Combining laser Doppler flowmetry with microdialysis: a novel approach to investigate the coupling of regional cerebral blood flow to neuronal activity

A comprehensive overview of diffuse correlation spectroscopy: theoretical framework, recent advances in hardware, analysis, and applications

Diffuse correlation spectroscopy (DCS) is a powerful tool for assessing microvascular hemodynamic in deep tissues. Recent advances in sensors, lasers, and deep learning have further boosted the development of new DCS methods. However, newcomers might feel overwhelmed, not only by the already-complex DCS theoretical framework but also by the broad range of component options and system architectures. To facilitate new entry to this exciting field, we present a comprehensive review of DCS hardware architectures (continuous-wave, frequency-domain, and time-domain) and summarize corresponding theoretical models. Further, we discuss new applications of highly integrated silicon single-photon avalanche diode (SPAD) sensors in DCS, compare SPADs with existing sensors, and review other components (lasers, sensors, and correlators), as well as data analysis tools, including deep learning. Potential applications in medical diagnosis are discussed and an outlook for the future directions is provided, to offer effective guidance to embark on DCS research.

NMDA attenuates the neurovascular response to hypercapnia in the neonatal cerebral cortex

Cortical spreading depolarization (SD) involves activation of NMDA receptors and elicit neurovascular unit dysfunction. NMDA cannot trigger SD in newborns, thus its effect on neurovascular function is not confounded by other aspects of SD. The present study investigated if NMDA affected hypercapnia-induced microvascular and electrophysiological responses in the cerebral cortex of newborn pigs. Anesthetized piglets were fitted with cranial windows over the parietal cortex to study hemodynamic and electrophysiological responses to graded hypercapnia before/after topically applied NMDA assessed with laser-speckle contrast imaging and recording of local field potentials (LFP)/neuronal firing, respectively. NMDA increased cortical blood flow (CoBF), suppressed LFP power in most frequency bands but evoked a 2.5 Hz δ oscillation. The CoBF response to hypercapnia was abolished after NMDA and the hypercapnia-induced biphasic changes in δ and θ LFP power were also altered. MK-801 prevented NMDA-induced increases in CoBF and the attenuation of microvascular reactivity to hypercapnia. The neuronal nitric oxide synthase (nNOS) inhibitor (N-(4 S)-4-amino-5-[aminoethyl]aminopentyl-N′-nitroguanidin) also significantly preserved the CoBF response to hypercapnia after NMDA, although it didn’t reduce NMDA-induced increases in CoBF. In conclusion, excess activation of NMDA receptors alone can elicit SD-like neurovascular unit dysfunction involving nNOS activity.

Exercise and cerebrovascular plasticity

Aging impairs cerebrovascular plasticity and subsequently leads cerebral hypoperfusion, which synergistically accelerates aging-associated cognitive dysfunction and neurodegenerative diseases associated with impaired neuronal plasticity. On the other hand, over two decades of researches have successfully demonstrated that exercise, or higher level of physical activity, is a powerful and nonpharmacological approach to improve brain function. Most of the studies have focused on the neuronal aspects and found that exercise triggers improvements in neuronal plasticity, such as neurogenesis; however, exercise can improve cerebrovascular plasticity as well. In this chapter, to understand these beneficial effects of exercise on the cerebral vasculature, we first discuss the issue of changes in cerebral blood flow and its regulation during acute bouts of exercise. Then, how regular exercise improves cerebrovascular plasticity will be discussed. In addition, to shed light on the importance of understanding interactions between the neuron and cerebral vasculature, we describe neuronal activity-driven uptake of circulating IGF-I into the brain.

Hippocampal functional hyperemia mediated by NMDA receptor/NO signaling in rats during mild exercise

Current studies have demonstrated that exercise increases regional cerebral blood flow (rCBF), an index of neuronal activity. However, neuronal regulation of the increased rCBF in the brain parenchyma is poorly understood. We developed a running model with rats for monitoring hippocampal cerebral blood flow (Hip-CBF) and found that mild treadmill running increases Hip-CBF in a tetrodotoxin-dependent manner, suggesting that functional hyperemia, an increase in rCBF in response to neuronal activation, occurs in the running rat's hippocampus (Nishijima T and Soya H. Neurosci Res 54: 186–191, 2006). To further support our hypothesis, it was important to discover the neurogenic pathways behind the increase in Hip-CBF that occurred during running. Here, we examine the possible role of N-methyl-d-aspartate (NMDA) receptor/nitric oxide (NO) signaling and group I metabotropic glutamate receptors in mediating the Hip-CBF increase. Hip-CBF during running was measured by laser-Doppler flowmetry. Intrahippocampal drug administration was performed by microdialysis. Mild treadmill running (10 m/min) increased Hip-CBF, which was remarkably attenuated by either NMDA receptor antagonists (1 mM MK-801) or NO synthase inhibitors (2 mM NG-nitro-l-arginine methyl ester). However, group I metabotropic glutamate receptor antagonists {1 mM 7-(hydroxyimino)cyclopropa[ b]chromen-1a-carboxylate ethyl ester + 1 mM 2-methyl-6-(phenylethynyl)pyridine hydrochloride} augmented the running-induced Hip-CBF increase. We also found that rCBF in the olfactory bulb was unchanged with running. These results strongly suggest that Hip-CBF during mild exercise is regulated locally under hippocampal neuronal activity, mediated mainly through NMDA receptor/NO signaling. Collectively, these results, together with our previous findings, support our hypothesis that mild exercise elicits neuronal activation, which then triggers functional hyperemia in the rat hippocampus.

Mechanisms involved in the cerebrovascular dilator effects of N-methyl-d-aspartate in cerebral cortex

Glutamate and its synthetic analogues N-methyl-d-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) are potent dilator agents in the cerebral circulation. The close linkage between neural activity-based release and actions of glutamate on neurons and the related decrease in cerebral vascular resistance is a classic example in support of the concept of tight coupling between increased neural activity and cerebral blood flow. However, mechanisms involved in promoting cerebral vasodilator responses to glutamatergic agents are controversial. Here we review the development and current status of this important field of research especially in respect to cerebrovascular responses to NMDA receptor activation.

Evidence of functional hyperemia in the rat hippocampus during mild treadmill running

Exercise appears to improve hippocampal plasticity and cognitive function, leading us to postulate that exercise may activate hippocampal neurons, which in turn increase hippocampal cerebral blood flow (Hip-CBF). Recent studies using laser-Doppler flowmetry (LDF) have shown that Hip-CBF increases with behavior and locomotion, but it has not been examined whether these changes are due to neuronal activation. We aimed to examine whether functional hyperemia, an increase in cerebral blood flow in response to neuronal activation, can occur in the exercising rat hippocampus. We applied a treadmill running model of rats and LDF combined with microdialysis. Prolonged mild treadmill running (10 m/min) resulted in an increase in Hip-CBF of 26+/-9% versus basal level. When tetrodotoxin was infused via microdialysis into the loci where Hip-CBF was monitored, the increased Hip-CBF with running was completely suppressed. These results provide evidence that functional hyperemia occurs in the rat hippocampus during mild treadmill running and suggest that our running animal model may be useful for examining the underlying mechanisms of exercise-induced hippocampal functional hyperemia.

Findings of brain 99mTc-ECD SPECT in high-functioning autism-3-dimensional stereotactic ROI template analysis of brain SPECT-

The aim of this study is confirmation of an abnormal regional cerebral blood flow (rCBF) pattern in high-functioning autism (HFA). Confirmation of an abnormal rCBF pattern in HFA may be useful for elucidate of its pathophysiology and a differential diagnosis, such as with attention-deficit/hyperactivity disorder (AD/HD). Brain 99mTc-ECD SPECT was performed in 16 cases of HFA. The HFA group consisted of 16 cases of HFA. They were all male, with an IQ of 76-126. They had normal brain MRI findings, and had an age of 9-14 years. We examined abnormal rCBF in HFA by comparing the results to those in the control group. The control group consisted of 1 male and 4 females cryptogenic epilepsy patients with normal intelligence. They have no problems in learning at school or mental or behavioral traits. They had normal brain MRI or SPECT findings, and had an age of 7-15 years. 3-dimensional stereotactic ROI template (3DSRT) was used to analyze SPECT data. We calculated the 'relative rCBF (%)' (RI count of each segment x 100/Sum of RI count of the corresponding hemisphere), and compared the values between the two groups. We found a significantly low 'relative rCBF (%)' in the left temporal region in the HFA group. We also calculated the 'L/R ratio' (the 'relative rCBF (%)' of a segment on the left side / the 'relative rCBF (%)' of the corresponding segment on the right side), and compared the value for each segment between the two groups. There were no significant differences in any segments between the two groups. We also checked for differences in the 'relative rCBF (%)' between segments on the right side and corresponding segments on the left side in both the HFA and control groups. We found significant right < left perfusion in the angular region and significant left < right perfusion in the pericallosal, thalamus, and hippocampus region in the HFA group. We also found significant right < left perfusion in the temporal region in the control group. Significant hypoperfusion in the left temporal region due to an unidentified underlying brain pathology and abnormal laterality in the angular, temporal (lack of right < left perfusion), pericallosal, thalamus, and hippocampus regions may influence the symptoms of autism.

Hemodynamics under Hippocampal Functional Hyperemia in Anesthetized Rat: A Greater Contribution of Red Blood Cell Velocity Compared to Its Concentration

It remains controversial which of the two regulators, red blood cell velocity (RBC-V) or concentration (RBC-C), is a main contributor to increasing flow (RBC-F) during functional hyperemia in the rat hippocampus induced by N-methyl-D-aspartate (NMDA). To address this, we monitored these parameters simultaneously under NMDA-infusion via microdialysis in the hippocampus of urethane-anesthetized rats and found a greater elevation in RBC-V than in RBC-C. This suggests that an RBC-V-dependent increase in RBC-F occurs under NMDA-induced functional hyperemia in the hippocampus as well as in the cortex.

Miniaturization: an overview of biotechnologies for monitoring the physiology and pathophysiology of rodent animal models

Recent advances in bioengineering technologies have made it possible to collect high-quality reproducible data quantitatively in a wide range of laboratory animal species, including rodents. Several of these technologies are incorporated into a plan called Miniaturization, which aims to design, develop, and maintain rodent animal models to study the pathophysiology and therapy of human diseases. Laser Doppler flowmetry, digital sonomicrometry, bioelectrical impedance, and microdialysis are some of the most widely used methods under the plan because they cause minimal pain and distress, reduce the number of animals used in biomedical research, and allow chronic, nonterminal assessment of physiological parameters in rodents. An overview of each of these technologies and their major applications in rodents used for biomedical research is provided.

N-methyl-D-aspartate-induced vasodilation is mediated by endothelium-independent nitric oxide release in piglets

N-methyl-D-aspartate (NMDA) elicits pial arteriolar dilation that has been associated with neuronal nitric oxide (NO) production. However, endothelial factors or glial P-450 epoxygenase products may play a role. We tested whether NMDA-induced pial vasodilation 1) primarily involves NO diffusion from the parenchyma to the surface arterioles, 2) involves intact endothelial function, and 3) involves a miconazole-sensitive component. Arteriolar diameters were determined using closed cranial window-intravital microscopy in anesthetized piglets. NMDA (10-100 microM) elicited virtually identical dose-dependent dilations in paired arterioles (r = 0.94, n = 15). However, NMDA- but not bradykinin (BK)-induced dilations of arteriolar sections over large veins were reduced by 31 +/- 1% (means +/- SE, P < 0.05, n = 4) compared with adjacent sections on the cortical surface. Also, 100 microM NMDA increased cerebrospinal fluid levels of NO metabolites from 3.7 +/- 1.0 to 5.3 +/- 1.2 microM (P < 0.05, n = 6). Endothelial stunning by intracarotid injection of phorbol 12,13-dibutyrate did not affect NMDA-induced vasodilation but attenuated vascular responses to hypercapnia and BK by approximately 70% (n = 7). Finally, miconazole (n = 6, 20 microM) pretreatment and coapplication with NMDA did not alter vascular responses to NMDA. In conclusion, NMDA appears to dilate pial arterioles exclusively through release and diffusion of NO from neurons to the pial surface in piglets.

Uncoupling of blood flow and metabolism in focal epilepsy

PURPOSE

Interictal measurements of cerebral blood flow are less helpful in localizing epileptic foci than are measurements of brain metabolism. This may be related to an uncoupling of blood flow and metabolism. In this study, brain metabolism and blood flow were compared in an acute experimental model of focal interictal epilepsy.

METHODS

Interictal epileptic foci were induced by an epicortical application of penicillin in rats. After 1 h, stereotyped interictal activity was initiated, lasting until the end of the experiment. Brain metabolism was determined with [14C]deoxyglucose, and cerebral blood flow with [14C]iodoan-tipyrine autoradiography.

RESULTS

In control experiments, metabolism and blood flow were coupled. In animals with focal interictal epileptic activity, the metabolism was strongly increased in the focus and reduced in areas lateral to the focus. In contralateral brain areas, blood flow and metabolism varied in a parallel fashion. Ipsilateral to the focus, however, blood flow and metabolism were altered disproportionately. In the focus, the increase of blood flow was less marked than the increase of metabolism, and the area with increased blood flow was larger than the area with increased metabolism. Lateral to the focus, in the area with a hypometabolism, blood flow was not concomitantly reduced.

CONCLUSIONS

The experiments show that blood flow and metabolism in focal epilepsy may be uncoupled in widespread regions. This is due neither to structural abnormalities nor to the duration or discharge pattern of epileptic activity. The results explain why interictal metabolic investigations have a higher predictive value in presurgical epilepsy evaluation than do interictal measurements of blood flow.

Inhibition of poly(ADP-ribose) polymerase: reduction of ischemic injury and attenuation of N-methyl-D-aspartate-induced neurotransmitter dysregulation

BACKGROUND AND PURPOSE

The nuclear enzyme poly(ADP-ribose) polymerase (PARP) may play a role in DNA repair. However, in cerebral ischemia, excessive PARP activation may lead to energy depletion and exacerbation of neuronal damage. We examined the effect of inhibiting PARP on (1) the degree of cerebral injury in a rat model of transient focal ischemia and (2) the degree of neurotransmitter dysregulation induced by local cortical perfusion of N-methyl-D-aspartate (NMDA).

METHODS

In experiment 1, rats were subjected to transient ischemia for 90 minutes by occlusion of the middle cerebral artery. After 22.5 hours of reperfusion, lesions were quantified by tetrazolium staining. Untreated rats were compared with those treated with the PARP inhibitor 3-aminobenzamide (10 mg/kg). In experiment 2, rats were implanted with microdialysis probes in the cortex, and 1 mmol/L NMDA was perfused for 2 hours. Extracellular concentrations of neurotransmitter and neuromodulator amino acids were measured. Untreated rats were compared with those given 10 mg/kg 3-aminobenzamide.

RESULTS

In experiment 1, PARP inhibition significantly reduced lesion volumes: 204+/-43 mm3 (untreated) versus 90+/-24 mm3 (treated). Neuroprotection was primarily manifested in the cortex. In experiment 2, NMDA perfusion resulted in large elevations of glutamate, taurine, and the lipid component phosphoethanolamine. Levels of the NMDA site modulator D-serine were reduced, and glycine levels appeared unchanged. 3-Aminobenzamide significantly attenuated the elevations in glutamate and phosphoethanolamine but had no effects on D-serine and glycine.

CONCLUSIONS

Inhibition of PARP reduced injury after transient focal ischemia in rats and attenuated NMDA-induced glutamate efflux and overall neurotransmitter dysregulation. The deleterious effects of excessive PARP activation may be related in part to amplification of excitotoxicity, possibly by cellular energy depletion and additional transmitter release and/or reduced reuptake.

Accumulation of cytotoxins during the development of seizures and edema after hypoxic-ischemic injury in late gestation fetal sheep

Several hours after an hypoxic-ischemic injury to the developing brain, hyperemia, then seizures, edema, and infarction can develop. The roles of nitric oxide (NO) synthesis and excitotoxin accumulation during these later phases of injury are not known. The time course of extracellular levels of amino acids within the parasagittal parietal cortex were measured with microdialysis during and for 3 d after 30 min of cerebral ischemia in nine chronically instrumented near-term fetal sheep (119-133 d). Cortical electroencephalographic (EEG) activity and extracellular space (ECS) were quantified simultaneously with real-time spectral analysis and cortical impedance measurements, respectively. Amino acid concentrations were measured using HPLC. During ischemia, citrulline (by-product of NO synthesis), glutamate, glycine, and gamma-aminobutyric acid (GABA) concentrations rose to 147 +/- 18%, 180 +/- 20%, 290 +/- 50% and 4800 +/- 1300% of baseline respectively (p < 0.05). The excitotoxic index ([glutamate] x [glycine]/[GABA]) decreased to 15 +/- 8%. Upon reperfusion, the cytotoxic edema and amino acid accumulation largely resolved within 1 h, and the EEG was depressed. Citrulline began to rise again by 4 h (p < 0.05), reaching a maximum (273 +/- 21%) at 32 +/- 2 h. Seizure activity developed at 7 +/- 2 h, and impedance plus the excitotoxic index then rose progressively and peaked at 32 +/- 2 h (480 +/- 170%). At 72 h, there was severe neuronal loss and laminar necrosis within the parasagittal cortex. These data suggest that, several hours after a severe hypoxicischemic injury, NO synthesis increased, then seizures arose, and edema developed concomitantly with the accumulation of excitotoxins.