Summary: Researchers directly measured oxygen levels in an intact brain and correlated those measurements with neural activity. Under normal conditions, roughly half of the oxygen consumed supports neuronal firing, while the other half supplies glial cells and basic metabolic needs of neurons.
Source: LMU
The brain has a high energy demand and is highly sensitive to oxygen availability. Neurobiologists at Ludwig-Maximilians-Universitaet (LMU) in Munich have, for the first time, directly linked oxygen consumption in the intact brain to the activity of specific nerve cells.
Although the brain represents a relatively small fraction of body mass, it consumes a disproportionately large share of metabolic resources. Most of this energy is produced by aerobic metabolism, relying on oxidative phosphorylation and substantial oxygen consumption. Consequently, local oxygen concentrations are a critical factor that affects the function of both neurons and glial cells. Until now, however, the precise relationship between oxygen use and neuronal activity in living brain tissue remained poorly defined because of technical limitations.
LMU neurobiologists Hans Straka, Suzan Özugur and Lars Kunz addressed this gap by directly measuring oxygen levels in the intact brain and correlating those measurements with neural activity. Their findings, reported in the journal BMC Biology, come from experiments using an established experimental model: tadpoles of the clawed frog Xenopus laevis. In these preparations the researchers employed sensitive electrochemical oxygen sensors to record concentrations both within the hindbrain tissue and inside a brain ventricle, under conditions that approximated the living animal.
The investigators controlled oxygen availability precisely by adjusting the oxygenation of the surrounding bath solution and manipulated neural activity pharmacologically. As a proxy for central neuronal activity, they recorded spike discharge from the trochlear nerve, which innervates the superior oblique eye muscle and therefore provides a defined, measurable output from a well-characterized motor circuit. By comparing oxygen signals with recorded spike activity, the team quantified how much oxygen consumption was attributable to ongoing neuronal firing.
One striking observation was that in an air-saturated bath—conditions comparable to normal atmospheric oxygen—the ventricle and adjacent hindbrain tissue exhibited oxygen concentrations close to zero. In other words, under typical conditions the available oxygen was immediately consumed by brain tissue. When oxygen levels in the bath were raised above twice atmospheric concentration, tissue oxygen rose and metabolic capacity became saturated, indicating an excess of available O2 beyond immediate metabolic needs. Pharmacological inhibition of neural activity reduced oxygen consumption by approximately 50%, demonstrating that about half of the oxygen budget in this preparation supports active neuronal signaling, while the remainder is used by glial cells and by baseline metabolic processes that maintain neuronal viability. Episodes of enhanced neuronal activity—spontaneous bursts in the trochlear nerve—produced transient increases in oxygen consumption that scaled with the magnitude and duration of those bursts.
These quantitative, controlled measurements provide a clearer picture of how oxygen availability and neuronal activity are coupled in an intact, functioning neural circuit. The results offer an empirical basis for understanding the brain’s energy budget and for future studies that aim to separate the oxygen demands of different cellular processes. Such insights are important for basic neuroscience and have potential clinical relevance—for example, improving interpretation of neuroimaging signals that depend on blood oxygenation, and better understanding the tissue-level consequences of hypoxia.
About this neuroscience research article
Source: LMU Munich
Media contact: Kathrin Bilgeri – LMU
Image source: Image in the public domain.
Original research (open access): “Relationship between oxygen consumption and neuronal activity in a defined neural circuit” by Suzan Özugur, Lars Kunz & Hans Straka. Published in BMC Biology. DOI: 10.1186/s12915-020-00811-6
Abstract
Relationship between oxygen consumption and neuronal activity in a defined neural circuit
Background
Neuronal computations related to sensory and motor activity, together with maintenance of spike discharge, synaptic transmission and other housekeeping processes, impose substantial energetic demands. Oxidative phosphorylation is the most efficient way for nervous tissue to generate large amounts of energy equivalents and therefore depends critically on O2 availability. As a result, local O2 levels influence neuronal function. Measurements of O2 consumption have long been used to estimate the cost of neuronal activity, but studying these metabolic relationships in vivo under defined experimental conditions has been limited by technical challenges.
Results
Using isolated but functionally intact Xenopus laevis tadpole preparations, we performed quantitative measurements of O2 concentrations in the hindbrain under in vivo-like conditions. Oxygen was recorded in the fourth ventricle and adjacent hindbrain tissue while simultaneously monitoring spike discharge of the trochlear nerve, which innervates the superior oblique eye muscle, as a proxy for central neural activity. In air-saturated bath Ringer solution, O2 levels in the ventricle and nearby hindbrain were near zero. Inhibition of mitochondrial respiration with potassium cyanide or fixation with ethanol raised ventricular O2 to bath levels, confirming that the tissue actively consumed accessible oxygen. Gradually increasing oxygenation of the bath produced corresponding rises in ventricular O2. Blocking spike discharge with the local anesthetic tricaine methanesulfonate reduced O2 consumption by roughly 50%, indicating a substantial fraction of oxygen use is tied to neuronal signaling. Conversely, spontaneous bursts of trochlear nerve spiking were accompanied by transient increases in O2 consumption that correlated with burst amplitude and duration.
Conclusions
By independently manipulating both oxygen availability and neuronal activity under controlled, in vivo-like conditions, we were able to quantitatively relate spike discharge magnitude in a defined motor circuit to local O2 consumption. The ability to separately adjust these parameters will allow further dissection of how metabolic and neuronal processes are coupled. These results provide quantitative empirical evidence linking physiologically relevant spontaneous neural activity with O2-dependent metabolism and demonstrate that isolated amphibian preparations are a promising model for investigating oxygen dynamics in relation to neural computations.