A local increase in cortical blood flow is widely accepted to report the presence and location of neuronal activity in the brain. This hemodynamic response is the basis of functional brain imaging methods such as functional Magnetic Resonance Imaging (fMRI). Yet the way that the brain controls these blood flow changes, and even the underlying purpose of the hemodynamic response are still unknown. There is growing evidence that impairments in regulation of the hemodynamic response may underlie age-related neurodegeneration, and can be linked to the progression of diseases such as Alzheimer's. A comprehensive model of the mechanisms underlying the coupling between neuronal and hemodynamic responses in the brain is urgently needed. The lack of a conclusive model to date is due, in part, to the difficulties faced in imaging the normal, functioning, intact brain in-vivo. In-vitro studies which investigate potential mechanisms in isolation are uniformly difficult to compare with other in-vitro results, and to reconcile with in-vivo observations. However, it is highly challenging to develop in-vivo imaging paradigms capable of achieving sufficient resolution, sensitivity and contrast to gain a complete picture of the neuronal, metabolic and vascular processes which together generate the hemodynamic response to activation. We have developed a suite of advanced optical imaging and microscopy tools for in-vivo examination of neurovascular coupling. We plan to exploit a wide range of optical contrasts including oxy- and deoxyhemoglobin, calcium sensitive dyes, cell-specific dyes, transgenic mice expressing fluorescent proteins, and intrinsically fluorescent substrates of energy metabolism such as NADH and FAD. We propose to refine and apply our imaging tools to address the following two fundamentally important questions: 'How is the cortical vasculature physically modulated?'and 'Why does the hemodynamic response happen?'Our multi-scale and multi-parametric imaging approaches offer the chance to both fully characterize the physical mechanisms which modulate and control the hemodynamic response, and to investigate the metabolic basis of the response and its relation to energy supply and demand at a cellular level. We believe that these studies will lead to a comprehensive model of neurovascular coupling. We also anticipate that the imaging techniques that we develop, and our careful characterization of the healthy brain, will clear the way for future in-vivo research into the cellular, metabolic and neurovascular underpinnings of disease.
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