Neurovascular Reactivity in the Aging Mouse Brain Assessed by Laser Speckle Contrast Imaging and 2-Photon Microscopy: Quantification by an Investigator-Independent Analysis Tool

Abstract

The brain has a high energy demand but little to no energy stores. Therefore, proper brain function relies on the delivery of glucose and oxygen by the cerebral vasculature. The regulation of cerebral blood flow (CBF) occurs at the level of the cerebral capillaries and is driven by a fast and efficient crosstalk between neurons and vessels, a process termed neurovascular coupling (NVC). Experimentally NVC is mainly triggered by sensory stimulation and assessed by measuring either CBF by laser Doppler fluxmetry, laser speckle contrast imaging (LSCI), intrinsic optical imaging, BOLD fMRI, near infrared spectroscopy (NIRS) or functional ultrasound imaging (fUS). Since these techniques have relatively low spatial resolution, diameters of cerebral vessels are mainly assessed by 2-photon microscopy (2-PM). Results of studies on NVC rely on stable animal physiology, high-quality data acquisition, and unbiased data analysis, criteria, which are not easy to achieve. In the current study, we assessed NVC using two different imaging modalities, i.e., LSCI and 2-PM, and analyzed our data using an investigator-independent Matlab-based analysis tool, after manually defining the area of analysis in LSCI and vessels to measure in 2-PM. By investigating NVC in 6–8 weeks, 1-, and 2-year-old mice, we found that NVC was maximal in 1-year old mice and was significantly reduced in aged mice. These findings suggest that NVC is differently affected during the aging process. Most interestingly, specifically pial arterioles, seem to be distinctly affected by the aging. The main finding of our study is that the automated analysis tool works very efficiently in terms of time and accuracy. In fact, the tool reduces the analysis time of one animal from approximately 23 h to about 2 s while basically making no mistakes. In summary, we developed an experimental workflow, which allows us to reliably measure NVC with high spatial and temporal resolution in young and aged mice and to analyze these data in an investigator-independent manner.

Aerobic exercise reverses aging-induced depth-dependent decline in cerebral microcirculation

“…Consistent with the small effect of exercise on cerebral microcirculation and oxygenation in superficial cortical areas, we observed no change in the relative peak amplitude or the latency of stimulus-induced hemodynamic response with exercise, assessed with OISI at 570 nm, which emphasizes the intrinsic signal originating from cerebral blood volume changes in the superficial cortical layers ( Figure 3 and Figure 3—figure supplement 1 ; Malonek et al, 1997 ; Tian et al, 2011 ). Young sedentary mice (7 months old) showed a significantly larger relative response amplitude than aged mice, consistent with the reduced cerebrovascular reactivity with age in healthy adults and rodents ( Bálint et al, 2019 ; Barnes, 2015 ; Cai et al, 2023 ; Jessen et al, 2017 ; Seker et al, 2021 ). The baseline CBF can have a strong effect on the magnitude of the hemodynamic response ( Buxton et al, 2004 ; Corfield et al, 2001 ).…”

Aerobic exercise reverses aging-induced depth-dependent decline in cerebral microcirculation

“…(Malonek et al, 1997; Tian et al, 2011) Young sedentary mice (7 months old) showed a significantly larger relative response amplitude than aged mice, consistent with the reduced cerebrovascular reactivity with age in healthy adults and rodents. (Bálint et al, 2019; J. N. Barnes, 2015; Seker et al, 2021) The baseline CBF can have a strong effect on the magnitude of the hemodynamic response. (Buxton et al, 2004; Corfield et al, 2001) While no statistically significant difference in the mean capillary RBC flux in the gray matter was found between the aged sedentary and exercise group (Figure 3b), the exercise group had larger capillary density in the gray matter (Figure 6b), suggesting that cortical blood perfusion was possibly also higher in the exercise group.…”

Capillary responses to functional and pathological activations rely on the capillary states at rest

“…For functional activation, constriction and dilation of brain capillaries have been consistently observed in in vivo optical imaging studies. 10,[38][39][40][41][42][43] However, some studies have reported no detectable changes in capillary diameters. 44,45 There could be many reasons for these discrepancies because of the heterogeneous nature and small changes in the capillary diameter responses to functional activations.…”