Frontiers in Synaptic Neuroscience最新文献

Epilepsy is a prevalent neurological condition, with underlying neuronal mechanisms involving hyperexcitability and hypersynchrony. Imbalance between excitatory and inhibitory circuits, as well as histological reorganization are relatively well-documented in animal models or even in the human hippocampus, but less is known about human neocortical epileptic activity. Our knowledge about changes in the excitatory signaling is especially scarce, compared to that about the inhibitory cell population. This study investigated the firing properties of single neurons in the human neocortex in vitro, during pharmacological blockade of glutamate receptors, and additionally evaluated anatomical changes in the excitatory circuit in tissue samples from epileptic and non-epileptic patients. Both epileptic and non-epileptic tissues exhibited spontaneous population activity (SPA), NMDA receptor antagonization reduced SPA recurrence only in epileptic tissue, whereas further blockade of AMPA/kainate receptors reversibly abolished SPA emergence regardless of epilepsy. Firing rates did not significantly change in excitatory principal cells and inhibitory interneurons during pharmacological experiments. Granular layer (L4) neurons showed an increased firing rate in epileptic compared to non-epileptic tissue. The burstiness of neurons remained unchanged, except for that of inhibitory cells in epileptic recordings, which decreased during blockade of glutamate receptors. Crosscorrelograms computed from single neuron discharge revealed both mono- and polysynaptic connections, particularly involving intrinsically bursting principal cells. Histological investigations found similar densities of SMI-32-immunopositive long-range projecting pyramidal cells in both groups, and shorter excitatory synaptic active zones with a higher proportion of perforated synapses in the epileptic group. These findings provide insights into epileptic modifications from the perspective of the excitatory system and highlight discrete alterations in firing patterns and synaptic structure. Our data suggest that NMDA-dependent glutamatergic signaling, as well as the excitatory synaptic machinery are perturbed in epilepsy, which might contribute to epileptic activity in the human neocortex.

Glia are as numerous in the brain as neurons and widely known to serve supportive roles such as structural scaffolding, extracellular ionic and neurotransmitter homeostasis, and metabolic support. However, over the past two decades, several lines of evidence indicate that astrocytes, which are a type of glia, play active roles in neural information processing. Astrocytes, although not electrically active, can exhibit a form of excitability by dynamic changes in intracellular calcium levels. They sense synaptic activity and release neuroactive substances, named gliotransmitters, that modulate neuronal activity and synaptic transmission in several brain areas, thus impacting animal behavior. This "dialogue" between astrocytes and neurons is embodied in the concept of the tripartite synapse that includes astrocytes as integral elements of synaptic function. Here, we review the recent work and discuss how astrocytes via calcium-mediated excitability modulate synaptic information processing at various spatial and time scales.

Cannabis exposure during gestation evokes significant molecular modifications to neurodevelopmental programs leading to neurophysiological and behavioral abnormalities in humans. The main neuronal receptor for Δ⁹-tetrahydrocannabinol (THC) is the type-1 cannabinoid receptor CB₁R, one of the most abundant G-protein-coupled receptors in the nervous system. While THC is the major psychoactive phytocannabinoid, endocannabinoids (eCBs) are the endogenous ligands of CB₁R and are known to act as retrograde messengers to modulate synaptic plasticity at different time scales in the adult brain. Accumulating evidence indicates that eCB signaling through activation of CB₁R plays a central role in neural development. During development, most CB₁R localized to axons of projection neurons, and in mice eCB signaling impacts axon fasciculation. Understanding of eCB-mediated structural plasticity during development, however, requires the identification of the precise spatial and temporal dynamics of CB₁R-mediated modifications at the level of individual neurons in the intact brain. Here, the cell-autonomous role of CB₁R and the effects of CB₁R-mediated eCB signaling were investigated using targeted single-cell knockdown and pharmacologic treatments in Xenopus. We imaged axonal arbors of retinal ganglion cells (RGCs) in real time following downregulation of CB₁R via morpholino (MO) knockdown. We also analyzed RGC axons with altered eCB signaling following treatment with URB597, a selective inhibitor of the enzyme that degrades Anandamide (AEA), or JZL184, an inhibitor of the enzyme that blocks 2-Arachidonoylglycerol (2-AG) hydrolysis, at two distinct stages of retinotectal development. Our results demonstrate that CB₁R knockdown impacts RGC axon branching at their target and that differential 2-AG and AEA-mediated eCB signaling contributes to presynaptic structural connectivity at the time that axons terminate and when retinotectal synaptic connections are made. Altering CB₁R levels through CB₁R MO knockdown similarly impacted dendritic morphology of tectal neurons, thus supporting both pre- and postsynaptic cell-autonomous roles for CB₁R-mediated eCB signaling.

The unitary postsynaptic response to presynaptic quantal glutamate release is the fundamental basis of excitatory information transfer between neurons. The view, however, of individual glutamatergic synaptic connections in a population as homogenous, fixed-strength units of neural communication is becoming increasingly scrutinized. Here, we used minimal stimulation of individual glutamatergic afferent axons to evoke single synapse resolution postsynaptic responses from central sensory lamina I neurons in an ex vivo adult rat spinal slice preparation. We detected unitary events exhibiting a NMDA receptor component with distinct kinetic properties across synapses conferred by specific GluN2 subunit composition, indicative of GluN2 subtype-based postsynaptic heterogeneity. GluN2A, 2A and 2B, or 2B and 2D synaptic predominance functioned on distinct lamina I neuron types to narrowly, intermediately, or widely tune, respectively, the duration of evoked unitary depolarization events from resting membrane potential, which enabled individual synapses to grade differentially depolarizing steps during temporally patterned afferent input. Our results lead to a model wherein a core locus of proteomic complexity prevails at this central glutamatergic sensory synapse that involves distinct GluN2 subtype configurations. These findings have major implications for subthreshold integrative capacity and transmission strength in spinal lamina I and other CNS regions.

For roughly the last 30 years, the notion that striatal dopamine (DA) depletion was the critical determinant of network pathophysiology underlying the motor symptoms of Parkinson's disease (PD) has dominated the field. While the basal ganglia circuit model underpinning this hypothesis has been of great heuristic value, the hypothesis itself has never been directly tested. Moreover, studies in the last couple of decades have made it clear that the network model underlying this hypothesis fails to incorporate key features of the basal ganglia, including the fact that DA acts throughout the basal ganglia, not just in the striatum. Underscoring this point, recent work using a progressive mouse model of PD has shown that striatal DA depletion alone is not sufficient to induce parkinsonism and that restoration of extra-striatal DA signaling attenuates parkinsonian motor deficits once they appear. Given the broad array of discoveries in the field, it is time for a new model of the network determinants of motor disability in PD.

Synapses are integral for healthy brain function and are becoming increasingly recognized as key structures in the early stages of brain disease. Understanding the pathological processes driving synaptic dysfunction will unlock new therapeutic opportunities for some of the most devastating diseases of our time. To achieve this we need a solid repertoire of imaging and molecular tools to interrogate synaptic biology at greater resolution. Synapses have historically been examined in small numbers, using highly technical imaging modalities, or in bulk, using crude molecular approaches. However, recent advances in imaging techniques are allowing us to analyze large numbers of synapses, at single-synapse resolution. Furthermore, multiplexing is now achievable with some of these approaches, meaning we can examine multiple proteins at individual synapses in intact tissue. New molecular techniques now allow accurate quantification of proteins from isolated synapses. The development of increasingly sensitive mass-spectrometry equipment means we can now scan the synaptic molecular landscape almost in totality and see how this changes in disease. As we embrace these new technical developments, synapses will be viewed with clearer focus, and the field of synaptopathy will become richer with insightful and high-quality data. Here, we will discuss some of the ways in which synaptic interrogation is being facilitated by methodological advances, focusing on imaging, and mass spectrometry.

Chronic pain is long-lasting pain that often persists during chronic diseases or after recovery from disease or injury. It often causes serious side effects, such as insomnia, anxiety, or depression which negatively impacts the patient's overall quality of life. Serotonin (5-HT) in the central nervous system (CNS) has been recognized as an important neurotransmitter and neuromodulator which regulates various physiological functions, such as pain sensation, cognition, and emotions-especially anxiety and depression. Its widespread and diverse receptors underlie the functional complexity of 5-HT in the CNS. Recent studies found that both chronic pain and anxiety are associated with synaptic plasticity in the anterior cingulate cortex (ACC), the insular cortex (IC), and the spinal cord. 5-HT exerts multiple modulations of synaptic transmission and plasticity in the ACC and the spinal cord, including activation, inhibition, and biphasic actions. In this review, we will discuss the multiple actions of the 5-HT system in both chronic pain and injury-related anxiety, and the synaptic mechanisms behind them. It is likely that the specific 5-HT receptors would be new promising therapeutic targets for the effective treatment of chronic pain and injury-related anxiety in the future.

The efficiency of neocortical information processing critically depends on the balance between the glutamatergic (excitatory, E) and GABAergic (inhibitory, I) synaptic transmission. A transient imbalance of the E/I-ratio during early development might lead to neuropsychiatric disorders later in life. The transgenic glutamic acid decarboxylase 67-green fluorescent protein (GAD67-GFP) mouse line (KI) was developed to selectively visualize GABAergic interneurons in the CNS. However, haplodeficiency of the GAD67 enzyme, the main GABA synthetizing enzyme in the brain, temporarily leads to a low GABA level in the developing brain of these animals. However, KI mice did not demonstrate any epileptic activity and only few and mild behavioral deficits. In the present study we investigated how the developing somatosensory cortex of KI-mice compensates the reduced GABA level to prevent brain hyperexcitability. Whole-cell patch clamp recordings from layer 2/3 pyramidal neurons at P14 and at P21 revealed a reduced frequency of miniature inhibitory postsynaptic currents (mIPSCs) in KI mice without any change in amplitude or kinetics. Interestingly, mEPSC frequencies were also decreased, while the E/I-ratio was nevertheless shifted toward excitation. Surprisingly, multi-electrode-recordings (MEA) from acute slices revealed a decreased spontaneous neuronal network activity in KI mice compared to wild-type (WT) littermates, pointing to a compensatory mechanism that prevents hyperexcitability. Blockade of GABA_B receptors (GABA_BRs) with CGP55845 strongly increased the frequency of mEPSCs in KI, but failed to affect mIPSCs in any genotype or age. It also induced a membrane depolarization in P14 KI, but not in P21 KI or WT mice. MEA recordings in presence of CGP55845 revealed comparable levels of network activity in both genotypes, indicating that tonically activated GABA_BRs balance neuronal activity in P14 KI cortex despite the reduced GABA levels. Blockade of GABA transporter 3 (GAT-3) reproduced the CGP55845 effects suggesting that tonic activation of GABA_BRs is mediated by ambient GABA released via GAT-3 operating in reverse mode. We conclude that GAT-3-mediated GABA release leads to tonic activation of both pre- and postsynaptic GABA_BRs and restricts neuronal excitability in the developing cortex to compensate for reduced neuronal GABA synthesis. Since GAT-3 is predominantly located in astrocytes, GAD67 haplodeficiency may potentially stimulate astrocytic GABA synthesis through GAD67-independent pathways.