Awake two‐photon fluorescence imaging in the TgF344‐AD Alzheimer’s disease rat model

Background Inextricably linked with the manifestations of Alzheimer’s disease (AD) is vascular compromise which arises early during disease progression. Vascular compromise leads to subsequent failure in neurovascular coupling, plaque clearance, and oxygen delivery. Previous imaging studies have relied on the use of anesthetics to study vascular compromise. It is widely recognized that anesthetics have a wide range of effects on normal brain physiology including dampening neurovascular coupling, slowing neuronal responses, and decreasing brain metabolism. Therefore, there is a need for better imaging models which examine vascular responses during the early stages of AD without the confounds of anesthesia. The aim of this study is to develop the methodology for non‐anesthetized 2‐photon fluorescence imaging using the TgF344‐AD rat model to examine vascular compromise in the ‘prodromal’ phase of AD. Method The TgF344‐AD rat recapitulates the age‐dependent nature of AD exhibiting a ‘prodromal’ phase wherein underlying neuropathological changes are occurring without overt cognitive deficits. A prominent change during this period is vascular compromise. Recent advancements in 2‐photon fluorescence microscopy (2PFM) have allowed for high depth imaging of live tissues and has been significantly used in deciphering neuro‐vascular responses within the brains of various animals. 2PFM allows for the direct measurement of blood flow changes within blood vessels in response to neuronal activation. A cranial window is implanted into 4‐month old rats to gain optical access to the barrel cortex. Result In order to track vascular responses along branching points of major diving vessels within the barrel cortex, a 1.5mm microprism is implanted adjacent to the barrel cortex providing sideways visualization into the vasculature within the region. A 3D‐printed motorized treadmill was constructed in order to calm and distract the conscious animal while imaging is undertaken. Preliminary results in anesthetized animals show that qualitative images can be obtained through the prism within the barrel cortex. Conclusion This method shows promise in providing a more naturalistic model for studying functional vascular responses over time. It provides a platform for generating novel insights into neurovascular coupling in disease and healthy conditions.