Microgravity dependence of excitable biological and physicochemical media.

Neuronal tissue and especially the central nervous system (CNS) is an excitable medium. Self-organisation, pattern formation, and propagating excitation waves as typical characteristics in excitable media consequently have been found in neuronal tissue. The properties of such phenomena in excitable media do critically depend on the parameters (i.e., electromagnetic fields, temperature, chemical drugs) of the system and on small external forces to which gravity belongs. The spreading depression, a propagating excitation depression wave of neuronal activity, is one of the best described of the those wave phenomena in the CNS. Especially in the retina as a true part of the CNS it can be easily observed with optical techniques due to the high intrinsic optical signal of this tissue. Another of such waves in neuronal tissue is the propagating action potential in nerve fibres. In this paper, data from our laboratories concerning the influence of gravity on the velocity of propagating waves in excitable media are summarized mainly in terms of the retinal spreading depression and propagating action potentials. Additionally, we have used waves in gels of the Belousov-Zhabotinsky reaction as the physicochemical model system of biological activity as the properties of these waves follow the same theories as the spreading depression and action potentials and they have some striking similarities in wave behavior. Thus propagating Belousov-Zhabotinsky waves are described by their gravity dependence.

Wave onset in central gray matter - its intrinsic optical signal and phase transitions in extracellular polymers.

The brain is an excitable media in which excitation waves propagate at several scales of time and space. "One-dimensional" action potentials (millisecond scale) along the axon membrane, and spreading depression waves (seconds to minutes) at the three dimensions of the gray matter neuropil (complex of interacting membranes) are examples of excitation waves. In the retina, excitation waves have a prominent intrinsic optical signal (IOS). This optical signal is created by light scatter and has different components at the red and blue end of the spectrum. We could observe the wave onset in the retina, and measure the optical changes at the critical transition from quiescence to propagating wave. The results demonstrated the presence of fluctuations preceding propagation and suggested a phase transition. We have interpreted these results based on an extrapolation from Tasaki's experiments with action potentials and volume phase transitions of polymers. Thus, the scatter of red light appeared to be a volume phase transition in the extracellular matrix that was caused by the interactions between the cellular membrane cell coat and the extracellular sugar and protein complexes. If this hypothesis were correct, then forcing extracellular current flow should create a similar signal in another tissue, provided that this tissue was also transparent to light and with a similarly narrow extracellular space. This control tissue exists and it is the crystalline lens. We performed the experiments and confirmed the optical changes. Phase transitions in the extracellular polymers could be an important part of the long-range correlations found during wave propagation in central nervous tissue.

Correlation of the electrical and intrinsic optical signals in the chicken spreading depression phenomenon.

This paper presents some results on the correlation between the electrophysiological and intrinsic optical signals (IOS) of spreading depression waves in chicken retinae. We first show that the peak of the time derivative of the electrophysiological wave occurs precisely when the optical signal reaches the electrode tip. Second, by comparing bath applications of propranolol and glycerol it can be shown that the slow potential shift is not directly correlated to the intrinsic optical signal. Propranolol depresses the amplitude of the electrical wave, although the intrinsic optical signal continues being visible. On the other hand, we observe total absence of the IOS under glycerol, while the electrical wave is always present. Correlations of this kind are relevant for a deeper understanding of the underlying mechanisms of the spreading depression phenomenon.

Biphasic effects of melatonin on the propagation of excitation waves in the chicken retina.

The retinal spreading depression (SD) is a propagating wave in an excitable medium, the neuronal tissue of the retina. Its velocity is about 3 mm/min and it is accompanied by a variety of changes in the tissue, including electrical and optical events. The pronounced intrinsic optical signal (IOS) of the retinal SD makes it an extremely versatile tool for the investigation of the action of drugs on neuronal tissue and more specific on propagating excitation waves in neuronal tissue. Furthermore, in the last decade increasing evidence has been collected, which shows that SD waves are the basic mechanism of the aura in classical migraine. We have investigated the influence of melatonin on the propagation of retinal SD waves as it has been postulated to have protective effects on neuronal tissue. The results demonstrate that melatonin indeed slows down the retinal SD, however, only in a defined concentration range. Additionally, it changes the IOS of the wave.

Mathematical model of the CA1 region of the rat hippocampus.

A mathematical transcription of the intrinsic circuit of the CA1 region of the rat dorsal hippocampus was made and the model parameters adjusted according to experimental data from intracellular recordings and single channel kinetics. This model was able to simulate well the profile of the field potentials recorded extracellularly and the well known phenomenon of the paired-pulse depression. The results suggest that the depression of the second pulse, often interpreted in the literature as resulting from GABA(A) inhibition, can also be due to 'shunting' effects on the CA1 pyramids' membrane. The rhythmic oscillations of the field potential (EEG) was obtained as an emergent property of the network dynamics. The frequency of the field oscillation followed the main synaptic input in the region (Schaffer collaterals).

Distribution of gap junctions in the chicken retina.

Coupling between cells of neuronal tissue can be due to electrical or chemical synapses. The molecular basis of an electrical synapse is the gap junction channel. Gap junctions have been found between neurones and glial cells, however, in some tissue their presence in the membranes of different cell types is still under discussion. In the retina of vertebrates, which is a true part of the CNS, the presence of gap junctions in the specialised glial cells of the retina, the Müller cells is not clear for chicken. Since these cells span the whole retina vertically, for some tasks, like spatial buffering of potassium, such gap junctions would not be required, in contrast to other parts of the CNS. The spatial buffering of potassium among others plays an important role in the propagation of excitation-depression waves in neuronal tissue, especially in the chicken retina. However, gap junctions could be involved in creating an electrical syncitium of glial cells, which might also contribute to excitation-depression wave propagation. In this paper we present an about complete screening of the presence of gap junctions in the chicken retina, including the proof that the Müller cells of this retina do not have gap junctions. This finding is discussed considering the highly specialised morphological structure of the Müller cells of the chicken retina, which have an extremely extended endfeet tree.

Pharmacological modulation of the refractory period of retinal spreading depression.

Spreading depression (SD) is a propagating wave of neuronal activity in the central nervous system and may play a role in triggering classical migraine. The retina serves as a model system for examining the phenomenon of SD and the influence of various drugs on it. After a SD wave passes a new wave can not be elicited in the absolute refractory period of the tissue (about 2 min), this is followed by a relative refractory phase of about 20 min before complete recovery. The aim of the present study was to describe the effects of Ba2+, a blocker of glial cell K+ channels, octanol, a gap junction blocker and diethylbarbiturate, a GABA(A) chloride channel-activating drug on the modulation of the refractory period of the retinal SD and to examine the possible mechanisms underlying this modulation. Two properties of SD, which are highly sensitive to any changes in the experimental conditions, are the propagation velocity of the wave and the accompanying slow negative potential shift. We measured the propagation velocity and the field potential amplitude in the chicken retina as a function of the recovery state of the tissue under control conditions and compared them with measurements in the presence of Ba2+, octanol or diethylbarbiturate. Under these conditions the manner of the recovery of the tissue changed significantly. Although after blocking the glial (Müller) cell K+ channels with Ba2+ (200 microM), the curve of recovery of the propagation velocity to its maximum value has the same shape as under control conditions, the propagation velocity is reduced in the whole recovery period and in the recovered retina to 84% of the control velocity. The importance of electrical coupling in the refractory phase and in the recovered tissue was examined by adding octanol (1 mM) to the perfusion solution. In this case the relative recovery phase was shortened and the field potential amplitude (110% of control) and propagation velocity (112% of control) are increased in the completely recovered retina. With the GABA(A)-chloride channel-activating drug diethylbarbiturate (800 microM) the propagation velocity (112% of control) and the amplitude of the field potential (111% of control) in the complete recovered retina are increased, but this seems to have no influence on the refractory state.

Exogenous application of gangliosides changes the state of excitability of retinal tissue as demonstrated by retinal spreading depression experiments.

Gangliosides are amphiphilic, sialic acid-containing glycosphingolipids which are found preferentially in complex composition in the cellular membranes of the nervous system of vertebrates, including the vertebrate retina as well as in other membranes. They are always exposed to the extracellular side of the membranes. By virtue of the negative charges they carry at their headgroup, they contribute to the surface charge of the membrane and may affect ion distribution, mainly that of protons and calcium ions, at the outer side of the membranes. Using retinal spreading depression (RSD) as a tool, we show in this study that the addition of exogenous gangliosides to the extracellular space can change the state of excitability of the retinal tissue. In RSD experiments it reduces the propagation velocity as well as the intrinsic optical signal of RSD waves. These effects are concentration dependent (IC50 about 20 microM) and increase with the increasing negative charge of the ganglioside headgroup. As a possible mechanistic basis of the changes found, the change of the calcium homeostasis of the extracellular space by the exogenously added gangliosides is discussed. Gangliosides have been reported to be useful in the treatment of some neuropathological syndromes, including migraine, although experimental verification has not been possible up to now. Taking into account that the retina is a true part of the CNS, our data may be interpreted as the requested verification.

Calcium waves in gray matter are due to voltage-sensitive glial membrane channels.

The retina is the most accessible piece of central gray matter in the vertebrate brain. Its wide dynamic operational range makes it the ideal neuronal network to study its excitability. Spreading depression waves in the retina are accompanied by strong intrinsic optical signals (IOS) and thus can be measured non-invasively with optical methods. Additionally, incubation with fluorescent dyes allows to follow calcium fluxes in parallel. The IOS can be divided into red and green scatter of light. We show that during spreading depression the red scatter signal precedes the green scatter signal and that the calcium signal matches the red scatter signal. Incubation of the retina with barium chloride leads to a reversible depression of red scatter and calcium signal whereas the green scatter signal is hardly effected. The wave propagation velocity is reduced, too. This supports the idea that the early red scatter signal is a direct visualisation of glial membrane potential and that glia cells in the chicken retina are involved in the control of extracellular calcium.

Self-sustained spreading depressions in the chicken retina and short-term neuronal-glial interactions within the gray matter neuropil.

The chicken retina is an accessible piece of intact gray matter in which a self-sustained form of the 'Spreading Depression' (SD) wave can be easily elicited and recorded for many hours with double barrel ion-sensitive electrodes in the extracellular space. The blockade of glial (Müller) cell potassium channels with barium chloride added to the perfusing Ringer depressed both the negative potential shift typical of SDs and the velocity of spread. Moreover, there was separation of the extracellular increase of potassium and the drop in the extracellular potential: the peak of the potassium wave was increased, as well as its duration whereas the potential wave could be depressed to zero or even inverted to positive. By contrast the transient extracellular calcium drop could not be separated from the extracellular potential wave but appeared related to it: no transient calcium drop was observed when the negative potential was completely depressed or inverted. Both, the amplitude of the extracellular potential and extracellular calcium activity appeared to be important factors controlling the velocity of spread.

The role of hippocampal commissures in the interhemispheric transfer of epileptiform afterdischarges in the rat: a study using linear and non-linear regression analysis.

The role of the forebrain commissures and the septal area in the interhemispheric transfer of hippocampal afterdischarges (ADs) was investigated in the rat under halothane anesthesia. Electrical seizures were elicited from the dorsal hippocampus before and after commissurotomy. The degree of relatedness between EEG signals recorded from homologous sites of both hippocampi was quantified using two approaches: (i) a time domain analysis considering an AD as a succession of discrete bursts; the onset times of such bursts were measured and used to estimate interhemispheric onset delays; (ii) using signal analysis the linear (r2) and non-linear (h2) regression coefficients between pairs of EEG signals were computed as a function of time shift between the two signals. In this way the values of association (linear and non-linear) and the corresponding time delays were measured. In general a tetanus applied unilaterally to the dorsal CA3 field resulted in bilaterally synchronous ADs. The estimated interhemispheric time delay was in most cases zero. This bilateral synchrony disappeared after section of a specific part of the ventral hippocampal commissure (VHC), the dorso-caudal third, but was not affected by section of other commissural fibers or by a lesion of the septal area. This study also allowed evaluation of different methods of quantification of the association between EEG signals, namely the linear (r2) and the non-linear (h2) regression coefficients. The latter was shown to be a more robust measure than the former and to yield values of association even in cases in which r2 was at noise level. The experimental findings allow the conclusion that ADs elicited from an epileptogenic focus spread to homologous sites in the contralateral hemisphere following commissural systems that may be strong enough to ensure the forming of one bilateral oscillating system.