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The glymphatic system is a brain-wide waste drainage system that promotes cerebrospinal fluid circulation through the brain to remove waste metabolites. Currently, the most common methods for assessing glymphatic function are ex vivo fluorescence microscopy of brain slices, macroscopic cortical imaging, and MRI. While all these methods have been crucial for expanding our understanding of the glymphatic system, new techniques are required to overcome their specific drawbacks. Here, we evaluate SPECT/CT imaging as a tool to assess glymphatic function in different anesthesia-induced brain states using two radiolabeled tracers, [111In]-DTPA and [99mTc]-NanoScan. Using SPECT, we confirmed the existence of brain state-dependent differences in glymphatic flow and we show brain state-dependent differences of CSF flow kinetics and CSF egress to the lymph nodes. We compare SPECT and MRI for imaging glymphatic flow and find that the two imaging modalities show the same overall pattern of CSF flow, but that SPECT was specific across a greater range of tracer concentrations than MRI. Overall, we find that SPECT imaging is a promising tool for imaging the glymphatic system, and that qualities such as high sensitivity and the variety of available tracers make SPECT imaging a good alternative for glymphatic research. [ABSTRACT FROM AUTHOR]

Respiratory brain pulsations have recently been shown to drive electrophysiological brain activity in patients with epilepsy. Furthermore, functional neuroimaging indicates that respiratory brain pulsations have increased variability and amplitude in patients with epilepsy compared to healthy individuals. To determine whether the respiratory drive is altered in epilepsy, we compared respiratory brain pulsation synchronicity between healthy controls and patients. Whole brain fast functional magnetic resonance imaging was performed on 40 medicated patients with focal epilepsy, 20 drug-naïve patients and 102 healthy controls. Cerebrospinal fluid associated respiratory pulsations were used to generate individual whole brain respiratory synchronization maps, which were compared between groups. Finally, we analyzed the seizure frequency effect and diagnostic accuracy of the respiratory synchronization defect in epilepsy. Respiratory brain pulsations related to the verified fourth ventricle pulsations were significantly more synchronous in patients in frontal, periventricular and mid-temporal regions, while the seizure frequency correlated positively with synchronicity. The respiratory brain synchronicity had a good diagnostic accuracy (ROCAUC = 0.75) in discriminating controls from medicated patients. The elevated respiratory brain synchronicity in focal epilepsy suggests altered physiological effect of cerebrospinal fluid pulsations possibly linked to regional brain water dynamics involved with interictal brain physiology. [ABSTRACT FROM AUTHOR]

Brain lactate concentration is higher during wakefulness than in sleep. However, it is unknown why arousal is linked to an increase in brain lactate and why lactate declines within minutes of sleep. Here, we show that the glymphatic system is responsible for state-dependent changes in brain lactate concentration. Suppression of glymphatic function via acetazo-lamide treatment, cisterna magna puncture, aquaporin 4 deletion, or changes in body position reduced the decline in brain lactate normally observed when awake mice transition into sleep or anesthesia. Concurrently, the same manipulations diminished accumulation of lactate in cervical, but not in inguinal lymph nodes when mice were anesthetized. Thus, our study suggests that brain lactate is an excellent biomarker of the sleep-wake cycle and increases further during sleep deprivation, because brain lactate is inversely correlated with glymphatic-lymphatic clearance. This analysis provides fundamental new insight into brain energy metabolism by demonstrating that glucose that is not fully oxidized can be exported as lactate via glymphatic-lymphatic fluid transport. [ABSTRACT FROM AUTHOR]

The glymphatic system has recently been shown to clear brain extracellular solutes and abnormalities in glymphatic clearance system may contribute to both initiation and progression of neurological diseases. Despite that diabetes is known as a risk factor for vascular diseases, little is known how diabetes affects the glymphatic system. The current study is the first investigation of the effect of diabetes on the glymphatic system and the link between alteration of glymphatic clearance and cognitive impairment in Type-2 diabetes mellitus rats. MRI analysis revealed that clearance of cerebrospinal fluid contrast agent Gd-DTPA from the interstitial space was slowed by a factor of three in the hippocampus of Type-2 diabetes mellitus rats compared to the non-DM rats and confirmed by florescence imaging analysis. Cognitive deficits detected by behavioral tests were highly and inversely correlated to the retention of Gd-DTPA contrast and fluorescent tracer in the hippocampus of Type-2 diabetes mellitus rats. Type-2 diabetes mellitus suppresses clearance of interstitial fluid in the hippocampus and hypothalamus, suggesting that an impairment of the glymphatic system contributes to Type-2 diabetes mellitus-induced cognitive deficits. Whole brain MRI provides a sensitive, non-invasive tool to quantitatively evaluate cerebrospinal fluid and interstitial fluid exchange in Type-2 diabetes mellitus and possibly in other neurological disorders, with potential clinical application. [ABSTRACT FROM AUTHOR]

Using in vivo two-photon imaging, we show that mice deficient in aquaporin-4 (AQP4) display increased fluorescence of nicotinamide adenine dinucleotide (NADH) when subjected to cortical spreading depression. The increased NADH signal, a proxy of tissue hypoxia, was restricted to microwatershed areas remote from the vasculature. Aqp4 deletion had no effects on the hyperemia response, but slowed [K+]o recovery. These observations suggest that K+ uptake is suppressed in Aqp4−/− mice as a consequence of decreased oxygen delivery to tissue located furthest away from the vascular source of oxygen, although increased oxygen consumption may also contribute to our observations. [ABSTRACT FROM AUTHOR]

Cerebral edema is a major contributor to morbidity associated with traumatic brain injury (TBI). The methods involved in most rodent models of TBI, including head fixation, opening of the skull, and prolonged anesthesia, likely alter TBI development and reduce secondary injury. We report the development of a closed-skull model of murine TBI, which minimizes time of anesthesia, allows the monitoring of intracranial pressure (ICP), and can be modulated to produce mild and moderate grade TBI. In this model, we characterized changes in aquaporin-4 (AQP4) expression and localization after mild and moderate TBI. We found that global AQP4 expression after TBI was generally increased; however, analysis of AQP4 localization revealed that the most prominent effect of TBI on AQP4 was the loss of polarized localization at endfoot processes of reactive astrocytes. This AQP4 dysregulation peaked at 7 days after injury and was largely indistinguishable between mild and moderate grade TBI for the first 2 weeks after injury. Within the same model, blood-brain barrieranalysis of variance permeability, cerebral edema, and ICP largely normalized within 7 days after moderate TBI. These findings suggest that changes in AQP4 expression and localization may not contribute to cerebral edema formation, but rather may represent a compensatory mechanism to facilitate its resolution. [ABSTRACT FROM AUTHOR]

Given the brain's uniquely high cell density and tissue oxygen levels bordering on hypoxia, the ability to rapidly and precisely match blood flow to constantly changing patterns in neural activity is an essential feature of cerebrovascular regulation. Locus coeruleus-norepinephrine (LC-NE) projections innervate the cerebral vasculature and can mediate vasoconstriction. However, function of the LC-mediated constriction in blood-flow regulation has never been addressed. Here, using intrinsic optical imaging coupled with an anesthesia regimen that only minimally interferes with LC activity, we show that NE enhances spatial and temporal aspects of functional hyperemia in the mouse somatosensory cortex. Increasing NE levels in the cortex using an α2-adrenergic receptor antagonist paradoxically reduces the extent of functional hyperemia while enhancing the surround blood-flow reduction. However, the NE-mediated vasoconstriction optimizes spatial and temporal focusing of the hyperemic response resulting in a sixfold decrease in the disparity between blood volume and oxygen demand. In addition, NE-mediated vasoconstriction accelerated redistribution to subsequently active regions, enhancing temporal synchronization of blood delivery. These observations show an important role for NE in optimizing neurovascular coupling. As LC neuron loss is prominent in Alzheimer and Parkinson diseases, the diminished ability to couple blood volume to oxygen demand may contribute to their pathogenesis. [ABSTRACT FROM AUTHOR]

Oxygen transport imposes a possible constraint on the brain's ability to sustain variable metabolic demands, but oxygen diffusion in the cerebral cortex has not yet been observed directly. We show that concurrent two-photon fluorescence imaging of endogenous nicotinamide adenine dinucleotide (NADH) and the cortical microcirculation exposes well-defined boundaries of tissue oxygen diffusion in the mouse cortex. The NADH fluorescence increases rapidly over a narrow, very low pO2 range with a p50 of 3.4±0.6 mm Hg, thereby establishing a nearly binary reporter of significant, metabolically limiting hypoxia. The transient cortical tissue boundaries of NADH fluorescence exhibit remarkably delineated geometrical patterns, which define the limits of tissue oxygen diffusion from the cortical microcirculation and bear a striking resemblance to the ideal Krogh tissue cylinder. The visualization of microvessels and their regional contribution to oxygen delivery establishes penetrating arterioles as major oxygen sources in addition to the capillary network and confirms the existence of cortical oxygen fields with steep microregional oxygen gradients. Thus, two-photon NADH imaging can be applied to expose vascular supply regions and to localize functionally relevant microregional cortical hypoxia with micrometer spatial resolution. [ABSTRACT FROM AUTHOR]

Recent reports based on a chemiluminescent enzymatic assay for detection of adenosine conclude that cultured astrocytes release adenosine during mildly hypoxic conditions. If so, astrocytes may suppress neural activity in early stages of hypoxia. The aim of this study was to reevaluate the observation using high-performance liquid chromatography (HPLC). The HPLC analysis showed that exposure to 20 or 120 minutes of mild hypoxia failed to increase release of adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine from cultured astrocytes. Similar results were obtained using a chemiluminescent enzymatic assay. Moreover, since the chemiluminescent enzymatic assay relies on hydrogen peroxide generation, release of free-radical scavengers from hypoxic cells can interfere with the assay. Accordingly, adenosine added to samples collected from hypoxic cultures could not be detected using the chemiluminescent enzymatic assay. Furthermore, addition of free-radical scavengers sharply reduced the sensitivity of adenosine detection. Conversely, use of a single-step assay inflated measured values due to the inability of the assay to distinguish adenosine and its metabolite inosine. These results show that cultured astrocytes do not release adenosine during mild hypoxia, an observation consistent with their high resistance to hypoxia. [ABSTRACT FROM AUTHOR]