Odor-evoked oxygen consumption by action potential and synaptic transmission in the olfactory bulb

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.

The olfactory bulb: A neuroendocrine spotlight on feeding and metabolism

Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in the external environment, before sight, taste or sound. Odour molecules activate olfactory sensory neurons that reside on the olfactory epithelium in the nasal cavity, which transmits this odour-specific information to the olfactory bulb (OB), where it is relayed to higher brain regions involved in olfactory perception and behaviour. Besides odour processing, recent studies suggest that the OB extends its function into the regulation of food intake and energy balance. Furthermore, numerous hormone receptors associated with appetite and metabolism are expressed within the OB, suggesting a neuroendocrine role outside the hypothalamus. Olfactory cues are important to promote food preparatory behaviours and consumption, such as enhancing appetite and salivation. In addition, altered metabolism or energy state (fasting, satiety and overnutrition) can change olfactory processing and perception. Similarly, various animal models and human pathologies indicate a strong link between olfactory impairment and metabolic dysfunction. Therefore, understanding the nature of this reciprocal relationship is critical to understand how olfactory or metabolic disorders arise. This present review elaborates on the connection between olfaction, feeding behaviour and metabolism and will shed light on the neuroendocrine role of the OB as an interface between the external and internal environments. Elucidating the specific mechanisms by which olfactory signals are integrated and translated into metabolic responses holds promise for the development of targeted therapeutic strategies and interventions aimed at modulating appetite and promoting metabolic health.

Identification of neural circuits controlling male sexual behavior and sexual motivation by manganese-enhanced magnetic resonance imaging

Introduction

Different techniques have been used to identify the brain regions that control sexual motivation and sexual behavior. However, the influence of sexual experience on the activation of these brain regions in the same subject is unknown. Using manganese-enhanced magnetic resonance imaging (MEMRI), we analyzed the activation of brain regions in the sexual incentive motivation (SIM) and the partner preference PP (tests) on weeks 1, 5, and 10 in male rats tested for 10 weeks. AIM. In experiment 1, we analyzed the possible toxic effects of 16 mg/kg of MnCl2 on male sexual behavior, running wheel, and motor execution. In experiment 2, subjects were tested for SIM and PP using MEMRI.

Methods

In both experiments, a dose of 16 mg/kg (s.c) of chloride manganese (MnCl2) was administered 24 h before subjects were tested and placed immediately thereafter in a 7-Tesla Bruker scanner.

Results

In experiment 1, the dose of 16 mg/kg of MnCl2 did not induce behavioral alterations that could interfere with interpreting the imaging data. In experiment 2, we found a clear preference for the female in both the SIM and PP tests. We found a higher signal intensity in the olfactory bulb (OB) in week 1 of the SIM test compared to the control group. We also found increased signal intensity in the socio-sexual behavior and mesolimbic reward circuits in the SIM test in week 1. In the PP test, we found a higher signal intensity in the ventral tegmental area (VTA) in week 10 compared to the control group. In the same test, we found increased signal intensity in the socio-sexual and mesolimbic reward circuits in week 5 compared to the control group. Cohen's d analysis of the whole brain revealed that as the subjects gained sexual experience we observed a higher brain activation in the OB in the SIM group. The PP group showed higher brain activation in the cortex and subcortical structures as they acquired sexual experience.

Discussion

As the subjects gain sexual experience, more structures of the reward and socio-sexual circuits are recruited, resulting in different, and large brain activations.

Modulating the Excitability of Olfactory Output Neurons Affects Whole-Body Metabolism

Metabolic state can alter olfactory sensitivity, but it is unknown whether the activity of the olfactory bulb (OB) may fine tune metabolic homeostasis. Our objective was to use CRISPR gene editing in male and female mice to enhance the excitability of mitral/tufted projection neurons (M/TCs) of the OB to test for improved metabolic health. Ex vivo slice recordings of MCs in CRISPR mice confirmed increased excitability due the targeted loss of Kv1.3 channels, which resulted in a less negative resting membrane potential (RMP), enhanced action potential (AP) firing, and insensitivity to the selective channel blocker margatoxin (MgTx). CRISPR mice exhibited enhanced odor discrimination using a habituation/dishabituation paradigm. CRISPR mice were challenged for 25 weeks with a moderately high-fat (MHF) diet, and compared with littermate controls, male mice were resistance to diet-induced obesity (DIO). Female mice did not exhibit DIO. CRISPR male mice gained less body weight, accumulated less white adipose tissue, cleared a glucose challenge more quickly, and had less serum leptin and liver triglycerides. CRISPR male mice consumed equivalent calories as control littermates, and had unaltered energy expenditure (EE) and locomotor activity, but used more fats for metabolic substrate over that of carbohydrates. Counter to CRISPR-engineered mice, by using chemogenetics to decrease M/TC excitability in male mice, activation of inhibitory designer receptors exclusively activated by designer drugs (DREADDs) caused a decrease in odor discrimination, and resulted in a metabolic profile that was obesogenic, mice had reduced EE and oxygen consumption (VO2). We conclude that the activity of M/TC projection neurons canonically carries olfactory information and simultaneously can regulate whole-body metabolism.SIGNIFICANCE STATEMENT The olfactory system drives food choice, and olfactory sensitivity is strongly correlated to hunger and fullness. Olfactory function thereby influences nutritional balance and obesity outcomes. Obesity has become a health and financial crisis in America, shortening life expectancy and increasing the severity of associated illnesses. It is expected that 51% of Americans will be obese by the year 2030. Using CRISPR gene editing and chemogenetic approaches, we discovered that changing the excitability of output neurons in the olfactory bulb (OB) affects metabolism and body weight stabilization in mice. Our results suggest that long-term therapeutic targeting of OB activity to higher processing centers may be a future clinical treatment of obesity or type II Diabetes.

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.

Olfaction in the context of obesity and diabetes: Insights from animal models to humans

The olfactory system is at the crossroad between sensory processing and metabolic sensing. In addition to being the center of detection and identification of food odors, it is a sensor for most of the hormones and nutrients responsible for feeding behavior regulation. The consequences of modifications in body homeostasis, nutrient overload and alteration of this brain network in the pathological condition of food-induced obesity and type 2 diabetes are still not elucidated. The aim of this review was first to use both humans and animal studies to report on the current knowledge of the consequences of obesity and type 2 diabetes on odorant threshold and olfactory perception including identification discrimination and memory. We then discuss how olfactory processing can be modified by an alteration of the metabolic homeostasis of the organism and available elements on pharmacological treatments that regulate olfaction. We focus on data within the olfactory system but also on the interactions between the olfactory system and other brain networks impacted by metabolic diseases.

Origins of 1/f-like tissue oxygenation fluctuations in the murine cortex

The concentration of oxygen in the brain spontaneously fluctuates, and the distribution of power in these fluctuations has a 1/f-like spectra, where the power present at low frequencies of the power spectrum is orders of magnitude higher than at higher frequencies. Though these oscillations have been interpreted as being driven by neural activity, the origin of these 1/f-like oscillations is not well understood. Here, to gain insight of the origin of the 1/f-like oxygen fluctuations, we investigated the dynamics of tissue oxygenation and neural activity in awake behaving mice. We found that oxygen signal recorded from the cortex of mice had 1/f-like spectra. However, band-limited power in the local field potential did not show corresponding 1/f-like fluctuations. When local neural activity was suppressed, the 1/f-like fluctuations in oxygen concentration persisted. Two-photon measurements of erythrocyte spacing fluctuations and mathematical modeling show that stochastic fluctuations in erythrocyte flow could underlie 1/f-like dynamics in oxygenation. These results suggest that the discrete nature of erythrocytes and their irregular flow, rather than fluctuations in neural activity, could drive 1/f-like fluctuations in tissue oxygenation.

Postsynaptic activity of inhibitory neurons evokes hemodynamic fMRI responses

Functional MRI responses are localized to the synaptic sites of evoked inhibitory neurons, but it is unknown whether, or by what mechanisms, these neurons initiate functional hyperemia. Here, the neuronal origins of these hemodynamic responses were investigated by fMRI or local field potential and blood flow measurements during topical application of pharmacological agents when GABAergic granule cells in the rat olfactory bulb were synaptically targeted. First, to examine if postsynaptic activation of these inhibitory neurons was required for neurovascular coupling, we applied an NMDA receptor antagonist during cerebral blood volume-weighted fMRI acquisition and found that responses below the drug application site (up to ~1.5 mm) significantly decreased within ~30 min. Similarly, large decreases in granule cell postsynaptic activities and blood flow responses were observed when AMPA or NMDA receptor antagonists were applied. Second, inhibition of nitric oxide synthase preferentially decreased the initial, fast component of the blood flow response, while inhibitors of astrocyte-specific glutamate transporters and vasoactive intestinal peptide receptors did not decrease blood flow responses. Third, inhibition of GABA release with a presynaptic GABAB receptor agonist caused less reduction of neuronal and blood flow responses compared to the postsynaptic glutamate receptor antagonists. In conclusion, local hyperemia by synaptically-evoked inhibitory neurons was primarily driven by their postsynaptic activities, possibly through NMDA receptor-dependent calcium signaling that was not wholly dependent on nitric oxide.

More than just summed neuronal activity: how multiple cell types shape the BOLD response

Functional neuroimaging techniques are widely applied to investigations of human cognition and disease. The most commonly used among these is blood oxygen level-dependent (BOLD) functional magnetic resonance imaging. The BOLD signal occurs because neural activity induces an increase in local blood supply to support the increased metabolism that occurs during activity. This supply usually outmatches demand, resulting in an increase in oxygenated blood in an active brain region, and a corresponding decrease in deoxygenated blood, which generates the BOLD signal. Hence, the BOLD response is shaped by an integration of local oxygen use, through metabolism, and supply, in the blood. To understand what information is carried in BOLD signals, we must understand how several cell types in the brain-local excitatory neurons, inhibitory neurons, astrocytes and vascular cells (pericytes, vascular smooth muscle and endothelial cells), and their modulation by ascending projection neurons-contribute to both metabolism and haemodynamic changes. Here, we review the contributions of each cell type to the regulation of cerebral blood flow and metabolism, and discuss situations where a simplified interpretation of the BOLD response as reporting local excitatory activity may misrepresent important biological phenomena, for example with regards to arousal states, ageing and neurological disease. This article is part of the theme issue 'Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity'.

Extrinsic neuromodulation in the rodent olfactory bulb

Evolutionarily, olfaction is one of the oldest senses and pivotal for an individual's health and survival. The olfactory bulb (OB), as the first olfactory relay station in the brain, is known to heavily process sensory information. To adapt to an animal's needs, OB activity can be influenced by many factors either from within (intrinsic neuromodulation) or outside (extrinsic neuromodulation) the OB which include neurotransmitters, neuromodulators, hormones, and neuropeptides. Extrinsic sources seem to be of special importance as the OB receives massive efferent input from numerous brain centers even outweighing the sensory input from the nose. Here, we review neuromodulatory processes in the rodent OB from such extrinsic sources. We will discuss extrinsic neuromodulation according to points of origin, receptors involved, affected circuits, and changes in behavior. In the end, we give a brief outlook on potential future directions in research on neuromodulation in the OB.

[18F]-FMISO PET/MRI Imaging Shows Ischemic Tissue around Hematoma in Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH), being the most severe cerebrovascular disease, accounts for 10-15% of all strokes. Hematoma expansion is one of the most important factors associated with poor outcome in intracerebral hemorrhage (ICH). Several studies have suggested that an "ischemic penumbra" might arise when the hematoma has a large expansion, but clinical studies are inconclusive. We performed a preclinical study to demonstrate the presence of hypoxic-ischemic tissue around the hematoma by means of longitudinal [¹⁸F]-fluoromisonidazole ([¹⁸F]-FMISO) PET/MRI studies over time in an experimental ICH model. Our results showed that all [¹⁸F]-FMISO PET/MRI images exhibited hypoxic-ischemic tissue around the hematoma area. A significant increase of [¹⁸F]-FMISO uptake was found at 18-24 h post-ICH when the maximum of hematoma volume is achieved and this increase disappeared before 42 h. These results demonstrate the presence of hypoxic tissue around the hematoma and open the possibility of new therapies aimed to reduce ischemic damage associated with ICH.

Reduced cerebral blood flow in an -synuclein transgenic mouse model of Parkinson's disease

There is increasing evidence that widespread cortical cerebral blood flow deficits occur early in the course of Parkinson's disease. Although cerebral blood flow measurement has been suggested as a potential biomarker for early diagnosis of Parkinson's disease, as well as a means for tracking response to treatment, the relationship of cerebral blood flow to α-synucleinopathy, a major pathological hallmark of Parkinson's disease, remains unclear. Therefore, we performed arterial spin-labeling magnetic resonance imaging and diffusion tensor imaging on transgenic mice overexpressing human wild-type α-synuclein and age-matched controls to measure cerebral blood flow and degenerative changes. As reported for early-stage Parkinson's disease, α-synuclein mice exhibited a significant reduction in cortical cerebral blood flow, which was accompanied by motor coordination deficits and olfactory dysfunction. Although no overt degenerative changes were apparent in diffusion tensor imaging images, magnetic resonance imaging volumetric analysis revealed a significant reduction in olfactory bulb volume, similar to that seen in Parkinson's disease patients. Our data, representing the first report of cerebral blood flow deficit in an animal model of Parkinson's disease, suggest a causative role for α-synucleinopathy in cerebral blood flow deficits in Parkinson's disease. Thus, α-synuclein transgenic mice comprise a promising model to study Parkinson's disease-related mechanisms of cerebral blood flow deficits and to investigate further its utility as a potential biomarker for Parkinson's disease.

Increased prefrontal oxygenation prior to and at the onset of over-ground locomotion in humans

We found using wireless near-infrared spectroscopy that prefrontal oxygenation increased before the onset of arbitrary over-ground walking, whereas the preexercise increase was absent when walking was suddenly started by cue. The difference in prefrontal oxygenation between start modes (considered related to central command) preceded heart rate response variances and demonstrated a positive relationship with the difference in heart rate. The central command-related prefrontal activity may contribute to cardiac adjustment, synchronized with the beginning of over-ground walking.

Development and validation of a real-time microelectrochemical sensor for clinical monitoring of tissue oxygenation/perfusion

Oxygen is of critical importance to tissue viability and there is increasing demand for its reliable real-time clinical monitoring in order to prevent, diagnose, and treat several pathological disorders, including hypoxia, stroke and reperfusion injury. Herein we report the development and characterisation of a prototype clinical O2 sensor, and its validation in vivo, including proof-of-concept monitoring in patients undergoing surgery for carpal tunnel release. An integrated platinum-based microelectrochemical device was custom designed and controlled using a miniaturised telemetry-operated single channel clinical potentiostat. The in vitro performance of different sensor configurations is presented, with the best sensor design (S2) displaying appropriate linearity (R2 = 0.994) and sensitivity (0.569 ± 0.022 nA μM-1). Pre-clinical validation of S2 was performed in the hind limb muscle of anaesthetised rats; tourniquet application resulted in a significant rapid decrease in signal (90 ± 27%, [ΔO2] ca. 140 ± 18 μM), with a return to baseline within a period of ca. 3 min following tourniquet release. Similar trends were observed in the clinical study; an immediate decrease in signal (39 ± 3%, [ΔO2] ca. 30 ± 20 μM), with basal levels re-established within 2 min of tourniquet release. These results confirm that continuous real-time monitoring of dynamic changes in tissue O2 can serve as an indicator of reperfusion status in patients undergoing carpal tunnel surgery, and suggests the potential usefulness of the developed microelectrochemical sensor for other medical conditions where clinical monitoring of O2 and perfusion is important.

Cerebral oxygenation during locomotion is modulated by respiration

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.

More homogeneous capillary flow and oxygenation in deeper cortical layers correlate with increased oxygen extraction

Our understanding of how capillary blood flow and oxygen distribute across cortical layers to meet the local metabolic demand is incomplete. We addressed this question by using two-photon imaging of resting-state microvascular oxygen partial pressure (PO₂) and flow in the whisker barrel cortex in awake mice. Our measurements in layers I-V show that the capillary red-blood-cell flux and oxygenation heterogeneity, and the intracapillary resistance to oxygen delivery, all decrease with depth, reaching a minimum around layer IV, while the depth-dependent oxygen extraction fraction is increased in layer IV, where oxygen demand is presumably the highest. Our findings suggest that more homogeneous distribution of the physiological observables relevant to oxygen transport to tissue is an important part of the microvascular network adaptation to local brain metabolism. These results will inform the biophysical models of layer-specific cerebral oxygen delivery and consumption and improve our understanding of the diseases that affect cerebral microcirculation.

On the existence of mechanoreceptors within the neurovascular unit of the mammalian brain

We describe a set of perivascular interneurons (PINs) with series of fibro-vesicular complexes (FVCs) throughout the gray matter of the adult rabbit and rat brains. PIN-FVCs are ubiquitous throughout the brain vasculature as detected in Golgi-impregnated specimens. Most PINs are small, aspiny cells with short or long (> 1 mm) axons that split and travel along arterial blood vessels. Upon ramification, axons form FVCs around the arising vascular branches; then, paired axons run parallel to the vessel wall until another ramification ensues, and a new FVC is formed. Cytologically, FVCs consist of clusters of perivascular bulbs (PVBs) encircling the precapillary and capillary wall surrounded by end-feet and the extracellular matrix of endothelial cells and pericytes. A PVB contains mitochondria, multivesicular bodies, and granules with a membranous core, similar to Meissner corpuscles and other mechanoreceptors. Some PVBs form asymmetrical, axo-spinous synapses with presumptive adjacent neurons. PINs appear to correspond to the type 1 nNOS-positive neurons whose FVCs co-label with markers of sensory fiber-terminals surrounded by astrocytic end-feet. The PIN is conserved in adult cats and rhesus monkey specimens. The location, ubiquity throughout the vasculature of the mammalian brain, and cytological organization of the PIN-FVCs suggests that it is a sensory receptor intrinsic to the mammalian neurovascular unit that corresponds to an afferent limb of the sensorimotor feed-back mechanism controlling local blood flow.

Mesoscopic and microscopic imaging of sensory responses in the same animal

Imaging based on blood flow dynamics is widely used to study sensory processing. Here we investigated the extent to which local neuronal and capillary responses (two-photon microscopy) are correlated to mesoscopic responses detected with fast ultrasound (fUS) and BOLD-fMRI. Using a specialized chronic olfactory bulb preparation, we report that sequential imaging of the same mouse allows quantitative comparison of odour responses, imaged at both microscopic and mesoscopic scales. Under these conditions, functional hyperaemia occurred at the threshold of neuronal activation and fUS-CBV signals could be detected at the level of single voxels with activation maps varying according to blood velocity. Both neuronal and vascular responses increase non-linearly as a function of odour concentration, whereas both microscopic and mesoscopic vascular responses are linearly correlated to local neuronal calcium. These data establish strengths and limits of mesoscopic imaging techniques to report neural activity.

Neuronal activity leads to a local increase in blood flow and volume, a process termed hyperaemia. Here, the authors employ multiple imaging approaches of neuronal and vascular activity at varying resolution to delineate the spatiotemporal dynamics of neurovascular coupling evoked by odours in the olfactory bulb.

Electronic nose using a bio-inspired neural network modeled on mammalian olfactory system for Chinese liquor classification

The simplification of data processing is the frontier domain for electronic nose (e-nose) applications, whereas there are a lot of manual operations in a traditional processing procedure. To solve this problem, we propose a novel data processing method using the bio-inspired neural network modeled on the mammalian olfactory system. Through a neural coding scheme with multiple squared cosine receptive fields, continuous sensor data are simplified as the spike pattern in virtual receptor units. The biologically plausible olfactory bulb, which mimics the structure and function of main olfactory pathways, is designed to refine the olfactory information embedded in the encoded spikes. As a simplified presentation of cortical function, the bionic olfactory cortex is established to further analyze olfactory bulb's outputs and perform classification. The proposed method can automatically learn features without tedious steps such as denoising, feature extraction and reduction, which significantly simplifies the processing procedure for e-noses. To validate algorithm performance, comparison studies were performed for seven kinds of Chinese liquors using the proposed method and traditional data processing methods. The experimental results show that squared cosine receptive fields and the olfactory bulb model are crucial for improving classification performance, and the proposed method has higher classification rates than traditional methods when the sensor quantity and type are changed.

On the Physics Underlying Longitudinal Capillary Recruitment

Numerous researchers have found that capillary vessel haematocrit depends on the vasodilatory state of the arterioles. At rest, vessel haematocrit is down to 15 %, suggesting a red blood cell velocity three times higher than the plasma velocity. This finding is analysed in the context of present understanding of propulsion of red blood cells (RBCs) and plasma by means of the arteriovenous pressure gradient. Interfacial forces between the red blood cells and the plasma are proposed as a rational explanation of the observed red blood cell velocities. While the arteriovenous pressure gradient across the capillaries propels the red blood cell and the plasma jointly, interfacial forces along the red blood cell membrane can propel RBCs at the cost of the plasma. Different options are explored for the physical origin of these interfacial forces and oxygen gradients are found to be the most probable source.

Vascular Compartmentalization of Functional Hyperemia from the Synapse to the Pia

Functional hyperemia, a regional increase of blood flow triggered by local neural activation, is used to map brain activity in health and disease. However, the spatial-temporal dynamics of functional hyperemia remain unclear. Two-photon imaging of the entire vascular arbor in NG2-creERT2;GCaMP6f mice shows that local synaptic activation, measured via oligodendrocyte precursor cell (OPC) Ca²⁺ signaling, generates a synchronous Ca²⁺ drop in pericytes and smooth muscle cells (SMCs) enwrapping all upstream vessels feeding the activated synapses. Surprisingly, the onset timing, direction, and amplitude of vessel diameter and blood velocity changes vary dramatically from juxta-synaptic capillaries back to the pial arteriole. These results establish a precise spatial-temporal sequence of vascular changes triggered by neural activity and essential for the interpretation of blood-flow-based imaging techniques such as BOLD-fMRI.

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Odor triggers rapid Ca²⁺ elevations in OPC process that are input specific

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All pericyte subtypes and SMCs respond to downstream synaptic activation

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Synchronous mural cell activation is associated with heterogeneous local hemodynamics

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The arteriole and first-order capillary dilate first and form the primary functional unit

The relative influence of hematocrit and red blood cell velocity on oxygen transport from capillaries to tissue

OBJECTIVE

Oxygen transport to parenchymal cells occurs mainly at the microvascular level and depends on convective RBC flux, which is proportional in an individual capillary to the product of capillary hematocrit and RBC velocity. This study investigates the relative influence of these two factors on tissue PO₂ .

METHODS

A simple analytical model is used to quantify the respective influences of hematocrit, RBC velocity, and RBC flow on tissue oxygenation around capillaries. Predicted tissue PO₂ levels are compared with a detailed computational model.

RESULTS

Hematocrit is shown to have a larger influence on tissue PO₂ than RBC velocity. The effect of RBC velocity increases with distance from the arterioles. Good agreement between analytical and numerical results is obtained, and the discrepancies are explained. Significant dependence of MTCs on RBC velocity at low hematocrit is demonstrated.

CONCLUSIONS

For a given RBC flux in a capillary, the PO₂ in the surrounding tissue increases with increasing hematocrit, as a consequence of decreasing IVR to diffusive oxygen transport from RBCs to tissue. These results contribute to understanding the effects of blood flow changes on oxygen transport, such as those that occur in functional hyperemia in the brain.

Quantitative vascular neuroimaging of the rat brain using superparamagnetic nanoparticles: New insights on vascular organization and brain function

A method called Quantitative Ultra-Short Time-to-Echo Contrast Enhanced (QUTE-CE) Magnetic Resonance Imaging (MRI) which utilizes superparamagnetic iron oxide nanoparticles (SPIONs) as a contrast agent to yield positive contrast angiograms with high clarity and definition is applied to the whole live rat brain. QUTE-CE MRI intensity data are particularly well suited for measuring quantitative cerebral blood volume (qCBV). A global map of qCBV in the awake resting-state with unprecedented detail was created via application of a 3D MRI rat brain atlas with 173 segmented and annotated brain areas. From this map we identified two distributed, integrated neural circuits showing the highest capillary densities in the brain. One is the neural circuitry involved with the primary senses of smell, hearing and vision and the other is the neural circuitry of memory. Under isoflurane anesthesia, these same circuits showed significant decreases in qCBV suggesting a role in consciousness. Neural circuits in the brainstem associated with the reticular activating system and the maintenance of respiration, body temperature and cardiovascular function showed an increase in qCBV with anesthesia. During awake CO2 challenge, 84 regions showed significant increases relative to an awake baseline state. This CO2 response provides a measure of cerebral vascular reactivity and regional perfusion reserve with the highest response measured in the somatosensory cortex. These results demonstrate the utility of QUTE-CE MRI for qCBV analysis and offer a new perspective on brain function and vascular organization.

Foundations of layer-specific fMRI and investigations of neurophysiological activity in the laminarized neocortex and olfactory bulb of animal models

Laminar organization of neuronal circuits is a recurring feature of how the brain processes information. For instance, different layers compartmentalize different cell types, synaptic activities, and have unique intrinsic and extrinsic connections that serve as units for specialized signal processing. Functional MRI is an invaluable tool to investigate laminar processing in the in vivo human brain, but it measures neuronal activity indirectly by way of the hemodynamic response. Therefore, the accuracy of high-resolution laminar fMRI depends on how precisely it can measure localized microvascular changes nearest to the site of evoked activity. To determine the specificity of fMRI responses to the true neurophysiological responses across layers, the flexibility to invasive procedures in animal models has been necessary. In this review, we will examine different fMRI contrasts and their appropriate uses for layer-specific fMRI, and how localized laminar processing was examined in the neocortex and olfactory bulb. Through collective efforts, it was determined that microvessels, including capillaries, are regulated within single layers and that several endogenous and contrast-enhanced fMRI contrast mechanisms can separate these neural-specific vascular changes from the nonspecific, especially cerebral blood volume-weighted fMRI with intravenous contrast agent injection. We will also propose some open questions that are relevant for the successful implementation of layer-specific fMRI and its potential future directions to study laminar processing when combined with optogenetics.

Activity-dependent calcium, oxygen, and vascular responses in a mouse model of familial hemiplegic migraine type 1

OBJECTIVE

Familial hemiplegic migraine type 1 (FHM1) is a subtype of migraine with aura caused by a gain-of-function mutation in the pore-forming α1 subunit of CaV 2.1 (P/Q-type) calcium channels. However, the mechanisms underlying how the disease is brought about and the prolonged aura remain incompletely understood.

METHODS

In the anesthetized FHM1 mouse model in vivo, we used two-photon microscopy to measure calcium changes in neurons and astrocytes during somatosensory stimulations and cortical spreading depression (CSD), the putative mechanism of the migraine aura. We combined it with assessment of local field potentials by electrophysiological recordings, cerebral blood flow by laser Doppler flowmetry, and oxygen consumption with measurement of the oxygen tissue tension.

RESULTS

During spreading depression, the evoked increase in cytosolic Ca(2+) was larger and faster in FHM1 mice than wild-type (WT) mice. It was accompanied by larger increases in oxygen consumption in FHM1 mice, leading to tissue anoxia, but moderate hypoxia, in WT mice. In comparison, before CSD, Ca(2+) and hemodynamic responses to somatosensory stimulations were smaller in FHM1 mice than WT mice and almost abolished after CSD. The CSD-induced Ca(2+) changes were mitigated by the CaV 2.1 gating modifier, tert-butyl dihydroquinone.

INTERPRETATION

Our findings suggest that tissue anoxia might be a mechanism for prolonged aura in FHM1. Reduced Ca(2+) signals during normal network activity in FHM1 as compared to WT mice may explain impaired neurovascular responses in the mutant, and these alterations could contribute to brain frailty in FHM1 patients. Ann Neurol 2016;80:219-232.

Energy and Potassium Ion Homeostasis during Gamma Oscillations

Fast neuronal network oscillations in the gamma frequency band (30-100 Hz) occur in various cortex regions, require timed synaptic excitation and inhibition with glutamate and GABA, respectively, and are associated with higher brain functions such as sensory perception, attentional selection and memory formation. However, little is known about energy and ion homeostasis during the gamma oscillation. Recent studies addressed this topic in slices of the rodent hippocampus using cholinergic and glutamatergic receptor models of gamma oscillations (GAM). Methods with high spatial and temporal resolution were applied in vitro, such as electrophysiological recordings of local field potential (LFP) and extracellular potassium concentration ([K(+)]o), live-cell fluorescence imaging of nicotinamide adenine dinucleotide (phosphate) and flavin adenine dinucleotide [NAD(P)H and FAD, respectively] (cellular redox state), and monitoring of the interstitial partial oxygen pressure (pO2) in depth profiles with microsensor electrodes, including mathematical modeling. The main findings are: (i) GAM are associated with high oxygen consumption rate and significant changes in the cellular redox state, indicating rapid adaptations in glycolysis and oxidative phosphorylation; (ii) GAM are accompanied by fluctuating elevations in [K(+)]o of less than 0.5 mmol/L from baseline, likely reflecting effective K(+)-uptake mechanisms of neuron and astrocyte compartments; and (iii) GAM are exquisitely sensitive to metabolic stress induced by lowering oxygen availability or by pharmacological inhibition of the mitochondrial respiratory chain. These findings reflect precise cellular adaptations to maintain adenosine-5'-triphosphate (ATP), ion and neurotransmitter homeostasis and thus neural excitability and synaptic signaling during GAM. Conversely, the exquisite sensitivity of GAM to metabolic stress might significantly contribute the exceptional vulnerability of higher brain functions in brain disease.

Mitochondrial Ultrastructure and Glucose Signaling Pathways Attributed to the Kv1.3 Ion Channel

Gene-targeted deletion of the potassium channel Kv1.3 (Kv1.3^−∕−) results in “Super-smeller” mice with a sensory phenotype that includes an increased olfactory ability linked to changes in olfactory circuitry, increased abundance of olfactory cilia, and increased expression of odorant receptors and the G-protein, G_olf. Kv1.3^−∕− mice also have a metabolic phenotype including lower body weight and decreased adiposity, increased total energy expenditure (TEE), increased locomotor activity, and resistance to both diet- and genetic-induced obesity. We explored two cellular aspects to elucidate the mechanism by which loss of Kv1.3 channel in the olfactory bulb (OB) may enhance glucose utilization and metabolic rate. First, using in situ hybridization we find that Kv1.3 and the insulin-dependent glucose transporter type 4 (GLUT4) are co-localized to the mitral cell layer of the OB. Disruption of Kv1.3 conduction via construction of a pore mutation (W386F Kv1.3) was sufficient to independently translocate GLUT4 to the plasma membrane in HEK 293 cells. Because olfactory sensory perception and the maintenance of action potential (AP) firing frequency by mitral cells of the OB is highly energy demanding and Kv1.3 is also expressed in mitochondria, we next explored the structure of this organelle in mitral cells. We challenged wildtype (WT) and Kv1.3^−∕− male mice with a moderately high-fat diet (MHF, 31.8 % kcal fat) for 4 months and then examined OB ultrastructure using transmission electron microscopy. In WT mice, mitochondria were significantly enlarged following diet-induced obesity (DIO) and there were fewer mitochondria, likely due to mitophagy. Interestingly, mitochondria were significantly smaller in Kv1.3^−∕− mice compared with that of WT mice. Similar to their metabolic resistance to DIO, the Kv1.3^−∕− mice had unchanged mitochondria in terms of cross sectional area and abundance following a challenge with modified diet. We are very interested to understand how targeted disruption of the Kv1.3 channel in the OB can modify TEE. Our study demonstrates that Kv1.3 regulates mitochondrial structure and alters glucose utilization; two important metabolic changes that could drive whole system changes in metabolism initiated at the OB.

Layer-Specific fMRI Responses to Excitatory and Inhibitory Neuronal Activities in the Olfactory Bulb

High-resolution functional magnetic resonance imaging (fMRI) detects localized neuronal activity via the hemodynamic response, but it is unclear whether it accurately identifies neuronal activity specific to individual layers. To address this issue, we preferentially evoked neuronal activity in superficial, middle, and deep layers of the rat olfactory bulb: the glomerular layer by odor (5% amyl acetate), the external plexiform layer by electrical stimulation of the lateral olfactory tract (LOT), and the granule cell layer by electrical stimulation of the anterior commissure (AC), respectively. Electrophysiology, laser-Doppler flowmetry of cerebral blood flow (CBF), and blood oxygenation level-dependent (BOLD) and cerebral blood volume-weighted (CBV) fMRI at 9.4 T were performed independently. We found that excitation of inhibitory granule cells by stimulating LOT and AC decreased the spontaneous multi-unit activities of excitatory mitral cells and subsequently increased CBF, CBV, and BOLD signals. Odor stimulation also increased the hemodynamic responses. Furthermore, the greatest CBV fMRI responses were discretely separated into the same layers as the evoked neuronal activities for all three stimuli, whereas BOLD was poorly localized with some exception to the poststimulus undershoot. In addition, the temporal dynamics of the fMRI responses varied depending on the stimulation pathway, even within the same layer. These results indicate that the vasculature is regulated within individual layers and CBV fMRI has a higher fidelity to the evoked neuronal activity compared with BOLD. Our findings are significant for understanding the neuronal origin and spatial specificity of hemodynamic responses, especially for the interpretation of laminar-resolution fMRI.

SIGNIFICANCE STATEMENT

Functional magnetic resonance imaging (fMRI) is a noninvasive, in vivo technique widely used to map function of the entire brain, including deep structures, in animals and humans. However, it measures neuronal activity indirectly by way of the vascular response. It is currently unclear how finely the hemodynamic response is regulated within single cortical layers and whether increased inhibitory neuronal activities affect fMRI signal changes. Both laminar specificity and the neural origins of fMRI are important to interpret functional maps properly, which we investigated by activating discrete rat olfactory bulb circuits.

Mapping oxygen concentration in the awake mouse brain

Although critical for brain function, the physiological values of cerebral oxygen concentration have remained elusive because high-resolution measurements have only been performed during anesthesia, which affects two major parameters modulating tissue oxygenation: neuronal activity and blood flow. Using measurements of capillary erythrocyte-associated transients, fluctuations of oxygen partial pressure (Po₂) associated with individual erythrocytes, to infer Po₂ in the nearby neuropil, we report the first non-invasive micron-scale mapping of cerebral Po₂ in awake, resting mice. Interstitial Po₂ has similar values in the olfactory bulb glomerular layer and the somatosensory cortex, whereas there are large capillary hematocrit and erythrocyte flux differences. Awake tissue Po₂ is about half that under isoflurane anesthesia, and within the cortex, vascular and interstitial Po₂ values display layer-specific differences which dramatically contrast with those recorded under anesthesia. Our findings emphasize the importance of measuring energy parameters non-invasively in physiological conditions to precisely quantify and model brain metabolism.

DOI:http://dx.doi.org/10.7554/eLife.12024.001

Brain cells need a constant supply of oxygen to fuel their activities. This oxygen is delivered by the flow of blood through the vessels in the brain. If the blood flow to brain tissue is cut off as happens in stroke, or if an individual stops breathing, the brain becomes deprived of oxygen and brain cells will be damaged and die. To better understand how the brain works in health and disease, scientists need to learn how much oxygen the blood must deliver to the brain tissue to adequately support the activities of brain cells.

Many studies have measured oxygen levels in the brain. However, these studies have looked only roughly and taken measurements from large areas of the brain, or they have involved animals receiving anesthesia, which can alter blood flow and oxygen use in the brain. Recently, scientists discovered that they could measure oxygen concentration at high detail in the brain of anesthetized rodents with a specialized microscope, by using molecules that emit light at a rate that depends on the local oxygen concentration.

Now, Lyons et al. have shown that this same technique can be used in mice that are awake. First, a piece of the skull was replaced with glass to create a small transparent window. Then, the animals were allowed to recover for a few weeks, and were trained to get them used to how they would be handled during the experiments. After this period, the oxygen concentrations and blood flow in different parts of the mouse brains were measured in fine detail using the microscope while the animals were awake and relaxed.

The experiments showed that oxygen levels in awake resting mice are actually lower than in anesthetized mice, and that oxygen levels differ between different parts of the mouse brain. This first detailed look at oxygen levels in the brain of awake animals will likely lead to more studies. For example, future studies may look at how quickly the brain uses oxygen under normal conditions and what happens in the brain during disease.

DOI:http://dx.doi.org/10.7554/eLife.12024.002

Sensory-Evoked Intrinsic Imaging Signals in the Olfactory Bulb Are Independent of Neurovascular Coupling

Functional brain-imaging techniques used in humans and animals, such as functional MRI and intrinsic optical signal (IOS) imaging, are thought to largely rely on neurovascular coupling and hemodynamic responses. Here, taking advantage of the well-described micro-architecture of the mouse olfactory bulb, we dissected the nature of odor-evoked IOSs. Using in vivo pharmacology in transgenic mouse lines reporting activity in different cell types, we show that parenchymal IOSs are largely independent of neurotransmitter release and neurovascular coupling. Furthermore, our results suggest that odor-evoked parenchymal IOSs originate from changes in light scattering of olfactory sensory neuron axons, mostly due to water movement following action potential propagation. Our study sheds light on a direct correlate of neuronal activity, which may be used for large-scale functional brain imaging.

Acute two-photon imaging of the neurovascular unit in the cortex of active mice

In vivo two-photon scanning fluorescence imaging is a powerful technique to observe physiological processes from the millimeter to the micron scale in the intact animal. In neuroscience research, a common approach is to install an acute cranial window and head bar to explore neocortical function under anesthesia before inflammation peaks from the surgery. However, there are few detailed acute protocols for head-restrained and fully awake animal imaging of the neurovascular unit during activity. This is because acutely performed awake experiments are typically untenable when the animal is naïve to the imaging apparatus. Here we detail a method that achieves acute, deep-tissue two-photon imaging of neocortical astrocytes and microvasculature in behaving mice. A week prior to experimentation, implantation of the head bar alone allows mice to train for head-immobilization on an easy-to-learn air-supported ball treadmill. Following just two brief familiarization sessions to the treadmill on separate days, an acute cranial window can subsequently be installed for immediate imaging. We demonstrate how running and whisking data can be captured simultaneously with two-photon fluorescence signals with acceptable movement artifacts during active motion. We also show possible applications of this technique by (1) monitoring dynamic changes to microvascular diameter and red blood cells in response to vibrissa sensory stimulation, (2) examining responses of the cerebral microcirculation to the systemic delivery of pharmacological agents using a tail artery cannula during awake imaging, and (3) measuring Ca(2+) signals from synthetic and genetically encoded Ca(2+) indicators in astrocytes. This method will facilitate acute two-photon fluorescence imaging in awake, active mice and help link cellular events within the neurovascular unit to behavior.

A dynamic model of oxygen transport from capillaries to tissue with moving red blood cells

Most oxygen required to support the energy needs of vertebrate tissues is delivered by diffusion from microvessels. The presence of red blood cells (RBCs) makes blood flow in the microcirculation highly heterogeneous. Additionally, flow regulation mechanisms dynamically respond to changes in tissue energy demand. These spatiotemporal variations directly affect the supply of oxygen to parenchymal cells. Due to various limiting assumptions, current models of oxygen transport cannot fully capture the consequences of complex hemodynamic effects on tissue oxygenation and are often not suitable for studying unsteady phenomena. With our new approach based on moving RBCs, the impact of blood flow heterogeneity on oxygen partial pressure (Po2) in the tissue can be quantified. Oxygen transport was simulated using parachute-shaped solid RBCs flowing through a capillary. With the use of a conical tissue domain with radii 19 and 13 μm, respectively, our computations indicate that Po2 at the RBC membrane exceeds Po2 between RBCs by 30 mmHg on average and that the mean plasma Po2 decreases by 9 mmHg over 50 μm. These results reproduce well recent intravascular Po2 measurements in the rodent brain. We also demonstrate that instantaneous variations of capillary hematocrit cause associated fluctuations of tissue Po2. Furthermore, our results suggest that homogeneous tissue oxygenation requires capillary networks to be denser on venular side than on arteriolar side. Our new model for oxygen transport will make it possible to quantify in detail the effects of blood flow heterogeneity on tissue oxygenation in realistic capillary networks.

Quantitative proteomics reveals olfactory input-dependent alterations in the mouse olfactory bulb proteome

Olfactory sensory information is processed and integrated by circuits within the olfactory bulb (OB) before being sent to the olfactory cortex. In the mammalian OB, neural activity driven by external stimuli can lead to experience-dependent changes in structures and functions. In this study, quantitative proteomics techniques were employed to study proteome-wide changes in the OB under four levels of neural activity (from low to high): devoid of peripheral input (using a transgenic model), wild-type control, and short-term and long-term odor exposures. Our results revealed that proteins related to various processes were altered in the OBs of odor-deprived and odor-stimulated mice compared to the wild-type controls. These changes induced by odor stimulation were quite different from those induced by a deficit in peripheral olfactory inputs. Detailed analysis demonstrated that metabolic process and synaptic transmission were the most commonly altered pathways and that the effects of peripheral deprivation were more profound. Our comparative proteomics analysis indicated that olfactory deprivation and odor exposure lead to different alterations in the OB proteome, which provides new clues about the mechanisms underlying the olfactory deprivation- or odor stimulation-induced plasticity of OB function and organization.

BIOLOGICAL SIGNIFICANCE

By combining quantitative proteomics, bioinformatics and WB/IHC analysis, this study reports the results of the first comparative study on proteome-wide changes in the olfactory bulb under different levels of olfactory input. Odor deprivation and stimulation induced proteomic changes clearly demonstrate significant metabolic shifts and alterations on synaptic transmission. This quantitative system biology study leads to a new level of understanding in the development of olfactory bulb plasticity induced by odor deprivation or stimulation, and provides many new clues for the olfactory bulb related functional studies.

Layer-dependent BOLD and CBV-weighted fMRI responses in the rat olfactory bulb

The olfactory bulb is a laminarized brain structure involved in odor sensation that has important implications to basic neuroscience research, like mechanisms for neurovascular coupling and early disease diagnosis. To investigate laminar-dependent responses to odor exposure, blood oxygenation level-dependent (BOLD) and cerebral blood volume weighted (CBVw) fMRI with iron oxide nanoparticle contrast agent were obtained with 110×110×500μm(3) resolution in urethane-anesthetized rats at 9.4T. The baseline total CBV is the largest at the olfactory bulb surface and midline, and decreases in the deeper layers, while a band of increased microvasculature density is observed at the glomerular, external plexiform and mitral cell layers. With odor exposure, CBVw fMRI is more sensitive and reproducible than BOLD. BOLD fMRI had the greatest activation on the bulb surface, midline, olfactory nerve and glomerular layers, while CBVw activation peaked in glomerular and external plexiform layers, but was still significant in mitral cell layer. Negative BOLD responses were observed in the bulb midline and near large blood vessels. CBVw laminar profiles are similar to the layer-dependent metabolic changes to the same odor exposure reported by previous glucose metabolism studies. Unique activation patterns for two different odor conditions were also differentiated with CBVw fMRI. Our study suggests that CBVw activation better represents the spatial location of metabolic activity in the olfactory bulb than BOLD.

The oxygen paradox of neurovascular coupling

The coupling of cerebral blood flow (CBF) to neuronal activity is well preserved during evolution. Upon changes in the neuronal activity, an incompletely understood coupling mechanism regulates diameter changes of supplying blood vessels, which adjust CBF within seconds. The physiologic brain tissue oxygen content would sustain unimpeded brain function for only 1 second if continuous oxygen supply would suddenly stop. This suggests that the CBF response has evolved to balance oxygen supply and demand. Surprisingly, CBF increases surpass the accompanying increases of cerebral metabolic rate of oxygen (CMRO2). However, a disproportionate CBF increase may be required to increase the concentration gradient from capillary to tissue that drives oxygen delivery. However, the brain tissue oxygen content is not zero, and tissue pO2 decreases could serve to increase oxygen delivery without a CBF increase. Experimental evidence suggests that CMRO2 can increase with constant CBF within limits and decreases of baseline CBF were observed with constant CMRO2. This conflicting evidence may be viewed as an oxygen paradox of neurovascular coupling. As a possible solution for this paradox, we hypothesize that the CBF response has evolved to safeguard brain function in situations of moderate pathophysiological interference with oxygen supply.

Anesthetic regimes modulate the temporal dynamics of local field potential in the mouse olfactory bulb

Anesthetized preparations have been widely used to study odor-induced temporal dynamics in the olfactory bulb. Although numerous recent data of single-cell recording or imaging in the olfactory bulb have employed ketamine cocktails, their effects on networks activities are still poorly understood, and odor-induced oscillations of the local field potential have not been characterized under these anesthetics. Our study aimed at describing the impact of two ketamine cocktails on oscillations and comparing them to awake condition. Anesthesia was induced by injection of a cocktail of ketamine, an antagonist of the N-methyl-d-aspartate receptors, combined with one agonist of α2-adrenergic receptors, xylazine (low affinity) or medetomidine (high affinity). Spontaneous and odor-induced activities were examined in anesthetized and awake conditions, in the same mice chronically implanted with an electrode in the main olfactory bulb. The overall dynamic pattern of oscillations under the two ketamine cocktails resembles that of the awake state. Ongoing activity is characterized by gamma bursts (>60 Hz) locked on respiration and beta (15-40 Hz) power increases during odor stimulation. However, anesthesia decreases local field potential power and leads to a strong frequency shift of gamma oscillations from 60-90 Hz to 100-130 Hz. We conclude that similarities between oscillations in anesthetized and awake states make cocktails of ketamine with one α2-agonist suitable for the recordings of local field potential to study processing in the early stages of the olfactory system.

Glucose sensitivity of mouse olfactory bulb neurons is conveyed by a voltage-gated potassium channel

The olfactory bulb has recently been proposed to serve as a metabolic sensor of internal chemistry, particularly that modified by metabolism. Because the voltage-dependent potassium channel Kv1.3 regulates a large proportion of the outward current in olfactory bulb neurons and gene-targeted deletion of the protein produces a phenotype of resistance to diet-induced obesity in mice, we hypothesized that this channel may play a role in translating energy availability into a metabolic signal. Here we explored the ability of extracellular glucose concentration to modify evoked excitability of the mitral neurons that principally regulate olfactory coding and processing of olfactory information. Using voltage-clamp electrophysiology of heterologously expressed Kv1.3 channels in HEK 293 cells, we found that Kv1.3 macroscopic currents responded to metabolically active (d-) rather than inactive (l-) glucose with a response profile that followed a bell-shaped curve. Olfactory bulb slices stimulated with varying glucose concentrations showed glucose-dependent mitral cell excitability as evaluated by current-clamp electrophysiology. While glucose could be either excitatory or inhibitory, the majority of the sampled neurons displayed a decreased firing frequency in response to elevated glucose concentration that was linked to increased latency to first spike and decreased action potential cluster length. Unlike modulation attributed to phosphorylation, glucose modulation of mitral cells was rapid, less than one minute, and was reversible within the time course of a patch recording. Moreover, we report that modulation targets properties of spike firing rather than action potential shape, involves synaptic activity of glutamate or GABA signalling circuits, and is dependent upon Kv1.3 expression. Given the rising incidence of metabolic disorders attributed to weight gain, changes in neuronal excitability in brain regions regulating sensory perception of food are of consequence.

Imaging local neuronal activity by monitoring PO₂ transients in capillaries

Two-photon phosphorescence lifetime microscopy (2PLM) has been used recently for depth measurements of oxygen partial pressure (PO(2)) in the rodent brain. In capillaries of olfactory bulb glomeruli, 2PLM has also allowed simultaneous measurements of PO(2) and blood flow and revealed the presence of erythrocyte-associated transients (EATs), which are PO(2) gradients that are associated with individual erythrocytes. We investigated the extent to which EAT properties in capillaries report local neuronal activity. We find that at rest, PO(2) at EAT peaks overestimates the mean PO(2) by 35 mm Hg. PO(2) between two EAT peaks is at equilibrium with, and thus reports, PO(2) in the neuropil. During odor stimulation, there is a small PO(2) decrease before functional hyperemia, showing that the initial dip in PO(2) is present at the level of capillaries. We conclude that imaging oxygen dynamics in capillaries provides a unique and noninvasive approach to map neuronal activity.

Oxidative phosphorylation, not glycolysis, powers presynaptic and postsynaptic mechanisms underlying brain information processing

Neural activity has been suggested to initially trigger ATP production by glycolysis, rather than oxidative phosphorylation, for three reasons: glycolytic enzymes are associated with ion pumps; neurons may increase their energy supply by activating glycolysis in astrocytes to generate lactate; and activity increases glucose uptake more than O₂ uptake. In rat hippocampal slices, neuronal activity rapidly decreased the levels of extracellular O₂ and intracellular NADH (reduced nicotinamide adenine dinucleotide), even with lactate dehydrogenase blocked to prevent lactate generation, or with only 20% superfused O₂ to mimic physiological O₂ levels. Pharmacological analysis revealed an energy budget in which 11% of O₂ use was on presynaptic action potentials, 17% was on presynaptic Ca²⁺ entry and transmitter release, 46% was on postsynaptic glutamate receptors, and 26% was on postsynaptic action potentials, in approximate accord with theoretical brain energy budgets. Thus, the major mechanisms mediating brain information processing are all initially powered by oxidative phosphorylation, and an astrocyte-neuron lactate shuttle is not needed for this to occur.

Two-photon microscopy as a tool to study blood flow and neurovascular coupling in the rodent brain

The cerebral vascular system services the constant demand for energy during neuronal activity in the brain. Attempts to delineate the logic of neurovascular coupling have been greatly aided by the advent of two-photon laser scanning microscopy to image both blood flow and the activity of individual cells below the surface of the brain. Here we provide a technical guide to imaging cerebral blood flow in rodents. We describe in detail the surgical procedures required to generate cranial windows for optical access to the cortex of both rats and mice and the use of two-photon microscopy to accurately measure blood flow in individual cortical vessels concurrent with local cellular activity. We further provide examples on how these techniques can be applied to the study of local blood flow regulation and vascular pathologies such as small-scale stroke.

Activity-dependent increases in local oxygen consumption correlate with postsynaptic currents in the mouse cerebellum in vivo

Evoked neural activity correlates strongly with rises in cerebral metabolic rate of oxygen (CMRO(2)) and cerebral blood flow (CBF). Activity-dependent rises in CMRO(2) fluctuate with ATP turnover due to ion pumping. In vitro studies suggest that increases in cytosolic Ca(2+) stimulate oxidative metabolism via mitochondrial signaling, but whether this also occurs in the intact brain is unknown. Here we applied a pharmacological approach to dissect the effects of ionic currents and cytosolic Ca(2+) rises of neuronal origin on activity-dependent rises in CMRO(2). We used two-photon microscopy and current source density analysis to study real-time Ca(2+) dynamics and transmembrane ionic currents in relation to CMRO(2) in the mouse cerebellar cortex in vivo. We report a direct correlation between CMRO(2) and summed (i.e., the sum of excitatory, negative currents during the whole stimulation period) field EPSCs (∑fEPSCs) in Purkinje cells (PCs) in response to stimulation of the climbing fiber (CF) pathway. Blocking stimulus-evoked rises in cytosolic Ca(2+) in PCs with the P/Q-type channel blocker ω-agatoxin-IVA (ω-AGA), or the GABA(A) receptor agonist muscimol, did not lead to a time-locked reduction in CMRO(2), and excitatory synaptic or action potential currents. During stimulation, neither ω-AGA or (μ-oxo)-bis-(trans-formatotetramine-ruthenium) (Ru360), a mitochondrial Ca(2+) uniporter inhibitor, affected the ratio of CMRO(2) to fEPSCs or evoked local field potentials. However, baseline CBF and CMRO(2) decreased gradually with Ru360. Our data suggest that in vivo activity-dependent rises in CMRO(2) are correlated with synaptic currents and postsynaptic spiking in PCs. Our study did not reveal a unique role of neuronal cytosolic Ca(2+) signals in controlling CMRO(2) increases during CF stimulation.

Hemodynamic responses evoked by neuronal stimulation via channelrhodopsin-2 can be independent of intracortical glutamatergic synaptic transmission

Maintenance of neuronal function depends on the delivery of oxygen and glucose through changes in blood flow that are linked to the level of ongoing neuronal and glial activity, yet the underlying mechanisms remain unclear. Using transgenic mice expressing the light-activated cation channel channelrhodopsin-2 in deep layer pyramidal neurons, we report that changes in intrinsic optical signals and blood flow can be evoked by activation of a subset of channelrhodopsin-2-expressing neurons in the sensorimotor cortex. We have combined imaging and pharmacology to examine the importance of glutamatergic synaptic transmission in this form of neurovascular coupling. Blockade of ionotropic glutamate receptors with the antagonists CNQX and MK801 significantly reduced forepaw-evoked hemodynamic responses, yet resulted in no significant reduction of channelrhodopsin-evoked hemodynamic responses, suggesting that stimulus-dependent coupling of neuronal activity to blood flow can be independent of local excitatory synaptic transmission. Together, these results indicate that channelrhodopsin-2 activation of sensorimotor excitatory neurons produces changes in intrinsic optical signals and blood flow that can occur under conditions where synaptic activation of neurons or other cells through ionotropic glutamate receptors would be blocked.

Multispectral reflectance imaging of brain activation in rodents: methodological study of the differential path length estimations and first in vivo recordings in the rat olfactory bulb

Dynamic maps of relative changes in blood volume and oxygenation following brain activation are obtained using multispectral reflectance imaging. The technique relies on optical absorption modifications linked to hemodynamic changes. The relative variation of hemodynamic parameters can be quantified using the modified Beer-Lambert Law if changes in reflected light intensities are recorded at two wavelengths or more and the differential path length (DP) is known. The DP is the mean path length in tissues of backscattered photons and varies with wavelength. It is usually estimated using Monte Carlo simulations in simplified semi-infinite homogeneous geometries. Here we consider the use of multilayered models of the somatosensory cortex (SsC) and olfactory bulb (OB), which are common physiological models of brain activation. Simulations demonstrate that specific DP estimation is required for SsC and OB, specifically for wavelengths above 600 nm. They validate the hypothesis of a constant path length during activation and show the need for specific DP if imaging is performed in a thinned-skull preparation. The first multispectral reflectance imaging data recorded in vivo during OB activation are presented, and the influence of DP on the hemodynamic parameters and the pattern of oxymetric changes in the activated OB are discussed.

Visualizing odor representation in the brain: a review of imaging techniques for the mapping of sensory activity in the olfactory glomeruli

The brain transforms clues from the external world, the sensory stimuli, into activities in neuroglial networks. These circuits are activated in specialized sensory cortices where specific functional modules are responsible for the spatiotemporal coding of the stimulus. A major challenge in the neuroscience field has been to image the spatial distribution and follow the temporal dynamics of the activation of such large populations in vivo. Functional imaging techniques developed in the last 30 years have enabled researchers to solve this critical issue, and are reviewed here. These techniques utilize sources of contrast of radioisotopic, magnetic and optical origins and exploit two major families of signals to image sensory activity: the first class uses sources linked to cellular energy metabolism and hemodynamics, while the second involves exogenous indicators of neuronal activity. The whole panel of imaging techniques has fostered the functional exploration of the olfactory bulb which is one of the most studied sensory structures. We summarize the major results obtained using these techniques that describe the spatial and temporal activity patterns in the olfactory glomeruli, the first relay of olfactory information processing in the main olfactory bulb. We conclude this review by describing promising technical developments in optical imaging and future directions in the study of olfactory spatiotemporal coding.

Simultaneous two-photon imaging of oxygen and blood flow in deep cerebral vessels

Uncovering principles that regulate energy metabolism in the brain requires mapping of partial pressure of oxygen (PO(2)) and blood flow with high spatial and temporal resolution. Using two-photon phosphorescence lifetime microscopy (2PLM) and the oxygen probe PtP-C343, we show that PO(2) can be accurately measured in the brain at depths up to 300 μm with micron-scale resolution. In addition, 2PLM allowed simultaneous measurements of blood flow and of PO(2) in capillaries with less than one-second temporal resolution. Using this approach, we detected erythrocyte-associated transients (EATs) in oxygen in the rat olfactory bulb and showed the existence of diffusion-based arterio-venous shunts. Sensory stimulation evoked functional hyperemia, accompanied by an increase in PO(2) in capillaries and by a biphasic PO(2) response in the neuropil, consisting of an 'initial dip' and a rebound. 2PLM of PO(2) opens new avenues for studies of brain metabolism and blood flow regulation.

In vivo visualization of olfactory pathophysiology induced by intranasal cadmium instillation in mice

Intranasal exposure to cadmium has been related to olfactory dysfunction in humans and to nasal epithelial damage and altered odorant-guided behavior in rodent models. The pathophysiology underlying these deficits has not been fully elucidated. Here we use optical imaging techniques to visualize odorant-evoked neurotransmitter release from the olfactory nerve into the brain's olfactory bulbs in vivo in mice. Intranasal cadmium chloride instillations reduced this sensory activity by up to 91% in a dose-dependent manner. In the olfactory bulbs, afferents from the olfactory epithelium could be quantified by their expression of a genetically encoded fluorescent marker for olfactory marker protein. At the highest dose tested, cadmium exposure reduced the density of these projections by 20%. In a behavioral psychophysical task, mice were trained to sample from an odor port and make a response when they detected an odorant against a background of room air. After intranasal cadmium exposure, mice were unable to detect the target odor. These experiments serve as proof of concept for a new approach to the study of the neural effects of inhaled toxicants. The use of in vivo functional imaging of the neuronal populations exposed to the toxicant permits the direct observation of primary pathophysiology. In this study optical imaging revealed significant reductions in odorant-evoked release from the olfactory nerve at a cadmium chloride dose two orders of magnitude less than that required to induce morphological changes in the nerve in the same animals, demonstrating that it is a more sensitive technique for assessing the consequences of intranasal neurotoxicant exposure. This approach is potentially useful in exploring the effects of any putative neurotoxicant that can be delivered intranasally.

Characterisation of carbon paste electrodes for real-time amperometric monitoring of brain tissue oxygen

Tissue O₂ can be monitored using a variety of electrochemical techniques and electrodes. In vitro and in vivo characterisation studies for O₂ reduction at carbon paste electrodes (CPEs) using constant potential amperometry (CPA) are presented. Cyclic voltammetry indicated that an applied potential of -650 mV is required for O₂ reduction at CPEs. High sensitivity (-1.49 ± 0.01 nA/μM), low detection limit (ca. 0.1 μM) and good linear response characteristics (R² > 0.99) were observed in calibration experiments performed at this potential. There was also no effect of pH, temperature, and ion changes, and no dependence upon flow/fluid convection (stirring). Several compounds (e.g. dopamine and its metabolites) present in brain extracellular fluid were tested at physiological concentrations and shown not to interfere with the CPA O₂ signal. In vivo experiments confirmed a sub-second response time observed in vitro and demonstrated long-term stability extending over twelve weeks, with minimal O₂ consumption (ca. 1 nmol/h). These results indicate that CPEs operating amperometrically at a constant potential of -650 mV (vs. SCE) can be used reliably to continuously monitor brain extracellular tissue O₂.