The oxygen initial dip in the brain of anesthetized and awake mice

Neurovascular coupling and CO2 interrogate distinct vascular regulations

Neurovascular coupling (NVC), which mediates rapid increases in cerebral blood flow in response to neuronal activation, is commonly used to map brain activation or dysfunction. Here we tested the reemerging hypothesis that CO2 generated by neuronal metabolism contributes to NVC. We combined functional ultrasound and two-photon imaging in the mouse barrel cortex to specifically examine the onsets of local changes in vessel diameter, blood flow dynamics, vascular/perivascular/intracellular pH, and intracellular calcium signals along the vascular arbor in response to a short and strong CO2 challenge (10 s, 20%) and whisker stimulation. We report that the brief hypercapnia reversibly acidifies all cells of the arteriole wall and the periarteriolar space 3-4 s prior to the arteriole dilation. During this prolonged lag period, NVC triggered by whisker stimulation is not affected by the acidification of the entire neurovascular unit. As it also persists under condition of continuous inflow of CO2, we conclude that CO2 is not involved in NVC.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Quantitative Optical Imaging of Oxygen in Brain Vasculature

The intimate relationship between neuronal activity and cerebral oxygenation underpins fundamental brain functions like cognition, sensation, and motor control. Optical imaging offers a noninvasive approach to assess brain oxygenation and often serves as an indirect proxy for neuronal activity. However, deciphering neurovascular coupling─the intricate interplay between neuronal activity, blood flow, and oxygen delivery─necessitates independent, high spatial resolution, and high temporal resolution measurements of both microvasculature oxygenation and neuronal activation. This Perspective examines the established optical techniques employed for brain oxygen imaging, specifically functional near-infrared spectroscopy, photoacoustic imaging, optical coherence tomography, and two-photon phosphorescent lifetime microscopy, highlighting their fundamental principles, strengths, and limitations. Several other emerging optical techniques are also introduced. Finally, we discuss key technological challenges and future directions for quantitative optical oxygen imaging, paving the way for a deeper understanding of oxygen metabolism in the brain.

Oxygen imaging of hypoxic pockets in the mouse cerebral cortex

Consciousness is lost within seconds upon cessation of cerebral blood flow. The brain cannot store oxygen, and interruption of oxidative phosphorylation is fatal within minutes. Yet only rudimentary knowledge exists regarding cortical partial oxygen tension (Po2) dynamics under physiological conditions. Here we introduce Green enhanced Nano-lantern (GeNL), a genetically encoded bioluminescent oxygen indicator for Po2 imaging. In awake behaving mice, we uncover the existence of spontaneous, spatially defined "hypoxic pockets" and demonstrate their linkage to the abrogation of local capillary flow. Exercise reduced the burden of hypoxic pockets by 52% compared with rest. The study provides insight into cortical oxygen dynamics in awake behaving animals and concurrently establishes a tool to delineate the importance of oxygen tension in physiological processes and neurological diseases.

Electro-metabolic signaling

Precise matching of energy substrate delivery to local metabolic needs is essential for the health and function of all tissues. Here, we outline a mechanistic framework for understanding this critical process, which we refer to as electro-metabolic signaling (EMS). All tissues exhibit changes in metabolism over varying spatiotemporal scales and have widely varying energetic needs and reserves. We propose that across tissues, common signatures of elevated metabolism or increases in energy substrate usage that exceed key local thresholds rapidly engage mechanisms that generate hyperpolarizing electrical signals in capillaries that then relax contractile elements throughout the vasculature to quickly adjust blood flow to meet changing needs. The attendant increase in energy substrate delivery serves to meet local metabolic requirements and thus avoids a mismatch in supply and demand and prevents metabolic stress. We discuss in detail key examples of EMS that our laboratories have discovered in the brain and the heart, and we outline potential further EMS mechanisms operating in tissues such as skeletal muscle, pancreas, and kidney. We suggest that the energy imbalance evoked by EMS uncoupling may be central to cellular dysfunction from which the hallmarks of aging and metabolic diseases emerge and may lead to generalized organ failure states-such as diverse flavors of heart failure and dementia. Understanding and manipulating EMS may be key to preventing or reversing these dysfunctions.

Single-microvessel occlusion produces lamina-specific microvascular flow vasodynamics and signs of neurodegenerative change

Recent studies have highlighted the importance of understanding the architecture and function of microvasculature, and dysfunction of these microvessels may underlie neurodegenerative disease. Here, we utilize a high-precision ultrafast laser-induced photothrombosis (PLP) method to occlude single capillaries and then quantitatively study the effects on vasodynamics and surrounding neurons. Analysis of the microvascular architecture and hemodynamics after single-capillary occlusion reveals distinct changes upstream vs. downstream branches, which shows rapid regional flow redistribution and local downstream blood-brain barrier (BBB) leakage. Focal ischemia via capillary occlusions surrounding labeled target neurons induces dramatic and rapid lamina-specific changes in neuronal dendritic architecture. Further, we find that micro-occlusion at two different depths within the same vascular arbor results in distinct effects on flow profiles in layers 2/3 vs layer 4. The current results reveal laminar-scale regulation distinctions in microinfarct response and raise the possibility that relatively greater impacts on microvascular function contribute to cognitive decline in neurodegenerative disease.

The oxygen initial dip in the brain of anesthetized and awake mice

Sensory stimulation generates a robust decrease in oxygen concentration (pO₂ initial dip) in brain tissue of anesthetized cats and rodents. This dip reports local activation of neurons much better than the delayed pO₂ increase associated with functional hyperemia. Here, we reinvestigated the issue in animals that recovered from acute surgery using two-photon lifetime microscopy. Targeting a distinct neuronal network that is the site of strong activation and energy consumption, we show that in anesthetized animals the pO₂ initial dip is present but extremely small in juxtasynaptic capillaries. In awake animals, it is no longer detectable in vessels or in the neuropil. This demonstrates that in healthy animals, neurovascular coupling is too fast and efficient to reveal a pO₂ initial dip.

An ongoing controversy in brain metabolism is whether increases in neural activity cause a local and rapid decrease in oxygen concentration (i.e., the “initial dip”) preceding functional hyperemia. This initial dip has been suggested to cause a transient increase in vascular deoxyhemoglobin with several imaging techniques and stimulation paradigms, but not consistently. Here, we investigate contributors to this initial dip in a distinct neuronal network, an olfactory bulb (OB) glomerulus most sensitive to a specific odorant (ethyl tiglate [ET]) and a site of strong activation and energy consumption upon ET stimulation. Combining two-photon fluorescence and phosphorescence lifetime microscopy, and calcium, blood flow, and pO₂ measurements, we characterized this initial dip in pO₂ in mice chronically implanted with a glass cranial window, during both awake and anesthetized conditions. In anesthetized mice, a transient dip in vascular pO₂ was detected in this glomerulus when functional hyperemia was slightly delayed, but its amplitude was minute (0.3 SD of resting baseline). This vascular pO₂ dip was not observed in other glomeruli responding nonspecifically to ET, and it was poorly influenced by resting pO₂. In awake mice, the dip in pO₂ was absent in capillaries as well as, surprisingly, in the neuropil. These high-resolution pO₂ measurements demonstrate that in awake mice recovered from brain surgery, neurovascular coupling was too fast and efficient to reveal an initial dip in pO₂.