Imaging separation of neuronal from vascular effects of cocaine on rat cortical brain in vivo

MRI techniques to study brain function assume coupling between neuronal activity, metabolism and flow. However, recent evidence of physiological uncoupling between neuronal and cerebrovascular events highlights the need for methods to simultaneously measure these three properties. We report a multimodality optical approach that integrates dual-wavelength laser speckle imaging (measures changes in blood flow, blood volume and hemoglobin oxygenation), digital-frequency-ramping optical coherence tomography (images quantitative 3D vascular network) and Rhod₂ fluorescence (images intracellular calcium for measure of neuronal activity) at high spatiotemporal resolutions (30μm, 10Hz) and over a large field of view (3×5mm²). We apply it to assess cocaine's effects in rat cortical brain and show an immediate decrease (3.5±0.9min, phase 1) in the oxygen content of hemoglobin and the cerebral blood flow followed by an overshoot (7.1±0.2min, phase 2) lasting over 20min whereas Ca²⁺ increased immediately (peaked at t=4.1±0.4min) and remained elevated. This enabled us to identify a delay (2.9±0.5min) between peak neuronal and vascular responses in phase 2. The ability of this multimodality optical approach for simultaneous imaging at high spatiotemporal resolutions permits us to distinguish the vascular versus cellular changes of the brain, thus complimenting other neuroimaging modalities for brain functional studies (e. g., PET, fMRI).