Brain activity affects dynamic but not static autoregulation.

A close correlation between neuronal activity and cerebral blood flow was documented (activation-flow coupling). The matter of an activity-related effect on autoregulation remains elusive. We modulated cortical activity by GABAergic inhibition in 10 pre-anestesized rats and compared effects on activation-flow coupling and autoregulation. Contralateral forepaw stimulation was used to assess activation-flow coupling. Somatosensory potentials as well as local flow velocity responses were recorded with surface electrodes and laser-Doppler flowmetry. Performing intermittent carotid compression we calculated from the laser-Doppler data the transient hyperemic response ratio as a parameter of dynamic cerebral autoregulation. Recovery of cerebral perfusion at the end of the compression phase served as an index of static autoregulation. Tests were performed at baseline and after four successive doses of midazolam (0.5 mg/kg), applied at 30 min intervals. Reversibility of the changes was evaluated by application of flumazenil (0.2 mg/kg). Resting flow velocity levels (162+/-52 vs. 110+/-62 U; p<0.001), evoked potentials (N2-P1-amplitude; 13+/-4 vs.6+/-3 microV; p< 0.005), and resultant flow velocity responses (28+/-12 vs. 4+/-3%; p< 0.0001) decreased after the first dose of midazolam and then remained stable. A dose-dependent decrease was found for the transient hyperemic response ratio (28+/-13 to 22+/-14 to 15+/-10 (p<0.05) to 9+/-5% (p<0.025)) but not static autoregulation. After antagonism, all changes were reversible. Dynamic but not static cerebral autoregulation depends from neuronal activity and thus metabolic demand of neurons.