptic transmission in VTA-DA neurons. However, how opioids depress GABAergic synaptic transmission in VTA-DA neurons remain to be studied. In the present study, we explored the mechanism of the inhibitory effect of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) on GABAergic synaptic transmission in VTA-DA neurons using multiple approaches and techniques. Our results showed that (1) DAMGO inhibits GABAergic inputs in VTA-DA neurons at presynaptic sites; (2) effect of DAMGO on GABAergic inputs in VTA-DA neurons is inhibited by potassium channel blocker 4-aminopyridine (4-AP) and Gi protein inhibitor N-ethylmaleimide (NEM); (3) phospholipase A2 (PLA2) does not mediate the effect of DAMGO on GABAergic inputs in VTA-DA neurons, but mediates it in the periaqueductal gray (PAG); (4) multiple downstream signaling molecules of μ receptors do not mediate the effect of DAMGO on GABAergic inputs in VTA-DA neurons. These results suggest that DAMGO depresses inhibitory synaptic transmission via μ receptor-Gi protein-Kv channel pathway in VTA-DA neurons, but via μ receptor-PLA2 pathway in PAG neurons.

Intranasally applied neuropeptide S shifts a high-anxiety electrophysiological endophenotype in the ventral hippocampus towards a "normal"-anxiety one

The neurobiological basis of pathological anxiety and the improvement of its pharmacological treatment are a matter of intensive investigation. Here, using electrophysiological techniques in brain slices from animals of the high anxiety-related behavior (HAB) and normal anxiety-related behavior (NAB) mouse model, we show that basal neurotransmission at ventral hippocampal CA3-CA1 synapses is weaker in HAB compared to NAB mice. We further demonstrate that paired-pulse facilitation (PPF) and long-term potentiation (LTP) at these synapses are more pronounced in slices from HAB animals. Based on previous findings, we also examined whether intranasal delivery of neuropeptide S (NPS), which increasingly emerges as a potential novel treatment option for anxiety symptoms occurring in a variety of diseases like anxiety disorders, posttraumatic stress disorder, and major depression, impacts on the high-anxiety electrophysiological endophenotype in HAB mice. Strikingly, we detected enhanced basal neurotransmission and reduced PPF and LTP in slices from NPS-treated HAB animals. Collectively, our study uncovers a multifaceted high-anxiety neurophysiological endophenotype in the murine ventral hippocampus and provides the first evidence that an intranasally applied neuropeptide can shift such an endophenotype in an anxiety-regulating brain structure towards a “normal”-anxiety one.

Achieving high-frequency optical control of synaptic transmission

The optogenetic tool channelrhodopsin-2 (ChR2) is widely used to excite neurons to study neural circuits. Previous optogenetic studies of synapses suggest that light-evoked synaptic responses often exhibit artificial synaptic depression, which has been attributed to either the inability of ChR2 to reliably fire presynaptic axons or to ChR2 elevating the probability of release by depolarizing presynaptic boutons. Here, we compare light-evoked and electrically evoked synaptic responses for high-frequency stimulation at three synapses in the mouse brain. At synapses from Purkinje cells to deep cerebellar nuclei neurons (PC→DCN), light- and electrically evoked synaptic currents were remarkably similar for ChR2 expressed transgenically or with adeno-associated virus (AAV) expression vectors. For hippocampal CA3→CA1 synapses, AAV expression vectors of serotype 1, 5, and 8 led to light-evoked synaptic currents that depressed much more than electrically evoked currents, even though ChR2 could fire axons reliably at up to 50 Hz. The disparity between optical and electrical stimulation was eliminated when ChR2 was expressed transgenically or with AAV9. For cerebellar granule cell to stellate cell (grc→SC) synapses, AAV1 also led to artificial synaptic depression and AAV9 provided superior performance. Artificial synaptic depression also occurred when stimulating over presynaptic boutons, rather than axons, at CA3→CA1 synapses, but not at PC→DCN synapses. These findings indicate that ChR2 expression methods and light stimulation techniques influence synaptic responses in a neuron-specific manner. They also identify pitfalls associated with using ChR2 to study synapses and suggest an approach that allows optogenetics to be applied in a manner that helps to avoid potential complications.

CGRP inhibits neurons of the bed nucleus of the stria terminalis: implications for the regulation of fear and anxiety

The bed nucleus of the stria terminalis (BNST) is thought to generate anxiety-like states via its projections to autonomic and neuroendocrine regulatory structures of the brain. However, because most BNST cells are GABAergic, they are expected to inhibit target neurons. In contrast with this, infusion of calcitonin gene-related peptide (CGRP) into BNST was reported to potentiate anxiety while activating BNST targets. The present study aimed to shed light on this paradox. The CGRP innervation of BNST originates in the pontine parabrachial nucleus and targets its anterolateral sector (BNST-AL). Thus, we investigated the effects of CGRP on BNST-AL neurons using patch recordings in vitro in male rats. CGRP did not alter the passive properties of BNST-AL cells but increased the amplitude of IPSPs evoked by stimulation of the stria terminalis (ST). However, IPSP paired-pulse ratios were unchanged by CGRP, and there was no correlation between IPSP potentiation and variance, suggesting that CGRP acts postsynaptically. Consistent with this, CGRP hyperpolarized the GABA-A reversal of BNST-AL cells. These results indicate that CGRP increases ST-evoked GABA-A IPSPs and hyperpolarizes their reversal potential through a postsynaptic change in Cl(-) homeostasis. Overall, our findings suggest that CGRP potentiates anxiety-like behaviors and increases neural activity in BNST targets, by inhibiting BNST-AL cells, supporting the conclusion that BNST-AL exerts anxiolytic effects.

Alzheimer's disease risk factor lymphocyte-specific protein tyrosine kinase regulates long-term synaptic strengthening, spatial learning and memory

The lymphocyte-specific protein tyrosine kinase (Lck), which belongs to the Src kinase-family, is expressed in neurons of the hippocampus, a structure critical for learning and memory. Recent evidence demonstrated a significant downregulation of Lck in Alzheimer's disease. Lck has additionally been proposed to be a risk factor for Alzheimer's disease, thus suggesting the involvement of Lck in memory function. The neuronal role of Lck, however, and its involvement in learning and memory remain largely unexplored. Here, in vitro electrophysiology, confocal microscopy, and molecular, pharmacological, genetic and biochemical techniques were combined with in vivo behavioral approaches to examine the role of Lck in the mouse hippocampus. Specific pharmacological inhibition and genetic silencing indicated the involvement of Lck in the regulation of neuritic outgrowth. In the functional pre-established synaptic networks that were examined electrophysiologically, specific Lck-inhibition also selectively impaired the long-term hippocampal synaptic plasticity without affecting spontaneous excitatory synaptic transmission or short-term synaptic potentiation. The selective inhibition of Lck also significantly altered hippocampus-dependent spatial learning and memory in vivo. These data provide the basis for the functional characterization of brain Lck, describing the importance of Lck as a critical regulator of both neuronal morphology and in vivo long-term memory.

Peroxynitrite alters GABAergic synaptic transmission in immature rat hippocampal slices

Increasing of peroxynitrite (ONOO(-)) production during ischemia in the immature brain was considered to be associated with impaired cognitive function. GABAergic synapses played an important role in memory formation including the induction of long-term potentiation (LTP) and long-term depression (LTD) in hippocampus. In the present study, we examined the effects of acute exposure of the ONOO(-) donor, SIN-1 on GABAergic synaptic transmission in immature rat hippocampal slices with whole-cell patch-clamp recordings. The results showed that SIN-1 increased the peak amplitude of evoked inhibitory postsynaptic currents (eIPSCs) and decreased paired pulse ratio via the formation of ONOO(-). In addition, it also increased the frequency of spontaneous (but not miniature) IPSCs in a dose-dependent manner without altering amplitudes or rise and decay times of both (sIPSCs and mIPSCs). It further demonstrated that the presynaptic action of SIN-1 was external calcium dependent and was not related to the changes of interneuron excitability. This study provides electrophysiological evidences from developing hippocampal slices to support that SIN-1 enhances action potential-dependent GABA release. It suggests that the potentiation effect of ONOO(-) may contribute to hyperexcitability and seizures and may underlie one of the mechanisms by which ischemia increases seizure susceptibility in the immature brain.

Anxiolytic actions of motilin in the basolateral amygdala

Motilin is a 22-amino-acid gastrointestinal polypeptide that was first isolated from the porcine intestine. We identified that motilin receptor is highly expressed in GABAergic interneurons in the basolateral nucleus (BLA) of the amygdala, the structure of which is closely involved in assigning stress disorder and anxiety. However, little is known about the role of motilin in BLA neuronal circuits and the molecular mechanisms of stress-related anxiety. Whole-cell recordings from amygdala slices showed that motilin depolarized the interneurons and facilitated GABAergic transmission in the BLA, which is mimicked by the motilin receptor agonist, erythromycin. BLA local injection of erythromycin or motilin can reduce the anxiety-like behavior in mice after acute stress. Therefore, motilin is essential in regulating interneuron excitability and GABAergic transmission in BLA. Moreover, the anxiolytic actions of motilin can partly be explained by modulating the BLA neuronal circuits. The present data demonstrate the importance of motilin in anxiety and the development of motilin receptor non-peptide agonist as a clear target for the potential treatment of anxiety disorders.

Loss of survivin in neural precursor cells results in impaired long-term potentiation in the dentate gyrus and CA1-region

In adult mammals, newborn neural precursor cells (NPCs) derived from either the subventricular zone (SVZ) or the subgranular zone (SGZ) migrate into the olfactory bulb and the dentate gyrus (DG), respectively, where some of them mature into excitatory and inhibitory neurons. There is increasing evidence that this neurogenesis process is important for some types of learning and synaptic plasticity and vice versa. Survivin, a member of the inhibitor-of-apoptosis protein (IAP) family, has been suggested to have a central role in the regulation of neurogenesis. The protein is abundantly expressed in nervous tissue during embryonic development while being restricted postnatally to proliferating and migrating NPCs in SVZ and SGZ. Here we examined adult Survivin(Camcre) mice with a conditional deletion of the survivin gene in embryonic neurogenic regions. Although the deletion of survivin had no effect on basic excitability in DG and CA1-region, there was a marked impairment of long-term potentiation (LTP) in these areas. Our data support a function of survivin in hippocampal synaptic plasticity and learning and underline the importance of adult brain neurogenesis for proper operation of the hippocampal tri-synaptic circuit and the physiological functions that depend on it.

Deficits in LTP induction by 5-HT2A receptor antagonist in a mouse model for fragile X syndrome

Fragile X syndrome is a common inherited form of mental retardation caused by the lack of fragile X mental retardation protein (FMRP) because of Fmr1 gene silencing. Serotonin (5-HT) is significantly increased in the null mutants of Drosophila Fmr1, and elevated 5-HT brain levels result in cognitive and behavioral deficits in human patients. The serotonin type 2A receptor (5-HT2AR) is highly expressed in the cerebral cortex; it acts on pyramidal cells and GABAergic interneurons to modulate cortical functions. 5-HT2AR and FMRP both regulate synaptic plasticity. Therefore, the lack of FMRP may affect serotoninergic activity. In this study, we determined the involvement of FMRP in the 5-HT modulation of synaptic potentiation with the use of primary cortical neuron culture and brain slice recording. Pharmacological inhibition of 5-HT2AR by R-96544 or ketanserin facilitated long-term potentiation (LTP) in the anterior cingulate cortex (ACC) of WT mice. The prefrontal LTP induction was dependent on the activation of NMDARs and elevation of postsynaptic Ca²⁺ concentrations. By contrast, inhibition of 5-HT2AR could not restore the induction of LTP in the ACC of Fmr1 knock-out mice. Furthermore, 5-HT2AR inhibition induced AMPA receptor GluR1 subtype surface insertion in the cultured ACC neurons of Fmr1 WT mice, however, GluR1 surface insertion by inhibition of 5-HT2AR was impaired in the neurons of Fmr1KO mice. These findings suggested that FMRP was involved in serotonin receptor signaling and contributed in GluR1 surface expression induced by 5-HT2AR inactivation.

Summation in the Hippocampal CA3-CA1 Network Remains Robustly Linear Following Inhibitory Modulation and Plasticity, but Undergoes Scaling and Offset Transformations

Many theories of neural network function assume linear summation. This is in apparent conflict with several known forms of non-linearity in real neurons. Furthermore, key network properties depend on the summation parameters, which are themselves subject to modulation and plasticity in real neurons. We tested summation responses as measured by spiking activity in small groups of CA1 pyramidal neurons using permutations of inputs delivered on an electrode array. We used calcium dye recordings as a readout of the summed spiking response of cell assemblies in the network. Each group consisted of 2–10 cells, and the calcium signal from each cell correlated with individual action potentials. We find that the responses of these small cell groups sum linearly, despite previously reported dendritic non-linearities and the thresholded responses of individual cells. This linear summation persisted when input strengths were reduced. Blockage of inhibition shifted responses up toward saturation, but did not alter the slope of the linear region of summation. Long-term potentiation of synapses in the slice also preserved the linear fit, with an increase in absolute response. However, in this case the summation gain decreased, suggesting a homeostatic process for preserving overall network excitability. Overall, our results suggest that cell groups in the CA3-CA1 network robustly follow a consistent set of linear summation and gain-control rules, notwithstanding the intrinsic non-linearities of individual neurons. Cell-group responses remain linear, with well-defined transformations following inhibitory modulation and plasticity. Our measures of these transformations provide useful parameters to apply to neural network analyses involving modulation and plasticity.

Apnea produces excitotoxic hippocampal synapses and neuronal apoptosis

Obstructive sleep apnea (OSA) results in the degeneration of neurons in the hippocampus that eventuates in neurocognitive deficits. We were therefore interested in determining the effects of apnea on monosynaptic excitatory processes in a hippocampal pathway (cornu ammonis 3-cornu ammonis 1, CA3-CA1) that has been shown to mediate the processing of cognitive information. In addition, to substantiate an anatomical basis for the cognitive dysfunction that occurs in OSA patients, we examined the effects of apnea with respect to neurodegenerative changes (apoptosis) in the same hippocampal pathway. In order to determine the effects of apnea, an automated system for the generation and analysis of single and recurrent periods of apnea was developed. Utilizing this system, the field excitatory postsynaptic potential (fEPSP) generated by pyramidal neurons in the CA1 region of the hippocampus was monitored in α-chloralose anesthetized rats following stimulation of glutamatergic afferents in the CA3 region. A stimulus-response (input-output) curve for CA3-CA1 synaptic activity was determined. In addition, a paired-pulse paradigm was employed to evaluate, electrophysiologically, the presynaptic release of glutamate. Changes in the synaptic efficacy were assessed following single episodes of apnea induced by ventilatory arrest (60 to 80 s duration, mean=72 s; mean oxygen desaturation was 53% of normoxia level). Apnea resulted in a significant potentiation of the amplitude (mean=126%) and slope (mean=117%) of the baseline CA1 fEPSP. This increase in the fEPSP was accompanied by a significant decrease in the amplitude (71%) and slope (81%) of normalized paired-pulse facilitation (PPF) ratios. Since the potentiation of the fEPSP is inversely proportional to changes in PPF ratio, the potentiated fEPSP accompanied by the reduced PPF reveals that apnea produces an abnormal increase in the preterminal release of glutamate that results in the over-activation (and calcium overloading) of hippocampal CA1 neurons. Thus, we conclude that individual episodes of apnea result in the development of excitotoxic processes in the hippocampal CA3-CA1 pathway that is critically involved in the processing of cognitive information. Morphologically, the deleterious effect of recurrent apnea was substantiated by the finding of apoptosis in CA1 neurons of apneic (but not normoxic) animals.

Altered synaptic transmission in the hippocampus of transgenic mice with enhanced central nervous systems expression of interleukin-6

Elevated levels of the inflammatory cytokine interleukin-6 (IL-6) occur in a number of CNS disorders. However, little is known about how this condition affects CNS neuronal function. Transgenic mice that express elevated levels of IL-6 in the CNS show cognitive changes, increased propensity for hippocampal seizures and reduced number of inhibitory interneurons, suggesting that elevated levels of IL-6 can cause neuroadaptive changes that alter hippocampal function. To identify these neuroadaptive changes, we measured the levels of protein expression using Western blot analysis and synaptic function using field potential recordings in hippocampus from IL-6 transgenic mice (IL-6 tg) and their non-transgenic (non-tg) littermates. Western blot analysis showed enhanced levels of the GFAP and STAT3 in the IL-6 tg hippocampus compared with the non-tg hippocampus, but no difference for several other proteins. Field potential recordings of synaptic transmission at the Schaffer collateral to CA1 synapse showed enhanced dendritic excitatory postsynaptic potentials and somatic population spikes in the CA1 region of hippocampal slices from IL-6 tg mice compared with slices from non-tg littermate controls. No differences were observed for several forms of short-term and long-term synaptic plasticity between hippocampal slices from IL-6 tg and non-tg mice. These results demonstrate that elevated levels of IL-6 can alter mechanisms involved in the excitability of hippocampal neurons and synapses, an effect consistent with recent evidence indicating that elevated production of IL-6 plays an important role in conditions associated with seizure activity and in other impairments observed in CNS disorders with a neuroinflammatory component.

Cholinergic-mediated response enhancement in barrel cortex layer V pyramidal neurons

Neocortical cholinergic activity plays a fundamental role in sensory processing and cognitive functions, but the underlying cellular mechanisms are largely unknown. We analyzed the effects of acetylcholine (ACh) on synaptic transmission and cell excitability in rat "barrel cortex" layer V (L5) pyramidal neurons in vitro. ACh through nicotinic and M1 muscarinic receptors enhanced excitatory postsynaptic currents and through nicotinic and M2 muscarinic receptors reduced inhibitory postsynaptic currents. These effects increased excitability and contributed to the generation of Ca(2+) spikes and bursts of action potentials (APs) when inputs in basal dendrites were stimulated. Ca(2+) spikes were mediated by activation of NMDA receptors (NMDARs) and L-type voltage-gated Ca(2+) channels. Additionally, we demonstrate in vivo that basal forebrain stimulation induced an atropine-sensitive increase of L5 AP responses evoked by vibrissa deflection, an effect mainly due to the enhancement of an NMDAR component. Therefore, ACh modified the excitatory/inhibitory balance and switched L5 pyramidal neurons to a bursting mode that caused a potent and sustained response enhancement with possible fundamental consequences for the function of the barrel cortex.

Group I mGluR antagonist rescues the deficit of D1-induced LTP in a mouse model of fragile X syndrome

BACKGROUND

Fragile X syndrome (FXS) is caused by the absence of the mRNA-binding protein Fragile X mental retardation protein (FMRP), encoded by the Fmr1 gene. Overactive signaling by group 1 metabotropic glutamate receptor (Grp1 mGluR) could contribute to slowed synaptic development and other symptoms of FXS. Our previous study has identified that facilitation of synaptic long-term potentiation (LTP) by D1 receptor is impaired in Fmr1 knockout (KO) mice. However, the contribution of Grp1 mGluR to the facilitation of synaptic plasticity by D1 receptor stimulation in the prefrontal cortex has been less extensively studied.

RESULTS

Here we demonstrated that DL-AP3, a Grp1 mGluR antagonist, rescued LTP facilitation by D1 receptor agonist SKF81297 in Fmr1KO mice. Grp1 mGluR inhibition restored the GluR1-subtype AMPA receptors surface insertion by D1 activation in the cultured Fmr1KO neurons. Simultaneous treatment of Grp1 mGluR antagonist with D1 agonist recovered the D1 receptor signaling by reversing the subcellular redistribution of G protein-coupled receptor kinase 2 (GRK2) in the Fmr1KO neurons. Treatment of SKF81297 alone failed to increase the phosphorylation of NR2B-containing N-methyl D-aspartate receptors (NMDARs) at Tyr-1472 (p-NR2B-Tyr1472) in the cultures from KO mice. However, simultaneous treatment of DL-AP3 could rescue the level of p-NR2B-Tyr1472 by SKF81297 in the cultures from KO mice. Furthermore, behavioral tests indicated that simultaneous treatment of Grp1 mGluR antagonist with D1 agonist inhibited hyperactivity and improved the learning ability in the Fmr1KO mice.

CONCLUSION

The findings demonstrate that mGluR1 inhibition is a useful strategy to recover D1 receptor signaling in the Fmr1KO mice, and combination of Grp1 mGluR antagonist and D1 agonist is a potential drug therapy for the FXS.

MT5-MMP, ADAM-10, and N-cadherin act in concert to facilitate synapse reorganization after traumatic brain injury

Matrix metalloproteinases (MMPs) influence synaptic recovery following traumatic brain injury (TBI). Membrane type 5-matrix metalloproteinase (MT5-MMP) and a distintegrin and metalloproteinase-10 (ADAM-10) are membrane-bound MMPs that cleave N-cadherin, a protein critical to synapse stabilization. This study examined protein and mRNA expression of MT5-MMP, ADAM-10, and N-cadherin after TBI, contrasting adaptive and maladaptive synaptogenesis. The effect of MMP inhibition on MT5-MMP, ADAM-10, and N-cadherin was assessed during maladaptive plasticity and correlated with synaptic function. Rats were subjected to adaptive unilateral entorhinal cortical lesion (UEC) or maladaptive fluid percussion TBI+bilateral entorhinal cortical lesion (TBI+BEC). Hippocampal MT5-MMP and ADAM-10 protein was significantly elevated 2 and 7 days post-injury. At 15 days after UEC, each MMP returned to control level, while TBI+BEC ADAM-10 remained elevated. At 2 and 7 days, N-cadherin protein was below control. By the 15-day synapse stabilization phase, UEC N-cadherin rose above control, a shift not seen for TBI+BEC. At 7 days, increased TBI+BEC ADAM-10 transcript correlated with protein elevation. UEC ADAM-10 mRNA did not change, and no differences in MT5-MMP or N-cadherin mRNA were detected. Confocal imaging showed MT5-MMP, ADAM-10, and N-cadherin localization within reactive astrocytes. MMP inhibition attenuated ADAM-10 protein 15 days after TBI+BEC and increased N-cadherin. This inhibition partially restored long-term potentiation induction, but did not affect paired-pulse facilitation. Our results confirm time- and injury-dependent expression of MT5-MMP, ADAM-10, and N-cadherin during reactive synaptogenesis. Persistent ADAM-10 expression was correlated with attenuated N-cadherin level and reduced functional recovery. MMP inhibition shifted ADAM-10 and N-cadherin toward adaptive expression and improved synaptic function.

Elevated interleukin-8 enhances prefrontal synaptic transmission in mice with persistent inflammatory pain

BACKGROUND

Interleukin-8 (IL-8) is known for its roles in inflammation and plays critical roles in the development of pain. Its expression increases in the brain after peripheral inflammation. Prefrontal cortex, including the anterior cingulate cortex (ACC), is a forebrain structure known for its roles in pain transmission and modulation. Painful stimuli potentiate the prefrontal synaptic transmission, however, little is known about the expression of IL-8 and its role in the enhanced ACC synaptic transmission in animals with persistent inflammatory pain.

FINDINGS

In the present study, we examined IL-8 expression in the ACC, somatosensory cortex (SSC), and the dorsal horn of lumbar spinal cord following hind-paw administration of complete Freund's adjuvant (CFA) in mice and its effects on the ACC synaptic transmission. Quantification of IL-8 at protein level (by ELISA) revealed enhanced expression in the ACC and spinal cord during the chronic phases of CFA-induced peripheral inflammation. In vitro whole-cell patch-clamp recordings revealed that IL-8 significantly enhanced synaptic transmission through increased probability of neurotransmitter release in the ACC slice. ACC local infusion of repertaxin, a non-competitive allosteric blocker of IL-8 receptors, notably prolonged the paw withdrawal latency to thermal radian heat stimuli bilaterally in mice.

CONCLUSIONS

Our findings suggest that up-regulation of IL-8 in the ACC partly attributable to the enhanced prefrontal synaptic transmission in the mice with persistent inflammatory pain.

Cholinergic suppression of excitatory synaptic transmission in layers II/III of the parasubiculum

Layer II of the parasubiculum (PaS) receives excitatory synaptic input from the CA1 region of the hippocampus and sends a major output to layer II of the medial and lateral entorhinal cortex. The PaS also receives heavy cholinergic innervation from the medial septum, which contributes to the generation of theta-frequency (4-12 Hz) electroencephalographic (EEG) activity. Cholinergic receptor activation exerts a wide range of effects in other areas of the hippocampal formation, including membrane depolarization, changes in neuronal excitability, and suppression of excitatory synaptic responses. The present study was aimed at determining how cholinergic receptor activation modulates excitatory synaptic input to the layer II/III neurons of the PaS in acute brain slices. Field excitatory postsynaptic potentials (fEPSPs) in layer II/III of the PaS were evoked by stimulation of either layer I afferents, or ascending inputs from layer V. Bath-application of the cholinergic agonist carbachol (0.5-10 μM) suppressed the amplitude of fEPSPs evoked by both superficial- and deep layer stimulation, and also enhanced paired-pulse facilitation. Constant bath-application of the GABA(A) antagonist bicuculline (10 μM) failed to eliminate the suppression, indicating that the cholinergic suppression of fEPSPs is not due to increased inhibitory tone. The muscarinic receptor antagonist atropine (1 μM) blocked the suppression of fEPSPs, and the selective M(1)-preferring receptor antagonist pirenzepine (1 μM), but not the M(2)-preferring antagonist methoctramine (1-5 μM), also significantly attenuated the suppression. Therefore, cholinergic receptor activation suppresses excitatory synaptic input to layer II/III neurons of the PaS, and this suppression is mediated in part by M(1) receptor activation.