Combining laser Doppler flowmetry with microdialysis: a novel approach to investigate the coupling of regional cerebral blood flow to neuronal activity.

Regional cerebral blood flow (rCBF) is directly coupled to neuronal activity; however, the mediators of this coupling have not been established. The characterization of these vasoactive substances requires a technique which enables sampling of locally released mediators together with the simultaneous monitoring of rCBF. The goal of this study was to establish such a technique by combining microdialysis and laser doppler flowmetry. Laser doppler and microdialysis probes were inserted into the dorsal hippocampal, CA1-dentate hilus, of rats. Animals received sequentially increasing concentrations of N-methyl-D-aspartate (NMDA). rCBF responded in a dose-dependent manner, increasing to 106.8 +/- 2.3%, 119.7 +/- 6.3%, 148.0 +/- 16.6%, and 191.4 +/- 20.4% of baseline at 50 microM, 100 microM, 200 microM, and 500 microM NMDA, respectively. All doses of NMDA produced an increase in extracellular concentrations of adenosine and citrulline, an indirect measure of nitric oxide generation. These results indicate that the combination of microdialysis and laser doppler flowmetry is a valuable tool to investigate the coupling of rCBF to neuronal activity. Moreover our data suggest two possible mediators of this coupling, nitric oxide and adenosine, which require further investigation.