John J. A. Marota

Evidence of a Cerebrovascular Postarteriole Windkessel With Delayed Compliance

A pronounced temporal mismatch was observed between the responses of relative cerebral blood volume (rCBV) measured by magnetic resonance imaging and relative cerebral blood flow measured by laser—Doppler flowmetry in rat somatosensory cortex after electrical forepaw stimulation, The increase of relative cerebral blood flow after stimulus onset and decrease after stimulus cessation were accurately described with a single exponential time constant of 2.4 ± 0.8 seconds. In contrast, rCBV exhibited two distinct and nearly sequential processes after both onset and cessation of stimulation. A rapid change of rCBV (1.5 ± 0.8 seconds) occurring immediately after onset and cessation was not statistically different from the time constant for relative cerebral blood flow. However, a slow phase of increase (onset) and decrease (cessation) with an exponential time constant of 14 ± 13 seconds began approximately 8 seconds after the rapid phase of CBV change. A modified windkessel model was developed to describe the temporal evolution of rCBV as a rapid elastic response of capillaries and veins followed by slow venous relaxation of stress. Venous delayed compliance was suggested as the mechanism for the poststimulus undershoot in blood oxygen-sensitive magnetic resonance imaging signal that has been observed in this animal model and in human data.

The Accuracy of Near Infrared Spectroscopy and Imaging during Focal Changes in Cerebral Hemodynamics

Near infrared spectroscopy (NIRS) can detect changes in the concentrations of oxy-hemoglobin ([HbO]) and deoxy-hemoglobin ([Hb]) in tissue based upon differential absorption at multiple wavelengths. The common analysis of NIRS data uses the modified Beer-Lambert law, which is an empirical formulation that assumes global concentration changes. We used simulations to examine the errors that result when this analysis is applied to focal hemodynamic changes, and we performed simultaneous NIRS measurements during a motor task in adult humans and a neonate to evaluate the dependence of the measured changes on detector-probe geometry. For both simulations and in vivo measurements, the wide range of NIRS results was compared to an imaging analysis, diffuse optical tomography (DOT). The results demonstrate that relative changes in [HbO] and [Hb] cannot, in general, be quantified with NIRS. In contrast to that method, DOT analysis was shown to accurately quantify simulated changes in chromophore concentrations. These results and the general principles suggest that DOT can accurately measure changes in [Hb] and [HbO], but NIRS cannot accurately determine even relative focal changes in these chromophore concentrations. For the standard NIRS analysis to become more accurate for focal changes, it must account for the position of the focal change relative to the source and detector as well as the wavelength dependent optical properties of the medium.

MRI measurement of the temporal evolution of relative CMRO2 during rat forepaw stimulation

This study reports the first measurement of the relative cerebral metabolic rate of oxygen utilization (rCMRO2) during functional brain activation with sufficient temporal resolution to address the dynamics of blood oxygen level‐dependent (BOLD) MRI signal. During rat forepaw stimulation, rCMRO2 was determined in somatosensory cortex at 3‐sec intervals, using a model of BOLD signal and measurements of the change in BOLD transverse relaxation rate, the resting state BOLD transverse relaxation rate, relative cerebral blood flow (rCBF), and relative cerebral blood volume (rCBV). Average percentage changes from 10 to 30 sec after onset of forepaw stimulation for rCBF, rCBV, rCMRO2, and BOLD relaxation rate were 62 ± 16, 17 ± 2, 19 ± 17, and −26 ± 12, respectively. A poststimulus undershoot in BOLD signal was quantitatively attributed to the temporal mismatch between changes in blood flow and volume, and not to the role of oxygen metabolism. Magn Reson Med 42:944–951, 1999.

Cortical depth-specific microvascular dilation underlies laminar differences in blood oxygenation level-dependent functional MRI signal

Changes in neuronal activity are accompanied by the release of vasoactive mediators that cause microscopic dilation and constriction of the cerebral microvasculature and are manifested in macroscopic blood oxygenation level-dependent (BOLD) functional MRI (fMRI) signals. We used two-photon microscopy to measure the diameters of single arterioles and capillaries at different depths within the rat primary somatosensory cortex. These measurements were compared with cortical depth-resolved fMRI signal changes. Our microscopic results demonstrate a spatial gradient of dilation onset and peak times consistent with “upstream” propagation of vasodilation toward the cortical surface along the diving arterioles and “downstream” propagation into local capillary beds. The observed BOLD response exhibited the fastest onset in deep layers, and the “initial dip” was most pronounced in layer I. The present results indicate that both the onset of the BOLD response and the initial dip depend on cortical depth and can be explained, at least in part, by the spatial gradient of delays in microvascular dilation, the fastest response being in the deep layers and the most delayed response in the capillary bed of layer I.

Cocaine Activation Discriminates Dopaminergic Projections by Temporal Response: An fMRI Study in Rat

We applied a sensitive new functional magnetic resonance imaging technique to identify the pattern and determinants of cocaine-induced brain activation in drug-naive rats. At doses greater than 0.1 mg/kg iv, cocaine produced robust activation throughout cortex with the largest magnitude increase in frontal neocortex. Additionally, we detected selective activation within dopamine-innervated subcortical regions including dorsomedial and ventrolateral striatum, nucleus accumbens region, and dorsal thalamus. Although dose response was similar among activated regions, temporal response differentiated regions along distinct anatomical boundaries with basal ganglia and limbic cortical structures, reaching maximum activation later than frontal neocortex. Pharmacological specificity was demonstrated by blocking cocaine-induced activation with SCH-23390, a selective D1 antagonist. Our data demonstrate the utility of fMRI to identify spatiotemporal patterns of cocaine-induced brain activation and implicate D1 dopaminergic mechanisms in acute cocaine action.

Design and evaluation of a continuous-wave diffuse optical tomography system

Diffuse optical tomography (DOT) can image spatial variations in highly scattering optical media. We have built an inexpensive and portable continuous-wave DOT system containing 18 laser diode sources (9 at 780nm and 9 at 830nm) and 16 silicon detectors, which can acquire 288 independent measurements in less than 4 seconds. These data can then be processed using a variety of imaging algorithms. We first discuss the design of diffuse imaging equipment in general, and then describe our instrument, along with the technical issues that influenced its design. The technical challenges involved in performing DOT over large optode areas are discussed. We also present rat brain measurements following electrical forepaw stimulation using DOT. These results clearly demonstrate the capabilities of DOT and set the stage for advancement to quantitative functional brain imaging.

Cerebrovascular Dynamics of Autoregulation and Hypoperfusion An MRI Study of CBF and Changes in Total and Microvascular Cerebral Blood Volume During Hemorrhagic Hypotension

BACKGROUND AND PURPOSE To determine how cerebral blood flow (CBF), total and microvascular cerebral blood volume (CBV), and blood oxygenation level-dependent (BOLD) contrast change during autoregulation and hypotension using hemodynamic MRI. METHODS Using arterial spin labeling and steady-state susceptibility contrast, we measured CBF and changes in both total and microvascular CBV during hemorrhagic hypotension in the rat (n=9). RESULTS We observed CBF autoregulation for mean arterial blood pressure (MABP) between 50 and 140 mm Hg, at which average CBF was 1.27+/-0.44 mL. g(-1). min(-1) (mean+/-SD). During autoregulation, total and microvascular CBV changes were small and not significantly different from CBF changes. Consistent with this, no significant BOLD changes were observed. For MABP between 10 and 40 mm Hg, total CBV in the striatum increased slightly (+7+/-12%, P<0.05) whereas microvascular CBV decreased (-15+/-17%, P<0.01); on the cortical surface, total CBV increases were larger (+21+/-18%, P<0.01) and microvascular CBV was unchanged (3+/-22%, P>0.05). With severe hypotension, both total and microvascular CBV decreased significantly. Over the entire range of graded global hypoperfusion, there were increases in the CBV/CBF ratio. CONCLUSIONS Parenchymal CBV changes are smaller than those of previous reports but are consistent with the small arteriolar fraction of total blood volume. Such measurements allow a framework for understanding effective compensatory vasodilation during autoregulation and volume-flow relationships during hypoperfusion.

Vascular Filters of Functional MRI: Spatial Localization Using BOLD and CBV Contrast

The spatial distributions of functional activation of rat somatosensory cortex by forepaw stimulation were quantitatively compared using blood oxygen level dependent (BOLD) signal and signal weighted by cerebral blood volume (CBV). The BOLD contrast to noise (CNR) distribution showed a significant dorsal shift with respect to the CBV method at fields strengths of 2 T (0.69 ± 0.09 mm) and 4.7 T (0.44 ± 0.15 mm). These shifts were attributed to the gradient of resting state blood volume across somatosensory cortex and the different CNR characteristics of the two image methods. The underlying principles suggest that the CBV method has a more uniform sensitivity to percent changes in functional indicators (blood volume or deoxygenated hemoglobin) across regions of variable resting state CBV. Magn Reson Med 42:591–598, 1999.

Regional sensitivity and coupling of BOLD and CBV changes during stimulation of rat brain

Functional MRI of rat brain was performed at 2 Tesla following intravenous injection of cocaine in order to 1) determine if changes in CBV and changes in BOLD signal were regionally coupled in brain parenchyma, and 2) compare the sensitivities of these imaging methods across different brain structures. Percent changes in CBV and BOLD relaxation rate were spatially and temporally coupled during this graded brain activation. The use of contrast agent increased functional sensitivity in all parenchymal brain structures, with a strong but predictable dependence on the resting‐state blood volume fraction. Magn Reson Med 45:443–447, 2001.

Nitrous Oxide Induces Preemptive Analgesia in the Rat That is Antagonized by Halothane

BackgroundNoxious stimulation-induced sensitization of the central nervous system has been proposed as a key element in the development of subsequent protracted pain. Accordingly, the authors used the formalin model of pain to test the hypothesis that general anesthesia can produce preemptive analgesia and thereby interfere with noxious stimulation-induced central sensitization. MethodsRats received 0.9% or 1.8% halothane, 30% or 75% nitrous oxide (N2O), or 75% N2O plus 0.9% halothane (n = 4 or 5 per group). Control rats (n = 5) received only 100% oxygen. Fifteen minutes after the induction of anesthesia, formalin was injected subcutaneously into a hind paw of each rat, and anesthesia was maintained for 5 more min. Because the behavioral pain response to formalin (i.e., flinching of the injected paw) is biphasic, these treatment groups were anesthetized only during phase 1 (acute phase). Another group (n = 5) received 75% N2O only during phase 2 (delayed phase). Reversibility of the N2O effect was tested by the administration of naloxone before phase 1 or naltrexone during phase 2 (n = 5 per group). Finally, additional rats anesthetized as described above (n = 4 or 5 per group) underwent tail-flick testing during anesthesia. ResultsAll anesthetics reduced phase 1 pain behavior, but only N2O produced antinociception on tail-flick testing. Thirty percent and 75% N2O, administered during phase 1, suppressed phase 2 flinching 29% and 49%, respectively, whereas nitrous oxide administered after phase 1 did not suppress phase 2 pain behavior. This effect of nitrous oxide was reversed by an opioid antagonist given during phase 1 but not phase 2. Halothane administered during phase 1 had no effect on phase 2 flinching, and it antagonized the effect of 75% N2O. ConclusionsNitrous oxide induces dose-dependent preemptive analgesia in this model that is reversed partially by naloxone, thus suggesting the involvement of endogenous opioids in this action. In contrast, halothane has no preemptive analgesic properties and even antagonizes the analgesic effect of nitrous oxide. Hence, the hypnotic potency of an anesthetic is a poor indication of its preemptive analgesic potential.