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

We measured the field potential and the changes in local cerebral blood flo w (LCBF) response during somatosensory activation (evoked LCBF) in alpha -c hloralose-anesthetized rats by laser-Doppler flowmetry under normocapnia (P aCO2=34.3 +/-3.8 mmHg) and hypercapnia (PaCO2=70.1 +/-9.8 mmHg). Somatosens ory 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 somatosensor y 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% h igher than that under normocapnia (p <0.01). The absolute response magnitud e 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 magnitud e at the same stimulus frequency between normocapnia and hypercapnia became large with increasing stimulus frequency (p <0.05). On the other hand, aft er normalization to each baseline level there was no significant difference in the response magnitude of the normalized evoked LCBF between normocapni a and hypercapnia, indicating that the normalized evoked LCBF reflects neur onal activity even when the baseline LCBF was changed by the PaCO2 level. T he peak time and termination time of LCBF response curves with respect to t he graded neuronal activity at 1 and 5 Hz stimulation increased significant ly under hypercapnia, compared with those under normocapnia (p <0.05), alth ough the rise time of 0.5 s was nearly constant. In conclusion, the results suggest a synergistic effect of the combined application of graded neurona l stimuli and hypercapnia on the LCBF response.