Vessel-branch-based analysis for local and global blood flow response in rat somatosensory cortex

Background and aims: The blood flow regulation system is of great interest for understanding energy demand and supply homeostasis in the brain. To investigate flow regulation system in vessels of various sizes, we developed a new method called vessel-branch-based analysis of spatiotemporal flow structures. The method was applied to characterize the hemodynamic response (plasma and RBC flow structure) under sodium nitroprusside (SNP) administration and electrical forepaw stimulation in individual vascular segments within the rat somatosensory cortex. Material and methods: Sprague-Dawley rats (7-9w) were anesthetized with isoflurane (5% for induction and 1.3-1.5% for experiments), and area (3 x 3 mm2) on the left parietal bone over the somatosensory cortex was thinned. The respiration rate was maintained at 0.87 Hz with mechanical ventilation. Time-lapse images of fluorescent signals were obtained in the rat somatosensory cortex, while a cocktail (0.02 ml) of plasma marker (Qdot-605, 1 µM) and FITC-labeled RBCs was injected at a rate of 2.23 ml/min from the internal carotid artery via external carotid artery. The image of RBC and plasma flow was simultaneously obtained through a band-pass filter (500-590nm and 595-615nm, respectively) with confocal microscopy at an excitation of 488 nm. The frame rate was 14.2 fps, the total measurement time was 18 s, and the field of view was 1.82x1.82 mm2. In vessel-branch-based analysis, the artery and vein regions were extracted semi-automatically. Then, the vessel regions were separated to each vessel branch. All vessel-branches were classified into 6 types of segments with referencing diameter of plasma flow in control data: small artery (SA), medium artery (MA), large artery (LA), small vein (SV), medium vein (MV) and large vein (LV), where diameter range are as follows: LA, >50; MA, ≦50, >25; SA, ≦25; SV, ≦50; MV, ≦50, >100; LV, >100 µm. The diameter and transit time of both plasma and RBC flow-structures were determined for each segment. SNP administration and forepaw stimulation were induced and changes from the baseline were measured. Results: Under SNP administration, RBC flow dilated more than plasma flow for all segment types (>5.1%). The venous transit time was longer than the arterial transit time for both plasma and RBC flow. The difference in diameter between RBC and plasma flow structure was maximum for the MA segment (MA, 26.4%; the others 0.26 sec) for most of the segments. The exception was the SA segment which unchanged in transit time (-0.08 sec) but increased in diameter (10.5%). The above results suggest that independent global and local flow regulation systems that depend on the vessel size may exist. Conclusions: We developed a vessel-branch-based analysis method for spatiotemporal blood flow structures, which allows us independent evaluation of vessel-by-vessel flow regulation system. Our approach will be further applicable to the analysis of flow structures in various disease models.Brain201