Critical reviews in neurobiology最新文献

We have previously described a new endogenous phosphorylation mechanism that maintains ionotropic gamma-aminobutyric acid receptor (GABAAR) function and have shown that the kinase involved is the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). This enzyme is closely associated with the receptor and phosphorylates the alpha1 subunit of the receptor. In a wealth of studies, a reduction in GABAergic neurotransmission has been suggested as a pathophysiological mechanism for human epilepsy. In this paper, we present evidence showing both reduced efficacy of this glycolysis-dependent GABAAR phosphorylation mechanism and of GABAergic inhibition in epileptogenic cortical tissue samples obtained during curative surgery of patients with partial seizures, as compared to non-epileptogenic human cortical tissue. This feature is not due to a reduction in the density of GABAAR alpha1 subunits in the epileptogenic tissue as evidenced by photoaffinity labeling. Maintaining the receptor in a phosphorylated state either by favoring the endogenous phosphorylation or by inhibiting a membrane-bound phosphatase sustains the GABAAR responses in the human epileptogenic cortex. The deficiency in endogenous phosphorylation and the associated decreased GABAAR function can account for transient failures of GABAergic inhibition and may favor seizure initiation and propagation. These findings suggest a functional link between epileptic pathology and the regional cerebral glucose hypometabolism observed in patients with partial epilepsies, since the dysfunction of the GABAergic mechanism is dependent on locally produced glycolytic ATP. They also point to new targets for developing molecules active in drug-resistant epilepsies.

The direction of plasticity at CA3-CA1 hippocampal synapses is determined by the strength of afferent stimulation. Weak stimuli lead to long-term depression (LTD) and strong stimuli to long-term potentiation (LTP), but both require activation of synaptic N-methyl-D-aspartate receptors (NMDARs). These receptors are therefore necessary and required for the induction of plasticity at CA3-CA1 synapses even though they carry little of the current responsible for the basal excitatory post-synaptic potential (EPSP). The influx of Ca(2+) via NMDARs triggers the subsequent and persistent changes in the expression of alpha-amino-3-hydroxy-5 methylisoxazole-4-proprionic acid receptors (AMPARs) and these receptors are responsible for the major part of the basal EPSP. The degree of activity of NMDARs is determined in part by extracellular Mg(2+) and by the co-agonists for this receptor, glycine and D-serine. During strong stimulation, a relief of the voltage-dependent block of NMDARs by Mg(2+) provides a positive feedback for NMDAR Ca(2+) influx into postsynaptic CA1 spines. In this review, we discuss how the induction of LTP at CA3-CA1 synapses requires further signal amplification of NMDAR activity. We discuss how the regulation of NMDARs by protein kinases and phosphatases is brought into play. Evidence is presented that Src family kinases (SFKs) play a "core" role in the induction of LTP by enhancing the function and expression of NMDARs. At CA3-CA1 synapses, NMDARs are largely composed of NR1 (NMDA receptor subunit 1)-NR2A or NR1-NR2B containing subunits. Recent, but controversial, evidence has correlated NR1-NR2A receptors with the induction of LTP and NR1-NR2B receptors with LTD. However, LTP can be induced by activation of either subtype of NMDAR and the ratio of NR2A:NR2B receptors has been proposed as an alternative determinant of the direction of synaptic plasticity. Many transmitters and signal pathways can modify NMDAR function and expression and, for a given stimulus strength, they can potentially lead to a change in the balance between LTP and LTD. As opposed to the "core" mechanisms of LTP and LTD, the resulting alterations in this balance underlie "meta-plasticity." Thus, in addition to their contribution to core mechanisms, we will also discuss how Src-family kinases could preferentially target NR1-NR2A or NR1-NR2B receptors to alter the relative contribution of these receptor subtypes to synaptic plasticity.

The mammalian thoracolumbar spinal cord contains all the necessary elements to generate a rhythmic oscillatory activity that is transformed into locomotor commands to agonist and antagonist limb muscles to produce gait at various speed. This motor program is produced by interneurons in the ventral horn and can be readily recorded even with in vitro spinal cord preparations isolated from rats or mice (once dorsal afferents are stimulated or excitatory neuronchemicals applied). The locomotor program is continuously modulated and refined by afferent sensory inputs and by signals descending from brain centers. Nevertheless, this program is not the only type of rhythmic discharge produced by spinal networks. In fact, activation of metabotropic group I glutamate receptors or block of certain K+ currents by 4-aminopyridine generates non-locomotor discharges, and, at the same time, facilitates evoked locomotor activity, which then suppresses any other interfering rhythmicity. These findings suggest that accessory networks, activated by suitable stimuli, might be exploited to restore locomotor activity damaged by a lesion, an obvious goal for neuro-rehabilitation purposes. The structure of the locomotor networks appears to include a rhythm-generating circuit that drives a pattern formation circuit, commanding motoneurons to discharge appropriate signals to skeletal muscles. Studies with the K+-channel blocker tetraethylammonium have indicated that this hierarchical arrangement is preserved in vitro. Hence, isolated spinal cord preparations represent an interesting experimental tool to investigate new mechanisms to upregulate various components of locomotor networks, especially after the induction of experimental lesions.

Once the tools for controlling calcium gradients became available to electrophysiologists, they began the quest for understanding the role of Ca2+ in the control of neuronal activity. In the early 1970s Paul Feltz and I spent a rich period in K. Krnjevic's laboratory in Montreal, and I was already involved in a research, which showed that an increase in intracellular Ca2+ concentration can lead to hyperpolarization of motoneurones. At about the same time, a potassium conductance activated by intracellular calcium injection was identified in mammals and snails. Since then, most of my work has dealt with the study of Ca2+ entry in neurons. Here I review the progress that led fi rst to the biophysical characterization and, later, to the molecular identification of T-type calcium channels. With the advent of new optical methods, in particular two-photon microscopy, we may be on the brink of a step forward in our understanding of how T channels play a role in the integrative processes that take place in a large cortical neuron such as the Purkinje cell.

Repetitive firing neuron or activation of synaptic transmission plays an important role in the modulation of synaptic efficacy, such as long-term potentiation (LTP) and long-term depression (LTD). These activity-dependent changes in synaptic efficacy are thought to be critical to learning and memory; however, the underlying mechanisms remain to be defined. Endogenous cannabinoids (eCBs) are diffusible modulators that are released from depolarized postsynaptic neurons and act on presynaptic terminals. Persistent release of eCBs can lead to long-term modulation of synaptic plasticity in the brain. Given a broad distribution of eCB receptors in the brain, the eCB signaling system could contribute to use-dependent modification of brain functions.

The sensory relay synapses in the thalamus undergo extensive refinement during early life. Disruptions of spontaneous activity, but not sensory deprivation, can induce large-scale re-organization of neuronal connections in the thalamus. Recent studies also reveal an extended period of synaptic refinement in the visual and somatosensory relay synapses, where sensory deprivation produces some unexpected effects on synaptic remodeling. This article aims to provide a brief overview of recent findings and current ideas about the refinement of relay synapses in the thalamus.

Behavioral sensitization is the augmented motor-stimulant response that occurs with repeated, intermittent exposure to most drugs of abuse, including cocaine. Sensitization, which is a long-lasting phenomenon, is thought to underlie drug craving and relapse to drug use. Much research has been conducted to determine the neural mechanisms of sensitization. The bulk of this effort has focused on the nucleus accumbens and ventral tegmental area (VTA) that comprise a portion of the mesolimbic dopamine system. Recently, studies have begun to also explore the role of the medial prefrontal cortex (mPFC) in sensitization, in part because this region provides glutamatergic innervation to the VTA and nucleus accumbens. The present review will coalesce these studies into a working hypothesis that states that cocaine sensitization results from a decrease in inhibitory modulation of excitatory transmission from the mPFC to the VTA and nucleus accumbens. The discussion will revolve around how repeated cocaine exposure alters dopamine, gamma-aminobutyric acid (GABA), and glutamate regulation of pyramidal cell activity. It will be proposed that cocaine-induced alterations in cortical transmission occur in two phases. During early withdrawal from repeated cocaine exposure, changes in neurotransmitter release are thought to underlie the decreased inhibitory modulation of pyramidal projection neurons. Following more prolonged withdrawal, the attenuation in inhibitory transmission appears to occur at the receptor level. A model will be presented that may serve to direct future studies on the involvement of the mPFC in the development of cocaine sensitization, which ultimately could lead to development of pharmacotherapies for cocaine addiction.

Recent advances in micro- and nano-fabrication techniques have led to the development of microfluidic platforms designed for in vitro biological studies. Based on their capability of precise control of the environment surrounding individual cells, these microfluidic platforms have been increasingly utilized to investigate physiologic responses at the single-cell level. It is likely that these devices will continue to gain popularity as a tool to study the behavior of individual cells as they are exposed to extrinsic agents and other cells. This article reviews microfluidic technology and its application to single-cell research, with emphasis on advances that are particularly useful for neuronal studies, such as platforms with patterned physical and chemical cues, integrated electrophysiology and other sensors, architecture for isolation of axons, and delivery of precisely controlled chemical factors.

Neuroblastoma cell lines have been used extensively to screen novel compounds for neurotoxic properties and associated mechanisms. Such transformed cell lines often display morphological, developmental, and signaling characteristics that are substantially different from the parental cell type. Consequently, the response of neuroblastoma cells to toxin exposure may differ from that of neurons. An appreciation of the pharmacological and functional differences between neurons and neuron-like cell lines is therefore essential when interpreting data derived from neuroblastoma-based assays. We have compared the effects of several neurotoxins on Ca2+ homeostasis and cell viability in cerebellar granule neurons (CGN) and a neuroblastoma cell line (Neuro-2a). To explore the mechanisms underlying differential sensitivity of intact neurons and neuroblastoma cells to neurotoxins, we also compared CGN and Neuro-2a cells for expression of voltage-gated sodium channels (VGSC) and N-methyl-D-aspartate receptors (NMDAR). Cytotoxic potency in neurons was several orders of magnitude greater for Caribbean-ciguatoxin-1 (C-CTX-1) than either domoate (Dom) or brevetoxin-2 (PbTx-2). In addition, the cytotoxic potency of C-CTX-1 was two orders of magnitude greater in CGN than in Neuro-2a cells. The effect of C-CTX-1 and Dom on calcium homeostasis was compared in fluo-3 loaded neurons. Dom caused an elevation in intracellular calcium ([Ca2+]i) at concentrations that paralleled the concentration/response relationship for cytotoxicity in CGN. Conversely, C-CTX-1 did not elevate [Ca2+]i within the dynamic concentration range for cell death. The discordance of the concentration/response relationships for C-CTX-1 induced cytotoxicity and [Ca2+]i elevation suggests that acute C-CTX-1 cytotoxicity may involve mechanisms other than Ca2+ load. C-CTX-1-induced elevation of [Ca2+]i in neurons was dependent on activation of NMDAR and the reverse mode of operation of the Na+/Ca2+ exchanger. These data demonstrate that, although C-CTX-1, domoate, and PbTx-2 share the ability to produce neurotoxicity and mobilize calcium, their respective molecular targets and mechanisms of neurotoxicity differ. Neuro-2a cells that were not pretreated with veratridine and ouabain were insensitive to C-CTX-1 and glutamatergic agonists. VGSC expression was 20-fold lower in Neuro-2a cells than in CGN, whereas NMDARs were not expressed in these neuroblastoma cells. It is therefore likely that the enhanced sensitivity of CGN, relative to Neuro-2a cells, to neurotoxins is a consequence of pronounced differences in VGSC and NMDAR expression. These results underscore the need to exercise caution in interpreting negative cytotoxicity data derived from the use of neuroblastoma cell lines.

Some 50 years have elapsed since Elliot et al. and MacIntosh & Oborin first reported a release of acetylcholine (ACh) from canine and feline cerebral cortices, respectively. In this review, subsequent developments in the field during the succeeding five decades are explored. The arrangement of material in the review is outlined in this abstract, concluding with some suggestions as to its potential significance. A number of technical advances during this period have contributed to a greater understanding of the role that ACh may play in the central nervous system. These include the relatively recent evolution of the microdialysis and transverse dialysis techniques that enabled investigators to explore ACh release in deep regions of the brain. Future studies will likely be refined with the use of microelectrode biosensors, which should allow real-time measurements of ACh concentrations at the synaptic level. Controversies arising from the use of cholinesterase inhibitors and muscarinic receptor antagonists to enhance release are being resolved as a result of a better understanding of the presynaptic actions of these agents. Future studies will also benefit from the recent development of clostridial and other neurotoxins to reduce ACh release in areas of the brain. The likelihood that ACh may act as a cotransmitter at synapses in conjunction with glutamic acid, nitric oxide, and adenosine triphosphate is also explored. Attention is focused on the elucidation of choline acetyl-transferase (ChAT)-containing pathways in the central nervous system using techniques such as immunohistochemistry, in situ hybridization, histochemistry of ChAT mRNA, acetylcholinesterase histochemistry, and the distribution of the vesicular ACh transporter. Such studies have defined several major groupings of cholinergic neurons in the brain, which provide ascending or descending projections to higher and lower central structures. A major section of the review is devoted to actual studies on ACh release in the brain and spinal cord. This presentation is in two sections. The text details some of the material that has been obtained in experiments over the past 50 years. In five Tables, the results obtained in the majority of release studies to date are summarized. Although the data obtained to date clearly support the hypothesis that ACh is involved in electroencephalographic activation associated with cerebral cortical arousal, this occurs while the animals appear to be awake with full postural control, suggesting that noncholinergic pathways to the cerebral cortex are also involved in such behavioral manifestations. The roles of acetylcholine in cognitive processes such as attention, learning, memory, responses to environmental changes, and motor activity still remain to be defined.