Cerebral Microcirculation: a Review Emphasizing the Interrelationship of Local Blood Flow and Neuronal Function

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Spatial flow-volume dissociation of the cerebral microcirculatory response to mild hypercapnia

The spatial and temporal response of the cerebral microcirculation to mild hypercapnia was investigated via two-photon laser-scanning microscopy. Cortical vessels, traversing the top 200 microm of somatosensory cortex, were visualized in alpha-chloralose-anesthetized Sprague-Dawley rats equipped with a cranial window. Intraluminal vessel diameters, transit times of fluorescent dextrans and red blood cells (RBC) velocities in individual capillaries were measured under normocapnic (PaCO2= 32.6 +/- 2.6 mm Hg) and slightly hypercapnic (PaCO2= 45 +/- 7 mm Hg) conditions. This gentle increase in PaCO2 was sufficient to produce robust and significant increases in both arterial and venous vessel diameters, concomitant to decreases in transit times of a bolus of dye from artery to venule (14%, P < 0.05) and from artery to vein (27%, P < 0.05). On the whole, capillaries exhibited a significant increase in diameter (16 +/- 33%, P < 0.001, n = 393) and a substantial increase in RBC velocities (75 +/- 114%, P < 0.001, n = 46) with hypercapnia. However, the response of the cerebral microvasculature to modest increases in PaCO2 was spatially heterogeneous. The maximal relative dilatation (range: 5-77%; mean +/- SD: 25 +/- 34%, P < 0.001, n = 271) occurred in the smallest capillaries (1.6 microm-4.0 microm resting diameter), while medium and larger capillaries (4.4 microm-6.8 microm resting diameter) showed no significant changes in diameter (P > 0.08, n = 122). In contrast, on average, RBC velocities increased less in the smaller capillaries (39 +/- 5%, P < 0.002, n = 22) than in the medium and larger capillaries (107 +/- 142%, P < 0.003, n = 24). Thus, the changes in capillary RBC velocities were spatially distinct from the observed volumetric changes and occurred to homogenize cerebral blood flow along capillaries of all diameters.

Relationship of spikes, synaptic activity, and local changes of cerebral blood flow

The coupling of electrical activity in the brain to changes in cerebral blood flow (CBF) is of interest because hemodynamic changes are used to track brain function. Recent studies, especially those investigating the cerebellar cortex, have shown that the spike rate in the principal target cell of a brain region (i.e. the efferent cell) does not affect vascular response amplitude. Subthreshold integrative synaptic processes trigger changes in the local microcirculation and local glucose consumption. The spatial specificity of the vascular response on the brain surface is limited because of the functional anatomy of the pial vessels. Within the cortex there is a characteristic laminar flow distribution, the largest changes of which are observed at the depth of maximal synaptic activity (i.e. layer IV) for an afferent input system. Under most conditions, increases in CBF are explained by activity in postsynaptic neurons, but presynaptic elements can contribute. Neurotransmitters do not mediate increases in CBF that are triggered by the concerted action of several second messenger molecules. It is important to distinguish between effective synaptic inhibition and deactivation that increase and decrease CBF and glucose consumption, respectively. In summary, hemodynamic changes evoked by neuronal activity depend on the afferent input function (i.e. all aspects of presynaptic and postsynaptic processing), but are totally independent of the efferent function (i.e., the spike rate of the same region). Thus, it is not possible to conclude whether the output level of activity of a region is increased based on brain maps that use blood-flow changes as markers.

Cortical "stress tests" in schizophrenia: regional cerebral blood flow studies

A total of 261 regional cerebral blood flow (rCBF) studies were carried out on 34 medication-free patients with chronic schizophrenia and 50 normal subjects. rCBF, an indicator of local cortical metabolism and activity, was measured during the resting state and also during four cognitive activation tasks or "cortical stress tests." The latter included the Wisconsin Card Sort (WCS), a test of prefrontal lobe function; a simple numbers matching task, and two versions of a visual Continuous Performance Task (CPT). Multivariate comparisons of the two subject groups were performed for each of the five testing conditions, and discriminant function analyses for each condition were carried out to define mathematical models that differentiated normal subjects from medication-free patients. The best such model was determined and was then applied to another group of patients who had diagnoses other than schizophrenia or for whom the diagnosis was unclear. This group included two patients with clinical "frontal lobe syndrome" and radiological evidence of frontal lobe damage. The most robust differences between the groups were seen in frontal rCBF during the WCS. In the discriminant function analysis, rCBF during the WCS was the best discriminator between the two groups, retrospectively classifying 85% of the subjects correctly. rCBF during the resting state and one of the CPTs correctly classified subjects at a rate only marginally better than chance. When the model derived from WCS rCBF was applied to a second group of patients, the two patients with known frontal lobe disease were classified as "schizophrenic" with 100% certainty. Three other patients with psychotic illnesses were also assigned to this group with greater than 80% certainty, whereas a patient with character disorder (rule-out affective disorder) was classified as "normal" with a high level of confidence. These data suggest (1) that schizophrenia is characterized by a deficit in prefrontal function that is revealed when regionally specific demand exceeds the physiological capacity, and (2) that functional brain imaging studies, such as rCBF, can best identify brain abnormalities during "cortical stress tests."

The terminal vascular bed in the superficial cortex of the rat. An SEM study of corrosion casts

The capillaries in the vascular bed of the rat brain have been investigated by means of scanning electron microscopy of corrosion casts. A technique is described that allowed the finer ramifications to be observed. A series of representative sites from the arteriovenous terminal pathway are described in detail. Contrary to previous reports, the dichotomic pattern of vessel distribution is shown to prevail over the network pattern. Arteriovenous shunts of discrete size were not seen. "Thoroughfare channels" could be recognized. The findings are considered in light of current physiological knowledge, and their significance for microcerebrovascular flow is indicated.

Quantitative capillary topography and blood flow in the cerebral cortex of cats: an in vivo microscopic study

In 50 anesthetized cats the microcirculation in intermediate and deeper layers of the cerebral cortex was visualized in vivo by microtransillumination, and documented by high-speed microcinephotography. The viability of the preparation was verified in a series of experiments demonstrating spontaneous vasomotion and responsiveness to chemical stimulation of pial arterioles and small arteries. Stereological methods for quantitative analysis of projected images of capillaries in a comparatively large tissue volume were employed to determine morphometric and topographical parameters of the asymmetric, highly tortuous intracortical capillary network. Capillary diameters (5.1 +/- 0.84 micrometer), radii of curvature (median 57 micrometer), total capillary lengths per tissue volume 939 +/- 338.2 mm/cu.mm), capillary volume fractions (2.1 +/- 0.51%), total capillary surface areas per tissue volume (15.3 +/- 4.85 sq.mm/cu.mm), and intercapillary distances (median 24.2 micrometer) showed significant interregional differences. The frequency distribution of the lengths of capillary segments (median 108 micrometer) was best described by a Weibull distribution. On the average 90% of all capillaries were continuously perfused. Capillary red cell flow (median velocity 1500 micrometer/sec) was predominantly unidirectional and conspicuously irregular. The variance of capillary red cell velocities (CRCVs) was significantly correlated (tau = 0.48) with capillary tortuosity. An extreme value distribution best described the observed frequency distribution of CRCVs. Flow irregularities represented both white noise and a significant stochastic periodicity at frequencies between 40 and 90 Hz.

Influence of respiratory stress and hypertension upon the blood-brain barrier

The effects of acute hypertension and respiratory stress induced by Aramine (metaraminol bitartrate) upon blood-brain barrier (BBB) permeability to horseradish peroxidase (HRP) were studied in adult inbred white rats. The BBB permeability was quantitated by slicing the brain of each animal into 500-mu thick sections, incubating the sections using the Reese-Karnovsky method, and counting all observed HRP perivascular exudates. No evidence of BBB compromise or significant elevation of blood pressure (BP) was observed in the following experimental groups: 1) control group of five animals; 2) hyperventilated group of five animals (final mean arterial blood gases; pO2, 104.2 mm Hg; pCO2, 24.8 mm Hg; pH, 7.53); 3) anoxic-stress group of five animals (final mean arterial blood gases; pO2, 31.4 mm Hg; pCO2, 58.2 mm Hg; pH 7.21). However, in a group of 15 animals subjected to anoxic stress followed by hyperventilation, in addition to extreme changes in the levels of arterial blood gases, a significant BP increase occurred (mean BP increase per second, 3.43 +/- 0.25 mm Hg; final mean BP, 163.3 +/- 3.18 mm Hg); as well as significant BBB opening (mean number of HRP exudates per animal, 12.2 +/- 0.85). Likewise, a final group of 10 animals given intravenous Aramine displayed a significant systemic BP elevation (mean BP increase per second, 6.9 +/- 0.38 mm Hg; final mean BP, 165.8 +/- 3.16 mm Hg), accompanied by BBB opening (mean number of exudates per animal, 51.5 +/- 5.95). The variable most strongly associated with the degree of barrier opening was the rate of BP rise (correlation coefficient = +0.84).

Adrenergic control of cerebral blood flow and energy metabolism in the rat

Studies in rats were designed to separate and define the roles of the intrinsic and extrinsic adrenergic neurons in the control of cerebral blood flow (CBF) and cerebral energy metabolism. The data suggest several conclusions: 1. Arterial sympathetic innervation plays a role in the autoregulation of cerebral circulation. 2. The central adrenergic neurons have several functions: a) they enhance cerebral vascular tone by action on alpha receptor sites. b) They play an important role in the metabolic control of CBF. The proton-sensitive receptor sites on blood vessel walls require beta-adrenergic input in order to function. c) They influence metabolic rate of brain tissue by acting on beta-receptor sites on the cell membrane.

Pial microcirculation in subarachnoid hemorrhage

Microsurgical and microscopic methods were employed in guinea pigs to expose, observe, and measure response characteristics of cerebral cortical pial microvessels and microcirculation to traumatic and nontraumatic experimental subarachnoid hemorrhage. Bleeding produced by vascular micropuncture was associated with a 44.3% arteriolar constriction. Topical application of homologous blood alone produced a 33.2% vasoconstriction. Observed microcirculatory flow characteristics subsequent to such microvascular changes were consistent with those known to be associated with cerebral cortical infarction. These changes could be prevented or reversed by topical application of the alpha adrenergic blocker, phenoxybenzamine. Topical pretreatment with the beta adrenergic blocker, propranolol, prevented blood-induced spasm, but did not reverse such spasm once it had been established. A chemo-mechanical mechanism is suggested as underlying the vasoconstriction association with rupture of pial microvessels. It is thought that consideration of such microvascular characteristics, in conjunction with those known to be associated with larger intracranial vessels, adds to current knowledge of the pathophysiology of subarachnoid hemorrhage and may be extrapolated to bear future clinical import.

Neurogenic control of cerebral blood flow in the baboon. Effects of alpha adrenergic blockade with phenoxybenzamine on cerebral autoregulation and vasomotor reactivity to changes in PaCO2

Cerebral autoregulation was studied in the baboon by increasing and decreasing cerebral perfusion pressure (CPP) before and after intravenous administration (1.5 mg per kilogram) of a long-acting alpha adrenergic blocker, phenoxybenzamine (PBZ). Likewise, cerebral vasomotor reactivity to changes of arterial carbon dioxide tension (Pa CO 2 ) was examined before and after PBZ.

In order to permit quantitative analysis, cerebral autoregulation (A.I.) and chemical vasomotor reactivity (C.I.) were expressed as indices where

A.I.=δCBF/δCPP and C.I.=δCBF/δPa CO 2 .

Following the intravenous injection of PBZ, cerebral autoregulatory vasoconstriction was impaired as CPP was increased. Cerebral vasomotor reactivity to changes in Pa CO 2 was altered both during hyperventilation hypocapnia (HV) and hypercarbia induced by inhalation of 5% carbon dioxide if alterations of CPP brought about by these procedures were taken into consideration. During hypocapnia C.I. was reduced 30% and during hypercarbia C.I. was increased 10%.

It is concluded that PBZ reduces the vasoconstrictor tonus of cerebral vessels during hypocapnia and raised CPP. It also enhances the vasodilator response to CO 2 inhalation.

The importance and relevance of studies of the pial microcirculation

Micropuncture data suggest that pial arterioles contribute only slightly less than parenchymal cerebral arterioles to total resting cerebrovascular resistance. Care must be taken in interpreting the micropuncture data, or the contribution of pial arterioles to cerebrovascular resistance may be erroneously underestimated. These considerations and the fact that pial arterioles have been shown to be highly reactive to a variety of physiological and abnormal stimuli suggest strongly that changes in pial arteriolar diameter should contribute importantly to control of flow to the underlying brain. In fact, parallels between changes in pial vascular diameter and regional blood flow have been observed. Moreover, since the responses of pial vessels to important vasoactive stimuli are qualitatively similar to those of the cerebral circulation as a whole when the latter are inferred from measurements of flow, the directly observable pial vessels may provide a model for the responses of the unseen parenchymal segments of the cerebrovascular bed. Such a model would be essential to our understanding of the control of cerebral blood flow, even if pial vessels themselves did not participate in the control of flow. Thus there are multiple reasons for continued study of the pial vessels, particularly with modern techniques developed for microcirculatory investigations.

Cholinergic Control of Blood Flow in the Cerebral Cortex of the Rat

Local blood flow was measured in the somatosensory cortex of urethanized rats by means of the hydrogen clearance method. The variations of cortical blood flow in the course of the anesthesia and the effects of the topical application of atropine, eserine and cholinomimetic drugs were studied. During urethan anesthesia, the electrical activity of the cerebral cortex fluctuated between a synchronized and a desynchronized state. During desynchronization, local cortical blood flow increased significantly. This increase in blood flow could be prevented by topical application of atropine and exaggerated by topical eserine. Topical application of arecoline, carbaminoylcholine, pilocarpine or acetylcholine with eserine significantly increased cortical blood flow. It is concluded that the increase in cortical blood flow that accompanies cortical desynchronization in the urethanized rat is mediated, at least in part, by a neurogenic mechanism that involves a cholinergic step at the cortical level.

Non-uniform response of regional cerebral blood flow to stimulation of cervical sympathetic nerve

Regional cerebral blood flow (CBF) was measured by an autoradiographic method in nine adult cats, using antipyrine-(14)C as a diffusible indicator. In seven of the cats, CBF measurements were made during stimulation of a cervical sympathetic trunk. Stimulation caused minor regional decreases of CBF in at least five of these seven cats. The decreases were non-uniform and occurred almost exclusively in cortical structures. Although constriction of cervical arteries probably accounts for some of the effects of sympathetic stimulation, the present study indicates that there is also an effect on cerebral regulatory arterioles. However, there is no convincing evidence that function of the autonomic nervous system is necessary for the normal regulation of the cerebral circulation.

Neurogenic control of cerebral circulation

A review of recent literature provides evidence that cerebral vessels are innervated with myelinated and nonmyelinated nerves. Among the latter, adrenergic nerves are readily demonstrated. Stimulation of cervical sympathetics produces a reduction in cerebral blood flow attributable to constriction of vessels in the neck and on the cerebral surface. In vitro experiments indicate that cerebral vessels possess a mechanism responsive to sympathomimetic agents. Evidence indicates that neurogenic influences can alter cerebrovascular diameter without necessarily playing a role in maintaining resting tone. Data are also available which suggest that neurogenic influences may modulate the response of cerebral vessels to other more potent stimuli. Cerebral vessels are less sensitive and less responsive to neurogenic influences than are vessels elsewhere. However, available data suggest that this characteristic of the cerebral vasculature reflects specialization of vascular muscle or of perivascular nerves, rather than a vestigial basis for the existence of these nerves.

Effects of reduced hematocrit on erythrocyte velocity and fluorescein transit time in the cerebral microcirculation of the mouse

The red blood cell velocity and the arteriole-to-venule transit time of sodium fluorescein were measured in the pial microcirculation of 15 mice immediately before and after phlebotomy and hemodilution. The acute anemic state produced by the phlebotomy and hemodilution was accompanied by an increase in the velocity of the red blood cells and of the fluorescent plasma. Such increments in velocity must be attributable to either an increased blood pressure or a fall in cerebrovascular resistance. Increases in pressure were generally absent, as was vasodilation, one cause of a decreased resistance. On the other hand, decreased blood viscosity accompanies a fall in hematocrit and could therefore account for a decreased vascular resistance in every animal. These data are consonant with earlier reports in animals and man of an increased cerebral blood flow accompanying anemia. Moreover, the data support those workers who attributed this increase in flow at least partly to a decrease in blood viscosity.

A method for studying isolated resistance vessels from rabbit mesentery and brain and their responses to drugs

A technique has been developed for studying the reactivity of single, isolated resistance vessels, 50 to 250 µ o.d., perfused at constant flow rate. The validity of the method is established because responses to a given stimulating agent are reproducible and stable over a reasonable period of time. The magnitude of the response is dependent on the perfusion pressure, being maximal at a physiological pressure level. Resistance vessels from mesentery and brain of normal rabbits were compared with respect to their threshold for response to several vasoconstrictors. The results reaffirmed the individuality of smooth muscle from different vascular beds. Cerebral and mesenteric vessels are alike in the dose required for threshold constrictor response to KCl, angiotensin and plasma, but differ in that cerebral vessels have a higher threshold for response to epinephrine, norepinephrine and serotonin, and a lower threshold for response to vasopressin, than mesenteric vessels. Since the differences in threshold between vessels from the two sources are not the same for all stimulating agents, it seems probable that smooth muscle of the cerebral vessels is not generally less sensitive to all stimuli than that of mesenteric vessels, but that vessels from the two sources differ in the "number of receptors" for the several vasoactive agents.