Neurovascular Physiology and Pathophysiology The physiological questions aim at understanding the molecular and biochemical mechanisms, by which the brain adapts local blood flow to neuronal activity and cellular metabolism with high temporal and spatial resolution. A tight relation exists between neuronal and glial cell activation, cerebral energy metabolism, and cerebral vasculature a phenomenon known as neurometabolic and neurovascular coupling. This tight coupling of electrical and metabolic activity and regional cerebral blood flow is characteristic for the brain. Increased neuronal and thereby metabolic activity of the cerebral cortex during functional activation is accompanied by a transient increase in local cerebral blood flow and by vascular hyperoxygenation [,2] (Figure ). In our recent work, we have shown that under physiological conditions there is no evidence for early hemoglobin-deoxygenation at the onset of increased neuronal activity [2,3]. It can be suggested that neurovascular coupling is tight and rapid, possibly serving to protect brain tissue from any period of hypo-oxygenation. Moreover, neurovascular coupling arouses fundamental questions about cerebral blood flow and metabolism: How does the vasculature serve brain function? How do neurons and blood vessels communicate? What specifically makes neurons run? Figure : Experimental model for the investigation of neurovascular coupling in rat or mouse Functional Magnetic Resonance Imaging (fmri) is widely used to visualize brain activity in humans and animals (Figure 2). Its powerful potential to generate 3-D activation maps of the working brain noninvasively has made fmri the predominant neuroscience method to explore human brain function. However, this method does not measure neuronal activity directly. Instead, it is based on the local adaptation of blood flow that occurs in areas of changed neuronal activity.
a) BOLD-fMRI Map b) BOLD Time Course e-7 e-8 e-9 - Delta-R2* [/s].5 stimulation p-value -.5 2 3 time [s] Figure 2: BOLD fmri in rat somatosensory cortex (forepaw stimulation) a) Superimposed on an anatomical image (grayscale, T2-weighted Spin Echo) is an activation map consisting of voxels with statistically significant intensity increase during forepaw stimulation (duration 8s) in a series of T2*- weighted Gradient Echo Images. There is activation in the contralateral somatosensory cortex. b) Time course of the BOLD-Signal within the area of activation, averaged across stimulations (gray box = Stimulation period). Forepaw Stimulation results in a local blood flow increase, inducing hyperoxygenation of the sample volume. Neurovascular coupling determines the relationship between neuronal activity and fmri brain maps. Its purpose and mechanism is far from being resolved. For a correct interpretation of fmri a precise understanding of this relationship between neuronal activity and local blood flow regulation is crucial. One of our projects aims at better understanding the quantitative relationship between neuronal activity, oxygen consumption and blood flow as the basis for functional brain imaging [4,5] (Figure 3). Only little is known about the mediators of neurovascular coupling. The involvement of the highly diffusible vasodilator bioradical nitric oxide (NO) in the regulation of regional cerebral blood flow is widely accepted. In our work, we have shown that NO acts as a modulator rather than a mediator of vascular relaxation due to functional activation in the cerebral cortex, permitting vasodilation mediated by other, still unknown substances. Potassium channels of vascular smooth muscle cells may be possible targets of NO modulation [6,7,8]. It has recently been suggested that the mediator of neurovascular coupling is released not by neurons or glial cells but by intravascular processes (Stamler et al., Science, 997). However we have now shown that blood-born mediators possibly released during hemoglobindeoxygenation do not play a significant role in neurovascular coupling [9]. In disease impaired cerebrovascular regulation leads to a mismatch of regional cerebral blood flow and metabolic demand which may itself contribute to tissue damage. We are investigating the 'fingerprints' and mechanisms of pathophysiological processes in vascular reactivity, cerebral blood flow or hemoglobin oxygenation as the basis for new diagnostic and therapeutic strategies in acute and chronic CNS disorders [,,2].
a) Whisker System b) Hb Activation Map D2 e- e-4 e-8 e-22 C3 e-7 e-9 c) Time Courses in D2 Whisker Barrel 2 4 6.2.3 p-value e- 2 4 6 [mmol] [mmol]. STIM Oxy-Hb Deoxy-Hb Total-Hb(CBV).2 STIM relative CBF time [s] relative CMRO2 2 4 6 Time [s] 2 4 6 Time [s] Figure 3: Optical imaging of whisker barrel cortex a) Somatotopy of the rat whisker barrel system: Each vibrissa projects to one barrel-shaped group of neurons in somatosensory cortex. b) Activation map: whisker D2 (top) and C3 (bottom) can be discriminated by the area of significant hemoglobin increase. c) Each cycle of the filterwheel delivers three absorption images (532, 57, 6nm) and one laser image. Time Courses of oxy-hemoglobin (Oxy-Hb) and deoxy-hemoglobin (Deoxy-Hb) and cerebral blood volume (Total-Hb) are obtained by spectral analysis (left). Laser Speckle Contrast Analysis measures cerebral blood flow (CBF, right top). From hemoglobin-oxygenation and CBF changes, the time course of relative CMRO 2 can be estimated (right bottom). The above mentioned topics are approached with different, independent methods to measure cerebral blood flow in relation to the underlying neuronal activity in rat and mouse models in vivo. Functional activation is performed by somatosensory stimulation (whisker deflection or electrical forepaw stimulation [,3]). We use high-field-fmri (7T Bruker Pharmascan Berlin Neuroimaging Centre, Figure 2), Optical Imaging (Filterwheel- Spectroscopy combined with LASCA, Figure 3), Microfiber-Spectroscopy, Laser Doppler Flowmetry, and Two-Photon-Laser-Scanning-Microscopy [4] to obtain information about CBF (cerebral blood flow) and CBO (cerebral blood oxygenation) changes and capillary perfusion in the rat or mouse somatosensory cortex. SEPs (somatosensory evoked potentials) are obtained at the same time to monitor neuronal activity. In order to address questions of vascular regulation directly we have established an isolated artery model (Figure 4)
Figure 4: Model of the isolated and cannulated cerebral artery from rat or mouse References. Lindauer U, Villringer A, Dirnagl U: Characterization of CBF response to somatosensory stimulation: model and influence of anesthetics. Am J Physiol 264: H223-H228, 993 2. Lindauer U, Royl G, Leithner C, Kuehl M, Gold L, Gethmann J, Kohl M, Villringer A, Dirnagl U: No evidence for early decrease in blood oxygenation in rat whisker cortex in response to functional activation. NeuroImage 3: 988-, 2 3. Kohl M, Lindauer U, Royl G, Kühl M, Gold L, Villringer A, Dirnagl U: Physical model for the spectroscopic analysis of cortical intrinsic optical signals. Phys Med Biol 45: 3749-3764, 2 4. Royl G, Leithner C, Sellien H, Müller JP, Megow D, Offenhauser N, Steinbrink J, Kohl- Bareis M, Dirnagl U, Lindauer U: Functional Imaging with Laser Speckle Contrast Analysis: Vascular Compartment Analysis and Correlation with Laser Doppler Flowmetry and Somatosensory Evoked Potentials. Brain Res 2: 95-3, 26 5. Royl G, Füchtemeier M, Leithner C, Megow D, Offenhauser N, Steinbrink J, Kohl-Bareis M, Dirnagl U, Lindauer U. Hypothermia effects on neurovascular coupling and cerebral metabolic rate of oxygen. NeuroImage 4: 523-532, 28 6. Dirnagl U, Lindauer U, Villringer A: Role of nitric oxide in the coupling of cerebral blood flow to neuronal activation in rats. Neurosci Lett 49: 43-46, 993 7. Lindauer U, Megow D, Matsuda H, Dirnagl U: Nitric oxide: a modulator, but not a mediator, of neurovascular coupling in rat somatosensory cortex. Am J Physiol 277: H799-H8, 999 8. Lindauer U*, Vogt J*, Schuh-Hofer S, Dreier JP, Dirnagl U: Cerebrovascular vasodilation to extraluminal acidosis occurs via combined activation of ATP-sensitive and Ca 2+ - activated potassium channels. J Cereb Blood Flow Metab 23: 227-238, 23 (* equal contribution) 9. Lindauer U*, Leithner C*, Kaasch H, Rohrer B, Foddis M, Füchtemeier M, Offenhauser N, Steinbrink J, Royl G, Kohl-Bareis M, Dirnagl U: Blood-borne mediators released during hemoglobin deoxygenation do not couple brain blood flow to metabolism. Manuscript in preparation 29 (* equal contribution). Seitz I, Dirnagl U, Lindauer U: Impaired vascular reactivity of isolated rat middle cerebral artery following cortical spreading depression in vivo. J Cereb Blood Flow Metab 24: 526-53, 24
. Windmüller O, Lindauer U, Foddis M, Einhäupl KM, Dirnagl U, Heinemann U, Dreier JP: Ion changes of spreading ischaemia induce rat middle cerebral artery constriction in the absence of NO. Brain 28: 242-25, 25 2. Leithner C, Royl G, Offenhauser N, Füchtemeier M, Kohl-Bareis M, Villringer A, Dirnagl U, Lindauer U: Pharmacological uncoupling of activation induced increases in CBF and CMRO 2. Manuscript in preparation 29 3. Lindauer U, Megow D, Schultze J, Weber JR, Dirnagl U: Nitric oxide synthase inhibition does not affect somatosensory evoked potentials in the rat. Neurosci Lett 26: 27-2, 996 4. Fernández-Klett F, Offenhauser N, Dirnagl U, Priller J*, Lindauer U*: Contractile capillaries do not mediate functional hyperemia in the mouse cerebral cortex. Manuscript in preparation 29 (* equal contribution)