John E. Fox

Effects of uniform extracellular DC electric fields on excitability in rat hippocampal slices in vitro

The effects of uniform steady state (DC) extracellular electric fields on neuronal excitability were characterized in rat hippocampal slices using field, intracellular and voltage‐sensitive dye recordings. Small electric fields (<|40| mV mm−1), applied parallel to the somato‐dendritic axis, induced polarization of CA1 pyramidal cells; the relationship between applied field and induced polarization was linear (0.12 ± 0.05 mV per mV mm−1 average sensitivity at the soma). The peak amplitude and time constant (15–70 ms) of membrane polarization varied along the axis of neurons with the maximal polarization observed at the tips of basal and apical dendrites. The polarization was biphasic in the mid‐apical dendrites; there was a time‐dependent shift in the polarity reversal site. DC fields altered the thresholds of action potentials evoked by orthodromic stimulation, and shifted their initiation site along the apical dendrites. Large electric fields could trigger neuronal firing and epileptiform activity, and induce long‐term (>1 s) changes in neuronal excitability. Electric fields perpendicular to the apical–dendritic axis did not induce somatic polarization, but did modulate orthodromic responses, indicating an effect on afferents. These results demonstrate that DC fields can modulate neuronal excitability in a time‐dependent manner, with no clear threshold, as a result of interactions between neuronal compartments, the non‐linear properties of the cell membrane, and effects on afferents.

Interaction dynamics of neuronal oscillations analysed using wavelet transforms.

This paper describes the use of a computational tool based on the Morlet wavelet transform to investigate the interaction dynamics between oscillations generated by two anatomically distinct neuronal populations. The tool uses cross wavelet transform, coherence, bi-spectrum/bi-coherence and phase synchronization. Using specimen data recorded from the hippocampus of a rat with experimentally induced focal epilepsy, linear and non-linear correlations between neuronal oscillations in the CA1 and CA3 regions have been computed. The results of this real case study show that the computational tool can successfully analyse and quantify the temporal interactions between neuronal oscillators and could be employed to investigate the mechanisms underlying epilepsy.

High-Frequency Network Activity, Global Increase in Neuronal Activity, and Synchrony Expansion Precede Epileptic Seizures In Vitro

How seizures start is a major question in epilepsy research. Preictal EEG changes occur in both human patients and animal models, but their underlying mechanisms and relationship with seizure initiation remain unknown. Here we demonstrate the existence, in the hippocampal CA1 region, of a preictal state characterized by the progressive and global increase in neuronal activity associated with a widespread buildup of low-amplitude high-frequency activity (HFA) (>100 Hz) and reduction in system complexity. HFA is generated by the firing of neurons, mainly pyramidal cells, at much lower frequencies. Individual cycles of HFA are generated by the near-synchronous (within ∼5 ms) firing of small numbers of pyramidal cells. The presence of HFA in the low-calcium model implicates nonsynaptic synchronization; the presence of very similar HFA in the high-potassium model shows that it does not depend on an absence of synaptic transmission. Immediately before seizure onset, CA1 is in a state of high sensitivity in which weak depolarizing or synchronizing perturbations can trigger seizures. Transition to seizure is characterized by a rapid expansion and fusion of the neuronal populations responsible for HFA, associated with a progressive slowing of HFA, leading to a single, massive, hypersynchronous cluster generating the high-amplitude low-frequency activity of the seizure.

The fully automatic ion-exchange and gel-permeation chromatography of neutral monosaccharides and oligosaccharides with a Jeolco JLC-6AH analyser

Experiences with a Jeolco JLC-6AH carbohydrate analyser used to provide a fully automated analysis service for neutral mono- and oligo-saccharides have been summarised. Modifications have been made to the equipment to extend its application, and a method for routine separations of oligosaccharides (d.p. 2--15) has been established. The degree of separation achieved is far superior to that which may be obtained by gel-filtration techniques under gravity- or peristaltic-pump-aided flow. The versatility and adaptability of equipment for use with assays other than the standard orcinol-sulphuric acid reaction shows that it is not too inconvenient to interchange between various assays and separations.

Reticulospinal neurones in the rat

Abstract Using an electrophysiological technique, reticulospinal neurones have been demonstrated in the rat. The fibres were found to originate in the medial reticular formation of the medulla, and pass through the ventral regions of the cord to the lower lumbar segments: both crossed and uncrossed components were identified. The conduction velocities were estimated and showed wide variation.

The effect of neuronal population size on the development of epileptiform discharges in the low calcium model of epilepsy

The CA1 region of the rat hippocampal slice generates spontaneous electrographic seizures (field bursts) when exposed to ACSF containing < or = 0.2 mM calcium. It has been proposed that, particularly during the early part of a field burst, synchronised activity in small independent aggregates of neurons results in low amplitude irregular population spikes and subsequent fusion of aggregates generates high amplitude, regular discharging spikes. In the present experiments, we have tested the hypothesis that progression from aggregate formation to aggregate fusion requires a critical mass of participating neurons. We found that isolated CA1 segments >2 mm are still able to generate high amplitude, regular discharging population spikes, but when segment length is reduced to 1-2 mm, only 29% generate spikes with these characteristics; in the remainder, the field burst shows a DC shift+/-low amplitude irregular population spikes. No field bursts were seen in segments < 0.7 mm or in 50% of those 0.7-1 mm in length (in the remaining 50%, only the DC component of the field burst was present). Exposing 1-2 mm segments to hypo-osmolar perfusate induced a return of high amplitude rhythmic discharging population spikes in the field burst. We interpret these observations by indicating that progression from aggregate formation to aggregate fusion requires a critical neuronal mass and can be enhanced by reducing osmolarity of the perfusate.

2008 Special Issue: Neuronal population oscillations of rat hippocampus during epileptic seizures

Neuronal population oscillations in the hippocampus have an important effect in the information processing in the brain and the generation of epileptic seizures. In this paper, we investigate the neuronal population oscillations in the hippocampus of epileptic rats in vivo using an empirical mode decomposition (EMD) method. A neuronal population oscillation can be decomposed into several relaxation oscillations, which possess a recovery and release phase, with the different frequencies that ranges from 0 to 600 Hz. The natures of relaxation oscillations at the pre-ictal, seizure onset and ictal states are distinctly different. The analysis of relaxation oscillations show that the gamma wave is a lead relaxation oscillation at the pre-ictal stage, then it moves to beta oscillation or theta oscillation while the ictal stage starts; the fast relaxation oscillations are associated with the slow relaxation oscillations in the CA1 or CA3, in particular, the fast relaxation oscillations are associated on the recovery phase of the slow relaxation oscillations during the pre-ictal interval, however move to the release phase of the slow relaxation oscillations during the ictal interval. Comparison of the relaxation oscillations in CA1 and CA3 shows that the neurons in the CA1 are more active during the epileptic seizures than during the pre-ictal stage. These findings demonstrate that this method is very helpful to decompose neuronal population for understanding the underlying mechanism of epileptic seizures.

Multiple peptide synthesis

The synthesis of large numbers of peptides can be very labor intensive and, if a conventional peptide synthesizer is used, only small numbers of peptides can be produced within a reasonable time. The techniques described below can make large numbers of different peptides simultaneously with varying degrees of mechanization, ranging from the wholly manual methods, to those involving complete mechanization of the whole synthesis process. Most of the multiple synthesis methods are primarily intended for small scale production ranging from microgram amounts up to a few tens of milligrams. All of the systems are economical in use of solvents and reagents, enabling cost-effective synthesis. The techniques described can also be used to prepare peptide libraries, containing several millions of peptide sequences, to enable the rapid screening of all possible permutations of amino acids within short peptides. However, it is considered that multiple synthesis methods are not particularly suited where extreme high purity or very long peptides are required.

Isolation, identification and synthesis of a novel tripeptide, methionyl-tyrosyl-lysine, from spinal cord and dorsal root ganglia of sheep

Abstract Low molecular weight peptides have been isolated from sheep dorsal root ganglia and sheep and cat spinal cord. The amino acid sequence of one of the peptides from sheep spinal cord was determined and it was identified as the tripeptide methionyl-tyrosyl-lysine (met-tyr-lys). Synthetic and extracted material showed identical chromatographic behaviour in several solvent systems. A chromatographically indistinguishable peptide was also found in sheep dorsal root ganglia and cat dorsal spinal cord. The presence of Met-Tyr-Lys in the spinal cord and its actions on dorsal horn neurones suggest that it may have a role in proprioception.

Frequency and synchrony of tetanically-induced, gamma-frequency population discharges in the rat hippocampal slice: the effect of diazepam and propofol

Fast, rhythmic discharges, at gamma frequencies, have been evoked in the CA1 region of the hippocampus by tetanic stimulation applied via pairs of electrodes placed close to the pyramidal layer. The discharges had a latency of 65-225 ms and a frequency of 60-100 Hz; they were synchronous (phase lag < 2 ms) when recorded from sites 0.4-0.8 mm apart. Diazepam (5-15 microM) and propofol (5 microM) reduced the frequency by 15-26%, but had no significant effect on synchrony or phase lag, suggesting that the generation and synchrony of this rhythm are pharmacologically distinct. 10 microM propofol had a more marked effect and, when frequency fell by approximately 50%, synchrony was also reduced.