Hemodynamic and oxygen-metabolic responses of the awake mouse brain to hypercapnia revealed by multi-parametric photoacoustic microscopy

Abstract

A widely used cerebrovascular stimulus and common pathophysiologic condition, hypercapnia is of great interest in brain research. However, it remains controversial how hypercapnia affects brain hemodynamics and energy metabolism. By using multi-parametric photoacoustic microscopy, the multifaceted responses of the awake mouse brain to different levels of hypercapnia are investigated. Our results show significant and vessel type-dependent increases of the vessel diameter and blood flow in response to the hypercapnic challenges, along with a decrease in oxygen extraction fraction due to elevated venous blood oxygenation. Interestingly, the increased blood flow and decreased oxygen extraction are not commensurate with each other, which leads to reduced cerebral oxygen metabolism. Further, time-lapse imaging over 2-hour chronic hypercapnic challenges reveals that the structural, functional, and metabolic changes induced by severe hypercapnia (10% CO₂) are not only more pronounced but more enduring than those induced by mild hypercapnia (5% CO₂), indicating that the extent of brain's compensatory response to chronic hypercapnia is inversely related to the severity of the challenge. Offering quantitative, dynamic, and CO₂ level-dependent insights into the hemodynamic and metabolic responses of the brain to hypercapnia, these findings might provide useful guidance to the application of hypercapnia in brain research.

Keywords: Photoacoustic microscopy; hemodynamics; hypercapnia; oxygen metabolism; vascular response.

Figure 1.

Time-lapse PAM of sO₂ and blood flow speed during the 120-minute hypercapnic challenge induced by 5% CO₂ inspiration. The yellow and white arrows highlight changes in the arteriole and venule, respectively. Scale bar: 500 µm.

Figure 2.

Time-lapse PAM of sO₂ and blood flow speed during the 120-minute hypercapnic challenge induced by 10% CO₂ inspiration. The yellow and white arrows highlight changes in the arteriole and venule, respectively. Scale bar: 500 µm.

Figure 3.

Changes in the average (a) arteriole diameter, (b) venule diameter, (c) flow speed, (d) arterial sO₂, (e) venous sO₂, and (f) C_Hb over the 120-minute hypercapnic challenge induced by 5% or 10% CO₂ inhalation. Data are presented as mean ± SD (n = 5). *,p < 0.05.

Figure 4.

Changes in (a) CBF, (b) OEF, and (c) CMRO₂ over the 120-minute hypercapnic challenge induced by 5% or 10% CO₂ inhalation. Data are presented as mean ± SD (n = 5). *, p < 0.05.

Figure 5.

Vessel type-dependent responses of (a) vessel diameter, (b) flow speed, and (c) volumetric flow to hypercapnia within the first 30-minute inhalation of 5% or 10% CO₂. These vessels (20 small arterioles, 14 large arterioles, 15 small venules, and 19 large venules) were from five mice. Data are presented as mean ± SD. *, p < 0.05.