The Impact of Altered Gravity on Neuronal Activity

Kurzfassung

Introduction: Learning and memory, cognitive processes and psychomotor functions are major key drivers to accomplish an optimal efficiency of human performance during manned spaceflight missions. Although astronauts are selected to be as the best candidates from a psychological point of view, it has been reported that spaceflight still has profound impacts on several aspects of human physiology. Especially the microgravity environment disturbs cognitive functions, such as spatial disorientation, disturbances of motor skills, visual illusions, impairments of concentration, and a general slowing-down of task performance. Furthermore, every aspect of human behavior, cognition and performance is based on the neuronal activity generated by billions of neurons, which is processed in the brain and transmitted via electrical activity through functional networks, where the neurons generate electro-chemical signals to relay information across synapses. Thus, altered synaptic neuronal transmission may lead to diminished human performance, while disruptions of neuronal transmissions are the main cause of a variety of mental disorders. Additionally, an excitatory input upon a neuron triggers an event called action potential (AP), which induces a rapid movement of ions across the cell membrane, creating an electrical signal which propagates down the axon causing neurotransmitter release at synaptic junctions. Previous experiments with patch-clamping (isolated leech neuron) in the Drop Tower have shown that APs are gravity dependent; demonstrating a tendency to increase the frequency of spontaneous activity under microgravity conditions, due to a shorter latency of APs, thus increasing the excitability of the membranes, and lowering the AP threshold. However such a technique is rather sensitive to external perturbances and does not reflect whether microgravity induce transient or long lasting changes. Therefore here we can ask ourselves: what is the impact of different gravity conditions on APs at a short- and long-term scale, and if excitability changes in neuronal networks show gravity dependence? Outline: The current project aims at studying neuronal transmission changes induced by different gravity conditions, using in vitro primary murine neurons, cultured directly atop an electrophysiological system, which is embedded with biocompatible multielectrode arrays (MEA), capable of record and stimulate extracellular electrical signals from excitable cells. The use of neurons in vitro, in particular primary neurons from the hippocampus of embryonic E17.5 mice provides an ideal model to study neuronal activity under different gravity conditions. Primary neurons are being used for this project, as they show a very similar development as compared to the physiological development in vivo in the brain, forming mature networks including functional synapses. Moreover, the focus of the project is on understanding the mechanistic contributions of the gravitational unloading on ion channels and synaptic signals to the control of neural networks in vitro. Therefore, we are interested in determining how microgravity influences astronaut performance at neuronal network level, as well as how hypergravity could be used as a countermeasure. Objectives: The overall objective of this project is to develop a platform for the investigation of network behavior in primary neurons under altered gravity. To achieve this, the project aims to: (1) Devise a neuronal culture protocol for MEA chips for optimal cell growth, to increase the physiological maturity, based upon either the use of astrocyte-neuron co-culture with astrocytes plated over coverslips or astrocyte-conditioned medium with a peristaltic pump. (2) Integrate the MEA system to various gravity-research platforms, such as parabolic flights, sounding rockets, drop towers, and the DLR Short-Arm Human Centrifuge. This will enable us to better understand the influence of micro- and hypergravity on neuronal activity for the implications on brain plasticity adaptation and astronaut performance. (3) Develop a pipeline for the analysis offline of the electrophysiological data from the MEA experiments. (4) Study neuronal activity throughout developmental stages - from early stages of synaptogenesis to synaptic network formation onwards, under hypergravity conditions. (5) Study neuronal activity at mature networks under microgravity conditions as well as combining electrical and pharmacological stimulations.