BOLD fMRI reflects both vascular and metabolic signals.

The hemodynamic and metabolic response of the cortex depends spatially and temporally on the activity of multiple cell types. Optogenetics enables specific cell types to be modulated with high temporal precision and is therefore an emerging method for studying neurovascular and neurometabolic coupling. Going beyond temporal investigations, we developed a microprojection system to apply spatial photostimulus patterns in vivo. We monitored vascular and metabolic fluorescence signals after photostimulation in Thy1-channelrhodopsin-2 mice. Cerebral arteries increased in diameter rapidly after photostimulation, while nearby veins showed a slower smaller response. The amplitude of the arterial response was depended on the area of cortex stimulated. The fluorescence signal emitted at 450/100 nm and excited with ultraviolet is indicative of reduced nicotinamide adenine dinucleotide, an endogenous fluorescent enzyme involved in glycolysis and the citric acid cycle. This fluorescence signal decreased quickly and transiently after optogenetic stimulation, suggesting that glucose metabolism is tightly locked to optogenetic stimulation. To verify optogenetic stimulation of the cortex, we used a transparent substrate microelectrode array to map cortical potentials resulting from optogenetic stimulation. Spatial optogenetic stimulation is a new tool for studying neurovascular and neurometabolic coupling.

Purpose:Previous work [1] modeling the metabolic flux between hyperpolarized [1‐13C]pyruvate and [1‐13C]lactate in magnetic resonance spectroscopic imaging (MRSI) experiments failed to account for vascular signal artifacts. Here, we investigate a method to minimize the vascular signal and its impact on the fidelity of metabolic modeling.Methods:MRSI was simulated for renal metabolism in MATLAB both with and without bipolar gradients. The resulting data were fit to a two‐site exchange model [1], and the effects of vascular partial volume artifacts on kinetic modeling were assessed. Bipolar gradients were then incorporated into a gradient echo sequence to validate the simulations experimentally. The degree of diffusion weighting (b = 32 s/mm2) was determined empirically from 1H imaging of murine renal vascular signal. The method was then tested in vivo using MRSI with bipolar gradients following injection of hyperpolarized [1‐13C]pyruvate (∼80 mM at 20% polarization).Results:In simulations, vascular signal contaminated the renal metabolic signal at resolutions as high as 2 × 2 mm2 due to partial volume effects. The apparent exchange rate from pyruvate to lactate (kp) was underestimated in the presence of these artifacts due to contaminating pyruvate signal. Incorporation of bipolar gradients suppressed vascular signal and improved the accuracy of kp estimation. Experimentally, the in vivo results supported the ability of bipolar gradients to suppress vascular signal. The in vivo exchange rate increased, as predicted in simulations, from kp = 0.012 s‐1 to kp = 0.020‐1 after vascular signal suppression.Conclusion:We have demonstrated the limited accuracy of the two‐site exchange model in the presence of vascular partial volume artifacts. The addition of bipolar gradients suppressed vascular signal and improved model accuracy in simulations. Bipolar gradients largely affected kp estimation in vivo. Currently, slow‐flowing spins in small vessels and capillaries are only partially suppressed, so further improvement is possible.Funding support: Seed Grant from the Radiological Society of North America, GE Healthcare, University of Wisconsin Graduate School

Understanding the biological mechanism and identifying biomarkers of hemorrhagic shock is important for diagnosis and treatment. We aim to use optical imaging to study how the cerebral blood circulation and metabolism change during the progression of severe hemorrhagic shock, especially the decompensatory stage. We used a multi-parameter (blood pressure (BP), cerebral blood flow (CBF), functional vascular density (FVD), blood oxygenation and mitochondrial NADH signal) cerebral cortex optical imaging system to observe brain hemodynamic change and metabolic alteration of rats in vivo for 4 h. Cerebral circulation and mitochondrial metabolism could be well preserved in the compensatory stage but impaired during the decompensatory stage. The changes of brain hemodynamics and metabolism may provide sensitive indicators for various shock stages including the transition from compensatory stage to decompensatory stage. Our novel imaging observations of hemodynamic and metabolic signals in vivo indicated that the rat brains under hemorrhagic shock suffered irreversible damage which could not be compensated by the autoregulation mechanism, probably due to injured mitochondria.

Cardiac fibrosis is a central pathological feature of adverse myocardial remodeling and a key contributor to the development and progression of heart failure. Increasing evidence indicates that fibrosis is not a passive consequence of injury but a dynamic, regulated process driven by complex interactions between immune cells, fibroblasts, and endothelial compartments. The objective of this review is to synthesize current mechanistic insights into immune–stromal regulation of cardiac fibrosis, with particular emphasis on fibroblasts as central integrators of inflammatory, mechanical, metabolic, and vascular signals. This narrative review integrates findings from experimental, translational, and clinical studies addressing immune cell–fibroblast crosstalk, neutrophil extracellular trap formation, endothelial plasticity including endothelial-to-mesenchymal transition, and epigenetic and metabolic mechanisms that stabilize fibroblast activation. The literature was thematically analyzed to construct a unified conceptual framework rather than to perform a quantitative synthesis. The reviewed evidence highlights macrophage–fibroblast interactions as a dominant regulatory axis governing fibrotic remodeling, with distinct immune cell subsets exerting divergent effects on fibroblast activation and extracellular matrix deposition. Neutrophil extracellular traps and endothelial dysfunction further amplify profibrotic signaling, while epigenetic and metabolic reprogramming preserves activated fibroblast phenotypes beyond the acute injury phase. Circulating biomarkers of fibrosis reflect these underlying biological processes but capture remodeling dynamics rather than fixed fibrotic burden. In conclusion, cardiac fibrosis should be viewed as the outcome of regulated immune–stroma–endothelium communication rather than irreversible scarring. Targeting key interaction nodes within these networks may enable more precise strategies to limit pathological remodeling while preserving essential reparative responses.

Functional brain imaging techniques such as positron emission tomography (PET) or functional magnetic resonance imaging (fMRI) provide a unique opportunity to study the brain at work. These techniques detect metabolic and vascular signals that are coupled to changes in neuronal activity, thus affording the possibility to localize brain areas, changing their level of activity during particular behavioral modalities (1). They also inform about basal and activated metabolic states, particularly PET, which can monitor with appropriate tracers the metabolic rates for glucose and oxygen, as well as blood flow. With fMRI, in addition to activity changes, it is possible to identify the degree of functional and anatomical connectivity existing between a given brain region, taken as a seed, and other brain areas.

In vivo optical imaging of cerebral blood flow (CBF) and metabolism did not exist 50 years ago. While point optical fluorescence and absorption measurements of cellular metabolism and hemoglobin concentrations had already been introduced by then, point blood flow measurements appeared only 40 years ago. The advent of digital cameras has significantly advanced two-dimensional optical imaging of neuronal, metabolic, vascular, and hemodynamic signals. More recently, advanced laser sources have enabled a variety of novel three-dimensional high-spatial-resolution imaging approaches. Combined, as we discuss here, these methods are permitting a multifaceted investigation of the local regulation of CBF and metabolism with unprecedented spatial and temporal resolution. Through multimodal combination of these optical techniques with genetic methods of encoding optical reporter and actuator proteins, the future is bright for solving the mysteries of neurometabolic and neurovascular coupling and translating them to clinical utility.

The adrenal gland provides an important function by integrating neuronal, immune, vascular, metabolic and endocrine signals under a common organ capsule. It is the central organ of the stress response system and has been implicated in numerous stress-related disorders. While for other diseases, regeneration of healthy organ tissue has been aimed at such approaches are lacking for endocrine diseases - with the exception of type-I-diabetes. Moreover, adrenal tumor formation is very common, however, appropriate high-throughput applications reflecting the high heterogeneity and furthermore relevant 3D-structures in vitro are still widely lacking. Recently, we have initiated the development of standardized multidimensional models of a variety of endocrine cell/tissue sources in a new multiwell-format. Firstly, we confirmed common applicability for pancreatic pseudo-islets. Next, we translated applicability for spheroid establishment to adrenocortical cell lines as well as patient material to establish spheroids from malignant, but also benign adrenal tumors. We aimed furthermore at the development of bovine derived healthy adrenal organoids and were able to establish steroidogenic active organoids containing both, cells of cortical and medullary origin. Overall, we hope to open new avenues for basic research, endocrine cancer and adrenal tissue-replacement-therapies as we demonstrate potential for innovative mechanistic insights and personalized medicine in endocrine (tumor)-biology.

PurposePositron emission tomography (PET) provides precise molecular information on physiological processes, but its low temporal resolution is a major obstacle. Consequently, we characterized the metabolic response of the human brain to working memory performance using an optimized functional PET (fPET) framework at a temporal resolution of 3 s.MethodsThirty-five healthy volunteers underwent fPET with [18F]FDG bolus plus constant infusion, 19 of those at a hybrid PET/MRI scanner. During the scan, an n-back working memory paradigm was completed. fPET data were reconstructed to 3 s temporal resolution and processed with a novel sliding window filter to increase signal to noise ratio. BOLD fMRI signals were acquired at 2 s.ResultsConsistent with simulated kinetic modeling, we observed a constant increase in the [18F]FDG signal during task execution, followed by a rapid return to baseline after stimulation ceased. These task-specific changes were robustly observed in brain regions involved in working memory processing. The simultaneous acquisition of BOLD fMRI revealed that the temporal coupling between hemodynamic and metabolic signals in the primary motor cortex was related to individual behavioral performance during working memory. Furthermore, task-induced BOLD deactivations in the posteromedial default mode network were accompanied by distinct temporal patterns in glucose metabolism, which were dependent on the metabolic demands of the corresponding task-positive networks.ConclusionsIn sum, the proposed approach enables the advancement from parallel to truly synchronized investigation of metabolic and hemodynamic responses during cognitive processing. This allows to capture unique information in the temporal domain, which is not accessible to conventional PET imaging.

The orexigenic gut-brain peptide, ghrelin and its G-protein coupled receptor, the growth hormone secretagogue receptor 1a (GHS-R1A) are pivotal regulators of hypothalamic feeding centers and reward processing neuronal circuits of the brain. These systems operate in a cooperative manner and receive a wide array of neuronal hormone/transmitter messages and metabolic signals. Functional magnetic resonance imaging was employed in the current study to map BOLD responses to ghrelin in different brain regions with special reference on homeostatic and hedonic regulatory centers of energy balance. Experimental groups involved male, ovariectomized female and ovariectomized estradiol-replaced rats. Putative modulation of ghrelin signaling by endocannabinoids was also studied. Ghrelin-evoked effects were calculated as mean of the BOLD responses 30 minutes after administration. In the male rat, ghrelin evoked a slowly decreasing BOLD response in all studied regions of interest (ROI) within the limbic system. This effect was antagonized by pretreatment with GHS-R1A antagonist JMV2959. The comparison of ghrelin effects in the presence or absence of JMV2959 in individual ROIs revealed significant changes in the prefrontal cortex, nucleus accumbens of the telencephalon, and also within hypothalamic centers like the lateral hypothalamus, ventromedial nucleus, paraventricular nucleus and suprachiasmatic nucleus. In the female rat, the ghrelin effects were almost identical to those observed in males. Ovariectomy and chronic estradiol replacement had no effect on the BOLD response. Inhibition of the endocannabinoid signaling by rimonabant significantly attenuated the response of the nucleus accumbens and septum. In summary, ghrelin can modulate hypothalamic and mesolimbic structures controlling energy balance in both sexes. The endocannabinoid signaling system contributes to the manifestation of ghrelin's BOLD effect in a region specific manner. In females, the estradiol milieu does not influence the BOLD response to ghrelin.

Dynamic activation model for a glutamatergic neurovascular unit

BackgroundPrevious studies have reported spatial dissociations between glucose metabolism and BOLD responses in the human posterior cingulate cortex (PCC)1, with an opposite influence of the fronto-parietal network (FPN) and the dorsal attention network (DAN) metabolism onto the PCC2. Until recently, a truly synchronized assessment of these influences in the temporal domain was limited due to the inherently low temporal resolution of positron emission tomography (PET) when compared to functional magnetic resonance imaging (fMRI).Aims & ObjectivesWe employed high temporal resolution functional PET (fPET) to synchronously investigate dynamic metabolic and hemodynamic responses during working memory performance in the PCC.MethodsThirty-five participants underwent either a simultaneous fPET/fMRI (n=19) or separate fPET and fMRI scans (n=16). During the scan, an n-back working memory paradigm was completed while acquiring [18 F]FDG fPET and BOLD fMRI data. fPET data were reconstructed to 3 s temporal resolution, whereas BOLD fMRI acquisition had a repetition time of 2 s.Participants were divided into three distinct groups based on the mentioned difference in task-specific metabolism between the FPN and DAN: low (metabolism of FPN <DAN, 25% of participants), balanced (FPN = DAN, 50%) and high (FPN >DAN, 25%). [18F]FDG and BOLD time series were extracted from the PCC region2 and these parameters were compared between the three groups using repeated measures ANOVA (groups-by-time interaction).ResultsIn the spatial domain, the PCC exhibited a negative BOLD response, while no change in metabolic demands was found during task performance (p<0.05 FWE-corrected cluster level following p<0.001 uncorrected voxel level).Temporal profile assessment of the PCC between the defined groups revealed an interaction effect (p = 0.0016) for the metabolic signal, with significant post-hoc differences between high vs. low participants (p <10-5) and low vs. balanced (p = 0.025). While the low group showed a constant decrease in PCC metabolism, followed by a subsequent increase, the high group exhibited a delayed increase. At the same time, the balanced group showed almost no variation in their metabolic response. BOLD time series of the PCC revealed robust decreases for all three groups without significant difference (interaction p = 0.9).Discussion & ConclusionSimultaneous BOLD fMRI and high resolution [18F]FDG fPET provide deeper insight into the divergence between glucose metabolism and BOLD deactivations in the PCC1 during working memory performance. Confirming the previous results of a dependence of PCC metabolic time course on energy demands of the corresponding task-positive networks2, the high-resolution dynamic assessment of the presented approach enabled distinguishing group differences with decreased vs. increased PCC metabolism. Given the involvement of impaired neurovascular coupling3 and widespread decreased metabolism4 in diseases like Alzheimer’ s, synchronous multimodal imaging at high temporal resolution could benefit the diagnosis of neuropsychiatric disorders at early stages by detecting minor deviations.

A common observation in fMRI studies using the BOLD signal is that older adults, compared with young adults, show overactivations, particularly during less demanding tasks. The neuronal underpinnings of such overactivations are not known, but a dominant view is that they are compensatory in nature and involve recruitment of additional neural resources. We scanned 23 young (20-37 years) and 34 older (65-86 years) healthy human adults of both sexes with hybrid positron emission tomography/MRI. The radioligand [18F]fluoro-deoxyglucose was used to assess dynamic changes in glucose metabolism as a marker of task-dependent synaptic activity, along with simultaneous fMRI BOLD imaging. Participants performed two verbal working memory (WM) tasks: one involving maintenance (easy) and one requiring manipulation (difficult) of information in WM. Converging activations to the WM tasks versus rest were observed for both imaging modalities and age groups in attentional, control, and sensorimotor networks. Upregulation of activity to WM-demand, comparing the more difficult to the easier task, also converged between both modalities and age groups. For regions in which older adults showed task-dependent BOLD overactivations compared with the young adults, no corresponding increases in glucose metabolism were found. To conclude, findings from the current study show that task-induced changes in the BOLD signal and synaptic activity as measured by glucose metabolism generally converge, but overactivations observed with fMRI in older adults are not coupled with increased synaptic activity, which suggests that these overactivations are not neuronal in origin.SIGNIFICANCE STATEMENT Findings of increased fMRI activations in older compared with younger adults have been suggested to reflect increased use of neuronal resources to cope with reduced brain function. The physiological underpinnings of such compensatory processes are poorly understood, however, and rest on the assumption that vascular signals accurately reflect neuronal activity. Comparing fMRI and simultaneously acquired functional positron emission tomography as an alternative index of synaptic activity, we show that age-related overactivations do not appear to be neuronal in origin. This result is important because mechanisms underlying compensatory processes in aging are potential targets for interventions aiming to prevent age-related cognitive decline.

The brain has evolved mechanisms to rapidly modify local vascular resistance near active neurons. This enables delivery of energy substrates and clearance of byproducts via the blood to meet the energetic demands of circuits engaged during cognitive and motor tasks. Collectively, the physiochemical processes governing the spatially precise blood flow changes that accompany neural activity are termed “neurovascular coupling”. Crucially, these blood flow changes are leveraged by functional brain imaging techniques in humans to evaluate disease processes and understand cognition. It is therefore imperative to understand in detail the interaction between neurons and vasculature in the brain. While several neurovascular coupling pathways have been identified and studied, there is still no consensus on the vascular signal transduction and transmission mechanisms that enable acute responses to vasoactive substances released by active neurons. Arteriolar diameter is viewed as a crucial control point for neurovascular coupling. The relatively sparse spatial arrangement of arterioles contrasts with the vast plexus of capillaries, suggesting that that the latter is well positioned to play a major role in sensing neural activity and transmitting signals to modify the state of contractile cells on upstream vessels. Thin-strand pericyte processes cover a large fraction of the capillary bed but the contributions of these cells to blood flow control are not understood and are controversial. Here, we provide evidence that thin strand pericytes in the deep capillary bed play a role in neurovascular coupling by sensing neural activity and rapidly relaying electrical signals to arterioles. We identify a KATP channel-dependent neurovascular signaling pathway that can be explained by the recruitment of capillary pericytes. We developed a vascular optogenetics approach to show that membrane currents generated in single thin-strand pericytes are rapidly sent over long distances to penetrating arterioles in vivo. We demonstrate that genetic disruption of the KATP channel reduces the neurovascular coupling arteriole diameter response and laser ablation of pericytes eliminates KATP-dependent neurovascular coupling. Overall, our work suggests that the thin-strand pericytes of the capillary bed actively sense neural activity and integrate this into electro metabolic signals that inform arterioles of local energy needs ensuring spatiotemporal specificity in the availability of energy substrates like glucose, lactate, and oxygen. This work was supported by the NIH T32 Interdisciplinary Training Program in Cardiovascular Disease at The University of Maryland School of Medicine (ITCVD-T32), and NIH grants 1R01AG066645, 5R01NS115401, and 1DP2NS121347. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.