Optical recording of rat entorhino-hippocampal system in organotypic culture

Neuronal avalanches in neocortical circuits

Networks of living neurons exhibit diverse patterns of activity, including oscillations, synchrony, and waves. Recent work in physics has shown yet another mode of activity in systems composed of many nonlinear units interacting locally. For example, avalanches, earthquakes, and forest fires all propagate in systems organized into a critical state in which event sizes show no characteristic scale and are described by power laws. We hypothesized that a similar mode of activity with complex emergent properties could exist in networks of cortical neurons. We investigated this issue in mature organotypic cultures and acute slices of rat cortex by recording spontaneous local field potentials continuously using a 60 channel multielectrode array. Here, we show that propagation of spontaneous activity in cortical networks is described by equations that govern avalanches. As predicted by theory for a critical branching process, the propagation obeys a power law with an exponent of -3/2 for event sizes, with a branching parameter close to the critical value of 1. Simulations show that a branching parameter at this value optimizes information transmission in feedforward networks, while preventing runaway network excitation. Our findings suggest that "neuronal avalanches" may be a generic property of cortical networks, and represent a mode of activity that differs profoundly from oscillatory, synchronized, or wave-like network states. In the critical state, the network may satisfy the competing demands of information transmission and network stability.

The dendritic layer-specific persistent enhancement of synaptic transmission induced by repetitive activation of protein kinase A

Synaptic plasticity, the cellular basis of brain memory, is established through at least two phases: short-term and long-term plasticity. It is assumed that the short-term plasticity instantaneously provoked in pre-existing synapses, as represented by a long-term potentiation (LTP) in the mammalian hippocampus, is converted to the long-term plasticity that develops slowly accompanying the formation of new synapses. However, this conversion has scarcely been analyzed primarily because of the lack of the model system. Recently, we found that a repeated activation of protein kinase A (PKA), but not a single activation of PKA, led to a slowly-developing long-lasting enhancement of synaptic strength coupled with synaptogenesis in cultured rat hippocampus and proposed that this phenomenon would serve as the required model system. In the present study, we investigated the geographical aspect of this phenomenon using a high-speed voltage-sensitive dye (VSD) imaging methodology. Before doing this, we had to overcome the difficulties in applying this methodology to the quantitative analysis on the cultured hippocampal slices. Those difficulties are multiple types of signal decay and a large variance in the number of cells among specimens. After resolving these problems we found that the enhancement of synaptic efficacy in the CA1 stratum radiatum occurred predominantly in the proximal dendritic layer.

Imaging spatio-temporal patterns of long-term potentiation in mouse hippocampus

Spatio-temporal patterns of neuronal activity before and after the induction of long-term potentiation in mouse hippocampal slices were studied using a real-time high-resolution optical recording system. After staining the slices with voltage-sensitive dye, transmitted light images and extracellular field potentials were recorded in response to stimuli applied to CA1 stratum radiatum. Optical and electrical signals in response to single test pulses were enhanced for at least 30 minutes after brief high-frequency stimulation at the same site. In two-pathway experiments, potentiation was restricted to the tetanized pathway. The optical signals demonstrated that both the amplitude and area of the synaptic response were increased, in patterns not predictable from the initial, pretetanus, pattern of activation. Optical signals will be useful for investigating spatio-temporal patterns of synaptic enhancement underlying information storage in the brain.

Effect of trichloroethylene on spatiotemporal pattern of LTP in mouse hippocampal slices

The effect of trichloroethylene (TCE) on long-term potentiation (LTP) was studied using both electrical and optical recording. The hippocampi from mice injected with 300 mg/kg or 1000 mg/kg TCE were sliced 24 h after administration. The field potential from the CAI was recorded. After the application of tetanus, population spikes (PS) were potentiated in all groups, but the post-per-pre ratio of PS was smaller in TCE groups than in the control. Optical recording was also carried out in 1000 mg/kg TCE-injected mice and a new analytical method using a high speed camera was employed. After the induction of tetanus, the optical signal was potentiated in both TCE and control groups. However, the post-per-pre ratio of the optical signals and response area were smaller in the TCE groups than in the control. It was suggested that the impairment of LTP is one of the mechanisms of the impairment of immediate memory after acute exposure to TCE in humans.

3-Hydroxykynurenine transaminase identity with alanine glyoxylate transaminase. A probable detoxification protein in Aedes aegypti

This study describes the functional characterization of a specific mosquito transaminase responsible for catalyzing the transamination of 3-hydroxykynurenine (3-HK) to xanthurenic acid (XA). The enzyme was purified from Aedes aegypti larvae by ammonium sulfate fractionation, heat treatment, and various chromatographic techniques, plus non-denaturing electrophoresis. The purified transaminase has a relative molecular mass of 42,500 by SDS-PAGE. N-terminal and internal sequencing of the purified protein and its tryptic fragments resolved a partial N-terminal sequence of 19 amino acid residues and 3 partial internal peptide sequences with 7, 10, and 7 amino acid residues. Using degenerate primers based on the partial internal sequences for PCR amplification and cDNA library screening, a full-length cDNA clone with a 1,167-bp open reading frame was isolated. Its deduced amino acid sequence consists of 389 amino acid residues with a predicted molecular mass of 43,239 and shares 45-46% sequence identity with mammalian alanine glyoxylate transaminases. Northern analysis shows the active transcription of the enzyme in larvae and developing eggs. Substrate specificity analysis of this mosquito transaminase demonstrates that the enzyme is active with 3-HK, kynurenine, or alanine substrates. The enzyme has greater affinity and catalytic efficiency for 3-HK than for kynurenine and alanine. The biochemical characteristics of the enzyme in conjunction with the profiles of 3-HK transaminase activity and XA accumulation during mosquito development clearly point out its physiological function in the 3-HK to XA pathway. Our data suggest that the mosquito transaminase was evolved in a manner precisely reflecting the physiological requirement of detoxifying 3-HK produced in the tryptophan oxidation pathway in the mosquito.

Effects of temperature increase on the propagation of presynaptic action potentials in the pathway between the Schaffer collaterals and hippocampal CA1 neurons

Effects of temperature increase on the neuronal activity of hippocampal CA2-CA1 regions were examined by using optical and electrophysiological recording techniques. Stimulation of the Schaffer collaterals at the CA2 region evoked depolarizing optical signals that spread toward the CA1 region at 32 degrees C. The optical signal recorded by 49 pixels was characterized by fast and slow components that were closely related to presynaptic action potentials and excitatory postsynaptic responses, respectively. The optical signal was depressed by temperature increase to 38-40 degrees C. The temperature increase to 38 degrees C produced a hyperpolarization and a depression of the excitatory postsynaptic potential (EPSP) in single hippocampal CA1 pyramidal neurons. The depression of the neuronal activity induced by temperature increase was attenuated by application of glucose (22 mM) or pyruvate (22 mM). Adenosine (200 microM) did not block the presynaptic action potential but strongly depressed the excitatory postsynaptic response. 8-Cyclopentyl-1,3-dimethylxanthine (8-CPT) (10 microM), an antagonist for adenosine A(1) receptors, attenuated the depression of the excitatory postsynaptic response but not the inhibition of the presynaptic action potential at 38 degrees C. These results suggest that adenosine mediates the high-temperature-induced depression of the excitatory synaptic transmission but not that of action potential propagation in rat CA1 neurons.

Extracellular ATP reduces optically monitored electrical signals in hippocampal slices through metabolism to adenosine

Electrical signals in rat hippocampal slices were optically monitored using a voltage-sensitive dye to determine whether extracellular ATP exhibits direct effects through its own receptors or indirect effects after its hydrolysis to adenosine. The dentate gyrus was stimulated and electrical signals in the CA1 and the CA3 region were analyzed. The signals were divided into two components: a transient component peaking within 10 ms (fast component) and a subsequent sustained component (slow component). ATP (10 to 100 microM) inhibited both the fast and the slow components in the CA1 region by about 30% and 70%, respectively. ADP, AMP and adenosine also inhibited the fast and the slow components. The inhibition by ATP was antagonized by aminophylline and other adenosine receptor antagonists, and by alpha,beta-methylene ADP, an inhibitor of 5'-nucleotidases. These results suggest that extracellular ATP inhibits neuronal electrical signals in hippocampal slices after its metabolism to adenosine.

Basic fibroblast growth factor and brain-derived neurotrophic factor promote survival and neuronal circuit formation in organotypic hippocampal culture

Neurotrophic effects in vitro have been generally related to promotion of differentiation, maturation and survival, but little is known about the effect on neuronal circuit formation. The organotypic culture system would be an available technique to investigate neuronal circuit formation and neuronal cell-cell interactions. As we reported previously, an optical recording system is a useful technique to comprehend neuronal activities and circuit from multi-points simultaneously. In this study, we investigated whether continuous application of basic fibroblast growth factor (bFGF or FGF-2) and brain-derived neurotrophic factor (BDNF) inhibited neuronal cell death induced by serum-deprivation in organotypic culture using propidium iodide staining, and we analyzed effects of bFGF and BDNF on the formation of neuronal circuits using the optical recording system. Continuous application of bFGF or BDNF significantly protected the slices from neuronal death. Optical recording also demonstrated that addition of 10 ng/ml bFGF or 50 ng/ml BDNF enhanced optical signals in all hippocampal areas significantly. These data strongly suggest that bFGF and BDNF promote the formation of neuronal circuits as well as survival and that optical recording of organotypic hippocampal slices would be a useful technique that enables us to analyze neuronal circuit formation easily.

Optical recording of trisynaptic pathway in rat hippocampal slices with a voltage-sensitive dye

Changes in membrane potentials were recorded from rat hippocampal slices with a voltage-sensitive dye using a real-time optical recording system, which had high spatial resolution of 128 x 128 points with a high time resolution of 0.6 ms. Serial excitatory propagation was recorded in the dentate gyrus. CA3 and CA1 after stimulation of the perforant pathway, and the optical signals were clearly divided into two components in the dentate gyrus adjacent to the stimulus site. The slow component was suppressed in Ca(2+)-free solution, but the fast component in the molecular layer was not affected. However, the application of 1 microM tetrodotoxin fully abolished both components. These results suggest that the fast and slow components mainly reflect Na(+)-dependent action potentials and excitatory postsynaptic potentials, respectively. The excitatory response duration in the stratum radiatum of CA3 was significantly longer than that in other hippocampal areas. The long-lasting excitation in CA3 is probably related to the CA3 associational projections, because direct stimulation of CA3 pyramidal cell layer also produced similar results. The long-lasting dendritic excitation is probably important to integrate synaptic transmission and may be related to epileptogenesis. When long-term potentiation was induced by a tetanic stimulation (100 Hz for 1 s), the onset latency in the stratum radiatum of CA1 was reduced to as much as 65%, suggesting an increase of excitatory propagation. The analysis of the spatial-temporal optical signals contributes to understanding information processes in the hippocampus, related to learning and memory including long-term potentiation.

The regional vulnerability to blockade of action potentials in organotypic hippocampal culture

We investigated how blockade of spontaneous action potentials influenced the synaptogenesis by measuring the field population spike using hippocampal organotypic cultures. Although respective blockade of inhibitory and excitatory neurotransmission by picrotoxin and CNQX did not significantly induce cell death in all hippocampal area, sodium channel blocking drugs (tetrodotoxin or lidocaine) caused specific and severe damage and affected the formation of functional synapse in CA1 and the entorhinal cortex but not in CA3. It is suggested that the spontaneous action potentials would play a critical role during synaptogenesis.