Daisuke Ishikawa

Interpyramid Spike Transmission Stabilizes the Sparseness of Recurrent Network Activity

Cortical synaptic strengths vary substantially from synapse to synapse and exhibit a skewed distribution with a small fraction of synapses generating extremely large depolarizations. Using multiple whole-cell recordings from rat hippocampal CA3 pyramidal cells, we found that the amplitude of unitary excitatory postsynaptic conductances approximates a lognormal distribution and that in the presence of synaptic background noise, the strongest fraction of synapses could trigger action potentials in postsynaptic neurons even with single presynaptic action potentials, a phenomenon termed interpyramid spike transmission (IpST). The IpST probability reached 80%, depending on the network state. To examine how IpST impacts network dynamics, we simulated a recurrent neural network embedded with a few potent synapses. This network, unlike many classical neural networks, exhibited distinctive behaviors resembling cortical network activity in vivo. These behaviors included the following: 1) infrequent ongoing activity, 2) firing rates of individual neurons approximating a lognormal distribution, 3) asynchronous spikes among neurons, 4) net balance between excitation and inhibition, 5) network activity patterns that was robust against external perturbation, 6) responsiveness even to a single spike of a single excitatory neuron, and 7) precise firing sequences. Thus, IpST captures a surprising number of recent experimental findings in vivo. We propose that an unequally biased distribution with a few select strong synapses helps stabilize sparse neuronal activity, thereby reducing the total spiking cost, enhancing the circuit responsiveness, and ensuring reliable information transfer.

2010 Special Issue: Fluorescent pipettes for optically targeted patch-clamp recordings

Targeted patch-clamp recordings are a promising technique that can directly address the physiological properties of a specific neuron embedded in a neuronal network. Typically, neurons are visualized through fluorescent dyes or fluorescent proteins with fluorescence microscopy. After switching to transmitted light microscopy, neurons of interest are re-identified and visually approached in situ with patch-clamp pipettes. Here we introduce a simpler method for neuron targeting. With fluorophore-coated pipettes, fluorescently labeled neurons and the pipette tips are simultaneously imaged at the same fluorescence wavelength in the same microscope field, so that the neurons and even their neurites are targeted without suffering from chromatic aberration or mechanical complication in optics. We did not find that the coated fluorophores affected the electric properties of pipettes or neurons. The novel technique will be wildly available for pipette micromanipulation under online visual control.

Operant Conditioning of Synaptic and Spiking Activity Patterns in Single Hippocampal Neurons

Learning is a process of plastic adaptation through which a neural circuit generates a more preferable outcome; however, at a microscopic level, little is known about how synaptic activity is patterned into a desired configuration. Here, we report that animals can generate a specific form of synaptic activity in a given neuron in the hippocampus. In awake, head-restricted mice, we applied electrical stimulation to the lateral hypothalamus, a reward-associated brain region, when whole-cell patch-clamped CA1 neurons exhibited spontaneous synaptic activity that met preset criteria. Within 15 min, the mice learned to generate frequently the excitatory synaptic input pattern that satisfied the criteria. This reinforcement learning of synaptic activity was not observed for inhibitory input patterns. When a burst unit activity pattern was conditioned in paired and nonpaired paradigms, the frequency of burst-spiking events increased and decreased, respectively. The burst reinforcement occurred in the conditioned neuron but not in other adjacent neurons; however, ripple field oscillations were concomitantly reinforced. Neural conditioning depended on activation of NMDA receptors and dopamine D1 receptors. Acutely stressed mice and depression model mice that were subjected to forced swimming failed to exhibit the neural conditioning. This learning deficit was rescued by repetitive treatment with fluoxetine, an antidepressant. Therefore, internally motivated animals are capable of routing an ongoing action potential series into a specific neural pathway of the hippocampal network.

Functional multineuron calcium imaging for systems pharmacology

Functional multineuron calcium imaging (fMCI) is a large-scale technique used to access brain function on a single-neuron scale. It detects the activity of individual neurons by imaging action potential-evoked transient calcium influxes into their cell bodies. fMCI has recently been used as a high-throughput research tool to examine how neuronal activity is altered in animal models of brain diseases, for example stroke, Alzheimer’s disease, and epilepsy, and to estimate how pharmacological agents act on normal and abnormal states of neuronal networks. It offers unique opportunities to discover the mechanisms underlying neurological disorders and new therapeutic targets.

Ex vivo cultured neuronal networks emit in vivo-like spontaneous activity.

Spontaneous neuronal activity is present in virtually all brain regions, but neither its function nor spatiotemporal patterns are fully understood. Ex vivo organotypic slice cultures may offer an opportunity to investigate some aspects of spontaneous activity, because they self-restore their networks that collapsed during slicing procedures. In hippocampal networks, we compared the levels and patterns of in vivo spontaneous activity to those in acute and cultured slices. We found that the firing rates and excitatory synaptic activity in the in vivo hippocampus are more similar to those in slice cultures compared to acute slices. The soft confidence-weighted algorithm, a machine learning technique without human bias, also revealed that hippocampal slice cultures resemble the in vivo hippocampus in terms of the overall tendency of the parameters of spontaneous activity.

Sound-induced modulation of hippocampal θ oscillations.

The mechanism of response of hippocampal neurons to a specific feature in sensory stimuli is not fully understood, although the hippocampus is well known to contribute to the formation of episodic memory in the multisensory world. Using in-vivo voltage-clamp recordings from awake mice, we found that sound pulses induced a transient increase in inhibitory, but not excitatory, conductance in hippocampal CA1 pyramidal cells. In local field potentials, sound pulses induced a phase resetting of the &thgr; oscillations, one of the major oscillatory states of the hippocampus. Repetitive sound pulses at 7 Hz (&thgr; rhythm) increased the &thgr; oscillation power, an effect that was abolished by a surgical fimbria–fornix lesion. Thus, tone-induced inhibition is likely of subcortical origin. It may segment hippocampal neural processing and render temporal boundaries in continuously ongoing experiences.

Normal learning ability of mice with a surgically exposed hippocampus.

In rats and mice, the hippocampus lies beneath higher than 1 mm of the neocortex. This anatomical feature makes it difficult to experimentally access the hippocampus from the surface of the brain in vivo. This problem may be solved by surgical removal of the cortical tissue above the hippocampus; however, it has not been examined whether this ‘hippocampal window’ surgery preserves the normal hippocampal function. We bilaterally aspirated the posterior parietal cortex above the dorsal hippocampus of adult male mice. These mice still exhibited normal local field potentials of the hippocampus, normal motor activity, and normal cognitive ability in the water-maze test and contextual fear conditioning, compared with intact or sham-operated controls. Thus, exposed hippocampal preparations provide a useful experimental model to study the physiology of the hippocampus.

Hollow cathode atomic source applicable to gas, liquid residue, and solid sample phases

ABSTRACT We have developed a compact DC-glow discharge hollow cathode atomic source without requiring extensive chemical pretreatment. This system is intended for laser absorption spectroscopy to function as the source of neutral atoms. Our investigative focus is on elements found in nuclear power related environments, particularly in the context of Fukushima Daiichi decommissioning. The elements constituting the samples are identified through investigating the emission produced by the relaxation of the sample atoms after being sputtered by Ne and Ar discharge. The emission spectrum was observed in pure substances and also compounds related to materials such as stainless steel, control material, cladding material, seawater, and concrete, which are expected to constitute the majority of radioactive wastes. In particular, brass and salt, compounds consisting of two elements, were sputtered to produce single atoms of each element. With a copper pipe as an example, the emission efficiency was also investigated, through which the optimal experimental conditions for sputtering were identified. It was found that sputtering would be likely to cause a reduction in pressure, and that emission intensity is related to the electronic temperature during discharge. In addition, this system is not restricted by sample shape or electronic conductivity, with only minor pretreatment.

Theta oscillations in isolated hippocampus

We analyzed proteins altered their levels after the gene ablation by twodimensional gel electrophoresis. Conclusions: Both Nurr1 and Nur77 gene were disrupted conditionally and simultaneously in a subset of neurons by administration of AAV-Cre. These mice provide a model to investigate the roles of Nurr1 and Nurr77 in the adult brain. Biochemical and histochemical changes in these mice are now under investigation.

Multiple whole-cell and field recordings from in toto hippocampal preparations

cell patch-clamp recordings in adult rat spinal cord slices. In this study, we examined the firing patterns of the SG neurons receiving CA-sensitive afferent fibers. In current-clamp mode, SG neurons discharged action potentials in response to a just supra-threshold current pulse. SG neurons tested were classified into five types: delayed firing, sustained repetitive firing, phasic firing, initial firing and other firing (e.g. single). Most (78%) of the CA-sensitive SG neurons were the delayed and sustained repetitive firing types, although CA-insensitive cells were also found in these two types of SG neurons. In the initial firing type, CA-sensitive neurons were not detected except for one cell. A previous study explored the relationship between the morphological class of SG neurons and their firing pattern, has shown that vertical cells exhibit the delayed or sustained firing type, and radial cells exhibit the phasic firing type. Moreover, central cells are reported to exhibit the initial firing pattern of discharge. In combination with previous studies, the present results suggest that activation of spinal TRPA1 presynaptically facilitates miniature excitatory synaptic transmission from primary afferents onto mainly delayed firing and sustained repetitive firing type neurons which are morphologically classified vertical and radial cells.