  Mammalian brain is a complex organ that contains billions of neurons. These
neurons form various neural circuits that control the perception, cognition,
emotion and behavior. Developing in vivo neuronal labeling and imaging
techniques is crucial for studying the structure and function of neural
circuits. In vivo techniques can provide true physiological information that
cannot be provided by ex vivo methods. In this study, we describe a new
strategy for in vivo neuronal labeling and quantification using MRI. To
demonstrate the ability of this new method, we used neurotropic virus to
deliver oatp1a1 gene to the target neural circuit. OATP1A1 protein is expressed
on the neuronal membrane and can increase the uptake of a specific MRI contrast
agent (Gd-EOB-DTPA). By using T1-weighted images for observation, labeled
neurons "light up" on MRI. We further use a dynamic-contrast-enhancement based
method to obtain measures that provide quantitative information of labeled
neurons in vivo.
