Propagation velocity of epileptiform activity in the hippocampus

The propagation of epileptiform burst activity was investigated in the CA1 area of the in-vitro hippocampal slice preparation of the guinea pig. This activity was provoked by 0.1 mM 4-aminopyridine in the bathing medium and was recorded in the pyramidal layer with an array of eight electrodes. The delay between the first population spike of a burst recorded with different electrodes was calculated using the cross-correlation function. The propagation velocity was estimated from the delays and the electrode intervals. It was found that the velocity of spontaneous and evoked epileptiform bursts varies between 0.15 and 5 m/s and is not confined to the range of conduction velocities of the fibre systems in CA1 (0.3–0.55 and 1.0–1.8 m/s). Different velocities can be present in different parts of the CA1 area and the initiation of spontaneous bursts is not confined to the CA2–3 areas, but can also occur in CA1. Burst activity also propagated in a low calcium-high magnesium medium. Different mechanisms of propagation are discussed and it is argued that the propagation velocity due to ephaptic interaction may vary largely. It is concluded that epileptiform activity can be propagated not only by synaptic connections at or near the pyramidal layer, but also by way of electrical field effects of population spikes

The double dipole model of theta rhythm generation: Simulation of laminar field potential profiles in dorsal hippocampus of the rat

A set of compartmental models of CA1 pyramidal, granular and polymorph cells of the dorsal hippocampus have been used to simulate membrane potentials generated by synaptic activation at various levels along these cells. From the membrane potential distributions the field potentials in dorsal CA1 and the dorsal blade of the dentate area have been simulated using a model based on volume conduction theory. Field potential profiles similar to laminar profiles, found experimentally in the dorsal hippocampus during theta rhythm, could only be simulated by assuming (almost) simultaneous synaptic excitation of the 3 cell types at given sites. The results lead to 2 alternative models for the simultaneous excitation of CA1 pyramidal cells and dentate granular cells during theta rhythm. Other electrophysiological evidence favours the model in which the two neuronal populations are activated distally near the fissure

Epileptic neuronal networks: methods of identification and clinical relevance

The main objective of this paper is to examine evidence for the concept that epileptic activity should be envisaged in terms of functional connectivity and dynamics of neuronal networks. Basic concepts regarding structure and dynamics of neuronal networks are briefly described. Particular attention is given to approaches that are derived, or related, to the concept of causality, as formulated by Granger. Linear and non-linear methodologies aiming at characterizing the dynamics of neuronal networks applied to EEG/MEG and combined EEG/fMRI signals in epilepsy are critically reviewed. The relevance of functional dynamical analysis of neuronal networks with respect to clinical queries in focal cortical dysplasias, temporal lobe epilepsies, and “generalized” epilepsies is emphasized. In the light of the concepts of epileptic neuronal networks, and recent experimental findings, the dichotomic classification in focal and generalized epilepsy is re-evaluated. It is proposed that so-called “generalized epilepsies,” such as absence seizures, are actually fast spreading epilepsies, the onset of which can be tracked down to particular neuronal networks using appropriate network analysis. Finally new approaches to delineate epileptogenic networks are discussed

Generation and propagation of epileptiform activity in the hippocampal slice preparation

For the investigation of epileptiform events in the hippo-campal CA1 field, in-vitro slices of the guinea-pig were used. After adding 0.1 mmol 4-aminopyridine to the bathing medium, field potentials were recorded with an electrode array, consisting of 8 semi-microelectrodes at spacings of 0.1 ram. A comparison was made between the spontaneously occurring field potentials (SFP) in CA I and those evoked by different inputs to the CA1 pyramidal cells, namely alveus, str. oriens and Schaffer collaterals. For this purpose the electrode array was placed in CA l, parallel to the axes for the pyramidal cells. The regularly occurring SEP's presented a similar distribution as the potentials evoked by stimulation of str. oriens or alveus of CAI, but differed from those evoked by stimulation of the chaffer colaterals. This indicates that in CA1 SFP's are generated in a similar way as field potentials evoked by alveus or str. oriens stimulation. It was also found that SFP's are propagated from CA3 and CAI at a velocity of 0.16-0.30 m/sec. Therefore pathways in alveus and str. oriens, connecting CA3 and CA1, may be important in propagating epileptiform activity. This was supported by experiments in which different pathways were sectioned

Presubiculum stimulation in vivo evokes distinct oscillations in superficial and deep entorhinal cortex layers in chronic epileptic rats

The characteristic cell loss in layer III of the medial entorhinal area (MEA-III) in human mesial temporal lobe epilepsy is reproduced in the rat kainate model of the disease. To understand how this cell loss affects the functional properties of the MEA, we investigated whether projections from the presubiculum (prS), providing a main input to the MEA-III, are altered in this epileptic rat model. Injections of an anterograde tracer in the prS revealed bilateral projection fibers mainly to the MEA-III in both control and chronic epileptic rats. We further examined the prS-MEA circuitry using a 16-channel electrode probe covering the MEA in anesthetized control and chronic epileptic rats. With a second 16-channel probe, we recorded signals in the hippocampus. Current source density analysis indicated that, after prS double-pulse stimulation, afterdischarges in the form of oscillations (20-45 Hz) occurred that were confined to the superficial layers of the MEA in all epileptic rats displaying MEA-III neuronal loss. Slower oscillations (theta range) were occasionally observed in the deep MEA layers and the dentate gyrus. This kind of oscillation was never observed in control rats. We conclude that dynamical changes occur in an extensive network within the temporal lobe in epileptic rats, manifested as different kinds of oscillations, the characteristics of which depend on local properties of particular subareas. These findings emphasize the significance of the entorhinal cortex in temporal lobe epilepsy and suggest that the superficial cell layers could play an important role in distributing oscillatory activity.status: publishe

Sunscreen Effects In Skin Analyzed By Photoacoustic Spectroscopy

In the photoacoustic technique, the signal is proportional to the heat produced in a sample as a consequence of modulated light absorption. This technique allows the spectroscopic characterization of multilayer systems: as the thermal diffusion length varies with the light modulation frequency, one can obtain the depth profile of the sample by analyzing the frequency-dependence of the signal. As the photoacoustic signal depends on thermal and optical properties of the sample, structural changes in the system under analysis account for signal variations in time. In this work, photoacoustic spectroscopy was used to characterize samples of sunscreen and the system formed by sunscreen plus skin. Measurements used a 1000W Xe arc lamp as light source, for wavelengths between 240nm and 400nm. This range corresponds to most of the UV radiation that reaches Earth. Skin samples were disks of about 0,5cm diameter. The absorption spectrum of sunscreen was obtained. Finally, photoacoustics was employed to monitor the absorption kinetics of the sunscreen applied to skin samples. This was done by applying sunscreen in a skin sample and recording the photoacoustic spectra in regular time intervals, up to 90 minutes after application. According to measurements, light absorption by the sunscreen plus skin system stabilizes between 25 and 45 minutes after sunscreen application. Results show that this technique can be utilized to monitor drug delivery and farmacokinetics in skin samples.5325143149Barja, P.R., (1996) Estudo da Indução Fotossintética Através da Técnica Fotoacústica: Efeitos de Saturação e Fotoinibição, , (master thesis), IFGW, UNICAMPRosencwaig, A., (1980) Photoacoustics and Photoacoustic Spectroscopy, pp. 219-231. , New York, John Wiley & SonsPucceti, G., Leblanc, R.M., A comparative study on chromophore diffusion inside porous filters by pulsed photoacoustic spectroscopy (1996) Journal of Membrane Science, 119, pp. 213-228Gutiérrez-Juárez, G., Vargas-Luna, M., Córdova, T., In vivo measurement of the human skin absorption of topically applied substances by photoacoustic technique (2002) Physiological Measurement, 23, pp. 1-12Bernengo, J.C., Photoacoustics as a tool for cutaneous permeation studies (1998) High Temperatures-high Pressures, 30, pp. 619-624Azevedo, J.S., UVA/UVB sunscreen determination by second-order derivative ultraviolet spectrophotometry (1999) II Farmaco, 54, pp. 573-578Rettberg, P., Horneck, G., Biologically weighted measurement of UV radiation in space on Earth with the biofilm technique (2000) Advanced Space Research, 26, pp. 2005-2014Van Der Leun, J.C., UV radiation from sunlight: Summary, conclusions and recommendations (1996) Journal of Photochemistry and Photobiology B: Biology, 35, pp. 237-2441Wolf, R., Sunscreens (2001) Clinics in Dermatology, 19, pp. 452-45