Uncoupling of absolute CBF to neural activity

Hemodynamics of local cerebral blood flow induced by somatosensory stimulation under normoxia and hyperoxia in rats

We observed changes in the local cerebral blood flow (LCBF), red blood cell (RBC) concentration and RBC velocity in alpha-chloralose anesthetized rats using laser-Doppler flowmetry during activation of the somatosensory cortex following electrical stimulation of the hind paw under hyperoxia (PaO(2)=513.5+/-48.4 mmHg; mean+/-S.D.) and normoxia (PaO(2)=106.4+/-8.4 mmHg). Electrical stimuli of 5 and 10 Hz (pulse width 0.1 ms) with an intensity of 1.5 mA were applied for 5 s (n=13 at 5 Hz, n=9 at 10 Hz). Baseline levels of LCBF and RBC concentration under hyperoxia were, respectively, 5.6+/-3.3 and 8.8+/-3.0% lower than those under normoxia (P<0.05), and that of RBC velocity under hyperoxia was slightly higher than that under normoxia (NS), suggesting mild vasoconstriction at rest under hyperoxia. At 5 Hz stimulation, after normalization to each baseline level, normalized response magnitudes of LCBF, RBC concentration and RBC velocity under hyperoxia were, respectively, 68.2+/-48.0, 71.1+/-65.5 and 66.0+/-56.3% greater than those under normoxia (P<0.05). At 10-Hz stimulation, normalized response magnitudes of LCBF and RBC concentration under hyperoxia were, respectively, 44.6+/-32.0 and 55.9+/-43.5% greater than those under normoxia (P<0.05), although a significant difference in the normalized response magnitude of RBC velocity was not detected between both conditions. The evoked LCBF under hyperoxia increased earlier, by approximately 0.15 s, than that under normoxia regardless of the stimulus frequency (P<0.05). These results suggest the involvement of oxygen interaction on the regulation of LCBF during neuronal activation.

Frequency dependence of local cerebral blood flow induced by somatosensory hind paw stimulation in rat under normo- and hypercapnia

We measured the field potential and the changes in local cerebral blood flow (LCBF) response during somatosensory activation (evoked LCBF) in alpha-chloralose--anesthetized rats by laser-Doppler flowmetry under normocapnia (PaCO(2)=34.3+/-3.8 mmHg) and hypercapnia (PaCO(2)=70.1+/-9.8 mmHg). Somatosensory activation was induced by electrical stimulation (0.2, 1, and 5 Hz with 1.5 mA for 5 s) of the hind paw. The neuronal activity of the somatosensory area of the hind paw was linear to the stimulus frequency, and there was no significant difference in the neuronal activity between hypercapnia and normocapnia. The baseline level of LCBF under hypercapnia was about 72.2% higher than that under normocapnia (p<0.01). The absolute response magnitude under hypercapnia was greater than that under normocapnia (p<0.05). The evoked LCBF under both conditions showed a frequency-dependent increase in the 0.2 to 5 Hz range, and the difference in the absolute response magnitude at the same stimulus frequency between normocapnia and hypercapnia became large with increasing stimulus frequency (p<0.05). On the other hand, after normalization to each baseline level there was no significant difference in the response magnitude of the normalized evoked LCBF between normocapnia and hypercapnia, indicating that the normalized evoked LCBF reflects neuronal activity even when the baseline LCBF was changed by the PaCO(2) level. The peak time and termination time of LCBF response curves with respect to the graded neuronal activity at 1 and 5 Hz stimulation increased significantly under hypercapnia, compared with those under normocapnia (p<0.05), although the rise time of 0.5 s was nearly constant. In conclusion, the results suggest a synergistic effect of the combined application of graded neuronal stimuli and hypercapnia on the LCBF response.

Evoked local cerebral blood flow induced by somatosensory stimulation is proportional to the baseline flow

The purpose of this study was to determine the relationship between the increase in local cerebral blood flow during neuronal activation (evoked LCBF) and the baseline flow level. We measured the hemodynamics in alpha-chloralose-anesthetized rats using laser-Doppler flowmetry during somatosensory stimulation under the hypocapnic, normocapnic and hypercapnic conditions. The baseline levels of LCBF and red blood cell (RBC) velocity under hypocapnia (PaCO(2)=26.4+/-1.1 mmHg) were, respectively, 10 and 11% lower than those under normocapnia (PaCO(2)=34.2+/-1.4 mmHg) (P<0.01). The evoked response magnitude of LCBF and RBC velocity under hypocapnia were, respectively, 22 and 18% lower than those under normocapnia. There was no significant difference in the baseline level and evoked response magnitude of RBC concentration. On the other hand, the baseline levels of LCBF, RBC velocity and RBC concentration under hypercapnia (PaCO(2)=73.4+/-13.3 mmHg) were, respectively, 47, 24 and 14% higher than those under normocapnia (PaCO(2)=34.7+/-2.5 mmHg) (P<0.01). The evoked response magnitude of LCBF, RBC velocity and RBC concentration under hypercapnia were, respectively, 96, 82 and 62% greater than those under normocapnia. After normalization with respect to each baseline level, there was no significant difference in normalized evoked response magnitude of LCBF, RBC velocity and RBC concentration, either between hypocapnic and normocapnic conditions or between hypercapnic and normocapnic conditions, indicating that evoked LCBF is proportional to the baseline flow. These results suggest that the amount of evoked LCBF is not determined by the demand for metabolic substrates.

Modulation of evoked cerebral blood flow under excessive blood supply and hyperoxic conditions

We measured the field potential and local cerebral blood flow (LCBF) using laser-Doppler flowmetry in alpha-chloralose anesthetized rats during activation of the somatosensory cortex by electrical stimulation of the hind paw under independent administration of additional carbon dioxide and oxygen. The aim of this study was to test the hypothesis that the increase in LCBF during activation of the cortex (evoked LCBF) is not directed toward supplying oxygen for oxidative metabolism. Under the hypercapnic condition (PaCO(2) = 74. 9 +/- 14.3 mmHg), the baseline LCBF was about 46.5% higher than that under the normocapnic condition (PaCO(2) = 35.7 +/- 2.1 mmHg) (p < 0. 001), but after normalization for each baseline (divided by the prestimulus level), there was no significant difference in the peak value and the rise time of normalized evoked LCBF. On the other hand, the baseline level of LCBF under the hyperoxic condition (PaO(2) = 479.4 +/- 77.2 mmHg) was about 5.0% lower than that under the normoxic condition (PaO(2) = 105.5 +/- 7.8 mmHg) (p < 0.01), suggesting mild vasoconstriction under the condition of hyperoxia at rest. The peak value of normalized evoked LCBF under the hyperoxic condition was about 6.5% higher than that under the normoxic condition (p < 0.05). In addition, the rise time of evoked LCBF was earlier under the hyperoxic condition (0.37 +/- 0.16 s) than that under the normoxic condition (0.52 +/- 0.12 s) (p < 0.01). The field potential measured during stimulation under hypercapnic and hyperoxic conditions was not significantly different when compared with that under normal gas conditions. These results support our hypothesis and suggest that the excess oxygen is involved in the mechanism underlying the regulation of LCBF.