Dynamics of Synaptic Transmission in the CNS: Contribution of Neuron-glia Interactions

The last 30 years have seen a growing appreciation of the importance for CNS functioning of the internal state of neural tissues, which is exquisitely reflective of the immediate-to-long-term history of the preceding neural activity experienced by those tissues. This dissertation comprises three research projects that together address different, but related aspects of dynamics of the state of neural tissues, with a focus on the roles played by astroglia and GABAergic synaptic transmission. The first two projects study the relationship between stimulus-evoked glial and neuronal activities within local networks of the dorsal horn of the spinal cord and sensitivity of GABAergic actions to the state of local glia and prior sensory stimulation, whereas the third project investigates the sensitivity of GABAergic actions to prior sensory stimulation in the neocortex. Project 1: Origins of Optical Intrinsic Signal and its significance. In rat spinal cord slice, repetitive electrical stimulation of the dorsal root at an intensity that activates C-fibers evokes a slow-to-develop and prolonged (30-50 s) change in light transmittance (OISDR) in the superficial part of the ipsilateral dorsal horn (DHs). Inhibition of astrocyte metabolism by bath-applied fluoroacetate and glutamine (FAc+Gln), or interference with glial and neuronal K+ transport by 4-aminopyridine (4-AP) lead to dissociation of the OISDR and the postsynaptic DHs response to a single-pulse dorsal root stimulus (P-PSPDR). The OISDR decreases under FAc+Gln, whereas the P-PSPDR remains unaltered; under 4-AP, the P-PSPDR increases, but the OISDR decreases. In contrast, both the OISDR and P-PSPDR increase when K+o is elevated. These observations indicate that the OISDR mainly reflects cell volume and light scattering changes associated with DHs astrocyte uptake of K+ and glutamate (GLU). In slices from subjects that received an intracutaneous injection of formalin 3-5 days earlier, both the OISDR and the response of the DHs to local application of K+ or GLU are profoundly reduced, and the normally exquisite sensitivity of the DHs to elevated K+o is decreased. Considered collectively, the observations raise the possibility that impaired regulation of DHs K+o and GLUo may contribute to initiation and maintenance of the CNS pain circuit and sensorimotor abnormalities that develop following intracutaneous formalin injection. Project 2: Effects of alteration of glia on neuronal plasticity. Transient (20min) exposure of the spinal cord slice to fluorocitrate (FC; a reversible inhibitor of glial energy production via the TCA cycle) is shown to be accompanied by a protracted decrease of the superficial dorsal horn (DHs) optical response to repetitive electrical stimulation of the ipsilateral dorsal root, and by a similarly protracted increase in the postsynaptic response of the DHs to single-pulse stimulation of the attached dorsal root (LTPFC). It also is shown that LTPFC does not occur in the presence of d-aminophosphopentanoic acid (APV), becomes progressively smaller as [K+]o in the perfusion solution is decreased from 3.0 mM (normal) to 0.0 mM, and is reduced or eliminated by bath application of 1 mM bicuculline. Somal whole-cell patch recordings were carried out to evaluate the effects of FC on the response of DHs neurons to puffer-applied GABA. The observations reveal that transient exposure of the slice to FC is reliably accompanied by a prolonged (>1 hr) depolarizing shift of the equilibrium potential for the DHs neuron transmembrane ionic currents evoked by GABA (average EGABApreFC: -75 mV ; EGABApostFC: -50 mV). Considered collectively, the findings demonstrate that LTPFC involves (1) elevation of [K+]o in the DHs, (2) NMDA receptor activation, and (3) conversion of the effect of GABA on DHs neurons from inhibition to excitation. It is proposed that a transient impairment of astrocyte energy production via the TCA cycle can trigger the cascade of dorsal horn mechanisms that underlies hyperalgesia and persistent pain. Project 3: Contribution of GABA to cerebral cortical dynamics. Imaging of the optical intrinsic signal (OIS), evoked in the rat sensorimotor cortical slice by 1s-long 20Hz electrical stimulation applied to locus at the layer VI/white matter junction, was used to delineate a column-shaped cortical region responding to a local thalamocortical input drive, and whole-cell patch clamp recordings were obtained from layer II-III pyramidal neurons residing in that region. Puffs of pressure-ejected GABA were released from a micropipette in a close vicinity of the recorded neuron's soma before and also immediately after conditioning electrical stimulation. Prior to conditioning stimulation, GABA puffs hyperpolarized the recorded neurons, whereas for ~15s subsequent to conditioning stimulation GABA puffs depolarized the same neurons. Two-photon Cl- imaging in cortical slices taken from CLM1 Clomeleon mice revealed that conditioning stimulation transiently elevates [Cl-]i in the stimulated cortical column; this increase is blocked by SR95531 (gabazine), a selective GABAA receptor antagonist. Next, two-photon Ca2+ imaging revealed that isoguvacine (GABAA receptor agonist) increases Ca2+ influx into neurons in the stimulated cortical column. Finally, OIS imaging in the presence of GABA antagonist bicuculline suggests that the depolarizing action of GABA is confined to the center of the stimulated cortical region, while at its margins GABA remains hyperpolarizing. Taken together, these findings suggest that synaptically released GABA can be either inhibitory or excitatory, depending on the activity state of the local network. Such activity dependence of GABA action can be expected to funnel stimulus-evoked activity in a cortical area into the central, most strongly driven cortical columns

Spin-Filtering Multiferroic-Semiconductor Heterojunctions

We report on the structural and electronic properties of the interface between the multiferoic oxide YMnO$_3$ and wide band-gap semiconductor GaN studied with the Hubbard-corrected local spin density approximation (LSDA+U) to density-functional theory (DFT). We find that the band offsets at the interface between antiferromagnetically ordered YMnO$_3$ and GaN are different for spin-up and spin-down states. This behavior is due to the spin splitting of the valence band induced by the interface. The energy barrier depends on the relative orientation of the electric polarization with respect to the polarization direction of the GaN substrate suggesting an opportunity to create magnetic tunnel junctions in this materials system.Comment: 4 pages, 4 figure

Interfacial Magnetoelectric Coupling in Tri-component Superlattices

Using first-principles density functional theory, we investigate the interfacial magnetoelectric coupling in a tri-component superlattice composed of a ferromagnetic metal (FM), ferroelectric (FE), and normal metal (NM). Using Fe/FE/Pt as a model system, we show that a net and cumulative interfacial magnetization is induced in the FM metal near the FM/FE interface. A carefully analysis of the magnetic moments in Fe reveals that the interfacial magnetization is a consequence of a complex interplay of interfacial charge transfer, chemical bonding, and spin dependent electrostatic screening. The last effect is linear in the FE polarization, is switchable upon its reversal, and yields a substantial interfacial magnetoelectric coupling.Comment: 5 pages, 6 figure

Magnetoelectric Coupling and Electric Control of Magnetization in Ferromagnet-Ferroelectric-Metal Superlattices

Ferromagnet-ferroelectric-metal superlattices are proposed to realize the large room-temperature magnetoelectric effect. Spin dependent electron screening is the fundamental mechanism at the microscopic level. We also predict an electric control of magnetization in this structure. The naturally broken inversion symmetry in our tri-component structure introduces a magnetoelectric coupling energy of $P M^2$. Such a magnetoelectric coupling effect is general in ferromagnet-ferroelectric heterostructures, independent of particular chemical or physical bonding, and will play an important role in the field of multiferroics.Comment: 5 pages including 3 figures and 1 tabl

Fluoride Induces a Volume Reduction in CA1 Hippocampal Slices Via MAP Kinase Pathway Through Volume Regulated Anion Channels

Regulation of cell volume is an important aspect of cellular homeostasis during neural activity. This volume regulation is thought to be mediated by activation of specific transporters, aquaporin, and volume regulated anion channels (VRAC). In cultured astrocytes, it was reported that swelling-induced mitogen-activated protein (MAP) kinase activation is required to open VRAC, which are thought to be important in regulatory volume decrease and in the response of CNS to trauma and excitotoxicity. It has been also described that sodium fluoride (NaF), a recognized G-protein activator and protein phosphatase inhibitor, leads to a significant MAP kinase activation in endothelial cells. However, NaF's effect in volume regulation in the brain is not known yet. Here, we investigated the mechanism of NaF-induced volume change in rat and mouse hippocampal slices using intrinsic optical signal (IOS) recording, in which we measured relative changes in intracellular and extracellular volume as changes in light transmittance through brain slices. We found that NaF (1~5 mM) application induced a reduction in light transmittance (decreased volume) in CA1 hippocampus, which was completely reversed by MAP kinase inhibitor U0126 (10 µM). We also observed that NaF-induced volume reduction was blocked by anion channel blockers, suggesting that NaF-induced volume reduction could be mediated by VRAC. Overall, our results propose a novel molecular mechanism of NaF-induced volume reduction via MAP kinase signaling pathway by activation of VRAC

Imiquimod enhances excitability of dorsal root ganglion neurons by inhibiting background (K2P) and voltage-gated (Kv1.1 and Kv1.2) potassium channels

<p>Abstract</p> <p>Background</p> <p>Imiquimod (IQ) is known as an agonist of Toll-like receptor 7 (TLR7) and is widely used to treat various infectious skin diseases. However, it causes severe itching sensation as its side effect. The precise mechanism of how IQ causes itching sensation is unknown. A recent report suggested a molecular target of IQ as TLR7 expressed in dorsal root ganglion (DRG) neurons. However, we recently proposed a TLR7-independent mechanism, in which the activation of TLR7 is not required for the action of IQ in DRG neurons. To resolve this controversy regarding the involvement of TLR7 and to address the exact molecular identity of itching sensation by IQ, we investigated the possible molecular target of IQ in DRG neurons.</p> <p>Findings</p> <p>When IQ was applied to DRG neurons, we observed an increase in action potential (AP) duration and membrane resistance both in wild type and TLR7-deficient mice. Based on these results, we tested whether the treatment of IQ has an effect on the activity of K⁺channels, K_v1.1 and K_v1.2 (voltage-gated K⁺channels) and TREK1 and TRAAK (K_2Pchannels). IQ effectively reduced the currents mediated by both K⁺channels in a dose-dependent manner, acting as an antagonist at TREK1 and TRAAK and as a partial antagonist at K_v1.1 and K_v1.2.</p> <p>Conclusions</p> <p>Our results demonstrate that IQ blocks the voltage-gated K⁺channels to increase AP duration and K_2Pchannels to increase membrane resistance, which are critical for the membrane excitability of DRG neurons. Therefore, we propose that IQ enhances the excitability of DRG neurons by blocking multiple potassium channels and causing pruritus.</p

Activation of protease activated receptor 1 increases the excitability of the dentate granule neurons of hippocampus

Protease activated receptor-1 (PAR1) is expressed in multiple cell types in the CNS, with the most prominent expression in glial cells. PAR1 activation enhances excitatory synaptic transmission secondary to the release of glutamate from astrocytes following activation of astrocytically-expressed PAR1. In addition, PAR1 activation exacerbates neuronal damage in multiple in vivo models of brain injury in a manner that is dependent on NMDA receptors. In the hippocampal formation, PAR1 mRNA appears to be expressed by a subset of neurons, including granule cells in the dentate gyrus. In this study we investigate the role of PAR activation in controlling neuronal excitability of dentate granule cells. We confirm that PAR1 protein is expressed in neurons of the dentate cell body layer as well as in astrocytes throughout the dentate. Activation of PAR1 receptors by the selective peptide agonist TFLLR increased the intracellular Ca2+ concentration in a subset of acutely dissociated dentate neurons as well as non-neuronal cells. Bath application of TFLLR in acute hippocampal slices depolarized the dentate gyrus, including the hilar region in wild type but not in the PAR1-/- mice. PAR1 activation increased the frequency of action potential generation in a subset of dentate granule neurons; cells in which PAR1 activation triggered action potentials showed a significant depolarization. The activation of PAR1 by thrombin increased the amplitude of NMDA receptor-mediated component of EPSPs. These data suggest that activation of PAR1 during normal function or pathological conditions, such as during ischemia or hemorrhage, can increase the excitability of dentate granule cells

Electrostatic capacitance of TiO_2 nanowires in a porous alumina template

Titanium oxide (TiO_2) nanowires were prepared for an electrolytic capacitor application by the automatic dipping technique using a porous alumina template. The automatic dipping technique allows us to exactly control the dipping rate so that we can obtain homogenous infiltration of nanowires in the porous alumina membrane, even though the solution is very acidic. From the TEM, SEM and XRD measurements, we confirmed that anatase phase TiO2 nanowires are highly infiltrated into the porous alumina template. In addition, the electrostatic capacitance of nanowires was measured and compared with a theoretical calculation using an effective thickness (delta e). We found that the effective thickness corresponds to the mean radius of nanowires and the experimental measurements were in good agreement with the calculations