Neurophotonics was launched in 2014 coinciding with the launch of the BRAIN Initiative focused on development of technologies for advancement of neuroscience. For the last seven years, Neurophotonics’ agenda has been well aligned with this focus on neurotechnologies featuring new optical methods and tools applicable to brain studies. While the BRAIN Initiative 2.0 is pivoting towards applications of these novel tools in the quest to understand the brain, in this article we review an extensive and diverse toolkit of novel methods to explore brain function that have emerged from the BRAIN Initiative and related large-scale efforts for measurement and manipulation of brain structure and function. Here, we focus on neurophotonic tools mostly applicable to animal studies. A companion article, scheduled to appear later this year, will cover diffuse optical imaging methods applicable to noninvasive human studies. For each domain, we outline the current state-of-the-art of the respective technologies, identify the areas where innovation is needed and provide an outlook for the future directions.
Photoacoustic imaging is an emerging technology capable of both functional and structural biological imaging. Absorption and scattering in tissue limit the penetration depth of conventional microscopy techniques to <1mm. Photoacoustic imaging however, can offer high-resolution and contrast at depths of several centimeters. Though functional imaging of endogenous contrast agents, such as hemoglobin, is widely implemented, currently photoacoustic imaging is unable to functionally report electrophysiological changes within cells. We aim to develop photoacoustic contrast agents to fulfill this need. Cells throughout the brain and body create electrical signals using ion channel proteins. These proteins undergo structural changes to regulate the flux of salt ions into the cell. We have recently developed ion channel activity tracers that dissociate from ion channels after the protein changes structure. By conjugating the tracer to dyes that are sensitive to changes in their chemical environment, we can detect tracer dissociation and therefore ion channel activity. We are exploring whether a similar mechanism can create photoacoustic signal intensity changes. To test if the environmental sensitivity of the dye is photoacoustically distinguishable, we imaged the dye in different solvent backgrounds. We report that manipulation of the chemical environment of the contrast dye results in robust changes in photoacoustic properties. We are working to capture photoacoustic signal changes that occur when ion channel proteins activate using live cell imaging. This technology could permit photoacoustic imaging of electrophysiological dynamics in deep tissue, such as the brain. Further optimization of this technology could lead to concurrent imaging of neural activity and hemodynamic responses, a crucial step towards understanding neurovascular coupling in the brain.
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