Effects of intrinsic and drug-induced neuromodulation on functional brain imaging

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Abstract Ascending neuromodulation associated with cognitive functions, such as arousal, attention, learning, memory, decision making, evaluation of reward, are active in any conscious human subject participating in a Blood Oxygenation Level Dependent (BOLD) functional Magnetic Resonance Imaging (fMRI) study. And yet, our understanding of how these systems affect the BOLD signal remains rudimentary. In fact, our current knowledge of neurovascular and neurometaboic mechanisms that underlie the BOLD signal has been derived almost exclusively from studies in anesthetized animals where the state of neuromodulation was uncertain. Recently, we have developed optical reporters for dopamine (DA), norepinephrine (NE), and acetylcholine (ACh) applicable for high-resolution imaging of brain function in awake behaving mice. In the proposed project, we will combine these reporters with an integrated suite of the BRAIN Initiative tools, developed by us and others, to investigate the microscopic makeup of “brain states” and their reflection in macroscopic BOLD fMRI signals. These tools (except fMRI) are only applicable to model organisms. Therefore, all experiments will be performed in awake behaving mice. Our Central Hypothesis is that ascending projections from one or more neuromodulatory systems contribute critically to generation of spontaneous (“resting-state”) hemodynamic fluctuations as well as task-induced hemodynamic responses. To test this hypothesis, we will investigate the relationship between neuronal, vascular and metabolic activity as a function of (i) intrinsic brain states (Aims 1-2), and (ii) exposure to cocaine – a common drug of abuse that acts by affecting neuromodulation (Aim 3). Brain states will be operationally defined based on the readout of DA, NE, and ACh reporters referenced to electrophysiological/imaging measures of local cortical dynamics. These studies will be performed in the context of resting-state hemodynamic fluctuations as well as task-induced hemodynamic responses in the primary somatosensory and frontal cortices. The proposed project will (i) provide a stronger physiological foundation for resting-state and task-induced fMRI in healthy individuals; (ii) place the relationship between the state of neuromodulation and energy expenditure (cerebral metabolic rate of O2, CMRO2) on a quantitative footing; and (iii) examine the effects of cocaine on neuronal and hemodynamic brain activity. This study will also generate further hypotheses about the ways in which substance exposure may affect fMRI readouts.