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Oxygen consumption in the nervous system
Oxygen consumption in the nervous system
Neuronal activity in the brain depends to a large extent on adenosine triphosphate generation and thus on the availability of oxygen. This makes oxygen a highly relevant readout for studying neuronal metabolism. To evaluate the dependency of neuronal activity on oxygen availability, semi-intact in vitro preparations of Xenopus laevis tadpoles with functional central and peripheral nervous systems were studied. Trochlear motor nerve spike discharge served as a physiological correlate for neuronal activity. O2-concentrations in the bath chamber and the brain were concurrently monitored using Clark-type oxygen microsensors during superfusion of Ringer solution with various concentration levels of oxygen. The O2-concentration was accurately set to a defined value by aeration with carbogen (95 % O2, 5 % CO2) or nitrogen. In air-saturated Ringer solutions (290 μmol/l O2), the IVth ventricle was devoid of oxygen due to consumption by adjacent brain tissue and an O2-concentration of zero was measured. At elevated oxygen bath concentrations of >290 μmol/l, the ventricular oxygen level was considerably augmented (> 0) while spontaneous burst discharge of the trochlear nerve caused a transient drop of the oxygen level within the IVth ventricle, indicating a neuronal activity-related increase in the demand for oxygen. In contrast, decreasing the concentration of oxygen in the Ringer solution below ∼40μmol/l completely ceased trochlear motor nerve activity. Oxygen delivery is limited by metabolic processes and diffusion, which are often impaired following injury in the brain, largely due to scar tissue formation or caused by associated diseases, such as lung impairments or during stroke. A good model for pathological condition are in vitro experiments, as oxygen delivery through the blood is absent. Therefore alternative delivery methods are required. Aiming at a spatially more accurate and faster means for the modulation of the oxygen level in the brain, the natural capability of algae and cyanobacteria to produce oxygen upon visible light illumination via photosynthesis was exploited. Injection of the green algae Chlamydomonas reinhardtii or the cyanobacteria Synechocystis sp. into the vascular system of Xenopus tadpoles prior to the generation of the semi-intact preparation distributed these single celled organisms throughout the vasculature of the entire brain. This new approach is termed ’Symbiotic Oxygen Supply’ (SOS). External induction of hypoxia caused an oxygen depletion within the IVth ventricle and a subsequent trochlear motor nerve activity abolishment. Illumination with visible light activated algal photosynthesis and increased the oxygen level in the brain, leading to a restart of motor nerve activity upon illumination within about 20 min. This suggests that SOS is sufficient to restore energy equivalents required for maintained neuronal activity in oxygen depleted environments. Accordingly, introduction of algae or cyanobacteria and illumination represents a promising method to augment the oxygen level in any diffusion-limited in vitro neuronal preparation devoid of a functional circulation and potentially also under in vivo conditions.
oxygen, photosynthesis, Xenopus laevis, oxidative phosphorylation, extraocular motoneurons
Özugur, Suzan
2021
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Özugur, Suzan (2021): Oxygen consumption in the nervous system. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)
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Abstract

Neuronal activity in the brain depends to a large extent on adenosine triphosphate generation and thus on the availability of oxygen. This makes oxygen a highly relevant readout for studying neuronal metabolism. To evaluate the dependency of neuronal activity on oxygen availability, semi-intact in vitro preparations of Xenopus laevis tadpoles with functional central and peripheral nervous systems were studied. Trochlear motor nerve spike discharge served as a physiological correlate for neuronal activity. O2-concentrations in the bath chamber and the brain were concurrently monitored using Clark-type oxygen microsensors during superfusion of Ringer solution with various concentration levels of oxygen. The O2-concentration was accurately set to a defined value by aeration with carbogen (95 % O2, 5 % CO2) or nitrogen. In air-saturated Ringer solutions (290 μmol/l O2), the IVth ventricle was devoid of oxygen due to consumption by adjacent brain tissue and an O2-concentration of zero was measured. At elevated oxygen bath concentrations of >290 μmol/l, the ventricular oxygen level was considerably augmented (> 0) while spontaneous burst discharge of the trochlear nerve caused a transient drop of the oxygen level within the IVth ventricle, indicating a neuronal activity-related increase in the demand for oxygen. In contrast, decreasing the concentration of oxygen in the Ringer solution below ∼40μmol/l completely ceased trochlear motor nerve activity. Oxygen delivery is limited by metabolic processes and diffusion, which are often impaired following injury in the brain, largely due to scar tissue formation or caused by associated diseases, such as lung impairments or during stroke. A good model for pathological condition are in vitro experiments, as oxygen delivery through the blood is absent. Therefore alternative delivery methods are required. Aiming at a spatially more accurate and faster means for the modulation of the oxygen level in the brain, the natural capability of algae and cyanobacteria to produce oxygen upon visible light illumination via photosynthesis was exploited. Injection of the green algae Chlamydomonas reinhardtii or the cyanobacteria Synechocystis sp. into the vascular system of Xenopus tadpoles prior to the generation of the semi-intact preparation distributed these single celled organisms throughout the vasculature of the entire brain. This new approach is termed ’Symbiotic Oxygen Supply’ (SOS). External induction of hypoxia caused an oxygen depletion within the IVth ventricle and a subsequent trochlear motor nerve activity abolishment. Illumination with visible light activated algal photosynthesis and increased the oxygen level in the brain, leading to a restart of motor nerve activity upon illumination within about 20 min. This suggests that SOS is sufficient to restore energy equivalents required for maintained neuronal activity in oxygen depleted environments. Accordingly, introduction of algae or cyanobacteria and illumination represents a promising method to augment the oxygen level in any diffusion-limited in vitro neuronal preparation devoid of a functional circulation and potentially also under in vivo conditions.