Therefore, these procedures may be even restricted to acute settings. In the experimental set-up, cerebral vessel monitoring and especially functional testing are often associated with large surgical procedures that need highly trained personnel, require longer anesthetics with the risk of further complications such as repetitive anesthesia for long-term observations, or call for highly demanding training of animals when used in awake settings (Hoover et al., 2021 Sciortino et al., 2021).
In addition, the obtained data are processed via complex algorithms, further limiting the use of bedside monitoring for research purposes (Nordström et al., 2017). The invasive monitoring can extend to a longer measuring period of several days to weeks however, information is often limited to a small local area of brain tissue of several centimeters. Although bedside monitoring of the brain function and cerebral vessels exists, it is invasive for most patients as they require surgery with potential adverse effects such as bleeding or wound infection. Furthermore, the results of the performed diagnostics just offer a snapshot of the current situation of cerebral vessels that can quickly change. As patients need to be transported to the diagnostic unit, they have a higher risk of complications and require extensive manpower (Donnelly et al., 2016).
In the clinical setup, the different imaging techniques for cerebral circulation are often time-consuming and expensive.
However, the limited accessibility of cerebral vessels complicates functional diagnostics for both physicians and fundamental researchers. The dysfunction of the neurovascular unit affects neurovascular, neurodegenerative, and neurooncological diseases. In summary, the procedures described in this protocol allow us to directly compare retinal and cerebral vascular beds and help to substantiate the role of the retina as a “window to the brain.” Finally, we demonstrate the robustness and variability of the reactivity of retinal vessels to hypercapnia at baseline as well as their reproducibility over time using two anesthetic protocols common for neurovascular research. We describe the experimental set-up in detail, outline the pitfalls of repeated retinal vessel analyses in an experimental set-up of several hours, and address issues that arise from the simultaneous use of two different assessment tools. Here, we present our protocol for the simultaneous assessment of retinal and cerebral vessels in an acute setting in anesthetized rats using a non-invasive retinal vessel analyzer and a superficial tissue imaging system for laser speckle contrast analysis via a closed bone window. The field of applications of retinal assessment could significantly be widened if more information about potential correlations between those two vascular beds and the feasibility of non-invasive retinal vessel analysis in acute neurovascular disease were available. To date, there is only a little information about potential simultaneous reactions of retinal and cerebral vessels in acute neurovascular diseases. Unlike cerebral vasculature, retinal blood vessels can be assessed non-invasively by retinal vessel analysis. Many recent research projects have described typical chronic changes in the retinal vasculature for diverse neurovascular and neurodegenerative disorders such as stroke or Alzheimer's disease.
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