BME PhD Dissertation Defense: Patrick Doran

Title: "Mesoscale Imaging of Local Neuronal Activity, Intrinsic Neuromodulation, and Quantitative Hemodynamics"

Advisory Committee: Anna Devor, PhD - BME (Research Advisor) Xue Han, PhD - BME (Chair) Brian DePasquale, PhD - BME Jeffrey Gavornik, PhD - Biology Sava Sakadzic, PhD - Massachusetts General Hospital Radiology Joseph Mandeville, PhD - Massachusetts General Hospital Radiology

Abstract: Ascending neuromodulatory systems, including the cholinergic basal forebrain and the noradrenergic locus coeruleus, are active in every conscious brain and exert direct effects on both neuronal activity and cerebral blood flow. Despite their ubiquity, the spatial organization of neuromodulatory signals at the mesoscale and their influence on the neurovascular coupling (NVC) relationship underlying blood-oxygen-level-dependent (BOLD) fMRI remain poorly understood. Misattributing neuromodulation-driven hemodynamic changes to local neuronal activity represents a fundamental confound for neuroimaging studies of cognition, arousal, and disease. An open source, widefield mesoscope was built for simultaneous two-color fluorescence and two-channel reflectance imaging on a single sCMOS detector, enabling concurrent measurement of GRAB sensors for acetylcholine (ACh) and norepinephrine (NE), neuronal calcium (jRGECO1a), and oxy-/deoxyhemoglobin in awake, head-fixed mice. A separate platform was developed to enable simultaneous BOLD fMRI and widefield fluorescence imaging at 9.4T using a custom MR-compatible fiber-optic bundle. Visual stimulation drives spatially structured, retinotopically organized ACh release in primary visual cortex, challenging the prevailing model of purely diffuse cholinergic broadcast signaling. NE, but not ACh, is a critical second determinant of hemodynamics: incorporating NE alongside neuronal calcium as a regressor dramatically improves hemodynamic prediction accuracy (r = 0.59 ± 0.03 vs. 0.32 ± 0.02 for calcium alone). Arousal-driven NE release causes region-specific vasoconstriction that reduces hemodynamic functional connectivity without corresponding changes in neuronal calcium coherence. Simultaneous BOLD fMRI and widefield imaging in awake mice reveals that neuronal calcium leads both BOLD and pupil dilation, while a hemodynamic impulse response function of widefield neuronal calcium data predicts cortical BOLD with limited accuracy (r = 0.15). Neuromodulatory state, particularly noradrenergic tone, is a significant, spatially heterogeneous determinant of cortical hemodynamics that cannot be ignored in hemodynamic brain imaging. The tools, datasets, and quantitative frameworks developed here provide a foundation for more mechanistic interpretation of BOLD fMRI signals in terms of their underlying neural and neuromodulatory origins.