Long-term adaptation of cerebral hemodynamic response to somatosensory stimulation during chronic hypoxia in awake mice

Abstract

Effects of chronic hypoxia on hemodynamic response to sensory stimulation were investigated. Using laser-Doppler flowmetry, change in cerebral blood flow (CBF) was measured in awake mice, which were housed in a hypoxic chamber (8% O₂) for 1 month. The degree of increase in CBF evoked by sensory stimulation was gradually decreased over 1 month of chronic hypoxia. No significant reduction of increase in CBF induced by hypercapnia was observed during 1 month. Voltage-sensitive dye (VSD) imaging of the somatosensory cortex showed no significant decrease in neural activation over 1 month, indicating that the reduction of increase in CBF to sensory stimulation was not caused by cerebrovascular or neural dysfunction. The simulation study showed that, when effective diffusivity for oxygen in the capillary bed (D) value increases by chronic hypoxia due to an increase in capillary blood volume, an increase in the cerebral metabolic rate of oxygen utilization during neural activation can occur without any increase in CBF. Although previous study showed no direct effects of acute hypoxia on CBF response, our finding showed that hemodynamic response to neural activation could be modified in response to a change in their balance to energy demand using chronic hypoxia experiments.

Figure 1

(A) Experiment I (laser-Doppler flowmetry (LDF) measurement and voltage-sensitive dye (VSD) imaging in awake animals) and experiment II (hematocrit measurement). LDF measurement during whisker stimulation was performed before (N=12) and 7 days (N=7), 14 days (N=7), and 1 month (N=7) after the start of chronic hypoxia. In five of these animals, LDF measurements during CO₂ inhalation and VSD imaging during whisker stimulation were performed before and 1 month after chronic hypoxia. In experiment II, hematocrit measurement was performed in a total of 12 animals (each measurement used three animals) before, at 7 days and 14 days during, and 1 month after chronic hypoxia. Hematocrit was estimated with a blood analyzer (I-STAT; Abbott). (B) Experimental protocols of whisker stimulation and 5% CO₂ inhalation. In whisker stimulation, 20 seconds of rectangular pulse air-puff stimulation (50-milliseconds pulse width and 100-milliseconds onset-to-onset interval, i.e., 10 Hz frequency) was given to the right whisker region of mice. Ten consecutive trials were repeated with an onset-to-onset interval of 120 seconds in each experiment. In CO₂ inhalation, 5% CO₂ gas was given to mice continuously for the same duration (20 seconds) and interval (120 seconds) as the sensory stimulation.

Figure 2

Time–response curves for normalized increase in cerebral blood flow (CBF) response to sensory stimulation during chronic hypoxia. Horizontal bars indicate the stimulation period. These data were normalized to baseline level (20 seconds before sensory stimulation). Each response curve represents the mean of all animals at each measurement day. Error bars indicate s.d.

Figure 3

Longitudinal cerebral blood flow (CBF) measurements under chronic hypoxia. Increase in CBF response to sensory stimulation was consistently observed at pre, 7 days, 14 days, and 1 month. Bold squares and line represent the mean of all animal data of average values at each measurement day. Error bars indicate s.d. *P<0.05, **P<0.01.

Figure 4

Increase in cerebral blood flow (CBF) evoked by 5% CO₂ inhalation. (A) Time–response curve of increase in CBF. Time–response curves were normalized to the baseline level (20 seconds before sensory stimulation) and shown for one representative animal. Horizontal bars indicate the stimulation period. (B) Mean percentage increase in CBF within 10 to 20 seconds 5% CO₂ inhalation (N=5). Error bars indicate s.d.

Figure 5

Neuronal activities evoked by sensory stimulation under chronic hypoxia condition. (A) Activation maps of voltage-sensitive dye (VSD) imaging experiment performed before (top) and 1 month (bottom) after chronic hypoxia, shown for one representative animal. Right and left frames showed VSD imagings before and immediately after sensory stimulation, respectively. (B) Summary of VSD results in five animals. There was no significant difference in the number of pixels between pre and 1 month.

Figure 6

Simulation result of Hyder's model. Solid and dashed lines indicated the relationship between cerebral metabolic rate of oxygen utilization (CMRO₂) and cerebral blood flow (CBF) among the respective D values.