Local cerebral blood flow (LCBF) changes with stimulation of whiskers on the rat's face. Responses are related to individual functional columns of neurons of which the barrels in layer IV are a part. Vascular modules that correspond to cortical barrels have been demonstrated. The general hypothesis we will test is that vascular modules are anatomically coincident with and functionally linked to columns in the cerebral cortex. In particular, focal increases in neural activity stimulates capillaries from which signals propagate upstream through electrically coupled endothelium to relax arteriolar smooth muscle and to change LCBF depending on the activity of the neuropil. The experiments are designed to examine this hypothesis directly. We will: l) Characterize the local vascular responses to controlled stimulation of single whiskers or single whisker rows during in vivo videomicroscopy through closed cranial windows in rats by multiple quantitative dynamic parameters. This tests the hypothesis that stimulation of a single whisker increases flow in a single cerebral arteriole. 2) Define directly the vascular module by microperfusion of single terminal arterioles the vascular units of rat barrel cortex. This tests the hypothesis that a single cerebral arteriole supplies a single cortical barrel. And 3) Demonstrate neuron to vessel signalling mechanisms and propagation in cortical slices. This tests the hypotheses that active neural elements in layer IV barrels stimulate capillaries that then transmit signals through electrical coupling to arterioles for control of LCBF. We propose that conjoint neural and vascular modules: (a) provide an optimal mechanism for precise control of LCBF, (b) occur more generally in the brain, and (c) share common features of local microcirculation control with other capillary beds. Understanding this mechanism in the cerebral cortex will improve our appreciation of functional studies (PET, MRI) in normal individuals that depend on changes in LCBF and of microvascular occlusions in stroke.
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