Nanosensors for sensitive brain-wide neurochemical imaging

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The large-scale dynamics of neural circuitry depend on interactions among numerous neurochemical spe- cies that play functionally distinct roles throughout the brain. Understanding the spatial and temporal character- istics of chemical signaling is thus crucial for building mechanistic models of brain function. Our laboratory has introduced paramagnetic neurotransmitter sensors that enable functional analysis of neurochemical phenomena over large fields of view by molecular-level functional magnetic resonance imaging (molecular fMRI). We have published applications of these sensors to spatiotemporal mapping of neurochemical phenomena in a series of substantial papers. The scope of such experiments has however been limited by the modest sensitivity provided by the existing probes, which must be applied at concentrations that substantially exceed physiological neuro- transmitter levels. The goal of this proposal is to establish a platform technology for noninvasive neurochemical imaging with substantially higher sensitivity, focusing initially on monoamine transmitters. Our approach is based on a novel principle for biochemical sensing in MRI that uses paramagnetic liposomes as responsive contrast agents. In this mechanism, the presence of neurotransmitter targets gates large contrast effects afforded by the liposomes, giving rise to a formidable amplification factor with respect to previous probes. Using this design, we predict that sensitivity to behaviorally relevant low-micromolar or submicromolar neurotransmitter concentrations will be achieved, with minimal potential for buffering effects. In addition, our preliminary studies suggest that wide-field brain delivery with these probes is achievable, and we also predict that perisynaptic cell type-specific readouts can be obtained by targeting the liposomes. Our work will address three Aims: In Aim 1, we will establish our liposome-based nanosensor (LBN) plat- form by combining lipid, polypeptide, and small molecular components to establish the new sensing mechanism we seek to exploit. We will use a variety of synthetic and molecular engineering methods to optimize this mech- anism for detection of behaviorally relevant interstitial dopamine and serotonin concentrations, with the goal of achieving sensitivity in the 0.1-1 µM range. In Aim 2, we will optimize strategies for brain-wide delivery of these probes, exploiting chemically-mediated blood-brain barrier disruption and infusion into cerebrospinal fluid. We will also implement a perisynaptic targeting approach. In Aim 3, we will validate liposome-based dopamine and serotonin LBNs by molecular fMRI in live rat brains, with reference to parallel neurochemical and hemodynamic fMRI measurements. In addition to establishing the novel neurochemical imaging platform we propose, these experiments will yield first-of-their-kind data about the wide-field distribution of dopamine and serotonin signaling in response to stimuli, as well as the relationship of these neurochemicals to conventional brain activity readouts. Although the technology we will develop will initially be applied in sedated rodents, we expect it to be applicable to many additional species and behavioral contexts.