Acute increases in forebrain blood flow during altering responses in conscious rabbits.

We have previously shown that alerting responses (documented by appearance of theta rhythm in the hippocampal EEG) are associated with a characteristically timed acute vasoconstriction in the ear artery bed of the conscious rabbit. We have now determined what happens to forebrain blood flow (Doppler probe chronically implanted around the internal carotid artery) during similar alerting responses in conscious rabbits, comparing forebrain flow to simultaneously measured ear flow. During an alerting response, forebrain flow increased by 31 +/- 8% of baseline (n = 6, P < 0.01), with the increase commencing within 1 s of the stimulus, at approximately the same time as the decrease in ear flow. Arterial pressure increased from 77 +/- 3 to 81 +/- 3 mmHg (P < 0.01), so that internal carotid conductance increased from 0.17 +/- 0.02 to 0.20 +/- 0.02 kHz/mmHg (P < 0.01). During a 1 h continuous recording period in the laboratory there was a negative correlation between forebrain and skin flow, with the Pearson coefficient in individual rabbits ranging from -0.18 to -0.62 (n = 6, all correlations P < 0.01). During this period, forebrain blood flow was just as variable, from second to second, as distal aortic flow, but not as variable as ear blood flow. Our study thus demonstrates that alerting responses in rabbits are associated with rapid increases in cerebral vascular conductance. We believe that this is the first demonstration of this phenomenon in the conscious experimental animal.