The concept of coupling blood flow to brain function: revision required?

Signal averaged laser Doppler measurements of activation-flow coupling in the rat forepaw somatosensory cortex

Regional alterations in cerebral blood flow (CBF) are widely used as a surrogate for neuronal function based on an intact coupling between changes in regional CBF and metabolism, activation-flow coupling (AFC). To further investigate parameters affecting AFC, we have implemented a rat model with electrical forepaw stimulation under alpha-chloralose anesthesia using laser Doppler (LD) measurements of flow parameters through thinned skull over contralateral somatosensory cortex. Signal averaging of the LD response was used to improve reproducibility. A characteristic flow response to electrical forepaw stimulation was reliably recorded from the somatosensory cortex using signal averaging. Stimulation at 5 Hz maximized the LD response, and constant current stimulation up to 1 mA did not induce changes in systemic blood pressure. The shape of the flow response consisted of an initial peak followed by a steady state plateau phase which was observed for stimulation durations longer than 4 s. When individual LD parameters of velocity, red blood cell concentration (CRBC), and cerebral blood flow (CBF) were compared, changes in LDCBF were primarily attributable to changes in LDvelocity rather than LDCRBC. This finding was also observed during hypercapnia. Characterization of AFC in the model provides a background for future studies of the effects of pharmacological manipulation or pathophysiological states.

Regional changes in forebrain activation during the early and late phase of formalin nociception: analysis using cerebral blood flow in the rat

This is the first neural imaging study to use regional cerebral blood flow (rCBF) in an animal model to identify the patterns of forebrain nociceptive processing that occur during the early and late phase of the formalin test. We measured normalized rCBF increases by an autoradiographic method using the radiotracer [99mTc]exametazime. Noxious formalin consistently produced detectable, well-localized and typically bilateral increases in rCBF within multiple forebrain structures, as well as the interpeduncular nucleus (Activation Index, AI = 66) and the midbrain periaqueductal gray (AI = 20). Structures showing pain-induced changes in rCBF included several forebrain regions considered part of the limbic system. The hindlimb region of somatosensory cortex was significantly activated (AI = 31), and blood flow increases in VPL (AI = 8.7) and the medial thalamus (AI = 9.0) exhibited a tendency to be greater in the late phase as compared to the early phase of the formalin test. The spatial pattern and intensity of activation varied as a function of the time following the noxious formalin stimulus. The results highlight the important role of the limbic forebrain in the neural mechanisms of prolonged persistent pain and provide evidence for a forebrain network for pain.

Global cerebral blood flow decreases during pain

Positron emission tomography studies have identified a common set of brain regions activated by pain. No studies, however, have quantitatively examined pain-induced CBF changes. To better characterize CBF during pain, 14 subjects received positron emission tomography scans during rest, during capsaicin-evoked pain (250 micrograms, intradermal injection), and during innocuous vibration. Using the H215O intravenous bolus method with arterial blood sampling, global CBF changes were assessed quantitatively. Painful stimulation produced a 22.8% decrease in global CBF from resting levels (P < 0.0005). This decrease was not accounted for by arterial PCO2 or heart rate changes. Although the exact mechanism remains to be determined, this pain-induced global decrease represents a previously unidentified response of CBF.

Reversal of ischemic-induced chronic memory dysfunction in aging rats with a free radical scavenger-glycolytic intermediate combination

Rats were subjected to bilateral carotid artery occlusion (2-VO) or sham occlusion (No-VO) and tested 12 weeks for visuo-spatial memory (VSM) function. After 14 weeks, 2-VO rats (N = 4) showing severe visuo-spatial memory impairment were given dimethyl sulfoxide (DMSO)-fructose 1,6-diphosphate (FDP) i.p. for seven days and retested on the water maze. After DMSO-FDP, a 54% improvement in their VSM was seen which nearly reached control No-VO values. Untreated 2-VO (N = 4) and No-VO (N = 8) rats showed no significant changes in their VSM. DMSO-FDP treatment was discontinued and rats were retested on the water maze but improvement was lost and VSM function regressed to pretreatment levels. Immunohistochemical examination showed minimal neuronal damage in all 2-VO rats and slight loss of microtubule associated protein-2. Glial fibrillary acidic protein immunostaining increase was observed only in untreated 2-VO rats. The results indicate that a DMSO-FDP combination improves VSM secondary to chronic brain hypoperfusion.

Autonomic and vasomotor regulation

The cerebellum not only modulates the systemic circulation, but also profoundly influences cerebral blood flow (rCBF) and metabolism (rCGU), and initiates long-term protection of the brain from ischemia. Electrical stimulation of the rostral ventral pole of the fastigial nucleus (FN), elevates arterial pressure (AP), releases vasoactive hormones, elicits consummatory behavioral and other autonomic events and site specifically elevates rCBF independently of changes in rCGU. Cerebral vasodilation results from the antidromic excitation of axons of brain stem neurons which innervate cerebellum and, through their collaterals, neurons in the rostral ventrolateral reticular nucleus (RVL). RVL neurons initiate cerebral vasodilation over polysynaptic vasodilator pathways which engage a population of vasodilator neurons in the cerebral cortex. In contrast, intrinsic neurons of FN, when excited, elicit widespread reductions in rCGU and, secondarily, rCBF, along with sympathetic inhibition. Electrical stimulation of FN can reduce the volume of a focal cerebral infarction produced by occlusion of the middle cerebral artery by 50%. This central neurogenic neuroprotection is long lasting (weeks) and is not due to changes in rCBF or rCGU. Rather, it appears to reflect alterations in neuronal excitability and/or downregulation of inflammatory responses in cerebral vessels. The FN, therefore, appears to be involved in widespread autonomic, metabolic, and behavioral control, independent of motor control. The findings imply that the FN receives inputs from neurons, probably widely represented in the central autonomic core, which may provide continuing information processing of autonomic and behavioral states. The cerebellum may also widely modulate the state of cortical reactivity to ischemia, hypoxia, and possibly other neurodegenerative events.

GABAergic regulation of cerebral microvascular tone in the rat

The role of GABA in regulating cerebral microvessels was examined in the parenchyma of the hippocampus and the surface of the neocortex. Microvessels were monitored in in vitro slices using computer-assisted videomicroscopy, and synaptically evoked field responses were simultaneously recorded. gamma-Aminobutyric acid (GABA) and the GABAA receptor agonist, muscimol, elicited vasodilation in hippocampal microvessels, whereas the GABAB receptor agonist, baclofen, elicited constriction. The muscimol-induced dilation persisted in the presence of the nitric oxide synthase inhibitor, N-nitro-L-arginine, indicating that this response is not mediated by nitric oxide. Inhibition of neuronal discharge activity with tetrodotoxin did not alter this dilation, but it fully blocked the constrictor response to baclofen. These data suggest that GABAB-mediated, but not GABAA-mediated, responses are dependent on action potential generation. The GABAA receptor antagonists, bicuculline and picrotoxin, elicited constriction, suggesting a tonic dilatory influence by endogenous GABA. Bicuculline-induced constriction was not attenuated by tetrodotoxin. In contrast, these vessels were unresponsive to the GABAB receptor antagonist, 2-hydroxysaclofen. Hippocampal microvessels dilated in response to moderate hypoxia, and this response persisted in the presence of bicuculline, indicating that the hypoxia-induced dilation is not mediated by an action at GABAA receptors. In arterioles located on the surface of the neocortex (i.e., not embedded in the parenchyma of the brain), muscimol elicited vasodilation, whereas bicuculline was ineffective. These results suggest that although these vessels are responsive to GABA, the local concentration of endogenous GABA is insufficient to elicit a tonic effect at rest. These findings raise the possibility that GABA plays a role in local neurovascular signaling in the parenchyma of the brain.

Local and propagated vascular responses evoked by focal synaptic activity in cerebellar cortex

We investigated the local and remote vascular changes evoked by activation of the cerebellar parallel fibers (PFs). The PFs were stimulated (25-150 microA, 30 Hz) in halothane-anesthetized rats equipped with a cranial window. The changes in arteriolar and venular diameter produced by PF stimulation were measured with the use of a videomicroscopy system. Cerebellar blood flow (BFcrb) was monitored by laser Doppler flowmetry and the field potentials evoked by PF stimulation were recorded with the use of microelectrodes. PF stimulation increased the diameter of local arterioles (+26 +/- 1%, mean +/- SE) in the activated folium (n = 10, P < 0.05). The vasodilation was greatest in smaller arterioles (16.5 +/- 0.8 microm), was graded with the intensity of stimulation, and was less marked than the vasodilation produced by hypercapnia in comparably sized vessels (+58 +/- 5%, CO2 pressure = 50-60 mmHg, n = 8). In addition, the vasodilation was greatest along the horizontal beam of activated PFs and was reduced in arterioles located away from the stimulated site in a rostrocaudal direction. The increases in vascular diameter were associated with increases in BFcrb in the activated area (+55 +/- 4%, n = 5). PF stimulation increased vascular diameter (+10 +/- 0.5%, n = 10) also in larger arterioles (30-40 microm) located in the folium adjacent to that in which the PFs were stimulated. Higher-order branches of these arterioles supplied the activated area. No field potentials were evoked by PF stimulation in the area where these upstream vessels were located. The data suggest that increased synaptic activity in the PF system produces a "local" hemodynamic response mediated by synaptic release of vasoactive agents and a "remote" response that is propagated to upstream arterioles from vessels residing in the activated folium. These propagated vascular responses are important in the coordination of segmental vascular resistance that is required to increase flow effectively during functional brain hyperemia.

A hypercapnia-based normalization method for improved spatial localization of human brain activation with fMRI

An issue in blood oxygenation level dependent contrast-based functional MRI is the accurate interpretation of the activation-induced signal changes. Hemodynamic factors other than activation-induced changes in blood oxygenation are known to contribute to the signal change magnitudes and dynamics, and therefore need to be accounted for or removed. In this paper, a general method for removal of effects other than activation-induced blood oxygenation changes from fMRI brain activation maps by the use of hypercapnic stress normalization is introduced. First, the effects of resting blood volume distribution across voxels on activation-induced BOLD-based fMRI signal changes are shown to be significant. Second, the effects of hypercapnia and hypoxia on resting and activation-induced signal changes are demonstrated. These results suggest that global hemodynamic stresses may be useful for non-invasive mapping of blood volume. Third, the normalization technique is demonstrated.

Assessment of functional hemispheric asymmetry by bilateral simultaneous cerebral blood flow velocity monitoring

The aim of this study was to investigate side-to-side differences of simultaneously measured middle cerebral artery (MCA) blood flow velocities during various hemisphere-specific tasks. Using a transcranial Doppler device, flow velocity changes of 24 healthy, right-handed subjects were monitored simultaneously in the left and right MCA during different hemisphere-specific tasks. Mean flow velocity (MFV) curves were averaged for each individual subject and task. Simultaneously, heart rate, blood pressure and end-tidal carbon dioxide (CO2) were measured in a subgroup of six subjects. When compared with the resting state, all stimuli produced significant (p < 0.001) bilateral MFV increases, ranging from 2.5-9.2%. A lateralization of MFV increases with a significantly (p < 0.001) more pronounced increase in MFV in the hemisphere contralateral to the performing band was observed both during simple sequential finger movements and a complex spatial task. During the complex spatial task, consistently higher MFV increases were observed in the right MCA (p < 0.001), regardless of the side of task performance. Recognition of pictorial material presented as part of a memory task, also resulted in a side-to-side difference of respective MFV increases (right > left, p < 0.001), whereas memorization did not. Whereas bilateral MFV elevations observed during stimulation with white noise were only discrete and not lateralized, exposure to overt speech produced significantly higher (p < 0.001) MFV increases in the left MCA. The time course of the MFV reaction showed a rapid increase with an initial maximum after 4-5 s. Heart rate, blood pressure, and end-tidal CO2 showed only subtle changes during the stimulation periods. In conclusion, the observed side-to-side differences of MFV reaction in the left and right MCA concur with current functional imaging data. Bilateral simultaneous repetitive transcranial Doppler monitoring is a sensitive method to detect cerebral perfusion asymmetries caused by hemisphere-specific activation, and thus may be helpful for noninvasive assessment of hemispheric dominance for language.

Regulation of cerebral microvessels by glutamatergic mechanisms

This study examined the role of glutamate receptor activation in the regulation of microvascular tone in the hippocampus and neocortex of the rat. Microvascular and neuronal responses were simultaneously recorded in brain slices using videomicroscopic analysis in conjunction with electrophysiological recording. Glutamate and other glutamate receptor agonists, including NMDA, kainic acid, and ACPD elicited dose-dependent dilation in preconstricted hippocampal microvessels. The lower concentrations of NMDA elicited dilation with an increase in neuronal excitability while dilatory responses to other agonists were associated with substantial depolarization. NMDA-mediated dilation was inhibited completely with a sodium channel blocker (TTX), an NOS inhibitor (L-NNA), or a specific inhibitor of neuronal NOS (7-NI). Inhibition of the GABA(A) or the A2 adenosine receptor did not attenuate the NMDA-induced dilation. The role of spontaneous glutamate receptor activation by endogenous glutamate in the regulation of resting dilatory tone was also examined. Blocking AMPA or metabotropic glutamate receptors did not induce significant responses in resting hippocampal vessels. However, the NMDA receptor antagonist, APV, elicited a dose-dependent constriction. In surface vessels of the neocortex, NMDA elicited a comparable dose-dependent dilation, and APV elicited a significantly smaller dose-dependent constriction. A 60 min period of hypoxia elicited a significant dilation of preconstricted hippocampal microvessels. APV did not significantly influence this dilatory response indicating that hypoxia-induced dilation is not mediated by NMDA receptor activation. Taken together, these results indicate that glutamate contributes to the dilatory tone of cerebral microvessels under physiologic conditions and that this effect is mediated by NMDA receptors. Glutamatergic vasodilation is dependent on neuronal discharge activity and the neuronal production of NO. The tonic influence is more pronounced in hippocampal microvessels than in neocortical vessels suggesting that the contribution of NMDA receptor activation to resting dilatory tone is dependent on the location of vessels within the brain.

Effects of indomethacin on cerebral blood flow and oxygen metabolism: a positron emission tomographic investigation in the anaesthetized baboon

The aim of this study was to clarify the controversy about the effects of indomethacin on the coupling of cerebral blood flow (CBF) to cerebral oxygen metabolism (CMRO2). CBF, blood volume (CBV), oxygen extraction fraction (OEF) and CMRO2 were measured by positron emission tomography (PET) in five anaesthetized baboons before and during an i.v. administration of indomethacin (bolus 20 mg/kg followed by perfusion 10 mg/kg.h). Administration of indomethacin resulted in a marked and homogenous decrease of CBF in every region analysed (-28% to -40%) and a moderate reduction in CBV (-8% to -16%). In contrast, CMRO2 displayed a small increase in thalamus and pons (+10% and +13%, respectively). OEF increased greatly in all structures studied (+59% to +96%). These findings show that the potent cerebrovascular effects of indomethacin are not related to a decrease in CMRO2 as measured through the use of PET.

Serotonin in the regulation of brain microcirculation

Manipulation of brainstem serotonin (5-HT) raphe neurons induces significant alterations in local cerebral metabolism and perfusion. The vascular consequences of intracerebrally released 5-HT point to a major vasoconstrictor role, resulting in cerebral blood flow (CBF) decreases in several brain regions such as the neocortex. However, vasodilatations, as well as changes in blood-brain barrier (BBB) permeability, which are blocked by 5-HT receptor antagonists also can be observed. A lack of relationship between the changes in flow and metabolism indicates uncoupling between the two variables and is suggestive of a direct neurogenic control by brain intrinsic 5-HT neurons on the microvascular bed. In line with these functional data are the close associations that exist between 5-HT neurons and the microarterioles, capillaries and perivascular astrocytes of various regions but more intimately and/or more frequently so in those where CBF is altered significantly following manipulation of 5-HT neurons. The ability of the microvascular bed to respond directly to intracerebrally released 5-HT is underscored by the expression of distinct 5-HT receptors in the various cellular compartments of the microvascular bed. Thus, it appears that while some 5-HT-mediated microvascular functions involve directly the blood vessel wall, others would be relayed through the perivascular astrocyte. The strategic localization of perivascular astrocytes and the different 5-HT receptors that they harbor strongly emphasize their putative pivotal role in transmitting information between 5-HT neurons and microvessels. It is concluded that the cerebral circulation has full capacity to adequately and locally adapt brain perfusion to changes in central 5-HT neurotransmission either directly or indirectly via the neuronal-astrocytic-vascular tripartite functional unit. Dysfunctions in these neurovascular interactions might result in perfusion deficits and might be involved in specific pathological conditions.

Evidence of cortical metabolic dysfunction in early Huntington's disease by single-photon-emission computed tomography

We compared perfusion of prefrontal, motor, and sensory cortices and basal ganglia in 29 Huntington's disease (HD) patients and nine controls. We found a significant reduction in perfusion in patients with HD of short (< 6 years, n = 10), medium (6-10 years, n = 8), and long duration (> 10 years, n = 11) compared with controls. Among short-duration patients, we observed decreases in cortical perfusion before evidence of atrophy on magnetic resonance imaging, suggesting that decreases in neuronal activity, as reflected by perfusion levels, precede gross structural changes. As expected, decreased perfusion was marked in basal ganglia. The extent of cortical perfusion correlated with clinical assessments of functional capabilities as well as with the duration of disease. Prefrontal perfusion correlated with cognitive measures, and motor cortical perfusion correlated with physical disability and activities of daily living scores. We found no significant clinical correlations with sensory cortical perfusion. Single-photon-emission computed tomography may be a sensitive method for assessing disease progression in clinical trials and pharmacologic intervention.

Propofol induces dilation and inhibits constriction in guinea pig basilar arteries

The present study investigated the direct action of propofol on guinea pig basilar arterial rings and the possible underlying mechanisms. Arterial rings, with and without endothelium, were mounted in an organ bath and connected to an isometric tension recording system. The effects of propofol (0.63-20 micrograms/mL) were compared with those of Intralipid (n = 13) after equilibration and precontraction by 5-hydroxytryptamine (5-HT). In another group (n = 8), after pretreatment with either propofol (5 micrograms/mL) or Intralipid, a contraction by 35 mM KCl was obtained and compared. Another group (n = 12) were incubated in Ca(2+)-free Krebs buffer and after depolarization by 45 mM KCl, a dose-response curve to CaCl2 was obtained to compare the effect of propofol (5 micrograms/mL) and Intralipid on the influx of Ca2+. Finally, in Ca(2+)-free Krebs buffer, the effect of Intralipid or propofol on 5-HT-evoked contractions (n = 6) were assessed. Propofol caused significant dilation with or without endothelium present, inhibited KCl-induced contraction, and significantly lowered the dose-response curve for CaCl2. In Ca(2+)-free Krebs buffer, propofol significantly inhibited 5-HT-evoked contraction, which is dependent on intracellular Ca(2+)-release. In conclusion, propofol inhibited vaso-constriction and induced vasodilation by mechanisms consistent with reduced extracellular calcium influx and suppressed intracellular calcium release.

Changes in local cerebral blood flow by neuroactivation and vasoactivation in patients with impaired cognitive function

Imaging of local cerebral blood flow (lCBF) may serve as an important supplementary tool in the aetiological assessment of dementias. In early or preclinical disease, however, there are less characteristic changes in lCBF. In the present study it was investigated whether vasoactivation or neuroactivation may produce more pronounced local lCBF deficits. Local CBF was investigated by using technetium-99m hexamethylpropylene amine oxime and single-photon emission tomography (SPET) in 80 patients (50 with mild cognitive impairment and 30 with dementia of Alzheimer type (DAT), all without evidence of cerebrovascular disease) at rest (baseline) and during activation. In 31 studies patients underwent vasomotor activation with acetazolamide, while 62 studies were performed under cognitive challenge (neuroactivation by labyrinth task). Cortical activity relative to that of cerebellum increased significantly in a right temporal region and tended to increase in other cortical regions upon vasoactivation. In contrast, neuroactivation reduced cortical activity relative to that of cerebellum in several left and right temporal and in left parietal regions. Visual classification of SPET images of patients with probable DAT by three observers resulted in a reduction of the number of definitely abnormal patterns from 9/12 to 4/12 by vasoactivation and an increase from 10/18 to 15/18 by neuroactivation. Correspondingly, abnormal ratings in patients with mild cognitive dysfunction were reduced from 7/19 to 5/19 by vasoactivation and were increased from 12/21 to 18/21 by neuroactivation. In conclusion, vasoactivation does not enhance local relative perfusion deficits in patients with cognitive impairment of non-vascular aetiology, whereas neuroactivation by labyrinth task produces more pronounced local flow differences and enhances abnormal patterns in lCBF imaging.

Assessment of the CO2 response by means of non diffusible contrast media and angio-CT in patients with cluster headache

We analyzed the possibility of assessing functional vasomotor changes by means of Arm-Brain Circulation Time (rABCT) and Vascular volume images (Vv) obtained with Angio-CT, in basal condition and following CO2 inhalation, in a sample of 48 patients with cluster headache. CO2 inhalation resulted in the appearance of local changes, which were detected in 28 regions. Analysis by indicator images of Vv-dependent rABCT distribution showed two main patterns: abnormal rABCT mostly evident at the smallest Vv pixels and abnormal rABCT dependent on abnormal Vv distribution. The former pattern was linked to abnormality at the circle of Willis; the latter to abnormal local vasomotor responses. Patients with cluster headache showed both patterns, which prompted us to conclude for the presence of low-degree stenosis in carotid arteries and vasomotor instability in peripheral brain vessels.

Pericapillary and distant axon terminals in the nuclei of the cat amygdala: a morphometric study

According to some ultrastructural studies, the pericapillary axon terminals in the central nervous system (CNS) are functionally connected with the capillary vessel wall. Thus, it may be expected that the population of pericapillary axon terminals will be morphologically distinct from the terminals at a further distance from the capillary walls. To test this hypothesis, morphometrical analysis of 3,048 axon terminals was performed, comparing terminals situated in the close vicinity of the capillary vessel with those at a distance from the vessels in the lateral, basal, medial, central and cortical nuclei of the amygdaloid body of eight cats. The cross-sectional area and circumference of each identified axon terminal profile were measured, and the shape of synaptic vesicles and the presence of synaptic contacts and granular vesicles were recorded. The statistical evaluation of results was performed by means of the Newman-Keuls' test, Wilcoxon's test, Fisher's contingency-table test and the test for two coefficients of structure. The morphometric examination revealed two ultrastructurally distinct groups of axon terminals, pericapillary and distant terminals, in all the nuclei of the amygdaloid body. The differentiating features were the shape of the synaptic vesicles, the number of synaptic contacts, and the size of the axon terminals. These results further support the hypothesis of a functional connection between axon terminals and the capillary vessel wall in the CNS.

Transcranial Doppler assessment of cerebral flow velocity at rest and during voluntary movements in young and elderly healthy subjects

Fifty-five healthy volunteers (thirty young and twenty-five elderly), all of them right-handed, were investigated by Transcranial Doppler monitoring mean flow velocity on the right and left middle cerebral artery at rest, during a motor task performed by the hand and in the following rest condition. In all cases the motor task induced an increase in bilateral mean flow velocity, but significantly greater on the opposite side middle cerebral artery in the task. The percentage increase in mean flow velocity compared to the rest condition observed during the motor task performed by the opposite side hand to the artery investigated was significantly greater in the young than the elderly. Moreover, in the young, this increase showed a slight prevalence during the left middle cerebral artery assessment relating to right hand preference. Furthermore, elderly subjects achieved the greatest mean flow velocity percentage increase compared to rest condition later than young subjects and returned to basal values more slowly.

Stimulus parameters influence characteristics of optical intrinsic signal responses in somatosensory cortex

Optical imaging of intrinsic signals was performed in the barrel cortex of the rat during whisker deflections of varying frequencies (1 to 20 Hz) and durations (0.1 to 5 s). A dose-response relationship was shown between these stimuli and the characteristics of the optically recorded intrinsic signal response. At constant frequencies, longer stimulus durations increased response magnitude, as defined by mean pixel value in statistically determined regions of interest. At constant durations, higher stimulus frequencies increased response magnitude. Response magnitude was also increased by greater numbers of deflections. When stimulus number was constant, there were no differences in response magnitude, regardless of stimulus frequency and duration. Spatial extent of responses, as defined by number of pixels in regions of interest, did not differ between stimulus frequencies, durations, or numbers. Comparison of the time to reach peak intrinsic signal response after stimulus onset ("time-to-peak") suggested that higher frequencies were associated with faster time-to-peak. Registration of intrinsic signal responses with cytochrome oxidase-stained whisker barrels demonstrated that responses were located over the barrel corresponding to the stimulated whisker. In summary, we have shown that the absolute number of stimuli delivered to the system is, at least for short stimulus periods (< or = 5 s), a determining factor for the magnitude of these responses, whereas stimulus frequency appears to influence time-to-peak response.

Regional 133xenon cerebral blood flow and cerebral 99mTc-HMPAO uptake in patients with obsessive-compulsive disorder before and during treatment

We previously reported increased regional cerebral cortical uptake and decreased caudate nucleus uptake of 99mTc-HMPAO in patients with obsessive-compulsive disorder(OCD) before treatment compared to matched normal controls. In the present study, we determined whether or not these changes persisted during treatment. Single-photon emission computed tomography was used to measure regional cerebral blood flow (rCBF) by 133Xe inhalation and cerebral uptake of 99mTc-HMPAO in eight adult male OCD patients before and during treatment with chlomipramine, and in eight age-matched normal male controls. With 133Xe, there were no significant differences in rCBF between patients with OCD and their matched controls, and no significant differences in rCBF in the patients before and during treatment. Significantly increased HMPAO uptake in the orbital frontal cortex, posterofrontal cortex, and high dorsal parietal cortex bilaterally occurred in the OCD patients before treatment compared to their matched controls, and there were significant reductions of HMPAO uptake, into the normal range, in all these areas in the patients during treatment. Significantly reduced HMPAO uptake in the caudate nucleus bilaterally occurred in the patients before treatment compared to their matched controls, and these reductions persisted during treatment. This study provides additional support for the involvement of both the orbital frontal cortex and the caudate nuclei in the pathophysiology of OCD.

Functional increases in cerebral blood volume over somatosensory cortex

We have examined the relationship between cerebral blood volume (CBV) and electrophysiology over primary somatosensory cortex (S-I) in the rat. We did this by comparing the spatial characteristics and time course of activity-related changes in plasma fluorescence, intrinsic optical reflectance signals, and single unit electrophysiology in S-I to identical stimuli. S-Is of urethane-anesthetized male Sprague-Dawley rats were exposed, and fluorescent Texas Red dextran dye (MW 70,000) was administered intravenously. Subsequently, foredigit electroshock or vibrissal deflection was associated with fluorescence increases over contralateral forelimb or posteromedial barrel subfield cortex. Fluorescence was delayed and prolonged, indicating that CBV increases at 1-1.5 s and peaks 2-2.5 s after the onset of stimulation in both regions. When stimulus intensity was adjusted to produce barely detectable fluorescence foci (10% above back-ground), significant electrophysiologic spiking was seen. At these parameters, fluorescence change overlay areas of increased cortical layer III cell firing on single unit recordings. However, surface boundaries of the smallest observable fluorescence foci at their peak spatial extents consistently overspilled electrophysiologic center receptive fields. Corresponding intrinsic optical reflectance decreases were seen at 610 and 850 nm, exhibiting similar timing and colocalizing closely with fluorescence increase at both wavelengths after identical stimuli. These signals similarly overspilled electrophysiologic activity. Thus, we observed delayed increases in vascular fluorescence (related to CBV) over activated cortex. The smallest detectable fluorescence changes overspilled the center receptive field boundaries and were associated with appreciable electrophysiologic firing. In addition, the striking spatial and temporal similarity between intrinsic optical reflectance and fluorescence activity suggests that changes in intrinsic cortical reflectance are strongly related to changes in CBV.

Regional differentiation of cortical activity in schizophrenia: a complementary approach to conventional analysis of regional cerebral blood flow

This study examines an alternative to a narrow locus-of-dysfunction cortical pathophysiology in schizophrenia, which in turn suggests a modified approach to the analysis of data from regional cerebral blood flow studies. The results provide qualified support for a model of impaired differentiation of cortical activity in schizophrenia. As an index of the differentiation of cortical fields, regional gradients of cortical blood flow were examined. Medicated patients (n = 15) failed to show the differentiation of networks in frontal areas during a verbal memory recognition task observed in normal comparison subjects (n = 15). Unmedicated patients (n = 15) at rest tended to lose normal lateralization and had increased gradients in the right frontal area compared with normal comparison subjects.

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.

Cerebrovascular parasympathetic innervation contributes to coupling of neuronal activation and blood flow in rat somatosensory cortex

We measured the increase of regional cerebral blood flow (rCBF) in the somatosensory cerebral cortex occurring in response to a standard stimulation of the L. side mystacial vibrissae (facial whiskers) in rats anaesthetised with halothane, in conjunction with blocking of activity in the R. side parasympathetic (PS) and sensory fibres innervating the cerebral vessels. Blocking was achieved reversibly and repeatedly by means of a cooling probe. When the PS fibres and the nasociliary nerve (NCN) were blocked together, but not when the NCN was blocked alone, the R. side rCBF increase occurring with whisker stimulation was significantly reduced. Our results indicate that, in addition to the intrinsic cortical factors demonstrated in earlier studies, the cerebrovascular PS innervation, but not the NCN, contributes to the increase in cortical rCBF associated with somatosensory cortical neuronal activation.

Enhanced cerebrovascular responsiveness to hypercapnia following depletion of central serotonergic terminals

Serotonin-containing nerve fibres innervate cerebral blood vessels, but the source of this innervation and the physiological effects of perivascular serotonin release remain controversial. The purpose of the present study was to examine the effects of central serotonergic depletion upon the relationship between CBF and glucose utilization under both normo- and hypercapnic conditions. To induce the loss of serotonergic terminals, rats were injected twice daily for 4 consecutive days with 20 mg/kg of the specific serotonergic neurotoxin methylenedioxyamphetamine (MDA). Between 4 and 6 weeks later, local CBF and glucose utilization were measured using the fully quantitative [14C]iodoantipyrine and [14C]2-deoxyglucose autoradiographic techniques, respectively, and the efficacy of the lesioning protocol was assessed using [3H]paroxetine radioligand binding analysis. In all animals treated with MDA, there was a significant decrease in serotonin uptake sites throughout the brain, falling from 223 +/- 20 to 40 +/- 16 fmol/mg tissue in parietal cortex, for example, although the raphe nuclei themselves were unaffected (300 +/- 20 fmol/mg tissue in controls and 291 +/- 18 in MDA-treated rats). In normocapnic rats, the effects of MDA pretreatment upon blood flow and glucose use were slight and focally concentrated. However, when the animals were rendered hypercapnic, CBF was significantly higher in MDA-treated rats than in normal controls, for example, increasing from 356 +/- 22 ml 100 g-1 min-1 in frontal cortex of hypercapnic controls to 700 +/- 81 ml 100 g-1 min-1 in MDA-pretreated rats with similar levels of hypercapnia. In some brain areas of hypercapnic MDA-pretreated rats, blood flows were too high (> 800 ml 100 g-1 min-1) to be accurately quantified.(ABSTRACT TRUNCATED AT 250 WORDS)

The intravascular contribution to fMRI signal change: Monte Carlo modeling and diffusion-weighted studies in vivo

Understanding the relationship between fMRI signal changes and activated cortex is paramount to successful mapping of neuronal activity. To this end, the relative extravascular and intravascular contribution to fMRI signal change from capillaries (localized), venules (less localized) and macrovessels (remote, draining veins) must be determined. In this work, the authors assessed both the extravascular and intravascular contribution to blood oxygenation level-dependent gradient echo signal change at 1.5 T by using a Monte Carlo model for susceptibility-based contrast in conjunction with a physiological model for neuronal activation-induced changes in oxygenation and vascular volume fraction. The authors compared our Model results with experimental fMRI signal changes with and without velocity sensitization via bipolar gradients to null the intravascular signal. The model and experimental results are in agreement and suggest that the intravascular spins account for the majority of fMRI signal change on T2*-weighted images at 1.5 T.

Age-related changes in vascular responses: a review

Normal aging is associated with different changes in the cardiovascular system that lead to an increase in pathological processes, such as hypertension, coronary artery disease, heart failure, and postural hypotension with enhancement of both morbidity and mortality. The vascular alterations consist of changes in the function and structure of the arteries, and increasing vascular stiffness, mainly when atherosclerosis is present, whose incidence is increased with age. The arteries accumulate lipids, collagen, and minerals. Cerebral perfusion may be reduced in the elderly, mainly regional cerebral blood flow, which leads to a deterioration of mental and physical functions. The degree of deterioration is increased when aging is associated with hypertension. Aging alters endothelial cells, which play an important role in vascular tone regulation. Such a process tends to reduce endothelium-dependent relaxations, and clearly reduces the vasodilation elicited by beta-adrenoceptor agonists. The contractions induced by different agents, such as 5-hydroxytryptamine, histamine, high potassium and angiotensin are barely affected with aging, whereas those elicited by noradrenaline or endothelin are usually reduced. However, plasma noradrenaline levels are increased with age, mainly due to a reduction in the sensitivity of presynaptic alpha 2-adrenoceptors and also of noradrenaline uptake. Sodium pump activity, that controls cellular ionic homeostasis, may be altered depending on animal species. Finally, vascular Ca2+ regulation appears to be altered and the extracellular Ca2+ dependence of contractile responses elicited by agonists is increased, which justifies the enhanced sensitivity to Ca2+ antagonists in senescence.

Inhibition of nitric oxide synthesis extends cerebrovascular autoregulation during hypertension

In anesthetized intact rats, cerebral blood flow is autoregulated until mean arterial blood pressure (MAP) exceeds 150 mmHg. At higher pressures cerebral blood flow breaks through autoregulation and rapidly increases. However, interruption of the arterial baroreceptor reflex eliminates breakthrough of autoregulation. Thus, breakthrough may reflect active rather than passive vasodilatation. We, therefore, sought to determine if breakthrough depends upon synthesis of the vasodilator nitric oxide. Thirty-eight anesthetized adult male Sprague-Dawley rats were studied. In all, MAP was raised by slow i.v. infusion of phenylephrine. In rats pretreated with the nitric oxide synthase inhibitor L-nitroarginine (L-NA; 22 mg/kg i.v.) or with a combination of L-NA plus D-arginine (D-Arg; 240 mg/kg i.v.), breakthrough did not occur even when MAP exceeded 185 mmHg (L-NA) and 165 mmHg (D-Arg). In contrast, breakthrough occurred in rats treated with L-NA plus L-arginine (L-Arg; 240 mg/kg i.v.) and in rats whose basal vascular tone had been increased by pretreatment with arginine vasopressin prior to infusion of phenylephrine. Removal of sympathetic innervation to cerebral vessels attenuated, but did not eliminate, effects of L-NA on breakthrough. Thus, vasodilatation seen with breakthrough of autoregulation depends upon release of nitric oxide or a nitric oxide donor.

Variations of blood flow at optic nerve head induced by sinusoidal flicker stimulation in cats

1. The present investigation explored, in thirty-four anaesthetized cats, the blood flow changes at the optic nerve head elicited by sinusoidally modulated photic stimuli. 2. The stimuli were achromatic, diffuse and had 30 deg diameter field size; the stimulus frequency was varied from 0 to 100 Hz, modulation depth from 0 to 100% and mean retinal illuminance up to 50,000 trolands (td); the blood flow was measured with a near-infrared (810 nm) laser Doppler flowmeter. 3. At various frequencies, modulation depths and mean retinal illuminance, sinusoidal flicker stimulation always caused an increase in blood flow at the optic nerve head relative to steady stimulation. 4. The frequency response and temporal contrast sensitivity function of the blood flow changes had a bandpass shape; the high-frequency slope of the frequency response was 3 decades (dec) per decade and that of the temporal contrast sensitivity function was 1.7 dec per dec, close to the slope for cat 'on' ganglion cells (2.6 dec per dec). 5. In most cats, the magnitude of the increase in blood flow was a sigmoidal function of modulation depth; in the remainder, the relationship was close to linear. 6. The threshold of blood flow changes varied with respect to mean retinal illuminance similar to Ferry-Porter's law and the photopic linear slope was 50 Hz dec-1. 7. In comparison with reported psychophysical and electrophysiological responses elicited by similar stimulations, the results of the present study resemble more those obtained from ganglion cells than those from electroretinograms, visual-evoked potentials and psychophysics. It is suggested that the blood flow changes at the optic nerve head are induced by the activity of ganglion cells.

[Is there still a place for routine deep hypocapnia in intracranial surgery?]

Deliberate hypocapnia during the anaesthetic management of the patient undergoing craniotomy has become an accepted standard of care. However there has been a resurgence of interest, in how hypocapnia should be applied in intra- and extra-operative settings. There are three possible therapeutic effects of hypocapnia, namely, (a) reduction of brain bulk through a reduction in cerebral blood volume, with a decrease cerebral blood flow; (b) developing an "inverse steal" by redistribution of blood from normal to ischaemic regions and (c) acting to offset cerebral acidosis by increasing pH in the extracellular space. In anaesthetic intraoperative practice, hypocapnia is used as a specific treatment of, or prophylaxis against, intracranial hypertension during induction of anaesthesia and the period before dural exposure. More commonly, hypocapnia is used for intraoperative brain relaxation (intracranial pressure = 0). Severe hypocapnia (< 20 mmHg) may result in cerebral production of lactate; however no studies have shown that a Paco2 in the range of 23-28 mmHg has deleterious effects. Recent studies in head-injured patients suggest that routine long-term hyperventilation, without an objective index of cerebral flow/metabolism coupling, may place the brain at risk for adverse outcome. The few data available for intraoperative management suggest that Paco2 figures of 30-35 mmHg result in acceptable operating conditions. Unless otherwise specifically indicated by surgical conditions or cerebral flow/metabolism coupling (e.g. jugular O2 saturation), routine application of profound (Paco2 < 28-30 mmHg) hyperventilation should probably be avoided and its use needs reevaluation.

Photic stimulation study of changing the arterial partial pressure level of carbon dioxide

To investigate the effect of the level of baseline cerebral blood flow (CBF) on local CBF augmentation by activation, we have used positron emission tomography to measure regional CBF (rCBF) in 12 normal volunteers with and without photic stimulation during hypocapnia, normocapnia, and hypercapnia. The increase in rCBF in the primary visual cortex by photic stimulation was 10.8 +/- 3.1, 18.6 +/- 9.3, and 19.5 +/- 6.1 ml 100 ml-1 min-1 in hypo-, normo-, and hypercapnia, respectively. The increase was significantly smaller in hypocapnia than in normocapnia (p < 0.005). The fractional CBF increase caused by the photic stimulation was the same in all breathing conditions. This result indicates that the magnitude of the CBF increase induced by neuronal activity correlates proportionally with the level of baseline CBF.

Role of the cerebellar fastigial nucleus in the physiological regulation of cerebral blood flow

Local cerebral blood flow (ICBF) was measured with [14C]iodoantipyrine in conscious, unrestrained rats during electrical stimulation of the fastigial nucleus (FN). Electrode position in the FN was determined by blood pressure (MABP) responses to stimulation under anesthesia. In nine rats in which MABP responses had been variable under anesthesia, bipolar stimulation (50 Hz, 0.5 ms, 1 s on/1 s off) with currents of 30-100 microA after recovery from anesthesia produced stereotypic behavior but little effect on MABP and ICBF. In seven other conscious rats currents could be raised to 75-200 microA without inducing seizures, resulting in sustained MABP elevations during the ICBF measurement and significantly increased ICBF in the sensory-motor (+45%), parietal (+31%), and frontal cortices (+56%) and the caudate-putamen (+27%) above control values (n = 9). Glucose utilization, measured with [14C]deoxyglucose, in rats similarly stimulated was significantly increased in six structures, including some of the above, indicating increases in ICBF due to metabolic activation. Unilateral or bilateral electrolytic lesions of the FN, placed 6-7 days before ICBF measurement, had negligible effects on resting ICBF and on autoregulation in conscious rats. These results fail to support a specific role for the FN in physiological regulation of cerebral blood flow in unanesthetized rats.

Functional magnetic resonance imaging (FMRI) of the human brain

Functional magnetic resonance imaging (FMRI) can provide detailed images of human brain that reflect localized changes in cerebral blood flow and oxygenation induced by sensory, motor, or cognitive tasks. This review presents methods for gradient-recalled echo-planar functional magnetic resonance imaging (FMRI). Also included is a discussion of the hypothesized basis of FMRI, imaging hardware, a unique visual stimulation apparatus, image post-processing and statistical analysis. Retinotopic mapping of striate and extrastriate visual cortex is discussed as an example application. The described echo-planar technique permitted acquisition of an image in 40 ms with a repetition rate of up to 2 per second. However, FMRI responses are slow compared to changes in neural activity. Onset of a visual checkerboard test pattern evoked a response that was delayed by 1-2 s and reached 90% of peak in 5 s. Return to baseline following stimulation was slightly slower. Alternating control (blank) and test (checkerboard) patterns every 20 s induced a cyclic response that was detected in the presence of noise using a cross-correlation technique that was verified by parametric statistics. FMRI revealed retinotopically organized patterns of visually evoked activity in response to annular stimuli that increased in visual field eccentricity. Retinotopy was also observed with test patterns rotated around the fixation point (center of gaze). Results from repeated tests 1 week apart were highly similar. Compared to passive viewing, an active visual discrimination task enhanced responses from extrastriate association cortex.

Neuropeptides in migraine and cluster headache

The cerebral circulation is invested by a rich network of neuropeptide Y (NPY) and noradrenaline containing sympathetic nerve fibers in arteries, arterioles and veins. However, the nerve supply of vasoactive intestinal peptide (VIP), substance P (SP) and calcitonin gene-related peptide (CGRP) containing fibers is sparse. While noradrenaline and NPY cause vasoconstriction, VIP, SP and CGRP are potent vasodilators. Stimulation of the trigeminal ganglion in cat and man elicits release of SP and CGRP. Subjects with spontaneous attacks of migraine show release of CGRP in parallel with headache. Cluster headache patients have release of CGRP and VIP during bouts. Treatment with sumatriptan aborts headache in migraine and cluster headache as well as the concomitant peptide release.

Scopolamine abolishes cerebral blood flow response to somatosensory stimulation in anesthetized cats: PET study

The effect of the cholinergic blocker, scopolamine on the cerebral blood flow (CBF) response to vibrotactile stimulation of a fore paw was studied using high-resolution positron emission tomography and H2 15O in 5 pentobarbital-anesthetized cats. Before scopolamine injection, the CBF response to the stimulation was found in the contralateral somatosensory cortex (mean ratio (contralateral/ipsilateral) control: stimulated 1.02 +/- 0.02: 1.17 +/- 0.05; P < 0.01). After intravenous injection of scopolamine (0.35 mg/kg), the CBF response was abolished. However, the cerebral metabolic rate of glucose (CMRGlu) response to the same stimulation was unchanged after scopolamine injection in the same cats. We concluded that scopolamine abolishes the CBF response but not neuronal response to stimulation. We suggest that cholinergic mechanisms may play an important role for mediating CBF coupling to neuronal activity during physiological stimulation.

Sensitivity and specificity of thallium-201 single-photon emission tomography in the functional detection and differential diagnosis of brain tumours

The aim of this retrospective study was to assess the contribution of thallium-201 single-photon emission tomography (SPET) in the detection and differential diagnosis of brain tumours. In 90 patients 201Tl SPET was performed because of clinical or radiological suspicion of tumoral invasion, completed by technetium-99m hexamethylpropylene amine oxime and 99mTc-sestamibi SPET in some patients. For all tumours, diagnosis was based on biopsy or autopsy. Other diagnoses were made only after clinical and radiological follow-up for at least 6 months. Histologically tumours consisted of astrocytoma stage I or II (number of patients, n = 6), astrocytoma stage III (n = 8), glioblastoma multiforme (n = 14) and oligodendroglioma (n = 3), brain metastasis (n = 14), lymphoma (n = 3), meningioma (n = 3), pituitary adenoma (n = 2), pineal tumour (n = 1), colloid cyst (n = 1) and craniopharyngioma (n = 1). False-negative studies included pineal tumour (n = 1), colloid cyst (n = 1), craniopharyngioma (n = 1), astrocytomas stage I or II (n = 6) and stage III (n = 3), oligodendroglioma (n = 2) and metastasis in the brain stem (n = 1). Additional metastases approximately < 1.5 cm were not detected in two patients and 201Tl SPET underestimated tumoral extent in one patient suffering from glioblastoma multiforme (n = 1). A false-positive study was obtained in a patient with skull metastasis (n = 1). All 15 patients who were finally shown to suffer from ischaemic infarction had a normal SPET study 9-28 days after the onset of symptomatology. Of five patients with haemorrhagic infarction, studied within 2 weeks, four were false-positive. Of six patients with intracranial haemorrhage, studied 9-39 days later, one showed focal 201Tl accumulation. Two further false-positive studies consisted of angioma and epidural haematoma. Finally, SPET studies were normal in six patients with definite diagnosis of (reactive) gliosis (n = 3), Binswanger's encephalopathy (n = 1), postinfectious encephalopathy (n = 1) and multiple sclerosis (n = 1). In the patient population presented, sensitivity of 201Tl SPET for supratentorial brain tumours was 71.7% and specificity was 80.9%. Clinical information and control SPET studies in combination with early, 30-min and 3- to 4-h delayed imaging may be expected to improve on these figures. On the other hand it seems that, in addition to tumoral histology, the presence of tumours in the fossa posterior and small volumes contribute to the occurrence of false-negative 201Tl SPET studies.

Diminished brain glucose metabolism is a significant determinant for falling rates of systemic glucose utilization during sleep in normal humans

Systemic glucose utilization declines during sleep in man. We tested the hypothesis that this decline in utilization is largely accounted for by reduced brain glucose metabolism. 10 normal subjects underwent internal jugular and radial artery cannulation to determine cerebral blood flow by N2O equilibrium technique and to quantitate cross-brain glucose and oxygen differences before and every 3 h during sleep. Sleep stage was graded by continuous electroencephalogram, and systemic glucose turnover was estimated by isotope dilution. Brain glucose metabolism fell from 33.6 +/- 2.2 mumol/100 g per min (mean +/- SE) before sleep (2300 h) to a mean nadir of 24.3 +/- 1.1 mumol/100 g per min at 0300 h during sleep (P = 0.001). Corresponding rates of systemic glucose utilization fell from 13.2 +/- 0.8 to 11.0 +/- 0.5 mumol/kg per min (P = 0.003). Diminished brain glucose metabolism was the product of a reduced arteriovenous glucose difference, 0.643 +/- 0.024 to 0.546 +/- 0.020 mmol/liter (P = 0.002), and cerebral blood flow, 50.3 +/- 2.8 to 44.6 +/- 1.4 cc/100 g per min (P = 0.021). Brain oxygen metabolism fell commensurately from 153.4 +/- 11.8 to 128.0 +/- 8.4 mumol/100 g per min (P = 0.045). The observed reduction in brain metabolism occurred independent of stage of central nervous system electrical activity (electroencephalographic data), and was more closely linked to duration of sleep. We conclude that a decline in brain glucose metabolism is a significant determinant of falling rates of systemic glucose utilization during sleep.

Pathogenetic mechanisms in vascular dementia

Vascular dementia accounts for approximately 20% of all cases of dementia and for about 50% in subjects over 80 years. Thromboembolism with multiple cerebral infarcts was considered to be almost the only pathogenetic pathway of vascular dementia, with multi-infarct dementia as its clinical manifestation. However, there is a great heterogeneity of vascular dementia syndromes and pathological subtypes, as documented by the number of pathogenetic mechanisms now known to underlie the clinical picture. They include thromboembolism and extracerebral and cerebral factors. Among the extracerebral factors are ischemic hypoxic dementia (i.e., dementia due to hypoperfusion), vasculitis, hyperviscosity and abnormalities of hemostasis. Among the cerebral factors are lipohyalinosis, cerebral amyloid angiopathy, disruption of the blood-brain barrier and altered regulation of cerebral blood flow. Therefore, the approach to vascular dementia should take the heterogeneity into account. In this context, the importance of non-infarct type should be considered; subcortical white matter disorder seems to be a noteworthy common pathway of vascular dementia produced by various vascular mechanisms. Finally, the heterogeneity of the vascular mechanisms involved in vascular dementia--namely hypoperfusion--might be a factor that can be positively influenced by targeted therapeutic intervention.

Histochemical localization of NADPH-dependent diaphorase (nitric oxide synthase) activity in vascular endothelial cells in the rat brain

This study investigated the localization of NADPH-dependent diaphorase activity within vascular endothelial cells in the rat brain. Light microscope observations showed that in addition to neurons and neuronal processes stained histochemically for NADPH-dependent diaphorase activity, endothelial cells in many medium to large diameter (20-100 microns) blood vessels were also stained. These vessels were either attached to the pial surface or contained within the substance of the tissue. In vascular endothelia, the formazan end-product of the diaphorase reaction was deposited as discrete clusters of darkly stained punctae that were located around the nucleus of these cells. Correlated light- and electron-microscopical examination revealed that the sites of formazan deposition occurred in regions of endothelial cytoplasm devoid of smooth and rough endoplasmic reticulum and of mitochondria. Since endothelial NADPH dependent diaphorase activity co-localizes with the activity of nitric oxide synthase (the synthetic enzyme for nitric oxide) these observations suggest that in vascular endothelial cells nitric oxide synthase may be a highly localized soluble cytosolic enzyme not structurally associated with any subcellular organelle. In addition, specific regions of the smooth muscle cells encircling the larger diameter blood vessels clearly demonstrated NADPH dependent diaphorase activity. Unmyelinated fibres and fibre-plexi surrounding blood vessels on the pial surface were also stained. The results of this study show specific NADPH dependent diaphorase activity in vascular endothelial cells in the rat brain. Therefore, together with neurons, endothelial cells may control nitric oxide-dependent vasodilation thereby regulating local blood flow in the brain.

Heterogeneity of vascular dementia: mechanisms and subgroups

Today, multiple, thromboembolically generated cerebral infarcts are regarded as the main pathogenetic pathway of vascular dementia (VAD), with multi-infarct dementia (MID) as its clinical counterpart. However, taking into account other vascular mechanisms that may influence the brain, such as vessel-wall damage (atherosclerosis, hyalinosis, amyloid angiopathy, or blood-brain barrier dysfunction), cerebrovascular insufficiency (disturbance of systemic circulation, perfusion vulnerability related to the vascular anatomy of the brain, or disturbance of autoregulation), and hyperviscosity, it is evident that MID is not the only VAD category. The diagnosis of MID ought to be reserved for the combination of progressive dementia associated with cerebral ischemic events and evidence of infarction that is mainly associated with the large cerebral arteries. Subcortical white-matter dementia characterized by frontosubcortical symptomatology, white-matter lesions, and small-vessel involvement with or without lacunes/infarcts--a combination of lacunar dementia and Binswanger's disease--appears to be another important VAD disease.

Pressure autoregulation is intact after arteriovenous malformation resection

The loss of autoregulatory control of cerebral perfusion to changes in perfusion pressure in tissue remote from an arteriovenous malformation (AVM) has been proposed as the mechanism underlying "normal perfusion pressure breakthrough." This study is the first direct test of this mechanism. Studies were performed during the resection of moderate to large AVMs in 25 patients undergoing 28 procedures under isoflurane anesthesia. Cerebral blood flow (CBF) was measured (xenon-133 method) in the hemisphere adjacent to the nidus before resection after dural exposure (pre), after AVM removal before dural closure at spontaneous systemic blood pressure (post), and, finally, with the mean arterial pressure increased by 20 mm Hg, using phenylephrine (post-BP). AVM resection resulted in a significant enhancement of perfusion in the adjacent hemisphere (30 +/- 2 vs. 25 +/- 1 ml/100g/min, P < 0.01), but no further increase of CBF occurred during increased perfusion pressure (30 +/- 2 ml/100g/min). One patient suffered a postoperative hemorrhage and another developed intraoperative brain swelling during the course of the resection that necessitated staging the procedure. These two patients had the highest increases in CBF, but intact pressure autoregulation. Preserved autoregulation to increased mean arterial pressure after resection does not support a hemodynamic mechanism for the observed increase in CBF from before the resection to after the resection. Pathological events, however, do appear to be related to increases in hemispheric perfusion.

Data analysis in behavioral cerebral blood flow activation studies using xenon-133 clearance

BACKGROUND AND PURPOSE

Three mainstream strategies exist to detect the responses of regional cerebral blood flow to functional activation. We tested the significance of changes in raw regional cerebral blood flow data, regional cerebral blood flow data normalized by division by global cerebral blood flow (dependent model of the regional-to-global cerebral blood flow relation), and regional cerebral blood flow data treating global cerebral blood flow as a covariate (independent model). Both latter models attempt to enhance regional sensitivity by removing global effects. We examined the sensitivity and pitfalls of these three strategies in behavioral activation studies.

METHODS

These three strategies of data analysis were applied to changes in regional cerebral blood flow induced by a visuospatial problem-solving task in 38 healthy subjects as measured by the intravenous xenon-133 method with 32 stationary detectors.

RESULTS

Mental activation increased blood flow in all regions of interest. Raw data were most sensitive and reliable to detect responses to mental stimulation. Both the independent and dependent models to remove global effects were less sensitive and falsely indicated deactivation in regions that were clearly stimulated.

CONCLUSIONS

In behavioral activation paradigms, safe data analysis should be restricted to using raw regional cerebral blood flow increases without normalization or separation of global from regional effects. Studies using complex stimulation tasks should be scrutinized for global cerebral blood flow effects confounding regional responses.

Role of nitric oxide in the coupling of cerebral blood flow to neuronal activation in rats

We tested the hypothesis that nitric oxide (NO) is a mediator in the coupling of cerebral blood flow to neuronal activation. The production of NO was blocked in anesthetized rats with the NO-synthase inhibitor N omega-nitro-L-arginine (L-NA). In controls, vibrissae stimulation for 60 s led to a fast (< or = 2 s), 17% increase in regional cerebral blood flow (rCBF) in the contralateral somatosensory cortex. Systemical (10 mg/kg) as well as topical (10(-3) M) application of L-NA reduced the response to stimulation by approximately 50%. Systemical application primarily attenuated the early component of the response, whereas topical application led to an attenuation throughout the whole 60-s stimulation interval. We conclude that NO is involved in rCBF coupling to neuronal activation.