In the brain, increased neural activity is correlated with increases of cerebral blood flow and tissue oxygenation. However, how cerebral oxygen dynamics are controlled in the behaving animal remains unclear. We investigated to what extent cerebral oxygenation varies during locomotion. We measured oxygen levels in the cortex of awake, head-fixed mice during locomotion using polarography, spectroscopy, and two-photon phosphorescence lifetime measurements of oxygen sensors. We find that locomotion significantly and globally increases cerebral oxygenation, specifically in areas involved in locomotion, as well as in the frontal cortex and the olfactory bulb. The oxygenation increase persists when neural activity and functional hyperemia are blocked, occurred both in the tissue and in arteries feeding the brain, and is tightly correlated with respiration rate and the phase of respiration cycle. Thus, breathing rate is a key modulator of cerebral oxygenation and should be monitored during hemodynamic imaging, such as in BOLD fMRI.

Functional ultrasound imaging (fUS) is an emerging technique that detects changes of cerebral blood volume triggered by brain activation. Here, we investigate the extent to which fUS faithfully reports local neuronal activation by combining fUS and two-photon microscopy (2PM) in a co-registered single voxel brain volume. Using a machine-learning approach, we compute and validate transfer functions between dendritic calcium signals of specific neurons and vascular signals measured at both microscopic (2PM) and mesoscopic (fUS) levels. We find that transfer functions are robust across a wide range of stimulation paradigms and animals, and reveal a second vascular component of neurovascular coupling upon very strong stimulation. We propose that transfer functions can be considered as reliable quantitative reporters to follow neurovascular coupling dynamics.

25In the brain, increased neural activity is correlated with an increase of cerebral blood flow and 26 increased tissue oxygenation. However, how cerebral oxygen dynamics are controlled in the 27 behaving animals remains unclear. Here, we investigated to what extent the cerebral oxygenation 28 varies during natural behaviors that change the whole-body homeostasis, specifically exercise. 29We measured oxygen levels in the cortex of awake, head-fixed mice during locomotion using 30 polarography, spectroscopy, and two-photon phosphorescence lifetime measurements of oxygen 31 sensors. We found that locomotion significantly and globally increases cerebral oxygenation, 32 specifically in areas involved in locomotion, as well as in the frontal cortex and the olfactory bulb. 33The oxygenation increase persisted when neural activity and functional hyperemia were blocked, 34 occurred both in the tissue and in arteries feeding the brain, and was tightly correlated with 35 respiration rate and the phase of respiration cycle. Thus, respiration provides a dynamic pathway 36 for modulating cerebral oxygenation. 3An adequate oxygen supply is critical for proper brain function 1 , and deficiencies in tissue oxygen 38 is a noted comorbidity in human diseases 2 and aging 3 . For these reasons, there has been a great 39 deal of interest in studying dynamics of cerebral oxygenation [4][5][6][7][8][9] . However, there is a gap in our 40 understanding of how behavior, such as natural exercises like locomotion, affects cerebral 41 oxygenation. In natural environments, animals and humans have evolved to spend a substantial 42 portion of their waking hours locomoting 10 . As exercise is known to have a positive effect on brain 43 health 11,12 , a better understanding of the basic brain physiology accompanying the behaviors can 44 give insight into how exercise can improve brain function. During movement, neuromodulator 45 release and neural activity in many brain regions is elevated [13][14][15][16][17][18][19][20] , and there is an increase in 46 cardiac output and respiratory rate. How these changes in local and systemic factors interact to 47 control cerebral oxygenation is a fundamental question in brain physiology but is not well 48 understood. Most cerebral oxygenation studies are performed in anesthetized animals 8,9,[21][22][23] (but 49 see 4 ), or non-invasively in humans. Anesthesia causes large disruptions of brain metabolism and 50 neural activity 24 , and non-invasive human studies are impeded by technical issues, making 51 accurate determination of any aspect of brain tissue oxygenation problematic. 52Here we investigated how and by what mechanisms voluntary exercise impacts brain 53 tissue oxygenation. We used intrinsic optical signal imaging 13,25 , electrophysiology, Clark-type 54 polarography 5,6,23 , and two-photon phosphorescent dye measurement 4,8,9 to elucidate how 55 vasodilation, neural activity, and systemic factors combine to generate changes in brain 56 oxygenation. All experiments were performed in awake mice ...

Nitric oxide (NO) is a gaseous signaling molecule that plays an important role in neurovascular coupling. NO produced by neurons diffuses into the smooth muscle surrounding cerebral arterioles, driving vasodilation. However, the rate of NO degradation in hemoglobin is orders of magnitude higher than in brain tissue, though how this might impact NO signaling dynamics is not completely understood. We used simulations to investigate how the spatial and temporal patterns of NO generation and degradation impacted dilation of a penetrating arteriole in cortex. We found that the spatial location of NO production and the size of the vessel both played an important role in determining its responsiveness to NO. The much higher rate of NO degradation and scavenging of NO in the blood relative to the tissue drove emergent vascular dynamics. Large vasodilation events could be followed by post-stimulus constrictions driven by the increased degradation of NO by the blood, and vasomotion-like 0.1-0.3 Hz oscillations could also be generated. We found that these dynamics could be enhanced by elevation of free hemoglobin in the plasma, which occurs in diseases such as malaria and sickle cell anemia, or following blood transfusions. Finally, we show that changes in blood flow during hypoxia or hyperoxia could be explained by altered NO degradation in the parenchyma. Our simulations suggest that many common vascular dynamics may be emergent phenomena generated by NO degradation by the blood or parenchyma.

Nitric oxide (NO) is a gaseous signaling molecule that plays an important role 1 in neurovascular coupling. NO produced by neurons diffuses into the smooth muscle 2 surrounding cerebral arterioles, driving vasodilation.However, the rate of NO 3 degradation in hemoglobin is orders of magnitude higher than in brain tissue, though how 4 this might impact NO signaling dynamics is not completely understood. We used 5 simulations to investigate how the spatial and temporal patterns of NO generation and 6 degradation impacted dilation of a penetrating arteriole in cortex. We found that the 7 spatial location of NO production and the size of the vessel both played an important role 8 in determining its responsiveness to NO. The much higher rate of NO degradation and 9 scavenging of NO in the blood relative to the tissue drove emergent vascular dynamics. 10Large vasodilation events could be followed by post-stimulus constrictions driven by the 11 increased degradation of NO by the blood, and vasomotion-like 0.1-0.3 Hz oscillations 12 could also be generated. We found that these dynamics could be enhanced by elevation 13 of free hemoglobin in the plasma, which occurs in diseases such as malaria and sickle 14 cell anemia, or following blood transfusions. Finally, we show that changes in blood flow 15 during hypoxia or hyperoxia could be explained by altered NO degradation in the 16 parenchyma. Our simulations suggest that many common vascular dynamics may be 17 emergent phenomenon generated by NO degradation by the blood or parenchyma. 18 19 20 21 22 23 fold faster than the surrounding tissue 73,75-78 . Because NO reacts with hemoglobin at 70 much higher rates than the tissue, the hemoglobin present inside a vessel plays an 71 appreciable role in shaping NO concentrations at the smooth muscle where it acts. Under 72 normal conditions, most hemoglobin in the blood in confined to red blood cells, with low 73 levels in the plasma. Due to fluid dynamics 79-81 , red blood cells will be excluded from the 74 few micrometer-thick cell free layer next to the endothelial cells, providing a measure of 75 spatial separation between the region of high NO degradation and the smooth muscles. 76However, if hemoglobin levels in the plasma rise (due to pathology or other processes) 82-77 89 , this will greatly increase the degradation rate of NO in the plasma, leading to decreased 78 NO levels in the smooth muscle 83,[90][91][92] . NO's diffusive properties and known reaction 79 rates lend themselves to computational approaches to understanding NO 80 signaling 38,59,75,78,[93][94][95][96][97][98] . While there have been detailed and informative models of NO 81 signaling from endothelial cells 59,91,96,99,100 showing that the size of the arteriole 75 and 82properties of the blood 96 are vital components to understanding NO signaling, the insight 83 from these models that the spatial location of blood plays an important role in the 84 degradation of NO has not been applied to neurovascular coupling or in a dynamic setting. 85 Intriguingly, in...

Understanding the relationships between biological processes is paramount to unravel pathophysiological mechanisms. These relationships can be modeled with Transfer Functions (TFs), with no need of a priori hypotheses as to the shape of the transfer function. Here we present Iliski, a software dedicated to TFs computation between two signals. It includes different pre-treatment routines and TF computation processes: deconvolution, deterministic and non-deterministic optimization algorithms that are adapted to disparate datasets. We apply Iliski to data on neurovascular coupling, an ensemble of cellular mechanisms that link neuronal activity to local changes of blood flow, highlighting the software benefits and caveats in the computation and evaluation of TFs. We also propose a workflow that will help users to choose the best computation according to the dataset. Iliski is available under the open-source license CC BY 4.0 on GitHub (https://github.com/alike-aydin/Iliski) and can be used on the most common operating systems, either within the MATLAB environment, or as a standalone application.

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1.AbstractUnderstanding the relationships between biological events is paramount to unravel pathophysiological mechanisms. These relationships can be modeled with Transfer Functions (TFs), with no need of a priori hypotheses as to the shape of the transfer function. Here we present Iliski, a software dedicated to TFs computation between two signals. It includes different pre-treatment routines and TF computation processes: deconvolution, deterministic and non-deterministic optimization algorithms that are adapted to disparate datasets. We apply Iliski to data on neurovascular coupling, an ensemble of biological events that link neuronal activity to local changes of blood flow, highlighting the software benefits and caveats in the computation and evaluation of TFs. We also propose a workflow that will help users to choose the best computation according to the dataset. Iliski is available under the open-source license CC BY 4.0 on GitLab (https://gitlab.com/AliK_A/iliski) and can be used on the most common operating systems, either within the MATLAB environment, or as a standalone application.