Florian P. M. Kohn

Self-organization and Pattern-formation in Neuronal Systems Under Conditions of Variable Gravity

Nonlinear Physical Science is a new book series co-published by Higher Education Press of China (HEP) and Springe. This book series will provide a forum to systematically summarize recent developments, discoveries and progresses on Nonlinear Physical Science for historical records. The aims of the book series are to provide the fundamental and frontier theories and techniques for nonlinear physical science; to stimulate more research interest on nonlinearity, synchronization and complexity in nonlinear science; and to directly pass the new knowledge to the young generation, scientists, engineers and students in the corresponding fi elds.

Gravity and neuronal adaptation, in vitro and in vivo—from neuronal cells up to neuromuscular responses: a first model

For decades it has been shown that acute changes in gravity have an effect on neuronal systems of human and animals on different levels, from the molecular level to the whole nervous system. The functional properties and gravity-dependent adaptations of these system levels have been investigated with no or barely any interconnection. This review summarizes the gravity-dependent adaptation processes in human and animal organisms from the in vitro cellular level with its biophysical properties to the in vivo motor responses and underlying sensorimotor functions of human subjects. Subsequently, a first model for short-term adaptation of neuronal transmission is presented and discussed for the first time, which integrates the responses of the different levels of organization to changes in gravity.

Preparing normal tissue cells for space flight experiments

ABSTRACT Deterioration of health is a problem in modern space flight business. In order to develop countermeasures, research has been done on human bodies and also on single cells. Relevant experiments on human cells in vitro are feasible when microgravity is simulated by devices such as the Random Positioning Machine or generated for a short time during parabolic flights. However, they become difficult in regard to performance and interpretation when long-term experiments are designed that need a prolonged stay on the International Space Station (ISS). One huge problem is the transport of living cells from a laboratory on Earth to the ISS. For this reason, mainly rapidly growing, rather robust human cells such as cancer cells, embryonic cells, or progenitor cells have been investigated on the ISS up to now. Moreover, better knowledge on the behavior of normal mature cells, which mimic the in vivo situation, is strongly desirable. One solution to the problem could be the use of redifferentiable cells, which grow rapidly and behave like cancer cells in plain medium, but are reprogrammed to normal cells when substances like retinoic acid are added. A list of cells capable of redifferentiation is provided, together with names of suitable drugs, in this review.

Behavior of Action Potentials Under Variable Gravity Conditions

The functional properties of neuronal tissue critically depend on cellular composition and intercellular communication. A basic principle of such communication found in various types of neurons is the generation of action potentials (action potentials) as discussed in Chapter 3 in some detail. These action potentials depend on the presence of voltage gated ion-channels (Fig. 7.1), especially sodium- and potassium channels, and propagate along cellular processes (e.g. axons) towards target neurons or other cells. It has already been shown in a previous chapter that the properties of ion-channels depend on gravity. To discover whether the properties of action potentials also depend on gravity, we examined the propagation of action potentials in earthworms (invertebrates) and isolated nerve fibers (i.e. bundles of axons) from earthworms under conditions of micro-and macro-gravity. In the second set of experiments we could verify our results on rat axons (vertebrates). Our experiments carried out during two parabolic flight campaigns revealed that micro-gravity slows action potential propagation velocity and macrogravity accelerates the transmission of action potentials. Additionally we looked at the behavior of spontaneously spiking neurons from leech in drop-tower experiments. The relevance of action potential behavior especially under microgravity for life science related questions is considerable, taking into account that altered gravity conditions might affect action potential velocity in man during space flight missions.

Interaction of Gravity with Molecules and Membranes

The basic to all ideas how gravity might interact with neuronal tissue is the cellular membrane being intrinsic part of any cells. It is known to be, with all its components and interactions, involved in all sensory processes. Ion-channels as integral membrane proteins are involved significantly in these mechanisms, and according to the question of gravity sensitivity they are of high interest based on two possible aspects. First, it might be possible that gravity directly interacts with single membrane based on proteins, including ion-channels; second, gravity might change its parameters instead of interacting with the thermodynamical system membrane, and thus affect the properties of ion-channels incorporated in the membrane indirectly. Changing physical parameters other than gravity in a variety of different experiments, for example temperature or pressure, has shown both mechanisms to be possible using a variety of techniques. Especially the investigation of mechano-sensitive ion-channels has contributed a lot to the understanding of how membranes can interact with mechanical and other weak external forces (i.e. Garcia-Anoveras and Corey, 1997; Sukharev, 1999).

Discussion and Perspectives

Throughout this text in about all chapters already some extended discussions about the presented results are given. Thus, at the end of the complete text only a short summary and general interpretation is given mainly to bring together results from different experimental sections. Additionally, something like an outlook for future experiments is given at some places. Especially according to technological aspects of this text, mainly the microgravity platforms, this necessarily includes highly political speculations about where manned space flight could go, and whether it is necessary at all.

Spreading Depression: A Self-organized Excitation Depression Wave in Different Gravity Conditions

The spreading depression is an excitation-depression wave that was first described by Leao in 1944 as a wave like spreading depression of neuronal activity in the central nervous system. The spreading depression waves travel over the cortex with a velocity of about 3mm/min, concomitant with a slow extra cellular negative potential shift about 10 to 30 mV. A very short hyper excitation at the wave front is followed by a complete suppression of electrical activity (Egert et al., 1995). The spreading depression itself is a fully reversible process without permanent damage of the neuronal tissue (Hansen und Nedergaard, 1988). Nearly at the same time as Leao discovered the cortical spreading depression, Lashley, a scientist suffering from migraine himself, described his own aura symptoms as patterns travelling over his field of vision. He postulated that these visual disorders are due to a neuronal inactivity which travels like a wave over the visual cortex at a velocity of about 3 mm/min (Lashley, 1941). For the first time in 1958 Milner linked these two phenomena (Milner, 1958). Not only the visual scotoma described by Lashley, but also other aura symptoms (e.g. somatosensory, somatomotory, auditive disorders) can be explained by spreading depression waves travelling over the corresponding cortex. Today the occurrence of spreading depression together with several neurological disorders is proven, e.g. for classical migraine (Welch et al., 1990; Welch et al., 1992), transient neurological disorders concomitant with ischemic attacks (Somjen et al., 1990), epilepsy (Marshall, 1959), transient global amnesia (Olesen et al., 1986) and brain traumata (Oka et al., 1977).

Fluorescence and Light Scatter Experiments to Investigate Cell Properties at Microgravity

Optical methods have been developed to measure a variety of biological relevant parameters of cells. Especially fluorescent dyes are meanwhile available for an increasing number of interesting features. In the described experiments we were mainly interested in dyes for membrane potential and variety of dyes for ionic concentrations. The intracellular calcium concentration is of specific interest here. Another question is that about changes in size and geometry of cells under variable gravity conditions, here light scatter experiments have been proven to be very useful. In the following both types of experiments will be presented, using the Bremen drop-tower as a microgravity platform.

The Brain Itself in Zero-g

In this study we could clearly show that microgravity and hyper gravity respectively lead to a neuromodulation in the cerebral cortex of humans. This implies different excitability of the neuronal networks and different arousal states of the subjects that might involve different states of attention and focus and therefore different mental and motor performance skills. Unfortunately the brain is characterized by all properties of complex system and therefore the processes at every level are chaotic, unstable and non-linear and unpredictable. Especially in our results with the slow cortical potentials this is expressed in the reaction of the brain to the altered gravity stimuli, where the polarity of the DC shifts to depend on each individual brain of the different subjects. Concerning the ambitions that brain machine interfaces are prospected for space system control, further research is essential about how the brain is influenced by microgravity conditions. Furthermore this study as a logical continuation of the above chapters shows that the fragmentation of complex systems in sub-systems that is conventionally used in biological research is very useful for the clarification of the underlying mechanisms but should always be verified in the whole system at best under the same experimental conditions.

Effects of Altered Gravity on the Actin and Microtubule Cytoskeleton, Cell Migration and Neurite Outgrowth

Human SH-SY5Y neuroblastoma cells were used to study the effects of altered gravity on the actin and microtubule cytoskeleton dynamics. A cholinergic stimulation of the cells during a 6-min period of changing gravity (3 parabolas) resulted in an enhanced actin-driven protrusion of evoked lamellipodia. Likewise, the spontaneous protrusive activity of non-activated cells was promoted during exposure to changing gravity (6 up to 31 parabolas). Ground-based experiments revealed a similar enhancement of the spontaneous and an evoked lamellar protrusive activity when the cells were kept at 2g hyper—gravity for at least 6 min. This gravity response was independent of the direction of the acceleration vector in respect to the cells. Exposure of the cells to “simulated weightlessness” (clinorotation) had no obvious influence on this type of lamellar actin cytoskeleton dynamics.