Multimodal study of the hemodynamic response to hypercapnia in anesthetized aged rats

• We compared the hemodynamic response to hypercapnia between young and old rats.
• Older rats have lower CBF increases in response to hypercapnia.
• Older rats have lower HbO and HbT increases in response to hypercapnia.
• Older rats have lower ΔCBV/ΔCBF ratio in response amplitudes to hypercapnia.
• Cerebral vascular reactivity and compliance appear to be lower in older animals.

With aging, the brain undergoes changes in metabolism and perfusion, both of which influence the widely used blood-oxygenation-level-dependent (BOLD) MRI signal. To isolate the vascular effects associated with age, this study measured the response to a hypercapnic challenge using different imaging modalities in 19 young (3 months-old) and 13 old (24 months-old) Long-Evans rats. Intrinsic optical imaging was used to measure oxy (HbO), deoxy (HbR) and total (HbT) hemoglobin concentration changes, laser speckle for cerebral blood flow (CBF) changes, and MRI for the BOLD signal. Older rats had smaller HbO (41% smaller), HbT (50%) and CBF (34%) responses, but the temporal dynamics did not exhibit significant age differences. The ratio of CBV to CBF responses was also smaller in older adults, potentially indicating a change in the compliance of vessels.