Alterations in perfused capillary morphometry in awake vs anesthetized brain

The search for true numbers of neurons and glial cells in the human brain: A review of 150 years of cell counting

For half a century, the human brain was believed to contain about 100 billion neurons and one trillion glial cells, with a glia:neuron ratio of 10:1. A new counting method, the isotropic fractionator, has challenged the notion that glia outnumber neurons and revived a question that was widely thought to have been resolved. The recently validated isotropic fractionator demonstrates a glia:neuron ratio of less than 1:1 and a total number of less than 100 billion glial cells in the human brain. A survey of original evidence shows that histological data always supported a 1:1 ratio of glia to neurons in the entire human brain, and a range of 40-130 billion glial cells. We review how the claim of one trillion glial cells originated, was perpetuated, and eventually refuted. We compile how numbers of neurons and glial cells in the adult human brain were reported and we examine the reasons for an erroneous consensus about the relative abundance of glial cells in human brains that persisted for half a century. Our review includes a brief history of cell counting in human brains, types of counting methods that were and are employed, ranges of previous estimates, and the current status of knowledge about the number of cells. We also discuss implications and consequences of the new insights into true numbers of glial cells in the human brain, and the promise and potential impact of the newly validated isotropic fractionator for reliable quantification of glia and neurons in neurological and psychiatric diseases. J. Comp. Neurol. 524:3865-3895, 2016. © 2016 Wiley Periodicals, Inc.

All brains are made of this: a fundamental building block of brain matter with matching neuronal and glial masses

How does the size of the glial and neuronal cells that compose brain tissue vary across brain structures and species? Our previous studies indicate that average neuronal size is highly variable, while average glial cell size is more constant. Measuring whole cell sizes in vivo, however, is a daunting task. Here we use chi-square minimization of the relationship between measured neuronal and glial cell densities in the cerebral cortex, cerebellum, and rest of brain in 27 mammalian species to model neuronal and glial cell mass, as well as the neuronal mass fraction of the tissue (the fraction of tissue mass composed by neurons). Our model shows that while average neuronal cell mass varies by over 500-fold across brain structures and species, average glial cell mass varies only 1.4-fold. Neuronal mass fraction varies typically between 0.6 and 0.8 in all structures. Remarkably, we show that two fundamental, universal relationships apply across all brain structures and species: (1) the glia/neuron ratio varies with the total neuronal mass in the tissue (which in turn depends on variations in average neuronal cell mass), and (2) the neuronal mass per glial cell, and with it the neuronal mass fraction and neuron/glia mass ratio, varies with average glial cell mass in the tissue. We propose that there is a fundamental building block of brain tissue: the glial mass that accompanies a unit of neuronal mass. We argue that the scaling of this glial mass is a consequence of a universal mechanism whereby numbers of glial cells are added to the neuronal parenchyma during development, irrespective of whether the neurons composing it are large or small, but depending on the average mass of the glial cells being added. We also show how evolutionary variations in neuronal cell mass, glial cell mass and number of neurons suffice to determine the most basic characteristics of brain structures, such as mass, glia/neuron ratio, neuron/glia mass ratio, and cell densities.

Study of temporal and perfusion physiology of skin capillaries in the dorsum of the foot

The aim of this study was twofold: firstly, to study the nature of any temporal variation in capillary numbers, and secondly to determine the proportion of perfused to total nutritional capillaries in normal skin. Using in vivo microscopy, the temporal behaviour of the number of visible capillaries in the skin of the dorsum of foot was observed over periods of time varying from 5 min to 55 days in 15 healthy subjects. Capillary perfusion was then studied by comparing capillary numbers before and after intravenous injection of sodium fluorescein. The mean percent difference in the number of visible capillaries over a mean period of 25.3 days was 5.5%. The percentage ratio of perfused to total capillaries was 54.2%. This study shows that there is little quantitative change in capillary numbers over periods of up to 50 days, and that under physiological conditions, about half of the nutritional capillaries of skin are not perfused.

Functional recruitment of red blood cells to rat brain microcirculation accompanying increased neuronal activity in cerebellar cortex

Scanning laser-Doppler flowmetry (SLDF) combines laser-Doppler flowmetry and laser scanning to provide images of cerebral blood flow (CBF) with high spatial and temporal resolution. We investigated the contribution of single vascular elements to the local increase of CBF accompanying increased neuronal activity in halothane-anesthetized rats. CBF was examined in the cerebellar cortex under control conditions and in response to electrical stimulation of parallel and climbing fibers. At rest, arterioles contributed 9%, venules 11-13% and small vessels (< 20 microm) 8-14%, while the background constituted 64-72% of the total SLDF signal. During activation the background signal decreased to 55-60% while the signal from arterioles increased to 11-12%, from venules to 14-15% and from small vessels to 14-19%. The signal increase in small vessels that did not give any laser-Doppler signal at rest was due to functional recruitment of red blood cells to the capillary bed. We conclude that functional recruitment may be an integral part of the hemodynamic response accompanying neuronal activity.

Brain acidosis, cerebral blood flow, capillary bed density, and mitochondrial function in the ischemic penumbra

Within the ischemic penumbra, there is a heterogeneous development of cortical intracellular acidosis that is associated with selective neuronal injury. This experiment, which used a rabbit model of moderate focal cerebral ischemia, examined the time course for changes in intracellular brain pH, cortical blood flow, capillary bed density, and mitochondrial function in the ischemic penumbra. After cortical annotation of regions of intracellular acidosis in the ischemic penumbra, the animals underwent transcardiac carbon black perfusion for measurement of capillary bed density. Analysis of variance and Pearson's correlation coefficients were used to determine the relationship between capillary bed density, brain intracellular pH, mitochondrial function, and cortical blood flow. Thirty minutes after the onset of ischemia, cortical blood flow declined from 46+/-2 to 22+/-1 mL/100gm/min (P<.01) in all groups. The overall cortical intracellular brain pH measured 6.78+/-.01 compared with a preischemic value of 6.98+/-.01 (P<.05). Within this moderately ischemic cortex, there were small regions (1,000 to 45,000 mum(2)) of increased acidosis, meauring 6.68+/-.01, not associated with focal changes in cortical blood flow, occurring within 15 minutes of ischemia and persisting throughout the ischemic period. Capillary bed density progressively declined with ongoing ischemia occurring after the development of acidosis. For example, capillary bed density in preischemic controls was 338+/-6/mm(2), whereas after 1 hour of ischemia, it measured 147+/-12/mm(2), at 3 hours 97+/-23/mm(2), and at 6 hours 92+/-16/mm(2). Mitochondrial function was reduced coinciding with the decrease in capillary bed density. These data support the hypothesis that cortical acidosis in the ischemic penumbra facilitates the development of perfusion defects that subsequently lead to mitochondrial dysfunction.

High-resolution mapping of discrete representational areas in rat somatosensory cortex using blood volume-dependent functional MRI

The present study documents the use of an iron oxide-based blood-pool contrast agent in functional magnetic resonance imaging to monitor activity-related changes in cerebral blood volume (CBV) resulting from peripheral sensory stimulation and the application of this technique to generate high-resolution functional maps. Rats, anesthetized with alpha-chloralose, were imaged during electrical stimulation (3 ms, 3 Hz, 3 V) of forelimb or hindlimb. Activation maps were generated by cross-correlation of the measured signal response and a square-wave function representative of the stimulus for each image pixel. Multislice imaging produced functional maps consistent with the known functional anatomy of rat primary somatosensory (S-I) cortex. Imaging with improved temporal resolution demonstrated rapid (<6 s) CBV increases which were sustained and relatively stable (coefficient of variation = 0.17 +/- 0.02) for forelimb stimulation periods of up to 5 min. Enabled by this sustained response we generated high-resolution (approximately 100 micrometer in-plane) functional maps showing discrete forelimb and hindlimb activation. This technique offers many advantages over other methods for the study of brain activity in the rat and has resolution sufficient to be useful in reorganization studies.

Brain capillary perfusion during sleep

Brain capillary perfusion was evaluated in the different states of the wake-sleep cycle-quiet wakefulness (QW), quiet sleep (QS), and active sleep (AS)-in rats. The extent of the perfused capillary network was determined by intravascular distribution of a fluorescent marker. Evans blue (EB); it remained unchanged across the three behavioral conditions, QW, QS, and AS. The anatomical network was assessed by alkaline phosphatase (AP) endothelial staining, which is known to underestimate the number of existing capillaries. The resulting number of AP profiles were, therefore, significantly lower than the number of EB profiles, but the percentage of AP-stained capillaries that were perfused (96%) was also unchanged across the behavioral conditions. The results indicate that no capillary recruitment accompanies the wake-sleep cycle. Capillary surface area is a relevant factor in determining exchanges across the blood-brain barrier. In the absence of capillary recruitment (relative constancy of the surface area), the CBF changes during sleep should preferentially affect flow-limited with respect to diffusion-limited transport.

Induced response to hypercapnia in the two-compartment total cerebral blood volume: influence on brain vascular reserve and flow efficiency

This study was undertaken to investigate the mechanisms of CBF increase as induced by hypercapnia. It was achieved in anesthetized rats by determining total cerebral blood volume (TCBV), parenchymal blood (CBV), plasma (CPV), erythrocyte (CEV) volumes and cerebral hematocrit (CHct) as well as CBF at about 40, 60, and 80 mm Hg PaCO2. TCBV was measured by a noninvasive blood dilution method using [99mTc]pertechnetate. CBV, CPV, and CEV were measured on isolated brain by 125I-serum albumin and 51Cr-erythrocytes. CBF was measured by both [131I/14C]iodoantipyrine and 57Co-microsphere extractions. The extraparenchymal blood volume (ECBV) was evaluated by subtracting CBV from TCBV. Under normocapnia, ECBV was 2.8 times larger than CBV. Under moderate hypercapnia, ECBV increased by 44%, CBV was not modified, and CBF increased by 52%. These results demonstrate that the main site of vasodilation is located in the extraparenchymal vasculature, which thus acts as a vascular reserve. By contrast, under severe hypercapnia, ECBV remained unchanged, whereas CBV then increased by 17%; CBF simultaneously showed an additional augmentation of either 52 or 309% when diffusible tracer or microspheres were used. This important increase in CBF cannot be explained either by capillary recruitment of closed capillaries or by active diameter lengthening of already open capillaries. The concomitant and great increase in capillary blood velocity was also shown to reduce cerebral flow efficiency, a situation consistent with a "luxury perfusion."

Capillary perfusion of the rat brain cortex. An in vivo confocal microscopy study

Confocal laser-scanning microscopy was used to visualize subsurface cerebral microvessels labeled with intravascular fluorescein in a closed cranial window model of the anesthetized rat. In noninvasive optical sections up to 250 microns beneath the brain surface, plasma perfusion and blood cell perfusion of individual capillaries were studied. Under resting conditions, in all cerebral capillaries the presence of plasma flow as demonstrated by the appearance of an intravenously injected fluorescent tracer within 20 seconds after injection. Plasma flow was verified even in capillaries that contained stationary erythrocytes or leukocytes; 91.1% of the capillaries contained flowing blood cells, 5.2% contained stationary blood cells, and no blood cells were seen in 3.6%. Mean blood cell velocity was 498.3 +/- 443.9 microns/s, and the mean blood cell supply rate was 35.75 +/- 28.01 cells per second. When capillaries were continuously observed for 1 minute, "on" and "off" periods of blood cell flow were noted. During hypercapnia (increase of PCO2 from 33.25 to 50.26 mm Hg), mean blood cell flux increased from 38.6 +/- 17.2 to 55.5 +/- 12.2 per second (P < .005, paired t test of mean values in six animals), and blood cell velocity increased from 519.5 +/- 254.8 to 828.5 +/- 460.8 microns/s (P = .074, paired t test of mean values in six animals). Homogeneity of blood cell flux increased as indicated by the coefficient of variation decreasing from 44.6% to 22.0%, and the portion of poorly perfused capillaries (blood cell flux, < 40 per second) decreased from 59.2% to 22.4%.(ABSTRACT TRUNCATED AT 250 WORDS)

Absent recruitment of capillaries in brain tissue recovering from stroke

The density of perfused capillaries (dCAP), defined as capillaries that transport glucose, as well as the volume fraction of these capillaries in the vascular bed (fCAP), and the mean transit time of blood through the capillaries (tCAP), were calculated from hemodynamic variables obtained in vivo by positron tomography of brains of six patients affected by stroke. Each patient was studied twice, within 38 hrs of the insult, and one week later. 38 ischemic and 38 contralateral mirror regions were compared. The metabolic rate for glucose (CMRglc) was determined on the basis of regional calculations of the lumped constant. No significant change of the lumped constant was observed in any region. In normal regions, no significant differences of any variables existed between the first and second studies. In the infarct regions of the first study, CMRglc and CMRO2 (cerebral metabolic rate for oxygen) were 30-50% of control (deactivation) and CBF (cerebral blood flow), capillary density, and the capillary diffusion capacity for fluorodeoxyglucose (K1) were similarly reduced, although the oxygen/glucose ratio was only 3.75 in the ischemic regions. While fCAP decreased, tCAP doubled. One week after the first study, blood flow returned to normal in the infarct regions despite continued depression of metabolism. Capillary density and diffusion capacity remained low, indicating absent recruitment of nutrition vessels (perfusion capillaries).

Effect of amphetamine on cerebral blood flow and capillary perfusion

The purpose of this study was to determine the cerebral regional microvascular and vascular responses to amphetamine sulfate at a dose (5 mg/kg) known to affect neuronal function. Cerebral blood flow (14C-iodoantipyrine method) and percent of perfused capillaries (fluorescein isothiocyanate-dextran and alkaline phosphatase staining method) were determined during control and after intravenous administration of amphetamine in conscious Long-Evans rats. Amphetamine caused an increase in blood pressure (34%) and heart rate (31%). There was a significant increase in averaged cerebral blood flow from 98 +/- 8 to 166 +/- 9 ml/min/100 g after amphetamine. This flow increase was significant in the cortex, basal ganglia, pons and medulla, however the increase was not significant in the hypothalamus. In control rats, there were approximately 325 +/- 17 capillaries/mm2 of brain tissue and 52 +/- 1% of them were perfused. Amphetamine increased the percent perfused significantly to 72 +/- 1% in all examined regions. There was a similar significant increase in the percent of perfused cerebral capillary volume fraction. There were both vascular and microvascular responses to amphetamine, increasing cerebral blood flow as well as reducing the diffusion distance for oxygen.

Pathophysiology of the human brain after stroke, monitored by positron emission tomography

Six stroke patients had positron tomograms both in the acute stage of the cerebrovascular accident (16 to 38 h after onset), and one week later. In and around the infarct, the studies revealed a wide range of metabolic states. In the healthy regions of the brain, all measured physiological variables, including the density of capillaries that transported glucose, blood flow, and oxygen and glucose metabolism, changed in parallel (recruitment). In the regions suffering the consequences of stroke, in the second study, the physiological couple between capillary density, metabolism, and flow was significantly impaired, and the impairment was proportional to the severity of ischemia in the first study. The research report of these findings appeared in the Journal of Cerebral Blood Flow and Metabolism (Gjedde et al. 1990).

Cortical functional architecture and local coupling between neuronal activity and the microcirculation revealed by in vivo high-resolution optical imaging of intrinsic signals

We have shown previously the existence of small, activity-dependent changes in intrinsic optical properties of cortex that are useful for optical imaging of cortical functional architecture. In this study we introduce a higher resolution optical imaging system that offers spatial and temporal resolution exceeding that achieved by most alternative imaging techniques for imaging cortical functional architecture or for monitoring local changes in cerebral blood volume or oxygen saturation. In addition, we investigated the mechanisms responsible for the activity-dependent intrinsic signals evoked by sensory stimuli, and studied their origins and wavelength dependence. These studies enabled high-resolution visualization of cortical functional architecture at wavelengths ranging from 480 to 940 nm. With the use of near-infrared illumination it was possible to image cortical functional architecture through the intact dura or even through a thinned skull. In addition, the same imaging technique proved useful for imaging and discriminating sensory-evoked, activity-dependent changes in local blood volume and oxygen saturation (oxygen delivery). Illumination at 570 nm allowed imaging of activity-dependent blood volume increases, whereas at 600-630 nm, the predominant signal probably originated from activity-dependent oxygen delivery from capillaries. The onset of oxygen delivery started prior to the blood volume increase. Thus, optical imaging based on intrinsic signals is a minimally invasive procedure for monitoring short- and long-term changes in cerebral activity.

Reduction of functional capillary density in human brain after stroke

The blood flow of brain tissue often returns to normal after an ischemic episode. As "luxury" rather than "reactive" reperfusion, this hyperemia is associated with low metabolism. It is not known to what extent the high blood flow accompanies a high, normal, or low density of capillaries. The resolution of this question may indicate whether the functional capillary density is variable and, if so, whether it is coupled to blood flow or metabolism. To answer these questions, we defined functional capillaries as capillaries that transport glucose. We then calculated the density of functional capillaries (Dcap) and the mean time of transit of blood through the capillaries (tcap) from hemodynamic variables obtained in vivo by positron tomography of five patients afflicted by cerebral ischemic stroke. Each patient was studied twice, within 36 h of the insult and 1 week later. We identified nominally "ischemic" regions in the first study as cortical gray matter regions, contiguous with the ischemic focus, in which the magnitude of blood flow did not exceed 20 ml 100 g-1 min-1. In these regions, values of metabolism and functional capillary density were proportionately low compared with normal values obtained in the contralateral hemisphere. The studies revealed a reduction of the functional density of exchange vessels in postischemic brain tissue as soon as 36 h after the insult. In "ischemic" regions, within 36 h of the insult, the net extraction of oxygen was inversely related to the capillary transit time and appeared to be limited mainly by the low functional density of the capillaries. Contrary to expectations, the reduced density persisted, even when more than adequate perfusion of the tissue returned. For these reasons, we concluded that changes of the capillary density were associated with changes of the metabolism of the tissue rather than with blood flow.

Congruence of total and perfused capillary network in rat brains

In awake normocapnic rats, the density of the total and of the perfused capillary network was determined in 10 brain areas. The density of perfused capillaries was measured by using fluorescein isothiocyanate (FITC) globulin or Evans blue as intravenous marker and by fluorescent microscopy. The density of morphologically existing capillaries was determined according to either the histochemical alkaline phosphatase method or a newly developed immunohistochemical fluorescent method that allows marking of the capillary wall constituent fibronectin with a primary antibody directed against fibronectin. This antibody is made visible by a second FITC-coupled antibody (indirect immunofluorescence). Comparison of perfused and existing capillary counts revealed high congruence when fluorescent results were compared. In contrast, the alkaline phosphatase technique yielded capillary counts that were consistently 30% lower than the fibronectin and the FITC globulin counts. The identity of the perfused and the morphologically existing capillary network could be confirmed by a newly developed double-staining technique. First, the perfused capillaries were quantified by intravascular Evans blue. Then, the existing capillaries were relocated in the same measuring field by the fibronectin technique. Such double staining resulted in identical capillary counts in 97% of all cases. The following conclusions have been reached: 1) Fluorescent methods show a perfusion of virtually all capillaries in the brain of the awake normocapnic rat. 2) The alkaline phosphatase technique appears to underestimate the capillary density in the rat brain.

Brain perfusion in acute and chronic hyperglycemia in rats

Recent studies show that acute and chronic hyperglycemia cause a diffuse decrease in regional cerebral blood flow and that chronic hyperglycemia decreases the brain L-glucose space. Since these changes can be caused by a decreased density of perfused brain capillaries, we used 30 adult male Wistar rats to study the effect of acute and chronic hyperglycemia on 1) the brain intravascular space using radioiodinated albumin, 2) the anatomic density of brain capillaries using alkaline phosphatase histochemistry, and 3) the fraction of brain capillaries that are perfused using the fluorescein isothiocyanate-dextran method. Our results indicate that acute and chronic hyperglycemia do not affect the brain intravascular space nor the anatomic density of brain capillaries. Also, there were no differences in capillary recruitment among normoglycemic, acutely hyperglycemic, and chronically hyperglycemic rats. These results suggest that the shrinkage of the brain L-glucose space in chronic hyperglycemia is more likely due to changes in the blood-brain barrier permeability to L-glucose.

Lack of capillary recruitment in the brains of awake rats during hypercapnia

The present study investigates the question of whether increases in CBF induced by hypercapnia in awake rats are accompanied by increases in the number of perfused capillaries. For the detection of perfused capillaries, gamma-globulin-coupled fluorescein isothiocyanate was injected intravenously. In 10 brain structures the density of perfused capillaries per square millimeter was determined from coronal sections using a highly sensitive fluorescent microscopical method that, in contrast to others, avoided air drying of the frozen brain sections. The results showed an inhomogeneous local distribution of the density of perfused capillaries during normo- and hypercapnia. The density of perfused capillaries was unchanged during hypercapnia compared with normocapnia, although blood flow was markedly increased. It is concluded that a capillary recruitment does not exist in the brain during the high-flow situation of hypercapnia.

Kinetics and distribution volumes for tracers of different sizes in the brain plasma space

Regional brain and plasma concentrations were determined for a series of radiotracers that differ in molecular weight and size in pentobarbital-anesthetized rats at 1, 5 and 30 min after i.v. injection. The tracers, [3H]inulin (mol. wt. 5000 Da, radius 1.5 nm), 5 [3H]dextrans (10,000-200,000 Da, 2.3-9.5 nm) and [51Cr]transferrin (79,000 Da, 3.8 nm), are not taken up into erythrocytes and do not measurably cross the blood-brain barrier in 30 min. Results were expressed as a brain distribution volume, defined as (dpm/g brain)/(dpm/ml plasma). Within 1 min after injection, all tracers attained an initial distribution volume which varied regionally from 0.4 to 1.6 X 10(-2) ml/g. The volumes remained constant between 1 and 30 min for tracers with radii greater than or equal to 3.8 nm, whereas the volumes increased up to 90% for tracers with radii less than or equal to 3.1 nm. Rates of equilibration for tracers with radii less than or equal to 3.1 nm were size dependent with smaller tracers equilibrating before larger tracers. These results indicate that the brain distribution volume for plasma tracers consists of two compartments: one which is quickly filled (less than or equal to 1 min) by all tracers and comprises approximately 60% of the total volume, and one which allows only tracers with radii less than or equal to 3.1 nm and comprises 40% of the total volume. The inverse relation between the rate of equilibration in the second compartment and molecular size may indicate a diffusion limitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of prazosin on microvascular perfusion during middle cerebral artery ligation in the rat

The purpose of this study was to evaluate the effects of prazosin, an alpha 1-adrenoceptor antagonist, on morphometric indexes of the total and perfused cerebral microvascular bed 1 hour after middle cerebral artery (MCA) ligation in pentobarbital-anesthetized rats. We hypothesized that this agent would prevent catecholamine-induced vasoconstriction in the ischemic brain. Cerebral blood flow (CBF) was determined with 14C-iodoantipyrine, and the perfused microvascular bed was visualized using fluorescein isothiocyanate-dextran. MCA occlusion did not alter systemic hemodynamic or blood gas parameters. CBF averaged 29 +/- 15 (mean +/- SD) ml/min/100 g in the MCA-ligated cortex and 49 +/- 18 in the other examined brain regions. Prazosin did not significantly alter these CBF values, averaging 26 +/- 14 and 48 +/- 10, respectively. There were no significant regional differences in total capillaries/mm2 in either group. The percent of the capillaries/mm2 perfused (51 +/- 6%) was similar in the two groups in all examined regions except the ischemic cortex. In the MCA-ligated cortex, 22 +/- 8% of the capillary volume was perfused in comparison with 49 +/- 8% in the prazosin-treated group. Prazosin-treated rats had an increased percentage of their microvasculature perfused despite a similarly reduced CBF. Prazosin appeared to reduce diffusion distances in the ischemic cortex. This might be due to its alpha 1-adrenoceptor blocking activity.

Determination of rat cerebral cortical blood volume changes by capillary mean transit time analysis during hypoxia, hypercapnia and hyperventilation

Changes in cerebral blood volume due to augmented or diminished numbers of blood-perfused capillaries can be studied in small animals by optical methods. Capillary mean transit time was determined by detection of the passage of a hemodilution bolus through a region of the parietal cerebral cortical surface, using a reflectance spectrophotometer through a small craniotomy in chloral hydrate-anesthetized rats. Local cerebral blood flow was determined in the same region by the butanol indicator-fractionation method. Blood volume was calculated from the product of blood flow and transit time. Normoxic, normocapnic values for these variables were blood flow = 144 ml/100 g/min; mean transit time = 1.41 s; and blood volume = 3.4 ml/100 g. Mean transit time reached a minimum (1.1 s) with moderate hypoxia or hypercapnia. Combined hypoxia and hypercapnia did not result in any further decrease in mean transit time although blood flow was much higher than either hypoxia or hypercapnia alone. The maximum blood volume recorded during hypercapnic hypoxia (12.1 ml/100 g) was 3.6 times greater than that at normoxic normocapnia, which suggests that under control conditions in the anesthetized rat considerably less than 100% of the cerebral capillaries were actively perfusing the tissue. These studies demonstrate that optical methods can be used to quantitatively measure blood volume. The data suggest that capillary recruitment is a physiologically significant phenomenon in rat cerebral cortex.

Perfused capillary morphometry in the senescent brain

This study compared quantitative indices of capillary morphometry in the perfused vs. total capillary bed of conscious young (8-10 month old) and senescent (28-33 month old) rat brains. The total capillary bed was identified through alkaline phosphatase staining of tissue sections cut from specific brain regions. The perfused capillary bed was identified by the presence of fluorescein isothiocyanate (FITC) dextran in the microvessels. Average capillary volume fraction, VV (mm3/mm3, mean +/- S.E.M.) was 0.040 +/- 0.002 and 0.033 +/- 0.001 in the total capillary bed of the young and old animals, respectively. These values were not statistically different. Perfused VV averaged 0.020 +/- 0.001 and 0.017 +/- 0.001 mm3/mm3 in the young and old animals, respectively. Average perfused VV was 50% in the young and 49% in the senescent rat brains. Young and senescent brains utilize similar proportions of their "capillary reserves." In the areas examined in the brains of young animals, differences were found in the percentage of capillary volume/mm3, VV, and surface area/mm3, SV, perfused which were not present in old animals. The structural and neurochemical changes noted by others in the brain during aging were not related to alterations in indices of average and regional total or perfused cerebral capillary bed morphometry. However, differences in percent perfused VV, SV, length, LV, and number, Na, present in the younger rat brains were not present in the senescent rat brains.

Regional studies of blood-brain barrier transport of glucose and leucine in awake and anesthetized rats

D-Glucose and L-leucine are transported across the blood-brain barrier (BBB) by two separate carrier-mediated facilitated diffusion mechanisms. In the awake rat there are regional differences in blood-to-brain glucose transport among the cerebral cortex, cerebellum, hippocampus, and striatum. To determine whether these are due to variations in the regional density or affinity of the glucose transporter moiety of brain capillaries or are secondary to regional tissue perfusion and capillary arrangement characteristics, we studied regional blood-to-brain transport of L-leucine in awake rats; regional blood-to-brain transport of both glucose and leucine under chloral hydrate anesthesia, a condition associated with altered regional brain blood flow (BF) and metabolism; and regional brain vascular volume, derived from the L-glucose and insulin spaces, in both awake and anesthetized rats. We found the same regional differences in blood-to-brain leucine transport in awake rats as we previously described for D-glucose transport. These regional differences in glucose and leucine transport disappear under chloral hydrate anesthesia, as regional differences in BF are abolished. However, we found regional differences in the brain vascular volumes, which are evident in wakefulness and persist during anesthesia. These results suggest that the regional differences in blood-to-brain transport are due mainly to local tissue perfusion and capillary arrangement characteristics rather than to intrinsic regional differences in the transport systems of the BBB.